1- Phenylethyl tosylates

A detailed and elegant study of the SnI solvolysis reactions of several substituted 1-phenylethyl tosylates in 50% aqueous TEE has enabled the rates of (1) separation of the carbocation-ion pair to the free carbocation, (2) internal return with the scrambling of oxygen isotopes in the leaving group, (3) racemization of the chiral substrate that formed the carbocation-ion pair, and (4) attack by solvent to be determined.122... 

Trifluoromethyl-l-phenylethyl tosylate has been used to differentiate as shown in Table 1, the solvolytic power of three fluorinated solvents and to compare these with formic and acetic acids The three fluorinated solvents are trifluoroacetic acid, trifluoroethanol, and 1,1,1,3,3,3-hexafluoroisopropyl alcohol [55]... 

Fig. 10 The Y-T plot for acetolysis of 2-phenylethyl tosylates [23] at 115°C r = 0.63. For interpretation of symbols, see Fig. 1. Reproduced with permission from Fujio et al. (1987b). Copyright 1987 Chemical Society of Japan.

The reaction scheme for aryl-assisted solvolysis in (13) is of course plausible except for the question of whether or not the second step is slow enough to be rate determining. However, in practice, this process should not be observed in the solvolysis of /3-phenylethyl tosylates, since the pre-equilibrium dissociation step into a primary carbocation cannot compete with the process (the Sn2 mechanism). 

Table 4.14. Extent of Aryl Rearrangement in 2-Phenylethyl Tosylate Solvolysis... 

Isotopically labeled carbons are scrambled to some extent during solvolysis of )8-phenylethyl tosylates. A bridged-ion intermediate or rapid rearrangement of a primary carbonium ion could account for the rearrangement. The extent of rearrangement increases as solvent nucleophilicity decreases. This increase is attributed... 

Both primary and secondary carbocations with )8-phenyl substituents usually give evidence of aryl participation. For example, isotopically labeled carbons are scrambled to some extent during solvolysis of j8-phenylethyl tosylates, A bridged-ion intermediate or rapidly reversible rearrangement of a primary carbocation could account for the randomization of the label. The extent of label scrambling increases as solvent nucleophilicity decreases. The data are shown in Table 5.19. This trend can be attributed to competition between Sn2 displacement by solvent and ionization with participation of the aryl group. While substitution in more nucleophilic solvents such as ethanol proceeds almost exclusively by direct displacement, the non-nucleophilic solvent trifluoroacetic acid leads to complete randomization of the label. 

Extent of aryl rearrangement in 2-phenylethyl tosylate solvolysis... 

The solitary exception in Table VIII, acetolysis of phenylethyl tosylate, is very probably not a clean SnI solvolysis, but belongs in the much explored but little known limbo between SnI and Sn2. 

These effects, determined with tertiary chlorides, unquestionably refer to 8n1 reactions so also do Lewis and Boozer s effects on acetolysis and formolysis—-if we can rely on the constancy of the effect of a-deuteration (Table VIII). The /3-effects then suggest that formolysis of the secondary bromide and solvolysis in aqueous ethanol of the tosylate are also SnI—or nearly so, unless bimolecular displacement reactions are also subject to similar isotope effects. Shiner (117) had however already shown that this was not so, since deuteration in the two methyl groups of isopropyl bromide did not lead to an experimentally significant effect on the displacement reaction with ethoxide ion in ethanol. It is thus also reasonable to interpret the very small effect (AAF" = 6 cal.) cited by Lewis (74f) for acetolysis of ethyl-2d8 brosylate as evidence that acetolysis of primary sulfonate esters is borderline if not Sn2. This conclusion, already suggested by the abnormally low a-effect for unassisted acetolysis of phenylethyl tosylate [Table VIII and text of Sec. VA, 2(b) 1 is supported by a similarly... 

Of the three acetolyses and three formolyses of /3-arylethyl tosylates investigated by Saunders and his collaborators (110,111) one, the acetolysis of /3-phenylethyl tosylate, has been classified as essentially bimolecular on the basis of both its a- and /9-effect. In the other five examples, the a-effects, listed in Table VIII, were shown to be quite normal for SnI solvolyses, and so give no indication that the reaction is assisted by S-phenyl migration. Such an indication is provided by the effect—or rather the lack of any effect within 2 or 3% —of /3-deuteration, also observed by Saunders and his co-workers. 

Saunders and co-workers (Amin et al., 1990) used E2 elimination reactions in the p-substituted 2-phenylethyl system to test the new criteria for tunnelling suggested by the above calculations. The actual substrates and base/solvent systems they used were (2-phenylethyl-2-f)-trimethylammonium bromide, [19], with sodium ethoxide in ethanol, 2-phenylethyl-2-f bromide, [20], with potassium t-butoxide in t-butyl alcohol and 2-(p-chlorophenyl)ethyl-2-f tosylate, [21], with potassium t-butoxide in t-butyl alcohol. When equation (57) was applied to the experimental secondary (kB/ S) KIEs in Table 39, the calculated /th h KIEs were 1.106 0.033 and 1.092 0.026 for [19] and [21],... 

Figure 2.5. Nucleophile selectivities determined from product analysis for the reactions of ring-suhstituted 1-phenylethyl derivatives (X-l-Y) with azide ion, acetate ion and methanol in 50 50 (v/v) water/trifluoroethanol. The selectivities are plotted against the appropriate Hammett substituent constant or a. Leaving group Y ( ) ring-suhstituted benzoates ( ) chloride (T) dimethyl sulfide (A) tosylate.

Probitm 16.95 Reaction of the (R)-tosyl ester of a-phenylethyl alcohol (C H,CHMeOTs) with CH,COOH yields a mixture of (S)- and rac-acetate. Explain. 

TOSYLAMIDO-2-PHENYLETHYL CHLOROMETHYL KETONE see THH450 6-(N-TOSYL)AMINOCAPROIC ACID DIAZOMETHYL KETONE see THH460... 

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