Oxophosphonium-Alkyne Cycloaddition Reactions: Reversible Formation of 1,2-Oxaphosphetes and Six-membered Phosphorus Heterocycles

Article abstract of DOI:10.1021/jacs.0c03494

While the metathesis reaction between alkynes and carbonyl compounds is an important tool in organic synthesis, the reactivity of alkynes with isoelectronic main-group R2E≠O compounds is unexplored. Herein, we show that oxophosphonium ions, which are the

Full text of DOI:10.1021/jacs.0c03494

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Article
Oxophosphonium-alkyne cycloaddition reactions: reversible formation
of 1,2-oxaphosphetes and six-membered phosphorus heterocycles
Pawel Löwe, Milica Feldt, Marius A. Wünsche, Lukas F. B. Wilm, and Fabian Dielmann
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c03494 • Publication Date (Web): 04 May 2020
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Oxophosphonium-alkyne cycloaddition reactions: reversible formation
of 1,2-oxaphosphetes and six-membered phosphorus heterocycles
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Pawel Löwe,a Milica Feldt,b Marius A. Wünsche,a Lukas F. B. Wilm,a Fabian Dielmanna*
a Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstraße 30,
48149 Münster, Germany.
b Theoretische Organische Chemie, Organisch-Chemisches Institut and Center for Multiscale Theory and
Computation, Westfälische Wilhelms-Universität Münster, Corrensstraße 40, 48149 Münster, Germany.
phosphorus cations – phosphorus heterocycles – cycloaddition reactions – metathesis reactions - Lewis acids
ABSTRACT: While the metathesis reaction between alkynes and carbonyl compounds is an important tool in organic
synthesis, the reactivity of alkynes with isoelectronic main group R2E=O compounds is unexplored. Herein, we show that
oxophosphonium ions, which are the isoelectronic phosphorus congeners to carbonyl compounds, undergo [2+2]
cycloaddition reactions with different alkynes to generate 1,2-oxaphosphete ions, which were isolated and structurally
characterized. The strained phosphorus-oxygen heterocycles open to the corresponding hetero-diene structure at elevated
temperature, which was used to generate six-membered phosphorus heterocycles via hetero Diels-Alder reactions. Insights
into the influence of the substituents at the phosphorus center on the energy profile of the oxygen atom transfer reaction
were obtained by quantum chemical calculations.
Introduction
oxetes, the four-membered metallacycles are formed
reversibly and they exhibit rich chemistry which
predominantly involves insertion reactions into the metal-
carbon bond.34–38 Moreover, thermolysis of I in the absence
of reactants leads to rearrangement reactions involving
the cyclopentadienyl ligands or to the corresponding
hydroxoacetylide II.33,39,40
The metathesis reaction between carbonyls and alkynes is
a powerful method for the synthesis of α,ß-unsaturated
ketones and has received much attention in the
construction of heterocycles and in natural product
synthesis.1–12 This well-understood reaction involves an
oxete intermediate which is unstable towards ring opening
and the formation of the enone product (Scheme 1a).13
Since carbonyl−alkyne [2+2] cycloadditions are orbital-
symmetry-forbidden in the ground state,14 either
photoirradiation15 or the use of a catalyst16,17 have been
established as avenues to overcome the kinetic barriers.
Nonetheless, the strained cyclic intermediate can be
stabilized by electron withdrawing substituents, which has
allowed the isolation and structural characterization of
many oxete derivatives.18–22
a
Our recent success in synthesizing Lewis base-free
oxophosphonium ions30 has inspired us to explore whether
they would undergo cycloaddition reactions with alkynes.
Regarding the analogy to the carbonyl−alkyne metathesis,
this reaction would either result in 1,2σ4λ4-oxaphosphete
four-membered rings or lead to the formation of 1-
phospha-4-oxa-butadiene ions (Scheme 1c).
Given the importance of 1,2σ5λ5-oxaphosphetanes C as
intermediates in the Wittig reaction41,42 and the rich
chemistry of 1,2σ3λ3-oxaphosphetanes B developed by the
group of Streubel,43–48 it is surprising that 1,2-
oxaphosphetes have not yet been isolated. Keglevich and
In contrast to ketones, isoelectronic main group element
E=O doubly bonded species are prone to spontaneous
dimerization or polymerization and thus require kinetic
stabilization. In 2012, Tamao and co-workers isolated the
first heavy ketone analogue with a terminal oxygen atom, a
germanone R2Ge=O.23 Subsequent discoveries of stable
silanones (R2Si=O),24–28 an anionic oxoborane ([R2B=O]-)29
and oxophosphonium ions ([R2P=O]+)30 by the groups of
Filippou, Kato, Inoue, Rieger, Iwamoto, Aldridge and by us
extended this series of heavy ketones. While the chemistry
of these compounds has been extensively explored, [2+2]
cycloaddition reactions with alkynes leading to the heavy
oxete analogues have not yet been encountered. However,
beyond main group elements, Bergman and coworkers
discovered that oxido complexes of titanium and
zirconium undergo cycloaddition reactions with alkynes to
give oxametallacyclobutenes I (Scheme 1b).31–33 Unlike
co-workers
postulated
1,2σ5λ5-oxaphosphete
intermediates in the reaction of phosphine oxides with the
particularly
electrophilic
alkyne
dimethylacetylenedicarboxylate.49 Mucsi and coworkers
showed that these species are unstable towards ring
opening
reactions
leading
to
stabilized
phosphoniumylides.50 Recently the first example of a
1,2σ3λ3-thiaphosphete E was synthesized by Ragogna and
coworkers via transfer of
a phosphinidene sulfide
intermediate to an alkyne.51 Herein, we report the reaction
of oxophosphonium ions with alkynes that gives access to
the first main group element heavy oxetes, namely 1,2σ4λ4-
oxaphosphete ions. Studies on the reactivity of this species
reveal analogies both to alkyne-carbonyl metathesis and to
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the chemistry of oxametallacyclobutenes. In addition,
of the parent oxete C3H4O (6.70 ppm)52 or the metallaoxete
Cp*2Ti(OCPhCH) (7.67 ppm).53 The effect can be
rationalized by an enhanced polarization of the C=C bond
of the four-membered ring as a results of the negative
hyperconjugation of π electron density from the carbon
atom into low lying σ* orbitals of the phosphorus atom.
1
2
3
4
5
6
7
8
computational methods were employed to elucidate the
influence of the substituents at phosphorus on the energy
profile of the reaction mechanism.
Scheme 1. a) Reaction of carbonyls with alkynes
triggered by irradiation or catalysts (cat. = Lewis or
Bronsted acid). b) Reaction of metallocene oxido
complexes
with
alkynes.
c)
Reaction
of
oxophosphonium ions with alkynes to give 1,2σ4λ4-
oxaphosphete cations. d) Examples of isolated four-
9
membered
heterocycles:
oxetane
(A),
1,2-
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oxaphosphetane (B, C), oxete (D), 1,2-thiaphosphete
(E). Dipp = 2,6-diisopropylphenyl.
a) Carbonyl-alkyne metathesis
R4
R3
h
or cat.
R2
R1
R2
electrocyclic
ring-opening
O
R1
O
O
+
R1
R2
[2+2]
R3
R4
R3
oxete
R4
b) Bergman
R2
R1
Cp*
Cp*
OH
O
M
Cp*

M
O
Ti
+
Cp*
Cp*
Cp*
M = Ti
R1 = H
[2+2]
R2
R1
R2
M = Ti, Zr
I
II
metallaoxete
c) This work
Figure 1. Solid-state structure of [2a][OTf]. Only one of the two
crystallographically independent molecules is depicted. Hydrogen
atoms, the OTf- anion and a disordered iPr group are omitted for
clarity. Ellipsoids are drawn at 50 % probability. Selected bond
lengths [Å] and angles [°]: P–N1 1.557(3), P–N4 1.557(3), P–O
1.677(2), P–C1 1.766(3), C1–C2 1.324(5), C2–O 1.413(4), P-O-
C2 88.6(2), N1-P-N4 109.0(2), P-C1-C2 87.9(2), C1-C2-O
105.2(3), O-P-C1 78.3(2).
Dipp
R3
R2
R1
R1
R1
N
O
P
R1
P
O
P
O
R1
=
+
N
R1
R1
[2+2]
N
R2
R3
R2
R3
Dipp
oxaphosphete
d)
O
P
O
P
O
O
P
S
Single crystals suitable for X-ray diffraction studies were
obtained for the triflate salt [2a][OTf]. Two
crystallographically independent molecules were found
with different orientation of the bulky R1 substituents
relative to the oxaphosphete unit leading to minor
differences in bond lengths and angles. The molecular
structure and selected structural parameters of one
oxaphosphete cation [2a]+ are depicted in Figure 1. The
oxaphosphete ring is perfectly planar (sum of angles: 360°)
and kite-like distorted, as evident by the small O-P-C1
angle of 78° and rather large C1-C2-O angle of 105°. The
C1–C2 bond (1.314 Å) and the C2–O bond (1.413 Å) are in
the range of typical C=C double and C–O single bonds,
respectively. The P–O bond of 1.677 Å is considerably
elongated compared to the similarly substituted phosphine
oxide tBu(iPrHN)2PO (1.489 Å)54 and to the acyclic
A
B
C
D
E
Results and Discussion
Reaction of oxophosphonium salts with alkynes. We
previously showed that the Lewis base-free
oxophosphonium salts [(R1)2P=O][X] (R1 = 1,3-bis(2,6-
diisopropylphenyl)imidazolin-2-ylidenamino, [X]- = triflate
[OTf]- or tetrakis[2,6-bis(trifluoromethyl)phenyl]borate
[BArF24]-) ([1][X]) can be prepared by reacting POCl3 with
two equivalents of R1-TMS via elimination of TMS-Cl and
subsequent chloride abstraction using Na[X].30 Heating a
fluorobenzene solution containing [1][BArF24] and
phenylacetylene to 120 °C gave the [2+2] cycloaddition
product [2a][BArF24] as an off-white, moisture sensitive
solid in quantitative yield (Scheme 2). The oxaphosphete
salt [2a][BArF24] shows a characteristic doublet at -14.6
ppm (2JPH = 17 Hz) in the 31P NMR spectrum, which
appears at lower frequency than the 31P NMR signal of the
oxophosphonium ion [1]+ (+59.0 ppm). The formation of
the four-membered heterocycle is further confirmed by
the 13C NMR spectrum, revealing a doublet at 99.9 ppm
(1JPC = 109 Hz) for the phosphorus-bound carbon atom and
a doublet at 163.5 ppm (2JPC = 28 Hz) of the adjacent
carbon atom which is deshielded by the oxygen atom. The
1H NMR signal of the ring proton appears at 3.99 ppm and
is significantly shifted to lower frequency compared to that
t
phosphonium salt [Me(Me2N)2POCH2 Bu][I] (1.546 Å)55,
which indicates a rather weak P–O bond in [2a]+. The
cycloaddition
reaction
between
[1][OTf]
and
phenylacetylene is less selective (cf. Figure S9), therefore
only the [BArF24]- anion was used in further studies.
Scheme 2. Synthesis of oxaphosphete salts [2a-
f][BArF24]. Dipp = 2,6-diisopropylphenyl.
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-
-
reaction56,57 and for the cycloaddition reaction between
BArF24
BArF24
R1
P
1
2
3
4
5
6
7
8
9
Dipp
R1
phenylacetylene derivatives and sterically encumbered
Ti(IV) oxido complexes.32,33,38,39 The less bulky Zr(IV) oxido
complexes, however, show the inverse regioselectivity,31,32
indicating the dominance of steric effects, which might also
control the selectivity in the case of oxophosphonium ion
[1]+.
O
P
O
N
+
H
R
N
R1
H
R
N
Dipp
1
2a-f
][BArF24]
[ ][BArF24
]
[
R1
Cycloreversion.
Previously,
Bergman
and
Hull
Table 1. Scope of terminal alkynes in [2+2]-
cycloaddition reactions with oxophosphonium salt
[1][BArF24]. aConversion according to 1H NMR
spectroscopy after 40 hours when the reaction was
stopped.
demonstrated that oxametallacyclobutenes I undergo
alkyne exchange reactions at room temperature
illustrating the reversibility of the metal oxido-alkyne
cycloaddition reaction.53,58 We found that oxaphosphete
ions show a similar behavior, however, the exchange
reaction requires harsher reaction conditions. Heating a
dichloromethane solution of the heterocycle [2a][BArF24]
with an equimolar amount of 4-ethynyltoluene in a sealed
NMR tube to 120 °C resulted in the slow conversion to the
oxaphosphete salt [2d][BArF24] (Scheme 3). 1H and 31P
NMR monitoring of the reaction progress revealed that the
maximum conversion of 57% was achieved after three
days (Table S1). Interestingly, the characteristic resonance
of the free oxophosphonium ion [1]+ can be detected at
59.1 ppm in the 31P NMR spectrum of the reaction mixture.
The generation of small amounts of [1]+ during the
reaction suggests a sequential alkyne elimination/addition
mechanism for the exchange reaction. Similarly,
phosphirenium salts undergo dissociative alkyne exchange
reactions via transient phosphenium ions.59
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Entry compou
nd
R
conditions
yield
1
2
3
4
5
[2b]+
[2c]+
[2d]+
[2a]+
p-Me2N-C6H4–
p-MeO-C6H4–
p-Me-C6H4–
Ph–
r.t., 30 min
60 °C, 3 h
99%
99%
99%
99%
35%a
100 °C, 12 h
120 °C, 12 h
3,5-(CF3)-
C6H3–
[2e]+
[2f]+
180 °C, 40 h
6
nPr–
60 °C, 19 h
79%
In order to explore possible substituent effects on the
[2+2] cycloaddition reaction, a series of sterically and
electronically different phenylacetylene derivatives were
reacted with oxophosphonium salt [1][BArF24] (Scheme 2,
Table 1), which gave the oxaphosphete salts [2b-
d][BArF24] in quantitative yield. The cycloaddition reaction
with electron rich alkynes is significantly faster than with
Scheme 3: Reaction of oxaphosphete [2a][BArF24] with
4-ethynyltoluene.
phenylacetylene, in particularly with
a
π-donating
-
-
dimethylamino (Entry 1) or a methoxy group (Entry 2) in
para position to the acetylene unit, suggesting a step-wise
reaction pathway for these substrates rather than a
concerted cycloaddition. By contrast, forcing reaction
conditions are required for the cycloaddition reaction with
BArF24
BArF24
R1
R1
P
R1
H
H
120°C, 75 h
CD2Cl2
P
O
O
R1
+
+
Ph
p-Tol
H
H
p-Tol
Ph
2d
[
][BArF24
57%
]
2a
[
][BArF24]
electron deficient alkynes (Entry 5). Heating
a
fluorobenzene solution of [1][BArF24] and 1-Ethynyl-3,5-
bis(trifluoromethyl)benzene in a sealed NMR tube at 180
°C for 40 hours resulted in only 35% conversion to the
oxaphosphete salt [2e][BArF24]. However, the reaction
might additionally be hampered by the increased steric
bulk of the alkyne compared with the other
phenylacetylene derivatives. The cycloaddition reaction
between [1][BArF24] and 1-pentyne has the same
regioselectivity as with aryl alkynes, but is slightly less
selective (Entry 6). The internal alkynes tolane and 1-
phenyl-1-propyne did not undergo cycloaddition reactions
with [1][BArF24] even at elevated temperature (180 °C)
indicating the importance of steric effects in the
cycloaddition reaction.
To prove this hypothesis, we heated a neat sample of
[2a][BArF24] with a heat gun at approximately 300 °C for
one minute. This gave the free oxophosphonium salt
[1][BArF24] and phenylacetylene in 73% yield, as
evidenced by NMR spectroscopy and gas chromatography
(Scheme 4, top). To further examine the thermal cleavage
of
[2a][BArF24]
in
solution,
we
used
N,N-dimethylaminopyridine (DMAP) to trap the generated
free oxophosphonium ion [1]+ (Scheme 4, bottom). Note
that DMAP readily reacts with [1]+ at room temperature to
give the Lewis base adduct [1(DMAP)]+,30 but it does not
react with the oxaphosphete ion [2a]+ at moderate
temperatures. In fact, heating a mixture of [2a][BArF24]
and DMAP in fluorobenzene at 100 °C for 16 hours gave
the Lewis base adduct [1(DMAP)]+ in less than 5% yield.
Complete alkyne elimination and formation of the trapping
product [1(DMAP)]+ required prolonged heating to 150 °C,
which is consistent with a high energy barrier for the
cycloreversion reaction.
Collectively, this study reveals that the [2+2] cycloaddition
reactions between the heavy main group carbonyl
analogue [1][BArF24] and terminal alkynes afford the
corresponding oxaphosphetes [2a-f][BArF24] with reaction
rates that strongly depend on the electronic properties of
the respective alkynes. The reaction is regioselective and
gives the heterocycle with the phenyl group adjacent to the
oxygen atom as the only product. The same regioselectivity
was observed for the carbonyl-alkyne metathesis
Scheme 4: Thermally induced cycloreversion of
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[2a][BArF24] neat (top) and in solution using DMAP as that of ketones (Figure 3). As model systems the reaction
1
trapping reagent (bottom).
of phenylacetylene with four different electrophiles was
2
3
4
5
6
7
8
9
considered: acetone (reaction b), an “unstabilized”
-
BArF24
R1
dimethyloxophosphonium
ion
(reaction
c),
an
H
O
P
300°C, 1 min
neat
oxophosphonium ion bearing N-heterocyclic imines (NHI)
as used experimentally (reaction d), an oxophosphonium
ion comprising one NHI and one N-heterocyclic olefin
(NHO) substituent as previously prepared30 (reaction e).
+
R1
-
Ph
BArF24
R1
1
[ ][BArF24
]
P
O
R1
73%
The energy profile of the reaction between
phenylacetylene and acetone (b) is similar to that of the
previously investigated uncatalyzed reactions between
formaldehyde and methoxyacetylene15 or acetaldehyde
and acetylene.70 As expected for the uncatalyzed reaction,
the [2+2] cycloaddition reaction has a high activation
barrier of 59.3 kcal/mol, which is significantly higher than
those of the oxophosphonium ions (c-e). The oxete
intermediate (CF-b) is unstable with respect to
electrocyclic ring opening via the second transition state
(ΔG‡ = 25.8 kcal/mol) to afford the α,ß-unsaturated ketone
(OF-b). The high energy first transition state (TS1) and the
fact that the open form is the thermodynamically stable
product are key differences to the energy profiles of the
heavier phosphorus congeners (reactions c-e). These
differences are particularly evident for the unstabilized
oxophosphonium ion [Me2P=O]+ (c), which forms the most
stable oxaphosphete in this series with a barrier of
36.8 kcal/mol to the open form. Moreover, the alkyne
addition constitutes a stepwise process due to the high
electrophilicity at phosphorus. Natural Bond Orbital
(NBO)71 analysis confirms that the reactant complex (RC-c)
is a σ-bonded species which is formed without a barrier.
The C⋯P distance (1.93 Å) is in the range of an elongated
C–P single bond and the geometries around the P and C
atom are significantly distorted from the previous planar
and linear structure, respectively (see Figure S67 for the
structure).
H
Ph
][BArF24
-
BArF24
N
DMAP
150°C, 16 h
H
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R1
P
2a
[
]
+
O
N
fluorobenzene
R1
Ph
1
[ (DMAP)][BArF24
]
99%
Computational studies. DFT methods60 at the PW6B95-
D3/def2-TZVPP61–63 level of theory were used to
determine the energy profile of the cycloaddition reaction
of [1]+ with phenylacetylene. The energy profile is shown
in Error! Reference source not found.. Separated
reactants (SR) were taken as reference. The computed
structure of the initial reactant complex (RC) shows an
approach of the terminal carbon atom of the alkyne to the
phosphorus atom with
a
distance of 2.94 Å. The
reaction barrier of
cycloaddition reaction has
a
12.6 kcal/mol (TS1) and results in the formation of
oxaphosphete [2a]+ (closed form, CF). CF is
thermodynamically favoured over the open form (OF) by
13.7 kcal/mol with an opening barrier of 22.6 kcal/mol
(TS2). The calculated energy profile agrees with the
experimentally observed stability and isolation of the
closed form ([2a]+). Nevertheless, the relatively low energy
barrier of the ring opening indicates that both open (OF)
and closed (CF) forms are in equilibrium at higher
temperatures.
Using the DLPNO-CCSD(T)/def2-TZVPP61,62,64–69 method,
we next examined the substituent effect in
oxophosphonium ions on the energy profile of the
cycloaddition reaction and compared the reactivity with
By employing π-donor substituents at the phosphorus
atom (reactions e and d), the energy difference between
the open and closed form gets smaller compared to the
simple model (c). Formal replacement of one NHI with a
NHO substituent has little influence on the overall reaction
Figure 2. Free energy profile of the cycloaddition reaction of [1]+ with phenylacetylene (reaction a) calculated at PW6B95-D3/def2-
TZVPP. Dipp-groups are shown in wireframe and hydrogens are omitted for clarity.
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path. However, it increases the first barrier by 6.8 kcal/mol
and reduces the second barrier by 8.1 kcal/mol. As a
result, the energy difference between CF-e and OF-e is
[2a][BArF24]. NMR spectroscopic studies (Table 2) reveal
that the 31P NMR resonances of the six-membered rings
1
2
3
4
5
6
7
8
2
appear at lower frequencies ([3a]+: -43.3 ppm, JPH = 3 Hz,
2
3
only 7.2 kcal/mol with
a
barrier of 11.0 kcal/mol,
[4]+: -20.0 ppm, JPH = 11 Hz, JPH = 12 Hz) than that of the
suggesting electrocyclic ring-opening and ring-closing
even at room temperature. The transition state geometries
of TS2-d and TS2-e are very similar (Figure S79) with P–O
bond distances of 2.47 Å (TS2-d) and 2.41 Å (TS2-e), while
the natural population analysis (NPA) gives a significantly
larger natural charge on phosphorus for TS2-d (2.022)
than for TS2-e (1.827) (cf. Table S8). Hence, the difference
among the transition state energy seems to be governed by
electronic effects rather than structural parameters.
four-membered
heterocycle
([2a]+:
-14.8 ppm,
2JPH = 18 Hz). The computed 31P NMR resonances of
selected model compounds agree with the experimental
data and show that the size of the heterocycle has
negligible influence on the phosphorus chemical shift (cf.
Figure S72, Table S9). As a result of the insertion into the
9
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P–O bond, the JCP coupling to the CO-carbon atoms reveal
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significantly smaller coupling constants ([4]+: 3 Hz) than in
[2a]+ (28 Hz). The ring expansion reaction was confirmed
by XRD studies on single crystals of [3a][BArF24] grown by
layering a saturated acetonitrile solution with n-pentane
(Figure 4). In the molecular structure, the
oxazaphosphininium ring is perfectly planar (sum of
angles: 720°) bearing C1–N1 (1.261 Å) and C3–C4 (1.336
Å) bond lengths in the range of double bonds and a
tetrahedrally coordinated phosphorus atom.
Scheme 5: [4+2] cycloaddition reactions involving
[2a][BArF24]. [BArF24]- anions are omitted.
CH3
R1
MeCN
120 °C, 16 h
R1
N
R1
R1
R1
P
O
P
O
P
O
R1
Ph
H
H
Ph
H
Ph
3a +
2a +
[
]
[
]
99%
H
O
Dipp
N
N
N
Dipp
Ph
R1
R1
PhCHO
O
R1
=
P
16 h, 120 °C
fluorobenzene
Ph
H
4 +
[ ]
99%
Table 2: Selected 31P and 13C NMR shifts and coupling
constants of oxaphosphete cation [2a]+ and its
Figure 3: Free energies for four different cycloaddition
reactions (b-e) calculated at the DLPNO-CCSD(T)/def2-TZVPP
level of theory.
a
cycloaddition products [3a]+, [4]+. Coupling constant
is below NMR resolution.
entry compound
δ (31P) [2JPH
]
δ(13C ,CP) δ(13C, CO)
[1JCP [2JCP
Reactivity of the oxaphosphete ion [2a]+. Our
computational results indicate that the acyclic 1-phospha-
4-oxa-butadiene state of [2a]+ is thermally accessible,
which prompted us to investigate whether it can be
trapped by [4+2] hetero Diels-Alder cycloaddition
reactions. Similar to α,β-unsaturated carbonyls, which are
important dienes in hetero Diels-Alder reactions for
material and natural product synthesis,72 phospha-
butadiene derivatives have been used in cycloaddition
reactions for the construction of phosphorus-containing
heterocycles.73–81 Furthermore, neutral σ3λ5-phosphoranes
containing the P=C–C=O system were postulated as
reactive intermediates based on their ability to undergo
[4+2] cycloaddition reactions with polar multiple
bonds.82,83
]
]
-14.8 ppm
[18 Hz]
99.9 ppm 163.5 ppm
[109 Hz] [28 Hz]
1
2
3
[2a]+
[3a]+
[4]+
-43.3 ppm
[3 Hz]
95.4 ppm 157.5 ppm
[133 Hz]
[-]a
-20.0 ppm
[11 Hz]
92.8 ppm 160.4 ppm
[152 Hz] [3 Hz]
Remarkably, the formation of [3a]+ is entirely reversible.
Heating of the neat samples with heat gun at
a
approximately 300 °C for one minute affords the free
oxophosphonium [1]+ ion via two consecutive
cycloreversion reactions in 82% yield according to NMR
analysis (Scheme 6). The cycloreversion of [3a]+ was
confirmed by a nitrile exchange experiment using 4-
methoxybenzonitrile (Scheme 6). Prolonged heating
resulted in the formation of a mixture of [3b]+ (62%) and
[3a]+ (26%) in addition to small amounts of [2a]+ (6%)
and [1]+ (3%). The thermolysis of [4][BArF24] is
unselective and gives a complex mixture containing [2a]+
The reaction of the oxaphosphete salt [2a][BArF24] with
acetonitrile and benzaldehyde gave the [4+2]
cycloaddition products [3a][BArF24] and [4][BArF24] as air-
stable solids in quantitative yield, respectively (Scheme 5).
The required high reaction temperature is consistent with
the 22.6 kcal/mol barrier (Figure 2) for the ring opening of
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and several other phosphorus species according to the 31P Oxophosphonium ion [1]+, which can be regarded as heavy
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4
5
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7
8
NMR analysis.
main group carbonyl analogue, undergoes [2+2]
cycloadditions with terminal alkynes to generate the first
oxaphosphete cations [2a-e]+. Alkyne exchange
experiments and trapping reactions reveal that the
formation of these strained four-membered heterocycles is
reversible, while the addition rate strongly depends on the
electronic properties of the alkynes. Quantum chemical
calculations show that the use of π-donor substituents at
the phosphorus atom triggers the electrocyclic ring
opening of the oxaphosphete ions to the 1-phospha-4-oxa-
butadiene structure at elevated temperature. This
property of [2a]+ was exploited to generate six-membered
heterocycles [3a]+ and [4]+ via [4+2] hetero Diels-Alder
reactions with acetonitrile and benzaldehyde, respectively.
Scheme 6: Nitrile exchange reaction (top) and
thermally induced cycloreversion (bottom). R’CN = p-
MeO-C6H4CN.
CH3
O
R'
O
N
N
R1
R1
R1
R1
140°C, 76 h
+ R'CN
+ MeCN
P
P
fluorobenzene
Ph
H
Ph
3b +
H
9
3a +
[
]
[
]
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11
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22
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24
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60
62%
O
P
300°C, 1 min
neat
+ MeCN + PhCCH
R1
R1
1 +
[ ]
Interesting analogies can be recognized with respect to the
cycloaddition reactions of oxophosphonium ion [1]+,
zirconium oxido complex [Cp*2Zr=O] and ketones. Both
[1]+ and [Cp*2Zr=O] afford four-membered heterocyclic
products in the [2+2] cycloaddition reaction with alkynes.
82%
The insertion of nitriles into the P–O bonds of
oxaphosphirane and oxaphosphetane complexes was
recently reported.47,84 However, unlike oxaphosphetes, the
saturated congeners require the acid-induced P–O bond
cleavage prior to the nitrile insertion. Beyond main-group
elements, ring expansion reactions have been reported for
oxazirconacyclobutenes I. The insertion of aldehydes
occurs selectively into the Zr–C bond rather than the Zr–O
bond, resulting in six membered heterocycles with O-Zr-O
structural motif.35,58
The reactions proceed without
a catalyst and are
reversible. In the case of carbonyls, however, acyclic α,β-
unsaturated carbonyl compounds are irreversibly formed
and a catalyst is required owing to the high energy barriers
associated with the reaction. With respect to the
subsequent reactivity of the cycloaddition products,
oxaphosphete ion [2a]+ resembles α,β-unsaturated
carbonyls as the analogous 1-phospha-4-oxa-butadiene
isomer is formed upon thermal cleavage of the P–O bond
and it can be used as diene in hetero Diels-Alder reactions.
In contrast, oxazirconacyclobutenes (I) undergo
rearrangement reactions at elevated temperature and
insertion reactions occur exclusively into the Zr–C bond of
the metallacycle. Considering the analogy to the carbonyl-
alkyne metathesis, the cycloaddition reaction between
oxophosphonium ions and alkynes is an intriguing
example of the diagonal relationship between carbon and
phosphorus in the periodic table. Furthermore, the analogy
to metal oxido complexes corroborates the notion that
ambiphilic main group elements such as the phosphenium
fragment (R1)2P+ in [1]+ can act as mimics for transition
metals.85–87
ASSOCIATED CONTENT
The supporting information is available free of charge at
https://pubs.acs.org/
Synthetic procedures, NMR spectra, mass spectrometry data,
crystallographic data and computational details.
Figure 4 Solid-state structure of [4a][BArF24]. Hydrogen
atoms (except H3) and the BArF24 anion are omitted for
clarity. Ellipsoids are displayed at 50 % probability. Dipp
groups are showed in wireframe for clarity. Only one part of
disordered Dipp and phenyl groups is depicted. Average
selected bond lengths [Å] and angles [°]: P–N1 1.671(2), P–C3
1.763(2), C3–C4 1.336(3), N1–C1 1.261(3), C1–O 1.375(2),
C4–O 1.384(2), N1-P-C3 102.34(9), P-C3-C4 121.6(2), C3-C4-O
123.2(2), C4-O-C1 121.5(2), C1-N1-P 124.0(2).
AUTHOR INFORMATION
Corresponding Author
Fabian Dielmann − Institut für Anorganische und Analytische
Chemie, Westfälische Wilhelms-Universität Münster,
Corrensstraße 30, 48149 Münster, Germany;
https://orcid.org/0000-0002-7025-5450; Email:
dielmann@uni-muenster.de
Summary
Authors
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Journal of the American Chemical Society
Pawel Löwe − Institut für Anorganische und Analytische
Chemie, Westfälische Wilhelms-Universität Münster,
Corrensstraße 30, 48149 Münster, Germany;
(11) McFarlin, A. T.; Watson, R. B.; Zehnder, T. E.;
Schindler, C. S. Interrupted Carbonyl-Alkyne Metathesis.
Adv. Synth. Catal. 2020, 362, 365–369.
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2
3
4
5
6
7
8
https://orcid.org/0000-0003-2275-552X
(12) Jin, T.; Yamamoto, Y. Gold-catalyzed intramolecular
carbocyclization of alkynyl ketones leading to highly
substituted cyclic enones. Org. Lett. 2007, 9, 5259–5262.
(13) Jayaprakash, S.; Jeevanandam, J.; Subramani, K. Ab
initio molecular orbital and density functional studies on
the ring-opening reaction of oxetene. J. Mol. Model. 2014,
20, 2494.
(14) Hoffmann, R.; Woodward, R. B. Conservation of
orbital symmetry. Acc. Chem. Res. 1968, 1, 17–22.
(15) Oblin, M.; Rajzmann, M.; Pons, J.-M. 2-H-
Methoxyoxete: a reactive intermediate en route to methyl
acrylate from methoxyacetylene and formaldehyde under
BF3 catalysis. An ab initio HF and DFT study. Tetrahedron
2001, 57, 3099–3104.
(16) Vieregge, H.; Bos, H. J. T.; Arens, J. F. Chemistry of
acetylenic ethers XLI. Condensation of acetylenic ethers
with carbonyl compounds under anhydrous conditions;
direct formation of α,β-unsaturated esters: (preliminary
communication). Recl. Trav. Chim. Pays-Bas 1959, 78, 664–
666.
(17) Oblin, M.; Pons, J.-M.; Parrain, J.-L.; Rajzmann, M.
Experimental evidence for a [2 + 2] mechanism in the
Lewis acid-promoted formation of α,β-unsaturated esters
from ethoxyacetylene and aldehydes. Synthesis and
characterisation of 4-ethoxyoxetes. Chem. Commun. 1998,
1619–1620.
(18) Aikawa, K.; Hioki, Y.; Shimizu, N.; Mikami, K. Catalytic
asymmetric synthesis of stable oxetenes via Lewis acid-
promoted 2+2 cycloaddition. J. Am. Chem. Soc. 2011, 133,
20092–20095.
(19) Aikawa, K.; Shimizu, N.; Honda, K.; Hioki, Y.; Mikami,
K. Effect of the trifluoromethyl group on torquoselectivity
in the 4π ring-opening reaction of oxetenes:
stereoselective synthesis of tetrasubstituted olefins. Chem.
Sci. 2014, 5, 410–415.
(20) Longchar, M.; Bora, U.; Boruah, R. C.; Sandhu, J. S. A
convenient synthesis of oxetene via [2 + 2]cycloaddition
reaction under microwave irradiation. Synth. Commun.
2002, 32, 3611–3616.
(21) Fan, H.; Wang, X.; Zhao, J.; Lib, X.; Gao, J.; Zhu, S.
Synthesis of trifluormethyl substituted oxetenes by the
reaction of acetylenedicarboxylate with trifluoromethyl
ketone in the presence of triphenylphosphine. J. Fluor.
Chem. 2013, 146, 1–5.
Milica Feldt − Theoretische Organische Chemie, Organisch-
Chemisches Institut and Center for Multiscale Theory and
Computation, Westfälische Wilhelms-Universität Münster,
Corrensstraße 40, 48149 Münster, Germany;
https://orcid.org/0000-0001-8314-9270
Marius A. Wünsche − Institut für Anorganische und
Analytische Chemie, Westfälische Wilhelms-Universität Münster,
Corrensstraße 30, 48149 Münster, Germany.
Lukas F. B. Wilm − Institut für Anorganische und Analytische
Chemie, Westfälische Wilhelms-Universität Münster,
Corrensstraße 30, 48149 Münster, Germany.
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Funding Sources
The authors gratefully acknowledge financial support from
the DFG (Emmy Noether program: DI 2054/1-1, IRTG 2027,
SFB 858).
ACKNOWLEDGMENT
We thank Dr. Alexander Hepp for performing the NMR
experiments and his help with assigning the data. Prof. F. E.
Hahn receives thanks for his generous support.
REFERENCES
(1) Becker, M. R.; Watson, R. B.; Schindler, C. S. Beyond
olefins: new metathesis directions for synthesis. Chem. Soc.
Rev. 2018, 47, 7867–7881.
(2) Liu, L.; Xu, B.; Hammond, G. B. Construction of cyclic
enones via gold-catalyzed oxygen transfer reactions.
Beilstein J. Org. Chem. 2011, 7, 606–614.
(3) Saito, A.; Tateishi, K. Syntheses of Heterocycles via
Alkyne-Carbonyl Metathesis of Unactivated Alkynes.
Heterocycles 2016, 92, 607–630.
(4) Saito, A.; Umakoshi, M.; Yagyu, N.; Hanzawa, Y. Novel
one-pot approach to synthesis of indanones through Sb(V)-
catalyzed reaction of phenylalkynes with aldehydes. Org.
Lett. 2008, 10, 1783–1785.
(5) Garkhedkar, A. M.; Senadi, G. C.; Wang, J.-J. ZnBr2-
Mediated Cascade Reaction of o-Alkoxy Alkynols: Synthesis
of Indeno1,2-cchromenes. Org. Lett. 2017, 19, 488–491.
(6) Ng, K.; Tran, V.; Minehan, T. A Single-Flask Synthesis of
α-Alkylidene and α-Benzylidene Lactones from
Ethoxyacetylene, Epoxides/Oxetanes, and Carbonyl
Compounds. Tetrahedron Lett. 2016, 57, 415–419.
(7) Saito, A.; Kasai, J.; Konishi, T.; Hanzawa, Y. Tandem
synthesis of 2,3-dihydro-4-iminoquinolines via three-
component alkyne-imine metathesis. J. Org. Chem. 2010,
75, 6980–6982.
(8) Jung, Y.; Kim, I. Total synthesis of brazilin. J. Org. Chem.
2015, 80, 2001–2005.
(9) Nayak, M.; Kim, I. Alkyne Carbonyl Metathesis As a
Means To Make 4-Acyl Chromenes: Syntheses of (±)-
Deguelin and (±)-Munduserone. J. Org. Chem. 2015, 80,
11460–11467.
(22) Asghari, S.; Habibi, A. K. Triphenylphosphine-
Catalyzed Synthesis of Stable, Functionalized 2H-Oxetes.
Phosphorus Sulfur Silicon Relat. Elem. 2005, 180, 2451–
2456.
(23) Li, L.; Fukawa, T.; Matsuo, T.; Hashizume, D.; Fueno,
H.; Tanaka, K.; Tamao, K. A stable germanone as the first
isolated heavy ketone with a terminal oxygen atom. Nat.
Chem. 2012, 4, 361–365.
(24) Alvarado-Beltran, I.; Rosas-Sánchez, A.; Baceiredo, A.;
Saffon-Merceron, N.; Branchadell, V.; Kato, T. A Fairly
Stable Crystalline Silanone. Angew. Chem. Int. Ed. 2017, 56,
10481–10485.
(25) Filippou, A. C.; Baars, B.; Chernov, O.; Lebedev, Y. N.;
Schnakenburg, G. Silicon-oxygen double bonds: a stable
(10) Ao, C.; Yang, X.; Jia, S.; Xu, X.; Yuan, Y.; Zhang, D.; Hu,
W. Zinc-Catalyzed Alkyne-Carbonyl Metathesis of
Ynamides with Isatins: Stereoselective Access to Fully
Substituted Alkenes. J. Org. Chem. 2019, 84, 15331–15342.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 16
silanone with a trigonal-planar coordinated silicon center.
Angew. Chem. Int. Ed. 2014, 53, 565–570.
Zirconium–Oxo-Mediated Alkyne–Aldehyde Coupling
Reaction. Organometallics 2015, 34, 1013–1016.
(38) Nguyen, T. T.; Kortman, G. D.; Hull, K. L. Synthesis,
Cycloaddition, and Cycloreversion Reactions of
Mononuclear Titanocene–oxo Complexes. Organometallics
2016, 35, 1713–1725.
(39) Pappas, I.; Chirik, P. J. Alkyne cycloaddition to a
titanocene oxide as a route to cyclopentadienyl
modification. Angew. Chem. Int. Ed. 2014, 53, 6241–6244.
(40) Nguyen, T. T.; Bertke, J. A.; Gray, D. L.; Hull, K. L. Facile
C–H, C–F, C–Cl, and C–C Activation by
Oxatitanacyclobutene Complexes. Organometallics 2015,
34, 4190–4193.
(41) Wittig, G.; Geissler, G. Zur Reaktionsweise des
Pentaphenyl-phosphors und einiger Derivate. Justus
Liebigs Ann. Chem. 1953, 580, 44–57.
(42) Wittig, G.; Schöllkopf, U. Über Triphenyl-phosphin-
methylene als olefinbildende Reagenzien (I. Mitteil. Chem.
Ber. 1954, 87, 1318–1330.
(43) Kyri, A. W.; Nesterov, V.; Schnakenburg, G.; Streubel,
R. Synthesis and reaction of the first 1,2-oxaphosphetane
complexes. Angew. Chem. Int. Ed. 2014, 53, 10809–10812.
(44) Kyri, A. W.; Brehm, P.; Schnakenburg, G.; Streubel, R.
Ring opening of a sterically crowded 1,2-oxaphosphetane
complex. Dalton Trans. 2017, 46, 2904–2909.
(45) Kyri, A. W.; Gleim, F.; García Alcaraz, A.;
Schnakenburg, G.; Espinosa Ferao, A.; Streubel, R. "Low-
coordinate" 1,2-oxaphosphetanes - a new opportunity in
coordination and main group chemistry. Chem. Commun.
2018, 54, 7123–7126.
1
2
3
4
5
6
7
8
(26) Kobayashi, R.; Ishida, S.; Iwamoto, T. An Isolable
Silicon Analogue of a Ketone that Contains an Unperturbed
Si=O Double Bond. Angew. Chem. Int. Ed. 2019, 58, 9425–
9428.
(27) Rosas-Sánchez, A.; Alvarado-Beltran, I.; Baceiredo, A.;
Saffon-Merceron, N.; Massou, S.; Hashizume, D.;
Branchadell, V.; Kato, T. Cyclic (Amino)(Phosphonium
Bora-Ylide)Silanone: A Remarkable Room-Temperature-
Persistent Silanone. Angew. Chem. Int. Ed. 2017, 56,
15916–15920.
(28) Wendel, D.; Reiter, D.; Porzelt, A.; Altmann, P. J.;
Inoue, S.; Rieger, B. Silicon and Oxygen's Bond of Affection:
An Acyclic Three-Coordinate Silanone and Its
Transformation to an Iminosiloxysilylene. J. Am. Chem. Soc.
2017, 139, 17193–17198.
(29) Loh, Y. K.; Porteous, K.; Fuentes, M. Á.; Do, D. C. H.;
Hicks, J.; Aldridge, S. An Acid-Free Anionic Oxoborane
Isoelectronic with Carbonyl: Facile Access and Transfer of
a Terminal B═O Double Bond. J. Am. Chem. Soc. 2019, 141,
8073–8077.
(30) Wünsche, M. A.; Witteler, T.; Dielmann, F. Lewis Base
Free Oxophosphonium Ions: Tunable, Trigonal-Planar
Lewis Acids. Angew. Chem. Int. Ed. 2018, 57, 7234–7239.
(31) Carney, M. J.; Walsh, P. J.; Hollander, F. J.; Bergman, B.
G. A reactive organometallic oxo intermediate, Cp*2Zr:O:
generation and subsequent trapping reactions forming
alkyne and nitrile addition products. J. Am. Chem. Soc.
1989, 8751–8753.
(32) Carney, M. J.; Walsh, P. J.; Bergman, R. G. Room
temperature generation of reactive intermediates Cp*Zr:O
and Cp*2Zr:S: trapping reactions with unsaturated organic
molecules and dative ligands. J. Am. Chem. Soc. 1990,
6426–6428.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(46) Kyri, A. W.; Schnakenburg, G.; Streubel, R. A novel
route to C-unsubstituted 1,2-oxaphosphetane and 1,2-
oxaphospholane complexes. Chem. Commun. 2016, 52,
8593–8595.
(47) Kyri, A. W.; Schnakenburg, G.; Streubel, R. Acid-
Induced Reactions of 1,2-Oxaphosphetane Complexes.
Organometallics 2017, 36, 3605–3612.
(48) Kyri, A. W.; Schnakenburg, G.; Streubel, R. C -
Trifluoromethyl-Substituted 1,2-Oxaphosphetane
Complexes: Synthetic and Structural Study.
(33) Polse, J. J.; Andersen, R. A.; Bergman, R. G.
Cycloaddition and Cycloreversion Reactions of a
Monomeric Ti(IV) Oxo Complex with Terminal and
Internal Alkynes. A Reversible
Oxametallacyclobutene/Hydroxoacetylide
Interconversion. J. Am. Chem. Soc. 1995, 117.
(34) Vaughan, G. A.; Sofield, C. D.; Hillhouse, G. L.;
Rheingold, A. L. Oxygen-atom transfer from nitrous oxide.
Identification of intermediates in the oxidation of
diphenylacetylene at Group 4 metal centers and the
structural characterization of (.eta.-
C5Me5)2Ti{N(O)NCPh:CPh}.cntdot.1/2C7H8. J. Am. Chem.
Soc. 1989, 111, 5491–5493.
(35) Vaughan, G. A.; Hillhouse, G. L.; Rheingold, A. L.
Syntheses, structures, and reactivities of unusual four-
membered metallacycles formed in insertion reactions of
N:N:O, N:N:NR, and N:N:CR2 with (.eta.5-
C5Me5)2Zr(C2Ph2). J. Am. Chem. Soc. 1990, 112, 7994–
8001.
(36) Carney, M. J.; Walsh, P. J.; Hollander, F. J.; Bergman, R.
G. Generation of the highly reactive intermediates
Cp*2Zr:O and Cp*2Zr:S: trapping reactions with alkynes,
nitriles, and dative ligands. Organometallics 1992, 761–
777.
Organometallics 2016, 35, 563–568.
(49) Keglevich, G.; Forintos, H.; Körtvélyesi, T.; Töke, L.
Inverse Wittig reaction of oxaphosphetenes formed by the
[2+2] cycloaddition of arylphosphine oxides and dimethyl
acetylenedicarboxylate (DMAD). J. Chem. Soc. 2002, 26–27.
(50) Mucsi, Z.; Kötvélyesi, T.; Viskolcz, B.; Csizmadia, I. G.;
Novák, T.; Keglevich, G. Can Four-Membered
Heterophosphete Structures Exist? The Contribution of
Phosphorus d Orbitals to Antiaromaticity. Eur. J. Org. Chem.
2007, 2007, 1759–1767.
(51) Graham, C. M. E.; Macdonald, C. L. B.; Boyle, P. D.;
Wisner, J. A.; Ragogna, P. J. Addressing the Nature of
Phosphinidene Sulfides via the Synthesis of P-S
Heterocycles. Chem. Eur. J. 2018, 24, 743–749.
(52) Martino, P. C.; Shevlin, P. B. Oxetene: synthesis and
energetics of electrocyclic ring opening. J. Am. Chem. Soc.
1980, 102, 5429–5430.
(53) Polse, J. L.; Andersen, R. A.; Bergman, R. G.
Cycloaddition and Cycloreversion Reactions of a
Monomeric Ti(IV) Oxo Complex with Terminal and
Internal Alkynes. A Reversible
(37) Kortman, G. D.; Orr, M. J.; Hull, K. L. Synthesis and
Reactivity of Dioxazirconacyclohexenes: Development of a
ACS Paragon Plus Environment

Page 9 of 16
Journal of the American Chemical Society
Oxametallacyclobutene/Hydroxoacetylide open-shell domain-based local pair natural orbital coupled
1
2
3
4
5
6
7
8
Interconversion. J. Am. Chem. Soc. 1995, 117, 5393–5394.
(54) Murugavel, R.; Pothiraja, R. Synthesis, spectral
characterization and crystal structures of
organophosphonic diamides: pyramidal nitrogen centers
and hydrogen bonding in [PhP(O)(NHtBu)2],
cluster singles and doubles theory. J. Chem. Phys. 2017,
146, 164105.
(69) Saitow, M.; Neese, F. Accurate spin-densities based on
the domain-based local pair-natural orbital coupled-
cluster theory. J. Chem. Phys. 2018, 149, 34104.
(70) Ghosh, A. K.; Datta, S.; Mukhopadhyay, A.;
[PhP(O)(NHDipp)2] (Dippꢀ=ꢀ2,6-iPr2C6H3) and
[tBuP(O)(NHiPr)2])(NHDipp)2] (Dipp = 2,6-iPr2C6H3)
and [tBuP(O)(NHiPr)2]. New J. Chem. 2003, 27, 968–974.
(55) Imrie, C.; Modro, T. A.; van Rooyen, P. H.; Wagener, C.
C. P.; Wallace, K.; Hudson, H. R.; McPartlin, M.; Nasirun, J.
B.; Powroznyk, L. Structural study on quasi-phosphonium
salts containing phosphorus-oxygen, phosphorus-oxygen,
phosphorus-nitrogen and phosphorus-sulphur bonds. J.
Phys. Org. Chem. 1995, 8, 41–46.
(56) Murai, K.; Tateishi, K.; Saito, A. Barluenga's reagent
with HBF4 as an efficient catalyst for alkyne-carbonyl
metathesis of unactivated alkynes. Org. Biomol. Chem.
2016, 14, 10352–10356.
(57) Rhee, J. U.; Krische, M. J. Alkynes as synthetic
equivalents to stabilized Wittig reagents: intra- and
intermolecular carbonyl olefinations catalyzed by Ag(I),
BF3, and HBF4. Org. Lett. 2005, 7, 2493–2495.
(58) Kortman, G. D.; Orr, M. J.; Hull, K. L. Synthesis and
Reactivity of Dioxazirconacyclohexenes: Development of a
Zirconium–Oxo-Mediated Alkyne–Aldehyde Coupling
Reaction. Organometallics 2015, 34, 1013–1016.
(59) Brasch, N. E.; Hamilton, I. G.; Krenske, E. H.; Wild, S. B.
π-Ligand Exchange on Phosphenium Ions: Reversible
Exchange between Free and Coordinated Alkynes in
Phosphirenium Salts. Organometallics 2004, 23, 299–302.
(60) For more detailed computational information see SI.
(61) Weigend, F.; Ahlrichs, R. Balanced basis sets of split
valence, triple zeta valence and quadruple zeta valence
quality for H to Rn: Design and assessment of accuracy.
PCCP 2005, 7, 3297–3305.
(62) Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs, R.
Auxiliary basis sets for main row atoms and transition
metals and their use to approximate Coulomb potentials.
Theor. Chem. Acc. 1997, 119–124.
(63) Zhao, Y.; Truhlar, D. G. Design of density functionals
that are broadly accurate for thermochemistry,
thermochemical kinetics, and nonbonded interactions. J.
Phys. Chem. A 2005, 109, 5656–5667.
(64) Hansen, A.; Liakos, D. G.; Neese, F. Efficient and
accurate local single reference correlation methods for
high-spin open-shell molecules using pair natural orbitals.
J. Chem. Phys. 2011, 135, 214102.
(65) Riplinger, C.; Neese, F. An efficient and near linear
scaling pair natural orbital based local coupled cluster
method. J. Chem. Phys. 2013, 138, 34106.
Chakraborty, T. Signatures of isomerization in
photodissociation of trans- crotonaldehyde probed by
multiphoton ionization mass spectrometry. J. Phys. Chem. A
2013, 117, 8710–8717.
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43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(71) Weinhold, F. Natural bond orbital analysis: a critical
overview of relationships to alternative bonding
perspectives. J. Comput. Chem. 2012, 33, 2363–2379.
(72) Tietze, L. F.; Kettschau, G. Hetero Diels-Alder
reactions in organic chemistry. In Stereoselective
Heterocyclic Synthesis I; Meijere, A. de, Houk, K. N., Lehn, J.-
M., Ley, S. V., Thiem, J., Trost, B. M., Vögtle, F., Yamamoto,
H., Metz, P., Eds.; Topics in Current Chemistry; Springer
Berlin Heidelberg: Berlin, Heidelberg, 1997; pp 1–120.
(73) Ohtsuki, K.; Walsgrove, H. T. G.; Hayashi, Y.;
Kawauchi, S.; Patrick, B. O.; Gates, D. P.; Ito, S. Diels–Alder
reactions of 1-phosphabutadienes: a highly selective route
to P=C-substituted phosphacyclohexenes. Chem. Commun.
2020, 56, 774–777.
(74) Appel, R.; Knoch, F.; Kunze, H. Niederkoordinierte
Phosphor-Verbindungen, 281). Monophosphabutadiene.
Chem. Ber. 1984, 117, 3151–3159.
(75) Miluykov, V.; Bezkishko, I.; Zagidullin, A.; Sinyashin,
O.; Lönnecke, P.; Hey-Hawkins, E. Cycloaddition Reactions
of 1-Alkyl-3,4,5-triphenyl-1,2-diphosphacyclopenta-2,4-
dienes. Eur. J. Org. Chem. 2009, 2009, 1269–1274.
(76) Möller, T.; Sárosi, M. B.; Hey-Hawkins, E. Asymmetric
phospha-Diels-Alder reaction: a stereoselective approach
towards P-chiral phosphanes through diastereotopic face
differentiation. Chem. Eur. J. 2012, 18, 16604–16607.
(77) Oshchepkova, E. S.; Zagidullin, A. A.; Miluykov, V. A.;
Sinyashin, O. G. Substituent effects in the asymmetric Diels-
Alder cycloaddition of 3,4,5-triaryl-1-(+)-neomenthyl-1,2-
diphospholes’ with maleic acid derivatives. Phosphorus
Sulfur Silicon Relat. Elem. 2016, 191, 1530–1532.
(78) Pavelka, L. C.; Baines, K. M. Facile synthesis of
luminescent benzo-1,2-dihydrophosphinines from a
phosphaalkene. Dalton Trans. 2012, 41, 3294–3301.
(79) Zagidullin, A. A.; Miluykov, V. A.; Krivolapov, D. B.;
Kharlamov, S. V.; Latypov, S. K.; Sinyashin, O. G.; Hey-
Hawkins, E. Intramolecular Cycloaddition Reactions of 1-
Alkenyl-3,4,5-triaryl-1,2-diphosphacyclopenta-2,4-dienes.
Eur. J. Org. Chem. 2011, 4910-4918.
(80) Zagidullin, A. A.; Oshchepkova, E. S.; Chuchelkin, I. V.;
Kondrashova, S. A.; Miluykov, V. A.; Latypov, S. K.; Gavrilov,
K. N.; Hey-Hawkins, E. P-Chiral 1,7-diphosphanorbornenes:
from asymmetric phospha-Diels-Alder reactions towards
applications in asymmetric catalysis. Dalton Trans. 2019,
48, 4677–4684.
(66) Riplinger, C.; Sandhoefer, B.; Hansen, A.; Neese, F.
Natural triple excitations in local coupled cluster
calculations with pair natural orbitals. J. Chem. Phys. 2013,
139, 134101.
(67) Riplinger, C.; Pinski, P.; Becker, U.; Valeev, E. F.;
Neese, F. Sparse maps—A systematic infrastructure for
reduced-scaling electronic structure methods. II. Linear
scaling domain based pair natural orbital coupled cluster
theory. J. Chem. Phys. 2016, 144, 24109.
(68) Saitow, M.; Becker, U.; Riplinger, C.; Valeev, E. F.;
Neese, F. A new near-linear scaling, efficient and accurate,
(81) Martin, G.; Ocando-Mavarez, E. First 1-phospha-1,3-
dienes unsubstituted in the carbon chain by pyrolysis of
diallylphosphines: A novel route to the phosphorus?carbon
double bond. Heteroatom Chem. 1991, 2, 651–654.
(82) Reiter, D.; Frisch, P.; Szilvási, T.; Inoue, S. Heavier
Carbonyl Olefination: The Sila-Wittig Reaction. J. Am. Chem.
Soc. 2019, 141, 16991–16996.
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(83) Konovets, A. I.; Kostyuk, A. N.; Pinchuk, A. M.;
Tolmachev, A. A.; Fischer, A.; Jones, P. G.; Schmutzler, R.
Dichloro(diisopropylamino)phosphonio[5(4H)oxopyrazol-
4-ylide-5-one]: Synthesis and properties. Heteroatom
Chem. 2003, 14, 452–458.
Page 10 of 16
1
2
3
4
(84) Helten, H.; Pérez, J. M.; Daniels, J.; Streubel, R. First
Brønsted Acid-Induced Ring Expansion of an
Oxaphosphirane Complex: A Combined Experimental and
DFT Study. Organometallics 2009, 28, 1221–1226.
(85) Martin, D.; Soleilhavoup, M.; Bertrand, G. Stable
singlet carbenes as mimics for transition metal centers.
Chem. Sci. 2011, 2, 389–399.
(86) Philip P. Power. Main-group elements as transition
metals. Nature 2010, 463, 171–177.
(87) Weetman, C.; Inoue, S. The Road Travelled: After
Main-Group Elements as Transition Metals. ChemCatChem
2018, 10, 4213–4228.
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