Enantioposition-selective asymmetric

Three types of reaction systems have been designed and applied for the enantioposition-selective asymmetric cross-coupling reactions so far. First example is asymmetric induction of planar chirality on chromium-arene complexes [7,8]. T vo chloro-suhstituents in a tricarhonyl("n6-o-dichlorobenzene)chromium are prochiral with respect to the planar chirality of the 7t-arene-metal moiety, thus an enantioposition-selective substitution at one of the two chloro substituents takes place to give a planar chiral monosubstitution product with a minor amount of the disubstitution product. A similar methodology of monosuhstitution can be applicable to the synthesis of axially chiral biaryl molecules from an achiral ditriflate in which the two tri-fluoromethanesulfonyloxy groups are enantiotopic [9-11]. The last example is intramolecular alkylation of alkenyl triflate with one of the enantiotopic alkylboranes, which leads to a chiral cyclic system [12], The structures of the three representative substrates are illustrated in Figure 8F.1. 

Asymmetric induction of central chirality at a carbon atom was achieved by an intramolecular enantioposition-selective asymmetric cross-coupling [12]. Treatment of the prochiral bisbo-rane 46, which was prepared from the alkenyl triflate 45 and 2 equiv. of 9-BBN with 20 mol % of Pd/(S)-(/ )-BPPFOAc (48) catalyst generated in situ in THF, brings about intramolecular Suzuki coupling. The following oxidative workup and p-nitrobcnzoylation affords the chiral cyclopentane derivative (R)-47 in 58% yield and 28% ee (Scheme 8F.14). 

Enantioposition-selective asymmetric cross-coupling has also been successfully applied to the synthesis of axially chiral biaryl molecules (Scheme 11).155],[56] Reaction of the achiral ditriflate 52 with 2 equiv of phenyhnagnesium bromide in the presence of lithium bromide and 5 mol % of PdCl2[(5)-phephos (10b)] at -30 °C for 48 h gave an 87% yield of the monophenylation product (S)-53, which is 93% ee, and a 13% yield of diphenylation product 54. The enantiomeric purity of the monophenylation product... 

Figure 8F.1. Substrates for enantioposition-selective asymmetric cross-coupUng.

Two precedent examples had been reported of the enantioselective [2+2+2] cycloaddition of alkynes. In one case, an enantioposition-selective intermolecular reaction of a triyne with acetylene generated an asymmetric carbon at the benzylic position of a formed benzene ring [19]. In the other case, an intramolecular reaction of a triyne induced helical chirality [20]. Both reactions were developed by chiral Ni catalysts. 

Pd-catalyzed asymmetric cross-coupling has been applied to asymmetric synthesis of planar chiral lricarbonyl(i7 -o-dichlorobenzene)chromium complexes and axially chiral biaryl molecules by an enantioposition-selective reaction. 

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