This document summarizes a study that used density functional theory calculations to examine how substituents and gold catalysts influence the reaction pathways of gold-catalyzed cycloisomerization of 1,5-enynes. The study found that with an AuCl catalyst and simplest 1,5-enyne substrate, the pathway involving intramolecular hydrogen shift from C3 to C2 to form a bicyclo[3.1.0]hexane product was both kinetically and thermodynamically favored. Introduction of different substituents or use of a [AuPH3]+ catalyst was found to affect the relative energies of reaction pathways.

1. DFT Studies on Gold-Catalyzed Cycloisomerization of 1,5-Enynes
Ting Fan, Xihan Chen, Jianwei Sun,* and Zhenyang Lin*
Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s
Republic of China
*S Supporting Information
ABSTRACT: Gold-catalyzed cycloisomerization of 1,5-
enynes has been investigated with the aid of density functional
theory calculations at the B3LYP level of theory. We have
examined how substituents influence the reaction paths in the
cycloisomerization of 1,5-enynes catalyzed by both AuCl and
[AuL]+
(L = phosphine).
Over the past few years, transition-metal complexes have
emerged as powerful catalysts for the transformation of
open-chain enynes to a variety of cyclic compounds.1
Among
the various transition-metal catalysts, gold and platinum
complexes have been widely studied and considered as the
most powerful catalysts for the activation of the triple bond of
enynes.2,3
For all of the catalyzed reactions of enynes,
cycloisomerization reactions of 1,6-enynes and 1,5-enynes
have attracted considerable interest,4−7
because the products
of these transformations contain five-membered or six-
membered rings which could be the precursors for many
subsequent synthetic applications. However, in contrast to 1,6-
enynes, whose cycloisomerization mechanisms have been
widely investigated,8−11
the cycloisomerization reactions of
1,5-enynes have been relatively less studied.12
In these limited
studies, many interesting results were reported. Equations 1−3
(Scheme 1) give examples of various reported cycloisomeriza-
tion reactions of 1,5-enynes catalyzed by gold complexes.5,13,14
Equations 1 and 2 seem to imply that the cycloisomerization
products are sensitive to the substituents present. When eq 3 is
included, the results suggest that different gold(I) precursors
containing different ligands give different products. Gold(I)
catalysts with phosphine ligands are popular catalysts to access
product C.
Although a number of reaction mechanisms have been
proposed to account for the formation of the observed
products, how different substituents and different gold(I)
precursors affect the reaction mechanism remains unknown. In
other words, the whole picture of the mechanisms is not fully
understood. Therefore, it is necessary to carry out a systematic
theoretical study on the mechanisms of cycloisomerization of
1,5-enyes in order to have a full picture of the reaction
mechanisms. Equations 1−3 do not include reactions of 1,5-
enynes having an acetoxyl at the C3 or C4 position. The
reaction mechanisms of these 1,5-enynes are unique and mainly
involve 1,2- or 1,3-migration of the acetoxyl group to the C2
position following coordination of the alkyne moiety on the
enyne under consideration to the metal center.11b,14a,15
Thus,
the mechanisms are not considered in this paper. In eqs 1−3,
the cycloisomerization products are labeled according to the
reaction paths from which they are derived. In the discussion
below, we will see that four distinct paths can lead to the
formation of different observed products. They are paths a1, a2,
b, and c, leading to the cycloisomerization products A1 (and
A1′), A2, B, and C, respectively. It should be noted that the
products C in eqs 2 and 3 contain different substituents. They
are labeled the same for the purpose of convenience only.
It is commonly accepted that cycloisomerization of enynes
catalyzed by a gold complex is initiated by coordination of the
enyne triple bond moiety to the gold metal center.16,17
After
coordination, the nucleophilic double bond in the enyne attacks
C1 of the enyne triple bond to form a cyclopropyl gold
carbene, which then undergoes further transformations
according to the different paths shown in Scheme 1.17
As shown in Scheme 2, the gold carbene intermediate 2
could undergo further transformation via paths a1, a2, b, and c
to form different products. Paths a1 and b lead to the formation
of six-membered-ring products, whereas paths c and a2 lead to
the formation of five-membered-ring products. Paths a1 and a2
involve cleavage of the C1−C6 bond in 2 as the first step,
which leads to the formation of the bicyclo[2.2.1]hexane
intermediate Aint. In path a1, the intermediate Aint then
undergoes further cleavage of both the C1−C2 and C5−C6
bonds to give A1int, from which a 1,2-hydrogen shift to C2
leads to two different cyclohexadiene products (3A1 and 3A1′),
depending on whether the migrating hydrogen comes from C3
or C6. Path a2 involves cleavage of the C5−C6 bond of the
bicyclo[2.2.1]hexane intermediate Aint to form the intermediate
A2int, which then isomerizes to give a cyclopentene product
(3A2). Path b involves cleavage of the C1−C5 bond in 2
accompanied by a hydrogen shift from C6 to C5, giving
cyclohexadiene 3B as the product, which is quite different from
the two cyclohexadiene products 3A1 and 3A1′ via path a1.
Path c involves a hydrogen shift from C3 to C2 to form a
bicyclo[3.1.0]hexane product (3C).
In this paper, we investigate how different substituents and
different gold(I) precursors affect the reaction mechanism with
Received: February 29, 2012
Published: May 17, 2012
Article
pubs.acs.org/Organometallics
© 2012 American Chemical Society 4221 dx.doi.org/10.1021/om300167u | Organometallics 2012, 31, 4221−4227

2. the aid of density functional theory (DFT) calculations. Early
theoretical calculations mainly focused on studies of a particular
reaction pathway for specific substrates.12,16,17
In the DFT
calculations reported here, we consider the effects of substrates
R1
, R3
, and R6
, because their electronic and steric properties
significantly affect the properties of the gold carbene
intermediate 2. Thus, we considered the model substrates
R1
CCCHR3
CH2CHCHR6
(R1
= H, OSiH3, Ph; R3
= H,
OBn; R6
= H, Et, CH2CH2Ph). The model catalysts AuCl and
[AuPH3]+
were employed. [AuPH3]+
was used to model
[AuPPh3]+
to save computational cost. In the systems studied
here, [AuPH3]+
is a good model for [AuPPh3]+
because the
steric effects are minimal due to the fact that the number of
ligands around the gold metal center is small (vide infra). In
addition, there are many previous works employing [AuPH3]+
as a model for the catalytically active species [AuPPh3]+
.18
Scheme 1
Scheme 2
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3. ■ COMPUTATIONAL DETAILS
All the molecular geometries of the model complexes were optimized
at the Becke3LYP (B3LYP) level of density functional theory.19
The
effective core potentials (ECPs) of Hay and Wadt with double-ζ
valence basis sets (LanL2DZ)20
were chosen to describe Au, P, Si, and
Cl. In addition, polarization functions were added for Au (ζf = 1.05), P
(ζd = 0.387), Si (ζd = 0.284), and Cl (ζd = 0.64).21
The 6-31g(d) basis
set was used for all the other atoms such as C, H, and O. Frequency
calculations at the same level of theory were carried out to verify all of
the stationary points as minima (zero imaginary frequency) and
transition states (one imaginary frequency) as well as to provide free
energies at 298.15 K including entropic contributions. Intrinsic
reaction coordinates (IRC)22
were carried out to identify transition
states indeed connecting two relevant minima. To obtain solvation-
corrected relative free energies, we employed a continuum medium to
do single-point calculations for all species studied, using UAKS radii
on the conductor polarizable continuum model (CPCM).23
Dichloro-
methane was employed as the solvent (according to reaction
conditions) in the CPCM calculations. All calculations were performed
with the Gaussian 03 software package.24
In all of our DFT calculations, we employed the LanL2DZ ECP
basis set as described above. One might suggest employing the
Stuttgart-Dresden ECP for gold to better take relativistic effects into
account.25
We used the triple-ζ SDD basis set with the Stuttgart-
Dresden ECP for Au to calculate the rate-determining barriers for the
three pathways shown in Figure 2. These additional calculations show
that the dependence on the basis set is insignificant. Using the
LanL2DZ basis set, the rate-determining barriers for the a1, b, and c
pathways are 16.2, 20.3, and 22.0 kcal/mol, respectively. Using the
SDD basis set, the barriers are 17.3, 20.7, and 22.2 kcal/mol,
respectively.
It should be noted that a very recent article comparing different
DFT methods for studying gold-catalyzed reactions of propargyl esters
suggested that M06 and B2PLYP perform better than other DFT
methods.26
The results reported in this work suggest that B3LYP is
applicable to the systems we studied here.
■ RESULTS AND DISCUSSION
AuCl as the Catalyst. We first employed the catalyst AuCl
and considered the simplest model 1,5-enyne substrate HC
CCH2CH2CHCH2, which has all hydrogen atoms as the
substituents, in our calculations. According to the four different
paths shown in Scheme 1, we calculated all the intermediates
and transition states. Figure 1 shows the free energy profiles
calculated for three possible paths found in the calculations. We
were not able to locate a transition state to complete path a2. In
addition, no intermediate was located connecting 2 and A1int as
shown in Scheme 1. In other words, the intermediate Aint
having a charge separation does not correspond to a stationary
point on the potential energy surface when the simplest model
substrate is considered.
The calculation results indicate that path c is both kinetically
and thermodynamically favorable, leading to the formation of
the bicyclo[3.1.0]hexane product 3C. The rate-determining
step is related to the hydrogen shift from C3 to C2 from the
intermediate 2. The calculation results are consistent with the
experimental observation given in eq 2 when both the
substituents at C1 and C6 are simply hydrogen atoms.
Path c is the most favorable path. This is expected because it
involves only the hydrogen shift from C3 to C2 whereas the
other two paths involve (also) the cleavage of carbon−carbon
bond(s) and have comparable reaction barriers.
Comparing eqs 1 and 2 (Scheme 1), we found that the
substituent at C1 plays a very important role in determining the
reaction products. When the substituent is OTIPS, cyclo-
hexadiene products A1 and A1′ were obtained. On the basis of
Scheme 2, one can see that in this case path a1 is operative. To
examine how the OTIPS substituent at C1 affects the
energetics of the reaction paths, we calculated the various
paths shown in Scheme 2 using (H3SiO)CCCH2CH2CH
Figure 1. Potential energy profile calculated for cycloisomerization of the 1,5-enyne HCCCH2CH2CHCH2 catalyzed by AuCl. The solvation-
corrected relative free energies and gas-phase relative free energies (in parentheses) are given in kcal/mol.
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4. CH2 as the model substrate, in which −OSiH3 was used to
model −OTIPS.
Figure 2 shows the free energy profiles calculated. Again, we
found three possible paths from the calculations. With a siloxyl
substituent at C1, we indeed found that path a1 is kinetically
the most favored. The siloxyl substituent is able to stabilize the
intermediate AintSiloxyl_H, which is a zwitterionic species. The
overall barrier for this path has an activation barrier of 16.2
kcal/mol involving a two-step process from the intermediate
2Siloxyl_H to AintSiloxyl_H and then to A1intSiloxyl_H. When the
substrate has all hydrogen substituents, path a1 is less favored.
It becomes the most favored path here because the lone pair
electrons associated with the oxygen atom of the siloxyl
substituent OSiH3 can stabilize the zwitterionic intermediate
AintSiloxyl_H and greatly reduce its overall activation energy
barrier (16.2 kcal/mol from 2Siloxyl_H to TS3A1Siloxyl_H). Figure
3 shows the two transition state structures (TS2A1Siloxyl_H and
TS3A1Siloxyl_H) calculated. We can see clearly the bond
breaking and formation processes from the calculated
structures.
For path b, the siloxyl substituent OSiH3 can stabilize the
transition state TS2BSiloxyl_H by conjugation between the lone
pair of O in a pure p-type orbital and the double bond to be
formed. Therefore, the activation energy barrier (20.3 kcal/
mol) is slightly smaller in comparison to the corresponding
activation energy barrier (23.9 kcal/mol) shown in Figure 1.
The siloxyl substituent OSiH3 does not affect path c, as the C1
substituent has no direct influence on the hydrogen shift from
C3 to C2. Therefore, the activation energy (22.0 kcal/mol)
found here is almost the same as that (21.9 kcal/mol) in the all-
hydrogen model. The results are consistent with the
experimental observation shown in eq 1 that introduction of
OTIPS at C1 gave cyclohexadiene products A1 and A1́.
In the discussion above, we have seen that eqs 1 and 2
(Scheme 1) give products derived from paths a1 and c,
respectively, according to the mechanism shown in Scheme 2.
Interestingly, eq 3 gives three different products which
correspond to the products derived from paths c, a2, and b,
respectively. We noticed that the catalyst used in eq 3 is
different from that used in eqs 1 and 2. We also noticed that the
substituents on the substrates in eq 3 are also different from
those on the substrates in eqs 1 and 2. Here, the interesting
results observed in eq 3 could be due to the different catalyst
used and/or the different substituents. We studied the
substituent effect first using again AuCl as the catalyst.
Figure 2. Potential energy profile calculated for cycloisomerization of the 1,5-enyne (H3SiOCCCH2CH2CHCH2 catalyzed by AuCl. The
solvation-corrected relative free energies and gas-phase relative free energies (in parentheses) are given in kcal/mol.
Figure 3. Calculated structures for TS2A1Siloxyl_H and TS3A1Siloxyl_H.
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5. Then we use PhCCCH2CH2CHCH2 having a phenyl
substituent at C1 as the substrate. The energy profiles are very
similar to those in Figure 1, and no intermediate Aint was found
in path a1 or a2. The relative energies calculated for the
relevant intermediates and transition states are given in Table 1
(entry 2). Three possible paths (paths a1, b, and c) were found.
Path a2 still cannot be found. Path c has the lowest overall
reaction barrier (Table 1, entry 2). The results obtained clearly
cannot explain the experimental observation of formation of the
A2 product given in eq 3 by simply introducing a phenyl
substituent at C1.
Then we use PhCCCHOBnCH2CHCH2 as the
substrate having the substituent OBn at C3 (a model for a
substrate in eq 3). The energy profiles calculated are again
similar to those in Figure 1, and the energetic results are given
in Table 1 (entry 3). No intermediate Aint is found in path a1
or a2. Again, path a2 cannot be found. Introduction of the
substituent OBn at C3 lowers significantly the barrier from the
intermediate 2 to 3C for path c. Clearly, the electron-donating
property of OBn stabilizes 3C and therefore promotes greatly
the 1,2-hydrogen shift from C3 to C2.
Then we calculated the potential energy profiles by
considering PhCCCH2CH2CHCHEt as the substrate to
see if introduction of an Et substituent at C6 has any effect. The
energetic results are given in Table 1 (entry 4). The path a2,
leading to the formation of the product A2 cannot be found.
Introduction of an Et substituent at C6 increases significantly
the barrier for path b that involves cleavage of the C1−C5 bond
accompanied by a hydrogen shift from C6 to C5. All the
attempts described above do not allow us to find the path a2
that leads to the formation of the cyclopentene product A2.
Clearly, it is not the substituents that lead to observation of the
different products shown in eq 3. It is likely that the cationic
catalyst [AuPPh3]+
plays a crucial role.
[AuPH3]+
as the Catalyst. Carefully examining the relevant
products in eq 3 (Scheme 1), we easily find that only when R1
is Ph was the product cyclohexadiene B obtained. Thus,
reaction of PhCCCH2CH2CHCH2 as the substrate
catalyzed by [AuPH3]+
was examined. The energy profiles
resemble those shown in Figure 2, in which the intermediate
Aint was located. The energetic results are given in Table 2
(entry 2). The three familiar paths a1, b, and c were found,
which have overall activation energies of 15.8, 14.3, and 18.3
kcal/mol and can lead to the formation of 3A or 3A′,
cyclohexadiene 3B, and [3.1.0]bicyclohexene 3C, respectively.
Table 1. Solvation-Corrected Relative Free Energies (kcal/mol) Calculated for Au(I)-Catalyzed Cycloisomerization of the 1,5-
Enyne R1
CCCHR3
CH2CHCHR6 a
system 1 TS1 2 TS2 1int/3
R1
= H, R3
= H, R6
= H, 0.0 12.8 −10.7 path a1 13.7 −11.8
path b 13.2 −22.0
path c 11.2 −19.1
R1
= Ph, R3
= H, R6
= H, 0.0 15.0 −3.2 path a1 18.5 −6.2
path b 20.2 −13.9
path c 18.1 −13.9
R1
= Ph, R3
= OBn, R6
= H 0.0 16.0 −0.5 path a1 17.0 −7.6
path b 20.6 −13.4
path c 7.5 −17.4
R1
= Ph, R3
= H, R6
= Et 0.0 13.3 −2.0 path a1 19.6 1.1
path b 27.1 −16.2
path c 19.5 −11.3
a
See Scheme 1 and Figure 1 for species labels. The catalyst is AuCl in all cases.
Table 2. Solvation-Corrected Relative Free Energies (kcal/mol) Calculated for Au(I)-Catalyzed Cycloisomerization of the 1,5-
Enyne R1
CCCHR3
CH2CHCHR6 a
system 1 TS1 2 TS2 int TS3 1int/2int 3
R1
= OSiH3, R3
= H, R6
= H; cat. AuCl 0.0 9.4 −9.7 path a1 −0.9 −1.7 6.5 −16.7
path b 10.6 −24.0 NA NA
path c 12.3 NA NA NA −18.8
R1
= Ph, R3
= H, R6
= H; cat. [AuPH3]+
0.0 9.8 −2.6 path a1 9.5 9.2 13.2 −2.7
path b 11.7 −14.2 NA NA
path c 15.7 NA NA NA −11.3
R1
= Ph, R3
= H, R6
= Et; cat. [AuPH3]+
0.0 7.2 −3.9 path a2 10.8 9.8 12.9 4.7
path b 16.1 −19.9 NA NA
path c 17.5 NA NA NA −10.8
R1
= Ph, R3
= OBn, R6
= Ph(CH2)2; cat. [AuPH3]+
0.0 10.0 −0.4 path a2 9.9 9.6 12 4.1
path b 16.9 −19.3 NA NA
path c 8.1 NA NA NA −18.9
R1
= Ph, R3
= OBn, R6
= Ph(CH2)2; cat. [AuPPh3]+
0.0 12.3 3.1 path a2 15.0 14 16.7 7.7
path b 22.5 −15.1 NA NA
path c 10.8 NA NA NA −20.4
a
See Scheme 1 and Figure 2 for species labels. NA represents not applicable, meaning that the corresponding species does not correspond to a
stational point on the potential energy surface.
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6. However, the path a2 leading to the formation of cyclopentenes
still cannot be found.
Thus, we considered PhCCCH2CH2CHCHEt as the
substrate by introducing an Et substituent at C6. Interestingly,
the path a2 leading to the formation of cyclopentenes can now
be achieved with an overall activation energy of 16.8 kcal/mol
(Table 2, entry 3). The Et substituent is essential for the
formation of the intermediate A2+
int, making path a2 possible.
In A2+
int, we observed a hyperconjugation between C6 and one
of the C−H bonds in the Et substituent (Figure 4). The C−C−
H angle in the hyperconjugation moiety was calculated to be
103°. The hyperconjugation together with the electron-
donating properties of the Et substituent stabilizes the
intermediate A2+
intPh_Et. In addition, when phosphine ligands
(instead of chloride) are introduced, there is a chance to
achieve a cationic catalyst. A cationic catalyst should promote
path a2 because charge separation (Scheme 2) no longer exists
in the intermediate A2+
intPh_Et.
Finally, we also calculated the energy profile for the substrate
PhCCCHOBnCH2CHCH(CH2)2Ph to see if the sub-
stituent R3
= OBn has any effect on the product formation. We
can see from the energetic results given in Table 2 (entry 4)
that the OBn substituent has little effect on the path a2 leading
to the formation of cyclopentenes. However, it significantly
stabilizes TS2C+
Ph_Et, making the path c the most favorable. All
the three paths have relatively low barriers, consistent with the
experimental observation that products A2, B, and C were all
observed.14b
However, it should be pointed out that the
experimentally observed product ratio (A2:B:C ≈ 6:3:1)
cannot be well accounted for by the theoretical calculations
here.
All the results presented above were based on the model
catalyst [AuPH3]+
having the relatively small phosphine model
ligand PH3. To examine the steric and electronic effects missed
because of the use of the model phosphine, we calculated the
energy profiles for the substrate PhCCCHOBnCH2CH
CH(CH2)2Ph using the realistic PPh3 ligand in the calculations
(Table 2, entry 5). Comparison of entries 4 and 5 in Table 2
indicates that the PH3 and PPh3 results are similar, except that
PPh3 gives slightly higher barriers than PH3 does. We also made
a comparison of the calculated structures (see the Supporting
Information). The structures calculated from the PH3 model
and the realistic PPh3 are also similar to each other. On the
basis of these results, we conclude that PH3 is a proper model
for PPh3 in the systems we studied here.18
The results are
understandable because of the low coordination number
around the Au metal center in the species involved in the
reactions.
■ SUMMARY
With all of the results presented above, we have the following
general observations.
(1) Formation of the cyclopropyl gold carbene intermediate
2, which is the first step in the reaction mechanism
shown in Scheme 2, is facile for all of the substrates
studied in this work. In this step, the gold catalyst acts as
a Lewis acid, coordinating with the enyne triple bond
unit, increasing the affinity of the triple bond unit for the
enyne double bond, and facilitating the formation of the
reactive cyclopropyl gold carbene intermediate for
further transformation.
(2) Among the four possible paths (Scheme 2), we do not
find a path that has an inaccessibly high overall reaction
barrier. This explains that various products can be
obtained from the cycloisomerization reactions and the
products formed are sensitive to the gold catalyst used
and the substituents on the enyne substrates.
(3) An electron-donating substituent at C3 significantly
reduced the reaction barrier for path c, which involves
a hydrogen shift from C3 to C2 in the cyclopropyl gold
carbene intermediate. This is because an electron-
donating substituent at C3 enhances the interaction
between the gold(I) metal center and the olefin unit
formed as a result of the hydrogen shift.
(4) The first intermediate Aint, which is formed in path a and
is a starting point for paths a1 and a2, does not
correspond to a local minimum unless the substituent R1
is very strongly electron-donating, such as alkoxy, or the
cationic catalyst [AuPR3]+
(note: phosphine ligands have
high affinity for gold(I) and allow formation of a cationic
gold(I) catalyst) is employed. In such a case, path a2
cannot be located unless at C6 there is a highly electron-
donating substituent such as an alkoxy group.
(5) On the basis of all of the calculations, we observed that
paths a1 and a2 are mutually exclusive. A strongly
electron-donating substituent at C6 is essential for path
a2 to exist and to be feasible. When both R1
and R6
are
strongly electron-donating substituents, path a2 is certain
to exist.
■ ASSOCIATED CONTENT
*S Supporting Information
Text giving the complete ref 24, figures giving energy profiles,
and tables giving Cartesian coordinates and electronic energies
for all of the calculated structures. This material is available free
of charge via the Internet at http://pubs.acs.org.
■ AUTHOR INFORMATION
Corresponding Author
*E-mail: sunjw@ust.hk (J.S.); chzlin@ust.hk (Z.L.).
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This work was supported by the Research Grants Council of
Hong Kong (HKUST603711 and HKU1/CRF08).
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Organometallics Article
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