Orbital symmetry requirements

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. ... 

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... 

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. 

According to Olah, the high intramolecular sensitivity arises from the orbital symmetry requirements of this transition state. The electrophile can only interact with two p orbitals that have the same sign in the highest occupied molecular orbital (83). Thus transition states 84 and 85 are possible, but 86 is not. 

Carbon-13 n.m.r. spectroscopy has been used to study benzylisoquinoline and tetrahydroisoquinoline alkaloids and their JV-methyl quaternary salts.30 N-Benzylpapaverinium bromide has been shown to undergo aerial oxidation in alkaline solution to 2-benzyl-6,7-dimethoxyisoquinolone and to give bases of general structure (10) with methylamine, benzylamine, and pyrrolidine.31 Chlorosulphonation of papaverinol affords the sulphonic acid ester (11), mild hydrolysis of which yields the acid (12), which with diazomethane is esterified and dehydrated to (13).32 N-Methyl-l,2-dihydropapaverine has been shown by kinetic studies and orbital-symmetry requirements to rearrange to the salt (14) by the route previously postulated.33... 

Thus far, orbital-symmetry requirements of any reaction have been met by changing the reaction conditions. Another possible approach is to change the orbital-symmetry requirements by adding a third reactant. Volger and Hogeveen (1967) effected the thermally forbidden 2 + 2 cycloaddition by adding transition metal catalysts,... 

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... 

The 2,5-dihydrofuran elimination of hydrogen is also Woodward-Hoffmann allowed. The reaction proceeds at relatively low temperatures ( 650 °K) with a low activation energy which is characteristic of such concerted processes. Conservation of orbital symmetry requires exclusive cw-2,5-hydrogen elimination accompanied by 7t-bond migration, viz. 

Conservation of orbital symmetry requires that 1,2-elimination be trans. This, of course, is sterically prohibited. In agreement with the rules, decomposition of 2,3-dihydrofuran occurs only at high temperatures, and gives CO and propene via a complex free radical path. 

Cycloadditions of ketenes with olefins follows orbital symmetry requirements.2 Knoche3 has reported the cycloaddition of dichloroketene to butyne-2 (yield 12%). 

We begin the discussion of concerted cycloaddition reactions by exploring how the orbital symmetry requirements distinguish between reactions that are favorable and those that are unfavorable. Cycloaddition reactions that occur through a pericyclic concerted mechanism can be written as a continuous rearrangement of electrons. If we limit consideration to conjugated systems with from two to six tt electrons, the reactions shown in Scheme 10.1 are conceivable. 

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. 

Orbital symmetry requirements, of course without violations but with quantitatively limited Influence ... 

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