Synthesis of Azaphosphinines by Directed Inverse-Electron-Demand Hetero-Diels–Alder Reactions with Na(OCP)

Article abstract of DOI:10.1002/chem.201802173

Herein, a straightforward synthetic approach towards azaphosphinines is reported. The synthesis of 1,3- and 1,4-aza-λ³-phosphinines as well as 1,2,4-diaza-λ³-phosphinines by inverse-electron-demand hetero-Diels–Alder reactions of sod

Full text of DOI:10.1002/chem.201802173

A Journal of
Accepted Article
Title: Synthesis of Azaphosphinines by Directed Inverse Electron
Demand Hetero-Diels-Alder Reactions with Na(OCP)
Authors: Max M. Hansmann
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Synthesis of Azaphosphinines by Directed Inverse Electron
Demand Hetero-Diels-Alder Reactions with Na(OCP)
Max M. Hansmann*
Abstract: Herein, a straightforward synthetic approach towards
azaphosphinines is reported. The synthesis of 1,3- and 1,4-aza-λ3-
phosphinines as well as 1,2,4-diaza-λ3-phosphinines by inverse
electron demand hetero-Diels-Alder reactions of sodium 2-
phosphaethynolate [NaOCP] with triazines and tetrazines is studied.
In the case of 1,2,4-triazines a hetero-D.A. reaction was developed
which relies on a new directing group approach based on the
complexation of sodium to form an ionic Do->Na-OCP tether. The
first X-ray characterization data for aza-λ3-phosphinines as well as
the first phosphinine triflates are presented.
Figure 1. Phosphinine (I), literature known examples of aza-λ3-phosphinines
λ3-Phosphinines (I), the higher homologs of pyridines, first
synthetized by Märkl in 1966,1 have found various applications
(II-IV) as well as P-heterocycles derived from NaOCP (V-VIII).
This approach raised my interest in the synthesis of novel
phosphorus ligands, namely azaphosphinines, by an inverse
electron demand hetero-Diels-Alder (D.A.) reaction with
subsequent retro-D.A. under loss of N2. Note, during the
preparation of this manuscript, Grützmacher et al. reported one
example of a phosphanaphthalene via N2 elimination, however,
the synthesis requires reaction times over several weeks to
months at 80°C.18 For accessing azaphosphinines triazines and
tetrazines should be ideal starting materials as they are electron-
deficient, known to undergo inverse electron demand hetero-D.A.
reactions with electron rich partners.19 Depending on the
regioselectivity, the reaction of NaOCP with the three isomers of
triazines, as well as tetrazines should lead to a variety of new
azaphosphinines (Scheme 1). Herein a straightforward synthesis
of some of these new heterocycles utilizing NaOCP is presented.
ranging from ligands, due to their excellent π-acceptor abilities,
to applications in material science.2 Based on the analogy
between pyridine and phosphinine, the mixed six-membered N/P
heteroaromatic systems, the aza-phosphinines, have been much
less investigated and remain a lab curiosity, presumably a result
of their challenging synthetic access. Shortly after the discovery
of phosphinines, Märkl described the first synthesis of a 1,4-aza-
λ3-phosphinine (II) by flash vacuum thermolysis (Figure 1).3
Besides an additional single example by Regitz in 1990,4 the
1,4-aza-λ3-phosphinine scaffold and its reactivity remained
unexplored.5 Furthermore, there have been only two reports on
the synthesis of 1,3-aza-λ3-phosphinines such as III starting from
1,3-azapyrylium compounds6 or an amino-phosphaalkyne.7
While 1,2-aza-λ5-phosphinines have found applications in
material science,8 only very few 1,2-aza-λ3-phosphinines such
as IV were synthesized by flash vacuum pyrolysis,9 or from
1,3,2-diazaphosphinine.10 Considering the importance of N and
P-heterocyclic compounds, it seems remarkable that no
coordination complex or X-ray structure has been reported for
any isomer of the aza-λ3-phosphinine.
The phosphaethynolate anion [PCO]- first described as its
lithium salt in 1992 by Becker et al.,11 remained unexplored until
the recent novel one-step preparation.12 Since then NaOCP has
found applications in the synthesis of a range of previously
unknown phosphorus heterocycles,13 such as V,14 VI,15 VII,16 or
VIII.17 Importantly, Grützmacher et al. could show that the
reaction of α-pyrone with NaOCP at 60°C affords the sodium salt
of 2-phosphininol (V) with concurrent loss of carbon dioxide.14a
Scheme 1. Targeted approach towards aza-phosphinines via hetero-D.A.
In the following aza-dienophiles are studied in the order of
increasing reactivity. While 31P {1H} NMR spectroscopy at room
temperature indicated no reaction between 3,5,6-triphenyl-1,2,4-
triazine (1a) with NaOCP in THF, elevated temperatures (90°C;
the reaction was performed in a closed pressure tube with large
headspace volume) and prolonged reaction times (5 days) led to
the generation of two new compounds with their 31P {1H} NMR
resonances at δ = 178 and 123 ppm in a 4:1 ratio (Scheme 2).
Both compounds were assigned based on 31P {1H} NMR as a
mixture of 1,4 (2a) and 1,3-azaphosphinines (3a) compared to
[a]
Dr. Max M. Hansmann
Georg-August Universität Göttingen
Institut für Organische und Biomolekulare Chemie
Tammannstraße 2, 37077 Göttingen (Germany)
E-mail: mhansma@uni-goettingen.de
Supporting information for this article is given via a link at the end of
the document.
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calculated data. DFT calculations of 2a and 3a at the GIAO-
PBE1PBE/6-311G(2d,2p)//B3LYP-D3BJ/def2-TZVP
2c and 2d [31P {1H} NMR: δ = 193.2 (2c); 192.8 ppm (2d)]. Note,
typically inverse electron D.A. reactions of triazines require
harsh conditions and are poorly regioselective.22 Interestingly,
switching to 1e further boosts the reaction rate (< 30 min at rt)
and selectivity to give 2e as single isomer. The proton in the 1,4-
level20
predict 31P {1H}: δ = 194 ppm (2a) and 137 ppm (3a) in good
agreement with the experimental data. Calculations indicate no
significant difference in thermodynamic product stabilities
between 1,4 and 1,3-regioisomers, slightly favoring the 1,3-
azaphosphinine 3a by 1.7 kcal/mol (B3LYP-D3BJ/def2-TZVP;
vide intra). As the reaction proceeded very slowly and reaction
times > 4d were required, we investigated enhancing the
reactivity by increasing the electron-deficiency of the triazine.
Indeed, 1,2,4-triazine 1b featuring a para-CF3 group reacts
faster with NaOCP (4 days at 65°C) but still leads to a poor
regioselectivity [4:1 mixture: 31P {1H}: δ = 185/130 ppm].21
3
azaphosphinine 2e shows a large JHP-coupling constant [1H
3
NMR (d8-THF): δ = 8.85 ppm (d, JHP = 31 Hz)]. The 13C for the
3-oxo carbon is at δ = 201.1 ppm (d, JCP = 44 Hz) significantly
downfield shifted compared to the other three carbon atoms in
the heterocycle [δ [ppm]: 147.2 (d, JCP = 53 Hz), 145.0 (d, JCP
=
19 Hz), 141.3 (d, JCP = 13 Hz)], indicating a partial phospha-acyl
contribution and disruption of the aromaticity (vide infra). The 1H-
15N HMBC spectrum allows assignment of the nitrogen atoms:
15N δ = -82.7 (pyridine) and -65.8 ppm (1,4-azaphosphinine; 2e);
-83.2 and -56.4 ppm (2d).23 In case of 2d it was possible to grow
single crystals suitable for X-ray diffraction (Figure 2).24
Scheme 2. Synthesis of aza-λ3-phosphinines from 1,2,4-triazines.
As a result of the poor regioselectivity and reactivity a directed
D.A. approach was investigated. The interaction of the sodium
cation with a nitrogen directing group was planned to generate
an ionic tether group I [py-Na-OCP] thereby increasing rate and
selectivity for the generation of the transient diazaphospha-
barrelene II (Scheme 3a).
Figure 2. X-ray solid state structure of 2d. Ellipsoids are drawn at the 50%
probability level. Selected bond parameters in [Å] and [°]: P1-C1 1.770(2); C1-
C2 1.442(2); C2-N1 1.342(2); N1-C3 1.339(2); C3-C4 1.399(2); C4-P1
1.731(2); C1-O1 1.288(2); P2-C5 1.769(2); C5-C6 1.437(2); C6-N2 1.344(2);
N2-C7 1.341(2); C7-C8 1.399(2); C8-P2 1.726(2); C4-P1-C1 102.83(8); C2-
N1-C3 124.6(1).
In the solid-state 2d contains two independent molecules in
the unit cell, forming a polymer bridged by sodium cations in
between the 1,4-azaphosphinin-3-olate and the bipyridine
moieties. Interestingly, C1-P1 [1.770(2) Å] is elongated
compared to P1-C4 [1.731(2) Å], reflecting some contribution of
the acyl/ketone resonance structure. Furthermore, the six-
membered ring is slightly distorted from the planar geometry
[deviation of C1 from the least-square plane by 0.062(1)Å].
While, previously it was shown that 2-oxo-phosphinine (V) could
be alkylated and protonated,14a we were curious to transform the
oxolate-moiety into
a
leaving group by triflation. Note,
phosphinine derived triflates are unknown compounds, which
however, should be valuable precursors for applications in
cross-coupling chemistry.25 Indeed, sodium salt 2c reacts with
Tf2O to afford the corresponding triflate, indicated by a downfield
shifted 31P {1H} NMR signal with fluorine coupling [2c: δ = 219.3
ppm (q, JPF = 20 Hz)]. Upon triflation the “acyl”-carbon atoms
shifts from δ = 201.4 ppm into a more typical aromatic region [δ
= 172.1 ppm (d, JCP = 54 Hz)]. Importantly, it was possible to
obtain single crystals of triflate 4c (Figure 3).24 In comparison to
2d the heterocyclic core in 4c is highly symmetrical and planar
[max. derivation from the least-square plane 0.024(1)] with equal
C-P [1.732(2) vs. 1.733(2)] and similar C-N [1.345(2) vs.
Scheme 3. Transient ionic [donor-Na-OCP] tether strategy for the inverse
electron demand D.A. reaction of 1,2,4-triazines.
Indeed 1c and 1d featuring an ortho-pyridine directing group
react with NaOCP at a dramatically higher reaction rate (14-16h
at 60°C) and excellent selectivity (> 15:1) leading to sodium salts
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orbitals (see Figure S1-2 in SI).26 Calculated nucleus
1.339(2)] bond lengths, while the C-O bond elongates upon
triflation [1.288(2) (2d) vs. 1.436(2) (4c)].
independent chemical shifts [NICS(1)]27 indicate
a
low
aromaticity for all the sodium salts with a slight increase from
phosphinin-2-olate (-4.8) to 1,3-azaphosphinin-4-olate (-5.0) to
1,4-azaphosphinin-3-olate (-5.2). Note, upon triflation or
alkylation the NICS values are shifted by ca. 4-5 ppm indicating
a large aromatic stabilization, although still lower than in
pyridine.28
1,2,3-trazines are known to be more reactive dienophiles than
1,2,4-triazines.29 Unfortunately, the reaction of NaOCP with the
parent 1,2,3-triazine as well as 5-phenyl-1,2,3-triazine led to
intense dark solutions with no new 31P {1H} NMR signal.
However, the more reactive 1,2,3-triazine 5, reacts with NaOCP
at -78°C to turn from a yellow to a bright red solution with
concurrent gas formation (Scheme 4). 31P {1H} NMR (δ = 123
ppm) indicates the formation of a single new product. Calculated
NMR shifts clearly favor the 1,3-isomer (127 ppm) over the 1,2-
isomer (278 ppm). Unfortunately, 6 shows poor solubility,
however, 1H NMR of a suspension in d8-THF shows for the
aromatic signals: δ [ppm] = 9.47 (dd, J = 32 Hz, 2.9 Hz), 8.57
(dd, J = 3.0 Hz, 2.9 Hz) in good agreement with 6.
Figure 3. X-ray solid state structure of 4c. Ellipsoids are drawn at the 50%
probability level. Selected bond parameters in [Å] and [°]: P1-C1 1.732(2); C1-
C2 1.399(2); C2-N1 1.345(2); N1-C3 1.339(2); C3-C4 1.411(2); C4-P1
1.733(2); C1-O1 1.436(2); C4-P1-C1 98.70(7); P1-C1-C2 127.2(1); C1-C2-N1;
C3-C4-P1 123.2(1).
Importantly, upon reaction of NaOCP with 1f, featuring a
pyridine directing group in 6- instead of the 3-position, at 60°C
the previously observed > 15:1 regioselectivity inverted to a 1:4
ratio [31P {1H} NMR: δ = 183: 138 ppm] favoring the 1,3-
azaphosphinine (Scheme 3c). Notably, 1g containing a H-atom
at the 3-position, reacts selectively (> 15:1) and fast (3h at rt) to
give the 1,3-azaphosphinine 3g (31P {1H} NMR: δ = 148.3 ppm).
The proton shows a large P-coupling constant [1H NMR: δ =
2
9.73 ppm (d, JHP = 51 Hz)], while both 13C NMR signals
adjacent to phosphorus are downfield shifted [210.1 ppm (d, JCP
= 46 Hz), 185.1 ppm (d, JCP = 70 Hz)]. The 15N NMR for the 1,3-
azaphosphinine (δ = -90.5 ppm) is downfield shifted by 25-34
ppm compared to the 1,4-isomers. To support the idea of the
ionic tethered hetero-D.A. the reaction to form 3f was performed
under identical conditions but in the presence of 15-crown-5
inhibiting coordination of the sodium cation to the directing
group. Indeed, a reversal in regioselectivity from 1:4 to 2.5:1 was
observed, favoring the 1,4-azaphosphinine. In line with the
tether strategy, DFT calculations show that the sodium cation
features short (py)-Na-(OCP) distances in the transition state
and in the diazaphospha-barrelene intermediate (Figure 4).
Scheme 4. Synthesis of 1,3-aza-phosphinine 6 and its analog without nitrogen.
Figure 5. X-ray solid state structure of 8. Ellipsoids are drawn at the 50%
probability level. Selected bond parameters in [Å] and [°]: P1-C1 1.792(1); C1-
C2 1.435(2); C2-C3 1.379(2); C3-C4 1.407(2); C4-C5 1.395(2); C5-P1
1.724(1); C1-O1 1.277(1); C5-P1-C1 101.79(5); P1-C1-C2 120.78(8); C4-C5-
P1 126.70(8).
Figure 4. DFT-calculated structures for the transition state (left) and
the .diazaphosphabarrelene intermediate (right) to form 2d.
In order to compare the electronic structure of 6 with its non-
aza derivative 8 the reaction of methyl coumalate (7) with
NaOCP was studied. Notably, from the two potential
regioisomers only 8 [31P {1H}: δ [ppm] = 149] is formed
selectively and fast (<10 min at rt) and could be structurally
characterized (Figure 5).24 In the solid-state structure two
Compared to phosphinin-2-olate and its methylated derivative,
introduction of nitrogen atoms has a significant influence on the
charge distribution, dipole moment, as well as on the frontier
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sodium cations are bridged between the phosphinin-2-olate and
the ester moieties forming a polymeric network. Note, the
reaction of NaOCP with pyridazine derivative 7’ does not lead to
8. Sodium salt 8 can be protonated (9) or triflated (10).
Interestingly, both compounds are remarkably stable and can be
purified by column chromatography in air.
reagents. We are currently exploring the scope of the directed
hetero-D.A. reactions as well as reactivity and coordination
chemistry of the new heterocycles. The functional moiety of a
phosphinine triflate should also pave the way for further
functionalization chemistry, such as cross-coupling strategies
with a wide array of applications.
1,2,4,5-tetrazines are known to be superiorly suited for inverse
electron demand D.A. reactions.30,31 Upon addition of NaOCP to
a solution of 1,2,4,5-tetrazine 11 the instant formation of a gas is
detected with a color change from pink to orange (Scheme 5).
31P {1H} NMR spectroscopy indicates a clean and rapid (<
10min) reaction to a new compound [31P {1H}: δ = 130 ppm]. An
X-ray diffraction study proved it to be the sodium salt of 1,2,4-
diazaphosphinin-5-olate (12), a previously unknown heterocycle
(Figure 6).31,32 In the solid-state structure sodium is coordinated
to the two nitrogen atoms of the 1,2-diaza moiety, while the
heterocyclic core is surprisingly planar [max. derivation 0.03(1) Å
at C1; calc. NICS value -5.3 ppm]. Similar to 2d the P-C bonds
are significantly different in length [1.776(2) vs. 1.743(2)], further
supported by 13C NMR data [δ [ppm] = 201.0 (JCP = 46 Hz);
190.2 (JCP = 72 Hz); 148.8 (JCP = 22 Hz)].
Acknowledgements
This work was supported by the Fonds der chemischen Industrie
(Liebig scholarship to M.M.H) and DFG (INST 186/1237-1). Dr.
Christopher Golz is acknowledged for the X-ray analysis. Prof.
Dr. Manuel Alcarazo is thanked for his continuous support.
Keywords: phosphinine • Diels-Alder reaction • P-heterocycles
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In summary, an inverse hetero-D.A. reaction with tri and
tetrazines was developed leading to new aza-λ3-phosphinines as
well as diaza-λ3-phosphinines, proven by the first X-ray data for
these compound classes. The concept of an ionic directed
hetero-D.A. was introduced and the directed reactions proceed
under remarkably mild conditions with high regioselectivity,
allowing to selectively access 1,4 and 1,3-azaphosphinines
depending on the position of the directing group. Even though
transient tether strategies for D.A. reactions are known,33 tethers
with tight ion contact pairs are unprecedented. This is especially
interesting as counter-cations are often neglected in reactions
with NaOCP. The here developed approach might be an
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10.1002/chem.201802173
Chemistry - A European Journal
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
A directed inverse electron
demand Diels-Alder reaction
of triazines and tetrazines
leads to novel aza-λ3-
phosphinines with concurrent
loss of N2. We introduce the
concept of an ionic bound
tether for D.A. reactions and
present the first x-ray
Max M. Hansmann*
Page No. – Page No.
Synthesis of
Azaphosphinines by
Directed Inverse Electron
Demand Hetero-Diels-Alder
Reactions with NaOCP
characterization data for
azaphosphinines as well as
the first (aza)phosphinine
triflates.
This article is protected by copyright. All rights reserved.