Tropylium-Catalyzed O-H Insertion Reactions of Diazoalkanes with Carboxylic Acids

Article abstract of DOI:10.1021/acs.orglett.0c04069

Herein, we describe the application of a nonbenzenoid aromatic carbocation, namely tropylium, as an organic Lewis acid catalyst in O-H functionalization reactions of diazoalkanes with benzoic acids. The newly developed protocol is applicable to a wide range of diazoalkane and carboxylic acid substrates with excellent efficiency (43 examples, up to 99% yield).

Full text of DOI:10.1021/acs.orglett.0c04069

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Letter
Tropylium-Catalyzed O−H Insertion Reactions of Diazoalkanes with
Carboxylic Acids
Claire Empel, Thanh Vinh Nguyen,* and Rene M. Koenigs*
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ABSTRACT: Herein, we describe the application of a nonbenzenoid aromatic carbocation, namely tropylium, as an organic Lewis
acid catalyst in O−H functionalization reactions of diazoalkanes with benzoic acids. The newly developed protocol is applicable to a
wide range of diazoalkane and carboxylic acid substrates with excellent efficiency (43 examples, up to 99% yield).
Scheme 1. O−H Functionalization of Diazoalkanes and
he O−H insertion reaction of diazoalkanes is a reaction
Tof fundamental interest in organic chemistry as it allows
the direct and concise synthesis of α-oxygenated carboxylic
acid derivatives.1,2 A classic approach toward O−H function-
alization is exemplified in the reaction of diazomethane with
carboxylic acids, which proceeds via protonation of diazo-
methane followed by nucleophilic substitution of the
diazonium ion intermediate to form methyl esters.3,4 This
method finds numerous applications in organic chemistry
ranging from method development to total synthesis.4 This
approach is, however, limited to simple diazoalkane substrates
like diazomethane because the crucial proton-transfer step is
significantly hampered in the case of structurally more
complicated and less Brønsted basic frameworks such as
aryldiazoacetates.5−7 Thus, O−H functionalization reactions of
this latter type of diazo compounds commonly require a
different activation process, with transition-metal catalysis
being one of the most advanced approaches in modern organic
synthesis (Scheme 1a).2,7 Despite the advances made in this
areain particular with regard to enantioselective O−H
functionalization reactions,7 costs and toxicity of either the
metal or its requisite ligand represent limitations of this
approach, thus making metal-free approaches toward O−H
functionalization very attractive. Approaches under metal-free
conditions constitute an appealing alternative to open up new
environmentally benign strategies toward this important
transformation. In this context, works by Jurberg and Davies
as well as He and Zhou reported the photochemical and
thermal O−H functionalization of carboxylic acids with
aryldiazoacetates (Scheme 1a).5 Similarly, organocatalysts
have recently found initial applications in the reaction of
diazo compounds to access electrophilic diazoniumion
intermediates.8 Our groups recently uncovered photoinduced
proton transfer reactions that allow for O−H functionalization
Reactivity of Organic Lewis Acid with Diazoalkanes
Received: December 9, 2020
Published: January 5, 2021
https://dx.doi.org/10.1021/acs.orglett.0c04069
© 2021 American Chemical Society
Org. Lett. 2021, 23, 548−553
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reactions of fluorinated alcohols and phenols with aryldiazoa-
cetates (Scheme 1b).6
Table 1. Reaction Optimization
Building on our interest in the chemistry of the non-
benzenoid aromatic tropylium ion9 and its application as an
organic Lewis acid catalyst,10 we believed that commercially
available and easy to handle tropylium salts can promote O−H
insertion reactions of diazoalkanes with hydroxyl-bearing
compounds (Scheme 1d). Interestingly, organic, or in a
broader sense, nonmetal Lewis acid catalyzed chemistry of
diazoalkanes has not been adequately explored despite being
rather conceptually simple. The most recent application of
organic Lewis acid catalysts in the reaction of diazoalkanes was
reported by Liu and Lv in 2019 on the decomposition of
aliphatic diazoalkanes in the presence of the tritylium cation to
give α,β-unsaturated esters (Scheme 1c).11 In a related context,
the strongly Lewis acidic B(C6F5)3 was recently reported by
Melen, Wirth, and Wilkerson-Hill as a viable main-group
catalyst to promote carbene transfer reactions.12,13 However,
the sensitivity of this borane and its analogues toward moisture
and air diminishes the practicality of this approach in standard
laboratories.14
We started our investigation by looking into tropylium-
promoted diazoalkane O−H functionalization with benzoic
acids (Scheme 1d). Methyl phenyldiazoacetate 1a was used as
our model substrate in reactions with a slight excess of benzoic
acid 6a (1.2 equiv) in the presence of 5 mol % tropylium
tetrafluoroborate (Trop(BF4)). Curiously, no reaction was
observed in polar solvents, such as acetonitrile, acetone, or
methanol, as methyl phenyldiazoacetate 1a remained un-
reacted in the reaction mixture even after 12 h of stirring at
room temperature (Table 1, entries 2−5). Moreover, no
reaction was observed in toluene either, which can be
attributed to the low solubility of Trop(BF4) in apolar
solvents. In tetrahydrofuran and 1,2-dichloroethane, the
desired product was obtained in low to moderate yield (entries
1 and 6). Only when dichloromethane was used did we obtain
a high yield of the O−H functionalization product 7a (entry
7). No significant improvement in reaction outcomes was
obtained with 2 equiv of either benzoic acid 6a or diazoacetate
1a. An increased loading of 10 mol % Trop(BF4) catalyst led to
the formation of the O−H functionalization product 7a in
excellent yield, while 1 mol % of catalyst gave a significantly
reduced yield of 7a (entry 11). It is important to note that the
related tritylium ion (entry 12) proved to be a less efficient
catalyst for this type of reaction. Only a reduced yield of the
reaction product was obtained with (Ph3C)BF4 under
otherwise identical conditions. Our control studies also
revealed that in the absence of benzoic acid, diazoacetate 1a
decomposed to form a diazine.15 Lastly, no reaction between
1a and 6a was observed in the absence of Trop(BF4).
a
b
entry
catalyst
solvent
yield (%)
1
2
3
4
5
6
7
Trop(BF4)
Trop(BF4)
Trop(BF4)
Trop(BF4)
Trop(BF4)
Trop(BF4)
Trop(BF4)
Trop(BF4)
Trop(BF4)
Trop(BF4)
Trop(BF4)
(Ph3C)BF4
Trop(BF4)
THF
29
MeCN
acetone
toluene
MeOH
1,2-DCE
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
no reaction
no reaction
no reaction
no reaction
44
70
78
65
92
64
76
dec
c
8
d
9
e
10
11
12
13
f
e
g
14
no reaction
a
Reaction conditions: To a mixture of catalyst (5 mol %) and benzoic
acid 6a (1.2 equiv) was added a solution of methyl phenyldiazoacetate
1a (0.2 mmol, 1.0 equiv) in 1.0 mL of solvent, and the resulting
mixture was stirred until the orange color of the diazoalkane
disappeared (approximately 10 min for DCM as solvent). Yields of
isolated products. 2 equiv of benzoic acid 6a. 2 equiv of 1a and 1
equiv of 6a. 10 mol % catalyst. 1 mol % catalyst. Reaction without
benzoic acid 6a. dec = decomposition of 1a.
b
c
d
e
f
g
halogens and electron-donating groups at the para- and
meta-position were well tolerated (7g−k, 7m), while a reduced
yield was observed in the case of ortho- substituents (7l). We
then proceeded to look at different diazoalkane substrates such
as trifluoromethyl phenyl diazomethane, diphenyl diazo-
methane and ethyl diazoacetate. All reactions worked, albeit
only in moderate yield in the case of (1-diazo-2,2,2-
trifluoroethyl)benzene (Scheme 2, 8). With ethyl diazoacetate,
rapid decomposition of the substrate occurred so the target
product was only obtained in low yield (10). On the other
hand, the donor/donor diphenyl diazomethane substrate
proved to be reactive even without the catalyst (Scheme 2,
9), presumably due to its enhanced Brønsted basicity as
discussed earlier.
Following the diazo substrate scope, we studied the
influence of the substitution pattern of the (hetero)aromatic
carboxylic acid 6 on the reaction (Scheme 3). Electron-
donating and -withdrawing groups such as alkyl, cycloalkyl,
halogen, trifluoromethyl, or alkoxy substituents were tolerated
in all positions of the aromatic ring, and the products were
obtained in moderate to high yields (7n−y). Pentafluor-
obenzoic acid worked well under our reaction conditions to
give product 7z in good yield. A heterocyclic system such as
thiophene was also tolerated to give product 7aa, albeit in low
yield. Functional groups that can interact with the tropylium
ion, such as the hydroxyl or nitro groups, were not tolerated
(6b and 6c). Carboxylic acids containing N-heterocyclic
frameworks such as pyridine or indole did not react, leaving
methyl phenyldiazoacetate untouched even upon an elongated
reaction time of 12 h (6e and 6f). This can be attributed to the
poisoning of tropylium ion catalyst by these N-heterocycles as
We next set out to investigate the applicability of this
reaction to different diazoalkanes 1 (Scheme 2). First, we
examined the influence of the alkyl group in the ester
functionality of the diazo compound to the reaction outcomes.
With benzoic acid 6a, the desired O−H functionalization
products were obtained in high isolated yield in the case of
linear alkyl esters (7a and 7b). The efficiency dropped slightly
with bulky groups such as cyclohexyl ester, (−)-menthyl ester,
tert-butyl ester, or benzyl ester (7c−f). In the presence of a
(−)-menthyl group, the O−H functionalization reaction to
yield 7d proceeded with low diastereoselectivity. The study
subsequently investigated the effect of aromatic substituents of
the aryldiazoacetate substrate on the reaction. Different
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Scheme 2. Scope of Different Diazoalkanes in the
Tropylium-Catalyzed O−H Functionalization
Scheme 3. Scope of Different (Hetero)aromatic and
Aliphatic Carboxylic acids
previously observed in our earlier works.10a,b Curiously, while
terepthalic acid (6d) was shown to be nonreactive, another
dicarboxylic acid, namely [1,1′-biphenyl]-2,2′-dicarboxylic
acid, reacted smoothly to give the double O−H functionaliza-
tion product 11 in high yield.
We subsequently investigated the scope of this tropylium-
catalyzed reaction with a range of aliphatic carboxylic acids. All
of the alkyl carboxylic acids accessible to us worked smoothly
to give the desired O−H functionalization products in good to
high yields (12a−j). Further studies in this context involved
the use of naturally occurring carboxylic acids, such as lipoic
acid or terpenoid-based acids (12k,l). In the case of lipoic acid,
only a poor yield of 25% was obtained (12k), which can be
explained by the interference of the nucleophilic disulfide
moiety to tropylium catalyst or resulting reactive intermediates.
Gratifyingly, terpenoid-based acids such as citronellic acid
provided the desired O−H functionalization products in high
yields (12l).
O−H functionalization reaction. It should be recalled at this
point that methyl phenyldiazoacetate rapidly decomposes in
the presence of Trop(BF4) to give the diazine byproduct (cf.
discussion of Table 1). When the reaction was carried out with
the deuterated benzoic acid d-6a, a ratio of ∼2:1 for
deuterium/hydrogen was observed in the reaction product d-
7a (Scheme 4a). The rather high content of hydrogen in the
reaction product d-7a might be reasoned by trace amounts of
We then performed a set of control experiments to gain
some insights into the mechanism of this tropylium-catalyzed
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overall reaction outcomes. Unfortunately, we were not able to
observe any covalent or noncovalent interaction in a 1:1
mixture of Trop(BF4) with methyl phenyldiazoacetate 1a in
CD3CN by spectroscopic analysis due to the rapid
decomposition of 1a within seconds. NMR spectroscopic
complexation studies of benzoic acid 6a and Trop(BF4)
Scheme 4. Control Experiments and Hypothesized
Mechanism
1
showed only a marginal perturbation of H and 13C NMR
shifts, which indicates only a very weak, if any, interaction of 6a
with the catalyst (for details, see the SI).10c,16 We also
attempted to observe the interaction between tropylium ion
and some other diazo compound such as diphenyldiazo-
methane; however, similar decompositions occurred. Yet, in
this case, tetraphenylethylene was observed as the major
decomposition product. Collectively, our studies hint at a
Lewis acid activation of the diazoalkane by the tropylium ion.
In light of the other Lewis acid catalytic reactions with methyl
phenyldiazoacetate,11−14 we hypothesize that the reaction
proceeds via the mechanistic pathway depicted in Scheme 5.
Scheme 5. Mechanistic Hypothesis
water in the reaction medium. When the standard reaction
with substrate 6a is performed in CD2Cl2, deuterium is not
incorporated in the reaction product. The acidic O−H proton
is important for the reaction, as control experiments with a
series of benzoate salts (13) did not lead to any product
formation with diazo 1a remaining untouched (Scheme 4a).
Presumably, the abundance of the nucleophilic benzoate ion,
which is not readily present in the weakly acidic benzoic acid,
in these reaction mixtures led to the formation of cyclo-
heptatrienyl benzoate and rendered the catalyst inactive. This
was also confirmed by a direct stoichiometric reaction between
sodium benzoate and Trop(BF4).16
The Lewis acid catalytic activity of tropylium ion in this
reaction might be shadowed by the Brønsted acid catalytic
activity of HBF4, which can potentially be formed from the
hydrolysis of Trop(BF4) if moisture is present (Scheme 4b).
Thus, we did some control studies with HBF4 as the reaction
catalyst to take this into account. Small amounts (0.5 mol %)
of HBF4 showed only marginal catalytic activity and trace
amounts of the reaction product could be observed after 24 h
reaction time, while the majority of the diazoalkane remained
untouched.16 When the amount of HBF4 was increased, the
yields of the reaction product improved but were still
significantly lower compared to the reaction with Trop(BF4),
even at complete conversion of diazoalkane 1a (Scheme 4b).
In a next set of experiments, we also studied the role of water
in the reaction mixture. Even in the presence of only 10 mol %
of water, the yield of the reaction product 7a was significantly
reduced. Unsurprisingly, addition of more water led to further
inhibition of the reaction. Similar results were observed in the
case of diazoalkanes that do not possess a neighboring
carbonyl group (cf. 8, 9 in Scheme 2 and SI).16 These studies
suggest that HBF4 or moisture only have little or a diminishing
effect on the reaction. Therefore, we can conclude that
Trop(BF4) indeed acts as a catalyst in this O−H functionaliza-
tion reaction and its hydrolysis, if any, does not contribute to
Lewis acid activation of diazoalkane 1a via coordination to the
Lewis basic carbonyl group (14) or the Lewis basic diazo
functional group (14′) furnishes an an activated carbene
intermediate (16), as suggested by the formation of the diazine
byproduct mentioned earlier, which presumably resulted from
the reaction between 16 and diazo 1a.
In summary, we have developed a new catalytic activity of
the nonbenzenoid aromatic tropylium ion, acting as an organic
Lewis acid catalyst, to promote O−H functionalization
reactions of aryldiazoacetates with carboxylic acids. This
operationally simple reaction proceeds under mild reaction
conditions with very short reaction times. This study features a
broad scope of diazoalkanes and carboxylic acid derivatives
including aromatic, heteroaromatic, and aliphatic carboxylic
acids to give α-functionalized esters with an efficiency of up to
99% yield. This work further enriches the versatile chemistry of
tropylium ion as well as opens up new pathways in the reaction
of diazoalkanes using environmentally benign organic Lewis
acid catalysts.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c04069.
■
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Experimental procedures, characterization, and analytical
data (PDF)
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AUTHOR INFORMATION
Corresponding Authors
■
Rene M. Koenigs − RWTH Aachen University, Institute of
Organic Chemistry, D-52074 Aachen, Germany;
orcid.org/0000-0003-0247-4384; Email: rene.koenigs@
rwth-aachen.de
Thanh Vinh Nguyen − School of Chemistry, University of
New South Wales, Sydney 2052, NSW, Australia;
orcid.org/0000-0002-0757-9970; Email: t.v.nguyen@
unsw.edu.au
Author
Claire Empel − RWTH Aachen University, Institute of
Organic Chemistry, D-52074 Aachen, Germany
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c04069
Notes
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(c) Zhang, Y.; Yao, Y.; He, L.; Liu, Y.; Shi, L. Rhodium(II)/Chiral
Phosphoric Acid-Cocatalyzed Enantioselective O-H Bond Insertion of
α-Diazo Esters. Adv. Synth. Catal. 2017, 359, 2754−2761. (d) Xie, X.-
L.; Zhu, S.-F.; Guo, J.-X.; Cai, Y.; Zhou, Q.-L. Enantioselective
Palladium-Catalyzed Insertion of α-Aryl α-diazoacetates into the O-H
Bonds of Phenols. Angew. Chem., Int. Ed. 2014, 53, 2978−2981.
(8) (a) Dumitrescu, L.; Azzouzi-Zriba, K.; Bonnet-Deplon, D.;
Crousse, B. Non-metal catalyzed insertion reactions of diazocarbonyls
to acid derivatives in fluorinated alcohols. Org. Lett. 2011, 13, 692−
695. (b) Zheng, H.; Dong, K.; Wherritt, D.; Arman, H.; Doyle, M. P.
Brønsted Acid Catalyzed Friedel-Crafts-Type Coupling and Dedini-
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Enantioselective Carbonyl 1,2- or 1,4-Addition Reactions of
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(d) Cleary, S. E.; Hensinger, M. J.; Brewer, M. Remote C−H
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
R.M.K. thanks the German Science Foundation and Dean’s
Seed Fund of RWTH Aachen Foundation for financial support.
C.E. thanks RWTH Aachen University for financial support.
We thank the Australian Research Council for funding
(FT180100260 to T.V.N. and DP200100063 to T.V.N. and
R.M.K.).
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B(C6F5)3·nH2O, the reaction required elevated temperature and long
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(15) The reaction of methyl phenyldiazoacetate 1a with Trop(BF4)
results in formation of a diazine:
.
(16) See the Supporting Information for more details.
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Org. Lett. 2021, 23, 548−553