Mechanistic studies of the thermolytic and photolytic rearrangement of [bis(diphenylphosphino)ethane]bis(neophyl)platinum(II)

Article abstract of DOI:10.1021/om00043a049

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(diphenylphoephino)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 (ΔS^≠_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.