A catalytic method for the reduction of secondary and tertiary phosphine oxides

Article abstract of DOI:10.1055/s-2007-982536

TMDS has been found to be an efficient hydride source for the reduction of tertiary and secondary phosphine oxides using a catalytic amount of Ti(Oi-Pr)₄. All classes of tertiary phosphine oxides, such as triaryl, trialkyl, and diphosphine, were effectively reduced. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-982536

LETTER
1545
A Catalytic Method for the Reduction of Secondary and Tertiary Phosphine
Oxides
R
eduction of Secondar
i
y
and
T
k
ertiary
P
hos
a
phine
O
xid
ë
es l Berthod,a Alain Favre-Réguillon,a,b Jahjah Mohamad,a Gérard Mignani,c Gordon Docherty,d Marc Lemaire*a
a
Laboratoire de Catalyse et Synthèse Organique, Institut de Chimie et Biochimie Moléculaires et Supramoléculaires (ICBMS), CNRS,
UMR5246, Université Lyon 1, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne, France
Fax +33(4)72448507; E-mail: marc.lemaire@univ-lyon1.fr
b
c
d
Laboratoire de Chimie Organique, UMR 7084, Conservatoire National des Arts et Métiers, 2 Rue Conté, 75003 Paris, France
Rhodia, Lyon Research Center, 85, Avenue des Frères Perret, BP 62, 69192 Saint-Fons Cedex, France
Rhodia, Novacare, P.O. Box 80, Trinity Street, Oldbury, B69 4LN, UK
Received 26 March 2007
Hydrosiloxane derivatives have emerged as potent reduc-
ing reagents. However, these compounds are not suffi-
ciently potent hydride sources themselves and require
activation. The combination of titanium(IV) isopropoxide
with triethoxysilane efficiently converted esters to the
corresponding primary alcohols.14 The first example of
catalytic reduction of phosphine oxides was described by
Coumbe et al.15 using this reducing system in THF. Two
equivalents of triethoxysilane are needed for successful
conversion. The proposed catalytic cycle involves reduc-
tion by a titanium hydride species generated by hydride
transfer from the silane to titanium via a s-bond meta-
thesis process.15,16 However, triethoxysilane can be dis-
proportionated by titanium(IV) isopropoxide to form
SiH4, a pyrophoric gas.17 Polymethylhydrosiloxane (PM-
HS)18 (Figure 1) which is a free-flowing liquid polymer,
soluble in most organic solvents and inert to air and mois-
ture, can be a suitable substitute for (EtO)3SiH, eliminat-
ing the risk of generating SiH4, but involved the used of a
stoichiometric amount of Ti(Oi-Pr)4.15 The low reactivity
of this system towards phosphine oxide reduction may be
explained by the polymeric nature of the hydride source.
Another disadvantage connected to the use of PMHS was
the formation of a gel during the workup that may limit the
recovery of the product.
Abstract: TMDS has been found to be an efficient hydride source
for the reduction of tertiary and secondary phosphine oxides using
a catalytic amount of Ti(Oi-Pr)4. All classes of tertiary phosphine
oxides, such as triaryl, trialkyl, and diphosphine, were effectively
reduced.
Key words: phosphorus, reductions, titanium, hydrosiloxane,
phosphine oxides
Transition-metal complexes incorporating phosphines
and diphosphines as the ligands are frequently used cata-
lysts in hydrogenation, hydroformylation, hydrocyana-
tion, allylic substitution, hydrosilylations, and palladium-
and nickel-catalysed coupling reactions.1 The reduction of
phosphine oxides in the final stage of a synthetic route
constitutes a widely used synthesis of phosphine ligands.
Indeed, because of the ease of interconversion of phos-
phine oxides and phosphines, it is possible to consider the
oxo function as a protecting group on phosphorus.2
Consequently, the search for safe and practical reducing
reagents of phosphine oxides is of great importance.
A survey of the literature3 showed a number of possible
methods for the reduction of phosphine oxide. The exist-
ing reducing agents, though efficient in producing high
yields of desired product, are pyrophoric such as metal hy-
dride,4 or harmful reagents such as silylhydride reagents.5
Furthermore, these reagents, which have remained the
reagent of choice for reduction of phosphine oxides, are
used at temperatures far above their boiling points.6 Thus,
alternative reductions have been studied using an oxygen
acceptor, such as chlorinated methyl disilanes,7 hexachlo-
rosilane,8 POCl3/(Et2N)3P,9 or a CO/Cl2/H2 system.10 It
was shown that the deoxygenation step with sterically hin-
dered or electron-deficient phosphine oxides can be im-
proved using an oxygen acceptor, such as Ph3P or (EtO)3P
in the presence of HSiCl3.11 Stoichiometric reagents, such
as SmI2/HMPA12 or transition-metal-based systems13
have also been used for the reduction of tertiary phosphine
oxides.
Me
Si
Me
Si
H
Me
Si
H
Si
Si
Me
Me
O
O
O
O
40
Me
Me
Me
Me
H
MeMe
Si
MeMe
Si
O
H
H
Figure 1 Structure of the polymethylhydrosiloxane (PMHS) and
tetramethyldisiloxane (TMDS)
It therefore appeared reasonable that by using hydrosil-
oxane oligomers activated by an efficient catalyst we
could significantly streamline the reduction of phosphine
oxides. By switching to lower molecular weight commer-
cially available hydrosiloxane, such as 1,1,3,3-tetrameth-
yldisiloxane (TMDS, Figure 1) we could expect a more
efficient catalytic process.19 This idea was tested in a
model study involving the reduction of triphenyl-
phosphine oxide in the presence of catalytic amounts of
Ti(Oi-Pr)4 in an aliphatic solvent (Table 1).
SYNLETT 2007, No. 10, pp 1545–1548
1
8
.0
6
.2
0
0
7
Advanced online publication: 07.06.2007
DOI: 10.1055/s-2007-982536; Art ID: G09407ST
© Georg Thieme Verlag Stuttgart · New York

1546
M. Berthod et al.
LETTER
Table 1 Reduction of Triphenylphosphine Oxide by Hydrosilane with a Catalytic Amount of Ti(Oi-Pr)4
hydroxysilane
Si–H:P=O = 2.5
Ph
Ph
Ph
Ph
O
P
Ph
P
Ph
Ti(Oi-Pr)4 (10 mol%)
Entry
Hydrosiloxane
PMHS
Solvent
Temp (°C)
67
Conditions
flask
Yield (%)a
1
2
THF
25
56
TMDS
PMHS
THF
67
flask
3
THF
67
sealed tube
sealed tube
sealed tube
sealed tube
sealed tube
sealed tube
sealed tube
sealed tube
flaskc
25
4
TMDS
TMDS
PMHS
THF
67
50
5
methylcyclohexane
methylcyclohexane
toluene
67
77
6
67
19
7
TMDS
PMHS
100
100
100
100
100
100
87
8
toluene
9
TMDS
PMHS
methylcyclohexane
methylcyclohexane
methylcyclohexane
100b
90
10
11
TMDS
100
a As calculated from 31P NMR.
b Isolated yield.
c TMDS was added in four portions.
As a prelude to that goal, we re-examined the reduction of titanium- and zirconium-based catalysts [Cp2TiCl2;
triphenylphosphine oxide using PMHS (Table 1, entry 1) Zr(Oi-Pr)4–HOi-Pr; Zr(OEt)4; TiCl4) but from our work
according to Coumbe et al.15 A 10 mol% catalytic amount no reduction of triphenylphosphine oxides could be
of Ti(Oi-Pr)4 refluxed in THF for 5 hours gave only mod- observed.
erate conversion (25%) justifying the use of the stoichio-
We decided to evaluate the scope of this new protocol
metric amount of Ti(Oi-Pr)4.15 Starting conditions using
with other phosphine oxides (Table 2).22 Diphosphine
TMDS (Table 1, entry 2) revealed encouraging reaction
oxides were effectively reduced to give diphosphine in
features, i.e. moderate yields (56%) under the same con-
excellent yields (Table 2, entries 1–3). We highlight the
ditions. Interestingly enough, no phosphinated side prod-
milder and safer conditions used for reducing phosphine
uct or intermediary was detected during the course of the
oxides employing HMDS/Ti(Oi-Pr)4 (110 °C) rather
reaction. Moreover, the boiling point of TMDS (71 °C) is
2
than HSiCl3/Et3N,23 PhSiH3/HSiCl3 or HSiCl3/Ph3P
close to the reaction temperature and therefore suspected
methods.11
TMDS evaporation. Thus, the reaction was done in a
This is particularly important in the reduction of the chiral
BINAPO compound (Table 2, entry 4) where no racemi-
sation was observed and the BINAP was obtained in high
yield.24 At the end of the reaction, the reaction mixture
was cooled and filtered to give the diphosphine as a pure
solid.21
Trialkylphosphine oxides25 (Table 2, entries 5 and 6) and
secondary phosphine oxides26 (Table 2, entries 7 and 8)
are also efficiently reduced by Ti(Oi-Pr)4/TMDS. 31P
NMR analyses showed a completed conversion after 10
hours at 100 °C in methylcyclohexane. Pure trialkylphos-
phines and secondary phosphines could be recovered by
distillation or could be isolated as borane complexes.25,26
At the end of the reaction, the reaction mixture was cooled
and BH3–THF was added carefully and aqueous workup
yielded pure borane complexes.21
sealed tube at 67 °C (Table 1, entries 3 and 4).20 As ex-
pected, no improvement was noticed with PMHS (entry
3). The reaction appeared to be solvent- and temperature-
driven (entries 5–10). Switching THF for methylcyclo-
hexane (entry 5) improved the conversion and increasing
the temperature to 100 °C (entry 9) yielded triphenyl-
phosphine in five hours, using 10 mol% catalyst and a
Si–H:P=O ratio of 2.5. The workup was straightforward.
The reaction was cooled and concentrated in vacuum. The
residue was suspended in pentane and insoluble triphen-
ylphosphine was filtrated and washed with pentane.21
Under those conditions, the conversion was 90% using
PMHS (entry 10). Numerous types of catalysts have been
used in combination with PMHS to reduce a wide range
of organic functional groups.16 The most frequently used
catalyst are titanium species. We evaluated different
Synlett 2007, No. 10, 1545–1548 © Thieme Stuttgart · New York

LETTER
Reduction of Secondary and Tertiary Phosphine Oxides
1547
(2) (a) Berthod, M.; Mignani, G.; Lemaire, M. Tetrahedron:
Asymmetry 2004, 15, 1121. (b) Berthod, M.; Saluzzo, C.;
Mignani, G.; Lemaire, M. Tetrahedron: Asymmetry 2004,
15, 639.
(3) (a) For reviews of the literature on organophosphorus
chemistry see all volumes of Organophosphorus Chemistry;
Royal Society of Chemistry: Cambridge, UK, 1970-2006.
(b) Engel, R. The Reduction of Quinquevalent Phosphorus to
the Trivalent State; Engel, R., Ed.; Marcel Dekker, Inc.: New
York, 1992.
Table 2 Reduction of Phosphine Oxides by TMDS with 10 mol%
of Catalyst Ti(Oi-Pr)4
TMDS
Si–H:P=O = 2.5
R1
R1
O
P
R2
P
R1
R1
R2
Ti(Oi-Pr)4 (10 mol%)
methylcyclohexane, 100 °C
Entry
Phosphine
Conversion Yield
(%)a
(%)b
(4) (a) LiAlH4, see: Henson, P. D.; Naumann, K.; Mislow, K.
J. Am. Chem. Soc. 1969, 91, 5645. (b) LiAlH4/CeCl3, see:
Imamoto, T.; Takeyama, T.; Kusumoto, T. Chem. Lett.
1985, 1491. (c) LiAlH4/MeOTf, see: Imamoto, T.; Kikuchi,
S.-I.; Miura, T.; Wada, Y. Org. Lett. 2001, 3, 87. Alane
(AlH3) prepared in situ by addition of concd H2SO4 to a
solution of LiAlH4 in THF, see: (d) Griffin, S.; Heath, L.;
Wyatt, P. Tetrahedron Lett. 1998, 39, 4405. (e) Bootle-
Wilbraham, A.; Head, S.; Longstaff, J.; Wyatt, P.
Tetrahedron Lett. 1999, 40, 5267. (f) DIBAL-H for sec-
phosphine oxides: Busacca, C. A.; Lorenz, J. C.; Grinberg,
N.; Haddad, N.; Hrapchak, M.; Latli, B.; Lee, H.; Sabila, P.;
Saha, A.; Sarvestani, M.; Shen, S.; Varsolona, R.; Wei, X.;
Senanayake, C. H. Org. Lett. 2005, 7, 4277. (g) Phosphine
boranes can be obtained by treatment of the reaction
mixtures with borane–THF or directly by reduction of
phosphine oxides with borane in THF. For the reduction of
tertiary phosphine oxides, see: Köster, R.; Morita, Y. Angew.
Chem., Int. Ed. Engl. 1965, 4, 593. (h) For the reduction of
secondary phosphine oxides, see: Stankevic, M.;
Pietrusiewicz, K. M. Synlett 2003, 1012. (i) Phosphine
boranes could be obtained directly from phosphine oxides by
using a mixture of LiAlH4/NaBH4/CeCl3, see: Imamoto, T.;
Oshiki, T.; Onozawa, T.; Kusumoto, T.; Sato, K. J. Am.
Chem. Soc. 1990, 112, 5244.
O
O
1
2
3
100c
95
P(Ph)2
(Ph)2P
O
O
100c
100c
91
95
(Ph)2P
P(Ph)2
O
O
P(Ph)2
(Ph)2P
O
100c
92
P(Ph)2
P(Ph)2
4
O
O
5
6
100d
100d
90e
95e
(n-C8H17)3P
O
(n-C4H9)3P
O
7
8
100d
100d
85e
89e
Ph
P
H
Ph
O
(5) (a) HSiCl3: Horner, L.; Balzer, W. D. Tetrahedron Lett.
1965, 21, 1157. (b) HSiCl3/NR3: Cremer, S. E.; Chorvat, R.
J. J. Org. Chem. 1967, 32, 4066. (c) PhSiH3: Marsi, K. L.
J. Org. Chem. 1974, 39, 265. (d) Ph2SiH: McKinstry, L.;
Livinghouse, T. Tetrahedron 1994, 50, 6145. (e) Me3SiCl/
LiAlH4: Kyba, E. P.; Liu, S. T.; Harris, R. L.
p-MeOC6H4
p-MeOC6H4
P
H
a As calculated from 31P NMR.
b Isolated yield.
c After 7 h at 100 °C.
Organometallics 1983, 2, 1877.
d After 10 h at 100 °C.
e Isolated as borane complexes.
(6) Trichlorosilane: fp –13 °C, bp 31 °C; phenylsilane: fp 8 °C,
bp 120 °C; diphenylsilane: fp 98 °C, bp 97 °C.
(7) Deleris, G.; Dunogues, J.; Calas, R. Bull. Soc. Chim. Fr.
1974, 3-4, 672.
(8) Naumann, K.; Zon, G.; Mislow, K. J. Am. Chem. Soc. 1969,
91, 7012.
(9) Timokhin, B. V.; Kazantseva, M. V.; Blazhev, D. G.;
Rokhin, A. V. Russ. J. Gen. Chem. 2000, 70, 1310.
(10) Masaki, M.; Kakeya, N. Angew. Chem., Int. Ed. Engl. 1997,
16, 552.
(11) Wu, H.-C.; Yu, J.-Q.; Spencer, J. B. Org. Lett. 2004, 6, 4675.
(12) Handa, Y.; Inanaga, J.; Yamaguchi, M. J. Chem. Soc., Chem.
Commun. 1989, 298.
In conclusion, we have shown that phosphine oxides are
readily converted into the corresponding phosphine in
high yield by the use of air-stable Ti(Oi-Pr)4 and TMDS.
TMDS is more reactive than PMHS and could be pro-
posed as a safer, easy-to-handle hydride source for the
catalytic reduction of secondary and tertiary phosphine
oxides.
(13) (a) Phosphine oxides have been reduced using
stoichiometric amount of MgCp2TiCl2 in boiling THF:
Mathey, F.; Maillet, R. Tetrahedron Lett. 1980, 21, 2525.
(b) Phosphine oxides have been reduced using two equiv of
Schwartz reagent: Zablocka, M.; Delest, B.; Igau, A.;
Skowronska, A.; Majoral, J.-P. Tetrahedron Lett. 1997, 38,
5997.
(14) Berk, S. C.; Buchwald, S. L. J. Org. Chem. 1992, 57, 3751.
(15) Coumbe, T.; Lawrence, N. J.; Muhammad, F. Tetrahedron
Lett. 1994, 35, 625.
References and Notes
(1) (a) Comprehensive Asymmetric Catalysis I–III; Jacobsen, E.
N.; Pfaltz, A.; Yamamoto, H., Eds.; Springer: Berlin, 1999.
(b) Buchwald, S. L.; Mauger, C.; Mignani, G.; Scholz, U.
Adv. Synth. Catal. 2006, 348, 23. (c) Berthod, M.; Mignani,
G.; Woodward, G.; Lemaire, M. Chem. Rev. 2005, 105,
1801.
(16) Lawrence, N. J.; Drew, M. D.; Bushell, S. M. J. Chem. Soc.,
Perkin Trans. 1 1999, 3381.
Synlett 2007, No. 10, 1545–1548 © Thieme Stuttgart · New York

1548
M. Berthod et al.
LETTER
(17) (EtO)3SiH should be used properly since it is volatile and is
known to cause blindness. Furthermore, incident using this
product have been reported, see ref. 12 in: (a) Berk, S. C.;
Kreutzer, K. A.; Buchwald, S. L. J. Am. Chem. Soc. 1991,
113, 5093. (b) Berk, S. C.; Buchwald, S. L. J. Org. Chem.
1993, 58, 3221.
(18) PMHS has already been used to reduce tertiary phosphine
oxides neat at 280–300 °C, see: Fritzsche, H.; Hasserodt, U.;
Korte, F.; Friese, G.; Adrian, K.; Arenz, H. J. Chem. Ber.
1964, 97, 1988.
(19) SAFETY: TMDS is quite stable and not generally
considered as a hazardous material, but under specific
conditions, TMDS can generate high volumes of hydrogen
gas in acid and basic conditions. Thus, all the necessary
precautions for the safe handling of flammable gases should
therefore be observed as mentioned in MSDS. Although
TMDS proved to be stable up to 250 °C in a glass vessel,
small exothermic reactions were observed with metallic
ones. Furthermore, the use of glassware always gives rise to
significantly higher conversion than the use of metal-
containing reactors. This is probably due to the low stability
of the titanium hydride intermediate. Further studies are
being done to confirm this.
yield, ee >98%). 1H NMR (300 MHz, CDCl3): d = 7.93 (2 H,
d, J = 8.5 Hz,), 7.88 (2 H, d, J = 8.1 Hz), 7.51 (2 H, d, J = 8.5
Hz), 7.39 (2 H, t, J = 7.9 Hz), 7.24–7.10 (20 H, m), 6.95 (2
H, dd, J = 8.1 Hz), 6.89 (2 H, d, J = 8.5 Hz). 31P NMR (81
MHz, CDCl3): d = –14.3. [a]D25 –224 (c 0.365, benzene); lit.
[a]D25 –229 (c 0.31, benzene). The ee was determined after
oxidation with H2O2 according to ref. 24b. Chiral column
[Daicel Chiralpak AD, 0.46 cm ∅ ꢀ 25 cm, 254 nm UV
detector, r.t., eluent 75:25 (n-heptane–2-PrOH), flow rate
0.5 mL/min], tR = 14.25 min for S and 18.3 for R. (b)Sekar,
G.; Nishiyama, H. J. Am. Chem. Soc. 2001, 123, 3603.
(25) General Procedure for the Reduction of
Trialkylphosphine Oxides
In a 50 mL dried round-bottomed flask fitted with a
magnetic stirrer and a condenser were placed the phosphine
oxide (5.2 mmol) and methylcyclohexane (5 mL). Then,
TMDS (1.14 mL, 6.5 mmol, 1.25 equiv) and Ti(Oi-Pr)4 (154
mL, 0.52 mmol) were added to the reaction vessel. The
heterogeneous mixture was stirred at 100 °C under an argon
atmosphere. After 10 h, 31P NMR analyses showed the
complete conversion of the starting reagent. The mixture
was cooled down to 0 °C and 1 M BH3·THF (10.4 mL, 10.4
mmol, 2 equiv) was added dropwise to the solution. The
mixture was allowed to warm to r.t. and stirred for 1 h. The
solution was again cooled down to 0 °C and a 3 N alcoholic
KOH solution (10 mL) was added dropwise to the reaction
vessel (caution: abundant foaming). After gas formation has
subsided, the resulting heterogeneous mixture was stirred at
50 °C under an argon atmosphere for 2 h. After cooling, the
mixture was washed with H2O (2 ꢀ 5 mL), a 10% aq solution
of HCl (5 mL), and a sat. aq solution of NaHCO3 (5 mL). The
resulting solution was dried upon MgSO4 and concentrated
under vacuum, yielding a pale liquid containing only pure
phosphine borane.
(20) Ace pressure tube Aldrich Ref. Z181099. We have checked
that under those conditions the internal pressure does not
exceed 1.2 bar.
(21) CAUTION: The TMDS remaining in the filtrate must be
destroyed by slow addition of a 3 M alcoholic solution of
KOH at r.t. TMDS decomposes on contact with bases,
forming hydrogen.
(22) General Procedure for the Reduction of dppe, dppp, and
dppb Oxides
In a 30 mL sealed tube with a magnetic stirrer were placed
the diphosphine (2.32 mmol) and methylcyclohexane (5
mL). Then, TMDS (1.03 mL, 5.8 mmol, 2.5 equiv) was
added to the reaction vessel followed by Ti(Oi-Pr)4 (0.07
mL, 0.23 mmol). The flask was heated at 100 °C. After 7 h,
the 31P NMR analysis showed the complete conversion of
the starting reagent. The heterogeneous mixture was cooled
at 0 °C, filtrated over porous glass and washed with 4 ꢀ 5
mL of pentane. The resulting white solid was dried under
vacuum, yielding the desired compound.
Tri-n-octylphosphine-borane (TOPB): yield 90%. 1H NMR
(300 MHz, CDCl3): d = 1.61–1.20 (42 H, m), 0.95–0.81 (9
H, m). 31P NMR (81 MHz, CDCl3): d = 15.6.
Tri-n-butylphosphine-borane (TBPB): yield 95%. 1H NMR
(300 MHz, CDCl3): d = 1.54–1.35 (18 H, m), 0.90 (9 H, t,
J = 7.1 Hz). 31P NMR (81 MHz, CDCl3): d = 15.6.
(26) General Procedure for the Reduction of Secondary
Phosphine Oxides
1,2-Bis(diphenylphoshino)ethane(dppe): yield 95%, mp
161 °C. 1H NMR (300 MHz, CDCl3): d = 7.32 (20 H, m),
2.13–2.08 (4 H, m). 31P NMR (81 MHz, CDCl3): d = –11.3.
1,3-Bis(diphenylphosphino)propane(dppp): yield 91%, mp
63 °C. 1H NMR (300 MHz, CDCl3): d = 7.39–7.33 (8 H, m),
7.30–7.28 (12 H, m), 2.21 (4 H, t, J = 7.5 Hz), 1.71–1.55 (2
H, m). 31P NMR (81 MHz, CDCl3): d = –16.3.
In a 50 mL dried round-bottomed flask fitted with a
magnetic stirrer and a condenser were placed secondary
phosphine oxide (2 mmol) and methylcyclohexane (5 mL).
Then, TMDS (0.44 mL, 2.5 mmol, 1.25 equiv) and Ti(Oi-
Pr)4 (59 mL, 0.2 mmol) were added to the reaction vessel.
The heterogeneous mixture was stirred at 100 °C under an
argon atmosphere overnight. After 10 h, 31P NMR analyses
showed the complete conversion of the starting reagent. The
mixture was cooled down to 0 °C and 2 M BH3·DMS (3 mL,
6 mmol, 3 equiv) was added dropwise to the solution. The
mixture was allowed to warm to r.t. and stirred for 2 h. The
crude material was concentrated under vacuum and purified
by flash chromatography EtOAc–cyclohexane (5:95) to give
the product as a white solid.
1,4-Bis(diphenylphosphino)butane(dppb): yield 95%, mp
135 °C. 1H NMR (300 MHz, CDCl3): d = 7.44–7.34 (8 H,
m), 7.33–7.30 (12 H, m), 2.04 (4 H, t, J = 7.5 Hz), 1.62–1.54
(4 H, m). 31P NMR (81 MHz, CDCl3): d = –14.9.
(23) Takaya, H.; Akutagawa, S.; Noyori, R. Org. Synth., Coll.
Vol. VIII 1993, 65-69, 57.
(24) (S)-2,2¢-Bis(diphenylphosphino)-1,1¢-binaphtyl
(BINAP) (a) In a tube with a magnetic stirrer was placed (S)-
BINAP oxide (1.44 g, 2.2 mmol, 1 equiv) in 5 mL of
methylcyclohexane. To this heterogeneous mixture was
added TMDS (0.97 mL, 5.5 mmol, 2.5 equiv) and Ti(Oi-Pr)4
(70 mL, 0,22 mmol, 0.1 equiv). The tube was sealed and the
mixture stirred and heated at 100 °C overnight. The hetero-
geneous mixture was cooled at 0 °C and filtrated over porous
glass, washing 4 to 5 times with cold pentane. The resulting
white solid was dried under vacuum, yielding 1.26 g (92%
Diphenylphosphine-borane: yield 85%. 1H NMR (300 MHz,
C6D6): d = 7.44–7.38 (4 H, m), 6.98–6.88 (6 H, m), 5.85 (1
H, d, J = 378 Hz), 2.40–1.36 (3 H, br m). 31P NMR (81 MHz,
C6D6): d = 2.9.
Bis(4-methoxyphenyl)phosphine-borane: yield 89%. 1H
NMR (300 MHz, CDCl3): d = 7.57 (4 H, dd, J = 8.9, 2.3 Hz),
6.96 (4 H, d, J = 8.7 Hz), 6.23 (1 H, d, J = 378 Hz). 31P NMR
(81 MHz, CDCl3): d = –1.54.
Synlett 2007, No. 10, 1545–1548 © Thieme Stuttgart · New York