Ylide compounds 1,3-elimination reactions

In addition there are certain other methods for the preparation such compounds. Upon heating of the thionocarbonate 2 with a trivalent phosphorus compound e.g. trimethyl phosphite, a -elimination reaction takes place to yield the olefin 3. A nucleophilic addition of the phosphorus to sulfur leads to the zwitterionic species 6, which is likely to react to the phosphorus ylide 7 via cyclization and subsequent desulfurization. An alternative pathway for the formation of 7 via a 2-carbena-l,3-dioxolane 8 has been formulated. From the ylide 7 the olefin 3 is formed stereospecifically by a concerted 1,3-dipolar cycloreversion (see 1,3-dipolar cycloaddition), together with the unstable phosphorus compound 9, which decomposes into carbon dioxide and R3P. The latter is finally obtained as R3PS ... 

Other Carbonyl Compounds. The reaction of alkylidenetriphenylphosphoranes with lactones (Scheme 8) affords betaines (32), which can be thermally decomposed to eliminate triphenylphosphine, giving lactones in which the alkylidene grouping of the starting ylide is incorporated into the ring.34... 

Formate esters behave as typical carbonyl compounds in reactions with a number of ylides, eliminating phosphine oxide and forming vinyl ethers, e.g. (33).35 Stabilized phosphoranes are able to condense with the carbonyl group of cyclic thioanhydrides (34).38 Quinoline derivatives, e.g. (35), are obtained from the condensation of dicar-boalkoxy-ylides with isocyanates.37 Benzoyl isothiocyanates and keto-phosphoranes give quantitative yields of (36), which are unreactive in Wittig reactions but can be readily oxidized by selenous acid.38 The products obtained from reactions (Scheme 9) with the triazolinedione (37) depend upon the stability of the ylide used.39... 

If the ylidic compound is a benzo fused system, alternative reaction schemes take over. 3,2-Sigmatropic shifts are no longer favoured, and proton abstraction with the formation of isomeric ylides, or elimination processes, become important. Equations (24) (77JOC3945)... 

Variants of this salt method include the use of 1,3-dihalogeno compounds, which undergo both substitution and elimination reactions to provide ylides. This method was used to prepare the cyclopentadienylide (30) (equation 24). ... 

Fulleropyrrolidines are synthesized through the addition of azomethine ylides as 1,3-dipolar compounds to fullerene (Scheme 3.7). This reaction is known as the Prato reaction. There are several methods of generating azomethine ylides. In the Prato reaction, a decarboxylation route is employed with the use of easily available aldehydes and a-amino acid derivatives. Both starting materials undergo dehydrative condensation to give 5-oxazolidinones, which generate azomethine ylides with elimination of CO2. Azomethine ylides react with Cgo and produce fulleropyrrolidines in moderate yields. 

The Julia-Lythgoc olefination operates by addition of alkyl sulfone anions to carbonyl compounds and subsequent reductive deoxysulfonation (P. Kocienski, 1985). In comparison with the Wittig reaction, it has several advantages sulfones are often more readily available than phosphorus ylides, and it was often successful when the Wittig olefination failed. The elimination step yields exclusively or predominantly the more stable trans olefin stereoisomer. 

Whereas phosphonium ylides normally react with carbonyl compounds to give alkenes, dimethylsulfonium methylide and dimethylsulfoxonium methylide yield epoxides. Instead of a four-center elimination, the adducts from the sulfur ylides undergo intramolecular displacement of the sulfur substituent by oxygen. In this reaction, the sulfur substituent serves both to promote anion formation and as the leaving group. 

The reaction of 196 with phenyl chlorocarbene 198 illustrates the synthesis of indolizines by cyclization of pyridinium ylides (Scheme 7). Cyclization of ylide rotamer 199 generates the intermediate product 200, which undergoes elimination of chloride to provide compound 201 <2005EJ01532>. 

As mentioned above (see Scheme 1), three main directions of the decomposition of intermediates that formed are possible when phosphorus and arsenic ylides react with compounds bearing C=X bonds 5,6,19,63,64,88 (i) elimination of R3E15=X to form olefins (Wittig type reaction) (ii) retro-Wittig type decomposition and (iii) elimination of R3E15 and formation of three-membered cycles (Corey-Chaykovsky type reaction). According to the data of Erker and coworkers,12,13,51 under kinetic control, the reaction of phosphorus ylides with thiocarbonyl compounds also affords phosphines and thiiranes, whose further transformations lead to olefins and R3PS under thermodynamic control. 

The preparation of novel phase transfer catalysts and their application in solving synthetic problems are well documented(l). Compounds such as quaternary ammonium and phosphonium salts, phosphoramides, crown ethers, cryptands, and open-chain polyethers promote a variety of anionic reactions. These include alkylations(2), carbene reactions (3), ylide reactions(4), epoxidations(S), polymerizations(6), reductions(7), oxidations(8), eliminations(9), and displacement reactions(10) to name only a few. The unique activity of a particular catalyst rests in its ability to transport the ion across a phase boundary. This boundary is normally one which separates two immiscible liquids in a biphasic liquid-liquid reaction system. 

When planning reactions of thiocarbonyl compounds with electrophilic carbene complexes it should be taken into aceount that thiocarbonyl compounds can undergo uncatalyzed 1,3-dipolar cycloaddition with acceptor-substituted diazomethanes to yield 1,3,4-thiadiazoles. These can either be stable or eliminate nitrogen to yield thiiranes or other products similar to those resulting from thiocarbonyl ylides [1338]. 

The second class of benzo-fused heterocycles accessible from benzofuroxans are benzimidazole oxides. In this case only one carbon from the co-reactant is incorporated in the product. With primary nitroalkanes 2-substituted l-hydroxybenzimidazole-3-oxides (46) are formed via displacement of nitrite, and / -sulfones behave similarly. The nitrile group of a-cyanoacetamides is likewise eliminated to alford 2-amide derivatives (46 R = CONRjX and the corresponding esters are formed in addition to the expected quinoxaline dioxides from acetoacetate esters. Under similar conditions secondary nitroalkyl compounds afford 2,2-disubstituted 2//-benzimidazole-1,3-dioxides (47). Benzimidazoles can also result from reaction of benzofuroxans with phosphorus ylides <86T3631>, nitrones (85H(23)1625>, and diazo compounds <75TL3577>. 

As previously reported, the radical addition of CF2Br2 on glycals (initiated by sodium dithionite) affords difluorobromomethylated compounds. These latter molecules are easily dehydrohalogenated in the presence of TBAF. Under such conditions, these difluorovinyl compounds can add a fluoride ion (from TBAF). The subsequent elimination of the acetate moiety yields trifluoromethyl unsaturated compounds. The double bond can then be reduced (Figure 6.37). The same kind of reaction occurs in the presence of DAST with gcm-difluor-omethylene compounds, which are obtained by addition of an ylide onto an ulose (Figure 6.37). 

Cyclization of a thiocarbonyl ylide with the C=C-bond of an aromatic ring was observed in the reaction of aryl biphenyl-2-yl ketones with di(tosyl)diazomethane in the presence of Rh2(OAc)4 (189). In the case where the aryl ring contains a 4-methoxy group, benzo[c]thiophene (164) was the only product formed. In contrast, when the aryl ring consists of a 2,4,6-trimethylphenyl group, compounds 165 and 166 were produced. It would seem that after 1,5-dipolar electrocyclization of the intermediate thiocarbonyl ylide occurs, aromatization then takes place by elimination of toluenesulfinic acid or methyl toluenesulfinate. 

Phosphites and 2,2-bis(trifluoromethyl)-5(2//)-oxazolone 71 react with elimination of carbon dioxide to give 2-aza-4-phospha-l,l-bis(trifluoromethyl)-l,3-butadiene 72 that can be used as a synthon for the previously unknown hydrogen-substituted nitrile ylide 72a in [3 + 2]-cycloaddition reactions. Examples of cycloadditions of 72a with dipolarophiles to give heterocyclic compounds 12t-ll are shown in Scheme 7.18. 

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