Synthesis and Reactivity of Boron-Functionalized C2B5-closo-Carboranes

Treatment of the nido-2,3-Et2C2B4H42−dianion (1) with monoboron reagents led to closo-C2B5 carborane derivatives with functional substituents at the inserted apical boron atom. The reactions of 1 with BX3 (X = Br, I) afforded the corresponding closo-1-X-2,3-Et2C2B5H4 (2a,b), and with PhCΞCBcat (cat = O2C6H4) produced the alkynyl-substituted closo-1-CΞCPh-2,3-Et2C2B5H4 (2c). Pd-catalyzed Negishi-type cross-coupling reactions of 2b with RCΞCZnCl at room temperature gave the corresponding closo-1-CΞCR-2,3-Et2C2B5H4 derivatives 2d-f, R = SiMe3, Me, and tBu, respectively. Compound 3 with two C2B5 moieties linked via a C=C unit was obtained by a similar boron incorporation reaction with cis-Cl2B5(Et)C=C(Et)BCl2. The reactions of 2c,d with Co2(CO)8 afforded the dicobaltatetrahedrane-substituted carboranes 4c and d, in which the clusters C2B5 and Co2C2 are connected by a B-C bond. Compounds 4c,d lost the apical boron on wet silica gel or sand to give the nido-C2B4-C2Co2 compounds 5c,d. Formation of the carboranyl-substituted (η5- C5H5)Co(cyclobutadiene) complex 6c was observed in the reaction of 2c with (η5-C5H5)Co(C2H4)2. The composition of the products follows from NMR and MS data.