Rhodium-catalyzed [6 + 2] cycloaddition of internal alkynes with cycloheptatriene: Catalytic study and DFT calculations of the reaction mechanism

Article abstract of DOI:10.1021/om4003736

A rhodium-catalyzed [6 + 2] cycloaddition of internal alkynes with cycloheptatriene is described. A series of substituted alkynes were cycloadded to cycloheptatriene through a [6 + 2] addition to give a variety of substituted bicyclic compounds in excellent yields. The optimal catalytic system for these transformations was a [Rh(COD)Cl]₂ (5.0 mol %) catalyst in combination with CuI (10 mol %) and PPh₃ (10 mol %). The proposed mechanism for this system includes an initial oxidative coupling reaction between the coordinated cycloheptatriene and the internal alkyne, followed by a [1,3]-shift of the Rh metal center and a reductive elimination from the Rh(III)-allyl complex to give the final product. Calculations using a model Rh(I) catalyst were also carried out to further understand this mechanism.

Full text of DOI:10.1021/om4003736

Article
pubs.acs.org/Organometallics
Rhodium-Catalyzed [6 + 2] Cycloaddition of Internal Alkynes with
Cycloheptatriene: Catalytic Study and DFT Calculations of the
Reaction Mechanism
Xinrui Zhang, Jingjing Wang, Hui Zhao, Haitao Zhao,* and Jianhui Wang*
Department of Chemistry, College of Science, Tianjin University, Tianjin 300072, People’s Republic of China
S
* Supporting Information
ABSTRACT: A rhodium-catalyzed [6 + 2] cycloaddition of
internal alkynes with cycloheptatriene is described. A series of
substituted alkynes were cycloadded to cycloheptatriene through a
[6 + 2] addition to give a variety of substituted bicyclic compounds
in excellent yields. The optimal catalytic system for these
transformations was a [Rh(COD)Cl]2 (5.0 mol %) catalyst in
combination with CuI (10 mol %) and PPh3 (10 mol %). The proposed mechanism for this system includes an initial oxidative
coupling reaction between the coordinated cycloheptatriene and the internal alkyne, followed by a [1,3]-shift of the Rh metal
center and a reductive elimination from the Rh(III)−allyl complex to give the final product. Calculations using a model Rh(I)
catalyst were also carried out to further understand this mechanism.
workers demonstrated a cobalt(I)-catalyzed [6 + 2] cyclo-
addition in which CHT was cyclized with terminal alkynes.16
Later, this cobalt(I)-catalyzed methodology was extended to [6
+ 2] cycloadditions of COTT to internal and terminal alkynes17
and to [6 + 2] cycloadditions of CHT to substituted allenes.18
Other groups have also achieved similar accomplishments for
[6 + 2] cycloadditions catalyzed by cobalt(I) complexes.19
These successes have paved the way for using the [6 + 2]
cycloaddition as a key reaction for the construction of ring
compounds in organic synthesis. In fact, [6 + 2] cycloaddition
reactions have also been applied to the synthesis of natural
products.20
Meanwhile, rhodium complexes have played an increasingly
important synthetic role in carbon−carbon bond formation.21
Rhodium-catalyzed cyclization/cycloaddition reactions, in
particular, have proven to be useful for constructing polycyclic
carbo- and heterocycles.22 In recent years many research groups
have achieved impressive breakthroughs in this area.23 Although
rhodium complexes exhibit high efficiency in these reported
cyclization reactions, there have been only a few reports on the
application of rhodium complexes to [6 + 2] cycloadditions.
For example, rhodium(I)-catalyzed [6 + 2] cycloadditions of
2-vinylcyclobutanones and alkenes have been used for the
synthesis of eight-membered rings,24 a [RhCl(dppp)2]-
catalyzed intramolecular [6 + 2] cycloisomerization reaction
between alkynes and allenylcyclobutane functionalities has been
reported for the efficient formation of bicyclo[6.4.0]-
dodecatriene frameworks under mild conditions,25 Rh(I)-
catalyzed [6 + 2] cycloadditions of 4-allenals with tethered
alkynes have also been reported,26 and the intermolecular [6 +
2] cycloaddition of 4-allenals with alkynes catalyzed by
INTRODUCTION
■
In recent years transition-metal catalysts have been widely used
in organic synthesis for the efficient preparation of organic
compounds.1 Transition-metal-mediated or -catalyzed higher-
order cycloaddition reactions are powerful methods for the
construction of a variety of medium-sized ring systems.2 The [6
+ 2] cycloaddition of cyclooctatetraene (COTT) in a
tricarbonyl(η4-COTT)iron(0) complex to internal alkynes to
give metal-free adducts under thermal conditions was first
reported by Kruerke.3 Later, Davis et al. demonstrated that the
[6 + 2] cycloaddition of 1,3,5-cycloheptatriene (CHT) in a
tricarbonyl(η4-CHT)iron(0) complex to internal alkynes
afforded bicyclo[4.2.1]nonatriene adducts under photoinduced
reaction conditions.4 Related chromium(0)-mediated [6 + 2]
cycloadditions of tricarbonyl (η6-COTT or η6-CHT)-
chromium(0) to alkynes have also been observed by Fischler
et al.,5 Rigby and co-workers,6 and Sheridan and co-workers.7
Tetracyclic adducts were observed when terminal alkynes were
replaced with internal alkynes in the chromium-promoted
cycloaddition through a stepwise [6 + 2] cycloaddition−homo
[6 + 2] cycloaddition sequence.8
A variety of stoichiometric [6 + 2] cycloadditions of
conjugated cyclic trienes with alkenes, dienes, and alkynes
have been intensively investigated, and it has been found that
[6 + 2] cycloadditions can be extended to other metal
complexes, such as iron,9 chromium,10 cobalt,11 molybdenum,12
ruthenium,13 and titanium complexes.14 A catalytic [6 + 2]
cycloaddition reaction using a Ziegler (TiCl4/Et2AlCl) catalyst
was first reported by Mach.15 A major side reaction, the
cyclotrimerization of alkynes, was found to compete with the [6
+ 2] cycloaddition reaction in this system.
Rigby and co-workers found that carbonylchromium(0)
complexes bearing labile ligands show good catalytic activity for
[6 + 2] cycloaddition reactions.10a−d Recently, Buono and co-
Received: April 29, 2013
© XXXX American Chemical Society
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Table 1. Optimization of Reaction Conditions for the [6 +2] Cycloaddition of CHT (1) to (4-Methylphenyl)phenylacetylene
(2a)
a
a
entry
catalyst
ligand
additive
yield (%)
entry
catalyst
ligand
additive
KI
yield (%)
b
d
1
(PPh3)3RhCl
0
12
13
14
15
16
17
18
19
20
21
22
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
63
d
2
(PPh3)3RhCl
64
n-Bu4NI
CuCl
CuBr
Cu(OAc)2
CuCl2
CuI
59
c
3
(PPh3)3RhCl
53
40
32
35
<5
0
4
(PPh3)3RhCl
CuI
CuI
75
60
71
62
80
69
64
88
5
(PPh3)3RhCl
PPh3
PPh3
6
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
7
CuI
8
Pd(OAc)2
PPh3
PPh3
PPh3
PPh3
CuI
0
9
PPh3 (5 mol %)
PPh3 (2 mol %)
PPh3
(PPh3)3RuCl2
Ru(COD)2Cl2
[RuCl2(p-cymene)]2
CuI
0
10
11
CuI
0
CuI
CuI
0
a
Reaction conditions: 1 (0.10 mmol), 2a (0.05 mmol), catalyst (5.0 mol % based on 2a), ligand (10 mol % based on 2a), additive (10 mol % based
b
c
d
on 2a), xylene (0.5 mL), 120 °C. The isolated yield is based on 2a. 60 °C. 140 °C. Some unidentified side products were also observed.
rhodium(I) was found to be a good method for the
construction of monocyclic eight-membered rings.27 However,
to the best of our knowledge, the direct [6 + 2] cycloaddition of
an alkyne to cycloheptatriene catalyzed by a rhodium catalyst
has never been reported.
Not only would the development of a rhodium-catalyzed [6
+ 2] cycloaddition system expand the cycloaddition method-
ology but it would also further our understanding of the
catalytic behavior of rhodium. With this in mind, rhodium-
catalyzed [6 + 2] cycloaddition reactions were explored using
cycloheptatriene and different internal alkynes as starting
materials. The rhodium-catalyzed [6 + 2] cycloaddition
proceeded smoothly to give the corresponding ring products
when [Rh(COD)Cl]2 was used in combination with PPh3 and
CuI as the catalyst. These results are reported herein.
mol %) was used as the additive, the yield of 3a decreased to
62% (Table 1, entry 7). However, the addition of PPh3 (10 mol
%) to the [Rh(COD)Cl]2 catalytic system led to a yield of 80%
(Table 1, entry 8), which is very different from that for the
(PPh3)3RhCl system. Reducing the amount of PPh3 additive to
5%, or 2%, gave yields of the desired products of 69% and 64%,
respectively (Table 1, entries 9 and 10). To our delight, when
[Rh(COD)Cl]2 was used in combination with CuI (10 mol %)
and PPh3 (10 mol %), the reaction gave 3a in 88% yield (Table
1, entry 11). However, when KI or n-Bu4NI was used as the
additive instead of CuI, the product yields dropped to 63% and
59%, respectively (Table 1, entries 12 and 13). Some
unidentified side products were also observed in the reaction
systems, indicating that CuI may play an important role in
controlling side reactions by activating the rhodium catalyst.
Other phosphine cleavage reagents, such as CuCl and CuBr,
were then tested as additives in combination with [Rh(COD)-
Cl]2 and PPh3 (10 mol %). In these reactions, 3a was produced
in low yields (Table 1, entries 14 and 15). When Cu(OAc)2 (10
mol %) was used in place of CuI as the additive, the
cycloaddition reaction was again suppressed and a yield of only
35% could be isolated after the reaction (Table 1, entry 16).
When CuCl2 (10 mol %) was added to the reaction system, the
cycloaddition reaction was inhibited, with less than 5% product
being obtained (Table 1, entry 17). These results indicate that
the effect of the additive depends on the valence state of the
copper ion and on the counterion. It should also be noted that
the cycloaddition reaction of 1 and 2a did not take place when
CuI was used alone (Table 1, entry 18). The above
experimental results indicate that CuI, a known reagent for
scavenging and reclaiming phosphines, plays a unique role in
accelerating the cycloaddition reaction and controlling the side
reactions. However, the specific role of CuI in this reaction is
still undetermined, since a reduced amount of PPh3 gave a
lower yield and other copper compounds produced different
results. Therefore, further investigations are needed to
determine exactly how CuI affects the reaction.
RESULTS AND DISCUSSION
■
In order to explore the possibility of a rhodium-catalyzed [6 +
2] cycloaddition reaction to form an eight-membered ring,
CHT (1) and (4-methylphenyl)phenylacetylene (2a) were
initially used as the cycloaddition reaction partners to optimize
the reaction conditions. The results are summarized in Table 1.
Using (PPh3)3RhCl as the catalyst at 60 °C did not result in a
reaction between 1 and 2a, and no [6 + 2] cycloaddition
product 3a was detected (Table 1, entry 1). However, when the
reaction temperature was increased to 120 °C, the
(PPh3)3RhCl exhibited better catalytic activity and gave 3a in
64% yield (Table 1, entry 2). Further elevating the reaction
temperature to 140 °C resulted in a mixture of the desired
product and unknown side products, with the isolated yield of
3a dropping to 53% (Table 1, entry 3).
Next, different additives were tested to improve the activity.
The addition of CuI (10 mol %), which is a phosphine cleavage
reagent,28 increased the reaction yield to 75% (Table 1, entry
4). However, using both CuI (10 mol %) and PPh3 (10 mol %)
with (PPh3)3RhCl gave an inferior result with only a 60% yield
(Table 1, entry 5).
When the catalyst was switched to [Rh(COD)Cl]2, 3a was
obtained in 71% yield (Table 1, entry 6), which is better than
that obtained with (PPh3)3RhCl (64%). When only CuI (10
Other metal compounds, such as Pd(OAc)2, (PPh3)3RuCl2,
Ru(COD)2Cl2, and [RuCl2(p-cymene)]2, did not exhibit
activity for this [6 + 2] cycloaddition reaction (Table 1, entries
19−22) under similar reaction conditions. This indicates that
B
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a b
,
Table 2. [6 + 2] Cyclization of Cycloheptatriene and Various Alkynes under the Optimal Conditions
a
Reaction conditions: cycloheptatriene 1 (0.5 mmol), alkynes 2 (0.25 mmol), [Rh(COD)Cl]2 (0.0125 mmol), PPh3 (0.025 mmol), CuI (0.025
b
mmol), xylene (0.5 mL), 120 °C, 18−24 h. Isolated yield based on 2.
rhodium metal has a unique reactivity for the formation of C−
C bonds. Thus, [Rh(COD)Cl]2 (5 mol %) in combination with
CuI (10 mol %) as the additive and PPh3 (10 mol %) as the
ligand was selected as the optimal catalytic system for the [6 +
2] cycloaddition reaction of 1 and 2a.
electronic effects play an important role in the [6 + 2]
cycloaddition reaction.
When naphthalene-substituted phenylacetylene (2l) was
subjected to the reaction, the product 3l was obtained in
medium yield (48%) (Table 2, entry 12), which may be due to
the strong steric effects in both 2l and 3l. This result supports
the inference that steric effects exert an important influence on
the cycloaddition process. Unfortunately, when phenylacety-
lenes bearing other groups were used as substrates, the yields of
the corresponding products were lower than those for the
aromatic group substituted substrates. For example, the
reaction of 1,4-diphenylbutadiyne (2m) and cycloheptatriene
gave 3m in low yield (40%) (Table 2, entry 13). Ethyl 3-
phenylpropiolate, a −COOEt substituted phenylacetylene,
displayed low reactivity in the [6 + 2] cycloaddition reaction
with 1 (Table 2, entry 14), indicating that the electronic effect
has a strong impact on the outcome of the reaction.
In general, the yield of the desired products was influenced
by the substituents on the phenylacetylene. Both aromatic
groups and aliphatic groups could be introduced as substituents
on the phenylacetylene, but the aromatic groups, including
both electron-donating and electron-attracting groups, reacted
more smoothly with cycloheptatriene to give better yields than
the aliphatic groups. It should be noted that the reaction can
tolerate many organic functional groups, such as −NO2,
−COCH3, −COOEt, and −Cl, which allows for the further
expansion of the products if needed.
Under the optimal conditions, a variety of 1,2-diphenyle-
thynes bearing different substituents were investigated for the
[6 + 2] cycloaddition reaction with 1, and the results are shown
in Table 2. When 1 was reacted with a diphenylacetylene
bearing an electron-donating group such as −CH3 (2a), −Et
(2c), −OEt (2d), or −OMe (2f) at the para- position of the
phenyl ring, the corresponding products 3a,c,d,f were obtained
in 88%, 85%, 87%, and 88% yields, respectively (Table 2,
entries 1, 3, 4, and 6). When 1 was reacted with
diphenylacetylene (2b) with no substituent, the product 3b
was obtained in 81% yield (Table 2, entry 2). Diphenylacety-
lene with −OMe at the ortho position of the phenyl ring (2e)
reacted with 1 to give the product 3e in 80% yield (Table 2,
entry 5). Since this is lower than the yield for 2f, this indicates
that the position of the substituent may affect the product yield.
On the other hand, diphenylacetylenes with an electron-
withdrawing group, such as −NO2 (2g,h) −CF3 (2i),
−COCH3 (2j), or −COOEt (2k), also reacted with 1 to give
the corresponding products 3g−k in 70%, 73%, 70%, 72%, and
67% yields, respectively (Table 2, entries 7−11). The yields of
the diphenylacetylenes with electron-withdrawing groups were
lower than those with electron-donating groups, indicating that
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On the basis of the above results and previous research on
cobalt(I)-catalyzed [6 + 2] cycloaddition,16,17 a plausible
catalytic mechanism is shown in Figure 1.
exchange of H with CHT, regenerating catalytic active complex
D to enter another catalytic cycle.
To gain a better understanding of the catalytic mechanism,
DFT calculations were carried out using the M06 functional.
The calculated energy profiles are shown in Figure 2. The
calculations show that ligand exchange of (Ph3P)Rh(COD)Cl
with a CHT to give complex II is an endothermic process with
an energy increase of 14.1 kcal/mol. The direct addition of an
alkyne ligand by II without the help of CuI gives the PPh3-
coordinated complex PIII with an increase in energy of about
10.0 kcal/mol. The subsequent intramolecular cyclization of
PIII to give the corresponding rhodacyclopentene goes through
a transition state with a very high energy barrier (45.4 kcal/
mol). Therefore, this reaction pathway is excluded.
Next, other pathways were calculated. The exchange of Ph3P
in complex II with an alkyne to give complex III is also an
endergonic process with an energy increase of 12.5 kcal/mol in
this stage. However, this process becomes exergonic if the
ligand exchange occurs in the presence of CuI, due to the
formation of (Ph3P)CuI,29 and the corresponding energy
decreases to −18.9 kcal/mol. Although the specific role of CuI
in this reaction is still undetermined, this calculation result is in
good agreement with the experimental result that the addition
of CuI may significantly accelerate the [6 + 2] cycloaddition
reaction.
Complex III immediately converts to rhodacyclopentene IV
after an oxidative cyclometalation through transition state
TSIII-IV with a barrier of 30.8 kcal/mol. The rhodium metal is
complexed with a π-allyl ligand in intermediate IV. A further
addition of an alkyne from the reaction solution gives
intermediate V, and the system energy slightly increases by
3.5 kcal/mol. Next V undergoes intermolecular reductive
elimination through transition state TSV-VI to finally give
complex VI, in which the rhodium atom is coordinated with the
two double bonds of the [6 + 2] cycloaddition product 3. A
ligand exchange of VI with another CHT substrate gives the
free product 3 and regenerates the catalytically active complex
III to enter another cycle. The energy decreased by about 0.1
kcal/mol in this regenerated step. The calculated results
indicate that the latter pathway is the kinetically and
thermodynamically favored reaction. The whole [6 + 2]
cycloaddition reaction is an exergonic process by 30.8 kcal/
mol, and the overall reaction barrier for the reaction is 29.7
kcal/mol.
Another possibility is that one PPh3 ligand coordinates to V
instead of one of the alkyne ligands to give V-P, and then there
is a reductive elimination from V-P to give the [6 + 2]
cycloaddition product. This possibility was considered, and the
profiles are shown in Figure 2. The PPh3 coordinates to the CuI
to give a PPh3−CuI complex in this scheme. From IV, a
transmetalation reaction between IV and PPh3−CuI occurs to
give intermediate V-P. Then, the reaction proceeds to form the
final product through a reductive elimination transition state
TSV-VI-P. This step (IV → TSV-VI-P) has a higher barrier of
44.8 kcal/mol than the same step (IV → TSV-VI) in our
proposed pathway.
Figure 1. Proposed mechanism for the rhodium-catalyzed [6 + 2]
cycloaddition of an internal alkyne to cycloheptatriene.
The mechanistic pathway for the formation of 3 involves the
following steps: initially, the catalytic precursor [Rh(COD)Cl]2
converts in situ to (Ph3P)Rh(COD)Cl (B) after a ligand
exchange with a triphenylphosphine in the presence of CuI, a
known reagent for scavenging and reclaiming phosphines. The
addition of triphenylphosphine and CuI in a mole ratio of 1:1
may also play an important role in controlling the ligand
exchange process to give complex B, instead of forming
complex B′, a less active species. Subsequently, the exchange of
the COD ligand in the rhodium complex B with CHT gives the
rhodium complex C, in which a CHT coordinates to the metal
center. Next the Ph3P is replaced with the alkyne in the
presence of CuI to give intermediate D, a proposed active
species in the catalytic cycle. Immediately, intermediate D
converts to rhodacyclopentene E, an σ,π-allyl complex, after
intramolecular oxidative cyclometalation. A further addition of
an alkyne from the solution gives the σ,π-allyl complex F.
Subsequent reductive elimination of F through the transition
state G produces intermediate H, in which the rhodium atom is
bonded with two double bonds, an acetylene, and a chloride.
The [6 + 2] cycloaddition product 3 is obtained after a ligand
From the calculations, the rate-determining step is III →
TSIII-IV (the oxidative coupling reaction). In this step, the
metal center gives two of its electrons to the ligand to form the
Me−C bond. Thus, a more electron-rich metal center should
facilitate the whole reaction. Additional calculations with
substituted 1,2-diphenylethyne models (III-OMe and III-
NO2 in Scheme 1) were carried out.
D
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Figure 2. Calculated energy profiles for the pathway of the [6 + 2] reaction catalyzed by Rh(I). The relative free energies and electronic energies (in
parentheses) are given in kcal/mol.
electrons to the metal center than the (4-nitrophenyl)-
phenylacetylene ligand. Analysis of the NBO charges that are
distributed over the alkyne ligands led to a similar conclusion.
The NBO analysis shows that the total charge in the alkyne
ligand of III-OMe is 0.077 and the total charge in the alkyne
ligand of III-NO2 is 0.044. The calculated reaction barriers
proved our assumptions (28.3 kcal/mol for TSIII-IVOMe and
31.0 kcal/mol for TSIII-IVNO2). The calculations explain why
the diphenylacetylene bearing an electron-donating group has a
higher yield and the diphenylacetylene bearing an electron-
withdrawing group has a lower yield.
Scheme 1. Substituted 1,2-Diphenylethyne Models
A common description of metal−alkyne bonding interactions
is the Dewar−Chatt−Duncanson (DCD) model.30 This
involves the donation of electrons from the CC π orbital
to an empty metal dσ orbital and the back-donation of electrons
from a metal dπ orbital into the CC π* orbital. The electron
richness of the metal center in these metal−alkyne complexes is
modulated by the donation and back-donation properties of the
coordinated alkynes.31 The total charge of the alkynes
determines how many electrons can be donated or accepted
from the alkyne ligands to the metal center.
A second-order perturbation theory analysis on the basis of
NBOs32 suggests that there is a significant donor−acceptor
interaction between these two moieties. For III-OMe, the
stabilization energy E(2) for the “doubly-occupied” Rh d
orbitals to the “vacant” π*(CC) orbitals is estimated to be
21.8 kcal/mol and the stabilization energy E(2) for the “doubly-
occupied” π(CC) orbitals to the “vacant” Rh d orbitals is
estimated to be 42.1 kcal/mol. For III-NO2, the relative
stabilization energies E(2) are 22.5 and 37.9 kcal/mol,
respectively. These results show that the (4-methoxyphenyl)-
phenylacetylene ligand in III-OMe is a better σ donor and a
poorer π acceptor in comparison with the (4-nitrophenyl)-
phenylacetylene ligand in III-NO2. Thus, the (4-
methoxyphenyl)phenylacetylene ligand can donate more
CONCLUSIONS
■
An efficient [Rh(COD)Cl]2/PPh3/CuI catalytic system for the
[6 + 2] cycloaddition of alkynes with cycloheptatriene has been
developed. A combination of PPh3 and CuI was the most
efficient additive in the [Rh(COD)Cl]2 catalysis system. High
yields were obtained for the cycloaddition of a series of
substituted alkynes with cycloheptriene. An electronic effect
was observed for substituted diphenylacetylenes, and this
reaction is tolerant to many organic functional groups, which
makes this method valuable for preparing a wide variety of
bicyclic compounds. A reaction mechanism including the
oxidative coupling between the coordinated cycloheptatriene
and the internal alkyne, followed by an intramolecular
migration of the C(sp3)−Rh bond and reductive elimination
from the Rh(III)−pentadienyl complex to give the product, was
proposed. DFT calculations using a model Rh(I) catalyst were
carried out to verify the proposed reaction mechanism. The
strong additive effect observed for the CuI in these reactions
was also supported by the DFT calculations.
E
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EXPERIMENTAL SECTION
AUTHOR INFORMATION
Corresponding Author
*E-mail: htzhao@tju.edu.cn (H.Z.); wjh@tju.edu.cn (J.W.).
Notes
■
■
General Information. All reactions were carried out under N2 and
monitored by analytical thin-layer chromatography on 0.20 mm silica
gel plates; spots were detected by UV absorption. Silica gel (200−300
mesh) was used for column chromatography. 1H and 13C NMR
spectra were recorded on Bruker AV 400 and 600 MHz spectrometers
with CDCl3 as the solvent. The chemical shifts are reported in ppm
The authors declare no competing financial interest.
1
ACKNOWLEDGMENTS
relative to CHCl3 (δ 7.26) for H NMR and relative to the central
■
CDCl3 resonance (δ 77.0) for 13C NMR. The NMR data of the known
This work was supported by grants from the NSFC (21072149
and 20872108) and the Innovation Foundation of Tianjin
University. We thank Professor Dr. J. L. Wynn for language
assistance.
compounds were in agreement with literature values. Coupling
1
constants (J) are quoted in Hz for H. Multiplicities are reported as
follows: singlet (s), doublet (d), doublet of doublets (dd), triplet (t),
quartet (q), and multiplet (m).
Materials and Methods. Unless otherwise noted, all reactions
were performed under an atmosphere of dry N2 with oven-dried
glassware. Reactions were monitored by analytical thin-layer
chromatography on 0.20 mm Anhui Liangchen silica gel plates, and
spots were detected by UV absorption. Silica gel (200−300 mesh)
(from Anhui Liangchen Chemical Co., Ltd.) was used for flash
chromatography. All substrates were synthesized using methods
reported in the literature.33 Other chemicals or reagents were obtained
from commercial sources.
General Procedure for the Rh-Catalyzed Cycloaddition
Reactions. A Schlenk tube equipped with a magnetic stir bar was
charged with an alkyne (0.25 mmol), cycloheptatriene (46 mg, 0.5
mmol), [Rh(COD)Cl]2 (6.2 mg, 0.0125 mmol), CuI (4.8 mg, 0.025
mmol), PPh3 (6.6 mg, 0.025 mol), and xylene (0.5 mL). The tube was
kept under an N2 balloon and stirred at 120 °C for the required time.
The reactor was then cooled to room temperature, and the crude
mixture (typically a brown slurry) was filtered through a short plug of
SiO2 using EtOAc as the eluent to remove the rhodium residue. After
further purification by SiO2 column chromatography, the products
were analyzed by NMR.
Computational Details. Molecular geometries of the model
complexes were optimized without constraints via DFT calculations
using the M06 functional,34 as implemented in the Gaussian 09 suite
of programs.35 The effective core potentials (ECPs) of Hay and Wadt
with a double-ζ valence basis set (LanL2DZ)36 were used in describing
Rh and I, whereas the 6-31G* basis set37 was used for all other atoms.
Polarization functions were added for Rh (ζf = 1.350) and I (ζd =
0.340).38 Frequency calculations at the same level of theory were also
performed to identify all of the stationary points as minima (zero
imaginary frequencies) or transition states (one imaginary frequency)
and to provide free energies at 298.15 K which include entropic
contributions by taking into account the vibrational, rotational, and
translational motions of the species under consideration. Transition
states were located using the Berny algorithm. Intrinsic reaction
coordinates (IRC)39 were calculated for the transition states to
confirm that such structures connected the two relevant minima.
Charge analysis was done with a stand-alone NBO 5.9 program.32
To examine the basis set dependence, a larger basis set, i.e., the
triple-ζ basis set of def2-TZVP for Rh and I40 and the TZVP basis set
for all other atoms,41 was also employed to carry out single-point
calculations for the intermediates and the transition states shown in
Figure 2. The additional calculations show that the dependence of the
basis set is insignificant. For instance, using the smaller basis set, the
relative electronic energy barrier heights of III → TSIII-IV, V → TSV-
VI, and PIII → TSPIII-IV were 28.4, 9.0, and 20.1 kcal/mol,
respectively. Using the larger basis set, the barrier heights were 30.5,
11.2, and 21.6 kcal/mol, respectively.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures, and tables giving experimental details and
1
characterization data, H NMR and 13C NMR spectra of all
new compounds, and computational details. This material is
available free of charge via the Internet at http://pubs.acs.org.
F
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