Boron Hydride Reduction - ACS Symposium Series (ACS Publications)

Publication Date (Web): November 30, 2016 ..... The coupling mixture was then directly treated with NaBH4 to afford aryl thiols in good yields (53). ...
0 downloads 7 Views 2MB Size
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.

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

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.

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

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.

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

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

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).

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

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

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.

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

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).

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

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

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.

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

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.

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

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

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

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

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

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.

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

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.

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

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.

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

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

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).

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

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

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).

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

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

Scheme 16. Reductive Alkylation of Malonitrile.

Scheme 17. Reduction of Hydroperoxide.

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

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

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).

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

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

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.

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

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

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.

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

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

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.

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

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).

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

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

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).

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

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

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.

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

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).

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

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.

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

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.

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

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.

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

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

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.

References Hajos, A. Studies in Organic Chemistry 1; Elsevier Scientific Publishing Co.: Amsterdam, 1979. 2. Hudlicky, M. Reductions in Organic Chemistry; John Wiley & Sons: New York, 1984. 3. Seyden-Penne, J. Reductions by the Alumino- and Borohydrides in Organic Synthesis; VCH Publishers, Inc.: New York, 1991. 4. Cox, L. R. Compounds of one saturated carbon-heteroatom bond - Alcohols; Thieme: Stuttgart, 2008; Vol. 36. 5. Daverio, P.; Zanda, M. Enantioselective reductions by chirally modified alumino- and borohydrides. Tetrahedron: Asymmetry 2001, 12, 2225–2259. 6. Fache, F.; Schulz, E.; Tommasino, M. L.; Lemaire, M. Nitrogen-Containing Ligands for Asymmetric Homogeneous and Heterogeneous Catalysis. Chem. Rev. 2000, 100, 2159–2231. 7. Jagdale, A. R.; Paraskar, A. S.; Sudalai, A. Cobalt(II) Chloride Hexahydrate-Diisopropylamine Catalyzed Mild and Chemoselective Reduction of Carboxylic Esters with Sodium Borohydride. Synthesis 2009, 660–664. 8. Matsumoto, Y.; Yonaga, M. One-Pot Sequential 1,4- and 1,2-Reductions of α,β-Unsaturated δ-Lactones to the Corresponding δ-Lactols with CuCl and NaBH4 in Methanol. Synlett 2014, 25, 1764–1768. 9. Morales-Serna, J. A.; Garcia-Rios, E.; Bernal, J.; E., P.; Gavino, R.; Cardenas, J. Reduction of Carboxylic Acids Using Esters of Benzotriazole as High-Reactivity Intermediates. Synthesis 2011, 1375–1382. 10. Rayati, S.; Bohloulbandi, E.; Zakavi, S. Sodium borohydride reduction of aldehydes catalyzed by an oxovanadium(IV) Schiff base complex encapsulated in the nanocavity of zeolite-Y. Inorg. Chem. Commun. 2015, 54, 38–40. 1.

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

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

11. Magano, J.; Dunetz, J. R. Large-Scale Carbonyl Reductions in the Pharmaceutical Industry. Org. Process Res. Dev. 2012, 16, 1156–1184. 12. Zeynizadeh, B.; Behyar, T. J. Braz. Chem. Soc. 2005, 16, 1200–1209. 13. Ianni, A.; Waldvogel, S. R. Reliable and Versatile Synthesis of 2-ArylSubstituted Cinnamic Acid Esters. Synthesis 2006, 2103–2112. 14. Cho, B. T.; Kang, S. K.; Kim, M. S.; Ryu, S. Y.; An, D. K. Solvent-free reduction of aldehydes and ketones using solid acid-activated sodium borohydride. Tetrahedron 2006, 62, 8164–8168. 15. Das, B.; Banerjee, J.; Chowdhury, N.; Majhi, A.; Holla, H. Remarkably Chemoselective Reduction of Unmodified Baylis-Hillman Adducts by InCl3/NaBH4: Application to the tereoselective Synthesis of Trisubstituted Alkenones Including Two Alarm Pheromones. Synlett 2006, 1879–1882. 16. Luche, J. L. Lanthanides in Organic Chemistry. 1. Selective 1,2 Reductions of Conjugated Ketones. J. Am. Che. Soc. 1978, 100, 2226–2227. 17. Gemar, A. L.; Luche, J. L. Lanthanoids in Organic Synthesis. 6. The Reduction of α-Enones by Sodium Borohydride in the Presence of Lanthanoid Chlorides: Synthetic and Mechanistic Aspects. J. Am. Chem. Soc. 1981, 103, 5454–5459. 18. Lannou, M. I.; Helion, F.; Namy, J. L. Applications of Lanthanide Trichloride Hydrates Prepared from Mischmetall in Luche-Type Reduction. Synlett 2007, 2707–2710. 19. Yeh, M. C. P.; Lee, Y. C.; Young, T. C. A Facile Approach to the Synthesis of Allylic Spiro Ethers and Lactones. Synthesis 2006, 3621–3624. 20. Bae, J. W.; Lee, S. H.; Jung, Y. J.; Yoon, C. O. M. Reduction of ketones to alcohols using a decaborane/pyrrolidine/cerium(III) chloride system in methanol. Tetrahedron Lett. 2001, 42, 2137–2139. 21. Filippini, D.; Loiseau, S.; Bakalara, N.; Dziuganowska, Z. A.; Van der Lee, A.; Volle, J. N.; Virieux, D.; Pirat, J. L. Dramatic effect of modified boranes in diastereoselective reduction of chiral cyclic a-ketophosphinates. RSC Adv. 2012, 2, 816–818. 22. Sato, M.; Gunji, Y.; Ikeno, T.; Yamada, T. Efficient Preparation of Optically Pure C2-Symmetrical Cyclic Amines for Chiral Auxiliary. Synthesis 2004, 1434–1438. Nagata, T.; Yorozu, K.; Yamada, T.; Mukaiyama, T. Enantioselective Reduction of Ketones with Sodium Borohydride, Catalyzed by Optically Active (β-Oxoaldiminato)cobalt(II) Complexes. Angew. Chem., Int. Ed. 1995, 34, 2145–2147. 23. He, P.; Liu, X.; Zheng, H.; Li, W.; Lin, L.; Feng, X. Asymmetric 1,2-Reduction of Enones with Potassium Borohydride Catalyzed by Chiral N,N′-Dioxide - Scandium(III) Complexes. Org. Lett. 2012, 14, 5134–5137. 24. Nishiyama, Y.; Han-ya, Y.; Yokoshima, S.; Fukuyama, T. Total Synthesis of (-)-Lepenine. J. Am. Chem. Soc. 2014, 136, 6598–6601. 25. Banfi, L.; Narisano, E.; Riva, R.; Baxter, E. W. e-EROS Encyclopedia of Reagents for Organic Synthesis; John Wiley & Sons: 2005; Lithium Borohydride. 26. Nozaki, K.; Kobori, K.; Uemura, T.; Tsutsumi, T.; Takaya, H.; Hiyama, T. A Chiral Bimetallic Lewis Acid as a Reaction Template. Asymmetric 268 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

27.

28.

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

29.

30.

31.

32.

33.

34.

35.

36. 37. 38.

39. 40.

41.

Reduction of Unsymmetric Ketones with LiBH4. Bull. Chem. Soc. Jpn. 1999, 72, 1109–1113. Suri, J. T.; Vu, T.; Hernandez, A.; Congdon, J.; Singaram, B. Enantioselective reduction of aryl ketones using LiBH4 and TarB-X: a chiral Lewis acid. Tetrahedron Lett. 2002, 43, 3649–3652. Kim, J.; Suri, J. T.; Cordes, D. B.; Singaram, B. Asymmetric Reductions Involving Borohydrides: A Practical Asymmetric Reduction of Ketones Mediated by (L)-TarB-NO2: A Chiral Lewis Acid. Org. Process Res. Dev. 2006, 10, 949–953. Tajbakhsh, M.; Lakouraj, M. M.; Shirini, F.; Habibzadeha, S.; Nikdoosta, A. Zirconium borohydride piperazine complex, an efficient, air and thermally stable reducing agent. Tetrahedron Lett. 2004, 45, 3295–3299. Xiang, S. H.; Xu, J.; Yuan, H. Q.; Huang, P. Q. Amide Activation by Tf2O: Reduction of Amides to Amines by NaBH4 under Mild Conditions. Synlett 2010, 1829–1832. He, L.; Yu, J.; Zhang, J.; Yu, X. α-Amidation of Cyclic Ethers Catalyzed by Simple Copper Salt and a Mild and Efficient Preparation Method for α,vAmino Alcohols. Org. Lett. 2007, 9, 2277–2280. Denolf, B.; Leemans, E.; De Kimpe, N. J. Asymmetric Synthesis of Aziridines by Reduction of N-tert-Butanesulfinyl α-Chloro Imines. J. Org. Chem. 2007, 72, 3211–3217. Brabandt, W. V.; Vanwalleghem, M.; D’hooghe, M.; De Kimpe, N. Asymmetric Synthesis of 1-(2- and 3-Haloalkyl)azetidin-2-ones as Precursors for Novel Piperazine, Morpholine, and 1,4-Diazepane Annulated b-Lactams. J. Org. Chem. 2006, 71, 7083–7086. Kison, C.; Meyer, N.; Opaatz, T. An Aldimine Cross-Coupling for the Diastereoselective Synthesis of Unsymmetrical 1,2-Diamines. Angew. Chem., Int. Ed. 2005, 44, 5662–5664. Caddick, S.; Judd, D. B.; Lewis, A. K.; Reich, M. T.; Williams, M. R. V. A generic approach for the catalytic reduction of nitriles. Tetrahedron 2003, 59, 5417–5423. Baxter, E. W.; Reitz, A. B. Reductive Aminations of Carbonyl Compounds with Borohydride and Borane Reducing Agents; Wiley: 2004; Vol. 59. Gribble, G. W. Encylopedia of Reagents for Organic Synthesis; John Wiley and Sons: 1995; New York, 1995; Vol. 7. Abdel-Magid, A. F.; Carson, K. G.; Harris, B. D.; Maryanoff, C. A.; Shah, R. D. Reductive Amination of Aldehydes and Ketones with Sodium Triacetoxyborohydride. Studies on Direct and Indirect Reductive Amination Procedures. J. Org. Chem. 1996, 61, 3849–3862. Gribble, G. W. Sodium borohydride in carboxylic acid media: a phenomenal reduction system. Chem. Soc. Rev. 1998, 27, 395–404. Abdel-Magid, A. F.; Mehrman, S. J. A Review on the Use of Sodium Triacetoxyborohydride in the Reductive Amination of Ketones and Aldehydes. Org. Process Res. Dev. 2006, 10, 971–1031. Cho, B. T.; Kang, S. K. Direct and indirect reductive amination of aldehydes and ketones with solid acid-activated sodium borohydride under solvent-free conditions. Tetrahedron 2005, 61, 5725–5734. Cho, B. T.; Kang, S. K. Clean 269 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

42.

43. 44.

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

45. 46.

47. 48.

49.

50.

51. 52.

53.

54. 55.

56.

57.

and simple chemoselective reduction of imines to amines using borric acidactivated sodium borohydride under solvent-free conditions. Synlett 2004, 1484–1488. Mohanazadeh, F.; Forozani, M.; Taheri, A. Reductive Amination of Aldehydes and Ketones with 2-(Tributylamino)- ethoxyborohydride. Monatshefte Chem. 2007, 138, 1187–1189. Taibakhsh, M.; Hosseinzadeh, R.; Alinezhad, H.; Ghahari, S.; Heydari, A.; Khaksar, S. Synthesis 2011, 490–496. Hutchins, R. O.; Natale, N. R.; Taffer, I. M. Cyanoborohydride supported on an anion exchange resin as a selective reducing agent. Chem. Commun. 1978, 1088–1089. Blackburn, L.; Taylor, R. J. K. In Situ Oxidation-Imine Formation-Reduction Routes from Alcohols to Amines. Org. Lett. 2001, 3, 1637–1639. Kouhkan, M.; B., Z. A New and Convenient Method for Reduction of Oximes to Amines with NaBH3CN In the Presence of MoCl5/NaHSO4·H2O System. Bull. Korean Chem. Soc. 2011, 32, 3323–3326. Scriven, E. F. V.; Turnbull, K. Azides: their preparation and synthetic uses. Chem. Rev. 1988, 88, 297–368. Ruoran Zhanga, R.; Edgara, K. J. Water-soluble aminocurdlan derivatives by chemoselective azide reduction using NaBH4. Carbohydr. Polym. 2015, 122, 84–92. Tayyari, F.; Wood, D. E.; Fanwick, P. E.; Sammelson, R. E. Monosubstituted Malononitriles: Efficient One-Pot Reductive Alkylations of Malononitrile with Aromatic Aldehydes. Synthesis 2008, 279–285. Frost, C. G.; Penrose, S. D.; R., G. A Practical Synthesis of α-Substituted tert-Butyl Acrylates from Meldrum’s Acid and Aldehydes. Synthesis 2009, 627–635. A. Stephen K. Hashmi, A. S.; Jaimes, M. C. B.; Schuster, A. M.; Rominger, F. J. Org. Chem. 2012, 77, 6394–6408. Chelucci, G.; Baldino, S.; Ruiu, A. Room-Temperature Hydrodehalogenation of Halogenated Heteropentalenes with One or Two Heteroatoms. J. Org. Chem. 2012, 77, 9921–9925. Jiang, Y.; Qin, Y.; Xie, S.; Zhang, X.; Dong, J.; Ma, D. A General and Efficient Approach to Aryl Thiols: CuI-Catalyzed Coupling of Aryl Iodides with Sulfur and Subsequent Reduction. Org. Lett. 2009, 11, 5250–5253. Shen, Y.; Sun, T.; Liu, X.; Jia, J. RSC Adv. 2012, 2, 4189–4197. Geiger, C.; Kreitmeier, P.; Reiser, O. Cobalt(II)-Azabis(oxazoline)Catalyzed Conjugate Reduction of α,β-Unsaturated Carbonyl Compounds. Adv. Synth. Catal. 2004, 347, 249–254. Ortiz-Marciales, M.; Rivera, L. D.; De Jesus, M.; Espinosa, S.; Benjamin, J. A.; Casanova, O. E.; Figueroa, I. G.; Rodrigues, S.; Correa, W. Facile Rearrangement of O-Silylated Oximes on Reduction with Boron Trifluoride/Borane. J. Org. Chem. 2005, 70, 10132–10134. Patra, P. K.; Nishide, K.; Fuji, K.; Node, M. Dod-S-Me and methyl 6-morpholinohexyl sulfide (MMS) as new odorless borane carriers. Synthesis 2004, 1003–1006. 270 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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

58. Hashimoto, T.; Yamamoto, K.; Maruoka, K. Catalytic enantioselective intramolecular cyclization of N-aryl diazoamides using a titanium-BINOLate complex. Chem. Commun. 2014, 50, 3220–3223. 59. Query, I. P.; Squier, P. A.; Larson, E. M.; Isley, N. A.; Clark, T. B. AlkoxideCatalyzed Reduction of Ketones with Pinacolborane. J. Org. Chem. 2011, 76, 6452–6456. 60. Eisenberger, P.; Bailey, A. M.; Crudden, C. M. Taking the F out of FLP: Simple Lewis Acid - Base Pairs for Mild Reductions with Neutral Boranes via Borenium Ion Catalysis. J. Am. Chem. Soc. 2012, 134, 17384–17387. 61. Lu, Z.; Bhongle, N.; Su, X.; Ribe, S.; Senanayake, C. H. Novel diacid accelerated borane reducing agent for imines. Tetrahedron Lett. 2002, 43, 8617–8620. 62. Li, D. R.; He, A.; Falck, J. R. Enantioselective, Organocatalytic Reduction of Ketones using Bifunctional Thiourea-Amine Catalysts. Org. Lett. 2010, 12, 1756–1759. 63. Bandini, M.; Bottoni, A.; Cozzi, P. G.; Miscione, G. P.; Monari, M.; Pierciaccante, R.; Umani-Ronchi, A. Chiral C2-Boron-Bis(oxazolines) in Asymmetric Catalysis - A Theoretical Study of the Catalyzed Enantioselective Reduction of Ketones Promoted by Catecholborane. Eur. J. Org. Chem. 2006, 4596–4608. 64. Zhang, Z.; Jain, P.; Antilla, J. C. Asymmetric Reduction of Ketones by Phosphoric Acid Derived Catalysts. Angew. Chem., Int. Ed. 2011, 50, 10961–10964. 65. Enders, D.; Rembiak, A.; Seppelt, M. Asymmetric organocatalytic reduction of ketimines with catecholborane employing a N-triflyl phosphoramide Brønsted acid as catalyst. Tetrahedron Lett. 2013, 54, 470–473. 66. Chakraborty, S.; Zhang, J.; Krause, J. A.; Guan, H. An Efficient Nickel Catalyst for the Reduction of Carbon Dioxide with a Borane. J. Am. Chem. Soc. 2010, 132, 8872–8873. 67. Itsuno, S. Enantioselective reduction of ketones; John Wiley & Sons: 1998; Vol. 52, pp 395−576. 68. Corey, E. J.; Helal, C. J. Reduction of Carbonyl Compounds with Chiral Oxazaborolidine Catalysts: A New Paradigm for Enantioselective Catalysis and a Powerful New Synthetic Method. Angew. Chem., Int. Ed. 1998, 37, 1986–2012. 69. Deloux, L.; Srebnik, M. Asymmetric boron-catalyzed reactions. Chem. Rev. 1993, 93, 763–784. 70. Martens, J.; Wallbaum, S. Asymmetric syntheses with chiral oxazaborolidines. Tetrahedron: Asymmetry 1992, 3, 1475–1504. 71. Cho, B. T. Recent development and improvement for boron hydride-based catalytic asymmetric reduction of unsymmetrical ketones. Chem. Soc. Rev. 2009, 38, 443–452. 72. Inoue, T.; Sato, D.; Komura, K.; Itsuno, S. Enantiomerically Pure 2-Piperazinemethanols as Novel Chiral Ligands of Oxazaborolidine Catalysts in Enantioselective Borane Reductions. Tetrahedron Lett. 1999, 40, 5379–5382. 271 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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

73. Fontaine, E.; Namane, C.; Meneyrol, J.; Geslin, M.; Serva, L.; Roussey, E.; Tissandie, S.; Maftouh, M.; Roger, P. Synthesis of optically-active benzylic amines; asymmetric reduction of ketoxime ethers with chiral oxazaborolidines. Tetrahedron: Asymmetry 2001, 12, 2185–2189. 74. Krzeminski, M. P.; Zaidlewicz, M. Asymmetric reduction of ketoxime derivatives and N-alkylketimines with borane-oxazaborolidine adducts. Tetrahedron: Asymmetry 2003, 14, 1463–1466. 75. Stepanenko, V.; De Jesus, M.; Correa, W.; Bermudez, L.; Vazquez, C.; Guzman, I.; Ortiz-Marciales, M. Chiral spiroaminoborate ester as a highly enantioselective and efficient catalyst for the borane reduction of furyl, thiophene, chroman, and thiochroman-containing ketones. Tetrahedron: Asymmetry 2009, 20, 2659–2665. Pakulski, M. M.; Mahato, S. K.; Bosiak, M. J.; Krzeminski, M. P.; Zaidlewicz, M. Enantioselective reduction of ketoxime ethers with boran-oxazaborolidines and synthesis of the key intermediate leading to (S)-rivastigmine. Tetrahedron: Asymmetry 2012, 23, 716–721. 76. Korenaga, T.; Nomura, K.; Onoue, K.; Sakai, T. Rational electronic tuning of CBS catalyst for highly enantioselective borane reduction of trifluoroacetophenone. Chem. Commun. 2010, 46, 8624–8626. 77. Stepanenko, V.; Huang, K.; Ortiz-Marciales, M. Org. Synth. 2015, XII, 1449–1457Collective volume. 78. Huang, K.; Ortiz-Marciales, M. Catalytic enantioselective borane reduction of benzyl oximes: Preparation of (S)-1-pyridine-3-yl-ethylamine bis-hydrochloride. Org. Synth. 2015, XII, 1458–1472 (Collective volume). 79. Brunel, J. M.; Pardigon, O.; Faure, B.; Buono, G. Enantioselective borane reduction of ketones catalysed by a chiral oxazaphospholidine-borane complex. J. Chem. Soc., Chem. Commun. 1992, 287–288. 80. Brunel, J. M.; Legrand, O.; Buono, G. Chiral (o-Hydroxyaryl)oxazaphospholidine Oxides: A New Class of Bifunctional Catalysts in the Enantioselective Borane Reduction of Ketones. Eur. J. Org. Chem. 2000, 3313–3321. 81. Gamble, M. P.; Studley, J. R.; Wills, M. New chiral phosphinamide catalysts for highly enantioselective reduction of ketones. Tetrahedron Lett. 1996, 37, 2853–3856. 82. Burns, B.; Gamble, M. P.; Simm, A. R. C.; Studley, J. R.; Alcock, N. W.; Wills, M. Tetrahedron: Asymmetry 1997, 8, 73–78. 83. Burns, B.; King, N. P.; Tye, H.; Studley, J. R.; Gamble, M. P.; Wills, M. Chiral phosphinamides: new catalysts for the asymmetric reduction of ketones by borane. J. Chem. Soc., Perkin Trans. 1 1998, 1027–1038. 84. Gamble, M. P.; Studley, J. R.; Wills, M. Design, synthesis and applications of a ketone reduction catalyst containing a phosphinamide combined with a dioxaborolidine unit. Tetrahedron: Asymmetry 1996, 7, 3071–3074. 85. Burns, B.; Studley, J. R.; Wills, M. Tetrahedron Lett. 1993, 34, 7105–7106. 86. Burns, B.; King, N. P.; Studley, J. R.; Tye, H.; Wills, M. Tetrahedron: Asymmetry 1994, 5, 801–804.

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

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

87. Gamble, M. P.; Simith, A. R. C.; Wills, M. A Novel Phosphinamide Catalyst for the Asymmetric Reduction of Ketones by Borane. J. Org. Chem. 1998, 63, 6068–6071. 88. Basavaiah, D.; Reddy, G. J.; Chandrashekar, V. A new chiral catalytic source with an N–P=O structural framework containing a proximal hydroxyl group for the borane-mediated asymmetric reduction of prochiral ketones. Tetrahedron: Asymmetry 2004, 15, 47–52. 89. Du, D.; Fang, T.; Xu, J.; Zhang, S. Structurally Well-Defined, Recoverable C3-Symmetric Tris(a-hydroxy phosphoramide)-Catalyzed Enantioselective Borane Reduction of Ketones. Org. Lett. 2006, 8, 1327–1330. 90. Martins, N.; Mateus, N.; Vinci, D.; Saidi, O.; Brigas, A.; Bacsa, J.; Xiao, J. Another side of the oxazaphospholidine oxide chiral ortho-directing group. Org. Biomol. Chem. 2012, 10, 4036–4042. 91. Smith, K.; Balakit, A. A.; Pardasani, R. T.; El-Hiti, G. A. J. Sulfur Chem. 2011, 32, 287–295. 92. Staubitz, A.; Robertson, A. P. M.; Sloan, M. E.; Manners, I. Amine- and Phosphine-Borane Adducts: New Interest in Old Molecules. Chem. Rev. 2010, 110, 4023–4078. 93. Staubitz, A.; Robertson, A. P. M.; Manners, I. Ammonia-Borane and Related Compounds as Dihydrogen Sources. Chem. Rev. 2010, 110, 4079–4124. 94. Bartori, G.; Bosco, M.; Bellucci, M. C.; Dalpozzo, R.; Marcantoni, E.; Sambri, L. TiCl4-Mediated Reduction of 1,3-Diketones with BH3-Pyridine Complex: A Highly Diastereoselective Method for the Synthesis of syn-1,3-Diols. Org. Lett 2000, 2, 45–47. 95. Sato, S.; Sakamoto, T.; Miyazawa, E.; Kikugawa, Y. One-pot reductive amination of aldehydes and ketones with α-picoline-borane in methanol, in water, and in neat conditions. Tetrahedron 2004, 60, 7899–7906. 96. Wang, X.; Xie, L.; Huang, K.-W.; Lai, Z. Chem. Commun. 2015, 51, 7610–7613. 97. Kalidindi, S. B.; Sanyal, U.; Jagirdar, B. R. Metal Nanoparticles via the Atom-Economy Green Approach. Inorg. Chem. 2010, 49, 3965–3967. 98. Hetzer, R. H.; Grais, H. J.; Raabe, G. Synthesis of Chiral α-(NSulfoximido) Phosphines, Phosphine Oxides, and Phosphonates through Hydrophosphination and Hydrophosphorylation of N-Vinyl Sulfoximines. Synthesis 2008, 1126–1132. 99. Burg, A. B.; Wagner, R. I. Chemistry of P-B Bonding: The Phosphinoborines and their Polymers. J. Am. Chem. Soc. 1953, 75, 3872–3877. 100. Ohff, M. Borane complexes of trivalent organophosphorus compounds. Versatile precursors for the synthesis of chiral phosphine ligands for asymmetric catalysis. Synthesis 1998, 1391–1415. 101. Brunel, J. M.; Faure, B.; Maffei, M. Phosphane-boranes: synthesis, characterization and synthetic applications. Cord. Chem. Rev. 1998, 178-180, 665–698. 102. McNulty, J.; Zhou, Y. A highly efficient general synthesis of phosphineborane complexes. Tetrahedron Lett. 2004, 45, 407–409. 103. Piers, W. E.; Chivers, T. Pentafluorophenylboranes: from obscurity to applications. Chem. Soc. Rev. 1997, 26, 345–354. 273 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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

104. Sajid, M.; Elmer, L. M.; Rosorius, C.; Daniliuc, C. G.; Grimme, S.; Kehr, G.; Erker, G. Facile Carbon Monoxide Reduction at Intramolecular Frustrated Phosphane/Borane Lewis Pair Templates. Angew. Chem., Int. Ed. 2013, 52, 2243–2246. 105. Courtemanche, M. A.; Legare, M. A.; Maron, L.; Fontaine, F. G. A Highly Active Phosphine-Borane Organocatalyst for the Reduction of CO2 to Methanol Using Hydroboranes. J. Am. Chem. Soc. 2013, 135, 9326–9329. 106. Courtemanche, M. A.; Legare, M. A.; Maron, L.; Fontaine, F. G. Reducing CO2 to Methanol Using Frustrated Lewis Pairs: On the Mechanism of Phosphine-Borane-Mediated Hydroboration of CO2. J. Am. Chem. Soc. 2014, 136, 10708–10717. 107. Ueng, S. H.; Brahmi, M. M.; Derat, E.; Fensterbank, L.; Lacote, E.; Malacria, M.; Curran, D. P. Complexes of Borane and N-Heterocyclic Carbenes: A New Class of Radical Hydrogen Atom Donor. J. Am. Chem. Soc. 2008, 130, 10082–10083. 108. Ueng, S. H.; Fensterbank, L.; Emmanuel Lacote, E.; Malacria, M.; Curran, D. P. Radical Deoxygenation of Xanthates and Related Functional Groups with New Minimalist N-Heterocyclic Carbene Boranes. Org. Lett. 2010, 12, 3002–3005. 109. Horn, M.; Mayr, H.; Lacote, E.; Merling, E.; Deaner, J.; Wells, S.; McFadden, T.; P., C. D. N-Heterocyclic Carbene Boranes are Good Hydride Donors. Org. Lett. 2012, 14, 82–85. 110. Taniguchi, T.; Curran, D. P. Silica Gel Promotes Reductions of Aldehydes and Ketones by N-Heterocyclic Carbene Boranes. Org. Lett. 2012, 14, 4540–4543. 111. Farrell, J. M.; Posaratnanathan, R. T.; Stephan, D. W. A family of N-heterocyclic carbene-stabilized borenium ions for metal-free imine hydrogenation catalysis. Chem. Sci. 2015, 6, 2010–2015.

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