Reactions of <i>a</i><i>rachno</i>-6,8-C₂B₇H₁₂^- with Electron Deficient Olefins: Syntheses of Cyano-Substituted Carboranes

The syntheses of new cyano-substituted derivatives of arachno-6,8-C2B7H13 have been achieved through the addition reactions of the arachno-6,8-C2B7H12- (1-) anion with cyano-activated olefins. The reaction of PSH+1- with tetracyanoethylene (TCNE) yielded the unusual bridging compound PSH+endo-6-endo-7-[μ2-(C(CN)2)2]-arachno-6,8-C2B7H12- (PSH+2-) resulting from cycloaddition of the TCNE at the C6−B7 edge of the anion. Consistent with its hypho skeletal electron count, an X-ray crystallographic study and DFT/GIAO calculations confirm 2- has a more open structure than 1-. The reaction of 1- with acrylonitrile resulted in the formation of endo-6-(NCCH2CH2)-arachno-6,8-C2B7H11- (3-), which, upon acidification, afforded endo-6-(NCCH2CH2)-arachno-6,8-C2B7H12 (3) in high yield. X-ray crystallographic and DFT/GIAO studies established that the cyanoethyl fragment in 3 is substituted at the endo-position of the C6 cage-carbon. Heating 3 in THF at 50 °C or in toluene at 110 °C resulted in the quantitative isomerization of the cyanoethyl-substituent from the endo- to the exo-position at C6 to yield exo-6-(NCCH2CH2)-arachno-6,8-C2B7H12 (4). This is the first example of an endo to exo isomerization to be observed at a cage-carbon of a carborane. While heating 3 resulted in isomerization to 4, heating 3- in the presence of a small amount of 3 yielded the new ethylene-bridged 10-vertex tricarbaborane μ6,9-(CH2CH2)-arachno-5,6,9-C3B7H11 (5) resulting from reduction of the 3- pendant nitrile group, followed by deammination and carbon insertion.