Abstract
A nickel/N-heterocyclic carbene (NHC) catalytic system has been developed for the borylation of aryl sulfoxides with B(2) (neop)(2) (neop=neopentyl glycolato). A wide range of aryl sulfoxides with different electronic and steric properties were converted into the corresponding arylboronic esters in good yields. The regioselective borylation of unsymmetric diaryl sulfoxides was also feasible leading to borylation of the sterically less encumbered aryl substituent. Competition experiments demonstrated that an electron-deficient aryl moiety reacts preferentially. The origin of the selectivity in the Ni-catalyzed borylation of electronically biased unsymmetrical diaryl sulfoxide lies in the oxidative addition step of the catalytic cycle, as oxidative addition of methoxyphenyl 4-(trifluoromethyl)phenyl sulfoxide to the Ni(0) complex occurs selectively to give the structurally characterized complex trans-[Ni(ICy)(2) (4-CF(3) -C(6) H(4) ){(SO)-4-MeO-C(6) H(4) }] 4. For complex 5, the isomer trans-[Ni(ICy)(2) (C(6) H(5) )(OSC(6) H(5) )] 5-I was structurally characterized in which the phenyl sulfinyl ligand is bound via the oxygen atom to nickel. In solution, the complex trans-[Ni(ICy)(2) (C(6) H(5) )(OSC(6) H(5) )] 5-I is in equilibrium with the S-bonded isomer trans-[Ni(ICy)(2) (C(6) H(5) )(SOC(6) H(5) )] 5, as shown by NMR spectroscopy. DFT calculations reveal that these isomers are separated by a mere 0.3 kJ/mol (M06/def2-TZVP-level of theory) and connected via a transition state trans-[Ni(ICy)(2) (C(6) H(5) )(η(2) -{SO}-C(6) H(5) )], which lies only 10.8 kcal/mol above 5.