Methylidene initiator

Tandem carbonyl olefmation—olefm metathesis utilizing the Tebbe reagent or dimethyl-titanocene is employed for the direct conversion of olefmic esters to six- and seven-mem-bered cyclic enol ethers. Titanocene-methylidene initially reacts with the ester carbonyl of 11 to form the vinyl ether 12. The ensuing productive olefm metathesis between titano-cene methylidene and the cis-1,2 -disubstituted double bond in the same molecule produces the alkylidene-titanocene 13. Ring-closing olefin metathesis (RCM) of the latter affords the cyclic vinyl ether 14 (Scheme 14.8) [18]. This sequence of reactions is useful for the construction of the complex cyclic polyether frameworks of maitotoxin [19]. 

Since the vinylcarbenes la-c and the aryl substituted carbene (pre)catalyst Id, in the first turn of the catalytic cycle, both afford methylidene complex 3 as the propagating species in solution, their application profiles are essentially identical. Differences in the rate of initiation are relevant in polymerization reactions, but are of minor importance for RCM to which this chapter is confined. Moreover, the close relationship between 1 and the ruthenium allenylidene complexes 2 mentioned above suggests that the scope and limitations of these latter catalysts will also be quite similar. Although this aspect merits further investigations, the data compiled in Table 1 clearly support this view. 

Although the molybdenum and ruthenium complexes 1-3 have gained widespread popularity as initiators of RCM, the cydopentadienyl titanium derivative 93 (Tebbe reagent) [28,29] can also be used to promote olefin metathesis processes (Scheme 13) [28]. In a stoichiometric sense, 93 can be also used to promote the conversion of carbonyls into olefins [28b, 29]. Both transformations are thought to proceed via the reactive titanocene methylidene 94, which is released from the Tebbe reagent 93 on treatment with base. Subsequent reaction of 94 with olefins produces metallacyclobutanes 95 and 97. Isolation of these adducts, and extensive kinetic and labeling studies, have aided in the eluddation of the mechanism of metathesis processes [28]. 

Reactions of titanocene-methylidene generated from titanacyclobutanes with acyl chlorides 55 [46] or acid anhydrides 56 [47] lead initially to the titanium enolates 57 (Scheme 14.24), which then afford aldols upon treatment with the carbonyl compounds. On the other hand, five-membered cyclic anhydrides are methylenated with dimethyltitanocene (Table 14.5, entry 7) [45]. 

In these reactions, it is not clear which alkene— the terminal alkene or the cycloalkene— reacts with methylidene carbene complex Ih in the initial step, and... 

Good evidence has been obtained that heterogeneous iron, ruthenium, cobalt, and nickel catalysts which convert synthesis gas to methane or higher alkanes (Fischer-Tropsch process) effect the initial dissociation of CO to a catalyst-bound carbide (8-13). The carbide is subsequently reduced by H2to a catalyst-bound methylidene, which under reaction conditions is either polymerized or further hydrogenated 13). This is essentially identical to the hydrocarbon synthesis mechanism advanced by Fischer and Tropsch in 1926 14). For these reactions, formyl intermediates seem all but excluded. 

Methylidenation of allylic thioethers. Methylidenation of an allylic phenyl-thioether with methylene iodide-diethylzinc is accompanied by a 2,3-sigmatropic rearrangement to a homologous allylic phenylthioether. The rearrangement is also initiated by ethylidene iodide. Cyclopropanation is not observed. The Simmons-Smith reaction with allylic sulfides results only in formation of an insoluble polymer. 

Titanacyclobutene complexes result from the reaction of dimethyltitanocene with alkynes (Scheme 29). The reaction can be envisioned to proceed either by initial methane extrusion to form the methylidene, followed by [2+2] cycloaddition, or by initial alkyne insertion to generate the vinylic intermediate followed by 7-elimination. The evidence suggests that the latter mechanism dominates. Diphenylacetylene and bis(trimethylsilyl)acetylene both provide the corresponding titanacyclobutene complex, preferentially abstracting an allylic 7-hydrogen over the... 

It is worthy to note here that the methylidene complex 11 is a poor initiator for olefin metathesis reactions at room temperature. Although this complex can undergo multiple catalytic turnovers, if it is intercepted by free phosphine ligand, it becomes incapable of reentering the metathesis catalytic cycle.32... 

Figure 17 Scheme representing possible surface reactions that occur to form the surface C2 species (probably CH2=CH(ad)) that initiate Fischer-Tropsch polymerization (a) by reaction of a methylidyne and a methylidene (b) via formation of a dicarbide which is then hydrogenated in the surface. 

In the gas phase reaction of Rh with c-CaH, among other processes, elimination of C2H4 has been observed and the resulting metal ion was shown to have the structure of a methylidene-rhodium complex (238) instead of a hydrido-methylidene species (239) . RhCHa (238) reacts readily with H2 and CH4 and represents the first example of methane activation by a cationic mononuclear transition-metal complex in the gas phase. Reactions of both Rh = CH2 and its analogues Fe-CH2 and C0-CH2 with cyclic hydrocarbons were studied, and it is assumed that in the initial step metallacycloalkanes are generated (Scheme 35). 

In addition to the effects of the phosphine and the L ligand, the carbene fragment has an impact on the metathesis activity of these complexes. As established above, phosphine dissociation can be the rate-limiting step in the initiation (first turnover), especially for the NHC complexes, and thus it is difficult to direcdy compare the metathesis activity of different carbenes in the absence of the influence of phosphine dissociation. One obvious trend is that the methylidenes are particularly less reactive than other carbenes for this catalyst series. This has great impact on the ADMET reaction since the methylidene is involved in each turnover. 

For reaction of the various ruthenium carbenes with either the terminal olefin 1-hexene [103] or ethyl vinyl ether [101], the rate of reaction increases in the order methylidene < vinyl carbene < benzylidene < alkylidene. The differences in rates were quite significant, especially for the methylidene, which is quite slow to initiate. The data also show that internal olefins are slower to react, but only by about an order of magnitude the ds isomers being more readily metathesized than the... 

The use of diazoalkanes instead of 3,3-diphenylcyclopropene provides a series of air-stable alkylidene complexes, Ru(=CHR)(Cl)2(PPh3)2 and Ru(=CHR) (Cl)2(PCy3)2 (R = Me, Et, Ph, p-CgHtCl), which are very efficient catalysts for the ROMP of norbomene and substituted cyclobutenes. Thus, for the Ru(=CHPh)(Cl)2(PPh3)2-initiated ROMP of norbomene, kj/kp = 9 (see Table 11.4). Reaction of Ru(=CHPh)(Cl)2(PCy3)2 with excess ethene gives Ru(==CH2)(Cl)2(PCy3)2, the first methylidene complex which has been isolated and shown to be an active metathesis catalyst (Schwab 1995, 1996). 

Grubbs-type initiators are well-defined ruthenium aUcylidenes. Compared to molybdenum- or tungsten-based Schrock catalysts, the reactivity of ruthenium-based Grubbs catalysts is somewhat different. In terms of polymer structure, ROMP of norbom-2-enes and norbomadienes using ruthenium-based systems generally results in the formation of polymers that, in most cases, predominantly contain frany-vinylene units. Polymerizations initiated by Grubbs-type initiators are best terminated by the use of ethyl vinyl ether, yielding methylidene-terminated polymers. 

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