Chirality in gold(III) homodimeric complexes

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Chirality in gold(III) homodimeric complexes

Huy-Dinh Vu, Jacques Renault, Thierry Roisnel, Philippe Uriac

To cite this version:

Huy-Dinh Vu, Jacques Renault, Thierry Roisnel, Philippe Uriac. Chirality in gold(III) homodimeric complexes. Tetrahedron Letters, Elsevier, 2020, 61 (39), pp.152323. �10.1016/j.tetlet.2020.152323�.

�hal-02957773�

2

Chirality in gold(III) homodimeric complexes

Huy-Dinh Vu

, Jacques Renault

, Thierry Roisnel

, and Philippe Uriac

———

*

Corresponding author. Fax: +33 2 23234425. E-mail: philippe.uriac@univ-rennes1.fr

aUniv Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) - UMR 6226, F-35000 Rennes, France

bDepartment of Chemistry, Vietnam National University of Forestry, Hanoi, Vietnam

cUniv Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226, ScanMat - UMS 2001, F-35000 Rennes, France

Numerous stable and reactive gold(III) complexes have been described;

however, most of them do not present efficient chirality because of their square-planar structure. Recently, the successful introduction of chirality (Fig. 1) has been achieved by two main strategies: the use of chiral BINOL

or

cyclometalated gold(III) complexes

(Wong) and the use NHC ligands

(Toste). Other complexes, based on bisoxazoline (BOX) and 2-pyridyl-(–) menthol ligands

(Fiksdahl) or isothioureas

have also been reported. The conception of these chiral gold(III) complexes has allowed catalytic developments.

Figure 1. Selected chiral gold(III) complexes.

In a previous paper,

we have reported the synthesis of aurocycles which are able to dimerize. Interestingly, these dimeric gold(III) complexes presented a chirality axis. In this paper we report how this chirality axis could be used to prepare chiral homodimeric complexes. Their catalytic activity was

illustrated by their ability to promote the cyclization of a representative N-Boc amino-ynone.

According to the literature procedure,

di-protected (S)-4- hydroxymethylpyrrolidin-2-one 1 was reacted with lithium phenyl acetylide to give

2, which was cyclized into ligand 3 in the presence of 5 equivalents of a 1M ZnCl

solution in ether (Scheme 1).

Scheme 1. Synthesis of ligand 3.

In the presence of 1 equivalent of AuCl

the vinylgold complex 4 was obtained in 30% yield (estimated purity = 90%) whereas the use of NaAuCl

led to 4’. As previously reported

HCl was generated

4 was treated with 1 equivalent of AuCl

in CDCl

the formation of aurocycle 5 was observed. In this case the HCl formed during the cyclisation process induced A B S T R A C T

Vinylgold complex 4 and aurocycle 5 were successively prepared from the protected (S)-2-hydroxymethyl-5-alkynyl-3,4-dihydro-2H-pyrrole using AuCl³. The dimerization of 5 in the presence of K²CO³ gave the chiral homodimers 6a(S,aR) + 6b(S,aS) which exhibited catalytic activity with AgSbF⁶. Using the X-ray data obtained for 5 and 6a, a transition state explaining the diastereoselectivity (de = 50 at –50 °C) was proposed.

Keywords:

Aurocycle Catalyst

Chiral cyclic imine

Chiral gold(III) homodimeric complexes

Scheme 2. Synthesis of aurocycle 5.

The NMR data of 5 were in accordance with the proposed structures as well as the HRMS data ([M-H]

; C

H

NO

Cl

Au : 469.9658). In the

C NMR spectra the loss of one aromatic CH as well as the

Hz) of the carbon in the

to the cyclisation. Finally, crystals of 5, suitable for X-ray study, were obtained by slow evaporation of the NMR solvent (CD

COCD

). In the crystal structure (Fig. 2a), as previously observed,

aurocycle 5 was almost planar because of the ionic interactions between the protonated nitrogen atom and the metallic center (distances: 2.953, 2.973 Å). Non-covalent interactions between two molecules of 5 could also be envisaged (Fig. 2b) involving π-stacking, electrostatic interactions (NH

- Au

distances: 4.139 and 3.986 Å) and hydrogen bonding (OH- Cl distances: 2.558-2.763 Å).

Figure 2a. Molecular structure of 5.

Figure 2b. Ionic interactions in the crystal of 5.

Finally, the dimerization of 5 leading to a mixture of the homodimeric complexes 6a (major) + 6b (minor) was quantitatively performed using the procedure previously reported (addition of K

CO

to a suspension of 5 in CDCl

at room temperature) (Scheme 3). The diastereoisomeric excess determined by

H NMR spectroscopy was low: de = 20 (Fig. 4).

When the reaction was performed on a solution of 5 in

The dimeric structure was determined by HRMS: [M-Cl]

: C

H

N

O

Cl

Au : 964.9619 and by NMR spectroscopy. For example, in the

H NMR spectra duplication of the signals of the heterocyclic CH

was observed. The number of carbon signals observed clearly indicated that only homodimers have been synthesized. Analysis of the crystals obtained by slow evaporation of the solvent in the NMR tube allowed us to determine the configuration of the chirality axis (Fig. 3) of the major diastereomer 6a: aR. In 6a the hydroxymethyl substituents were in exo-positions.

Figure 3. Molecular structure of 6a.

Figure 4.

H NMR determination of the de.

This stereoselectivity could be explained by a concerted HCl

elimination involving deprotonation of the iminium by K

CO

followed by ligand exchange (nitrogen in place of Cl

) on the 3

gold atom. Two transition states I and II (Fig. 5) were possible depending on the rotation along the chirality axis. The interactions of the chlorine atoms with the hydroxymethyl substituent could stabilize I

and assist the Cl

elimination to give homodimer 6a as the major product.

Figure 5. Transition states I (aR) and II (aS).

Finally, the catalytic activity of the mixture 6a + 6b (60/40) was examined using

previously synthetized in our laboratory.

The conversion into fluorescent pyrrolidinone 8 was complete after 12 h (Scheme 4) using 5% 6 in the presence of AgSbF

(1 eq.).

Scheme 4. Catalytic activity of 6a-b.

Conclusion

This preliminary result prompted us to design and prepare other complexes involving pyrrolidine or piperidine rings

with various substituents on the chiral C-2 in order to control the diastereoselectivity and obtain enantiopure chiral homodimeric gold(III) complexes. Such complexes will be tested on the model reactions proposed in the literature.

Acknowledgments

The authors are deeply grateful to P. Jéhan for MS spectrometry (CRMPO, Scanmat, University of Rennes1) and to A. Bondon for NMR spectroscopy (PRISM, University of Rennes1).

References and notes

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Supplementary Material

Supplementary data associated with this article can be found, in the online version, at http….

CCDC 1996822-1976823 contain the supplementary crystallographic data for 5 and 6a. This data can be obtained free of charge

emailing

Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Highlights

homodimeric complexes

Graphical Abstract