Enantioface discrimination

The earliest example of enantioface discrimination of a planar-stabilized carbanion, the second type of reaction, was the reaction of allylic phenyl ether and 2-acetyl-1 -tetralone aided by PdCl2(diop) and sodium phenoxide and resulted in up to 10% optical yield (203). 

Hydrogenation of 2,2,2-trifluoroacetophenone and its derivatives with a mixture of trans-RuCl2[(S)-xylbinap][(S)-daipen] and (CH3)3COK in 2-propanol gives the S alcohols quantitatively with a high optical purity (Scheme 1,64) [258]. Unlike with many chiral borane reagents [264], the sense of enantioface discrimination is the same as in hydrogenation of acetophenone. The electronic effects of 4 -substituents on the enantioselectivity are small. These chiral fluorinated alcohols are useful as components of new functionalized materials [265]. 

The same transition-state model can be used to correlate the results for the enantioface-discriminating hydrocarbalkoxylation discussed above with the few results obtained in the enantiomer-discriminating hydrocarbalkoxylation, as will be discussed elsewhere (32). 

Reliability of the Predictions of Isomeric Composition and Enantiomeric Excess in Enantioface-Discriminating Hydroformylation. 106... 

Asymmetric hydroformylations of all the above types have been achieved with rhodium catalysts enantioface- and enantiomer-discriminating hydroformylations also occur with cobalt and platinum catalysts whereas with ruthenium or iridium complexes only enantioface-discriminating synthesis has been reported up to now (see Sect. 2.1.4.). 

In asymmetric reactions in which only one isomer is formed, enantioface discrimination corresponds to asymmetric induction. However, in reactions in which two isomers are formed if both isomers are chiral, as in the hydroformylation of (Z)-2-hexene, the optical purities of the two isomers are in general different and enantioface discrimination does not correspond quantitatively (and sometimes not even quali-... 

For instance, when a-[2H]-styrene is hydroformylated with the Rh/(—)-DIOP catalytic system, the two reaction products obtained have almost identical optical purity and opposite absolute configuration (Fig. 2)51). Therefore, in this case, the enantiomeric excess measured indicates both the type (the re-re enantioface reacts preferentially) and extent of enantioface discrimination ( 15%) occurring during the reaction. 

In the case of the deuterioformylation of 1-pentene in the presence of [(—)-DIOP] Pt(SnCl3) Cl as the catalyst precursor, on the other hand, the two chiral reaction products (Fig. 3) mainly arise from the two opposite enantiofaces 43). Therefore, in 1-pentene hydroformylation, the enantiomeric excess found in the chiral reaction product does not correspond in type and extent to enantioface discrimination. 

Table 14. Correlation between enantioface-discriminating and enantiomer-discriminating hydroformylation with Rh/(—)-DIOP catalytic systems"...

The simplest case which can be used to explore the factors influencing the difference in the energies of the diastereomeric transition states, which determine asymmetric induction, is the hydroformylation of (Z)-2-butene with the Rh/(—)-DIOP or Pt/ (—)-DIOP catalytic system. In this case, the asymmetric induction cannot be connected with enantioface discrimination in the step leading to the Tt-complex because this olefin has no enantiofaces 68). In the first step of the reaction it is assumed that a Tt-complex is formed by interaction between substrate and catalyst. This Tt-complex, depending on its geometry, can exist in two different conformations arising from the rotation of the olefin around the metal-olefin Tt-system-bond axis. 

CH/7T hydrogen bonds in organic reactions have been reviewed, including major sections on diastereoface- and enantioface-discriminating reactions.223 g)... 

The Cp() ,f )-Ti[All] and Cp(S,S)-Ti[All] reagents have been condensed with a variety of aldehydes always with good enantios-electivities (eqs 4-11). The degree of enantioface discrimination of these allyltitanium reagents is very high. The Si face attack is preferred for the Cp(/f,/f)-Ti[All] reagent and the Re face attack is preferred for the Cp(S,S)-Ti[All] reagent. 

Allyltitanium compounds and Ti enolates derived from mono-Gp chloro titanium complexes with two chiral alkoxo ligands add to aldehydes with high enantioface discrimination.973... 

The synthesis of dihydrofurans with an additional ester moiety and one or two quaternary centers are prepared from titanium enolates formed by reactions of 3,4-dienoates with Cp2TiCl2. Titanium enolate derivatives TiCl3[CH2CH2C(=0)0Et] and the Tebbe reagent compound have been applied in the synthesis of pumilio-toxin.1915 Chiral allyl and mono-Gp chloro enolato titanium compounds add with high enantioface discrimination to aldehyde.973... 

The degree and sense of enantioface discrimination are highly dependent... 

The former reaction discriminates the enantiofaces of the donor, and the asymmetric center is formed on the donor carbon atom. The latter reaction proceeds via enantioface discrimination of the Michael acceptor generating a chiral carbon center on the acceptor. Although both reactions are known, their mecha-... 

These malonate and nitroalkane reactions gave the adducts with the predicted absolute configurations (i )-adducts were obtained from cyclic (Z)-enones and (S)-adducts from acyclic ( )-enones when (S)-21 was employed. The stereochemical outcome can be summarized as M(a)-attack. The involvement of the a-enantioface-discriminating mechanism suggests that the chiral catalysts are located in the vicinity of the enone carbonyl group at the transition state. The reaction of the primary nitroalkane mentioned above also supports this explanation. 

A C2-symmetric homochiral diol 13 (DHPEX) is a chiral proton source developed by Takeuchi et al., for samarium enolates which are readily prepared by Sml2-mediated allylation of ketenes [25,26]. In the stoichiometric reaction using DHPEX 13, they found that -45 C was the best reaction temperature for the enantioface discrimination, e.g., when methyl (1-methyl-l-phenylethyl)ketene 55 was used as a substrate, the product exhibited 95% ee [27]. The catalytic reaction was carried out using trityl alcohol as an achiral proton source which was added to a mixture of in situ generated samarium enolate 56 and DHPEX 13 (0.15 equiv) slowly so as not to exceed the ratio of the achiral proton source to DHPEX 13 of more than 0.7. The highest ee (93% ee) of product 57 was gained when the achiral proton source was added over a period of 26 h (Scheme 8) [27]. 

Based on these results, asymmetric induction in the hydroformylation of conjugated dienes is interpreted to arise from secondary hydrogenation of the primary achiral unsaturated aldehyde. Thus (at least in the above case), enantioface-discriminating hydrogenation of an unsaturated aldehyde (which is easily formed) and not enantiofacc discriminating hydroformylation of the primary diene or an intermediate alkene is responsible for asymmetric induction18-45. 

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