Catalytic Hydrophosphination of Alkenylalkyl Ethers

Article abstract of DOI:10.1055/s-2003-42092

Hydrophosphination of alkenylalkyl ethers catalyzed by Pd- and Ni-complexes is described. The reaction is regioselective giving only Markovnikov product isolated as α-alkoxyalkyldiphenyl oxide after oxidation by H₂O₂.

Full text of DOI:10.1055/s-2003-42092

LETTER
2155
Catalytic Hydrophosphination of Alkenylalkyl Ethers
Catalytic
H
ydrophosph
a
ination of
A
r
lkenyla
i
lkyl Et
n
hers a A. Kazankova, Mstislav O. Shulyupin, Irina P. Beletskaya*
Department of Chemistry, Moscow State University (MSU), Leninskie Gory 1, building 3, 119992 Moscow, Russia
Fax +7(95)9393618; E-mail: beletska@org.chem.msu.ru
Received 4 August 2003
Abstract: Hydrophosphination of alkenylalkyl ethers catalyzed by
Pd- and Ni-complexes is described. The reaction is regioselective
giving only Markovnikov product isolated as a-alkoxyalkyldiphe-
nyl oxide after oxidation by H2O2.
Key words: nickel, palladium, alkenes, hydrophosphination, al-
kenylalkyl ethers
Equation 1
The results of the model reaction of vinylbutyl ether with
Ph2PH [conversion (%) of Ph2PH measured by 31P NMR]
are given in Table 1. It was shown that the yield of the
product was equal to conversion.
The reaction of diphenylphosphine with alkenes is a sim-
ple and convenient way to alkyldiphenylphosphines and
particularly to functionalized phosphines of this type.1
Hydrophosphination of alkenes is usually carried out us-
ing radical initiators,2 or in the presence of strong bases3
or Brønsted acids.4 In all these reactions R2P-group is usu-
ally directed in b-position of terminal alkenes. Inter- or in-
rtamolecular hydrophosphination of alkenes under the
action of PH3 or R2PH can be catalyzed by platinum and
palladium complexes,5 as well as by complexes of lan-
thanides.6 However Pt- and Pd-complexes catalyze only
the reaction with the activated alkenes, having electron
withdrawing groups (EWG) and R2P-group is always
added to the terminal carbon atom as in the case of radical
reactions.
As can be seen from Table 1 Pd(0)- or Ni(0)-complexes
are inefficient (entries 1, 2, 11, 12). The PdCl2(PPh3)2
complex is more active than PdCl2(CH3CN)2 (entries 5
and 7). But Ni2+-complexes NiBr2(PPh3)2 (entries 14, 15)
and ‘ligandless’ NiBr2 (entry 17) are the most active. The
activity of NiBr2 can be apparently concerned with the
formation of NiBr2(Ph2PH)2. The efficiency of Pd-com-
plexes having secondary phosphine as a ligand has been
recently demonstrated.8
The high activity of Ni2+-complexes allowed us to use
lower concentration of the catalyst and to reduce the reac-
tion temperature and reaction time compared to Pd2+-com-
plexes. The reaction can be carried out even at room
temperature but needs 7 days for completion (entry 14).
The anion in Ni2+-complexes plays an important role be-
cause Ni(acac)2 does not catalyze the reaction (entry 13).
Recently we have performed hydrophosphination of sty-
renes and heteroanalogs of styrene catalyzed by Pd- and
Ni-complexes and have shown that Ni-complexes are
more active than Pd-complexes.7 In all reaction only anti-
Markovnikov products – b-isomers were formed. Appar-
ently aryl groups activate the double bond because alkyla-
lkenes failed to react with Ph2PH under these conditions.
We suppose that this reaction can be considered as a cata-
lytic analog of Michael addition.
We did not observe any significant influence of solvents.
The reaction proceeds equally well in benzene, THF or
MeCN (entry 3, 9, 10).
The fact that this reaction is not catalyzed by zerovalent
nickel complexes, as the reaction of styrenes and alkenes
with EWG, suggests that they have different mechanisms.
Earlier we have shown that the mechanism and regiose-
lectivity of hydrophosphination of alkynes changes in the
presence of acids.9 It is obvious that acids can be formed
by the reduction of PdCl2 and NiBr210 and in such way can
catalyze the addition to nucleophilic alkenes. However we
have shown that in the presence of acids (HCl or
CF3COOH) (entries 19, 20) reaction proceeds very slowly
and the result is the same as for Ni(0)–HCl (entry 21).
A different situation is observed when HCl is added to
Ni(acac)2 (entry 22), probably because of NiCl2 for-
mation.
In this report we have shown that hydrophosphination
catalyzed by nickel and palladium complexes proceeds
easily with alkenes having an electron-donating group –
alkenylalkyl ethers (Equation 1). However in contrast to
hydrophosphination of alkenes with EWG-groups this
reaction is not catalyzed by Pd(0)-and Ni(0)-complexes
but proceeds in the presence of Pd2+ or Ni2+ complexes.
The regioselectivity of the reaction is also different, the
reaction leads to Markovnikov products – a-alkoxyalkyl-
phosphines.
SYNLETT 2003, No. 14, pp 2155–2158
0
6
.1
1
.2
0
0
3
Advanced online publication: 15.10.2003
DOI: 10.1055/s-2003-42092; Art ID: G19803ST
© Georg Thieme Verlag Stuttgart · New York

2156
M. A. Kazankova et al.
LETTER
Table 1 The Influence of the Catalyst Nature and Temperature on
the Reaction of Ph2PH with Vinylbutyl in Benzene
Entry Catalyst
Mol%
catalyst [time (h)]
Temp °C
Conver-
siona
1
2
Pd(PPh3)4
2
100 (8)
120 (8)
100 (8)
80 (15)
100 (8)
100 (8)
100 (8)
100 (8)
100 (8)
100 (8)
120 (8)
80 (8)
0
<1
Pd(PPh3)4
5
3
PdCl2(CH3CN)2
PdCl2(CH3CN)2
PdCl2(CH3CN)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(CH3CN)2
PdCl2(CH3CN)2
Ni(cod)2
5
100
>90
<50
100
90
4
5
5
2
6b
7
2
2
Scheme 1
8
5
95
9c
10d
11
12
13
14
15
16
17
18
19
20
21
22
5
100
100
0
Notice that besides MHal2, HalMPPh2 or M(PPh2)2
formed by the exchange reaction between MHal2 and
Ph2PH, may also participate in the catalytic cycle. The
mechanism explains the formation of Markovnikov
product in the reaction. Proving of this mechanism means
that Wacker-type activation of alkenes can be performed
by Ni2+.
5
5
Ni[P(OEt)3]4
Ni(acac)2
5
0
5
120 (8)
80 (2)
0
NiBr2(PPh3)2
NiBr2(PPh3)2
NiBr2(PPh3)2
NiBr2
5
100
100
100
100
0
The optimum conditions for the hydrophosphination of
vinylbuthyl ether are 2 mol% of NiBr2(PPh3)2 in benzene
at 80 °C. Under these conditions the hydrophosphination
of several alkenylalkyl ethers having alkyl substituents at
a- or b-carbon was carried out (Table 2). As can be seen
from Table 2 the introduction of alkyl group in both posi-
tions has little influence on the yield, which remains mod-
erately high (entries 2–5). The only exception is the cyclic
ether dihydro-2H-pyrane, the reaction of which requires
more rigid conditions (100 °C) and gives the product in a
lower yield (entry 6).
2
80 (2)
2
r.t. (7 d)
80 (2)
2
NiBr2(PPh3)2/Et3N
CF3COOH
5/10
80 (2)
10
10
5/10
5/10
80 (2)
<10
25
HCl
80 (2)
Ni(cod)2/HCl
Ni(acac)2/HCl
80 (2)
25
The synthesized phosphines are readily oxidized upon
isolation and for obtaining good NMR spectra and analyt-
ical data all phosphines (2a–f) were oxidized by 30%
H2O2 to corresponding phosphine oxides (3a–f).
80 (2)
100
a Determined by NMR 31P.
b 1.5 Molar excess of vinyl ether.
c Reaction in THF.
It should be noted that conventional syntheses of a-
alkoxyalkylphosphines are multi step or require reagents
easily hydrolyzed with formation of HCl.12
d Reaction in CH3CN.
The synthesized phosphines (3a–f) and their oxides (4a–
f) are not well-studied compounds. However, their appli-
cation in Horner–Wittig reaction for stereoselective syn-
thesis of functionalized alkenes13 or as complexing agents
for extraction of uranium and thorium salts from acidic
solutions is known.14 Apart from this, these phosphines
having chiral carbon atom can be considered as potential-
ly interesting ligands for transition metal catalysis.
We can assume that the mechanism of this reaction in-
volves activation of double bond by M2+ with subsequent
nucleophilic attack (as at the first step of Wacker process).
But at the next step the protonolysis (the acid may be
complexed by Ph2PH) of carbon-metal bond takes place
instead of b-hydrogen elimination. Probably this reaction
pathway is due to the stabilization of organometallic inter-
mediate by intramolecular M-P bond.11 The necessity of
protonolysis allows to explain the absence of the reaction
in the presence of Ni(acac)2. Another proof of this as-
sumption is the absence of reaction catalyzed by
NiBr2(PPh3)2 in the presence of Et3N (entry 18). The fol-
lowing catalytic cycle can be suggested (Scheme 1).
Conclusion: we proposed a simple and efficient method
for the synthesis of a-alkoxyalkyldiphenylphosphines by
hydrophosphination of alkylvinyl ethers in the presence of
Ni-salts.15
Synlett 2003, No. 14, 2155–2158 © Thieme Stuttgart · New York

LETTER
Catalytic Hydrophosphination of Alkenylalkyl Ethers
2157
Table 2 Hydrophosphination of Vinylethers with Ph2PHa
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Entry
1
Ether
Phosphine oxide
Yield
(%)b
(4) (a) See ref.2d (b) Dombek, B. D. J. Org. Chem. 1978, 43,
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(5) (a) Nagel, U.; Reigel, B.; Bublewitz, A. J. Organomet.
Chem. 1989, 370, 223. (b) Hoye, P. A. T.; Pringle, P. G.;
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(c) Pringle, P. G.; Brewin, D.; Smith, M. B.; Worboys, K. In
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1a
83
85
3a
2
3
4
1b
1c
3b
3c
3d
80
76
1d
1e
Organometallics 2000, 19, 950.
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5
70
40
(b) Douglass, M. R.; Stern, C. L.; Marks, T. J. J. Am. Chem.
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3e
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6c
(8) Schnyder, A.; Aemmer, T.; Indolese, A. F.; Pittelkow, U.;
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1f
3f
a Conditions: vinyl ether(1.65 mmol), Ph2PH (1.5 mmmol) and
Ni(PPh3)2Br2 (2% mol) in benzene (3 mL), 2 hours, 80 °C.
b Isolated yield.
c 8 Hours, 100 °C
Acknowledgment
We thank Trostyanskaya I.G. and Kirichenko E. for the preparation
of starting compounds and The Grant of Russian Federation
President supporting Leading Scientific Schools – 1611.2003.3 for
financial support.
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Synlett 2003, No. 14, 2155–2158 © Thieme Stuttgart · New York

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LETTER
(15) Experimental Conditions: In a 8 mm tube filled with argon
were placed Ni(PPh3)2Br2 (55 mg, 5 mol%), Ph2PH (279 mg,
1,5 mmol), alkenyl ether (1.65 mmol) and dry benzene (3
mL). The tube was sealed and placed in a hot bath. Reaction
time and temperature are given in Table 2. After cooling the
tube was opened and the solvent was evaporated. The
target compound was recrystallized from hexane or purified
by column chromatography (Al2O3, Et2O). 1-Butoxy-
ethyl(diphenyl)phosphine oxide (3a) was recrystallized
from hexane (cooling up to –18 °C), colorless oil. 1H NMR
(400 MHz, CDCl3): d = 0.83–0.85 (t, 3 H), 1.22–1.26 (m, 2
H), 1.45–1.51 (m, 5 H), 3.19–3.25 (m, 1 H), 3.58–3.64 (m, 1
H), 4.19–4.23 (m, 1 H), 7.45–7.58 (m, 6 H), 7.88–7.92 (m, 2
H), 8.05–8.09 (m, 2 H). 31P{1H} NMR (162.6 MHz, CDCl3):
d = 30.0. Anal. Calcd: C, 71.52; H, 7.61. Found: C, 71.35; H,
7.45.
residue was dissolved in 15 mL of Et2O and 2 ml 30% H2O2
was added. The mixture was stirred until the organic phase
discoloured. The organic phase was separated, washed with
water, dried under MgSO4 and evaporated in vacuum. The
Synlett 2003, No. 14, 2155–2158 © Thieme Stuttgart · New York