7147 [PtP2H(olefin)] which subsequently loses olefin, and reacts further with R X (Scheme I). &Migration in [PtP3R]. seems less likely since a 19-electron intermediate would be Later in the reaction sequence, however, 3a is predominantly formed from 2a. The formation of 4a in the latter part of the reaction results from the reaction of 3a with further alkyl halide, i.e., trans-PtHBr(PEt&
+ RBr
--f
trans-PtBr~(PEt&
+ RH
This processz3 is accelerated by radical initiators and a chain mechanism again appears operative. However, for some reactive halides (e.g., a-bromoesters, benzyl bromide), 4a is produced by a radical nonchain process.6 Although our studies on the palladium analog l b are less extensive, closely similar behavior is observed, e.g., 5 duroquinone causes marked inhibition of additions to l b . Many other organic halides do not react via the chain mechanism shown in Scheme I, since radical scavengers are i n e f f e c t i ~ e . ~Recently ,~~ a nonchain mechanism has been invoked for addition of CH31, C6H5CH2Br,and C2HJ to Pt(P(C6H5)3)3based on trapping experiments using t-BuNO. 2 5 Although detailed discussion must await more extensive experimental data, we find that the reaction of C2HJ with Pt(P(CGH5)3)3 occurs predominantly by the chain mechanism as evidenced by strong inhibition using radical scavengers, the nonchain process providing perhaps only the initiation step in this case.
Acknowledgments. This work has been supported by the Alfred P. Sloan Foundation (J. A. O.), donors of the Petroleum, Research Fund, administered by the American Chemical Society, and by the National Science Foundation in the form of a generous grant (GP32317) for the XL 100-MHz nmr spectrometer. (22) Note the relative stability of the alkyl-titanium species, (TCaHahTi(CzH5)
