Metallocarbene intermediate

Most cyclic monomers of interest in the field of composites happen to be heterocyclic in nature. Polymerizability of some monomers is summarized in Table 1.1. Ring opening polymerizations invariably follow ionic mechanisms, although a few are known to proceed via the free radical route and some via metathesis involving metallocarbene intermediates. 

Fused bicyclic morpholones 145 and 146 have been synthesized in good yields but with modest diastereoselectivity from 1,3-oxazolidines 144 via reaction of the nitrogen of the oxazolidine with a metallocarbene intermediate followed by a benzyl migration (Scheme 37) <2003TA917>. 

These fluorinated catalysts were also employed in the functionalization of the first members of the series of alkanes, from methane to butane. To that end, the reaction was carried out in supercritical carbon dioxide, mixed with the alkane, in a fluid mixture that also dissolved the catalyst and EDA (Scheme 14). In this manner, a significant amount of methane (and the other alkanes) was available to react with the in situ generated metallocarbene intermediate, with no other C—H bond in the reaction mixture. Methane has been converted into ethyl propionate with turnover numbers up to 750 at 40 °C, and a total pressure of 250 atm. 

It is accepted that these catalytic reactions occur via a metallocarbene intermediate. The structure suggested for this species is shown in Scheme 6. Rho-... 

The metallocarbene intermediates are most often formed from thermal, photolytic, or metal-catalyzed deconposition of diazocarbonyl compounds, with concomitant loss of dinitrogen. Under transition metal catalysis, the initially formed species is a metallocarbene rather than a free carbene, and this is usually desirable due to the moderated reactivity (and, hence, fewer undesired side reactions) of the metal-complexed carbene. The two most common methods for introduction of the diazo group are acylation of diazoalkanes with suitably activated carboxylic acid derivatives and diazo transfer reactions in the case of more acidic active methylene substrates fScheme 16.12T... 

Reaction Mechanism. It is generally accepted that the mechanism of the metathesis reaction involves a metallocarbene intermediate (Fig. 39). Decomposition of the intermediate proceeds to generate either the productive or degenerative product. Additional evidence for this type of reaction mechanism has been provided by the isolation of the reaction intermediates formed in homogeneous metathesis reactions. 

Extensive studies have established that the stability and reactivity of the transient metallocarbene intermediates is controlled by several factors including the nature of (i) the metal center, (ii) auxiliary ligands, and (iii) substituents on the... 

The role of the rhodium is probably two-fold. Initially due to its Lewis acidity it reversibly forms a complex with the nitrile nitriles are known to complex to the free axial coordination sites in rhodium(II) carboxylates as evidenced by the change of colour upon addition of a nitrile to a solution of rhodium(II) acetate, and by X-ray crystallography. Secondly the metal catalyses the decomposition of the diazocarbonyl compound to give a transient metallocarbene which reacts with the nitrile to give a nitrile ylide intermediate. Whether the nitrile ylide is metal bound or not is unclear. 

An NMR and structural study characterized the intermediates generated from diimine catalysts on reaction with diazodiphenylmethane.193 The dominant species in solution is dinuclear, but a monomeric metallocarbene species can be detected. 

The mechanistic similarity between olefin and alkane metathesis, even if intuitive, has needed time to become obvious in the laboratory for various reasons olefin formation starting from alkanes is thermodynamically disfavored, especially at low temperature, and, as a consequence, the proposal of olefinic intermediates was not obvious. Several facts lead to the metallocarbenic mechanistic assertion ... 

At present the accepted mechanism is a nonconcerted, nonpairwise process involving metallocarbenes that reversibly react with alkenes to yield metallacyclo-butane intermediates 48,64,68... 

A similar reaction of ylide 200 can also be carried out under thermal conditions or in the presence of catalytic amounts of Cu(acac)2 [143]. The carbenoid reactions of iodonium ylides can also be effectively catalyzed by rhodium(II) complexes [144, 145]. The product composition in the rhodium(II) catalyzed reactions of iodonium ylides was found to be identical to that of the corresponding diazo compounds, which indicates that the mechanism of both processes is similar and involves metallocarbenes as key intermediates as it has been unequivocally established for the diazo decomposition [144]. 

Metallocarbenes derived from diazoacetates and the appropriate transition metal are generally thought to be electrophilic in nature that is, they are susceptible to nucleophilic addition. This reactivity manifold has been exploited with the formation of ylide species on reaction with a number of nucleophiles providing intermediates that can undergo either rearrangements (e.g., 41 > 42, Scheme 8.8)25 or cycloadditions (e.g., 43 > 44, Scheme 8.8)26 depending on the type of ylide formed... 

Insertion by copper(II) into a diazo derivative such as (43), thus forming an intermediate metallocarbene, leads to cyclized products with high stereoselectivity (e.g. 44 after decarboxylation).4 Diazo ketone (45) produced quino-lizidines (46) and (47), which arise from initial metallocarbene... 

Catalysts comprising a metal-carbon double bond (metallocarbenes, or metallocenes) are efficient. With these initiators, the polymerization mechanism appears to involve coordination of the C=C double bond in the cycio- or dicycloalkene at a vacant d orbital on the metal. The metallocyclobutane intermediate which is formed decomposes to produce a new metal carbene and a new C=C bond. Propagation consists of repealed insertions of cycloalkenes at the metal carbene. 

Metallocarbene formation by hydrogen shift explains the observed selectivity in the 1,5-dehydrocyclization of 3-methylhexane on Pt/AljOj (41). Three cyclic intermediates may be formed from this molecule, 1,2-dimethylcyclopentane (4), 1,3-dimethylcylopentane (5), and ethylcyclopentane (6). By using several selectively C-labeled 3-methylhexanes, the contribution of each parallel pathway both in cyclic type isomerization and in dehydro-cylization to gaseous cyclic molecules was determined. Relative rates of 3 2 1 were observed for 1-5, 2-6, and 6-7 ring closure (giving 5, 4, and 6, respectively) (Scheme 49 and Table VII), whatever the dispersion of the platinum (2-10%) and the temperature (32O°-38O°C). 

This reaction probably involves the participation of more than two carbon atoms and the extension of a delocalized n-electron system. The results are readily explained by the unequal stabilities of the two possible intermediates, involving carbene formation on the methyl group and on the alkyl group, respectively (Scheme 74). In the first case, demethylation easily occurs by metallocarbyne formation, while in the second case, stabilization by carbene-olefin isomerization (very fast compared to hydrocracking) prevents dealkylation. However, when the a-carbon atom in the alkyl group cannot dehydrogenate to give a metallocarbene, as for 1-methyl-1-terf-butylcyclohexane, dealkylation prevails over demethylation. 

The interaction of metallocarbene (or alkylidene) complexes with unsaturated compounds via the formation of a metallacyclobutane intermediate plays an important role in several stoichiometric and catalytic reactions, e.g., metathesis, cycloaddition and polymerization. 

Fig. 10.3-10 Disulfide modification using alkylate nearby nucleophiles. These rhodium carbenoids. (a) Disulfide bonds pathways are demonstrated for (b) a cyclic react with metallocarbenes to form ylide-like peptide hormone and (c) a protein. The intermediates 27a-c (where X = a negative disulfide in (c) is represented by the yellow charge, coordinated rhodium, or a proton), spheres, and the N-terminus is indicated by These can undergo a sigmatropic the yellow arrow,...

Alkene metathesis occurs by way of an intermediate metallacycle 87, followed by ring opening to give either the starting materials or one of the new alkenes and a new metallocarbene complex (2.111). Further metallocycle formation using another alkene and ring-opening provides the other product alkene and recovered catalyst to continue the cycle. 

Carbenes ( CR2) are the most reactive organic intermediates. The Simmons-Smith reaction (see chapter 6) tamed the extraordinarily reactive methylene ( CH2) in the form of a metal-carbenoid ( ICH2ZnI ). In 1964, Ernest Otto Fischer made the first metallocarbenes [e.g., (C0)5W=CCH3(0CH3)] and they were stable enough for X-ray diffraction studies on crystalline samples. Although unsuspected at the time, metallo-carhenes would hold the secret to the magic of olefin metathesis. 

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