American chemists have obtained an N-heterocyclic carbene that is stable in aqueous solution. It is not protonated by water and does not react with nucleophiles due to its bulky carborane substituents. The study was published in Science Advances.
Carbenes are organic compounds with a divalent carbon atom in their structure. This carbon atom usually has an unshared electron pair and an empty orbital, making carbenes very reactive toward nucleophiles and electrophiles. N-heterocyclic carbenes are more stable than others due to the electronic effects of the two nitrogen atoms located near the carbene center. But even they quickly decompose in air.
Chemists led by Vincent Lavallo of the University of California, Riverside, recently showed that N-heterocyclic carbenes can be stable not only in air, but even in aqueous solution.
To obtain such a carbene, scientists introduced two bulky cluster substituents (carborane) into the molecule of the N-heterocyclic carbene precursor, in which nine boron atoms and one carbon atom were linked together. In this case, each boron formed a bond with other boron atoms, and the only carbon atom in the cluster was attached to the carbene fragment. Then, to increase the stability of the future carbene, chemists chlorinated these substituents. Now each boron atom was linked to a chlorine atom.
The chemists mixed the resulting chlorinated precursor with lithium hexamethyldisilazide, a strong base. This formed an N-heterocyclic carbene. The scientists characterized it using NMR spectroscopy and X-ray structural analysis. During their work, they noticed that if a loosely closed NMR ampoule with a carbene solution was left in the air, its NMR spectrum did not change, meaning that the carbene did not decompose. Then they added distilled water to the carbene solution in tetrahydrofuran. To the scientists' surprise, even after adding water, the NMR spectrum did not change at all. Thus, they found out that the resulting carbene was stable in an aqueous solution.
The scientists then showed that their carbene was not only stable in water, but could also be obtained in aqueous solution. They mixed a carbene precursor and lithium hydroxide in a mixture of water and tetrahydrofuran, resulting in pure carbene with no signs of decomposition.
Thus, chemists obtained a carbene stable in water for the first time. Moreover, it did not react not only with water, but also with complexes of various transition metals, heterocumulenes and elemental selenium. The authors of the article suggest that the low reactivity of the obtained carbene is associated with acceptor and large carborane substituents, which "protect" the carbene from other molecules.
Earlier we talked about how chemists first obtained crystalline nitrene that was stable at room temperature.