Regioselective Synthesis of Indenols via Nickel-Catalyzed Carbocyclization Reaction.

2-Halophenyl ketones 1a-e (1a, o-IC[sub 6]H[sub 4]COCH[sub 3]) undergo carbocyclization with alkyl propiolates (2a, CH[sub 3](CH[sub 2])[sub 4]C≡CCO[sub 2]CH[sub 3]; 2b, TMSC≡CCO[sub 2]Et 2c, CH[sub 3]C≡CCO[sub 2]CH[sub 3]; 2d, CH[sub 3]OCH[sub 2]C≡CCO[sub 2]CH[sub 3]; 2e, CH[sub 3](CH[sub 2])[sub 3]C≡CCO[sub 2]CH[sub 3]; 2f, PhC≡CCO[sub 2]CH[sub 3]; and 2g, (CH[sub 3])[sub 3]C≡CCO[sub 2]CH[sub 3]) in the presence of Ni(dppe)Br[sub 2] and zinc powder in acetonitrile at 80 °C to afford the corresponding indenol derivatives 3a-m with remarkable regioselectivity in good to excellent yields. The nickel-catalyzed carbocyclization reaction was successfully extended to other simple disubstituted alkynes. Thus, the reaction of 2-halophenylketones 1a-e with disubstituted alkynes (2h, PhC≡CPh; 2i, CH[sub 3]C[sub 6]H[sub 4]C≡CC[sub 6]H[sub 4]CH[sub 3]; 2j, CH[sub 3]CH[sub 2]C≡CCH[sub 2]CH[sub 3]; 2k, PhC≡CCH[sub 3]; 21, TMSC≡CCH[sub 3]; and 2m, PhC≡C(CH[sub 2])[sub 3]CH[sub 3]) proceeded smoothly to afford the corresponding indenols 4a-t in good to excellent yields. For unsymmetrical alkynes 2k-m, the carbocyclization gave two regioisomers with regioselectivities ranging from 1:2 to 1:12 depending on the substituents on the alkyne and on the aromatic ring of halophenyl ketone. A possible mechanism for this nickel-catalyzed carbocyclization reaction is also proposed. [ABSTRACT FROM AUTHOR]