Halides, aryl, with active enantioselectivity

For the performance of an enantioselective synthesis, it is of advantage when an asymmetric catalyst can be employed instead of a chiral reagent or auxiliary in stoichiometric amounts. The valuable enantiomerically pure substance is then required in small amounts only. For the Fleck reaction, catalytically active asymmetric substances have been developed. An illustrative example is the synthesis of the tricyclic compound 17, which represents a versatile synthetic intermediate for the synthesis of diterpenes. Instead of an aryl halide, a trifluoromethanesul-fonic acid arylester (ArOTf) 16 is used as the starting material. With the use of the / -enantiomer of 2,2 -Z7w-(diphenylphosphino)-l,F-binaphthyl ((R)-BINAP) as catalyst, the Heck reaction becomes regio- and face-selective. The reaction occurs preferentially at the trisubstituted double bond b, leading to the tricyclic product 17 with 95% ee. °... 

The redox reaction between diphenylbenzidine and thiosulfate is of first order with respect to thiosulfate, diphenylbenzidine, and hydrogen ions.301 Zinc-diamine-catalysed reduction of various ketones with polymethylhydrosiloxane in protic conditions show moderate enantioselectivities. Probable mechanisms are proposed.302 Reduction of non-activated aryl and alkyl halides by a neutral ground-state organic molecule (13) afforded the corresponding indolines in excellent yields. A tentative mechanism has been suggested (Scheme 13).303... 

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