The gold(I)- and silver(I)-catalyzed nicholas reaction

Article abstract of DOI:10.1021/om3011257

The Au(I) and Ag(I) catalytic Nicholas type reaction has been developed for oxygen and carbon nucleophiles. This process occurs with high reaction yields and selectivity, avoiding the main shortcomings of the acid-promoted standard Nicholas reaction. A catalytic reaction pathway involving trimetallic complex intermediates is proposed.

Full text of DOI:10.1021/om3011257

Communication
pubs.acs.org/Organometallics
The Gold(I)- and Silver(I)-Catalyzed Nicholas Reaction
Carolina Valderas,†,‡ María C. de la Torre,*,‡ Israel Fernandez,† María Paz Munoz,‡,§
́
̃
and Miguel A. Sierra*,†
†
́
́
Departamento de Quımica Organica, Facultad de Quımica, Universidad Complutense, 28040-Madrid, Spain
́
‡
́
́
Instituto de Quımica Organica General, Consejo Superior de Investigaciones Cientıficas (CSIC), Juan de la Cierva 3, 28006-Madrid,
́
Spain
S
* Supporting Information
ABSTRACT: The Au(I) and Ag(I) catalytic Nicholas type reaction has been
developed for oxygen and carbon nucleophiles. This process occurs with high
reaction yields and selectivity, avoiding the main shortcomings of the acid-
promoted standard Nicholas reaction. A catalytic reaction pathway involving
trimetallic complex intermediates is proposed.
he trapping of hexacarbonyldicobalt cluster stabilized
propargylic cations by nucleophiles (the Nicholas
C−heteroatom bond forming reactions.9 Succeeding in this
T
endeavor would be of great interest, since the use of acid
conditions in the Nicholas reaction would be precluded. This
would be important, for example, to produce complex organic
architectures using sensitive natural products.10 Reported
herein is the successful attempt to develop a catalytic (gold
or silver),11 room-temperature version of the Nicholas reaction.
Yields and selectivities are considerably higher than those
obtained under standard (Lewis acid promoted) conditions.
Addition of benzyl alcohol to complex 1 was tested with
different gold catalysts, in the presence or in the absence of
silver salts, and at two different temperatures, 0 °C and room
temperature (Scheme 2). Table 1 compiles the results of the
different experiments conducted. Three different products were
isolated in variable amounts in these experiments. Compound 2
is the expected Nicholas-type product, since it is known that
these reactions occurred with allylic rearrangement.12 Com-
pound 3 is the elimination product, and compound 4 arises
reaction)1 is a powerful and widely used tool in organic
synthesis.2 The strong stabilization provided by the Co cluster
to the carbocation allows the nucleophile to react without the
formation of allene byproducts. This carbocation is generated
by acid treatment of a preformed propargyl−Co2(CO)6
complex. The acid used is usually a Lewis acid, but protic
acids or clays have been successfully used in the carbocation
generation (Scheme 1).3 Modifications have been introduced to
Scheme 1. The Nicholas Reaction
Scheme 2. Reaction of Compound 1 with Au and Ag
Catalysts
avoid the main shortcomings of the Nicholas reaction, namely
the use of acid media and the instability of propargyl−
Co2(CO)6 at temperatures above −40 °C.4−6
The ability of gold catalysts to promote SN-like reactions
using benzylic alcohols reported by Asensio and his co-workers7
led us to investigate the possibility of using gold catalysts to
promote the Nicholas reaction. It is known that gold is able to
act as a Lewis acid8 as well as an appealing catalyst in C−C and
Received: November 22, 2012
Published: February 1, 2013
© 2013 American Chemical Society
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Table 1. Study of the Reaction of Compound 1 with Different Gold and Silver Catalysts
c
yield (%)
a
b
entry
Au cat. (amt (mol %))
cocat. (amt (mol %))
amt of BnOH (equiv)
time (h)
2
3
4
1
1
AuCl3 (5)
NaAuCl4·2H2O (5)
Ph3PAuCl (10)
Ph3PAuCl (10)
Ph3PAuCl (10)
Ph3PAuCl (5)
none
none
1
1
3
3
3
3
3
3
3
3
3
3
48
48
48
12
4
0
0
0
100
100
66
0
2
none
0
0
0
2
0
0
20
0
3
none
4
AgOTf (15)
AgSbF6 (15)
NaBArF (7.5)
AgSbF6 (5)
AgSbF6 (15)
AgSbF6 (10)
AgOTf (15)
HOTf (15)
NaBArF
49
57
55
77
68
68
48
0
5
12
9
4
0
6
3
3
0
7
4
0
6
0
8
none
4
0
4
0
9
none
2
0
12
4
0
10
11
12
none
48
3
0
0
none
100
0
0
0
none
48
34
0
49
a
The reactions in entries 3−6 and 9−13 were also carried out with 1 equiv of BnOH. Yields were considerably lower than those reported in the table
b
c
using 3 equiv of BnOH. The reactions were carried out at room temperature. In isolated product.
from the addition of water competing with the addition of the
benzyl alcohol.13 As depicted in Table 1, salts of Au(III)
(entries 1 and 2) were ineffective in promoting the addition of
benzylic alcohol. Complex 1 reacts with neutral Ph3PAuCl
(entry 3), but products derived from the incorporation of
benzylic alcohol were not isolated. Instead, elimination product
3 (minor product) and rearranged alcohol 4 were isolated
together with unreacted starting material. The use of Ph3PAuCl
in the presence of AgOTf as cocatalyst produces a substantial
amount (49% isolated yield) of the desired product 2 (entry 4).
Similarly, the mixture Ph3PAuCl/AgSbF6 (entry 5) affords
compound 2 in shorter reaction times (4h vs 12 h) and in
higher yields (57% vs 49%). To rule out Ag(I) as the species
responsible for these reactions (see below), NaBArF was tested
next as cocatalyst for the Au(I) complex (entry 6). Under these
conditions, which allowed a decrease in the catalyst load, 2 was
formed with times and yields comparable to those of the
AgSbF6 experiment.
studied next (Scheme 3). Both the mixture Ph3PAuCl (5%)/
NaBArF (7.5%) and AgSbF6 (5%) produced excellent yields of
the corresponding adducts 5a,b with primary alcohols (MeOH
and EtOH). Indole and 1,3,5-trimethoxybenzene were also
Scheme 3. Au and Ag Catalytic Nicholas Reaction with
diverse O and C Nucleophiles
Even more satisfactory was the reaction of complex 1 with
AgSbF6 in the absence of Au catalysts (entries 7−10). Now,
compound 2 was obtained in 77% isolated yield (entry 7) using
AgSbF6 (5% mol). Increasing the load of AgSbF6 does not
improve either the yields or the reaction times (entries 8 and
9). The use of AgOTf produced compound 2 in lower yields
(entry 10). Product 4, derived from water incorporation, was
obtained in variable amounts in these reactions.13 To discard
the possibility of traces of acid in the catalysts being responsible
for the observed reactivity, the reaction was carried out with
TfOH under standard conditions (entry 11). Elimination
product 3 was obtained as the sole reaction product due to a
fast E1 reaction, as expected.12 This experiment rules out the
possibility of traces of TfOH formed in situ from the AgOTf
catalysts (in the presence or absence of [Au]) being the active
catalysts of the Nicholas reaction. These results could be
extrapolated to the hypothetical role of HSbF6 as promoter of
the reaction when AgSbF6 salt is used.
Finally, the use of NaBArF in the absence of Au(I) was
tested. Compound 2 was isolated in 34% yield after 48 h of
reaction at room temperature (entry 12, Table 1), while the
analogous reaction in the presence of Au(I) takes place in 3 h
under the same conditions, leading to compound 2 isolated in
55% yield.
Having demonstrated the possibility of carrying out a Ag(I)-
and Au(I)-catalyzed Nicholas reaction, other nucleophiles were
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reactive under these reaction conditions, providing good yields
of compounds 6 (derived from the attack of the C3 indole
carbon atom) and 7, respectively. Minor amounts of
elimination and water addition products (3 and 4) were
obtained in some cases. The less reactive 1,2,3-trimethox-
ybenzene and 1,4-dimethoxybenzene were unreactive toward
the addition. Elimination and water addition products in
variable amounts were the reaction products, together with
unreacted starting material. Although Ag(I)-catalyzed reactions
were usually faster and cleaner, no significant differences in
reactivity between both Au(I) and Ag(I) catalytic systems were
found.
species. To shed some light on the above processes, aware that
considerable additional experimentation and computation
would be required before fully understanding the mechanism
of this reaction, we first computed the geometry adopted by
complex 11 upon coordination of Co2(CO)8 to the triple bond
of compound 9 (DFT calculations were carried out at the
PCM(CH2Cl2)-M06/def2-SVP//B3LYP/def2-SVP level; see
the Supporting Information). Conformer I (Figure 1), where
However, the differences in reactivity between Au(I) and
Ag(I) were exacerbated when allyltrimethylsilane was reacted
with substrate 1. Now, while gold catalyst was inactive, a 60%
yield of complex 8 was obtained when AgSbF6 (5%) was the
catalyst. While the Co-mediated Hosomi−Sakurai type
cyclization is sensitive to the Lewis acid used,14a the
intramolecular Hosomi−Sakurai reaction in propargylic alco-
hols has been carried out efficiently both in its Co-mediated
version using TMSOTf as promotor and in the absence of Co
using Au(I) catalysts.14b The dramatic difference in the Au(I)
vs Ag(I) reactions observed for complex 8 pointed to a reaction
mechanism different from the classical cationic pathway (see
below).
Figure 1. Possible conformations adopted by compound 11. The
relative free energy (ΔG298, computed at 298 K at the PCM(CH2Cl2)-
M06/def2-SVP//B3LYP/def2-SVP level) is given in kcal/mol.
the isopropenyl substituent and the metal fragment are placed
in pseudoequatorial and pseudoaxial positions, respectively, is
6.2 kcal/mol more stable than conformer II, having the
isopropenyl group in a pseudoaxial position. This suggests that
the reaction with the gold(I) catalyst occurs mainly from
conformer I.
Thus, the coordination of the gold(I) catalyst to the double
bond of the more stable conformer I may lead to two possible
trimetallic cationic species: i.e. compound I-A, where the
gold(I) moiety and the dicobalt fragment are anti, and
compound I-B, where both metallic moieties are syn (Figure
2a). The species I-A lies 2.2 kcal/mol below I-B. This is in part
due to reduced steric hindrance in this complex and,
additionally, to the presence of a stabilizing intramolecular
To demonstrate that the mechanism of the catalyzed reaction
differs from the classical cation formation, the stereochemistry
of the catalyzed process in comparison with the standard
Nicholas reaction was studied next using the alcohol 9 derived
from (R)-(−)-carvone (Scheme 4).15 Whereas benzyl ether 10
Scheme 4. Reactions of Propargylic Alcohol 9 with Different
Catalysts
was obtained as a 60/40 diastereomeric mixture in the reaction
of the preformed propargyl−Co2(CO)6 complex 11 with
BF3·Et2O at −20 °C for 15 min (standard Nicholas process),
a remarkable increase in the diastereoselection was observed in
the reaction with Ph3PAuCl/NaBArF (80/20 mixture) or in the
analogous transformation using AgSbF6 (91/9 mixture).16 The
stereochemistry of the major reaction product 10 was
spectroscopically determined as the trans diastereoisomer.17
Results in Scheme 4 clearly demonstrate that the role of the
Ag(I) or Au(I) in promoting the reaction is different from the
conventional, Lewis acid promoted Nicholas reaction and that
the role of these catalysts is not to act as mere cation-forming
Figure 2. (a) Possible species formed upon coordination of gold(I) to
conformer I. (b) NBO orbitals involved in the stabilization of species
I-A. Relative free energies (ΔG298, computed at 298 K at the
PCM(CH2Cl2)-M06/def2-SVP//B3LYP/def2-SVP level) are given in
kcal/mol.
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Scheme 5. Proposed Reaction Pathway for the Catalyzed Au(I) Nicholas’ Reaction
donor−acceptor interaction between the oxygen atom of the
alcohol substituent and the gold(I) fragment (Au···O distance
of 3.14 Å). Indeed, the second-order perturbation theory
(SOPT) of the NBO method locates a two-electron stabilizing
delocalization from the lone pair of this oxygen atom to an
empty p atomic orbital of the transition metal with a significant
associated SOPT energy of ΔE(2) = −5.2 kcal/mol (Figure
2b). Evidently, the stereoselectivity of the reaction would be
defined in the coordination step, which is in agreement with the
observed experimental results.
Scheme 6. Ag(I)-Catalyzed Reaction of No-Conjugated
Propargylic Alcohols
From species I-A and I-B the OH group should migrate to
Ag(I) or Au(I), forming new allyl−gold(I) or −silver(I)
complexes.18 Nucleophilic addition to carbon atom C3 would
yield the new intermediates 14 and 15, from which the
demetalation occurs, giving the reaction products 10 and
regenerating the catalyst. The moderate to strong increase in
selectivity observed in the catalytic processes respect to the
Lewis acid promoted “classical reaction” is congruent with the
participation of polymetallic intermediates such as 12 and 13
(Scheme 5).
Finally, the necessity of the presence of a double bond allylic
to the alcohol for the reaction above to occur was addressed.
Co2(CO)6 complex 16, lacking this structural feature, was
prepared from 1-phenyl-2-propyn-1-ol (17) and reacted with
benzylic alcohol in the presence of Ph3PAuCl/NaBArF and
AgSbF6 (5% loads of catalyst related to the alcohol 16).
Interestingly, the complex 16 was inert toward the Au(I)
catalyst, and it was recovered unaltered, while the expected
benzyl alcohol adduct 18 was isolated in 63% yield in the
presence of Ag(I) catalyst (Scheme 6).19 Therefore, the
catalytic processes developed through this communication do
not require an allylic rearrangement to occur.
In summary, an Au(I) and Ag(I) catalytic Nicholas type
reaction has been developed. The reaction proceeds smoothly
at room temperature in good to excellent yields, minimizing the
formation of elimination products. Oxygen and carbon
nucleophiles can be used as well. Experimental data obtained
from the reactions of chiral carvone derivatives toward Au(I),
Ag(I), and BF3·Et2O pointed to the involvement of trimetallic
cationic species in these processes, the coordination of the
catalyst to the substrate determining the stereoselectivity. The
findings that the Nicholas reaction can be catalyzed by Au(I)
and Ag(I) catalysts and that these species participate in the
process open the doors to the possibility of effecting
asymmetric chiral catalysis. Efforts to fully develop this new
methodology are now in progress.
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures, and tables giving full experimental procedures,
spectroscopic data, and 1H, 13C NMR spectra for all synthesized
compounds mentioned in the text as well as additional NMR
information for selected compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mc.delatorre@csic.es (M.C.d.l.T.); sierraor@quim.
ucm.es (M.A.S.).
Present Address
§School of Chemistry, University of East Anglia, Earlham Road,
Norwich NR4 7T, U.K.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
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Organometallics
Biography
Communication
(10) Representative examples: (a) de la Torre, M. C.; Deometrio, A.
́
́
M.; Alvaro, E.; Garcıa, I.; Sierra, M. A. Org. Lett. 2006, 8, 593.
́
(b) Sierra, M. A.; Torres, R. M.; de la Torre, M. C.; Alvaro, E. J. Org.
́
Chem. 2007, 72, 4213. (c) Montenegro, H. E.; Ramırez-Lop
́
ez, P.; de
la Torre, M. C.; Asenjo, M.; Sierra, M. A. Chem. Eur. J. 2010, 16, 3798.
́
́
(d) Ramırez-Lopez, P.; de la Torre, M. C.; Montenegro, H. E.; Asenjo,
M.; Sierra, M. A. Org. Lett. 2008, 10, 3555.
(11) The use of stoichiometric amounts of Ag(I) salts to promote the
Nicholas reaction from propargyl chlorides has been reported. See the
following. (a) AgBF4: Vizniowski, G. S.; Green, J. R.; Breen, T. L.;
Dalacu, A. V. J. Org. Chem. 1995, 60, 7496. (b) AgSbF6: Tumanov, V.
V.; Zatonsky, G. V.; Smit, W. A. Tetrahedron 2010, 66, 2156.
́
(12) Alvaro, E.; de la Torre, M. C.; Sierra, M. A. Chem. Eur. J. 2006,
12, 6403.
(13) Alternatively, formation of compound 4 may occur through a
metal-assisted HO-intramolecular process analogous to those reported,
among others, for gold-/silver-catalyzed reactions of 3-silyloxy-1,4-
enynes with alcohols13a or Baylis−Hillman acetates into 2-(acetox-
ymethyl)alk-2-enoates.13b See: (a) Haug, T. T.; Harschneck, T.;
Duschek, A.; Lee, C.-U.; Binder, J. T.; Menz, H.; Kirsch, S. F. J.
Organomet. Chem. 2009, 694, 510. (b) Liu, Y.; Mao, D.; Qian, J.; Lou,
S.; Xu, Z.; Zhang, Y. Synthesis 2009, 1170.
(14) (a) Hayashi, Y.; Yamaguchi, H.; Toyoshima, M.; Okado, K.;
Toyo, T.; Shoji, M. Org. Lett. 2008, 10, 1405. and the pertinent
references therein (b) Tsao, K.-W.; Cheng, C.-Y.; Isobe, M. Org. Lett.
2012, 10, 5274.
This article is based upon a poster presented by Carolina Valderas
(pictured here with Dr. John Gladysz) at the first Organometallics
Symposium at the ACS meeting in Philadelphia, Pennsylvania on
August 21, 2012.20 Carolina Valderas received her B.S. at Universidad
Autonoma de Madrid. During this time, she carried out an
́
undergraduate collaboration at Radboud University of Nijmegen
(Netherlands) in the group of Prof. Floris Rutjes. She got her M.S. in
́
Organic Chemistry at Universidad Autonoma de Madrid. In 2010 she
began her Ph.D. at Universidad Complutense de Madrid under the
supervision of Prof. Miguel A. Sierra and Dr. M. C. de la Torre, the
corresponding authors of this paper.
(15) The stereochemistry at the formed tertiary carbinol center of
alkyne 9 was established by comparison of the 13C NMR data of the
terpenic domain with those of related derivatives; see: Zhao, L.;
Burnell, D. L. Tetrahedron Lett. 2006, 47, 3291.
(16) It is known that the Nicholas reaction may be reversible under
“conventional” Lewis acid conditions. See: (a) Kihara, N.; Kidoba, K.
ACKNOWLEDGMENTS
■
Financial support by the Spanish Ministerio de Economia y
Competitividad (MINECO), grants CTQ2010-20714 and
Consolider-Ingenio 2010 (CSD2007-0006), and the Comuni-
dad de Madrid (CAM), grant P2009/PPQ1634-AVANCAT, is
acknowledged. I.F. and M.P.M. thank the MINECO for Ramon
y Cajal fellowships.
́
Org. Lett. 2009, 11, 1313. (b) Asenjo, M.; de la Torre, M. C.; Ramırez-
Lopez, P.; Sierra, M. A. To be submitted for publication. Therefore,
́
control experiments to discard the possibility that the diasterose-
lectivity obtained in reaction 11 with Au(I) or Ag(I) was the result of
thermodynamic control were carried out. Thus, 68 mg (0.12 mmol) of
a 60/40 mixture of diasteromers of adduct 10 was reacted with 4.2 mg
(0.012 mmol, 10%) of AgSbF6 in 2 mL of DCM at room temperature .
After 2 h the crude reaction product was analyzed by 1H NMR.
Appreciable decomposition of the starting material was observed
together with unreacted 10. Integration of the signals corresponding to
the terminal alkyne hydrogens show that the initial ratio of
diasteromers remained unaltered. A second experiment was carried
out using 21 mg (0.038 mmol) of a 60/40 mixture of diasteromers of
adduct 10 and 0.6 mg (0.0019 mmol, 5%) of AgSbF6 in 1 mL of DCM
at room temperature. The reaction mixture was kept at room
temperature for 48 h. Compound 10 disappeared, forming a complex
reaction mixture lacking any complexed alkyne moiety (1H NMR). We
can conclude safely that, at least under the conditions tested, the
intercoversion between diastereomers of compound 10 does not
happen.
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