One‐Pot Synthesis of 1,6‐Diiodo‐2,3,4,5‐tetracarba‐ nido ‐hexaboranes(6) and Mechanistic Studies on the Reaction System Alkynes/BI 3 /NaK 2.8

Attempts to prepare the tetracarba‐ nido ‐octaborane(8) derivative (EtC) 4 (BI) 4 ( 4 ) by treatment of 3‐hexyne with BI 3 (2 equiv.) and NaK 2.8 led to 2,3,4,5‐tetraethyl‐1,6‐diiodo‐2,3,4,5‐tetracarba‐ nido ‐hexaborane(6) ( 2a ) as the predominant product with trace quantities of 4 as well as 2,3‐diethyl‐1,4,5,6,7‐pentaiodo‐2,3‐dicarba‐ closo ‐heptaborane(7) ( 5 ). Starting at −78 °C, reactions of 3‐hexyne, 2‐butyne or diphenylacetylene, respectively, with BI 3 (1 equiv.) and NaK 2.8 afforded the corresponding 1,6‐diiodo‐2,3,4,5‐tetracarba‐ nido ‐hexaboranes(6) 2a − c as the single carborane products. In the cases of 3‐hexyne and diphenylacetylene, the formation of hexaorganylbenzene derivatives was also detected. To confirm the formation of 4 and 5 , the independent dehalogenation of cis ‐3,4‐bis(diiodoboryl)‐3‐hexene ( 3 ) with NaK 2.8 has been studied, which indeed affords 4 and 5 , with the former being the predominant product. A mechanism for the formation of 2a has been proposed, which involves iodoboration of the alkyne to give the borylalkene 1a . This is followed by deiodination, presumably yielding the iodoborirene derivative 7 , which easily dimerizes to form the 1,4‐diboracyclo‐2,5‐hexadiene derivative 8 . Subsequent rearrangement leads to the final C 4 B 2 ‐ nido ‐carborane 2a . To determine the stereochemistry of 1a , its pyridine adduct 9 (and the isomer 9 ′) and catechol derivative 10 (and the isomer 10 ′), and the diisopropylamino derivative 11 have also been prepared. The new compounds have been characterized by MS and NMR spectroscopy, and the structures of 2a , 3 , 9 and 9 ′ were confirmed by single‐crystal X‐ray analyses. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)