Carba-arachno-pentaboranes lO

Carbaboranes with an arachno-CB4 skeleton, that is derivatives of 1-carba-arachno-pentaborane(lO), were identified as side products (Scheme 3.2-11) 18 in the reaction of dichloroethylborane with elemental Li [29], and 19 was obtained from pro-pyne and tetraborane(lO) [30], The reaction of dimethyl(dipropyn-l-yl)silane with a large excess of tetraethyldiborane(6) [ hydride bath which contains Et2BH2BEt2, other ethyldiboranes(6) and triethylborane] [31, 32] resulted in the formation of 20 in good yield [33]. 

Scheme 3.2-12. Exchange of chlorine for hydrogen with LiBH4 in the diborabenzofulvene 21 followed by hydroboration and coordination with BH3 as well as protonation to give 1-carba-arachno-pentaborane(lO) 22.

Scheme 3.2-13. Hydroboration of diethyl (propyn-l-yl)borane in the hydride bath via 23 and 24, followed by Et2BH-catalyzed condensation and self-assembly, leads to the 1-carba-arachno-pentaborane(lO) 25, having the Et2B substituent at the exopolyhedral carbon atom.

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