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 pi-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.
Lim
Chultack
C
Department of Chemistry, College of Natural Sciences, Kyungpook National University, Taegu 702-701, South Korea.
Lee
Hee Soon
HS
Kwak
Young-Woo
YW
Choi
Cheol Ho
CH
D008956:Models, Chemical; D008962:Models, Theoretical; D017646:Organosilicon Compounds / Q000737:chemistry*; D011720:Pyrazoles / Q000737:chemistry*; D012821:Silanes / Q000737:chemistry*
C509548:3-trimethylsilyl-1-pyrazoline; D017646:Organosilicon Compounds; D011720:Pyrazoles; D012821:Silanes