Inverted solvolysis products

The contact ion pair R R2HC LBr , in contrast to a free carbenium ion R R2HC , is chiral. Starting from enantiomerically pure (R)-2-bromooctane, the contact ion pair first produced is also a pure enantiomer. In this ion pair, the bromide ion adjacent to the carbenium ion center partially protects one side of the carbenium ion from the reaction by the nucleophile. Consequently, the nucleophile preferentially reacts from the side that hes opposite the bromide ion. Thus, the solvolysis product in which the configuration at the reacting C atom has been inverted is the major product. To a minor extent the solvolysis product with retention of configuration at the reacting C atom is formed. 

Let us first concentrate on the mixture of solvolysis products 3. Isomer 3E (the major isomer) retains the stereochemistry of the starting material, whereas isomer 3Z (the minor isomer) inverts the stereochemistry of the starting material. In addition, the labeling experiments carried out with substrate 5 indieate serambling of D/H in the retained ii-isomers 6, whereas the position of the label is not modified in the invertedZ-isomer (Scheme 35.5). 

Section 3, p. 36). Also, the theoretical definition does not specify the stereochemistry of the products, which are very rarely (if ever) found to be the 50/50 mixture of inverted and retained configurations predicted by classical theory (Doering and Zeiss, 1953 Okamoto et al., 1975a, b Bone et al., 1975). We shall use the term Sn 1 to refer to mechanisms in which k j is rate determining and the term classical SN 1 to refer to the mechanism (possibly defunct) in which rate-determining formation of a free carbocation is envisaged. However, kx (Fig. 2) is only rate determining if k2 > k l when internal ion pair return is important (k2 < k x) a more complex mechanistic notation is required (e.g. for solvolysis of t-butyl chloride in trifluoroethanol, p. 36). 

Deuterium isotope effects in the solvolysis of 5a-cholestan-3a-yl and -3/3-yl brosylates are interpreted as evidence for a rate-determining ionization step, with ring A in a fairly rigid chair conformation. Product analyses, however, suggest a conformational change towards half-chair in the transition state leading from the carbocation to products of inverted configuration. There is also evidence of a... 

For (58) and (60) keq was found to equal krac80,82. Thus it appears that internal return in allylic systems results in racemization and complete oxygen equilibration. However, this result is also consistent with an ion pair (63) which retains coordination of the ether oxygen to the carbon atom from which it departed. (63) would produce an equimolar mixture of (62a) and (62b). Determination of the 180 distributions of both recovered enantiomers from the solvolysis of (60) revealed identical scrambling, i.e. rapid interconversion of ion pairs (63) and (64)S2 With (55), however, predominant product formation from (63) was indicated by excess carbonyl-180 in the inverted ester and excess ether-180 in the ester of retained configuration80. ... 

Reed found that crystalline prenyltransferase could solvolyze the allylic substrates. This reaction required inorganic pyrophosphate and had a velocity of about 2% of the normal reaction rate [6]. Examination of the allylic product, either dimethylallyl alcohol or geraniol, revealed that C-1 had inverted and the csu-binol oxygen had come from water. Since the normal reaction involves inversion of C-1 and scission of C-O bond, the solvolysis seemed to be mimicking the normal reaction, with H2O replacing the organic portion of isopentenyl pyrophosphate in the catalytic site. This indicates that ionization of the allylic pyrophosphate is the first event, followed by condensation to form a new bond, then a hydrogen elimination from C-2 of the former isopentenyl moiety. Thus, there is an ionization-condensation-elimination sequence of events. 

The evidence on the stereochemistry of the solvolysis was obtained from optically active C6H5CH(OTf)CF3 (Va), which gave ratios of Zca, the polar-imetric rate constant, and kuv, the rate of product formation, of 7.3, 27, 1.2, 1.1, and 1.0 in TFA, HFIP, TFE, HOAc, and CH3CH2OH, respectively (11). The acetolysis product from ( —)-Va was 41% inverted and 59% racemized (11). 

Treatment of (15R)-PGA2 methyl ester (208 R = Me, R = H) from coral with methanesulphonyl chloride and solvolysis of the resulting mesylate (212) in acetone-water gave the C-15-inverted product (213), which after acetylation [to (209b)] was similarly converted into PGE2 and PGF2 (Scheme 45). 

To understand the results obtained in the acetolysis of ii-styryl(phenyl)iodo-nium tetrafluoroborate 1 is a bit more complicated. As we can see in Scheme 35.1, the reaction affords a mixture of retained (3E) and inverted (3Z) solvolysis (substitution) products, together with iodobencene (the leaving group) and phenylace-tylene (clearly an elimination product). It is almost impossible to formulate a single mechanism that could explain the formation of all the reaction products. In consequence, several mechanistic pathways should be considered in this case. 

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