Unexpected Pd/C-catalyzed room temperature and atmospheric pressure hydrogenation of 2-methylenecyclobutanones

Article abstract of DOI:10.1016/j.mcat.2019.110450

Pd/C-catalyzed hydrogenation reaction of 2-methylenecyclobutanones (2-MCBones)occurs at their exocyclic C[dbnd]C sites regio-specifically to produce 2-substituted cyclobutanones, which are useful building blocks for the synthesis of drug and pesticide int

Full text of DOI:10.1016/j.mcat.2019.110450

MolecularCatalysis474(2019)110450
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Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Unexpected Pd/C-catalyzed room temperature and atmospheric pressure
hydrogenation of 2-methylenecyclobutanones
Yufan Yanga, Mengxuan Lia, Hongen Caoa,b,c, Xu Zhanga,⁎, Lei Yua,⁎
a School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, China
b State Key Laboratory Breeding Base of Green Pesticide and Agricultural Bioengineering, Key Laboratory of Green Pesticide and Agricultural Bioengineering, Ministry of
Education, Research and Development Center for Fine Chemicals, Guizhou University, Guiyang, Guizhou 550025, China
c Institute of Pesticide of School of Horticulture and Plant Protection, Yangzhou University, Yangzhou, Jiangsu 225009, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Pd/C-catalyzed hydrogenation reaction of 2-methylenecyclobutanones (2-MCBones) occurs at their exocyclic
C]C sites regio-specifically to produce 2-substituted cyclobutanones, which are useful building blocks for the
synthesis of drug and pesticide intermediates and are key intermediates in the total synthesis of many natural
products. Surprisingly, atmospheric pressure H2 in balloon can be used as the cheap, clean and safe reductant
and the reaction occurs smoothly at room temperature (25 °C). Quantitative calculation demonstrates that the
release of the intramolecular ring strain caused by the combined effect of the four-membered ring and exocyclic
C]C bond in 2-MCBones provides additional dynamic driving forces for the reaction. The higher hydrogenation
energy releasing of the exocyclic C]C than C]O was also found to be the causation for good regio-selectivity of
the reaction. These new findings may broaden the understandings on the catalytic hydrogenation reactions of
strained organic molecules.
Hydrogenation
Selective reaction
2-Methylenecyclobutanone
Cyclobutanone
Palladium
1. Introduction
cyclobutanones via the organoselenium-catalyzed oxidative ring ex-
pansion reaction of MCPs [17]. The method employed H2O2 as the
2-Substituted cyclobutanones are useful building blocks in synthetic
organic chemistry that have been comprehensively employed for the
synthesis of a series of drug and pesticide intermediates such as lac-
tones, cyclopentanones, benzodiquinanes, naphthalenes, 9,10-dihy-
drobenzocycloocten-7(8H)-ones, etc [1–4]. They are also key inter-
mediates in the total synthesis of many natural products such as α-
Bisabolol, (-)-Debromofiliformin, (-)-Filiformin, ( )-Scirpene, ( )-a-
Cuparenone, ( )-Herbertene, etc [5–9]. Generally, 2-substituted cyclo-
butanones are synthesized via the ring-expansion reactions of cyclo-
propyl-contained compounds [5–17]. Among reported works, the oxi-
dative ring expansion of methylenecyclopropanes (MCPs, Scheme 1, Eq.
(1)) may be one of the most practical methods because of the concise
preparation procedures of the starting materials from commercially
available reagents [13–24]. The reactions once employed large amount
of chemical oxidants such as m-chlorobenzoperoxoic acid (MCPBA),
diisopropyl azodicarboxylate (DIAD), diethyl azodicarboxylate (DEAD)
or cerium (IV) ammonium nitrate (CAN), which might generate solid
wastes in large-scale production (Eq. (1)) [13–16]. Recently, during our
continuous investigations on organoselenium catalysis [25–30], we
clean oxidant, but its substrate scope was limited to disubstituted MCPs
to produce 2,2′-bis-substituted cyclobutanones, while the reactions with
mono-substituted MCPs only led to a mixture of decomposed by-pro-
ducts. Moreover, because MCPs were prepared via the Wittig-reactions
of cyclopropylidene ylid with carbonyls [18–20], the protocol in-
evitably generated the high molecular weight by-product triphenyl-
phosphine oxide (Ph3P(O)) that might lead to a lot of solid wastes in
practical applications. Therefore, developing more atom-economic
procedures for 2-substituted cyclobutanone synthesis is still crucial in
the field.
(1)
On the other hand, since cyclobutanone is a commercially available
chemical that can be purchased at an acceptable price [31], it may
serve as a good building block in organic synthesis for introducing four-
membered carbocycles into the molecules. In 2014, we reported that,
by using a simple condensation of cyclobutanone with aldehydes, once
developed
a novel method for the synthesis of 2-substituted
⁎ Corresponding authors.
E-mail addresses: zhangxu@yzu.edu.cn (X. Zhang), yulei@yzu.edu.cn (L. Yu).
https://doi.org/10.1016/j.mcat.2019.110450
Received 12 February 2019; Received in revised form 26 May 2019; Accepted 28 May 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

Y. Yang, et al.
MolecularCatalysis474(2019)110450
2.2. General procedure for the hydrogenation reaction of 2-MCBones
25 mg of Pd/C (containing 0.0235 mmol of Pd) and a magnetic bar
were added into a reaction tube charged with N2. A solution of
1.0 mmol of 2-MCBone 1 (synthesized via the method in ref. [40] in
10 ml of anhydrous THF/EtOH (volume ratio = 4:1) was injected into
the reaction tube, which was then equipped with a balloon charged
with H2. The mixture was magnetically stirred at room temperature
(25 °C) for 6 h. The solvent was evaporated under vacuum and the re-
sidue was separated by flash column chromatogram (eluent: petroleum
ether-EtOAc 20:1) to give the 2-substituted cyclobutanones 2 in related
yields (vide infra).
2.3. Method for the determination and calculation of C-mass balance
C-mass balance was calculated by the following equation:
C-mass balance = (carbon weight in the product + carbon weight in
the by-products)/carbon weight in the reactant
Carbon weight in the by-products was determined by elemental
analysis. For example, in the reaction of 1a (Table 3, entry 1), after
getting the desired product 2a by flash column chromatogram, acetone-
methanol 1:1 was used as high-polar solvent to wash out the rest by-
products on silica gel chromatographic column. Solvent of the com-
bined solution of the by-products was removed under vacuum and the
residue was weight and sent to elemental analysis to determine the
carbon content. For the reaction of 1a (Table 3, entry 1), 133.0 mg of
product 2a (containing 82.46% of C) and 30.4 mg of by-products was
obtained. Elemental analysis indicated the by-products contained
72.51% of C. Therefore, the C-mass balance could be calculated as:
Scheme 1. Synthesis of 2-substituted cyclic ketones 2a and 4a-c and calcula-
tion of the reaction energy.
C-mass balance
=
(133.0 mg × 82.46% +30.4 mg × 72.51%)/
[11 mmol (each molecular of 1a contained 11 C)×12.01 mg/mmol
(atom weight of C)] = 99.7%
hardly obtained 2-methylenecyclobutanones (2-MCBones) could be
directly synthesized (Scheme 1, Eq. (2)) [32]. In the reaction, cheap,
low-loading (10 mol%) and removable Ca(OH)2 was employed as cat-
alyst and no waste was generated other than the water [33–35]. Re-
cently, we were surprised to find that the molecules were so reactive
that their catalytic hydrogenation reaction could be performed at at-
mospheric pressure and under room temperature. Different from the
hydrogen transfer reactions [36–38], the reaction employed H2 (just in
a balloon) as an efficient reductant. It did not require expensive metal
complexes [39], while the simple, commercial available and recyclable
Pd/C was the preferable catalyst. In regardless of the multiple reaction
sites in 2-MCBones as well as the possible cyclobutyl ring opening side-
reactions, the reaction occurred at the exocyclic C]C bond regio-spe-
cifically. Herein, we wish to report our findings.
3. Results and discussion
3.1. Catalyst screenings
Catalyst screenings were initially performed and for a 1 mmol-scale
reaction, 5 mg of catalyst was employed (Table 1). The 2-MCB reactant
dissolved well in THF, while alcohols, such as EtOH, were good solvents
Table 1
Catalyst screeningsa.
(2)
Entry
Catalyst
2a yield (%)
2. Experimental
1
2
3
4
5
6
7
8
Raney Ni
10%Pt/C
10%Ru/C
10%Pd/C
Pd@PANI
Ru@PANI
Cu@PANI
Pd@Al
Pd&Cu@Al
Cu&Fe@Al
PdCl2
Pd(OAc)2
Pd(PPh3)4
trace
trace
trace
21
11
0
2.1. General methods
Solvents were of analytical purity (AR) and were purchased from
reagent merchant. 2-MCBones were synthesized via the literature
method [40]. NMR spectra were recorded on Bruker Avance instru-
ments (600 or 400 MHz for 1H NMR and 150 or 100 MHz for 13C NMR)
using CDCl3 as solvent and Me4Si as internal standard. Chemical shifts
for 1H NMR were relative to internal Me4Si (0 ppm) and J-values were
shown in Hz. Theoretical calculations were conducted by GaussView 5
software (Gaussian09 package) using semi-empirical molecular orbital
method (PM6). Elemental analysis was performed on an Elementar
Vario EL cube instrument.
0
13
15
0
5
8
9
10
11
12
13
7
a
1 mmol of 1a, 5 mg of catalyst (or a 1 × 1 piece of catalyst for Pd@Al, Pd&
Cu@Al and Cu&Fe@Al) and 10 ml of THF/EtOH (4:1) were employed.
2

Y. Yang, et al.
MolecularCatalysis474(2019)110450
for catalytic hydrogenation reaction [37]. Therefore, THF/EtOH (4:1)
was employed as the reaction solvent. We first tested the Raney Ni-
catalyzed hydrogenation of 2-MCBone 1a with atmospheric pressure H2
in a balloon under room temperature (rt =25 °C), but almost no reac-
tion occurred (Table 1, entry 1). Other commercially available regular
catalysts, such as Pt/C, Ru/C and Pd/C (catalytic metal weight contents
were all 10%) were then examined, and Pd/C was screened out to be
the most active one, affording 2a in 21% yield (Table 1, entries 4 vs.
1–3). Pd@PANI [40–44], Ru@PANI [45], Cu@PANI [46], Pd@Al [47],
Pd&Cu@Al [48] and Cu&Fe@Al [49], which were novel catalysts de-
veloped by our group recently, were employed but failed to optimize
the reaction any more (Table 1, entries 5–10). Homogeneous Pd cata-
lysts, such as PdCl2, Pd(OAc)2 and Pd(PPh3)4 were tested, but resulted
in poor 2a yield and a series of unidentified by-products were observed
in thin-layer chromatography (TLC). The above results demonstrated
that only Pd catalysts worked for the reaction (Table 1, entries
4,5,8,9,11–13 vs. 1–3,6,7,10), and Pd/C was preferable not only for the
higher product yield, but also for the good availability. Moreover, it
was found that the product yield could be enhanced by increasing the
Pd/C catalyst amount, and 25 mg of Pd/C for 1 mmol scale reaction was
the inflection point of the curve, where 2a was obtained in 81% yield
(Fig. 1). Further increasing Pd/C amount could not improve the reac-
tion (Fig. 1), which was probably because of the limitation of the
contact of the two-phase reactants of 1a with H2.
Table 2
Solvent screeningsa.
Entry
Solvent
2a yield (%)
1
2
3
4
5
6
7
8
toluene
cyclohexane
CH2Cl2
CHCl3
THF
1,4-dioxane
MeCN
DMSO
DMF
MeOH
EtOH
trace
trace
trace
trace
12
trace
7
trace
trace
29
9
10
11
30
a
1 mmol of 1a, 25 mg of Pd/C and 10 ml of solvent were employed.
Table 3
Substrate extensionsa.
3.2. Solvent screenings
Solvent screenings for the reaction were performed and the results
were summarized in Table 2. The reaction did not occur in non-polar
solvents such as toluene and cyclohexane, or chloro-contained solvents
such as CH2Cl2 and CHCl3 (Table 2, entries 1–4), while polar solvents
such as THF, 1,4-dioxane, MeCN, DMSO or DMF also led to very poor
2a yields (Table 2, entries 5–9). Alcohol solvents, such as MeOH and
EtOH, were more preferable, affording 2a in 29–30% yields (Table 2,
entries 1011). Interestingly, stirring 1a in EtOH could produce 2a in 8%
yield even without using H2 balloon as the hydrogen source, showing
that alcohol might participate the reaction as H-donor in the presence
of Pd catalyst and the generated acetaldehyde might be reduced to
EtOH again by H2. This process could accelerate the hydrogen process
because it occurred in a homogeneous solution, other than the hydro-
genation with H2 occurred between two-phase reactants [37]. Based on
this finding, EtOH was added in THF (which could dissolve 2-MCBone
well) to improve the reaction solvent and parallel experimental results
demonstrated that THF/EtOH = 4:1 was the best volume ratio, giving
2a in 81% yield (Fig. 2).
Entry
R (1)
2: yield (%)b
C-mass balance (%)c
1
2
Ph (1a)
Ph (1a)
2a: 83
99.7
–
2a: 80d, 81d,e, 78d,e
76d,e
,
3
4
5
6
7
8
9
10
11
4-MeC6H4 (1b)
4-ButC6H4 (1c)
4-MeOC6H4 (1d)
3-MeOC6H4 (1e)
2-MeOC6H4 (1f)
4-FC6H4 (1 g)
4-CF3C6H4 (1 h)
1-C10H7 (1i)
2b: 78
2c: 56
2d: 77
2e: 86
2f: 81
2g: 71
2h: 69
2i: 75
99.6
99.5
99.0
99.2
99.1
99.4
99.3
99.5
99.1
2-C4H3S (2-thiophene,
1 j)
c-C6H11 (1k)
2 j: 91
12
a
2k: 46
99.5
1 mmol of 1a, 25 mg of Pd/C and 10 ml of solvent were employed.
Isolated yields based on 1.
Method of C-mass balance determination and calculation were given in
b
c
experimental section 2.3.
d
Reaction in 10 mmol scale.
Reaction with recycled Pd/C catalyst.
e
3.3. Investigations on the effects of reaction temperature and time
Owing to the high reactivity of the 2-MCBone reactant, the hydro-
genation reaction occurred at room temperature smoothly in high yield
and turnover frequency. As shown in Fig. 3, the product yield of the
hydrogenation reaction was almost constant in the range of 25–100 °C,
but for energy saving consideration, room temperature reaction con-
dition, as we initially used, should be preferable. The reaction afforded
2a in 43% yield within 1 h. The yield gradually increased and reached
its peak at 6 h, which should be the most favorable reaction time
(Fig. 4). Moreover, after heating 2a at 100 °C under the standard re-
action conditions (1 mmol of 2a, 25 mg of Pd/C, 10 ml of THF/EtOH,
6 h, H2), more than 94% of the chemical could be recovered again by
flash column chromatography separation, showing that the produced 2-
substituted cyclobutanones was stable under the reaction conditions
and it did not decompose at the temperature below 100 °C. Thus, the
slight decrease in yields upon increasing the reaction temperature
Fig. 1. Catalyst amount screenings [reaction conditions: 1 mmol of 2-MCBone,
10 ml of THF/EtOH (4:1), 25 °C, 6 h, H2].
3

Y. Yang, et al.
MolecularCatalysis474(2019)110450
3.4. Substrate extensions
A series of 2-substituted cyclobutones were then synthesized via this
hydrogenation reaction under the optimized reaction conditions
(Table 3). The product yield did not decrease in a 10 times scale-up
reaction (Table 3, entries 2 vs. 1), and 1.28 g (in 80% yield) of 2-ben-
zylcyclobutan-1-one (2a) was synthesized smoothly. The catalyst could
be recovered by centrifugal separation and was reusable for at least
three times without deactivation (Table 3, entry 2). Both electron-en-
riched and -deficient 2-MCBones were favorable substrates for the re-
action (Table 3, entries 3–9), and the electron-enriched substrates were
generally more preferable than the electron-deficient ones (Table 3,
entries 3, 5–7 vs. 8,9). Interestingly, 2-(2-methoxybenzyl)cyclobutan-1-
one (2f) could be synthesized in even higher yield than that of 2d, in
regardless of the higher steric hindrance in substrate caused by the
ortho-substituent on aryl (Table 3, entries 7 vs. 5). Similarly, (E)-2-
(naphthalen-1-ylmethylene)cyclobutan-1-one (1i), as an example of
substrate bearing bulky group, was also smoothly converted into the
related product 2i in good yield (Table 3, entry 10). It was notable that
the reaction was tolerant to sulfur-contained group, which might cause
the Pd catalyst poisoning in many reactions, and the reaction of sub-
strate 1j led to product 2j in excellent yield (Table 3, entry 11). The
reaction was also applicable for aliphatic substrate, such as 1k, and
produced the desired 2-substituted cyclobutanone 2k in moderate yield
(Table 3, entry 12). C-mass balance of the reaction was calculated and it
was generally high (> 99%) in all of the examples (Table 3, entries
1,3–12).
Fig. 2. THF/EtOH ratio screenings (reaction conditions: 1 mmol of 2-MCBone,
25 mg of Pd/C, 10 ml of THF/EtOH, 25 °C, 6 h, H2).
3.5. Mechanism study
A series of methylene-substituted cyclic ketones 1a and 3a-c were
employed in the hydrogenation reaction for comparison to judge the
effect of substrate ring sizes on the reaction (Scheme 1). Using (E)-2-
benzylidenecyclopentan-1-one (3a) or (E)-2-benzylidenecyclohexan-1-
one (3b) as substrate resulted in decreased product yield (Scheme 1,
runs 2,3 vs. 1). Bearing a very large eight-membered ring in substrate,
the hydrogenation reaction of 2-benzylcyclooctan-1-one (3c) was dif-
ficult to occur and only traces of the desired product were observed on
thin-layer chromatographic (TLC) plate (Scheme 1, run 4). To reveal
the reasons for the above experimental results, quantitative calculations
were performed with GaussView 5 software using semi-empirical mo-
lecular orbital method (PM6). It was found that the reaction of 1a to 2a
released ca 110 kJ/mol of energy (Scheme 1, run 1). With the in-
creasing of the substrate ring sizes, the released energies of the reac-
tions of 3a to 4a and 3b to 4b decreased to 96 and 92 kJ/mol respec-
tively (Scheme 1, runs 2–3). In the reaction of the eight-membered ring-
contained substrate 3c to 4c, only 6 kJ/mol of energy was released
(Scheme 1, run 4). The reduced reaction energy releasing might be the
reason of the decreased product yields, in good accordance with the
results of parallel experiments. Therefore, it was demonstrated that the
hydrogenation reaction of the C]C bond of 2-MCBones could partially
release the high intramolecular ring strain caused by the combined
effect of the small ring and exocyclic C]C bond of the substrate and
provide additional driving forces to facilitate the reaction processes
under mild conditions. Chemo-selective hydrogenation reactions of
C]C versus C]O bonds over noble metals may attribute to the grater
affinity of the metals for C]C [50–52]. Moreover, the quantitative
calculation results have also indicated that, the hydrogenations of
carbonyls in 2a, 4a and 4b to produce alcohols 5 released less energy
than the related C]C bond hydrogenation (Scheme 1, runs 5 vs. 1, 6 vs.
2, 7 vs. 3), and these results could well explain the reason of the high
regio-selectivities of the reactions.
Fig. 3. Effects of the reaction temperature (reaction conditions: 1 mmol of 2-
MCBone, 25 mg of Pd/C, 10 ml of THF/EtOH, 6 h, H2).
Fig. 4. Effects of the reaction time (reaction conditions: 1 mmol of 2-MCBone,
25 mg of Pd/C, 10 ml of THF/EtOH, 25 °C, H2).
(Fig. 3) may be rather attributed to catalyst deactivation or to the oc-
currence of other side-effects other than the product decomposition.
This is supported by the fact that reactions do not come to completion
even for very long reaction times (Fig. 4).
4. Conclusions
In conclusion, an unexpected Pd/C-catalyzed room temperature and
4

Y. Yang, et al.
MolecularCatalysis474(2019)110450
atmospheric pressure hydrogenation reaction of 2-methylenecyclobu-
tanones was found to be an efficient method for the synthesis of 2-
substituted cyclobutanones, which were useful synthetic intermediates
for many drug and pesticide intermediates as well as the total synthesis
of natural products. The method employed commercially available
terminal starting materials such as cyclobutanone and aldehydes and
could be performed under mild reaction conditions using atmospheric
pressure H2 in balloon as the cheap, clean and safe reductant.
Quantitative calculations demonstrated that the release of the in-
tramolecular ring strain caused by the combined effect of the four-
membered ring and exocyclic C]C bond in the intermediate 2-
MCBones provided additional dynamic driving forces for the reaction
and the high regio-selectivity of the reaction was caused by the higher
hydrogenation energy releasing of C]C than C]O. These new findings
have broadened the understandings on the catalytic hydrogenation
reactions of strained organic molecules.
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Acknowledgements
We thank National Key Research and Development Program of
China (2018YFD0200100), National Natural Science Foundation of
China (21202141), Natural Science Foundation of Jiangsu Province
(BK20181449), Jiangsu Provincial Six Talent Peaks Project (XCL-090),
and Priority Academic Program Development of Jiangsu Higher
Education Institutions (PAPD) for financial support.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2019.110450.
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