Probing ring contraction and decarboxylation of Rhodizonate and the influence of Cu (II) using surface‐enhanced Raman Scattering

• Published in: Journal of Raman spectroscopy Vol. 51; no. 2; pp. 256 - 263
• Main Authors: Milán‐Garcés, Erix A., Georgopoulos, Stéfanos Leite, Barros Santos, Patrícia, Sato, Fernando, Sant'Ana, Antonio Carlos, Andrade, Gustavo F.S., Oliveira, Luiz Fernando Cappa
• Format: Journal Article
• Language: English
• Published: Bognor Regis Wiley Subscription Services, Inc 01-02-2020
• Subjects: Carboxyl group; Catalytic converters; Chemical reactions; Contraction; Conversion; Decarboxylation; Ions; Organic chemistry; Raman scattering; Raman spectra; Raman spectroscopy; Reaction kinetics; Rhodizonate; ring contraction; SERS spectroscopy; Species; Spectroscopy
• ISSN: 0377-0486; 1097-4555
• DOI: 10.1002/jrs.5767

Understanding ring contraction and decarboxylation reactions is critical to different fields of chemistry. The reaction of conversion of rhodizonate (Rho) to croconate (Cro) in solution was proposed to occur by initially α‐oxo‐alcohol ring contraction followed by decarboxylation reaction. In the present study, SERS (surface‐enhanced Raman scattering) spectroscopy was used for real‐time label free measurement of the kinetics of this reaction in solution. It was observed that Rho remained as the predominant species for the first two hours, and after that different intermediate and product species were detected via spectral characterization. In 24 hours Rho was consumed and the SERS spectrum was assigned to a intermediate species, which in combination with DFT calculation was unambiguously identified as a previously proposed five‐membered ring intermediate. It was probed the catalytic influence of Cu (II) ions in the rapidly conversion of such intermediate to Cro and/or Cu (II)‐Cro complex. The results suggest that Cu (II) ions catalyze the decarboxylation step of reaction probably by initially attacking the carboxyl group of the intermediate species. This study shows that SERS can be broadly applied to study slow ring contraction of Rho and catalyzed decarboxylation reactions in solution with detection of intermediate species which are not easily accessible by other techniques.
Surface‐enhanced Raman scattering spectroscopy was used to probe the spontaneous conversion of Rhodizonate to Croconate in water. It was possible to monitor the ring contraction reaction producing an intermediate specie followed by a decarboxylation reaction to Croconate. It was found that Cu (II) ions catalyze the reaction only in the presence of the intermediate.