Sequential Hydrogen Tunneling in o Tolylmethylene

o Tolylmethylene 1 is a metastable triplet carbene that rearranges to o xylylene 2 even at temperatures as low as 2.7 K via [1,4] H atom tunneling. Electron paramagnetic resonance EPR and electron nuclear double resonance ENDOR spectroscopical techniques were used to identify two conformers of 1 anti and syn in noble gas matrices and in frozen organic solutions. Conformer specific kinetic measurements revealed that the rate constants for the rearrangements of the anti and syn conformers of 1 are very similar. However, the orbital alignment in the syn conformer is less favorable for the hydrogen transfer reaction than the orbital configuration in the anti conformer. Our spectroscopic and quantum chemical investigations indicate that anti 1 and syn 1 rapidly interconvert via efficient quantum tunneling forming a rotational pre equilibrium. The subsequent second tunneling reaction, the [1,4] H migration from anti 1 to 2, is rate limiting for the formation of 2. We here present an efficient strategy for the study of such tunneling equilibri