Diels-Alder reaction orbital analysis

A TT MOLECULAR ORBITAL ANALYSIS OF THE DIELS-ALDER REACTION... 

Frontier orbital analysis is a powerful theory that aids our understanding of a great number of organic reactions Its early development is attributed to Professor Kenichi Fukui of Kyoto University Japan The application of frontier orbital methods to Diels-Alder reactions represents one part of what organic chemists refer to as the Woodward-Hoffmann rules a beautifully simple analysis of organic reactions by Professor R B Woodward of Harvard University and Professor Roald Hoffmann of Cornell University Professors Fukui and Hoffmann were corecipients of the 1981 Nobel Prize m chemistry for their work... 

The same conclusions are drawn by analysis of the frontier orbitals involved in cycloadditions. For the most common case of the Diels-Alder reaction, which involves dienophiles with electron-attracting substituents, the frontier orbitals are l/2 of the diene (which is the HOMO) and n of the dienophile (which is the LUMO). Reaction occurs by interaction of the HOMO and LUMO, which can be seen from the illustration below to be allowed. 

Orbital Interaction Analysis. An orbital interaction diagram for the Diels-Alder reaction is shown in Figure 12.5a. The geometry of approach of the two reagents which ensures a maximum favorable interaction between the frontier MOs (dashed lines) preserves a plane of symmetry at all separations. The MOs are labeled according to whether they are symmetric (S) or antisymmetric (A) with respect to reflection in the plane. Simultaneous overlap of both HOMO-LUMO pairs is a necessary feature of all peri-... 

The photochemical dimerization of unsaturated hydrocarbons such as olefins and aromatics, cycloaddition reactions including the addition of 02 ( A ) to form endoperoxides and photochemical Diels-Alders reaction can be rationalized by the Woodward-Hoffman Rule. The rule is based on the principle that the symmetry of the reactants must be conserved in the products. From the analysis of the orbital and state symmetries of the initial and final state, a state correlation diagram can be set up which immediately helps to make predictions regarding the feasibility of the reaction. If a reaction is not allowed by the rule for the conservation of symmetry, it may not occur even if thermodynamically allowed. 

Diels-Alder reaction, 169-170 aromatic TS, 151 benzyne, 160 butadiene + ethylene, 169 diastereoselectivity, 292 interaction diagram, 169 orbital analysis, 169-170 orbital correlation diagram, 198, 201 reverse demand, 169 substituent effects, 169-170 Diethyl tartrate, 11 Difluorocarbene ( CF2), 115... 

A third reason for the interest in the 2+2, in particular, is that it allows for a logical analysis of orbital symmetry arguments. Few photochemical reactions are as predictable as the Diels-Alder reaction, but the 2+2 photocycloaddition is a good starting point for comparison. 

One reason that the Diels-Alder reaction goes so well is that the transition state has six delocalized 71 electrons and thus is aromatic in character, having some of the sPec stabilization of benzene. You could look at it as a benzene ring having all its 7t bonds but missing two o bonds. This simple picture is fine as far as it goes, but it is incomplete. We shall retuf 110 a more detailed orbital analysis when we have described the reaction in more detail. 

We have recently argued [39] that facial selectivity in the Diels-Alder reactions of 5-substituted cyclopenta-1,3-dienes is a reflection of hyperconjugative effects a frontier orbital analysis is shown in Fig. 6-10 [40], The molecule adopts a conformation where the better electron-donating group (C-Me rather than C-X) hinders approach of the dienophile from that face. Overlap of the C-C (T-bond (a better donor than C-X) increases the energy of the diene HOMO. Furthermore overlap of the C-C (T -orbital with the diene LUMO yP reduces the energy of the... 

A TT Molecular Orbital Analysis of the Diels-Alder Reaction... 

Frontier orbital analysis of a [4 -I- 2] cycloaddition reaction shows that overlap of in-phase orbitals to form the two new a bonds requires suprafacial orbital overlap (Figure 29.5). This is tme whether we use the LUMO of the dienophile (a system with one TT bond Figure 29.1) and the HOMO of the diene (a system with two conjugated rr bonds Figure 29.2) or the HOMO of the dienophile and the LUMO of the diene. Now we can understand why Diels-Alder reactions occur with relative ease (Section 8.8). 

Diels-Alder reaction, 382, 392—393 of benzyne, 931-932 orbital symmetry analysis of, 388—390 Dienes. See also Alkadienes... 

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