Scaffolding catalysts: Highly enantioselective desymmetrization reactions

Article abstract of DOI:10.1002/anie.201103470

Ex-changing places: A highly enantioselective desymmetrization of 1,2-diols has been developed in which the catalyst utilizes reversible covalent bonding to the substrate to achieve both high selectivity and rate acceleration (see scheme, PMP=pentalmethyl

Full text of DOI:10.1002/anie.201103470

DOI: 10.1002/anie.201103470
Organocatalysis
Scaffolding Catalysts: Highly Enantioselective Desymmetrization
Reactions**
Xixi Sun, Amanda D. Worthy, and Kian L. Tan*
The use of reversible covalent bonding is a common design
feature in synthetic catalysts. For most synthetic catalysts,
reversible covalent bonding is used to form a reactive
intermediate thereby affording an enhanced rate of reaction
(e.g. enamine, iminium, and N-heterocyclic carbene cataly-
sis).[1,2] A less developed area of reversible covalent bonding
catalysis uses this bonding to transiently tether reagents
(Figure 1).[3] A key aspect of this mode of catalysis is that a
phosphorylation reactions. These strategies stand out as
synthetically practical methods because they afford syntheti-
cally useful monoprotected products with no limitation on the
theoretical yield. We decided to investigate this reaction as a
means of testing a new catalyst motif that incorporates a
reversible covalent bond between the catalyst and substrate.
Ostensibly, the selectivity in peptide catalysis is due, at least in
part, to noncovalent interactions between the substrate and
the catalyst that serve to organize the intermediate complex.
In comparison, we considered that reversible covalent bond-
ing catalysis might involve a more intimate association
between the catalyst and the substrate, and thus lead to
enhanced reactivity and/or selectivity. Similar to our phos-
phine ligand design, we synthesized a bifunctional molecule
to serve as a silyl transfer catalyst (Figure 2). The scaffolding
Figure 1. Catalytic cycle for scaffolding catalysis.
part of the acceleration arises from the entropic advantage
gained through intramolecularity.[4] We have recently
reported the development of phosphine ligands that use
reversible covalent bonds between a substrate and the ligand
to control regio-, diastereo-, and enantioselectivity in hydro-
formylation.[5–7] Herein we extend this catalysis design
strategy to organic catalysts. In particular, we report the
catalytic desymmetrization of meso diols in up to 97% ee with
high yield at room temperature.
The use of chiral organic compounds as electrophile-
transfer catalysts has successfully allowed the kinetic reso-
lution and desymmetrization of alcohol substrates in a variety
of transformations; such reactions include acylation,[8,9]
phosphorylation,[10] sulfonylation,[11] and silylation.[12,13]
Desymmetrization of diol and triol substrates has been
accomplished by peptide-catalyzed silylation, acylation, and
Figure 2. Catalyst design.
catalyst contains a substrate-binding site, which uses rever-
sible covalent bonding, and incorporates an imidazole group
as the catalytically active residue (Figure 2). The aim is to
reversibly bind the diol, then, through an intramolecular
transfer or deprotonation, functionalize the free alcohol
(Scheme 1). A unique feature of this catalytic cycle is that
[*] X. Sun, A. D. Worthy, Prof. K. L. Tan
Department of Chemistry, Boston College
2609 Beacon St., Chestnut Hill, MA 02467-3860 (USA)
E-mail: kian.tan.1@bc.edu
[**] We thank Professors Marc Snapper and Amir Hoveyda for helpful
discussions. We thank Bo Li for X-ray structure analysis of 6. We
thank the ACS-PRF (DNI-5001400) and NIGMS (RO1GM087581)
for funding this research. Mass spectrometry instrumentation at
Boston College is supported by funding from the NSF (DBI-
0619576).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201103470.
Scheme 1. Proposed catalytic cycle for silylation
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Communications
Table 2: Substrate scope with TBSCl.
the binding of the catalyst to the substrate is entropically
Entry[a]
Product
Yield [%]
79
ee [%]
neutral (because an alcohol is released upon binding of
substrate). Consequently, the subsequent silylation step is
intramolecular but does not have to pay an entropic penalty
for bringing the catalyst and diol substrate together. Further-
more, the enantioselectivity in the reaction could be derived
from the binding step, functionalization step, or a combina-
tion of the two.
1[b]
89
2[c]
3[b]
4[c]
87
88
86
90
95
92
To initiate these studies, we developed a catalyst structure
that is easy to synthesize and modify. The catalysts in Table 1
are readily synthesized in two steps from amino alcohols. As
depicted in Table 1, when employing catalyst 1 in the
desymmetrization of 1,2-cyclopentane diol, the monopro-
tected product forms in 17% yield with À9% ee (Table 1,
entry 1). Using 2 and 3, which are derived from valine and
tert-leucine, respectively, affords the product in modest yield
and improved enantioselectivity (Table 1, entries 2 and 3). By
optimizing the less expensive valine core structure, it was
found that addition of a substituent adjacent to the imidazole
ring dramatically affects the selectivity of the reaction.
Silylation with 4a and 4b show a significant matched and
mismatched relationship, with 4b yielding a product with
excellent enantioselectivity (Table 1, entry 5);[14] and 4a
furnishes the opposite product enantiomer in low selectivity
(Table 1, entry 4). The high enantioselectivity in the silylation
reaction is notable given the fact that the reaction is
performed at room temperature. An explanation for the
high selectivities at noncryogenic temperatures is the rigid
nature of covalent bonding.
5[d]
6[e]
7[f]
82
93
78
90
86
90
[a] All reactions performed with 20 mol% 4b and 3 mol% PMP·HCl, and
are an average of two runs. [b] TBSCl (4 equiv), PMP (1.2 equiv), RT,
24 h. [c] TBSCl (2 equiv), PMP (1.2 equiv), RT, 12 h. [d] TBSCl (4 equiv),
PMP (2 equiv), 48C, 24 h. [e] TBSCl (4 equiv), PMP (2 equiv), RT, 24 h.
[f] TBSCl (4 equiv), PMP (2 equiv), 48C, 36 h.
hexane-1,2-diol, 1,2,3,4-tetrahydro-2,3-naphthalenediol, and
cyclohexene-1,2-diol provide comparable yields and enantio-
selectivities to those of the five-membered ring substrates
(Table 2, entries 2–4). Seven- and eight-membered rings also
yield the desired product in 90% and 86% ee, respectively
(Table 2, entry 5 and 6). The substrate scope was expanded to
an acyclic substrate, butane-2,3-diol, thus affording the
product in 90% ee and 78% yield (Table 2, entry 7). The
silyl transfer onto both trans-cyclohexane-1,2-diol and (R,S)-
pentane-2,4-diol using 4b as the catalyst yields the products in
less than 5% yield; furthermore, cis-4-cyclopentane-1,3-diol
affords the desired product in 26% yield and 15% ee. Given
that the proposed mechanism involves intramolecular trans-
fer or deprotonation, a strong proximity effect might account
for these observations. Though constraining in terms of
substrate scope, a catalyst that recognizes both chirality and
proximity has the potential to be a powerful reagent for site-
selective reactions.
Having optimized the catalyst structure, we investigated
the substrate scope.
A tetrahydrofuran-based substrate
affords the product in good yield and enantioselectivity
(Table 2, entry 1). The six-membered ring substrates, cyclo-
Table 1: Optimization of silyl transfer reaction.
With these encouraging results, we also investigated the
generality of the silyl reagents that can be transferred. The
smaller triethylsilyl group is transferred effectively with
excellent enantioselectivity (90% ee) and yield (94%;
Table 3, entry 1) after a reaction time of 1 hour at room
temperature. The sterically larger silyl reagent tert-butyldi-
phenyl silyl chloride (TBDPSCl) affords the product in
90% ee and good yield (75%; Table 3, entry 2), although
extended reaction times are necessary to achieve high
conversion. The reaction time can be shortened by increasing
the concentration of the reaction mixture (1.0m in diol), the
result being only a small decrease in the ee value (Table 3,
entry 2). Silylation with the very reactive dimethylphenyl silyl
chloride (DMPSCl) results in a decrease in enantioselectivity
to 79% (Table 3, entry 3); presumably, the lowered selectivity
is a result of background silylation.
Entry
Catalyst
Yield [%][a]
ee [%][b]
1
2
3
4
1
2
3
4a
4b
4b
17
19
20
25
À9
34
40
À16
97 (94)[c]
92
5
84 (92)[c]
76
6[d]
[a] Reaction performed at 0.20 mmol substrate (0.4m). Yields deter-
mined by GC analysis using an internal standard, 1,3,5-trimethoxyben-
zene. [b] Enantiomeric excess (ee) determined by GC analysis. [c] Reac-
tion performed at 0.40 mmol substrate. Yield of product given in
parentheses. [d] Reaction performed 0.2m of diol in tBuOH.
PMP=pentamethylpiperidine, TBS=tert-butyldimethylsilyl, THF=te-
trahydrofuran.
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Table 3: Electrophile scope.
being important for both rate acceleration and enantioselec-
tivity. In support of exchange occurring under the reaction
conditions, isopropyl alcohol was added to 4b in the presence
1
of 10 mol% PMP·HCl and monitored by H NMR spectros-
Entry
Electrophile
Yield [%][a]
ee [%][a]
copy. Under these reaction conditions, an equilibrium is
established with a Keq of 0.12 (Scheme 4).[15] Collectively,
1[b]
2[c]
3[e]
TESCl
TBDPSCl
DMPSCl
94
90
75 (76)[d]
71
90 (86)[d]
79
these observations strongly support
a mechanism that
involves reversible covalent substrate–catalyst bonding.
[a] Yields and ee values are an average of two runs. [b] TESCl (1.2 equiv),
0.2m in diol, RT, 1 h [c] TBDPSCl (4 equiv), 0.2m in diol, RT, 48 h.
[d] 1.0m diol, RT, 24 h. [e] DMPSCl (1.2 equiv), 0.2m in diol, RT, 1 h.
DMPS=dimethylphenylsilyl, TES=triethylsilyl, TBDPS=tert-butyldi-
phenylsilyl.
The results with TESCl are a practical improvement over
the original TBSCl conditions, because the reaction time is
shorter and less silyl chloride is employed. We therefore
applied these conditions to butane-2,3-diol, which with TBSCl
conditions required a 36 hour reaction time at 08C to obtain
high enantioselectivity (Table 2, entry 7). With the TESCl
procedure, the product is formed with improved selectivity
and yield (84%, 92% ee) after a 4 hour reaction time at room
temperature (Scheme 2). Previous attempts with TBSCl to
desymmetrize acyclic substrates with vinyl or aryl substitution
afforded poor conversion; however, with the new procedure
these substrates react with comparable levels of selectivity to
the cyclic substrates in 4–8 hours (Scheme 2).
Scheme 4. Exchange of 4b with isopropanol and substrate.
To learn more about the step in the mechanism that is
responsible for selectivity, the structure and stability of the
catalysts as well as the catalyst–substrate complexes were
examined. First, catalyst 4b forms only a single diastereomer
of catalyst as observed by 1H NMR spectroscopy.[16] Exchange
of 4-bromobenzyl alcohol onto 4b allowed an X-ray structure
to be obtained. The X-ray structure shows that the alcohol
binds syn to the iPr group on the oxazoline ring (6; Figure 3).
We believe that the two isopropyl groups serve the dual
purpose of gearing the substrate binding site in this syn
relationship and to conformationally restrict the location of
the imidazole group. In a second experiment, cis-1,2-cyclo-
pentane-diol exchanges with 4b to form two new species, and
reaches equilibrium in approximately 3 hours (Scheme 4).[17]
We have tentatively assigned these compounds as being
derived from the binding of the enantiotopic alcohols to 4b.
The two new compounds form in a 60:40 ratio, thus suggesting
that there is little binding stereoselectivity. The poor selec-
tivity in binding is consistent with the enantioselectivity of the
reaction originating from the functionalization step. One
explanation for the observed stereoinduction of the catalyst is
that when the R alcohol binds to the catalyst it places the free
alcohol in close proximity to the catalytic site, thus facilitating
intramolecular transfer. Conversely, if the S alcohol binds, the
Scheme 2. Silylation with TESCl.
In terms of a mechanism for catalysis and stereoinduction,
it is conceivable that either noncovalent bonding (hydrogen
bonding) or reversible covalent bonding is responsible for the
substrate–catalyst organization. To differentiate these possi-
bilities, hydrogen-bonding solvents were examined. When
1,2-cyclopentane diol was silylated using tBuOH as the
solvent the product forms in 92% ee (Table 1, entry 6),
which is inconsistent with a hydrogen-bonding mechanism.
To further validate a reversible covalent bond between the
substrate alcohol and the catalyst, we synthesized control
catalyst 5, which lacks a substrate-binding site. Silylation with
5 delivers an almost racemic mixture of the product in low
yield (Scheme 3), which is consistent with substrate binding
Figure 3. X-ray structure of 6.[18]ok Thermal ellipsoids shown at 50%
Scheme 3. Control reaction.
probability.
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free alcohol is placed away from the catalytic residue, thereby
preventing intramolecular processes. In this way restricting
the spatial orientation of the imidazole ring is essential for
stereoinduction.
In summary, we have developed a new organic catalyst
that uses reversible binding to alcohol substrates to both
accelerate and control the selectivity in a desymmetrization
reaction. The catalyst is readily synthesized from inexpensive
building blocks; furthermore, the modular nature of the
synthesis allows derivatives to be rapidly accessed. The rigid
covalent bonding between the substrate and catalyst gener-
ates a highly enantioselective silylation reaction for a range of
1,2-diols. The ability of the scaffolding catalyst to function-
alize substrates through both stereoselectivity and proximity
is encouraging for developing these types of catalysts for
complex molecule modifications where site-selective catalysis
is required. We are continuing to develop these catalysts for
other electrophile-transfer reactions, and we envision design-
ing a series of second-generation structures that specifically
functionalize anti 1,2-diols, syn- and anti 1,3-diols, and syn-
and anti 1,4-diols.
Experimental Section
A solution of cis-1,2-cyclopentanediol (41 mg, 0.40 mmol), 4b (22 mg,
0.080 mmol, 20 mol%), and 1,2,2,6,6-pentamethylpiperidine hydro-
chloride (2.3 mg, 0.012 mmol, 3 mol%) in anhydrous THF (0.50 mL)
was added to an oven-dried glass reaction vial. The reaction mixture
was stirred at room temperature for 10 min. 1,2,2,6,6-pentamethylpi-
peridine (87 mL, 0.48 mmol, 1.2 equiv) was added, followed by
addition of a solution of tert-butylchlorodimethylsilane (120 mg,
0.80 mmol, 2.0 equiv) in anhydrous THF (0.50 mL). After stirring at
room temperature for 4 h, reaction mixture was quenched by addition
of N,N-diisopropylethylamine (200 mL) and methanol (60 mL). The
mixture was stirred at room temperature for 10 min, and was then
concentrated under reduced pressure. Flash column chromatography
on silica gel (hexanes/EtOAc = 20/1) afforded pure product as
colorless oil (80 mg, 92%, 94% ee). GLC analysis using a chiral
stationary phase (Supelco Beta Dex 120 (30 ꢀ 0.15 mm ꢀ 0.25 mm film
thickness) was performed using the following conditions: 788C for
100 min, 208CminÀ1 to 1808C, 1808C for 20 min, 15 psi.
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Published online: July 7, 2011
Keywords: asymmetric catalysis · desymmetrization ·
.
organocatalysis · silanes · synthetic methods
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[14] In the desymmetrization < 5% the bis(silylated) product is
observed. Also a control reaction showed no silyl migration
during the reaction (see the Supporting Information for details).
[15] The value of this Keq is similar to the value determined for our
phosphorous ligand in Figure 2. See Ref. [5a] for details.
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1.9:1 and 5.7:1. See the Supporting Information for details of
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faster than isopropanol. The exchange with isopropanol and
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