Olefin absolute configuration

Considering an olefinic functionality as a chromophore, the absolute configuration of cyclic allylic alcohols can be determined using a method that involves the conversion of the alcohol to the corresponding benzoate.60 This can also be extended to acyclic alcohols where the conformations are dynamic (see Fig. 117). Interested readers may consult the literature for details.61... 

Introduction of a double bond between the triple bond and the leaving group leads to enyne electrophiles 45, which would give access to vinylallenes 46 if the attack of the nucleophile takes place at the triple bond in an SN2" (1,5) substitution reaction (Scheme 2.16). In addition to the regioselectivity, two types of stereoselectivity also have to be considered in this transformation, i.e. the configuration of the olefinic double bond of the vinylallene and the (relative or absolute) configuration of the allenic chirality axis. 

In the first step, a cyclic intermediate is generated by a suprafacial addition, followed by a SN2-type ring opening (e.g., halogenation or epoxidation ring opening). In this manner, olefin 1 may either be converted into diastereomer 2 or 3, which may be optically active or racemic. Mechanism control thus means that the relative, but not the absolute, configuration of the two vicinal centers is defined. 

In this case the absolute configuration at (at least) one of the chiral units needs to be determined. This situation is encountered, for example, in enantioselective addition reactions of olefins with known configuration when two stereogenic centers are created and the olefin geometry is translated to the product in a predictable way. For example, the enantioselective vicinal hydroxylation of (ZT)-l,2-disubstituted olefins97 98, such as tert-butyl (ZT)-3-(methoxycar-bonyl)-2-propenylcarbamate (for assignment, see p 439). 

Scheme 81 shows a highly enantioselective C—C bond formation in the BINAP-Pd(II) diacetate-catalyzed reaction of aryl triflate and 2,3-dihydiofuran (193). A BINAP-Pd(0) species generated by the action of a tertiary amine on the Pd(II) complex is the actual catalyst. Enhancement of the enantioselectivity through kinetic resolution of the intermediate is indicated by the double-bond isomer having opposite absolute configuration at the arylated carbon. / -Substituted 2-pyrrolines may also be used as olefinic substrates. 

Table 7. Relationship between absolute configuration of some a-olefins and optical activity sign of their polymers...

As shown in Table 13, the sign of the optical activity of polymers of alkyl- or cycloalkyl-vinyl-ethers at 589 mp, in hydrocarbon solvent, can in general be foreseen on the basis of the absolute configuration of the asymmetric carbon atom that is the nearest to the ethereal group. A positive sign is observed when the asymmetric carbon atom of the monomer has absolute configuration (S), as in poly-a-olefins. 

In the case of poly-a-olefins a more precise idea about the influence exerted by the absolute configuration of the asymmetric carbon atom of the lateral chains on the conformation of the principal chain was... 

According to the above data and to preliminary determinations of the polymers optical activity in the solid state (112) it was suggested (105) that the principal chain of optically active poly-a-olefins, which in general posses a helix type conformation in the solid state, consists of sections having helix conformation also in the molten state or in dilute solution. Most sections of the principal chain are spiraled in one screw sense only, depending on the absolute configuration of the asymmetric carbon atom of the lateral chains. 

Olefin polymerization using heterogeneous catalysts is a very important reaction and stereochemical aspects have been studied extensively. For a review on this topic see Pino et al. [9], Briefly, the origin of stereoregularity in polyolefins (47) is explained by the chiral nature of the acdve site during polymerization. If the absolute configuration of the first intermediate can be controlled by chiral premodification then we should obtain a non-racemic mixture of R - and "S"-chains. This has indeed been observed e.g. with catalyst M4 for the polymerization (partial kinetic resolution) of racemic 3,7-dimethyl-l-octene (ee 37%) and also for the racemic monomer 46 using Cd-tartate M5. 

In addition, saturated tertiary hydrocarbons [108] also act as a chiral source of asymmetric autocatalysis to give the pyrimidyl alkanol, with the absolute configurations corresponding to that of the chiral alkanes (Scheme 21). The correlation between the absolute configuration of the hydrocarbon and that of the obtained alkanol is reproducible. Therefore, the asymmetric autocatalysis is sufficiently applicable as a chiral sensor for saturated tertiary hydrocarbons. Various chiral hydrocarbons with zr-electrons such as 1,1 -binaphthyls [110], helicenes [82], olefins [83], allenes [111], and [2.2]paracy-clophanes [112,113] serve as chiral initiators in this asymmetric autocatalysis. 

---