Metathesis vs. Polymerization of terminal acetylenes over [W(CBut)(OBut)3]

Abstract

Terminal acetylenes RC-CH are transformed catalytically into metathesis products RC=CR and C2H2 on the carbyne complex [W(CBU~)(OSU~)~], a process which is rapidlyfollowed by a polymerization of the alkyne; in the case of sterically hindered ButC-CH, a stoichiometric reaction is observed, by which a dinuclear compound p-C2H2[W(OBut)& 1 and the deprotiotungstenacyclobutadiene [ W C 3 ( B ~f ) 2 ( 0 B ~t ) 2 ] 2 are formed.

Selective Terminal Alkyne Metathesis: Synthesis and Use of a Unique Triple Bonded Dinuclear Tungsten Alkoxy Complex Containing a Hemilabile Ligand

“…[13] The length of the W W triple bond, 2.430 (8) , is comparable to that in similar complexes in the literature. [14] The ether function of one of the ligands coordinates the other tungsten atom with a W À O length of 2.257(3) , vs. 1.957(3) , 1.897(3) , and 1.920(3) for the three W À O bonds to non-bridging ligands.…”

“…[7] Using the same catalyst we have demonstrated some promising results on alkyl-monosubstituted alkynes using diethyl ether as solvent. [8] More recent results in our group have shown that the addition of an external ligand such as quinuclidine to complex 1 could improve the selectivity: an 80 % metathesis selectivity was attained using hept-1-yne as substrate. [9] Based on the idea that coordination at the metal centre was responsible for this improvement in selectivity, we decided to look at the synthesis of new catalysts where ligand modification would give rise to variable coordination within the first coordination sphere of the metal by using a hemilabile ligand.…”

“…[7] Accordingly,such species synthesized from complex 1 and tert-butylacetylene have been found to be extremely reactive towards phenylacetylene polymerization at room temperature. [8] In our case, using hemilabile alkoxy ligands, the presence of the extra coordinating moiety in the coordination sphere undoubtedly plays a major role in the selectivity enhancement: DFT calculations on these metallacyclic intermediates and transition states are currently underway to determine the precise role of these ligands in the deprotonation process.…”

Alkyne metathesis catalysts: Scope and future

“…A general mechanistic scheme related to this chemistry is given below (scheme 10). In 1993 however, attempts at homogeneous metathesis of terminal alkynes haves been successful, on the well defined (tBuO) 3 W CtBu complex [34]. The use of alkyl monosubstituted alkynes rather than aryl ones led to several turnovers in metathesis at room temperature, which were increased at high temperature (80°C) and upon using diethyl ether as solvent.…”

Alkyne Metathesis Catalysts: Scope and Future

“…These deprotonated metallacycles are highly active catalysts for terminal alkynes polymerization as they are prone to be transformed into metallacarbenes (scheme 10) [34]. Scheme 10: Polymerization of phenylacetylene over deprotonated metallacyclobutadiene [34].…”