1,3-cationic alkylidene migration of nonclassical carbocation: A density functional theory study on gold(I)-catalyzed cycloisomerization of 1,5-enynes containing cyclopropene moiety

Article abstract of DOI:10.1021/ja410734e

For quite a long time, nonclassical carbocations have only been regarded as special intermediates with limited cases in solvolysis reactions. However, the present work shows that in common reaction, typical nonclassical carbocations may be involved in and have significant effects on the reaction mechanisms. In this work, DFT studies have been performed on the mechanism of gold(I)-catalyzed cycloisomerization of 1,5-enynes containing cyclopropene moiety at PBE1PBE/6-31+G/SDD level. An unprecedented pathway containing two consecutive 1,3-cationic alkylidene migrations of nonclassical carbocation intermediates derived from norbornenyl cation, rather than the generally considered Wagner-Meerwein 1,2-alkyl migrations, was found. Detailed structural analysis shows the nature of this 1,3-cationic alkylidene migration: it is promoted by strong cation-π interaction between the cationic center and the double bond. Topological analysis shows that for certain nonclassical carbocation intermediates (1c′-A and 1c′-F), there do exist bond critical point and bond path between the cationic center and the double bond. On the basis of the mechanisms proposed, the product selectivity controlled by the substituent effects was also rationalized.

Full text of DOI:10.1021/ja410734e

Article
pubs.acs.org/JACS
1,3-Cationic Alkylidene Migration of Nonclassical Carbocation:
A Density Functional Theory Study on Gold(I)-Catalyzed
Cycloisomerization of 1,5-Enynes Containing Cyclopropene Moiety
Qinghai Zhou and Yuxue Li*
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345
Lingling Road, Shanghai 200032, People’s Republic of China
S
* Supporting Information
ABSTRACT: For quite a long time, nonclassical carbocations
have only been regarded as special intermediates with limited
cases in solvolysis reactions. However, the present work shows
that in common reaction, typical nonclassical carbocations may
be involved in and have significant effects on the reaction
mechanisms. In this work, DFT studies have been performed
on the mechanism of gold(I)-catalyzed cycloisomerization of
1,5-enynes containing cyclopropene moiety at PBE1PBE/6-
31+G**/SDD level. An unprecedented pathway containing
two consecutive 1,3-cationic alkylidene migrations of non-
classical carbocation intermediates derived from norbornenyl cation, rather than the generally considered Wagner−Meerwein
1,2-alkyl migrations, was found. Detailed structural analysis shows the nature of this 1,3-cationic alkylidene migration: it is
promoted by strong cation−π interaction between the cationic center and the double bond. Topological analysis shows that for
certain nonclassical carbocation intermediates (1c′-A and 1c′-F), there do exist bond critical point and bond path between the
cationic center and the double bond. On the basis of the mechanisms proposed, the product selectivity controlled by the
substituent effects was also rationalized.
Scheme 1. (A) Normal 1,5-Enyne and 1,5-Enyne Containing
Cyclopropene Moiety; and (B) Typical Conditions and
Substrates for the Gold(I)-Catalyzed Cycloisomerization of
1,5-Enynes Containing Cyclopropene Moiety
INTRODUCTION
■
In the past few years, gold catalysis1 has been intensively
studied for its advantages such as mild reaction conditions
under room temperature, no need for inert atmosphere, and
diverse reaction modes. Among gold-catalyzed reactions,
cycloisomerization of enynes2−5 has drawn much attention, as
this type of reaction transforms linear, simple, and easy-to-get
substrates into various useful synthons in synthetic chemistry
with high yield and atom economy. Generally, the gold(I)-
catalyzed cycloisomerization of normal 1,5-enynes gives [3.1.0]
bicyclic compounds or cyclohexadiene compounds, and it is
commonly accepted that the Wagner−Meerwein 1,2-alkyl
migrations are often involved in the rearrangements of the
carbocation skeleton.2−4,6
Recently, Wang and co-workers7 connected C4 and C6 in
the normal 1,5-enyne skeleton with a single bond (Scheme 1A),
introducing cyclopropene moiety into the substrate. The typical
reaction conditions and substrates are listed in Scheme 1B.8
This gold(I)-catalyzed cycloisomerization of enyne 1 affords
benzene derivatives rapidly in high yields at room temperature.
The product distribution can be controlled by the substituents,
involving cleavages of both the carbon−carbon triple (C1
C2) and the double (C5C6) bonds (product 3) or neither of
them (product 2) to afford mixed products (for 1a) or solo
product (for 1b−1d), respectively.
Subsequently, the reaction diverges into Path-1 and Path-2.
Along Path-1, ring-enlargement of A and 1,2-H migration lead
to product 2. Path-2 accounts for the formation of the double-
cleavage product 3. The back-donation of the gold atom in A
Mechanisms were proposed7 as shown in Scheme 2. The
initial cyclization affords the vinyl gold intermediate A.
Received: October 21, 2013
© XXXX American Chemical Society
A
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proof of nonclassical 2-norbornyl cation was obtained.10o
However, for quite a long time, people used to think that
nonclassical carbocations are only special intermediates with
limited cases. The present work indicates that typical
nonclassical carbocations possibly exist in common chemical
reactions, acting as an important intermediate, and exerting
significant effects on the reaction mechanisms.
Scheme 2. Mechanisms Proposed for the Gold(I)-Catalyzed
Cycloisomerization of 1,5-Enynes Containing Cyclopropene
Moiety in Ref 7
MODELS AND METHODS
■
To disclose the reaction mechanisms, DFT13 studies using the
PBE1PBE14 method with Gaussian 0915 program have been
performed. Recent test calculations show that PBE0 is one of the
best functionals to calculate gold complexes.16 For C, H, O, and P, the
6-31+G** basis set was used; and for the Au atom, the SDD basis set
with effective core potential (ECP)17 was used. Harmonic vibrational
frequencies were calculated to identify all stationary points to be either
minima (with zero imaginary frequencies) or transition states (with
one imaginary frequency). Selected transition states have been checked
by intrinsic reaction coordinate (IRC) calculations.8 The Hirshfeld
atomic charges were calculated by the Multiwfn program.8,18 The
solvent effect was estimated by the IEFPCM19 method with UAHF
radii in dichloromethane (ε = 8.93) using the Gaussian 0320 program.
RESULTS AND DISCUSSION
■
Cyclization Pathways. First, substrate 1a was used to
leads to C or D. 1,2-Alkyl migration of D then yields E, bond
breaking of E generates F, and another 1,2-migration forms the
Dewar benzene-type intermediate G. Ring-enlargement of G
leads to H, and subsequent 1,2-H migration affords the final
product 3. Alternatively, D may transform to I via 1,3-alkyl
migration, and subsequently yield H.
explore the whole reaction pathways.8 As shown in Scheme 3
Scheme 3. Calculated 5-endo-dig and 4-exo-dig Cyclization
Pathways of the Reactant Complex 1a-R
a
However, our test calculations show that the key
intermediate G is not a stationary point on the potential
energy surface, and I is not a minimum, but like a transition
structure between F and J.8 These results indicate that there
must be new pathways between intermediate A and product 3.
Kozmin et al. suggested that the double-cleavage products of
cycloisomerization of siloxy-substituted 1,5-enynes are gen-
erated via 1,2-alkyl migrations. This mechanism has been
supported by DFT calculations.3,4 The double-cleavage
products of cycloisomerization of 1,6-enynes resulted from a
concerted homoallylic cation rearrangement via a four-
membered ring transition state, which has been well studied
by Echavarren and Yu et al.5 Although several theoretical
studies4 have been done on the cycloisomerization of normal
1,5-enynes, the mechanism of the present case to form the
double-cleavage product is indeed intriguing.
In this Article, we proposed a mechanism characterized by
two consecutive 1,3-cationic alkylidene migrations of typical
nonclassical carbocation intermediates derived from norbor-
nenyl cation. It is apparently different from the commonly
called 1,3-carbon and 1,3-alkylidiene migration,9a−c because the
migrated group here is a carbocation and the cationic center
remains unchanged during the process. Nonclassical carboca-
tions have been proposed to participate in the skeletal
rearrangement in metal-catalyzed isomerization of enynes.9g−j
However, detailed theoretical studies and clear structural
analyses are seldom.9j Here, this idea was confirmed further
by our study.
a
A′ is a cyclization intermediate of normal 1,5-enyne taken from ref 4c
(reoptimized).8 The relative free energies in dichloromethane (298.15
K, 1.0 atm) are in kcal/mol, calculated at the PBE1PBE/6-31+G**/
SDD level.
and Figure 1, the gold catalyst coordinates with substrate 1a
forming the reactant complex 1a-R. The π-electron of the
double bond in the cyclopropene moiety then attacks the C
C triple bond. The energy barrier of 4-exo-dig cyclization is
much higher than that of 5-endo-dig cyclization (10.2 kcal/mol
vs 3.4 kcal/mol). The 5-endo-dig cyclization transition state 1a-
TS-5-endo-dig contains a five-membered ring. The C2−C3−
C4 angle is 105.8°, which is close to the ideal bond angle in an
sp3-hybridized carbon atom (109.5°), whereas in the 4-exo-dig
cyclization transition state 1a-TS-4-exo-dig, there is a four-
membered ring with a C2−C3−C4 angle of 94.9°. Obviously,
the ring strain is a main factor controlling the selectivity in the
cyclization step.
Since the discovery of norbornyl and norbornenyl cation by
Winstein and Trifan in 1949 and 1955, so many famous groups
have been dedicated to the study of nonclassical carboca-
tions.10−12 Schleyer, Olah, Houk, etc., did profound theoretical
studies. After more than a half century, a breakthrough has been
made recently by Krossing et al., when the crystallographic
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Figure 1. The optimized structures on the cyclization pathways with selected bond lengths (in Å), bond angles (in degrees), and Hirshfeld atomic
charges (positive in red, negative in blue). The structure of A′ is also given. The relative free energies in solvent ΔGsol (298.15 K, 1.0 atm) are in
kcal/mol. The details of the Ph groups and the PPh3 ligand are not shown for clarity. Calculated at the PBE1PBE/6-31+G**/SDD level.
Scheme 4. Calculated Direct Ring-Enlargement (Path-1) and 1,3-Cationic Alkylidene Migration (Path-3) Pathways of the Vinyl
a
Gold Intermediate 1a-A
a
The relative free energies in dichloromethane ΔGsol (298.15 K, 1.0 atm) are in kcal/mol. Calculated at the PBE1PBE/6-31+G**/SDD level.
For normal 1,5-enynes,4c in the bicyclo intermediate A′
(Scheme 3 and Figure 1), C1 forms a full C−C bond with both
C5 and C6 (1.546 and 1.556 Å), and the C1−C2 bond is
elongated to 1.435 Å. The formal positive charge localizes on
C2 and the Au atom, and may promote the subsequent 1,2-
alkyl migrations. However, in 1a-A, the C4−C6 bond prevents
C1 and C5 from forming a full σ-bond, and the C1−C5
distance is much longer (1.717 Å). Thus the formal positive
charge localizes on C5, and the C1−C2 bond largely remains a
double bond (1.397 Å). These differences between 1a-A and A′
lead to different pathways in the subsequent reactions.
Ring-Enlargement Pathway (Path-1). As shown in the
left part of Scheme 4 and Figure 2, along Path-1, the original
C4−C6 bond breaks (1a-1-TS1), leading to intermediate 1a-B.
Subsequent 1,2-H migration (1a-1-TS2) affords the final
product 2a, accompanied by the regeneration of the gold
catalyst. This pathway is similar to that of normal 1,5-enynes,
which breaks the C1−C5 bond in the ring-enlargement process
and leads to the cyclohexadiene products.4c
1,3-Cationic Alkylidene Migration Pathway (Path-3).
In our test calculations, only the intermediates A, F, J, and H in
Path-2 (Scheme 2) were successfully located.8 Inspired by
these findings, we proposed a new reaction pathway to go
C
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Figure 2. The optimized structures on Path-1 and Path-3 with selected bond lengths (in Å) and Hirshfeld atomic charges (in red). The relative free
energies in solvent ΔGsol (298.15 K, 1.0 atm) are in kcal/mol. Details of the Ph groups and the PPh3 ligand are not shown for clarity. Calculated at
the PBE1PBE/6-31+G**/SDD level.
through these possible intermediates. As shown in Scheme 5,
the initial cyclization intermediate A undergoes 1,3-cationic
process, the π-electrons of 1a-F on C1 attack C5, forming C5−
C1 bond, and the C1−C6 double bond migrates to C6−C4,
leading to intermediate 1a-J. Next, the concerted4c,8 ring-
enlargement and 1,2 H-migration (1a-3-TS3) over a barrier of
6.7 kcal/mol leads to the double-cleavage product 3,
accompanied by the regeneration of the gold catalyst. It is
interesting that both 1a-A and 1a-J have bicyclo[3.1.0]hexene
backbones, and undergo a similar ring-enlargement process to
form the final products. The largest barrier of this reaction is
12.3 kcal/mol, which is well consistent with the fact that the
reaction is completed within 5 min.
Scheme 5. New Mechanism Proposed for the
Cycloisomerization (Path-3)
Nonclassical Carbocation Intermediates in the Re-
action. The intermediates 1a-A and 1a-F bring to mind the
famous nonclassical 2-norbornenyl carbocation (Scheme
6A).10−12 They have similar structures in which the cationic
center is stabilized by π-donation of the neighboring double
bond. Furthermore, 1a-F is very similar to the bicyclo[2.1.1]-
hex-2-en-5-ylium cation (M).10k,l,11a In the 2-norbornenyl
cation, the distances between the cationic center and the
carbon atoms of the double bond are 1.88 and 1.87 Å,
respectively.10j,l In M these distances are 1.741 and 1.744 Å,
respectively.10k,l In 1a-A, the cationic center C5 faces the
double bond unsymmetrically. Therefore, C5 has a stronger
interaction with the alkene carbon C1 (C5−C1 = 1.717 Å)
than with C2 (C5−C2 = 2.414 Å). 1a-F has more symmetrical
structure (C5−C6 = 1.799 Å, C5−C1 = 1.664 Å), and is 7.2
kcal/mol more stable than 1a-A. In 1a-F, a styrene fragment
comes into being. The conjugation of the phenyl group with
the double bond, and better electron donation to the cationic
center C5 may account for its stability. However, in 1a-J, C5−
C4 and C5−C6 are 2.778 and 2.403 Å, respectively, indicating
alkylidene migration, breaking the C5−C6 bond and forming
the C5−C2 bond to afford F, and another 1,3-cationic
alkylidene migration breaking the C5−C4 bond and forming
the C5−C1 bond leads to J. The following ring-enlargement of
J breaks the C1−C2 bond, yielding the final double-cleavage
product 3. This process can be envisioned as a tertiary
carbocation walking and turning around on a pentadiene
framework.
As shown in Scheme 4 and Figure 2, in the first 1,3-cationic
alkylidene migration (1a-3-TS1), the π-electrons of the C1
C2 double bond in 1a-A attack the cationic center C5. Over a
small barrier of 1.5 kcal/mol, the C5−C2 bond forms, the C5−
C6 bond breaks, and the C1−C2 double bond simultaneously
migrates to C1−C6, leading to bicyclo[2.1.1]hexene inter-
mediate 1a-F. Thus the cationic center C5 migrates from C6 to
the Au-linked carbon C2. The C5−C6 bond break corresponds
to the CC double bond break in substrate 1a. Subsequently,
1a-F undergoes a further 1,3-cationic alkylidene migration via
transition state 1a-3-TS2 over a barrier of 12.3 kcal/mol. In this
D
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Scheme 6. (A) Famous Nonclassical 2-Norbornenyl Cation,
Bicyclo[2.1.1]hex-2-en-5-ylium Cation (M), and
Intermediates 1a-A and 1a-F in the Isomerization Reaction;
and (B) Classical Carbocation Resonance Structures of 1a-A
and 1a-F
that there are only weak interactions between the cationic
center C5 and the double bond. This may result from that the
Au-linked carboanion C2 is bonded with C5 and thus stabilizes
the carbocation. As shown in Scheme 6B, the resonance
structures indicate that A, D, and C shown in Scheme 2 are
actually one species, and so are F and E. The atoms in
molecules (AIM)12,18d,e,21 topological analysis on intermediates
A and F for substrates 1a, 1b, and 1c′ (using methyl groups
instead of nBu groups in 1c) has been done (Figure 3). The
results show that for 1c′-A and 1c′-F, there do exist bond path
and bond critical points between C5 and C1, which are similar
to a 1,2,4,7-antitetramethyl-2-norbornyl cation with a C−C
bond distance of 1.710 Å.12a The larger Wiberg bond indices
(WBI)21d of C1−C5 in 1c′-A and 1c′-F also indicate stronger
interaction.
Figure 3. The molecule graphs and the Laplacian of the electron
density distribution (∇2ρ) of the positive C5 and the double bond
C1−C2/C6−C1 plane in intermediates A and F for substrates 1a, 1b,
and 1c′ (R2 = R3 = Me). The blue points indicate the bond critical
points, and the black lines between the atom pair are bond paths. The
selected bond lengths are in antstroms. The Wiberg bond indices
(WBI) of C1−C5 are given in italic.
Nature of the 1,3-Cationic Alkylidene Migration. To
disclose the nature of this 1,3-cationic alkylidene migration, the
1,3-alkyl migration of neutral molecule 5-exo-methylbicyclo
[2.1.1] hex-2-ene (K) to 6-exo-methylbicyclo [3.1.0] hex-2-ene
(L) was used as a reference system (Scheme 7A).9a−c The
framework rearrangement of K to L is similar to that of 1a-F to
1a-A or 1a-J, both involving isomerization between [2.1.1] and
[3.1.0] bicyclic structures. Theoretical studies22 show that the
isomerization of K to L may involve a diradical mechanism, and
the reaction barrier is higher than 30 kcal/mol, which is
consistent with the high reaction temperature of 120 οC. In the
transition state TS-KL, the bridging carbon C5 is far away from
both C2 and C6 (2.470 and 2.424 Å, respectively);22b that is,
the old bond is broken, but the new one is not formed yet. This
may account for the high reaction barrier.
The isomerization of 1a-A to 1a-F and 1a-F to 1a-J is very
easy, and the barriers are only 1.5 and 12.3 kcal/mol,
respectively. However, the barrier of the isomerization of L
to K is higher than 54.1 kcal/mol. In transition state 1a-3-TS1,
the C5−C6 bond is not fully broken (1.476 Å), and C5 has
strong interaction with C1 (1.658 Å) and C2 (2.176 Å). In
transition state 1a-3-TS2, the C5−C4 bond is nearly broken
(2.068 Å), but the C5−C1 bond is nearly fully formed (1.489
Å). A depiction of the orbital interactions in 1,3-cationic
alkylidene migration between 1a-A and 1a-F is proposed in
Scheme 7B based on an inspection of the MOs of these
species.8 For 1a-A, one p orbital and one sp2 orbital on C5 are
involved in the 1,3-cationic alkylidene migration: the p orbital
containing the positive charge attacks the double bond,
overlaps with the p orbital on C2, and evolves into a sp2
orbital, forms the new C5−C2 σ bond; the C5−C6 σ bond
breaks, the sp2 orbital on C5 evolves into the p orbital
containing the positive charge, and the sp2 orbital on C6
evolves into the p orbital of the new double bond. Thus, the
easy 1,3-cationic alkylidene migration of A to F and F to J may
be ascribed to the strong interaction between the carbocation
C5 and the double bond on the five-membered ring. To test
this hypothesis, the isomerization of L+, which is formed by
abstracting a hydride from C5 in L, was calculated. As expected,
the isomerization barrier of L+ to K+ is only 1.6 kcal/mol, which
is similar to that of 1a-A to 1a-F. Therefore, this easy 1,3-
migration is promoted by the strong cation−π interaction in
the nonclassical carbocations.
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Scheme 7. (A) Depiction of Orbital Interactions in the 1,3-
Alkyl Migration of K to L Based on Ref 22b; (B) Depiction
of Orbital Interactions in the 1,3-Cationic Alkylidene
Migration of the Nonclassical Carbocation 1a-A to 1a-F; and
a
(C) 1,3-Cationic Alkylidene Migration of L+ to K+
a
The data in (B) and (C) were calculated at the PBE1PBE/6-
31+G**/SDD level; the relative free energies in solvent ΔGsol (298.15
K, 1.0 atm) are in kcal/mol. Here, K, 1a-A, and L+ are used as the zero
energy reference points, respectively.
Figure 4. The transition states of the rate-determining steps along
Path-1 and Path-3 of substrates 1a, 1b, and 1c′. Here, 1a-F, 1b-F, and
1c′-F were used as the zero energy reference points, respectively. The
details of the Ph groups and the PPh3 ligand are not shown for clarity.
The selected bond lengths are in angstroms. The relative free energies
in solvent ΔGsol (298.15 K, 1.0 atm) are in kcal/mol. The green
numbers indicate the favorable transition states. Calculated at the
PBE1PBE/6-31+G**/SDD level.
In Wagner−Meerwein rearrangement, the nucleophilic alkyl
group moves to an adjacent carbocation via 1,2-migration to
generate another more stable carbocation species, and at the
position where the alkyl group left forms the new cationic
center (Scheme 2). However, in the 1,3-cationic alkylidene
migration here, it is the cationic center C5 that migrates, and
the cationic center remains on C5 in the whole process. The
1,4-carbon migration of cation obtained by removing the
hydride on C3 in L has been studied by Winstein et al.9d−f In
this structure, the positive charges delocalize on the five-
membered ring. Recently, we also found 1,3-cationic alkylidene
migration in 1,6-enyne cycloisomerization reactions.23 There-
fore, this reaction pattern might be expected to have some
broader applications.
Origin of the Product Selectivity of 2 and 3. As shown
in Scheme 4, the rate-determining steps of Path-1 and Path-3
are the ring-enlargement process (1a-1-TS1) and the second
1,3-cationic alkylidene migration process (1a-3-TS2), respec-
tively. As compared to these two rate-determining steps, the
barrier of the first 1,3-cationic alkylidene migration (1a-3-TS1)
is much lower and therefore very fast, and so the intermediate
1a-A should transform to 1a-F quickly. Therefore, 1a-F is the
“real reactant” for the competing reactions along Path-1 and
Path-3. For convenience, in this section, 1a-F, 1b-F, and 1c′-F
were used as the zero energy reference points, respectively. The
calculated results of transition states 1-TS1 and 3-TS2 of
substrates 1a, 1b, and 1c′ (Figure 4) are well consistent with
the experimental observations (see Scheme 1). 1a-1-TS1 and
1a-3-TS2 have nearly identical relative energies, indicating that
both 2a and 3a can be equally obtained (2a:3a = 1:1). For
substrate 1b, 1b-3-TS2 is 3.1 kcal/mol higher in energy than
1b-1-TS1, indicating that only 2b can be obtained (2b:3b =
1:0), whereas for substrate 1c′, 1c′-3-TS2 is 4.8 kcal/mol lower
in energy than 1c′-1-TS1, which means only 3c′ can be
obtained (2c:3c = 0:1), and thus the product selectivity is
reversed. We found that the product selectivity can be
rationalized by the electronic effects of the substituents. In
the transition states, the hydroxyl groups are attached to sp3-
hybridized carbon atom C3. Therefore, only the electron-
withdrawing inductive effect is working. The phenyl group has a
much stronger electron-donating conjugation effect than do the
alkyl groups.
In the direct ring-enlargement step (1-TS1), the positive
charges on C5 will delocalize into the forming six-membered
ring. Therefore, electron-donating groups on the six-membered
ring will facilitate this process. As compared to 1a-1-TS1, the
absence of the electron-withdrawing hydroxyl group in 1b-1-
TS1 accelerates the ring-enlargement. However, in 1c′-1-TS1,
the displacement of two phenyl groups with methyl groups
makes the ring-enlargement much more difficult.
The 1,3-cationic alkylidene migration transition states (3-
TS2) all partially possess nonclassical carbocation properties. In
these structures, the electron density on the CC double bond
determines the ability to stabilize carbocation C5. The electron-
withdrawing hydroxyl group will decrease the stabilizing effect
to some degree; thus 1a-3-TS2 is higher in energy than 1b-3-
TS2. In 1c′-3-TS2, the two phenyl groups are changed to
methyl groups, and the barrier rises as expected. The charge
donation from the double bond to the cationic center C5 is
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evidenced by the C4−C6 bond elongation; that is, the longer is
the bond, the lower is the energy. In 1b-3-TS2, 1a-3-TS2, and
1c′-3-TS2, the C4−C6 bond lengths are 1.398, 1.396, and
1.385 Å, respectively. Possibly due to that the cationic center
C5 is stabilized by the partially bonded carboanion C2, 3-TS2
is less sensitive to the electronic effects of the substituents. This
leads to the reversal of the product selectivity, although the
energy change of 1-TS1 and 3-TS2 has the same trend.
Unsymmetrical Substrate 1d. For the unsymmetrical
substrate 1d, only the TMS group migrated product 3d was
obtained (see Scheme 1B).7 Therefore, 1d is a good case to
further test with the mechanism proposed in Scheme 4. The
key intermediate and transition states were calculated using
model 1d′ in which R2 = Me. As shown in Scheme 8, the ring-
Scheme 8. Calculated Key Intermediate and Transition
a
States for Substrate 1d Using 1d′ (R2 = Me)
Figure 5. Structures of 1d′-3-TS2-TMS and 1d′-3-TS2-Me with
selected bond lengths (in Å) and bond angles (in degrees). The
relative free energies in solvent ΔGsol (298.15 K, 1.0 atm) are in kcal/
mol. Calculated at the PBE1PBE/6-31+G**/SDD level.
cationic center and the double bond. These results show that in
a common reaction, typical nonclassical carbocation may be
involved in and have significant effects on the reaction
mechanisms. The selectivity of the initial cyclization is
controlled by ring strain, and the product distribution is mainly
controlled by the electronic effects of the substituents.
a
The relative free energies in solvent ΔGsol (298.15 K, 1.0 atm) are in
kcal/mol. Calculated at the PBE1PBE/6-31+G**/SDD level. 1d′-R is
ASSOCIATED CONTENT
* Supporting Information
used as the zero energy reference point.8
■
S
Computational details, the relaxed potential energy surface
(PES) scan, selected IRC calculations, selected molecular
orbitals, and the calculated total energies and geometrical
coordinates. This material is available free of charge via the
Internet at http://pubs.acs.org.
enlargement processes are much higher in energy than the 1,3-
migrations. The 1,3-migration of the TMS-substituted cationic
alkylidene (1d′-3-TS2-TMS) is 6.1 kcal/mol more favorable
than that of 1,3-migration of the methyl group-substituted
cationic alkylidene (1d′-3-TS2-Me). These results are well
consistent with the experimental observations and can be
rationalized by strong hyperconjugation of the C8−Si bond of
the TMS group to the cationic center C5. As shown in Figure 5,
in 1d′-3-TS2-TMS, the C9−Si−C5 bond angle is 109.8°.
However, the C8−Si−C5 bond angle decreases to 102.9°, and
the C8−Si bond obviously bends to C5, whereas in 1d′-3-TS2-
Me the smallest bond angle H−C7−C6 is 106.1°. The Si−C5
bond in 1d′-3-TS2-TMS is shorter than that in 1d′-3-TS2-Me
(1.903 vs 1.922 Å), indicating stronger interaction between C5
and the TMS group in 1d′-3-TS2-TMS.
AUTHOR INFORMATION
Corresponding Author
liyuxue@sioc.ac.cn
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This project is supported by the National Basic Research
Program of China (973 Program, no. 2009CB825300) and the
Natural Science Foundation of China (Grant nos. 21172248,
21372249, 21121062).
CONCLUSION
■
DFT studies have been performed on the mechanism of
gold(I)-catalyzed cycloisomerization of 1,5-enynes containing
cyclopropene moiety at the PBE1PBE/6-31+G**/SDD level.
An unprecedented pathway containing two consecutive 1,3-
cationic alkylidene migrations of nonclassical carbocation
intermediates was proposed. This 1,3-cationic alkylidene
migration is promoted by strong cation−π interaction between
the cationic center and the double bond. Topological analysis
shows that for certain nonclassical carbocation intermediates,
there do exist bond critical point and bond path between the
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