SYNTHESIS OF COBALT-SUBSTITUTED 1,3-DIENE COMPLEXES WITH UNUSUAL STRUCTURES AND THEIR EXO-SELECTIVE DIELS-ALDER REACTIONS

The synthesis and characterization (including crystallographic data) o f several substituted-pyridine (Rpyr) cobalt bis(dimethylglyoxime) 1,3 -butadiene complexes (R = H, tBu, 3,5-diMe, and N,N-dimethylamino) and their Diels-Alder reactions with a variety of dienophiles are reporte d here. The cobalt-carbon bonds in the Diels-Alder cycloadducts can be cleaved so that cobalt complexes as well as functionalized organic cy cloadducts are recovered. Through these cobalt-carbon bond cleavage re actions, cobalt-diene complexes can serve as synthons for a variety of 1,3-dienes such as 1,3-butadiene, 2-(trimethylsiloxy)-1,3-butadiene, iodoprene, (E)-1-methoxy-3-(trimethylsiloxy)-1,3-butadiene (Danishefsk y's diene), and 1,2-dichloro-1,3-butadiene. The preparation of several cobalt-substituted 1,2- and 1,3-pentadiene complexes and highly exo-s elective Diels-Alder reactions of the 1,3-pentadiene complexes are the n discussed followed by demetalation reactions of these more highly su bstituted cobalt cycloadducts. These demetalation reactions maintain t he stereochemical integrity found in the metal cycloadducts and also l ead to cobalt recovery.