Examination of a Series of Ir and Rh PXL Pincer Complexes as (Pre)catalysts for Aromatic C–H Borylation

• Published in: Organometallics Vol. 40; no. 7; pp. 1004 - 1013
• Main Authors: Hung, Ming-Uei, Press, Loren P, Bhuvanesh, Nattamai, Ozerov, Oleg V
• Format: Journal Article
• Language: English
• Published: American Chemical Society 12.04.2021
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/acs.organomet.1c00081

This work follows a previously published study of high-turnover aromatic C–H borylation by Ir complexes supported by POCOP-type ligands incorporating −PiPr2 side donors (L1–L3) using HBpin in the presence of olefins. A variety of pincer-supported Ir and Rh complexes have been tested as precatalysts in the analogous aromatic C–H borylation. The ligands in this study included a number of aryl/bis­(phosphine) pincers of the PCP type, as well as PCS, PNP, PNSb, PSiP, and PBP ligands. The syntheses primarily targeted precursors of the (PXL)­M­(H)­(Cl) type (M = Rh, Ir); exceptions included complexes (L15)­Ir­(H)­(OAc), (L17)­Ir­(COE), (L18)­Ir­(H)4, and (L16)­Rh­(H2). The catalytic competence was tested using C6D6 as the substrate (and solvent) with HBpin and 1-hexene as reagents. C–H borylation and hydroboration of 1-hexene were the two competing catalytic reactions. None of the Rh complexes tested displayed any C–H borylation activity. Among the Ir complexes, only those possessing a central aryl site in the pincer showed significant C–H borylation activity with C6D6. The most promising precatalysts (L3)­Ir­(H)­(Cl), (L4)­Ir­(H)­(Cl), (L7)­Ir­(H)­(Cl), (L11)­Ir­(H)­(Cl), (L13)­Ir­(H)­(Cl), and (L15)­Ir­(H)­(OAc) were tested in the C–H borylation of C6H5F and C6H5CF3. All of these catalysts showed a roughly statistical preference for the borylation of only meta and para sites in C6H5CF3. For C6H5F borylation of all three sites (ortho, meta, para) was observed. Catalysts supported by the ligands L3 and L11 showed the highest rate of reaction and higher selectivity for C–H borylation vs hydroboration. Borylation of m-ClC6H4Me catalyzed by (L3)­Ir­(H)­(Cl) and (L11)­Ir­(H)­(Cl) resulted in only the borylation of only one aromatic C–H site (meta to both substituents), <10% of borylation of the benzylic site, and no borylation of the C–Cl moiety.