EXOSELECTIVE DIELS-ALDER REACTIONS OF AMINOCARBENE COMPLEXES

The first examples of intermolecular Diels-Alder reactions of aminocarbene complexes are described. Various members of both the alkylamino and acylamino families of complexes are investigated. The E-isomer of [trans-propenyl(methyl-amino)methylene]pentacarbonyltungsten (22) will react with Danishefsky's diene and 1-methoxy-1,3-butadiene, but the Z-isomer will not. The yields are good only for the more reactive Danishefsky's diene, but all reactions with acyclic dienes are completely selective for the exo cycloadduct (greater-than-or-equal-to 96:4). The [trans-propenyl(N-methyl-N-benzoylamino)methylene]pentacarbonyltungsten(0) (35) is more reactive than 22, and if it is converted into the chelated complex 42, where the oxygen of the benzoyl group replaces a carbon monoxide ligand on the metal, a highly reactive dienophile is obtained. Complex 42 will react with Danishefsky's diene within minutes at room temperature, and although the yields are low (approximately 30%) due to competing dinuclear carbene complex formation, the cycloaddition again occurs with complete exo selectivity (greater-than-or-equal-to 97:3). The cycloaddition of 42 with cyclopentadiene is not stereoselective and gives approximately a 1:1 mixture of endo and exo isomers. Possible sources of the unique reactivity and high exo selectivity of these complexes with acyclic dienes are discussed in terms of their solution spectra and the solid-state structures of complexes 22-E, 22-Z, and 34.