Lego
(Newbee)
05-23-03 21:36
No 435007
      Zn(BH4)2/Pyridine reduces nitro compounds
(Rated as: excellent)
    

Nobee seems to bee interested in Chinese Journals but why?



First and Efficient Method for Reduction of Aliphatic and Aromatic Nitro Compounds with Zinc Borohydride as Pyridine Zinc Tetrahydroborato Complex: A New Stable Ligand-Metal Borohydride

Journal of the Chinese Chemical Society, 2003, 50, 267-271


Fulltext as PDF (http://nr.stic.gov.tw/ejournal/ChiChemSociety/2003/EJ52-2003-267.pdf)

[...]
 
Pyridine zinc tetrahydroborate, [(Py)Zn(BH4)2], as a new stable ligand-metal borohydride, is prepared quantitatively by complexation of 1:1 zinc borohydride and pyridine at room tem perature. This reagent efficiently reduces different aromatic and aliphatic nitro compounds to their primary amines in refluxing THF. In addition, the reduction shows chemoselectivity for aliphatic nitro compounds over the aromatic nitro compounds.

Keywords: Reduction; Zinc borohydride; Nitro compound; Chemoselectivity; Amine.

[...]

(Py)Zn(BH4)2 is stable, easy to handle and is quantitatively prepared by complexation of 1:1 zinc borohydride and pyridine in dry ether at room temperature.

[...]

This reducing agent could be stored for months without losing its activity.

[...]


It is noted that carboxylic acid and amides were reduced faster than the ni tro group; there fore chemoselective reduction of such functional groups in the presence of nitro group is feasible. (Entries 6, 7, 10 & 11). On the other hand our at tempts to selective reduction of this functional group in the presence of carboxylic acid or amido group were unsatisfactory under different conditions. The reducing ability of the reagent for the reduc tion of dinitro groups in the substrates is demonstrated by ready reduction of 2,5-dinitrochlorobezene to the corresponding amine with 8 molar equivalents in refluxing THF (95% yield, Entry 9). Heterocyclic compounds such as 2-chloro-4-nitropyridine that con tain a nitro group could also be reduced to their corresponding amine faster than the aromatic ones (Entry 12).
A comparison in Table 1, shows that with respect to the aromatic and heterocyclic nitro compounds, primary, secondary and tertiary aliphatic nitro compounds were rapidly reduced to their corresponding amines in excellent yields under reflux condition (En tries 13-15) whereas sodium borohydride-transition metal salts systems18,19,25d for reduction of aliphatic nitro compounds usually give 60-80% yields. This advantage is shown by a comparison in the reduction of 2-methyl-2-nitrobutane with (Py)Zn(BH4)2 and BER/Ni(OAc)2 (Table 2, Entry 6).


Table 1. Reduction of Nitro Compounds to Their Amines with (Py)Zn(BH4)2a
Entry Substrate Product Molar Ratio Reag./Subs. Time (h) Yield (%)b
1











Molecule: a1 ("c1(ccccc1)[N+]([O-])=O")













Molecule: a1a ("c1(ccccc1)N")

4 7.5 95
2











Molecule: a2 ("c1(ccc(cc1)N)[N+]([O-])=O")













Molecule: a2a ("c1(ccc(cc1)N)N")

4 5 96
3











Molecule: a3 ("c1(cc(ccc1)N)[N+]([O-])=O")













Molecule: a3a ("c1(cc(ccc1)N)N")

4 6 80
4











Molecule: a4 ("c1(cc(ccc1)N)[N+]([O-])=O")













Molecule: a4a ("c1(ccc(cc1)O)N")

4 1.5 97
5











Molecule: a5 ("c1(c(cccc1)O)[N+]([O-])=O")













Molecule: a5a ("c1(c(cccc1)O)N")

4 2 93
6











Molecule: a6 ("c1(ccc(cc1)C(=O)O)[N+]([O-])=O")













Molecule: a6a ("c1(ccc(cc1)CO)[N+]([O-])=O")

4 2.2 95
7











Molecule: a7 ("c1(ccc(cc1)C(=O)O)[N+]([O-])=O")













Molecule: a7a ("c1(ccc(cc1)CO)N")

8 4.5 92
8











Molecule: a8 ("c1(cccc2c1cccc2)[N+]([O-])=O")













Molecule: a8a ("c1(cccc2c1cccc2)N")

4 3.5 98
9











Molecule: a9 ("c1(cc(ccc1Cl)[N+]([O-])=O)[N+]([O-])=O")













Molecule: a9a ("c1(cc(ccc1Cl)N)N")

8 7 95
10











Molecule: a10 ("c1(ccc(cc1)NC(C)=O)[N+]([O-])=O")













Molecule: a10a ("c1(ccc(cc1)N)[N+]([O-])=O")

4 2 91
11











Molecule: a11 ("c1(ccc(cc1)NC(C)=O)[N+]([O-])=O")













Molecule: a11a ("c1(ccc(cc1)N)N")

8 3.1 94
12











Molecule: a12 ("c1cnc(cc1[N+]([O-])=O)Cl")













Molecule: a12a ("c1cnc(cc1N)Cl")

4 2 96
13











Molecule: a13 ("CCCC[N+]([O-])=O")













Molecule: a13a ("CCCCN")

4 0.25 94
14











Molecule: a14 ("C([N+]([O-])=O)(C)C")













Molecule: a14a ("C(N)(C)C")

4 0.8 94
15











Molecule: a15 ("C([N+]([O-])=O)(C)(C)C")













Molecule: a15a ("C(N)(C)(C)C")

4 0.67 96

a All reactions were performed in THF under reflux condition.
b Yields referred to isolated products.


Table 2. Comparison Reduction of Nitro Compounds with (Py)Zn(BH4)2 and Some Other Reported Reagents

Molar Ratio (Reag./Subs.)a/Time (h)/Yield (%)

Entry Substrate Product I II28 III17 IV17
1











Molecule: b1 ("c1(ccccc1)[N+]([O-])=O")













Molecule: b1a ("c1(ccc(cc1)N)N")

4(7.5)(95) (5:0.1)(1)(94) (8:1.5)(2)(86) (5:2)(0.5)(90)
2











Molecule: b2 ("c1(ccc(cc1)N)[N+]([O-])=O")













Molecule: b2a ("c1(cc(ccc1)N)N")

4(5)(96) ---- (8:1.5)(1.5)(85) (5:2)(0.33)(91)
3











Molecule: b3 ("c1(cc(ccc1)N)[N+]([O-])=O")













Molecule: b3a ("c1(ccc(cc1)O)N")

4(6)(80) (5:0.1)(1)(96) (8:1.5)(1.5)(90) (5:2)(0.17)(85)
4











Molecule: b4 ("c1(c(cccc1)O)[N+]([O-])=O")













Molecule: b4a ("c1(c(cccc1)O)N")

4(2)(93) [td]---- (8:1.5)(1.5)(95) (5:2)(0.17)(88)
5











Molecule: b5 ("c1(cccc2c1cccc2)[N+]([O-])=O")













Molecule: b5a ("c1(cccc2c1cccc2)N")

4(3.5)(98) (5:0.1)(1)(96) ---- ----
6











Molecule: b6 ("C([N+]([O-])=O)(C)(C)C")













Molecule: b6a ("C(N)(C)(C)C")

4(0.67)(96) (6:0.1)(1)(96) ---- ----

I (Py)Zn(BH4)2; II BER/Ni(OAc)2; III NaBH4/BiCl3; IV NaBH4/SbCl3.
a In the case of reported reagents, molar ratios are: NaBH4 or BER/halides or salts/substrate.
BER = borohydride exchange resin

Preparation of Pyridine Tetrahydroborato Zinc Complex: [(Py)Zn(BH4)2]
An ethereal solution of Zn(BH4)2 (0.16 M, 250 mL) was prepared from ZnCl2 (5.452 g, 0.04 mol) and NaBH4 (3.177 g, 0.084 mol) according to an established procedure in the literature29 Then, pyridine (3.164 g, 0.04 mol) in ether (50 mL) was added dropwise to the ethereal solution of Zn(BH4)2 and stirred for 30 min. Careful evaporation of the solvent under vacuum at room temperature gave [(Py)Zn(BH4)2] in almost quantitative yield (6.83 g, 98%).13

Reduction of Nitrobenzene with [(Py)Zn(BH4)2]: A Typical Procedure
In a round-bottom flask equipped with magnetic stirrer and condenser, was placed nitrobenzene (0.123 g, 1 mmol) and THF (8 mL), also the reducing agent (0.7 g, 4 mmol) was added. The reaction mixture was stirred magnetically under reflux condition for 7.5 h with TLC monitor of the progress of the reaction. After completion of the reaction, methanol (3 mL) was added to the reaction mixture and magnetically stirred for 15 min. The solvent was evaporated and the resulting crude material was purified by a silica gel column chromatography with appropriate eluent. Evaporation of the solvent affords aniline as a pure liquid compound (0.088 g, 95% yield).


References:
[...]
13. Faraji, F. M.Sc. Thesis, Urmia University, March 2002, Iran.
[...]
17. Ren, P. D.; Pan, S. F.; Dong, T. W.; Wu, S. H. Synth.
Commun. 1995, 25, 3799.
18. Yoo, S. E.; Lee, S. H. Synlett 1990, 419.
19. Petrini, M.; Ballini, R.; Rosini, G. Syn the sis 1987, 713.
[...]
25. (a) Osby, J. O.; Ganem, B. Tetrahedron Lett. 1985, 26, 6413.
(b) Nose, A.; Kudo, T. Chem. Pharm. Bull. 1981, 29, 1159.
(c) Sarma, J. C.; Borbaruah, M.; Sharma, R. P. Tetrahedron Lett. 1985, 26, 4657. (d) Sarma, D. N.; Sharma, R. P. Tetrahedron Lett. 1985, 26, 2581.
28. Yoon, N. M.; Choi, J. Synlett 1993, 135 and the references cited therein.
29. (a) Gensler, W. J.; Johnson, F.; Sloan, A. D. B. J. Am. Chem. Soc. 1960, 82, 6074. (b) Crabbe, P.; Garcia, G. A.; Rius, C. J. Chem. Soc, Perkin Trans. 1 1973, 810. See also Post 394840 (demorol: "Preparation of zinc borohydride solution", Novel Discourse) and Post 435004 (Lego: "A slight variation", Novel Discourse)

The candle that burns twice as bright burns half as long
 
 
 
 
    Rhodium
(Chief Bee)
05-23-03 23:31
No 435027
      Journal Xenophobia     

Great post!

I think the reason why very few are interested in chinese journals is because they are too lazy to scan trough the table-of-contents of lesser known journals to find novel things of interest, they merely follow references from other journal articles they happen to read - and as asian journals seldom are quoted (for precisely the reason above), people never read them.
 
 
 
 
    Rhodium
(Chief Bee)
05-24-03 00:32
No 435043
      Zn(BH4)2/Pyridine reduces carbonyl compounds
(Rated as: excellent)
    

(Pyridine)(tetrahydroborato)zinc Complex, [Zn(BH4)2(py)], as a New Stable, Efficient and Chemoselective Reducing Agent for Reduction of Carbonyl Compounds
Bull. Korean Chem. Soc. 24(4), 453-459 (2003) (http://journal.kcsnet.or.kr/publi/bul/bu03n4/453.pdf)

Abstract

(Pyridine)(tetrahydroborato)zinc complex, [Zn(BH4)2(py)], as a stable white solid, was prepared quantitatively by complexation of an equimolar amount of zinc tetrahydroborate and pyridine at room temperature. This reagent can easily reduce variety of carbonyl compounds such as aldehydes, ketones, acyloins, -diketones and ,-unsaturated carbonyl compounds to their corresponding alcohols in good to excellent yields. Reduction reactions were performed in ether or THF at room temperature or under reflux conditions. In addition, the chemoselective reduction of aldehydes over ketones was accomplished successfully with this reducing agent.
 
 
 
 
    dioulasso
(Newbee)
01-09-04 18:31
No 481426
      N-methylation of amines with Zn(BH4)2
(Rated as: excellent)
    

Some may find this interresting (posted it here, since this was the most closely related thread still open...):




Use of Zinc Borohydride in Reductive Amination: An Efficient and Mild Method
for N-Methylation of Amines

Sukanta Bhattacharyya et. al.
J. Chem. Soc. Perkin Trans. 1, pp. 1-2 (1994) (http://bobo_bee.tripod.com/Hive/papers/2_red_am.djvu)

An efficient method for the reductive methylation of amines using paraformaldehyde, zinc chloride and zinc borohydride is described.




This can be an alternative to:

../rhodium/chemistry /amphetamine.methylation.html

PiHKAL #109 MDMA (http://www.erowid.org/library/books_online/pihkal/pihkal109.shtml)

Tihkal #50. NMT (http://www.erowid.org/library/books_online/tihkal/tihkal50.shtml)

With this Zn(BH4)2 method nPr-NH2 is monomethylated w/ 70% yield.


Reductive amination w/ Zn(BH4)2 was discussed in:

Post 122811 (dormouse: "reductive amination with NaBH4 -- why *does* it work?  -rev drone", Serious Chemistry)

Post 69528 (spric: "reductive amination with Zn(BH4)2", Methods Discourse)


Preparations of Zn(BH4)2


Post 394840 (demorol: "Preparation of zinc borohydride solution", Novel Discourse)

" Unorthodox cooking, illicit cooking. A bit of real science, in fact. "
 
 
 
 
    Lego
(Hive Bee)
01-31-04 03:26
No 485626
      Zn(BH4)2/Pyridine also reduces COOH, Pt. 1
(Rated as: excellent)
    

J. Chem. Res., 2003, 8, 522-525
No DOI found

Mild and convenient method for reduction of aliphatic and aromatic carboxylic acids and anhydrides with (pyridine)(tetrahydroborato)zinc complex as a new stable ligand-metal tetrahydroborate agent

Behzad Zeynizadeh and Karam Zahmatkesh

Department of Chemistry, Faculty of Sciences, Urmia University, Urmia 57159-165, Iran


Abstract: Structurally different aliphatic and aromatic carboxylic acids and anhydrides are efficiently reduced to their corresponding alcohols with a new modified zinc tetrahydroborate agent, (pyridine)(tetrahydroborato)zinc complex, [(Py)Zn(BH4)2], in refluxing THF.


Keywords: reduction, zinc tetrahydroborate, carboxylic acids, anhydrides, pyridine


Although, there are numerous literature references to the synthetic applications of various metal borohydrides,1 only sodium borohydride has gained commercial status in spite of its poor selectivity and lesser reactivity in organic solvents. Moreover, this reagent is invariably used in excess quantities. In order to affect the reactivity of sodium borohydride, many modifications have been made upon the reagent and so various modified borohydride agents are introduced for different reduction purposes. Recently, the preparation and applications of these reagents in organic synthesis have been reviewed.2
In fact, the reducing ability of borohydrides can be modified by: (a) hydride exchange with other substituents,3 (b) changing of sodium cation to other metal cations,4 quaternary ammonium5 and phosphonium cations,6 (c) a concurrent cation and hydride exchange,7 (d) preparation of ligand-metal borohydrides,8 (e) combination of borohydrides with metals, metal salts, Lewis acids;9 and, finally, (f) supporting on polymers.10
Among the reported transition metal borohydrides, zinc tetrahydroborate, Zn(BH4)2, is the only reagent which has been used frequently in the reduction reactions.11 However, because of its requirement for storage in a cold place and instability in solid state or solution for a long time, it should always be used as its freshly prepared ethereal solutions and this situation puts some restrictions on its uses. Recently new modifications of zinc tetrahydroborate in the form of ligand–metal complexes such as [(pyz)Zn(BH4)2]n12, [(dabco)Zn(BH4)2]2c,6b and [(Ph3P)x- Zn(BH4)2] (x=1, 2)13 have been introduced and their applications in the reduction of different functional groups have been reported. These successes prompted us to prepare (pyridine)-(tetrahydroborato)zinc complex, [(Py)Zn(BH4)2], as a new modified and stable zinc tetrahydroborate agent and investigate its reducing abilities.14 In our preliminary investigation, we reduced structurally different aromatic and aliphatic nitro compounds efficiently to their corresponding amines.14b In continuation of our experiments, now we wish to report an efficient method for the reduction of aliphatic and aromatic carboxylic acids and anhydrides to their corresponding alcohols in refluxing THF (Scheme 1).

Synthetic transformation of carboxylic acids to alcohols is one of the most important reactions in organic synthesis and this goal has been achieved by several borohydride combination systems.15-23 Zinc tetrahydroborate24 alone or its combination system with trifluoroacetic anhydride (TFAA)25 are the only reports of transition metal borohydrides that can reduce different aliphatic and aromatic carboxylic acids to their alcohols.



RCO2H/(RCO)2O ---(Py)Zn(BH4)2/THF/Reflux/85-97%--> RCH2OH
R: Alkyl, Aryl, Heteroaryl

Scheme 1


However, reduction reactions with these systems as mentioned above have some drawbacks. In the line of our interests for the preparation of new stable reducing agents on the base of zinc tetrahydroborate2c,12,13, we synthesised (Py)Zn(BH4)2. This reagent is easily prepared in almost quantitative yield by the complexation of 1:1 pyridine and Zn(BH4)2 in ethereal solution at room temperature (Eqns. (1) and (2)).

ZnCl2 + 2NaBH4 ---Dry ether--> Zn(BH4)2 + 2NaCl                   (1)
Zn(BH4)2 + Pyridine --> Pyridine.Zn(BH4)2                   (2)


This white stable reducing agent can be stored for months in a sealed bottle without losing its activity. It is not hygroscopic or light sensitive. Structural determination of the reagent was accomplished by the atomic absorption technique, volumetric assay and elemental analysis leading to its formula as (Py)Zn(BH4)2.
Investigation of reduction reactions with this system showed that the reagent can easily reduce variety of aliphatic, aromatic and heteroaromatic carboxylic acids to their corresponding alcohols with 2–4 molar equivalents of the reagent under reflux condition. Because of the better efficiency and reactivity of the reagent in THF, this was a solvent of choice. The results of this facile reduction are summarised in Table 1 and the corresponding alcohols were obtained in high to excellent yields (90–97%). As it is evident from Table 1, the reducing agent tolerates a number of other functional groups like halogen, methoxy and nitro groups in the molecules. Reduction of cinnamic acid gives the corresponding a,ß-unsaturated alcohols leading to the olefinic groups being unaffected (Entry 8). It is interesting to note that this contrasts with the behaviour of this substrate with LiAlH4 where it gives 1-phenylpropanol,26 by Zn(BH4)2/TFAA25 where there is no reduction and by Zn(BH4)224 alone where the major product with accompanying double bond reduction is a mixture of 1,2-diol and 1,3-diol (ratio: 3/2) of 3-phenylpropanediol (Table 2, Entry 4). Moreover, the olefinic groups are not affected when away from the carboxylic groups. For example oleic acid on reduction with this reagent gives oleyl alcohol (entry 16). It has been reported that dicarboxylic acids react with borane reagents27 to give the polymeric insoluble intermediates leading to incomplete reductions. However, in some cases the corresponding lactone is the major product.28 Our reducing agent completely reduces diacids such as terephthalic acid and trans-1,4-cyclohexanedicarboxylic acid to the correspon-ding diols in high yields (Entries 13 and 15). To highlight the limitations and advantages of our method, we compared some of our results against those of reported reagents (Table 2).
In other attempt, we also compared the efficiency of isolated (Py)Zn(BH4)2 with the addition effect of pyridine to the tetrahydrofuran solution of Zn(BH4)2 in the reduction of benzoic acid. The obtained results show that at a similar time in both procedures, the isolated reagent has higher rate and efficiency in comparison to the in situ procedure.

In exploring further the synthetic utility of this reducing agent we also found that symmetric aliphatic and aromatic anhydride carboxylic acids are efficiently reduced with 2–4 molar equivalents of the reagent in refluxing THF. The corresponding alcohols were obtained in high to excellent yields (85–96%).

The results of this type of reduction are summarised in Table 3 and we see that the aliphatic anhydrides are reduced faster than aromatic ones with low molar ratios of the reagent.

In conclusion, the usefulness of this methodology lies in the fact that pyridine stabilises Zn(BH4)2 in the form of (Py)Zn(BH4)2 for easy handling and storage. This new stable ligand-metal borohydride can efficiently reduce a variety of aliphatic, aromatic and heteroaryl carboxylic acids under mild condition. Reduction of diacids and conjugated or non-conjugated compounds to their alcohols was easily achieved without affecting the olefinic bond and formation of polymeric compounds. This reagent also efficiently reduces anhydride carboxylic acids with fast reduction of aliphatic compounds relative to the aromatic ones. Stability of the reagent relative to Zn(BH4)2, a simple procedure, mild reaction conditions, easy work-up of the reaction mixture and the high yields of the products are noteworthy advantages of this system and make it a useful addition to the present methodologies in this area.

The tendency is to push it as far as you can
 
 
 
 
    Lego
(Hive Bee)
01-31-04 03:29
No 485628
      Zn(BH4)2/Pyridine also reduces COOH, Pt. 2
(Rated as: good read)
    

Table 1: Reduction of carboxylic acids to their alcohols with (Py)Zn(BH4)2a
Entry Substrate Product Molar ratio
reag./subs.
Time/h Yield/%b M.p. or B.p./°C
(found)
M.p. or B.p./°C
(reported29)
1 Benzoic acid Benzyl alcohol 2 1.5 96 205-206 205
2 4-Chlorobenzoic acid 4-Chlorbenzyl alcohol 4 3.6 93 69-71 70-72
3 4-Methylbenzoic acid 4-Methylbenzyl alcohol 3 5.1 97 60-61 59-61
4 4-Methoxybenzoic acid 4-Methylbenzyl alcohol 3 6.6 96 258-259 259
5 4-Hydroxybenzoic acid 4-Hydroxybenzyl alcohol 4 3.6 92 118-120 118-122
6 2-Hydroxybenzoic acid (salicylic acid) 2-Hydroxybenzyl alcohol, salicyl alcohol 3 1.5 93 82-84 83-85
7 4-Aminobenzoic acid 4-Aminobenzyl alcohol 3 2 95 63-65 60-65
8 3-Phenylpropionic acid, cinnamic acid 3-Phenylprop-2-en-1-ol, cinnamyl alcohol 3 4.1 90 32-34 33-35
9 3-Bromobenzoic acid 3-Bromobenzyl alcohol 3 3 96 165/16 165/16 mm Hg
10 4-Nitrobenzoic acid 4-Nitrobenzyl alcohol   2.3 97 92-93 92-94
11 Pyridine-3-carboxylic acid, nicotinic acid Pyridin-3-ylmethanol 2 0.7 90 154/28 154/28 mm Hg
12 Phenylacetic acid 2-Phenylethanol 3 1.5 97 219-220 220
13 Phenyl-1,4-dicarboxylic acid, terephthalic acid 4-Methanolbenzyl alcohol 4 7 95 117-119 117-119
14 3-Methylbutyric acid, isovalerianic acid 3-Methylbutanol 3 0.35 90 129-130 130
15 Cyclohexane-1,4-dicarboxylic acid [4-(hydroxymethyl)cyclohexyl]methanol 4 0.4 96 283 283
16 CH3(CH2)7=CH(CH2)7CO2H CH3(CH2)7=CH(CH2)7CH2OH 2 0.5 94 205-206 207

a All reactions were performed in THF under reflux conditions.
b Yields refer to isolated pure products.

Table 2: Comparison of reduction of carboxylic acids to their alcohols with (Py)Zn(BH4)2 and other reported reagents
Entry Substrate Molar ratioa, Time/h
Yield/% (I)
Molar ratioa, Time/h
Yield/% (II)
Molar ratioa, Time/h
Yield/% (III)
Molar ratioa, Time/h
Yield/% (IV)
Molar ratioa, Time/h
Yield/% (V)
Molar ratioa, Time/h
Yield/% (VI)
1 Benzoic acid 2(1.5)(96) 3.3(6)(90) MRb(1)(74) MRc(1.3)(93) MRd(5)(92) 4(1)(89)
2 4-Nitrobenzoic acid 3(2.3)(97) 3.3(4)(90) MRb(1)(75) --- MRd(2.5)(94) ---
3 2-Hydroxybenzoic acid 3(1.5)(93) 4.4(4)(0) --- MRc(1.3)(92) --- 5(3)(92)
4 Cinnamic acid 3(4.1)(90) 4.4(5)(90)e MRb(1)(80) MRc(1.3)(97) --- ---
5 Terephthalic acid 4(7)(95) 6.6(5)(70) --- --- --- ---
6 Phenylacetic acid 3(1.5)(97) 3.3(3)(95) --- MRc(1.3)(98) MRd(4)(88) ---
7 4-Chloroacetic acid 4(3.6)(93) --- MRb(1.3)(88) MRc(1.3)(98) --- ---
8 4-Methoxybenzoic acid 3(6.6)(96) --- MRb(1)(93) --- MRd(4.5)(97) ---
9 4-Hydroxybenzoic acid 4(3.6)(92) --- MRb(1)(83) --- --- ---
10 Nicotinic acid 2(0.7)(90) --- MRb(2)(85) --- --- ---
11 4-Aminobenzoic acid 3(2)(95) --- --- --- --- 8(4.5)(80)

I) (Py)Zn(BH4)2
II) Zn(BH4)2
III) BER/Ni(OAc)2
IV) NaBH4/I2
V) PhCH2NEt3BH4/Me3SiCl
VI) BH3-THF;
a Molar Ratio as Reagent/Substrate
b BER/Ni(OAc)2/Substrate: 3/0.08/1
c NaBH4/I2/Substrate: 1.2/0.5/1
d BH4-/Me3SiCl/Substrate: 2/2/1
e Product as mixture of 1,2-diol and 1,3-diol (ratio: 3/2) of 3-phenylpropanediol

Table 3: Reduction of anhydride carboxylic acids to their alcohols with (Py)Zn(BH4)2a
Entry Substrate Product Molar ratio
reag./subs.
Time/h Yield/%b M.p. or B.p./°C
(found)
M.p. or B.p./°C
(reported)29
1 Acetic anhydride Ethanol 2 1.5 90 78-79 78-79
2 Trifluoroacetic anhydride Trifluoroethanol 2 1 85 77-78 77-80
3 Isobutyric anhydride, Isobutyl alcohol, 2-methylpropanol 2 2.5 92 108-109 108
4 Cyclohexan-1,2-dicarboxylic acid anhydride [2-(hydroxymethyl)cyclohexyl]methanol 3 0.8 93 44-45 43-45
5 Phthalic acid 4-Methanolbenzyl alcohol 4 6 96 64 63-65

a All reactions were performed in THF under reflux conditions.
a Yields referred to isolated pure products.





Experimental
All products were characterised by a comparison of their physical data with those of authentic samples (IR, 1H NMR and m.p. or b.p.). All yields referred to isolated products. TLC accomplished the purity determination of the substrates, products and reactions monitoring over silica gel PolyGram SILG/UV 254 plates.

Preparation of (pyridine)(tetrahydroborato)zinc complex;
(Py)Zn(BH4)2: an ethereal solution of Zn(BH4)2 (0.16 M, 250 ml) was prepared from ZnCl2 (5.452 g, 0.04 mol) and NaBH4 (3.177 g, 0.084 mol) according to an established procedure in the literature.30 Then, a solution of pyridine (3.164 g, 0.04 mol) in ether (50 ml) was added drop wisely to the ethereal solution of Zn(BH4)2 and stirred for 30 min. Evaporation of the solvent under vacuum at room temperature gave (Py)Zn(BH4)2 in nearly quantitative yield (6.83 g, 98%) which was decomposed to the dark material at 106-108°C.14
Found: C, 34.33; H, 7.61; N, 7.99; Zn, 37.44 %. Calculated for C5B2H13NZn: C, 34.48; H, 7.52; N, 8.04; Zn, 37.54 %.

Reduction of benzoic acid to benzyl alcohol with (Py)Zn(BH4)2: a typical Procedure:
In a round-bottom flask equipped with magnetic stirrer and condenser, a solution of benzoic acid (0.122 g, l mmol) in THF (5 ml) was prepared. Then the reducing agent (0.35 g, 2 mmol) was added and the reaction mixture was stirred under reflux condition for 1.5 h. TLC monitors the progress of the reaction. After completion of the reaction, the mixture was quenched with dilute HCl (5 ml, 3 N) and the aqueous layer was extracted with CH2Cl2 (3×10 ml). The combined organic extracts was washed with NaOH (1N, 10 ml), water, brine solution and dried over anhydrous Na2SO4. Evaporation of the solvent and column chromatography of crude material over silica gel with eluent of CCl4/ether: 5/2 affords pure liquid benzyl alcohol (0.088 g, 96% yield).

Reduction of phthalic anhydride to 1,2-benzenedimethanol with (Py)Zn(BH4)2: a typical procedure:
In a round-bottom flask equipped with magnetic stirrer and condenser, a solution of phthalic anhydride (0.148 g, l mmol) in THF (5 ml) was prepared. Then the reducing agent (0.71 g, 4 mmol) was added and the reaction mixture was stirred under reflux condition for 6 h. The progress of reaction was monitored by TLC. After completion of the reaction, the mixture was quenched with dilute HCl (5 ml, 3 N) and the aqueous layer was extracted with CH2Cl2 (3×10 ml). The combined organic extracts was washed with NaOH (1N, 10 ml), water, brine solution and dried over anhydrous Na2SO4. Evaporation of the solvent and column chromatography of crude material over silica gel with eluent of CCl4/Ether: 5/2 affords pure crystals of 1,2-benzenedimethanol (0.1 g, 96% yield).




The authors gratefully acknowledge supporting of this work by the research council of Urmia University.



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The tendency is to push it as far as you can
 
 
 
 
    Rhodium
(Chief Bee)
05-31-04 03:26
No 510437
      Nitroalkene Reduction with Zinc Borohydride
(Rated as: good read)
    

Reduction of Conjugated Nitroalkenes with Zinc Borohydride.
A Mild Method for Converting Monosubstituted Nitroalkenes to Nitroalkanes and Disubstituted Ones to Oximes

Brindaban C. Ranu and Rupak Chakraborty
Tetrahedron 48(25), 5317-5322 (1992) (../rhodium/pdf /znbh4.nitroalkenes2oximes.pdf)

Abstract
Mono-β-substituted conjugated nitroalkenes are readily reduced by zinc borohydride in 1,2-dimethoxyethane to the corresponding nitroalkanes, whereas the disubstituted ones furnish the corresponding oximes in excellent yields.

The Hive - Clandestine Chemists Without Borders
 
 
 
 
    Rhodium
(Chief Bee)
10-18-04 03:59
No 536276
      Reductive Amination with Zinc Borohydride
(Rated as: good read)
    

Reductive Amination with Zinc Borohydride. Efficient, Safe Route to Fluorinated Benzylamines
Sukanta Bhattacharyya, Arindam Chatterjee and John S. Williamson
Synthetic Communications 27(24), 4265-4274 (1997) (../rhodium/pdf /redamin.znbh4.fluorobenzylamines.pdf)

Abstract
Fluorinated benzylamines are synthesized in high yields by reductive alkylation of secondary amines with appropriate fluoroaldehydes using a combination of zinc chloride and zinc borohydride. The present method method offers an alternative to toxic sodium cyanoborohydride and is adaptable to multigram-scale preparations.

The Hive - Clandestine Chemists Without Borders