Amine boranes secondary

Properties and Reactions. Amine boranes are usually colodess, crystalline compounds, which exhibit sharp melting points and thermal stability when pure. Primary and secondary amine boranes are generally soHds at ambient temperatures. With the exception of trimetbylamine borane, the ahphatic /-amine boranes are Hquids. The nature of the bonding in amine boranes has been the subject of theoretical investigations (21—23). 

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)]. 

Acylation of amines. Reaction of a primary or secondary amine, borane-trime-thylamine, and a carboxylic acid in the molar ratio 1 1 3 in refluxing xylene results in formation of an amide in yields generally of 70-95% (equation I). 

Amine>Boranes are complexes of boranes (BH,) with amines. Most of the secondary and tertiary amines form these complexes. 

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... 

Scheme 11.14 Catalytic dehydrocoupling of secondary amine-borane adducts.

Reaction scope Although there have been extensive investigations into the stoichiometric reactions of group 2 complexes with ammonia-borane [119, 147-153], secondary amine-boranes, and primary amine-boranes and their thermal decomposition products, only a handful of catalytic reactions have been reported. The reaction products depend on the substitution of the amine-borane. For example. Harder and coworkers have reported that the sterically demanding primary amine-borane ArNH2 BH3 (Ar = 2,6-di-/so-propylphenyl) reacts to... 

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,... 

Reductions using Boron Compounds.—The advantages of lithium triethylboro-hydride as a very powerful reducing agent, and of amine—boranes derived from primary and secondary amines as mild, chemoselective reducing agents for aldehydes and ketones, have been extolled. Several reagents are recom-... 

Scheme 5 Dehydrogenation of secondary amine boranes at group 9 metal centers leading to the formation of a coordinated aminoborane (upper) ora linear borazane chain (lower). ...

Organoboranes react with a mixture of aqueous NH3 and NaOCl to produce primary amines. It is likely that the actual reagent is chloramine NH2CI. Chloramine itself,hydroxylamine-O-sulfonic acid in diglyme, and trimethyl-silyl azide " also give the reaction. Since the boranes can be prepared by the hydroboration of alkenes (15-16), this is an indirect method for the addition of NH3 to a double bond with anti-Markovnikov orientation. Secondary amines can be prepared by the treatment of alkyl- or aryldichloroboranes or dialkylchlorobor-anes with alkyl or aryl azides. 

Secondary amines are formed by reaction of trisubstituted boranes with alkyl or aryl azides. The most efficient borane intermediates are monoalkyldichloroboranes, which are generated by reaction of an alkene with BHCl2 Et20.190 The entire sequence of steps and the mechanism of the final stages are summarized by the equation below. 

Reaction of the salts of primary and secondary alky Initio compounds with diborane in THF solution at 25 °C yields the corresponding hydroxylamines.117 Kabalka has reported the reduction of nitroalkenes to hydroxylamines or amines with a variety of borane and borohydride reagents (Eq. 6.61).118... 

XII), which is then reduced with borane in THF to produce (VI). The trimethoxy secondary amine (VI) is demethylated by refluxing its solution in glacial acetic acid and HBr to yield dobutamine hydrobromide... 

The reaction has broad applications and a large number of secondary and especially tertiary amines was prepared in isolated yields ranging from 60% to 84% [1136]. Although the mechanism of this reaction is not clear it is likely that the key step is reduction of the acid by borane, generated in situ from sodium borohydride and the acid, to an aldehyde which reacts with the amine as described in the section on reductive amination (p. 134-136). 

The oxidation by amine oxides provides a basis for selection among non-equivalent groups on boron. In acyclic organoboranes, the order of reaction is tertiary > secondary > primary. In cyclic boranes, stereoelectronic factors dominate. With 9-BBN derivatives, for example, preferential migration of a C—B bond which is part of the bicylic ring structure occurs. 

The reduction of amides with borane leads to the formation of borane-amine adducts, which can be resistant towards acylating agents or hydrolysis. Such borane complexes can be cleaved either by treatment with a secondary amine (e.g. piperidine, 60 °C [180]), or oxidatively, by treatment with iodine (Entry 3, Table 10.11 [181,182]). 

Few examples of the preparation of hydrazines or hydroxylamines on insoluble supports have been reported (Table 10.17). Hydrazines have been prepared by the reduction of aromatic diazonium salts or /V-nitroso amines (prepared from secondary amines by treatment with tert-butyl nitrite [340]), and by the N-amination of support-bound amines (Entry 3, Table 10.17). The direct reduction of hydrazones with borane to yield hydrazines on solid phase has not been reported, and appears to be difficult because of the ease with which the N-N bond of hydrazines is cleaved by reducing agents [340]. 

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