Borate pinacol [(Bpin)2], I believe everyone is familiar with it. It is one of the raw materials for making borate esters catalyzed by palladium. What I want to share is not its application in coupling chemistry, but another aspect of it. A potential role: reducing agent for nitrogen oxides.
Nitrogen oxides are still widely used in organic synthesis, such as pyridine nitrogen oxides and tertiary aliphatic amine nitrogen oxides, some of which we deliberately prepare for the next translocation or rearrangement reaction. Some nitrogen oxides require us to remove oxygen to obtain reduction products. The reagent used is (Bpin)2, which is cheap and common. The key is that it exhibits good functional group selectivity during the reduction process (J. Org. Chem. 2011, 76, 7842–7848). Aliphatic tertiary amine oxides can be reduced at room temperature, while pyridine nitrogen oxides need to be heated to 70 degrees before they can react.
Let’s take a look at the substrate universality of this reaction. As can be seen from the figure below, aliphatic nitrogen oxides only need room temperature and can be reduced in 5 minutes with a quantitative yield. Pyridine nitrogen oxide requires an increase in temperature to complete.
This reaction can be carried out not only in organic solvents, but also in water, and the yield of aminotetrahydrofuran is still very good.
Let’s take a look at the reaction mechanism of this reaction. As can be seen from the figure below, the oxygen anions of nitrogen oxides attack boron to form a transition state similar to a three-membered ring, and finally generate the target reduction product and sodium carbonate (pinB)2O .
