An abnormal spin‐spin coupling of vinylic‐allylic protons generated by a constraint of configuration

Abstract

Abstract-In a system where a rigid or frozen conformation constrains an allylic proton to orient nearly perpendicular to the plane of the olefinic bond, the allylic spin coupling may become comparable to, or greater than, the vicinal spin coupling. This anomalous situation is demonstrated in three series of compounds: the adducts of diazoalkanes to 1,2-diazepines, Cpiperidinocyclcalk-2-en-1 -one oximes and heterobicyclo [3,2,0]hept-3,6-dienes, where the NMR spectra have been well established by decoupling experiments.SPIN-spin coupling in NMR spectroscopy is one of the most descriptive physico-chemical phenomena, which allows structural chemists to conceive the shape of a molecule. The relation of the magnitude of the coupling constant with respect to the dihedral angle, as defined by the two hydrogen nuclei under consideration, has developed along a dual approach by empirical correlation as well as by theoretical calculations.2 The applications of these results to the determination of the configuration and of the conformation of a molecule have been widely practised by organic chemists. The early theoretical predictions, based on valence bond calculations, and molecular orbital interaction^,^ culminated in the emergence of the well known Karplus rule: which defines the dependence of coupling constants through 3-and 4-bonds on the dihedral angle, among other factors such as electronegativities of substituents, hybridisation of carbon atoms, and bond angles and lengths, etc. The Karplus rule has undergone various theoretical since its inception and the validity of the rule is convincingly confirmed by the experimental data2 in various types of organic molecules.Garbish4 has estimated 3-and 4-bond vinylic-allylic couplings by considering (r and rr-bond contributions to both types of coupling as a function of the allylic proton conformation. According to this empirical relation, for the 3-(JI2) and 4-bond coupling (113) constants relative to the dihedral angle (see the stereorelationship A), the following set of equations is obtained.From the theory, the term of cos2+ represents the (rbond contribution and the term of sin2+ the rr-bond contribution. Experimentally it is known that a12 is far larger than ~1 3 , since an interaction through four o-bonds is noticeable only in a W-plan (+ = 0"). It is also logical to expect that the term of sin24 becomes important at 4 = 90" where the rr-orbital and the allylic C-H, bond are held parallel, at which configuration (or conformation) the overlap between the orbitals * Professeur associt of CUHR 1971 to 1972; on leave of absence froin Simon Fraser University, Burnaby 2, B.C., Canada.is the best. Although the signs of the coupling constants are opposite in the vicinity of 4 = go", the experimentally observable coupling constants J12 and J13 are determined by the numerical value of b,, and b13. Depending on the factors such as electronegativities of substituents and bond lengths and angles, etc., a situation may arise in which the numerical value of b13 is bigger than b,, in t...