Atypical and Asymmetric 1,3-P,N Ligands: Synthesis, Coordination and Catalytic Performance of Cycloiminophosphanes

Article abstract of DOI:10.1002/chem.202101921

Novel seven-membered cyclic imine-based 1,3-P,N ligands were obtained by capturing a Beckmann nitrilium ion intermediate generated in situ from cyclohexanone with benzotriazole, and then displacing it by a secondary phosphane under triflic acid promotion. These “cycloiminophosphanes” possess flexible non-isomerizable tetrahydroazepine rings with a high basicity; this sets them apart from previously reported iminophophanes. The donor strength of the ligands was investigated by using their P-κ¹- and P,N-κ²-tungsten(0) carbonyl complexes, by determining the IR frequency of the trans-CO ligands. Complexes with [RhCp*Cl₂]₂ demonstrated the hemilability of the ligands, giving a dynamic equilibrium of κ¹ and κ² species; treatment with AgOTf gives full conversion to the κ² complex. The potential for catalysis was shown in the Ru^II-catalyzed, solvent-free hydration of benzonitrile and the Ru^II- and Ir^I-catalyzed transfer hydrogenation of cyclohexanone in isopropanol. Finally, to enable access to asymmetric catalysts, chiral cycloiminophosphanes were prepared from l-menthone, as well as their P,N-κ²-Rh^III and a P-κ¹-Ru^II complexes.

Full text of DOI:10.1002/chem.202101921

Accepted Article
Accepted Article
Title: Atypical And Asymmetric 1,3-P,N-Ligands: The
Synthesis, Coordination and Catalytic Performance of
Cycloiminophosphanes
Authors: Mark K. Rong, Eduard O. Bobylev, Flip Holtrop, Martin
Nieger, Andreas W. Ehlers, J. Chris Slootweg, and Koop
Lammertsma
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To be cited as: Chem. Eur. J. 10.1002/chem.202101921
Link to VoR: https://doi.org/10.1002/chem.202101921
01/2020

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Atypical And Asymmetric 1,3-P,N-Ligands: The Synthesis,
Coordination and Catalytic Performance of
Cycloiminophosphanes
Mark K. Rong,[a] Flip Holtrop,[a] Eduard O. Bobylev,[a] Martin Nieger,[b] Andreas W. Ehlers,[a,c,d] J. Chris
Slootweg[a,d] and Koop Lammertsma*[a,c]
[a]
M.K. Rong, F. Holtrop, E.O. Bobylev, Dr. A.W. Ehlers, Dr. J.C. Slootweg, Prof. Dr. K. Lammertsma
Department of Chemistry and Pharmaceutical Sciences,
Vrije Universiteit Amsterdam,
De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands
E-mail: k.lammertsma@vu.nl
[b]
[c]
[d]
Dr. M. Nieger
Department of Chemistry,
University of Helsinki,
A. I. Virtasen aukio 1, P.O. Box 55, Helsinki, Finland.
Dr. A.W. Ehlers, Prof. Dr. K. Lammertsma
Department of Chemistry,
University of Johannesburg,
Oakland Park 2006, Johannesburg, South Africa
Dr. A.W. Ehlers, Dr. J.C. Slootweg
Present address:
Van 't Hoff Institute for Molecular Sciences,
University of Amsterdam,
P.O. Box 94157, 1090 GD Amsterdam, The Netherlands.
Supporting information for this article is given via a link at the end of the document.
Abstract: Novel 7-membered cyclic imine-based 1,3-P,N-ligands
were obtained by capturing an in situ generated Beckmann nitrilium
ion intermediate from cyclohexanone with benzotriazole, and then
displacing it by a secondary phosphane under triflic acid promotion.
These ‘cycloiminophosphanes’ possess flexible non-isomerizable
tetrahydroazepine rings with a high basicity, which sets them apart
from previously reported iminophophanes. The donor strength of the
ligands was investigated using their P-κ1- and P,N-κ2-tungsten(0)
carbonyl complexes, by determining the IR frequency of the trans-CO
ligands. Complexes with [RhCp*Cl2]2 demonstrated the hemilability of
the ligands, by giving a dynamic equilibrium of κ1- and κ2-species;
treatment with AgOTf gives full conversion to the κ2-complex. The
potential for catalysis was shown in the RuII-catalyzed solvent-free
hydration of benzonitrile and the RuII- and IrI-catalyzed transfer
hydrogenation of cyclohexanone in isopropanol. Finally, to enable
access to asymmetric catalysts, chiral cycloiminophosphanes were
prepared from L-menthone, as well as their P,N-κ2-RhIII- and a P-κ1-
RuII-complexes.
binding modes (N-κ1, P-κ1, P-κ1η2, κ2, and μ; Figure 1) and their
complexes have found application in homo/heterogeneous
catalysis, bio-inorganic chemistry, and photoluminescence.[3-7]
Most of these 1,3-P,N-complexes are based on pyridyl- and
imidazolyl-based ligands A and B, which have structural
limitations that are inherent to their syntheses.[4c,5a,b] Recently we
reported on the highly tunable iminophosphanes C and their
tautomers phosphaamidines
D that can be independently
substituted on the P, C, and N-atoms.[6,7] These 1,3-P,N-ligands
are readily accessible from (base-stabilized) nitrilium triflate
precursors and even though they are obtained as (dynamic) E/Z
isomer mixtures, the equilibrium shifts to the desired Z-conformer
on coordination to metals (Figure 2).[6] The hapticity in
κ1/κ2-[(P,N)RhCp*Cl2]-complexes and the favorable performance
Introduction
Ligand-based reactivity can enhance the activity of transition
metal catalysts,[1] as is the case for hybrid ligands,[2] which
combine the properties of different heteroatoms to enable
hemilability, redox non-innocence, proton-shuttling, and substrate
coordination.[1,2] 1,3-P,N-ligands are particularly subject to diverse
Figure 1. 1,3-P,N ligands and their diverse transition metal complexes.
1
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Figure 5. Non-isomerizable cycloiminophosphanes.
cyclohexanone precursors are readily available from terpenoids
used in, for instance, the flavoring and perfume industry.[12]
In the present study we report on both the synthesis of these
novel cyclic 1,3-P,N-ligands and their surprising electronic
properties that sets them apart from non-cyclic iminophosphanes.
We explore their coordination to early and late transition metals in
κ1/κ2 complexes and assess their performance in catalytic nitrile
hydration and transfer hydrogenation. Crystal structures are
provided for the (a)symmetric tetrahydroazepine synthons,
ligands, and a W complex.
Figure 2. Synthesis of iminophosphanes and hemilabile metal-coordination.
Results and Discussion
The synthesis of the cyclic ligands is discussed first, followed by
an analysis of their donor capacity using IR- and 31P NMR
spectroscopy on W0 carbonyl complexes and dynamic RhIII
complexes, respectively. Next, RuII species are examined to
evaluate the ligands’ performance in homogeneous catalysis. We
also discuss analogous asymmetric ligands with a natural
product-derived backbone.
Figure 3. Catalyzed nitrile hydration.
in (κ1-P,N)-Ru(II)-catalyzed nitrile hydration correlated with the
electronic properties of the ligands[6d] and the basicity of the
nitrogen-donor (Figure 3).[3,4]
Precursor synthesis
Our synthetic strategy is based on reacting phosphanes with a
seven-membered ring nitrilium ion, i.e., the 3,4,5,6-tetrahydro-
azepinium ion.[10] Because this ion could not be accessed by our
established methodology in which amides are reacted to imidoyl
halides with subsequent halide to triflate exchange,[6] as the
activating agents converted ε-caprolactam to thick intractable
mixtures of presumably protonated imidoyl chloride, dimers
and/or nylon like polymers,[13,14] we decided to generate the
desired nitrilium ion in situ using the Beckmann rearrangement
and trap it with benzotriazole.[11]
Treating neat cyclohexanone with hydroxylamine·HCl salt
by grinding them together in a mortar, while slowly adding NaOH,
yielded the corresponding pure oxime conveniently, even on large
scale (120 mmol, 82%; Scheme 1).[15] Next, under an atmosphere
of nitrogen, the oxime was activated in situ as the corresponding
methylsulfonate with methylsulfonyl chloride and triethylamine in
MeCN at 0 °C.[11c,16] Benzotriazole was added and the mixture
was heated to reflux for 2 h to facilitate the ring expansion and
trap the nitrilium ion. The reported work-up[11c] was significantly
simplified by adding water to the crude mixture to precipitate pure
4 as a white solid in good yield (60%); alternatively, evaporation,
extraction into Et2O and filtration over neutral alumina also
provides 4 (58%). Single crystals suitable for X-ray diffraction
analysis were obtained from Et2O and revealed a remarkably flat
Iminophosphanes
C are accessed from nitrilium ion
precursors, which are known both as reactive synthons and
intermediates.[5,8] Illustrative is the Beckmann rearrangement of
cyclohexanone oxime to the ring-expanded seven-membered
nitrilium intermediate (with an iminium resonance form), which
hydrolyzes to the ε-caprolactam that is used as the building block
for the commercial production of nylon-6 (Figure 4, top).[9,10] The
intermediate can also be trapped by nucleophiles, for instance by
benzotriazole,[11] which under Lewis acid promotion can be
displaced by other nucleophiles.[11a] This reactivity mirrors our
reported nitrilium triflate approach for the synthesis of C and might
be suitable to access novel 7-membered cyclic iminophosphanes
(Figure 4, E). Such ‘cycloiminophosphanes’ would be
conformationally locked, i.e., unable to undergo E/Z-isomerization
(Figure 5), may carry chiral groups as the required chiral
conformation [N1-C1-N2-N3
=
179.54(8); C6-C1-N2-N3
=
−0.37(12)], presumably due to N1-
-
-H11 and
N3- - -H6A hydrogen bonding (2.50 and 2.29 Å, respectively;
Figure 6). The C1-N1 bond length of 1.2643(13) Å is typical for an
imine bond and the C1-N2 bond of 1.4334(12) Å is similar to those
of other N-heterocycle stabilized imines.[6c]
Figure 4. Cycloiminophosphane synthesis from the Beckmann nitrilium
intermediate.
2
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Table 1. Lewis acid induced exchange of the benzotriazolyl group of 4 for
PPh2.[a]
Loading
[mol%]
Con-
version
to E [%]
Selectivity
for E
[%][b]
t
Entry
Promotor
Solvent
[min.]
1[c]
2
AlCl3
AlCl3
AlCl3
AlCl3
AlCl3
SnCl2
SnCl2
SnCl2
SnCl4
10%
10%
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
Toluene
CHCl3
CHCl3
CHCl3
10
90
25
40
57
64
99
70
72
36
50
76
100
100
95
Scheme 1. Benzotriazolyl-tetrahydroazepine synthesis.
3
10%
300
90
88
4
25%
93
5
100%
100%
100%
100%
100%
100%
100%
90
99[d]
95
6
90
7
300
300
300
300
10
97
8
63
Figure 6. Displacement ellipsoid plot of benzotriazolyl tetrahydroazepine 4 at
the 50% probability level. Hydrogen atoms are omitted for clarity, with exception
of H6A and H11. Selected bond lengths (Å) and angles (°): C1-N1 =1.2643(13),
C1-N2 = 1.4334(12), N1-C2 =1.4693(12), C1-C6 = 1.5046(13), N2-N3 =
1.3746(11), N3-N4 = 1.2947(12), N2-C12 = 1.3780(12), N3 - - -H6A =2.29, N1
- - -H11 = 2.50, N1-C1-N2 = 114.81(9), N1-C1-C6 = 129.01(9), N2-C1-C6 =
116.18(8), C1-N1-C2 = 118.88(9), N1-C1-N2-N3 = 179.54(8), C6-C1-N2-N3 =
−0.37(12).
9
50
.
10
11[e]
BF3 OEt2
92
HOTf
100
[a] Conditions: 1 equiv. Ph2PH, solvent (0.35M), reflux. [b] Determined by 31P
NMR spectroscopy. [c] T =80°C, μW conditions, 0.25M. [d] Conversion to the
AlCl3 adduct of 6. [e] RT, 0.5M.
Phosphane introduction
Treatment of 4 with 1 equiv. of triflic acid at 0 °C resulted
instantly in a suspension from which its iminium form 5 could be
isolated by filtration (Scheme 3). Whereas 5 is subject to
decomposition over time, both in solution and as an isolated solid,
it reacted cleanly upon immediate resuspension with phosphanes
to give 6 within minutes. After work-up, diphenyl derivative 6a was
obtained in 77% as an air-stable solid. The aliphatic derivatives
6b,c (conversion 85% (n-Bu) and 66% (Cy)) could not be purified
satisfactorily until after the subsequent deprotonation-step (vide
infra). Bulky substituents may hinder the formation of 6, as
suggested by the lower selectivity found for 6c. Crystals suitable
for X-ray diffraction could be obtained for 6a and 6c by slow
diffusion of pentane into a THF solution. The molecular structures
show a chair conformation for the tetrahydroazepine rings, with
the P-lone pair facing away from the imine (Figure 7). Generally,
the structures are comparable to those reported for non-cyclic
iminophosphane ligands.[6a,b] The imine bond lengths are in the
expected range [6a: N1-C1 = 1.2858(16) Å; 6c: N1-C1 =
1.2892(19) Å], as are the P-C bonds [6a: P1-C1 = 1.8269(13) Å;
6c: P1-C1 = 1.8310(16) Å].[18]
In analogy to the formation of 4, we tried to capture the in situ
generated 3,4,5,6-tetrahydro-azepinium ion directly with
a
phosphane to obtain the desired cycloiminophosphane ligand E,
but to no avail. Instead, the benzotriazole group of 4 could be
replaced for diphenylphosphane using Lewis acid promotion,
which we examined under a variety of conditions (Scheme 2,
Table 1). We started with a microwave reaction employed in
related displacements by N-nucleophiles,[11a] but this reaction
using 10 mol% AlCl3 (entry 1) proved to be less effective than
regular heating under reflux (entries 2 and 3). The still modest
conversion, probably due to Lewis pair interaction with the
phosphane,[17] could be enhanced by increasing the amount of
AlCl3 (entries 4, 5). Using equimolar amounts, AlCl3 proved to be
more selective than SnCl2, SnCl4, and BF3 (entries 5-10) and
resulted in 99% conversion to an Al-adduct of the desired ligand,
which on treatment with water gave the protonated ligand and a
mixture of oxy aluminum anions. However, the by far most
effective and convenient manner to obtain the protonated ligand
was found to be the direct activation of 4 through protonation with
triflic acid (entry 11).
Scheme 2. The activation of 4 for Ph2PH introduction.
Scheme 3. Acid-facilitated activation of 4 to access ligands 6 and 7.
3
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previously reported ligands.[6] The unexpected high basicity of
7previously reported ligands.[6] The unexpected high basicity of 7
seems akin to that of structurally related 1,3-N,N-bases such as
DBU.[21]
Coordination Chemistry and Catalysis
The donor strength of P,N-ligands affects their efficiency in
cooperative reactions.[3,4,6] To examine the influence of 7 in
transition metal complexes, we synthesized the tungsten carbonyl
complexes 10, 11 as the IR frequency of their trans-CO ligands
reflect the ligand’s P,N-donor strength.[22] Reacting ligand 7a with
either [W(CO)5(MeCN)] or [(COD)W(CO)4] provided, respectively,
κ1-complex 10 (82%) and κ2-complex 11 (83%; Scheme 5). IR
spectroscopic analysis of the trans-CO ligands indicated
increasing electron donation to the metal for 11 (2069, 2008
(trans), 1869, 1825 cm-1) with respect to 10 (2069 (trans), 1904,
1873 cm-1). Compared to the analogous W-complexes of the
widely applied Ph2PPy ligand (Figure 1, A; κ1: 2050, 1980, 1920
cm-1; κ2: 2017, 1890, 1870, 1826 cm-1)[22a] 7a appears to be a far
stronger N-donor. Of note is that the CO-stretches for κ2-complex
11 are weaker than those of κ1-complex 10, which illustrates that
the additional coordination of the strong N-donor provides a more
electron rich metal center with stronger W(CO) backdonation.
Single crystals of 10 were obtained by cooling a toluene solution.
The molecular structure (Figure 9) shows a tight bond of the metal
Figure 7. Displacement ellipsoid plot of phosphaazepinium triflates 6a (left) and
6c (right) at the 50% probability level. C-H hydrogen atoms and the triflate
anions are omitted for clarity. Selected bond lengths (Å) and angles (°) of 6a:
P1-C1 = 1.8269(13), P1-C7 = 1.8249(13), P1-C13 = 1.8223(13), C1-N1 =
1.2858(16), N1-C2 = 1.4839(16), N1-H1 = 0.869(13), N1-C1-P1 = 123.06(10)
N1-C1-C6 = 119.70(11), C1-N1-C2 = 125.03(11); Selected bond lengths (Å)
and angles (°) of 6c: P1-C1 = 1.8310(16), P1-C7 = 1.8478(15), P1-C13 =
1.8697(16), C1-N1 = 1.2892(19), N1-C2 = 1.4779(19), N1-C1-P1 = 124.44(12),
N1-C1-C6 = 120.25(14), C1-N1-C2 = 125.72(13).
Compounds 6a-c could be readily deprotonated in THF by
using NaHMDS as base at −78 °C and then extracted into
pentane to provide the desired novel ligands (7a: 92%; 7b: 83%;
7c: 76% yield). Surprisingly, and in contrast to 6, the products
show sets of two distinct 31P NMR signals [7a: δ 6.0 (14%), 5.4
(86%); 7b: δ −10.7 (95%), −17.5 (5%); 7c: δ 17.5 (88%), 13.5
(12%)]. Conformational flips of the aliphatic rings, known as
flippamers, are known to be observable by NMR spectroscopy,[19]
and this may also be the case for the tetrahydroazepine ring in 7
(Figure 8, with the presumed major conformer in the center).
center with the trans-CO [W1-C19
=
2.0058(18) Å;
Scheme 5. Analysis donor capacity of 7a using W(CO)n complexes.
Figure 8. Possible tetrahydroazepine-ring conformations (flippamers) for 7a-c.
Two axial protons are marked for clarity.
A striking property of 7 is its relative basicity. For example,
dissolving 7 in CDCl3 led to its instant decomposition, presumably
due to protonation (chloroform pKa > 16). Compared to known
iminophosphanes, the seemingly higher basicity of 7 may be
attributable to its C- and N-alkyl substituents. For comparison, we
synthesized non-cyclic C,N-dimethyl iminophosphane 9 from the
nitrilium ion obtained by methylation of acetonitrile (8, 91%)[20] and
diphenylphosphane (Scheme 4). Also in this case, a strong base
(n-BuLi) was needed to generate the 1,3-P,N-product (93%;
δ(31P) = 6.7 (E), −13.2 (Z)), whereas NEt3 (pKa ≈ 11) sufficed for
Figure 9. Displacement ellipsoid plot of W carbonyl complex 10 at 50%
probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths
(Å) and angles (°): W1-P1 = 2.5333(4), W1-C19 = 2.0058(18), C19-O1 =
1.142(2), P1-C13 = 1.8216(15), P1-C7 = 1.8234(16), P1-C1 = 1.8617(16), C1-
N1 = 1.265(2), C19-W1-P1 = 175.60(5), O1-C19-W1 = 177.95(15), N1-C1-P1 =
117.22(12), C1-P1-W1-C21 = −15.71(8), C13-P1-W1-C22 = 14.32(7).
Scheme 4. Synthesis of the non-cyclic 7a-analogue 8.
4
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W-Cavg. = 2.046 Å] and a slightly weakened CO triple bond [C19-
O1 = 1.142(2) Å; C-Oavg. = 1.139 Å]. The W1-P1 distance
[2.5333(4) Å] is comparable to the one in analogous PPh3 and
Ph2PPy complexes.[22a,23] Further parameters of the ligand are
similar to those for 6a.
The influence of P-substituents[5c] was apparent in
[(7)RhIII(Cp*)Cl2] complexes, which can equilibrate between P-κ1
and P,N-κ2 forms with characteristic 31P NMR chemical shifts and
coupling constants, as has been shown for related
iminophosphanes (generally: 1JP,Rhκ1 ≈ 146 Hz; 1JP,Rhκ2 ≈ 114 Hz;
e.g. [(Me-NC(Ph)P(3-Me-Ph)2)RhCp*Cl2] (P-κ1: δ 34.7 ppm, 1JP,Rh
1
= 144.6 Hz; P,N-κ2: (δ −12.1 ppm, JP,Rh = 114.7 Hz); see also
Figure 2).[6b,d] To obtain the complexes, 7 was reacted with 0.5
equiv. [Rh(III)Cp*Cl2]2 in DCM (Scheme 6) to give exclusively P-κ1
complexes for the n-butyl substituted ligand (31P NMR: 12b: δ
29.7 ppm, 1JP,Rh = 134.2 Hz, major (98%); 21.3 ppm, 1JP,Rh = 136.2
Hz, minor (2%)) and mainly the P,N-κ2 complex for the
dicyclohexyl ligand (12c: δ −1.5 ppm, 1JP,Rh = 104.5 Hz, major
(66%); 41.0 ppm, 1JP,Rh = 133.4 Hz, minor (18%); 31.2 ppm, 1JP,Rh
= 130.6 Hz, minor (16%)). The observation of two P-κ1
resonances suggests the presence of flippamers or rotamers; the
absence of 31P-31P couplings rules out bridged complexes
(μ-coordination).
Figure 10. 31P NMR spectra at room temperature and at −94 °C for complex
12a.
For comparison, we synthesized the Rh-complexes of non-cyclic
ligand 9 (Scheme 4), which is similarly substituted as 7a. The low
temperature 31P NMR spectrum showed the P-κ1 complex (δ 39.1
1
1
ppm, JP,Rh = 144.2 Hz (55%); δ 27.7 ppm, JP,Rh = 132.8 Hz
(29%)) together with small amounts of both the N-κ1 complex
(δ 33.9 ppm, JP,Rh = 152.3 Hz (4%)) and the P,N-κ2 complex
1
(δ −15.0 ppm, 1JP,Rh = 115.0 Hz (6%)) before AgOTf converted it
fully to the κ2 complex (72% yield, δ(31P) −15.5 (d, 1JP,Rh = 114.6
Hz). Even though the amount of observed N-coordination is lower
for 9 than for 7a, these results highlight the influence of the
C,N,P-substituents of 7 on the P,N-coordination mode.
All Rh-complexes 12a-c could be fully converted to the
bidentate complex 13 upon chloride abstraction with AgOTf
(Scheme 6; 13a: 82% yield, δ(31P) −16.8 ppm, 1JP,Rh = 113.9 Hz;
13b: 83% yield, δ(31P) −19.7 ppm, 1JP,Rh = 110.0 Hz; 13c: 76%
yield, δ(31P) −2.6 ppm, 1JP,Rh = 106.8 Hz).
Next, we explored the coordination to RuII (Scheme 6) and
the catalytic activity[4a] of the resulting complexes. Reacting 7a,b
with [Ru(p-cym)Cl2]2 (p-cym = p-cymene) provided the P-κ1
complex 14b (66%, δ(31P) 27.3 ppm). Complex 14a could not be
isolated from the reaction mixture that showed the presence of
two products (δ(31P) 31.2 (27%), 23.4 (73%) ppm), which we
tentatively assign to the N-κ1 and P-κ1 complexes, respectively, in
analogy to RhIII complex 12a (vide supra).
Scheme 6. The coordination of ligands 7 to RhIII and RuII.
Surprisingly, Rh-complexation of phenyl ligand 7a did not give a
clean P-κ1/κ2 mixture. The 31P NMR spectrum of complex 12a
showed a mixture of two broad signals (δ 36.9 ppm, d, 1JP,Rh
Next, three RuII complexes of ligands 7a-c were
preliminarily tested for their effectiveness as catalysts in the
=
142.5 Hz, (42%); δ 30.1 ppm, br. s, (58%)) that resolved into three
doublets on lowering the temperature to −94 °C (Figure 10). Their
coupling constants suggests a mixture of two P-κ1complexes
(δ 38.0 ppm, 1JP,Rh = 139.3 Hz (31%); δ 36.5 ppm, 1JP,Rh = 137.7
Hz (54%)) and possibly an N-κ1 complex, as it has a strikingly
different P-Rh coupling and other coordination modes are unlikely
due to the lack of additional P- and/or Rh couplings. (δ 27.5 ppm,
JP,Rh = 153.9 Hz, (15%)). The N-monodentate coordination mode
of 1,3-P,N-ligands has been reported only for MnII and FeII,[3,24]
and not for rhodium. Apparently, the two P-κ1 and one N-κ1
bonding modes interchange rapidly at room temperature.
The competing P-κ1/N-κ1 coordination modes for 12a can be
attributed to the C,N-dialkyl-P-phenyl substitution pattern of ligand
7a. Its aryl groups reduce the donating property of the phosphane
group as compared to 7b and 7c, and the cyclic alkyl chain makes
its imine a stronger donor than in reported iminophosphanes.[6]
o
solvent-free, closed-vessel hydration of benzonitrile[25] at 180 C
for 3 h (Table 2). Surprisingly, in situ generated 14a with the
phenyl substituted ligand 7a proved to be quite an active catalyst,
yielding 79% product. In situ generated 14b with the n-butyl
substituted ligand 7b afforded a somewhat lower yield of 59%,
which could be enhanced to 70% by preforming the catalyst
(entries 2 and 3, respectively). The least effective catalyst was the
RuII complex of ligand 7c, giving a hydration yield of only 15% that
may have its origin in the more limiting steric factors. Even though
the catalytic conditions were not optimized in this brief screen, it
is rewarding that a hydration yield as high as 79% was obtained
for P-κ1-RuII-complex 14a, which resembles the highest yield of
82% found for the comparable Ru-catalyst with an acyclic
iminophosphane.[25] Both perform much better than the analogous
Ru-complex of the established Ph2PPy ligand, which gives a
hydration yield of 6%.[6d]
5
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situ generation of the catalyst, at 3 mol% substantially more active
in presence of KOH (entries 4-5). Even catalyst loading the
conversions were slow (up to 20 h; entries 1-3), but similar to the
catalyst of Jalón et al.,[26] the catalyst was the corresponding κ2-
complex of 14b, obtained by ion exchange with NaBF4, was
active under these conditions (entry 7). With 0.5 mol% catalyst
and 2.5 mol% KOH, a reaction time of 2 h still resulted in the
quantitative hydrogenation of cyclohexanone (entry 8). Last, as
iridium(I) complexes are generally very active hydrogenation
catalysts,[28] we also explored the in situ generation of
[(7b)Ir(COD)Cl], which showed a similar trend as the RuII
complexes (entries 9-11).
Table 2. Solvent-free catalyzed hydration of benzonitrile with precatalyst
[Ru(p-cym)Cl2].[a]
Entry
Ligand (L)
T (°C)
t (h)
Yield[b] (%)
1
2
3
4
5
7a
7b
180
180
180
180
180
3
3
3
3
3
79
59
7b[c]
7c
70
15
PyPPh2
6[6d]
These preliminary screenings demonstrate that the
conveniently in situ generated κ1 and κ2 complexes of RuII and IrI
with cyclic 1,3-P,N-ligand 7 are active catalysts that warrant
further scrutiny.
[a] Reaction conditions: Ph-C≡N (3.6 mmol), H2O (7.2 mmol), 1.4 mol%
[Ru(p-cym)Cl2] and ligand 7. [b] Determined by GC. [c] Preformed catalyst
(14b).
As complex 14b performed only modestly in the hydration
of benzonitrile, we chose to further screen its potential by
preliminarily exploring the transfer hydrogenation of
cyclohexanone in i-PrOH, under conditions adapted from of Jalón
Chirality
As, to the best of our knowledge, no asymmetric
1,3-P,N-ligand-based catalysts have been reported,[4] the
synthesis of such ligands may be valuable. With a synthetic route
toward cyclic iminophosphanes at hand, we pursued substituting
the backbone with a chiral group by using an inexpensive
terpenoid as asymmetric starting point. The readily available
terpenoid L-menthone[12] is well-suited for this purpose, since its
sizeable 2S-i-Pr group is expected to be favorable for asymmetric
induction.[29] Following the synthesis of the chiral ligands, we
report their Ru and Rh complexes and briefly reflect on their
catalytic potential.
et
al.,
who
used
the
analogous
complex
of
2-PPh2-1-methylimidazole to obtain a hydrogenation yield of 21%
on using a KOH/catalyst ratio of 333:1 and a substrate/catalyst
ratio of 2000:1.[26,27] Table 3 summarizes the effect of changes in
catalyst loading, reaction time, and the addition of KOH. After in
Table 3. Catalytic transfer hydrogenation of cyclohexanone.
The asymmetric derivatives of 7 were pursued in analogy to
the parent compound, albeit that the solventless oxime synthesis
was not effective, but reacting L-menthone with the
hydroxylamine·HCl salt in an EtOH/H2O mixture did provide
oxime 16 as a colorless liquid after purification via crystallization
at 5°C (73%; Scheme 7).[30] Subsequent treatment with MsCl,
NEt3 and benzotriazole induced the Beckmann rearrangement via
17 to the desired benzotriazolyl azepine adduct 19, which was
isolated as an orange liquid (90%).
Whereas the ring expansion could have generated either or
both regioisomers 18 and 19 (Scheme 7, pathways A and B), the
NMR spectra showed only a single set of signals for the Me and
i-Pr CH3-groups (δ(1H) 1.11, 1.08, 1.06 ppm; δ(13C) 24.2, 20.1,
17.7 ppm), indicating the formation of a single isomer. Based on
the reported selectivity of related asymmetric cyclohexanone
Cat.
t
Yield[a]
(%)
Loading
(mol%)
Entry
precatalyst (M)
L
Additives
(h)
1
2
3
4
5
[Ru(p-cym)Cl2]2
[Ru(p-cym)Cl2]2
[Ru(p-cym)Cl2]2
[Ru(p-cym)Cl2]2
[Ru(p-cym)Cl2]2
7b
7b
7b
7b
7b
3
3
3
1
1
2
3
3
20
-
-
-
-
20
20
4
quant.
0
quant.
5 mol%
KOH
6
7
[Ru(p-cym)Cl2]2
[Ru(p-cym)Cl2]2
7b
7b
1
1
4
4
0
1 mol%
NaBF4
quant.
5 mol%
KOH,
1 mol%
NaBF4
8
[Ru(p-cym)Cl2]2
7b
0.5
2
quant.
2.5 mol%
KOH
9
[Ir(COD)Cl]2
[Ir(COD)Cl]2
7b
7b
1
1
4
4
2
-
10
quant.
5 mol%
KOH
11
[Ir(COD)Cl]2
7b
1
4
84
5 mol%
KOH,
1 mol%
NaBF4
Reaction conditions: cyclohexanone in i-PrOH (2M), reflux. [a] Determined by
GC.
Scheme 7. Chiral benzotriazolyl-tetrahydroazepine synthesis.
6
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10.1002/chem.202101921
Chemistry - A European Journal
FULL PAPER
substrates In the Beckmann rearrangement,[31] we expected the
formation of 19 to be favored. Whereas crystals of 19, grown in a
MeCN solution at −20 °C, were too temperature sensitive to
isolate for X-ray crystallography, protonation with HOTf in CHCl3
gave a thermally stable salt (20, 79%) in sharp contrast to the
highly unstable unsubstituted azepinium triflate 5. Crystals
suitable for X-ray analysis were obtained by slow diffusion of
pentane into a DCM solution of 20. Its molecular structure (Figure
11) concurs with the anticipated (2S,5R)-2-i-Pr-5-Me-regioisomer
19 with both alkyl groups in equatorial positions. Compared to the
non-protonated 4 (Figure 6), the benzotriazolyl group of 20 is
rotated by 155o and tilted with respect to the imine group
[20: N1-C1-N2-N3 = −25.9(3); 4: N1-C1-N2-N3 = 179.54(8)].
Clearly, protonation of the imine group prevents the
intramolecular H-bonding that facilitated the planar conformation
of 4; the N1-H1 hydrogen interacts only with the triflate anion
[H1- - -O1 = 1.97 (2) Å]. The positively charged N1 assumedly
causes the slightly tighter benzotriazolyl-bonding [20: C1-N2 =
1.384(3), N2-N3 = 1.403(3) Å; 4: C1-N2 = 1.4334(12), N2-N3 =
1.3746(11) Å]. The parameters of the iminium group [C1-N1 =
1.280(3) Å, N1-C1-C6 = 122.68(19)°, C1-N1-C2 = 125.01(19)°]
are comparable to those for 6a and 6c (Figure 6).
shows a tetrahydroazepine chair similar to the one in 20 with the
2S-i-Pr and 5R-Me indeed in equatorial positions and confirms
that the stereochemical information of the L-menthone is retained
over the synthesis. The conformation and bonding parameters of
22 compare closely to that of the achiral, non-substituted 6a
(Figure 7) [22: C1-N1 = 1.289(2) Å, P1-C1 = 1.8252(18) Å,
N1-C1-P1 = 121.79(14)°; 6a: N1-C1 = 1.2858(16) Å, P1-C1 =
1.8269(13) Å, N1-C1-P1 = 123.06(10)°].
Scheme
8.
Access
of
asymmetric
ligands
21
from
chiral
benzotriazolyltetrahydroazepine 19.
Figure 11. Displacement ellipsoid plot of 1-benzotriazolyl-(2S,5R)-2-isopropyl-
5-methyl tetrahydroazepinium triflate 20 at the 50% probability level. The triflate
anion and C-H hydrogen atoms are omitted for clarity, with exception of H2 and
H5. Selected bond lengths (Å) and angles (°): C1-N1 = 1.280(3), C1-N2 =
1.384(3), N1-C2 = 1.487(3), N1-H1 = 0.89(2), C1-C6 = 1.490(3), N2-N3 =
1.403(3), N3-N4 = 1.278(3), N2-C11 = 1.390(3), N1-C1-N2 = 117.2(2), N1-C1-
C6 = 122.68(19), N2-C1-C6 = 120.1(2), C1-N1-C2 = 125.01(19), N1-C1-N2-N3
= −25.9(3), C6-C1-N2-N3 = 153.6(2).
Figure 12. Displacement ellipsoid plot of 2S,5R-phosphaazepinium triflate 22
at the 50% probability level. The triflate anion and C-H hydrogen atoms are
omitted for clarity, with exception of H2 and H5. Selected bond lengths (Å) and
angles (°): P1-C1 = 1.8252(18), P1-C11 = 1.8217(19), P1-C17 = 1.8259(18),
C1-N1 = 1.289(2), N1-C2 = 1.490(2), N1-C1-P1 = 121.79(14), N1-C1-C6 =
120.82 (16), C1-N1-C2 = 124.57(16).
The introduction of the phosphane group on the chiral ring
could not be achieved in analogy with the synthesis of the
non-chiral ligands 7 (Scheme 3): surprisingly, treatment of the
protonated precursor 20 with diphenylphosphane yielded its
dehydrocoupling product tetraphenyldiphosphane.[32-34] Instead,
the phosphane group was introduced by treating 19 with lithium
phosphides LiPR2 (R = Ph, n-Bu) in THF to give the desired chiral
cycloiminophosphane 21 in 53%, after purification via an
acid/base work-up involving salt 22 (Scheme 8). The 31P NMR
spectrum showed a single resonance at δ 6.6, indicating the
absence of flippamers, which was attributed to the reduced
flexibility of the ring on which the i-Pr and Me substituents favor
equatorial positions.
Single crystals of 22 suitable for an X-ray structure
determination were obtained by slow diffusion of pentane into a
saturated DCM solution. The molecular structure (Figure 12)
Scheme 9. The coordination of ligands 21 to RuII and RhIII.
7
This article is protected by copyright. All rights reserved.

10.1002/chem.202101921
Chemistry - A European Journal
FULL PAPER
Coordination of chiral ligand 21 to RuII gave the
corresponding P-κ1 complexes 23 (Scheme 9; 19%, δ(31P) 24.8
ppm). Likewise, coordination of 21 to RhIII afforded κ1-RhIII
complex 24 that showed, akin to complex 12a (vide supra), a
broad 31P NMR signal at room temperature, which resolved at
−90°C into a series of doublets with two major P-κ1 resonances
(δ 34.3 ppm, 1JP,Rh = 140.9 Hz, 38%; δ 22.8 ppm, 1JP,Rh = 142.5
Hz, 42%; Figure 13) and four minor ones with couplings indicative
of P-κ1 and κ2 bonding (δ 29.9 ppm, 1JP,Rh = 140.9 Hz, 7%; δ 26.5
ppm, 1JP,Rh = 137.7 Hz, 7%; δ 24.7 ppm, 1JP,Rh = 137.7 Hz, 4%;
δ 20.7 ppm, 1JP,Rh = 115.0 Hz, 2%). The P-κ1 signals likely reflect
different rotamers, as the absence of 31P-31P couplings rules out
µ-complexation. In contrast to 12a, no N-κ1 signal was detected
for 24, presumably because the adjacent i-Pr group discourages
coordination at this site. Chloride abstraction converted 24 and its
indicative of dynamic conformational behavior of the
tetrahydroazepine ring. The ligands coordinate both in P-κ1- and
P,N-κ2-fashion to W(carbonyl) complexes, which were analyzed
by IR spectroscopy to quantify the ligands donor strength.
Coordination to [RhCp*Cl2]2 gave a dynamic mixture of κ1- and
κ2-complexes, that on treatment with silver triflate culminate in
only the κ2-complexes. The reaction with [Ru(p-cym)Cl2]2
selectively provides P-κ1-complexes, which were also effective
catalysts for the hydration of benzonitrile (1.4 mol%, 180 oC, 3 h,
up to 79%) and the transfer hydrogenation of cyclohexanone (0.5
o
mol%, 83 C, 2 h, quant.); for the latter reaction also iridium(I)
could be used (1 mol%, 83 oC, 4 h, quant.). Finally, as a preamble
to asymmetric catalysis, a chiral cycloiminophosphane could be
accessed from the natural precursor L-menthone via a selective
Beckmann rearrangement. It was characterized by X-ray
crystallography, and used to access RhIII- and RuII-complexes.
These chiral ligands form promising candidates for the study of
asymmetric 1,3-P,N-catalysis in the future.
1
isomers to κ2-25 (δ(31P, DCM) −11.3 ppm, JP,Rh = 106.6 Hz),
which was calculated to be energetically favored by 3.1 kcal mol-1
over its epimer 25* (ωB97XD/6-31+G(d,p), Def2-TZVP for
Rh).[6b,d,35-39] The obtained chiral transition metal complexes might
be useful for asymmetric catalytic reactions, but such
investigations were outside the scope of the present study. Based
on the performance of ligands 7 (vide supra), asymmetric transfer
hydrogenation seems a promising starting point.
Experimental Section
Preparation of Compounds. The syntheses and full characterization of
4-7, 9-14, 19-25, the Lewis acid catalyst screening of 4, and the P-κ1 and
κ2 Rh(III) complexes of 9 are described in full detail (14 pages) in the
Supporting Information, which also contains their 1H-, 13C{1H}-, 19F{1H}-,
and 31P-NMR spectra (37 pages).
Computational Procedure. Density functional calculations were
performed at the ωB97X-D[36] level of theory using Gaussian09, revision
A.02.[37] Geometry optimizations were performed using the 6-31+G(d,p)[38]
basis set (Def2-TZVP for Rh)[39] and the nature of each stationary point
(see Supporting Information) was confirmed by frequency calculations.
X-ray crystallography. Deposition numbers CCDC 2084404 (for 4),
2084405 (for 6a), 2084406 (for 6c), 20844071433432 (for 10), 2084408
(for 20), and 2084409 (for 22) contain the supplementary crystallographic
data for this paper. These data are provided free of charge from the
Cambridge
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cif. Their single-crystal X-ray structure
determinations are described in the Supporting Information.
Figure 13. 31P NMR spectra of 24 at room temperature and −90 °C.
Acknowledgements
Conclusion
This work was supported by the Council for Chemical Sciences of
The Netherlands Organization for Scientific Research (NWO/CW).
We thank T. van Dijk for measuring high resolution mass-spectra
and for fruitful discussions. M. K. Jongkind and R. Hoogendoorn
are acknowledged for contributing to the synthesis of 9. A. Chirila
and M. M. Heeren contributed to screening suitable Beckmann
rearrangement substrates.
This study reports the synthesis of cyclic 2-phospha-
tetrahydroazepines as novel 1,3-P,N-ligands with the intend to
open new opportunities in coordination chemistry and catalysis.
This
class
of
cycloiminophosphanes
comprises
a
seven-membered aliphatic imine ring with which it complements
other classes of 1,3-P,N-ligands, including the aromatic
2-pyridyl- and 2-imidazolyl-phosphanes, as well as the recently
reported acyclic iminophosphanes and phosphaamidines. The
ligands were readily obtained in a one pot process, through the
Beckmann rearrangement of cyclohexanones to reactive nitrilium
ion intermediates, which were then trapped with benzotriazole.
Keywords: N,P ligands • Coordination modes • Cooperative
effects • Homogeneous catalysis • Ligand design
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The benzotriazole was then quantitatively replaced for
a
secondary phosphane (R = Ph, n-Bu, Cy), facilitated by triflic acid
activation. With respect to other 1,3,-P,N ligands, these
cycloiminophosphanes distinguish themselves by their high
N-basicity and their flexible backbone, as 31P NMR spectroscopy
of the neutral ligands reveals the presence of flippamers,
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10.1002/chem.202101921
Chemistry - A European Journal
FULL PAPER
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10.1002/chem.202101921
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10.1002/chem.202101921
Chemistry - A European Journal
FULL PAPER
Entry for the Table of Contents
1,3-P,N hybrid ligands are broadly applied in cooperative catalysis, but show limited electronic variation. We report novel analogues
with a high N-basicity, based on 7-membered cyclic imines. Their transition metal complexes demonstrated their donor strength (W),
hemilabile coordination (Rh), and catalytic activity for nitrile hydration (Ru) and transfer hydrogenation (Ru, Ir). Also chiral derivatives
were obtained, synthesized from L-menthone.
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