First determination of absolute rate constants for the reaction of aroyl-substituted benzyl carbanions in water and DMSO

Article abstract of DOI:10.1021/ja027624k

The prompt generation of carbanions II and III within the duration of the nanosecond laser pulse provides a way of evaluating absolute rate constants for their two decay pathways, protonation and cyclization, the latter resulting from an intramolecular nu

Full text of DOI:10.1021/ja027624k

Published on Web 11/26/2002
First Determination of Absolute Rate Constants for the
Reaction of Aroyl-Substituted Benzyl Carbanions in Water
and DMSO
Laura Llauger, Gonzalo Cosa, and J. C. Scaiano*
Contribution from the Department of Chemistry, UniVersity of Ottawa, 10 Marie-Curie,
Ottawa, Ontario K1N 6N5, Canada
Received July 9, 2002
Abstract: The prompt generation of carbanions II and III within the duration of the nanosecond laser pulse
provides a way of evaluating absolute rate constants for their two decay pathways, protonation and
cyclization, the latter resulting from an intramolecular nucleophilic carbanion displacement of iodide tethered
at the end of the lateral alkyl chain. Absolute rate constants are given for both carbanions (II and III) and
show that the intra-SN2 reaction is favored in aprotic media, such as dimethyl sulfoxide (DMSO), while
protonation is the dominant reaction in basic aqueous media.
Scheme 1
Introduction
The relative reactivities of carbanions, deeply influenced by
the solvent,1,2 have been extensively studied.3-7 However, only
a few examples of the determination of absolute rate constants
for their reactions,3-6 mostly hydrolysis, have been reported.
Competitive product studies are not necessarily under kinetic
control and may be influenced by parameters other than the
actual reaction of the carbanion, such as protonation-deproto-
nation, and other pathways contributing to reversibility in
product generation. In this paper we report the first absolute
rate constants for carbanions participating in nucleophilic
reactions other than solvolysis.
Scheme 2
Formation of reactive carbanions in solution using pulsed
techniques make it possible to observe them directly on a short-
time scale and to determine absolute rate constants for many
of their elementary reactions. Dorfman et al.3,4 determined rate
constants for protonation reactions of the benzyl anion using
pulse radiolysis. A decade ago, Craig et al.5,6 characterized
carbanions derived from the photodecarboxylation of p-nitro-
phenylacetate. Following flash photolysis, they studied the decay
modes of the p-nitrobenzyl carbanion in aqueous media. More
recently, photochemical studies of ketoprofen, a nonsteroidal
antiinflammatory drug, have shown that it undergoes decar-
boxylation to form a carbanion in the subnanosecond time
scale.8-14 This species is responsible for the formation of
3-ethylbenzophenone, as illustrated in Scheme 1.
The rapid generation of carbanion I provides an interesting
intermediate on which to perform mechanistic studies. The decay
of this carbanion (I) in water is fast (τ ) 215 ns), making it
unlikely for reactions other than protonation to compete. Suitably
substituted carbanions may however undergo intramolecular
nucleophilic substitution reactions (see Scheme 2). With this
in mind, we examined compounds 1 and 2, which contain an
* Corresponding author. E-mail: tito@photo.chem.uottawa.ca.
(8) Costanzo, L. L.; De Guidi, G.; Condorelli, G.; Cambria, A.; Fama, M.
Photochem. Photobiol. 1989, 50, 359-365.
(1) Bernasconi, C. F.; Brunnell, R. D. J. Am. Chem. Soc. 1988, 110, 2900-
2903.
(9) Budac, D.; Wan, P. J. Photochem. Photobiol. A: Chem. 1992, 67, 135-
(2) Pliego, J. R., Jr.; Riveros, J. M. Phys. Chem. Chem. Phys. 2002, 4, 1622-
166.
1627.
(10) Bosca´, F.; Miranda, M. A.; Carganico, G.; Mauleo´n, D. Photochem.
Photobiol. 1994, 60, 96-101.
(3) Bockrath, B.; Dorfman, L. M. J. Am. Chem. Soc. 1974, 96, 5708-5715.
(4) Bockrath, B.; Dorfman, L. M. J. Am. Chem. Soc. 1975, 97, 3307-3310.
(11) Mart´ınez, L. J.; Scaiano, J. S. J. Am. Chem. Soc. 1997, 119, 11066-11070.
(12) Cosa, G.; Mart´ınez, L. J.; Scaiano, J. C. Phys. Chem. Chem. Phys. 1999,
1, 3533-3537.
(5) Craig, B. B.; Weiss, R. G.; Atherton, S. J. J. Phys. Chem. 1987, 91, 5906-
5912.
(6) Craig, B. B.; Atherton, S. J. J. Chem. Soc., Perkin Trans. 2 1988, 1929-
(13) Borsarelli, C. D.; Braslavsky, S. E.; Sortino, S.; Marconi, G.; Monti, S.
Photochem. Photobiol. 2000, 72, 163-171.
1935.
(7) Richard, J. P.; Amyes, T. L.; Toteva, M. M. Acc. Chem. Res. 2001, 34,
981-988.
(14) Cosa, G.; Llauger, L.; Scaiano, J. C.; Miranda, M. A. Org. Lett. 2002, 4,
3083-3085.
9
15308
J. AM. CHEM. SOC. 2002, 124, 15308-15312
10.1021/ja027624k CCC: $22.00 © 2002 American Chemical Society

Absolute Rate Constants for Aroyl-Substituted Benzoyl Carbanions
A R T I C L E S
Scheme 3
iodine atom at the end of an alkyl chain. Upon photoexcitation,
and subsequent decarboxylation, they were expected to render
their respective carbanions14 (II and III) which could either react
with water or undergo an intramolecular SN2 reaction (see
Scheme 2).
Figure 1. Transient absorption spectra following 355 nm laser excitation
of (A) a N2O-saturated 0.1 M KOH solution of 1 (10 mM) 64 ns after the
laser pulse and (B) an Ar-saturated 10 mM solution of 1 in NaH/DMSO
72 ns after the pulse.
To gain insight into the reactivity of these carbanions, we
carried out comparative studies of the photodecarboxylation of
compounds 1 and 2 in a 0.1 M KOH aqueous solution and in
dimethylsufoxide (DMSO), to which an excess of NaH was
added to generate the corresponding carboxylate forms.
Results and Discussion
Carboxylic acids 1 and 2 were synthesized by R-alkylation
of the corresponding tert-butyl esters 11 and 12, followed by
hydrolysis of the tert-butyl esters 13 and 14 with titanium
tetrachloride15 (see Scheme 3). Ester 12 was prepared by
treatment of ketoprofen (10) with tert-butyl alcohol and 1-ethyl-
3-[3-(dimethylamino)propyl]carbodiimide hydrochloride (EDCI)
in the presence of a catalytic amount of 4-(dimethylamino)-
pyridine (DMAP). This methodology did not work for the
esterification of acid 9. Takeda et al.16 reported novel esterifi-
cation reagents of the active carbonate type. The reaction of
dicarbonates and carboxylic acids in the presence of DMAP as
a catalyst gave esters in good yield accompanied by the
evolution of carbon dioxide. Indeed, ester 11 was obtained
quantitatively by reaction of 9 and di-tert-butyl dicarbonate
(Boc2O) in the presence of a catalytic amount of DMAP. The
introduction of the alkyl chain was achieved by reaction of
enolates from esters 11 and 12, generated by LDA at low
temperature, with 1,5-diodopentane. Removal of the tert-butyl
group from esters 13 and 14 was performed by titanium
tetrachloride at low temperature, yielding acids 1 and 2 in good
yields.
Figure 2. Normalized transient absorption spectra obtained following 355
nm laser excitation of N2O-saturated 0.1 M KOH solutions of 1 (10 mM):
(O) aqueous solution; (b) DMSO mole fraction 0.21; (0) 0.43, and (9) 10
mM 1 in NaH/DMSO solution, recorded 64, 32, 24, and 72 ns after laser
pulse, respectively.
around 390 nm and at approximately 800 nm; both bands
showed the same kinetics and are assigned to carbanion II (see
Figure 1).
The red-shift of the carbanion II band from 590 nm in water
(protic) to 800 nm in DMSO (aprotic) was expected (vide infra).
Pronounced solvent effects on carbanion spectra are known.8,12,19
To verify the shift of carbanion II band with solvent, we
irradiated 10 mM 1 in basic aqueous solutions with different
DMSO mole fractions. Figure 2 illustrates the progressive red-
shift with increasing mole fraction of DMSO.
In pure DMSO, the decay was slower than that in aqueous
0.1 M KOH, with a lifetime of 180 ns.20 The increased lifetime
is mainly due to the reduction of carbanion protonation in the
anhydrous conditions employed.
The transient absorption spectrum obtained following 355 nm
laser excitation of an aqueous 0.1 M KOH solution of compound
1 was similar to that for ketoprofen,12,14,17 with a broad
maximum wavelength at ca. 590 nm (see Figure 1); similar
spectra were recorded for 2 (not shown). The decay kinetics
for II, recorded at 600 nm, appeared first order with a lifetime
of 150 ns (kobs ) 6.73 × 106 s-1).18
When a 0.1 M KOH solution of 2 was excited with a 308
nm laser, the decay of the resulting carbanion (III), recorded at
600 nm, was slower than that obtained for carbanion II, with a
lifetime of 390 ns (kobs ) 2.59 × 106 s-1). Thus, introduction
of one additional methyl group in structure 1 increases the
lifetime of the resulting tertiary carbanion (III) by a factor of
2.6, under conditions where protonation is the dominant
pathway. We recently reported on these trends in rate con-
stants,14 which are highly dependent on carbanion substitution.
From a series of ketoprofen-derived carbanions, we observed
Laser excitation of a solution of 1 in DMSO, containing an
excess of NaH, gave a transient absorption spectrum with bands
(15) Valencic, M.; van der Does, T.; de Vroom, E. Tetrahedron Lett. 1998, 39,
1625-1628.
(16) Takeda, K.; Akiyama, A.; Nakamura, H.; Takizawa, S.-I.; Mizuno, Y.;
Takayanagi, H.; Harigaya, Y. Synthesis 1994, 1063-1066.
(17) Monti, S.; Sortino, S.; De Guidi, G.; Marconi, G. J. Chem. Soc., Faraday
Trans. 1997, 93, 2269-2275.
(19) Encinas, S.; Miranda, M. A.; Marconi, G.; Monti, S. Photochem. Photobiol.
1998, 67, 420-425.
(20) The decay kinetics were recorded at 390 nm because the intensity of the
signal at 800 nm was wery weak.
(18) This value was measured following 308 nm laser excitation since the signal
associated to the carbanion, generated in a basic aqueous solution, is better
defined.
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A R T I C L E S
Llauger et al.
Table 1. Absolute Rate Constants for Protonation and Cyclization
of Carbanions II and III
C-
solvent
fC
fW
kobs (s-1
)
kC (s-1
)
kW (s-1 a
)
II DMSOb 0.82 0.18 5.97 ( 0.06 × 106 4.9 × 106 (1.1 × 106)
waterc
0.07 0.93 6.73 ( 0.05 × 106 4.7 × 105 6.3 × 106
0.03 0.97 2.59 (0.01 × 106 7.7 × 104 2.5 × 106
III DMSOb 0.98 0.02 1.97 ( 0.02 × 106 1.9 × 106 (4.0 × 105)
water
a The values of kW in DMSO are given in parentheses since some
variation is anticipated due to day-to-day fluctuations in the water content
of “dry” DMSO. b Dried with CaH2, distilled under low pressure, and kept
in sieves (base used NaH).
that their reactivities were controlled by the number and size
of the alkyl groups attached to the carbanion center, an example
of steric control of protonation.
Figure 3. Plot of first-order rate constants for decay of carbanion II vs
(b) H2O and (9) D2O concentrations in DMSO, under N2 atmosphere.
Inset: transient carbanion decay at [H2O] ) 0 (τ ) 180 ns).
Photolysis of 4 mM 1 and 2 in 0.1 M KOH aqueous solutions
gave predominantly the protonated compounds 3 (93%) and 4
(97%), respectively (see Scheme 2). Quantum yields of decom-
position were approximately 0.8 for both compounds. The cyclic
compounds 5 (7%) and 6 (3%) were obtained in low yields.
They are formed by intramolecular cyclization (intra-SN2).
Under these experimental conditions, the intra-E2 products (7
and 8) were not detected. To favor intramolecular SN2 over
protonation, we changed the solvent for DMSO, leading to a
less solvent-encumbered anion. Further, DMSO cannot stabilize
the carbanion by hydrogen bonding, leading to a more nucleo-
philic anion.2 When we irradiated a 4 mM solution of 1 in
DMSO, in the presence of NaH, to high conversion, the
photoproducts contained 82% cyclization (5) and the remaining
18% was the protonated compound 3. Also, we isolated dimer
15, thermodynamic product of a (dark) nucleophilic substitution
between two carboxylic molecules. In one case, with freshly
dried DMSO, we were able to completely preclude formation
of 3; this suggests that formation of some 3 in DMSO is due to
traces of water. This is further supported by the high reactivity
of water in DMSO (vide infra). When a 4 mM solution of 2 in
DMSO with an excess of NaH was irradiated, we obtained
compound 6 in 98% yield. This shows that the tertiary carbanion
center is still capable of an intramolecular SN2 cyclization. No
reactions were observed when basic aqueous or NaH/DMSO
solutions were kept in the dark, except for the formation of 15
in solutions of 1 in NaH/DMSO.
their respective fractions. To evaluate the water effect on the
photolysis of 1 in DMSO, we examined the quenching of
carbanion II by water. The rate constant for carbanion decay
was plotted against the concentration of H2O or D2O. The slope
of the plot yields a bimolecular rate constant [kq(H2O)] of 5.1
× 107 M-1 s-1. We also determined the rate constant for
quenching by D2O, which led to kq(D2O) of 1.1 × 107 M-1 s-1
(Figure 3). This leads to an isotope effect [kq(H2O)/kq(D2O) ∼
5]. These results show that water (H2O or D2O), acts as a proton
source, favoring the formation of 3.
We also examined the quenching of carbanion III by water.
The slope of the plot (not shown) yields a bimolecular rate
constant [kq(H2O)] of 8.8 × 106 M-1 s-1. This low value, in
comparison to the one obtained from quenching of carbanion
II, corroborates again that the steric effect, due to an additional
methyl group directly attached to the carbanion center, decreases
the reactivity of the carbanion by a factor of 5.8 in DMSO. For
comparison, the values of kW in pure water for both carbanions
(II and III, see Table 1) yield a ratio of 2.5; i.e., hydrogen
bonding among water molecules seems to decrease its selectiv-
ity.
Extrapolation of the water quenching plot to 55 M water
predicts a value for kobs of 2.8 × 109 s-1 (τ = 0.4 ns). Water is
therefore ca. 400-fold more reactive in DMSO than in aqueous
media. This, together with the values of kC in the two solvents,
places the well-known fact that SN2 reactions are faster in polar
aprotic than in protic solvents on an absolute rate scale.
The fact that in one case we were able to achieve quantitative
cyclization of 1 in DMSO suggests that in the majority of cases,
when some protonation was observed, the formation of products
3 and 4 in DMSO is due to traces of water. The slope from
Figure 3 and the kW value in DMSO (e.g., see value in
parentheses in the top line of Table 1) allows an estimation of
the water content of “dry” DMSO. Thus, the value of 1.0 ×
106 s-1 suggests a ∼0.03% water content.
Considering that cyclization and protonation are the only two
decay pathways available to carbanions II and III, the observed
decay rate constants can be defined as:
kobs ) kW + kC
In conclusion, our studies of 1 and 2 render the first absolute
rate constants for reactions of carbanions II and III in solution.
The results provide insights into some established patterns of
nucleophilic behavior of carbanions, such as steric and solvent
effects. Further, it is clear that photodecarboxylations of
carboxylates attached to a suitable chromophore provide a useful
probe in this type of studies. Carbanions produced in this manner
are formed essentially instantaneously in an irreversible process,
thus making their transient decays, and the consequent formation
kW ) fW × kobs
kC ) fC × kobs
where kW is the pseudo-first-order protonation rate constant, kC
is the cyclization rate constant, and fW and fC are the fractions
of carbanions undergoing protonation and cyclization, respec-
tively. Table 1 gives the observed rate constants as well as the
calculated absolute rate constants for protonation (kW) and
cyclization (kC) of both carbanions (II and III), according to
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Absolute Rate Constants for Aroyl-Substituted Benzoyl Carbanions
A R T I C L E S
135.4 (C quaternary), 128.6-133.7 (Ph), 81.5 (C quaternary), 42.7
(CH2), 28.4 (CH3); MS m/z 296 (M+, 0.4), 196 (M+ - C5H9O2, 65).
of products, a kinetically controlled reaction, ideal for mecha-
nistic studies.
tert-Butyl 2-(3-Benzoylphenyl)propionate (12).23 A solution of
compound 10 (5 g, 20 mmol), DMAP (1.2 g, 9.8 mmol), and t-BuOH
(2 mL, 22 mmol) in 80 mL of methylene chloride was cooled with
stirring in an ice bath. 1-Ethyl-3-[3-(dimethylamino)propyl]carbodiimide
hydrochloride (EDCI, 4.2 g. 22 mmol) was added, and the reaction
mixture was stirred at 0°C for 2 h and at room temperature overnight.
The solution was concentrated to dryness in vacuo, and the residue
was taken up in acetyl acetate and water. The organic layer was
evaporated, washed with saturated sodium bicarbonate and water, and
dried (Na2SO4). The solvent was removed in vacuo to yield 12 (5.1 g,
Experimental Section
Compound 2-(3-benzoylphenyl)acetic acid was purchased from Karl
Industries Inc. (Aurora, OH). All the other starting materials used in
this work were purchased from Sigma-Aldrich Canada (Oakville, ON)
and used without further purification. Solvents were dried up by the
usual methods. DMSO was distilled from CaH2 and stored over
molecular sieves. Column chromatography was carried out with Merck
silica gel for flash columns, 230-400 mesh. Analytical thin-layer
chromatography (TLC) was performed on E. Merck precoated silica
gel 60 F254. All NMR spectra were recorded at 25°C at 300 MHz (1H
and 13C) and chemical shifts are reported relative to internal TMS. All
reactions were conducted under an inert atmosphere (N2 or Ar).
1
84%). H NMR (300 MHz, CDCl3) δ 7.41-7.80 (9H, Ph), 3.66 (q, J
) 7.2 Hz, 1H, CH), 1.45 (d, J ) 7.2 Hz, 3H, CH3), 1.38 (s, 9H, CH3);
13C NMR (75 MHz, CDCl3) δ 196.9 (CO), 173.6 (COO), 141.8 (C
quaternary), 138.1 (C quaternary), 137.9 (C quaternary), 128.6-132.8
(Ph), 81.1 (C quaternary), 46.7 (CH), 28.4 (CH3); MS m/z 210 (M+
-
Photoproducts. We prepared 4 mM solutions of acids 1 and 2 in
DMSO and an excess of approximately 8 equiv of 95% NaH. After
radiation, the mixtures were quenched with water and extracted with
diethyl ether. The organic layer was washed with brine, dried (Na2-
SO4), and concentrated in vacuo.
C5H9O2, 25).
tert-Butyl 2-(3-Benzoylphenyl)-7-iodoheptanoate (13). A solution
of 11 (500 mg, 1.69 mmol) in anhydrous THF (17 mL) was cooled in
a dry ice-acetone bath (-78 °C). To this solution were added LDA
(0.93 mL, 1.86 mmol) and 1,5-diodopentane (0.78 mL, 5.08 mmol).
After 10 min, the dry ice-acetone bath was removed and the mixture
was stirred, warming up to room temperature under an atmosphere of
nitrogen for one and a half hour. The reaction mixture was then
quenched with water and extracted with ethyl acetate (2 × 25 mL).
The organic layer was washed with brine, dried (Na2SO4), and
concentrated in vacuo to give a residue, which was purified by flash
chromatography column (Si2O, 95:5 hexane/AcOEt) to furnish 13 (470
Product Studies. Steady-state photolysis studies were carried out
in a prototype of a Luzchem photoreactor equipped with seven UV
lamps. The 4 mM NaH/DMSO solutions of 1 and 2 were deaerated
under an Ar atmosphere and irradiated for 20 min at 300 nm. The
photolysis mixture was analyzed by HPLC (Varian instrument using a
reverse phase 4.6 × 250 mm analytical Zorbax SB-C18 column; the
mobile phase used was 15:85 water:methanol, and the flow rate
employed was 0.5 mL/min. The detection used a Varian 9065
Polychrom diode array detector). For quantum yield determinations of
1 and 2, deaerated 4 mM 0.1 M aqueous KOH solutions of 1 and 2
were analyzed in the same conditions for 15 min of radiation.
Ketoprofen in 0.1 M KOH was employed as a standard (the quantum
yield of photodecarboxylation is 0.75 at pH 7.4).8 To quantify the
starting material, as well as the photoproducts, a calibration curve was
made in the HPLC with different KP standard solutions, registering
the signal at 254 nm.
1
mg, 57%). H NMR (300 MHz, CDCl3) δ 7.37-7.77 (9H, Ph), 3.47
(t, J ) 7.8 Hz, 1H, CH), 3.11 (t, J ) 6.9 Hz, 2H, CH2I), 2.00 (m, 2H,
CH2), 1.75 (m, 4H, CH2CH2), 1.37 (s, 9H, CH3), 1.27 (m, 2H, CH2);
13C NMR (75 MHz, CDCl3) δ 196.9 (CO), 173.1 (COO), 140.3 (C
quaternary), 138.1 (C quaternary), 137.9 (C quaternary), 128.7-132.7
(Ph), 81.3 (C quaternary), 52.8 (CH), 33.7 (CH2), 30.5 (CH2), 28.3
(CH3), 26.8 (CH2), 7.3 (CH2I); MS m/z 392 (M+ - C5H9O2, 54).
tert-Butyl 2-(3-Benzoylphenyl)-7-iodo-2-methylheptanoate (14).
We followed the same procedure as that for 13. The ester 12 (1 g, 3.2
mmol) gave compound 14 in a 66% (1.1 g). 1H NMR (300 MHz,
CDCl3) δ 7.37-7.7 (9H, Ph), 3.11 (t, J ) 7 Hz, 2H, CH2I), 1.97 (m,
2H, CH2), 1.77 (m, 2H, CH2), 1.48 (s, 3H, CH3), 1.35 (s, 3H, CH3),
1.23 (m, 4H, CH2CH2); 13C NMR (75 MHz, CDCl3) δ 197.1 (CO),
175.1 (COO), 145.3 (C quaternary), 138.0 (C quaternary), 137.9 (C
quaternary), 127.9-132.8 (Ph), 82.2 (C quaternary), 51.2 (C quater-
nary), 39.3 (CH2), 33.6 (CH2), 31.3 (CH2), 28.2 (CH3), 24.1 (CH2),
23.3 (CH3), 7.2 (CH2I); MS m/z 406 (M+ - C5H9O2, 43).
2-(3-Benzoylphenyl)-7-iodoheptanoic Acid (1).15 A stirred solution
of ester 13 (470 mg, 1 mmol) in dichloromethane (24 mL) under a
nitrogen atmosphere was cooled to -10 °C. Titanium tetrachloride (3.8
mL, 3.8 mmol) was slowly added, and the temperature was brought to
0 °C. After 3 h of stirring, a chilled 2 M solution of HCl (20 mL) was
added. The organic phase was separated, washed with 2 M HCl (3 ×
25 mL) brine (2 × 25 mL), and concentrated under reduced pressure
to yield compound 1 (247 mg, 60%) as yellow oil. 1H NMR (300 MHz,
CDCl3) δ 7.77 (s, 1H, Ph), 7.75 (d, J ) 1.4 Hz, 2H, Ph), 7.66 (m, 1H,
Ph), 7.55 (m, 2H, Ph) 7.45 (t, J ) 7.4 Hz, 3H, Ph), 3.61 (t, J ) 7.6 Hz,
1H, CH), 3.11 (t, J ) 6.9 Hz, 2H, CH2I), 2.09 (m, 1H, CH2), 1.76 (m,
3H, CH2), 1.31 (m, 4H, CH2CH2); 13C NMR (75 MHz, CDCl3) δ 196.8
(CO), 179.9 (COOH), 139.0 (C quaternary), 138.3 (C quaternary), 137.7
(C quaternary), 128.7-132.7 (Ph), 51.2 (CH), 33.0 (CHCH2), 32.8 (CH2-
CH2I), 30.1 (CH2), 26.4 (CH2), 7.2 (CH2I); MS m/z 263 (M+ - I, 9).
Nanosecond Laser Flash Photolysis. The laser flash photolysis
system has been previously described.21,22 To obtain kinetics of 0.1 M
KOH solutions of 1 and 2, the samples were excited with a Lumonics
EX-530 laser with a Xe-HCl-Ne mixture generating pulses at 308
nm of approximately 6 ns and 100 mJ output at the source. Kinetics
from 10 mM NaH/DMSO solutions of 1 and 2, as well as all the spectra
(aqueous and organic samples), were obtained by exciting with the third
harmonic of a Surelite Nd:YAG laser generating pulses at 355 nm of
8 ns duration and 20 mJ output. The signals from the monochromator/
photomultiplier system were initially captured by a Tektronix 2440
digitizer and transferred to a PowerMacintosh computer that controlled
the experiment with a software developed in the LabVIEW 5.1
environment form National Instruments. All the transient spectra and
kinetics were recorded by employing 7 × 7 mm2 Suprasil quartz flow
cells. Samples were deaerated by Ar (DMSO) or N2O (KOH). Use of
N2O eliminates any possible interference from hydrated electrons that
are otherwise long-lived in aqueous systems. The quenching rate
constants were obtained with static samples.
tert-Butyl 2-(3-Benzoylphenyl)acetate (11).16 To a solution of di-
tert-butyl dicarbonate (Boc2O, 1 g, 4.40 mmol) and 2-(3-benzoyl-
phenyl)acetic acid (9, 0.5 g, 2.08 mmol) in t-BuOH (21 mL) was added
4-(dimethylamino)pyridine (DMAP, 76 mg, 0.62 mmol) at room
temperature. After 50 min, the solvent was removed in vacuo to give
11 (0.69 g, 99%). 1H NMR (300 MHz, CDCl3) δ 7.37-7.78 (9H, Ph),
3,56 (s, 2H, CH2), 1.41 (s, 9H, CH3); 13C NMR (75 MHz, CDCl3) δ
196.8 (CO), 170.7 (COO), 138.1 (C quaternary), 138.0 (C quaternary),
2-(3-Benzoylphenyl)-7-iodo-2-methylheptanoic Acid (2). We fol-
lowed the same procedure as that for 1. The ester 14 (1 g, 3.2 mmol)
(21) Scaiano, J. C. J. Am. Chem. Soc. 1980, 102, 7747.
(22) Scaiano, J. C.; Tanner, M.; Weir, D. J. Am. Chem. Soc. 1985, 107, 4396.
(23) Dhaon, M. K.; Olsen, R. K.; Ramasamy, K. J. Org. Chem. 1982, 47, 1962-
1965.
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A R T I C L E S
Llauger et al.
1
gave compound 2 in a 66% (1.1 g). H NMR (300 MHz, CDCl3) δ
7.40-7.83 (s, 9H, Ph), 3.11 (t, J ) 6.9 Hz, 2H, CH2I), 1.99 (m, 2H,
CH2), 1.77 (q, J ) 7.5 Hz, 2H, CH2), 1.59 (s, 3H, CH3), 1.38 (m, 2H,
CH2), 1.23 (m, 2H, CH2); 13C NMR (75 MHz, CDCl3) δ 197.0 (CO),
182.4 (COOH), 143.5 (C quaternary), 138.0 (C quaternary), 137.8 (C
quaternary), 128.2-132.9 (Ph), 50.4 (CH), 39.1 (CH2), 33.5 (CH2),
31.2 (CH2), 24.0 (CH2), 7.3 (CH2I); MS m/z 450 (M+, 1.7), 406 (M+
- CO2H, 10), 373 (M+ - I, 1); HRMS m/z calcd for C21H23O3I
450.0692, found 450.0679.
Ph), 2.55 (tt, J ) 11.6 and 3.3 Hz, 1H, CH), 1.85 (m, 4H, CH2), 1.73
(bd, J ) 13 Hz, 1H, CH2), 1.40 (m, 5H, CH2); 13C NMR (225 MHz,
CDCl3) δ 196.0 (CO), 148.3 (C quaternary), 137.9 (C quaternary), 137.6
(C quaternary), 127.8-132.2 (Ph), 44.5 (CH), 34.3 (R-CH2), 26.8 (â-
CH2), 26.0 (γ-CH2); MS m/z 264 (M+, 65); HRMS m/z calcd for
C19H20O 264.1514, found 264.1515.
3-(1-Methylcyclohexyl)phenyl Phenyl Ketone(6). This compound
was formed following photodecarboxylation of 2 in NaH/DMSO
solutions. 1H NMR (300 MHz, CDCl3) δ 7.37-7.83 (m, 9H, Ph), 1.99
(m, 2H, CH2), 1.56 (m, 2H, CH2), 1.46 (m, 2H, CH2), 1.19 (s, 3H,
CH3); 13C NMR (75 MHz, CDCl3) δ 197.6 (CO), 150.7 (C quaternary),
138.2 (C quaternary), 137.8 (C quaternary), 127.8-132.7 (Ph), 38.5
(C quaternary), 38.2 (R-CH2), 30.1 (γ-CH2), 26.7 (CH3), 23.0 (â-CH2);
MS m/z 278 (M+, 52); HRMS m/z calcd for C20H22O 278.1671, found
278.1605.
3-(6-Iodohexyl)phenyl Phenyl Ketone (3). This compound was
formed following photodecarboxylation of 1 in a 0.1 M KOH solution.
1H NMR (300 MHz, CDCl3) δ 7.35-7.80 (m, 9H, Ph), 3.15 (t, J ) 7
Hz, 2H, CH2I), 2.65 (t, J ) 7.7 Hz, 2H, CH2Ph), 1.80 (quin, J ) 7 Hz,
2H, CH2CH2I), 1.63 (quin, J ) 7.7 Hz, 2H, CH2CH2Ph), 1.38 (m, 4H,
CH2CH2); 13C NMR (75 MHz, CDCl3) δ 197.4 (CO), 143.2 (C
quaternary), 138.1 (C quaternary), 138.0 (C quaternary), 128.1-133.0
(Ph), 36.0 (R-CH2), 33.7 (â-CH2), 31.5 (CH2CH2I), 30.7 (γ-CH2), 7.5
(CH2I); MS m/z 392 (M+, 7); HRMS m/z calcd for C19H21OI 392.0637,
found 392.0637.
3,11-Bis-(3-benzoylphenyl)-1,9-dioxa-cyclohexadecane-2,10-di-
one (15). Thermodynamic byproduct isolated only from a dark reaction
1
of 1 in NaH/DMSO solutions. H NMR (300 MHz, CDCl3) δ 7.34-
7.79 (m, 9H, Ph), 4.10 (m, 2H, CH2O); 3.70 (dt, J ) 12 and 3.5 Hz,
1H, CH), 2.26 (m, 1H, CHCH2), 1.23-1.44 (m, 7H, CH2); 13C NMR
(75 MHz, CDCl3) δ 196.9 (CO), 173.8-173.9 (COO), 140.4-140.5
(C quaternary), 138.2 (C quaternary), 133.8 (C quaternary), 128.7-
132.9 (Ph), 64.5-65.0 (CH2O), 51.4-52.0 (CH), 34.8-35.0 (CHCH2),
28.6-29.2 (OCH2CH2), 27.5-27.9 (CH2), 26.0-26.2 (CH2); MS m/z
616 (M+, 22), 308 (M+ - C20H20O3, 6); HRMS m/z calcd for monomer
C20H20O3 308.1412, found 308.1412.
3-(6-Iodo-1-methylhexyl)phenyl Phenyl Ketone (4). This com-
pound was formed following photodecarboxylation of 2 in a 0.1 M
KOH solution. 1H NMR (300 MHz, CDCl3) δ 7.36-7.80 (m, 9H, Ph),
3.12 (t, J ) 7 Hz, 2H, CH2I), 2.74 (q, J ) 7 Hz, 1H, CH), 1.75 (quin,
J ) 7 Hz, 2H, CH2), 1.58 (q, J ) 7.3 Hz, 2H, CH2), 1.2-1.3 (m, 4H,
CH2CH2), 1.24 (s, 3H, CH3); 13C NMR (75 MHz, CDCl3) δ 197.4 (CO),
148.2 (C quaternary), 138.1 (C quaternary), 138.0 (C quaternary),
128.4-132.7 (Ph), 40.2 (CH), 38.4 (CH2), 33.7 (CH2), 30.9 (CH2), 27.0
(CH2), 22.6 (CH3), 7.6 (CH2I); MS m/z 406 (M+, 9); HRMS m/z calcd
for C20H23OI 406.0794, found 406.0765.
Acknowledgment. J.C.S. thanks NSERC (Canada) for gener-
ous financial support. G.C. thanks the Ontario Graduate
Scholarship Program for a postgraduate scholarship.
3-Cyclohexylphenyl Phenyl Ketone (5). This compound was
formed following photodecarboxylation of 1 in NaH/DMSO solutions.
1H NMR (500 MHz, CDCl3) δ 7.78 (m, 2H, Ph), 7.65 (m, 1H, Ph),
7.56 (m, 2H, Ph), 7.47 (m, 2H, Ph), 7.42 (m, 1H, Ph), 7.37 (m, 1H,
JA027624K
9
15312 J. AM. CHEM. SOC. VOL. 124, NO. 51, 2002