Tetraalkyldiboranes and 9-Borabicyclo nonane Dimer

In contrast to trialkylboranes BR3, the mono- and dialkylboranes R2BH and RBH2 are dimers with structures related to diborane(6) i.e., they are alkyldiboranes. With a few exceptions (borolane dimers and 9-borabicyclo[3.3.1]nonane dimer) they may also be in equilibrium at room temperature with small amounts of monomer. Characteristically there is rapid R/H exchange. Hence alkyldiboranes are in most cases mixtures of boranes in various degrees of alkylation. Tetraalkyldiboranes are therefore mixtures of alkyldiboranes and trialkylboranes, and the mixtures have mean compositions corresponding to a tetraalkylated diborane  

The chemistry of the alkyldiboranes is largely covered in recent review articles.1 5 In practically all the preparative methods for organohydroboranes, the BH3/BR3 redistribution is important. 

Alkyldiboranes can also be obtained from trialkylboranes by hydrogenation under pressure at elevated temperatures.9 

For preparation of the well-known10,11 alkyldiboranes with ethyl, propyl, and butyl groups, it is convenient to react the available trialkylboranes1,6 with diborane(6). 

Diborane(6) is best generated in accord with the first equation in a separate apparatus (Fig. 4), and the gas is passed into the tri-alkylborane R3B (R = C2H5, C3H7) at room temperature. The redistribution reaction (second equation) proceeds extremely sluggishly in pure trialkylborane. However, after addition of a little diethyl ether12 or, better, some previously prepared alkyldiborane, diborane(6) reacts with heat evolution practically quantitatively with trialkylboranes. 

---