Ammonia-borane adduct

The formation of individual cycloborazanes can be achieved, in some cases, by transition metal-catalysed dehydrocoupling reactions. By using this strategy, secondary amine-boranes are converted to the four-membered ring (H2BNMe2)2 under mild conditions, whereas primary amine-boranes produce borazines. The cyclic pentamer is obtained as the exclusive product from ammonia-borane adduct H3B NH3 employing an iridium(fll) catalyst [see eqn (2.13) in Section 2.3)]. 

Figure 4.24. Potential energy curves for the dissociation a) of the C—N bond in protonated methylamine and b) of the B—N bond in the ammonia-borane adduct. Results of large-scale Cl calculations with all geometric parameters except the bond distance R held fixed (by permission from Michl and BonaCie-Kouteck, 1990).

Borazine, B3N3H6, was first prepared by thermolysis of the diborane ammonia adduct [(BH2)(NH3)2] [BH4] . More convenient procedures for the laboratory preparation of this important ring system in multigram quantities involve either (a) the decomposition of ammonia- borane [eqn (9.1)] or (b) the reaction between ammonium sulfate and sodium borohydride [eqn (9.2)]. The latter method provides a convenient and economical synthesis of borazine. 

In 2006, Stephan introduced the concept of frustrated Lewis pairs , systems in which steric congestion precludes the formation of a Lewis acid-base adduct [25,26]. These combinations offer novel reactivity involving the activation of a variety of small molecules including amines, alkenes, alkynes, and molecular hydrogen [94]. While initially phosphines were the Lewis base of choice for FLP reactions, Al-heterocyclic carbenes were soon proven to be viable candidates as well. For instance, an FLP of ItBu and tris(pentafluorophenyl)borane was shown to react with molecular hydrogen in the usual FLP fashion, forming an imidazo-lium borate salt. This FLP was also used to activate ammonia, aniline, and diphe-nylamine, by addition of the NHC to the amine-borane adduct as shown in Scheme 15.5 [95]. 

Scheme 15.5 N-H bond cleavage of amine-borane adducts by IfBu to produce imidazolium borate salts (R2NH = ammonia, diphenylamine, aniline Ar = pentafluorophenyl) [95].

Other methods for the synthesis of AB have been described by Shore and coworkers at Ohio State University involving reactions of ammonia with borane adducts of appropriate amines [104]. 

Softer bases will also react with diborane and cause symmetric cleavage. A typical preparation for ammonia borane from diborane that seeks to avoid the salt structure would be to cleave the diborane symmetrically with a soft base, forming a BH3 Soft Base adduct. The soft base is then displaced by ammonia to yield the product, in the desired BH3NII3 form [53]. 

Borazine has been known since the pioneering work of Alfred Stock early in this century. Stock s work was important in two regards He was the first to study compounds such as the boranes. silanes, and other similar nonmetal compounds, and he perfected vacuum line techniques for the handling of au- and moisture-sensitive compounds, invaluable to the modern inorganic chemist.67 Stock synthesized borazine by heating the adduct of diborane and ammonia 66... 

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