Studies on [PtCl2]- or [AuCl]-catalyzed cyclization of 1-(indol-2-yl)-2,3-allenols: The effects of water/steric hindrance and 1,2-migration selectivity

Article abstract of DOI:10.1002/chem.201402423

The [PtCl₂]- or [AuCl]-catalyzed reaction of 1-(indol-2-yl)-2,3-allenols occurred smoothly at room temperature to afford a series of poly-substituted carbazoles efficiently. Compared with the [PtCl ₂]-catalyzed process, the [AuCl]-catalyzed reaction represents a significant advance in terms of the scope and the selectivity. Selective 1,2-alkyl or aryl migration of the gold carbene intermediate was observed: compared with the methyl group, the isopropyl, cyclopropyl, cyclobutyl, and cyclohexyl groups migrate exclusively; the cyclopropyl group shifts selectively over the ethyl group; the 1,2-migration of a non-methyl linear alkyl is faster than methyl group; the phenyl group migrates exclusively over methyl or ethyl group. DFT calculations show that water makes the elimination of H₂O facile requiring a much lower energy and validates the migratory preferences of different alkyl or phenyl groups observed.

Full text of DOI:10.1002/chem.201402423

DOI: 10.1002/chem.201402423
Full Paper
&
Homogeneous Catalysis
Studies on [PtCl2]- or [AuCl]-Catalyzed Cyclization of 1-(Indol-2-yl)-
2,3-Allenols: The Effects of Water/Steric Hindrance and 1,2-
Migration Selectivity
Youai Qiu,[a] Chunling Fu,[a] Xue Zhang,*[b] and Shengming Ma*[a, b]
Abstract: The [PtCl2]- or [AuCl]-catalyzed reaction of 1-
(indol-2-yl)-2,3-allenols occurred smoothly at room tempera-
ture to afford a series of poly-substituted carbazoles effi-
ciently. Compared with the [PtCl2]-catalyzed process, the
[AuCl]-catalyzed reaction represents a significant advance in
terms of the scope and the selectivity. Selective 1,2-alkyl or
aryl migration of the gold carbene intermediate was ob-
served: compared with the methyl group, the isopropyl, cy-
clopropyl, cyclobutyl, and cyclohexyl groups migrate exclu-
sively; the cyclopropyl group shifts selectively over the ethyl
group; the 1,2-migration of a non-methyl linear alkyl is
faster than methyl group; the phenyl group migrates exclu-
sively over methyl or ethyl group. DFT calculations show
that water makes the elimination of H2O facile requiring
a much lower energy and validates the migratory preferen-
ces of different alkyl or phenyl groups observed.
Introduction
of gold and platinum carbenes is far less understood: despite
several reported examples involving 1,2-migration of alkyl, aryl,
and halogens, there remains some limitations: 1) The yields are
not very good in 1,2-migration of methyl group; 2) It is non-se-
lective with 1,2-migration of the ethyl group versus the phenyl
group; 3) The order of 1,2-migration of the alkyl groups has
not been established; 4) Various factors on the 1,2-migration of
Transformations of metal carbene complexes have been dem-
onstrated to be powerful tools of synthetic organic chemistry
in the past several decades.[1] Traditionally, metal carbene com-
plexes have often been generated from organic diazo com-
pounds[2] or the reaction of olefin metathesis.[3] Interestingly,
several non-traditional approaches for metal carbene formation
have emerged: a-Oxo gold carbenes may be generated from
the reaction of Au+-coordinated alkynes with oxygen-centered
nucleophiles bearing a protective leaving group, such as sulfur,
amine, imine, pyridine, and quinoline [Eq. (1)];[4] gold or plati-
num carbene may also be generated by such intramolecular
reactions with nucleophiles bearing a leaving group [Eq. (2)];[5]
in addition, intramolecular alkene unit in 1,5-enynes may also
act as the nucleophile to generate cyclopropyl gold or plati-
num carbene [Eq. (3)],[6] moreover, such reactions may also be
observed with allenes: Metal carbene intermediates may be
formed through intramolecular nucleophilic attack of carbonyl
oxygen lone pair or olefinic double bond on the metal-activat-
ed allenic double bond [Eq. (4)].[7–8] However, the 1,2-migration
[a] Y. Qiu, Prof. Dr. C. Fu, Prof. Dr. S. Ma
Laboratory of Molecular Recognition
and Synthesis, Department of Chemistry
Zhejiang University, Hangzhou 310027, Zhejiang (P.R. China)
E-mail: masm@sioc.ac.cn
[b] Prof. Dr. X. Zhang, Prof. Dr. S. Ma
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
345 Lingling Lu, Shanghai 200032 (P.R. China)
Fax: (+86)21-626-093-05
E-mail: xzhang@sioc.ac.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402423.
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gold and platinum carbene[9] have not been established sys-
tematically. Thus, the migratory aptitude in gold and platinum
carbene remains to be elucidated.
nols[21]. Herein, we present the results of DFT calculations that
support the originally proposed 1,2-migration of the Pt–car-
bene intermediates, and importantly, provide impetus for the
discovery of the more efficient [AuCl] for the selective 1,2-alkyl
migration of 4,4-dialkyl-substituted allenols, which show very
low reactivities under the [PtCl2]-catalyzed protocol.[14] Finally,
the nature of the migratory preferences of different alkyl
groups have also been established experimentally and ex-
plored by DFT calculations.
On the other hand, the tricyclic carbazole ring system is the
core structure for a wide range of alkaloids featuring a variety
of biological activities.[10] In addition, carbazole derivatives are
also widely used as important building blocks for the construc-
tion of polymers that exhibit interesting physical properties.[11]
The important potentials of these carbazole derivatives have
made them attractive targets for organic synthesis.[12] Starting
from 2009, we have also observed that cyclic alkyl metal car-
bene intermediates may be afforded through the reaction of Results and Discussion
coordinated allene with an indole, furan, thiophene, or benzo-
Theoretical studies on mechanisms
furan unit followed by elimination of
[Eq. (5)].[13]
a leaving group
To shed light on the mechanism of the above mentioned
[PtCl2]-catalyzed cycloisomerization reaction,[14] DFT calculations
have been performed first, using the 4-non-substituted 1-
(indol-2-yl)-2,3-allenols 2a as the model substrate.
Computational methodology
All calculations were performed with the Gaussian 09 pro-
gram.[15] Geometries have been fully optimized with the densi-
ty functional theory of B3LYP method.[16,17] The 6-311G (d, p)
basis set was used for carbon, hydrogen, nitrogen, and oxygen
atoms and LANL2DZ basis set[18] with effective core potential
(ECP) for platinum and gold atoms. Harmonic vibration fre-
quency calculations were carried out for all the stationary
points to confirm each structure being either a minimum (no
imaginary frequency) or a transition structure (one imaginary
frequency). The solvent effect has been considered by using
the CPCM[19] (UAHF atomic radii) model based on the gas-
phase-optimized structures. The
In the [PtCl2]-catalyzed cycloisomerization of 1-(indol-2-yl)-
2,3-allenols, based on deuterium-labeling experiments, the
mechanistic assumption involves a 1,2-migration on a Pt–car-
bene intermediate. Nonetheless, the detailed mechanism of
this reaction has not been established. In 2012, we reported
the exclusive 1,2-aryl migration over methyl through the Pt-
catalyzed cyclization of polysubstituted 1-(indol)-2,3-allenols
(Scheme 1).[14] However, the reaction of non-equivalent 4,4-di-
reported relative energies are
free energies at 298 K (DG298
)
and the zero-point energies cor-
rected electronic energies (DE0),
both in gas phase, and free en-
ergies (DGsol) in toluene (dielec-
tric constant e=2.37) for the
[PtCl2]-catalyzed process and in
1,2-dichloroethane (DCE) (e=
10.13) for the [AuCl]-catalyzed
process, respectively.
Computational results and
discussion
The computed energy surface
for the [PtCl2]-catalyzed cycloiso-
Scheme 1. Cyclization of 1-indol-2,3-allenols 2.
merization of 2a is provided in
Figure 1. The initial formation of
alkyl-substituted allenols afforded a mixture of regioisomers
with very low yields, and the reaction of allenol with phenyl
and ethyl groups cannot occur under Pt-catalyzed conditions.
These facts promoted us to identify a more efficient catalyst to
realize the selective 1,2-alkyl migration of 1-(indol)-2,3-alle-
a complex 2a_PtCl2 (selected as free energy reference) in-
volves coordination of [PtCl2] to the distal allenic double bond,
which would be followed by nucleophilic attack of the indolyl
C3 to provide the zwitterionic cyclic species Int1. This 6-endo-
trig cyclization step is computed to be exergonic (DG298K
=
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to the metal, features the high-
est relative energy barrier of the
profile (18.1 kcalmolÀ1, TS5), and
may, thus, be rate-determining.
Having established the mech-
anism for [PtCl2]-catalyzed cyclo-
isomerization, DFT calculations
have also been performed for
the [AuCl]-catalyzed reaction.[21]
The computed potential energy
surfaces are qualitatively similar
to that of [PtCl2], but less energy
demanding especially in the 1,2-
H-shift step (11.3 vs. 18.1 kcal
molÀ1 for PtCl2, TS5). The Wiberg
bond index shows the bond
order of CÀM in Int4_PtCl2 is
1.09[22] (Figure 2), much larger
than the value in Int4_AuCl
(0.77). This is in accordance with
the well accepted higher ability
of Pt compared with Au of form-
ing and stabilizing carbonic C=M
bonds.[23] It is due to the forma-
tion of a stronger carbeneÀPt
bond (1.894 ꢁ, Figure 2) com-
pared to carbeneÀAu bond
(1.990 ꢁ, Figure 2) that denotes
the relative stability of Pt-car-
bene intermediate Int4, hence,
showing lower reactivity for the
subsequent 1,2-H-migration.
Figure 1. DFT-calculated energy surfaces of cycloisomerization of 2a catalyzed by [PtCl2] and [AuCl].
À10.6 kcalmolÀ1) and requires only a 5.0 kcalmolÀ1 activation
barrier (TS1, Figure 1). The expected platinum carbene Int4
could be formed by direct elimination of H2O from the zwitter-
ionic intermediate Int1. However, the activation free energy of
the elimination of H2O from Int1 to Int4 is computed to be
31.0 kcalmolÀ1 (TS2), indicating that this step is not kinetically
favorable. Considering the fact that these reactions are usually
conducted at room temperature, there should be other possi-
bilities. Interestingly, when water is taken into account, a more
kinetically favorable pathway of the elimination of H2O from
Int1 has been discovered: Abstraction of the proton from Int2
by water cluster[20] forms the hydroxonium intermediate Int3,
followed by protonation of the hydroxyl group and elimination
of H2O, affording the platinum carbene Int4. Calculations show
that the presence of water makes the direct elimination of H2O
a reversible stepwise proton–abstraction–donation process,
which is highly exergonic by 29.8 kcalmolÀ1 and requires only
2.4 kcalmolÀ1 in total. Thus, the water cluster may act as
a proton shuttle in this process, abstracting the proton and
subsequent protonating the hydroxyl group. The last step,
consisting of a 1,2-H-shift on the resulting carbene with simul-
taneous coordination of the newly formed CÀC double bond
Figure 2. Optimized geometries of Int4_PtCl2 and Int4_AuCl with corre-
sponding carbene–metal bond length [ꢁ] and bond order.
Experimental studies
The DFT calculated results prompted us to investigate the via-
bility of using [AuCl] as the catalyst in this cycloisomerization
reaction. Indeed, when allenol 2b was treated with 5 mol% of
[AuCl], the desired carbazole product was obtained in 87%
yield after just 30 min in DCE. Furthermore, even the 4,4-di-
methyl-substituted 2,3-allenol 2c could be conducted to 9-
ethyl-3,4-dimethylcarbazole in 88% yield within 1 hour under
the catalysis of [AuCl], as compared to the reaction catalyzed
by [PtCl2] in toluene (Scheme 2).[14]
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Table 1. Optimization of the catalyst and concentration for the reaction
of 1-(1-benzyl-1H-indol-2-yl)-4-methylhexa-2,3-dien-1-ol.[a]
Entry
Catalyst
Solvent
t
[h]
Yield
3d+3d’ [%][b]
Ratio
3d/3d’
1[c]
2
[PtCl2]
[PtCl2]
[AuCl]
[AuCl]
[AuCl]
[AuCl]
[PtCl4]
[AuCl(PPh3)]
[AuCl3]
toluene
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
48
42
0.5
0.7
0.5
48
17
18
18
34
23
33
73
84
57
18
8
91:9
91:9
91:9
92:8
92:8
91:9
94:6
–
3
Scheme 2. [AuCl]-catalyzed cyclization reaction of 2b and 2c.
4[d]
5[e]
6[e,f]
7
Optimization of the reaction conditions with [AuCl] as catalyst
8[g]
9
25
88:12
The excellent reactivity profile of [AuCl] led us to test its per-
formance with the substrates that performed very poorly
under the standard [PtCl2]-catalyzed conditions. Our initial in-
vestigation was focused on the reaction of 1-(1-benzyl-1H-
indol-2-yl)-4-methylhexa-2,3-dien-1-ol (2d). When the reaction
was conducted under the catalysis of [PtCl2] (5 mol%) in tolu-
ene, the ethyl-migrated carbazole 3d was formed in 31% yield
(analyzed by using NMR spectroscopy), together with the
methyl-migrated product 3d’ in 3% yield (entry 1, Table 1). In-
terestingly, compared with the reaction under the catalysis of
[PtCl2], the reaction of 2d with [AuCl] (5 mol%) in DCE afford-
ed 3d in a similar yield with a much faster rate (entry 3,
Table 1). Fortunately, the combined yield of 3d and 3d’ was
improved to 84% when the reaction was conducted in a dilut-
ed solution (entry 5, Table 1). When 1 mol% of [AuCl] was
used, the combined yield of 3d and 3d’ was only 57%, with
2d being recovered in 24% (entry 6, Table 1). The use of other
catalysts such as [PtCl4], [AuCl(PPh3)]/[AgSbF6], or [AuCl3] led to
less than 50% of conversion (entries 7–9, Table 1), indicating
that the use of [AuCl] as a catalyst is the critical for the success
in this reaction.
[a] The reaction was conducted with 2d (0.2 mmol) and catalyst (5 mol%)
in solvent (1 mL). [b] Determined by NMR spectroscopic analysis using di-
bromomethane as the internal standard. [c] The volume of solvent was
1.5 mL. [d] The volume of solvent was 5 mL. [e] The volume of solvent
was 10 mL. [f] [AuCl] (1 mol%) was used, and the recovery of 2d was
23%. [g] [AgSbF6] (5 mol%) was added.
Table 2. Effect of temperature and solvent of the reaction of 1-(1-benzyl-
1H-indol-2-yl)-4-methylhexa-2,3-dien-1-ol.[a]
Entry
Solvent
t
[h]
Yield
3d+3d’ [%][b]
Ratio
3d/3d’
1
DCE
DCE
DCE
toluene
CH2Cl2
CH3CN
dioxane
THF
0.5
3.5
16.5
24
1
10
10
12
84
63
58
46
76
20
52
16
92:8
92:8
93:7
91:9
2[c]
3[d]
4[e]
5
Furthermore, the effect of temperature and solvent were
considered in the reaction of 2d under the catalysis of [AuCl]
(5 mol%). At a lower temperature (108C or 08C) in DCE, the
combined yield of 3d and 3d’ was slightly lower (entries 2 and
3, Table 2). Several solvents were also tested for the [AuCl]-cat-
alyzed reaction of 2d at room temperature with DCE still
being the best (entries 4–8, Table 2).
92:8
6
7
8
85:15
90:10
88:12
[a] The reaction was conducted with 2d (0.2 mmol), [AuCl] (5 mol%), and
solvent (10 mL). [b] Determined by NMR spectroscopy using dibromome-
thane as internal standard. [c] The temperature was 108C, [d] The temper-
ature was 08C. [e] The recovery of 2d was 23%.
[AuCl]-Catalyzed cyclization reaction of 4,4-dialkyl-1-(indol-2-
yl)-2,3-allenols
Under the optimized [AuCl]-catalyzed conditions, cycloisomeri-
zation of 4,4-dimethyl substituted allenols 2c and 2e–2g pro-
ceeded smoothly to provide better yields of carbazoles 3c and
3e–3g than the previous [PtCl2]-catalyzed[14] process (en-
tries 1–4, Table 3). In addition to being a methyl group, R1 can
also be an ethyl and propyl group (entries 5 and 6, Table 3). It
can be easily concluded that the yield was lower as the in-
creasing of steric hindrance of R1. 4,4-Trimethylenelbuta-2,3-
dien-1-ol (2j) underwent smooth cyclization with ring expan-
sion to afford fused carbazole 3j in 71% yield (entry 7,
Table 3).
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[AuCl]-catalyzed cyclization reaction of non-equivalent 4,4-di-
alkyl-substituted-1-(indol-2-yl)-2,3-allenols
Table 3. [AuCl]-catalyzed cyclization reaction of 4,4-dialkyl-1-(indol-2-yl)-
2,3-allenols.[a]
[AuCl] also catalyzes the cycloisomerization reaction of a series
of different 4-methyl-4-alkyl-1-(indol-2-yl)-2,3-allenols 2. The se-
lective 1,2-alkyl migration of non-equivalent 4,4-dialkyl substi-
tuted allenols were observed and the results were shown in
Table 5. Different carbazoles were formed in moderate to good
yields and selectivity, with the longer alkyl group migrating;
the structure was confirmed by the X-ray diffraction study of
3p (Figure 3).[24] In addition to ethyl group (entry 1, Table 5), R1
can also be a propyl, pentyl, and nonyl group (entries 2–5,
Table 5). Moreover, from the synthetic view-point, the ratio of
3 and 3’ may be improved to ꢀ97:3 by simple recrystallization
(entries 1–5, Table 5). The reaction can be easily conducted in
a scale of 3 mmol of 2p (1.0770 g) in a slightly higher yield
(entry 4, Table 5).
Entry R1/R2/R3/R4/R5
t
Isolated yield Yield/t
[h] (3) [%]
in Ref. [14] [%/h]
1
2
3
4
5[b]
6[b,c]
7[b]
Me/H/H/Et/H (2c)
Me/H/H/Et/7-Me (2e)
Me/Me/H/Et/H (2 f)
Me/H/Me/Et/H (2g)
Et/H/H/Bn/H (2h)
Pr/H/H/Bn/H (2i)
1
1
1
1
4
88 (3c)
85 (3e)
69 (3 f)
87 (3g)
57 (3h)
73/10
72/16
55/12
–
–
–
–
24 34 (3i)
À(CH2)3À/H/H/Bn/H (2j) 23 71 (3j)
[a] Reaction conditions: A mixture of 2 (0.2 mmol) and [AuCl] (5 mol%) in
DCE (10 mL) under N2. [b] DCE (20 mL) was used. [c] [AuCl] (15 mol%)
was used.
Table 5. [AuCl]-catalyzed cyclization reaction of 4,4-dialkyl-1-(indol-2-yl)-
2,3-allenols.[a]
[AuCl]-Catalyzed cyclization reaction of 4-alkyl-4-phenyl-1-
(indol-2-yl)-2,3-allenols
Furthermore, we have also tested the reaction of 4-alkyl-4-
phenyl-1-(indol-2-yl)-2,3-allenols. Selective 1,2-migration of
phenyl over methyl group occurred in allenols 2k to give 3k
in 75% yield (entry 1, Table 4). Electronic effects of substituents
on the phenyl ring showed no obvious impact on the yields
(entries 2 and 3, Table 4); both meta- and para-substituted sub-
strates could form the corresponding carbazoles. It is worth
noting that the phenyl group migrate exclusively over the
ethyl group (entry 4, Table 4), which cannot be realized in pre-
vious [PtCl2]-catalyzed process.[14]
Entry
R1/R2
t
[h]
Yield
Yield [%][d]
(Ratio) 3/3’
3 and 3’ [%][b,c]
1
2
3
4[e]
5
Me/Et (2d)
Me/Pr (2o)
Me/Pent (2p)
Me/Pent (2p)
Me/Nona (2q)
2
24
4
4
4
75 (92:8) (3d/3d’)
76 (88:12) (3o/3o’)
68 (90:10) (3p/3p’)
71 (90:10) (3p/3p’)
51 (92:8) (3q/3q’)
46 (98:2)
49 (97:3)
55 (98:2)
60 (98:2)
38 (97:3)
[a] Reaction conditions: A mixture of 2 (0.2 mmol), and [AuCl] (5 mol%) in
DCE (10 mL) under N2. [b] Yield of isolated 3 and 3’. [c] The ratio of 3/3’ is
reported in the parenthesis. [d] Yield and ratio after recrystallization.
[e] The reaction was conducted with a 3 mmol (1.0770 g) scale of 2p.
Table 4. [AuCl]-catalyzed cyclization reaction of 4-alkyl-4-phenyl-1-(indol-
2-yl)-2,3-allenols.[a]
Entry R1/R2
t
Isolated yield Yield/t reported in ref. [14]
[h] (3) [%]
[%/h]
1
2
3
4
Me/Ph (2k)
1
1
1
3
75 (3k)
88 (3l)
79 (3m)
53 (3n)
67/24
77/25
75/12
NR[b]
Me/p-FC6H4 (2l)
Me/m-MeC6H4 (2m)
Et/Ph (2n)
Figure 3. ORTEP representation of 3p.
[a] Reaction conditions: A mixture of 2 (0.2 mmol) and [AuCl] (5 mol%) in
DCE (10 mL) under N2. [b] No reaction.
It is worth noting that exclusive 1,2-isopropyl migration over
the methyl group occurred in allenol 2s to give 3s in 35%
yield (entry 1, Table 6). Cyclopropyl, cyclobutyl, and cyclohexyl
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groups have also exclusively migrated in good yields (en-
tries 2–8, Table 6), which was established by the X-ray diffrac-
tion study of 3x (Figure 4).[25] The 1-position of indoles (R3)
may be substituted with methyl (entry 8, Table 6), ethyl (en-
tries 1 and 3–7, Table 6), and benzyl groups (Table 6, entry 2).
Substituents on the 5 position of indoles (R4) could be methyl
(entries 3 and 8, Table 6), or methoxy (entry 5, Table 6) groups.
A substituent may also be introduced into the 1 position of
the carbazole ring in 3x by installing R2 groups in the starting
allenols.
peting with ethyl group, the cyclopropyl group also migrates
highly selectively (95:5) [Eq. (6)].
Table 6. [AuCl]-catalyzed cyclization reaction of 4-methyl-4-alkyl-1-(indol-
2-yl)-2,3-allenols.[a]
Experimental study on the effect of the in situ-generated
water
The excellent reactivity of [AuCl] enables us to demonstrate
the effect of water experimentally. Molecular sieves (4 ꢁ MS)
were added to the reaction mixture of 2b to remove some of
the in situ-generated water. As expected, compared with the
reaction in the absence of 4 ꢁ MS, the reaction proceeded in
a lower rate (Scheme 3), thus confirming the co-catalytic role
of water shown by DFT calculations.
Entry
R1/R2/R3/R4
t
[h]
Yield of
isolated 3 [%]
1[b]
2
3
4
5
6
7
8[c]
iPr/H/Et/H (2s)
20
4
2
3
5
3
2
3
35 (3s)
71 (3t)
86 (3u)
70 (3v)
86 (3w)
76 (3x)
69 (3y)
31 (3z)
cyclopropyl/H/Bn/H (2t)
cyclopropyl/H/Et/5-Me (2u)
cyclopropyl/H/Et/H (2v)
cyclopropyl/H/Et/5-OMe (2w)
cyclopropyl/Me/Et/H (2x)
cyclobutyl/H/Et/H (2y)
cyclohexyl/H/Me/5-Me (2z)
[a] Reaction conditions: A mixture of 2 (0.2 mmol) and [AuCl] (5 mol%) in
DCE (10 mL) under N2. [b] DCE (20 mL) and [AuCl] (10 mol%) were used.
[c] [AuCl] (10 mol%) was applied.
Scheme 3. The effect of in situ-generated water on the reaction.
Theoretical studies on the selectivity
The above mentioned selectivity arises primarily from the pref-
erence for either group shift in the gold-stabilized carbene in-
termediate (Int4 in Figure 1). To validate the selectivity, the rel-
ative activation barriers for the competing migrations have
been calculated based on several model substrates 4,4-disub-
stituted-1-(1-methyl-indol-2-yl)-2,3-allenols 2_1.
Figure 4. ORTEP representation of 3x.
As can be deduced from the energetic data shown in
Table 7, the free-energy barrier of phenyl group migration is
computed to be lower than that of the competing methyl and
ethyl group migration (entries 1 and 2, Table 7). These calculat-
In addition, we observed that the reaction of 1-(1-ethyl-1H-
indol-2-yl)-4-cyclopropylhexa-2,3-dien-1-ol (2aa) in DCE under
the catalysis of [AuCl] gave 3aa and 3aa’ in 81% yield; com-
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lease of steric hindrance through the migration of
the more sterically hindered group accounts for the
migratory preference observed here. In addition, sol-
vent also seems to play a role in enhancing such a se-
lectivity. Of course, there may also be an electronic
effect.[27]
Table 7. Calculated free-energy barriers [kcalmolÀ1
steps (TS5 in Figure 1).
] for competitive 1,2-migration
Conclusion
We have reported a simple and mild Au-catalyzed re-
action of 1-(indol-2-yl)-2,3-allenols, providing an effi-
cient route to differently substituted carbazoles,
which proceeds by a selective 1,2-alkyl or aryl shift
via a gold-carbene intermediate. Compared with the
previous [PtCl2]-catalyzed process, the [AuCl]-cata-
lyzed reaction represents a big step forward in terms
of the scope and the selectivity. Migratory aptitude
of 1,2-alkyl or aryl migration has been established:
compared with methyl group, the isopropyl, cyclo-
propyl, cyclobutyl, and cyclohexyl groups migrate ex-
clusively; the cyclopropyl group shifts selectively over
ethyl group; the 1,2-migration of a non-methyl linear
Entry
R1/R2
1,2-R1 shift
DG298 (DGsol
1,2-R2 shift
DG298 (DGsol
DDG298
(DDGsol)
)
)
1
2
3
4
5
6
Me/Ph (2k_1)
Et/Ph (2n_1)
Me/Et (2d_1)
Me/nPr (2o_1)
Me/iPr (2s_1)
Me/Cyclopropyl
(2t_1 or 2v_1)
Et/Cyclopropyl
(2aa_1)
19.0 (18.7)
17.5 (15.8)
17.8 (17.2)
17.7 (17.0)
20.6 (20.7)
22.6 (22.0)
14.4 (12.6)
14.3 (12.6)
17.0 (14.5)
17.2 (14.1)
12.4 (8.8)
15.7 (9.5)
4.6 (6.1)
3.2 (3.2)
0.8 (2.7)
0.5 (2.9)
8.2 (11.9)
6.9 (12.5)
7
21.1 (17.1)
15.4 (9.1)
5.7 (8.0)
ed results are in accordance with the generally accepted rate
of migratory aptitude to a free carbenic center, H>Ph>Me.[26]
The free-energy barrier of methyl group migration is com-
puted to be higher than that of the competing ethyl, propyl,
isopropyl, and cyclopropyl migration (entries 3–6, Table 7). The
barrier difference between the migration of regiocompetitive
alkyl groups increases roughly with the difference of their
steric hindrance, so the bulkier alkyl substituent is
calculated to migrate preferentially, thus, capturing
the experimental trends, although not quantitatively.
The transition structures of the gold carbene 1,2-alkyl
migration is responsible for the migratory aptitude.
The transition structures of competitive 1,2-isopropyl
and 1,2-methyl migrations (entry 5, Table 7), which
are denoted as TS_iPr and TS_Me (Figure 5), respec-
tively, are taken as a typical example. The structures
of TS_iPr and TS_Me largely resemble the planar car-
bazole products (top view, Figure 5), except that the
migrating group bridges the migration original (C1)
and terminal (C2) carbon atoms. These two transition
states are very late in terms of the C=C bond forma-
tion, but are very early in terms of the migration of
the alkyl group. As a consequence, the non-migrating
isopropyl group in TS_Me (side view, Figure 5),
roughly in the tricyclic plane, causes a repulsive steric
interaction with gold atom (Au···H distance is only
2.558 ꢁ, Figure 5, TS_Me, side view) and with the in-
dolyl moiety (H···H distance is only 1.995 ꢁ, Figure 5,
TS_Me, side view; compare the side view of TS_Me
with that of TS_iPr). The angle a in TS_Me (115.88) is
farther from the normal 1208 as compared with that
of TS_iPr (117.98) (side view, Figure 5) to avoid such
alkyl is faster than methyl group; the phenyl group migrate ex-
clusively over the methyl or ethyl groups. DFT calculations sup-
port the originally hypothesized 1,2-migration on the metal
carbene intermediate and discover the co-catalytic role of the
in situ-presented water, which makes the elimination of H2O
facile, requiring a much lower energy. Importantly, the calcula-
tions suggest that the migratory preference of more bulky
Figure 5. The side and top views of the calculated transition structures of competitive
1,2-isopropyl (TS_iPr) and 1,2-methyl (TS_Me) migrations with selected structural param-
steric interactions, resulting in the less stability of TS_
Me relative to TS_iPr. Thus, we reasoned that the re- eters [ꢁ].
Chem. Eur. J. 2014, 20, 10314 – 10322
www.chemeurj.org
10320
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Full Paper
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alkyl group is a consequence of releasing the steric effect. Fur-
ther studies in this area are being pursued in this laboratory.
Experimental Section
Typical procedure for 3b
[AuCl] (2.5 mg, 0.01 mmol, weighed in a glovebox), compound 2b
(62.0 mg, 0.20 mmol), and DCE (10 mL) were added sequentially to
a dry Schlenk tube under N2. After continuous stirring for 0.5 h at
RT, the reaction was complete as monitored by TLC. Filtration
through a short pad of silica gel (eluent: Et2O (20 mLꢂ3)), evapora-
tion and column chromatography on silica gel (petroleum ether
(30–608C)/dichloromethane=10:1) afforded 3b[13] (51.1 mg, 87%)
as a liquid. 1H NMR (300 MHz, CDCl3): d=7.92 (s, 1H, ArH), 7.42–
7.13 (m, 4H, ArH), 6.98 (d, J=7.2 Hz, 1H, ArH), 4.30 (q, J=7.2 Hz,
2H, NCH2), 3.20 (t, J=7.8 Hz, 2H, ArCH2), 2.55 (s, 3H, ArCH3), 1.93–
1.72 (m, 2H, CH2), 1.62–1.45 (m, 2H, CH2), 1.44–1.27 (m, 7H, 2ꢂ
CH2 +CH3), 0.91 ppm (t, J=6.9 Hz, 3H, CH3); 13C NMR (75 MHz,
CDCl3): d=140.4, 138.6, 138.1, 127.8, 126.1, 125.2, 123.0, 122.8,
120.5, 119.2, 107.8, 105.9, 37.4, 34.5, 31.8, 29.7, 29.5, 22.7, 21.6,
14.1, 13.7; IR (neat): n˜ =2954, 2929, 2858, 1598, 1579, 1499, 1477,
1379, 1347, 1331, 1309, 1248, 1150, 1078 cmÀ1; MS (70 ev, EI): m/z
(%): 294 [M+ +1] (25.47), 293 (100).
Acknowledgements
Financial support from the National Natural Science Founda-
tion of China (21232006) and the National Basic Research Pro-
gram (2011CB808700) is greatly appreciated. Xue Zhang is re-
sponsible for the DFT calculation. Shengming Ma is a Qiu Shi
Adjunct Professor at Zhejiang University. We thank Mr. W. Fan
in our group for reproducing the results presented in entry 4
of Table 3; entry 4 of Table 4; entry 2 of Table 5; and entry 5 of
Table 6.
Keywords: allenes · carbenes · density functional calculations ·
gold · platinum
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[24] Crystal data for compound 3p: C25H27N, Mw =341.48, Triclinic, space
group PÀ1, Final R indices [I>2s(I)], R1=0.0671, wR2=0.2111, R indi-
ces (all data) R1=0.0996, wR2=0.1817, a=5.8176(5), b=11.7814(11),
c=15.1647(14) ꢁ, a=107.077(8), b=96.708(7), g=93.760(7)8, V=
981.28(16) ꢁ3, T=293 (2) K, Z=2, reflections collected/unique: 6119/
3583 (Rint =0.0178), number of observations [>2s(I)]2309, parameters:
237. CCDC-980888 (3p) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
[25] Crystal data for compound 3x: C19H21N, Mw =263.37, Monoclinic, space
group P21/c, Final R indices [I>2s(I)], R1=0.0547, wR2=0.1503, R indi-
ces (all data) R1=0.1016, wR2=0.1241, a=4.9183(3), b=21.9564(18),
c=14.1548(11) ꢁ, a=90, b=98.836(7), g=90, V=1510.41(19) ꢁ3, T=
293 (2) K, Z=4, reflections collected/unique: 6006/2752 (Rint =0.0339),
number of observations [>2s(I)]1638, parameters: 184. CCDC-980889
(3x) contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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[27] For such a report on the rearrangement involving cationic intermedi-
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Received: February 28, 2014
Published online on July 23, 2014
Chem. Eur. J. 2014, 20, 10314 – 10322
www.chemeurj.org
10322
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim