CHROMENES THROUGH METAL-CATALYZED REACTIONS OF STYRENYL ETHERS - MECHANISM AND UTILITY IN SYNTHESIS

An efficient metal-catalyzed process that converts styrenyl ethers to 2-substituted chromenes is described. This class of reactions may be c arried out on either terminal or disubstituted styrenyl substrates. De pending on the level of substitution of the olefins, the Ru-carbene ca talyst may initiate reaction either by interaction with the styrenyl o r the carbocyclic alkene. Metal-catalyzed rearrangements, carried out under an atmosphere of ethylene, afford excellent yields of monomeric products. With disubstituted styrene ethers, the presence of ethylene is also critical to reaction efficiency. Mechanistic data that rationa lize these observations are provided. Although Ru complexes (PCy3)(2)C l2Ru=CHCH-CPh2 or (PCy3)(2)Cl2Ru=CHPh effectively serve as catalysts, with the more functionalized substrates, higher yields are obtained wh en o(CHCMe2Ph)(N(2,6-(i-Pr)(2)C6H3))(OCMe(CF3)(2))(2) is used. A varie ty of starting materials for the metal-catalyzed chromene synthesis (d isubstituted styrenes) are available in the optically pure form throug h the Zr-catalyzed kinetic resolution protocol, allowing several 2-sub stituted chromenes to be prepared in high enantiomeric purity. However , a number of functionalized substrates cannot be efficiently resolved by the latter method, indicating that more effective methods that add ress this deficiency are required.