Frontier orbitals ketene

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.

Frontier Orbital Interactions in the Transition State of One-Step [2+2]-Cycloadditions Involving Ketenes... 

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... 

Fig. 15.13. Frontier orbital interactions in the transition state of the one-step [2+2]-cycloaddition of ketene and ethene.

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 energies and coefficients of the frontier orbitals of ketene are shown in Fig. 6.39. The regioselectivity in the reaction between cyclopentadiene and dichloroketene giving the cyclobutanone 6.249 is explained by the overlap from the large LUMO coefficient on the central atom of the ketene and the larger coefficient at C-l in the HOMO of the diene. 

The cycloaddition of ketenes to carbonyl compounds also shows the expected regioselectivity. Both HOMO,keI() c/LlJMO(kctcnc) and LUMO(ketone)/ HOMO(ketcnc) interactions may be important, but they lead to the same conclusions about regioselectivity. Lewis acid catalysis is commonly employed in this reaction presumably the Lewis acid lowers the energy of the LUMO of the ketene (or that of the ketone) in the same way that it does with dienophiles. Ketenes also dimerise with ease, since they are carbonyl compounds. The regiochemistry, whether it is forming a /3-lactone 6.256, 6.257 or a 1,3-cyclobutanedione 6.258, is that expected from the frontier orbitals of Fig. 6.39. 

The regiochemistry of the reaction between an n. T-unsaturated ketone and a ketene is the opposite, even in an intramolecular reaction 8.53 —> 8.54, which involves substantial twisting and ring strain. This time it is in the sense easily explicable by the frontier orbitals. The LUMO of the unsaturated ketone and the LUMO of the ketene show that the initial bonding will be between the [3 carbon of the enone and the carbonyl carbon of the ketene, and there is an orthogonal orbital on the other carbon atom of the ketene able to complete the cycloaddition, just as we saw earlier (see pages 212 and 253) for thermal reactions. 

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. 

Ketenes also dimerise with ease, since they are carbonyl compounds, and the regiochemistry, whether it is forming a /3-lactone 6.386 or a 1,3-cyclobutanedione 6.387, is that expected from the frontier orbitals of Fig. 6.51.886... 

Site selectivity in ketene cycloadditions is also explained by the frontier orbitals. Diphenylketene reacts with isoprene 6.397 mostly at the more substituted double bond to give the cyclobutanone 6.398 as the major product.890 In contrast, it reacts with cw-piperylene 6.399891 and with cw-butadiene-l-nitrile 6.400890 at the less substituted double bond. In all three cases the site of attack is the double bond having the largest coefficient in the HOMO. 

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,... 

Thermal [2+2]-cycloaddition reactions are less common, but photochemical [2+2]-cycloaddition reactions are very common. This fact can be explained by analyzing these cycloaddition reactions using Woodward-Hofifmann selection rules. In frontier orbital approach, the thermal reaction of two ethene molecules (one is HOMO and other is LUMO) is orbital symmetry forbidden process for its suprafacial-suprafacial [7t s+7t s]-cycloaddition, but a suprafacial-antarafacial [jt s+jt a]-cycloaddilion reaction is symmetry allowed process (Fig. 3.1). It signifies that the cycloaddilion of one two-7t electron system with another two-ji electron system will be a thermally allowed process when one set of orbitals is reacting in a suprafacial mode and other set in an antarafacial mode ( s means suprafacial and a means antarafacial). Thermal [7t s+Ji a]-reactions usually occur in the additions of alkenes to ketenes, when alkene is in the ground state and ketene in the excited state [1] (Fig. 3.2). 

Fig. 3.2 Frontier orbital interactions of thermally allowed antarafacial interaction of a ketene (LUMO) and an olefin (HOMO)...

The frontier-orbital analyses of 12 types of ketene 2 - - 2-cycloadditions have been reported. All the transition-state geometries obtained in this study are similar but the extent of the formation of the two covalent bonds differs appreciably. The... 

Reactive Enophile in [4 + 2] Cycloadditions. Vinylketenes are not effective as dienes in Diels-Alder reactions because they undergo only [2 + 2] cycloaddition with alkenes, as predicted by frontier molecular orbital theory. However, silylketenes exhibit dramatically different properties from those found for most ketenes. (Trimethylsilyl)vinylketene (1) is a relatively stable isolable compound which does not enter into typical [2 + 2] cy do additions with electron-rich alkenes. Instead, (1) participates in Diels-Alder reactions with a variety of alkenic and alkynic dienophiles. The directing effect of the carhonyl group dominates in controlling the regiochemical course of cycloadditions using this diene. For example, reaction of (1) with methyl propiolate produced methyl 3-(trimethylsilyl)sahcylate with the expected regiochemical orientation. ProtodesUylation of this adduct with trifluoroacetic acid in chloroform (25 °C, 24 h) afforded methyl salicylate in 78% yield (eq 2). 

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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