Stereochemistry antarafacial process

Similarly, the rearrangement of the corresponding alcohol with anti configuration affords a 1 19 mixture of 12 and 13. Since both 12 and 13 have E configuration, the minor products must come from a net antarafacial process, probably by a nonconccrted, heterolytic mechanism384. This procedure has been used to establish the stereochemistry of the molecular fossil 13,16-dimethyloctacosane-l,28-diol. 

The two modes of addition and the associated stereochemistry resemble other addition reactions we studied earlier. The suprafacial addition is a concerted syn addition to one of the ti systems. The antarafacial addition corresponds to a concerted anti addition. Although anti addition reactions are common in the chemistry of alkenes, the two groups that add to the alkene are not bonded to each other in the transition state. In cycloaddition reactions, both atoms of the molecule that bond to the terminal atoms of the second molecule are also connected to each other. Thus, only if the number of atoms in each of the two molecules is quite large can one molecule add to the other in an antarafacial process. 

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,... 

According to the Woodward-Hoffmann rales, five concerted transition states are possible for the Claisen as well as the closely related Cope rearrangements chair, boat, twist, cross and plane (Table 6). Only the chair and boat TS have to be considered, as twist, cross and plane are antarafacial-anta-rafacial processes and require highly elevated temperatures. - For the correct prediction of product stereochemistry it is nevertheless crucial to know the preference for chair- or boat-like transition state in the actual 3,3-sigmatropic shift. 

A cycloaddition reaction can be classified not only by the number of electrons in the individual components, but also by the stereochemistry of the reaction with regard to the plane of the tt system of each reactant. For each component of the reaction, there are two possibilities the reaction can take place on only one side of the plane or across opposite faces of the plane. If the reaction takes place across only one face, the process is called suprafacial if across both faces, antarafacial. The four possibilities are shown in the following diagram ... 

These terms resemble the familiar ones syn and anti, but with this difference. Syn and anti describe the net stereochemistry of a reaction. We have seen anti addition, for example, as the overall result of a two>step mechanism. Suprafacial and antarafacial, in contrast, refer to actual processes the simultaneous making (or breaking) of two bonds on the same face or opposite faces of a component. 

The stereochemistry of such a process is termed conrotatory or antarafacial if the substituents at the interacting termini of the conjugated system both rotate in the same sense, for example,... 

If these reactions occur in uncatalyzed processes where bond breaking and bond formation are taking place concertedly, and not in two-step pathways via ionic or diradical intermediates, then the stereochemistry of these sigmatropic shifts can be predicted in a qualitative manner 1 -4. According to the Woodward-Hoffmann rules the thermally allowed reaction should take place in an antarafacial fashion across the allylic framework. The shifting hydrogen has to move from one side of the allylic plane to the other as depicted below. 

We may further extend the analysis of pericyclic reactions by considering that a single p orbital, denoted by the symbol m, can be a participant in a pericyclic reaction. In this analysis, one lobe of the p orbital makes up the top face of a one-atom n system, while the other lobe makes up the bottom face. The participation of a single p orbital is suprafacial if both cycloaddition processes involve only one of the two lobes of the p orbital, and it is antarafacial if the cycloaddition involves both. We may thus predict that the conrotatory opening of the cyclopropyl anion to an allyl anion (Figure 11.72) should take place via an -F 2 ] pathway. Conversely, the opening of the cation would be a -F 2 ] process, giving the opposite stereochemistry in the product." ... 

Among the cycloaddition reactions that have been shown to have general synthetic utility are the [2 + 2] cycloadditions of ketenes and alkenes. The stereoselectivity of ketene-alkene cycloaddition can be analyzed in terms of the Woodward-Hoffmann rules. To be an allowed process, the [ 2 + 2] cycloaddition must be suprafacial in one component and antarafacial in the other. Figure 6.5 illustrates the transition state. The ketene, utilizing the low-lying LUMO, is the antarafacial component and interacts with the HOMO of the alkene. The stereoselectivity of ketene cycloadditions can be rationalized in terms of steric effects in this transition state. Minimization of interaction between the substituents R and R leads to a cyclobutanone in which these substituents are cis. This is the stereochemistry observed in these reactions. 

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