Synthesis of 1-[bis(trifluoromethyl)phosphine]-1'- oxazolinylferrocene ligands and their application in regio- and enantioselective Pd-catalyzed allylic alkylation of monosubstituted allyl substrates

Article abstract of DOI:10.3762/bjoc.10.126

A class of novel, easily accessible and air-stable 1-[bis(trifluoromethyl) phosphine]-1'-oxazolinylferrocene ligands has been synthesized from ferrocene. It became apparent that these ligands can be used in the regio- and enantioselective Pd-catalyzed allylic alkylation of monosubstituted allyl substrates in a highly efficient manner. Excellent regio- and enantioselectivity could be obtained for a wide range of substrates.

Full text of DOI:10.3762/bjoc.10.126

Synthesis of 1-[bis(trifluoromethyl)phosphine]-1’-
oxazolinylferrocene ligands and their application in
regio- and enantioselective Pd-catalyzed allylic
alkylation of monosubstituted allyl substrates
Zeng-Wei Lai1,2, Rong-Fei Yang3, Ke-Yin Ye1, Hongbin Sun*2
and Shu-Li You*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 1261–1266.
1State Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences, 345
Lingling Lu, Shanghai 200032, China, 2State Key Laboratory of
Natural Medicines and Center of Drug Discovery, China
Pharmaceutical University, 24 Tongjia Xiang, Nanjing 210009, China
and 3Process Development and Manufacturing Department,
Pharmaron (Beijing) Co. Ltd, 6 Taihe Road, BDA, Beijing, 100176,
China
doi:10.3762/bjoc.10.126
Received: 05 January 2014
Accepted: 05 May 2014
Published: 30 May 2014
This article is part of the Thematic Series "Organophosphorus chemistry".
Guest Editor: P. R. Hanson
Email:
Hongbin Sun* - hbsun2000@yahoo.com; Shu-Li You* -
slyou@sioc.ac.cn
© 2014 Lai et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
allylic substitution; enantioselectivity; ferrocene; organophosphorus;
palladium; regioselectivity
Abstract
A class of novel, easily accessible and air-stable 1-[bis(trifluoromethyl)phosphine]-1’-oxazolinylferrocene ligands has been synthe-
sized from ferrocene. It became apparent that these ligands can be used in the regio- and enantioselective Pd-catalyzed allylic alkyl-
ation of monosubstituted allyl substrates in a highly efficient manner. Excellent regio- and enantioselectivity could be obtained for a
wide range of substrates.
Introduction
The palladium-catalyzed asymmetric allylic alkylation (AAA) substrates, the palladium-catalyzed allylic substitution reaction
reaction is now becoming an efficient method for the construc- prefers to give linear products rather than the branched ones
tion of carbon–carbon bonds [1-5]. Despite extensive investi- [6-9] (Scheme 1). Accordingly, the regio- and enantioselective
gation and noteworthy advances in this field, several challenges allylic substitution reaction of monosubstituted allylic substrates
remain to be solved. For instance, with monosubstituted allyl to preferably obtain the branched products is one of the contin-
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Beilstein J. Org. Chem. 2014, 10, 1261–1266.
uing challenges. To our knowledge, there are several cases in highly efficient for the Pd-catalyzed allylic alkylation reactions
which high levels of both regio- and enantioselectivity have of monosubstituted allyl substrates. Herein, we report the syn-
been realized by introducing special ligands [10-34] (Figure 1). thesis of 1-[bis(trifluoromethyl)phosphine]-1’-oxazolinylfer-
Hayashi and coworkers reported a sterically bulky chiral rocene ligands and their application in Pd-catalyzed allylic
monophosphine ligand (MeO-MOP) could be used for the alkylation reactions of monosubstituted allyl substrates with
Pd-catalyzed alkylation of branched monosubstituted allyl excellent regio- and enantioselectivity.
acetate favoring the branched products. However, linear prod-
ucts were favored when the linear allyl substrates were Results and Discussion
employed [23,24]. The chiral oxazoline–phosphite ligands As depicted in Scheme 2, ligands L1a–L1d were synthesized
introduced by Pfaltz and coworkers proved to be highly effi- from known compounds 3, which were obtained from ferrocene
cient for regio- and enantiocontrol in the Pd-catalyzed allylic in three steps according to the reported procedures [39-41]. The
alkylation reaction. Excellent results were obtained for the commercially available ferrocene was dilithiated with n-BuLi
bulky and electron-rich aryl allyl substrates [25-27]. In 2001, and then quenched with dibromoterafluoroethane to give dibro-
Dai, Hou and their coworkers synthesized a new class of 1,1’- moferrocence 1. Treatment of 1 with n-BuLi at −20 °C fol-
ferrocene-based P,N-ligands, namely SiocPhox. The applica- lowed by trapping with CO2 afforded compound 2. Treatment
tion of these SiocPhox ligands in the Pd-catalyzed allylic substi- of compound 2 with (COCl)2 and then chiral amino alcohols
tution led to excellent regio- and enantioselectivities for a wide yielded the amide intermediates which were transformed to
range of substrates in both allylic alkylation and amination reac- their corresponding 1-bromo-1’-oxazolinylferrocenes 3. Even-
tions despite of the electronic properties of the allylic substrates tually, lithium–bromide exchange of 3 with n-BuLi at −78 °C,
[28-33]. Recently, Shen and co-workers reported an elegant followed by quenching with P(OPh)3, provided the phospho-
synthesis of bis(perfluoroalkyl)phosphine-oxazoline ligands nite intermediates which were used without further purification.
where small but strongly electron-withdrawing substituents Subsequently, trifluoromethylation provided the ligands L1a–d
were introduced at the phosphorus [34]. 1,2-Ferrocene based in moderate yields, upon treatment with Ruppert’s reagent
P,N-ligands were synthesized and gave excellent regio- and (TMSCF3) and CsF [42-45]. Notably, ligands L1a–d are mois-
enantioselectivities in the Pd-catalyzed allylic alkylation reac- ture and air-stable, and their NMR spectra show no change even
tions of monosubstituted allylic substrates. Inspired by these after being stored over six months under ambient atmosphere.
pioneering studies above and as our continuing interests in the
transition metal-catalyzed asymmetric allylic alkylation reac- To test the suitability of these 1-[bis(trifluoromethyl)phos-
tion [35-38], we envisaged that the 1-[bis(trifluoromethyl)phos- phine]-1’-oxazolinylferrocene ligands in Pd-catalyzed allylic
phine]-1’-oxazolinylferrocene ligands, a straightforward combi- alkylation reactions, we began our study by choosing methyl
nation of the features of SiocPhox and Shen’s ligand, should be cinnamyl carbonate and dimethyl malonate as the model
Figure 1: Representative ligands developed for the regio- and enantioselective Pd-catalyzed allylic alkylation.
Scheme 1: Transition metal-catalyzed allylic substitution reactions with monosubstituted allyl substrates.
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Beilstein J. Org. Chem. 2014, 10, 1261–1266.
Scheme 2: Preparation of 1-[bis(trifluoromethyl)phosphine]-1’-oxazolinylferrocene ligands. Reagents and conditions: (a) (i) n-BuLi, TMEDA, Et2O, rt;
(ii) (BrCF2)2, −78 °C. (b) n-BuLi, CO2, THF, −20 °C. (c) (i) (COCl)2, DCM, rt; then TEA, amino alcohol, DCM, rt; (ii) Ph3P, CCl4, TEA, CH3CN, rt.
(d) (i) n-BuLi, TMEDA, P(OPh)3, Et2O, −78 °C; (ii) TMSCF3, CsF, Et2O, rt.
substrates, along with the catalysts derived from Pd2(dba)3 and at 0 °C resulted in an increased enantioselectivity (b/l: 96/4,
ligands 1a–d. The results are summarized in Table 1. Ligands 88% ee, entry 9, Table 1). When the reaction was run at −30 °C,
L1a–d were screened in the reaction using bis(trimethyl- only a trace amount of product was formed. As for the leaving
silyl)acetamide (BSA) as the base and LiOAc as the additive. groups of allyl substrates, the cinnamyl acetate could also be
The results suggested that ligands L1a–d were effective for this tolerated to give a similar level of regio- and enantioselectivity
reaction with full conversion and high selectivities (entries 1–4, (entry 11, Table 1). The absolute configuration of the product
Table 1). The catalyst derived from L1d gave the highest selec- was assigned as (S) by comparing the sign of the optical rota-
tivities [b/l (branched/linear): 95/5, 82% ee; entry 4, Table 1]. tion with that reported in literature [28].
With ligand L1d, different reaction parameters including the Pd
precursor and solvent were further optimized. The utilization of Under the optimized reaction conditions (2 mol % of Pd2(dba)3,
[Pd(C3H5)Cl]2 as Pd precursor or DCM as solvent resulted in 4 mol % of L1d, 300 mol % of CH2(COOMe)2, 300 mol % of
slightly lower selectivities (entries 5–6, Table 1). Further BSA and 3 mol % of NaOAc in DCE at 0 °C; entry 9, Table 1),
screening of the additives revealed that NaOAc was the optimal the substrate scope was examined to test the generality of the
one (b/l: 97/3, 85% ee, entry 7, Table 1). Running the reaction reaction (Table 2). We first compared the reaction of branched
Table 1: Evaluation of the ligands and optimization of the reaction conditions.a
entry
4 or 4a’
[Pd]
L1
Additive
Solvent
T (°C)
Yield (%)b
6a/7ac
ee (%)d
1
2
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a’
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
[Pd(C3H5)Cl]2
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
L1a
L1b
L1c
L1d
L1d
L1d
L1d
L1d
L1d
L1d
L1d
LiOAc
LiOAc
LiOAc
LiOAc
LiOAc
LiOAc
NaOAc
KOAc
DCE
DCE
DCE
DCE
DCE
DCM
DCE
DCE
DCE
DCE
DCE
rt
rt
95
96
84/16
93/7
85/15
95/5
90/10
83/17
97/3
91/9
96/4
nd
68
68
80
82
76
76
85
82
88
nd
87
3
rt
91
4
rt
95
5
rt
93
6
rt
91
7
rt
90
8
rt
91
9
NaOAc
NaOAc
NaOAc
0
95
10e
11e
−30
rt
trace
80
95/5
aReagents and conditions: 2.0 mol % Pd2(dba)3, 4.0 mol % ligand, 0.2 mmol allyl substrate, 0.6 mmol dimethyl malonate, 0.6 mmol BSA, 3.0 mol %
additive, solvent (2 mL). bIsolated yield after 12 h. cDetermined by 1H NMR of the crude reaction mixture. dDetermined by HPLC. eReaction for 24 h.
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Beilstein J. Org. Chem. 2014, 10, 1261–1266.
Table 2: Regio- and enantioselective allylic alkylation of monosubstituted allyl substrates.a
entry
R
T (°C)
Yield (%)b
6/7c
ee (%)d
1
2
4a, Ph
0
0
0
0
0
0
rt
0
0
rt
0
0
0
0
95
93
95
93
96
94
90
91
90
90
95
91
90
96
96/4
95/5
99/1
95/5
95/5
98/2
83/17
96/4
99/1
93/7
99/1
97/3
93/7
81/19
88
87
92
85
82
70
65
83
83
81
92
94
88
ND
5
3
4b, 1-naphthyl
4c, 4-MeC6H4
4d, 4-MeOC6H4
4e, 2-thienyl
4f, 2-furyl
4
5
6
7
8
4g, 4-ClC6H4
4h, 4-BrC6H4
4i, 2-FC6H4
4j, 2-MeOC6H4
4k, 2-MeC6H4
4l, 3-ClC6H4
4m, methyl
9
10
11
12
13
14e
aReagents and conditions: 2.0 mol % Pd2(dba)3, 4.0 mol % L1d, 0.5 mmol allyl substrate, 1.5 mmol dimethyl malonate, 1.5 mmol BSA, 3.0 mol %
NaOAc, DCE (5 mL). bIsolated yield after 12 h. cDetermined by 1H NMR of the crude. dDetermined by HPLC. e[Pd(C3H5)Cl]2 as the Pd precursor.
substrate 5 with the linear substrate 4a. Nearly identical results We conducted some control experiments to probe the effect of
were obtained indicating that the reaction proceeds via the for- the bis(trifluoromethyl) group in the ligands (Scheme 3). With
mation of the same Pd-π-allyl intermediate. Substrates bearing ferrocence-based biphenyl phosphine-oxazoline L2 as the
either an electron-donating group or electron-withdrawing ligand, the Pd-catalyzed allylic alkylation of cinnamyl
group on the aromatic ring of the aryl allyl carbonates all carbonate with dimethyl malonate afforded the linear product as
proceeded smoothly in full conversion within 12 h. In all cases, the major product (b/l: 40/60). Whereas the corresponding
the reactions gave excellent regioselectivity favoring the forma- ligand L1d with two CF3 groups (instead of two phenyl groups)
tion of the branched products in good to excellent enantio- at the P atom improved the regioselectivity significantly (b/l:
selectivity (b/l: 93/7–99/1, 81–94% ee). It is known that the 96/4). A preliminary explanation was described in Figure 2. In
regioselectivity could be strongly influenced by electronic prop- addition to the effect of different metals, there are at least two
erties of the allyl substrates and the formation of branched prod- additional factors controlling the regioselectivity of the allylic
ucts was dramatically reduced for substrates bearing electron- alkylation reaction. The steric factor favors path a since the
withdrawing groups [21]. Fortunately, with our catalytic terminal allylic carbon is less hindered. In contrast, when the R
system, substrates bearing electron-withdrawing groups were group has the ability to stabilize the carbocation, the electronic
well tolerated with excellent regioselectivity and preferred for- factor would favor the formation of the branched product (path
mation of the branched products (b/l: 93/7–99/1, entries 8–10, b). The phosphorus atom has a stronger trans effect comparing
and 13, Table 2). Reactions of sterically hindered 1-naphthyl with the oxazoline nitrogen, indicating that the carbon trans to
allyl carbonate, 2-MeO and 2-Me-substituted cinnamyl carbon- phosphorus atom bears more electropositivity [46]. This fact
ates occurred smoothly to give excellent regio- and enantio- may be responsible for the preferred placement of the substi-
selectivity (b/l: up to 99/1, up to 94% ee, entries 3, 11, 12, tuted allylic carbon in the trans position to the phosphorus atom
Table 2). In addition, heteroaryl allyl carbonates 4e and 4f also to better stabilize the electropositivity of the carbon. When the
gave good regioselectivity with slightly lower enantio- nucleophile attacks the more electropositive substituted allylic
selectivity (entries 6 and 7, Table 2). Good regioselectivity (b/l: carbon terminus, a branched product will be formed. The intro-
81/19) was obtained with 2-buten-3-yl carbonate as a substrate duction of the CF3 group on the phosphorus atom further
(entry 14, Table 2).
increases the trans influence of the P(CF3)2 moiety and
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Beilstein J. Org. Chem. 2014, 10, 1261–1266.
Scheme 3: Comparison of the effect of ligands in the reaction.
dation of China (21025209, 21121062, 21332009), and the
Chinese Academy of Sciences for generous financial support.
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