Carbocations, 1-phenylethyl

Stabilization of a carbocation intermediate by benzylic conjugation, as in the 1-phenylethyl system shown in entry 8, leads to substitution with diminished stereosped-ficity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. The system has been analyzed in terms of the fate of the intimate ion-pair and solvent-separated ion-pair intermediates. From this analysis, it has been estimated that for every 100 molecules of 1-phenylethyl chloride that undergo ionization to an intimate ion pair (in trifluoroethanol), 80 return to starting material of retained configuration, 7 return to inverted starting material, and 13 go on to the solvent-separated ion pair. 

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-... 

Table 2 Rate constants, equilibrium constants, and estimated Marcus intrinsic barriers for the formation and reaction of ring-substituted l-phenylethyl carbocations X-[6+] (Scheme 8)°... 

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. 

In retrospect, it should have been clear to me - as I am sure it was to Bill Jencks -that the rate and equilibrium constants for addition of solvent to 1-phenylethyl carbocation intermediates of solvolysis of 1-phenylethyl derivatives would serve as the first step in the characterization of the dynamics of the reactions of their ion pair intermediates. Therefore, this earlier work has served as a point of departure for our experiments to determine relative and absolute barriers to the reactions of ion pair intermediates of solvolysis. 

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. 

The carbocation intermediate must exist in water long enough to allow for racemization of the carbocation-anion or ion dipole pair to occur, so that k < 1 X 10 s for addition of solvent (Scheme 15). In the case of solvolysis of (R)-l-phenylethanol in H2 0, which proceeds by a stepwise mechanism through a 1-phenylethyl carbocation intermediate that is captured by water... 

The 13 C NMR chemical shifts of the para carbon for various phenyl-substituted sp2-and sp-hybridized carbocations (Figure 15) indicate that the demand for rr-aryl delocalization of the positive charge for the -silyl-substituted vinyl cation 393 is lower compared with that in a-phenylethyl (78), a-methyl-a-phenylpropyl (397) and cumyl (292) cations150 153. The stabilizing effect of the /)-silyl group in 393 is comparable to that of the cyclopropyl substituent in 1-cyclopropylbenzyl cation (398). The para carbon shift in the /)-cr-silyl... 

Cram s original studies287 established, based on kinetic and stereochemical evidence, the bridged ion nature of (3-phenylethyl cations in solvolytic systems. Spectroscopic studies (particularly1H and 13C NMR)288-291 of a series of stable long-lived ions proved the symmetrically bridged structure and at the same time showed that these ions do not contain a pentacoordinate carbocation center (thus are not nonclassical ions ). They are spiro[2.5]octadienyl cations 111 (spirocyclopropylbenzenium ions)—in... 

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... 

First-Order Reactions First-order nucleophilic substitution requires ionization of the halide to give a carbocation. In the case of a benzylic halide, the carbocation is resonance-stabilized. For example, the 1-phenylethyl cation (2°) is about as stable as a 3° alkyl cation. 

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... 

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). 

Controlled polymerization requires that the initiation rate is at least comparable to that of propagation. Initiation in controlled/living carbocationic systems is usually carried out using models of growing species in their dormant state (e.g., the adducts of a monomer with protonic acids). This enables a similar set of equilibria to be established between carbocations and dormant species for initiation and for propagation. For example, 1-phenylethyl halides have similar reactivity as the macromolecular dormant species in styrene polymerizations, and I-alkoxyethyl derivatives are as reactive as the macromolecular species in the polymerization of vinyl ethers [Eq. (38)] ... 

Photoheterolysis of benzylic chlorides [204] yielded results signifying that simple benzyl cations, such as cumyl and 1-phenylethyl cations, can exist in the solution as free ions radicals arising from a competing photohomolysis are also observed frequently. Haloalkyl-carbocations are studied by heterolysis of the corresponding dihalides in super acid media [205]. NMR chemical shifts are interpreted as evidence for an interaction between the vacant orbital of cationic center of the haloalkyl carbocations with a lone electron pair of the halogen atom. 3-chloro-l-methylcyclopentyl cation 73, thermally eliminates hydrogen chloride and yields l-methyl-2-cyclopentyl cation 74, a similar behavior reported for y-chloroalkyl carbocations [206] (Scheme 5). 

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