Theoretical studies vinyl substitution

Despite the undeniable synthetic value of the benzannulation reaction of aryl and alkenyl Fischer carbene complexes, the details of its mechanism at the molecular level remain to be ascertained. Indeed, although a relatively large number of theoretical studies have been directed to the study of the molecular and electronic structure of Fischer carbene complexes [22], few studies have been devoted to the analysis of the reaction mechanisms of processes involving this kind of complexes [23-30]. The aim of this work is to present a summary of our theoretical research on the reaction mechanism of the Dotz reaction between ethyne and vinyl-substituted hydroxycarbene species to yield p-hydroxyphenol. 

A controversial issue of heteroatom-stabilized cations is the relative stabilization of carbocationic centers adjacent to oxygen and sulfur.541 In solution studies, a-O-substituted carbocations were found to be stabilized more than a-iS -substituted carbocations.677 Gas-phase studies reached an opposite conclusion,678 679 whereas subsequent theoretical studies (high-level ab initio methods) supported the findings of solution chemistry. Recent results, namely, basicities of various vinylic compounds (365-370) measured in the gas phase also support this conclusion.680 Although monoheteroatom-substituted compounds 365 and 366 were found to have similar proton affinities, an additional a-methyl group increased the stability of the carbenium ion derived from 367 more than that of the sulfur counterpart 368. Even larger differences were found between proton affinities of the bis-heteroatom-substituted compounds 369 and 370. 

Silyl-substituted carbenium ions have attracted considerable experimental interest because they are believed to be intermediates in electrophilic additions to vinyl, ethynyl and aryl silanes, in solvolytic reactions and in cationic cyclization reactions1 -4,322. Despite this wide interest, knowledge concerning the effect of silyl substitution on the stabilities of carbenium ions was rather qualitative and only recently more quantitative data became available. Theoretical studies centered mainly around a- and / -silyl-substituted carbenium ions, but y-silyl effects have been also studied. 

Both experimental and theoretical studies of the peracid epoxidation of ethene and alkyl-substituted olefins have suggested that a spiro transition structure is favored over a planar one. For example, DFT calculations indicated that the transition structure for the reaction of peroxyformic acid and ethene has the geometry shown in Figure 9.45. In this case the two C-0 distances are identical, indicating synchronous formation of the two C-O bonds. If the olefin is substituted with a methyl, methoxy, vinyl, or cyano group, however, the transition structure can become asynchronous, meaning that one C-0 bond has been formed to a greater extent than the other in the transition structure. ... 

The carbon NMR spectrum of the vinyl-substituted vinyl cation, CH2=C -CH=CHMe, in superacid solution at low temperature, was reported by Siehl and co-workers. It consists of five pairs of signals revealing that two isomers had been formed, most probably the E- and Z-isomers. This assumption was confirmed by the Joint theoretical study. Furthermore... 

The proton-transfer reactions of the vinyl cation H3CC=CH2 have been studied in a flow tube and theoretically. The lifetimes and UV-VIS absorption spectra of many aryl-substituted vinyl cations are reported. The solvent effects on the protonation of acetylene and ethylene are the subject of continuum solvent quantum-chemical calculations. The structures chosen were the symmetrically bridged non-classical vinyl and ethyl cations apparently the different hydration energies of these structures affect the energetics of their protonation in water. ... 

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