Proton-Sponge-Initiated Reactions of 6,8-Dicarba-arachno-nonaborane(13) and 6,7-Dicarba-arachno-nonaborane(13) with Methyl Propynoate: One-Step Syntheses of the 6-(MeOOCCH2)-5,6,7-Tricarba-arachno-decaborane(12) and 6-(MeOOCCH2)-5,6,10-Tricarba-nido-decaborane(10) Tricarbaboranes

Abstract

The reaction of arachno-6,8-C2B7H13 (1) with methyl propynoate in the presence of only 0.1 equivalent of 1,8-bis(dimethylamino)naphthalene (Proton Sponge®, PS) results in the one-step formation of the neutral tricarbaborane 6-(MeOOCCH2)-arachno-5,6,7-C3B7H12 (2). Similarly, only 0.1 equivalent of PS initiates the reaction of the isomeric arachno-6,7-C2B7H13 (3) carborane with methyl propynoate to yield the new tricarbaborane, 6-(MeOOCCH2)-nido-5,6,10-C3B7H10 (4). These results suggest a cyclic reaction sequence involving initial deprotonation of the starting dicarbaborane by PS, followed by reaction of the resulting anion with methyl propynoate to produce a new anion. Protonation of this second anion by another equivalent of the starting dicarbaborane then produces the final neutral tricarbaborane product and the cycle begins again. While 4 is unreactive toward NaH or KH, it readily added n-BuLi to give, following acidification with HCl-Et2O, a new product tentatively identified as 6-(MeOOCCH2)-Bu-arachno-5,6,10-C3B7H11 (6). The proposed structures of 4 and 6 are strongly supported by ab initio/IGLO nuclear shielding calculations.