Conformation and planar chirality

In this chapter, we have described the conformations and planar chirality of pillar[ ]arenes. In many cases, 1,4-dialkoxybenzene units in pillar[n]arenes... 

As described above, the stereochemical course of the reaction was proven to be accompanied by inversion of configuration. The most probable explanation is that the substrate adopts a planar conformation at some stage of the reaction, and the chirality of the product is determined by the face of this intermediate that is approached by a proton. If this assumption is correct and the conformation of the substrate in the active site of the enzyme is restricted in some way, the steric bulk of the o-substituents will have some effect on the reactivity. Thus, studies of the o-substituted compounds will give us information on the stereochemistry of the intermediates. 

The rather obscure term vicinal is used to denote ligand-based sources of chirality, other than those due to conformation.181 These sources may, be the donor atom itself (the free ligand may or may not be chiral), or the framework of the ligand, in which case central, axial or planar chirality may be involved.16-115116 129 As noted previously, vicinal and conformational effects are often interdependent, since the substitution of a ligand required to introduce chirality may simultaneously result in conformational locking. 

One of the most interesting developments in the stereochemistry of organic compounds in recent years has been the demonstration that trans-cyclooctene (but not the cis isomer) can be resolved into stable chiral isomers (enantiomers, Section 5-IB). In general, a Wa/w-cycloalkene would not be expected to be resolvable because of the possibility for formation of achiral conformations with a plane of symmetry. Any conformation with all of the carbons in a plane is such an achiral conformation (Figure 12-20a). However, when the chain connecting the ends of the double bond is short, as in trans-cyclooctene, steric hindrance and steric strain prevent easy formation of planar conformations, and both mirror-image forms (Figure 12-20b) are stable and thus resolvable. 

Figure 12-20 Representation of (a) achiral and (b) chiral conformations of frans-cycloalkenes, using frans-cyclooctene as a specific example. For frans-cyclooctene, the achiral state is highly strained because of interference between the inside alkenic hydrogen and the CH2 groups on the other side of the ring. Consequently the mirror-image forms are quite stable. With frans-cyclononene, the planar state is much less strained and, as a result, the optical isomers are much less stable. With frans-cyclodecene, it has not been possible to isolate mirror-image forms because the two forms corresponding to (b) are interconverted through achiral planar conformations corresponding to (a) about 1016 times faster than with frans-cyclooctene.

Like in other chiroptical switches (Section 5.3.1), solvent polarity was found to play an important role. Diastereoselective cyclization was observed in THF and toluene, but not in nonpolar solvents such as n-hexane. Upon photoexcitation, diarylethenes 24 (Scheme 11) can adopt a planar and a twisted conformation, and photocyclization only proceeds through the planar conformation. In the case of chiral diarylethene 27a, there are two diastereomeric planar conformations leading to the diastereomers of the cyclic product 27b. The stereoselectivity in the photocyclization process is enhanced because of a decrease in the excited state energy of the unreactive twisted form, providing a relaxation pathway for the less favorable planar diastereoisomer in more polar solvents. Chiral photochromic diarylethenes are among the most prominent photoswitches known today, featuring nondestructive read-out, excellent reversibility, and the potential for construction of switchable molecular wires and modulation of liquid crystalline phases (see Section 5.5.3).[40,411... 

An NOE experiment of cyclic ether 40 with irradiation at the methyl group on C-3 showed 3% enhancement in the signal of the vinyl proton at C-8. This result along with the molecular modeling suggests that the C(3)-C(4) and C(7)-C(8) olefinic moieties of 40 form stereogenic planes in the most stable conformation, and proves its planar chiral nature <2005JA12182>. 

Solid-state X-ray structural studies of the tetrakis HCl salt of 8.5 (i.e., 8.5b) revealed that the molecule does not exist in a planar, circular conformation as represented in Scheme 8.1.1. Rather, as depicted in Figures 8.1.2 and 8.1.3, the molecule adopts a helical twist and overall figure-eight conformation. This twisting makes 8.5b chiral by virtue of conformation and serves to define a pair of enantiomeric atropisomers, at least in the solid state. In solution, detailed H... 

Because many chelate rings are not planar, they can have different conformations in different molecules, even in otherwise identical molecules. In some cases, these different conformations are also chiral. The notation used also requires using two lines to establish the handedness and the lower case labels X and 8. The first line connects the atoms bonded to the metal. In the case of ethylenediamine, this line connects the two nitrogen atoms. The second line connects the two carbon atoms of the ethylenediamine, and the handedness of the two rings is found by the method described in Section 9-3-5 for separate rings. A counterclockwise rotation of the second line is called X (lambda) and a clockwise rotation is called 8 (delta), as shown in Figure 9-16. Complete description of a complex then requires identification of the overall chirality and the chirality of each ring. 

Katsuki has studied asymmetric epoxidation of non-functionalized olefins catalyzed by chiral Mn(salen) complex. Recently they proposed that the ligands of Mn(salen) complexes take non-planar stepped conformation and the direction of the folding ligands is strongly related to the sense of chirality in the asymmetric epoxidation (Eq. (7.26)) [71]. On the basis of this proposal, conformational con-... 

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