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Polar crystals can be driven into collective oscillations by optical fields tuned to precise resonance frequencies. As the amplitude of the excited phonon modes increases, novel processes scaling non-linearly with the applied fields begin to contribute to the dynamics of the atomic system. Here we show two such optical nonlinearities that are induced and enhanced by the strong phonon resonance in the van der Waals crystal hexagonal boron nitride (hBN). We predict and observe large sub-picosecond duration signals due to four-wave mixing (FWM) during resonant excitation. The resulting FWM signal allows for time-resolved observation of the crystal motion. In addition, we observe enhancements of third-harmonic generation with resonant pumping at the hBN transverse optical phonon. Phonon-induced nonlinear enhancements are also predicted to yield large increases in high-harmonic efficiencies beyond the third.

 

The activation of C–O bonds in aryl methyl ethers is a fundamental method for the cross-coupling of carbon–oxygen bonds; however, this process is highly challenging due to the high dissociation energy compared with other phenol derivatives. Herein, we report a mild Ru(0)-catalyzed cleavage of C(aryl)–O bonds enabled by a combination of a Ru 3 (CO) 12 catalyst and an imine auxiliary. This method offers rapid entry to synthetically valuable biaryl aldehydes from abundant anisoles. Broad functional group tolerance is observed using this strategy, including unprecedented tolerance towards aryl bromides. The synthetic utility of this strategy has been demonstrated in sequential processes to construct complex biaryls, exploiting the orthogonal selectivity of C–O bond activation. DFT studies were conducted to provide insight into the selectivity of C–O bond cleavage. This method establishes the mildest approach to C–OMe cross-coupling reported to date.