1-Phenylethyl derivatives , nucleophilic

We have examined the competing isomerization and solvolysis reactions of 1-4-(methylphenyl)ethyl pentafluorobenzoate with two goals in mind (1) We wanted to use the increased sensitivity of modern analytical methods to extend oxygen-18 scrambling studies to mostly aqueous solutions, where we have obtained extensive data for nucleophilic substitution reactions of 1-phenylethyl derivatives. (2) We were interested in comparing the first-order rate constant for internal return of a carbocation-carboxylate anion pair with the corresponding second-order rate constant for the bimolecular combination of the same carbocation with a carboxylate anion, in order to examine the effect of aqueous solvation of free carboxylate anions on their reactivity toward addition to carbocations. 

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.

We have assumed that the values of Kas for formation weak encounter complexes between nucleophile and substrate, and between nucleophile and carbocation are similar. This is supported by the observation of similar values of Kas [see above] for formation of encounter complexes between neutral substrate and anionic nucleophile (0.7 M-1),2 between cationic substrate and anionic nucleophile (0.2 M 1),27 and between neutral substrate and neutral nucleophile (0.3 M-1).20 We use the value of k-d= 1.6 X 10los-1 that can be calculated from Xas = 0.3 M-1 formation of encounter complexes with 1-phenylethyl derivatives and kA = 5 X 109 M-1 s-1 (equation (2)).20 The uncertainty in this value for fc d is approximately equal to the range of experimental values for Xas (0.2-0.7 m 1) 2-20-27... 

For the recombination of a-phenylethyl cations with strong nucleophiles, Richard and Jencks (1984a,b,c) obtained good Y-T correlations with the same r value of 1.15 as observed for the solvolysis (Fig. 35). The p value of -2.7 for the bimolecular substitution reaction of azide ion with 1-phenylethyl derivatives is significantly more positive than the value of p = -5.7 for the solvolysis reaction. This shows that there is a smaller development of positive charge in the transition state for the reaction of azide ion than for solvolysis. It is consistent with a coupled concerted reaction with a transition state in which positive charge development at the benzylic carbon is neutralized by bonding to azide ion. 

Studies of the solvolysis of 1-phenylethyl chloride and its p-substituted derivatives in aqueous trifluorethanol containing azide anion as a potential nucleophile provide details relative to the mechanism of nucleophilic substitution in this system. 

The first method, resolution, is unattractive unless both enantiomers are useful in synthesis. In some cases, such as the resolution of dienecarboxylic acid derivatives mentioned earlier (via the phenylethyl-ammonium salt), the resolution is efficient and provides optically pure materials in good yield.39 60,63 In certain cases, the dienyliron complex can be treated with a chiral nucleophile to give a mixture of dia-stereomers which are separated and then reconverted to enantiomerically pure dienyl complex.64 An example of this method is the resolution of complex (27 Scheme 33), via the menthyl ethers (195) and... 

The presence of an aryl group at the /7-carbon atom of an alkyl halide influences both its photochemical behaviour and its photoreactivity (for reviews, see Section VII of Reference 11 and Reference 12). The irradiation of 2-phenylethyl bromide and iodide (40) in nucleophilic solvents such as methanol (equation 44)64 yields simpler and different product mixtures than those of 1-bromo- and 1-iodooctane, described in equation 325 in Section II. A. The only ion-derived product is 41, while for the 1-halooctanes the main ionic... 

In the 1-phenylethyl system, 5 nI and 5 n2 pathways could occur in the same solution concerted reactions occurred when the carbenium ion did not have a meaningful lifetime in the presence of a nucleophile. This finding had important applications to the reactions of glycosyl derivatives in water, where the reality or otherwise of glycosyl cation intermediates is now known to depend on the leaving group. 

The choice of monomers is limited not only by the order of nucleophilicity. The first block should carry a living end capable to initiate the polymerization of the second monomer by addition. Up to now, only a few systems are known to meet these requirements. Thus, in the presence of stable counterions, living polymerization of DXL 109), DXP 109), XHF 110,111,112), N-t-butylaziridine 113), 1,3,3-trimethylazeti-dine 1W), conidine 115) and l-(2-phenylethyl)-2-methylaziridine 116) may proceed. The last four monomers are highly nucleophilic cyclic amines and there is not too much chance that the active species derived from this class of monomers would initiate polymerization of other, less nucleophilic monomers. Thus, attempts directed toward the preparation of block copolymers of cyclic ethers or acetals were employed... 

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