Transition-Metal-Free Cross-Coupling of Indium Organometallics with Chromene and Isochroman Acetals Mediated by BF3·OEt2

Article abstract of DOI:10.1021/acs.orglett.6b02058

A transition-metal-free coupling of triorganoindium reagents with benzopyranyl acetals mediated by a Lewis acid has been developed. The reaction of R₃In with chromene and isochroman acetals in the presence of BF₃·OEt₂ afforded 2-substituted chromenes and 1-substituted isochromans, respectively, in good yields. The reactions proceed with a variety of triorganoindium reagents (aryl, heteroaryl, alkynyl, alkenyl, alkyl) using only 50 mol % of the organometallic, thus demonstrating the efficiency of these species. Preliminary mechanistic studies indicate the formation of an oxocarbenium ion intermediate in the presence of the Lewis acid.

Full text of DOI:10.1021/acs.orglett.6b02058

Letter
pubs.acs.org/OrgLett
Transition-Metal-Free Cross-Coupling of Indium Organometallics
with Chromene and Isochroman Acetals Mediated by BF3·OEt2
Jose M. Gil-Negrete, Jose Perez Sestelo,* and Luis A. Sarandeses*
́
́
́
Centro de Investigaciones Científicas Avanzadas (CICA) and Departamento de Química Fundamental, Universidade da Coruna,
̃
E-15071 A Coruna, Spain
̃
S
* Supporting Information
ABSTRACT: A transition-metal-free coupling of triorganoin-
dium reagents with benzopyranyl acetals mediated by a Lewis
acid has been developed. The reaction of R3In with chromene
and isochroman acetals in the presence of BF3·OEt2 afforded
2-substituted chromenes and 1-substituted isochromans,
respectively, in good yields. The reactions proceed with a
variety of triorganoindium reagents (aryl, heteroaryl, alkynyl,
alkenyl, alkyl) using only 50 mol % of the organometallic, thus
demonstrating the efficiency of these species. Preliminary
mechanistic studies indicate the formation of an oxocarbenium ion intermediate in the presence of the Lewis acid.
α-Substituted oxygen heterocycles comprise an important
structural motif in chemistry, biology, and medicine.1 In
particular, benzopyrans such as 2H-chromene and isochroman
with α-functional groups constitute a large family of natural and
synthetic products with interesting biological activities. For
example, iclaprim is an antibiotic in phase III clinical trials for
the treatment of hospital-acquired pneumonia;2 acolbifene is
employed in the treatment of breast cancer;3 sonepiprazole is
an isochroman analog with activity as a selective dopamine
receptor antagonist that has also been investigated as an
antipsychotic;4 and penidicitrinin B is a natural product with
antioxidant activity.5 Additionally, other analogs with these
heterocyclic nuclei have found applications as photochromic
materials and as precursors of flavylium dyes (Figure 1).6
Therefore, the development of efficient methods for the
synthesis of α-substituted benzopyrans such as 2H-chromene
and isochroman is of great interest.7
The synthesis of α-substituted benzopyrans and α-
substituted ethers in general can be achieved by nucleophilic
addition to an in situ generated oxocarbenium ion formed by
Figure 1. Representative α-substituted chromenes and isochromans.
the reaction between acetals and Brønsted acids,8 Lewis acids,9
or under transition metal catalysis.10 The oxocarbenium ions
recently due to the drawbacks associated with the use of
play an important role in the synthesis of natural products and
palladium or nickel in coupling reactions, such as the toxicity,
bioactive molecules and, in particular, in the chemistry of
cost, and ligand design.19
carbohydrates and polycyclic ethers.11 Furthermore, in recent
years the oxidative α-functionalization of ethers has also been
developed.12 The nucleophilic counterpart in the addition to
oxocarbenium ions is usually a soft nucleophile: e.g., enolates
and derivatives,13 active methylene compounds,14 electron-rich
alkenes and arenes,15 and alkynes under metal catalysis.16
Organometallic species have found limited applications in these
reactions due to the low compatibility with the reaction
conditions, and mainly organoboron derivatives have been used
in this respect.17,18 In addition, transition-metal-free coupling
reactions of organometallic species have attracted great interest
The applications of indium in organic synthesis have
increased steadily in the past few years.20 Among these
applications, we discovered transition-metal-catalyzed cross-
coupling reactions using organoindium reagents.21,22 The high
efficiency, versatility, and selectivity of these reagents make
them useful alternatives to other organometallics in coupling
reactions. In addition, the soft nucleophilicity of organoindium
Received: July 14, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02058
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
reagents and the ongoing interest in novel methods led us to
explore the application of these compounds in the reaction with
acetals in the presence of Lewis acids. In this communication
we report a novel transition-metal-free coupling reaction of
triorganoindium reagents with oxocarbenium ion precursors.
Our study started with an assessment of the reaction of an
organoindium compound with an acetal in the presence of a
Lewis acid. The paramount importance of α-arylated
benzopyrans led us to start by investigating the reaction of
triphenylindium with 2-ethoxy-2H-chromene (1, Table 1) in
Scheme 1. Scope of the BF3-Mediated Reaction of
Triorganoindium Reagents with Chromene Acetal 1
Table 1. Reaction Optimization
mol % of
Ph3In
Lewis acid
(mol %)
yield
a
entry
conditions
(%)
1
2
3
4
5
6
7
8
9
100
40
40
50
50
50
50
50
50
BF3·OEt2 (200)
BF3·OEt2 (120)
BF3·OEt2 (120)
BF3·OEt2 (120)
BF3·OEt2 (20)
Cu(OTf)2 (120) THF, 80 °C
TMSOTf (120) THF, 80 °C
Yb(OTf)3 (120) THF, 80 °C
InBr3 (120) THF, 80 °C
DCE/THF (1:1), rt
THF, 0 °C
THF, 80 °C
THF, 80 °C
THF, 80 °C
92
56
65
91
10
−
−
33
6
a
Isolated yields.
a
b
Reactions performed at room temperature. Major regioisomer
c
isolated; 65% overall yield, ratio C-2/C-4 = 65:35. Major regioisomer
isolated; 65% overall yield, ratio C-2/C-4 = 62:38.
the presence of different Lewis acids and solvents. The
chromene 1 was chosen based on its stability and ease of
preparation. Other leaving groups different to the alkoxide,
such as the acetate, were not considered since they have not
been reported in the chemistry of chromenes. Since
triorganoindium compounds are normally prepared from the
corresponding organolithium reagents by transmetalation with
InCl3 in THF solution, and to avoid an evaporation step that
could lead to decomposition of the organometallic,23 the use of
this solvent in the coupling reactions is preferred (or as
cosolvent). These considerations prompted us to start our
study by testing BF3·OEt2 as the Lewis acid in the reactions.
Initial experiments on the reaction of 1 with Ph3In (100 mol %)
and BF3·OEt2 (200 mol %) in DCE/THF (1:1) at rt afforded
the coupling product 2 in an excellent 92% yield (Table 1, entry
1). Further experiments were performed to reduce the amount
of Lewis acid needed and to check the possibility of the indium
reagent transferring more than one aryl group in the reaction,
as occurs in the transition-metal-catalyzed coupling reactions of
these reagents.20 In this way the reaction with 40 mol % of
Ph3In and 120 mol % of BF3·OEt2 in THF at 0 °C afforded 2 in
56% yield (entry 2). When the reaction was performed at 80
°C, the yield increased to 65% while the use of 50 mol % of
Ph3In led to an excellent 91% yield (entries 3 and 4,
respectively). Furthermore, the use of lower quantities of
BF3·OEt2 (20 mol %) gave a lower yield (entry 5), and finally,
the use of other Lewis acids [Cu(OTf)2, TMSOTf, Yb(OTf)3,
InBr3] proved to be ineffective for the production of 2 in good
yields (entries 6−9), probably due to the incompatibility of
THF as a solvent because it is a donor compound that
deactivates the acid character of the salt.
the optimized reaction conditions (Scheme 1). In general,
reactions of 1 with triarylindium reagents afforded good yields
of the corresponding 2-aryl chromenes 2−6 (67−91%). These
reactions proceeded efficiently overnight at 80 °C except for
the reactions with electron-rich arylindium reagents, such tri(4-
methoxyphenyl)indium and tri(p-tolyl)indium, which took
place at room temperature with high yields. Additionally,
heteroarylindium compounds such as tri(2- thienyl)indium also
reacted efficiently with 1 to give compound 7 in 74% yield.
When the reaction was performed with trialkynylindium
reagents, the desired 2-alkynyl chromenes 8 and 9 were
obtained in good yields (85% and 89%, respectively). The
alkynylation of acetals is a key approach to deliver α-carbon
substituents to oxygenated functional groups due to the
possibility of further elaboration of the triple bond to a range
of substituents.9b,16,24 The results obtained in this study
demonstrate the utility of indium reagents in the alkynylation
of acetals in the presence of Lewis acids. Groups other than aryl
and alkynyl were also studied to demonstrate the versatility of
indium reagents in these reactions. In this sense, the reaction of
1 with an alkenylindium compound afforded 2-alkenyl
chromene 10 in an excellent 92% yield without isomerization.
Finally, the reaction of 1 with tributyl- and trimethylindium
gave lower yields of the products and a lack of regioselectivity,
with 2-substituted chromenes 11 and 12 obtained along with
the 4-substituted isomers (65% yield, ratio C-2/C-4 65:35 to
62:38). Interestingly, the reaction involving tricyclopropyl-
indium afforded the 2-substituted chromene 13 regioselectively
in 84% yield, a result that contrasts with those obtained with
the other alkylindium reagents, and thus demonstrating the
special reactivity of the cyclopropyl derivatives. Remarkably, all
Encouraged by these results, we evaluated the scope of this
reaction by employing different triorganoindium reagents under
B
DOI: 10.1021/acs.orglett.6b02058
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Letter
of these reactions give good yields on using 50 mol % of the
organoindium reagent, which indicates that the triorganoin-
dium reagents transfer more than one group attached to the
metal. Nevertheless, when these reactions were performed with
100 mol % of R3In, good yields of the products were obtained
even at room temperature, a fact that could indicate that the
transfer to the electrophile of the first group attached to the
metal is faster than the others.
Once the reaction of R3In with chromene acetals in the
presence of BF3·OEt2 was probed, we turned our attention to
the study of the reactivity of isochroman derivatives. Isochro-
man is a relevant structural motif in natural products and
biologically active compounds, as well as being a building block
in synthetic organic chemistry.1b Since reactions of chromene
acetals are expected to proceed through an oxocarbenium ion
intermediate, the use of isochroman acetals as the starting
electrophiles should give rise to the same type of intermediate.
Reaction of 1-methoxyisochroman (14, Scheme 2) with
Ph3In (50 mol %) under the previously described conditions
We found that the reaction of 14 with electron-deficient
arylindiums such as tri(4-fluorophenyl)indium also occurs
efficiently to afford 18 in a good 88% yield (Scheme 2).
Heterocyclic nucleophiles such the 2-thienyl ring can also be
added to isochroman acetal 14 to obtain 19 in 71% yield under
the optimized conditions. Additionally, 1-alkynyl isochromans
were also efficiently prepared by the BF3·OEt2-mediated
addition of trialkynylindium reagents to 14 (70−82%). These
results show the utility of indium reagents in the Lewis acid
mediated reactions with isochroman acetals and constitute a
new entry in the reactivity of these compounds.
Although the mechanism of these reactions was not studied,
a plausible proposal could include the generation of an
intermediate oxocarbenium ion by reaction between the acetals
and the Lewis acid, followed by nucleophilic addition of the
triorganoindium reagents. In an attempt to obtain additional
information about this process, the chiral chromene acetals 22a
and 22b were prepared.25 Interestingly, the reaction of 22a or
22b, independently, with an excess of Ph3In (100 mol %) in the
presence of BF3·OEt2 at room temperature afforded, after 6 h of
reaction, the racemic chromene 2 in good yields (92−94%) in
both cases (Scheme 3). These results are consistent with the
Scheme 2. Scope of the BF3-Mediated Reaction of
Triorganoindium Reagents with Isochroman Acetal 14
Scheme 3. Results of BF3-Mediated Reaction of
Triphenylindium with Chiral Acetals
formation of an achiral oxocarbenium intermediate during the
course of the reaction that gives rise to the reaction products by
further reaction with the organoindium reagent.
In summary, we have developed a new transition-metal-free
reaction of organoindium reagents with chromene and
isochroman acetals mediated by BF3·OEt2. The reaction can
be performed with a variety of triorganoindium reagents to
afford 2-substituted chromenes and 1-substituted isochromans
in good yields. These results show the versatility of organo-
indium reagents in carbon−carbon bond-forming reactions and
constitute a new example of the wide synthetic utility of these
reagents in addition to their classical reactions under transition
metal catalysis. Further studies to expand the synthetic scope of
these processes and investigations into the mechanism are in
progress and will be reported in due course.
(120 mol % of BF3·OEt2, THF, 80 °C) afforded 1-
phenylisochroman (15) in an excellent 92% yield after 12 h.
Compared with the chromene acetal 1, isochroman 14 proved
to be slightly less reactive, and it required longer reaction times
for the reaction to reach completion, even on using an excess of
the indium counterpart. The probable reason for this is that
while in 1 the formation of the transient oxocarbenium ion is
favored by the supplementary stabilization of the double bond
at C-3 that gives an aromatic intermediate, in 14 the
oxocarbenium ion is only stabilized by the aromatic ring.
The BF3·OEt2-mediated coupling with isochroman acetal was
studied using other organoindium reagents. The reaction with
tri(p-tolyl)indium afforded 16 in 90% yield, and the coupling
with the electron-rich derivative tri(4-methoxyphenyl)indium,
under the same conditions, gave isochroman 17 in 84% yield.
The lower reactivity of isochroman acetal is revealed by the
need to heat the reaction mixture under reflux in these two
examples, while in the reactions with chromene acetal 1 the
reactions with electron-rich R3In occur at room temperature.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b02058.
Experimental procedures, compound characterization
data, and copies of NMR spectra for all compounds
(PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: sestelo@udc.es.
*E-mail: luis.sarandeses@udc.es.
C
DOI: 10.1021/acs.orglett.6b02058
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
P.; Yap, G. P. A.; Watson, M. P. J. Org. Chem. 2015, 80, 4003−4016.
(c) Haidzinskaya, T.; Kerchner, H. A.; Liu, J.; Watson, M. P. Org. Lett.
2015, 17, 3857−3859.
Notes
The authors declare no competing financial interest.
(17) (a) Moquist, P. N.; Kodama, T.; Schaus, S. E. Angew. Chem., Int.
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W. J. Am. Chem. Soc. 2011, 133, 14082−14089. (c) Baxter, M.;
Bolshan, Y. Chem. - Eur. J. 2015, 21, 13535−13538.
ACKNOWLEDGMENTS
■
́
We are grateful to the Spanish Ministerio de Economia y
Competitividad (MINECO, Grant Numbers CTQ2012-31200
and CTQ2015-68369-P) and the EDRF funds for financial
support.
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Germain, N.; Baltaze, J.-P.; Coste, A.; Hallonet, A.; Laurea
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no, H.;
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