Phosphaketenes as building blocks for the synthesis of triphospha heterocycles

Article abstract of DOI:10.1002/chem.201403419

Unsaturated phosphorus compounds, such as phosphaalkenes and phosphaalkynes, show a versatile reactivity in cycloadditions. Although phosphaketenes (R-P=C=O) have been known for three decades, their chemistry has remained limited. Herein, we show that heteroatom-substituted phosphaketenes, R₃E-P=C=O (E=Si, Sn), are building blocks for silyl- and stannyl-substituted five-membered heterocycles containing three phosphorous atoms. The structure of the heterocyclic anion depends on the nature of the tetrel atom involved. Although the silyl analogue [P₃C ₂(OSiR₃)₂]^- is an aromatic 1,2,4-triphospholide, the stannyl compound [P(CO)₂(PSnR ₃)₂]^- is a 1,2,4-triphosphacyclopenta-3,5- dionate with a delocalized OCPCO fragment. Because of their anionic character, these compounds can easily be used as building blocks, for example, in the preparation of a silyl-functionalized hexaphosphaferrocene or the parent 1,2,4-triphosphacyclopenta-3,5-dionate [P(CO)₂(PH)₂] ^-. NMR spectroscopic investigations and computations have shown that the heterocycle-formation reactions presented herein are remarkably complex. Cycle time! The reaction of Na(OCP) with tetrelhalides, R₃EX (E=Si, Sn), gives silyl- and stannyl-substituted five-membered heterocycles containing three phosphorous atoms. Phosphaketenes, R₃E-P=C=O, are intermediates, which react with Na(OCP) as a P^- transfer reagent to the final products (see scheme; Tf=triflucromethanesulfonyl).

Full text of DOI:10.1002/chem.201403419

DOI: 10.1002/chem.201403419
Communication
&
Reaction Mechanisms
Phosphaketenes as Building Blocks for the Synthesis of
Triphospha Heterocycles
[a]
[a]
[a, b]
˝
Dominikus Heift, Zoltꢀn Benko,* and Hansjçrg Grꢁtzmacher*
as kinetically favored isomer, which slowly isomerizes to the
phosphaketene) were found to be stable for several months as
solids or dissolved in solvents having poor coordinating char-
acter (e.g., THF, hexane, benzene). However, when the phos-
phaketene Ph3SnꢀP=C=O (1a) was dissolved in a coordinating
solvent, such as dimethyl sulfoxide (DMSO), pyridine, or
(Me2N)3PO (HMPTA), the color of the solution slowly changed
from yellow to orange, and the formation of gas was observed.
According to NMR spectroscopic data, 1a decomposes cleanly
to form one species [Ph3Sn(DMSO)x][2a] under the evolution of
carbon monoxide (Scheme 1).
In the 31P NMR spectrum of a DMSO solution, [Ph3Sn-
(DMSO)x][2a] was identified by a triplet and a doublet signal at
d=124.5 and ꢀ68.3 ppm, respectively [2J(P,P)=20.9 Hz]. The
119Sn NMR spectrum exhibited a multiplet resonance at d=
ꢀ129.3 ppm, which can be assigned to the stannyl groups
bonded to phosphorus nuclei. Furthermore, two singlets at
d=ꢀ224.2 and ꢀ235.0 ppm were observed, which are in the
range of solvent-coordinated triphenyltin cations.[6] Presuma-
bly, the Lewis base Me2Sd+ꢀOdꢀ coordinates to the stannyl
group of the phosphaketene 1a and displaces the (OCP)ꢀ
anion via a pentacoordinate tin species: Ph3SnꢀP=C=O+
Abstract: Unsaturated phosphorus compounds, such as
phosphaalkenes and phosphaalkynes, show a versatile re-
activity in cycloadditions. Although phosphaketenes (Rꢀ
P=C=O) have been known for three decades, their chemis-
try has remained limited. Herein, we show that hetero-
atom-substituted phosphaketenes, R3EꢀP=C=O (E=Si, Sn),
are building blocks for silyl- and stannyl-substituted five-
membered heterocycles containing three phosphorous
atoms. The structure of the heterocyclic anion depends on
the nature of the tetrel atom involved. Although the silyl
analogue [P3C2(OSiR3)2]ꢀ is an aromatic 1,2,4-triphospho-
lide, the stannyl compound [P(CO)2(PSnR3)2]ꢀ is a 1,2,4-tri-
phosphacyclopenta-3,5-dionate with a delocalized OCPCO
fragment. Because of their anionic character, these com-
pounds can easily be used as building blocks, for example,
in the preparation of a silyl-functionalized hexaphospha-
ferrocene or the parent 1,2,4-triphosphacyclopenta-3,5-
dionate [P(CO)2(PH)2]ꢀ. NMR spectroscopic investigations
and computations have shown that the heterocycle-for-
mation reactions presented herein are remarkably com-
plex.
Phosphaketenes have a high affinity to dimerize and
form 1,3-diphosphatane-2,4-diones.[1] Only steric pro-
tection,[2] or as has been demonstrated more recently
hetero substitution[3] on the phosphorous atom,
allows sufficient stabilization to make the monomeric
form
isolable.[4]
Sodium
phosphaethynolate,
Na(OCP),[5] can be synthesized in large quantities
from simple chemicals and provides an easy access
to the tetrel-substituted heterophosphaketenes (R3Eꢀ
P=C=O, E=Si, Ge, Sn, Pb) by nucleophilic substitu-
tion.[3b] Especially, the germyl and stannyl com-
pounds, which exist as phosphaketene isomer exclu-
sively (with silyl groups, R3SiꢀOꢀCꢁP was observed
Scheme 1. Formation of the heterocyclic anions [2a,b]ꢀ from 1a,b.
˝
[a] Dr. D. Heift, Dr. Z. Benko, Prof. Dr. H. Grꢀtzmacher
DMSO![Ph3Sn(DMSO)]+ +(OCP)ꢀ. This implies that (OCP)ꢀ
anions are involved in the conversion 1a![Ph3Sn(DMSO)][2a].
Consequently, [NaOCPꢂ(dioxane)2.5] was reacted in THF with
two equivalents of R3SnꢀP=C=O (1; 1a: R=Ph, 1b: R=Cy)
and indeed these reactions cleanly deliver very similar com-
pounds Na[2a] (R=Ph) and Na[2b] (R=Cy; Scheme 1).
Department of Chemistry and Applied Biosciences
ETH Zurich, CH-8093 Zurich (Switzerland)
E-mail: benkoe@inorg.chem.ethz.ch
hgruetzmacher@ethz.ch
[b] Prof. Dr. H. Grꢀtzmacher
Lehn Institute of Functional materials (LIFM)
Sun Yat-Sen University, 510275 Guangzhou (P.R. China)
Colorless single crystals of the sodium salts of [2a]ꢀ and
[2b]ꢀ were obtained from saturated DME and THF solutions,
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403419.
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Communication
respectively. The results of the X-
ray diffraction studies are shown
in Figure 1a and b.[7]
The anions [2a]ꢀ and [2b]ꢀ
are almost planar 1,2,4-triphos-
phacyclopenta-3,5-dionates. The
PꢀP bonds [2.197(1); 2.189(7) ꢄ]
show
normal
single-bond
lengths and the tricoordinate
phosphorus atoms reside in
trigonal pyramidal coordination
spheres with the two stannyl
substituents in trans position.
The OCPCO units exhibit p de-
localization similar to previously
reported structures.[5b,8] The CO
bonds (1.24–1.26 ꢄ) are slightly
elongated compared to the CO
double bond in the related
[(Me3Sn)2P](Ad)C=O (1.217 ꢄ),[9]
whereas the PꢀC bonds are
slightly shorter than in this refer-
ence compound (1.877 ꢄ). Fur-
thermore, natural bond orbital
(NBO) calculations[10] on the sim-
plified anion [2c]ꢀ (H instead of
Ph or Cy groups) indicated de-
localization from the p-type lone
pair of the P atom into the two
Figure 1. a) ORTEP plot of {[Na(DME)2][2a]}2 ꢂ(DME) (thermal ellipsoids are drawn at 50% probability level). Hydro-
gen atoms and sodium cations with solvent molecules (DME) are omitted for clarity. Selected bond lengths [ꢄ]
and angles [8]: P1ꢀP2 2.197(1), P1ꢀSn1 2.498(9), P1ꢀC1 1.891(3), C1ꢀO1 1.241(4), C1ꢀP3 1.790(3), P3ꢀC2 1.778(3),
C2ꢀO2 1.257(4), C2ꢀP2 1.876(3), P2ꢀSn2 2.502(9); P1-C1-P3 119.4(2), C1-P3-C2 100.0(2), P3-C2-P2 120.2(2), C2-P2-
P1 96.6(1), P2-P1-C1 96.8(1), S8(P1)=296.58, S8(P2)=287.08. b) ORTEP plot of zigzag array {[Na(THF)3][2b]} (ther-
1
mal ellipsoids are drawn at 50% probability level). Hydrogen atoms and cyclohexyl rings at the tin atoms are
omitted for clarity. Selected bond lengths [ꢄ] and angles [8]: P1ꢀP2 2.189(7), P1ꢀSn1 2.522(5), P1ꢀC1 1.875(2), C1ꢀ
O1 1.244(2), C1ꢀP3 1.780(2), P3ꢀC2 1.784(2), C2ꢀO2 1.248(2), C2ꢀP2 1.878(2), P2ꢀSn2 2.530(5); P1-C1-P3 121.1(1),
C1-P3-C2 100.6(9), P3-C2-P2 120.9(1), C2-P2-P1 98.1(7), P2-P1-C1 98.3(7), S8(P1)=289.78, S8(P2)=294.18.
adjacent
p*(CO)
orbitals,[11]
which is in agreement with the
observed bond lengths. Al-
though in {[Na(DME)2][2a]}2 ꢂ
(DME) (Figure 1a) only one
oxygen atom of the heterocyclic
anion [2a]ꢀ coordinates to one
Na+ (see Figure S1 in the Sup-
porting Information for details),
{[Na(THF)3][2b]}
(Figure 1b)
1
forms a regular polymeric zigzag
chain, in which the quasi C2 sym-
metric anion connects two sol-
vent coordinated sodium cations
in a m2-(k1O, k1O) fashion.
We followed the reaction be-
tween two equivalents of
Ph3SnꢀP=C=O and one equiva-
lent of Na(OCP) in [D8]THF by 31P
and 119Sn NMR spectroscopy at
a temperature of ꢀ408C. The re-
action is remarkably complex,
and three intermediates [IM1]ꢀ–
Figure 2. Observed intermediates [IM1]ꢀ, [IM2]ꢀ, [IM3]ꢀ, and the final product [2a]ꢀ (bottom) and their
162.0 MHz 31P NMR spectra ([D8]THF, ꢀ408C, top). Selected chemical shifts (d in ppm) and coupling constants (J in
Hz) (chemical shifts were calculated at the (B3LYP/aug-cc-pVDZ(-PP) level are given in parentheses): [IM1]ꢀ: P1
2
290.9 (289), P2 78.2 (152); J(P1,P2)=8.2; [IM2]ꢀ: P3 118.8 (116), P4 60.4 (59), P5 ꢀ228.2 (ꢀ214);
1
1
2J(P3,P4)=3J(P3,P5)=17.0, J(P4,P5)=319.4; [IM3]ꢀ: P6 224.2 (262), P7 3.7 (36), P8 ꢀ167.1 (ꢀ120); J(P6,P7)=401.7,
2
2J(P6,P8)=135.8; [2a]ꢀ: P9 132.2 (147), P10 ꢀ70.4 (14); J(P9,P10)=20.2.
[IM3]ꢀ were identified. The 31P NMR spectra of the different
species are shown in the upper section of Figure 2. The struc-
tures and the assignment of the 31P NMR signals is based on
comparison with literature data and DFT calculations on model
compounds,[10] which leave little doubt about the identity of
the intermediates (see bottom of Figure 2 and the Supporting
Information). Selected simulated spectra are included in the
Supporting Information.
The reaction between Ph3SnꢀP=C=O (1a) and Na(OCP) is
fast, and within the mixing time, all Na(OCP) is consumed, only
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Communication
the anionic four-membered ring [IM1]ꢀ and Ph3SnꢀP=C=O
were detected at ꢀ408C. Although the concentrations of
[IM1]ꢀ and Ph3SnꢀP=C=O decreased continuously, the parallel
formation of intermediates [IM2]ꢀ and [IM3]ꢀ and the final
product [2a]ꢀ was observed. Intermediate [IM2]ꢀ was formed
in very low concentrations and contained a phosphaketene
group with a chemical shift comparable to Mes*ꢀP=C=O (d=
ꢀ206.9 ppm).[2] Intermediate [IM3]ꢀ, which was present in
much higher concentrations compared with [IM2]ꢀ, features
a five-membered ring consisting of two OCP units (cf. a similar
dipole, the heterocycle [B]ꢀ is formed in a [3+2] cycloaddition
with stannylphosphaketene 1c. With the Pb(d+)-Cb-Pa(dꢀ) dipole
the cycloaddition delivers [D]ꢀ. In both processes, loss of
carbon monoxide (CaOa) occurs, and a PꢀP bond is formed.[13]
The cycloaddition [A]ꢀ +1c![D]ꢀ is the entry to the mini-
mum-energy reaction pathway (MERP (1)) shown in black in
Figure 3 and requires the crossing of an activation barrier E° =
17.0 kcalmolꢀ1. The conversion of [D]ꢀ to the final product pro-
ceeds in one further step over a barrier of similar height (E° =
16.3 kcalmolꢀ1). The formation of the heterocycle [B]ꢀ from
[A]ꢀ and 1c on the MERP (2) shown in grey in Figure 3 pro-
ceeds through a higher activation barrier (E° =26.0 kcalmolꢀ1),
but leads via [C]ꢀ to the observed intermediates [IM2]ꢀ and
[IM3]ꢀ. Intermediate [IM2]ꢀ may also rearrange to [IM3]ꢀ in-
volving first a 1,3-stannyl-group migration to form [E]ꢀ and
subsequently a ring closure to obtain [IM3]ꢀ. The activation
barriers encountered on MERP (2) for the conversion of [B]ꢀ to
[C]ꢀ to [IM2]ꢀ to [E]ꢀ to [IM3]ꢀ are rather similar and do not
exceed 15 kcalmolꢀ1. The intermediate [IM2]ꢀ is at the cross
point between the two MERPs and via an activation barrier of
15.2 kcalmolꢀ1 the heterocycle [D]ꢀ on MERP (1) is reached
from MERP (2). We did not expect the accuracy of the compu-
tational model to be better than ꢂ5 kcalmolꢀ1. Under this as-
sumption, [B]ꢀ, [C]ꢀ, [D]ꢀ, [IM2]ꢀ, [E]ꢀ, and [IM3]ꢀ may be in
equilibrium, in which [IM2]ꢀ and especially [IM3]ꢀ as the most
stable intermediate serve as reservoirs for [D]ꢀ. Heterocycle
[D]ꢀ is continuously consumed to form irreversibly the thermo-
dynamically most stable compound [2c]ꢀ (E° =29.8 kcalmolꢀ1
for the reverse reaction [2c]ꢀ![D]ꢀ). Because all bar-
riers leading to [D]ꢀ have about the same height as
the one to the final product [2c]ꢀ, which consumes
[D]ꢀ, and [D]ꢀ has a higher energy than [IM2]ꢀ and
[IM3]ꢀ, only very small concentrations of [D]ꢀ will be
present preventing its detection. As such, the compu-
tational model is in agreement with the experimental
observations.
1
1,3,4-oxadiphospholonide[12] d=159.2 and ꢀ35.5 ppm, J(P,P)=
361.0 Hz] and a distannylphosphino moiety, which shows
a characteristic chemical shift (d=ꢀ127.1 ppm) and satellite
pattern (cf. [(Me3Sn)2P](Ad)C=O[9] ꢀ120.4 ppm, 1J(Sn,P)=
617.2 Hz). Intermediates [IM2]ꢀ and [IM3]ꢀ are structural iso-
mers of the final product [2a]ꢀ and stannyl group migrations,
and ring opening/closing steps must be involved in their trans-
formation to [2a]ꢀ. To propose a mechanism involving the ex-
perimentally observed species, DFT calculations were per-
formed with a model reaction system 1c+1c+Na(OCP), in
which 1c is H3SnꢀP=C=O (Figure 3).[10]
In the first reaction step, the (OCP)ꢀ anion attacks the elec-
trophilic carbon atom of stannylphosphaketene 1c to form
[A]ꢀ, which is supposedly in an fast equilibrium with the more
stable [IM1]ꢀ (ꢀ6.9 kcalmolꢀ1) given the small barrier between
[A]ꢀ and [IM1]ꢀ of only 1.7 kcalmolꢀ1. Intermediate [IM1]ꢀ is
observed on the NMR time scale and serves as reservoir for the
reactive species [A]ꢀ. When [A]ꢀ reacts as a Pa(d+)-Cb-Ob(dꢀ)
The germanium analogue of cycle [2a]ꢀ is not ac-
cessible from Ph3GeꢀP=C=O and Na(OCP). At ꢀ508C,
the germanium analogue of intermediate [A]ꢀ is
formed, which slowly isomerizes at temperatures
above ꢀ108C to the sodium salt of 1-germyl-1,2-di-
phosphetane-3,4-dione (see the Supporting Informa-
tion). When trimethylsilyl chloride, triflate, or best
azide is reacted with Na(OCP) in a ratio of 2:3, the
31P NMR spectrum exhibits again a triplet and a dou-
blet resonance. However, the chemical shifts (d=
186.1 ppm (t), 173.4 ppm (d), 2J(P,P)=55.2 Hz) indi-
cate the quantitative formation of a different species,
which was unequivocally identified as the sodium
salt of
a 3,5-bis(trimetylsiloxy)-1,2,4-triphospholide
(Na[3], Scheme 2). In this case, when the reaction
was followed by 31P NMR spectroscopy at low tem-
peratures, only two intermediates were detected,
which are assigned to the dimer of the silyl-substitut-
ed phosphaketene ((Ph3SiꢀP=C=O)2; d=176 ppm (s))
and the silyl analogue of [IM1]ꢀ with 31P resonances
at d=309 (s) and 78.9 ppm (s).
Figure 3. Possible calculated reaction pathways leading to 2c from two molecules of Rꢀ
P=C=O (1c) and Na(OCP) (R=SnH3) at B3LYP/aug-cc-pVDZ(-PP) level of theory. The rela-
tive energies of the intermediates and transition-state structures are given compared to
that of [A]ꢀ +RꢀP=C=O (1c). Sodium cations were included in the calculations but are as
the additional carbon monoxide molecules (in the case of [B]ꢀ, [C]ꢀ, [IM2]ꢀ, [D]ꢀ, [E]ꢀ,
[IM3]ꢀ and [2c]ꢀ) omitted for clarity.
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Communication
[II]ꢀ is the more stable one. Conversely, this means that the
stability evoked by the aromatic p system in the anionic P3C2
ring[17] is relatively weak with respect to that of the “linear”
OCPCO p system (cf. the CO p bond is exceptionally strong[16]),
which places the anionic charge on the two most electronega-
tive oxygen atoms.
In summary, the reaction between heteroatom-substituted
phosphaketenes Ph3EꢀP=C=O (E=Si, Sn) and sodium phos-
phaethynolate, Na(OCP), gave an easy, high-yielding access to
new five-membered 1,3,4-triphospha-heterocycles. Compound
[{(TMSO)C}2P3]ꢀ can be further used as a building block for
functionalized metallocene derivatives. Lucky circumstances al-
lowed us to follow one reaction closely by NMR spectroscopy,
and in combination with DFT computations, the remarkable
complexity of these reactions was revealed. The identification
of several intermediates, which rearrange to one final product,
gives insight into the reactivity of the “heavy” pseudohaloge-
nide (OCP)ꢀ toward phosphaketenes. Furthermore, this and
previous work demonstrates that the reported anionic organo-
phosphorus compounds profit strongly from p delocalization
over an OCPCO fragment.
Scheme 2. Formation and reactivity of anion [3]ꢀ.
Similar to the tert-butyl analogue [(tBuC)2P3]ꢀ, which, howev-
er, was only obtained by multistep syntheses in moderate
yields,[14] the anion [{(TMSO)C}2P3]ꢀ can be used for the synthe-
ses of metal complexes.[15] The reaction of Na[3] with [FeBr2-
(THF)2] cleanly affords the sandwich complex 4 (1,1’,2,2’,4,4’-
hexaphosphaferrocene) as brown powder, which was fully
characterized by NMR, elemental analysis, and HRMS data. In
the reaction with water, the hydrolysis of the silyl groups deliv-
ers anion [5]ꢀ, which is stable in THF solution for several days.
Anion [5]ꢀ, which can be regarded as the simplest heavier ana-
logue of the urazolide anion [(HN)2(CO)2N]ꢀ, is alternatively ac-
cessible from [2a]ꢀ and H2O. The protonation takes place on
the two neighboring phosphorous centers and not on the
oxygen atoms.
Acknowledgements
This work was supported by the Swiss National Science Foun-
dation (SNF) , and the ETH Zꢁrich.
Keywords: NMR spectroscopy · phosphaketenes · phosphorus
heterocycles · reaction mechanisms · sandwich complexes
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1
RO
RP
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SnH3
SiH3
H
0.0 (0.0) [ISn]ꢀ
0.0 (0.0) [ISi]ꢀ
0.0 (0.0) [IH]ꢀ
32.9 (21.0) [IISn]ꢀ
ꢀ9.2 (ꢀ14.2) [IISi]ꢀ
17.0 (11.0) [IIH]ꢀ
55.9
66.8
84.4
85.8
114.8
117.5
29.9
48.0
33.1
[10] For details on the calculations, see the Supporting Information.
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Communication
[11] The second-order perturbation theory analysis of Fock matrix in NBO
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42.5 kcalmolꢀ1
.
˝
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-
[17] L. Nyulꢀszi, Z. Benko in Aromaticity in Heterocyclic Compounds, Vol. 19
˝
Cb-Pa(dꢀ) as a dipole and the P=C bond of 1c as a dipolarophile. Howev-
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´
Received: June 24, 2014
Published online on July 23, 2014
Chem. Eur. J. 2014, 20, 11326 – 11330
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ꢃ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim