A Tautomeric λ3/λ5-Phosphane Pair and Its Ambiphilic Reactivity

Article abstract of DOI:10.1021/acs.inorgchem.9b00076

The central phosphorus atom of a novel hydroxyl-functionalized triarylphosphane was shown to reversibly insert into one of the molecule's O-H bonds, which forms the basis for a tautomeric λ³/λ⁵-phosphane equilibrium. For the first time, this equilibrium was detected for a λ³-triarylphosphane and the underlying dynamic process was elucidated by NMR spectroscopy. On the basis of reactivity studies, a nucleophilic character was assigned to the minor species present in solution, the λ³-phosphane. Upon methylation, for example, the λ³-form was selectively removed from the equilibrium and converted to the corresponding phosphonium salt. However, upon generation of an alkoxide via proton abstraction, the electrophilic character of the λ⁵-phosphane in the equilibrium became evident since the alkoxide was found to attack the molecule's phosphorus atom. This intramolecular reaction led to the selective formation of a new anionic λ⁶-hydridospirophosphane.

Full text of DOI:10.1021/acs.inorgchem.9b00076

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
A Tautomeric λ3/λ5‑Phosphane Pair and Its Ambiphilic Reactivity
̈
Jean-Marc Morsdorf, Hubert Wadepohl, and Joachim Ballmann*
̈
Anorganisch-Chemisches Institut, Ruprecht-Karls-Universitat Heidelberg, Im Neuenheimer Feld 276, 69120 Heidelberg, Germany
S
* Supporting Information
ABSTRACT: The central phosphorus atom of a novel
hydroxyl-functionalized triarylphosphane was shown to rever-
sibly insert into one of the molecule’s O−H bonds, which
forms the basis for a tautomeric λ3/λ5-phosphane equilibrium.
For the first time, this equilibrium was detected for a λ3-
triarylphosphane and the underlying dynamic process was
elucidated by NMR spectroscopy. On the basis of reactivity
studies, a nucleophilic character was assigned to the minor
species present in solution, the λ3-phosphane. Upon methyl-
ation, for example, the λ3-form was selectively removed from
the equilibrium and converted to the corresponding phosphonium salt. However, upon generation of an alkoxide via proton
abstraction, the electrophilic character of the λ5-phosphane in the equilibrium became evident since the alkoxide was found to
attack the molecule’s phosphorus atom. This intramolecular reaction led to the selective formation of a new anionic λ6-
hydridospirophosphane.
Chart 1. Ambiphilic Phosphorus Containing Compounds A
and B (D = Donor, A = Acceptor) together with the System
Reported Herein (D)
INTRODUCTION
■
Ambiphilic molecules, i.e., compounds that combine electro-
philic and nucleophile moieties within the same framework, are
known to exhibit unusual reactivities, which may be dominated
either by their electrophilic or by their nucleophilic subunits or
by both of them.1 Over the past decades, frustrated Lewis
acid/base pairs containing a rigid backbone to separate the
nucleophilic donor and electrophilic acceptor units (commonly
phosphines and boranes) have been explored in detail.2 More
recently, ambiphilic reactivity patterns have been discovered
for a number of organophosphorus compounds incorporating
donor and acceptor units that are directly bound to each
other.3 In the case of the diazadiphosphapentale A (see Chart
1), the amido-substituted phosphorus atom was found to react
as an electrophile, for example, upon σ-bond metathesis with
NH3,3a while the carbon-substituted phosphorus atom was
found to exhibit nucleophilic character, for example, upon
alkylation with MeI.3d,4 A similar ambiphilicity was also
reported for the structurally related phosphoramidite B (see
Chart 1).3e Recently, the groups of Kano and Morokuma
reported on the [OPO]-coordinated boronate C (see Chart
1)5 and its preparation starting from BH3, [C7H7]BF4 (i.e., a
hydride abstractor) and Martin’s well-known ring-closed λ4-
phosphoranide anion.6 In the synthesis of C, a unique
rearrangement via a two-fold ring expansion has been
exploited,5 which demonstrates that coordinatively stabilized
λ3-phosphanes are actually readily accessible starting from the
corresponding λ5-phosphanes. Considering that numerous λ5-
dioxyspirophosphanes with fascinating stereochemical proper-
ties are known,7it is not surprising that this research field is
rapidly emerging at presentin particular with respect to the
development of novel organic transformations.8 In view of the
unusual structure of C5 and its novelty in main group
a
a
In the inset, the [OPO]-coordinated boronate C is shown with its
P−B bond represented as dative D → A interaction.
chemistry, it needs to be noted that formally related [ONO]-
coordinated metal complexes have been explored in detail by
Veige and co-workers.9 However, there is a fundamental
difference between these [OPO]- and [ONO]-systems, which
becomes obvious upon inspection of the respective protonated
Received: January 9, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b00076
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Article
forms. In H3[ONO], hypervalency does not play a role in
general,10 which is also true for the deprotonated forms (e.g.,
for [ONO]3−) and for the related [NNN]-systems.11 A similar
situation is found for protonated [NPN]-derivatives, while
H2[OPO]-frameworks are known to form ring-closed λ5-
phosphanes, as the central phosphorus atom tends to insert
into one of the O−H bonds.12 Most often, the ring-closed λ5-
[OPO]-phosphanes are thermodynamically favored over the
corresponding open-chain λ3-[OPO]-phosphanes, although a
few exceptions have been reported.13 Interestingly, the reverse
reaction, i.e., the ring-opening of λ5-[OPO]-phosphanes, is not
easily achieved and the yet known λ5-hydrido-triarylphos-
phanes are not even converted to the open-chain λ3-form upon
deprotonation, as λ4-phosphanide anions are generated
preferentially.6 Nearly half a century ago, Burgada and co-
workers have shown that cyclic λ5-tetraoxophosphanes may
coexist in an equilibrium with the corresponding open-chain
λ3-phosphites.14 However, equilibria of this type have not been
established for hydroxyl-functionalized λ3-triarylphosphanes.
Herein, a de novo synthesis of the λ3-[OPO]-system depicted in
Figure 1 was developed, which was found to coexist in a
Scheme 1. Synthesis of (S)-5/(R)-5 and Equilibrium
between (S)-5/(R)-5 and 5i in Solution
PhPCl2, the acetal impeded a premature formation of an air-
sensitive cyclic λ5-phosphane, which significantly simplified the
workup procedure. After purification, the λ3-phosphane 4 was
easily converted to the cyclic λ5-phosphane 5 via deprotection
with methanolic HCl. The desired product was isolated as a
racemic mixture of (S)-5 and (R)-5. Upon crystallization from
MeOH, single crystals of 5 were obtained and subjected to
XRD (see Figure 1). As expected, 5 was found to crystallize as
a racemic pair in a centrosymmetric space group (P21/n).
Routine NMR spectroscopic measurements suggested that an
equilibrium between the λ5-phosphane 5 and the λ3-
triarylphosphane 5i was present in THF-d8 (see inset in
Scheme 1). For the λ5-phosphanes (S)-5 and (R)-5, a singlet at
−52.6 ppm was observed in the 31P{1H} NMR spectrum. In
the corresponding 19F{1H} NMR spectrum, the four chemi-
cally inequivalent CF3 groups in 5 gave rise to four multiplet
signals in a 1:1:1:1 ratio. For 5i, a singlet was found in the
31P{1H} NMR spectrum at −7.9 ppm, while two broad equally
intense signals were detected in the 19F{1H} NMR spectrum,
due to the diastereotopic, but otherwise equivalent, CF3
groups. To substantiate that both species are indeed in
equilibrium with each other, variable-temperature 31P{1H} and
19F{1H} NMR spectra were recorded over a temperature range
from −60 to +20 °C.16 The thermodynamic state variables
ΔRH, ΔRS, and ΔRG (293 K) were determined by variable-
temperature 31P{1H} NMR spectroscopy using Ph3PO as
internal standard. Fitting the van’t Hoff plot for 5 → 5i
revealed that ΔRH = 7.68 0.20 kJ/mol, ΔRS = 10.72 0.77
J/(K·mol), and ΔRG (293 K) = 10.82 0.30 kJ/mol, which is
in line with an endergonic ring-opening of 5. A dynamic P−H/
Figure 1. ORTEP plot of the molecular structure of (S)-5
(displacement ellipsoids set at 50% probability level, all carbon-
bound hydrogen atoms omitted). Selected bond lengths (Å) and
angles (deg): P−H 1.335(19), P−O1 1.8463(10), O2−H2 0.87(3),
O1−P−H 174.2(8).
tautomeric equilibrium with its λ5-alkoxy-hydrido-triarylphos-
phane (cf. equilibrium D in Chart 1). Given that equilibria of
this type were unprecedented for triarylphosphanes, the novel
λ3/λ5-phosphane pair was examined in detail, in particular with
respect to its dynamics in solution and with respect to its
(ambiphilic) reactivity.
RESULTS AND DISCUSSION
■
The aforementioned tautomeric λ3-triarylphosphane/λ5-al-
koxy-hydrido-triarylphosphane couple 5/5i was prepared in
five steps starting from the commercially available alcohol 1
(see Scheme 1).
Employing a well-documented ortho-lithiation procedure,15
1 was converted to the corresponding dilithium derivative and
quenched with an excess of I2 to afford 2. The hydroxyl group
in 2 was then protected with ClCH2OEt/DIPEA and the acetal
3 isolated in 82% yield. The use of this particular protective
group allowed for the desired lithium-halogen exchange via
reaction with PhLi at 0 °C, which was completed within a few
minutes due to the ortho-directing influence of the acetal
functionality. After reaction of the lithiated intermediate with
1
O−H exchange was observed in the H−1H EXSY spectrum,
which agrees well with a fully reversible ring-opening/ring-
closure process, i.e., with the equilibrium between 5 and 5i.
Upon dissolution of 5 in MeOH-d4, a rapid and quantitative
(>98%) deuterium exchange reaction was observed. Within 1
B
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min, this H/D exchange led to the formation of 5-d2, which
was shown unambiguously by 31P NMR spectroscopy (see top-
row spectra in Figure 2). As expected, the deuterated species
Scheme 2. Synthesis of 6, 7, and 8 Exploiting the
Nucleophilic Reactivity of 5i
Figure 2. Top: 31P NMR spectrum of 5/5i in THF-d8 at 25 °C
(spectrum 1) and 31P{1H} NMR spectrum of 5-d2/5i-d2 in THF-d8 at
25 °C (spectrum 2). The doublet and triplet patterns at −52.6 ppm
Figure 3. ORTEP plot of the molecular structure of 6 as its Et2O
adduct (displacement ellipsoids set at 50% probability level, all
carbon-bound hydrogen atoms omitted). Selected bond lengths (Å)
and angles (deg): P−Li2 2.504(4), Li2−O1 1.865(4), Li2−O2
1.914(4), Li1−O2 1.822(4), Li1−O1 1.889(4), Li1−O2−Li2
88.12(16), Li2−O1−L1 87.61(16).
and −53.5 ppm, respectively, are due to spin−spin coupling (1JH,P
=
1
308.6 Hz and JD,P = 47.5 Hz). Bottom: 19F−19F EXSY/NOESY
spectrum of 5-d2/5i-d2 in THF-d8 at 25 °C.
5i-d2 and 5-d2 were in equilibrium as well, which was shown by
19F−19F EXSY spectroscopy. In the correlation spectrum (see
Figure 2, bottom), exchange cross-peaks were detected for the
interchanging and therefore mutually related CF3 groups of 5i-
d2 and 5-d2. Thus, an equilibrium between 5i-d2 and 5-d2
needs to be present in solution.
structure of 7). A similar reactivity was found upon alkylation
of 5i with [Me3O]BF4], which led to the isolation of the
corresponding phosphonium salt 8 (see Scheme 2). In the 1H
NMR spectrum of 8, the resonance of the P-bound methyl
group was unambiguously identified on the basis of its
2
characteristic JH,P coupling constant (2JH,P = 14.0 Hz).
To further probe the equilibrium between the λ3/λ5-
phosphane couple 5/5i, reactivity studies were conducted.
Upon reaction with LiHMDS, the equilibrium, which lies on
the side of the cyclic λ5-phosphane, was shifted selectively to
the open-chain λ3-phosphane by selectively removing the λ3-
phosphane as its doubly deprotonated derivative 6 (see
Scheme 2). Note that 6 was isolated in a solvent-free form,
but crystallized as its etherate (see Figure 3). A λ4-phosphanide
anion, similar to the one reported by Martin and co-
workers,6c,d i.e., deprotonation of the P−H group in 5, was
not observed in our case.
With the nucleophilicity of the phosphorus atom in the λ3-
triarylphosphane 5i exposed, we posed the question whether
an electrophilic character was found for the corresponding λ5-
phosphane 5. This type of reactivity was indeed observed upon
treatment of the λ3/λ5-phosphane couple with 1,8-bis(di-
methylamino)naphthalene (BDMAN, “proton sponge”). In
Et2O, a proton was abstracted from the hydroxyl group in 5,
generating an alkoxide, which was found to attack the
electrophilic phosphorus atom of the λ5-phosphane. This
intramolecular reaction proceeded selectively and led to the
bicyclic λ6-hydridospirophosphane (R)-9/(S)-9 (see Scheme
3).
Akin to metalation, protonation was found to expose the
nucleophilic reactivity of the ambiphilic couple as well. Upon
reaction with HNTf2 in CH2Cl2, 5i was removed from the
tautomeric equilibrium and isolated in form of its phospho-
nium salt 7 (see SI for an ORTEP plot of the molecular
In the 31P NMR spectrum of 9, a characteristically8b,17
1
upfield shifted doublet with a large JH,P coupling constant of
705.7 Hz was detected at −139.2 ppm. In the crystalline state,
the λ6-spirophosphane anion in 9 was found to adopt a
C
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Scheme 3. Synthesis of (R)-9/(S)-9 Exploiting the
Electrophilic Reactivity of (R)-5/(S)-5
distorted octahedral structure with an H1−P−C1 angle of
168.0° between the axially oriented atoms (see Figure 4).
Figure 5. Optimized structures and Gibbs free energy profile for the
ring-opening reaction 5 → 5i taking one molecule of THF into
account (B3LYP/6-311+G(d,p), DFT-D3 dispersion, and PCM
solvation correction for THF, 5trans O‑P‑H set to 0.0 kcal/mol). The
barrier for the Berry-pseudorotation was not determined as indicated
by the dashed line interconnecting the energy levels between
Figure 4. ORTEP plot of the anion in 9 (displacement ellipsoids set
at 50% probability level, all carbon-bound hydrogen atoms omitted).
Selected bond lengths (Å) and angles (deg): P−H1 1.325(19), P−O1
1.8386(14), P−O2 1.8415(14), P−C10 1.8760(19), P−C19
1.895(2), P−C1 1.8780(19), O1−P−O2 84.59(6), C10−P−O2
85.71(7), C19−P−O1 92.00(7), C19−P−C10 97.64(9), O1−P−
C1 85.32(7), O2−P−C1 90.62, C10−P−C1 94.74(8), C19−P−C1
95.43(8), H1−P−C1 168.0(8).
5trans O‑P‑H and 5cis O‑P‑H
.
corresponding cis-configured species, presumably via Berry-
pseudorotationan isomerization pathway, which is well-
documented for λ5-phosphanes.7a,21 Next, the transition state
containing a six-membered ring with an O···H···P motif is
reached, before the O−H bond in 5i is formed. In the
optimized structure of 5i, hydrogen bonds between the two
hydroxyl groups were detected in silico and the THF molecule
was found to serve as a hydrogen bonding acceptor in all the
optimized structures. This is in line with the experimental
finding that 5 was isolated as its THF solvate (and crystallized
as MeOH solvate). The aforementioned significantly more
positive ΔRG values, which were calculated without an
explicitly included THF molecule, are considered meaningful
as well: Upon dissolution of rigorously dried samples of 5 in
CD2Cl2, no signals corresponding to 5i were detected by NMR
spectroscopy, indicating that intermolecular hydrogen bonding
interactions to the surrounding solvent are of crucial
importance for the detection of the λ3/λ5-phosphane
equilibrium described in this Article.
Interestingly, an aryl-substituent is trans to H1 in 9, while an
alkoxide was found in trans-position to the corresponding
hydrogen atom in 5. As expected, 9 crystallized as a racemic
mixture in a centrosymmetric space group (P1
the case for 5 (P21/n) (vide supra).
̅
), which is also
With the reactivity of the tautomeric λ3-triarylphosphane/λ5-
alkoxy-hydrido-triarylphosphane pair explored in detail, light
was shed on the underlying ring-opening reaction 5 → 5i by
modeling the reaction pathway computationally.18 At the
B3LYP/6-311+G or B3LYP/6-311++G-(2df, 2p) level of
theory,18a,b endergonic reaction profiles were found, but
exceedingly large ΔRG values were obtained, which varied
between 6.9 and 10.6 kcal/mol (exp. ΔRG ≈ 2.6 kcal/mol)
depending on the basis set and the employed solvation model
(no solvation, polarizable continuum model, or SMD
model).18c,d This situation improved significantly when
dispersion corrections were taken into account,18f although
overcompressed structures were obtained,19 which was
corrected by including one THF molecule into the model.
This approach led to reasonable ΔRG values of 4.0 kcal/mol
and a barrier of approximately 14.6 kcal/mol at the B3LYP/6-
311+G(d,p) level of theory (see Figure 5).20
CONCLUSIONS
■
In summary, two molecules with opposing reactivities were
found to coexist in an equilibrium, namely, the hydroxyl-
functionalized λ3-phosphane 5i and the λ5-alkoxy-hydrido-
triarylphosphane 5. While intra- and intermolecular equilibria
between tri- and penta-coordinated phosphorus species are
well-known for phosphites and phosphoramidites (e.g.,
compound B, cf. Chart 1), equilibria of this type were hitherto
undocumented for hydroxyl-functionalized triarylphosphanes.
Starting from the thermodynamic minimum, i.e., from 5 with
the P-bound oxygen and hydrogen atoms mutually trans-
positioned, the system was found to isomerize to the
D
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The experimental results and the insights gained from DFT
calculations both suggest that the choice of the solvent plays an
important role for the equilibrium between 5 and 5i, as
hydrogen bonding interactions significantly alter the thermo-
dynamics of the ring-opening reaction 5 → 5i, which is
reflected by the ratio between both species in solution
(CD2Cl2 vs THF-d8). Reactivity studies revealed that the
minor species in equilibrium (5i) may be either deprotonated
or selectively protonated (or methylated) to afford either the
dilithium salt 6 or the phosphonium salt 7 (or 8). To
supplement the latter P-nucleophilic reactivities of the
tautomeric 5/5i pair, a proton was selectively abstracted
from the major species in equilibrium (5), which exposed the
electrophilicity of 5 and led to the preparation of the new
anionic λ6-hydridospirophosphane 9.
support, fruitful discussions, and continued interest in our
work.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00076.
■
S
Experimental procedures for the preparation of 2−9,
selected NMR and IR spectra, detail on DFT methods
and optimized coordinates for selected structures,
crystallographic data and ORTEP plot of the molecular
structures of 7 (PDF)
Accession Codes
CCDC 1863225, 1863226, 1863230, and 1863231 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: joachim.ballmann@uni-heidelberg.de. Phone: (+49)
6221-54-8596.
■
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group elements, see: Chu, T.; Nikonov, G. I. Oxidative Addition and
Reductive Elimination at Main-Group Element Centers. Chem. Rev.
2018, 118, 3608−3680.
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Joachim Ballmann: 0000-0001-6431-4197
Author Contributions
The experiments were designed and carried out by J.-M.M.;
DFT calculations were carried out and analyzed by J.B.; H.W.
conducted all the crystallographic studies. The manuscript was
written through contributions of all authors. All authors have
given approval to the final version of the manuscript.
Funding
We thank the Bernthsen Foundation (University of Heidel-
berg) for funding of this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank Prof. Dr. Markus Enders and Dr. Ju
their support with NMR measurements. Patrick W. Antoni is
gratefully acknowledged for his help with the DFT calculations.
The authors acknowledge support by the state of Baden-
■
̈
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(b) Kajiyama, K.; Yoshimune, M.; Nakamoto, M.; Matsukawa, S.;
Kojima, S.; Akiba, K.-y. Highly Selective One-Pot Synthesis of
Wurttemberg through bwHPC and the German Research
̈
Foundation (DFG) through grant no. INST 40/467-1 FUGG
(JUSTUS cluster). We thank Prof. Dr. L. H. Gade for generous
E
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