Thermal concerted reactions 1,7-hydrogen shifts

The above selection rules, therefore, predict that [1, 5] hydrogen shifts in neutral polyenes would be thermally allowed and the reaction would be facile, but thermal [1, 3] and [1, 7] shifts must go by an antarafacial process and they will be difficult to attain because of the geometric strain. This is also confirmed by many experimental observations. Thus concerted uncatalysed [1, 3] hydrogen shifts have not been seen in the diene of the following type, [1, 5] shifts are well known. 

If these reactions occur in uncatalyzed processes where bond breaking and bond formation are taking place concertedly, and not in two-step pathways via ionic or diradical intermediates, then the stereochemistry of these sigmatropic shifts can be predicted in a qualitative manner 1 -4. According to the Woodward-Hoffmann rules the thermally allowed reaction should take place in an antarafacial fashion across the allylic framework. The shifting hydrogen has to move from one side of the allylic plane to the other as depicted below. 

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]. 

Many of the reagents that react with NR have been investigated with the above criteria in mind. The conclusion reached is that the most successful will operate by what has become known as the thermal ene addition. This involves the 1,3-cycloadditionr of an unsaturated group X=Y to an olefinic double bond in which there is a transfer of an allylic hydrogen and a shift of the double bond in probably a concerted reaction. The general reaction is illustrated in Fig.l. 

In connection with some other work, the optically active lactone sulphoxides (83) and (84) have been observed to undergo a rapid, thermally induced racemization in boiling chloroform. Deuterium-labelling experiments indicate that the reaction probably involves the intermediacy of the sulphenic acids, e.g. (85), that are formed by a concerted hydrogen shift. Such a sigmatropic process finds analogy in the thermal isomerization of... 

Concerted thermochemical [1,7] hydrogen shifts are unknown in cyclo-heptatrienes or similar cyclic molecules. The best known example is given by the equilibration of vitamin Dj and precaliferol (Equation 6,98). Presumably the reaction centres adopt a gentle spiral conformation, as indicated, to facilitate the transfer of the H-atom from the top face at one terminus to the bottom face at the other terminus. The necessity for this type of conformation makes the rarity of thermal [1,7] H-shifts rather more understandable. The [1,5] shifts are usually available as competing processes. Thus O-deuteriated... 

A concerted [1,3] thermal shift of hydrogen to convert 7-31 to 7-32 is not a reasonable mechanism because such shifts are ruled out by the selection rules for pericyclic reactions (see Chapter 6). 

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