(-)-β-pinene-derived N-heterocyclic carbenes: Application to highly enantioselective intramolecular stetter reaction

Article abstract of DOI:10.1021/cs500207d

Novel chiral N-heterocyclic carbenes derived from (-)-β-pinene have been synthesized and used as efficient catalysts for the intramolecular Stetter reaction. Mono-, di-, and trisubstituted 4-chromanone derivatives have been obtained almost quantitatively

Full text of DOI:10.1021/cs500207d

Letter
pubs.acs.org/acscatalysis
(−)-β-Pinene-Derived N‑Heterocyclic Carbenes: Application to Highly
Enantioselective Intramolecular Stetter Reaction
Zbigniew Rafinski,*,† Anna Kozakiewicz,‡ and Katarzyna Rafinska†
́
́
†Department of Organic Chemistry and ‡Department of Crystallochemistry and Biocrystallography, Nicolaus Copernicus University,
́
7 Gagarin Street, 87-100 Torun, Poland
S
* Supporting Information
ABSTRACT: Novel chiral N-heterocyclic carbenes derived
from (−)-β-pinene have been synthesized and used as efficient
catalysts for the intramolecular Stetter reaction. Mono-, di-,
and trisubstituted 4-chromanone derivatives have been
obtained almost quantitatively and with high enantioselectivity
up to 99:1 er.
KEYWORDS: Stetter reaction, N-heterocyclic carbenes, organocatalysis, terpenoids, chromanones
he enormous potential of N-heterocyclic carbenes
T(NHCs) as organocatalysts has become evident in recent
years and plays an important role in the growing area of
asymmetric organocatalysis. The phenomenal success of NHCs
can be attributed primarily to their ability to invert the typical
electrophilic nature of aldehydes (“umpolung”).1 In this
context, the Stetter reaction is undoubtedly one of the most
important umpolung processes that allows formation of unique
1,4-dicarbonyl compounds and related derivatives, such as
ketophosphonates, nitroketones, or ketonitriles.2 Various chiral
NHCs have been developed, and the triazolium salts 1−3 have
proved to be the most efficient precatalysts in numerous
asymmetric applications (Figure 1).3
Figure 2. Chiral triazolium salts derived from (1R)-camphor.
have been effective in highly enantioselective reactions. Many
catalysts suffer from a relatively high cost of enantiopure
starting materials. Consequently, the design of new NHC
catalysts of improved reactivity profiles derived from a readily
available chiral precursor is a great challenge.
Except for camphor, no other monoterpenes were used for
the synthesis of triazolium carbene catalysts. Therefore, we
decided to undertake the synthesis of new terpenyl NHCs. β-
Pinene was chosen as a low-priced and readily available chiral
precursor, commercially available in both enantiomeric forms.5
Chiral auxiliaries and ligands with the pinane backbone are
widely used in asymmetric synthesis.6 In this communication,
we report our preliminary results on the synthesis of novel
triazolium salts 8a−c from (−)-β-pinene and their application
to the enantioselective intramolecular Stetter reaction, provid-
ing chromanone products in almost quantitative yields and
excellent enantioselectivity.
Figure 1. Chiral bi- and tetracyclic triazolium salts.
Deprotonation of the salts generates free carbenes, which
react in situ with aldehydes by nucleophilic attack.
We started our investigation by synthesis of triazolium salts
from (−)-β-pinene, as outlined in Scheme 1. (−)-β-Pinene was
transformed into cis-amino alcohol 5 according to the reported
method.7 Annulation to the lactam 7 proceeded in an overall
In 2008, (1R)-camphor-derived triazolium salts 4 (Figure 2)
were applied as efficient catalysts for the intramolecular crossed
benzoin condensations and the Michael and Stetter reactions.4
To date, these are the only terpenyl triazolium salts
combining the reactivity of the NHCs with the selectivity
effect of the terpene moiety. Despite the significant progress,
only a limited number of reported NHCs backbone scaffolds
Received: November 3, 2013
Revised: March 21, 2014
Published: March 25, 2014
© 2014 American Chemical Society
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Letter
Scheme 1. Synthesis of Triazolium Salts from (−)-β-Pinene
Table 1. Screening of the Triazolylidene Catalysts Derived
from (−)-β-Pinene and cis-1-Amino-2-indanol
a
d
e
entry
catalyst
yield (%)
er
1
2
3
4
5
8a
8b
8c
8d
8e
8b
8b
NR
99
31
10
74
92
95
97:3
99:1
95:5
96:4
96:4
95:5
two-step process, using our own procedure based on the
reaction of 5 with chloroacetyl chloride, followed by cyclization
in the presence of potassium tert-butoxide. In the initial
attempts, using the literature procedure, the yield of annulation
did not exceed 20%.4a With the lactam 7 in hand, the procedure
developed by Rovis8 was used for its conversion into triazolium
salts 8a−c (Scheme 1). Their preparation does not require
chromatographic purification. Evaporation of the solvent
followed by washing with diethyl ether or toluene provides
pure products that are air- and water-stable solids. The
structure of 8a was established by X-ray crystallographic
analysis, as shown in Figure 3.9
b
6
c
7
a
Reactions were performed in the presence of 8a−e (8 mol %),
DIPEA (8 mol %) in 0.1 M in toluene, and at rt. 8b (5 mol %) and
DIPEA (5 mol %) were used. The reaction was performed at 0 °C.
Isolated yields. Determined by HPLC using a chiral column
b
c
d
e
(Chiracel Daicel OD-H).
and triazolylidenes derived from 8b−e, high selectivity was
observed with a catalyst loading of 8 mol %; however, in the
case of 8c, the Stetter product was obtained in low yield. It is
worth noting that the reaction catalyzed by amino indanol
derived triazolium salts 8d,e and DIPEA gave the desired
product 10a with excellent enantioselectivity, albeit in low
yields (entries 4−5, Table 1). Gratifyingly, the new NHC
catalyst generated from 8b allowed a complete conversion to
the desired product with excellent enantioselection of 97:3 er.
Lowering the amount of 8b from 8 to 5 mol % and the reaction
temperature to 0 °C also gave excellent yields of the desired
cyclization products but with a small erosion of the er value.
The reaction parameters were further examined in the
presence of 8 mol % of triazolium salt 8b. As shown in Table 2,
several conventional organic and inorganic bases, such as
dicyclohexylethylamine (DCyEA), DBU, Et3N, KHMDS,
DMAP, tert-BuOK, and also the commercially available
phosphazene bases P2-Et and P2-tert-Bu, were tested and gave
the reaction product in excellent yield and high enantiomeric
ratio. Cesium carbonate led to a racemic mixture. A possible
explanation of the slight difference in selectivity can be
competitive epimerization under the reaction conditions. We
also surmise that in the case of cesium carbonate, there is
incomplete conversion of triazolium salt to carbene so that the
free base might racemize the product. To confirm this, the
catalytic process outlined in Table 2, entry 6 was repeated with
enantioenriched (R)-chromanone 10a (96% ee) replacing 9a as
the starting material. After stirring for 16 h, the chromanone
10a was recovered in greatly reduced enantiomeric excess (4%
ee) (Scheme 2).
Figure 3. ORTEP drawing of molecular structure of 8a. Thermal
ellipsoids are set at the 30% probability level.
The intramolecular Stetter reaction was chosen as the model
reaction to test the catalytic activity of the new triazolium salts.
The reaction is highly interesting because it provides an access
to 4-chromanone derivatives, widely occurring in many
bioactive natural products and used as versatile synthons in
organic synthesis.10
Our initial exploration focused on the examination of
triazolium salts 8a−e in the Stetter cyclization of 9a. As
shown in Table 1, the carbene generated from triazolium salt 8a
proved to be inactive. Using diisopropylethylamine (8 mol %)
Screening of the various solvents, such as o-xylene, CH2Cl2,
DMF, THF, dimethoxyethane (DME), cyclopentyl methyl
ether (CPME), tert-amyl methyl ether (TAME), and tert-butyl
methyl ether (MTBE) showed that they are well tolerated in
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Letter
a
the economical aspect, further study was carried out with tert-
butyl methyl ether.
Table 2. Screening of Different Bases
After indentifying both the optimal triazolylidene catalyst and
conditions, the reaction scope was examined using a wide range
of substrates with different substitution patterns of the aromatic
ring. Various O-tethered mono-, di-, and trisubstituted
salicylaldehyde-derived substrates bearing either electron-
donating or electron-withdrawing groups were tested (Table
4). For substrates 9b−d, bearing the electron-donating groups
on the salicylaldehyde-structure (5-OMe, 4-OMe, 5-Me, entries
2−4), the products were obtained in excellent yields and with
remarkably high enantioselection. Surprisingly, 5-fluoro, and 3-
iodo derivatives 9f and 9g, were also suitable reaction partners,
and the resulting O-tethered chromanones 10f−g were
obtained in a highly selective fashion, albeit for the 3-fluoro
9e derivative slight erosion of the er value without affecting the
reactivity was observed. The enantioselectivity remains at the
same level when the substituent on the aryl moiety is either an
electron-rich or electron-deficient group (entries 1−4, 6, and 7,
Table 4). Di- or trisubstituted substrates led to the formation of
di- or trialkylated or dihalogenated chromanones. Cyclization of
4,6-dichloro 9k also occurred efficiently to give 10k in a nearly
quantitative yield with 98:2 er. Unfortunately, the di-tert-butyl-
substituted 9h gave the cyclization product with only 62:38 er,
although the yield remained excellent. Selectivity was also
affected, when the 5-tert-butyl group of 9i was replaced by the
methyl group (entry 9, Table 4). Replacement of the spatially
large tert-butyl substituents in 9h by the methyl groups in 9j
significantly enhanced the enantioselectivity. Comparing
chromanones 10h−10j shows that bulky substituents in the 3
and 5 positions decreased the selectivity of the reaction.
Moreover, trimethyl-substituted O-tethered substrate 9l was
also synthesized and tested in the intramolecular Stetter
reaction. The desired product 10l was obtained in high yield
and enantioselectivity, 94:6 er. It is worth noting that 4-
chromanone derivatives 10e−10l are unknown in the literature.
After successfully performing a highly enantioselective
intramolecular Stetter reaction on a broad range of aromatic
aldehydes, we turn our attention to the potentially more
difficult aliphatic aldehyde bearing acidic hydrogens α to the
aldehyde. Under the conditions used for the previous aromatic
aldehydes 9a−l, 11 failed to give the reproducible results, and
the desired pyrolidinone 12 was obtained in an excellent yield
of 98% with a moderate enantioselectivity of 35% ee (Scheme
3).
b
c
entry
base
yield (%)
er
1
2
DIPEA
DCyEA
DBU
99
99
99
99
97
91
97
99
97
98
97:3
96:4
91:9
92:8
88:12
50:50
92:8
91:9
95:5
95:5
3
4
Et3N
5
KHMDS
Cs2CO3
tBuOK
DMAP
P2-Et
6
7
8
9
10
P2-tBu
a
Reactions were performed in the presence of 8b (8 mol %), base (8
mol %) in 0.1 M in toluene, and at rt. Isolated yields. Determined by
HPLC using chiral column (Chiracel Daicel OD-H).
b
c
Scheme 2. Racemization of 10a under the Reaction
Condition
this reaction (Table 3); however, the reactions in DMF and
DME were not so efficient as in other solvents and provided
the product in only 52% and 36% yield, respectively (Table 3,
entry 4 and 7). Finally, both the reaction in tert-butyl methyl
ether and cyclopentyl methyl ether led to the optimal
combination of 99% yield and 98:2 er. Taking into account
a
Table 3. Screening of Different Solvents
Furthermore, we also investigated the utility of triazolium
salts 8b,c in the construction of all-carbon quaternary
stereogenic centers2b,11 (which poses a great challenge) and
cyclopentene-forming annulation reaction, respectively.12
Cyclization of salicylaldehyde-derived substrate 13 bearing
β,β-disubstituted Micheal acceptor occurred very efficiency to
give the coumaranone 14 in a excellent yield with 95% ee.
Unfortunately, application of triazolium salt 8c in hydro-
acylation of an unactivated double bond of 15 failed to catalyze
the reaction. To our great delight, with 10 mol % β-pinene-
derived triazolium salt 8c bearing a mesityl group, an
enantioselective benzoin-oxy-Cope reaction proceed smoothly,
affording the cyclopentene product 16 in 71% yield with 96%
ee and 8:2 dr (Scheme 4).
b
c
entry
solvent
yield (%)
er
1
2
3
4
5
6
7
8
9
toluene
o-xylene
THF
99
99
98
99
97
91
97
99
97
97:3
96:4
96.5:3.5
88.5:11.5
90:10
93:7
DMF
CH2Cl2
DME
MTBE
CPME
TAME
98:2
98:2
97:3
a
Reactions were performed in the presence of 8b (8 mol %), DIPEA
b
c
In conclusion, a series of novel tetracyclic triazolium salts was
prepared from the readily available (−)-β-pinene. They were
used to generate NHC catalysts for the enantioselective
synthesis of mono-, di-, and trisubstituted 4-chromanones.
(8 mol %) in 0.1 M in solvent, and at rt. Isolated yields. Determined
by HPLC, using chiral column (Chiracel Daicel OD-H). MTBE =
methyl tert-butyl ether, CPME = cyclopentyl methyl ether, TAME =
tert-amyl methyl ether.
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a
Table 4. Enantioselective Intramolecular Stetter Reaction
Scheme 3. Enantioselective Intramolecular Stetter Reaction
of 11
Scheme 4. Application of Triazolium Salts 8b,c in Other
Reactions
opportunities in terms of structural selection and modification
of future catalysts. Further applications of these (−)-β-pinene-
derived triazolium salts in other asymmetric reactions are
currently under way.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, characterization, and crystallographic
data for the products. This material is available free of charge
via the Internet at http://pubs.acs.org.
a
Reaction conditions: 8b (8 mol %) and DIPEA (8 mol %) in 0.1 M in
b
MTBE (0.1M) for 20 h, rt. The reaction was conducted for 48 h.
c
d
Isolated yields. Determined by HPLC, using chiral columns
AUTHOR INFORMATION
(Chiracel Daicel OD-H, OJ, Lux Cellulose-1).
■
Corresponding Author
*E-mail: payudo@chem.umk.pl.
The chiral triazolium salt 8b and DIPEA generates a highly
efficient and selective carbene catalyst for the intramolecular
Stetter reaction, affording the 4-chromanones in excellent yields
with up to 99:1 enantiomeric ratio utilizing relatively low
catalyst loadings (8 mol %).
Structural diversity of enantiomerically pure monoterpenes,
an access to both enantiomeric forms, commercial availability,
and a relatively low cost of chiral precursor offers great
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the National Science Centre (Grant
No. UMO-2011/03/D/ST5/05626) for financial support.
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