Reactivity and Mechanism in the Oxidative Addition of Allylic Halides to a Dimethylplatinum(II) Complex

The complex [PtMe2(2,2′-bipyridine)], 1, reacts with allyl chloride and allyl bromide, to give [PtXMe2(CH2CHCH2)­(2,2′-bipyridine)], 2, X = Cl; 3, X = Br, with 2-methylallyl chloride to give [PtClMe2(CH2CMeCH2)­(2,2′-bipyridine)], 4, and with crotyl chloride (a mixture of trans- and cis-MeCHCHCH2Cl), to give a mixture of [PtClMe2(trans-CH2CHCHMe)­(2,2′-bipyridine)], 5, and [PtClMe2(cis-CH2CHCHMe)­(2,2′-bipyridine)], 6. The complexes are formed mostly by trans oxidative addition and, for the crotyl complexes, without allylic rearrangement. Complex 1 reacts with CH2CHCHMeCl, mostly with allylic rearrangement, to give complex 5, with complexes 6 and [PtClMe2(CHMeCHCH2)­(2,2′-bipyridine)], 7, as minor products. The reactions of 1 with CH2CHCH2X (X = Cl or Br) and MeCHCHCH2Cl follow second-order kinetics (first-order in both reagents), but 1 reacts with CH2CHCHMeCl by third-order kinetics (first-order in 1, second-order in CH2CHCHMeCl) and with CH2CMeCH2Cl by a mixture of second- and third-order kinetics. The second-order reactions are proposed to occur by the SN2 mechanism, and potential mechanisms of the third-order reactions are discussed.