Organometallic Enantiomeric Scaffolding: General Access to 2-Substituted Oxa- and Azabicyclo3.2.1octenes via a Brønsted Acid-catalyzed 5+2 Cycloaddition Reaction

摘要

6-Substituted TpMo(CO)2(η-2,3,4-pyranyl)- and TpMo(CO)2(η-2,3,4-pyridinyl) scaffolds (Tp = hydridotrispyrazolylborato) function as reaction partners in an efficient regio- and stereocontrolled synthesis of functionalized oxa- and azabicyclo[3.2.1]octenes through a novel Brønsted acid-catalyzed [5+2] cycloaddition reaction. Excellent exo-selectivities are obtained and the reaction gives products with complete retention of enantiomeric purity when carried out with chiral, non-racemic scaffolds. The substituent at C-6 of the η³-coordinated heterocyclic scaffold not only influences [5+2] reactivity but also plays a critical role in the demetalation step directing the reaction to only one of two possible products.