CHENG-TUNG LIN and WEN-JEI HSU. Can. J. Chern. 67, 2153 (1989). The regioselectivity of cycloaddition reactions of trichloronitrosomethane (1) with 2-alkyl (R = Me, Et, i-Pr, t-Bu, CF3, Bz, and C1) and 2-aryl (Ar = Ph, 4-CH30Ph, 4-CH3Ph, 3-CH3Ph, and 4-ClPh) 1,3-butadienes is described. The orientation of cycloaddition is substituent dependent, producing 2,5-disubstituted (para) and 2,4-disubstituted (meta) 3,6-dihydro-1,2-oxazines from 2-alkyl-and 2-aryl-l,3-butadienes respectively. Increasing the bulk of the substituent from methyl to tert-butyl in 2-alkyl-l,3-butadienes increases the regioselectivity. Kinetic studies in various solvents indicate that plots of log k m / k H vs. Hammett u + values give straight lines with the p values lying between -0.60 and -0.91 for the reactions with 2-aryl-1,3-butadienes. A concerted reaction mechanism is proposed and the orientation preference is consistent with frontier molecular orbital predictions for 2-alkyl-l,3-butadienes. In the reactions with 2-aryl-l,3-butadienes, a transition state leading to a spin-paired diradical, which then converts by partial electron transfer to zwitterionic structure, is proposed. de la cycloaddition depend du substituant; i partir des 2-alkyl-et des 2-aryl-buta-l,3-diknes, on obtient respectivement des 3,6-dihydro-l,2-oxazines disubstitukes en 2,5 (para) et en 2,4 (mtta). Avec les 2-alkyl-buta-l,3-dibnes, une augmentation du volume du substituant (en allant du mkthyle au tert-butyle) provoque une augmentation de la rkgiosklectivitt. Des ttudes cinktiques dans divers solvants suggkrent que les courbes du log k m / k H vs. les valeurs u + de Hammett donnent des droites avec des valeurs de p variant de -0,60 A -0,91 pour les rkactions des 2-aryl-buta-l,3-dibnes. Pour les 2-alkyl-buta-l,3-diknes, on suggkre un mtcanisme rkactionnel concert6 et la prtfkrence de I'orientation est en accord avec les prkdictions des orbitales molkculaires frontikres. Dans les reactions des 2-aryl-buta-l,3-dibnes, on propose une ktat de transition conduisant A un diradical dont les spins sont pairks qui se transforme ensuite en une structure zwitterionique par un transfert partiel d'klectrons.
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