Inductive, CC‐Hyperconjugative, and Frangomeric Effects in Solvolytic Fragmentation Reactions

Abstract

l-Azabicyclo[3.2.l]octyl endo-4-p-toluenesulfonate (la), 1azabicyclo[3.3.1 Inonyl endo-4-p-toluenesulfonate (2a), and the equatorial''] 4-bromo-I-azaadamantane (3a) undergo quantitative fragmentation in 80% ethanol to give the methy-~ (' 1 Prof.7n8 leneimmonium salts (7), (S), and (9), respectively[*! Moreover, they react some lo3, lo4, and lo5 times faster than the nitrogen-free parent compounds (4a), {5a), and (6a). This comparison demonstrates the pronounced frangomeric acceleration of the ionization of (la), (2a), and (3a)"l. The azabicyclics thus react by a concerted mechanism, as was to be expected from the antiperiplanar arrangement of the C(4)-R' bond and the electron pair at N(l) relative to the C(2)-C (3) bond131.On the other hand, the axial''] epimer ( l b ) in which this stereoelectronic condition is not fulfilled (cf. ref.[31) reacts slower than the parent compound (4b), giving the substitution and elimination products (lc), (Id), and (12), respectively, along with the fragmentation product (7). The fivefold slower reaction of (16) relative to (4b) and the formation of several products suggest the intermediacy of the cation (lo), whose formation is rendered more difficult by the electronwithdrawing inductive ( -0 effect of the nitrogen atom (SOH = solvent). Surprisingly, the likewise axial"] epimer (2b) reacts twice as fast, and (3b) even 1230-times as fast as (Sb) and (6a)I5], respectively, although the stereoelectronic conditions for a concerted fragmentation are again not fulfilled141. In the case of (2b) and (3b), fragmentation forming (8) and (9), respectively, is accompanied by substitution giving (2c) and (3c), and with (2b) by elimination giving the olefin (13); this indicates occurrence of the cationic intermediates (11) and (14), respectively. This assumption is supported by the observation that addition of ten mol. equiv. of sodium azide to an aqueous solution of (3b) hardly affects the reaction rate, while the yield of (9) falls from 75 to 58%. The question thus arises as to how the nitrogen atom accelerates the ionization of the axial bromide (3b) to form (14).