Other Carbonyl and Imine Activation Chemistries

Akiyama and coworkers noticed that treatment of diallylisopropylsilane (108) and a,P-unsaturated ketones with Bp3 OEt2 results in a Michael addition of one of the allyl moieties, followed by an intramolecular allylation onto the P-silyl carbocation intermediate to furnish a domino allylation product, (109) (Equation 65) [68]. 

Xi and coworkers have reported that by using a combination of zirconocene (llO)-mediated C-C bond-forming reactions and BF3 OEt2 promoted carbonyl activation, it is possible to prepare highly substituted cyclopentadienes and tetrahy-droindenes in an efficient one-pot synthetic route (Equation 66) [69]. 

Wang and coworkers have reported an example of chemoselective nucleophilic attack on a,p-unsaturated ketones promoted by BF3 OEt2. In their example, the titanium enolate (111) of a p-keto a-diazo compound adds in a 1,2-fashion to the enone substrate (Equation 67) [70]. The low yield was attributed to a significant recovery of starting material. 

Ishihara and coworkers have demonstrated that fluorinated alkynylamines (112) react smoothly with a variety of aldehydes or ketones, in the presence of a catalytic amount of Bp3 OEt2 and of 4A molecular sieves, to give the corresponding a-(trifluoromethyl)-a,P-unsaturated amides (113) in good yields and high Z E selectivity (Equation 68) [71]. 

Nakama and coworkers have shown that in the presence of a combination of BF3 OEt2 and zinc (11) iodide catalyst system, aldoximes react with a,P-unsaturated carbonyl compounds at ambient temperature, affording N-alkylated nitrones (114) in excellent yields (Equation 69) [72]. 

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