Synthesis of axially chiral oxazoline-carbene ligands with an N-naphthyl framework and a study of their coordination with AuCl·SMe2

Article abstract of DOI:10.3762/bjoc.8.81

Axially chiral oxazoline-carbene ligands with an N-naphthyl framework were successfully prepared, and their coordination behavior with AuCl·SMe2 was also investigated, affording the corresponding Au(I) complexes in moderate to high yields.

Full text of DOI:10.3762/bjoc.8.81

Synthesis of axially chiral oxazoline–carbene ligands
with an N-naphthyl framework and a study of their
coordination with AuCl·SMe2
Feijun Wang*1, Shengke Li1, Mingliang Qu1, Mei-Xin Zhao1, Lian-Jun Liu1
and Min Shi*1,2
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 726–731.
1Key Laboratory for Advanced Materials and Institute of Fine
Chemicals, East China University of Science and Technology,
130 MeiLong Road, Shanghai 200237, P. R. China and 2State Key
Laboratory of Organometallic Chemistry, Shanghai Institute of
Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin
Road, Shanghai 200032, P. R. China
doi:10.3762/bjoc.8.81
Received: 02 March 2012
Accepted: 19 April 2012
Published: 11 May 2012
Associate Editor: I. Marek
Email:
Feijun Wang* - feijunwang@ecust.edu.cn; Min Shi* -
Mshi@mail.sioc.ac.cn
© 2012 Wang et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
axially chiral ligand; gold; N-heterocyclic carbenes; N-naphthyl
framework
Abstract
Axially chiral oxazoline–carbene ligands with an N-naphthyl framework were successfully prepared, and their coordination behav-
ior with AuCl·SMe2 was also investigated, affording the corresponding Au(I) complexes in moderate to high yields.
Introduction
During the past decade, with an explosive growth of asym- effective ligands for a number of homogeneous gold catalyzes
metric homogeneous gold catalysis in C–C, C–O, or C–N bond [1-8]. However, during our ongoing survey of chiral
formations, the design and synthesis of chiral gold complexes NHC–Au(I) complexes in the literature, we only found a few
has received wide attention. Compared with the more unique papers of relevance. Tomioka and co-workers disclosed
commonly used air-sensitive phosphine ligand, N-heterocyclic the first chiral NHC–Au(I) complex 1 (Figure 1), which was
carbenes (NHCs), with intrinsic characteristics such as strong applied to catalyze the asymmetric cyclization of 1,6-enynes
δ-donor but poor π-acceptor abilities, ease of preparation, air giving the corresponding cyclopentane derivatives with
and thermal stability of their metal complexes, and the con- moderate enantioselectivity up to 59% [9,10]. Iglesias and
venient introduction of chiral elements, have also emerged as co-workers reported a type of NHC–Au(I) complexes 2
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Beilstein J. Org. Chem. 2012, 8, 726–731.
Figure 1: Chiral NHC–Au(I) complexes.
containing C2-symmetric bis(NHC)-ligands with two to achieve the kinetic resolution of Morita–Baylis–Hillman
imidazolin-2-ylidene moieties on a chiral dioxolane backbone, adducts, affording up to 99% ee of the (E)-allylation products
produced in up to 95% ee by hydrogenation of a prochiral and 92% ee of the recovered Morita–Baylis–Hillman adducts.
alkene [11]. Recently, Toste and co-workers reported a novel These intriguing results stimulated us to further develop the
family of axially chiral (acyclic diaminocarbene) gold(I) axially chiral oxazoline–carbene ligands 7 with an N-naphthyl
complexes 3 derived from 3,3′-substituted 1,1′-binaphthalenyl- framework and to evaluate their coordination with AuCl·SMe2.
2,2′-diamine, and their application in the dynamic kinetic asym-
Results and Discussion
metric transformation of propargyl esters, giving the corres-
ponding substituted chromenes in up to 99% ee [12]. Our group Synthesis of axially chiral ligands
also developed a new family of axially chiral NHC–Au(I) Initially, we attempted to synthesize the desired axially chiral
complexes (4–6) with a binaphthyl or biphenyl framework ligands 7, and the synthetic route is shown in Scheme 1. Using
[13,14]. These Au(I) complexes were applied to catalyze the methyl 1-hydroxy-2-naphthoate (9) as the starting material, tri-
asymmetric cyclization of 1,6-enynes or allene in up to 70% ee, fluoromethylation with Tf2O in the presence of pyridine
and the asymmetric intramolecular hydroamination of allene in afforded its trifluoromethanesulfonate 10 in 95% yield, which
up to 44% ee.
was made to react with 2-nitroaniline in the presence of
Pd(OAc)2/DPE-phos catalytic system to give the corresponding
We previously reported a novel type of axially chiral ligand 7 coupling compound 11 in 98% yield (Scheme 1). Subsequent
with an N-naphthyl framework (Figure 2) instead of traditional reduction of compound 11 with Pd/C under a hydrogen atmos-
binaphthyl framework [15]. Their palladium complexes 8 phere produced the desired compound 12 in 99% yield. Cycliza-
showed high stereoselectivities in asymmetric allylic arylations tion of 12 in the presence of triethyl orthoformate and a
catalytic amount of TsOH at 80 °C gave the corresponding
benzimidazole derivative 13 in 76% yield, which was further
treated with (S)-2-amino-2-phenylethanol in the presence of
Cs2CO3 in toluene to afford the corresponding amide 14a as a
mixture of diastereomeric isomers successfully in 91% yield.
To our delight, the two diastereomeric isomers with a chiral
N-naphthyl axis, (Sa,S)-15a and (Ra,S)-15a, were synthesized
from amide 14a according to the classical synthetic method for
the preparation of an oxazoline ring, and were easily isolated by
Figure 2: Axially chiral oxazoline–carbene ligands and their palladium
complexes.
silica gel column chromatography in 37% yield and 44% yield,
respectively. Similarly, using L-valinol, (Sa,S)-15b and (Ra,S)-
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Beilstein J. Org. Chem. 2012, 8, 726–731.
Scheme 1: Synthesis of axially chiral benzimidazole derivatives.
15b could also be synthesized. Quaternization of the benzimida- (Sa,S)-8 (R2 = Me) (Figure 3C), the chemical shifts of the
zole ring of (Sa,S)-15 and (Ra,S)-15 with R2I (R2 = Me, Et) gave protons of the coordinated oxazoline group changed signifi-
the corresponding benzimidazolium salts 7, respectively.
cantly. In order to confirm this hypothesis, complex (Sa,S)-16aa
was recrystallized from CH2Cl2/petroleum ether (1/3, v/v), and
its structure was established by single-crystal X-ray diffraction
Coordination study with AuCl·SMe2
The coordination behavior of ligand 7 with Pd(OAc)2 has been studies (Figure 4; Supporting Information File 2). The crystal
disclosed in previous work. However, only (Sa,S)-7 could give data of Au(1)–C(7) (1.991(5) Å) is a typical Au–CNHC bond
the corresponding Pd-complex (Sa,S)-8, while the Pd(II)-com- length, in-line with those of other reported examples [16-21].
plex with R-geometry of the chiral N-naphthyl axis could not be Furthermore, the angle of I(1)–Au(1)–C(7) = 173.12(16)°
obtained from (Ra,S)-7. With the NHC precursors 7 in hand, suggests a nearly linear coordination geometry around the
their coordination with AuCl·SMe2 in the presence of NaOAc in gold(I) center, which is also a typical feature for known gold(I)
acetonitrile was further examined. Au(I)-complexes were complexes. Moreover, no coordinated oxazoline group in the
isolated by flash column chromatography. Comparing the complexes (Sa,S)-16 and (Ra,S)-16 could be further coordinated
chemical shifts of protons on the oxazoline ring of the Au(I)- with other metal atoms such as Pd and Cu to furnish dual-metal
complex (Sa,S)-16aa (R1 = Ph, R2 = Me) in the 1H NMR spec- catalysts, which is a hot research field in asymmetric catalysis
trum (Figure 3B) with those of the NHC precursor (Sa,S)-7aa [22-24].
(Figure 3A), we found that the chemical shifts of these protons
did not change much, suggesting that the chiral oxazoline group Au(I)-complexes (Sa,S)-16 with different N-substituents or
may not coordinate with the Au atom. However, according to groups on the oxazoline ring were synthesized from the corres-
the analysis of the 1H NMR spectrum of the Pd(II)-complex ponding NHC precursors 7. The yields are summarized in
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Beilstein J. Org. Chem. 2012, 8, 726–731.
Figure 4: Solid-state molecular structure of (Sa,S)-16aa with thermal
ellipsoids at the 30% probability level. Selected bond distances (Å) and
angles (°): Au1–I1 2.5260(5), Au1–C7 1.991(5), C7–N1 1.334(7),
C7–N2 1.357(7), N2–C14 1.427(6), I1–Au1–C7 173.12(16),
Au1–C7–N2 124.5(4), C7–N2–C14–C15 105.8(6).
Table 1. It was found that the geometry the of chiral N-naph-
thyl axis had a significant influence on the yields of the Au(I)-
complexes. The salts (Sa,S)-7 with an S-geometry of the chiral
N-naphthyl axis gave higher yields of the corresponding Au(I)-
complexes than did the salts (Ra,S)-7, presumably due to the
steric repulsion of the phenyl group on the oxazoline ring [25-
Figure 3: The coordination study of the NHC precursor (Sa,S)-7aa with
metal salts by 1H NMR analysis of the chemical shifts of protons on the
oxazoline ring. (A) The 1H NMR spectrum of NHC precursor (Sa,S)-
7aa. (B) The 1H NMR spectrum of Au(I)-complex (Sa,S)-16aa. (C) The
1H NMR spectrum of Pd(II)-complex (Sa,S)-8 (R2 = Me).
Table 1: The coordination study with AuCl·SMe2.
entry
compound 15
R2I
salt 7
Au(I)-complex
yield (%)a
1
2
3
4
5
6
(Sa,S)-15a (R1 = Ph)
(Ra,S)-15a (R1 = Ph)
(Sa,S)-15a (R1 = Ph)
(Ra,S)-15a (R1 = Ph)
(Sa,S)-15b (R1 = iPr)
(Ra,S)-15b (R1 = iPr)
MeI
MeI
EtI
(Sa,S)-7aa
(Ra,S)-7aa
(Sa,S)-7ab
(Ra,S)-7ab
(Sa,S)-7ba
(Ra,S)-7ba
(Sa,S)-16aa
(Ra,S)-16aa
(Sa,S)-16ab
(Ra,S)-16ab
(Sa,S)-16ba
(Ra,S)-16ba
95
50
89
48
90
56
EtI
MeI
MeI
aisolated yield.
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Beilstein J. Org. Chem. 2012, 8, 726–731.
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only 50% yield (Table 1, entry 2).
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Conclusion
NHC-oxazoline bidentate ligands with an axially chiral N-naph-
thyl framework were synthesized, and their coordination
manners with AuCl·SMe2 were investigated. Diastereomeric
NHC precursors 7, (Sa,S)-7 and (Ra,S)-7, gave the corres-
ponding axially chiral NHC–Au complexes bearing a noncoor-
dinated oxazoline group. However, while the ligands (Sa,S)-7
gave their corresponding Au complexes (Sa,S)-16 in excellent
yields, only modest yields of complexes (Ra,S)-16 were
obtained for ligands (Ra,S)-7. Further studies focusing on the
preparation of conformationally stable and enantiomerically
pure transition-metal catalysts with an axially chiral N-aryl
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Acknowledgements
Financial support from the National Natural Science Founda-
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