Ruthenium-catalyzed one-pot synthesis of (E)-(2-arylvinyl)boronates through an isomerization/cross-metathesis sequence from allyl-substituted aromatics

Article abstract of DOI:10.1002/ejoc.201402192

We described the efficient preparation of (E)-(2-arylvinyl)boronates from allylbenzene derivatives on the basis of an isomerization/cross-metathesis sequence catalyzed by a modified Hoveyda-Grubbs catalyst. The implementation of the experimental procedure was simple and compatible with a large variety of substrates. This methodology provides a new chemical transformation not described to date. Allyl-substituted aromatics can thus be converted into diversely functionalized compounds, such as (E)-stilbene derivatives or (E)-vinyl azides, in only two steps. Copyright

Full text of DOI:10.1002/ejoc.201402192

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201402192
Ruthenium-Catalyzed One-Pot Synthesis of (E)-(2-Arylvinyl)boronates through
an Isomerization/Cross-Metathesis Sequence from Allyl-Substituted Aromatics
Rémy Hemelaere,[a] Frédéric Caijo,[b] Marc Mauduit,[c] François Carreaux,*[a] and
Bertrand Carboni[a]
Keywords: Homogeneous catalysis / Domino reactions / Metathesis / Isomerization / Boron
We described the efficient preparation of (E)-(2-arylvinyl)-
boronates from allylbenzene derivatives on the basis of an
a large variety of substrates. This methodology provides a
new chemical transformation not described to date. Allyl-
substituted aromatics can thus be converted into diversely
functionalized compounds, such as (E)-stilbene derivatives or
(E)-vinyl azides, in only two steps.
isomerization/cross-metathesis sequence catalyzed by
a
modified Hoveyda–Grubbs catalyst. The implementation of
the experimental procedure was simple and compatible with
ation of the terminal double bond.[11] Borylation of styryl
bromide with a less expensive and more atom economical
boron source, pinacol borane (HBpin), was developed by
using a PdCl2(CH3CN)2/SPhos system [SPhos = 2-(dicyclo-
hexylphosphino)-2Ј,6Ј-dimethoxybiphenyl].[12]
Introduction
The synthetic versatility of alkenylboronates has made
them key intermediates in organic synthesis.[1] Besides their
aptitude to react efficiently in transition-metal-catalyzed
cross-coupling reactions for C–C bond formation,[2] it has
been shown that alkenylboronates can also be coupled
stereospecifically to organic heteronucleophiles in the pres-
ence of copper catalysts.[3] In addition to their wide applica-
tions in organic chemistry as anionic or cationic building
blocks, (E)-styrylboronates 4 constitute an interesting fam-
ily of bioactive compounds, in particular because of their
neuroprotective activities.[4]
This class of compounds can be synthesized from the
corresponding terminal alkynes by uncatalyzed[5] or metal-
catalyzed[6] hydroboration through a syn-addition approach
of the boron reagents (Scheme 1).[7] The single boronation
reaction of alkynes with bis(pinacolato)diboron (B2pin2)
was also performed by using copper[8] or palladium[9] spe-
cies as catalysts. Other alternative procedures have been de-
veloped from terminal alkenes, first by the rhodium-cata-
lyzed dehydrogenative borylation of vinylarenes with diox-
aborolane hydrides.[10] Thereafter, different reports have de-
alt with the use of B2pin2 and transition-metal catalysts to
overcome the unwanted hydrogenation and/or hydrobor-
Scheme 1. Access routes to (E)-(2-arylvinyl)boronates.
Several other catalytic methods employing pinacol or 2-
methyl-2,4-pentanediol ethylene boronic esters have been
also reported.[13]Among these, cross-metathesis (CM) is
probably one of the most elegant strategies, as it allows the
production of alkenylboronates starting from styrenes with
a high selectivity in favor of the (E) isomers in the case of
ruthenium catalysts.[14,15]
Using this approach, we recently developed a route to
new alkenylboronates from allyl-substituted aromatics.[16]
During this work, we observed that modified Hoveyda–
Grubbs catalyst M71-SIPr[17] induced the formation of by-
product 4, which may have resulted from the isomerization
of starting material 1 followed by cross-metathesis with vin-
ylboronate pinacol ester 2 (Scheme 1). This result likely im-
plies the in situ formation of a ruthenium hydride species by
partial decomposition of the catalyst[18] given the absence of
an additive in the implementation.[19] To the best of our
knowledge,[20] only one example of such a domino isomer-
[a] Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-
Université de Rennes 1,
Campus de Beaulieu, 35042 Rennes Cedex, France
E-mail: francois.carreaux@univ-rennes1
http://www.scienceschimiques.univ-rennes1.fr/en/home/teams/
icmv/
[b] Ωcat SYSTEM®, Ecole Nationale Supérieure de Chimie de
Rennes,
11 allée de Beaulieu, CS50837, 35708 Rennes Cedex, France
[c] Ecole Nationale Supérieure de Chimie de Rennes,
11 allée de Beaulieu, CS50837, 35708 Rennes Cedex, France
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201402192.
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Eur. J. Org. Chem. 2014, 3328–3333

Ruthenium-Catalyzed Synthesis of (E)-(2-Arylvinyl)boronates
Figure 1. Ruthenium catalysts used in this study; Cy = cyclohexyl.
ization/cross-metathesis by using Hoveyda–Grubbs catalyst at reflux was necessary to produce the (E) isomer of 4a
HG-II with allyltrimethylsilane as the cross partner has stereoselectively in 62% yield after purification.[27,28] From
been reported.[21] The isomerization reaction can be fol- these results, it appears that the cross-metathesis was the
lowed by ring-closing metathesis (RCM), which adds an ad- slow kinetic step in the sequential reaction. Under these
ditional quantity of catalyst (tandem reaction) as described experimental conditions, the unusual ambivalent behavior
by Arisawa and van Otterlo for the synthesis of hetero- of the M71-SIPr catalyst is highlighted if we compare with
cycles.[22,23] Given that the stereoselective chemical transfor- more conventional ruthenium olefin metathesis catalysts
mation of allyl aromatics into (E)-(2-arylvinyl)boronates is such as G-II and HG-II (Table 1, entries 4 and 5). With
novel and could be useful in organic synthesis, we searched these two catalysts, there was no formation of desired prod-
to optimize and generalize this catalytic process. In this pa- uct 4a, although the isomerization reaction took place. In
per, we report our results with this goal in mind. A struc- the case of HG-II, only isomerized product 3 was ob-
ture–activity relationship study was realized with different served.[29]
ruthenium catalysts to identify the influence of the frag-
To investigate if the amino carbonyl group present in
ments on the product distribution (Figure 1). We also show M71-SIPr was essential for its catalytic activity, we tried an-
that allyl aromatics can be converted easily into diversely other Hoveyda–Grubbs-type complex, that is, HG-SIPr
functionalized compounds.
(Table 1, entry 6). A slight drop in the conversion to 4a was
observed, which indicated that this function was not crucial
for the efficiency of the catalyst, unlike the bulkiness of the
N-heterocyclic carbene ligand. This was confirmed by the
Results and Discussion
The effects of varying the catalyst on the isomerization/ use of the M71-SIMes catalyst, which showed a significantly
cross-metathesis sequence were explored by using 4-allyl- lower efficiency for the production of 4a compared to M71-
1,2-dimethoxybenzene (1a) as the model substrate (Table 1). SIPr (Table 1, entry 7). A possible explanation could be the
This compound was chosen for the optimization of the se- increased thermal stability of the catalyst, thanks to the
quential reaction owing to its high propensity to undergo bulky SIPr ligand.[30] By heating at reflux, the supposed for-
double-bond isomerization in the presence of ruthenium– mation of a ruthenium hydride species is thus limited while
carbene catalysts.[24] The reported results were obtained by safeguarding a certain quantity of the catalyst for an ef-
using pinacol vinylboronate[25] (2, 2 equiv.) in toluene, and ficient cross-metathesis reaction.[31] Other catalysts, such as
starting material 1a was completely consumed in all studied M832[17b] and M23, were investigated with less success
catalytic processes. The increase in temperature in the pres- (Table 1, entries 8 and 9). In particular, the replacement of
ence of M71-SIPr was a determinant factor that favored the the benzylidene ligand by an indenylidene ligand (see M23)
formation of the cross-metathesis product resulting from had a detrimental effect on the “one-pot” sequential reac-
the isomerization reaction (Table 1, entries 1–3).[26] Heating tion, and a mixture of 3 and 6 was produced, the latter of
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R. Hemelaere, F. Caijo, M. Mauduit, F. Carreaux, B. Carboni
SHORT COMMUNICATION
Table 1. Optimization of the sequential process with 1a by using single (E) stereoisomer with modest to good yields ranging
different ruthenium catalysts.[a]
from 47 to 80%. The presence of different electron-rich and
electron-deficient substituents on the aromatic core had no
apparent effect on the successful progress of this sequential
reaction, and 4d was obtained with a nonprotected phenolic
group (Scheme 2).
The usefulness of this chemical transformation was em-
phasized by proceeding to replace the boron substituent by
various functional groups. (E)-Vinyl iodides 7g and 7i were
prepared by a two-step synthesis from the corresponding
allyl aromatics (Scheme 3). After evaporation of toluene,
the crude mixture of the one-pot isomerization/cross-me-
tathesis reaction was treated directly with an iodine solution
Entry Catalyst
T [°C]
Ratio 3/4a/5/6[c] Yield of 4a [%][d]
under basic conditions (I2, NaOH)[32] to afford the desired
compounds in 60 and 50% yield, respectively, over two
steps. The boron substituent can be also replaced by an
azide group under mild conditions as reported in the litera-
ture.[33] Using this catalytic procedure (NaN3, CuSO4), (E)-
alkenyl azides 8f and 8h were obtained in good yields from
the corresponding allyl aromatics.
1
2
3
4
5
6
7
8
9
M71-SIPr
M71-SIPr
M71-SIPr
G-II
50[b]
1:2.5:0:0
1:3.4:0:0
1:5.9:0:0
1:0:0:1.7
1:0:0:0
60
72
70[b]
reflux
reflux
reflux
reflux
reflux
reflux
reflux
84 (62)[e]
n.d.
HG-II
n.d.
HG-SIPr
M71-SIMes
M832
1:4:0:0
78
1:1.7:0:0
1.7:1:0:0
1.4:0:0:1
n.d.
n.d.
n.d.
As another illustration of the synthetic interest of this
methodology, we next turned our attention towards the
transformation of allyl aromatics into stilbene derivatives 9
by including a Suzuki cross-coupling reaction in the synthe-
sis (Scheme 4). For instance, we planned the synthesis of
3,5,3Ј,4Ј,5Ј-pentamethoxystilbene (9a), a methoxylated ana-
log of resveratrol, which has been described to exert more
potent inhibition of cell growth than resveratrol and other
methoxylated derivatives in the human breast carcinoma
M23
[a] 0.2 m in toluene. [b] The catalyst in solution was heated for
5 min at reflux before the addition of 1a and 2 at the indicated
temperature. [c] Determined by H NMR spectroscopy by analysis
of the product mixtures prior to purification. Only the (E) isomer
of 4a was observed. [d] Determined with 3-fluoro-4-nitrotoluene as
an internal standard; n.d.: not determined. [e] Yield of the product
after isolation by flash chromatography on silica gel.
1
which resulted from the cross-metathesis of starting mate- cell line MCF-7.[34] Compound 9a was thus prepared
rial 1a. stereoselectively in 40% yield overall from 1a. Alkenyl-
With these results in hand, we then envisaged the exten- boronates 4 can also serve as building blocks for a one-step
sion of this optimized catalytic process to various allyl aro- multicomponent process such as the Petasis reaction.[35] By
matics. In all cases, alkenylboronates 4 were obtained as a using salicylaldehyde and morpholine, allylamines 10b and
Scheme 2. Extending the sequential reaction to other allyl-substituted aromatics.
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Ruthenium-Catalyzed Synthesis of (E)-(2-Arylvinyl)boronates
Scheme 3. Synthetic transformations including the replacement of the boron substituent by functional groups.
Scheme 4. Synthetic transformations including a carbon–carbon bond-formation reaction.
10e were obtained in acceptable yields (60 and 58%, respec-
tively) taking into account that the trivalent boron nucleo-
Experimental Section
General Procedure for the Isomerization/Cross-Metathesis Sequen-
phile is a pinacol boronate ester instead of a boronic
tial Reaction: 4,4,5,5-Tetramethyl-2-vinyl-1,3,2-dioxaborolane (2,
0.60 mmol) was added to a solution of 1 (0.30 mmol) in anhydrous
toluene (0.2 m) under an inert atmosphere followed by M71-SIPr
(3 mol-%). The resulting mixture was heated at reflux for 1 h in a
preheated oil bath. Thereafter, toluene was removed under reduced
pressure, and the crude product was purified by column
chromatography by using the appropriate eluent to afford desired
compound 4 as a single (E) isomer.
acid.[36]
Conclusions
We described in this paper the stereoselective synthesis of
(E)-(2-arylvinyl)boronates 4 through a ruthenium-catalyzed
isomerization/cross-metathesis sequence from allyl-substi-
tuted aromatics. M71-SIPr is the more efficient catalyst for
this process, probably owing to its thermal stability. We
showed that this simple procedure is reliable with differently
substituted substrates. This new chemical transformation
could open attractive opportunities in organic synthesis in
view of the fact that allylbenzene derivatives can be con-
verted into diversely functionalized compounds in only two
steps. Work is underway in our laboratory with the idea to
transpose this strategy to other cross-metathesis partners.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details and copies of the NMR spectra for key
intermediates 4a–j and final products 7–10.
Acknowledgments
This work was supported by the University of Rennes 1 and the
Centre National de la Recherche Scientifique (CNRS). R. H.
thanks the Région Bretagne for a research fellowship.
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SHORT COMMUNICATION
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[11]
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The use of a preheated oil bath instead of gradual warming to
reflux was crucial for good reproducibility.
[12]
[13]
[27]
[28]
[29]
The loss of product during chromatography on silica was no-
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[18]
For the pioneering contributions of Grubbs that showed that
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Published Online: April 15, 2014
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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