Copper-Catalyzed Decarboxylative Hydrophosphinylation of α-Acyl-α-Diazoacetates

Article abstract of DOI:10.1002/ejoc.202001326

A simple copper-catalyzed decarboxylation–denitrogenation C–P coupling reaction of α-acyl-α-diazoacetates with hydrophosphoryl compounds is reported. The reaction may proceed via a process involving the generation of (diazomethyl)ketones after hydrolysis in the presence of water and the hydrophosphinylation of the copper carbene intermediates. This finding may suggest the potential use of the relatively more readily available α-acyl-α-diazoacetates as replacement of (diazomethyl)ketones in some cases.

Full text of DOI:10.1002/ejoc.202001326

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Copper-Catalyzed Decarboxylative Hydrophosphinylation of α-
Acyl-α-Diazoacetates
Authors: Can Zhang, Chao Dong, Xin Wang, and Ruwei Shen
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202001326
Link to VoR: https://doi.org/10.1002/ejoc.202001326
01/2020

10.1002/ejoc.202001326
European Journal of Organic Chemistry
COMMUNICATION
Copper-Catalyzed Decarboxylative Hydrophosphinylation of α-
Acyl-α-Diazoacetates
Can Zhang,Chao Dong, Xin Wang, Ruwei Shen*
C. Zhang, C. Dong, X. Wang, Prof. Dr. R. Shen
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering
Nanjing Tech University
Nanjing 211816, China
E-mail: shenrw@njtech.edu.cn
Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
Abstract:
A
simple
copper-catalyzed
decarboxylation-
Cu-catalyzed decarboxylative hydrophosphinylation reaction of α-
acyl-α-diazoacetates with hydrophosphoryl compounds, leading
to the formation of α-phosphinyl ketones (Scheme 1c).[13] It is
denitrogenation C–P coupling reaction of α-acyl-α-diazoacetates with
hydrophosphoryl compounds is reported. The reaction may proceed
via a process involving the generation of (diazomethyl)ketones after
hydrolysis in the presence of water and the hydrophosphinylation of
the copper carbene intermediates. This finding may suggest the
potential use of the relatively more readily available α-acyl-α-
noted
that
the
transition-metal
(TM)
catalyzed
hydrophosphinylation reaction of diazo compounds has been
studied to some extent.[14] Nevertheless, most of the reactions
proceed through the P(O)–H insertion into the metal-carbene
diazoacetates as replacement of (diazomethyl)ketones in some cases. intermediates, thus resulting in the normal denitrogenative C–P
coupling products (Scheme 1b). To the best of our knowledge,
such a decarboxylative hydrophosphinylation process has not
Diazo compounds represent an important class of highly reactive
been reported. In addition, although recent reports have already
synthetic intermediates frequently used in organic synthesis.[1]
shown a wide range of new transformations for the synthesis of
However, most diazo compounds are known to be hazardous
α-phosphinyl arylketones, alkyl-substituted products (R1 = Alkyl
group, Scheme 1c) have been seldom obtained with these
chemicals with relatively low stability and explosive property.[1,2]
Thus, there is a continuous effort on the search for new and safer
methods.[13]
surrogates to replace the commonly used unstable diazo
compounds,[3,4] and alternatively, the development of new
methods and technologies that allow for safer preparation and use
of these compounds.[5] However, the drawbacks remain. The use
of (diazomethyl)ketones is such a case in point. These
compounds are usually prepared from the well-known
Arndt−Eistert reaction, namely, a reaction of an acyl chloride or
mixed anhydride with an excess of the most hazardous and
explosive diazomethane (Scheme 1a, left).[6] A much safer
protocol might be the deformylative diazo transfer reaction,
originally developed by Wolf and Yates,[7] through alkali treatment
of diazo-diketone derivatives (Scheme 1a, right). The improved
variation of the protocol offered by Danheiser using a one-pot
trifluoroacetyl activation and diazo transfer procedure further
expands the scope of the method to allow for the synthesis of vinyl
α-diazoketones.[8] But regardless, alternative protocols for
generation and use of these compounds remain highly desirable.
During our recent study on new synthetic methods towards
organophosphorus compounds,[9, 10] we came to consider that the
relatively more available and stable α-acyl-α-diazoacetates may
undergo hydrolysis under suitable conditions to in-situ form
terminal diazoketones, which would be captured by H-heteroatom
compounds upon transition metal catalysis leading to α-
functionalized ketones via the formation of C-heteroatom bond.[11]
Scheme 1. The decarboxylative hydrophosphinylation of α-acyl-α-
Accordingly, α-acyl-α-diazoacetates may be regarded as
convenient precursors of some labile (diazomethyl)ketones.[12]
Along this line and in keeping with our interest in C–P bond
formation process,[10] we disclose herein for the first time a new
diazoacetates.
1
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Table 1. Optimization on the copper-catalyzed reaction of 1a and 2a.[a]
Entry
Catalyst
CuSO4
Solvent
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
THF
Additive
K2CO3
3a (yield%)[b]
4a (yield%)[b]
1
2
3
4
5
6
7
8
9
-
-
[c]
CuSO4
K2CO3
-
-
-
CuSO4
NaOH
H2O
H2O
H2O
H2O
H2O
H2O
trace
CuSO4
59
75
74
44
54
74
-
-
-
-
-
-
Cu(OAc)2
CuCl2
CuI
Cu(MeCN)4PF6
Cu(OAc)2
10
Cu(OAc)2
toluene
H2O
43
20
11
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
DCE
H2O
H2O
H2O
H2O[e]
42
50
-
16
12
MeCN
EtOH
dioxane
H2O
trace
13[d]
14
-
-
-
72
-
15
[a] Unless otherwise noted, a mixture of 1a (0.2 mmol), 2a (1.2 equiv), [Cu] (5 mol%), solvent (1 mL) and additive (2 equiv) was heated in a Schlenk tube under N2.
[b] Isolated yields. [c] 2 equiv of H2O were added. [d] ethyl diphenylphosphinate was obtained as the product. [e] 8 equiv of H2O.
α-Acyl-α-diazoacetates are isolated stabilized diazo
compounds, and can be readily obtained from acylacetic esters
via a diazo transfer reaction using sulfonyl azide reagents.[15] We
entry 9), whereas the reactions in other solvents including toluene,
DCE and MeCN gave a mixture of 3a and 4a (Table 1, entries 10-
12). When EtOH was used as a solvent, no C–P coupled product
was detected, but ethyl diphenylphosphinate was obtained as the
major product (Table 1, entry 13). In addition, the expected
product was not formed either, when water was used as a solvent.
This was attributed to the fact that the copper catalyst
decomposed as the formation of bronze copper metal was
observed in the reaction (Table 1, entry 15). Therefore, the best
result was obtained when dioxane was used as the solvent; a
clean transformation was achieved, and 3a was obtained in 75%
yield (Table 1, entry 5).
Table 2 summarized the results of the Cu(OAc)2-catalyzed
decarboxylative hydrophosphinylation reactions of a variety of α-
acyl-α-diazoacetates with diphenylphosphine oxide (2a) under
the optimal conditions. First, a series of different alcohol-derived
α-acetyl-α-diazoacetates 1a-1e served as good precursors for the
reaction, giving rise to the expected product 3a in good to high
yields (entries 1-5). Amongst, 4-bromobenzyl α-acetyl-α-
diazoacetate 1e showed a relatively higher reactivity, and the
reaction proceeded more efficiently and completed in 1 h to give
initially
chose
α-acetyl-α-diazoacetate
(1a)
and
diphenylphosphine oxide (2a) as model substrates to examine the
reaction under various conditions. We questioned if the presence
of an inorganic base would trigger the occurring of the
decarboxylation process. However, the experiments showed that
the use of inorganic bases such as NaOH and K2CO3 were
incompatible with the copper-catalyzed conditions (Table 1,
entries 1-3).[16] Interestingly, when two equivalents of water were
added instead, the reaction gave rise to 3a in 59% yield efficiently
in the presence of 5 mol% CuSO4 (Table 1, entry 4). With these
encouraging results, we then examined the catalytic activity of
other bench-available copper catalysts in the presence of water.
The reaction catalyzed by Cu(OAc)2 and CuCl2 gave the product
3a in much better the yields (Table 1, entries 5 and 6). However,
copper(I) salts such as CuI and Cu(MeCN)4PF6 were proven less
effective and gave 3a in moderate yields (Table 1, entries 7 and
8). Particularly, a considerable amount of unknown precipitate
was observed in the reaction using CuI as the catalyst.
Furthermore, the transformation appears to be solvent-dependent. 3a in 71% yield, probably due to the ease of hydrolysis of the 4-
The reaction performed in THF gave 3a in a similar yield (Table 1, bromobenzyl group. Notably, p-tolyl α-acetyl-α-diazoacetate (1f)
2
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10.1002/ejoc.202001326
European Journal of Organic Chemistry
COMMUNICATION
also participated well in the reaction although a somewhat lower
yield was obtained (entry 6). Other α-acyl-α-diazoacetates by
varying the substituent R1 (1g: n-Bu, 1h: Bn, 1i: neopentyl, 1j:
cyclohexyl, and 1k: cyclopropyl) were also synthesized, and
subjected to reaction with 2a under the optimal conditions. The
corresponding products 3b-3f were all effieciently generated in
good to high yields (entries 7-11). In addition, dimethyl 2-
diazomalonate 1l and diisopropyl 2-diazomalonate 1m were also
suitable substrates for this transformation, delivering the products
3g and 3h in 70% and 83% yields, respectively. When an
asymmetric diazomalonate 1n was employed, a mixture of
products 3i and 3j were formed as expected (entry 14).
[a] Unless otherwise noted, a mixture of 1 (0.2 mmol), 2a (1.2 equiv), [Cu] (5
mol%), solvent (1 mL) and water (2 equiv) was heated to 100 oC for 3 h in a
Schlenk tube under N2. [b] Isolated yields. [c] 1 h.
The reaction was successfully extended to include some other
hydrophosphoryl compounds as substrates (Table 3).
Diarylphosphine oxides bearing electron-donating and electron-
withdrawing substitutes on the phenyl rings all reacted smoothly
with 1a to provide the expected products 3k-3o in moderate to
high yields. A thiophene-substituted H-phosphine oxide also
participated well in the reaction to give 3p in 61% yield. In addition,
cyclohexyl(phenyl)phosphine
oxide
and
cyclohexyl
phenylphosphinate were also applicable to this transformation,
yielding 3q and 3r in 55% and 54% yield, respectively. The
reaction of diethyl phosphonate however did not give any
identifiable product.
Table 2. The reaction scope with respect to diazodicarbonyl compounds[a]
Table 3. The reaction scope with respect to hydrophosphoryl compounds[a]
[a] Unless otherwise noted, a mixture of 1a (0.2 mmol), 2 (0.24 mmol), Cu(OAc)2
(5 mol%), water (0.4 mmol) and dioxane (1 mL) was heated in a sealed Schlenk
tube for 3 h. Isolated yields are given. [b] No identifiable product was obtained.
It is worth mentioning that the reaction of α-benzoyl-α-
diazoacetate 1o and 2a did not generate decarboxylative C–P
coupling product under the present conditions, but the normal
insertion product 5 was obtained in a high yield (Scheme 2, eq.1).
The reactions conducted at higher concentrations or in the
presence of more equivalents of water also gave 5 in high
yields.[17] In contrast, the reaction of ethyl 2-diazo-2-tosylacetate
(1p) remained to deliver the decarboxylative C–P coupling
product 6 in 68% yield (Scheme 2, eq. 2). These results indicated
a substituent effect of the reaction, the reason of which however
remains to be clarified.
To obtain more insight into the reaction mechanism, we also
prepared two diazodicarbonyl compounds bearing more reaction
3
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points to probe possible competitive reactions, i.e., an allyl 2-
diazo-3-oxobutanoate (1q) and ethyl 2-diazo-3-(methyl(p-
tolyl)amino)-3-oxopropanoate (1r). Although substrate 1q was
10 and a deamidative product 3i (Scheme 2, eq. 4). The results
suggest the competative intramolecular arylation of the possible
copper-carbene intermediate 11 may take place in the reaction.
Indeed, the reaction of 1r catalyzed by 5 mol% Cu(OAc)2 in the
presence of 2 equiv of water without adding 2a delievered the
product 9 cleanly in 65% yield (Scheme 2, eq. 5). Finally, we
performed the catalytic reaction of 1a and 2a using D2O instead
of H2O (Scheme 2, eq. 6). Interestingly, in the presence of 2 equiv
of D2O, the reaction afforded the product 3a-D in 73% yield with
a deuteration of ca. 73% at the γ-C position (the calculated
deuteration rate refers to one H atom of the methyl group). While
the loss of deuterium atoms at the α-C position was probably due
to deuterium-hydrogen exchange during the purification process
on silica gel column chromatography, we reasoned that the
incorporation of deuterium at the γ-C position indicated the
existance of an enol-keto tautomerization with respect to the
acetyl moiety during the reaction.
known as
a
good precusor for the intramolecular
cyclopropanation reaction under the copper or rhodium
catalysis,[18] the reaction with 2a under the present conditions
Based on the obtained results and literature precedents,[14]
a
tentative mechanism is proposed in Scheme 3. Firstly, the
hydrolysis of α-acyl-α-diazoacetate 1 takes place to give an α-
diazoacetoacetic acid intermediate
A under the reaction
conditions.[20] The tautomerization between A and the enol form
A’ may exist to some extent because of activation of the carbonyl
group by intramolecular hydrogen bond. Decarboxylation of the
intermediate A then gives the (diazomethyl)ketone B, which
reacts with the copper catalyst to generate the copper-carbene
intermediate C. The subsequent hydrophosphinylation reaction of
C finally gives the product 3 and regenerates the copper catalyst
(Scheme 3).
Scheme 3. The proposed mechanism.
In conclusion, we have disclosed
a copper-catalyzed
decarboxylative hydrophosphinylation reaction between α-acyl-α-
diazoacetates and hydrophosphoryl compounds. The study does
not only provide a new alternative way to generate (α-
alkylphosphinyl)ketones, but may also suggest the possible use
of relatively more readily available and stable α-acyl-α-
diazoacetates instead of α-diazoketones in some cases. Further
study on the applications of the method as well as the reaction
mechanism will be pursued.
Scheme 2. The control experiments.
only gave the decarboxylative C–P coupling product 3a in 74%
yield with high selectivity, and the [2+1] cycloadduct 8 was not
formed (Scheme 2, eq. 3).[19] These results indicate that the
hydrolysis of the ester group may take place faster under the
present conditions. On the other hand, the reaction of 1r turned
out to be much more complicated, yielding a mixture of the
cyclized product 9, with the decarboxylative C–P coupling product
Acknowledgements
4
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10.1002/ejoc.202001326
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[11] For a review on catalytic X-H insertion reactions of carbenoids, see: D.
Gillingham, N. Fei, Chem. Soc. Rev. 2013, 42, 4918–4931.
Financial support from the NSFC (21302095), scientific research
innovation project for postgraduates in Jiangsu province
(SJCX20_0361) and Nanjing Tech University is acknowledged.
[12] Ref (1a), Chapter 3, pp 96-165.
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Wang, J. Sun, M. D. Zhou, Asian J. Org. Chem. 2018, 7, 1839–1843; (d)
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Keywords: copper catalysis • hydrophosphinylation •
decarboxylation • C–P bond • α-acyl-α-diazoacetates
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yield. In addition, the reactions of 1o and 2a catalyzed by 5 mol%
Cu(OAc)2 in the presence of 4 and 8 equiv of water, also only gave 5 in
88% and 89% yield, respectively.
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[19] A comparative result was also observed from the reaction of 1q and 2a
catalyzed by 2.5 mol% of Rh2(OAc)4 instead of Cu(OAc)2 in the presence
of water giving 3a in 66% yield.
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10.1002/ejoc.202001326
European Journal of Organic Chemistry
COMMUNICATION
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10.1002/ejoc.202001326
European Journal of Organic Chemistry
COMMUNICATION
Entry for the Table of Contents
Key Topic: Decarboxylative hydrophosphinylation
A simple Cu-catalyzed decarboxylative hydrophosphinylation of α-acyl-α-diazoacetates with hydrophosphoryl compounds is developed
via a process of the generation of (diazomethyl)ketones and the subsequent C–P coupling. The findings imply the potential use of the
relatively more readily available α-acyl-α-diazoacetates as replacement of (diazomethyl)ketones in some cases.
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