Achiral reactant

In this as m other reactions m which achiral reactants yield chiral products the product IS formed as a racemic mixture and is optically inactive Remember for a substance to be optically active not only must it be chiral but one enantiomer must be present m excess of the other... 

Similarly, the two faces at a trigonal earbon in a molecule containing a stereogenic center are diastereotopie. Both ehiral and achiral reactants can distinguish between these diastereotopie faces. Many examples of diastereotopie transformations of sueh eompounds are known. One of the cases that has been examined elosely is addition reactions at a trigonal center adjacent to an asymmetric carbon. Particular attention has been given to the case of nucleophilie addition to carbonyl groups. 

As a general rule, formation of a new chirality center by reaction betweer two achiral reactants always leads to a racemic mixture of enantiomeri<... 

As a general rule, the reaction of a chiral reactant with an achiral reactant leads to unequal amounts of diastereomeric products. If the chiral reactant is optically active because only one enantiomer is used rather than a racemic mixture, then the products are also optically active. 

In cases that chiral products are formed from achiral reactants, racemic mixtures of products will be produced in the absence of chiral influence (reagent, catalyst, or solvent). 

Acetylpyridine, cathodic pinacolisation, 40 165 Achiral reactants hydrogenation with, 42 489-498 Acid-base catalysis... 

A nice analysis of non-linear effects in Rh-chiral diamine-catalysed transfer hydrogenation has been performed that reinforces the need to consider the kinetics of all of the steps in reaction manifolds (e.g. reversible formation of diastereomeric precursors and their subsequent interaction with achiral reactants). ... 

In principle, the approach outlined above for the a-oxoamides can be applied to any reaction, ground or excited state, which converts an achiral reactant into a chiral product, and Toda, Tanaka, and coworkers have investigated a wide variety of such processes [ 15,16]. A complete discussion of their work is beyond the scope of this review, and we illustrate the general approach taken with one final example. As shown in Scheme 4, irradiation of crystalline complexes of ene-diones 20a-f with chiral host (R,R)-(-)-9b led to cyclized products 21a-f in the variable yields and ee values indicated in Table 1 [22]. Remarkably, for reasons that were not clear (there was no accompanying X-ray crystallography), the R=n-propyl derivative 20g was found to give a completely different photoproduct, spiro compound 22 (69% yield, 97% ee, stereochemistry unknown), a result that once again illustrates the rather capricious nature of the use of chiral hosts for asymmetric induction. 

The process by which a stereochemically inactive center is converted to a specific stereoisomeric form. In most cases, the reacting center is prochiral. Such processes can occur with reactions involving an optically active reagent, solvent, or catalyst (eg., an enzyme). The reaction produced by such a process is referred to as an enantioselective reaction. In principle, use of circularly polarized light in photochemical reactions of achiral reactants might also exhibit asymmetric induction. However, reported enantioselectivities in these cases have been very small. 

Asymmetric induction is the use of a chiral reagent or catalyst to convert an achiral reactant to a chiral product having an excess of one enantiomer. In biochemistry the chiral catalyst is often en enzyme. For example. 

Thus, in the example of the chiral olefin 3, there is simple diastereoselectivity of (m4a + m4b)/(m4c + m4d) and induced diastereoselectivities of m4a/m4b and m4c/m4d. It is not strictly necessary, but conversely there is no harm, in applying the term simple diastcrcosclcc-tivity to the first case, i.e., to a diastereoselective reaction of achiral reactants. In this volume the presentation of a given reaction type always begins with simple diastereoselectivity of achiral reactants. 

Additive-Induced Stereoselectivity Stereoselectivity in a reaction between achiral reactants is additive induced when (a) stereogenic unit(s) is (are) stereoselectively generated under the influence of a noncovalently bound chiral compound (e.g., solvent, catalyst). 

In the Michael addition of the ( -cnethiolatc to ( )-3-penten-2-one, the reaction occurs at a center that is prostereogenic in each of the two achiral reactants. One (racemic) diastereomer is formed as the main adduct (see pp469 and 472 for the determination of configuration)8-89. 

On many occasions, a reaction carried out with achiral reactants results in the formation of a chiral product. In the absence of any chiral influence, the outcome of such reactions is the formation of a racemic form. For example, hydrogenation of ethylmethylketone yields a racemic mixture of 2-hydro-xybutane. 

Many instances of stereospecific selection of enantiotopic groups or faces may be found in nature. One such is extracted from the tricarboxylic acid cycle and is shown in Exercise 1.6. At each step, achiral reactants are transformed to achiral products with high stereospecificity ... 

The enantiomeric excesses In chiral products reported from circularly polarized Irradiation of achiral reactants In achiral solvents are usually less than 0.2-0.3% (77, 79). The largest enantiomeric excess reported Is 1.6% for an unrelated rearrangement (80). Furthermore, It Is known that circularly polarized Irradiation of JBN In dlchloromethane yields very low (<0.2%) atroplsomerlc excesses (81). Therefore, the 1.1% atroplsomerlc excess of BN observed upon Irradiation of solutions In mixture k cannot be ascribed to circularly polarized excitations. 

Use of chiral single crystals to convert achiral reactants to chiral products in high optical yield application to die di-Jt-methane and Norrish type II photorearrangements, J. Am. Chem. Soc., 108, 5648-5649. (b) Chen, J., Pokkuluri, P. R., Scheffer, J. R., and Trotter J. (1990) Absolute asymmetric induction differences in dual pathway photoreactions, Tetrahedron Lett., 31, 6803-6806. (c) Fu, T. Y., Liu, Z., Scheffer, J. R., and Trotter, J. (1993) Supramolecular photochemistry of crystalline host-guest assemblies absolute asymmetric photorearrangement of the host component, J. Am. Chem. Soc., 115, 12202-12203. (d) Leibovitch, M.,... 

Small fluctuations in the ratio of the two enantiomers are considered to be present in racemic mixtures of chiral molecules [14,101]. We thought that, when a reaction system involves asymmetric autocatalysis with amplification of ee, the initial small fluctuation of ee in racemic mixtures that arises from the reaction of achiral reactants can produce an enantiomerically enriched product. We anticipated that when z-P Zn was treated with pyrimidine-5-carbaldehydes without adding any chiral substance, extremely slight enan-tioenrichment would be induced statistically in the initially formed zinc alkoxide of the alkanol, and that the subsequent amplification of chirality by asymmetric autocatalysis would produce the pyrimidyl alkanol with detectable enantioenrichment (Scheme 19). 

We consider a production of chiral enantiomers R and S from an achiral substrate A in a closed system. Actually, in the Soai reaction, chiral molecules are produced by the reaction of two achiral reactants A and B as A + R or A + B -> S. But in a closed system a substrate of smaller amount controls... 

Scheme 1. Asymmetric catalysis from achiral reactants A and B under the influence of a chiral catalyst (cat/ or cat5).

Pt A), 63-84 (c) Evans, S.V., Garcia-Garibay, M., Omkaram, N., Scheffer, J.R., Trotter, J., and Wireko, F. (1986) Use of chiral single crystals to convert achiral reactants to chiral products in high optical yield application to the di-Jt-methane and Norrish type II photorearrangements. 

The solution to this problem is straightforward either chemically attach a homochiral auxiliary to the achiral reactant or cocrystallize the achiral reactant with an external homochiral auxiliary. In botlreases, the crystals formed must be non-centrosymmetric, i.e., chiral, because it is not possible for homochiral objects to pack with a center of symmetry between them. In our research, we have pioneered the use of internal, built-in chiral auxiliaries to generate chiral... 

In principle, any of the photoproducts shown in Table 4 could have been prepared in enantiomerically pure form by irradiating their achiral precursors in solution to form a racemate and then separating the enantiomers by means of the classical Pasteur resolution procedure [36]. This sequence is shown in the lower half of Fig. 3. The top half of Fig. 3 depicts the steps involved in the solid-state ionic chiral auxiliary method of asymmetric synthesis. The difference between this approach and the Pasteur method is one of timing. In the ionic chiral auxiliary method, salt formation between the achiral reactant and an optically pure amine precedes the photochemical step, whereas in the Pasteur procedure, the photochemical step comes first and is followed by treatment of the racemate with an optically pure amine to form a pair of diastereomeric salts. The two methods are similar in that the crystalline state is crucial to their success. The Pasteur resolution procedure relies on fractional crystallization for the separation of the diastereomeric salts, and the ionic chiral auxiliary approach only gives good ees when the photochemistry is carried out in the crystalline state. 

---