Ylids, dimerization

Chiefly in an hydrophobic medium, a base can extract the proton on position 2 leading to a reactive intermediate (able to give subsequent condensation) that could be an ylid (35, 36) or a carbene (37), though no dimer has ever been isolated as is the case with benzothiazolium (32, 38). Two mechanisms have been proposed for explaining the particular reactivity of thiazolium ... 

Phosphorus ylids react with carbon dioxide to give the isolable salts (48), which can be hydrolyzed to the carboxylic acids (49) (thus achieving the conversion RR CHX RR CHCOOH) or (if neither R nor R is hydrogen) dimerized to... 

Nitrenes have a short lifetime (only several microseconds)86- 8 and undergo stabilization by the following reactions isomerization to imines, dimerization to azo compounds, hydrogen abstraction followed by ring closure to heterocyclic compounds, bimolecular insertion into C-H bonds to secondary amines, addition to solvent yielding ylids, and addition to unsaturated systems yielding heterocyclic compounds. Table 117-106 includes the reaction products and references for the different classes of nitrenes. 

When A.A-dimethylisoindolinium bromide is treated with phenyl-lithium, it gives V-methylisoindole via the ylid (54).2 59,60 An attempt to prepare benzo[c]thiophene via the analogous ylid (55) failed. Thus, when l,3-dihydrobenzo[c]thiophene methylsulfonium iodide was treated with phenyllithium, it gave a mixture of methyl phenyl sulfide, spiro[5.5]-l-methylthio-2,3-benzo-6-methylthio-methyleneundeca-7,9-diene (56), and 3,4-bis(methylthio)-l,2 5,6-dibenzo-l,5-cyclooctadiene (57).59,60 The formation of methyl phenyl sulfide may be explained by the formation and ring cleavage of compound 58, and compounds 56 and 57 arise by Diels-Alder dimerization of the o-quinodimethane (59) formed by ring cleavage of the ylid (55). 

Two proposed mechanisms for the polymerization of a,a -bis(dialkyl-sulfonio)-p-xylene dihalides (463) are shown in Fig. 70 [302]. Both mechanisms begin with the abstraction of an a-proton to produce ylid 464. The 1,6-elimination of dialkyl sulfide produces the p-xylylene pseudo-diradical 465. One mechanism involves the formation of polymer from this species via the dimerization of 465 to give the dication diradical 466. This species was proposed to grow rapidly to high molecular weight polymer (467) by head-to-tail additions to both ends [301]. An alternative mechanism involving polymer formation from 465 via a putative anionic mechanism has been proposed [302]. There were two main factors in the proposal of... 

Thioketenes are stable isolable compounds only if the heterocumulene system is protected against dimerization by bulky substituents or perfluoroalkyl groups. 4 in these cases they are suitable educts for addition of hydroxylic compounds. 4. 4.ii5 reaction is also possible for the ylid (SO). The resulting thioxoester (51) does not however react with p-nitrobenzaldehyde in a Wittig reaction. ... 

If this rearrangement occurs, polymerization and dimerization reactions of the ylid or the bromoalkyllithium reagent can be considered in the following way. 

Y MS-stilbene is found as one of the minor products of the reaction 83). Although a cleavage of the benzyl ylid to give phenyl carbene followed by dimerization could account for the stilbene, this reaction is considered to be less likely than a stepwise process which initially involves a displacement on 9 by the benzyl ylid followed by an a -(3 elimination reaction 83). 

The reaction of triethyl phosphite with maleic anhydride has been re-investi-gated. The product is not the dimeric phosphorane (11), but the ylid (12). Reactions of alkyl or aryl diphenylphosphinites with 1-nitro-l-alkenes have been studied in detail. Radical ion-pairs are involved as shown by e.s.r. and P CIDNP experiments. Allene may be hydrophosphorylated in a radical-initiated process to give allyl- and isopropenyl-phosphonates. A phosphorane (14) has been observed during the reaction of ethyl diethylthiophosphinite (13) with methacrylic acid. ... 

In light of the known reactivity of the thiophene S,N-ylids, a detailed examination of the reaction of thiophene with ethyl azidoformate has been undertaken. A careful product analysis revealed the presence of previously unreported products, such as 68, 70, and 75, which can be explained satisfactorily only in terms of the intermediacy of the thiophene S,N-ylid 66 (Scheme 12) (86TL1105). Thus, if the initial reaction of thiophene with the ethoxycarbonylnitrene generates 66, the products 68-70 may be rationalized in terms of a Diels-Alder dimerization to 67. Cheleotropic elimination of 69 from 67, followed by aromatization, would result in the formation of 68. Alternatively, 66 could undergo rearrangement by way of a bicyclic transition state (71) (cf. 33, Scheme 7) to a dipolar intermediate 72 (analogous to 35, Scheme 7). Proton transfer in 72 would then furnish 73 analogous to the 2-... 

Das Ylid 617 und Fluorenon 625 reagieren zum Allen 626, das nur als sein Dimeres 627 gefaBt werden kann i 3). 

The ylid-type ligands such as Me3PCH2 furnish dimeric complexes containing bridges ... 

All performed in protic media, the extensive exchange experiments discussed above carry intriguing implications for aprotic solvents. Here ylid concentrations are not suppressed ndi. are their dimers cleaved as a consequence of solvent acidity, so it mould be possible to prepare a host of yet-unknown heteroatom-substituted eAylenes by the ylid route. In addition to the peraminoethylenes synthesized via ylids, in fact, several 1,2-diamino-1,2-dithioethylenes and tetrathioethylenes have already been prepared by deprotonation of thiazoUum and 1,3-dithiolium salts, respectively. 

Finally, mechanisms for dimer formation from ylids deserve comment. Since Breslow s investigations it has been clear that the kinds of ylids under discussion are good nucleophiles, so attack of an ylid on its cationic precursor, followed by proton loss, is an attractive scheme (path a). It is not yet known how effectively simple coupling of pairs of ylids (path b) can compete with path (a) (though work of Quast and Hiinig (Section III.B.3) has shed much light on this... 

For a variety of reasons, among them the relatively high thermodynamic stability of such ylids and the marked tendency for tetra-aminoethylenes to give reaction products derived from only half of the molecule, the dissociation theme pervades most of the literature on these compounds in the form either of an assumption or of a question. Since nearly all of the many transformations of the Wanzlick dimer (5) have been interpreted in terms of initial dissociation into the... 

Alkyl methacrylates fumarate copolymerization, 279 MA copolymer-alcohol reactions, 284 MA copolymer-amine reactions, 284 MA copolymerization, 520-531, 535, 542 maleate copolymerization, 279 Alkyloxyamines, reaction with MA, 83 A -Alkyl-2-pyridine ylids, MA adducts, 217 7V-Alkylpyrroles, MA alkylation, 237 Alkyl thioglycolates, reaction with maleates, 49 Alkylthiosuccinic acids, 52, 55 ii/-Alkylthiosuccinic anhydrides, melting points, 50 Alkylureas, reaction with MA, 87 Allene, dimer-MA Diels-Alder diadduct, 109 N-Allylacetamide, MA copolymerization, 312 Allyl acetate... 

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