Stereoselective thiirane desulfurization controlled by a bridging or terminal acyl ligand: Concerted vs. SN2 pathways

An acyl-bridged heterodinuclear Pt/Mn complex [(dppe)Pt(μ-C(O)Np-1κC,2κO)–Mn(CO)4] (Np = neopentyl, –CH2CMe3) has been prepared and unambiguously characterized. Upon reaction with thiiranes, this species is able to facilitate the direct desulfurization of three-membered S-heterocycles to produce olefins with complete retention of stereochemistry, with kinetic details supporting a concerted mechanism for thiirane desulfurization. Conversion of this species to a non-bridged, opened Pt/Mn complex [(dppe)(NpC(O))Pt–Mn(CO)5] can be effected by reaction with CO(g) in benzene solution. These non-bridged species display the opposite reactivity with thiiranes, generating the isomer with an inverted stereochemistry. Isolation of key thiamanganacyclic intermediates indicate that inversion occurs in the first C–S bond cleavage event, while the second desulfurization step proceeds with retention to produce the olefin. As a result, control of stereoselectivity in this process appears to be mediated by the existence or absence of forced Pt/Mn metal–metal proximity.