Abstract
1,4‐ and 1,5‐diols undergo cyclodehydration upon treatment with cationic N‐heterocyclic carbene (NHC)–IrIII complexes to give tetrahydrofurans and tetrahydropyrans, respectively. The mechanism was investigated, and a metal‐hydride‐driven pathway was proposed for all substrates, except for very electron‐rich ones. This contrasts with the well‐established classical pathways that involve nucleophilic substitution.
Highlights
NHC–Ir complexes (NHC = N-heterocyclic carbene) have proven to be excellent catalysts in numerous processes, in dehydrogenations and transfer hydrogenations.[1–3, 5a,c,d,e] NHCs can be relatively functionalized to provide the desired reactivity
Mechanistic investigations indicated that the oxygen functionality on the NHC ligand was involved in proton transfer steps, which enables reactions to be performed under base-free conditions.[3b]. The binfunctional nature of the NHC–Ir complexes (1) was explored in the acceptorless dehydrogenation of alcohols[2] (Scheme 1, top)
Were intrigued by the possibility that a similar hydrogen-transfer mechanism could be operating in the case of the diols, and we have studied the cyclodehydration reactions of 1,4- and 1,5-diols catalyzed by NHC–iridium complexes 1 a–c
Summary
We observed that, when two 1,4-diols, 1-phenyl-1,4-pentanediol (2 a) and 1,4-diphenyl-1,4-butanediol (2 j), were reacted with catalyst 1 a, tetrahydrofuran products were formed in very good yields (Scheme 1, bottom) instead of the expected products derived from a dehydrogenation process (Scheme 1, top) The synthesis of this type of cyclic ether from diols is a well-established procedure that can be mediated by Brønsted[5] or Lewis acids,[6] and mechanisms that involve nucleophilic substitution have been proposed.[7] Cyclizations under basic conditions have been reported.[8] when transition-metal complexes were used, the possibility that an alternative hydrogen-borrowing (or hydrogen-autotransfer) mechanism could be operating was not investigated; this motivated us to study the mechanism of these formal cyclodehydration reactions.[9] We found that the mechanism for the dehydrogenation of benzylic alcohols by catalyst 1 a involved an initial hydrogen-transfer step with concomitant formation of an iridium–hydride species.[2] The hydroxy/alkoxide functionality on the carbene ligand participated in proton-transfer steps. We propose mechanistic pathways that are dependent on the electronic properties of the diols as well as on whether the substrate is a 1,4- or a 1,5-diol
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