A^(1,3) interaction and conformational energy of axial–axial 1,3-dimethyl interaction

Abstract

type strain was demonstrated directly from the nuclear magnetic resonance studies of the acyl and nitroso derivatives of 2-methylpiperidine (2 series) and 2,6-cis-dimethylpiperidine (1 series). In these two series, the bulky 2-methyl groups are forced to assume the axial conformation due to a severe A('s3) interaction in the equatorial conformation. The free energy of activation (AG*) for the internal rotation around the N-C and N-N bonds in la-lg was determined. The comparison of these AG* values with those of the corresponding derivatives of dimethylamine indicates the decreases are in the range of 3.64.7 kcal/rnole which is in fair agreement with the conformational energy of 1,3-diaxial CH3/CH3 interaction. An argument was presented to show that this interaction energy corresponds t o the decrease of AG* in l a -l g .C a n a d~a n Journal of Chemistry, 46, 2821Chemistry, 46, (1968 The stereochemical coilsequence of reactions parently represent quantitatively the extent of of 2-alkylcyclohexane derivatives (A S B) has the delocalization energy and also qualitatively led Johnson to conclude that there is a severe the extent of the planar configuration a t N=C non-bonded interaction between the group X 0 (or X') and an equatorial alkyl group (1). This and N=N bonds in 1 and 2 (where -C // interaction was termed A(''3) strain (1) and the \ D energy was approximated with 1,3-diaxial CH3/ CH, interaction (3.7 kcal/mole). During our studies on the photochemical behavior of nitrosamines we became aware of some unusual reactivities (2) and nuclear magnetic resonance (n.m.r.) characteristics of N-nitroso-cis-2,6-dimethylpiperidine (lg, N-NO for NCOR) that might be related to A('.,) strain. The n.m.r. study on the conformational aspects of the acyi derivatives of cis-2,6-dimethylpiperidine (1 series) and of 2-methylpiperidine (2 series) was undertaken. The purpose of this communication is to demonstrate directly the presence of the A(133) interaction by n.m.r. spectroscopy in systems 1 and 2 and further to provide a method of estimating the conformational energy of 1,3-diaxial CH,/CH, interaction in 1. . ; i systems due to an extensive delocali-(N=N ) zation of the lone pair electrons of nitrogen with the hetero n electron system has been well documented by n.m.r. spectroscopy (3, 4). The rotational barriers due to the partial double bond character (15-21 kcal/mole for amide and 23.3 kcal/mole for dimethyl nitrosamine) apn is either acyl or nitroso group). The structure of these acyl derivatives, if the A('v3) interaction is ignored for a moment, must be represented by the nearly trigonal configuration at N as in 1 and 1'. With a planar or a near planar structure at the N=C bond, it is expected, and can be shown by Dreiding Models, that A('z3) interaction between 0 (and/or R) and a-equatorial CH, is severe in 1' and that 1 is the predominant conformation. Since the delocalization energy (15 N 23 kcal/mole) is several times higher than the 1,3-diaxial CH,/CH, interaction, the amount of conformer 1' will be negligibly s...

Photochemistry of nitroso compounds in solution. XV. Configuration and conformation of the enone oximes obtained from photoaddition of N-nitrosopiperidine to conjugated dienes

“…The syn-cyclohexenone oxime 86 must have the forced conformation 14 with the axial orientation of the piperidine ring since a severe Alp3 interaction (17,18) would not allow an equatorial orientation of piperidine ring in the alternative conformation. Though the severe A l s 3 interaction no longer exists in anti-oxime 7b, the coupling For personal use only.…”

Nuclear magnetic resonance studies on the configurations and conformations of heterocyclic nitrosamines

“…The rigid framework of trans-decahydroquinoline makes the corresponding syrz configuration of 15 inconceivable due to a severe A". 3, interaction (16,17). The only possible isomer, for the sole nitrosation product obtained, must possess anti configuration (15) which in turn, requires that the signals at z 7.63 and 4.90 be assigned to the cis-axial and the cis-equatorial protons respectively since they are geminally coupled to each other (J = 13 Hz).…”

Stereoselectivity in the Reactions of Nitrosopiperidine Carbanions. Steric vs. Stereoelectronic Control

“…For example, Chow et al (16) estimated the strain in the 1,3-diaxial conformation of cis-2,6-dimethylpiperidine to be 3.9 kcal/mol, the difference in activation energies for the barrier to rotation of its nitrosamine (19.4) relative to that of dimethylnitrosamine (23.3). A better model for comparison is now available from the detailed study of Cooney et al (17).…”