Pro-chiral atoms

Isotactic macromolecule - A tactic macromolecule, essentially comprising only one species of repeating unit which has chiral or pro chiral atoms in the main chain in a unique arrangement with respect to its adjacent constitutional units. [8]... 

A pro-chiral molecule may turn chiral after undergoing a symmetry breaking chemical reaction on a surface. An example where covalent bonds to surface atoms are formed is the reaction of frans-2-butene with Si(100) [30]. As with purely adsorption-induced chirality, the relative alignment of the prochiral reactant with respect to the surface plane determines the handedness of the adsorbate complex (Scheme la). 

Draw the structural formula of 2-methyl-2-(3-oxobutyl)cyclopentane-l,3-dione and determine the pro chirality centres and the topicities of the hydrogen atoms. 

This slightly less symmetrical molecule is not chiral but prochiral. The carbon atom is a pro-chiral (or prostereo genic) centre. The plane of the paper is still a plane of symmetry, but the yellow plane containing the two H atoms is not and the hydrogen atoms are enantiotopic. They are magnetically equivalent and can be distinguished only by humans, enzymes, and other asymmetric reagents. 

Most of the known asymmetric synthesis reactions equilibrating from pro-chiral racemic mixtures are photocyclizations where ring systems that include one or two asymmetric C atoms are created as a first or the final product. 

The R/S system is used for a description of chiral molecules. Designations also exist for pro-chiral molecules, (where replacement of one group will produce a chiral center), pro-R, pro-S, as well as faces of planes containing atoms, re and... 

The molecules in the original crystal need not be chiral they can be pro-chiral and still give an analogous effect. Glycine, which lacks an asymmetric carbon atom, crystallizes in the a form as bipyramids with well-developed [110] and [Oil] sets of faces.These crystals are mono-... 

Most interestingly, it has been observed by the same authors that some pro-chiral substrates can also be processed by these enzymes [162]. Thus, in the case of such compounds, the regioselective attack at one single carbon atom of the oxirane ring affords the corresponding diol in optically enriched (or even enantiopure) form. The outstanding interest in these two last cases is of course the fact that, in contrast to a classical resolution process which is limited to a 50% yield, the theoretically possible product yield is 100% (Fig. 14). 

Katsuki and coworkers examined enantiotopic selective hydroxylation of pro-chiral substrates with chiral (salen)manganese complexes as catalysts [13]. This reaction also proceeds via a radical intermediate [13a]. The kinetic isotopic effect (kn/kD = 4.6) observed in the hydroxylation of ethylbenzene with complex lib supports the idea that hydrogen atom abstraction is the rate-determining step [13b]. In the reaction using chiral (salen)manganese complexes which have no chiral cavity, radical decay should occur less selectively and should deteriorate the enan-tioselectivity of hydroxylation. A solvent of intense viscosity constitutes a strong... 

Both in the case of Mezey et al. and Boon et al. the calculation of involves coalescence of the chiral atom in both molecules, similar to the atoms-in-mole-cules work by Cioslowski et mentioned earlier. This coalescence pro-... 

Note that (pro) -chirality is a property of the entire molecule, not just a particular atom within the molecule. Consider the example of triptycene. 111, in Figure 2.46. The structure can be desymmetrized by several pathways, one of which will produce a chiral structure in only one step. However, the desymmetrization can be accomplished in at most three steps if each step involves a different symmetry element, so it is said to be (prop-chiral. Table 2.3 lists the (prop-chirality of molecules as a function of their symmetry. ... 

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