Iron-salt-promoted highly regioselective α and β hydrophosphination of alkenyl arenes

Article abstract of DOI:10.1002/chem.201301417

Iron(ic) phosphination: The iron-promoted hydrophosphination of alkenyl arenes with diphenylphosphine as the phosphinating agent has been established. This method provides efficient access to diarylalkylphosphines. The regioselectivity of the reaction can

Full text of DOI:10.1002/chem.201301417

COMMUNICATION
DOI: 10.1002/chem.201301417
Iron-Salt-Promoted Highly Regioselective a and b Hydrophosphination
of Alkenyl Arenes
Lucie Routaboul,[a] Fabien Toulgoat,[a] Jꢀrꢀmie Gatignol,[a] Jean-FranÅois Lohier,[a]
Brigitte Norah,[a] Olivier Delacroix,[a] Carole Alayrac,[a] Marc Taillefer,*[b] and
Annie-Claude Gaumont*[a]
Dedicated to Professor Irina Petrovna Beletskaya
The catalytic addition of phosphines to alkenes (hydro-
phosphination) is an attractive process (Scheme 1).[1] It is a
100% atom-economical reaction that uses widely available
and inexpensive starting materials. It offers an access to al-
kylphosphines, which are useful ligands,[2] organocatalysts,[3]
valuable a adduct A, with formation of a stereogenic centre,
is still a challenging problem.[11] In addition, examples of
styrenic hydrophosphination are scarce. Addition to styrene
was reported under basic,[9b,c,d] and radical[12] conditions and
more recently by using transition-metal catalysts (Ni, Cu).[13]
All these procedures afforded anti-Markovnikov adducts.
Inexpensive and environmentally friendly iron salts[14]
have recently emerged as powerful tools in organic synthe-
sis.[15] Their efficiency has been demonstrated in several ad-
dition reactions to carbon–carbon double bonds (hydroami-
nation,[16] hydrothiolation,[17] hydroboration,[18] hydrosilyla-
tion,[19] hydroarylation,[20] and oxyphosphorylation[21]) and
during the course of our studies in the double hydrophosphi-
nation of terminal arylacetylenes.[22] As part of our ongoing
studies of the control of the regioselectivity of hydrophos-
phination reactions,[7a,b,23] herein we report the iron-promot-
ed hydrophosphination reaction of styrenes, which offers se-
lective access to either the b adduct B or the a adduct A
(Schemes 1 and 2).[24]
Scheme 1. Formation of Markovnikov and anti-Markovnikov regioisom-
ers by catalytic addition of phosphines to alkenes.
and reagents in organic synthesis.[4] However, due to the
À1 [5]
À
high energy of the P H bond (E=77 kcalmol ), the reac-
tion usually requires activation by radical initiators.[1c,6]
Thermal-,[7] acid-,[8] and base-promoted[9] reactions have also
been applied, although less frequently. Metal- and lantha-
nide-catalyzed processes were also reported recently.[1c,10]
Whatever the method, the addition usually proceeds in an
anti-Markovnikov way leading to the b adduct B (addition
of the phosphorus atom to the terminal carbon atom of a
terminal alkene). In contrast, the selective formation of the
Scheme 2. Regioselective iron-catalyzed hydrophosphination of alkenyl
arenes.
[a] Dr. L. Routaboul, Dr. F. Toulgoat, J. Gatignol, J.-F. Lohier,
Dr. B. Norah, Dr. O. Delacroix, Dr. C. Alayrac,
Prof. Dr. A.-C. Gaumont
Laboratoire de Chimie Molꢀculaire et Thioorganique
UMR CNRS 6507, INC3M FR3038
ENSICAEN & Universitꢀ de Caen Basse-Normandie
6 bvd Marꢀchal Juin, 14050 CAEN (France)
Fax : (+33)2-31-45-28-77
As a model reaction, we investigated the iron-catalyzed
addition of diphenylphosphine to styrene (1a) (Scheme 2,
Ar=Ph). A preliminary set of experiments was arbitrarily
carried out at 608C in acetonitrile. Without catalyst, the re-
action was very slow (18% conversion of Ph2PH after 5 h)
leading to the formation of the b adduct (11%) along with a
mixture of unidentified byproducts (Table 1, entry 1). Use of
0.3 equivalents of FeCl2 afforded 46% of the b adduct 2a[13b]
with a 47% conversion of Ph2PH after 5 h at 608C (Table 1,
entry 2). Finally, running the reaction for 12 h led to full
conversion of Ph2PH and to 97% of the corresponding
phosphine 2a (Ar=Ph)[25] (Table 1, entry 3). No trace of the
E-mail: annie-claude.gaumont@ensicaen.fr
[b] Dr. M. Taillefer
Institut Charles Gerhardt Montpellier UMR 5253 CNRS
ENSCM, 8 Rue de l’Ecole Normale
34296 Montpellier Cedex 5 (France)
Fax : (+33)4-67-14-43-19
E-mail: marc.taillefer@enscm.fr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301417.
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hypothetical a adduct 3a could be detected. When the cata-
lyst loading was decreased to 0.1 equivalents an incomplete
conversion of 68% was observed after 12 h. The full conver-
sion was not reached, even after an extended reaction time
of 24 h. Other iron salts such as [FeACTHUNGTREN(NUG acac)3] and K3[Fe(CN)6]
also led to the selective formation of the anti-Markovnikov
b adduct 2a, but the best promoter remains FeCl2.
A different result was observed when FeCl3 was the cata-
lyst. Indeed, a new signal was observed in the 31P NMR
spectrum of the crude medium. Isolation and full characteri-
Table 1. Screening of iron catalysts for the hydrophosphination of styr-
ene (1a, Ar=Ph) with diphenylphosphine.[a]
zation suggested the formation of the
a adduct 3a
Catalyst
Time
[h]
Ph2PH Conv.
[%][b]
b adduct 2a
a adduct 3a
(Scheme 2 and Table 1, entry 6), by comparison with an au-
thentic sample prepared by an alternative route.[20] This pre-
liminary result led us to investigate the possibility of tunable
control of the regioselectivity of hydrophosphination by
careful selection of the iron catalyst (Scheme 2).
[%][c]
[%][c]
1
2
3
4
5
6
none
FeCl2
FeCl2
5
5
12
12
12
12
18
47
99
66
70
11
46
97
66
68
–
–
–
–
–
–
Fe
N
K3Fe(CN)6
FeCl3
100
92
Table 3. Regioselective FeCl3-mediated a hydrophosphination of various
[a] Reaction conditions: Iron salt (0.3 equiv), CH3CN, Ph2PH (1 equiv),
styrene (2 equiv), 608C. [b] Conversion (consumption of Ph2PH) deter-
mined by 31P NMR spectroscopy. [c] 31P NMR spectroscopic yield.
alkenyl arenes with diphenylphosphine.[a]
Table 2. Regioselective FeCl2-mediated b hydrophosphination of various
alkenyl arenes with diphenylphosphine.[a]
Alkenyl arene
Product
Yield [%]
T
[8C]
31P[b]
Iso-
lated[c]
1
2
60
60
85
92
73
85
Alkenyl arene
Product
Yield [%]
31P[b]
Iso-
lated[c]
1
2
97
95
87
3
60[d]
84
74
85
84
4
5
60
60
62
44
3
4
100
100
<5
<5
84[d]
6
60
<5
<5
5
6
100
100
87
82
7
8
90
90
90
90
96
100
90
76
76
72
53
7
90
74
9
8
9
47
17
30[e]
10
79
[e]
–
[a] Reaction conditions: FeCl3 (0.3 equiv), CH3CN, Ph2PH (1 equiv), al-
kenyl arene (2 equiv), 60–908C. [b] 31P NMR yield. [c] Isolated yield of
the corresponding 3·BH3. [d] Heating was performed by microwave irra-
diation (3ꢂ10 min). A lower 31P NMR yield of 55% was obtained by
classical heating.
[a] Reaction conditions: FeCl2 (99% purity, 0.3 equiv), CH3CN, Ph2PH
(1 equiv), alkenyl arene (2 equiv), 608C. [b] 31P NMR yield. [c] Isolated
yield of the corresponding 2·BH3. [d] Reaction performed for 48 h.
[e] Reaction performed at 908C.
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a and b Hydrophosphination of Alkenyl Arenes
COMMUNICATION
The scope of these processes
for different alkenyl arenes is
shown in Tables 2 and 3. FeCl2
was selected for the preparation
of the anti-Markovnikov b ad-
ducts 2a–i and FeCl3 for the
Markovnikov a adducts 3a–c
and 3e–k.
With FeCl2 (Table 2), all of
the alkenyl arenes tested af-
forded the expected b adduct
with 0.3 equivalents of FeCl2 at
608C in acetonitrile. The scope
of the reaction ranges from
electron-rich to electron-poor
derivatives (Table 2, entries 2–5
and 6–7). Substrates bearing
ortho substituents also reacted
in high yields (Table 2, entries 3
Figure 1. X-ray structures of 3b·BH3 (left) and 3h·BH3 (right).
and 7). Addition to a 1,1-disub-
stituted double bond proved to
be more difficult (Table 2, en-
tries 8 and 9), even at higher temperature (908C).[1c,27]
The scope of the FeCl3-catalyzed a-hydrophosphination
process was evaluated (Table 3) at temperatures between 60
and 908C. Compared to the FeCl2 promoted formation of b-
adducts, the substituents of double bond play a less impor-
tant role in the a addition. Indeed, the hydrophosphination
adducts of a-methylstyrene, 1,1-diphenylethene and a-
methyl-p-methylstyrene were formed in very good yields (90
to 100% 31P NMR yield; Table 3, entries 7–9). On the other
hand, the electronic effects play a major role. Whereas elec-
tron-donating groups are well tolerated (entries 2–4), elec-
tron-withdrawing groups seem to inhibit the hydrophosphi-
nation process (conversion <5%, entries 5–6). This tenden-
cy is in agreement with the results reported for the hydroa-
mination of styrenes with FeCl3.[16c] Interestingly, when mon-
osubstituted alkenes were used (entries 1–4), the
temperature could be lowered to 608C. The possibility of
lowering the catalyst loading to 0.1 equiv was checked with
substrate 1b. After 12 h, the conversion to 3b was only
42%. Because extending the reaction time may favor the
thermal formation of the b adduct (Table 1, Entry 1), a cata-
lyst loading of 0.3 equivalents appears more convenient for
the formation of the a adduct.
the a adduct.[8] By analogy, HCl, which could be released by
FeCl3, may be responsible for the transformation. However,
replacing FeCl3 by HCl[28] under other similar conditions led
to poor conversion of the alkenyl arenes (as an example,
with 1,1-diphenylethene, less than 7% of the expected a
adduct was formed).
Impurities in FeCl3 could also be suspected to promote
the addition reaction, notably copper oxide traces.[29] Thus,
metal contaminants (MnCl2, ZnCl2, CuCl2, Cu2O) commonly
found in FeCl3 were tested in the reaction under the opti-
mized conditions and none are efficient promoters of the
formation of the a adduct (see Table 4, entry 1 and the Sup-
porting Information). Moreover, whatever the quality of
FeCl3 (Table 4, entries 2–4), similar results were obtained.
Table 4. Influence of the quality of FeCl3 on the a hydrophosphination
of 1,1 diphenylethene[30] to form compound 3i.[a]
Catalyst
Ph2PH Conv. [%][b]
a adduct[b]
1
2
3
4
Cu2O
25
100
100
93
1
94
92
81
FeCl3 (98%)
FeCl3 (99.99%)
FeCl3·6 H2O
[a] Reaction conditions: FeCl3 (0.3 equiv), CH3CN, Ph2PH (1 equiv), 1,1-
diphenylethene (2 equiv), 608C, 20 h. [b] 31P NMR yield.
Single-crystals of the addition products 3b·BH3 and
3h·BH3 obtained with p-methoxystyrene and a-methylstyr-
ene, respectively, were grown by slow diffusion of pentane
into a CH2Cl2 solution. X-ray structures furnished the defini-
tive proof of the a addition (Figure 1).
The difference encountered in the scope and limitation of
the two processes clearly indicates a potential difference in
mechanism. From a general point of view, the observation
of the a regioisomer in the hydrophosphination process is
rare in the literature, regardless of the substrates involved.
Only stoichiometric amounts of a strong acid (CH3SO3H) in
harsh conditions were reported to allow the formation of
Although the mechanism of the original a addition is still
under investigation, the scope of the reaction can give some
clues. A likely catalytic cycle would start from the initial ac-
tivation of the double bond of the alkenyl arene by the
Lewis acid (FeCl3) leading to a polarized p complex (or
transient carbocation), which is stabilized by electron-donat-
ing substituents. Subsequent addition of diphenylphosphine
to the activated double bond and release of hydrogen chlor-
ide could be the next step. Lastly, protonation of the carbon
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M. Taillefer, A.-C. Gaumont et al.
yl-(1-phenyl-ethyl)phosphine borane (3a·BH3) was isolated as a colorless
oil (129 mg, 0.42 mmol) in 73% yield.
Acknowledgements
We gratefully acknowledge financial support from the Agence Nationale
de la Recherche, projet no. ANR-09-CP2D-13-Phoscat, the Ministꢃre de
la Recherche et des Nouvelles Technologies, the CNRS (Centre National
de la Recherche Scientifique), the Rꢀgion Basse-Normandie and the Eu-
ropean Union (FEDER Funds). COST ACTION CM0802 “PHOSCI-
NET” is also acknowledged for support.
Scheme 3. Plausible reaction mechanism for the FeCl3 catalyzed a hydro-
phosphination of alkenyl arenes.
Keywords: alkenes · hydrophosphination · homogeneous
catalysis · iron · phosphanes · regioselectivity
metal bond would lead to the formation of the final product
and would release the initial iron complex (Scheme 3).
The fact that only FeCl3 and not FeCl2 is able to promote
the a addition is not clear. Nevertheless, the difference in
Lewis acidity of these iron salts can probably account for
this difference.[31] With regard to the formation of the b
adduct, iron(II) could simply favor a radical process through
activation of traces of O2.[21] Addition of a radical inhibitor
such as tert-butylcatechol or performing the reaction in the
dark lowered the yield by 20 to 30%, but did not fully sup-
press the reaction. Similar tests performed in the presence
of FeCl3 did not affect the course of the reaction. Thus, at
this stage, it is difficult to draw concrete conclusions. Ra-
tionalization of these experimental results is currently in
progress.
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In conclusion, we have developed an iron-promoted hy-
drophosphination of alkene derivatives with diphenylphos-
phine as the phosphinating agent. Compared to other hydro-
phosphination methodologies, our approach uses a low-cost
and low-toxicity transition metal and offers highly selective
access to both the a and the b adducts just by selecting the
nature of the iron salt (FeCl3 or FeCl2 respectively). Unpre-
cedented access to the a adduct through substrate independ-
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methodology to various substrates other than styrenes and
to asymmetric reactions is currently under investigation, as
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Experimental Section
Typical procedure: FeCl3-catalyzed a hydrophosphination of styrene (1a)
with diphenylphosphine (Table 3, entry 1): Styrene (1a, 120 mL,
1.04 mmol) and diphenylphosphine (100 mL, 0.57 mmol) were successive-
ly added to
a solution of FeCl3 (30 mg, 0.18 mmol) in acetonitrile
(150 mL). The reaction mixture was then heated at 608C for 12 h. After
cooling to 08C, BH3·SMe2 (63 mL, 0.7 mmol) was added dropwise and the
reaction mixture stirred at RT for 30 min. Volatiles were eliminated
under reduced pressure then the crude product was purified by silica gel
chromatography with a CH2Cl2/pentane mixture (2:3) as eluent. Diphen-
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a and b Hydrophosphination of Alkenyl Arenes
COMMUNICATION
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Received: April 15, 2013
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ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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M. Taillefer, A.-C. Gaumont et al.
Hydrophosphination
L. Routaboul, F. Toulgoat, J. Gatignol,
J.-F. Lohier, B. Norah, O. Delacroix,
C. Alayrac, M. Taillefer,*
A.-C. Gaumont* .............. &&&& — &&&&
Iron(ic) phosphination: The iron-pro-
moted hydrophosphination of alkenyl
arenes with diphenylphosphine as
phosphinating agent has been estab-
lished. This method provides efficient
access to diarylalkylphosphines. The
regioselectivity of the reaction can be
fine-tuned by the choice of the iron
salts, offering highly selective access to
both the b and the a adducts (see
scheme).
Iron-Salt-Promoted Highly Regioselec-
tive a and b Hydrophosphination of
Alkenyl Arenes
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ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!