Transition metal promoted cyclization reactions of nitrile ligands: Stereochemically controlled synthesis and structural investigation of some chiral oxazoline platinum(II) complexes

Abstract

The reactions of the nitrile complexes cis- and trans-[PtCl2(NCR)2] with S(+)-2-chloro-1-propanol in the presence of 2 equiv. of n-BuLi afford the chiral oxazoline complexes cis- and trans-[PtCl2{(R)-N=C(R)OCH2CH(Me)}2] [cis, R=Me (1), cis, R=Ph (2); trans, R=Ph (3)]. The structures of 1 and 2 were determined by X-ray analysis which allowed to establish the R configuration of the oxazoline rings. The reactions of cis- and trans-[PtCl2(NCR)2] with S(+)-2-chloro-1-phenylethanol and R(−)-2-chloro-1-phenylethanol in the presence of 2 equiv. of n-BuLi afford the oxazoline complexes cis- and trans-[PtCl2{(S)-N=C(R)OCH(Ph)CH2}2] [cis, R=Me (4), cis, R=Ph (5); trans, R=Ph (6)] and cis- and trans-[PtCl2{(R)-N=C(R)OCH(Ph)CH2}2] [cis, R=Me (7), trans, R=Me (8); trans, R=Ph (9)]. The structure of 6 was determined by an X-ray investigation which allowed to establish the S configuration of the oxazoline rings. The proposed mechanism for the nitrile to chiral oxazoline conversion entails nucleophilic attack of the alkoxide, generated by deprotonation reaction of the corresponding alcohol with n-BuLi, followed by intramolecular cyclization. This latter step proceeds with complete inversion of configuration at the chiral carbon atom for 1–3, while with retention of configuration for 4–9.