Ylides, rearrangement

Electron deficient species can attack the unshared electron pairs of heteroatoms, to form ylides, such as in the reaction of thietane with bis(methoxycarbonyl)carbene. The S —C ylide rearranges to 2,2-bis(methoxycarbonyl)thiolane (Section 5.14.3.10.1). A"-Ethoxycar-bonylazepine, however, is attacked by dichlorocarbene at the C=C double bonds, with formation of the trans tris-homo compound (Section 5.16.3.7). 

The major limitation of asymmetric sulfur ylide epoxidations is that only aromatic vinylepoxides can be formed efficiently and with high selectivity. When an aliphatic aldehyde is allowed to react with a semistabilized or nonstabilized sulfur ylide, poor diastereoselectivities and yields are observed, due to problems in controlling the ylide conformation and competing ylide rearrangement reactions [71]. However, some racemic, aliphatic vinylepoxides have been successfully formed by sulfur ylide epoxidations, although varying diastereoselectivities were observed [78-80],... 

The ylide rearrangement is a sigmatropic change of the order (3, 2) since the new o bond is two and one atoms away from the original position. 

Rhodium(II) carboxylate dimers and their carboxamide counterparts have been demonstrated to be exceptionally useful catalysts for carbene transfer processes involving diazocarbonyl substrates [1]. Doyle s seminal work identified Rh2(OAc)4 as the catalyst of choice for a variety of cyclopropanation, C-H insertion, and ylide rearrangement transformations using diazoketones or diazoesters [2]. Important contributions by Taber [3], Padwa [4], and Davies [5] further established the superior catalytic activity of dirho-dium catalysts and the excellent selectivity of rhodium-[Pg.417]

The products formed in the reaction of diphenylcyclopropen-one or -thione and 1-methyl-2-methylthiopyrroline have been shown to be the l,2,7,8-tetrahydroazocin-2-one (87) and the thione, respectively, evidently arising from a C,N-insertion pathway involving an ylide rearrangement (72HCA1759). 

The first example of an ammonium ylide rearrangement was encountered in 1928 by Stevens and co-workers who observed that a novel [l,2]-benzyl shift occurred when ammonium salt 1 was heated in dilute... 

Fig. 106 Rhodium(II)-catalyzed oxonium ylide rearrangements of diazo ketones 445...

Fig. 107 Onium ylide rearrangements catalyzed by rhodium(II) carboxylates...

Cu(II) Catalysis [l,2]-Onium Ylide Rearrangements (Stevens Rearrangements) 406... 

While [2,3]-onium ylide rearrangements are proceeding most often by concerted pathways (review [388]), [l,2]-onium ylide rearrangements (Stevens rearrangements) often involve radical intermediates (recent reviews [389, 390]). A study aimed at the comparison of Rh(II) and Cu(II)-catalyzed oxonium ylide reactions of oxygenated diazoketones using 3 mol% of the former and 15 mol% of the latter showed that the copper-catalyzed process provided better yields and selectivities for [1,2]-rearrangement products (see Part 2, Sect. 6, Fig. 107) [391, 392]. 

Lactam antibiotics, new syntheses using S-ylide rearrangement 89YZ345. 

As with the rearrangements already discussed, sulfonium ylide rearrangements have been used to solve constitutional problems and as intermediates in stereoselective reactions. Therefore, the remainder of the chapter will be... 

In the course of a sulfonium ylide rearrangement a maximum munber of three stereogenic units can be generated (Scheme 87). The resulting homoal-lylic sulfide 354 contains up to two stereogenic centers and one stereogenic double bond. Based on the work of Houk [204] with R = H, the following simphfied transition-state models can be discussed (Scheme 88). 

Scheme 97 Application of cobalt salen complexes for sulfonium ylide rearrangements...

Scheme 98 Studies concerning the question of the intermediacy of metal-bound ylides in the sulfonium ylide rearrangement...

The allylic position of the sulfide group in the cycloadducts may be employed for 2,3-sigmatropic reactions, as demonstrated by the alkenation/sulfur ylide rearrangement sequence (144) (145). A... 

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