Carbenes diazo compound catalysis

Rhodium(II) acetate was found to be much more superior to copper catalysts in catalyzing reactions between thiophenes and diazoesters or diazoketones 246 K The outcome of the reaction depends on the particular diazo compound 246> With /-butyl diazoacetate, high-yield cydopropanation takes place, yielding 6-eco-substituted thiabicyclohexene 262. Dimethyl or diethyl diazomalonate, upon Rh2(OAc)4-catalysis at room temperature, furnish stable thiophenium bis(alkoxycarbonyl)methanides 263, but exclusively the corresponding carbene dimer upon heating. In contrast, only 2-thienylmalonate (36 %) and carbene dimer were obtained upon heating the reactants for 8 days in the presence of Cul P(OEt)3. The Rh(II)-promoted ylide formation... 

Among transition-metal compounds that are effective for metal carbene transformations, those of Cu and Rh have received the most attention [7-10]. Cu catalysis for reactions of diazo compounds with olefins has been known for more than 90 years [11], but the first report of Rh catalysis, in the form of dirhodium(II) tetraacetate, has been recent [12], Although metal carbene intermediates with catalytically active Cu or Rh compounds have not yet been observed, those... 

Transition-metal catalysis, especially by copper, rhodium, palladium and ruthenium compounds, is another approved method for the decomposition of diazo compounds. It is now generally accepted that short-lived metal-carbene intermediates are or may be involved in many of the associated transformations28. Nevertheless, these catalytic carbene transfer reactions will be fully covered in this chapter because of the close similarity in reaction modes of electrophilic carbenes and the presumed electrophilic metal-carbene complexes. 

Cycloaddition of the carbene derived from 205 to bis(trimethylsilyl)acetylene yields the expected cyclopropene in low yield both photochemically (20%) and under catalysis by copper triflate at 80 °C (10-13%)119. The latter version of the reaction is accompanied by [3 + 2] cycloaddition of the diazo compound to the alkyne, and the photochemical route yields a by-product which obviously comes from carbenic C,H insertion at a SiMe3 group of the alkyne. 

The diazo-compound decomposes to gaseous nitrogen and a carbene under catalysis by Cu(II). " established the... 

For comprehensive reviews that provide numerous examples of cyclopropanation based on Rh- and Cu-carbene catalysis, see M. P. Doyle, M. A. McKervey, and T. Ye, Modem Catalytic Methods for Organic Synthesis with Diazo Compounds, Wiley New York, 1998 M. P. Doyle and D. C. Forbes, Chem. Rev., 1998, 98, 911 and M. P. Doyle, J. Org. Chem., 2006, 71, 9254. 

The understanding of this catalysis started in 1952, shortly after the concept of carbenes was introduced (see Sect. 8.1). Yates postulated that transition-metal catalysts react with diazo compounds by formation of transient electrophilic metal carbenes, because that complex can be depicted as a metal-stabilized carbocation (8.104). Doyle (1986 a) proposed the catalytic cycle (8-46) for the formation of the carbenoid 8.104 and its reaction with an electron-rich substrate S . The reagent S is, first of all, an alkene in cyclopropanation, but can also belong to other groups of compounds, to be discussed later in this section. 

Ylide generation from diazo compounds by reaction of carbenoids is a better method than photochemical or thermal dediazoniation in the presence of organic substrates containing heteroatoms, because these dediazoniations without metal catalysis yield, in most cases, not very selective carbenes. Here again, the copper-catalyzed route is in most cases inferior to that with rhodium catalysts. The diazoketo ester with a terminal thioalkyl group (8.145) can be obtained from the... 

The diazo compound decomposes to gaseous nitrogen and a carbene under catalysis by Cu(Il). Insertion into the exposed alkene gives the three-membered ring. The stereochemistry partly comes from the tether —the hnkage between the carbene and the rest of the molecule that delivers the carbene to the bottom face of the alkene. The rest comes from the inevitable cis fusion between the five- and three-membered rings. 

Duroquinone reacts with diazomethane to give adducts that under the action of acid catalysis, heat, or light lose nitrogen to give products (Scheme 5.36). Addition of carbene to CN double bonds has also been observed. The addition of dichlorocarbene to diazo compounds gives olefins (Scheme 5.37). 

Diazo compounds, with or without metal catalysis, are well-known sources of carbenes. For synthetic purposes a metal catalyst is used. The diazo compounds employed are usually a- to an electron-withdrawing group, such as an ester or a ketone, for stability. In the early days, copper powder was the catalyst of choice, but now salts of rhodium are favoured. The chemistry that results looks very like the chemistry of free carbenes, involving cyclopropanation of alkenes, cyclopropenation of alkynes, C-H insertion reactions and nucleophilic trapping. As with other reactions in this chapter, free carbenes are not involved. Rhodium-carbene complexes are responsible for the chemistry. This has enormous consequences for the synthetic applications of the carbenes - not only does the metal tame the ferocity of the carbene, but it also allows control of the chemo-, regio- and stereoselectivity of the reaction by the choice of ligands. 

Phenylthiomethyl)trimethylsilyl carbene (10) has been generated via two independent methods, either from the diazo compound through copper catalysis (eq 14), or from the chloro-(phenylthiomethyl)silane by base-induced o -elimination. The generation of carbene 10 was verified by [2 +1] cycloadditions with olefins affording cyclopropanes (eq 15) in low to moderate yields (12-61%). ... 

The use of transition metal species can lower appreciably the decomposition temperature of ot-diazo-carbonyl compounds they can also alter the reactivity of the carbene intermediate (resulting from the initial nitrogen elimination see Section 3.9.2.1) by complex formation. Hence, the Wolff rearrangement may occur with difficulty or, usually, not at all. Thus, some copper species (excepting, for example, Cul), or Rh and Pd catalysts are inappropriate. Freshly prepared silver(I) oxide has been used most frequently, but silver salts (especially silver benzoate) are sometimes preferred.Silver-based catalysts are usually employed in combination with an alkaline reagent e.g. sodium carbonate or a tertiary amine). Even under silver catalysis competing reactions may be observed, and sometimes the products of Wolff rearrangement may not be obtained (see Section 3.9.2.3). 

The diazo reactions in this chapter are characterized by processes run either in the gas phase, in relatively inert matrices, or in — typically, but not exclusively — aprotic and comparatively apolar solvents, either thermally or photolytically or with transition metal catalysis of various types. The metastable intermediates are carbenes (RR C ), i. e., neutral, apparently divalent, carbon compounds, or their transition metal complexes (coined carbenoids, see later in this section). It is interesting to recall that the synthesis of a compound that we now call a carbene, namely methylene (H2C ), was already attempted in the early 19th century, i.e., before the tetravalency of carbon was established. Dumas (1835) and Regnault (1839) thought then that it should be possible to obtain a compound consisting of one carbon and two hydrogen atoms by dehydration of methanol (a compound of which only the atomic ratio 1C 4H lO was then known). ... 

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... 

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