ISSN:
0947-3440
Keywords:
Aziridines, 2-imino-, chiral, non-racemic
;
Amidines, N,N′-dialkyl-2-halo-
;
1,3-Dehydrohalogenation, regio-selectivity and diastereoselectivity of
;
E/Z Diastereomerisation
;
[2 + 1] Cycloreversion to isocyanides and imines
;
Valence isomerisation
;
Calculations, high-level ab initio, RHF, UHF, MPn, CCSD(T)
;
2H-Azirine, 3-amino
;
Diaziridine, 3-methylene
;
Diazatrimethylenemethanes, diastereomeric, closed-shell and open-shell
;
Reorganisations, transition states of thermal
;
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
2-Iminoaziridine 19 and the pairs of isomers 22/23 and 32/33 can be obtained in high yields by base-induced 1,3-dehydro-halogenation of the corresponding α-halo amidines. Regio-selectivity of the ring closure reaction of 21 is achieved by treatment with potassium hydride as base in the presence of 18-crown-6 to afford almost exclusively 23. At low temperatures, the cyclisation of 21a under the influence of potassium tert-butoxide occurs diastereoselectively yielding (Z)-22 and (Z)-23 (87:13). Resolution of racemic 21a can be attained by single recrystallisation of the nicely crystallised mandelates of the like configuration, thus providing an efficient route to (R)- and (S)-23 of high enantiomeric purity. Chiroptical data are reported for 21a and 23. - (E)- and (Z)-22 equilibrate faster by more than one order of magnitude than (E)- and (Z)-23 but almost the same preference for the E diastereomer is found in both cases. - Only first-order decomposition into isocyanides 6 and imines is observed on thermolysis of 19, 22, 23, and 32 with activation parameters depending on the substitution pattern. In contrast, the thermal valence isomerisation 35 → 36 is fast enough to compete with the [2 + 1] cycloreversion of 35. Thermal racemisation of (R)-23 does not occur.Quantum-chemical calculations were performed on the parent iminoaziridines (E)- and (Z)-38, 3-amino-2H-azirine (39), the methylenediaziridines cis- and trans-40, and diastereomeric closed-shell and open-shell planar (41) and (open-shell) „mono-orthogonal“ diazatrimethylenemethanes 42, 43. Complete geometry optimisations were appropriately performed with the RHF/6-31 + G** and the UHF/6-31 + G** basis sets. Energies of the closed-shell states were calculated on the RHF, MP2, MP4SDTQ, and CCSD(T) levels, those of open-shell states on the UHF, UMP2, UMP4SDTQ, and CCSD(T)/UHF levels. - (E)-38, being lowest in energy of the parent iminoaziridines (Erel = 0.0), equilibrates with (Z)-38 via an almost linear transition state (Erel = 113.4 kJ mol-1) and decomposes into hydrogen isocyanide and formaldimine in a one-step cheletropic process [Erel = 179.8 kJ mol-1, CCSD(T)/RHF] with a highly unsymmetrical transition state in which the N1-C2 bond is almost completely broken whereas the C2-C3 bond is still strong. The same is true for the cheletropic decomposition of (Z)-38 the transition state of which (Erel = 168.2 kJ mol-1) is even lower by 11.6 kJ mol-1. - Energy-rich zwitterionic transition states are found with the RHF method among which 1A′-(E,Z)-41 is lowest in energy [Erel = 203.0 kJ mol-1, CCSDT(T)]. Planar singlet (= transition states of CN-bond rotation) and triplet diazatri-methylenemethane diradicals possess energies in the range of Erel = 140-160 and 100-120 kJ mol-1 [CCSD(T)], respectively. Complete UHF optimisation of the singlet structures without symmetry constraint yielded five minima 42, 43 of mono-orthogonal geometry with similar or slightly lower energies than the planar UHF singlets. In the transition state of the valence isomerisation of (E)-38 [Erel = 170 kJmol-1, CCSD(T)/UHF], the N1-C3 bond is elongated while the plane of the methylene group is still orthogonal to the C(2) = NH plane. An activation energy of (185 ± 20 kJmol-1) is estimated for ring opening of 38 involving species with high diradical character.
Additional Material:
6 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/jlac.1995199512303
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