Carbenoids reactions with alkynes

The Fritsch-Buttenberg-Wiechell rearrangement of the carbenoid gave the alkyne 131, which was further metallated in the reaction mixture with the excess of the afkyhnetal to... 

In order to outline the scope of this chemistry, Sections 4.8.2 and 4.8.3 will discuss the catalysts and carbenoid precursors used. This will be followed by reactions of caibenoids with ir-systems, organized according to the ir-system involved, alkenes (Section 4.8.4), alkynes (Section 4.8.5), benzenes and electron-rich heterocycles (Section 4.8.6). Particular emphasis will be placed on the stereochemical outcome of these reactions with reference to applications in organic synthesis. 

Functionalized cyclopropenes are viable synthetic intermediates whose applications [99.100] extend to a wide variety of carbocyclic and heterocyclic systems. However, advances in the synthesis of cyclopropenes, particularly through Rh(II) carboxylate—catalyzed decomposition of diazo esters in the presence of alkynes [100-102], has made available an array of stable 3-cyclopropenecarboxylate esters. Previously, copper catalysts provided low to moderate yields of cyclopropenes in reactions of diazo esters with disubstituted acetylenes [103], but the higher temperatures required for these carbenoid reactions often led to thermal or catalytic ring opening and products derived from vinylcarbene intermediates (104-107). 

Turning to carbene related reactive species, alkylidene carbenoids like 26 (X = halogen, OR, NR2) are particularly valuable for preparative purposes since they can undergo cycloaddition reactions with olefins (to methylenecyclopropanes), isomerizations (to alkynes by the so-called Fritsch-Buttenberg-Wiechell rearrangement), and dimerization (to [3]cumulenes). Although carbenoids have been studied extensively by NMR spectroscopy [23], the first X-ray structural analysis of a stable carbenoid, 27, as a TMEDA 2THF complex has been reported only recently [24]. 

Furans may be formed in the reaction with metal carbenoids derived from diazocarbonyl compounds, if alkynes are used instead of alkenes. Furan formation is particularly favored when the carbenoid is a 3-diazo-2-oxopropionate (e. g., 8.110, Wenkert et al., 1983) or contains two electron-withdrawing groups (see Davies and Romines, 1988) and when electron-donating groups are present in the alkyne. Davies... 

As cyclopropenyl esters are formed in analogous reactions of metal carbenoids with alkynes, it may be that the final products are furan derivatives, as it is known (Komendantov et al., 1975) that such rearrangements (8-74) take place easily in the presence of copper catalysts. 

The carbenes or carbenoids can be generated in a variety of ways. It is not always clear whether the reaction is concerted or stepwise and whether the carbene behaves as an electrophilic, nucleophilic or radical species. For instance, a carbenoid generated from bismuthonium ylide (19) in the presence of copper(I) chloride would behave as a triplet and add as a radical to a terminal alkyne" (equation 17). The dicarbonyl structure and the absence of reaction with methyl propionate to a furane might well indicate electrophilic character of this carbene. 

Volume 9 deals with the majority of addition and elimination reactions involving aliphatic compounds. Chapter 1 covers electrophilic addition processes, mainly of water, acids and halogens to olefins and acetylenes, and Chapter 2 the addition of unsaturated compounds to each other (the Diels-Alder reaction and other cycloadditions). This is followed by a full discussion of a-, y- and S-eliminations (mainly olefin and alkyne forming) and fragmentation reactions. In Chapter 4 carbene and carbenoid reactions, and in Chapter 5 alkene isomerisation (including prototropic and anionotropic, and Cope and Claisen rearrangements), are discussed. 

An unprecedented cyclopropenation reaction of alkynes catalyzed by ZnCl2 was reported. While Simmons-Smith-type carbenoids failed in the [2 + 1]-cycloaddition with alkynes, the use of enynones as the carbene source enabled the preparation of substituted 2-furyl cyclopropene derivatives with remarkable scope (14OL5780). 

Cyclopropanation. Decomposition of dimethyl diazomalonate by direct photolysis or by transition metal catalysis in the presence of alkenes leads to cyclopropanation (eq 1). The use of alkynes to trap the carbenoid species affords cyclopropenes (eq 2). Rhodium(II) acetate-catalyzed reaction with allenes allows ready access to methylenecyclopropanes, which form the basis for a methylenecyclopentane annulation protocol (eq 3). ... 

Triphenylbismuthonium ylide reacted with terminal alkynes in the presence of a catalytic amount of copper(I) chloride to form furan derivatives (Scheme 11) [27]. Although the yields were low, the products were obtained regioselectively. The reaction was sensitive to steric factors, and internal alkynes did not provide the product. A carbenoid intermediate was probably involved in the reaction. 

In 1993, Satoh and coworkers reported the preparation of lithium- and magnesium-aUtylidene carbenoids from 1-chlorovinyl phenyl sulfoxides by sulfoxide-metal exchange reaction at low temperature (Scheme 6). 1-Chlorovinyl phenyl sulfoxide (128) is easily synthesized from the corresponding aldehyde and chloromethyl phenyl sulfoxide in high yield. Sulfoxide 128 was treated with f-BuLi in THF at —78 °C to give the terminal alkyne 131. Obviously, the intermediate of this reaction was the alkylidene carbenoid 129. 

Highly Lewis basic and nucleophilic functional groups are not compatible with zinc carbenoids. The methylation or ylide formation of heteroatoms is one of the most important side reactions of these reagents. For example, amines, thioethers and phosphines readily react with the zinc reagents to generate ammonium salts", sulfonium" and phosphonium ylides" ". Terminal alkynes generally lead to a large number of by-products". ... 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

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