HETEROCYCLES

Communication | Special issue | Vol. 80, No. 2, 2010, pp. 933-939
Received, 9th October, 2009, Accepted, 4th December, 2009, Published online, 1st February, 2010.
DOI: 10.3987/COM-09-S(S)134

■ Helical Chirality Control of Tropos Sandwich-Shaped L2M3 Complexes with C3-Symmetric Tris(diphenylphosphinophenyl)benzene Ligand

Kazuki Wakabayashi and Koichi Mikami*

Department of Applied Chemistry, Graduate School of Science and Technology, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan

Abstract

The L2M3 complexes with tropos C3-symmetric ligands interconvert rapidly between the chiral propeller (P)- and (M)-helicity of the sandwich-shaped L2M3 complexes at room temperature and are chirally controlled to adopt a single helical structure upon complexation with a chiral diamine. The L2M3 complexes chirally controlled can be employed for asymmetric transfer hydrogenation.

Various asymmetric catalysts with atropisomeric (atropos in Greek)1 ligands have been developed to attain high enantioselectivity.2 In contrast, we have reported that chirally flexible (tropos)1 benzophenone-derived ligands can be controlled to a single chiral conformation by a chiral activator and to attain higher enantioselectivity.3,4 For example, tropos benzophenone-derived ligands, 2,2’-diphenylphosphinobenzophenones (DPBP) could be chirally controlled to a single conformation with chiral diamines such as 1,2-dipenylethylenediamine (DPEN) to provide higher enantioselectivity in the Ru complex-catalyzed asymmetric hydrogenation of simple ketone substrates (up to >99%, 99% ee).3a,b DPBP can also be employed to give much higher enantioselectivity than the enantiopure atropos BINAP in the Rh complex-catalyzed asymmetric transfer hydrogenation of simple ketone substrates (up to >99%, 99% ee).4 Furthermore, DPBP is now commercially available from Sigma-Aldrich Co. (Catalog No. 845821-92-3). Other tropos ligands also adopt a chiral conformation even in a solution phase and exhibit advantageous properties over atropos ligands.5 In modification of the benzophenone-derived diphenylphosphine ligand (DPBP), the introduction of one more diphenylphosphinophenyl part was executed to construct C3-symmetric tropos ligands (Scheme 1) which could adopt a chiral propeller conformation. We report here that the C3-symmetric tropos ligand can also be controlled to a single chiral conformation upon addition of a chiral diamine.

The C3-symmetric tropos ligand consists of the three coordination parts and the central core (Y). Just like the benzophenone (DPBP) ligand, the rotational barrier around the single bond between the coordinating part and the core (Y) should be low. We synthesized the more stable C3-symmetric tropos ligand with the coordinating 3-(diphenylphosphino)phenyl part and the benzene core (Y = Ph) (Scheme 2). The 1,3,5-tris(3’-diphenylphosphinophenyl)benzene (C3-(diphenylphosphino)phenylbenzene: C3-DPPB) was synthesized from 1,3,5-tris(3’-hydroxyphenyl)benzene according to the synthetic method of BIPHEP from biphenol.5b 1,3,5-Tris(3’-hydroxyphenyl)benzene was prepared from 1,3,5-tribromobenzene and 3-methoxyphenylboronic acid by the Suzuki-Miyaura coupling.6

The C3-DPPB ligand has three freely rotational single bonds between the phenyl core and, hence, interconverts rapidly between the helical conformations ((P) and (M)). The C3-symmetric triphosphine ligands with metal sources (M = Pd, Rh) gave the sandwich-shaped L2M3 complexes,7,8 which rapidly interconverted between (P)- and (M)-helical conformations (Scheme 2).
The X-ray structural analysis of Pd3C16(C3-dppb)2 showed that the L2M3 complex with the C3-symmetric ligand adopted D3-symmetric conformation (Figure 1).9 The top view of Pd3C16(C3-dppb)2 showed the C3-helical conformation. On the other hand, the side view of Pd3C16(C3-dppb)2 showed the C2-symmetric conformation around the Pd metal.

The chirality control of the L2M3 complexes (L = C3-DPPB (1: Ar = phenyl) and C3-DM-DPPB (1: Ar = 3,5-xylyl), M = Pd and Rh) was examined upon addition of (S,S)-DPEN. The L2M3 complex with Rh ([Rh3(nbd)3(C3-dm-dppb)2](SbF6)3)10 was instantaneously controlled in a single chiral conformation upon complexation with (S,S)-DPEN (Scheme 3); The Rh-C3-DM-DPPB complex with (S,S)-DPEN could form two diastereomers, ((P)/(S,S) and (M)/(S,S)) but the Rh3(C3-dm-dppb)2[(S,S)-dpen]3 complex was instantly controlled in a single diastereomer.11 The 31P NMR spectrum of the Rh complex only showed the doublet peak for the single diastereomer: 31P NMR (CDC13, 162 MHz) δ 49.80 (d, ЈRh-P = 132.4 Hz). The Rh3(C3-dppb)2 complex with (S,S)-DPEN was also controlled to a single chiral conformation: 31P NMR (CDC13, 162 MHz) δ 50.31 ppm (d, ЈRh-P = 133.6 Hz)). Unfortunately, Pd3C16(C3-dm-dppb)2 were not coordinated with DPEN.

The helicity of diphenylphosphine complexes is thus controlled by chiral diamines where the steric interaction is operative between the equatorial amine protons of the chiral diamines and the phenyl groups on the phosphine ligands;3-5,12 In Figure 2, the C3-DPPB metal complex is exemplified in the (M)-conformation. With the equatorial amine protons of (R,R)-DPEN (Figure 2b), the phenylphosphine groups in the (M)-conformation exhibit the repulsive interaction. Therefore, the C3-DM-DPPB-Rh complexes with (S,S)-DPEN are deduced to adopt the (M)-conformation as shown in Figure 2a.

The Rh complexes with the C3-symmetric ligands thus chirally controlled to the single (M)-conformation can be used as asymmetric catalysts in the asymmetric transfer hydrogenation.13,14 Under the reaction conditions, the Rh complex with C3-DPPB and (S,S)-DPEN was not so stable. To stabilize the C3-DPPB complex, the bulky C3-DM-DPPB ligand was employed for the transfer hydrogenation of aromatic ketone (Table 1). The C3-DM-DPPB-Rh complex with (S,S)-DPEN gave the hydrogenation product with 82% ee (entry 1). The enantioselectivity thus obtained is higher than that obtained with the enantiopure (R)-BINAP4 (entry 3). The C3-DM-DPPB-Rh complex was also chirally controlled to a single helical conformation with (S)-diaminobinaphthyl (DABN) instead of DPEN but did not provide the hydrogenation product because of the deactivating nature of DABN12b (entry 2). The C3-DM-DPPB-Rh complex with DPEN thus gave the transfer hydrogenation product with 82% ee. The enantioselectivity with the tropos C3-DM-DPPB-Rh complex is higher than that obtained with the atropos and enantiopure BINAP counterpart.

We have thus reported the chirality control of tropos C3-symmetric triphosphine ligands. The C3-symmetric DPPB ligand gave the corresponding tropos L2M3 complexes of which the helicity can be controlled by chiral diamines such as DPEN to the single helical structure. The tropos L2M3 complexes thus chirally controlled can be used in the asymmetric transfer hydrogenation of a ketone substrate to attain higher enantioselectivity than the atropos and enantiopure BINAP counterpart.

References

1. The word atropos consists of “a” meaning “not” and “tropos” meaning “turn” in Greek. Therefore, the chirally rigid or flexible nature of a ligand can be called atropos or tropos, respectively. K. Mikami, K. Aikawa, Y. Yusa, J. J. Jodry, and M. Yamanaka, Synlett, 2002, 1561; CrossRef Also see: W. Kuhn, 'Stereochemie', ed. by K. Freudenberg, Franz Deuticke, Leipzig, 1933, pp. 803-824.
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