Exciton Coupling Analysis and Enolization Monitoring by Vibrational Circular Dichroism Spectra of Camphor Diketones

Abstract

The keto-enol tautomerization of (1R,3R,4R)-(+)-3-benzoylcamphor in solution was traced by the infrared (IR) and vibrational circular dichroism (VCD) spectra, reflecting the Boltzmann populations of the isomers. To investigate the exciton coupling of the carbonyl vibrations in the region 1800-1600 cm(-1), VCD spectra of a series of camphor derived β-diketones were analyzed with the support of density functional theory (DFT) calculations. The results confirm the importance of the exciton chirality for VCD and manifest that the VCD spectroscopy is a convenient technique to investigate the keto-enol tautomerization equilibria in chiral diketones.

Structural Analysis Using Chiroptical Spectroscopy: Insights and Cautions

“…The coefficients c 1 and c 2 can be determined by diagonalizing the 2 × 2 matrix composed of transition wavenumbers as the diagonal elements and interaction energy as the off‐diagonal elements. These points have been illustrated recently using the VCD spectra of spiroindicumides and benzoylcamphor …”

“…These points have been illustrated recently 74 using the VCD spectra of spiroindicumides 80 and benzoylcamphor. 81 As another illustration of the NDEC method, the VCD spectrum of (-)-(6R,7R,8S,10R,2´Z)-centratherin (see Fig. 3), a natural product, 82 containing a positive bisignate VCD couplet (minus VCD band at 1716 and plus VCD band at 1705 cm -1 with equal peak intensities) associated with C = O stretching vibrations (see fig.…”

Electronic and vibrational exciton coupling in oxidized trianglimines

“…Recently, Taniguchi and Monde expanded the exciton chirality concept also on vibrational circular dichroism and formulated, so‐called, vibrational circular dichroism exciton chirality (VCDEC) rule . This approach to solving stereochemical problems seems to be very attractive and has been applied to study stereochemistry of natural products and dynamic systems . However, Abbate et al recently subjected to critical analysis capabilities of the VCDEC method.…”

New Pyrazole‐ and Benzimidazole‐derived Ligand Systems

“…Formation of ligand 3 was a rather slow reaction at reflux in EtOH, requiring that 3‐(hydroxymethylene)camphor and 2‐hydrazinopyridine be refluxed for 2.5 days. The keto‐enol tautomerism of 3‐(hydroxymethylene)camphor has been explored both in solution and in the solid state ; for 3‐(hydroxymethylene)camphor, the condensation can be envisioned as proceeding via the diketone to form the hydrazone and then the pyrazole. This ligand contains CH, CH 2 , and CH 3 groups that are observed in 1 H NMR at 7.48, 1.92, and 0.96 ppm, respectively.…”