Hydrogen Peroxide Promoted Mizoroki-Heck Reactions of Phenyldiazenes with Acrylates, Acrylamides, and Styrenes

Article abstract of DOI:10.1021/acs.orglett.6b00449

Mizoroki-Heck reactions, which are well-known for aryldiazonium salts and which have recently been described for arylhydrazines, have now been extended to phenyldiazenes. In situ generation of phenyldiazenes from azocarboxylates allowed clean and selective reactions with styrenes, acrylates, and acrylamides using palladium(II) acetate in the presence of silver(I) acetate or hydrogen peroxide as oxidant. Hydrogen peroxide was thereby shown to be a cheap and broadly applicable alternative for the established palladium-silver(I) system.

Full text of DOI:10.1021/acs.orglett.6b00449

Letter
pubs.acs.org/OrgLett
Hydrogen Peroxide Promoted Mizoroki−Heck Reactions of
Phenyldiazenes with Acrylates, Acrylamides, and Styrenes
Roman Lasch, Stefanie K. Fehler, and Markus R. Heinrich*
Department of Chemistry and Pharmacy, Pharmaceutical Chemistry, Friedrich-Alexander-Universitat Erlangen-Nurnberg,
̈
̈
Schuhstraße 19, 91052 Erlangen, Germany
S
* Supporting Information
ABSTRACT: Mizoroki−Heck reactions, which are well-known for aryldiazonium salts and which have recently been described
for arylhydrazines, have now been extended to phenyldiazenes. In situ generation of phenyldiazenes from azocarboxylates
allowed clean and selective reactions with styrenes, acrylates, and acrylamides using palladium(II) acetate in the presence of
silver(I) acetate or hydrogen peroxide as oxidant. Hydrogen peroxide was thereby shown to be a cheap and broadly applicable
alternative for the established palladium−silver(I) system.
izoroki−Heck1 coupling reactions of aryl residues with
piece between aryldiazonium ions and hydrazines in terms of
oxidation state, have not yet been described as precursors or
intermediates in Heck reactions. Besides this formal gap to be
closed, our interest in the role of phenyldiazenes in Heck-type
reactions was due to successful trapping experiments of these
often short-lived intermediates12 and attractive future applica-
tions in 18F-radiosynthesis, which would be enabled by the high-
yielding access to 18F-fluorophenylazocarboxylates.13 Several
series of preliminary experiments were carried out with
phenylazocarboxylic acid tert-butyl esters11,12 and methyl
acrylate or styrene (see Tables S1−S8, Supporting Information).
All variations, including solvents, bases, ligands, palladium
sources, and various additives, did, however, give the desired
cinnamic ester or stilbene only in yields lower than 46%. Whereas
the change from tert-butyl to more reactive methyl azocarbox-
ylates (see Table S9)14 did not lead to significant improvements,
a breakthrough could be achieved with phenylazocarboxylate salt
2a in the presence of silver acetate in acetic acid (Table 1).15
Under acidic conditions, phenylazocarboxylate salts are known
to be converted to phenyldiazenes.12 Preliminary experiments
with 2a were performed on a 0.175 mmol scale, and then
optimization was continued on a 0.5 mmol scale (see Tables S10
and S11). Selected results from the final attempts with
azocarboxylate 2a, which was in all cases quantitatively obtained
from the corresponding tert-butyl ester 1a, are summarized in
Table 1. Addition of triphenylphosphine to the standard reaction
mixture led to a slight improvement (entries 1, 2), whereas a
higher reaction temperature of 60 °C significantly decreased the
M
alkenes have been known since 1971 and represent one of
the most important palladium-catalyzed carbon−carbon bond-
forming processes in the field of organic chemistry.2,3 Besides
commonly used aryl iodides, bromides, and chlorides, various
other aryl precursors such as triflates,4 diazonium salts,5,6 boronic
acids,7 hydrazines,8 and sulfonyl halides9 have been employed
(Scheme 1), and catalysts other than palladium have also been
Scheme 1. Mizoroki−Heck Reaction and Related
Transformations: Nitrogen-Based Leaving Groups
found useful.7b,10 Among the aryl precursors, diazonium salts5,6
have gained particular interest in a subtype which is generally
referred to as the Matsuda−Heck reaction and which can often
be conducted under comparably mild conditions.
The broad variety of studies on diazonium salts is therefore
nicely complemented by recent work on arylhydrazines by Loh,8
which showed that other aryl precursors may also enter the
catalytic cycle in combination with a loss of dinitrogen.
Interestingly, phenyldiazenes,11 which represent the missing
Received: February 16, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b00449
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Optimization of Reaction Conditions
Table 2. Evaluation of Substrate Scope and Limitations:
Aromatic Substitution Pattern
a
b
entry
variation of conditions
yield of 4aa (%)
a,
b
c
1: R1 =
1a: 4-Br
method
yield of 4 (%)
1
2
3
4
5
72
75
53
83
0
entry
PPh3 (10 mol %) added
reaction at 60 °C
H2O2 (2 equiv) instead of AgOAc
absence of Pd
1
A
B
B
A
B
A
B
B
A
B
B
B
B
B
B
4aa (75)
4aa (80)
4ba (60)
4ca (77)
4ca (73)
4da (75)
4da (78)
4ea (61)
4fa (48)
4fa (36)
4ga (56)
4ha (49)
4ia (56)
4ja (88)
4ka (traces)
2
1a: 4-Br
3
1b: 2-Br
4
1c: 4-F
a
Reaction conditions: tetrabutylammonium hydroxide (0.75 mmol),
1a (0.5 mmol), CH3CN (1.4 mL), 5 min; then slow addition of 2a
(over 45 min) to 3a (2.5 mmol), AgOAc (1.0 mmol), Pd(OAc)2 (0.05
mmol), AcOH (3 mL), rt. Yields determined by H NMR using
1,3,5-trimethoxybenzene as internal standard.
5
1c: 4-F
6
1d: 4-Cl
7
1d: 4-Cl
b
1
8
1e: 2,4-Cl2
1f: 4-I
9
10
11
12
13
14
15
1f: 4-I
1g: 4-CN
1h: 4-NO2
1i: 4-OMe
1j: 4-O(4-FC6H4)
1k: 4-NMe2
yield (entry 3). A screening of oxidants (see Table S12) with the
aim of replacing silver acetate revealed hydrogen peroxide as
most suitable reagent (entry 4).
To the best of our knowledge, hydrogen peroxide is known to
be formed in oxidative Heck reactions16 but has so far not been
described as a cheap additive to promote such palladium-
catalyzed carbon−carbon bond formations. A final experiment in
the absence of palladium ruled out a mechanism via a free aryl
radical, which could basically lead to the same product 4aa.17
After the synthetic applicability of the reaction reported in
Table 1 (entry 4) was demonstrated on a 3 mmol scale (86%
yield), we started to determine scope and limitations. A first
series of experiments was carried out with various substituents on
the aromatic core of the azocarboxylic ester 1 (Table 2).
Halogenated (entries 1−10) as well as most donor- and acceptor-
substituted aryl residues (entries 11−14) were tolerated, with the
strongly electron-donating dimethylamino group being the only
exception (entry 15). A comparison of the known combination
of palladium with silver acetate (method A)15 with the new Pd/
H2O2 system was carried out among the halogenated azo esters
1a,c,d,f. Thus, only the reactions with the 4-iodo compound 1f
showed a certain preference for method A (entries 9 and 10). In
the next step, variations of the alkene were investigated in
combination with azocarboxylic esters 1a and 1c (Table 3). The
azo esters 1a and 1c were chosen as both had given comparable
yields in the previous study (Table 2) and the brominated
compound would allow further substitution, whereas the
fluorinated compound is useful to evaluate the applicability for
18F-radiolabeling. Increased steric demand of the ester (entries
1−3) as well as additional substituents on the alkene (entries 4−
6) were tolerated except for the α-substituted acrylate 3d (entries
4 and 5), which preferably gave the isomeric acrylate 4ad′
(Figure 1) instead of the fully conjugated product 4ad.18
Experiments with acrylamides 3f and 3g led to the highest yields
in this series (entries 7−9). Methyl vinyl ketone (3i) initially
provided a mixture of the desired product 4ch and its reduced
analogue 4ch′ (entry 10, see also Figure 1). Through a
replacement of acetic acid by formic acid with omission of the
oxidant the product ratio could be fully shifted to 4ch′ (78%)
(entry 11).19
a
Conditions A: tetrabutylammonium hydroxide (0.75 mmol), 1 (0.5
mmol), CH3CN (1.4 mL), 5 min; then slow addition of 2 (over 45
min) to 3a (2.5 mmol), AgOAc (1.0 mmol), PPh3 (0.05 mmol),
Pd(OAc)2 (0.05 mmol), AcOH (3 mL), rt. Conditions B:
tetrabutylammonium hydroxide (0.75 mmol), 1 (0.5 mmol),
CH3CN (1.4 mL), 5 min; then slow addition of 2 (over 45 min) to
3a (2.5 mmol), H2O2 (1.0 mmol), Pd(OAc)2 (0.05 mmol), AcOH (3
b
c
mL), rt. Yields after purification by column chromatography.
before.20 A competition experiment with azo ester 1a and equal
amounts of methyl acrylate (3a) and acrylonitrile under
conditions B provided only 4ca (47%), thereby underlining the
low reactivity of the nitrile. In the series of alkenes, 3d (entries 4
and 5) and 3f (entries 7 and 8) can be used to compare methods
A and B, which did not show significant differences. In contrast to
that, a comparison of methods A and B in reactions with styrene
(3i) clearly demonstrated that only the conditions including
hydrogen peroxide are suitable for this type of alkene (Table 4,
entries 1 and 2). Whereas additional substituents on the aromatic
core did not have much influence on the reaction (entries 3−5),
the introduction of methyl groups on the double bond led to
differentiated results, which are comparable to those obtained for
acrylates 3d and 3e (cf., Table 3, entries 4−6). Likewise,
methylation in the α-position provided an isomeric product
4dm′ (Table 4, entry 6, see also Figure 1) and methylation in the
β-position was tolerated to give 4dn, but at a below average yield
of 49% (entry 7). Potential applicability in radiosynthesis was
demonstrated in a reaction of the fluorinated azoester 1c with
styrene 3k (entry 8).
A two-step sequence demonstrating the synthetic utility is
shown in Scheme 2. Nucleophilic substitution of N-acetyl-L-
tyrosine (5) with azo ester 1h could be achieved at room
temperature, and diphenyl ether 6 was then further converted
under the new Heck-type conditions to give 7. As known
bifunctional 1,4-disubstituted benzenes do not allow such a
sequence under comparably mild conditions,21 Heck reactions of
Surprisingly, no conversion could be achieved in attempts with
acrylonitrile, which has successfully been used in Heck reactions
B
DOI: 10.1021/acs.orglett.6b00449
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 3. Evaluation of Substrate Scope and Limitations:
Acrylates, Acrylamides, and Methyl Vinyl Ketone
Scheme 2. Two-Step Functionalization of N-Acetyl-L-tyrosine
a,b
entry
1: R1 =
3: R2, R3, R4 =
method
yield of 4 (%)
1
2
3
4
1c: F
1a: Br
1c: F
1a: Br
3a: H, H, OMe
3b: H, H, OEt
3c: H, H, OtBu
3d: H, Me, OMe
A
A
A
A
4ca (76)
4ab (78)
4cc (77)
A plausible reaction mechanism is depicted in Scheme 3. The
intermediacy of phenyldiazene 8, which is formed under acidic
c
4ad (13)
Scheme 3. Plausible Reaction Mechanism and Trapping
Experiments
c
4ad′ (42)
5
1a: Br
3d: H, Me, OMe
B
4ad (15)
4ad′ (50)
4ce (68)
4cf (87)
4cf (83)
4ag (92)
4ch (34)
4ch′ (48)
6
1c: F
1c: F
1c: F
1a: Br
1c: F
3e: Me, H, OMe
3f: H, H, NH2
3f: H, H, NH2
3g: H, H, NHC4H9
3h: H, H, Me
B
A
B
B
B
7
8
9
10
d
11
1c: F
3h: H, H, Me
B
4ch′ (78)
a
b
For conditions A and B, see footnotes of Table 2. Yields after
purification by column chromatography. Not isolated. Yield
determined by H NMR using 1,3,5-trimethoxybenzene as internal
standard. Reaction in HCOOH in the absence of H2O2.
c
1
d
conditions from azocarboxylate 2,22 was proven in a trapping
experiment12 with 2,5-dimethylfuran (9) yielding a pyridazinium
salt 10 (see the Supporting Information). Although all reactions
were conducted under air,23 which usually induces a quick decay
of phenyldiazenes24 into aryl radicals, no biphenyls were found in
a test reaction containing a large excess of benzene. We therefore
assume that phenyldiazene 8 can directly enter the catalytic
cycle.25,26 For hydrogen peroxide mediated reactions, an
alternative pathway via fragmentation of azo peracid 11 can
currently not be excluded.27 This pathway would circumvent the
intermediacy of diazene 8.
Figure 1. Additional products.
Table 4. Evaluation of Substrate Scope and Limitations:
Styrenes
After entering the cycle, the oxidants silver(I) acetate and
hydrogen peroxide are likely to ensure an adjustment of the
oxidation state of the palladium complex, which is necessary
since the oxidation state of phenyldiazene is one level lower than
that of a diazonium ion, which can be used in Heck reactions
without additional oxidant. The oxidation by hydrogen peroxide
appears to be much faster than by silver(I) ions, since the
peroxide (method B) tolerates a fast addition of azocarboxylate 2
to the reaction mixture (64% of 4aa, cf. entry 2, Table 2; see also
the Supporting Information), whereas a similar modification of
method A led to a complex product mixture. Whether this
oxidation step occurs before, as assumed in Scheme 3, or after
coordination of the Pd complex to the alkene is currently difficult
to assess. In this context, the unprecedented selectivity to acrylic
esters over acrylonitrile20 may point to coordination of a PdI
rather than a PdII species to the alkene. The fact that isomeric
product mixtures were obtained from reactions with 2-
methacrylic acid methyl ester (3d) (Table 3, entries 4 and 5),
as has been observed under classical reaction conditions,18a,b
a,b
entry
1: R1 =
3: R2, R3, R4 =
method
yield of 4 (%)
1
2
3
4
5
6
1d: Cl
1d: Cl
1d: Cl
1d: Cl
1d: Cl
1d: Cl
3i: H, H, H
3i: H, H, H
3j: H, H, F
A
B
B
B
B
B
4di (0)
4di (71)
c
4dj (70)
3k: H, H, Cl
3l: H, H, CN
3m: H, Me, H
4dk (80)
4dl (61)
4dm (−)
4dm′ (14)
4dn (49)
7
8
1d: Cl
1c: F
3n: Me, H, H
3k: H, H, Cl
B
B
c
4ck (73)
b
a
For conditions A and B, see footnotes of Table 2. Yields after
c
purification by column chromatography. Compounds 4dj and 4ck are
identical.
phenylazocarboxylates via phenyldiazenes could become val-
uable tools for the broad functionalization of peptides.
C
DOI: 10.1021/acs.orglett.6b00449
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
5826. (h) Schmidt, B.; Elizarov, N.; Schilde, U.; Kelling, A. J. Org. Chem.
2015, 80, 4223.
(6) Reactions via in situ generation of diazonium ions from anilines:
supports the assumption that the late stages of the mechanism
are identical to those of common Heck reactions.
In summary, a new version of the Mizoroki−Heck reaction
proceeding via phenyldiazenes has been developed. Represent-
ing the missing piece between known reactions of arylhydrazines
and aryldiazonium ions, it is useful to convert a broad variety of
olefinic substrates under very mild conditions at room
temperature and under air with full E/Z selectivity. Beneficially,
cheap H2O2 can be used as oxidant, which is so far unknown in
palladium-catalyzed C−C bond formations. Ongoing research is
directed toward further applications of the Pd−H2O2 system
including combinatorial chemistry and radiochemical reactions
based on readily available [18F]-4-fluorophenylazocarboxylic acid
tert-butyl ester ([18F]-1c).13
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b00449.
■
S
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(b) Cohen, S. G.; Nicholson, J. J. Org. Chem. 1965, 30, 1162.
(c) Hoffmann, R. W. Chem. Ber. 1965, 98, 222.
(12) Fehler, S. K.; Pratsch, G.; Heinrich, M. R. Angew. Chem., Int. Ed.
2014, 53, 11361.
General experimental methods, characterization data, and
NMR spectra (PDF)
(13) Fehler, S. K.; Maschauer, S.; Hofling, S.; Bartuschat, A.;
̈
Tschammer, N.; Hubner, H.; Gmeiner, P.; Prante, O.; Heinrich, M. R.
Chem. - Eur. J. 2014, 20, 370.
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AUTHOR INFORMATION
Corresponding Author
■
(14) (a) Hofling, S. B.; Bartuschat, A. L.; Heinrich, M. R. Angew. Chem.,
̈
Int. Ed. 2010, 49, 9769. (b) Jasch, H.; Hofling, S.; Heinrich, M. R. J. Org.
̈
Chem. 2012, 77, 1520.
*E-mail: markus.heinrich@fau.de. Tel: +49-9131-85-24115. Fax:
+49-9131-85-22585.
(15) (a) Laha, J. K.; Jethava, K. P.; Dayal, N. J. Org. Chem. 2014, 79,
8010. (b) Anand, M.; Sunoj, R. B.; Schaefer, H. F. ACS Catal. 2016, 6,
696.
Notes
(16) Gligorich, K. M.; Sigman, M. S. Angew. Chem., Int. Ed. 2006, 45,
6612.
The authors declare no competing financial interest.
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̈
ACKNOWLEDGMENTS
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(b) Brunner, H.; Bluchel, C.; Doyle, M. P. J. Organomet. Chem. 1997,
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This work was supported by the Deutsche Forschungsgemein-
schaft (GRK 1910, “Medicinal Chemistry of Selective GPCR
Ligands”, subproject B3 and HE5413/3-3). We thank Daniel
541, 89.
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experimental assistance.
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Org. Lett. XXXX, XXX, XXX−XXX