Enantioselective rhodium-catalyzed addition of arylboronic acids to trifluoromethyl ketones

Article abstract of DOI:10.1002/adsc.201201125

A new C₂-symmetrical, chiral bisphosphorus ligand proved to be efficient for the rhodium-catalyzed nucleophilic addition of arylboronic acids to trifluoromethyl ketones, providing a series of chiral trifluoromethyl- substituted tertiary alcohols in high yields (up to 93%) and excellent enantioselectivities (>99%). Copyright

Full text of DOI:10.1002/adsc.201201125

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DOI: 10.1002/adsc.201201125
Enantioselective Rhodium-Catalyzed Addition of Arylboronic
Acids to Trifluoromethyl Ketones
Renshi Luo,a,b Ke Li,a Yuling Hu,a and Wenjun Tanga,*
a
State Key Laboratory of Bioorganic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese
Academy of Sciences, 345 Ling Ling Road, Shanghai 200032, Peopleꢀs Republic of China
E-mail: tangwenjun@sioc.ac.cn
School of Pharmaceutical Sciences, Gannan Medical University, Yixueyuan Road, Ganzhou, Jiangxi 341000, Peopleꢀs
b
Republic of China
Received: December 25, 2012; Revised: March 23, 2013; Published online: April 30, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201201125.
Abstract: A new C2-symmetrical, chiral bisphospho-
rus ligand proved to be efficient for the rhodium-
catalyzed nucleophilic addition of arylboronic acids
to trifluoromethyl ketones, providing a series of
chiral trifluoromethyl-substituted tertiary alcohols
in high yields (up to 93%) and excellent enantiose-
lectivities (>99%).
excellent enantioselectivities. In contrast, the highly
enantioselective addition of arylboronic acids to ke-
tones or trifluoromethyl ketones remains an underde-
veloped area. Lu[17] and Lam[18] reported an intramo-
lecular addition of arylboronic acids to ketones with
excellent ees. However, progress on its intermolecular
version was scarce and, only recently, were moderate
ees achieved.[19] To the best of our knowledge, only
two reports are available on the enantioselective addi-
tion of arylboronic acids to trifluoromethyl ketones.[7]
Minnaard and co-worker reported the addition of ar-
ylboronic acids to 2,2,2-trifluoroacetophenone with
only moderate enantioselectivities (up to 83% ee).[7a]
Low ees (<54%) were also observed when a chiral
phosphite ligand was employed.[7b] A highly enantio-
Keywords: arylboronic acids; chiral phosphorus li-
gands; nucleophilic addition; rhodium; trifluoro-
methyl ketones
Syntheses of chiral tertiary alcohols have gained sig- selective addition of arylboronic acids to trifluoro-
nificant interest due to their structural relevance in methyl ketones has yet to be developed. We herein
biologically active natural products and pharmaceuti- report an efficient synthetic method for trifluoro-
cals.[1] Consequently, the preparations of chiral tri- methyl-substituted chiral tertiary alcohols by the rho-
fluoromethylated tertiary alcohols have become an dium-catalyzed addition of arylboronic acids to tri-
important subject as an increasing number of thera- fluoromethyl ketones and excellent enantioselectivi-
peutic reagents contain such structures.[2] One major ties have been achieved.
synthetic strategy is the trifluoromethylation of ke-
The development of chiral catalysts with tunable
tones which remains very challenging in its asymmet- steric and electronic properties plays a pivotol role in
ric versions.[3] An important alternative is the asym- the realization of efficient asymmetric catalytic reac-
metric addition of carbon nucleophiles to trifluoro- tions. We have recently reported a series of tunable
methyl ketones or pyruvates[4] by alkynyl,[5] alkyl,[6] al- and operationally convenient chiral bisphosphorus li-
kenyl[5d] or aryl[7] additions, as well as aldol,[8] nitroal- gands[20] on the basis of a 2,3-dihydrobenzo[d]-
dol,[9] carbonyl-ene,[10] Friedel–Crafts[11] or other types
ACHTUNGTREN[NGNU 1,3]oxophosphole framework (Figure 1) and their ex-
of reactions.[12] The addition of boronic acids to tri- cellent applications in asymmetric hydrogenation[20a,c]
fluoromethyl ketones to construct chiral trifluorome- and propargylation.[20b] By variation of the R groups
thylated tertiary alcohols is particularly attractive due in this structure, a series of C2-symmetrical bisphos-
to the ready availability, good stability, and non-toxic phorus ligands with various steric and electronic prop-
nature of various boronic acids as the starting materi- erties can be developed. For the rhodium(I)-catalyzed
als. Since Miyaura[13] developed the first Rh-catalyzed addition of arylboronic acids to trifluoromethyl ke-
addition of arylboronic acids to aldehydes in 1998, tones, a chiral bisphosphorus ligand with a deep chiral
significant progress[1g] has been achieved on transition pocket is required in order to provide a well-defined
metal-catalyzed asymmetric additions of arylboronic chiral environment when coordinated to a rhodium
acids to aldehydes,[14] a-keto esters[15] or isatins[16] with center. The chiral bisphosphorus ligand 4 containing
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tiary alcohol was isolated in 79% yield and 96% ee
(entry 8). Among various inorganic and organic bases
screened, potassium carbonate provided a best yield
(entries 8–11).
Under optimized reaction conditions, the Rh-4 cat-
alyst proved to be efficient for the syntheses of a wide
array of trifluoromethyl-substituted chiral tertiary al-
cohols. As can be seen in Table 2, a variety of
trifluoroACTHNUTRGENNmUG ethyl-substituted bisACHUTNGTRENN(NGU aryl) carbinols were
prepared in good yields and excellent enantioselectiv-
ities. With 2,2,2-trifluoro-1-phenylethanone as the
substrate, arylboronic acids with either electron-do-
Figure 1. Tunable chiral bisphosphorus ligands.
2’,6’-dimethoxyphenyl substituents has proven to be nating or electron-withdrawing substituents at the 4-
an efficient ligand for this transformation (Table 1). position afforded excellent yields and enantioselectiv-
An initial study of various BIBOP ligands for the ities (entries 1–4). 2-Substituted arylboronic acid was
rhodium-catalyzed addition of 4-methoxyphenylbor- also applicable to provide a high ee (entry 5). Various
onic acid to 2,2,2-trifluoroacetophenone provide substituted phenylboronic acids (entries 6–16) were
promising results (Table 1). With [Rh
ACHTUNGTRENNUNG
(1.5 mol%) as the catalyst precursor and BIBOP (1) fluorophenyl)AHCTUNTGRENNUNG
or MeO-BIBOP (2) (3.6 mol%) as the ligand, the ad- achieved in all cases except for 2-methoxyphenylbor-
dition proceeded at 608C for 20 h to provide the de- onic acid (entry 17). Other substituted 2,2,2-trifluoro-
sired product in a moderate yield and ee (entries 1 1-phenylACHTNUTRGNEeNUG thanones were also applicable (entries 18–
and 2). With phenyl groups at the R positions, ligand 24, 27, and 28). Addition to 2’-methoxytrifluoroaceto-
3 led to a much improved enantioselectivity (79% ee, phenone also provided an excellent ee albeit with
entry 3). A good ee (85%) was achieved when ligand a low yield (entry 27). A high ee and good yield were
4 containing 2,6-dimethoxyphenyl substituents was also achieved with 3’-trifluoromethoxytrifluoroaceto-
employed (entry 4). Optimization of the solvent fur- phenone (entry 28). Heteroaryl substrates such as 2-
ther improved the yield and enantioselectivity (en- thiophenyl derivatives could also be employed to pro-
tries 5–8). With MTBE as the solvent, the desired ter- vide good to excellent ees (entries 25 and 26).
Table 1. Optimization of reaction conditions.
Entry[a]
Ligand
Base
Solvent
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
1
2
3
4
4
4
4
4
4
4
4
4
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
KF
toluene
toluene
toluene
toluene
dioxane
DCM
71
70
61
68
78
40
56
79
23
71
<10
16
45
40
79
85
92
89
96
96
92
96
91
93
THF
MTBE
MTBE
MTBE
MTBE
MTBE
9
10
11
12
K3PO4
KOH
Et3N
[a]
The reactions were carried out at 608C in solvent (1.5 mL) for 20 h with 2,2,2-trifluoroacetophenone (0.25 mmol), para-
methoxyphenylboronic acid (0.5 mmol), and base (0.75 mmol) in the presence of [Rh(C2H4)Cl]2 (1.5 mol%) and ligand
AHCTUNGTRENNUNG
(3.6 mol%); the absolute configurations were assigned on the basis of the absolute configuration of 10k (table 2,
entry 11) through a similar stereochemical model.
Isolated yield after column chromatography.
[b]
[c]
Determined by chiral HPLC on a Chiralcel OD-H column.
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Enantioselective Rhodium-Catalyzed Addition of Arylboronic Acids to Trifluoromethyl Ketones
Table 2. Addition of arylboronic acids to trifluoromethyl aryl ketones.
Entry[a]
R’
R“
Yield [%] of 10[b]
ee [%][c]
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Ph
4-MeO-C6H4
4-Me-C6H4
4-Cl-C6H4
4-Ph-C6H4
3-MeO-C6H4
4-MeO-C6H4
Ph
79 (10a)
78 (10b)
83 (10c)
71 (10d)
83 (10e)
81 (10f)
61 (10g)
85 (10h)
86 (10i)
73 (10j)
91 (10k)
91 (10l)
93 (10m)
87 (10n)
61 (10o)
73 (10p)
37 (10q)
83 (10r)
86 (10s)
81 (10t)
81 (10u)
83 (10v)
47 (10w)
58 (10x)
81 (10y)
61 (10z)
31 (10aa)
79 (10ab)
95
99
97
95
95
95
95
97
97
97
98
96
97
99
95
99
81
93
96
95
95
96
93
96
81
89
96
96
Ph
4-F-C6H4
4-F-C6H4
4-F-C6H4
4-F-C6H4
4-F-C6H4
4-F-C6H4
4-F-C6H4
4-F-C6H4
4-F-C6H4
4-F-C6H4
4-F-C6H4
4-F-C6H4
4-Cl-C6H4
4-Cl-C6H4
4-Br-C6H4
4-Br-C6H4
4-Br-C6H4
4-Me-C6H4
4-Me-C6H4
2-thiophenyl
2-thiophenyl
2-OMe-C6H4
3-OCF3-C6H4
4-Me-C6H4
4-Cl-C6H4
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
4-Ph-C6H4
4-TMS-C6H4
4-Bu-C6H4
4-i-Pr-C6H4
3-CF3O-C6H4
2-naphthyl
2-F-C6H4
2-MeO-C6H4
4-MeO-C6H4
4-Me-C6H4
4-MeO-C6H4
4-Me-C6H4
4-Cl-C6H4
4-MeO-C6H4
4-i-Pr-C6H4
4-MeO-C6H4
4-Cl-C6H4
C6H5
4-MeO-C6H4
[a]
The reactions were carried out at 608C in MTBE (1.5 mL) for 20 h with ketone (0.25 mmol), boronic acid (0.5 mmol),
and K2CO3 (0.75 mmol) in the presence of [Rh(C2H4)Cl]2 (1.5 mol%) and ligand 4 (3.6 mol%). The absolute configura-
AHCTUNGTRENNUNG
tion of 10k was determined by X-ray crystallography (Figure 3) and all the others were assigned on the basis of similar
stereochemical models.
Isolated yields.
[b]
[c]
Determined by chiral HPLC on a Chiralcel OD-H, Chiralcel AD-H, or Lux Amylose-2 column.
Scheme 1. Rhodium-catalyzed addition of 4-methoxyphenylboronic acid to acetophenone
Encouraged by the high enantioselectivity observed
The X-ray structure[21] of the [Rh(4)
ACHTNGUTER(NNUG nbd)]BF4 com-
with trifluoromethyl ketones, the addition of 4-me- plex revealed a deep chiral pocket of the C2-symmet-
thoxyphenylboronic acid to acetophenone was also rical Rh-4 catalyst, providing a structural insight for
tested under similar reaction conditions. An excellent the high enantioselectivities observed in this transfor-
ee was also achieved albeit with a low conversion and mation. As depicted in Figure 2, the rhodium center
yield (Scheme 1). Our further study is focusing on im- is buried deep in the structure of ligand 4, and the
proving the reactivity of this catalytic system.
two 2’,6’-dimethoxyphenyl substituents of the ligand
extend parallel towards the coordination of norborna-
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Figure 2. The X-ray structure of [Rh(4)ACHTNUGRTNEUNG(nbd)]BF4 [hydrogen atoms, BF4 anion, and inclusion solvent (dichloromethane) are
omitted for clarity].
Figure 3. The X-ray structure of the chiral alcohol 10k (the
absolute configuration of the chiral center in 10k is S).
diene, providing the major steric forces for substrate
coordination. The distance between the rhodium
center and the ipso carbon of 2’,6’-dimethoxyphenyl
substituents is ~4.3 ꢂ, well short of a length of
a phenyl substituent (~5.2 ꢂ). Thus, the orientation
of an aromatic substrate could be well defined when
coordinating to the Rh-4 catalyst.
A proposed mechanism for this Rh(I)-catalyzed
transformation is illustrated in Figure 4. Under basic
conditions, a hydroxyrhodium(I) complex 14 is gener-
ated from the [RhACHTUNTRGNENG(U CH2CH2)2Cl]2 precursor and
a chiral bisphosphorus ligand. Transmetallation of 11
with arylboronic acid 9 provides an arylrhodium(I)
Figure 4. The stereochemical model of the Rh-4 catalyst
during the addition of arylboronic acids to trifluoromethyl
species 15. Substrate coordination of a trifluoromethyl ketones
aryl ketone 8 to 15 adopts two possible coordination
modes A and B. A greater steric interaction is expect-
ed between the aryl group of the trifluoromethyl ance with the absolute configuration observed in the
ketone and the ligand in mode A, whereas the more addition product 10k (Figure 3).[23]
favorable mode B undergoes insertion and transme-
In conclusion, we have disclosed an efficient
tallation to form the chiral trifluoromethyl-substituted method for the rhodium-catalyzed addition of aryl-
diarylmethanol 10 and regenerate the arylrhodium(I) boronic acids to trifluoromethyl ketones, leading to
species 15.[22] This stereochemical model is in accord- the formation of a series of chiral trifluoromethyl-sub-
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Enantioselective Rhodium-Catalyzed Addition of Arylboronic Acids to Trifluoromethyl Ketones
[3] For reviews, see: a) T. Billard, B. R. Langlois, Eur. J.
stituted tertiary alcohols in high yields and excellent
enantioselectivities. The new chiral bisphosphorus
ligand 4 with a deep chiral pocket is the key to the
success of this transformation. Applications of this
methodology for syntheses of biologically active mol-
ecules and further improvement of the current cata-
lyst system are currently under investigation and
progress will be reported in due course.
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Experimental Section
General Procedure
[4] For a recent review, see: J. Nie, H.-C. Guo, D. Cahard,
To a mixture of trifluoromethyl ketone 5 or 8 (0.25 mmol,
1 equiv.), arylboronic acid 6 or 9 (76 mg, 0.5 mmol, 2 equiv.),
K2CO3 (100 mg, 0.75 mmol, 3 equiv.), ligand 4 (5.9 mg,
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0.009 mmol, 3.6 mol%) and [RhACHTNUGTRNE(UNG C2H4)2Cl]2 (1.5 mg,
0.00375 mmol, 1.5 mol%) was added MTBE (1.5 mL). The
mixture was stirred at 608C for 20 h and then concentrated.
The residue was directly subjected to silica gel column chro-
matography [dichloromethane/hexanes (7:3 v/v) as the
eluent] to afford the desired alcohol product 7 or 10. The
enantioselectivity was determined by chiral HPLC on a Chir-
alcel OD-H, Chiralcel AD-H, or Lux Amylose-2 column.
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Acknowledgements
We are grateful to NSFC (21272254), the “Thousand Plan”
Youth program, the Innovation Fund of State Key Laborato-
ry of Bioorganic and Natural Products Chemistry, and gener-
ous support from Boehringer Ingelheim.
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[21] CCDC 901956 {[Rh(4)ACTHNUTRGNE(NUG nbd)]BF4} contains the supple-
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Adv. Synth. Catal. 2013, 355, 1297 – 1302