The tautomerization and ring closure in the Claisen rearrangement: A DFT study

Abstract

Elementary processes of the aromatic Claisen rearrangement were investigated by DFT calculations. First, rearrangements of four substrates Ph—O—CH2—CHCH2 [A], Ph—O—CH2—CHCH(OMe) [B], Ph—O—CH2—CHCH2····BF3 [C], and Ph—O—CH—CHCH(OMe)····BF3 [D] were examined. In these systems, the tautomerization is initiated by the intermolecular proton transfer involving the transient ion‐pair intermediate. An ignition‐propagation chain‐reaction mechanism in the tautomerization was suggested. For [A], the (ortho‐allyl phenol → α‐methyl‐dihydrobenzofuran (α‐methyl‐cumarane)) process was found to be ready and the product of the Claisen rearrangement seems to be the cumarane rather than the phenol. In [D] (activated both by the terminal methoxy group and by the BF3 catalyst), not the [3,3]‐sigmatropic shift but the tautomerization is the rate determining step. Second, the parent system, Ph—O—CH2—CHCH2, was investigated with (H2O) n (n = 2, 4, 6, and 10) systematically. The tautomerization takes place by the proton transfer via the water dimer or trimer. Except n = 2, similar changes of Gibbs free energies were obtained from the ether substrate to the cumarane.

Density functional theory and ab initio studies on hyperfine coupling constants of phosphinyl radicals

“…Among them are electronic (UV/Vis), vibrational (infrared and Raman), nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR) spectroscopy. The latter—also called electron spin resonance (ESR) spectroscopy—has proved to be indispensable in studies of radical species, which attract the attention of researchers due to their important roles in physics, chemistry, biochemistry, environmental, and material science …”