Vinylidene from metal allenylidenes

Highly reactive organic vinylidene and allenylidene species can be stabilized upon coordination to a metal center [1]. In 1979, Bruce et al. [2] reported the first ruthenium vinylidene complex from phenylacetylene and [RuCpCl(PPh3)2] in the presence of NH4PF6. Following this report, various mthenium vinylidene complexes have been isolated and their physical and chemical properties have been extensively elucidated [3]. As the a-carbon of ruthenium vinylidenes and the a and y-carbon of ruthenium allenylidenes are electrophilic in nature [4], the direct formation of ruthenium vinylidene and ruthenium allenylidene species, respectively, from terminal alkynes and propargylic alcohols provides easy access to numerous catalytic reactions since nucleophilic addition at these carbons is a viable route for new catalysis (Scheme 6.1). 

In parallel, since the first preparation of allenylidene-metal complexes in 1976, the formation of these carbon-rich complexes developed rapidly after the discovery, in 1982, that allenylidene-metal intermediates could be easily formed directly from terminal propargylic alcohols via vinylidene-metal intermediates. This decisive step has led to regioselective catalytic transformations of propargylic derivatives via carbon(l)-atom bond formation or alternately to propargylation. Due to their rearrangement into indenylidene complexes, metal-allenylidene complexes were also found to be catalyst precursors for olefin and enyne metathesis. 

Allenylidenes could be considered as divalent radicals derived from allenes. In a similar way to vinylidenes, allenylidenes can be stabilized by coordination with transition metals and again ruthenium is one of the most widely used metals. Metal-allenylidene complexes can be easily obtained from terminal propargylic alcohols by dehydration of the initially formed metal-hydroxyvinylidenes, in which the reactivity of these metal complexes is based on the electrophilic nature of Ca and Cy, while Cp is nucleophilic. Catalytic processes based on nucleophilic additions and pericyclic reactions involving the it system of ruthenium allenylidenes afford interesting new structures with high selectivity and atom economy. 

In contrast to many studies on cycloaromatization via transition metal-vinylidene complexes as key reactive intermediates, only one example of such a reaction via transition metal-allenylidene complexes has been reported to date. In 2008, Yada et al. reported the formation of substituted fiirans 78 from 3-butyne-l,2-diols 77 in the presence of a catalytic amount of thiolate-bridged diruthenium complex (Scheme 21.33) [45]. This methodology was also applied to the formation of a substituted pyrrole 80 from l-amino-2-butyn-2-ol 79. It is noteworthy that thiolate-bridged diruthenium complexes worked as effective catalysts toward cyclization involving both ruthenium-allenylidene and ruthenium-vinylidene complexes as key reactive intermediates. 

Cadiemo V, Gamasa MP, Gimeno J (2001) Eur J Inorg Chem 571-591 Bmneau C, Dixneuf PH (eds) (2008) Metal vinylidenes and allenylidenes in catalysis from reactivity to applications in synthesis. Wiley, Weinheim Cadiemo V, Gimeno J (2009) Chem Rev 109 3512-3560... 

Bruneau C, Dixneuf PH (eds) (2008) Metal vinylidenes and allenylidenes in catalysis from reactivity to applications in synthesis. Wiley, Weinheim... 

Fc3Pd) jU4-vinylidene complex with a butterfly metal array has been reported. " Formation of //-allenylidene complexes 50 from propargyl alcohols with Fe3(CO)i2 may involve a dehydration process/ ... 

Only a few examples have been obtained through the classical methodologies followed in group 6 metal chemistry. Most rf -Cs Fischer-type ruthenium and osmium carbenes arise from the nucleophilic additions of alcohol and amino groups at the electrophilic carbenic Ca-atom of both allenylidene and vinylidene complexes. The fate of the reaction depends on the electrophilicity as well as the steric hindrance around the Ca-atom, which can control its accessibility, especially for bulky nucleophiles. These features have been thoroughly discussed in a recent review. ... 

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