Photochemical allyl shift

Scheme 16 Photochemical 1,2-allyl shifts on pyrrolo[2,3-i>]indole [83]...

Compoimd 63 probably undergoes 5 1 with allylic shift, but rearrangement of (4-)-a-deutero-64 leads to extensive racemisation (proved to occur during reaction), and the kinetic effects of ring substituents, e.s.r. measurements, and the photochemical induction of reaction, support a radical-pair mechanism . Kinetics of the gas-phase reaction have led to similar conclusions , but it is disturbing that the activation energy is much lower than the bond energy of G(i)-N. These should be approximately equal for a radical non-chain process. 

Reactions.—Full details are now available" of the photochemical and thermal [1,3] phenylthio shifts of allyl phenyl sulphides [(73)- (74)]. Oxidation of such sulphides can lead to allyl alcohols through the known sulphoxide-sulphenate equilibrium with sulphenate trapping. 

It is seen from the figure that in thermal reaction, a bonding interaction (in the same phase) can be maintained only in the antarafacial mode of shift. Therefore, thermal 1,3-suprafacial shift of hydrogen is forbidden from orbital symmetry considerations. The antarafacial shift is orbital symmetry allowed process and will be a concerted process. Photochemically, [l,3]-suprafacial shift of hydrogen is a symmetry allowed process because the bonding intemction takes place in the same phase of allyl group [1, 2]. 

Photochemical [1,3] sigmatropic shifts occur in some allylic systems. Draw the structures of the products resulting from the following alkene. Which product should predominate ... 

Aoyama, H., Photochemical reaction of thiobenzamides bearing an allylic substituent on the nitrogen atom double-bond migration via tandem 1,4- and 1,6-hydrogen shift, /. Chem. Soc., Perkin Trans. 1, 1851,1997. 

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