Ketenes reaction geometry with alkenes

Reactions of Ketenes, AUenes and Carbenes which Appear to be Forbidden. Some [2 + 2] cycloadditions only appear to be forbidden. One of these is the cycloaddition of ketenes to alkenes. These reactions have some of the characteristics of pericyclic cycloadditions, such as being stereospecifi-cally syn with respect to the double bond geometry, and hence suprafacial at least on the one component, as in the reactions of the stereoisomeric cyclo-octenes 6.110 and 6.112 giving the diastereoisomeric cyclobutanones 6.111 and 6.113. However, stereospecificity is not always complete, and many ketene cycloadditions take place only when there is a strong donor substituent on the alkene. An ionic stepwise pathway by way of an intermediate zwitterion is therefore entirely reasonable in accounting for many ketene cycloadditions. 

There is evidence that the reactions can take place by all three mechanisms, depending on the structure of the reactants. A thermal [ 2s + 2J mechanism is ruled out for most of these substrates by the orbital symmetry rules, but a lA + A] mechanism is allowed (p. 1212), and there is much evidence that ketenes and certain other linear molecules ° in which the steric hindrance to such an approach is minimal can and often do react by this mechanism. In a [ 2s + 2a] -cycloaddition the molecules must approach each other in such a way (Fig. 15.12a) that the + lobe of the HOMO of one molecule (1) overlaps with both + lobes of the LUMO of the other (11), even although these lobes are on opposite sides of the nodal plane of 11. The geometry of this approach requires that the groups S and U of molecule 11 project into the plane of molecule 1. This has not been found to happen for ordinary alkenes, ° but if... 

In contrast with the photochemical cycloaddition reaction of two alkenes, the [2+2] cycloaddition of a ketene and an alkene occurs under thermal conditions. The ketene is formed typically from an acid chloride and a mild base such as EtsN, or from an a-halo-acid chloride and zinc. Cycloaddition with an alkene occurs stereospecifically, such that the geometry of the alkene is maintained in the cyclobutanone product. The regioselectivity is governed by the polarization of the alkene, with the more electron-rich end of the alkene forming a bond to the electron-deficient central carbon atom of the ketene. Thus, the product from cycloaddition of dimethylketene with the enol ether Z-171 is the cyclobutanone m-172, whereas with -171, the isomer trans-lll is formed (3.116). ... 

The experimental observations relative to the cycloaddition of ethoxyketene to olefins are in accord with stereochemical predictions that can be made from analysis of Fig. 10.15. Isomeric products are obtained from cis- and trans-2-butene, indicating that the addition is stereospecific and consistent with a concerted reaction. The substituent on the olefin always becomes vicinal and cis to the ethoxy group in the cyclobutanone product. This is exactly the stereochemistry predicted from the geometry shown in Fig. 10.15 if the ethoxy group on the ketene and the largest substituent on the alkene are placed as far apart as possible ... 

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