SYNTHESIS AND REACTIVITY STUDIES OF THE SMALL CARBORANE CLOSO-2,3-ET2C2B5H5

Three new viable synthetic routes to the small closo-carborane 2,3-Et2C2B5H5(I) are reported based on cluster expansion reactions of the small nido-carborane 2,3-Et2C2B4H6utilizing Lewis base-borane adducts. Thus, reaction of nido-2,3-Et2C2B4H6with Et3N-BH3at 140 °C resulted in the capping of the open face of the nido-carborane by a BH group derived from the amine-borane and produced I in 50–60% yield. Reactions employing Et3N·l0BH3confirmed that the inserted BH group is derived from the amine-borane. Reactions of nido-2,3-Et2C2B4H6with Et3N-BH3at higher temperature (170 °C) resulted in the isolation of a mixture of close-carboranes including I, closo-1,7-Et2C2B6H6(II), and closo-l,7-Et2C2B7H7(III). II and III were also formed by reaction of I with Et3N-BH3at 190 °C. Alternatively, I may be prepared bv the reaction of Li+[nido-2,3-Et2C2B4H5]−with (CH3)2S-BH3or reaction of Na+Li+[nido-2,3-Et2C2B4H4]12−with (CH3)2S-BHBr2in yields of of 44% and 49%, respectively. Reaction of Na+Li+[nido-2,3-Et2C2B4H4]2−with PhBCl2or MeBBr2produced the mono-B-substituted compounds closo-1-Ph-2,3-Et2C2B5H4(IV) (41%), and closo-l-Me-2,3-Et2C2B5H4(V) (46%). Thermolysis of I at ~320 °C in the gas phase resulted in the high-yield (81%) isomerization of the carborane to closo−2,4-Et2C2B5H5(VI). Halogenation of I with elemental bromine or with Br2/AlBr3at room temperature results in the exclusive formation of closo-5-Br-2,3-Et2C2B5H4(VII). Reaction of I with 1 equiv of Li(BEt3)H resulted in the formation of the first seven-vertex nido-carborane anion [nido-3,4-Et2C2B5H6]−(VIII), which was shown to have a structure based on a dodecahedron missing one five-connected vertex, and is thus the first 18-electron, seven-vertex nido polyhedral borane for which this geometry has been confirmed. The analogous neutral, seven-vertex 18-electron species nido-6-Me3P-3,4-Et2C2B5H5(IX) was formed in the reaction between I and Me3P at temperatures above −40 °C. © 1990, American Chemical Society. All rights reserved.