Activation of propargylic alcohols by dimolybdenum tris(μ-thiolate) complexes: Influence of the substituents R in HC [tbnd]CCR 2(OH)-vinylidene/allenylidene transformation. Reactivity of allenylidene complexes

Abstract

Reaction of the bis(nitrile) complex [Mo 2Cp 2(μ-SMe) 3(NCMe) 2](BF 4) ( 1) with dimethylpropargylic alcohol, HC CCMe 2(OH), at room temperature in dichloromethane produced good yields of the μ-alkynol species [Mo 2Cp 2(μ-SMe) 3{μ-CHCCMe 2(OH)}](BF 4) ( 2a) through replacement of the two acetonitrile ligands in 1 by the alkynol. The NMR spectra of 2a indicate a μ-η 1:η 1 coordination mode for the alkyne which is thereby incorporated into a dimetallacyclobutene ring like that found here by X-ray diffraction (XRD) analysis of the related complex [Mo 2Cp 2(μ-SMe) 3(μ-η 1:η 1-CHCCO 2Me)](BPh 4) ( 2b). When 2a was stirred with Et 3N at room temperature in dichloromethane, deprotonation gave high yields of the μ-3-hydroxyalkynyl derivative [Mo 2Cp 2(μ-SMe) 3{μ-η 1:η 2-CCCMe 2(OH)}] ( 3), together with small amounts of the already-known vinylacetylide [Mo 2Cp 2(μ-SMe) 3{μ-η 1:η 2-CCC(Me)CH 2}] ( 4) resulting from dehydration of 3. Treatment of 3 with 1 equiv. of HBF 4 · OEt 2 in diethyl ether at room temperature gave the 3-hydroxyvinylidene derivative [Mo 2Cp 2(μ-SMe) 3{μ-η 1:η 2-CCHCMe 2(OH)}](BF 4) ( 5) as the major product, together with other minor products [Mo 2Cp 2(μ-SMe) 3{μ-η 1:η 2-CCHC(Me)CH 2}](BF 4) ( 6), [Mo 2Cp 2(μ-SMe) 3(μ-η 1:η 2-CCCMe 2)](BF 4) ( 7), [Mo 2Cp 2(μ-SMe) 3(μ-η 1:η 2-CCH 2)](BF 4) ( 8), [Mo 2Cp 2(μ-SMe) 3{μ-η 1:η 2-CCH(CHMe 2)}](BF 4) ( 9) and [Mo 2Cp 2(μ-SMe) 3(μ-O)](BF 4) ( 10). The vinylidene ( 6) and allenylidene ( 7) species resulted from dehydration of the 3-hydroxyvinylidene complex 5 whereas the vinylidene derivative 8 was formed by deketonisation of 5. When 3 reacted with a large excess of HBF 4 · OEt 2 in dichloromethane, the 3-isopropylvinylidene complex 9 was obtained nearly quantatively via a H radical process. When left for several days CD 2Cl 2 solutions of 5 afforded mainly the vinylidene species 8 by deketonisation and the side-oxoproduct [Mo 2Cp 2(μ-SMe) 3(μ-O)](BF 4) ( 10) by hydrolysis or reaction with oxygen. Addition of nucleophiles (H −, OMe −, OH −, SMe −) to the allenylidene complex [Mo 2Cp 2(μ-SMe) 3(μ-η 1:η 2-CCCPh 2)](BF 4) ( 11) resulted in the formation of the corresponding μ-acetylide derivatives [Mo 2Cp 2(μ-SMe) 3(μ-η 1:η 2-CCCRPh 2)] [R = H ( 12), OMe ( 16a), OH ( 17), SMe ( 16b)], which by further reaction with tetrafluoroboric acid afforded either the vinylidene species [Mo 2Cp 2(μ-SMe) 3{μ-η 1:η 2-CCH(CRPh 2)}](BF 4) when R = H ( 13), or the starting complex 11 when R is a leaving group (OMe). Reaction of 13 with Na(BH 4) gave the μ-alkylidyne complex [Mo 2Cp 2(μ-SMe) 3(μ-η 1-CCH 2CPh 2H)] ( 14) by nucleophilic attack of H − at the C β carbon atom of the vinylidene chain. Proton addition at C α in 14 led to the formation of a μ-vinylidene compound 15 containing an agostic C–H bond. New complexes have been characterised by elemental analyses and spectroscopic methods, supplemented for 2b and 3 by X-ray diffraction studies.