Rhodium(II)- or Copper(I)-Catalyzed Formal Intramolecular Carbene Insertion into Vinylic C(sp2)?H Bonds: Access to Substituted 1H-Indenes

Article abstract of DOI:10.1002/anie.201709375

A rhodium(II)- or copper(I)-catalyzed formal intramolecular carbene insertion into vinylic C(sp²)?H bonds is reported herein. This method provides straightforward access to 1H-indenes with high efficiency and excellent functional-group compatibility. Mechanistically, the reaction is proposed to involve the following sequence: metal carbene formation, intramolecular nucleophilic addition of the double bond to the electron-deficient carbene carbon atom, dearomatization, and finally a 1,5-H shift.

Full text of DOI:10.1002/anie.201709375

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Rh(II)- or Cu(I)-Catalyzed Formal Intramolecular Carbene
Insertion into Vinylic C(sp2)-H Bond: An Access toward
Substituted 1H Indenes
Authors: Jianbo Wang, Qi Zhou, Shichao Li, and Yan Zhang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201709375
Angew. Chem. 10.1002/ange.201709375
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10.1002/anie.201709375
Angewandte Chemie International Edition
DOI: 10.1002/anie.200((will be filled in by the editorial staff))
Carbene Insertion
RhII- or CuI-Catalyzed Formal Intramolecular Carbene Insertion
into Vinylic C(sp2)-H Bond: An Access toward Substituted 1H
Indenes**
Qi Zhou, Shichao Li, Yan Zhang, Jianbo Wang*
Abstract: A RhII or CuI-catalyzed formal intramolecular carbene
insertion into vinylic C(sp2)-H bonds is reported herein. This
method provides straightforward access to 1H-indenes with high
efficiency and excellent functional group compatibility.
Mechanistically, the reaction is proposed to involve the following
sequence: metal carbene formation, intramolecular nucleophilic
addition of the double bond to electron-deficient carbenic carbon,
dearomatization and finally 1,5-H shift.
Transition-metal-catalyzed carbene insertion into C-H bonds
represents one of the characteristic transformations of metal
carbene species.[1] The previous studies for carbene insertion into
Scheme 1. Formal RhII carbene insertion into C(sp2)-H bond.
C-H bonds have been focused in aliphatic C(sp3)-H bonds. For
example, Davies and co-workers have demonstrated remarkable
reported the CoII-catalyzed formal CoIII-carbene insertion into
regio- and stereoselectivities for intermolecular C(sp3)-H bond
vinyl C-H bond for the synthesis of substituted 1H-indenes.[9]
insertions with donor-acceptor carbene precursors and chiral
The reaction is proposed to proceed through a radical process.
Rh(II) catalysts.[2] With RuII-porphyrin complexes as the
Interestingly, in their work Rh2(OAc)4 failed to catalyse the
catalysts, Che and co-workers have demonstrated the selective
reaction, while the CuI-catalyzed reaction gave only poor results.
carbene insertion into primary C-H bonds of alkanes.[3] Pérez and
Herein we demonstrate the highly efficient RhII or CuI-catalyzed
co-workers reported a Ag-catalyzed carbene insertion into
intramolecular carbene insertions into vinylic C-H bond for the
methane C-H bond in supercritical CO2.[4] Transition-metal-
synthesis of substituted 1H indenes. Mechanistic studies indicate
catalyzed carbene insertion into C(sp3)-H bonds is considered as
the reaction follows a stepwise pathway involving carbon cation
an unique method for inert C(sp3)-H bond functionalization,
intermediate, which is distinct from the radical process in de
which has attracted significant attentions in recent years.
Bruin’s CoIII carbene radical system.
In addition to C(sp3)-H bond insertions, carbene insertion into
Since N-tosylhydrazones have been extensively explored as
aromatic C(sp2)-H bonds are also well-documented.[5]
Mechanistically, aromatic C(sp2)-H bond insertion is different
the precursors for in situ generation of non-stabilized diazo
substrates,[10] we began our study with o-vinyl N-tosylhydrazone
1a as the model substrate. When Rh2(OAc)4 was used as the
from the corresponding C(sp3)-H bond insertion: the former
follows stepwise electrophilic substitution mechanism, while the
latter usually proceeds through concerted insertion mechanism.
For electron-deficient aromatic C-H bonds, formal C-H insertion
may also follows a mechanism involving C-H bond metalation
and carbene migratory insertion.[6] Regardless of the differences
in reaction pathways, the carbene aromatic C(sp2)-H bond
insertions are well established as unique approaches for direct
aromatic C-H bond functionalizations.[7] In the contrast, the
insertion into vinylic C-H bonds by carbene remains much less
developed, presumably attributed to the competing
cyclopropanation of alkene as an uncontainable side reaction.[8]
Such side reaction may be minimized in intramolecular
reactions. In this context, de Bruin and co-workers have recently
o
catalyst in dichloroethane (DCE) at 100 C, the expected vinylic
C-H bond insertion product 2a could be obtained in 38% isolated
yield (Table 1, entry 1). Further experiments show that toluene is
the optimal solvent. Carrying out the reaction at lower
temperature led to decrease of the yields (Table 1, entries 4-6).
The effect of bases was then examined and LiOtBu was found to
afford optimal results (Table 1, entries 8-10).
Subsequently, a series of metal catalysts were screened with
slightly decreased loading of LiOtBu (Table 1, entries 11-17).
The highest yield was obtained by using Rh2(Oct)4 as the catalyst
(Table 1, entry 12). It is noteworthy that CuI as a cheap catalyst
could also exhibit good activity (Table 1, entry 16). Finally,
control experiment suggested that in the absence of transition-
metal catalyst, the reaction under otherwise identical conditions
failed to give the product 2a (Table 1, entry 18).
Additionally, it was noted that a regio-isomer of 2a was
formed in the reaction and the ratio was found to be related to the
reaction time. The substrate 1a could be transformed into a
mixture of 2a and 2a' in a 5.9:1 ratio with a NMR yield of 84%
in 5 min. Prolonging the reaction time leads to a higher ratio of
2a to 2a' with a little change of the yield, which suggests that 2a'
could be slowly transformed into 2a under the reaction
conditions.
[] Q. Zhou, S. Li, Y. Zhang, Prof. Dr. J. Wang
Beijing National Laboratory of Molecular Sciences (BNLMS)
and Key Laboratory of Bioorganic Chemistry and Molecular
Engineering of Ministry of Education, College of Chemistry,
Peking University, Beijing 100871 (China); Email:
wangjb@pku.edu.cn
Prof. Dr. J. Wang
The State Key Laboratory of Organometallic Chemistry,
Shanghai Institute of Organic Chemistry, Chinese Academy
of Sciences, Shanghai 200032 (China)
Supporting information for this article is available on the
WWW under http://www.angewandte.org or from the author.
1
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10.1002/anie.201709375
Angewandte Chemie International Edition
Table 1: Optimization of the reaction conditions.[a]
the substrate 1a. In this experiment, the loading of catalyst
Rh2(Oct)4 could be reduced to 0.5 mol% and the indene product
yield
(%)[b]
entry
cat.(mol%)
base(eq)
solvent
T (oC)
1[c]
2[c]
3[c]
4
Rh2(OAc)4(1)
Rh2(OAc)4(1)
Rh2(OAc)4(1)
Rh2(OAc)4(1)
Rh2(OAc)4(1)
Rh2(OAc)4(1)
Rh2(OAc)4(1)
Rh2(OAc)4(1)
Rh2(OAc)4(1)
Rh2(OAc)4(1)
Rh2(TFA)4(1)[d]
Rh2(Oct)4(1)[e]
Cu(OTf)2(5)
Pd(OAc)2(5)
Co(acac)2(5)
CuI(5)
LiOtBu(1.2 eq)
LiOtBu(1.2 eq)
LiOtBu(1.2 eq)
LiOtBu(1.1 eq)
LiOtBu(1.1 eq)
LiOtBu(1.1 eq)
LiOtBu(1.1 eq)
NaH(1.1 eq)
DCE
100
100
38
74
80
dioxane
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
100
70
30
49
78
78
45
36
28
24
88
15
15
0
80
5
90
6
110
100
100
100
100
100
100
100
100
100
100
100
7
8
NaOMe(1.1 eq)
Cs2CO3(1.1 eq)
LiOtBu(1.05 eq)
LiOtBu(1.05 eq)
LiOtBu(1.05 eq)
LiOtBu(1.05 eq)
LiOtBu(1.05 eq)
LiOtBu(1.05 eq)
LiOtBu(1.05 eq)
LiOtBu(1.05 eq)
9
10
11
12
13[c]
14[c]
15[c]
16[c]
17[c]
18[c]
81
trace
0
AuCl3(5)
none
[a] If not otherwise noted, the reaction was carried out with 1a (0.2 mmol) in
toluene (2 mL). [b] The yields refer to the isolated products. [c] The reaction
was carried out with 1a (0.1 mmol) in solvent (1 mL). [d] TFA = CF3CO2. [e]
Oct = CH3(CH2)6CO2.
With the optimized reaction conditions in hand, we then
started to investigate the substrate scope of the reaction. The
substrates with electron-withdrawing groups in the para-position
of the aromatic ring (Ar) all afforded the corresponding products
in good to excellent yields (Scheme 2, 2b-e). When the para-
position was substituted by a methoxy group, the reaction led to a
mixture of 2f and its isomer 2f' in 0.5 h, while almost a single
product 2f could be obtained by extending the reaction time to 12
h. The substrates with meta-substituents offered the
corresponding products in good to excellent yields (Scheme 2,
2g-i). The substrate 1j produced a mixture of 2j and 2j' with the
ratio varying from 4.3:1 to 5.9:1, depending on the reaction time
(Scheme 2, 2j).
We then investigated the influence of aromatic substituent of
the vinylic double bond (R = Ar). The substrates bearing para,
meta, ortho-substituents on the phenyl ring could all afford the
corresponding products in good yields (Scheme 2, 2k-r). The
substrates bearing naphthyl and thienyl substituent could also
give good yields of the corresponding products (Scheme 2, 2s, t).
It was noted for some substrates, long reaction time (12 h) was
required in order to obtain the products in high isomeric ratio.
When the R substituent of the vinylic double bond is a hexyl
group, the isomeric products 2u and 2u' were formed in a ratio of
1.4:1. In the case where R is H, only single indene product 2w
could be formed. Finally, in the cases where both R and R' are H,
the corresponding isomeric indene products were obtained in 1:1
ratio in each case (Scheme 2, 2x, 2y). When neither R nor R' is H,
the corresponding products were obtained in good yields
(Scheme 2, 2z, 2za).
Scheme 2. Reaction scope of the N-tosylhydrazones. If not otherwise noted,
the reaction conditions are as following: N-tosylhydrazone 1a-y (0.2 mmol),
Rh2(Oct)4 (1 mol%), LiOtBu (1.05 equiv), toluene (2 mL), 100 oC, 0.5 h. All
the yields refer to the isolated indene products. The numbers in the
parentheses refer to the reaction time and the ratio of the isomeric indene
products. [a] N-tosylhydrazone (0.1 mmol), Rh2(Oct)4 (1 mol%), LiOtBu (1.05
equiv), toluene (1 mL), 100 oC.
2a could be obtained in 95% yield under identical reaction
conditions (eq. 1).
To further demonstrate the practical usefulness of this reaction
in synthesis, a scale-up experiment was carried out with 2 mol of
2
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10.1002/anie.201709375
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hygrogen shift can afford both 2a and 2a'. Under prolonged
reaction at 100 C, 2a' isomerizes to thermodynamically more
stable 2a.
To substantiate the proposed reaction mechanism, some
mechanistic experiments were carried out. Firstly, deuterium-
labelling experiments were conducted with the substrates 1a-d
and 1a-d' (eq. 2 and eq. 3). The experimental results showed that
deuterium atoms were nearly all retained in the products. Thus,
the results do not support path b, where there should be H/D
exchange in the deprotonation/protonation process.
As shown in Table 1 (entry 16), the vinylic C-H bond also
occurs smoothly with CuI as the catalyst. Since copper has the
advantage of being abundant and cheap, we then investigated the
substrate scope of this transformation using CuI as the catalyst.
Thus, the CuI-catalysed reaction with a series of substituted N-
tosylhydrazone substrates could lead to the corresponding indene
products in good yields (Scheme 3). However, it was noted that
the reaction with the substrate bearing alkyl substituent in the 2-
position of the vinylic double bond only afforded a low yield of
18% (Scheme 3, 2w), while under RhII-catalyzed condition, the
same substrate 1w afforded the indene product 2w in 66% yield.
It was also noted that the reaction of the N-tosylhydrazone
bearing no substituent in the vinylic double bond gave the
corresponding isomeric indene products 2y and 2y' in moderate
yield with essentially no selectivity (Scheme 3). The reaction
with the substrate which both positions of the vinyl group are
substituted afforded a moderate yield (Scheme 3, 2z).
o
Scheme 4. Proposed reaction pathways.
Scheme 3. The scope of the CuI-catalyzed reaction of N-tosylhydrazones. If
not otherwise noted, the reaction conditions are as following: N-
tosylhydrazone (0.1 mmol), CuI (10 mol%), LiOtBu (1.05 equiv), toluene (1
mL), 100 oC, 12 h. All the yields refer to the isolated indene products.
We have proposed three reaction pathways to account for this
vinylic C(sp2)-H bond insertion, as shown in Scheme 4. In the
presence of base, diazo intermediate A is generated from N-
tosylhadrazone 1a, which undergoes dediazoniation to form RhII
carbene B intermediate. From RhII carbene B there are three
possible pathways. For path a, direct insertion of RhII carbene B
into vinylic C-H bond occurs in a concerted manner through
transition state C. Although RhII carbene insertion into C(sp3)-H
bond are well established as concerted process,11 the
corresponding concerted insertion into vinylic C(sp2)-H bond is
less likely in view of the fact that the analogous aromatic C(sp2)-
H bond insertion is believed to follow stepwise electrophilic
substitution mechanism.[12] Moreover, concerted insertion does
not account for the formation of isomeric product 2a'.
Secondly, the KIE experiments were conducted by using the
substrates 1a and 1a-d (eq. 4). No evident primary KIE was
observed in both intermolecular competition experiments and
parallel experiments. The results are not supportive to path a, in
which concerted C-H bond insertion is expected to show certain
level of primary KIE. The lack of primary KIE also suggests that
C-H bond cleavage is not involved in the rate-limiting step.[13]
For path b and c, the nucleophile attack of the vinylic double
bond to the electron-deficient carbenic carbon center of RhII
carbene A leads to intermediate D. In path b, the intermediate D
releases a proton to form the intermediate E, which undergoes
protonation to afford product 2a. However, neither path b
explains the formation of 2a'. Thus, the plausible pathway is path
c, in which the intermediate D first undergoes the release of RhII
catalyst to give a dearomatized intermediate F. From F, 1,5-
3
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10.1002/anie.201709375
Angewandte Chemie International Edition
Micouin, T. Lecourt, Synlett 2013, 24, 2477; j) J. Egger, E. M.
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Hammett linear free energy correlation study (Scheme 5).
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reaction constant of = 0.47 (r = 0.94). The relatively small
positive reaction constant suggests that for path c, the formation
of intermediate D is not likely to be the rate-limiting step.
Otherwise, a negative reaction constant with large value should
be expected, because in this step a positive charge is developed at
the carbon adjacent to the substituted aromatic ring. The
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In conclusion, we have developed an efficient method for the
synthesis of substituted 1H-indenes via intramolecular RhII- or
CuI-catalyzed carbene insertion into vinylic C(sp2)-H bonds. This
work represents a rare example of a formal carbene insertion into
a vinylic C(sp2)-H bonds.[9] The reaction occurs smoothly with
high yields and good functional group tolerance, thus providing a
straightforward method for the synthesis of 1H-indene
derivatives.[14] Mechanistic study suggests that this formal vinylic
C(sp2)-H insertion is analogous to the corresponding aromatic
C(sp2)-H bond insertion.
Acknowledgements
The project is supported by National Basic Research Program of
China (973 Program, No. 2015CB856600) and NSFC (Grant
21332002).
Conflict of interest
The authors declare no conflict of interest.
[13] Large KIE is expected for thermal [1,5]-H shift, see: Y. Itou, S.
Mori, T. Udagawa, M. Tachikawa, T. Ishimoto, U. Nagashima, J.
Phys. Chem. A 2007, 111, 261.
Keywords: diazo compounds · insertion reaction· metal carbene ·
N-tosylhydrazone · indene
[14] For a recent review of indene synthesis, see: B. Gabriele, R.
Mancuso, L. Veltri, Chem. - Eur. J. 2016, 22, 5056.
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10.1002/anie.201709375
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Carbene Coupling
Qi Zhou, Shichao Li, Yan Zhang, Jianbo
Wang* _____Page – Page
Formal carbene C(sp2)-H bond insertion: The RhII- or CuI-catalyzed formal carbene
insertion into vinylic C-H bond has been developed. The reaction provides a
straightforward method to 1H-indenes with high efficiency and good functional
group compatibility. The reaction is proposed to follow a stepwise mechanism
involving nucleophilic addtion of double bond to metal carbene and
dearomatization/aromatization processes.
5
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