FULL PAPER * E-mail: chemistry_arshadi@yahoo.com; Tel.: 0098-152-6433775; Fax: 0098-152-6432231 Received December 17, 2010; revised February 25, 2011; accepted March 18, 2011. Chin. J. Chem. 2011, 29, 1347—1352 © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1347 New Insights on the Mechanism of Thermal Cleavage of Unsaturated Bicyclic Diaziridines: A DFT Study Arshadi, S.* ,a Bekhradnia, A. R. b Ahmadi, S. a Karami, A. R. c Pourbeyram, S. a a Department of Chemistry, Payame Noor University, Tehran, Iran b Pharmaceutical Sciences Research Center, Department of Medicinal Chemistry, Mazandaran University of Medical Sciences, Sari, Iran c Shahid Rajaee Teacher Training University, Tehran, Iran A DFT calculations are carried out at UB3LYP/6-311＋＋G (3df, 2p) levels of theory to study electrocyclic thermal cleavage of four (R) derivatives of unsaturated bicyclic diaziridines, 1 X-R , to produce corresponding (Z) and (E) azomethine imides (2 X-Z , 2 X-E , 3 X-Z and 3 X-E ), where X＝H, Me, t-Bu and Ph. Cleavage of 1 X-R series to form the most stable 3 X-Z product, (path 2) is found the favored procedure because of delocalized negative charge on five atoms and lower steric effect in related transition state. According to IRC calculations in paths 1 and 2, C 6 —N 1 bond is cleaved before the rate determinating step (transition state). The stability of unsaturated bicyclic diaziridines and their corresponding (Z) and (E) azomethine imides is in the following order in gas phase and chloroform, tetra- hydrofuran, and acetone solvents: 3 X-Z ＜3 X-E ＜2 X-Z ＜2 X-E ＜1 X-R ＜1 X-S . Keywords unsaturated bicyclic diaziridines, azomethine imides, electrocyclic thermal cleavage, conrotatory motion, molecular modeling, conformational analysis Introduction Diaziridines were first synthesized in 1959 by three groups of researchers who employed analogous meth- ods. 1-3 Throughout the first 20 years, the most funda- mental contribution to this chemistry was made by E. Schmitz. 4,5 Diaziridines are proved to be unique chem- ical objects as: (1) They possess nitrogen atoms which are configuration-stable under trivial conditions (inver- sion barriers 20—27 kcal/mol), and consequently, have been extensively applied to investigate stereochemistry of nitrogen. 6-8 (2) Diaziridine derivatives are also of interest as neurotropically active compounds. 9-11 (3) They have high formation enthalpy due to the hydrazine fragment and the three-membered strained cycle. This, along with their low toxicity, makes it become ideal candidate to replace hydrazine derivatives in rocket propellants. (4) Diaziridines are prone to ring expansion reactions with electrophilic reagents (ketenes, iso- cyanates, isothiocyanates, and acylating reagents). 12-16 In continuation of our research program dealing with molecular modeling, 17 thermal cleavage of unsaturated diaziridines was truly discussed and compared with pre- vious publication on their saturated counterparts. 18 The present work will mainly focus on two points mentioned in previous articles: 19 Synthesize of bicyclic allyl anion at room temperature by rearrangement of tri-cyclic an- ion, seemingly in a forbidden disrotatory process (Scheme 1) and the initial conrotatory motion which is proposed to operate in the ring opening of cyclopropyl anion. When the change in the sense of this rotation comes to pass; the system has already crossed the tran- sition state. Unlike bicyclic allyl anion, only one possi- ble product is expected. Hence, here, unsaturated di- aziridines are selected which give four possible products via four different paths. Results confirm that in both of the most preferred reaction paths (paths 1 and 2), C—N 1 bond cleavage occurs before reaching transition state (detrminating reaction rate). Scheme 1 Rearrangement of cyclopropyl anions fused to bi- cyclic structures The possibility of pericyclic reactions can be com- monly established based on the Woodward-Hoffmann rules. In thermally electrocyclic reactions, in which a ring is opened (or formed), systems with 4n＋2 and 4n number of electrons are predicted to undergo disrotatory and conrotatory movements, respectively. 19 Azomethine imides are the product of diaziridines cleavage and five memberd rings are formed in 1,3-di-