Evidence for the concerted mechanism of the Diels-Alder reaction of butadiene with ethylene.

Diels-Alder reactions of substituted dienes and dienophiles have been found to cccur by both concerted and stepwise mechanisms.’.* The nature of the mechanism of the prototype Diels-Alder reaction of butadiene with ethylene to form cyclohexene is not only of fundamental interest but has generated fervid fulminations for favorite We wish to report new experimental evidence that is compatible only with the concerted mechanism, as well as further theoretical evidence for the synchronicity of this concerted mechanism.6 The reactions of 1,1,4,4-tetradeuterio-1,3-butadiene with cisor trans-1,2-dideuterioethylene were carried out at 185 “ C for 36 h at a pressure of 1800 psi in a stainless steel b~mb.~-’O Infrared spectra of the unreacted dideuterioethylenes indicate that they do not isomerize under the conditions of the reaction. Each cyclohexene product was separated from butadiene dimers by preparative GLC” and was then epoxidized with m-chloroperbenzoic acid (Figure 1). The N M R spectrum of cyclohexene oxide (Figure 2A) has resonances for the protons at C, and C, at 6 1.19 and 1.38, corresponding to protons that are cis and trans to the epoxide oxygen.12 The proton NMR spectra of the epoxides obtained from the reactions of cis and trans-dideuterioethylenes are clearly different. The DH couplings were partially eliminated by irradiation of the C3 and C6 deuteriums to give the spectra shown in Figure 2. The cis-dideuterio compound has a singlet resonance at d 1.19, due to the product with C4 and Cs hydrogens cis to the epoxide, and a second singlet a t 6 1.38, due to the