Reactivities of some norbornene- and sesquinorbornene-type double bonds

Abstract

Some of the factors which govern the stereochemical course of Diels-Alder cycloadditions to norbornene and norbornadiene were investigated. Treatment of bicyclo{2.2.1}hepta-2,5-diene-2,3-dicarboxylic acid anhydride (11) and bicyclo{2.2.1}hept-2-ene-2,3-dicarboxylic acid anhydride (16) with furan leads to a predominance of the Alder's rule adducts 56 and 58. This is in contrast to the results obtained by Bartlett and Watson and also Edman and Simmons wherein they found that when cyclopentadiene was treated with either 11 or 16, the major product was the result of anti-Alder's rule addition. Also, it was found that treating 7-oxabicyclo-{2.2.1}hept-2-ene-2,3-dicarboxylic anhydride (72) with cyclopentadiene and furan now results in a reversal of the Alder/anti-Alder product ratios from those found by treating 11 or 16 with these same two dienes. A mechanism is postulated in which bridging group interactions leading to the Alder addition product are considered to be responsible for the observed product ratios. Treatment of the furan 29 with benzyne results in endo addition to give the topologically interesting olefin 102. X-ray crystallographic analysis revealed a double bond dihedral angle of 157.9(DEGREES), the largest bend from planarity yet observed. Treatment of the series of olefins 26, 30, 47 and 102 with various electrophiles resulted in exclusive exo addition with no rearrangement of the initial product observed in any of the thermal reactions. Despite the sterically hindered nature of the central bond in olefins 26, 47 and 102, these molecules react with singlet oxygen to yield unstable dioxetanes. In the case of 102, treatment with dichlorocarbene yields the adduct 125. Under photochemical conditions, 102 is capable of direct-excitation to yield the endo,endo-dihydro adduct (133).