Substrate regulation of product distribution in the reactions of arylchromium carbene complexes with alkynes
摘要:
The reactions of arylcarbene complexes with alkynes were examined for six of the nine possible substitution patterns for mono- and dioxygenated aryl substituents of the carbene carbon. The product distributions were found to be highly dependent on a number of factors, including solvent, temperature, concentration of alkyne, and the nature of the aryl substituent. The product distributions were determined in nearly all cases for phenol and indene products and in some cases for furans, cyclobutenones, and cyclopentenediones, which were minor products in these reactions. The product distribution for the reaction of each arylcarbene complex was determined as a function of both temperature and alkyne concentration, since the combined product distribution profiles provided a much more sensitive measure of the relative influences of the aryl substituents on the reaction outcome. Furthermore, this distribution profile was determined for the reactions with 3-hexyne and I-pentyne for each carbene complex. A series of monosubstituted arylcarbene complexes were examined to identify the effects of oxygen substituents at various positions on the aryl ring. The m-methoxy group has no effect on the product distribution, whereas the o-methoxy group influences the distribution by its ability to chelate to the metal center and the p-methoxy group influences the distribution by its ability to donate electrons by resonance. The product distributions from the reactions of the 2,3-, 2,4-, and 2,5-dimethoxy complexes followed the profile expected from the simple sum of the profiles of the monomethoxyl complexes. In all cases where an effect was observed, higher concentrations of alkyne led to a higher selectivity for phenol over indene products. The dependence of the product distribution on the concentration of the alkyne substrate is suggested to be due to a process in which a second molecule of alkyne coordinates to the metal center and determines the chemical outcome of an intermediate that has covalently incorporated the first alkyne. It is further suggested that the special ability of an alkyne to display this effect is related to the ability of an alkyne to readily switch from a 2 to a 4 e- donor. This phenomenon of substrate regulation of product distribution is termed the allochemical effect, and a mechanistic explanation is developed that features this proposed process and that is refined to accommodate the observed effects of solvent, temperature, chelation, and steric and electronic effects that have been observed for the reaction of carbene complexes and alkynes.
Palladium-Catalyzed Reductive Cross-Coupling Reaction of Aryl Chromium(0) Fischer Carbene Complexes with Aryl Iodides
作者:Kang Wang、Yu Lu、Fangdong Hu、Jinghui Yang、Yan Zhang、Zhi-Xiang Wang、Jianbo Wang
DOI:10.1021/acs.organomet.7b00657
日期:2018.1.8
The first palladium-catalyzed reductive cross-couplings of aryl chromium(0) carbene complexes with aryliodides have been realized. This coupling reaction shows excellent functional group tolerance and high efficiency. Mechanistically, aryl chromium(0) carbene complexes undergo transmetalation with arylpalladium species to generate palladium(II) carbene intermediates, which is followed by migratory
C–H Bond Functionalization of Benzoxazoles with Chromium(0) Fischer Carbene Complexes
作者:Fangdong Hu、Jinghui Yang、Ying Xia、Chen Ma、Haiping Xia、Yan Zhang、Jianbo Wang
DOI:10.1021/acs.organomet.6b00008
日期:2016.5.23
An efficient C-H bond functionalization of benzoxazoles with chromium(0) Fischer carbene complexes under catalyst-free conditions has been developed. A series of benzoxazoles and chromium(0) carbene complexes are compatible with the reaction. This transformation provides an alternative way for C-H bond alkylation of benzoxazoles. In the reaction mechanism the elimination of the Cr(CO)(5) fragment seems more favored than the elimination of an alkoxy group, which is in sharp contrast to the previous reports on the reaction of organolithium reagents with chromium(0) Fischer carbene complexes.
Optically Active (<i>aR</i>)- and (<i>aS</i>)-Linear and Vaulted Biaryl Ligands: Deracemization versus Oxidative Dimerization
作者:Gang Hu、Daniel Holmes、Brina Fay Gendhar、William D. Wulff
DOI:10.1021/ja903820m
日期:2009.10.14
The copper-mediated deracemization of the C2-symmetric vaulted biaryl ligands VANOL and VAPOL has been investigated. In the course of the studies that have led to a more reliable procedure for this process, an unprecedented oxidative dimerization of these ligands has been uncovered. The structures of these oxidative dimerization products were elucidated by a series of NMR experiments, and these assignments were supported by other spectroscopic techniques as well as their chemical reactivity. This oxidative dimerization process was not observed for the linear biaryl ligands BANOL and BINOL, although the new deracemization procedure was effective for the generation of BINOL with high optical purity. The (aS)-enantiomers of BINOL, VANOL and VAPOL were accessible with a copper complex of (-)-sparteine, and the (aR)-enantiomeric series were accessible with a copper complex of O'Brien's diamine. Both sparteine and O'Brien's diamine give higher optical purities with VANOL and VAPOL than with BINOL, and this is consistent with the steric congestion present in the matched and mismatched copper complexes of these diamines with the biaryl ligands.