C-H Borylation Mediated by Group 9 Pincer Complexes and Synthesis of Iridium Triflates and Triarylmethyl Species

Abstract

The importance of organoboron compounds in chemical synthesis is apparent from the Nobel Prizes awarded to Herbert C. Brown (1979) and Akira Suzuki (2010) for their contribution to the development of new synthetic methodologies involving C-B bonds. Recently, dehydrogenative C-H borylation has emerged as an attractive approach to organoboron compounds. Multiple examples of catalytic conversion of unactivated C-H bonds in alkanes, benzylic and allylic C(sp3)-H bonds, as well as C(sp2)-H bonds in alkenes have been reported. Notably, the dehydrogenative C(sp)-H borylation of terminal alkynes (abbreviated as DHBTA) is missing in the list above.

Here we describe the first example of DHBTA catalyzed by a new SiNN iridium pincer complex. The reaction is strictly chemoselective and can be performed under very mild conditions with terminal alkynes in high yield. We then explore Rh analogs of these SiNN iridium complexes. Although the new Rh compounds turned out to be inactive as DHBTA catalysts, they brought to light the unusual dual non-innocence behavior of the SiNN ligand. The (SiNN)Rh system also turned out to be a modest benzene borylation catalyst.

Later, impressive TONs of 6500 for DHBTA were achieved by a new unsymmetric PNP iridium complex from ligand screening. The synthesis and characterization of the possible intermediates in DHBTA are also discussed. We also present the discovery of a new selective dehydrogenative diboration of the DHBTA products, alkynylboronates. The independent synthesis and characterization of the possible catalytic intermediates in the diboration are also described.

Electrophilic iridium complexes have shown their great ability to activate strong bonds or serve as strong Lewis acids to catalyze reactions. We describe synthetic approaches to a highly electrophilic iridium triflate complex supported by a PNP pincer ligand and its equilibrium with ether solvents.

The value of making new triarylmethyl cations is relevant to both hydrodefluorination of C(sp3)-F bonds and organometallic chemistry. We illustrate the synthesis of selected triarylmethanol and triarylmethyl chloride bearing electron withdrawing groups. Preliminary attempts to generate highly electrophilic triarylmethyl cations are also shown.