The oxidation of a series of aryl tert-butyl sulfoxides (4-X-C6H4SOC(CH3)3: 1, X = OCH3; 2, X = CH3; 3, X = H; 4, X = Br) photosensitized by 3-cyano-N-methylquinolinium perchlorate (3-CN-NMQ+) has been investigated by steady-state irradiation and nanosecond laser flash photolysis (LFP) under nitrogen in MeCN. Products deriving from the C–S bond cleavage in the radical cations 1+•–4+• have been observed in the steady-state photolysis experiments. By laser irradiation, the formation of 3-CN-NMQ• (λmax = 390 nm) and 1+•–4+• (λmax = 500–620 nm) was observed. A first-order decay of the sulfoxide radical cations, attributable to C–S bond cleavage, was observed with fragmentation rate constants (kf) that decrease by increasing the electron donating power of the arylsulfinyl substituent from 1.8 × 106 s–1 (4+•) to 2.3 × 105 s–1 (1+•). DFT calculations showed that a significant fraction of the charge is delocalized in the tert-butyl group of the radical cations, thus explaining the small substituent effect on the C–S bond cleavage rate constants. Via application of the Marcus equation to the kinetic data, a very large value for the reorganization energy (λ = 62 kcal mol–1) has been calculated for the C–S bond scission reaction in 1+•–4+•.
Structural Effects on the C−S Bond Cleavage in Aryl tert-Butyl Sulfoxide Radical Cations
DEL GIACCO, Tiziana;
2013
Abstract
The oxidation of a series of aryl tert-butyl sulfoxides (4-X-C6H4SOC(CH3)3: 1, X = OCH3; 2, X = CH3; 3, X = H; 4, X = Br) photosensitized by 3-cyano-N-methylquinolinium perchlorate (3-CN-NMQ+) has been investigated by steady-state irradiation and nanosecond laser flash photolysis (LFP) under nitrogen in MeCN. Products deriving from the C–S bond cleavage in the radical cations 1+•–4+• have been observed in the steady-state photolysis experiments. By laser irradiation, the formation of 3-CN-NMQ• (λmax = 390 nm) and 1+•–4+• (λmax = 500–620 nm) was observed. A first-order decay of the sulfoxide radical cations, attributable to C–S bond cleavage, was observed with fragmentation rate constants (kf) that decrease by increasing the electron donating power of the arylsulfinyl substituent from 1.8 × 106 s–1 (4+•) to 2.3 × 105 s–1 (1+•). DFT calculations showed that a significant fraction of the charge is delocalized in the tert-butyl group of the radical cations, thus explaining the small substituent effect on the C–S bond cleavage rate constants. Via application of the Marcus equation to the kinetic data, a very large value for the reorganization energy (λ = 62 kcal mol–1) has been calculated for the C–S bond scission reaction in 1+•–4+•.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
