DuPhos explained

DuPhos is a class of organophosphorus compound that are used ligands for asymmetric synthesis. The name DuPhos is derived from (1) the chemical company that sponsored the research leading to this ligand's invention, DuPont and (2) the compound is a diphosphine ligand type. Specifically it is classified as a C₂-symmetric ligand, consisting of two phospholanes rings affixed to a benzene ring.

The ligand was introduced in 1991 by M.J. Burk^{[1] [2]} and first demonstrated in asymmetric hydrogenation of certain enamide esters to amino acid precursors:

Other chiral diphosphine ligands were known at the time of invention, e.g. DIOP, DIPAMP, CHIRAPHOS, but DuPhos was found to be more effective.

Description

The ligand consists of two 2,5-alkyl-substituted phospholane rings connected by a 1,2-phenyl bridge. The alkyl group can be methyl, ethyl, propyl, or isopropyl. In the closely related bis(dimethylphospholano)ethane or BPE ligand ^{[3] [4]} the o-phenylene bridge is replaced by a 1,2-ethylene bridge. Both compounds can be obtained from the corresponding chiral diol through conversion to the cyclic sulfate and reaction with lithiated phenylbisphosphine. In DuPhos the phosphorus atoms are electron-rich making the resulting metal complexes reactive. The phosphorus atoms also introduce a kind of pseudo-chirality making enantioselection independent of the overall chemical conformation^[5]

Another early application is the synthesis of unnatural chiral amino acids in a formal reductive amination^[6] for example starting from benzophenone and the hydrazone of benzoyl chloride:^[7]

In the original scope the metal catalyst was rhodium but catalysis by ruthenium was introduced in 1995 ^[8] with the hydrogenation of the ketone group in β-keto esters:

Applications

An application of an asymmetric synthesis with a DuPhos ligand is the hydrogenation of dehydrowarfarin to warfarin:^[9]

Duphos is also applied in the synthesis of tryptophan derivatives.^[10]

In polymerization catalysis

DuPhos ligands are used in metal catalyzed alpha-olefin/carbon monoxide copolymerization to form chiral isotactic polyketones. The first publication in this field dates back to 1994 with catalyst system [Pd(Me-DuPhos(MeCN)₂)](BF4)₂^[11]

BozPhos ligand

Mono oxidation of (R,R)-Me-Duphos using borane dimethylsulfide as protective group and hydrogen peroxide as oxidizing agent gives bozPhos This ligand is useful in copper-catalyzed asymmetric addition of diorganozinc reagents to N-diphenylphosphinoylimines.

Notes and References

1. C₂-symmetric bis(phospholanes) and their use in highly enantioselective hydrogenation reactions. Mark J. Burk. J. Am. Chem. Soc.. 1991. 113. 22 . 8518–8519. 10.1021/ja00022a047.
2. Preparation and use of C₂-symmetric bis(phospholanes): production of .alpha.-amino acid derivatives via highly enantioselective hydrogenation reactions . Mark J. Burk . John E. Feaster . William A. Nugent . Richard L. Harlow . Journal of the American Chemical Society . 1993. 115. 22 . 10125–10138. 10.1021/ja00075a031.
3. New electron-rich chiral phosphines for asymmetric catalysis Mark J. Burk, John E. Feaster, Richard L. Harlow Organometallics, 1990, 9 (10), pp 2653–2655
4. New chiral phospholanes; Synthesis, characterization, and use in asymmetric hydrogenation reactions Tetrahedron: Asymmetry, Volume 2, Issue 7, 1991, Pages 569-592 Mark J. Burk, John E. Feaster, Richard L. Harlow
5. Recent Developments in Catalytic Asymmetric Hydrogenation Employing P-Chirogenic Diphosphine Ligands Karen V. L. Crépy, Tsuneo Imamoto Advanced Synthesis & Catalysis Volume 345 Issue 1-2, Pages 79 - 101 2003
6. Enantioselective hydrogenation of the C:N group: a catalytic asymmetric reductive amination procedure Mark J. Burk, John E. Feaster J. Am. Chem. Soc., 1992, 114 (15), pp 6266–6267
7. Catalytic asymmetric reductive amination of ketones via highly enantioselective hydrogenation of the C=N double bond Mark J. Burk, Jose P. Martinez, John E. Feaster and Nick Cosford Tetrahedron Volume 50, Issue 15, 11 April 1994, Pages 4399-4428
8. Practical asymmetric hydrogenation of β-keto esters at atmospheric pressure using chiral Ru (II) catalysts J. P. Genêt, V. Ratovelomanana-Vidal, M. C. Caño de Andrade, X. Pfister, P. Guerreiro and J. Y. Lenoir Tetrahedron Letters Volume 36, Issue 27, 3 July 1995, Pages 4801-4804
9. The first practical asymmetric synthesis of R and S-Warfarin Andrea Robinson and Hui-Yin Li John Feaster Tetrahedron Letters Volume 37, Issue 46, 11 November 1996, Pages 8321-8324
10. A highly enantioselective asymmetric hydrogenation route to β-(2R,3S)-methyltryptophan R. Scott Hoerrner, David Askin, R.P. Volante and Paul J. Reider Tetrahedron Letters Volume 39, Issue 21, 21 May 1998, Pages 3455-3458
11. Palladium(II)-Catalyzed Isospecific Alternating Copolymerization of Aliphatic .alpha.-Olefins with Carbon Monoxide and Isospecific Alternating Isomerization Cooligomerization of a 1,2-Disubstituted Olefin with Carbon Monoxide. Synthesis of Novel, Optically Active, Isotactic 1,4- and 1,5-Polyketones Zhaozhong Jiang, Ayusman Sen J. Am. Chem. Soc., 1995, 117 (16), pp 4455–4467