Electron-rich alkyl group

The most electron-rich alkyl group (more substituted carbon) migrates first. The general migration order ... 

The electronic character of aromatic groups at the a-C of nitrones affects both, the yields and the stereoselections (Table 2.28). Electron-rich aromatic groups increase enantioselectivities but decrease the yields (entries 1-3). Electron-deficient ones slightly decrease enantioselectivities but increase the reaction rates (entries 4 and 5). The electronic properties of A-bound aromatic groups of nitrones has almost no obvious impact on the enantio-selections (entries 6-10). Both, electron-deficient and electron-rich aromatic groups afford good enantioselectivities and diastereoselectivities. Nitrone with a A-bound furyl group furnishes, in moderate yield, the best ee but the lowest diastereoselectivity (entry 9). a-Alkyl and A-alklyl nitrones fail to react (entries 11 and 12). 

The reaction was successfully applied to both electron-rich and electron-poor 4-nitrophenyl carboxylates among them, the conversion of the electron-deficient esters was found to be faster and more efficient. Many functional groups are tolerated on both the side of the carboxylic ester (halo, keto, formyl, ester, cyano, nitro and protected amino groups, heterocyclic and a,-unsaturated carboxylic esters) and of the alkene (electron-rich alkyl-substituted alkenes, electron-poor acrylate derivatives, trimethylvinylsilane as an ethylene surrogate). The cinnamate derivatives could become particularly useful substrates, since the availability of the synthetically equivalent vinyl halides is rather limited. In analogy to conventional Mizoroki-Heck chemistry, linear (Zi)-substituted alkenes are predominantly but not exclusively obtained. Selected examples are shown in Table 4.1. 

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