C–H and C–F Bond Activations at a Rhodium(I) Boryl Complex: Reaction Steps for the Catalytic Borylation of Fluorinated Aromatics

Abstract

Treatment of the rhodium(I) boryl complex [Rh(Bpin)(PEt3)3] (1, pin = pinacolato = O2C2Me4) with pentafluorobenzene, 1,3,5-trifluorobenzene, 1,3-difluorobenzene, or 3,5-difluoropyridine led to C–H activation reactions to give the aryl complexes [Rh(C6F5)(PEt3)3] (4), [Rh(2,4,6-C6F3H2)(PEt3)3] (5), [Rh(2,6-C6F2H3)(PEt3)3] (6), and [Rh{4-(3,5-C5NF2H2)}(PEt3)3] (8). For 5, 6, and 8 consecutive reactions with in situ generated HBpin occurred to yield [Rh(H)(PEt3)3] (7) and boronic esters. The boryl complex 1 gave with hexafluorobenzene or perfluorotoluene the C–F activation products [Rh(C6F5)(PEt3)3] (4) and [Rh(4-C6F4CF3)(PEt3)3] (9), respectively. The complexes 5, 6, and 9 react with B2pin2 to yield 1 and boronic ester derivatives. On the basis of these stoichiometric reactions catalytic C–H and C–F borylation reactions using 1 or 7 were developed to generate 2-Bpin-1,3,5-C6F3H2, 2-Bpin-1,3-C6F2H3, and 4-Bpin-C6F4CF3 from 1,3,5-trifluorobenzene, 1,3-difluorobenzene, or perfluorotoluene and B2pin2. On treatment of pentafluoropyridine with B2pin2 in the presence of 1 or 7 as catalyst 2-Bpin-C5NF4 was synthesized by C–F borylation at the 2-position. Using 2,3,5,6-tetrafluoropyridine, B2pin2, and catalytic amounts of 7 led to a C–H borylation reaction at the 4-position. 4-Bpin-C5NF4 can also be prepared by the reaction of 2,3,5,6-tetrafluoropyridine with stoichiometric amounts of HBpin or by the reaction of pentafluoropyridine with an excess of HBpin in the presence of 7, whereas the reaction of pentafluoropyridine with stoichiometric amounts of HBpin and 5 mol % 7 resulted in the formation of 2,3,5,6-tetrafluoropyridine via hydrodefluorination reaction at the 4-position. This regioselectivity contrasts the borylation of pentafluoropyridine at the 2-position with 1 as catalyst. Overall, the obtained fluorinated aryl boronic ester derivatives might serve as versatile building blocks.