Publications by authors named "Norbert Hoffmann"

33 Publications

A novel unknown-input and single-output approach to extract vibration patterns via a roving continuous random excitation.

ISA Trans 2021 Dec 20. Epub 2021 Dec 20.

Imperial College London, Mechanical Engineering, Exhibition Road, SW7 2AZ London, United Kingdom.

Operating deflection shape analysis allows investigating the dynamic behaviour of a structure during operation. It normally requires simultaneous, multi-point measurements to capture the response from an unknown excitation source (unknown-input and multiple-output), which can complicate its usage for structures without ease of access. A novel vibration pattern testing method is proposed based on a roving continuous random excitation employing a small robotic Hexbug device and a single-point measurement. The Hexbug introduces a random excitation in consecutive locations while roaming over the structure. The resulting multi-modal, time and location dependent response of the system is captured in a single location, and then analysed with a newly developed method based on empirical wavelet transform, multiscale morphological filtering and optimization to extract the excited vibration patterns. The efficiency of the proposed method is experimentally demonstrated on a free-free and a cantilevered beam with comparison to mode shapes extracted by hammer test. The validation highlights its ability to extract several vibration patterns from a long slender structure with good accuracy and robustness, with the general ability to expand the usability of an operating deflecting shape analysis.
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http://dx.doi.org/10.1016/j.isatra.2021.12.011DOI Listing
December 2021

Enantioselective synthesis of heterocyclic compounds using photochemical reactions.

Authors:
Norbert Hoffmann

Photochem Photobiol Sci 2021 Dec 25;20(12):1657-1674. Epub 2021 Nov 25.

ICMR, Equipe de Photochimie, UFR Sciences, CNRS, Université de Reims Champagne-Ardenne, B.P. 1039, 51687, Reims, France.

Different methods for the direct enantioselective photochemical synthesis of heterocycles are presented. Currently, asymmetric catalysis with templates involving hydrogen bonds or metal complexes is intensively investigated. Enzyme catalysis can be simplified under photochemical conditions. For example, in multi enzyme systems, one or more enzyme catalytic steps can be replaced by simple photochemical reactions. Chiral induction in photochemical reactions performed with homochiral crystals is highly efficient. Such reactions can also be carried out with crystalline inclusion complexes. Inclusion of a photochemical substrate and an enantiopure compound in zeolites also leads to enantioselective compounds. In all these methods, the conformational mobility of the photochemical substrates is reduced or controlled. Memory of chirality is a particular case in which a chiral information is temporally lost but the rigid conformations stabilize the molecular structure which leads to the formation of enantiopure compounds. Such studies allows a profound understanding on how particular conformations determine the configuration of the final products.Graphical abstract.
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http://dx.doi.org/10.1007/s43630-021-00135-6DOI Listing
December 2021

Photocycloadditions of Arenes Derived from Lignin.

J Org Chem 2021 10 22;86(19):13310-13321. Epub 2021 Sep 22.

ICMR, Equipe de Photochimie, UFR Sciences, CNRS, Université de Reims Champagne-Ardenne, 51687 Reims, France.

Intramolecular photocycloaddition reactions of 3,4-alkoxybenzonitriles derived from vanillin with alkenes have been investigated. In contrast to previous reports on photochemical reactions with these compounds, mainly [2 + 3] cycloaddition has been observed. A competing [2 + 2] photocycloaddition plays a minor role. Most probably, these additions occur at the singlet state S. In the case of a triplet reaction, a different regioselectivity of the [2 + 2] cycloaddition would be observed. Linear and angular [2 + 3] cycloadducts are formed as major products. The first isomer is transformed in the second one by a photochemical vinyl-cyclopropane rearrangement, which increases the selectivity of the reaction. The influence of the substitution pattern on the reactivity and the selectivity has also been investigated.
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http://dx.doi.org/10.1021/acs.joc.1c01361DOI Listing
October 2021

Studies on The Application of The Paternò-Büchi Reaction to The Synthesis of Novel Fluorinated Scaffolds.

Chemistry 2021 Nov 4;27(63):15722-15729. Epub 2021 Oct 4.

ICMR, Equipe de Photochimie, CNRS, Université de Reims Champagne-Ardenne, 1 UFR Sciences, B.P. 1039, 51687, Reims, France.

In the context of new scaffolds obtained by photochemical reactions, Paternò-Büchi reactions between heteroaromatic, trifluoromethylphenyl ketone and electron rich alkenes to give oxetanes are described. A comprehensive study has then been carried out on the reaction of aromatic ketones with fluorinated alkenes. Depending on the substitution pattern at the oxetane ring, a metathesis reaction is described as a minor side process to give mono fluorinated alkenes. Overall, this last reaction corresponds to a photo-Wittig reaction and yield amid isosteres. In order to explain the uncommon regioselectivity of the Paternò-Büchi reaction with these alkenes, electrostatic-potential derived charges (ESP) have been determined. In a second computational study, the relative stabilities of the typical 1,4-diradical intermediates of the Paternò-Büchi reaction have been determined. The results well explain the regioselectivity. Further transformations of the oxetanes or previous functionalization of the fluoroalkenes open perspectives for oxetanes as core structures for biologically active compounds.
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http://dx.doi.org/10.1002/chem.202102621DOI Listing
November 2021

Photochemical radical cyclization reactions with imines, hydrazones, oximes and related compounds.

Chem Soc Rev 2021 Jul;50(13):7418-7435

CNRS, Université de Reims Champagne-Ardenne, ICMR, Equipe de Photochimie, UFR Sciences, B.P. 1039, 51687 Reims, France.

Photochemical reactions are a key method to generate radical intermediates. Often under these conditions no toxic reagents are necessary. During recent years, photo-redox catalytic reactions considerably push this research domain. These reaction conditions are particularly mild and safe which enables the transformation of poly-functional substrates into complex products. The synthesis of heterocyclic compounds is particularly important since they play an important role in the research of biologically active products. In this review, photochemical radical cyclization reactions of imines and related compounds such as oximes, hydrazones and chloroimines are presented. Reaction mechanisms are discussed and the structural diversity and complexity of the products are presented. Radical intermediates are mainly generated in two ways: (1) electronic excitation is achieved by light absorption of the substrates. (2) The application of photoredox catalysis is now systematically studied for these reactions. Recently, also excitation of charge transfer complexes has been studied in this context from many perspectives.
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http://dx.doi.org/10.1039/d1cs00196eDOI Listing
July 2021

Directional soliton and breather beams.

Proc Natl Acad Sci U S A 2019 May 26;116(20):9759-9763. Epub 2019 Apr 26.

Department of Ocean Technology Policy and Environment, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Chiba 277-8563, Japan.

Solitons and breathers are nonlinear modes that exist in a wide range of physical systems. They are fundamental solutions of a number of nonlinear wave evolution equations, including the unidirectional nonlinear Schrödinger equation (NLSE). We report the observation of slanted solitons and breathers propagating at an angle with respect to the direction of propagation of the wave field. As the coherence is diagonal, the scale in the crest direction becomes finite; consequently, beam dynamics form. Spatiotemporal measurements of the water surface elevation are obtained by stereo-reconstructing the positions of the floating markers placed on a regular lattice and recorded with two synchronized high-speed cameras. Experimental results, based on the predictions obtained from the (2D + 1) hyperbolic NLSE equation, are in excellent agreement with the theory. Our study proves the existence of such unique and coherent wave packets and has serious implications for practical applications in optical sciences and physical oceanography. Moreover, unstable wave fields in this geometry may explain the formation of directional large-amplitude rogue waves with a finite crest length within a wide range of nonlinear dispersive media, such as Bose-Einstein condensates, solids, plasma, hydrodynamics, and optics.
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http://dx.doi.org/10.1073/pnas.1821970116DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6525507PMC
May 2019

Photochemically Induced Intramolecular Radical Cyclization Reactions with Imines.

J Org Chem 2018 02 26;83(4):1867-1875. Epub 2018 Jan 26.

CNRS, Université de Reims Champagne-Ardenne, ICMR, Equipe de Photochimie, UFR Sciences, B.P. 1039 , 51687 Reims, France.

The photochemically induced intramolecular hydrogen abstraction or hydrogen atom transfer in cyclic imines 8a,b followed by a cyclization is investigated. Two types of products are observed, one resulting from the formation of a C-C bond, the other from the formation of a C-N bond. A computational study reveals that hydrogen is exclusively transferred to the imine nitrogen leading to a triplet diradical intermediate. After intersystem crossing, the resulting zwitterionic intermediate undergoes cyclization leading to the final product.
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http://dx.doi.org/10.1021/acs.joc.7b02810DOI Listing
February 2018

Load-separation curves for the contact of self-affine rough surfaces.

Sci Rep 2017 07 31;7(1):6900. Epub 2017 Jul 31.

Politecnico di BARI. Department of Mechanics, Mathematics and Management, V Gentile 182, 70126, Bari, Italy.

There are two main approximate theories in the contact of rough solids: Greenwood-Williamson asperity theories (GW) and Persson theories. Neither of them has been fully assessed so far with respect to load-separation curves. Focusing on the most important case of low fractal dimension (D  = 2.2) with extensive numerical studies we find that: (i) Persson's theory describes well the regime of intermediate pressures/contact area, but requires significant corrective factors: the latter depend also on upper wavevector cutoff of the roughness; hence, (ii) Persson's theory does not predict the correct functional dependence on magnification; (iii) asperity theories in the discrete version even neglecting interaction effects are more appropriate in the range of relatively large separations, also to take into consideration of the large scatter in actual realization of the surface.
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http://dx.doi.org/10.1038/s41598-017-07234-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5537336PMC
July 2017

Studies in organic and physical photochemistry - an interdisciplinary approach.

Org Biomol Chem 2016 Aug 6;14(31):7392-442. Epub 2016 Jul 6.

James Cook University, College of Science and Engineering, Townsville, QLD 4811, Australia.

Traditionally, organic photochemistry when applied to synthesis strongly interacts with physical chemistry. The aim of this review is to illustrate this very fruitful interdisciplinary approach and cooperation. A profound understanding of the photochemical reactivity and reaction mechanisms is particularly helpful for optimization and application of these reactions. Some typical reactions and particular aspects are reported such as the Norrish-Type II reaction and the Yang cyclization and related transformations, the [2 + 2] photocycloadditions, particularly the Paternò-Büchi reaction, photochemical electron transfer induced transformations, different kinds of catalytic reactions such as photoredox catalysis for organic synthesis and photooxygenation are discussed. Particular aspects such as the structure and reactivity of aryl cations, photochemical reactions in the crystalline state, chiral memory, different mechanisms of hydrogen transfer in photochemical reactions or fundamental aspects of stereoselectivity are discussed. Photochemical reactions are also investigated in the context of chemical engineering. Particularly, continuous flow reactors are of interest. Novel reactor systems are developed and modeling of photochemical transformations and different reactors play a key role in such studies. This research domain builds a bridge between fundamental studies of organic photochemical reactions and their industrial application.
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http://dx.doi.org/10.1039/c6ob00842aDOI Listing
August 2016

Optical and photophysical properties of anisole- and cyanobenzene-substituted perylene diimides.

Phys Chem Chem Phys 2016 Feb;18(6):4924-41

Dipartimento di Scienze della Vita, Università di Modena e Reggio Emilia, via, Campi 183, 41125 Modena, Italy.

One- and two-photon absorption cross-sections and spectra and the photophysical properties of eight perylenetetracarboxy-3,4:9,10-diimide (PDI) derivatives are reported and analyzed. The investigated compounds are characterized by direct binding of the phenyl rings of the substituents to the bay positions of the perylene core. They have been designed to test the effects of differences in the electronic nature - electron donating (anisole) or accepting (cyanobenzene) - and binding topology (cis or trans, meta or para disubstitution or tetrasubstitution) of the bay substituents on the above optical and photophysical observables. (TD)DFT and Hückel MO calculations have provided theoretical information on the ground-state geometries, the MOs and the electronic spectra of several model compounds. For tetrasubstituted and cis disubstituted derivatives, strong steric interactions in the bay area determined the preferred conformations, with perylene cores distorted near the substituted bay(s) and a 42-44° twisting of the substituent rings relative to the core, quite irrespective of the electronic nature of the substituents. On the other hand, in trans-disubstituted PDI steric hindrance in the bay areas was much weaker and similar in the cyanobenzene and the anisole derivatives. So, the large differences found in the conformational preferences were completely attributable to electronic effects. With electron-accepting cyanobenzene, the substituent rings were found normal to the central planar perylene core, thus enabling the assignment of the moderate spectroscopic effects to inductive interactions. The DFT analysis of the PDI trans-disubstituted with electron-donating anisoles gave quite strongly distorted perylene-core geometries and less twisted (59°) substituent rings. The corresponding increased substituent/core conjugative interactions resulted in new CT allowed electronic transitions and an extremely pronounced solvent-polarity dependence of the emission spectra and intensities. All anisole substituted PDI feature a very fast radiationless decay path in polar solvents, likely related to a relaxation to a charge-separated configuration in the lowest excited-state.
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http://dx.doi.org/10.1039/c5cp07758cDOI Listing
February 2016

Photoorganocatalysed and visible light photoredox catalysed trifluoromethylation of olefins and (hetero)aromatics in batch and continuous flow.

Chem Commun (Camb) 2016 Feb 6;52(12):2493-6. Epub 2016 Jan 6.

Institute of Organic Chemistry, RWTH Aachen, Landoltweg 1, 52074, Aachen, Germany.

Trifluoromethylation of olefins and (hetero)aromatics with sodium triflinate as CF3 source and readily accessible benzophenone derivatives as photosensitisers has been developed in batch and flow. The use of an iridium-based photocatalyst enables the trifluoromethylation to proceed under visible light irradiation.
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http://dx.doi.org/10.1039/c5cc09881eDOI Listing
February 2016

The influence of stem design on critical squeaking friction with ceramic bearings.

J Orthop Res 2013 Oct 27;31(10):1627-32. Epub 2013 Jun 27.

Tribology Research Institute, Southwest Jiaotong University, Chengdu, 610031, China.

Ceramic-on-ceramic hip joints have been reported to squeak, a phenomenon that may occur in compromised lubrication conditions. One factor related to the incidence of in vivo squeaking is the stem design. However, it has not yet been possible to relate stem design to squeaking in deteriorating lubrication conditions. The purpose of this study was to determine critical friction factors for different stem designs. A hip simulator was used to measure the friction factor of a ceramic bearing with different stem designs and gradually deteriorating lubrication represented by evaporation of a volatile fluid lubricant. The critical squeaking friction factor was measured at the onset of squeaking for each stem. Critical friction was higher for the long cobalt chrome (0.32 ± 0.02) and short titanium stems (0.39 ± 0.02) in comparison with a long titanium stem (0.29 ± 0.02). The onset of squeaking occurred at a friction factor lower than that measured for dry conditions, in which squeaking is usually investigated experimentally. The results suggest that shorter or heavier stems might limit the possibility of squeaking as lubrication deteriorates. The method developed can be used to investigate the influence of design parameters on squeaking probability.
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http://dx.doi.org/10.1002/jor.22413DOI Listing
October 2013

Vibration transfer in the ball-stem contact interface of artificial hips.

Med Eng Phys 2013 Oct 23;35(10):1513-7. Epub 2013 May 23.

TUHH - Hamburg University of Technology, Institute of Biomechanics, Denickestrasse 15, 21073 Hamburg, Germany.

Audible squeaking has put the reputation of ceramic bearings for total hip replacements into question. Inter-articular friction induces vibrations in the ceramic head which are transferred through the head-taper interface to the femoral stem. If excited to sufficient amplitudes, squeaking can be emitted by the stem. Hence, the damping and amplification properties of this interface have a crucial influence on stem vibrations. The aim of this study was to determine the vibration transfer behavior between the head and the taper of a femoral stem and its dependence on the assembly force, in order to assess its influence on the development of audible squeaking. A ceramic head was assembled on a titanium femoral stem taper with high and low forces. Frequency response functions characterizing the head-stem interface were determined experimentally. The interface demonstrated negligible influence on vibration transfer in the squeaking frequency range (1-5 kHz). However its damping effect in lower and higher frequency ranges was significant and some areas of amplification were also found. Although squeaking vibration was not influenced by the particular interface studied, the method established can be utilized to trace taper designs with dynamic properties that decrease the susceptibility to squeaking.
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http://dx.doi.org/10.1016/j.medengphy.2013.04.001DOI Listing
October 2013

Parallel microflow photochemistry: process optimization, scale-up, and library synthesis.

Org Lett 2012 Sep 20;14(17):4342-5. Epub 2012 Aug 20.

Dublin City University, School of Chemical Sciences, Dublin 9, Ireland.

A novel, multimicrocapillary flow reactor (MμCFR) was constructed and applied to a series of sensitized photoadditions involving 2(5H)-furanones. The reactor allowed for rapid and energy-, time-, and space-efficient sensitizer screening, process optimization, validation, scale-up, and library synthesis.
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http://dx.doi.org/10.1021/ol301773rDOI Listing
September 2012

Photochemical reactions of aromatic compounds and the concept of the photon as a traceless reagent.

Authors:
Norbert Hoffmann

Photochem Photobiol Sci 2012 Nov;11(11):1613-41

Institut de Chimie Moléculaire de Reims, UMR 7312 CNRS et Université de Reims Champagne-Ardenne, Equipe de Photochimie, UFR Sciences, B.P. 1039, F-51687 Reims, cedex 02, France.

Electronic excitation significantly changes the reactivity of chemical compounds. Compared to ground state reactions, photochemical reactions considerably enlarge the application spectrum of a particular functional group in organic synthesis. Multistep syntheses may be simplified and perspectives for target oriented synthesis (TOS) and diversity oriented synthesis (DOS) are developed. New compound families become available or may be obtained more easily. In contrast to common chemical reagents, photons don't generate side products resulting from the transformation of a chemical reagent. Therefore, they are considered as a traceless reagent. Consequently, photochemical reactions play a central role in the methodology of sustainable chemistry. This aspect has been recognized since the beginning of the 20th century. As with many other photochemical transformations, photochemical reactions of aromatic, benzene-like compounds illustrate well the advantages in this context. Photochemical cycloadditions of aromatic compounds have been investigated for a long time. Currently, they are applied in various fields of organic synthesis. They are also studied in supramolecular structures. The phenomena of reactivity and stereoselectivity are investigated. During recent years, photochemical electron transfer mediated reactions are particularly focused. Such transformations have likewise been performed with aromatic compounds. Reactivity and selectivity as well as application to organic synthesis are studied.
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http://dx.doi.org/10.1039/c2pp25074hDOI Listing
November 2012

Homogeneous photocatalytic reactions with organometallic and coordination compounds--perspectives for sustainable chemistry.

Authors:
Norbert Hoffmann

ChemSusChem 2012 Feb 27;5(2):352-71. Epub 2012 Jan 27.

CNRS, Université de Reims Champagne-Ardenne, Institut de Chimie Moléculaire de Reims (UMR 6229), Equipe de Photochimie, UFR Sciences, Reims, France.

Since the time of Giacomo Ciamician at the beginning of the 20th century, photochemical transformations have been recognized as contributing to sustainable chemistry. Electronic excitation significantly changes the reactivity of chemical compounds. Thus, the application of activation reagents is frequently avoided and transformations can be performed under mild conditions. Catalysis plays a central role in sustainable chemistry. Stoichiometric amounts of activation reagents are often avoided. This fact and the milder catalytic reaction conditions diminish the formation of byproducts. In the case of homogeneous catalysis, organometallic compounds are often applied. The combination of both techniques develops synergistic effects in the sense of "Green Chemistry". Herein, metal carbonyl-mediated reactions are reported. These transformations are of considerable interest for the synthesis of complex polyfunctionalized compounds. Copper(I)-catalyzed [2+2] photocycloaddition gives access to a large variety of cyclobutane derivatives. Currently, a large number of publications deal with photochemical electron-transfer-induced reactions with organometallic and coordination compounds, particularly with ruthenium complexes. Several photochemically induced oxidations can easily be performed with air or molecular oxygen when they are catalyzed with organometallic complexes. Photochemical reaction conditions also play a certain role in C-H activation with organometallic catalysts, for instance, with alkanes, although such transformations are conveniently performed with a variety of other photochemical reactions.
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http://dx.doi.org/10.1002/cssc.201100286DOI Listing
February 2012

Friction-induced whirl vibration: root cause of squeaking in total hip arthroplasty.

J Biomech 2012 Jan 17;45(2):297-303. Epub 2011 Nov 17.

Institute of Mechanics and Ocean Engineering, Hamburg University of Technology, Hamburg, Germany.

Squeaking is reported for ceramic-on-ceramic hip arthroplasty, and risk factors leading to this phenomenon have been investigated empirically in the past, this way giving hints to when this phenomenon occurs. The aim of this study is to present an experimentally validated explanation for the dynamical mechanism underlying the squeak, i.e. a description of what happens when noise is generated. First the kinematics of the ceramic bearing couple in relative motion are reconsidered. The relative motion at the contact zone can be understood as superposition of relative rotation and translation. The relative weight of both components depends substantially on the instantaneous load vector, which primarily determines the position of the contact area, and the instantaneous relative rotation vector. For the investigated gait scenarios, both load vector and rotation axis vary strongly during the gait cycle. Second, experimental vibration analysis during squeak is performed. A pronounced micrometer scale elliptical motion of the ball inside the liner is found. It is shown that the rotational component of the relative kinematics during gait indeed leads to friction induced vibrations. We show that a generic whirl type friction induced flutter instability, also known from similar (non bio-) mechanical systems, is the root cause of the emitted squeaking noise. Based on the identified mechanism, the role of THA system parameters (materials, design), patient risk factors, as well as the role of the gait cycle, will have to be reconsidered and linked in the future to develop effective measures against squeaking.
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http://dx.doi.org/10.1016/j.jbiomech.2011.10.025DOI Listing
January 2012

Photochemical electron transfer mediated addition of naphthylamine derivatives to electron-deficient alkenes.

J Org Chem 2011 Sep 10;76(17):7104-18. Epub 2011 Aug 10.

Equipe de Photochimie, Institut de Chimie Moléculaire de Reims, UMR 6229 CNRS et Université de Reims Champagne-Ardenne, UFR Sciences, B.P. 1039, 51687 Reims, France.

Using photochemical electron transfer, N,N-dimethylnaphthylamine derivatives are added to α,β-unsaturated carboxylates. The addition takes place exclusively in the α-position of electron-deficient alkenes and mainly in the 4-position of N,N-dimethylnaphthalen-1-amine. A minor regioisomer results from the addition in the 5-position of this naphthylamine. A physicochemical study reveals that the fluorescence quenching of N,N-dimethylnaphthalen-1-amine is diffusion-controlled and that the back electron transfer is highly efficient. Therefore no transformation is observed at lower concentrations. To overcome this limitation and to induce an efficient transformation, minor amounts of water or another proton donor as well as an excess of the naphthylamine derivative are necessary. A mechanism involving a contact radical ion pair is discussed. Isotopic labeling experiments reveal that no hydrogen is directly transferred between the substrates. The hydrogen transfer to the furanone moiety observed in the overall reaction therefore results from an exchange with the reaction medium. An electrophilic oxoallyl radical generated from the furanone reacts with the naphthylamine used in excess. Concerning some mechanistic details, the reaction is compared with radical and electrophilic aromatic substitutions. The transformation was carried out with a variety of electron-deficient alkenes. Sterically hindered furanone derivatives are less reactive under standard conditions. In a first experiment, such a compound was transformed using heterogeneous electron transfer photocatalysis with TiO(2).
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http://dx.doi.org/10.1021/jo201080mDOI Listing
September 2011

Microphotochemistry: a reactor comparison study using the photosensitized addition of isopropanol to furanones as a model reaction.

Photochem Photobiol Sci 2011 Sep 18;10(9):1399-404. Epub 2011 Feb 18.

School of Chemical Sciences, Dublin City University, Ireland.

Three types of micro-photoreactor setups were investigated using DMBP-sensitized additions of isopropanol to furanones as model reactions. The results were compared to experiments using a conventional batch reactor. Based on conversion rates, reactor geometries and energy efficiency calculations the microsystems showed superior performances over the batch process. Of the three micro setups examined, the LED-driven microchip gave the best overall results.
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http://dx.doi.org/10.1039/c1pp05024aDOI Listing
September 2011

The influence of component design, bearing clearance and axial load on the squeaking characteristics of ceramic hip articulations.

J Biomech 2011 Mar 4;44(5):837-41. Epub 2011 Feb 4.

TUHH, Hamburg University of Technology, Institute of Biomechanics, Denickestrasse 15, 21073 Hamburg, Germany.

Squeaking of hip replacements with ceramic-on-ceramic bearings has put the use of this material into question despite its superior wear behavior. Squeaking has been related to implant design. The purpose of this study was to determine the influence of particular acetabular cup and femoral stem designs on the incidence of squeaking and its characteristics. The dynamic behavior of the stem, head and stem assembled with head was investigated by determining their eigenfrequencies using experimental and numerical modal analysis. Four different stem and three different cup designs were investigated. Operational system vibrations resulting in audible squeaking were reproduced in a hip simulator and related to the respective component eigenfrequencies. The applied joint load and bearing clearance were varied in the clinically relevant range. Stems with lower eigenfrequencies were related to lower squeaking frequencies and increased acoustic pressure (loudness), and therefore to a higher susceptibility to squeaking. Higher load increased the squeaking frequency, while the acoustic pressure remained unchanged. No influence of the clearance or the cup design was found. Stem design was found to have an important influence on squeaking characteristics and its incidence, confirming and explaining similar clinical observations. Cup design itself was found to have no major influence on the dynamic behavior of the system but plays an important indirect role in influencing the magnitude of friction: Squeaking only occurs if the friction in the joint articulation is sufficient to excite vibrations to audible magnitudes. If friction is low, no squeaking occurs with any of the designs investigated.
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http://dx.doi.org/10.1016/j.jbiomech.2010.12.012DOI Listing
March 2011

Photosensitized addition of isopropanol to furanones in a 365 nm UV-LED microchip.

Photochem Photobiol Sci 2010 Dec 8;9(12):1601-3. Epub 2010 Oct 8.

School of Chemical Sciences, Dublin City University, Dublin 9, Ireland.

The DMBP-sensitized addition of isopropanol to furanones was studied in a novel LED-driven microchip reactor. Complete conversions were achieved after just 2.5 to 5 min of irradiation with 6 × 365 nm high-power LEDs. The results were compared to analogous experiments using a conventional batch reactor.
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http://dx.doi.org/10.1039/c0pp00223bDOI Listing
December 2010

Deformation characteristics and eigenfrequencies of press-fit acetabular cups.

Clin Biomech (Bristol, Avon) 2011 Jan;26(1):46-51

TUHH, Hamburg University of Technology, Biomechanics Section, Denickestrasse 15, 21073 Hamburg, Germany.

Background: elastic deformation of press-fitted acetabular cups during implantation provides primary stability. Excessive deformation can lead to chipping or improper seating of ceramic inlays and is dictated by cup stiffness, which also affects its vibrational characteristics. Purpose was to investigate the influence of cup design on deformation during press-fitting and on vibration properties.

Methods: deformation of ten acetabular cups (with and without ceramic inlay) was tested for radial loads clinically occurring during press-fitting (0-2000N). Eigenfrequencies were measured using experimental modal analysis and related to mass and stiffness.

Findings: the first eigenfrequency of the shells varied greatly (4-9kHz); insertion of inlays caused an increase (16-33 kHz). The range of shell stiffness was high (2.7-48.4kN/mm), increasing due to inlay insertion (124.7-376.2kN/mm). Stiffness and mass were sufficient predictors for eigenfrequencies (p<0.001,R²=0.94).

Interpretation: the cups investigated represent a large stiffness range. Lower cup stiffness can increase primary stability but jeopardize inlay seating, and a suitable balance must be achieved by the designer. Eigenfrequencies also decrease with decreasing stiffness but were all found to lie considerably above clinically observed squeaking frequencies, indicating that these cup designs play no predominant role in the squeaking phenomenon. The observed relation between eigenfrequencies and the quotient of stiffness and mass might be used in the development of new thin walled cup designs so that their contribution to system vibrations is prevented. Presently, surgeons should be aware of the deformation characteristics of cups in order to select a suitable press-fit magnitude.
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http://dx.doi.org/10.1016/j.clinbiomech.2010.08.015DOI Listing
January 2011

Squeak in hip endoprosthesis systems: An experimental study and a numerical technique to analyze design variants.

Med Eng Phys 2010 Jul 16;32(6):604-9. Epub 2010 Mar 16.

Institute of Mechanics and Ocean Engineering, Hamburg University of Technology, Eissendorfer Str. 42, 21073 Hamburg, Germany.

Hip endoprosthesis systems are analyzed with respect to their susceptibility to self-excited vibrations and sound or noise generation. Experimental studies reveal that certain configurations can become unstable causing exponentially growing regular high-frequency oscillations that asymptotically approach a limit-cycle with considerable amplitude. Ultimately the vibrations do also lead to the emission of sound that is perceived as squeaking or squeal. To identify dominant influence factors and critical parameters, stability analyses were conducted on the basis of finite-element modeling. The resulting numerical approach, based on the determination of complex eigenvalues and eigenvectors, is shown to be an effective tool to analyze and show differences between endoprosthesis designs with respect to their susceptibility to develop squeaking phenomenons.
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http://dx.doi.org/10.1016/j.medengphy.2010.02.006DOI Listing
July 2010

Stereoselective triplet-sensitised radical reactions of furanone derivatives.

Chemistry 2010 Mar;16(11):3341-54

Université de Reims Champagne Ardenne, Institut de Chimie Moléculaire de Reims, UMR CNRS N degrees 6229, Equipe de Photochimie, UFR Sciences, B.P. 1039, 51687 Reims, France.

The stereo- and regioselectivity of triplet-sensitised radical reactions of furanone derivatives have been investigated. Furanones 7a,b were excited to the (3)pipi* state by triplet energy transfer from acetone. Intramolecular hydrogen abstraction then occurred such that hydrogen was transferred from the tetrahydropyran to the beta position of the furanone moiety. Radical combination of the tetrahydropyranyl and the oxoallyl radicals led to the final products 8a,b. In the intramolecular reaction, overall, a pyranyl group adds to the alpha position of the furanone. The effect of conformation was first investigated with compounds 9a,b carrying an additional substituent on the tether between the furanone and pyranyl moiety. Further information on the effect of conformation and the relative configuration at the pyranyl anomeric centre and the furanone moiety was obtained from the transformations of the glucose derivatives 12, 14, 17 and 18. Radical abstraction occurred at the anomeric centre and at the 5'-position of the glucosyl moiety. Computational studies of the hydrogen-abstraction step were carried out with model structures. The activation barriers of this step for different stereoisomers and the abstraction at the anomeric centre and at the 6'-position of the tetrahydropyranyl moiety were calculated. The results of this investigation are in accordance with experimental observations. Furthermore, they reveal that the reactivity and regioselectivity are mainly determined in the hydrogen-abstraction step. Intramolecular hydrogen abstraction (almost simultaneous electron and proton transfer) in (3)pipi* excited furanones only takes place under restricted structural conditions in a limited number of conformations that are defined by the relative configuration of the substrates. It is observed that in the biradical intermediate, back-hydrogen transfer occurs leading to the starting compound. In the case of glucose derivatives, this reaction led to epimerisation at the anomeric centre.
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http://dx.doi.org/10.1002/chem.200903045DOI Listing
March 2010

Development of agriculture left-overs: fine organic chemicals from wheat hemicellulose-derived pentoses.

Top Curr Chem 2010 ;294:79-115

Agro Industrie Recherches et Développements, Route de Bazancourt, 51110 Pomacle, France.

This review is dedicated to wheat hemicelluloses and its main components D-xylose and L-arabinose as raw materials for fine organic chemistry. The context of the wheat agro-industry, its by-products, and extraction and hydrolysis of hemicelluloses to produce the pentoses are considered. The straightforward preparation of pentose-based surfactants, their properties, and their situation in the field of carbohydrate-based surfactants are addressed. Multistep transformations of pentoses are also described, first from a methodology point of view, with the aim of producing multifunctional enantiopure building-blocks, then considering targeted natural and/or bioactive products. Selected reactions of furfural, an important dehydration product of pentoses, are also presented.
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http://dx.doi.org/10.1007/128_2010_54DOI Listing
June 2011

Photochemical key steps in the synthesis of surfactants from furfural-derived intermediates.

ChemSusChem 2009 ;2(12):1130-7

Institut de Chimie Moléculaire de Reims, UMR 6229 CNRS et Université de Reims Champagne-Ardenne, Equipe de Photochimie, UFR Sciences, B.P. 1039, 51687 Reims, Cedex 02, France.

Furfural is oxidized to 2[5H]-furanone by using hydrogen peroxide or to 5-hydroxy-2[5H]-furanone by using photo-oxygenation. An amine function is introduced by photochemically induced radical addition of tertiairy amines, some of which carry an n-alkyl side chain as hydrophobic moiety. These amines are produced from fatty aldehydes and cyclic secondary amines. The resulting adducts are transformed into amphoteric surfactants possessing an ammonium and a carboxylate function. Amphoteric (pK(N) and isoelectric point) and surfactant properties such as the critical micelle concentration and the adsorption efficiency are determined.
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http://dx.doi.org/10.1002/cssc.200900150DOI Listing
April 2010

Visceral artery aneurysms--follow-up of 23 patients with 31 aneurysms after surgical or interventional therapy.

Langenbecks Arch Surg 2009 Nov 12;394(6):1093-100. Epub 2009 Mar 12.

Department of Vascular Surgery and Kidney Transplantation, University Hospital, Heinrich-Heine-University, Düsseldorf, Germany.

Purpose: Visceral artery aneurysms (VAA) are rare forms of vascular pathology, with an incidence of 0.1% to 0.2% in routine autopsies. They frequently present as a life-threatening, often fatal, emergency, if associated with rupture and intra- or retroperitoneal bleeding. The clinical symptoms, natural history, and mortality of VAAs vary depending on the vessels involved. The mortality rates range from 8.5% up to 25% and, in pregnant women, up to 75%. A retrospective analysis of all VAAs diagnosed at our institution from 1991 to 2006 was performed. The presentation, management, and outcome of therapy was evaluated for each patient.

Materials And Methods: Twenty-three patients (12 men, 11 women, mean age 55.8 years) with 31 VAAs were identified. The anatomical involvement concerned seven regions: celiac (CT) nine, superior mesenteric (SMA) seven, splenic (SA) five, hepatic (HA) six, gastroduodenal (GDA) two, pancreatoduodenal (PDA) one, and one branch of the superior mesenteric artery. Fourteen patients presented symptoms attributable to their aneurysms, which included a total of four ruptures. Nine patients had no symptoms. The etiology of VAAs was atherosclerosis (67.8%), mycotic embolization (12.9%), trauma (9.7%), Marfan Syndrome (3.2%), Klippel-Trenaunay-Weber syndrome (3.2%), and giant cell arteritis (3.2%). Open surgery was performed for 29 aneurysm in 21 patients: partial resection and tailoring in 13 cases (41.9%), resection of the aneurysm with additional autologous vein graft interposition in nine cases and prosthetic graft interposition in 2 cases (35.5%), aneurysm exclusion by ligation in three cases (9.6%) and aneurysm ligation combined with additional autologous bypass procedure in two cases (6.5%). Two patients (6.5%) were treated interventionally with embolization, in one case each with a right hepatic artery aneurysm and in the other with splenic artery aneurysm.

Results: No deaths were observed. The morbidity rate associated with surgical treatment was low. After treatment, a total of 17 patients were followed up for a period ranging from 3 to 154 months (mean 54.6 months). Fifteen patients required no additional procedures. The patency rate of the reconstructed visceral arteries was 90.4%. Six patients were lost for follow-up.

Conclusions: Surgical and interventional therapy of VAAs can be life-saving treatments for the patient with a low periprocedural morbidity. The success rate, defined as the exclusion of VAA rupture and the absence of abdominal discomfort, in our material was 88.2% after a mean follow-up of 54.6 months.
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http://dx.doi.org/10.1007/s00423-009-0482-zDOI Listing
November 2009

Photochemical reactions as key steps in organic synthesis.

Authors:
Norbert Hoffmann

Chem Rev 2008 Mar 27;108(3):1052-103. Epub 2008 Feb 27.

Laboratoire des Réactions Sélectives et Applications, UMR 6519 CNRS et Université de Reims Champagne-Ardenne, UFR Sciences, BP 1039, F-51687 Reims, Cedex 02, France.

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http://dx.doi.org/10.1021/cr0680336DOI Listing
March 2008

Photoinduced-electron-transfer chemistry: from studies on PET processes to applications in natural product synthesis.

Acc Chem Res 2007 Feb 27;40(2):128-40. Epub 2007 Jan 27.

Institute of Organic Chemistry, University of Cologne, Greinstr. 4, 50939 Köln, Germany.

The application of photoinduced electron transfer (PET) for the construction of heterocyclic ring systems is an appealing route in synthetic organic photochemistry. Electronically excited carbonyl chromophors in ketones, aldehydes, amides, or imides are strong electron acceptors that oxidize alkenes, amines, thioethers, or carboxylates. In subsequent steps, the radical anions formed thereof either are operating as secondary electron donors and initiate a photon-driven chain reaction or combine with electrophilic species and form products. These reactions are applied in the synthesis of heterocyclic compounds. The basic structures of these target molecules are bicyclic tertiary amines from the pyrrolizidine, benzopyrrolizidine, and indolizidine families, cyclic oligopeptides, macrocyclic ring systems, and many more.
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http://dx.doi.org/10.1021/ar068148wDOI Listing
February 2007

Chlorodifluoromethyl-substituted monosaccharide derivatives--radical activation of the carbon-chlorine-bond.

Carbohydr Res 2006 Nov 20;341(16):2641-52. Epub 2006 Sep 20.

Institut für Chemie, Organische Chemie, Universität Rostock, Albert-Einstein-Strasse 3a, D-18059 Rostock, Germany.

The dithionite-mediated addition of BrCF(2)Cl to 3,4-di-O-pivaloyl-D-xylal (1) generated preferably 1-CF(2)Cl-substituted products, that is, (2-bromo-2-deoxy-3,4-di-O-pivaloyl-beta-D-xylopyranosyl)-chlorodifluoromethane and (2-deoxy-3,4-di-O-pivaloyl-beta-D-threo-pentopyranosyl)-chlorodifluoromethane. Selected chlorodifluoromethyl-substituted monosaccharide derivatives were hydrodechlorinated or alkylated at the CF(2)Cl-group using tin reagents under radical reaction conditions. Thus, hydrodechlorinations of (2,3,4-tri-O-acetyl-6-deoxy-alpha-L-galactopyranosyl)-chlorodifluoromethane and of methyl 3,4-di-O-acetyl-2-C-chlorodifluoromethyl-2,6-dideoxy-alpha/beta-L-glucopyranoside are reported using tri-n-butyltin hydride initiated by AIBN. UV-initiated allylations are reported for reactions of (2-deoxy-3,4-di-O-pivaloyl-beta-D-threo-pentopyranosyl)-chlorodifluoromethane, (2,3,4-tri-O-acetyl-6-deoxy-alpha-L-galactopyranosyl)-chlorodifluoromethane, 1,3,4,6-tetra-O-acetyl-2-C-chlorodifluoromethyl-2-deoxy-alpha-D-glucopyranose, 1,3,4,6-tetra-O-acetyl-2-C-chlorodifluoromethyl-2-deoxy-alpha-D-mannopyranose and methyl 3,4-di-O-acetyl-2-C-chlorodifluoromethyl-2-deoxy-alpha/beta-D-rabinopyranoside with allyltri-n-butyltin.
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http://dx.doi.org/10.1016/j.carres.2006.08.023DOI Listing
November 2006
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