TRANSITION-METAL MEDIATED EXO SELECTIVE DIELS-ALDER REACTIONS - PREPARATION OF 2-COBALT-SUBSTITUTED 1,3-DIENES CONTAINING C-2 SYMMETRICAL 2,3-DIBENZOBICYCLO[2.2.2]OCTANEDIONE DIOXIME EQUATORIAL LIGANDS AND THEIR USE IN THERMAL AND LEWIS-ACID CATALYZED 4-CYCLOADDITIONS(2)

The preparation of C-2 symmetric 2,3-dibenzobicyclo[2.2.2]octanedione dioxime is reported. This ligand is then used in the preparation of a (pyridine)(glyoxime)(2)cobalt(III) chloride complex. The chloride is r educed to a cobaloxime anion which reacted with allenic electrophiles to produce (pyridine)(glyoxime)(2)cobalt-1,3-dienyl complexes. Thermal and Lewis acid catalyzed Diels-Alder reactions of these dienyl comple xes as well as the complexes with glyoxime = diphenylglyoxime and dime thylglyoxime are reported. In most cases these Diels-Alder reactions a re anti (exo) selective, and in many cases diastereoselectivities are >20:1. Cycloadduct demetalation reactions are also reported which pres erve cycloaddition stereochemistry and provide cobalt complexes which can be recycled into the starting dienyl complex.