Ligand Effects in Regio‐ and Enantioselective Cobalt‐Catalyzed Intramolecular [4+2]‐ and [2+2]‐Cycloaddition Reactions of Unactivated 1,3‐Diene‐8‐ynes and 1,3‐Diene‐8‐enes

Abstract

Abstract Intramolecular Diels–Alder reactions have been extensively used for the syntheses of biologically active compounds including natural products. We have identified Co(I) complexes and optimal reaction conditions to accomplish highly diastereo‐ and enantioselective intramolecular [4+2]‐cycloaddition reactions of 1,3‐diene‐ynes and 1,3‐diene‐enes. These catalysts, readily prepared from an earth‐abundant metal and tunable ligands, serve as practical replacements for the prohibitively expensive Rh‐complexes previously used for similar reactions. The cationic complexes are readily prepared by the reaction of L CoX 2 with Zn in the presence of NaBARF. Both neutral and cationic Co(I)‐complexes of ( S )‐BINAP and ( S )‐ H 8 ‐BINAP ligands effect the cyclization of 1,3‐diene‐ynes giving the expected [4+2]‐addition products in excellent selectivities. The [4+2]‐cycloadditions of the less reactive 1,3‐diene‐enes proceed only with the cationic complexes and 1,2‐ bis‐ [(2 S ,5 S )‐2,5‐diphenylphospholan‐1‐yl]ethane (PhBPE) and appropriately substituted phosphinooxazoline (PHOX) are the most optimal ligands for these reactions. Depending on the ligands, 1,3‐diene‐enes bearing an aromatic substituent at the terminal position undergo an intramolecular [2+2]‐cycloaddition, giving cyclobutane products in very good yields and enantioselectivities. The absence of the corresponding [2+2]‐cycloaddition in the 1,3‐diene‐yne substates may be related to the enhanced strain in the expected bicyclo[3.2.0]heptene products. A unified mechanism involving Co(I)/Co(III)‐redox cycle, consistent with all available evidence, is proposed for these reactions.