Activation of olefin metathesis complexes containing unsymmetrical unsaturated N-heterocyclic carbenes by copper and gold transmetalation

Abstract

The activation of ruthenium-indenylidene complexes containing two unsymmetrical unsaturated N-heterocyclic carbenes (u2-NHCs) by a transmetalation process is reported. The use of copper(I) or gold(I) chlorides promotes the rapid trapping of one NHC ligand that releases the catalytic active Ru-species. Impressive initiation rates with full-conversions are observed within one minute. This practical protocol demonstrates excellent catalytic performances in various ring-closing metathesis (RCM) and self-metathesis (SM).In a few decades, olefin metathesis has rapidly become one of the most efficient synthetic tools in organic chemistry This attractiveness was mainly due to the development of efficient, well-defined, air stable and easy to handle rutheniumarylidene complexes with high tolerance towards many organic functions. 1 Since the pioneer works describing the phosphine-based Grubbs first-generation catalyst in early 1990s, 2 the research efforts to furnish more robust and powerful complexes have drastically increased.

Cyclic (Alkyl)(amino)carbenes (CAACs) in Ruthenium Olefin Metathesis

“…The propensity of ruthenium bis-NHC alkylidenes to generate active catalytic species has been discussed. ,,, To determine whether strongly σ donating and π accepting CAACs could impede or facilitate the activation of bis­(CAAC)Ru indenylidene complexes, Skowerski and co-workers next examined their reaction with a doubly chelating olefin . As shown in Scheme , reaction of Ru-43 with 1-isopropoxy-2-(propenyl)­benzene at 60 °C in toluene led to the clean formation of the Hoveyda–Grubbs derivative Ru-11 , which was isolated in 86% yield.…”

CuI-Catalyzed Coupling Reactions of 4-Iodopyrazoles and Alcohols: Application toward Withasomnine and Homologs

“…Alternatively, CuI-assisted RCM [24,35] of 13 in CH 2 Cl 2 under milder conditions using MW-aided heating at 80 • C for 1 h successfully afforded pyrrole-[1,2-b] pyrazole 14 (63%), which was immediately hydrogenated under a hydrogen gas atmosphere with Pd/C in MeOH to give penultimate product 6 in 90% yield. As the transformation from 6 to 7 via a Suzuki-Miyaura coupling reaction has already been reported [22,23], the present approach constitutes a formal total synthesis of withasomnine (7).…”

Alternative Transformations of N-Heterocyclic Carbene Complexes of the Group 11 Metals in Transmetalation Reactions (A Review)

“…Remarkably, such route as the "reverse" transmetalation can be used for the activation of the bis-NHC complexes of ruthenium via the conversion into the corresponding mono-NHC complex revealing high activity under the conditions of olefins metathesis owing to the binding of the second carbene ligand with CuCl or AuCl [74]. For example, the reaction of complex 7 containing two unsaturated N-heterocyclic carbenes with CuCl or AuCl in CH 2 Cl 2 in the presence of 1-isopropoxy-2-vinylbenzene has occurred with the formation of ruthenium complex 8 and carbene complexes of copper 9 or silver 10, formally the products of "reverse" transfer of carbene (Scheme 4), CuCl being more reactive than AuCl under the transformation conditions.…”

“…The presence of CAAC in the structure of 171 likely favored the enhanced stability of this catalyst system with respect to ethylene. The possibility of the use of "reverse" transmetalation for the activation of bis-NHC complex of ruthenium 176 (1 mol %) in situ via the addition of CuCl or AuCl under conditions of metathesis of diethyl 2,2-diallylmalonate into diethyl cyclopent-3-ene-1,1-dicarboxylate in toluene at temperature 30 or 80°С has been demonstrated in [74]. Complex 176 (Scheme 55) has revealed low catalytic activity with respect to the studied diene at 30°C: conversion of the starting substrate as low as 10% within 2 h has been observed (67% after 24 h).…”