Design and synthesis of quasi-diastereomeric molecules with unchanging central, regenerating axial and switchable helical chirality via cleavage and formation of Ni(II)-O and Ni(II)-N coordination bonds

Article abstract of DOI:10.3762/bjoc.8.223

We describe herein the design and synthesis of asymmetric, pentadentate ligands, which are able to coordinate to Ni(II) cations leading to quasi-diastereomeric complexes displaying two new elements of chirality: stereogenic axis and helix along with configurational stabilization of the stereogenic center on the nitrogen. Due to the stereocongested structural characteristics of the corresponding Ni(II) complexes, the formation of quasi-diastereomeric products is highly stereoselective providing formation of only two, (R_a^*,M_h^*, R_c^*) and (R_a^*,P _h^*,R_c^*), out of the four possible stereochemical combinations. The reversible quasi-diastereomeric transformation between the products (R_a^*,M _h^*,R_c^*) and (R _a^*,P_h^*,R _c^*) occurs by intramolecular trans-coordination of Ni-NH and Ni-O bonds providing a basis for a chiral switch model.

Full text of DOI:10.3762/bjoc.8.223

Design and synthesis of quasi-diastereomeric
molecules with unchanging central, regenerating
axial and switchable helical chirality via cleavage and
formation of Ni(II)–O and Ni(II)–N coordination bonds
Vadim A. Soloshonok*1,2, José Luis Aceña1, Hisanori Ueki3 and Jianlin Han4
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 1920–1928.
1Department of Organic Chemistry I, Faculty of Chemistry, University
of the Basque Country, 20018 San Sebastián, Spain, 2IKERBASQUE,
Basque Foundation for Science, 48011 Bilbao, Spain, 3International
Center for Materials Nanoarchitectonics (MANA), National Institute for
Materials Science (NIMS), 1-1, Namiki, Tsukuba, Ibaraki 305-0044,
Japan and 4School of Chemistry and Chemical Engineering, Nanjing
University, Nanjing, 210093, China
doi:10.3762/bjoc.8.223
Received: 09 August 2012
Accepted: 29 October 2012
Published: 13 November 2012
This article is part of the Thematic Series "Molecular switches and cages".
Guest Editor: D. Trauner
Email:
Vadim A. Soloshonok* - vadym.soloshonok@ehu.es
© 2012 Soloshonok et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
axial chirality; central chirality; chiral switches; coordination bonds;
functional materials; helical chirality; modular structural design;
molecular devices
Abstract
We describe herein the design and synthesis of asymmetric, pentadentate ligands, which are able to coordinate to Ni(II) cations
leading to quasi-diastereomeric complexes displaying two new elements of chirality: stereogenic axis and helix along with configu-
rational stabilization of the stereogenic center on the nitrogen. Due to the stereocongested structural characteristics of the corres-
ponding Ni(II) complexes, the formation of quasi-diastereomeric products is highly stereoselective providing formation of only
two, (Ra*,Mh*,Rc*) and (Ra*,Ph*,Rc*), out of the four possible stereochemical combinations. The reversible quasi-diastereomeric
transformation between the products (Ra*,Mh*,Rc*) and (Ra*,Ph*,Rc*) occurs by intramolecular trans-coordination of Ni–NH and
Ni–O bonds providing a basis for a chiral switch model.
Introduction
The design and synthesis of organic molecules for which the versible diastereomeric transformations are considered as the
changing of their three-dimensional structure or function can be most promising models for the development of molecular
predicted, is a fast-growing multidisciplinary field of research switches with nondestructive read out of the optical informa-
[1-13]. In particular, organic compounds that can undergo re- tion [14-18]. Taking into account the issue of fatigue resistance
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Beilstein J. Org. Chem. 2012, 8, 1920–1928.
or durability of a potential molecular switch, the newly In this context, we subsequently envisioned the design and syn-
emerging design strategies make use of the formation and thesis of structurally related complexes in which the chiral
cleavage of metal–ligand coordination bonds as a basis for a switch will be based on trans-coordination of nonsymmetric
chiral conformational switch in the conformationally restricted ligands (Scheme 2).
molecular backbone [19-23]. Here, we would like to describe a
new design and synthesis of quasi-diastereomeric molecules
with unchanging central, regenerating axial, and switchable
helical chirality, through cleavage/formation of Ni(II)–O and
Ni(II)–N coordination bonds.
Recently, we introduced a new approach to the design of
organic molecules with switchable chirality by simple cleavage
and formation of metal–ligand coordination bonds, as a poten-
tially useful and conceptually new model for the development
of a new generation of organic chiroptical molecular switches
[24,25]. Thus, achiral C2-symmetric pentadentate ligands 1
were able to coordinate with d8 metals [Ni(II) or Pd(II)] to form
tetra-coordinated diastereomeric complexes (Ra*,Ph*,Sc*)-2 and
(Ra*,Mh*,Rc*)-3, which contain at least three elements of
chirality, namely stereogenic center, axis and helix (Scheme 1).
The structure of these complexes incorporated two five- and
one six-membered ring displaying a square-planar geometry. It
is worth mentioning that only two out of the four possible
diastereomeric complexes were accessed in a highly stereose-
Scheme 2: General design of asymmetric pentadentate ligands 4 and
chiroptically switchable quasi-diastereomeric molecules 5 and 6,
possessing elements of central, axial and helical chirality.
lective manner, as a result of the intrinsic steric hindrance asso- The major difference of the general design presented in
ciated with the design of achiral ligands 1. In addition, crystal- Scheme 2 with the previously reported models (Scheme 1) is
lization of the diastereomeric mixtures resulted in the forma- that the starting pentadentate ligand 4 is asymmetric and there-
tion of a single diastereomer (Ra*,Ph*,Sc*)-2 or (Ra*,Mh*,Rc*)- fore already possesses the stereogenic center at the coordina-
3 in the solid state, depending on the nature of the chelating tion site C. It is assumed that the pentadentate ligand 4 upon
metal (Ni or Pd) employed.
coordination with d8 metals will give rise to tetra-coordinated
square-planar complexes 5 and 6. The presence of coordinated
and noncoordinated groups A and E, positioned up or down
relative to the metal coordination plane, will create an element
of axial chirality, directed through the coordination sites D in
complex 5 and B in 6, and the corresponding adjacent methyl-
enes of the chelating five-membered rings. The second, newly
created element of chirality may be provided by steric repulsive
interactions in the arms BA/DE, which will twist the six-
membered chelating ring resulting in a screw sense of helical
chirality. The transcoordination motion in these complexes is
expected to be controlled in a way that noncoordinated sites,
i.e., E in 5 and A in 6, will move up and down, respectively,
resulting in the interconversion of complexes 5 and 6. This
assumed concurrent and unidirectional motion of arms ED and
AB is expected to have the following desired stereochemical
consequences: Thus, the axial chirality on the arm CDE in 5
will disappear with simultaneous generation of a new chiral axis
on the arm CBA in 6, of the same stereochemical sense. Next,
this envisioned mode of transcoordination must proceed also
with a loss and regeneration of helical chirality, but, in contrast
to the loss/regeneration of axial chirality, with the inversion of
Scheme 1: Previous design of diastereomeric molecules 2 and 3 with
switchable chirality starting from achiral, C2-symmetric pentadentate
ligands 1.
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Beilstein J. Org. Chem. 2012, 8, 1920–1928.
the stereochemical sense, providing quasi-diastereomeric rela- were reacted [39] with “acid” module 8 to form the corres-
tionships between complexes 5 and 6 [26].
ponding amides 9 in quantitative chemical yield. Application of
compounds 9 for bis-alkylation of “amine” module 10 was
As one can expect, these desired structural and stereochemical conducted in two separate steps including isolation and purifica-
considerations can be seriously compromised by the presence of tion of the intermediate products 11. The undesired formation of
central chirality in the starting ligand 4. For instance, applica- the corresponding quaternary ammonium salts was not observed
tion of racemic ligands 4 will give rise to at least four diastereo- [40] and the target ligands 12 were prepared in high overall
meric products rendering the designed diastereomeric inter- yield (96%).
transformation with switchable chirality simply pointless.
Furthermore, application of enantiomerically pure ligands 4 can
present a problem of mismatched stereochemical preferences
between the existing central and newly generated set of axial
and helical chirality. On the other hand, if the stereogenic center
in ligands 4 is configurationally flexible (can easily undergo (S)
to (R) transformation) one may expect the opposite trend, as the
combined stereochemical preferences of axial and helical
chirality will impose the corresponding matching configuration
of the stereogenic center. Thus, the geometric difference
between the designed quasi-diastereomers 5 and 6 is that the
noncoordinated arm CDE and the substituent R on the stereo-
genic coordination site C in 5 are on the same side of the metal
chelation plane, while in structure of 6 the arm CBA and the
substituent R are on the opposite sides. In contrast, the
geometric relations between the off-plane six-membered chelate
rings, giving rise to helical chirality, and the substituent R are
the same in both quasi-diastereomers 5 and 6. Considering these
geometric features, one may expect that the actual absolute con-
figuration of the stereogenic center C, determined by the stereo-
chemical priority of the groups A and E, has no importance for
the quasi-diastereomeric relationships between 5 and 6. Conse-
quently, there is a good chance that the geometric position of
the substituent R, and therefore the relative configuration of the
stereogenic center on C, may be efficiently controlled by the
Scheme 3: Preparation of imino–carbonyl ligands 13 by desym-
metrization of achiral carbonyl–carbonyl ligands 12.
matching stereochemical preferences between the axial and
helical chirality in the quasi-diastereomers 5 and 6. However, to
realize this possibility the stereogenic center C in ligands 4
should be configurationally unstable and easily adaptable to the Surprisingly, we found that the seemingly simple transforma-
developing stereochemical environment upon its configura- tion of 12 to 13 can present a significant synthetic problem.
tional stabilization in complexes 5 and 6.
Thus, our attempts to prepare the desired mono-imino ligands
13 from compounds 12, using one equivalent of various amines,
resulted in the formation of complex mixtures of inseparable
Results and Discussion
To build the real molecules to this design, we explored the products. Possible reasons for this outcome are the presence of
preparation of imino–carbonyl ligands 13 (Scheme 3) by two potentially reactive amide groups and the multifunctional
desymmetrization of achiral carbonyl–carbonyl ligands 12. nature of compounds 12. In particular, preparation of the most
Compounds 12 were prepared in the framework of our recently desirable NH-containing (R’ = H) derivatives 13, by reaction of
described methodology for applying a new generation of 12 with ammonia or its derivatives, was not possible at all.
nucleophilic glycine equivalents [27-32] to a general asym- Modification of the synthetic scheme with installation of the
metric synthesis of α-amino acids [33-38]. In this manner, imino functionality on intermediate stages also gave inaccept-
modular assembly of achiral C2-symmetric pentadentate ligands able results, probably due to hydrolytic liability of the corres-
12 was carried out by using three inexpensive and readily avail- ponding imino derivatives of, for instance, compounds 9 and
able starting structural units. First, the “phenone” modules 7 11.
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Beilstein J. Org. Chem. 2012, 8, 1920–1928.
While these unexpected synthetic obstacles can, probably, be To receive more precise data about the structure of complexes
overcome with a more extensive search for optimized condi- 14 and 15 we performed crystallographic analysis of compound
tions, we decided to take advantage of our experience in the 14a [54] (Figure 1). Quite surprisingly, we found that in the
chemistry of Ni(II)-chelated amino acid Schiff bases [41-48] solid state (monoclinic crystal system, space group P21/n) com-
and to develop an indirect approach for the preparation of asym- plex 14a has three elements of chirality: stereogenic center
metric ligands of type 13. To this goal, we studied the reactions [N(2)], axis [N(1)–C(8) bond] and helix (off-plane position of
of biscarbonyl compounds 12 with glycine.
the chelate rings). While these three elements of chirality can
give up to four possible diastereomeric combinations, only one
As shown in Scheme 4, heating of pentadentate ligands 12 in diastereomer, as a pair of (Rc,Ra,Ph) and (Sc,Sa,Mh) enan-
acetonitrile in the presence of a Ni(II) source [NiCl2 or tiomers, was found in the crystallographic unit cell. This fact
Ni(NO3)2], glycine and a base, lead to the formation of the quite clearly suggested that the observed set of stereochemical prefer-
unusual complexes 14. In a similar manner, by using PdCl2 ences is a result of a mutual match between these three elements
instead of Ni(II), the Pd-chelated complex 15 was obtained. As of chirality giving rise to the most stable diastereomeric stereo-
it follows from these results, only one carbonyl group in the chemistry.
starting ligands 12 was involved in the formation of the corres-
ponding Schiff base with glycine, similar to the reactivity
observed for tridentate ligands with Ni(II) [49-53]. In the 1H
and 13C NMR spectra of compounds 14 and 15, resonances of
aromatic and aliphatic hydrogens and carbons appear as sharp
peaks indicating that the extra bidentate moiety in complexes 14
and 15 acts merely as a substituent and does not compete for
coordination with Ni(II). Complexes 14 are red-colored, 15 is
yellow, and both are neutral, highly crystalline compounds (mp
> 250 °C), good solubility in organic solvents, and can be easily
purified by regular column chromatography on SiO2.
Figure 1: Crystallographic structure of complex 14a.
The distances of the Ni–O(5) [1.8651(13) Å] and Ni–N(4)
[1.8569(16) Å] bonds of the five-membered chelate ring of the
glycine Schiff base fragment were relatively similar, while the
Scheme 4: Preparation of complexes 14a,b and 15 by reactions of
ligands 12 with glycine.
bonds Ni–N(3) [1.8417(16) Å] and Ni–N(2) [1.9535(16) Å]
were noticeably shorter and longer, respectively. The square-
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Beilstein J. Org. Chem. 2012, 8, 1920–1928.
planar structure of compound 14a is slightly distorted with however, plausible key transformations can be deduced based
angles N(4)–Ni–N(2) of 173.48(7)° and N(3)–Ni–O(5) of on the closely related results on the well-established general
175.15(6)°. This deviation from planarity gives compound 14a oxidation of enolates [55-64] and glycine-derived enolates [65-
a bowl-like shape and renders it inherently chiral. For instance, 70], in particular, under similar conditions. Thus, the ionized
the torsion angle C(19)–N(3)–C(20)–C(21) of 32.0(3)° sets the form of α-hydroxy intermediate 17 can undergo the cleavage of
element of helical chirality, while the axial chirality is located the glycine C–N bond [65,68-70] resulting in the formation of
through the N(1)–C(8) bond with the torsion angle neutral complex 18. Previously, we demonstrated that in situ
C(9)–N(1)–C(8)–C(7) of −139.5(2)°. The central chirality is formed derivatives of type 18, without the pendant side chain,
located on N(2) as, coordinated to Ni(II), nitrogen is configura- undergo further stabilization through dimerization, giving rise
tionally stable and has four different substituents. As mentioned to the corresponding binuclear complexes [48]. In this case, the
above, the absorptions of hydrogens and carbons in the NMR presence of the bidentate substituent allows for a different,
spectra, recorded at ambient temperature, are sharp and there- intramolecular stabilization mechanism leading to the forma-
fore do not suggest any diastereomeric transformations. Accord- tion of target complexes 19/20.
ingly, one may assume that in CHCl3 solution compounds 14
and 15 also exist in one diastereomeric form possibly stabilized It is interesting to note that the acetophenone module-derived
by apical coordination of the amidic carbonyl of the pendent compound 14a gave the expected enolate 16a; however, its
side chain (distance of Ni–O(2) = 2.665 Å).
further oxidation lead mostly to decomposition, suggesting
higher instability of intermediate derivatives of type 17 and 18
With complexes 14 and 15 in hand, we next studied the oxi- [48]. Also, the benzophenone-derived, Pd(II)-containing
dation of 14 as presented in Scheme 5. Formation of the corres- analogue 15 (Scheme 4) failed to produce the target complexes
ponding enolates 16, in the presence of relatively strong bases of type 19/20, pointing to some limitation of this synthetic alter-
(KOt-Bu), is an established step in general methods for homolo- native. Nevertheless, this oxidative pathway allowed for con-
gation of the glycine moiety by alkyl halide alkylation [27,50- venient preparation of otherwise inaccessible complexes 19/20
53], aldol [29,33,36,37] and Michael [28,31,34,35,38,39,43-47] formally derived from asymmetric carbonyl/imine ligands 13
addition reactions. The following oxidation step of the enolates (Scheme 3) and Ni(II). In contrast to typical Ni(II) complexes,
16 in the presence of atmospheric oxygen, is less studied [48]; such as 14a,b, compounds 19/20 display an unusual dark-brown
Scheme 5: Oxidation of enolates 16 and formation of complexes 19 and 20.
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Beilstein J. Org. Chem. 2012, 8, 1920–1928.
color. Nevertheless, their high solubility in polar organic
solvents allows for their straightforward purification by column
chromatography. Initial assignment of the structures of com-
pounds 19/20 by NMR was not possible because of the very fast
exchange between Ni–O=C and Ni–NH=C chelation. Thus, due
to fast interconversion between compounds 19 and 20, only
broad signals were observed in the 1H NMR spectra recorded at
ambient temperature. However, variable temperature experi-
ments (−50 °C) produced a notable sharpening in both aromatic
and aliphatic protons. Ultimately, elucidation of their structure
was carried out by X-ray analysis, which was feasible due to
their high crystallinity [54].
We found that in the solid state, compounds 19 and 20 exist as
the sole NH–Ni coordinated quasi-diastereomer (Ra*,Mh*,Rc*)-
19 (Figure 2), containing a pair of (Ra,Mh,Rc) and (Sa,Ph,Sc)
enantiomers in the crystallographic unit cell (triclinic,
space
group). Surprisingly, the distance of the Ni–NH bond was found
to be the shortest [1.8518(15) Å] in the Ni(II)-coordinated
plane, followed by the Ni–N(3) [1.8524(14) Å] of the same
five-membered chelate ring. Two other Ni–N(1) [1.8760(14) Å]
and Ni(1)–N(2) [1.9076(15) Å] were noticeably longer. The
square-planar coordination plane in compound 19 was substan-
tially more distorted, as compared with complex 14a or carbon-
yl-coordinated analogues 2 and 3 [24,25] (Scheme 1), with the
angles N(3)–Ni–N(1) of 171.94(6)° and N(4)–Ni–N(2) of
169.80(6)° rendering it inherently chiral. Consequently, the
torsion angle C(24)–N(3)–C(25)–C(26) of 34.4(2)° gives rise to
an element of helical chirality in compound 19. The element of
axial chirality is generated by the restricted rotation around
the N(1)–C(13 ) bo n d with th e to rsio n an g le
C(14)–N(1)–C(13)–C(12) of −119.59(18)°. The third element
of chirality, a stereogenic center, is located on N(2).
Figure 2: Crystallographic structure of complex 19.
is an obvious preference for Ni–NH over Ni–O coordination.
The flexibility of this modular design, in which the three major
Conclusion
In conclusion, we have shown that the newly prepared Ni(II) “phenone”, “acid” and “amine” modules can be selected by
complexes derived from asymmetric, pentadentate ligands exist using many structural modifications, provides a very practical
as quasi-diastereomers, generating two new elements of tool for the preparation of a wide variety of Ni(II) complexes,
chirality (stereogenic axis and helix) to the pre-existing configu- which may result in a higher sensitivity to chemical properties
rationally stable stereogenic nitrogen. The stereoselective for- such as temperature, solvent or pH of the reaction medium.
mation of only two, (Ra*,Mh*,Rc*) and (Ra*,Ph*,Rc*), out of
Experimental
the four possible stereochemical combinations is most remark-
able, arising due to the highly stereocongested structural charac- General
teristics of these Ni(II) complexes. The quasi-diastereomeric 1H and 13C NMR were performed on Varian Unity-300 (299.94
transformation between products (Ra*,Mh*,Rc*) and MHz) and Gemini-200 (199.98 MHz) spectrometers with TMS
(Ra*,Ph*,Rc*) occurs by intramolecular transcoordination of and CDCl3 as internal standards. High-resolution mass spectra
Ni–NH and Ni–O bonds. This transcoordination results in the (HRMS) were recorded on a JEOL HX110A instrument.
loss and regeneration of axial and helical chirality with reten- Optical rotations were measured on a JASCO P-1010
tion and inversion, respectively, of the stereochemical sense. polarimeter. Melting points (mp) are uncorrected and were
The stereogenic center in this case remains unchanged. As it obtained in open capillaries. All reagents and solvents, unless
follows from the crystallographic studies, in the solid state there otherwise stated, are commercially available and were used as
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Beilstein J. Org. Chem. 2012, 8, 1920–1928.
received. Unless otherwise stated, yields refer to isolated yields 1H), 6.92–7.01 (m, 2H), 7.13–7.22 (m, 2H), 7.25–7.88 (m,
of products of greater than 95% purity as estimated by 1H and 16H), 8.47 (dd, J = 8.79, 1.18 Hz, 1H), 8.58 (dd, J = 8.50, 1.18
13C NMR spectrometry. All new compounds were character- Hz, 1H), 11.0 (bs, 1H); 13C NMR (75 MHz, CDCl3) δ 61.1,
ized by 1H, 13C NMR and HRMS. For preparation of 12a,b see 63.1, 64.0, 65.3, 121.2, 122.4, 123.4, 123.6, 125.0, 126.0, 126.2,
[25,39].
126.4, 128.3, 128.8, 129.4, 129.5, 129.7, 129.7, 129.9, 131.5,
132.3, 132.7, 133.0, 133.1, 133.8, 134.2, 134.9, 137.9, 138.6,
142.2, 165.0, 169.2, 176.6, 177.8, 198.9; HRMS–ESI (m/z): [M
Synthesis of complexes 14a,b and 15
General procedure. To a suspension of compound 12, glycine + H]+ calcd for C39H33N4O5Pd, 743.1436; found, 743.1501.
(5 equiv), and MX2 (NiCl2, Ni(NO3)2 or PdCl2, 2 equiv) in
MeOH, NaOH (7 equiv) was added, and the reaction mixture Complexes 19/20. To a solution of 14b (1 equiv) in CH3CN,
was stirred at 60–70 °C for 4 h. Then, the reaction mixture was was added KOt-Bu (4 equiv) at rt and the reaction mixture was
poured over a slurry of ice and 5% AcOH. After complete stirred at rt under aerobic conditions until the starting com-
precipitation, the solid was filtered, washed with water, and pound was consumed completely, as confirmed by TLC. After
dried. The product was purified by silica-gel column chroma- evaporation of the solvent, water and a calculated amount of 5%
tography (CHCl3/acetone = 4:1).
AcOH aq. (4 equiv) was added and extracted with CH2Cl2 three
times. The combined organic layers were dried over MgSO4,
Complex 14a (46% yield). Mp 277.2 °C (dec); 1H NMR and the product was purified on a short-path flash silica-gel
(300 MHz, CDCl3) δ 2.34 (s, 3H), 2.68 (s, 3H), 2.89 (d, J = column. Due to the fast exchange between complexes 19/20,
15.4 Hz, 1H), 3.30 (d, J = 16.5 Hz, 1H), 3.83 (d, J = 15.4 Hz, NMR spectra were not properly recorded. Mp 264.5 °C (dec);
1H), 3.85 (d, J = 16.2 Hz, 1H), 3.86 (d, J = 18.8 Hz, 1H), 3.94 HRMS–ESI (m/z): [M + Na]+ calcd for C37H30N4NaNiO3,
(d, J = 12.6 Hz, 1H), 4.43 (d, J = 19.0 Hz, 1H), 4.54 (d, J = 12.8 659.1569; found, 659.1547.
Hz, 1H), 6.92 (ddd, J = 8.30, 7.13, 1.18 Hz, 1H), 7.16–7.24 (m,
2H), 7.30–7.36 (m, 1H), 7.37–7.46 (m, 2H), 7.50 (dd, J = 8.30,
Supporting Information
1.46 Hz, 1H), 7.64 (ddd, J = 8.69, 7.13, 1.56 Hz, 1H), 7.93 (dd,
J = 8.01, 1.56 Hz, 1H), 8.06 (dd, J = 8.59, 1.17 Hz, 1H),
Supporting Information File 1
8.09–8.15 (m, 2H), 8.97 (dd, J = 8.49, 1.17 Hz, 1H); 13C NMR
(75 MHz, CDCl3) δ 18.9, 28.5, 59.8, 60.0, 60.9, 64.4, 121.2,
121.3, 122.1, 123.3, 124.8, 126.8, 128.7, 128.9, 129.4, 131.4,
131.5, 131.7, 132.1, 135.2, 139.9, 141.4, 166.7, 168.8, 176.1,
176.8, 203.2; HRMS–ESI (m/z): [M + Na]+ calcd for
C29H28N4NaNiO5, 593.1311; found, 593.1321.
NMR spectra of compounds 14b and 19/20.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-8-223-S1.pdf]
Acknowledgements
We thank IKERBASQUE, Basque Foundation for Science, for
financial support. The crystallographic studies were performed
by Dr. D. R. Powell, University of Oklahoma.
Complex 14b (83% yield). Mp 259.3 °C (dec); 1H NMR
(300 MHz, CDCl3) δ 2.83 (d, J = 16.5 Hz, 1H), 3.37 (d, J =
16.4 Hz, 1H), 3.65 (d, J = 19.8 Hz, 1H), 3.84 (d, J = 12.2 Hz,
1H), 3.88 (d, J = 13.2 Hz, 1H), 3.89 (d, J = 19.8 Hz, 1H), 4.14
(d, J = 12.8 Hz, 1H), 4.70 (d, J = 13.1 Hz, 1H), 6.72 (ddd, J =
8.30, 6.84, 1.27 Hz, 1H), 6.84 (dd, J = 8.20, 1.76 Hz, 1H),
7.00–7.74 (m, 17H), 7.90–8.00 (m, 2H), 8.47 (d, J = 8.79 Hz,
1H), 8.83 (d, J = 7.91 Hz, 1H), 11.1 (bs, 1H); 13C NMR (75
MHz, CDCl3) δ 59.0, 60.5, 61.2, 63.9, 120.7, 122.6, 123.3,
124.2, 125.1, 125.5, 125.9, 126.3, 128.2, 128.9, 129.1, 129.4,
129.4, 129.6, 129.8, 131.1, 132.0, 132.1, 132.5, 133.0, 133.1,
133.9, 134.7, 138.1, 138.7, 142.4, 166.5, 171.0, 176.7, 177.1,
199.2; HRMS–ESI (m/z): [M + H]+ calcd for C39H33N4NiO5,
695.1804; found, 695.1801.
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