Temperature and Para-Substituent Effects on the Face Selectivity of 1,3-Dipolar Cycloaddition Reactions of Benzonitrile Oxides with 5-Substituted Adamantane-2-thiones, N-Benzyladamantyl-2-imines, and 2-Methyleneadamantanes

The 1,3-dipolar cycloaddition reactions of para-substituted benzonitrile oxides (5-Y) with 5-fluoroadamantane-2-thione (2-F) and -2-methyleneadamantane (3-F) as well as with variously 5-substituted-N-benzyladamantyl-2-imines (4-X) were examined. They produce two geometrically isomeric Δ²-1,4,2-oxathiazolines (7-F,Y), Δ²-isoxazolines (8-F,Y), and Δ²-1,2,4-oxadiazolines (11-X,Y), respectively. The face selectivity in the latter reaction was found to be ∼1:1 regardless of the variations in 5-substituent and the temperature. For the former two reactions, the para-substituent was varied from electron-withdrawing (Y = F, Cl, Br, CN, or NO₂) to -releasing (Y = Me, or OMe). The face selectivity was measured in all cases. The differences Δρ for the reactions of 2- and 3-F with 5-Y were obtained from linear Hammett plots; they are +0.12 and 0.0, respectively. These low values and information from previous studies imply a concerted one-step mechanism with very little charge distribution differences in the transition states. These effects of temperature on the Z/E product ratios provide us, for the first time, with activation parameter differences between the syn- and anti transition states; their values are discussed. The product bias resulting from the favored attack of nitrile oxide on the zu face is discussed in terms of transition state hyperconjugation based on the experimental results and AM1 calculations.