Asymmetric synthesis with chiral cyclic phosphorus auxiliaries.
A review presents phosphoryl-stabilized a-, b-, and g-carbanions which can be used in stereoselective alkylations, allylations, aminations and rearrangements as well as in Michael addn. and cyclopropanation reactions. Stoichiometric protocols are also used in asym. Wittig-Horner olefinations. The use of the P:O group as a Lewis base opens the path to catalytic carbonyl addns. of various organometallic reagents. With trichlorosilylenolethers a new catalytic version of the aldoladdn. was invented. Phosphinamide borane complexes are the new brother of the famous CBS process for the asym. redn. of ketones. Epoxides can be ring-opened by SiCl4/P:O-Lewis base or as vinyl oxiranes with organocuprates, catalyzed by phosphoramidites. The latter method found widespread use for the conjugate addn. to a,b-unsatd. systems. Interesting enough, similar phosphites and phosphoramidites can also be used as ligands in hydrogenations and hydroformylations. Finally, the predecessors of many of these ligands were introduced as chiral derivatizing and shift reagents.
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