Steric course of zwitterionic ketene cycloaddition

The initial stabilizing interaction is between the HOMO of the ethylene and the LUMO of the ketene, the localized ttJq orbital that lies in the same plane as the substituents on its terminal C atom. [58, Fig. 3, p. 360] Of the four such mutual orientations, only the one depicted at the left of Fig. 6.8 fulfils two requirements  

1) The electropositively substituted carbon atom of the alkene (Ci) and the oxygen atom of the ketene are diagonally disposed across the rectangle  

2) The ketene s less bulky substituent (S), as expressed by a less negative value of Taft s steric parameter [59], [60, Chap. 4], faces its reaction 

As the ketene glides in plane relative to its reaction partner, it executes an additional motion that can be described as a superposition of two displacements (i) an out-of-plane translation to the side of the less bulky of the two substituents on Ci and (ii) rotation about its own CC bond to further reduce repulsion between these two substituents and, at the same time, to allow progressive localization of negative charge on the substituted carbon atom of the ketene moiety, and thus to stabilize the zwitterion in the proper conformation for its eventual closure. 

There is an additional steric effect that affects the rate of formation of the intermediate. As the bond to C2 closes, the carbonyl oxygen atom is brought against the substituent cis to the smaller of the two substituents on Ci in our example it is the one trans to the alkoxy group. As a result, when its steric requirements are greater than that of the second substituent on C2 the cis-substituted vinyl ether reacts more rapidly than the trans an effect that increases with increasing bulk of R . 

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Figure 6.8. Steric course of zwitterionic ketene cycloaddition and...

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