Amine-borane adducts

Transition metal-catalyzed approaches to boron-nitrogen compounds are potentially of significant importance. Very interestingly. Manners et ah have reported the catalytic dehydrocoupling of primary or secondary amine-borane adducts 

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Amine—borane adducts have the general formula R3N BX where R = H, alkyl, etc, and X = alkyl, H, halogen, etc. These compounds, characterized by a coordinate covalent bond between boron and nitrogen, form a class of reducing agents having a broad spectmm of reduction potentials (5). 

Amine-borane adducts have the general formula R3NBX3 where R = alkyl, H, etc., and... 

Reactions of Enamines.—Hydroboronation of the 2-enamine (397) gave a product (amine-borane adduct ) from which the 3a- and 3 -pyrrolidino-derivatives (398 3a 3)5 = 82 18) were liberated by refluxing methanol. The stereochemistry of this reduction is quite unusual, and not fully explained. Reaction of the 3,5-dienamine (399) with dichlorocarbene led to ring expansion, giving the A-homo-4-chloro-dienone (400). " Condensations of the enamine... 

Scheme 11.13 Catalytic dehydrocoupling of primary amine-borane adducts.

Scheme 11.14 Catalytic dehydrocoupling of secondary amine-borane adducts.

Jaska CA, Temple K, Lough AJ, Manners I (2003) Transition metal-catalyzed formation of boron-nitrogen bonds catalytic dehydrocoupling of amine-borane adducts to form aminoboranes and borazines. J Am Chem Soc 125 9424—9434... 

Clark TJ, Russell CA, Manners I (2006) Homogeneous, titanocene-catalyzed dehydrocoupling of amine-borane adducts. J Am Chem Soc 128 9582-9583... 

Amine-borane adducts have great interest as they are promising candidates for hydrogen storage since evolution of hydrogen is observed when amine-borane adducts are exposed to titanium complexes (Scheme 5.10). 

Scheme 5.10 Titanium-catalysed dehydrogenation reactions of amine-borane adducts.

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].

Primary and secondary amines react with 1,2-alkyl or aryldiboranes to give amine-borane adducts which, with appropriate thermal encouragement, evolve hydrogen and yield the corresponding aminoboranes. Amino-boranes generally exhibit a tendency to disproportionate by reversibly exchanging R (or Ar) for H on boron,... 

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