Radical fragmentations. CIDNP

The complex photochemistry of cysteine derivatives sensitized by 4-carboxy-benzophenone 15 has been unravelled by CIDNP. The initially formed sulphur-centred (see. Figure 19) radical decarboxylates rapidly to give an a-amino alkyl radical, which in turn cleaves into a thiyl radical R-S and a vinylamine in competition with being oxidized to an imine by surplus sensitizer all these resulting species are unstable themselves and undergo further reactions. The rates of the radical fragmentations and the radical oxidation were obtained from the CIDNP experiments. 

Further evidence for the formation of alkene radical cations derives from the work of Giese, Rist, and coworkers who observed a chemically induced dynamic nuclear polarization (CIDNP) effect on the dihydrofuran 6 arising from fragmentation of radical 5 and electron transfer from the benzoyl radical within the solvent cage (Scheme 6) [67]. 

Scheme 6 Observation of a CIDNP effect on fragmentation of radical 5...

The electron-transfer mechanism for electrophilic aromatic nitration as presented in Scheme 19 is consistent with the CIDNP observation in related systems, in which the life-time of the radical pair [cf. (87)] is of particular concern (Kaptein, 1975 Clemens et al., 1984, 1985 Keumi et al., 1988 Morkovnik, 1988 Olah et al., 1989 Johnston et al., 1991 Ridd, 1991 Rudakov and Lobachev, 1991). As such, other types of experimental evidence for aromatic cation radicals as intermediates in electrophilic aromatic nitration are to be found only when there is significant competition from rate processes on the timescale of r<10 los. For example, the characteristic C-C bond scission of labile cation radicals is observed only during the electrophilic nitration of aromatic donors such as the dianthracenes and bicumene analogues which produce ArH+- with fragmentation rates of kf> 1010s-1 (Kim et al., 1992a,b). 

The mechanism of the cycloaddition appears to be concerted for various reagents however, for several cases, radical cation cycloaddition-cycloreversions have a stepwise component. For example, CIDNP effects observed during the PET induced dimerization of spiro[2.4]heptadiene (97) identify a dimer radical cation with spin density only on two carbons of the dienophile fragment this intermediate must be a doubly linked radical cation ( 99 + 282,283 pulsed laser experiment at high concentrations of 97 supports a second dimer radical cation at high... 

Observation of emission and absorption in the NMR benzylic proton signal from an irradiated solution of diphenyldiazomethane in toluene was the first example showing the importance of the CIDNP technique. Triplet diphenylcarbene generated by photolysis of diphenyldiazomethane in toluene abstracts a hydrogen atom to generate the triplet radical pair, which either recombines to give 31, or diffuses apart, ultimately to produce dimers of each fragment (32, 33, Scheme 9.7,... 

The electron transfer induced reaction of this diene system results in rapid [4 + 2]dimerization conversely, the dimer rapidly undergoes cycloreversion upon electron transfer. Both reactions result in strong CIDNP effects. The monomer polarization supports a radical cation with a spin density distribution like those of the butadiene or fulvene radical cations. The dimer polarization identifies a dimer radical cation with appreciable spin density only on two carbons of the dienophile fragment this species can only be the doubly linked radical cation D [135, 136], Significantly, a second dimer radical cation is implicated in a pulsed... 

The assignment of an antisymmetrical cyclopropane SOMO to the radical cation of 105 is based on a comparison of CIDNP effects (Fig. 18) with those for cis-1,2-diphenylcyclopropane. While the nuclei of the aromatic segments show identical or very similar polarization, the cyclopropane protons show characteristic differences. This suggests significantly different spin density distributions for the cyclopropane moieties of the two species and, thus, different structures [229]. The benzonorcaradiene radical cation should owe its structure to the symmetry of the fragment FMOs at the points of union. The styrene HOMO is antisymmetric at the positions of attachment, suggesting preferred interaction with the antisymmetric cyclopropane HOMO (as shown below). 

Chemically induced dynamic nuclear polarization (CIDNP) has been used to discriminate radical and nonradical processes in cycloadditions of electron-rich alkenes and electron-poor carbonyl components <1998MI9>. Spectroscopic observation of the fragmentation of an oxetane radical anion revealed the generation of the most stable alkene radical <2003JOC10103, 2006PPS51>. Transient absorption spectroscopy has been employed to monitor the Paterno-Biichi cycloaddition of benzophenone and furan <2004JA2838>. 

The El-induced fragmentation of gaseous [4 + 2]- and [2 + 2]dicyclopentadiene radical cations has been studied by Roth and coworkers using Fourier transform ion cyclotron resonance mass spectrometry, and compared to the cleavage of these ions in solution using chemically induced dynamic nuclear polarization (CIDNP). Both in the gas and in the liquid phase, the isomers of the molecular ions formed by single C—C bond cleavage were observed. It is noteworthy that these distonic ions were termed non-vertical radical cations. 

Good indirect evidence for radical cation formation as the key step in the fragmentation of a model nucleotide C4 radical (9) was obtained by the groups of Giese and Rist, who observed a CIDNP signal in the enol ether product 10 (Scheme 3) [10]. 

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