Vinylidene from acyl complexes

Acyl complexes can also result from the reaction of terminal alkynes with cationic, hydrated complexes of iron (Entry 4, Table 2.7) [47]. An electrophilic vinylidene complex is probably formed as intermediate this then reacts with water and tautomerizes to the acyl complex. 

Water also attacks the electrophilic a carbon of the ruthenium vi-nylidene complex 80. The reaction does not yield the ruthenium acyl complex, however, as is found for the reaction with the similar iron vinylidene complex [(i75-C5H5)(CO)2Fe=C=CHPh]+ (56), but rather 91 is the only isolated product (78). The mechanism for this transformation most reasonably involves rapid loss of H+ from the initially formed hydroxycarbene to generate an intermediate acyl complex (90). Reversible loss of triphenyl-phosphine relieves steric strain at the congested ruthenium center, and eventual irreversible migration of the benzyl fragment to the metal leads to formation of the more stable carbonyl complex (91) [Eq. (86)]. 

Several hundred examples of vinylidene complexes have been prepared. Vinylidene complexes have been prepared by rearrangement of alkyne complexes, additions of acid or base to acetylide complexes, by deprotonation of carbyne complexes, by dehydration of acyl complexes, and by ot-hydrogen shifts from vinyl complexes. Syntheses from alkjme and from acetylide complexes are most common. A complex of a terminal alkyne and a transition metal can exist as an alkyne complex or as a vinylidene complex. Although the free vinylidene is much higher in energy than the free alkyne, the vinylidene complex is often more stable tlnan the alkyne complex. Vinylidene complexes are most often obtained with late transition metals because this tautomer possesses less repulsion between the filled (i-orbitals of the metal and the filled ir-orbitals of the ligand. 

Scheme 21). It involves the initial formation of anionic acyl complexes by addition of Li[GH2R] reagents to the carbonyl ligand, which, by reaction with acetyl chloride, generate the vinylidenes via spontaneous loss of MeC02H from intermediate acyl(oxy) carbenes. " ... 

Formation of metal acyl or carbonyl complexes from 1-alkynes in the presence of water is often assumed to proceed via attack on an intermediate vinylidene complex to give a hydroxycarbene complex (Equation 1.24) ... 

The acylate anion from Mn(CO)3(j/-C5H5) and LiMe reacts similarly reaction with l,8-bis(dimethylamino)naphthalene (proton sponge) affords the vinylidene, although in this reaction, it is the binuclear complex which is isolated as the final product (38) ... 

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