In SituGeneration of Molybdenum-Based Catalyst for Alkyne Metathesis: Further Developments and Mechanistic Insights

Joaquin Geng Lopez,Piotr Pawluc,Maciej Zaranek,André Mortreux,Régis M Gauvin,H Olivier-Bourbigou

doi:10.2516/ogst/2015046

Abstract

Molybdenum-based catalysts are among the best candidates to achieve alkyne metathesis. They can be either well-defined carbynes, previously synthesized before their use, but also prepared in situ upon using stable molybdenum carbonyl complexes, or high oxidation state molybdenum salts that need a previous alkylation, both type of precursors being “activated” by hydroxyl-containing compounds such as phenols and silanols. This paper is presenting studies made on these systems, directed towards the knowledge of the reaction paths leading to the active species, and in particular to define the essential role of hydroxyl-containing co-catalyst in the formation of the active species, still ill-defined. From an analysis of the byproducts formed during the reaction, as well as of the initial products, reaction paths to access catalytic carbyne species is suggested, where the ligand environment consists of phenoxy (or siloxy) groups, typically required and identified to lead to alkyne metathesis in the case of well-defined catalysts.

Highlights

In 1976 was held the first metathesis symposium in Mainz, where our Nobel Laureate Yves Chauvin presented some work on tungsten-based catalysts generated from the welldefined methoxyphenyl tungsten pentacarbonyl carbene complex and TiCl4 for cyclopentene ring-opening polymerization [1]
Within the frame of this paper, we will first present and compare catalytic results that have been observed on catalytic systems based on high oxidation state dioxo and dinitrosyl molybdenum/AlEt3/ZOH catalysts for alkyne metathesis, with regard to the use of various and selected ligand environment
A second part will be devoted to analysis of the byproducts of these reactions, as well as those observed on molybdenum hexacarbonyl/ArOH catalysts

Summary

Introduction

In 1976 was held the first metathesis symposium in Mainz, where our Nobel Laureate Yves Chauvin presented some work on tungsten-based catalysts generated from the welldefined methoxyphenyl tungsten pentacarbonyl carbene complex and TiCl4 for cyclopentene ring-opening polymerization [1]. Since our former, seminal work on molybdenum carbonyl as catalyst precursor, our interest on the alkyne metathesis reaction has been focused in the early 80s on the use of higher oxidation state molybdenum complexes, which when associated with an alkylating/reducing agent, and again a phenol as co-catalyst, have been shown to be two orders of magnitude more reactive than the former ones. This has been exemplified in a short series of papers where MoO2(acac)2/AlEt3/PhOH combinations were shown to be active at room temperature [8] and could be applied to

Results

Discussion

Conclusion