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

Paul A Morton; Abigayle L Boyce; Anamarija Pišpek; Lennox W Stewart; Daniel J Ward; Bengt E Tegner; Stuart A Macgregor; Stephen M Mansell

doi:10.1021/acs.organomet.4c00025

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

Organometallics. 2024 Mar 28;43(9):974-986. doi: 10.1021/acs.organomet.4c00025. eCollection 2024 May 13.

Authors

Paul A Morton¹, Abigayle L Boyce¹, Anamarija Pišpek¹, Lennox W Stewart¹, Daniel J Ward¹, Bengt E Tegner¹, Stuart A Macgregor¹, Stephen M Mansell¹

Affiliation

¹ Institute of Chemical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, U.K.

Abstract

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.