A planar chiral [2.2]paracyclophane derived N-heterocyclic stannylene

Article abstract of DOI:10.1039/c2dt31497e

The reaction of pseudo-ortho-4,12-N,N′-diphenyldiamino-[2.2] paracyclophane ((±)-3) with Sn[N(SiMe₃)₂] ₂ results in the formation of the monomeric planar chiral N-heterocyclic stannylene (±)-4, featuring a unique [2.2]para

Full text of DOI:10.1039/c2dt31497e

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COMMUNICATION
A planar chiral [2.2]paracyclophane derived N-heterocyclic stannylene†‡
Isabel Piel,a Julia V. Dickschat,b Tania Pape,b F. Ekkehardt Hahn*b and Frank Glorius*a
Received 9th July 2012, Accepted 13th August 2012
DOI: 10.1039/c2dt31497e
emerged.11 Some NHGes and NHSns bearing chiral N-substitu-
The reaction of pseudo-ortho-4,12-N,N′-diphenyldiamino-
[2.2]paracyclophane (( )-3) with Sn[N(SiMe3)2]2 results in
the formation of the monomeric planar chiral N-heterocyclic
ents have also been described.12
Combining the generally higher stability11 of the diamido EII
derivatives with the interesting properties of expanded ring
NHCs13 we initiated a program to prepare cyclic diamido SnII
compounds where two atoms of the paracyclophane backbone
are amino functionalized and these two nitrogen atoms are
bound to the same tin atom. Such derivatives are not only chiral
but would also feature a very large (10-membered) heterocycle.
The corresponding heterocyclic diaminocarbenes are not known
yet. Herein we report on the first planar chiral N-heterocyclic
stannylene ( )-4 (Scheme 1) bearing a [2.2]paracyclophane
backbone architecture.
stannylene ( )-4, featuring a unique [2.2]paracyclophane
backbone, which has been characterized by an X-ray diffrac-
tion study.
The [2.2]paracyclophane motif and heterocyclic compounds
derived thereof have been known since the 1960s, but there has
been a resurgence of interest in such compounds in the past
decade.1 The motivation for most studies in this field has been
the desire to create heterocycles possessing either planar chiral-
ity, or the capacity for long-distance electronic communication,
although this latter property has been discussed conversely in the
literature.2 The [2.2]paracyclophane motif offers the possibility
for the generation of topological chirality which constitutes a
key principle in overcoming the limits of traditional central
chirality.3
Our investigation started from the readily available unsubsti-
tuted [2.2]paracyclophane (1). Initial bromination of 1 followed
by a rearrangement under microwave conditions using a
modified literature procedure resulted in the smooth formation of
pseudo-ortho-dibromide ( )-2 (Scheme 1, see also ESI‡).14
A
N-Heterocyclic carbenes (NHCs) have attracted tremendous
interest as ligands in organometallic chemistry4 and as organo-
catalysts.5 A number of NHCs bearing chiral substituents6 but
also planar chiral NHCs7 are known, amongst which some cyclo-
phane derived ones show promising properties in organocata-
lysis.7e The key to success seems to be the restriction of
conformational flexibility, as Fürstner demonstrated with a
carbene center being implemented in the stereogenic unit.7f
While NHC-substituted [2.2]paracyclophanes have been described,
no NHC is known in which the cyclophane moiety serves as the
backbone, linking the two NHC ring nitrogen atoms with each
other.
Buchwald–Hartwig amination of the pseudo-ortho-dibromide
( )-2 with aniline led to the corresponding pseudo-ortho-
diamine ( )-3,15 which in a final step could be converted into the
targeted NHSn ( )-4 using Sn[N(SiMe3)2]2.16
Compared to the large body of chemistry dealing with NHCs,
their heavier germanium(II) (NHGe),8 tin(II) (NHSn)9 and lead(II)
(NHPb)10 analogues received less attention although some of
these N-heterocyclic derivatives were known years before NHCs
aOrganisch-Chemisches Institut, Corrensstr. 40, D-48149 Münster,
Germany. E-mail: glorius@uni-muenster.de; Fax: +49-251-8333202;
Tel: +49-251-8333248
bInstitut für Anorganische und Analytische Chemie, Westfälische
Wilhelms-Universität Münster, Corrensstr. 28/30, D-48149 Münster,
Germany. E-mail: fehahn@uni-muenster.de; Fax: +49-251-8333108;
Tel: +49-251-8333110
†Dedicated to Professor Klaus Jurkschat on the occasion of his 60th
birthday.
‡Electronic supplementary information (ESI) available: Experimental
details of the synthesis and full characterization for all new compounds.
CCDC 891264. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt31497e
Scheme 1 Preparation of stannylene ( )-4.
13788 | Dalton Trans., 2012, 41, 13788–13790
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and the N–Sn–N angle observed in ( )-4 (10-membered hetero-
cycle) fits perfectly in this series of expanding angles at the tin
atom depending on the size of the heterocycle.
In spite of the monomeric nature of ( )-4 in the solid state, the
molecular structure determination indicates that the Lewis-acidic
tin(II) center might still be accessible for small basic molecules.
This is also indicated by the high-field resonance in the 119Sn
NMR spectrum at δ = 41.8 ppm (in THF-d8). This resonance is
significantly shifted high-field when compared to the resonances
recorded for related cyclic diaminostannylenes where chemical
shifts of about 200 ppm have been recorded for various deriva-
tives.9 The chemical shift in the 119Sn NMR spectra has been
shown to depend on the solvent used with basic solvents like
THF leading to an upfield shift relative to hydrocarbon solvents
like benzene or toluene. The upfield shift in THF results from
coordination of the solvent molecules to the Lewis-acidic
p-orbital at the tin center which has been demonstrated by X-ray
diffraction for some derivatives.9e,f We assume that the upfield
resonance recorded for ( )-4 results from the combination of an
electron-rich backbone and the use of THF as the solvent for the
NMR experiment. Unfortunately, ( )-4 is not soluble enough to
record the 119Sn NMR spectrum in benzene or toluene.
While we were able to prepare the planar chiral stannylene
( )-4, so far all attempts to synthesize the closely related amidi-
nium salt precursors or the NHC ( )-5 from diamine ( )-3 were
unsuccessful (Scheme 1). The distance between the two amino
groups of the pseudo-ortho-diamine ( )-3 is presumably too
large to convert this diamine into the corresponding carbene pre-
cursor, whereas the larger tin(II) atom is more suitable for ring
formation leading to ( )-4.
Fig. 1 Top: Molecular structure of one enantiomer of ( )-4. Ellipsoids
are drawn at 50% probability and hydrogen atoms have been omitted for
clarity; bottom part of the molecular structure of ( )-4 with the N-phenyl
substituents omitted to show the planar chirality. Selected bond lengths
(Å) and angles (°): Sn–N1 2.1296(14), Sn–N2 2.1186(13), N1–C1
1.401(2), N1–C13 1.417(2), N2–C7 1.410(2), N2–C23 1.416(2); N1–
Sn–N2 96.69(5), Sn–N1–C1 118.07(11), Sn–N1–C13 121.32(10), C1–
N1–C13 120.17(14), Sn–N2–C7 115.30(10), Sn–N2–C23 125.42(11),
C7–N2–C23 118.62(13).
1
Analysis by H NMR spectroscopy provides evidence for the
formation of stannylene ( )-4.‡ For example, the 1H NMR spec-
trum of diamine derivative ( )-3 in THF-d8 shows a resonance
for the N–H protons at δ = 5.48 ppm, which is absent in the
1H NMR spectrum of stannylene ( )-4. The 119Sn NMR spec-
trum ( )-4 features a resonance at δ = 41.8 ppm compared to δ =
766 ppm (in C6D6)17a and δ = 601 ppm (in THF-d8)17b for
di{[bis(trimethylsilyl)]amido}tin(II).
In summary we have prepared and characterized a [2.2]para-
cyclophane derived N,N′-disubstituted N-heterocyclic stannylene
( )-4 by 1H, 13C and 119Sn NMR and X-ray diffraction tech-
niques. While the sterically demanding substituents prevent
intermolecular interactions, the tin centre in ( )-4 is most likely
still accessible for Lewis bases like THF.
Unequivocal evidence for the formation of a monomeric di-
aminostannylene with
a paracyclophane backbone and a
Acknowledgements
10-membered heterocycle comes from an X-ray diffraction
analysis (Fig. 1).§ As expected for a racemic mixture, ( )-4 crys-
tallizes in the centrosymmetric space group P21/n with both
enantiomers present in the same crystal. Due to the steric protec-
tion of the tin atom, the individual stannylenes in the crystal
lattice show no intermolecular interactions of the type often
observed for diaminostannylenes.9 The N1–Sn–N2 angle
(96.69(5)°) in the cyclic compound ( )-4 is only slightly smaller
than the N–Sn–N angle found in acyclic diaminostannylenes
which fall in the range of about 100°–110°.16c,17a,18 Only one
acyclic diaminostannylene (Sn(NHAr)2, Ar = C6H2-2,4,6-tBu3,
N–Sn–N 89.6(5)°)19 features a smaller N–Sn–N angle than
( )-4. Among the known cyclic diaminostannylenes the N–Sn–
N angle expands with the size of the heterocycle. Stannylenes
derived from a four-membered heterocycle feature smaller N–
Sn–N angles (73.2(4)° and 74.62(10)°)20 than those obtained
from five-membered heterocycles (range 77.12(7)° to 78.5
(2)°).9b–d A further expansion of the N–Sn–N angle to 92.63(6)°
was found for a cyclic diaminostannylene derived from a
six-membered heterocycle.20b A distannylene derived from a
12-membered heterocycle is also known (angle N–Sn–N 98.3(3)°)21
We are grateful to Ms Karin Gottschalk for skillful technical
assistance and to Plasma Parylene Systems GmbH for the
donation of [2.2]paracyclophane (1). Generous financial support
by the Deutsche Forschungsgemeinschaft (SFB 858) and the
NRW Graduate School of Chemistry (fellowship for I.P.) is
gratefully acknowledged. The research of F.G. was supported by
the Alfried Krupp Prize for Young University Teachers of the
Alfried Krupp von Bohlen und Halbach Foundation.
Notes and references
§Crystal data for ( )-4: C28H24N2Sn, M = 507.18, T = 153(2) K, λ =
0.71073 Å, μ(Mo-Kα) = 1.205 mm−1, monoclinic, a = 11.6317(3), b =
10.4920(3), c = 17.6728(5) Å, β = 93.7083(4)°, V = 2152.27(10) Å3,
space group P21/n, Z = 4, 25 504 measured intensities, 6557 unique
intensities, all reflections used in refinement against F2, R = 0.0238,
wR = 0.0611 (I ≥ 2σ(I)), wR2 = 0.0643 (for all data).
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13790 | Dalton Trans., 2012, 41, 13788–13790
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