Abstract
The hydrogen isotope exchange rates of α-deuterio- and α-tritiocyclopropanes with electronwithdrawing substituents and the corresponding open 2-substituted propanes have been measured in solutions of sodium methoxide in methanol. Substituents fall in two classes: CF 3, CN and ∅SO 2 make the α-hydrogen in cyclopropanes more acidic than in the open chain analogues. On the other hand acyl groups (t-C 4H 9CO, iso-C 3H 7CO, C 6H 5CO) make the cyclopropanes less acidic than the corresponding 2-substituted propanes. An explanation based on inductive and mesomeric stabilisation of carbanions is given. A large isotope effect has been found for bulky isopropyl t-butyl ketone, suggesting rate limiting hydrogen abstraction by base. In most other cases (Table 2) isotope effects are small, presumably because anions remain strongly associated with methanol by hydrogen bonding (e.g. R − … DOCH 3) and consequently solvent exchange (with MeOH) becomes rate limiting for D/H (T/H) exchange.
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