REACTIONS OF A TERTIARY CARBON CARRYING A TERT-BUTYL GROUP - ACID-CATALYZED ALCOHOLYSES OF ACTIVATED AZIRIDINES WITHOUT AND WITH SOLVENT ASSISTANCE

Acid-catalyzed ring opening of tosylated and acylated 2-tert-butylaziridines 1 (2-phenyl) and 2 (2-benzyl) generates carbenium ions 18 (from 1) and 19 (from 2) which rearrange by a neopentyl rearrangement to carbenium ions 20 (18 --> 20) and 21 (19 --> 21). Reactions of 18-21 farm the final products: (1) deprotonation yields allylamides, homoallylamides, and enamides; (2) external trapping of 18 or 19 by a solvent molecule yields ethers; (3) internal trapping (only acylated 1 and 2) yields oxazolines and dihydrooxazines. The amount of external trapping with 1a,b depends on the activation: 1a (tosyl activation) yields more methyl ether (75% vs 51%) than 1b (benzoyl), but 1a yields less isopropyl ether (0% vs 25%) than 1b. The latter finding is in accord with an intramolecular interaction of the carbenium center of 18a with one of the tosyl oxygen atoms provided that this interaction distorts the carbenium plane to a pyramid which sterically retards external trapping. The former finding is not observed with 2a and 2b. This points to at least some methanol-assisted ring opening of 1a-H+ that must be supported by a benzylic effect. The required conformation of the phenyl ring is fixed in 1a-H+ (N-protonation and bulky SO2 cis to phenyl) but not in 1b-H+ (O-protonation and planar nitrogen conformation).