Bis(pinacolato)diboron: Synthesis and applications

General description

Bis(pinacolato)diboron is an important organic intermediate. In Suzuki reaction, it has the advantages of high reaction selectivity, mild conditions, and high yield. For some compounds with unstable substrates or requiring improved reaction selectivity, using Bis(pinacolato)diboron to react with aryl halides is a good choice. It can be used to prepare an allyl boron ester, including a reaction system of substrate copper salt catalyst, ligand, base, and Bis(pinacolato)diboron, in which the substrate undergoes olefin dehydroboroesterification reaction with Bis(pinacolato)diboron. In addition, it can also be used to prepare 3,4-dimethoxy-5-methylphenylboronic acid pinacol. Its appearance is as follows:

Figure 1 Appearance of Bis(pinacolato)diboron.

Synthesis

A 2-L, three-necked flask fitted with a mechanical stirrer, a dropping funnel, and a reflux condenser connected to nitrogen source and a bubbler is flushed with nitrogen . To the flask are added 64.4 g (0.325 mol) of tetrakis(dimethylamino)diboron and 600 mL of toluene, and then a solution of 76.9 g (0.652 mol) of pinacol in 400 mL of toluene. The flask is immersed in an ice-water bath and a 6.6 M ethereal solution of hydrogen chloride (200 mL, 1.32 mol) is dropwise added during 2 hr. As soon as the addition is started, a white precipitate of dimethylamine hydrochloride appears. The slurry is stirred at room temperature for additional 4 hr. The precipitate is removed by filtration in a Buchner funnel with suction, and the filtrate is concentrated on a rotary evaporator to give a white solid. The solid is dissolved in ca. 700 mL of pentane and the remaining solid is again removed by filtration. The filtrate is washed three times with 500 mL of water and dried over anhydrous magnesium sulfate.

The drying agent is removed by filtration and the filtrate is then concentrated to ca. 150 mL. The flask is heated to dissolve the resulting precipitate, allowed to cool to room temperature, and then thoroughly chilled in a freezer (-30°C). The first crop is collected by filtration and is washed twice with 30 mL of cold pentane. The mother liquor is again concentrated to give another crop of crystal. The procedure is repeated twice. Combined crystals are dried under reduced pressure (0.1 mm) for 16 hr at room temperature to give 75.4 g (91%) of Bis(pinacolato)diboron as colorless plates, mp 138°C, lit.4 mp 138°C [1]. 1H-NMR (300 MHz, CDCl3) δ 7.51 (dd, 4H, J= 8.4 Hz, J 13.2 Hz), 7.44 (s, 2H), 1.92 (m, 4H), 1.14 (m, 12H), 0.78 (t, 6H, J= 6.6 Hz), 0.59 (t, 4H, J= 6.6 Hz); 13C-NMR (50 MHz, CDCl3) δ 152.75, 139.26, 130.35, 126.37, 121.66, 121.34, 55.89, 40.42, 31.67, 29.79, 23.85, 22.79, 14.22.

Applications

In 1995, Miyaura introduced bis(pinacolato)diboron as a reagent to convert aryl halides to arylboronic esters under mild conditions [2]. In 2004, Bräse et al. used t bis(pinacolato)diboron to synthesize symmetrical biaryls in-situ, but it was not until 2000 that this in-situ borylation was used during a polymerization reaction [3]. The use of bis(pinacolato)diboron for the synthesis of biarenes was further investigated by Wu et al. They showed that after optimization of the reaction, using a stronger base (K3PO4) and a differend catalyst (Pd(dppf)Cl2), several biarenes could be obtained in good yields [4]. The bis(pinacolato)diboron can be used for the synthesis of regiosymmetric conjugated polymers, and the high polydispersity of these polymers indicates that the use of bis(pinacolato)diboron does not result in a catalyst- transfer type polymerization. The results presented by Reynolds show that the use of bis(pinacolato)diboron for the synthesis of regiosymmetric conjugated polymers with moderate molecular weights and a defined structures is very successful. There is still a lot of improvement possible and more research is needed. In addition, Bis(pinacolato)diboron is used for synthesis of functional polymers.

References

[1]Ishiyama T, Murata M, Ahiko T, et al. Bis (pinacolato) diboron[J]. Organic Syntheses, 2000, 77: 176-185.

[2]Ishiyama, T.; Murata, M.; Miyaura, N. J. Org. Chem. 1995, 60, 7508-7510.

[3]Nising, C. F.; Schmid, U. K.; Nieger, M.; Bra?se, S. J. Org. Chem. 2004, 69, 6830-6833.

[4]Ma, N.; Zhu, Z.; Wu, Y. Tetrahedron 2007, 63, 4625-4629.