Photochemical concerted reactions 1,7-hydrogen shifts

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]. 

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