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

Abstract

The strongly electrophilic borane derivative amino-CHCHCH-B(CF)6 was α-CH deprotonated with LiTMP to give the borata-alkene {[amino-(CH)-CH[double bond, length as m-dash]B(CF)][Li]}9 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-CH-CH-B(CF) boranes (conveniently generated by hydroboration of terminal alkenes R-CH[double bond, length as m-dash]CH with Piers' borane [HB(CF)]) were analogously deprotonated to give the respective borata-alkenes 16a-e (R: Ph-CH-, CH, Bu, Cy, PhCHCH-). They underwent "non-classical" borata-Wittig olefination reactions with ethylformate to give the respective enolether carbonylation products, or their C-elongated aldehydes (after hydrolysis). The borata-alkene [Ph-(CH)-CH[double bond, length as m-dash]B(CF)] [LiHTMP] (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.

A Boratafulvene

“…[11c, 12a,15, 30] Preliminary analysis of the reaction mixtures by NMR spectroscopy indicates the formation of an oxaboretane intermediate C as the dominant species of amixture after afew hours. [15] Themethylene CH 2 6), C3-C4 1.346( 8), C4-C5 1.525( 6), C5-C6 1.354( 5), C6-B1 1.617( 9), C2-Sn1 2.215(3);S n1-CH 3 2.073( 5), 2.148( 6), 2.180 (9). Scheme 5.…”

The Boron‐Wittig Olefination of Aldehydes and Ketones with Bis[(pinacolato)boryl]methane: an Extended Reaction Scope

“…The DFT computational study proposed that in the case of monosubstituted bis[(pinacolato)boryl]methane reagents, formation of the six‐membered intermediates 11 was more consistent with experimental results than four‐membered adducts of the type 12 . Nevertheless, because the formation of 1,2‐oxaboretanide complexes 12 by the boron‐Wittig reaction was confirmed by X‐ray diffraction studies for analogous species with B(Mes) 2 , and B(C 6 F 5 ) moieties, the latter reaction pathway should not be completely omitted from the consideration. These models provide straightforward explanation for the E‐ stereoselectivity observed in the reactions of aldehydes 9k – n and 1 , since the corresponding conformations 13a are favourable over 13b when R 1 =H (Scheme , B ).…”

Borata‐Alkene Species as Nucleophilic Reservoir

“…Compound 51 underwent facile [2+2] cycloaddition reactions with benzophenone or fluorenone to yield the respective 1,2‐oxaboretanides 52 and 53 , that could also be crystallized (Scheme 25). [69] The electron withdrawing properties of the B(C 6 F 5 ) 2 favoured the borata‐alkene stabilization although did not prevent its nucleophilic attack to the ketone reagents. Complementarily, Tomioka and co‐workers conducted the boron‐Witting reaction between borata‐alkene [( i Pr 2 N) 2 B=C(H)(CN)] − and aldehydes, with the concomitant formation of stereoselective ( Z )‐alkenes [70] .…”