Reactions acetylene—vinylidene

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... 

That the substitution mechanism depends on the nature of the nucleophile is shown by the formation of the ketene acetals (151) from the reaction of vinylidene chloride with alkoxide ions. It was suggested that two consecutive eliminations-additions take place, and that in both cases the alkoxide attacks the acetylene at the substituted carbon (Kuryla and Leis, 1964). Since chloroacetylene (132) is also an inter-... 

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). 

Vinylidene fluoride, CH2=CF2, is obtained by the pyrolysis of 1,1-difluoro- 1-chloroethane, which in turn is produced from acetylene, vinylidene chloride, or 1,1,1-trichloroethane by reaction with hydrogen fluoride. Because of its low boiling temperature, —84°C, vinylidene fluoride is suspension or emulsion polymerized under pressure. Considerable head-head linkage quantities are produced in these polymerizations. 

The second reaction pathway involves the isomerisation of acetylene to the vinylidene radical followed by further reaction with the acetylene to form ben-zyne and then the diphenyl radical, as shown in Figure 5.14. Addition of acetylene to the phenyl radical in a further four steps forms two fused benzene rings called naphthalene. 

Ru-vinylidene complexes can be easily prepared by reaction of low-valent ruthenium complexes with terminal acetylenes. Treatment of the Ru(ii) complex 117 with phenylacetylene gave the Ru(iv)-vinylidene complex 118 in 88% yield (Scheme 41 ).60 The reaction of 118 with C02 in the presence of Et3N afforded selectively the Ru-carboxylate complex 120, probably via the terminal alkynide intermediate 119. 

DFT calculations confirmed the similarities with the alkyne/vinylidene transformation but have revealed that additional parameters were essential to achieve the isomerization [8, 20-23]. The hydride ligand on the 14-electron fragment RuHC1L2 opens up a pathway for the transformation similar to that obtained for the acetylene to vinylidene isomerization. However, thermodynamics is not in favor of the carbene isomer for unsubstituted olefins and the tautomerization is observed only when a re electron donor group is present on the alkene. Finally the nature of the X ligand on the RuHXL2+q (X = Cl, q=0 X = CO, q=l) 14-electron complex alters the relative energy of the various intermediates and enables to stop the reaction on route to carbene. 

The mechanism for the ethylene to methylcarbene reaction has been calculated at the DFT (B3PW91) level with the model system RuHCl(PH3)2 [8, 21]. As in the case of the acetylene to vinylidene reaction, the starting complex was assumed to be the 14-electron complex RuHCl(PH3)2 generated in situ. The reaction path is very similar to that obtained with C2H2. They differ mainly in the overall direction of the energy pattern downhill for acetylene and uphill for ethylene. 

Aryl acetylenes undergo dimerization to give 1-aryl naphthalenes at 180 °C in the presence of ruthenium and rhodium porphyrin complexes. The reaction proceeds via a metal vinylidene intermediate, which undergoes [4 + 2]-cycloaddition vdth the same terminal alkyne or another internal alkyne, and then H migration and aromatization furnish naphthalene products [28] (Scheme 6.29). 

Intermolecular reactions of propargylic alcohols with a-methylstyrene gave the corresponding 1-hexene-5-ynes in moderate yields with complete regioselectivity (Scheme 7.30). The incorporation of a deuterium atom at the C-6 position (acetylenic terminal carbon) of the product and a substantial isotope effect (kH/fco = 4) were observed when a-methylstyrene-methyl-dj was used in place of a-methylstyrene. It is considered that the Cp-Cy double bond of an allenylidene complex reacts with a-methylstyrene, where the allenylidene complex works as an enophile, to afford the corresponding vinylidene complex via an allenylidene-ene reaction, as shown in Scheme 7.30. 

This was explained by the involvement of a vinylidene complex that is also in agreement with the migration of the acetylenic hydrogen to C-2 observed by deuterium labeling. The stereoselective reaction requires the use of EtjN and a slight excess of the alkyne. 

Terminal acetylenes (1-alkynes) undergo a 1,2-hydrogen shift in reactions with many metal centers to give vinylidene complexes. These reactions may proceed via an intermediate tp-alkyne complex, which has been isolated or detected spectroscopically in some cases (11-14,18-20). 

The reaction between acetylene and RhfCOXi CjH i -QH,) [which acts as a source of the Rh(COXf/5-C9H7) fragment] affords 33 in 50% yield (61). The reaction is supposed to proceed via oxidative addition of the alkyne to the rhodium fragment, followed by isomerization to the vinylidene complex which then interacts with a second rhodium fragment ... 

Metal cluster complexes containing vinylidene ligands have been considered as models of species present when olefins or alkynes are chemisorbed on metal surfaces (114). Vinylidene has been detected in reactions of ethylene or acetylene with Fe(100), Ni( 111), and Pt(l 11) surfaces (115), and was shown to be an intermediate by theoretical studies on a manganese surface (116). The facile cleavage of C-H bonds which occurs in these systems, together with hydrogen addition or abstraction, also occurs on metal clusters. Typical of the reactions considered is the hydrogen transfer reaction... 

The hydrido-/i2-q, q2-vinyl complex (79), which is obtained from reactions between H2Os3(CO),0 and acetylene (124) or ethylene (125) under mild conditions, loses a molecule of CO on heating to give the pale yellow 3-vinylidene derivative (80). 

Thermal decomposition of Os3(CO),[Me2As(CH=CH2)] at 96°C gives a/i-vinyl complex Os3(/i-AsMe2)(u-CH=CH2)(CO),0 (82), which at higher temperatures affords successively the white / -vinylidene complex (83) and then the/t 3-alky ne derivative (84) (129). This sequence of reactions provides the first authenticated report of the interconversion of cluster-bound vinylidene to acetylene. 

Addition of the acetylenic alcohols HC=C(CH2) OH (x = 3,4) to 1 affords a one-pot synthesis of the cyclic carbene complexes (88). The reaction proceeds via initial formation of the vinylidene complexes, followed by an intramolecular attack of the terminal alcohol function on the a carbon [Eq. (84)] (85). Combining the nucleophilicity at the /3 carbon of... 

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