Insertion reactions of benzyne-nickel(0) complexes with acetylenes

Article abstract of DOI:10.1021/om00003a029

Complexes of nickel(0) containing η²-4,5-difluorobenzyne, Ni((1,2-η)-4,5-F₂C₆H₂)(PEt ₃)₂ (10) and Ni((1,2-η)-4,5-F₂C₆H₂)(dcpe) (11; dcpe = 1,2-bis(dicyclohexylphosphino)ethane, Cy₂PCH₂-CH₂PCy₂) have been synthesized by alkali-metal reduction of the appropriate (2-halogenoaryl)nickel(II) halides. Spectroscopic measurements (¹³C NMR, FAB-MS) indicate that 10, 11, and the parent benzyne complex Ni((1,2-η)-C₆H₄)(PEt₃)₂ (2) are monomeric, analogous to the structurally characterized species Ni((1,2-η)-C₆H₄)(Cy₂PCH₂CH ₂PCy₂). Complexes 2 and 10 undergo rapid intermolecular exchange with PEt₃ at room temperature and react with disubstituted acetylenes by double insertion into the metal-benzyne bond to form 1,2,3,4-tetrasubstituted naphthalenes. With electrophilic acetylenes (MeO₂CC₂CO₂Me, MeC₂-CO₂Me, HC₂CO₂Me, and CF₃C₂CF₃) an aromatic cyclotrimer is also formed; exceptionally, hexafluorobut-2-yne also gives with 10 a phenanthrene derived from two benzyne units and the acetylene. The unsymmetrical acetylenes tert-butylacetylene and methyl 2-butynoate give rise to good regioselectivity in the resulting naphthalenes, the favored isomers being very dependent on the steric and electronic influence of the substituents. The dcpe complexes react similarly but more slowly with acetylenes, and with MeO₂CC₂CO₂Me the monoinsertion complexes Ni{C(CO₂Me)=C(CO₂Me)C₆H₂R ₂-o}(dcpe) (R = H, F) can be observed.