Synthesis, Structures and Reactivity of Mono‐ and Diborylacetylenes

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AbstractThe reaction of [bis(diisopropylamino)boryl]acetylene with 2 equiv. of HCl and 1 equiv. of dithiocatechol or 2‐hydroxythiophenol yields the corresponding monoborylacetylenes 1 and 2, respectively. Diborylacetylenes 3 and 4 are obtained from the reactions of bis(trimethylstannyl)acetylene with 2 equiv. of bromoboranes. Treatment of chlorobis(diisopropylamino)borane with LiC≡C−EMe3 (E = C, Si) leads to the corresponding element‐substituted [bis(diisopropylamino)boryl]acetylenes 5a,b. The syntheses of monoborylacetylenes 6a,b−8a,b are achieved by the reaction of 5a,b with 2 equiv. of HCl and 1 equiv. of catechol, dithiocatechol or 2‐hydroxythiophenol, respectively. Stoichiometric amounts of [CpCo(CO)2] react with 1−4 in refluxing toluene to give the corresponding (η4‐cyclobutadiene)cobalt complexes 9−12. Analogous isomeric mixtures of (η4‐cyclobutadiene)cobalt complexes 13a/13a′−15a/15a′ are obtained from 1‐tert‐butyl‐2‐borylacetylenes 6a−8a and [CpCo(C2H4)2], whereas isomeric mixtures of (η4‐cyclohexatriene)(η5‐cyclopentadienyl)cobalt complexes 13b/13b′−15b/15b′ are formed with 1‐boryl‐2‐silylacetylenes 6b−8b under similar conditions. Hydrolysis of 13a−15a yields the (η4‐1,3‐di‐tert‐butylcyclobutadiene)cobalt complex 16. Catalytic trimerizations of mono‐ (1, 2) and diborylacetylenes (3, 4) with Co2(CO)8 lead to isomeric mixtures of triboryl‐ (17/17′, 18/18′) and hexaborylbenzene derivatives (19, 20), respectively. The new compounds are characterized by NMR spectroscopy and mass spectrometry as well as by X‐ray structure analyses for 1b, 5b, 6b, 14a, 15a and 16. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)