Generation of low-valent alkoxy niobium from Nb(OEt)5 and Grignard reagents and their use as catalysts in the cyclotrimerization of isocyanates

A new highly active low-valent alkoxyniobium species has been developed following the reaction of Nb(OEt)(5) with Grignard reagent such as i-PrMgCl or EtMgCl, with the material behaving as an efficient catalyst for cyclotrimerization reaction of isocyanates. In this reagent system, the existence of a low-valent niobium species was confirmed by the formation of the niobium-alkyne complex prepared from the Nb(OEt)(5)/Grignard reagent system and an alkyne. Furthermore, the hydrolysis and diallylation reactions of the niobium-alkyne complex provided further confirmation of its existence, with corresponding (Z)-alkene and diallylated products being isolated in good yields. (C) 2013 Elsevier B.V. All rights reserved.