Aryl-Vinyllithium Compounds

The configurational lability 158 159 of a-aryl-vinyllithium species in etheral solvents was first observed by Curtin 112 and more recently studied kinetically by Knorr and coworkers U3). The investigations show nicely the influence of different donor solvents on the rate of the rearrangement of the following organolithium compounds. 

The activation parameters and the pseudo first-order rate constants k of the configurational inversion 158 a-c 159 a-c are listed in Table 6. 

The results of Table 6 have been tentatively explained as follows The balance for the transformation of 160 into the solvent-separated ion pair 162 should amount to 

The donor dependence of k+ in Table 6 agrees with current notions except for TMEDA. Moderate amounts of HMPA cause an impressive rate enhancement unfortunately, the decomposition temperature of 158a in the presence of HMPA drops from around 80 to below 0 °C, precluding a precise study. 

Whereas 158d with P-CH3 groups does not invert faster than 158a, E/Z topomeriza-tion of 158e is rapid in THF even at —25 °C (k+ = 7.4 s-1 for CH3 coalescence). In the case of 164 with even bulkier t-alkyl p-substituents nonequivalences were detected 

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Numerous functionalized aryl- ", alkyl- , and vinyllithium " compounds have been used as acylating reagents for the preparation of carbene complexes mainly of Cr and W. 

The cyclization of a 4-substituted-5-hexenyllithium proceeds also by a 5> n-addition of the CHj-Li moiety to the triple bond (Scheme 7-11). By contrast, vinyllithium compounds bearing aryl or silyl substituents on the lithiated sp carbon are known to undergo facile cis-trans isomerization [15]. 

An example prepared by tin-lithium transmetallation is compound 637, which reacts with enolizable ketones, after transmetallation with cerium(III) chloride895. This intermediate was transformed into the corresponding vinylzinc reagent and, after palladium(O)-catalyzed cross-coupling reactions with aryl iodides, was used in the synthesis of the antitumor antibiotic rineomycinone B2 methyl ester940,941. The vinyllithium 627 has also been transformed into the corresponding vinyl iodide by stannylation followed by reaction with iodine. The arylation has been performed in this case by a palladium(0)-catalyzed... 

Vinyllithium is tetrameric in THF solution. Alkenyllithium compounds containing a-hydrogen or-aUcyl substituents do not isomerize readily however, the presence of donor solvents promotes (Z)-to-( ) isomerization. Those compounds containing a-aryl substituents isomerize readily. The configurational stabilities of HDC=C(Li)X, where X is CN or C02Me, and RHC=C(Li)Y, where Y is OR or MesSi, have been investigated. 

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