Vinylidene reactions with nucleophiles

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). 

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... 

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). 

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... 

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-... 

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 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)]. 

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. 

Contrary to the previous pathway of P-H addition to alkyne - that is, via alkyne insertion into the M-P bonds - this reaction has been shown to proceed via the nucleophilic attack of the phosphine to a ruthenium-vinylidene intermediate to yield the anti-Markovnikov product with a predominant (Z -stereoisomer (Scheme 8.36). Indeed, it has been shown that [Cp RuL2] X intermediate gives vinylidene species with propargyl alcohols. The (Z)-isomer is formed as the major product, but iso-merizes easily into the ( )-isomer upon isolation by chromatography over silica gel. 

This chapter will focus on the nucleophilic addition reactions of transition metal vinylidenes and allyl complexes with Grignard reagents. Reactions with transition metal vinylidenes will be discussed initially, and then a brief review of allyl complexes with Grignard reagents will conclude the chapter. The synthesis and some general reactions of these vinylidene and allyl complexes will be presented. A more detailed description of the chemistry of these metal complexes can be found in the literature [1]. 

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). ... 

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. 

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. 

Our attempts to prepare useful materials by chemical modification of poly(vinylidene bromide) (Ref. 50) were not very successful, although, in reactions with the anions of methyl mercaptan or 2-mercaptoethanol under phase transfer conditions, degrees of substitution as high as 55% were obtained, while a reaction with the less nucleophilic sodium methyl sulfinate gave only 46% conversion. The reactions were accompanied by extensive elimination and the final modified polymers were always highly colored. 

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... 

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. 

---