sp³–sp² vs sp³–sp³ C–C Site Selectivity in Rh-Catalyzed\nRing Opening of Benzocyclobutenol: A DFT Study

Abstract

The\nC_sp³–C_sp² vs C_sp³–C_sp³ site\nselectivity in the C–C bond activation in Rh-catalyzed ring\nopening of benzocyclobutenol was systematically investigated using\ndensity functional theory (DFT). The catalytic cycle includes three\nelementary steps: the proton transfer from the substrate to a rhodium\nhydroxide, the C–C cleavage, and the proton transfer from water\nonto a carbon forming the final product with regeneration of the rhodium\nhydroxide. The site selectivity is determined by the C–C cleavage\nstep; the C_sp³–C_sp² cleavage is favored over the C_sp³–C_sp³ cleavage because the former transition state\nis stabilized by an interaction between the benzene ring of the substrate\nand Rh. DMSO, a more polar solvent, reduces the site selectivity as\nthe more polar C_sp³–C_sp³ transition state (TS) is stabilized more than the C_sp³–C_sp² TS and decreases\nthe advantage of the latter TS. DPPF ligand is bulky, and the steric\nrepulsion on the tighter C_sp³–C_sp² TS causes the loss of the site selectivity. For the\neven more crowded Rh­(P­(<i>t</i>-Bu)₃)₂ catalyst, one phosphine has to dissociate before the C–C\ncleavage reaction takes place, and the advantage of the C_sp³–C_sp² TS is regained for\nthe less crowded RhP­(<i>t</i>-Bu)₃ active catalyst.