Further Observations on the Formation of Naphthalenes by Double Insertion of Acetylenes into Benzyne−Nickel(0) Complexes

The reactions of acetylenes with benzyne−nickel(0) complexes Ni((1,2-η)-4,5-X 2 C 6 H 2 )L 2 (X = H, L 2 = 2PEt 3 ( 1 ), dcpe ( 2 ); X = F, L 2 = 2PEt 3 ( 3 ), dcpe ( 4 )) to give substituted naphthalenes have been investigated further in an effort to understand the factors that control the successive insertions. Complex 1 reacts with a mixture of tert -butylacetylene and 3-hexyne in a 1:3 molar ratio to give approximately equal amounts of 1,2-diethyl-3- tert -butylnaphthalene ( 11 ), the exclusive product of cocyclization, and 1,2,3,4-tetraethylnaphthalene ( 10 ), which results from double insertion of 3-hexyne. Double insertion of 1,7-octadiyne into the benzyne−nickel bond of 1 occurs exclusively in an intramolecular fashion to give 1,2,3,4-tetrahydroanthracene ( 14 ) in 48% isolated yield. These observations are consistent with the following postulates: (1) the first insertion occurs under steric control and forms a nickelaindene in which the substituted carbon atom is bound to nickel; (2) the second insertion occurs in the nickel−vinyl bond of this species, not the nickel−aryl bond. The second insertion is also under steric control, but the regiospecificities for tert -butylacetylene and 1,7-octadiyne are opposite. Addition of PEt 3 or PPh 3 suppresses (though not completely) the catalytic cyclotrimerization of methyl propiolate in the presence of 3 in favor of stoichiometric formation of a mixture of dimethyl 6,7-difluoronaphthalenedicarboxylates, the ratio of the 1,3- and 2,3-isomers, 8 and 9, being 1:3 (PEt 3 ) and 1:4 (PPh 3 ). For tert -butyl propiolate in the presence of PPh 3, the ratio of 1,3- to 2,3-isomer changes to 1.6:1. These observations support the idea that for acetylenic esters both steric and electronic factors influence the second insertion. The isolated nickelaindene Ni{C 6 H 4 C(CO 2 Me) C(CO 2 Me)}(dcpe) ( 18 ) reacts with methyl propiolate, probably under electronic control, to give trimethyl 1,2,3-naphthalenetricarboxylate as the only product of cocyclization.