Infoscience

Journal article

Chiral Phosphites and Phosphoramidites Based on the Tropane Skeleton and Their Application in Catalysis

Chiral BINOL phosphorochloridites were esterified and amidated with enantiopure 6-hydroxytropinone [6-hydroxy-8-azabicyclo[3.2.1]octan-3-one] cyclic acetal to yield chiral bidentate and monodentate phosphite-phosphoramidite ligands for copper-catalyzed asym. conjugate addn. of diethylzinc to cyclic enones. Condensation of N-protected pyrrole with tetrabromoacetone with subsequent acetalization and asym. hydroboration gave (6S)-6-hydroxy-8-azabicyclo[3.2.1]octan-3-one ethylene glycol acetal (8), which was treated with (2S)- and (2R)-BINOL phosphorochloridite esters (7a,b), affording chiral bidentate phosphite-phosphoramidite ligands, (S,S)-9a and (R,R)-9a; their octahydro analogs (S,S)-9b and (R,R)-9b were also prepd. Monodentate chiral phosphoramidite (6S)-6-methoxymethoxy-N-[(2S)-BINOL-phosphino]-8-azabicyclo[3.2.1]octan-3-one acetal [(S)-13] and 8-(2-methylthioethyl)-substituted tropinone acetal (2S)-BINOL phosphite esters (11a, b) were prepd. by phosphination of the corresponding N- or O-protected 8 with BINOL- or octahydro-BINOL phosphorochloridites. When applied to Cu-catalyzed asym. 1,4-addns. of dialkylzinc to cyclic enones, ranging from 2-cyclopentenone to 2-cyclooctenone (14a-d) and acyclic enones RCH:CHCOR1 (16a-d; R = Bu, Ph, 2-thienyl; R1 = Me, Ph), the bidentate phosphoramidite (S,S)-9a bearing two (S)-binaphthol units worked selectively in most cases affording the alkylation products with up to 90% ee. Ligand (S,S)-9a was also tested in Rh-mediated asym. hydrogenation of di-Me itaconate and Me acetamidocinnamate, exhibiting 85% ee and 95% ee selectivity superior to the majority of other ligands. In both reactions, a matched/mismatched case was obsd. for (S,S)-9a and its diastereomer (R,R)-9a.

Fulltext

Related material

Contacts

EPFL authors