Abstract
Essential to drug discovery, molecular libraries are used to screen small-molecule compounds for biological activity. An emphasis on compound quantity has led to the overuse of established synthetic methods, yielding compounds with similar structural properties, often dominated by planar (sp2) character. In contrast, complex small molecules enriched in tetrahedral (sp3) character and increased chirality demonstrate greater protein-binding specificity and offer higher-quality drug candidates. The Harman Research Laboratory at the University of Virginia has explored dearomatization and stereocenter formation via dihapto-coordination (η2) to an electron-rich tungsten fragment, {W(Tp)(NO)(PMe₃)} (Tp = trispyrazolylborate, NO = nitrosyl, PMe₃ = trimethylphosphine). π-Backbonding from the metal donates electron density into the organic ligand, increasing the nucleophilicity of unbound alkenes and activating them toward electrophiles in diverse organic transformations. Electrophilic addition reactions, such as protonation, generate tungsten-stabilized carbocation (hyperdistorted η2-allyl) intermediates that subsequently undergo stereoselective nucleophilic additions. The resulting organic molecules can be liberated from the metal complex via an oxidation reaction. The organometallic methodology described above has been applied to the dearomatization of pyridine, enabling the synthesis of a library of piperidinyl-substituted methylphenidate (RitalinTM) analogs. While historically focused on six-membered aromatic systems, the technique of tungsten dihapto-coordination has been extended to carbocycles of other sizes, including the stereoselective transformation of five-, seven-, and eight-membered carbocycles into highly functionalized cyclopentene, cycloheptene, and cyclooctene cores rich in sp3 and stereogenic character.
