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Reaction of amides with NaBH4/I2 system in THF gives the corresponding amines in 70-76% yields. Reduction of nitriles yields the corresponding amines in 70-75% yields. The I2/NaBH4 system is useful in the hydroboration of olefins and the corresponding alcohols are obtained in 78-92% yields after H2O2/OH- oxidation. The reagent system is also useful for the reduction of carboxylic esters and acids to the corresponding alcohols in 60-90% yields.
The NaBH4 does not reduce amides, nitriles, carboxylic esters and acids under ambient conditions. The efforts to increase the reactivity of NaBH4 towards esters using Lewis acids lead to the discovery of hydroboration of olefins1 and methods for generation of diborane which is now commercially available in various forms (e.g. BH3·THF, BH3·SMe2 and BH3·NR3). However, efforts are still continuing for developing convenient methods for the reduction of functional groups using NaBH4 along with other additives. For example, it has been recently reported that carboxylic acid esters and amides are reduced to the corresponding alcohols and amines using NaBH4/ZnCl2 in THF in the presence of a tertiary amine under reflux conditions.2 It has been also reported that a mixture of R2SeBr2 (R = CH2CH2Br or C2H5) and NaBH4 reduces amides and nitriles to the corresponding amines.3 it has been shown that the R2SeBr2/NaBH4 system in THF gives borane. We describe here convenient procedures for the reduction of amides and nitriles to amines, carboxylic esters and acids into alcohols and also hydroboration of alkenes using the NaBH4/I2 system.
Although, it has been known for a long time that pure diborane can be obtained by the reaction of I2 with NaBH4 in diglyme4, this readily accessible reagent system did not receive much attention in synthetic applications. We have reported that the I2/NaBH4 system is useful for the generation of diborane for utilization in the preparation of amine boranes.5 Addition of I2 to RCOOH/NaBH4 mixture leads to selective reduction of carboxylic acid group to alcohol.6 We wish to report here that the addition of I2 (10 mmol) into a mixture of an amide (or) imide (10 mmol) and NaBH4 (23 mmol) in THF for 2.5h at 0°C followed by refluxing for 3h gives the corresponding amine in 70-76% yield (entries 1-4 in Table 1). The yields are comparable to those reported using BH3·SMe2,7 NaBH4/ZnCl2/N,N-diethylaniline2 in THF and NaBH4/R2SeBr2.3 lt has been reported that hydroboration of olefins with I2/NaBH4 in THF followed by oxidation with NaBO3 give alcohols.8
We have observed that addition of I2 (2.8 mmol) to NaBH4 (7 mmol) in THF (25 ml) at 0°C followed by olefin (15 mmol); stirring for 2h at room temperature and oxidation with H2O2/NaOH or H2O2/NaOAc gives the corresponding alcohols in 78-92% yields (entries 1-5 in Table 2). The regio- and stereoselectivities observed here are similar to those reported previously using BH3·THF,9 BH3·SMe29 and BH3·N,N-diethylaniline5 complexes for the hydroboration of alkenes.
Selective hydroboration of olefinic group can be achieved when it is present along with an ester group as illustrated by the hydroboration-oxidation of methyl-10-undecenoate (entry 5, Table 2).
However, carboxylic esters can be reduced to the corresponding alcohols in 86-89% by the addition of I2 (5 mmol) and NaBH4 (12 mmol) in THF (30 ml) at 0°C for 2.5h and refluxing the mixture for 0.5h after the addition of carboxylic esters (entries 1 and 3, Table 3). We have also observed that addition of I2 (5 mmol) to NaBH4 (12 mmol) in THF (30 ml) at 0°C for 2.5h followed by the addition of Ph3P (10 mmol) in THF (20 ml) gives Ph3P·BH3 in 94% yield, indicating the formation of BH3·THF in the reaction of NaBH4 with I2.
In all cases , we have used NaBH4, more than required by the stoichiometry, since it is known that the "BH3" complexes also react with iodine to give R2B-I species which are known to cleave ethers.10 Also, the I2 in THF was added slowly in portions to NaBH4 in THF at 0°C in order to avoid possible cleavage of THF. However, the addition time can be reduced to 0.5h at 0°C without significant change in results.
We have recently reported that successive addition of RCOOH and I2 into NaBH4 in THF leads to reduction of carboxylic acids. This method gives good results in 10 mmol scale. When the reduction of carboxylic acid was attempted in >50 mmol scale substantial amounts of products (I and/or II) derived from THF cleavage were also obtained.
However, this problem can be circumvented by the addition of I2 into NaBH4 at 0°C followed by the addition of carboxylic acid. The corresponding alcohols are obtained in good yields after work up (entries 3 and 4, Table 3). However, the chemoselectivity observed in the reduction of olefinic acids is lost by performing the reaction in this way. For example, the reduction of 10-undecenoic acid gives 10-undecenoic in 89% yield by the addition of the acid into NaBH4 in THF at room temperature followed by addition of I2 at 0°C.6 However, addition of I2 into NaBH4 at 0°C followed by addition of carboxylic acid leads to the hydroboration of the double bond and a mixture of 1,11-undecanediol (59%) and 11-hydroxy-undecanoic acid (20%) (entry 5 in Table 3) are obtained after oxidation with H2O2/OH- and protolysis.
In conclusion, the readily accessible I2/NaBH4 reagent system is useful in the reduction of amides, nitriles, carboxylic esters and acids and also for hydroboration of alkenes.
General
Tetrahydrofuran distilled freshly over benzophenone-sodium was used for all the experiments. Infrared spectra were recorded on a Perkin-Elmer lR spectrometer 1310 with polystyrene as reference. NMR spectra were recorded on a JEOL-FX-100 spectrometer in deuterated chloroform using tetramethylsilane as internal standard. The chemical shifts (δ) are expressed in ppm downfield from the signal for internal Me4Si. For TLC plates coated with silica gel were run in hexane/ethyl acetate mixture and spots were developed in iodine chamber. For column chromatographic purification under gravity, column grade silica gel (100-200 mesh size) was employed.
Reduction of imide
Imide (1.7 g, 5 mmol) in dry THF (15 ml) was added to a slurry of NaBH4 (1 g, 27 mmol) in dry THF (15 ml) in a two-neck septum capped round-bottom flask. 12 (3 g, 12 mmol) in dry THF (20 ml) was added under nitrogen atmosphere at 0°C for 2.5h. The mixture was refluxed for 6h; cooled to 0°C and the excess hydride was carefully destroyed with 3N HCl (5 ml). After the gas evolution ceased, it was neutralized using 3N NaOH (8 ml). The organic layer was separated and aqueous layer was extracted with ether (3 x 10 ml). The combined organic extracts were washed with water, brine and dried over anhydrous MgSO4. The solvent was evaporated and the amine borane residue was treated with BF3·Et2O followed by aq. NaOH to liberate the free amine. The product (entry 1, Table 1) was purified by chromatography on silica gel column (hexane/ethyl acetate 85:15). Yield: 1.2g (76%).
Reduction of N-methylacetanilide
The amide (1.49 g, 10 mmol) and NaBH4 (0.88 g, 23 mmol) were taken in dry THF (70 ml) in a two-neck septum capped round-bottom flask. Iodine (2.54 g, 10 mmol) in dry THF (20 ml) was added in portions under nitrogen atmosphere at 0°C for 2.5h. The reaction mixture was refluxed (70°C) for 3h; cooled to 0°C, the excess hydride destroyed by careful addition of 3N HCl (6 ml). After the gas evolution ceased, it was neutra lized using 3N NaOH (8 ml). The organic layer was separated and the aqueous layer was extracted with ether (2x15 ml). The combined organic extract was washed with water, brine and dried over anhydrous MgSO4. The solvent was removed and the product was purified by column chromatography on silica gel (hexane/ethylacetate 95:5). Yield: 1 g (74%).
Reduction of 3,5-dimethylbenzylcyanide
ln a two-neck septum-capped round-bottom flask, NaBH4 (0.88 g, 23 mmol) and 3,5-dimethylbenzylcyanide (1.45g, 10 mmol) were taken in dry THF (30 ml). 12 (2.54 g,10 mmol) in dry THF (20 ml) was added under nitrogen atmosphere at 0°C for 2.5h. The reaction mixture was refluxed (70°C) for 3h. lt was cooled to 0°C, 6N HCl (8 ml) was added slowly and the contents were refluxed for 0.5h. The mixture was cooled to 0°C and 3 g of NaOH was added. The organic layer was separated and the aqueous layer was extracted with ether (3x10 ml). The combined organic extract was washed with water, brine and dried over anhydrous MgSO4. lt was concentrated and chromatographed on silica gel column (hexane/ethylacetate 60:40). Yield: 1.1 g (74%).
Hydroboration of styrene
In a two-neck septum-capped round-bottom flask NaBH4 (0.27 g, 7 mmol) was taken in dry THF (70 ml). Iodine (0.71 g, 2.8 mmol) in dry THF (15 ml) was added under nitrogen atmosphere over 2.5h at 0°C. Styrene (1.5 g, 15 mmol) was added and the reaction mixture was stirred for 2h at 70°C. It was quenched with water (2 ml), THF (20 ml) was added and oxidized using H2O2 (30%, 30 ml)/NaOH (3N, 30 ml). The organic layer was separated and the aqueous layer was extracted with ether (3x10 ml). The combined organic extract was washed with water, brine and dried over anhydrous MgSO4. On evaporation of solvent and purification by chromatography on silica gel column (hexane/ethylacetate 90:10), 1.68 g (90%) of alcohols were isolated.
Reduction of 10,11-dibromoundecanoic acid (50 mmol scale) using NaBH4/I2
NaBH4 (2.1 g, 60 mmol) in dry THF (120 ml) was taken in two-necked round-bottom flask. To the slurry I2 (6.4 g, 70 mmol) in THF (60 ml) was added slowly during 2.5h through a pressure equalizer at 0°C. To this 10,11-dibromoundecanoic acid (17.2 g, 50 mmol) in THF (30 ml) was added through a cannula. The contents were further stirred for 1h at 70°C. Dil. HCl (20 ml, 3N) was carefully added. The aqueous layer was extracted with ether (3 x 20 ml). The combined organic layer was washed with 3N NaOH solution, brine and dried over MgSO4. Evaporation of solvent afforded 10,11-dibromoundecanol. Yield: 15 g (90%).
No. |
Substrate |
Conditions Temp.(Time) |
Productd |
Yield |
1 | 70°C (6h)b |
76% |
||
2 | PhNCH3COCH3 | 70°C (3h)c |
PhNCH3COCH3 | 74% |
3 | PhNHCOCH3 | 70°C (3h)c |
PhNHC2H5 | 75% |
4 | PhCONH2 | 70°C (3h)c |
PhCH2NH2 | 70% |
5 |
3,5-Me2PhCH2CN | 70°C (3h)c |
3,5-Me2Ph(CH2)2NH2 | 74% |
6 | PhCH2CN | 70°C (3h)c |
PhCH2CH2NH2 | 72% |
7 | PhCN | 70°C (3h)c |
PhCH2NH2 | 70% |
8 | CH3(CH2)7CN | 70°C (3h)c |
CH3(CH2)7CH2NH2 | 75% |
No. |
Substrate | Conditions Temp.(Time) | Producte | Yield |
1 |
PhCH=CH2 | 25°C (2h)b | PhCH2CH2OHf | 90% |
2 |
C8H17CH=CH2 | 25°C (2h)b | C8H17CH2CH2OH | 92% |
3 |
C6H10 | 25°C (4h)c | C6H11OH | 85% |
4 |
25°C (4h)c | 81% | ||
5 |
CH2=CH(CH2)8COOMe | 25°C (2h)d | HO(CH2)10COOMe | 78% |
No. |
Substrate | Conditions Temp.(Time) | Product(s)d | Yield |
1 |
PhCH2COOEt | 70°C (0.5h)b | PhCH2CH2OH | 85% |
2 |
CH3(CH2)8COOCH3 | 70°C (0.5h)b | CH3(CH2)8CH2OH | 89% |
3 |
CH3(CH2)8COOH | 25°C (1h)c | CH3(CH2)8CH2OH | 90% |
4 |
BrCH2CHBr(CH2)8COOH | 25°C (1h)c | BrCH2CHBr(CH2)8CH2OH | 86% |
5 |
CH2=CH(CH2)8COOH | 25°C (1h)c | HOCH2CH2(CH2)8CH2OH | 59% |
| HOCH2CH2(CH2)8COOH | 20% |