Acyloxy groups, electronic effects

Asymmetric Diels-Alder Reaction of Unsaturated Aldehydes . The boron atom of acyloxyborane is activated by the electron-withdrawing acyloxy groups, and consequently acyloxyborane derivatives are sufficiently Lewis acidic to catalyze certain reactions. Thus, asymmetric Diels-Alder reactions of a,p-enals with dienes using (1) as a Lewis acid catalyst have been developed. For example, the reaction of cyclopentadiene and methacrolein gives the adduct in 85% yield (endo exo= 11 89) and 96% ee (major exo isomer) (eq 3). Some additional examples are listed in Figure 1. The a-substituent on the dienophile increases the enantioselectivity, while p-substitution dramatically decreases the selectivity. In the case of a substrate having substituents in both a- and p-positions, high enantioselectivity is observed thus the a-substituent effect overcomes that of the p-substituent. 

From a consideration of detailed results on the conformational equilibria of aldopentopyranose derivatives, it has been pointed out92- 9 that a more sophisticated model is required before conformational populations can be reliably predicted, at least with acylated derivatives. Even with adjustment of the original parameters in order to take revised values for the anomeric equilibrium of D-lyxopyranose tetraacetate and the conformational equilibrium of /3-D-arabinopyranose tetraacetate into account, the observed data cannot be accommodated within the framework of this model, except on a very broad, qualitative basis. Other possible factors that should be considered " include polar contributions from substituents other than that on C-1, attractive interactions between syn-diaxial acyloxy groups, non-bonded interactions between atoms that have unshared pairs of electrons,repulsive interactions between gauche-vicinal groups, the effect of solvent pressure, and differences between the molar volume of conformers. 

It was also shown that the initiating performance of compounds, like l-chloro-thioxanthene-9-one, is enhanced by the presence of an acyloxy group in the four position. This was attributed to the greater triplet n Ti activity because of the electron withdrawing effect of the 4-acetoxy group. This high... 

More recent B-NMR studies at room temperature have shown that (acyloxy)diethylboranes are monomeric also in non-polar solvents like chloroform [(5( B) - 60 ppm] [46]. Under the same conditions the corresponding 9-BBN derivatives are monomeric (both types I and II) or dimeric, depending on the electron-donating or -withdrawing effect of the substituent on the carboxyl group, the temperature, and the concentration of the... 

Bimolecular reactions of aniline with /V-acyloxy-/V-alkoxyamides are model Sn2 processes in which reactivity is dictated by a transition state that resembles normal Sn2 processes at carbon. Electronic influences of substituents support a non-synchronous process which has strong charge separation at the transition state and which is subject to steric effects around the reactive centre, at the nucleophile but not on the leaving group. The sp3 character of nitrogen and disconnection between the amino group and the amide carbonyl renders these reactions analogous to the displacement of halides in a-haloketones. 

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