Orbital symmetry requirements for

This compound is less stable than 5 and reverts to benzene with a half-life of about 2 days at 25°C, with AH = 23 kcal/mol. The observed kinetic stability of Dewar benzene is surprisingly high when one considers that its conversion to benzene is exothermic by 71 kcal/mol. The stability of Dewar benzene is intimately related to the orbital symmetry requirements for concerted electrocyclic transformations. The concerted thermal pathway should be conrotatory, since the reaction is the ring opening of a cyclobutene and therefore leads not to benzene, but to a highly strained Z,Z, -cyclohexatriene. A disrotatory process, which would lead directly to benzene, is forbidden. ... 

The products consistently exhibit retention of the original olefin stereochemistry and are probably formed in a concerted manner. An exciplex formed from singlet excited benzene and the ground state olefin (allowing relaxation of the orbital symmetry requirements for concerted 1,3- and 1,4-cycloaddition) has been proposed to account for these products/126 Srinivasan and Hill reported an unusual photochemical addition to benzene to form cycloadduct (52)<74) ... 

This crossing is, in turn, an orbital symmetry requirement for the fragmentation to follow the allowed process through path [1 ], as shown by the level diagram of Fig. 12b. 

In view of the Importance In this case of unquestionable knowledge of the configuration, an X-ray analysis was made of toxlsterol Cl dlnltrobenzoate (96) (Figure 22). This analysis established beyond doubt structure I, anticipated for toxi-sterol Cl on the basis of Its stability and FMR data. Its stereochemistry is consistent with formation via an overall 4lla + 2 a pathway that is, not In agreement with the orbital symmetry requirements for a concerted process. 

How do orbital symmetry requirements relate to [4tc - - 2tc] and other cycloaddition reactions Let us constmct a correlation diagram for the addition of butadiene and ethylene to give cyclohexene. For concerted addition to occur, the diene must adopt an s-cis conformation. Because the electrons that are involved are the n electrons in both the diene and dienophile, it is expected that the reaction must occur via a face-to-face rather than edge-to-edge orientation. When this orientation of the reacting complex and transition state is adopted, it can be seen that a plane of symmetry perpendicular to the planes of the... 

Linear constraints include various simple cases such as forcing a single coefficient to zero (in which case C has only one non-zero coefficient) or setting pairs of coefficients equal or opposite. However, more complicated conditions, such as symmetry requirements for orbitals, as considered in the next Section, are also included in this category. For example, Eq. (38) in Section 3.3, leads to m-n( ) linear constraints, correspond-... 

The valence bond picture for six-coordinate octahedral complexes involves dispi hybridization of the metal (Fig. I i.lc. d). The specific d orbitals that meet the symmetry requirements for the metal-ligand o bonds are the four-coordinate d complexes discussed above, the presence of unpaired electrons in some octahedral compounds renders the valence level ( — l)J orbitals unavailable for bonding. This is true, for instance, for paramagnetic [CoFJ3- (Fig. I I.lc). In these cases, the VR model invokes participation of -level dorbitals in the hybridization. 

It must be admitted that most of the preceding examples of additions (not eliminations) which appear to obey the symmetry rules are probably not concerted. Are they therefore fraudulent, deceptive, misleading, etc Probably not. For most of these appear to be oases in which the shape originally impressed by orbital symmetry requirements is retained to the point at which a succeeding reaction step produces the stereoselective result. The mixed anti products found in some halogenations (157) suggest that there is an intermediate for which anions can compete... 

All two-center, two-electron integrals were set to the integral over s symmetry orbitals, a requirement for rotational invariance discussed above ... 

We recognize immediately that some of these combinations would encounter strain and/or entropic restrictions. However, orbital symmetry considerations provide a fundamental insight into the electronic nature of the cycloaddition reactions and allow us to see that some of the TS structures are electronically favorable, whereas others are not. Woodward and Hoffmann formulated the orbital symmetry principles for cycloaddition reactions in terms of the frontier orbitals. An energetically accessible TS requires overlap of the frontier orbitals to permit smooth formation of the new... 

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