ChemInform Abstract: Steric and Conjugational Electronic Effects on the Configuration of Acylated Hydrazines.

Abstract

The crystal structures of twelve N,N'-acylated cyclic hydrazines are described. In five adducts of 1,2,44riazolidine-3,Sdione with various cyclopentadiene derivatives, it is shown that the configuration of the triazolidinedione moiety with respect to the bicycloheptyl rings is highly sensitive to very small differences in steric effects. The degree of flattening of the N atoms at the N-N bond is determined by the steric repulsions of the substituents. The average valence angle (a,,) at the nitrogens ranges between 110.5 and 112.0'. Increasing the size of N,N' rings and replacing the urea-type nitrogen of the triazolinedione ring by carbon is shown to cause significant flattening at the hydrazine nitrogens, and five of the other seven structures described here are nearly flat at nitrogen (aav greater than 119'). The transition state for nitrogen inversion is achieved when the nitrogen atom becomes planar. In acyclic hydrazines, conformations with the lone pairs a t nitrogen perpendicular are electronically favored, and the nitrogens invert one at a time, with a substantial increase in nitrogen inversion barrier when the noninverting nitrogen lone pair orbital is forced to be coplanar with that of the inverting nitrogen.' When acyl groups or other mesomerically electron-withdrawing groups are attached to the nitrogens of an acyclic hydrazine, the nitrogens flatten substantially, and the nitrogen inversion barrier is too low to measure by NMR.2 When the nitrogens of an N,N'-diacylhydrazine are linked by cyclic alkyl groups, however, the nitrogen lone pairs are forced to assume angles far from 90°, and there is significant lone pair-lone pair interaction, resulting in substantially pyramidal acylated nitrogen atoms.3 Hydrazines linked N,N' by two rings small enough to hold the CN,N'C angles in both rings near 0' may well be required to flatten both nitrogens at once to accomplish nitrogen inversion. The crystal structures of a set of compounds containing the 1,2,4-triazolidine-3,5-dione ring fused to a bicyclic ring system show that in most of them, the hydrazine nitrogens adopt significantly pyramidal configurations? despite the presence of an acyl group on each nitrogen and absence of the steric inhibition to resonance which occurs when the nitrogens of 2,3-diazabicyclic hydrazines are acylated by reagents which do not link the acyl groups in a small ring3 Each carbonyl group of the triazolidinedione ring is conjugated to N 4 as well as to one of the hydrazine nitrogens, which should lead to a decrease in the resonance interaction of the hydrazine nitrogen with the carbonyl group attached to it compared to that of an alkylated carbonyl group, presumably causing more facile bending at the hydrazine nitrogen. The lack of molecules having planar hydrazine nitrogens in the triazolidinedione series previously studied4 prevented examination of extrapolated geometrical parameters near the transition state for double nitrogen inversion. We were therefore prompted to study the structural features necessary to achieve flattening at the nitrogen of hydrazines in which the nitrogen lone pairs are held near coplanarity. Two possible ways to induce flattening, increasing the steric hindrance in bent structures and substituting the N 4 group, are discussed. A. Steric Effects on the Configuration of the Adducts 1,2,4Triazolidine-3,5-dione with Various Cyclopentadiene Derivatives. Molecular Structure and Discussion. Schematic drawings with atom numberingS and stereoscopic drawings down the N-N hydrazine bond are shown in I-V and Figures 1-5, respectively. Comparisons of bond lengths and angles are given in Tables I and Israel Institute of Technology. f University of Wisconsin-Madison. Table I. Comparison between Bond Lengths (A) and Bond Angles (deg) in I and I1