# High-Speed Reduction of Triarylpyrylium Salts Using Zn(BH4)2/SiO2 as an Efficient and Regiospecific Reducing Reagent

Authors: Arash Mouradzadegun; Ali Reza Kiasat; Somayeh Elahi
Journal: Journal of Chemistry
 (2013)

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## Abstract

The regiospecific reduction of some triarylpyrylium salts in the presence of modified hydride donors was investigated. Among these reagents, Zn(BH4)2/SiO2 performed the best results. The major advantages of this reagent are the cheapness, availability, simplicity in operation,  very short reaction time and much improved regioselectivity in comparison with the other reducing reagents.


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## Body

## 1. Introduction

The selective reduction is a real challenge in organic synthesis. The selectivity is generally achieved by the use of modified reducing reagents which are formed by the replacement of hydride with sterically bulky substituents or electron-withdrawing groups [1–4]. Thus to achieve this goal, currently the use of modified hydride donors has been expanded [5–8].The reduction of trialkyl and triphenylpyrylium salts in the presence of hydride donors especially NaBH4 has been extensively studied [9–12]. In addition to reduction by hydride transfer, a one-electron reduction by zinc is possible leading exclusively to 4,4′-bis-pyran dimmers [13]. Hydride donors can attack to pyrylium salts either in the α-position, giving rise to a corresponding dienone (I), or in γ-position, leading to 4H-pyran (II) (Scheme 1). To the best of our knowledge no other example of selective reduction of triarylpyrylium salts has been published to date.Scheme 1
Reduction of trialkyl and triphenylpyrylium salts in the presence of NaBH4.So in connection with this trend and in continuation with our studies to develop selective, preparative, and synthetically useful methodology for preparation [14], application [15, 16], and other  transformations  of various pyrylium and thiopyrylium salts [17–20], here we wish to report regiospecific reduction of triarylpyrylium salts carrying electron-donor or withdrawing groups on phenyl substitutions of 2,4,6-positions of pyrylium ring with some of the modified hydride donors such as Zn(BH4)2, Zn(BH4)2/SiO2, borohydride supported on ion exchange resin (BER), and sulfureted borohydride ion exchange resin (SBER). We do hope that these new reagents could perform more efficiency beside facile methodology for regiospecific reduction of triarylpyrylium salts.


## 2. Experimental

Chemicals were purchased from Fluka, Merck, and Aldrich chemical companies. Monitoring of the reactions was accomplished by TLC. IR spectra were obtained on a  Bomen  MB: 102 FT IR spectrophotometer.1HNMR spectra were recorded on 400 MHz Brucker using CDCl3 as the solvent and TMS as the internal standard.


### 2.1. Syntheses

All triarylpyrylium  perchlorates  were synthesized from the corresponding aldehydes and ketones by the method previously described [21, 22].


### 2.2. Preparation of Reducing Reagents

#### 2.2.1. Borohydride Supported on Ion Exchange Resin

Amberlite IRA-400 (chloride form) (1 g) was washed several times with distilled water to remove foreign materials. The resin was stirred in a 20% aqueous sodium borohydride solution (100 mL) for 20 min. It was then filtered and washed several times with water. The resin was finally dried over anhydrous P2O5 for 5 h under vacuum at 50°C.


#### 2.2.2. Sulfurated Borohydride Ion Exchange Resin

Sulfur power (1 mmol) and borohydride exchange resin (prepared by the previous procedure) was added in methanol (5 mL) and stirred until colour of resin becomes red (15–20 min), ensuring that BER changed to sulfurated borohydride ion exchange resin (SBER).


#### 2.2.3. Zinc Borohydride

To sodium borohydride (0.8 g) in redistilled diethylether (25 mL) was added recently fused zinc chloride (1.7 g). The mixture was stirred at 0–5°C. After filtration, the clear solution was used immediately.


#### 2.2.4. Zinc Borohydride Supported on Silica Gel

A solution of zinc borohydride (3 mmol) in diethylether was added to silica gel (1 g) and stirred at room temperature for 30 min. Solvent was then evaporated off under vacum to give the supported reagent which was used for reduction of substrate on the same day.


### 2.3. General Procedure

(1)
The reaction of 1 mmol triphenylpyrylium percholorate in the presence of 1 mmol of various reducing reagents was investigated in THF and CH3OH (2 mL). The reaction followed by TLC until conversion was completed, then reaction mixture was filtered, and the product was worked up by evaporating the solvent.
(2)
The reaction of other triarylpyrylium perchlorates (1 mmol) was done with zinc borohydride supported on silica gel (1 mmol), as the best reagent, in THF (2 mL). The reaction followed by TLC until conversion was completed, then reaction mixture was filtered, and the product was worked up by evaporating the solvent. The structure of these compounds was confirmed by IR,1HNMR, and physical data (m.p.).


## 2.1. Syntheses

All triarylpyrylium  perchlorates  were synthesized from the corresponding aldehydes and ketones by the method previously described [21, 22].


## 2.2. Preparation of Reducing Reagents

### 2.2.1. Borohydride Supported on Ion Exchange Resin

Amberlite IRA-400 (chloride form) (1 g) was washed several times with distilled water to remove foreign materials. The resin was stirred in a 20% aqueous sodium borohydride solution (100 mL) for 20 min. It was then filtered and washed several times with water. The resin was finally dried over anhydrous P2O5 for 5 h under vacuum at 50°C.


### 2.2.2. Sulfurated Borohydride Ion Exchange Resin

Sulfur power (1 mmol) and borohydride exchange resin (prepared by the previous procedure) was added in methanol (5 mL) and stirred until colour of resin becomes red (15–20 min), ensuring that BER changed to sulfurated borohydride ion exchange resin (SBER).


### 2.2.3. Zinc Borohydride

To sodium borohydride (0.8 g) in redistilled diethylether (25 mL) was added recently fused zinc chloride (1.7 g). The mixture was stirred at 0–5°C. After filtration, the clear solution was used immediately.


### 2.2.4. Zinc Borohydride Supported on Silica Gel

A solution of zinc borohydride (3 mmol) in diethylether was added to silica gel (1 g) and stirred at room temperature for 30 min. Solvent was then evaporated off under vacum to give the supported reagent which was used for reduction of substrate on the same day.


## 2.2.1. Borohydride Supported on Ion Exchange Resin

Amberlite IRA-400 (chloride form) (1 g) was washed several times with distilled water to remove foreign materials. The resin was stirred in a 20% aqueous sodium borohydride solution (100 mL) for 20 min. It was then filtered and washed several times with water. The resin was finally dried over anhydrous P2O5 for 5 h under vacuum at 50°C.


## 2.2.2. Sulfurated Borohydride Ion Exchange Resin

Sulfur power (1 mmol) and borohydride exchange resin (prepared by the previous procedure) was added in methanol (5 mL) and stirred until colour of resin becomes red (15–20 min), ensuring that BER changed to sulfurated borohydride ion exchange resin (SBER).


## 2.2.3. Zinc Borohydride

To sodium borohydride (0.8 g) in redistilled diethylether (25 mL) was added recently fused zinc chloride (1.7 g). The mixture was stirred at 0–5°C. After filtration, the clear solution was used immediately.


## 2.2.4. Zinc Borohydride Supported on Silica Gel

A solution of zinc borohydride (3 mmol) in diethylether was added to silica gel (1 g) and stirred at room temperature for 30 min. Solvent was then evaporated off under vacum to give the supported reagent which was used for reduction of substrate on the same day.


## 2.3. General Procedure

(1)
The reaction of 1 mmol triphenylpyrylium percholorate in the presence of 1 mmol of various reducing reagents was investigated in THF and CH3OH (2 mL). The reaction followed by TLC until conversion was completed, then reaction mixture was filtered, and the product was worked up by evaporating the solvent.
(2)
The reaction of other triarylpyrylium perchlorates (1 mmol) was done with zinc borohydride supported on silica gel (1 mmol), as the best reagent, in THF (2 mL). The reaction followed by TLC until conversion was completed, then reaction mixture was filtered, and the product was worked up by evaporating the solvent. The structure of these compounds was confirmed by IR,1HNMR, and physical data (m.p.).


## 3. Results and Discussion

In efforts to regiospecific reduction of triarylpyrylium salts to corresponding dienones (III) as only product, these transformations were studied by various hydride donors (Scheme2).Scheme 2
Regiospecific reduction of triarylpyrylium salts in the presence of various hydride donors.Initially to achieve optimal conditions, the transformation of 1 mmol triphenylpyrylium perchlorate (as the model compound) was investigated with various hydride donors (1 mmol) in different solvent such as THF and CH3OH according to the hydride donor (Table 1).Table 1
Comparing the ability of different hydride donors for reduction of triphenylpyrylium salts in various solvent.
Entry
Reducing reagent
Solvent
Time
Results
1
NaBH4
THF
1 h
Dienone and 4H-pyran
2
BER
THF
4 h
Dienone and 4H-pyran
3
SBER
CH3OH
4 h
Dienone and 4H-pyran
4
Zn (BH4)2
THF
Imm.
Dienone
5
Zn (BH4)2/SiO2
THF
5 min
Dienone
Imm: immediately (1 min).As shown in Table1, selectivity has not been observed in cases of 1 to 3, and both products, dienone and 4H-pyran, were obtained. This may be described by considering the fact that selectivity of hydride ion depends on the counter ion.The results reveal that Zn(BH4)2 and Zn(BH4)2/SiO2 are both potentially more selective  reagents,  because these new hydride donors, attack  to triphenylpyrylium salt only in α-position,  leading to corresponding 2H-pyran which is converted to dienone as only product. Between these two reagents, Zn(BH4)2 showed less stability, so, in spite of shorter reaction time, Zn(BH4)2/SiO2 was selected as the best choice.The generality of this process was illustrated with various triarylpyrylium perchlorate carrying electron-donor or withdrawing groups on phenyl substitutions of 2,4,6-positions of pyrylium ring in the presence of Zn(BH4)2/SiO2 in which corresponding dienones with good yields and very short reaction times were synthesized (Table 2).Table 2
Reaction of pyrylium salts with Zn(BH4)2/SiO2.
Entry
Substrate
Product
Yield (%)
Time ( min)
1
98
5
2
85
3
3
92
3
4
97
Imm.
5
79
Imm.
6
92
Imm.
7
89
Imm.
8
90
Imm.
9
82
Imm.
Imm: immediately (1 min).It is necessary to note that such modified hydride donor that exhibited longer reaction time for model compound allows to reach unusual shorter reaction time for other triarylpyrylium salts ( entries   4–9).The stereochemistry of the ring opening product was found to be in the trans configuration as determined from the coupling constants associated with the1H NMR spectral resonances of the ring  protons in.  Because of actually not characterized form of modified reagent the only H- was used for selective attacking (Scheme 3).Scheme 3
Stereochemistry of product.


## 4. Conclusions

Zinc borohydride supported on silica gel provide an inexpensive and efficient methodology for the regiospecific reduction of triarylpyrylium salts. Moreover, the mildness, convenience, stability, and high yield will make these simple reagents more useful and attractive in this methodology.


## 5. Physical and Spectral Data

(1)
1,3,5-Triphenyl-penta-2,4-dien-1-one. Yellow crystals, m.p.: 120°C (from EtOH); yield 98%; IR (neat): νCO (1646 cm−1). 1HNMR (CDCl3, 400 MHz): δ=6.74 (H1, d, J=16 Hz), 6.89 (H3, s), (7.31–7.57), 8.03 (15H, m, Ar-H), 8.53 (H2, d, J=16 Hz).(2)
1,5-Diphenyl-3-p-tolyl-penta-2,4-dien-1-one. Oil, yield 85%; IR (neat): νCO (1638 cm−1). 1HNMR (CDCl3, 400 MHz): δ=2.48 (3H, s, CH3), 6.71 (H1, d, J=16 Hz), 6.82 (H3, s), (6.89–7.49), 8.02 (14H, m, Ar-H), 8.44 (H2, d, J=16 Hz).(3)
1,5-Bis-(4-methoxy-phenyl)-3-phenyl-penta-2,4-dien-1-one. Oil, yield 92%; IR (neat): νCO (1638 cm−1). 1HNMR (CDCl3, 400 MHz): δ=3.88 (6H, s, OCH3), 6.82 (H1, d, J=16 Hz), 6.91 (H3, s), 7.02, (7.34–7.57), 8.01 (13H, m, Ar-H), 8.26 (H2, d, J=16 Hz).(4)
3-(4-Methoxy-phenyl)-1,5-diphenyl-penta-2,4-dien-1-one. Yellow crystals, m.p.: 110°C (from EtOH); yield 92%; IR (neat): νCO (1648 cm−1). 1HNMR (CDCl3, 400 MHz): δ=3.86 (3H, s, OCH3), 6.82 (H1, d, J=16.2 Hz), 6.90 (H3, s), 7.02, (7.34–7.57), 8.01 (14H, m, Ar-H), 8.47 (H2, d, J=16.2 Hz).(5)
1,5-Bis-(4-methoxy-phenyl)-3-p-tolyl-penta-2,4-dien-1-one. Oil, yield 79%; IR (neat): νCO (1635 cm−1). 1HNMR (CDCl3, 400 MHz): δ=2.46 (3H, s, CH3), 3.87 (6H, s, OCH3), 6.71 (H1, d, J=16.4 Hz), 6.82 (H3, s), (6.86–7.49), 8.02 (12H, m, Ar-H), 8.44 (H2, d, J=16.4 Hz).(6)
1,3,5-Tris-(4-methoxy-phenyl)-penta-2,4-dien-1-one. Oil, yield 92%; IR (neat): νCO (1638 cm−1). 1HNMR (CDCl3, 400 MHz): δ=3.88 (9H, s, OCH3), 6.82 (H1, d, J=16.2 Hz), 6.91 (H3, s), 7.02, (7.28–7.57), 8.02 (12H, m, Ar-H), 8.26 (H2, d, J=16.2 Hz).(7)
3-(4-Dimethylamino-phenyl)-1,5-diphenyl-penta-2,4-dien-1-one. Red crystals, m.p.: 130°C (from EtOH); yield 89%; IR (neat): νCO (1637 cm−1), 1HNMR (CDCl3, 400 MHz): δ=2.46 (6H, s, N(CH3)2), 6.71 (H1, d, J=16.2 Hz), 6.89 (H3, s), 7.37–7.49, 8.02 (14H, m, Ar-H), 8.44 (H2, d, J=16.2 Hz).(8)
3-(4-Chloro-phenyl)-1,5-diphenyl-penta-2,4-dien-1-one. Yellow crystals, m.p.: 112°C (from EtOH); yield 90%; IR (neat): νCO (1649 cm−1). 1HNMR (CDCl3, 400 MHz): δ=6.71 (H1, d, J=16.4 Hz), 6.72 (H3, s), 7.33–7.58, 8.01 (14H, m, Ar-H), 8.48 (H2, d, J=16.4 Hz).(9)
3-(4-Nitro-phenyl)-1,5-diphenyl-penta-2,4-dien-1-one. Brown crystals, m.p.: 115°C (from EtOH); yield 82%; IR (neat): νCO (1649 cm−1), NO2 (1345, 1518 cm−1). 1HNMR (CDCl3, 400 MHz): δ=6.71 (H1, d, J=16.2 Hz), 6.89 (H3, s), 7.28–7.58, 8.01 (14H, m, Ar-H), 8.48 (H2, d, J=16.2 Hz).



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*Source: 938714-2012-10-30.xml*