Intermolecular oxygen atom?π interaction in the crystal packing of chiral amino alcohol bearing a pentafluorophenyl group

Article abstract of DOI:10.1016/S0022-1139(03)00089-7

Presence of an unique atom-to-face alcoholic oxygen atom?π interaction between a pentafluorophenyl group and alcoholic oxygen (O?π) has been demonstrated by X-ray analysis of the novel chiral amino alcohol instead of the well-known interaction between usu

Full text of DOI:10.1016/S0022-1139(03)00089-7

Journal of Fluorine Chemistry 122 (2003) 201–205
Intermolecular oxygen atomꢀ ꢀ ꢀp interaction in the crystal packing of
chiral amino alcohol bearing a pentafluorophenyl group
Toshinobu Korenaga*, Hikaru Tanaka, Tadashi Ema, Takashi Sakai*
Department of Applied Chemistry, Faculty of Engineering, Okayama University, Tsushima-naka, Okayama 700-8530, Japan
Received 29 January 2003; received in revised form 10 March 2003; accepted 14 March 2003
Abstract
Presence of an unique atom-to-face alcoholic oxygen atomꢀ ꢀ ꢀp interaction between a pentafluorophenyl group and alcoholic oxygen
(Oꢀ ꢀ ꢀp) has been demonstrated by X-ray analysis of the novel chiral amino alcohol instead of the well-known interaction between usual
aromatic ring and alcoholic hydrogen atom (OHꢀ ꢀ ꢀp).
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Aromatic interaction; Attractive interaction; Chiral ligand; Amino alcohol; Asymmetric synthesis
1. Introduction
Intermolecular aromatic interactions (e.g. p–p [1],
XHꢀ ꢀ ꢀp [2], cation-p [3], etc.) are relatively weak in com-
parison with typical hydrogen bonds, although these forces
plays a significant role in molecular recognition [4] in both
biological [5] and chemical systems.1,2 Under such circum-
stance, perfluorophenyl groups construct occasional inter-
Fig. 1. Interaction of water with benzene (a), and with hexafluorobenzene
(b).
actions with electron-rich aromatic rings. Thus, the mixture
of hexafluorobenzene and benzene forms a 1:1 co-crystal
with a face-to-face stacked arrangement [8], while the
preferential geometry of the benzene dimer is edge-to-face
[1,9] (i.e. CHꢀ ꢀ ꢀp interaction). Theoretical studies3 suggest
that hexafluorobenzene interacts with several electron-rich
atoms (atom-to-face interaction), in contrast to the prefer-
ential XHꢀ ꢀ ꢀp or cation-p interaction of benzene. Recently,
Gallivan and Dougherty investigated the mode of interaction
between hexafluorobenzene and a water molecule using ab
initio calculations [11] (Fig. 1). The lone pair electrons of
oxygen is situated over the face of the p system [11] (oxygen
atomꢀ ꢀ ꢀp interaction, i.e. Oꢀ ꢀ ꢀp interaction (Fig. 1b)),
whereas the hydrogen atom of a water molecule interacts
with the benzene plane [12] (OHꢀ ꢀ ꢀp interaction (Fig. 1a)).
Recently, Oꢀ ꢀ ꢀp interactions between perfluorophenyl
groups and an oxygen atom of amide carbonyl were dis-
closed by X-ray analysis of carbonic anhydrase II inhibi-
tors.4
Here, we wish to report the first observation of an Oꢀ ꢀ ꢀp
interaction between a pentafluorophenyl group and alcoholic
oxygen in the crystal packing of the novel chiral amino
alcohol, (1R,2S)-2-amino-1-(pentafluorophenyl)-2-pheny-
lethanol (1) (other chiral diol or amino alcohol bearing a
pentafluorophenyl group [14], fluorinated aromatic ligand
for asymmetric catalyst [15]), which we prepared as a
promising new chiral ligand [16]. This finding of the
Oꢀ ꢀ ꢀp interaction supports Dougherty’s prediction [11]
(Fig. 1b), because the alcoholic oxygen is similar to that
* Corresponding authors. Tel.: þ81-86-251-8092;
fax: þ81-86-251-8092.
E-mail address: tsakai@cc.okayama-u.ac.jp (T. Sakai).
1 Effect of an attractive interaction between a pentafluorophenyl group
and an enolate anion on the diastereoselective reactions [6].
2 Examples of aromatic interaction in asymmetric synthesis, see [7].
3 The interactions in X-ray crystals have also been investigated by
searching the Cambridge structural database (CSD). However, tangible
examples have not been shown, and moreover, alcoholic compounds have
not been contained in these searches [10].
4 The sp2 ! pà charge-transfer complexation dominate over the
interaction between amide carbonyl and pentafluorophenyl groups,
although the electrostatic interaction is favored in the case of oxygen
atom (sp3) of a water molecule [13].
0022-1139/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0022-1139(03)00089-7

202
T. Korenaga et al. / Journal of Fluorine Chemistry 122 (2003) 201–205
Scheme 1. Synthesis of (1R,2S)-1.
of a water molecule. Elucidation of such intra- or inter-
molecular interactions (intramolecular interaction between a
pentafluorophenyl group and a nitrogen atom [17]) would be
significant for the development of a new molecular recogni-
tion system, especially for asymmetric synthesis.1,2
[14c]. (R)-Cyanohydrin 3 was treated with PhLi in the
presence of tetramethylsilane (TMS)-Cl (addition of
TMS-Cl was essential in this reaction [18]) in Et2O at
À85 8C to give the corresponding (R)-imine 4, which with-
out purification was reduced with NaBH4 in EtOH at 0 8C to
give amino alcohol 5 in 90% de. After purification by silica-
gel column chromatography, the major diastereomer
(1R,2S)-5 was obtained in >99% ee and 63% yield (two
steps). Treatment with 1.1 eq. of tetrabutylammonium fluor-
2. Results and discussion
Synthesis of the desired amino alcohol 1 was performed
as shown in Scheme 1. Enantiomerically pure (R)-cyanohy-
drin 2 [14c], which was prepared by enzymatic optical
resolution, was protected with tert-butyldimethylsilyl tri-
fluoromethanesulfonate (TBS-OTf) without racemization
ide (TBAF) gave enantiomericallly pure 1 in 94% yield
24
(½aŠ ¼ þ37 (c 0.49, EtOH)). The relative configuration of 1
D
was determined to be erythro by X-ray analysis (Fig. 2). The
diastereoselectivity is consistent with that in the reduction of
similar imines derived from chiral cyanohydrins [14a,19].
Fig. 2. X-ray analysis of (1R,2S)-1.

T. Korenaga et al. / Journal of Fluorine Chemistry 122 (2003) 201–205
203
O
˚
{[sum of the van der Waals radii [21] (3.22 A) of oxygen rw
C
˚
˚
(1.52 A) and carbon rw (1.70 A)] Â 1.05} (Fig. 3) [2].
Existence of the Oꢀ ꢀ ꢀp interaction was judged by compar-
ison of a couple of conditions in which (a) all the distances of
Dpln, Datm, and Dlin are shorter than Dmax, and (b) angle o
[O–(C6–C7)–Ar plane] is smaller than 908 [2]. In fact, all the
distances Dpln, Datm, Dlin were shorter than Dmax as well as
the sum of the van der Waals radii of oxygen rwO and carbon
rwC. In addition, o was smaller than 908. These data satisfied
Nishio’s definition for the presence of an Oꢀ ꢀ ꢀp interaction.
The position of the oxygen in close proximity to the pen-
tafluorophenyl group is reasonably explained not only by the
assistance of the two interactions of O–Hꢀ ꢀ ꢀN and N–Hꢀ ꢀ ꢀp
butalsothatofanOꢀ ꢀ ꢀpinteraction,because(a)thestrengthof
the N–Hꢀ ꢀ ꢀp interaction (<2 kcal molÀ1) [22] is estimated to
be similar to that of an Oꢀ ꢀ ꢀp interaction for a water molecule
[11] and is not enough to parallelize both aromatic rings, and
(b) the stronger O–Hꢀ ꢀ ꢀN interaction is not directed to the
pentafluorophenyl group to regulate the geometry.
Fig. 3. Analysis of location of alcohol’s oxygen toward the pentafluor-
ophenyl group in the crystal packing of 1 for confirmation of Oꢀ ꢀ ꢀp
interaction.
We initially expected that the intra- and/or intermolecular
p–p interaction(s) between the pentafluorophenyl and phe-
nyl groups [1,8] operates predominantly to regulate the
crystal packing of the amino alcohol 1, because we had
observed a similar p–p stacking between pentafluorophenyl
and naphthyl groups [14c]. However, those interactions were
not observed in this crystal structure. As shown in Fig. 2a,
the dihedral angle (C3–C1–C2–C9) is 1728, which indicates
that pentafluorophenyl and phenyl groups are in the anti
conformation.
3. Conclusion
As shown in Fig. 2b (side view), the two molecules are
stacked head-to-head, and the same distances of C6 ꢀ ꢀ ꢀ C6
0
The observed distance of the Oꢀ ꢀ ꢀAr (C6F5) plane (i.e.
0
and C12 ꢀ ꢀ ꢀ C12 (5.35 A) indicate the slipped-parallel orien-
˚
˚
Dpln ¼ 3:05 A) was rather shorter than that calculated for a
˚
water oxygen and hexafluorobenzene (3.20 A) [11], despite
tation. The crystal packing (up and down) seems to be
regulated principally by the O–Hꢀ ꢀ ꢀN hydrogen bond5
(Fig. 2b (I)), which, however, would not be sufficient for
the parallelization of the two stacked molecules. Judging
from the packing structure, the atomꢀ ꢀ ꢀp interactions could
assist the parallelization, one of which would be an N–Hꢀ ꢀ ꢀp
interaction6 (Fig. 2b (II)), which is known to be weak [2b].
The other example we discuss here is the interaction
between alcoholic oxygen and the pentafluorophenyl group
(Oꢀ ꢀ ꢀp interaction (III)). The distance between the oxygen
the lower electron density of the former alcoholic oxygen
located nearby the pentafluorophenyl group. These results
conclude us that the pentafluorophenyl group has the poten-
tial to interact with an alcoholic oxygen atom. These find-
ings will be significant in the design of novel ligands for
asymmetric synthesis and/or supramolecular systems for
advanced materials.
˚
4. Experimental
and the ring centroid is 3.08 A (Fig. 2b (IV)), which is
slightly longer than that for the reported amide oxygen
4
2
˚
4.1. General methods
(2.9 A), probably due to the different nature of sp and
sp3 oxygen atoms.
All reactions were carried out under a nitrogen atmo-
sphere with dry and freshly distilled solvents under anhy-
drous conditions, unless otherwise noted. Tetrahydrofuran
(THF) was distilled from sodium, and dichloromethane
(CH2Cl2) was distilled from calcium hydride. Reactions
were monitored by thin-layer chromatography with pre-
coated silica-gel plates (Merck 60 F254, plate length
40 mm). As a usual workup procedure, the reaction mixture
was extracted with ethyl acetate (EtOAc). The organic layer
was dried over MgSO4, filtrated with suction, and then
concentrated in vacuo. Preparative column chromatography
was carried out by using silica-gel (Fuji Silysia BW-127 ZH,
100–270 mesh). 1H and 13C NMR spectra were measured at
200 MHz (or 300 MHz) and 50 MHz (or 75 MHz), respec-
tively, and chemical shifts are given relative to tetramethyl-
silane. 19F NMR spectra were measured at 188 MHz
(or 282 MHz) and chemical shifts are given relative to C6F6.
The conventional method, based on only the distance
between the oxygen atom and the aromatic ring,3,4 was
insufficient. Thus, the presence of such an Oꢀ ꢀ ꢀp interaction
was examined at several distances, which were defined by
Nishio et al. for survey of XHꢀ ꢀ ꢀp (especially CHꢀ ꢀ ꢀp)
interactions [2].7 The obtained distances are Dpln (Oꢀ ꢀ ꢀAr
plane), Datm (Oꢀ ꢀ ꢀC7), Dlin (Oꢀ ꢀ ꢀC7–C8), and Dmax (3.38 A)
˚
5
˚
The distance between hydrogen and nitrogen atoms is ca. 1.8 A and
˚
that for oxygen and nitrogen atoms is 2.83 A which is within a range of
general O–H ꢀ ꢀ ꢀN hydrogen bonds [20].
6 The distance between the hydrogen atom and the nearest carbon atom
˚
in the phenyl group is ca. 2.6 A (Fig. 2b (II)). This distance is within a
range of general N–H ꢀ ꢀ ꢀAr interaction [2b].
7 In addition to distance parameters, an angle parameter y was
considered in the CH ꢀ ꢀ ꢀp interaction [2]. However, atomic natures,
especially electronic distribution, differ vastly between hydrogen and
oxygen atoms. Therefore, angle parameter y was excluded here.

204
T. Korenaga et al. / Journal of Fluorine Chemistry 122 (2003) 201–205
4.2. (R)-2-(tert-Butyldimethylsiloxy)-2-
(pentafluorophenyl)acetonitrile ((R)-3)
for C19H21F5NOSi: C, 57.54; H, 5.79; N, 3.36. Found: C,
57.18; H, 5.83; N, 3.50.
For (1R,2R)-5. Yellow oil; Rf ¼ 0:36 (hexane/EtOAc
1
TBS-OTf, 2.60 ml, 11.3 mmol was slowly added to a
solution of cyanohydrin (R)-2 (1.67 g, 7.50 mmol, >99%
ee) (obtained by the lipase LIP-catalyzed transesterification
of cyanohydrin (Æ)-2 as reported in [14c]) and 4-(dimethy-
lamino)pyridine (DMAP, 1.85 g, 10.0 mmol) in CH2Cl2
(20 ml) under cooling in an ice bath. After being stirred
for 2.5 h, the mixture was acidified (pH 4) by addition of 3%
aqueous HCl. The resulting mixture was treated in a usual
manner. The residual mixture was purified by column
chromatography (SiO2, hexane/EtOAc (10:1)) to give (R)-
3 (2.62 g, 96% yield, 99% ee) as a colorless oil: Rf ¼ 0:58
(SiO2, hexane/EtOAc (4:1)); 1H NMR (200 MHz, CDCl3) d
0.13 (s, 3H), 0.26 (s, 3H), 0.90 (s, 9H), 5.77 (s, 1H); 13C
NMR (50 MHz, CDCl3) d À5.4, À5.3, 18.1, 25.4, 54, 111.2
(4:1)); H NMR (CDCl3) d À0.17, (s, 3H), 0.07 (s, 3H),
0.88 (s, 9H), 1.48 (d, 2H), 4.38 (d, J ¼ 8:4 Hz, 1H), 4.98 (d,
J ¼ 8:4 Hz, 1H), 7.20–7.27 (m, 5H).
4.4. (1R,2S)-2-Amino-1-(pentafluorophenyl)-2-
phenylethanol (erythro-1)
To a solution of amine (1R,2S)-5 (240 mg, 0.57 mmol) in
THF (2.0 ml) was added tetra-n-butylammonium fluoride
(TBAF, 1 M in THF, 0.57 ml, 0.57 mmol), and then the
mixture was stirred for 1 h at 0 8C. The resulting mixture
was concentrated in vacuo, and the residual viscous oil was
purified by column chromatography (SiO2, EtOAc) to give
(1R,2S)-1 (161 mg, 94% yield) as a white solid. The optical
purity was determined by HPLC analysis after acetylation
(see below): mp, 174–175 8C, Rf ¼ 0:53 (SiO2, EtOAc); 1H
NMR (300 MHz, CDCl3) d 1.85 (br, 3H), 4.32 (d,
J ¼ 7:5 Hz, 1H), 5.10 (d, J ¼ 7:5 Hz, 1H), 7.27–7.34 (m,
5H); 19F NMR (282 MHz, CDCl3) d À0.36 À0.18 (m, 2F),
7.06 (t, J ¼ 21:0 Hz, 1F), 19.9 (dd, J ¼ 7:1, 21.0 Hz, 2F);
13C NMR (75 MHz, CDCl3) d 60.0, 71.1, 114.5 (m), 126.7,
128.2, 128.7, 135.6 (m), 138.8 (m), 141.1, 142.1 (m), 143.2
(m), 146.6 (m); IR (KBr) 3365, 3304 (OH), 3107 (NH2),
1523, 1499, 995 cmÀ1. Anal. Calcd for C14H10F5NO: C,
(m), 116.6, 135.2 (m), 140.4 (m), 142.4 (m), 147.5 (m); 19
F
NMR (188 MHz, CDCl3) d 1.7–2.1 (m, 2F), 11.6 (t,
J ¼ 20:3 Hz, 1F), 20.0–20.2 (m, 2F); IR (neat) 2958,
2934, 2862, 1513, 1003 cmÀ1. Anal. Calcd for C14H16NOSi:
C, 49.84; H, 4.78; N, 4.15. Found: C, 49.47; H, 4.47; N, 4.49;
HPLC analysis: retention time ¼ 10.7 and 11.5 min for (S)-
and (R)-3 (0.5:99.5), respectively. The conditions of HPLC
are as follows: Daicel CHIRALCEL OD-H, 4:6 mm Â
25 cm, hexane/i-PrOH (300:1), 0.5 ml minÀ1, UV 254 nm;
28
D
½aŠ ¼ þ30:1 (c 0.87, CHCl3).
55.45; H, 3.32; N, 4.62. Found: C, 55.69; H, 3.35; N, 4.61;
24
4.3. (1R,2S)-2-(tert-Butyldimethylsiloxy)-2-
(pentafluorophenyl)-1-phenylethylamine (erythro-5)
½aŠ ¼ þ37 (c 0.49, EtOH).
D
4.5. Acetylation of (1R,2S)-1 for HPLC analysis
A solution of TBS ether (R)-3 (1.69 g, 5.00 mmol) in Et2O
(17 ml) was stirred at À85 8C for 10 min. After addition of
chlorotrimethylsilane (TMS-Cl, 0.95 ml, 7.5 mmol), a solu-
tion of phenyllithium (1.8 M in cyclohexane–ether (7:3),
3.7 ml, 7.50 mmol) was added dropwise over 10 min. After
the mixture being stirred for additional 1.5 h at the tem-
perature, a suspension of NaBH4 (284 mg, 7.50 mmol) in
EtOH (10 ml) was added at 0 8C. After being stirred for
additional 1.5 h at the temperature, the combined mixture
was acidified (pH 4) with 3% aqueous HCl, and then
saturated aqueous NaHCO3 solution was added to become
pH 9. The resulting mixture was treated in a usual manner.
The residual oil was purified by column chromatography
(SiO2, hexane/EtOAc (20:1) to (5:1)) to give (1R,2S)-5
(1.30 g, 63% yield) and (1R,2R)-5, (1R,2S)-5/(1R,2R)-5 ¼
Pyridine (0.5 ml) was added to a solution of (1R,2S)-1
(20 mg, 0.066 mmol) and AcCl (45 ml, 0.66 mmol) in
toluene (1.0 ml) at 0 8C. After removal of a cooling bath,
the reaction mixture was stirred for 4 h. The mixture was
acidified (pH 4) with 3% aqueous HCl, and extracted with
ethyl acetate. The extract was treated in a usual manner. The
residual solid was purified by column chromatography
(SiO2, hexane/EtOAc (2:1)) to give diacetate of 1 (21 mg,
0.055 mmol, 83% yield, 99.4% ee) as a white solid. The
optical purity was determined by HPLC analysis: retention
time ¼ 15.8 and 48.4 min for diacetate of (1R,2S)-1 and
(1S,2R)-1 (99.7:0.3), respectively. The conditions of HPLC
are as follows: column; Daicel CHIRALCEL OD-H, f
4:6 mm  25 cm, hexane/i-PrOH (19:1)), 0.8 ml minÀ1
,
24
UV 254 nm; ½aŠ ¼ þ17 (c 0.15, EtOH).
1
95:5, which was determined by H NMR spectra.
D
For (1R,2S)-5. Colorless oil; Rf ¼ 0:35, (hexane/EtOAc
(4:1)); 1H NMR (200 MHz, CDCl3) d À0.39, (s, 3H), À0.29
(s, 3H), 0.66 (s, 9H), 1.43 (s, 2H), 4.31 (d, J ¼ 8:2 Hz, 1H),
4.94 (d, J ¼ 8:2 Hz, 1H), 7.26–7.35 (m, 5H); 19F NMR
(188 MHz, CDCl3) d À0.43 (s, 2F), 6.72 (t, 1F), 20.4–20.9
(m, 2F); 13C NMR (50 MHz, CDCl3) d À5.9, À5.6, 17.9,
25.5, 60.6, 72.5, 116.4 (m), 127.5, 127.9, 128.4, 135.0 (m),
138.1 (m), 140.0 (m), 142.7 (m), 142.9, 147.6 (m); IR (neat)
3394 (NH2), 2955, 2858, 1520, 1503, 999 cmÀ1. Anal. Calcd
4.6. X-ray analysis of (1R,2S)-1
The single crystal growth was carried out in a dichlor-
omethane–hexane mixed solvent at room temperature.
Crystal data for (1R,2S)-1: C14H10F5NO, Fw ¼ 303:23,
triclinic, space group P1, a ¼ 6:365ð2Þ, b ¼ 10:207ð4Þ,
˚
c ¼ 5:348ð2Þ A, a ¼ 99:539ð5Þ8, b ¼ 100:06ð3Þ8, g ¼
3
˚
101:774ð2Þ8, V ¼ 327:6ð2Þ A , T ¼ 150:2 K, Z ¼ 1,

T. Korenaga et al. / Journal of Fluorine Chemistry 122 (2003) 201–205
205
Dcalc ¼ 1:537 g cmÀ3, R ¼ 0:077 for 1205 observed reflec-
tions [I > 3:00sðIÞ] and 206 variable parameters. All mea-
surements were made on a Rigaku RAXIS-IV imaging plate
area detector with Mo Ka radiation. Crystallographic data
for the structure reported in this paper have been deposited
with the Cambridge Crystallographic Data Center as sup-
plementary publication no. CCDC-205107.
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Acknowledgements
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This work was partially supported by the Ministry of
Education, Science, Sports and Culture of Japan. We are
grateful to Dr. Toshiyuki Oshiki in Okayama University for
his kind assistance in X-ray analysis of 1, and to Toyobo Co.
Ltd. (Osaka, Japan) for giving us lipase LIP. We also thank
the SC-NMR Laboratory of Okayama University for H,
13C, and 19F NMR measurements and the Venture Business
Laboratory of Okayama University for X-ray analysis.
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