1,2-CF bond activation of perfluoroarenes and alkylidene isomers of titanium. DFT analysis of the C-F bond activation pathway and rotation of the titanium alkylidene moiety

Abstract

Isomeric alkylidene complexes syn- and anti-(PNP)Ti=[C(t)Bu(C(6)F(5))](F) (1) and (PNP)Ti=[C(t)Bu(C(7)F(7))](F) (2) have been generated from C-F bond addition of hexafluorobenzene (C(6)F(6)) and octafluorotoluene (C(7)F(8)) across the alkylidyne ligand of transient (PNP)Ti=C(t)Bu (A) (PNP(-)=N[2-P(CHMe(2))(2)-4-methylphenyl](2)), which was generated from the precursor (PNP)Ti=CH(t)Bu(CH(t)(2)Bu). Two mechanistic scenarios for the activation of the C-F bond by A are considered: 1,2-CF addition and [2 + 2]-cycloaddition/beta-fluoride elimination. Upon formation of the alkylidenes 1 and 2, the kinetic and thermodynamic alkylidene product is the syn isomer, which gradually isomerizes to the corresponding anti isomer to ultimately establish an equilibrium mixture (when using 1, 65/35) if the solution is heated in benzene to 105 degrees C for 1 h. Single crystal X-Ray crystallographic data obtained for the two isomers of 2 (and syn isomer of 1) are in good agreement with computed DFT-optimized models. Our calculations suggest convincingly that the isomerization process proceeds via a concerted rotation involving a heterolytic bond cleavage about the alkylidene bond. The two rotamers are thermodynamically very close in energy and interconvert with an estimated barrier of similar to 26 kcal/mol. The electronic reason for this unexpectedly low barrier is investigated. (C) 2011 Elsevier B.V. All rights reserved.