Highly efficient D ₂ generation by dehydrogenation of formic acid in D ₂O through H ⁺/D ⁺ exchange on an iridium catalyst: Application to the synthesis of deuterated compounds by transfer deuterogenation

Deuterated compounds have received increasing attention in both academia and industrial fields. However, preparations of these compounds are limited for both economic and practical reasons. Herein, convenient generation of deuterium gas (D ₂) and the preparation of deuterated compounds on a laboratory scale are demonstrated by using a half-sandwich iridium complex with 4,4'-dihydroxy-2,2'-bipyridine. The "umpolung" (i.e., reversal of polarity) of a hydrogen atom of water was achieved in consecutive reactions, that is, a cationic H ⁺/D ⁺ exchange reaction and anionic hydride or deuteride transfer, under mild conditions. Selective D ₂ evolution (purity up to 89%) was achieved by using HCO ₂H as an electron source and D ₂O as a deuterium source; a rhodium analogue provided HD gas (98%) under similar conditions. Furthermore, pressurized D ₂ (98%) without CO gas was generated by using DCO ₂D in D ₂O in a glass autoclave. Transfer deuterogenation of ketones gave α-deuterated alcohols with almost quantitative yields and high deuterium content by using HCO ₂H in D ₂O. Mechanistic studies show that the H ⁺/D ⁺ exchange reaction in the iridium hydride complex was much faster than β-elimination and hydride (deuteride) transfer. Cheap D ₂ gas! Convenient generation of deuterium gas (D ₂) and preparation of deuterated compounds on a laboratory scale are demonstrated using a half-sandwich iridium complex with 4,4'-dihydroxy-2,2'- bipyridine (see scheme). The "umpolung" of a hydrogen atom of water was achieved in consecutive reactions (cationic H ⁺/D ⁺ exchange reaction and anionic hydride (deuteride) transfer) under mild conditions.