The β effect of silicon in phenyl cations

Article abstract of DOI:10.1021/ja074778x

Irradiation of chloroanisoles, phenols, and N,N-dimethylanilines bearing a trimethylsilyl (TMS) group in the ortho position with respect to the chlorine atom caused photoheterolysis of the Ar-Cl bond and formation of the corresponding ortho-trimethylsilylphenyl cations in the triplet state. The β effect of silicon on these intermediates has been studied by comparing the resulting chemistry in alcoholic solvents with that of the silicon-free analogues and by computational analysis (at the UB3LYP/6-311+G(2d,p) level in MeOH). TMS groups little affect the photophysics and the photocleavage of the starting phenyl chlorides, while stabilizing the phenyl cations, both in the triplet (ca. 4 kcal/mol per group) and, dramatically, in the singlet state (9 kcal/mol). As a result, although triplet phenyl cations are the first formed species, intersystem crossing to the more stable singlets is favored with chloroanisoles and phenols. Indeed, with these compounds, solvent addition to give aryl ethers (from the singlet) competed efficiently with reduction or arylation (from the triplet). In the case of the silylated 4-chloro-N,N- dimethylaniline, the triplet cation remained in the ground state and trapping by π nucleophiles remained efficient, though slowed by the steric bulk of the TMS group. In alcohols, the silyl group was eliminated via a photoinduced protiodesilylation during the irradiation. Thus, the silyl group could be considered as a directing, photoremovable group that allowed shifting to the singlet phenyl cation chemistry and was smoothly eliminated in the same one-pot procedure.

Full text of DOI:10.1021/ja074778x

Published on Web 12/04/2007
The â Effect of Silicon in Phenyl Cations
Valentina Dichiarante, Andrea Salvaneschi, Stefano Protti, Daniele Dondi,
Maurizio Fagnoni,* and Angelo Albini
Department of Organic Chemistry, UniVersity of PaVia, Viale Taramelli 10, 27100 PaVia, Italy
Received June 29, 2007; E-mail: fagnoni@unipv.it
Abstract: Irradiation of chloroanisoles, phenols, and N,N-dimethylanilines bearing a trimethylsilyl (TMS)
group in the ortho position with respect to the chlorine atom caused photoheterolysis of the Ar-Cl bond
and formation of the corresponding ortho-trimethylsilylphenyl cations in the triplet state. The â effect of
silicon on these intermediates has been studied by comparing the resulting chemistry in alcoholic solvents
with that of the silicon-free analogues and by computational analysis (at the UB3LYP/6-311+G(2d,p) level
in MeOH). TMS groups little affect the photophysics and the photocleavage of the starting phenyl chlorides,
while stabilizing the phenyl cations, both in the triplet (ca. 4 kcal/mol per group) and, dramatically, in the
singlet state (9 kcal/mol). As a result, although triplet phenyl cations are the first formed species, intersystem
crossing to the more stable singlets is favored with chloroanisoles and phenols. Indeed, with these
compounds, solvent addition to give aryl ethers (from the singlet) competed efficiently with reduction or
arylation (from the triplet). In the case of the silylated 4-chloro-N,N-dimethylaniline, the triplet cation remained
in the ground state and trapping by π nucleophiles remained efficient, though slowed by the steric bulk of
the TMS group. In alcohols, the silyl group was eliminated via a photoinduced protiodesilylation during the
irradiation. Thus, the silyl group could be considered as a directing, photoremovable group that allowed
shifting to the singlet phenyl cation chemistry and was smoothly eliminated in the same one-pot procedure.
Scheme 1. â-Stabilizing Effect of Silicon on the Generation of (a)
Alkyl and Vinyl Cations and (b) Phenyl Cations; (c) Application to
the Case of Photogenerated Triplet Phenyl Cations
Introduction
Silicon-based substituents (e.g., the trimethylsilyl (TMS)
group) are well-known to stabilize a positive charge present on
a â position1 and to facilitate the formation of alkyl and vinyl
cations (Scheme 1a). Twenty years ago, this stabilization was
exploited by Sonoda et al. for the generation of an elusive
intermediate such as a phenyl cation in solution.2 Thus, although
the triflate of the 2-(trimethylsilyl)phenol was stable upon
heating in TFE in the presence of 2,6-lutidine at 150 °C, the
2,6-bis(trimethylsilyl)-substituted analogue gave, under the same
conditions, the corresponding trifluoroethyl ether quantitatively
(Scheme 1b).2,3 The success of this reaction was ascribed to
the hyperconjugative stabilization of the cation by the C-Si σ
bonds, as confirmed by more recent calculations,4 as well as to
the good leaving group.2,5
Dinitrogen is a better leaving group than triflate anion, and
both thermal and photochemical generation of phenyl cations
from diazonium salts are documented, although the competition
of radical paths may make the recognition of the mechanism
not obvious.6 Indeed, the effect of the TMS groups was
calculated to be so strong that the 2,6-bis(trimethylsilyl)phenyl
diazonium cation was predicted to decompose spontaneously
at room temperature.2,7a The focus of experimental and com-
putational studies has been to individuate a substituent that
(1) (a) Lambert, J. B. Tetrahedron 1990, 46, 2677-2689. (b) Lambert, J. B.;
Zhao, Y.; Emblidge, R. W.; Salvador, L. A.; Liu, X.; So, J.-H.; Chelius, E.
C. Acc. Chem. Res. 1999, 32, 183-190.
(2) Himeshima, Y.; Kobayashi, H.; Sonoda, T. J. Am. Chem. Soc. 1985, 107,
5286-5288.
(3) Phenyl triflate too was stable under harsh solvolytic conditions. See: (a)
Subramanian, L. R.; Hanack, M.; Chang, L. W.; Imhoff, M. A.; Schleyer,
P. v. R.; Effenberger, F.; Kurtz, W.; Stang, P. J.; Dueber, T. E. J. Org.
Chem. 1976, 41, 4099-4103. (b) Laali, K.; Szele, I.; Yoshida, K. HelV.
Chim. Acta 1983, 66, 1710-1720.
(4) Laali, K. K.; Rasul, G.; Prakash, G. K. S.; Olah, G. A. J. Org. Chem.
2002, 67, 2913-2918.
(5) Apeloig, Y.; Arad, D. J. Am. Chem. Soc. 1985, 107, 5285-5286.
(6) (a) Gasper, S.; Devadoss, C.; Schuster, G. B. J. Am. Chem. Soc. 1995,
117, 5206-5211. (b) Milanesi, S.; Fagnoni, M.; Albini, A. J. Org. Chem.
2005, 70, 603-610. (c) Zollinger, H. Diazo Chemistry; VCH: Weinheim,
Germany, 1994; Vol. 1. (d) Ussing, B. R.; Singleton, D. A. J. Am. Chem.
Soc. 2005, 127, 2888-2899.
(7) (a) Nicolaides, A.; Smith, D. M.; Jensen, F.; Radom, L. J. Am. Chem. Soc.
1997, 119, 8083-8088. (b) Aschi, M.; Harvey, J. N. J. Chem. Soc., Perkin
Trans. 2 1999, 1059-1061. (c) Hori, K.; Sonoda, T.; Harada, M.;
Yamazaki-Nishida, S. Tetrahedron 2000, 56, 1429-1436. (d) Ambroz, H.
B.; Kemp, T. J. Chem. Soc. ReV. 1979, 8, 353-365.
9
10.1021/ja074778x CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 15919-15926
15919

A R T I C L E S
Dichiarante et al.
Chart 1. Preparation of Silylated Phenyl Chlorides 1-4a
Results
Four silylated phenyl chlorides were studied in this work,
namely, 2-chloro-5-methoxyphenyltrimethylsilane (1), 2-chloro-
5-methoxy-1,3-bis-trimethylsilanylbenzene (2), 4-chloro-3-tri-
methylsilanylphenol (3), and (4-chloro-3-trimethylsilanylphenyl)-
dimethylamine (4) (Chart 1). Silanes 1-4 were prepared from
the corresponding phenyl halides via coupling of the Grignard
reagent with trimethylsilylchloride (TMSCl) as illustrated in
Chart 1.
Preparative Irradiation. Compounds 1-4 were irradiated
in protic solvents (methanol, 2,2,2-trifluoroethanol (TFE), and
water/acetonitrile mixtures) (i.e., under the conditions found to
favor the heterolytic dissociation of the Ar-Cl bond with the
silicon-free analogues).9 The experiments were carried out both
in neat solvents and in the presence of a π nucleophile, either
an alkene (allyltrimethylsilane, ATMS) or an aromatic (ben-
zene). Since acidity was liberated in the reaction, an equivalent
amount of a buffering agent such as cesium carbonate was added
in some cases. This avoided undesired thermal desilylation either
of the starting compounds or of the end products, which
otherwise took place to some extent. Most of the irradiations
were carried out until a high or complete consumption of the
starting material was reached, but irradiations at shorter times
were also performed to check for the occurrence of secondary
processes. The product mixture was analyzed both as such and
after full desilylation by treatment with Bu4NF.
Compounds 1-4 showed a very weak fluorescence (ΦF <
0.05) both in MeCN and in TFE solutions, similar to that of
the corresponding silicon-free analogues.11,12 Phosphorescence
at 77 K in glass was likewise similar in shape and somewhat
less intensive.
Photochemistry of 2-Chloro-5-methoxyphenyltrimethyl-
silane (1). Irradiation of compound 1 (0.05 M) in all of the
solvents tested caused an efficient dechlorination (Table 1). In
MeOH in the presence of Cs2CO3, photoreduction to form
anisole 5 and, mainly, solvolysis to give dimethoxy derivative
7 took place. Prolonged irradiation (up to 16 h) caused a partial
desilylation of 7 to give 7a (Scheme 2). In the presence of
benzene (1 M), some arylation took place and biphenyl 12a
(10%) was formed, though photosubstitution remained by far
the main path. Irradiation for a shorter time evidenced the
formation of (4-methoxybiphenyl-2-yl)trimethylsilane 12 as an
intermediate (Scheme 3).
a (a) TMSCl, Mg, THF reflux; (b) SO2Cl2, CHCl3, room temperature
60% for the two steps; (c) BBr3, CH2Cl2 -78 f 25 °C 2h, 54%; (d) NaNO2,
HCl, 0 °C, then CuCl, 52%; (e) SnCl2, ethanol/ethylacetate 5:2, 63%; (f)
NaBH4, CH2O, 51%.
would make generation of such an unstable species possible,
with little interest in the ensuing reactions, which were actually
limited to solvolysis.
As a matter of fact, the synthetic application of phenyl cations
has been little explored because general and simple methods
for their thermal generation are not available.8 Recently,
however, there has been an increased interest in the photoin-
duced generation of this intermediate, and a novel chemistry
has been revealed. The reactions observed depend on which
spin state of the cation is formed, and this in turn depends on
the precursor and on the mode of generation. To summarize,
phenyldiazonium ion fragments and gives either the singlet or
the triplet phenyl cation, according to the substituents, while
the irradiation of electron-donating substituted phenyl chlorides,
phosphate, or sulfonate esters in a polar solvent offers a
straightforward access to triplet phenyl cations (Scheme 1c).9
The singlet is a localized cation with a vacant σ orbital at the
dicoordinated carbon atom,7b while the triplet state has a
diradical character with single occupancy of the σ orbital and
the charge delocalized on the aromatic ring.4,7
Singlet phenyl cations are unselective electrophiles, and
reaction via this path mostly results in solvolysis, forming Ar-O
or Ar-N bonds when the reaction is carried out in alcohols or
acetonitrile, respectively.6,10 Triplet phenyl cations react selec-
tively with π nucleophiles (olefins and aromatics) forming new
Ar-C bonds.9 With this large scope access, it becomes feasible
to study the effect a silyl group has on the chemistry of the
phenyl cation, in addition to that on its formation.
In TFE, reduction to give anisoles 5 and 5a (the latter
desilylated) took place along with solvolysis to trifluoroethyl-
ether 9a. The silicon containing ether 9 was detected at a shorter
irradiation time (Scheme 2). With 1 M benzene, 12a was isolated
in a satisfactory yield (60%) at the expense of the other
photoproducts (Scheme 3). Furthermore, allylated derivatives
14 and 14a were obtained in a poor yield in the presence of
ATMS (0.5 M).
In water/acetonitrile (1:5) mixture, acetanilide 15 (67% yield)
was the main product (accompanied by 5) and the product
distribution did not change with 1 M benzene (Scheme 2).
Photochemistry of 2-Chloro-5-methoxy-1,3-bis-trimethyl-
silanylbenzene (2). Chloride 2 (0.025 M, a saturated solution)
As a contribution toward this issue, we report here the
photocleavage of some phenyl chlorides that are structurally
related to the above-mentioned thermally labile phenyl triflates
(one or two TMS group(s) in ortho with respect to the leaving
group; Chart 1). The comparison of the photochemistry of these
phenyl chlorides with that of the silicon-free analogues allows
assessing the â effect of silicon.
(8) Stang, P. J. In Dicoordinated Carbocations; Rappoport, Z., Stang, P. J.,
Eds.; Wiley: New York, 1997; pp 451-460.
(9) (a) Fagnoni, M.; Albini, A. Acc. Chem. Res. 2005, 38, 713-721. (b)
Fagnoni, M. Lett. Org. Chem. 2006, 3, 253-259.
(11) Manet, I.; Monti, S.; Fagnoni, M.; Protti, S.; Albini, A. Chem.-Eur. J.
2005, 11, 140-151.
(10) Dichiarante, V.; Dondi, D.; Protti, S.; Fagnoni, M.; Albini, A. J. Am. Chem.
Soc. 2007, 129, 5605-5611.
(12) Freccero, M.; Fagnoni, M.; Albini, A. J. Am. Chem. Soc. 2003, 125, 13182-
13190.
9
15920 J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007

â
Effect of Silicon in Phenyl Cations
A R T I C L E S
Table 1. Photolysis of 1 (0.05 M) and 2 (0.025 M) in the Presence of Various Nucleophilesa
products (yield %)b
substituted
aryl chloride
solvent
nucleophile
tirr, h (conversion, %)
reduced
arylated
1
MeOH
none
9 (78)
16 (100)
5 (19)
5 (28)
7 (48)
7 (55)
7a (15)
7 (46)
9a (44)
1
1
MeOH
TFE
C6H6 1 M
none
6 (78)c
7 (100)
5 (20)
5 (23)
5a (18)
5a (3)
5 (22)
12a (10)
1
1
TFE
TFE
C6H6 1 M
ATMS 0.5 M
7 (100)
12 (100)
9a (25)
9a (53)
12a (60)
14 (11)
14a (8)
1
1
2
2
H2O/MeCN 1:5d
H2O/MeCN 1:5d
MeOH
none
C6H6 1 M
none
23 (90)
20 (86)
16 (100)
1 (100)c
5 (21)
5 (14)
15 (67)
15 (65)
8 (51)
9a (3.5)
10 (94)
9a (100)
10 (73)
9a (4)
TFE
none
8 (100)
1.5 (100)
2 (88)c
2
2
TFE
TFE
C6H6 1 M
ATMS 0.5 M
10 (75)
9a (38)
10 (23)
4 (100)c
a Cesium carbonate (0.025 M for 1 and 0.0125 M for 2) added. b Yields were based on the initial amount of reagent and determined by GC. c Sample
treated with Bu4NF before analysis. d No base added.
Scheme 2. Irradiation of Aryl Halides 1-3 in Neat Solvents
Scheme 3. Irradiation of Aryl Halides 1-3 in the Presence of π
Nucleophiles
6 exclusively in MeOH and mainly ether 11a in TFE. Biphenyl
13a was a minor product (8% yield) in the presence of benzene,
where at low conversion silylated 13 was also detected.
Photochemistry of (4-Chloro-3-trimethylsilanylphenyl)-
dimethylamine (4). Aniline 4 (Table 2) gave silylated aniline
16, along with a small amount of 16a in MeOH, with no change
in the presence of benzene (Scheme 4). In TFE, 16 and 16a
were likewise formed, although in a lower yield, but in this
solvent arylation took place to a small extent with benzene (18a)
and in a good yield with ATMS (17a, 62%). Again, aniline 16
was the only product in MeCN/H2O mixed solvents. Notewor-
thy, no solvolysis took place upon irradiation of aniline 4,
contrary to the case of anisoles and phenols 1-3.
Steady State Measurements. The decomposition quantum
yields (Φ-1) of the silyl derivatives 1-4 (5 × 10-3 M) were
determined by irradiation in MeOH and TFE at 254 nm. The
results are gathered in Table 3. The Φ-1 values ranged from
0.1 to 0.3 in MeOH and 0.2 to 0.4 in TFE. The solvent effect
on quantum yields was small, except that in the case of 1, where
the value in TFE was three times as much as that in MeOH. In
Table 3, these values are compared to those measured for the
was completely consumed within 5 h when irradiated in neat
MeOH, and the disilylated derivative 8 was formed as the only
isolated product in 51% yield (Table 1, Scheme 2). In TFE,
irradiation caused likewise solvolysis of the chlorine atom and
both 10 and 9a were formed (10 was completely converted to
9a upon prolonged irradiation). Solvolysis to 10 remained the
main process, in the presence of either 1 M benzene or ATMS
with no competing C-arylation, and also in this case full
desilylation occurred upon doubling the irradiation time from
2 to 4 h.
Photochemistry of 4-Chloro-3-trimethylsilanylphenol (3).
Photolysis of compound 3 in alcohols (Table 2) gave reduced
9
J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007 15921

A R T I C L E S
Dichiarante et al.
Table 2. Photolysis of 3 and 4 (0.05 M) in the Presence of Various Nucleophilesa
products (yield %)b
substituted
aryl chloride
solvent
nucleophile
tirr, h (conversion, %)
reduced
arylated
3
3
MeOH
TFE
none
none
4 (100)
20 (100)
6 (97)
6 (13)
6a (9)
6 (9)
11a (52)
11a (38)
3
4
4
4
TFE
C6H6 1 M
none
20 (100)
1 (100)
1.5 (100)
1 (65)
13a (8)
6a (6)
MeOH
MeOH
TFE
16 (86)
16a (12)
16 (87)
16a (6)
16 (33)
16a (7)
16 (40)
16a (10)
16a (10)
16 (16)
16a (32)
16 (60)
C6H6 1 M
none
3 (90)
4
4
TFE
TFE
ATMS 0.5 M
C6H6 1 M
6 (100)
17a (62)
18a (7)
3 (100)c
4
H2O/MeCN 1:5d
none
18 (100)
a -d See Table 1.
Scheme 4. Irradiation of Aniline 4
Table 3. Quantum Yield of Photodecomposition (Φ-1) of Phenyl
Trimethylsilanes 1-4 in Alcohols
a
quantum yields (Φ-
)
1
Ar−SiMe3
MeOH
TFE
1
2
3
4
0.13 (0.1)b
0.27 (0.1)b
0.43 (0.07)b
0.29 (0.07)b
0.20 (0.46)c
0.30 (0.5)d
0.17 (0.66)c
0.32 (0.95)d
Figure 1. Geometries, Mulliken atomic charges, and spin density (in
parentheses) on the Cl atom calculated in MeOH bulk at the CPCM-
a In parentheses, the quantum yield of the silicon-free analogue. b From
ref 10. c This work. d From ref 13.
3
UB3LYP/6-311+G(2d,p)//UB3LYP/6-311+G(2d,p) level for (a) 1 (Ar-
Cl bond: 1.88 Å) and (b) the same upon stretching the Ar-Cl bond up to
4.00 Å.
corresponding silicon-free aryl chlorides. The effect caused by
introducing a silyl group was not uniform, with an enhancement
with anisoles 1 and 2 and a marked decrease with chlorophenol
3 and aniline 4.
Calculations. Further insight was sought by using DFT
calculations, known from previous studies to be suitable for the
generation and chemistry of phenyl cations.13 The geometries
of the intermediates studied were optimized by using a B3LYP
functional at the 6-311+G(2d,p) level (Supporting Information).
Frequency calculations were performed at the same level of
theory to check the minima. The solvent effect was evaluated
by carrying out single-point calculations on the optimized
geometries in vacuo through the CPCM polarizable conductor
calculation model14 by using methanol as solvent.
The properties of the triplet state of the precursor phenyl
chlorides were examined for the case of 31 (in MeOH). Figure
1 shows the lowest-energy geometry of this state. As one may
see, the Ar-Cl bond is virtually cleaved (1.88 Å), C4 sticks
out of the molecule plane (C4-C3-C2-C1 dihedral angle of
9°), and the C4-Cl bond is further bent upward by an angle of
31° with the C4-C5-C3 plane. This state cleaves heterolytically
(14) (a) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995-2001. (b)
Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003,
24, 669-681.
(13) Guizzardi, B.; Mella, M.; Fagnoni, M.; Freccero, M.; Albini, A. J. Org.
Chem. 2001, 66, 6353-6363.
9
15922 J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007

â
Effect of Silicon in Phenyl Cations
A R T I C L E S
1
+
Figure 2. Relative energies of phenyl cations 19-21 by taking C6H5 ) 0.
as indicated by the fact that, stretching the C4-Cl bond up to
4 Å, the chlorine atom acquired a full negative charge (-0.914)
and only a 4% spin density.
The energy of the phenyl cations (19+-21+) was then
calculated in MeOH for both the singlet and the triplet states.
The isodesmic reaction in eq 1 was used to quantify the
stabilization of the cation by both the ortho-TMS group and
the para electron-donating group with respect to the parent
singlet phenyl cation taken as the reference. The results are
graphically shown in Figure 2, along with the data for the
silicon-free cations (for details, see Supporting Information). It
is apparent that the stabilization induced by the TMS group is
dramatic and often larger than that exerted by the groups in
para. It is noteworthy that the effect of the TMS is considerably
greater for singlet than for triplet states. Thus, the 4-methox-
yphenyl cation (19a+) is less stable than the parent cation in
both the singlet and the triplet states, with the singlet slightly
more stable. Introduction of a silyl group (cation 19+) brings
about a stabilization of 11.1 kcal/mol for the singlet and 4.3
kcal/mol for the triplet, so that the former is now by far (ca. 9
kcal/mol) the lowest state. A second TMS group as in the case
of 20+ further increases the singlet-triplet gap (∆EST ) -14.7
kcal/mol), and 120+ is stabilized by 17.1 kcal/mol with respect
to 1C6H5+. Analogously, with the aminophenyl cation 21+,13,15
introduction of a silyl group stabilized the singlet with respect
to 21a+, although the triplet remained the lowest state. To
summarize, each silyl group stabilized the triplet by 3-4 kcal/
mol and the singlet by 9 kcal/mol.
Figure 3. Stationary points on the PES for the addition of 319+ (X )
3
OMe) and 21+ (X ) NH2) to ethylene.
about 30 kcal/mol below the reactants, was found for both 19+
and 21+ (E1 ) 1.0 and 2.0 kcal/mol, respectively), and a
transition state was located on the PES. In the case of singlet
cations, a strongly stabilized (60 kcal/mol) closed shell adduct
cation (a phenonium ion) was formed through a barrierless
process.
Discussion
Photogeneration of the Phenyl Cation. According to the
literature, the effect of a TMS group on the photophysics of
aromatics is varied.17 With polynuclear derivatives such as
silylated naphthacenes,18a pyrenes,18b and anthracenes,18c
a
marked enhancement of both fluorescence and phosphorescence
quantum yield (proportional to the number of the silyl groups)
was observed. However, in other cases (e.g., for porphyrin
derivatives), both the fluorescence and phosphorescence quan-
tum yield did not appreciably change.19 Closer to the compounds
considered here, the photophysical parameters of trimethyl-p-
tolylsilane did not significantly differ from those of toluene,
Calculations on the Cation Reactivity. The same approach
was used for predicting the reactivity of phenyl cations 19+
and 21+ with π nucleophiles, by calculating the potential energy
surface (PES) for the reaction with ethylene (Figure 3).16 In
the case of the triplets, a path leading to a distonic radical cation,
(17) For a review on organosilane photochemistry, see: Steinmetz, M. G. Chem.
ReV 1995, 95, 1527-1588.
(18) (a) Kyushin, S.; Ishikita, Y.; Matsumoto, H.; Horiuchi, H.; Hiratsuka, H.
Chem. Lett. 2006, 64-65. (b) Maeda, H.; Inoue, Y.; Ishida, H.; Mizuno,
K. Chem. Lett. 2001, 1224-1225. (c) Kyushin, S.; Ikarugi, M.; Goto, M.;
Hiratsuka, H.; Matsumoto, H. Organometallics 1996, 15, 1067-1070.
(19) Horiuchi, H.; Tanaka, T.; Yoshimura, K.; Sato, K.; Kyushin, S.; Matsumoto,
H.; Hiratsuka, H. Chem. Lett. 2006, 662-663.
(15) 4-Amino-3-trimethylsilylphenyl cation was used as a model for 4-N,N-
dimethylamino-3-trimethylsilylphenyl cation.
(16) Mella, M.; Coppo, P.; Guizzardi, B.; Fagnoni, M.; Freccero, M.; Albini,
A. J. Org. Chem. 2001, 66, 6344-6352.
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A R T I C L E S
Dichiarante et al.
Scheme 5. Overall Mechanism in the Irradiation of 1-4
Table 4. Comparison of the Product Distribution between
4-Chloroanisole and Phenyl Chlorides 1, 2, and 4 upon Irradiation
in Alcohols
reduction/substitution
arylation/reduction
+
substitution
MeOH
(neat)
TFE
TFE
TFE
aryl chloride
(neat)
(benzene 1 M)
(ATMS 0.5 M)
4-chloroanisole
78/22a
29/71
0/100
100/0
73/27a
48/52
0/100
100/0
92/8a
93/7a
23/77
0/100
86/14
1
2
4
68/32 (0/100)b
0/100
14/86
a From ref 21. b In MeCN/water 5:1 as the solvent.
except for a slight red shift of the fluorescence and phospho-
rescence spectrum.20
As a matter of fact, compounds 1-4 exhibited both fluores-
cence and phosphorescence similar to those of the corresponding
silicon-free aromatic chlorides. It is reasonable to think that ISC
is as efficient as with the parent phenyl chlorides (ΦISC g 0.7)
and their photoreaction (Φ-1 ) 0.13 to 0.43) in polar media
proceeds from the triplet. Under these conditions, the process
taking place is heterolytic fragmentation, as supported by DFT
calculations in MeOH. This evidences that cleavage of 31
localizes the charge on the leaving Cl anion (Figure 1). It can
be thus safely concluded that the phenyl cation is generated in
the triplet state.
In parallel, the diagnostic triplet trapping by π nucleophiles
is complete with chloroanisole, partial with 1 and insignificant
with 2.
With the silylated derivatives, trapping by π nucleophiles is
3
intrinsically disfavored by the TMS group and 19+ is in part
reduced. Figure 3 shows that triplet addition to ethylene to form
a diradical adduct that then evolves to the arylated product is
3
viable for the silylated cation. However, in contrast to 19a+
where the process is barrierless, a transition state was detected
Although the silyl group affects the efficiency of the
fragmentation, the process remain qualitatively the same, and
any difference in the photoproduct distribution from phenyl
halides 1-4 occurring upon introduction of TMS group(s) must
be related to an effect on the cation rather than on the
photochemical step. The main effect is on the energy of such
an intermediate. The trimethylsilyl group stabilizes the singlet
much more than the triplet (Figure 2) so that the former state is
9 and 17 kcal/mol below the latter in cations 19+ and 20+,
respectively, whereas the triplet remains the lowest state with
the aminophenyl cation 21+, although the gap decreases.
3
3
with 19+ and 21+ (1.0 and 2.0 kcal/mol, respectively). This
can be reasonably attributed to the steric effect of the TMS
group. Notice that addition to ethylene remains barrierless for
the silylated singlets (Figure 3, right part), although, due to the
unselective nature of these states, arylation via the singlet does
not compete with solvolysis.
Some effect of the medium can be noticed. A more nucleo-
philic medium facilitates solvolysis both vs reduction (compare
the experiments in neat alcohols, where solvolysis has a greater
role in MeOH with respect to TFE) and vs arylation of benzene
(see the results for 1 in MeCN-H2O with respect to TFE, Table
4).
Effect of the TMS Group on the Chemistry of Phenyl
Cations. Previous evidence shows that the main fate of triplet
phenyl cation is reduction either by H transfer from the solvent
or by electron transfer from the starting aryl chloride, while
the singlet adds nucleophilic solvents.9,13,16,21 The triplets are
further distinguished by the selective attack to π nucleophiles.
The effect of Si is apparent in Table 4, where the behavior of
silylated and non-silylated phenyl chlorides is compared.
As hinted above, the picture does not apply to the irradiation
3
of aniline 4. Triplet cation 21+ is stabilized by 3.5 kcal/mol
with respect to the silicon-free cation, but this is not sufficient
3
for reversing the order of the states. Triplet 21+ remains the
ground state cation, and reduction is the only path in neat
solvents (Table 4). In the presence of a π nucleophile trapping
occurs, inefficiently with benzene but in a good yield with
ATMS. Apparently, steric hindrance by the TMS group inhibits
the reaction at least with the former nucleophile (note the barrier
in this process as shown in Figure 3).
The products distribution from 2-trimethylsilyl-4-hydrox-
yphenyl cation 22+ was similar to that obtained from 20+ in
TFE, although the material balance was less satisfactory in the
presence of π traps, while in MeOH reduction highly predomi-
nated. Deprotonation of 22+ to yield a 4-oxocyclohexa-2,5-
dienylidene carbene11 might take place in this case, and part of
the product might arise from that intermediate.
In the case of chloroanisole, where the singlet is slightly
below the triplet, reduction predominates 3 to 1 over solvolysis.
This can be attributed to the initial formation of the triplet 19a+
that in part is reduced by reaction with the solvent and in part
intersystem crosses to the singlet forming ethers by solvent
addition.10,21 The sequential introduction of TMS groups makes
solvolysis as important as reduction (with one group) and the
sole process (with two groups), in accord with the fact that in
silylated phenyl cations the singlet is strongly stabilized and
the large exothermicity arguably increases the role of ISC.22
Summing up, the aryl-Cl cleavage in polar media is little
affected and proceeds from the triplet as in the silicon-free
analogues, thus giving the triplet phenyl cation (Scheme 5). The
stabilization of the cation by the TMS group is much larger for
the singlets, however. With the 4-methoxyphenyl cation, this
makes the singlet by far the more stabilized state and the
(20) Shizuka, H.; Obuchi, H.; Ishikawa, M.; Kumada, M. J. Chem. Soc., Faraday
Trans. 1 1984, 80, 383-401.
(21) Protti, S.; Fagnoni, M.; Mella, M.; Albini, A. J. Org. Chem. 2004, 69,
3465-3473.
(22) Harvey’s calculations23 support that with phenyl cation the barrier to ISC
is negligible when the process is strongly exothermic.
(23) Harvey, J. N.; Aschi, M.; Schwarz, H.; Koch, W. Theor. Chem. Acc. 1998,
99, 95-99.
9
15924 J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007

â
Effect of Silicon in Phenyl Cations
A R T I C L E S
Scheme 6. Photoinduced Protiodesilylation of Substituted Phenyl
Trimethylsilanes
6). As an example, introduction of a TMS group makes the
preparation of phenyl ethers from phenyl chlorides easy, and
under the same conditions the TMS group is removed.
Experimental Section
General Procedure for the Photochemical Reactions of 1-4. A
solution of the starting chlorides 1-4 (0.025-0.05 M) and an equivalent
amount of Cs2CO3 (when required) was placed in quartz tubes, flushed
with nitrogen, and irradiated. The solvent was removed from the
photolyzed solutions, and the residue was purified by column chro-
matography (cyclohexane/ethyl acetate mixtures as eluant). Trapping
experiments were carried out by adding the selected nucleophile (0.5-
1.0 M) to the initial mixture.
Irradiation of 1 in Methanol. A solution containing 322 mg (1.5
mmol, 0.05 M) of 1 and 244 mg (0.75 mmol, 0.025 M) of Cs2CO3 in
30 mL of MeOH was irradiated for 16 h. Purification by column
chromatography (cyclohexane as the eluant) afforded 80 mg of 5
(colorless oil, 30% yield), 22 mg of dimethoxybenzene (7a, 11% yield),
and 88 mg of (2,5-dimethoxyphenyl)trimethylsilane (7, colorless oil,
28% yield).
chemical reactions involve that intermediate. With the 4-ami-
nophenyl cation, the triplet remains the ground state and the
chemistry observed arises from it.
Loss of the TMS Group. An important point is that, as
shown in Tables 1 and 2, irradiation led to partial or complete
desilylation concurrently with reaction at the chlorinated
carbon. Elimination of the Me3Si+ is due to a photoinduced
protiodesilylation process in analogy to the previously reported
photodesilylation of 2-trimethylsilyl-1,3-dimethoxybenzene in
1,1,1,3,3,3-hexafluoroisopropanol.24 This process is another
manifestation of the â-silyl effect in the stabilization of the
cyclohexadienyl cation (Scheme 6). Note that the protiodesi-
lylation took place also thermally, but only in more acidic
media.25
With the present compounds 1-4, neither dechlorination nor
desilylation was observed in blank experiments (quartz tubes
covered with aluminum foil during the irradiation). However,
if the base was omitted, the acidity (HCl) liberated upon
photolysis caused a thermal desilylation. On the other hand,
when there was irradiation in the presence of a buffering agent
(e.g., cesium carbonate or even water), desilylation took place
as a secondary photochemical process. The proportion of the
desilylation depended on the group replacing the chlorine atom
in the reaction. Thus, a trimethylsilyl group in ortho either to a
methoxy or to a trifluoroethoxy group was rapidly lost, whereas
with C-substituted (allyl, phenyl) desilylation was slower, and
it was noteworthy that ortho-trimethylsilyl acetanilides were
totally recalcitrant to (photo)desilylation.
1
5: H NMR (CDCl3) δ: 0.35 (s, 9H), 3.90 (s, 3H), 6.90-6.95 (m,
1H), 7.10-7.20 (m, 2H), 7.30-7.40 (m, 1H); 13C NMR (CDCl3) δ:
-1.6 (CH3), 54.6 (CH3), 113.4 (CH), 118.5 (CH), 125.1 (CH), 128.5
(CH), 141.7, 158.5; IR (neat) ν/cm-1: 838, 1046, 1228, 1247, 1409,
1570, 2955; MS (m/z): 180 (M+, 20), 165 (100), 135 (25), 121 (5),
107 (5), 91 (10), 77 (5), 51 (2), 43 (5); Anal. Calcd for C10H16OSi: C,
66.61; H, 8.94. Found: C, 66.7; H, 8.8.
1
7: H NMR (CD3COCD3)26 δ: 0.25 (s, 9H), 4.75 (s, 3H), 4.80 (s,
3H), 6.80-6.95 (m, 3H); 13C NMR (CD3COCD3) δ: -0.7 (CH3), 56.2
(CH3), 56.4 (CH3), 112.0 (CH), 116.0 (CH), 122.0 (CH), 129.8, 155.0,
159.9; IR (neat), υ/cm-1: 837, 1221, 1480, 2953, 3421; Anal. Calcd
for C11H18O2Si: C, 62.81; H, 8.63. Found: C, 62.9; H, 8.5.
Irradiation of 1 in TFE in the Presence of ATMS. Quantities of
322 mg (1.5 mmol, 0.05 M) of 1, 244 mg (0.75 mmol, 0.025 M) of
Cs2CO3, and 2.4 mL (15 mmol, 0.5 M) of ATMS in 30 mL of TFE
were irradiated for 7 h. Purification by column chromatography (neat
cyclohexane) afforded 14 mg of 5 (colorless oil, 6% yield) and a mixture
of 20 mg of 2-allyl-5-methoxyphenyltrimethylsilane (14, 7% yield) and
30 mg of 14a (16%).
1
14: H NMR (CDCl3, from the mixture) δ: 0.35 (s, 9H), 3.50 (d,
2H, J ) 6.3 Hz), 3.80 (s, 3H), 5.00-5.15 (m, 2H), 5.90-6.05 (m,
1H), 6.80-6.85 (m, 2H), 7.05 (d, 1H, J ) 2.9 Hz); 13C NMR (CDCl3,
from the mixture) δ: 0.1 (CH3), 39.1 (CH2), 55.0 (CH3), 113.4, 115.5
(CH2), 120.3 (CH), 125.4, 130.2 (CH), 132.0 (CH), 138.2 (CH), 157.0;
IR (from the mixture) υ/cm-1: 837, 989, 1245, 1512, 1610, 2951.
Spectroscopic data of compound 14a in accordance with the
literature.27
Conclusions
The â-silicon effect individuated two decades ago as a tool
for the thermal generation of phenyl cations due to the large
stabilization of the singlet state is of general application. In
particular, this has been extended to a series of 2-trimethylsi-
lylphenyl cations bearing an electron-donating group in 4 that
are photochemically generated from the corresponding phenyl
chlorides and exhibit a dramatic stabilization of the singlet, much
less of the triplet state. In the most clear-cut cases, this leads to
interchange of the reactiVe spin state of the cation and
substitution of the singlet chemistry (solvolysis, and thus O- or
N-arylation) for the triplet chemistry (reduction or C-arylation).
Since (photo)desilylation then occurs smoothly, one may think
of the silyl group as a directing, photoremoVable group (Scheme
Irradiation of 1 in MeCN/Water 5:1. A quantity of 322 mg (1.5
mmol, 0.05 M) of 1 in 30 mL of MeCN/water 5:1 was irradiated for
23 h (90% of 1 consumption). Purification by column chromatography
(cyclohexane/ethyl acetate from 99:1 to 7:3 as the eluant) afforded 57
mg of 5 and 238 mg of 2-trimethylsilyl-4-methoxyacetanilide (15,
colorless oil, 67% yield based on consumed 1).
1
15: H NMR (DMSO-d6) δ: 0.25 (s, 9H), 2.00 (s, 3H), 3.80 (s,
3H), 6.90-7.05 (m, 3H), 9.10 (bs, 1H); 13C NMR (DMSO-d6) δ: -0.7
(CH3), 22.9 (CH3), 55.1 (CH3), 114.5 (CH), 119.6 (CH), 129.9 (CH),
135.2, 138.6, 157.0, 169.2; IR (neat) ν/cm-1: 838, 1041, 1475, 1651,
2930, 3226; Anal. Calcd for C12H19NO2Si: C, 60.72; H, 8.07. Found:
C, 61.0; H, 7.6.
Irradiation of 2 in Methanol. A quantity of 216 mg (0.75 mmol,
0.025 M) of 2 in 30 mL of MeOH was irradiated for 5 h. Purification
(24) Lew, C. S. Q.; McClennan, R. A. J. Am. Chem. Soc. 1993, 115, 11516-
11520.
(26) Crowther, G. P.; Sundberg, R. J.; Sarpeshkar, A. M. J. Org. Chem. 1984,
49, 4657-4663.
(27) Gomes, P.; Gosmini, C.; Pe´richon, J. Org. Lett. 2003, 5, 1043-1045.
(25) Benkenser, R. A.; Krysiak, H. R. J. Am. Chem. Soc. 1953, 75, 4528-
4531.
9
J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007 15925

A R T I C L E S
Dichiarante et al.
by column chromatography (neat cyclohexane as the eluant) yielded
78 mg of 2,5-dimethoxy-1,3-bis(trimethylsilyl)benzene (8, colorless oil,
37% yield).
Hz), 115.7 (CH), 116.1 (CH), 122.9 (CF3, q, J ) 276 Hz), 150.5, 151.0;
IR (neat) υ/cm-1: 1076, 1163, 1512, 1666, 3332; m/z (%): 192 [M+]
(100), 109 (80), 81 (55), 63 (5), 53 (15); Anal. Calcd for C8H7F3O2:
C, 50.01; H, 3.67. Found: C, 50.2; H, 3.65.
1
8: H NMR (CDCl3) δ: 0.35 (s, 18H), 3.70 (s, 3H), 3.85 (s, 3H),
7.00 (s, 2H); 13C NMR (CDCl3) δ: -0.0 (CH3), 55.4 (CH3), 63.6 (CH3),
121.7 (CH), 132.9, 154.8, 164.9; IR (neat) υ/cm-1: 837, 1207, 1247,
1379, 1577, 2953; Anal. Calcd for C14H26O2Si2: C, 59.52; H, 9.28.
Found: C, 59.6; H, 9.1.
Irradiation of 4 in TFE in the Presence of ATMS. Quantities of
342 mg of 4 (1.5 mmol, 0.05 M), 2.4 mL of ATMS (15 mmol, 0.5 M),
and 244 mg (0.75 mmol, 0.025 M) of Cs2CO3 in 30 mL of TFE were
irradiated for 6 h. Purification by column chromatography (cyclohexane/
ethyl acetate 99:1 as the eluant) afforded 150 mg of 17a (colorless oil,
62% yield). Spectroscopic data of compound 17a in accordance with
literature data.6b MS (m/z): 161 (M+, 100), 160 (85), 145 (11), 134
(39), 117 (23), 91 (12).
Irradiation of 2 in TFE in the Presence of Benzene. Quantities
of 216 mg (0.75 mmol, 0.025 M) of 2, 122 mg (0.375 mmol, 0.0125
M) of Cs2CO3, and 3 mL (30 mmol, 1 M) of benzene in 30 mL of
TFE were irradiated for 2.5 h. Purification by column chromatography
(cyclohexane as the eluant) afforded 192 mg of 2-(2,2,2-trifluoroet-
hoxy)-5-methoxy-1,3-bis(trimethylsilyl)benzene (10, colorless oil, 73%
yield).
10: 1H NMR (CD3COCD3) δ: 0.40 (s, 18H), 3.80 (s, 3H), 4.30 (q,
2H, J ) 8.6 Hz), 7.05 (s, 2H); 13C NMR (CD3COCD3) δ: 0.4 (CH3),
56.0 (CH3), 73.0 (CH2, q, J ) 33 Hz), 123.5 (CH), 124.9 (CF3, q, J )
276 Hz), 134.2, 157.3, 162.4; IR (neat) υ/cm-1: 839, 1073, 1163, 1280,
1381, 1576, 2955; Anal. Calcd for C15H25F3O2Si2: C, 51.40; H, 7.19.
Found: C, 51.5; H, 7.1.
Acknowledgment. Partial support of this work by MIUR,
Rome, is gratefully acknowledged. We acknowledge Consorzio
Interuniversitario Lombardo per l’Elaborazione Automatica
(CILEA) for the free access to their supercomputer facilities.
Supporting Information Available: Details of the preparation
of compounds 1-4, 6, 16; NMR spectra of compounds 1-8,
10, 11a, 15, 16; and details of the calculations on intermediates.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Irradiation of 3 in TFE. Quantities of 300 mg (1.5 mmol, 0.05 M)
of 3 and 244 mg (0.75 mmol, 0.025 M) of Cs2CO3 in 30 mL of TFE
were irradiated for 18 h. Purification by column chromatography
(cyclohexane/ethyl acetate 95:5 as the eluant) yielded 108 mg of 11a
(pale yellow oil, 38% yield).
JA074778X
1
11a: H NMR (CDCl3)28 δ: 4.30 (q, 2H, J ) 8.2 Hz), 5.10 (bs,
(28) Endo, Y.; Shudo, K.; Okamoto, T. J. Am. Chem. Soc. 1982, 104, 6393-
1H), 6.75-6.90 (m, 4H); 13C NMR (CDCl3) δ: 66.5 (CH2, q, J ) 35
6397.
9
15926 J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007