Cumulene ylides

Many of the compounds of this type have been prepared by interaction of cumulene ylides witli heterocumulenes and these are covered in a review by Bestmann <77AG(E)349>. The oxetane system 5 results from reaction of Ph3P=C=C=S with aryl isocyanates, ArNCO <77AG(E)349>. An unusual approach to an oxetane ylide is the reaction of DMAD with CO2 and two equivalents of methyl phosphite which gives 6 <81CC607>. 

The other major approach to systems of this type is cycloaddition of cumulene ylides with vinyl isocyanate and vinyl isothiocyanate. Thus, Ph3P=C=C=0 reacts with these two reagents to give 160 and 161 respectively while the corresponding reactions of Ph3P=C=C=NPh afford 162 and 163 <88T543>. By using styryl isothiocyanate with the ketene ylide, 164 was obtained. 

The addition of mole equivalent of hydrogen chloride to a solution of the hetero-cumulene ylides (8) leads to the formation of cyclobutane-1,3-dione derivatives, which, when treated with base, give the bis-ylides (9). Compounds (9) react as expected with />-nitrobenzaldehyde, and can be oxidized with ozone to the cyclo-butanetrione derivative (10) (Scheme 1). ... 

The five-carbon carbene has been used in a synthesis of the monoterpene artemisia ketone (5). Reaction of the sulfide 3 with 2 gives a cumulenic ylide (a), which decomposes to a mixture of four isomeric products (547 yield). The major product (387 yield) is an allenic thioether (4) formed by a [2.3]sigmatropic shift. On hydrolysis 4 is converted into the terpene 5. 

The basic NRMS method and the auxiliary techniques outlined have enabled the discovery and characterization of a large variety of reactive neutrals that had eluded experimental studies due to their unusual structures and reactivities. The species studied so far are summarized in Table 1 and include radicals, diradicals, carbenes, nitrenes, cumulenes, ylides, hypervalent species, and weak intermolecular complexes. 

Many examples of ylides possessing the P=C=P functionality have been synthesized and characterized. Symmetric, asymmetric, cyclic and acyclic examples are known36-43. The known crystal structures of these ylides are listed in Table 4. Whereas most small cumulenes are linear, cumulenes having more than two double bonds are frequently bent48 - 50. The P=C=P angle is the most characteristic feature of these types of ylides. 

Phosphacumulene Ylides are Phosphorus Ylides and at the Same Time Cumulenes H. J. 

III. YLIDE AND SMALL-RING FORMATIONS BY SILYLENE REACTIONS WITH HETEROATOMS AND CUMULENES... 

Co-condensation of pyrrole 1020 + atomic carbon + CO2 at 77 K followed by addition of methanolic HCI generates ion 1023 and A -methylpyridinium 3-carboxylic acid 1027 in a 1.2 1 ratio (Scheme 200) <1997JA5091>. The observation of acid 1027 provides strong chemical evidence for the ylide 1021 rather than cumulene 1022 (Scheme 198) which would not be expected to react with CO2. However, acid 1027 is not generated when CO2 is added to 77 K matrix formed upon reaction of atomic carbon with pyrrole 1020. 

Allenes can be easily prepared using the 3,2-sigmatropic rearrangement of ylidic intermediates. This methodology also provides a general route towards the synthesis of cumulenes. Some examples using sulfonium and ammonium ylides are collected in Table 5. ... 

Abstract Phosphacumulene ylides of the general formula Ph3P=C=C=X [X=0, S, NR, (0R)2] are versatile C2-building blocks. They can act either as C-nudeophiles-only in a manner typical of phosphorus ylides, or as cumulenes undergoing [2 + n -cycloadditions with other cumulenes such as CO2, COS, RNCO etc. Most prominent is their tandem addition-Wittig alkenation of hydroxy- or amino-substituted carbonyl compounds. With aptly chosen reaction partners all these pathways may lead to heterocyclic products. Some recent applications of these methods to the syntheses of azetidines, five-membered lactams, lactones, tetramates, tetronates and pyrroles as well as to six-membered quinolones and to macrolides are delineated. 

A study directed to obtain evidence for the interrelationships between het-arylnitrenes, their ring-expanded cyclic carbodiimide isomers, and ring-opened nitrile ylides (Scheme 69) and a review of cumulene rearrangements of ketene-ketene, isocyanate-isocyanate, thioketene-ketene, imidoylketene-ketenimine,... 

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