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Metal (or Ion) Induced Reductive Coupling of Aromatic Aldehydes and Ketones in Aqueous Media

Metal (or Ion) Induced Reductive Coupling of Aromatic Aldehydes and Ketones in Aqueous Media

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Some Applications of Ultrasound Irradiation in Pinacol Coupling…



111



were prepared in only 11% and 16% yield respectively. Whereas under ultrasound irradiation,

they were increased to 49.6% and 51.5% respectively in the same time, 1,2-bis(o,pdichlorophenyl)-1,2-ethanediol was up to 77.5% yield in this procedure. Aromatic ketones are

much less efficient for this reaction. Aromatic ketones with electron donating groups such as

m-aminoacetophenone and p-methoxylacetophenone, and steric hindered ketones such as

dibenzyl ketone and benzophenone did not give any pinacol products.

In recent years, catalysts and reagents supported on inorganic substrates have received

increasing attention because of their high level of chemoselectivity and environmental

compatibility as well as simplicity of operation. When certain chemicals are absorbed onto

solid supports their reactivity is enhanced over the reagent itself. Montmorillonite K10 is

known to behave as Bronsted acids in organic reactions. The use of K10 as solid support has

become very useful in synthetic organic chemistry because of its enhanced selectivity due to

its lamellar swelling structure and large surface area. Up to now, the catalyst has been used as

acidic catalyst for many organic reactions. The advantages of the catalyst are easy handing,

chemical inertness, and lower cost, environmentally friendly and easy modification of acidity

by exchanging the cations in the interlayer space. We examined the pinacol coupling

catalyzed by ZnCl2 supported on Montmorillonite K10 instead of ZnCl2, some aromatic

aldehydes such as o,p-dichlorobenzaldehyde, m-chlorobenzaldehyde, cinnamaldehyde and

furfural gave the desired 1,2-diols with 87%, 74%, 75% and 61% yield respectively.

Compared with the reaction catalyzed by ZnCl2 only, the pinacols yield increased about 10%30%, and the supported reagent was very easy separation and recycling [121]. In the present

system, the ratio of dl and meso of the corresponding 1,2-diols is about 1:1. The K10-ZnCl2

could be recycled for 3 times without significant losing activity.

Pinacol coupling of aromatic aldehydes in aqueous H2NSO3H or H3PO4 mediated by zinc

powder under ultrasound irradiation could lead to the corresponding pinacols in 14%~88%

yields within 2.5 h. Aromatic aldehydes with electronwithdrawing groups increase the

reactivity. For system Zn-H2NSO3H(aq., 1N), when the substrates are o-chlorobenzaldehyde,

m-chlorobenzaldehyde and m-bromobenzaldehyde, the corresponding pinacol products were

obtained in 70%, 74% and 63% yield respectively for 2.5 h ultrasound irradiation; while using

Zn-H3PO4(aq., 3N), the yield of pinacols were 85%, 88% and 79% respectively, which also

indicated that higher meso-stereoisomer can be obtained and higher yield of pinacols also

obtained under system Zn-H3PO4 when compared to those of Zn-H2NSO3H [122].

If Zn-(COOH)2 (aq.) instead of H2NSO3H or H3PO4 during the reactions, high yields of

pinacol could be obtained when o,p-dichlorobenzaldehyde (78%) and p-chlorobenzaldehyde

(65%) as the substrate compared with the reaction using H2NSO3H or H3PO4 aqueous (the

corresponding yields are 54% and 42% using H2NSO3H, 42% and 34% using H3PO4,

respectively) after 2.5 h ultrasound irradiation [123]. Ultrasound irradiation frequency had

little effect on this reaction system. No coupling of m-chlorobenzaldehyde was observed

when Zn powder was replaced by Mg or Al powder.

Zinc is a amphoteric metal,the reductive coupling of aromatic aldehydes in a basic

system such as Zn-NaOH (10% aq.)-MeOH under ultrasonic irradiation was observed. The

results showed that the basic situation not only further improved the pinacols yield of

aromatic aldehydes with electron-withdrawing substituents in the benzene ring, but also

aromatic aldehydes with electron-donating substituents in the benzene ring such as pmethylbenzaldehyde and p-methoxybenzaldehyde yielded the pinacols in 92% and 80%



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Zhi-Ping Lin and Ji-Tai Li



respectively [124]. Compared with the acidic medium, the coupling in basic medium was

significantly improved on the chemoselectivity and shortened the reaction time from 2.5-3 h

to 25-60 min. When piperonaldehyde was used as substrate, the meso-pinacol was obtained

only, while other aldehydes as the substrates, the ratio of dl and meso was about 1:1.



2) Reductive Coupling of Aromatic Aldehydes and Ketones Induced by Magnesium

Powder in Aqueous Media

In 1999, Zhang and Li reported a simple and effective method for pinacol coupling

reactions of various aromatic aldehydes and ketones in aq. NH4Cl (0.1 N) mediated by

magnesium with stirring [125]. The yields of pinacols were 56-96%, but it needed a long

reaction time (12-24 h) and the molar ratio of ArCHO:Mg turning as high as 1:20. Mečiarová

et al. [109] studied the influence of reaction time, quality and quantity of magnesium, and

reaction conditions on pinacol coupling of benzaldehyde by Mg-NH4Cl (or H2O). They

decreased the molar ratio of PhCHO:Mg to 1:10, and found that ultrasound irradiation can

accelerate the pinacolization of benzaldehyde using magnesium turning, the conversion of

benzaldehyde up to 100% determined by 1H NMR and the corresponding pinacol was

obtained in 95% yield within 90 min. It is clear that the ultrasound can accelerate the metalinduced pinacolization of benzaldehyde, but the pinacolization of other aldehydes did not

involve in the paper. They proved also that reaction could be carried out in pure water

(without addition of ammonium chloride) in very good yields. Later on they found also that

very good yields of pinacols are formed with zinc powder in aqueous NH4Cl, but no reaction,

even under sonication, was observed with iron, nickel and tin powders.

We studied the coupling of aromatic aldehydes induced by magnesium powder in the

NH4Cl (0.1M) aqueous within 3 h at room temperature under ultrasonic irradiation. The data

were shown that this system was very effective for the aromatic aldehydes with electrondonating substituents in the benzene ring such as p-methylbenzaldehyde and pmethoxybenzaldehyde yielded the pinacols both in 95% and the ratio of dl/meso of

corresponding pinacol were 9/1 and 2/1 respectively. When benzaldehyde and

piperonaldehyde were used as the substrates, the corresponding pinacols were obtained in

75% and 72% respectively, while the ratios of dl/meso were about 1:1 [127].

The pinacol coupling of aromatic aldehydes and ketones was carried out in 20-62% and

10-91% yield respectively with Mg and Mg-MgCl2 in water under ultrasound irradiation at

r.t. for 3-4 h. For example, 1,2-diphenyl-1,2-ethanediol, 1,2-di(p-methylphenyl)-1,2ethanediol and 1,2-di(p-methoxyphenyl)-1,2-ethanediol were obtained with 85%, 90% and

91% yield respectively using Mg-MgCl2 at r.t for 3 h under ultrasound and higher than those

in stirring condition. Furthermore, when aromatic aldehydes with electron-withdrawing such

as the p-chlorobenzaldehyde, m-chlorobenzaldehyde and furfural were used as the substrates,

the pinacols were obtained in 70%, 65% and 60% respectively, while in Mg-NH4Cl the yield

of pinacols decreased dramatically. By comparison, the reaction activity induced by Mg in

pure water significantly lower than in MgCl2 aqueous [128]. The coupling of aromatic

ketones showed very lower reactivity in the above two systems. While insteaded to MgNH4Br (aq.), the coupling of acetophenone was successful coupling in 66% yield for 3 h with

sonication. The ratio of dl and meso of the corresponding 1,2-diols is 71/29, and no alcohol

was found during the reaction. The similar coupling yields of aromatic aldehydes were

obtained in the Mg-MgCl2 (aq.) syntem, but the meso-isomer was increased [129]. It was



Some Applications of Ultrasound Irradiation in Pinacol Coupling…



113



shown that Lewis acid not only affects the chemoselectivity of the reaction, but also the

diastereoselectivity of the pinacol.

At room temperature, the Mg-MgI2 system has high chemoselectivity especially for

benzophenone, furaldehyde, α-acetonaphthone and β-acetonaphthone, to give the desired 1,2diols with 99%, 96%, 90% and 91% yields respectively within 20-60 min in a mixed solvent

of ether-benzene. The dl/meso ratio of the coupling products of α-acetonaphthone and βacetonaphthone were 8/2 and 7/3. We also observed the effect of different frequency of

ultrasound irradiation on this reaction. When the frequency was 25 kHz, the coupling of

benzaldehyde resulted the desired product in 56% yield in ethanol, while under 40 kHz and

59 kHz ultrasound irradiation, the pinacolization were completed with 49% and 35% yields

respectively [130]. It is shown that lower frequency of ultrasound irradiation improved the

result. The reason may be that the lower frequency condition creates the better cavitation.



3) Reductive Coupling of Aromatic Aldehydes and Ketones Induced by Aluminum

Powder in Aqueous Media

Khurana et al. reported pinacol coupling of aromatic aldehydes and ketones promoted by

aluminum in KOH aqueous solution in 1994 and 1996 respectively, and the corresponding

1,2-diols were produced in high yields [131]. But excess amount of alkali (the molar ratio of

substrate /KOH = 1/9) was used during the reaction. In 1999, Mečiarová et al. reported that

benzaldehyde can coupled into pinacol at a lower concentration of KOH aqueous solution

using ultrasonic probes [109]. The reaction time was shortened but the yield did not meet

Khurana‘s result. Furthermore, the yield was determined by 1H NMR analysis, not the

isolated yield.

Under ultrasound irradiation the pinacol coupling reaction of aromatic aldehydes and

ketones was carried out in 60%-98% yield with aluminum in aqueous NaOH-MeOH at r.t.

within 20-30 min [132]. Among them, benzaldehyde, p-methoxybenzaldehyde and pmethylbenzaldehyde were reacted smoothly with aluminum in aqueous NaOH-MeOH. The

corresponding pinacol coupling products were obtained in 91%, 89% and 88% yield

respectively, m-chlorobenzaldehyde and p-chlorobenzaldehyde gave nearly quantitative yield

of 1,2-diols. But o,p-dichorobenzaldehyde gives 61% pinacol only. The reason may be that

the steric hindrance around carbonyl group inhibits the coupling during the reaction. When

aromatic ketones such as p-methoxyacetophenone and p-chloroacetophenone were used as the

substrates, less pinacols and more alcohols were obtained, while the reaction with

cinnamaldehyde was unsuccessful in the same conditions.

Metal aluminum has a low first ionization potential (5.986 eV) and the presence of trace

fluoride ion can accelerate corrosion of aluminum. In 2000, Chen et al. reported that the

conversion of benzaldehyde was 100% (measured by 1H NMR) by stirring for 16 h in the AlKF aqueous solution, and yield of pinacol was obtained in 87% [36d]. While under

ultrasound irradiation, benzaldehyde conversion will reach 99% within 1.5 h, 1,2-diphenyl1,2-ethanediol yield can reach 82%. But the coupling product of other aromatic aldehyde was

not as good as stirring conditions [198].



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Zhi-Ping Lin and Ji-Tai Li



4) Manganese Powder Induced Reductive Coupling of Aromatic Aldehydes and

Ketones in Aqueous Media

The pinacol coupling of aromatic aldehydes was carried out in 40-90% yield with

manganese in aqueous NH4Cl at r.t. for 2 h under ultrasound irradiation. The reactions in MnMnCl2/THF:H2O(1:4) system gave pinacols in 30-95% yield at r.t. for 2-3 h under ultrasound

[133]. Compared with classical method, the main advantages of the present procedure are

shorter reaction time, less reagent quantity and higher yield. For example, 1,2-diphenyl-1,2ethanediol, and 1,2-di(p-chlorophenyl)-1,2-ethanediol were prepared previously in 65% and

64% yield respectively using manganese in aqueous NH4Cl at r.t. for 16 h, whereas under

ultrasound irradiation, 1,2-diphenyl-1,2-ethanediol and 1,2-di(p-chlorophenyl)-1,2-ethanediol

were obtained in 70% and 90% yield respectively at r.t. for 2 h. Compared with Mn-NH4Cl

(aq.)-THF system, the Mn-MnCl2 (aq.)-THF system can lead to the higher chemoselectivity.

It is indicated that the aromatic aldehydes with electron-withdrawing substituents in the

benzene ring show higher reactivity and higher yield than those electron-donating

substituents. The coupling has lower stereoselectivity in the Mn-MnCl2 (aq.)-THF system and

the ratio of dl/meso is about 3/2.

The Mn-HOAc-H2O system was less effective in pinacol coupling as the abovementioned two systems [199].

5) Reductive Coupling of Aromatic Aldehydes Induced by Vanadium (II) in Aqueous

Solution

Vanadium (II) complexes have been recognized to be highly capable of one–electron

reduction, thereby including radical reactions such as reduction of several organic substrates

[200] and pinacol-type reductive coupling [201]. In 1926, Conant reported the dimolecular

reduction of carbonyl compounds by vanadium and chromous salts [202], but some aromatic

aldehydes were slowly reduced by vanadium salts in the presence of acid, alcohol or acetone.

In 1989, Pedersen reported the stereoselective coupling of two different types of substrates,

yet electronically similar aldehydes employing the well-characterized vanadium (II) complex,

[V2Cl3(THF)6]2[Zn2Cl6]; the major diastereomer in all of the cross coupling reactions is a

threo diol [203]. Hirao reported highly diastereoselective pinacol coupling of secondary

aliphatic aldehydes induced by Cp2VCl3/R3SiCl/Zn [69a]; and using VOCl3/Me3SiCl/Al

system [69c], six aromatic aldehydes gave desired pinacols in 49%-89% yields, their

diastereoselectivities were high (dl:meso≧9:1).

Pinacol coupling of aromatic aldehydes by aqueous vanadium (II) solution under

ultrasound irradiation at 15-35 oC can lead to the corresponding pinacols in 78%-93% yields

within 15-30 min. The substituent group in the benzene ring has no significant effection on

the reactivity but the stereoselectivity. For example, when p-methylbenzaldehyde, pmethoxybenzaldehyde, and piperonaldehyde as substrate, the ratio of dl and meso of 1,2-diols

was dl isomer, 91/9 and 92/8. While benzaldehyde and m-chlorobenzaldehyde as the

substrate, the ratio of dl and meso was about 72/38 and 67/33 respectively [134].

The optimization reaction condition and yield of pinacol coupling in aqueous under

ultrasound irradiation were summarized in Table 1.

Should be clear is that eithor the classical method or ultrasound, Sn can not reduce

aromatic aldehydes and ketones to vicinal diol. When the aldehydes and ketones with nitrogroup in the benzene ring as substrates, there were no pinacol obtained, but the nitro was



Some Applications of Ultrasound Irradiation in Pinacol Coupling…



115



reduced to amino group. The pinacol coupling was difficult to go on under the above

mentioned conditions when the aldehydes and ketones with amino groups in the benzene ring.

Table 1. The optimization reaction condition and yield of pinacol coupling in aqueous

under ultrasound irradiation

Entry



ArCHO



1



C6H5CHO



Met/ (aq.)/ time



Al/NaOH-MeOH(aq.)/20 min

VCl2/EtOH(aq.)/15 min

Mg/MgCl2(aq.)/3 h

2

4-CH3OC6H4CHO Al/NaOH-MeOH(aq.)/20 min

Mg/NH4Cl(aq.)/3 h

VCl2/EtOH(aq.)/30 min

Mg/MgCl2(aq.)/3 h

3

4-CH3C6H4CHO

Al/NaOH-MeOH(aq.)/20 min

Mg/NH4Cl(aq.)/3 h

VCl2/EtOH(aq.)/30 min

Zn/NaOH-MeOH(aq.)/30 min

Mg/MgCl2(aq.)/3 h

4

4-ClC6H4CHO

Al/NaOH-MeOH(aq.)/20 min

VCl2/EtOH(aq.)/25 min

Zn/NaOH-MeOH(aq.)/30 min

Mn/MnCl2/THF(aq.)/2 h

5

3-ClC6H4CHO

Al/NaOH-MeOH(aq.)/20 min

VCl2/EtOH(aq.)/15 min

Zn/NaOH-MeOH(aq.)/30 min

Mn/MnCl2/THF(aq.)/2 h

6

2-ClC6H4CHO

VCl2/EtOH(aq.)/15 min

7

2,4-Cl2C6H3CHO

Zn/K10-ZnCl2(aq.)/THF/3 h

8

3-BrC6H4CHO

VCl2/EtOH(aq.)/15 min

9

piperonaldehyde

VCl2/EtOH(aq.)/25 min

Zn/NaOH-MeOH(aq.)/40 min

10

furaldehyde

Mg-I2/ether-benzene/60 min

11

PhCOPh

Mg-I2/ether-benzene/30 min

12

α-acetonaphthone

Mg-I2/ether-benzene/60 min

β-acetonaphthone

Mg-I2/ether-benzene/60 min

*dl/meso were determined by 1H NMR.



Isolated

yield, %

91[132]

91[134]

85[128]

89[132]

95[127]

89[134]

91[128]

88[132]

95[127]

85[134]

92[124]

90[128]

98[132]

92[134]

97[124]

95[133]

98[132]

86[134]

85[124]

86[133]

88[134]

87[121]

93[134]

87[134]

79[124]

96[130]

99[130]

90[130]

91[130]



*dl/meso

72/28

31/69



91/9

7/93

71/29

90/10

100/0

51/49

92/8

71/29

43/57

50/50

67/33

55/45

61/39

61/39

meso

37/63

79/21

0/100

57/43

79/21

70/30



2. Reductive Coupling of Aromatic Aldehydes and Ketones Using LowValent Titanium

Low valent titanium is a highly reactive reagent and attracts increasing interest in

carbonyl-coupling reactions. High valent titanium reagent or complexes could be reduced by

some metal to corresponding low valent titanium complexes, which can induce some

aromatic aldehydes and ketones occurred the pinacol coupling reaction. In 1973, Mukaiyama



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Zhi-Ping Lin and Ji-Tai Li



firstly reported that TiCl4-Zn reduced aromatic aldehydes and ketones to produce the

corresponding 1,2-diols in high yield [135], but the stereoselectivity was not reported. With

improved of the preparation of low-valent titanium, the in-depth study was underwent and

there are many reports on the synthesis of 1,2-diol using low-valent titanium complex in

recent years [22, 39, 42, 43, 51, 136]. However, in spite of their potential utility, some of the

reported methods suffer from drawbacks such as expensive catalysts and critical reduction

conditions.

McMurry et al. [33] reported that coupling reaction induced by low valent titanium gave

pinacols at 0 oC, but at reflux temperature, it gave alkenes through deoxygenation. In the

presence of ultrasonic irradiation, the coupling was carried out at room temperature, gave

pinacol in high yield, and improved the chemoselectivity and stereoselectivity. Besides, the

competing Cannizzaro reaction, giving alcohol and carboxylic acid, was not observed and

there was also no olefin formation arising from McMurry reactions.



1) Reductive Coupling of Aromatic Aldehydes Induced by TiCl4-M (Zn, Mg, Al)-THF

in CH2Cl2

In 2001, Yamamoto et al. reported diastereoselective pinacol coupling of aldehydes

promoted by monomeric titanocene (III) complex Cp2TiPh [61]. Five aromatic aldehydes

given desired pinacol in 54-96% yields within 1-4 h. In 2000, Li et al. reported the 1,2-diols

were obtained in pinacol coupling mediated by TiCl4-Mg with a high stereoselectivity [42].

These systems of TiCl4-THF-Zn, TiCl4-THF-Al, TiCl4-THF-Mg can quickly reduce a

number of aromatic aldehydes to pinacol with high yields and high stereoselectivity under

ultrasound. Without ultrasound, the pinacols were obtained in lower yield. For example, using

TiCl4-THF-Zn under Ar stirring for 30 min, 1,2-diphenyl-1,2-ethanediol was previously

prepared in 57% yield [51]; using TiCl4-Et2O-Al and stirring for 38 h gave 1,2-diphenyl-1,2ethanediol in 50% yield [43]. Whereas under ultrasonication for only 5 min, in the presence

of TiCl4-THF-Zn, replacement of Ar by N2, 1,2-diphenyl-1,2-ethanediol was obtained in 96%

yield. TiCl4-THF-Al system provided 1,2-diphenyl-1,2-ethanediol in 90% yield for 20 min

[137]. Unfortunately, this method applies only to aromatic aldehydes with electron-donating

substituents in the benzene ring.

As shown in Table 2, the type of reduce metal has some effects on the reaction speed,

yield and product stereoselectivity. Al was proven to be more diastereoselective (dl/meso)

than Zn. The reaction in TiCl4-THF-Zn reduction system can be carried out in higher yields

and shorter reaction time, but lower stereoselectivity. While in TiCl4-THF-Al system, pinacol

coupling can be carried out in higher yields within 15-20 min, and the stereoselectivity was

also improved. Compared with the previous two systems, TiCl4-THF-Mg system was not

efficient to the reaction [137].

2) Reductive Coupling of Aromatic Aldehydes Induced by TiCl3-M (Al, Mg, Mn, Zn)EtOH

In 1982, Clerici et al. reported pinacol coupling of aromatic aldehydes and ketones

promoted by aqueous titanium trichloride in basic media [138]. The reaction was completed

in few minutes, but the reducing power of Ti3+/Ti4+ system is strongly pH dependent, the

method has some limitations with respect to some aromatic aldehydes and ketones.



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