Borane-amine complex

Diborane [19287-45-7] the first hydroborating agent studied, reacts sluggishly with olefins in the gas phase (14,15). In the presence of weak Lewis bases, eg, ethers and sulfides, it undergoes rapid reaction at room temperature or even below 0°C (16—18). The catalytic effect of these compounds on the hydroboration reaction is attributed to the formation of monomeric borane complexes from the borane dimer, eg, borane-tetrahydrofuran [14044-65-6] (1) or borane—dimethyl sulfide [13292-87-0] (2) (19—21). Stronger complexes formed by amines react with olefins at elevated temperatures (22—24). 

Borane complexes are the most widely used commercial boron compounds, after sodium borobydride. Examples used in organic synthesis are amine borane complexes and borane complexes of tetrahydrofuran and dimethyl sulfide. 

The synthesis of a number of other amine—borane complexes from THF BH [14044-65-6] have also been described (10). 

Experimental results are consistent with this representation nucleophilic reagents always attack B in amine—borane complexes, electrophilic reagents preferentially attack N. 

Borane, 1-methylbenzylaminocyanohydropyrrolyl-, 3, 84 Borane, thiocyanato-halogenohydro-, 3,88 Borane, trialkoxy-amine complexes, 3, 88 Borane, triaryl-guanidine complexes, 2,283 Borane, trifluoro-complexes Lewis acids, 3,87 van der Waals complexes, 3, 84 Borane complexes aminecarboxy-, 3,84 aminehalogeno-, 3, 84 amines, 3, 82, 101 B-N bond polarity, 3, 82 preparation, 3, 83 reactions, 3, 83 bonds B-N, 3, 88 B-O, 3, 88 B-S, 3, 88 Jt bonds, 3, 82 carbon monoxide, 3, 84 chiral boron, 3, 84 dimethyl sulfide, 3, 84 enthalpy of dissociation, 3, 82... 

Tetrasubstituted phosphinous amides of the type R2NPPh2 have been successfully arylated at phosphorus by the action of bromobenzene, in a process catalyzed by NiBr2, to give the aminophosphonium bromides [R2NPPh3] Br [109]. Other representative members of this class form phosphane-borane complexes [62], are aminated at phosphorus by chloramine to yield bis(amino)phos-phonium salts [110] and have been claimed to be protonated at phosphorus by ethereal tetrafluoroboric acid, as determined by NMR analysis [111]. 

Amine-borane complexes are not very reactive toward hydroboration, but the pyridine complex of borane can be activated by reaction with iodine.160 The active reagent is thought to be the pyridine complex of iodoborane. 

Reduction of aldehydes and ketones. Earlier work on amine borane reagents was conducted mainly with tertiary amines and led to the conclusion that these borane complexes reduced carbonyl compounds very slowly, at least under neutral conditions, and that the yield of alcohols is low. Actually complexes of borane with primary amines, NHj or (CH3)3CNH2, reduce carbonyl compounds rapidly and with utilization of the three hydride equivalents. BH3 NH3 is less subject to steric effects than traditional complex hydrides. A particular advantage is that NH3 BH3 and (CH3)3CNH2 BH3 reduce aldehyde groups much more rapidly than keto groups, but cyclohexanone can be reduced selectively in the presence of aliphatic and aromatic acyclic ketones. 

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]). 

Ebden, M. R. Simpkins, N. S. Fox, D. N. A. Activation of benzylic amines towards regioselective metalation by borane complex formation. Tetrahedron Lett. 1995, 36, 8697—8700. Ariffin, A. Blake, A. J. Ebden, M. R. H, W.-S. Simpkins, N. S. Fox, D. N. A. The diastereose-lective and enantioselective substitution reactions of an isoindoline-borane complex. 

One of the novel exiting developments in the field of tin-free radical chain reactions is the development of complexes of borane with NHCs as HAT reagents [6]. Borane (BH3) itself has a BDE of 106.6 kcal mol 1 which is much too high for its use in organic radical chain reactions. The group of Roberts and others have demonstrated that complexes of boranes with amines and phosphines have a reduced BDE and that they can be used in radical chain reactions [7]. However, the reduction is only moderate and too low to make these complexes generally applicable. 

Even without the directing effect of coordination to O, incorporation of the amine lone pairs into an amine-borane complex 106 allows the easy deprotonation at the benzylic site to give 107.73 Refluxing ethanol deprotects the nitrogen. 

Unsubstituted isoxazolium salts reacted with borohydride to give isoxazolines (68) (and their borane complexes) 3-substituted-5-unsubstituted isoxazolium salts gave isoxazolines and, finally, isoxazolidines.181 Most other nucleophiles (e.g., ethoxide, hydroxide, and amines) gave only products of ring cleavage, though in the case of piperidine and morpholine the adducts 69 could be isolated at low temperatures.180 It is thus possible that in these... 

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