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Accepted Article
Title: Gold(I)-Catalyzed Intramolecular C(sp3)−H Insertion by
Decarbenation of Cycloheptatrienes
Authors: Antonio M. Echavarren, Xiang Yin, Giuseppe Zuccarello, and
Cristina García-Morales
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To be cited as: Chem. Eur. J. 10.1002/chem.201900919
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Gold(I)-Catalyzed Intramolecular C(sp3)H Insertion by
Decarbenation of Cycloheptatrienes

Abstract: A novel synthesis of indanes and dihydronaphtalenes
based on the intramolecular insertion into C(sp3)H bonds of gold(I)
carbenes generated by retro-Buchner reaction (decarbenation) has
been developed. Deuterium-labeling and kinetic isotope effect
experiments, DFT calculations, and generation of the proposed
carbene intermediate from a well-characterized gold(I) carbenoid
support the involvement of a three-centered concerted mechanism for
the C(sp3)H functionalization process.

Introduction
The direct functionalization of aliphatic C(sp3)H bonds by metal
carbene insertion is a powerful strategy for the straightforward
access of complex structures in an atom economical fashion. [1,2]
Despite the low reactivity of C(sp3)H bonds, metal-catalyzed
bond insertion reactions have been described using Cu, Rh, and
Ru carbenes prepared in situ from diazo compounds.[3] In contrast,
the Au-mediated versions are scarce.[3a,4] The group of Pérez
reported the first C(sp3)H insertion of gold(I) carbenes derived
from ethyl diazoacetate to functionalize linear alkanes.[5,6] Later,
the groups of Toste[7a] and Malacria[7b] demonstrated that gold(I)
carbenes generated in enyne cyclizations undergo CH insertions
intramolecularly. The groups of Zhang[ 8 ], Hashmi[ 9 ] and
Malacria[10] also found that gold(I) vinylidene intermediates or oxo gold(I) vinylidenes undergo CH insertion reactions to access
complex
hydrocarbon
skeletons
and
functionalized
cyclopentanones. Other CH functionalizations have been
reported from gold(I) carbenes generated from diazo
compounds[11,12] or by different precursors.[13,14]
We have demonstrated that 7-substituted-1,3,5-heptatrienes
undergo retro-Buchner reaction in a formal decarbenation
process to generate gold(I) carbenes which undergo
cyclopropanation,
cycloaddition,
and
Friedel-Crafts
annulations.[15] Our method is a safer and convenient alternative
to the most common generation of metal carbenes from
diazocompounds,[ 16 ] since cycloheptatriene derivatives are
thermally stable molecules that can be stored indefinitely under
ordinary conditions. Encouraged by our previous findings, we
wondered whether gold(I) carbenes generated by retro-Buchner

[a]

Xiang Yin, Giuseppe Zuccarello, Cristina García-Morales, Prof. Dr.
A. M. Echavarren
Institute of Chemical Research of Catalonia (ICIQ), Barcelona
Institute of Science and Technology (BIST)
Av. Països Catalans 16, 43007 Tarragona (Spain)
E-mail: aechavarren@iciq.es
Departament de Química Orgànica i Analítica
Universitat Rovira I Virgili
C/ Marcel·lí Domingo s/n, 43007 Tarragona (Spain)
Supporting information for this article is given via a link at the end of
the document.

reaction would also undergo C(sp3)H bond insertions. However,
we have previously observed that cycloheptatriene 1 reacts in
presence of a gold(I) catalyst at 120 ºC in 1,2-dichloroethane via
decarbenation through norcaradiene 2 losing benzene to
generate gold(I) carbene 3, which reacts to give biscyclopropane
4 as the major product by the intermolecular cyclopropanation of
2 (Scheme 1a).[15a] In addition, small amounts of indane 5 (8%)
were obtained by intramolecular CH insertion at the benzylic
position. We postulated that the intramolecular CH insertion
could become the major pathway exploiting the Thorpe-Ingold
effect, placing the CH bond closer to the reactive gold(I) carbene
carbon atom. To do so, we envisioned to introduce a quaternary
carbon center adjacent to the C(sp3)H bond in cycloheptatriene
6 (Scheme 1b). This would result in contracting (angle)  in
gold(I) carbene 7 to ensure close proximity of the C(sp3)H bond
to the gold(I) carbene enabling the formation of functionalized
indane derivatives 8.

Scheme 1. Precedent and substrate design for C(sp3)–H bond insertion. C6H6
= benzene.

Here we describe the scope of this C(sp3)H bond
functionalization for the synthesis of a wide diversity of indanes
and provide support of the mechanism by deuterium-labeling
experiments, determination of the kinetic isotope effects, DFT
calculations, as well as by generation of the proposed carbene
intermediate under stoichiometric conditions from a wellcharacterized gold(I) carbenoid.

Results and Discussion
We first studied the reaction of 7-(2(tert-butyl)phenyl)cyclohepta1,3,5-triene (6a) to give indane 8a with different Au(I) complexes
A-G at 120 ºC in 1,2-dichloroethane (Table 1, entries 1-7). Among
the screened catalysts, complex F proved to be the most suitable
for this transformation (Table 1, entry 6). Lowering the reaction
temperature to 100 ºC led to indane 8a in excellent yield (Table 1,

This article is protected by copyright. All rights reserved.

Accepted Manuscript

Xiang Yin,[a] Giuseppe Zuccarello,[a] Cristina García-Morales,[a] and Antonio M. Echavarren*[a]

10.1002/chem.201900919

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Table 1. Synthesis of indane 8a via C(sp3)H bond insertion.[a]
[Au] (5 mol %)
solvent, T, h
- C6H6
6a

8a

entry

[Au]

solvent

T (ºC)

t [h]

8a yield [%][b]

1

A

DCE

120

7/18

51/52

2

B

DCE

120

7/18

55/60

3

C

DCE

120

7/18

18/22

4

D

DCE

120

7/18

15/15

5

E

DCE

120

7

80

6

F

DCE

120

7

86

7

G

DCE

120

7

trace

8

F

DCE

100

7

93 (85)[c]

9

F

DCE

80

7

72

10

F

toluene

100

7

F

EtOAc

100

7

F

1,4-dioxane

100

7

Activated C–H bonds alpha to an oxygen atom also participated
in the retro-Buchner/CH insertion reaction to yield 2-alkylethersubstituted indanes (8k-q) (Scheme 3). The insertion reaction in
compound 6m takes place with complete site selectivity at the C2 position rather than at the benzylic methylene. This result
confirms the preferential formation of 5-membered over larger
sized rings in metal-catalyzed intramolecular CH insertion
reactions.[ 17 ] 2-Silylether-substituted indanes 8n-o were only
obtained in moderate yields presumably due to the concomitant
cleavage of the silicon protecting group. Indeed, when bulky TIPS
and TBS protecting groups were replaced by the more labile TMS
group, no CH bond insertion was observed. Instead, only
deprotected primary alcohol was recovered. Replacing the gemdimethyl moiety by spiro-fused cyclopentane ring resulted in the
formation of 8q/8q’ as a 1.4:1 mixture. Although the -oxygen
methylene CH bonds are still the most reactive, functionalization
of the cyclopentane CH methylene bonds competes leading to
significant amounts of 8q’.

50

12

[a]

44

11

Scheme 2. Synthesis of indanes 8b-j by insertion into C(sp3)H bonds.
Reactions at 120 °C for 12 h.

30

[a] 6a = 0.2 mmol scale. 0.025 M. [b] Yields determined by 1H NMR using
1,3,5-trimethoxybenzene as internal standard. [c] Isolated yields in
parentheses. DCE = 1,2-dichloroethane.

Both para- and meta-substituted cycloheptatrienes afforded
the corresponding indane products (8b-g) in excellent yields
under the optimized reaction conditions (100 ºC, 7 h in 1,2dichloroethane) (Scheme 2). Indane 8h was isolated in slightly
lower yield due to the competing cyclopropanation of a second
molecule of starting material by the gold(I) carbene. [15a] The
formation of indanes 8i and 8j required higher temperatures
(120 ºC) and longer reaction times (12 h). In these cases, the
ortho-substituent or the benzo-fused ring provides enough
steric hindrance to slow down the retro-Buchner process.

Scheme 3. Synthesis of indanes 8k-q by insertion into C(sp3)H bonds with an
-oxygen.

The fine tuning between electronic, steric and conformational
factors for the preferred site and regioselectivity of CH bond
insertion reactions becomes more evident in the reactions of
substrates 6r-x (Scheme 4). The methyl CH and the methylene
C–H bonds compete in the reaction with the gold(I) carbene
leading to indanes 8r-x and 8r’-x’ in excellent 75-86% overall
yields. In the case of R = Me or Bn, although 8r’-s’ were obtained
as the major products, considering statistical factors, insertion into
methylene C–H bonds is 2.7-2:1 faster than that at the methyl
CH bonds. In the reaction of 6t-u bearing esters with electrondonating groups, the preference for methylene CH bond
functionalization is even more pronounced (ca. 3-5 times faster).
On the other hand, esters with electron-withdrawing groups slow
down the CH bond insertion at the methylene leading to the
preferred formation of 8v’-x’.

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entry 8). Changing the solvent to toluene, ethyl acetate or 1,4dioxane led to lower yields of 8a (Table 1, entries 10-12).

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OMe
R

Ar

H

F (5 mol %)
R
DCE, 100 ºC, 0.5-18 h
- C6H6, - MeOH

9a4, 9b-d

R
Ar

Ar

10b-d/10b’-d’

MeO
R

R

MeO
OMe

Ar

OMe

Scheme 4. Insertion into competing methylene and methyl C(sp3)H bonds.

o-Cyclohexylphenyl cycloheptatriene 6y undergoes insertion
preferentially into the axial C–H bond to form cis-fused 8y rather
than trans-fused 8y’ (Scheme 5).

Scheme 5. Insertion into competing axial and equatorial C(sp3)H bonds.

To expand the scope of the CH insertion of gold(I) carbenes, we
attempted the synthesis of 6-membered rings. Thus, we studied
the reaction of substrates of type 9 bearing a benzylic CH bond
in which further activation was provided by an -OR group (Table
2). Substrates 9a1-a2 with hydroxy or acetate groups failed to give
any product of CH functionalization (Table 2, entries 1-2).
However, substrate 9a3 with a TBS-protected secondary alcohol
afforded tetrahydronaphthalene 10a in 30% yield by CH
insertion at the benzylic position, followed by ROH elimination
(Table 2, entry 3). Methyl ether 9a4 proved to be a better substrate,
leading to 10a in 71% of yield (Table 2, entry 4).
Table 2. Synthesis of dihydronaphtahlene 10a via C(sp3)H bond insertion.
LG
MeO
MeO
Ar
A (5 mol %)
H
MeO

DCE, 100 ºC
- C6H6, - LGH

OMe

MeO

9a1-a4: Ar = pMeOC6H4

Scheme 6. Gold(I)-catalyzed synthesis of dihydronaphtalenes by CH insertion
reaction and elimination of methanol.

To shed light into the mechanism of the gold(I)-catalyzed retroBuchner/C–H insertion reaction we performed deuterium labelling
experiments and measured the kinetic isotope effects (KIE). [18]
Firstly, double deuterated cycloheptatriene 6k-d2 was subjected
to the gold(I)-catalyzed C(sp3)H functionalization reaction
conditions (Scheme 7a). Both deuterium labels were incorporated
in the final indane product 8k-d2 according to an intramolecular
reaction pathway. The KIE of the CH insertion step was
determined on monodeuterated cycloheptatriene 6k-d1 (Scheme
7b). Under identical reaction conditions, compound 6k-d1 gave a
3.4:1:1 mixture of indanes 8ka-d1, 8kb-d1 and 8kc-d1 which
corresponds to an overall KIE value of 1.7:1, which is in
agreement with the concerted three-centered mechanism for
metal-carbene C–H insertion reactions that usually shows KIE in
the order of 1-2.5.[19]
a

OMe
D

LG

t [h]

10a yield [%][a]

1

9a1

OH

18

9a2

OAc

18

9a3

OTBS

18

DCE, 100 ºC, 7 h
- C6H6

9a4

OMe

2.5

71

8ka-d1
H
OMe

+

6k-d1

H D
8kb-d1

30

4

F (5 mol %)

n.d.

3

D

H
OMe

D +
OMe

OMe

n.d.

2

D

b

10a

9

D
OMe

8k-d2 ( 90%, 1:1 dr)

6k-d2

OMe

entry

F (5 mol %)

D

DCE, 100 ºC, 7 h
- C6H6

H

OMe

Ar

10b/b’: R = H, Ar = C6H5 (73%, 8:1)
10c/c’: R = H, Ar = pClC6H4 (53%, 13:1)
10d/d’: R = CF3, Ar = C6H5, (61%, 8:1)

[a] Yields determined by

1

H NMR using 3,5-dimethylpyrazol as internal

standard. LG = leaving group.

Further optimization of the reaction conditions revealed that gold
complex F was the best catalyst allowing the isolation of
dihydronaphtalene 10a in 89% yield as a single double bond
isomer (Scheme 6). Products 10b-d were obtained in good yields
(53-73%) together with small amounts of their double bond
isomers 10b’-d’.

H D
8kc-d1
8ka-d1 + 8kb-d1 + 8kc-d1 (82%, 3.4:1:1)
KIE = 1.7 : 1

Scheme 7. a) Deuterium labelling and b) KIE experiments on the CH insertion
of 6k.

Recently, we have described the generation and characterization
of aryl gold(I) carbenes from their corresponding gold(I)
carbenoids.[ 20 ] These reactive carbenes were found to be the
genuine intermediates in the cyclopropanation of olefins by
gold(I)-catalyzed
retro-Buchner/cyclopropanation
reaction.
Hence, we synthesized gold(I) carbenoid complex 11 bearing a
di-tert-butylphenyl group,[ 21 ] which was subjected to chloride
abstraction with GaCl3 at -90 ºC (Scheme 8). The CH insertion
took place immediately upon activation of gold(I) carbenoid 11
with GaCl3 at -90 ºC yielding cleanly indane 8f. Gold(I) carbene
12 could not be detected by NMR because of the very fast
intramolecular C–H insertion, although it was detected by ESI-MS
of gold(I) carbenoid 11.

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10a (89%)

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tBu

tBu
Au

GaCl3

H
Cl

tBu
tBu P

CD2Cl2
-90 ºC

Au
tBu

Scheme 10. Calculated reaction profiles for the competitive CH insertions into
methyl (blue pathway, leading of product 8’) and methylene (red pathway,
leading to products 8).

H
tBu

tBu
8f (95%)[a]

12
detected by ESI in gas phase

11

Scheme 8. Activation of gold(I) carbenoid 11 and formation of indane 8f.
NMR yield calculated with diphenylmethane as internal standard.

[a] 1

H

The insertion of gold(I) carbenes into C(sp3)H bonds was further
investigated theoretically by DFT calculations at M06, 6-31G(d)
(C, H, N, O) and SDD (Au) level of theory, representing 1,2dichloroethane solvent with PCM and starting from optimized
gold(I) carbenes.[15c] First, we studied the selective formation of
indane 8k from 6k (see Scheme 3). After the retro-Buchner
reaction, highly reactive gold(I) carbene I could insert
intramolecularly into the CH bonds adjacent to oxygen (Scheme
9, red pathway) and/or CH bonds of the methyl groups (Scheme
9, blue pathway) through three-centered transition states TSI-II or
TSI-III, respectively.[22,23] Due to the donation of electron density
from OMe to the electron-deficient carbon center in the transition
state, the calculated free energy of activation to reach TSI-II (G‡
= 1.2 kcal/mol) was found to be much lower than that for TSI-III
(G‡ = 9.4 kcal/mol). These results are consistent with the
experimental selective formation of indane 8k from 6k by carbene
CH insertion. In addition, structures similar to intermediate II with
gold(I) bound to the aromatic ring in a 1-fashion, which was
found to be the minimum (-48.8 kcal/mol) for the red pathway or
in the methylene C–H insertion, were previously isolated and
confirmed by single crystal diffraction by our group.[24]

Next, we studied the C–H insertion reaction with substrates 6r-t
and 6v containing electron withdrawing ester substituents, which
experimentally delivered the corresponding indanes as a mixture
of 8r-t, 8v by carbene insertion into the methylene C(sp3)H
adjacent to the ester group, and products 8r’-t’, 8v’ by carbene
insertion into the C–H of the methyl groups (see Scheme 4).
Gold(I) carbenes IV and V are rotamers which can easily
interconvert into each other and respectively form (1-indane)gold(I) intermediate VI through TSIV-VI (blue pathway) leading to
products 8’ or VII via TSV-VII (red pathway) to give products 8
(Scheme 10).
The nature of substituent R has a direct effect on the relative rates
of the CH insertion. For the acetate (R = Me), insertion into
methyl CH bond is slightly favored (G‡IV-VI = 10.6 kcal/mol vs
G‡V-VII = 11.3 kcal/mol) (Table 3, entry 1), whereas for the
benzoate, nearly identical free energies were found supporting a
less selective formation of mixture 8s/8s’ (Table 3, entry 2). As
expected, the electron donating p-MeOC6H4 substituent has a
stabilizing effect on TSV-VII favoring insertion into the methylene
C(sp3)H bond (Table 3, entry 3). In contrast, the electron
withdrawing p-NO2C6H4 substituent clearly inverts the selectivity
(G‡IV-VI ´< G‡V-VII) (Table 3, entry 4). These theoretical values
are in qualitative agreement with the experimentally observed
ratios of the formation of products 8 and 8’ (Table 3, columns 5
and 6).
Table 3. Energies for the competitive CH insertions into methyl (blue
pathway, product VI ≈ 8’) and methylene (red pathway, product VII ≈ 8).[a]
entry

R

G‡V-VII
(8)

G‡IV-VI
(8’)

Cal. 8/8’[b]

Exp. 8/8’[c]

1

Me

11.3

10.6

1:7.8

1:1.5

2

Ph

10.4

10.6

1:2.3

1:1.2

3

p-MeOC6H4

9.8

11.0

1.6:1

1.7:1

4

p-NO2C6H4

11.4

10.6

1:8.7

1:5

[a]

Free energy barriers in kcal/mol. [b] Product ratio calculated at 100 ºC.
Product ratio obtained experimentally at 100 ºC.

[c]

Conclusions
TSI-III

TSI-II

Scheme 9. Calculated (DFT) reaction profile for the CH insertion. L = IPr. Free
energies in kcal/mol.
O
R

O

O

O
H
H
AuL+

H
AuL+

R

V (1.8-3.6)

IV (0.0)
TSIV-VI

TSV-VII

O
O

O

R

R
O

LAu+

VI

8’

LAu+

VII

8

In summary, gold(I) carbenes generated by decarbenation of 7aryl-1,3,5-heptatrienes via retro-Buchner reaction undergo
C(sp3)H bond insertion reactions to give access to highly
functionalized indanes and dihydronapthalenes. Deuterium
labelling experiments and DFT calculations support a threecenter transition state and the origin of insertion selectivity. In
accordance with previous reports, electron-donating substituents
adjacent to the CH bond facilitate the insertion reaction. The
CH bond insertion has also been observed in a reaction of a
gold(I) carbene generated from a well-characterized gold(I)
carbenoid, which further supports the conclusion that the species
involved in gold(I)-catalyzed retro-Buchner reactions belong to
family of electrophilic metal carbenes.

Acknowledgements

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tBu
tBu P

10.1002/chem.201900919

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Keywords: gold catalysis • C–H insertion • carbenes •
decarbenation • indane

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This article is protected by copyright. All rights reserved.

Accepted Manuscript

We thank the Agencia Estatal de Investigación (AEI)/FEDER, UE
(CTQ2016-75960-P and FPI predoctoral fellowship to C. G.-M.),
the AGAUR (2017 SGR 1257), and CERCA Program/Generalitat
de Catalunya for financial support.

10.1002/chem.201900919

Chemistry - A European Journal

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R2
[LAuL´]+
- C6H6

Indanes
R2
R1

AuL+

detected in gas phase

C–insertion R1
H

R2
22 examples
44%–90%

Dihydronaphtalenes
KIE
Detection of intermediates

Xiang Yin,[a] Giuseppe Zuccarello,[a]
Cristina García-Morales,[a] and Antonio
M. Echavarren*[a]

DFTs

A novel synthesis of indanes and dihydronaphtalenes based on the intramolecular
insertion into C(sp3)H bonds of gold(I) carbenes generated by retro-Buchner
reaction (decarbenation) has been developed. Deuterium-labeling and kinetic isotope
effect experiments, DFT calculations, and generation of the proposed carbene
intermediate from a well-characterized gold(I) carbenoid support the involvement of
a three-centered concerted mechanism for the C(sp3)H functionalization process.

Page No. – Page No.
Gold(I)-Catalyzed
Intramolecular
C(sp3)–H Insertion by Decarbenation
of Cycloheptatrienes

This article is protected by copyright. All rights reserved.

Accepted Manuscript

R1