Carbon-carbon bond activation of 2,2,6,6-tetramethyl-piperidine-1-oxyl by a RhII metalloradical:: A combined experimental and theoretical study

Competitive major carbon-carbon bond activation (CCA) and minor carbon-hydrogen bond activation (CHA) channels are identified in the reaction between rhodium(II) meso-tetramesitylporphyrin [Rh-II(tmp)] (1) and 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) (2). The CCA and CHA pathways lead to formation of [Rh-III(tmp)Me] (3) and [Rh-III(tmp)H] (5), respectively. In the presence of excess TEMPO, [Rh-II(tmp)] is regenerated from [Rh-III(tmp)H] with formation of 2,2,6,6-tetramethyl-piperidine-1-ol (TEMPOH) (4) via a subsequent hydrogen atom abstraction pathway. The yield of the CCA product [Rh-III(tmp)Me] increased with higher temperature at the cost of the CHA product TEMPOH in the temperature range 50-80 degrees C. Both the CCA and CHA pathways follow second-order kinetics. The mechanism of the TEMPO carbon-carbon bond activation was studied by means of kinetic investigations and DFT calculations. Broken symmetry, unrestricted b3-lyp calculations along the open-shell singlet surface reveal a low-energy transition state (TS1) for direct TEMPO methyl radical abstraction by the Rh-II radical (S(H)2 type mechanism). An alternative ionic pathway, with a somewhat higher barrier, was identified along the closed-shell singlet surface. This ionic pathway proceeds in two sequential steps: Electron transfer from TEMPO to [Rh-II(por)] producing the [TEMPO](+)[Rh-I(por)](-) cation-anion pair, followed by net CH3+ transfer from TEMPO+ to Rh-I with formation of [Rh-III(por)Me] and (DMPO-like) 2,2,6-trimethyl-2,3,4,5-tetrahydro-1-pyridiniumolate. The transition state for this process (TS2) is best described as an S(N)2-like nucleophilic substitution involving attack of the d(z)2 orbital of [Rh-I(por)](-) at one of the C-Me-C-ring alpha* orbitals of [TEMPO]+. Although the calculated barrier of the open-shell radical pathway is somewhat lower than the barrier for the ionic pathway, R-DFT and U-DFT are not likely comparatively accurate enough to reliably distinguish between these possible pathways. Both the radical (S(H)2) and the ionic (S(N)2) pathway have barriers which are low enough to explain the experimental kinetic data.