Vinylidene reactions with electrophiles

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. 

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). 

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. 

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... 

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)... 

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 vast majority of work exploring the reactivity of ruthenium viny-lidene complexes has focused on the attack of alcohols at the electrophilic a carbon of monosubstituted vinylidenes, resulting in the formation of ruthenium alkoxycarbene complexes. Bruce and co-workers have determined, for example, that the phenylvinylidene complex 80 is slowly transformed in refluxing MeOH to the methoxycarbene complex 82 in good yield (73,83). The mechanism for this reaction must involve initial attack of the alcohol at the electrophilic Ca to form a transient vinyl intermediate 81 which is rapidly protonated at the nucleophilic Cp, generating the product carbene 82 [Eq. (79)]. In contrast to monosubstituted vinylidene complexes, disubstituted vinylidene complexes are generally unreactive to nucleophiles even the relatively small dimethylvinylidene complex 83 shows no reaction with MeOH after 70 hours at reflux [Eq. (80)]. 

Water also attacks the electrophilic a carbon of the ruthenium vi-nylidene complex 80. The reaction does not yield the ruthenium acyl complex, however, as is found for the reaction with the similar iron vinylidene complex [(i75-C5H5)(CO)2Fe=C=CHPh]+ (56), but rather 91 is the only isolated product (78). The mechanism for this transformation most reasonably involves rapid loss of H+ from the initially formed hydroxycarbene to generate an intermediate acyl complex (90). Reversible loss of triphenyl-phosphine relieves steric strain at the congested ruthenium center, and eventual irreversible migration of the benzyl fragment to the metal leads to formation of the more stable carbonyl complex (91) [Eq. (86)]. 

Most efforts to explore the reactivity of ruthenium carbene complexes have employed the alkoxycarbene species so readily synthesized from the inter- or intramolecular reaction of vinylidene complexes with alcohols. These electrophilic alkoxycarbene complexes exhibit only limited reactivity at Ca, primarily with hydride reagents. For example, treatment of the 2-oxacyclopentylidene complex 97 with NaAlH2(OCH2CH2OMe)2 affords the neutral 2-tetrahydrofuranyl complex (98) [Eq. (89)] (55), as was anticipated from similar reductions of iron carbene complexes (87). 

It should be pointed out that the electrophilic [2 + 2] cycloaddition of CH2=0 and fluoroolefins carried out in HF is rather limited in the scope. For example, reaction of formaldehyde with hexafluoropropylene exclusively affords CF3CF(CF3)CH20H, and in reaction with vinylidene fluoride, the ether (CF3CH2CH2)20 is formed as the major product. More on the electrophilic reactions of fluoroolefins in anhydrous HF can be found in detailed review. ... 

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. 

The reluctance of the carbyne carbon to react with nucleophiles is revealed by the reaction with LiEt3BH (see Scheme 6). Here the most electrophilic site is not the carbyne carbon but the ipara position of the aryl ring in the carbyne substituent Both ruthenium and osmium five coordinate, cationic, carbyne complexes undergo this reaction. The structure of a representative example, the osmium compound derived from the p-tolyl carbyne complex, has been determined by X-ray crystallography [16]. The unusual vinylidene complex reacts with HCl to produce a substituted benzyl derivative. The reaction may proceed through the intermediate a-vinyl complex depicted in Scheme 6 although there is also the possibility that the vinylidene compound is in equilibrium with the carbene tautomer as shown below. 

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. 

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