2.3- Pentadiene, enantiomers

The nonplanarity of allenes has an interesting stereochemical consequence 1 3 Disubstituted allenes are chiral they are not superimposable on their mirror images Even an allene as simple as 2 3 pentadiene (CH3CH=C=CHCH3) has been obtained as sep arate enantiomers... 

Examine models of both enantiomers of 5 2 3 pentadiene to verify that they are non superimposable... 

A chiral molecule need not contain a chiral center. Examine the two enantiomers of 2,3-pentadiene (A and B). 

The enantiomers shown are related as a right-hand and left-hand screw, respectively. Chiral allenes are examples of a small group of molecules that are chiral, but don t have a chirality center. What they do have is a chirality axis, which in the case of 2,3-pentadiene is a line passing through the three carbons of the allene unit (carbons 2, 3, and 4). The Cahn-Ingold-Prelog R-S notation has been extended to chiral allenes and other molecules that have a chirality axis. Such compounds are so infrequently encountered, however, we will not cover the rules for specifying their stereochemistry in this text. 

P(CH3)3, P(OCH3)3], For each of the [Cr(CO)3(j/4 CH-diene)] complexes 39g, 37n, and 37o two enantiomers are expected. As a consequence of the C—H—Cr bridge from C-5 or C-6 of the 1,3-cyclohexadiene, and from C-5 or C-7 of the 1,3-cycloheptadiene, both ligands lose their inherent mirror planes. The situation is somewhat different with the 2,4-dimethyl-1,3-pentadiene complex (37n). Here only the 4Z methyl group is able to form the C—H—Cr bridge. Due to the two coordination modes of the diene, R or S, a pair of enantiomers is present for 37n. Finally, 37r forms two diastereomeric pairs of enantiomers, because the C—H—Cr bridge can be formed from C-5 or C-6 and the diene can be coordinated in the R or S mode. 

In the presence of (+)-ephedrine, ( )-l-acetylcyclo-octene 59dE photoisomerizes to enantiomeric (Z)-isomer, which is trapped by cyclo-pentadiene to give the Diels-Alder adduct 62d in 22% ee (Sch. 21) [118]. Direct irradiation at >280 nm of doubly bridged (Z)-l(10)-bicyclo[8.8.0] octadecen-2-one 87Z in diethyl (+)-tartrate as a solvent affords the (E)-isomer 87E in favor of the levorotatory enantiomer where the EjZ ratio is 7 (Sch. 31) [162]. 

Problem 8.41 The allene 2,3-pentadiene (CH3CH=C=CHCH3) does not have a chiral C but is resolved into enantiomers, (a) Draw an orbital picture that accounts for the chirality [see Problem 5.23(What structural features must a chiral allene have -4... 

Bao and Wulff compared catalysts prepared from vaulted biaryls and from bromo-borane dimethylsulfide with those generated from linear biaryls with regard to their capacity to provide enantioselective induction in the Diels-Alder reaction of cyclo-pentadiene and methacrolein (Eqs 6 and 7) [7]. Because the (5) enantiomers of vaulted biaryls result in induction opposite to that resulting from use of the (5) enantiomer of binaphthol, and because effective catalysts cannot be generated from binaphthol and phenylboron dichloride, suggest that the catalysts obtained from vaulted biaryls do not have the same structure as the Cs-symmetrical catalyst produced from binaphthol. 

The best results before organic catalysis were with amides 125 and Lewis acid catalysts based on Al, Ti, and Cu(II) with C2-symmetric ligands. Corey s aluminium complex 127 derived from the diamine whose resolution was described in chapter 22 works well with substituted cyclo-pentadienes 124 and the product 126 was used in prostaglandin synthesis.28 There are three aspects of stereoselectivity in this reaction which diastereotopic face of 124 is attacked (that anti to the CH2OBn group), is the product exo or endo (endo) and which endo product is formed, 126 or its enantiomer Only for the last question is asymmetric catalysis necessary, though Lewis acid catalysis of any kind enhances endo/exo selectivity. 

Epoxidation of prochiral dialkenyl carbinols 28 provides the anti-epoxide 29 of extremely high optical purity [68, 69, 70]. The first epoxidation occurs in an enantiotopic selective manner while the second one proceeds in an enantiomer-differentiating manner (kinetic resolution). In the second step, the minor (R)-monoepoxides 30 are consumed faster than the major (S)-enantiomers 29 and therefore the enantiomeric excess of the major anfi-monoepoxide 29 increases as the reaction proceeds. If a reaction proceeds with a kfas(/ksio, =104, anti-syn selectivity for the fast reaction=98 2 and for the slow reaction=38 62, as observed in the epoxidation of racemic ( )-l-cyclohexyl-2-buten-l-ol,the enantiomeric excess of anti-29 on calculation is 99.4,99.96, and 99.994% ee at 50,99, and 99.9% conversion, respectively. Yields of anti-29 are 48,93 (maximum), and 91% at the respective conversions. Actually, in the epoxidation of 1,4-pentadien-... 

The diastereoselectivity inherent to the Diels-Alder reaction can be seen in most of the examples in preceding reactions. The reaction is not, however, enantioselective since there is no facial control for intermolecular reactions (some facial control is available for intramolecular reactions). The ortho rule, the endo rule (secondary orbital interactions), and steric interactions provide some orientational control but facial control is also required for enantioselectivity. When ethyl acrylate reacts with 2-methyl-1,3-pentadiene, it can approach from the bottom as in 247A or from the top as in 247B. Clearly, the two products (248A and 248B) are mirror images and enantiomers. This lack of facial selectivity leads to racemic mixtures in all Diels-Alder cyclizations discussed to this point. 

Carbon atom 3 is the sp hybrid allene carbon atom. Carbons 2 and 4 are both sp and planar, but their planes are perpendicular to each other. None of the carbon atoms is attached to four different atoms, so there is no chiral carbon atom. Nevertheless, 2,3-pentadiene is chiral, as you should see from your models and from the following drawings of the enantiomers. 

The 2,3-pentadiene from the left of Figure 4.35 can be viewed (from end-on) as in Figure 4.36. Since carbon will take priority over hydrogen, the sense of helicity (using the usual priority rules) as shown in Figure 4.36 is clearly anticlockwise and thus the enantiomer represented is S. 

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