Catalyst Design for Rh-Catalyzed Arene and Alkane C–H Borylation: The NHC Affects the Induction Period, and Indenyl is Superior to Cp

In order to establish design criteria for Rh C–H
borylation
catalysts, analogues of the successful catalyst [Rh(Ind)(SIDipp)(COE)]
(Ind = η5-indenyl, SIDipp = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene,
and COE = cis-cyclooctene) were synthesized by changing
the indenyl and carbene ligands. [RhCp(SIDipp)(COE)] (1) formed alongside the C–C activated, cyclometalated byproduct
[RhCp(κ2CAr,Ccarbene-SIDipp′)(iPr)] (rac-2; SIDipp′ = 1-(6-isopropylphenyl)-3-(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene).
Computational modeling of COE dissociation showed that both C–C
and C–H activation of the SIDipp aryl group is thermally attainable
and reversible under experimental conditions, with the C–C
activation products being the more thermodynamically stable species.
Oxidative addition of 1 with SiH(OEt)3 gave
the Rh silyl hydride [RhCp(H){Si(OEt)3}(SIDipp)] (rac-3). [Rh(Ind)(IDipp)(COE)] (4; IDipp = 1,3-bis(2,6-diisopropylphenyl)-imidazole-2-ylidene), the
carbonyl analogue [Rh(Ind)(IDipp)(CO)] (5; νCO = 1940 cm–1, cf. 1944 cm–1 for [Rh(Ind)(SIDipp)(COE)]), and [Rh(Ind)(IMe4)(COE)]
(6; IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene)
were also characterized, but attempts to synthesize Rh carbene complexes
with fluorenyl or 1,2,3,4-tetrahydrofluorenyl ligands were not successful.
For the catalytic C–H borylation of benzene using B2pin2, 1 was inactive at 80 °C, and [Rh(Ind)(SIDipp)(COE)]
was superior to all other complexes tested due to the shortest induction
period. However, the addition of HBpin to precatalyst 4 eliminated the induction period. Catalytic n-alkane
C–H borylation using [Rh(Ind)(NHC)(COE)] gave yields of up
to 21% alkylBpin, but [RhCp*(C2H4)2] was the better catalyst.