Reestimation of the Taft's substituent constant of the pentafluorophenyl group

Article abstract of DOI:10.1021/jo048939g

The Taft's substituent constant of the pentafluorophenyl group (σ*C₆F₅)) was reestimated to be 1.50 by correlation between IR spectral data (v_C=O) and σ* constants for a series of esters (involving the pentafluorobenzyl group) of 3-phenylpropanoic acid and butanoic acid. The possibility of the disturbance of the correlation by the intramolecular π-π interaction between C ₆F₅ and C₆H₅ groups in pentafluorobenzyl 3-phenylpropanoate was excluded by ab initio and DFT calculations of the stable conformations and their carbonyl frequencies. The reestimated σ* (C₆F₅) value was used for calculation of the pK_a value of pentafluorobenzyl alcohol [14.5 (or 14.3)].

Full text of DOI:10.1021/jo048939g

Reestim a tion of th e Ta ft’s Su bstitu en t
Con sta n t of th e P en ta flu or op h en yl Gr ou p
Toshinobu Korenaga,* Koichi Kadowaki,
Tadashi Ema, and Takashi Sakai*
Department of Applied Chemistry, Faculty of Engineering,
Okayama University, 3-1-1 Tsushima-naka,
Okayama 700-8530, J apan
korenaga@cc.okayama-u.ac.jp
Received J une 23, 2004
Abstr a ct: The Taft’s substituent constant of the pentafluo-
rophenyl group (σ*(C6F5)) was reestimated to be 1.50 by
correlation between IR spectral data (vCdO) and σ* constants
for a series of esters (involving the pentafluorobenzyl group)
of 3-phenylpropanoic acid and butanoic acid. The possibility
of the disturbance of the correlation by the intramolecular
π-π interaction between C6F5 and C6H5 groups in pen-
tafluorobenzyl 3-phenylpropanoate was excluded by ab initio
and DFT calculations of the stable conformations and their
carbonyl frequencies. The reestimated σ*(C6F5) value was
used for calculation of the pKa value of pentafluorobenzyl
alcohol [14.5 (or 14.3)].
F IGURE 1. Estimation of the Taft’s σ*(C6F5) value by using
the carbonyl frequency of ester 1a .
points in which reexamination is required in their
estimation processes.9 Both cases required the precise
pKa values of C6F5 group-containing alcohols or carboxylic
acid; however, those were measured by potentiometric
titrations under basic conditions. The method is unsuit-
able for the compounds containing a C6F5 group because
of the high sensitivity at the 4-position toward oxy
anions.10 To avoid such experimental ambiguity, we here
reestimated the σ*(C6F5) value by correlation between
their IR spectral data (vCdO) and σ* constants using a
series of esters, Ph(CH2)2CO2CH2R (1) (method 1) and
n-C3H7CO2CH2R (2) (method 2), in which pentafluo-
robenzyl (R ) C6F5) esters are involved.11 The method
should be more suitable for estimation of the σ* value
for substituents (R) sensitive to basic conditions. The
σ*(C6F5) value (1.50) thus estimated was applied to give
a reestimated pKa value [14.5 (or 14.3)] for pentafluo-
robenzyl alcohol (3).
A pentafluorophenyl (C6F5) group is a distinctive and
useful substituent in organic synthesis,1 because of the
π-π stacking ability with an electron-rich aromatic ring2
and of the enhanced electron-withdrawing property. For
example, tris(pentafluorophenyl)borane highly acceler-
ates the aldol-type reaction of ketene silyl acetal and
aldehyde.3,4 The increasing utilities of the C6F5 group for
fine-tuning of the properties of advanced materials come
to demand a precise index, i.e., the Taft’s substituent
Resu lts a n d Discu ssion . We first examined adapting
the carbonyl frequency (vCdO) of pentafluorobenzyl 3-
phenylpropanoate12 (1a ) (Figure 1) to Cohen’s equation
derived by the correlation between vCdO and σ* values
(method 1).11 The vCdO value of the prepared ester 1a was
constant σ*, for the electron-withdrawing property.5,6
A
couple of Taft’s constant values for the C6F5 group
(σ*(C6F5)) have been reported to be 1.1 by Chang et al.7
and 1.96 by Brink,8 although both of them involve the
measured in CCl4 to show 1750.8 cm-1 13
and its adapta-
,
(1) Chiral diol, amino alcohol, or diamine bearing pentafluorophenyl
groups: (a) Sakai, T.; Kubo, K.; Kashino, S.; Uneyama, K. Tetrahe-
dron: Asymmetry 1996, 7, 1883. (b) Sakai, T.; Takayama, T.; Ohkawa,
T.; Yoshio, O.; Ema, T.; Utaka, M. Tetrahedron Lett. 1997, 38, 1987.
(c) Sakai, T.; Miki, Y.; Tsuboi, M.; Takeuchi, H.; Ema, T.; Uneyama,
K.; Utaka, M. J . Org. Chem. 2000, 65, 2740. (d) Korenaga, T.; Tanaka,
H.; Ema, T.; Sakai, T. J . Fluorine Chem. 2003, 122, 201. (e) Sakai, T.;
Korenaga, T.; Washio, N.; Nishio, Y.; Minami, S.; Ema, T. Bull. Chem.
Soc. J pn. 2004, 77, 1001.
(2) Review for interaction between the C6F5 group and aromatic
rings: Meyer, E. A.; Castellano, R. K.; Diederich, F. Angew. Chem.,
Int. Ed. 2003, 42, 1210.
(3) Review for applications of pentafluorophenylboranes: Piers, W.
E.; Chivers, T. Chem. Soc. Rev. 1997, 26, 345.
tion to Cohen’s equation (eq 1, see Figure 1) gave the
σ*(C6F5) value of 1.50 (Figure 1).
(7) Chang, I. S.; Price, J . T.; Tomlinson, A. J .; Willis, C. J . Can. J .
Chem. 1972, 50, 512.
(8) Brink, H. Acta Pharm. Suec. 1980, 17, 233.
(9) Chang’s value (σ*(C6F5) ) 1.1) was proposed by adaptation of
the pKa values of a couple of alcohols containing a C6F5 group to the
standard correlation equation between the pKa values and the O-H
frequency for nonfluorinated alcohols. However, the observed pKa
values for C6F5 group-containing alcohols were deviated from the
standard correlation line, showing smaller pKa values.7 On the other
hand, Brink’s value (σ*(C6F5) ) 1.96) was derived from the pKa value
(4) Ishihara, K.; Hanaki, N.; Funahashi, M.; Miyata, M.; Yamamoto,
H. Bull. Chem. Soc. J pn. 1995, 68, 1721.
(5) Taft, R. W., J r. J . Am. Chem. Soc. 1953, 75, 4231.
of C6F5COOH.8 However, two different pKa values (3.389a or 1.759b
)
had been reported independently, and the latter was used for estima-
tion of the σ*(C6F5) value of 1.96:8 (a) Chambers, R. D.; Drakesmith,
F. G.; Musgrave, W. K. R. J . Chem. Soc. 1965, 5045. (b) Ryan, M. T.;
Berner, K. J . Spectrochim. Acta, Part A 1969, 25, 1155.
(10) (a) Birchall, J . M.; Haszeldine, R. N. J . Chem. Soc. 1961, 3719.
(b) Burdon, J .; Hollyhead, W. B.; Patrick, C. R.; Wilson, K. V. J . Chem.
Soc. 1965, 6375.
(11) The method was established by Cohen et al., who reported the
evaluation of the σ* values for a series of substituents (R) by using an
experimentally driven correlation equation between vCdO and σ*(R)
values using Ph(CH2)2CO2CH2R: Takahashi, S.; Cohen, L. A.; Miller,
H. K.; Peake, E. G. J . Org. Chem. 1971, 36, 1205.
(6) More than 50 articles containing discussions using σ* constants
were published in 1999-2003. The articles published in 2003-2004
are shown here: (a) Babij, C.; Poe¨, A. J . J . Phys. Org. Chem. 2004, 17,
162. (b) El-Taher, S.; El-Azhary, A. A. Int. J . Quantum Chem. 2004,
98, 502. (c) Hamza, M. S. A.; Felluga, A.; Randaccio, L.; Tauzher, G.;
Eldik, R. Dalton Trans. 2004, 287. (d) Valentic, N. V.; Uscumlic, G.
S.; Radojkovic-Velickovic, M. Indian J . Chem., Sect. B: Org. Chem.
Incl. Med. Chem. 2003, 42B, 1137. (e) Lin, G.; Lai, C.; Liao, W.; Liao,
P.; Chan, C. J . Chin. Chem. Soc. 2003, 50, 1259. (f) Drmanic, S. Z.;
J ovanovic, B. Z.; Marinkovic, A. D.; Misic-Vukovic, M. M. J . Serb.
Chem. Soc. 2003, 68, 515. (g) Cherkasov, A.; Sprous, D. G.; Chen, R.
J . Phys. Chem. A 2003, 107, 9695.
(12) Davis, B. Anal. Chem. 1977, 49, 832.
(13) The deviation is (0.25 cm-1
.
10.1021/jo048939g CCC: $27.50 © 2004 American Chemical Society
Published on Web 09/21/2004
7340
J . Org. Chem. 2004, 69, 7340-7343

F IGURE 2. Confirmation of possible conformations for ester 1a .
Here, we should discuss the stability of the possible
conformations A and B for ester 1a (Figure 2), because
conformation B may be stabilized by the intramolecular
π-π (C6F5-C6H5) interaction,2,14 and it must exhibit an
unsuitable vCdO for the correlation. We confirmed that
conformation A is more stable than B15 by ab initio and
DFT calculations. The calculations of the optimized
geometries of the ester 1a were performed at the B3LYP/
6-31G* level16 using the Gaussian 03W.17 The relative
energy between A and B (∆E ) EA - EB) was estimated
to be -4.2 kcal mol-1 at the MP2/6-31G** level (Figure
2), which suggests a sufficient stability of conformation
A. In addition, the carbonyl frequencies (vCdO) of confor-
mations A and B were calculated at the B3LYP/6-31G*
level (Figure 2). After being scaled by 0.9613,18 the vCdO
of conformation A was estimated to be 1753 cm-1, which
was very close to the experimental value (1750.8 cm-1).
Thus, the abundance ratio of conformation B with vCdO
(1767 cm-1) is negligible.19
Then, we estimated the σ*(C6F5) value by using non-
aromatic carboxylic acid esters (2), without the possibility
of intramolecular π-π interaction (method 2). The vCdO
values of 12 kinds of esters20 (2a -l) were measured
(Table 1), and the correlation equation (eq 2) was derived
by using the least-squares method.
σ* ) 0.094630vCdO - 164.03
(14) The interaction energy between C6F6 and C6H6 was estimated
to be ca. -3.5 kcal mol-1: (a) Williams, J . H. Acc. Chem. Res. 1993,
26, 593. (b) West, A. P., J r.; Mecozzi, S.; Dougherty, D. A. J . Phys.
Org. Chem. 1997, 10, 347.
(r2 ) 0.993, RMS error ) 0.07) (2)
(15) The most stable conformation of the non-C6F5 compounds is
conformation A: Oakes, R. E.; Beattie, J . R.; Moss, B. W.; Bell, S. E.
J . J . Mol. Struct. (THEOCHEM) 2002, 586, 91.
(16) Frequency calculation performed on optimized structures at the
same level produced no imaginary frequencies.
(18) Wong, M. W. Chem. Phys. Lett. 1996, 256, 391.
(19) To demonstrate the accuracy of the calculated vCdO value, the
calculations of the vCdO of Ph(CH2)2CO2CH2CF3 and Ph(CH2)2CO2CH2-
CCl3 were carried out at the B3LYP/6-31G* level. After being scaled
by 0.9613, the former was estimated to be 1761 cm-1 (experimental
value 1763 cm-1) and the later was estimated to be 1760 cm-1
(experimental value 1760 cm-1).
(20) Ester 2c is a new compound. Esters 2a , 2b, 2i, 2k , and 2l were
commercially available. 2d : (a) Movsumzade, E. M.; Kulieva, D. A.;
Mamedov, M. G.; Nasirov, Ya. F. Izv. Vyssh. Uchebn. Zaved., Khim.
Khim. Tekhnol. 1985, 28, 12. 2e: (b) Dominguez, E.; Carretero, J . C.;
Fernandez-Mayoralas, A.; Conde, S. Tetrahedron Lett. 1991, 32, 5159.
2f: (c) Sapozhkova, N. D.; Smirnova, N. P. Metody Poluch. Khim.
Reakt. Prep. 1965, 13, 23. 2g: (d) Zeinalov, B. K.; Nasirov, A. B.;
Gadzhiev, T. P. Azerb. Khim. Zh. 1968, 6, 122. 2h : (e) Pelter, A.;
Colclough, E. Tetrahedron 1995, 51, 811. 2j: (f) Tsujimoto, H. J apanese
Kokai Tokkyo Koho J P 2004047183, 2004.
(17) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J . R.; Montgomery, J . A., J r.; Vreven, T.;
Kudin, K. N.; Burant, J . C.; Millam, J . M.; Iyengar, S. S.; Tomasi, J .;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J .; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J . E.; Hratchian, H. P.; Cross, J . B.; Adamo,
C.; J aramillo, J .; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin,
A. J .; Cammi, R.; Pomelli, C.; Ochterski, J . W.; Ayala, P. Y.; Morokuma,
K.; Voth, G. A.; Salvador, P.; Dannenberg, J . J .; Zakrzewski, V. G.;
Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J . B.; Ortiz, J . V.; Cui,
Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J .; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J .;
Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challa-
combe, M.; Gill, P. M. W.; J ohnson, B.; Chen, W.; Wong, M. W.;
Gonzalez, C.; Pople, J . A. Gaussian 03, Revision B.04; Gaussian, Inc.:
Pittsburgh, PA, 2003.
(21) The correlation coefficient for Ph(CH2)2CO2CH2R was not
described in ref 11. We calculated the correlation coefficient from their
data by the least-squares method to give r2 ) 0.997 (RMS error ) 0.05),
which was slightly higher than that of n-C3H7CO2CH2R. Therefore,
we selected the σ*(C6F5) value of 1.50 derived from eq 1.
J . Org. Chem, Vol. 69, No. 21, 2004 7341

TABLE 1. Ca r bon yl F r equ en cies of Bu tyr ic Acid Ester s a n d σ* Va lu es
a
b
Reference 11. Calculated by using eq 2.
calculations. Consequently, we concluded that the σ*(C6F5)
value is 1.50,21 which is larger than Chang’s value
(σ*(C6F5)
(σ*(C6F5) ) 1.96).
) 1.1) and smaller than Brink’s value
Finally, we utilized the σ*(C6F5) value for estimation
of the pKa value of alcohol 3. The σ*(C6F5) value (1.50)
estimated above was adapted to the pKa-σ*(R) correla-
tion eqs 38 or 411 to give the pKa values of 14.5 or 14.3
for 3, respectively (Figure 3). These values must be more
reliable as compared with the reported value (13.7) that
was calculated by eq 3, using Brink’s σ*(C6F5) value
(1.96).8
F IGURE 3. Estimation of the pKa value for pentafluorobenzyl
alcohol 3.
In conclusion, Taft σ* values were widely used in a
variety of areas, for instance, kinetic or thermodynamic
studies, designing of advanced materials, and so on.6
Figure 4 shows the order of inductive effect for a series
of fluorinated or aromatic substituents.22 The pentafluo-
rophenyl group is found to possess the moderate electron-
withdrawing ability between 3,5-dinitrophenyl and di-
fluoromethyl groups.
Satisfactory correlation between the σ*(R) and vCdO
values for esters 2a -l was observed.21 The σ*(C6F5) value
was thus estimated to be 1.54 by adaptation of the
measured vCdO value (1749.7 cm-1) for ester 2m 12 in CCl4
to eq 2, and found that the value is consistent with that
(1.50) determined by using ester 1a . These results also
suggest that influence of the intramolecular π-π interac-
tion in ester 1a is negligible as was confirmed by the
F IGURE 4. Taft’s σ* values for the fluorinated or aromatic compounds.
7342 J . Org. Chem., Vol. 69, No. 21, 2004

0.6, 13.3, 17.7, 35.4, 63.8, 172.2; IR (CCl4) 981, 1091, 1165, 1249,
1743, 2876, 2941, 2967 cm-1
Exp er im en ta l Section
.
P en ta flu or oben zyl 3-P h en ylp r op a n oa te (1a ).12 To a solu-
tion of pentafluorobenzyl alcohol (0.59 g, 3.6 mmol) and pyridine
(0.71 g, 9.0 mmol) in Et2O (6.0 mL) was added 3-phenylpropanoyl
chloride (0.51 g, 3.0 mmol) at 0 °C. After being stirred for 15 h,
the mixture was acidified (pH <4) by 10% dilute HCl, and an
organic layer was extracted with ether (3 × 4 mL) and treated
in the usual manner. The product was purified by column
chromatography (SiO2, hexane/EtOAc (8:1)) to give 1a (0.90 g,
2.7 mmol, 91% yield) as a colorless oil. 1H NMR (200 MHz,
CDCl3) δ 0.94 (t, J ) 7.4 Hz, 3H), 1.66 (m, 2H), 2.31 (t, J ) 7.4
Hz, 2H), 5.19 (s, 2H); 19F NMR (282 MHz, CDCl3) δ -84.8 (m,
2F), -76.0 (t, J ) 21 Hz, 1F), -65.4 (m, 2F); IR (CCl4) 942, 1058,
2-P h en oxyeth yl Bu tyr a te (2g).20d Ester 2g was prepared
from 2-phenoxyethanol (0.83 g, 6.0 mmol), butyryl chloride (0.53
g, 5.0 mmol), and pyridine (1.2 g, 15 mmol) quantitatively (1.0
g, 5.0 mmol). Colorless oil; 1H NMR (200 MHz, CDCl3) δ 0.95 (t,
J ) 7.4 Hz, 3H), 1.67 (m, 2H), 2.34 (t, J ) 7.4 Hz, 2H), 4.17 (t,
J ) 4.6 Hz, 2H), 4.44 (t, J ) 4.6 Hz, 2H), 6.89-7.0 (m, 3H),
7.26-7.33 (m, 2H); 13C NMR (50 MHz, CDCl3) δ 12.9, 17.7, 35.1,
61.8, 65.1, 113.8, 120.3, 128.8, 157.9, 172.2; IR (CCl4) 955, 1087,
1172, 1245, 1456, 1495, 1589, 1601, 1743, 2876, 2955, 2967 cm-1
.
2-Meth oxyeth yl Bu tyr a te (2h ).20e Ester 2h was prepared
from 2-methoxyethanol (0.73 g, 9.6 mmol), butyryl chloride (0.85
g, 8.0 mmol), and pyridine (2.1 g, 26 mmol) in 73% yield (0.85 g,
5.8 mmol). Colorless oil; 1H NMR (200 MHz, CDCl3) δ 0.95 (t,
J ) 7.4 Hz, 3H), 1.67 (m, 2H), 2.33 (t, J ) 7.4 Hz, 2H), 3.39 (s,
3H), 3.60 (t, J ) 4.8 Hz, 2H), 4.23 (t, J ) 4.8 Hz, 2H); IR (CCl4)
1135, 1311, 1507, 1522, 1751, 2850, 2917, 3030 cm-1
.
2,2,2-Tr ibr om oeth yl Bu tyr a te (2c). Ester 2c was prepared
from 2,2,2-tribromoethanol (1.0 g, 3.6 mmol), butyryl chloride
(0.32 g, 3.0 mmol), and pyridine (0.71 g, 9.0 mmol) in 50% yield
(0.54 g, 1.5 mmol) by the procedure described for ester 1a . Brown
1034, 1094, 1132, 1179, 1252, 1739, 2877, 2934, 2967 cm-1
.
2-Eth oxyeth yl Bu tyr a te (2j).20f Ester 2j was prepared from
2-ethoxyethanol (0.65 g, 7.2 mmol), butyryl chloride (0.64 g, 6.0
mmol), and pyridine (1.4 g, 18 mmol) in 56% yield (0.54 g, 3.4
1
oil; H NMR (500 MHz, CDCl3) δ 1.02 (t, J ) 7.5 Hz, 3H), 1.76
(m, 2H), 2.47 (t, J ) 7.5 Hz, 2H), 4.93 (s, 2H); 13C NMR (50 MHz,
CDCl3) δ 13.6, 18.1, 35.7, 36.0, 76.4, 171.2; IR (CCl4) 1012, 1043,
1091, 1152, 1244, 1757, 2851, 2876, 2921, 2966 cm-1. Anal. Calcd
for C6H9Br3O2: C, 20.42; H, 2.57. Found: C, 20.23; H, 2.52.
2-Cya n oeth yl Bu tyr a te (2d ).20a Ester 2d was prepared from
2-cyanoethanol (0.60 g, 8.4 mmol), butyryl chloride (0.75 g, 7.0
mmol), and pyridine (1.7 g, 21 mmol) in 50% yield (0.49 g, 3.5
1
mmol). Colorless oil; H NMR (200 MHz, CDCl3) δ 0.95 (t, J )
7.6 Hz, 3H), 1.22 (t, J ) 7.0 Hz, 3H), 1.66 (m, 2H), 2.33 (t, J )
7.6 Hz, 2H), 3.54 (q, J ) 7.0 Hz, 2H), 3.63 (t, J ) 4.6 Hz, 2H),
4.23 (t, J ) 4.6 Hz, 2H); 13C NMR (50 MHz, CDCl3) δ 13.4, 14.9,
18.2, 35.8, 63.2, 66.3, 68.2, 173.3; IR (CCl4) 1128, 1180, 1253,
1739, 2875, 2971 cm-1
.
P en ta flu or oben zyl Bu tyr a te (2m ).12 Ester 2m was pre-
pared from pentafluorobenzyl alcohol (0.71 g, 3.6 mmol), butyryl
chloride (0.38 g, 3.6 mmol), and pyridine (0.85 g, 10.8 mmol) in
1
mmol). Colorless oil; H NMR (200 MHz, CDCl3) δ 0.97 (t, J )
7.4 Hz, 3H), 1.68 (m, 2H), 2.35 (t, J ) 7.4 Hz, 2H), 2.71 (t, J )
6.4 Hz, 2H), 4.29 (t, J ) 6.4 Hz, 2H); 13C NMR (50 MHz, CDCl3)
δ 12.6, 17.1, 34.8, 57.8, 116.6, 171.9; IR (CCl4) 1092, 1168, 1249,
1
56% yield (0.54 g, 2.0 mmol). Colorless oil; H NMR (200 MHz,
1557, 1748, 2850, 2877, 2918, 2966 cm-1
.
CDCl3) δ 0.94 (t, J ) 7.4 Hz, 3H), 1.66 (m, 2H), 2.31 (t, J ) 7.4
Hz, 2H), 5.19 (s, 2H); 19F NMR (282 MHz, CDCl3) δ -84.8 (m,
2F), -76.0 (t, J ) 21 Hz, 1F), -65.4 (dd, J ) 6.8, 21 Hz, 2F); IR
(CCl4) 943, 1058, 1134, 1163, 1312, 1507, 1522, 1750, 2876, 2934,
2-Ch lor oeth yl Bu tyr a te (2e).20b Ester 2e was prepared from
2-chloroethanol (0.58 g, 7.2 mmol), butyryl chloride (0.64 g, 6.0
mmol), and pyridine (1.4 g, 18 mmol) in 84% yield (0.76 g, 5.1
2968 cm-1
.
1
mmol). Colorless oil; H NMR (200 MHz, CDCl3) δ 0.97 (t, J )
7.4 Hz, 3H), 1.68 (m, 2H), 2.34 (t, J ) 7.4 Hz, 2H), 3.68 (t, J )
6.0 Hz, 2H), 4.34 (t, J ) 6.0 Hz, 2H); 13C NMR (50 MHz, CDCl3)
δ 12.7, 17.7, 35.0, 41.1, 63.1, 171.9; IR (CCl4) 1095, 1170, 1247,
Ack n ow led gm en t. This work was partially sup-
ported by the Ministry of Education, Science, Sports and
Culture of J apan. We thank the SC-NMR Laboratory
of Okayama University for 1H, 13C, and 19F NMR
measurements.
1311, 1746, 2877, 2935, 2967 cm-1
.
2-Iod oeth yl Bu tyr a te (2f).20c Ester 2f was prepared from
2-iodoethanol (0.83 g, 4.8 mmol), butyryl chloride (0.43 g, 4.0
mmol), and pyridine (0.95 g, 12 mmol) in 91% yield (0.88 g, 3.6
mmol). Brown oil; 1H NMR (200 MHz, CDCl3) δ 0.97 (t, J ) 7.4
Hz, 3H), 1.68 (m, 2H), 2.33 (t, J ) 7.4 Hz, 2H), 3.30 (t, J ) 6.6
Hz, 2H), 4.33 (t, J ) 6.6 Hz, 2H); 13C NMR (50 MHz, CDCl3) δ
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
spectra of compound 2c and Cartesian coordinates of 1a . This
material is available free of charge via the Internet at
http://pubs.acs.org.
(22) Taft’s σ* data for several fluorine substituents: Bra¨ndstro¨m,
A. J . Chem. Soc., Perkin Trans. 2 1999, 1855.
J O048939G
J . Org. Chem, Vol. 69, No. 21, 2004 7343