Surmounting Alkoxide Trap Strategy: N-Heterocyclic Carbene Chromium(0)-Catalyzed C-Alkylation between Alcohols

Abstract

The crucial role of the alkoxide trap problem and the impact of the oxidation state of the chromium center were demonstrated on the catalytic activity due to the d-wall issue. Through the strategy of surmounting the alkoxide trap, a Cr(0) catalyst was presented herein for efficient C-alkylation between alcohols via borrowing hydrogen/hydrogen autotransfer. The synthesized bis-(N-heterocyclic carbene)-Cr(0) system shows an efficient catalytic performance (40 examples, up to 96% yield). Only a catalytic loading of a cheap and readily available base NaOH is effective enough for the reaction. Compared with Cr(III)/Cr(II), Cr(0) can well avoid the d-p π interaction in the key metal-alkoxide intermediate, thus overcoming the thermodynamic sink due to the alkoxide trap problem. It is plausible that the Cr(III) systems need to be reduced to Cr(II) for weakening the alkoxide trap effect and enhancing activity by using reductive strong bases. This surmounting alkoxide trap strategy should be helpful for the development of efficient and nonprecious transition metal catalysts.

CrCl2-catalyzed α-alkylation of carbonyl compounds via a borrowing hydrogen approach

“…On the basis of our experimental observations and previous literature reports, 16,17 a plausible reaction pathway is proposed as shown in Scheme 7. In the presence of LiOH, PPh 3 and benzyl alcohol, the Cr( ii ) precursor would afford the corresponding intermediate Cr-1 .…”

“…16 Very recently, Ke and co-workers reported a Cr(0) system for C -alkylation using alcohols (Scheme 1c). 17 To date, the Cr catalysts involved in borrowing-hydrogenation reactions mainly utilized the triazinyl-core PN 5 P ligand, the tetraphenylporphyrin-core ligand, and the bis(imino)pyridine ligand. Usually, these ligands under base-metal catalysis are not only more expensive, but also more difficult to prepare and handle under standard laboratory conditions than nonprecious metals.…”

Recent advances in cross-coupling of alcohols via borrowing hydrogen catalysis

“…Ke and co-workers, in 2023, employed bis-NHC Cr(0)-catalyst ( Cr-3 ) for the β-alkylation of alcohols. 55 A series of catalytic experiments including DFT-studies were performed, which established the enormous impact of the low oxidation state of chromium in overcoming the alkoxide trap problem.…”

Hydrogenation of Esters Catalyzed by Bis(N-Heterocyclic Carbene) Molybdenum Complexes

“…Recently, the group of Ke reported hydrogen borrowing reactions catalyzed by bis(NHC)Mo(CO) 4 complexes 2 and bis(NHC)Cr(CO) 4 complexes featuring strongly electrondonating, bidentate bis(NHC) ligands. 23,24 Given that Mn complexes bearing these bis(NHC) ligands catalyze the hydrogenation of esters, 25 we conjectured that comparable Mo complexes could also act as hydrogenation catalysts. In this respect, it is worth noting that very recently, the first molecular W-based system for the hydrogenation of esters has been reported by Topf and co-workers.…”