TMSCl-promoted electroreduction of triphenylphosphine oxide to triphenylphosphine

Article abstract of DOI:10.1055/s-0030-1259529

Direct reductive transformation of triphenylphosphine oxide to triphenylphosphine was performed successfully by electrolysis with TMSCl in an acetonitrile/BuBr/(Zn anode)-(Pt cathode)/undivided cell/constant current electrolysis system. A plausible ECEC mechanism involving the formation of silylated phosphorus radical is proposed. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0030-1259529

582
LETTER
TMSCl-Promoted Electroreduction of Triphenylphosphine Oxide to
Triphenylphosphine
Hideo Tanaka,*a Tomotake Yano,a Kazuma Kobayashi,a Syogo Kamenoue,a Manabu Kuroboshi,a Hiromu
Kawakubo*b
a
The Graduate School of Natural Science and Technology, Okayama University, Tsushima-Naka 3-1-1, Okayama 700-8530, Japan
Fax +81(86)2518079; E-mail: tanaka95@cc.okayama-u.ac.jp
API Department, Asahi Kasei Chemicals Corporation, Kanda Jinbocho 1-105, Chiyoda-ku, Tokyo 101-8101, Japan
b
Received 23 November 2010
Electroreduction12 of 2 would be a promising alternative
without use of such reductants; thereby, providing a
cheap, environmentally benign, and straightforward ac-
cess to 1. Though a few reports on the electroreduction of
Abstract: Direct reductive transformation of triphenylphosphine
oxide to triphenylphosphine was performed successfully by elec-
trolysis with TMSCl in an acetonitrile/Bu4NBr/(Zn anode)–(Pt
cathode)/undivided cell/constant current electrolysis system. A
plausible ECEC mechanism involving the formation of silylated 2 have been reported so far,13 the desired product 1 could
phosphorus radical is proposed.
not be obtained efficiently: C–P bond cleavage mainly oc-
curred to give diphenylphosphine oxide (4), diphe-
nylphosphine (5), phenylphosphine oxide (6), benzene,
etc. (Scheme 2).
Key words: reduction, electron transfer, phosphorus, silicon
Triphenylphosphine (1) is an important reagent for vari-
ous modern organic synthesis, such as Wittig reaction,1
Mitsunobu reaction,2 Mukaiyama–Corey lactonization,3
Appel reaction,4 and Staudinger reaction.5 In these reac-
tions, 1 is turned into stable and flame-resistant triphe-
nylphosphine oxide (2); as the result, a significant amount
of 2 is stored as a troublesome waste. From the view point
of treatment of the waste as well as saving phosphorus re-
sources, recycle use of 1 via reduction of 2 is a keen
project (Scheme 1).
t-BuOH, [Bu3NMe]+OMs–
Ph
H2NCH2CH2NH2
+
2
H
P
O
(GC)–(GC), undivided cell
CPE (50 V), 4 F/mol
Ph
4 (2%)
Ph
P
Ph
benzene (25%)
cyclohexadiene (35%)
cyclohexene (1%)
+
+
H
H
P
O
Ph
H
5 (4%)
6 (8%)
Scheme 2 Direct electroreduction of 2
Wittig reaction, Mitsunobu reaction, etc.
Recently, we reported indirect transformation of 2 to 1 via
triphenylphosphine dichloride (3) by chemical14 and elec-
trochemical reduction (Scheme 1).15 In our continuing
study on the reduction of 2 to 1, we found that direct elec-
troreduction of 2 proceeded smoothly in the presence of
trimethylsilyl chloride (TMSCl) to give 1 in good yield
(Scheme 3). Herein is described the first practically sig-
nificant procedure for direct electroreductive transforma-
tion of 2 to 1.
aluminium hydrides,
Ph
Ph
Ph
Ph
silyl hydrides, etc.
:
P
P
O
Ph
Ph
1
2
Ph
Cl
+ 2 e–
(COCl)2
Ph
P
Cl
Ph
3
Scheme 1 Recycle use of triphenylphosphine
Ph
Ph
+ e–, Me3SiCl (6 mmol)
Ph
Ph
Ph
P
P
O
Reduction of 2 to 1 has been performed with several re-
ductants, such as metal hydrides (hydrosilanes,6 alumini-
um hydrides,7 etc.), low-valent metals (SmI2/HMPA,8
TiCp2Cl2/Mg,9 etc.), and organic reductants (hydrocar-
bon/activated carbon,10 hexaethylphosphorus triamide/
phosphoryl trichloride,11 etc.). These procedures are,
however, not practical since they always require stoichio-
metric amount of reducing agents which are expensive,
explosive, and/or not easy to handle.
Bu4NBr (1 mmol), MeCN (5 mL)
(Zn)–(Pt), undivided cell
25 mA, 2–3 F/mol
Ph
1
2
67–89%
0.5 mmol
Scheme 3 TMSCl-promoted electroreduction of 2
A typical procedure for the electroreduction of 2 is as
follows: A mixture of 2 (0.5 mmol), TMSCl (6 mmol),
tetrabutylammonium bromide (Bu4NBr, 1 mmol), and ac-
etonitrile (MeCN, 5 mL) was placed in an undivided cell
fitted with a zinc anode and a platinum cathode (1.5 × 1
cm2, each). The mixture was electrolyzed under constant-
current conditions (25 mA) at room temperature. After
passage of 2 F/mol of electricity, the desired product 1
SYNLETT 2011, No. 4, pp 0582–0584
x
x.
x
x.
2
0
1
1
Advanced online publication: 08.02.2011
DOI: 10.1055/s-0030-1259529; Art ID: U10210ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Electroreduction of Triphenylphosphine Oxide to Triphenylphosphine
583
2
was obtained in 67% yield together with 4 (13%) and re-
covered 2 (13%). The yield of 1 increased up to 89% when
3 F/mol of the electricity was passed, whereas 12% of 5
was obtained by passage of 4 F/mol of electricity. Proper
choice of the anode material is significant. Among the an-
ode materials thus far examined, magnesium and alumin-
ium anodes were also used effectively to give 1 in 59%
and 55% yields (2 F/mol), respectively, whereas no appre-
ciable amount of 1 was obtained with platinum, nickel,
tin, and SUS304 anodes.
+ e–
O–
Ph
Ph
C–P bond cleavage
Ph P–
•
O
Ph
Ph
P
•
8
Ph
7
Me3Si Cl
4 + benzene
Ph
•
Ph
P
O
SiMe3
SiMe3
Ph
5 + 6
+ cyclohexadiene
+ cyclohexene
9
Presence of TMSCl is essential for the reduction of 2
(Figure 1); thus, in the absence of TMSCl, diphenylphos-
phine oxide (4) was obtained almost as a sole product
(78%). The presence of 0.5–3 mmol of TMSCl increased
the yield of 1 from 9% to 70% and decreased that of 4
from 48% to 2% yields. Further addition of TMSCl result-
ed in no appreciable change of the products. Similar re-
sults were obtained with an Mg anode.
+ e–
O
Ph
Ph P–
Ph
Me3Si
P–O bond cleavage
Me3Si SiMe3
Cl
10
1
+
O
Scheme 4 A plausible mechanism of direct electroreduction of 2 to
1 in the presence of TMSCl
silyl oxyphosphorus radical 9 and further one-electron re-
duction followed by P–O bond cleavage is proposed. Ap-
plication of the TMSCl-promoted electroreduction to
reductive P–O bond cleavage of other phosphorus com-
pounds is under our investigation.17
Acknowledgment
Figure 1 Electroreduction of 2 to 1 in the presence of TMSCl.
Reagents and conditions: 2 (0.5 mmol), TMSCl (as shown), Bu4NBr
(1 mmol), MeCN (5 mL), (Zn anode)–(Pt cathode), undivided cell, 3
F/mol, r.t.
The SC-NMR Laboratory of Okayama University is thanked for
recording 31P NMR.
References and Notes
In place of TMSCl, silylating reagents such as tert-
butyldimethylsilyl chloride, triisopropylsilyl chloride,
and trimethylsilyl triflate (6 mmol) were used efficiently
to give 1 in 65%, 62%, and 66% yields, respectively, after
2 F/mol of electricity was passed. On the other hand,
p-toluenesulfonyl chloride, mesyl chloride, benzoyl chlo-
ride, and acetic anhydride gave no appreciable amout of 1.
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H. Tanaka et al.
LETTER
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Synlett 2011, No. 4, 582–584 © Thieme Stuttgart · New York

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