Orbital interactions ketenes + alkenes

The psudoexcitation preferentially occurs in ketenes. The energy gap is smaller between and of ketenes than between n and it of alkenes. The orbital of ketene is raised in energy by the interaction with the n orbital on the carbonyl oxygen above n of alkenes. The orbital of ketenes is lower in energy than n of alkenes. The pseudoexcitation is preferred in ketenes and occurs through the interaction. The [2h-2] cycloaddition reactions take place across the C=C bond of ketenes rather than C=0 bond. 

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.

In contrast to the [4+2]-additions of butadiene to ethene or acetylene (Figures 15.8 and 15.9), the two HOMO/LUMO interactions stabilize the transition state of the [2+2]- addition of ketenes to alkenes to a very different extent. Equation 15.2 reveals that the larger part of the stabilization is due to the LUMOketene/HOMOethene interaction. This circumstance greatly affects the geometry of the transition state. If there were only this one frontier orbital interaction in the transition state, the carbonyl carbon of the ketene would occupy a position in the transition state that would be perpendicular above the midpoint of the ethene double bond. The Newman projection of the transition state (Figure 15.11) shows that this is almost the case but... 

Cycloadditions with reactive ketenes therefore can he observed only when they are prepared in situ and in the presence of the alkene to which they shall he added. Dichloroketene generated in situ is the best reagent for intermolecular [2+2]-cycloadditions. Dichloroketene is poorer in electrons than the parent ketene and therefore more reactive toward the relatively electron-rich standard alkenes. The reason is that the dominating frontier orbital interaction between these reactants involves the LUMO of the ketene, not its HOMO (see Section 15.2.4). 

The frontier orbital treatment for vinyl cation cycloadditions, such as those of ketenes, has some merits. It satisfyingly shows that the bond forming between C-l and C-l develops mainly from the interaction of the LUMO of the ketene (n of the C=0 group) and the HOMO of the alkene 6.178, and that the bond between C-2 and C-2 develops mainly from the interaction of the HOMO of the ketene (i/j2 of the 3-atom linear set of orbitals analogous to the allyl anion) and the LUMO of the alkene 6.179. 

An interesting situation arises in the case of [tt s + rr a] cycloaddition, where the ground state interaction allows the reaction to proceed under thermal conditions. The reaction is suprafacial with respect to the HOMO component and antarafacial with respect to the LUMO component. Although this reaction is favorable in terms of overlap of the orbitals, it is not a common reaction because of geometric reasons. The requirement of substituents on one alkene to be oriented directly toward the molecular plane of the second alkene is rather a sterically unfavorable situation (Figure 4.9). It occurs in some very specific cases (for example, ketenes) where steric congestion is reduced. 

Although the applicability of the frontier orbital theory is very broad indeed, it is nevertheless necessary to be aware of the fact that the nature of this approach is still only approximate so that in certain cases some exceptions cannot be ruled out. The origin of these eventual failures was thoroughly discussed by Dewar in the study [51], where it was demonstrated that the greatest potential weakness of the approach consists in the very assumption attributing the decisive role only to interactions between the HOMO and LUMO orbitals of the individual components. It appears, namely, that this assumption need not be satisfied in all cases, and if this happens, the predictions of frontier orbital theory may fail. The typical example in this respect is, e.g., the addition of electron rich alkenes to ketenes, which is not, as demonstrates the formation of cyclobutanone instead of the expected a-methyleneoxetane,... 

Frontier molecular orbital (FMO) treatment of these reactions indicates that bond formation between C -1 and C -1 of ketene and olefin is due to interaction of HOMO of alkene and LUMO of Ketene. At the same time bond formation between C - 2 of olefin and C - 2 of ketene is by the coupling of HOMO of ketene and LUMO of alkene as shown below ... 

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