Stereoselective [2 + 2 + 2] cocyclotrimerization of oxa- and azabenzonorbornadienes with alkynes catalyzed by nickel complexes: first transition metal-mediated synthesis of isobenzofuran and isoindole precursors

Abstract

Oxabenzonorbornadiene (1a) reacted with various alkynes in the presence of Ni(PPh₃)₂Cl₂, PPh₃ and zinc powder in toluene to give two common products 2a and 2b regardless of the alkyne used. The formation of 2a and 2b are proposed to be from the Diels–Alder reaction of 1a and isobenzofuran. The latter is generated from the retro Diels–Alder reaction of [2 + 2 + 2] cycloadducts of 1a and alkynes catalyzed by the nickel system. A series of nickel-catalyzed [2 + 2 + 2] cycloadducts (4a–j and 5a–d) of oxa- and azabenzonorbornadienes (1a–d) with terminal alkynes were isolated at temperatures −5–18 °C. Similarly, hepta-1,6-diyne and octa-1,7-diyne reacted with 1a and 1d to give novel pentacyclic [2 + 2 + 2] cycloadducts 6a–d in 62–75% yields. These products are convenient isobenzofuran and isoindole precursors that react with various dienophiles to afford the corresponding Diels–Alder adducts. In addition, the [2 + 2 + 2] cycloadducts may be used as precursors for the synthesis of aromatic compounds. For example, the reaction of 1a with methyl but-2-ynoate catalyzed by the nickel system provided aromatic compound 3 in 94% yield. In this reaction, 1a is a ‘masked acetylene’ providing an acetylene moiety to 3.