Superelectrophilic Fe(III)-Ion Pairs as Stronger Lewis Acid Catalysts for (E)-Selective Intermolecular Carbonyl-Olefin Metathesis

Article abstract of DOI:10.1021/acs.orglett.0c00917

An intermolecular carbonyl-olefin metathesis reaction is described that relies on superelectrophilic Fe(III)-based ion pairs as stronger Lewis acid catalysts. This new catalytic system enables selective access to (E)-olefins as carbonyl-olefin metathesis products. Mechanistic investigations suggest the regioselective formation and stereospecific fragmentation of intermediate oxetanes to be the origin of this selectivity. The optimized conditions are general for a variety of aryl aldehydes and trisubstituted olefins and are demonstrated for 28 examples in up to 64% overall yield.

Full text of DOI:10.1021/acs.orglett.0c00917

pubs.acs.org/OrgLett
Letter
Superelectrophilic Fe(III)−Ion Pairs as Stronger Lewis Acid Catalysts
for (E)‑Selective Intermolecular Carbonyl−Olefin Metathesis
Haley Albright,† Hannah L. Vonesh,† and Corinna S. Schindler*
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ABSTRACT: An intermolecular carbonyl−olefin metathesis re-
action is described that relies on superelectrophilic Fe(III)-based
ion pairs as stronger Lewis acid catalysts. This new catalytic system
enables selective access to (E)-olefins as carbonyl−olefin meta-
thesis products. Mechanistic investigations suggest the regioselec-
tive formation and stereospecific fragmentation of intermediate
oxetanes to be the origin of this selectivity. The optimized conditions are general for a variety of aryl aldehydes and trisubstituted
olefins and are demonstrated for 28 examples in up to 64% overall yield.
formation of (E)-olefins as products is thermodynamically
favored, a mixture of both diastereomers is often observed.2 In
recent years, carbonyl−olefin metathesis reactions have seen
increased interest as a result of their ability to directly form
carbon−carbon bonds between carbonyl and olefin function-
alities.5−8 Lewis-acid-catalyzed approaches have been devel-
oped as part of these efforts and undergo an initial [2 + 2]-
cycloaddition to form intermediate oxetanes and a subsequent
retro-[2 + 2]-cycloreversion to yield the corresponding
carbonyl−olefin metathesis products.9 On the basis of this
design principle, viable procedures for carbonyl−olefin ring-
closing and ring-opening metatheses, as well as transannular
carbonyl−olefin metathesis, have been reported that proceed
through oxetane intermediates.10−13 Additional approaches to
intermolecular carbonyl−olefin cross-metathesis exist, relying
on either zeolites14 or carbocations as organocatalysts.12,15,16
In comparison with olefin−olefin cross-metathesis, the
currently available protocols for carbonyl−olefin cross-meta-
thesis reactions remain significantly underdeveloped. We
herein report studies toward an intermolecular carbonyl−
olefin cross-metathesis reaction between aromatic aldehydes 6
and olefins 7 that relies on superelectrophilic Fe(III)−ion
pairs17 as stronger Lewis acid catalysts. The reaction is found
to be selective for one of three possible products via four
diastereomeric oxetane intermediates (cis- and trans-8 and -9)
(Figure 1B).
lefin−olefin cross-metathesis reactions are powerful tools
Ofor direct carbon−carbon bond formation to access
more complex olefins from simple olefin precursors.1 Upon
reaction with a metal alkylidene catalyst, olefins 1 and 2 are
converted to the corresponding heterodimerization product 3
or homodimerization products 4 and 5 (Figure 1A).2 The
selectivity between products can be controlled depending on
the choice of substrates, the relative ratio of the two olefins,
and the metal alkylidene catalyst employed.3,4 Although the
The mechanistic studies reported in this Letter are
consistent with regiospecific oxetane formation and the
Received: March 11, 2020
Figure 1. (A) Olefin−olefin cross-metathesis. (B) (E)-Selective
carbonyl−olefin cross-metathesis relying on Fe(III)−ion pairs.
https://dx.doi.org/10.1021/acs.orglett.0c00917
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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subsequent stereospecific oxetane fragmentation, which
accounts for the high selectivity in products. Insights gained
from this work are expected to guide future reaction
development and catalyst design to expand and improve the
synthetic utility of available protocols.
The initial studies toward the development of an
intermolecular carbonyl−olefin metathesis reaction focused
on benzaldehyde 12 and 2-methyl-2-butene 13 as substrates
(Table 1). Early efforts identified benzaldehyde 12 and 2-
pairs,17 resulting upon halide abstraction from neutral metal
salts (MXn) with silver salts (AgX),18 can function as Lewis
acidic superelectrophiles for catalytic carbonyl−olefin meta-
thesis reactions. The addition of catalytic amounts of silver
salts in combination with FeCl3 as the Lewis acid resulted in
increased yields of the desired carbonyl−olefin metathesis
product 14 (entries 9−13, Table 1). Specifically, AgBF4 was
identified as the superior silver salt together with FeCl3,
providing (E)-olefin in 51% yield (entry 13, Table 1).
Stoichiometric amounts of AgBF4 under otherwise identical
conditions resulted in diminished yields of 14 in 28% (entry
14, Table 1). Decreasing loadings of AgBF4 of 10 and 20 mol
% also proved inferior and formed 14 in 27 and 20%,
respectively (entries 15 and 16, Table 1). GaCl3 (which had
similar yields to FeCl3) was evaluated in combination with 30
mol % of AgBF4; however, the transformation provided a
diminished yield of 35% (entry 17, Table 1). Importantly, (E)-
olefin 14 was observed as the exclusive carbonyl−olefin
metathesis product for all Lewis acids and reaction conditions
evaluated (Table 1).
Subsequent efforts focused on obtaining experimental
support for heterobimetallic ion pairs as the active catalytic
species under the optimal reaction conditions. Several distinct
Lewis acidic species could be operative as the active catalyst:
FeCl3 (A), AgBF4(B), heterobimetallic ion pairs
[FeCl2]+[BF4]− (C), and [Fe]3+3[BF4]− (D), resulting from
chloride abstraction, or FeCl2F (E), FeF3 (F), and BF3 (G),
formed via fluoride transfer or the decomposition of C and D
(Table 2). As previously demonstrated, substoichiometric
amounts of FeCl3 formed metathesis product 14 in 19% yield,
whereas the sole use of AgBF4 failed to promote the desired
carbonyl−olefin metathesis reaction (entries 1 and 2, Table 2).
Equimolar loadings of 10 mol % FeCl3 and AgBF4 were also
able to catalyze the reaction, although in a low yield of 20%
Table 1. Reaction Optimization for Intermolecular
a
Carbonyl−Olefin Metathesis
entry
Lewis acid
additive
mol %
solvent
yield 14 (%)
b
1
BF3·Et2O
FeCl3
FeCl3
FeCl3
GeCl3
AlCl3
Fe(OTf)3
Sc(OTf)3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
30
30
30
30
30
30
30
30
30
30
30
DCM
DCM
DCE
28
19
16
2
17
0
30
26
0
31
24
36
51
28
27
20
35
27
19
b
2
b
3
b
4
toluene
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
b
5
b
6
b
7
b
8
c
9
AgOTS
AgAsF6
AgSbF6
AgPF6
AgBF4
AgBF4
AgBF4
AgBF4
AgBF4
AgBF4
AgBF4
c
10
11
12
13
14
15
16
17
c
c
d
c
c
c
c
e
a
18
19
Table 2. Determination of Active Catalytic Species
f
a
Conditions: All reactions were performed using 5.0 equiv of the
substrate 12 and 1 equiv of 13 in DCM (0.3 M) at 25 °C for 24 h.
b
c
Yields are reported based on NMR analysis. Yields are reported
d
based on GC analysis. Yields are reported based on isolated yield.
e
f
2.0 equiv of 12 was used. 1.0 equiv of 12 was used.
methyl-2-butene 13 in a 5:1 ratio as optimal for the
transformation. (See the Supporting Information for additional
́
details.) Franzen and coworkers were able to achieve 60% yield
of 14 with 20 mol % of TrBF4 as the cationic catalyst and
otherwise identical conditions as entry 1, Table 1.15 Catalytic
amounts of BF3·OEt2 and FeCl3 promoted the desired
intermolecular carbonyl−olefin metathesis reaction in 28 and
19% yield, respectively (entries 1 and 2, Table 1). In
comparison, decreased yields of 14 were observed with other
solvents, including dichloroethane and toluene, under
otherwise identical reaction conditions (entries 3 and 4,
Table 1). GaCl3 was analogous to FeCl3 and resulted in the
desired metathesis product in 17% yield (entry 5, Table 1),
whereas stronger Lewis acids, such as AlCl3, proved ineffective
in promoting the desired carbonyl−olefin metathesis reaction
(entry 6, Table 1). Promising results were also obtained with
catalytic amounts of metal triflates, Fe(OTf)3 and Sc(OTf)3,
resulting in the formation of (E)-olefin 14 in increased yields
of 30 and 26%, respectively (entries 7 and 8, Table 1).
Recently, we were able to show that heterobimetallic ion
entry
species
Lewis acid
additive
X mol %
yield 14 (%)
b
1
A
B
C
D
E
F
FeCl3
−
FeCl3
FeCl3
FeCl3
FeCl3
FeF3
−
AgBF4
AgBF4
AgBF4
AgF
AgF
−
−
30
10
30
10
30
−
19
0
20
51
4
9
0
28
c
2
de
,
3
4
5
6
7
8
c e
,
c
c
b
b
F
G
BF3·Et2O
−
−
a
Conditions: All reactions were performed using 5.0 equiv of the
substrate 12 and 1.0 equiv of 13 in DCM (0.3 M) at 25 °C for 3 h.
Yields were determined via NMR with PhMe3Si as an internal
standard. Yields were based on isolated yield. Yields were reported
based on GC analysis. Formation of AgCl was observed.
b
c
d
e
B
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presence of F− ions from the AgBF4 additive. Furthermore,
when the reaction was conducted with BF3·Et2O (10 mol %),
the desired metathesis product 14 was observed in a
diminished yield of 28% (entry 8, Table 2). Collectively,
these results suggest the formation of [FeCl2]+[BF4]− and
[Fe]3+3[BF4]− as heterobimetallic ion pairs that serve as the
active catalytic species under optimal conditions for the
intermolecular carbonyl−olefin metathesis.
Table 3. Olefin Evaluation for Intermolecular Carbonyl−
d
Olefin Metathesis
The olefin substrate scope for intermolecular carbonyl−
olefin cross-metathesis reactions relying on superelectrophilic
Fe(III)−ion pairs was next investigated (Table 3). Specifically,
the substitution of longer aliphatic chains on the olefin
substrate, including ethyl, isobutyl, and n-heptyl, was found to
be compatible with the optimal reaction conditions and
formed the respective metathesis products in up to 40% yield
(19−21, Table 3). 2-Ethyl-2-pentene, 22, was also found to be
reactive and provided a 23% yield of the corresponding
metathesis product, whereas styrene derivative 23 proved
unreactive for metathesis. Importantly, the corresponding (E)-
olefins were the exclusive metathesis products observed over
the course of these transformations. Following the inves-
tigation into the olefin substrates, the aldehyde substrates were
evaluated upon their ability to undergo the desired
intermolecular carbonyl−olefin metathesis reaction (Table
4). para-Substituted aryl aldehydes with both electron-
withdrawing and electron-donating groups proved viable
under the reaction conditions, resulting in up to a 62% yield
of the metathesis products (24−33, Table 4). Polyaromatic
substrates including phenanthrene- and fluorene-derived aryl
aldehydes effectively promoted the metathesis in low to
moderate yields of 27 and 51%, respectively (34 and 35, Table
4). ortho-, meta-, para-, and multi-substituted aldehydes were
compatible with the optimized conditions for intermolecular
carbonyl−olefin metathesis and formed the desired products in
yields of up to 64% (36−41, Table 4). Additionally, 2-
a
b
With benzaldehyde as aldehyde. With 4-chlorobenzaldehyde as
c
aldehyde. Yields denote the metathesis product formation of each
olefin substrate displayed above. Conditions: All reactions were
performed with 5.0 equiv of 6 and 1.0 equiv of olefin substrates 16
with 10 mol % FeCl3 and 30 mol % AgBF4 in DCM (0.3 M) at 25 °C
for 3 h.
d
(entry 3, Table 2). In comparison, the optimal reaction
conditions relying on FeCl3 (10 mol %) and AgBF4 (30 mol
%) provided the desired product 14 in 51% yield (entry 4,
Table 2). The quantitative formation of AgCl, as a white solid,
was observed over the course of this transformation.19 Utilizing
FeCl3 (10 mol %) and AgF (10 mol %) also resulted in the
formation of product, suggesting that FeCl2F may be formed
under these conditions, albeit 14 was observed in only 4% yield
(entry 5, Table 2). Similarly, 30 mol % AgF together with
substoichiometric amounts of FeCl3 resulted in only 9% yield
of the desired metathesis product (entry 6, Table 2).
Additionally, FeF3 was not an active catalyst for this
transformation, confirming that it does not form in the
Table 4. Evaluation of the Aldehyde Substrate Scope for Intermolecular Carbonyl−Olefin Metathesis Relying on Fe(III)−Ion
a
Pairs
a
Conditions: All reactions were performed with 5.0 equiv of aldehyde and 1.0 equiv of olefin substrates FeCl3 (30 mol %) and AgBF4 (30 mol %)
in dichloromethane (0.3 M) at 25 °C for 3 h.
C
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Figure 2. Experiments in support of regioselective oxetane formation and stereospecific oxetane fragmentation under the optimal reaction
conditions for intermolecular carbonyl−olefin metathesis.
naphthaldehyde substrates provided moderate yields of up to
43% (43−45, Table 4). In accordance with the previous
observations made during the initial reaction optimization and
the investigation of the olefin substrate scope, the correspond-
ing (E)-olefins were the only observed products for this
transformation.
The next efforts aimed to determine the origin of the high
(E)-selectivity observed in this transformation. Upon the
addition of aryl aldehydes 6 and olefins 7, four distinct oxetane
stereoisomers could form (Figure 2). cis- and trans-oxetanes 9
would be expected as the major isomers as a result of the
carbonyl oxygen atom adding to the more electrophilic carbon
of the olefin, and cis- and trans-oxetanes 8 are predicted to be
the minor products formed. The fragmentation of these
oxetane intermediates could result in three distinct metathesis
products: (E)-11 formed upon the fragmentation of trans-9,
(Z)-11 resulting from cis-9, and trisubstituted alkene 10 as the
product obtained from both cis- and trans-8. trans-4620 was
̀
synthesized independently via the Paterno−Buchi reaction and
̈
subjected to the optimal conditions to gain insight into
whether oxetane formation proceeds regioselectively (Figure
2I). The sole product formed was trisubstituted olefin 23,
which proved stable under reaction conditions. This olefin is
not observed under the optimal conditions for intermolecular
carbonyl−olefin metathesis, which suggests that regioisomeric
oxetanes cis- and trans-8 are not formed as reactive
intermediates in this reaction, confirming the lack of observed
olefin product 10. To probe whether the fragmentation of
oxetane trans-9 proceeds stereospecifically (Figure 2II),
̀
Figure 3. NMR fragmentation study of oxetane stereoisomers.
product 23 was observed in 78% yield together with (E)-
olefin 14 in 47% yield, resulting upon the fragmentation of
trans-46 and trans-47, respectively. Importantly, the corre-
sponding stereoisomer (Z)-11 was not observed in this
transformation. On the basis of these results, the isomerization
of (Z)-11 could also rapidly proceed under the optimized
reaction conditions. To test this hypothesis, the isomerization
of (Z)-14 and benzaldehyde (12) in a 1:4 ratio, respectively,
was monitored (Figure 2III). Isomerization from (Z)-14 to
(E)-14 was observed, however, in only 23% yield over the
oxetane trans-4720 was accessed via Paterno−Bu
̈
chi reaction
protocols as a mixture of isomers (Figure 2II). trans-47,
together with oxetane trans-46, was characterized as a mixture
in a 1:2.6 ratio and subjected to the optimal reaction
conditions (Figures 2II and 3). As expected, metathesis
D
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course of the reaction. Together, these results suggest the
regioselective formation and the subsequent stereospecific
fragmentation of an oxetane intermediate to result in the
exclusive formation of (E)-olefin products.
ACKNOWLEDGMENTS
■
We thank the NIH/National Institute of General Medical
Sciences (R01-GM118644), the Alfred P. Sloan Foundation,
and the David and Lucile Packard Foundation for financial
support. H.A. thanks the National Science Foundation Science
Foundation for a predoctoral fellowship.
The intermolecular carbonyl−olefin metathesis reaction of
benzaldehyde (6) and trisubstituted olefin 23 was monitored
1
via H NMR. (See the Supporting Information for details.)
The formation of the (E)-olefin metathesis product is evident
within the first 5 min and becomes the major product in the
solution after 3 h. Additionally, studies have been performed to
determine if competing carbonyl-ene reaction pathways are
responsible for the diminished overall yield observed in
intermolecular carbonyl−olefin metathesis reactions.21 How-
ever, no byproducts resulting via carbonyl-ene intermediates
could be isolated, which is in stark contrast with the previously
developed GaCl3-catalyzed carbonyl−olefin ring-opening
metathesis reactions developed in our group. Consequently,
the diminished yields are hypothesized to be the result of
competing decomposition pathways during either oxetane
formation or fragmentation (Figures 2II and 3).
The studies of Lewis-acid-catalyzed intermolecular carbon-
yl−olefin metathesis reactions have revealed significant insights
into the reaction pathway. Specifically, the metathesis reaction
proceeds via a distinct regioisomer of the four possible oxetane
intermediates to result in the selective formation of the (E)-
olefin metathesis product. The lower yields observed in this
method are determined to be due to competing decomposition
pathways during oxetane formation and the subsequent
fragmentation. The insights presented herein are expected to
enable the development of more efficient catalyst systems to
promote this transformation and to develop this reaction
design into a platform of general synthetic utility.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
Corresponding Author
■
Corinna S. Schindler − Department of Chemistry, Willard
Henry Dow Laboratory, University of Michigan, Ann Arbor,
Michigan 48109, United States; orcid.org/0000-0003-
4968-8013; Email: corinnas@umich.edu
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Authors
Haley Albright − Department of Chemistry, Willard Henry Dow
Laboratory, University of Michigan, Ann Arbor, Michigan
48109, United States
Hannah L. Vonesh − Department of Chemistry, Willard Henry
Dow Laboratory, University of Michigan, Ann Arbor, Michigan
48109, United States
̈
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00917
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Author Contributions
†H.A. and H.L.V. contributed equally.
Notes
The authors declare no competing financial interest.
E
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