Borane as reducing agent

The catalytic enantioselective reduction of 1-ketophosphonates has recently been developed. This approach takes advantage of a development in the enantioselective reduction of prochiral ketones to chiral alcohols by means of catalytic amounts of oxazaborolidines with borane as reducing agent. Thus, the enantioselective reduction of 1-ketophosphonates is accomplished by treatment with different boranes, BH. THF (0.9 eq), BII3Me2S (0.66 eq),5 545 qj- catccholborane (1.1 eq)5° 5 6 in different solvent systems in the presence of a catalytic amount of freshly prepared B-n-butyloxazaborolidine, (5) or (R) (Scheme 7.93). The reaction is complete in about 5 h and produces the expected dialkyl 1-hydroxyalkylphosphonates in satisfactory yields (53-98%). 

Corey et al. [4] have investigated enantioselective reduction of ketones with THF-BHa and (5 )-diphenyl prolinol-borane adduct as catalyst. They further introduced modification of oxazaborolidines in which R = CH3, n-butyl as catalyst 1 and used along with other boranes as reducing agents (Scheme 1). The catalysts 1 are known as CBS catalysts and... 

H. Frank, H. J. Chaves das Neves, and E. Bayer. Use of borane as reducing agent in sequence analysis of peptides by gas chromatography-mass spectrometry. J, Chromatog., 1978, 152, 357. 

Okinaka Y (1970) Electroless gold deposition using borohydride of dimethylamine borane as reducing agent. Plating 57 914—920... 

Certain base adducts of borane, such as triethylamine borane [1722-26-5] (C2H )2N BH, dimethyl sulfide borane [13292-87-OJ, (CH2)2S BH, and tetrahydrofuran borane [14044-65-6] C HgO BH, are more easily and safely handled than B2H and are commercially available. These compounds find wide use as reducing agents and in hydroboration reactions (57). A wide variety of borane reducing agents and hydroborating agents is available from Aldrich Chemical Co., Milwaukee, Wisconsin. Base displacement reactions can be used to convert one adduct to another. The relative stabiUties of BH adducts as a function of Group 15 and 16 donor atoms are P > N and S > O. This order has sparked controversy because the trend opposes the normal order estabUshed by BF. In the case of anionic nucleophiles, base displacement leads to ionic hydroborate adducts (eqs. 20,21). 

Azo, azoxy, and hydrazo compounds can all be reduced to amines. Metals (notably zinc) and acids, and Na2S204, are frequently used as reducing agents. Borane reduces azo compounds to amines, though it does not reduce nitro compounds. " Lithium aluminum hydride does not reduee hydrazo compounds or azo compounds, though with the latter, hydrazo compounds are sometimes isolated. With azoxy compounds, LiAHLj gives only azo compounds (19-48). 

Alkyl derivatives of borane and alane can function as reducing agents in a similar fashion. Two reagents of this group, disiamylborane and diisobutylaluminum hydride (DIBA1H), are included in Table 5.3. The latter is an especially useful reagent. 

Sodium borohydride or dimethyl amine borane have found limited use as reducing agents because of expense. In addition, bath stability, plating rate, and deposit properties are inferior to those of formaldehyde-reduced baths. The deposit is a copper—boron alloy. Copper—hypophosphite baths have been investigated, but these are poorly autocatalytic, and deposit only very thin coatings. 

It is more interesting that reduction of complex ketones could be dramatically improved by optimization of the reaction temperature. In the case of the phenoxyphenylvinyl methyl ketone (6), a 5-lipoxygenase inhibitor synthesis intermediate, we were able to improve the enantiomeric excess from the 80-85% range up to 96% by selection of the optimal temperature for the reduction. In this case, the temperature range for an acceptable enantiomeric excess is very narrow. Generally, Me-CBS is the best catalyst with borane complexes as reducing agent. 

Radical reactions of substrate 27 were initially conducted at low temperature in toluene with triethyl borane as initiator and tributylstannane as reducing agent. Under these conditions no conversion was detected. Increasing the temperature to ambient temperature led to 99% yield and a diastereomeric ratio (d.r.) of 47/53 in favour of the cis compound cis-29. Reactions in the presence of the chiral complexing agent... 

Optically active P-hydroxysulfoximines which catalyze the asymmetric borane reduction of ketones [110], also catalyze the same reaction with sodium borohydride/trimethylsilyl chloride system as reducing agent [126]. Reduction of a protected a-hydroxyacetophenone afforded the alcohol with 90% ee. 

Chiral boranes can be used as reducing agents for carbonyl compounds. 30 Brown prepared a variety of chiral boranes in connection with extensive studies of hydroboration reactions (sec. 5.4.B). Reaction of diborane and a-pinene, for example, gave (-)-( / ,25,3/ ,5/ )-diisopinocampheylborane (206), which reduced carbonyl derivatives to alcohols with high asymmetric induction.33 Trialkylboranes are also capable of... 

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). ... 

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