Zirconocene-mediated preparation of 1,3-, 1,4-, and 2,3-dibora-1,3-butadienes: Their isolation and characterization and use in Suzuki-Miyaura coupling

Hydrozirconation of 1-alkynyl pinacolboronates, 1, with HZrCp(2)Cl provides gem-borazirconocenes 2. The latter when treated with CuBr gives the homocoupled (1E,3E)-2,3-dibora-1,3-butadienes, 3, in good yield (62-67%). The reaction works even for hindered 2 (R = t-Bu). Structure 3 was assigned on the basis of NMR. Suzuki-Miyaura coupling of 3a (R = n-Bu) with PhI in the presence of CsF leads to the replacement of both boron groups by phenyl and hydrogen to give 6 in 76% yield. Zirconocene-mediated coupling of 1 leads to diastereomeric products: (1E,3E)-1,3-dibora-1,3-butadienes, 4, in 7-26% isolated yields and (1E,3E)-1,4-dibora-1,3-butadienes, 5, in 17-34% isolated yields. The two isomers can be separated by selective precipitation of 5 from the reaction mixture in pentane (-20 degrees C) followed by silica gel chromatography to give pure 4. The reaction does not work when R in 1 is the t-Bu group. Assignments of structures for 4 and 5 were done on the basis of 1D and 2D NMR experiments. In addition, a single-crystal X-ray analysis of 5a showed it to be a highly planar and linearly oriented molecule. Suzuki-Miyaura coupling of 4a proceeded to replace the terminal boron group exclusively, while the internal boron group of 4a remained intact.