Strained Allenes as Dienophiles in the Diels-Alder Reaction: An Experimental and Computational Study.

Strained allenes, such as 1,2-cyclohexadiene, undergo facile Diels-Alder reactions with otherwise unreactive dienes. Substituted cyclohexa-1,2-dienes add to furan via a Diels-Alder reaction, forming only two of four possible regioisomers and stereoisomers. Alkyl cyclohexa-1,2-diene carboxylates yield the nonconjugated endo adduct as the major product. However, chiral cyclohexa-1,2-dienecarboxylates, such as l-menthyl and l-bornyl cyclohexa-1,2-dienecarboxylate, show no diastereoselectivity in [4 + 2] cycloadditions. DFT (B3LYP/6-31G) calculations were performed in order to explain these results. A comparison to the calculated B3LYP/6-31G transition structures and intermediates along the reaction paths of 1,2-cyclohexadiene with 1,3-butadiene and with furan (as well as propadiene with butadiene) show that the diradical stepwise pathways are preferred over the concerted paths. At the same time, the concerted transition structures are extremely asynchronous. A QM/MM study indicates a minimal influence of the chiral auxiliary even in the concerted scenario.