Properties of Substituted Tetrahydroborates

The reaction of lithium monoalkyltrihydroborates with 1 equivalent of acetic acid in tetrahydrofuran proceeds rapidly, evolving one equivalent of H2, and generating Li(RBH3), RB(0C(0)CH3)2, and Li(RB(0C(0)CH3)3). In diethyl ether or n-pentane (25 C, 15 minutes), Li(0C(0)CH3) precipitates from the solution and, thus, the monoalkylborane is obtained [1]  

The reaction of CH3I with Li(RBH3) is rapid in tetrahydrofuran at 0°C but impractically slow in / -pentane and diethyl ether. The reaction rate accelerates if methyl iodide is used in excess and 10% tetrahydrofuran is added. In both cases the monoalkylborane is formed Li(RBH3) + CH3l- RBH2 + LiI + CH4 [1]. 

Li(RBH3) reacts rapidly with a 20% excess of (CH3)3SiCl in tetrahydrofuran, diethyl ether, or n-pentane, to form the monoalkylborane and H2 with concurrent precipitation of LiCl. In diethyl ether or n-pentane, redistribution of the monoalkylborane is less than in tetrahydrofuran [1]. 

The monoalkyltrihydroborates may be regarded as convenient stabilized forms of monoal-kylboranes. Thus, treatment of the species with either HCl, CH3I, or (CH3)3SiCl, depending on the solvent required, in the presence of a substrate such as a hydroboration or reduction target, is a convenient way to effect such chemistry [2]. Similarly, dialkyldihydroborates are convenient sources of dialkylboranes and, thus, are useful precursors in typical dialkylborane chemistry [1, 2]. 

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