Reactions of Ketenes and Alkenes

Ketenes are especially reactive in [2 + 2] cycloadditions and an important reason is that they offer a low degree of steric interaction in the TS. Another reason is the electrophilic character of the ketene LUMO. As discussed in Section 10.4 of Part A, there is a large net charge transfer from the alkene to the ketene, with bond formation at the ketene sp carbon mnning ahead of that at the sp2 carbon. The stereoselectivity of ketene cycloadditions is the result of steric effects in the TS. Minimization of interaction between the substituents R and R leads to a cyclobutanone in which these substituents are cis, which is the stereochemistry usually observed in these reactions. 

163 For reviews, see W. T. Brady, in The Chemistry of Ketenes, Allenes, and Related Compounds, S. Patai, ed., Wiley-Interscience, New York, 1980, Chap. 8 W. T. Brady, Tetrahedron, 37, 2949 (1981) J. Hyatt and R. W. Reynolds, Org. React., 45, 159 (1994) T. T. Tidwell, Ketenes, Wiley, New York, 1995. 

Concerted Cycloadditions, Unimolecular Rearrangements, and Thermal Eliminations 

The best yields are obtained when the ketene has an electronegative substituent, such as halogen. Simple ketenes are not very stable and must usually be generated in situ. The most common method for generating ketenes for synthesis is by dehydrohalo-genation of acyl chlorides. This is usually done with an amine such as triethylamine.167 Other activated carboxylic acid derivatives, such as acyloxypyridinium ions, have also been used as ketene precursors.168 Ketene itself and certain alkyl derivatives can be generated by pyrolysis of carboxylic anhydrides.169 

Intramolecular ketene cycloadditions are possible if the ketene and alkene functionalities can achieve an appropriate orientation.170 

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Cl2C=C=0 is -9.15 eV. Clearly, the presence of these groups raises the energy of the HOMO and would be expected to increase the rate of a cycloaddition driven by the LUMO of an alkene. This has been noted in many intermolecular reactions of ketenes and alkenes.259 xhis rate enhancement is apparent in the attempted intramolecular cyclization (see below) of 316 (X = H), which failed. When the a-chloro analog (316, X = Cl) was prepared, however, treatment with triethylamine led to 317, which cyclized to give a 55% yield of the [2+2]-cycloadduct (318), along with 19% of the ene adduct (319-see sec. 11.13) where X = Cl. 

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