Selective dimerization of 1,6-diynes catalyzed by ionic liquid-supported nickel complexes in an ionic liquid/toluene biphasic system

Abstract

A NiCl 2 -6H 2 O/Zn reagent with a 2-aryliminomethylpyridine ligand catalyzed cyclization/polymerization of 1,6-diynes mainly to yield the corresponding poly(1,3-butadienylene) compounds, 10 whereas, NiBr 2 -3H 2 O/Zn with use of 3-(2-(2-methoxyethoxy)-ethyl)-1-methyl-1H-imidazol-3-ium bromide as a ligand converted 1,6-diynes to the corresponding annulated 1,3,5,7-cyclooctatetraenes in a biphasic solvent system consisting of toluene and an ionic liquid. 15For homogeneous metal-catalyzed reactions, a bimetallic or bimolecular cooperative mechanism has often been proposed, where a dimerization of the metal species or dimeric interaction of the metal species plays a critical role: 1 Many cases have been postulated in which a bimetallic or bimolecular mechanism is 20 involved as a major course for effective, selective production of the desired compound. Meanwhile, the reverse case has also been reported in which dimerization of the metal species deactivates the catalyst by forming stable, coordinatively saturated, inert complexes and furthers decomposition by disproportionation. In 25 this context, isolating the catalyst to a solid phase from a solution phase by immobilization onto polymeric supports or inorganic materials is a versatile means not only for avoiding the formation of dimeric metal molecules thereby deactivating and decomposing the catalyst but also for increasing the catalyst 30 reactivity and selectivity via a bimetallic or bimolecular mechanism. 2 Herein we propose a new strategy for controling the reactions which proceed through formation of a dimeric metal intermediate: Isolation (or immobilization) of the metal-catalyst 35 into an ionic liquid (IL) 3 phase in an IL/usual organic solvent biphasic system which can separate the catalyst from the employed substrate and maintain a high catalyst concentration in the IL phase throughout the reaction, a situation which might be favorable for reactions via a dimeric metal intermediate and corresponding COTs catalyzed by a (DME)NiBr 2 /Zn [DME = 1,2-dimethoxyethane] reagent in THF-H 2 O, where reaction with a large amount of catalyst loading (20~40 mol%) realized highly selective production of COTs against co-production of [2+2+2] cycloadducts. 6 The results were rationalized by assuming the 55 dimeric mechanism that Wilke suggested.Cyclooctatetraenes (COTs) are of interest as ligands capable of coordinating to various metal atoms and electron transporting components in organic light-emitting devices, and especially as precursors for polyacetylenes in the ring-opening metathesis 60 polymerization (ROMP) reaction. 7 Efforts, therefore, have been made in developing methods for synthesizing COTs. 8 Of the existing methods, the transition metal-catalyzed [2+2+2+2] cycloaddition of alkynes can be considered one of the most attractive and direct. 65During the course of studying the transition metal-catalyzed cycloisomerization of alkynes, 9 we found that a 2-(aryliminomethyl)pyridine/NiCl 2 -6H 2 O/Zn reagent catalyzed cycloaddition of diethyl dipropargylmalo...

From the Development of Catalysts for Alkyne and Alkyne-Nitrile [2+2+2] Cycloaddition Reactions to Their Use in Polymerization Reactions

“…45 The reaction of 1,6-diynes with 2, 5, or 10 mol% of dipimp-nickel(II) chloride hexahydrate in the presence of zinc powder gave polymers with an M n of around 2 × 10 3 in all cases. The use of 10 mol% of the catalyst gave the highest yield of polymer (75% at 100% conversion).…”

“…This was effective in achieving selective formation of the corresponding COTs from 1,6-diynes with a catalyst loading of 2−5 mol%. 45…”

Transition‐Metal‐Catalyzed Cycloadditions for the Synthesis of Eight‐Membered Carbocycles

“…When aromatic or heteroaromatic diynes were used as substrates, the ratio of COT/benzene derivatives was as high as 20:1. Recently, Okamoto and co‐workers extended Wender’s [(2+2)+(2+2)] reaction protocol by using ionic liquid supported nickel complexes in an ionic liquid/toluene biphasic system 11c…”

Biphasic synthesis of “block” copolymers and star‐shaped polymers via monomer‐selective cationic copolymerization in organic/fluorinated solvents

“…In biphasic systems, the components, such as monomers, catalysts, and additives, are separated into two phases, and a reaction can proceed in either phase. Thus, reactions in biphasic systems possess a variety of advantages, such as simple separation of the products and catalyst, reuse of the catalyst, and high selectivity . A wide range of solvents that can be employed for biphasic systems, such as organic solvents, water, fluorinated solvents, ionic liquids, and supercritical CO 2 , enables the construction of these systems for a variety of reactions.…”