(Bicyclo[2.2.1]hepta-2,5-diene)[(R,R)-2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane]rhodium Tetrafluoroborate1

(BF4-)

[67650-23-1]  · C38H40BF4O2P2Rh  · (Bicyclo[2.2.1]hepta-2,5-diene)[(R,R)-2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane]rhodium Tetrafluoroborate  · (MW 780.39) (ClO4-)

[81445-14-9]  · C38H40ClO6P2Rh  · (Bicyclo[2.2.1]hepta-2,5-diene)[(R,R)-2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane]rhodium Perchlorate  · (MW 793.04) (PF6-)

[132014-70-1]  · C38H40F6O2P3Rh  · (Bicyclo[2.2.1]hepta-2,5-diene)[(R,R)-2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane]rhodium Hexafluorophosphate  · (MW 838.55)

(chiral catalyst precursor for the asymmetric hydrogenation of amino acid precursors as well as for hydrosilations, hydroborations, hydroformylations, alkene migration, and hydrogen transfer reactions2)

Alternate Name: (bicyclo[2.2.1]hepta-2,5-diene)(DIOP)rhodium tetrafluoroborate; (norbornadiene)(DIOP)rhodium tetrafluoroborate.

Physical Data: while physical data for the title complex are sparse, the analogous cod complex has been characterized fully. Physical data for [Rh(1,5-cod)(DIOP)]ClO4: mp 183-184 °C (dec); 1H NMR (CDCl3) d 1.09 (s, 6H, CH3), 2.37 (m, 8H, CH2), 2.71 (m, 4H, CH2), 3.67 (m, 2H, CH), 4.54 (m, 4H, CH=), 5.26-8.00 (m, 20H, Ph) ppm.

Solubility: sol EtOH, MeOH, CH2Cl2; sparingly sol THF; insol Et2O.

Form Supplied in: cationic rhodium-DIOP complexes are usually prepared in situ or from the procedure given below. In reactions employing the various rhodium-DIOP complexes, the diene ligand is frequently hydrogenated prior to catalysis to afford the active solvated catalyst precursors [RhS2(DIOP)]+.

Preparative Method: to an acetone (30 mL) solution of [Rh(1,5-cod)Cl]2 (0.27 g, 0.55 mmol) is added Silver(I) Perchlorate (0.233, 1.15 mmol) and the mixture is allowed to stir for 1 h at ambient temperature. Colorless precipitates are removed by filtration through filter paper and washed with dry acetone. To the pale yellow filtrate and washings is added DIOP ((2,3-O-Isopropylidene)-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane)1,3 (0.50 g, 1.15 mmol) in acetone. The crude complex is recrystallized from an acetone-ether mixture to give deep-orange crystals (0.37 g, 45%).4

Handling, Storage, and Precautions: cationic rhodium-DIOP complexes are air-sensitive and should be handled and stored under an inert atmosphere.

Hydrogenation.

Asymmetric hydrogenation catalyzed by chiral transition metal complexes is an active research area of synthetic organic chemistry, largely owing to its utilization in Monsanto's L-DOPA synthesis.2b Rhodium(I) complexes containing chiral phosphine ligands are used extensively; the standard for evaluating the efficiency of these catalyst precursors is the asymmetric hydrogenation of a-(N-acylamino)acrylates.5 Using the cationic rhodium-DIOP systems, hydrogenation of various precursors of alanine, phenylalanine, tyrosine, DOPA and leucine proceed almost quantitatively with optical yields approaching 95% ee.6-8 For example, hydrogenation of prochiral thienyl-substituted acylaminoacrylates gives the corresponding saturated propanoic acid derivatives as shown in eq 1.9

The naturally occurring amino acid phosphinothricin can be prepared by the asymmetric hydrogenation of a-acylamidoacrylate precursors with enantiomeric excesses reaching 70%.10 Likewise, Leu-enkephalins and other small peptides can be prepared from asymmetric hydrogenations, although the reactions are only moderately stereoselective (23% ee) using cationic rhodium-DIOP complexes.11 Although carbonyl functionalities can be stereoselectively reduced to the corresponding alcohols, optical yields are generally much lower than analogous reactions employing neutral rhodium-DIOP complexes.12 For instance, methyl 2-acetamido-3-oxobutyrate is hydrogenated en route to an enantioselective synthesis of D- and L-threonine in 90% yield but with enantiomeric excesses of only 18-25% when using cationic rhodium-DIOP systems (eq 2).13

The asymmetric catalytic hydrogenolysis of epoxides can also be accomplished with chiral rhodium complexes. Unfortunately, optical yields using cationic DIOP complexes are low (30%) and epoxides without carboxy functionalities are not hydrogenated under similar conditions.14 The addition of amines to the catalyst system has been found to enhance yields in the catalyzed hydrogenation of certain prochiral alkenes. Drastic increases in optical yields are observed in the asymmetric hydrogenation of 3-phenyl-3-butenoic acid upon addition of catalytic Triethylamine (eq 3).15 Finally, as is generally the case for all asymmetric catalyzed hydrogenations, optical yields are disappointingly low for the reduction of simple alkenes (ca. 15%).16

Modified DIOP Ligands.

Alkylated17 and sulfonated18 analogs of the DIOP ligand, as well as amine19 and boron20 derivatives, have been prepared recently. The corresponding cationic rhodium complexes are active catalysts for the asymmetric hydrogenation of amino acid precursors, generally proceeding with higher optical yields than reactions employing parent catalyst systems.

Alkene Migration.

Cationic rhodium-DIOP complexes are moderately effective for allylic hydrogen migration of secondary and tertiary allylamines to give the corresponding imines and (E)-enamines respectively (eq 4).4 Higher yields are found, however, in reactions employing neutral rhodium systems containing different chiral phosphine ligands.

Miscellaneous.

Cationic rhodium-DIOP complexes are active catalyst precursors for a number of asymmetric reactions including hydrosilation,21 hydroformylation,22 hydrogen transfer,23 and hydroboration (eq 5).24 Optical yields, however, are generally much lower than reactions employing other chiral phosphines (or their neutral phosphinorhodium counterparts) and are, therefore, not synthetically useful.


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Stephen A. Westcott

University of North Carolina, Chapel Hill, NC, USA



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