Facile phosphorus-carbon bond formation using a tungsten-coordinated phosphirenyl cation

Article abstract of DOI:10.1021/om301266x

Reaction of K₂[W(CO)₅] with Cl₂PN-i- Pr₂ in the presence of diphenylacetylene leads to the tungsten aminophosphirene complex [W(CO)₅{P(N-i-Pr₂)C(Ph)C(Ph)}] (1), which can be converted to th

Full text of DOI:10.1021/om301266x

Communication
pubs.acs.org/Organometallics
Facile Phosphorus−Carbon Bond Formation using a Tungsten-
Coordinated Phosphirenyl Cation
Arumugam Jayaraman and Brian T. Sterenberg*
Department of Chemistry and Biochemistry, University of Regina, 3737 Wascana Parkway, Regina, Saskatchewan, Canada S4S 0A2
S
* Supporting Information
ABSTRACT: Reaction of K2[W(CO)5] with Cl2PN-i-Pr2 in
the presence of diphenylacetylene leads to the tungsten
aminophosphirene complex [W(CO)5{P(N-i-Pr2)C(Ph)C-
(Ph)}] (1), which can be converted to the chlorophosphirene
complex [W(CO)5{P(Cl)C(Ph)C(Ph)}] (2) by reaction with
HCl. Chloride abstraction from 2 with excess AlCl3 leads to
the tungsten-complexed phosphirenyl cation [W(CO)5{PC-
(Ph)C(Ph)}][AlCl4] (3). Compound 3 reacts with PPh3 to form a phosphoniophosphirene complex and undergoes electrophilic
aromatic substitution with ferrocene to form a ferrocenylphosphirene complex. Chloride abstraction from 2 with silver triflate
leads to a phosphirenyl triflate complex, which reacts with PPh3 and ferrocene in the same fashion as 3 but also reacts cleanly
with a wider range of substrates, including phenylacetylene and allyltrimethylsilane, to form respectively a phenyl-
alkynylphosphirene complex and an allylphosphirene complex.
hosphorus−carbon bond formation is a fundamental step
P
in the synthesis of organophosphorus compounds.
Nucleophilic attacks on phosphorus electrophiles constitute a
significant proportion of P−C bond forming reactions.1
Phosphorus electrophilicity can be enhanced via anion
abstraction, broadening the range of potential nucleophiles.
For example, chloride abstraction from P−Cl bonds has been
used extensively to form cationic, electron-deficient phosphorus
species including phosphenium ions2 and transition-metal
phosphinidene complexes,3 both of which can be used in P−
C bond forming reactions. We were interested in extending this
methodology to chloride abstraction from metal-coordinated
chlorophosphines, to form metal-coordinated phosphenium
ions. Although halide abstraction has been applied to the
generation of metal-coordinated phosphenium ions, examples
are limited to heteroatom-stabilized species, and very little is
known about their reactivity.4 Metal coordination has the
potential to stabilize otherwise unstable phosphenium ions, and
by generating them in the coordination sphere of a metal
complex we may be able to observe otherwise unstable species.
Metal coordination is also expected to alter the reactivity by
protecting the phosphorus lone pair and by altering electro-
philicity through P to M donation or M to P back-donation,
depending on the nature of the metal fragment. Our interest
was drawn to the phosphirenyl cation (Figure 1a), because the
ring unsaturation provides potential aromatic stabilization,5
which may allow us to form stable phosphenium complexes
without the need for heteroatom stabilization. Transition-
metal-coordinated phosphirenyl cations were first proposed as
intermediates in anion exchange reactions of phosphirene
complexes.6,7 The first stable transition-metal-coordinated
phosphirenyl cation was an η3 nickel complex obtained by
condensation of vaporized atomic nickel with tert-butylphos-
phaalkyne (Figure 1b).8 A pentacarbonyltungsten-complexed
Figure 1. Phosphirenyl cation and phosphirenyl cation metal
complexes.
phosphirenyl cation was prepared by abstraction of triflate from
a phosphirenyl triflate complex using B(OTf)3 in liquid SO2 at
−78 °C (Figure 1c).9 However, the reactivity of these metal-
coordinated phosphirenyl cations remains completely unex-
plored, likely in part due to the difficult synthetic routes used to
form them. We report here the facile generation of a tungsten-
coordinated phosphirenyl cation and its versatile reactivity
toward P−C bond formation.
To generate a coordinated phosphirenyl cation, we chose the
known chlorophosphirene complex [W(CO)5{P(Cl)C(Ph)C-
(Ph)}] (2)7,10 as an ideal precursor. Synthesis of 2 was
achieved in two steps. Reaction of K2[W(CO)5] with Cl2PN-i-
Pr2 and diphenylacetylene leads to the aminophosphirene
complex 1 (Scheme 1). This reaction involves a (1 + 2)-
cycloaddition between diphenylacetylene and an electrophilic
aminophosphinidene complex generated in situ from the
tungsten dianion and Cl2PN-i-Pr2. This route to 1 is simpler
and more efficient than other reported methods7,10 and is
similar to the method used for the generation of a transient iron
aminophosphinidene complex from Collman’s reagent and
Received: December 31, 2012
Published: January 23, 2013
© 2013 American Chemical Society
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Communication
the optimized geometry is shown in Figure 3.13 Shortened
a
Scheme 1
phosphirenyl ring P−C distances (1.738 vs 1.784 Å in
a
Reagents and conditions: (i) Cl2PN-i-Pr2, PhCCPh, THF, room
temperature; (ii) HCl, Et2O, room temperature.
Cl2PN-i-Pr2.11 Reaction of 1 with hydrogen chloride gave the
desired chlorophosphirene complex 2.
Treatment of 2 with 4 equiv of AlCl3 resulted in an
immediate color change to deep red. Note that reaction of 2
with 1 equiv of AlCl3 does not generate a detectable reaction.6
The 31P NMR spectrum of the resulting solution showed a
single phosphorus resonance at δ 171.7, deshielded by 280.7
ppm from the precursor, consistent with previously observed
phosphirenyl chemical shifts.9 The 13C NMR spectrum showed
a peak for the phosphirene ring carbons at δ 180.5, similarly
deshielded by 37.3 ppm from 2 (143.2 ppm). The electrospray
mass spectrum showed an ion cluster centered at m/z 533,
which matches the pattern predicted for a cation with the
formula C19H10O5PW. These data are consistent with
formulation of the new compound as [W(CO)5{PC(Ph)C-
(Ph)}][AlCl4] (3), a tungsten complex of a phosphirenyl
cation, formed by chloride abstraction from 2 (Scheme 2).
Figure 3. Optimized structures of 2 and 3. Selected distances (Å): 2,
W−P = 2.541, P−C = 1.784, CC = 1.343, C−Ph = 1.449 Å; 3, W−P
= 2.378 Å, P−C = 1.738 Å, CC = 1.393 Å, C−Ph = 1.434 Å.
precursor 2), along with a longer CC distance (1.393 vs
1.343 Å), show that the empty phosphorus p orbital is
stabilized by resonance delocalization. A shortened P−W bond
(2.378 vs 2.541 Å in 2) also indicates that W-to-P π back-
donation is significant. However, this distance is much longer
than W−P double bonds14 and cannot be considered a true
double bond with full phosphido character. Shortened C−Ph
bonds suggest that charge is also being delocalized onto the
phenyl rings. In the optimized structure, the phenyl rings are
nearly coplanar with the PC2 ring, providing further evidence
for this delocalization. Optimization of both η1- and η3-
phosphirenyl coordination shows that η1 coordination is more
stable by 11.0 kcal/mol than η3 coordination, which again
demonstrates that the phosphirenyl cation must be η1
coordinated. The calculated parameters suggest that all three
of the forms shown in Figure 2 make a significant contribution
to 3.
a
Scheme 2
Attempts to isolate and crystallize 3 resulted in reversion to 2
as excess AlCl3 crystallizes from solution. As a result, it was
trapped with PPh3, resulting in phosphine coordination to the
phosphirenyl phosphorus to form the P−P-bonded adduct 4
(Scheme 2), which was characterized by X-ray crystallography
(Figure 4). This reactivity demonstrates that 3 is electrophilic at
phosphorus. Compound 3 also undergoes rapid electrophilic
aromatic substitution with ferrocene to form the ferrocenyl-
phosphirene complex 5 in 51% isolated yield (Scheme 2), again
illustrating the strongly electrophilic phosphorus center.
Clearly, 3 reacts as a phosphenium ion.
Reaction of 3 with other substrates, including arenes, alkynes,
and alkenes, led to decomposition and no observable
phosphorus-containing products. One possible reason for the
decomposition is the excess AlCl3 needed to form 3, which may
react with both the substrate and the product. Similarly, the
HCl generated as the byproduct of an electrophilic substitution
reaction may be contributing to product decomposition. As a
result, we looked for alternative chloride abstractors. Reaction
of 2 with silver tetrafluoroborate resulted in chloride
abstraction; however, the observed product was the unreactive
fluorophosphirene complex [W(CO)5{P(F)C(Ph)C(Ph)}].
Silver triflate led to the phosphirenyl triflate complex 6
(Scheme 3). Compound 6 reacts rapidly with ferrocene to
give 5, the same product formed in the reaction of 3 with
ferrocene, in 86% yield (Scheme 3). On the basis of this
reactivity, we suggest that 6 is in equilibrium with the
a
Reagents and conditions: CH2Cl2, room temperature; (i) AlCl3, 4
equiv; (ii) PPh3; (iii) Cp2Fe. [W] = W(CO)5.
Furthermore, the large 392 Hz W−P coupling and the lack of
W coupling in the ring carbon resonances indicate that the
phosphirenyl cation is coordinated to W in an η1 fashion
through P.
The bonding between the transition metal and phosphorus
in planar M−PR2 complexes can range between two extremes,
phosphenium and four-electron phosphido (Figure 2), depend-
ing on the extent of metal-to-P π back-donation.12 For the
phosphirenyl cation specifically, an additional contributing form
is delocalized (aromatic) overlap of the empty phosphorus p
orbital with the alkenyl π bond. A computational study has
been carried out to address the nature of the bonding in 3, and
Figure 2. Bonding between transition metal and phosphorus: (a) four-
electron phosphido; (b) phosphenium. (c) aromatic phosphirenyl
cation.
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Communication
are applicable to any W(CO)5(PClR2) complex and will
constitute a powerful new method for P−C bond formation.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and a CIF file giving experimental details
and compound characterization data, computational details,
energies, and Cartesian coordinates of optimized geometries,
and crystallographic data for 4. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: brian.sterenberg@uregina.ca.
■
Notes
Figure 4. ORTEP diagram of the cation of compound 4. The
counterion and hydrogen atoms have been omitted for clarity. Selected
bond lengths (Å) and angles (deg): W1−P1 = 2.4612(4), P1−P2 =
2.2526(6), P1−C6 = 1.796(2), P1−C7 = 1.792(2), C6−C7 =
1.319(2); W1−P1−P2 = 124.90(2), W−P1−C6 = 129.22(6), W1−
P1−C7 = 124.86(6), C6−P1−C7 = 43.14(8), P1−C7−C6 = 68.6(1),
P1−C6−C7 = 68.3(1).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Bob McDonald and Mike Ferguson (University of
Alberta) for X-ray data collection, Compute Canada (Westgrid)
for computing resources, and the University of Regina for
funding.
a
Scheme 3
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a
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