Alkylidyne formation

It is apparent that Pt is rather generally the least reactive of the group VIII (IUPAC 8-10) metals, as its reaction-completion temperatures are substantially the highest. Only the temperature of alkylidyne formation is an exception to this generalization, where Pt is not notably different from the other metals. 

Palladium seems to be a particularly effective dehydrogenation metal in alkyl dehydrogenation on evacuation or, for the C4 species, alkylidyne formation from... 

With the failure of the diisopropylamino group to favour formation of alkylidyne iron complexes we next turned our attention to the possibility of using steric factors to favour alkylidyne formation. The reaction of [Fe(CO)5] with simple aryl or alkyl lithium reagents (LiR R = Me, tBu, C6H4Me-4, C6H40Me-4) followed by trifluoroacetic anhydride and triphenylphosphine lead in all cases to the exclusive formation of [Fe(CO)3(PPh3)2] in high yield (Scheme 9). 

Thus far, all examples of C—C activation and/or C—C formation have employed alkynes, either alone or in concert with cluster-bound alkylidyne or CO. 

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. 

The difference in reactivity of metal clusters and metal surfaces has also been well illustrated in these iridium-based systems [205]. A lack of reactivity of alkyli-dyne species on Ir4/y-Al203 with H2 is observed meanwhile, the chemisorption of H2 is not hindered. This behavior contrasts with that of metallic surfaces, which allow the reaction between alkylidyne species and H 2. It is inferred that over metallic clusters the reaction of H2 with alkyklidyne is not allowed because of the lack of available adjacent metal sites, which are necessary for the formation of the intermediates [205]. 

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). 

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. 

The most recent method developed for the nA —> An approach relies on dynamic covalent bond formation using a metathesis reaction. In this case, reactions are typically under thermodynamic control, providing the potential for increased selectivity in product formation. The initial examples using alkyne metathesis toward the formation of SPMs were reported by Adams, Bunz, and coworkers using the precatalyst [Mo(CO)6] [27,28], but rather low yields of the desired products (4) limited general applicability (Scheme 6.2). Recent efforts by Moore and coworkers using a Mo(VI)-alkylidyne catalyst, however, have refined this process such that precipitation-driven reactions now provide moderate to excellent results (see Scheme 6.24) [29]. 

Scheme 17. Formation of supracyclopentadienyl derivatives from the alkylidyne moiety in the dimeric tantalum complex and alkynes (13). Drawings of molecules are schematic.

The formation of alkylidene and alkylidyne compounds was noted above. Quite a number of these are now known. The use of electron rich alkenes to make carbene complexes was pioneered by Lappert131 as in the reaction... 

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