Gold-Catalyzed Formal C?C Bond Insertion Reaction of 2-Aryl-2-diazoesters with 1,3-Diketones

Article abstract of DOI:10.1002/asia.201800934

The transition-metal-catalyzed formal C?C bond insertion reaction of diazo compounds with monocarbonyl compounds is well established, but the related reaction of 1,3-diketones instead gives C?H bond insertion products. Herein, we report a protocol for a gold-catalyzed formal C?C bond insertion reaction of 2-aryl-2-diazoesters with 1,3-diketones, which provides efficient access to polycarbonyl compounds with an all-carbon quaternary center. The aryl ester moiety plays a crucial role in the unusual chemoselectivity, and the addition of a Br?nsted acid to the reaction mixture improves the yield of the C?C bond insertion product. A reaction mechanism involving cyclopropanation of a gold carbenoid with an enolate and ring-opening of the resulting donor–acceptor-type cyclopropane intermediate is proposed. This mechanism differs from that of the traditional Lewis-acid-catalyzed C?C bond insertion reaction of diazo compounds with monocarbonyl compounds, which involves a rearrangement of a zwitterion intermediate as a key step.

Full text of DOI:10.1002/asia.201800934

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Gold-Catalyzed Formal C-C Bond Insertion Reaction of 2-Aryl-2-
diazoesters with 1,3-Diketones
Authors: Yuan-Yuan Ren, Mo Chen, Ke Li, and Shou-Fei Zhu
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10.1002/asia.201800934
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Gold-Catalyzed Formal C–C Bond Insertion Reaction of 2-Aryl-2-
diazoesters with 1,3-Diketones
Yuan-Yuan Ren, Mo Chen, Ke Li, and Shou-Fei Zhu*
Abstract: The transition-metal-catalyzed formal C–C bond insertion
reaction of diazo compounds with monocarbonyl compounds is well
established, but the related reaction of 1,3-diketones instead gives C–
H bond insertion products. Herein, we report a protocol for a gold-
catalyzed formal C–C bond insertion reaction of 2-aryl-2-diazoesters
with 1,3-diketones, which provides efficient access to polycarbonyl
compounds with an all-carbon quaternary center. The aryl ester
moiety plays a crucial role in the unusual chemoselectivity, and the
addition of a Brønsted acid to the reaction mixture improves the yield
of the C–C bond insertion product. A reaction mechanism involving
cyclopropanation of a gold carbenoid with an enolate and ring-
opening of the resulting donor–acceptor-type cyclopropane
intermediate is proposed. This mechanism differs from that of the
traditional Lewis-acid-catalyzed C–C bond insertion reaction of diazo
compounds with monocarbonyl compounds, which involves
rearrangement of a zwitterion intermediate as a key step.
a
Diazo compounds have found a wide variety of applications in
synthetic organic chemistry.1 For example, they have been used
as carbene precursors in transition-metal-catalyzed carbene
transfer reactions.2 In addition, because diazo compounds have
a partial negative charge on the carbon connected to the diazo
group, they can undergo formal C–C bond insertion reactions with
carbonyl compounds in the presence of Lewis acid catalysts
(Scheme 1a).3 Several widely used name reactions,4 including the
Arndt–Eistert reaction, the Buchner–Curtius–Schlotterbeck
reaction, the Tiffeneau–Demjanov rearrangement, and the
Roskamp reaction, are formal C–C bond insertion reactions of
diazo compounds with various aldehydes or ketones (also known
as homologation reaction) and are believed to proceed via
nucleophilic addition of the diazo compounds to the carbonyl
compounds followed by rearrangement of the resulting
zwitterionic intermediate via TS-1 (Scheme 1a). Despite this
considerable progress, most of the above-mentioned reactions
involve monocarbonyls as electrophiles.
Scheme 1. Metal-catalyzed reaction of diazo compounds with carbonyl
compounds
Although 1,3-diketones are commonly used in organic
synthesis, catalytic C–C bond insertion reactions of 1,3-diketones
with diazo compounds remain unknown. In 2014, Shi and co-
workers5 reported that gold-catalyzed reactions of 1,3-diketones
and α-aryl diazoesters produce C–H bond insertion products in
good yields (Scheme 1b). Herein we report a protocol for a gold-
catalyzed formal C‒C insertion reaction of aryl diazoesters 1 with
1,3-diketones 2 to form polycarbonyl compounds 3 with an all-
carbon quaternary center (Scheme 1c). The aryl ester moiety of
the diazoesters plays a key role in the unusual chemoselectivity,
and the addition of a Brønsted acid to the reaction mixture
improves the yield of the C–C bond insertion product. While we
were preparing this manuscript, Bi and coworkers6 reported an
elegant AgOTf-catalyzed C–C bond insertion reaction of 2-aryl-2-
diazoacetates with aryl 1,3-ketones 2 (where R1 or R2 is an aryl
group). Compared with this silver-catalyzed system, our gold-
catalyzed system exhibits higher activity (reaction time: 1 h for
gold versus 6 h for silver; catalyst loading: as low as 0.5 mol% for
gold versus 10 mol% for silver) and can be extended to alkyl 1,3-
ketones (where R1, R2, or both are alkyl).
[*]
Y.-Y. Ren, M. Chen, K. Li, Prof. S.-F. Zhu
State Key Laboratory and Institute of Elemento-Organic Chemistry,
College of Chemistry, Nankai University
Tianjin 300071 (China)
Initially, we studied the reaction of methyl 2-diazo-2-
phenylacetate and 1,3-diketone 2a with (ArO)3PAuNTf2 (Ar = 2,4-
tBu2C6H3) as a catalyst (Table 1, entry 1). As described by Shi and
coworkers,5 this reaction afforded the C‒H insertion product in
high yield (83%); however, we also isolated unexpected C‒C
bond insertion product 3 in 11% yield. To our delight, further
experiments revealed that the distribution of the C–H and C–C
bond insertion products was strongly affected by the ester moiety
E-mail: sfzhu@nankai.edu.cn
Prof. S.-F. Zhu
Collaborative Innovation Center of Chemical Science and
Engineering (Tianjin), Tianjin 300071 (China)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/asia.201800934
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of the diazo compound (entries 1–7). In particular, the use of an
aryl ester substantially increased the ratio of C‒C bond insertion
to C‒H insertion (to >95:5, entries 6 and 7). In addition, the
presence of an acidic additive affected both the yield and the
product distribution, and the addition of 2 mol% H3PO4 increased
the C‒C insertion product yield to 89% (entries 8‒11). The solvent
also strongly influenced the outcome of the reaction. The use of
CHCl3 or toluene lowered the yields as well as the selectivity for
C‒C insertion (entries 12 and 13). The use of Et2O slightly
affected the yield and selectivity but remarkably slowed down the
reaction (entry 14). Finally, we explored other metal catalysts that
have been successfully used in transformations of diazo
compounds. Rh2(OAc)4 and Pd(OAc)2 failed to give the desired
product (entries 15 and 16). Cu(PF6)(MeCN)4 and AgOTf afforded
the desired C‒C bond insertion product but in a much lower yield
than that obtained with a gold catalyst (entries 17 and 18). It is
worth mentioning that the reaction could be performed on a gram
scale at a gold catalyst loading as low as 0.5 mol% without
compromising the yield or selectivity (entries 19 and 20).
were explored (3aa‒3al). We found that the reaction outcome
depended strongly on the electronic and steric properties of the
phenyl-group substituents. Ketones with electron-donating
groups (3aa‒3ag, 3ai‒3al) gave higher yields than a ketone with
an electron-withdrawing group (3ah). Unlike ketones having para-
or meta-substituted phenyl groups, more sterically hindered
ketones with ortho-substituted phenyl groups failed to give the
desired C‒C bond insertion products (data not shown). In addition
to phenyl-substituted ketones, 1,3-ketones with naphthyl (3am),
piperonyl (3an), and thienyl (3ao and 3ap) groups smoothly
underwent the C‒C bond insertion reactions and gave
satisfactory yields. More importantly, we found that even aliphatic
1,3-diketones underwent C‒C bond insertion reactions with good
yields (3aq‒3as, 70‒79% yield). It is worth noting that the AgOTf-
catalyzed reaction of methyl 2-diazo-2-phenylacetate with a
typical aliphatic 1,3-ketone (2q) under the optimal conditions
described by Bi and coworkers6 gave a poor yield (18%) of the C–
C insertion product. The structure of 3ab was confirmed by means
of single-crystal X-ray analysis.7
Table 1. Metal-catalyzed reaction of 2-diazo-2-phenylacetate 1 with 1,3-
diketone 2a: optimization of reaction conditions.[a]
Entry
R
[M]
[Au][c]
[Au]
[Au]
[Au]
[Au]
[Au]
Acid
none
none
none
none
none
none
Solv.
t (h)
1
3 (%)[b]
4 (%)[b]
83
Me
1
2
3
4
5
6
CH2Cl2
11
28
11
2
Et
iPr
CH2Cl2 0.5
50
CH2Cl2
CH2Cl2
1
1
38
tBu
Bn
Ph
Ar[d]
37
CH2Cl2 0.5
CH2Cl2 0.5
9
50
61
2
7
8
[Au]
[Au]
none
PhCO2H
TsOH
CH2Cl2
1
71
65
3
19
9
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
CH2Cl2 0.5
9
[Au]
CH2Cl2
1
1
1
1
73
10
11
12
13
14
15
16
17
18
19[h]
20[i]
[Au]
(D)-CSA[e] CH2Cl2
74
3
[Au]
H3PO4
H3PO4
H3PO4
H3PO4
H3PO4
H3PO4
H3PO4
H3PO4
H3PO4
H3PO4
CH2Cl2
CHCl3
89
4
[Au]
65
24
24
4
[Au]
toluene 12
Et2O 12
36
[Au]
74
Rh2(OAc)4
Pd(OAc)2
[Cu][g]
AgOTf
[Au]
CH2Cl2 0.5
CH2Cl2 12
ND[f]
ND
49
ND
ND
2
CH2Cl2
CH2Cl2
CH2Cl2
1
1
3
30
1
84
3
Scheme 2. Gold(I)-catalyzed formal C‒C bond insertion reactions of 2-diazo-
2-phenylacetate 1a with various 1,3-diketones 2. The isolated yields were
given.
[Au]
CH2Cl2 12
83
8
[a] Reaction conditions: diazoester 1 (0.2 mmol), 2a (1.0 mmol), solvent (1.0
mL), [M] (0.004 mmol), acid additive (0.004 mmol), 25 °C. [b] 1H NMR yield with
1,3,5-trimethoxybenzene as internal standard. [c] [Au] is (ArO)3PAuNTf2, Ar =
2,4-tBu2C6H3. [d] Ar = 2,6-Cl2C6H4. [e] (D)-camphorsulfonic acid. [f] Not detected.
[g] [Cu] = Cu(PF6)(MeCN)4. [h] Performed with 1 mol% catalyst at gram scale
(3.3 mmol 1 was used and 1.4 g 3aa was isolated). [i] Performed with 0.5 mol%
catalyst.
Next, we investigated the effects of the substituents on the Ar
group of diazoesters 1b–1h (Scheme 3). All the tested reactions
ran smoothly and generally gave the desired products in good
yields. In reactions of aromatic 1,3-diketone 2a, 2-aryl-2-
diazoacetates having para- and meta-methoxy substituents (1c
and 1f) afforded relatively low yields. The reactions of aliphatic
We next evaluated the reactivities of various 1,3-diketones
with 1a under the optimal conditions (Scheme 2). First, the effects
of substituents on the phenyl groups of the aromatic 1,3-diketones
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1,3-diketone 2q with various 2-aryl-2-diazoesters also ran
smoothly but gave lower yields of the C‒C bond insertion products
(3bq–3hq) compared with the corresponding reactions of
aromatic 1,3-diketone 2a.
products 3at–3av in good yields (Scheme 4). The insertion
occurred regiospecifically at the alkyl ketone side. Moreover, even
an unsymmetrical aryl 1,3-diketone with a strongly electron-
withdrawing group (–CF3) on one of the phenyl rings and a
strongly electron-donating group (–OMe) on the other underwent
the C‒C bond insertion exclusively at the electron-deficient
ketone side and produced a single product (3aw) in 74% yield.
However, the reactions with the 1,3-diketones having two different
substituents with similar electron properties (2x) exhibited poor
regioselectivity. The structures of 3at–3aw were confirmed by
single-crystal X-ray analysis.7
1,4-Diketones like the ones generated in this study have
diverse potential applications in organic synthesis.8 For example,
simple treatment of 3aa with NH2NH2·H 2O in EtOH at reflux gave
4,5-dihydro-3(2H)-pyridazinone 5 (81% yield), which is the core
structure of various pharmaceuticals9 (Scheme 5, eq. a).10
Furthermore, 3aa could be readily converted into 5-hydroxy-1,5-
dihydro-2H-pyrrol-2-one 6 (in 86% yield), which is also a core
structure of several bioactive compounds11 (Scheme 5, eq. b).12
Scheme 3. Gold(I)-catalyzed formal C‒C bond insertion reactions of 2-aryl-2-
diazoacetates 1 with 1,3-diketones 2a or 2q. The isolated yields were given.
Scheme 5. Transformations of product 3aa
Scheme 4. Gold(I)-catalyzed formal C‒C bond insertion reactions of 2-diazo-
2-phenylacetate 1a with unsymmetrical 1,3-diketones 2t–2x. The isolated
yields were given.
Scheme 6. Proposed mechanism
Interestingly, unsymmetrical phenyl alkyl 1,3-diketones 2t–2v
were also suitable substrates, giving desired C–C bond insertion
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On the basis of the above-described experimental results
and the known reactivity of metal carbenoids,1 we propose the
reaction mechanism shown in Scheme 6. The reaction starts with
the generation of electrophilic carboncation gold species I via
decomposition of 1a promoted by the cationic gold complex.13
Gold carbenoid I then undergoes a cyclopropanation reaction with
the enol isomer of the 1,3-diketone to afford cyclopropane
intermediate II.14 The phosphoric acid additive may promote the
transformation of the 1,3-diketone to its enol isomer.15
Acknowledgements
We thank the National Natural Science Foundation of China
(21625204), the “111” project (B06005) of the Ministry of
Education of China, the National Program for Special Support of
Eminent Professionals, and the Fundamental Research Funds for
the Central Universities for financial support.
Keywords: Gold catalysis • C–C bond insertion • diazo
compounds • 1,3-diketones • quaternary carbons
Intermediate II, which is
a typical donor–acceptor type
cyclopropane, easily undergoes a ring-opening reaction involving
a proton transfer.16 Both bond a and bond b of II are polar bonds
and can undergo cleavage: cleavage of bond a gives the formal
C–C insertion product 3aa, whereas cleavage of bond b gives the
formal C–H insertion product 4aa. To gain additional insight into
the chemoselectivity, we performed density functional theory
calculations on cyclopropane intermediate II at the B3LYP/6-31G*
level. (Note that the most stable isomer of this intermediate is
shown in Scheme 6; the energies of the other isomers are
included in the supplementary information.) Compared with
methyl ester III, aryl ester II has a shorter bond b and a slightly
longer bond a. The H3PO4 additive may further increase the length
of bond a of II. It is reasonable to assume that the longer bond,
that is, bond a, is more easily broken, to give formal C–C bond
insertion product 3aa.
In summary, we have developed a protocol for a gold-
catalyzed formal C–C insertion reaction of diazo compounds with
1,3-diketones. Both aryl and alkyl 1,3-diketones smoothly
underwent the reaction, and even unsymmetrical diketones
bearing substituents with different electronic characteristics gave
good yields and excellent selectivities toward the C–C bond
insertion. This protocol provides a new method for constructing
polycarbonyl compounds with an all-carbon quaternary center,
which have a variety of potential applications. A reaction
mechanism involving cyclopropanation of a gold carbene with an
enolate and ring-opening of the resulting donor–acceptor-type
cyclopropane was proposed. The aryl ester moiety and the acidic
additive control the ring-opening direction so that the C–C bond
insertion product is formed. This mechanism differs from that of
the traditional Lewis-acid-catalyzed C–C bond insertion reaction
of diazo compounds with monocarbonyl compounds, which
involves a rearrangement of a zwitterion intermediate as a key
step.
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Experimental Section
The (2,4-tBu2C6H3O)3PAuNTf2 (4.5 mg, 0.004 mmol, 2 mol%) and 2a
(224.3 mg, 1.0 mmol) were introduced into an oven-dried Schlenk tube in
an argon-filled glovebox. After H3PO4 (0.4 mg, 0.004 mmol, 2 mol%) in 0.2
mL CH2Cl2 was injected into the Schlenk tube, the mixture was stirred at
25 °C. A solution of 1a (61.4 mg, 0.2 mmol) in 0.8 mL CH2Cl2 was
introduced into the mixture in one portion. The reaction accomplished in 1
hour. Then the reaction mixture was concentrated and purified by flash
chromatography on silica gel (petroleum ether/ethyl acetate = 10:1, v/v) to
give 3aa as a white solid.
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Yuan-Yuan Ren, Mo Chen, Ke Li, and
Shou-Fei Zhu*
Gold-Catalyzed Formal C–C Bond
Insertion of α-Aryl Diazoesters with
1,3-Diketones
Text for Table of Contents
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