New synthetic routes to azacarborane clusters: Nitrile insertion reactions of nido-5,6-C2B8H11- and nido-B10H13-

Article abstract of DOI:10.1021/ja960787m

A synthetic sequence involving the initial nucleophilic attack of the isoelectronic nido-5,6-C₂B₈H₁₁^- or nido-B₁₀H₁₃^- anions at a nitrile carbon, followed by nitrile hydroboration and cage-insertion, has been fouud to yield new azacarborane clusters in good yields. Thus, the reaction of nido-5,6-C₂B₈H₁₁^- with refluxing acetonitrile gave the azatricarbaborane anion arachno-7-CH₃-5,7,14,12-C₃NB₈H₁₁^- (1^-) in 65% yield, while nido-B₁₀H₁₃^- reacted with acetonitrile, benzyl cyanide, or CH₃¹³CN to give the azamonocarbaborane anions arachno-7-CH₃-7,12-CNB₁₀H₁₃^- (2a^-), arachno-7-Bn-7,12-CNB₁₀H₁₃^- (2b^-), and arachno-7-CH₃-7,12-¹³CNB₁₀H₁₃^- (2a^--¹³C), respectively. Single-crystal X-ray studies of the isoelectronic clusters, 1^- and 2a^-, showed that hydroboration of the nitrile occurred, with the resulting imino group inserting into the cage framework in a position bridging the B2 and B11 borons. Consistent with their arachno skeletal electron counts, 1^- and 2a^- have cage frameworks containing two six-membered open faces that may be derived from a bicapped hexagonal antiprism by removal of two non-adjacent five-coordinate vertices. Alternatively, 1^- and 2a^- may be considered as having 10-vertex arachno frameworks with exopolyhedral bridging imine substituents. Acidification of 1^- resulted in loss of one boron and the imine nitrogen and formation of the known tricarbaborane nido-6-CH₃-5,6,9-C₃B₇H₁₀. In contrast, acidification of 2^- led to loss of only one boron to yield the new azamonocarbaborane hypho-12-R-12,13-CNB₉H₁₅ (3). A single-crystal X-ray study confirmed that 3 has an 11-vertex hypho structure, containing two six-membered open faces, that is based on a 14-vertex closo polyhedron missing three vertices. In 3, further hydroboration of the CN group occurred, such that the carbon contains an additional hydrogen and the nitrogen is connected to two borons. Deprotonation of 3 with Proton Sponge, 1,8-bis(dimethylamino)naphthalene, initially yielded hypho-12-R-12,13-CNB₉H₁₄^- (3^-), which subsequently underwent a skeletal rearrangement to yield the isomeric anion hypho-12-R-12,11-CNB₉H₁₄^- (4^-). A single-crystal X-ray study of 4^- confirmed that it has an 11-vertex hypho structure with one seven-membered and one five-membered open face. Subsequent acidification of 4^- resulted in loss of one additional boron to give hypho-8-R-8,13-CNB₈H₁₄ (5). Deprotonation with Proton Sponge gave the new anion hypho-8-R-8,13-CNB₈H₁₃^- (5^-). Based upon spectral and computational data, 5 and 5^- are proposed to have the CN unit incorporated into a 10-vertex hypho structure.