Free, Hydrogen-Bonded, and Cation-Stabilized Carbanions. Alpha. to a Cyano Group in a Cyclobutane Ring

The carbanion α to the cyano group of 3-alkoxycyclobutanecarbonitrile (3) was prepared by three different methods: (a) addition of MeO- in MeOH and i-PrO in i-PrOH to bicyclobutanecarbonitrile (2); (b) deprotonation of the cis and trans isomers of 3 in MeO--MeOH and in i-PrO-i-PrOH in the presence of crown ether; (c) deprotonation of 3 under ion-pairing conditions (i-PrONa-i-PrOH). The different product distribution (cis-trans ratio) obtained in each case indicates that each method yields a different type of carbanion. In method (a), a “free” carbanion whose inversion rate is faster than the reorganization of the surrounding solvent molecules is obtained. The observed cis-trans ratio of 3.5 reflects the relative ease of protonation of an equatorial position as compared to protonation of an axial one. Method (b) yields a hydrogen-bonded carbanion which exhibits some retention of configuration, while the paired sodium cation in method (c) induces equivalent amounts of retention and inversion. In the latter case, the inversion is accompanied by some isoinversion. A near-unity H/D kinetic isotope effect in the deprotonation reactions points to a preequilibrium formation of the carbanion with the subsequent step being rate limiting. The elimination of HC1 from 3-chlorocyclobutanecarbonitrile under non-ion-pairing conditions is intrepreted accordingly as a monomolecular elimination from the hydrogen-bonded conjugated base (ElcBhb).