Pinacol Coupling Reaction of Benzaldehyde Mediated by TiCl3-Zn in Basic Media under Ultrasound Irradiation

Pinacol coupling of benzaldehyde mediated by TiCl3-Zn in basic media under ultrasound irradiation can lead to the corresponding pinacol rapidly. The optimum reaction condition is chosen. .


Introduction
The pinacol coupling, which was described in 1859, is still a useful tool for the synthesis of vicinal diols 1 .1, 2-Diols have been used as intermediates for the construction of biologically important natural product skeletons and asymmetric ligands for catalytic asymmetric reaction 2 .In particular, the pinacol coupling has been employed as a key step in the construction of HIV-protease inhibitors 3 .The formation of 1, 2-diol has been attempted using a number of regents such as Mg 4 , Mn 5 , Al 6 , In 7 , and so on.
Low valent titanium is a highly reactive reagent for carbonyl-coupling reactions 8 .In 1973, Mukaiyama firstly reported that TiCl 4 -Zn reduced aromatic aldehydes and ketones to produce pinacols in high yield 9 , but the stereoselectivity was not reported.In 1982, Clerici reported pinacol coupling of aromatic aldehydes and ketones promoted by aqueous TiCl 3 in basic media 10 .
Ultrasound (u.s) has increasingly been used in organic synthesis.Me•arová 11 reported the pinacol coupling reaction in aqueous media under u.s and found that ultrasound considerably accelerated the benzaldehyde conversion.Our laboratory has also reported the pinacol coupling of aromatic aldehydes and ketones in aqueous media under u.s 12 .Herein, we wish to report the result of the pinacol coupling of benzaldehyde by TiCl 3 -Zn in aqueous NaOH under ultrasound irradiation (Scheme 1).

Experimental
Benzaldehyde was distilled before use.IR spectra were recorded on Bio-Rad FTS-40 spectrometer (KBr).MS were determined on a VG-7070E spectrometer (EI, 70 eV). 1 H NMR spectra were measured on Bruker AVANCE 400 (400 MHz) spectrometer using TMS as internal standard and CDCl 3 as solvent.Sonication was performed in Shanghai Branson-CQX ultrasonic cleaner (with a frequency of 25kHz and a nominal power 250W) and SK 250 LH ultrasonic cleaner (with a frequency of 40 kHz and a nominal power 250 W; Shanghai Kudos Ultrasonic Instrument Co., Ltd.).The reaction flasks were located in the maximum energy area in the cleaner, where the surface of reactants is slightly lower than the level of the water.The reaction temperature was controlled by addition or removal of water from ultrasonic bath.

Procedure for the pinacol coupling of benzaldehyde under u.s
A 50 ml round flask was charged with Zn power (1.8 mmol) and 15% TiCl 3 solution (1.5 mmol).The mixture was irradicated in the water bath of an ultrasonic cleaner at r.t., after 15 minutes, a solution of benzaldehyde (1 mmol) in 2mL EtOH was added, and then the 10% NaOH solution (4.5 ml) was added.After the completion of the reaction, the mixture was extracted with ethyl acetate ( 3×15 mL ).The combined organic layers were washed with saturated aqueous NaHCO 3 solution and brine, dried over anhydrous MgSO 4 for 12 h and filtered.Ethyl acetate was evaporated under reduced pressure to give the crude product, which was separated by column chromatography on silica (200-300 mesh), eluted with a mixture of petroleum ether and diethyl ether.1, 2-diphenyl-1, 2-ethanedio was confirmed by MS, IR, 1 HNMR spectral data.

Results and Discussion
Low valence titanium has stronger reducing power in basic media.As shown in Table 1, the effect of the amount of 10% NaOH has been investigated.Without addition of the aqueous NaOH solution, 2 was obtained in 41% yield within 60 min (Entry a).Increasing the amount of NaOH, the yield of 2 increased to 48%, 52%, 58% and 59% respectively (Entry b, c, d and e).
We also investigated the effect of frequency of ultrasound irradiation and stirring on the reaction.While under 40 KHz ultrasound irradiation and stirring, the yields of 1, 2-diol were 58% and 51% respectively (entry o, p), which are lower than 78% (entry l, the yield of 2 under 25 KHz ultrasound irradiation).It is clear that ultrasound can accelerate the pinacolization, and the higher yield can be obtained when the frequency of ultrasound irradiation was 25 kHz.
The pinacol coupling reaction can form dl and meso stereoisomers.In the reaction system, the ratio of dl and meso of 2 is about 3/2.

Conclusion
Pinacol coupling of benzaldehyde mediated by TiCl 3 -Zn in basic media under ultrasound irradiation can lead to the corresponding pinacol rapidly.When the molar ratio of C 6 H 5 CHO:TiCl 3 : Zn was 1:1.5:1.8, the mount of 10% NaOH was 4.5 mL, and the frequency of ultrasound irradiation was 25 kHz, the yield of pinacol coupling of benzaldehyde was 78%.The ratio of dl and meso of 1, 2-diol was about 3/2.

Scheme 1
Scheme 1 Pinacolization of benzaldehyde under ultrasound irradiation

Table 1
Effects of the reaction condition on pinacolization of benzaldehyde under u.s