The NMR spectra of the cycloaddition adducts between tetracyanoethylene oxide (TCNEO) and norbornadiene derivatives have been studied. It was shown that there was significant long-range coupling between 7-anti and endu hydrogens in the adducts and that this long-range coupling was solvent dependent. Based on the present study it is concluded that the long-range coupling between endu and anti hydrogens can be as high as 4.1 Hz when an electronegative substituent is attached to the carbon which bears endu hydrogen.
TETRACYANOETHYLENE oxide (TCNEO) can undergo [2 + 31 cycloaddition reactions with alkenes and even aromatic compounds1.* under thermal conditions. In addition to its mechanistic interest, this reaction may be useful in the synthesis of tetrahydrofuran derivatives. We wish to report the results of some of the interesting aspects exhibited by the NMR spectra of the cycloaddition adducts. When norbornadiene and TCNEO were refluxed for 9 hours in toluene the exo adduct l2 was isolated in 30% yield. The NMR spectrum in acetone-d, showed two vinyl protons as a triplet at 66.47, two protons as a doublet (J = 1.45 Hz) at 63.71, two bridgehead protons as a quintet at 63.49 and two bridge protons as a multiplet at approximately 61.70.
The doublet at 63.71 suggested that the adduct may be the endo isomer rather than 1 since it is well documented3" that the endo hydrogen (H5) does not couple with a bridgehead hydrogen (H4) in the exo isomer of norbornanes and norbornenes. One should not rule out the possibility, however, that the doublet with a small coupling constant of 1.45 Hz is due to the long-range between H, and H,. This was shown to be the case by carrying out a decoupling experiment. When the lower field bridge proton H, (-61-70) was irradiated, the doublet at 63.71 collapsed to a sharp singlet. This decoupling experiment also enabled us to assign the lower field first order doublet bridge proton absorption as H, and the higher field absorption as H,. This suggests that H, is experiencing a larger magnetic shielding effect by the cyano groups as compared to H, by the double bond.3e The NMR spectrum of 1 is shown in Fig. 1.
The fact that the exo isomer 1 was the product was further substantiated by the isolation of 2 from the reaction of 7-t-butoxynorbornadiene and TCNEO. The NMR of 2 (Spec. No. 0320) showed two vinyl protons as a triplet at 66.31, one bridge proton as a broad singlet at 83-84, two protons (H5, H,) as a sharp singlet at 63.79, two bridgehead protons as a quartet at 63.32 and nine t-butoxy protons as a singlet at 61.31 (Table 1). The appearance of H,, H, as a sharp singlet is consistent with the absence of a bridge anti-hydrogen. In addition, compound 3 was obtained from the