Structure, Synthesis, and Properties of Some Persubstituted 1,2‐Dintroethanes. In Quest of Nitrocyclopropyl‐Anion Derivatives

Abstract

Abstract Attempts to deprotonate nitrocyclopropane led to solutions which showed strong ESR. signals ( Fig. 1 )and from which 1‐nitro‐ l′‐nitroso‐bicyclopropyl ( 3 ) and 1,1′‐dinitro‐bicyclopropyl ( 2 ) were isolated. The activation energy for rotation about the central C, C‐bond of 2 is estimated to be about 12 kcal/mol ( 1 H‐NMR. spectra in Fig. 2 ). In contrast, the oven‐chain analogue 2,3‐dimethyl‐2,3‐dinitrobutane ( 1 ) shows a methyl singlet down to −70° C. Low‐temperature X‐ray analyses of 1, 2, 3 , and also of 1,1′‐dinitro‐bicyclobutyl ( 4 ) show that all four molecules have gauche‐conformations but reveal striking structural differences between the openchain and the cyclic derivatives ( Fig. 4–6 ): the central C, C‐bond is long in 1 (1.575 Å), short in 2 (1.479 Å); the C, N‐bonds are long in 1 (1.549 Å), short in 2 (1.488 Å); the orientation of the nitro groups is bisected in 2 and perpendicular in 1. The crystal structure of the nitro‐nitroso compound 3 is isomorphous with that of the dinitro compound 2 and thus disordered ( Fig. 15–16 ). The effect of the nitro group as π‐electron acceptor on the molecular conformations and bond lengths is discussed. From analysis of the anisotropic vibrational parameters of 2 the root‐mean‐square librational amplitude of the nitro groups about their C, N‐bonds is estimated to be about 5.8° at 95 K, corresponding to a rotational barrier of about 9 kcal/mol, i. e. the same order of magnitude as the NMR. estimate of about 12 kcal/mol for C, C‐rotation.