Alkenes transition state symmetry

In contrast with the thermal [4 + 2] Diels-Alder reaction, the 2 + 2 cycloaddition of two alkenes to yield a cvclobutane can only be observed photo-chemically. The explanation follows from orbital-symmetry arguments. Looking at the ground-state HOMO of one alkene and the LUMO of the second alkene, it s apparent that a thermal 2 + 2 cycloaddition must take place by an antarafacial pathway (Figure 30.10a). Geometric constraints make the antarafacial transition state difficult, however, and so concerted thermal [2 + 2j cycloadditionsare not observed. 

The oxidation of the simplest symmetrically substituted alkene, ethylene, is noteworthy in that an asymmetric spiro transition state is observed. When constrained to Cs symmetry with eqnal forming carbon-oxygen bond lengths, the energy increases by only 0.1 kcalmol. The spiro TS has the plane of the HO—ONO (or peracid) at right angles to the axis of the C=C bond. In an idealized spiro TS this angle is exactly 90°. While the formation of snlfoxides from snlhdes by peroxynitrons acid is well-established , epoxidations have not yet been observed in solution. 

The syn-periplanar eliminations by pyrolysis of esters, xanthates, sulfoxides and amine oxides are symmetry-allowed. With respect to the alkene portion of the transition state, the centers presumably are or... 

The carbon-silicon bond has two important effects on the adjacent alkenc. The presence of a high-energy filled CT orbital of the correct symmetry to interact with the n system produces an alkene that is more reactive with electrophiles, due to the higher-energy HOMO, and the same ff orbital stabilizes the carbocation if attack occurs at the remote end of the alkene. This lowers the transition state for electrophilic addition and makes allyl silanes much more reactive than isolated alkenes. 

In a study of reactivity and stereoselectivity in norbornenes and related alkenes, the observation of pyramidalized alkene carbons led Ken to the discovery of a general pattern — alkenes with no plane of molecular symmetry pyramidalize so as to give a staggered arrangement about the allylic bonds. Subsequent studies showed that there is a similar preference for staggering of bonds in transition states. 

The four-membered cyclic transition state is not allowed by orbital symmetry theory and parity rules. It requires inversion of configuration at the a-carbon and trans addition to the alkene by a conrotatory process, which is sterically impossible [261,263]. The six-membered transition state is allowed by parity rules, but the relative contributions of this pathway and that by unimolecular ionization depends on their relative rate constants and therefore their free energies of activation. Since the transition state of electrophilic addition to alkenes proceeds with a very late transition state requiring an electrophile with a highly developed charge, covalent species are not sufficiently polarized to react directly with alkenes. Thus, the reaction should occur in two steps rather than by a concerted addition [264],... 

The Eigen model requires two different hydrogen-bonded intermediates eorresponding to the two alkene produets that are formed. Eaeh intermediate leads to a different transition state, structures 2 and 3 drawn in Scheme 2. The former, 2, leads ultimately to 1-methylcyclopentene as the final product. This transition state has no overall molecular symmetry, but retains the methyl rotor as well as free rotation about the carbon-nitrogen single bond for a net symmetry number a = 9. Because transition state 2 is chiral, the reaction path degeneracy is equal to (18/ ) = 4. 

Consideration of the HOMO-LUMO interactions also indicates that the [2Tr+2Tr] addition is allowed photochemically. The HOMO in this case is the excited alkene tt orbital. The LUMO is the tt of the ground state alkene, and a bonding interaction is present between both pairs of carbons where new bonds must be formed. Similarly, the concept of aromatic transition states shows that the reaction has an antiaromatic 4tt combination of basis set orbitals, which predicts an allowed photochemical reaction. Thus, orbital symmetry considerations indicate that photochemical [2tt- -2tt] cycloaddition of alkenes is feasible. 

The addition of alkenes to dienes is a very useful method for the formation of six-membered carbocyclic rings. The reaction is known as the Diels-Alder reaction. The concerted nature of the mechanism was generally agreed on and the stereospecificity of the reaction was firmly established even before the importance of orbital symmetry was recognized. In the terminology of orbital-symmetry classification, the Diels-Alder reaction is a [ 4,+ 2 ] cycloaddition, an allowed process. The stereochemistry of both the diene and the alkene (the alkene is often called the dienophile) is retained in the cyclization process. The transition state for addition requires the diene to adopt the s-cis conformation. The diene and alkene approach... 

Cycloadditions of alkenes are rare and likely proceed in a stepwise fashion rather than by the concerted process implied in the equation. Figure 10.11 shows the interaction between the HOMO of one ethylene molecule and the LUMO of another. The carbons that are to become o-bonded to each other experience an antibonding interaction during cycloaddition, which raises the activation energy. The reaction is symmetry-forbidden. Reaction, were it to occur, would take place slowly and by a mechanism in which the two new a bonds are formed in separate steps rather than one involving a single transition state. 

The philicity of a carbene directly depends on the structure of the transition state of an addition reaction. The rules of orbital symmetry conservation forbid the least-motion C2v-symmetry reaction path [41]. For electrophilic carbenes, characterized by predominance of the n — p interaction, preferable is the so-called 7r-approach (Fig. 8.3). In the case of nucleophilic carbenes, optimum conditions for the overlap between the (Tcxy 7r -orbitals are provided by the asymmetrical cr-approach (Fig. 8.3b). By making use of certain assumptions, Rondan, Houk, and Moss [44, 45] calculated the overlap integrals Sjj between the corresponding frontier orbitals of carbene and alkene for the n- and the (7-approaches. Then, having computed the energies of those orbitals, they obtained the energies of stabilization of the composite system arising in two... 

To account for this transalkylidenation process a concerted pairwise exchange of alkylidene moieties was initially proposed, a quasi cyclobutane transition state being envisaged. According to Woodward—Hoffman rules, the synchronous formation and breaking of double bonds in this manner is thermally forbidden. However, the interaction of the metal d orbitals with the alkene orbitals in the transition state, was once thought to reduce the symmetry requirements of such a transformation. 

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