Boron Hydride Reduction - American Chemical Society

Chapter 8

Boron Hydride Reduction

Downloaded by UNIV OF FLORIDA on December 6, 2016 | http://pubs.acs.org Publication Date (Web): November 30, 2016 | doi: 10.1021/bk-2016-1236.ch008

Shinichi Itsuno* Department of Environmental & Life Sciences, Toyohashi University of Technology, Toyohashi, 441-8580 Japan *E-mail: [email protected]

Boron hydrides, comprising Lewis acidic borane derivatives and basic metal borohydride derivatives, are versatile reducing agents that have wide applications in chemistry, both in the laboratory and on an industrial scale. This chapter presents an overview of the importance of boron hydride derivatives including MBH4 and BH3 in the field of reduction of carbonyl, imine and other functionalities as both chiral and achiral versions.

Introduction Sodium borohydride (NaBH4) is a typical reducing agent used widely in organic synthesis. The B-H bond of NaBH4 is polarized toward hydrogen. The hydrogen is, therefore, electron rich and behaves like a hydride (H–). NaBH4 was discovered in 1943 by H. I. Schlesinger and H. C. Brown. NaBH4 and its derivatives are the most versatile and useful reducing agents, and are used in a number of industrial processes. Their specific reactivities and selectivities have been explored and reviewed (1–3). Practical synthetic methods for reductions with borohydrides have also been reviewed (4). Chirally modified borohydrides have also been developed for the asymmetric reduction of ketones and imines (5, 6). Various other modified metal borohydrides have also been developed for the selective reduction of specific functional groups. This chapter focuses on borohydride reductions developed in the last two decades.

Reduction of Carbonyl Compounds with Borohydride Compared with the reduction of aldehydes and ketones, esters require stronger reducing agents such as lithium aluminum hydride. The complete © 2016 American Chemical Society Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


reduction of α,β-unsaturated esters to saturated alcohols can be performed with modified borohydride reagents. In the presence of CoCl2 and diisopropylamine, NaBH4 reduced α,β-unsaturated ester 3 to the corresponding saturated alcohol 4 (Scheme 1) (7). The same reaction system also successfully reduced aromatic ester 5 and lactone 7. This procedure provides a promising alternative to LiAlH4 reductions.

Scheme 1. Borohydride Reduction of Esters.

The reduction of lactones 3 to lactols 4 can be achieved using diisobutylaluminum hydride. The use of CuCl (0.5 equiv) and NaBH4 (10 equiv) also enabled the efficient, highly chemoselective one-pot synthesis of δ-lactols from α,β-unsaturated δ-lactones in methanol (Scheme 2) (8).

242 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 2. Borohydride Reduction of Lactone to Lactol.

Diverse reduction strategies have been developed for the borohydride reduction of carboxylic acid. Cardenas reported a simple and practical protocol for the reduction of carboxylic acids 5 via in situ formation of hydroxybenzotriazole esters 6 and 7, followed by NaBH4 reduction (Scheme 3) (9). Optically active amino acid derivatives were also reduced to amino alcohols in high yields. The reduction of α,β-unsaturated carboxylic acids gave allylic alcohols in high yields.



Scheme 3. Borohydride Reduction of Carboxylic Acid to Alcohol. Chemoselective reduction is an important technique in synthesis. The chemoselective NaBH4 reduction of aldehyde 9 in the presence of ketone 10 was achieved using an oxovanadium (IV) Schiff base complex encapsulated in the nanopores of zeolite Y (Equation 1) (10).



Selective reduction occurred within a few minutes at room temperature to give the primary alcohol 12 in quantitative yield. NaBH4 reduction is the preferred method for transforming aldehydes to alcohols in large-scale synthesis (11). NaBH4 reduction of aldehydes is typically carried out in THF, alcohols, or a combination thereof. Solvent-free systems have also been applied to the reduction of aldehydes and ketones with NaBH4. In the presence of wet SiO2 under solvent-free conditions, benzaldehyde was reduced to benzyl alcohol in 100% yield within 1 min at room temperature (12). The chemoselective reduction of ketones in α-ketoesters was also possible using NaBH4 at low temperature in ethanol. Using this method, mandelates 15 were prepared by reduction of α-ketoesters 14 (Scheme 4) (13).

Scheme 4. Borohydride Reduction of α-Ketoester.

Solvent-free reduction of ketones has also been developed. NaBH4 activated by solid acids such as boronic acid, benzoic acid, and p-toluenesulfonic acid showed efficient reducing ability in ketone reduction (14). The chemoselective reduction of unmodified Baylis-Hillman adducts using InCl3-NaBH4 was also achieved (15), allowing the convenient synthesis of trisubstituted E-alkenones. Pheromone synthesis has also been demonstrated using this method. Luche reported the selective 1,2-reduction of conjugated ketone 16 with NaBH4 and lanthanide salts (LnCl3-nH2O) in methanol (16). In many cases, this system gave a high yield of allylic alcohol 17 uncontaminated with the product of 1,4-reduction (17). Recent developments and modifications of the Luche system are as follows. An inexpensive alloy of light lanthanides called Mischmetall was 245 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


used to prepare a mixture of lanthanide trichloride hydrates, which was applied successfully in the Luche-type reduction of conjugated ketones (Scheme 5) (18).

Scheme 5. Borohydride Rreduction of α,β-Unsaturated Ketone. The Luche reduction was also applied to the synthesis of allylic spirolactones 20, as shown in Equation 2 (19). Decaborane (B10H14) was utilized instead of borohydride for the reduction of conjugated ketones to allylic alcohols (20).

The Luche reagent has been used for the diastereoselective synthesis of phosphinosugars. α-Ketophosphinate 21 was reduced diastereoselectively to 23 using NaBH4, proline and CeCl3-7H2O (Equation 3) (21).



Asymmetric catalytic reduction of prochiral ketones was performed with a NaBH4-chiral cobalt complex. Diketone 24 was easily reduced with NaBH4 in the presence of a catalytic amount of the chirally modified cobalt complex (Equation 4) (22).

In addition to NaBH4 and its derivatives, KBH4 and LiBH4 have been successfully utilized in carbonyl reductions. Asymmetric 1,2-reduction of conjugated ketones was performed using a Luche reduction system. A chiral N,N′-dioxide–Sc(III) complex was an efficient catalyst for the 1,2-reduction of conjugated ketones such as 28 to chiral allylic alcohols 29 using KBH4 (Equation 5) (23). Excellent enantioselectivities (up to 95% ee) were achieved with the chiral Luche reduction. The Luche reduction has been used as a key step in the total synthesis of naturally occurring compounds such as (–)-Lepenine (24).

LiBH4 is a stronger reducing agent compared with NaBH4 and highly soluble in ethers (25). LiBH4 can reduce esters to alcohols and primary amides to amines; substrates unaffected by NaBH4. Prochiral ketones 30 were reduced 247 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


asymmetrically with LiBH4 in the presence of chiral bisboronate 31 (Scheme 6) (26).

Scheme 6. Asymmetric Reduction of Ketones with Chirally Modified LiBH4.

Another chiral cyclic boronate, TarB-X (34), was also efficient at inducing chirality in borohydride reductions (27). Acetophenone was reduced with a chiral agent prepared from TarB-X and LiBH4 to give a chiral alcohol with high enantioselectivity (Scheme 7) (28). Zr(BH4)2Cl2 efficiently reduces aldehydes, as shown in Scheme 8 (29). The same reagent also reduces benzophenone but requires a longer reaction time. A zirconium borohydride piperazine complex (Ppyz)Zr(BH4)2Cl2 is a stable and selective reducing agent. (Ppyz)Zr(BH4)2Cl2 regioselectively reduces α,β-unsaturated carbonyl groups (Scheme 8) (29).

Amine Synthesis by Borohydride Reduction The reduction of amides to amines is an important transformation in organic synthesis. LiAlH4 and borane complexes are the most widely used reducing agents for this reaction. An alternative practical method was developed, using NaBH4 to reduce amides 38 via activation with Tf2O (30). In the presence of Tf2O, various kinds of amides and lactams were reduced by NaBH4 to corresponding amines 40 in high yields (Scheme 9). 248 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 7. Asymmetric Reduction of Acetophenone with LiBH4 and TarB-X.

Scheme 8. Aldehyde Reduction with Zr(BH4)2Cl2.



Scheme 9. Borohydride Reduction of Amide to Amine. The reduction of tosylamidated cyclic ethers with NaBH4 gave the corresponding α,ω-amino alcohols (31). Reduction of (RS)-N-tert-butanesulfinyl α-halo imines 41 with NaBH4 in THF followed by cyclization with KOH afforded the corresponding (RS, S)-N-tert-butylsulfinyl)aziridines 44 in quantitative yields. In contrast, using LiBHEt3 as a reducing agent afforded the epimer, (RS, R)-N-(tert-butylsulfinyl)aziridines 45 (Scheme 10) (32).

Scheme 10. Asymmetric Synthesis of Aziridine. Amines can be easily obtained from the reduction of imines. For example, 4-imidoyl-(ω-haloalkyl)-β-lactams were reduced with NaBH4 followed by cyclization to give the corresponding bicyclic β-lactams (33). Reduction of diimines 49 gave vicinal diamines using NaBH3CN (Scheme 11) (34). Both syn 250 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


and anti-configurations of vicinal diamines 48 could be obtained, depending on the reducing agent used, as shown in Scheme 11. Nitriles 50 are also reduced with NaBH4 in the presence of nickel boride catalyst, with the products isolated as the N-Boc derivative of amines 51 (Scheme 12) (35).

Scheme 11. Vicinal Diamine Synthesis.

Scheme 12. Borohydride Reduction of Nitriles. 251 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Reductive amination is an important tool for the construction of carbon-nitrogen bonds. Various kinds of aldehydes, ketones, and amines, in the presence of a reducing agent, can be used in reductive amination. An excellent review article on the reductive amination of carbonyl compounds is available (36). Sodium cyanoborohydride and sodium acetoxyborohydride are most commonly used as reducing agents for reductive amination. Sodium triacetoxyborohydride (NaBH(OAc)3) (37–39) is one of the typical reducing agent useful for reductive amination. A review article on the reductive amination of ketones and aldehydes is available (40). Other than these reducing agents, some acid-activated sodium borohydrides, such as NaBH4-H3BO3, have been successfully applied to reductive amination (41). Under solvent-free conditions, benzaldehyde and aniline with NaBH4-H3BO3 produced N-phenyl benzylamine in 94% yield after 15 min. 2-(Tributylamino)-ethoxyborohydride is another effective reducing agent for reductive amination, and was used to transform ketones 30 into the corresponding secondary amines 53 (R = H) in high yield (Scheme 13) (41b, 42).

Scheme 13. Reductive Amination with an Ammonium Borohydride.

Sodium borohydride in 2,2,2-trifluoroethanol is also a powerful and efficient reducing agent for reductive amination (Scheme 14) (43). In most cases, secondary amines 53 were obtained in high yield within a few minutes.



Scheme 14. Reductive Amination with NaBH4 in 2,2,2-Trifluoroethanol.

The in situ oxidation of alcohols to imines has been achieved using manganese dioxide. The reduction of the resultant imine was conducted using polymer-immobilized cyanoborohydride (44). The one-pot synthesis of secondary and tertiary amines from alcohols and primary amines was achieved using a combination of MnO2 and polymer-immobilized cyanoborohydride (45). With NaBH3CN, optimum conditions afforded the amine in a maximum 40% yield, while NaBH(OAc)3 gave no amine. Reduction of oximes is an alternative method for preparing amines. NaBH3CN in combination with a MoCl5/NaHSO4-H2O system gave high yields of amines from oximes (46). Amines are obtained by the reduction of azides. NaBH4 is a useful reducing agent in this transformation (47). Azide reduction with NaBH4 has been used in aminopolysaccharide synthesis. The reduction of 6-azido-6-deoxy-2,4-di-Otrioxadecanoylcurdlan 55 with excess NaBH4 unexpectedly gave a water-soluble product, shown to be the amide 6-trioxadecanamido-6-deoxycurdlan 56 (Scheme 15) (48).



Scheme 15. Borohydride Reduction of Azides 54 and 55.

Reductive Alkylation The reductive alkylation of malononitrile with aromatic aldehydes was successfully performed using NaBH4. For example, benzylmalononitrile 58 was prepared from malononitrile 57 and benzaldehyde in ethanol followed by reduction with NaBH4. Various kinds of aldehydes 36 were used to prepare the malononitriles in a one-pot synthesis (Scheme 16) (49). A similar reductive alkylation of Meldrum’s acid was reported using NaBH4 (50). The as-obtained 5-monosubstituted Meldrum’s acids were easily converted to α-substituted acrylates.

Borohydride Reduction of Other Functionalities NaBH4 is a useful agent for the reduction of hydroperoxides to alcohols. Heteroaromatic oxazoles 60 containing a hydroperoxide moiety, prepared from alkylydeneoxazolines 59, were readily reduced with NaBH4 to give the corresponding alcohols 61 in high yields (Scheme 17) (51).



Scheme 16. Reductive Alkylation of Malonitrile.

Scheme 17. Reduction of Hydroperoxide.



Boron hydrides have been utilized as efficient hydride sources for other reactions, such as the hydrodehalogenation of halo-heteropentalenes 62 and 64. For example, NaBH4-TMEDA was a hydride source for the hydrodehalogenation of 2-bromo-5-phenylthiophene 62 in the presence of Pd catalyst (Scheme 18) (52).

Scheme 18. Reduction of Aromatic Halide.

NaBH4 reduction has been efficiently used for the synthesis of aromatic thiols from aryl iodides. The CuI-catalyzed coupling reaction of aryl iodides and sulfur powder occurred in the presence of K2CO3 in DMF. The coupling mixture was then directly treated with NaBH4 to afford aryl thiols in good yields (53). The desulfurization of coal water slurry plays an important and practical role in air pollution control. A rapid desulfurization method for industrial coal water slurry was achieved using ultrasound-assisted borohydride reduction (54). Conjugate reduction of α,β-unsaturated carbonyl compounds was performed with NaBH4 in the presence of cobalt complex. Chiral azabis(oxazoline)s are efficient ligands for enantioselective transfer of hydrogen to prochiral carboncarbon double bonds of α,β-unsaturated esters. Highly enantioselective conjugate reduction of 66 was achieved by using cobalt complex of chiral azabis(oxazoline) 65 (Equation 6) (55).



Reduction with Borane and Its Derivatives Most metal hydrides have poor hydride donating abilities. They are not useful reducing agents. The group 13 elements including boron have Lewis acidity. Molecules with Lewis basic properties coordinate with trivalent boron. A hydride attached to a boron atom is able to smoothly transfer to activate coordinated substrates. Aldehydes and ketones are easily hydroborated to give alcohols after hydrolytic workup. Imines and oximes are also reduced by BH3-dimethyl sulfide (DMS). In the presence of boron trifluoride, aromatic O-triisopropylsilyl ketoximes were rearranged to cyclic aniline derivatives (56). Carboxylic acids were also reduced with borane to give alcohols. The first step of this reaction was the formation of a borate of carboxylic acid. The boron atom accepts electrons from the borate oxygen. The electrons of the ester oxygen do not flow into the carbonyl group. Thus, the carbonyl group of the carboxylic acid borate behaves like a ketone, which is able to be reduced with boron hydride. Another important functionality reduced by borane is amides. For example, dodecyl methyl sulfide-borane complex is an odorless borane complex used for amide reduction (57). Borane-THF has been useful for the reduction of optically active oxindole to indoline without deterioration of the enantiopurity (58). Some modified boranes have been developed as reducing agents. Ketones are reduced by pinacolborane 68 in the presence of NaOtBu catalyst (Scheme 19) (59). Pinacolborane 68 was also used to reduce imines using a catalyst prepared from DABCO and B(C6F5)3 (60).

Scheme 19. Pinacolborane Reduction of Ketones.



Diacids can act as accelerating agents for the borane reduction of imines. A reducing agent prepared from borane and phthalic acid reduced imines at low temperature (–78 to –25 °C) to give the corresponding amines in high yields (61). Prochiral ketones were reduced to enantioenriched secondary alcohols using catecholborane in the presence of a thiourea-amine chiral organocatalyst (62). Propiophenone was asymmetrically reduced by catecholborane 69 with chiral organocatalyst 70 (Equation 7) to give the chiral secondary alcohol in 99% ee. Chiral C2-boron-bis(oxazolines) were efficient catalysts in the asymmetric reduction of ketones with catecholborane (63). Chiral phosphoric acids can also act as chiral organocatalysts in the asymmetric reduction of aromatic ketones (64). A chiral phosphoric acid 73-DMAP complex may be the precatalyst for asymmetric reduction (Equation 8). Catecholborane has been used as a reducing agent in the asymmetric synthesis of chiral secondary amines.



Aromatic ketimines such as 74 were reduced asymmetrically with catecholborane in the presence of chiral N-triflyl phosphoramides 76 to give the corresponding chiral secondary amines with good enantiomeric ratios (up to 86:14) and excellent yields under mild reaction conditions (Equation 9) (65).

Catecholborane had been used to reduce CO2. In the presence of Ni catalyst 77 (Figure 1), catalytic reduction of CO2 to the methoxide level was achieved (66).

Figure 1. Ni catalyst for catecholborane reduction of CO2.

Asymmetric catalytic reduction of prochiral ketones to give enantiomerically enriched alcohols remains a fundamentally asymmetric transformation. Many asymmetric catalysts have been developed for the reduction of ketones by borane, most notably oxazaborolidines, which have been extensively reviewed (67–71). C=N containing compounds such as imines and oximes are also efficiently reduced, producing respective chiral amines (72–75). Chiral oxazaborolidines, such as 78, are easily prepared from corresponding amino alcohols and borane, and are efficient catalysts for asymmetric reductions, as shown in Scheme 20. Even highly reactive substrates such as trifluoroacetophenone, which is susceptible to non-catalytic reduction by BH3, was reduced to a chiral alcohol by electronic tuning of the boron atom by the chiral oxazaborolidine catalyst 81 (Equation 10) (76). 259 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 20. Asymmetric Reduction of Ketones and Imines with Oxazabororidine.

Oxazaborolidine-like spiroborate esters 84 and 85 were prepared from 1,2-amino alcohol, ethylene glycol and triisopropyl borate (77). The chiral spiroborate esters were successfully applied for asymmetric borane reduction of ketones and oxime ethers. Most of the ketones were reduced asymmetrically to give the corresponding secondary alcohols in high yields with very high enantioselectivities (Scheme 21). The chiral spiroborate ester 84 was also efficiently used as a catalyst for asymmetric reduction of pyridyl ketoximes (79a and 79b). Highly optically active pyridlyl amines (80a and 80b) were synthesized by this method (78).



Scheme 21. Asymmetric Reduction with Chiral Spiroborate Ester Catalysts.

Buono introduced catalysts not based on an oxazaborolidine structure, but on an oxazaphospholidine 90-borane complex (Equation 11) (79, 80). Another useful chiral catalyst containing an N–P=O structural framework for asymmetric borane reduction was developed by Wills (81–87). Other chiral N–P=O ligands have also been developed (Equations 12 and 13) (88–90).



Reduction with Amine-Borane Complexes Borane adducts such as BH3-THF and BH3-DMS are widely used in organic synthesis, mainly in the hydroboration reaction of alkenes or alkynes. Polymeric sulfide-borane complexes have also been used as convenient hydroborating and reducing reagents (91). Borane has unique properties in reduction due to its Lewis acid character, as discussed earlier. Amine complexes of borane are stable reducing agents, and are, typically, a more stable source of borane. This stability, along with solubility and ease of handling, make them attractive borane 262 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


sources in many reactions (92, 93). They can be used in aqueous, alcoholic or even acidic solvents, and are somewhat less reactive than borane complexes of ethers or sulfides. Aminoboranes selectively reduce carbonyl groups such as aldehydes and ketones. They are inert towards carboxylic acids, esters, and other functional groups. Using this catalyst can improve processes dramatically. Some amine-borane complexes, such as pyridine-borane, trimethylamine-borane, and t-butylamine-borane, are commercially available. The reduction of 1,3-diketones 95 to the corresponding syn-1,3-diols 96 was achieved in high yield and with excellent diastereoselectivity using pyridine-borane complex in the presence of TiCl4 (Scheme 22) (94).

Scheme 22. Reduction of Diketone with Pyridine-Borane Complex.

α-Picoline-borane is another useful amine-borane complex used for the reductive amination of aldehydes and ketones to amines (95). This reaction has been carried out in methanol, water, or neat in the presence of small amounts of acetic acid. The metal organic framework (MOF)-amino-borane complex, namely UiOAB, prepared from UiO-66-NH2 and borane was used as a reducing agent in a size-selective reduction (96). Ammonia-borane was used to reduce a metal salt to give metal nanoparticles. Metal nanoparticles of Cu, Ag, and Au have been prepared using this 263 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

atom-economical green approach (97). BNHx polymers generated during the reduction of metal ions by ammonia borane stabilize the nanoparticles. Phosphine-boranes have not found use as reducing agents in organic synthesis. One exception is the reduction of N-vinylsulfoximine (98). The synthesis of phosphine-borane complexes was first reported by Burg and Wagner (99), and their applications in synthesis have been reviewed (100, 101). The efficient general synthesis of phosphine-borane complexes is achieved using phosphine, sodium borohydride, and acetic acid (102).


The Use of Frustrated Lewis Pair in Reduction The reduction of carbon monoxide with borane was achieved with Piers’ borane 98 [HB(C6F5)2] (103) as a phosphane/borane frustrated Lewis pair (FLP) (104). The reduction product 99 was obtained as a colorless solid in 63% yield (Equation 14).

Phosphine-borane 100, a frustrated Lewis pair, was found to be a highly active organocatalyst in the reduction of CO2 to methanol using hydroboranes (Scheme 23) (105, 106).

Scheme 23. Reduction of CO2 Using FLP System.

N-Heterocyclic Cabenes In addition to hydride reductions, radical hydrogen transfer reactions have been developed using N-heterocyclic carbine (NHC)-borane complexes (107). The reduction of xanthate 105 using an NHC-BH3 complex and AIBN gave 106 (Scheme 24). Reduction also occurred smoothly under Et3B/O2 conditions. Curran et al. further developed 1,3-dimethylimidazol-2-ylideneborane, which achieved a higher yield in the xanthate reduction (108). 264 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 24. Reduction of Xanthate Using NHC-BH3. N-Heterocyclic carbene-borane complexes can be used as hydride reducing agents as well as in radical reductions. Reductions of highly electron-poor C=N and C=C bonds provide hydrogenation products along with new, stable borylated products (109). NHC-boranes, such as 1,3-dimethylimidazol-2-ylidine trihydridoborane 107, are also efficient reducing agents for aldehydes and ketones in the presence of silica gel (110). The reduction of ketones and imines using NHC boranes 107 and 108 are summarized in Scheme 25. Chemoselective reduction of aldehydes in the presence of ketones is also possible with 107. When a mixture of equal amounts of 4-bromobenzaldehyde and 4-bromoacetophenone was 265 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


treated with 107 and silica gel, the reduction of 4-bromobenzaldehyde occurred exclusively to give the corresponding primary alcohol in excellent yield along with the unreacted ketone. Recently N-heterocyclic carbene-stabilized borenium ions were found to be efficient catalysts in imine hydrogenation reactions (111).

Scheme 25. Reduction of Ketones and Imines with NHC-BH3.



Conclusion Borohydride reductions of functional groups including C=O and C=N double bonds are important methods in organic synthesis. NaBH4 is clearly the most versatile reducing agent and is widely used in synthesis. Its reactivity can be precisely controlled using additives and alternate reaction conditions. More than 70 years after the discovery of NaBH4, new reduction system are still being reported in the literature. Compared with basic NaBH4, Lewis acidic borane and its derivatives have different reducing capabilities. Various kinds of catalytic asymmetric reductions have also been developed, including chiral oxazaborolidine and oxazaphospholidine-borane systems. They provide extremely highly efficient enantioselective syntheses of chiral alcohols and amines. Some chiral organocatalysts have also been applied to borane reductions. The use of recently developed NHC-boranes and FLP systems allows wider applications of the reduction system. More specific reduction systems will continue to be discovered using boron-containing reagents and catalysts.

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