Acceptorless Alcohol Dehydrogenation: OH vs NH Effect in Bifunctional NHC–Ir(III) Complexes

Abstract

Bifunctional complexes bearing N-heterocyclic carbene (NHC) ligands functionalized with hydroxy or amine groups were synthesized to measure the beneficial effect of different modes of metal–ligand cooperation in the acceptorless dehydrogenation of alcohols. In comparison to complexes with an amine moiety, hydroxy-functionalized iridium catalysts showed superior activity. In contrast to alcohols, 1,4-diols underwent cyclization to give the corresponding tetrahydrofurans without involving dehydrogenation processes. Mechanistic investigations to rationalize the “OH effect” in these types of complexes have been undertaken.

Nonclassical Mechanism in the Cyclodehydration of Diols Catalyzed by a Bifunctional Iridium Complex

“…We tested a series of NHC–Ir III complexes in the cyclodehydration reaction of 1‐phenyl‐1,4‐pentanediol ( 2 a ; Table ) . The optimized reaction conditions for the acceptorless alcohol dehydrogenation (AAD) reaction (Scheme , top) had previously been tested on diol 2 a (i.e., iridium complex 1 a in a mixture of toluene and t ‐butanol (2.6:1, v/v) heated at reflux), and under these conditions, tetrahydrofuran 3 a was formed in excellent yield (91 %, Table , entry 1) .…”

“…We tested a series of NHC–Ir III complexes in the cyclodehydration reaction of 1‐phenyl‐1,4‐pentanediol ( 2 a ; Table ) . The optimized reaction conditions for the acceptorless alcohol dehydrogenation (AAD) reaction (Scheme , top) had previously been tested on diol 2 a (i.e., iridium complex 1 a in a mixture of toluene and t ‐butanol (2.6:1, v/v) heated at reflux), and under these conditions, tetrahydrofuran 3 a was formed in excellent yield (91 %, Table , entry 1) . In contrast, neutral iridium dichloride complex 1 b did not promote the cyclization; instead, mono‐ and dioxidized linear compounds 4 a and 5 a , as well as deoxygenated ketone 6 a (see the Supporting Information) were detected in the crude mixture at 80 % conversion of substrate 2 a (entry 2).…”

“…However, 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 . 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 . The hydroxy/alkoxide functionality on the carbene ligand participated in proton‐transfer steps.…”

“…NHC–Ir complexes (NHC=N‐heterocyclic carbene) have proven to be excellent catalysts in numerous processes, particularly in dehydrogenations and transfer hydrogenations . ,,, NHCs can be relatively easily 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 . The binfunctional nature of the NHC–Ir complexes ( 1 ) was also explored in the acceptorless dehydrogenation of alcohols (Scheme , top). Here, 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 , bottom) instead of the expected products derived from a dehydrogenation process (Scheme , top).…”