X-ray evidence for contrasting stereochemistry in related diene addition products

Abstract

2,3-Dichloro-5,6-dicyanobenzoquinone(DDQ) reacts readily with 1,5,5-trimethyl-3-methylenecyclohexene to give an adduct having endo-stereochemistry which crystallises in the monoclinic space group P2₁/n with lattice parameters a= 9·06 ± 0·01, b= 16·10 ± 0·01, c= 12·00 ± 0·01 Å, β= 96° 42′± 5′, Z= 4. In contrast the addition of cyclopentadiene to DDQ under similar conditions yields an exo-stereomer as the principle product which is monoclinic, space group p2₁/n with unit-cell dimensions a= 8·051 ± 0·005, b= 11·14 ± 0·01, c= 13·55 ± 0·01 Å, β= 94° 28′± 6′, Z= 4. Both structures were solved by centrosymmetric symbolic addition and refined by least-squares calculations to R= 0·112 for 955 reflections and R= 0·082 for 1225 reflections respectively.