Cycloadditions and Other Reactions Leading to Cyclobutanes

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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Cyclobutanes may be converted to alkenes thermally, the reverse of the [2 + 2] cycloaddition reaction. These retroaddition or cycloreversion reactions have important synthetic applications and offer further insights into the chemical behavior of the 1,4-diradical intermediates involved they may proceed to product alkenes or collapse to starting material with loss of stereochemistry. Both observations are readily accommodated by the diradical mechanism. Generation of 1,4-tetramethylene diradicals in other ways, such as from cyclic diazo precursors, results in formation of both alkenes and cyclobutanes, with stereochemical details consistent with kinetically competitive bond rotations before the diradical gives cyclobutanes or alkenes. From the tetraalkyl-substituted systems (5) and (6), cyclobutane products are formed with very high retention stereospecificity,while the diradicals generated from the azo precursors (7) and (8) lead to alkene and cyclobutane products with some loss of stereochemical definition. ... 

Two major side reactions compete with the coupling reaction protonation of the radical anion followed by further reduction and protonation leading to the saturated dihydro product, and polymerization induced by the basic dianion formed by coupling of two radical anions. Other, less typical reaction pathways include reaction between a radical anion and a molecule of substrate. Scheme 2, dimerization of two radicals formed by protonation of the initial radical anion. Scheme 3, or, infrequently, cleavage of the radical anion followed by coupling. However, for radical anions derived from monoactivated alkenes, the pathway in Scheme 2 has only been unequivocally established as a major pathway in a few cases in which the final zero-electron product is a cyclobutane, that is, a cycloaddition product. 

Cycloreversion Reactions A cycloreversion reaction is the reverse of a cycloaddition reaction and leads to the formation of the starting reactants through the cleavage of two bonds in the ring [18], A typical example is the formation of C2H4+ and neutral C2H4 from the cyclobutane radical cation. As shown in reaction (6.37), this reaction proceeds through the intermediacy of a distonic ion. The radical cations of a variety of other four-membered cyclic compounds, such as cyclobutanones (3), diketene (4), oxetane (5), cyclobutylamine (6), and thiocyclobutane (7), are known to participate in cycloreversion reactions [27]. 

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