Isolable phosphaalkenes bearing 2,4,6-trimethoxyphenyl and 2,6-Bis(trifluoromethyl)phenyl as P-substituents

Article abstract of DOI:10.1021/acs.joc.0c01514

The multistep synthesis and characterization of new P=C analogues of olefins from readily available starting materials is reported. Specifically, the phosphaalkenes TMOP-P=CPh2 (1a: TMOP = 2,4,6-trimethoxyphenyl) and ArF-P=CPh2 [1b: ArF = 2,6-bis(trifluor

Full text of DOI:10.1021/acs.joc.0c01514

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Article
Isolable Phosphaalkenes Bearing 2,4,6-Trimethoxyphenyl
and 2,6-Bis(trifluoromethyl)phenyl as P-Substituents
Zeyu Han, David Röhner, Kerim Samedov, and Derek P Gates
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.0c01514 • Publication Date (Web): 21 Sep 2020
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Page 1 of 11
The Journal of Organic Chemistry
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Isolable Phosphaalkenes Bearing 2,4,6-Trimethoxyphenyl and 2,6-
Bis(trifluoromethyl)phenyl as P-Substituents
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Zeyu Han, David Röhner, Kerim Samedov, Derek P. Gates*
ABSTRACT: The multistep synthesis and characterization of new P=C analogues of olefins from readily available starting
materials is reported. Specifically, the phosphaalkenes TMOP–P=CPh2 (1a: TMOP = 2,4,6-trimethoxyphenyl) and ArF–P=CPh2
[1b: ArF = 2,6-bis(trifluoromethyl)phenyl] have been prepared, isolated and characterized. In addition, synthetically
challenging intermediates, such as the corresponding pyrophoric primary phosphines and bis(trimethylsilyl)phosphines
have been isolated and characterized. The title compounds, TMOP–P=CPh2 (1a) and ArF–P=CPh2 (1b), along with TMOP–PH2
(3a) have been characterized by X-ray crystallography. Importantly, the successful synthesis and isolation of phosphaalkenes
1a and 1b provides a foundation for future investigations of their polymerization, by analogy to the known polymerization of
Mes–P=CPh2.
+
INTRODUCTION
R2
R2
R
P
R
R
S
N
S
N
C
P
C
C
P
The synthesis and study of heavy atom analogues of alkenes
and alkynes has played a prominent role in the growth of
interest in p-block chemistry and continues to be a very
active area of study.1 Historically, organophosphorus
compounds have played a key role in the advancement of
multiple bond chemistry, owing in part to the close analogy
between phosphorus and carbon.2 The history of multiple
bonding between phosphorus and carbon illustrates the
important role that thermodynamic and/or kinetic
stabilization imparts on compound isolability. For instance,
thermodynamic stabilization by delocalization was
employed to afford the first isolable phosphamethine
R1
R1
R = H, Ph
C
A
B
OSiMe3
C
P
C
P
P
P
D
E
F
5
cynanines (Chart 1: A)3 or phosphinines (B).4,
Chart 1. Key examples in the development of isolable
Alternatively, kinetic stabilization by employing sterically
compounds featuring multiple bonding involving phosphorus
as facilitated by the kinetic and thermodynamic stabilization
strategies of increasing steric bulk and electron delocalization.
encumbering substituents (t-Bu and Mes) permitted the
isolation of the first phosphaalkynes (C)6 and
phosphaalkenes (D, E).7,
A breakthrough in bulky
8
As growth in multiple bond chemistry broadened to
heavier elements, there has simultaneously been
considerable developments in phosphaalkene chemistry
centering on incorporating sophisticated chemically
functional substituents to the P=C bond in addition to
sterically hindering ones.11 As a result, phosphaalkenes
have increasingly gained attention for catalysis, polymers
and materials science. We have been interested in
extending the analogy between olefins and phosphaalkenes
to the polymerization of P=C bonds. A key challenge to
substituent design came with the employment of the
“supermesityl” substituent (Mes* 2,4,6-tri-tert-
=
butylphenyl) to permit the isolation of the diphosphene
(F).9 In recent years, the development of larger and more
encumbering substituents has facilitated the isolation of
low-coordinate and multiply bonded compounds featuring
main group elements and transition metals that were
previously deemed unattainable.10
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polymerizing phosphaalkenes lies in balancing the kinetic trimethoxyphenyl-
Page 2 of 11
2,6-
(TMOP)
and
the
1
2
3
4
5
6
7
8
and thermodynamic factors needed to stabilize the P=C
bond. By analogy to ethylene, we imagined that a
polymerizable P=C monomer must be kinetically stable,
thereby permitting isolation and purification, yet
bis(trifluoromethyl)phenyl- (ArF) substituents (Figure 1: 1a
and 1b, respectively). We note that TMOP has been
successfully employed in the synthesis of isolable
disilenes,17 yet has not been employed to any great extent in
low-coordinate P-chemistry beyond its use with zirconium
phosphinidenes.18 In contrast, ArF has been quite popular as
an unreactive bulky substituent and has been used to
prepare phosphaalkenes [ArFP=CMe(R), R = Me, Ph],19
diphosphenes (ArF-P=PAr),20, 21 phosphenium cations (ArF-
thermodynamically
unstable
with
respect
to
polymerization. Thus, we sought phosphaalkenes that
possessed kinetically stabilizing substituents that were
large enough to permit monomer purification and isolation
but not too large to inhibit polymerization with initiators or
catalysts. We became intrigued by the isolable
phosphaalkene E which appeared to lie at the interface of
kinetic/thermodynamic stability given that o-Tol-P=CPh2
was not isolable as a monomer and reportedly gave only an
ill-defined “polymeric material”.8 We later showed that o-
Tol-P=CPh2 and Ph-P=CPh2 afford monomer-dimer
equilibria in solution12 and Ott and co-workers have
postulated that such equilibria may also consist of higher
oligomers and/or polymers.13
9
+
PNR2 )22 and a primary phosphide (ArF-PH–).23 That said,
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our attempts to polymerize ArFP=CMe(R) (R = Me, Ph) have
thus far been unsuccessful, likely due to reactivity at the C–
Me substituent. Thus, we have sought routes to 1a and 1b
which bear only C–Ph substituents.
Herein we report the multi-step synthesis of two novel
isolable phosphaalkenes, TMOP-P=CPh2 (1a) or ArF-P=CPh2
(1b), starting from readily available precursors. The
present work provides a foundation for the fundamental
study of P=C bond chemistry and for the further study of the
initiation of P=C bonds towards polymerization.
Monomer E and related P-Mes monomers may be
polymerized to afford functional phosphine-containing
polymers, poly(methylenephosphine)s, which have
OCH3
CF3
15
attractive chemical and physical properties.2d,14,
Of
Ph
Ph
Ph
P
C
P C
particular interest to us has been the mechanism of
polymerization. For instance, we discovered the radical-
Ph
H3CO
OCH3
CF3
initiated
polymerization
of
MesP=CPh2
using
1a
1b
Ph(Me)CH·TEMPO proceeds via an unexpected addition-
isomerization mechanism involving C-H activation of the o-
Me group of the P-Mes substituent (Scheme 1).16 In addition,
the homo- and co-polymerization (anionic or radical) of
ArP=C(Ph)R (Ar = Mes, m-Xyl; R = Naph, Phen, CMe2Ox, 3-
C6H4Ox, Pyrenyl) appears to follow the same addition-
isomerization mechanism.15 Although it is likely that the
addition-isomerization mechanism applies to all P-Mes-
containing phosphaalkenes, there is evidence for small
amounts “normal” enchainment at least in some
copolymers.
Figure 1. Two targeted phosphaalkenes in this work.
RESULTS AND DISCUSSION
Synthesis and Characterization of TMOP-P=CPh2
(1a). Prior to the present work, the compounds TMOP-PCl2
and TMOP-PH2 were mentioned by Majoral et al. as
precursors for zirconocene phosphinidene complex
trapping reactions.18 In their report, a detailed synthetic
protocol was not described, nor was full characterization of
either compound given. Therefore, we now report full
details of the synthesis and characterization of these
phosphines (Scheme 2). In contrast to Mes-H and Mes*-H,
which require halogenation before lithiation,9, 24-26 TMOP-H
can be directly lithiated by treatment with n-BuLi (1 equiv)
in Et2O solution. Subsequent addition to PCl3 (5 equiv)
afforded the desired 2a as judged by the presence of a
singlet resonance at 156 ppm in the 31P{1H} NMR spectrum
(cf. PCl3: P = 219). The strict control of TMOP–H:n-BuLi
stoichiometry is crucial to avoid double or triple lithiation
of TMOP–H. Importantly, the resultant TMOP-Li must be
added dropwise to an excess of PCl3 at –78 °C to prevent
significant formation of TMOP2PCl (P = 85) and TMOP3P (P
= −7.7). By following these procedures carefully, near
quantitative formation of 2a (98%) over TMOP2PCl (1%)
and TMOP3P (1%) was obtained.
Scheme 1. Addition Polymerization of Phosphaalkene
MesP=CPh2 to Afford Poly(methylenephosphine)
Ph
H3C
H3C
P
C
Ph
C
[init.]
Ph
P
CH2
MesPh
X
n
H
H
P
CPh2
[init.]
Ph
C
H3C
CH2
P
MesPh
y
CH3
x
n
(x >> y)
In an effort to favor the “normal” head-to-tail addition
polymerization of P=C bonds (i.e. the polymer has
predominantly –P–C–P–C– enchainment) we sought
isolable monomers with analogous steric encumbrance to E
but being less prone to intramolecular activation than Mes.
In the present study, our two target monomers are the
unprecedented phosphaalkenes bearing the 2,4,6-
Scheme 2. Synthetic Route to TMOP-P=CPh2 (1a)
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These bond lengths are typical of the 16 structures of
1) n-BuLi
OCH3
LiAlH4
2) PCl3
PCl2
1
2
PH2
primary arylphosphines of type Ar(PH2)n found on the
Cambridge Crystallographic Database [range in Å: 1.23(4) –
1.42(4)].30, 31
TMOP
TMOP
H3CO
OCH3
3
TMOP-H
2a
3a
4
5
6
7
8
9
Ph2CO
KOH (7 mol%)
OCH3
1) n-BuLi
2) TMSCl
Ph
Ph
P
C
P(TMS)2
TMOP
- O(Me3Si)2
H3CO
OCH3
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4a
1a
The desired product 2a was isolated as a pale-yellow
solid in 70% yield after the removal of residual TMOP-H by
sublimation. It is crucial to remove TMOP–H prior to
reduction of 2a to 3a due to the similar properties of TMOP-
H and 3a rendering them difficult to separate. 31P NMR
spectroscopic analysis of the product showed only a singlet
resonance at 156 ppm. The identity of the product as 2a was
confirmed by further analysis with H and 13C{1H} NMR
1
spectroscopy along with ESI-MS. Unlike MesPCl2, which is
liquid at room temperature, solid TMOP-PCl2 can more
conveniently be weighed and handled at room temperature
under inert atmosphere. For longer periods, it is advisable
to store pure compound 2a in a freezer inside of a glovebox
as we have observed degradation at room temperature after
a week. Analysis of a sample of 2a stored for several weeks
at room temperature using 31P NMR spectroscopy revealed,
in addition to the major signal for 2a at 156 ppm, two small
resonances at 150 and 100 ppm. These “impurities” were
not identified or analyzed further and are not present in
samples of 2a stored at –35 °C in a glovebox.
Figure 2. Structure of 3a showing closest intermolecular
contacts.
Following
standard
reduction
protocols
for
phosphines,27 a solution of pure 2a in Et2O was slowly
added to a stirred suspension of LiAlH4 (0.7 equiv) in Et2O
at –78 °C. Analysis of an aliquot removed from the reaction
mixture using 31P NMR spectroscopy revealed only a triplet
resonance at –174 ppm (1JPH = 204 Hz), indicative of
complete conversion to 3a. Notably, this is amongst the
highest field shifts reported for a primary phosphine (cf. Ph-
CC-PH2: P = –176).28, 29 In the unlikely event that reduction
was incomplete, additional LiAlH4 was added to complete
the reduction. After workup, the desired phosphine 3a was
isolated as colorless crystals (Yield = 40%). The formulation
1
and purity of primary phosphine 3a was confirmed by H
and 13C{1H} NMR spectroscopy, ESI-MS, elemental analysis
and X-ray crystallography. We caution that compound 3a is
pyrophoric upon air exposure and due care must be taken
when handling samples of the compound or combustible
items the compound may have come into contact with.
The molecular structure of 3a is shown in the
supporting information Figure S33 and the hydrogen atoms
bound to phosphorus were located and refined
isotropically. Compound 3a crystallizes as discrete
molecules and there are no intermolecular contacts which
would be consistent with P–H…P hydrogen bonding. The
closest intermolecular contacts are between the P–H moiety
and the o-OMe groups of two nearest neighbor molecules as
shown in Figure 2 [dHH = 2.21 Å; cf. rvdw = 2.40 Å; dHO = 2.68
Å; cf. rvdw = 2.70 Å]. The P–H bond lengths, P(1)–H(1) and
P(1)–H(2), were 1.284(1) and 1.395(1) Å, respectively.
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1
2
3
Table 1. Important Metrical Parameters for Phosphaalkenes of Type Ar*P=C(Ar)(Ar') (Ar* = Mes, Mes* and 2,6-
Mes2C6H4)
4
5
6
1a
1b
Compound
Mes-P=C(Ar)(Ar')[1]
Average Range
Ar*-P=C(Ar)(Ar')[2]
Average
Range
7
8
9
Bond lengths (Å)
1.689(2
1.700(2
1.692(1
0)
1.832(1
1.682(2) –
1.7082(13)
1.821(3) –
1.690(1
3)
1.843(1
4)
1.487(1
5)
1.479(1
6)
1.677(5) –
1.701(6)
1.832(8) –
1.857(5)
1.483(7) –
1.497(6)
1.47(1) –
1.500(8)
P=C
P–CMes/Ar*
C–Ctrans
C–Ccis
)
)
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1.822(2
1.868(2
)
)
0)
1.490(1
3)
1.485(1
2)
1.8418(13)
1.481(3) –
1.500(2)
1.476(4) –
1.498(2)
1.495(3
1.490(3
)
1.482(3
)
)
1.479(3
)
Bond angles (˚)
107.84(
1)
115.37(
1)
126.52(
1)
117.87(
2)
104.74(
1)
112.89(
2)
130.47(
2)
116.60(
2)
106.71(
3)
116.64(
104.20(1) –
108.8(13)
114.2(1) –
105.69(
1)
116.90(
1)
132.55(
1)
110.47(
1)
102.2(2) –
108.89(6)
112.9(2) –
119.7(3)
125.7(4) –
136.5(2)
105.3(4) –
117.58(1)
∠CMes/Ar*-P=C
∠P=C–Ctrans
∠P=C–Ccis
3)
126.62(
2)
116.66(
2)
119.3(2)
123.9(1) –
129.2(1)
115.1(1) –
119.0(3)
∠Ccis–C–Ctrans
N/A
N/A
15 structures in CCDC
11 structures in CCDC
# in CCDC
Ref.
This work
15c,d,f,32i,34
15d,35
[1] Ar, Ar’ = aryl. [2] Ar* = Mes* or 2,6-Mes2C6H3; Ar, Ar’ = aryl.
To obtain the corresponding silylphosphine 4a, a THF
solution of 3a was cooled to –78 °C and treated with n-BuLi
(2 equiv). It is important not to exceed 2 equiv of n-BuLi
since the TMOP- group in 3a can also be lithiated. After
allowing the reaction mixture to slowly warm to room
temperature (ca. 16 h), the yellow solution was again cooled
to –78 °C whereupon Me3SiCl (3 equiv) was added to
complete the silylation. Once the reaction mixture had
warmed to room temperature, analysis of a small aliquot by
31P NMR spectroscopy showed a singlet resonance at –179
ppm which is in the typical range for silylphosphines [cf.
MesP(SiMe3)2 δP = –162].32a Pyrophoric 4a was isolated as a
pale-yellow oil which did not crystallize upon standing nor
upon cooling solutions in common organic solvents to –35
°C (e.g. THF, hexane, Et2O, toluene, etc.). Furthermore,
attempts to perform vacuum distillation led to degradation
of the product. Thus, this intermediate could not be purified
to an analytical standard. Nevertheless, the identity of 4a as
dark yellow/orange oil. Crystals suitable for X-ray analysis
could be obtained by cooling a concentrated solution of the
oil in Et2O to –35 °C. Additional characterization by 1H and
13C{1H} NMR spectroscopy, mass spectrometry and
elemental analysis confirmed the formulation and purity of
1a. 31P NMR spectroscopic analysis of crystals of 1a that had
been exposed to air atmosphere for 24 h, and subsequently
were dissolved in dry CDCl3, suggested that ca. 45% of the
product remained with unidentified signals (P = 60 to –
130).
The molecular structure of phosphaalkene 1a is shown
in the supporting information Figure 3(a) and, for ease in
comparison, Table 1 shows selected metrical parameters
for 1a and the average and range for crystallographically
characterized phosphaalkenes of type Ar*P=C(Ar)(Ar') (Ar*
= Mes, Mes* and 2,6-Mes2C6H4, Ar/Ar' = phenyl or other
aryl) found in the Cambridge Crystallographic Database.
Remarkably, the metrical parameters are overwhelmingly
similar, perhaps implying that in triarylphosphaalkenes the
P-substituent has little effect provided it is bulky enough to
inhibit dimerization. The P=C bond length of 1a [1.689(2)
Å] is at the longer end of the range typical for C-substituted
phosphaalkenes (1.61-1.71 Å)33 and does not differ
considerably from those of P-Mes phosphaalkenes (1.68 -
1.71 Å)15c,d,f,32i,34 or P-Ar* (Ar* = Mes* or 2,6-Mes2C6H3)
phosphaalkenes (1.68 - 1.70 Å).15d,35 The P-CTMOP bond for
1a [1.822(2) Å] is at the shorter end of the range typical for
P-Mes phosphaalkenes (range: 1.82 - 1.84 Å)15c,d,f,32i,34 and P-
Ar* phosphaalkenes (range: 1.83 - 1.86 Å).15d,35 A slight
elongation of the P=C bond and shortening of the P-CTMOP
bond has been observed for related phosphaalkenes)15f,32i
the predominant species was confirmed by H and 13C{1H}
1
NMR spectroscopy and mass spectrometry. The impurities
include TMOP-PH(SiMe3) (δP = –176) and did not hinder the
next step.
With silylphosphine 4a in hand, the base-catalyzed
phospha-Peterson reaction32 was attempted by treating its
THF solution with benzophenone in the presence of KOH (8
mol %). The reaction mixture was stirred overnight.
Subsequently, a small aliquot was analyzed by 31P NMR
spectroscopy to reveal that the signal assigned to 4a (P = –
179) had been replaced by a new singlet resonance at 210
ppm. Such a downfield chemical shift is clearly indicative of
successful P=C bond formation. Crude 1a was obtained as a
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and may reflect π-conjugation between the TMOP group Inspired by these reports, we developed a route to 4b
1
2
3
4
5
6
7
8
and the P=C bonds. On the other hand, the angle between
the plane of the P=C bond and the plane of the aromatic ring
of the TMOP-group in 1a is 76.42(7)° and suggests minimal
π-conjugation. For comparison, the analogous angle is ca.
71˚ for MesP=CPh2,34a,b 79.51˚ for Mes*P=CPh235a and 78.04˚
for 2,6-Mes2C6H3-P=CPh2.35g
which is outlined in Scheme 3. A solution of primary
phosphine 3b in Et2O was treated with Me3SiOTf (2 equiv)
and NEt3 (2 equiv). After 24h, the progress of reaction was
evaluated by recording a 31P NMR spectrum of an aliquot
removed from the reaction mixture which revealed that the
triplet of septets assigned to 3b (P = –140, 1JPH = 213 Hz, 4JPF
= 30 Hz) had been replaced by a doublet of septets which
was assigned to secondary phosphine 3b'(δP = –134, 1JPH
=
219 Hz, 4JPF = 61 Hz). In the 1H NMR spectrum, the presence
of a doublet resonance at 3.89 ppm with a 216 Hz coupling
constant (1JPH) and a doublet resonance at 0.03 ppm with a
6 Hz coupling constant (4JHH), integrating to 1:9, provided
clear support for the identity of the product as monosilane
3b'.Given the extreme air sensitive and pyrophoric nature
of the product, it was used in the next step without further
purification.
9
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Attempts to further silylate 3b' with Me3SiOTf were
unsuccessful. Moreover, treating 3b' with MeLi, KH or
LiNiPr2 followed by Me3SiCl did not result in the desired
product. Finally, employing the weak and sterically
hindered base KN(SiMe3)2 to deprotonate 3b' provided a
route to doubly-silylated 4b after treatment of the
intermediate {presumably K[ArFPSiMe3]} with Me3SiCl.
Analysis of the reaction mixture by using 31P NMR
spectroscopy revealed a broad resonance at –135 ppm
which lacked P–H coupling. The crude product was isolated
as a colorless liquid in 70% yield after purification by
Figure 3. Molecular structures of: (a) TMOP-P=CPh2 (1a)
and (b) ArF-P=CPh2 (1b). Thermal ellipsoids are drawn at the
50 % probability level; all hydrogen atoms are omitted for
clarity.
Synthesis and Characterization of ArF-P=CPh2 (1b).
The synthesis of 2,4,6-tris(trifluoromethyl)phenyl-
substituted phosphaalkene 1b (ArF-P=CPh2) turned out to
be very challenging. Literature reports of both ArF-PCl2
(2b)19, 36, 37 and ArF-PH2 (3b)23, 36, 37 provided a few possible
routes to these key compounds. For the present study, we
followed most closely the procedures of Foss and
coworkers for 2b37 and Bertrand and coworkers for 3b.23
With pure 3b in hand, our attention now turned to the
silylation step which for ArPH2 and RPH2 (Ar = aryl, R =
alkyl) typically involves metalation with n-BuLi or MeLi
followed by silylation with Me3SiCl.38,32i,27 Unfortunately, it
is known that treating 3b with RLi (R = Me or n-Bu) affords
LiArF and RPH2 rather than the desired Li[ArFPH] and R–H.23
Thus, we had to explore other possibilities to silylate 3b. It
is known that 3b can be singly deprotonated with KH to
afford K[ArFPH].23 In addition, the silylation of PH3 and
related RPH2 (R = Me, t-Bu, Ph) with Me3SiOTf and NEt3 has
been reported to afford P(SiMe3)3 or RP(SiMe3)2,
respectively.39, 40
1
vacuum distillation in an oil bath. H NMR spectroscopic
analysis of a C6D6 solution of the product (Figure 4) revealed
a doublet resonance at 7.43 ppm and triplet resonance at
6.61 ppm in 2:1 ratio (3JHH = 8 Hz) assigned to the m-H and
p-H of the ArF moiety, respectively. In addition, a doublet
resonance (3JPH = 8 Hz) which integrated to 18 H was
observed at 0.26 ppm and was assigned to the SiMe3
moieties of 4b. A small signal at 0.18 ppm was assigned to
N(SiMe3)341 and, based on its integrated ratio, suggests that
ca. 4 % of this impurity was present in 4b. Due to the
extreme pyrophoric and malodorous nature of 4b, analysis
by mass spectrometry could not be performed.
Scheme 3. Synthetic Route to ArF-P=CPh2 (1b)
1) n-BuLi, TMEDA
2) PCl3
F3C
CF3
LiAlH4
PCl2
PH2
ArF
ArF
2b
3b
ArF-H
Me3SiOTf
Et3N
1) KN(SiMe3)2
2) Me3SiCl
PH(TMS)
P(TMS)2
Figure 4. 1H NMR (400 MHz, C6D6) spectrum of 4b. *
indicates residual C6HD5 and † indicates N(SiMe3)3.
ArF
ArF
3b'
4b
With 4b in hand, the base-catalyzed phospha-Peterson
reaction32 was attempted. Specifically, a solution of 4b and
benzophenone in THF was treated with catalytic KOH.
Analysis of the reaction mixture by 31P NMR revealed no
signals that could be attributed to phosphaalkene 1b. No
evidence for reaction was observed when the solution was
CF3
Ph
Ph
Ph2CO
KOt-Bu (1 mol%)
P
C
- O(Me3Si)2
CF3
1b
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heated for extended time periods. The regular phospha-
remained in the case of 1b whilst only 45% remained in the
case of 1a.
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7
8
Peterson reaction involving treatment of 4b with MeLi
followed by addition of benzophenone was also
unsuccessful. The Lewis-acid-mediated phospha-Peterson
reaction was also attempted.42 Thus, a solution of 4b in
CH2Cl2 was treated with benzophenone (1 equiv) in the
presence of AlCl3 (1 equiv). This too did not afford any of the
desired phosphaalkene 1b as judged from 31P NMR
spectroscopy.
The successful formation of 1b was finally achieved
when a THF solution of 4b and benzophenone (1 equiv) was
treated with soluble base, KOt-Bu (1 mol%). After 2 h at
room temperature, an aliquot was removed from the
reaction mixture and analyzed by 31P NMR spectroscopy.
The spectrum revealed that the signal assigned to 4b (P = –
135) was no longer present and a new septet resonance was
observed at 207 ppm (4JPF = 61 Hz). Such a downfield
resonance is highly indicative of P=C bond formation. The
product was isolated by recrystallization from hexanes at –
35 °C. Unfortunately, the crystals melt below room
temperature, presumably due to the presence of an
impurity (vide infra). Additional support for the
formulation of the product as 1b was obtained from the
13C{1H} NMR spectrum which displayed a doublet of septets
at 194.5 ppm (1JPC = 44 Hz, 5JCF = 3 Hz) that was assigned to
the P=C bond. For comparison, related phosphaalkenes
have very similar chemical shifts for the P=C bond: 193.6
ppm (d, 1JPC = 42 Hz; MesP=CPh2);34a 193.1 (d, 1JPC = 40.5 Hz;
1a).
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Figure 5. H NMR (400 MHz, CDCl3) spectrum of 1b: (a)
crude 1b and (b) pure 1b. † indicates N(SiMe3)3 and * indicates
residual CHCl3.
SUMMARY
In closing, we have designed efficient synthetic routes
from readily available starting materials to two novel
phosphaalkenes (1a and 1b). The key P=C bond forming
step involved the base-catalyzed phospha-Peterson
reaction
of
a
bis(trimethylsilyl)phosphine
and
benzophenone. To our knowledge, phosphaalkene 1a
represents the first example of a P-TMOP substituted
phosphaalkene and illustrates the effectiveness of this
readily available, but little used, substituent in low-
coordinate p-block chemistry. In addition, fluroaryl-
substituted 1b is a rare example of an ArF-phosphaalkene
and is a structural analogue to the polymerizable
MesP=CPh2 (E) but bears chemically inert CF3 in place of
CH3 moieties. Although both routes pose considerable
synthetic challenge, 1a and 1b are isolable and show no
degradation upon storage for extended periods under inert
atmosphere. Thus, we believe they may be suitable
monomers for reactions with potential initiators for
polymerization, a subject that will be of future study.
Unfortunately, samples of 1b were consistently
contaminated with N(SiMe3)3 (δH = 0.18, δC = 5.6)41 which
remained from the preparation of 4b. Despite repeated
recrystallizations, the phosphaalkene always contained ca.
12% N(SiMe3)3 from 1H NMR spectroscopic analysis [Figure
5(a)]. Attempts to distill 1b resulted in decomposition
which is likely a consequence of its projected high boiling
point. We imagined that hydrolysis of N(SiMe3)3 to easily
removed NH3 and (Me3Si)2O might provide a means to
purify 1b. We have used aqueous extraction successfully to
purify other phosphaaalkenes.32i,42 Thus, a CH2Cl2 solution
of crude 1b was extracted with H2O. The organic layer was
separated and evaporated to dryness (83 % recovery).
Analysis of the colorless liquid by using 1H NMR
spectroscopy revealed that the signal assigned to N(SiMe3)3
was no longer present and the spectrum was otherwise
unchanged [Figure 5(b)]. Likewise, the 13C{1H} NMR
spectrum confirmed that N(SiMe3)3 had successfully been
removed. The 31P and 19F NMR spectra were also unchanged
following purification and high-resolution mass
spectrometric analysis and elemental microanalysis
confirmed the formulation and purity of 1b. Moreover,
crystals of 1b were obtained by slow evaporation of a
hexanes solution and X-ray crystallographic analysis
confirmed its molecular structure [Figure 3(b)]. Structural
details are given in the supporting information and the key
metrical parameters (Table 1) are similar to those of 1a and
related phosphaalkenes. Crystals of 1b that were exposed
to air for 24 h showed less degradation to those of 1a,
suggesting a higher air stability for the former. Specifically,
31P NMR spectroscopic analysis of solutions of the air-
exposed crystals revealed that ca. 90% of phosphaalkene
EXPERIMENTAL SECTION
All manipulations of air- and/or water-sensitive
compounds were performed under a nitrogen atmosphere
using standard Schlenk or glovebox techniques. 1H, 31P and
13C{1H} NMR spectra were recorded at room temperature
on Bruker Avance 300 MHz or 400 MHz spectrometers.
Chemical shifts are reported relative to: residual CHCl3 (δH
= 7.26 for 1H); residual C6HD5 (δH = 7.16 for 1H); 85 % H3PO4
as an external standard (δP = 0.00 for 31P); CFCl3 as an
external standard (δF = 0.00 for 19F); CDCl3 (δC = 77.16 for
13C{1H}); C6D6 (δC = 128.06 for 13C{1H}). Mass Spectra were
acquired using Kratos MS 50 instrument in EI mode (70 eV).
Elemental analyses were performed in the University of
British Columbia Chemistry Microanalysis Facility.
Hexanes, diethyl ether and dichloromethane were
deoxygenated with nitrogen and dried by passing through a
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The Journal of Organic Chemistry
column containing activated basic alumina.43 Prior to use,
combined. In the end, the solvent was removed under
1
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3
4
5
6
7
8
THF was freshly distilled from sodium/benzophenone ketyl
and N,N,N′,N′-tetramethylethylenediamine (TMEDA) was
freshly distilled from sodium. Benzophenone (Aldrich) was
sublimed prior to use. KOH was recrystallized from EtOH
and heated in vacuo following a modified literature
procedure for the purification of sodium hydroxide.44
vacuum to obtain 3a as a white solid. This compound was
employed in the next step without further purification.
Crystals of 3a suitable for X-ray crystallography analysis
were obtained from slow evaporation of its saturated Et2O
solution at –35˚C in the glovebox freezer. Yield: 1.04 g
(40%).
Trimethoxybenzene,
trimethylsilyl
chloride
were
1
31P NMR (162 MHz, CDCl3, δ): –173.5 (t, JPH = 211 Hz);
purchased from Aldrich and used as received. Compounds
1H NMR (400 MHz, CDCl3, δ): 6.16 (s, 2H), 3.86 (s, 6H), 3.84
2b37 and 3b23 were made following literature procedures.
1
(s, 3H), 3.70 (d, JPH = 211 Hz, 2H); 13C{1H} NMR (101 MHz,
9
CDCl3, δ): 164.6 (s), 164.4 (s), 107.0(d, 1JPC = 71 Hz), 91.2(s),
56.3 (s), 55,8 (s); MS (ESI+) m/z: [M+H]+ 201; Anal. Calcd
for C9H13O3P: C, 54.00; H, 6.55; Found: C, 54.23; H, 6.55.
Warning. Primary phosphines (3a and 3b) and
silylphosphines (3b', 4a, and 4b) are pyrophoric and
malodorous. Thus, caution must be exercised to avoid
exposures of the compounds to air or any combustibles (e.g.
tissues) that may have come into contact with the
compounds.
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Synthesis of TMOP-P(SiMe3)2 (4a). Primary phosphine
3a (0.89 g, 4.46 mmol) was dissolved in THF (80 mL) and
cooled to –78 °C. To this stirred solution was slowly added
n-BuLi in hexanes (3.6 mL, 2.5 M, 8.92 mmol). The colorless
solution turned to dark red during the addition. The
solution was stirred for another 16 h while the cooling bath
slowly warmed from –78 °C to room temperature. Over this
time, the solution color changed from dark red to yellow.
The flask was then cooled to –78 °C again and Me3SiCl (1.7
mL, 13.4 mmol) was added. The mixture was stirred for
additional 3 h at –78 °C and slowly warmed to room
temperature. Upon reaction completion, an aliquot of the
reaction mixture was taken for 31P NMR spectroscopy
analysis, which showed a chemical shift at –179 ppm. THF
was removed under vacuum and the product was extracted
with hexanes (3 × 50 mL) to remove LiCl. The hexane phases
were collected and combined inside a separate Schlenk
flask and the solvent was removed under vacuum. This
compound could be used for the next step without further
purification as long as it was the predominant species (see
supporting information for its 31P NMR spectrum). In the
end 4a was obtained as pale-yellow oil. Yield: 0.51 g (33%).
Synthesis TMOP-PCl2 (2a). Inside a 100 mL Schlenk
flask, TMOP-H (8.4 g, 50 mmol) was dissolved in a minimum
amount of Et2O (10 mL). The solution was cooled to 0 °C and
Et2O (50 mL) was added until the precipitation was
dissolved again. A solution of n-BuLi in hexanes (20 mL, 2.5
M, 50 mmol) was added dropwise over 2 h while the
solution was stirred at 0 °C. The reaction mixture was kept
stirring at 0 °C and slowly warmed up to room temperature
over the course. After 12 h, the resultant white suspension
was diluted with cold Et2O (50 mL, 0 °C) and added
dropwise into a flask containing PCl3 (21.8 mL, 250 mmol)
in Et2O (200 mL) at –78 °C. The mixture was stirred for
additional 5 h at –78 °C and slowly warmed up to room
temperature. Analysis of the reaction by 31P NMR
spectroscopy indicated the formation of 2a (δP = 156). The
Et2O layer was then transferred into another flask through
a filter stick and the product was further extracted with
Et2O (3 x 50 mL). After combining the Et2O layers, the
volatiles were removed under vacuum and very pale-yellow
solids were obtained. This product should be further
purified by sublimation under vacuum at 65°C for 12 h to
remove any unreacted TMOP-H. The resultant TMOP-PCl2
seems to be unstable at ambient temperature, even under
inert atmosphere in the glovebox. Therefore, storage of 2a
in the glovebox freezer is necessary for longer period. Yield:
9.4 g (70%).
1
31P NMR (121 MHz, CDCl3, δ): –179.4 (s); H NMR (300
MHz, CDCl3, δ): 6.12 (s, 2H), 3.82 (s, 3H), 3.78 (s, 6H), 0.20
(s, 9H), 0.18 (s, 9H); 13C{1H} NMR (75 MHz, CDCl3, δ): 163.5
(s), 161.2 (s), 98.3(d, 1JPC = 20 Hz), 90.5 (s), 55.2 (s), 55.1 (s),
1.81 (s); MS (EI) m/z: [M]+ 344; HRMS (EI) m/z: [M]+ Calcd
for C15H29O3PSi2 344.1393; Found 344.1385.
Synthesis of TMOP-P=CPh2 (1a). In a 25 mL Schlenk
flask, a mixture of 4a (175 mg, 0.51 mmol), benzophenone
(93 mg, 0.51 mmol) and KOH (2 mg, 0.036 mol) were
dissolved in THF (9 mL) and stirred for 12 h at room
temperature. The solution turned yellow over time.
Analysis of the reaction by 31P NMR spectroscopy indicated
the formation of 1a (δP = 210). THF was then removed
under vacuum and 1a was obtained as dark yellow/orange
oil. Crystals of 1a were obtained by cooling a concentrated
Et2O solution (100 mg 1a/1 mL Et2O) to –35 ˚C inside a
1
31P NMR (162 MHz, CDCl3, δ): 156.5 (s); H NMR (400
MHz, CDCl3, δ): 6.10 (s, 1H), 6.09 (s, 1H), 3.87 (s, 6H), 3.84
(s); 13C{1H} NMR (101 MHz, CDCl3, δ): 164.4 (s), 164.2 (s),
106.8 (d, 1JPC = 71 Hz), 91.0 (s), 56.1 (s), 55,6 (s); MS (ESI+)
m/z: [M+H]+ 270.
Synthesis of TMOP-PH2 (3a). LiAlH4 (0.35 g, 9.2 mmol)
was dissolved in Et2O (50 mL) and cooled to –78 °C. 2a (3.5
g, 13 mmol) was dissolved in Et2O (50 mL) in a second flask
and added to the LiAlH4/Et2O solution via cannula. The
mixture was stirred for 4 h at –78 °C and slowly warmed to
room temperature. Afterwards degassed H2O (10 mL) was
added into the reaction mixture dropwise at 0˚C via a
syringe to quench any unreacted LiAlH4 (Caution: this
reaction is exothermic and evolves H2. If H2O is added too
rapidly, explosion may occur.). The Et2O layer on the top
was then transferred into another flask via filter cannula.
The leftover solid (mostly LiCl and AlCl3) was washed with
Et2O (3 x 30 mL) and the Et2O layers were collected and
glovebox. After 12 h, yellow crystals suitable for X-ray
crystallography analysis were harvested. Yield: 120 mg
(65%).
1
31P NMR (121 MHz, CDCl3, δ): 210.4 (s); H NMR (300
MHz, CDCl3, δ): 7.56 (m, 2H), 7.30 (m, 3H), 7.09 (m, 3H), 6.96
(m, 2H), 5.93 (s, 2H), 3.78 (s, 3H), 3.60 (s, 6H); 13C{1H} NMR
1
(75 MHz, CDCl3, δ): 193.1 (d, JPC = 40 Hz, P=C), 162.8 (s),
161.3 (d, 2JPC = 4 Hz), 144.3 (d, 2JPC = 26 Hz), 143.8 (d, 2JPC=
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14 Hz), 137.2 (s), 132.1 (s), 129.6 (s), 129.1 (d, 3JPC = 6 Hz),
water layer was removed by pipette. The solvent was
1
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128.0 (s), 127.7 (s), 126.7 (s), 108.5 (d, JPC = 45 Hz), 55.1
removed from the remaining organic layer in vacuo to
afford 1b free from N(SiMe3)3. Recovered: 0.10 g (83%).
Crystals of 1b suitable for X-ray crystallography analysis
were obtained from slow evaporation of its saturated
hexanes solution at room temperature in the glovebox.
(s), 54.8 (s); MS (EI) m/z: [M]+ 364; HRMS (EI) m/z: [M]+
Calcd for C22H21O3P 364.1228; Found 364.1227; Anal. Calcd
for C22H21O3P: C, 72.52; H, 5.81; Found: C, 72.60; H, 5.88.
Synthesis of ArF-PH(SiMe3) (3b'). Primary phosphine
3b (2.5 g, 10.1 mmol), Me3SiOTf (4.5 g, 20.2 mmol) and NEt3
(2.1 g, 20.2 mmol) were dissolved in Et2O (50 mL) and the
mixture was stirred at room temperature for 24 h. An
aliquot of the reaction mixture was taken and checked by
31P NMR spectroscopy to confirm the formation of 3b'(δP
= –134). Upon reaction completion, the solvent was
removed under vacuum to obtain the product as a liquid.
This product was used in the next step without further
purification. Yield: 2.6 g (80%).
31P NMR (162 MHz, CDCl3, δ): 208 (sept, 4JPF = 61 Hz); 19
F
4
1
NMR (376 MHz, CDCl3, δ): –56.6 (d, JFP = 26 Hz); H NMR
(400 MHz, CDCl3, δ): 7.80 (d, 3JHH = 8 Hz, 2H), 7.56 (m, 2H),
7.46 (m, 4H), 7.09 (m, 3H), 6.93 (m, 2H); 13C{1H} NMR (101
MHz, CDCl3, δ): 194.5 (d of sept, 1JPC = 44 Hz, 5JCF = 3 Hz, P=C),
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140.9 (d, 1JPC = 76 Hz), 129.6 (q, 2JCF = 29 Hz), 129.1 (d, 3JPC
=
7 Hz), 128.9 (s), 128.5 (s), 128.4 (s), 128.3 (s), 128.1 (s),
1
127.7 (s), 127.4 (s), 123.8 (q, JCF = 276 Hz). MS (EI) m/z:
[M]+ 410; HRMS (EI) m/z: [M]+ Calcd for C21H13F6P
410.0659; Found 410.0672; Anal. Calcd for C21H13F6P: C,
61.47; H, 3.19; Found: C, 61.57; H, 3.71.
31P NMR (162 MHz, C6D6, δ): –134.0 (d of sept, 1JPH = 219
Hz, 4JPF = 61 Hz); 19F NMR (376 MHz, C6D6 , δ): –57.8 (d, 4JFP
1
3
= 26 Hz); H NMR (400 MHz, C6D6, δ): 7.36 (d, JHH = 8 Hz,
2H), 6.58 (t, 3JHH = 8 Hz, 1H), 3.89 (d, 1JPH = 216 Hz, 1H), 0.03
(d, 4JHH = 6 Hz, 9H).
ASSOCIATED CONTENT
Supporting Information.
These materials are available free of charge via the Internet
at http://pubs.acs.org.
Synthesis of ArF-P(SiMe3)2 (4b). Secondary phosphine
3b'(0.50 g, 1.6 mmol) was dissolved in THF (20 mL) and
added to THF solution of KN(SiMe3)2 (0.38 g, 1.9 mmol) at –
78 °C. The mixture was stirred and warmed slowly to room
temperature. Afterwards Me3SiCl (0.41 g, 3.8 mmol) was
added slowly into the solution at –78 °C. The reaction was
warmed to room temperature and the mixture was checked
by 31P NMR spectroscopy to confirm the formation of 4b (δP
= –136). Upon reaction completion, the solvent was
removed under vacuum and the product was distilled under
full vacuum (110-120 ˚C, 0.01mmHg) in an oil bath.
Silylphosphine 4b was obtained as colorless malodorous
liquid. This synthesis could be repeated in order to
accumulate enough 4b for the next step. Yield: 0.43 g (70%).
X-ray crystallographic data as CIF files.
Copies of NMR spectra of the compounds from each
synthetic step and X-ray single-crystal diffraction data for
3a, 1a and 1b.
AUTHOR INFORMATION
Corresponding Author
Derek P. Gates Chemistry Department, University of
British Columbia, 2036 Main Mall, Vancouver, Canada V6T 1Z1;
E-mail: dgates@chem.ubc.ca.
31P NMR (162 MHz, C6D6 , δ): –136.0 (m, broad signal);
19F NMR (376 MHz, C6D6 , δ): –55.5 (d, 4JFP = 30 Hz); 1H NMR
(400 MHz, C6D6 , δ): 7.43 (d, 3JHH = 8 Hz, 2H), 6.61 (t, 3JHH = 8
Authors
Zeyu Han Chemistry Department, University of British
Columbia, 2036 Main Mall, Vancouver, Canada V6T 1Z1
3
Hz, 1H), 0.26 (d, JPH = 8 Hz, 18H); 13C{1H} NMR (101 MHz,
2
2
C6D6 , δ): 138.9 (d of q, JPC = 10 Hz, JCF = 27 Hz), 133.4 (d,
David Röhner Chemistry Department, University of British
Columbia, 2036 Main Mall, Vancouver, Canada V6T 1Z1
1JPC = 52 Hz), 129.5 (m), 126.4 (s), 124.2 (q, JCF = 274 Hz),
1
1.7 (d, 2JPC = 18 Hz).
Kerim Samedov Chemistry Department, University of
British Columbia, 2036 Main Mall, Vancouver, Canada V6T 1Z1
Synthesis of ArF-P=CPh2 (1b). Silylphosphine 4b (0.78
g, 2.0 mmol), benzophenone (0.36 g, 2.0 mmol) and KOt-Bu
(2.2 mg, 0.02 mmol) were mixed and dissolved in THF (20
mL). The solution was stirred at room temperature for 2 h.
Analysis of the reaction by 31P NMR spectroscopy indicated
the formation of 1b (δP = 207). Upon reaction completion,
the solvent was removed under vacuum and the product
was purified by crystallization from hexanes in the glovebox
freezer (–35˚C) multiple times. Phosphaalkene 1b was
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We are grateful to the NSERC of Canada for financial
support of this work. ZH thanks UBC Graduate Scholarship
– Research Excellence Award for support. DR thanks the
Deutscher Akademischer Austauschdienst (DAAD) for an
exchange fellowship.
1
obtained as colorless liquid at ambient temperature. H
NMR spectroscopic analysis showed that the sample of 1b
contained ca. 12% N(SiMe3)3. Yield: 0.57 g (70%). A 10 mL
Schlenk flask was charged with crude 1b (0.12 g) and
dissolved in CH2Cl2 (2 mL) in a glovebox. Under a stream of
N2 on a Schlenk line (the solution should not be exposed to
O2), degassed water (2 mL) was added to the above solution
and the resultant mixture was stirred. After 10 min, the
stirring was stopped and once the layers had separated the
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