Achiral intermediate

The product is chiral but is formed as a racemic mixture because it anses from an achiral intermediate (the enol) it is therefore not optically active... 

Figure 11.10 Stereochemistry of the S j1 reaction. Because the reaction goes through an achiral intermediate, an enantiomeri-cally pure reactant should give a racemic product.

The Gabriel synthesis represents another indirect but highly valuable approach to amines. Trost has demonstrated a method for the asymmetric ring-opening of butadiene monoepoxide by use of one equivalent of phthalimide, 7t-allylpalladium chloride dimer, and the chiral bisphosphine 22 (Scheme 7.37). The dynamic kinetic asymmetric transformation proceeded through a putative achiral intermedi-... 

Racemization takes place whenever the reaction causes chiral molecules to be converted to an achiral intermediate. 

Therefore, these reactions in which the chiral molecules are first converted into an achiral intermediate are examples of SN1 reactions because the leaving group departs from a chiral carbon and they lead to complete recemisation. 

Much attention has been devoted to the acid- and nucleophile-catalyzed racemization of thiosulfinates. As a result of the extensive studies by Kice and his co-workers (112) and by Fava (281), it is clear now that the easy racemization of thiosulfinates caused by acids and bases (e.g., pyridine) is related to the scission of the sulfur-sulfur bond and the formation of sulfenic acid or its anion as an achiral intermediate. As expected, introduction of steric hindrance... 

Attempts to achieve an asymmetric 1,3-proton shift reaction of (/ )-33, obtained from ethyl 3,3,3-trifhioro-2-oxopropanoate and (f )-l-phenylethanamine in 81 % yield, resulted in conversion into 34 in 89% yield, but without any reliably delectable enantiomeric excess.26 Even at 10% conversion, the Shiff base 34 formed is completely racemic. Imine 34 undergoes isotopic exchange in triethylamine/methanoI-r/4 at a rate 10 times slower than the isomerization of 33 to 34. The authors reason that if a 1.3-proton shift mechanism is operating, some enantiomeric excess would have to be observable in product 34 at low conversion. Since this is not the ease, a 1,5-proton shift to the carbonyl oxygen, via stabilized anion 37, to form achiral intermediate enol 38, was proposed.26... 

Discrimination can readily be observed between the two possible modes of attack on a carbonium ion (195 196 and 195 197 when the nucleophile is part of the substrate. In such cases, the phenomenon of neighboring group participation is observed (for a review, see ref. 69). For example, solvolysis of the erythro-tosylate isomer 202 in acetic acid gave largely the erythro-acetate isomer 204 vi a the chiral bridged ion 203, whereas the threo isomer 205 yielded a racemic mixture of threo products 207A and 207B via the achiral intermediate 206 (70). 

Donor atoms at the d and e sites are also found in the chelate complexes [M(CO)2 (CH3)2PC2H4P(CH3)2 (v4-diene)] [32a-32f (27), 34a, 34b, 34d, and 34e (28)]. From the 1H-NMR spectra of 32a and 34a, one can infer a preference for the ad and ae enantiomers (Table V). They are intercon-verted via an achiral intermediate. For complexes with unsymmetrically substituted diene ligands (32b, 32c, 32e, 32f, 34b, and 34e), two dia-stereomeric pairs of enantiomers are detected. Here, too, a mutual transformation of the diastereomers is observed, but the complexes remain chiral, even during fast isomerization. Again, rotation of the diene ligands satisfactorily explains the experimental results. Isomers with the chelate ligand at... 

Magnetization transfer was used to detect fluxionality in the bimetallic complex 347 (Scheme 45).138 Both metals move in this case, and the reaction occurs at a considerably lower temperature than is required for 343. An intramolecular mechanism via an achiral intermediate (or transition state) as pictured in 358 was proposed based on selective isotopomer irradiation as described for 343. Stronger bonding of the Mo(CO)3 moiety to the tropylium ion in the transition state than to the distorted cycloheptatriene... 

Not so with the other diastereoisomer of this compound Now, the phenonium ion is symmetrical with a plane of symmetry—it is therefore achiral, and the same whichever enantiomer we start from. Attack on each end of the phenonium ion gives a different enantiomer, so whichever enantiomer of starting material we use we get the same racemic mixture of products. You can compare this reaction with the loss of stereochemical information that occurs during an SjnjI reaction of enantiomerically pure compounds. Both reactions pass through an achiral intermediate. 

The intermediate oxonium ion.is delocalized and achiral. If a single enantiomer of the starting material is used, racemic product is formed through this achiral intermediate. Attack at one carbon atom gives one enantiomer attack at the other gives the mirror image. 

Initial addition of the electrophile CF (from CI2) occurs from either side of the planar double bond to form the bridged chloronium ion. In this example, both modes of addition (from above and below) generate the same achiral intermediate, so either representation can be used to draw the second step. 

FIGURE 2 Conversion of an unlabeled asymmetric tetrahedron (left) into its mirror image (right) by continuous deformation (small arrows) of the geometric figure along an achiral pathway. The achiral intermediate (center) has C, symmetry. 

The existence of chiral pathways in this molecule is made possible by the existence of the two independent degrees of freedom that govern internal motion, rotation, and inversion. As molecular complexity increases, the number of degrees of freedom also increases and, unless an achiral pathway is energetically much preferred, it becomes more and more likely that enantiomerization proceeds by a chiral pathway. For example, it is extremely improbable that reversal of helicity in a polymeric chain involves an achiral intermediate or transition state. There is a strong resemblance here to the stochastic achirality of ensembles of achiral molecules discussed previously. 

Fig. 12 represent particular stationary points on the multidimensional potential energy surface and are chirally connected through a continuous reaction pathway that involves no achiral intermediates this is evidently the case because the product of the reaction sequence is not racemic. Hence, they cannot be partitioned into R and L homochirality classes. 

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