POP–Rhodium-Promoted C–H and B–H Bond Activation and C–B Bond Formation

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.