Vinylidene with electrophiles

Protonation of alkenyl complexes has been used [56,534,544,545] for generating cationic, electrophilic carbene complexes similar to those obtained by a-abstraction of alkoxide or other leaving groups from alkyl complexes (Section 3.1.2). Some representative examples are sketched in Figure 3.27. Similarly, electron-rich alkynyl complexes can react with electrophiles at the P-position to yield vinylidene complexes [144,546-551]. This approach is one of the most appropriate for the preparation of vinylidene complexes [128]. Figure 3.27 shows illustrative examples of such reactions. 

As in alkynyl complexes, Cp is electron rich and thus reacts readily with electrophiles. These reactions proceed more readily with neutral rather than cationic vinylidenes. 

Much of the chemistry of the vinylidene group was established with Group 8 complexes of the type [M(=C=CRR )(L )(P) Cp ]+ and several of the ubiquitous reactions of vinylidenes with 0-, S- and N-nudeophiles and with electrophiles have been mentioned above. 

The activation of alkynes to metal-vinylidenes with transition metal complexes of Groups 6-9, essentially, provides reactive intermediates with an electrophilic... 

For the reactions of cumulenylidenes, a picture has begun to emerge wherein nucleophiles attack at Ca, Cy, Ce, etc., whilst electrophilic attack occurs at Cp, C6, etc. (Figure 5.53). Thus, the reactions of vinylidenes with nucleophiles at Ca and electrophiles at Cp fits within this scheme. 

Subsequent reaction with a nucleophile affords a metal-vinylidene complex. This subject has been reviewed by Bruce. Reactions with electrophilic alkenes initially lead to a cyclobutenyl complex in a two-step process via a paramagnetic intermediate. Subsequently, the ring opens in a concerted fashion to a butadiene derivative. 

Ionic copolymerizations are only possible under three conditions. The first is that the ionic chain end can induce the polymerization of the other monomer at any given time. For example, in the anionic copolymerization of ethylene oxide with vinyl compounds CH2=CHR, alkoxide ions—CH2—CH2—O" may be formed. These alkoxide ions can only add monomers with electrophilic double bonds, i.e., monomer containing electron-attracting substituents R (e.g., acrylonitrile, vinylidene cyanide). 

Vinylidene complexes may also be formed by the reaction of ii -alkynyl complexes with electrophiles (Scheme 8.49). Again, if an alcohol is present, a carbene complex will be formed. In this case, the carbene complex 8.184 was converted to a gem-dimethyl group by reaction with a Grignard reagent. 

Another approach toward C-O bond formation using alkynes that has been pursued involves the intermediacy of transition metal vinylidenes that can arise from the corresponding y2-alkyne complexes (Scheme 13). Due to the electrophilicity of the cr-carbon directly bound to the metal center, a nucleophilic addition can readily occur to form a vinyl metal species. Subsequent protonation of the resulting metal-carbon cr-bond yields the product with anti-Markovnikov selectivity and regenerates the catalyst. 

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. 

Electrophilic vinylidene complexes, which can be easily generated by a number of different methods [128], can react with non-carbon nucleophiles to yield carbene complexes (Figure 2.9 for reactions with carbon nucleophiles, see Section 3.1). 

Terminal alkynes readily react with coordinatively unsaturated transition metal complexes to yield vinylidene complexes. If the vinylidene complex is sufficiently electrophilic, nucleophiles such as amides, alcohols or water can add to the a-carbon atom to yield heteroatom-substituted carbene complexes (Figure 2.10) [129 -135]. If the nucleophile is bound to the alkyne, intramolecular addition to the intermediate vinylidene will lead to the formation of heterocyclic carbene complexes [136-141]. Vinylidene complexes can further undergo [2 -i- 2] cycloadditions with imines, forming azetidin-2-ylidene complexes [142,143]. Cycloaddition to azines leads to the formation of pyrazolidin-3-ylidene complexes [143] (Table 2.7). 

Electrophilic vinylidene complexes, capable of reacting with non-carbon nucleophiles to yield Fischer-type carbene complexes, can be obtained by addition of electrophiles to alkynyl complexes (Figure 2.11, Table 2.7, Entries 11, 12) [134,144]. 

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 alkenyl(amino)aUenylidene complex 41 is also prone to undergo electrophilic additions at the Cp atom of the cumulenic chain. Thus, treatment of 41 with HBF4 OEt2 led to the spectroscopically characterized dicationic vinylidene complex 65 (Fig. 10) [52, 53]. Related Cp-protonations of complexes 35 (Fig. 6) have also been described [49]. 

The electrophilic reactivity of lithium carbenoids (reaction b) becomes evident from their reaction with alkyl lithium compounds. A, probably metal-supported, nucleophilic substitution occurs, and the leaving group X is replaced by the alkyl group R with inversion of the configuration . This reaction, typical of metal carbenoids, is not restricted to the vinylidene substitution pattern, but occurs in alkyl and cycloalkyl lithium carbenoids as well ". With respect to the a-heteroatom X, the carbenoid character is... 

A similar type of substitution, which clearly shows the electrophilic character, occurs in vinylidene carbenoids. In an early example of this reaction, Kobrich and AnsarP observed that the aUcene 70 results when the fi-configurated vinyl lithium compound 68 is treated with an excess of butyllithium and the fithioafkene 69 formed thereby is protonated (equation 41). Obviously, the nucleophilic attack of the butyl residue on the carbenoid takes place with inversion of the configuration. 

While protonation affords the vinylidene expected by H migration from the original 1-alkyne, use of other electrophiles provides a convenient route to disubsti-tuted vinylidenes. The stereospecificity of this reaction with Re(C=CR)(NO)(PPh3)... 

Cp has been discussed [170b]. Alkylation with haloalkanes (often iodoalkanes), triflates (alkyl, benzyl, cyclopropyl), or [RsO] (R = Me, Et) is often the best entry to vinylidenes of any particular system. Other common electrophiles, such as halogens (Cl, Br, I), acylium ([RCO] ), azoarenes ([ArN2] ), tropylium ([C7H7] ), triphenylcarbenium (trityl, [CPhs] ), arylthio (ArS) and arylseleno (ArSe) have also been used. 

An early approach to vinylidenes was by the formal dehydration of metal acyls, which is best achieved by treatment with an electrophile, often the proton in the form of a non- or weakly-coordinating strong acid. The reaction appears to proceed stepwise via a hydroxycarbene formed by protonation of the acyl, subsequent dehydration of which affords the vinylidene. Occasionally, mixtures of the two complexes are obtained, again suggesting the intermediacy of the carbene. 

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