Enantioselective reactions stereodifferentiation

When the aromatic substrates to be coupled do not contain any chiral information, stereodifferentiation may be achieved in those reactions that are mediated by chirally modified metal complexes. Thus, 2-naphthalenol is atropo-enantioselectively dimerized by oxidation with a chiral copper complex20,21. 

The double stereodifferentiation is also found in the reactions of acyclic nucleophiles (Scheme 8E.42). Whereas the alkylation of W-imino(methylphosphonates) affords the simple allylated product in poor enantioselectivity (11% ee) using ligand 38a, excellent enantioselec-tivity and good diastereoselectivity can be obtained from the alkylation with the 1,3-diphenyl-allyl system [188],... 

The versatility of the methodology is illustrated in Eq. (80) it is possible to prepare syn or anti propargylic adducts or an allenyl adduct from a single allenic stannane by appropriate modification of reaction conditions. Both additions proceed with excellent diastereo- and enantioselectivity. Addition to an enantioenriched a-methyl-/3-OBn aldehyde proceeds with excellent stereodifferentiation (Eq. 81). 

Mechanistic implications of these reactions and the structure of the active rhodium species have been discussed110. The effect of double stereodifferentiation is also known, when this catalyst and chiral diazoacetates with auxiliaries such as (-tf)-pantolactone are employed111. The most effective (/ )- or (S)-5-oxopyrrolidine-2-carhoxylate ligand has also been compared with other chiral cyclic ligands, which generally provide cyclopropanes in lower enantioselectivities 112 this type of complex can be bound to polyethylene113. The enantioselectivities of inter-and intramolecular cyclopropanations are apparently the same as with the free" complexes. 

The binuclear copper complex 4 is also an effective catalyst for enantioselective cyclo-propanation of simple olefins76. The major tram-isomer is produced by the -enantiomer of 4 with predominant (15) configuration. For these reactions the influence of the menthyl ester chirality is not negligible otherwise the relatively high difference in the ee values when the R-or S-catalyst is used cannot be rationalized. Thus, double stereodifferentiation seems to operate to some extent. 

When applied to the Mukaiyama reaction, the principles of stereodifferentiation can lead to excellent dias-tereoselection and enantioselection. When silyl ketene acetal 465 was reacted with chiral aldehyde 466 in the presence of TiCl4, a 75% yield of 467 and 468 was obtained as a 98 2 mixture, favoring 467. ... 

Double stereodifFerentiation was used to achieve enantioselective enolborination of chiral ketones by reaction with chlorobis(isopinocampheyl)borane (DIPCl) in the presence of a chiral amine [69]. As an example, reaction of excess ( )-94 with IPC2BCI and sparteine at —78°C produced 95 regioselectively with a 90% ee (evaluated after oxidation into diadds). 

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