Alkenes conjugated polyenes

There are a variety of methods for the computation of the MOs that interact in the transition states of [4+2]-cycloadditions. The LCAO method (linear combination of atomic orbitals) is often employed, and the basic idea is as follows. The MOs of the -systems of alkenes, conjugated polyenes, or conjugated polyenyl cations, radicals, or anions all are built by so-called linear combinations of 2p AOs. In a somewhat casual formulation, one might say that the MOs of these -systems are constructed with the help of the 2pz AOs. These AOs are centered at the positions of the n C atoms that are part of the -system. LCAO computations describe a conjugated -electron system that extends over n s/ 2-hybridized C atoms by way of n Ji-type MOs. 

General reactivity trends for alkenes were established for hydrozirconahon by way of qualitative studies terminal alkene > internal alkene > exocyclic alkene > cyclic alkene trisubshtuted alkene. The rate of hydrozirconahon decreases with increasing substitution on the alkene. This property was used for selechve monohydrozir-conation of conjugated and non-conjugated polyene derivahves (Scheme 8-8) [84-86]. 

Conjugated acyclic dienes (in contrast to cyclic dienes. Section 9.1.3.2), like alkenes conjugated to electron-withdrawing groups, give only linear oligomerization, resulting in acyclic polyene products. 

The hydrogenation of alkenes conjugated to aromatic systems is usually much less difficult than partial reduction of polyenes, as the aromatic ring is less susceptible to reduction. Many catalysts of Group 10 metals have been used for this transformation, including Raney nickel, Pd adsorbed on carbon, Pd adsorbed on calcium carbonate, Pd on barium sulfate, metallic palladium, PtOa, chloroplatinic acid or platinum metal. Normally, the stereochemistry of the reduced product arises from syn addition of a hydrogen molecule to the less-hindered face of the double bond, as exemplified in the catalytic hydrogenation of a steroidal styrene over metallic palladium (Scheme 93). ... 

Conjugated Polyenes Table 3.4 contains examples of conjugated alkenes with the major site of reaction indicated. This table also included some examples of functionalized polyenes. 

Simple alkenes do not absorb ambient light, but may react upon photosensitization by an added sensitizer (which may be an impurity or a component of the drug formulation). As an example, photosensitized oxidation may be quite efficient. Conjugated alkenes and polyenes absorb strongly in the UV (in some cases also in the visible) and photoreact. 

The photochemistry of alkenes, dienes, and conjugated polyenes in relation to orbital symmetry relationships has been the subject of extensive experimental and theoretical studyThe analysis of concerted pericyclic reactions by the principles of orbital symmetry leads to a complementary relationship between photochemical and thermal reactions. A process that is forbidden thermally is allowed photochemically and vice versa. The complementary relationship between thermal and photochemical reactions can be illustrated by considering some of the reaction types discussed in Chapter 10 and applying orbital symmetry considerations to the photochemical mode of reaction. The case of [2Tr- -2Tr] cycloaddition of two alkenes, which was classified as a forbidden thermal reaction (see Section 10.1), can serve as an example. The correlation diagram (Figure 12.17) shows that the ground state molecules would lead to a doubly excited state of cyclobutane, and would therefore involve a prohibitive thermal activation energy. 

In the absence of other factors, therefore, the adsorption energies of a group of isomeric conjugated polyenes or phenyl alkenes should increase as the number of Irons double bonds increases or the number of cis double bonds decreases. Jacques and Kagan 41) have summarized the relative adsorption affinities of several compounds of this type ... 

For distinguishing cis/trans isomers of conjugated polyenes or of alkenes, Randic et and then Pogliani started from the observation that in an s-cis butadi-... 

Collisional energy transfer is frequently called sensitization. It is an important tool in photochemistry because it provides access to excited triplet states of compounds whose singlets do not intersystem cross efficiently (such as alkenes and conjugated polyenes). It also provides access directly to the triplet states of compounds for determination of the multiplicity of the state responsible for a photochemical reaction. 

Molecular Orbitals of Alkenes and Conjugated Polyene Systems Molecular Orbitals of Conjugated Ions or Radicals... 

MOLECULAR ORBITALS OF ALKENES AND CONJUGATED POLYENE SYSTEMS... 

In order to understand and explain the results of the various pericychc reactions on the basis of different theoretical models, a basic understanding of the molecular orbitals of the molecules, particularly those of alkenes and conjugated polyene systems and their symmetry properties, is required. 

There are two independent symmetry elements, viz., mirror plane, m, and twofold axis, C2, that are used to characterize various molecular orbitals of alkenes or conjugated polyene systems. 

As shown in Figure 1.13, irradiation of an alkene or conjugated polyene system promotes an electron from its ground state HOMO to the ground state LUMO, which then becomes the highest occupied molecular orbital in the excited state, for example, W3 of butadiene becomes HOMO upon excitation of an electron from W2 to W3 on irradiation. 

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