Vinylidene with nucleophiles

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. 

Similar to vinylidene complexes, carbyne complexes can also react with nucleophiles to yield heteroatom-substituted carbene complexes (Figure 2.14) [122,181-184],... 

The formation of metal vinylidene complexes directly from terminal alkynes is an elegant way to perform anti-Markovnikov addition of nucleophiles to triple bonds [1, 2], The electrophilic a-carbon of ruthenium vinylidene complexes reacts with nucleophiles to form ruthenium alkenyl species, which liberate this organic fragment on protonolysis (Scheme 1). 

In view of their capacity for Michael reactions with nucleophiles to give intermediate vinylidene-iodonium ylides, alkynyliodonium ions might be expected to behave as 1,3-dipolarophiles. Cycloadducts in which the nucleophilic end of the dipole is bound to the / -carbon atom of the starting alkynyliodonium ion (i.e. the / -adduct) might also be anticipated (equation 136). 

As discussed in Section II.D, the ability of alkynyliodonium salts to undergo Michael additions with nucleophilic reagents provides access to / -functionalized vinyliodonium salts (equation 177). However, this approach will not succeed unless the intermediate vinylidene-iodonium ylides can be captured by protonation. Thus, the best results are obtained when the nucleophile bears an acidic hydrogen or when the reactions are conducted in an acidic medium. 

The catalyst [RuC12(CO)3]2 also promotes the electrophilic activation of the C=CH bond of co-arylalk-l-ynes. The intramolecular cycloisomerization takes place with nucleophilic addition of the aryl group to the activated /1-carbon of the alkyne bond, thus eliminating a vinylidene intermediate [68]. 

The cationic vinylidene complexes (56 and 54) are electrophilic at the a carbon and react with nucleophiles to generate the r/-vinyl complex... 

The 7c-acidity of vinylidene ligands also points towards reactions with nucleophiles at Ca. This will be particularly true if the metal centre is positively charged or co-ligated by other strong 7t-acids, which competitively compromise M— Ca retrodonation. In the case of monobasic... 

Another route to heteroatom stabilized carbenes involves first the interaction of a terminal alkyne with an iridium complex to yield an iridium vinylidene compound. The vinylidene complex then reacts with nucleophiles, typically alcohols, to form oxa stabilized carbene complexes. O Connor extended this strategy to form a carbene ligand on the same iridium center as a metallacycle (76) and was also able to... 

The vinylidene complexes 8 also react with nucleophiles such as water and methanol. Hydration of the unsubstituted vinylidene complex 8a leads to the formation of the rj -acetyl complex 13, whereas the corresponding reactions of 8b and 8c result in the C-C bond cleavage, giving the cationic carbonyl complex 14 and organocarbonyl products R COMe(R = OMe or Me). On the other hand, the reactions of 8 with methanol afford the methoxycarbene complex 15 and the vinyl complex 16, depending upon the substituent of the vinylidene complexes 8. Intramolecular nucleophilic attack takes place in the reaction of 4 with 3-butyn-l-ol, giving the cyclic alkoxycarbene complex [Cp RuCl(p-SPr%Ru =C(CH2)30 Cp ](OTf). ... 

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. 

The proposed mechanism begins with the dissociation of the chloride to afford the starting Ru precatalyst, which upon coordination with the corresponding aUcyne would give rise to the key vinylidene intermediate A (Scheme 15). Nucleophilic attack by the pendant alcohol to the vinylidene with concurrent removal of a proton by the amine would provide alkenyl ruthenium species B, which after protonolysis... 

Cyclizations can be initiated by a nucleophilic attack, e.g. by H2O or a carboxylic acid, to a catalytic ruthenium vinylidene followed by trapping with an electrophile. Lee and coworkers described the Ru-catalyzed hydrative cyclization of 1,5-enynes (Scheme 33) to give functionalized cyclopentanones [147]. Treatment of 1,5-enynes bearing an internal Michael acceptor with a catalytic amount of [Ru3Cl5(dppm)3]PF6 in the presence of water initially afforded the corresponding ruthenium vinylidene species. Nucleophilic anti-Markovnikov addition of water... 

Nishibayashi and Sakata recently described the Ru-catalyzed [3+2] cycloaddition of ethynylcyclopropanes bearing two carboxy groups at the homopropargyUc position with aldehydes and aldimines to afford 2-ethynyltetrahydrofurans and pyrrolidines (Scheme 52) [179]. The proposed mechanism requires the formation of the ruthenium allenylidene species II by isomerization of the initially formed vinylidene I. Nucleophilic attack of species II to the aldehyde or aldimine, which are activated by BF3-OEt2, would afford allenylidene III. Final nucleophilic attack on the Cy by the oxygen or nitrogen followed by tautomerization of the vinylidene... 

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. 

Etherification using a metal vinylidene has also been combined with G-G bond formation through the reaction of an alkynyl tungsten complex with benzaldehyde (Scheme 14). The addition of an internal alcohol to the incipient /3,/Udialkylvinylidene that is generated leads to dehydration and the formation of a Fischer-type alkylidene complex. Further reactions of this carbene with a range of nucleophiles have provided access to various furan derivatives.374,375... 

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 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. 

The nucleophilic attack of f-butyllithium on lithium vinylidene carbenoids has also been used for synthetic purposes in as far as the reaction permits to generate sterically hindered alkenes. Thus, treatment of the dibromoalkene 78 generated from adamantanone with an excess of f-butyllithium results in the formation of the alkene 79 that contains three bulky substituents at the double bond (equation 43) . In an analogous way, a f-butyl residue is introduced into chloroenamine 80 (equation 44) . 

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