Chiral salen ligands designed to form polymetallic complexes

Article abstract of DOI:10.1016/j.tet.2011.10.084

Chiral salen ligands capable of forming polymetallic complexes have been designed. The ligands possess substituents in the 4,4′-positions, but have no substituent in the 3,3′-positions to allow a second metal ion access to the salen oxygen atoms. Ligands in which a polyether chain links the 4,4′-positions were prepared and complexed to copper. In addition, acyclic ligands with potential metal coordinating substituents in the 4,4′-positions were prepared and complexed to copper and cobalt. The crystal structure of one of the cobalt complexes shows it to be a trimetallic complex in which a Co(II)(OAc)₂ group coordinates to the salen oxygen atoms of two Co(III)(salen)(OAc) units. In contrast, the crystal structure of a Co(salen) complex with tert-butyl groups attached to the 3,3′-positions is found to be mononuclear. All of the complexes were tested as asymmetric phase transfer catalysts for the asymmetric alkylation of an alanine methyl ester, forming (R)-α-methyl phenylalanine methyl ester with up to 85% ee.

Full text of DOI:10.1016/j.tet.2011.10.084

Tetrahedron 68 (2012) 133e144
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Chiral salen ligands designed to form polymetallic complexes
*
Thierry R.J. Achard, William Clegg, Ross W. Harrington, Michael North
School of Chemistry and University Research Centre in Catalysis and Intensified Processing, Newcastle University, Bedson Building, Newcastle upon Tyne, NE1 7RU, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 July 2011
Received in revised form 7 October 2011
Accepted 24 October 2011
Available online 31 October 2011
Chiral salen ligands capable of forming polymetallic complexes have been designed. The ligands possess
substituents in the 4,40-positions, but have no substituent in the 3,30-positions to allow a second metal
ion access to the salen oxygen atoms. Ligands in which a polyether chain links the 4,40-positions were
prepared and complexed to copper. In addition, acyclic ligands with potential metal coordinating sub-
stituents in the 4,40-positions were prepared and complexed to copper and cobalt. The crystal structure
of one of the cobalt complexes shows it to be a trimetallic complex in which a Co(II)(OAc)2 group co-
ordinates to the salen oxygen atoms of two Co(III)(salen)(OAc) units. In contrast, the crystal structure of
a Co(salen) complex with tert-butyl groups attached to the 3,30-positions is found to be mononuclear. All
of the complexes were tested as asymmetric phase transfer catalysts for the asymmetric alkylation of an
Keywords:
Salen
Bimetallic
Crown-ether
Copper
alanine methyl ester, forming (R)-
a-methyl phenylalanine methyl ester with up to 85% ee.
Ó 2011 Elsevier Ltd. All rights reserved.
Cobalt
1. Introduction
Once formed, salen ligands are known to form mononuclear com-
plexes with over 40 metals from lithium11 to uranium12 and these
complexes can have various geometries13 including square-planar
(with varying degrees of distortion towards tetrahedral),14 square-
based pyramidal15,16 and octahedral.16,17 In the latter case, the salen
ligand can occupy four coplanar coordination sites (giving a trans-
geometry), or it can occupy four coordination sites, which are not all
Salen ligands of general structure 1 are amongst the most versatile
ligands available.1 They are constructed by a modular synthesis from
salicylaldehydes and diamines and the structure of both of these
components can readily be extensively varied, thus allowing the steric,
electronic and physical properties of the ligand to be tuned2 and fa-
cilitating the synthesis of immobilizedandrecyclablesalencomplexes.3
Chiral versions of the ligands are available by a number of methodol-
ogies, most commonly by the use of a diamine containing one or more
stereocentres4 or a stereoaxis,5 though the introduction of axial6 or
planar7 chirality within the salicylaldehyde unit is also possible. Most
commonly, the two salicylaldehyde units are identical and the diamine
is C2-symmetrical so that the ligand is also C2-symmetrical.8 However,
C1-symmetrical salen ligands can also be prepared by the use of a C1-
symmetrical diamine9,10 and/or two different salicylaldehydes.9
coplanar, resulting in the formation of cis-
a and cis-b geometries,
which are intrinsically chiral at the metal ion.18
The versatility of salen ligands is further enhanced by their
ability to form bi- or poly-metallic complexes of various forms.
Most simply, this can be achieved by coordination of one19 or
both20 of the salen oxygen atoms to a second metal, giving com-
plexes of general structure 2a and 2b, respectively, a situation,
which is most common when M1 forms a four-coordinate salen
complex. In addition, the metals may be coordinated through one
or two ancillary ligands giving structures 3a,b depending on
whether M1 and M2 share one16,21 or two bridging ligands.22 It is
also possible to use the ancillary ligand to bridge between a met-
al(salen) unit and a different metal(ligand) unit.23 Finally, addi-
tional functional groups may be introduced onto the salen ligand to
coordinate to a second metal. Examples of this include polyether
macrocycles 424 and 525 and phenoxy-derivatives in which the
ortho-oxygen atoms may be alkylated26 6a or deprotonated27 6b.
Simultaneous use of more than one of the above coordination
modes to form bimetallic complexes is also possible.28
Much of the interest in metal(salen) complexes in recent years
has focused on their applications as asymmetric cata-
lysts,2,3,5,6,7a,8,9,11,18,26,27,29 and they display such high levels of
* Corresponding author. Tel.: þ44 191 222 7128; fax: þ44 191 222 6929; e-mail
address: Michael.north@ncl.ac.uk (M. North).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.10.084

134
T.R.J. Achard et al. / Tetrahedron 68 (2012) 133e144
catalytic activity in a wide range of reactions that they are referred
to as privileged catalysts.30 In a number of cases the ability of salen
ligands to form bimetallic complexes has been found to be critical
to the high level of catalytic activity they display.26,27,31e38 How-
ever, the ability of salen ligands to form polymetallic complexes
also gives them other applications in the construction of supra-
molecular architectures with potential receptor and materials sci-
ence applications.39 In view of our previous work on metal salen
complexes of type 2b where the central metal is copper31,32 or
cobalt,32,33 type 3a where both metals are aluminium,34 titanium16
or vanadium,35 and type 3b where both metals are titanium36 or
one is titanium and the other vanadium,37 we decided to in-
vestigate the synthesis of salen ligands designed specifically to bind
to two or more metal ions and in this paper we report the results of
this study.
2. Results and discussion
2.1. Macrocyclic salen ligands with a polyether bridge
Our starting point for this project was to develop salen ligands,
which also contain polyether macrocycles. Previous work has
concentrated on complexes 4 in which the polyether bridges the
ortho-positions of the salen ligand24 or 5 in which the polyether
linkages do not bridge the salen aromatic rings.25 However, struc-
ture 5 offers limited opportunities for metalemetal interactions
and our previous work on copper(salen) and cobalt(salen) com-
plexes has shown that substituents in the ortho-positions can
prevent bimetallic complex formation.32,33 Therefore, we decided
to target polyether macrocycles 7 in which the polyether linkages
are in the meta-positions of the salen aromatic rings. Retrosynthetic
analysis of structure 7 (Scheme 1) indicated that the macrocyclic
structure could be constructed from fragments 8e10 by formation
of bonds ‘a’ and ‘b’. Since imine formation is a reversible process, it
was decided that formation of the salen unit should be the final
step of the synthesis to minimize problems due to subsequent
hydrolysis and equilibration.
Scheme 1. Retrosynthetic analysis of macrocycles 7.
Therefore, the starting point for the synthesis of ligands 7 be-
came aldehydes 9. Three different aldehydes 9aec were chosen as
starting materials to allow the influence of substituents on the ar-
omatic rings to be investigated. Compound 9a was commercially
available and 9b was prepared by a literature procedure.40 Alde-
hyde 9c was prepared in 50% yield by the formylation of the
known41 dihydroxybenzene derivative 11 as shown in Scheme 2.
Scheme 2. Synthesis of aldehyde 9c.
Rather than rely on a lengthy protection/deprotection strategy
for the selective alkylation of the 4-phenoxy group in aldehydes
9aec, it was decided instead to rely on alkylation occurring selec-
tively at the less hindered 4-phenoxy group. Therefore, aldehydes
9aec were treated with sodium hydride in DMSO followed by the

T.R.J. Achard et al. / Tetrahedron 68 (2012) 133e144
135
addition of a bis-tosylate 10aec to give a set of five bis-aldehydes
12aee (Scheme 3). In all cases, the yields of bis-aldehydes 12aee
were low (7e25%) due to formation of byproducts in which alkyl-
ation had occurred at the 2-phenoxy group, but the desired product
could be separated from the regioisomers by column chromatog-
raphy. Notably, the lowest yield was obtained for compound 12e,
which is probably due to the large tert-butyl group in the 5-position
hindering the desired alkylation of the 4-phenoxy group in this
case. In the case of compounds 12a,c, the regiochemistry of the
alkylation was proven by NOESYexperiments, which showed cross-
peaks between the ArOCH2CH2 hydrogens and the protons in both
positions 3 and 5 of the aromatic ring.
isolated in 10e43% yield, which probably reflects the poor solu-
bility of both the ligands and the copper complexes. The electro-
spray mass spectra of complexes 13a,c,d all showed MþNa peaks,
suggesting that the polyether chain can coordinate to a sodium ion
resulting in the formation of a heterobimetallic complex. In addi-
tion, the mass spectrum of complex 13c showed peaks corre-
sponding to 2MþNa and 3MþNa, providing further evidence for
the formation of cyclic dimer and trimer from ligand 7c in solution.
2.2. Salen ligands with potential metal-complexing groups in
the 4 and 40-positions
In view of the poor solubility encountered for ligands 7aed and
their corresponding copper complexes 13aed, it was decided to
investigate a different approach to the synthesis of salen ligands
capable of binding two metal ions. This is shown in Fig. 1 and in-
volves the introduction of flexible groups onto the 4-position of the
salen aromatic rings, terminated by a donor group capable of co-
ordinating to a metal ion. Five potential metal-complexing groups
were selected. Three of these have a methoxy group at the end of
a long alkyl chain (containing 8e12 carbon atoms) and are based on
the known ability of methoxy groups in complexes of type 6a to
coordinate to a metal ion.26 The introduction of a benzylic alcohol
into the 4-position was also investigated by analogy with com-
plexes 6b,27 and the introduction of a polyether chain was felt to
mimic macrocycles 4 and 7.24
Fig. 1. Design of non-macrocyclic, bimetallic salen complexes.
Aldehydes 14aed were prepared by the alkylation of 2,4-dihy-
droxybenzaldehyde using potassium hydrogen carbonate as base43
as shown in Scheme 4. Aldehyde 14d was included in this study as
a control compound lacking a potential metal-coordinating group
at the end of the alkyl chain. Known aldehyde 14e was prepared by
the literature procedure44 and aldehyde 14f was prepared by the
formylation45 of 3-hydroxymethylphenol in 1,2-dichlorobenzene
(Scheme 5). Each of aldehydes 14aef was then converted into the
corresponding salen ligand 15aef in 88e99% yield by treatment
with compound 8 in the presence of sodium methoxide (Scheme 6).
Ligands 15aef were each complexed to copper by treatment with
Scheme 3. Synthesis of copper complexes 13aed.
The low yield obtained for the production of bis-aldehyde 12e
prevented its further manipulation. However, treatment of bis-al-
dehydes 12aed with (R,R)-1,2-diaminocyclohexane dihydro-
chloride42 8 in refluxing methanol and ethanol gave macrocyclic
salen ligands 7aed in 73e95% yield. Salen ligands 7aed were found
to have only low solubility in a range of organic solvents. The solu-
bility was not improved by alkylation of the aromatic ring (7b), but
did increase as the size of the polyether chain increased. This per-
mitted a study of the stability of ligands 7c and 7d in chloroform and
in both cases, the ligands were found to be unstable in solution,
equilibrating to a mixture of cyclic monomers, dimers and trimers,
through reversible hydrolysis of the imine bonds.
Only a preliminary study of the ability of ligands 7aed to form
metal complexes has been carried out. However, each of ligands
7aed was found to form a paramagnetic copper(II) complex 13aed
when treated with copper(II) acetate. Complexes 13aed were
Scheme 4. Synthesis of aldehydes 14aed.

136
T.R.J. Achard et al. / Tetrahedron 68 (2012) 133e144
groups in the 3 and 30-positions of the salen ligand was also pre-
pared by metallation of the known22f,46 salen ligand using the same
methodology. Complexes 17a and compound 18 were found to
form crystals suitable for analysis by X-ray crystallography. 18 is
a mononuclear square-planar complex with a structure that is
essentially identical to those of other Co(II)(salen) complex-
es13b,32a,47 and other square-planar M(II)(salen) complexes pre-
viously reported. The structure of one of the four independent
molecules of the asymmetric unit is shown in Fig. 2. The four
molecules are related in pairs by pseudo-inversion symmetry, the R
chirality of all the ligand chiral centres leading to conformations of
the cyclohexane rings that break this pseudo-symmetry. Two of the
molecules show significantly greater twisting and folding de-
viations from ideal square-planar geometry than the other two, but
in other respects the molecules have essentially identical geometry.
Scheme 5. Synthesis of aldehyde 14f.
Scheme 6. Synthesis of ligands 15aef and their metal(II) complexes.
Fig. 2. The structure of one molecule of complex 18, with 40% probability displace-
ment ellipsoids and selective atom labelling.
copper(II) acetate to give copper complexes 16aef. The complexes
were isolated in good yield (67e82%) except for complex 16f, which
was isolated in only 20% yield due to the high solubility of the
copper complex.
Ligands 15aed were also converted into the corresponding
Co(salen) complexes 17aed by treatment with anhydrous cobalt(II)
acetate. For comparison, Co(II)(salen) complex 18 with tert-butyl
In contrast, the structure of complex 17a is much more complex
(Fig. 3). The molecular structure is trimetallic, consisting of two
Co(III)(salen)(acetate) units joined by a Co(II)(acetate)2 unit. Each
cobalt ion is octahedral and the Co(salen) units adopt the trans-
geometry. The central cobalt ion is coordinated by the salen oxygen
atoms of both salen ligands (cf. 2b), and two of the acetate units
bridge between the central cobalt and the Co(salen) units while the
Fig. 3. Molecular structure of complex 17a from X-ray crystallography, with 40% probability displacement ellipsoids. H atoms are omitted for clarity. The selective atom labelling
shown emphasizes the pseudo-inversion symmetry of the molecule, broken principally by the identical rather than opposite absolute configurations of the two cyclohexanediamine
units. While the view chosen for Fig. 2 is perpendicular to the salen coordination plane, here it is at right-angles, looking in the salen coordination plane of both the outer cobalt
atoms.

T.R.J. Achard et al. / Tetrahedron 68 (2012) 133e144
137
Table 1
Synthesis of
other two are bonded terminally as monodentate ligands to the
outer cobalt atoms. The acetate units present in the complex come
from the cobalt(II) acetate used for its synthesis. The molecule has
almost exact inversion symmetry, the R,R chirality of the two salen
ligands being almost the only exception. A simplified representa-
tion of the structure is shown in Fig. 4. This type of trimetallic
structure for Co(salen) complexes has been reported before, but
only for complexes with no substituents in the 3- and 30-positions
of the salen aromatic rings,48 and can give rise to complexes with
urease inhibitory activity.48e Complex 17a is also the only known
chiral example of this type of structure. Thus, the contrasting
structures of complexes 17a and 18 clearly illustrate how sub-
stituents in the 3- and 30-position of a salen ligand prevent the
coordination of a second metal to the oxygen atoms of an M(salen)
complex.
a-methyl phenylalanine methyl ester 20
Entry
Catalyst
Yield
eea
1
2
3
4
5
6
7
8
13a
13b
13c
13d
16a
16b
16c
16d
16e
16f
41
30
30
71
94
73
74
98
62
61
72
42
48
78
2 (R)
7 (R)
7 (R)
6 (R)
85 (R)
79 (R)
75 (R)
79 (R)
57 (R)
6 (R)
53 (R)
55 (R)
38 (R)
74 (R)
9
10
11
12
13
14
17a
17b
17c
17d
a
Determined by 1H NMR analysis of the diastereomeric ureas formed on treat-
ment of product 20 with (S)-phenylethylisocyanate.
depends on the length of the methoxyalkyl chain, with complex
16a bearing the shortest chain giving the highest asymmetric in-
duction (Table 1, entry 5). Lengthening the methoxyalkyl chain
(Table 1, entries 6 and 7) or removing the methoxy group (Table 1,
entry 8), results in a decrease in the asymmetric induction. These
results support the supposition that the methoxy groups can fa-
cilitate formation of a heterobimetallic complex as shown in Fig. 1,
though the effect on the catalytic activity of the complexes is not
dramatic. Complex 16e with a polyether chain is a less enantiose-
lective catalyst (Table 1, entry 9), and catalyst 16f with hydroxy-
methyl substituents displays negligible asymmetric induction.
Cobalt complexes 17aed all display moderate levels of catalytic
activity (Table 1, entries 11e14). Interpretation of these results is
complicated since complex 17a is known to exist as a mixed Co(II)/
Co(III) trimetallic complex in the solid state, and how, if at all, this
dissociates in situ is not known. The solid state structures of com-
plexes 17bed have not been determined.
Fig. 4. Representation of the trimetallic molecular structure of complex 17a.
R¼O(CH2)8OMe.
2.3. Catalytic activity of the metal(salen) complexes
We have previously reported that Cu(salen) and Co(salen)
complexes make highly effective solideliquid asymmetric phase
transfer catalysts for the alkylation of amino acid enolates, using
sodium hydroxide as a base and leading to enantiomerically
3. Conclusions
enriched a,a
-disubstituted amino acids.31,32 The highest levels of
catalytic activity were displayed by catalysts, which lacked sub-
stituents in the 3 and 30-positions of the salen ring, suggesting that
the formation of heterobimetallic complexes of type 2a or 2b may
be important to the catalytic activity. Therefore, copper complexes
13aed and 16aef along with cobalt complexes 17aed were
screened as catalysts for the alkylation of substrate 19, leading to a-
methyl-phenylalanine methyl ester 20 as shown in Scheme 7, and
the results are tabulated in Table 1.
A series of macrocyclic and acyclic salen complexes bearing
substituents at the 4 and 40-positions, but with no substituents at
the 3 and 30-positions, have been prepared. The substituents were
designed to facilitate the formation of polymetallic complexes and
the X-ray crystal structure of a cobalt complex of one of the acyclic
complexes confirms the formation of a trimetallic complex. The
copper and cobalt complexes of the salen ligands were tested as
asymmetric phase transfer catalysts and a copper complex with
O(CH2)8OMe substituents in the 4 and 40-positions was found to
display the highest level of asymmetric induction.
4. Experimental
4.1. General
Chromatographic separations were performed with silica gel 60
(230e400 mesh) and thin-layer chromatography was performed
on polyester-backed sheets coated with silica gel 60 F254, both
supplied by Merck. Melting points were determined with Barn-
stead Electrothermal apparatus No IA9100. Optical rotations were
recorded on a PerkineElmer 343 or a PoLARR 2001 polarimeter in
a thermostated cell of length 1 dm at 20 ꢀC using the sodium D-line,
and a suitable solvent that is reported along with the concentration
(in g/100 ml). Infrared spectra were recorded on a PerkineElmer
FT-IR Paragon 1000 spectrometer from a thin film between NaCl
plates in the reported solvent, or from KBr discs. The characteristic
absorption is reported as strong (s), medium (m) or weak (w).
Scheme 7. Use of M(salen) complexes as asymmetric phase transfer catalysts.
Macrocyclic complexes 13aed were all found to show negligible
levels of asymmetric induction (Table 1, entries 1e4) and the
product that was formed in these reactions probably arises from the
uncatalysed background reaction. The lack of catalytic activity
displayed by complexes 13aed is probably due to their poor solu-
bilities. In contrast, copper complexes 16aed with substituents in
the 4 and 40-positions of the salen rings are highly enantioselective
catalysts (Table 1, entries 5e8). Notably, the asymmetric induction

138
T.R.J. Achard et al. / Tetrahedron 68 (2012) 133e144
1H NMR and 13C NMR spectra were recorded on a Bruker Avance
by column chromatography (Et2O) to give two fractions. The first
was compound 12a obtained as a white solid and the second was
a mixture of compound 12a and the 2,2-dialkylated regioisomer,
which could be separated by recrystallization from CHCl3/Et2O to
give compound 12a (350 mg, 25%) as a white solid. Rf (Et2O) 0.40;
360 spectrometer, (1H 360 MHz, 13C 90 MHz) or a Jeol-Lambda (1H
500 MHz, 13C 125 MHz) spectrometer. All spectra were recorded at
room temperature and the solvent for a particular spectrum is
given in parentheses. 1H and 13C NMR spectra were referenced to
TMS and chemical shift (
d
) values, expressed in parts per million
nmax (CHCl3) 1634 (s) and 1505 (m) cmꢁ1
; dH (CDCl3) 3.68 (4H, s,
(ppm), are reported downfield of TMS. The multiplicity of signals is
reported as singlet (s), doublet (d), triplet (t), quartet (q), multiplet
(m), broad (br) or a combination of any of these. Coupling constant
values (J) are given in hertz. Low- and high-resolution mass spectra
were recorded at the EPSRC national service at the University of
Wales, Swansea, or on a Bruker Apex III FTMS or Jeol AX505 W
spectrometer within the Chemistry Department at King’s College
London. The sample was ionized by electron ionization (EI),
chemical ionization (CI), fast atom bombardment (FAB) or elec-
trospray ionization (ESI). The major fragment ions are reported and
only the molecular ions are assigned.
ArOCH2CH2OCH2), 3.81 (4H, t J 4.6 Hz, ArOCH2CH2), 4.10 (4H, t J
4.6 Hz, ArOCH2), 6.35 (2H, d J 2.3 Hz, ArH), 6.48 (2H, dd J 8.6, 2.3 Hz,
ArH), 7.34 (2H, d J 8.6 Hz, ArH), 9.64 (2H, s, CHO), 11.39 (2H, s, OH);
dC (CDCl3) 66.3, 69.8, 71.3, 101.7, 109.1, 115.7, 135.6, 164.8, 166.3,
194.8; m/z (CI, NH3) 391 (MHþ, 100), 408 (52), 392 (24); HRMS (ES):
MþNaþ, found 413.1204. C20H22NaO8 requires 413.1206.
4.2.3. O,O0-(3-Hydroxy-4-formyl-5-methylphenyl) triethylene glycol
(12b). To a suspension of NaH (0.30 g, 7.66 mmol) in dry DMSO
(6 mL) under nitrogen,
a solution of 2,4-dihydroxy-6-methyl-
benzaldehyde40 9b (0.53 g, 3.48 mmol) in dry DMSO (5 mL) was
added dropwise with vigorous stirring. The temperature was kept
under25 ꢀC andtheresultingbrownsolutionwasstirredfor1 h. Then
a solution of triethylene glycol ditosylate 10a (0.80 g, 1.74 mmol) in
dry DMSO (8 mL) was added in one portion and the resulting mixture
was stirred for 20 h. Subsequently, water (150 mL) was added, and
the resulting brown solution was acidified with 6 M HCl to pH 1,
whereupon the solution became milky yellow. The aqueous mixture
was extracted with CHCl3 (5ꢂ40 mL). The combined organic layers
were washed with a solution of 1 M HCl (150 mL) and dried (MgSO4).
The solvent was removed in vacuo and the residue was purified by
column chromatography (5/1: Et2O/CHCl3) to give compound 12b
(320 mg, 10%) as a white solid. Rf (17% CHCl3/Et2O) 0.51; nmax (KBr)
3435 (br), 3019 (s), 2400 (w) and 1630 cmꢁ1 (m); dH (CDCl3) 2.53 (6H,
s, CH3), 3.76 (4H, s, ArOCH2CH2OCH2), 3.87 (4H, t J 4.5 Hz,
ArOCH2CH2), 4.16 (4H, t J 4.5 Hz, ArOCH2), 6.26 (2H, d J 2.3 Hz, ArH),
6.26 (2H, d J 2.3 Hz, ArH), 10.10 (2H, s, CHO), 12.46 (2H, s, OH); dC
(CDCl3) 18.7, 68.0, 69.8, 71.3, 99.6, 111.3, 113.8, 144.2, 166.3, 166.8,
193.3; m/z (ES) 441 (MþNaþ, 100), 353 (19); HRMS (ES): MþNaþ,
found 441.1517. C22H26NaO8 requires 441.1519.
X-ray single-crystal diffraction data were collected for complex
18 using a Bruker SMART 1K diffractometer with Mo K
a
radiation
¼0.71073 A) at 150 K. The small and weakly diffracting crystals of
complex 17a required the use of synchrotron radiation at station 9.8
ꢀ
(l
ꢀ
of the SRS at Daresbury Laboratory (
l
¼0.6893 A), measurements
being made at 120 K. Standard Bruker control and integration
software was used, and the structures were solved and refined with
SHELXTL.49 Data have been deposited at the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge, CB2 1EZ (see
http://www.ccdc.cam.ac.uk/products/csd/request/) with reference
numbers CCDC 835278 and 835279.
4.2. Synthesis of polyethers 12a–e
4.2.1. 2,4-Dihydroxy-5-tert-butylbenzaldehyde
(9c). Phosphorus
oxychloride (1.4 mL) was added dropwise to DMF (37 mL) with
rapid stirring over 30 min whilst keeping the reaction temperature
below 10 ꢀC. A solution of 3-hydroxy-4-tert-butylphenol41 11
(14.1 g, 85.0 mol) in DMF (38 mL) was then slowly added, keeping
the temperature below 10 ꢀC. The reaction was warmed to room
temperature and stirred for 1 h. Then ice and 10% aqueous NaOH
were added to adjust the pH to 9e10, resulting in formation of
a solid. This mixture was heated to reflux for 10 min and then ad-
justed to pH 3 with 10% aqueous HCl. After cooling to room tem-
perature, the mixture was left overnight. The solid product was
collected by filtration, washed with water until neutral, dried and
purified by column chromatography using hexane/CHCl3 (70:30) to
give compound 9c (1.495 g, 50%) as a soft orange solid. Rf (11% Et2O/
hexane) 0.22; nmax (KBr) 3330 (br), 2970 (w), 1644 (s) and
1510 cmꢁ1 (w); dH (CDCl3) 1.32 (9H, s, (CH3)3), 5.75 (1H, s, OH), 6.23
(1H, s, ArCH), 7.31 (1H, s, ArCH), 9.64 (1H, s, CHO), 11.17 (1H, s, OH);
dC (CDCl3) 30.0, 34.5, 105.4, 115.1, 130.2, 133.4, 162.7, 163.3, 195.3; m/
z (CI, NH3) 212 (MþNHþ4 , 68) 195 (MHþ, 100) 179 (19); HRMS (ES):
MþNaþ, found 217.0840. C11H14NaO3 requires 217.0835.
4.2.4. O,O0-(3-Hydroxy-4-formylphenyl)
tetraethylene
glycol
(12c). To a suspension of NaH (6.33 g,159.28 mmol) under nitrogen
in dry DMSO (125 mL), a solution of 2,4-dihydroxybenzaldehyde 9a
(10.00 g, 72.40 mmol) in dry DMSO (70 mL) was added dropwise
with vigorous stirring. The temperature was kept below 25 ꢀC and
the resulting brown solution was stirred for 1 h. Then a solution of
tetraethylene glycol ditosylate 10b (18.19 g, 36.20 mmol) in dry
DMSO (40 mL) was added in one portion and the resulting mixture
was stirred for 20 h. Subsequently, water (700 mL) was added, and
the resulting brown solution was acidified with 6 M HCl to pH 1,
whereupon the solution became milky yellow. The aqueous mixture
was extracted with CHCl3 (4ꢂ200 mL). The combined organic layers
were washed with a solution of 1 M HCl (400 mL) and dried
(MgSO4). The solvent was removed in vacuo and the residue was
purified by column chromatography (Et2O followed by Et2O/CHCl3
(5:1)) to give compound 12c (3.70 g, 23%) as an orange oil; Rf (17%
CHCl3/Et2O) 0.23; nmax (CHCl3) 2252 (w), 1635 (m) and 1507 cmꢁ1
(m); dH (CDCl3) 3.71e3.85 (8H, m, ArOCH2CH2OCH2CH2), 3.88 (4H, t J
4.6 Hz, ArOCH2CH2), 4.18 (4H, t J 4.6 Hz, ArOCH2), 6.43 (2H, d J 2.3 Hz,
ArH), 6.57 (2H, dd J 8.7, 2.3 Hz, ArH), 7.44 (2H, d J 8.7 Hz, ArH), 9.73
(2H, s, CHO), 11.48 (2H, s, OH); dC (CDCl3) 68.3, 69.7, 71.1, 71.3, 101.7,
109.1, 115.7, 135.6, 164.8, 166.4, 194.8; m/z (EI) 434 (Mþ, 100), 297
(29), 252 (37), 226 (44), 208 (52), 164 (61), 139 (30), 105 (16); HRMS
(ES): MþNaþ, found 457.1475. C22H26NaO9 requires 457.1469.
4.2.2. O,O0-(3-Hydroxy-4-formylphenyl) triethylene glycol (12a). To
a suspension of NaH (0.47 g, 15.92 mmol) in dry DMSO (10 mL)
under nitrogen,
a solution of 2,4-dihydroxybenzaldehyde 9a
(1.00 g, 7.24 mmol) in dry DMSO (6 mL) was added dropwise with
vigorous stirring whilst keeping the temperature below 25 ꢀC. The
resulting brown solution was stirred for 1 h. Then a solution of
triethylene glycol ditosylate 10a (1.65 g, 3.62 mmol) in dry DMSO
(5 mL) was added in one portion and the resulting mixture stirred
for 20 h. Subsequently, water (200 mL) was added, and the result-
ing brown solution was acidified with 6 M HCl to pH 1, whereupon
the solution became milky yellow. The aqueous mixture was
extracted with CHCl3 (5ꢂ40 mL). The combined organic layers were
washed with a solution of 1 M HCl (200 mL) and dried (MgSO4). The
solvent was removed in vacuo and the crude product was purified
4.2.5. O,O0-(3-Hydroxy-4-formylphenyl)
pentaethylene
glycol
(12d). To a suspension of NaH (6.33 g, 159.28 mmol) under nitrogen
in dry DMSO (125 mL), a solution of 2,4-dihydroxybenzaldehyde 9a
(10.00 g, 72.40 mmol) in dry DMSO (70 mL) was added dropwise with

T.R.J. Achard et al. / Tetrahedron 68 (2012) 133e144
139
vigorous stirring. The temperature was kept below 25 ꢀC and the
resulting brown solution was stirred for 1 h. Then a solution of pen-
taethylene glycol ditosylate 10c (19.79 g, 36.20 mmol) in dry DMSO
(40 mL) was added in one portion and the resulting mixture was
stirred for 20 h. Subsequently, water (700 mL) was added, and the
resulting brown solution was acidified with 6 M HCl to pH 1, where-
upon the solution became milky yellow. The aqueous mixture was
extracted with CHCl3 (4ꢂ200 mL). The combined organic layers were
washed with a solution of 1 M HCl (400 mL) and dried (MgSO4). The
solvent was removed invacuo and the residuewas purified by column
chromatography (CHCl3/EtOAc, 1:1) to give compound 12d (3.45 g,
20%) as a soft orange oil; Rf (50% CHCl3/EtOAc) 0.26; nmax (CHCl3) 3019
(w), 2400 (w),1630 (m) and 1508 cmꢁ1 (s); dH (CDCl3) 3.62e3.77 (12H,
m, ArOCH2CH2OCH2CH2OCH2), 3.87 (4H, t J 4.9 Hz, ArOCH2CH2), 4.18
(4H, t J 4.9 Hz, ArOCH2), 6.43 (2H, d J 2.2 Hz, ArH), 6.57 (2H, dd J 8.6,
2.2 Hz, ArH), 7.74 (2H, d J 8.6 Hz, ArH), 9.72 (2H, s, CHO), 11.47 (2H, s,
OH); dC (CDCl3) 68.3, 69.7, 71.0, 71.0, 71.3, 101.7, 109.1, 115.6, 135.6,
164.8, 166.4, 194.8; m/z(CI, NH3)496(MþNHþ4 ,100), 376 (28), 358 (12),
332 (7); HRMS (ES): MþNaþ, found 501.1674. C24H30NaO10 requires
501.1686.
ArOCH2CH2), 4.06 (4H, br, ArOCH2), 6.21e6.32 (4H, m, 2ꢂ ArH), 6.97
(2H, d J 8.7 Hz, ArH), 8.08 (2H, s, CH]N), 13.81 (2H, s, OH); dC
(CDCl3) 24.6, 33.1, 67.5, 69.6, 71.0, 71.4, 101.8, 102.4, 106.6, 106.8,
132.9, 133.3, 162.6; m/z (ES) 469 (MHþ,100), 341 (32), 295 (27), 259
(38), 255 (73); HRMS (ES): MHþ, found 469.2334. C26H33N2O6 re-
quires 469.2333.
4.3.2. Compound 7b. Obtained as a yellow gel in 73% yield. ½a D20
ꢃ
ꢁ578 (c 0.1, CHCl3); nmax (CHCl3) 2938 (m), 2360 (m), 2253 (s), 1614
(s) and 1457 cmꢁ1 (m); dH (CDCl3) 1.54e2.68 (8H, m, NCHCH2CH2),
2.13 (6H, s, CH3), 3.21e3.25 (2H, m, NCH), 3.71 (4H, s, ArOCH2
-
CH2OCH2), 3.82 (4H, t J 4.9 Hz, ArOCH2CH2), 4.06 (4H, t J 4.9 Hz,
ArOCH2), 6.09 (2H, s, ArH), 6.17 (2H, s, ArH), 8.27 (2H, s, CH]N),14.63
(2H, s, OH); dC (CDCl3) 19.1, 24.6, 33.2, 67.5, 69.9, 71.1, 71.2, 100.3,
109.2, 110.4, 141.1, 161.9, 163.3, 168.4; m/z (CI, NH3) 497 (MHþ, 6);
HRMS (ES): MHþ, found 497.2638. C28H37N2O6 requires 497.2646.
4.3.3. Compound 7c. Obtained as a yellow oil in 95% yield. ½a D20
ꢃ
ꢁ320 (c 0.017, CHCl3); nmax (CHCl3) 3019 (br), 2938 (w), 1627 (s) and
1515 cmꢁ1 (w); dH (CDCl3) 1.51e2.03 (8H, m, NCHCH2CH2),
3.21e3.26 (2H, m, NCH), 3.70e3.80 (8H, m, ArOCH2CH2OCH2CH2),
3.80 (4H, t J 5.0 Hz, ArOCH2CH2), 4.07 (4H, t J 4.5 Hz, ArOCH2),
6.31e6.40 (4H, m, 2ꢂ ArH), 6.98 (2H, d J 8.5 Hz, ArH), 8.08 (2H, s,
CH]N), 13.81 (2H, s, OH); dC (CDCl3) 24.6, 33.6, 67.8, 69.9, 71.0, 71.2,
72.1, 102.1, 107.1, 112.6, 133.1, 162.9, 164.1, 165.1; m/z (CI, NH3) 513
(MHþ, 39), 402 (23), 369 (28), 254 (33), 153 (48) 115 (100); HRMS
(ES): MþNaþ, found 535.2435. C28H36N2NaO7 requires 535.2414.
4.2.6. O,O0-(2-tert-Butyl-4-formyl-5-hydroxy phenyl) pentaethylene
glycol (12e). To a suspension of NaH (0.13 g, 3.40 mmol) in dry
DMSO (4 mL) under nitrogen, a solution of 2,4-dihydroxy-5-tert-
butyl-benzaldehyde 9c (0.30 g, 1.54 mmol) in dry DMSO (1.5 mL)
was added dropwise with vigorous stirring. The temperature was
kept below 25 ꢀC and the resulting brown solution was stirred for
1 h. Then a solution of pentaethylene glycol ditosylate 10c (0.42 g,
0.77 mmol, 1 equiv) in dry DMSO (4 mL) was added in one portion
and this mixture was stirred for 20 h. Subsequently, water (40 mL)
was added, and the resulting brown solution was acidified with 6 M
HCl to pH 1, whereupon the solution became milky yellow. The
aqueous mixture was extracted with CHCl3 (5ꢂ10 mL). The com-
bined organic layers were washed with a solution of 1 M HCl
(40 mL) and dried (MgSO4). The solvent was removed in vacuo and
the residue was purified by column chromatography (Et2O/hexane:
70/30) to give compound 12e (20 mg, 7%) as a soft green oil; Rf (30%
hexane/Et2O) 0.51; nmax (CHCl3) 3019 (m), 2963 (w), 2400 (w), 1639
(s) and 1582 cmꢁ1 (w); dH(CDCl3) 1.29 (18H, s, (CH3)3), 3.51e3.61
4.3.4. Compound 7d. Obtainedasayellowoilin86%yield. ½a D20
ꢁ724
ꢃ
(c 0.021, CHCl3); nmax (CHCl3) 3019 (br), 2937 (w), 1627 (s) and
1515 cmꢁ1 (w); dH (CDCl3) 1.33e2.01 (8H, m, NCHCH2CH2), 3.21e3.27
(2H, m, NCH), 3.60e3.67 (12H, m, ArOCH2CH2OCH2CH2OCH2), 3.81
(4H, t J 4.8 Hz, ArOCH2CH2), 4.08 (4H, t J 4.4 Hz, ArOCH2), 6.31e6.39
(4H, m, 2ꢂ ArH), 6.98 (2H, d J 8.4 Hz, ArH), 8.09 (2H, s, CH]N),13.82
(2H, s, OH); m/z (CI, NH3), 558 (MHþ, 22), 511 (14), 441 (14), 403 (14),
349 (16), 274 (16), 222 (17),126 (25) and 115 (100); HRMS (ES):
MþNaþ, found 579.2675. C30H40N2NaO8 requires 579.2676.
4.4. General procedure for the synthesis of copper complexes
(8H, m, ArOCH2CH2OCH2CH2OCH2), 3.63 (4H, t J 2.4 Hz, ArOCH2
13aed
-
CH2OCH2), 3.83 (4H, t J 4.5 Hz, ArOCH2CH2), 4.09 (4H, t J 4.5 Hz,
ArOCH2), 6.31 (2H, s, ArH), 7.28 (2H, s, ArH), 9.63 (2H, s, CHO), 11.29
(2H, s, OH); dC (CDCl3) 30.1, 34.7, 67.9, 69.7, 71.0, 71.1, 71.1, 100.4,
114.4, 131.5, 131.9, 163.2, 165.4, 195.2; m/z (CI, NH3) 608 (MþNHþ4 ,
100), 413 (11), 69 (15); HRMS (ES): MþNaþ, found 613.2987.
C32H46NaO10 requires 613.2983.
Solutions of salen ligand 7aed (0.83 mmol) in EtOH (20 mL) and
Cu(OAc)2$H2O (0.17 g, 0.83 mmol) in water (2 mL) were mixed and
refluxed with vigorous stirring for 2 h. The resulting solution was
then cooled to room temperature, filtered and the precipitate
washed successively with H2O, MeOH and Et2O (3ꢂ10 mL) to pro-
vide copper complexes 13aed.
4.3. General procedure for the synthesis of ligands 7aed
To a stirred mixture of bis-aldehyde 12aed (10.0 mmol) and
4.4.1. Compound 13a. Obtained as a black solid in 42% yield.
Mp>400 ꢀC; nmax (KBr) 3435 (br),1605 (s) and 1524 cmꢁ1 (m); HRMS
(ES): MþNaþ, found 552.1294. C26H30CuN2NaO6 requires 552.1297.
(R,R)-1,2-diaminocyclohexane
dihydrochloride42
8
(1.87 g,
10.0 mmol) in MeOH (37 mL) and EtOH (37 mL) was added NaOMe
(1.26 g, 23.3 mmol) dissolved in MeOH (5 mL) at room temperature.
The resulting bright yellow solution was stirred under reflux
overnight. Subsequently, the reaction mixture was allowed to cool
to room temperature and then filtered and evaporated in vacuo.
The yellow residue was taken up in CH2Cl2 (80 mL), filtered and
then the organic layers were washed with water (2ꢂ30 ml) and
brine (1ꢂ30 ml). The combined aqueous layers were back-extracted
with CH2Cl2. The combined organic layers were dried (MgSO4) and
evaporated to dryness to leave ligands 7aed.
4.4.2. Compound 13b. Obtained as a brown/green solid in 30% yield.
Mp>400 ꢀC; nmax (KBr) 2865 (br),1605 (s) and 1522 cmꢁ1 (m); HRMS
(ES): MþNaþ, found 580.1619. C28H34CuN2NaO6 requires 580.1611.
4.4.3. Compound 13c. Obtained as a black solid in 23%. Mp>400 ꢀC;
½
a 2D0
ꢃ
ꢁ320 (c 0.0056, DMSO); nmax (KBr) 2859 (br), 1605 (s), 1525 (s)
and 1428 cmꢁ1 (m); m/z (ES) 596 (MþNaþ, 48), 367 (100), 301 (57),
242 (50), 177 (82); HRMS (ES): MþNaþ, found 596.1592.
C28H34CuN2NaO7 requires 596.1554. 2MþNaþ, found 1169.3477.
C56H68Cu2N4NaO14 requires 1169.3222. 3MþNaþ, found 1742.5299.
C84H102Cu3N6NaO21 requires 1742.4884.
4.3.1. Compound 7a. Obtained as a hard yellow gel in 87% yield.
½
a 2D0
ꢃ
ꢁ567 (c 0.035, CHCl3); nmax (CHCl3) 3435 (br), 3019 (m), 1627
(s) and 1515 cmꢁ1 (w); dH (CDCl3) 1.50e2.05 (8H, m, NCHCH2CH2),
3.20e3.33 (2H, m, NCH), 3.70 (4H, s, ArOCH2CH2OCH2), 3.80 (4H, br,
4.4.4. Compound 13d. Obtained as a black solid in 10% yield.
Mp>400 ꢀC; ½a 2D0
ꢁ263 (c 0.0095, DMSO); nmax (KBr) 3436 (br),
ꢃ

140
T.R.J. Achard et al. / Tetrahedron 68 (2012) 133e144
2862 (w), 1606 (s), 1527 (s) and 1485 cmꢁ1 (w); m/z (ES) 640
(MþNaþ, 95), 337 (100); HRMS (ES): MþNaþ, found 640.1780.
C30H38CuN2NaO8 requires 640.1816.
32.2 mmol), tin(IV) chloride (0.37 mL, 3.22 mmol) and tributyl-
amine (3.07 mL, 12.9 mmol). The mixture was stirred for 20 min at
room temperature, then paraformaldehyde (2.12 g, 70.9 mmol) was
added. The resulting yellowish solution was heated to 100 ꢀCꢄ5 ꢀC
for 8 h. After cooling, the reaction mixture was poured into water
(10 mL) and the aqueous phase was extracted with CHCl3
(3ꢂ30 ml). The combined organic layers were washed with brine
(30 mL), dried (MgSO4) and evaporated in vacuo. The residue was
purified by column chromatography (CHCl3/Et2O, 1:1) to give
compound 14f (740 mg, 15%) as a white solid. Mp 78e80 ꢀC; Rf (50%
CHCl3/Et2O) 0.21; nmax (CHCl3) 3434 (br), 3020 (s), 2977 (w), 2400
(m), 1655 (s), 1575 (w) and 1508 cmꢁ1 (w); dH (CDCl3) 4.77 (2H, s,
CH2), 7.02e7.05 (2H, m, 2ꢂ ArH), 7.57 (1H, d J 7.7 Hz, ArH), 9.90 (1H,
s, CHO), 11.10 (1H, s, OH); dC (CDCl3) 64.4, 114.9, 117.7, 119.8, 134.0,
151.1, 161.9, 196.1; m/z (CI) 152 (MHþ, 100), 135 (16), 123 (63), 95
(60), 77 (78); HRMS (ES): Mþ, found 152.0469. C8H8O3 requires
152.0468.
4.5. General procedure for the synthesis of aldehydes 14aed
2,4-Dihydroxybenzaldehyde (1.99 g, 14.5 mmol) and the ap-
propriate bromoalkane50 (14.5 mmol) were dissolved in DMF
(20 mL) and KHCO3 (1.45 g, 14.5 mmol) was added. The mixture
was heated at reflux for 3 h, then cooled to room temperature and
poured into an aqueous 6 N HCl solution (200 mL). The organic
layer was separated and the aqueous layer was extracted with
diethyl ether (3ꢂ60 mL). The combined organic layers were dried
(MgSO4) and the solvent was removed in vacuo. The residue was
purified by column chromatography (hexane/acetone: 99/1) to give
compounds 14aed.
4.5.1. Compound 14a. Obtained as a colourless oil in 30% yield. Rf
(10% acetone/hexane) 0.21; nmax (CHCl3) 3019 (m), 2932 (w), 2860
(w), 2400 (w), 1630 (m), 1575 (w) and 1508 cmꢁ1 (w); dH (CDCl3)
1.28e1.53 (10H, m, (CH2)5), 1.68e1.75 (2H, m, ArOCH2CH2), 3.26
(3H, s, OCH3), 3.30 (2H, t J 6.5 Hz, MeOCH2), 3.92 (2H, t J 6.5 Hz,
ArOCH2), 6.34 (1H, d J 2.2 Hz, ArH), 6.45 (1H, dd J 8.7, 2.2 Hz, ArH),
7.34 (1H, d J 8.7 Hz, ArH), 9.63 (1H, s, CHO), 11.41 (1H, s, OH); dC
(CDCl3) 26.2, 26.4, 29.3, 29.6, 29.7, 30.0, 58.4, 69.0, 73.3, 101.4, 109.2,
115.4, 135.6, 164.9, 166.8, 194.7; m/z (CI, NH3) 281 (MHþ, 100);
HRMS (ES): MHþ, found 281.1745. C16H25O4 requires 281.1747.
4.6. General procedure for the synthesis of ligands 15aef
To a stirred mixture of aldehyde 14aef (1.24 mmol) in MeOH
(10 mL) was added a solution of (R,R)-1,2-diaminocyclohexane
dihydrochloride42 (0.13 g, 0.69 mmol) and sodium methoxide
(0.074 g, 1.37 mmol) in MeOH (10 mL) at room temperature. The
resulting bright yellow solution was stirred at reflux for 2 h. Sub-
sequently, the reaction was allowed to cool to room temperature
and the solvent evaporated in vacuo. The residue was dissolved in
CHCl3 (35 mL) and washed with water (5ꢂ8 mL). The combined
aqueous phase was extracted with CHCl3 (8 mL). The combined
organic layers were washed with brine (8 mL), dried (MgSO4) and
the solvent was removed in vacuo to give ligands 15aef.
4.5.2. Compound 14b. Obtained as a colourless oil in 40% yield. Rf
(10% acetone/hexane) 0.33; nmax (CHCl3) 3417 (br), 3155 (w), 2930
(s), 2856 (w), 2253 (w), 1630 (s), 1576 (w), 1506 (w) and 1467 cmꢁ1
(w); dH (CDCl3) 1.26e1.60 (14H, m, (CH2)7), 1.76e1.84 (2H, m,
ArOCH2CH2), 3.34 (3H, s, OCH3), 3.38 (2H, t J 6.6 Hz, MeOCH2), 4.01
(2H, t J 6.6 Hz, ArOCH2), 6.42 (1H, d J 2.3 Hz, ArH), 6.54 (1H, dd J 8.7,
2.3 Hz, ArH), 7.43 (1H, d J 8.7 Hz, ArH), 9.72 (1H, s, CHO), 11.49 (1H,
s, OH); dC (CDCl3) 26.3, 26.5, 29.3, 29.7, 29.8, 29.85, 29.9, 30.0, 59.0,
69.0, 73.3, 101.4, 109.2, 115.4, 135.6, 164.9, 166.9, 194.7; m/z (EI) 308
(Mþ, 100); HRMS (ES): MHþ, found 309.2064. C18H29O4 requires
309.2060.
4.6.1. Compound 15a. Obtained as a yellow oil in 90% yield. ½a D20
ꢃ
ꢁ528 (c 0.0125, CHCl3); nmax (CHCl3) 3019 (s), 2935 (m), 2960 (w),
1626 (s) and 1515 cmꢁ1 (w); dH (CDCl3) 1.25e1.89 (32H, br,
(CH2)6þCHCH2CH2), 3.13e3.16 (2H, m, NCH), 3.25 (6H, s, OCH3),
3.28 (4H, t J 6.6 Hz, MeOCH2), 3.84 (4H, t J 6.5 Hz, ArOCH2), 6.22 (2H,
dd J 8.5, 2.3 Hz, ArH), 6.25 (2H, d J 2.3 Hz, ArH), 6.90 (2H, d J 8.5 Hz,
ArH), 8.00 (2H, s, CH]N), 13.73 (2H, br, OH); dC (CDCl3) 24.9, 26.6,
26.7, 29.7, 29.9, 30.0, 30.3, 33.7, 59.2, 68.6, 72.2, 73.5, 102.2, 107.3,
112.7, 133.3, 163.6, 164.3, 165.5; m/z (CI), 639 (MHþ, 90), 377 (90),
280 (26), 253 (27), 178 (59), 158 (35), 115 (100); HRMS (ES): MHþ,
found 639.4376. C38H59N2O6 requires 639.4368.
4.5.3. Compound 14c. Obtained as a colourless oil in 25% yield. Rf
(10% acetone/hexane) 0.29; nmax (CHCl3) 3019 (s), 2929 (m), 2857
(w), 2400 (w), 1630 (s) and 1508 cmꢁ1 (w); dH (CDCl3) 1.29e1.60
(18H, m, (CH2)9), 1.77e1.84 (2H, m, CH2), 3.35 (3H, s, OCH3), 3.38
(2H, t J 6.6 Hz, MeOCH2), 4.02 (2H, t J 6.6 Hz, ArOCH2), 6.42 (1H, d J
2.2 Hz, ArH), 6.54 (1H, dd J 8.7, 2.2 Hz, ArH), 7.43 (1H, d J 8.7 Hz,
ArH), 9.72 (1H, s, CHO), 11.50 (1H, s, OH); dC (CDCl3) 26.0, 26.2, 26.4,
29.0, 29.1, 29.4, 29.5, 29.6, 29.7, 29.8, 58.9, 69.0, 73.4, 101.4, 109.2,
115.4, 135.6, 164.9, 166.9, 194.7; m/z (CI, NH3) 337 (MHþ, 100);
HRMS (ES): MHþ, found 337.2377. C20H33O4 requires 337.2373.
4.6.2. Compound 15b. Obtained as a yellow oil in 93% yield. ½a D20
ꢃ
ꢁ530 (c 0.01, CHCl3); nmax (CHCl3) 3435 (br), 2930 (m), 2856 (w),
1623 (s) and 1512 cmꢁ1 (w); dH (CDCl3) 1.21e1.89 (40H, br,
(CH2)8þCHCH2CH2), 3.12e3.15 (2H, m, NCH), 3.25 (6H, s, OCH3),
3.28 (4H, t J 6.7 Hz, MeOCH2), 3.83 (4H, t J 6.6 Hz, ArOCH2), 6.23 (2H,
dd J 8.5, 2.3 Hz, ArH), 6.25 (2H, d J 2.3 Hz, ArH), 6.90 (2H, d J 8.5 Hz,
ArH), 8.00 (2H, s, CH]N), 13.73 (2H, br, OH); dC (CDCl3) 24.4, 26.1,
26.2, 29.2, 29.4, 29.52, 29.58, 29.6, 29.8, 33.2, 58.9, 68.4, 72.0, 73.3,
102.9, 107.0, 112.4, 133.1, 163.4, 164.1, 165.2; m/z (ES), 695 (MHþ,
100); HRMS (ES): MHþ, found 695.4991. C42H67N2O6 requires
695.4994.
4.5.4. Compound 14d43. Obtained as an orange gel in 38% yield. Rf
(10% EtOAc/heptane) 0.44; nmax (CHCl3) 3019 (m), 2932 (w), 2860
(w), 2400 (w), 1630 (m), 1575 (w) and 1508 cmꢁ1 (w); dH (CDCl3)
0.92 (3H, t J 7.0 Hz, CH3), 1.31e1.50 (6H, m, (CH2)3), 1.77e1.85 (2H,
m, CH2), 4.00 (2H, t J 6.5 Hz, OCH2), 6.42 (1H, d J 2.3 Hz, ArH), 6.54
(1H, dd J 8.6, 2.3 Hz, ArH), 7.44 (1H, d J 8.6 Hz, ArH), 9.72 (1H, s,
CHO), 11.50 (1H, s, OH); dC (CDCl3) 14.4, 22.9, 26.0, 29.3, 31.9, 69.0,
101.4, 109.2, 115.4, 135.6, 164.9, 166.9, 194.2; m/z (ES) 223 (MHþ,
100); HRMS (ES): MHþ, found 223.1326. C13H19O3 requires
223.1329.
4.6.3. Compound 15c. Obtained as a yellow oil in 97% yield. ½a D20
ꢃ
ꢁ451 (c 0.0175, CHCl3); nmax (CHCl3) 3400 (br), 2930 (s), 2855 (m),
2252 (w),1627 (s) and 1512 cmꢁ1 (w); dH (CDCl3) 1.19e1.97 (48H, br,
(CH2)10þCHCH2CH2), 3.13e3.16 (2H, m, NCH), 3.26 (6H, s, OCH3),
3.29 (4H, t J 6.7 Hz, MeOCH2), 3.84 (4H, t J 6.5 Hz, ArOCH2), 6.23 (2H,
dd J 8.5, 2.3 Hz, ArH), 6.26 (2H, d J 2.3 Hz, ArH), 6.91 (2H, d J 8.5 Hz,
ArH), 8.00 (2H, s, CH]N), 13.73 (2H, br, OH); dC (CDCl3) 24.3, 26.1,
26.2, 29.2, 29.4, 29.60, 29.65, 29.66, 29.68, 29.75, 29.8, 33.2, 58.9,
68.4, 72.0, 73.4, 102.9, 107.1, 112.4, 133.1, 163.4, 164.1, 165.2; m/z (ES)
4.5.5. 2-Hydroxy-4-hydroxymethylbenzaldehyde (14f). To a 50 mL
four-necked round-bottom flask equipped with a reflux condenser,
mechanical stirrer, thermometer and a nitrogen inlet were added
1,2-dichlorobenzene (20 mL), 3-hydroxymethylphenol (3.99 g,

T.R.J. Achard et al. / Tetrahedron 68 (2012) 133e144
141
751 (MHþ, 100); HRMS (ES): MHþ, found 751.5627. C46H75N2O6
requires 751.5620.
(s), 2937 (m), 2861 (w), 1607 (s), 1529 (m) and 1429 cmꢁ1 (w); m/z
(ES) 606 (MþNaþ, 59), 584 (MHþ, 100); HRMS (ES): MHþ, found
584.2667. C32H45CuN2O4 requires 584.2670.
4.6.4. Compound 15d. Obtained as a yellow solid in 99% yield. Mp
60e64 ꢀC; ½a 2D0
ꢃ
ꢁ598 (c 0.03, CHCl3); nmax (CHCl3) 3019 (w), 2225
4.7.5. Compound 16e. Obtained as a dark purple solid in 68% yield.
(m), 1626 (s) and 1513 cmꢁ1 (w); dH (CDCl3) 0.90 (6H, t J 6.7 Hz,
CH3), 1.30e1.98 (24H, m, (CH2)4þCHCH2CH2), 3.19e3.24 (2H, m,
NCH), 3.93 (4H, t J 6.6 Hz, OCH2), 6.32 (2H, dd J 8.6, 2.2 Hz, ArH),
6.35 (2H, d J 2.2 Hz, ArH), 6.99 (2H, d J 8.6 Hz, ArH), 8.09 (2H, s, CH]
N), 13.82 (2H, s, OH); dC (CDCl3) 14.4, 23.0, 24.6, 26.0, 29.4, 31.9,
33.5, 68.4, 72.0, 101.9, 107.0, 112.4, 133.1, 163.4, 164.0, 165.2; m/z (ES)
523 (MHþ, 100); HRMS (ES): MHþ, found 523.3532. C32H47N2O4
requires 523.3530.
Mp>400 ꢀC; ½a 2D0
ꢁ320 (c 0.0197, CHCl3); nmax (KBr) 2929 (br), 1628
ꢃ
(s), 1535 (s) and 1448 cmꢁ1 (m); m/z (ES) 614 (MþNaþ, 100), 592
(MHþ, 80); HRMS (ES): MþNaþ, found 614.1663. C28H36CuN2NaO8
requires 614.1665.
4.7.6. Compound 16f. Obtained as a brown solid in 20% yield.
Mp>400 ꢀC; ½a 2D0
ꢁ1020 (c 0.007, CHCl3); nmax (CHCl3) 3424 (br),
ꢃ
3018 (s), 2398 (w), 1632 (s), 1520 (w) and 1422 cmꢁ1 (w); m/z (ES)
466 (MþNaþ, 31), 444 (MHþ, 100); HRMS (ES): MHþ, found
444.1100. C22H25CuN2O4 requires 444.1105.
4.6.5. Compound 15e. Obtained as a yellow oil in 96% yield after
extraction using EtOAc rather than CHCl3. ½a D20
ꢁ533 (c 0.046,
ꢃ
CHCl3); nmax (CHCl3) 3019 (s), 2400 (w), 1628 (m) and 1515 cmꢁ1
(w); dH (CDCl3) 1.43e1.97 (8H, m, CHCH2CH2), 3.20e3.28 (2H, m,
NCH), 3.37 (6H, s, OCH3), 3.50e3.56 (4H, br, MeOCH2), 3.71e3.82
(4H, br, MeOCH2CH2), 5.25 (4H, s, OCH2O), 6.47 (2H, dd J 8.5, 2.3 Hz,
ArH), 6.54 (2H, d J 2.3 Hz, ArH), 7.04 (2H, d J 8.5 Hz, ArH), 8.15 (2H, s,
CH]N), 13.75 (2H, s, OH); dC (CDCl3) 24.6, 33.5, 59.4, 68.2, 71.9,
72.4, 93.4, 104.1, 107.5, 113.7, 133.1, 161.0, 164.13, 164.16; m/z (CI) 531
(MHþ, 100), 444 (12), 323 (60); HRMS (ES): MþNaþ, found
553.2565. C28H38N2NaO8 requires 553.2520.
4.8. General procedure for the synthesis of cobalt complexes
17aed and 18
Salen ligand (5.0 mmol) and anhydrous cobalt(II) acetate (1.25 g,
5.0 mmol) were added to ethanol (16 mL) and the solution stirred
at 70 ꢀC for 1 h under an argon atmosphere. During this time an
orange solid precipitated. The reaction mixture was allowed to cool
to room temperature and then filtered and washed with hexane
under a nitrogen atmosphere. The orange precipitate was purified
by suspension in refluxing ethanol under an argon atmosphere,
followed by filtration and washing with hexane.
4.6.6. Compound 15f. Obtained as a yellow oil in 88% yield. ½a D20
ꢃ
ꢁ125 (c 0.02, CHCl3); nmax (CHCl3) 2930 (w), 1629 (m) and
1560 cmꢁ1 (s); dH (THF-d8) 1.31e1.90 (8H, m, CHCH2CH2), 3.11e3.18
(2H, m, NCH), 4.25 (4H, s, CH2), 6.50 (2H, d J 7.8 Hz, ArH), 6.60 (2H, s,
ArH), 6.90 (2H, d J 7.8 Hz, ArH), 8.10 (2H, s, CH]N), 10.67 (2H, s,
OH), 12.80 (2H, s, ArOH); dC (CDCl3) 24.2, 33.0, 64.8, 72.6, 114.7,
116.7,121.0,131.6,145.6,161.3,164.3; m/z (ES) 405 (MþNaþ, 27), 383
(MHþ,100); HRMS (ES): MHþ, found 383.1965. C22H27N2O4 requires
383.1964.
4.8.1. Compound 17a. Obtained as a dark brown solid in 98% yield.
Crystals suitable for X-ray analysis were obtained by re-
crystallization from CHCl3/hexane. Mp 160e167 ꢀC; ½a D20
þ103 (c
ꢃ
0.0258, CHCl3); nmax (CHCl3) 3019 (s), 2933 (m), 2859 (w), 2400 (w),
1606 (s), 1526 (m) and 1468 cmꢁ1 (w); m/z (FAB) 695 (Co(salen)þ,
100); HRMS (ES): Co(salen)þ, found 695.3470. C38H56CoN2O6 re-
quires 695.3465. Selected crystallographic data: C84H124Co3N4O20
,
4.7. General procedure for the synthesis of copper complexes
16aef
M¼1686.7, triclinic, space group P1, a¼10.799(4), b¼13.577(5),
¼77.983(4)ꢀ,
ꢀ
c¼15.613(6) A,
a
¼87.211(4),
V¼2144.0(14) A , Z¼1, T¼120 K, R (F, F >2
b
¼73.262(4),
g
3
ꢀ
2
s
)¼0.081, Rw (F2, all
Solutions of salen ligand 15aef (0.83 mmol) in EtOH (20 mL)
and Cu(OAc)2$H2O (0.17 g, 0.83 mmol) in water (2 mL) were mixed
and refluxed with vigorous stirring for 2 h. The resulting solution
was then cooled to room temperature, filtered and the precipitate
washed successively with H2O, MeOH and Et2O (3ꢂ10 mL) to pro-
vide copper complexes 16aef.
data)¼0.237. The structure of the molecule is close to centrosym-
metric, the pseudosymmetry being broken mainly by the R chirality
of all the chiral centres of the two ligand cyclohexanediamine
groups; this absolute configuration is satisfactorily confirmed from
anomalous scattering effects,51 though these are relatively weak
because the heavy Co atoms have a centrosymmetric arrangement.
Refinement required the use of similarly restraints on the geometry
of the ligands and restraints on displacement parameters, and there
is undoubtedly unresolved disorder in the side-chains of the li-
gands, as indicated by high displacement parameters and short
intermolecular contacts.
4.7.1. Compound 16a. Obtained as a brown solid in 71% yield. Mp
212e213 ꢀC; ½a 2D0
ꢁ837 (c 0.0245, CHCl3); nmax (KBr) 3430 (br), 3019
ꢃ
(s), 2936 (m), 2861 (w), 1607 (s) and 1529 cmꢁ1 (m); m/z (FAB) 722
(MþNaþ, 24), 700 (MHþ, 100); HRMS (ES): MHþ, found 700.3512.
C38H57CuN2O6 requires 700.3507.
4.8.2. Compound 17b. Obtained as a dark brown solid in 93% yield.
4.7.2. Compound 16b. Obtained as a dark brown solid in 81% yield.
Mp 148e156 ꢀC; ½a D20
þ380 (c 0.0316, CHCl3); nmax (CHCl3) 3019 (s),
ꢃ
Mp 208e210 ꢀC; ½a D20
ꢃ
ꢁ645 (c 0.0165, CHCl3); nmax (CHCl3) 3420
2931 (m), 2858 (w), 2400 (w), 1606 (s), 1606 (m), 1531 (w) and
1430 cmꢁ1 (w); m/z (FAB) 751 (MHþ, 100); HRMS (ES): MHþ, found
752.4166. C42H65CoN2O6 requires 752.4169.
(br), 3019 (s), 2932 (m), 2857 (w), 2359 (s), 2341 (s), 1607 (s) and
1528 cmꢁ1 (w); m/z (ES) 756 (MHþ, 100); HRMS (ES): MHþ, found
756.4136. C42H65CuN2O6 requires 756.4133.
4.8.3. Compound 17c. Obtained as a dark brown solid in 93% yield.
4.7.3. Compound 16c. Obtained as a dark brown solid in 67% yield.
Mp 114e118 ꢀC; ½a D20
þ577 (c 0.0142, CHCl3); nmax (CHCl3) 3019 (s),
ꢃ
Mp 202e204 ꢀC; ½a D20
ꢃ
ꢁ587 (c 0.015, CHCl3); nmax (CHCl3) 3019 (s),
2929 (w), 2857 (w), 2400 (w), 1607 (m), 1519 (m) and 1428 cmꢁ1
(w); m/z (FAB) 808 (MHþ, 90), 807 (100); HRMS (ES): MHþ, found
808.4801. C46H73CoN2O6 requires 808.4795.
2929 (m), 2856 (w), 2359 (s), 2341 (s), 1607 (s) and 1527 cmꢁ1 (w);
m/z (ES) 812 (MHþ, 100); HRMS (ES): MHþ, found 812.4758.
C46H73CuN2O6 requires 812.4759.
4.8.4. Compound 17d. Obtained as an orange solid in 15% yield.
4.7.4. Compound 16d. Obtained as a brown solid in 82% yield. Mp
Mp>350 ꢀC; ½a 2D0
ꢁ823 (c 0.011, CHCl3); nmax (KBr) 3019 (s), 2939
ꢃ
261e267 ꢀC; ½a 2D0
ꢃ
ꢁ787 (c 0.015, CHCl3); nmax (KBr) 3419 (br), 3019
(m), 2861 (w), 1599 (m), 1523 (m) and 1431 cmꢁ1 (w); m/z (ES) 579

142
T.R.J. Achard et al. / Tetrahedron 68 (2012) 133e144
(MHþ, 100); HRMS (ES): MHþ, found 579.2631. C32H45CoN2O4 re-
quires 579.2628.
References and notes
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Science Books: Sausalito, 2009.
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4.8.5. Compound 18. Obtained as a red solid in 53% yield. Crystals
~
suitable for X-ray analysis were obtained by recrystallization from
3. (a) Canali, L.; Sherrington, D. C. Chem. Soc. Rev. 1999, 28, 85e93; (b) Baleizao, C.;
CH2Cl2. Mp 291e297 ꢀC; ½a D20
ꢁ125 (c 0.0386, CHCl3); nmax (CHCl3)
ꢃ
Garcia, H. Chem. Rev. 2006, 106, 3987e4043; (c) Leung, A. C. W.; MacLachlan, M.
J. J. Inorg. Organomet. Polym. Mater. 2007, 17, 57e89; (d) Madhavan, N.; Jones, C.
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9814e9821; (c) Ciogli, A.; Cort, A. D.; Gasparrini, F.; Lunazzi, L.; Mandolini, L.;
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3019 (s), 2946 (m), 2400 (m), 1597 (m), 1535 (m) and 1420 cmꢁ1
(w); m/z (ES) 491 (MHþ, 100), 210 (70); HRMS (ES): MHþ, found
491.2112. C28H37CoN2O2 requires 491.2103. Selected crystallo-
graphic data: C28H36CoN2O2$0.25CH2Cl2, M¼512.8, monoclinic,
ꢀ
space groupꢀ C2, a¼30.229(3), b¼18.755(2), c¼19.046(2) A,
3
2
ꢀ
b
¼103.959(2) , V¼10,479.2(19) A , Z¼16, T¼150 K, R (F, F >2 )¼
s
0.047, Rw (F2, all data)¼0.114. The asymmetric unit of the crystal
structure contains four crystallographically independent molecules
of the complex, together with half each of two dichloromethane
solvent molecules, one of which is disordered; further, highly dis-
ordered solvent molecules may also be present, as indicated by the
identification of small voids in the structure,52 but these could not
be modelled with discrete atoms. The structure overall is close to
centrosymmetric, the pseudosymmetry being broken mainly by the
R chirality of all the chiral centres of the ligand cyclohexanediamine
groups; this absolute configuration is confirmed from anomalous
scattering effects.51 The four molecules are related in pairs by
pseudo-inversion centres.
9. Kleij, A. W. Eur. J. Inorg. Chem. 2009, 193e205.
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245e255; (c) Evans, D. J.; Junk, P. C.; Smith, M. K. Polyhedron 2002, 21,
2421e2431; (d) Cort, A. D.; Mandolini, L.; Schiaffino, L. Chem. Commun. 2005,
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13. (a) Pasini, A.; Gullotti, M.; Ugo, R. J. Chem. Soc., Dalton Trans. 1977, 346e356; (b)
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(e) Bharara, M. S.; Tonks, S. A.; Gorden, A. E. V. Chem. Commun. 2007,
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14. For representative examples see: (a) Bernardo, K.; Leppard, S.; Robert, A.;
Commenges, G.; Dahan, F.; Meunier, B. Inorg. Chem. 1996, 35, 387e396; (b)
White, D. J.; Laing, N.; Miller, H.; Parsons, S.; Coles, S.; Tasker, P. A. Chem.
Commun. 1999, 2077e2078; (c) Miller, H. A.; Laing, N.; Parsons, S.; Parkin, A.;
Tasker, P. A.; White, D. J. J. Chem. Soc., Dalton Trans. 2000, 3773e3782; (d) Szyk,
E.; Surdykowski, A.; Barwiolek, M.; Larsen, E. Polyhedron 2002, 21, 2711e2717;
(e) Coxall, R. A.; Lindoy, L. F.; Miller, H. A.; Parkin, A.; Parsons, S.; Tasker, P. A.;
White, D. J. J. Chem. Soc., Dalton Trans. 2003, 55e64; (f) Zhang, F.; Bai, S.; Yap, G.
P. A.; Tarwade, V.; Fox, J. M. J. Am. Chem. Soc. 2005, 127, 10590e10599; (g) Wang,
F.; Zhang, H.; Li, L.; Hao, H.-Q.; Wang, X.-Y.; Chen, J.-G. Tetrahedron: Asymmetry
2006, 17, 2059e2063; (h) Correia, I.; Dornyei, A.; Jakusch, T.; Avecilla, F.; Kiss, T.;
Costa Pessoa, J. Eur. J. Inorg. Chem. 2006, 2819e2830; (i) Shimazaki, Y.; Yajima,
T.; Tani, F.; Karasawa, S.; Fukui, K.; Naruta, Y.; Yamauchi, O. J. Am. Chem. Soc.
2007, 129, 2559e2568; (j) Storr, T.; Verma, P.; Pratt, R. C.; Wasinger, E. C.;
Shimazaki, Y.; Stack, T. D. P. J. Am. Chem. Soc. 2008, 130, 15448e15459; (k)
Constable, E. C.; Zhang, G.; Housecroft, C. E.; Neuburger, M.; Schaffner, S.;
Woggon, W.-D. New J. Chem. 2009, 33, 1064e1069; (l) Orio, M.; Jarjayes, O.;
Kanso, H.; Philouze, C.; Neese, F.; Thomas, F. Angew. Chem., Int. Ed. 2010, 49,
4989e4992.
15. For representative examples see: (a) Srinivasan, K.; Kochi, J. K. Inorg. Chem.
1985, 24, 4671e4679; (b) Samsel, E. G.; Srinivasan, K.; Kochi, J. K. J. Am. Chem.
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nadies, J. A.; Raymond, K. N. Inorg. Chem. 1986, 25, 154e160; (d) Bonadies, J.
A.; Butler, W. M.; Pecoraro, V. L.; Carrano, C. J. Inorg. Chem. 1987, 26,
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1995, 14, 17e18; (f) Cornman, C. R.; Geiser-Bush, K. M.; Rowley, S. P.; Boyle, P.
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W. T.; Kolis, J. W. Acta Crystallogr. 1998, C54, 1263e1265; (h) Hoshina, G.;
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G.; Tsuchimoto, M.; Ohba, S. Acta Crystallogr. 1999, C55, 1082e1084; (j) Liang,
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Chem. 2010, 49, 6664e6672.
4.9. Synthesis of a
-methyl-phenylalanine methyl ester31 (20)
(ꢄ)-N-para-Chlorobenzylidene alanine methyl ester31 19 (0.20 g,
1.05 mmol) was dissolved in dry toluene (2.5 mL) under an argon
atmosphere. To this solution, catalyst (0.02 mmol), powdered so-
dium hydroxide (0.15 g, 3.66 mmol) and benzyl bromide (0.15 mL,
1.26 mmol) were added sequentially. The mixture was stirred for
19 h under an argon atmosphere, then MeOH (2 mL) and AcCl
(0.5 mL) were added, the latter added dropwise. The reaction was
stirred for a further 4 h, then the solvents were removed in vacuo.
The residue was purified by column chromatography (EtOAc then
EtOH/EtOAc 1:4) to give compound 20 as a colourless oil. dH(CDCl3)
1.35 (3H, s, CH3C), 2.12 (2H, br s, NH2), 2.76 (1H, d J 13.2 Hz, CH2),
3.07 (1H, d J 13.2 Hz, CH2), 3.63 (3H, s, OCH3), 7.02e7.33 (5H, m,
ArCH); dC (CDCl3) 28.3, 48.6, 53.7, 60.5, 128.6, 130.0, 131.6, 138.2,
179.2. To determine the enantiomeric excess, (S)-phenyl-
ethylisocyanate (1 or 2 drops) was added to an NMR sample of
compound 20 and allowed to react overnight until there was no
unreacted amino ester present. The de of the resulting urea, which
corresponds to the ee of compound 20 was determined by in-
tegration of the PhCH2 protons of the resulting diastereomeric
ureas.
Acknowledgements
The authors thank Newcastle University for financial support.
We also thank EPSRC for an equipment grant (X-ray diffractometer)
and for funding of the National Crystallography Service (synchro-
tron component operated from Newcastle), and CCLRC (now STFC)
for access to synchrotron facilities at Daresbury Laboratory SRS.
16. Belokon’, Y. N.; Carta, P.; Gutnov, A. V.; Maleev, V.; Moskalenko, M. A.; Yashkina,
L. V.; Ikonnikov, N. S.; Voskoboev, N. V.; Khrustalev, V. N.; North, M. Helv. Chim.
Acta 2002, 85, 3301e3312.
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J. Chem. Soc., Dalton Trans. 1972, 503e508; (b) Schaefer, W. P.; Waltzman, R.;
Huie, B. T. J. Am. Chem. Soc. 1978, 100, 5063e5067; (c) Gheller, S. F.; Bradbury, J.
R.; Mackay, M. F.; Wedd, A. G. Inorg. Chem. 1981, 20, 3899e3904; (d) Jurisson, S.;
Lindoy, L. F.; Dancey, K. P.; McPartlin, M.; Tasker, P. A.; Uppal, D. K.; Deutsch, E.
Inorg. Chem. 1984, 23, 227e231; (e) Che, C.-M.; Cheng, W.-K.; Mak, T. C. W. Inorg.
Chem. 1988, 27, 250e253; (f) Tararov, V. I.; Hibbs, D. E.; Hursthouse, M. B.;
Ikonnikov, N. S.; Malik, K. M. A.; North, M.; Orizu, C.; Belokon’, Y. N. Chem.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tet.2011.10.084. This data in-
cludes MOL files and InChiKeys of the most important compounds
described in this article

T.R.J. Achard et al. / Tetrahedron 68 (2012) 133e144
143
ˇ
€
ꢄ
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