Cobalt-Mediated Cyclooligomerization Reactions of Borylacetylenes

Reaction of bis(diethylamino)acetylene (1) with lithium and chlorobis(dimethylamino)borane yields 1-[bis(dimethylamino)boryl]-2-(diethylamino)acetylene (2). Treatment of 1-aryl-2-(trimethylstannyl)acetylenes 3 with chlorobis(diisopropylamino)borane results in the quantitative formation of 1-aryl-2-[bis(diisopropylamino)boryl]acetylene compounds 4. The 1-aryl-2-borylacetylene derivatives 5, 6, and 7 are obtained by the reaction of 3 with the appropriate haloborane, or by reacting 4 with two equivalents of HCl and one equivalent of catechol, dithiocatechol and 2-hydroxythiophenol. The catalytic cyclotrimerizations of 5a−d with [CpCo(CO)2] or [Co2(CO)8] lead to isomeric mixtures of the triborylbenzene derivatives 8a−d and 8a′−d′, whereas 6a−d and 7a−b undergo cyclotrimerization only with catalytic amount of [Co2(CO)8], to give isomeric mixtures of the triborylbenzene derivatives 12a−d/12a′−d′ and 13a−b/13a′−b′, respectively. The reaction of 2 with a stoichiometric amount of (η5-cyclopentadienyl)bis(ethene)cobalt leads to (η5-cyclopentadienyl)(η4-cyclobutadiene)cobalt complexes 9. The analogous complexes 10 and 11 are obtained by the reactions of 6 and 7 with a stoichiometric amount of (η5-cyclopentadienyl)dicarbonylcobalt. Treatment of Co2(CO)8 with 5a, 6a, and 7a yields the dicobaltatetrahedranes 14a, 15a and 16a. Compound 15a reacts with two equivalents of diphenylacetylene and 1-(1,3,2-benzodioxaborolyl)-2-phenylacetylene (5a), to furnish (1,3,2-benzodithiaborolyl)pentaphenylbenzene (17) and bis(1,3,2-benzodioxaborolyl)(1,3,2-benzodithiaborolyl)triphenylbenzene (18), respectively. The new compounds have been characterized by NMR spectroscopy and mass spectrometry, as well as by X-ray structural analyses for 4a, 5a, 6a, and 10a. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)