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

Abstract

Experimental and theoretical studies on equilibria between iridium hydride alkylidene structures, [(TpMe2)Ir(H){C(CH2R)ArO}] (TpMe2=hydrotris(3,5‐dimethylpyrazolyl)borate; R=H, Me; Ar=substituted C6H4 group), and their corresponding hydride olefin isomers, [(TpMe2)Ir(H){R(H)CC(H)OAr}], have been carried out. Compounds of these types are obtained either by reaction of the unsaturated fragment [(TpMe2)Ir(C6H5)2] with o‐C6H4(OH)CH2R, or with the substituted anisoles 2,6‐Me2C6H3OMe, 2,4,6‐Me3C6H2OMe, and 4‐Br‐2,6‐Me2C6H2OMe. The reactions with the substituted anisoles require not only multiple CH bond activation but also cleavage of the MeOAr bond and the reversible formation of a CC bond (as revealed by 13C labeling studies). Equilibria between the two tautomeric structures of these complexes were achieved by prolonged heating at temperatures between 100 and 140 °C, with interconversion of isomeric complexes requiring inversion of the metal configuration, as well as the expected migratory insertion and hydrogen‐elimination reactions. This proposal is supported by a detailed computational exploration of the mechanism at the quantum mechanics (QM) level in the real system. For all compounds investigated, the equilibria favor the alkylidene structure over the olefinic isomer by a factor of between approximately 1 and 25. Calculations demonstrate that the main reason for this preference is the strong Ir–carbene interactions in the carbene isomers, rather than steric destabilization of the olefinic tautomers.

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).…”

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

“…Moreover, pure individual samples of, for example, C or 3 remain unaltered when heated under the conditions in which they form (although as discussed in the following paper, [11] 3 equilibrates with its hydride alkene tautomer 6 at somewhat higher temperatures). Therefore, they must form through different, competitive reaction pathways, one involving CÀH bond activation (for B and C) and the other requiring more complex reactivity to be discussed in due course [11] (yielding 2 and 3).…”

“…[12] In our system, the CÀO bond cleavage is accompanied by the formation of a new CÀC bond, which gives rise in appropriate cases to hydride alkylidene and hydride alkene iridium tautomers that exist in equilibrium. This facet of the chemistry of the alkyl aryl ethers investigated here will be dealt with in the following paper in this issue, [11] taking advantage of the observation of such a reactivity in the reactions of 1 with 2,6-methylanisole and related molecules. Part of this work has been published in a preliminary form.…”

“…Only the diastereomer shown in Scheme 4 has been detected. Coordination of the other enantiotopic face of the alkene [15] appears to be sterically hindered, but as revealed by theoretical calculations to be discussed in the following paper, [11] the corresponding isomer can be accessed upon heating. Spectroscopic data for 6 are collected in the Experimental Section and provide full support for its structure.…”

“…[8][9][10] To complete these studies, we have also investigated the reactivity of alkyl aryl ethers A, toward the unsaturated [(Tp Me2 )IrA C H T U N G T R E N N U N G (C 6 H 5 ) 2 ] fragment (1) (Tp Me2 = hydrotris(3,5-dimethylpyrazolyl)borate) and present corresponding results in this and the following paper. [11] Firstly we address the CÀH bond activation itself, by studying the reactivity of 1 toward anisole, phenetole (ethyl phenyl ether, C 6 H 5 OEt), and some of their halo or other derivatives, and analyze by DFT methods the aliphatic and aromatic C À H bond activation. Clearly, metal coordination to the ring p system, as well as to the oxygen atom, may facilitate activation of the CA C H T U N G T R E N N U N G (sp 2 )ÀH bonds, whereas only IrÀO coordination can assist that of the CA C H T U N G T R E N N U N G (sp 3 )ÀH bonds.…”