Carbocation Catalysis: Oxa-Diels-Alder Reactions of Unactivated Aldehydes and Simple Dienes

Article abstract of DOI:10.1002/ejoc.201501112

The versatility of the trityl cation (TrBF₄) as a highly efficient Lewis acid organocatalyst is demonstrated in the oxa-Diels-Alder reaction of various unactivated aromatic and aliphatic aldehydes and simple unactivated dienes, such as isoprene and 2,3-dimethylbutadiene. The transformation proceeds smoothly to give 3,6-dihydropyrane adducts in high to moderate yields with catalyst loadings down to 1.0 mol-% under mild reaction conditions. In contrast to most previously reported strategies, this protocol does not require substrate functional group activation, neither by electron-deficient aldehydes (2-oxo aldehydes) or electron-rich dienes (methoxy or amino-butadiene).

Full text of DOI:10.1002/ejoc.201501112

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201501112
Carbocation Catalysis: Oxa-Diels–Alder Reactions of Unactivated Aldehydes
and Simple Dienes
Mahmoud Abd El Aleem Ali Ali El Remaily,[a,b] Veluru Ramesh Naidu,[a] Shengjun Ni,[a] and
Johan Franzén*[a]
Keywords: Carbocations / Trityl ion / Cycloaddition / Organocatalysis
The versatility of the trityl cation (TrBF4) as a highly efficient
Lewis acid organocatalyst is demonstrated in the oxa-Diels–
Alder reaction of various unactivated aromatic and aliphatic
aldehydes and simple unactivated dienes, such as isoprene
and 2,3-dimethylbutadiene. The transformation proceeds
smoothly to give 3,6-dihydropyrane adducts in high to mod-
erate yields with catalyst loadings down to 1.0 mol-% under
mild reaction conditions. In contrast to most previously re-
ported strategies, this protocol does not require substrate
functional group activation, neither by electron-deficient
aldehydes (2-oxo aldehydes) or electron-rich dienes (meth-
oxy or amino-butadiene).
However, Matsubara et al. recently reported two strate-
gies that exceed the previous work in this area. They
showed that 5.0 mol-% iron(III)porphyrin {[Fe(TPP)]-
Introduction
Functionalized pyran rings are found in a large number
of naturally occurring compounds and these heterocycles
have attracted much interest among both chemists and biol-
ogists because they are challenging synthetic targets and
have diverse biological activity.[1] Over the years, a variety
of synthetic strategies for the preparation of functionalized
pyrans has been developed and among these, the oxa-Diels–
Alder reaction has evolved as the most straightforward
strategy.[2,3]
BF4}[12a] (TPP
=
meso-tetraphenylporphyrinato) or
iron(IV)corrole[12b] complexes efficiently catalyzed the oxa-
Diels–Alder reaction of unactivated dienes with unactivated
aldehydes under mild reaction conditions in high yields
with a relatively broad substrate scope (Scheme 1, c).
However, in contrast to the corresponding Diels–Alder
reaction of electron deficient alkenes, the oxa-Diels–Alder
reaction of aldehydes is much more challenging due to the
substantially higher LUMO level of the πC–O bond com-
pared with an electron-deficient πC–C bond. This strongly
limits the general applicability of the oxa-Diels–Alder reac-
tion in synthetic applications to the use of either highly acti-
vated dienes, such as Danishefsky’s,[3b,4] Brassard’s[3b,5] or
Rawal’s diene[6] or highly activated aldehydes containing
electron-withdrawing groups, e.g., glyoxylates.[7] Thus, there
is a challenging demand for the development of oxa-Diels–
Alder reactions of unactivated dienes with unactivated alde-
hydes. The few scattered strategies reported so far require
elevated temperatures and/or strong Brønsted acids,[8]
Lewis acid[9,10] or transition metal catalysis[11] and are in
general restrained by low yields, harsh reaction conditions,
high catalyst loadings and poor substrate scope.
Scheme 1. a) TrBF4-catalyzed Diels–Alder, b) TrBF4-catalyzed
aza-Diels–Alder, c) oxa-Diels–Alder catalyzed by [Fe(TPP)]BF4
and trityl ion.
[a] Department of Chemistry, Organic Chemistry, Royal Institute
of Technology (KTH),
Teknikringen 30, SE-100 44 Stockholm, Sweden
E-mail: jfranze@kth.se
In our group we have recently applied carbocations as
novel Lewis acid organocatalysts for various transforma-
tions, often with remarkably low catalyst loading and high
yields.[13,14] In the course of our work we found that the
www.kth.se/en/che/divisions/orgkem/research/JohanFranzen
[b] Department of Chemistry, Faculty of Science, Sohag University,
82524 Sohag, Egypt
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201501112.
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Carbocation Catalysis: Oxa-Diels–Alder Reactions
trityl ion [e.g. triphenylcarbenium tetrafluoroborate Table 1. Optimization of reaction conditions for the hetro-Diels–
Alder reaction.[a]
(TrBF4)] is highly efficient for the activation of α,β-alde-
hydes and tosyl imines in the Diels–Alder reaction[14a,14b]
and the aza-Diels–Alder reaction,[14b] respectively
(Scheme 1, a and b).
Here we show the expansion of our research in this area
by applying trityl tetrafluoroborate (TrBF4) as a catalyst
for the oxa-Diels–Alder reaction of unactivated dienes with
unactivated aldehydes under mild reaction conditions
(Scheme 1, c). The reaction shows high efficiency with low
catalyst loadings and constitutes a cheap, commercially
available and metal free complement to Matsubara’s pre-
viously reported iron complexes.[12]
Entry
Cat. [mol-%]
Solvent
t [h]
Yield[b] [%]
1
2
3
4
5
6
7
8
TrBF4 (5.0)
TrBF4 (1.0)
TrBF4 (0.5)
CH2Cl2
CH2Cl2
CH2Cl2
1
85
83
22
40
54
30
29
0
61
32
5
0
0
24
48
24
24
24
24
24
24
24
24
24
24
(p-MeOPh)(Ph)2CBF4 CH2Cl2
BF3OEt2 (5.0)
TiCl4 (5.0)
HBF4 (5.0)
AlCl3 (5.0)
TrBF4 (1.0)
TrBF4 (1.0)
TrBF4 (1.0)
TrBF4 (1.0)
TrBF4 (1.0)
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
DCE
PhMe
Et2O
CH3NO2
CH3CN
Results and Discussion
9
10
11
12
13
The positively charged carbon atom of the trityl ion is
isoelectronic to boron and owes its Lewis acidity to its low-
lying empty pC-orbital capable of accepting electrons and
thereby activating an interactive species. We therefore antic-
ipated that the free electron pair on benzaldehyde would
interact with the empty pC-orbital of the trityl ion to give
oxonium ion intermediate A (Scheme 2). This would lower
the LUMO of the carbon–oxygen bond facilitating the
cycloaddition with unactivated dienes giving adduct B. Re-
lease of the 3,6-dihydropyran will regenerate the trityl ion
catalyst.
[a] The catalyst was added to a solution (0.3 m) of benzaldehyde
and 2,3-dimethylbutadiene. [b] Determined by 1H NMR spec-
troscopy using 1-methylnaphthalene as internal standard; DCE =
dichloroethene.
tronic properties of the pendant aromatic rings.[14a,14b] As
expected, the rate of reaction decreased as electron density
of the aromatic ring increased (e.g., decreasing Lewis acid-
ity of the carbocation), going from hydrogen to methoxy
(Table 1, entry 4).
To evaluate the catalytic ability of the trityl ion, 5.0 mol-
% of a series of Lewis and Brønsted acids were screened as
catalyst for the oxa-Diels–Alder reaction. BF3·OEt2,
HBF4·OEt2, and TiCl4[9b] were found to be potent catalysts,
although substantially less efficient than the trityl ion
(Table 1, entries 5–7 vs. entry 1). Furthermore, AlCl3 was
completely inactive as a catalyst for this reaction (Table 1,
entry 8).
Performing the reaction in other solvents had a negative
effect on the yield and less polar solvents DCE and toluene
gave 61 and 32% yield of the product after 24 h, respec-
tively (Table 1, entries 9 and 10). More polar solvents
Scheme 2. Trityl ion catalyzed oxa-Diels–Alder reaction.
Gratifyingly, in our initial experiments we found that in (Et2O, CH3NO2 and CH3CN) almost completely sup-
the presence of only 1.0 mol-% of trityl tetrafluoroborate pressed the reactivity, most likely due to blocking of the
(TrBF4), benzaldehyde (1a) and 2,3-dimethylbutadiene (2a) Lewis acidic site of the carbocation, and only starting mate-
smoothly converted into the corresponding 3,6-dihydro- rial was recovered (Table 1, entries 11–13).
pyran adduct 3a in 83% yield within 24 h at room tempera-
After establishing the optimal reaction conditions for the
ture (Table 1, entry 2). Increasing of the catalyst loading to oxa-Diels–Alder reaction we started to investigate the reac-
5.0 mol-% had no effect on the yield; however, it substan- tion of various benzaldehydes 1a–p with 2,3-dimethylbuta-
tially decreased the reaction time from 24 h to full conver- diene (2a; Table 2). Thus, under the standard reaction con-
sion within one hour (Table 1, entry 1).
ditions (1.0 mol-% TrBF4, CH2Cl2, room temp.) 2-naphth-
On the other hand, when the catalyst loading was re- aldehyde 1b gave the corresponding 3,6-dihydropyran 3b in
duced to 0.5 mol-% the conversion dropped dramatically 93% yield (90% isolated yield, Table 2). Halogenated benz-
and the reaction proceeded to 22% yield over 48 h before aldehydes were also found to be very good substrates for
stopping, most likely due to catalyst deactivation (Table 1, this transformation and gave the corresponding cycload-
entry 3).
dition adducts 3c–h in high to excellent yields (Table 2). p-
The trityl ion constitutes a rather unique center of Lewis Nitro- and o-nitrobenzaldehyde 1i and 1j gave the cyclo-
acidity that can be easily tuned by alteration of the elec- addition products 3i and 3j in 95 and 99% yield, respec-
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M. A. E. A. A. A. El Remaily, V. R. Naidu, S. Ni, J. Franzén
SHORT COMMUNICATION
tively. In contrast to the inhibiting effect of using aceto- tion conditions (1.0 mol-% TrBF4, CH2Cl2, room temp.)
nitrile as the solvent (cf. Table 1, entry 13), cyanobenzalde- and resulted in low yield and poor conversion. However, by
hyde derivatives (1k and 1l) were well tolerated to give the increasing the catalyst loading to 2.0 mol-% full conversion
corresponding products 3k and 3l in good yields with only of starting material was observed within 24 h and 3q was
a slight decrease compared with the corresponding halogen isolated as a single regioisomer in 71% yield (Table 2). For
and nitro substituents.
the reaction of 2-naphthaldehyde 1b and p-chlorobenzalde-
hyde with isoprene 2b the catalyst loading had to be in-
creased to 3.0 mol-% to obtain full conversion and under
these reaction conditions the corresponding pyrans 3r and
3s were isolated as single regioisomers in excellent yields
(Table 2).
Table 2. Trityl catalysed hetro-Diels–Alder of aromatic alde-
hydes.[a,b]
The more sterically demanding 2,3-dibenzylbutadiene
(2c) gave the corresponding adduct 3t in high yield in the
presence of 2.0 mol-% TrBF4. Unfortunately, 1,3-cyclohexa-
diene (1u), cyclopentadiene (1v), 1,4-dipheylbutadiene (1w)
and 2,4-hexadiene (1x) did not undergo oxa-Diels–Alder re-
actions, even after extensive optimization in terms of cata-
lyst loading, reaction temperature or screening of different
aromatic aldehydes (Table 2).
Although aromatic aldehydes provided satisfactory re-
sults in the oxa-Diels–Alder reactions, the use of alkyl alde-
hydes turned out to be more challenging. Thus, dihydro-
cinnamic aldehyde (4a) underwent an oxa-Diels–Alder reac-
tion with 2,3-dimethylbutadiene (2a) under the optimized
reaction conditions (1.0 mol-% TrBF4, CH2Cl2, room
temp.) to give pyran 5a in only 44% yield after 5 days. After
further optimization of the reaction conditions the yield of
5a was improved to 61% by using 2.0 mol-% TrBF4 and
24 h reaction time (Table 3). For the corresponding reaction
with the less reactive diene, isoprene (2b), the catalyst load-
ing had to be further increased to 3.0 mol-% to give di-
hydropyran 5e in 69% yield. The more sterically demanding
cyclohexanecarbaldehyde (4b) reacted very smoothly with
2,3-dimethylbutadiene (2a) in the presence of 2.0 mol-%
Table 3. Trityl-catalyzed oxa-Diels–Alder reactions of aliphatic al-
dehydes.[a,b]
[a] TrBF4 (1.0 mol-%) was added to a solution (0.3 m) of aldehyde
1
(1.5 equiv.) and diene (1.0 equiv.). [b] Yield was determined by H
NMR using 1-methylnaphthalene as internal standard. Yields in
brackets are isolated yields. [c] 2.0 mol-% TrBF4 was used.
[d] 3.0 mol-% TrBF4 was used. [e] 48 h reaction time. [f] After 72 h
reaction time only decomposition of diene was observed.
In contrast to what could be expected, increasing the
electron density on the aromatic ring of the aldehyde [Me-,
MeO- (3m–p)] had only minor effect, both on yield and rate
of the reaction. Thus, electron-donating moieties such as
methyl (1m and 1n) or methoxy groups (1o and 1p) on the
benzaldehyde gave high but in general somewhat lower
yields than the electron-withdrawing groups and pyrans
3m–p could be isolated in 73–82% yield. Following this, we
turned our attention towards different dienes. Isoprene (2b)
turned out to be less reactive towards benzaldehyde (1a)
[a] TrBF4 (2.0 mol-%) was added to a solution (0.3 m) of aldehyde
(1.5 equiv.) and diene (1.0 equiv.). [b] Yield was determined by H
NMR using 1-methylnaphthalene as internal standard. Yields in
brackets are isolated yields. [c] 3.0 mol-% TrBF4 and 48 h reaction
1
than 2,3-dimethylbutadiene (2a), under the optimized reac- time was used.
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Carbocation Catalysis: Oxa-Diels–Alder Reactions
TrBF4, to give pyran 5b in moderate yield over 24 h
(Table 3). We were also intrigued to find that the acid sensi-
tive acetaldehyde (4c) smoothly underwent an oxa-
Diels–Alder reaction in the presence of 2.0 mol-% TrBF4
to give 2,4,5-trimethyl-3,6-dihydro-pyran (5c) in 63% yield
(Table 3). Interestingly, cyclopropanecarbaldehyde (4d)
underwent the cycloaddition both with 2,3-dimethylbuta-
diene (2a) and isoprene (2b) to give 5d and 5f, respectively,
in moderate yields without affecting the α-cyclopropyl
group.
Acknowledgments
This work was made possible by grants from the Swedish Research
Council (VR). V. R. N. thanks the Wenner-Gren Foundations for
a grant. S. N. thanks the Chinese Scholarship Council (CSC) for a
grant. M. A. A. E.-R. thanks the Erasmus Mundus Action 2, Wel-
come project for a grant.
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Conclusions
In conclusion we have developed a trityl cation (TrBF4)
catalyzed oxa-Diels–Alder reaction of unactivated dienes
and unactivated aldehydes that provides easy access to dif-
ferent 3,6-dihydropyran derivatives in good to excellent
yields under mild reaction conditions and remarkably low
catalyst loadings. In contrast to most previously reported
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activation of the substrates, neither by electron-deficient al-
dehydes (2-oxo aldehydes) or electron-rich dienes (methoxy
or amino-butadiene) and this overcomes a severe limitation
often associated with the oxa-Diels–Alder reaction. Further
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Experimental Section
General: All reactions were performed in pre-dried solvents under
nitrogen atmosphere (for further details see the Supporting Infor-
mation).
Preparation of 4,5-Dimethyl-2-phenyl-3,6-dihydro-2H-pyran (3a):
Benzaldehyde (1a; 192 mg, 1.8 mmol) and 2,3-dimethylbutadiene
(2a; 100 mg, 1.2 mmol) were added to a solution of TrBF4 (4 mg,
0.012 mmol) in CH2Cl2 (4 mL). After full conversion of the starting
material (determined by 1H NMR), the reaction mixture was
quenched with a saturated NaHCO3 aqueous solution. The mixture
was extracted with CH2Cl2 (ϫ3), the combined organic phases
were dried with MgSO4 and the solvent was distilled off at ambient
temperature. The residue was purified by flash chromatography
(pentane/ethyl acetate; 95:5) to give the title compound (181 mg,
80% isolated yield). Spectral data were in accordance with those
previously reported.[12a]
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures for the synthesis of all compounds.
Characterization and spectroscopic data for all new compounds.
Eur. J. Org. Chem. 2015, 6610–6614
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M. A. E. A. A. A. El Remaily, V. R. Naidu, S. Ni, J. Franzén
SHORT COMMUNICATION
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Received: August 27, 2015
Published Online: September 18, 2015
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