Mechanisms Generating Optical Activity

The dipole coupling requires two chromophoric groups having transitions with significant electric transition moments that couple via dipole-dipole interactions. 

The /z-m mechanism is similar in concept, but assumes interaction of a transition dipole within one group with a transition quadrupole (magnetic moment) within the other group. It is much more difficult to recognize and to apply, n-n transitions coupling with n-n transition in oligopeptide molecules depict such a situation [81, 82]. 

The above mechanisms generating optical activity are not exactly quantitative and they are not additive. They can serve as a great way to explain simple chiral situations. They have many limitations and must be used carefully. Tltey cannot be used for chromophores that are inherently chiral. However, complete methods of calculations for rather complex molecules based on this concept and a detailed 

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Much work has also been conducted on the hydrosilylation of ketones and imines (Equation 16.16). The products from these reactions are silyl ethers and sdylamines. These additions of silanes across C-X ir-bonds have been conducted predominantly for the purpose of generating optically active alcohols and amines after hydrolysis. Because the mechanism of these reactions is less defined than the mechanism of alkene hydrosilylation, and this chemistry lies outside the theme of this chapter, the hydrosilylation of ketones and imines is presented only briefly. Instead, this chapter provides an overview of the scope and motivation for the hydrosilylation of alkenes and alkynes and provides details on the mechanisms of these reactions catalyzed by complexes of various metals. Several comprehensive reviews of the scope of these reactions have been published. ... 

In addition, the results of such reactions have suggested plausible models for the mechanism of abiotic generation of optical activity, including an autocatalytic feedback mechanism (261). The latter involves random development of chiral crystals from achiral starting material, and solid-state reaction leading to products in which one enantiomer is in excess and thus can bias subsequent further crystallization (262). 

Methods for the generation and use of enantiomerically pure vinylketene complexes have been well developed in recent years. This has allowed stereocontrol to be exerted over subsequent reactions to yield optically active organic or organometallic products. It has also allowed investigations to be carried out into the reaction mechanisms of several fundamental processes. This area is currently ripe for further exploration and would be of particular interest in its application to other metal systems. We trust that the work presented here will serve as a platform for further new and exciting discoveries. 

In the first step, a cyclic intermediate is generated by a suprafacial addition, followed by a SN2-type ring opening (e.g., halogenation or epoxidation ring opening). In this manner, olefin 1 may either be converted into diastereomer 2 or 3, which may be optically active or racemic. Mechanism control thus means that the relative, but not the absolute, configuration of the two vicinal centers is defined. 

Now lets consider the rotational strength of the n-n transition for one example generating the optical activity through the mechanism. Thus the rotational strength is given by ... 

An important consequence of the mechanism of the rearrangement step is that if the migrating group R is attached to the amide carbonyl via a chiral carbon atom the configration is retained in the product, thus generating a fully optically active amine from optically active starting material (e.g. 2-phenylpropanamide). 

Carbocations contain sp hybridized orbitals and thus have planar structures. S 1 mechanisms proceed via a carbocation intermediate, so a nucleophile attack is equally possible from either side of the plane. Therefore, a pure, optically active alkyl halide undergoing an S 1 substitution reaction will generate a racemic mixture as a product, as shown in Figure 3-6. 

Because the geometry of the 9-double bond was not clear at that time, Corey et al. 75> tried to prepare the (Z)-9-isomer as well as the ( )-9-isomer of leukotriene-A (78 and 86). In the synthesis of the former isomer the tribenzoyl derivative of D-(—)-ribose (79) was converted in 8 stepy into the optically active epoxyaldehyde 71 and the latter to 72. 72 was olefmated with ylide 82, generated by treatment of the corresponding phosphonium mesylate with lithium diisopropylamide in THF/ HMPA75) (Scheme 15). In the first olefination step 72+82- 78, however, similar to the first method, a A9-isomer mixture was formed. The loss of (Z)-selectivity of the Wittig reaction is due to the use of conjugated unsaturated, i.e. moderate ylides of type 82, and had to be expected because of the mechanism of the Wittig reaction (see Sect. 2). 

In contrast to the trans-disubstituted phenylcyclopropanes cis-disubstituted phenylcyclopropanes, such as 23 and 30, do not show any detectable CD signals a. This is reasonable, if the optical activity of the two lowest energy electronic bands of phenylcyclopropanes are generated essentially by the coupled-oscillator mechanism (exciton mechanism for the degenerate case, such as 19). In cis-phenylcyclopropanes the interacting transition moments will lie (approximately) in the same plane. Hence, in the coupled-oscillator mechanism there will be only a very small Cotton effect (which is zero, if... 

The relative extent of dialkylation depends on the electrophilicity of RX (and the nucleophilicity of AR ) when a realtively fast SET (AE i/2 < 0.5 V) is the primary reaction. Other mechanisms may also satisfactorily explain the distribution of products. For instance, adduct formation between the alkyl radical and the mediator (acting as a radical trap) is possible and must be considered in such a case, further reduction of AR may take place, either by electron transfer or by abstraction of a hydrogen atom from the solvent. However, let us keep in mind that radical anions or dianions may act as nucleophiles, since a partial inversion of configuration of some optically active secondary RX compounds has been found [222] after workup under experimental conditions similar or identical to those of the electrolyses. Table 8 exemplifies alkylation reactions following a SET. The reaction scheme may be complicated by the fact that reduced forms of the mediator may act as a reducing nucleophile toward RX. The SET may then be assumed as the rate-determining step in aliphatic nucleophilic substitutions [223], and/or R generated in solution may be added to an electrophilic mediator, such as an activated ketone [224]. 

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