β-Ketophosphonates formation via deesterification or deamidation of cinnamyl/alkynyl carboxylates or amides with H-phosphonates

Article abstract of DOI:10.1039/c5ra23950h

We report here an unprecedented Fe/Cu synergistically catalyzed deesterificative or deamidative oxyphosphorylation of unsaturated carboxylates or amides with H-phosphonates. The valuable β-ketophosphonates were obtained along with chemoselective cleavage of C_sp²-C(CO) or C_sp-C(CO) bonds in good yields under oxygen atmosphere with a wide substrate scope.

Full text of DOI:10.1039/c5ra23950h

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b-Ketophosphonates formation via deesterification
or deamidation of cinnamyl/alkynyl carboxylates or
amides with H-phosphonates†
Cite this: RSC Adv., 2015, 5, 103977
a
a
b
b
b
ac
Received 28th October 2015
Accepted 24th November 2015
*
Yao Zhou, Mingxin Zhou, Ming Chen, Jihu Su, Jiangfeng Du and Qiuling Song
DOI: 10.1039/c5ra23950h
www.rsc.org/advances
We report here an unprecedented Fe/Cu synergistically catalyzed rearrangement. In term of deamidation reaction, even less
deesterificative or deamidative oxyphosphorylation of unsaturated examples have been reported.2 a,b-Unsaturated (including C]C
carboxylates or amides with H-phosphonates. The valuable b-keto- bonds and C^C bonds) compounds are an important class of
phosphonates were obtained along with chemoselective cleavage of compounds in organic synthesis, they usually serve as Michael
sp –C(CO) or Csp–C(CO) bonds in good yields under oxygen atmo- acceptors to lead to 1,4-adduct.4 Recently, some elegant exam-
2
C
sphere with a wide substrate scope.
ples emerged on selective cleavage of C(sp3)–C(CO) bonds in
aryl alkyl ketones;5 intriguingly, selective cleavage of C(sp2) or
C(sp)–C(CO) single bond of a,b-unsaturated compounds is
scant.6 and the known methods mainly focused on decarbox-
ylative C–C cleavage of a,b-unsaturated carboxylic acids, such as
decarboxylation of cinnamic acids7 or alkynyl carboxylic acids.8
To our knowledge, deesterication or deamidation via chemo-
selective cleavage of C(sp2)–C(CO) or C(sp)–C(CO) bond in
cinnamyl/alkynyl esters or amides has not been reported thus
far. Therefore it is a challenge and would be highly desirable if
efficient deesterication or deamidation of cinnamyl/alkynyl
esters or amides could be developed.
Organophosphorus compounds are of great importance and
widely applied in organic synthesis, material sciences, biology
and pharmaceuticals.9 It is always appealing for organic
chemists to develop new and highly efficient strategy for
construction of C–P bonds.10 In 2011, Yang and his coworkers
rstly reported a Cu-catalyzed decarboxylative phosphonation
of cinnamic acids and arylalkynyl carboxylic acids with diphe-
nylphosphine oxide (Scheme 1a).7a Very recently we reported an
efficient method converting cinnamic acids or alkynyl carbox-
ylic acids into b-ketophosphonates with H-phosphonates via
decarboxylative oxyphosphorylation reaction (Scheme 1b).7d,8c
As our continuous interests in activation of C–C single bond11/
functionalization of C–C multiple bonds,7d,8c herein, we report
an unprecendent Fe/Cu cocatalyzed deesterication and dea-
midation of cinnamyl/alkynyl esters or amides via chemo-
selective cleavage of C(sp2)–C(CO) or C(sp)–C(CO) bond as well
as difunctionalization of C]C or C^C bonds to construct new
C–P bonds under mild conditions. Compared with other
precious noble metals, Fe and Cu are inexpensive, nontoxic,
earth-abundant and user friendly for academia as well as for
industry. Currently, Cu/Fe-cocatalyzed reactions have gained
Esters and amides are pivotal structural motifs in organic
chemistry, pharmaceuticals and biologically active compounds,
signicant explorations have been made in the synthesis of
these two types of compounds, however, surprisingly, direct
utilization of esters or amides as synthetic blocks via deester-
ication1 or deamidation2 is very rare despite the wide existence
and ready accessibility of these two types of compounds. There
are three known ways for cleavage of C–C(COOR) bond: (a)
direct cleavage of C–C(COOR) bond, rendering COOR as
a leaving group;1e–1g (b) the two-step strategy: hydrolysis of ester
to carboxylic acids rst, then subsequent decarboxylation;1a–1d
(c) decarboxylation of esters by releasing CO2 gas.3 The second
strategy is step- and time-consuming, which is not suitable for
late-stage modication when acid or base sensitive functional
groups existing on the molecules; and the third one, in theory,
belongs to decarboxylation; only the rst strategy is the most
straight-forward and efficient method to generate a new moiety
by releasing COOR as a leaving group. However, very few
precedent literatures have reported such transformation yet and
the known deesterication was followed by a structural
aInstitute of Next Generation Matter Transformation, College of Chemical Engineering,
College of Materials Science & Engineering at Huaqiao University, 668 Jimei Blvd,
Xiamen, Fujian, 361021, P. R. China. E-mail: qsong@hqu.edu.cn; Fax: +86-592-
6162990
bHefei National Laboratory for Physical Sciences at Microscale, Department of Modern
Physics, University of Science and Technology of China, Hefei, Anhui 230026, P. R.
China
cBeijing National Laboratory for Molecular Sciences, Beijing, 100190, P. R. China
† Electronic supplementary information (ESI) available: Full experimental and
optimization details, characterization of the synthesized products. See DOI:
10.1039/c5ra23950h
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Table 1 Optimization of the reaction parametersa
Entry [Cu] cat.
[Fe] cat.
Base (1 equiv.) Yieldb (%)
1
2
3
4
CuOTf (5 mol%) FeCl3 (10 mol%) Et3N
CuOTf (5 mol%) FeCl2 (10 mol%) Et3N
CuOTf (5 mol%) FeBr2 (10 mol%) Et3N
CuOTf (5 mol%) FeCl3 (10 mol%) (iPr)2NEt
CuOTf (5 mol%) FeCl3 (10 mol%) (iPr)2NEt
CuOTf (5 mol%) FeCl3 (10 mol%) (iPr)2NEt
CuOTf (5 mol%) FeCl3 (5 mol%) (iPr)2NEt
CuOTf (10 mol%) FeCl3 (10 mol%) (iPr)2NEt
CuOTf (5 mol%) FeCl3 (10 mol%) (iPr)2NEt
CuOTf (5 mol%) FeCl3 (10 mol%) (iPr)2NEt
(69)c
67
54
(70)c
(79)c
(74)c
58
5d
6e
7
Scheme
1 Transition-metal-catalyzed phosphonation reactions
between a,b-unsaturated carbonyl compounds and H-phosphonates.
8
65
—
47
7
9f
10g
11
12
much attention due to their superior combined bimetallic
catalytic reactivities.12 In this strategy, environmentally benign
Cu/Fe cocatalysts were for the rst time employed to efficiently
cleave C(sp2)–C(CO) or C(sp)–C(CO) bond via deesterication or
deamidation protocol in cinnamate or alkynyl carboxylates or
amides under 1 atm dioxygen atmosphere and valuable b-
ketophosphonates, which are key intermediates for the total
synthesis of various natural products13 and building blocks for
a myriad of organic transformations,14 were generated in good
yields with a broad range of substrate scope in a one-pot
protocol.
—
FeCl3 (10 mol%) (iPr)2NEt
(iPr)2NEt
CuOTf (5 mol%)
—
—
a
Reaction conditions: ethyl cinnamate 1a (0.5 mmol), H-diethyl
phosphate 2a (1.5 mmol), Cu catalyst and Fe cocatalyst in DMSO
(1 mL), Et3N (0.5 mmol), O2 atmosphere, 90 ꢀC, 17 h, in a sealed tube.
b
c
d
e
GC yield. Isolated yield in parenthesis. 24 h. 1.3 mmol of 2a
f
was used for 24 h. The reaction was performed under N2
atmosphere. g The reaction was performed under air.
We commenced our study with ethyl cinnamate (1a) and H-
diethyl phosphonate (2a) as substrates using 5 mol% CuOTf
ꢀ
together with 10 mol% FeCl3 as the cocatalyst system at 90 C.
To our delight, the desired product b-ketophosphonate was
formed in 69% isolated yield in the presence of Et3N in DMSO
within 17 h via deesterication. Other iron salts, such as FeCl2
or FeBr2 gave lower yields (Table 1, entries 2–3). When the base
was changed from Et3N to (iPr)2NEt, the isolated yield was
slightly increased to 70% (entry 4). Reaction time was also
important for the improvement of isolated yields: when the
reaction time prolonged from 17 h to 24 h, the yield was
increased to 79% (entry 5), yet when H-diethyl phosphonate (2a)
was decreased from 3 equiv. to 2.6 equiv., the yield of desired
product 3aa was only slightly dropped to 74% (entry 6). Given
the difficulty on the purication of nal product 3aa with excess
amount of H-diethyl phosphonate (2a), we decided to take entry
6 as the optimal conditions. Control experiments suggested that
molecular dioxygen, CuOTf and FeCl3 were all indispensable to
this reaction (entries 9–12).15
Encouraged by the above results, we next explored the substrate
scope (Scheme 2). Gratifyingly, a variety of ethyl cinnamate deriv-
atives which bear either electron-donating (methyl, p-t-Bu, p-MeO)
or electron-withdrawing (p-NO2) groups on the aromatic rings of
cinnamates were well tolerable in this transformation, affording
the desired products in moderate to good yields (3ba–3fa, 3ma).
Ortho-, meta- and para-substituted groups gave the desired product
in decent yields with ortho one in a relative low yield owing to the
steric hindrance (3ba–3da). Halo-substituted cinnamates such as
uorine, chlorine and bromine were well compatible, leading to
the corresponding products in 62–68% (3ga–3ja). Polyphenylene
Scheme 2 Cooperative deesterificative tandem oxophosphorylation
of cinnamates (1) with H-phosphonates under O2 atmosphere.
Reaction conditions: 1 (0.5 mmol), 2 (1.3 mmol), CuOTf (5 mol%), FeCl3
(10 mol%), (iPr)2NEt (0.5 mmol), DMSO (1 mL), O2 atmosphere, 90 ꢀC,
24 h. [a]Methyl ester was used. [b]28 h. [c]2 (1.5 mmol).
acrylate, for instance (E)-ethyl-3-([1,10-biphenyl]-4-yl)acrylate and 1-
naphthylacrylate are also good candidates for this transformation
and gave the desired b-ketophosphonates in 71% and 58% yields
respectively (3ka and 3la). Heteroaromatic acrylate, like 2-thio-
pheneacrylate worked smoothly under standard conditions to give
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3na in 65% yield. It was noteworthy that di-n-butyl (2b), diisopropyl
(2c), and diphenyl phosphoxide (2d) as well as ethyl phosphonate
(2e) were all suitable substrates in addition to diethyl phosphonate
(2a), generating b-ketophosphonates 3ab–3ae in good to excellent
yields.
To evaluate the generality of this tandem oxyphosphory-
lation reaction, a variety of other a,b-unsaturated systems were
further employed under the standard conditions (Scheme 3). In
addition to various cinnamates (1a–1e) (deesterication), to our
delight, cinnamides (1f–1m) were also good substrates for this
transformation via deamidation pathway regardless of primary,
secondary or tertiary amides (entries 6–13). Meanwhile, 3-
phenyl-propiolate (1n–1o) and 3-phenylpropiolamide (1p–1q)
were both compatible for this transformation via chemo-
selective cleavage of C(sp)–C(CO) bonds and lead to the desired
products in good yields (entries 14–17).
It is noteworthy that the reaction can be readily scaled up
without loss of the efficiency: when 1a (8 mmol) and 2a
(20.8 mmol) were exposed to the standard conditions, 68%
(1.39 g) of desired product 3aa was obtained (eqn (1)), which
demonstrated the potential utility of this transformation in
synthetic community, since b-keto-phosphonate is a key inter-
mediate for the synthesis of various natural products such as
(ꢁ)-blepharocalyxin D,13a (ꢁ)-diospongin B,13b 1-phosphono-3-
azabicyclo[3.1.0] hexanes.13c
(1)
(2)
To understand the origination of the carbonyl oxygen atom of
b-ketophosphonates, isotope labeling experiments were per-
formed. Water scrambling experiment was rstly conducted
with the product 16O–3aa under the standard conditions in the
presence of H218O (5 equiv.) to gure out the possibility of
oxygen atom exchange with water, it turned out that 42% of
product was incorporated by 18O (eqn (2)), indicating that oxygen
exchange with water occurred in the standard conditions.
Subsequently, substrates 1a and 2a were performed under
18O2 under the standard conditions, 82% of 18O–labeled
product 3aa was detected (eqn (3)). When the reaction was
conducted in the presence of 5.0 equiv. of H218O under O2, 36%
of 18O–3aa and 64% of unlabeled products were detected with
ethyl cinnamate 1a (eqn (4)). From this reaction, we could tell
that the oxygen of the carbonyl group should come from
molecular oxygen. If the oxygen was from water, the ratio of
18O–3aa : 16O–3aa should be 5 : 1, that is ca. 82% 18O–3aa
should be detected since one equivalent of water was from the
reaction, yet there was only 36% of 18O–3aa was detected in the
real system. Moreover, 38% 18O-labeled and 62% unlabeled
products were detected when the reaction of 1a and 2a was
conducted in the presence of H2O (5 equiv.) under 18O2 (eqn
(5)). On the basis of all above 18O labeled experiments (eqn (2)–
(5)), we can determine that the oxygen in carbonyl group should
be exclusively from dioxygen, the low level of 18O in desired
product 3aa in eqn (5) comes from the water scrambling
reaction.
(3)
(4)
Scheme
3 a,b-Unsaturated compound scope. Unless otherwise
(5)
noted, all reaction were run with 1 (0.5 mmol), 2a or 2c (1.3 mmol),
CuOTf (5 mol%). FeCl3 (10 mol%), (iPr)2NEt (0.5 mmol), DMSO (1 mL) in
a sealed tube under O2 atmosphere at 90 ꢀC for 24 h. [a]80 ꢀC, 2a (1.5
mmol).
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To elucidate and conrm our hypothesis for this oxidative Based on our hypothesis, we have reasons to believe that the
radical involving tandem reaction, EPR (Electron Para- peroxide signal in Fig. 1b is hydrogen peroxide, yet the
magnetic Resonance) experiments were further conducted peroxide signal in Fig. 1c should be cOOR.
under various reaction conditions with the addition of free
On the basis of all above results and previous reports,7d,8c,16
a
radical spin trapping agent DMPO (5,5-dimethyl-1-pyrroline tentative mechanism for this tandem deestericative or dea-
N-oxide). Firstly, the P-centered radical was determined by midative oxyphosphorylation is illustrated in Scheme 4: single
EPR since an EPR signal of phosphorous species (AP ¼ 46.7 G, electron transfers from iron(III) species to HP(]O)(OR)2 in the
AN ¼ 14.3 G, AHb ¼ 18.9 G) was detected in the reaction of 2a, presence of molecular oxygen, forming dialkyl phosphonate
FeCl3 and (iPr)2NEt in DMSO (Fig. 1a). Interestingly, no same cation radical I. The H+ from the dialkyl phosphonate cation
phosphorous signal was detected in the reaction of 2a, CuOTf radical I is grabbed by the base leading to dialkyl phosphonyl
and (iPr)2NEt in DMSO (Fig. 1b). These results suggested that radical II. This radical is trapped by cinnamate 1 to afford an in
FeCl3 could promote diethyl H-phosphonate (2a) to generate situ generated C-centered radical, which further reacted with
P-centered radical, while the formation of the P-centered Cu(II)–(cO–OH) species under dioxygen atmosphere to form
radical is independent on CuOTf. When DMPO was added hydroperoxide species III, trace amount of byproduct E was
to the reaction of 1a, 2a, CuOTf, FeCl3 and (iPr)2NEt in DMSO, detected on GCMS, which might come from the b-hydrogen
an EPR signal was observed (AN ¼ 13.9 G, AHb ¼ 11.9 G) elimination. Aer reduction of the peroxide (one equivalent of
(Fig. 1c), data analysis illustrated that it is different from the H-phosphonate serves as reductant), a-ester-b-ketophospho-
signal in Fig. 1b (AN ¼ 12.9 G, AHb ¼ 10.3 G, AHg ¼ 1.2 G). nate IV was generated, whose existence was also proven by
GCMS analysis, since a trace amount of (OR)2PCH2COOEt G was
detected on GCMS. Another one equivalent excess H-
phosphonate acts as nucleophile to attack the carbonyl group
in the ester, which also explains why ca. 3 equivalent of H-
phosphonate was needed in order to get the optimized results
(Table 1). Eventually the desired product 3 was afforded with the
help of base.
In conclusion, a novel copper/iron-cocatalyzed aerobic
deestericative or deamidative oxyphosphorylation via cleavage
of cinnamate or alkynyl carboxylate with H-phosphonates has
been developed, this reaction leads to very valuable b-keto-
phosphonates in good yields with a broad spectrum of substrate
scope. Environmentally benign catalysts system (copper and
iron salts) and the use of O2 as the oxidant plus readily available
substrates make this transformation sustainable and practical.
A putative mechanism is proposed on the basis of mechanistic
studies, isotope labeling and EPR studies, suggesting that FeCl3
was essential to generate P-centered radical which was
responsible to initiate the whole process, yet the formation of
the radical is independent on CuOTf, interestingly, CuOTf
denitely has responsibility on the activation of molecular
oxygen which was proven to be the source of the oxygen in the
carbonyl group of b-ketophosphonates. Further exploration on
the synthetic applications is under the way in our laboratory.
Fig. 1 The electron paramagnetic resonance (EPR) spectra of the Scheme 4 Plausible reaction mechanism for oxyphosphorylation of
reaction mixture under various conditions.
cinnamates.
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Acknowledgements
Financial support from the National Science Foundation of China
(21202049), the Recruitment Program of Global Experts (1000
Talents Plan), Fujian Hundred Talents Plan, program of Innova-
tive Research Team of Huaqiao University and Graduate inno-
vation fund of Huaqiao Universityare gratefully acknowledged.
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