Ylide compounds metal-mediated reaction

The mechanism was also supported by the observation of quantitative formation of an insertion (initiation) product F2B-CH2-F as a result of the reaction of BF3 with diazomethane at -40°C to around -60°C [18], As described later in this review for the Pd-mediated polymerization of diazocarbonyl compounds and the B-mediated polymerization of dimethylsulfoxonium ylides, this type of mechanism is significant as a general one for PSMS with a combination of coordination (nucleophilic addition) and 1,2-migration for the metal-mediated reactions. 

Transition-metal mediated carbene transfer from 205 to benzaldehyde generates carbonyl ylides 211 which are transformed into oxiranes 216 by 1,3-cyclization, into tetrahydrofurans 212, 213 or dihydrofurans 214 by [3 + 2] cycloaddition with electron-deficient alkenes or aUtynes, and 1,3-dioxolanes 215 by [3 + 2] cycloaddition with excess carbonyl compound (equation 67). Related carbonyl ylide reactions have been performed with crotonaldehyde, acetone and cyclohexanone (equation 68). However, the ylide generated from cyclohexanone could not be trapped with dimethyl fumarate. Rather, the enol ether 217, probably formed by 1,4-proton shift in the ylide intermediate, was isolated in low yield . In this respect, the carbene transfer reaction with 205 is not different from that with ethyl diazoacetate, whereas a close analogy to diazomalonates is observed for the other carbonyl ylide reactions. 

Dipolar cycloaddition reactions constitute a powerful and convergent tool for the preparation of various heterocyclic compounds, which have been widely applied in the synthesis of numerous natural products, pharmaceuticals, and functional materials. The chemistry of 1,3-dipolar cycloaddtion reactions has been well documented in a number of reviews [3]. In this section the focus is on transition-metal-mediated 1,3-dipolar cycloaddition reactions with some important 1,3-dipoles, including azides, diazoalkanes, carbonyl ylides, and azomethine ylides, rather than a full review of the reactions of all types of 1,3-dipoles. 

Similar to carbonyl ylides, azomethine ylides are normally generated as transient species in situ, and a number of protocols have been established so far [22]. Among them, the transition-metal-mediated procedures have enj oyed great privileges in terms of milder reaction conditions, better selectivities, and broad functional group tolerance. This part focuses on the recent development of transition-metal-mediated in situ generation of (metal-containing) azomethine ylides as well as their applications for the synthesis of aromatic compounds. 

In catalytic epoxidation reactions an alternative to the ylide generation method via alkylation/deprotonation is the transition metal-mediated carbene transfer from diazo compounds to sulfide catalysts. In 1994, Aggarwal and coworkers employed this method in the enantioselective catalytic epoxidation of aldehydes [25]. Using 20mol% of non-racemic sulfide 17 and lmol% of Rh2(OAc)4 together with the slow addition of PhCHN2, a 58% yield and 11% ee were obtained in the epoxidation of benzaldehyde (Scheme 20.10). The enantioselectivity was similar to the results obtained by Breau and Durst using preformed sulfonium salts [26]. 

The ylide, of general structure (4.132), is formed by reaction of the corresponding sulphide (4.133) with a metallocarbenoid resulting from transition metal salt-mediated decomposition of a diazo compound (4.134) or N-tosyl hydrazone salt (4.135). Reaction of the sulfonium ylide (4.132) with aldehyde (4.136) yields the trans-epoxide (4.137) as the major product and sulfide (4.133) which is then returned to the catalytic cycle (Figure 4.10). 

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