Rearrangement ylidic

Another type of ylidic rearrangement, the Sommelet-Hauser rearrangement, was discovered later, in 1937, by Sommelet and studied extensively by Sommelet and by Hauser. This transposition involves the base-promoted rearrangement of non stabilized ammonium and sulfonium ylides. Thus, the ammonium salt (11), when heated with alkali, gave cleanly the substituted diphenylmethane derivative (12 Scheme 5). Wittig found that the dibenzyl ammonium salt (13) reacted with phenyllithium giving two... 

In 1969, Ollis recognised that the rearrangement of ally lie ammonium and sulfonium y tides was a facile process of considerable synthetic interest, and Baldwin reported his generalization of the sigma-tropic rearrangement processes. Since then, numerous examples of the synthetic utility of ylidic rearrangements have been recorded in the chemical literature and this number is growing continuously. 

With some substrates, several types of ylidic rearrangements can occur simultaneously, though by judicious choice of experimental conditions, this competition can be controlled. This is of paramount importance in the utilization of these rearrangements in synthesis. The two examples described below illustrate this point. 

Despite the pioneering work of Ando and Doyle, few synthetic applications of oxonium ylidic rearrangements have been reported. However, three examples of synthetic relevance appeared recently (Schemes 60 to 62). When a-allyloxyacetic esters are reacted with trimethylsilyl triflate and a base, the transposed material (252), resulting from a 3,2-sigmatropic rearrangement of the transient ylide (251), could be isolated in good yields. ° The same product ratio was also obtained upon treatment of the ketene acetal (253) with trimethylsilyl triflate. This second variant involves the direct formation of ylide (251) followed by its transformation into (252 Scheme 60). 

Scheme 3. Ylidic rearrangement via radical pair intermediates...

The second expansion is very much less stereoselective and produces a mixture of alkenes in the ratio (E,E)/(E,Z)-5 l. Here should be noted the use of a stabilized ylide in the second stage of Scheme 3. Generation requires only an amidine base (DBU) but the rearrangement proceeds at room temperature, some 70 °C above the most favoured unstabilized system. The ylidic carbon may also be part of the thiopyran ring, in which case, as equation (11) shows, the products are alicyclic (79JOC3230). 

The ylidic 1,3,4-benzothiadiazine (158) also undergoes rearrangements involving the carbon substituent on sulfur (81JCS(P1)2245>. The products, which depend on the nature of R, arise from competing [1,2], [1,4] and [2,3] shifts (Scheme 11). A further pathway involves loss of the substituent from sulfur to give (159). 

Allylic phenyl tellurides, prepared in situ from the corresponding halides and diphenyl ditelluride, were converted by Phi = NTs, through a [2,3] sigmatropic rearrangement of ylidic intermediates, into A-tosylated allylic amines, e.g. [49] ... 

In the majority of examples the ylids are insufficiently stable and further rearrangements occur. Attack at C-2 of the thiophene ring by the ylidic car-... 

Phosphorodiamidites, e.g. (48), and M-chloroamines (49) gave exclusively di-amidochloridates, e.g. (50), in methylene chloride, whereas in petroleum ether some amidate, e.g. (51), was formed as well. The reactions of carbon tetrachloride with alkylbis(dialkylamino)phosphines (52) have been further studied this year. The remarkable rearrangement (53) to (54) occurs only with large isopropyl substituents on nitrogen, and with alkyl or hydrogen on the ylidic carbon. 

Rearrangements of ylidic intermediates proceed with allylic transposition. This useful property is the key feature of one of the best preparations of 1, S-dienes (Scheme 9). Trost has used this approach to synthesize yomogi alcohol (165) in a particularly simple way by 5-alkylation of sulfide (31) and solvolysis of the resulting sulfonium ion (Scheme 37). ... 

One of the landmarks in the early use of the 3,2-rearrangement of sulfur ylides is the squalene synthesis reported by Ollis and already discussed in Section 3.10.4. This approach used benzyne to generate the ylidic intermediate. Frequently, however, benzyne produces a mixture of products (Scheme 38). The sulfide (168) may be formally regarded as the expected 3,2-sigmatropic rearrangement product, whereas (169) results from a competitive 1,4-rearrangement. Use of carbenes instead of benzyne gener-... 

Vedejs has iso discussed the inefficiency of copper catalysts in ylidic 3,2-sigmatropic processes. Optimization of ring expansion reactions using diazomalonates were found to be unsatisfactory and analogous reactions using diazoketones were totally unsuccessful. Takano has taken advantage of a rhodium-promoted ylide formation followed by a 3,2-rearrangement, in a useful synthesis of y,8-unsaturated carbonyl compounds (Scheme 45). ... 

Julia, in a very elegant approach, has used sulfur ylidic 3,2-rearrangements to produce p.y-unsaturated aldehydes (Scheme 46). Following the same idea, Kociensky also used ylidic 3,2-sigmatropic rearrangements to prepare P,7-unsaturated aldehydes he employed sulfur ylides in conjunction with silicon substituents (Scheme 47). In contrast, Mander used the nitrogen ylidic 3,2-rearrangement as an easy route towards the synthesis of P,y-unsaturated aldehydes (Scheme 48). ° It is noteworthy that better... 

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