Heck-Matsuda arylation of olefins through a bicatalytic approach: Improved procedures and rationalization

Article abstract of DOI:10.1002/adsc.201301144

The scope of the palladium-catalyzed Heck-Matsuda reaction, proceeding through the catalytic activation of anilines into the corresponding diazonium salts, has been considerably extended and is now working with deactivated electrophiles. Two different procedures, using catalytic amounts of both palladium and acid, have been optimized allowing the concept of bicatalysis to cover the complete electronic range of anilines. These environmentally friendly procedures proceed under very mild conditions, at room temperature in methanol, and only generate tert-butyl alcohol, water and nitrogen as by-products. Rationalization of reaction outcomes encountered in this work has been discussed with the support of computational studies.

Full text of DOI:10.1002/adsc.201301144

UPDATES
DOI: 10.1002/adsc.201301144
Heck–Matsuda Arylation of Olefins Through a Bicatalytic
Approach: Improved Procedures and Rationalization
Nicolas Oger,a FranÅois Le Callonnec,a Denis Jacquemin,a,c,* Eric Fouquet,b
Erwan Le Grognec,a and FranÅois-Xavier Felpina,c,*
a
Universitꢀ de Nantes; UFR des Sciences et des Techniques, CNRS UMR 6230, CEISAM, 2 rue de la Houssiniꢁre, 44322
Nantes Cedex 3, France
Fax : (+33)-251-125-402; phone: (+33)-251-125-422; e-mail: fx.felpin@univ-nantes.fr
Universitꢀ de Bordeaux, UMR CNRS 5255, ISM, 351 cours de la Libꢀration, 33405 Talence Cedex, France
Institut Universitaire de France, 103 blvd St Michel, 75005 Paris Cedex 5, France
b
c
Fax : (+33)-251-125567; phone: (+33)-251-125-564; e-mail: Denis.Jacquemin@univ-nantes.fr
Received: December 18, 2013; Published online: March 12, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201301144.
Abstract: The scope of the palladium-catalyzed
Heck–Matsuda reaction, proceeding through the
catalytic activation of anilines into the correspond-
ing diazonium salts, has been considerably extended
and is now working with deactivated electrophiles.
Two different procedures, using catalytic amounts
of both palladium and acid, have been optimized al-
lowing the concept of bicatalysis to cover the com-
plete electronic range of anilines. These environ-
mentally friendly procedures proceed under very
mild conditions, at room temperature in methanol,
and only generate tert-butyl alcohol, water and ni-
trogen as by-products. Rationalization of reaction
outcomes encountered in this work has been dis-
cussed with the support of computational studies.
An industrial application has also been developed
at Syngenta for the synthesis of the herbicide Prosul-
furonꢂ.[6]
Thereby, aryldiazonium salts, easily prepared from
inexpensive anilines, compete favourably with less-re-
active and more-expensive aryl halides but remain
much less employed. Although their reputation as
hazardous compounds can partly explain their limited
use, recent studies have demonstrated that several
crystalline salts can be stable up to 6008C[7] and can
be handled under safe conditions even on a multi-
kilogram scale.[8] However, to avoid any unpredictable
safety issues, in-situ preparations have been occasion-
ally proposed[9,10] and are privileged for scale-up prep-
aration.[11] The use of in-situ prepared diazonium salts
also avoids the handling of extensive quantities of sol-
vents required for the purification of the crystalline
form. Another issue with diazonium salts, which is
also encountered with traditional aryl halides, is the
side-formation of a stoichiometric amount of a mostly
inorganic salt originating from the counterion. Whilst
this issue is rather harmless on a laboratory scale, the
Keywords: anilines; arenediazonium salts; Heck–
Matsuda reaction; palladium; sustainable chemistry
The use of aryldiazonium salts as aryl halide surro- neutralization and the treatment of aqueous phases
gates for palladium-catalyzed reactions has recently might become compulsory for large-scale applications.
attracted a growing interest from the synthetic com-
In a preliminary communication, we proposed an
munity, especially for Heck- and Suzuki-type transfor- unprecedented approach for the coupling of aryldia-
+
mations.[1] The weak C N2 bond dissociation energy zonium salts with acrylates that addressed most of the
À
(~130 kJ·molÀ1) allows oxidative palladium addition issues listed above. Indeed, we developed the first
under very mild conditions, without requiring addi- Heck–Matsuda reaction using a catalytically in-situ
tional ligand or, in some cases, base. For instance, the
coupling of aryldiazonium salts with olefins,[2] the so-
called Heck–Matsuada reaction, has found important
developments in the synthesis of materials,[3] hetero-
cycles[4] and natural products (Scheme 1).[5]
Scheme 1. General representation of the Heck–Matsuda re-
action.
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led to disappointing results [Scheme 3, Eq. (1)]. By
contrast, when an electron-withdrawing group was
added (aniline 1b) or when the bromine atom was
moved to the 2-position (aniline 1c) the correspond-
ing coupling products 3b and 3c were isolated in good
yields.
We recently rationalized this behaviour by an inter-
nal stabilizing coordination of cationic palladium spe-
cies with the nitro group, suppressing the formation
of unwanted side-products.[14] Clearly, the success of
this approach depends on a subtle mix of activation
with electron-withdrawing groups [Eq. (1) vs. Eq. (2)]
and/or stabilization with ortho-coordinating groups
[Eq. (1) vs. Eq. (3)].
In this Update, we report our latest results that ulti-
mately led to the development of two different proce-
dures working with either electron-poor or electron-
rich anilines without the intervention of any base or
ligand. Moreover, a rationalization of the results pre-
sented in this contribution has been performed with
quantum chemical approaches.
In order to grasp the unwanted behaviour when 4-
bromoaniline 1a was used as starting material (see
Scheme 3), we carefully monitored the evolution of
Scheme 2. Strategy for the Heck–Matsuda reaction through
a double catalytic cycle.
1
the reaction by H NMR. Instead of the expected sty-
rene 3a, we observed the stealthy formation of the tri-
generated diazonium salt through the double catalytic azene 5, which rapidly degraded into a collection of
cycle depicted in Scheme 2.[12] This approach features unidentified side-products in a few hours. The forma-
a number of key advances: (i) the procedure does not tion of the triazene 5 started immediately after the
require the isolation of potentially hazardous crystal- acid addition and was quantitative after a few minutes
line salts; (ii) the diazonium salt is in-situ catalytically (Scheme 4). Interestingly, with ortho-substituted and/
generated through the reaction advancement; (iii) or electron-poor anilines the formation of the corre-
only benign by-products such as N2, t-BuOH and H2O sponding triazene was not observed, highlighting the
are generated; and (iv) the reaction is carried out influence of the nucleophilic contribution of the ani-
under mild and ligand-free conditions.
line vs. electrophilic contribution of the diazonium
Whilst we initially worked with electron-poor ani- salt (vide infra).
lines, especially those bearing a nitro group since this
Unfortunately, the sensitivity of 5 toward acidic
methodology was envisaged as an extension of the conditions excludes any reversible process[15] and
Heck–reduction–cyclization (HRC) strategy recently thereby stops the catalytic cycle. Actually, the triazene
developed in our laboratory for the synthesis of het- formation is the result of the coexistence in the reac-
erocycles,[13] we later observed that 4-bromoaniline 1a tor of the diazonium salt 4 in catalytic amount
Scheme 3. Influence of the ortho-coordinating group on the reaction yield.
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Heck–Matsuda Arylation of Olefins
gave inconsistent results (entry 2), even in the pres-
ence of anisole[14] as stabilizing agent (entry 3). All at-
tempts to reduce the aniline addition time (entry 4) as
well as the stirring time after addition (entry 5) signif-
icantly degraded the reaction yields. The use of an
excess of acid does not significantly improve the reac-
tion yield, highlighting both the efficiency and the ro-
bustness of our newly designed process (entry 6).
Moreover from a safety point of view, we can argue
that the reaction can be stopped at any time by turn-
ing off the syringe pump.
Scheme 4. Mechanism for formation of the triazene 5.
Having elaborated a novel protocol that avoided
(ꢀ20 mol%) with the unreacted free aniline. Indeed, the formation of the unwanted triazene, we further
N
in the absence of acid, t-BuONO is essentially unreac- explored its scope with a collection of anilines. The
tive with anilines in methanolic solutions as we deter- “syringe pump” approach was systematically com-
mined by 1H NMR spectroscopy. Although at this pared to the procedure described previously with
stage, the factors governing the triazene formation electron-poor anilines (Table 2).[12] As expected, a sim-
versus the Pd oxidative addition remained unclear, we ilar reaction pattern was obtained with a variety of
reasoned that a process where the diazonium salt and neutral and electron-rich anilines for which no cou-
the aniline do not coexist together in solution would pling product was observed when the syringe pump
circumvent the triazene formation. Accordingly, we was omitted (entries 1–6). However, the reaction
designed a novel procedure by which the aniline is yields were spectacularly improved for anilines bear-
slowly added via a syringe pump and immediately ing halogen (compound 3a), alkyl groups (compounds
transformed into the corresponding diazonium salt 3d and 3e) and methoxy (compounds 3f and 3g) with
(Table 1). After extensive optimizations, we deter- our newly designed procedure. Even the sterically
mined that 0.3 equiv. of MeSO3H was required to- hindered 2,4,5-trimethylaniline 3e and the very elec-
gether with a slow addition of the aniline over 24 h tron-rich 3,4,5-trimethoxyaniline 3g were successfully
(entry 1). Decreasing the amount of acid to 0.2 equiv, reacted with methyl acrylate. For the record, the mild
conditions developed for the preparation of 3f con-
trast with literature precedents reporting, under ho-
mogeneous catalysis, the use of high pressure
Table 1. Optimization studies.
(4000 bar, 908C)[16] or extended refluxing times (120–
1608C, 12 h).[17] We also found that meta-substituted
anilines were poorly reactive without the syringe
pump, while this work allowed synthetically useful
yields of the expected styrene (compounds 3h and 3i).
The low yields obtained with method A for com-
pounds 3h and 3i were attributed to the formation of
the corresponding triazene as observed in the crude
1H NMR. As already noted during the preliminary
optimization studies, the use of anisole as additive did
not affect the reaction outcome. This result suggests
that palladium species bearing electron-rich aryls are
sufficiently stable in MeOH and are not prone to al-
ternate pathways. For comparison, we also focused on
electron-deficient anilines, efficiently coupled with
methyl acrylate by using our “old” procedure (en-
tries 9–14). Unexpectedly, modest yields were ob-
tained with the “syringe-pump” procedure. This result
suggests that the low concentration of the diazonium
salt induced by the slow addition of aniline over 24 h,
dramatically slows down the reaction rate. With elec-
tron-deficient substituents, aryldiazonium salts and ar-
ylpalladium species are much more reactive and
highly prone to decomposition, especially in MeOH
due to redox pathways. With this set of results, a reac-
tivity-procedure relationship can be established. With
Entry Variation from the “standard” condi-
tions
Yield
[%][a]
1[b]
2
none
0.2 equiv. MeSO3H
80
39–77[c]
41
3
4
5
6
0.2 equiv. MeSO3H, anisole as additive
addition over 12 h instead of 24 h
stirring time after addition 12 h
excess MeSO3H, no syringe pump
59
75
82
[a]
[b]
Yields of isolated products.
Reaction conditions: aniline (0.2 mmol), t-BuONO
(1.25 mmol) and MeSO3H (0.3 mmol) in MeOH (8 mL)
were stirred for 10 min at 258C. Then, a solution of
methyl acrylate (2.2 mmol) and PdACTHNUTRGNEUNG(OAc)2 (2.2 mol%) in
MeOH (2 mL) were added to the reaction mixture fol-
lowed by the remaining aniline (0.8 mmol) in MeOH
(10 mL) via syringe pump over 24 h. After addition, the
mixture was stirred for an additional 24 h.
[c]
Irreproducible results.
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Table 2. Procedure-efficiency relationship.
[a]
Isolated yield.
[b]
Reaction conditions: aniline (2 mmol), t-BuONO (2.5 mmol) in MeOH (10 mL) were stirred for 30 min at 08C.
Then, methyl acrylate (4.4 mmol) in MeOH (8 mL), Pd(OAc)2 (2.2 mol%), and MeSO3H (0.4 mmol) in MeOH
AHCTUNGTRENNUNG
(4 mL) and, eventually, anisole (1 mmol) were added dropwise at 08C. The reaction mixture was allowed to
reach 258C and stirred for 48 h.
Reaction conditions: aniline (0.2 mmol), t-BuONO (1.25 mmol) and MeSO3H (0.3 mmol) in MeOH (8 mL) were
[c]
stirred for 10 min at 258C. Then, a solution of methyl acrylate (2.2 mmol) and PdACTHNUTRGNEUNG(OAc)2 (2.2 mol%) in MeOH
(2 mL) were added to the reaction mixture followed by the remaining aniline (0.8 mmol) in MeOH (10 mL) via
syringe pump over 24 h. After addition, the mixture was stirred for an additional 24 h.
[d]
[e]
Pd
ACHTUNGTRENNUNG(OAc)2 (4.4 mol%) was used.
Anisole (0.5 mmol) was used as additive.
strongly deactivated anilines bearing electron-rich wanted redox processes. The only exception to this
substituents, the formation of triazene was systemati- rule was the 4-cyanoaniline 1n which was much more
cally detected without the syringe pump and the ex- efficiently coupled to methyl acrylate with the syringe
pected coupling product was obtained, at best, in low pump procedure. This result can be rationalized con-
yields. In order to avoid the triazene formation, the sidering the high coordination properties of the cyano
slow addition of aniline through the syringe pump is group. With concentrated solution of aniline 1n, the
mandatory. On the other hand, with electron-poor coordination sphere of palladium is likely saturated,
anilines, the high reactivity of the corresponding di- dropping down its catalytic activity. On the other
azonium salts favoured the Pd-catalyzed pathway and hand, with a slow addition, the solution is no longer
the triazene formation was not detected. In this case, saturated with aniline 1n, and palladium species keep
concentrated solutions are preferred to avoid any un- their high reactivity. Whatever the procedure used,
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Heck–Matsuda Arylation of Olefins
ing support to develop an enantioselective version of
this double catalytic arylation.
As depicted in Scheme 7, our procedure also
worked for the arylation of acrylonitrile 10 with 4-me-
thoxyaniline 1f, giving the corresponding olefin 11 as
a mixture of E/Z isomers (E/Z=78/22).
In order to understand the factors governing the tri-
azene formation, we have performed DFT calcula-
Scheme 5. Preparation of carbocycle 8.
tions using the SMD-M06-2X/6-311+ +GACTHNURGTNE(UNG d,p) ap-
proach (see the Supporting Information for details).
We have first considered the reaction between 1a and
4 to yield 5 (Scheme 4) and determined the related
energetic profile (Figure 1). For the attack of the lone
pairs of the amino group of 1a on the diazonium
moiety of 4, we found a clear transition state to form
the protonated triazene (see the Supporting Informa-
tion for a movie clip of the representative imaginary-
frequency mode). The barrier amounts to 17.4 kcal·-
molÀ1 in vacuum and 21.6 kcal·molÀ1 in methanol, this
compound can of course exist under different confor-
mations of extremely close energy (the energy differ-
ences within the errors bars of the calculations). For
the deprotonation step, we have considered a reaction
of the protonated triazene with 1a acting as a base.
Despite our efforts, we could not locate a transition
Scheme 6. Arylation of cyclic olefin 8.
Scheme 7. Arylation of acrylonitrile 10.
we observed a perfect stereocontrol towards the ex- state in methanol (see the Supporting Information for
clusive formation of (E)-olefins.
a representation of the vacuum transition state),[20] in-
With the aim of developing an asymmetric version dicating a barrierless process to form the triazene and
of our double catalytic concept, we also preliminary the ammonium. The calculations therefore indicate,
explored the reactivity of cyclic olefin 8 under race- beyond reasonable doubt, that the rate-limiting step is
mic conditions.[18] The carbocycle 8 was prepared on the attack of the amine on the diazonium. Starting
a multi-gram scale by LiH-mediated alkylation of with the corresponding nitro derivative, 1j, the pro-
methyl malonate 7 with (Z)-1,4-dichlorobut-2-ene 6 in tonated triazene generated by this reaction could not
DMF (Scheme 5).[19]
be obtained, despite several attempts. Indeed, only an
We observed that our newly designed procedure unreactive p-stacked structure was reached irrespec-
allows the coupling of the cyclic olefin 8 with both tive of the selected protocol or starting geometry
electron-poor and electron-rich anilines in useful (Figure 2). These conclusions are clearly in the line of
yields (Scheme 6). These results provide an encourag-
Figure 1. Computed free energy profile (in methanol) for the formation of the triazene of the bromo derivative.
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Nicolas Oger et al.
tron-withdrawing groups. Both environmentally
friendly procedures feature very mild conditions, al-
lowing reactions at room temperature without any
ligand, and only generate t-BuOH, H2O and N2 as by-
products. We rationalized the reaction outcomes with
the support of computational studies, confirming the
experimental observations. We believe that these
robust procedures are now mature for a wide use in
synthetic chemistry
Figure 2. Unreactive p-stacked structures bearing a p-NO2
substitutent.
Experimental Section
the experimental findings that show triazene forma-
tion for 1a but not for 1j.
General Information
We reasoned that the triazene formation could be
directly related to the nucleophilicity of the anilines
toward the corresponding diazonium salts. To classify
the relative nucleophile strengths of the amines, we
computed, with conceptual DFT, the chemical poten-
tial (m) and electrophilicity (w) of a panel of six para-
substituted anilines using the same SMD-M06-2X/6-
All commercial reagents were used as received. Silica gel
(40–63 mm) was used in flash column chromatography. Ana-
lytical thin-layer chromatography (TLC) was performed on
silica gel plates (TLC silica gel 60 F254), visualized with
a Spectroline UV254 lamp, and stained with a basic solution
of KMnO4. Solvent systems and flash column chromatogra-
phy are reported as percent by volume values. 1H and
13C NMR were recorded at 250 MHz or 300 MHz and 62.5
MHz or 75 MHz, respectively. Proton chemical shifts were
internally referenced to the residual proton resonance in
CDCl3 (d=7.26 ppm). Carbon chemical shifts were internal-
ly referenced to the deuterated solvent signals in CDCl3
(d=77.0 ppm). FT-IR spectra were recorded with samples
loaded as KBr plates.
311+ +GACHTUNGTRENNUNG(d,p) level of theory. In the conceptual DFT
framework, it is usual to consider that the most nucle-
ophilic molecule presents the smallest chemical po-
tential (m) and electrophilicity (w) indices.[21] Results
are collected in Table 3. As can be seen, with the
most nucleophilic anilines (p-MeO, p-t-Bu, p-Br) we
always obtained the formation of the corresponding
triazene, while for the less nucleophilic ones (p-CN,
p-CO2Me, p-NO2), this outcome was never observed.
In conceptual DFT, the obtained parameters account
for all (within the selected level of theory) electronic
effects, but no steric effect is considered as it origi-
nates from the interaction of two molecules. Howev-
er, from an experimental point of view, the successful
coupling of 2-bromoaniline 1c with both procedures
suggests an important contribution of the steric hin-
drance in the formation of the triazene.
In summary, we have described in this full account
an expanded scope of the Heck–Matsuda reaction
using a substoichiometric amount of diazonium salt
through the intervention of a bicatalytic process. Our
studies ultimately led to the development of two dif-
ferent procedures allowing the use of variously deco-
rated anilines with either electron-donating or elec-
General Procedure
To a solution of aniline (0.2 mmol) in MeOH (8 mL) was
added t-BuONO (1.1 mmol) and methanesulfonic acid
(30 mol%) at 258C. The resulting mixture was stirred for
10 min at 258C. Methyl acrylate (2.2 mmol) and palladium
acetate (2.2 mol%) in MeOH (2 mL) were added to the so-
lution. A solution of aniline (0.8 mmol) in MeOH (10 mL)
was added to the mixture via a syringe pump (RazelScientif-
ic R99-E, index 01, flow rate 0.397 mL·hÀ1) for 24 h at 258C.
After addition, the solution was stirred for 24 h at 258C.
Then the solvent was removed and the crude product was
purified by flash chromatography to give the corresponding
cross-coupled product.
Acknowledgements
F.-X.F gratefully acknowledges the University of Nantes, the
Centre National de la Recherche Scientifique (CNRS), the
Rꢀgion Pays de la Loire in the framework of a recruitment
sur poste stratꢀgique, and the Agence Nationale de la Recher-
che (ANR JCJC 7141) for the financial support of this proj-
ect. D.J. acknowledges the European Research Council and
the Rꢀgion des Pays de la Loire for financial support in the
framework of a Starting Grant (Marches – 278845) and a re-
crutement sur poste stratꢀgique, respectively. This research
used resources of 1) the GENCI-CINES/IDRIS, 2) the
CCIPL (Centre de Calcul Intensif des Pays de Loire) and 3)
a local Troy cluster. F.-X.F. and D.J. are members of the Insti-
tut Universitaire de France (IUF).
Table 3. Computed chemical potential (m) and electrophilici-
ty (w) indices with conceptual DFT.
Amine subtitution
m
w
Triazene occurrence
p-NO2
p-CO2Me
p-CN
p-Br
p-MeO
p-t-Bu
À4.61 3.35
À3.78 1.59
À3.70 1.44
À3.43 1.17
À3.16 1.06
À3.18 1.01
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