Stereoselective synthesis of ring C-hexasubstituted trianglamines

Article abstract of DOI:10.1039/c004873a

The addition of organolithium reagents to the trianglimine derived from (R,R)-1,2-diaminocyclohexane and terephthalaldehyde gave the corresponding trianglamine with complete stereocontrol and the R configuration of all six newly formed stereocenters. The structure of the hexaphenyl-substituted macrocyle was determined by X-ray crystallographic study. The new trianglamines were tested as ligands in enantioselective catalytic reactions.

Full text of DOI:10.1039/c004873a

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Stereoselective synthesis of ring C-hexasubstituted trianglamines†
Diego Savoia,*a Andrea Gualandia and Helen Stoeckli-Evansb
Received 6th April 2010, Accepted 26th May 2010
First published as an Advance Article on the web 28th June 2010
DOI: 10.1039/c004873a
The addition of organolithium reagents to the trianglimine derived from (R,R)-1,2-diaminocyclohexane
and terephthalaldehyde gave the corresponding trianglamine with complete stereocontrol and the R
configuration of all six newly formed stereocenters. The structure of the hexaphenyl-substituted
macrocyle was determined by X-ray crystallographic study. The new trianglamines were tested as
ligands in enantioselective catalytic reactions.
Table 1 Synthesis of compounds 3a–c from macrocyclic hexaimine 1
Introduction
RM
Product
Yield (%)
Chiral perazamacrocycles and their metal complexes have found
applications in biomedical research, diagnosis, anion sensing,
molecular recognition, enantiomeric discrimination, asymmetric
catalysis, and materials chemistry.1
MeLi
n-BuLi
PhLic
3a
3b
3c
98 (89)a
94 (86)b
97 (92)a
The condensation of enantiopure 1,2-diaminocyclohexane with
aromatic dialdehydes allows for the preparation of symmetric
polyimine macrocycles, which incorporate equal amounts of both
reaction partners and whose size can be tuned by the proper choice
of experimental conditions, such as the use of a metal template,
temperature and solvent. Although the [2+2] cyclocondensation
products are generally thermodynamically favoured, the forma-
tion of the [3+3] hexaiminomacrocycles, called trianglimines, is
favored by the rigidity of both reaction partners.2 Reduction of
the imine functions afforded the corresponding perazamacrocyles
containing secondary amine groups (trianglamines). Compound
1, and the reduced analogue 2 derived from terephthalaldehyde,
are typical examples of such compounds.3
Although ring substituents can be easily introduced at the
nitrogen atoms of the secondary amines,4 alkylation of the imine
moieties in macrocyclic polyimines has never been described.
This would also be an alternative procedure to introduce lateral
functionalities in the macromolecules and expand their potential
for a variety of applications. Owing to the relatively poor
electrophilicity of the imine function, we envisioned that the use of
strong nucleophiles, such as organometallic reagents, would allow
the achievement of this goal (Scheme 1).
a After crystallization (MeOH). b After column chromatography. c No
reaction occurred upon addition of PhMgBr in the same conditions.
aromatic aldehydes.5 We were pleased to observe that, regardless of
the nature of the organolithium reagent (methyl, n-butyl, phenyl),
a single diastereomer was produced in excellent yields in the
reactions affording 3a–c (Table 1). In fact, traces of different
diastereomers could not be detected by NMR analyses of the
crude reaction mixtures. The C3-symmetry of these compounds
1
was evident in their H-NMR spectra. On the other hand the
addition of the corresponding Grignard reagents proved to be
unsuccessful, as no reaction was observed (MeMgBr, PhMgBr), or
a complex mixture was obtained with low conversion (allylMgCl).
Based on our previous studies,5 the R configuration of all
the newly formed stereocenters was assumed. Later, this and
the C3 symmetry, was confirmed by the X-ray crystallographic
study of the Ph-substituted compound 3c (Fig. 1).† In the
crystal the molecules stack along [001] creating columns with
the channels filled with acetonitrile molecules (see ESI†). The
structure of this compound displays alternating up and down
spatial position of the phenyl substituents around the almost
planar ring. The stereochemical outcome can be explained by
the preferred attack of the nucleophile to the less hindered face
Results and discussion
With this aim, we performed the addition of organolithium
reagents to the trianglimine 1, using our previous experience and
knowledge concerning the addition of organolithium reagents
to the diimines derived from (R,R)-1,2-diaminocyclohexane and
aDipartimento di Chimica “G. Ciamician”, Universita` di Bologna, via Selmi
2, 40126 Bologna, Italy; Fax: +39 051 2099456; Tel: +39 051 2099571.
E-mail: diego.savoia@unibo.it.
bInstitut de Physique, Universite´ de Neuchaˆtel, Rue Emile-Argand 11, CP
158, CH-2009 Neuchaˆtel, Switzerland; Fax: (+41) 327182511
† Electronic supplementary information (ESI) available: X-Ray crystallog-
raphy and supplementary figures. CCDC reference number 759763. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c004873a
Fig. 1 Views of the molecular structure of 3c∑4(CH3CN) solvate; the
C-bound H-atoms have been omitted for clarity.
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Scheme 1 Stereoselective synthesis of hexasubstituted trianglamines.
Table 2 Copper-catalyzed enantioselective Henry reaction
of each imine group, i.e. syn to the C1-H and C2-H hydrogen
atoms of the cyclohexane rings. The complete diastereoselectivity
is probably due to the rigidity of the substrate in the first step, and,
after each individual step, by the stereochemistry of the previously
introduced substituents.
Ligand
Yield of 4a
Configuration
E.e. (%)b
3a
78 (76)c
74 (72)c
93
R
S
R
S
46
19
84
82
3c
2
It should be underlined that this communication is the first
report on C–C bond forming organometallic reactions performed
on macrocyclic polyimines. Instead, reduction of the macro-
cyclic polyimine 1 by sodium borodeuteride has been reported
affording the quasi-enantiomeric d6-derivatives of compound 2
with complete diastereoselectivity.6 Also relevant to the present
communication is the report on the reduction of the bisketimine
moiety of the MnCl2-complex of a pentaazamacrocycle derived
from 2,6-diacetylpyridine and an ethylene-tethered bis[(S,S)-1,2-
diaminocyclohexane]: the sense of asymmetric induction and the
degree of diastereoselectivity were found dependent on the type of
reduction (Pd-catalyzed hydrogenation with ammonium formate
vs. borohydride addition).7
ent-2d
96 (78)c
a Calculated by 1H NMR. b Calculated by HPLC analysis on chiral column
Chiralpak OD. c Yield of isolated, pure product (%). d Data taken from the
literature,8 obtained using ent-2-Cu(OAc)2 (3 mol%), 10 equiv. MeNO2
and no solvent.
of acetophenone by silanes (Scheme 3). The outcomes of these
reactions are reported in Tables 2 and 3, respectively.
Owing to the ability of compound 2 to act as a ligand in metal-
catalyzed enantioselective organic synthesis,8,9 we were interested
to verify if the stereocontrolled introduction of substituents in
the macrocyclic ring would modify the coordination behavior of
the ligand towards the metal center, so affecting and hopefully
improving the enantioselectivity in the catalytic process. Therefore,
the macrocylic ligands 3 were tested in two typical reactions where
the unsubstituted ligand 2 displayed moderate enantioselectiv-
ity, namely, the Cu(OAc)2-catalyzed Henry reaction10,11 between
nitromethane and benzaldehyde (Scheme 2)8 and the reduction
Scheme 3 Enantioselective catalytic reduction of propiophenone.
The reactions between nitromethane and benzaldehyde per-
formed in the presence of ligand 3a and 3c were not satisfactory,
considering that the use of the unsubstituted ligand 2 in the same
experimental conditions provided a better yield and enantioselec-
tivity for product 4.12 It is surprising, however, that 3c provided
the opposite sense of asymmetric induction as compared to 3a
Scheme 2 Enantioselective catalytic Henry reaction.
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Table 3 Zinc-catalyzed enantioselective reduction of propiophenone
using PMHS, a higher e.e. was obtained with the open ligand 2
(86% vs. 83%). In our hands, working with ligand 6 and Ph2SiH2
the yield of 5 could be increased to 93%, but the same level of
enantioselectivity (83% e.e.) was obtained.
Ligand
Silane
Yield (%) of 5a
E.e. (%)b
3a
3a
3a
3b
3c
3a
3a
3a
3a
2
PMHS
Et3SiH
71
11
94
89
86
93
91
90
92
70
71
73
70
90
93
60
60
64
54
25
66
67
67
71
86
83
74
78
83
83
The lower enantioselectivity exhibited by the macrocyclic ligand
3a (e.e. ≤71%) with respect to the analogous acyclic ligand 6 (83%
e.e.) might be explained by its markedly reduced ability to undergo
rotation around the benzylic C*–N bonds, contrary to 6 which can
assume the most stable conformation in the transition state.
In conclusion, the introduction of ring substituents in the
macrocyclic ligand caused a diminished enantioselectivity in the
hydrosilylation of propiophenone as well as in the Henry reaction.
In the latter case inversion of the sense of asymmetric induction
was observed with the macrocyclic ligand bearing the more bulky
substituent. Therefore, it is conceivable that the diastereomeric
macrocyclic ligands with the same configuration of the cyclo-
hexanediamine moieties but the opposite configuration of the
substituted stereocenters can give more satisfactory performances.
Hence, attempts will be made to prepare the hexamethyl derivative
through the reduction of the precursory macrocyclic hexaketimine
derived from 1,4-diacetylbenzene. The introduction of functional-
ized or heteroaromatic substituents would also increase the interest
for such macrocycles for their potential use in the wide field of
supramolecular chemistry.
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
PMHSg
Ph2SiH2
Ph2SiH2
Ph2SiH2
PMHSh
Ph2SiH2
c
d
e
f
2
d
f
2
2
6
6
a After chromatographic purification. b Calculated by HPLC analysis on a
chiral column Chiralpak OD. c 7.0 mol% of Et2Zn was used. d 10.5 mol%
of Et2Zn was used. e 1.5 mol% of 3a and 4.5 mol% of Et2Zn were used.
f 10.5 mol% of Et2Zn was used starting from -78 ◦C. g Data taken from the
literature.9 h Data taken from the literature,9 where the use of ent-6 gave
(R)-5.
and 2, affording product 4 with the S configuration and very low
enantioselectivity.
Similar results were obtained in the reduction of propiophenone
by different silanes in the presence of catalytic amounts of
diethylzinc and ligands 3a–c. By using the methyl-substituted
ligand 3a, the effect of the nature of the reducing agent was
examined under identical experimental conditions: 1.2 equiv.
reducing agent and 3.5 mol% of either Et2Zn and ligand. It was
observed that higher yield and e.e. of (S)-1-phenylpropanol 5 were
obtained using diphenylsilane (94%, 64% e.e.) with respect to poly-
methylhydrogensiloxane (PMHS, 71%, 60% e.e.) and triethylsilane
(11%, 60% e.e.).
Successively, the role of the macrocyclic ring substituent was
investigated with the n-butyl and phenyl-substituted macrocycles
3b and 3c, respectively, with Ph2SiH2 as the reducing agent. The
adverse effect of the increased size on the enantioselectivity was
assessed: in fact, the e.e. of (S)-1-phenylpropanol 5 decreased: 3a,
64% e.e.; 3b, 54% e.e.; 3c: 25%. Further experiments were carried
out using the best performing ligand 3a and vaying the amounts
of Et2Zn: two- and three-fold increases of the amount of Et2Zn
allowed the enhancement of the e.e. to 66% and 67%, respectively,
and the same 67% e.e. was obtained even reducing the amounts
of Et2Zn (4.5 mol%) and ligand 3a (1.5 mol%). Finally, a slightly
better 71% e.e. and 92% yield of 5 was obtained by performing
the reduction at -78 ◦C and allowing the mixture to slowly reach
room temperature.
A comparison was then made between the unsubstituted
macrocyclic ligand 2 and the methyl-substituted analogue 3a,
using Ph2SiH2. It was shown that 2 provided a higher level of
enantioselectivity (83% e.e.), which was only slightly lower than
that reported in the literature9 using PMHS as the reducing agent
(86% e.e.).
To understand the importance of the ligand macrocyclic struc-
ture on the stereocontrol, these results should also be compared
with those previously obtained with the open ligand 6,5,13 whose
skeleton is a fragment of the macrocyclic ligand 3a. Although a
higher yield of 1-phenylpropanol 5 was obtained with ligand 6
Experimental
General Information
Melting points are uncorrected. Optical rotations were measured
on a digital polarimeter in a dm cell and [a]D-values are given
in 10-1 deg cm3 g-1. H NMR spectra were recorded on Varian
1
Mercury and Gemini instruments for samples in CDCl3 which
was stored over Mg: 1H chemical shifts are reported in ppm
relative to CHCl3 (dH 7.27), J-values are given in Hz. Infrared
spectra were recorded on a Nicolet FT-380 spectrometer and IR
assignments are reported in wave numbers (cm-1). MS spectra were
taken at an ionising voltage of 70 eV on a Agilent Technologies
5975 spectrometer with GLC injection (using HP-5 column,
30 m, ID 0.25 mm). Molecular weights were determined on
an Agilent Technologies MS 1100 instrument. Analytical high
performance liquid chromatography (HPLC) was performed on
Agilent Technologies 1200 instrument equipped with a variable
wave-length UV detector, using a Daicel Chiralpak OD column
(0.46 cm I.D. x 25 cm). HPLC grade isopropanol and n-hexane
were used as the eluting solvents. Chromatographic separations
were performed on columns of SiO2 (Merck, 230–400 mesh) at
medium pressure. All the organic, inorganic and organometallic
reagents and anhydrous solvents were purchased from Aldrich.
The products 1,3 2,3a 5,14 6,5,15 were previously described.
Synthesis of imine 1
To the suspension of (R,R)-1,2-diaminocyclohexane L-tartrate
(2.35 g, 8.9 mmol) in MeOH (25 mL), terephtalaldehyde (1.19 g,
8.9 mmol) and triethylamine (3.10 mL, 22.3 mmol) were added.
The reaction mixture was stirred for 48 h and the solvent was
evaporated at reduced pressure. A saturated aqueous solution
of NaHCO3 (20 mL) was added and the organic material was
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extracted with dichloromethane (3 ¥ 30 mL). The collected organic
layers were washed with brine (20 mL), dried over Na2SO4 and
concentrated to leave a white solid, which was crystallized from
AcOEt to give pure 1 (1.77 g, 2.8 mmol, 94%).
cyclohexane/AcOEt, 9 : 1) gave (R)-4: 0.046 g (86%); 84% e.e. was
determined by chiral HPLC (Chiralpak OD; 2-propanol/hexane
1 : 9, 1.0 mL min-1.; 214 nm): retention times 11.5 min (S, minor
enantiomer) and 13.8 min (R, major enantiomer).
Organometallic additions to imine 1. Typical procedure
Enantioselective hydrosilylation reaction. Typical procedure
Phenyllithium (0.5 M in Et2O, 14.1 mL, 7.06 mmol) was added to a
magnetically stirred solution of the imine 1 (0.500 g, 0.78 mmol) in
THF (20 mL) cooled at -78 ◦C. After 60 min the reaction mixture
was slowly warmed up until room temperature was reached and
stirring was continued for 24 h. The mixture was quenched with a
saturated aqueous solution of NaHCO3 (10 mL) at 0 ◦C, then the
organic material was extracted with ethyl acetate (3 ¥ 30 mL). The
collected ethereal layers were washed with brine (20 mL), dried
over Na2SO4 and concentrated to leave a white solid, which was
crystallized from MeOH to give pure 3c (0.792 g, 0.72 mmol, 92%).
Et2Zn (1.0 M in toluene, 68 mL, 0.07 mmol) was slowly added to
a solution of 3a (0.050 g, 0.07 mmol) in toluene (2 mL). After
30 min propiophenone (252 mL, 1.89 mmol) and diphenyl silane
(419 mL, 2.27 mmol) were added at 0 ◦C and the reaction was
warmed at room temperature. After 4 h a 1 M solution of NaOH
◦
in MeOH (1 mL) was slowly added at 0 C and the mixture was
stirred for 30 min. The organic phase was extracted with Et2O
(3 ¥10 mL). The organic layer was dried over Na2SO4 and the
solvents were evaporated to dryness. Column chromatography
(SiO2, cyclohexane/AcOEt, 9 : 1) gave (S)-5: 0.165 g (94%);
64% e.e. was determined by chiral HPLC (Chiralpak OD; 2-
propanol/hexane 1 : 99 to 5 : 95 in 10 min., 0.5 mL min-1.; 214 nm):
retention times 20.7 min (R, minor enantiomer) and 21.4 min (S,
major enantiomer).
3a
White solid; 89% (0.509 g, 0.69 mmol) from 0.78 mmol (0.500 g)
of 1; mp 87–88 ◦C (from MeOH); [a]20 -150.4 (c 0.99 in CHCl3);
D
nmax (KBr)/cm-1 3228, 3301, 2962, 2925, 2854, 1448, 1367, 1125,
1059, 1016, 932, 757; dH (200 MHz; CDCl3) 0.92–1.12 (6 H, m),
1.25 (18 H, d, J 6.4), 1.74–1.88 (m, 6 H), 2.21 (6 H, m), 2.49 (6
H, m), 4.01 (6 H, q, J 6.4), 7.37 (12 H, s); dC (50 MHz; CDCl3)
22.2, 24.9, 31.2, 52.8, 57.1, 126.5, 145.9; m/z (ES) 734.0 (40%, [M
+ H]+), 367.5 (100, [1/2 M + H]+); 245.4 (21, [1/3 M + H]+).
Acknowledgements
We are grateful to the XRD Application LAB, Microsystems
Technology Division, Swiss Center for Electronics and Microtech-
nology, Neuchaˆtel, for access to the X-ray diffraction equipment.
The research was supported by MIUR (PRIN Project: “Sintesi e
stereocontrollo di molecole organiche per lo sviluppo di tecnologie
innovative”).
3b
Yellow slurry oil; 86% (0.525 g, 0.53 mmol) from 0.62 mmol (0.400
g) of 1; [a]20 -54.6 (c 1.10 in CHCl3); nmax(neat)/cm-1 3302, 2854,
D
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