Abstract
We have prepared a range of alkynyl benzoates in high yields and have investigated their reactivities with the strong Lewis acid B(C6F5)3. In such molecules both σ-activation of the carbonyl and π-activation of the alkyne are possible. In contrast to the reactivity of propargyl esters with B(C6F5)3 which proceed via 1,2-addition of the ester and B(C6F5)3 across the alkyne, the inclusion of an additional CH2 spacer switches off the intramolecular cyclization and selective σ-activation of the carbonyl group is observed through adduct formation. This change in reactivity appears due to the instability of the species which would be formed through B(C6F5)3 activation of the alkyne.
Highlights
The intrinsic Lewis acidity of Group 13 compounds has led to their broad application in Lewis acid- catalyzed reactions in synthetic chemistry
Addition of the Lewis acid B(C6F5)3 to 1 at ambient temperature resulted in adduct formation between the ester oxygen atom and the vacant orbital at boron, evidenced by 11B-NMR data which displayed a broad peak consistent with other carbonyl adducts of B(C6F5)3 [43]
The 11B- and 19F-NMR spectra are dependent upon both the concentration of the reaction and on the mole ratio of B(C6F5)3 to alkynyl benzoate
Summary
The intrinsic Lewis acidity of Group 13 compounds has led to their broad application in Lewis acid- catalyzed reactions in synthetic chemistry. In our research we have focused on the activation of alkynes by Molecules 2015, 20 electrophilic boron reagents towards nucleophilic attack This reactivity is well-precedented in FLP chemistry and many examples have been reported in which the FLP adds in a 1,2-manner across the alkyne. Formation of versatile boron allylation reagents from propargyl esters (Scheme 1) [41] In all cases these intramolecular cyclization reactions involve the 1,2-addition of the carbonyl oxygen atom from the ester or amide and the borane across the alkyne [40,41,42]. Π-activation appears to be entirely suppressed in favor of σ-adduct formation between the carbonyl group and the Lewis acid Such differences in reactivity between these alkynyl benzoate substrates and the related propargyl esters and amides are discussed
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