Hydrogen shift photochemically symmetry allowed

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]. 

Analysis of a 1,7-hydrogen shift process indicates that the suprafacial hydrogen shift is symmetry forbidden in a thermal reaction. Photochemically, [1,7] suprafacial shift of hydrogen is symmetry allowed (Fig. 4.4) [1,2]. 

In thermal reaction, bonding interaction is maintained in the suprafacial mode of 1,5-shift and hence this process is symmetry allowed, while the antarafacial shift is symmetry forbidden. The suprafacial shift also corresponds to a favorable six-electron Huckel-type transition state in thermal reaction, whereas Huckel-type TS for suprafacial [l,3]-sigmatropic hydrogen shift is antiaromatic and is a forbidden process (Fig. 4.2) [1, 2]. Photochemically, [l,5]-hydrogen shift in the suprafacial mode is a symmetry forbidden process, but antarafacial shift is a symmetry allowed process (Fig. 4.3). 

Just as there are secondary interactions in cycloadditions, so too are there ancillary orbital-symmetry effects in sigmatropic reactions. In process (79), Jones and Jones (1967) find no products of [1,3] hydrogen or methyl shifts, e.g. 1,4,7-trimethylheptatriene, which ostensibly (Table 5) are photochemically allowed. They point out that in the first... 

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