MECHANISTIC STUDIES OF THE THERMOLYTIC AND PHOTOLYTIC REARRANGEMENT OF [BIS(DIPHENYLPHOSPHINO)ETHANE]BIS(NEOPHYL)PLATINUM(II)

Comparative mechanistic studies are presented of the thermolytic and photolytic behavior of the bis-(neophyl)platinum(II) derivative Pt(CH2CMe2Ph)2(dppe) (neophyl = 2-methyl-2-phenylpropyl; dppe = 1,2-bis(diphenylphosphino)ethane). Thermolytic rearrangement is less facile than for monodentate P-donor analogues and affords the platinaindan Pt(2-C6H4CMe2CH2)(dppe) by intramolecular aromatic C-H activation and H-transfer to eliminated tert-butylbenzene. The kinetic isotope effect on metallacyclization (k(obs)H/k(obs)D = 2.40) and the negative activation entropy (DELTA-S(double dagger)obs = -13 (+/-4) J.K-1.mol-1) suggest a pathway in which scission of one Pt-P is mechanistically significant but C-H addition to Pt has the most energetic transition state. Photolytic rearrangement in toluene-d0 proceeds by two major paths, both of which involve primary Pt-C homolysis. The resultant neophyl radical may then provide a destination for migrating hydrogen in a cyclometalation of the residual 17-electron organoplatinum species, leading to Pt(2-C6H4CMe2CH2) (dppe). Alternatively, H-abstraction by the neophyl fragment within the solvent cage produces a benzyl radical which recombines with the metal moiety to give Pt(CH2Ph)(CH2Me2Ph)(dppe). This benzylplatinum complex is also photolabile, giving ultimately platinaindan via benzyl radical expulsion. This "indirect' solvent metalation is not evident during photolysis in toluene-d8 and in benzene.