Triphenylphosphaalkenes in Chemical Equilibria

Article abstract of DOI:10.1002/ejic.201801322

Triphenylphosphaalkenes 1a–c were prepared in good to excellent yields in a modified phospha-Peterson reaction between PhP(Li)TMS and benzophenones with different para-substituents at the C-phenyl groups (a: R = H, b: R = O-octyl, c: R = F). Owing to the

Full text of DOI:10.1002/ejic.201801322

DOI: 10.1002/ejic.201801322
Communication
Phosphaorganic Chemistry
Triphenylphosphaalkenes in Chemical Equilibria
Nicolas D'Imperio,[a] Anna I. Arkhypchuk,*[a] Juri Mai,[a] and Sascha Ott*[a]
Abstract: Triphenylphosphaalkenes 1a–c were prepared in reversible dimerization and oligomerization reactions, some of
good to excellent yields in a modified phospha-Peterson reac- which are not detectable by 31P NMR monitoring. The dimers
tion between PhP(Li)TMS and benzophenones with different and oligomers are in chemical equilibria with monomeric 1a–c
para-substituents at the C-phenyl groups (a: R = H, b: R = O- and can be converted quantitatively to phosphinites 4a–c by
octyl, c: R = F). Owing to the low kinetic stabilization that is the irreversible addition of methanol across the P=C double
provided by the P-phenyl group, compounds 1a–c engage in bond.
Introduction
improved synthetic procedure towards P-phenylphosphaalk-
enes, strategies to prevent the formation of unwanted side
products during their formation, as well as an in-depth study
of their dimerization and oligomerization behavior. It is shown
that the products of the latter two processes are in chemical
equilibria with the monomeric phosphaalkene.
In the field of low-coordinate main group chemistry, kinetic sta-
bilization is an often encountered strategy to prevent decom-
position of the target compounds, and other side reactions
such as dimerizations, oligo- and polymerization.[1] For exam-
ple, several sterically bulky substituents have been developed
over the years to stabilize the P=C double bond in phosphaalk-
enes.[2] Amongst these, the supermesityl group (2,4,6-tBu3Ph,
Mes*)[3] is particularly popular as it allows the purification of
phosphaalkenes by column chromatography.[4] Decreasing the
steric bulk of the protecting group at the phosphorus atom
decreases the kinetic stabilization, and thus results in synthetic
challenges, as illustrated for example by the fact that phos-
phaalkenes with smaller mesityl groups [2,4,6-(CH3)3Ph, Mes]
are fewer in the literature.[5] P-phenyl substituted phosphaalk-
enes are even less stable, are often not isolable, and are re-
ported to rapidly dimerize to the corresponding 1,2-diphos-
phetanes.[6] On the contrary, highly reactive phosphaalkenes
are appealing if they are intermediates in downstream chemical
transformations. We have recently reported such a process in
which phosphaalkenes are key intermediates for the reductive
cross-coupling of aldehydes to alkenes.[7] By using Mes*[8] and
Mes[9] to stabilize the P=C double bonds, 1,2-disubstituted ole-
fins can be synthesized in a one-pot reaction. P-phenyl substi-
tuted phosphaalkenes are most likely even more reactive and
may allow an even broader substrate scope in terms of carbonyl
compounds that could be used for the reductive carbonyl-to-
alkene cross-coupling chemistry.
Results and Discussion
We recently reported a modified protocol[9] for the phospha-
Peterson reaction[5,10] to synthesize P-Mes-phosphaalkenes.
Prior to our work, the starting material MesP(Li)TMS was made
in situ by treating MesP(TMS)2 with one equivalent of MeLi in
THF.[11] The so-formed MesP(Li)TMS was then reacted in THF
with ketones to afford the corresponding P-Mes-phosphaalk-
enes.[12] In our hands, this procedure was plagued by irrepro-
ducibility and the formation of various undesirable side prod-
ucts. We hypothesized that small amounts of unreacted MeLi
or other organolithium products thereof may be responsible for
this behavior, in particular, since it is known that such species
initiates the anionic polymerization of phosphaalkenes.[13] As
we could show in our adjusted protocol,[9] this undesired reac-
tivity can be overcome by using LiOEt[14] as desilylating agent
instead of MeLi. Thus, MesP(Li)TMS was prepared from
MesP(TMS)2 by treatment with one equivalent of LiOEt to afford
MesP(Li)TMS which reacts smoothly at room temperature in
Et2O with several aldehydes to afford Mes-phosphaalkenes as
the only products.[9]
Encouraged by these results, we were interested to see
whether a similar procedure would also be applicable for the
preparation of P-phenyl-phosphaalkenes. Thus, PhP(Li)TMS was
prepared by treating PhP(TMS)2 with one equivalent of LiOEt in
THF at room temperature. Removal of the solvent afforded a
yellow solid which was dissolved in water-free Et2O and used as
prepared for all subsequent reactions. The reactivity of ethereal
solution of PhP(Li)TMS towards benzophenone was tested.
In analogy to a report by Gates et al.,[6] three species were
expected when benzophenone is added to an ethereal solution
Unfortunately, the current approaches to P-phenyl-phos-
phaalkenes are somewhat unreliable, and their stability and re-
activity not systematically investigated. Herein, we report an
[a] Uppsala University, Department of Chemistry - Ångström Laboratory,
Box 523, 75120 Uppsala, Sweden
E-mail: Anna.Arkhypchuk@kemi.uu.se
Sascha.Ott@kemi.uu.se
http://www.kemi.uu.se/research/synthetic-molecular-chemistry/research-
groups/ott-group/
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201801322.
Eur. J. Inorg. Chem. 0000, 0–0
1
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
of PhP(Li)TMS (Scheme 1). The 31P NMR spectrum of the reac-
tion mixture (Figure 1a) shows these three compounds, i.e. the
desired phosphaalkene 1a (δ = +233 ppm), its head-to-head
dimer 2a (δ = +5 ppm) and the diphosphirane 3a (δ =
–118 ppm).
Scheme 2. Reaction between benzophenone and one equivalent of
PhP(Li)TMS gives rise to phosphaalkene 1a. The latter can be reacted further
with a second equivalent of PhP(Li)TMS to form 3a.
With this stoichiometry of the reagents, the formation of 3a
is immediate and quantitative, as illustrated by the 31P NMR
spectrum in Figure 1b. During the reaction, phosphaalkene 1a
is formed, but the second equivalent of PhP(Li)TMS attacks the
P=C double bond of 1a, resulting in the formation of 3a. Having
understood how 3a is formed, we set out to develop a proce-
dure that would suppress its formation, and allow high yields
of the desired phosphaalkene 1a. In this procedure, precaution
was taken to minimize the exposure of already formed 1a to
unreacted PhP(Li)TMS. This can be achieved easily by simply
reversing the order of addition of the two reagents. Thus, one
equivalent of PhP(Li)TMS was added quickly to the ethereal
benzophenone solution instead (Scheme 2). The reaction mix-
ture was monitored by 31P NMR spectroscopy which showed
the selective formation of 1a (Figure 1c) without any 3a being
detected. Finally, to confirm the mechanism proposed above
for the formation of 3a, a second equivalent of PhP(Li)TMS was
added to the reaction mixture from Figure 1c, resulting in the
clean conversion of 1a into 3a, as shown in Figure 1d. Unfortu-
nately, all attempts to isolate 3a were unsuccessful. So were
attempts to coordinate 3a to tungsten [W(CO)5CNMe] or
molybdenum [Mo(CO)5CNMe] fragments. Decomposition to
multiple phosphorus containing products was observed in all
of the cases.
Scheme 1. Reaction between equimolar amounts of PhP(Li)TMS and benzo-
phenone leading to the formation of 1a, 2a, and 3a. In this example an
ethereal solution of the ketone was added to a solution of PhP(Li)TMS in
water-free Et2O at room temperature.
Figure 1. 31P NMR spectra of the reactions depicted in Scheme 1 and
Scheme 2. a) Reaction monitoring for the conditions reported in Scheme 1
showing a mixture of 1a, 2a and 3a. b) Selective formation of 3a obtained
by reacting one equivalent of benzophenone and two equivalents of
PhP(Li)TMS. c) Selective synthesis of 1a, achieved by adding one equivalent
of a Et2O solution of PhP(Li)TMS to an ethereal solution of benzophenone.
d) Transformation of 1a to 3a by addition of a second equivalent of
PhP(Li)TMS to the mixture shown in Figure 1c.
The reliable and high-yielding synthetic procedure that was
developed for 1a was then used for the preparation of two
other triphenylphosphaalkenes that carry either an electron-
donating (1b) or -withdrawing (1c) substituent in the para-posi-
tions of their C-phenyl groups. The latter two compounds were
prepared by reacting ketone b (R = O-octyl) and c (R = F) with
PhP(Li)TMS as described above (Scheme 3). We were interested
to see how these substituents effect the synthesis of the phos-
phaalkenes, as well as their stability and reactivity.
While monitoring the equilibrium between 1a and 2a, it was
noticed that the concentration of 3a remained constant. Thus,
3a is formed as a side product during the formation of 1a but
is not in equilibrium with either 1a or 2a. Mechanistically, we
hypothesized that 3a may however be formed from 1a. As the
P=C double bond is polarized[15] with the P-center being par-
tially positively charged,[16] there could be the possibility of a
nucleophilic attack of PhP(Li)TMS on the phosphaalkene 1a. In
order to prove this hypothesis, a test reaction between two
The main differences between the three ketones may be in
equivalents of PhP(Li)TMS and one equivalent of benzophen- their reactivity towards PhP(Li)TMS and in the stabilities of the
one was performed, as depicted in Scheme 2. subsequently formed phosphaalkenes 1a–c. From a mechanis-
Scheme 3. Synthesis of 1a–c from ketones a–c and PhP(Li)TMS, followed by the dimerization equilibrium to form 2a–c.
Eur. J. Inorg. Chem. 0000, 0–0 www.eurjic.org © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2

Communication
tic viewpoint, one would expect that a ketone with electron
withdrawing groups (EWG) such as c would be a more reactive
electrophile than the electron rich analogue b, with unsubsti-
tuted benzophenone a being in between. Also, the electronic
nature of the P=C double bonds in 1a–c will vary, which could
have a noticeable impact on the stability of these compounds.
In order to get quantitative information from the 31P NMR spec-
troscopic monitoring of the reactions, an internal standard was
used. We decided to use a Mes*-phosphaalkene, more specifi-
cally [(E)-(4-methoxybenzylidene) (2,4,6-tri-tert-butylphenyl)-
phosphane] as internal standard since it has been shown to
have a high chemical stability[8] and a similar relaxation time as
phosphaalkenes 1a–c. A typical example of such a 31P NMR
reaction monitoring starting from ketone a is shown in Figure 2.
After their selective formation, phosphaalkenes 1a–c slowly
decrease in concentration due to their dimerization to the cor-
responding head-to-head dimers 2a–c. This reaction is not a
simple decomposition pathway, as evidenced by the fact that a
steady state concentration of both compounds is reached on
timescales of hours to days. 1 and 2 are thus in a chemical
equilibrium. For 1,2a, this equilibrium is reached after 48 hours
when a ratio between 1a:2a of 1:2.4 is established. The reac-
tions of b and c, as well as the dimerizations of 1b,c to 2b,c
were studied analogously, the results of which are summarized
in Table 1 (see Supporting Information for details). In the pres-
ence of electron rich substituents, the equilibrium is shifted to-
wards the phosphaalkene 1b. Even after four days, the ratio
between 1b:2b is 3.9:1. In the opposite case with the electron
poor ketone, the equilibrium is shifted toward the dimer 1c and
the ratio between 1c:2c is found to be 1:2.1.
The internal standard that was used in the reaction monitor-
ing in Figure 2 allowed us to reveal another interesting aspect
of the reactivity of 1. During the time that was needed for 1
to equilibrate with 2, it was noticed that the total amount of
phosphorus species that could be detected by 31P NMR spec-
Figure 2. 31P NMR investigation of the equilibrium between 1a and 2a (condi-
tions as in Scheme 3) with an internal standard (*) and after MeOH addition
with formation of 4a (conditions as in Scheme 4). a) First measurement of
the reaction after 14 min. The mmol of 1a (δ = +233 [ppm]) and 2a (δ =
+5 [ppm]) can be obtained from the ratio of the integrals of their signals and
the internal standard * (δ = +244 [ppm]) (see Supporting Information for
details). b) after 2 hours. c) after 48 hours. d) First measurement (several
minutes) after MeOH addition, showing formation of 4a (δ = +125 [ppm])
from 1a, with 2a being still present in the reaction mixture. e) The second
measurement after MeOH addition was performed after 30 hours, f) 7 days
after MeOH addition.
1
troscopy decreased markedly. H NMR spectroscopic monitor-
ing gives a similar result and shows that the total amount of
1a and 2a decreases on these timescales, while the total num-
ber of aromatic protons stays unchanged (see Supporting Infor-
mation). These observations can only be explained by the as-
sumption of a second process that consumes some of the phos-
phaalkene. As this process does not produce any defined phos-
phorus species that could be identified by 31P spectroscopy, we with 1 and 2. In the example of the triphenylphosphaalkene 1a
suggest this process to be the formation of higher oligomers/ in Figure 2, all chemical equilibria are established after 48 hours
polymers of 1 (Scheme 4). Interestingly, the formation of such (Figure 2c), and no further changes are observed. If this hypoth-
species does not deplete the solution of 1 and 2 completely, esis is correct, it should be possible to recover all phosphorus
as one would expect from an irreversible process. In fact, we species by an irreversible quenching experiment that would re-
propose that also the oligomers are in chemical equilibrium move 1 from all equilibria. Following Le Châtelier's principle,
Table 1. Investigation of equilibria between 1a–c, 2a–c and higher oligomers.
NMR yield
of 1a–c[a]
[%]
NMR yield
of 4a–c[a]
[%]
1a–c at
equilibrium
[%]
2a–c at
equilibrium
[%]
oligomers at
equilibrium
[%]
a: R = Ph
b: R = p-C6H4-O-octyl
c: R = p-C6H4-F
97
65
40
87
73
54
18
64
14
45
17
31
37
19
55
[a] Determined with Mes*-phosphaalkene as internal standard, see Supporting Information for details.
Eur. J. Inorg. Chem. 0000, 0–0 www.eurjic.org © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3

Communication
even the species that are not detectable by 31P NMR spectro- is thus a clear correlation in that the more electron-deficient
scopy could thereby be recovered and possible to quantify. We the phosphaalkene, the more it is engaged in oligomerization.
decided on the addition of methanol across the P=C double In contrast, the electron rich phosphaalkene 1b prevails largely
bond of the phosphaalkenes as a suitable trapping reaction. as the monomeric species.
Thus, methanol was added to the reaction mixtures that had
reached equilibrium (Scheme 4).
Conclusions
In summary, we were able to modify previously reported
phospha-Peterson reactions and adopt them for the synthesis
of P-phenyl-phosphaalkenes. The new protocol relies on the
clean formation of lithium phenyl(trimethylsilyl)phosphanide
PhP(Li)TMS which is obtained from PhP(TMS)2 by cleavage of
one TMS group through lithium ethanolate. PhP(Li)TMS that is
obtained in this way reacts smoothly with benzophenone (a) as
well as electron-rich (b) and –deficient (c) analogues to produce
phosphaalkenes 1a–c in 87, 73 and 54% yield, respectively. It is
shown that the phosphaalkene can be reacted further with a
second equivalent of PhP(Li)TMS under the formation of di-
phosphirane 3, a compound whose origin has been unclear
prior to this work. Phosphaalkenes 1a–c engage in various
chemical equilibria, most pronounced in a head-to-head dimeri-
zation to 2a–c that can be observed by 31P NMR spectroscopy.
Scheme 4. Proposed equilibria between 1, 2 and higher oligomers and their
reaction with MeOH.
In general, it emerges that the equilibrium lies more on the
monomer side for electron-rich phosphaalkene 1b. In addition
to the dimerization, quantitative quenching experiments show
that phosphaalkenes 1a–c engage in another chemical equilib-
rium reaction that we assign to a reversible oligomerization.
This process does not form a defined monodisperse species and
has previously escaped detection as it is not visible by 31P NMR
spectroscopy. Oligomerization is most pronounced for phos-
phaalkene 1c with 55% of all phosphorus containing species
being in the oligomer form, with less oligomers being detected
for 1a and 1b (37% and 19%, respectively). The mixtures of
phosphaalkenes, their dimers and oligomers showed a surpris-
ingly high stability over timescales of days with negligible levels
of irreversible decomposition. All species that are derived from
phosphaalkenes 1a–c can be channeled into one compound
by an irreversible quenching step, in this case the addition of
As expected, methanol reacts fast with the P=C double bond
in 1a–c to generate the corresponding phosphinites 4a–c,[17]
while dimers 2a–c are not directly affected (Figure 2d). On a
timescale of hours, however, also 2a–c are converted to phos-
phinites 4a–c through their equilibrium reaction with 1a–c (Fig-
ure 2e). The concentration of phosphinites 4a–c increases even
after all 2a–c is converted and reaches a final maximum con-
centration within a few days (Figure 2f).
Since the addition of MeOH to the P=C bond can be ex-
pected to be quantitative, the final concentration of 4a–c corre-
sponds to the total yield of the initial phospha-Peterson reac-
tion, which is otherwise difficult to determine as some product
1a–c may have already reacted further to oligomers that are
not visible by 31P NMR spectroscopy.
With this analysis, it turns out that the yield of phosphaalk- methanol across the P=C double bond of 1a–c to afford phos-
ene formation in case of 1a and 1b is remarkably high (87 and phinites 4a–c. This behavior presents a new possibility in the
73%, respectively). Somewhat lower yields were found for 1c field of low valent phosphorus chemistry, as it shows that P-
(54%) which we attribute to the generally high reactivity of phenyl phosphaalkenes with poor kinetic stabilization can nev-
electron deficient ketone such as c towards nucleophiles in ertheless be prepared in high yields and used as intermediates
general. Interestingly, the yields for 1 that were obtained by the in subsequent chemical transformations.
indirect method from quantification of 4 are very similar to
those obtained by 31P NMR analysis shortly after completion of
Acknowledgments
the phospha-Peterson reaction. Dimerization and oligomeriza-
tion are thus comparably slow processes relative to formation
of 1, which is an important finding if 1 is envisaged as an inter-
mediate in other chemical transformations.
Financial support for this work was provided by the Swedish
Research Council.
The combined knowledge of all analyses above allows the
quantification of all species that are in equilibrium, i.e. the phos-
phaalkenes 1, their dimers 2, as well as their oligomers that are
invisible by 31P NMR spectroscopy (Table 1). Phosphaalkene 1c
is most prone to oligomerization with 55% residing in the oligo-
meric state. This reactivity is less severe for 1a and 1b where
oligomerization occurs only in 37% and 19%, respectively. There
Keywords: Chemical equilibria · Phosphaalkenes ·
Reversibility · Cross-coupling · Synthesis design
[1] T. C. Klebach, R. Lourens, F. Bickelhaupt, J. Am. Chem. Soc. 1978, 100,
4886–4888.
[2] M. Yoshifuji, J. Organomet. Chem. 2000, 611, 210–216.
Eur. J. Inorg. Chem. 0000, 0–0
www.eurjic.org
4
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
[3] a) M. Yoshifuji, I. Shima, N. Inamoto, K. Hirotsu, T. Higuchi, J. Am. Chem.
Soc. 1981, 103, 4587–4589; b) R. Appel, C. Casser, M. Immenkeppel, F.
Knoch, Angew. Chem. Int. Ed. Engl. 1984, 23, 895–896; Angew. Chem.
1984, 96, 905.
[4] K. Esfandiarfard, A. I. Arkhypchuk, A. Orthaber, S. Ott, Dalton Trans. 2016,
45, 2201–2207.
[11]
[12]
S. C. Serin, F. S. Pick, G. R. Dake, D. P. Gates, Inorg. Chem. 2016, 55, 6670–
6678.
B. W. Rawe, C. M. Brown, M. R. MacKinnon, B. O. Patrick, G. J. Bodwell,
D. P. Gates, Organometallics 2017, 36, 2520–2526.
[13]
[14]
[15]
C.-W. Tsang, M. Yam, D. P. Gates, J. Am. Chem. Soc. 2003, 125, 1480–1481.
F. Uhlig, R. Hummeltenberg, J. Organomet. Chem. 1993, 452, C9–C10.
[5] M. Yam, J. H. Chong, C.-W. Tsang, B. O. Patrick, A. E. Lam, D. P. Gates,
Inorg. Chem. 2006, 45, 5225–5234.
[6] S. Wang, K. Samedov, S. C. Serin, D. P. Gates, Eur. J. Inorg. Chem. 2016,
2016, 4144–4151.
[7] K. Esfandiarfard, J. Mai, S. Ott, J. Am. Chem. Soc. 2017, 139, 2940–2943.
[8] J. Mai, A. I. Arkhypchuk, A. K. Gupta, S. Ott, Chem. Commun. 2018, 54,
7163–7166.
M. François, Angew. Chem. Int. Ed. 2003, 42, 1578–1604; Angew. Chem.
2003, 115, 1616.
[16]
[17]
W. W. Schoeller, J. Chem. Soc., Chem. Commun. 1985, 334–335.
T. A. Van der Knaap, T. C. Klebach, R. Lourens, M. Vos, F. Bickelhaupt, J.
Am. Chem. Soc. 1983, 105, 4026–4032.
[9] A. I. Arkhypchuk, N. D'Imperio, S. Ott, Org. Lett. 2018, 20, 5086–5089.
[10] H. Kawanami, K. Toyota, M. Yoshifuji, J. Organomet. Chem. 1997, 535, 1–
5.
Received: October 30, 2018
Eur. J. Inorg. Chem. 0000, 0–0
www.eurjic.org
5
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Phosphaorganic Chemistry
Triphenylphosphaalkenes were pre-
pared in good to excellent yields in a
modified phospha-Peterson reaction
between PhP(Li)TMS and different
benzophenones. Depending on the
electronics of the C-substituents, the
title compounds engage in reversible
dimerization and oligomerization reac-
tions, some of which are not detect-
able by 31P NMR monitoring but can
be quantified by quenching experi-
ments.
N. D'Imperio, A. I. Arkhypchuk,*
J. Mai, a. S. Ott* .................................. 1–6
Triphenylphosphaalkenes in Chemi-
cal Equilibria
DOI: 10.1002/ejic.201801322
Eur. J. Inorg. Chem. 0000, 0–0
www.eurjic.org
6
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim