Stereoselectivity in organometallic reactions: Oxidative addition of alkyl halides to platinum(II)

The ligands cis- and trans-1,2-(N = CHC6H5)(2)C6H10 (trans, 1; cis, 2) underwent cyclometalation of one phenyl substituent on reaction with [Pt2Me4(mu-SMe2)(2)] (3) to give CH4 and chiral platinum(II) complexes [PtMe{1-(N=CHC6H4)-2-(N=CHC6H5)C6H10}], containing new N,N,C-donor tridentate ligands (trans, 4; cis, 5); complex 4 was structurally characterized. Reaction of racemic 4 and 5 with primary alkyl halides (iodomethane, iodoethane, 1-iodopropane, and benzyl bromide) gave platinum(IV) products by oxidative addition, often with a high degree of stereoselectivity. The absolute configuration of the major stereoisomer was determined for the following representative platinum(IV) products by X-ray crystal structure analyses: [PtIMe2{cis-1-(N=CHC6H4)-2-(N=CHC6H5)C6H10}] (7a), [PtIMeEt{trans-1-(N=CHC6H4)-2-(N=CHC6H5)C6H10}] (8a), and [PtBrMeBz{trans-1-(N=CHC6H4)-2-(N=CHC6H5)- C6H10}] (13a; Bz = benzyl). In other cases, structural assignment was made by NMR (including NOE effects in selected cases) and supported by molecular mechanics calculations. The first step of the oxidative addition occurs primarily at the face syn and anti to the cyclohexyl group in 4 and 5, respectively, and the initial oxidative addition is trans. In the case of methyl iodide addition, but in no others, subsequent isomerization can occur to give products which appear to arise from cis oxidative addition, and the basis for this trend is elucidated. Hydrolysis of [PtIMeEt{cis-1-(N=CHC6H4)-2-(N=CHPh)C6H10}] (9b) gives PhCH=O and [PtIMeEt{cis-1-(N=CHC6H4)-2-(NH2)C6H10}], with a change in stereochemistry at platinum.