Further studies of substitution reactions of stannyl and germyl anionoids with alkyl bromides. Rearrangement of the 6-hepten-2-yl moiety

Abstract

The stereochemical outcomes of reactions of (trimethyltin)lithium, (dimethylphenyltin)lithium, (methyldiphenyltin)lithium, and (triphenyltin)lithium in tetrahydrofuran with trans- and cis-2-, 3-, and 4-methylcyclohexyl bromides have been determined on the basis of 1H and 13C NMR spectroscopy. The (C6H5)3SnLi reactions proceed stereospecifically with inversion at carbon, while the (CH3)3SnLi reactions are nonstereospecific, as observed previously in some other systems, cis- and trans-2-methoxybromocyclohexanes and -cyclopentanes were also reacted with (CH3)3SnLi, and low yields of (2-methoxycyclohexyl)- and (2-methoxycyclopentyl)trimethylstannanes were isolated. On the basis of 13C NMR spectra and deoxystannylation reactions, the former is largely (∼90%) trans while the latter is exclusively trans. The pronounced stereochemical distinction between reactions of (CH3)3SnLi and (C6H5)3SnLi with cyclohexyl bromides is not observed in corresponding reactions of (CH3)3GeLi and (C6H5)3GeLi; both are nonspecific. Certain reactions of cyclopropylcarbinyl bromide and 6-bromo-1-hexene with R3SnLi and R3GeLi (R = CH3 or C6H5) were also studied. Rearranged product (allylcarbinyl) was observed in the reaction of cyclopropylcarbinyl bromide with (CH3)3SnLi, but cyclopentylmethyl products (from cyclization of any hex-5-enyl free radical) was not observed in any case. However, with the secondary 6-bromo-1-heptene all reagents studied (with the exception of (C6H5)3SnLi) afforded rearranged (2-methylcyclopentyl)methyl products, consistent with the intervention of the free radical, which cyclizes rapidly. Some further estimates of the conformational A values of R3Ge and R3Sn are reported, and the triphenyl derivatives have significantly larger values.