Syntheses and surprising regioselectivity of 5- and 6-substituted decaboranyl ethers via the nucleophilic attack of alcohols on 6- and 5-halodecaboranes

The selective syntheses of new classes of decaboranyl ethers containing a range of functional groups substituted at the B5 or B6 positions were achieved through the reaction of alcohols with halodecaboranes. The surprising regioselectivity of the reaction, where the reaction of the 6-halodecaboranes (6-X-B(10)H(13)) with alcohols yielded the 5-substituted decaboranyl ethers (5-RO-B(10)H(13)) and the reaction with 5-halodecaboranes (5-X-B(10)H(13)) gave the 6-substituted decaboranyl ethers (6-RO-B(10)H(13)), was confirmed by NMR and X-ray crystallographic analyses. The crystallographic determinations also showed that the decaboranyl ethers had shortened B-O bonds and apparent sp(2) hybridization at oxygen indicating significant π-backbonding from oxygen to the cage boron. A possible substitution mechanism was computationally identified involving: (1) initial nucleophilic attack by the alcohol-oxygen at a site adjacent to the 5- or 6-halo-substituted boron, (2) movement of the terminal hydrogen at the point of attack to a bridging position, (3) formation of a 5-membered (B-O-H-Cl-B) cyclic transition state allowing the acidic methanolic-hydrogen to bond to the halogen, (4) release of HX, and finally (5) movement of a bridging hydrogen into the vacated terminal position. Deuterium labeling studies confirmed the movement of hydrogen from a bridging position of the halodecaborane into the halogen-vacated terminal position on the decaboranyl ether product. The relative reaction rates of the 6-X-B(10)H(13) compounds (X = F, Cl, Br, I) with alcohols were likewise found to be consistent with this mechanism.