Bifunctional Water Activation for Catalytic Hydration of Organonitriles

Treatment of [Rh­(COD)­(μ-Cl)]2 with excess tBuOK and subsequent addition of 2 equiv of
PIN·HBr in THF afforded [Rh­(COD)­(κC2-PIN)­Br]
(1) (PIN = 1-isopropyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)­imidazol-2-ylidene,
COD = 1,5-cyclooctadiene). The X-ray structure of 1 confirms
ligand coordination to “Rh­(COD)­Br” through the carbene
carbon featuring an unbound naphthyridine. Compound 1 is shown to be an excellent catalyst for the hydration of a wide
variety of organonitriles at ambient temperature, providing the corresponding
organoamides. In general, smaller substrates gave higher yields compared
with sterically bulky nitriles. A turnover frequency of 20 000
h–1 was achieved for the acrylonitrile. A similar
Rh­(I) catalyst without the naphthyridine appendage turned out to be
inactive. DFT studies are undertaken to gain insight on the hydration
mechanism. A 1:1 catalyst–water adduct was identified, which
indicates that the naphthyridine group steers the catalytically relevant
water molecule to the active metal site via double hydrogen-bonding
interactions, providing significant entropic advantage to the hydration
process. The calculated transition state (TS) reveals multicomponent
cooperativity involving proton movement from the water to the naphthyridine
nitrogen and a complementary interaction between the hydroxide and
the nitrile carbon. Bifunctional water activation and cooperative
proton migration are recognized as the key steps in the catalytic
cycle.