Gold(I)-Catalyzed Conia-ene Cyclization of Internal ?-Acetylenic β-Ketoesters under High Pressure.

Article abstract of DOI:10.1002/cctc.201701388

The influence of pressure on the gold(I)-catalyzed Conia-ene cyclization of ?-acetylenic-β-ketoesters bearing an internal alkyne moiety was investigated for the first time. The reaction catalyzed by commercially available PPh₃AuNTf₂

Full text of DOI:10.1002/cctc.201701388

DOI: 10.1002/cctc.201701388
Full Papers
Gold(I)-Catalyzed Conia-ene Cyclization of Internal
e-Acetylenic b-Ketoesters under High Pressure.
[a]
Wojciech Chaładaj,*[a] Agata Kołodziejczyk,[a, b] and Sylwester Domanski
´
The influence of pressure on the gold(I)-catalyzed Conia-ene
cyclization of e-acetylenic-b-ketoesters bearing an internal
alkyne moiety was investigated for the first time. The reaction
catalyzed by commercially available PPh3AuNTf2 complex,
which is sluggish at ambient pressure owing to steric reasons,
proceeds smoothly under high-pressure conditions (6 kbar).
Mechanistic aspects of the reaction proceeding via 5-exo-dig
and 6-endo-dig cyclization pathways were also studied employ-
ing DFT methods. Differences in the reactivity of substrates
containing terminal and internal alkynes is discussed.
Introduction
Thermal cyclization of carbonyl compounds bearing
CÀC multiple bonds was described by Conia in the
late 1960s.[1] Although the classic reaction requires
extremely harsh conditions, it has attracted much at-
tention owing to its atom efficiency and the potential
synthetic utility of the products.[2] As a consequence,
many catalytic procedures that enable the cyclization
of acetylenic b-ketoesters, malonates, and related
compounds have been developed.[3] Cationic phos-
phine gold complexes[4] were shown to be extraordi-
narily active catalysts for the Conia-ene cyclization of
e-acetylenic b-ketoesters.[5] The reaction, however, is
sluggish for substrates bearing an internal alkyne
moiety, which was attributed to steric interactions of
the substituent at the alkyne moiety with the enolate
approaching from the opposite side relative to the
coordinated gold complex. In contrast, a shorter ho-
molog, cyclizing via 5-endo-dig mode, and thus with-
out such unfavorable interaction, underwent smooth
reaction under gold catalysis (Scheme 1).[6]
Scheme 1. Gold-catalyzed Conia-ene cyclization of acetylenic b-ketoesters.
In 2008, Balme reported an effective protocol for
the 5-exo-dig Conia-ene cyclization of substrates
bearing an internal alkyne moiety, employing a copper com-
plex as catalyst under microwave irradiation (Scheme 2a).[7] As
a result of the postulated mechanism that involves the coordi-
nation of the catalyst to both alkyne and b-ketoester moieties,
the configuration of the product was opposite to that
expected from the gold-catalyzed reaction. This class of com-
pounds can also be accessed via tandem Pd-catalyzed cycliza-
tion-coupling of terminal e-acetylenic-b-ketoesters and (heter-
o)aryl bromides.[8] In 2010, the problem of the inefficiency of
the 5-exo-dig cyclization under gold catalysis was approached
by Sawamura and coworkers through the application of com-
plexes with bulky semihollow phosphine ligands as catalysts
(Scheme 2b).[9] A mixture of two products, originating from 5-
exo-dig and 6-endo-dig cyclization, was obtained. The high effi-
ciency of the catalyst was explained by the bonding of the
alkyne substrate in the ligand pocket, which creates a steric
environment that makes the CÀC bond formation entropically
more favorable despite steric repulsion. Despite the conceptual
elegance of this solution, the use of a sophisticated ligand of
large molecular weight is a disadvantage. We anticipated that
a similar effect could be achieved by the application of gold
´
[a] Dr. W. Chaładaj, A. Kołodziejczyk, S. Domanski
Institute of Organic Chemistry
Polish Academy of Sciences
Kasprzaka 44/52, 01-224 Warsaw (Poland)
E-mail: wojciech.chaladaj@icho.edu.pl
[b] A. Kołodziejczyk
Institute of Physical Chemistry
Polish Academy of Sciences
Kasprzaka 44/52, 01-224 Warsaw (Poland)
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under https://doi.org/10.1002/
cctc.201701388.
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ganometallic chemistry and transition metal cataly-
sis.[15,16] In particular, the influence of pressure on re-
actions involving the activation of simple CÀC multi-
ple bonds by carbophilic Lewis acids (e.g. gold com-
plexes) is a virtually unexplored field. Notably, in the
early 2000s, as part of research that led to seminal
papers which initiated the boom of gold catalysis,[17]
Hashmi et al. conducted control experiments to ex-
clude the influence of high pressure (in kbar region),
among other methods.[18] Nevertheless, as additions
and cycloadditions to multiple bonds are generally
associated with negative volumes of activation, it is
to be expected that many reactions triggered by car-
bophilic activation of alkynes by AuI complexes
would also exhibit a negative volume of activation
and thus be accelerated by pressure.
Results and Discussion
To probe the feasibility of the concept, the influence
of pressure on the efficiency of gold-catalyzed Conia-
ene reaction was investigated. The cycloisomerization
of b-ketoester 1a, catalyzed by PPh3AuNTf2, was
chosen as a benchmark reaction. This reaction is slug-
gish at ambient pressure, providing only traces of the
expected products after 4 h, if catalyzed by 1 mol%
of PPh3AuNTf2 (Table 1, Entry 1). A mixture of isomeric
2a and 3a was obtained, resulting from 5-exo-dig
and 6-endo-dig cyclization, respectively (8:2 ratio of
2a and 3a). In contrast, the reaction run at 6 or 10
kbar exhibited considerably higher, but still unsatis-
factory, yields (Table 1, Entries 2–3). The regioselectivi-
ty was only slightly affected by pressure (76:24 and
74:26 ratio of 2a and 3a at 6 and 10 kbar, respective-
ly). Interestingly, better efficiency was observed if the
process was conducted at 6, rather than at 10 kbar.
The decrease in yield at higher pressure, in spite of a
Scheme 2. Various approaches to Conia-ene cyclization of internal e-acetylenic b-ketoest-
ers.
complexes ligated to simple phosphine ligands (e.g. PPh3Au)
under high hydrostatic pressure (Scheme 2c).
slightly higher conversion, could be attributed to pressure ac-
celeration of product degradation (Table 1, Entries 2 and 4 vs.
3 and 5). Further improvement was achieved by an increase in
catalyst loading (Table 1, Entries 4–6). Similarly, extending the
reaction time resulted in the enhancement of both conversions
and yields (Table 1, Entries 7–9).
Pressure is rarely considered a variable of interest during the
search for satisfactory reaction conditions, unless a gaseous re-
agent is employed. The application of hydrostatic pressure in
liquid systems, however, provides an interesting alternative to
heat, aggressive reagents (e.g. strong Lewis acids) or other
“non-classical” reaction conditions to overcome high activation
barriers.[10] Although not part of the classical toolbox of syn-
thetic organic chemistry, high pressure techniques reveal their
potential in some transformations, providing reactivities or se-
lectivities otherwise unreachable.[11] For instance, a Diels–Alder
reaction of thiophene run at 15 kbar is successful, illustrating
the power of the high-pressure approach, whereas this sub-
strate is otherwise virtually unreactive.[12] This strategy was also
found particularly efficient for reactions that are sluggish
owing to steric reasons.[13] The influence of pressure has been
studied for many organic transformations, including key steps
of natural product syntheses.[14] Considerably less attention has
been paid to the application of high pressure techniques in or-
Next, the scope of the reaction with respect to structure of
the substrate was investigated. An increase in the steric bulk
of the substituent at the alkyne moiety from Me to Et and iPr
resulted in the improvement of the selectivity to 90:10 and
95:5, respectively (Table 2, Entries 1–3). However, the enhance-
ment of the selectivity was accompanied by a drop in the
yield. In the latter case, higher catalyst loading was necessary
to obtain full conversion. An analogous substrate bearing a
bulky tBu group was unreactive, even if higher catalyst loading
(4 mol%) and pressure (up to 10 kbar) were applied. A phenyl
substituent on the alkyne terminus is tolerated under the reac-
tion conditions, leading to a mixture of isomeric products,
albeit with low selectivity (Table 2, Entry 4). Interestingly, the
selectivity was reversed towards the formation of the six-mem-
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Table 1. Influence of pressure on the model reaction.[a]
Table 2. Reaction scope.[a]
Entry
Cat. loading
[mol%]
Pressure, Time
[kbar, h]
Conversion[b]
[%]
Yield[b]
[%]
Entry
Substrate
Cat. loading Yield
Ratio 2:3
[mol%]
[%]
1
2
3
4
5
6
7
8
9
1
1
1
2
2
3
1
1
2
0.001, 4
6, 4
10, 4
6, 4
10, 4
6, 4
6, 24
10, 24
6, 24
6
39
43
67
74
90
79
80
99
3
25
21
55
40
76
55
40
80
1
2
74
76:24
2
3
4
2
4
4
63
52
58
90:10
95:5
[a] Conditions: Compound 1 (0.1 mmol), PPh3AuNTf2 (1–3 mol%), dodec-
ane (25 ml), CH2Cl2 (0.5 mL). [b] Determined by GC with dodecane as an
internal standard.
bered product, which could be attributed to electronic reasons
(stabilization of positive charge a to the aromatic substituent).
Change of the phenyl to a 4-cyanophenyl substituent on the
acetylene resulted in a significant drop in reactivity (Table 2,
Entry 5). The reaction run with 4 mol% of catalyst led to the
formation of the desired product in 34% yield and starting ma-
terial remained. Further increase in catalyst loading to 5 mol%
and pressure to 10 kbar gave the expected product with excel-
lent yield (96%) and selectivity (95:5). In contrast, the introduc-
tion of a 4-methoxyphenyl substituent at the alkyne terminus
caused instability of the substrate under the reaction condi-
tions, even under ambient pressure (Table 2, Entry 6). Variation
of the substituent in the ketoester moiety is also tolerated
under the reaction conditions (Table 2, Entries 7–8). Change of
the methyl substituent to the considerably more sterically de-
manding isopropyl group in the ester moiety had little if any
influence on the reaction outcome (c.f. Table 2, Entries 1 and
7). In contrast, a similar modification on the ketone moiety re-
sulted in a drop of reactivity, caused by steric interactions.
Higher catalyst loading (4 mol%) was necessary to reach full
conversion. Compounds containing an aromatic ring incorpo-
rated in the tether linking the ketoester and alkyne groups, as
well as lactone-derived ketoesters underwent cyclization under
the reaction conditions. In both cases 6-endo-dig was found to
be the major cyclization pattern.
40:60
5[b]
5
2
96
95:5
6
decomp.
–
7
8
2
4
68
59
80:20
80:20
9
2
4
76
82
37:63
29:71
To shed more light on the nature of gold-catalyzed Conia-
ene cyclization, especially of substrates bearing internal al-
kynes, the reaction was investigated computationally. To the
best of our knowledge, there is only one previous report of a
DFT study of the discussed reaction.[19] In this account, 5-exo-
dig cyclization of terminal alkynes was investigated. However,
the presented mechanistic picture is inconsistent. In the
schemes, the authors depicted the expected anti addition of
the nucleophile relative to the gold moiety, whereas the calcu-
lated structures of the transition state for cyclization and of
the vinyl-gold intermediate both show the ketoester and gold
10
[a] Conditions: e-Acetylenic-b-ketoester (0.5 mmol), PPh3AuNTf2 (2 or
4 mol%), CH2Cl2 (4.5 mL), RT, 6 kbar. [b] run at 10 kbar.
fragments on the same side of the double bond (correspond-
ing to the unreasonable syn carbometalation). This contrasts
with experimental data that supports the widely accepted
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mechanism proceeding via anti addition. Moreover, in the dis-
cussed report,[19] the calculations were performed on the E-
enol form of the b-ketoester, whereas the Z form is considera-
bly more stable owing to stabilization by an internal hydrogen
bond, which was left without comment.[20]
Consequently, a nucleophilic addition of the enol form of
the b-ketoester moiety to activated alkyne occurs, producing a
vinyl-Au intermediate (Figure 2),[24] which easily undergoes sub-
sequent protodeauration,[25] liberating the product and the cat-
ionic gold complex. This crucial carbometalation step deter-
mines the regio- (5- vs. 6-membered product) and stereoselec-
tivity (anti addition) of the reaction. Reaction pathways for 5-
exo-dig and 6-endo-dig cyclization were compared for sub-
strates bearing terminal and methyl-capped alkyne moieties
(blue and red profiles in Figure 3, respectively).[20] For the ter-
minal alkyne series, the energy barrier is significantly lower for
5-exo-dig than for 6-endo-dig cyclization. Such decrease could
originate in the slippage of gold upon coordination to the
alkyne moiety, causing desymmetrization of p and p* orbitals
and the development of a positive charge on one terminus of
the CÀC multiple bond. Consequently, the reaction leading
cleanly to the formation of the 5-membered product proceeds
smoothly. In contrast, the energy barriers of the cyclization of
compounds bearing internal alkyne moieties are relatively high
and comparable for both reaction modes. The calculated barri-
er for 5-exo-dig cyclization is considerably higher for substrates
containing a methyl-substituted alkyne than for a terminal one
(76.3 vs. 25.4 kJmolÀ1, 87.2 vs. 36.6 kJmolÀ1 including solva-
tion). Thus, the main contribution could be attributed to steric
hindrance caused by the presence of the methyl group at the
alkyne or to symmetric coordination of the metal center to the
alkyne. The transition state is also later for the reaction
The reaction starts with the coordination of the cationic
gold complex to the alkyne moiety of the substrate, which is
energetically highly favorable.[21] Interestingly, the geometry of
the coordination of gold to compounds bearing terminal and
internal alkynes differs significantly (Figure 1). Compound 4a
forms a complex exhibiting significant slippage of gold to-
wards the terminal carbon atom (AuÀCsp distances are 2.190
and 2.541 Š), whereas in 4b the metal is situated almost sym-
metrically (AuÀCsp distances are 2.298 and 2.320 Š). This phe-
nomenon is also reflected in the charge distribution and in the
shape of CÀC p and p* bonds which, according to Natural
Bond Orbitals analysis, are almost symmetrical (ꢀ50% contri-
bution of p orbitals from both C atoms) and noticeably polar-
ized (ꢀ60% contribution of p orbitals from one of C atoms)
for internal and terminal alkynes, respectively.[20] Furthermore,
calculated shielding of acetylenic carbons revealed a downfield
shift for both carbons (by 15.4 and 13.4 ppm) upon coordina-
tion of the internal alkyne. It contrast, complexation of a termi-
nal alkyne is reflected in a slight upfield shift of the carbon
atom situated closer to the metal center (0.8 ppm) and strong
downfield shift of the other one (50.1 ppm), which clearly indi-
cates an dissymmetric position of the metal. A similar phenom-
enon has been noticed for calculated PPh3Au+ complexes with
simple terminal and internal alkynes (propyne and 2-butyne)
lacking a neighboring nucleophile (e.g. a ketoester moiety).[20]
Such a dichotomy in the bonding of terminal and internal al-
kynes is fully consistent with the available NMR spectra of cat-
ionic gold(I) complexes with terminal[22] and internal[23] alkynes.
Dissymmetric gold coordination to the terminal alkyne moiety,
resulting in desymmetrization of p and p* orbitals of alkyne,
and formation of a significantly positive charge on one side of
the CÀC multiple bond could be one of the reasons for the
high preference towards 5-exo-dig cyclization of terminal e-ace-
tylenic-b-ketoesters.
Figure 1. Comparison of the coordination of a cationic gold-phosphine com-
plex to terminal and internal alkyne moieties, calculated at B3LYP/6-31g(d)
(SSD for gold) level of theory.
Figure 2. Structures of the transition states for AuPPh3+-mediated 5-exo-dig
and 6-endo-dig cyclization of terminal and Me-capped e-acetylenic-b-ketoest-
ers, calculated at B3LYP/6-31g(d) (SSD for gold) level of theory.
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and cycloadditions to CÀC multiple bonds, for exam-
ple, for Michael and Diels–Alder reactions.[27] The more
negative value obtained for the reaction of the more
sterically congested substrate is consistent with the
fact that the effect of pressure is more pronounced
for systems involving steric or strain factors.[13]
Conclusions
We demonstrated that pressure strongly accelerates
gold-catalyzed Conia-ene cyclization of internal e-ace-
tylenic-b-ketoesters, which is otherwise sluggish
owing to steric reasons. This is the first example of
the application of high-pressure techniques to reac-
tions triggered by carbophilic activation of alkynes
with p-acidic transition metal complexes. Theoretical
investigations of the title reaction are in good agree-
ment with the difference in the reactivity of sub-
strates bearing terminal and internal alkynes via 5-
exo-dig and 6-endo-dig cyclization modes. The calcu-
lated highly negative volume of activation is in line
with the significant pressure acceleration observed
for the Au-catalyzed cycloisomerization of internal e-
acetylenic-b-ketoesters.
Figure 3. Comparison of the energy profiles for 5-exo-dig and 6-endo-dig cyclization (car-
boauration) of terminal (blue) and Me-capped (red) e-acetylenic-b-ketoesters, calculated
at B3LYP/6-311+ +g(d,p)//B3LYP/6-31g(d) (Def2-TZVP//SSD for gold) level of theory.
Values in parentheses include solvation (CH2Cl2, PCM model).
Experimental Section
involving the internal alkyne than for the terminal one, which
is reflected in the CÀC distance of the forming bond (2.097 vs.
2.295 Š) and slightly in the reacting CÀC multiple bond (1.297
vs. 1.277 Š). Later transition states are generally expected for
reactions involving more sterically congested substrates.[13] In
the case of the 6-endo-dig cyclization, a significant part of the
barrier height could be ascribed to the inherent barrier associ-
ated with the formation of a 6-membered ring. Similarly to 5-
exo-dig cyclizations, in this case the transition state is also later
for the cyclization involving an internal alkyne, as indicated by
the CÀC distance of the forming bond (2.193 vs. 2.395 Š).
Finally, to rationalize the positive impact of the pressure on
the reaction rate, we have attempted to determine volumes of
the calculated structures. Unfortunately, the accuracy of the
volume calculation (inside a contour of 0.001 electronsBohrÀ3)
turned out to be insufficient, providing implausible values of
molar volumes and, as a consequence, activation and reaction
volumes. Such calculation of reaction volumes has been previ-
ously employed to support the mechanism involving pressure
acceleration of the [4+4] cycloaddition of [2.2] (9,10)anthraceno-
phane.[26] A considerably better approximation was achieved
employing the volume of the cavity calculated using the Polariz-
able Continuum Model (PCM) of solvation. The cavity is con-
structed employing a sum of interlocking van der Waals-spheres
centered at atomic positions. The calculated values of activa-
tion volumes for 5-exo-dig cyclization were À13.5 and
À35.9 cm3 molÀ1 for substrates bearing terminal and methyl
capped alkynes, respectively. These highly negative values agree
well with experimental observations of significant pressure ac-
celeration of the investigated reaction. Such highly negative
values (down to À40 cm3 molÀ1) are characteristic of additions
General procedure for gold-catalyzed Conia-ene cyclization
of internal e-acetylenic b-ketoesters.
e-acetylenic b-ketoester (0.5 mmol) was weighed in a 5-mL Teflon
ampoule. Then, a solution of PPh3AuNTf2 (typically 2 or 4 mol%) in
CH2Cl2 (ca. 3 mL) was added. The ampoule was closed with a
Teflon screw cap (with a small hole) fitted with a rubber O-ring.
Then, solvent (CH2Cl2) was introduced through a syringe to fill the
ampoule completely, which was then tightly sealed with a Teflon
screw (bolt) equipped with a rubber O-ring. The ampoule was then
placed in a high pressure chamber filled with petroleum ether and
compressed to 6 kbar. The procedure is illustrated in the Support-
ing Information.[20] After 24 h, the system was decompressed, the
reaction mixture was transferred to a flask, concentrated and sub-
jected to chromatography (silica gel, ca. 15 g, hexane-AcOEt 95:5
to 9:1).
A mixture of (Z)-methyl 1-acetyl-2-ethylidenecyclopentanecarboxy-
late (2a) and methyl 1-acetyl-2-methyl-2-cyclohexenecarboxylate
(3a) (2a/3a=75/25) was prepared through the reaction of methyl
2-acetyloct-6-ynoate 1a following the general procedure with
2 mol% of the catalyst. The mixture was isolated in 74% yield as a
1
colorless oil. H NMR of 2a (400 MHz, CDCl3): d=5.73–5.64 (m, 1H),
3.72 (s, 3H), 2.52–2.37 (m, 3H), 2.20 (s, 3H), 2.12–2.00 (m, 1H),
1.77–1.63 (m, 2H), 1.63–1.50 ppm (m, 3H); 13C NMR of 2a
(101 MHz, CDCl3): d=204.81, 172.23, 139.99, 122.91, 69.11, 52.27,
1
37.17, 35.16, 26.31, 24.48, 15.37 ppm; indicative signals in H NMR
of 3a (400 MHz, CDCl3): d=5.75 (brs, 1H), 3.74 (s, 3H), 2.35–2.25
(m, 1H), 2.16 (s, 3H), 1.96–1.85 (m, 1H), 1.74 ppm (s, 3H); 13C NMR
of 3a (101 MHz, CDCl3): d=206.16, 172.35, 129.63, 128.28, 65.49,
52.12, 29.89, 26.60, 25.04, 21.65, 18.59 ppm; IR (CH2Cl2): n=2952,
2877, 2842, 1737, 1710, 1435, 1356, 1261, 1234, 1136, 1085 cmÀ1
HRMS (ESI), m/z: calcd for: C11H16O3Na (M+Na+): 219.0997 Found
;
219.0995.
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All calculations were performed with Gaussian 09 package (Revi-
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SI) for gold and for all other atoms, respectively. For all stationary
points found, frequency calculations were performed to confirm
their nature (no and one imaginary frequency for minimum and
transition state, respectively) and to calculate corrections to ther-
modynamic functions. Single point solvation (dichloromethane) en-
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Acknowledgements
Financial support from the Polish National Science Centre (Grant
decision DEC-2013/11/D/ST5/02979) and Polish Ministry of Sci-
ence and Higher Education (stipend for W.C.) is gratefully ac-
knowledged. The authors wish to thank Professor Janusz Jurczak
for providing access to high pressure apparatus and Artur Uli-
kowski for his help and valuable discussion during manuscript
preparation.
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[20] See the Supporting Information for more details.
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Conflict of interest
The authors declare no conflict of interest.
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Keywords: Conia-ene
calculations · gold · high-pressure · homogeneous catalysis
reaction
·
density
functional
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Manuscript received: August 25, 2017
Revised manuscript received: October 16, 2017
ChemCatChem 2017, 9, 4334 –4339
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