ChemInform Abstract: CONFORMATIONS OF FOUR‐MEMBERED RING HYDRAZINES AND HYDRAZINE RADICAL CATIONS

Abstract

Photoelectron spectra of 1,2-dialkyl- 1,2-diazetidines show that the 1,2-dimethyl compound (1) exists overwhelming- ly in the diequatorial trans conformation, while a significant amount of the diaxial trans conformation is detected for the diisopropyl compound (2), and the tricyclic structure holds the diazetidine ring in nearly eclipsed geometry for 3,4-dimethyl- 3,4-diazatricyclo(4.2.1 .02~5)non-7-ene (4) and its saturated analogue (3). Although N-dimethylaminoazetidine (5), N-dipro- pylaminoazetidine (6), and biazetidine (8) are predominantly in gauche conformations, the anti conformation predominates for N-piperidinylazetidine (7). The rates of double nitrogen inversion for 3 and 4 are nearly identical and increase slightly with increasing solvent polarity, and AG*298 is about 3.8 kcal/mol lower than for 1. The ESR splitting constants for the cations from 1,2, and 8 are reported, and compared to those for other tetraalkylhydrazine radical cations. Although many techniques have been used to study the conformations of tetraalkylhydrazines,2 two of the most useful are NMR and photoelectron (PE) spectroscopy. NMR allows accurate quantitation of the relative amounts of conformations if their interconversion is slow on the NMR time scale, and also determination of the rates for conformational equilibration. NMR does not directly yield information on the dihedral angle between the nitrogen long pairs (e), but the cis or trans dis- position of the N-alkyl groups attached to a ring may be es- tablished. This allows inferring 0 to within several degrees from a knowledge of the CN,NC endocyclic torsional angle allowed by the ring system in bicyclic233 and monocyclic The PE technique measures the vertical ionization potentials for the lone pair combination orbitals. Their separation is rather sensitive to 0, experimentally varying from a maximum of 2.3 eV near 0 = 180 and 0°5,6 to a minimum of about 0.5 eV near 0 = 90O.' The PE time scale is exceedingly short, so even if conformational interconversiuri ;S iuo rapid to be frozen out by NMR spectroscopy, the PE spectrum observed is the su- perposition of those for the conformations present. Although the relative amounts of different 0 value conformations thus detected cannot be nearly as accurately quantitated as in NMR experiments, the qualitative predominance of conformations in six-ring hydrazines determined by PE experiments has al- ways agreed with that determined by 13C NMR in solu- tion. Conformational information on tetraalkylhydrazine radical cations has come almost exclusively from ESR studies,* which have established that the barrier for double nitrogen inversion is quite low, and that for some cases in which the planar form is strained, the equilibrium geometry is not planar at nitro- gen.