Syntheses and Surprising Regioselectivity of 5- and 6-Substituted Decaboranyl Ethers via the Nucleophilic Attack of Alcohols on 6- and 5-Halodecaboranes

Abstract

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.

Synthesis, Structural Characterization, and Reactivity Studies of 5-CF3SO3-B10H13

“…The pentane was vacuum-evaporated to give 2 (45.0 mg, 0.30 mmol, 80%) as a white solid. The 11 B and 1 H NMR spectra of 2 were consistent with those previously reported …”

“…The B5–O1 distance (1.447(4) Å) in 1 is slightly longer than that found in 6-TfO-B 10 H 13 (1.433(4) Å) but is significantly longer than those found in 5-RO-B 10 H 13 (R = Me (1.370(3) Å), ClC 2 H 4 OC 2 H 4 (1.3604(19) Å), and CH 3 CCCH 2 (1.3828(17) Å)) . These differences are consistent with less O1 → B5 π-bonding in 1 , as is expected for the strongly electron-withdrawing triflate group.…”

“…Nucleophilic attack of alcohols on 5-X-B 10 H 13 haloboranes was previously shown to result in halogen substitution by an alkoxide to yield 6-RO-B 10 H 13 ethers . Likewise, when 1 was reacted with methanol, clean conversion to 6-MeO-B 10 H 13 ( 2 ) was observed (eq ) with the 80% isolated yield of 2 being significantly higher than that from the analogous reaction of methanol with 5-Br-B 10 H 13 (54% yield).…”

“…The formation of the 6-substituted 6-RO-B 10 H 13 derivatives starting from 5-TfO-B 10 H 13 is consistent with the observed formation of 6-RO-B 10 H 13 products in the reaction of alcohols with 5-X-B 10 H 13 (X = Cl, Br, I). DFT studies of the MeOH/5-Cl-B 10 H 13 reaction indicated that this reaction proceeds by initial nucleophilic attack of the alcohol at B6 to form a transition state in which the B5–B6 bridging hydrogen moves to an endo position on B5. Then as HCl is eliminated from B5, the B6 hydrogen moves to the B5–B6 bridging position to produce 6-MeO-B 10 H 13 .…”

Boron, aluminium, gallium, indium and thallium

“…A new class of decabolanyl ethers containing an array of functional groups substituted at the B5 or B6 positions of the nido-B 10 clusters have been described and fully characterised by Sneddon and co-workers. 38 A new synthesis of the per-chloro borane Cs 2 [B 12 Cl 12 ] from the chlorine gas free reaction of SO 2 Cl 2 with Cs 2 [B 12 H 12 ] has been reported. 39 Reaction of the trityl salt [Ph 3 C] 2 [B 12 Cl 12 ] with AlMe 3 , GaEt 3 or InEt 3 has been shown to result in the formation of the chloro-borane stabilisation of dialkylmetal cation fragments.…”