Thiophenium ylides

Ylides other than acceptor-substituted diazomethanes have only occasionally been used as carbene-complex precursors. lodonium ylides (PhI=CZ Z ) [1017,1050-1056], sulfonium ylides [673], sulfoxonium ylides [1057] and thiophenium ylides [1058,1059] react with electrophilic transition metal complexes to yield intermediates capable of undergoing C-H or N-H insertions and olefin cyclopropanations. 

Thermolysis of the ylide (15) in thiophene results in ready rearrangement to dimethyl thiophene-2-malonate (16). The same product is obtained if the thermolysis is carried out in the presence of 2-methylthiophene or cyclohexene, proving that the rearrangement occurs by an intramolecular process (78CC85). However, when 2,5-disubstituted thiophenium ylides are thermolyzed, dissociation to carbenoid species seems to occur. This reaction is further discussed in Section 3.14.2.9. 

The rhodium- or copper-catalyzed interaction of dimethyl diazomalonate with 2-(l-al-kenyl)thiophenes yields either a dimethyl 2-(2-thienyl)cyclopropane-l,l-dicarboxylate, a thiophenium ylide, or a dimethyl [5-(l-alkenyl)-2-thienyl]malonate, depending on the substituents at the alkene bond and the heterocyele. ... 

Even thiophene itself will react with carbenes, at sulfur, to produce isolable thiophenium ylides, and in these, the sulfur is definitely tetrahedral. The rearrangement of thiophenium bis(ethoxycarbonyl) methylide to the 2-substituted thiophene provides a rationalisation for the reaction of thiophene with ethyl diazoacetate, which produces what appears to be the product of carbene addition to the 2,3-double bond perhaps this proceeds via initial attack at sulfur followed by S C-2 rearrangement, then collapse to the cyclopropane. Acid catalyses conversion of the cyclopropanated compound into a thiophene-3-acetic ester. ° 2,5-Dichlorothiophenium bis(methoxycarbonyl)methylide has been used as an efficient source of the carbene simply heating it in an alkene results in the transfer of (Me02C)2C to the alkene. ... 

Even thiophene itself will react with carbenes, at sulfur, to produce isolable thiophenium ylides, and in these, the sulfur is definitely tetrahedral. The... 

Thiophenium ylides. The reaction of dimethyl diazomalonate with thiophene with conventional copper catalysts is extremely slow, but does result in dimethyl thiophene-2-malonate (2) in about 35% yield. When rhodium(II) aceta is used as catalyst, the reaction at 20° is still slow (18 hours), but results in thiophenium bismethoxycarbonylmethylide (1) in 95% yield. This reaction is facilitated by halogen substituents the ylides are not formed from thiophenes substituted with cyano, formyl, or acetyl groups. ... 

The use of ylides as carbene precursors constitutes a novel original approach to control or extend carbenic reactions. The thiophenium ylide XXVIII prepared by catalytic decomposition of methyl diazomalonate in the presence of dichloro-2,5-thiophene has been successfully applied as a carbene equivalent or precursor to the cyclopropanation of olefins, the OH insertion and the C-H insertion reaction in activated arenes [112, 113] the catalyst being rhodium(II) acetate or copper(II) acetylaceto-nate. 

Sulfur ylides (such as thiophenium ylides) may act as carbene precursors in the presence of rhodium or copper catalysts. As the reported examples refer to their preparation from diazomalonates, this approach appears as a sophisticated development of the more classical carbene generation from diazoesters. 

Thiophenium fluorosulfonate, 1,2,3,4,5-tetramethy 1-ylide, 4, 724 Thiophenium salts aromaticity, 4, 724 proton abstraction, 4, 766 pyramidal inversion barrier, 4, 724 structure, 4, 715 synthesis, 4, 723-724 Thiophenium salts, 1-alkyl-solvolysis, 4, 766 UV spectra, 4, 766 Thiophenium salts, aryl-synthesis, 4, 726... 

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... 

Thiophenium bis(alkoxycarbonyl)methylides (44) are obtained in high yield by rhodium(II) carboxylate-catalyzed reaction of diazomalonate esters with thiophene derivatives (88JCS(P1)1023). Likewise, ylides from benzo[b]thiophene and dibenzothiophene (e.g. 45) have also been reported by tram-ylidation using phenyliodonium bis(phenylsulfonyl)methylide (88JHC1599). 

Proton abstraction from the pentamethylthiophenium ion (13) gave the ylide (141) reaction of this with p-nitrobenzaldehyde gave the epoxide (142) (74TL75). Thiophenium salts, in contrast to thiophene 1-oxides (Section 3.14.4.1), do not seem to undergo Diels-Alder additions (74TL75). 

In the third article, Porter describes the chemistry of thiophenium salts and ylids, to which he has contributed extensively. The chemistry of 1,4-diazocines is presented by Perlmutter this article follows an earlier review by the same author on azocines in Volume 31 of Advances in Heterocyclic Chemistry. The final article of this volume, and the second contribution from Japan, is by Tsuge and Kanemasa and deals with recent advances in azomethine ylide chemistry. 

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