Rearrangement vinylidene

Alkylidenecyclopentene-l,4-diones (cf. 12, 130).2 The cobaltacyclopente-nedione (2) formed on reaction of a cyclobutenedione with 1, when complexed with dimethylglyoxime, reacts with 1-alkynes to form 5-alkylidenecyclopentene-1,4-diones (3). The reaction involves an alkyne-vinylidene rearrangement. Ben-zoquinones are usually formed in minor amounts. 

Another theoretical study also showed that the third pathway (bl +b3+b4), 1,3 hydrogen shift, through a hydrido-alkynyl intermediate could compete with the 1,2 hydrogen shift pathway (bl+b2) when the metal center is electron-rich enough [29, 30]. Indeed several hydrido-alkynyl intermediates have been detected or even isolated during the q -l-alkyne-to-vinylidene rearrangement on electron-rich metal centers, such as Co(I), Rh(I) and Ir(I) [73-78]. The ab initio M P2 calculations by Wakatsuki, Koga and their coworkers on the transformation of the model complex RhCl(PH3)2(HC=CH) to the vinylidene form RhCl(PH3)2(C=CH2) indicated that the transformation proceeded via the oxidative addition intermediate RhCl(PH3)2(H) (C CH) [30]. 

Often, selectivity for a vinylidene-mediated pathway is heavily dictated by substrate structure. It is especially true in the case of hetero-atom substituted alkynes that Jt-alkyne/vinylidene rearrangement is driven by a reduction in steric interactions at the metal center. 

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... 

Figure 5.48 Alkyne-vinylidene rearrangements E = H, SiR3, SnR3, SR, SeR...

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 former product is a retro ene isomer, but the latter was proposed to occur by a retro vinylidene rearrangement (see Chapter 3, Section 1). 

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). 

Stepwise double alkyne to vinylidene tautomerization is the key step responsible for the formation of the 77 -butadienyl iridium(in) complex [Ir K -0,C -O=G(Me)CH=CPh (77 -PhCH=CHC=CHPh)(PPh3)2]SbF6 579 346 proposed mechanism, which is illustrated in Scheme 82, involves an alkyne to vinylidene rearrangement (I —> II) followed by a hydride insertion (II — III), a second alkyne to vinylidene rearrangement (III — V), and a migratory insertion of the vinyl to the vinylidene (V — VI) resulting in the G-C bond formation. 

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