Transition-metal-free arylations via photogenerated triplet 4-alkyl- and 4-trimethylsilylphenyl cations

Article abstract of DOI:10.1021/jo4007046

The irradiation in protic solvents of 4-chloroalkylbenzenes and 4-chlorophenyltrimethylsilane caused the heterolytic cleavage of aryl-chlorine bonds to give the corresponding triplet phenyl cations. These were exploited for transition-metal-free arylation reactions under mild conditions to give allylbenzenes, γ-benzyl lactones, 3-arylacetals (ketals), and biaryls in moderate to good yields. The path followed was supported by DFT calculations at the UB3LYP/6-311+G(2d,p) level.

Full text of DOI:10.1021/jo4007046

Article
pubs.acs.org/joc
Transition-Metal-Free Arylations via Photogenerated Triplet 4‑Alkyl-
and 4‑Trimethylsilylphenyl Cations
Hisham Qrareya, Carlotta Raviola, Stefano Protti,* Maurizio Fagnoni,* and Angelo Albini
PhotoGreen Lab, Department of Chemistry, University of Pavia, V. Le Taramelli 12, 27100 Pavia, Italy
S
* Supporting Information
ABSTRACT: The irradiation in protic solvents of 4-chloroalkylbenzenes and 4-chlorophenyltrimethylsilane caused the
heterolytic cleavage of aryl−chlorine bonds to give the corresponding triplet phenyl cations. These were exploited for transition-
metal-free arylation reactions under mild conditions to give allylbenzenes, γ-benzyl lactones, 3-arylacetals (ketals), and biaryls in
moderate to good yields. The path followed was supported by DFT calculations at the UB3LYP/6-311+G(2d,p) level.
Chart 1
INTRODUCTION
■
The synthesis of (poly)functionalized alkyl(hetero)arenes is of
considerable importance in organic chemistry due to the
possible elaboration of the alkyl chains.1 A typical approach
involves the activation of Ar−X bonds in substituted alkyl
aromatics under metal-catalyzed conditions.2 However, in-
novative protocols for transition-metal-free arylations have
recently emerged and involve relatively weak Ar−X bonds, as in
the thermal decomposition of diaryliodonium salts3a or in the
visible light photocatalytic reduction of arenediazonium salts.3b
The metal-free activation of the Ar−X bond in alkyl aryl halides
(or esters) may be envisaged for a larger scope application in
the preparation of biphenyls by reaction with unactivated
arenes. This approach has been sparsely reported in the
literature,4,5 but the use of a strong base (e.g., KOtBu) and of a
high temperature (up to 200 °C), as well as of a large excess of
the arene (from 100 equiv up to neat) were required.
Furthermore, these reactions are efficient on phenyl iodides,
bromides, and triflates, while being limitedly applied to easily
available chlorides,4a,c,d contrary to that reported for related
transition-metal-catalyzed cross-coupling reactions.6 An alter-
native may be sought in the photochemical cleavage of the
aryl−chlorine bond. However, the photochemical behavior of
n-chloroalkylbenzenes has not been extensively documented,7,8
Increasing attention has been paid to triplet phenyl cations
(3Ar+) formed by the photoinduced heterolytic fragmentation
of the Ar−Cl bond. Examples are the photoreduction of 4-
chloro-n-butylbenzene (2) in 2-propanol11 and the arylations
occurring upon irradiation of 2 in the presence of π-bond
nucleophiles.12 The formation of a phenyl cation has been also
invoked in the photolysis of 2-chloroisopropylbenzene in 2,2,2-
trifluoroethanol (TFE), where a mixture of ethers, alkenes,
propylbenzene, and indane was obtained.13
In view of the above results, further exploration of the
potential of phenyl cations photogenerated from n-chloroalkyl-
benzenes for transition-metal-free arylations appeared worth-
while. In parallel with the experimental study, a computational
analysis was carried out, since the only data available dealt with
1 in the gas phase (CASSCF level of theory) where a homolytic
photodissociation takes place.14 A combined experimental and
computational study of the photochemistry of chlorides 1−4
(see Chart 1) as well as of 4-chlorotrimethylsilylbenzene (5)
was thus performed, as reported below.
and application in synthesis is limited to photostimulated SRN
1
reactions of 4-chlorotoluene (1, Chart 1) with diphenyl
arsenide9a and triphenylstannyl anion9b in liquid ammonia
(but the same reaction was sluggish in DMSO).10
Received: April 3, 2013
© XXXX American Chemical Society
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Figure 1. Geometries, spin densities, and ESP atomic charges (in parentheses) calculated in bulk MeOH at the CPCM-UB3LYP/6-311+G(2d,p)//
3
3
UB3LYP/6-311+G(2d,p) level of theory for (a) 1 (Ar−Cl bond length: 1.84 Å) and (b) 1 upon stretching the Ar−Cl bond up to 4.00 Å. Bond
lengths (Å), angles (∠, in degrees), and dihedral angles (∠, in degrees) for (c) 1+ and (d) 1+.
1
3
Table 1. Calculated ET and ΔEstretch of Triplet Aryl Chlorides
RESULTS
■
Considered in This Study
Density functional theory (DFT) at the UB3LYP/6-311+G-
(2d,p) level was adopted in this work to optimize the absolute
a
b
aryl chloride
ET
ΔEstretch
minima of phenyl chlorides 1−5, their triplets 1−35, and the
3
1
73.1
73.4
73.1
72.9
73.6
76.3
15.6
15.4
15.0
15.2
17.7
18.7
resulting cations 1,31+−1,35+ (see the Supporting Information
for details), analogously to previous studies on substituted
chlorobenzenes.15 For simplicity, the ethyl derivative 2′ was
taken as a model for the butyl analogue 2, and the calculations
were extended to chlorobenzene (Ph-Cl), in order to assess the
effect of the alkyl groups. Solvent effects (bulk MeOH) were
included at the same level of theory by single-point calculations
using the C-PCM method (conductor-like polarizable con-
2′
3
4
5
Ph−Cl
a
Triplet energy (kcal mol−1) calculated in bulk MeOH at the CPCM-
UB3LYP/6-311+G(2d,p)//UB3LYP/6-311+G(2d,p) level of theory;
b
3
Energy required to stretch the Ar−Cl bond up to 4 Å for 1−35.
tinuum model).15 As for parent triplet 1, computational data
3
1
singlets 1+−15+ were planar with a cumulative double-bond
character at the C3−C4−C5 moiety (see Figures 1c and S4a,
Supporting Information), while the corresponding triplets
31+−35+ were planar and close to a regular hexagon (Figure
1d), with the only exception of 4-trimethylsilylphenyl cation
(35+), where a small out-of-plane displacement (∠C4−C5−C6−
C1 = 2.9°) of the Ar−Si bond occurred (Figure S4b, Supporting
Information).
obtained in the gas phase essentially confirmed what was
previously reported,14 that is the aryl−chlorine bond was
elongated and stuck out from the ring plane. In bulk methanol,
calculations revealed a significant partial negative charge at the
chlorine atom (−0.22, Figure 1a), with the spin density mainly
localized on C4 (44%) and only 8% on Cl. A further stretching
of the Ar−Cl bond in 31 up to 4.00 Å (Figure 1b) resulted in a
marked charge separation (Cl −0.79; C4 0.03). The spin
localization on the chlorine atom remained almost unchanged
(10%), while energy increased by ca. 15 kcal mol−1 with respect
to the equilibrium geometry. Similar data were obtained for
triplet aromatics with more branched alkyl substituents viz.
ethyl, i-Pr, and t-Bu (see Figures S1−S2, Supporting
The isodesmic reaction shown in eq 1 was adopted to
evaluate the energy of the phenyl cations formed (calculated in
3
Information and Table 1). The results for silyl derivative 5
were similar to those obtained for the isosteric t-Bu-substituted
benzene (34, see Figure S2, Supporting Information, for
comparison). The calculated triplet energies (ET, 72.9−73.6
kcal mol−1) and ΔEstretch (the energy required to stretch the
Ar−Cl bond up to 4 Å) for 31−35 are reported in Table 1. With
Ph-Cl, ΔEstretch = 18.7 kcal mol−1 (the highest in the series), the
charge separation is less pronounced (C4 = −0.01, Cl = −0.60),
and 20% of the spin density remains localized on the chlorine
atom (see Figure S3, Supporting Information, and Table 1).
Further studies were likewise performed on both singlet and
triplet phenyl cations. The geometry was characteristic. Thus,
MeOH, see Supporting Information) with respect to the
reduced form 1a−5a. In all cases, alkyl and trimethylsilyl groups
imparted a slight stabilization to the singlets with respect to the
parent singlet phenyl cation taken as the reference point and a
larger stabilization (>4 kcal mol−1) to the triplets (Figure 2).
Singlets are the ground state of these cations, with a triplet/
singlet energy gap ranging from ca. 14 (1+−4+) to 19.2 kcal
mol−1 (5+).
B
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been tested for the synthesis of allylbenzenes (by reaction with
ATMS),16 γ-benzyl lactones (by using 4-pentenoic acid as the
nucleophile),12b 3-arylacetals and ketals (by reaction with vinyl
ethers),12a and biaryls (in the presence of aromatics).16
Photolysis of 1−5 has been carried out at 254 nm for direct
irradiation, applied with alkenes, and at 310 nm under
sensitized conditions (acetone 20%v/v) when a trap absorbing
at 254 nm was employed (e.g., benzene, mesitylene). The use
of a base (Et3N or Cs2CO3) to buffer the HCl released17 was
also tested. The phenyl chlorides mostly reacted quantitatively,
except when benzene and mesitylene were used as the
nucleophiles (see below and the Experimental Section).
Gratifyingly, the results obtained (see Tables 2−4) demon-
strated that smooth arylation reactions starting from alkyl(silyl)
substituted phenyl chlorides under metal-free conditions were
feasible.
Irradiation of a mixture of 1−5 (0.05 M) and ATMS (0.5 M)
in water/acetonitrile 1/5v/v gave the corresponding 4-
allylarenes 6−10 in a satisfactory yield (53−72%, Table 2a).
In the preparation of allylated 6 and 7, yields were lower when
shifting to 2,2,2-trifluoroethanol as the solvent. Aqueous
acetonitrile was the chosen medium for the arylation of 4-
pentenoic acid, and the corresponding γ-benzyl lactones 11−15
were isolated in satisfying amounts (Table 2b). A partial ring-
opening of lactones 14−15 to the corresponding 5-phenyl-4-
hydroxypentenoic acids has been observed after the isolation.
A three-component reaction was likewise accomplished by
irradiation of 1−5 in methanol at 254 nm in the presence of
ethyl vinyl ether or 2-methoxypropene. This gave 3-aryl acetals
(16−20) or ketals (21−25), respectively (Table 3). In the
synthesis of 16 and 17, a lower excess of the nucleophile (0.25
M) was likewise tested, and the yields were still satisfactory at
least in the first case. A 3-arylketal (a protected ketone) was the
only product observed in the reaction between 1 and 2-
methoxypropene. However, addition of aqueous HCl (equi-
molar to 1) to the photolyzed solution resulted in the efficient
one-pot preparation of the corresponding α-aryl ketone 21′
(Scheme 1).
Finally, biaryls 26−35 were synthesized in discrete yields by
photoreaction between the chosen chlorides and benzene or
mesitylene (Table 4). In this case, a longer irradiation time (50
h) was required to achieve a significant consumption of the
substrate (particularly when benzene was used), and TFE was
required to have a satisfying arylation yield. However, a faster
reaction roughly maintaining the same yields can be obtained
by lowering the concentration of chlorides (see Table 4 and the
Experimental Section).
Figure 2. Relative Gibbs free energies (see Tables S1 and S2,
Supporting Information) of singlet (red) and triplet (blue) phenyl
cations in solution (MeOH) according to the isodesmic reaction in eq
1.
Turning to the experiment, compounds 1−5 exhibit low
fluorescence (see Table S3, Supporting Information). Irradi-
ation of 1 in MeOH (254 nm) led to a mixture of the
dechlorinated toluene (1a, 80% yield based on the
consumption of 1) and 4-methylanisole (1b, 18%). In order
to assess whether both products arose from the same
intermediate, the reaction was repeated in the presence of
increasing amounts of allyltrimethylsilane (ATMS), a known
trap of phenyl cations.16 The irradiation time required for the
conversion did not change significantly, while as shown in
Figure 3, the yields of both photoproducts depended on
[ATMS] and decreased in a parallel way with the increase of
the trapping product, 4-allyltoluene (6).
The successful trapping by ATMS fostered a more extended
investigation on the reaction of the examined chlorides with π-
bond nucleophiles and its synthetic potential. The phenyl
chlorides employed were commercially available or easily
accessible from the corresponding 4-alkylanilines as in the
case of 3 and 4 (see the Experimental Section). Irradiation has
DISCUSSION
■
The photochemical behavior of PhCl and of the corresponding
alkylated derivatives depends on the medium used. Accord-
ingly, irradiation of PhCl in alkanes caused an efficient
dechlorination via homolysis of the Ar−Cl bond.18a On the
contrary, the consumption of 1 when photolyzed in 9:1
acetonitrile−water mixture was negligible (Φr = 3.7 × 10−3),18b
whereas it was consistently larger in MeOH (Φr = 0.09) where
both toluene and 4-methylanisole were formed.19
Triplet phenyl cations as synthetic intermediates were
initially characterized upon photolysis of chloro- and (less
efficiently) fluoroanilines.20 Later, it was demonstrated that the
photoheterolytic cleavage was likewise feasible for a wide range
of electron-rich aromatic halides and esters.12 On the basis of
the present results, it appears that the reaction is rather general,
Figure 3. Ratio between the yield of 4-allyltoluene (6) and that of the
other photoproducts (toluene 1a and 4-methylanisole 1b) from the
irradiation of 4-chlorotoluene (1) in MeOH in the presence of an
increasing amount of allyltrimethylsilane (ATMS).
C
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a
Table 2. Synthesis of (a) Allylbenzenes 6−10 and (b) γ-
Table 3. Synthesis of 3-Arylacetals and 3-Arylketals 16−25
a
Benzyl Lactones 11−15
a
Reaction conditions: 0.05 M solution of 1−5 and 0.05 M Cs2CO3 in
MeOH irradiated at 254 nm for 18 h in the presence of 0.5 M ethyl
vinyl ether (products 16−20) or 2-methoxypropene (products 21−
b
25); Reaction carried out by using 0.25 M of ethyl vinyl ether.
Scheme 1. One-Pot Synthesis of α-Aryl Ketone 21′
a
Reaction conditions: 0.05 M solution of 1−5 in water/acetonitrile 1/
5v/v irradiated at 254 nm for 18 h in the presence of the chosen
nucleophile (ATMS or 4-pentenoic acid, 0.5 M); arylation yields based
on the consumption of the phenyl chloride. Reaction carried out in
2,2,2-trifluoroethanol, Cs2CO3 0.05 M. See ref 12b. Partial
hydrolysis to the corresponding 5-aryl-4-hydroxypentanoic acid has
been observed.
b
c
d
was successful, although a longer irradiation time (ca. 50 h) was
required. Photoheterolysis of the Ar−Cl bond from the triplet
3
states 1−35 (path b) is supported by computational results
provided that a polar protic solvent 5:1 acetonitrile−water
mixture, MeOH, or TFE is used. The presence of a mild
electron-donating substituent (an alkyl or to a lesser extent a
SiMe3 group, compare the σ values for Et = −0.15; Me =
−0.17; SiMe3 = −0.07)21 contributes to drive the reaction
toward heterolysis. Actually, calculations confirmed that the
energy required to stretch the Ar−Cl bond in PhCl (ca. 19 kcal
mol−1) is 3 kcal mol−1 higher than those of alkyl-substituted 1−
4 (Table 1).
Intersystem crossing in 1−5 is known to be quite efficient
(Scheme 2, path a),7,14 and direct irradiation is a viable choice
provided that the trap does not absorb competitively. If this is
the case (aromatics), acetone sensitization (ET = 80 kcal mol−1)
(see Figure 1).
Singlet phenyl cations 11+−15+ are planar and have a
structure similar to those reported for analogues bearing a
moderate donating substituent, e.g., CH(OR)2,15 and different
from cations substituted with more electron-rich groups, such
as OMe or NMe2, where a puckering of the ring was
observed.22 The triplets 31+−35+ has a regular hexagonal
shape as in other (substituted) triplet phenyl cations. Singlets
11+-15+, are more stable than the corresponding triplets (by at
least 14 kcal mol−1) and are accessible from them by
intersystem crossing (ISC, Scheme 2, path c). Thus, in neat
solvent, the chemistry occurring results from the competition
D
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a
Table 4. Synthesis of Biphenyls 26−35
a
Reaction conditions: 0.05 M solution of 1−5 and 0.05 M Et3N in TFE/acetone 8/2 mixture irradiated at 310 nm for 50 h in the presence of 1.0 M
b
benzene (products 25−30) or mesitylene (products 31−35). Yields based on the consumption of phenyl chlorides. 0.025 M 1−5 were used.
Scheme 2. Irradiation of Phenyl Chlorides 1−5 in Neat Protic Solvents and in the Presence of π-Bond Nucleophiles
E
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between hydrogen abstraction (path e) and ISC to the singlets
leading to solvolysis (path d).
EXPERIMENTAL SECTION
■
1H and 13C NMR spectra were recorded on a 300 MHz spectrometer.
The attributions were made on the basis of 1H and 13C NMR, as well
as DEPT-135 experiments; chemical shifts are reported in ppm
downfield from TMS. The photochemical reactions were performed
by using nitrogen-purged solutions in quartz tubes. Irradiations were
performed in a multilamp reactor fitted with ten 15 W phosphor
coated lamps (maximum of emission 310) or with four low pressure
Hg lamps (maximum of emission at 254 nm). The π bond
nucleophiles (allyltrimethylsilane (ATMS), ethyl vinyl ether, 2-
methoxypropene, 4-pentenoic acid, benzene, and mesitylene) were
commercially available and were used as received, except for ethyl vinyl
ether, which was freshly distilled before use. Aryl chlorides 1, 2, and 5
were commercially available. Toluene (1a) and 4-methylanisole (1b)
have been characterized by comparison with authentical samples, and
their yield calculated by means of calibration curves. Compounds 3
(oil) and 4 (oil) have been synthesized from the corresponding
anilines through a Sand-Mayer reaction by following a known
procedure35 and obtained in 40% (665 mg) and 43% (801 mg)
yield, respectively.
On the other hand, in the presence of a sufficiently high
concentration (0.5 M) of a π-bond nucleophile, trapping of the
triplet cation (Scheme 2, path f) and formation of an Ar−C
bond was exclusive. The efficiency of the photoreaction in neat
solvents and in the presence of a trap did not vary, supporting
the idea that the cation is formed through a unimolecular
fragmentation, not requiring the intermediacy of an exciplex, at
least under the present conditions.23 Moreover, the inter-
mediacy of an aryl radical (Ar•) formed by photohomolysis of
the Ar−X bond24 or from cleavage of Ar−X•− in turn obtained
by a photoinduced electron transfer reaction (PET) between
Ar−Cl* and the nucleophile24 is likewise ruled out. The former
path has a role only in nonpolar solvents18a or when using aryl
halides having a more labile Ar−halogen bond such as Ar−I or
Ar−Br.24 A PET reaction is reasonably excluded since the
reaction between 31 and ATMS (the most oxidizable
nucleophile among those tested) was calculated to be slightly
endoergonic (ΔGET = +3.3 kcal mol−1).25 In addition, the
attempted reaction between mesitylene and a substituted
phenyl radical was recently reported but the arylation failed
due to the high steric hindrance of the alkyl aromatic.28
Thus, all of the previously found classes of addition by using
electron-rich substituted phenyl cations have been carried out
successfully also with halides 1−5 (alkene addition followed by
elimination with ATMS, or by intramolecular nucleophilic
attack with pentenoic acid, or by intermolecular trapping, with
enol ethers and methanol; arylation via benzenium ion
intermediate with benzene or mesitylene).16
A peculiar case is represented by silane 5 that bears two
potential leaving groups (viz. nucleofugal Cl and electrofugal
SiMe3). Both of these groups were activated under metal-
catalyzed conditions. Thus, the Ar−Cl bond in 5 was cleaved in
the Pd-catalyzed α-arylation of aliphatic aldehydes29 whereas
the Ar−SiMe3 bond was activated in gold-catalyzed cross-
coupling reactions with unactivated arenes to form biaryls.30
We recently studied the chemistry of photogenerated phenyl
cations having 14b group-based substituents (viz. a trimethyl-
silyl or trimethylstannyl) in the ortho position with respect to
the positive charge.22,31 A β-silicon (or β-tin) effect operated
and intersystem crossing of the triplet cation to the singlet was
favored also in the presence of π bond nucleophiles. In the
present case, however, neither experimental nor computational
data showed a significant difference between silane 5 and
alkylbenzenes 1−4, except for a lower stabilization imparted to
the triplet cation (that had the higher triplet-singlet gap of the
series, ca. 19 kcal mol−1, Figure 2). As expected, no δ-silicon
effect operated, which allowed us to prepare substituted
arylsilanes prone to further elaboration through metal
catalysis.30,32,33
1
3: H NMR (CDCl3) δ 7.30−7.15 (AA′BB′, J = 8 Hz, 4H), 2.90−
2.75 (m,1H), 1.3−1.25 (d, J = 7 Hz, 6H); 13C NMR (CDCl3) δ 147.2,
131.2, 128.3 (CH), 127.7 (CH), 33.5 (CH), 24.0 (CH3); IR (neat) ν/
cm−1 2961, 1493, 1092, 1013, 825. Anal. Calcd for C9H11Cl: C, 69.90;
H, 7.17. Found: C, 69.8; H, 7.3.
4: spectroscopic data were in accordance with the literature.36 Anal.
Calcd for C10H13Cl: C, 71.21; H, 7.77. Found: C, 71.1; H, 7.9.
General Procedure for the Synthesis of 4-Alkyl-1-allylben-
zenes 6−10. Method A. A solution of the aryl chlorides (1−5, 1.5
mmol, 0.05 M) and allyltrimethylsilane (ATMS, 15 mmol, 0.5 M) in
MeCN/H2O 5:1 (30 mL) was nitrogen purged in quartz tubes, serum
capped, and irradiated at 254 nm for 18 h until complete consumption
of the aromatic. The photolyzed mixture was extracted with diethyl
ether (3 × 20 mL), the organic phases were reunited, dried over
MgSO4, evaporated, and the residue was purified by column
chromatography (eluant: n-hexane).
Method B. A solution of the aryl chlorides (1−5, 1.5 mmol, 0.05
M), allyltrimethylsilane (ATMS, 15 mmol, 0.5 M), and Cs2CO3 0.05
M (1.5 mmol) in TFE (30 mL) was nitrogen purged in quartz tubes,
serum capped, and irradiated at 254 nm for 18 h until complete
consumption of the aromatic (except where indicated). The
photolyzed solution were evaporated under vacuum, and the residue
was purified by column chromatography (eluant: n-hexane).
1-Allyl-4-methylbenzene (6): oil, 105 mg, 53% yield from method
A and 68 mg, 38% yield (based on the 90% consumption of 1) from
method B. The spectroscopic data of compound 6 were in accordance
with the literature.37 Anal. Calcd for C10H12: C, 90.85; H, 9.15. Found:
C, 90.8; H, 9.1.
1-Allyl-4-butylbenzene (7): oil, 188 mg, 72% yield from method A
and 180 mg 69% yield from method B. The spectroscopic data of
compound 7 were in accordance with the literature.38 Anal. Calcd for
C13H18: C, 89.59; H, 10.41. Found: C, 89.6; H, 10.4.
1-Allyl-4-isopropylbenzene (8): oil, 168 mg, 70% yield from
39
1
method A; H NMR (CDCl3) δ 7.25−7.15 (AA′BB′, 4H, J = 8
Hz), 6.05−6.00 (m, 1H), 5.20−5.10 (m, 2H), 3.45−3.40 (d, 2H, J =
6.5 Hz), 3.00−2.95 (m, 1H), 1.35−1.30 (d, 6H, J = 7 Hz); 13C NMR
(CDCl3) δ 146.5, 137.6 (CH), 137.3, 128.4 (CH), 126.2 (CH), 115.5
(CH2), 39.8 (CH2), 36.6 (CH), 23.9 (CH3); IR (neat) ν/cm−1 2963,
1515, 1248, 1064, 836. Anal. Calcd for C12H16: C, 89.94; H, 10.06.
Found: C, 89.9; H, 10.3.
CONCLUSIONS
■
The results presented here (see Tables 2−4) again strengthen
the role of triplet phenyl cations as intermediates for transition-
metal-free arylations under mild conditions. The generation of
the cations, previously limited to electron-rich aromatics,16 is
now extended to easily available alkyl (or silyl) substituted
phenyl chlorides. Although an excess of a nucleophile has to be
used, particularly in the preparation of biphenyl derivatives, the
method proposed competes with transition-metal-free reac-
tions4,5 where the nucleophile was mostly used as the reaction
medium.
1-Allyl-4-tert-butylbenzene (9): oil, 188 mg, 72% yield from
method A. The spectroscopic data of compound 9 were in accordance
with the literature.40 Anal. Calcd for C13H18: C, 89.59; H, 10.41.
Found: C, 89.6; H, 10.3.
(4-Allylphenyl)trimethylsilane (10): oil, 151 mg, 53% yield from
method A. The spectroscopic data of compound 10 were in
accordance with the literature.41 Anal. Calcd for C12H18Si: C, 75.71;
H, 9.53. Found: C, 75.6; H, 9.6.
General Procedure for the Photochemical Synthesis of γ-
Benzyl Lactones 11−15. A solution of aryl chlorides (1−5, 1.5
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mmol, 0.05 M) and 4-penten-1-oic acid (15 mmol, 0.5 M) in MeCN/
H2O 5:1 (30 mL) was purged for 5 min with nitrogen, serum capped,
and irradiated with four 15-W Hg lamps (emission centered at 254
nm) for 18 h. The photolyzed mixture was thus neutralized with
aqueous NaHCO3 5% and extracted with diethyl ether (3 × 20 mL).
The organic phase were collected, dried over MgSO4, evaporated, and
the residue purified by column chromatography (eluant: cyclohexane/
ethyl acetate mixture).
74.2; H, 9.3. The same reaction has been carried out in the presence of
0.25 M ethyl vinyl ether affording 277 mg of 16 (95% yield).
1-Butyl-4-(2-ethoxy-2-methoxyethyl)benzene (17): oil, 259 mg,
73% yield (after purification by column chromatography, eluant: neat
cyclohexane). The spectroscopic data of compound 17 were in
accordance with the literature.12b Anal. Calcd for C15H24O2: C, 76.23;
H, 10.24. Found: C, 76.3; H, 10.2. The same reaction has been carried
out in the presence of ethyl 0.25 M vinyl ether affording 202 mg of 17
(57% yield).
5-(4-Methylbenzyl)dihydrofuran-2(3H)-one (11): oil, 200 mg, 70%
1-(2-Ethoxy-2-methoxyethyl)-4-isopropylbenzene (18): oil, 200
mg, 60% yield (after purification by column chromatography, eluant:
cyclohexane/ethyl acetate 98:2); 1H NMR (CDCl3) δ 7.30−7.20
(AA′BB′, 4H, J = 7 Hz), 4.65−4.55 (t, 1H, J = 6 Hz), 3.75 −3.60 (AB
system, 1H), 3.55−3.45 (AB system, 1H), 3.35 (s, 3H), 3.00−2.95 (d,
2H, J = 7 Hz), 2.95−2.90 (m, 1H), 1.40−1.30 (d, 6H, J = 7 Hz),1.30−
1.25 (m, 3H); 13C NMR (CDCl3) δ 146.7, 134.3, 129.3 (CH), 126.5
(CH), 104.5 (CH), 61.7 (CH2), 53.0 (CH3), 39.6 (CH2), 33.6 (CH),
23.9 (CH3), 15.2 (CH3); IR (neat) ν/cm−1 2958, 1513, 1226, 1127,
1051, 822. Anal. Calcd for C14H22O2: C, 75.63; H, 9.97. Found: C,
75.6; H, 9.9.
yield (after purification by column chromatography, eluant: cyclo-
hexane/ethyl acetate 75:25); H NMR (CD3COCD3) δ 7.20−7.10
1
(AA′BB′, J = 8 Hz, 4H), 4.80−4.70 (qui, J = 7 Hz, 1H), 2.95−2.80
(dd, J = 6 and 14 Hz, 2H), 2.55−2.40 (m, 2H), 2.30 (s, 3H), 2.30−
2.25 (m, 1H), 2.00−1.90 (m, 1H); 13C NMR (CD3COCD3) δ 177.5,
137.2, 135.2, 130.6 (CH), 130.2 (CH), 81.8 (CH), 41.9 (CH2), 29.3
(CH2), 28.2 (CH2), 21.4 (CH3); IR (neat) ν/cm−1 2925, 1773, 1516,
1179, 919, 807. Anal. Calcd for C12H14O2: C, 75.76; H, 7.42. Found:
C, 75.7; H, 7.4.
5-(4-Isopropylbenzyl)dihydrofuran-2(3H)-one (13): oil, 151 mg,
46% yield (after purification by column chromatography, eluant:
1
1-tert-Butyl-4-(2-ethoxy-2-methoxyethyl)benzene (19): oil, 211
cyclohexane/ethyl acetate 8:2); H NMR (CD3COCD3) δ 7.30 (s,
mg, 71% yield based on 84% consumption of 4 (after purification by
column chromatography, eluant: cyclohexane/ethyl acetate 98:2); H
4H), 4.80−4.70 (qui, J = 7 Hz, 1H), 3.00−2.90 (m, 3H), 2.50−2.20
(m, 3H), 2.00−1.95 (m, 1H), 1.25 (d, J = 6 Hz, 6H); 13C NMR
(CD3COCD3) δ 177.5, 148.3, 135.7, 130.6 (CH), 127.6 (CH), 81.9
(CH), 41.9 (CH2), 34.8 (CH), 29.3 (CH2), 28.3 (CH2), 24.7 (CH3);
IR (neat) ν/cm−1 2960, 1776, 1514, 1178, 1020, 920, 818. Anal. Calcd
for C14H18O2: C, 77.03; H, 8.31. Found: C, 77.0; H, 8.3.
1
NMR (CDCl3) δ 7.35−7.20 (AA′BB′, 4H, J = 8 Hz), 4.65−4.55 (t,
1H, J = 6 Hz), 3.80−3.70 (AB system, 1H), 3.55−3.45 (AB system,
1H), 3.30 (s, 3H), 3.0−2.90 (d, 2H, J = 6 Hz), 1.35 (s, 9H), 1.30−1.20
(t, 3H, J = 7 Hz); 13C NMR (CDCl3) δ 149.0, 134.0, 129.0 (CH),
125.4 (CH), 104.4 (CH), 61.7 (CH2), 53.0 (CH3), 39.5 (CH2), 34.3,
31.3 (CH3), 15.1 (CH3); IR (neat) ν/cm−1 2964, 1514, 1364, 1269,
1121, 1064. Anal. Calcd for C15H24O2: C, 76.23; H, 10.24. Found: C,
76.2; H, 10.2.
5-(4-tert-Butylbenzyl)-dihydrofuran-2(3H)-one (14): oil, 164 mg,
47% yield (after purification by column chromatography, eluant:
1
cyclohexane/ethyl acetate 8:2); H NMR (CD3COCD3) δ 7.40−7.20
(AA′BB′, J = 8 Hz, 4H), 4.80−4.70 (qui, J = 7 Hz, 1H), 3.00 (dd, J = 6
and 11 Hz, 2H), 2.50−2.40 (m, 2H), 2.30−2.10 (m, 2H), 2.05−1.95
(m, 2H), 1.30 (s, 9H); 13C NMR (CD3COCD3) δ 177.5, 150.5, 135.3,
130.3 (CH), 126.6 (CH), 81.9 (CH), 41.8 (CH2), 34.1, 32.0 (CH3),
29.3 (CH2), 28.3 (CH2); IR (neat) ν/cm−1 2962, 1774,1512, 1364,
1178, 832. Anal. Calcd for C15H20O2: C, 77.55; H, 8.68. Found: C,
77.6; H, 8.9. After isolation a partial hydrolysis of 14 to 5-(4-tert-
butylphenyl)-4-hydroxypentenoic acid has been observed as pointed
out by the signals at 4.20−4.30 (m, 1H) in the 1H NMR spectra and at
64.8 (CH) in the 13C NMR.
(4-(2-Ethoxy-2-methoxyethyl)phenyl)trimethylsilane (20): oil, 248
mg, 72% yield based on 91% consumption of 5 (after purification by
column chromatography, eluant: neat cyclohexane); 1H NMR
(CDCl3) δ 7.50−7.20 (AA′BB′, 4H, J = 8 Hz), 4.65−4.55 (t, 1H, J
= 6 Hz), 3.75−3.65 (AB system, 1H), 3.50−3.40 (AB system, 1H),
3.35 (s, 3H), 3.00−2.95 (d, 2H, J = 6 Hz), 1.30−1.20 (t, 3H, J = 7
Hz), 0.40 (s, 9H); 13C NMR (CDCl3) δ 137.9, 137.7, 133.2 (CH),
128.8 (CH), 104.3 (CH), 61.7 (CH2), 53.0 (CH3), 40.0 (CH2), 15.2
(CH3), −1.2 (CH3); IR (neat) ν/cm−1 2955, 1248, 1110, 1064, 838.
Anal. Calcd for C14H24O2Si: C, 66.61; H, 9.58. Found: C, 66.6; H, 9.6.
1-(2,2-Dimethoxypropyl)-4-methylbenzene (21): oil, 192 mg, 66%
yield (after purification by column chromatography, eluant: neat
5-(4-(Trimethylsilyl)benzyl)-dihydrofuran-2(3H)-one (15): oil, 220
mg, 59% yield (after purification by column chromatography, eluant:
1
cyclohexane/ethyl acetate 7:3); H NMR (CD3COCD3) δ 7.55−7.30
1
cyclohexane); H NMR (CDCl3) δ 7.25−7.05 (AA′BB′, 4H), 3.20 (s,
(AA′BB′, J = 8 Hz, 4H), 4.80−4.70 (qui, J = 7 Hz, 1H), 3.05−2.95 (m,
2H), 2.50−2.25 (m, 3H), 2.15−2.0 (m, 1H). 0.3 (s, 9H); 13C NMR
(CD3COCD3) δ 177.4, 139.2, 139.1, 134.6 (CH), 130.1 (CH), 81.7
(CH), 42.3 (CH2), 29.4 (CH2), 28.4 (CH2), −0.66 (CH3); IR (neat)
ν/cm−1 2954, 1772, 1602, 1248, 1178, 839, 692. Anal. Calcd for
C14H20O2Si: C, 67.70; H, 8.12. Found. C, 67.7; H, 8.3. After isolation a
partial hydrolysis of 15 to 5-(4-trimethylsilylphenyl)-4-hydroxypente-
noic acid has been observed as pointed out by the signals at 4.20−4.30
6H), 2.85 (s, 2H), 2.35 (s, 3H), 1.25 (s, 3H); 13C NMR (CDCl3) δ
136.5, 135.9, 131.4 (CH), 129.7 (CH), 102.8, 48.6 (CH3), 43.3
(CH2), 21.8 (CH3), 21.4 (CH3); IR (neat) ν/cm−1 2944, 1515, 1376,
1126, 1051. Anal. Calcd for C12H18O2: C, 74.19; H, 9.34. Found: C,
74.2; H, 9.3.
One-Pot Synthesis of 1-p-Tolylpropan-2-one (21′). From 179 μL
of 4-chloro toluene (1, 0.05 M, 1.5 mmol), 488 mg of Cs2CO3 (0.05
M, 1.5 mmol), and 1.45 mL of 2-methoxypropene (0.5 M, 15 mmol)
in MeOH (30 mL). A 100 μL portion of 37% aq HCl was added to the
photolyzed solution, allowed to stay for 1 h at rt, and evaporated.
Purification by column chromatography (eluant: hexane/ethyl acetate
98:2) afforded 124 mg of 21′ (oil, 56% yield). Spectroscopic data of
21′ were in accordance with the literature.42 Anal. Calcd for C10H12O:
C, 81.04; H, 8.16. Found: C, 81.1; H, 8.2.
1-Butyl-4-(2,2-dimethoxypropyl)benzene (22): oil, 262 mg, 74%
yield (after purification by column chromatography, eluant: neat
hexane); 1H NMR (CDCl3) δ 7.20−7.05 (AA′BB′, 4H, J = 8 Hz), 3.30
(s, 6H), 2.90 (s, 2H), 2.70−2.60 (t, 2H, J = 8 Hz), 1.7−1.6 (m, 2H),
1.45−1.35 (m, 2H), 1.20 (s, 3H), 1.0−0.90 (s, 3H); 13C NMR
(CDCl3) δ 140.7, 134.4, 129.9 (CH), 128.7 (CH), 101.8, 48.2 (CH3),
42.2 (CH2), 35.2 (CH2), 33.6 (CH2), 22.3 (CH3), 21.0 (CH2), 13.9
(CH3); IR (neat) ν/cm−1 2931, 1513, 1377, 1127, 1052. Anal. Calcd
for C15H24O2: C, 76.23; H, 10.24. Found: C, 76.2; H, 10.1.
1-Isopropyl-4-(2,2-dimethoxypropyl)benzene (23): oil, 200 mg,
60% yield (after purification by column chromatography, eluant: neat
hexane); 1H NMR (CD3COCD3) δ 7.30−7.15 (m, 4H), 3.25 (s, 6H),
1
(m, 1H) in the H NMR spectra and at 64.6 (CH) in the 13C NMR.
General Procedure for the Synthesis of 3-Arylacetals or
Ketals (16−25. A solution of the aryl chloride (1−5, 1.5 mmol, 0.05
M), ethyl vinyl ether, or 2-methoxypropene (15 mmol, 0.5 M) and
Cs2CO3 (1.5 mmol, 0.05 M) in methanol (30 mL) was nitrogen
purged in quartz tubes and irradiated at 254 nm until complete
consumption of the aromatic (except where indicated). The solvent
was thus removed under vacuum and the residue purified by column
chromatography (hexane/ethyl acetate mixtures with 1% Et3N as the
eluant).
1-(2-Ethoxy-1-methoxyethyl)-4-methylbenzene (16): oil, 239 mg,
82% yield (after purification by column chromatography, eluant: neat
1
cyclohexane); H NMR (CDCl3) δ 7.20−7.05 (AA′BB′, J = 8 Hz,
4H), 4.65−4.60 (t, J = 6 Hz, 1H), 3.65−3.55 (AB system, 1H), 3.50−
3.40 (AB system, 1H), 3.35 (s, 3H), 2.95−2.85 (d, 2H, J = 6 Hz), 2.35
(s, 3H), 1.25−1.15 (t, 3H, J = 7 Hz); 13C NMR (CDCl3) δ 135.7,
134.0, 129.2 (CH), 128.9 (CH), 104.6 (CH), 61.8 (CH2), 53.1 (CH3),
39.7 (CH2), 20.9 (CH3), 15.2 (CH3); IR (neat) ν/cm−1 2927, 1516,
1124, 1064. Anal. Calcd for C12H18O2: C, 74.19; H, 9.34. Found: C,
G
dx.doi.org/10.1021/jo4007046 | J. Org. Chem. XXXX, XXX, XXX−XXX

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Article
2.95 (s, 2H), 2.95−2.90 (m,1H), 1.30−1.25 (d, 6H, J = 7 Hz), 1.2 (s,
3H); 13C NMR (CD3COCD3) δ 147.6, 136.3, 131,4 (CH), 127.0
(CH), 102.8, 48.6 (CH3), 43.3 (CH2), 34.8 (CH), 24.8 (CH3), 21.8
(CH3); IR (neat) ν/cm−1 2959, 1513, 1376, 1123, 1064, 819. Anal.
Calcd for C14H22O2: C, 75.63; H, 9.97. Found: C, 75.6; H, 9.9.
1-tert-Butyl-4-(2,2-dimethoxypropyl)benzene (24): oil, 230 mg,
65% yield (after purification by column chromatography, eluant: neat
hexane); 1H NMR (CD3COCD3) δ 7.35−7.15 (AA′BB′, 4H), 3.25 (s,
6H), 2.85 (s, 2H), 1.35 (s, 9H), 1.10 (s, 3H); 13C NMR
(CD3COCD3) δ 149.8, 135.9, 131.2 (CH), 125.9 (CH), 102.7, 48.6
(CH3), 43.2 (CH2), 35.2 (CH3), 21.0 (CH3); IR (neat) ν/cm−1 2961,
1516, 1126, 1052, 826. Anal. Calcd for C15H24O2: C, 76.23; H, 10.24.
Found: C, 76.2; H, 10.2.
(4-(2,2-Dimethoxypropyl)phenyl)trimethylsilane (25): oil, 239 mg,
63% yield (after purification by column chromatography, eluant: neat
hexane); 1H NMR (CDCl3) δ 7.45−7.25 (AA′BB′, 4H, J = 9 Hz), 3.25
(s, 6H), 2.85 (s, 2H), 1.15 (s, 3H), 0.25 (s, 9H); 13C NMR (CDCl3) δ
139.7, 138.5, 134.1 (CH), 131.0 (CH), 102.7, 48.7 (CH3), 43.8
(CH2), 21.9 (CH3), −0.6 (CH3); IR (neat) ν/cm−1 2944, 1515, 1376,
1126, 1051. Anal. Calcd for C14H24O2Si: C, 66.61; H, 9.58. Found: C,
66.6; H, 9.6.
General Procedure for the Synthesis of Biaryls 26−35. A
solution of the aryl chloride (1−5) (1.5 mmol, 0.05 M), benzene, or
mesitylene (30 mmol 1.0 M), 20% acetone, and TEA 0.05 M (1.5
mmol) in 30 mL of TFE was nitrogen purged in quartz tubes and
irradiated at 310 nm for 50 h. The solvent was removed under vacuum
and the residue purified by column chromatography (eluant: neat
hexane).
4-Methylbiphenyl (26): colorless solid, 105 mg, 52% yield based on
the 80% consumption of 1 (after purification by column
chromatography, eluant: neat hexane), mp 41−43 °C (lit.43 mp 42−
43 °C). The spectroscopic data of compound 26 were in accordance
with the literature.44 Anal. Calcd for C13H12: C, 92.81; H, 7.19. Found:
C, 92.8; H, 7.2. The same irradiation has been carried out on a 0.025
M solution of 1 affording 71 mg of 26 (62% yield, 90% consumption
of 1).
4′-Butyl-2,4,6-trimethylbiphenyl (32): oil, 193 mg, 51% yield (after
purification by column chromatography, eluant: neat hexane); 1H
NMR (CDCl3) δ 7.30−7.05 (AA′BB′, 4H, J = 8 Hz), 6.95 (s, 2H),
2.75−2.65 (t, 2H, J = 8 Hz), 2.30 (s, 3H), 2.10 (s, 6H), 1.80−1.70 (m,
2H),1.50−1.40 (m, 2H), 1.05−0.95 (t, 3H, J = 7 Hz); 13C NMR
(CDCl3) δ 140.9, 139.0, 138.1, 136.3, 136.1, 129.0 (CH), 128.2 (CH),
127.9 (CH), 35.3 (CH2), 33.5 (CH2), 22.4 (CH2), 20.9 (CH3), 20.7
(CH3), 13.9 (CH3); IR (neat) ν/cm−1 2927, 1478, 1377, 1005, 850.
Anal. Calcd for C19H24: C, 90.42; H, 9.58. Found: C, 90.4; H, 9.6.
4′-Isopropyl-2,4,6-trimethylbiphenyl (33): oil, 179 mg, 50% yield
(after purification by column chromatography, eluant: neat hexane);
1H NMR (CDCl3) δ 7.35−7.10 (AA′BB′, 4H, J = 8 Hz), 7.00 (s, 2H),
3.05−2.95 (m, 1H), 2.40 (s, 3H), 2.10 (s, 6H), 1.40−1.35 (d, 6H, J =
7 Hz); 13C NMR (CDCl3) δ 146.8, 139.0, 138.2, 136.3, 129.0 (CH),
127.9 (CH), 126.2 (CH), 33.7 (CH), 24.0 (CH3), 20.9 (CH3), 20.7
(CH3); IR (neat) ν/cm−1 2960, 1611, 1479, 1056, 850, 833. Anal.
Calcd for C18H22: C, 90.70; H, 9.30. Found: C, 90.7; H, 9.3.
4′-tert-Butyl-2,4,6-trimethylbiphenyl (34): colorless solid, 150 mg,
61% yield based on the 65% consumption of 4 (after purification by
column chromatography, eluant: neat hexane); mp 108−110 °C; IR
(neat) ν/cm−1 2924, 2854, 1059, 1056, 854. The spectroscopic data of
34 were in accordance with the literature.50 Anal. Calcd for C19H24: C,
90.42; H, 9.58. Found: C, 90.4; H, 9.6.
(2′,4′,6′-Trimethylbiphenyl-4-yl)trimethylsilane (35): colorless
solid, 208 mg, 60% yield based on the 86% consumption of 5 (after
purification by column chromatography, eluant: neat hexane); mp 70−
72 °C; 1H NMR (CDCl3) δ 7.50−7.20 (AA′BB′, 4H, J = 8 Hz), 7.0 (s,
2H), 2.45 (s, 3H), 2.1 (s, 6H), 0.4 (s, 9H); 13C NMR (CDCl3) δ
141.3, 138.9, 138.1, 136.4, 135.9, 133.2 (CH), 128.5 (CH), 127.9
(CH), 20.9 (CH3), 20.7 (CH3), −1.1(CH3); IR (neat) ν/cm−1 2918,
1605, 1458, 1376, 1033, 849, 690. Anal. Calcd for C18H24Si: C, 80.53;
H, 9.01. Found: C, 80.5; H, 9.0.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra for compounds 6−11, 13−16, and
18−35; details of the calculations on intermediates. This
material is available free of charge via the Internet at http://
pubs.acs.org.
4-Butylbiphenyl (27): oil, 137 mg, 63% yield, based on the 69%
conversion of 2 (after purification by column chromatography, eluant:
neat hexane). The spectroscopic data of compound 27 were in
accordance with the literature.45 Anal. Calcd for C16H18: C, 91.37; H,
8.63. Found: C, 91.4; H, 8.4. The same irradiation has been carried out
on a 0.025 M solution of 2 affording 77 mg of 27 (49% yield).
4-Isopropylbiphenyl (28): oil, 136 mg, 55% yield based on the 84%
consumption of 3, (after purification by column chromatography,
eluant: neat hexane). The spectroscopic data of 28 were in accordance
with the literature.44 Anal. Calcd for C15H16: C, 91.78; H, 8.22. Found:
C, 91.8; H, 8.2. The same irradiation has been carried out on a 0.025
M solution of 3 affording 100 mg of 28 (68% yield).
AUTHOR INFORMATION
■
Corresponding Author
*Tel: +390382987316. Fax: +390382987323. E-mail: (M.F.)
fagnoni@unipv.it, (S.P.) prottistefano@gmail.com.
Notes
4-tert-Butylbiphenyl (29): colorless solid, 133 mg, 67% yield based
on the 63% consumption of 4, (after purification by column
chromatography, eluant: neat hexane), mp 45−47 °C (lit.46 mp 49−
51 °C). The spectroscopic data of compound 29 were in accordance
with the literature.46 Anal. Calcd for C16H18: C, 91.37.; H, 8.63.
Found: C, 91.4; H, 8.6. The same irradiation has been carried out on a
0.025 M solution of 4 affording 102 mg of 29 (72% yield, 90%
consumption of 4).
Biphenyl-4-yltrimethylsilane (30): colorless solid, 142 mg, 76%
yield based on the 55% consumption of 5 (after purification by column
chromatography, eluant: neat hexane), mp 48−50 °C, (lit.47 mp 52
°C). The spectroscopic data of compound 30 were in accordance with
the literature.47 Anal. Calcd for C15H18Si: C, 79.58; H, 8.01. Found: C,
79.6.; H, 8.0. The same irradiation has been carried out on a 0.025 M
solution of 5 affording 41 mg of 30 (48% yield, 50% consumption of
5).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.P. acknowledges MIUR, Rome (FIRB-Futuro in Ricerca 2008
project RBFR08J78Q), for financial support. This work has
been supported by the Fondazione Cariplo (Grant No. 2011-
1839). We thank Dr. Simone Lazzaroni (University of Pavia)
for preliminary experiments.
REFERENCES
■
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(25) A PET reaction between excited Ar-Cl and the nucleophile
1
could arise either from Ar-Cl or from 3Ar-Cl. The singlet has no role
since ISC in compounds 1−5 is very efficient and we did not observe a
significant quenching of the fluorescence of 1 in the presence of
3
ATMS. The ΔGET of the PET reaction between 1 and ATMS was
then calculated by means of the Rehm−Weller equation26a where E(1/
1
•−) = −2.29 V vs SCE26b,c and E(3Ar-Cl/Ar-Cl•−) = 1.30 eV (value
calculated from the phosphorescence spectrum of 126d). The E(R•+/
R) of the nucleophiles ranged from 1.50 V vs SCE (for ATMS) to 2.35
vs SCE (for benzene).27
I
dx.doi.org/10.1021/jo4007046 | J. Org. Chem. XXXX, XXX, XXX−XXX