Alkynyl complexes, reaction with electrophiles

The hydride-methyl complex OsH(Me)(CO)2(P Pr3)2 reacts with electrophilic reagents. The reaction products depend on the nature of the reagent (Scheme 39). Whereas the reaction with iodine gives almost quantitatively the diiodide OsI2(CO)2(P,Pr3)2, the reaction with a five-fold excess of phenylacetylene does not lead to the formation of the previously mentioned bis-alkynyl complex... 

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. 

The alkynyltitanium complexes TiCI((C=CR) (R typically = alkyl groups) are prepared in situ by reacting 1-alkynes with TiCU in the presence of Et3N. The synthetic potential of these compounds has been illustrated in reactions with a number of electrophiles. In the presence of terminal alkynes, symmetrical 1,3-diynes are formed. The reaction with chlorodiphenylphosphine produces the corresponding alkynyl phosphine, while treatment with triethylphosphite affords tris(phenylethynyl) phosphine by substitution of the three alkoxo groups. The reaction with trimethyl orthoformate gives acetals.6-9... 

Shaw and coworkers have studied the anodically induced formation of the a,7i complex [ CpFe(CO)2 2(p-ri ri -C=CPh)] from Cp(CO)2Fe-C=CPh by infrared reflectance spectroelectrochemistry (IRRAS). This process goes along with the cleavage of the Fe-alkynyl bond and attack of the CpFe(CO)2 electrophile thus formed onto the parent alkynyl complex. Such processes are commonly dubbed as mother-daughter (or father-son) reactions. The identity of the electrogenerated product was established by comparing its IR absorbances to those of the independently prepared product. 

The great majority of o-acetylide transition metal complexes are prepared by interaction of a metal halide with acetylide, RC C", or the formal oxidative addition of terminal alkynes or alkynyl stannanes to the metal center. As amply demonstrated in the previous section, alkynyliodonium salts may serve as electrophilic acetylene equivalents. In other words, transition metal complexes may act as nucleophiles in reactions with alkynyliodonium species. Indeed, the reaction [81] of the square planar Vaska s complex, 106, and its Rh analog, 107, with a variety of alkynyliodonium triflates in toluene results in 89-96% isolated yield of the hexa-coordinate o-acetylide complexes, 108 and 109 [Eq. (58)]. Reaction is essentially instantaneous and occurs with retention of stereochemistry around the metal center. 

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. 

Disubstituted neutral vinylidenes 220 have been prepared by reaction of the cr-alkynyl complex [Ru(G=GPh)Gp(PPh3) P(OMe)3 ] 219 with organic halides XGH2R via electrophilic addition of the methylenic GH2R unit to the G j-atom of the phenylacetylide chain and subsequent Arbuzov-like dealkylation of the phosphite ligand (Scheme 22). The formation of minor amounts of vinylidenes 221 in the reactions of 219 with isothiocyanates has also been reported. ... 

Ooi and Maruoka found that aluminum alkoxide (83) prepared from aluminum complex (82) and tertiary propargyl alcohol reacts with highly electrophilic aldehydes, such as chloral and pentafluorobenzaldehyde, to give alkynyl transfer products in good yield (Scheme 6.65) [84]. Bisphenol structure of ligands on aluminum center is the most important feature to obtain the alkynylated products, and thus, in reactions with (84) or (85) instead of (83) dramatical decrease in the yield of alkynyl product was observed. Same group also reported that cyanide transfer reaction with acetone cyanohydrin as a cyanide donor catalyzed by complex (82) [84]. 

---