Stereochemistry formation, racemic mixture

The stereochemistry of a solvolysis reaction can be affected if the substrate has a substituent that can donate a pair of electrons to the developing carbocation center. For example, treatment of ( )-t/zreo-3-bromo-2-butanol (19) with HBr gave only the racemic 2,3-dibromobutane (20). There was none of the meso compound that would have been expected if the reaction involved protonation, loss of water, and formation of a free carbocation intermediate. Similarly, reaction of ( )-eri/tizra-3-bromo-2-butanol with HBr gave only meso-2,3-dibromobutane. The reaction of 19 seems best explained by nucleophilic participation of the bromine on the adjacent atom in concert with departure of the water. The result is a bridged intermediate (21) that is the same bromonium ion expected from the electrophilic addition of Br2 to cis-2-butene (Figure 8.13). Back-side attack by bromide ion on either carbon atom involved in the three-membered bromonium ring is equally likely, so a racemic mixture results. 

The hypothesis of stereochemical control linked to catalyst chirality was recently confirmed by Ewen (410) who used a soluble chiral catalyst of known configuration. Ethylenebis(l-indenyl)titanium dichloride exists in two diaste-reoisomeric forms with (meso, 103) and C2 (104) symmetry, both active as catalysts in the presence of methylalumoxanes and trimethylaluminum. Polymerization was carried out with a mixture of the two isomers in a 44/56 ratio. The polymer consists of two fractions, their formation being ascribed to the two catalysts a pentane-soluble fraction, which is atactic and derives from the meso catalyst, and an insoluble crystalline fraction, obtained from the racemic catalyst, which is isotactic and contains a defect distribution analogous to that observed in conventional polypropylenes obtained with heterogeneous catalysts. The failure of the meso catalyst in controlling the polymer stereochemistry was attributed to its mirror symmetry in its turn, the racemic compound is able to exert an asymmetric induction on the growing chains due to its intrinsic chirality. 

A final proof for the stereochemistry present in alapyridaine was performed by CD spectroscopy. As shown in Figure 8, the alap5a idaine sample isolated from heated Maillard mixtures did not show any CD effect, thus confirming the formation of racemic alap)o idaine during thermal treatment as suggested by the optical rotation of 0° measured by polarimetry. In contrast, the synthetic ( S)-(+)-enantionier showed a pronounced CD effect, in particular, when measured at pH 9, thus confirming the enantiopurity of that sample. 

The action of a chiral Homer-Wittig type reagent on a racemic aldehyde has been mentioned in Section 2.3.7. In the E/Z mixture of products arising from olefination of 109, the -stereoisomer is mainly formed from one enantiomer of the aldehyde 109 while the Z-isomer is derived from the other enantiomer. Here, each enantiomer gives a defined stereochemistry for the double bond that is created (69). A new stereogenic unit is formed, which is conceptually similar to the formation of a new asymmetric centre in a racemic substrate. 

Needless to say that the absolute stereochemistry of each chiral center of vicinal diols 34 as well as the diastereomeric and enantiomeric excess is very important. For example, diol 34a was a mixture of four stereoisomers, as ilR S) lS,2R) (lR,2R) (lS 2S) = 89 0 6 (or 5) 5 (or 6) [81]. The hydroxy ketone formation occurred in a highly enantioselective manner [79,80] as described in Eq. (23). Equation (24) explains the whole results initially formed (/f)-33a was reduced predominantiy by an enzyme ( L-enzyme ) to afford (i/ ,25)-34a (89% of the total mixture), while another minor enzyme ( D-en-zyme ) seemed to be responsible for the formation of the (IR R) isomer (5-6%). The (/ )-33a would be partially racemized in whole cells of ye t [79] to give the (5)-33a, and this would be further reduced in a similar manner by the two enz5mes to give the (1S,2S) (5-6%) and (1S,2R) isomers, the formation of the latter t ing almost undetectable. In this case, single recrystallization of the corresponding dibenzoate of 3 provided the diaste-reomerically and enantiomerically pure (1R,2S) compound [69,72,81]. In other cases also, recrystallization was very effective to obtain diastereomerically and enantiomerically pure materials [69,84,88]. 

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