Reduction of λ5-Phosphinines

Article abstract of DOI:10.1002/ejic.201500856

A convenient method to easily prepare parent λ³-phosphinine from easily accessible λ⁵-precursors was developed. A series of λ⁵-phosphinines bearing heteroatom substituents OMe, SMe, and/or NMe₂ at the phosphorus

Full text of DOI:10.1002/ejic.201500856

DOI: 10.1002/ejic.201500856
Communication
CLUSTER
ISSUE
Synthesis of Phosphinines
Reduction of λ5-Phosphinines
Aleksandr Savateev,[a] Yurii Vlasenko,[a] Nataliya Shtil,[a] and Aleksandr Kostyuk*[a]
Abstract: A convenient method to easily prepare parent λ3- methoxy-λ5-phosphinine was determined by an X-ray diffrac-
phosphinine from easily accessible λ5-precursors was devel- tion analysis. A series of reducing agents were tested in order
oped. A series of λ5-phosphinines bearing heteroatom substitu- to prepare λ3-phosphinine. 1,1-Dimethoxy-λ5-phosphinine was
ents OMe, SMe, and/or NMe2 at the phosphorus atom were reduced by LiAlH4. The method of choice appeared to be the
prepared by electrocyclization of phosphahexatrienes gener- reduction of bis(dimethylamino)-λ5-phosphinine with diisobut-
ated in situ. Reaction conditions for the synthesis of λ5-phos- ylaluminium hydride (DIBAL-H) in 30 % overall yield starting
phinines were optimized. The molecular structure of 1,1-di- from vinyl ethyl ether.
Introduction
Analogous electrocyclization of hexaphosphatrienes featur-
ing a trivalent phosphorus atom was also attempted. Mathey
et al. have demonstrated that flash vacuum pyrolysis (FVP) of
tris(allyl)phosphane or vinyl(diallyl)phosphane gave phosphin-
ines probably via hexaphosphatrienes.[11] An attempt to pre-
pare phosphahexatrienes featuring trivalent phosphorus or
their metal complexes did not produce the targeted λ3-phos-
phinines.[12]
In the literature, synthesis of λ3-phosphinines was described
by reduction of λ5-phosphinines featuring dimethoxy groups at
the phosphorus atom.[9] Thus, reduction of λ5-phosphinines to
λ3-phosphinines might be a solution, provided they are readily
available by an alternative route. In this work we describe the
synthesis of λ5-phosphinines featuring OMe, SMe, and/or NMe2
groups at the phosphorus atom and their reduction into parent
λ3-phosphinine.
Even after half a century after the publication by Märkl[1] on
the synthesis of the first stable λ3-phosphinine, many of these
compounds remain accessible only with difficulty. This is espe-
cially true for the parent λ3-phosphinine prepared by Ashe[2]
and phosphinines featuring basic functional groups. Methods
for the synthesis and properties of phosphinines are well-docu-
mented.[3] Two main methods for functionalized phosphinines
have been developed so far. The first one is based on the reac-
tion of pyrylium salts with phosphine or its equivalents.[4] The
method is well-developed, but provides access to 2,4,6-substi-
tuted derivatives. The second method is the reaction of 1,3,2-
diazaphosphinines with disubstituted alkynes through a cyclo-
addition/cycloreversion sequence.[5] Both methods have limited
opportunities for varying substitution patterns. Other syntheses
of functionalized phosphinines are considered as milestones,
for example, those of 2-hydroxyphosphinine[6] and aldehyde
phosphinine.[7] Among the latest approaches to λ3-phos-
phinines, an iron-catalyzed [2+2+2] cycloaddition reaction of
diynes with phosphaalkynes should be mentioned.[8]
Results and Discussion
1.1. Synthesis of λ5-Phosphinines
λ3-Phosphinines can be prepared by reduction of the corre-
sponding λ5-phosphinines.[9] Nevertheless, the approach is not Amide 2 was prepared by slow addition of chlorophosphine 1
considered as a practical one, as λ5-phosphinines are mainly into a solution of allylmagnesium chloride in THF with vigorous
synthesized from λ3-phosphinines.
stirring (Scheme 1). Phosphinite 3 was obtained upon treat-
ment of amide 2 with methanol at room temperature.
Previously we have shown that λ5-phosphinines can easily
be prepared by starting from common building blocks by elec-
trocyclization of variously substituted hexaphosphatrienes.[10]
The procedure allows synthesis of both parent and highly sub-
stituted phosphinines. All λ5-phosphinines synthesized have
aryl, alkyl, dialkyl amino substituents or a mixture of those sub-
stituents at the phosphorus atom.
[a] Organophosphorus Department, Institute of Organic
Chemistry, <orgName>National Academy of Sciences of Ukraine,
Murmanska 5, 02660, Kyiv-94 Ukraine
Scheme 1. Synthesis of amide 2 and phosphinite 3.
All compounds bearing 2-ethoxyvinyl groups, for example 1,
2, 3, 4a–c, exist predominantly as trans-isomers, but cis-isomers
(ca. 2 %) are also present as evidenced by 1H and 31P NMR
spectra. Signals of all cis-isomers in 31P NMR spectra are 3.6–
E-mail: a.kostyuk@yahoo.com
www.ioch.kiev.ua
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejic.201500856.
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9.1 ppm upfield with respect to trans-isomers. Compounds 2
and 3 were oxidized with a hydrogen peroxide urea complex,
elemental sulfur or selenium giving air-stable distillable com-
pounds 4a–d (Scheme 2).
Table 1. Reaction conditions for the deprotonation of phosphonium salts 5a–
c,e and yields of the corresponding phosphinines 6a–c,e.
Entry
Salt
Base[a]
T [°C]
Isolated yield [%]
1
2
3
4
5
6
7
8
5a
5a
5a
5a
5a
5b
5c
5e
5e
5e
5e
5e
LiHMDS
LiHMDS
LiHMDS (1 equiv.)
nBuLi
–75
0
62
traces[b]
< 27[b]
26
Alkylation of compounds 4a–d with methyltriflate was ac-
complished at –70 °C in dichloromethane. Salts 5a–d are air-
–75
–75
–70
–70
–80
–75
–70
–75
–80
–78
1
sensitive viscous oils. In their H NMR spectra, a distinctive pat-
tBuOK
traces[b]
78
76
tern of allyl methylene protons with JP, H coupling constant val-
ues ranging from 18.3 Hz for 5b to 61.0 Hz for 5c and 100.8 Hz
for 5d was observed. In their 31P NMR spectra, the signals of
phosphonium salts ranged from δ = 77.1 ppm for 5a, δ =
68.2 ppm for 5b, δ = 70.5 ppm for 5c, and δ = 64.4 ppm with
LiHMDS
LiHMDS
LiHMDS
tBuOK
tBuOK
nBuLi
67
9
82
10
11
12
48[c]
26
1
a coupling constant of JPSe = 467 Hz for 5d.
nBuLi
45[10a]
In a previous work, electrocyclization of phosphahexatrienes
to prepare λ5-phosphinines in situ by deprotonation of phos-
phonium salts with n-butyllithium was established quite well
by some of us.[10a]
[a] The reaction was carried out in THF, and the ratio of base to phosphonium
salt was 2.1:1 unless otherwise specified. [b] Judging by 31P NMR spectro-
scopy. [c] CH2Cl2 was used as a solvent.
In order to optimize the reaction conditions and maximize
the yield of λ5-phosphinines (Scheme 3), we tested different
bases and varied some other reaction conditions (Table 1).
It was found that LiHMDS is a universal base: it gave symmet-
ric and unsymmetric phosphinines in moderate to good yields
(entries 1, 6, 7, 8), while tBuOK worked well only with phos-
phonium salt 5e, which has bis(dimethylamino) groups at the
phosphorus atom (entry 9). The reaction requires at least
2 equiv. of the base and should be carried out at low tempera-
ture. When only 1 equiv. of LiHMDS was used, the yield dropped
markedly (entry 3). Also we found that in the phosphine synthe-
sis the more concentrated the solution, the lower were the
yields (entry 11). This is probably due to polymerization of
phosphahexatrienes.
Upon treatment with LiHMDS, phosphonium salt 5d showed
totally different chemical properties from those of salts 5a–c,e
(Scheme 4).
Scheme 4. Deprotonation of phosphonium salt 5d.
Under these conditions, amide 2 was separated as a major
(78 %) product (Table 2, entry 1). Such an observation can be
rationalized if LiHMDS is considered as a nucleophile attacking
All λ5-phosphinines 6a–c,e are air-stable compounds, which the SeMe group of salt 5d (Figure S2 in the Supporting Informa-
slowly darken upon storage even under argon. In 31P NMR spec- tion). In addition, a compound with tentative structure 7 was
tra, signals of phosphinines are upfield with respect to the cor- separated. An opposite tendency was observed when tBuOK
responding phosphonium salts.
was employed as a base (Table 2, entry 2).
Scheme 2. Synthesis of phosphonium salts 5a–d.
Scheme 3. Synthesis of phosphinines 6a–c,e.
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Scheme 5. Interconversions between phosphinines.
Table 2. Deprotonation of phosphonium salt 5d; composition (%) of the reac-
tion mixture.
comparison with the average value of 1.80 Å for Car–PV.[13] In
6a they are both 1.705(2) Å. The C–C bond lengths in the cen-
tral cycle vary in the narrow range of 1.384(3) to 1.391(3) Å.
Dimroth showed that under acidic conditions phosphinines
could easily exchange substituents at the phosphorus atom or
hydrolyze to the corresponding phosphinic acid derivatives.[14]
We found that, on being dissolved in concentrated hydrochloric
acid, phosphinine 6e was readily transformed into phosphinic
acid 8 (Scheme 5). Under the same conditions, phosphinine 6b
gave phosphinic ester 9.
Compound 9 was also obtained from 6e by treatment with
trifluoromethanesulfonic acid in methanol. Alkylation of ester 9
with MeOTf probably gave phosphonium salt 10, which, upon
deprotonation with Et3N, yielded phosphinine 6a. This provides
an alternative route to phosphinine 6a.
Entry
Base
T [°C]
2
7
1
2
3
LiHMDS
tBuOK
tBuOK
–80
–80
–20
78
37
59
22
63
41
For compound 6a we were able to grow crystals suitable for
single-crystal X-ray diffraction analysis. Recently we reported
the X-ray structure of the phosphinine bearing morpholyl
groups at the phosphorus atom.[10a] The molecular structure of
compound 6a has been determined by a single-crystal X-ray
diffraction analysis. The perspective view of molecule 6a and
selected geometrical parameters are given in Figure 1. The cen-
tral six-membered cycle is planar within 0.004 Å. It is worth
to note that both P1–C1 and P1–C1′ bonds are shortened in
1.2 Reduction of λ5-Phosphinines to λ3-Phosphinine
In the literature there are few examples for the reduction of
dimethoxy-λ5-phosphinines to λ3-phosphinines with various re-
ducing agents.[9] We decided to explore a few commonly avail-
able reducing agents for the reduction of λ5-phosphinines 6a,e
(Scheme 6).
Scheme 6. Reduction of λ5-phosphinines 6a,e.
Lithiumaluminium tetrahydride is a convenient reagent for
the reduction of dialkyl alkylphosphonates to RP(O)H2 com-
pounds.[15] We successfully applied LiAlH4 in boiling ether for
the reduction of phosphinine 6a (Table 3, entry 1). After work-
up, a parent phosphinine, compound 11, was separated and
characterized by its 1H, 13C, 31P NMR spectra,[16] which were
identical to those reported previously.
Figure 1. The perspective view of 6a. Selected bond lengths [Å] and angles
[°]: P1–C1′ 1.705(2), P1–O1 1.591(2), P1–O2 1.597(2), P1–C1 1.705(2), C1–C2
1.391(3), C2–C3 1.384(3); C1′–P1–O1 114.61(8), C1′–P1–O2 115.35(8), O1–P1–
O2 91.4(1), C1′–P1–C1 105.6(2), O1–P1–C1 114.61(8), O2–P1–C1 115.35(8).
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Communication
λ3-Phosphinine (11)
Table 3. Reduction of λ5-phosphinines.
Method A: A solution of dimethoxyphosphinine 6a (0.59 g,
3.7 mmol) in diethyl ether (8 mL) was added dropwise to ice-cooled
LiAlH4 (0.15 g, 3.9 mmol). The resulting suspension was stirred at
+35 °C for 68 h. The precipitate was separated by filtration (a filter
with fine pores was used) and washed with diethyl ether. The ether
was distilled off at atmospheric pressure, and the residue was dis-
tilled in vacuo (0.02 Torr) into a receiver cooled with liquid nitrogen.
The product was obtained as a colorless liquid. Yield: 54 mg, 15 %.
Entry
Substrate Reducing agent
T
[°C]
Solvent Yield
[%]
1
2
4
5
6
6a
6a
6e
6e
6f
LiAlH4 (1.06 equiv.)
35
Et2O
THF
15
LiAlH4/Me3SiCl (3.6 equiv.) 35
72[a]
DIBAL-H (2.1 equiv.)[b]
DIBAL-H (2.9 equiv.)
DIBAL-H (2.9 equiv.)
r.t.[b]
r.t.
r.t.
toluene 84[a]
–
–
86
0
[a] Conversion according to 31P NMR of the reaction mixture. [b] DIBAL-H:
diisobutylaluminium hydride; r.t.: room temperature.
Method B: To a stirred solution of N,N,N′,N′-tetramethyldiamino-
phosphinine 6e (0.3 g, 1.6 mmol) in toluene (1.5 mL) cooled in an
ice bath was added dropwise a toluene solution of DIBAL-H (2.1 mL,
3.4 mmol, 1.65 mol L–1). The ice bath was removed, and the result-
ing solution was maintained at +16 °C for 24 h. A toluene solution
of phosphabenzene was distilled in vacuo (0.02 Torr) into a receiver
cooled with liquid nitrogen and used for further transformations.
Alane, prepared in situ from equimolar amounts of
chlorotrimethylsilane and LiAlH4 in diethyl ether (entry 2), was
found to be less efficient. To move the reaction forward a con-
tinuous addition of new portions of alane was necessary.
Phosphinine 6e was found to be inert towards LiAlH4 but
was successfully reduced with diisobutylaluminium hydride
(DIBAL-H) in toluene at room temperature (entry 4). A solution
of λ3-phosphinine 11 in toluene was distilled in vacuo into a
low-temperature trap, and this distillate was used for further
transformations.
To obtain pure λ3-phosphinine 11, we conducted the reduc-
tion of 6e under solvent-free conditions. As a result, we sepa-
rated pure phosphinine 11 in 86 % yield. When we applied the
same reduction conditions to phosphinine 6f,[10a] a more steri-
cally hindered analogue of phosphinine 6e, the reaction did
not run at all. Thus, we conclude that the reaction is very sensi-
tive to steric factors. Studies of the possibility of applying this
reaction to substituted phosphinines are underway in our labo-
ratory, and the results will be reported in due course.
Method C: N,N,N′,N′-Tetramethyldiaminophosphinine 6e (2.0 g,
11 mmol) was added to cooled pure solid DIBAL-H (4.53 g,
32 mmol). The resulting mixture was warmed slowly to room tem-
perature with stirring and maintained at room temperature over-
night. Phosphabenzene was distilled in vacuo (0.02 Torr) into a re-
ceiver cooled with liquid nitrogen, affording the target compound
as a colorless liquid. B.p. 93–94 °C (760 Torr). Yield: 0.90 g, 86 %.
1H NMR (300 MHz, C6D6): δ = 7.09 (dt, J = 3.6, J = 8.1 Hz, 1 H, CH),
7.44 (qd, J = 8.2, J = 2.2 Hz, 2 H, CH), 8.50 (dd, JP, H = 37.5, JH,H
=
10.2 Hz, 2 H, CH) ppm. 13C{1H} NMR (126 MHz, C6D6): δ = 129.5 (d,
JC,P = 22.7 Hz, CH), 134.4 (d, JC,P = 15.1 Hz, CH), 155.0 (d, JC,P
=
=
1
54.2 Hz, CH) ppm. 31P NMR (202 MHz, C6D6): δ = 207.0 (tt, JP, H
37.4, JP, H = 8.4 Hz) ppm.
CCDC-1055114 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Keywords: Phosphinines · Phosphahexatrienes ·
Conclusions
Electrochemistry · Cyclization · Reduction
A set of novel parent λ5-phosphinines have been synthesized
in high yields. The procedures can be run in a typical laboratory.
A new method for the synthesis of parent λ3-phosphinine from
available and inexpensive reagents starting from vinyl ethyl
ether in five steps in a total yield of 30 % was developed. The
current approach can probably be extended to other readily
available λ5-phosphinines known in the literature.
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Experimental Section
31P NMR spectra were recorded with either a Gemini-200 (81 MHz)
or Bruker Avance drx 500 (202 MHz) spectrometer with 85 % H3PO4
as an external standard. 1H NMR spectra were recorded with Varian
VXR-300 (300 MHz) or Bruker Avance drx 500 (500 MHz) spectrome-
ters. 13C NMR spectra were recorded with a Bruker Avance drx 500
(126 MHz) spectrometer. In 1H and 13C NMR spectra, chemical shifts
are given with respect to deuterated solvents with residual peaks
at δ = 7.26 and 77.2 ppm for CDCl3, δ = 7.16 and 128.4 ppm for
C6D6. Attached proton test (APT) experiments were used to assign
signals in 13C NMR spectra to different carbon atoms. All manipula-
tions with chemicals were conducted under an atmosphere of
argon by using Schlenk techniques. All experiments were repeated
at least two times to ensure of their reproducibility.
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Received: July 31, 2015
Published Online: October 13, 2015
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