Substituted 1-phenylethyl

An example with the characteristics of the coupled displacement is the reaction of azide ion with substituted 1-phenylethyl chlorides. Although the reaction exhibits second-order kinetics, it has a substantially negative p value, indicative of an electron deficiency at the transition state. The physical description of this type of activated complex is the exploded S 2 transition state. 

Tertiary aliphatic carbocations 85 Ring-substituted 1-phenylethyl carbocations 86 Cumyl and di-orf/io-methylcumyl carbocations 91 a,a-Diphenyl carbocations 95 Oxocarbenium ions 95... 

The rate and equilibrium constants for the reactions of ring-substituted 1-phenylethyl carbocations (X-[6+]) in 50/50 (v/v) trifluoroethanol/water (Table 2 and Scheme 8),13 14 17 43, and for interconversion of ring-substituted 1-phenyl-... 

Fig. 5 Logarithmic plots of rate-equilibrium data for the formation and reaction of ring-substituted 1-phenylethyl carbocations X-[6+] in 50/50 (v/v) trifluoroethanol/water at 25°C (data from Table 2). Correlation of first-order rate constants hoh for the addition of water to X-[6+] (Y) and second-order rate constants ( h)so1v for the microscopic reverse specific-acid-catalyzed cleavage of X-[6]-OH to form X-[6+] ( ) with the equilibrium constants KR for nucleophilic addition of water to X-[6+]. Correlation of first-order rate constants kp for deprotonation of X-[6+] ( ) and second-order rate constants ( hW for the microscopic reverse protonation of X-[7] by hydronium ion ( ) with the equilibrium constants Xaik for deprotonation of X-[6+]. The points at which equal rate constants are observed for reaction in the forward and reverse directions (log ATeq = 0) are indicated by arrows.

Table 5. Rate and equilibrium constants for the formation and reaction of cyclic benzylic carbocations [18 + ] and [20+ ] and analogous ring-substituted 1-phenylethyl carbocations (Scheme 15)°... 

The partitioning of simple tertiary carbocations, ring-substituted 1-phenylethyl carbocations, and cumyl carbocations between deprotonation and nucleophilic addition of solvent strongly favors formation of the solvent adduct. The more favorable partitioning of these carbocations to form the solvent adduct is due, in part, to the greater thermodynamic stability of the solvent... 

The intrinsic barrier for the addition of solvent to an a-alkoxy benzyl carbocation is several kcal mol-1 smaller than that for the corresponding reaction of ring-substituted 1-phenylethyl carbocations. This result is consistent with the conclusion that these nucleophile addition reactions become intrinsically easier as stabilizing resonance electron donation from an a-phenyl group to the cationic center is replaced by electron donation from an a-alkoxy group. 

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... 

C KIE in the base-promoted elimination reactions (equation 257) of 2,2-diaryl-1,1,1-trichloroethanes (388), substituted 2-phenylethyltrimethylammonium bromides (389), substituted 2-phenylethyl chlorides (390) and substituted 1-phenylethyl chlorides (391), labelled successively at the a and the p carbons, have been measured 70 and it was concluded that these reactions proceed according to the E2 mechanism with transition states having varying degrees of Elcb or El character. 

Dixon and Schuster (1979, 1981) have investigated both the thermal and electron-donor induced reactions of 1-phenylethyl peroxyacetate [28] and a series of substituted 1-phenylethyl peroxybenzoates [29a-29e]. They report the direct generation of electronically excited states from unimolecular thermo-lyses, as well as generation of light by the chemically initiated electron-exchange luminescence mechanism. 

Fig. 35 The Y-T plots for the reactions of ring substituted 1-phenylethyl chlorides in 20% acetonitrile in water at ionic strength 0.8 (NaC104) at 25°C r = 1.15. Reproduced with permission from Richard et al. (1984b). Copyright 1984 American Chemical...

Fig. 37 More O Ferrall-Jencks diagram for the Menschutkin reactions of 1-phenylethy] and benzyl chlorides with pyridine. The structures of transition states were optimized by ab initio MO calculation (RHF/b-Sf G ). O, substituted 1-phenylethyl chlorides with pyridine , benzyl chlorides with pyrindine , with 4-nitropyridine O, methyl and A, ethyl chlorides with pyridine (Fujio et al, unpublished).

Further studies on the pyrolysis of chlorinated and brominated hydrocarbons have been reported by Maccoll et al. for 3-bromopentane , menthyl and neo-menthyl chlorides , r-alkyl chlorides , dimethylallyl chlorides , a-chloro-o-xylene , and substituted 1-phenylethyl chlorides . Other workers have reported on the thermal reactions of ethyl chloride , monochloropentanes , 1-... 

The displacement reaction at the substituted 1-phenylethyl centre has a substantial p y value of -0.55 which indicates strong interaction between leaving group and nucleophile as in a four-centre transition structure (Scheme 2). The additional methyl group would assist the mechanistic change from in-line to front-side owing to its stabilisation effect on an incipient carbenium ion. 

Figure 2. Estimated rate constants for reactions of nucleophiles with substituted 1-phenylethyl carbocations, plotted against the effective Hammett constant of the ring substituent with r+ = 2.1 (+) trifluoroethanol (A) methanol acetate anion ( ) trifluoroethoxide anion propanethiol (o) azide. (Reproduced from reference 17.

That C -X heterolysis is less developed in the transition state for thermolysis of esters than alkyl halides is supported by recent studies on substituted 1-phenylethyl chlorides . At 608°K a Hammett reaction constant of—1.36 was observed, this being much smaller than that of—4.95 at 318°K observed for the solvolysis of the same substrates in 80% aqueous acetone. Although part of the difference between these latter two values is attributable to the difference in reaction temperature, the results suggest that C -X bond breaking is less developed in pyrolyses than in solvolytic reactions. 

The reaction scheme for the deracemization of (substituted) 1-phenylethyl amine derivatives by the combination of a monoamine oxidase (MAO) with an co-TA ATA (amino transferase) denotes the commercially available co-TAs from Codexis. 

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