No abstract

The rate constants (log k ) for solvolysis of 2-, 3-, and 4-substituted bicyclo[2.2.2]octyl p-nitrobenzenesulfonates 10, 11, and 12, respectively, correlate linearly with the corresponding inductive substituent constants u?. The formation ofthe ion pairs 9 is, therefore, controlled by the [effect of neighboring substituents. It follows from the corresponding reaction constants pI of -I .54, -1.12, and -1.22 that inductivity is highest at the positions a to C(1). It is lower and practically equal at the p-and y-positions. Therefore, charge dispersal is similar to that previously observed in the quinuclidinium ion 7.The observation that the solvolysis rates of many bi-and tricyclic arenesulfonates are controlled by the inductive ( I ) effect of neighboring substituents has led to a new method to gauge charge dispersal in carbocations [I].This method makes use of the Hammett-type equation log klk, = plop, where k and k, are the rate constants for the substituted and unsubstituted compounds, respectively. or are the inductive constants [2] for the substituents used as probes for charge dispersal, and pI are the derived reaction constants that gauge the inductivity of the C-atoms to which the substituents are attached.For instance, the pI values for the substituted C-atoms in the solvolysis of the 2-norbornyl, 2-bicycl0[2.2.2]octyl and 2-adamantyl p-toluenesulfonates (tosylates) 1, 2, and 3 are -2.00 [3], -1.50 [4], and -0.80 [5], respectively. Inductivity thus differs widely, although these tosylates possess the common partial structure 4'). It was, therefore, concluded that p I values reflect directional electron mobility, a property which is evidently subject to constraints imposed by the entire molecule [l]. In the solvolysis of substituted 2-ex0 -and 2-end0 -norbornyl tosylates 5 and 6, respectively, in 80 % (v/v) EtOH positive charge is mainly dispersed to C(6); but substantial charge is transmitted to other neighboring C-atoms [612). The same applies to the ')' )It was ascertained in each case that solvolysis proceeds by the unimolecular S,l mechanism, i.e. without appreciable nucleophilic solvent participation. The strong participation of the dorsal C(6) in the ionization of 5 and the lesser involvement of the dorsal C(7) in 6 are considered to be the reason for the high rxoiendo rate ratio for unsubstituted 5 and 6, respectively [l].

A comparison of the solvolysis rates of the substituted 2‐exo‐ and 2‐endo ‐norbornyl p‐toluenesulfonates 1, 2, 3 and 4 and the substituted 1‐ and 2‐adamantyl sulfonates 9 and 10, respectively, in 80% ethanol and 97% trifluoroethanol has shown that the sensitivity of rates to the I‐effect of substituents, i.e. the inductivity of these compounds, varies strongly with structure, configuration and solvent. In 97% trifluoro‐ethanol, a solvent of low nucleophilicity and high ionizing power, the inductivities of the 2‐endo‐norbornyl p‐toluenesulfonates 2 and 4 as well as the inductivities of the adamantyl derivatives 9 and 10 were larger than in 80% ethanol. In contrast, the inductivity of the 2‐exo‐norbornyl p‐toluenesulfonates 1 was practically unchanged. It was, therefore, concluded that the transition states for the former compounds are not, or only weakly, bridged, whereas the transition states for the 2‐exo‐norbornyl p‐tolu‐enesulfonates 1 involve graded bridging by C (6). These results confirm that, due to differential bridging strain, 2‐norbornyl cations are anisotropic to polar effects.

Correlation of the rate constants for solvolysis of (I)‐(III) in 80% ethanol with the inductive substituent constants leads to the reaction constants p = ‐ 1.54 (‐ 1.12, ‐ 1.22).

Die Darstellung der Adamantyltosylate (Ib) erfolgte aus den 1‐Brom‐adamantanen (Ia) mit Ag‐tosylat in Acetonitril (22‐80°C, 1‐96 h).