Copper(I)-catalyzed hydrophosphination of styrenes

Article abstract of DOI:10.1016/j.jorganchem.2010.09.069

Hydrophosphination of styrenes has been accomplished with metal salts for the first time. (CuOTf)₂·toluene complex is the catalyst of choice, but CuCl can also be used. "In-situ" EPR and NMR studies suggest Cu(I) as the catalytically active metal species, giving exclusively the anti-Markovnikov product. Phosphine oxides or β-ketophosphine oxides can be prepared in one-pot by oxidation with molecular oxygen.

Full text of DOI:10.1016/j.jorganchem.2010.09.069

Journal of Organometallic Chemistry 696 (2011) 362e367
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Copper(I)-catalyzed hydrophosphination of styrenes
Antonio Leyva-Pérez a, Jose A. Vidal-Moya a, Jose R. Cabrero-Antonino a, Salem S. Al-Deyab b,
,b,
Saud I. Al-Resayes b, Avelino Corma a
*
a Instituto de Tecnología Química, Universidad Politécnica de Valencia-Consejo Superior de Investigaciones Científicas, Avda. de los Naranjos s/n, 46022, Valencia, Spain
b Chemistry Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
Hydrophosphination of styrenes has been accomplished with metal salts for the first time. (CuOTf)2$
toluene complex is the catalyst of choice, but CuCl can also be used. “In-situ” EPR and NMR studies
suggest Cu(I) as the catalytically active metal species, giving exclusively the anti-Markovnikov product.
Received 22 September 2010
Accepted 29 September 2010
Available online 8 October 2010
Phosphine oxides or
oxygen.
b
-ketophosphine oxides can be prepared in one-pot by oxidation with molecular
Keywords:
Ó 2010 Elsevier B.V. All rights reserved.
Copper(I) catalysis
Hydrophosphination
Styrenes
Phosphine oxides
b-Ketophosphines
Tertiary phosphines
1. Introduction
conditions [7]. Therefore, there is not still a simple metal salt that
catalyzes the hydrophosphination of alkenes. Furthermore, the use
of a metal could allow designing heterogeneous catalysts for a fully
sustainable system.
Addition of XeH bonds (X ¼ carbon, heteroatom, semi-metallic
or metallic) across unsaturated CeC bonds is a 100% atom-
economical reaction that fits the requirements of modern sustain-
able chemistry [1e3]. In particular, phosphines R2P-H add to
alkenes to give tertiary alkyl phosphines, useful as ligands and
intermediates in organic synthesis (Scheme 1) [4,5].
The addition proceeds usually in anti-Markovnikov way. The
reaction can be carried out in the presence of a strong base in
stoichiometric amount but a catalytic process is always preferred
provided that acceptable conversion and selectivity are achieved.
Radical initiators such as AIBN are generally used as catalyst [4a].
Lewis acids could also act as catalyst but, unfortunately, examples
are scarce [6e8]. Only two metallic systems based on ytterbium [6]
and nickel complexes [7] have been successfully employed for the
metal-catalyzed intermolecular hydrophosphination of unactivated
alkenes while organolanthanide species can act as catalysts for the
intramolecular process [8]. All these metallic catalysts are quite
expensive and toxic and, in some cases, operate under harsh
First-row transition metals have attracted the interest of
chemists in the last years for their use in catalysis, since new or old
coinage-metal-catalyzed reactions can now be accomplished. In
contrast to second- and third-, first row transition metals are
inexpensive, non-toxic and environmentally friendly. In particular,
iron [9] and copper [10] have regained attention for their ability to
activate unsaturated CeC bonds. During the course of our inves-
tigations in the iron(III)-catalyzed dimerisation of styrenes [2], we
found that Fe(III) coordinates so poorly to triphenylphosphine that
liquid 31P NMR measurements of the complex FeCl3(PPh3)2 only
recorded the peak for free PPh3
(d
¼ ꢀ6 ppm), regardless the
solvent used. In contrast, 31P MAS-CP solid NMR of the solid gave
the expected downshifted signal for the coordinated phosphine
(
d
¼ 336 ppm). Although variable temperature experiments in
solution should be performed to confirm the stability of the
complex, results infer that Fe(III) hardly interacts with phosphines
PR2
[cat.]
R2P
anti-Markovnikov
+
R2P-H
+
R
R
R
* Corresponding author. Instituto de Tecnología Química, Universidad Politécnica
de Valencia-Consejo Superior de Investigaciones Científicas, Avda. de los Naranjos
s/n, 46022, Valencia, Spain. Tel.: þ34 963877800; fax: þ34 963877809.
E-mail address: acorma@itq.upv.es (A. Corma).
Markovnikov
Scheme 1. Hydrophosphination of alkenes.
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.09.069

A. Leyva-Pérez et al. / Journal of Organometallic Chemistry 696 (2011) 362e367
363
O
PPh2
catalyst
PPh2
HPPh2
1eq
+
+
anhydrous
1,4-dioxane
Cl
Cl
Cl
1eq
3
2
4
1
Scheme 2. Hydrophosphination of 4-chlorostyrene 1 with diphenylphosphine 2.
in solution and, given the high affinity of phosphines to bound (and
poisoning) coinage metals, we envisage Fe(III) and, by analogy,
highly cationic first-row transition metals, could act as alternative
catalysts to coinage metals for those reactions were phosphines are
used as reactants, it is the case for the hydrophosphination of
alkenes. Therefore, we carried out an extended study on the
addition of phosphines to alkenes catalyzed by different metal
salts. We can anticipate that, although unfortunately Fe(III) salts do
not work as catalysts, other metal salts do work. Finally Cu(I) was
found the catalyst of choice in terms of activity, non-toxicity and
price.
Preliminary catalytic results for different metal salts in dioxane
at 80 ꢁC are shown in Table S1. It was found that PdCl2, Ru(OTf)3
and FeCl3 (entries 2e4) did not give any conversion to products.
However, the exchange of chlorides in FeCl3 with triflate or tri-
flimide anions significantly improves the yield to the hydro-
phosphination product 3 and the phosphine oxide 4 (entries 5, 6)
[11,12]. Iron(III) works better than iron(II) (compare entries 5 and
7). The Fe(OTf)3-catalyzed hydrophosphination of 1 with 2 (entry
5) was followed in d8-dioxane by in-situ 1H, 13C, 31P and DEPT,
NMR spectroscopy. The NMR measurements showed that 3 is
a primary product and 4 was formed later. However, when a high
purity FeCl3 (99.99%) was exchanged with triflate anions and used
as catalyst, the yield to 3 and 4 dropped to the blank level (entry 8).
This result suggests that iron is not the real catalyst but impurities
of other metals, copper being a plausible candidate [13]. Indeed,
copper(II) salts give similar or even better yields to 3 and 4 than
the corresponding iron species, Cu(OTf)2 in particular (entries
9e11). Brønsted acid catalysis proceeds in low extent (entry 12)
but silver(I) [3,14] works well (entries 13, 14). Anyway, a second
series of experiments were carried out, increasing the amount of
metal catalyst, reaction temperature and reactants concentration
(Table 1).
2. Results and discussion
2.1. Formation of tertiary phosphines
The hydrophosphination of 4-chlorostyrene 1 with diphenyl-
phosphine 2 was used as reaction test (Scheme 2).
Table 1
Hydrophosphination of 4-chlorostyrene 1 with diphenylphosphine 2 in the pres-
ence of different metal species. Conditions: A) anhydrous DMF (2 M), 140 ꢁC; B)
anhydrous 1,4-dioxane (2 M), 100 ꢁC. If not specified, the triflate salts are not pre-
formed but pure compounds.
1,4-Dioxane (at 100 ꢁC) is better reaction medium than DMF (at
140 ꢁC) for the reaction (compare entries 1 and 6) [2] but, never-
theless, Cu(OTf)2 showed complete conversion and selectivity for 3
in DMF (entry 2). CuCl2 also gave a good conversion and selectivity
(entry 3) and the corresponding Fe(III) catalysts worked poorly
(entries 4, 5). In dioxane, a slight decrease in activity was observed
Entry Conditions Catalyst
Conversion (%)a 3 (%)a 4 (%)a
1
2
3
4
5
6
7
8
A
non-catalyzed
Cu(OTf)2
CuCl2
Fe(OTf)3
FeCl3
non-catalyzed
Cu(OTf)2
CuCl2
10
100
81
60
25
88
81
77
100
92
96
64
100
40
87
39
72
91
5
e
e
4
e
e
7
95
71
40
e
B
51
72
61
95
87
77
50
77
7
9
17
e
e
6
4
6
6
5
e
e
e
5
9c
10b
11c
12d
13
14
15e
16
17
18
19f
(CuOTf)2$toluene
CuCl
FeCl3
Fe(OTf)3
Fe(NTf2)2
Ag2CO3
AgOTf
70e
33
32
86
80
MnCl2$4H2O þ 2AgOTf 100
a
NMR yield, double-checked with GC-MS yield. Both techniques differentiate
quantitatively 3 and 4.
b
Run with 10 mol% of catalyst.
catalyst 5 mol%.
0.5 mol%.
c
d
e
26% of 3 corresponding to another isomer, assigned by GC-MS to the Markov-
nikov product.
Fig. 1. EPR spectra of Cu(OTf)2 in dioxane (a) before and (b) after addition of 2 at room
temperature.
f
Catalyst pre-formed by stirring at rt during 30 min before addition of reactants.

364
A. Leyva-Pérez et al. / Journal of Organometallic Chemistry 696 (2011) 362e367
for the copper(II) salts (entries 7, 8). At this point, we performed in-
situ EPR and NMR experiments of the catalytic system Cu(OTf)2/
dioxane (Fig. 1). The EPR spectrum of Cu(OTf)2 shows a para-
magnetic signal corresponding to Cu(II). The signal disappears after
the addition of 2 at room temperature, the product 5 being
concomitantly formed as indicated by 31P NMR spectroscopy
(signal at w20 ppm). These data suggest that the formation of Cu(I)
and 5 is the result of a redox process between Cu(OTf)2 and 2. In this
sense, it has been reported that In(C5H5)3 and 2 react to produce the
corresponding In(C5H5) complex and Ph2P-PPh2 as oxidation
product [15,16]. Therefore, EPR experiments show that Cu(I) should
be the true catalyst in the phosphination reaction.
2.2. Formation of phosphine oxides
Shifting the selectivity from the phosphine 3 to the phosphine
oxide 4 would be of interest since phosphine oxides have biological
Table 3
Scope and limitations of the hydrophosphination of alkenes catalyzed by
(CuOTf)2$toluene complex.
Accordingly, (CuOTf)2$toluene and CuCl showed an excellent
activity as catalysts for the hydrophosphination of 1 with 2 (Table 1,
entries 9e13) even at room temperature (entry 10) [17]. Iron salts
worked poorly in general (entries 14e16) but, curiously, an isomer
of 3 was obtained in significant amount (entry 15). GC-MS and 31P
NMR suggests that this isomer could be the Markovnikov product,
in accordance with the superior ability of Fe(III) to activate benzylic
positions [2,18]. Anyway, FeCl3 and Fe(II) worked poorly again
(entries 14 and 16) as catalyst. Although, perhaps, Fe(III) could be
a real catalyst under these new reaction conditions, it cannot be
concluded. Silver triflate worked well as catalyst (entry 17),
although was less effective than Cu(I) (compare entries 9 and 17).
Silver carbonate did not work.
Entry
Alkene
Phosphine
Product yield (%)a
95 [83]
1b
1
6
2
3
92/83
7
F
98/92 [72]
ICP-AES analysis of Fe(OTf)3 and Fe(NTf2)2 [2,19] were per-
formed (Table 2) and they shows that the copper and silver
impurities are extremely low in both salts. Mn(II), the main metal
impurity of Fe(OTf)3 by ICP-AES analysis and detected by EPR
measurements (six signals with relative intensity 1:1:1:1:1:1 at
g ¼ 2.0066 and hyperfine coupling constant of 96G, IMn ¼ 5/2), and
not present in Fe(NTf2)2, also works as catalyst but worse than Cu(I)
(compare entries 9 and 19). Therefore, results point out to
(CuOTf)2$toluene as the catalyst of choice. The amounts of 4 are low
in all cases. Compounds 3 and 4 were isolated in order to assure the
correctness of NMR assignations, since both products have only
slight differences [20].
8
F
C
4
3
85/88
9
MeO
5
6
>99/>99 [80]
10
83/89
Ph2PH
The scope of the Cu(I)-catalyzed hydrophosphination of alkenes
was then studied (Table 3). Styrenes 1, 6e11 react well with 2 to
afford exclusively the anti-Markovnikov product in good to excel-
lent yields, regardless their electronic nature (compare entries 1e5)
or the position of the substituent on the ring (entries 6, 7). 1H- and
31P NMR yields fit, confirming that no polymerization of the styrene
derivative occurs under the reaction conditions.
2
11
7b, c
87
NO
2
8
19
16
12
Disubstituted alkenes react sluggishly (entries 8 and 9) or do not
react at all (entry 10). 1-Octene 15 also reacts poorly (entry 11).
However, dialkyl phosphine 16 reacts with styrene derivatives in
moderate yields (entries 12, 13) and no oxidized product were
observed [20]. Finally, when both counterparts are alkylic, no
reaction is observed (entry 14).
9d
13
10e
11e
<5
Table 2
14
ICP-AES analysis of the salts Fe(OTf)3 and Fe(NTf2)2.
Metal
Fe(OTf)3
Fe(NTf2)2
wt% (x 104)
% respect to Fe
wt% (x 104)
% respect to Fe
15
16
Fe
Mn
Na
In
108727
377
294
170
69
100
0.35
0.27
0.16
0.06
0.03
e
110591
5
100
e
12b
13e
14e
1
9
15.
62
29
<5
Cy2PH
854
246
105
9
<1
<1
792
<1
1939
0.77
0.22
0.09
e
16
Al
Cu
Ga
Co
Zn
Ag
K
27
7
1
<1
<1
<1
e
31P NMR/1H NMR yields; between brackets, isolated yields.
31P NMR yield, confirmed by GC-MS.
72% phosphine oxide product.
a
e
e
b
c
e
0.71
e
e
d
e
1H NMR yield, confirmed by GC-MS.
31P NMR yield.
e
1.75

A. Leyva-Pérez et al. / Journal of Organometallic Chemistry 696 (2011) 362e367
365
Table 4
Metal-catalyzed hydrophosphination of 4-chlorostyrene 1 with diphenylphosphine
2 under an oxygen atmosphere.
Entry Catalyst
Conversiona 3a 4a
(CuOTf)2$toluene 100 69
20 13b
5a 17a 18a Othersa
1
e
e
e
27
20
4
18
50
e
2b
3
Fe(OTf)3
non-catalyzed
100
100
29
20
e
5
12 13
a
GC-MS yield.
b
Tentatively assigned to the Markovnikov addition product.
O2 (2-3 eq.)
5
1
A)
+
18
4
+
1,4-dioxane (2 M)
100 ºC, 24 h
(1 eq.)
(1 eq.)
Fig. 2. Kinetic plot-yield for the hydrophosphination of 1 with 2 in the presence (A) or
not (B) of (CuOTf)2$toluene (20 mol%).
8 %
6 %
O2 (1.5 eq.)
alkyldiaryl phosphines is of interest [25] and
b-ketophosphine
1,4-dioxane (0.5 M)
oxides have been studied by the Warren’s group and others and are
useful intermediates for the preparation of ketones [26,27], asym-
metric cyclopropanations [28], electrophile addition [29] and more
[30]. We were wondering if 18 comes from a double oxidation of 3
(in accordance to the in-situ NMR experiments, see entry 5 in
Table S1) or, alternatively, from a possible addition of Ph2P(O)H 5 to
1, to form 4, and later benzylic oxidation occurs, since Hirai and Han
have reported an air-induced hydrophosphorylation to alkenes
[23]. Phosphine oxide 5 would be formed in-situ from the oxidation
of 1. To check these hypotheses, two new experiments were carried
out (Scheme 3).
The reaction of 1 with 5 was performed under similar conditions
to those in Table 4 (Scheme 3, A). The hydrophosphorylation of 1
only proceeds in minor extent, showing that 18 does not come from
a previous addition of 5 to 1 [23]. Moreover, 4 was completely
recovered after treatment with oxygen (Scheme 3, B), which also
discards a benzylic oxidation of 4. The oxidation of phosphines [31]
is a more favoured process than the benzylic oxidation under non-
B)
4
18
80 ºC, 16 h
NO REACTION
Scheme 3. Study on the formation of the b-ketophosphine oxide 17.
and medicinal properties [21,22]. Although secondary phosphine
oxides can add directly to alkenes (hydrophosphorylation)
[21,23,24], the corresponding phosphines are usually less expen-
sive. Thus, the metal-catalyzed hydrophosphination of 1 with 2 was
performed under a stoichiometric amount of O2 to oxidise in-situ 3
to 4 (Table 4).
Indeed, the use of (CuOTf)2$toluene in catalytic amount allows
obtaining the phosphine oxide 4 in good yields, although benzylic
oxidation to form 17 competes to some extent (entry 1). b-Keto-
phosphine oxide 18 is formed in the presence of Fe(OTf)3 (entry 2)
and also under non-catalyzed conditions (entries 2, 3), although the
product distribution is different. The synthesis of b-functionalized
Table 5
Hydrophosphination of 4-chlorostyrene 1 with diphenylphosphine 2 under different reaction conditions.
Entry
(CuOTf)2$toluene
Catechol (eq.)
Comments
Conversion (%)a
Product 3 (%)b
1
2
3
4
5
6
7
8
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
e
e
e
1
Standard conditions
N2 þ deaerated solvent
77
85
73
47
60
87
66
94
53
40
30
82
62
54/62
75/73
58/73
36/38
40/45
78/73
47/63
81/90
33/36
24/22
16/22
60/57
42/38
Darkness þ radical inhibitor
Darkness þ deaerated solvent
Radical inhibitor
1
e
e
1
1
3
3
1
1
9
10
11
12
13
D2O (1 eq.) þ radical inhibitor
a
31P NMR conversion.
b
31P/1H NMR yields.

366
A. Leyva-Pérez et al. / Journal of Organometallic Chemistry 696 (2011) 362e367
10 mol%) was placed in a 1.5 mL vial and anhydrous 1,4-dioxane
(0.25 mL), 4-chlorostyrene 1 (60.0 L, 0.5 mmol) and diphenyl-
m
phosphine 2 (86.0 L, 0.5 mmol) were sequentially added. The vial
was sealed and the mixture was placed in a pre-heated oil bath at
100 ꢁC and magnetically stirred for 18e24 h. An aliquot was taken
for GC-MS and the rest was concentrated under reduced pressure,
re-dissolved in CDCl3 and analysed by NMR. When d8-1,4-dioxane
was used, the mixture was directly analysed by NMR. For isolation,
the mixture was passed through a pad of silica, eluted with diethyl
ether and the volatiles were removed under vacuum. When the
products were chromatographied on silica under nitrogen,
a meaning loss of yield was observed (nearly 70% from the expected
Fig. 3. Kinetic plot-yield for the hydrophosphination of the styrene derivatives 9 (A), 6
(B), 1 (C) and 8 (D) with 2 in the presence of (CuOTf)2$toluene (20 mol%) For reaction
conditions see Fig. 2.
by NMR). 31P assignments (
d
, ppm, H3PO4 as reference): ꢀ41
(2) [32], ꢀ10.3 (3, isolated, see Supplementary material data)
[25], þ31.7 (4, isolated, see Supplementary material data), þ18 (5)
[33], þ26.8 (17, isolated, see Supplementary material data) [26],
þ25.8 (PH2POOH) [17], ꢀ13.6 (Ph2P-PPh2) [15]. For those experi-
ments where iron or manganese catalysts were used, the mixture
was passed through a pad of silica (AcOEt/ether (20 vol%) as eluent)
after the reaction. With degassed solvent: d8-1,4-dioxane was
degassed by bubbling dry N2 during 15 min. Under darkness: the
vial was wrapped with aluminium foil. With radical inhibitor: 4-tert-
butylcatechol was added together with the catalyst.
Under oxygen atmosphere: the reaction was performed at half-
scale in a double-walled conical 2 mL vial equipped with a sealable
cap and connected to a manometer. Oxygen was purged three times
at room temperature and finally loaded to 5e6 atm. (e0.6 mmol,
2.2 eq.). After reaction, the oxygen was removed and the mixture
analysed as above.
catalyzed conditions, so the formation of 18 from 17 seems quite
disfavoured. Thus, we can only speculate about a radical mecha-
nism promoted by molecular oxygen.
2.3. Insights into the mechanism
In order to elucidate if copper(I) is active as a catalytic Lewis
acid or is simply activating traces of O2 for radical catalysis [23],
a set of experiments were carried out (Table 5 and Figs. 2 and 3).
d8-1,4-dioxane was employed as solvent to avoid any further
manipulation.
Degassing of the solvent, reaction under darkness, use of 4-tert-
butylcatechol as radical inhibitor or combination of these condi-
tions lead generally to a decrease in the conversion and product
yield. Remarkably, if Cu(I) was used as catalyst, the yield is
systematically 20e40% higher (compare entries 1e11). Addition of
water (as D2O) has a minor influence on the reaction (compare
entries 8, 9 and 12, 13). Kinetic experiments were then carried out
(Figs. 2 and 3). It can be clearly seen that the presence of Cu(I) in the
reaction medium (containing the radical inhibitor) boosts the
initial rate of the reaction in nearly two orders of magnitude (TOFs:
9.5 hꢀ1 vs 0.3 hꢀ1), giving a better final yield.
Kinetics. (CuOTf)2$toluene complex (128 mg, 20 mol%) and
4-tert-butylcatechol (415.5 mg, 2.5 mmol) were placed in a 10 mL
round-bottomed flask. Air was evacuated under vacuum and
nitrogen was introduced. A septum rubber was finally fitted with
a N2 balloon and anhydrous 1,4-dioxane (1,25 mL), the corre-
sponding styrene derivative (2.5 mmol) and diphenylphosphine
(430
mL, 2.5 mmol) were added by syrinnge. The mixture was
placed in a pre-heated oil bath at 100 ꢁC and magnetically stirred.
Aliquots of 0.15 mL were periodically taken, cooled with 0.5 mL of
CDCl3 and analysed by 31P NMR.
The influence of the electronic nature of the substituent on the
aryl ring of the styrene derivative was also studied (Fig. 3), showing
that electron-donor groups accelerate the reaction while electron-
withdrawing groups retard it.
Acknowledgments
These results suggest a charged intermediate in the transition
state, which would also confirm the catalytic activity of Cu(I) as
Lewis acid. When representing the initial rates in a Hammett plot
the correlations are not lineal for any value of s, which infers than
contribution to the reaction state of the radical pathway occurs
concomitantly.
A. L.-P. thanks CSIC for a contract under the JAE-doctor program.
J. R. C.-A. thanks MICINN for the concession of a FPU pre-doctoral
fellowship. Financial support by the Consolider-Ingenio 2010
(proyecto MULTICAT) abd by the PLE2009 project from MCIINN are
acknowledged.
3. Conclusions
Appendix A. Supplementary material data
The first metal salt-catalyzed hydrophosphination of styrenes
has been accomplished. (CuOTf)2$toluene is the catalyst of choice,
although the more stable CuCl can also be used. The Cu(I) active
species is generated from Cu(OTf)2 by a redox reaction with the
phosphine. Mn(II) and Ag(I) are also active catalysts. The anti-
Markovnikov products are exclusively formed in good yields.
General methods, characterisation and NMR copies of
compounds 3, 4 and 17. Supplementary data related to this article
can be found online at doi:10.1016/j.jorganchem.2010.09.069.
References
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molecular oxygen by adding or not (CuOTf)2$toluene in catalytic
amounts. Kinetic studies show that the electronic density of the
ring influences the rate of the reaction.
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Supplementary material data). (CuOTf)2$toluene complex (51.4 mg,

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