Electronic Control of Stereoselectivity. 8. The Stereochemical Course of Electrophilic Additions to Aryl-Substituted 9-Isopropylidenebenzonorbornenes

Article abstract of DOI:10.1021/ja00414a014

A series of aryl-substituted 9-isopropylidenebenzonorbornenes (1-4) and the parent 7-isopropylidenenorbornene (5) have been synthesized, and the stereochemistry of addition of numerous electrophiles to their exocyclic double bond has been determined.For assistance in interpreting the results, photoelectron spectroscopic investigations and extensive ZDO and MNDO/3 calculations were also undertaken.When weak electophiles reagents such as singlet oxygen, m-chloroperbenzoic acid, N-bromosuccinimide, N-methyltriazolinedione, and tert-butyl hypochlorite were studied, all gave product distributions which greatly favored anti addition when the aryl ring was unsubstituted or substituted by a pair of methoxyl groups.The placement of chlorine or fluorine groups on the aromatic ring was accompanied by a substantial enhancement in the relative amount of syn product.For strong electrophiles such as protonated tert-butyl hypochlorite, dichlorocarbene, the acetylinium cation, and protonated formaldehyde, syn attack was greatly favored or dominated exclusively.These marked crossovers in syn/anti stereoselection, which serve as a convenient tool with which to assess relative electrophilicity, may be explained in terms of the involvment of bridged or open ion pathways.Where bridged ions develop (weak electrophiles), long-range homoaromatic charge delocalization to the aromatic ring develops, with the result that anti attack becomes kinetically dominant.When powerfully electrophilic species are involved, this phenomenon is not important and transient aryl complex formation appears controlling.