Transition states photochemical

The initial discussion in this chapter will focus on addition reactions. The discussion is restricted to reactions that involve polar or ionic mechanisms. There are other important classes of addition reactions which are discussed elsewhere these include concerted addition reactions proceeding through nonpolar transition states (Chapter 11), radical additions (Chapter 12), photochemical additions (Chapter 13), and nucleophilic addition to electrophilic alkenes (Part B, Chi iter 1, Section 1.10). 

The actual reported results bear out this analysis. Thus a thermal [1,3] migration is allowed to take place only antarafacially, but such a transition state would be extremely strained, and thermal [1,3] sigmatropic migrations of hydrogen are unknown." On the other hand, the photochemical pathway allows suprafacial [1,3] shifts, and a few such reactions are known, an example being " ... 

The thermal and photochemical reactions of Ir111 complexes (438) and (439), respectively, yield the same product species (440), R2 = Ph2, Me2, HPh.692 Kinetic studies indicate that the reaction involves the direct reductive transfer of the methyl to the phosphide via a three-centered transition state. The crystal structures of (438) and (439) (R2 = Ph2) are reported. 

The alternate approach of Dewar and Zimmerman can be illustrated by an examination of the 1,3,5-hexatriene system.<81,92> The disrotatory closure has no sign discontinuity (Hiickel system) and has 4n + 2 (where n = 1) ir electrons, so that the transition state for the thermal reaction is aromatic and the reaction is thermally allowed. For the conrotatory closure there is one sign discontinuity (Mobius system) and there are 4u + 2 (n = 1) ir electrons, so that the transition state for the thermal reaction is antiaromatic and forbidden but the transition state for the photochemical reaction is aromatic or allowed (see Chapter 8 and Table 9.8). If we reexamine the butadiene... 

The alternative electrophilic mechanism for the nitration of ESE with TNM requires a close approach of a hindered ESE to a N02 group on the quaternary carbon center of TNM. However, this transition state is sterically very demanding, and it will not readily account for the observed reactivity. Furthermore, the observed lack of regioselectivity in the nitration of the isomeric enol silyl ethers of 2-methylcyclohexanone that leads to the same 2-methyl-2-nitrocyclohexanone (in thermal as well as photochemical nitration) is not readily reconciled by a concerted (electrophilic) mechanism (equation 18). 

Applying these rules in pericyclic reactions it has been shown and a generalization given that thermal reactiom occur via aromatic transition states while photochemical reactions proceed via antiaromatic transition state. A cyclic transition state is considered to be aromatic or isoconjugate with the corresponding aromatic system if the member of conjugated atoms and that of the n... 

These and other experiments imply that even reactions that proceed via bulky transition states can take place in the clathrate if the initiation is photochemical, perhaps as a result of local deformation of the host lattice, and presumably as a result of the large energy dissipation involved. However, even here there is some lattice control of the reaction pathway. 

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