Acetalization allows the photoheterolysis of the Ar-Cl bond in chlorobenzaldehydes and chloroacetophenones

Article abstract of DOI:10.1021/jo3016264

The nonaccessibility of phenyl cations by irradiation of electron-poor aryl chlorides was circumvented by transforming the carbonyl group of aromatic ketones or aldehydes into the corresponding 1,3-dioxolanes and the carboxyl group of benzoate ester into

Full text of DOI:10.1021/jo3016264

Article
pubs.acs.org/joc
Acetalization Allows the Photoheterolysis of the Ar−Cl Bond in
Chlorobenzaldehydes and Chloroacetophenones
Carlotta Raviola, Stefano Protti, Davide Ravelli, Mariella Mella, Angelo Albini, and Maurizio Fagnoni*
PhotoGreen Lab, Department of Chemistry, University of Pavia, Viale Taramelli 12, 27100 Pavia, Italy
S
* Supporting Information
ABSTRACT: The nonaccessibility of phenyl cations by
irradiation of electron-poor aryl chlorides was circumvented
by transforming the carbonyl group of aromatic ketones or
aldehydes into the corresponding 1,3-dioxolanes and the
carboxyl group of benzoate ester into an orthoester
functionality. This transformation allowed the heterolytic
photoactivation of the Ar−Cl bond in protic media and the
generation of phenyl cations. In the presence of π-bond nucleophiles, arylated products were obtained in good to excellent yields.
leaving group such as nitrogen in diazonium salts (Scheme
1b).6 However, disadvantages of this choice are the poor
stability of such precursors, the large excess of nucleophile
required for the arylation (from 20 up to 150-fold the amount
of salt used), and the byproducts formed in some cases.6 This
makes general use of the diazonium salts unsatisfactory. In one
instance, the use of diaryl iodonium salts was reported, but a
complex mixture resulted.7 A strategy for activating the
heterolysis of the Ar−X bond is thus desirable, and this issue
is confronted here for the case of benzaldehyde and
acetophenone derivatives. As is well-known,8 the photo-
chemistry of these compounds generally involves the triplet
manifold and results in some of the earliest discovered and
INTRODUCTION
■
Phenyl cations, useful intermediates in organic chemistry,1 are
obtained upon heterolysis of an aryl−heteroatom bond under
thermal1 or photochemical2,3 conditions. In recent years, it has
been demonstrated that the second approach is superior for the
mild conditions required3 and because it allows the selective
generation of phenyl cations in the triplet state. These
intermediates (3Ar+, π5σ1 electronic structure) effectively add
onto π-bond nucleophiles (e.g., alkenes, alkynes, and
aromatics), offering a metal-free alternative to transition-
metal-catalyzed aryl−carbon bond-forming reactions.2,3 These
cations are generated by heterolysis of the Ar−X bond in the
triplet state of aromatic derivatives (X = chloride, phosphate, or
sulfonate)2,3 in polar or protic media. The generation of phenyl
cations is favored by the presence of electron-donating
groups,2,3 such as −NH2, −OR, −SMe, or alkyl groups on
the ring3,4 (Scheme 1a), although it occurs also with parent
chlorobenzene in a protic solvent such as 2,2,2-trifluoroethanol
(TFE), while homolysis of the Ph−Cl bond remains the main
photoprocess in apolar solvents.5
most widely investigated reactions, such as the Paterno−Buchi
̀
̈
synthesis of oxetanes9,10 and the reductive dimerization of
carbonyls to pinacols.11,12 As for halogenated derivatives, the
photochemical activation of the carbonyl hydrogen in
(substituted) benzaldehydes toward C−C13 and C−O14 bond
formation was likewise feasible, whereas the activation of aryl−
Br15 or of −Cl16 bonds by a SRN1 reaction17 has been reported
only in a couple of cases. Apart from the latter examples, the
carbonyl moiety generally prevents any photochemical
activation of the other functional groups in the molecule. A
possible way around this limitation involves the transformation
of the carbonyl into a different functional group (from which it
can be regenerated) that is able to promote the activation of the
desired chemical bond.18 A straightforward strategy in the case
considered is the acetalization of the carbonyl group that also
has the advantage of introducing a substituent that exerts only a
small electron-withdrawing effect,23 thus facilitating aryl−
chlorine photoheterolysis. As for the carbonyl groups, the
protection/deprotection protocol is widely used in synthetic
planning, but this strategy has been used so far only in thermal
chemistry.24 Thus, chlorinated aromatic aldehydes and ketones
This reaction does not extend to electron-poor aryl chlorides,
though, where it is limited to compounds bearing an excellent
Scheme 1
Received: August 1, 2012
Published: September 10, 2012
© 2012 American Chemical Society
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protected as acetals have been successfully employed in
transition-metal-catalyzed reactions where the reactivity of the
original carbonyl group prevented any cross-coupling reaction
with aryl Grignard reagents.25 Furthermore, the acetal moiety in
protected acetophenone derivatives was able to activate
selectively the Ar−H bond present in the ortho position in
lithiation processes.26 In other cases, the carbonyl protection
markedly increased the reactivity of the aryl−chlorine bond in
4-chlorobenzaldehyde in the decarboxylative cross-coupling of
isopropyl phthalate with the 1,3-dioxolane derivative of the
chloroaromatic, a key step in the synthesis of angiotensin II
receptor antagonist telmisartan.27 A related activation of the
Ar−Cl bond was reported in the lithiation of various protected
chlorinated benzaldehydes.28
optimize the absolute minimum of these species (see the
Supporting Information for details), as in previous studies on
phenyl chlorides.29,32 Solvent effects (MeOH bulk) were
included at the same level of theory by single-point calculations
using the CPCM method (conductor-like polarizable con-
tinuum model).33 The elongation of the Ar−Cl bond (up to
4.00 Å) has been evaluated as well. Figure 2 gathers the results
3
3
obtained for 1 (Figure 2a,b) and 1a (Figure 2c,d).
RESULTS
■
In this work, we chose acetals 1−3 in comparison with the
corresponding nonprotected derivatives 1a−3a (see Figure 1)
Figure 2. Geometries, spin densities, and ESP atomic charges (in
parentheses) calculated in MeOH bulk at the CPCM-UB3LYP/6-
311+G(2d,p)//UB3LYP/6-311+G(2d,p) level of theory for: (a) 31
(Ar−Cl bond length: 1.82 Å); (b) 31 upon stretching the Ar−Cl bond
3
3
up to 4.00 Å; (c) 1a (Ar−Cl bond length: 1.75 Å); (d) 1a upon
stretching the Ar−Cl bond up to 4.00 Å.
Figure 1. Chloroaryl acetals (1−3, 3′) and their nonprotected
derivatives (1a−3a, 3′a) considered in the present work.
3
As is apparent from Figure 2a, in 1 the symmetry of the
aromatic backbone is lowered with the C4 atom sticking out of
the molecule plane and a partial negative charge is present at
the chlorine atom (−0.18) that moves toward the partially
positively charged π system. This is in fair agreement with
previous findings concerning electron-rich aromatic chlor-
ides.29,32 Moreover, the spin density is largely localized at C4
(43%) and C1 (26%). Stretching the C4−Cl bond up to 4.00 Å
led to a marked charge separation (Cl −0.65; C1 and Cα +0.28
and +0.30, respectively) and only to a minor spin localization
(Cl 18%, see Figure 2b). On the contrary, no ring deformation
as suitable models for exploring the derivatization strategy for
recovering the carbon-chlorine photoactivation. A combined
experimental and computational investigation was carried out.
In the case of the meta derivative 3′ the investigation was
limited to the computational aspect, since we previously
demonstrated that arylations were less successful with meta-
substituted cations.29
Computational Results. The photochemistry of both
aromatic carbonyls8 and aromatic halides30 proceeds via their
lowest energy triplet state. The value of the triplet energy of the
protected aryl carbonyl derivatives considered in this study
were calculated and reported in Table 1. The triplet energy of
4-chlorobenzaldehyde (1a) was likewise calculated for the sake
of comparison.
3
was observed in 1a, the Ar−Cl bond lying in the plane of the
aromatic ring, with the spin density mainly localized at the
carbonyl site (27% on Cα and 49% on the oxygen atom; see
Figure 2c). Furthermore, the elongation of the Ar−Cl bond up
to 4.00 Å resulted in no significant charge formation and a spin
distribution mainly localized at the C4 and Cl atoms (ca. 50%
each; see Figure 2d). The energy increase accompanying the
The most relevant properties of 31 (chosen as model
compound) were compared with those of the unprotected
aldehyde (31a). Density functional theory (DFT) at the
UB3LYP/6-311+G(2d,p) level of theory was adopted to
above stretch accounted to 18.2 kcal mol−1 for 1 and to 30.6
3
kcal mol−1 for 1a. Thus, heterolytic dechlorination in 1 is
viable, analogously to other aryl chlorides, while dechlorination
of 31a confronts a barrier 12 kcal mol−1 higher and, in any case,
would proceed via a homolytic pathway. Similar results have
been obtained when comparing 3 with 3a and 3′ with 3′a (see
Figures S3 and S4, Supporting Information).
3
3
Table 1. Calculated Triplet Energy of Selected Aryl
Chlorides Considered in This Study
a
aryl chloride
ET
1
1
1
+
It is worthy of note that cations 1+, 3+, and 3′ were
planar, and their structures resembled that of the parent singlet
phenyl cation (see the Supporting Information). On the
contrary, in electron-donating substituted phenyl cations a
puckering of the ring and a small out-of-plane displacement
were observed.29 Figure 3 shows the structure of 1,31+ as
representative cases where it is apparent that the C3−C4−C5
moiety has some character of a cumulene in the singlet,
whereas the triplet has a geometry close to a regular hexagon as
in other substituted triplet cations.29
1
72.2
b
1a
3
65.8 (71.7)
72.1
3′
19
73.6
74.2
a
Triplet energy (kcal mol−1) calculated in MeOH bulk at the CPCM-
UB3LYP/6-311+G(2d,p)//UB3LYP/6-311+G(2d,p) level of theory
b
(see the Supporting Information). Value measured in nonpolar
solvent; see ref 31.
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dechlorination to give products 4 and 5 as the only process.
Irradiation of 1−3 (0.05 M) in methanol and in TFE caused
the formation of the corresponding phenyl cations that were
efficiently trapped by a series of π-bond nucleophiles (0.5 M,
Table 2). It should be noted that the irradiation of the
corresponding deprotected derivatives 1a−3a under the same
conditions led to a significant consumption of these
compounds, but neither reductive dechlorination nor arylation
in the presence of π-bond nucleophiles were observed.
As shown in Tables 2 and S2 (Supporting Information), in
most cases, the arylation yield was between 70% and
quantitative, with only a few percent of the reduction products.
The end products included allylbenzenes 6, 12, and 17 by
irradiation of acetals 1 and 3 and ketal 2 in the presence of
allyltrimethylsilane (ATMS) in TFE (yields 63 to 99%) as well
as γ-benzyl lactones 7 (quantitative from 1 and 4-pentenoic
acid, entry 2) and 13 from the corresponding reaction of 2
(79%) (entry 9). A series of mixed acetals were obtained from 1
with ethyl vinyl ether in MeOH and TFE (8 and 9, 78 and 68%
yields, respectively) and with 2-methoxypropene in TFE (10 in
78% yield), as well as from 2 (14−16 with ethyl vinyl ether or
2-methoxypropene in the two alcohols, yields from 43 to 77%)
and 3 (18 with 2-methoxypropene in TFE, 70%). Biphenyl 11
was obtained by irradiation of 1 (0.025M) in the presence of
benzene (the consumption of the starting material was limited
to 70% after 15 h irradiation in this case and the reaction was
too slow with a larger concentration of the starting acetal). The
photoreaction of compound 2 in the presence of ATMS was
likewise tested under different conditions, namely in the
presence of oxygen, by using a longer wavelength (310 nm)
and a triplet sensitizer (acetone). In every case, the arylation
yield for the same irradiation time was lower (entries 7 and 8).
Moreover, the crude mixture obtained from the photoreaction
between 2 and ATMS was treated with p-toluenesulfonic acid
and deprotected 4-allylacetophenone (12′) was then isolated in
80% yield (see the Experimental Section).
́
Figure 3. Bond lengths (Å), angles (∠, in degrees) and dihedral angles
1
3
(∠, in degrees) for 1+ (left) and 1+ (right).
The isodesmic reaction reported in eq 1 (see the Supporting
Information for details) was then calculated in solution
(MeOH) and used to evaluate the energy of the isomeric
phenyl cations 1+, 3+, 3′ and, accordingly, the (de)stabilization
+
imparted by the 1,3-dioxolane group with respect to the parent
singlet phenyl cation taken as the reference point. Figure 4
In view of the positive results obtained for protected
aromatic aldehydes and ketones, the photoreactivity of 1-(4-
chlorophenyl)-4-methyl-2,6,7-trioxabicyclo[2.2.2]octane (19,
Scheme 2), where a −COOR group has been protected as
orthoester, was next examined. Computational investigations
Figure 4. Relative Gibbs free energies (see Tables S4 and S5,
Supporting Information, for details) of singlet (red) and triplet (blue)
3
performed on 19 (in MeOH bulk, except where otherwise
+
phenyl cations 1+, 3+, and 3′ in solution (MeOH) according to the
noted) predicted a situation similar to that of 31, with the C−Cl
bond sticking out of the aromatic plane (see Figure 5a).
Moreover, stretching of the Ar−Cl bond up to 4.00 Å (Figure
5b) resulted in the development of a partial negative charge at
the chlorine atom (from −0.18 to −0.57) with the positive
charge mainly localized at C1 and Cα (0.26 and 0.56,
respectively). An energy increase of 20.4 kcal mol−1, somewhat
larger than that found for 31, has been observed upon
stretching of the C−Cl bond. The influence of the solvent
has also been tested; TFE and water gave similar results to
those observed in methanol (see the Supporting Information).
The occurrence of heterolytic cleavage of the Ar−Cl bond in
orthoester 19 was experimentally confirmed by irradiation in
TFE in the presence of ethyl vinyl ether and ATMS (Scheme
2). The corresponding 3-aryl acetal 20 and allylated derivative
22 were deprotected during workup, and the corresponding
hydroxy esters 21 and 23 were obtained in a modest yield (24
and 20%, respectively).
isodesmic reaction reported in eq 1.
shows the relative energy of the six isomeric phenyl cations. In
all of the cases, the singlets are the lower energy states and their
energy is close to that of the unsubstituted phenyl cation. The
triplets were displaced upward by a relevant amount (ca. 20
+
kcal mol−1) only in the case of the meta derivative (3′ ).
Experimental Results. The absorption spectra of 1−3 are
blue-shifted with respect to the corresponding 1a−3a (see
Figures S1 and S2 in the Supporting Information), and acetals
1−3 exhibit low fluorescence (see Table S1, Supporting
Information). Irradiation experiments were carried out in the
presence of an equimolar amount of base (Cs2CO3 or Et3N) in
order to avoid that the HCl liberated in the process34 caused
the deprotection of the carbonyl group.35 The dioxolanes
reacted only slowly in nonprotic solvents, as demonstrated by
the reaction of 1 in ethyl acetate (not shown), and fast in
methanol (see Φ−1 in Table 2), in any case with reductive
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a
Table 2. Irradiation of 1−3 in Neat Solvent and in the Presence of π-Bond Nucleophiles
a
b
ArCl 0.05 M in the presence of the chosen nucleophile (0.5 M) and Cs2CO3 (0.03 M) irradiated at λ = 254 nm. Quantum yields of
photodecomposition in neat MeOH measured by irradiating at λ = 254 nm a 5 × 10−3 M solution of 1−3 in the presence of an equimolar amount of
Et3N. Yield based on the consumption of ArCl; see the Supporting Information for details. 1 (0.025 M) and benzene 1 M were used. Reaction
carried out under aerated conditions. Irradiated at λ = 310 nm. Acetone (20% v/v) added.
c
d
e
f
g
in Scheme 3), whereas in the presence of π-bond nucleophiles
DISCUSSION
■
(Nu(Y)) efficient trapping occurs (path AR) leading to the
corresponding arylated products (6−18, 20, and 22)3,4 as the
only or (in a few cases) main products. With a poor trap (see
benzene in the reaction of 1) or when a somewhat hindered
cation29 (such as the o-chlorophenyl derivative from 3) was
generated, trapping was less efficient and the amount of the
reduction product 4 was more significant.
In conclusion, we extended the generation of phenyl cations
to electron-poor aromatics. Although the photochemistry of
chlorobenzaldehyde or chloroacetophenone derivatives in-
volves the carbonyl function exclusively, an expeditious
transformation into acetals reintroduces the heterolytic photo-
fragmentation of the aryl−Cl bond. The phenyl cation
chemistry is thus accessible and clean arylation reactions can
be obtained. Similar results were obtained when a chlor-
obenzoate orthoester was used.
The above results support the reasoning that transforming
carbonyl and carboxyl groups into acetals and orthoesters shifts
the barrier to heterolytic cleavage of the aryl−chlorine bond as
supported by both computational and experimental results. As
is apparent from Table 1 for the case of 4-chlorobenzaldehyde,
protection of the carbonyl group significantly increases the
triplet energy (up to 72 kcal mol−1), and the three dioxolane
isomers and the orthoester have similar triplet energies that
marginally differ from that of chlorobenzene (experimental 81.5
kcal mol−1).36 Apparently, ISC is effective also in these
compounds (no advantage from attempted acetone sensitiza-
tion, experimental ET acetone =78.9 kcal mol−1).36 In protic
solvents, such as alcohols, the hoped for fragmentation occurs
with a reasonable quantum yield (3−17%), comparable to that
observed with chloroanisoles.3 The triplet phenyl cation is thus
formed, and the reactions expected from this intermediate do
occur under these conditions. Thus, in neat alcohols hydrogen
abstraction to give dechlorinated 4 and 5 takes place (path RD
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grade were employed in the photochemical reactions. Quantum yields
were measured at 254 nm (1 Hg lamp, 15 W). The acetals 1−3 (or 4−
5) were prepared from the corresponding carbonyl derivatives 1a−3a
(benzaldehyde or acetophenone) by azeotropic water elimination from
a toluene-ethylene glycol solution in the presence of p-toluenesulfonic
acid monohydrate and redistillation (or column chromatography
purification for 2).
Scheme 2. Irradiation of 19 in TFE in the Presence of Ethyl
Vinyl Ether and Allyltrimethylsilane ATMS
Synthesis of 1-(4-Chlorophenyl)-4-methyl-2,6,7-trioxa-
bicyclo[2.2.2]octane (19). Orthoester 19 was prepared from 4-
chlorobenzoyl chloride by following a procedure used in the synthesis
of 1-n-amyl-4-methyl-2,6,7-trioxabicyclo[2.2.2]octane.37
Step a. 4-chlorobenzoyl chloride (2 g, 11.4 mmol) was added to a
solution of (3-methyloxetan-3-yl)methanol (1.20 mL, 11.4 mmol) and
pyridine (0.90 mL, 11.5 mmol) in CH2Cl2 (5 mL) at 0 °C. The
resulting mixture was stirred for 5 h at 0 °C. The mixture was then
diluted with CH2Cl2 and the organic layer washed with brine and
water, dried with Na2SO4, and then concentrated to afford 2.02 g of
(3-methyloxetan-3-yl)methyl 4-chlorobenzoate (19′, white solid, 74%
yield, mp = 47−48 °C). The crude ester was employed for the
following step without further purification. 19′: 1H NMR
(CD3COCD3) δ 1.40 (s, 3H), 4.35−4.40 (d, 2H, J = 6 Hz), 4.45 (s,
2H), 4.55−4.60 (d, 2H, J = 6 Hz), 7.55−8.05 (AA′BB′, 4H, J = 11
Hz); 13C NMR (CD3COCD3) δ 21.7, (CH3), 40.4, 70.4 (CH2), 80.0
(CH2), 130.1 (CH), 130.3, 132.4 (CH), 140.2, 166.4; IR (KBr), ν/
cm−1 2924, 1730, 1264, 1102, 759.
Step b. Crude 19′ (766 mg, 3.18 mmol) was dissolved in 5 mL of
dry CH2Cl2. The resulting solution was cooled at −15 °C and boron
trifluoride etherate (100 μL, 0.80 mmol) was added. After 24 h GC
analysis revealed a 90% consumption of the starting material.
Triethylamine (TEA, 0.44 mL, 3.18 mmol) was added and the
reaction mixture was diluted with diethyl ether (20 mL) and filtered to
remove the resulting TEA-BF3 complex. The filtrate was concentrated
and the residue purified by chromatographic separation by using silica
gel (eluant: CH2Cl2 with 0.2% v/v TEA) to afford 306 mg of 19
(white solid, 40% yield, mp = 102−105 °C). 19: 1H NMR
(CD3COCD3) δ 0.90 (s, 3H), 4.05 (s, 6H), 7.35−7.55 (AA′BB′,
4H, J = 8.5 Hz); 13C NMR (CD3COCD3) δ 14.6 (CH3), 31.5, 74.1
(CH2), 108.1, 128.9 (CH), 129.0 (CH), 135.4, 138.8; IR (KBr) ν/
cm−1 2924, 1788, 1594, 1094, 831, 760. Anal. Calcd for C12H13ClO3:
C, 59.88; H, 5.44. Found: C, 59.9; H 5.4.
Figure 5. Geometries, spin density, and ESP atomic charges (in
parentheses) calculated in MeOH bulk at the CPCM-UB3LYP/6-
311+G(2d,p)//UB3LYP/6-311+G(2d,p) level for: (a) 319 (Ar−Cl
3
bond length: 1.82 Å); (b) 19 upon stretching the Ar−Cl bond up to
4.00 Å.
Scheme 3. Photoreactivity of Phenyl Chlorides 1−3 and 19
Preparative Irradiations. A solution of 1−3 or 19 (1.5 mmol,
0.05 M except where otherwise indicated), Cs2CO3 (0.9 mmol, 0.03
M), and π-bond nucleophiles (15 mmol, 0.5 M) in the chosen alcohol
(TFE or methanol, 30 mL) was nitrogen purged in quartz tubes and
irradiated by means of a multilamp reactor equipped with 4 Hg lamps,
15 W (emission centered at 254 nm). The reaction course was
followed by GC analysis. GC yields of compounds 4 and 5 have been
determined by comparison with authentic samples. The photolyzed
solution was concentrated in vacuo at 80−100 Torr, and the resulting
residue purified by silica gel column chromatography (eluant:
pentane/diethyl ether mixture with 0.2% v/v triethylamine or
pentane/dichloromethane mixture).
EXPERIMENTAL SECTION
■
General Methods. NMR spectra were recorded on a 300 MHz
spectrometer. The attributions were made on the basis of H and 13C
1
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 in a
multilamp reactor equipped with 4 Hg lamps, 15 W (emission
centered at 254 nm) for the irradiation (or 10 phosphor coated lamps,
15 W each, emission centered at 310 nm, see Table 2). The reaction
course was followed by GC analyses. Workup of the raw photolyzed
mixtures involved concentration in vacuo (80−100 Torr) and
chromatographic separation using silica gel. 4-Chlorobenzaldehyde
(1a), 4-chloroacetophenone (2a), 2-chlorobenzaldehyde (3a), 4-
chlorobenzoyl chloride, and all of the π-bond nucleophiles are
commercially available and were used as received, except for ethyl vinyl
ether, which was freshly distilled before use. Solvents of HPLC purity
2-(4-Allylphenyl)-1,3-dioxolane (6): 6 h irradiation, 78% con-
sumption of 1; eluant: pentane/diethyl ether 9:1, oil, 67% yield based
1
on the consumption of 1; H NMR (CD3COCD3) δ 3.40−3.45 (d,
2H, J = 7 Hz), 3.95−4.10 (m, 4H), 5.00−5.10 (m, 2 H), 5.70 (s, 1H),
5.90−6.05 (m, 1H), 7.20−7.40 (AA′BB′ system, 4H, J = 8 Hz); 13C
NMR (CD3COCD3) δ 40.9 (CH2), 66.2 (CH2), 104.7 (CH), 116.4
(CH2), 128.0 (CH), 129.5 (CH), 137.8, 138.8 (CH), 142.2; IR (neat)
ν/cm−1 2890, 1702, 1605, 1084, 838. Anal. Calcd for C12H14O2: C,
75.76, H, 7.42. Found: C, 75.6; H, 7.6.
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5-(4-(1,3-Dioxolan-2-yl)benzyl)dihydrofuran-2(3H)-one (7): 6 h
irradiation, 90% consumption of 1; eluant: from pentane/diethyl ether
9:1 to pentane/diethyl ether 7:3. oil, 100% yield based on the
in accordance with the literature.40 Anal. Calcd for C11H12O: C, 82.46;
H, 7.55. Found: C, 82.5; H, 7.6.
5-(4-(2-Methyl-1,3-dioxolan-2-yl)benzyldihydrofuran-2(3H)-one
(13): 19 h irradiation, 100% consumption of 2; eluant: from pentane/
diethyl ether 9:1 to pentane/diethyl ether 5:5, oil, 79% yield; 1H NMR
(CD3COCD3) δ 1.55 (s, 3H) 1.95−2.00 (m, 1H), 2.30−2.35 (m, 1H),
2.40−2.50 (m, 2H) 2.95−3.00 (m, 2H), 3.70−4.00 (m, 4H), 4.70−
4.80 (qui, 1H, J = 7 Hz), 7.25−7.40 (AA′BB′, 4H, J = 8 Hz); 13C
NMR (CD3COCD3) δ 28.3 (CH3), 28.4 (CH2), 29.3 (CH2), 42.0
(CH2) 65.5 (CH2), 81.8 (CH), 109.6, 126.6 (CH), 130.4 (CH), 137.9,
143.5, 177.4; IR (neat) v/cm−1 2928, 1773, 1175, 1035. Anal. Calcd
for C15H18O4: C, 68.68; H, 6.92. Found: C, 68.7; H, 6.9.
1
consumption of 1; H NMR (CD3COCD3) δ 1.95−2.00 (m, 1H),
2.30−2.35 (m, 1H), 2.40−2,45 (m, 2H), 3.00−3.05 (m, 2H), 3.95−
4.10 (m, 4H), 4.70−4.80 (qui, 1H, J = 6.5 Hz), 5.70 (s, 1H), 7.30−
7.40 (AA′BB′ 4H, J = 8 Hz); 13C NMR (CD3COCD3) δ 28.3 (CH2),
29.3 (CH2), 42.0 (CH2), 66.2 (CH2), 81.6 (CH), 104.6 (CH), 128.0
(CH), 130.5 (CH), 138.4, 139.2, 177.4; IR (neat) ν/cm−1 2925, 1774,
1176, 1080. Anal. Calcd for C14H16O4: C, 67.73; H, 6.50. Found: C,
67.7; H, 6.5.
2-(4-(2-Ethoxy-2-methoxyethyl)phenyl)-1,3-dioxolane (8): 20 h
2-(4-(2-Ethoxy-2-methoxyethyl)phenyl)-2-methyl-1,3-dioxolane
irradiation, 100% consumption of 1; eluant: pentane/diethyl ether 9:1,
1
(14): 20 h irradiation, 100% consumption of 2; eluant: pentane/diethyl
oil, 78% yield; H NMR (CDCl3) δ 1.15−1.20 (t, 3H, J = 7 Hz),
1
ether 99:1, oil, 77% yield; H NMR (CD3COCD3) δ 1.10−1.15 (t,
2.90−2.95 (d, 2H, J = 6 Hz), 3.35 (s, 3H), 3.40−3.70 (m, 2H), 4.00−
4.15 (m, 4H), 4.55−4.60 (t, 1H, J = 6 Hz), 5.80 (s, 1H), 7.25−7.40
(AA′BB′ system, 4H, J = 8 Hz); 13C NMR (CDCl3) δ 15.1 (CH3),
40.0 (CH2), 53.2 (CH3), 61.9 (CH2), 65.2 (CH2), 103.6 (CH), 104.4
(CH), 126.3 (CH), 129.4 (CH), 135.8, 138.2; IR (neat), ν/cm−1
2888, 1275, 1080, 820. Anal. Calcd for C14H20O4: C, 66.65; H, 7.99.
Found: C, 66.7; H, 8.0.
3H, J = 7 Hz), 1.55 (s, 3H), 2.85−2.90 (d, 2H, J = 6 Hz), 3.30 (s, 3H),
3.40−3.50 (m, 1H), 3.60−3.65 (m, 1H), 3.70−4.00 (m, 4H), 4.60−
4.65 (t, 1H, J = 6 Hz), 7.25−7.40 (AA′BB′, 4H, J = 8 Hz); 13C NMR
(CD3COCD3) δ 15.9 (CH3), 28.4 (CH3), 40.7 (CH2), 53.4 (CH3),
62.5 (CH2), 65.4 (CH2), 105.5 (CH), 109.6, 126.2 (CH), 130.5 (CH),
138.2, 142.9; IR (neat) ν/cm−1 2979, 1199, 1124, 1041, 737. Anal.
Calcd for C15H22O4: C, 67.64; H, 8.33. Found: C, 67.6; H, 8.3.
2-(4-(2-Ethoxy-2-(2,2,2-trifluoroethoxy)ethyl)phenyl)-2-methyl-
1,3-dioxolane (15): 19 h irradiation, 100% consumption of 2; eluant:
2-(4-(2-(2,2,2-Trifluoroethoxy)-2-ethoxyethyl)phenyl)-1,3-dioxo-
lane (9): 20 h irradiation, 100% consumption of 1; eluant: from
pentane/diethyl ether 99:1 to pentane/diethyl ether 96:4, oil, 68%
1
1
pentane/diethyl ether 98:2, oil, 52% yield; H NMR (CD3COCD3) δ
yield; H NMR (CD3COCD3) δ 1.10−1.15 (t, 3H, J = 7 Hz), 2,95−
1.10−1.15 (t, 3H, J = 7 Hz), 1.55 (s, 3H), 2.95−3.00 (d, 2H, J = 6
Hz), 3.50−3.60 (m, 2H), 3.70−4.10 (m, 6H), 4.85−4.90 (t, 1H, J = 6
Hz), 7.25−7.40 (AA′BB′, 4H, J = 8.5 Hz); 13C NMR (CD3COCD3) δ
15.7 (CH3), 28.3 (CH3), 40.7 (CH2), 63.4 (CH2), 63.5 (q, CH2, J =
33 Hz), 65.5 (CH2), 105.1 (CH), 109.6, 125.9 (q, CF3, J = 275 Hz),
126.4 (CH), 130.5 (CH), 137.4, 143.2; IR (neat) ν/cm−1 2981, 1280,
1161, 1077, 1040, 870. Anal. Calcd for C16H21F3O4: C, 57.48; H, 6.33.
Found: C, 57.5; H, 6.3.
3.00 (d, 2H, J = 6 Hz), 3.45−3.50 (m, 1H) 3.70−3.75 (m, 1H), 3.95−
4.15 (m, 6H), 4.85−4.90 (t, 1H, J = 6 Hz), 5.70 (s, 1H), 7.30−7.40
(AA′BB′, 4H, J = 8 Hz); 13C NMR (CD3COCD3) δ 15.7 (CH3), 40.9
(CH2), 63.0 (CH2), 63.5 (q, CH2, J = 34 Hz), 66.2 (CH2), 104.7
(CH), 105.0 (CH), 127.8 (CH), 127.4 (q, CF3, J = 240 Hz), 130,6
(CH), 138.2, 138.8; IR (neat) ν/cm−1 2889, 1280, 1162, 1088, 968,
823. Anal. Calcd for C15H19F3O4: C, 56.25; H, 5.98. Found: C, 56.2;
H, 5.9.
2-(4-(2-(2,2,2-Trifluoroethoxy)-2-methoxypropyl)phenyl)-2-meth-
yl-1,3-dioxolane (16): 12 h irradiation, 89% consumption of 2; eluant:
from pentane/diethyl ether 98:2 to pentane/diethyl ether 1:1, oil, 43%
yield based on the consumption of 2; 1H NMR (CD3COCD3) δ 1.15
(s, 3H), 1,55 (s, 3H), 2.95−3.00 (d, 2H, J = 6 Hz), 3.30 (s, 3H), 3.70−
3.80 (m, 2H) 3.90−4.00 (m, 4H), 7.25−7.35 (AA′BB′, 4H, J = 8 Hz);
(CD3COCD3) δ 22.1 (CH3), 28.3 (CH3), 43.7 (CH2), 49.4 (CH3),
59.8 (CH2, q, J = 34 Hz), 65.5 (CH2), 104.0, 109.6, 126.1 (CH), 128,0
(q, CF3, J = 275 Hz), 131.2 (CH), 137.6, 143.3; IR (neat) ν/cm−1
2933, 1283, 1162, 1077, 1043, 972, 870. Anal. Calcd for C16H21F3O4:
C, 57.48; H, 6.33. Found: C, 57.5; H, 6.3.
2-(2-Allylphenyl)-1,3-dioxolane (17): 6 h irradiation, 88% con-
sumption of 3; eluant: pentane/diethyl ether 9:1, oil, 63% yield based
on the consumption of 3; The spectroscopic data of 17 were in
accordance with literature data.41 Anal. Calcd for C12H14O2: C, 75.76;
H, 7.42. Found: C, 75.8; H, 7.4.
2-(4-(2-(2,2,2-Trifluoroethoxy)-2-methoxypropyl)phenyl)-1,3-di-
oxolane (10): 6 h irradiation, 77% consumption of 1; eluant: pentane/
1
diethyl ether 98:2, oil, 78% yield based on the consumption of 1; H
NMR (CD3COCD3) δ 1.20 (s, 3H), 3.00−3.05 (d, 2H, J = 3 Hz), 3.30
(s, 3H), 3.95−4.10 (m, 6H), 5.70 (s, 1H), 7.30−7.40 (AA′BB′, 4H, J =
8 Hz); 13C NMR (CD3COCD3) δ 22.1 (CH3), 43.9 (CH2), 49.4
(CH3), 59.8 (CH2, q, J = 35 Hz), 104.0, 104.7 (CH), 127.6 (CH),
129.0 (CF3, q, J = 275 Hz), 131.3 (CH), 138.2, 139.2; IR (neat) ν/
cm−1 2952, 1162, 1083, 1050, 971, 868. Anal. Calcd for C15H19F3O4:
C, 56.25; H, 5.98. Found: C, 56.2; H, 5.9.
2-(Biphenyl-4-yl)-1,3-dioxolane (11): 15 h irradiation, 70%
consumption of 1; eluant: pentane/diethyl ether 95:5, colorless
solid, 87% yield based on the consumption of 1, mp = 53−54 °C (lit.38
mp 57 °C). Spectroscopic data of 11 are in accordance with the
literature.38 Anal. Calcd for C15H14O2: C, 79.62; H, 6.24. Found: C,
79.6; H, 6.2.
2-(4-Allylphenyl)-2-methyl-1,3-dioxolane (12): 18.5 h irradiation,
2-(2-(2-Methoxy-2-(2,2,2-trifluoroethoxy)propyl)phenyl)-1,3-di-
oxolane (18): 8 h irradiation, 73% consumption of 3; eluant: pentane/
77% consumption of 2; eluant: from neat pentane to pentane/diethyl
1
1
diethyl ether 99:1, oil, 70% yield based on the consumption of 3: H
ether 7:3, oil, 99% yield based on the consumption of 2: H NMR
NMR (CD3COCD3) δ 1.20 (s, 3H), 3.20 (s, 2 H), 3.30 (s, 3H), 3−
90−4.00 (m, 4H), 4.00−4.10 (m, 2H), 6.10 (s, 1H), 7.25−7.30 (m,
2H), 7.35−7.40 (m, 1H), 7.55−7.60 (dd, 1H, J = 7, 2 Hz); 13C NMR
(CD3COCD3) δ 22.0 (CH3), 39.4 (CH2), 49.3 (CH3), 60.2 (CH2, q, J
= 34 Hz), 66.1 (CH2), 102.7 (CH), 104.4, 127.5 (2 CH), 129.3 (CH),
129.5 (CF3, J = 280 Hz), 132.2 (CH), 136.9, 138.4; IR (neat) ν/cm−1
2891, 1282, 1162, 1084, 970, 758. Anal. Calcd for C15H19F3O4: C,
56.25; H, 5.98. Found: C, 56.2; H, 5.9.
(CD3COCD3) δ 1.50 (s, 3H), 3.35−3.40 (d, 2H, J = 7 Hz), 3.70−4.00
(m, 4H), 5.00−5.10 (m, CH2), 5.90−6.00 (m, CH), 7.15−7.40
(AA′BB′, 4H, J = 8 Hz); 13C NMR (CD3COCD3) δ 28.4 (CH3), 40.8
(CH2), 65.4 (CH2), 109.6, 116.3 (CH2), 126.5 (CH), 129.4 (CH),
138.9 (CH), 140.8. 142.9; IR (neat) ν/cm−1 2953, 1685, 1638, 1268,
1158, 1041, 958, 917, 839. Anal. Calcd for C13H16O2: C, 76.44; H,
7.90. Found: C, 76.3; H, 8.0.
The reaction was repeated under the same conditions, and crude 12
was deprotected in situ by adding p-toluenesulfonic acid.39 In detail,
water (0.17 mL), p-toluensulfonic acid monohydrate (17 mg, 0.09
mmol), and acetone (1.7 mL) were added to the photolyzed solution.
The resulting mixture was stirred overnight at room temperature and
diluted with toluene and a saturated sodium hydrogen carbonate
solution. The toluene phase was separated and washed with brine,
dried over MgSO4, and concentrated in vacuo at 80−100 Torr. The
resulting residue was purified by silica gel column chromatography
(eluant: hexane with 0.2% v/v triethylamine) affording 148 mg of 4-
allylacetophenone (12′, oil, 80% yield). Spectroscopic data of 12′ are
Irradiation of 19 in TFE in the presence of ethyl vinyl ether: 14.5
h irradiation, 100% consumption of 19. Arylated compound 20 thus
formed was not stable during the purification procedure (by column
chromatography eluant: CH2Cl2) and hydrolyzed to 3-hydroxy-2-
(hydroxymethyl)-2-methylpropyl 4-(2-ethoxy-2-(2,2,2-
1
trifluoroethoxy)ethyl)benzoate (21, 141 mg, oil, 24% yield). 21: H
NMR (CD3COCD3) δ 1.10−1.15 (m, 6H), 3.05−3.10 (d, 2H, J = 6
Hz), 3.50−3.55 (m, 1H), 3.60−3.65 (m, 4H) 3.70−3.75 (m, 1H),
3.80−3.85 (t, 2H, OH, J = 5 Hz), 4.10−4.15 (m, 2H), 4.20 (s, 2H),
4.95−5.00 (t, 1H, J = 6 Hz), 7.40−8.00 (AA′BB′, 4H, J = 9 Hz); 13C
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Article
NMR (CD3COCD3) δ 15.7 (CH3), 17.5 (CH3), 41.0 (CH2), 41.8
(CH2), 63.2 (CH2, q, J = 33 Hz), 66.3 (CH2), 62.9 (CH2), 104.7
(CH), 127.7 (CF3, q, J = 275 Hz), 130.1, 130.5 (CH), 131.1 (CH),
132.3, 167.2. IR (neat) ν/cm−1 3446, 2980, 1715, 1266, 1165, 739.
Anal. Calcd for C18H25F3O6: C, 54.82; H, 6.39. Found: C, 54.8; H, 6.4.
Irradiation of 19 in TFE in presence of ATMS: 24 h irradiation,
100% consumption of 19. As in the former case, 3-hydroxy-2-
(hydroxymethyl)-2-methylpropyl 4-allylbenzoate (23, oil, 79 mg, 20%
yield) was isolated after workup (column chromatography, eluant:
CH2Cl2/pentane 9:1) in place of compound 22. 23: 1H NMR
(CDCl3) δ 0.95 (s, 3H), 2.50 (bs, 2H), 3.45−3.50 (d, 2H, J = 6.5 Hz),
3.60−3.65 (m, 4H), 4.45 (s, 2H), 5.10−5.20 (m, 2H), 5.90−5.95 (m,
1H), 7.30−8.00 (AA′BB′, 4H, J = 9.5 Hz). 13C NMR (CDCl3) δ 16.8
(CH3), 40.0 (CH2), 41.0 (CH2), 67.0 (CH2), 67.6 (CH2), 116.5
(CH2),128.7 (CH), 129.9 (CH), 136.1, 140.0, 167.4. IR (neat) ν/
cm−1 3412, 2955, 1715, 1277, 840. Anal. Calcd for C15H20O4: C,
68.16; H, 7.63. Found: C, 68.2; H, 7.7.
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ASSOCIATED CONTENT
■
S
* Supporting Information
́
(17) See for reviews: Rossi, R. A.; Pierini, A. B.; Penenory, A. B.
̃ ̃
1H and 13C NMR spectra for compounds 1−3, 6−19, 21, and
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available free of charge via the Internet at http://pubs.acs.org.
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(18) This may be considered a particular case of the “remote
activation” of a chemical bond several atoms away from the modified
center developed by Breslow19 and recently applied to a variety of
reactions and based either on coordination20 or on (temporary)
covalent transformation of a molecule moiety.21,22
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: fagnoni@unipv.it.
(19) Breslow, R. Acc. Chem. Res. 1980, 13, 170−177.
Notes
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D’Acchioli, J. S.; Carpenter, G. B.; Sweigart, D. A. Organometallics
2002, 21, 1262−1270. Oh, M.; Yu, K.; Li, H.; Watson, E. J.; Carpenter,
G. B.; Sweigart, D. A. Adv. Synth. Catal. 2003, 345, 1053−1060.
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Vatele, J.-M.; Hanessian, S. Tetrahedron 1996, 52, 10557−10568.
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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 was
funded by the CINECA Supercomputer Center, with computer
time granted by ISCRA COMPDHT (HP10CZEHG6) project.
We thank Miss Jone Mendigutxia Manrique for preliminary
experiments.
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