EFFECT OF STRUCTURAL FEATURES OF TRI-FUNCTIONALIZED ARYL PHOSPHATE ISOMERS IN THE S_RN1 BEHAVIOUR. A DFT STUDY

Our group has been involved in searching new synthetic routes to organostannanes by the S_RN1 mechanism.¹ Based on our experience we consider of interest to study the scope of S_RN1 in the polystannylation of an aromatic ring. Now we inform the experimental results obtained in the reaction of trimethylstannylsodium (1) with aryl phosphates supporting three nucleofugal groups, in liquid ammonia as solvent, and a DFT study which support the notable differences observed between isomeric substrates. In order to synthesize 1,2,4-tris(trimethylstannyl)benzene (2) we use as starting substrates 1,3-bis(diethoxyphosphoryloxy)-4-chlorobenzene (3) and 1,4-bis(diethoxyphosphoryloxy)-3-chlorobenzene (4). Under photostimulation 3 and 4 led to entirely different products distributions. Meanwhile 3 afforded 2 together with 1,3- and 1,4-bis(trimethylstannyl)benzene in a 63:23:10 ratio, 4 yielded a mixture of 2 and 1,4- bis(trimethylstannyl)benzene in a 32:68 ratio. Starting substrates were almost recovered in dark reactions supporting an S_RN1 mechanism. These results were unexpected since the substrates would have similar electron affinities. We have previously demonstrated that differences in spin density on radical anions affect products distributions,² so, we started a systematic theoretical study over 3 and 4 with the B3LYP functional, the LACVP pseudopotential (Sn) and the 6-31+G* (C, H, O, P) with GAUSSIAN03. We observed meaningful differences in SOMOs MO, spin density distribution and activation energies of the corresponding radical anions intermediate, which support the experimental outcome and allow us to interpret the distribution of stannylated end-products in terms of the structural features of the starting isomers. ¹ Silbestri, G. F.; Lo Fiego, M. J.; Lockhart, M. T.; Chopa, A. B. J. Organometal. Chem. 2010, 695, 2578-95 and references therein. ² Dorn, V. B.; Badajoz, M. A.; Lockhart, M. T.; Chopa, A. B.; Pierini, A. B. J. Organometal. Chem. 2008, 693, 2458-62.