Preparation of Butadienylpyridines via Iridium-NHC Catalyzed Alkyne Hydroalkenylation and Quinolizine Rearrangement.

Iridium(I) NHC complexes of formula Ir(κ 2 O,O' -BHetA)(IPr)( η 2 -coe) {BHetA = Bis-Heteroatomic Acidato, acetylacetonate or acetate; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-carbene; coe = cyclooctene} have been prepared by reaction of Ir(κ 2 O,O' -BHetA)( η 2 -coe) 2 complexes with IPr. These complexes react with 2-vinylpyridine to afford the hydrido-iridium(III)-alkenyl cyclometalated derivatives IrH(κ 2 O,O' -BHetA)(κ 2 N , C -C 7 H 6 N)(IPr) through the iridium(I) intermediates Ir(κ 2 O,O' -BHetA)(IPr)( η 2 -C 7 H 7 N). The cyclometalated IrH(κ 2 O,O' -acac)(κ 2 N,C -C 7 H 6 N)(IPr) complex efficiently catalyzes the hydroalkenylation of aromatic and aliphatic terminal alkynes and enynes with 2-vinylpyridine to afford 2-(4-R-butadienyl)pyridines with Z , E -configuration as the major reaction products (yield up to 89%). In addition, unprecedented ( Z )-2-butadienyl-5-R-pyridine derivatives have been obtained as minor reaction products (yield up to 21%) from the elusive 1 Z ,3gem-butadienyl hydroalkenylation products. These compounds undergo a thermal 6π-electrocyclization to afford the bicyclic 4 H -quinolizine derivatives that, under reaction conditions, tautomerize to 6 H -quinolizine to afford the ( Z )-2-(butadienyl)-5-R-pyridine by a retro-electrocyclization reaction.