Reducing agents catecholborane

Falck and coworkers introduced secondary amine-thiourea 64 as a highly enantioselective catalyst for the reduction of ketones (Scheme 19.74). Among a number of reducing agents, catecholborane afforded the best results leading to enantioenriched alcohols in good to high yields and from moderate to excellent enantioselectivities. 

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. 

Borane complexes, borane-THF and borane dimethylsulfide, are generally the appropriate reducing agents in this reaction. Borane thioxane has been used, but it suffers from the inconvenience that thioxane is quite expensive. Diborane gas has been tried by Callery and has proved to be very efficient. The use of this reagent could open new industrial developments for this technology. Catecholborane has shown some advantages at low temperature when selectivity is needed.40... 

Organoboranes have proven to be excellent reducing agents where the functional group is preferentially reduced, without the concomitant hydroboration of carbon-carbon multiple bonds. For instance, citronellal is reduced to the unsaturated alcohol with catecholborane (Eq. 133) 221... 

Catecholborane 1.8 (on page 11) is a mild reducing agent that is not sensitive to moisture [KB7]. It can be used without solvent or in CHCI3, and it reduces aldehydes, ketones, hydrazones, and acetals. It also reduces acids if used in excess at room temperature. Esters are reduced in refluxing THE, and alkenes are hydrobo-rated in similar conditions. 

Reduction of indoles to indolinesf This reduction requires an acidic medium and was chosen for a detailed study of this new reducing agent. Isolated 1 in TFA reduces 2 to 3 in 867o yield but the same yield is attainable (in 2 minutes) by the combination BHs-THF in TFA. Other somewhat less effective reagents for this reduction are BHa-pyridine in TFA, BHa (CHa)aS-THF in TFA, NaBHaCN in TFA, and catecholborane in TFA. 

A very useful variant of the hydrazone reduction is the deoxygenation of aldehydes and ketones via the hydride reduction of tosylhydrazones (Caglioti reaction) The method is mild, convenient and widely applicable. While sodium borohydride was used in the earlier procedures, considerable improvements have been achieved through the uses of sodium cyanoborohydride, catecholborane, diborane, his-benzoyloxy borane and copper borohydride as reducing agents and HMPA, DMF, sulpholane, etc. as solvents. Use of the sterically crowded 2,4,6-triisopropyl tosylhydrazone derivative has greatly facilitated the reduction in some cases (equations 61-64). ... 

In the area of preparative chemistry, there are reviews of the functional-group selectivity of complex hydride reducing agents, the reduction of organic compounds with diborane, the formation of C—C bonds using organoboranes," and a report on the preparation, properties, and synthetic applications of catecholborane. ... 

The mechanistic studies of a chiral Brpnsted acid-catalysed asymmetric reduction of ketones with catecholborane as the reducing agent to give highly enantioselective chiral secondary alcohols indicated that phosphoryl catechol borate, derived from reaction of the Brpnsted acid with catecholborane, acts as the active catalyst. ... 

Catecholborane is one of the widely used hydroborating and reducing agents with a very rich chemistry (27,22). It can be readily made in situ by passing B2H6, prepared using NaBRi and I2 through a solution of catechol in benzene (eq 4) (25,24). 

Catecholborane is a versatile reducing agent for many selective reductions. It exhibits properties which are unique and complementary to those of other subst. boranes, such as thexylborane, disiamylborane, or 9-borabicyclononane. - E 5% excess catecholborane added drop wise at 25° under N2 to a stirred mixture of palmitic anhydride and chloroform, refluxed 3 days, extracted with water, then 6 times with 1 N NaOH-soln. to remove catechol hexadecanol. Y 96%. F. e., reductions, and limitations s. G. W. Kabalka, J. D. Baker, Jr., and G. W. Neal, J. Org. Chem. 42, 512 (1977). 

Catecholborane or sodium borohydride in acetic acid can also be used as reducing agents in this reaction. 

The first example of highly enantioselective reduction of ketones (320) with catecholborane (321) as a reducing agent, catalysed by a chiral phosphoric acid derivative (323), gave chiral secondary alcohols (322) with high enantioselectivities (Scheme 84). ... 

A number of functional groups do not react with catecholborane These include organohalogen, alcohols and thiols, ethers, amides, nitro groups, and sulfones. Slowly reducing groups include acid chlorides, esters and nitriles. Aldehydes, ketones, imines, and sulfoxides are reduced. A hydroborating agent. 

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