Hydroxy- and aminophenyl radicals from arenediazonium salts

Article abstract of DOI:10.1002/chem.201003713

Arenediazonium salts are well-known sources of aryl radicals; however, the hydroxy- and amino-substituted derivatives 1, 2, and 3, which lead to the respective formation of radicals 4, 5, and 6, have rarely been employed in synthetic organic chemistry so far. New synthetic applications of these species have been found, and the properties that may have previously hindered their successful use have been investigated. Copyright

Full text of DOI:10.1002/chem.201003713

DOI: 10.1002/chem.201003713
Hydroxy- and Aminophenyl Radicals from Arenediazonium Salts
Gerald Pratsch,[a] Christian A. Anger,[b] Katharina Ritter,[a] and Markus R. Heinrich*[a]
Arenediazonium salts and their synthetic applications
have a long history in both ionic and radical chemistry.[1]
Starting with the pioneering work by Peter Griess in the
1860s,[2] arenediazonium salts became well-known through
several prominent name reactions among which the Sand-
meyer,[3a] Meerwein,[3b] Pschorr,[3c] Gomberg-Bachmann,[3d]
and the Japp-Klingemann[3e] transformations. Given the
large number of reports published in this field in the past
150 years, it is surprising that there is one group of diazoni-
um ions for which radical reactions remained practically un-
known. Driven by the interest to find out whether this ex-
clusion is due to the unique reactivity of the hydroxy-substi-
tuted aryl radicals 1[4] and 2,[5,6] or is rather a consequence
of special properties of the related arenediazonium ions 3
and 4 (e.g., the possible formation of quinonediazides), we
revisited the chemistry of these compounds and their reac-
tive intermediates. Herein, we summarized our first results.
by Victor Meyer,[15] dating back to the year 1887, gives a
hint to the generation of the 2-hydroxyphenyl radical (1)
from 3. An alternative access to radicals 1 and 2 has been
described by Crich et al.,[4a,b] who reacted iodobenzenes in
the presence of diphenyl diselenide, AIBN, and Bu3SnH.
Our first experiments with diazonium salt 3 already re-
vealed a part of the unique behavior of aryl radical 1
À
(Scheme 1). Whereas the successful formation of C C
Scheme 1. Unique behavior of 2-hydroxyphenyl radical 1 derived from
the diazonium salt 3.
bonds has usually been observed in the addition of the di-
azonium salt to a mixture of the substrate (e.g., alkene, aro-
matic) and the reductant (e.g., TiCl3, FeSO4),[16] now only
phenol (5) was isolated as product—even in the presence of
the highly reactive aryl radical scavenger furan (6, path A;
Scheme 1).[16c] Surprisingly, the reverse addition, which we
believed to be far less favorable owing to the occurring of
homocoupling (addition of aryl radical 1 to precursor 3),[17]
gave the desired product 7 in good yield.
For comparison, the conventional preparation of 2-(2-fur-
yl)phenol (7) by organometallic methods requires more
elaborate starting materials, palladium catalysis, and a final
deprotection of the hydroxy group.[18] Moreover, the diazo-
nium-based synthesis of biaryl 7 also compares favorably
with the known radical access to 7 from 2-iodobenzene.[4a]
Further experiments were aimed at an extension to the 4-
hydroxy derivative 4 as well as to other reaction types such
as allylation (Table 1, entries 2, 3, and 7)[19] and carboamino-
hydroxylation (entry 4).[20] It became apparent that the “re-
verse addition”, as shown in Scheme 1, is only necessary for
The required diazonium ions 3 and 4 are readily available
as chloride salts from the corresponding 2- and 4-aminophe-
nols in the presence of isoamylnitrite and hydrochloric
acid.[7,8] Up to date, the salts of 3 and 4 have mainly been
utilized as precursors for quinone diazides[9] and carbenes,[10]
in Schiemann-type reactions[11] or for the purpose of surface
modification.[12] Palladium-catalyzed cross-coupling reac-
tions with arenediazonium salts, although they have been
known for more than two decades,[13] have just recently
been extended to the hydroxy-substituted derivatives 3 and
4.[14] In the field of radical chemistry, only one report on the
preparation of 2-hydroxybenzonitrile from 2-aminophenol
[a] G. Pratsch, K. Ritter, Prof. Dr. M. R. Heinrich
Department fꢀr Chemie und Pharmazie, Pharmazeutische Chemie
Friedrich-Alexander-Universitꢁt Erlangen-Nꢀrnberg
Schuhstraße 19, 91052 Erlangen (Germany)
Fax : (+49)9131-85-22585
E-mail: Markus.Heinrich@medchem.uni-erlangen.de
titaniumACTHNUTRGNE(UNG III)-mediated reactions but not for those conduct-
ed with iron(II) sulfate as reductant. We currently assign
this difference to the fact that water can act as a hydrogen-
[b] C. A. Anger
Department fꢀr Chemie und Biochemie
Organische Chemie I
Technische Universitꢁt Mꢀnchen
Lichtenbergstrasse 4, 85747 Garching b. Mꢀnchen (Germany)
radical donor in the presence of titaniumACHTNUTRGNEUNG(III) ions, but not
in the presence of iron(II).[21] In the case of diazonium salt
3, the undesired reduction to phenol (5) is probably en-
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COMMUNICATION
Chang.[25b] Since the major (and often only) by-product in
all the reactions was phenol (5), deuteration experiments
were carried out to determine whether the transferred hy-
drogen atom originated from the solvent or from the phe-
nolic hydroxy groups[26] of the reactants (3 or 4), the radicals
(1 or 2), and the respective products. The results obtained
from this study however clearly show that the free hydroxy
functionalities only play a minor role as hydrogen-atom
donors (see the Supporting Information).[27] We further as-
sumed that the behavior of radicals 1 and 2 could be deci-
sively influenced by their uncommon hydrophilicity com-
pared with most of the other aryl radicals that contain non-
polar substituents. With the intention to employ such special
properties for the purpose of selectivity, we conducted sever-
al two-phase reactions. For each of the experiments, we
chose two competing substrates, one of which was exclusive-
ly present in the aqueous and the other solely in the organic
phase. From a first attempt with equal amounts of tert-butyl
iodide (15) and glycerol (16), we obtained phenol (5) as
product formed by hydrogen transfer from 16. The fact that
2-iodophenol (17) did not emerge from the nearly diffusion-
controlled iodine-transfer[28] to the hydroxyphenyl radical
(1) indicates that 1 was unable to enter the organic phase
(Scheme 2). The replacement of 3 by its 4-hydroxy deriva-
tive 4 under otherwise identical conditions led to a 10:1-mix-
ture of phenol (5) and 4-iodophenol.
Table 1. Radical reactions of diazonium salts 3 and 4.
Entry Arenediazonium salt Substrate Product
Yield
[%]
1
58[a]
3
3
3
8
2
3
71[b]
9
54[b]
10
4
5
48[c]
3
4
11
12
61[a]
6
7
48[a]
Scheme 2. Hydrogen transfer to the 2-hydroxyphenyl radical 1 in the
presence of tert-butyl iodide (15).
4
4
13
14
61[b]
Similarly, protonated furfurylamine (18) was arylated to
yield 19 unaffected by the presence of tert-butyl iodide (15)
(Scheme 3, upper reaction). The two phase set-up also led
to a prefect selectivity between 2-methylfuran (20) and fur-
furylamine 18, as only 19 and no 21 was formed (Scheme 3,
lower reaction).
[a] Conditions A: substrate (10 equiv), TiCl3 (3 equiv), HCl/H2O;
[b] Conditions B: substrate (6 equiv), FeSO4 (9 equiv), DMSO/H2O (5:2);
[c] Conditions C: substrate (3 equiv), FeSO4 (3 equiv), TEMPO (2 equiv),
DMSO/H2O (2:1).
Further evidence for the extraordinary hydrophilicity of
the 2- and 4-hydroxyphenyl radicals 1 and 2 could be gained
from attempts to synthesize 2- and 4-hydroxybiphenyl under
Gomberg-Bachmann conditions using undiluted benzene as
organic layer.[29] Although benzene can be considered as
more polar than cyclohexane,[30] still almost no transfer of
radicals 1 or 2 from the aqueous to the organic phase was
observed.[31] As a control experiment, we investigated the
behavior of a “common” diazonium salt, such as 22, under
hanced by the increased electrophilicity of the related s-
type radical 1 caused by its ortho-hydroxy group.[22,23]
Regioisomers of products 8, 12, and 13 were not detected
1
by H NMR analysis of the crude reaction mixtures. Compa-
rably high regioselectivities in intermolecular radical aryla-
tions of furan and pyridine have before been observed by
Johnson,[24] Abramovitch and Saha,[25a] and Lynch and
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M. R. Heinrich et al.
Scheme 5. Radical allylation of the protonated 4-amino-substituted diazo-
nium salt 27 leading to 28.
were possible when suitable „one-phase“ reaction conditions
were applied. As an example, the allylation of the bistetra-
fluoroborate 27[35] gave 28 in a good yield (79%). In a two-
phase set-up such as the one shown Scheme 3, the aryl radi-
cal 26 completely remained in the aqueous phase.[36]
In summary, we have described the reaction conditions
under which the hydroxy- and aminophenyl diazonium ions
3, 4, and 27 can successfully undergo radical reactions that
À
lead to the formation of C C bonds. The fact that free hy-
droxy or amino groups were well tolerated under aqueous
“one-phase conditions” renders the previously required pro-
tective-group strategies needless. Favorably, the 2-hydroxy-
phenyl radical 1 is not stabilized through inter- or intramo-
lecular transfer of hydrogen. The distinct hydrophilicity of 1
and 2 can be used to attain the selectivity in two-phase reac-
tions given that suitable reductants (FeSO4 or TiCl3 at low
concentrations) are employed. The titanium-mediated aryla-
tions further gives a hint that especially in the field of radi-
cal chemistry, the reverse addition of reagents may be a
more general concept useful to improve hitherto challenging
reactions.
Scheme 3. Two-phase experiments demonstrating the hydrophilicity of
the 2-hydroxyphenyl radical (1).
two-phase reaction conditions (Scheme 4). In contrast to all
the reactions shown above, we now exclusively obtained the
iodinated product 23 and no bromobenzene (24), which
points to an effective phase-transfer of the lipophilic bromo-
phenyl radical 25 to the organic phase.[32] The 4-methoxy-
phenyl radical, which can be considered to possess a degree
of hydrophilicity between those of radicals 1 and 25, reacted
unselectively and led to a 5:1 mixture of anisole and 4-
iodoanisole.
Experimental Section
Preparation of 4-(2-chloroallyl)phenol (14) as a representative example:
A
solution of 4-hydroxyphenyl diazonium chloride (3; 2.00 mmol,
313 mg) in DMSO/water (5:2 v/v, 2.5 mL) was added dropwise over
10 min to a degassed and vigorously stirred mixture of DMSO/water (5:2
v/v, 6 mL), iron(II)-sulfate heptahydrate (18.0 mmol, 5.00 g), and 2,3-di-
chloro-1-propene (12.0 mmol, 1.11 mL). After stirring for further 15 min,
water was added and the reaction mixture was extracted with ethyl ace-
tate (3ꢃ50 mL). The combined organic phases were washed with satd.
aqueous sodium chloride and dried over sodium sulfate. Concentration in
vacuo and purification by column chromatography (silica gel, hexane/
EtOAc=2:1) gave 4-(2-chloroallyl)phenol (14; 1.23 mmol, 208 mg, 61%)
1
as a orange oil. Rf =0.8 (hexane/EtOAc=2:1) [UV]; H NMR (600 MHz,
CDCl3, 258C, TMS): d=3.55 (s, 2H; CH2), 5.11 (q, J=1.2 Hz, 1H;
CHH), 5.23 (td, J=0.5 Hz, J=1.2 Hz, 1H; CHH), 6.80 (d, J=6.7 Hz,
2H;
2 ꢃ CH), 7.10 ppm (d, J=6.7 Hz, 2H; 2 ꢃ
CH); 13C NMR
(91 MHz, CDCl3, 258C, TMS): d=44.6 (CH2), 113.2 (CH2), 115.4 (2 ꢃ
CH), 129.2 (Cq), 130.3 (2 ꢃ CH), 142.0 (Cq), 154.4 ppm (Cq); IR (NaCl):
n˜ =3355 (m), 2915 (w), 1632 (m), 1613 (w), 1514 (s), 1443 (m), 1368 (w),
1235 (m), 1173 (m), 1123 (w), 886 (m), 851 (w), 830 cmÀ1 (m); MS (EI):
m/z (%): 170 (32) [37Cl-M]+, 169 (42), 168 (100) [35Cl-M]+, 133 (73), 132
(18), 131 (55), 107 (12), 105 (16), 85 (23), 83 (41), 77 (27); HRMS (EI):
m/z calcd for C9H9ClO: 168.0342, found: 168.0345.
Scheme 4. Control experiment demonstrating the phase transfer of the 4-
bromophenyl radical (25).
Even more polar aryl radicals, such as 26, can be generat-
ed from arenediazonium salts that contain a protonated
amino group (Scheme 5).[33,34] Comparable to the hydroxy-
phenyl radicals 1 and 2, reactions with lipophilic substrates
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À
Radical-Mediated Formation of C C Bonds
COMMUNICATION
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Acknowledgements
The generous financial support of our research by the Deutsche For-
schungsgemeinschaft (DFG), the Dr. Otto-Rçhm-Gedꢁchtnisstiftung,
and the FAU Erlangen-Nꢀrnberg is gratefully acknowledged. We would
like to thank J. Mersini for the experiments conducted during her
summer internship at the TU Mꢀnchen.
À
Keywords: allylation · arenediazonium salts · biaryls · C C
coupling · radical reactions
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Received: December 23, 2010
Revised: January 28, 2011
Published online: March 14, 2011
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Chem. Eur. J. 2011, 17, 4104 – 4108