Dimethoxycarbene: Conformational Analysis of a Reactive Intermediate.

Hans Peter Reisenauer, Jaroslaw Romanski, Grzegorz Mloston, and Peter R. Schreiner,

Eur. J. Org. Chem. 2006, 4813. Download

    Abstract.  Dimethoxycarbene was prepared from an oxadiazoline precursor under high-vacuum flash pyrolysis (HVFP) conditions and was trapped at low temperatures by matrix isolation techniques (Ar, 10 K). The excellent agreement between the computed [CCSD(T)/cc-pVDZ] IR spectrum of the mixture of conformers of dimethoxycarbene and the experimentally measured IR absorptions allows a detailed analysis ofthe conformational preference of dimethoxycarbene. ItsUV spectrum is in agreement with earlier studies and our TD-B3LYP/6-311+G(d,p) computations. The computed [CCSD(T)/cc-pVDZ] rotational profile is rather steep and separates the s-trans,s-trans and s-cis,s-trans conformers by a 16 kcal mol^-1 barrier, whilst the s-cis,s-cis conformer is too high-lying to be observable (+17 kcal mol^-1). In marked contrast with the gauche,gauche minimum of dimethoxymethane, the s-trans,s-trans conformer of dimethoxycarbene is slightly preferred (0.5 kcal mol^-1). The s-cis,s-trans conformer equilibrates at the high temperatures required during HFVP generation and both conformers can be identified in the IR spectrum of the argon matrix at 10 K. The conformational preference is partly due to the minimization of the overall dipole moment in the s-trans,s-trans conformer.