Alkenes with carbenes

One of the earliest reported thermal reactions of Fischer carbene complexes was the reaction with olefins to give cyclopropanes [127]. More recently it has been shown that photolysis accelerates inter molecular cydopropanation of electron-poor alkenes [128]. Photolysis of Group 6 imine carbenes with alkenes... 

Table 25 Photo-driven reaction of Group 6 imine carbenes with alkenes ...

The Reaction of Carbenes with Alkenes in Aqueous Medium... 

The pyridine ylide method also allows determination of the rate constants for the intermolecular reactions of carbenes with alkenes, alcohols, or other carbene... 

After a great deal of research on the mechanism of this reaction, it now appears likely that the crucial step is the formation of carbene metal complexes and that the products are formed by recombination of the carbenes with alkene in the various possible ways ... 

Ferracyclic carbenes, with alkenes, 6, 133 Ferracyclopentendiones, preparation, 6, 82-83 Ferraoxetene, synthesis, 6, 82 Ferricenium, characteristics, 12, 449 Ferrocene-acetylene polymers, with metallocene units,... 

Optically active metal complexes have been recognized as excellent catalysts for the enantioselective cyclopropanation of carbenes with alkenes. Normally, diazo compounds react under metal catalysts in the dark to afford carbenoid complexes as key intermediates. Katsuki et al. have reported the ds-selective and enantioselective cyclopropanation of styrene with a-diazoacetate in the presence of optically active (R,R)-(NO + )(salen)ruthenium complex 80, supported under illumination (440 nm light or an incandescent bulb) [59]. The irradiation causes dissociation of the apical ligand ON + in 80, and thus avoids the splitting of nitrogen from the a-diazoacetate. 

The reactions of carbenes with alkenes, as illustrated in this section, are the best methods for the synthesis of cyclopropanes. 

The ability of a p-carbene to react with an unsaturated hydrocarbon and form an enlarged dimetallocycle encourages speculation over their role in such processes as alkene metathesis and Fischer-Tropsch synthesis. In Scheme 6 a possible mechanism for metathesis initiated by a p-carbene is presented, owing much to other workers (T7,22). Reactions of p-carbenes with alkenes are under investigation in our laboratory. Recently Pettit has observed that the p-methylene complex [Fe2(C0)8(p-CH2)] generates propene when subjected to a pressure of ethene and has also suggested the intermediacy of a three-carbon dimetallocycle (23). 

This section only covers [2+ l]-cycloaddition reactions performed with carbenes. The synthesis of trifiuoromcthyl-subsliluted three-membered-ring compounds has been achieved by reaction of carbciies with trifiuoronicthyl-subslituled unsaturated systems and by reaction of trifluoro-melhyl-substituled carbenes with alkenes. 

As already mentioned for rhodium carbene complexes, proof of the existence of electrophilic metal carbenoids relies on indirect evidence, and insight into the nature of intermediates is obtained mostly through reactivity-selectivity relationships and/or comparison with stable Fischer-type metal carbene complexes. A particularly puzzling point is the relevance of metallacyclobutanes as intermediates in cyclopropane formation. The subject is still a matter of debate in the literature. Even if some metallacyclobutanes have been shown to yield cyclopropanes by reductive elimination [15], the intermediacy of metallacyclobutanes in carbene transfer reactions is in most cases borne out neither by direct observation nor by clear-cut mechanistic studies and such a reaction pathway is probably not a general one. Formation of a metallacyclobu-tane requires coordination both of the olefin and of the carbene to the metal center. In many cases, all available evidence points to direct reaction of the metal carbenes with alkenes without prior olefin coordination. Further, it has been proposed that, at least in the context of rhodium carbenoid insertions into C-H bonds, partial release of free carbenes from metal carbene complexes occurs [16]. Of course this does not exclude the possibility that metallacyclobutanes play a pivotal role in some catalyst systems, especially in copper-and palladium-catalyzed reactions. 

The stereospecific addition to alkenes is consistent with (difluoromethyl)fluorocarbene being generated in the singlet state, Adducts of this carbene with alkenes were observed during the pyrolysis of trimethyl(l,l,2,2-tetrafluoroethyl)silane. ... 

Singlet and triplet fluorenylidenes have been generated and observed spectroscopically as transient products of irradiation of diazofluorene in acetonitrile. As a result, it has proved possible to calculate the rate of intersystem crossing and the rate constants for reaction of both carbenes with alkenes and alcohols. Reaction of singlet fluorenylidene (67) with alkenes affords cyclopropanes as the major product, as shown for methyl methacrylate in Scheme 9. Surprisingly,... 

Several useful inter- and intramolecular reactions of chromium, molybdenum, tungsten, iron, or manganese Fischer carbenes with alkenes to form cyclopropanated products have... 

Table 1. Relative Reaetivities of Carbenes with Alkenes (25 °C)...

Intramolecular reactions of carbenes with alkenes have been exploited in synthesis. The sesquiterpene cycloeudesmol was prepared using, as a key step, the intramolecular cyclopropanation of the diazoketone 122 (4.97). The cyclopropana-tion reaction occurs stereoselectively to give the tricyclic product 123, which was subsequently converted into the natural product. A synthesis of sesquicarene was achieved using the copper(I)-catalysed decomposition of the diazo compound 125, itself prepared by oxidation of the hydrazone 124 (4.98). 

The generally accepted catalytic cycle for this change partners dance was proposed by Yves Chauvin and is believed to involve metallocyclobutane intermediates that result from reaction of metal alkylidenes (also called metal carbenes) with alkenes. The catalysts themselves are metal alkylidenes, in fact. Chauvin s catalytic cycle for olefin metathesis is summarized here. 

The reactivity of most singlet carbenes with alkenes is dominated by the electrophilic character of the carbene (the empty p orbital). Thus, the more electron rich the alkene, the faster the carbene addition. Increasing alkyl group substitution on alkenes increases the rate of addition. This trend parallels the reactivity for the addition of other electrophiles with alkenes, such as acids, Xj, and borane. Dialkylcarbenes are less selective than dihalocarbenes, whereas carbenes with neighboring O or N atoms are resonance stabilized (see margin) and are highly selective. This trend tracks the reactivity-selectivity principle (see Chapter 7), where the more stable carbenes are the more selective. 

Regardless of the mechanism, the reactions of carbenes with alkenes provide the most widely used synthetic route to cyclopropanes. 

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