HETEROCYCLES

Communication | Special issue | Vol. 86, No. 1, 2012, pp. 139-146
Received, 9th June, 2012, Accepted, 20th July, 2012, Published online, 30th July, 2012.
DOI: 10.3987/COM-12-S(N)30

■ DESIGN AND SYNTHESIS OF A C2-SYMMETRIC CHIRAl 1,2-BIS(DIPHENYLPHOSPHINO)BENZENE LIGAND VIA RHODIUM-CATALYZED INTRAMOLECULAR [2+2+2] CYCLOADDITION

Fumiya Mori, Keiichi Noguchi, and Ken Tanaka*

Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo184-8588, Japan

Abstract

The design and synthesis of a C2-symmetric chiral 1,2-bis(diphenylphosphino)benzene ligand via the rhodium-catalyzed intramolecular [2+2+2] cycloaddition followed by reduction are disclosed. This new chiral dppbz-type ligand could be employed for the rhodium-catalyzed asymmetric alkene hydrogenation.

1,2-Bis(diphenylphosphino)benzene (dppbz) has been widely employed as a useful ligand for a number of transition-metal-catalyzed reactions.1 As dppbz is an achiral ligand, asymmetric variants of transition-metal-catalyzed reactions, in which the dppbz ligand is indispensable, cannot be developed. Therefore, the design and synthesis of chiral dppbz-type ligands are important subjects.2 On the other hand, the cationic rhodium(I)/biaryl bisphosphine complex-catalyzed [2+2+2] cycloadditions3–5 involving phosphorus-substituted alkynes are useful methods for the synthesis of phosphorus-substituted benzenes.6,7 In order to synthesize dppbz-type ligands, we attempted the reactions of terminal and internal 1,6-diynes 1a,b with bis(diphenylphosphinoyl)acetylene (2) in the presence of a cationic rhodium(I)/rac-BINAP complex (10 mol %), while the expected 1,2-bis(diphenylphosphinoyl)benzenes 3a,b were not obtained at all even at the elevated temperature (80 °C) (Scheme 1).

Thus, we designed the intramolecular [2+2+2] cycloaddition of triyne diphosphine oxide 4a, which would furnish tricyclic 1,2-bis(diphenylphosphinoyl)benzene 5a (Scheme 2). In this reaction, the use of chiral propargyl alcohol-derived triyne diphosphine oxide 4b would furnish C2-symmetric chiral tricyclic 1,2-bis(diphenylphosphinoyl)benzene 5b (Scheme 2). In this Communication, we disclose the synthesis of a new chiral 1,2-bis(diphenylphosphino)benzene ligand as well as achiral one via the rhodium-catalyzed intramolecular [2+2+2] cycloaddition.

We first investigated the [2+2+2] cycloaddition of achiral triyne 4a (Table 1). Pleasingly, the desired benzene 5a was obtained in good yield at room temperature by using a cationic rhodium(I)/BIPHEP complex (entry 1). Screening of biaryl bisphosphine ligands (Figure 1, entries 1–4) revealed that the use of BINAP furnished 5a in the highest yield (entry 2). Thus obtained diphosphine oxide 5a was reduced to the corresponding diphosphine ligand 6a in moderate yield by treatment with HSiCl3 and Me2NC6H5 (Scheme 3).

Next, we examined the synthesis of chiral triyne diphosphine oxide 4b starting from commercially available chiral propargylic alcohol (R)-7. Although the double etherification of dibromide 8 with racemic (±)-7 leading to triyne 9 was first examined, complete decomposion of 8 was observed (Scheme 4).

Therefore, the stepwise etherification was examined as shown in Scheme 5. The etherification of bromide 108 with (R)-7 (98% ee) followed by desilylation afforded diyne (R)-11 in good yield. Bromination of (R)-11 furnished bromide (R)-12 in good yield. The second etherification of (R)-12 with (R)-7 also proceeded to give the desired triyne (R,R)-9 in high yield. The subsequent copper(I)-catalyzed double diphenylphosphination9 followed by oxidation with H2O2 furnished chiral triyne diphosphine oxide (R,R)-(+)-4b in high yield. In order to optimize the [2+2+2] cycloaddition step, the reaction of diastereomeric mixture of triyne 4b, prepared from racemic propargylic alcohol (±)-7, was examined by using various biaryl bisphosphine ligands (Table 2). Among the ligands examined (Figure 1, entries 1–4), the use of BIPHEP furnished the desired benzene 5b with the highest selectivity at room temperature for 16 h, although 4b was still remained in 24% (entry 1). Pleasingly, when the reaction of chiral triyne (R,R)-(+)-4b was conducted at room temperature for 24 h, complete conversion of 4b was observed to give (R,R)-(+)-5b in good yield (Scheme 5). Thus obtained diphosphine oxide (R,R)-(+)-5b could be reduced to the corresponding diphosphine (R,R)-(+)-6b (98% ee) by treatment with HSiCl3 and iPr2NH, although the product yield was low.

The ORTEP diagrams of [Rh(cod)((+)-6b)]SbF6 are shown in Figure 2.10 In the front view, four phenyl groups on the phosphorus atoms are distorted C2-symmetrically due to steric repulsions between the methyl and phenyl groups. However, in the side view, four phenyl groups on the phosphorus atoms are not distorted presumably due to the high rigidity of the ligand backbone.

In order to evaluate a potential applicability of the chiral diphosphine ligand (R,R)-6b (98% ee), its application to the rhodium-catalyzed asymmetric hydrogenation of alkenes was examined briefly (Table 3). For comparison purpose, the same reactions were conducted using (S)-BINAP and (S,S)-Chiraphos, possessing the distorted backbone, as a ligand under the same reaction conditions. In the hydrogenation of sterically less demanding α-substituted acrylates 13a,b, the ee values using (R,R)-6b (entries 1 and 4) were significantly lower than those using (S)-BINAP7d and (S,S)-Chiraphos (entries 2, 3, 5, and 6). On the contrary, in the hydrogenation of sterically demanding α,β-disubstituted acrylamide 13c, the conversion and/or ee value using (R,R)-6b (entry 7) were markedly higher than those using (S)-BINAP and (S,S)-Chiraphos (entries 8 and 9). Although there remains room for improvement in the enantioselectivity, these results would demonstrate the potential utility of the new C2-symmetric chiral dppbz-type ligand for asymmetric catalyses.

In conclusion, we have disclosed the design and synthesis of a C2-symmetric chiral 1,2-bis(diphenylphosphino)benzene ligand via the rhodium-catalyzed intramolecular [2+2+2] cycloaddition. This new chiral dppbz-type ligand could be employed for the rhodium-catalyzed asymmetric alkene hydrogenation. Future work will focus on further tuning of chiral ligand structures and their application to asymmetric catalyses.

ACKNOWLEDGEMENTS
This work was supported partly by a Grant-in-Aid for Scientific Research (No. 20675002) from MEXT, Japan. We thank Takasago International Corporation for the gift of chiral propargylic alcohol (R)-7, and Umicore for generous support in supplying of a rhodium complex.

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