t-BuOK-catalyzed addition phosphines to functionalized alkenes: A convenient synthesis of polyfunctional phosphine derivatives

Article abstract of DOI:10.1016/S0040-4039(02)01177-2

The use of t-BuOK in DMSO allows a smooth addition of Ph₂PH, Cy₂PH and Ph₂P(O)H to various functionalized alkenes leading to polyfunctional phosphines in good yields. This method has been used to prepare precursors for P,P- and P,N-ligands.

Full text of DOI:10.1016/S0040-4039(02)01177-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 5817–5819
t-BuOK-catalyzed addition phosphines to functionalized alkenes:
a convenient synthesis of polyfunctional phosphine derivatives
Tanasri Bunlaksananusorn and Paul Knochel*
Ludwig-Maximilians-Universita¨t Mu¨nchen, Institut fu¨r Organische Chemie, Butenandtstrasse 5-13, Haus F, D-81377 Mu¨nchen,
Germany
Received 24 April 2002; revised 14 June 2002; accepted 17 June 2002
Abstract—The use of t-BuOK in DMSO allows a smooth addition of Ph2PH, Cy2PH and Ph2P(O)H to various functionalized
alkenes leading to polyfunctional phosphines in good yields. This method has been used to prepare precursors for P,P- and
P,N-ligands. © 2002 Elsevier Science Ltd. All rights reserved.
The preparation of functionalized phosphines is an
important synthetic goal since these can act as ligands
for many metallic salts used for the catalysis of organic
reactions.1 Recently, we have found that catalytic
amounts of t-BuOK in N-methylpyrrolidinone (NMP)
considerably facilitate the addition of various carbonyl
compounds to styrenes2 and other olefins bearing
weakly activating groups like silyl, thio or phosphorus
functions.3 The addition of phosphines to alkenes has
been studied and requires either radical initiation,4
transition metal catalysis5 or strong basic conditions.6
The use of phosphine–borane complexes is also possible
and allows selective hydrophosphination.7
diphenylphosphine (2a) or an aliphatic dialkylphos-
phine like dicyclohexylphosphine (2b), leading to func-
tionalized phosphines of type 3 (Scheme 1 and Table 1).
Thus, styrene (1a) reacts very fast with Ph2PH (2a) (rt,
1 h), leading to 3a in 83% yield (entry 1 of Table 1).
Slightly activated alkenes like the vinylic thioether 1b,
the vinylic silane 1c and the vinylic phosphine 1d react
under similar conditions, furnishing the polyfunctional
phosphines 3b–d (entries 2–4) in 80–90% yield. Hetero-
cyclic compounds bearing a vinyl group also react with
diphenylphosphine leading to potential P,N-ligands
(3e–g; 63–68%, entries 5–7). Interestingly, whereas 2-
vinylpyridine (1e), bearing an unsaturated vinylic imine
unit was expected to react well, we have observed that
the isomeric cross-conjugated 3-vinylpyridine (1f) reacts
equally as fast (rt, 1 h, entry 6). Triethoxyvinylsilane
(1h) reacts in the presence of EtOK (used instead of
t-BuOK in order to avoid alkoxide scrambling), leading
to the phosphine 3h, which can be used to attach a
phosphine unit on silica gel.8 The reaction with Cy2PH
proceeds similarly. However its addition to styrene
produces the phosphine oxide 4. The sensitive interme-
diate dicyclohexylphosphine adduct is oxidized by
DMSO leading to 4 in 73% yield (entry 9).
Herein, we wish to report that catalytic amounts of
t-BuOK (20 mol%) in DMSO allow the hydrophosphi-
nation of functionalized alkenes of type 1 using either
t-BuOK (20 mol %)
R1
+
R22PH
R1
R22P
R2 = Cy
Cy2P
DMSO, rt to 40 °C
1 h
1
2a-b
3
DMSO
R1
R1 = Ph, SPh, SiPh3, PPh2
R1 = 2-Pyridyl, 3-Pyridyl,
O
4
R1 =
We found that these basic conditions also allow addi-
tion of the cyclic hydrogen phosphonate9 5 to styrene
(1a), leading to the adduct 6 in 87% yield (Scheme 2).
Attempts to add diphenylphosphine (2a) to trisubsti-
tuted unsaturated pyridines like 7 led to long reaction
times and the mixtures of the phosphine oxide adduct 9
N
Boc
Scheme 1.
Keywords: addition reaction; phosphine ligands; diphosphines; func-
tionalized phosphines.
and
the
corresponding
phosphine.
However,
diphenylphosphine oxide 8 reacts faster and affords
only the product trans-9 in 50% yield as a single
* Corresponding author. Tel.: +49-89-2180-7679; fax: +49-89-2180-
7680; e-mail: paul.knochel@cup.uni-muenchen.de
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01177-2

5818
T. Bunlaksananusorn, P. Knochel / Tetrahedron Letters 43 (2002) 5817–5819
Table 1. Functionalized phosphines 3a–h and phosphine
oxide 4 obtained by the t-BuOK catalyzed addition of
phosphines 2a–b to functionalized alkenes 1a-h in DMSO
at rt
Scheme 3.
Finally, addition–elimination reactions are also possi-
ble. Remarkably, the reaction of Ph2PH (2a) with b-
methoxystyrene (14) leads to 15, the product of the
substitution of the methoxy groups by the
diphenylphosphine oxide moiety in 86% yield uniquely
as the trans-product.13 We tentatively propose that this
reaction occurs via an addition–elimination mechanism
(Scheme 4).
In summary, we have developed a very convenient
procedure for adding diphenylphosphine to a variety of
functionalized alkenes under mild conditions. Applica-
tions to asymmetric catalysis are underway.14
Acknowledgements
We thank the DFG (Leibniz-Program) and the Fonds
der Chemischen Industrie for generous financial sup-
port. We thank Chemetall GmbH (Frankfurt), the
Degussa AG (Hanau) and BASF AG for the generous
gift of chemicals.
diastereoisomer10 (Scheme 2). 1,2-Diphosphine oxides
can be easily prepared by this method. Thus, the addi-
tion of 8 to the trisubstituted cyclohexenylphosphine
oxide (10) leads to the C2-symmetrical phosphine oxide
1111 in 89% yield (trans:cis>99:1). A similar addition of
8 to the phosphine oxide 12 furnishes the diphenylphos-
phine oxide of rac-renorphos12 (13) in 69% yield
(trans:cis>99:1) (Scheme 3).
Scheme 4.
Scheme 2.

T. Bunlaksananusorn, P. Knochel / Tetrahedron Letters 43 (2002) 5817–5819
5819
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