Triptycene-Based Chiral and meso-N-Heterocyclic Carbene Ligands and Metal Complexes

Article abstract of DOI:10.1002/chem.201601474

Based on 1-amino-4-hydroxy-triptycene, new saturated and unsaturated triptycene-NHC (N-heterocyclic carbene) ligands were synthesized from glyoxal-derived diimines. The respective carbenes were converted into metal complexes [(NHC)MX] (M=Cu, Ag, Au; X=Cl, Br) and [(NHC)MCl(cod)] (M=Rh, Ir; cod=1,5-cyclooctadiene) in good yields. The new azolium salts and metal complexes suffer from limited solubility in common organic solvents. Consequently, the introduction of solubilizing groups (such as 2-ethylhexyl or 1-hexyl by O-alkylation) is essential to render the complexes soluble. The triptycene unit infers special steric properties onto the metal complexes that enable the steric shielding of selected areas close to the metal center. Next, chiral and meso-triptycene based N-heterocyclic carbene ligands were prepared. The key step in the synthesis of the chiral ligand is the Buchwald–Hartwig amination of 1-bromo-4-butoxy-triptycene with (1S,2S)-1,2-diphenyl-1,2-diaminoethane, followed by cyclization to the azolinium salt with HC(OEt)₃. The analogous reaction with meso-1,2-diphenyl-1,2-diaminoethane provides the respective meso-azolinium salt. Both the chiral and meso-azolinium salts were converted into metal complexes including [(NHC)AuCl], [(NHC)RhCl(cod)], [(NHC)IrCl(cod)], and [(NHC)PdCl(allyl)]. An in situ prepared chiral copper complex was tested in the enantioselective borylation of α,β-unsaturated esters and found to give an excellent enantiomeric ratio (er close to 90:10).

Full text of DOI:10.1002/chem.201601474

DOI: 10.1002/chem.201601474
Full Paper
&
N-Heterocyclic Carbenes
Triptycene-Based Chiral and meso-N-Heterocyclic Carbene Ligands
and Metal Complexes
Roman Savka,[a] Marvin Bergmann,[a] Yuki Kanai,[a] Sabine Foro,[b] and Herbert Plenio*[a]
Abstract: Based on 1-amino-4-hydroxy-triptycene, new satu-
rated and unsaturated triptycene-NHC (N-heterocyclic car-
bene) ligands were synthesized from glyoxal-derived dii-
mines. The respective carbenes were converted into metal
complexes [(NHC)MX] (M=Cu, Ag, Au; X=Cl, Br) and
[(NHC)MCl(cod)] (M=Rh, Ir; cod=1,5-cyclooctadiene) in
good yields. The new azolium salts and metal complexes
suffer from limited solubility in common organic solvents.
Consequently, the introduction of solubilizing groups (such
as 2-ethylhexyl or 1-hexyl by O-alkylation) is essential to
render the complexes soluble. The triptycene unit infers spe-
cial steric properties onto the metal complexes that enable
the steric shielding of selected areas close to the metal
center. Next, chiral and meso-triptycene based N-heterocyclic
carbene ligands were prepared. The key step in the synthesis
of the chiral ligand is the Buchwald–Hartwig amination of 1-
bromo-4-butoxy-triptycene with (1S,2S)-1,2-diphenyl-1,2-dia-
minoethane, followed by cyclization to the azolinium salt
with HC(OEt)3. The analogous reaction with meso-1,2-diphen-
yl-1,2-diaminoethane provides the respective meso-azolinium
salt. Both the chiral and meso-azolinium salts were converted
into
metal
complexes
including
[(NHC)AuCl],
[(NHC)RhCl(cod)], [(NHC)IrCl(cod)], and [(NHC)PdCl(allyl)]. An
in situ prepared chiral copper complex was tested in the
enantioselective borylation of a,b-unsaturated esters and
found to give an excellent enantiomeric ratio (er close to
90:10).
well established.[2] Anilines are very convenient precursors for
N-heterocyclic carbenes (NHC).[8] The same should hold true for
amino-triptycenes. Peris et al.[9] and Bielawski et al.[10] reported
on triptycene-based NHC ligands. However, the respective
metal complexes do not utilize the special steric properties of
the triptycene backbone,[11] because the NHC unit is attached
symmetrically through the respective 2,3-positions of tripty-
cene. Consequently, the two remaining triptycene wings are
far away from the metal center, and exert only little effect on
the coordination sphere of the metal ion. Our strategy is to
connect the heterocyclic carbene unit to the 1-position of the
triptycene. The steric bulk on the two sides of each aryl unit in
the triptycene-based N,N’-diaryl NHC ligand should be very dif-
ferent, thus offering the chance for stereoselective catalysis in
the respective transition metal complexes.[12] Quadrant dia-
grams are simple but useful representations of the stereo-
chemical properties of ligands in the coordination sphere of
transition metals (Scheme 1).[13] Depending on the steric prop-
erties of the ligand, certain quadrants can be sterically inacces-
sible. Controlling the orientation of substrates undergoing cat-
alytic transformations at the metal center may lead to a defined
stereochemistry in the reaction product.[14] To benefit from the
stereodirecting effect of a ligand, the coordination sphere of
the metal center has to be modulated in a well-defined
manner. Certain ligands seem to be highly capable in this re-
spect, and are useful for a large number of different chemical
transformations. This led Jacobsen to coin the term “privileged
chiral ligands.”[15] Several of those privileged ligands are phos-
Introduction
Triptycene, being the first representative of the iptycene
family,[1] was first synthesized in the early 1940s[2] and is charac-
terized by an unusual D3h symmetry. In the 1980s, iptycene
chemistry was rediscovered by Hart et al., and only then did
the field evolve at a faster pace.[3] Recently, iptycenes have
found numerous applications in polymer chemistry,[4] molecu-
lar machines,[5] material science, and in host–guest chemistry.[6]
The special geometric properties of triptycene motivated their
use as backbones for modified ligands in organometallic
chemistry. The chemistry of triptycene-based phosphines was
pioneered by Gelman et al., and a number of trans-chelating
1,8-phosphine-substituted triptycenes have been synthesized
as well as the catalytic properties of the respective metal com-
plexes studied.[7] The Bartlett synthesis of triptycene relies on
the reaction of anthracene and benzoquinone and, based on
this approach, the synthesis of 1-aminotriptycenes has been
[a] Dr. R. Savka, M. Sc. M. Bergmann, Y. Kanai, Prof. Dr. H. Plenio
Organometallic Chemistry, TU Darmstadt
Alarich-Weiss-Strasse 12, 64287 Darmstadt (Germany)
E-mail: plenio@tu-darmstadt.de
[b] S. Foro
Department of Materials Science, TU Darmstadt
Alarich-Weiss-Strasse 2, 64287 Darmstadt (Germany)
1
Supporting information, including general experimental, full sets of H and
13C NMR spectra, additional synthetic procedures, determination of enantio-
meric ratios, cyclic voltammograms, and mass spectra, as well as the
ORCID identification number for the corresponding author of this article
can be found under http://dx.doi.org/10.1002/chem.201601474.
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Results and Discussion
Synthesis of bistriptycene-N-heterocyclic carbene ligands
and metal complexes
Triptycene quinone 3 was synthesized according to a modified
literature procedure from Bartlett et al.,[2] and then converted
into the known (9s,10s)-4-amino-9,10-dihydro-9,10-[1,2]benze-
noanthracene-1-ol (Scheme 3, 5) based on a modified proce-
dure from Yang et al.[24] The synthesis of aminophenol 5
proved to conveniently work on a large scale, as it relies on in-
expensive starting materials. Furthermore, the purification of
the intermediates does not require chromatography. In our
hands, the five-step synthesis provides an overall yield of 43%,
with the lowest yielding step in the synthesis being the selec-
tive conversion of the quinone into the monooxime (yield
75%).
Scheme 1. Prototypical chiral bistriptycene-NHC metal complexes in quad-
rant diagrams; conformational control is possible once R is a sterically de-
manding group.
phines,[16] but the closely related NHC ligands are much less
prominent in this context.[17]
One problem of NHC ligands[18] in stereoselective synthesis
is that stereodirecting groups at the ligand tend to be remote
from the catalytically active metal center. Several strategies
have been employed to overcome such problems, and to
obtain NHC ligands suitable for asymmetric catalysis[12a,19]
(Scheme 2):[12b] i) to convey chiral information from the 4,5-po-
sition to the active site by increasing the steric bulk of the N-
substituents through a gearing effect;[20] ii) the introduction of
a rigid chiral framework;[21] iii) bicyclic or tricyclic NHC ligands
with blocked NÀR rotation[22] or iv) bidentate NHC ligands.[23]
Scheme 3. Synthesis of (9s,10s)-4-amino-9,10-dihydro-9,10-[1,2]benzenoan-
thracene-1-ol. Reagents and conditions: a) anthracene, 1,4-benzoquinone,
xylene, reflux; b) acetic acid (AcOH), HBr, reflux; c) KBrO3, AcOH, H2O, reflux;
d) NH2OH·HCl, THF, H2O, 508C; e) hydrazine hydrate, Pd/C, THF, reflux.
The amino group in 5 can be used to build an NHC ligand
following the standard reaction sequence (Scheme 4).[25] The
phenolic-OH in 5 is helpful, since it has two advantages: it is
electron-donating, leading to a more electron-rich NHC ligand,
and it provides a handle for the functionalization with long-
chain alkyl groups. This alkylation step is very important to im-
prove the otherwise poor solubility of the triptycene deriva-
tives in common organic solvents.[26] However, the selective al-
kylation of the -OH group in 5 was not successful in the pres-
ence of the -NH2 unit. Therefore, 5 was first converted into the
diimine 6 with glyoxal (Scheme 4).[27] This diimine is poorly
soluble in most solvents other than DMSO. Reactions of this
diimine with various alkyl halides RX (R=2-Et-hexyl, Et, nBu)
led to the respective O-alkylated diimines 7a, 7b, and 7c. Dii-
mine 7b (R=Et) displays poor solubility (insufficient for record-
Scheme 2. Four types of chiral NHC-metal complexes.
The primary objective of the present study is to synthesize
triptycene-based NHC ligands as well as stereochemically de-
fined (chiral and meso) triptycene based NHC-metal complexes.
The rigid geometry of the bulky triptycene unit[1] should
enable the deep penetration of the space close to a catalytically
active metal, prevent the MÀC(NHC) rotation, and thus, may
enable stereoselective catalysis (Scheme 2). For such ligands an
efficient gearing mechanism based on the C3-symmetric tripty-
cene is likely.[6c] Steric bulk at the imidazole ring acts on one
triptycene wing, orienting the second one, and shielding the
space close to the metal to enable steric control at the metal
center.
1
ing decent H NMR spectra), that of 7c (R=Bu) is modest, but
the solubility of 7a (R=2-Et-hexyl)[28] in common solvents is
good. However, the 2-ethyl-hexyl group also has drawbacks
because the chirality of the side chain will cause problems
when attempting to obtain single crystals of the diastereo-
meric mixtures. 7b and 7c were converted into the respective
imidazolium salts 8b·HCl and 8c·HCl employing the estab-
lished Hintermann procedure.[29] Alternatively, diimine 7a was
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Scheme 5. Reagents and conditions: a) [AuCl(SMe2)], NaOCMe2Et, THF,
À788C to RT; b) [CuBr(SMe2)], NaOCMe2Et, THF, RT; c) [RhCl(cod)]2, NaOC-
Me2Et, THF, RT; d) CO, CH2Cl2, RT.
Scheme 4. Reagents and conditions: a) glyoxal (aq. 40% wt), HCOOH, MeCN,
608C; b) 2-ethylhexyl bromide, K2CO3, KI, DMF, 708C or alkyl iodide, K2CO3,
DMF, 708C; c) LiAlH4, THF, À108C to RT; d) HC(OEt)3, HCOOH, 1208C;
e) EtOCH2Cl, 1008C; f) (CH2O)n, Me3SiCl, EtOAc, 708C.
first reduced to the diamine 9a and then cyclized to the imida-
zolinium salt 10a·HCl (Scheme 4).[30]
Scheme 6. Reagents and conditions: a) Ag2O, CH2Cl2, 408C; b) [MCl(cod)]2
(M=Rh, Ir), CH2Cl2, 408C.
Despite the limited solubility of the azolium chlorides 8 and
10, several metal complexes could be synthesized. The respec-
tive carbene was generated from 10a·HCl and sodium amylate,
followed by reaction with suitable metal precursors
[AuCl(Me2S)], [CuBr(Me2S)], and [RhCl(cod)]2 (Scheme 5). The re-
action of [RhCl(cod)(10a)] with CO led to the formation of
[RhCl(CO)2(10a)] in almost quantitative yield (Scheme 5). The
unsaturated imidazolium salts 8b·HCl and 8c·HCl could be
converted into the respective metal complexes [AgCl(8b)],
[AgCl(8c)], [IrCl(cod)(8b)], [IrCl(cod)(8c)], and [RhCl(cod)(8c)]
(Scheme 6). Complexes of the b and c series with ethoxy or
butoxy groups are characterized by modest solubility, leading
to 13C NMR spectra with poor signal-to-noise ratios. This pri-
marily concerns the carbene carbon atoms, which are charac-
terized by long relaxation times.[31]
1
and 5-positions of the heterocyclic carbene.[33] In the H and
the 13C NMR spectra of [IrCl(cod)(8b)] there is no evidence for
different isomers (syn and anti). The detailed analysis of NOESY
(signals of the cod ligand) and HMBC spectra did not provide
evidence for an exclusive syn-isomer at room temperature.
Based on this, we conclude that the solution complex [IrCl(-
cod)(8b)] exists as a mixture of rapidly interconverting isomers.
Furthermore, it is hard to see any stereodirecting effect, which
could lead to the formation of a single (syn) isomer. Conse-
quently, the exclusive formation of the syn isomer in the crystal
appears to be a solid state phenomenon.
The most important property of the bistriptycene NHC
ligand is that the aryl flaps are reaching into the space above
and below the plane of the N-aryl group bonded to the imida-
X-ray crystal structure analysis of [IrCl(cod)(8b)]
Single crystals of this complex were grown by recrystallization
from isopropanol/CH2Cl2. The crystal structure is highly indica-
tive concerning the role of the triptycene ligand (Figure 1). Iri-
dium displays a typical square-planar coordination geometry
with a crystallographic mirror plane bisecting the molecule in
the ClÀIrÀC(NHC) plane; bond lengths within the coordination
sphere of Ir are normal.[32] In the solid state, the two triptycene
flaps are oriented in a syn-orientation. However, based on NMR
data, the triptycene unit in the metal complex should be able
to rotate rapidly around the (aryl)CÀN single bond, as was
shown for related NHC-metal complexes, in which rotation was
only blocked in the presence of bulky substituents in the 4-
Figure 1. ORTEP-plots of the X-ray crystal structure[39,51] of complex [IrCl(-
cod)(8b)] (view onto the plane of the heterocycle and view perpendicular to
the square-planar coordination sphere). Important bond lengths (pm) and
angles (8): IrÀC(NHC) 206.1(15), IrÀCl 237.7(4), IrÀC(cod) 207.3(9), 218.3(12),
Cl-Ir-C(NHC) 93.1(4).
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zolylidene. According to the quadrant model, two neighboring
quadrants in the syn-complex are occupied by the bulky tripyt-
cene units, whereas the two other quadrants lack sterically de-
manding groups. This offers the chance for stereoselective cat-
alysis; however, this is restricted to the isomerically pure syn-
or anti -complexes, both of which possess bulky groups to pre-
vent isomer interconversion by rotation.
recording the 1H NMR spectra in [D6]DMSO, the interconversion
of the rotamers is fast on the NMR timescale and consequently,
only a single set of isomers is observed, which considerably
simplifies the NMR spectra. The cyclization of 14 provides
good yields of the respective imidazolinium salts 15, which are
excellent precursors for the synthesis of chiral transition-metal
complexes. The synthesis of the meso-azolinium salt was done
in a similar manner utilizing 13c and meso-1,2-diphenyl-1,2-di-
aminoethane (Scheme 8).
A typical feature of the N-aryl and the imidazolylidene rings
in SIMes-type ligands is the orthogonality of the two ring
planes.[32,34] However, in the crystal structure of [IrCl(cod)(8b)],
those two planes are tilted with an angle of approximately
608, which appears to result from the presence of the two
bulky triptycene groups and the cod ligand. This tilting has
two consequences. The obvious one being the partial removal
of the triptycene flaps from the space surrounding the metal
center, leading to an alleviation of the steric hindrance close to
the metal center. This might be important concerning the po-
tential stereoselective catalysis with related complexes. On the
other hand, in the absence of a bulky cod ligand, the tripty-
cene wing could rotate towards the metal. Furthermore, the
presence of bulky groups in the 4- and 5-positions of the het-
erocycle (which are needed for the inhibition of rotation) will
push the triptycene flaps back towards the metal center.
The other consequence of this tilting is the proximity of iridi-
um to the ortho-carbon on the other side of the N-aryl group.
This IrÀC distance is only 360 pm, whereas that of the other
ortho-carbon is 440 pm. As a consequence, the respective CÀH
bond is oriented towards the metal center, possibly facilitating
CÀH-activation reactions. The packing of the rigid triptycene
units in the crystal leads to the formation of extended pores,
something which has been observed also for related iptycene
structures.[35] As a consequence of this solid state property, it
can be very difficult to completely remove solvent from solid
iptycene derivatives. To better understand the steric bulk of
NHC 8, we have determined the buried-volume[36] (based on
the crystal structure of [IrCl(cod)(8b)]) as 30.9%. This is slightly
smaller than N-mesityl-substituted NHCs.[37,38]
Scheme 7. Synthesis of chiral azolium salts as NHC precursors. Reagents and
conditions: a) 50% aq. H3PO2, tBuONO, THF, 408C; b) n-hexyl iodide, K2CO3,
DMF, 708C or 2-ethylhexyl bromide or octylbromide, K2CO3, KI, DMF, 708C;
c) N-bromosuccinimide (NBS), DMF, 1008C; d) (1S,2S)-1,2-diphenylethylenedi-
amine, Pd2(dba)3 (dba=dibenzylideneacetone), rac-BINAP, sodium tert-pent-
oxide, toluene, 1008C; e) HC(OEt)3, NH4BF4, HCOOH, 1208C.
Synthesis of chiral and meso-NHC ligands
To enable stereocontrol in both the NHC ligands and derived
metal complexes (and potentially in catalytic reactions with
such metal complexes), substituents at the C2-backbone of the
azolinium unit are necessary. Based on (1S,2S)-1,2-diphenyle-
thylenediamine and meso-1,2-diphenyl-1,2-diaminoethane, it is
possible to establish the desired stereochemistry.
Scheme 8. Synthesis of meso-NHC ligands and metal complexes. Reagents
and conditions: a) meso-1,2-diphenyl-1,2-diaminoethane, Pd2(dba)3, rac-
BINAP, sodium tert-pentoxide, toluene, 1008C, (R=C6H17); b) HC(OEt)3,
NH4BF4, HCOOH, 1208C; c) [AuCl(SMe2)] or [RhCl(cod)]2 or [IrCl(cod)]2 or
[PdCl(allyl)]2 with K2CO3, acetone, 608C.
The synthesis of new chiral NHC·HBF4 salts is summarized in
Scheme 7. The known aminophenol 5[2,24] was converted into
the diazonium salt and reduced in situ to phenol 11. Etherifica-
tion of the hydroxyl group (leading to the 12 series) offers the
chance to introduce solubilizing groups. The bromination of
12a, 12b, and 12c produces the respective bromophenols
13a, 13b, and 13c. The bromine substituent enables introduc-
tion of the chiral (1S,2S)-1,2-diphenylethylenediamine by Buch-
wald–Hartwig amination,[40] an established method involved in
the synthesis of chiral NHC ligands.[20a,33b] In CDCl3 solvent, the
chiral diamines 14 exist as distinct rotamers. However, when
Synthesis of chiral and meso-NHC-metal complexes
The reaction of 15a·HBF4 with [AuCl(SMe2)] and K2CO3 in ace-
tone[41] provides the respective gold complex in good yield
(Scheme 9). The complexes with Rh were synthesized accord-
ing to a related procedure.[42] The meso-NHC-metal complexes
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were prepared in a similar manner (Scheme 8). In general, the
meso-complexes are less well behaved than the chiral com-
plexes, and the yields for the formation of the respective metal
complexes are lower. All attempts to obtain single crystals suit-
able for X-ray crystal structure analysis of a chiral or a meso
metal complex were unsuccessful.
Catalytic properties
Preliminary tests of the chiral NHC ligands in enantioselective
catalysis were performed. The conjugate addition of heteroa-
tom nucleophiles is a valuable tool for the introduction of
a functional group into the b-position of a,b-unsaturated ac-
ceptors.[45] We therefore tested the asymmetric borylation of
a,b-unsaturated esters as a preliminary probe of the enantio-
directing effect of the triptycene-NHC ligands. Several chiral
NHC-copper complexes have been employed successfully for
this reaction.[45,46] The in situ formed Cu complexes with chiral
NHC 15a induces the enantioselective borylation of three dif-
ferent a,b-unsaturated esters[21b,46e] (Scheme 10). The respective
products are formed in virtually quantitative yields. The enan-
tioselectivity of the borylation reaction was determined follow-
ing the oxidative conversion of the borylated products into
the respective chiral alcohols and the synthesis of the respec-
tive mandelic acid esters. The mandelic ester method is an es-
tablished procedure for the determination of enantiomeric
Scheme 9. Synthesis of chiral NHC-metal complexes. Reagents and condi-
tions: a) [AuCl(SMe2)], K2CO3, acetone, 608C; b) [RhCl(cod)]2, K2CO3, acetone,
608C.
1
excess by NMR spectroscopy using the H NMR integrals of the
respective resonances in the diastereomeric esters.[47,48] For the
three reactions studied, er values of close to 90:10 were ob-
served.[49] These data show that enantioselective catalysis with
triptycene-based NHC ligands is possible with excellent chiral
induction comparable to that of other NHC-Cu complex-
es.[21b,46e] More (flexible) bulk at the periphery of the triptycene
wings might be helpful to even better control the chiral space
around the transition metal to improve chiral induction.
The NMR spectra of chiral NHCs 15a and 15b and of the
meso NHC 17 reveal significant differences between the two
isomers (i.e., chiral vs. meso). For example, the azolinium
proton in the chiral 15a·HBF4 resonates at 9.19 ppm, the
equivalent proton in 17·HBF4 at 9.69 ppm. The two bridgehead
CH units of the triptycene (inside and outside) are observed at
5.82 and 6.48 ppm in [AuCl(17)], but at 5.38 and 5.91 ppm in
[AuCl(15a)]. Most importantly, the respective metal complexes
of both the meso and the chiral compounds each consist of
a single isomer only. There is no evidence whatsoever for any
representative of the two classes of compounds to be mixtures
of isomers. This clearly shows that the orientation of the tripty-
cene wings is controlled by the given orientation of the
phenyl groups at the backbone of the heterocyclic unit. This
was expected, since there are several examples in the literature
for which this orientation control has been shown.[12b,20a,33d,43]
Scheme 10. Enantioselective borylation of a,ß-unsaturated esters. Reagents
and conditions: a) 15a·HBF4 (5 mol%), CuCl (5 mol%), THF, sodium tert-pent-
oxide; B2(pin)2, 18 h, T=À208 C; b) THF, NaBO3·4 H2O, water, 3 h, RT; c) (R)-O-
acetylmandelic acid, DMAP, CH2Cl2, DCC, 24 h, RT; (DMAP=4-(dimethylami-
1
no)-pyridine, DCC=dicyclohexylcarbodimide). The H NMR integrals of the
protons in the diastereomeric esters were utilized for enantiomeric ratio (er)
determination.
Donor properties of the bistriptycene-NHC
The redox potentials of [RhCl(cod)(8a)] (E1/2 = +0.66 V) and
[IrCl(cod)(8a)] (E1/2 = +0.71 V) were determined. The IrI/IrII
redox potential falls into the expected range, which is ob-
served for related complexes with IMes ligands.[44] The n˜(CO) of
NHC 10a in [IrCl(CO)2(10a)] was determined and found to be
3 cmÀ1 (Tolman electronic parameter 2055 cmÀ1) higher than
that of the reference complex [IrCl(CO)2(SIMes)]. Both results
are indicative of a slightly lower electron-donating capacity of
ligand 10a. The donor properties of the chiral and meso-NHC
ligands were also evaluated by considering the RhI/RhII redox
potential of [RhCl(cod)(15a)] (E1/2 =0.739 V) and [RhCl(cod)(17)]
(E1/2 =0.677 V). This value is more cathodic than that of the re-
lated [(RhCl(cod)(SIMes)] species with 4-alkyoxy substituents
(E1/2 =0.785 V),[44] and suggests stronger electron-donating
properties of NHC 15a and 17 compared to the alkoxy-substi-
tuted SIMes.[25a]
Conclusions
We have successfully up-scaled the synthesis of triptycene ami-
nophenol 5 to provide large amounts of this compound from
cheap starting materials. This triptycene aminophenol 5 was
converted (via the glyoxal-derived diimines) into the respective
azolium salts, which are convenient precursors for NHC-metal
complexes. The inherently poor solubility of triptycene deriva-
tives was overcome by installing solubilizing long alkyl groups
on the phenolic OH group (leading to the 7 series). Based on
this, several (soluble) metal complexes of the bistriptycene-
NHC ligand with Cu, Ag, Au, Rh, and Ir were synthesized (com-
plexes with 8b, 8c, or 10a). The NMR spectra and the crystal
structure of [IrCl(cod)(8b)] with a bistriptycene-NHC ligand re-
vealed two limitations of this approach: i) despite the signifi-
cant bulk of the triptycene group, the free rotation around the
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crystalline solid (9.5 g, 75% yield). Spectroscopic data are in ac-
cordance with literature data.[24]
NÀC bond results in the formation of isomeric metal com-
plexes, and ii) the lack of steric bulk in the 4- and 5-positions
of the heterocycle allows a 608 tilt of the triptycene wings,
thus relieving steric pressure from the space close to the metal
center.
Synthesis of aminophenol 5:[24] To a stirred solution of 4 (9.5 g,
31.7 mmol) were added Pd/C (0.95 g, 10 wt%) and hydrazine hy-
drate (65% aq., 168.5 mmol) in THF (250 mL), followed by refluxing
for 2 h, and then filtered. Both flask and filter were washed with
DCM. The filtrate was concentrated under reduced pressure, and
the residue washed with pentane. The precipitate was collected by
filtration and dried in vacuo (8.2 g, 91% yield). Spectroscopic data
are in accordance with literature data.[24]
To obtain stereochemically defined NHC ligands, the Buch-
wald–Hartwig amination of bromotriptycene 13 with (1S,2S)-
1,2-diphenylethylenediamine or meso-1,2-diphenyl-1,2-diami-
noethane, followed by the cyclization of the diamine with
HC(OEt)3, were utilized. The respective reactions of the meso-
and of the chiral azolium salt with suitable metal precursors
provide the respective stereochemically defined chiral or meso-
complexes [(NHC)ML] with 15a, 15b, or 17 for M=Rh, Ir, Au,
Pd. Preliminary catalytic tests of the respective copper com-
plexes with chiral NHC in the enantioselective borylation pro-
vide excellent yields and enantioselectivities.
Synthesis of 11: To a solution of 1 (5.00 g, 17.52 mmol) in distilled
THF (280 mL) under nitrogen was added 50% aq. H3PO2 (5.75 mL,
52.56 mmol). The solution was heated to 408C, and then a solution
of tert-butyl nitrite (3.47 mL, 90%, 26.28 mmol) in THF (20 mL) was
added dropwise. The mixture was kept at 408C for 24 h. The THF
was then removed under reduced pressure. The residue was dis-
solved in CH2Cl2 and washed with brine. The organic layer was sep-
arated, dried over MgSO4, and the filtrate concentrated under re-
duced pressure. Silica gel column chromatography, with ethyl ace-
tate/cyclohexane (1:10) as eluent, afforded 2 as a yellowish solid
1
(3.05 g, 65% yield). H NMR (300 MHz, CDCl3): d=7.46–7.35 (m, 4H,
Experimental Section
HAr), 7.04–6.96 (m, 5H, HAr), 6.84 (dd, J=8.0, 7.4 Hz, 1H, HAr), 6.45
(dd, J=8.1, 0.9 Hz, 1H, HAr), 5.84 (s, 1H, CHAr3), 5.42 (s, 1H, CHAr3),
4.77 ppm (s, 1H, OH); 13C NMR (75 MHz, CDCl3): d=150.3, 148.1,
145.7, 145.2, 131.2, 126.1, 125.3, 123.8, 123.8, 116.8, 113.1, 54.4,
47.1 ppm. HRMS (EI): m/z: calcd for C20H14O: 270.10345 [M+H]+;
found 270.1044.
Synthesis of quinone 1:[2] Anthracene (51.18 g, 315.9 mmol) (re-
crystallized from xylene) and 1,4-benzoquinone (34.14 g,
315.9 mmol) (recrystallized from cyclohexane) were heated under
reflux in xylene (300 mL) for 2 h. Next, the mixture was cooled to
room temperature and the precipitated solid collected by filtration.
The solid was first washed with a small amount of xylene, then
thoroughly washed with hot water, and finally, with ethanol. After
drying in vacuo, the product was obtained as pale yellow rhom-
boids (69.7 g, 85% yield). 1H NMR (300 MHz, CDCl3): d=7.40 (dd,
J=5.4, 3.2 Hz, 2H), 7.23–7.14 (m, 4H), 7.08 (dd, J=5.4, 3.2 Hz, 2H),
6.31 (s, 2H), 4.87 (s, 2H), 3.14 ppm (s, 2H).
Synthesis of 12a: To a solution of 11 (1.5 g, 5.5 mmol) in DMF
(10 mL) was added K2CO3 (2.3 g, 16.6 mmol). The mixture was
stirred for 5 min at room temperature, and then butyl iodide
(1.06 mL, 7.2 mmol) was added in one portion. The reaction mix-
ture was stirred at 708C for 24 h. The mixture was then cooled to
room temperature, and water (150 mL) was added. The product
was extracted a few times with diethyl ether, the extract was
washed three times with water, followed by three times with brine,
dried over MgSO4, and filtered. The solvent was removed under re-
duced pressure, and the obtained solid washed with cold metha-
nol to give alkylated phenol 12a (1.45 g, 74% yield). 1H NMR
(300 MHz, CDcl3): d=7.48–7.39 (m, 4H, HAr), 7.08–7.05 (m, 1H, HAr),
7.04–7.00 (m, 4H, HAr), 6.96 (dd, J=8.2, 7.3 Hz, 1H, HAr), 5.96 (s, 1H,
CHAr3), 5.45 (s, 1H, CHAr3), 4.03 (t, J=6.4 Hz, 2H, OCH2, hexyl),
1.95–1.85 (m, 2H, CH2, hexyl), 1.66–1.54 (m, 2H, CH2, hexyl), 1.50–
1.42 (m, 4H, CH2, hexyl), 1.05–0.98 ppm (m, 3H, CH3, hexyl);
13C NMR (75 MHz, CDCl3): d=154.1, 147.6, 146.0, 145.6, 133.5,
126.0, 125.2, 125.1, 123.8, 123.7, 116.4, 109.9, 68.8, 54.5, 47.1, 31.8,
29.5, 26.0, 22.9, 14.2 ppm; HRMS (EI): m/z: calcd for C26H26O:
354.1978 [M]+; found: 354.1977.
Synthesis of hydroquinone 2:[2] To a solution of 1 (50.0 g,
174.82 mmol) in glacial acetic acid (600 mL) at the boiling point,
four drops of 40% aq. HBr were added. An exothermic reaction fol-
lowed, and a fine cream-colored solid precipitated over a few mi-
nutes. After another 30 min at the boiling point, the reaction mix-
ture was cooled to room temperature, and the solid collected by
filtration. The solid was washed with acetic acid, then ethanol, and
1
dried in vacuo (40.23 g, 80% yield). H NMR (300 MHz, [D6]DMSO):
d=8.81 (s, 2H), 7.45–7.34 (m, 4H), 7.02–6.92 (m, 4H), 6.32 (s, 2H),
5.81 ppm (s, 2H).
Synthesis of quinone 3:[2] Hydroquinone 2 (12.0 g, 41.96 mmol)
was dissolved in hot glacial acetic acid (900 mL), and then a solu-
tion of potassium bromate (2.69 g, 16.11 mmol) in 245 mL of hot
water was added. A deep orange color developed immediately.
The solution was heated under reflux for two minutes, hot water
(250 mL) was added, and then boiling continued for a few minutes.
The solution was cooled to room temperature and a yellow solid
collected by filtration. The quinone was washed with acetic acid,
water, and ethanol, and finally, dried in vacuo (10.93 g, 92% yield).
1H NMR (300 MHz, CDCl3): d=7.47–7.40 (m, 4H), 7.07–7.00 (m, 4H),
6.60 (s, 2H), 5.80 ppm (s, 2H).
Synthesis of monooxime 4:[24] To a stirred solution of quinone 3
(12.1 g, 42.5 mmol) was added NH2OH·HCl solution (15.3 g,
220.1 mmol dissolved in 35 mL water) in THF (350 mL). The reac-
tion mixture was heated for 12 h at 508C and then cooled to room
temperature. The volatiles were removed under reduced pressure,
then the residue dissolved in CH2Cl2 and was washed with water.
The organic layer was dried over anhydrous MgSO4, and the filtrate
was evaporated under reduced pressure. The residue was recrystal-
lized from ethanol/water, and the product obtained as a yellow
Synthesis of 13a and 13b: The respective alkylated phenol 12a
or 12b (4.18 mmol) was dissolved in DMF (25 mL). Next, NBS
(1.48 g, 8.36 mmol) was added in few portions, and the solution
stirred for 24 h at 1008C. After that, the reaction mixture was
cooled to room temperature, and water (250 mL) was added. The
product was extracted a few times with diethyl ether, the extract
was washed three times with water, and then three times with
brine, dried over MgSO4, and filtered. The solvent was removed
under reduced pressure, and solid residue washed with cold meth-
anol to give bromophenol 13a (1.49 g, 82% yield), 13b (1.48 g,
77% yield), or 13c (1.63 g, 84%) as an off-white solid.
13a: 1H NMR (500 MHz, CDCl3): d=7.47–7.43 (m, 2H, HAr), 7.43–
7.39 (m, 2H, HAr), 7.09 (d, J=8.7 Hz, 1H, HAr), 7.04–7.00 (m, 4H, HAr),
6.46 (d, J=8.8 Hz, 2H, HAr), 5.92 (s, 1H, CHAr3), 5.87 (s, 1H, CHAr3),
3.96 (t, J=6.4 Hz, 2H, OCH2, hexyl), 1.89–1.82 (m, 2H, CH2, hexyl),
1.58–1.51 (m, 2H, CH2, hexyl), 1.44–1.39 (m, 4H, CH2, hexyl), 0.99–
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0.95 (m, 3H, CH3, hexyl); 13C NMR (75 MHz, CDCl3): d=153.4, 146.4,
145.4, 145.0, 136.2, 129.0, 125.5, 125.3, 124.2, 123.9, 111.6, 110.1,
69.1, 53.6, 47.6, 31.7, 29.4, 26.0, 22.8, 14.2 ppm; HRMS (EI): m/z:
calcd for C26H25OBr: 432.1083 [M]+; found: 432.1078.
[MÀC28H31NO]+; found: 575.3180; calcd for C35H35ON: 485.2713
[MÀC35H37NO]+; found: 485.2712.
Synthesis of 15a·HBF4 and 15b·HBF4: The respective diamine 14a
or 14b (0.981 mmol), NH4BF4 (103 mg, 0.982 mmol), CH(OEt)3
(9 mL), and a catalytic amount of formic acid (2 drops) were stirred
at 1208C overnight. Next, the resulting suspension was cooled to
room temperature, and diethyl ether (20 mL) was added. The pre-
cipitate was collected by filtration and washed a few times with di-
ethyl ether. The corresponding azolium salts 15a·HBF4 (757 mg,
76% yield) and 15b·HBF4 (662 mg, 63% yield) were obtained as
off-white solids.
13b: 1H NMR (500 MHz, CDCl3): d=7.46–7.42 (m, 2H, HAr), 7.41–
7.37 (m, 2H, HAr), 7.08 (d, J=8.7 Hz, 1H, HAr), 7.03–6.99 (m, 4H, HAr),
6.46 (d, J=8.7 Hz, 1H, HAr), 5.89 (s, 1H, CHAr3), 5.86 (s, 1H, CHAr3),
3.85 (dd, J=5.6, 1.2 Hz, 2H, OCH2, 2-EtHex), 1.81 (hept, J=6.1 Hz,
1H, CH, 2-EtHex), 1.63–1.45 (m, 4H, CH2, 2-EtHex), 1.43–1.36 (m,
4H, CH2, 2-EtHex), 1.00 (t, J=7.5 Hz, 3H, CH3, 2-EtHex), 0.98–
0.94 ppm (m, 3H, CH3, 2-EtHex); 13C NMR (126 MHz, CDCl3): d=
153.5, 146.39, 145.36, 145.0, 136.2, 129.0, 125.4, 124.2, 123.9, 110.0,
71.4, 53.6, 47.7, 39.6, 31.0, 29.3, 24.4, 23.3, 14.3, 11.5 ppm; HRMS
(EI): m/z: calcd for C28H29OBr: 460.1396 [M]+; found: 460.1399.
15a·HBF4: [a]2D0 =À188.0 (c=0.505 in DMSO); 1H NMR (500 MHz,
[D6]DMSO): d=9.18 (s, 1H, NCHN), 7.66–7.62 (m, 6H, HAr), 7.49–
7.43 (m, 6H, HAr), 7.39 (d, J=7.2 Hz, 2H, HAr), 7.38–7.33 (m, 4H, HAr),
7.32–7.28 (m, 2H, HAr), 7.12–7.05 (m, 6H, HAr), 7.01 (td, J=7.5,
1.2 Hz, 2H, HAr), 6.88 (d, J=8.9 Hz, 2H, HAr), 6.38 (s, 2H), 6.24 (s,
2H), 5.92 (s, 2H), 4.05 (t, J=6.4 Hz, 4H, OCH2, hexyl), 1.82–1.75 (m,
4H, CH2, hexyl), 1.54–1.46 (m, 4H, CH2, hexyl), 1.39–1.33 (m, 8H,
CH2, hexyl), 0.94–0.90 ppm (m, 6H, CH3, hexyl); 13C NMR (126 MHz,
[D6]DMSO): d=157.8, 153.4, 144.8, 144.6, 144.4, 144.2, 142.3, 134.6,
129.6, 129.3, 128.0, 125.4, 125.3, 125.0, 124.4, 124.0, 123.8, 123.6,
123.4, 110.6, 74.6, 68.4, 47.1, 46.1, 30.9, 28.5, 25.2, 22.1, 13.8 ppm;
HRMS (EI): m/z: calcd for C53H50O2N2: 746.3867 [MÀC14H13ÀBF4]+;
found: 746.3872.
1
13c: H NMR (300 MHz, CDCl3): d=7.56–7.32 (m, 4H), 7.16–6.92 (m,
5H), 6.45 (d, J=8.8 Hz, 1H), 5.91 (s, 1H), 5.86 (s, 1H), 3.95 (t, J=
6.4 Hz, 2H), 1.97–1.75 (m, 2H), 1.59–1.50 (m, 2H), 1.47–1.28 (m,
8H), 1.02–0.82 ppm (m, 3H); 13C NMR (75 MHz, CDCl3): d=153.4,
146.4, 145.3, 145.0, 136.2, 129.0, 125.5, 125.3, 124.2, 123.9, 111.6,
110.1, 69.0, 53.6, 47.5, 32.0, 29.5, 29.4, 26.3, 22.9, 14.3 ppm; HRMS:
m/z: calcd for C28H29OBr: 460.1401; found: 460.14093.
Synthesis of 14a and 14b: Under nitrogen atmosphere, to a solu-
tion of Pd2(dba)3 (55.4 mg, 0.06 mmol) and BINAP (90.4 mg,
0.145 mmol) in toluene (15 mL) was added sodium tert-pentoxide
(2.5m in THF, 1.16 mL 2.9 mmol), and the resulting mixture stirred
for 30 min at room temperature. After this time, 2.42 mmol of 13a
or 13b (respectively) and (1S, 2S)-1,2-diphenylethylenediamine
(256 mg, 1.2 mmol) were added, and the reaction mixture was
heated to 1008C for 24 h. After cooling to room temperature, the
reaction mixture was filtered through a plug of silica, and the plug
washed with CH2Cl2. The filtrate was evaporated under reduced
pressure, and the residue purified by column chromatography
(silica gel, cyclohexane/ethyl acetate, 10:1, v/v) affording chiral dia-
mine 14a (888 mg, 80% yield) or 14b (1.0 g, 85% yield) as a white
solid.
1
15b·HBF4: [a]2D0 =À175.2 (c=0.448 in (CH3)2CO); H NMR (300 MHz,
[D6]DMSO): d=9.19 (s, 1H, NCHN), 7.66 (d, J=7.4 Hz, 6H, HAr),
7.53–7.42 (m, 6H, HAr), 7.41–7.26 (m, 8H, HAr), 7.16–6.99 (m, 8H,
HAr), 6.90 (d, J=8.9 Hz, 2H, HAr), 6.40 (s, 2H), 6.26 (s, 2H), 5.91 (s,
2H), 3.95 (d, J=5.4 Hz, 4H, OCH2, 2-EtHex), 1.78 (hept, J=6.2 Hz,
2H, CH, 2-EtHex), 1.63–1.44 (m, 8H, CH2, 2-EtHex), 1.43–1.30 (m,
8H, CH2, 2-EtHex), 1.02–0.88 ppm (m, 12H, CH3, 2-EtHex); 13C NMR
(75 MHz, [D6]DMSO): d=157.8, 153.6, 144.8, 144.6, 144.4, 144.3,
142.3, 134.6, 134.6, 129.6, 129.3, 128.0, 125.5, 125.4, 125.0, 124.4,
124.1, 123.7, 123.6, 123.6, 110.4, 74.7, 70.8, 47.1, 46.3, 30.2, 28.5,
23.6, 23.6, 22.6, 13.9, 11.1 ppm; HRMS (EI): m/z: calcd for
C57H58O2N2: 802.4493 [MÀC14H13ÀBF4]+; found: 802.4488.
14a: [a]2D0 =À34.7 (c=0.614 in CHCl3); 1H NMR (500 MHz,
[D6]DMSO): d=7.45 (d, J=7.1 Hz, 2H, HAr), 7.41 (d, J=7.1 Hz, 2H,
HAr), 7.36 (d, J=7.0 Hz, 4H, HAr), 7.21 (d, J=7.4 Hz, 4H, HAr), 7.13–
7.02 (m, 10H, HAr), 7.02–6.94 (m, 4H, HAr), 6.43 (d, J=8.7 Hz, 2H,
HAr), 6.31 (d, J=8.7 Hz, 2H, HAr), 6.01 (s, 2H, CHAr3), 5.92–5.88 (m,
2H), 5.81 (s, 2H, CHAr3), 4.70–4.65 (m, 2H), 3.89–3.81 (m, 4H, OCH2,
hexyl), 1.71 (quint, J=6.5 Hz, 4H, CH2, hexyl), 1.48 (quint, J=6.9 Hz,
4H, CH2, hexyl), 1.38–1.31 (m, 8H, CH2, hexyl), 0.92 ppm (t, J=
6.5 Hz, 6H, CH3, hexyl); 13C NMR (126 MHz, [D6]DMSO): d=146.5,
145.7, 145.6, 145.4, 141.7, 137.2, 134.2, 133.6, 127.7, 126.7, 124.9,
124.7, 124.5, 123.5, 123.4, 123.3, 112.6, 111.2, 69.2, 65.3, 46.8, 46.7,
31.0, 28.8, 25.3, 22.1, 13.9 ppm; HRMS (EI): m/z: calcd for C33H31ON:
457.2400 [MÀC33H33NO]+; found: 457.2399; calcd for C33H32ON:
458.2478 [MÀC33H32NO]+; found: 458.2454.
Synthesis of [AuCl(15a)]: Azolium salt 15a·HBF4 (110 mg,
0.108 mmol) was dissolved in a mixture of acetone (6 mL) and
methanol (2 mL). To the resulting solution was added Amberlite
IRA-410 (chloride form) ion-exchange resin (600 mg, washed with
methanol prior to use). The mixture was vigorously stirred at room
temperature for 24 h and then filtered. The solvent was removed
in vacuo. To the obtained white solid was first added [AuCl(SMe2)]
(31.9 mg, 0.108 mmol) and acetone (3 mL), followed 10 min later
by powdered K2CO3 (44 mg, 0.318 mmol). The resulting suspension
was stirred at 608C overnight in the dark. After this time, the sol-
vent was removed in vacuo, and dichloromethane was added. The
mixture was filtered through a pad of Celite, and the pad was then
washed with dichloromethane. The solvent was concentrated and
the solid residue purified by column chromatography (silica gel,
pentane/diethyl ether, 10:1, v/v) to provide the desired gold com-
14b: [a]2D0 =À34.8 (c=0.035 in CHCl3); 1H NMR (500 MHz,
[D6]DMSO): d=7.44 (d, J=6.9 Hz, 2H, HAr), 7.38 (d, J=7.1 Hz, 2H,
HAr), 7.35 (d, J=7.0 Hz, 2H, HAr), 7.32 (d, J=7.0 Hz, 2H, HAr), 7.21–
7.18 (m, 4H, HAr), 7.12–7.02 (m, 10H, HAr), 7.02–6.94 (m, 4H, HAr),
6.43 (d, J=8.7 Hz, 2H, HAr), 6.29 (d, J=8.8 Hz, 2H, HAr), 5.99 (s, 2H,
CHAr3), 5.89–5.84 (m, 2H), 5.78 (s, 2H, CHAr3), 4.68–4.64 (m, 2H),
3.78–3.70 (m, 4H, OCH2, 2-EtHex), 1.69 (hept, J=12.1, 6.1 Hz, 2H,
CH, 2-EtHex), 1.57–1.39 (m, 8H, CH2, 2-EtHex), 1.37–1.31 (m, 8H,
CH2, 2-EtHex), 0.96–0.89 ppm (m, 12H, CH3, 2-EtHex); 13C NMR
(126 MHz, [D6]DMSO): d=146.7, 145.7, 145.6, 145.4, 141.7, 137.1,
133.9, 133.6, 127.7, 126.7, 124.9, 124.2, 124.5, 123.5, 123.4, 123.3,
112.6, 110.6, 71.3, 65.3, 46.8, 46.7, 38.8, 38.8, 30.2, 30.1, 28.5, 23.6,
22.6, 13.9, 11.0 ppm; HRMS (EI): m/z: calcd for C42H41ON: 575.3183
1
plex as a white solid (78 mg, 62% yield). H NMR (500 MHz, CDCl3):
d=7.60–7.42 (br, 12H, HAr), 7.40 (d, J=6.8 Hz, 2H, HAr), 7.37 (d, J=
7.2 Hz, 2H, HAr), 7.24–7.13 (br, 2H, HAr), 7.09–7.02 (m, 4H, HAr),
7.00–6.72 (m, {t, 2H, J=7.7 Hz + t, 2H, J=7.2 Hz + br, 2H}, HAr),
6.48 (d, J=8.8 Hz, 2H, HAr), 5.88 (s, 2H, CHAr3), 5.72–5.49 (br, 2H,
CH, backbone), 5.39 (s, 2H, CHAr3), 3.99–3.89 (m, 2H, OCH2, hexyl),
1.85 (quint, J=6.5 Hz, 4H, CH2, hexyl), 1.58–1.50 (m, 4H, CH2,
hexyl), 1.46–1.38 (m, 8H, CH2, hexyl), 0.97 ppm (t, J=7.0 Hz, 6H,
CH3, hexyl); 13C NMR (126 MHz, CDCl3): d=194.81 (C-Au), 154.0,
145.6, 145.2, 144.5, 144.2, 143.8, 137.5, 135.1, 129.9, 129.8, 126.9,
125.9, 125.6, 125.5, 125.3, 124.3, 124.2, 124.1, 110.0, 78.5, 68.7, 49.9,
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47.2, 31.7, 29.4, 26.0, 22.8, 14.2 ppm; HRMS (EI): m/z: calcd for
C53H50O2N2: 746.3867 [MÀC14H12ÀAuCl]+; found: 746.3863.
General procedure for the asymmetric b-borylation of a,b-unsa-
turated esters (according to a modified literature procedure):[46c]
Imidazolinium salt 6a·HBF4 (30.36 mg, 5 mol%) and CuCl (2.82 mg,
5 mol%) were placed into a flame-dried Schlenk tube under nitro-
gen. Next, THF (2 mL) and sodium tert-pentoxide (2.5m in THF,
29.5 mL, 13 mol%) were added. The mixture was stirred for 15 min
at room temperature and then cooled to À208C. After this, bis(pi-
nacolato)diboron (158.4 mg, 0.623 mmol), the respective a,b-unsa-
turated ester (0.567 mmol), and MeOH (49.5 mL, 1.13 mmol) were
added. The mixture was stirred for 18 h (time is not optimized) at
À208C and subsequently, filtered through a silica pad. The silica
pad was washed with Et2O, and the resulting filtrate was concen-
trated under reduced pressure. In all cases, complete conversion of
a,b-unsaturated ester was observed by 1H NMR analysis of the
crude mixture.
General procedure for the synthesis of [RhCl(cod)(15a)] and
[RhCl(cod)(15b)]: A vial was charged with the corresponding
NHC·HBF4 (1 equiv), [RhCl(cod)]2 (0.5 equiv), and K2CO3 (3 equiv).
The resulting mixture was suspended in acetone (2.0 mL), and
stirred for 20 h at 608C. After this time the solvent was removed in
vacuo and dichloromethane was added (3 mL). The mixture was fil-
tered through a pad of silica. The pad of silica was then washed
with dichloromethane until the filtrate became colorless, and the
solvent was removed in vacuo. The solid residue was washed with
cold methanol. The respective rhodium complexes [RhCl(cod)(15a)]
(74 mg, 69% yield) and [RhCl(cod)(15b)] (60 mg, 65% yield) were
obtained as yellow solids.
Oxidation procedure (according to a modified protocol):[21a] The
resulting crude mixture was dissolved in THF (5 mL). Next,
NaBO3·4H2O (436.2 mg, 2.84 mmol) and water (5 mL) were added,
and the obtained suspension stirred for 3 h at room temperature
After this, the mixture was diluted with water (20 mL), and the
product was extracted with Et2O. The combined extracts were
washed with brine and dried over MgSO4. The solvent was evapo-
rated under reduced pressure and the residue purified by column
chromatography (silica, cyclohexane/ethyl acetate, 7:1, v/v).
[RhCl(cod)(15a)]: 1H NMR (500 MHz, CDCl3): d=8.54 (d, J=7.0 Hz,
1H, HAr), 7.91 (d, J=7.5 Hz, 2H, HAr), 7.60–7.52 (m, 5H, HAr), 7.48–
7.43 (m, 2H, HAr), 7.42–7.39 (m, 1H, HAr), 7.38–7.33 (m, 5H, HAr),
7.32–7.29 (m, 2H, HAr), 7.04–7.00 (m, 2H, HAr), 6.99 (d, J=7.3 Hz,
1H, HAr), 6.97–6.92 (m, 2H, HAr), 6.89 (t, J=7.3 Hz, 2H, HAr), 6.81 (t,
J=7.4 Hz, 1H, HAr), 6.67 (d, J=8.8 Hz, 1H, HAr), 6.65–6.59 (br, 1H),
6.54 (d, J=8.7 Hz, 1H, HAr), 6.33 (d, J=7.2 Hz, 1H, HAr), 5.91 (s, 1H),
5.83 (s, 1H), 5.65 (s, 1H), 5.38 (s, 1H), 5.24–5.21 (m, 1H), 5.13–5.10
(m, 1H), 4.92–4.85 (m, 1H, Hcod), 4.46 (q, J=7.2 Hz, 1H, Hcod), 4.08–
4.02 (m, 1H), 4.02–3.92 (m, 3H), 3.85–3.79 (m, 1H, Hcod), 2.74–2.68
(m, 1H, Hcod), 2.19–2.10 (m, 1H), 1.96–1.80 (m, 5H), 1.79–1.70 (m,
1H, Hcod), 1.60–1.49 (m, 7H), 1.46–1.36 (m, 9H), 1.01–0.93 (m, 6H,
CH3, hexyl), 0.93–0.86 (m, 1H, Hcod), 0.49–0.39 ppm (m, 1H, Hcod);
13C NMR (126 MHz, CDCl3): d=217.8 (d, J(C-Rh) =47 Hz, Ccarbene), 153.4,
153.1, 146.6, 146.0, 145.5, 145.4, 144.8, 144.6, 144.5, 144.5, 144.3,
142.9, 140.1, 138.8, 134.8, 134.0, 130.8, 129.7, 129.6, 129.2, 129.2,
128.8, 128.7, 127.5, 127.3, 126.3, 126.2, 125.5, 125.2, 125.1, 124.9,
124.88, 124.86, 124.8, 124.2, 123.9, 123.8, 123.6, 123.0, 122.8, 122.6,
109.5, 109.0, 99.9, 97.5, 97.5, 78.5, 78.2, 69.8 (d, J=13.6 Hz), 68.9,
68.6, 67.8 (d, J=13.5 Hz), 49.7, 49.5, 47.3, 47.3, 33.0, 31.80, 31.75,
30.9, 29.6, 29.4, 28.4, 27.8, 26.1, 26.0, 22.8, 14.2 ppm; HRMS (EI): m/
z: calcd for C67H61O2N2Rh: 1028.3783 [MÀHCl-C8H12]+; found:
1028.3802.
Ethyl 3-hydroxybutanoate: 63 mg, 84% yield. 1H NMR (300 MHz,
CDCl3): d=4.18 (q, J=7.2 Hz, 2H), 2.98 (br, 1H), 2.54–2.36 (m, 2H),
1.28 (t, J=7.1 Hz, 3H), 1.23 ppm (d, J=6.3 Hz, 3H).
1
Ethyl 3-hydroxy-3-phenylpropanoate: 100 mg, 91% yield. H NMR
(500 MHz, CDCl3): d=7.40–7.34 (m, 4H), 7.31–7.27 (m, 1H), 5.14 (dt,
J=8.8, 3.5 Hz, 1H), 4.19 (q, J=7.2 Hz, 2H), 3.22 (d, J=3.5 Hz, 1H),
2.79–2.69 (m, 2H), 1.27 ppm (t, J=7.2 Hz, 3H).
Ethyl 3-hydroxyoctanoate: 95 mg, 89% yield. 1H NMR (500 MHz,
CDCl3): d=4.17 (q, J=7.2 Hz, 2H), 4.03–3.96 (m, 1H), 2.89 (br, 1H),
2.50 (dd, J=16.4, 3.1 Hz, 1H), 2.40 (dd, J=16.4, 9.0 Hz, 1H), 1.56–
1.49 (m, 1H), 1.48–1.38 (m, 2H), 1.38–1.25 (m, 8H), 0.89 ppm (t, J=
6.9 Hz, 3H). All spectral data match those reported in ref. [50].
Synthesis of O-acetylmandelates for the determination of enan-
tiomeric ratio: b-Alcohol product (0.0818 mmol), (R)-O-acetylman-
delic acid (20.6 mg, 0.106 mmol), and a catalytic amount of DMAP
were placed into a flame-dried Schlenk tube under nitrogen. Then
CH2Cl2 (2 mL) was added, and the obtained solution cooled to 08C.
Next, DCC (21.9 mg, 0.106 mmol) was added and the mixture
stirred at room temperature for 24 h. The obtained white suspen-
sion was filtered to remove dicyclohexylurea. The filtrate was dilut-
ed with CH2Cl2 (10 mL), washed with sodium bicarbonate solution,
brine, and dried over MgSO4. The solvent was evaporated under re-
duced pressure, and the obtained crude mixture was analyzed by
1H NMR spectroscopy (500 MHz, CDCl3, no internal integration stan-
dard was used) to determine the enantiomeric ratio.
[RhCl(cod)(15b)]: 1H NMR (500 MHz, CDCl3): d=8.54 (d, J=7.3 Hz,
1H, HAr), 7.92 (d, J=7.6 Hz, 2H, HAr), 7.60–7.52 (m, 5H, HAr), 7.48–
7.44 (m, 2H, HAr), 7.40–7.32 (m, 6H, HAr), 7.28 (d, J=7.4 Hz, 2H, HAr),
7.03–7.00 (m, 2H, HAr), 6.99 (d, J=7.4 Hz, 1H, HAr), 6.97–6.92 (m,
2H, HAr), 6.88 (t, J=7.3 Hz, 2H, HAr), 6.80 (t, J=7.4 Hz, 2H, HAr), 6.67
(d, J=8.8 Hz, 1H), 6.64–6.59 (br, 1H), 6.55 (d, J=8.6 Hz, 1H), 6.32
(d, J=7.2 Hz, 1H), 5.89 (s, 1H), 5.81 (s, 1H), 5.64 (s, 1H), 5.36 (s,
1H), 5.21 (s, 1H), 5.13–5.11 (m, 1H), 4.92–4.86 (m, 1H, Hcod), 4.45 (q,
J=7.2 Hz, 1H, Hcod), 3.98–3.92 (m, 1H, Hcod), 3.91–3.80 (m, 4H),
2.72–2.66 (m, 1H, Hcod), 2.20–2.11 (m, 1H, Hcod), 1.96–1.87 (m, 1H,
Hcod), 1.86–1.79 (m, 2H, CH, 2-EtHex), 1.78–1.71 (m, 1H, Hcod), 1.65–
1.46 (m, 11H), 1.46–1.34 (m, 9H), 1.05–0.92 (m, 12H, CH3, 2-EtHex),
0.92–0.83 (m, 2H, Hcod), 0.46–0.36 ppm (m, 1H, Hcod); 13C NMR
(126 MHz, CDCl3): d=217.82 (d, J(C-Rh) =47 Hz, Ccarbene), 153.5, 153.2,
153.2, 146.6, 146.0, 145.5, 145.3, 144.8, 144.6, 144.5, 144.4, 144.3,
143.0, 140.2, 138.8, 134.9, 133.99, 133.95, 130.8, 129.7, 129.6,
129.22, 129.18, 128.73, 128.68, 127.4, 127.3, 126.3, 126.2, 125.5,
125.2, 125.1, 125.0, 124.9, 124.8, 124.2, 123.9, 123.6, 123.0, 122.8,
122.6, 109.4, 109.3, 108.8, 99.9, 97.4, 78.5, 78.2, 71.3, 71.0, 69.8 (d,
J=14.1 Hz), 67.8 (d, J=15.3 Hz), 49.6, 49.5, 47.6, 47.4, 39.8, 39.6,
39.5, 33.06, 31.13, 31.09, 31.03, 30.95, 30.8, 29.9, 29.42, 29.37, 29.34,
29.29, 28.4, 27.7, 24.52, 24.48, 24.44, 24.35, 23.31, 14.27, 11.62,
11.60, 11.5, 11.4 ppm; HRMS (EI): m/z: calcd for C71H70O2N2ClRh:
1120.4175 [MÀC8H12]+; found: 1120.4135; calcd for C71H69O2N2Rh:
1084.4409 [MÀHClÀC8H12]+; found: 1084.4440.
Acknowledgements
This work was supported by the DFG via grant Pl 178/13-2. We
thank the VLM Korrosions-Prꢁftechnik, Labortechnik & Dien-
stleistungen GmbH, for a gift of activated alumina. We wish to
thank Prof. Dr. M. Reggelin for help with the mandelic acid
method.
Keywords: chiral · copper · homogeneous catalysis · iridium ·
N-heterocyclic carbene
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Received: March 30, 2016
Published online on && &&, 0000
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FULL PAPER
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N-Heterocyclic Carbenes
R. Savka, M. Bergmann, Y. Kanai, S. Foro,
H. Plenio*
&& – &&
Triptycene-Based Chiral and meso-N-
Heterocyclic Carbene Ligands and
Metal Complexes
From triptycene to enantioselective
catalysis: New triptycene-based chiral
and meso-N-heterocyclic carbenes were
synthesized. The triptycene unit infers
favorable steric properties onto the re-
spective NHC-metal complexes, ena-
bling the shielding of selected areas
close to the metal center.
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Chem. Eur. J. 2016, 22, 1 – 10
www.chemeurj.org
10
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!