Highly Regio- and Stereoselective Cocyclotrimerization and Linear Cotrimerization of α,β-Unsaturated Carbonyl Compounds with Alkynes Catalyzed by Nickel Complexes

Abstract

Cyclic enones 2-cyclohexen-1-one (1a), 4,4-dimethyl-2-cyclohexen-1-one (1b), 2-cyclopenten-1-one (1c), and 2-cyclohepten-1-one (1d) react with octa-1,7-diyne (2) in THF in the presence of Ni(PPh₃)₂I₂, ZnI₂, and Zn powder at 62 °C to give [2 + 2 + 2] cycloaddition−dehydrogenation products 3a − d in 32−80% yields. α,β-Unsaturated lactone 5a (5,6-dihydro-2H-pyran-2-one) undergoes [2 + 2 + 2] cycloaddition with 2 to give both the corresponding cyclohexadiene product 6 (29%) and dehydrogenation product 7 (39%). Under similar reaction conditions, 3-buten-2-one reacts with 2 and various substituted hepta-1,6-diynes 9a − c to give [2 + 2 + 2] cycloaddition−dehydrogenation products 11a − d in 68−80% yields. Diphenylacetylene also reacts with 1a − d, 5a, and 2(5H)-furanone (5b) to afford the corresponding [2 + 2 + 2] cocyclotrimerization products 13a − d and 14a − b. No dehydrogenation of products 13 and 14 was observed under the reaction and workup conditions. The reactions of acrylates with alkynes catalyzed by nickel complexes give products that depend greatly on the reaction conditions. Treating ethyl acrylate (15a) with 1-phenyl-1-propyne (16) in the presence of Ni(PPh₃)₂Cl₂ and Zn at 90 °C in toluene affords cocyclotrimerization product 19a as the major product (54% yield). However, treatment of CH₂CHCOOR (R = Et and t-Bu) with mono alkynes 16 and 12 in the presence of Ni(PPh₃)₂X₂ (X = Cl and I) and Zn powder in toluene at 60 °C affords the corresponding conjugated trienes 17a − c in 82−92% yields. The MS data of 17 firmly support an adduct of two molecules of alkyne and a molecule of acrylate. Similarly, the reaction of 15a with octa-1,7-diyne in the presence of Ni(PPh₃)₂I₂, ZnI₂, and zinc gives triene derivative 21 in 68% yield. NOE and X-ray results indicate that in these trienes the substituents from each alkyne and alkene moiety are cis to each other. The unique stereoselectivity can be attributed to the exclusive formation of seven-membered nickelacycloheptadiene intermediate 25 during the catalytic reaction.