Formation of Iminoboranes

Since iminoboranes are thermodynamically unstable with respect to their oligomerization (Section IV,A), they can be isolated under laboratory conditions by making the oligomerization kinetically unfavorable. Low temperature, high dilution, bulky groups R and X, or a combination of these is necessary. 

The parent compound HBNH was the first iminoborane to be identified unequivocally (8). It was formed by photolysis of solid H3BNH3 and trapped, together with further products, in a rare gas matrix which provided low temperature and high dilution for this extremely unshielded molecule. Evidence for the iminoborane in the mixture came from a vibrational analysis, all four atoms in turn being isotopi-cally labeled. 

The first iminoborane that could be handled at — 30°C in a liquid 1 1 mixture with Me3SiCl was (FjCglBNfBu (9). It was synthesized by the general elimination reaction in Eq. (1) (Hal = halide) from the corresponding aminoborane. A hot tube procedure must be employed for 

Thermal decomposition of azidoboranes [Eq. (3)], and of [trimethyl-silyl(trimethylsilyloxy)amino]boranes [Eq. (4)], permits a simple synthesis of symmetric iminoboranes RBNR (17-19). A hot tube procedure at about 300°C and 10 Torr turned out to be useful. The iminoborane iPrBNiPr, for example, was prepared at a rate of 10 g/hour by the azidoborane method. No separation problems are met with this method. Handling the liquid reactants of Eqs. (3) and (4) is hazardous. 

The violence of uncontrolled decompositions decreases markedly with the bulk of R, and explosions are more violent in the case of azidoboranes. Hot tube experiments at 400°C, aiming at the preparation of aminoiminoboranes XBNR [X = Me3Si(iBu)N, R = iPr, Bu] from the corresponding azidoboranes XRBN3, have been unsuccessful insofar as the cyclodimers of XBNR were the only isolable products (14). 

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The most widely used method for the synthesis of iminoboranes involves the 1,2-addition of boron-element bonds such as boron-hydrogen, boron-halogen, boron-carbon, or boron-sulfur bonds across the C=N bond of nitriles thereby producing variously substituted iminoboranes (Eq. (2)). The formation of iminoboranes as well as the stability of the products depends on the substituent on the nitrile group, the nature of the boron-element bond to be cleaved during the 1,2-addition across the C N bond, and to a lesser extent on the non-reacting boron substituents 26T... 

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