Molecular orbital theory Alkenes

Correlation of the effect of substituents on the rates of reactions with early transition states often is best accomplished in terms of perturbational molecular orbital theory, and polar effects can play a major role for such reactions [100, 101]. Essentially this theory states that energy differences between the highest occupied molecular orbital (HOMO) of one reactant and the lowest unoccupied molecular orbital (LUMO) of the other reactant are decisive in determining the reaction rate the smaller the difference in energy, the faster the predicted rate of reaction [102,103]. Since the HOMO of a free radical is the SOMO, the energy difference between the SOMO and the alkene HOMO and/or LUMO is of considerable importance in determining the rates of radical additions to alkenes [84],... 

In the beginning, Ken created a frontier molecular orbital (FMO) theory of regioselectivity in cycloadditions. In particular, his classic series of papers showed how FMO theory could be used to understand and predict the regioselectivity of 1,3-dipolar cycloadditions. Ken s generalizations about the shapes and energies of frontier molecular orbitals of alkenes, dienes, and 1,3-dipoles, are in common use today and they appear in many texts and research articles. 

The classical Dewar-Chatt-Duncanson model for metal-alkene bonding has been revisited with a combination of X-ray structural data (see Diffraction Methods in Inorganic Chemistry) and DFT calculations (see Molecular Orbital Theory), particularly on complexes of the type (acac)Rh(alkene)2. These indicate the existence of distortions from idealized geometry involving a twist (127), where the axis of the double bond is no longer perpendicular to the molecular plane and a roll (128), where the line... 

The rates of 1,3-dipolar cycloadditions of diazoalkanes to alkenes and alkynes have been determined electron-attracting substituents in the latter increase the rate, in accordance with frontier molecular orbital theory, which predicts that these reactions are controlled by the interaction of the highest occupied molecular orbital of the diazo-compound with the lowest unoccupied molecular orbital of the dipolarophile " the kinetics of the reactions of methyl diazoacetate or phenyl diazomethanesulphonate, on the other hand, give rise to U-shaped activity functions, which is also explained by the theory. Diazomethane or... 

C=C unit of an alkene by what is known as 1,3-dipolar addition (a cycloaddition reaction—a pericyclic reaction governed by molecular orbital theory, which is introduced in Chapter 24, Section 24.1). A discussion of this mechanism will not be presented, but to explain dihydroxylation, it is necessary to understand that the initial reaction converts the alkene to the cyclic product, 126, by a concerted process. 

The names associated with this reaction are taken from the two scientists who developed it, Otto Diels (Germany 1876-1954) and Kru-t Alder (Germany 1902-1958). They were awarded the Nobel Prize in 1950, and the reaction is called the Diels-Alder reaction to honor their work. Many years of work led to a new mechanism that explains reactions of this type, based on what is known as frontier molecular orbital theory. This mechanism is based on the interaction of Tt-electrons in the diene with those of the alkene. Kenichi Fukui (Japan 1918-1998) was awarded the Nobel Prize in 1981 for his contributions in this area. 

Clearly, the reaction of 2 and butadiene is much faster than the reaction of ethene because it does not require high temperatures and high pressure the reaction with 5 is slower. According to frontier molecular orbital theory, a mechanistic explanation of these experimental observations must include a discussion of the molecular orbitals of 1,3-butadiene and ethene (see Figure 24.2) compared with the molecular orbitals of 2 and 5. Figure 24.2 shows the energy level of the molecular orbitals but does not show the actual orbitals. Indeed, only the energy value is required once it is known that the HOMO of the diene reacts with the LUMO of each alkene. 

The lone electron of the allyl radical is associated with the rr-nonbonding MO, which places electron density on carbons 1 and 3 only. This localization is shown clearly in the unpaired electron density map in Figure 8.6. Thus, both the resonance model and molecular orbital theory are consistent in predicting radical character on carbons 1 and 3 of the allyl radical but no radical character on carbon 2, consistent with the experimental observation. Importantly, when there is a difference, the reaction will occur to generate the alkene product that is most stable—in other words, with the more highly substituted double bond. 

We have noted several times in this book that resonance structures are inherently a valence bond theory (VBT) concept. Molecular orbital theory (MOT) does not require such structures. Hence, there are MOT bonding concepts that describe the bonding pictures given above for alkenes, alkynes, and CO. A simple MOT picture is given in the following Going Deeper highlight. 

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). 

The protocol developed by Jacobsen and Katsuki for the salen-Mn catalyzed asymmetric epoxidation of unfunctionalized alkenes continues to dominate the field. The mechanism of the oxygen transfer has not yet been fully elucidated, although recent molecular orbital calculations based on density functional theory suggest a radical intermediate (2), whose stability and lifetime dictate the degree of cis/trans isomerization during the epoxidation <00AG(E)589>. 

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