1- Adamantyl chloride, solvolysis

Gajewski, J.J., Is the tert butyl chloride solvolysis the most misunderstood reaction in organic chemistry Evidence against nucleophihc solvent participation in the tert butyl chloride transition state and for increased hydrogen bond donation to the 1 adamantyl chloride solvolysis transition state, /. Am. Chem. Soc., 2001, 123(44), 10877-10883. 

That maximum acceleration occurs when the vacant p orbital is parallel to the plane of the cyclopropyl ring can be seen from the solvolysis of spiro[cyclo-propane-l,2 -adamantyl] chloride (71). The carbocation formed by departure of Cl is unable to adopt the geometry of the bisected cyclopropylcarbinyl cation, but can orient its empty p orbital properly to form the bicyclobutonium ion. This compound solvolyzes 103 times more slowly than 1-adamantyl chloride.82 On the other hand, 72 solvolyzes 10s times faster than 73. The cation from 72 does have its p orbital parallel to the plane of the ring as in the bisected cyclopropylcarbinyl cation.83... 

The method should work better for adamantyl chlorides or norbomyl chloride for which Sn2, and, presumably, preassociation processes are precluded. Speculative values of pATr1 and pATR for the relevant carbocations 54-56 are listed below based on a solvolysis rate constant in water for the... 

Having noted that two correlations (Figs. 10 and 11), which appeared at first sight to support fully the choice of t-butyl chloride as a model compound, cannot be completely explained, it is not surprising that rates of a solvolysis of 1-adamantyl chloride do not correlate well with Y as shown in Fig. 12 (Morten, 1975). Similar... 

Equation (50) correlates rate constants for a number of solvolysis reactions and Figure 13 illustrates the Grunwald-Winstein plot for the solvolysis of z-butyl chloride against a K value from 1-adamantyl chloride where w = 0.78. 

Construct a Grunwald-Winstein plot and comment briefly on the magnitude of the slope. The y, Ad parameter is determined from the solvolysis of 1 -adamantyl chloride. 

In this correlation the solvolysis of 1-adamantyl chloride is employed as the reference reaction (7,). The slope m is consistent with a transition state with about 1/3 carbenium ion character of that of the reference reaction (where full carbenium ion character is assumed to be expressed). 

Kevill and co-workers first address the much-debated issue of nucleophilic involvement in solvolysis of tert-butyl derivatives. Interestingly, the tert-butyl sulfonium salt shows more rate variation with solvent changes than does the 1-adamantyl salt. In particular, the tert-butyl salt shows a rate increase in aqueous TFEs (where both Y and N increase) that is not found for 1-adamantyl. Because a variation in Y cannot explain the result, Kevill argues that the tert-butyl derivative is receiving nucleophilic solvent assistance. On the basis of the available evidence, Harris et al. (Chapter 17) propose that tert-butyl chloride is inaccurately indicated by some probes to receive nucleophilic solvent assistance because the model system (1-adamantyl chloride) has a different susceptibility to solvent electrophilicity. Kevill and coworkers disagree with this proposal, noting that essentially the same tert-butyl to 1-adamantyl rate ratio is found for the chlorides and the sulfonium salts if solvent electrophilicity were important in one case but not the other, then the rate ratio should vary. 

Figure 2. Correlation of logarithms of solvolysis rates for 1-adamantyl chloride versus tert-butyl chloride at 25 °C (7). E represents ethanol, T represents trifluoroethanol, and HF represents hexafluoro-2-propanoL...

Because the solvolysis of adamantyl chloride has been used as a standard reaction, we list here the results of applying the conversion equations to this standard, using the equation given by Swain to represent the adamantyl rates. R of equation 5 was again set equal to 1 for acetic and formic acids. The yA value (equation 5) was found to be 0.04684, compared with 0.0627 when tert-butyl chloride solvolysis was the standard reaction for obtaining Y. 

There are five papers on carbamate chemistry of interest.6,84-87 The mechanism of the reaction in MeCN of /V-mcthyI-/V-phcnyIcarbamoyI chlorides (94) with benzyl-amines is believed to be Sf2 based on Hammett p values, a cross-interaction constant pxy of —0.14, ka/ko values for the /V-dcutcriatcd benzylamines all <1, and low activation enthalpies.84 The aminolysis of /7-nitrophcnyl /V-phenylcarbamatcs in acetonitrile involving the T1 (7) was discussed earlier.6 Solvolysis-decomposition of N- -adamantyl-/V-/7-tolylcarbamoyl chloride (95) in hydroxylic solvents involves a facile slow ionization (SWl mechanism) giving a cation which eliminates ArNCO to... 

Winstein (1957c). They argued that during solvolysis of neophyl substrates [46, X = Cl, Br] internal return from the proposed phenonium ion-pair intermediate [47] would yield the tertiary derivative [48] (Fig. 13) and this would solvolyse rapidly. Thus, the titrimetric rate constants should correspond to the ionization rate constant k j (Fig. 2). Later neophyl tosylate [46, X = OTs] and its p-methoxy derivative were used (Smith et al., 1961 Diaz et al., 1968b Yamataka et al., 1973), and recently Schadt et al. (1976) defined as a scale of solvent ionizing power for tosylates, designated F2-AdOTs or Yqt, using eqn (5) but based on 2-adamantyl tosylate [6] instead of t-butyl chloride. A correlation of the rates of solvolysis of neophyl tosylate with F0 Xs (Fig. 14) is satisfactory (correlation coefficient... 

The extent to which a solvent stabilises ions could be measured by comparing the equilibrium constant for a standard dissociation with the value for that in a standard solvent. The Grunwald-Winstein equation (Equation 48) uses as its standard the solvolysis of /-butyl chloride (Equation 49) and assumes that the transition state has almost complete carbenium ion character and that the formation of the ion-pair is rate determining. The rate constant would therefore measure the energy of formation of the free carbenium ion. Equation (48) defines the solvent parameter, K, where is the solvolysis rate constant of /-butyl chloride in the solvent (S) and the standard solvent (ss) is 80% EtOH/H2O. Other solvolyses can be used as standards and those of 1-adamantyl species has largely supplanted the original one for reasons given later. 

Grunwald-Winstein-type analyses (using ici) for the solvolysis of extremely crowded tertiary alkyl chlorides having a neopentyl or a (l-adamantyl)methyl group at the reaction centre show that they behave as fcc substrates. The solvolytic behaviour of various crowded alkyl derivatives and the origins of dispersions of data points of aqueous organic solvents in Grunwald-Winstein type correlations are discussed in detail. A possible role of hydrophobic solvation is tentatively raised. 

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