Reaction of Trimethyl- and Triphenylstannate, (CH3)3Sn(-) and Ph3Sn(-), with Optically Active 2-Octyl Bromide, Chloride, and Tosylate

Article abstract of DOI:10.1021/ja00374a022

The reaction of lithium, sodium, and potassium trimethylstannate, (CH3)3SnLi, -Na, and -K, and triphenylstannate, Ph3SnLi, -Na, and -K, with optically active 2-octyl tosylate, chloride, and bromide is reported.Alkylation of (CH3)3SnM with 2-octyl tosylate proceeds with complete inversion at carbon.The corresponding reactions with 2-octyl chloride and especially 2-octyl bromide show considerably greater variations in reaction stereoselectivity.These variations depend on a variety of reaction parameters, including the order of reagent addition, i.e., normal vs. inverse addition of reagents, reaction temperature, solvent, gegenion, concentration, and additives.Thus, for example, at 0 deg C the normal addition reaction of 2-octyl chloride with (CH3)3SnM in THF proceeds with 100percent inversion of configuration, while the equivalent reactions with 2-octyl bromide proceed with 34percent (M = Na), 33percent (M = K), and 53percent (M = Li) net inversion.The latter reactions are more stereoselective when carried out at lower temperatures: at -70 deg C the corresponding values are 62percent (M = Na) and 83percent (M = Li) net inversion.Stereoselectivity in these reactions is dramatically influenced by the order of reagent addition.Thus, under inverse-addition conditions, for example, the reaction of (CH3)3SnM with 2-octyl bromide in THF at 0 deg C occurs with high stereoselectivity (97percent and 98percent net inversion, respectively, for M = Li and Na) and with minor adjustment of other parameters can be made to occur stereospecifically.Both the normal and indirect addition of Ph3SnLi and -Na to 2-octyl bromide occur with complete inversion of configuration at carbon.A comparison of the reactivity of (CH3)3SnM and Ph3SnM reveals that the latter is significantly less reactive than the former; triphenylstannate, however, is the more stereoselective reagent, its reaction with optically active 2-bromooctane being essentially stereospecific, irrespective of the order of reagent addition.The results of this study and those from earlier investigations involving the cis- and trans-4-tert-butylcyclohexyl and cyclopropylcarbinyl systems are compared and discussed as well as the reliability of recently reported trapping experiments for discerning between the S_N2 and free radical components of such reactions.