Metal-free cross-coupling reactions of aryl sulfonates and phosphates through photoheterolysis of aryl-oxygen bonds

Article abstract of DOI:10.1002/anie.200461444

Photochemical cleavage of Ar-O bonds in phenyl esters substituted with electron-donating groups offers a convenient method for the arylation of alkenes and arenes. The reaction proceeds by an S_N1 mechanism via a triplet phenyl cation to form allylbenzene or biphenyl derivatives under mild conditions (see scheme).

Full text of DOI:10.1002/anie.200461444

Communications
Synthetic Methods
Metal-Free Cross-Coupling Reactions of Aryl
Sulfonates and Phosphates through
Photoheterolysis of Aryl–Oxygen Bonds**
Marco De Carolis, Stefano Protti, Maurizio Fagnoni,*
and Angelo Albini*
Metal-mediated cross-coupling reactions have been exten-
sively developed for the formation of aryl–carbon bonds. In
these reactions, an aryl–halogen (mostly Br or I) or an aryl–
oxygen bond is activated in the starting compound, for
example, an aryl sulfonate[1] can be used in the Suzuki
ꢀ
Miyaura procedure for the synthesis of biphenyls.[2,3]
A
limitation of these syntheses is the severe experimental
conditions that must be employed, such as an inert atmos-
phere, strictly anhydrous solvents, and the use of a base or a
high temperature. Furthermore, although triflates are the
most reactive precursors for the aryl sulfonate series they are
less easily handled and are relatively expensive. Efforts to
extend the reaction to the less reactive mesylates[6] and
tosylates have met with some success.[1,6,7] Another class of
reagents with a potential O-leaving group is the aryl
phosphates, of which to our knowledge only a couple of
examples have been reported. These involve coupling the
phosphate with Grignard reagents in the presence of phos-
phane–nickel(ii) complexes.[8]
We have recently identified an alternative cross-coupling
reaction, in which the aryl-substituted bond in the starting
compound is cleaved heterolytically, rather than activated, by
a photochemical reaction. For example, the photosensitized
decomposition of diazonium salts allowed general access to a
triplet phenyl cation[9] (Scheme 1) which in turn reacted with
olefins and aromatic compounds[10] to generate aryl–carbon
Scheme 1. Photochemical generation of triplet phenyl cations.
[*] M. De Carolis, S. Protti, Dr. M. Fagnoni, Prof. A. Albini
Department of Organic Chemistry
University of Pavia
Via Taramelli 10, 27100 Pavia (Italy)
Fax: (+39)0382-507-323
E-mail: fagnoni@unipv.it
angelo.albini@unipv.it
[**] Partial support of this work by Murst, Rome (Italy) is gratefully
acknowledged.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200461444
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Angewandte
Chemie
bonds. The electrophilic character of these
salts limits the choice of nucleophiles that
can be used in the photoarylation reactions,
since there will be competing thermal reac-
tions. However, aryl cations could also be
photogenerated starting from electron-
donating substituted aryl halides, such as
chloro- or fluoroanilines,[11] chlorophenols,
and chloroanisoles[12] (Scheme 1). Delocali-
zation of the positive charge on the electron-
donating substituents in the resulting aryl
cation as well as the use of a polar medium
resulted in efficient photoheterolysis.
Extending the scope of this mild, metal-
free cross-coupling reaction would greatly
increase its synthetic significance.
Scheme 2. Arylation reactions by the irradiation of phenyl esters in the presence of allyltrimethylsilane
or benzene.
We reasoned that the incorporation of
O-leaving groups might also be beneficial in
the aryl cation precursor, thus enabling the
reaction to be applied to the readily avail-
able and versatile family of aryl esters.
There are no precedents for such processes,
and what is known about these compounds
Table 1: Photolysis of 4-N,N-dimethylamino esters 1–4 (0.05m) in the presence of ATMS (1m) or
benzene (1m).
Entry Ester Solvent
Nucleophile
Consumption [%]
tIRR [h]
Yield[a] [%]
phenylated 11 12
1
2
3
4
5
6
7
8
9
1
1
1
1
2
2
MeCN
TFE
ATMS
100
100
100
100
73
4
4
4
4
5
5
5
5
5
5
5
9, 95
–
–
–
–
81
56
–
–
–
–
–
–
–
–
5
ATMS[b]
9, 100
9, 100
10, 100
9, 15
ꢀ
points to homolytic cleavage of the ArO X
H2O/MeCN ATMS[c]
bond, as exemplified by the (radical) photo-
Fries reaction of organic esters.[13] The
photochemistry of their inorganic counter-
parts (aryl sulfonates and phosphates) has
also not been extensively studied. Aryl
tosylates were reported to undergo the
photo-Fries reaction in polar solvents,[14]
but aryl triflates eliminated the triflate
group both on irradiation in acetone[15a]
and under photoelectron transfer conditions
(for example, from pentafluorophenyl tri-
flate[15b]), although this is known for a very
TFE
TFE
benzene[b]
ATMS
TFE
benzene
ATMS or benzene
ATMS
84
10, 19
3[d]
4
4
4
4
29–85
100
100
100
85
[d]
[d]
MeCN
TFE
9, 82
16
–
–
ATMS
9, 100
9, 100
10, 96
10
11
H2O/MeCN ATMS[c]
–
–
TFE
benzene
4
[a] Yields determined by gas chromatography (GC) analysis based on consumed ester; for preparative
results see the Experimental Section. [b] 0.2m nucleophile. [c] ATMS at 0.5m for solubility reasons.
[d] No photoproducts detected by GC analysis under any of the conditions used for the other esters.
limited number of cases. Furthermore, the nickel-mediated
synthesis of biphenyl starting from phenyl triflate was favored
in UV light.[15c] The photolability of the aryl tosylates, like the
aryl phosphates, has been known for 50 years and represents
one of the first examples of photoinduced nucleophilic
substitution.[16] However, this photolability has remained
limited to photohydrolysis in water,[17] apart from the intra-
molecular process reported for alken-1-ylarylmethyl phos-
phates[18a] and of bi- and triaryl phosphates,[18b] where aryl–
vinyl or aryl–aryl bonds are formed. Conversely, photo-
found that cross-coupling reactions had taken place in all
cases and gave the corresponding allylated and phenylated
derivatives. The product yield depended on the reagent used
and on the solvent (MeCN, 2,2,2-trifluoroethanol (TFE), or
H2O/MeCN 1:5). Since acid was liberated in the process, the
solutions were buffered by an equimolar amount of triethyl-
amine (except when water was present in solution). Control
experiments showed that thermal processes were not involved
in the formation of the phenyl–carbon bonds.
Table 1 shows the results with dimethylaminophenyl
esters as the substrates. In particular, photolysis of mesylate
1 in the presence of ATMS (1m) provided 4-(2-propenyl)-
N,N-dimethylaniline (9) in almost quantitative yield.[19] The
formation of 4-phenyl-N,N-dimethylaniline (10) was quanti-
tative in TFE in the presence of benzene; water/MeCN
afforded the next highest yield (48%), although photoreduc-
tion to N,N-dimethylaniline (12) was not negligible. However,
triflate 2 gave a low yield of both 9 and 10 (15 and 19%,
respectively, in TFE), with deprotection to yield 4-hydroxy-
N,N-dimethylaniline (11) being the main pathway. Tosylate 3
was more resistant to photodecomposition and no products
were detected by gas chromatographic analysis (Table 1,
entry 7).
ꢀ
fragmentation of the P OAr bond occurred through resonant
two-photon reactions on irradiation of the 4-methoxyphenyl-
diethyl phosphate in cyclohexane.[18c]
Therefore, we decided to investigate the photochemistry
of N,N-dimethylamino- and methoxy-substituted phenyl
sulfonates 1–3 and 5–7 as well as the phosphates 4 and 8.
These compounds are readily prepared from the correspond-
ing phenols (see the Supporting Information) by irradiation in
an argon-flushed polar solution in the presence of either
allyltrimethylsilane (ATMS) or benzene (1m). The N,N-
dimethylamino derivatives were found to decompose quanti-
tatively (or close to) within 4–5 hours while the methoxy-
derivatives needed 20–32 hours (Scheme 2, Table 1). We
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The performance of diethylphosphate 4 was very close to
see Table 2, entry 4). Furthermore, the presence of oxygen
only slightly affected the yield of 13 from 6 when the reaction
was carried out in water/MeCN (Table 2, entry 5).
We then tested 4-(N,N-dimethylamino)phenyl (17) and 4-
methoxyphenyl acetate (18) as representative organic esters
(Scheme 3). Both compounds underwent photohomolysis of
that of mesylate 1. Allylated derivative 9 was produced in a
quantitative yield when 4 was irradiated in TFE or MeCN/
water while phenylated 10 was produced in a yield of 96% in
TFE and about 50% in the other solvents. Phenylation was
also examined in MeOH and no significant difference was
observed when MeCN was used. The precursor that gave the
best results was mesylate 1, which gave equally good yields
when lower amounts of both nucleophiles were employed
(see Table 1, entries 2 and 4).
Since the conditions were different from those used in the
original study on haloanilines,[11] we tested the cross-coupling
reactions of 4-chloro-N,N-dimethylaniline under these con-
ditions for comparison. Formation of allylated 9 was quanti-
tative in this case (ATMS could effectively be used at 0.2m
concentration), while phenylation of benzene occurred in a
low to moderate yield (24–51%). Again, reduction to N,N-
dimethylaniline was the main competing process.
Scheme 3. Photochemical reactions of phenyl acetates.
Methoxyphenyl esters 5–8 inefficiently absorbed the UV-
B light used and reacted only sluggishly.[22] Acetone (0.9m)
sensitization proved efficient even if irradiation times were
four to six times longer than those of esters 1–4 (ꢁ 20–
30 hours).[23,24] Photoreaction of mesylate 5 in the presence of
ATMS yielded 4-(2-propenyl)anisole (13) and the yield
increased from 17% in acetonitrile to 100% in aqueous
acetonitrile[25] (Scheme 2, Table 2). Phenylation of benzene
the acetyl group, and phenols 11 (14–45%) and 15 (close to
quantitative) were produced, as well as the photo-Fries
product 19 (19–56% yield) in the former case. Carrying out
the irradiation in the presence of either ATMS (1m) or
benzene (1m) did not affect the product distribution and no
phenylation products were obtained.
Finally, precursors lacking electron-donating substituents
were tested. Thus phenyl mesylate and phenyl triflate were
photolyzed both in TFE and water/MeCN
(the solvents which gave the best results in
Table 2: Photolysis of 4-methoxy esters 5–8 in the presence of nucleophiles.
the previous cases) in the presence of the
above nucleophiles, but neither allylben-
zene nor biphenyl were formed.
Entry Ester Solvent
Nucleophile Consumption [%]
tIRR [h]
Yield[a] [%]
phenylated
15 16
1
2
3
4
5
6
7
8
9
5
5
5
6
6
6
6
7
7
8
8
TFE
ATMS
79
100
100
100
100
100
100
71
30
24
24
30
24
28
24
22
22
30
24
13, 73
–
6
–
8
–
9
–
9
–
–
The above data show that various
phenyl sulfonates and phosphates with elec-
tron-donating substituents are suitable pre-
cursors for the photochemical phenylation
of alkenes and arenes. Both mesylate 1 and
phosphate 4 could be used for synthesizing
N,N-dimethylamino derivatives 9 and 10 in
quantitative yields, although concurrent
paths dramatically lowered the arylation
yield in the photodecomposition of triflate 2
and tosylate 3. Also in the cases of the meth-
oxyphenyl esters, triflate 6 was the best
precursor for obtaining both 13 and 14. The
latter compound was also obtained in high
yield from mesylate 5 and phosphate 8,
H2O/MeCN ATMS[b]
H2O/MeCN benzene
13, 100
14, 58
–
–
–
–
TFE
H2O/MeCN ATMS[b,d]
TFE benzene
H2O/MeCN benzene
TFE
TFE
TFE
TFE
ATMS
13, 100 (48)[c]
13, 90
14, 97
14, 74
13, 23
14, 15
13, 44
14, 96
–
–
21
12
–
ATMS
benzene
ATMS
benzene
85
100
100
10
11
12
–
–
[a] Yields determined by gas chromatographic (GC) analysis based on consumed ester; for preparative
results see the Experimental Section. [b] ATMS at 0.5m for solubility reasons. [c] 0.2m nucleophile.
[d] Under nondeaerated conditions.
was less satisfactory, since the maximum yield of 4-phenyl-
anisole (14) was 58% both in water/MeCN and TFE. Triflate
6 gave moderate to excellent yields of both 13 and 14,
especially in TFE—a much better performance than both
aminophenyl triflate 2 and methoxyphenyl mesylate 5.
Tosylate 7 gave complex mixtures with poor yields of the
phenylation products (not exceeding 23%), with deprotection
being by far the main pathway. Again, TFE was the best
solvent for the arylation reactions starting from phosphate 8,
and photoproducts 13 and 14 were isolated in 44 and 96%
yields, respectively.
while tosylate 7 gave complex mixtures. Phenyl esters
performed better than phenyl halides in the arylation of
arenes,[26] where the latter precursors underwent reduction to
a considerable extent (see below). The initiating photoheter-
olysis step liberated mineral acids during the reaction. The use
of triethylamine as a buffer prevented the irradiated solutions
from darkening and gave a clean reaction. Particularly
noteworthy is that a mixed solvent containing water exerted
a sufficient buffering effect.[27]
The photoreactions of phenyl halides substituted with
electron-donating groups[11,12] and of benzenediazonium
salts[10] supports the theory that the arylation of esters results
from the triplet state of the precursor. In this case, two
Of the methoxyphenyl esters examined, triflate 6 gave the
best results. The reaction proceeded fairly efficiently in TFE
with a 0.2m concentration of nucleophile (48% with ATMS,
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Chemie
competitive cleavage paths operate (Scheme 4), with photo-
solvent) and avoids the use of bases or other aggressive
ꢀ
homolysis of the O X bond forming a phenyloxy radical and
reagents. In addition, a relatively low concentration of the
p nucleophile is efficient (often 0.2m, a 4:1 excess with respect
to the ester), and expensive and often unstable catalysts are
not required. The success of the reaction is based on the
smooth photogeneration of the triplet phenyl cations and
their selective reaction with p nucleophiles. Mechanistic
studies on this previously unreported efficient photoheterol-
ysis of the phenyl–oxygen bond of these esters are now
underway.
ꢀ
photoheterolysis of the phenyl O bond yielding a phenyl
cation in the triplet state. The first pathway accounts both for
Experimental Section
Typical procedure of the phenylation reactions through photoheter-
olysis of phenyl esters: A solution (18–30 mL) of ester 1–8 (0.05m),
triethylamine (TEA, 0.05m), and benzene or ATMS (0.2 to 1m) in the
chosen solvent was poured in a quartz tube[33] and purged for 10 min
with argon, septum capped, and irradiated with six 15 W phosphor-
coated lamps (range of emission 295–335 nm, maximum at 310 nm).
The solution was concentrated in vacuo and purified by column
chromatography (cyclohexane/ethyl acetate as eluants). In the
reactions involving esters 5–8, acetone (100 mL per mL of solvent,
0.9m) was added to the reaction mixture. When water/MeCN was
used as the solvent, no TEA was added. The yields of isolated
products consistently approached those determined by gas chromato-
graphic analysis (Table 1 and Table 2); for example, compound 6!13,
80%; 6!14, 85%.
Received: July 27, 2004
Revised: October 13, 2004
Published online: January 12, 2005
Scheme 4. Mechanistic paths in the photodegradation of esters 1–8.
Keywords: arylation · aryl cations · aryl esters · cross-coupling ·
photochemistry
.
the formation of deprotected phenols 11 and 15 (important in
the photolysis of triflate 2) and for the photo-Fries reaction
that occurs predominantly with acetate 17. This route also
probably has a role in the reactions with tosylates. The triplet
phenyl cation formed in the second pathway adds selectively
to p nucleophiles. Benzene rearomatization leads to biphen-
yls 10 and 14, while elimination of a trimethylsilyl group by
ATMS leads to allylbenzenes 9 and 13.[28] A competing
reaction for the triplet cation is reduction by the solvent.[31]
The importance of this pathway depends on the starting
material (and thus on the nature of the counterion XOꢀ) and
of the p trap, as well as the solvent chosen. Among the
aminophenyl esters reduction is significant particularly with
the phosphate, while among the methoxyphenyl esters
reduction is particularly significant with the mesylate and
the triflate. However, in almost all cases it is possible to
choose a solvent where reduction is minimized and arylation
occurs in high or quantitatively yield, with TFE and MeCN/
water (5:1) often being a good choice.
In summary, the photolysis of phenyl sulfonates and
phosphates substituted with electron-donating groups is an
appealing method for the phenylation of alkenes and arenes.
The method is general and, when all the previous findings
with phenyl halides and benzenediazonium salts are consid-
ered, compares favorably with metal catalysis[32] in that mild
conditions are employed. These reactions are also insensitive
to oxygen and moisture (indeed, water can be used as a mixed
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Communications
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[28] Attack of allyltrimethylsilane by cations is well known. The
alternative formation of compounds 9 and 13 by a photo-
generated aryl radical is excluded on two grounds. First, it has
been reported that such a path operates only with strongly
electrophilic aryl radicals (for example, 3,5-dinitrophenylradi-
cal[29]) and, then, only with a poor yield. Moreover, b fragmen-
tation of the adduct radical leading to allylated derivatives is
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alkyl- or aryl-substituted silanes.[30] Second, purposely designed
experiments disfavor this hypothesis. Thus, generation of an
authentic 4-methoxyphenyl radical by photolysis of 4-bromoa-
nisole in the presence of ATMS gave no allylated compounds but
only anisole. Likewise, substituting 2,3-dimethylbutene for
ATMS gave 2,3-dimethyl-3-(4-methoxyphenyl)-1-butene, 2,3-
dimethyl-3-(4-methoxyphenyl)-2-(2,2,2-trifluoroethoxy)butane,
2,3-dimethyl-2-(4-methoxyphenyl)butane, and 2-(4-methoxy-
phenyl)-2-(2,2,2-trifluoroethoxy)-3,3-dimethylbutane from 5 in
CF3CH2OH/Cs2CO3, in the same proportion as from 4-chloro-
anisole (see reference [12]). This reaction is difficult to envisage
other than involving loss of a proton from (or solvent addition
to) an adduct cation. (Further evidence is the fact that a cationic
Wagner–Meerwein rearrangement has taken place in the last
product).
[29] F. Ek, L.-G. Wistrand, T. Frejd, J. Org. Chem. 2003, 68, 1911 –
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phenyl cation has been previously demonstrated, see Refs. [10–
12].
[32] In rare instances, the Suzuki coupling occurred under microwave
irradiation in the absence of a catalyst, see: N. E. Leadbeater, M.
Marco, Angew. Chem. 2003, 115, 1445 – 1447; Angew. Chem. Int.
Ed. 2003, 42, 1407 – 1409.
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[19] Palladium-catalyzed arylation of allyltrimethylsilane starting
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selectivities according to the conditions used,[20] whereas aryl
triflates afforded arylated allylsilanes.[21]
[20] T. Jeffery, Tetrahedron Lett. 2000, 41, 8445 – 8449.
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5076 – 5079.
[22] Methoxyphenyl esters exhibited the long-wavelength band at
275–280 nm, with virtually no absorption at 310 nm, thus poorly
matching the spectrum of the lamp (see Experimental Section).
In contrast, aminophyenyl esters strongly absorbed at 310 nm
(e ꢁ 1200–2500 molꢀ1 dm3 cmꢀ1).
[33] Quartz allowed the absorption of all of the light emitted by the
lamp.
[23] Some irradiations with mesylate 5 in the presence of ATMS were
carried out under nonsensitized conditions at 254 nm and gave
the same product distribution, although product 13 was not
stable under prolonged irradiation. The use of the lamps at a
wavelength of 310 nm prevented both product photodegradation
and competititve absorption by benzene when this was used as
the trap. A possible wavelength dependence of the reaction is
under investigation.
[24] In these acetone-sensitized reactions in the presence of ATMS,
the Paternꢁ-Bꢀchi oxetane was found as a by-product in a
variable amount, particularly in the case of phosphate 8.
[25] The presence of a protic polar medium (for example, water)
could dramatically favor the photoheterolytic process, see: M.
Freccero, M. Fagnoni, A. Albini, J. Am. Chem. Soc. 2003, 125,
13182 – 13190.
[26] 4-Methoxybiphenyl was obtained only in 7% yield in aprotic
solvent after irradiation of 4-chloroanisole in the presence of
benzene (1m). In TFE the yield did not exceed 70%.
[27] Triethylamine (0.05m) buffers the acid liberated in the photolysis
of the esters and avoids degradation and polymerization of the
photoproducts, as indicated by experiments using cesium
carbonate instead which led to the same product distribution.
1236
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2005, 44, 1232 –1236