HOMO-LUMO interactions in the 2 2 cycloaddition of an alkene and a ketene

Ketenes are especially reactive in 2 + 2 cycloadditions because they offer a low degree of steric hindrance at one center—the carbonyl group—and a low-energy LUMO. 

Cyclobutanes can also be formed by nonconcerted processes involving zwit-terionic intermediates. The combination of an electron-rich alkene (enamines, enol ethers) and an electrophilic one (nitroalkenes, cyano-substituted alkenes) is required for such processes. 

In reactions with tetracyanoethylene, the stereochemistry of the double bond of an enol ether is retained in the cyclobutane product when the reaction is carried out in nonpolar solvents. In polar solvents, cycloaddition is nonstereospecific in accordance with a longer lifetime for the zwitterionic intermediate.  

Photochemical cycloadditions provide a method that is often complementary to thermal cycloadditions with regard to the types of compounds that can be prepared. The theoretical basis for this complementary relationship between thermal and photochemical modes of reaction lies in orbital-symmetry relationships and was discussed in detail in Part A, Chapter 10. 

The reaction types permitted by photochemical excitation that are particularly useful for synthesis are 2 + 2 additions between two carbon-carbon double bonds, and 2 + 2 addition of alkenes with carbonyl compounds leading to oxetanes. Many 

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Fig. 6.13. HOMO-LUMO interactions in the [2 + 2] cycloadditions of an alkene and a ketene (a) frontier orbitals of the alkene and ketene (b) [2tts + 2ttJ representation of suprafacial addition to the alkene and antarafacial addition to the ketene (c) [2tts + (2tts + 2tts)] alignment of orbitals.

Figure 7.4. HOMO-LUMO interactions in the 2 + 2 cycloaddition of an alkene and a ketene.

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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