Preparation of Butadienylpyridines by Iridium‐NHC‐Catalyzed Alkyne Hydroalkenylation and Quinolizine Rearrangement. Issue 46 (21st June 2021)

• Record Type: Journal Article
• Authors: Azpíroz, Ramón; Greger, Ingo; Oro, Luis A.; Passarelli, Vincenzo; Castarlenas, Ricardo; Pérez‐Torrente, Jesús J.
• Is Part Of: Chemistry. Volume 27:Issue 46(2021)
• Page Start: 11868
• Page End: 11878
• Publication Date: 2021-06-21
• Subjects: C−H activation -- C−C coupling -- catalysis -- hydroalkenylation -- iridium -- N-heterocyclic carbenes; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.202101414
• Languages: English
• ISSNs: 0947-6539

Abstract

Abstract: Iridium(I) N‐heterocyclic carbene 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 treating 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‐ C7 H6 N)(IPr) through the iridium(I) intermediate Ir(κ 2 O, O' ‐BHetA)(IPr)( η 2 ‐C7 H7 N). The cyclometalated IrH(κ 2 O, O' ‐acac)(κ 2 N, C– C7 H6 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, 3 gem ‐butadienyl hydroalkenylation products. These compounds undergo a thermal 6π‐electrocyclization to afford bicyclic 4 H ‐quinolizine derivatives that, under catalytic reaction conditions, tautomerize to 6 H ‐quinolizine to afford the ( Z )‐2‐(butadienyl)‐5 R ‐pyridine by a retro‐electrocyclization reaction. Abstract : Magic watchmaker . The Ir‐NHC catalyst efficiently hydroalkenylates alkynes with 2‐vinylpyridine. The interplay of the NHC and acac ligands drives the reaction via Ir III intermediates allowing the selective formation of expected and unexpected butadienylpyridine derivatives. The unusual 5‐substituted 2‐butadienylpyridines result from a complex stepwise mechanism involving quinolizine intermediates.