Transition Metal Vinylidene- and Allenylidene-Mediated Catalysis in Organic Synthesis

Abstract

With their mechanistic novelty and various modalities of reactivity, transition metal unsaturated carbene (alkenylidene) complexes have emerged as versatile intermediates for new reaction discovery. In particular, the past decade has witnessed remarkable advances in the chemistry of metal vinylidenes and allenylidenes, leading to the evolution of a diverse array of new catalytic transformations that are mechanistically distinct from those developed in the previous two decades. This review aims to provide a survey of the recent achievements in the development of organic reactions that make use of transition metal alkenylidenes as catalytic intermediates and their applications to organic synthesis.

Further Evidence for ‘Extended’ Cumulene Complexes: Derivatives from Reactions with Halide Anions and Water

“…From a mechanistic perspective the addition of water to the putative quinoidal cumulene provides further insight into the ruthenium‐mediated hydration of alkynes . In the case of simple alkynes, there are two principle outcomes from such a reaction, either Markovnikov addition to give a methyl ketone (thought to proceed through attack of water to an η 2 ‐alkyne intermediate) or anti‐Markovnikov addition via a vinylidene complex which affords an aldehyde . The regiochemical outcome of the reaction is, therefore, dictated by the balance between the alkyne and vinylidene tautomers of the coordinated alkyne, which is in turn controlled by the choice of ligands within the coordination sphere of the metal .…”

Acylruthenium Complexes from Reactions of RuCl2(PPh3)3 with Terminal Alkynes

“…Complexes Ru(‐C(CH 2 Ph)=CHPh)Cl(CO)(PPh 3 ) 2 ( 4 ) and RuHCl(CO) 2 (PPh 3 ) 2 ( 5 ) (major) (with minor RuCl 2 (CO) 2 (PPh 3 ) 2 ( 6 )) are produced quantitatively by heating the solution of complexes 2 and 3 in toluene, respectively (Scheme ). The formation of complex 4 is proposed through a benzyl carbonyl intermediate from decarbonylation of the acyl ligand . Interestingly, complex 4 is formed as a thermodynamic product, from the migratory insertion of benzyl group onto vinylidene ligand; in contrast, the insertion occurs on carbonyl CO instead of terminal methylene =CH 2 ligand, giving complexes Ru(=CH 2 )(‐C(=O)R)Cl(PPh 3 ) 2 (R=Ph, o ‐tolyl, p ‐tolyl) in previous reports .…”

“…The formation of complex 4 is proposed through a benzyl carbonyl intermediate from decarbonylation of the acyl ligand. [12,18] Interestingly, complex 4 is formed as a thermodynamic product, from the migratory insertion of benzyl group onto vinylidene ligand; in contrast, the insertion occurs on carbonyl CO instead of terminal methylene = CH 2 ligand, giving complexes Ru(= CH 2 ) (-C(= O)R)Cl(PPh 3 ) 2 (R = Ph, o-tolyl, p-tolyl) in previous reports. [16] Complex 4 is characterized by solution NMR, 1 H NMR spectrum displays the characteristic C=CHPh vinyl proton signal at 6.29 ppm as a triplet and CH 2 of benzyl group at 4.96 ppm, and the 13 C resonance of the carbonyl group is at 202.2 ppm.…”

“…The involvement of metal vinylidene intermediate in anti‐Markovnikov hydration of terminal alkynes is well studied . The formation of complexes 2 and 3 is proposed through a bis‐vinylideneruthenium intermediate (Scheme ), followed by nucleophilic addition of one H 2 O molecular on the C ɑ next to Ru of one vinylidene group (when R=Ph), giving acyl vinylideneruthenium complex 2 .…”

“…The involvement of metal vinylidene intermediate in anti-Markovnikov hydration of terminal alkynes is well studied. [11,12,18] The formation of complexes 2 and 3 is proposed through a bis-vinylideneruthenium intermediate (Scheme 2), [19] followed by nucleophilic addition of one H 2 O molecular on the C � next to Ru of one vinylidene group (when R = Ph), giving acyl vinylideneruthenium complex 2. Whereas decarbonylation occurs at the acyl group when it has an alkyl substituent (R = (CH 2 ) 5 CH 3 ), [12c] and further hydration on the other vinylidene ligand yields complex 3.…”