Alkylidyne complex

Byers, P.K, Carr, N. and Stone, F.G.A. (1990) Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 106. Synthesis and reactions of the alkylidyne complexes [M ( CR)(CO)2 (C6F5)AuC(pz)3 j (M = W or Mo, R — alkyl or aryl, pz — pyrazol-l-yl) crystal structure of pjC PtAu(C6F5)( l3-CMe)(CO)2(PMe2Ph)2 (C6F5)AuC(pz)3 ]. Journal of the Chemical Society, Dalton Transactions, (12), 3701—3708. 

On the basis of the fact that tungsten(VI) alkylidene complexes will metathesize olefins one might predict that acetylenes should be metathesized by tungsten(VI) alkylidyne complexes (29). Acetylene metathesis is not unknown, but the catalysts are inefficient and poorly understood (30, 31). 

W(CCMe3)(0CMe3)3 reacts rapidly with symmetric acetylenes to give the new alkylidyne complexes shown in equation 13. W(CPh)(0CMe3)3 is orange and W(CCH2CH2CH3)(0CMe3)3 1s white. Both can be sublimed. The latter is an important species since it... 

R.R. Schrock We believe so, but the t-but oxide ligand seems to be a unique alkoxide. We have prepared tungstenacyclobutadiene complexes containing other alkoxide ligands which are stable toward decomposition to an alkylidyne complex. 

R.R. Schrock There are bands at ca. 1300 cm in the IR spectra of several alkylidyne complexes which might be assigned to W C stretching modes analogous to those observed by Fischer in complexes of the type (X)(C0)5W=CR (X = halide), but we have not done the appropriate labelling or Raman studies to confirm the assignments. 

Finally, one can readily visualize various duodectet-rule-conforming alkyli-dene/alkylidyne complexes with more extensive multiple bonding, such as... 

Ostensibly minor variations of a synthetic procedure sometimes can have significant consequences. For example, substituting KOCMe(CF3)2 for LiOC-Me(CF3)2 is believed [85] to lead to formation of the alkylidyne complex shown in Eq. 16 instead of the known [83] Mo(CH-f-Bu)(NAd)[OCMe(CF3)2]2 (Ad=ad-amantyl). A proton is likely to be transferred before formation of the final product, since it has been known for some time that both W(CH-f-Bu)(NAr)[OC-Me(CF3)2]2 and W(C-f-Bu)(NHAr)[OCMe(CF3)2]2 are stable species that cannot be interconverted in the presence of triethylamine [41]. In such circumstances the nucleophilicity of the alkoxide ion and the nucleophilicity and acidity of the alcohol formed upon deprotonation of the alkylidene will be crucial determinants of whether the imido nitrogen atom is protonated at some stage during the reaction. At this stage few details are known about side reactions in which amido alkylidyne complexes are formed. 

In the last several years tungsten alkylidyne complexes [60], W(CCMe3) (CH2CMe3)3 and W(CCMe3)Cl2(dimethoxyethane) in particular, have been a source of alkylidene complexes bound to oxide surfaces as a consequence of protonation of the alkylidyne ligand by a surface-bound hydroxyl group [112-114]. 

Alkylidyne complexes can be used as catalysts for the metathesis of alkynes. For a classic review see Schrock [18],... 

The chemistry of alkylidene and alkylidyne complexes of early transition metals was developed by Schrock and co-workers and these complexes turned out to be of crucial importance to alkene and alkyne metathesis. Initially their research focused on tantalum complexes of the type CpTaCEIE, which after a-elimination (Figure 16.6) led to alkylidene complexes Cp(R)Cl2Ta=CHR [11]. 

At the same time, Filrstner used tungsten alkyUdene complex 150 developed by Schrock for ring-closing alkyne metathesis. He compared the reactivities of tungsten alkylidyne complex 150 and Mo(CO)6-p-ClC6H40H (Table 6.4) and showed that both catalysts work well, although a higher reaction temperature is required in the case of Mo(CO)6-p-chlorophenol. 

Although the structure of the complex arising from I52/CH2CI2 is not clear, this catalyst is excellent in terms of ease of preparation. The catalyst is very active for formation of cycloalkynes with ring sizes different from those of diynes (Table 6.5). In contrast to tungsten alkylidyne complex 150, catalyst 152/ CH2CI2 is sensitive toward an acidic proton such as amide proton and exhibited remarkable tolerance towards many polar functional groups (Table 6.5). 

Schrock, R. R. The Discovery and Development of High Oxidation State Alkylidyne Complexes for Alkyne Metathesis. In Handbook of Metathesis, 1st ed. Grubbs, R. H., Ed. Wiley-VCH Weinheim, 2002 Vol. 1, pp 173-175. 

Alkyne Metathesis with Tungsten Alkylidyne Complex 301... 

Alkyne Metathesis Using Molybdenum Alkylidyne Complex 304... 

This mechanism was later confirmed experimentally in 1981 by Schrock and others, who reported the first example of alkyne metathesis by tungsten(vi)-alkylidyne complex. They have prepared tungsten alkylidyne complex 120 (Equation (21)) and found that it reacts with diphenylacetylende to give tungsten alkylidyne complex 121 and another alkyne 122 (lequiv.) (Equation (22)). Furthermore, complex 121 works as a catalyst for the alkyne metathesis reaction. 

At the same time, Fiirstner and others used Schrock s tungsten alkylidyne complex 120 for ring-closing alkyne metathesis. They compared 120 with the Mo(CO)6-/>-ClC6H40H system (Table 4) in reactivity and found that... 

To identify the truly active species for the alkyne metathesis, various experiments are carried out for ring-closing alkyne metathesis of diynes (Table 5). Activation of complex 140 with CH2CI2 and evaporation of all the volatiles is shown to yield Mo[N(Ar)( Bu)]3Cl 141a and alkylidyne complex 141c as major components. The former complex 141a, that is also accessible by treatment of 140 with CI2 (Equation (24)), had an equal catalytic activity (entry 6), but... 

The alkylidyne complexes are best prepared by cleavage of a W=W triple bond92 as depicted... 

W=W triple bond and formation of both nitrido and alkylidyne complexes of WV1. Crystal structure determination94 of [(OBu )3W=N] and [(OBu )3W IMe]2 has shown that the nitrido complex is polymeric with unsymmetrical W—N—W bridges. The alkylidyne complex is dimeric with a short W—C bond length of 1.76 A. As expected, this distance is significantly shorter than the one observed for the W—C double bond. 

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