Cyclisations of alkynoic acids using copper(I) arylspiroborate complexes

In this study we have studied the synthesis, characterization and catalytic activity of phosphinocopper(I) complexes [Cu(PPh 3 ) 3 (NCCH 3 )][B(O 2 C 6 H 4 -4-R) 2 ] (3a: R = H; 3b: R = Me; 3c: R = NO 2 ) and [Cu(PPh 3 ) 3 (NCCH 3 )][B(O 2 C 6 H 3 -3,5-di- t Bu) 2 ] (3d) containing weakly coordinating arylspiroborate ligands bearing various electronic and sterically defined characteristics. All new compounds have been characterized fully including single crystal X-ray diffraction studies for 3a and 3c and confirm that the arylspiroborate ligands do not coordinate to the copper atoms. Using these new metal complexes as precatalysts in the cyclisation of short chain alkynoic acids gave the corresponding exo-dig cyclic lactones exclusively.