Experimental and Computational Studies on the Iridium Activation of Aliphatic and Aromatic CH Bonds of Alkyl Aryl Ethers and Related Molecules

Abstract

Reaction of the Ir(III) complex [(Tp(Me2))Ir(C(6)H(5))(2)(N(2))] (1N(2)) with ortho-cresol (2-methylphenol) occurs with cleavage of the O-H and two C(sp(3))-H bonds of the phenol and formation of the electrophilic hydride alkylidene derivative [(Tp(Me2))Ir(H){=C(H)C(6)H(4)-o-O}] (2). The analogous reaction of 2-ethylphenol gives a related product 3. Both 2 and 3 have been shown to be identical to the minor, unidentified products of the already reported reactions of 1 with anisole and phenetole, respectively. Thus, in addition to the route that leads to the known heteroatom-stabilized hydride carbene [(Tp(Me2))Ir(H){=C(H)OC(6)H(4)-o-}] (B), anisole can react with 1 with cleavage of the O-CH(3) bond and formation of a new carbon-carbon bond. In contrast, only C-H bond-activation products with structures akin to B result from 1N(2) and 3,5-dimethylanisole (complex 8) or 4-fluoroanisole (9). Using anisole as a model, a computational study of the triple C-H bond activation (two aliphatic C-H bonds plus an ortho-metalation reaction) that is responsible for the formation of these heteroatom-stabilized hydride carbenes has been undertaken.

Synthetic, Mechanistic, and Theoretical Studies on the Generation of Iridium Hydride Alkylidene and Iridium Hydride Alkene Isomers

“…It has been shown in the preceding paper [9] that the reaction of 1·N 2 and 2-ethylphenol at 60 8C gives a kinetic mixture of the alkylidene and alkene isomers, 2 c/2 o, of approximately 20:1 (Scheme 2). However, if the reaction is performed at higher temperatures (!…”

“…Taking into consideration that the reaction of 1·N 2 with anisole and phenetole gives rise to two different hydride carbene products, [9] the formation of which requires aromatic ortho-C À H bond activation, we have investigated the analogous reactions of the 2,6-dimethyl-substituted anisoles shown in Scheme 4. Quite unexpectedly, the hydride olefin complexes 4 o-6 o are formed in high spectroscopic yields (!…”

“…[8a] Also very recently Caulton et al found analogous equilibria between the p-olefin and carbene isomers of norbornene in the reaction of this alkene with some rhenium complexes of pincer ligands of type [ReH 4 A C H T U N G T R E N N U N G (PNP R )] (PNP R = N(SiMe 2 CH 2 PR 2 ) 2 ). [8b] As discussed in the preceding paper in this issue, [9] we have observed that the reaction of the complex [(Tp Me2 )Ir-A C H T U N G T R E N N U N G (C 6 H 5 ) 2 (N 2 )], 1·N 2 (Tp Me2 = hydrotris(3,5-dimethylpyrazolyl)borate) with 2-ethylphenol occurs in the way represented in Scheme 2, to give the hydride alkylidene complex 2 c as the major product accompanied by minor amounts (ca. 5 %) of its hydride alkene tautomer, 2 o (c and o will be employed throughout this paper as abbreviations for carbene and olefin, respectively).…”

Cyclometalation Using d-Block Transition Metals: Fundamental Aspects and Recent Trends

“…The alkyl group is transferred to the aryl ring, and affords a mixture of alkylidene and alkene iridium hydride complexes. 381,382 Iridium hydrides have also been generated via cyclometalation of pyridyl-functionalized carbenes. Upon reaction of the free carbene 234 with [IrCl(cod) 2 ] 2 , C pyridyl -H bond oxidative addition occurs to yield the C,C-iridacycle 235 rather than pyridine N-coordination (Scheme 98).…”

Carbenes and Nitrenes

“…Nitrene nickel complexes (97) have been added to alkyl and aryl azides to provide unsymmetrical azo compounds (99). 133 The mechanism has been shown to involve a metallospirobicycle (98 …”