Synthesis of highly 1,3-proton shift transferable N-benzyl imines reaction conditions

Article abstract of DOI:10.1016/j.jfluchem.2003.11.032

The standard reaction conditions commonly used for the condensation of carbonyl compounds with amines were found to be synthetically inefficient for preparation of the imines derived from trifluoroacetophenone and benzylamines owing to the susceptibility

Full text of DOI:10.1016/j.jfluchem.2003.11.032

Journal of Fluorine Chemistry 125 (2004) 603–607
Synthesis of highly 1,3-proton shift transferable N-benzyl imines of
trifluoroacetophenone under the ‘‘low-basicity’’ reaction conditions
Dmitrii O. Berbasov, Ifeyinwa D. Ojemaye, Vadim A. Soloshonok*
Department of Chemistry and Biochemistry, University of Oklahoma, 620 Parrington Oval, Room 208, Norman, OK 73019, USA
Received 21 October 2003; accepted 17 November 2003
Abstract
The standard reaction conditions commonly used for the condensation of carbonyl compounds with amines were found to be synthetically
inefficient for preparation of the imines derived from trifluoroacetophenone and benzylamines owing to the susceptibility of these imines to
1,3-proton shift. Application of a ‘‘low-basicity’’ method, using instead of free benzylamines their salts formed from acetic acid (AA),
allowed synthesis of the target compounds in chemically pure form and excellent chemical yields.
# 2004 Elsevier B.V. All rights reserved.
Keywords: 1,3-Proton shift reaction; Imines; Operationally convenient conditions; Fluorine and compounds
1. Introduction
catalysts for general asymmetric synthesis [2b,3,4e,4f,4h].
For instance, taking into account a uniquely vide range of
various synthetic applications of a-phenylethylamine
(Fig. 2) and its derivatives in the modern asymmetric
synthesis [5], the synthetic potential offered by its enantio-
merically pure trifluoro-analog 1, wherein the trifluoro-
methyl might play the role of a stereodirecting group, is
enormous [2b,4,6].
Taking into account the importance of a-trifluoromethyl
(and more generally, perfluoroalkyl/aryl) containing amines,
as discussed above, it is not surprising that the development
of synthetic methods for preparing these compounds has
been the focus of numerous research groups [4,7–14,16,17]
(see also footnote 2). The vast variety of the approaches
available in the literature can be divided into the following
methods: (a) a conventional (with application of external
reducing reagents) reductive amination of the corresponding
trifluoromethyl ketones [7]; (b) a nucleophilic addition
of alkyl groups (Grignard reagents, sulfoxide-stabilized
carbanions) to a-trifluoromethyl-imines [8] or hydrazones
[9]; (c) reactions of the trifluoromethyl-imines [10]; (d) a
nucleophilic addition of trimethyl(trifluoromethyl)silane to
nitrones [11] or imines [12]; (e) a reduction or nucleophilic
opening of fluoral-derived 1,3-oxazolidines [13]; (f) ela-
boration of trifluoroacetimidoyl halides [14].
a-Trifluoromethyl containing amines A (Fig. 1) represent
one of the most synthetically powerful fluorine-containing
building block-synthons in modern organic chemistry. For
instance, the SciFinder search for the fixed structure B
(Fig. 1) yielded more than 11,466 compounds and 2204
references, which is truly remarkable considering that all of
these derivatives are purely synthetic. This unique interest in
chemistry of a-trifluoromethyl containing amino-com-
pounds is driven mostly by agricultural and pharmaceutical
industries1 using the structural motif B as a pharmacophore
unit in the design of new generations of fungicides,
pesticides, herbicides, desiccants, defoliants, insecticides,
arthropodicides, microbicides, selective antibacterial agents,
therapeutic agents/probes, enzyme inhibitors, enzyme
receptor antagonists/agonists [1–4]. There is also significant
continued interest in fluorinated amino-compounds in the
area of material science, in particular, in the development of
new electroluminescent devices and liquid crystals [1–4].
An emerging and quite exciting area, so far stemming from
a purely academic interest in fluorinated amino-compounds,
is the application of their chiral and enantiomerically pure
derivatives as chiral internal/external ligands, auxiliaries, and
* Corresponding author. Tel.: þ1-405-325-8279; fax: þ1-405-325-6111.
E-mail address: vadim@ou.edu (V.A. Soloshonok).
1 More than 90% of the references retrieved by SciFinder are patents, a
fact manifesting the remarkable industrial interest in this class of
compounds.
For quite some time, we have been developing a new
method conceptually different from the methods listed
above. This method avoids the use of conventional reducing
reagents but results in a product corresponding to a reductive
0022-1139/$ – see front matter # 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2003.11.032

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D.O. Berbasov et al. / Journal of Fluorine Chemistry 125 (2004) 603–607
F
tion of this biomimetic approach for preparation of fluorine-
F
F
F3C
R
C
containing amines [16], a- and b-amino acids [17] starting
from readily available fluorinated aldehydes and ketones, or
a- and b-keto carboxylic acids, respectively. Our most recent
achievement in this area is the development of double-PSR
methodology for a direct, one-pot conventional reducing
reagent-free transformation of perfluoroalkyl-carboxylic
acids to the corresponding a,a-dihydroperfluoroalkylamines
[18]. Of particular interest are the results reported by other
research groups on application of the PSR methodology for
transamination of fluorine-free carbonyl compounds to the
corresponding amino-derivatives [19] as well as the pre-
paration of fluorine-containing phosphorus analogs of a- and
b-amino acids [20].
NH2
N
(A)
(B)
Fig. 1. General structure a-trifluoromethyl containing amino compounds
(A), and fixed structure (B) used for the SciFinder search.
stereodirecting group
H3C
Ph
NH2
F3C
Ph
NH2
1
Fig. 2. a-Phenylethylamine its trifluoro-analog 1.
2. Results and discussion
Ar
NH
Base-Catalized
1,3-Proton Shift
2
Rf
R
Rf
R
R
Rf
As a part of our current project on the development of
catalytic version of the asymmetric PSR method, we needed
a series of non-enaminolizable N-benzyl imines of trifluoro-
acetophenone 8a–d (Scheme 2). The method previously
reported by us [16b] for preparation of imine 7a involves
the standard condensation reaction conditions between tri-
fluoroacetophenone (6) and benzylamine (7a) using p-tolyl-
sulphonic acid (p-TSA) as a catalyst, toluene as a solvent
and a Dean–Stark trap to remove the produced water. While
this method is operationally simple it is, unfortunately, not
synthetically efficient, allowing preparation of the target
imine 8a in only moderate chemical yield (up to 68%) and
involving tedious purification by flash-chromatography. The
major drawback of this reaction conditions is that, during the
reaction, the imine 8a easily undergoes 1,3-proton shift
isomerization to give Schiff base 9a (Table 1, entry 1).
Due to the similarity between structures 8a and 9a their
separation is difficult. Moreover, some 10% of other bypro-
ducts were observed in the crude mixture by 19F NMR.
Similar results were obtained in the reactions of ketone 6
with benzylamines 7b–d, having electron releasing (entries
2 and 3) and electron-withdrawing (entry 4) substituents on
the phenyl ring. As expected, [16c,16d] the largest amount
of the isomerized product 9d was observed in the reaction of
the trifluoromethyl derivative 7d, giving rise to an almost 1:1
mixture of imines 8d and 9d. The isomerization of 8a–d to
9a–d could be catalyzed by the benzylamine remaining in
the reaction mixture acting as a base, or, taking into account
the reaction high temperature, the transformation may be a
Ar
N
O
Ar
N
3
2
Hydrolysis
Rf = Fluoroalkyl, fluoroaryl
R = Alkyl, Aryl, COOAlk, CH -COOAlk
R'= H, Me; Ar = Ph, 3-, 4-Pyridyl
Rf
R
R'
O
2
+
NH
2
Ar
4
5
Scheme 1. Biomimetic, conventional reducing reagent-free reductive
amination of carbonyl compounds.
amination of fluorocarbonyl compounds to the correspond-
ing fluorine-containing amines and amino acids (Scheme 1).
This approach, mimicking biological transamination [15],
represents the most ideal solution to the reductive amination
of carbonyl compounds (Scheme 1) making use of the
intramolecular reduction–oxidation process via a base-cat-
alyzed 1,3-proton shift in the azaallylic system of azo-
methines (imines) 2. We were the first to demonstrate that
the presence of electron-withdrawing perfluoroalkyl or per-
fluoroaryl groups in a-position to the imine function in
derivatives 2 makes their base-catalyzed isomerization to
Schiff bases 3 virtually irreversible and thus synthetically
useful. Products 3 can be easily hydrolyzed under mild
acidic conditions giving rise to a readily separable mixture
of the target fluorinated amino 4 and carbonyl 5 compounds.
The synthetic advantage of this biomimetic approach over
the literature methods is its generality and operational
convenience.2 Previously we reported an efficient applica-
F3C
Ph
F3C
Ph
H2N
Ar
F3C
Ph
7a-d
2 A concept of the ‘‘Atom Economy’’ introduced by Professor Barry M.
Trost has found quick and unanimous understanding, support and
appreciation in the chemistry community as a philosophical guideline
for the development of organic synthesis in 21st century. On the other
hand, in the current literature one can notice another trend shaping a
paradigm of the synthetic methodology of the future, which is simplicity of
experimental conditions, or as we prefer to call it, operationally convenient
reaction conditions.
+
N
N
O
Table 1
Ar
Ar
6
8a-d
9a-d
Ar = C6H5 (a); 4-CH3O-C6H4 (b);
3,4-(CH3O)2-C6H3 (c); 4-CF3-C6H4 (d)
Scheme 2.

D.O. Berbasov et al. / Journal of Fluorine Chemistry 125 (2004) 603–607
605
Table 1
First, we conducted the reaction of ketone 6 with acetic
acid (AA) salts of benzylamines 7a,c (7a-AA) using toluene
as a solvent. We found that the reactions occurred with
remarkably increased rates and chemical yields (entries 7
and 8) giving rise to a mixture of 8a,c and 9a,c, however.
Even though we did not obtain the desired result, preparation
of pure imines 8, the substantially improved ratio of products
8a,c and 9a,c was quite encouraging. Using benzene as a
solvent for the reactions of ketone 6 with salts 7a–d-AA
decreased the reaction rates, but further increased the ratio of
8a–d and 9a–d (entries 9–12) to an almost satisfactory level
of >98/2, except for the reaction of the trifluoromethyl
derivative 9d (entry 12). Finally, we carried out the reactions
in chloroform. To our satisfaction in all cases (entries 13–16)
we observed clean and complete conversion of ketone 6
giving rise to only the target imines 8a–d, which could be
isolated chemically pure and in excellent yields.
Reactions of trifluoroacetophenone 6 with amines 7a–d. Synthesis of
imines 8a–d (Scheme 2)a
Entry
7a–d
Solvent
Methodb
T
(h)
Yield
%
Ratioc
8/9
1
2
a
b
c
Toluene
Toluene
A
A
A
A
A
A
B
B
B
B
B
B
B
B
B
B
9
9
87
85
86
41
90
96
96
97
94
95
96
92
97
94
97
96
71/29
67/33
66/34
56/44
61/39
74/26
85/15
97/3
3
Toluene
9
4
d
a
a
a
c
Toluene
9
5
Benzene
Chloroform
Toluene
16
22
1
6
7
8
Toluene
1
9
a
b
c
Benzene
Benzene
Benzene
Benzene
Chloroform
Chloroform
Chloroform
Chloroform
3
>98/2
>98/2
>98/2
90/10
>99/1
>99/1
>99/1
>99/1
10
11
12
13
14
15
16
3
3
d
a
b
c
3
20
20
20
20
d
a All reactions were conducted at reflux in the indicated solvent.
b Method A: ketone 6 (1 equivalent), amine 7 (1.1 equivalent), p-TSA
(0.1 equivalent). Method B: ketone 6 (1 equivalent), amine 7 (1.1
equivalent), acetic acid (1.1 equivalent).
3. Conclusion
In summary, we found that the standard reaction condi-
tions commonly used for the condensation of carbonyl
compounds with amines are synthetically inefficient for
preparation of the target imines derived from trifluoroace-
tophenone and benzylamines. On the other hand, application
of the ‘‘low-basicity’’ method, using instead of free benzyl-
amines their salts with acetic acid, allowed synthesis of the
target compounds in chemically pure form suitable for
catalytic and kinetic studies.
c Determined by 19F NMR (300 MHz) analysis of the crude reaction
mixtures.
thermally induced process. To determine the cause of the
1,3-proton shift under these reaction conditions we con-
ducted two experiments. Thus, after refluxing of pure imine
8a in toluene for several hours it was isolated intact, while
addition of benzylamine (catalytic amounts: 10 mol%) to a
solution of 8a in toluene, even at room temperature, resulted
in fast isomerization of imine 8a to Schiff base 9a. Further-
more, even though we found that the reaction temperature
had a noticeable effect on the rate of imine 8a formation, its
influence on the isomerization and ratio of products 8a and
9a was rather insignificant. Thus, the reactions between
ketone 6 and benzylamine 7a, conducted in benzene (entry
5) and chloroform (entry 6) were much slower giving rise to
a mixture of 8a and 9a in a ratio comparable with that
observed in reaction conducted in toluene (entry 1).
Having analyzed the data obtained, we concluded that the
p-TSA-catalyzed condensation conditions are incompatible
with the high susceptibility of the target imines 8 to 1,3-
proton-shift. Therefore, we decided to try the ‘‘low-basi-
city’’ reaction conditions recently reported by us [21]. These
conditions, consisting in the reaction of a carbonyl com-
pound with a salt of amine and acetic or trifluoroacetic acid,
were originally developed for the preparation of imines
derived form highly electrophilic and polyfunctional fluori-
nated carbonyl compounds. For example, application of the
corresponding salts of benzylamine, instead of benzylamine
and catalytic amounts of p-TSA, allowed us to exclude the
haloform-type decomposition of polyfluorinated carbonyl
compounds as well as to dramatically improve the chemo-
and regio-selectivity in the reactions of fluorinated a- and
b-keto esters.
4. Experimental
4.1. General
Unless otherwise noted, all reagents and solvents were
obtained from commercial suppliers and used without
further purification. All the reactions were carried out in
a regular atmosphere without any special caution to exclude
air. Unless indicated, H, 19F, and 13C NMR spectra, were
1
taken in CDCl3 solutions at 299.95, 282.24, and 75.42 MHz,
respectively, on an instrument in the University of Oklahoma
NMR Spectroscopy Laboratory. Chemical shifts refer to
TMS and CFCl3 as the internal standards.
Yields refer to isolated yields of products of greater than
95% purity as estimated by 1H and 19F NMR spectrometry.
All new compounds were characterized by 1H, 19F, 13C
NMR and mass spectrometry.
4.2. Typical procedure for preparing imines 8a–d
4.2.1. N-(1-Phenyl-2,2,2-trifluoroethylidene)benzylamine
(8a) [16b]
A solution of ketone 6 (5.000 g, 28.88 mmol) in 5 ml of
chloroform was added at room temperature to a solution of

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D.O. Berbasov et al. / Journal of Fluorine Chemistry 125 (2004) 603–607
benzylamine (3.714 g, 34.66 mmol) and acetic acid
(2.080 g, 34.66 mmol) in chloroform (20 ml). The resultant
mixture was refluxed until the reaction was completed (20 h,
monitored by 19F NMR).
4.2.7. N-40-Trifluoromethybenzylidene-1-phenyl-2,2,2-
trifluoroethylamine (9d)
1H NMR: d 8.43 (s, 1H), 7.95 (d, J ¼ 7:8 Hz, 2H), 7.69 (d,
J ¼ 7:5 Hz, 2H), 7.54 (d, J ¼ 5:1 Hz, 2H), 7.40 (m, 3H),
4.84 (q, J ¼ 6 Hz, 1H). 19F NMR: d ꢀ63.43 (s), ꢀ74.36 (d,
J ¼ 7:9 Hz). 13C NMR: d 164.38, 138.29, 134.51, 129.41,
129.12, 129.00, 128.83, 128.74, 128.69, 128.62 (q,
J ¼ 272:0 Hz), 125.65, (q, J ¼ 3:7 Hz), 75.05 (q,
J ¼ 28:7 Hz).
Yields of imines 8a–d are listed in Table 1 (entries 13–16).
4.2.2. N-(10-Phenyl-20,20,20-trifluoroethylidene)-4-
methoxybenzylamine (8b)
1H NMR: d 7.47 (m, 3H), 7.28 (m, 2H), 7.16 (d,
J ¼ 8:1 Hz, 2H), 6.86 (d, J ¼ 8:1 Hz, 2H), 4.54 (s, 2H),
3.77 (s, 3H). 19F NMR: d ꢀ71.36 (s). 13C NMR: d 158.76,
158.55 (q, J ¼ 32:3 Hz), 131.94, 130.17, 130.03, 128.84,
128.80, 127.63, 119.71 (q, J ¼ 278:7 Hz), 113.95, 56.31,
55.17.
Acknowledgements
This work was supported by the start-up fund provided by
the Department of Chemistry and Biochemistry, University
of Oklahoma.
4.2.3. N-(10-Phenyl-20,20,20-trifluoroethylidene)-3,4-
dimethoxybenzylamine (8c)
1H NMR: d 7.51 (dd, J ¼ 2:1, 4.8 Hz, 2H), 7.28 (m, 3H),
6.83 (d, J ¼ 3 Hz, 1H), 6.80 (s, 1H), 6.74 (dd, J ¼ 2:1,
7.8 Hz, 1H), 4.55 (s, 2H), 3.87 (s, 3H), 3.86 (s, 3H). 19F
NMR: d ꢀ71.40 (s). 13C NMR: d 158.71 (q, J ¼ 33:9 Hz),
149.00, 148.17, 130.50, 130.21, 129.99, 128.86, 127.65,
119.68 (q, J ¼ 278:7 Hz), 119.67, 111.12, 110.94, 56.55,
55.87, 55.79.
References
[1] (a) For recent, excellently edited and comprehensive monographs on
synthesis and application of fluoro-organic compounds in general and
fluorine-containing amino-compounds in particular, see: R. Filler, Y.
Kobayashi, L.M. Yagupolskii (Eds.), Organofluorine in Medicinal
Chemistry and Biochemical Applications, Elsevier, Amsterdam, The
Netherlands, 1993;
4.2.4. N-(10-Phenyl-20,20,20-trifluoroethylidene)-4-
(trifluoromethyl)benzylamine (8d)
(b) I. Ojima, J.R. McCarthy, J.T. Welch (Eds.), Biomedical Frontiers
of Fluorine Chemistry, American Chemical Society, Washington,
DC, 1996;
1H NMR: d 7.59 (d, J ¼ 8:1 Hz, 2H), 7.52 (m, 3H), 7.39
(d, J ¼ 8:1 Hz, 2H), 7.27 (m, 2H), 4.65 (s, 2H). 19F NMR: d
ꢀ63.00 (s), ꢀ71.47 (s). 13C NMR: d 164.37, 159.80 (q,
J ¼ 34:3 Hz), 142.03, 130.46, 129.99, 129.04, 127.78,
127.49, 125.52 (q, J ¼ 3:7 Hz), 125.51 (q, J ¼ 272:1 Hz),
119.58 (q, J ¼ 278:6 Hz), 56.20.
(c) R.D. Chambers (Ed.), Topics in Current Chemistry—Organo-
fluorine Chemistry: Techniques and Synthons, vol. 193, Springer,
Berlin, Germany, 1997;
(d) R.E. Banks, B.E. Smart, J.C. Tatlow, Organofluorine Chemistry:
Principles and Commercial Applications, Plenum Press, New York,
1994;
(e) M. Hudlicky, A.E. Pavlath, Chemistry of Organic Fluorine
Compounds II. A Critical Review, ACS Monograph 187, American
Chemical Society, Washington, DC, 1995.
Schiff bases 9a–d, isolated as by-products, can be easily
prepared from 8a–d under the general conditions described
previously for synthesis 9a [16b]. Spectral characteristics of
9b–d are listed below.
[2] (a) For comprehensive reviews on fluorine-containing amino acids
and asymmetric synthesis of fluorinated compounds, see: V.P.
Kukhar, V.A. Soloshonok, (Eds.) Fluorine-Containing Amino Acids,
Wiley, Chichester, UK, 1994;
4.2.5. N-40-Methoxybenzylidene-1-phenyl-2,2,2-
trifluoroethylamine (9b)
(b) V.A. Soloshonok (Ed.), Enantiocontrolled Synthesis of Fluoro-
Organic Compounds: Stereochemical Challenges and Biomedicinal
Targets, Wiley, Chichester, UK, 1999.
1H NMR: d 8.25 (s, 1H), 7.74 (d, J ¼ 9:0 Hz, 2H), 7.55 (d,
J ¼ 6:3 Hz, 2H), 7.33 (m, 3H), 6.88 (d, J ¼ 9:0 Hz, 2H),
4.73 (q, J ¼ 7:5 Hz, 1H), 3.75 (s, 3H). 19F NMR: d ꢀ74.28
(d, J ¼ 7:9 Hz). 13C NMR: d 164.99, 162.37, 135.24,
130.44, 128.78, 128.74, 128.50, 128.27, 124.81 (q,
J ¼ 280:7 Hz),113.98, 74.96 (q, J ¼ 28:2 Hz), 55.23.
[3] (a) T. Hayashi, V.A. Soloshonok (Eds.), Enantiocontrolled Synthesis
of Fluoro-Organic Compounds, Tetrahedron: Asymmetry Special
Issue, Tetrahedron: Asymmetry 5 (6) (1994). (b) G. Resnati, V.A.
Soloshonok, (Eds.), Fluoroorganic Chemistry: Synthetic Challenges
and Biomedical Rewards, Tetrahedron Symposium-in-Print No. 58.
Tetrahedron 52 (1996) 1–330.
[4] (a) For recent leading publications on synthesis and application of a-
trifluoromethyl containing amines, see:J. Legros, F. Meyer, M.
Coliboeuf, B. Crousse, D. Bonnet-Delpon, J.-P. Begue, J. Org. Chem.
68 (2003) 6444;
4.2.6. N-30,40-Dimethoxybenzylidene-1-phenyl-2,2,2-
trifluoroethylamine (9c)
1H NMR: d 8.25 (s, 1H), 7.55 (m, 3H), 7.37 (m, 3H), 7.19
(dd, J ¼ 1:8, 8.4 Hz, 1H), 6.82 (d, J ¼ 11:1 Hz, 1H), 4.76
(q, J ¼ 8:1 Hz, 1H), 3.91 (s, 3H), 3.86 (s, 3H). 19F NMR: d
ꢀ74.17 (d, J ¼ 5:9 Hz). 13C NMR: d 165.14, 152.04,
149.21, 135.09, 131.52, 128.65, 128.51, 128.43, 124.21
(q, J ¼ 281:0 Hz), 123.96, 110.20, 109.02, 74.81 (q,
J ¼ 28:2 Hz), 55.74.
(b) F. Gagosz, S.Z. Zard, Org. Lett. 5 (2003) 2655;
(c) B. Torok, S.G.K. Prakash, Adv. Synth. Catal. 245 (2003) 165;
(d) Y. Gong, K. Kato, H. Kimoto, Bull. Chem. Soc. Jpn. 75 (2002)
2637;
(e) K. Funabiki, M. Nagamori, M. Matsui, D. Enders, Synthesis 17
(2002) 2585;
(f) A. Volonterio, P. Bravo, W. Panzeri, C. Pesenti, M. Zanda, Eur. J.
Org. Chem. 19 (2002) 3336;

D.O. Berbasov et al. / Journal of Fluorine Chemistry 125 (2004) 603–607
607
(g) Y. Gong, K. Kato, J. Fluorine Chem. 116 (2002) 103;
(h) T. Katagiri, Y. Fujiwara, S. Takahashi, N. Ozaki, K. Uneyama,
Chem. Commun. 9 (2002) 986;
(c) E.E. Snell, A.E. Braunstein, E.S. Severin, Y.M. Torchinsky
(Eds.), Pyridoxal Catalysis: Enzymes and Model Systems, Inter-
science, New York, 1968;
(i) W.B. Motherwell, L. Storey, J. Synlett 4 (2002) 646;
(j) N. Lebouvier, C. Laroche, F. Huguenot, T. Brigaud, Tetrahedron
Lett. 43 (2002) 2827;
(d) W.P. Jencks, Catalysis in Chemistry and Enzymology, McGraw-
Hill, New York, 1969, pp. 143–145;
(e) B.M. Guirard, E.E. Snell, in: M. Florkin, E.H. Stotz, (Eds.)
Comprehensive Biochemistry, vol. 15, Elsevier, New York, 1964,
Chapter 5;
(k) F. Levrat, H. Stoeckli-Evans, N. Engel, Tetrahedron: Asymmetry
13 (2002) 2335.
[5] (a) For reviews on the use of (R)- and (S)-a-phenylethylamine in
´
asymmetric synthesis, see:E. Juaristi, J. Escalante, J.L. Leon-Romo,
(f) T.C. Bruice, S.J. Benkovic, Bioorganic Mechanisms, vol. 2, W.A.
Benjamin, New York, 1966, Chapter 8.
A. Reyes, Tetrahedron: Asymmetry 9 (1998) 715;
´
(b) E. Juaristi, J.L. Leon-Romo, A. Reyes, Escalante J. Tetrahedron:
[16] (a) For preparation of fluorine-containing amines using PSR
methodology see:V.A. Soloshonok, A.G. Kirilenko, V.P. Kukhar, G.
Resnati, Tetrahedron Lett. 35 (1994) 3119;
Asymmetry 10 (1999) 2441.
[6] (a) Steric bulk and stereocontrolling properties of trifluoromethyl
group, as well as fluorine-containing substituents, are still con-
troversial issues. For some discussions, see:V.A. Soloshonok, D.V.
Avilov, V.P. Kukhar, Tetrahedron 52 (1996) 12433;
(b) V.A. Soloshonok, A.D. Kacharov, D.V. Avilov, T. Hayashi,
Tetrahedron Lett. 37 (1996) 7845;
(b) T. Ono, V.A. Soloshonok, Kukhar, J. Org. Chem. 61 (1996) 6563;
(c) V.A. Soloshonok, T. Ono, Synlett 9 (1996) 919–921;
(d) V.A. Soloshonok, T. Ono, Tetrahedron 52 (1996) 14701;
(e) V.A. Soloshonok, T. Ono, J. Org. Chem. 62 (1997) 3030.
[17] (a) For preparation of fluorine-containing a- and b-amino acids
using PSR methodology see:V.A. Soloshonok, A.G. Kirilenko, V.P.
Kukhar, G. Resnati, Tetrahedron Lett. 34 (1993) 3621;
(b) V.A. Soloshonok, A.G. Kirilenko, S.V. Galushko, V.P. Kukhar,
Tetrahedron Lett. 35 (1994) 5063;
(c) V.A. Soloshonok, A.D. Kacharov, D.V. Avilov, K. Ishikawa, N.
Nagashima, T. Hayashi, J. Org. Chem. 62 (1997) 3470.
[7] (a) K. Kato, Y. Gong, T. Saito, H. Kimoto, Enantiomer 5 (2000)
521;
(c) V.A. Soloshonok, A.G. Kirilenko, N.A. Fokina, I.P. Shishkina,
S.V. Galushko, V.P. Kukhar, V.K. Svedas, E.V. Kozlova, Tetrahedron
Asymmetry 5 (1994) 1119;
(b) S. Watanabe, T. Fujita, M. Sakamoto, H. Hamano, T. Kitazume,
T. Yamazaki, J. Fluorine Chem. 83 (1997) 15.
[8] (a) A. Ishii, K. Higashiyama, K. Mikami, Synlett 12 (1997) 1381;
(b) P. Bravo, F. Guidetti, F. Viani, M. Zanda, A.L. Markovsky, A.E.
Sorochinsky, I.V. Soloshonok, V.A. Soloshonok, Tetrahedron 54
(1998) 12789.
(d) V.A. Soloshonok, A.G. Kirilenko, N.A. Fokina, S.V. Galushko,
V.P. Kukhar, V.K. Svedas, G. Resnati, Tetrahedron: Asymmetry 5
(1994) 1225;
(e) V.A. Soloshonok, V.P. Kukhar, Tetrahedron 52 (1996) 6953;
(f) V.A. Soloshonok, V.P. Kukhar, Tetrahedron 53 (1997) 8307;
(g) V.A. Soloshonok, T. Ono, I.V. Soloshonok, J. Org. Chem. 62
(1997) 7538;
[9] D. Enders, K. Funabiki, Org. Lett. 3 (2001) 1575.
[10] I. Kumadaki, S. Jonoshita, A. Harada, M. Omote, A. Ando, J.
Fluorine Chem. 97 (1999) 61.
[11] D.W. Nelson, R.A. Easley, B.N.V. Pintea, Tetrahedron Lett. 40
(1999) 25.
(h) V.A. Soloshonok, I.V. Soloshonok, V.P. Kukhar, V.K. Svedas, J.
Org. Chem. 63 (1998) 1878.
[12] (a) J.C. Blazejewski, E. Anselmi, M.P. Wilmshurst, Tetrahedron
Lett. 40 (1999) 5475;
[18] V.A. Soloshonok, H. Ohkura, K. Uneyama, Tetrahedron Lett. 43
(2002) 5449.
(b) G.K.S. Prakash, M. Mandal, G.A. Olah, Synlett 1 (2001) 77;
(c) G.K.S. Prakash, M. Mandal, G.A. Olah, Angew. Chem. Int. Ed.
40 (2001) 589;
[19] (a) For application of PSR methodology for preparing fluorine-free
amino-compounds see:W.G.H. Johannes, V.G. Johannes, N.J.M.
Roeland, Z. Binne, Tetrahedron Lett. 36 (1995) 3917;
(b) G. Cainelli, D. Giacomini, A. Trere, P.P. Boyl, J. Org. Chem. 61
(1996) 5134;
(d) G.K.S. Prakash, M. Mandal, G.A. Olah, Org. Lett. 3 (2001)
2847.
[13] (a) A. Ishii, F. Miyamoto, K. Higashiyama, K. Mikami, Tetrahedron
Lett. 39 (1998) 1199;
(c) A. Hjelmencrantz, U. Berg, J. Org. Chem. 67 (2002) 3585.
[20] (a) For application of PSR methodology for preparing fluorinated P-
amino acids see:Y. Chengye, Youji Huaxue 21 (2001) 862;
(b) X. Jingbo, Y. Chengye, Heteroatom Chem. 11 (2000) 541;
(c) X. Jingbo, Z. Xiaomei, Y. Chengye, Heteroatom Chem. 11
(2000) 536.
(b) A. Ishii, F. Miyamoto, K. Higashiyama, K. Mikami, Chem. Lett.
2 (1998) 119.
[14] K. Uneyama, J. Fluorine Chem. 97 (1999) 11.
[15] (a) A.E. Braunstein, M.G. Kritsmann, Biochimiya 2 (242) (1937)
859;
[21] (a) For application of PSR methodology for preparing fluorinated P-
amino acids see:H. Ohkura, D.O. Berbasov, V.A. Soloshonok,
Tetrahedron 59 (2003) 1647;
(b) E.E. Snell, in: P.M. Fasella, A.E. Braunstein, A. Rossi-Fanelli,
(Eds.), Chemical and Biological Aspects of Pyridoxal Catalysis,
Macmillan, New York, 1963, pp. 1–12;
(b) D.O. Berbasov, V.A. Soloshonok, Synthesis 13 (2003) 2005.