Weak absolute helicity direction in Ni-salen by trans-cyclohexane-(1R,2R)- diamine

Article abstract of DOI:10.1016/j.ica.2010.07.044

Helical Ni-salen foldamers were synthesized from enantiomerically pure cyclohexane-(1R,2R)-diamine, N,N′-bis-(N-phenyl (4-diphenylphosphine)-3- salicylidenato carboxamide)-(1R,2R)-cyclohexanediaminato-nickel(II). X-ray structural characterization of the absolute helicities observed confirms our earlier assertions, based on solution spectroscopic evidence, that trans-cyclohexane-diamine, a common component of chiral salen catalysts, is a weak director of absolute helicity for Ni-salen.

Full text of DOI:10.1016/j.ica.2010.07.044

Inorganica Chimica Acta 364 (2010) 259–260
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Inorganica Chimica Acta
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Note
Weak absolute helicity direction in Ni–salen by trans-cyclohexane-(1R,2R)-diamine
*
Laural A. Fisher, Fan Zhang, Glenn P.A. Yap , Joseph M. Fox
University of Delaware, Department of Chemistry and Biochemistry, 236 Brown Laboratory, Newark, DE 19716, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 22 July 2010
Helical Ni–salen foldamers were synthesized from enantiomerically pure cyclohexane-(1R,2R)-diamine,
N,N0-bis-(N-phenyl (4-diphenylphosphine)-3-salicylidenato carboxamide)-(1R,2R)-cyclohexanediamina-
to-nickel(II). X-ray structural characterization of the absolute helicities observed confirms our earlier
assertions, based on solution spectroscopic evidence, that trans-cyclohexane-diamine, a common compo-
nent of chiral salen catalysts, is a weak director of absolute helicity for Ni–salen.
Dedicated to Prof. Arnold L. Rheingold
Keywords:
Absolute helicity
Salen
Ó 2010 Elsevier B.V. All rights reserved.
Ni Catalyst
Enantioselective
Cyclohexane-diamine
Crystal structure
Chiral salens are broadly useful ligands for enantioselective
catalysis, [1] and the origin of asymmetric induction in reactions
catalyzed by chiral-metal salens has been a topic of considerable
interest and debate. In some models for asymmetric induction, it
has been argued that helical chirality plays a critical role [1c–
e,2]. Implicit to many of these arguments is that an element of cen-
tral chirality—typically a chiral diamine—creates a bias for a unique
sense of folding in metal–salen complexes.
prior study, the assignment of structures 1(M) and 1(P) were made
on the basis of spectroscopic studies in solution [6c]. However, we
were not able to obtain a crystal structure of 1 and to confirm that
the mixture of helical diastereomers also exists in the solid state.
Herein, we describe the crystallographic properties of a Ni–salen
5, N,N0-bis-(N-phenyl (4-diphenylphosphine)-3-salicylidenato
carboxamide)-(1R,2R)-cyclohexanediaminato-nickel(II),
a
fold-
amer that crystallizes as a mixture of helical diastereomers, even
though it is derived from enantiomerically pure (1R,2R)-trans-
cyclohexanediamine.
Recently, our group has described how salen-based metallo-
foldamers [3–5] (e.g., 1, 2) can be used as tools for studying the
influence of central chirality on helical chirality [6]. The intercon-
version between the helical diastereomers of such foldamers can
be observed on the timescale of NMR spectroscopy, which provides
a convenient spectroscopic handle for characterization [4,5]. A sur-
prising outcome of the study was that trans-cyclohexane-1,2-dia-
mine—a common component of chiral salen catalysts—is a only
weak director of absolute helicity in Ni–salen foldamers such as
1 [4c]. For complex 1 (Scheme 1), nearly equal amounts of two con-
formers of opposite helicity interconvert on the 1H NMR timescale
(400 MHz) with coalescence at ꢀ10 °C. Furthermore, in complex
2—where the helical bias of the diamine and the endgroup are mis-
matched—it was observed that the absolute configuration is dom-
inated by the endgroup, and not by the diamine [4c].
We prepared foldamer 5 from acid chloride 2 [4a,7] as shown in
Scheme 2. Thus, acyl transfer by 4-iodoaniline was followed by
ozonolysis to provide salicylaldehyde 3, which coupled to diphen-
ylphosphine under Pd-catalyzed conditions to provide 4. Combina-
tion of 4 with (1R,2R)-cyclohexanediamine and Ni(OAc)2 gave 5 in
92% yield. Complex 5 was crystallized from benzene, and the struc-
ture1 is displayed in Fig. 1. The Ni atoms have square planar coordi-
nation with the steric bulk at the ligand arms causing a molecular
twofold resulting in helices.
Four symmetry-unique Ni–salen molecules are located in the
unit cell in M–M and P–P helical pairs. Each same-helicity pair is
roughly twofold symmetrical with the major deviations from sym-
metry arising from single bond rotation of the diphenyl P to car-
boxamide phenyl C bond. The M–M and P–P helical pairs are
Levy and co-workers have studied Zn+2 and Fe+2/+3 metallohelic-
enes based on cyclohexane-(1R,2R)-diamine, and observed that
such complexes crystallize as a 1:1 mixture of helical diastereo-
mers. Spectroscopic studies on these metallohelicenes suggested
that the M-conformers were predominant in solution [7]. In our
1
Crystal data: triclinic, P1, a = 13.679(2) Å, b = 20.954(3) Å, c = 21.951(3) Å,
a
= 85.816(3)°, b = 73.507(3)°,
c
= 74.794(3)°, V = 5821.7(14) Å3, Z = 2, T = 218(2) K,
Dcalc = 1.303 g cmꢀ3
,
R(F) = 4.86% for 24,990 observed independent reflections
(2.80 < 2h < 56.62°). Eight severely disordered, cocrystallized benzene solvent mole-
cules per unit cell were treated as diffused contributions with Squeeze/Platon (A.L.
Spek, J. Appl. Crystallogr. 36 (2003) 7).
* Corresponding author.
E-mail address: gpyap@udel.edu (G.P.A. Yap).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.07.044

260
L.A. Fisher et al. / Inorganica Chimica Acta 364 (2010) 259–260
Me
C
H
C H
12 25
12 25
O
Acknowledgments
O
O
O
C
H
12 25
O
O
O
Me
Me
O
O
O
H
H
H
N
N
O
N
O
O
For financial support of this work, we thank the NSF (CHE-
0547865). NMR spectra were obtained with instrumentation sup-
ported by NSF CRIF:MU grant: CHE 0840401.
H
O
N
O
O
N
H
N
H
O
H
N
N
O
O
N
H
H
O
O
Ni
N
H
N
N
O
O
Ni
Ni
H
H
O
O
N
N
N
H
H
H
N
O
N
H
N
O
H
H
O
O
C
O
12 25
1(P)
O
O
O
H
O
O
O
1(M)
H
2
Me
Me
Appendix A. Supplementary data
H
C
C
25 12
25 12
Me
Supplementary data (synthetic details, multinuclear NMR and
high-resolution mass spectra) associated with this article can be
found, in the online version, at doi:10.1016/j.ica.2010.07.044.
Scheme 1. For Ni–salen foldamers where trans-cyclohexanediamine is the only
source of central chirality, there is practically no bias on absolute helicity. (b) Chiral
endgroups completely overwhelm the helical preference of an internally trans-
cyclohexanediamine.
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related by pseudo inversion. A true inversion center would require
the equimolar presence of S, S enantiomer. In the crystalline state,
5 consist of equal amounts of P-helical and M-helical diastereo-
mers in the unit cell. This observation provides direct crystallo-
graphic evidence for the conclusions reached from spectroscopic
studies on 1: trans-cyclohexanediamine is, by itself, only a poor
director of absolute helicity in Ni–salen complexes.