Vinylidene anions

The acetylene anion radical undergoes autodetachment of the electron, but the vinylidene anion can be generated easily [83]. Since the calculated isomerization barrier is -45 kcal/mol, the 2B2 ground-state vinylidene anion radical is predicted to be stable with respect to the 1,2-hydrogen shift [30, 84, 85]. As mentioned before, the vinylidene anion radical was used as the precursor for the generation of the singlet vinylidene in Lineberger s experimental studies. 

Scheme 53. Electrophilic attack at p-carbon of vinylidene anion. Reagents (i) NaN(SiMe3)2, —78 °C,...

Deprotonation of the alkyl carbynes Tp M( = CCH2R)(CO)2 with BuLi or KO Bu at Cp forms synthetically versatile vinylidene anions [Tp W( = C = CHR)... 

Aldehydes and ketones can also be used as electrophiles in reactions with the vinylidene anion [Tp M( = C = CH2)(CO)2] . Low-temperature reaction of [Tp M( = C = CH2)(CO)2] with RR C = 0 (R = Ph, Pr, R = H R = Ph, R = Me) followed by protonation yields Tp W = CCH2C(OH)RR (CO)2. When R = Ph and R = H, one equivalent of base leads to deprotonation and hydroxide elimination to form the conjugated vinyl carbyne complex Tp W (= CCH = CHPh)(CO)2 (as the E isomer) in 53% yield two equivalents of base produces a 1 1 mixture of the vinyl carbyne and the ethylidyne complex. With base, Tp W = CCH2C(OH)PhMe (CO)2 simply regenerates the starting ethylidyne complex and ketone,reminiscent of the tendency of propargylic alcohols to eliminate aldehyde or ketone under basic conditions. 

Treatment of the carbyne MoL(=CCH2Bu )Cp [L = P(OMe)3] with BuLi gives an equilibrium mixture of the carbyne anion [MoL (HCCHBu )Cp] and the vinylidene anion [M0L2(=C=CHBu )Cp]. ... 

Reaction of CO2 with the acetylide anion [W(C=CR)(CO)3(dppe)] results in formation of the vinylidene anion... 

This reaction can proceed by 1,1-proton abstraction to form a carbene radical anion, but can also occur by l,n-abstraction to form the negative ion of a diradical. Thus, reaction of O with methylene chloride results in the formation of CCI2 (Eq. S.Sa), reaction with ethylene gives vinylidene radical anion, H2CC (Eq. 5.8b), and the reaction with acetonitrile gives the radical anion of cyanomethylene, HCCN (Eq. 5.8c) Investigations of these ions have been used to determine the thermochemical properties of dichlorocarbene, CCI2, vinylidene, and cyanomethylene. ... 

Closely related to the a-addition of nucleophiles is the P-deprotonation of electrophilic carbyne complexes. In many of the examples reported [143,530,531] the resulting vinylidene complexes could not be isolated but were generated in situ and either oxidized to yield stable carbene complexes [532] or used as intermediates for the preparation of other carbyne complexes [527]. Cationic carbyne complexes can be rather strong acids and undergo quick deprotonation to vinylidene complexes with weak bases [143]. An interesting example of the use of anionic vinylidene complexes as synthetic intermediates is sketched in Figure 3.24. 

The question of configurational stability has been investigated first for vinylidene carbenoids and, more recently, for alkylcarbenoids. Vinyl anions are usually considered to be configurationally stable" ° the calculated inversion barrier of the ethenyl anion 10 (R = H) is about 35 kcal mol (equation 4)" . Concerning lithioalkenes, this configurational stability has been confirmed experimentally for a-hydrogen, a-alkyl and a-aryl substituted derivatives . The inversion of vinylidene lithium carbenoids was already... 

This is the most common route to vinylidene complexes and occurs in reactions of the 1 -alkynes with metal complexes, preferably with labile neutral or anionic ligands, which give neutral or cationic complexes, respectively. In the latter case, halide is commonly extracted, either by spontaneous displacement by a polar solvent, or by using sodium, silver or thallium salts. 

In fact, the first isolation ofthe vinylidene pentacarbonyltungsten complexes was reported by Mayr et al. in 1984 [6]. The vinylidene complexes 16 were obtained by the alkylation of anionic pentacarbonyltungsten t-butylacetylide complex 15, obtained by the reaction of [Et4N ] [W(CO)5Cl ] with lithium acetylide, with FS03Me or [Et30 ] [Bp4 ]. Further protonation with CF3SO3H in the presence of Me4N P afforded a unique method for the preparation of carbyne complexes 17 (Scheme 5.4). 

The stoichiometric interaction of an enyne and [RuCl(PCy3)(pcymene)]B(Ar )4 XVIIIa containing a bulky non-coordinating anion B(ArF)4 showed by NMR at —30 ° C the formation of the alkenyl alkylidene ruthenium complex and acrolein. This formation could be understood by the initial formation of a vinylidene intermediate and transfer of a hydride from the oxygen a-carbon atom to the electrophilic vinylidene carbon, as a retroene reaction step (Scheme 8.13) [54]. 

The formation of vinylcarbamates is restricted to secondary amines and also to terminal alkynes, which is in line with the formation of a metal vinylidene intermediate. It is noteworthy that even starting from secondary amines, the presence of a hydroxy group in propargylic alcohols drove the reaction towards the formation of fi-keto carbamates, resulting from initial Markovnikov addition of the carbamate anion to the triple bond followed by intramolecular transesterification [10]. The proposed general catalytic cycle which applies for the formation of vinylic carbamates is shown in Scheme 10.2. 

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