Synthesis and structure of salts of a sterically shielded, lipophilic, C2-symmetric, fluxional aluminate

Article abstract of DOI:10.3998/ark.5550190.p008.404

Multi-gram amounts of halogen-free lipophilic aluminate salts have been prepared, featuring two sterically demanding chelating ligands derived from a methane-2,2'-bisphenolate. The ligand is prepared by condensation of two equivalents of 2,4-di-tert-butyl

Full text of DOI:10.3998/ark.5550190.p008.404

General Papers
ARKIVOC 2014 (iv) 296-318
Synthesis and structure of salts of a sterically shielded, lipophilic,
C2-symmetric, fluxional aluminate
Timo Söhner, Ina K. Küppers, Wiebke Wackerow, Frank Rominger, and Bernd F. Straub*
Universität Heidelberg, Organisch-Chemisches Institut, Im Neuenheimer Feld 270, 69120
Heidelberg, Germany
E-mail: Straub@oci.uni-heidelberg.de
Abstract
Multi-gram amounts of halogen-free lipophilic aluminate salts have been prepared, featuring two
sterically demanding chelating ligands derived from a methane-2,2´-bisphenolate. The ligand is
prepared by condensation of two equivalents of 2,4-di-tert-butylphenol with acetone induced by
boron fluoride etherate. The C2-symmetry of this “almebate” anion implies helical chirality.
Substantial steric shielding of the aluminate core results in high stability towards aqueous bases.
Almebate salts hydrolyze in aqueous acetic acid at room temperature within days.
Tetraphenylphosphonium and tetrabutylammonium salts have been prepared by salt metatheses.
Sodium almebate can be activated thermally, generating electrophilic sodium(I) with a low
coordination number. It eliminates chloride ligands from an N-heterocyclic carbene gold chloride
complex, proven by a single-crystal X-ray diffraction study. The barrier of molecular
racemization by a degenerate rearrangement of the intrinsically chiral almebate anion in acetone
1
has been determined to be ꢀG‡ 53.6 ± 2 kJ mol-1 from four H NMR coalescence phenomena
(500 MHz; 258 K, 273 K, 283 K, 290 K).
Keywords: Aluminate, fluxional molecule, gold, helical chirality, racemization, salt metathesis,
sodium, symmetry, weakly coordinating anion
Introduction
Weakly coordinating anions (WCAs)1 have attracted wide interest due to their use in lithium
batteries and as counterions for highly electrophilic cations. Examples for using WCAs in
applied chemistry are the lithium(I)-catalyzed reactions2-5 as well as transition metal-catalyzed
reactions6-9. Applications comprise olefin polymerization10 and electrochemistry.11-13 There are
numerous examples for the stabilization of reactive cationic electrophiles.14-16
Most weakly coordinating anions, however, contain either C-Cl bonds or persistent C-F-
bonds. These substituents are beneficial for characteristics such as low nucleophilicity, chemical
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inertness, and high solubility in polar organic solvents. The most prominent anions are
tetrakis(pentafluorophenyl)borate [B(C6F5)4]-,17 tetrakis[3,5-bis(trifluoromethyl)phenyl]borate
[B(ArF)4]-,18,19 a “chemically inert but explosive” carboranate with twelve CF3 substituents,20
tetrakis(trifluoromethyl)borate,21 and the perfluorinated alkoxyaluminate {Al[OC(CF3)3]4}-,22 as
presented in Figure 1.
Figure 1. Well-known weakly-coordinating anions (WCA): BArF20,17 Kobayashi’s and
Brookhart’s BArF24,18,19 the King-Michl carboranate,20 Willner’s [B(CF3)4]-,21 and Krossing’s
“Teflon ball” PFTB.22
There are only few examples of halogen-free lipophilic anions with low nucleophilicity. One
class of these halogen-free anions are carboranates such as the [CB11H12]- anion (Figure 2).23
These carboranates are unstable towards oxidation, and their expensive synthesis limits their use.
Another halogen-free anion was recently reported by our group.24 This “altebate” anion (Figure
2) is based on an AlO4 core, which is sterically shielded by two tetra-tert-butyl substituted 2,2’-
biphenolate ligands. The major benefits of this compound are the ease of its preparation and the
low costs of the starting materials. However, its instability in protic environments limits the
number of potential applications.
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Me3C
CMe3
Me3C
CMe3
H
Me
H
C
Me
C
B
H
B
H
H
Me
Me
Me
Me3C
Me3C
CMe3
CMe3
Me3C
Me3C
CMe3
CMe3
O
O
O
O
O
O
B
B
B
B
B
H
H
B
H
B
B
B
B
H
Me
B
B
Me
B
B
B
Al
Me
O
O
B
B
B
B
B
B
H
Me
Me
H
Me
H
Me
Me3C
CMe3
Me3C
CMe3
Figure 2. Lipophilic halogen-free anions: carboranates [CB11H12]-,23 [CB11Me12]-,25,26
“altebate”,24 and “bortebate”.27
We aimed at preparing a cost-effective, lipophilic, halogen-free and therefore
environmentally benign anion with weakly coordinating character and adequate hydrolytic
stability. Herein we report the synthesis of a new aluminate based on two C1-linked phenols with
alkali tetrahydrido alanates MAlH4 (M = Li, Na). According to the naming of the altebate anion,
we propose the name “almebate” for the new anion, derived from aluminate with
dimethylmethylene linker and eight tert-butyl groups (Figure 3).
Figure 3. New aluminate anion (“almebate”) in a flat and in a stereochemical representation.
Results and Discussion
Synthesis of the bisphenol
We synthesized 6,6´-(propane-2,2-diyl)bis(2,4-di-tert-butylphenol) (1)28,29 following an
optimized protocol of Meier et al. for less sterically crowded bisphenols.30 Acetic acid as solvent
was not mandatory to obtain a homogeneous reaction mixture. Thus, neat acetone and boron
trifluoride etherate were used stoichiometrically as reactants (Scheme 1). After a reaction time of
16 hours, the crude product was recrystallized from diethyl ether / methanol. Single-crystals
suitable for a X-ray diffraction study were obtained by recrystallization in acetic acid (Figure 4).
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OH
OH
OH
Me2
C
Me3C
Me3C
CMe3
O
BF3(OEt2)
neat, 16 h, r.t.
50 %
Me
Me
CMe3
CMe3
CMe3
1
Scheme 1. Synthesis of the ligand precursor 1.
Other Lewis acids result in lower yields: By using concentrated sulfuric acid, the isolated
yield decreased to 12 %. AlCl3 or operating at higher temperature lead to dealkylation of ortho-
tert-butyl substituents as described by Sartori et al.31
Figure 4. ORTEP ellipsoid-and-stick model of the single-crystal X-ray structure of 6,6´-
(propane-2,2-diyl)bis(2,4-di-tert-butylphenol) (1). Color code: C black, H gray, O red.32,33
Synthesis of alkali metal almebate complexes
The synthesis of the new compounds Li(thf)2 almebate and Na(thf)2 almebate was inspired by the
standard literature procedure of the Pan group.34 Instead of AlMe3, we used LiAlH4 and NaAlH4
for the synthesis of the aluminate complexes (Scheme 2). The reaction time was increased to
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48 hours due to the higher steric demand of the substrate 6,6´-(propane-2,2-diyl)bis(2,4-di-tert-
butylphenol) (1) as compared to Pan’s ethylidenebis-2,2´-(4,6-di-tert-butylphenol). Our attempts
to synthesize the analogous borate species (hypothetical “bormebates”) from MBH4 have not
been successful so far. Apparently, the lower polarity of the B-H bond and the smaller radius of
the boron center lead to significantly higher reaction barriers and increased steric repulsion.
Scheme 2. Synthesis of Li(thf)2 almebate (2) and Na(thf)2 almebate (3).
As a result of almebate’s symmetry, we expected four CH signal sets in the aromatic region
of the spectrum and six singlets in the aliphatic region. The almebate anion comprises two
diastereotopic pairs of homotopic methyl groups and two constitutionally different
1
diastereomeric pairs of two homotopic tert-butyl groups. The H NMR spectrum of sodium
almebate in acetone-d6 at room temperature provides evidence for the aluminate’s fluxionality:
the proton signal of the propane-2,2-diyl bridge is a broad coalescing peak with at a chemical
shift of 1.85 ppm. A signal at 1.26 ppm with the intensity of four tert-butyl groups features a low
full width at half maximum. The other tert-butyl signal at 1.06 ppm is again very broad,
indicating a signal coalescence due to an intramolecular exchange process (Figure 5 and vide
infra).
The thf ligands at the sodium ion can be easily exchanged by acetone. Dissolving Na(thf)2
almebate (3) in acetone and removal of the solvent in vacuo quantitatively yields Na(acetone)2
almebate (4) (Scheme 4). Single-crystals were obtained by recrystallization from
acetone/pentane (Figure 6). The complex crystallizes in the monoclinic space group C2/c, and
the adduct features C2 symmetry. The aluminium’s coordination geometry is distorted
tetrahedral. One pair of oxygen atoms bridges a sodium atom and an aluminium atom, the other
pair of phenolates is coordinated only to the aluminium center.
The coordinated solvent molecules can be eliminated thermally. By heating Na(thf)2
almebate (3) at 150 °C and ca. 1 mbar for 24 hours, the thf ligands are removed quantitatively. In
analogy to acetone complex 4, we expect that sodium in the resulting aluminium adduct 5 is
coordinated by two oxygen atoms of the almebate’s backbone and by two methyl groups via
agostic interactions (Scheme 3). The sodium atom’s low coordination number in structure 5 is
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synthetically advantageous for the abstraction of halide ligands from uncharged transition metal
precatalysts (vide infra).
Figure 5. 1H NMR spectrum of Na(thf)2 almebate (3) in acetone-d6 (300.5 MHz, 300 K).
Scheme 3. Substitution (left) and elimination (right) of ligated solvent molecules.
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Figure 6. ORTEP ellipsoid-and-stick model of the single-crystal X-ray structure of the acetone
adduct of sodium almebate (4). Color code: C black, H gray, O red, Al blue, Na yellow. Selected
bond lengths (Å): Al-O2 1.7181(18), Al-O1 1.7742(18), Na-O2 2.344(2), Na-O1 2.244(3), Na-
C1 2.830(17).32,33
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Salt metathesis and slow hydrolysis
In salt metathesis reactions, the lithium or sodium cation can easily be exchanged by organic
cations. We synthesized tetraphenylphosphonium almebate (6) with 95 % yield and tetrabutyl-
ammonium almebate (7) with 99 % yield (Scheme 4). Single-crystals of tetraphenylphosphonium
almebate were obtained by recrystallization from dichloromethane/pentane (Figure 7).
Compound 7 is structurally characterized as a salt with separated ions. Tetraphenylphosphonium
almebate is stable in a mixture of THF-d8 and D2O (4:1) at room temperature for several days.
The rate of hydrolysis amounts to 2.5 % per week. The same result was observed with a tenfold
excess of NaOH or NEt3 in the same solvent system. In weakly acidic media, however, the
hydrolysis proceeded faster than in the presence of base. Nevertheless, hydrolysis was much
slower than with the analogous altebate salts that do not comprise the 2,2-propanediyl linker.19 In
a mixure of THF-d8 and 1 M acetic acid in D2O, 10 % of the almebate was decomposed after
21 hours and 45 % after a week. Upon addition of trifluoroacetic acid, the decomposition
proceeded rapidly and the almebate anion was completely hydrolyzed within 30 minutes. This is
consistent with increased steric hindrance towards nucleophilic attack at the aluminium center,
while protonation-induced hydrolysis remains favorable.
Scheme 4. Salt metathesis of sodium almebate with phosphonium and ammonium bromides.
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Figure 7. ORTEP ellipsoid-and-stick model of the single-crystal X-ray structure of
tetraphenylphosphonium almebate (6). Color code: C black, H gray, O red, Al blue, P
magenta.32,33
Degenerate rearrangement and five NMR coalescences
The inversion of the bowl-shaped bisphenolate aluminium fragment of the intrinsically chiral
almebate anion leads to the formation of its enantiomer (Scheme 5). The almebate anion does not
comprise stereogenic centers, but it features helical chirality.35,36 The degenerate racemization
reaction proceeds rapidly at room temperatures and below. We used the tetraphenylphosphonium
salt 6 to rule out the influence of electrophilic alkali metal cations. We observed five coalescence
1
phenomena by temperature-dependent H NMR spectroscopy in acetone-d6: two pairs of
aromatic CH fragments, two CMe2 groups and two pairs of tert-butyl substituents interconvert
with an experimentally determined Gibbs free activation energy of ꢀG‡ 53.6 ± 2 kJ mol-1 (Table
1).
We were unable to use one particular coalescence of two pairs of aromatic CH fragment
signals for the calculation of the racemization barrier due to the signals’ apparently temperature-
dependent chemical shifts even at low temperatures (Figure 8). Each of the five coalescence
temperatures was determined at 500.13 MHz with an accuracy of 5 K, resulting in a computed
barrier with low or no temperature-dependence (Figures 9, 10, and 11).
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Scheme 5. Molecular origin of almebate’s dynamic properties: racemization by a degenerate
rearrangement.
Figure 8. Front view on a temperature-dependent series of 1H NMR spectra, aromatic signals of
tetraphenylphosphonium almebate 6 in acetone-d6, 500.13 MHz.
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Figure 9. Side view on a temperature-dependent series of 1H NMR spectra, aromatic signals of
tetraphenylphosphonium almebate 6 in acetone-d6, 500.13 MHz.
Figure 10. Side view on a temperature-dependent series of 1H NMR spectra, CMe2 signals of
tetraphenylphosphonium almebate 6 in acetone-d6, 500.13 MHz.
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Figure 11. Side view on a temperature-dependent series of 1H NMR spectra, tert-butyl signals of
tetraphenylphosphonium almebate 6 in acetone-d6, 500.13 MHz.
1
Table 1. Coalescence temperatures, interconverting molecular fragments, H NMR chemical
shift differences at 193 K, calculated exchange rates at the respective coalescence temperature,
and Gibbs free activation barriers at the respective coalescence temperature for almebate salt 6 in
acetone-d6
Tc (± 5 K)
Fragment
Chemical shift
difference [ppm]
T-dependent
0.107
kc [s-1]
ꢀGc‡ [kJ mol-1]
(± 1 kJ mol-1)
250 K
258 K
273 K
283 K
290 K
CHaromatic
tert-Bu
CHaromatic
CMe2
-
-
119
378
668
881
52.6
53.2
53.9
54.6
0.341
0.602
0.794
tert-Bu
Based on its facile rearrangement-racemization, the almebate ion or its derivatives might be
suitable for applications in supramolecular chemistry and for chiroptic materials.37
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Halide ligand elimination
In this section, we provide evidence for the weakly-coordinating nature of the almebate anion.
Cationic gold complexes are potent electrophiles and excellent catalysts for alkyne
transformation reactions.38-43 The synthesis of IPrAu(SMe)2 almebate (8) has been accomplished
in high yield by the reaction of the “activated” sodium almebate (5) with IPrAuCl (Scheme 6). In
a single-crystal X-ray structure, the almebate’s almost tetrahedral AlO4 core features Al-O bond
lengths in the range of 1.734 - 1.741 Å (Figure 12).
Scheme 6. Chloride ligand elimination by thermally activated sodium almebate 5.
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Figure 12. ORTEP ellipsoid-and-stick model of the single-crystal X-ray structure of
IPrAu(SMe2) almebate 8. Color code: C black, H gray, N blue, O red, S yellow, Au gold, Al
cadet blue. Selected bond lengths (Å): Au-S 2.293(5), Au-C1 1.997(6), N1-C1 1.343(8), N2-C1
1.348(7), Al-O1 1.733(4), Al-O2 1.733(4), Al-O3 1.737(4), Al-O4 1.741(4). Selected bond
angles: S1-Au-C1 176.0°.32,33
Limitations of increasing the steric demand
We tried to formally exchange the CMe2 linker by more bulky fragments, to further increase the
steric shielding of the anionic AlO4 core. The intention was to improve the hydrolytic stability, to
weaken the aluminate’s nucleophilicity and to limit its ability to coordinate to electrophilic metal
cations. Other ketone substrates such as pentan-3-one or cyclopentanone, however, did not yield
the desired bisphenols (Scheme 7), presumably due to inhibitive steric crowding in the
intermediates.44 Even a multi-step approach with isolated 1-hydroxyalkylphenol intermediates
failed. Apparently, the accumulation of bulky substituents is not only a cause for higher
activation energies in the final step of the synthesis of lipophilic aluminates, but already poses a
challenge for the preparation of their respective ligand starting materials.
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BF3(OEt2)
or H2SO4, r.t.
pentanꢀ3ꢀone or cyclopentanone
or hexachloroacetone
OH
OH
OH
R2
C
Me3C
Me3C
CMe3
2
one-pot approach
CMe3
CMe3
OH
CMe3
multi-step
approach
Br2 , CH2Cl2
0°C, 1 h
BF3(OEt2)
or
Me3C
H2SO4
CMe3
1.) nꢀBuLi, Et2O,
ꢀ78°C to 0°C
OH OH
OH
R
R
Me3C
Me3C
Br
2.)
O
R
R
CMe3
CMe3
Et2O, ꢀ 78°C to r.t.
R = Et
49s% 9
R2 = (CH2)4 34<% 10
88 %
Scheme 7. Two failed strategies for the synthesis of even more sterically shielded bisphenols.
Summary
Salts of a new halogen-free, thermally stable aluminate anion have been synthesized from cost-
efficient starting materials. The dimethylmethylene units that each link two phenolate ligand
fragments enforce a C2 symmetry of the aluminate anion. The dynamics of the racemization-
rearrangement have been determined quantitatively by a temperature-dependent NMR study. The
hydrolytic stability is significantly higher than that of other lipophilic aluminates described in
literature. The thermally activated sodium salt readily eliminates chloride ligands from transition
metal chloride complexes. The “almebate” anion coordinates to alkali metal cations, but has a
low affinity for sterically demanding electrophiles such as a cationic NHC gold complex.
Experimental Section
General procedures. Chemicals were supplied by Acros, Aldrich, and TCI, and were used
without further purification. Reactions involving air-sensitive reagents were carried out under N2
or argon by using standard Schlenk techniques. Solvents were dried in an Mbraun MB SCS-800
solvent purification system. NMR spectra were recorded at 300 K by using Bruker ARX-250,
Bruker Avance 300, Bruker Avance 500, or Bruker Avance 600 spectrometers. Chemical shifts
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are reported in ppm relative to TMS, and were determined by reference to the 13C or 1H residual
solvent peaks. Melting points were determined by using a Gallenkamp hot-stage microscope.
THF was dried by an MBRAUN solvent purification system MB SPS-800.
6,6´-(Propane-2,2-diyl)bis(2,4-di-tert-butylphenol) (1).28,29 Under inert gas conditions, boron
trifluoride diethyl etherate (5.6 ml, 46 mmol, 1 eq) was slowly added to a solution of 2,4-di-tert-
butylphenol (19.0 g, 92 mmol, 2 eq) in acetone (2.6 g, 46.0 mmol, 1 eq) at room temperature.
The reaction mixture was stirred at room temperature for 16 h. Methanol at 0°C was added and
stirred for 1 h. The resulting suspension was filtered, and the crude product was washed with
methanol at 0°C. The crude product was purified by recrystallization from acetic acid.
Colorless crystals, yield 50 %, 10.5 g, mp: 191 °C (literature 174-175 °C)28; IR (KBr cm-1): ῦ
3465, 2963, 2909, 2872, 1467, 1438, 1418, 1392, 1362, 1331, 1296, 1272, 1250, 1220, 1199,
1
1163, 876, 821, 796, 771. H NMR (600.2 MHz, CDCl3): δΗ 1.30 (18H, s, C(CH3)3) 1.37 (18H,
4
s, C(CH3)3), 1.75 (6H, s, CH3), 5.00 (2H, s, OH), 7.32 (2H, d, JHH 2.3 Hz, HPh), 7.43 (2H, d,
13
4JHH 2.3 Hz, HPh). C{1H} NMR (75.4 MHz, CDCl3): δC 28.9 (C(CH3)2) 29.5 (C(CH3)3), , 31.7
(C(CH3)3), 34.7 (q- C(CH3)3), 35.3 (q- C(CH3)3), 40.4 (q- C(CH3)2), 119.7 (CPh), 124.0 (CPh),
130.7 (q-CPh) 137.4 (q-CPh), 143.0 (q-CPh) 151.8 (q-CPh) ppm. HR-MS (EI), m/z (%) for
+
C31H48O2 : calcd. 452.3654, found 452.3662; Anal. Calcd. for C31H48O2 C 82.24; H 10.69%;
Found: C 82.05; H 10.72%. CCDC 958107.
Bis(tetrahydrofuran)lithium almebate (2). Under inert gas a solution of 6,6´-(propane-2,2-
diyl)bis(2,4-di-tert-butylphenol) (1) (13.6 g, 30.0 mmol, 2 eq) in 90 ml dry THF was added
slowly to a solution of NaAlH4 (0.87 g, 15.0 mmol) in 10 mL dry THF at room temperature. The
reaction mixture was heated under reflux conditions for 2 d, and filtered through Celite. After
removal of the solvent in vacuo, the resulting colorless foam was recrystallized from
THF/pentane.
Colorless powder, yield: 75 %, 1.60 g, mp: 141°C. IR (KBr cm-1): ῦ 3472, 2957, 2870, 1623,
1477, 1435, 1390, 1360, 1331, 1295, 1271, 1237, 1202, 1178, 1044, 880, 830, 783, 775, 660,
1
611. H NMR (300.5 MHz, acetone-d6): δ 0.72-1.32 (36 H, br s, C(CH3)3) 1.25 (36 H, s,
C(CH3)3), 1.75-1.82 (8 H, m, thf), 1.67- 2.01 (12H, br s, C(CH3)2), 3.56-3.68 (m, 8 H, thf), 6.82-
6.95 (4H, br s, HPh), 7.27 (4H, d, 4JHH 2.5 Hz, HPh)) ppm. 13C{1H} NMR (75.4 MHz, acetone-d6):
δ 26.1 (thf), 28.9 (C(CH3)3), 29.5 (C(CH3)3) 32.3 (C(CH3)2), 34.5 (q-C(CH3)3) 35.6 (q-C(CH3)3),
42.8 (q-C(CH3)2), 68.0 (thf), 120.8 (CPh), 123.2 (CPh), 136.1 (q-CPh), 137.3 (qCPh), 138.6 (q-CPh),
-
157.2 (qCPh) ppm. HR-MS (ESI), m/z (%) for C62H92AlO4 calcd. 927.6811, found 927.6791.
Bis(tetrahydrofuran)sodium almebate (3). Under inert gas a solution of 6,6´-(propane-2,2-
diyl)bis(2,4-di-tert-butylphenol) (1) (13.6 g, 30.0 mmol, 2 eq) in 90 ml dry THF was added
slowly to a solution of NaAlH4 (0.87 g, 15.0 mmol, 1 eq) in 10 mL dry THF at room
temperature. The reaction mixture was heated under reflux conditions for 2 d, and filtered
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through Celite. After removal of the solvent in vacuo, the resulting colorless foam was
recrystallized from THF/pentane.
Colorless powder, yield: 70 %, 11.5 g, mp: 237°C decomp. with gas evolution. IR (KBr cm-1): ῦ
3472, 2961, 2870, 1621, 1477, 1434, 1389, 1360, 1271, 1236, 1202, 1179, 1049, 885, 832, 783,
1
775, 751, 660, 610. H NMR (300.5 MHz, acetone-d6): δ 0.65-1.32 (36 H, br s, C(CH3)3) 1.26
(36 H, s, C(CH3)3), 1.75-1.82 (8 H, m, thf), 1.67- 2.01 (12H, br s, C(CH3)2), 3.56-3.68 (m, 8 H,
4
13
thf), 6.78-6.92 (4H, br s, HPh), 7.27 (4H, d, JHH 2.5 Hz, HPh)) ppm. C{1H} NMR (75.5 MHz,
acetone-d6): δ 26.1 (thf), 28.9 (C(CH3)3), 29.5 (C(CH3)3) 32.3 (C(CH3)2), 34.5 (q-C(CH3)3) 35.6
(q-C(CH3)3), 42.8 (q-C(CH3)2), 68.0 (thf), 120.8 (CPh), 123.3 (CPh), 136.1 (q-CPh), 137.3 (qCPh),
-
138.7 (q-CPh), 157.2 (q-CPh) ppm. HR-MS (ESI), m/z (%) for C62H92AlO4 calcd. 927.6811,
found 927.6793. Anal. Calcd. for C70H108AlNaO6 C 76.74; H 9.94%; found C 76.52; H 9.54%.
Bis(acetone)sodium almebate (4). The ligand exchange at sodium was performed by dissolving
bis(tetrahydrofuran)sodium
bis[6,6´-(propane-2,2-diyl)bis(2,4-di-tert-butylphenolato)-
]aluminate(III) (3) in acetone. Removal of the solvent in vacuo and recrystallization from
acetone/pentane gave the product quantitatively.
Colorless powder, yield: 99 %, 1.51 g, mp: 253°C. IR (KBr cm-1): ῦ 3413, 2961, 2868, 1710
(C=O), 1620, 1478, 1433, 1389, 1360, 1271, 1233, 1202, 1157, 928, 878, 832, 783, 776, 660. 1H
NMR (300.5 MHz, acetone-d6): δ 0.65-1.31 (36 H, br s, C(CH3)3) 1.25 (36 H, s, C(CH3)3), 1.75-
4
2.08 (12H, br s, C(CH3)2), 2.08 (s, 6 H,CO(CH3)2), 6.80-6.95 (4H, br s, HPh), 7.27 (4H, d, JHH
13
2.5 Hz, HPh)) ppm. C{1H} NMR (75.4 MHz, acetone-d6): δ 30.2 (CH3), 30.5 (C(CH3)3), 31.4
(C(CH3)3), 32.2 (C(CH3)2), 34.4 (q-C(CH3)3) 35.4 (q-C(CH3)3), 42.6 (q-C(CH3)2), 120.6 (CPh),
123.1 (CPh), 136.0 (q-CPh), 137.1 (qCPh), 138.5 (q-CPh), 157.1 (q-CPh), 209.9 (C=O) ppm. HR-MS
-
(ESI), m/z (%) for C62H92AlO4 calcd. 927.6811, found 927.6791. Anal. Calcd. for
C70H108AlNaO6:C 76.51; H 9.82%; found: C 76.44; H 10.02%. CCDC 958109.
Sodium almebate (5). This compound was obtained by elimination of the thf ligands at 120 °C
and 0.1 mbar over 24 h in the solid state. The product was obtained as a colorless solid in
quantitative yield.
Colorless powder, mp: 274°C. IR (KBr cm-1): ῦ 3472, 2961, 2870, 1621, 1477, 1434, 1389,
1
1360, 1271, 1236, 1202, 1179, 1049, 885, 832, 783, 775, 751, 660, 610. H NMR (600.2 MHz,
acetone-d6): δ 0.4-1.33 (72H, m, C(CH3)3), 1.62- 2.04 (12H, br s, C(CH3)2), 6.78-6.96 (4H, br s,
HPh), 7.27 (4H, d, 4JHH 2.4 Hz, HPh)) ppm. 1H-NMR (400.1 MHz, CD2Cl2: δ 0.10-1.05 (18 H, br
s, C(CH3)3), 1.28 (36 H, s, C(CH3)3), 1.06-1.72 (18 H, br s, C(CH3)3), 1.75- 2.45 (12H, br s,
C(CH3)2), 6.82-7.18 (4H, br s, HPh), 6.82-7.18 (4H, br s, HPh)) ppm. 13C{1H} NMR (150.9 MHz,
acetone-d6): δ 30.3 (C(CH3)3), 31.5 (C(CH3)3) 32.3 (C(CH3)2), 34.5 (q-C(CH3)3) 35.5 (q-
C(CH3)3), 42.7 (q-C(CH3)2), 120.7 (CPh), 123.2 (CPh), 136.0 (q-CPh), 137.2 (qCPh), 138.5 (q-CPh),
157.1 (q-CPh) ppm. 13C{1H}-NMR (100.6 MHz, CD2Cl2): δ 29.5-30.5 (C(CH3)3 + (C(CH3)2), 31.9
(C(CH3)3) 34.5 (q-C(CH3)3) 35.3 (br s, q-C(CH3)3), 43.0 (q-C(CH3)2), 124.4 (2 C, br s, CPh),
136.9 (br s, q-CPh),, 138.5 (q-CPh), 139.5 (br, s, q-CPh), 153.7 (br s, q-CPh) ppm.
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-
HR-MS (ESI), m/z (%) for C62H92AlO4 calcd. 927.6811, found 927.6797.
Tetraphenylphosphonium almebate (6). Under inert gas, tetraphenylphosphonium bromide
(100 mg, 0.25 mmol) in 4 mL dichloromethane was slowly added to a solution of
bis(tetrahydrofuran)sodium
bis[6,6´-(propane-2,2-diyl)bis(2,4-di-tert-butylphenola-
to)]aluminate(III ) (4) (270 mg, 0.25 mmol) in 4 mL dichloromethane whilst stirring. A colorless
solid (NaBr) precipitated. After stirring for 1 h at room temperature, the suspension was filtered
through Celite and the solvent was removed in vacuo. Recrystallization from dichloromethane /
diethyl ether gave the title product.
Colorless solid, yield: 95%, 300 mg, mp >300°C. IR (KBr): ῦ 2950, 2903, 2866, 1477, 1437,
1
1386, 1358, 1296, 1271, 1238, 1201, 1109, 876, 853, 786, 779, 724, 619, 528. H NMR (300.5
MHz, acetone-d6): δ 0.72-1.41 (36 H, br s, C(CH3)3), 1.26 (36 H, s, C(CH3)3) 1.65 – 2.05 (12H,
4
br s, C(CH3)2), 6.78-6.96 (4H, br s, HPh), 7.27 (4H, d, JHH= 2.2 Hz, HPh), 7.80-7.90 (16 H, m,
HPh), 7.94-8.04 (4H, m, HPh) ppm. 13C{1H} NMR (150.9 MHz, acetone-d6): δ 31.8 (C(CH3)3),
32.4 (C(CH3)2), 34.7 (q-C(CH3)3), 35.8 (q-C(CH3)3), 42.9 (q-C(CH3)2), 118.5 (q-CPh), 119.7 (q-
CPh), 121.0 (CPh), 123.4 (CPh), 131.5 (q-CPh), 131.6 (q-CPh), 135.7 (CPh), 136.3 (CPh), 136.5 (CPh),
136.6 (CPh), 137.5 (CPh), 138.8 (CPh), 157.4 (q-CPh) ppm. 31P NMR (121 MHz, acetone-d6): 23.0
-
ppm. HR-MS (ESI), m/z (%) for C62H92AlO4 calcd. 927.6816, found 927.6809; for C24H20P+
calcd. 339.1297, found 339.1293. Anal. Calcd. for C86H112AlO4P: C 81.48, H 8.90; found: C
81.43, H 9.02. CCDC 958108.
Tetrabutylammonium almebate (7). Under inert gas tetrabutylammonium bromide (80 mg,
0.25 mmol) in
4
mL dichloromethane was slowly added to
a
solution of
bis(tetrahydrofuran)sodium
bis[6,6´-(propane-2,2-diyl)bis(2,4-di-tert-butylphenola-
to)]aluminate(III) (4) (270 mg, 0.25 mmol) in 4 mL dichloromethane whilst stirring. A colorless
solid (NaBr) precipitated. After stirring for 1 h at room temperature, the suspension was filtered
through Celite and the solvent was removed in vacuo. Recrystallization from diethyl ether /
pentane gave the product.
Colorless solid, yield: 99%, 290 mg, mp 210 °C. IR (KBr): ῦ 3423, 2961, 1478, 1433, 1386,
1
1358, 1295, 1271, 1238, 1201, 1157, 925, 875, 806, 786, 778, 767, 661, 619. H NMR
(600.2 MHz, acetone-d6): δ (0.72-1.41 (36 H, br s, C(CH3)3) 0.98 (12H, t, 3JHH 7.4 Hz, CH3) 1.25
(36 H, s, C(CH3)3), 1.40-1.48 (8 H, m, CH2), 1.69-2.02 (m, 20 H, CH2, C(CH3)2), 3.44-3.49 (8 H,
m, CH2), 6.80-6.99 (4H, br s, HPh), 7.28 (4H, d, 4JHH 2.6 Hz, HPh) ppm. 13C{1H} NMR (75 MHz,
acetone-d6): δ 13.8 (CH3), 20.3 (CH2), 24.4, (CH2), 31.6 (C(CH3)3), 32.4 (C(CH3)2), 34.6
(q-C(CH3)3) ), 35.6 (q- C(CH3)3), 42.8 (q-C(CH3)2), 59.4 (N-CH2), 120.8 (CPh), 123.2 (CPh),
136.1 (q-CPh), 137.4 (q-CPh), 138.7 (q-CPh), 157.3 (q-CPh) ppm. HR-MS (ESI), m/z (%) for
-
C62H92AlO4 calcd. 927.6816, found 927.6810; for C16H36N+ calcd. 242.2842, found 282.2840.
Anal. Calcd. for C78H112AlNO4 C 80.01, H 11.02; N 1.20 found: C 79.56, H 11.05, N 1.22.
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[Bis-1,3-(2,6-diisopropylphenyl)imidazole-2-ylidene](dimethylsulfide)gold almebate (8).
Under inert gas a solution of mono(acetone)sodium bis[6,6´-(propane-2,2-diyl)bis(2,4-di-tert-
butylphenolato]aluminate(III) (5) (88 mg 0.08 mmol) in 10 mL dry dichloromethane was added
to a solution of (IPr)AuCl (50 mg, 0.09 mmol) and dimethylsulfide (0.1 ml, 0.14 mmol) at room
temperature whilst stirring. A colorless solid (NaCl) precipitated. The resulting suspension was
stirred for 1 h at room temperature. The suspension was filtered through Celite and the solvent
was removed under vacuum. Recrystallization from dichloromethane / pentane gave the title
product.
Colorless solid, yield: 91 %, 115 mg, mp 198°C. IR (KBr): ῦ 3445, 3158, 3112, 3075, 2961,
2869, 1595, 1550, 1458, 1433, 1386, 1358, 1295, 1271, 1238, 1201, 1180, 1157, 1129, 1059,
1
1033, 987, 948, 925, 875, 856, 804, 786, 778, 758, 704, 661, 619. H NMR (600.2 MHz,
acetone-d6): δ 0.39-1.02 (36 H, br s, C(CH3)3) 1.17 - 1.21 (60 H, m, C(CH3)3, Me), 1.51-2.03
3
(12H, br s, C(CH3)2), 2.13 (6 H, s, S(CH3)2), 2.38 (4H, pseudo-sept, JHH 6.9 Hz, CH(CH3)2),
6.76-6.85 (4H, br s, HPh), 7.16 (4H, d, 4JHH= 2.6 Hz, HPh), 7.20 (2H, s, NHC=CHN), 7.29 (4H, d,
3
13
3JHH 7.8 Hz, HPh), 7.50 (2H, t, JHH= 7.8 Hz, HPh) ppm. C{1H} NMR (75 MHz, acetone-d6): δ
24.2 (CH(CH3)2), 25.1 (CH(CH3)2), 29.8 (CH(CH3)2), 30.5 (S(CH3)2), 31.8 (C(CH3)3), 32.6 (2 C,
C(CH3)2, (C(CH3)3), 34.7 (q-C(CH3)3), 35.8 (q-C(CH3)3), 43.0 (q-C(CH3)2), 120.8 (CPh), 123.2
(CPh), 125.4 (CPh), 126.4 (CPh), 132.2 (CPh), 136.3 (CPh), 137.6 (CPh), 138.9 (CPh), 146.9, (q-CPh),
-
157.4 (q-CPh), 177.4 (N2CAu) ppm. HR-MS (ESI), m/z (%) for C62H92AlO4 calcd. 927.6816,
found 927.6792; for C29H42NAuS+ calcd. 647.2728, found 647.2720. Anal. Calcd. for
C91H134AlAuN2O4 C 69.30, H 8.63; N 1.78 S 2.03 found: C 69.09, H 8.70, N 1.78 S 2.10. CCDC
958110.
Synthesis of 2-bromo-4,6-di-tert-butylphenol. The synthetic protocol of Akai et al. was used.45
A solution of bromine (0.50 mL, 9.7 mmol) in dichloromethane (10 mL) was added dropwise to
a solution of 2,4-di-tert-butylphenol in dichloromethane (2 mL) at 0°C. After stirring of the
orange solution for an additional hour at this temperature, water was added and the phases were
separated. The organic layer was washed six times with water and dried over anhydrous
magnesium sulfate. The solvent was removed by evaporation under reduced pressure and
purified by silica gel column chromatography with pentane as eluent.
Colorless powder, yield 88 %, 2.4 g, mp 63 °C. IR (KBr cm-1): ῦ 3509, 2997, 2965, 2870, 1786,
1567, 1476, 1447, 1403, 1394, 1364, 1334, 1280, 1253, 1202, 1178, 1137, 1090, 934, 896, 870,
840, 820, 745, 714, 645, 607, 545, 511. 1H NMR (300.5 MHz, CDCl3): δΗ 1.28 (9H, s, C(CH3)3),
1.40 (9H, s, C(CH3)3), 5.64 (1H, s, OH), 7.24 (2H, d, 4JHH 2.4 Hz, HPh), 7.32 (2H, d, 4JHH 2.4 Hz,
13
HPh). C{1H} NMR (75.5 MHz, CDCl3): δC 29.5 (C(CH3)3), 31.6 (C(CH3)3), 34.5 (q-C(CH3)3),
35.7 (q-C(CH3)3), 112.0 (q-CPh), 123.8 (CPh), 126.4 (CPh), 136.8 (q-CPh), 143.8 (q-CPh), 148.1 (q-
CPh) ppm. HR-MS (ESI), m/z (%) for C14H20BrO+: calcd. 283.0703, found 283.0702.
Synthesis of 2,4-di-tert-butyl-6-(3-hydroxypent-3-yl)phenol (9). In a flame dried flask 3.3 mL
n-Buli (1.6 M in hexane, 5.4 mmol, 2 eq) was added at -78°C to a solution of 2-bromo-4,6-di-
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tert-butylphenol (0.70 g, 2.7 mmol, 1 eq) in 50 mL of dry diethyl ether. The solution was
warmed to 0°C and stirred for 30 min. After re-cooling to -78°C a solution of 0.25 g pentan-2-
one (2.8 mmol, 1.1 eq.) in 6 mL diethyl ether was added dropwise. After warming to room
temperature and stirring for two hours saturated aqueous ammonium chloride solution was
added. Additional diethyl ether was added the organic layer was separated. The organic layer
was washed three times with water and saturated aqueous sodium chloride solution, dried over
anhydrous magnesium sulfate and filtered. The solvent was removed by evaporation under
reduced pressure and further purified by column chromatography on a silica gel column
Light yellow powder, yield 49 %, 0.39 g, m.p. 55 °C. IR (KBr cm-1): ῦ 3424, 3115, 2961, 2880,
1768, 1614, 1440, 1391, 1373, 1361, 1278, 1234, 1202, 1159, 1135, 1122, 961, 879, 816, 764,
1
3
722. H NMR (300.5 MHz, CD2Cl2): δΗ 0.89 (6H, t, JHH 7.4 Hz, CH2CH3), 1.28 (9H, s,
4
C(CH3)3), 1.41 (9H, s, C(CH3)3), 1.83 -2.06 (4H, m, CH2), 2.33 (1H, s, OH), 6.83 (2H, d, JHH
2.4 Hz, HPh), 7.20 (2H, d, JHH 2.4 Hz, HPh), 9.58 (1H, s, OH). 13C{1H} NMR (75.5 MHz,
4
CDCl3): δC 8.2 (CH3), 29.9 (C(CH3)3), 31.7 (C(CH3)3), 33.7 (q-C(CH3)3), 34.4 (q-C(CH3)3), 35.4
(CH2), 82.9 (1H, s, C-OH), 122.2 (CPh), 123.1 (CPh), 126.2 (q-CPh), 136.7 (q-CPh), 140.2 (q-CPh),
+
154.3 (q-CPh) ppm. HR-MS (ESI), m/z (%) for C19H31O2 : calcd. 291.2329, found 291.2327.
Synthesis of 2,4-di-tert-butyl-6-(1-hydroxycyclopentyl)phenol (10). In a flame dried flask 3.3
mL n-Buli (1.6 M in hexane, 5.4 mmol, 2 eq) was added at -78°C to a solution of 2-bromo-4,6-
di-tert-butylphenol (0.70 g, 2.7 mmol, 1 eq) in 50 mL of dry diethyl ether. The solution was
warmed to 0°C and stirred for 30 min. After re-cooling to -78°C a solution of 0.24 g
cyclopentanone (2.8 mmol, 1.1 eq.) in 6 mL diethyl ether was added dropwise. After warming to
room temperature and stirring for two hours saturated aqueous ammonium chloride solution was
added. Additional diethyl ether was added the organic layer was separated. The organic layer
was washed three times with water and saturated aqueous sodium chloride solution, dried over
anhydrous magnesium sulfate and filtered. The solvent was removed by evaporation under
reduced pressure and further purified by column chromatography on a silica gel column and
sublimation.
Light yellow powder, yield 34 %, 0.27 g, mp 125 °C. IR (KBr cm-1): ῦ 3500, 3266, 3001, 2964,
2874, 1479, 1445, 1395, 1363, 1333, 1244, 1204, 1187, 1159, 1077, 991, 876, 834, 815, 719. 1H
NMR (300.5 MHz, CD2Cl2): δΗ 1.28 (9H, s, C(CH3)3), 1.41 (9H, s, C(CH3)3), 1.72-2.02 (4H, m,
4
CH2), 2.07 -2.15 (4H, m, CH2), 2.24 (1H, s, OH), 7.03 (2H, d, JHH 2.4 Hz, HPh), 7.23 (2H, d,
4JHH 2.4 Hz, HPh), 8.89 (1H, s, OH). 13C{1H} NMR (75.5 MHz, CDCl3): δC 23.3 (CH2) 29.8
(C(CH3)3), 31.7 (C(CH3)3), 34.6 (q-C(CH3)3), 35.4 (q-C(CH3)3), 39.8 (CH2), 86.7 (1H, s, q-C-
OH), 120.0 (CPh), 123.6 (CPh), 129.1 (q-CPh), 136.8 (q-CPh), 140.8 (q-CPh), 153.7 (q-CPh) ppm.
+
HR-MS (ESI), m/z (%) for C19H29O2 : calcd. 289.2173, found 289.2170.
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Acknowledgements
Financial support by the Universität Heidelberg is gratefully acknowledged. We thank Dr. Jürgen
Graf for the NMR measurements.
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