Rules for photochemical reactions

The rule for photochemical reactions is simply the reverse of the rule for thermal reactions ... 

These results are in accord with orbital symmetry principles. Indeed, examples found in the study of vitamin D provided the initial examples of the dichotomy between thermal and photochemical processes that led to development of the concepts underlying the Woodward-Hoffmann rules for photochemical reaction. It was found that the triene precalciferol gave ergosterol on photocyclization, but the stereoisomer lumisterol on heating. 

The rule may then be stated A thermal pericyclic reaction involving a Hiickel system is allowed only if the total number of electrons is 4n + 2. A thermal pericyclic reaction involving a Mobius system is allowed only if the total number of electrons is 4n. For photochemical reactions these rules are reversed. Since both the 2 + 4 and 2 + 2 cycloadditions are Hiickel systems, the Mdbius-Hiickel method predicts that the 2 + 4 reaction, with 6 electrons, is thermally allowed, but the 2 + 2 reaction is not. One the other hand, the 2 + 2 reaction is allowed photochemically, while the 2 + 4 reaction is forbidden. 

Crudely, but adequately for now, we may state rule governing which cycloadditions can take place and which not. A thermal pericyclic cycloaddition is allowed if the total number of electrons involved can be expressed in the form (4n+2), where n is an integer. If the total number of electrons can be expressed in the form 4n it is forbidden. Another way of saying the same thing is that reactions with an odd number of curly arrows are allowed and those with an even number are forbidden. This rule needs to be qualified, as we shall see shortly, and in due course in Chapter 3 made more precise, along with the rules for all the other kinds of pericyclic reaction, in one all-encompassing rule. For now, we need to introduce the rule for photochemical pericyclic cycloadditions. 

Frontier orbitals also explain why the rules change so completely for photochemical reactions. In a photochemical cycloaddition, one molecule has had one electron promoted from the HOMO to the LUMO, and this excited-state molecule reacts with a molecule in the ground state. The interacting orbitals that most effectively lower the energy of the transition structure are... 

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. 

In this chapter, recent developments in the regioselective, site-selective, and stereoselective preparation of oxetanes have been summarized. The relative nudeophilicity of the alkene carbons was seen to be important for regioselectivity, in addition to the well-known radical stability rule. Likewise, the three-dimensional structures of the triplet 1,4-biradicals were seen to play an important role in stereoselectivity. For photochemical reactions that proceed via radical ion pairs, the spin and charge distributions are crucial determinants of regioselectivity. It follows that the concepts used in selective oxetane synthesis should stimulate future investigations into the mechanistically and synthetically fascinating Paterno-Bitchi-type reactions. 

The easiest explanation is based on the frontier orbitals—the highest occupied molecular orbital (HOMO) of one component and the lowest unoccupied orbital (LUMO) of the other. Thus if we compare a [2 + 2] cycloaddition 6.133 with a [4 + 2] cycloaddition 6.134 and 6.135, we see that the former has frontier orbitals that do not match in sign at both ends, whereas the latter do, whichever way round, 6.134 or 6.135, we take the frontier orbitals. In the [2 + 2] reaction 6.133, the lobes on C-2 and C-2 are opposite in sign and represent a repulsion—an antibonding interaction. There is no barrier to formation of the bond between C-l and C-l, making stepwise reactions possible the barrier is only there if both bonds are trying to form at the same time. The [4 + 4] and [6 + 6] cycloadditions have the same problem, but the [4 + 2], [8 + 2] and [6 + 4] do not. Frontier orbitals also explain why the rules change so completely for photochemical reactions, as we shall see in Chapter 8. 

For thermal reactions, with 4n electrons in the transition state the conrotatory process is allowed, and with [4n -1- 2] electrons in the transition state the disrotatory process is allowed. For photochemical reactions, rules are usually reversed. 

Let there be light When an electron is promoted by light from the HOMO to the LUMO, the HOMO of the excited state is now the old LUMO. The HOMO has moved up one MO in energy, and the phase of the ends have reversed. We expect the rules to reverse, and they do. The An photochemical electrocyclic reactions go dis, and the 4n+2 photochemical electrocyclic reactions go con, as predicted. This all boils down to an easy-to-remember rule An electrocyclics go thermal con, with the rules reversing for An+2, and also reversing for photochemical reactions. 

For photochemical reactions, HOMO-MHQMG" and lUMO-TUMO 1 interactions dominate, in contrast to the HOMO-LUMO interactions involved in thermal reactions, The rules tor numbers of electrons involved in photochemical pericydic reactions are the reverse of those for thermal reactions. 

The Woochvard-Huflimnn rules are a general expression of this. A pericyclic reaction which is entirely suprafacial is allowed thermally if 4n + 2 electrons are involved (Hiickel transition state), but forbidden for 4n electrons. If there is one antarafacial component, the reaction will be allowed thermally if 4n electrons are involved (Mobius transition state), but forbidden for 4n + 2 electrons. For photochemical reactions, these rules are reversed. Roald Hoffmann shared the Nobel prize for Chemistry with Kenichi Fukui in 1981 for his contribution to this concept Robert Burns Woodward had already won the prize in 1965. 

We have seen that the stereochemistry of an electrocyclic reaction depends on the mode of ring closure, and the mode of ring closure depends on the number of conjugated 7T bonds in the reactant and on whether the reaction is carried out under thermal or photochemical conditions. What we have learned about electrocyclic reactions can be summarized by the selection rules listed in Table 29.1. These are also known as the Woodward-Hoffmann rules for electrocyclic reactions. 

In the field of inorganic photochemistry, Zink has presented an interesting molecular orbital analysis of the photochemical reactions of ds and d compounds which complements his previous ligand-field approach and provides predictions in accord with experimental findings. Exceptions to Adamson s empirical rules for photochemical ligand release continue to appear (Kirk and Kelly). Endicott et al. have provided a critical examination of models for photoredox reactions of transition-metal ammine complexes. They stress the role of the solvent in relaxation of the Franck-Condon excited state to the primary radical-pair products. 

Therefore, thermal electrocyclic reactions of four IT electrons can only occur in a conrotatory manner, while photochemical electrocyclic reactions of four t electrons can only occur in a disrotatory manner. This is summarized by the Woodward-Hoffman rules for electrocyclic reactions (given in Table 1 below). 

These two processes give the same product (cyclobutene) for the prototype reaction, but different stereoisomers if the butadiene is asymmetrically substituted with methyl groups. The purpose of the Woodward-Hofifmann rules for such reactions is to rationalise the stereochemistry of the products for both thermal and photochemical reactions. 

From a more detailed analysis of MO and state correlation diagrams. Woodward and Hoffmann presented a set of selection rules for cycloaddition reactions, which are summarized in Table 11.1. ° Here p and q are the number of electrons in the two n systems imdergoing the cycloaddition reaction. When the sum of p and g is a member of the 4n series, then the reaction is thermally allowed to be suprafacial with respect to one of the n components and antarafacial with respect to the other one. When the sum of p and qisa member of the 4n -h 2 series, then Ihe reaction is thermally allowed when it is either suprafacial with respect to both components or antarafacial with respect to both. As usual, the selection rules are reversed for photochemical reactions. 

Therefore, photochemical interconversion is allowed in the conrotatory pathway. These generalizations are true for all the systems containing (4n - - 2) TT-electrons, where n = 0, 1, 2, etc. Thus, Woodward—Hoffmann rules for electrocyclic reactions may be summed up as given in Table 2.1. 

According to the Woodward-Hoffmann rules for electrocychc reactions, a 6k electrocychzation is thermally allowed in a disrotatory manner and photochemically allowed in a conrotatory manner. However, in the present context of synthesis of aromatic compounds as final products, which are devoid of any stereocenters, the stereochemical aspects of the substituents in the intermediate dihydroaromatic compound should not matter. The photochemical 6x electrocychzation of c/s-stilbene derivatives followed by oxidation of the dihydroaromatic intermediate provides access to angularly fused polycyclic aromatic compounds (Scheme 16.2) [4]. 

We noted in Chapter 15 that, for the most part, the orbital symmetry rules are not directly applicable to photochemistry. However, some photochemical reactions of simple tt systems do give products that are consistent with expectations based on orbital symmetry, although this does not prove that these are concerted, pericyclic processes, The photochemical selection rules for pericyclic reactions are opposite of those for thermal pericyclic reactions. For example, there are many examples of [1,3] and [1,7] sigmatropic shifts that appear to go by the photochemically "allowed" suprafacial-suprafacial pathway Eqs. 16.22 and 16.23 show two (recall that the thermal reactions would be suprafacial-antarafacial). These reactions occur upon direct irradation, while sensitized photolysis produces products more consistent with biradical-type reactions. 

Table 20.2 summarizes all An and 4m + 2 reactions. Other 4m processes will follow the rules for the 2 + 2 dimerization of a pair of alkenes, and 4m + 2 processes will resemble the 4 + 2 cycloaddition we know as the Diels-Alder reaction. Perhaps you can see the relationship to aromatidty (4m + 2) that plays a role in this analysis. The transition state for these cycloaddition reactions is cyclic and will be allowed only in the cases where the number of electrons makes the transition state aromatic, 4m + 2 electrons for thermal processes and 4m for photochemical reactions. 

---