Ethene nucleophilic substitution reactions

Roschenthaler and co-workers have reported a new methodology for the synthesis of ort/ o-CF2X-substituted arylphosphonates (750) via the Diels-Alder reaction of selected 1,3-butadienes (749). The reactivity of (750) was then examined to give the respective phosphine oxides (751), carboxylic acid (754), -ethens of type ArCF = CFAr (752) and nucleophilic substitution reaction products (753) with various electrophiles (Scheme 191). " " ... 

The first step consisted of the Mitsunobu etherification of an allyl alcohol with the resorcinol monoester, which afforded 102. Cleavage of the benzoyl protecting group released the phenol, which was then attached to the solid support. One-step cleavage of the THP group and bromination was achieved with PPhs/C Br4 to furnish 103. Nucleophilic substitution of the bromide with benzylamine was followed by acylation of the secondary amine wtith N-Boc-allylglycine 104, which resulted in the precursor 105, ready for the metathesis reaction this was performed with catalyst 101 to yield the final product 106. Either 1-octene or ethene was employed to generate 101. 

The main direction of the reaction of pyrroles with l-alkylthio-2-chloroacetylenes (30°C-35°C, 1.5 h) in the system KOH/DMSO is nucleophilic substitution of chlorine atom with pyrrolate anions to afford N-(alkylthioethynyl)pyrroles in 14%-42% yield (Scheme 2.50) [513]. Apart from these pyrroles, the reaction mixture contains 2-alkylthio-l,l- w- (45%-51%) and l-alkylthio-l,2-fciy-(pyrrol-l-yl)ethenes (3%-5%). When pyrrole-acetylene molar ratio is 2 1,2-alkylthio-l,l-fciy (pyrrol-l-yl) ethenes become major reaction products (up to 51% yield) (Table 2.5). 

The only side products of the reaction, 2-benzoyl-l,l-di(indol-3-yl)ethenes (5%-6% yields), are probably formed via the nucleophilic substitution of bromine atom with indole in the intermediates, 3-(2-benzoyl-l-bromovinyl)indoles. 

Jug and co-workers investigated the mechanism of cycloaddition reactions of indolizines to give substituted cycl[3,2,2]azines <1998JPO201>. Intermediates in this reaction are not isolated, giving evidence for a concerted [8+2] cycloaddition, which was consistent with results of previous theoretical calculations <1984CHEC(4)443>. Calculations were performed for a number of substituted ethenes <1998JPO201>. For methyl acrylate, acrylonitrile, and ethene, the concerted [8+2] mechanism seems favored. However, from both ab initio and semi-empirical calculations of transition states they concluded that reaction with nitroethene proceeded via a two-step intermolecular electrophilic addition/cyclization route, and dimethylaminoethene via an unprecedented two-step nucleophilic addition/cyclization mechanism (Equation 1). 

As expected for an electrophilic addition, the reaction rate increases as alkyl groups are substituted on the double bond. The electron-donating alkyl groups make the alkene more nucleophilic. Table 11.1 lists the relative rates of bromination of a series of alkenes. As can be seen from this table, replacing all four of the hydrogens of ethene with methyl groups results in an increase in the rate of the reaction by a factor of 2 million. 

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