Mechanisms of the Au- and Pt-catalyzed intramolecular acetylenic schmidt reactions: A DFT study

Article abstract of DOI:10.1021/jo1017469

The reaction mechanisms of the PtCl₄- and Au(I)-catalyzed intramolecular acetylenic Schmidt reactions were analyzed by means of hybrid density functional calculations at the B3LYP/6-31G*(LANL2DZ) level of theory for better understanding of the

Full text of DOI:10.1021/jo1017469

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Mechanisms of the Au- and Pt-Catalyzed Intramolecular Acetylenic
Schmidt Reactions: A DFT Study
Yuanzhi Xia*,† and Genping Huang†,‡
†College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035,
People’s Republic of China, and ‡Qinghai Institute of Salt Lakes, Chinese Academy of Sciences,
Xining 810008, People’s Republic of China
xyz@wzu.edu.cn; xiayz04@mails.gucas.ac.cn
Received September 6, 2010
The reaction mechanisms of the PtCl4- and Au(I)-catalyzed intramolecular acetylenic Schmidt reac-
tions were analyzed by means of hybrid density functional calculations at the B3LYP/6-31G*(LANL2DZ)
level of theory for better understanding of the acceleration effect of ethanol solvent in PtCl4-catalyzed
reaction and the different catalytic activities of Au and Pt catalysts. Calculations indicate the rate of
the PtCl4-catalyzed reaction in noncoordinative solvent of 1,2-dichloroethane is limited by isomer-
ization of the relatively stable chelate complex to the reactive π-complex of PtCl4 with the acetylenic
moiety of homopropargyl azide substrate, which requires an activation energy of 29.6 kcal/mol. All nucle-
ophilic cyclization, dinitrogen elimination, and 1,2-H shift of metal-carbene steps are quite facile. The
generation of 2H-pyrrole intermediate in PtCl4-catalyzed reaction is completed by a ligand substitu-
tion reaction, and the final 2H-pyrrole to 1H-pyrrole isomerization is an intermolecular process with
another 2H-pyrrole as a proton shuttle. When in ethanol solution, the favorable coordination of
solvent molecules with PtCl4 could inhibit the chelation of PtCl4 with the homopropargyl azide. Besides,
the alcohol coordination also facilitates the generation of 2H-pyrrole intermediate and the intermolec-
ular isomerization of 2H-pyrrole to 1H-pyrrole. Consequently, the overall activation barrier of PtCl4-
catalyzed reaction in ethanol solution is lowered to 21.5 kcal/mol, determined by the H-abstraction step
of the intermolecular 2H-pyrrole to 1H-pyrrole isomerization. The basic steps in the Au(I)-catalyzed
reaction are similar to those in the PtCl4-catalyzed one. However, no chelate complex could be formed
from PR3AuSbF6 and homopropargyl azide, and the 2H-pyrrole generation step is much more favorable,
indicating weaker interactions of Au(I) catalyst with the homopropargyl azide and the C-C double bond
of 2H-pyrrole.
1. Introduction
chemistry,1 material science,2 and polymer synthesis.3 Fur-
thermore, they are useful building blocks in the synthesis of
many physiologically interesting natural products as well as
Pyrroles are one of the most important classes of hetero-
cyclic compounds due to their wide appearance in medicinal
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Published on Web 10/27/2010
DOI: 10.1021/jo1017469
r
2010 American Chemical Society

Xia and Huang
JOCArticle
in heterocyclic chemistry.4 Therefore, many methods exist for
the organic synthesis of pyrrole derivatives. The classical
methods for pyrrole synthesis include the Paal-Knorr con-
densation reaction,5 Knorr reaction,6 Hantzsch reaction,7 and
Barton-Zard reaction.8 Recently, the development of metal-
catalyzed one-, two-, or multi-component reactions for synthe-
sis of highly substituted pyrroles has attracted tremendous
interest.9
SCHEME 1
Over the past decade, organic chemists have witnessed a
significant advance in the π-acidic transition metal-catalyzed
cyclization of unsaturated precursors for the synthesis of carbo-
and heterocycles,10 and new methodologies based on plati-
num and gold catalysis have grown into a major field of exper-
imental11 as well as theoretical research.12 In this respect, a
variety of methods for the synthesis of pyrroles are described
by using gold catalysts,13 while the employment of platinum
complexes is rare.14 The recent Schmidt reaction of homo-
propargyl azides from the Toste group and the Hiroya group
represents a rare example of pyrrole synthesis in which both
Au and Pt complexes are capable catalysts (Scheme 1).15a,b
Despite the convenient synthesis of substituted pyrrole by
both methods, it is interesting to find that different reaction
conditions were employed in the reactions. While Toste et al.
found that the combination of (dppm)Au2Cl2 with AgSbF6
could efficiently promote the conversion of homopropargyl
azide into substituted pyrrole in dichloromethane solution
(Condition A), the efficiency of this transformation was more
condition-dependent in the Pt-catalyzed reaction of Hiroya
and co-workers. Namely, the PtCl4-catalyzed reactions were
rather sluggish in 1,2-dichloroethane solution. Interestingly,
great improvement of the reaction rate was observed if the
PtCl4 catalyst was stirred in ethanol solution for 1 h at 50 °C
before adding the substrate (Condition B). Different catalyt-
ic activities of Au and Pt complexes have been observed in
many reactions, and the understanding of such difference from
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The plausible mechanism of these metal-catalyzed Schmidt
reactions is outlined in Scheme 2. In this mechanism, the
carbophilic transition metal Pt or Au first activates the triple
bond of A and triggers the intramolecular nucleophilic addi-
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to carbene intermediate D, which could produce the 2H-pyrrole
(E) and regenerate the active catalyst via a formal 1,2-H shift.
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SCHEME 2
Finally, the 1H-pyrrole product (F) is formed from the tauto-
merization of 2H-pyrrole. In addition, another possible mech-
anism initiated by metal-promoted decomposition of the azide
for generation of a nitrene-like intermediate was not ruled out
by the experiments.15a,b
studies of gold chemistry have revealed that such H migra-
tions could be assisted by alcohol, water, or ligands.17 Third,
the 2H-pyrrole (D) has been proposed as a key intermediate
in many pyrrole syntheses;18 however, the tautomerization
of 2H-pyrrole to 1H-pyrrole is difficult, as theoretical studies
at different levels have shown the activation barrier is over
34 kcal/mol.19 The details of this tautomerization process in
Au- and Pt-catalyzed intramolecular acetylenic Schmidt reac-
tions are unclear. Moreover, an in-depth comparison of the
mechanism of the Au-catalyzed reaction with that of the Pt-
catalyzed one is important to understand the different catalytic
activity of Au and Pt.
The detailed catalytic cycle of Au and Pt catalysts and the
solvent effect on Pt-catalyzed reaction are not easily acces-
sible by experiment. Therefore, we report herein a DFT com-
putational study on the Au- and Pt-catalyzed intramolecular
acetylenic Schmidt reactions, aiming to better understand
the experimental observations and to reveal the origin of dif-
ferent catalytic activity of the two catalysts at a molecular
level. The experimental results are satisfactorily explained
from the discussion of detailed potential energy surfaces and
We are very interested in the different catalytic activity of
Au and Pt catalysts in the intramolecular Schmidt reac-
tions of homopropargyl azides, particularly how the PtCl4-
catalyzed reaction is accelerated in ethanol solution and why
an induction period is necessary. According to the experimen-
tal facts and the possible mechanism, several aspects of these
reactions are worthy of investigation. First, since ethanol is a
coordinative solvent, we hypothesized that the first coordi-
nation of solvent molecule to the platinum catalyst may occur
during the induction period. But why further reactions could
be facilitated by the ethanol coordination is unknown. Second,
the mechanism in Scheme 2 suggests that the 1,2-H shift is
involved in D to E transformation, thus the possibility of
ethanol-catalyzed H-shift should be considered. Several recent
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the geometric structures of intermediates and transition states
involved in these transformations. While recent works mainly
focused on the importance of alcohol (and water) in H-migra-
tionsingoldcatalysis,17 the present study uncovered the signif-
icance of solvent coordination with catalyst in the PtCl4-
catalyzed reaction, that the precoordination of alcohol solvent
could inhibit the chelation of PtCl4 catalyst with homoprop-
argyl azide substrate and thus facilitates the following intra-
molecular nucleophilic cyclization. The influence of alcohol
ligand on other steps was also investigated. The computa-
tional results indicated the generation of 1H-pyrrole product
from 2H-pyrrole intermediate is an intermolecular process in
both Au- and Pt-catalyzed reactions, in which the 2H-pyrrole
itself acts as proton shuttle to catalyze the 1,5-H shift, and great-
ly lowers the tautomerization barrier to about 20 kcal/mol.
Furthermore, the feature of Au(I) catalyst in the concerned
reaction was demonstrated by comparing the energy of each
fundamental step with that of the Pt catalyst.
in numerous theoretical simulations of the mechanisms of Pt-
and Au-catalyzed reactions.12,17,24 The vibrational frequencies
were computed at the same level of theory as that for the geom-
etry optimization to check whether the optimized geometrical
structure is at an energy minimum (no imaginary frequency) or
a transition state (only one imaginary frequency). Intrinsic reac-
tion coordinate (IRC) calculations were used to confirm that the
transition states connected the related reactants and products.25
Bulk solvent effectswere computed by the Polarizable Continuum
Model (PCM) with UAO radii at the B3LYP/6-31G*(LANL2DZ)
level using the gas-phase optimized structures.26 The dielectric
constant in the PCM calculations was set to ε=10.360, 24.550,
and 8.930 for simulation of dichloroethane (DCE), ethanol (EtOH),
and dichloromethane (DCM), respectively, the solvents used in
experiments. Solvation free energies (ΔGDCE,ΔGEtOH,andΔGDCM
)
were calculated by adding the solvation energies to the computed
gas-phase relative free energies (ΔG298). All relative energies were
referred to the energy sum of free reactant and catalyst. Unless
otherwise stated, only the energy and geometry of the lowest
energy conformer were provided for a compound that had more
than one conformation.
In the experiments of Toste et al. the bidentate phosphine digold(I)
complexes (2.5% (dppm)Au2Cl2 and 5% AgSbF6) were found
tobetheoptimal catalysts;however, thecombinationof 5%PPh3-
AuCl and 5% AgSbF6 could also produce the 2,5-disubstituted
pyrrole in 72% yield.15a Previous works considered cationic
[PPh3Au]þ species as the active catalyst in such systems,24a,b
whereas calculations indicated that the [PPh3Au]þ moiety has
strong interaction with the SbF6- ligand in reactions.17b,c,g,27
The simplification of the experimentally used PPh3 ligand to PH3
was adopted in many theoretical studies.17a,b,24a,b Thus, it is rea-
sonable to use PH3AuSbF6 for simulation of the gold(I) catalyst
in the present study. To fully understand the solvent effect in
PtCl4-catalyzed reactions, the possibilities of solvent molecules
as ligands and as proton shuttles were considered. In these cases
the explicit ethanol and dichloroethane solvents were modeled
by methanol and chloromethane, respectively, for minimizing
the CPU time.
2. Computational Method
All of the DFT calculations were performed with the Gauss-
ian 09 program package.20 The geometry optimizations of all
minima and transition states involved were performed at the
B3LYP levels of theory.21 The 6-31G* basis set was used for C,
H, O, P, Cl, and N atoms,22 while the LANL2DZ basis set was
used for Pt and Au.23 These methods have been proven reliable
(20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson,
G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.;
Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.;
Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.;
Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.;
Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.;
Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.;
Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.;
Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.;
Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth,
G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas,
O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision A.02; Gaussian, Inc., Wallingford, CT, 2009.
(21) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Becke, A. D.
J. Chem. Phys. 1993, 98, 1372. (c) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B
1988, 37, 785.
(22) (a) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,
2257. (b) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.
(23) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt,
W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt, W. R.
J. Chem. Phys. 1985, 82, 299.
3. Results and Discussion
In this section, we will first present the PtCl4-catalyzed intra-
molecular Schmidt reaction of homopropargyl azides in 1,2-
dichloroethane solution to reveal why the reaction was slug-
gish under this condition, and then the rate acceleration effect
of ethanol solvent will be analyzed. Next, the details of 2H-
pyrrole to 1H-pyrrole tautomerization will be discussed. Final-
ly the potential energy surface of the PR3AuSbF6-catalyzed
reaction is depicted and comparison is made to highlight the
difference between Pt and Au catalysts.
(24) Calculations at the B3LYP/6-31þþG**(LANL2DZþp) level indi-
cate the relative energy values in the current systems are not sensitive to the
basis set. Details are given in the Supporting Information. For recent DFT
3.1. Computational Results for the PtCl4-Catalyzed Acet-
ylenic Schmidt Reaction in 1,2-Dichloroethane Solution. Accord-
ing to the proposed mechanism in Scheme 2, the detailed poten-
tial energy surface of the PtCl4-catalyzed Schmidt reaction of
homopropargyl azide 1 in 1,2-dichloroethane solution is depicted
in Figure 1. Geometric structures of selected transition states and
intermediates are collected in Figure 2, and the computed struc-
tures for all other species are given in the Supporting Information.
The possible nitrene pathway is higher in energy and is given in the
Supporting Information.
ꢀ
ꢀ
studies of Au and Pt catalysis, see: (a) Perez, A. G.; Lopez, C. S.; Marco-
Contelles, J.; Faza, O. N.; Soriano, E.; Lera, A. R. d. J. Org. Chem. 2009,
74, 2982. (b) Lee, Y. T.; Kang, Y. K.; Chung, Y. K. J. Org. Chem. 2009, 74,
7922.(c) Soriano, E.; Marco-Contelles, J. J. Org. Chem. 2007, 72, 1443.
(d)Soriano,E.;Marco-Contelles,J.Organometallics2006, 25, 4542. (e)Nevado,
C.; Echavarren, A. M. Chem.;Eur. J. 2005, 11, 3155. (f) Nieto-Oberhuber, C.;
~
Munoz, M. P.; Bunuel, E.; Nevado, C.; Cardenas, D. J.; Echavarren, A. M.
Angew. Chem., Int. Ed. 2004, 43, 2402. (g) Nevado, C.; Cardenas, D. J.;
~
ꢀ
ꢀ
ꢀ
~
Echavarren, A. M. Chem.;Eur. J. 2003, 9, 2627. (h) Mendez, M.; Munoz,
M. P.; Nevado, C.; Cardenas, D. J.; Echavarren, A. M. J. Am. Chem. Soc. 2001,
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123, 10511. (i) Martın-Matute, B.; Cardenas, D. J.; Echavarren, A. M. Angew.
Chem., Int. Ed. 2001, 40, 4754. (j) Lemiere, G.; Gandon, V.; Cariou, K.; Hours,
A.; Fukuyama, T.; Dhimane, A.-L.; Fensterbank, L.; Malacria, M. J. Am.
Chem. Soc. 2009, 131, 2993. (k) Cheong, P. H.-Y.; Morganelli, P.; Luzung,
M. R.; Houk, K. N.; Toste, F. D. J. Am. Chem. Soc. 2008, 130, 4517. (l) Nieto-
ꢀ
ꢁ
This cyclization reaction starts with the coordination of
PtCl4 catalyst with substrate 1. Figure 1 shows that the
ꢀ
Oberhuber, C.; Perez-Galan, P.;Herrero-Gomez, E.; Lauterbach, T.; Rodrıguez,
C.; Lopez, S.; Bour, C.; Rosellon, A.; Cardenas, D. J.; Echavarren, A. M. J. Am.
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
~
Chem. Soc. 2008, 130, 269. (m) Nieto-Oberhuber, C.; Lopez, S.; Munoz, M. P.;
(25) (a) Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1989, 90, 2154.
(b) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523.
(26) Tomasi, J.; Persico, M. Chem. Rev. 1994, 94, 2027.
(27) See the Supporting Information for details.
ꢀ
~
Cardenas, D. J.; Bunuel, E.; Nevado, C.; Echavarren, A. M. Angew. Chem., Int.
Ed. 2005, 44, 6146. (n) Soriano, E.; Marco-Contelles, J. J. Org. Chem. 2005, 70,
9345.
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FIGURE 1. Potential energy surface for the PtCl4-catalyzed acetylenic Schmidt reaction in 1,2-dichloroethane solution.
FIGURE 2. Geometric structures for selected intermediates and transition states of the PtCl4-catalyzed reaction in 1,2-dichloroethane solu-
tion; selected distances are in angstroms.
formation of chelate complex 2 is exergonic by 37.3 kcal/mol.
In this complex, both the carbon-carbon triple bond and the
proximal nitrogen atom of the homopropargyl azide are coor-
dinated with the Pt center, and the Pt-N, Pt-C1, and Pt-C2
7846 J. Org. Chem. Vol. 75, No. 22, 2010

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SCHEME 3
distances are 2.164, 2.610, and 2.358 A, respectively. To
effect the intramolecular cyclization, complex 2 could first
rearrange into isomeric complex 3 via dissociation of the Pt-N
coordination. The transition state corresponding to this pro-
cess is TS1, which has a much more elongated Pt-N distance
of 4.328 A and is 29.6 kcal/mol higher in solvation free energy
than 2. The resulting intermediate 3 is a η2-π-complex of PtCl4
with 1 with the distances between Pt and two sp-hybridized
carbon atoms of 2.405 and 2.436 A. The formation of 3 from 2
is endergonic by 28.3 kcal/mol in 1,2-dichloroethane solution.
The subsequent intramolecular nucleophilic cyclization from 3
is quite facile with an activation energy of only 4.2 kcal/mol in
the gas phase via transition state TS2, in which the forming
C1-N distance is 2.310 A and Pt is connected to C2 with a
distance of 2.140 A. The generated cyclic intermediate 4 is
about 50 kcal/mol lower in energy than 3. In 4, the Pt-C2
distance is shortened to 1.966 A. The solvation free energy of
the following dinitrogen elimination transition state TS3 is
only 5 kcal/mol higher than that of 4. Upon the formation of
Pt-carbene 5, the exergonicity is increased to 109.9 kcal/mol,
indicating the nucleophilic cyclization and dinitrogen elimina-
tion steps are highly energetically favorable. Geometries show
that the breaking N-N distance in TS3 is only slightly elon-
gated by 0.11 A compared to that distance in 4, and the Pt-C2
distance is further shortened to 1.905 A in 5.
The following step is a formal 1,2-H migration in the Pt-
carbene intermediate 5. The activation barrier of this step
(via TS4) is about 13.1 kcal/mol in 1,2-dichloroethane solu-
tion. In TS4, the shifting hydrogen atom is closer to C2, with
the H-C2 and H-C3 distances of 1.221 and 1.517 A, respec-
tively. IRC calculation indicates the intermediate resulting
from 1,2-H shift is 6, which is 12.2 kcal/mol more stable than
5. In intermediate 6, C2 and C3 are σ-bonded with the Pt
atom and one Cl ligand, respectively, with the C3-Cl and
Pt-C2 distances of 1.881 and 2.095 A, resulting from the
addition of one Pt-Cl bond across the C2-C3 double bond.
The Pt-Cl distance in 6 is 2.536 A, being about 0.2 A longer
than the other three normal Pt-Cl distances. To generate the
2H-pyrrole intermediate, 6 could first isomerize to π com-
plex 7 via TS5. This step requires an activation energy of
16.4 kcal/mol, and 7 is 17 kcal/mol less stable than 6. In TS5,
the Pt-Cl and C2-C3 distances are shortened to 2.417 and
1.444 A, respectively, and the Pt-C2 nad C3-Cl distances
are elongated to 2.130 and 2.631 A. The C2-C3 distance in 7
is 1.433 A, significantly longer than the normal C-C double
bond distance.28 This implies the strong interaction between
C2-C3 and PtCl4. As a result, the dissociation of 7 into PtCl4
and 2H-pyrrole 8 is endergonic by 18.2 kcal/mol. Thus, the
direct formation of 8 is unfavorable due to the large energy
gap (35.2 kcal/mol) from 6 to 8 and PtCl4. We envisioned a
ligand exchange reaction between substrate 1 and complex 7
to be more feasible. Indeed, the free energy is increased by
only 1 kcal/mol for the formation of complex 9 from the
coordination of 1 with 7. The interaction between Pt and the
C2-C3 double bond is weakened in complex 9, as evidenced
by the obviously elongated Pt-C2 and Pt-C3 distances of
2.547 and 2.465 A, respectively. Then the dissociation of
9 into 8 and 3 is endergonic by 8.2 kcal/mol. Accordingly, the
energy required for generation of 2H-pyrrole 8 from 6 is 26.2
kcal/mol via the intermediacy of complexes 7 and 9.
The final step of the reaction is tautomerization of 2H-
pyrrole 8 to 1H-pyrrole 10. The 1,5-H shift transition state
TS6 is 62.5 kcal/mol higher in energy than intermediate 6.
Alternatively, the coordination of PtCl4 with the N site of 8 is
exergonic by 33 kcal/mol, leading to intermediate 11, which
is the global minimum on the potential energy surface. How-
ever, the activation barrier for the 1,5-H shift via TS7 is still
(28) (a) McMurry, J. Organic Chemistry, 5th ed.; Thomson Learning:
Pacific Grove, CA, 1999. (b) Wang, S. C.; Troast, D. M.; Conda-Sheridan,
M.; Zuo, G.; LaGarde, D.; Louie, J.; Tantillo, D. J. J. Org. Chem. 2009, 74,
7822. (c) Zhao, Y.-L.; Suhrada, C. P.; Jung, M. E.; Houk, K. N. J. Am. Chem.
Soc. 2006, 128, 11106.
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FIGURE 3. Geometric structures for complexes 13, 14, and 30; selected distances are in angstroms.
SCHEME 4
as high as 49.9 kcal/mol. These results indicate that the
tautomerization of 2H-pyrrole 8 to 1H-pyrrole 10 via an
intramolecular 1,5-H migration is not feasible kinetically.29
In fact, later results of this study find the intermolecular
H-migrations are more favorable, and the isomerization of
chelate complex 2 to 3 is the rate-limiting step of the PtCl4-
catalyzed reaction in 1,2-dichloroethane solution.
Thus, the computational results in Figure 1 reveal that all
the nucleophilic cyclization, dinitrogen elimination, and 1,2-
H shift of Pt-carbene steps are relatively easy for the PtCl4-
catalyzed reaction in 1,2-dichloroethane solution. However,
the reaction efficiency is limited by several factors, including
(1) the formation of chelated complex 2 retards the following
intramolecular cyclization, as the isomerization of 2 to 3
requires an activation energy of 29.6 kcal/mol, (2) the energy
for generation of 2H-pyrrole 8 from σ-bonded complex 6 is
about 26.2 kcal/mol, indicating this step is also energetically
demanding, and (3) the intramolecular 1,5-H shift for con-
verting 2H-pyrrole intermediate to 1H-pyrrole product is not pos-
sible due to the very high activation barrier of 49.9 kcal/mol
of this process.
PCM solvation model calculations. However, only marginal
variation of the potential energy surface is detected. Con-
sidering the fact that an induction period was necessary for
rate acceleration of Hiroya’s reactions in ethanol solution,
we propose the first complexation of PtCl4 catalyst with
alcohol solvent may occur during this period. Besides, the
coordination of PtCl4 with alcohol solvent may ease the 2H-
pyrrole generation step, as Figure 1 has demonstrated the
promoting effect of substrate coordination on this step. In
addition, the presence of alcohol as proton shuttle to facil-
itate the 2H-pyrrole to 1H-pyrrole tautomerization is also
possible. Therefore, the influences of alcohol solvent on the
key steps of PtCl4-catalyzed reaction are detailed here.
The coordination energy of alcohol solvent (explicit sol-
vent is exemplified by MeOH) to PtCl4 catalyst is given in
Scheme 3. The formation of 13 from PtCl4 and one MeOH
is exergonic by 24.9 kcal/mol, and the solvation free energy
is further decreased to -34.1 kcal/mol upon the formation
of hexacoordinated complex 14 (Figure 3). This energy is
comparable to the formation energy of chelate complex 2.
Hence, it is most likely that complex 14 is formed first during
the induction period. In reactions, equilibration of 14 to 13
by losing one alcohol ligand is a facile process with the solva-
tion free energy increased by only 9.2 kcal/mol. Coordination
of 13 to the carbon-carbon triple bond of substrate 1 is
almost thermodynamically neutral, as 30 is only 0.4 kcal/mol
higher in energy than 13. Then from 30, the following nucle-
ophilic cyclization and dinitrogen elimination steps are quite
facile and irreversible (vide infra). These results demonstrate
that the precoordination of alcohol solvent prevents the
3.2. HowDoesAlcohol Solvent Accelerate the PtCl4-Catalyzed
Reaction? The above results disclose the factors that may
influence the PtCl4-catalyzed reaction in 1,2-dichloroethane.
We next focus on the acceleration effect of alcohol solvent on
the reaction. We first change the solvent to ethanol in the
(29) (a) Hashmi, A. S. K.; Pankajakshan, S.; Rudolph, M.; Enns, E.;
Bander, T.; Rominger, F.; Freyb, W. Adv. Synth. Catal. 2009, 351, 2855.
€
ꢀ
(b) Hashmi, A. S. K.; Buhrle, M.; Salathe, R.; Batsb, J. W. Adv. Synth. Catal.
2008, 350, 2059.
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FIGURE 4. Geometric structures for 60, TS50, and 70; selected distances are in angstroms.
SCHEME 5
aActivation free energies in ethanol solution. Detailed potential energy surfaces are depicted in the Supporting Information.
chelation of PtCl4 with homopropargyl azide 1, and provides a
low-energy-lying pathway to the reactive reactant complex 30.30
Although the coordination of 13 with the proximal nitrogen
of 1 to give 2a0 is slightly exergonic, calculations suggest the
following reaction from this intermediate is kinetically
unfavorable.27
In 1,2-dichloroethane solution, the energy needed for
generation of 2H-pyrrole 8 from 6 is about 26.2 kcal/mol via
the ligand exchange reaction. The influence of alcohol ligand
on this process is given in Scheme 4, and optimized geome-
tries are depicted in Figure 4. The free energy is increased by
7.3 kcal/mol for the complexation of 6 with MeOH to give 60.
Then via TS50 the σ-bonded complex 60 is transformed to
π-bonded complex 70. Due to endergonic formation of 60
from 6 and MeOH, the activation energy for β-Cl elimina-
tion via TS50 is 15.4 kcal/mol, similar to that of the pro-
cess without alcohol ligand. In contrast to the unfavor-
able dissociations of complexes 7 and 9, the dissociation of 70
into 13 and 8 is almost energetically neutral. Thus, the gen-
eration of 2H-pyrrole intermediate 8 from 6 is facilitated by
the coordination of alcohol solvent, and the barrier of this
process is decreased to 15.4 kcal/mol, which is determined by
the energy gap between 6 and TS50.
According to Figure 1, the tautomerization of 2H-pyrrole
to 1H-pyrrole via an intramolecular 1,5-H shift (TS6 or TS7)
is unfeasible, thus the activation energies of possible inter-
molecular H migrations are calculated. The possible path-
ways of alcohol solvent-assisted H-migrations are illustrated
in Scheme 5, which shows the involvement of two MeOH
molecules could efficiently lower the activation barrier to
around 30 kcal/mol. When starting from 2H-pyyrole 8,
the tautomerization could be realize via a seven-membered
ring transition state (TS8) with an activation free energy of
28.4 kcal/mol. However, the formation of 8 from 6 with the
assistance of alcohol coordination is endergonic by 11.2 kcal/
mol (Scheme 4), thus making the tautomerization via TS8
unfavorable as the overall barrier is about 40 kcal/mol.
Alternatively, the formation of 11 from 2H-pyrrole and PtCl4
is highly exergonic in catalytic reaction (Figure 1). Calcula-
tions indicate the 1,5-H shift of 11 could be catalyzed by a
(30) As expected, the coordination of PtCl4 with 1,2-dichloroethane solvent
is rather weak. The exergonicities for formations of complexes PtCl4(CH3Cl)
and PtCl4(CH3Cl)2 from PtCl4 and CH3Cl are only 4.6 and 8.1 kcal/mol,
respectively.
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SCHEME 6
aActivation free energies in ethanol solution. Detailed potential energy surfaces are depicted in Supporting Information. bActivation energy for the
first step.
cluster containing two methanol molecules via a stepwise
generation of intermediate 16. In this zwitterionic species the
protonated 2H-pyrrole moiety interacts with the delocalized
negative charge via H-bonding interactions. Geometries in
Figure 5 show the H-C1, H-C2, H-C3, H-C4, and H-N
distances in 16 are 2.396, 2.384, 2.093, 1.926, and 2.181 A,
respectively. As a result of the charge separation, intermedi-
ate 16 is highly unstable and could be transformed to 10 and 8
via TS12 with an activation energy of only 0.8 kcal/mol. In
fact, TS12 is not a typical hydrogen transfer transition state.
The imaginary vibrational mode shows in TS12 the cationic
moiety is moving toward the nitrogen atom of the pyrrole
anion. The geometry of TS12 reveals the H-C2, H-C3, and
H-C4 are elongated to 2.548, 2.361, and 1.943 A, respec-
tively, while the H-C1 and H-N distances are shortened to
2.295 and 1.954 A. As mentioned above, however, the for-
mation of 2H-pyrrole 8 from intermediate 6 is endergonic by
11.2 kcal/mol with an alcohol solvent coordinated to PtCl4,
and even higher endergonicity is predicted without the alcohol
ligand. Thus, in reactions the intermolecular tautomeriza-
tion of 2H-pyrrole via TS11 and TS12 is not possible.
fashion (eq 2, Scheme 5). In this process, the H-abstraction
transition state TS9 is 31.1 kcal/mol higher in energy than 11,
leading to zwitterionic intermediate 15 endergonically, by
25.6 kcal/mol. The activation barrier is determined by the
second step, and the free energy of the H-donation transition
state TS10 is 33.8 kcal/mol relative to that of 11. Despite the
dramatic lowering of the 2H-pyrrole to 1H-pyrrole tauto-
merization activation barrier by methanol cluster, the 33.8
kcal/mol activation energy is still too high, considering the
reactions were carried out at 50 °C or in refluxing ethanol.
Besides, how the reaction occurred in solvents other than
ethanol is still unclear.
3.3. Intermolecular 2H-Pyrrole to 1H-Pyrrole Isomeriza-
tion with a 2H-Pyrrole Proton Shuttle. The previous calcula-
tions indicate that 2H-pyrrole intermediate could be gener-
ated in both 1,2-dichloroethane and ethanol solutions. We
hypothesize the final tautomerization of 2H-pyrrole to 1H-
pyrrole could be realized via the 2H-pyrrole-assisted H
migrations.31 Scheme 6 shows the activation energies of the
pathways in which one 2H-pyrrole acts as a proton shuttle to
facilitate the 1,5-H shift. The reaction between two 2H-pyrrole
8 is a two-step proton transfer process (eq 1, Scheme 6). The
first step consists of the deprotonation of 2H-pyrrole via TS11,
requiring an activation energy of 27.7 kcal/mol. In TS11 the
N0-H and C4-H distances are 1.175 and 1.567 A, respectively
(Figure 5). The free energy is increased by 24.5 kcal/mol for
According to the favorable formation of 11 from the coor-
dination of PtCl4 with the nitrogen site of 2H-pyrrole, the
2H-pyrrole-assisted H migrations of 11 are shown in eq 2,
Scheme 6, and the influence of alcohol solvent coordination on
this process is given in eq 3. In eq 2, the H-abstraction by 2H-
pyrrole via TS13 has an activation energy of 21.2 kcal/mol,
and the energy of the generated zwitterionic intermediate
17 is 5.7 kcal/mol higher than the energy sum of 11 and 8.
(31) Lui, K.-H.; Sammes, M. P. J. Phys. Org. Chem. 1990, 3, 555.
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FIGURE 5. Geometric structures for selected intermediates and transition states in 2H-pyrrole-catalyzed 2H-pyrrole to 1H-pyrrole isomer-
izations; selected distances are in angstroms.
All attempts to locate a transition structure corresponding to
the H-donation (TS14) failed. This may be attributed to the
decreased basicity of the nitrogen atom of pyrrole anion, which
is coordinated to PtCl4 in 17. Such a problem is not encoun-
tered when one alcohol is incorporated as a ligand (eq 3,
Scheme 6). Calculations indicate the complexation of MeOH
with 11 to form 110-b is exergonic by about 10 kcal/mol (vide
infra). The energy barrier for the H-abstraction via TS130 is
21.5 kcal/mol, which is similar to that without the MeOH
ligand. Zwitterionic intermediate 170 is 9.1 kcal/mol higher
in energy than 110-b, and the activation energy for the H-
donation via TS140 is 11.5 kcal/mol. Thus the activation free
energy for 2H-pyrrole-catalyzed H-migration of 11 in alco-
hol solution is 21.5 kcal/mol, determined by the H-abstrac-
tion transition state TS130. The geometric structures of related
intermediate and transition states are depicted in Figure 5.
For the PtCl4-catalyzed reactions in 1,2-dichloroethane
solution, another 2H-pyrrole could coordinate to the Pt cat-
alyst in place of alcohol solvent as a σ-ligand. In this case, the
activation energy of the 2H-pyrrole-assisted H-migration is
about 24.9 kcal/mol.27
3.4. Overall Catalytic Cycle of the PtCl4-Catalyzed Reac-
tion in Ethanol Solution. The energy profile of the overall cat-
alytic cycle of PtCl4-catalyzed reaction in ethanol solution is
depicted in Figure 6. The reaction starts with the ligand
exchange of complex PtCl4(MeOH)2 14 with substrate 1, yield-
ing reactant complex 30 with an endergonicity of 9.6 kcal/mol.
Intermediate 40 could be formed easily from the intramolecu-
lar cyclization of 30. After an energetically favorable dissocia-
tion of the MeOH ligand, intermediate 4 undergoes a facile
dinitrogen elimination step, leading to platinum-carbene 5
highly exergonically. The following 1,2-H shift of 5 has an
activation energy of only about 11 kcal/mol. This exergonic
step leads to intermediate 6, in which the Pt atom and one Cl
ligand are σ-bonded with C2 and C3 of the heterocyle moiety,
respectively. The generation of 2H-pyrrole 8 from 6 is facili-
tated upon the incorporation of one MeOH ligand. In the
process, a first formation of 60 from the coordination of
MeOH with 6 is slightly endergonic. Then π-complex 70 is
formed from a β-Cl elimination (via TS50) and the liberation
of 8 is realized by a ligand dissociation step. The final for-
mation of 1H-pyrrole product from 2H-pyrole intermediate is
an intermolecular process featuring a 2H-pyrrole proton
shuttle to catalyze the H-migration. Coordination of 13 to
the nitrogen site of 8 is exergonic by 23.9 kcal/mol, generat-
ing hexacoordinated complex 110-a. Prior to the incorpora-
tion of another 2H-pyrrole, 110-a would isomerize to a more
thermodynamically stable isomer 110-b, in which both 2H-
pyrrole and MeOH ligands occupy the axial positions.32
Formation of H-bonding complex 180 from 110-b and 8 is
endergonic by 14.3 kcal/mol. The H-abstraction by 2H-
pyrrole represents the rate-limiting step of the whole
reaction, and transition state TS130 lies 21.5 kcal/mol
above 110-b. The resulting zwitterionic intermediate 170
is 9.1 kcal/mol less stable than 110-b. The catalytic cycle is
completed by a relatively facile H-donation step (via TS140),
from which the 1H-pyrrole product is formed in company
(32) The 2H-pyrrole-assisted H-migration starting from 110-a is slightly
higher in energy. The barrier for 110-a to 110-b isomerization is 18.9 kcal/mol.
The detailed potential energy surfaces are given in the Supporting Informa-
tion. Such isomerization is quite facile via a dissociation, isomerization, and
recoordination process. See: (a) Braga, A. A. C.; Ujaque, G.; Maseras, F.
Organometallics 2006, 25, 3647. (b) Wang, M.; Cheng, L.; Hong, B.; Wu, Z.
Organometallics 2009, 28, 1506.
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FIGURE 6. Potential energy surface for the overall catalytic cycle of PtCl4-catalyzed acetylenic Schmidt reaction in alcohol solution, R = Me.
with the regeneration of complex 110-b. The overall exer-
gonicity of this reaction is about 150 kcal/mol.
the H-abstraction transition state TS19 and intermediate 23.
This is also the highest energy barrier on the reaction path-
way. The 1H-pyrrole product and intermediate 23 are gen-
erated with a relatively lower activation barrier by passing
the final H-donation step, increasing the overall exergonicity
to 112.6 kcal/mol.
3.5. Mechanism of the Au(I)-Catalyzed Acetylenic Schmidt
Reaction. As shown in Figure 7, the mechanism of the Au(I)-
catalyzed reaction is similar to that of the PtCl4-catalyzed
one. The potential energy surface of the PH3AuSbF6-cata-
lyzed reaction indicates all the cyclization (via TS15), dini-
trogen elimination (via TS16), and 1,2-H shift of Au-carbene
intermediate (via TS17) steps are easy to occur with relatively
low energy barriers. After the cyclization and dinitrogen
elimination steps, carbene intermediate 21 is formed irrever-
sibly with an exergonicity of 55.8 kcal/mol. The 1,2-H shift
step leads to π complex 22, further increasing the exergoni-
city to 80.4 kcal/mol. Dissociation of π complex 22 into PH3-
AuSbF6 and 2H-pyrrole 8 is exergonic by 6.5 kcal/mol, sug-
gesting the interaction between Au(I) and the C-C double
bond of 2H-pyrrole is rather weak. The coordination of 2H-
pyrrole nitrogen site to Au(I) is quite favorable, and the
resulting intermediate 23 is 101.3 kcal/mol lower than the
energy sum of 1 and PH3AuSbF6. Similarly, the direct 1,5-H
shift of 23 via TS18 is unfeasible due to the insurmount-
able energy barrier, whereas the intermolecular process with
the assistance of a 2H-pyrrole proton shuttle is more favor-
able. H-bonding complex 24 is first formed from intermedi-
ate 23 and 2H-pyrrole, followed by the H-abstraction and
H-donation steps. The activation barrier of this process is
20.1 kcal/mol, corresponding to the energy difference between
Accordingly, the different catalytic activity of Au(I) and
PtCl4 catalysts originates from the different coordination
behavior of the two catalysts with the substrate and the inter-
mediates. The PtCl4 catalyst is inclined to form a stable hexa-
coordinated complex with homopropargyl azide 1, in non-
coordinating solvent, thus retarding the following reactions.
Whereas the Au(I) is more selective to the C-C triple bond
of substrate 1, forming complex 19 with the solvation free
energy increased by 4.0 kcal/mol. No chelate complex like 2
could be formed from PH3AuSbF6 and 1.33 In PtCl4-cata-
lyzed reaction, a relatively stable organometallic intermedi-
ate 6 is generated from the 1,2-H shift of Pt-carbene. This
makes the generation of 2H-pyrrole intermediate difficult,
because the barrier required for isomerization of 6 to π com-
plex 7 and the following dissociation of 7 is 35.2 kcal/mol
(Figure 1). In contrast, the generation of 2H-pyrrole inter-
mediate in Au(I)-catalyzed reaction is quite facile. Dissocia-
tion of π complex 22 to PH3AuSbF6 and 2H-pyrrole is
energetically favorable.
(33) The coordination of Au(I) to the proximal nitrogen of the azide is
unfavorable, see the Supporting Information for details.
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JOCArticle
FIGURE 7. Potential energy surface for the Au(I)-catalyzed acetylenic Schmidt reaction in dichloromethane solution, [Au]=PH3AuSbF6.
4. Conclusions
in the presence of one 2H-pyrrole proton shuttle and one 2H-
pyrrole σ-ligand.
In summary, the reaction mechanisms of PtCl4- and Au(I)-
catalyzed intramolecular acetylenic Schmidt reaction have
been studied with the DFT method at the B3LYP/6-31G*-
(LANL2DZ) level of theory. The PtCl4-catalyzed reactions
in 1,2-dichloroethane and ethanol solutions, acceleration effect
of ethanol solvent, details of the intermolecular 2H-pyrrole
to 1H-pyrrole isomerization, and the different catalytic activ-
ity of Au and Pt catalysts are presented. These results will
provide guidance for the future design of new cyclization
reactions in the ever-growing field of gold- and platinum-
based catalysis.10-12,16
For the PtCl4-catalyzed reaction in 1,2-dichloroethane solu-
tion, a relatively stable chelate complex is formed irreversibly
from PtCl4 and homopropargyl azide. The rate of the whole
reaction is limited by isomerization of this complex to the
reactive π complex, which requires an activation barrier of
29.6 kcal/mol. Via the following facile steps of nucleophilic
cyclization, dinitrogen elimination, and 1,2-H shift of Pt-
carbene, a σ-adduct from the addition of one Pt-Cl bond to
the C-C double bond of 2H-pyrrole is obtained highly
exergonically. Because this σ-adduct is 17 kcal/mol more
stable than the π-complex of 2H-pyrrole and PtCl4 and the
dissociation of the latter via an initial incorporation of another
substrate is endergonic by 9.2 kcal/mol, the energy required
for generation of 2H-pyrrole intermediate is 26.2 kcal/mol.
While the activation energy for the intramolecular 1,5-H
shift of 2H-pyrrole is too high to overcome, the intermole-
cular processes with a 2H-pyrrole proton shuttle are more
favorable. The energy for the isomerization of 2H-pyrrole to
1H-pyrrole in 1,2-dichloroethane solution is 24.9 kcal/mol,
When the solvent is changed to ethanol and an induction
period is used, formation of hexacoordinated complex PtCl4-
(ROH)2 is exergonic by 34.1 kcal/mol. Then the energy
required for the formation of the reactive π complex is only
9.6 kcal/mol, via substitution of one alcohol ligand by the
C-C triple bond of homopropargyl azide substrate. The
involvement of one alcohol ligand to PtCl4 does not affect
much the kinetic and thermodynamic parameters of the nucle-
ophilic cyclization, dinitrogen elimination, and 1,2-H shift of
Pt-carbene steps. However, the activation energies for the
generation of 2H-pyrrole from the σ-adduct and the inter-
molecular 2H-pyrrole to 1H-pyrrole isomerization are reduced
to 15.4 and 21.5 kcal/mol, respectively. Thus, the acceleration
effect of ethanol solvent in PtCl4-catalyzed reaction is the result
of solvent coordination to the catalyst.
The mechanism of the Au(I)-catalyzed reaction is similar
to that of the PtCl4-catalyzed one. However, much weaker
interactions of PR3AuSbF6 with homopropargyl azide and
the C-C double bond of 2H-pyrrole are calculated, as no
chelate complex could be formed from Au(I) and substrate
and the generation of 2H-pyrrole by dissociation of the π com-
plex is much easier. This accounts for the different catalytic
activity of Au(I) and PtCl4 in noncoordinative solvents. The
final isomerization of 2H-pyrrole to 1H-pyrrole in Au(I)-cat-
alyzed reaction is also assisted by a 2H-pyrrole proton shuttle.
The H-abstraction step of this intermolecular process is the
rate-controlling step of the overall reaction, with an activation
energy of 20.1 kcal/mol.
Acknowledgment. This work is supported by National
Natural Science Foundation of China (Grant No. 21002073),
J. Org. Chem. Vol. 75, No. 22, 2010 7853

Xia and Huang
JOCArticle
Wenzhou Science & Technology Bureau (Grant No.
Y20100003), and Wenzhou University (Startup funding
to Y.X.).
the possible nitrene pathways, the intermolecular 2H-pyrrole
to 1H-pyrrole isomerizations, and Au(PH3)þ-catalyzed reac-
tions, geometries in the PtCl4- and Au(I)-catalyzed reaction,
validation of computational method, and IRC plots for select-
ed transition states. This material is available free of charge via
the Internet at http://pubs.acs.org.
Supporting Information Available: Energies and Cartesian
coordinates of all stationary points, energetic profiles for
7854 J. Org. Chem. Vol. 75, No. 22, 2010