Carbene-catalyzed desymmetrization of 1,3-diols: Access to optically enriched tertiary alkyl chlorides

Article abstract of DOI:10.1039/c6cc03345h

The introduction of a chlorine atom to a carbon center in an enantioselective manner via conventional C-Cl bond formation is difficult. Here we report a new approach to this class of tertiary alkyl chlorides with high optical purities. Instead of forming a new C-Cl bond, our approach involves carbene-catalyzed desymmetrization of 2-chloro-1,3-diols as the key step to set up the chiral carbon center with excellent enantiomeric excess.

Full text of DOI:10.1039/c6cc03345h

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DOI: 10.1039/C6CC03345H
Journal Name
ARTICLE
Carbene-catalyzed desymmetrization of 1,3-diols: access to
optically enriched tertiary alkyl chlorides
Bao-Sheng Li,a,‡ Yuhuang Wang,a,‡ Rupert S. J. Proctor,a Zhichao Jina and Yonggui Robin Chia,b*
Received 00th January 20xx,
Accepted 00th January 20xx
The introduction of a chlorine atom to a carbon center in an enantioselective manner via conventional C-Cl bond
formation is difficult. Here we report a new approach to this class of tertiary alkyl chlorides with high optical purities.
Instead of forming a new C-Cl bond, our approach involves carbene-catalyzed desymmetrization of 2-chloro-1,3-diols as
DOI: 10.1039/x0xx00000x
www.rsc.org/
the key step to set up
the chiral carbon center
with excellent enantiomeric excess.
bears an alcohol functional group from the substrate and can
readily undergo further transformations.
Introduction
The chlorine atom is widely found in natural products and
synthetic molecules with interesting properties.1 As such, its
introduction to molecules, especially in an enantioselective
fashion, has received considerable attention. In particular, it
remains challenging to prepare optically enriched tertiary alkyl
chlorides with the chlorine atom connected to a stereogenic
carbon center. To date the dominant approach has been to
form a new carbon-chlorine bond via asymmetric catalysis
(Scheme 1a). For instance, asymmetric chlorination of carbonyl
compounds (such as
-keto esters) via enol or enolate
intermediates is the most well-established method.2,3 Chiral
Lewis acid catalysts2 and amine3a or phosphoric acid3b organic
catalysts have been explored for this type of reaction, as
disclosed by, Shibata,2a Shibatomi,2b,c Bartoli,3a and Antilla.3c
Our laboratory is interested in exploring carbene organic
catalysts for rapid access to functional molecules.4 Herein, we
report a new approach for effective access to optically
enriched tertiary alkyl chlorides via N-heterocyclic carbene
(NHC)-catalyzed desymmetrization of 1,3-diols.5 Our method
utilizes a readily available substrate with a chlorine atom pre-
installed on a pre-stereogenic carbon center (Scheme 1b). The
postulated reaction pathway involves selective formation of an
Scheme 1 The synthesis of optically enriched chiral tertiary alkyl chlorides
Notably, NHCs as versatile organic catalysts are most often
used for new bond formations.10 The use of NHCs for kinetic
resolution has also received considerable attention in recent
years.11 In contrast, desymmetrization through NHC
organocatalysis12 has thus far received much less study. One
nice work in this direction was reported by Scheidt, in which
addition of an azolium enolate intermediate to one of the
ketone moieties of a symmetric 1,3-diketone compound led to
enantiomerically enriched products.12a,b Rovis12c and You have
ester at one of the two alcohol units of substrate
1
via the
The
catalytic generation of key azolium intermediate I 6
.
intramolecular hydrogen bonding between the two alcohol
groups of the 1,3-diol substrate7,8 likely plays an important role
in this process.9 The optically enriched alkyl chloride product
3
a.Division of Chemistry & Biological Chemistry, School of Physical & Mathematical
Sciences, Nanyang Technological University, Singapore, 637371 (Singapore, E-
mail: robinchi@ntu.edu.sg
reported desymmetrization through
a Stetter reaction,
respectively.12d In the area of diol desymmetrization,
conversion of non-chlorine-containing diols has mainly been
achieved with chiral Lewis acid catalysts13 and more recently
with chiral phosphoric acid,14 quinidine-derived15a or sulfide15b
organic catalysts.
b.State Key Laboratory Breeding Base of Green Pesticide and Agricultural
Bioengineering Ministry of Education, Guizhou University, Guiyang, 550025
(China)
‡
These authors contributed equally to this work.
† Electronic Supplementary Information (ESI) available: Characterization data
and experimental procedures. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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(entry 12). Increasing or decreasing the reactionVtieewmAprtieclreaOtnulirnee
both led to decreased ee (see SI), suggesDtiOnIg: 1t0h.1a0t3t9h/Ce6hCyCd0r3o3g4e5Hn
bonding might play important roles in our desymmetrization
reactions.
Results and discussion
Table 1 Optimization of reaction conditionsa
Table 2 Examples of Ar substituents for the diol substratesa
entry
1
cat.
R
3 yield (%)c
3 erd
84:16
80:20
76:24
72:28
84:16
89:11
90:10
77:23
87:13
87:13
95:5
A
C6H5
95
90
92
85
87
93
95
90
93
94
95
90
2
A
4-Cl-C6H5
4-CH3O-C6H5
C6H5CH=CH
2-CH3-C6H5
1-Naphthyl
1-Naphthyl
1-Naphthyl
1-Naphthyl
1-Naphthyl
1-Naphthyl
1-Naphthyl
3
A
4
A
5
A
6
A
7
B
a
Conditions as in Table 1, entry 11 unless otherwise specified;
8
C
isolated yield after column chromatography. bThe corresponding
fluorine atom-containing substrate was used.
9
D
The scope of the reaction was then examined. We first
evaluated the diol substrates by replacing the phenyl group of
1a with other (hetero) aryl substituents (Table 2). Installing
10
11
12
E
F
F (2 mol%)b
95:5
various substituents on the phenyl ring 3b-3j of the diols had
no significant effect on the reaction outcomes, and the
corresponding products were obtained with excellent yields
and enantiomeric ratios. The phenyl unit of 1a could also be
replaced by a 1-naphthyl 3h or 2-thiophenyl 3i group.
Furthermore, we found that when the Cl atom was replaced
with an F atom, the reaction worked effectively to give 3j with
95% yield and a somewhat lower 85:15 er under the same
conditions without further optimization.
We next evaluated diol substrates bearing non-Ar
substituents (olefins and alkynes), in order to provide rich
opportunities for further functionalization. As indicated in
Table 3, products 4a-4h containing alkene units with a variety
of substitution patterns could be obtained with excellent
a All reactions of 1a (0.10 mmol, 20.1 mg) with aldehyde 2 (0.2
mmol, 31.2 mg) were carried out with 5 mol% A-F and 10 mol%
b
Cs2CO3 in THF (1.0 mL) for 16 h. Catalyst F of 2 mol% was
d
used (48 h). c Isolated yield. Er of 3 was determined by chiral
HPLC analysis. Mes, 1,3,5-trimethylbenzene.
Results of our condition optimization are briefed in Table 1,
using diol 1a as the model substrate. Triazolium NHC pre-
catalyst A
, developed by Bode,16 was first used to identify a
suitable aldehyde to be the acylation agent (entries 1-12). We
were very encouraged to find the formation of 3a in 95% yield
and 84:16 er when benzaldehyde was used (entry 1, R = Ph).
Further studies of aldehyde substrates found that other aryl
aldehydes and enals could effectively lead to the products 3a-
3j in good yield and a variety of enantiomeric ratios. The use of
alkyl aldehydes, however, led to little formation of the desired
product. Replacing the phenyl group of benzaldehyde with a
naphthyl unit led to better results than with other aldehydes
examined here (entry 6). We then proceeded to use naphthal
as the acylation reagent to search for optimal NHC catalysts
enantioselectivities and yields. Substrates with
substituent on the double bond terminus gave the
corresponding products 4i 4l with excellent yields and
a cyclic
-
selectivities. For products 4i and 4j, containing a benzocycle, a
higher enantioselectivity was achieved with the five-
membered ring than with its six-membered analogue. When a
benzopyran-like structure was incorporated, the product 4k
was obtained with good results as well. A cyclohexene-
containing substrate was then examined and the expected
product 4m was afforded with good yield and er. Likewise,
substrates with a Z-olefin substituent gave the desired product
4n with similar yield and selectivity. Moreover, when the
(entries 7-12), and found that the use of NHC pre-catalyst
first developed by Rovis,17 afforded 3a with 95% yield and 95:5
er (entry 11). Decreasing the catalyst loading of to 2 mol%
F,
F
led to no apparent changes in er, albeit a longer reaction time
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alkene moiety of the diol substrate was replaced with an
ARTICLE
The optically enriched products from our reaction can
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DOI: 10.1039/C6CC03345H
alkyne, the product 4p was formed with yield and er readily undergo further transformations (Scheme 2). For
comparable to those of the alkene-containing products such as example, oxidation of the free alcohol unit of 3a (to form
4a. However, the substrates with the alkyl sunstituent gave followed by intramolecular Friedel-Crafts acylation effectively
the monoester products (4q 4s) with moderate er. Our afforded indanone 6, which is a common scaffold in functional
5)
-
attempt to prepare 1,3-diol substrate by replacing the allylic molecules. Our phenol containing product 3e can undergo
substituent with a aryl unit was unsuccessful (as below) due to oxidation followed by intramolecular addition/dearomitization
the the tertiary benzyl or allylic chloride is sensitive for to form spirocyclic ether adduct 10 (Scheme 2b).19
reduction of 1,3-diester to 1,3-diol. The absolute configuration
of the products was confirmed by optical rotation comparison
of a derivative of 3j with a known compound.18
Table 3 Examples of diol substrates containing alkene or alkyne
substituentsa
Scheme 2 Synthetic transformations
Conclusions
In summary, we have developed a new approach for
effective access to tertiary alkyl chlorides with high
enantiomeric excess. Instead of forming a new C-Cl bond, we
obtain the sterically challenging chiral carbon center
connected to a chlorine atom through carbene-catalyzed
desymmetrization of 1,3-diols. The optically enriched products
from our reaction can readily undergo further transformations.
Since our method bypasses the difficulties in forming
challenging C-Cl bonds enantioselectively, we expect a broad
set of chiral alkyl chlorides to be easily obtainable by using our
metal-free organic catalytic methods.
Acknowledgements
We acknowledge support from Singapore’s National Research
a
Conditions as in Table 1 entry 11 unless otherwise specified;
isolated yield after column chromatography.
Foundation (NRF), Ministry of Education (MOE), Nanyang
Technological University (NTU), and China’s National Key
Notably, further catalytic esterification of the product 3a
Program for Basic Research (No. 2010CB 126105), Thousand
under the standard condition could occur slowly to form
Talent Plan, National Natural Science Foundation (No.
diester adduct 3aʹʹ (in 20% yield). This additional esterification
21132003; No. 21472028), Guizhou Province Returned
did not lead to change of ee for 3a. This result suggested that
Oversea Student Science and Technology Activity Program, and
the second acylation reaction (to form 3a’’) was not a kinetic
Guizhou University.
resolution process. The enantiomeric excess of 3a obtained in
our reaction exclusively came from catalytic desymmetrization
of 1a (not from kinetic resolution of the mono-ester 3a).
Intramolecular transesterificatioin of 3a (to form the other
enantiomer 3aʹ) was not observed (eq. 1), indicating ester 3a is
stable (no ester-exchange) under our catalytic conditions.
Notes and references
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