Solvolysis in Carboxamides. II. Solvolytic Elimination of 7β-Methylbicyclo[3.3.1]non-3β-yl Tosylate in Carboxamides

Abstract

Abstract Solvolysis of 7β-methylbicyclo[3.3.1]non-3β-yl tosylate (1) was carried out in a series of carboxamides, N-methylformamide (NMF), N-methylacetamide (NMA), N,N-dimethylformamide (DMF), and N,N-dimethyl-acetamide (DMA), as solvents. The rate (k1 first-order rate constant) of 1 decreases in the order NMF>NMA>DMF>DMA, a linear free-energy relationship being found between the rates of 1 and those of 1-adamantyl tosylate in acetic acid and 80% ethanol as well as the carboxamides. The products consist of olefin (≥96%) (exo-7-methylbicyclo[3.3.1]non-2-ene (4) and 3-methylbicyclo[3.3.1]non-2-ene (5)) and small amounts of esters (acetates or formates, ≤4%). The content of rearranged olefin 5 is considerably less than that of acetolysis (55%) and ethanolysis (55%), decreasing in the order NMF (34%)>NMA(22%)>DMF(13%)>DMA(9%). A linear reactivity-selectivity relationship holds between log k1 and log (%5/%4) for the four carboxamide solvents. The observed kinetic isotope effect (kH/kD=1.01±0.03) for 7α-deuterium-tagged derivative (1-7α-d) in NMF indicates that the σ(C(7)–H)-assisted ionization process for the solvolysis of 1 in the amides is absent. The results are in line with the stepwise mechanism which involves the formation of an intimate ion-pair intermediate followed by competitive processes such as 1,5-hydride shift, syn-El reaction, and nucleophilic substitution by the amide molecules. Substitution by the amide molecule leads to the formation of a cationic imidate, the hydrolysis of which affords the carboxylic esters. It is proposed that the syn-El process via an intimate ion-pair intermediate is affected by the tosylate anion reactivity in each amide solvent and suppressed by the hydrogen bonding of the each amide with the tosylate anion.