Arylsulfonylacetylenes as alkynylating reagents of C sp 2 -H bonds activated with lithium bases

Article abstract of DOI:10.1002/anie.201107821

Chameleon: A new strategy for the synthesis of a wide variety of alkynyl derivatives by the reaction of substituted arylsulfonylacetylenes with organolithium species is described (see scheme). The high yields, the simplicity of the experimental procedure, the broad scope of this reaction, and the formation of C_sp-C sp 2 bonds without using transition metals are the main features of this methodology. Copyright

Full text of DOI:10.1002/anie.201107821

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DOI: 10.1002/anie.201107821
Synthetic Methods
À
Arylsulfonylacetylenes as Alkynylating Reagents of Csp2 H Bonds
Activated with Lithium Bases**
Josꢀ Luis Garcꢁa Ruano,* Josꢀ Alemꢂn,* Leyre Marzo, Cuauhtꢀmoc Alvarado, Mariola Tortosa,
Sergio Dꢁaz-Tendero, and Alberto Fraile
Dedicated to Dr. Amelia Tito.
In recent years, acetylene chemistry has become an increas-
ingly attractive topic for chemists because of its importance
in the synthesis of bioactive natural products and new
materials as well as in biochemistry.[1] A variety of new
approaches have appeared for incorporating alkyne moieties
into organic molecules. The most common methods for the
being the availability of the starting halides. Moreover,
specialized reaction conditions are often necessary to improve
the poor results obtained in some couplings involving
electron-rich Csp2 components (which interfere with the
oxidative addition step) or electron-poor alkynes (which do
not readily form the copper acetylide intermediates).[4]
A
À
formation of Csp Csp2 bonds to provide aryl acetylenes and
more recent strategy for the formation of aryl alkynes is the
conjugated enynes are illustrated in Scheme 1,[2] the most
powerful being the well-known Sonogashira cross-coupling
inverse-Sonogashira-type reaction (Scheme 1b)[5] in which
the alkyne source is typically a bromo alkyne and aromatic
[5b]
C H activation is achieved using palladium, nickel,[5a] or
À
copper[5c] catalysts. This method is generally restricted to
À
substrates susceptible to C H activation (azoles or activated
aromatic rings). A third, but even less general approach is
based on a gold-catalyzed Friedel–Crafts-type addition of
highly electron-rich aromatic rings[6] (e.g., 2,4,6-trimethoxy-
phenyl groups,[6a] thiophenes[6b] or indoles[6c]) to electron-poor
terminal alkynes or alkynyl iodonium salt derivatives
(Scheme 1c). Because all these methods suffer from problems
associated with the use of expensive catalysts and harsh
reaction conditions, the development of alternative methods
able to circumvent these limitations would be highly desir-
able.
À
Scheme 1. Approaches for the synthesis of Csp2 Csp bonds.
We have recently reported reactions of ortho-sulfinyl
benzylcarbanions with b-monosubstituted vinylsulfones as
the solution for creating carbon skeletons containing two
adjacent chiral centers.[7] To evaluate the applicability of this
methodology to the synthesis of quaternary centers by
reaction with b-disubstituted vinylsulfones, we decided to
synthesize 3aA (Scheme 2, left) by Michael addition of the
reaction (Scheme 1a).[3] It starts from aryl or alkenyl halides
and terminal alkynes and requires the presence of a palla-
dium(0) catalyst and a copper source (cocatalyst). The scope
of this reaction is quite general, with its primary limitation
[*] Prof. Dr. J. L. Garcꢀa Ruano, Dr. J. Alemꢁn, L. Marzo, Dr. C. Alvarado,
Dr. M. Tortosa, Dr. A. Fraile
Departamento de Quꢀmica Orgꢁnica (C-1)
Universidad Autꢂnoma de Madrid, Cantoblanco, 28049 Madrid
(Spain)
E-mail: joseluis.garcia.ruano@uam.es
jose.aleman@uam.es
Dr. S. Dꢀaz-Tendero
Departamento de Quꢀmica (Modulo 13)
Universidad Autꢂnoma de Madrid, Cantoblanco, Madrid (Spain)
Scheme 2. Alkynylation of anisole with arylsulfonylacetylenes.
[**] Financial support from the Spanish Government (CTQ-2009-12168)
and CAM (“programa AVANCAT CS2009/PPQ-1634”) is gratefully
acknowledged. J.A., M.T., and S.D.T thank the MICINN for “Ramon
y Cajal” contracts, C.A. thanks the “Consejo Nacional de Ciencia
y Tecnologꢀa de Mꢃxico” for a postdoctoral fellowship, and L.M.
thanks the Ministerio de Educaciꢂn y Ciencia for a predoctoral
fellowship. We gratefully acknowledge computational time provided
by the “Centro de Computaciꢂn Cientꢀfica” at the Universidad
Autꢂnoma de Madrid CCC-UAM.
orthometalated anisole (obtained with nBuLi) to the sulfo-
nylacetylene 1a. Unexpectedly, only the anti-Michael[8]
addition product 4aA was obtained (Scheme 2, right).
Based on this unexpected behavior, we hypothesized
À
about a new approach for the synthesis of Csp2 Csp bonds
based on the use of sulfonyl acetylenes as general alkynylating
reagents of aromatic positions activated with lithium bases to
form ArLi. Taking into account that the conditions for this
reaction (5 min at À788C) are milder than those required by
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107821.
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[a]
other methods[9] (Scheme 1), and the fact that no transition
metals are necessary, we thought this method could be used as
an alternative to the methodologies indicated at Scheme 1.
Herein, we demonstrate the scope and generality of this
method for the alkynylation of a wide variety of Csp2-lithium
substrates.
À
Table 1: Alkynylations of Ar H activated by tBuLi or nBuLi.
Under optimized reaction conditions, ortho lithiation of
anisole (2A) using nBuLi or tBuLi at room temperature with
subsequent addition of the sulfonylacetylene 1a at À788C,
afforded 4aA in 5 minutes (81% yield; entry 1, Table 1).
Under similar reaction conditions, 2A reacted with the TIPS-
substituted sulfonylacetylene 1b to afford alkynyl derivative
4bA in 80% yield (entry 2, Table 1). This result is particularly
remarkable because of the potential synthetic utility of 4bA
as an intermediate in the preparation of many other alkynyl
derivatives (by previous deprotection of the terminal alkyne),
indicating that the anti-Michael behavior of the sulfonylace-
tylenes is not exclusive of 2-aryl derivatives.[10] Other aryl-
acetylenesulfones (1c–e) bearing electron-donating and elec-
tron-withdrawing groups at the aromatic ring, also reacted
with similar yield, thus affording the corresponding acety-
lenes (entries 3–5, Table 1). Reaction of 2B with 1a exclu-
sively provided 4aB in 85% yield (entry 6, Table 1), which
was increased up to 92% (entry 7) on a 4.0 mmol scale. The
absence of dialkynylated products indicates that 4aB is less
reactive than 2B, as would be expected from the deactivating
influence of the alkynyl groups on the formation of the
corresponding aryl lithium species. The reaction of 2C was
interesting because of its regioselectivity (entry 8, Table 1).
Working under the usual reaction conditions (ortho lithiation
at RT), the reaction affords a 80:20 mixture of 4aC and its
regioisomer 4-alkynyl-1,3-dimethoxybenzene, which were
easily separated by chromatography (56% yield of 4aC).
The lower susceptibility of 2D to ortho lithiation, is likely
a result of both electronic (high electronic density of the ring)
and steric factors, which lead to the need to perform the
reaction at room temperature (other reactions were nearly
instantaneous at À788C). Under these reaction conditions,
decomposition of 1a was very fast,[11] and the use of a large
excess of the nucleophile (8 equiv) activated by TMEDA
(8 equiv) was necessary to achieve its complete alkynyla-
tion[11] to obtain 4aD[12] in 80% yield (entry 9, Table 1). A
particularly interesting result was obtained from 2-methoxy-
naphthalene (2E), which upon lithiation by nBuLi and
addition of 1a afforded a 95:5 mixture of the two possible
regioisomers. After separation, the major isomer, 4aE, was
obtained in 70% yield (entry 10, Table 1). The observed
regioselectivity (attack at C3 is favored) can be explained by
assuming that electronic and steric factors both inhibit C1
lithiation and subsequent alkynylation.
Entry
Substrate
1 (R1)
Product
Yield
[%][b]
1
2
3
4
5
1a
1b
1c
1d
1e
4aA
4bA
4cA
4dA
4eA
81
80
55
53
73
2A
6
7
1a
1a
85
2B
2C
4aB
4aC
92[c]
8
1a
56[d]
9
2D
2E
1a
1a
4aD
4aE
80[e]
10
70
11
12
1a
1b
4aF
4bF
99
90
2F
13
14
2G
2H
1a
1a
4aG
4aH
86
67
The use of other ortho-directing groups was also success-
ful. The N,N-diisopropylcarboxamide 2F underwent reaction
with 1a and 1b to give 4aF and 4bF, respectively (entries 11
and 12, Table 1). The N,N-diethylcarbamate 2G also under-
went reaction with 1a, thus affording 4aG in 86% yield
(entry 13, Table 1). If the carbamate occupies the b position
of a naphthalene ring, we can expect a similar behavior to that
observed for 2E (entry 10, Table 1). In this sense, the estradiol
derivative 5 reacts with nBuLi and 1a to afford 6 in 89% yield
[a] All the reactions were performed with 1 (0.2 mmol) and 2 (0.4 mmol).
[b] Yield of isolated product after flash chromatography. [c] Reaction
carried out at 4.0 mmol scale. [d] A mixture 4:1 regioisomers was
obtained. [e] Full conversion requires 8 equivalents of anion, RT, and
TMEDA and hexanes were used to increase the reactivity of the
nucleophile. THF=tetrahydrofuran, TIPS=triisopropylsilyl, TMEDA =
N,N,N’,N’-tetramethylenediamine.
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with complete regioselectivity (Scheme 3). This transforma-
tion is particularly interesting given that alkyne moieties have
previously been used in steroid chemistry as delivery vectors
method would allow a very simple and direct introduction of
an alkyne in such positions. Thus, the a position of furan and
thiophene was alkynylated to afford 4aI and 4aJ, respectively,
in high yields (entries 1 and 2, Table 2) without giving any
double alkynylation, which had been observed using other
methods. N-methylpyrrole gave the expected product 4aK,
although an excess of the nucleophile (8 equiv) at room
temperature, and [12]crown-4 ether were required (entry 3,
Table 2). Interestingly, N-methylindole (2L) gave alkynyla-
tion at C2 (entry 4, Table 2),[15] whereas the TIPS derivative
2M afforded the C3-alkynylated product 4aM after removal
of the silyl protecting group (entry 5). These compounds had
been previously prepared using both catalytic[15a–c] and non-
catalytic methods[15d] that were generally long synthetic
sequences. Analogously, the mild reaction conditions used
for obtaining 4aN in 65% yield (À788C, 5 min; entry 6,
Table 2) by metalation of benzothiazole and then reaction
with 1a contrast with the harsh conditions (toluene, 1008C,
24 h) previously used for achieving the alkynylation of
benzothiazoles.[5d]
Scheme 3. Alkynylation of the estradiol derivative 5.
to enhance the activity of different biologically active
molecules.[13] For this purpose, alkynes have traditionally
been introduced at C17 through acetylide addition. The
reaction illustrated in Scheme 3 opens the possibility of an
entirely new site for this type of alkyne linker.
Next, we focused our attention on the alkynylation of
ferrocene 2H because this reaction could be useful in the
preparation of new materials.[14] To our knowledge, all the
methods available for introducing alkynyl groups at ferrocene
cores are indirect or give poor yields.[14b] Our method allows
the synthesis of 4aH (67%) by direct reaction of ferrocene
with tBuLi and 1a (entry 14, Table 1).
From the above results, it seemed obvious that alkynyla-
tion of organolithiums generated by conventional methods
involving lithium–halogen interchange, would also be possi-
ble with sulfonylacetylenes.[16] In this work, we have illus-
trated this reaction with five examples (Scheme 4). The first
Alkynylated heteroaromatic rings are important inter-
mediates in the preparation of bioactive molecules (Table 2).
À
Given that many Csp2 H bonds of aromatic heterocycles are
sufficiently activated to undergo lithiation with RLi, our
[a]
À
Table 2: Alkynylations of Ar H activated by tBuLi or nBuLi.
Entry
Substrate
Product
Yield [%][b]
1
2
3
2I (X=O)
2J (X=S)
2K (X=NMe)
4aI
4aJ
4aK
89
87
69[c]
Scheme 4. Alkynylation of pyridines and double bonds.
two involve the synthesis of 2-alkynyl pyridines[17] (lithiation
of pyridines cannot be achieved by ortho metallation).
Reaction of 2-bromopyridine (2O) with nBuLi and then
addition of either 1a or 1b under the standard reaction
conditions (in these cases, both Br/Li interchange and
reaction with the sulfonylethylene are performed at À788C)
afforded 4aO and 4bO, respectively, in very high yields
(Eq (1), Scheme 4). In a similar way, the alkynylation of
iodoalkenes would provide a simple and new entry to enynes,
which are commonly found in naturally occurring com-
pounds.[18] Thus, reaction of (E)-1-iodo-1-octene (2P) with
nBuLi and then addition of 1a and 1b allows the synthesis of
the enynes 4aP and 4bP, respectively, in almost quantitative
yield with retention of the E configuration at the double bond
(Eq.(2), Scheme 4).[19] Finally, a more elaborate method for
synthesizing enynes is shown in Equation(3), Scheme 4. It has
4
2L
4aL
80
5
6
2M
2N
4aM
4aN
32[d]
64
[a] All the reactions were performed with 1 (0.2 mmol) and 2 (0.4 mmol).
[b] Yield of isolated product after flash chromatography. [c] Full con-
version requires 8 equiv of anion, RT, and [12]crown-4 ether (1 equiv).
[d] TMEDA and hexanes as the solvent was used to increase the reactivity
of the nucleophile.
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been reported that ortho-alkynyl anisoles react with I2 to
afford 3-iodobenzofurans.[20] Under these conditions, 4aB
(entry 3, Table 1) was transformed into 7, which reacted
in situ with nBuLi and then with alkyne 1b, thus yielding 3-
alkynylbenzofurane 8 in 65% yield (two steps from 4aB). The
three-step sequence from 2B to deliver 8 illustrates an
efficient method for the synthesis of 3-alkynylbenzofuranes
that involves two alkynylation reactions.
To understand the unexpected electrophilic behavior of
substituted sulfonylacetylenes, we first performed theoretical
calculations using density functional theory (DFT; Gaus-
sian09[21] using the B3LYP functional[22]) on the relative
electrophilic character of the a and b carbon atoms of 1a (for
details see the Supporting Information). However, these
calculations revealed that the b-carbon atom is the most
electrophilic position, thus suggesting that any intermolecular
nucleophilic attack should take place through a more typical
Michael-type approach. Therefeore, we assume that the
formation of species A (Figure 1), with the lithium associated
The attack at the a position shows a smaller energy barrier,
and the DE with respect to the b attack is so large
(4.40 kcalmolÀ1) that it would explain the complete selectivity
observed in these reactions.
In summary, we have explored a new strategy for the
À
alkynylation of a wide variety of Csp2 H bonds activated by
lithium bases with 2-aryl- and 2-TIPS-substituted sulfonyl-
acetylenes, based on the unexpected anti-Michael addition.
The main advantage of this method with respect to those
À
usually employed for the construction of the Csp2 Csp bonds,
derives from the mild reaction conditions (À788C, 5 min) and
the absence of transition metals. In addition, the broad scope
of the reaction, the high yields, and the simple experimental
manipulation make this procedure an excellent alternative to
the formation of aryl and vinyl alkynes.[24]
Received: November 7, 2011
Revised: November 26, 2011
Published online: February 1, 2012
Keywords: alkynes · arenes · computational chemistry ·
.
organolithium compounds · synthetic methods
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À
alkyne moiety instead of formation of a Csp Csp2 bond.
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Figure 1. Energies of the transition states for the a and b attacks on
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[9] For Sonogashiraꢂs reaction, temperature ranges between RTand
1008C and reaction times from several hours to several days,
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considered is based on the evolution of the anion resulting in the
Michael addition into the carbene by elimination of the sulfonyl
group and then rearrangement of the Ar groups. It was also
rejected because the TS for this evolution is less stable than that
observed for anti-Michael addition (see Figure 1). Additionally,
we could not observe any difference in the reaction rate by
changing the nature of the migrating group (aryl, heteroaryl or
vinyl) which should be the case if the rearrangement (carbene
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[24] Although this reaction may not appear to be atom economical,
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