Synthesis, Structures and Reactivity of Mono‐ and Diborylacetylenes

The 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−EMe 3 (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 (η 4 ‐cyclobutadiene)cobalt complexes 9 − 12 . Analogous isomeric mixtures of (η 4 ‐cyclobutadiene)cobalt complexes 13a/13a′ − 15a/15a′ are obtained from 1‐ tert ‐butyl‐2‐borylacetylenes 6a − 8a and [CpCo(C 2 H 4 ) 2 ], whereas isomeric mixtures of (η 4 ‐cyclohexatriene)(η 5 ‐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 (η 4 ‐1,3‐di‐ tert ‐butylcyclobutadiene)cobalt complex 16 . Catalytic trimerizations of mono‐ ( 1 , 2 ) and diborylacetylenes ( 3 , 4 ) with Co 2 (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)