Nucleophilic substitution diene selective formation

Zirconium tetrachloride promotes a tandem nucleophilic addition and aldol-type condensation reaction of methyl propynoate, or /V,/V-dimethylpropynamidc, with aldehydes, or ketones, in the presence of tetra-n-butylammonium iodide (Scheme 6.13) [8] with a high selectivity towards the formation of Z-isomers. A similar reaction occurs between aliphatic and aromatic aldehydes and penta-3,4-dien-2-one to yield 1-substituted 2-acetyl-3-iodobut-3-enols (50-75%) [9]. 

Since the Lewis acid-promoted reactions of the oxidized products with nucleophiles give the corresponding N-acyl-a-substituted amines efficiently, the present reactions provide a versatile method for selective C-H activation and C-C bond formation at the a-position of amides [138]. Typically, TiCl4-promoted reaction of a-t-butyldioxypyrrolidine 66, which can be obtained by the ruthenium-catalyzed oxidation of l-(methoxycarbonyl)pyrrolidine with f-BuOOH, with a silyl enol ether gave keto amide 67 (81%), while the similar reaction with less reactive 1,3-diene gave a-substituted amide 68 (Eq. 3.80). 

The data for the reactions of four substituted styrene radical cations with selected dienes are summarized in Table 8. - As discussed above for the reaction of styrene radical cations with nucleophiles, the interpretation of these data is complicated by the possibility that two competing reactions are responsible for the observed quenching of the radical cation. One of these is electron transfer from the alkene to the styrene radical cation to generate the neutral styrene and the radical cation of the alkene (Eq. 29). In this case, the quenching rate constant is that for electron transfer, and does not provide any information on the kinetics for the initial addition, although the secondary radical calion/neutral pair may in some cases lead to adduct formation. The other reaction is addition of the alkene to the radical cation to generate an adduct radical cation that is the precursor of the final cyclobutanation and Diels-Alder products (Eq. 30). 

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