Synthesis of vinylphosphines and unsymmetric diphosphines: Iron-catalyzed selective hydrophosphination reaction of alkynes and vinylphosphines with secondary phosphines

Article abstract of DOI:10.1039/c5cc10185a

Iron complex-catalyzed regioselective single hydrophosphination of terminal arylalkynes with secondary phosphines was achieved. Unsymmetric 1,2-bis(phosphino)ethanes with different phosphino groups were obtained by using our catalytic systems. The structu

Full text of DOI:10.1039/c5cc10185a

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Synthesis of vinylphosphines and unsymmetric
diphosphines: iron-catalyzed selective
Cite this:DOI: 10.1039/c5cc10185a
hydrophosphination reaction of alkynes and
vinylphosphines with secondary phosphines†‡
Received 11th December 2015,
Accepted 14th January 2016
Masumi Itazaki,* Shinya Katsube, Masahiro Kamitani§ and Hiroshi Nakazawa*
DOI: 10.1039/c5cc10185a
www.rsc.org/chemcomm
Iron complex-catalyzed regioselective single hydrophosphination
of terminal arylalkynes with secondary phosphines was achieved.
Unsymmetric 1,2-bis(phosphino)ethanes with different phosphino
groups were obtained by using our catalytic systems. The structures of
the obtained vinylphosphine, unsymmetric 1,2-bis(phosphino)ethane,
and iron catalyst precursors were confirmed by single crystal X-ray
diffraction studies.
Scheme 1 Hydrophosphination of secondary phosphines with alkynes.
complex (Scheme 2).9 In this case, reactions of terminal arylalkynes
with 2 equiv. of secondary arylphosphines (PHR2) afforded the
corresponding diphosphines as a result of double hydrophosphina-
tion via vinylphosphines being single hydrophosphination products.
As vinylphosphines produced in this catalytic system are readily
converted into the double hydrophosphination product, we
could not isolate the vinylphosphines in high yield. In this
communication, we present the first iron complex-catalyzed
regioselective synthesis of vinylphosphines and unsymmetric
1,2-bis(phosphino)ethanes with different phosphino groups
using our catalytic reactions.
We firstly examined the catalytic activity of CpFe(CO)2(Me)
and Cp*Fe(CO)2(Me) (Cp* stands for Z5-C5Me5) for the reaction of
equimolar amounts of PHPh2 and phenylacetylene, and found that
CpFe(CO)2(Me) showed low selectivity between single and double
hydrophosphination and Cp*Fe(CO)2(Me) showed low catalytic
activity. After several trials, we found that a methyl(pyridine)iron
complex, Cp*Fe(CO)(py)(Me), exhibited good catalytic activity. The
reaction of equimolar amounts of PHPh2 with phenylacetylene in a
1 : 1 molar ratio at 110 1C for 48 h in toluene in the presence of
10 mol% of the iron complex produced single and double hydro-
phosphination compounds in a 4 : 1 molar ratio. Under optimized
conditions (PHPh2 (174 mL, 1.0 mmol), 1.5 equiv. of PhCRCH
(164 mL, 1.5 mmol), Cp*Fe(CO)(py)(Me) (32 mg, 0.1 mmol)),
Vinylphosphines have been gaining importance as synthetic
intermediates for the preparation of polyphosphines and ligands
for transition metal catalysts in organic synthesis.1 One of the
most efficient methods for the synthesis of vinylphosphines is
the catalytic addition of a P–H bond in trivalent phosphine to a
CRC triple bond (Scheme 1).
However, hydrophosphination of a P–H bond with alkynes
catalyzed by a transition metal complex is generally difficult to
be accomplished because the starting phosphine and/or the
product (vinyl phosphine) usually coordinate to a catalytically
active species to form a coordinatively-saturated complex having
less or no catalytic activity. Examples of catalytic addition of a P–H
bond of a P(III) compound to a CRC triple bond are quite limited:
one example of Co,1f Cu,2 and Zr3 and two examples of Ca,4 Ni,5
Pd,5 La,6 and Yb4,7 have been reported. Among many kinds
of transition metals, iron is a highly abundant, inexpensive,
and relatively non-toxic metal. These advantages of iron make it
highly attractive as a catalyst. Previously, we found catalytic trans-
selective hydrogermylation of terminal and internal alkynes by an
iron complex.8 In addition, we reported regioselective double
hydrophosphination of terminal arylalkynes catalyzed by an iron
Department of Chemistry, Graduate School of Science, Osaka City University,
3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan.
E-mail: mitazaki@sci.osaka-cu.ac.jp, nakazawa@sci.osaka-cu.ac.jp
† Dedicated to Professor Kohtaro Osakada on the occasion of his 60th birthday.
‡ Electronic supplementary information (ESI) available. CCDC 1441570–1441576,
1441672 and 1441673. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c5cc10185a
Scheme 2 Regioselective double hydrophosphination of terminal aryl-
alkynes catalyzed by an iron complex.
§ Present address: Department of Chemistry, School of Science, Kitasato University,
1-15-1 Kitasato, Minami-ku, Sagamihara, Kanagawa 252-0373, Japan.
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Table 1 Reaction of secondary phosphines with terminal alkynes catalyzed
by a methyl(pyridine)iron complex Cp*Fe(CO)(py)(Me)a
NMR yieldb,c Isolated
Time/h % (E/Z)
Entry
R
R0
yieldd
%
1
2
3
4
5
6
7
8
Ph
p-Me-C6H4
p-MeO-C6H4 Ph
Ph
Ph
48
48
48
48
48
48
48
72
72
48
90 (10/90)
65 1a
59 1b
25 1c
65 1d
30 1e
42 1f
22 1g
32 1h
46 1i
65 1j
60 1k
—
79 (20/80)
73 (15/85)
72 (9/91)
74 (12/88)
85 (17/83)
42 (28/72)
69 (13/87)
72 (28/72)
75 (10/90)
72 (9/91)
NRg
Fig. 1 ORTEP drawings of (a) 1b, (b) 1d, (c) 1i and (d) 1j with 50% thermal
ellipsoidal plots. Hydrogen atoms except vinyl protons and amine protons
are omitted for simplicity.
p-NH2-C6H4
p-F-C6H4
Ph
Ph
Ph
Ph
p-tBu-C6H4
2-Pyridyl
3-Thiophenyl Ph
The molecular structures of 1b, 1d, 1i, and 1j were deter-
mined using single crystal X-ray diffraction (Fig. 1). The CQC
bond portion apparently possesses the Z configuration in these
compounds. The CQC bond distance (1.342(4) Å for 1b,
1.341(2) Å for 1e, 1.346(6) Å for 1i, 1.345(3) Å for 1j) is similar
to that for Z-diphenylstyrylphosphine (1.339(2) Å) reported
previously,10 but is longer than a typical vinylic C(sp2)QC(sp2)
bond distance (1.299 Å)11 and the CQC bond distance found in
C2H4 (1.3142(3) Å).12
9
Ferrocenyl
p-NH2-C6H4
p-NH2-C6H4
Alkyle
Ph
p-Me-C6H4
p-MeO-C6H4 48
10
11
12
13
Ph
48
48
Ph
Alkyl f
NRg
—
a
0
Reaction conditions: 110 1C, 10 mL Schlenk tube. [PHR2 ] : [acetylene]:
[Cp*Fe(CO)(py)(Me)]: [toluene] = 1 : 1.5: 0.1: 10. Based on 31P NMR
b
using triphenylphosphine oxide P(QO)Ph3 as an internal standard. c Based
on 31P NMR. d Z isomer only. e Alkyl = nHex, cHex, tBu. Alkyl = cHex, nBu.
f
g
No reaction.
The proposed catalytic cycle for hydrophosphination of
only vinylphosphine was produced in 90% yield as a 10 : 90 terminal alkynes is shown in Scheme 3. We reported a similar
E/Z mixture, according to the 31P NMR measurement. After catalytic cycle for double hydrophosphination8 and it was sup-
purification, pure [(1Z)-2-phenylethenyl]diphenylphosphine (1a) ported by DFT calculations.13 In Scheme 3, Cp*Fe(CO)(py)(Me)
was isolated in 65% yield based on PHPh2 (Table 1, entry 1). reacts with PHPh2 to give Cp*Fe(CO)(PHPh2)(Me) (A). Conversion
This catalytic system to obtain vinylphosphines is applicable for of A into Cp*Fe(CO)(PPh2) (B) occurs either via P–H oxidative
para-substituted phenylacetylenes whether the substituent has addition and MeH (CH4) elimination or via concerted MeH
electron-donation or -withdrawing character (entries 2–6). An elimination from the coordinated PHPh2 and the Me ligand.
alkyne derivative with an aromatic substituent other than a The alkyne coordinates to the iron center of B in an Z2-fashion
phenyl derivative such as a 2-pyridyl, 3-thiophenyl, or ferrocenyl (B - C). This species C undergoes an insertion of the coordinated
group could be converted into the corresponding vinylphosphine alkyne into the Fe–P bond (C - D). The corresponding alkyne
(entries 7–9). Reaction of 4-ethynyl-benzenamine with PHR2 insertion into an M–P bond was proposed by DFT calculation for
(R = p-MeC6H4, p-MeOC6H4) also afforded the corresponding Ni, Pd, Pt, and Rh complexes.14 The phosphorus atom in D may
vinylphosphines (entries 10 and 11). In contrast, the hydrophos- coordinate to the iron center to give a four-membered metalla-
phination did not take place when alkylalkynes, RCRCH (R = nHex, cycle complex E, and they may be in equilibrium.15,16 Complex D
cHex, and tBu), or alkyl phosphines, PHR2 (R = cHex, nBu), were is converted into D0 when vinyl isomerization (D - D0) takes
used as substrates (entries 12 and 13). In addition, internal place. Similar isomerization has been reported for hydrometala-
alkynes such as diphenyl and methylphenyl acetylene were not tion of alkynes.8,17 The coordination of PHPh2 to the Fe center
converted into hydrophosphination products.
of D0 affords F and then the C–H bond formation from F yields
Scheme 3 Proposed catalytic cycle.
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Scheme 4 Synthesis of the phosphine complex
phosphine(phosphide) complex G.
A
and the
Fig. 3 ORTEP drawing of E with 50% thermal ellipsoidal plots. Hydrogen
atoms except methylene protons are omitted for simplicity.
the Z-vinylphosphine and reinstates an intermediate B to com-
plete the catalytic cycle. On the other hand, E-vinylphosphine is
obtained by the reaction of D with PHPh2. Although iron complex
B is a very important intermediate in the catalytic cycle, the
complex is too reactive to be isolated due to a 16e species.
A 16e phosphide complex B has a possibility of reacting with
PHPh2 present in solution to give a phosphine(phosphide)iron
complex, Cp*Fe(CO)(PHPh2)(PPh2). Therefore, we attempted the
isolation of the phosphine(phosphide)iron complex. Reaction of
the methyl(pyridine)iron complex Cp*Fe(CO)(py)(Me) with PHPh2
led to the isolation of Cp*Fe(CO)(PHPh2)(Me) (A). Reaction of the
isolated A with PHPh2 (1 : 1 molar ratio) in toluene at 110 1C for
1 h led to the isolation of the phosphine(phosphide)iron complex,
Cp*Fe(CO)(PHPh2)(PPh2) (G) (Scheme 4).
The ORTEP drawings of Cp*Fe(CO)(py)(Me), A and G are
shown in Fig. 2. Structural disorder in Cp*Fe(CO)(py)(Me) was
observed between CO and Me ligands. Complexes A and G have
Z5-Cp*, CO, and PHPh2 ligands on the central iron and a methyl
ligand for A and a diphenylphosphide ligand for B, and adopt a
typical three-legged piano-stool structure. The Fe–P1(phosphine)
bond distance (2.1575(6) Å for A, 2.1808(4) Å for G) is shorter
than the Fe–P2(phosphide) bond distance (2.3081(4) Å).
Scheme 5 Catalytic activity for hydrophosphination of complexes A, E,
and G.
The Fe–PPh2 bond distance of E (2.2067(11) Å) is shorter than
that of G (2.3081(4) Å) because of ring strain for small internal
bond angle. The C(24)–C(25) bond distance (1.351(5) Å) is a
typical CQC double bond distance and the metallated carbon
P
(
C(25)+ = 359.91) is planar.
We checked the catalytic activity for hydrophosphination of
complexes A, E, and G. Reaction of PHPh2 with phenylacetylene
in the presence of 10 mol% of A, E, or G at 110 1C yielded the
desired vinylphosphine in high yield (Scheme 5).
It is considered that complexes A and G can readily form
the active species Cp*Fe(CO)(PPh2) B and the dissociation of
coordinated P atoms from iron center in E forms D. These
results show that complexes A, E, and G are the catalyst
precursors in our reaction.
In the reaction of PHAr2 with Ar0CRCH, CpFe(CO)2(Me) was
reported to serve as a precatalyst for double hydrophosphination9
and this work revealed that Cp*Fe(CO)(py)(Me) served as an effective
precatalyst for single hydrophosphination. Thus, we examined
the reaction of Z-vinylphosphine (1a or 1j) obtained in this work
with secondary diarylphosphines catalyzed by CpFe(CO)2(Me)
at 110 1C for 24 h with the hope of formation of unsymmetric
1,2-bis(phosphino)ethanes with different phosphine substituents.
The results are shown in Table 2. Unsymmetric diphosphines
2a–2e were obtained in high yields (entries 1–5). The molecular
structure of 2e was confirmed by X-ray diffraction (Fig. 4).
As G corresponds to a resting state of the catalytic cycle in
Scheme 3, we expected that G would react with phenylacetylene.
The reaction of G with phenylacetylene in the 1 : 1 molar ratio at
110 1C for 1 h in toluene afforded a metallaphosphacyclobutene
complex E as a result of a regioselective insertion of phenyl-
acetylene into the Fe–PPh2 bond (see Scheme 3) (eqn (1)).
(1)
Fig. 3 shows the molecular structure of E by X-ray diffraction
studies. Complex E contains a planar four-membered ring with a
P–Fe–C angle (68.34(10)1) that is larger than that of a corresponding
Ru complex, [Ru(Z5-indenyl)(k2-PhCQCHPPh2)] (65.63(6)1).16
Table 2 Synthesis of unsymmetric diphosphines catalyzed by CpFe(CO)2(Me)a
Entry
Vinylphosphine
R00
Isolated yield %
1
2
3
4
5
1a
1a
1a
1a
1j
Ph
84 2a
85 2b
83 2c
69 2d
88 2e
p-Me-C6H4
p-MeO-C6H4
p-F-C6H4
Ph
Fig. 2 ORTEP drawings of (a) Cp*Fe(CO)(py)(Me), (b) A, and (c) G with 50%
thermal ellipsoidal plots. Hydrogen atoms except the hydrogen atom on
the P atom are omitted for simplicity.
a
Reaction conditions: 110 1C, 10 mL Schlenk tube, [PHR2] : [vinylphosphine] :
[Fe cat.] = 1:1:0.05.
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This work was supported by a Grant-in-Aid for Scientific
Research from JSPS (Category C, no 25410073 (M. I.)), by a
Challenging Exploratory Research Grant (no 25620048 (H. N.
and M. I.)), and by a Grant-in-Aid for Scientific Research on
Innovation Area ‘‘Stimuli-responsive Chemical Species’’ (No.
25109538 (H. N.)) from MEXT, Japan.
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Chem. Commun.
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