Borata-Wittig olefination reactions of ketones, carboxylic esters and amides with bis(pentafluorophenyl)borata-alkene reagents

Abstract

The strongly electrophilic borane derivative amino–CH₂CH₂CH₂–B(C₆F₅)₂6 was α-CH deprotonated with LiTMP to give the borata-alkene {[amino–(CH₂)₂–CHB(C₆F₅)₂⁻][Li⁺]}₂9 which underwent facile [2 + 2] cycloaddition reactions with benzophenone or fluorenone to yield the respective 1,2-oxaboretanides 11a,b. Compounds 9 and 11 were characterized by the X-ray diffraction. Thermolysis or hydrolysis of compounds 11a,b gave the corresponding borata-Wittig olefination products 12a,b. A variety of R–CH₂–CH₂–B(C₆F₅)₂ boranes (conveniently generated by hydroboration of terminal alkenes R–CHCH₂ with Piers’ borane [HB(C₆F₅)₂]) were analogously deprotonated to give the respective borata-alkenes 16a–e (R: Ph–CH₂–, ⁿC₄H₉, ^tBu, Cy, PhCH₂CH₂–). They underwent “non-classical” borata-Wittig olefination reactions with ethylformate to give the respective enolether carbonylation products, or their C₁-elongated aldehydes (after hydrolysis). The borata-alkene [Ph–(CH₂)₂–CHB(C₆F₅)₂⁻] [Li⁺HTMP] (16a) gave the respective “non-classical” borata-Wittig olefination products, the enolethers 25a,b and 27, respectively, upon treatment with methyl- or ethyl acetate or γ-butyrolactone.