Stereoselective racemic chiral olefins

Fig. 3.25. Thought experiment I Products from the addition of a racemic chiral dialkylborane to a racemic chiral olefin. Rectangular boxes previously discussed reference reactions for the effect of substrate control (top box reaction from Figure 3.20) or reagent control of stereoselectivity [leftmost box reaction from Figure 3.24 (rewritten for racemic instead of enantiomerically pure reagent)]. Solid reaction arrows, reagent control of stereoselectivity dashed reaction arrows, substrate control of stereoselectivity red reaction arrows (kinetically favored reactions), reactions proceeding with substrate control (solid lines) or reagent control (dashed lines) of stereoselectivity black reaction arrows (kinetically disfavored reactions), reactions proceeding opposite to substrate control (solid lines) or reagent control (dashed lines) of stereoselectivity.

An isotactic stereospecific polymerization arises essentially from the favored complexation of one prochiral face of the a-olefin, followed by a stereospecific process. The stereospecific insertion process and the stereospecific polymerization of racemic a-olefins giving isotactic polymers may be expected to be stereoselective whenever the asymmetric carbon atom is in an a- or /3-position relative to the double bond, and when the interaction between the chirality center of the olefin and the chiral catalytic site is negligible. 

As in stereoselectivity, the degree of stereoelection is reduced as the chiral centre is positioned further away from the C = C bond only racemic a-olefins with a chiral carbon atom in the a- or -position to the double bond have been stereoselectively polymerised. Note that, in general, stereoelectivity is lower than stereoselectivity. 

As mentioned in Section 9.12.2.1.1, the boron-zinc exchange can be performed stereoselectively if diisopropyl-zinc instead of diethylzinc is used. For example, hydroboration of the chiral, racemic endocyclic olefin 134 with diethylzinc, followed by twofold transmetallation and electrophilic capture of the resulting copper intermediate with allyl bromide was used for the highly diastereoselective formation of the stereotriad in product 136 (Scheme 35).35,35a 103 QorreSp0nding enantioselective transformations were carried out with chiral boranes and catalytic amounts of copper salts (see Section 9.12.2.2.2).36... 

Chiral olefins (racemic or enantiomerically pure) can provide two diastcreomers when carbene (carbenoid) CX2 is added, or four diastereomers when carbene (carbenoid) CXY is employed. Cyclic alkenes (on-ring stereoselection) are considered before acyclic olefins (acyclic stcrcosclcction) are discussed. 

Stereoselectivity has also been observed in the copolymerization of racemic a-olefins with ethylene (187,188) and in the copolymerization of racemic a-olefins with an optically active a-olefin (189,190). In the latter case a copolymer of the optically active a-olefin and of the antipode with the same chirality of the racemic monomer, together with a homopolymer of the remaining antipode, was obtained (Scheme 22). 

Stereoselectivity and stereoelectivity in the polymerization of racemic a-olefins have probably the same origin as in the polymerization of epoxides (Section 3.2.2.), auid are determined by the chiral character of the catalysts used. Chiral catalytic centers of a given configuration attack prevailingly one face of the double bond of the monomer and centers of the opposite configuration attack prevailingly the other face. In the case of chiral olefins, the two diastereofaces of a monomer molecule have different reactivities, and whether the re-re face or the si-si face of this molecule is more reactive depends on the type of chirality of the asymmetric carbon atom. For instance, on the basis of the investigation on Pt-complexes (179), the si-si face is the more reactive face in an (S)-olefin. Thus chiral catalytic centers which attack the si-si face may prefer the (S)-antipode, whereas those which attack the re-re face may prefer the (R)-antipode (Scheme 23). 

Studies on stereoselective polymerization of racemic olefins also support this view.338 Polymerization of 3,7-dimethyl-l-octene (the chiral center is in a position to the double bond) took place with 90% stereoselectivity yielding an equimolar mixture of homopolymers of the two enantiomers. No stereoselectivity was observed in the polymerization of 5-methyl-1-heptene (the chiral center is in y position to the double bond). The conclusion is that the ability of a catalytic center to distinguish between the two enantiomers of a monomer required for stereoselective polymerization must arise from its intrinsic asymmetry. The first-ever chiral polypropylene synthesized using a chiral zirconium complex with aluminox-ane cocatalyst is the latest evidence to testify the role of the catalyst center in isotactic polymerization.339... 

Optically active diisopinocamphenylborane can be used to resolve racemic olefins. The reagent adds to one enantiomer, and the other is unchanged. Optical purities on the order of 37-65% are possible. Chiral ally lie alcohols can be resolved with chiral epoxidizing agents derived from tartrate complexes of titanium. One enantiomer is epoxidized and the other is not. Thus, die two alcohol enantiomers can be separated, one as the unsaturated alcohol and one as the epoxy alcohol. Use of die other tartrate isomer reverses die stereoselectivity. Selectivities on die order of >100 are possible with this method. As in any kinetic resolution, however, only one enantiomer can be recovered. The other is converted to a different chiral product. 

It should be noted that the related imine-oxaziridine couple E-F finds application in asymmetric sulfoxidation, which is discussed in Section 10.3. Similarly, chiral oxoammonium ions G enable catalytic stereoselective oxidation of alcohols and thus, e.g., kinetic resolution of racemates. Processes of this type are discussed in Section 10.4. Whereas perhydrates, e.g. of fluorinated ketones, have several applications in oxidation catalysis [5], e.g. for the preparation of epoxides from olefins, it seems that no application of chiral perhydrates in asymmetric synthesis has yet been found. Metal-free oxidation catalysis - achiral or chiral - has, nevertheless, become a very potent method in organic synthesis, and the field is developing rapidly [6]. 

Thus, for both chiral and prochiral a-olefins, the isotactic sequence of the stereogenic tertiary carbon atom of the backbone is due to the enantioselectivity of the chiral active sites to the prochiral carbon atom of the monomer. The stereoselectivity (namely the selection, among the enantiomers, of a racemic... 

Applying these methods for the epoxidation of prochiral olefins without additional measures racemic epoxides are obtained. In most cases, the idea is to make the method stereoselective and thus obtain pure or enriched enantiomers of epoxides by using chiral reagents or by addition of optically active auxiliaries. Some of the results obtained by various groups will be discussed. 

Chiral nitrones were used in [3+2] qrcloadditions for the resolution of olefinic compounds. 1,3-Dipolar addition of an enantiomericaUy pure 3,4-dihydroxypyrrolidine-derived nitrone 61 onto racemic 2,3-dihydro-l-phenyl-IH-phosphole 1-oxide 62 led to a regio- and stereoselective formation of cycloadducts 63 and 64 combined with an efficient resolution of the phosphole oxide 62 [59]. The nitroxide 61a afforded a 66% yield of a 10 1 diastereomeric ratio for 63 64, the recovered phosphole 62 (21% yield, calculated on l.Sequiv phosphole initially involved) showing 59% ee. An increased (96%) ee for unreacted 62 was obtained from the reaction of 61b at higher conversion (91% yield of 63 64, calculated on nitrone). However, under these latter conditions, the yield of recovered (S)-62 was around 20% and the diastereoselectivity for 63 64 only 2.9 1. 

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