Stereoselective Synthesis of all Possible Phosferrox Ligand Diastereoisomers Displaying Three Elements of Chirality: Stereochemical Optimization for Asymmetric Catalysis
2020
All four possible
diastereoisomers of phosphinoferrocenyloxazoline
(Phosferrox type) ligands containing three elements of chirality were
synthesized as single enantiomers. The Sc configured oxazoline moiety (R = Me, i-Pr) was
used to control the generation of planar chirality by lithiation,
with the alternative diastereoisomer formed by use of a deuterium
blocking group. In each case subsequent addition of PhPCl2 followed by o-TolMgBr resulted in a single P-stereogenic diastereoisomer (Sc,Sp,Sphos and Sc,Rp,Rphos, respectively). The alternative
diastereoisomers were formed selectively by addition of o-TolPCl2 followed by PhMgBr ((Sc,Sp,Rphos and Sc,Rp,Sphos, respectively). Preliminary application
of these four ligand diastereoisomers, together with (Sc,Sp) and (Sc,Rp) Phosferrox (PPh2), to palladium catalyzed allylic alkylation of trans-1,3-diphenylallyl acetate revealed a stepwise increase/decrease
in ee, with the configuration of the matched/matched diastereoisomer
as Sc,Sp,Sphos (97% ee).
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