Acetylenic-vinylidene rearrangement

Acetylene-vinylidene rearrangements of silylacetylene-iron carbonyl complexes have been observed,537 while iron-acetylide hydride complexes of the type [Fe(H)(C=CR)(dmpe)2], where dmpe=l,2-bis(dimethylphosphino)ethane, have been found to react with anions to afford substituted alkenyl complexes. It has been proposed538 that a likely reaction course for this latter rearrangement involves initial protonation of the cr-bound acetylide ligand at the carbon (I to the metal centre to form a vinylidene complex. Metal-to-carbon hydride migration in this vinylidene complex with attack by the anion would then lead to the neutral complex (see Scheme 106). A detailed mechanistic investigation has been carried out539 on the novel metathetical... 

Kasai, P.H. ESR study of sodium atom acetylene complex evidence for acetylene-vinylidene rearrangement. J. Phys. Chem. 1982, 86, 4092-4094. 

An organometallic approach to 938, in which there is an acetylenic—vinylidene rearrangement of a chiral jS-hydroxylactone without loss of stereochemical integrity, provides an interesting alternative to the synthesis of this natural product. Treatment of 40, derived from 39, with 2.2 equivalents of lithium acetylide-ethylenediamine complex provides crystalline diol 939 in 57% yield. However, this synthesis is problematic, because 40 is not easily prepared or isolated. A more practical route involves in situ generation of the bis-mesylate 39 and trapping with lithium acetylide-ethylenediamine complex to afford 939 in 87% overall... 

The tungsten carbene complex [(CO)5W=CHAr] (Ar = Ph, p-tol) reacts with 2-butyne to give stilbenes and [(CO)5W(MeC=CMe)]. Terminal alkynes, such as Bu C=CH, are however polymerized. Vinylidene complexes are proposed as intermediates since the acetylene-vinylidene rearrangement has ample precedence in stoichiometric reactions (Scheme 26). 

The conversion of l,3-hexadien-5-yne to benzene can occur by another mechanism, namely, an acetylene to vinylidene rearrangement followed by insertion - cyclization into a vinyl C-H bond (Scheme 7.23). 

The vinylidene ligand, M=C=CHR, readily formed from terminal acetylenes by a 1,2-migration of the H atom, is another intermediate character carbene. Although no heteroatom is present, the double bond adjacent to the carbene carbon clearly stabilizes the vinylidene relative to a Schrock alkylidene. A vinylidene is very prone to insertion Eq. 11.37 shows a case where a double acetylene-to-vinylidene rearrangement is accompanied by an insertion into M—H. The final producL most likely formed by a further vinylidene in.sertion, is 5-coordmate, stabilized by a weak bond to a vinyl carbon (dotted line). 

Silvestre and Hoffmann have considered the conversion of ruthenium acetylide complexes to the corresponding vinylidene species using extended Hiickel molecular orbital calculations (69). Although the rearrangement of free acetylene to its vinylidene isomer is thermodynamically disfavored, their results indicate that the transformation becomes thermodynamically... 

An intermolecular version of this rearrangement involving dissociation of the acidic proton on Ca of the slipped acetylene, followed by reprotonation of an intermediate acetylide (discussed in Section VI,C), must also be considered as a potential route to the cationic ruthenium vinylidene species (Scheme 7). Unfortunately, to date this mechanism has not been addressed experimentally or theoretically. 

Smith, B.J. Smernik, P. Radom, L. The vinylidene-acetylene rearrangements. A phantom minimum on the MP2 potential energy surface. Chem. Phys. Lett. 1992, 188, 589-594. 

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