Catalyst-Dependent Chemoselective Formal Insertion of Diazo Compounds into C?C or C?H Bonds of 1,3-Dicarbonyl Compounds

Article abstract of DOI:10.1002/anie.201802834

A catalyst-dependent chemoselective one-carbon insertion of diazo compounds into the C?C or C?H bonds of 1,3-dicarbonyl species is reported. In the presence of silver(I) triflate, diazo insertion into the C(=O)?C bond of the 1,3-dicarbonyl substrate leads

Full text of DOI:10.1002/anie.201802834

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International Edition: DOI: 10.1002/anie.201802834
German Edition: DOI: 10.1002/ange.201802834
Silver Catalysis
Catalyst-Dependent Chemoselective Formal Insertion of Diazo
À
À
Compounds into C C or C H Bonds of 1,3-Dicarbonyl Compounds
Zhaohong Liu, Paramasivam Sivaguru, Giuseppe Zanoni, Edward A. Anderson, and Xihe Bi*
Abstract: A catalyst-dependent chemoselective one-carbon
described a formal insertion of rhodium carbenoids derived
À
À
À
insertion of diazo compounds into the C C or C H bonds of
1,3-dicarbonyl species is reported. In the presence of silver(I)
from diazo compounds into the C C bonds of strained
benzocyclobutenols to form quaternary carbon centers
=
À
À
triflate, diazo insertion into the C( O) C bond of the 1,3-
dicarbonyl substrate leads to a 1,4-dicarbonyl product con-
taining an all-carbon a-quaternary center. This reaction
constitutes the first example of an insertion of diazo-derived
through sequential C C bond cleavage, carbene migratory
insertion, and intramolecular aldol reaction.[8]
We targeted a new method to achieve the insertion of
diazo-derived carbenoids into acyclic C C bonds.[9] Based on
À
À
carbenoids into acyclic C C bonds. When instead scandium-
(III) triflate was applied as the catalyst, the reaction pathway
switched to formal C H insertion, affording 2-alkylated 1,3-
dicarbonyl products. Different reaction pathways are proposed
to account for this powerful catalyst-dependent chemoselectiv-
ity.
achievements in the silver-catalyzed activation of diazo
compounds by our group[10] and others,[11] we questioned
whether inexpensive silver catalysts could be used to mediate
À
À
C C insertion. Although coinage metals have been exten-
[12]
2
[13]
À
À
sively exploited in the alkylation of C(sp) H, C(sp ) H,
and C(sp ) H bonds with diazo compounds[15] (e.g., Shiꢀs
3
[14]
À
report on an alkylation reaction; Figure 1a),[16a] equivalent
À
T
he construction of all-carbon quaternary centers by one-
C C insertions are unknown. Herein, we describe the
À
carbon insertions into C C bonds is a formidable challenge,
realization of this process, which offers a new strategy to
install all-carbon quaternary centers in acyclic systems.[3c] In
addition, the reaction pathway can be altered so that either
not only because of the difficulty of achieving the selective
cleavage of inert C C bonds, but also because of the
[1]
À
À
À
subsequent formation of a congested quaternary carbon
center.[2] Progress in this field has been especially limited
with acyclic systems,[3] where insertions of diazo compounds
into carbonyl compounds represent one approach to con-
struct quaternary carbon centers.[4] In such processes, the
diazo compound invariably acts as an ambiphilic species,
undergoing a sequential nucleophilic addition/1,2-rearrange-
ment cascade.[5] Whereas diazo compounds have also been
widely explored as a source of carbenoids in transition-metal
C C or C H insertion occurs depending on the choice of
catalyst (Figure 1b).[17]
catalysis,[6] the insertion of diazo-derived metal carbenoids
into acyclic C C bonds is unknown. To the best of our
knowledge, only one report by Wang and co-workers has
[7]
À
Figure 1. Insertions of diazo compounds into 1,3-dicarbonyl species.
[*] Z. Liu, Dr. P. Sivaguru, Prof. X. Bi
Department of Chemistry
Northeast Normal University
Changchun 130024 (China)
E-mail: bixh507@nenu.edu.cn
Homepage: http://www.bigroup.top/
Initial studies revealed that the use of silver(I) triflate
(10 mol%) as the catalyst in CH2Cl2 at 408C exclusively
À
afforded the C C insertion product 3a in 89% yield
(Figure 2).[18] When we performed the reaction in the
Prof. X. Bi
State Key Laboratory of Elemento-Organic Chemistry
Nankai University, Tianjin 300071 (China)
Prof. E. A. Anderson
Chemistry Research Laboratory
University of Oxford
12 Mansfield Road, Oxford, OX1 3TA (UK)
Prof. G. Zanoni
Department of Chemistry
University of Pavia
Viale Taramelli 12, 27100 Pavia (Italy)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201802834.
Figure 2. Initial results on the gold- and silver-catalyzed insertion of
diazo compounds into 1,3-dicarbonyl species.
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presence of a gold catalyst,[16a] 3a and 4a were obtained in
50% and 28% yield, respectively. While the reason for the
generation of this mixture is unclear, it highlights the
connectivity of 3r was confirmed by single-crystal X-ray
analysis.[29] The present method was not restricted to
a-diazoesters, with diazophosphonates 1s and 1t also reacting
efficiently to afford products 3s and 3t, featuring a phospho-
rus-substituted a-quaternary center.[20] Relatively non-stabi-
lized diazo compounds, such as (1-diazo-2,2,2-trifluoroethyl)-
benzene (1u), the sterically hindered substrate diphenyldia-
zomethane (1v), and tricyclic 9-diazo-9H-fluorene (1w), were
also compatible with the reaction conditions, delivering 3u–
3w in 45–77% yield. A variety of symmetric 1,3-dicarbonyl
substrates could be employed, including heteroaryl diketones,
affording the expected 1,4-dicarbonyls 3aa–3af in good yields
upon reaction with 1e. With an unsymmetric diaryl ketone,
a 2:1 mixture of the isomers 3ag and 3ag’ was obtained in
82% yield, illustrating the moderate electronic influence over
=
À
challenge of achieving selective C( O) C insertion over
competing C H and O H[19] insertions; complete selectiv-
ity for C C insertion of diazo-derived carbenoids into 1,3-
dicarbonyl species, as observed with AgOTf as the catalyst,
[16]
À
À
À
has not been reported previously.
=
À
The scope of this silver-catalyzed chemoselective C( O)
C insertion reaction was investigated next (Scheme 1). A
variety of aryl diazoacetates, bearing either electron-poor
(with F, Cl, Br, CO2Me, or CF3 substituents) or electron-rich
(OMe, CH3, t-Bu) arenes, were successfully converted into the
À
desired C C insertion products 3a–3k in good to excellent
yields. Aside from methyl and ethyl esters, isopropyl (1l),
benzyl (1m), allyl (1n), and 3-hexenyl (1o) phenyldiazoace-
tates were also suitable substrates, affording the correspond-
ing products 3l–3o in 75–96% yield. 2-Naphthyl- (1p),
2-thienyl- (1q), and 3-indolyl-substituted (1r) diazoacetates
also proved to be suitable reaction partners, and the
À
the position of formal C C insertion, while phenyl methyl 1,3-
diketone delivered 3ah exclusively in 58% yield as the
=
À
reaction occurred selectively at the C( O) C bond next to
the alkyl ketone. These two observations provided early
insight into factors affecting the mechanism of the insertion
process.
Achieving catalyst control over the site of reactions such
as insertion processes is an attractive goal.[17] To our delight,
=
À
À
we found that a switch from C( O) C insertion to C H
insertion into the 1,3-dicarbonyl species could be accom-
plished by employing Sc(OTf)3 as the catalyst instead of
À
AgOTf; at 808C, complete selectivity for the C H insertion
was observed for substrate 1a and diketone 2a (to give 4a in
80% yield; Scheme 2).[18,21] The scope of this complementary
procedure proved to be equally as broad as that of the silver-
catalyzed process, with the reactions of various ethyl aryl
diazoacetates, bearing either electron-withdrawing or elec-
À
tron-donating groups, affording the C H insertion products
4b–4j in good to excellent yields. The structure of compound
4c was unambiguously assigned by X-ray analysis.[29] Isopro-
pyl, benzyl, and allyl phenyldiazoacetate were also productive
substrates, giving the expected products 4k–4m in good
yields. 2-Naphthyl ethyl diazoacetate and cyclic 4-diazoiso-
À
chroman-3-one also smoothly underwent selective C H
insertion to furnish 4n and 4o in 80% and 65% yield,
respectively. A diazophosphonate again proved to be a suit-
able substrate, affording 1,3-diketone 4p in 52% yield.
Excitingly, non-stabilized diazo compounds such as diphenyl,
aryl, and alkenyl diazomethanes were accommodated, giving
the products 4r–4v in 48–76% yield. Variation of the 1,3-
dicarbonyl compound in this scandium-catalyzed insertion
was evaluated with ethyl phenyldiazoacetate and ethyl
4-methoxyphenyldiazoacetate. Pleasingly, the reaction pro-
ceeded smoothly irrespective of the electronic character of
the diaryl dicarbonyl compound: Electron-donating and
-withdrawing groups (at various positions of the aryl rings)
À
and a dithiophene substrate all afforded the C H insertion
products in excellent yields (4aa–4ag). An alkyl 1,3-diketone
and an aryl b-ketoester were also tested as substrates, and
afforded the expected products 4ah and 4ai in 68% and 42%
yield, respectively.
The reaction of methyl phenyldiazoacetate and 1,3-cyclo-
hexanedione using either AgOTf or Sc(OTf)3 as the catalyst
offered insight into the mechanisms of these different
=
À
Scheme 1. Scope of the C( O) C insertion. Reaction conditions:
1 (0.3 mmol) in CH2Cl2 (5.0 mL) was added dropwise (1 h) to a mixture
of AgOTf (10 mol%) and 2 (0.45 mmol) in CH2Cl2 (1.0 mL) at 408C.
Yields of isolated products are given.
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=
À
À
the C( O) C and C H insertion reactions of acyclic 1,3-
dicarbonyl substrates.
On the basis of these results and related precedents,[7,23]
a plausible mechanism for the chemoselective one-carbon
insertion of diazo compounds into acyclic 1,3-dicarbonyl
species is proposed (Scheme 4). In the case of the silver-
À
Scheme 4. Proposed reaction mechanisms for the silver-catalyzed C C
À
and the scandium-catalyzed C H insertions.
=
À
catalyzed C( O) C insertion, 1,3-diketone 2 is first converted
by AgOTf into silver enolate A, which engages with diazo
compound 1 to give the electrophilic silver carbenoid B
following loss of molecular nitrogen. The cyclopropanation of
B leads to intermediate C,[24] which then undergoes a retro-
aldol fragmentation to afford intermediate D.[25] Upon pro-
tonation by HOTf, D releases the 1,4-diketone product 3, with
concomitant regeneration of the silver catalyst. In contrast to
existing catalytic methods for the insertion of nucleophilic
diazo compounds into ketones,[4] the transient formation of an
electrophilic silver carbenoid (B) and the subsequent cyclo-
propanation would represent a novel mode of carbenoid
reactivity. Notably, the formation of product 3ah (Scheme 1)
would be consistent with such a mechanism, where cyclo-
propanation occurs at the double bond of the predominant
enol form (i.e., that of the methyl ketone), rather than
a pathway that relies on migrating group ability, in which
migration of the C2 carbon atom would likely be disfavored.
À
Scheme 2. Scope of the C H insertion. Reaction conditions:
1 (0.6 mmol) in DCE (5.0 mL) was added dropwise (1 h) to a mixture
of 4 ꢀ M.S. (40 mg), Sc(OTf)3 (30 mol%), and 2 (0.3 mmol) in DCE
(1.0 mL) at 808C. Yields of isolated products are given. DCE=1,2-
dichloroethane, M.S.=molecular sieves.
insertion processes (Scheme 3). To our surprise, both catalysts
exclusively furnished the O H insertion product 5c in 80%
and 54% yield, respectively, without formation of the
anticipated C( O) C or C H insertion products 5a and 5b.
We considered that 5c might arise from coordination of the
enol form of the cyclic 1,3-diketone to the metal ion (or to
À
=
À
À
À
À
a metal carbenoid), subsequently generating the formal O H
In the complementary scandium-catalyzed C H insertion,
insertion product. 1,3-Cyclohexanedione differs from the
previous substrates in only being able to form a trans enol,[22]
which may suggest that the formation of a cis enol, potentially
as a bidentate complex with the metal, is critical in mediating
protonation of the diazo compound 1 by HOTf could yield the
diazonium ion B’, which displays carbocation-like reactiv-
ity,[23] undergoing facile nucleophilic attack by the scandium
À
enolate A’ to give complex C’. The C H insertion product 4 is
then released, with regeneration of Sc(OTf)3. In the absence
of this chelate A’ (Scheme 4), direct reaction on the more
reactive enolate oxygen atom may instead by possible (to give
5c). The divergent reactivity of the diazo compounds under
different modes of catalysis is therefore rationalized by the
putative formation of an electrophilic silver carbenoid species
in the presence of AgOTf, whereas in the presence of
Sc(OTf)3, the diazo compound behaves as a carbocation
source.
Scheme 3. Control experiments.
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To test the robustness of this chemoselective one-carbon
C C insertion, a multigram-scale experiment was performed
(Scheme 5). When 10 mmol of 1e and 2a were subjected to
Acknowledgements
À
X.B. and E.A.A. thank the NSFC (21522202, 21502017) and
the EPSRC (EP/M019195/1), respectively, for support.
Conflict of interest
The authors declare no conflict of interest.
À
Keywords: 1,3-dicarbonyl compounds · C C insertion ·
À
C H insertion · diazo compounds · silver catalysis
À
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À
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À
effecting a formal C H insertion and leading to products of
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À
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Manuscript received: March 7, 2018
Version of record online: && &&, &&&&
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Communications
Silver Catalysis
Z. Liu, P. Sivaguru, G. Zanoni,
E. A. Anderson, X. Bi*
&&&& — &&&&
Catalyst-Dependent Chemoselective
Formal Insertion of Diazo Compounds
À
À
=
À
into C C or C H Bonds of 1,3-Dicarbonyl
Compounds
Catalyst control: The catalyst-dependent
chemoselective insertion of diazo com-
pounds into 1,3-dicarbonyl species is
reported. In the presence of AgOTf, the
diazo compounds selectively insert into
the C( O) C bond, affording 1,4-dicar-
bonyl products with an all-carbon a-
quaternary center. In contrast, in the
À
presence of Sc(OTf)3, C H insertion
results in a-C H alkylation products.
À
6
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