Experimental and Computational Studies on the Mechanism of N-Heterocycle C−H Activation by Rh(I)

Abstract

Evidence is presented for a proposed mechanism of C-H activation of 3-methyl-3,4-dihydroquinazoline (1) by (PCy(3))(2)RhCl. One intermediate (3), a coordination complex of 1 with (PCy(3))(2)RhCl, was identified along the path to the Rh-N-heterocyclic carbene product of this reaction (2). Isotopic labeling and reaction-rate studies were used to demonstrate that C-H activation takes place intramolecularly on the reaction coordinate between 3 and 2. Computational studies corroborate the proposed mechanism and suggest that the rate-limiting step is oxidative addition of the C-H bond to the metal center. The consequences of this mechanism for coupling reactions of N-heterocycles that occur via Rh-catalyzed C-H bond activation are discussed.

Theoretical studies of iridium-mediated tautomerization of substituted pyridines

“…the conversion of a Rh(III) pyridyl hydride complex to the Rh(I) protic NHC product, and the formal 1,3-shift of the hydride proceeds via the b-insertion mechanism [32]. In our system, however, this mechanism is unlikely in that the HeIreCeN fragment cannot adopt the required syn coplanar conformation.…”

“…1) [33e35]. This tautomerization process is important not only in biological processes [36]but also in useful catalytic CeC coupling reactions, where the Rh and Ru protic NHC complexes are authenticated as active catalysts [32,33,37]. Interconversions between protic NHC complexes and N-bound complexes or the metal heteroaryl hydride complexes should greatly modify the electronic effects of the metal center, leading to desirable features in catalysis and in molecular recognition.…”

“…Mechanistic studies on this tautomerization process, however, are rare [32,38,39]. Bergman, Ellman, and coworkers reported the first studies on the tautomerization of 3-methyl-3,4-dihydroquinazoline, and a rhodium(III) hydride intermediate was established as an intermediate, which, on the basis of theoretical studies, undergoes bÀhydride insertion into the proximal heteroaryl group to give the NHC product (Scheme 1) [32].…”

“…Bergman, Ellman, and coworkers reported the first studies on the tautomerization of 3-methyl-3,4-dihydroquinazoline, and a rhodium(III) hydride intermediate was established as an intermediate, which, on the basis of theoretical studies, undergoes bÀhydride insertion into the proximal heteroaryl group to give the NHC product (Scheme 1) [32]. We recently reported the microscopic reverse process, and 1,3-shift of the NH proton to the iridium(I) center was observed upon anion exchange, leading to iridium(III) hydrides (Scheme 2) [39].…”

Metallacyclic Pyridylidene Structures from Reactions of Terminal Pyridylidenes with Alkenes and Acetylene

“…[7][8][9] In work based on the use of [Tp'Ir(R)(R')] fragments (Tp' = hydrotris(pyrazolyl)borate groups; R,R' = alkyl or aryl groups), we observed isomerization of pyridine, 2-substituted pyridines, and polypyridines to NHCs. [10] As a continuation of this work, we have studied the reactivity of pyridylidene complexes 1 toward ethene, propene, and acetylene to yield novel iridacyclic pyridylidene structures that stem from CÀN bond formation.…”