A theoretical study of thermal [1,3]‐sigmatropic rearrangements of 3‐trimethylsilyl‐1‐pyrazoline: Concerted vs. stepwise mechanisms

Possible reaction mechanisms of 1,3‐silyl and 1,3‐hydrogen thermal rearrangements of trimethylsilyl‐1‐pyrazoline and its model systems were theoretically explored using B3LYP, MP2, CR‐CCSD(T), CASSCF(6,5), and MRMP2(6,5) theories. Nitrogen substitution at the center position of allylic moiety turned out to have a special stabilizing effect on diradical intermediates, allowing a stepwise pathway. This substitutional effect was attributed to the nitrogen lone pair electrons, which form strong π‐conjugations with diradicals. The second nitrogen substitution at the terminal allylic position selectively reduces the reaction barrier of antarafacial retention pathway, creating a competition between concerted and stepwise channels. The introduction of a five‐membered ring imposes ring strain on the allylic moiety and increases steric hindrance, allowing no antarafacial channels. The combined effect of the nitrogen substitution and the five‐membered ring further removes the possibility of concerted reaction pathways. Therefore 1,3‐silyl migrations of 3‐trimethylsilyl‐1‐pyrazoline were found to occur only through stepwise mechanisms, implying that the Woodward–Hoffmann rule is not operative. The 1,3‐hydrogen migration also occur via a stepwise mechanism; however, it would not occur easily because its reaction barrier is much higher than that of 1,3‐silyl migrations. Current study shows that a stepwise mechanism can be the dominant reaction pathway of some particular [1,3]‐sigmatropic rearrangements. © 2005 Wiley Periodicals, Inc. J Comput Chem 27: 228–237, 2006

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