Electronic Control of Stereoselectivity. 11. Long-Range Modulation of Stereoselection in Diels-Alder Cycloadditions of N-Methyltriazolinedione to Aryl-Substituted 9-Butadienylidenebenzonorbornenes

Article abstract of DOI:10.1021/jo00341a017

Three 9-butadienylidenebenzonorbornenes (4a-c) have been synthesized and their ability to enter into Diels-Alder reaction examined.Although these substances are unreactive to a host of dienophiles, tetracyanoethylene enters into <2 + 2> addition with 4b and 4c.However, these reactions do not allow for examination of the stereoselectivity question.When recourse was made to the more reactive N-methyltriazolinedione reagent, <4 + 2> adducts 9 and 10 were produced in the following syn/anti ratios: 4a, 49:51; 4b, 24:76; 4c, 24:76.The chemical shifts of the N-methyl groups in the urazole segments of these adducts proved quite divergent and conducive to reliable structural assignment.The results are interpreted in terms of zwitterion intervention, with charge delocalization into the aromatic ring where this is feasible (the tetrafluoro example excluded).Because of this long-range homoaromatic stabilization, closure of the second C-N bond from the anti direction is kinetically favored.