Different leaving group

Data obtained for three different leaving groups are presented in Table 6.7. The results show that syn elimination is extensive for quatemaiy ammonium salts. With better leaving groups, the extent of syn elimination is small in the polar solvent DMSO but quite significant in benzene. The factors which promote syn elimination will be discussed shortly. 

For E2 eliminations in 2-phenylethyl systems with several different leaving groups, both the primary isotope effect and Hammett p values for the reactions are known. Deduce from these data the relationship between the location on the E2 transition state spectrum and the nature of the leaving group i.e., deduce which system has the most El-like transition state and which has the most Elcb-like. Explain your reasoning. 

A number of other, related compounds along with detailed experimental procedures are reported by Atkins, Richman and Oettle. Rings of other sizes and different leaving groups distinguish the latter work from that previously cited. In this case, as in the pre-... 

Although the rates were greatly different (as expected with such different leaving groups), the product ratios were the same, within 1 %. If this had taken place by a second-order mechanism, the nucleophile would not be expected to have the same ratio of preference for attack at the P hydrogen compared to... 

Direct support for a stepwise pathway is, however, provided by comparison of the rates of reaction of a series of substrates, having different leaving groups, with the same nucleophile, e.g. 2,4-dinitro-halogenobenzenes (86) with piperidine (87) ... 

The Taniaphos catalyst (6) has been found to give S 2 products in excellent yields (90-98%) with a high enantioselectivity (90-98% ee) in the allylic substitution of (7e) several different leaving groups by Grignard reagents in the presence of CuBr.SMe2 in r-BuOMe at -78 °C.9. 

Fig. 11.14 The value of kcat for substrates of /J-glucosidase possessing a common glycosyl group and different leaving groups at C-1.

Scheme6.1. Displacement of different leaving groups by fluoride [5-8].

The rate of reaction of (467), (468) and (469), having different leaving groups L, with sodium thiophenoxide has been measured (71JHC537). While the reactivity ratio fc(469)A(468) was always greater than unity, the ratio (467)A(468) depended on the leaving group. When L was p-nitrophenoxy the value was 0.5, whereas when L was Br it was 2.0. 

Grunwald-Winstein eqn (9) with OTs values derived from 2-adamantyl tosylate (Section 3, p. 36) and with iV0Ts values based on methyl tosylate (Section 5), Schadt et al. (1976) have correlated a variety of solvolyses of other tosylates. Whilst their work is more extensive in that Sn 2 type solvolyses were studied (Table 9), only tosylates have been examined and solvation effects caused by different leaving groups are thus minimized. However, the subtlety of solvation effects is emphasized by the dependence of the relative rates of p-bromobenzenesulphonates and tosylates on solvent electro-... 

Traditionally, relative stabilities of carbocations have been derived from the comparison of the rates of solvolysis reactions following the SN1 mechanism, for which the designation Dm + An has recently been proposed [36], The comparison of solvolytic rate constants for substrates of a large structural variety is hampered by the fact that the published solvolysis rates refer to different solvents, different temperatures, and precursors with different leaving groups. Dau-Schmidt has, therefore, converted solvolysis rates of a manifold of alkyl chlorides and bromides to standard conditions, i.e., soiv of RC1 in 100% EtOH at 25° C (Scheme 6) [37]. Although from a theoretical point of view, ethanol is not an ideal solvent for observing unassisted SN 1-type reactions (nucleophilic solvent participation), it has been selected as the reference solvent because most available experimental data have been collected in solvents of comparable nucleophilicity, a fact which made conversions to 100% ethanol relatively unproblematic [38],... 

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