Methoxycarbonyl carbene

Electron deficient species can attack the unshared electron pairs of heteroatoms, to form ylides, such as in the reaction of thietane with bis(methoxycarbonyl)carbene. The S —C ylide rearranges to 2,2-bis(methoxycarbonyl)thiolane (Section 5.14.3.10.1). A"-Ethoxycar-bonylazepine, however, is attacked by dichlorocarbene at the C=C double bonds, with formation of the trans tris-homo compound (Section 5.16.3.7). 

IR bands for the both states. For example, singlet and triplet states of 2-naphthyl-(methoxycarbonyl)carbenes (NMCs, 17) show very different IR spectra (Scheme 9.4) (1590, 1625, 1640 cm for NMC and 1660 cm for NMC). Both states are observable in this case. In the singlet state, the carbomethoxy group assumes a conformation perpendicular to the naphthylcarbene plane to avoid destabilization of the empty carbenic 2p atomic orbital by the electron-withdrawing carbonyl group, while in the triplet, the methoxycarbonyl group is in the same plane to delocalize an unpaired electron. For this reason, there is a barrier between the two states and hence both of them are observable under these conditions. 

Fluorenylidene and its substituted derivatives are typical examples of the other class. Other carbenes showing similar behavior are monoarylcarbenes such as mononaphthylcarbenes (12) and phenyl(methoxycarbonyl)carbene (53, AGst = 0.2 kcal/mol). 

Thietane reacts with bis(methoxycarbonyl)carbene to give a S+-C ylide which rearranges to 2,2-bis(methoxycarbonyl)thiolane (CHEC 5.14.3.10.1). 

Carbenic fragmentation is formally the reverse of addition of carbenes to the sulfur atom of the thiophene ring, and has been observed only with 2,5-dichlorothiophenium bis(alkoxycarbonyl)methylides (30). When 30 (R = CHj) is heated at 110°C in refluxing toluene in the presence of rhodium(II) acetate or copper(II) acetylacetonate, fragmentation of the ylid to 2,5-dichlorothiophene and the carbenoid occurs. The bis(methoxycarbonyl)-carbene has been trapped with alkenes to produce high yields of the cyclopropanated products (78CC641). Since the ylid is a stable crystalline solid with a long shelf life, it represents a convenient source of bis-(methoxycarbonyl)carbene. 

Bis(methoxycarbonyl)carbene generated in this way is a highly electrophilic species and readily reacts with mesomerically activated aromatic substrates to give the corresponding aryl malonates in fair to excellent yields. 

Thietanone 423 is obtained by desulfurization of cyclic disulfide 424 under the action of bis(methoxycarbonyl)carbene or dkphenylcarbene, generated from the corresponding diazo compounds (85TL5187). This transformation takes place only with disulfides containing bulky substituents. Otherwise, carbene insertion into the S—S bond proceeds with ring expansion (Section VII,C,1). 

Naphthyl(methoxycarbonyl)carbene (29) has been investigated in low temperature matrices, and in flash-photolysis experiments. " In each of these complementary studies the carbene was generated from the a-diazo ester, methyl 2-diazo-(2-naphthyl)acetate. Matrix photolysis of the precursor at 450 nm yields primarily the triplet ground-state carbene, which has been... 

On the other hand, cycloaddition of cyclobutene by bis(methoxycarbonyl)carbene yielded dimethyl bicyclo[2.1.0]pentane-5,5-dicarboxylate (10) as the sole product (55% yield) in the direct photolysis, whilst the triplet carbene reaction in addition to 10 produced two vinylcy-clopropanes 11 and 12 which probably arise from the same intermediate 1,3-diradical. 

Difiuorocarbene underwent addition to quadricyclane to give enr/u-6-(2,2-difluorovinyl)bi-cyclo[3.1.0]hex-2-ene together with three other products in 3% yield.The proposed formation of an insertion product into one of the cyclopropane C — C bonds was not verified in this reaction, but was observed in the reaction with other carbenes such as dichlorocarbene and bis(methoxycarbonyl)carbene. The major product with these carbenes possessed a bi-cyclo[3.2.1]octa-2,6-diene structure as the result of the insertion of the carbene into the cyclopropane bond next to the methylene bridge. ... 

Methylene, bis(methoxycarbonyl)carbene, and difluorocarbene reacted with bicyclobutane with the cleavage of two bonds, but the yields were generally low. > 09 Difluorocarbene underwent... 

Thus, dimethyl diazomalonate (274) yields bis(methoxycarbonyl)carbene (275) in a thermal decomposition. While 275 does undergo a Wolff rearrangement, it is also re-ported to add to naphthalene to yield the cyclopropane 276 in 30-40% yield. When the... 

A general examination of the addition of bis(methoxycarbonyl)carbene, generated under homogeneous and heterogeneous conditions by the copper- or silver-catalysed decomposition of dimethyl diazomalonate, to olefins has been made. Cyclohexene was found to react five times faster than 1-methylcyclohexene using an iodo(trimethyl phosphite)copper(i) catalyst to give the cyclopropane derivative. 

Addition of Carbene and Carbenoids to Olefins.—The addition of carbenes and carbenoids to olefins continues to be one of the most popular routes used for cyclopropane synthesis. A review of carbenes (and nitrenes) based on the 1973 literature has appeared during the year, and methoxycarbonyl carbene has been the subject of a theoretical study. ... 

Synthetic Uses of Thiophen Derivatives.—The reaction between dimethyl diazo-malonate and thiophen is catalysed by rhodium salts the product is a stable ylide (36) which can be used as an equivalent of bis(methoxycarbonyl)carbene. Thus the heating of (36) with cyclo-octene under reflux affords the bicyclononane (37) in... 

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