POP–Rhodium-Promoted C–H and B–H Bond Activation and C–B Bond Formation
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The square-planar monohydride complex RhH{xant(PiPr2)2} (1; xant(PiPr2)2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) activates C–H bonds of arenes. Heating of benzene solutions of 1 at 80 °C affords RhPh{xant(PiPr2)2} (2). Under the same conditions, toluene gives Rh(m-tolyl){xant(PiPr2)2} (3a) and Rh(p-tolyl){xant(PiPr2)2} (3b) in a 78:22 molar ratio, whereas m-xylene leads to Rh(C6H3-3,5-Me2){xant(PiPr2)2} (4). At room temperature, fluorobenzene and 1,3-difluorobenzene generate Rh(C6H4-2-F){xant(PiPr2)2} (5) and Rh(C6H3-2,6-F2){xant(PiPr2)2} (6), respectively. Complex 1 also promotes the B–H bond activation of pinacolborane (HBpin) and catecholborane (HBcat). The reactions initially give the trans-dihydride derivatives RhH2(BR2){xant(PiPr2)2} (BR2 = Bpin (7), Bcat (8)), which lose H2 to afford the square-planar Rh(BR2){xant(PiPr2)2} (BR2 = Bpin (9), Bcat (10)). Complex 2 reacts with HBpin to regenerate 1 and to give Ph-Bpin. Similarly, complex 9 regenerates 1 and gives Ph-Bpin by reaction with benzene. In agreement with these transformations, complex 1 catalyzes the direct C–H borylation of arenes. The selectivity of the process appears to be governed by the kinetic of the C–H bond activations of the arenes. Benzylic borylation is not observed for methylbenzenes. The first X-ray structure of a square-planar rhodium–boryl complex is also reported.
