Borane carbonyls complex hydrides

The domain of hydrides and complex hydrides is reduction of carbonyl functions (in aldehydes, ketones, acids and acid derivatives). With the exception of boranes, which add across carbon-carbon multiple bonds and afford, after hydrolysis, hydrogenated products, isolated carbon-carbon double bonds resist reduction with hydrides and complex hydrides. However, a conjugated double bond may be reduced by some hydrides, as well as a triple bond to the double bond (p. 44). Reductions of other functions vary with the hydride reagents. Examples of applications of hydrides are shown in Procedures 14-24 (pp. 207-210). 

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. 

Various polymer-supported hydrides have been applied successfully to reductions of both carbonyl and olefin groups. Rajasree and Devaky13 describe a cross-linked polystyrene-supported ethylenediamine borane reagent for the selective reduction of aldehydes in the presence of ketones (entry 9). This borane reagent is easily prepared and can be recycled after completion of the reaction. This is a practical alternative to standard borane reagents such as diborane, borane-amine, or borane-sulfide complexes. 

It is postulated that the reaction involves a coordination of the carbonyl to the boron of the oxazaborolidine, which activates the carbonyl towards hydride addition. Presumably, complexation occurs syn to the smaller group of the ketone, followed by reduction by an intramolecular hydride transfer from the borane which is complexed to the amine. 

An accepted pathway for the reduction of cyclohexanone with borane (B2H6) in oxacyclopentane (THF) is presented in Scheme 9.16. As shown there, it is presumed that, initially, the borane THF complex is converted to a carbonyl group borane complex (i.e., C=0 BH3) and that hydride donation from the complexed borane to the carbon of the carbonyl leads to a monoalkoxyborane. 

The rapid reaction between carboxylic acids and borane is related to the electrophilicity of borane. An acyloxyborane is recognized to be an initial intermediate. The carbonyl group in this molecule, which is essentially a mixed anhydride, is activated by the electronegative nature of the trivalent boron atom. This fact intrigued us greatly and then awoke us interest in whether the acyloxyborane can accept the attack of molecule other than a simple hydride. Addition of 1/3 equiv of borane-TBT complex to the acrylic acid dissolved in dichloromethane followed by diene at low temperature resulted in the formation of Diels-Alder adduct in good yield. Further, the reaction could progress satisfactory even with a catalytic amount of borane. 

Chiral C2-symmetric boron bis(oxazolines) act as enantioselective catalysts in the reduction of ketones promoted by catecholborane.321 DFT calculations indicate that the stereochemical outcome is determined by such catalysts being able to bind both the ketone and borane reducing agent, activating the latter as a hydride donor, while also enhancing the electrophilicity of the carbonyl. X-ray structures of catalyst-catechol complexes are also reported. 

There is a second effect. In the transition state in which the stronger Lewis acid complexes the carbonyl oxygen, the carbonyl group is a better electrophile. Therefore, it becomes a better hydride acceptor for Brown s chloroborane than in the hydride transfer from Alpine-Borane. Reductions with Alpine-Borane can actually be so slow that decomposition of this reagent into a-pincnc and 9-BBN takes place as a competing side reaction. The presence of this 9-BBN is problematic because it reduces the carbonyl compound competitively and of course without enantiocontrol. 

The hydride nucleophile prefers to attack the carbonyl group of the borane complex IBBU2 from the top face following the Biirgi-Dunitz trajectory.3 The... 

The Lewis basic carbonyl group forms a complex with the empty p orbital of the Lewis acidic borane. Hydride transfer is then possible from anionic boron to electrophilic carbon. The resulting tetrahedral intermediate collapses to an iminium ion that is reduced again by the borane. 

Ths Houk Modsl [CL4, CL5, HP3, HW1, PR1 ] The intervention of a favored conformation through which the reduction occurs can depend on the stereo-electronic interactions in a Lewis acid-base complex that is formed between a tricoordinated reducing agent (boranes, DIBAH) and a carbonyl compound. The complex associates in a way that minimizes the different repulsive interactions, and the transfer of the hydride takes place in second stage. Accordingly, the conforma-... 

---