CIDNP sulfoxide

Two more recent studies of the nature of the sulfur-carbon bond cleavage in sulfoxide photolyses have been conducted, using ESR and CIDNP detection methods, respectively. In the latter case there is some evidence for a triplet process being involved in the photocleavage of aryl methyl sulfoxides. 

Jarvis et al. found that heating a-azidobenzyl phenyl sulfoxide (75) at 70 °C resulted in the formation of 78 in 33% yield.52 In addition to the sulfinimine, minor amounts of benzonitrile (15%), benzaldehyde (20%), PhS02SPh, and PhSSPh were detected. A radical mechanism involving radical pairs 76 and 77 is suggested and was supported by an NMR CIDNP effect. 

The photochemistry of this series of benzyl/allyl systems 34 was studied by chemically induced dynamic nuclear polarization (CIDNP) experiments [31]. Contrary to the assertion made here, the authors concluded that the formation of the (unisolated) sulfme 35 was an electrocyclic process, under either direct or sensitized irradiation, because no CIDNP signals were observed. On the other hand, CIDNP results for the acyclic 38 suggest that homolytic cleavage occurs on either side of the sulfoxide. Products were not identified. 

A series of papers on the photochemistry of a set of aromatic sulfoxides by German and Israeli authors relied very heavily on CIDNP results. A few of the conclusions reached in these papers may be over-reaching in retrospect, perhaps because of the limitations of the CIDNP technique, some imknown-until-later triplet energies, and the lack of product analysis in some cases. Nonetheless, this work gives very good evidence for the participation of ho-molytic a-cleavage. 

The second mechanism in the literature was suggested several years later on the basis of CIDNP investigation of the photochemistry of aryl methyl sulfoxides [31,45]. It will be referred to as the sulfinyl mechanism. The key step in the sulfinyl mechanism is the reaction of a free sulfinyl radical (formed by a-cleavage) with another radical to effect O atom transfer. 

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