Catalyst design for Rh-catalysed 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 borylationcatalysts, 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 andcarbene ligands. [RhCp(SIDipp)(COE)] (1) formed alongside the C–Cactivated, 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–Hactivation of the SIDipp aryl group is thermally attainable and reversibleunder experimental conditions, with the C–C activation products being the morethermodynamically stable species. Oxidative addition of 1 withSiH(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) werealso characterized, but attempts to synthesize Rh carbene complexes withfluorenyl or 1,2,3,4-tetrahydrofluorenyl ligands were not successful. For thecatalytic C–H borylation of benzene using B₂pin₂, 1 wasinactive at 80 °C, and [Rh(Ind)(SIDipp)(COE)] was superior to all othercomplexes tested due to the shortest induction period. However, the addition ofHBpin to precatalyst 4 eliminated the induction period.Catalytic n-alkane C–H borylation using [Rh(Ind)(NHC)(COE)] gaveyields of up to 21% alkylBpin, but [RhCp*(C₂H₄)₂]was the better catalyst.