Oxiranes carbonyl ylide generation

Interestingly, sulfonium ylides generated from electrophilic carbene complexes and sulfides can react with carbonyl compounds, imines, or acceptor-substituted alkenes to yield oxiranes [1320-1325], aziridines [1321,1326,1327] or cyclopropanes [1328,1329], respectively. In all these transformations the thioether used to form the sulfonium ylide is regenerated and so, catalytic amounts of thioether can be sufficient for complete conversion of a given carbene precursor into the... 

Imine ylides 7 and carbonyl ylides 8 are not stable but may be generated in situ by pyrolysis of suitably substituted aziridines and oxiranes. The energy of the HOMO, and therefore the nucleophilicity of the parent 18-electron dipoles, decreases from very high to very low across the series 7-18. In the same series, the electrophilicity increases from moderate to high, being consistently higher when the central atom is oxygen. 

Whereas the thermal ring-opening reaction of oxiranes and aziridines is frequently used for generation of carbonyl ylides and azomethine ylides, the analogous procedure starting with thiiranes does not produce the expected thiocarbonyl ylides (8). However, in the case of tetraaryl-substituted thiiranes, the photolytically mediated reaction with tetracyanoethylene (TCNE) is believed to occur via a single electron transfer (SET) mechanism, also involving a thiocarbonyl ylide as a likely intermediate (75,76) (Scheme 5.14). 

In recent years there has been a growing interest in the use of carbonyl ylides as 1,3-dipoles for total synthesis.127-130 Their dipolar cycloaddition to alkenic, alkynic and hetero multiple bonded dipolaro-philes has been well documented.6 Methods for the generation of carbonyl ylides include the thermal and photochemical opening of oxiranes,131 the thermal fragmentation of certain heterocyclic structures such as A3-l,3,4-oxadiazolines (141) or l,3-dioxolan-4-ones132-134 (142) and the reaction of carbenes or car-benoids with carbonyl derivatives.133-138 Formation of a carbonyl ylide by attack of a rhodium carbenoid... 

Evidence for carbonyl ylide formation has also been reported in bridged diphenyloxirans,65 and a particularly stable, highly colored ylide (75) is generated on irradiation of the fused oxiran (76), both in rigid matrices at... 

Transition-metal mediated carbene transfer from 205 to benzaldehyde generates carbonyl ylides 211 which are transformed into oxiranes 216 by 1,3-cyclization, into tetrahydrofurans 212, 213 or dihydrofurans 214 by [3 + 2] cycloaddition with electron-deficient alkenes or alkynes, and 1,3-dioxolanes 215 by [3 + 2] cycloaddition with excess carbonyl compound120 (equation 67). Related carbonyl ylide reactions have been performed with crotonaldehyde, acetone and cyclohexanone (equation 68). However, the ylide generated from cyclohexanone could not be trapped with dimethyl fumarate. Rather, the enol ether 217, probably formed by 1,4-proton shift in the ylide intermediate, was isolated in low yield120. In this respect, the carbene transfer reaction with 205 is not different from that with ethyl diazoacetate121, whereas a close analogy to diazomalonates is observed for the other carbonyl ylide reactions. 

Carbonyl ylides, often generated from a carbene and an aldehyde or ketone, are in general reactive intermediates that can isomerize to stable products (oxiranes, enolethers) or can be tn ped by [3+2] cycloaddition reactions [18]. As Scheme 8 shows, silyldiazoacetic esters can also be used to generate carbonyl ylides [19]. 

Thus, metal-catalyzed decomposition of 10a in the presence of an equimolar amoimt of benzaldehyde generates the carbonyl ylide 21 which can be trapped with a suitable electron-poor dipolarophile to give tetrahydrofiirans 23, 24, and dihydrofuran 25, respectively. These heterocycles were obtained as sole products in 41-62 % yield when Rh2(pfb)4 or [Ru2(CO)4(OAc)2] were used as catalysts, whereas catalysis by CuOTf afforded mainly the oxirane 22 (38 %) besides some cycloaddition product (23 11.5% 24 16%). 

Direct observation of the carbonyl ylide (49) at 10 K in an argon matrix has also been reported the ylide is obtained by direct photolysis of (o-carbomethoxyphenyl)diazomethane (50) and undergoes reversible photoisomerization to the oxirane (51) as shown in Scheme 4. Competing intramolecular and intermolecular 0-H bond insertion pathways have been observed on analogous photochemically induced generation of [2-(hydroxymethyl)-... 

The photolysis of oxirans generates carbonyl ylides which can be trapped by dipolarophiles to give tetrahydrofurans (Scheme 22). ... 

Arnold first discovered that PET reactions of cis- and rrans-2,3-diphenyloxiranes (1 and 2) with electron-accepting sensitizers such as DCN, 1,4-dicyanobenzene, dimethyl terephthalate, and methyl 4-cyanoben-zoate produce the isomeric tetrahydrofuran derivatives 7 and 8 (Scheme 2). The proposed mechanism involves the CC bond cleavage of the oxirane radical cations to give ylide radical cations 3 and 4, followed by back electron transfer (BET) to generate carbonyl ylides 5 and 6. The resulting carbonyl ylides are trapped with dipolarophiles, such as acrylonitrile, maleonitrile, and fumaronitrile, to produce the observed products. In the absence of dipolarophiles, C)S-/ir s-isomerization (1/2 = 0.19-0.28) was observed, which is consistent with the formation of carbonyl ylides 5 and 6, as well as their radical cations 3 and 4. This method was successfully apphed to the synthesis of various tetrahydrofurans (Scheme 3) and dihydrofurans (Scheme 4). ... 

Phase transfer catalysis, or more correctly, two-phase systems, were widely applied in the synthesis of oxiranes via generation of sulfonium and sulfoxo-nium ylides and their subsequent reactions with carbonyl compounds.67... 

The first examples of reactions of non-stabilized selenium ylides with enolisable carbonyl compounds have been reported. Ylide (45) was generated in situ from Me2 ePh MeS04 and NaH in the presence of R COR [R = Ph, 4-O2NC6H4, Bu, or hexyl R = H, Me, or Et or R R = (CH2)s] to give the corresponding oxirans (76-94%). ... 

A variety of substituted sulphonium ylides have been used in the oxiran-forming reaction with carbonyl compounds. Diphenylsulphonium-allylide (6), generated from the salt with t-butyl-lithium, afforded the expected oxiran in 80% yield upon reaction with cyclohexanone. Use of n-butyl-lithium as the generating base resulted in lower yields and contamination by the oxiran formed from a butylide, the latter probably... 

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