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(κ²C_Ar,C_carbene-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; ν_CO = 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.