Generation of dipolar intermediates from small rings

Among the cycloaddition reactions that have been shown to have some generality and synthetic utility are the 2 + 2 cycloadditions of ketenes with alkenes. These 

Illustrations of other 1,3-dipoles that are believed to form from ring-opening of strained rings are given in Table 7.4. 

For a review, see W. T. Brady, in The Chemistry of Ketenes Allenes, and Related Compounds, S. Patai (ed.), John Wiley and Sons, New York (1980), Chap. 8. 

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.  

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Scheme 6.5. Generation of Dipolar Intermediates from Small Rings... 

Generation of dipolar intermediates from small rings... 

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