Aryl-vinyl bridging reactions

This scheme represents an alkyne-bromine complex as an intermediate in all alkyne brominations. This is analogous to the case of alkenes. The complex may dissociate to a inyl cation when the cation is sufficiently stable, as is the case when there is an aryl substituent. It may collapse to a bridged bromonium ion or undergo reaction with a nucleophile. The latta is the dominant reaction for alkyl-substituted alkynes and leads to stereospecific anti addition. Reactions proceeding through vinyl cations are expected to be nonstereospecific. 

When one of the components of the molecule undergoing reaction is a double bond, an alternative reaction mode is operative. Thus it can be seen that 417 follows the usual path involving aryl-aryl bridging affording biphenyl and a carbene that is trapped by solvent. However, in addition a vinyl-aryl di-jr-methane process is also operative and affords the... 

Alkynes react with bromine via an electrophilic addition mechanism. A bridged bromonium ion intermediate has been postulated for alkyl-substituted acetylenes, while vinyl cations are suggested for aryl-substituted examples.119 1-Phenylpropyne gives mainly the anti addition product in acetic acid, but some of the syn isomer is formed.120 The proportion of dibromide formed and stereoselectivity are enhanced when lithium bromide is added to the reaction mixture. 

Other activated sulfoxides. This alkylative elimination reaction has been extended by Trost and Bridges to a one-pot synthesis of alkenes, vinyl sulfides, a, l-unsaturated sulfoxides, and a,j3-unsaturated nitriles. The sulfoxides (1-4) are converted into the anions by lithium N-isopropylcyclohexylamide or sodium hydride and are then alkylated at 20° in THE or DME elimination is then effected by raising the temperature to reflux. In some cases trimethyl phosphite is added as a scavenger for phenylsulfenic acid. Typical results are formulated in the equations. The elimination reaction is facilitated by an aryl, thioaryl, or... 

Aside from the aforementioned demanding examples, the 6-exo-trig Mizoroki-Heck cyclization also allows for the formation of bridged polycyclic compounds (Scheme 5.27). Vinyl iodide 144 was cyclized in good yield and produced 145 as a 9 1 mixture of doublebond isomers (minor isomer not shown) (144 145) [21]. The analogous aryl iodide (not shown) furnished in the presence of 5 mol% (Ph3P)4Pd the product in 81% yield, but in a 7 3 mixture of double-bond isomers. Coe et al. [69] used the Mizoroki-Heck reaction... 

This reaction appears to be more difficult than the insertion into a vinylic C-H bond, which occurs in the synthesis of indoles from 2-nitrostyrenes (see paragraph 5.3.). The known chemical inertness of aromatic C-H bonds towards nitrene insertion [13] and the stabilisation of the intermediate aryl nitrene triply bridged in the ruthenium cluster are probably responsible for the poor yield of the heterocyclic product. 

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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