Highly regio- and stereoselective cocyclotrimerization and linear cotrimerization of alpha,beta-unsaturated carbonyl compounds with alkynes catalyzedby nickel complexes

Cyclic enones 2-cyclohexen-1-one (1a), 4,4-dimethyl-2-cyclohexen-1-one (Ib) , 2-cyclopenten-1-one (Ic), and 2-cyclohepten-1-one (Id) react with octa-1, 7-diyne (2) in THF in the presence of Ni(PPh3)(2)I-2, ZnI2, and Zn powder a t 62 degrees C to give [2 + 2 + 2] cycloaddition-dehydrogenation products 3 a-d in 32-80% yields. alpha,beta-Unsaturated lactone 5a (5,6-dihydro-2H-pyr an-2-one) undergoes [2 + 2 + 2] cycloaddition with 2 to give both the corre sponding cyclohexadiene product 6 (29%) and dehydrogenation product 7 (39%) . Under similar reaction conditions, 3-buten-2-one reacts with 2 and variou s substituted hepta-1,6-diynes 9a-c to give [2 + 2 + 2] cycloaddition-dehyd rogenation products 11a-d in 68-80% yields. Diphenylacetylene also reacts w ith 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 pro ducts 13 and 14 was observed under the reaction and workup conditions. The reactions of acrylates with alkynes catalyzed by nickel complexes give prod ucts that depend greatly on the reaction conditions. Treating ethyl acrylat e (15a) with 1-phenyl-1-propyne (16) in the presence of Ni(PPh3)(2)Cl-2 and Zn at 90 degrees 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)(2)X-2 (X = Cl and I) and Zn powder in toluene at 60 degrees 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)(2)I-2 , ZnI2, and zinc gives triene derivative 21 in 68% yield. NOE and X-ray res ults indicate that in these trienes the substituents from each alkyne and a lkene moiety are cis to each other. The unique stereoselectivity can be att ributed to the exclusive formation of seven-membered nickelacycloheptadiene intermediate 25 during the catalytic reaction.