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11.
Prof. Michael North Marta Omedes‐Pujol Courtney Williamson 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(37):11367-11375
The asymmetric addition of trimethylsilyl cyanide to aldehydes can be catalysed by Lewis acids and/or Lewis bases, which activate the aldehyde and trimethylsilyl cyanide, respectively. It is not always apparent from the structure of the catalyst whether Lewis acid or Lewis base catalysis predominates. To investigate this in the context of using salen complexes of titanium, vanadium and aluminium as catalysts, a Hammett analysis of asymmetric cyanohydrin synthesis was undertaken. When Lewis acid catalysis is dominant, a significantly positive reaction constant is observed, whereas reactions dominated by Lewis base catalysis give much smaller reaction constants. [{Ti(salen)O}2] was found to show the highest degree of Lewis acid catalysis, whereas two [VO(salen)X] (X=EtOSO3 or NCS) complexes both displayed lower degrees of Lewis acid catalysis. In the case of reactions catalysed by [{Al(salen)}2O] and triphenylphosphine oxide, a non‐linear Hammett plot was observed, which is indicative of a change in mechanism with increasing Lewis base catalysis as the carbonyl compound becomes more electron‐deficient. These results suggested that the aluminium complex/triphenylphosphine oxide catalyst system should also catalyse the asymmetric addition of trimethylsilyl cyanide to ketones and this was found to be the case. 相似文献
12.
The recently reported LR-HSQMBC experiment has been optimized for 1H–15N long-range heteronuclear couplings. Several previously unreported four-bond correlations, consistent with the predicted by DFT calculations (0.2–0.3 Hz 4JNH couplings), have been observed for strychnine using 2 Hz optimization of the LR-HSQMBC experiment. This experiment offers an advantage over accordion-optimized experiments such as IMPEACH and CIGAR for the observation of long-range 1H–15N correlations in that the experiment is refocused and employs a CLIP pulse sequence element to bring the long-range correlations into phase, allowing broadband X-decoupling to be employed during acquisition. 相似文献
13.
Ronald C. Crouch Jeffrey G. Raab Marius Pelmuş Evgeny Tischenko Michael Frey Alexei V. Buevich Mikhail Reibarkh R. Thomas Williamson Gary E. Martin 《Magnetic resonance in chemistry : MRC》2022,60(2):210-220
Modification of the recently reported 19F-detected 1,1-ADEQUATE experiment that incorporates dual-optimization to selectively invert a wide range of 1JCC correlations in a 1,n-ADEQUATE experiment is reported. Parameters for the dual-optimization segment of the pulse sequence were modified to accommodate the increased size of 1JCC homonuclear coupling constants of poly- and perfluorinated molecules relative to protonated molecules to allow broadband inversion of the 1JCC correlations. The observation and utility of isotope shifts are reported for the first time for 1,1- and 1,n-ADEQUATE correlations. 相似文献
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Julian M. Menter George D. Williamson Kimberly Carlyle Cyril L. Moore Isaac Willis 《Photochemistry and photobiology》1995,62(3):402-408
Although previous studies have demonstrated that the predominant photochemistry of type I collagen under 254 nm irradiation may be attributed either to direct absorption by tyrosine/phenylalanine or to peptide bonds, direct collagen photochemistry via solar UV wavelengths is much more likely to involve several age- and tissue-related photolabile collagen fluorophores that absorb in the latter region. In this study, we compare and contrast results obtained from irradiation of a commercial preparation of acid-soluble calf skin type I collagen in solution with UVC (primarily 254 nm), UVA (335–400nm) and broad-band solar-simulating radiation (SSR; 290^1–00nm). Excitation spectroscopy and analysis of photochemically induced disappearance of fluorescence (fluorescence fading) indicates that this preparation has at least four photolabile fluorescent chromophores. In addition to tyrosine and L-3,4-dihydroxyphenylalanine, our sample contains two other fluorophores. Chromophore I, with emission maximum at 360 nm, appears to be derived from interacting aromatic moieties in close mutual proximity. Chromophore II, with broad emission at430–435 nm, may be composed of one or more age-related molecules. Collagen fluorescence fading kinetics are sensitive to excitation wavelength and to conformation. Under UVC, chromophore I fluorescence disappears with second-order kinetics, indicating a reaction between two proximal like molecules. Adherence to second-order kinetics is abrogated by prior denaturation of the collagen sample. A new broad, weak fluorescence band at400–420 nm, attributable to dityrosine, forms under UVC, but not under solar radiation. This band is photolabile to UVA and UVB wavelengths. Amino acid analysis indicates significant destruction of aromatic amino acids under UVC, but not under UVA or SSR. When properly understood, collagen fluorescence fading phenomena may act as a sensitive molecular probe of structure, conformation and reactivity. 相似文献
17.
Abstract Cellulose carbamate and ester derivatives were synthesized in homogeneous solutions of lithium chloride (LiCl)/N,N-dimemyl-acetamide (DMAc) by the reaction of cellulose with ethyl 4-isocyanatobenzoate and the activated esters of N,N-dimethyl-aminobenzoic acids. Comparative reactions were performed with phenyl isocyanate and the activated ester of benzoic acid. All reactions were followed spectroscopically by FTIR, 1H NMR, and 13C NMR. Degrees of substitution were calculated utilizing UV spectroscopy. The isocyanate reactions are facile allowing controllable degrees of substitution and high yields. By contrast, the activated ester pathway inherently results in lower degrees of substitution and lower yields due in part to undesirable side reactions. 相似文献
18.
Saikat Banerjee Waqas Rasheed Ruixi Fan Dr. Apparao Draksharapu Dr. Williamson N. Oloo Prof. Dr. Yisong Guo Prof. Dr. Lawrence Que Jr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(41):9608-9613
The [FeIV(O)(Me3NTB)]2+ (Me3NTB=tris[(1-methyl-benzimidazol-2-yl)methyl]amine) complex 1 has been shown by Mössbauer spectroscopy to have an S=1 ground state at 4 K, but is proposed to become an S=2 trigonal-bipyramidal species at higher temperatures based on a DFT model to rationalize its very high C−H bond-cleavage reactivity. In this work, 1H NMR spectroscopy was used to determine that 1 does not have C3-symmetry in solution and is not an S=2 species. Our results show that 1 is unique among nonheme FeIV=O complexes in retaining its S=1 spin state and high reactivity at 193 K, providing evidence that S=1 FeIV=O complexes can be as reactive as their S=2 counterparts. This result emphasizes the need to identify factors besides the ground spin state of the FeIV=O center to rationalize nonheme oxoiron(IV) reactivity. 相似文献
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20.
M. A. Williamson J. D. B. Smith P. M. Castle R. N. Kauffman 《Journal of polymer science. Part A, Polymer chemistry》1982,20(7):1875-1884
The laser-initiated polymerization of charge-transfer monomer complexes was investigated with a pulsed nitrogen laser and an argon laser. Several donor-acceptor monomer charge-transfer systems were screened for polymerization in different solvents. Polymerization by laser initiation was achieved in two of these systems; that is, 2-vinylnaphthalene/fumaronitrile and 9-vinylanthracene/fumaronitrile. The best polymer yields were obtained with the 2-vinylnaphthalene/fumaronitrile system in sulfolane solvent. The influences on the polymer yield and composition of solvents, varying focal path length, glass conditioning, initiation sources, environment, and monomer feed ratios were evaluated. Infrared (IR) spectral studies, gel permeation chromatography (GPC), and chemical analyses were performed to characterize the polymer products arising from the polymerization of 2-vinylnaphthalene/fumaronitrile in sulfolane. The polymer contained a high percentage of sulfolane (ca. 1/3 mol fraction), presumably arising from solvent transfer to the growing polymer chains during the propagation phase of the polymerization. 相似文献