A highly selective ferrocene-based planar chiral PIP (Fc-PIP) acyl transfer catalyst for the kinetic resolution of alcohols

Article abstract of DOI:10.1021/ja108238a

Novel planar chiral ferrocene nucleophilic catalysts (Fc-PIP) containing both central and planar chiral elements were designed and synthesized for catalytic enantioselective acyl transfer of secondary alcohols. A remarkably efficient catalyst with high selectivity factors (up to S = 1892) was identified. Comparing the combination of central and planar chirality revealed a strong requirement for the "matched" chiral elements, indicating that the stereogenic center of the imidazole rings should present itself on the same face as the ferrocenyl fragment; otherwise, the catalyst is completely inactive. An exclusively stacked transition state that accounts for the high selectivity of the kinetic resolution of secondary alcohols is proposed. Notably, this newly designed catalyst family is suitable for the catalytic kinetic resolution of bulky arylalkyl carbinols, producing esters with extremely high ee (>99%).

Full text of DOI:10.1021/ja108238a

Published on Web 11/08/2010
A Highly Selective Ferrocene-Based Planar Chiral PIP (Fc-PIP)
Acyl Transfer Catalyst for the Kinetic Resolution of Alcohols
Bin Hu,† Meng Meng,† Zheng Wang,† Wenting Du,† John S. Fossey,*,†,‡
Xinquan Hu,§ and Wei-Ping Deng*,†
School of Pharmacy, East China UniVersity of Science and Technology, 130 Meilong Road,
Shanghai 200237, China, School of Chemistry, UniVersity of Birmingham, Edgbaston,
Birmingham B15 2TT, U.K., and College of Chemical Engineering and Materials Science,
Zhejiang UniVersity of Technology, Hangzhou 310014, China
Received September 17, 2010; E-mail: j.s.fossey@bham.ac.uk; weiping_deng@ecust.edu.cn
Abstract: Novel planar chiral ferrocene nucleophilic catalysts (Fc-PIP) containing both central and planar
chiral elements were designed and synthesized for catalytic enantioselective acyl transfer of secondary
alcohols. A remarkably efficient catalyst with high selectivity factors (up to S ) 1892) was identified.
Comparing the combination of central and planar chirality revealed a strong requirement for the “matched”
chiral elements, indicating that the stereogenic center of the imidazole rings should present itself on the
same face as the ferrocenyl fragment; otherwise, the catalyst is completely inactive. An exclusively stacked
transition state that accounts for the high selectivity of the kinetic resolution of secondary alcohols is
proposed. Notably, this newly designed catalyst family is suitable for the catalytic kinetic resolution of bulky
arylalkyl carbinols, producing esters with extremely high ee (>99%).
In view of the importance of chiral bulky arylalkyl carbinols
in asymmetric organic synthesis and their synthetic difficulty,1
the catalyst reported herein provides important access to bulky
alcohols in high optical purity.
Since the pioneering work of Vedejs in nonenzymatic
catalytic kinetic resolution (KR)2 of secondary alcohols using
chiral phosphine2a and chiral 4-dimethylaminopyridine (DMAP)
equivalents2b as nucleophilic acyl transfer catalysts, asymmetric
nucleophilic acyl transfer catalysis for catalytic KR has been
further developed.3,4 Fu’s planar chiral DMAP equivalent is an
exemplary system of exquisite design (Figure 1)4a,b that is highly
effective for catalytic KR, with selectivity factors (S) ranging
from 32 to 95; the highest S value (S ) 95) was obtained when
bulky (rac)-phenyl-tert-butyl carbinol was employed as a
substrate. These ferrocenyl-based systems rapidly became the
gold standard in catalytic KR.4c Thereafter, several chiral
DMAP-like catalysts incorporating elements of central,4d,k,5
axial,6 and pseudoplanar7 chirality have been developed that
show selectivity factors comparable to those of the earlier planar
chiral DMAP equivalents. An important breakthrough was
Figure 1. Representative nucleophilic catalysts and the design of planar
chiral PIP 1.
† East China University of Science and Technology.
‡ University of Birmingham.
presented by Birman and co-workers,8 who described another
class of simple nucleophile design that incorporates a dihy-
droimidazole moiety along with the parent pyridine to generate
the 2,3-dihydroimidazo[1,2-a]pyridine (DHIP) scaffold. Among
those reported, the catalyst CF3-PIP8a proved to be particularly
effective in the catalytic KR of benzylic alcohols with excellent
S values (up to 85). The high selectivity factors were reasoned
to be due to the π-π and cation-π interactions between the
pyridinium ring of the N-acylated catalyst and the arene ring
of the alcohol. On the basis of this hypothesis, catalysts Cl-
PIQ8b and BTM8c with extended π systems were developed
(Figure 1), and the enhanced π-π and cation-π interactions9
§ Zhejiang University of Technology.
(1) (a) Ohkuma, T.; Sandoval, C. A.; Srinivasan, R.; Lin, Q.; Wei, Y.;
Muniz, K.; Noyori, R. J. Am. Chem. Soc. 2005, 127, 8288–8289. (b)
Diaz-Valenzuela, M. B.; Phillips, S. D.; France, M. B.; Gunn, M. E.;
Clarke, M. L. Chem.sEur. J. 2009, 15, 1227–1232.
(2) (a) Vedejs, E.; Daugulis, O.; Diver, S. T. J. Org. Chem. 1996, 61,
430–431. (b) Vedejs, E.; Chen, X. H. J. Am. Chem. Soc. 1996, 118,
1809–1810.
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1560–1638. (b) Vedejs, E.; Jure, M. Angew. Chem., Int. Ed. 2005,
44, 3974–4001. (c) Dalko, P. I.; Moisan, L. Angew. Chem., Int. Ed.
2004, 43, 5138–5175. (d) Jarvo, E. R.; Miller, S. J. In ComprehensiVe
Asymmetric Catalysis, Supplement 1; Jacobsen, E. N., Pfaltz, A.,
Yamamoto, H., Eds.; Springer-Verlag: Berlin, 2004.
9
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J. AM. CHEM. SOC. 2010, 132, 17041–17044 17041

A R T I C L E S
Hu et al.
were found to be extremely effective improvements for the
catalytic KR of various secondary alcohols, with selectivity
factors up to 117 and 355 respectively. Since cation-π
interactions have been invoked to explain the selectivity of
nucleophilic catalysis5b,d,10 and demonstrated to exert intramo-
lecular control in flexible-sensor systems,11,12 it is reasonable
to expect that such phenomena may be operating here, although
Houk and co-workers13 elucidated a subtle interplay between
steric and electronic effects that revealed CH-π interactions
to play an important role in some catalytic systems as well.
While there have been reports dedicated to uncovering the
origins of enantioselectivity in catalytic KR, the goal of unam-
biguous elucidation of the origin of enantioselectivity still
remains.14
Herein, we describe these new nonenzymatic nucleophilic
catalysts 1 and their application to catalytic KR of arylalkyl
carbinols with selectivity factors up to an impressive value of
1892, which is comparable to that achievable with enzymes.
This new catalyst construct also serves as a good model for the
elucidation of the experimental rationale of the importance of
the π-π and cation-π interactions in delineating the control
of stereoselection. Unlike enzymes, the general class of synthetic
catalyst defined here can operate over a wide temperature range,
is tolerant of a variety of solvent conditions, and offers wider
substrate scope. The catalyst system described herein offers
hitherto unachievable enzyme-like selectivity factors from a
synthetic catalyst, and in view of the advantages of synthetic
catalysts, this system can truly supplant enzyme-mediated re-
actions.
Encouraged by the success of the combination of planar
chirality and central chirality in asymmetric catalysis,15 we set
out to develop the new class of planar chiral ferrocene
nucleophilic catalysts 1, which combine aspects from planar
chiral DMAP and chiral imidazole catalysts (Figure 1).
Birman14,16 has discussed how the phenyl group on the
stereogenic center of their imidazole rings exerts its stereo-
chemical influence primarily through discriminating between
the two faces of the carbonyl group of the in situ-generated
amide in its transition state of acyl transfer. Although good
empirical evidence was offered, it is generally difficult to ascribe
the level of stereoselection to any one factor, since it is
inherently difficult to corroborate computational and experi-
mental observations.14a As with existing rationales for stereo-
selection in planar chiral nucleophilic catalysis,4a it is apparent
that our system also benefits from the steric influence of the
pentaphenyl floor, i.e., the “bottom” face is completely blocked
by the η5-C5Ph5 moiety.
The synthesis of desired planar chiral PIP 1 was accomplished
as shown in Figure 2. In the case of 1a as an example, the
catalyst synthesis was conducted as follows: The preparation
of racemic 3 according to Fu’s protocol4a followed by the Pd-
catalyzed C-N coupling of (S)-4-phenyloxazolidin-2-one to 3
gave two chromatographically separable diastereoisomers, (S,Sp)-
and (S,Rp)-4a in 45 and 47% yield, respectively (92% overall
yield). Hydrolysis of the resultant isomers in the presence of
methanolic sodium hydroxide afforded the corresponding al-
cohols, which were then treated with MsCl/Et3N without further
purification to furnish the two diastereoisomers (S,Sp)- and
(S,Rp)-1a in 83 and 85% yield, respectively. The relative
configurations of two isomers were assigned on the basis of
the X-ray diffraction analysis of the isomer (S,Sp)-1a (Figure 2
inset).
(4) (a) Ruble, J. C.; Latham, H. A.; Fu, G. C. J. Am. Chem. Soc. 1997,
119, 1492–1493. (b) Ruble, J. C.; Tweddell, J.; Fu, G. C. J. Org. Chem.
1998, 63, 2794–2795. (c) Fu, G. C. Acc. Chem. Res. 2000, 33, 412–
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121, 5813–5814. (i) Vedejs, E.; Daugulis, O. J. Am. Chem. Soc. 2003,
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With the eight planar chiral PIP derivatives Sp-1a-d and Rp-
1a-d in hand, we proceeded to investigate their catalytic
behavior in the catalytic KR of (()-1-phenylpropanol under
Birman’s optimal resolution conditions at 0 °C;8a the results
are summarized in Table 1. Surprisingly, only one compound,
namely, Rp-1a (S ) 27), gave catalytic results with S values
comparable to those of the systems of Birman and Fu. In
contrast, Sp-1a was completely catalytically inactive for acyl
transfer even after a prolonged reaction time. These findings
clearly indicated that the phenyl group attached to the stereo-
genic center of Sp-1a is bulky enough to shield the entire top
face from effective acyl transfer.
(8) (a) Birman, V. B.; Uffman, E. W.; Hui, J.; Li, X. M.; Kilbane, C. J.
J. Am. Chem. Soc. 2004, 126, 12226–12227. (b) Birman, V. B.; Jiang,
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A Highly Selective Ferrocene-Based Acyl Transfer Catalyst
A R T I C L E S
Figure 2. Synthesis of planar chiral PIP catalysts 1a-d.
Table 1. Screening and Optimization of Planar Chiral PIP
Catalysts 1a-d in the KR of (()-1-Phenylpropanola
entry catalyst
solvent
t (h) 6a ee (%)b 5a ee (%)b CHPLC (%)c Sc
1
2
3
4
5
6
7
8
9
Sp-1a CHCl3
Sp-1a CHCl3
10
24
7
10
7
10
7
10
7
7
7
7
7
NDd
NDd
77.2
NDd
35.4
NDd
1
-
-
-
-
-
27
-
7
-
1
1
10
20
27
31
36
53
58
92
125
53
23
28
-
Rp-1a CHCl3
Sp-1b CHCl3
Rp-1b CHCl3
Sp-1c CHCl3
Rp-1c CHCl3
Sp-1d CHCl3
Rp-1d CHCl3
94.2
-
95.9
-
4.4
55
-
73
-
81.5
15.8
48.7
52.5
55.0
18.9
50.8
43.5
53.3
36.5
31.5
45.6
47.3
56.3
3.8
0.7
68.5
76.4
77.2
92.5
85.2
92.3
85.7
96.3
97.5
91.6
82.2
75.1
64.9
84.4
94.2
21.5
88.0
71.1
97.9
55.2
44.9
76.7
74.0
96.7
Figure 3. Possible transition state for catalysts (S, Rp)-1a interacting with
(S)-5a.
10 Rp-1a CH2Cl2
11 Rp-1a CHCl3
12 Rp-1a tert-amyl alcohol
13 Rp-1a THF
plausible transition state, TS I, is shown in Figure 3; such a
transition state exhibits a favorable cation-π interaction and
minimal steric interactions and is in complete agreement with
the experimental observations of this report. As such, a stacked
transition state can be considered as the exclusive reaction
manifold, as supported by the fact Sp-1a was completely
catalytically inactive. The use of this rationale and these ob-
servations serves to further elucidate Birman’s PIP,8a PIQ,8b
and BTM8c systems: where similar facial selectivity can now
be deduced, the π-π and cation-π interactions of the stacked
conformer are the main factors for attaining the high selectivity.
The one face is essentially completely blocked by the phenyl
substituent, and the selectivity results from favoring a stacked
transition state over a splayed one. However, these interpreta-
tions suggest that any bulky group in place of the phenyl group
in Rp-1 would give the same levels of selectivity factor, but
catalysts Rp-1c,d bearing alkyl groups exhibited drastically
14 Rp-1a Et2O
7
7
13
24
7
15 Rp-1a toluene
16 Rp-1a toluenee
17 Rp-1a toluenef
18 Rp-1a tolueneg
19 Rp-1a tolueneh
20 Rp-1a toluenei
7
7
a Conditions: 0.4 M 1-phenylpropanol and 5 mol % catalyst at 0 °C.
b Determined by chiral HPLC. c Calculated from the ee of the ester and
unreacted alcohols. d Not determined. e The reaction was conducted at
-20 °C. f The reaction was conducted at -40 °C. g Using 2 mol %
catalyst Rp-1a instead. h Using 2 mol % catalyst Rp-1a and acetic
anhydride instead. i Using 2 mol % catalyst Rp-1a and isobutyric anhy-
dride instead.
According to Birman’s computational study,14a a splayed
transition state in the system they studied was found to be 7.6
kcal/mol less stable than that of a stacked transition state. A
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J. AM. CHEM. SOC. VOL. 132, NO. 47, 2010 17043

A R T I C L E S
Hu et al.
Table 2. Scope and Generality of the Rp-1a-Catalyzed KR of
tested. Selectivity factors ranging from 31 to 534 were observed,
and notably, this is the first example of a catalytic KR of
secondary alcohols with a selectivity factor S > 500 where the
resultant ester was obtained in >99% ee, which is comparable
only to enzymatic catalysis. Furthermore, it was found that our
new catalyst has a significant steric effect on the alkyl part of
alcohols, and bulkier alkyl groups gave higher selectivity factors
(Table 2, entries 1-4). To our delight, exploiting the improved
selectivity at lower temperatures further raised the S value to
801 at -20 °C and amazingly to 1892 at -40 °C (Table 2,
entries 5 and 6).
Secondary Alcohols
entry
R1
R2
t (h) 6 ee (%) 5 ee (%) CHPLC (%)
Sc
1
2
3
4
Ph
Ph
Ph
Ph
Me
Et
i-Pr
t-Bu 10
t-Bu 18
t-Bu 24
8
7
8
83.0
85.7
92.4
99.1
99.5
99.8
94.1
86.9
86.1
86.1
85.1
82.5
85.2
87.6
97.9
92.3
81.1
72.1
62.0
54.5
90.1
83.9
94.1
92.7
94.6
36.1
51.3
53.3
50.0
45.0
42.0
38.3
36.7
50.9
49.3
52.2
52.1
53.4
29.7
31 (33)
58 (41)
84 (59)
534 (117)
801
1892
57
44
35
47
41
37
According to Birman’s computational study,14a steric interac-
tions between the alkyl group and the virtually coplanar ortho
hydrogen of the phenyl group can account for the observed
stereochemical outcome, thus explaining the enantioselectivity
trends we observed with the size of the alkyl group.
5a Ph
6b Ph
7
8
o-OMePh Me
m-OMePh Me
p-OMePh Me
1-naphthyl Me
p-ClPh
p-BrPh
styryl
8
8
8
7
8
8
8
9
10
11
12
13
In conclusion, we have developed a remarkably effective
planar chiral PIP catalyst for the enantioselective acyl transfer
of secondary alcohols to achieve selectivity factors up to S )
1892. Notably, this newly designed catalyst, Rp-1a, is suitable
for the catalytic KR of bulky arylalkyl carbinols to afford the
corresponding esters with with high ee (99.8%) and S values
(up to 1892). It is notable that the enzymatic catalytic KR of
bulky arylalkyl carbinols has not been reported.17 Our new
catalytic system provides a competitive and attractive synthetic
method for obtaining highly optical pure bulky alcohols.1 Further
applications of Rp-1a to various substrates and catalytic
processes, especially involving bulky arylalkyl carbinols, are
ongoing in our laboratory.
Me
Me
Me
18
a Reaction run using 5 mol % Rp-1a at -20 °C. b Reaction run using
5 mol % Rp-1a at -40 °C. c Data in parentheses are S values for
Birman’s Cl-PIQ.14
reduced selectivity factors, suggesting a noninnocent role of the
aromatic appendage, such as the difference in forming the en-
antioselectively favored conformation upon acylation of the
catalyst.16
While newly synthesized planar chiral catalyst Rp-1a was an
effective catalyst for the KR of (()-1-phenylpropanol, the
structural novelty as well as the potential application in model
studies to elucidate origins of the enantioselectivity prompted
us to further optimize the resolution conditions.
Screening of solvents (Table 1, entries 10-15) revealed that
toluene gives the best selectivity factor (S ) 58), which is higher
than the value S ) 41 obtained with Cl-PIQ.8b Further
optimization (Table 1, entries 15-17) revealed that a much
higher selectivity factor S ) 125 was obtained when the reaction
was performed at -40 °C, although a concomitant loss in
activity was apparent (Table 1, entry 17). Decreasing the catalyst
loading to 2 mol % gave a similar selectivity and slightly lower
conversion (Table 1, entry 18). Use of acetic anhydride as the
electrophile source led to reduced selectivity (Table 1, entry
19), and in contrast to Birman’s result,8c the bulkier isobutyric
anhydride also resulted in lower selectivity (Table 1, entry 20).
The above optimization led us to established that 2 mol %
Rp-1a/(EtCO)2O (0.75 equiv)/i-Pr2NEt (0.75 equiv)/toluene/0 °C
were the optimal reaction conditions for catalytic KR. With these
conditions, the generality and substrate scope were probed. A
series of arylalkyl carbinols were screened, and from Table 2 it
can be seen that kinetic resolution proceeded smoothly under
the optimal reaction conditions for all of the arylalkyl carbinols
Acknowledgment. W.-P.D. is thankful for financial support
from the New Century Excellent Talents in University, the Ministry
of Education, China (NCET-07-0283), the Shanghai Committee of
Science and Technology (Grant 06PJ14023), and the “111” Project
(B07023). J.S.F. thanks ECUST for a Visiting Professorship, ERDF
AWMII, and the University of Birmingham for support. W.-P.D.
and J.S.F. thank the CAtalysis and Sensing for our Environment
(CASE) network and the CASE09 Workshop at ECUST for net-
working opportunities.
Supporting Information Available: Synthetic procedures for
planar chiral PIPs 1a-d and corresponding spectral data; X-ray
analysis data for (S,Sp)-1a (CIF); general experimental proce-
dures; and HPLC data analysis for all kinetic resolutions of rac-
5. This material is available free of charge via the Internet at
http://pubs.acs.org.
JA108238A
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