Scope and Limitations of the s-Block Metal-Mediated Pudovik Reaction

Article abstract of DOI:10.1002/chem.201905565

The hydrophosphorylation of phenylacetylene with di(aryl)phosphane oxides Ar₂P(O)H (Pudovik reaction) yields E/Z-isomer mixtures of phenylethenyl-di(aryl)phosphane oxides (1). Alkali and alkaline-earth metal di(aryl)phosphinites have been studi

Full text of DOI:10.1002/chem.201905565

A Journal of
Accepted Article
Title: Scope and Limitation of the s-Block Metal-Mediated Pudovik
Reaction
Authors: Matthias Westerhausen, Benjamin E. Fener, Philipp Schüler,
Nico Ueberschaar, Peter Bellstedt, Helmar Görls, and Sven
Krieck
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Scope and Limitation of the s-Block Metal-Mediated Pudovik
Reaction
Benjamin E. Fener,[a] Philipp Schüler,[a] Nico Ueberschaar,[b] Peter Bellstedt,[c] Helmar Görls,[a] Sven
Krieck,[a] and Matthias Westerhausen*[a]
Abstract: The hydrophosphorylation of phenylacetylene with
di(aryl)phosphane oxides Ar2P(O)H (Pudovik reaction) yields E/Z-
isomer mixtures of phenylethenyl-di(aryl)phosphane oxides (1).
Alkali and alkaline-earth metal di(aryl)phosphinites have been
studied as catalysts for this reaction with increasing activity for the
heavier s-block metals. The Pudovik reaction can only be mediated
for di(aryl)phosphane oxides whereas P-bound alkyl and alcoholate
substituents impede the P-H addition across alkynes. The
demanding mesityl group favors the singly hydrophosphorylated
products 1-Ar whereas smaller aryl substituents lead to the doubly
hydrophosphorylated products 2-Ar. Polar solvents are beneficial for
an effective addition. Increasing concentration of the reactants and
the catalyst accelerates the Pudovik reaction. However, Mes2P(O)H
does not form the bis-phosphorylated product 2-Mes but activation
of an ortho-methyl group and cyclization occurs yielding 2-benzyl-1-
mesityl-5,7-dimethyl-2,3-dihydrophosphindole 1-oxide (3).
The addition of P-H bonds of phosphane oxides across
unsaturated hydrocarbons (Pudovik reaction) like alkynes
yielding alkenylated and alkylated phosphane oxides (Scheme
1) represents
a versatile procedure for the formation of
phosphorus-carbon bonds. Depending on the precatalyst,
radical and polar reaction mechanisms have been discussed.
Thus, diverse catalyst systems have been investigated such as
e.g. Al2O3/KOH,[2] palladium(0),[3] ytterbium(II),[4] rhodium(I),[5,6]
nickel,[7] potassium,[8,9] and bases in general.[10] In the presence
of air the product range deviates and radical mechanisms
account for the observed products and by-products.[11,12]
Scheme 1. Singly and doubly hydrophosphorylated alkynes, yielding
alkenylphosphane oxides 1-R and 1,2-bis(phosphorylated) alkanes 2-R.
1. Introduction
s-Block metal-catalyzed processes are experiencing a vastly
growing interest due to their beneficial properties including low
toxicity (with highly toxic beryllium being an exception), global
abundance and availability. Furthermore, tuning of reactivity
succeeds by the choice of the metals (hardness, size, Lewis
acidity), coligands (denticity, Lewis basicity, donor strength) and
solvents (polarity, denticity). Therefore, research with respect to
hydrofunctionalization[1] of alkenes and alkynes focusses
especially on the essential metals sodium, potassium,
magnesium, and calcium. However, the redox-inert s-block
metals show a different catalytic mechanism than redox-active
transition metal-based catalysts which also allow oxidative
addition and reductive elimination steps contrary to the s-block
metals.
Potassium-mediated addition of dimesitylphosphane oxide (R =
Mes) across alkynes yielded varying mixtures of Z- and E-
isomeric alkenyl-dimesitylphosphane oxides 1-R with the isomer
ratio depending on the solvent.[9] Furthermore, this study
revealed that these alkenes that formed during the Pudovik
reaction were inert under the applied reaction conditions.
Suitable substituents R’ included aryl, ester and bulky trialkylsilyl
groups.
Here we elucidated the dependency of the Pudovik reaction on
nature and size of the s-block metal, the P-bound substituent R
and the used solvent. Furthermore, we studied a unique
cyclization of the alkenylphosphane oxide to 5,7-dimethyl-2,3-
dihydrophosphindole 1-oxide if dimesitylphosphane oxide was
applied in the Pudovik reaction. As precatalysts we chose the
bis(trimethylsilyl)amides (hexamethyldisilazanides, hmds) of the
alkali and alkaline-earth metals because these complexes are
easily accessible on large scale and highly soluble in common
organic reagents guaranteeing homogeneous reaction
conditions. In addition, the s-block metals sodium, potassium,
magnesium and calcium are essential non-toxic metals which
are globally abundant and very inexpensive.
[a]
B. E. Fener, P. Schüler, Dr. S. Krieck, Prof. Dr. M. Westerhausen
Institute of Inorganic and Analytical Chemistry
Friedrich-Schiller-University Jena
Humboldtstr. 8, D-07743 Jena, Germany
E-mail: m.we@uni-jena.de
[b]
[c]
Dr. Nico Ueberschaar
Mass Spectrometry Platform
Friedrich Schiller University Jena, Germany
Humboldtstr. 8, D-07743 Jena
Dr. Peter Bellstedt
NMR platform
Friedrich Schiller University Jena, Germany
Humboldtstr. 8, D-07743 Jena
2. Results and Discussion
2.1 Influence of the metal on the Pudovik reaction
Supporting information for this article is given via a link at the end of
the document.
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Due to the fact that alkali metal diarylphosphinites (M-O-PR2)
degrade and/or dismutate to the soluble phosphanides M-PR2
and to the sparingly soluble phosphinates MO2PR2,[13,14] the
activity of stock solutions of M-O-PR2 slowly decreases. Hill and
coworkers[15] already observed under reductive conditions the
formation of diphenylphosphanide in solutions containing
calcium diphenylphosphinite species. Nevertheless, [(thf)4Ca(O-
PPh2)2] is stable in the crystalline state and hence, the single
crystal structure could be determined.[15] Degradation and
disproportionation again yield phosphanide and phosphinate
species the latter being very sparingly soluble due to the
formation of strand-like polymeric structures with bridging
phosphinate ligands.[16]
Therefore freshly prepared s-block metal diarylphosphinites
have to be used for reliable studies and the following unified
procedure was applied: The s-block metal phosphinites M-O-
PR2 were prepared in situ via metalation of HP(O)R2 with the
hexamethyldisilazanides (hmds) of the alkali and alkaline-earth
metals yielding the corresponding metal diarylphosphinites of
the type M-OPAr2. Quantitative conversion was verified by 31P
NMR spectroscopy. At room temperature, 5 mol-% of the amide
were combined with dimesitylphosphane oxide in THF and then
a slight excess amount of phenylacetylene was added. After one
hour, an aliquot was protolyzed with ethanol and the product
composition was investigated by 31P NMR spectroscopy.
below in chap. 2.6). No other products were formed under these
reaction conditions. Steric pressure is much more pronounced in
the Z-isomer of 1-Mes than in E-1-Mes. Therefore, the second
hydrophosphorylation step occurred preferentially at Z-1-Mes to
significantly lower steric strain via formation of less strained 2-
Mes decreasing the concentration of Z-1-Mes. It can be
excluded in either case that the isomeric ratio changed after
complete conversion regardless of the fact if catalyst is present
or not.
Table 1. Dependency of the Pudovik reaction (addition of
dimesitylphosphane oxide across phenylacetylene) on the s-block metal
of the applied bis(trimethylsilyl)amide precatalyst [Scheme 2, THF,
c(HPOMes2) ~ 0,05 mol/l].
Entry
Metal
After 1 h
Conv./%
After 27 h
Conv./%
E/Z
E/Z
1
2
3
4
5
6
7
8
9
Li
Na
K
0
31
93
>99
>99
0
-
3/1
5/1
7/1
11/1
-
8
>99
> 99
>99
>99
0
5/1
31/1[a]
5/1
7/1
13/1
-
Rb
Cs
Mg
Ca
Sr
0
-
0
-
3
3/1
6/1
0
-
Ba
85
95
22/1
[a] The amount of Z-1-Mes is lowered due to a preferred second Na-
mediated hydrophosphorylation yielding 2-Mes (see text).
Scheme 2. s-Block metal-mediated hydrophosphorylation of phenylacetylene
with dimesitylphosphane oxide, using M(hmds) and M(hmds)2 of the alkali and
alkaline-earth metals, respectively, as precatalysts; c(HPOMes2) ~ 0.05 mol/l.
2.2 Substituent effects on the Pudovik reaction
The outcome of this study is summarized in Table 1. The
precatalysts with the small metals such as Li(hmds) (entry 1),
Mg(hmds)2 and Ca(hmds)2 (entries 6 and 7) could not mediate
this addition reaction whereas fast conversion was observed for
the complexes of the heavier metals (entries 3-5 and 9). The
formation of doubly hydrophosphorylated products is
disadvantageous due to intramolecular steric strain between the
phenyl group and the P-bound mesityl substituents. The E/Z-
isomer ratio that was observed after one hour remains
unchanged and very comparable values are determined after 27
For the following experiments, K(hmds) was applied as
precatalyst in the Pudovik reaction as depicted in Scheme 3. In
a typical experiment 5 mol-% of K(hmds) were combined with
di(organyl)phosphane oxide in THF and then phenylacetylene
was added to this solution at room temperature to elucidate the
influence of the substitution pattern on the formation of singly
and doubly hydrophosphorylated phenylacetylene. The
conversion (decline of the R2P(O)H concentration) and the
amount of singly (1-R) and doubly hydrophosphorylation
products (2-R) were determined by 31P NMR spectroscopy.
hours.
A
distinctive
exception
pertains
to
the
hydrophosphorylation products via catalysis by Na(hmds) (entry
2). After approx. one day the resonance of the Z-isomer
vanished nearly completely, leading to an E/Z-ratio of 31/1.
Contrary
to
the
other
metal
catalysts,
doubly
hydrophosphorylated 2-Mes was observed (7.5% of 2-Mes and
92.5% of E/Z-1-Mes) and unequivocally identified by the
characteristic 31P NMR parameters of two doublets at δ(31P) =
3
37 and 44 ppm with a J(P,P) coupling constant of 47.7 Hz (see
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Scheme 3. Potassium-mediated hydrophosphorylation of phenylacetylene
with di(organyl)phosphane oxide, using K(hmds) as precatalyst; c(HPOMes2)
~ 0.05 mol/l.
10
OPh
0
-
-
0
-
-
The outcome of this study is summarized in Table 2. The
conversion was determined via the decline of starting
phosphane oxide (conversion). The product ratio of 1-R and 2-R
was estimated by integration of the 31P NMR spectra. As
discussed above, dimesitylphosphane oxide only added once
2.3 Proposed mechanism for the Pudovik reaction
The proposed mechanism of the Pudovik reaction is depicted in
Scheme 4. The s-block metal di(aryl)phosphinite adds across
the C≡C functionality. Protolysis of this intermediate with
Ar2P(O)H regains the catalyst MOPAr2 and yields E/Z-mixtures
of mono-phosphorylated 1-Ar. The newly formed alkene moiety
is reactive enough to add another phosphinite leading to bis-
phosphorylated compounds. Now protolysis yields very sparingly
soluble bis-phosphorylated 2-Ar which precipitates from the
reaction mixture and hence, is removed impeding a possible
equilibrium interconverting 2-Ar back into 1-Ar. Due to
intramolecular shielding and steric strain only the mono-
hydrophosphorylation product was isolated as an isomeric
mixture of 1-Mes.
yielding
phenylethenyl-dimesitylphosphane
oxide
(styryl-
di(mesityl)phosphane oxide, 1-Mes (entry 1) with an E/Z-ratio of
5/1. Removal of one ortho-methyl substituent and use of ortho-
tolyl substituents decelerates the conversion and after 1 h also
doubly hydrophosphorylated phenylacetylene (2-oTol) was
observed (entry 2). After 22 h, the mono-hydrophosphorylation
product 1-oTol was completely converted to the doubly
phosphorylated ethane derivative 2-oTol. Similar steric
hindrance can be assumed if one mesityl group is replaced by a
phenyl substituent (entry 3). Removal of all methyl groups and
application of diphenylphosphane oxide accelerates the second
hydrophosphorylation step and only product 2-Ph was detected
despite the fact that the conversion was incomplete after one
hour (entry 4). The benzyl-substituted phosphane oxide (entry 5)
proved to be more reactive than the phenyl congener. Bis(4-
dimethylaminophenyl)phosphane oxide (entry 6) added
significantly faster across the alkyne unit and only the doubly
hydrophosphorylated product 2-R was found. Alkyl- (entries 7
and 8), alkoxide (entry 9) and phenolato groups (entry 10)
suppressed the potassium-mediated hydrophosphorylation of
phenylacetylene under these reaction conditions.
Table 2. Dependency of the Pudovik reaction on the substituents of the
applied secondary phosphane oxides R2P(O)H [Scheme 3, THF, c(HPOR2) ~
0,05 mol/l].
Entry
R
After 1 h
After 22 h
Conv.
(%)
A (E/Z)
B
Conv.
(%)
A (E/Z)
B
1
2
3
Mes
oTol
93
55
76
100
(83/17)
0
>99
>99
>99
100
(83/17)
0
66
(100/0)
34
64
< 1
>99
84
Scheme 4. Simplified proposed mechanism for the formation of the bis-
phosphorylated products 2-Ar. The equilibrium between 1-Ar and the bis-
phosphorylated compounds was deduced from NMR spectroscopic
observation of intermediate 2-Mes during formation of 2-benzyl-1-mesityl-5,7-
dimethyl-2,3-dihydrophosphindole 1-oxide 3 (see below).
Mes/Ph
36
16
(100/0)
(100/0)
4
5
6
Ph
Bz
14
64
< 1
< 1
< 1
>99
>99
>99
36
< 1
< 1
< 1
>99
>99
>99
>99
>99
C6H4-4-
NMe2
>99
2.4 Solvent effects on the Pudovik reaction
7
8
9
Cy
tBu
OEt
0
0
0
-
-
-
-
-
-
0
0
0
-
-
-
-
-
-
The initial study of the potassium-mediated hydro-
phosphorylation of phenylacetylene with dimesitylphosphane
oxide showed a strong dependency of the E/Z-ratio on the
nature of the solvent.[9] Therefore we performed this catalytic
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addition reaction (Scheme 2) in various solvents, including
aliphatic and aromatic hydrocarbons, ethers, amines, and
acetonitrile. In some of these solvents (pentane, 1,4-dioxane,
NEt3, methyl tert-butyl ether, and diethyl ether), the solubility of
the hydrophosphorylation product is rather poor, leading to
suspensions toward the end of the conversions. Nevertheless, in
all studied solvents a complete conversion was observed within
one day.
The dependency of the potassium-mediated hydro-
phosphorylation of phenylacetylene with di(mesityl)phosphane
oxide on the polarity of the solvent is summarized in Table 3.
Several scales have been investigated to characterize polarity of
solvents with larger values for more polar molecules.[17] The ET
values were determined with a pyridinium-N-phenolate-derived
dye with the color depending on the polarity of the medium. The
dielectric constant εr and the dipole moment µ of the solvent
molecules are given as well in Table 3 and correspond
satisfactory to the ET values.[17]
strongest Brønsted acid and hence, interference with the
catalytic cycle cannot reliably be excluded.
2.5 Concentration effects on the Pudovik reaction
In another experiment, we investigated the influence of the
concentration on the potassium-mediated Pudovik reaction
according to Scheme 2. Thus, the molar ratio remained
unchanged (THF solution, 5 mol-% of catalyst, equimolar ratio of
di(mesityl)phosphane oxide and phenylacetylene), but the
concentration of di(mesityl)phosphane oxide varied from 0.035
M to 0.06 M and finally to 0.175 M. The E/Z ratio of the mono-
phosphorylation product styryl-di(mesityl)phosphane oxide 1-
Mes was determined after 30 minutes via protolysis of the
reaction mixture (in order to inactivate the catalyst) and the E/Z
ratio of styryl-di(mesityl)phosphane oxide 1-Mes was elucidated
by 31P NMR spectroscopy. The E/Z ratio decreased from 4.2/1
over 3.3/1 to 1.6/1 for the 0.035 M, 0.06 M and 0.175 M
solutions, respectively, with increasing amounts of the Z-isomer
with increasing concentration. In addition, only the diluted
solution still contained small amounts of starting Mes2P(O)H
whereas all di(mesityl)phosphane oxide had already been
consumed after 30 minutes in more concentrated solutions. The
E/Z ratio of product 1-Mes did not change for the diluted
solutions if prolonged reaction times were applied. However, the
concentrated reaction mixture behaved differently. After one
hour, the E/Z ratio changed to 1.8/1 in favor of the E-isomer and
after 3 days, only the E-isomer was observed. However, two
new resonances with δ(31P) values of approx. 53 and 59 ppm
with an intensity ratio of 14/1 were observed. These resonances
were only observed in Lewis basic solvents like ethers and
amines whereas in non-donor solvents these resonances were
absent. We showed earlier that E- and Z-isomers interconvert
into each other rather slowly suggesting that the Z-1-Mes
preferably reacted to these new products.
After isolation of this new unexpected compound we determined
the molar weight with 1H-DOSY NMR experiments (Stalke
method).[18] As an internal standard adamantane with a molar
weight of 136.24 g mol-1 was added to a [D8]toluene solution as
depicted in Figure S10 (see Supporting Information).
Comparison of the diffusion coefficients as described earlier[18]
gave a molar weight of 407 g mol-1 supporting a structure of a
similar mass than 1-Mes.
To elucidate the exact product composition and the reaction
mechanism HR-MS and X-ray diffraction studies as well as NMR
experiments were performed. The highly resolved mass of the
M+ species verified a formula of C26H29OP which is identical to
the mass of the mono-phosphorylated product (verifying a
rearrangement product of 1-Mes). Spectroscopic data and X-ray
diffraction studies verified the formation of 2-benzyl-1-mesityl-
5,7-dimethyl-2,3-dihydrophosphindole 1-oxide (3) as shown in
Scheme 5; the product formed formally via addition of a C-H
bond of an ortho-methyl group of the mesityl substituent onto the
newly formed alkene moiety in 1-Mes.
Table 3. Dependency of the Pudovik reaction on the solvent [Scheme 2,
reaction time of 1 h, c(HPOR2) ~ 0.05 mol/l].
[a]
[b]
Entry
Solvent
ET
31
εr
µ [c]
Conv. (%)
E/Z
1
Pentane
Dioxane
Benzene
Toluene
NEt3
1.84
2.27
2.4
0
70
63
70/30
88/12
67/33
74/26
67/33
94/6
2
36
0.45
0
3
34.3
32.1
32.1
34.7
36.5
34.5
67
4
2.43
2.45
0.31
0.07
52
5
82
6
MeOtBu
2-MeTHF
Et2O
> 99
> 99
91
7
3.05
4.42
5.03
5.68
7.2
75/25
93/7
8
1.15
9
Me4THF
THP
> 99
> 99
> 99
> 99
> 99
82/18
93/7
10
11
12
13
36.2
35.8
37.4
45.6
1.63
1.71
1.75
3.53
DME
60/40
83/17
24/76
THF
7.47
36.0
MeCN
Increasing polarity and donor strength of the solvent increases
the rate with quantitative conversions already after one hour for
ethereal solvents (entries 6-12 with diethyl ether, entry 8, being
an exception) and acetonitrile (entry 13). The E/Z ratio varies
strongly generally in favor of the E-isomer. It is noteworthy that
in acetonitrile the Z-isomer is the major product. In this table
acetonitrile is not only the most polar molecule but also the
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Figure 1. Molecular structure and atom labeling scheme of 2-benzyl-1-mesityl-
5,7-dimethyl-2,3-dihydrophosphindole 1-oxide (3). The ellipsoids represent a
probability of 30 %, H atoms are neglected for clarity reasons. Selected bond
lengths (pm): P1-O1 148.92(11), P1-C1 184.05(14), P1-C4 179.53(15), P1-
C18 182.65(14), C1-C2 154.7(2), C2-C3 151.2(2), C3-C4 140.17(19), C1-C11
154.1(2), C11-C12 151.3(2); bond angles (deg.): O1-P1-C1 114.66(6), O1-P1-
C4 113.25(7), O1-P1-C18 110.42(6), C1-P1-C4 94.44(7), C1-P1-C18
108.98(6), C4-P1-C18 110.42(6).
Scheme
5.
Equilibrium
between
E-
and
Z-isomeric
styryl-
di(mesityl)phosphane oxide and subsequent cyclization yielding 2-benzyl-1-
mesityl-5,7-dimethyl-2,3-dihydrophosphindole 1-oxide (3).
For the investigation of the fragmentation pattern ESI-MS
experiments were performed. The fragmentation experiments
revealed that the compound decayed predominantly into
fragments with masses of 119, 269, and 283 as depicted in
Scheme S1 (see Supporting Information). Stable isotope
labeling and assignment of the observed masses was achieved
with either deuterium oxide (D2O) as cosolvent during the MS
study or with the use of D-C≡C-Ph during the Pudovik reaction.
Interestingly both independently executed experiments showed
the same fragments. The most plausible explanation therefore is
that the protons are exchangeable under conditions used for
ESI-MS investigations. This finding can be explained by a
reversible 1,3-hydrogen shift from C1 to O1 of 2-benzyl-5,7-
dimethyl-1-mesityl-2,3-dihydro-1H-phosphindole 1-oxide (3, see
Figure 1) leading to a P1=C1 double bond and a P-bound
hydroxyl functionality.
Selected NMR parameters of the racemate of the major (R,S)-
and (S,R)-isomers of 2-benzyl-1-mesityl-5,7-dimethyl-2,3-
dihydrophosphindole 1-oxide (3) are summarized in Table 4, the
numbering scheme is identical to the atom labeling of the
molecular structure (Figure 2). The chemical shift of the
phosphorus atom δ(31P) = 53 ppm lies in the region of 1-phenyl-
2-phospholene 1-oxide (CHCl3, 58.5 ppm; benzene, 57.2
ppm).[23] The resonance of the minor racemic (R,R)- and (S,S)-
components shows a chemical shift of δ(31P) = 59 ppm.
The X-ray diffraction studies at a single crystal verified the
formation
of
2-benzyl-1-mesityl-5,7-dimethyl-2,3-dihydro-
phosphindole 1-oxide (3) as depicted in Scheme 5. Molecular
structure and atom labeling scheme are depicted in Figure 1.
This molecule contains the two stereogenic centers P1 and C1
but due to the centrosymmetric monoclinic space group C2/c the
crystalline state consists of a racemate of (R,S)- and (S,R)-
isomeric molecules. The P1=O1 bond length of 148.92(11) pm is
very similar to values of simple unstrained diarylphosphane
oxides (Ph2P(O)H: 148.81(11)[19] Mes2P(O)H: 148.54(13) pm[20]
)
and of alkenyl-di(mesityl)phosphane oxides like E-1-Mes
(148.61(11) pm).[9] These short P=O bonds are commonly
explained by hyperconjugation from the negatively charged
oxygen atom into antibonding σ*(P-C) bonds leading to slight
lengthening of the P-C bonds.[21] Deprotonation and formation of
potassium diarylphosphinites of the type [(L)KOPAr2]n with three-
coordinate phosphorus atoms leads to an elongated P-O
bond.[14,22] Contrary to the very similar P=O distances in various
phosphane oxides the P-Caryl bond lengths to the aryl
substituents depend on the steric pressure induced by bulky
groups. Thus, the P-Caryl distances increase in the row
Ph2P(O)H (180.0 pm),[19] Mes2P(O)H (181.6 pm)[20] and E-1-Mes
(182.7 pm).[9] The latter value is comparable to the P1-C18
distance in 3 (182.65(14) pm) verifying a similar intramolecular
steric pressure. The endocyclic P1-C4 bond length of 179.53(15)
in compound 3 is significantly smaller than the P-Caryl distances
but similar to the P-C bond to the styryl group in E-1-Mes
(179.68(15) pm) [9] suggesting a slight delocalization within the
P1-C4-C3 fragment.
Table 4. NMR data (chemical shifts δ [ppm] and coupling constants J [Hz])
of 2-benzyl-1-mesityl-5,7-dimethyl-2,3-dihydrophosphindole 1-oxide.
Fragment[a]
δ(1H)
J(P,H)[b]
δ(13C{1H})
41.2
J(P,C)[c]
1J = 69.3
2J = 6.8
2J = 28.7
1J = 102.3
2J = 8.7
3J = 9.5
4J = 2.3
3J = 11.3
3J = 4.2
-
1
2
2.81
n.d.
3.00
3J = 12.6
34.4
3
-
-
146.1
131.8
140.9
130.0
143.1
124.9
19.6
4
-
-
-
5
-
6
6.90
-
4J = 3.1
7
-
8
6.90
2.37
2.34
3.42/2.81
4J = 3.1
9
-
10
11
-
21.7
3J = 6.9
34.2
2J = 1.7
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intermediate formation of 2-Mes). Thereafter, the intensity of the
resonances of KOPMes2 and 2-Mes decreased and the
resonances at δ(31P) = 53 and 59 ppm, indicative of the
12
13,17
14,16
15
-
-
-
-
-
-
-
-
-
-
-
140.3
128.9
128.5
128.9
126.6
142.6
131.2
141.3
23.1
3J = 12.9
7.23
-
formation
of
2-benzyl-1-mesityl-5,7-dimethyl-2,3-
7.23
-
dihydrophosphindole 1-oxide (3), appeared. After 125 min, an
E/Z-isomeric ratio of 2.8:1 was observed for 1-Mes. While the
intensity of the singlets of the isomers of 3 increased, the
resonances of Z-1-Mes vanished (E/Z-ratio of 4.5:1 after 10
hours) and finally, only the singlets of 3 at δ(31P) = 53 and 59
ppm and of E-1-Mes at 23 ppm were visible. This time
dependency is depicted in Figure 2.
7.23
-
18
-
1J = 91.0
19,23
20,22
21
-
6.87
-
3J = 11.0
-
4J = 2.8
24,26
25
1.58/2.81[d]
2.28
-
-
21.0
[a] The numbering of the CH fragments is identical with the atom labeling
scheme of the molecular structure as depicted in Figure 1. [b] Coupling
constants between hydrogen atoms and P1. [c] Coupling constants between
carbon atoms and P1. [d] Chemical shifts of these methyl groups were
determined at 243 K due to coalescence at r.t. caused by hindered rotation
of the mesityl group around the P1-C18 bond.
Temperature-dependent NMR experiments allow the elucidation
of the barriers of hindered rotation for an exchange between two
equally occupied positions using the Gutowsky-Holm
equation.[24] At room temperature, the resonances of the ortho-
methyl (C24 and C26) and of the meta-CH groups (C20 and
C22) of 3 are broad due to hindered rotation around the P1-C18
bond. At low temperature a splitting of these signals is observed
whereas at high temperature a rotation of the mesityl group that
is fast on the NMR time scale leads to single resonances (Figure
S11, Supporting Information). Determination of the coalescence
temperature (Tc = 253 K for the ortho-methyl groups between 22
and 24 ppm in the 13C{1H} NMR spectrum) allows the elucidation
of a rotational barrier of 46 kJ mol-1.
2.6 Mechanism of phosphindole 1-oxide formation
In order to gain mechanistic insight we performed this catalytic
addition of Mes2P(O)H across phenylacetylene in THF. Thus we
added 5 mol-% of K(hmds) to di(mesityl)phosphane oxide
producing a mixture of KOPMes2 and Mes2P(O)H with a molar
ratio of 0.05:0.95. To this solution we added an equimolar
amount of Ph-C≡CH, leading to a final mixture of 0.05:0.95:1 for
the molar ratio of KOPMes2, Mes2P(O)H and Ph-C≡CH. Initially,
the formation of 1-Mes was observed as described above and
after 15 min, 1-Mes (E/Z-ratio of 0.9:1) and Mes2P(O)H coexist
in solution (Figure 4). After approx. 50 min also the resonances
Figure 2. 31P NMR spectroscopic monitoring at 161.98 MHz of the potassium-
mediated hydrophosphorylation of phenylacetylene with di(mesityl)phosphane
oxide in [D8]THF at room temperature.
These findings shed light on the mechanism of the potassium-
mediated Pudovik reaction and the subsequent cyclization
process
yielding
2-benzyl-1-mesityl-5,7-dimethyl-2,3-
dihydrophosphindole 1-oxide (3) if di(mesityl)phosphane oxide
has been applied. The reaction of Mes2P(O)H with
phenylacetylene or [D]phenylacetylene yields the E- and Z-
isomers of 1-Mes as depicted in Scheme 4. Despite the fact that
the acidity of di(aryl)phosphane oxide (pKa of Ph2P(O)H: 20.7)
and phenylacetylene (pKa: 21-23.2 depending on solvent) are
quite similar the deuterium is bound at the carbon atom of the
styryl group adjacent to the P-atom. Subsequent protonation
of bis-phosphorylated 2-Mes appeared with
a rather low
intensity which can unequivocally be recognized at the two
doublets at δ(31P) = 37 and 44 ppm with a 3J(P,P) coupling
constant of 47.7 Hz. After one hour di(mesityl)phosphane oxide
was completely consumed and a very small singlet of KOPMes2
could be detected at δ(31P) = 95 ppm. The E/Z-ratio of 1-Mes
changed in favor of the E-isomer to 2.1:1 (probably via an
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occurs with Mes2P(O)H and the amount of doubly deuterated
congeners due to protolysis with D-C≡C-Ph is negligible.
A second (reversible) addition of Mes2P(O)H onto the newly
precatalyst in the addition of dimesitylphosphane oxide onto
phenylacetylene, we applied the bis(trimethylsilyl)amides (hmds)
of the alkali and alkaline-earth metals. The lighter s-block metal
congeners Li(hmds) as well as the M(hmds)2 derivatives of
magnesium, calcium and strontium show no activity. The most
reactive species are Rb(hmds) and Cs(hmds), but the reactivity
of K(hmds) and Ba(hmds)2 is only slightly lower. The E/Z-ratio of
the resulting diaryl-alkenylphosphane oxides also depends on
the radius and softness of the metal ions. In summary, the alkali
metal hmds complexes are more reactive than the alkaline-earth
metal congeners with increasing catalytic reactivity with
increasing size of the metal ions.
These s-block metal hmds precatalysts are able to promote and
accelerate the addition of aryl-substituted phosphane oxides
across phenylacetylene whereas alkyl-, alkoxy- and aryloxy-
substituted phosphane oxides cannot by added onto alkynes
under these reaction conditions. Phosphane oxides Ar2P(O)H
with smaller aryl groups show a two-step addition yielding firstly
the mono-addition products, diaryl-styrylphosphane oxides 1-R,
and then the bis-phosphorylated derivatives, 1-phenyl-1,2-
bis(diarylphosphoryl)ethane 2-R. Thereafter, we used the bulky
dimesitylphosphane oxide to decelerate the addition reaction for
time-dependent studies and to avoid the second addition step,
ensuring quantitative formation of the mono-phosphorylated
congener, dimesityl-styrylphosphane oxide 1-Mes.
formed
alkenyl
moiety,
yielding
2-Mes,
is
highly
disadvantageous and only very low concentrations have been
observed. The steric strain in Z-1-Mes is more pronounced than
in the E-isomer. Therefore, cyclization occurs much faster for Z-
isomeric 1-Mes. Due to the fact that both Mes2P(O)H and H-
C≡C-Ph are consumed yielding quantitatively 1-Mes, the
catalyst KN(SiMe3)2 is reformed. This base can deprotonate an
ortho-methyl group of the mesityl substituent in 1-Mes. This
carbanion undergoes an intramolecular nucleophilic attack at the
alkenyl substituent leading to cyclization. Protonation with
HN(SiMe3)2 finally leads to the formation of 2-benzyl-1-mesityl-
5,7-dimethyl-2,3-dihydrophosphindole 1-oxide (3) regaining the
catalytic species KN(SiMe3)2.
The nature of the solvent plays a minor role in the s-block metal-
mediated Pudovik reaction. In ethereal solvents the reaction is
faster than in aliphatic and aromatic hydrocarbons with diethyl
ether and triethylamine showing a mediocre support of this
reaction. Ethereal solvents stabilize the intermediate metal
phosphinites much more effectively via coordination to the metal
ions and deaggregation of the catalytic species whereas in
hydrocarbons larger aggregates can easily be realized as
previously shown for crystalline complexes.
Without changing the molar ratios of dimesitylphosphane oxide,
phenylacetylene and precatalyst, the amount of solvent has
been reduced. In increasingly concentrated solutions the E/Z-
ratio of targeted dimesityl-styrylphosphane oxide decreases.
Furthermore, a second unique reaction sequence has been
observed. After complete consumption of the starting substrates,
the reformed precatalyst K(hmds) now slowly deprotonates an
ortho-methyl group of the P-bound mesityl substituent.
Thereafter, cyclization occurs yielding 2-benzyl-1-mesityl-5,7-
dimethyl-2,3-dihydrophosphindole 1-oxide (3). Due to two chiral
atoms four isomers (two enantiomeric pairs) are observed.
The mechanism of the catalytic cycle starts with the metal
diarylphosphinite, Ar2P-OM, formed via metalation of
diarylphosphane oxide with M(hmds). This phosphinite adds
onto the alkyne functionality of Ar'-C≡C-H yielding diaryl-
alkenylphosphane oxides 1-Ar after protonation with Ar2P(O)H.
Labeling experiments with Ph-C≡C-D verify that the phosphane
oxide is the proton source in this reaction step. A second
Scheme 7. Proposed mechanism for the potassium-mediated cyclization
process of 1-Mes yielding quantitatively 2-benzyl-1-mesityl-5,7-dimethyl-2,3-
dihydrophosphindole 1-oxide (3). Formation of very minor amounts of 2-Mes
has intermediately been observed.
Conclusions
The addition of phosphane oxides across alkynes (Pudovik
reaction) requires a catalyst to overcome the disadvantageous
entropic influence and the approach of two electron-rich
molecules. Catalysis with s-block metal reagents is one
established procedure to activate the phosphane oxide partner
by metalation and formation of the more reactive phosphinite. As
addition
occurs
yielding
the
insoluble
1-phenyl-1,2-
bis(diarylphosphoryl)ethane 2-Ar which precipitate from the
reaction mixture. If bulky dimesitylphosphane oxide is applied,
the second addition step remains irrelevant. Instead of that the
ortho-methyl group of the mesityl substituent is deprotonated by
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10.1002/chem.201905565
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Hz), 140.3 (d, J = 12.9 Hz), 131.8 (d, J = 102.3 Hz), 130.0 (d, J = 9.5 Hz),
128.9 , 128.5 , 126.6 (d, J = 91.0 Hz), 124.9 (d, J = 11.3 Hz), 77.4 (d, J =
11.6 Hz), 77.2 , 76.8 , 41.2 (d, J = 69.3 Hz), 34.4 (d, J = 6.8 Hz), 34.2 (d,
J = 1.7 Hz), 23.1 , 21.7 , 21.0 , 19.6 (d, J = 4.2 Hz); 31P-NMR (161.99
MHz, CDCl3, 297 K) δ 53.6 (s); IR (ATR): (cm-1) = 3024 (w), 2917 (w),
2843 (w), 1604 (m), 1405 (m), 1188 (s), 1179 (vs), 1088 (m), 877 (s), 756
(s), 662 (vs), 637 (s); MS: m/z (%) = 777 [2M]+ (10), 388 [M]+ (50), 373
[M-CH3]+ (60), 297 [M-Bn]+ (100), 270 [M-Mes]+ (20), 119 [Mes]+ (8), 91
[Bn]+ (10); HRMS (ESI) ([M+H]+, C26H30OP): calcd: 389.2034; found:
389.2029; Elemental analysis: C26H29OP (388 g/mol) calc. C 80.38 % H
7.52 %; found C 80.11 % H 7.51 %
M(hmds), followed by the above mentioned cyclization to the
phosphindole oxide derivative 3.
In summary, the Pudovik reaction can effectively be promoted
by the heavy alkali metal bis(trimethylsilyl)amides M(hmds) with
M = K, Rb, and Cs, leading to mono- and bis-phosphorylated
compounds. Bulky P-bound mesityl groups hinder the second
addition reaction. Mono-phosphorylated 1-Mes is cyclized after
deprotonation of an ortho-methyl group of the P-bound mesityl
substituent by catalytic amounts of K(hmds) finally giving
phosphindole oxide 3.
Mass spectrometric experiments. High resolution mass spectrometry was
carried out using a THERMO (Bremen, Germany) QExactive plus Orbitrap
mass spectrometer coupled to a heated electrospray source (HESI).
Flow was set to 15ꢀµL/min using a syringe pump. For monitoring a full
scan mode was selected with the following parameters. Polarity: positive;
scan range: 100 to 1500 m/z; resolution: 280,000; AGC target: 3ꢀ×ꢀ106;
maximum IT: 512ꢀms. General settings: sheath gas flow rate: 15; auxiliary
gas flow rate 5; sweep gas flow rate: 0; spray voltage: 3.0ꢀkV; capillary
temperature: 250ꢀ°C; S-lens RF level: 90; vaporizer temperature:
auxiliary gas heater temperature: 105ꢀ°C. For MS2 experiments
normalized collision energy was set to 60 and the mass selection window
was set to ± 0.2 m/z of the protonated molecule to prevent isotope
fragmentation using a mass range from 100 to 500 m/z. Compound 3
was picked with a 200ꢀµL Eppendorf tip and dissolved in 0.5ꢀmL of HPLC-
MS grade acetonitrile and diluted with HPLC-MS grade water 1:1, (v/v).
Experimental Section
General remarks. All manipulations were carried out under an inert
nitrogen atmosphere using standard Schlenk techniques, if not otherwise
noted. The solvents were dried over KOH and subsequently distilled over
sodium/benzophenone under
a nitrogen atmosphere prior to use.
Deuterated solvents were dried over sodium, distilled, degassed, and
stored under nitrogen over sodium. 1H, 31P and 13C{1H} NMR spectra
were recorded on Bruker Avance III 400 and Fourier 300 spectrometers.
Chemical shifts are reported in parts per million relatively to SiMe4 (1H,
13C) or H3PO4 31P) as an external standard referenced to the solvents
(
residual proton signal. ASAP-HSQC-DEPT,[25] 1H{13P, 1H}[26] und
1H{1H}[26] were recorded using the published pulse sequences. All
substrates were purchased from Alfa Aesar, abcr, Sigma Aldrich or TCI
and used without further purification. [Na(hmds)],[27] [Rb(hmds)],[28]
[Cs(hmds)],[28] [Ca(hmds)2(thf)2],[29] and secondary phosphane oxides (o-
tolyl)2POH,[30] (C6H4-4-NMe2)2POH,[31] (PhCH2)2POH,[32] Cy2POH,[33]
tBu2POH[33] were prepared according to literature protocols. [Li(hmds)]3
and [Mg(hmds)2] were prepared by reaction of H(hmds) with nBuLi or
Mg(nBu)2 respectively. [Sr(hmds)2] and [Ba(hmds)2] were prepared
starting from the metal by reaction with liquid ammonia and metalation of
H(hmds) afterwards. The yields given are not optimized. Purity of the
compounds was verified by NMR spectroscopy.
X-Ray structure determination of 3. The intensity data were collected on
a Nonius KappaCCD diffractometer, using graphite-monochromated Mo-
K radiation. Data were corrected for Lorentz and polarization effects;
absorption was taken into account on a semi-empirical basis using
multiple-scans.[34-36] The structure was solved by direct methods
(SHELXS)[37] and refined by full-matrix least squares techniques against Fo2
(SHELXL-97).[37] The hydrogen atoms of compound 3 were included at
calculated positions with fixed thermal parameters. All non-hydrogen atoms
were refined anisotropically.[37] XP[38] and POV-Ray[39] program packages
were used for structure representations. Crystal Data and refinement details
for 3: C26H29OP, M = 388.46 g mol-1, colorless prism, size 0.124 x 0.112 x
0.088 mm3, monoclinic, space group C2/c, a = 22.4180(5), b = 8.5492(2), c
= 22.5499(5) Å,  = 99.657(1)°, V = 4260.58(17) Å3 , T = -140 °C, Z = 8,
calcd. = 1.211 g cm-3, µ(Mo-K) = 1.43 cm-1, multi-scan, transmin: 0.7190,
transmax: 0.7456, F(000) = 1664, 15088 reflections in h(-28/28), k(-11/11),
General protocol for catalytic studies. Secondary phosphane oxide (200
mg, 1.0 eq) was placed in a Schlenk flask and dissolved in THF (5 mL). A
solution of K(hmds) in THF (c = 0.18 mol/L, 0.19 mL, 0.05 eq) was added
via syringe and stirred for five minutes at room temperature.
Phenylacetylene (0.08 mL, 0.7 mmol, 1.1 eq.) was injected into the
reaction vessel in one portion. The reaction mixture was stirred for one
hour. The conversion of secondary phosphine oxide was tracked via 31P-
NMR spectroscopy of a hydrolyzed aliquot.
l(-28/29), measured in the range 1.84°    27.48°, completeness 
max
= 99.3%, 4854 independent reflections, Rint = 0.0334, 4326 reflections
with Fo > 4(Fo), 258 parameters, 0 restraints, R1obs = 0.0421, wR2obs
=
0.0994, R1all
= 0.0482, wR2all = 0.1037, GOOF = 1.053, largest
difference peak and hole: 0.399 / -0.335 e Å-3.
Synthesis of 2-benzyl-1-mesityl-5,7-dimethyl-2,3-dihydrophosphindole 1-
oxide (3). Dimesitylphosphane oxide (3.0 g, 10.5 mmol, 1.0 eq) was
placed in a Schlenk-flask and dissolved in THF (30 mL). A solution of
[K(hmds)] in THF (c = 0.27 mol/L, 3.9 mL, 1.1 mmol, 0.1 eq) was added.
To this yellow solution was added phenylacetylene (1.2 g, 11.6 mmol, 1.1
eq). After a reaction time of two hours the mixture was quenched with
ethanol. The solvents were evaporated under reduced pressure and the
residue was purified by flash column chromatography (SiO2, PE:EtOAc
1:1). The phospholane oxide 3 (3.5 g, 9.0 mmol, 86 %) was isolated as
light yellow pure product. Suitible crystals for the single crystal x-ray
diffraction were obtained by recrystallisation in n-heptane/acetonitrile
(2:0.1) at room temperature. m.p.: 102 - 104 °C; Rf (SiO2, PE:EtOAc 1:1):
0.4; 1H-NMR (400.13 MHz, CDCl3, 297 K) δ 7.33 – 7.15 (m, 5H), 6.90 (d,
4JPH = 3.2 Hz, 2H), 6.89 – 6.84 (m, 2H), 3.46 – 3.35 (m, 1H), 3.02 – 2.92
(m, 2H), 2.88 – 2.72 (m, 2H), 2.54 – 2.07 (m, 6H), 2.37 (s, 3H), 2.34 (s,
3H), 2.28 (s, 3H); 13C{1H}-NMR (100.61 MHz, CDCl3, 297 K) δ 146.1 (d, J
= 28.7 Hz), 143.1 (d, J = 2.3 Hz), 141.3 (d, J = 2.8 Hz), 140.9 (d, J = 8.7
Acknowledgements
We acknowledge the valuable support of the NMR
(www.nmr.uni-jena.de/) and mass spectrometry service
platforms (www.ms.uni-jena.de/) of the Faculty of Chemistry and
Earth Sciences of the Friedrich Schiller University Jena,
Germany. The mass spectrometer used for high resolution mass
spectrometric investigations was financed by the German
Research Foundation (DFG, Bonn, Germany) in the frame of the
Collaborative Research Center SFB 1127 (ChemBioSys). P.S. is
very grateful to the German Environment Foundation (Deutsche
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metals • catalysis • phosphane oxide addition
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10.1002/chem.201905565
Chemistry - A European Journal
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The Pudovik reaction is an atom-
economic addition of phosphane
oxides across alkynes, strongly
depending on the nature of the s-block
metal, the solvent, the bulkiness of P-
bound groups, and the concentration.
Bulky mesityl groups can interact with
the newly formed C=C bond of the
alkenylphosphane oxides.
B. E. Fener, P. Schüler, N. Ueberschaar,
P. Bellstedt, H. Görls] S. Krieck, M.
Westerhausen*
Page No. – Page No.
Scope and Limitation of the s-Block
Metal-Mediated Pudovik Reaction
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