The ¹H NMR spectra of some norbornene derivatives, a LIS study

Abstract

Abstract-The 'H N M R spectra of 2-exo-hydroxyiiiethyl-3-endomethylnorbornene and the corresponding 2-endu-3-exo isomer have been completely assigned with the aid of lanthanide reagents. This enabled a full analysis of the unusual spectrum of 2-exo-hydroxymethyl-3-endo-methylnorbornene to be undertaken, confirming the proposed structure. The AEu values for 2-exu-hydroxymethyl-3-endo-methylnorbornene and the 2-endo-3-exo-isomer have been used to test the effect of rotational averaging on the calculated pseudo contact shifts. Good agreement is obtained for a dynamic model in which the Eu atom exchanges between two sites on the oxygen atom of the OH bond, and in which the rotational equilibrium about the CH-CH,OH bond is explicitly considered.THE SYNTHESIS and properties of 2, 3 dimethylnorbornadiene (5) are of considerable current interest. A projected synthesis of this elusive compound was from cyclopentadiene and crotonaldehyde involving the Diels Alder reaction of trans-crotonaldehyde and cyclopentadiene to give a mixture of aldehydes 1 and 2 which, with NaBH,, afforded the primary alcohols 3 and 4. Whilst alcohol 4 gave the expected IH spectrum, that of the compound we thought to be 3 was not immediately recognisable. In particular, the spectrum (Fig. 1) did not possess the expected C-3 methyl doublet, but at fir\t sight merely a single peak at hl.1. In order to confirm the identity of this and the related alcohol 4 a full analysis of the spectrum was undertaken, and also a complete structural study of both 3 and4 using lanthanide shift reagents.The general validity of the McConnell-Robertson equation1 for pseudo contact shifts of all * H nuclei in saturated alcohols, except the OH and possibly the CH-OH protons, is now well However, most previous determinations of molecular geometry by this method have assumed a rigid shift reagentsubstrate complex without consideration of possible rotational isomerism both in the substrate and in the complex, though the necessity of averaging the equation over all populated rotamers has been stressed4 and recently some attempts have been made to include this averaging.5*fi Compounds 3 and 4 provide ideal examples for testing this procedure in that the rigid bicyclo[2.2.1]-heptyl skeleton is attached to the conformationally mobile CH,OH group.We shall show that when rotational averaging is included, the lanthanide induced shift (LIS) study gives both structural and configurational information on 3 and 4. RESULTS AND DISCUSSIONThe identification of aldehydes 1 and 2, and thus of alcohols 3 and 4, was based on their N M R spectra and in particular on the well known shielding of the endo substituents (C,--Me in 2 and C,-CHO in 1) compared to the analogous exo substituents (C,-Me in 1 and C,-CHO in 2).7-9 Compound 3The I H spectrum of 3 was assigned partly on the basis of the chemical shifts and coupling constants and completed (Table I ) by the LIS using Eu(dpm),. The plots of shift reagent-substrate molar ratio vs induced shift were strictly linear except for the hydrogen atoms ...

Proton nuclear magnetic resonance spectra of cyclic monoenes: hydrocarbons, ketones, heterocycles, and benzo derivatives

“…The power of the lanthanide shift reagents for the simplification of spectra and for structure elucidation was nicely demonstrated in the case of 5-exo-methyl-6-endo-hydroxymethyl-76 and its 5-endo-6-exoisomer. 262 Of importance is the finding that only a dynamic model for th • complex which considered rotamers about the C-0 as well as the C-CH2 O bond could adequately explain the experimental results.…”