Facile formation of azines from reactions of singlet ...

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Title Facile formation of azines from reactions of singlet methylene and dimethylcarbene with precursor diazirines: theoretical explorations
Author(s) G. V. Shustov, Michael T. H. Liu, K. N. Houk
Journal Canadian Journal of Chemistry / Revue Canadienne De Chimie
Date 1999
Volume 77
Issue 5-6
Start page 540
End page 549
Abstract The reactions of the singlet methylene (la) and dimethylcarbene (1b), with their diazirine precursors, diazirine (2a), and dimethyldiazirine (2b), have been studied theoretically using ab initio and density functional theory. The reaction has no activation barriers for the parent system (la + 2a) and proceeds via a reactive complex and a transition state with a small negative enthalpy of activation (Delta H-298(not equal) = -1.1 kcal mol(-1), Delta S-298(not equal) = -34.4 cal mol(-1) K-1 Delta G(298)(o) = 9.2 kcal mol(-1)) for the dimethyl derivatives (1b + 2b). The formation of N-methylene diazirinium ylides (3a,b) is exothermic by 64-80 kcal mol(-1). The isomer, 1,3-diazabicyclo[1.1.0]butane (4a), is more stable (5-12 kcal mol(-1)) than isomer 3a, but can neither be formed by direct thermal reaction of 1a with 2a nor undergo the direct rearrangement into formaldazine (5a). The rearrangement of ylides 3a,b into azines 5a,b proceeds by conrotatory C-3(-) N-1 ring opening. The predicted activation barrier of ca. 15 kcal mol(-1) for the ring opening in ylide 3b is in excellent agreement with experimental data. The formation of pyridinium ylides from carbenes and pyridine is also studied.

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