Enantiopure encaged Verkade's superbases: Synthesis, chiroptical properties, and use as chiral derivatizing agent

Article abstract of DOI:10.1002/chir.23156

Verkade's superbases, entrapped in the cavity of enantiopure hemicryptophane cages, have been synthesized with enantiomeric excess (ee) superior to 98%. Their absolute configuration has been determined by using electronic circular dichroism (ECD) spectros

Full text of DOI:10.1002/chir.23156

Received: 26 August 2019
DOI: 10.1002/chir.23156
Revised: 14 October 2019
Accepted: 4 November 2019
S P E C I A L I S S U E A R T I C L E
Enantiopure encaged Verkade's superbases: Synthesis,
chiroptical properties, and use as chiral derivatizing agent
Jian Yang1 | Bastien Chatelet1 | Damien Hérault1 | Véronique Dufaud2 |
Vincent Robert3 | Stéphane Grass4 | Jérôme Lacour4
| Nicolas Vanthuyne1 |
Marion Jean1 | Muriel Albalat1 | Jean‐Pierre Dutasta5 | Alexandre Martinez1
1 Aix Marseille Univ, CNRS, Centrale
Abstract
Marseille, iSm2, Marseille, France
Verkade's superbases, entrapped in the cavity of enantiopure hemicryptophane
cages, have been synthesized with enantiomeric excess (ee) superior to 98%.
Their absolute configuration has been determined by using electronic circular
dichroism (ECD) spectroscopy. These enantiopure encaged superbases turned
out to be efficient chiral derivatizing agents for chiral azides, underlining that
the chirality of the cycloveratrylene (CTV) macrocycle induces different
magnetic and chemical environments around the phosphazide functions.
2 Laboratoire de Chimie, Catalyse,
Polymères, Procédés CNRS, UMR 5265,
Université Claude Bernard Lyon1,
Villeurbanne cedex, France
3 Laboratoire de Chimie Quantique
Institut de Chimie, UMR CNRS 7177,
Université de Strasbourg, Strasbourg,
France
4 Department of Organic Chemistry, Quai
Ernest Ansermet 30, University of Geneva,
Geneva, Switzerland
5 Laboratoire de Chimie École Normale
Supérieure de Lyon, CNRS, UCBL, 46
Allée d'Italie, Lyon F‐69364, France
KEYWORDS
chiral derivatizing agent, ECD, enantiopure cages, Verkade's superbase
Correspondence
Alexandre Martinez, Aix Marseille Univ,
CNRS, Centrale Marseille, iSm2,
Marseille, France.
Email: alexandre.martinez@centrale‐
marseille.fr
1 | INTRODUCTION
Wadhwa and Verkade,5 Raders and Verkade,6 and Yang
et al7). New aspects of pro‐azaphosphatranes have recently
Pro‐azaphosphatranes 1 (Chart 1), also named Verkade's
superbases, were discovered in 1989 by Verkade.1,2 These
species are highly basic with a pKa around 32 in CH3CN.3
In contrast with phosphazene bases (Schwesinger's bases),
protonation occurs on the phosphorus atom of the pro‐
azaphosphatrane. The high pKa value was attributed
to the high stability of its conjugated acid, the
azaphosphatrane cation. Pro‐azaphosphatranes have been
extensively used as basic and nucleophilic catalysts in
many transformations, providing high yields of the desired
compounds under mild conditions while avoiding most of
the side reactions encountered with other systems (for a
review, see Verkade et al4; for more recent examples, see
been developed. Verkade reported their use as ligands for
palladium‐catalyzed reactions, for instance, in Suzuki cou-
pling or Hartwig‐Buchwald reaction.8,9 Yang et al nicely
pursued this study by complexing other metals like Ni
and Rh to pro‐azaphosphatranes and investigated the
stereoelectronic properties of this ligand.10-12 In link with
this research, our group described the synthesis and
structures of Verkade's superbase‐gold complexes.13
Another elegant application of Verkade superbases is
their use as Lewis base to create “reversed frustrated
Lewis acid/base” or true frustrated Lewis pairs (FLP)
systems.14-17 Alternatively, our group studied the behavior
of Verkade's superbases in the confined space of
Chirality. 2019;1–8.
wileyonlinelibrary.com/journal/chir
© 2019 Wiley Periodicals, Inc.
1

2
YANG ET AL.
2.2 | Synthesis of enantiopure caged
azaphosphatranes (P)‐3 and (M)‐3
In an ice‐bath cooled round‐bottom flask, bis
(dimethylamino)chlorophosphine (0.154 mL, 1.0 mmol,
1.0 Equiv.) was dissolved in acetonitrile (6 mL), to which
a solution of hemicryptophane (P)‐2 or (M)‐2 (952 mg, 1
mmol, 1.0 Equiv.) in acetonitrile (15 mL) was then added
drop‐wise. The reaction mixture was vigorously stirred at
0°C for 0.5 h. The mixture was then brought to room tem-
perature overnight. The solvent was then removed under
vacuum and the residue was purified on silica gel by flash
chromatography (CH2Cl2/MeOH; 15/2) to give pure
azaphosphatrane (P)‐3 or (M)‐3 as a white solid (884
CHART
methoxybenzyl)
1
Structure of Verkade's superbases (PMB = p‐
hemicryptophanes,18,19 which are chiral host molecules
built from a cyclotriveratrylene unit bound to another C3‐
symmetry group.20,21 The confinement was found to simul-
taneously increase the thermodynamic basicity of the pro‐
azaphosphatrane and decrease the rate of proton transfer.
When tested as catalyst in a Diels‐Alder basico‐catalyzed
reaction, the encaged Verkade's superbase led to an
improvement of the diastereoselectivity, when compared
with its model compound that is devoid of cavity.22 The
endohedral functionalization of the cage also allowed
building FLP system benefiting from the strong isolation
of the Lewis basic and acid partners induced by the
cavity.23
Herein, we wish to report on the synthesis of the
first Verkade's superbase encaged in enantiopure
covalent cages. The absolute configuration was assigned
by ECD spectroscopy, and these encapsulated pro‐
azaphosphatranes were found to act as efficient chiral
derivatizing agents for chiral azides.
1
mg, 87%). H NMR (400 MHz, CDCl3) δ 7.32 (s, 3H),
6.97 (s, 3H), 6.15 (d, J = 8.27 Hz, 6H), 6.01 (d, J = 8.27
Hz), 4.82‐4.93 (m, 3H), 4.89 (d, J = 13.6 Hz, 3H), 4.45
1
(d, JP‐H = 491 Hz, 1H, P‐H), 4.42‐4.19 (m, 9H), 3.87‐
3.74 (m, 3H), 3.64 (d, J = 13.5 Hz, 3H), 3.57 (s, 9H),
3.36‐3.47 (m, 3H), 3.31‐3.17 (m, 9H), 2.68‐2.76 (m, 3H);
13C NMR (101 MHz, CDCl3) δ 158.27, 147.20, 146.45,
132.03, 131.71, 131.31, 129.01, 114.95, 114.61, 112.74,
70.23, 65.65, 55.12, 50.38, 47.63, 42.54, 36.80; 31P NMR
(121 MHz, CDCl3) δ −31.98. These data are consistent
with those reported in the literature.25
25
ðþÞ − ðPÞ − 3:½αꢀD ¼ þ73 ðc 0:1; CH2Cl2Þ
2 | MATERIALS AND METHODS
2.1 | General
25
ð−Þ − ðMÞ − 3:½αꢀD ¼ −74 ðc 0:1; CH2Cl2Þ
All commercial reagents and starting materials were used
directly as received without further purification. All dry
solvents were purified prior to use through standard pro-
cedures or obtained from a solvent drying system (MB‐
SPS‐800). All the reactions were carried out under an
atmosphere of argon, unless otherwise noted. Flash col-
umn chromatography was performed using silica gel 60
(230‐400 mesh). Thin‐layer chromatography was per-
formed on aluminum‐coated plates with silica gel 60
F254 and was visualized with a UV lamp or by staining
with potassium permanganate. NMR spectra were
recorded at either 300 or 400 MHz on BRUKER Avance
III nanobay spectrometers. Chemical shifts δ are reported
in ppm and coupling constants J in Hz. ECD and UV
spectra were measured on a JASCO J‐815 spectrometer
equipped with a JASCO Peltier cell holder PTC‐423 to
2.3 | Synthesis of enantiopure caged
proazaphosphatranes (P)‐4 and (M)‐4
Under an atmosphere of argon, in a flame‐dried Schlenk
flask, azaphosphatrane (P)‐3 or (M)‐3 (350 mg, 0.35
mmol, 1.0 Equiv.) was dissolved in dried THF (2 mL), t‐
BuOK (98 mg, 0.875 mmol, 2.5 Equiv.) was then added,
and the reaction mixture was stirred at room temperature
for 2 h. The solvent was removed under vacuum, and tol-
uene (5 mL) was added. The reaction mixture was stirred
for another 0.5 h, and then the suspension was filtered
under argon through a two‐necked fritted glass funnel.
The filtrate was recovered, and the solvent was removed
under vacuum to give enantiopure proazaphosphatrane
1
(P)‐4 or (M)‐4 as a white solid (200 mg, 58%). H NMR
(300 MHz, Toluene‐d8) δ 7.31 (s, 3H), 7.04 (d, J = 6.6
Hz, 6H), 6.91 (s, 3H), 6.73 (d, J = 8.5 Hz, 6H), 4.80 (d, J
= 13.6 Hz, 3H), 4.27‐4.11 (m, 12H), 4.10‐3.93 (m, 6H),
3.84‐3.71 (m, 3H), 3.69 (s, 9H), 3.60 (d, J = 13.2 Hz,
3H), 3.15‐2.99 (m, 3H), 2.99‐2.90 (m, 6H), 2.88‐2.74 (m,
3H); 31P NMR (121 MHz, Toluene‐d8) δ 121.29; 13C
maintain the temperature at 25.0
0.2°C. High‐
resolution mass spectra (HRMS) were performed at
Spectropole Analysis Service of Aix Marseille University.
Enantiopure hemicryptophanes (P)‐2 and (M)‐2 were syn-
thesized according to our previous reported procedure.24

YANG ET AL.
3
NMR (101 MHz, CDCl3) δ 157.35, 149.68, 147.27, 134.13,
133.57, 132.03, 128.56, 118.57, 115.13, 114.82, 68.78, 67.26,
54.83, 54.41, 52.42, 48.83, 36.15. These data are in agree-
ment with those reported in the literature for the racemic
mixture.18,19
which was purified by silica gel column chromatography
(petroleum ether: ethyl acetate = 10:1) to give pure 2‐
azido‐1‐isopropyl‐4‐methylcyclohexane (6) as a colorless
1
oil, 603 mg, 25% yield. H NMR (400 MHz, CDCl3) δ
3.99 (q, J = 3.3 Hz, 1H), 2.08‐1.98 (m, 1H), 1.80‐1.63 (m,
3H), 1.60‐1.46 (m, 1H), 1.31‐1.11 (m, 2H), 0.99‐0.82 (m,
11H). 13C NMR (101 MHz, CDCl3) δ 60.54, 47.39, 38.95,
34.85, 29.48, 26.51, 24.89, 22.14, 20.86, 20.64. HRMS
(ESI‐TOF) m/z: calcd for C10H19N3Ag+ [M + Ag]+,
288.0624; found 288.0623.26
25
ðþÞ − ðPÞ − 4:½αꢀD ¼ þ78 ðc 0:1; CH2Cl2Þ
25
ð−Þ − ðMÞ − 4:½αꢀD ¼ −76 ðc 0:1; CH2Cl2Þ
2.4 | Procedure for the synthesis of
2‐azido‐1‐isopropyl‐4‐methylcyclohexane
(6)26
2.5 | Procedure for the synthesis of (1‐
azido‐2‐methoxyethyl)benzene (7)27
2.5.1 | 2‐methoxy‐1‐phenylethan‐1‐ol
2.4.1 | 2‐Isopropyl‐5‐methylcyclohexyl
methanesulfonate
To a solution of 2‐methoxy‐1‐phenylethan‐1‐one (10
mmol) in 50 mL methanol was added gradually NaBH4
(20 mmol) at 0°C, then reaction mixture was brought to
room temperature and kept stirring for another 2 h until
the reaction completed monitored by TLC. Then solvent
was removed under vacuum, and 50 mL water was added
and extracted by EtOAc (3 × 50 mL). Organic phases
were collected and dried over anhydrous Na2SO4 and fil-
tered and concentrated to give the crude product, which
was purified by silica gel column chromatography (petro-
leum ether: ethyl acetate = 5:1) to give pure 2‐methoxy‐1‐
To a solution of racemic menthol (12.8 mmol) in dichlo-
romethane (50 mL) was added NEt3 (2.7 mL, 19 mmol).
The mixture was stirred for 1 h, then it was brought to
0°C, and a solution of methanesulfonyl chloride (MsCl)
(1.5 mL, 19 mmol) in dichloromethane was added
dropwise. Then the reaction mixture was warmed to
room temperature and kept stirring for another 2 h until
the reaction completed monitored by TLC. The reaction
was quenched with saturated NH4Cl solution (50 mL).
The mixture was extracted by dichloromethane (3 × 50
mL). Organic phases were collected and dried over anhy-
drous Na2SO4, filtered and concentrated to give the crude
product, which was purified by silica gel column chroma-
tography (petroleum ether: ethyl acetate = 20:1) to give
pure 2‐isopropyl‐5‐methylcyclohexyl methanesulfonate
1
phenylethan‐1‐ol as a colorless oil, 1.3 g, 86% yield. H
NMR (400 MHz, CDCl3) δ 7.49‐7.24 (m, 5H), 4.92 (dd, J
= 8.9, 2.8 Hz, 1H), 3.69‐3.36 (m, 5H), 3.04‐2.88 (m, 1H).
13C NMR (101 MHz, CDCl3) δ 140.34, 128.38, 127.81,
126.14, 78.22, 72.63, 59.00. HRMS (ESI‐TOF) m/z: calcd
for C9H12O2Na+ [M + Na]+, 175.0730; found 175.0731.27
1
as a colorless oil, 2.85 g, 95% yield. H NMR (400 MHz,
CDCl3) δ 4.57 (td, J = 10.9, 4.6 Hz, 1H), 3.02 (s, 3H),
2.37‐2.21 (m, 1H), 2.14‐2.02 (m, 1H), 1.79‐1.65 (m, 2H),
1.58‐1.38 (m, 2H), 1.36‐1.20 (m, 1H), 1.16 – 1.01 (m,
1H), 0.95 (dd, J = 6.8, 3.4 Hz, 6H), 0.93‐0.87 (m, 1H),
0.85 (d, J = 6.9 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ
83.37, 77.21, 47.46, 42.25, 39.11, 33.78, 31.64, 25.83,
23.15, 21.83, 20.79, 15.70.26
2.5.2 | (1‐Bromo‐2‐methoxyethyl)benzene
To a solution of 2‐methoxy‐1‐phenylethan‐1‐ol (6.6
mmol) in 50 mL chloroform was added gradually PBr3
(10 mmol) at 0°C, then reaction mixture was brought to
room temperature and kept stirring for another 2 hours
until the reaction completed monitored by TLC. Then
the reaction was quenched by adding saturated K2CO3
(50 mL). The mixture was extracted by chloroform (3 ×
50 mL). Organic phases were collected and dried over
anhydrous Na2SO4 and filtered and concentrated to give
the crude product which was purified by silica gel column
chromatography (petroleum ether: ethyl acetate = 10:1)
to give pure (1‐bromo‐2‐methoxyethyl)benzene as a color-
2.4.2 | 2‐Azido‐1‐isopropyl‐4‐
methylcyclohexane (6)26
To a solution of racemic 2‐isopropyl‐5‐methylcyclohexyl
methanesulfonate (13.3 mmol) in DMF (50 mL) was
added NaN3 (80 mmol). The mixture was stirred at 80°C
for 48 h. And then DMF was evaporated under vacuum,
100 mL of water was added, and the mixture was
extracted by dichloromethane (3 × 50 mL). Organic
phases were collected and dried over anhydrous Na2SO4
and filtered and concentrated to give the crude product,
1
less oil, 965 mg, 68% yield. H NMR (300 MHz, CDCl3) δ
7.39‐7.13 (m, 5H), 4.98 (t, J = 7.0 Hz, 1H), 3.92‐3.71
(m, 2H), 3.30 (s, 3H). 13C NMR (75 MHz, CDCl3) δ
139.11, 128.75, 128.71, 127.84, 76.86, 58.97, 51.64. HRMS

4
YANG ET AL.
(ESI‐TOF) m/z: calcd for C9H15NOBr+ [M + NH4]+,
2.6 | General procedure for reactions of
enantiopure caged proazaphosphatrane
232.0332; found 232.0330.27
(M)‐4 with chiral azides 6 and 7
Under an atmosphere of argon, to a solution of (M)‐4
(0.05 mmol) in toluene in a flame‐dried Schlenk tube
was added azide compound 6 or 7 (0.05 mmol). The mix-
ture was stirred at 50°C overnight. The solvent was evap-
orated under vacuum, and the residue was dissolved in
CDCl3 for NMR analysis.27
2.5.3 | (1‐azido‐2‐methoxyethyl)benzene
(7)
To a solution of (1‐bromo‐2‐methoxyethyl)benzene (6.53
mmol) in 50 mL DMF was added gradually NaN3 (9.8
mmol), then the reaction mixture was heated at 80°C
overnight until the reaction completed monitored by
TLC. Then 100 mL of water was added and extracted by
chloroform (3 × 50 mL). Organic phases were collected
and dried over anhydrous Na2SO4 and filtered and con-
centrated to give the crude product, which was purified
by silica gel column chromatography (petroleum
ether: ethyl acetate = 20:1) to give pure (1‐azido‐2‐
methoxyethyl)benzene (7) as a colorless oil, 856 mg,
3 | RESULTS AND DISCUSSION
3.1 | Synthesis of enantiomerically pure
encaged Verkade's superbases
The synthesis of the first enantiopure hemicryptophanes
was described in 2005 by Crassous and Dutasta,28 and
more recently, we reported a convenient large‐scale syn-
thesis of these supramolecular structures.24 Enantiopure
hemicryptophanes (P)‐2 and (M)‐2 were thus synthesized
in four steps following our previous reported procedure
(Scheme S1).24 The corresponding azaphosphatranes (P)‐
1
74% yield. H NMR (300 MHz, CDCl3) δ 7.39‐7.20 (m,
5H), 4.62 (t, J = 6.4 Hz, 1H), 3.53 (d, J = 6.7 Hz, 2H),
3.34 (s, 3H). 13C NMR (75 MHz, CDCl3) δ 136.71,
128.78, 128.48, 127.05, 76.45, 65.14, 59.11. HRMS (ESI‐
TOF) m/z: calcd for C9H11N3ONa+ [M
200.0794; found 200.0794.27
+
Na]+,
SCHEME 1 (A) Synthesis of enantiomerically pure encaged Verkade's superbases (P)‐4 and (M)‐4. (B) the CTV 5 that has been resolved on
gram scale24

YANG ET AL.
5
3 and (M)‐3 were then obtained by reaction of, respec-
tively, (P)‐2 and (M)‐2 with (Me2N)2PCl in acetonitrile at
room temperature overnight (Scheme 1). Our previously
reported synthesis of the two enantiomers (P)‐3 and (M)‐
3 is of limited utility here due to the tedious optical reso-
lution of the racemate 3 by chiral HPLC. Chromatography
on these species required a solvent mixture in the mobile
phase that degraded the stationary phase, leading to very
low product recovery from the process.25 In the present
report, we chose to resolve the racemate of the key CTV
intermediate 5 (Schemes 1 and S1), which can be effec-
tively done on a large scale. In this manner, enriched
(P)‐3 and (M)‐3 can be obtained at the hundred mg scale.
As CTV units can racemize before the ring closure of the
cage (the energy barrier of racemization of CTV com-
pound being around 112 kJ·mol−1 at 298 K),24 the enan-
tiomeric purity of these compounds was checked by
adding chiral hexacoordinated phosphorus TRISPHAT
anion (Tris (tetrachlorobenzenediolato)phosphate(V)
anion, Figure 1).25,29-38 This anion presents a right‐
handed or left‐handed propeller geometry of Δ or Λ con-
figurations, respectively, and is a useful chiral solvating
agent (CSA) for the NMR discrimination of enantiomers
of chiral cationic and certain neutral organometallic
species. In a typical experiment, 1.0 equivalent of
[cinchonidinium][Δ‐TRISPHAT] was added to a solution
of [(M)‐3][Cl−], [(P)‐3][Cl−], or rac‐3 in CDCl3. The
resulting 31P NMR spectra displayed respectively one signal
at −30.82 ppm, one signal at −31.63 ppm, and two signals at
−30.74 and −31.63 ppm (Figure 1), giving rise to an
enantiomeric excess (ee) superior to 98% for both
3.2 | Assignment of the absolute
configuration
The ECD spectra of both enantiomers of 4 were recorded
in CH2Cl2 at 293 (Figure 2). As previously
K
observed with cyclotriveratrylene, cryptophane, and
1
hemicryptophane derivatives, the La band of the spectra
(230 to 270 nm) is poorly sensitive to substituents and
allows for the assignment of the absolute configuration
of the CTV moiety.39,40 A characteristic positive‐negative
bisignate (from low to high energies) is observed in the
ECD spectrum of (+)‐4 at around 250 nm, which can
be attributed to the P configuration, whereas the M
configuration was assigned to the (–)‐4 enantiomer,
which exhibits a negative‐positive bisignate (Figure 2).
3.3 | Use of enantiopure encaged
Verkade's superbase as chiral derivatizing
agent for chiral azides
In 2000, Verkade demonstrated that enantiopure
proazaphosphatranes could be used as chiral derivatizing
agents for chiral azides.27 As a follow‐up of this work, we
thus carried out the reaction of the enantiopure
encapsulated Verkade's superbase (M)‐4 with two differ-
ent racemic chiral azides rac‐6 or rac‐7, in toluene
at 50°C overnight to afford the corresponding encapsu-
lated phosphazide diastereomers (1S,2R,5S)‐(M)‐8 and
(1R,2S,5R)‐(M)‐8 and (S)‐(M)‐9 and (R)‐(M)‐9 (Scheme 2).
An excellent diastereomeric peak separation of around 98
Hz was observed in the 1H‐decoupled 31P NMR spectra of
both species (Figure 3), allowing for an accurate quantita-
tive determination of the diastereomeric ratios. If the 31P
enantiomers
[(–)‐(P)‐3][Cl–]
and
[(+)‐(M)‐3][Cl–].
Azaphosphatranes (M)‐3 and (P)‐3 were then deprotonated
with t‐BuOK in THF, affording the enantiopure encaged
Verkade's superbases (M)‐4 and (P)‐4 with 58% yield
(Scheme 1). It can be highlighted that the synthesis of a
Verkade's superbase, confined in an enantiopure cavity is,
to the best of our knowledge unprecedented.
1
NMR revealed only two sets of signals, the H NMR was
FIGURE 1 31P NMR spectra (CDCl3, 298 K, 121.5 MHz) of
hemicryptophane rac‐3 and the enantiomers (–)‐(P)‐3 and (+)‐(M)‐
3 in the presence of 1 Equiv. of [cinchonidinium] [Δ‐TRISPHAT]
FIGURE 2 ECD spectra of enantiomerically pure encaged
Verkade's superbases (+)‐(P)‐4 (red) and (–)‐(M)‐4 (green) in
CH2Cl2 at 293 K

6
YANG ET AL.
SCHEME 2 Synthesis of encapsulated phosphazides 8 and 9
FIGURE 3 1H‐decoupled 31P NMR spectra (CDCl3, 298 K, 162 MHz) of encaged phosphazides (A) 8 and (B) 9
much more complicated, probably because of the
expected C1 symmetry of each diastereomer (Figures S12
and S14). Although no diastereomeric ratio was reached
when the azides derivatives were used in excess, these
results underline that once covalently bound to the
phosphorus of the encaged Verkade's superbase, the
two enantiomers of a chiral molecule are in strongly
different magnetic and chemical environments even if
the inherently chiral CTV unit is remote from the
phosphorus center.
4 | CONCLUSIONS
In conclusion, we have described the unprecedented syn-
thesis of enantiopure encaged Verkade's superbases. Our
synthetic pathway provides the desired compounds in
six steps from vanillyl alcohol with an overall yield of
6%. The assignment of the absolute configuration has
been achieved by comparison of ECD spectra of
enantiopure (M)‐4 and (P)‐4 with other CTV derivatives.
Moreover, these compounds could act as efficient chiral

YANG ET AL.
7
13. Chatelet B, Nava P, Clavier H, Martinez A. Synthesis of gold (I)
complexes bearing Verkade's superbases. Eur J Inorg Chem.
2017;2017(37):4311‐4316.
derivatizing agents when reacting with racemic azides,
leading to encapsulated chiral phosphazides.
14. Mummadi S, Unruh DK, Zhao J, Li S, Krempner C. “Inverse”
frustrated Lewis pairs–activation of dihydrogen with
organosuperbases and moderate to weak Lewis acids. J Am
Chem Soc. 2016;138(10):3286‐3289.
ORCID
Jérôme Lacour https://orcid.org/0000-0001-6247-8059
Alexandre Martinez
5734
https://orcid.org/0000-0002-6745-
15. Mummadi S, Kenefake D, Diaz R, Unruh DK, Krempner C.
Interactions of Verkade's superbase with strong Lewis acids:
from labile mono‐and binuclear lewis acid–base complexes to
phosphenium cations. Inorg Chem. 2017;56(17):10748‐10759.
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