Cyclooctane-1,5-diyl dihydroborate

Many metal (cyclooctane-1,5-diyl)dihydroborates(l-) can be prepared by reaction of the easily prepared 9-borabicyclo [3.3.1]nonane dimer (9-BBN)5,6 with metal-organic compounds.7 The reactions of 9-BBN with alkali metal hydrides in ethers are particularly smooth. 

The preparations of lithium and sodium (cyclooctane-1,5-diyl)dihydro-borates(l-) in tetrahydrofuran proceed via isolable, stable etherates. These can be made solvent-free simply by heating under vacuum. 9-Borabicyclo[3.3.1 ]-nonane dimer (9-BBN) can easily be prepared from cycloocta-1,5-diene2 by reaction with tetraethyldiborane(6), tetrahydrofuran-borane8,9 or dimethyl sulfide-borane.10 The synthesis of alkali metal (cyclooctane-1,5-diyl)dihydroborates is achieved by addition of 9-BBN to a suspension of the alkali metal hydride in tetrahydrofuran. Lithium hydride reacts more slowly than sodium or potassium hydride. The reactions are brought to completion by heating under reflux. 

Tetrahydrofuran is dried by refluxing over Na/K and finally distilling from sodium tetraethylaluminate. The apparatus consists of a 500-mL three-necked flask equipped with an internal thermometer, a magnetic stirrer, and a reflux condenser capped with an inert gas (pure N2, argon) bypass. The apparatus is evacuated and then filled with inert gas. 

Two hundred mL of tetrahydrofuran and (3.4 g 430 mmole) LiH powder are transferred to the 500-mL flask. A total of (24.5 g, 100 mmole) 9-BBN-dimer is added in portions in 1 hr to the well-stirred suspension by transferring it from a 100-mL flask via a 15-cm-long PVC tube (0.9 i.d.) by occasional shaking. The mixture is then heated 3 hr under reflux. After cooling to room temperature, the excess LiH is separated by filtration. 

The filtrate is evaporated to dryness under vacuum ( 14 torr). The tetra-hydrofuran solvate is freed of solvent by heating to 150° under vacuum (1CT3 torr). The salt cake is broken up with a glass rod and then pulverized to yield 25.2 g (97%) of tetrahydrofuran-free powder. 

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The air-stable alkali metal (cyclooctane-1,5-diyl)dihydroborates(l-) are very moisture-sensitive. They must be stored and handled in an inert gas atmosphere. The salts are insoluble in aliphatic solvents and poorly soluble in aromatic solvents such as mesitylene. 

TABLE I Spectroscopic Parameters for Alkali Metal (Cyclooctane-1,5-diyl)dihydroborates( 1- )... 

The hydride content of both the ether-containing and solvent-free alkali metal (cyclooctane-1,5-diyl)dihydroborate is (1-) can be determined volumet-rically by measuring the hydrogen evolved after treatment with an alcohol (e.g., 2-ethyl-l-hexanol) at room temperature. 

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