Synthesis and stereochemical assignments of diastereomeric Ni(II) complexes of glycine Schiff base with (R)-2-(N-{2-[N-alkyl-N-(1-phenylethyl)-amino] acetyl}amino)benzophenone; A case of configurationally stable stereogenic nitrogen

Article abstract of DOI:10.3762/bjoc.10.41

A family of chiral ligands derived from a-phenylethylamine and 2-aminobenzophenone were prepared by alkylation of the nitrogen atom. Upon reaction with glycine and a Ni(II) salt, these ligands were transformed into diastereomeric complexes, as a result of the configurational stability of the stereogenic nitrogen atom. Different diastereomeric ratios were observed depending on the substituent R introduced in the starting ligand, and stereochemical assignments were based on X-ray analysis, along with NMR studies and optical rotation measurements.

Full text of DOI:10.3762/bjoc.10.41

Synthesis and stereochemical assignments of
diastereomeric Ni(II) complexes of glycine Schiff
base with (R)-2-(N-{2-[N-alkyl-N-(1-phenylethyl)-
amino]acetyl}amino)benzophenone; a case of
configurationally stable stereogenic nitrogen
Hiroki Moriwaki*1,2, Daniel Resch1, Hengguang Li1, Iwao Ojima1,
Ryosuke Takeda2, José Luis Aceña3 and Vadim A. Soloshonok*3,4
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 442–448.
1Department of Chemistry, Institute of Chemical Biology & Drug
Discovery, State University of New York at Stony Brook, Stony Brook,
New York 11794-3400, United States, 2Hamari Chemicals Ltd. 1-4-29
Kunijima, Higashi-Yodogawa-ku, Osaka, Japan 533-0024,
3Department of Organic Chemistry I, Faculty of Chemistry, University
of the Basque Country, 20018 San Sebastián, Spain and
4IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain
doi:10.3762/bjoc.10.41
Received: 30 September 2013
Accepted: 17 January 2014
Published: 19 February 2014
Associate Editor: V. M. Dong
Email:
© 2014 Moriwaki et al; licensee Beilstein-Institut.
License and terms: see end of document.
Hiroki Moriwaki* - hiroki-moriwaki@hamari.co.jp;
Vadim A. Soloshonok* - vadym.soloshonok@ehu.es
* Corresponding author
Keywords:
amino acids; asymmetric synthesis; Ni(II) complexes; Schiff bases;
stereogenic nitrogen
Abstract
A family of chiral ligands derived from α-phenylethylamine and 2-aminobenzophenone were prepared by alkylation of the nitrogen
atom. Upon reaction with glycine and a Ni(II) salt, these ligands were transformed into diastereomeric complexes, as a result of the
configurational stability of the stereogenic nitrogen atom. Different diastereomeric ratios were observed depending on the
substituent R introduced in the starting ligand, and stereochemical assignments were based on X-ray analysis, along with NMR
studies and optical rotation measurements.
Introduction
Since the beginning of organic chemistry, α-amino acids RCH(NH2)COOH units have been explored in great detail.
(α-AAs) have been an attractive target for synthetic chemists. Structurally complex natural or tailor-made [1] α-AAs can be
The reported synthetic methods for assembling the prepared using the currently available methodologies [2-30];
442

Beilstein J. Org. Chem. 2014, 10, 442–448.
however, the chemistry of α-AAs continues to evolve [31-38]. tion, here we describe the preparation of Ni(II) complexes of
Many of the reported synthetic methodologies have shown pref- glycine Schiff base with α-phenylethylamine-derived ligands.
erence for adjusting stoichiometry or developing catalytic reac- One unusual feature of these new glycine-Ni(II) complexes is
tions to achieve a specific chemical and stereochemical that they, along with the chiral amine residue, have a configura-
outcome [2-30]. On the other hand, a measure of the value of a tionally stable stereogenic nitrogen, leading to formation of two
synthetic method, regardless of its stoichiometric or catalytic diastereomers. Consequently, stereochemical assignments of the
requirements, is the cost of the final product. In this regard, the diastereomeric products, based on crystallographic data, and
synthetic preparation of α-AAs is far behind the biocatalytic sense of stereochemical preferences, were an important part of
methods that dominate their industrial production [39,40]. It this work.
was emphasized, in relatively recent reviews [39-41], that
currently available, purely chemical methods for preparation of Results and Discussion
α-AAs are prohibitively expensive. The advantage of biocat- Taking advantage of the recently developed new generation of
alytic processes is that they can be conducted under opera- modular ligands/complexes 3 [73,74], we have designed ligands
tionally convenient conditions [42,43] and therefore are more 4a–f (Scheme 1) derived from α-phenylethylamine [75,76], as
economical. Consequently, the current emphasis in developing the source of chirality. As one can see from Scheme 1, the syn-
synthetic procedures for preparation of α-AAs is focused on thesis of ligands 4a–f involves a set of very simple reactions,
simplicity of the experimental procedures and cost of the target occurring with virtually quantitative yields [77,78].
α-AAs.
Among various chiral nucleophilic glycine equivalents, the
Ni(II) complex of glycine Schiff base 1 (Figure 1) possesses
some attractive characteristics that underscore its potential
commercial application [44-47]. In particular, homologation of
Ni(II) complex 1 via alkyl halide alkylation [48-50], aldol [51-
53], Mannich [54,55] and Michael [56-58] addition reactions
can be conducted at room temperature and without specially
controlled conditions. Achiral derivatives of 1, compounds 2
[59,60] and 3 [61-63], show even greater features of practi-
cality and found application in the convenient synthesis of
symmetrically α,α-disubstituted α-AAs [64,65], homologation
under asymmetric PTC [66,67] and Michael addition reactions
[68-72]. Nevertheless, the commercial application of chiral
complex 1 is rather limited. The major shortcomings of com-
pound 1 are: problematic scale up of its synthesis, partial
racemization of the N-(benzyl)proline moiety and undesirable
Scheme 1: Synthesis of chiral ligands 4a–f.
stereochemical outcomes. To overcome these deficiencies we
initiated a project aiming to design a novel and advanced struc-
tural type of Ni(II) complexes using an inexpensive and To prepare the target Gly-Ni(II) complexes, ligands 4a–f were
nonracemizable chiral auxiliary. As the first step in this direc- heated in MeOH in the presence of glycine, Ni(OAc)2 and
K2CO3 (Scheme 2). To compare the results, all reactions were
conducted under the same conditions. The reaction mixtures
were poured into water and the products were simply filtered
off.
The chemical and stereochemical outcomes of the reactions,
presented in Table 1, were rather unexpected. First of all, one
can see a clear tendency that the increase in steric bulk of the
substituent R has a significant effect on the reaction rates. In the
case of R = H and Me (4a,b; Table 1, entries 1 and 2) the reac-
Figure 1: A family of chiral and achiral equivalents of nucleophilic
glycine.
tions were completed in 1 hour and the corresponding Ni(II)
complexes were obtained in high yields. Under the same condi-
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Beilstein J. Org. Chem. 2014, 10, 442–448.
that the complex(es) obtained in the reaction of NH (bearing
ligand R = H) had unprecedentedly low solubility. During the
complex formation reaction, the product(s) precipitated from
the reaction mixture and were isolated by filtration. Solvents
such as DMF, DMSO, and acetic acid were used in an attempt
to dissolve the product without success. The stereochemical
outcome in this reaction remains unclear.
Since the reactions were conducted in open atmosphere, forma-
tion of byproducts (S)-7a–f (5–20%) were observed in all cases.
Compounds (S)-7a–f result from the reaction of atmospheric
oxygen with the corresponding enolates derived from glycine
complexes 5 and 6 and the basic reaction conditions [79,80].
Formation of 4-phenylquinazoline derivatives (S)-7a–f can be
prevented by conducting the reactions in oxygen-free atmos-
phere. Another general trend is apparent when considering the
observed optical rotations of the diastereomeric products 5 and
6. In all cases, the major diastereomers 5 had low-magnitude
rotations as compared with high-magnitude dextrorotatory rota-
tions of 6. This trend can be used to assign the configurations of
products 5 and 6 and related compounds. It is interesting to note
that the difference in optical rotation between diastereomers 5e
Scheme 2: Preparation of diastereomeric Ni(II) complexes 5a–f and
6a–f.
tions, the reactions of Et- and n-Pr-containing ligands (4c,d; and 6e was the least pronounced in the series. Most likely, some
Table 1, entries 3 and 4) proceeded with noticeably slower rates similarity in the structure of N-benzyl and N-α-(methyl)benzyl
affording the corresponding products 5 and 6 in lower yields. groups results in low impact on the optical rotation of diastere-
Still slower reaction rates were observed in the case of Bn (4e) omers 5e and 6e.
and 2-(phenyl)ethyl (4f) bearing ligands (Table 1, entries 5 and
6). These results revealed that the steric bulk of the amine’s Knowledge of one of the stereocenters in diastereomers 5 and 6
nitrogen plays a significant role and should be minimized to allowed us to use X-ray crystallography to determine the
maintain practically sounding reactions rates. On the other absolute configuration of the major product 5b [81]. The struc-
hand, the diastereoselectivity in the formation of 5 and 6 was ture presented in Figure 2 shows its (SCRN) absolute configur-
increasing along with the steric bulk of the substituent R, ation. Accordingly, the second product, diastereomer 6b, has
reaching the maximum of ~75/25 in the case of R = n-Pr and Bn (SCSN) configuration. The X-ray structure of 5b revealed the
groups (Table 1, entries 4 and 5). Another surprising result is complex is not fully planar as expected. The benzophenone
chelate ring system deviates from planarity by 14.4°
(0.16 e.s.d.). The torsion angle of Ni2–O5–C10–C29 is −10.9°
Table 1: Preparation of diastereomeric Ni(II) complexes 5a–f and
further indicating a deviation from planarity. One of the rings in
the benzophenone chelate is rotated 85.8° (0.36 e.s.d.) out of the
plane presumably due to sterics. This rotation minimizes the
Ar–H···H–Ar repulsion as well as repulsion with methylene
group of the glycine moiety and the Ar–H.
6a–f.
Entry
R
5/6
ratioa
Yield
(%)
[α]Db
5
6
1
2
3
4
5
6
H (a)
N/Ac
87.4
96.9
67.7
70.1
31.5d
12.1d
N/Ac
N/Ac
Me (b)
Et (c)
57/43
61/39
74/26
76/24
+88.6
+1701.8
As we have mentioned above, there is a great degree of simi-
larity in the chiroptical properties of complexes 5b–e and 6b–e,
sufficient to make the corresponding stereochemical assign-
ments. Moreover, there is also another general trend observed in
the 1H NMR spectra of compounds 5 and 6. In particular, as it
is evident form Figure 3, the α-phenylethylamine moiety's
methyl group is located in relatively close proximity to the
Ni(II) atom, causing the methyl protons to shift more down-
field [82-85]. Accordingly, in all 1H NMR spectra of com-
−150.8 +1226.4
−55.0 +1033.3
+303.6 +361.5
N/Ae N/Ae
n-Pr (d)
Bn (e)
Ph(CH2)2 (f) 67/33
aDetermined by 1H NMR analysis on the crude reaction mixtures.
bMeasured in dichloromethane, 25 ºC, c = ~1. cThe compound could
not be dissolved in any solvent. dThe reaction was incomplete and the
starting ligand was also recovered. eThe diastereomers could not be
separated.
444

Beilstein J. Org. Chem. 2014, 10, 442–448.
dation of the corresponding enolates. 5) The (SCRN) stereo-
chemistry of the Ni(II) complexes is generally preferred.
Drawing from these results, an improved design of new ligands
is currently under investigation in our laboratories.
Experimental
General procedure for the preparation of
ligands 4b–f
In analogous manner as previously reported [77], secondary
amine 4a [78] (1 equiv), N,N-diisopropylethylamine
(1.5 equiv), alkyl halide (1.5–3.5 equiv) and 10 mL of MeCN
were placed in a round bottom flask and stirred at room
temperature under nitrogen. After completion of the reaction
(monitored by TLC), the reaction mixture was evaporated to
dryness under vacuum. The residue was dissolved in 5 mL of
CH2Cl2 and washed with 5 mL of water. The aqueous layer was
washed with 3 × 5 mL fractions of CH2Cl2. The collected
organic fractions were dried over MgSO4 and the solvent was
removed under vacuum to yield the crude product. Analytically
pure samples of 4b–f were obtained by column chromatog-
raphy (Hex/EtOAc).
Figure 2: Crystallographic structure of (SCRN)-5b.
pounds 5b–e this methyl group is shifted downfield
(2.5–2.9 ppm) as compared with the chemical shift of the same
methyl in diastereomers 6b–e (2.2–1.75 ppm).
(S)-N-(2-benzoylphenyl)-2-(methyl-(1-phenylethyl)-
amino)acetamide (4b): From 4a (6.14 g, 17.23 mmol) and MeI
(4.3 mL, 68.52 mmol), 4.61 g (72.4%) of 4b. White solid. Mp
80–82 °C; [α]D25 +23.7 (c 1.55, CHCl3); 1H NMR (300 MHz,
CDCl3) δ 1.54 (d, J = 6.9 Hz, 3H), 2.44 (s, 3H), 3.12 (d, J =
16.8 Hz, 1H), 3.25 (d, J = 16.8 Hz, 1H), 3.82 (q, J = 6.6 Hz,
1H), 6.87–6.92 (m, 1H), 7.15–7.22 (m, 1H), 7.25–7.28 (m, 2H),
7.50–7.72 (m, 7H), 7.73–7.92 (m, 2H), 8.69 (d, J = 8.7 Hz, 1H),
11.67 (br, 1H); 13C NMR (75 MHz, CDCl3) δ 18.4, 40.8, 58.7,
63.5, 121.5, 122.1, 125.1, 127.1, 127.6, 128.2, 130.0, 132.3,
132.5, 133.2, 138.4, 139.0, 142.8, 171.1, 197.9; HRMS: [M +
H]+ calcd for C24H25N2O2, 373.1916; found, 373.1923.
Figure 3: Crystallographic structure of (SCRN)-5b showing an expo-
sure of the methyl moiety of α-phenylethylamine to the Ni(II).
Combining together the crystallographic, chiroptical and
General procedure for the preparation of
1H NMR data of compounds 5 and 6, we can confidently assign
the (SCRN) absolute configuration to all products 5b–e and Ni(II) complexes 5b–f and 6b–f
(SCSN) stereochemistry to the complexes 6b–e.
To a flask containing a methanol solution of ligand 4b–f
(1 equiv), Ni(OAc)2·4H2O (2 equiv) and glycine (5.0 equiv),
was added K2CO3 (9 equiv), and the reaction mixture was
Conclusion
In conclusion, this exploratory work has revealed that: 1) Appli- stirred at 60–70 °C. The progress of the reaction was monitored
cation of α-phenylethylamine as the source of stereochemistry by TLC and upon completion (consumption of the reagent 4),
information in the novel Ni(II) complexes, results in the forma- the reaction mixture was poured into ice water. The target prod-
tion of diastereomeric glycine Schiff base products due to the uct was extracted three times with CH2Cl2. The combined
configurational stability of the stereogenic nitrogen. 2) The organic layer was dried over anhydrous MgSO4 and evaporated
steric bulk of the substituent R on the α-phenylethylamine under vacuum. After evaporation of the solvents, the target
residue has a negative effect on the reaction rates and a positive complexes 5b–f and 6b–f were obtained in diastereomerically
influence on the stereochemical outcome. 3) The optimal size of pure form by separation on silica-gel column chromatography.
the R group is n-Pr or Bn. 4) Synthesis of this type of Ni(II)
complexes should be conducted under oxygen-free conditions 5b and 6b: From 4b (512 mg, 1.375 mmol), 647.3 mg (96.9 %)
to avoid the formation of byproducts, resulting from the oxi- of a 57:43 mixture of 5b and 6b. Data of 5b: Red solid. Mp
445

Beilstein J. Org. Chem. 2014, 10, 442–448.
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265–267 °C; [α]D25 +88.6 (c 1.0, CH2Cl2); 1H NMR (300
MHz, CDCl3) δ 2.49 (d, J = 6.9 Hz, 3H), 2.57 (d, J = 16.8 Hz,
1H), 2.83 (s, 3H), 3.65–3.85 (m, 2H), 3.90–4.00 (m, 1H), 3.97
(d, J =16.8 Hz, 1H), 6.77–7.05 (m, 4H), 7.26–7.40 (m, 6H),
7.51–7.53 (m, 3H), 8.63 (d, J = 9.0 Hz, 1H). 13C NMR (75
MHz, CDCl3): 19.7, 47.5, 58.9, 61.3, 64.6, 121.1, 124.2, 125.1,
125.8, 126.0, 128.5, 129.0, 129.5, 129.6, 130.1, 132.7, 133.5,
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1H NMR (300 MHz, CDCl3) δ 2.27 (s, 3H), 2.34 (d, J = 6.9 Hz,
3H), 3.35 (d, J = 16.5 Hz, 1H), 3.60–3.72 (m, 3H), 4.32 (q, J =
6.9 Hz, 1H), 6.75–6.93 (m, 3H), 7.13–7.18 (m, 1H), 7.30–7.51
(m, 9H), 8.48 (d, J = 8.1 Hz, 1H); 13C NMR (75 MHz, CDCl3)
δ 17.6, 42.2, 61.1, 62.2, 66.2, 121.3, 124.3, 125.5, 125.6, 126.2,
128.7, 129.1, 129.3, 129.7, 129.9, 130.2, 132.5, 133.4, 134.5,
134.8, 142.3, 171.9, 176.0, 177.2. HRMS: [M + H]+ calcd for
C26H26N3O3Ni, 486.1328; found, 486.1337.
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Supporting Information
Supporting Information File 1
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Experimental data for compounds 4c–f, 5c–e and 6c–e.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-41-S1.pdf]
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Acknowledgements
We thank IKERBASQUE, Basque Foundation for Science; the
Basque Government (SAIOTEK S-PE12UN044), Spanish
Ministry of Science and Innovation (CTQ2010-19974) and
Hamari Chemicals (Osaka, Japan) for generous financial
support. The Stony Brook University single-crystal diffrac-
tometer was obtained through the support of the National
Science Foundation (NSF) grant CHE-0840483. JLA and VAS
are very grateful to Professor Dieter Seebach (ETH-Zürich) for
fruitful and inspirational discussions.
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