Abstract
A kinetic and product study of the 3‐cyano‐N‐methyl‐quinolinium photoinduced monoelectronic oxidation of a series of β‐hydroxysulfoxides has been carried out to investigate the competition between Cα‐S and Cα‐Cβ bond cleavage within the corresponding cation radicals. Laser flash photolysis experiments unequivocally established the formation of sulfoxide cation radicals showing their absorption band (λ max ≈ 520 nm) and that of 3‐CN‐NMQ• (λ max ≈ 390 nm). Steady‐state photolysis experiments suggest that, in contrast to what previously observed for alkyl phenyl sulfoxide cation radicals that exclusively undergo Cα‐S bond cleavage, the presence of a β‐hydroxy group makes, in some cases, the Cα‐Cβ scission competitive. The factors governing this competition seem to depend on the relative stability of the fragments formed from the two bond scissions. Substitution of the β‐OH group with ‐OMe did not dramatically change the reactivity pattern of the cation radicals thus suggesting that the observed favorable effect of the hydroxy group on the Cα‐Cβ bond cleavage mainly resides on its capability to stabilize the carbocation formed upon this scission.
Highlights
It is a great honor for us for having been invited to contribute to the Special Issue of Photochemistry & Photobiology dedicated to celebrating the career of Dr Edward Clennan
On the reactivity of aryl sulfide cation radicals bearing a hydroxy group in β position, we showed that the OH group determines the Cα-Cβ instead of the Cα-S bond cleavage [35]
No products were observed in the oxidation of 1, a result that can be attributed to the low fragmentation rate for the cation radical 1+ that favors the competitive and unproductive back electron transfer (BET) process as shown by the laser flash photolysis (LFP) experiments
Summary
It is a great honor for us for having been invited to contribute to the Special Issue of Photochemistry & Photobiology dedicated to celebrating the career of Dr Edward Clennan. It is well known that electron transfer (ET) processes play a fundamental role in many biological and organic processes For this reason, an increasing number of studies have been focused on the reactivity and the properties of the radical ions, the primary species obtained from these processes [1,2]. Among the classes of organic compounds whose reactivity in ET process have been the subject of intense investigation, sulfides have attracted a special interest [3,4,5,6,7,8] since their monoelectronic oxidation is involved in many biological processes [9], in organic synthesis [10] and in the initiation of radical polymerization [11]. The redox potentials of methyl phenyl sulfoxide and thioanisole are 2.01 [21] and 1.47 V [22] (vs SCE), respectively
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