Mechanism of the phospha-wittig-horner reaction

Article abstract of DOI:10.1002/anie.201301469

Doing the phosphate dance: The phospha-Wittig-Horner reaction proceeds through stepwise P-P cleavage of an oxadiphosphetane intermediate, followed by a [2,3]-sigmatropic rearrangement that paves the way for the final E2 elimination to form 1-phosphaallenes. The mechanism is thus greatly different to that of its carbon analogue, that is, the Horner-Wadsworth-Emmons reaction.

Full text of DOI:10.1002/anie.201301469

Angewandte
Chemie
DOI: 10.1002/anie.201301469
Organophosphorus Chemistry
Mechanism of the Phospha-Wittig–Horner Reaction**
Anna I. Arkhypchuk, Yurii V. Svyaschenko, Andreas Orthaber, and Sascha Ott*
Phosphaalkenes are prominent ligands in homogenous catal-
ysis due to their p-acceptor capacity[1] and are appealing
building blocks in polymer science.[1b,2] Methods for their
preparation are however often limited by small substrate
scopes, or residual OTMS substituents that stem from certain
synthetic methods.[3] The phospha-Wittig–Horner (pWH)
reaction, as first coined by Mathey and co-workers,[4] converts
aldehydes and relatively unreactive ketones into C-mono-
P-terminated cumulenes, which have however been syntheti-
cally highly challenging with only sporadic reports in the
literature.[7]
Despite of the synthetic versatility of the pWH reaction, it
has been a greatly underexplored method for the preparation
of phosphaalkenes. At the same time, the reaction mechanism
has hitherto been unknown. Formally, the pWH reaction is
the phosphorus analogue to the Horner–Wadsworth–
Emmons reaction (HWE), which allows preparation of
alkenes and allenes from carbonyl compounds and ketenes,
respectively.[8] The mechanism of the HWE reaction has been
studied in great detail over the last decades, with the crucial
and
C,C-disubstituted
phosphaalkenes,
respectively
(Scheme 1, top). Furthermore, the substituent at the phos-
À
À
step being a simultaneous C P and C O bond cleavage in an
intermediately formed oxaphosphetane to afford the desired
alkene and the phosphate byproduct (Scheme 1, bottom).[9] In
the spirit of the frequently drawn analogy between phospho-
rus and carbon,[10] it was assumed that the pWH reaction
proceeds along a similar pathway. In the present report, we
show for the first time that this assumption is wrong and that
the pWH reaction proceeds in a more complex, stepwise
fashion. The mechanism of the pWH reaction is elucidated
with the aid of spectroscopically and crystallographically
characterized reaction intermediates.
Scheme 1. The phospha-Wittig–Horner reaction (top), for which the
mechanism is unknown, and the carbon-analogue Horner–Wadsworth–
Emmons reaction (bottom).
The present study is based on two pWH reagents that are
stabilized by {W(CO)5} fragments but differ in the PIII
substituent (1, 2),[5a] and two substrate ketenes, one with
phenyl substituents (3), the other with a fused fluorenyl core
(4). As for all HWE and pWH reactions, the reactions are
initiated by the addition of base. Thus, stoichiometric
amounts of organic base (1,8-diazabicyclo[5.4.0]undec-7-ene,
DBU) were added to 1 or 2 followed by the addition of either
ketene 3 or 4. The products that were obtained after 30 min
and aqueous workup were however not the expected phos-
phaallenes, but previously unknown phosphinophosphates
5a–d (Scheme 2).
Complexes 5a–d are obtained as pale yellow solids in
good to excellent yields. They feature characteristic 31P NMR
chemical shifts of À9.8 ppm for the PV, and 127 ppm (R = Ph,
5a,c) or 148 ppm (R = tBu, 5b,d) for the PIII center. All of the
NMR studies as well as HRMS data of 5a–d are in agreement
with their proposed structures. Unambiguous evidence for the
structure of 5 was obtained by single-crystal X-ray diffraction
of compound 5c. As shown in Figure 1, the PIII exhibits
phorus center can be chosen rather freely, although smaller
groups demand additional stabilization by metal coordina-
tion.[5] In context of our previous work on the incorporation of
low-valent phosphorus into unsaturated carbon scaffolds,[6]
we were interested to see whether the scope of the pWH
reaction could be extended to ketene substrates from which 1-
phosphapropadienes would become accessible. Such phos-
phaallenes are the shortest members of an intriguing class of
[*] A. I. Arkhypchuk, Dr. Y. V. Svyaschenko, Dr. A. Orthaber, Dr. S. Ott
Department of Chemistry, ꢀngstrçm Laboratories
Uppsala University, Box 523, 751 20 Uppsala (Sweden)
E-mail: sascha.ott@kemi.uu.se
[**] The Swedish Research Council, the Gçran Gustafsson Foundation,
COST action 0802 PhoSciNet, and Uppsala University (U3MEC
molecular electronics priority initiative) is greatly acknowledged for
financial support. A.O. would like to thank the Austrian Science fund
(FWF) for an Erwin Schrçdinger fellowship (J 3193-N17), Y.V.S. the
Swedish Institute for a scholarship in the Visby Program (382/
00386/2012).
À
a tetrahedral coordination sphere with a P1 C1 single bond
À
(1.793(8) ꢀ) and a C1 C2 double bond (1.344(10) ꢀ).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301469.
Extending the times for the reaction between 1,2 and 3,4
from 30 min to a couple of days leads to the formation of the
desired phosphaallenes 6a–d (Scheme 2). Changing from an
organic to an organolithium base (lithium diisopropylamide
(LDA) or tBuOLi) accelerates the reaction significantly, and
phosphaallenes 6a–d are obtained within one hour without
detection of any reaction intermediates. Phosphaallenes 6a–d
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KGaA. This is an open access article under the terms of the Creative
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use, distribution and reproduction in any medium, provided the
original work is properly cited and is not used for commercial
purposes.
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Scheme 3. Isolated products from the solvolysis of phosphaallenes
6a–d.
sequence. Considering that the pWH reaction is relatively fast
for simple ketones, such as acetone, and advances without any
detectable reaction intermediates,[5a] it must be the presence
Scheme 2. Reaction between 1,2 and 3,4 using DBU as base. Reagents
and conditions: i) For ketene 3: 1 equiv DBU, THF, RT, 30 min; ketene
4 was generated in situ from the corresponding acid chloride: 2 equiv
of DBU, THF, RT. ii) 1 equiv DBU, THF, RT, 2–5 days. iii) 1) catalytic
amounts of DBU, 5 min, 2) H2O.
=
of the C C double bond in 5a–d that impedes the elimination
step.
Further examination of the pWH reaction between 1 and
2 and ketene 3 allowed the isolation and characterization of
another unprecedented reaction intermediate. When the
reaction is conducted with sub-stoichiometric amounts of
base, stable new products 7a,b can be detected under the
reaction conditions (Scheme 2), and isolated and character-
ized by NMR spectroscopy and HRMS. Compounds 7a,b are
addition products of 3 with 1 and 2, respectively, with
a phosphate and PIII group at the former internal ketene
carbon.
The crystal structure of 7a (Figure 1) shows a distorted
tetrahedral geometry around the phosphorus center P1 with
À
À
similar P1 C1 and C1 C2 bond distances as in 5c (1.828(6) ꢀ
and 1.325(9) ꢀ, respectively). Also, compound 7 can re-enter
the pWH reaction simply by the addition of base. Its
transformation to 5 occurs on a timescale of minutes, while
the final elimination to afford 6 proceeds on a longer
timescale, as described above.
The discovery of compounds 5 and 7 allows for a detailed
mechanistic proposal for the pWH reaction that is different to
that commonly believed for the HWE reaction (Scheme 1).
The reaction starts with the formation of the salts of pWH
Figure 1. Crystal structures of compound 5c (left) and 7a (right).
Ellipsoids set at 50% probability; all hydrogen atoms are omitted for
clarity, except for that at the vinyl moiety of 5c, and that of the PIII
center in 7a. Only one of the disordered ethyl groups in 7a is
displayed.
are characterized by typical 31P NMR chemical shifts of
57 ppm (6a), 98 ppm (6b), 60 ppm (6c), and 94 ppm (6d),[7c]
are however highly moisture sensitive which hampers their
isolation and purification. In the absence of extensive kinetic
stabilization by large substituents at phosphorus, such behav-
ior is not uncommon.[7a] Water (8) and methanol (9) addition
products of 6 could however be isolated and characterized, in
case of 8b and 8c even by single-crystal X-ray studies
(Scheme 3 and Supporting Information).
Isolated compound 5 was re-exposed to the basic reaction
conditions to confirm that 5, or a deprotonated form of 5, is
a genuine intermediate in the pWH reaction, and not just
a decomposition product. Indeed, exposure of 5a–d to DBU
results in its conversion into the corresponding phosphaal-
lenes 6a–d on similar timescales as those required in the
original one-pot reaction (Scheme 2). It thus seems that the
elimination of the phosphate group with the concomitant
formation of the phosphaallene is the rate-limiting step of the
reagents 1 and 2 which have partial P P double bond
=
character but also bear significant negative charge on the
PIII center.[5a,11] Attack of the PIII nucleophile at the ketene
leads to the formation of intermediate A (Scheme 4). In
analogy to the HWE reaction and the formation of oxaphos-
phetane intermediates, subsequent intramolecular nulceo-
philic ring closure occurs also on A, and the corresponding
oxadiphosphetane B is obtained. In contrast to what is
apparent for the HWE reactions, however, intermediate B
does not undergo a single-step phosphate elimination, but
À
exclusive cleavage of the P P single bond. The thus-formed
intermediate C contains the newly formed phosphate group
and bears negative charge on the PIII center. In case the
reaction is performed with sub-stoichiometric amounts of
base, intermediate C acts as a base and will deprotonate
residual 1 or 2. Supporting this notion, it was found in
a separate experiment that 1 is a stronger acid than 7, as the
former exchanges its proton in anhydrous [D4]MeOH, while
2
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mechanistic picture of the pWH reaction for the first time.
À
Exclusive P P bond cleavage of the oxadiphosphetane,
followed by a [2,3]-sigmatropic rearrangement, establishes
the phosphate leaving group at the PIII. Subsequent E2
elimination is rate-limiting and establishes the 1-phosphaal-
lenes. We believe that the increased mechanistic understand-
ing provided herein will revive the pWH reaction and
popularize its use for the preparation of phosphaalkenes in
catalysis and materials science.
Experimental Section
General procedure for compounds 5: A solution of 1 or 2 in THF
(1 mL) was added to a solution of 1 equiv of ketene 3 or 4 in THF
(5 mL) at 208C. 1 equiv of of DBU in THF (1m) was added dropwise,
and the resulting reaction mixture was stirred for 5–30 min, during
which time it was monitored by 31P NMR spectroscopy. Aqueous
work-up was followed by purification by silica gel column chroma-
tography using diethyl ether as eluent. 5c: 66% yield of isolated
product (see the Supporting Information for details). 31P NMR
(CDCl3): d = 127.2 (d, 1JPP = 49 Hz, 1JPW = 293 Hz, PIII), À9.9 ppm
(d, PV). 1H NMR (CDCl3): d = 8.1–7.93 (m, 2H, Ph), 7.83 (d, J = 7 Hz,
1H, Ar), 7.63 (d, J = 7 Hz, 1H, Ar), 7.59 (d, J = 7.6 Hz, 1H, Ar), 7.54–
Scheme 4. Proposed mechanism of the pWH reaction.
that of the latter remains unchanged. In the presence of
sufficient amount of base, intermediate C remains in its
anionic state und undergoes a [2,3]-sigmatropic rearrange-
ment that leads to the final intermediate D. E2 elimination of
the phosphate gives rise to the desired phosphaallenes 6a–d.
7.46 (m, 4H, Ph, HC =), 7.39 (t, 3JHH = 7 Hz, 1H, Ar), 7.35 (t, 3JHH
=
7 Hz, 1H, Ar), 7.28–7.20 (m, 1H, Ar), 6.91–9.83 (m, 2H, Ar), 4.15–
3.93 (m, 2H, OCH2CH3), 3.87–3.59 (m, 2H, OCH2CH3), 1.28 (t,
3JHH = 7 Hz, 3H, OCH2CH3), 1.05 ppm (t, 3JHH = 7 Hz, 3H,
OCH2CH3). 13C NMR (CDCl3): d = 198.4 (d, J = 28 Hz), 195.8 (d,
JCW = 126 Hz, JCP = 8 Hz), 146.7 (d, J = 6 Hz), 142.2 (s), 140.4 (d, J =
À
As indicated above, products of exclusive P C bond
1 Hz), 138.2 (d, J = 13.5 Hz), 134.7 (d, J = 3 Hz), 134.6 (d, J = 37 Hz),
132.9 (d, J = 2 Hz), 132.6 (d, J = 16 Hz), 130.1 (d, J = 25 Hz), 129 (d,
J = 11 Hz), 129 (d, J = 2 Hz), 127.7 (s), 126.5 (s), 123 (dd, J = 40 Hz,
J = 3 Hz), 121.7 (s), 120 (s), 119.9 (s), 64.4 (d, J = 6 Hz), 64.3 (d, J =
6 Hz), 16. (d, J = 7 Hz), 15.80 ppm (d, J = 7 Hz). HRMS (solution in
CHCl3 with addition of AgTFA): calcd for C29H24O9P2WAg,
[M+Ag]+ 870.94683, found 870.94751.
General procedure for compounds 7: A solution of 1 or 2 in THF
(1 mL) was added to a solution of 1 equiv of ketene 3 or 4 in THF
(5 mL) at 208C. One drop of a 1m solution of DBU in THF was added.
The reaction mixture was stirred for 5 min and then directly poured
onto a silica gel column (short plug), and eluted with diethyl ether.
Pure products were obtained by chromatography on silica gel with
cleavage of the oxaphosphetane intermediate in the HWE
reaction have never been observed, and intermediates of type
5 and 7 are encountered only in the pWH reaction. Compar-
À
ing the two reactions, it can be noted that the P P bond in B is
À
weaker than the corresponding C P bond in oxaphosphe-
tanes and can therefore be expected to be cleaved more easily.
At the same time, the sp -C O bond in B is stronger than the
sp -C O bond in the oxaphosphetane in Scheme 1. With B
thus possessing a relatively weak P P bond and a stronger C
2
À
3
À
À
À
O bond, it is not surprising any longer that selective cleavage
À
of the P P bond is observed. The negative charge in the
resulting intermediate C is delocalized and thus best de-
scribed as an allyl anion. The [2,3]-sigmatropic rearrangement
to form intermediate D is the most puzzling finding in the
reaction mechanism. Related [3,3] rearrangements have
however precedence in the isomerization of allylic phos-
phates.[12] The driving force for the formation of D is the
CH2Cl2 as eluent. 7a: 100 mg (0.176 mmol) of
1 and 36 mg
(0.186 mmol) of ketene 3 was used for reaction. Yield: 130 mg;
1
97%. 31P NMR (CD2Cl2): d = À6.2 (d, PV), À26.3 ppm (d, JPW
=
232 Hz, PIII). 1H NMR (dcm-d2): d = 7.94–7.66 (m, 2H, Ph), 7.54–
7.40 (m, 3H, Ph), 7.39–7.21 (m, 10H, Ph), 6.64 (d, 1JHP = 365 Hz, 1H,
PH), 3.78–3.36 (m, 4H, OCH2CH3), 1.09 (td, 3JHH = 7 Hz, JHP = 1 Hz,
3H, OCH2CH3), 0.98 ppm (td, 3JHH = 7 Hz,
J
HP = 1 Hz, 3H,
OCH2CH3). 13C NMR (dcm-d2): d = 198.8 (d, J = 23 Hz), 196.2 (d,
CW = 126 Hz, JCP = 7 Hz), 142.6 (dd, J = 14 Hz, J = 6 Hz), 141.3 (dd,
À
creation of a new P O bond and the formation of a relatively
J
stable vinyl carbanion. Furthermore, intermediate D is
kinetically stabilized as the final E2 elimination to form 6
requires an s-trans arrangement of the vinyl lone pair and the
phosphate leaving group. Such a conformation demands the
J = 57 Hz, J = 9 Hz), 138.8 (dd, J = 2 Hz, J = 2 Hz), 138.2 (dd, J =
5 Hz, J = 2 Hz), 134.8 (d, J = 13 Hz), 131.2 (d, J = 2 Hz), 129.4 (d,
J = 1 Hz), 129 (s), 128.8 (d, J = 10.6 Hz), 128.5 (s), 128.2 (s), 128.1 (s),
127.7 (d, J = 42 Hz), 64.2 (d, J = 6 Hz), 63.9 (d, J = 6 Hz), 15.7 (d, J =
7 Hz), 15.6 ppm (d, J = 7.3 Hz). HRMS (solution in CHCl3/MeCN
with addition of CF3COOAg): calcd for C29H26O9P2WAg, [M+Ag]+
872.96248, found 872.96292.
=
co-planarity of the entire POPC C portion of the molecule
which inevitably leads to severe steric clashes between the
phosphate and the Rꢁ groups. The steric argument is also
reflected in the long reaction times that are observed for the
transformation of D to 6a–d.
In summary, we have investigated the reactivity of pWH
reagents 1 and 2 towards ketenes and were able to establish
a reliable route to 1-phosphaallenes. The use of ketene
substrates allowed the identification of unique reaction
intermediates 7a,b as well as 5a–d, which offer a detailed
Received: February 19, 2013
Revised: April 3, 2013
Published online: && &&, &&&&
Keywords: ketenes · phosphaallenes · phospha-Wittig–
.
Horner reaction · reaction mechanisms
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4
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Organophosphorus Chemistry
A. I. Arkhypchuk, Y. V. Svyaschenko,
A. Orthaber, S. Ott*
&&&& — &&&&
Mechanism of the Phospha-Wittig–
Horner Reaction
Doing the phosphate dance: The phos-
paves the way for the final E2 elimination
pha-Wittig–Horner reaction proceeds
to form 1-phosphaallenes. The mecha-
nism is thus greatly different to that of its
carbon analogue, that is, the Horner–
Wadsworth–Emmons reaction.
À
through stepwise P P cleavage of an
oxadiphosphetane intermediate, followed
by a [2,3]-sigmatropic rearrangement that
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5
These are not the final page numbers!