Synthesis
The general procedure for the synthesis of 5-methylthiophene-2-boronic acid pinacol ester from 2-methylthiophene and pinacolborane was carried out as follows: cobalt complex (0.01 mmol, selected from 1-4), 2-methylthiophene (1 mmol), and pinacolborane (1 mmol) were added to scintillation vials fitted with magnetic stirring bars according to the reaction scheme shown in Figure 2(a). The reaction progress was monitored by analyzing an aliquot of the mixture by GC-FID. The reaction mixture was stirred at room temperature until the reaction was complete and subsequently quenched by exposure to air. The resulting solid was dissolved in CDCl3, filtered through a Pasteur pipette fitted with a silicone plug and subsequently analyzed by 1H and 13C NMR spectroscopy without further purification. The above reaction can also be carried out in 2 ml of tetrahydrofuran (THF) if required. Figure 2(a) illustrates the percentage conversion of cobalt complexes 1-4, with the values in parentheses being the isolated yields. In addition, Figure 2(b) details other boronation products achieved using cobalt complexes 2 and 3 according to the previously described reaction conditions.
References
[1] Organic Letters, 2012, vol. 14, # 16, p. 4266 - 4269
[2] Patent: WO2015/89119, 2015, A1. Location in patent: Page/Page column 34
[3] Chemistry - An Asian Journal, 2010, vol. 5, # 7, p. 1657 - 1666
[4] Tetrahedron, 2008, vol. 64, # 26, p. 6103 - 6114
[5] Organometallics, 2015, vol. 34, # 19, p. 4732 - 4740
