Aryl alkyl acetylene, regioselective

Scheme 2 Regioselective hydrofluorination of aryl alkyl acetylene...

Of more specialized interest is the reaction of alkynes with Co2(CO)s in polar solvents under conditions of elevated temperature and CO pressure. The major products are the (E) and (Z) isomers of so-called bifurandiones, the products of formal (but not actual) carbene dimerization. An intermediate, a complex of this lactone-derived carbene bridging a Co2(CO>7 moiety, may be isolated from reactions of alkyl-, aryl- or dialkyl-acetylenes with Co2(CO)8. Its formation is completely regioselective even for ethyl vj. methyl, placing the larger group a to the lactone carbonyl. - In the subsequent formation of the bifurandione itself regioselectivity in alkyne incorporation into the second ring is incomplete and in the opposite direction, an observation which remains unexplained (Scheme 11). ... 

For the alkylation and alkenylation of C-H bonds, olefins and acetylenes are used as reactants. This type of coupling protocol is not applicable to arylation. Recently, a nitrogen-directed arylation of aromatic C-H bonds, leading to biaryl compounds has been developed. In 2001, Oi demonstrated that ruthenium(II)-phosphine can be used as a catalyst in the regioselective arylation of 2-arylpyridines using aryl halides (Eq. 9.34) [4 ]. The predominant ortho selectivity indicates that the coordination of... 

Recently, a new regioselective synthesis of 2-alkyl and 2-aryl 5-substituted pyridines was reported128. It involved the chemoselective reaction of commercially available 2,5-dibromopyridine with terminal acetylenes or phenylzinc chloride in the presence of Pd(0) (equation 110). However, this method does not allow the preparation of 2-methyl-5-... 

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