CIDNP Chemically Induced alkenes

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]. 

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>. 

Cycloaddition reactions of triplet excited 1,4-quinones to ground-state alkenes occur either via a triplet exciplex intermediate, which collapses to a triplet biradical,1000 or via separated radical ion intermediacy.990 The existence of biradical intermediates has been proven by measurements of chemically induced dynamic nuclear polarization (CIDNP) (Special Topic 5.3), for example in the reaction of 1,4-benzoquinone (313) with norbomadiene (314) yielding two products, the spiro-oxetane 315 and the spiro-oxolane 316 (Scheme 6.139).1001 Interestingly, quadricyclane (317) provides the same reaction as norbomadiene. 

Stoicheiometric hydroformylation of alkenes with [MnH(CO)s] has recently been reported. Treatment of 1,2-diphenyl 3,3-dimethylcyclo-propene in hexane under CO with [MnH(CO)s] at 55 °C gave a mixture of cis-(S7%) and trans-(l37o) aldehydes together with some alkane. The proposed mechanism is shown in Scheme 3. A chemically induced dynamic nuclear polarization (CIDNP) effect was observed in the H n.m.r. spectrum during the reaction and this was ascribed to the initial formation of an alkyl radical (10). The final step, reaction of a metal hydride with a metal acyl to give free aldehyde, is similar to that proposed in Co-catalysed hydro-... 

Consequently, all the alkene 12 detected under such conditions was formed by the Cope-type elimination (b). On the other hand, alkene formed in the absence of thiophenol was generated by both paths. Recently, Marque et al showed that thermolysis of alkoxyamines in the presence of the scavenger PhSH induces chemically induced dynamic nuclear polarization (CIDNP). The CIDNP effect, detected by H-NMR, indicates that the intermolecular H-transfer occurs, in contrast to earlier reports,already in the spin correlated geminate (cage) radical pair, formed immediately after the cleavage of the NO C bond. 

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