Solvent effects adamantyl

For the ABBB monomer, two diastereoisomeric chiral capsules were possible, with a diastereoisomeric ratio that varies from 6 1 in cyclohexane-di2 to 1 1 in chloroform-d, with a strong solvent effect the latter was also the guest and therefore influenced the stability of the capsules both from the inside and outside of the capsule. The ABAB monomer gave rise to only one chiral racemic capsule, while for AABB two regioisomeric structures were possible, with a relative amount from 1 1 to 2 1 increasing the steric difference between A and B from n-hexyl to adamantyl residues. Moreover, the equilibrium between the regioisomeric forms could be shifted entirely on one side if two adjacent A and two B groups are covalently connected. 

Separation of Nucleophilic and Electrophilic Contributions to Solvation Effects. The discussion on the appropriate choice of m value for the N0Ts scale (equation 7) is part of a more general problem of dissecting the nucleophilic contributions (corresponding to the IN term in equation 5) and electrophilic contributions (included in the mY term in equation 5) to solvent effects. When a substrate reacts by a nucleophilically solvent assisted pathway, m decreases and l increases, often in a uniform manner (equation 12). Schadt et al. proposed (3) that an increase in nucleophilic assistance (increase in l) caused delocalization of positive charge, which led to a decrease in m. All of the deviations from the rates expected for SN1 (kc) reactivity were attributed to nucleophilic solvent assistance (3) Schadt et al. (3) assumed that m values for kc processes would be the same as for 2-adamantyl (II), and more recent data for 1-adamantyl (I), 1-adamantylmethylcarbinyl, and 1-bicyclo[2.2.2]octyl tosylates support this assumption (4, 53). 

Solvent Effects on Competing Nucleophilically Solvent Assisted (ks) and Anchimerically Assisted (JtA) Processes. Schleyer et al.s work (8) on solvolyses of 2-adamantyl (II) arose from earlier studies (71) of the competition between ks and fcA processes in (3-arylalkyl systems (XII and XIII). Apparently, no crossover occurred between the two processes, because rate-product correlations were observed. Hence, any cationic intermediates in the ks process must be sufficiently strongly solvated to prevent attack by... 

A mechanistic model involving nucleophilic assistance, but not taking into account the variable electrophilic assistance in different solvents, has been proposed (54, 55) for the solvolysis of tert-butyl halides. The analysis was based on a comparison of solvent effects on the solvolysis rates of tert-butyl and adamantyl substrates. The solvent properties were analyzed in terms of parameters N and Y the electrophilic assistance was incorporated into Y (54, 56). Such an approximation had been acceptable in the original wor4c (14-16), which dealt mostly with aqueous alcohols as solvents. This approximation is no longer permissible when materials like TFA and fluori-nated alcohols are used as solvents. In fact, Fainberg and Winstein (56) pointed out that different solvent mixtures could not be placed on the same correlation line. 

The acid-catalysed hydrolysis of three iV-phenylalkanesulfinamides (49, R = i-Pr, t-Bu, 1-adamantyl) in aqueous mineral acids (HCl, HBr, H2SO4, HCIO4) was found to proceed via a slow spontaneous (uncatalysed) pathway, an A2 acid-catalysis pathway, and an acid-dependent nucleophilic catalysis pathway, the last of which predominates in hydrobromic and hydrochloric acid solutions (Scheme 16a). A mechanistic switchover from A2 to A1 (Scheme 16b) was detected for the isopropyl and t-butyl compounds in concentrated sulfuric acid. Order of catalytic activity, effect of added salts, Arrhenius parameters, kinetic solvent isotope, and solvent effects were all consistent with the proposed mechanisms. ... 

The reason the adamantyl system is much more sensitive to the substitutions of CH3 for H is that its cage structure prevents solvent participation whereas the i-propyl system has much stronger solvent participation. The internal stabilizing effect of the methyl substituent is therefore more important in the adamantyl system. 

Fig. 9 Comparison of polar and steric effects of alkyl groups on bromination rates of linear ( ), branched (O) and adamantyl (A) alkenes in acetic acid and in methanol (Ruasse and Zhang, 1984 Ruasse et al., 1990). Polar effects are identical in both solvents [full line, eq. (24)], but steric effects differ. Deviations of branched alkenes are attributed to steric inhibition of nucleophilic solvation by methanol.

Catalyst 329, prepared from trimethylaluminum and 3,3/-bis(triphenylsily 1)-1,1 /-bi-2-naphthol, allowed the preparation of the endo cycloadduct (2S )-327 with 67% ee. The use of non-polar solvents raised the ee, but lowered the chemical yield213. Recently, it was reported that the reaction to form 327 exhibited autoinduction when mediated by catalyst 326214. This was attributed to a co-operative interaction of the cycloadduct with the catalyst, generating a more selective catalytic species. A wide variety of carbonyl ligands were tested for their co-operative effect on enantioselectivity. Sterically crowded aldehydes such as pivaldehyde provided the best results. Surprisingly, 1,3-dicarbonyl compounds were even more effective than monocarbonyl compounds. The asymmetric induction increased from 82 to 92% ee when di(l-adamantyl)-2,2-dimethylmalonate was added while at the same time the reaction temperature was allowed to increase by 80 °C, from -80 °C to 0°C. 

The development of these various solvent parameters and scales has been accompanied by the realization that there are uncertainties in the physical property of the solvent that is correlated by a particular parameter in cases where systematic changes in solvent structure affect several solvent properties. Consider a reaction that shows no rate dependence on the basicity of hydroxylic solvents, and a second reaction that proceeds through a transition state in which there is a small transition state stabilization from a nucleophilic interaction with the hydroxyl group. The rate constants for the latter reaction will increase more sharply with changing solvent nucleophilicity than those for the former, and they should show a correlation with some solvent nucleophilicity parameter. This trend was observed in a comparison of the effects of solvent on the rate constants for solvolysis of 1-adamantyl and ferf-butyl halides, and is consistent with a greater stabilization of the transition state for reaction of the latter by interaction with nucleophilic solvents. ... 

The limiting nature of the solvolysis reactions of 2-adamantyl solvolyses apparently arises as a result of steric inhibition of rearside nucleophilic solvent participation by the axial hydrogens shown in 95. Such steric effects are absent... 

Grunwald-Winstein eqn (9) with OTs values derived from 2-adamantyl tosylate (Section 3, p. 36) and with iV0Ts values based on methyl tosylate (Section 5), Schadt et al. (1976) have correlated a variety of solvolyses of other tosylates. Whilst their work is more extensive in that Sn 2 type solvolyses were studied (Table 9), only tosylates have been examined and solvation effects caused by different leaving groups are thus minimized. However, the subtlety of solvation effects is emphasized by the dependence of the relative rates of p-bromobenzenesulphonates and tosylates on solvent electro-... 

Alkyl radicals are readily generated from azoalkanes by photochem-ically induced elimination of nitrogen. 1-Adamantyl radicals can be obtained in this way by photolysis of azo-1,1 -adamantane in acetonitrile the products are derived principally by hydrogen abstraction from the solvent although radical addition to the nitrile group is also important. The effect of solvent viscosity... 

Allard, B., Casadevall, A., Casadevall, E. and Largeau, C. (1980) The acid-enhancing effect of HFIP as solvent. Carbocation rearrangemebt during solvolysis of 2-adamantyl tosylate in anhydrous HFIP. Nouv. J. Chim., 4, 539-545. 

---