Carbene from carbyne complexes

Table 2.10. Heteroatom-substituted carbene complexes prepared from carbyne complexes.

The discussed reactions of carbene and carbyne complexes show that they have essential significance as catalysts or unstable transient intermediate compounds in such catalytic processes as metathesis of olefins and other unsaturated compounds, Fischer-Tropsch synthesis, syntheses of cyclopropanes from diazoalkanes and olefins, and polymerization of olefins and alkynes as well as in organic synthesis. Except for alkynes [reaction (5.132) ] some compounds containing double bonds react with carbon monoxide and carbene ligands to form bonds with those groups. Examples of such compounds are enamines, ynamines, and Schiff bases. The JV-vinylpyrrolidone (enamine), methoxyphenylcarbene, and excess of CO (higher pressure) react to furnish enaminoketone. 

Compared to their carbene cousins, carbyne complexes, containing a metal-carbon triple bond, have seen very little development. One example is an interesting phenol synthesis by the reaction of a diyne 8.478 with a carbyne complex (Scheme 8.128). The reaction is remarkable for the low temperature at which it proceeds. " As in the Dotz reaction, the aromatic carbon atom bearing the hydroxyl substituent is derived from a carbon... 

H. Fischer, J. Organomet. Chem., 1980,195, 55. From carbene to carbyne complexes kinetic study of the C, Cr migration of the SnPha group in [(OC)5Cr(CSnPii3NEt2)]. 

An impressive series of carbene- and carbyne-bridged complexes of platinum have been formed by reaction of metal carbene complexes with a platinum complex (equation 141). These complexes have been verified by X-ray crystallography, and the synthetic method appears to be one of some generality. Among the complexes of this type formed from carbenes are ones having metal frameworks with Pt—W,409-413 Pt—Mn414-418 and Pt—Cr413,417 bonds. 

Fischer carbyne complexes are usually generated from carbene complexes by 1,2-elimination of an alcohol under the influence of boron trihalides204 or other Lewis acids.205... 

These play an enormous role in organometallic chemistry and will be mentioned frequently. The nature of the M to C bonding is very much dependent on the nature of M and R. At one extreme there are compounds in which M is in a high valence state and the R group(s) not of jr-donor character. In these cases, the bonds are comparable to those just discussed for M=NR and M=N. For these types of compounds, the terms alkylidene (M=CR2) and alkylidyne (M=CR) have been favored. On the other hand, when the metal is in a low valence state and the substituents on carbon are n donors, the M—C bonds are not full double or triple bonds and the systems are rendered stable by the migration of charge from the substituents (such as OR or NR) onto the carbon atom, as shown in resonance terms in (16-VI). For these compounds it is customary to use the terms carbene complex and carbyne complex. The chemistry is qualitatively different for the two classes. 

It was from such carbene complexes that, again, E. O. Fischer (1973) prepared the first carbyne complexes, as illustrated by the following reactions 132... 

The migration of a triphenyltin group from a carbene (CO)5Cr C(SnPh3)NEt2, on treatment with PPh3 with loss of CO, leads to a carbyne complex, trans-[(CO)4Cr(SnPh3)( CNEt2)] the reaction follows a first order kinetics . ... 

These Fischer-type carbene complexes can be readily converted to Fischer-type carbyne complexes, and this material has been reviewed. Methods to remove the a-alkoxy group include treatment with BBrs, oxide abstraction from acyl ligands, and electrophihc attack on the jS-carbon of... 

Several groups have completed computational studies on the relative stabilities of osmium carbyne, carbene, and vinylidene species. DFT calculations on the relative thermodynamic stability of the possible products from the reaction of OsH3Cl(PTr3)2 with a vinyl ether CH2=CH(OR) showed that the carbyne was favored. Ab initio calculations indicate that the vinylidene complex [CpOs(=C=CHR)L]+ is more stable than the acetylide, CpOs(-C=CR)L, or acetylene, [CpOs() -HC=CR)L]+, complexes but it doesn t form from these complexes spontaneously. The unsaturated osmium center in [CpOsL]+ oxidatively adds terminal alkynes to give [CpOsH(-C=CR)L]+. Deprotonation of the metal followed by protonation of the acetylide ligand gives the vinylidene product. 

Fischer first prepared carbyne complexes (1973) by electrophilic abstraction of methoxide ion from a methoxy methyl carbene. 

If trans-subsituted carbene complexes containing ligands which have a greater (T-donor/n-acceptor ratio than CO (e.g., PRj, AsRj, SbRj or a Jt-aromatic ) are reacted with electron-pair acceptor acids, these ligands remain in the product complex. Only the heteroatom-containing substituent is removed from the carbene carbon and cationic carbyne complexes are formed ... 

The reactions of Fischer carbene complexes with alkynes can under certain conditions lead to products that result from the incorporation of two alkynes, the carbene ligand and a carbon monoxide. In inter-molecular reactions, this is most commonly observed for acetylene itself or for sterically unhindered al-kynes. °2 As can be anticipated by the mechanism in Scheme 36, two-alkyne incorporated products of the type (258) are also favored for high alkyne concentration. Synthetically, the two-alkyne reactions are most useful in intramolecular reactions, two of which have been reported and are exemplified by the reactions in Scheme 43. The typical product from the reaction of a Fischer carbene complex with a diyne, such as (308), is a bicyclic phenol of the type (309). ° These products are apparently the result of the assembly of pieces indicated by (311). Under some conditions, dienones of the type (310) and (314) can be isolated, and it is thought they are the immediate precursors of the phenol products via an in situ reduction by a chromium(O) species. This reaction is completely regioselective with diyne (308) and the phenol (309) results from incorporation of the terminal alkyne of (308) before the disubstituted alkyne. Phenols of the type (309) have also been observed from the reaction of diynes with carbyne complexes. ... 

The tungsten thiocarbyne complex 171 is attacked by PRj (R = Me, Et) at the carbyne carbon atom (164). Templeton and co-workers found that with PEtj the phosphonium carbyne complex 172 is obtained [Eq. (141)] whereas reaction with PMcj gives the bisphosphonium carbene complex 173 [Eq. (142)]. One PMej appears to be able to dissociate from 173, and... 

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