Ester stereoelectronic effects

Steric and stereoelectronic effects control the direction of approach of an electrophile to the enolate. Electrophiles approach from the least hindered side of the enolate. Numerous examples of such effects have been observed. In ketone and ester enolates that are exocyclic to a conformationally biased cyclohexane ring there is a slight... 

Taira, K. and Gorenstein, D.C., Experimental tests of the stereoelectronic effect at phosphorus Michaelis-Arbuzov reactivity of phosphite esters, Tetrahedron, 40, 3215, 1984. 

P. Deslongchamps, Stereoelectronic Control in the Cleavage of Tetrahedral Intermediates in the Hydrolysis of Esters and Amides , Tetrahedron 1977, 31, 2463 - 2490 P. Deslongchamps, Stereoelectronic Effects in Organic Chemistry , Pergamon Press, Oxford, 1983. 

One further point is worthy of brief mention. While we have focused on lone pair/lone pair repulsive interactions that destabilize transition state C, it is conceivable that A is actually stabilized relative to C by a favorable charge-charge interaction between the ester carbonyl (5 ) and the aldehydic carbonyl carbon (5+) owing to the proximity of these groups in A. While it is not yet possible to resolve the relative contributions of these distinct stereoelectronic effects, it is clear that our mechanistic proposal e)mlains the experimental results only if the dioxaborolane and the C-COaiPr bonds exist in the conformations indicated in B. Any conformational infidelity at either site would be expected to lead to diminished enantioselectivity. 

Treatment of a cyclic 23- or 3,4-ortbo-ester in a pyranoside under mild acidic conditions will cause regioselective cleavage so that a free equatorial OH is produced adjacent to an axial O-acyl group [14] (Scheme 1). The reasons for this have engendered considerable theoretical treatment based on considerations of stereoelectronic effects [IS]. 

This Chapter deals with the stereoelectronic effects which control the cleavage of tetrahedral intermediates during the formation or the hydrolysis of an ester. Since these effects are also operative in the ester function itself, a discussion of the functional group will first be presented. 

Intermediate 69 can either yield aZ (hydroxy-ester) or an E (lactone) ester. Intermediate 70 can only yield an E ester (lactone) whereas intermediate 7 can produce two esters, the hydroxy-ester having an E conformation and the lactone. Thus, primary stereoelectronic effects allow the cleavage of intermediates 69-71 to produce either the hydroxy-ester or the lactone prod-... 

The relationship between the conformation of the acetal function and its reactivity towards ozone has been described in detail (cf p. 41 ). It was shown that the insertion of ozone into the C —H bond of the acetal function to form the corresponding hydrotrioxide tetrahedral intermediate is subject to stereoelectronic control. This section deals with the next step of this reaction, i,e., the decomposition of the hydrotrioxide tetrahedral intermediate to yield the ester product. Experimental results will be presented to show that this step is also controlled by stereoelectronic effects. These results can therefore be used as evidence for the principle of stereoelectronic control in ester formation. 

Lehn and Wipff (72) and Gorenstein and co-workers (73-80) have proposed on the basis of molecular orbital calculations that stereoelectronic effects similar to those observed in esters and amides play also an important role in the hydrolysis of phosphate esters. For instance, calculations suggest that the axial P — OR bond in the trigonal bipyramid conformation 120 is weaker than that in the conformation 121 because in the former, the oxygen atom of the equatorial OR group has an electron pair anti peri planar to the axial P — OR bond. Experimental results tend to support this interesting proposal but additional experiments are needed before unambiguous conclusions can be reached (81). 

High catalytic activity of dimeric complex 1 in the hydrosilylation of allyl esters and allyl ethers (as well allyl glycidyl ether) has been shown in Ref. [18]. The catalytic activity of rhodium-siloxide-phosphine complexes depends on the steric effects of the siloxy group and stereoelectronic effects of the trisubstituted phosphine [14]. Comparison of catalytic activity of dimeric (1) and monomeric, phosphine (2, 3) and non-phosphine (4) rhodium siloxide complexes in the examined reaction is presented in Table 1. 

The cleavage of ring ct( 0 bond leads to the formation of 63, wherein the carboxylic acid function is in Z-configuration. A Z-carboxylic acid benefits from two stereoelectronic effects, whereas an. E-ester such as 62 derives advantage from only one stereoelectronic effect, vide infra. For this reason, the transition state energy for the... 

We shall now consider the cleavage of each of the above hemi-orthoester con-formers under stereoelectronic control just as we dealt previously with the cleavage of the important orthoester conformers first, 96a will cleave to the hydroxy Z-ester 98, as shown in Eq. 25 second, 96b will not cleave and constitute the neutral conformer third, 96c will cleave to the hydroxy E-ester 99, as shown in Eq. 26. Interestingly, neither of 96a and 96c can cleave to a lactone because no ring oxygen electron pair orbital is in stereoelectronic effect with the equatorial aC-OMe bond. Thus, in instances where the tetrahydropyran ring cannot easily undergo chair inversion, lactone will not be formed. Additionally, since the Z-ester is more stable than the E-ester, hydrolysis will take place preferentially via the conformer 96a and the hydroxy Z-ester 98 will be formed. 

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