Preparation of Butadienylpyridines by Iridium‐NHC‐Catalyzed Alkyne Hydroalkenylation and Quinolizine Rearrangement

Abstract

Iridium(I) N‐heterocyclic carbene complexes of formula Ir(κ2O,O’‐BHetA)(IPr)(η2‐coe) [BHetA=bis‐heteroatomic acidato, acetylacetonate or acetate; IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐carbene; coe=cyclooctene] have been prepared by treating Ir(κ2O,O’‐BHetA)(η2‐coe)2 complexes with IPr. These complexes react with 2‐vinylpyridine to afford the hydrido‐iridium(III)‐alkenyl cyclometalated derivatives IrH(κ2O,O’‐BHetA)(κ2N,C‐C7H6N)(IPr) through the iridium(I) intermediate Ir(κ2O,O’‐BHetA)(IPr)(η2‐C7H7N). The cyclometalated IrH(κ2O,O’‐acac)(κ2N,C–C7H6N)(IPr) complex efficiently catalyzes the hydroalkenylation of aromatic and aliphatic terminal alkynes and enynes with 2‐vinylpyridine to afford 2‐(4R‐butadienyl)pyridines with Z,E configuration as the major reaction products (yield up to 89 %). In addition, unprecedented (Z)‐2‐butadienyl‐5R‐pyridine derivatives have been obtained as minor reaction products (yield up to 21 %) from the elusive 1Z,3gem‐butadienyl hydroalkenylation products. These compounds undergo a thermal 6π‐electrocyclization to afford bicyclic 4H‐quinolizine derivatives that, under catalytic reaction conditions, tautomerize to 6H‐quinolizine to afford the (Z)‐2‐(butadienyl)‐5R‐pyridine by a retro‐electrocyclization reaction.