Ruthenium complex disubstituted vinylidene

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)]. 

Noncarbon-based electrophiles also react with ruthenium acetylides to give disubstituted vinylidene complexes. Addition of halogens to the phenyl acetylide complex 63 occurs rapidly and quantitatively to form /3-halovinylidene complexes [Eq. (76)] (71,74). Facile bromination of the... 

Treatment of the disubstituted vinylidene complex 124 with base in wet methanol cleaves the vinylidene bond to give the cationic ruthenium carbonyl complex 126 and bibenzyl in good yield. This reaction presumably proceeds via initial formation of the acyl complex 125, which decomposes to give the products [Eq. (106)] (78). Other mono- and disubstituted vinylidene complexes, however, do not give identifiable decomplexation... 

The formation of a ruthenium vinylidene is proposed as the key intermediate in the regioselective addition of hydrazine to terminal alkynes [55]. This reaction, which proceeds via addition of the primary amino group of a 1,1-disubstituted hydrazine followed by deamination, provides an unprecedented access to a variety of aromatic and aliphatic nitriles. The tris(pyrazolyl)borate complex RuCl(Tp)(PPh3)2 gave the best catalytic activity in the absence of any chloride abstractor (Scheme 10.17). 

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