'Synthetic prospecting' using an electrospray ionisation mass spectrometry directed survey of the alkylation and arylation chemistry of [Pt2(μ-S)2(PPh3)4]

Abstract

Electrospray ionisation mass spectrometry (ESI-MS) has been used as an analytical tool in a wide-ranging scoping study of the alkylation and arylation reactions of [Pt2(μ-S)2(PPh3)4]. From these experiments, the factors that influence the formation of different product species - formed by mono- or di-alkylation - are determined. If the alkylating agent is an alkyl chloride or sulfate, monoalkylation followed by dialkylation of the two sulfido groups occurs, dependent on the alkylating power of the reagent used. For example, n-butyl chloride gives solely [Pt2(μ-S)(μ-SBu)(PPh3)4]+ while dimethyl sulfate gives [Pt2(μ-SMe)2(PPh3)4]2+. This species, previously unisolated is stable in the absence of good nucleophiles, but the addition of potassium iodide results in rapid conversion to [Pt2(μ-SMe)2(PPh3)3I]+. This iodo complex is also observed from the reaction of [Pt2(μ-S)2(PPh3)4] with excess MeI, after the initial formation of mono- and di-methylated species. In these reactions, the iodide presumably displaces a phosphine ligand, which is then quaternised by excess alkylating agent. Changing the alkylating agent to a longer chain alkyl iodide or methyl bromide decreases the rate of alkylation of the sulfide in the initially formed [Pt2(μ-S)(μ-SR)(PPh3)4]+. Mixed-thiolate species of the type [Pt2(μ-SMe)(μ-SR)(PPh3)4]2+ are easily generated by reaction of [Pt2(μ-S)(μ-SR)(PPh3)4]+ with excess Me2SO4 and is also dependent on the avoidance of nucleophiles. Reactions towards α,ω-dialkylating agents are surveyed; the chain length is found to have a dramatic effect on the rate of the second intramolecular cyclisation process, illustrated by a competitive reactivity study involving a mixture of Br(CH2)4Br and Br(CH2)5Br; on completion of the reaction the former gives [Pt2{μ-S(CH2)4S}(PPh3)4]2+ while the latter predominantly gives monoalkylated[Pt2(μ-S){μ-S(CH2)5Br}(PPh3)4]+. The reactivity of o- and p-dihaloxylenes has been explored, with the reaction with p-BrCH2C6H4CH2Br giving the bridged species [(PPh3)4Pt2(μ-S)(μ-SCH2C6H4CH2S)(μ-S)Pt2(PPh3)4]2+. Arylation reactions of [Pt2(μ-S)2(PPh3)4] with halobenzenes and 2-bromoheterocyclic compounds (pyridine, thiophene) are also described. © 2005 Elsevier B.V. All rights reserved.