Pairwise hydrogenation of styrene catalyzed by Grubbs-type ruthenium catalysts: parahydrogen-induced polarization evidence and DFT mechanistic study

초록

We report the first observation of parahydrogen-induced polarization (PHIP) catalyzed by Grubbs-type ruthenium complexes, including Grubbs 1st generation (G1), Grubbs 2nd generation (G2), and Hoveyda-Grubbs 1st generation (HG1) catalysts. The characteristic antiphase NMR signal patterns of ethylbenzene product, with signal enhancements of up to similar to 1143-fold for G1, similar to 27-fold for G2, and similar to 22-fold for HG1, provide unambiguous evidence for pairwise hydrogen addition from a single parahydrogen (p-H2) molecule. This represents a fundamentally new catalyst class for PHIP, expanding the scope of parahydrogen-based hyperpolarization beyond conventional Rh- and Ir-based systems. DFT calculations reveal the most plausible pathway for pairwise hydrogen addition. The computational results confirm a pairwise mechanism through oxidative addition of H2, sequential migratory insertion, and reductive elimination, wherein both hydrogen atoms originate from a single H2 molecule, consistent with the observed PHIP enhancement. These findings reveal the pairwise nature of the hydrogenation mechanism of broadly applied Grubbs catalysts and open new avenues for catalyst discovery in the rapidly growing field of parahydrogen-based NMR signal enhancement and hyperpolarized molecular imaging.

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