Photochemical reactions sigmatropic rearrangements

In this primer, Ian Fleming leads you in a more or less continuous narrative from the simple characteristics of pericyclic reactions to a reasonably full appreciation of their stereochemical idiosyncrasies. He introduces pericyclic reactions and divides them into their four classes in Chapter 1. In Chapter 2 he covers the main features of the most important class, cycloadditions—their scope, reactivity, and stereochemistry. In the heart of the book, in Chapter 3, he explains these features, using molecular orbital theory, but without the mathematics. He also introduces there the two Woodward-Hoffmann rules that will enable you to predict the stereochemical outcome for any pericyclic reaction, one rule for thermal reactions and its opposite for photochemical reactions. The remaining chapters use this theoretical framework to show how the rules work with the other three classes—electrocyclic reactions, sigmatropic rearrangements and group transfer reactions. By the end of the book, you will be able to recognize any pericyclic reaction, and predict with confidence whether it is allowed and with what stereochemistry. 

Sometimes thermal or photochemical reactions involve rearrangements in which a a-bond flanked by one or more 7C-electrons are transferred to new positions / to /. They are known as sigmatropic rearrangements of the order (i, j). Here the system (/, ) is numbered by starting with atoms from which o bond is to be migrated, i.e.,i represents initial position ofo-bond, whereas, j represents the position to which bond has migrated. For example ... 

Sandmeyer reaction, 306 Sandwich compoimds, 275 Sawhorse projections, 7 Saytzev elimination, 249, 256 Schiff bases, 221 Schmidt rearrangement, 122 Selectivity, 156, 169, 326 a, 362 a, 370 372 aj,385 a bonds, 6 o complexes, 41,131 Sigmatropic rearrangements, 352-357 antarafacial, 353 carbon shifts, 354 hydrogen shifts, 352 orbital symmetry in, 352 photochemical, 354 suprafadal, 353 thermal, 353... 

A similar analysis of [1,5] sigmatropic rearrangements shows that in this case the thermal reaction must be suprafacial and the photochemical process antarafacial. For the general case, with odd-numbered /, we can say that [1,/] suprafacial migrations are allowed thermally when j is of the form 4n + 1, and photochemically when j has the form An - 1 the opposite is true for antarafacial migrations. 

Thus, as predicted by the orbital symmetry rules, this thermal suprafacial [1,3] sigmatropic reaction took place with complete inversion at C-7. Similar results have been obtained in a number of other cases.426 However, similar studies of the pyrolysis of the parent hydrocarbon of 103, labeled with D at C-6 and C-7, showed that while most of the product was formed with inversion at C-7, a significant fraction (11 to 29%) was formed with retention.427 Other cases of lack of complete inversion are also known.428 A diradical mechanism has been invoked to explain such cases.429 There is strong evidence for a radical mechanism for some [1,3] sigmatropic rearrangements.430 Photochemical suprafacial [1,3] migrations of carbon have been shown to proceed with retention, as predicted.431... 

For analysis of the photochemical reaction, the interaction of the hydrogen Is orbital with -tt3 of the allyl system is used. The interaction is bonding at both the migration origin and terminus, so the [ 1,3] sigmatropic rearrangement is photochemically allowed. 

Classify these reactions as electrocyclic reactions, [x + y] cycloadditions, or [/,/] sigmatropic rearrangements and explain whether each is allowed thermally or photochemically. 

After your experience with cycloadditions and sigmatropic rearrangements, you will not be surprised to learn that, in photochemical electrocyclic reactions, the rules regarding conrotatory and disrotatory cyclizations are reversed. 

Several cases of photochemical reactions, for which the thermal equivalents were forbidden, are shown below. In some cases the reactions simply did not occur thermally, like the [2 +2] and [4 +4] cycloadditions, and the 1,3- and 1,7-suprafacial sigmatropic rearrangements. In others, the photochemical reactions show different stereochemistry, as in the antarafacial cheletropic extrusion of sulfur dioxide, and in the electrocyclic reactions, where the 4-electron processes are now disrotatory and the 6-electron processes conrotatory. In each case,... 

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