Dual ReVCatalysis in One-Pot Consecutive Meyer–Schuster and Diels–Alder Reactions

Article abstract of DOI:10.1002/ejoc.201600926

A readily available oxo-Re^Vcatalyst has been shown to promote a one-pot multistep process that involves the Meyer–Schuster rearrangement of alkynols followed by the Diels–Alder cycloaddition of the resulting enones. These reactions were performed under mild conditions in the presence of 5 mol-% of the catalyst to afford the cycloadducts in good yields with almost 100 % endo stereoselectivity. In-depth computational studies of the cycloaddition mechanism provided the preferred geometry of the rhenium complex in the transition state (TS) and corroborated the experimental endo selectivity.

Full text of DOI:10.1002/ejoc.201600926

DOI: 10.1002/ejoc.201600926
Full Paper
Rhenium(V) Catalysis
Dual ReV Catalysis in One-Pot Consecutive Meyer–Schuster and
Diels–Alder Reactions
Serena Bugoni,[a] Alessio Porta,*[a] Zuleykha Valiullina,[a,b] Giuseppe Zanoni,[a] and
Giovanni Vidari*[a]
Abstract: A readily available oxo-ReV catalyst has been shown yields with almost 100 % endo stereoselectivity. In-depth com-
to promote a one-pot multistep process that involves the
Meyer–Schuster rearrangement of alkynols followed by the
Diels–Alder cycloaddition of the resulting enones. These reac-
tions were performed under mild conditions in the presence of
5 mol-% of the catalyst to afford the cycloadducts in good
putational studies of the cycloaddition mechanism provided
the preferred geometry of the rhenium complex in the transi-
tion state (TS) and corroborated the experimental endo select-
ivity.
Introduction
almost always the first choice in the synthetic design of mono-
and polycyclic scaffolds that contain six-membered rings.[4,5]
This reaction generally proceeds with good regio- and stereo-
selectivity in the creation of up to four stereogenic centers.[5]
Typically, the preparation of enones and the cycloaddition reac-
tions are performed separately in different flasks, which requires
the workup and purification of the enone.
Herein, we demonstrate that the readily available rhenium
complex [ReOCl3(OPPh3)·(SMe2)] (1)[7,8] is an efficient Lewis acid
catalyst for Diels–Alder processes. Moreover, we describe the
first examples of consecutive Meyer–Schuster (M–S) rearrange-
ments of propargylic alcohols and in situ Diels–Alder reactions
of the resulting α,ꢀ-unsaturated ketones, with both reactions
catalyzed by the same transition-metal complex 1.[7,8] Indeed,
at the onset of our project, we were unaware of other examples
The development of new, efficient, and atom-economical pro-
cedures to shorten synthetic routes and maximize the conver-
sion of starting materials into products is one of the main goals
of sustainable chemistry.[1] In this context, simple addition reac-
tions and prototropic rearrangements, with only catalytic
amounts of other reagents, are model examples of atom-eco-
nomical processes.[1] Moreover, the combination of two reac-
tions of this type, which enables the formation of multiple
bonds in a one-pot process, can result in the realization of
unusual selectivity and/or a rapid increase in molecular com-
plexity.[2]
Two powerful and widely used synthetic transformations are
the Meyer–Schuster (M–S) rearrangement[3] and Diels–Alder
(D–A) reactions.[4,5] The first one gives rise to conjugated en-
ones, which are among the most versatile substrates for subse-
quent C–C bond-forming reactions such as cycloaddition proc-
esses.[4] Indeed, the M–S reaction has been shown to be a valid
alternative to other traditional preparations of enones, includ-
ing aldol-like condensations and Wittig-like olefination meth-
ods.[6] These well-known procedures, however, have drawbacks
such as the use of strongly basic conditions, the generation of
noxious byproducts, and an overall low atom economy. On the
other hand, the exceptionally practical Diels–Alder reaction is
in which
a single catalyst promoted a Meyer–Schuster
rearrangement followed by a C–C bond-forming cycloaddition
reaction.[9] This task seemed to be quite ambitious, as sterically
hindered chalcones are sluggish dienophiles and poorly coordi-
nate to sterically demanding metallic complexes.[10]
Results and Discussion
Recently, we developed a new catalytic procedure that employs
the rhenium complex [ReOCl3(OPPh3)·(SMe2)] (1)[8] in the rapid
and efficient M–S rearrangement of free secondary and tertiary
propargylic alcohols.[7] During these studies, the rapid and
clean (Z) to (E) isomerization of the enones was observed. We
envision that this process is promoted by the Lewis acid-like
coordination of the oxo-ReV catalyst 1 to the carbonyl moiety
(Scheme 1).[7] This Lewis acid character of the rhenium complex
could then be exploited to not only induce the double bond
isomerization but also promote addition reactions such as
Diels–Alder cycloadditions to conjugated olefins (Scheme 1). In
fact, it is well known that Lewis acids, such as AlCl3 and TiCl4,
dramatically accelerate Diels–Alder cycloadditions[11] and allow
[a] Dipartimento di Chimica, Sezione Chimica Organica, Università degli Studi
di Pavia,
Via Taramelli 12, 27100 Pavia, Italy
E-mail: alessio.porta@unipv.it
vidari@unipv.it
www.b2chem.eu
[b] Institute of Organic Chemistry, Ufa Scientific Centre of the Russian
Academy of Sciences,
prosp. Oktyabrya 71, 450054 Ufa, Russia
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201600926.
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Table 1. Oxo-ReV- 1 or Cu(OTf)2-catalyzed Diels–Alder reactions.
[a] Calculated yield is based on recovered starting enone (n.r.: no reaction, nAm = n-amyl).
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them to proceed at reasonable rates at low temperature, which computational study. On the basis of the known geometry of
typically results in high endo selectivity and fewer side prod- rhenium complex 1 described by Abu-Omar and collabora-
ucts.[12] This enhancement in the rate can be explained by fron- tors,[8b] we found two possible geometries (i.e., A and B, Fig-
tier molecular orbital (FMO) theory.[13–15]
ure 1) of ligands around the rhenium center. In structure A, the
phenyl group of the unsaturated side chain was stacked with
one phenyl group of the POPh3 ligand, whereas in geometry B,
the two phenyl groups were anti oriented with the phenyl
group of the ligand parallel to the acetyl group. The calculated
energy between the two structures is 2.69 kcal mol–1 (gas
phase),[17] and therefore only geometry A was considered to be
significantly populated.
Scheme 1. Suggested mechanism for the oxo-rhenium(V)-catalyzed (Z) to (E)
isomerization of unsaturated carbonyl compounds and the extension towards
a successive Diels–Alder cycloaddition (Ln = ligand).
In preliminary experiments, the efficiency of the oxo-Re com-
plex 1 to catalyze the D–A cycloaddition of enones and cyclo-
pentadiene was compared with that of the well-known Lewis
acid Cu(OTf)2 (OTf = trifluoromethanesulfonate, Table 1).[16]
Both Lewis acids were inactive in solvents such as 1,2-di-
methoxyethane (DME, Table 1, Entries 3, 5, 7, 9, and 11),
whereas in noncoordinating solvents such as toluene and di-
chloromethane (DCM), both catalysts delivered the desired
cycloadducts in good (Table 1, Entries 2 and 4) to excellent
yields (Table 1, Entries 1, 6, 8, and 10), surprisingly even at low
temperature. Thus, DCM was the solvent of choice.
Moreover, 5 mol-% of catalyst 1 proved to be slightly more
active than 10 mol-% Cu(OTf)2 (Table 1, Entries 1 and 6 vs. 4
and 8, respectively). In striking contrast, the uncatalyzed ther-
mal reactions that were carried out in toluene under forcing
conditions afforded no cycloadducts but instead gave the re-
covered starting enones and polymers of cyclopentadiene. With
regard to the stereoselectivity, Cu(OTf)2 afforded product 3a
(Table 1, Entry 4) as a 10:1 mixture of endo/exo adducts,
whereas the same reaction catalyzed by complex 1 (Table 1,
Entry 1) delivered 3a with complete endo selectivity (according
to HPLC and NMR analysis). The optimized reaction conditions
for the employment of oxo-Re-catalyst 1 were then extended
to the Diels–Alder reaction between cyclopentadiene and dif-
ferent enones (Table 1, Entries 13–16). The expected cycload-
ducts were obtained in moderate to high yields with complete
endo selectivity (according to NMR analysis). To further increase
the appeal of this new catalytic procedure, the sterically hin-
dered tert-butyl acrylate (2h) and oxazolidinone 2i were suc-
cessfully submitted to the Diels–Alder reaction (Table 1, En-
tries 17 and 18).
Figure 1. Modeled geometries A and B of the complex between oxo-rhenium
complex 1 and enone 2c.
Subsequently, the possible transition state (TS) structures for
the oxo-Re 1 catalyzed Diels–Alder reaction of 2c with cyclo-
pentadiene in DCM were calculated at the M062X[18] level of
theory by using a differentiated basis set with a polarizable
continuum model (PCM).[19] The C and H atoms were described
with the 6-31G(d,p) basis set. The Cl, O, and P atoms were de-
scribed with the 6-31+G(d,p) basis set, and the Re atom was
described with the LANL2DZ[20,21] relativistic effective core po-
tential (ECP) basis set by using the Gaussian 09 software
suite.[22] All degrees of conformational freedom were consid-
ered, and different possible orientations of the oxo-Re 1–dieno-
phile 2c complex were considered on the basis of the geome-
tries of A and B. A similar analysis that was carried out at the
B3LYP level of theory was inconsistent with the experimental
data. Moreover, the real TSs could not be located for all of the
considered geometries, and the intrinsic reaction coordinate
(IRC) plot showed complete asynchronous bond formation. In
contrast, the M062X theory allowed us to locate both the endo
and exo transition states (Figure 2) on the reaction coordinate
(see Supporting Information for IRC plot), which showed that
only complex A was involved in the TSs. The calculated energy
difference between the endo and exo TSs (C and D, respectively,
Figure 2) is 2.87 kcal mol–1 (gas phase),[23] which is in accord-
ance with the complete endo selectivity observed for the Diels–
Alder reaction.
Modeling Studies of the Oxo-Re-Catalyzed Diels–Alder
Reaction
To shed some light on the complete endo stereoselectivity ob-
served in the oxo-Re complex 1 catalyzed Diels–Alder reaction
(Table 1), the geometry of the C=O–oxo-Re complex 1 involved
in the catalytic cycle was modeled by using a DFT approach.
Simple enone 2c was modeled as a dienophile to facilitate the
Figure 2. 3D plot of the endo (i.e., C) and exo (i.e., D) transition states involved
in the Diels–Alder reaction between cyclopentadiene and enone 2c catalyzed
by the oxo-Re complex 1.
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One-Pot Protocols
catalyst. The expected products were formed (Table 2, Entries 1
and 3) in rather low yields and accompanied by byproducts,
After demonstrating the remarkably high catalytic activity of most likely the result of DCM being a poor solvent for the M–S
oxo-Re complex 1 in the Diels–Alder reactions of enones and process.[7] The initial M–S rearrangement was then performed
esters, we then explored the feasibility of a one-pot two-step in DME, which proved to be the best solvent.[7] Avoiding the
protocol for the oxo-Re-catalyzed M–S rearrangement of prop- workup procedure, we then replaced the DCM with DME, and
argylic alcohols to enones and subsequent in situ Diels–Alder the Diels–Alder reaction was carried out in the same flask after
reaction with oxo-Re complex 1 to catalyze both processes. In- the addition of an excess amount of the diene. An additional
deed, a combination of the two reactions has been reported in aliquot of catalyst 1 or another catalyst were not required to
a previous publication by our group,[24] but under the reported accelerate the D–A reaction, which smoothly proceeded at 0 °C
conditions, complex 1 showed no catalytic activity in the cyclo- to deliver the endo cycloadducts in good to high yields with
addition step, which required the addition of BF3·Et2O complete stereoselectivity (Table 2, Entries 2 and 4–8).
(1.1 equiv.) to proceed.[24]
To further increase the appeal of this new method, we ex-
In initial experiments, the M–S and the D–A reactions were plored the possibility of an aldehyde olefination to give a conju-
carried out in DCM with 5 mol-% of complex 1 as the sole gated (E)-enone[24] followed by an in situ Diels–Alder reaction.
Table 2. Consecutive one-pot M–S and D–A reactions of propargylic alcohols both promoted by the catalysis of oxo-rhenium(V) complex 1.
[a] Calculated yield is based on recovered enone.
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Scheme 2. Six-step reaction sequence including aldehyde olefination and Diels–Alder reactions.
Table 3. Multistep transformation from alkyne to Diels–Alder cycloadduct.
[a] The deprotonation of the free alkyne (Entries 1–3) was carried out with nBuLi in dry DME at –78 °C for 1 h. [b] The addition of the lithium alkynylide to
the aldehyde was performed at –78 °C for 1 h (Entry 4), 2 h (Entries 1 and 2), or 3 h (Entry 3). [c] The quenching of the alkoxide (Entries 1–4) by Amberlite®
IR 120H (1 h) was followed by exposure to 5 mol-% of 1 at 55 °C for 17 h (or 19 h for Entry 3). [d] The Diels–Alder reaction with cyclopentadiene (16 equiv.)
was carried out in dry DCM at 0 °C for 4 d. [e] Calculated yield is based on recovered enone.
In essence, our ambitious plan was to perform the six-step reac- dented atom-economical procedure for the olefination of carb-
tion sequence in succession and avoid the isolation of interme- onyl compounds followed by the in situ Diels–Alder reaction of
diates (Scheme 2). In this process, complex 1 would serve as the intermediate enones to exclusively afford the endo cyclo-
the catalyst three times, namely, in the promotion of the adducts. This remarkable endo selectivity was fully supported
Meyer–Schuster rearrangement of propargylic alcohol 7, in the by DFT calculations. We believe that these new procedures add
(Z) to (E) isomerization of intermediate enone 2, and in the final to other useful protocols of modern organic chemistry for C–C
Diels–Alder reaction to give cycloadduct 3.
bond-forming processes and further illustrate the elegance and
efficiency of one-pot consecutive reaction processes.[2]
In the event, the initial preparation of enone 2 from alkyne
5 and aldehyde 6 was performed in DME, according to our
optimized procedure,[24] After a solvent exchange from DME to
DCM, compound 2 was then was exposed to an excess amount
of cyclopentadiene at low temperature to deliver the expected
Diels–Alder endo cycloadduct. The gratifying results of this new
procedure are reported in Table 3.
Experimental Section
Diels–Alder Cycloaddition with 2a as Dienophile: Complex
[ReOCl3(OPPh3)(SMe2)][1] (1, 5 mol-%) was added to a solution of
enone 2a (216 mg, 1 mmol, 1.0 equiv.) in anhydrous DCM (10 mL,
0.1 M) that was precooled to 0 °C. After stirring at 0 °C for 20 min,
freshly distilled cyclopentadiene (685 μL, 8.0 equiv.) was added. The
mixture was stirred at 0 °C for 1 d followed by the addition of
another portion of freshly distilled cyclopentadiene (8.0 equiv.).
After stirring for an additional 1 d, the reaction was quenched by
the addition of Et2O and H2O. The aqueous phase was extracted
with Et2O (3 × 8 mL), and the combined organic layers were washed
with H2O and brine, dried with Na2SO4, and then evaporated under
Conclusions
In summary, we have demonstrated the efficient Lewis acid-like
catalytic activity of oxo-Re catalyst 1 in the Diels–Alder reaction
of different carbonyl derivatives. Moreover, on the basis of the
catalytic role of 1 in both the Meyer–Schuster rearrangement
and the cycloaddition reaction, we have developed an unprece- reduced pressure. The residue was purified by silica gel chromatog-
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raphy [hexanes/methyl tert-butyl ether (MTBE), from 99:1 to 95:5]
to afford endo cycloadduct 3a (228 mg, 81 % yield) as a colorless
viscous oil; Rf = 0.21 (2 % MTBE/hexane). 1H NMR (300 MHz, CDCl3):
δ = 7.12–7.20 (m, 4 H), 6.41 (dd, J = 5.0, 3.3 Hz, 1 H), 6.02 (dd, J =
5.5, 2.7 Hz, 1 H), 3.32 (s, 1 H), 3.18 (d, J = 4.9 Hz, 1 H), 3.07 (dd, J =
5.0, 3.5 Hz, 1 H), 3.00 (s, 1 H), 2.44 (t, J = 7.5 Hz, 2 H), 2.35 (s, 3 H),
1.87 (d, J = 8.5 Hz, 1 H), 1.53–1.65 (m, 3 H), 1.20–1.37 (m, 5 H), 0.90
(t, J = 7.0 Hz, 3 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 210.3 (s),
141.4 (s), 139.1 (d), 135.3 (s), 133.1 (d), 129.1 (d), 127.2 (d), 60.2 (d),
48.6 (d), 47.5 (t), 46.5 (d), 45.0 (d), 41.7 (t), 31.4 (t), 23.4 (t),
NMR spectroscopic data, HRMS, IR, and HPLC analyses are in com-
plete agreement with previously reported data for compound 3a.
Acknowledgments
The authors thank the Italian Ministero dell'Università e della
Ricerca (MIUR) (funds PRIN 2011) for the financial support and
Professor Mariella Mella for the NMR spectral measurements.
Keywords: Synthetic methods · Rearrangment ·
Cycloaddition · Rhenium · Enones
22.4 (t), 20.8 (d), 13.8 (q) ppm. IR (film): ν = 2954, 1711, 1490, 1322,
˜
693 cm–1. HRMS (EI): calcd. for C20H26O 282.1984; found 281.1989.
General Procedure for One-Pot M–S and D–A Reaction Se-
quence: (Table 2, Entry 4). Complex [ReOCl3(OPPh3)(SMe2)] (1,
5 mol-%) was added to a solution of propargylic alcohol 4b
(111 mg, 0.5 mmol, 1.0 equiv.) in anhydrous 1,2-dimethoxyethane
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(10 mL, 0.05
(for times, see Table 2), the solvent was removed under vacuum,
and anhydrous DCM (5 mL, 0.1 ) was added. After cooling the
M) at room temperature. After stirring at 55 °C for 3 h
M
solution to 0 °C, freshly distilled cyclopentadiene (342 μL, 8.0 equiv.)
was added. The mixture was stirred at 0 °C for 1 d followed by
another addition of freshly distilled cyclopentadiene (8 equiv.). After
stirring for an additional 2 d, the reaction was quenched by the
addition of Et2O and H2O. The aqueous phase was extracted with
Et2O (3 × 15 mL), and the combined organic layers were washed
with H2O and brine, dried with Na2SO4, and evaporated under re-
duced pressure. The residue was purified by silica gel chromatogra-
phy (hexanes/MTBE, from 98:2 to 96:4) to afford endo cycloadduct
3b (100 mg, 70 % yield) as a colorless viscous oil; Rf = 0.28 (2 %
MTBE/hexane). 1H NMR (300 MHz, CDCl3): δ = 8.06–8.09 (m, 2 H),
7.54–7.70 (m, 3 H), 7.10–7.34 (m, 4 H), 6.59 (dd, J = 5.5, 3.2 Hz, 1 H),
5.99 (dd, J = 5.5, 2.7 Hz, 1 H), 4.03 (dd, J = 4.9, 3.4 Hz, 1 H), 3.58 (d,
J = 4.8 Hz, 1 H), 3.47 (s, 1 H), 3.24 (s, 1 H), 2.45 (s, 3 H), 2.17 (d, J =
8.5 Hz, 1 H), 1.71–1.79 (m, 1 H) ppm. 13C NMR (75 MHz, CDCl3): δ =
199.9 (s), 144.9 (s), 139.1 (d), 138.0 (s), 137.1 (s), 132.7 (d), 132.6 (d),
128.4 (d), 128.4 (d), 128.3 (d), 128.3 (d), 126.6 (d), 124.2 (d), 56.1 (d),
48.5 (t), 48.4 (d), 48.0 (t), 45.5 (d), 21.4 (q) ppm. IR (film): ν = 2956,
˜
1712, 1491, 1327, 698 cm–1. HRMS (EI): calcd. for C20H26O 288.1514;
found 288.1519.
General One-Pot Multistep Procedure: (Table 3, Entry 1). A solu-
tion of BuLi (2.5
wise to a solution of alkyne 5a (210 mg, 1.18 equiv.) in anhydrous
DME (17 mL, 0.1 ) that was precooled to –78 °C under argon. After
M in hexane, 805 μL, 1.15 equiv.) was added drop-
M
stirring for 1 h at –78 °C, commercially available aldehyde 6a
(210 mg, 1.75 mmol, 1.0 equiv.) was added, and the mixture was
stirred for 2 h (for times, see Table 3) at –78 °C until the starting
material was completely consumed. The mixture was then warmed
to room temperature, and Amberlite® IR-120 (H) (2.2 equiv.) was
added. After the mixture was vigorously stirred for 1 h at room
temperature, the solution was decanted into another dry flask, and
complex [ReOCl3(OPPh3)(SMe2)] (1, 5 mol-%) was added. The result-
ing mixture was then stirred at 55 °C for 17 h, and the solvent was
removed under vacuum. Anhydrous DCM (17 mL, 0.1M) was added.
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[15] In the literature, there is only one example of a Diels–Alder cycloaddition
promoted by methylrhenium trioxide (MTO), but this type of catalyst is
After the solution was cooled to 0 °C, freshly distilled cyclopentadi-
ene (1.2 mL, 8.0 equiv.) was added. The mixture was stirred at 0 °C
for 2 d, and then an additional portion of freshly distilled cyclopen-
tadiene (1.2 mL, 8 equiv.) was added. After stirring for an additional
2 d, the reaction was quenched by the addition of Et2O and H2O.
The aqueous phase was extracted with Et2O (3×), and the combined
organic layers were washed with H2O and brine, dried with Na2SO4,
and evaporated under reduced pressure. The residue was purified
by silica gel chromatography (hexanes/MTBE, from 98:2 to 96:4) to
exclusively afford endo cycloadduct 3a (113 mg, 45 % yield). The
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Received: June 27, 2016
Published Online: ■
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Rhenium(V) Catalysis
We have reported a simple and effi-
cient one-pot protocol for a Meyer–
Schuster rearrangement of alkynols
followed by a Diels–Alder cycloaddi-
tion of the resulting enones. A readily
available oxo-rhenium(V) catalyst pro-
moted both transformations under
mild conditions to afford a variety of
products in good overall yields with al-
most complete endo stereoselectivity.
S. Bugoni, A. Porta,* Z. Valiullina,
G. Zanoni, G. Vidari* ......................... 1–8
Dual ReV Catalysis in One-Pot Con-
secutive Meyer–Schuster and Diels–
Alder Reactions
DOI: 10.1002/ejoc.201600926
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