1,3-Dipolar Cycloadditions to Bicyclic Olefins. III. The Effects of Aromatic Solvents on the exo/endo Product Ratios for the 1,3-Dipolar Cycloadditions of Nitrile Oxides to Norbornadienes

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Abstract
When no hydrogen-bonded complex is formed between the precursors (α-chloro-α-hydroxyiminoacetophenone (5a) and α-chloro-α-hydroxyiminoacetone (5b)) of nitrile oxides (phenylglyoxylonitrile oxide (2a) and methylglyoxylonitrile oxide (2b)) and norbornadienes(2,3-bis(methoxycarbonyl)norbornadiene (1) and norbornadiene (6)) in solvents, the exo/endo product ratios for the cycloadditions of 2a and 2b to 1 or 6 increase or decrease with increasing temperature. When the hydrogen-bonded complex between 5a and 1 is formed in a solvent cage, however, the ratio keeps a constant value until the temperature is elevated up to a certain temperature, at which the ratio jumps up. After the jumping, the ratio has another constant value, regardless of the temperature. Similar results are obtained in the cycloaddition of 2b to 1 in the aromatic solvents used.