IRREVERSIBLE ADDITION OF CARBON NUCLEOPHILES TO ETHYLENE IN CATIONIC PLATINUM(II) COMPLEXES

The cationic complex [Pt(eta(2)-C2H4)Cl(tmen)]+ 1 (tmen = N,N,N',N'-tetramethylethylenediamine) reacts with carbon nucleophiles [an acidic carbon substrate (HL) in the presence of a base (Na2CO3); HL = acetylacetone, HL1; ethyl acetoacetate, HL2; or diethyl malonate, HL3] to give the corresponding addition products [Pt(CH2CH2L)Cl(tmen)] (L = L1, 3; L2, 4; or L3, 5) and [{(tmen)CIPtCH2CH2}2(L1 - H)] 6. Compound 3 exhibits a keto-enol tautomerism. The keto tautomer is kinetically and thermodynamically favoured and in solution it represents about 90% of the equilibrium mixture; on the contrary, the enol tautomer is the favoured form in the solid. In compound 4 the carbon atom of the nucleophile which has bound to ethylene is chiral and, as a consequence, the methylene protons exhibit a diastereotopic splitting which is greater for the methylene group next to platinum than for that next to the asymmetric carbon. Treatment of compounds 3-6 with acids (HCl or HClO4) does not reverse the carbon to carbon coupling but cleaves the platinum-carbon bond, forming the ethylated nucleophile and [PtCl2(tmen)] or [{PtCl(tmen)}2][ClO4]2 depending on the acid used. The cationic complex [Pt(eta(2)-C2H4) (NO2) (tmen)]+ 2 reacts with carbon nucleophiles in a similar way to 1. Treatment of the addition products with HClO4 under an ethylene atmosphere gives the ethylated nucleophile and the starting complex 2.