Synthesis and Resolution of 3,3′-Disubstituted xylBINAP Derivatives and Their Application in Rhodium-Catalyzed Asymmetric Hydrogenation

Abstract

A novel class of 3,3′-disubstituted xylBINAP ligands have been synthesized and tested in the hydrogenations of substituted olefins. This new substitution pattern has demonstrated that the 3,5-dialkyl meta effect and 3,3′-disubstitution can operate in a synergistic fashion in Rh-catalyzed hydrogenation of dehydroamino acids. Notably, (S _ax )-8 outperforms BINAP, xylBINAP and previously reported 3,3′-disubstituted BINAP derivatives in the hydro­genation of methyl N-acetamido cinnamate.

Key words

ligands - hydrogenations - asymmetric catalysis - homogeneous catalysis - rhodium

References and Notes

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8

This alcohol is available from standard LiAlH₄ reduction
of (+)-(S)-2-(6-methoxynaphthalen-2-yl)propanoic acid [(S)-naproxen].

9

Attempts at forming the ether linkage via alkylative methods failed and resulted in elimination rather than substitution

10

(S _ax )-8, R _f  = 0.35; (R _ax )-7, R _f  = 0.25 (EtOAc-hexanes, 35%)

11

Cleavage of the chiral auxiliary followed by alkylation with MeI successfully afforded 3,3′-(MeO)₂-xylBINAP(O). However, exposing 3,3′-(MeO)₂-xylBINAP(O) to the reduction conditions caused demethylation. Use of alternative reduction conditions (AlH₃˙THF) resulted in decomposition.

17

General Procedure for Rh-Catalyzed Asymmetric Hydrogenations: Rh(nbd)₂BF₄ (3.7 mg, 0.01 mmol), ligand (0.011 mmol) and MeOH (1 mL) were combined under an inert atmosphere and stirred for 30 min at room temperature. Olefin (1 mmol) was added with an additional aliquot of MeOH (1 mL). The reaction mixture was then subjected to three consecutive freeze/pump/thaw cycles on a double manifold (pump time = 10 min). Upon warming to room temperature, the reaction was placed under a hydrogen atmosphere (2 atm) for 7 h. Upon completion, volatiles were removed in vacuo and the remaining residue was passed through a plug of silica gel (hexanes-EtOAc, 1:1). Upon solvent evaporation, the isolated product was used directly for either chiral GC or chiral HPLC analysis.
Chiral GC data for N-(1,2,3,4-tetrahedronaphthalen-1-yl)acetamide(21): Cyclodex B column; isothermal 170 ˚C; t _R1 [(R)-21] = 23.5 min, t _R2 [(S)-21] = 24.4 min.
Chiral HPLC data for methyl α-acetamido cinnamate (23): Chiral HPLC, Chiralcel OD; hexane-i-PrOH, 9:1; 1.0 mL/min; t _R1 [(R)-23] = 12.4 min, t _R2 [(S)-23] = 16.3 min.