An expedient reduction of sec-phosphine oxides to sec-phosphine-boranes by BH3·SMe2

Article abstract of DOI:10.1055/s-2003-39304

Secondary phosphine oxides can be expeditiously converted into secondary phosphine-boranes by treatment with an excess of BH₃·SMe₂ at room temperature. Selectivity of the conversion towards the formation of secondary phosphine-boranes is greatly improved by the addition of a small amount of water to the reaction mixture.

Full text of DOI:10.1055/s-2003-39304

1012
LETTER
An Expedient Reduction of sec-Phosphine Oxides to sec-Phosphine-boranes by
BH3·SMe2
A
a
n
E
xpedient Reductio
a
n
of se
c
-Ph
r
osphine
e
Oxide
s
to
k
BH
3·StMe ankevič,a K. Michał Pietrusiewicz*a,b
b
Fax +48(22)63266 81; E-mail: kmp@icho.edu.pl
Received 22 March 2003
and that at least three equivalents of borane are necessary
for the complete conversion of the starting 1a (approxi-
mately 1st/3rd and 2nd/3rd of the substrate was converted
with 1 equivalent and 2 equivalents of borane, respective-
Abstract: Secondary phosphine oxides can be expeditiously con-
verted into secondary phosphine-boranes by treatment with an ex-
cess of BH3·SMe2 at room temperature. Selectivity of the
conversion towards the formation of secondary phosphine-boranes
is greatly improved by the addition of a small amount of water to the ly). Prolonged reaction time was usually detrimental to
reaction mixture.
the yield of the isolated secondary phosphine-borane and
monitoring of the progress of the reaction by TLC was
critical and allowed to establish the optimal reaction time
at ca. 45 min. which secured both ca. 100% conversion
and 100% yield of the desired secondary phosphine-bo-
rane in the model reaction.
Key word: phosphorus, boron, complexes, reductions, stereoselec-
tivity
Phosphine-boranes are an important class of compounds,
which are especially useful in the elaboration of phos-
phine structures.1–3 BH3 in phosphine-boranes serves not
only as a valuable and easy to remove protecting group for
the phosphine functionality, but it also brings about the
useful reactivity features of a typical tetracoordinated
phosphorus derivative as well as its configurational stabil-
ity. The use of phosphine-boranes in the synthesis of
phosphorus compounds is thus complementary to the use
of the most commonly employed phosphine oxides. This
is especially true for secondary phosphine boranes
R1R2P(BH3)H and secondary phosphine oxides
R1R2P(O)H, which both are very useful precursors to ter-
tiary phosphine derivatives,4 including the nonracemic
ones.5–8 Phosphine-boranes are typically obtained by the
reaction of a free phosphine and BH3·THF or BH3·SMe2
complex,1,9 or directly from phosphine oxides
R1R2R3P(O) by the in situ reduction with LiAlH4 in pres-
ence of NaBH4 and CeCl3.10,11 Recently, more straightfor-
ward conversions of selected five-membered ring
phosphine oxides were also shown possible by the use of
an excess of BH3 complexes alone.12,13
The developed procedure was then used for the conver-
sion of a series of structurally diversified symmetrical and
unsymmetrical secondary phosphine oxides, H-phospho-
nates and H-phosphinate into the corresponding phos-
phine-boranes. The results of this screening are presented
in Table 1.
Scheme 1
As shown in Table 1, the conversion of secondary phos-
phine oxides in reactions with BH3·THF complex was
practically always complete and the yields of 2
(Scheme 1) were typically high (entries 1–5) except for
the sterically crowded substrates (entries 6–11). In the lat-
ter cases the competitive formation of sizeable amounts of
hydroxyphosphine-boranes (boranate phosphinous acids)
3 was observed (Scheme 2) and these compounds did not
undergo any further conversion under the reaction condi-
tions even after addition of additional equivalents of
BH3·THF complex.
In this paper, we wish to report that BH3·THF or
BH3·SMe2 can effectively convert secondary phosphine
oxides into secondary phosphine-boranes in high yield
under mild conditions and with complete avoidance of
handling of obnoxious secondary phosphines.
The exploratory experiments and optimization of the re-
duction process were first performed with BH3·THF com-
plex and with t-butylphenylphosphine oxide (1a) as a
model >P(O)H substrate. It was found that the reduction
takes place readily at room temperature in THF solutions
Scheme 2
Synlett 2003, No. 7, Print: 02 06 2003.
Art Id.1437-2096,E;2003,0,07,1012,1016,ftx,en;G05903ST.pdf.
© Georg Thieme Verlag Stuttgart · New York

LETTER
An Expedient Reduction of sec-Phosphine Oxides to sec-Phosphine-boranes by BH3·SMe2
1013
Table 1 Reduction of >P(O)H Compounds with BH3·THF Complex14
Entry
Time (min)
Comp.
Substituents
Isolated yield (%)
R1
Ph
R2
t-Bu
Ph
1
–
2
3
1
2
a
b
c
d
e
f
45
120
45
100
74a
84
87
71
74
64
76
18
40
41
36
–
0
Ph
–
15
–
3
Ph
PhCH2
1-NpCH2
2-NpCH2
c-Hex
i-Pr
–
b
b
4
Ph
45
–
–
5
Ph
45
–
–
6
Ph
45
–
25
33
20
68
52
56
–
7
g
h
i
Ph
45
–
8
Ph
p-Tol
45
–
9
Ph
o-Tol
45
–
10
11
12
13
14
j
Ph
o-An
45
–
k
l
c-Hex
Ph
c-Hex
i-PrO
45
–
15c
22 h
16 h
60
87
89
m
n
EtO
PhO
EtO
–
PhO
–
–
a 67% yield after 45 min.
b Np = naphthyl.
c Prolongation of the reaction time led to rapid decrease of yield
H-phosphinate gave poor conversion (entry 12) and H- As can be seen from Table 2 under these conditions even
phosphonates were found non reactive under these condi- crowded secondary phosphine oxides gave high yields of
tions (entries 13 and 14).
the isolated products 2 (entries 6–8). The only exceptions
were di-n-hexylphosphine oxide (1s) (entry 14) 1-naphth-
ylderivatives 1q, 1r (entries 12, 13) which gave low con-
versions.
Further optimization experiments led us to discover that
replacement of BH3·THF with BH3·SMe2 together with an
addition of a small amount of water to the reaction mix-
ture made the reduction process more selective and result- Interestingly, phosphine-boranes 2q,r containing P-1-
ed in the complete suppression of the formation of 3 under naphthyl substituents appeared to be unstable compounds.
these conditions (Scheme 3). The beneficial suppressive Attempts to isolate these compounds by chromatography
effect of added water was however accompanied by a were always accompanied by their partial decomposition.
marked decrease in yields of isolated 2. Use of a larger ex- Also a longer storage of these compounds at room temper-
cess of BH3·SMe2 resulted in increased yields of 2 to a ature resulted in slow loss of the borane protecting group
high level while keeping the formation of 3 completely and the formation of the free phosphines.
suppressed. The required excess amount of BH3·SMe2
The role of the added water is currently unclear. Similar
varied from case to case (between 3 equiv and 10 equiv)
addition of MeOH or EtCOOH instead of water did not
but it could be generalized at the level of 10 equivalents to
show any suppressive effect on the formation of 3
(Scheme 4).
secure the highest yields of 2 in all cases without any op-
timization trials. The results of the reductions performed
Also, water admixed to BH3·THF reagent had no apparent
with BH3·SMe2 under generalized conditions are present-
effect on the hydroxyphosphine-borane formation
(Scheme 4).
ed in Table 2.15
Use of BH3·SMe2 without added water resulted in the for-
mation of mixtures of 2 and 3 similar to those obtained
with BH3·THF.
In order to establish a stereochemical course of the studied
conversion, a reduction of the enantiomerically pure (R)-
(+)-1a6 with BH3·SMe2 was also carried out under the de-
Scheme 3
Synlett 2003, No. 7, 1012–1016 ISSN 1234-567-89 © Thieme Stuttgart · New York

1014
M. Stankevič, M. Pietrusiewicz
LETTER
Table 2 Reduction of >P(O)H Compounds with BH3·SMe2 Complex in the Presence of Water15
Entry
Comp.
Time (h)
Substituent
Isolated yield (%)
R1
Ph
R2
t-Bu
1
–
2
1
2
a
c
f
1,5
1,5
94a
96
86
73
81
84
80
81
73
70
68c
20d
26
40
Ph
PhCH2
c-Hex
i-Pr
–
3
Ph
1,5
–
4
g
h
i
Ph
1,5
–
5
Ph
p-Tol
1,5
–
6
Ph
o-Tol
1,5
–
7
j
Ph
o-An
1,5
–
8
k
l
c-Hex
Ph
c-Hex
i-PrO
1,5
–
9
0,5 he
1,5
–
10
11
12
13
14
o
p
q
r
s
p-An
p-An
–
3,5-Me2C6H3
3,5-Me2C6H3
1-Npb
1,5
–
Ph
Ph
1,5
–
2-Me-1-Npb
n-Hex
1,5
67
51
n-Hex
8 mine
a Use of 3 equiv. of BH3·SMe2 led to 80% yield of 3a.
b Np = naphthyl.
c The corresponding secondary phosphine, was also isolated in this case in 6% yield.
d The corresponding secondary phosphine, was also isolated in this case in 18% yield.
e Prolongation of the reaction time led to rapid decrease in yield.
veloped reaction conditions. In this experiment the gener- MeOH) with the known literature value for (R)-(–)-4a of
ated 2a was in situ transformed into 4a by treatment with 93% optical purity ([a]D = – 10, c = 1, MeOH)7 allowed us
n-BuLi at low temperature followed by methyl iodide to conclude that the studied reduction occurred with pre-
(Scheme 5). It was already known16 that low-temperature dominant retention of configuration at phosphorus albeit
lithiation and alkylation of secondary phosphine boranes with low overall stereoselectivity (ca. 18%).
is a stereoselective process, which takes place with reten-
The observed low stereoselectivity of the studied conver-
tion of configuration at P so the comparison of the specific
sion is probably due to the likely intermediacy of the con-
rotation value of the isolated 4a ([a]D = –1.49, c = 0.975,
figurationally unstable secondary t-butylphenylphosphine
(Scheme 6). According to the mechanistic proposal by
Scheme 4
Scheme 5
Synlett 2003, No. 7, 1012–1016 ISSN 1234-567-89 © Thieme Stuttgart · New York

LETTER
An Expedient Reduction of sec-Phosphine Oxides to sec-Phosphine-boranes by BH3·SMe2
1015
Keglevich12 the reduction of the P=O group by borane oc-
curs through an intramolecular hydride delivery from the
borane coordinated to the phosphoryl oxygen followed by
the dissociation of the P-O bond in the resulting pentaco-
ordinate intermediate and formation of a protonated phos-
phonium intermediate, which is then converted into
phosphine and phosphine-borane. In our case this would
inevitably lead to the formation of a symmetrical phos-
phonium intermediate and eventually to racemic 4a
(Scheme 6, path a). The isolation of the optically active 4a
precludes the involvement of the symmetrical phosphoni-
um species in our case and it thus appears more likely that
the pentacoordinate intermediate undergoes ligand
coupling17 yielding the phosphine directly (Scheme 6,
path b).
(10) Imamoto, T.; Kusumoto, T.; Suzuki, N.; Sato, K. J. Am.
Chem. Soc. 1985, 107, 5301.
(11) Imamoto, T.; Oshiki, T.; Onozawa, T.; Kusumoto, T.; Sato,
K. J. Am. Chem. Soc. 1990, 112, 5244.
(12) (a) Keglevich, G.; Chuluunbaatar, T.; Ludanyi, K.; Tőke, L.
Tetrahedron 2000, 56, 1. (b) Keglevich, G.; Fekete, M.;
Chuluunbaatar, T.; Dobó, A.; Harmat, V.; Tőke, L. J.Chem.
Soc., Perkin Trans. 1 2000, 4451. (c) Keglevich, G.; Fekete,
M.; Chuluunbaatar, T.; Dobó, A.; Böcskei, Z.; Tke, L. Synth
Commun. 2000, 30, 4221.
(13) For the first observation of reduction of phosphine oxides
with boron compounds see: (a) Köster, R.; Morita, Y.
Angew. Chem., Int. Ed. Engl. 1965, 4, 593. (b) Köster, R.;
Tsay, Y.-H.; Synoradzki, L. Chem. Ber. 1987, 120, 1117.
(14) Typical procedure for the reduction of secondary phosphine
oxides with BH3·THF complex:
To a solution of secondary phosphine oxide (0.5 mmol) in 10
mL of dry THF was added through a syringe 1.5 mmol of
BH3·THF complex. The mixture was stirred at room
temperature for 15 min–2 h (see Table 1). Then reaction
mixture was evaporated to dryness and the crude product
was purified by column chromatography (hexane:ethyl
acetate, 6:1, then hexane:ethyl acetate, 2:1).
Phosphine-boranes 2a, 2b, 2j were known compounds.8a
Data for new compounds follow:
Benzylphenylphosphine-borane (2c). Yield 84%; 1H NMR
(CDCl3) d = 0.12–1.90 (br m, 3 H), 3.11–3.50 (m, 2 H), 5.53
(dq, JP-H = 376 Hz, 1 H), 6.96–7.10 (m, 2 H), 7.22–7.31 (m,
3 H), 7.37–7.60 (m, 5 H) ppm; 13C NMR (CDCl3) d = 31.94,
32.56, 125.02, 125.55, 127.03, 127.08, 128.54, 128.60,
128.81, 129.09, 129.18, 131.71, 131.76, 132.97, 133.14,
133.25 ppm; 31P NMR (CDCl3) d = 8.10 ppm (m); Anal for
C13H16BP Calcd: C 72.95, H 7.53 Found: C 72.95, H 7.60.
(1-Naphthylmethyl)phenylphosphine-borane (2d). Yield
87%; 1H NMR (CDCl3) d = 0.20–1.90 (br m, 3 H), 3.47–4.02
(m, 2 H), 5.58 (dq, JP-H = 374 Hz, 1 H), 7.02–7.11 (m, 1 H),
7.28–7.69 (m, 8 H), 7.77–7.98 (m, 3 H) ppm; 13C NMR
(CDCl3) d = 29.49, 30.10, 123.01, 124.33, 125.11, 125.17,
125.87, 126.41, 127.77, 127.87, 127.96, 128.02, 128.50,
128.70, 129.00, 129.52, 129.72, 130.92, 131.68, 131.74,
132.96, 133.12, 133.85 ppm; 31P NMR (CDCl3) d = 5.48
ppm (m); Anal for C17H18BP Calcd: C 77.31, H 6.87 Found:
C 77.21, H 6.99.
Scheme 6
In summary, we have developed an efficient procedure for
the direct conversion of readily available secondary phos-
phine oxides into secondary phosphine-boranes by
BH3·SMe2 under mild conditions and with complete
avoidance of handling of secondary phosphines.
References
(2-Naphthylmethyl)phenylphosphine-borane (2e). Yield
71%; 1H NMR (CDCl3) d = 0.15–1.85 (br m, 3 H), 3.30–3.68
(m, 2 H), 5.60 (dq, JP-H = 374 Hz, 1 H), 7.12–7.17 (m, 1 H),
7.34–7.60 (m, 8 H), 7.68–7.89 (m, 3 H) ppm; 13C NMR
(CDCl3) d = 32.19, 32.80, 125.87, 126.25, 127.03, 127.10,
127.41, 127.55, 127.85, 127.97, 128.27, 128.61, 128.81,
130.54, 130.72, 131.72, 131.77, 132.19, 132.97, 133.14
ppm; 31P NMR (CDCl3) d = 7.94 ppm (m); Anal for
C17H18BP Calcd: C 77.31, H 6.87. Found: C 77.41, H 6.93.
c-Hexylphenylphosphine-borane (2f). Yield 74%; 1H NMR
(CDCl3) d = –0.05–1.65 (br m, 3 H), 1.12–1.46 (m, 5 H),
1.63–2.10 (m, 6 H), 5.24 (dq, JP-H = 364 Hz, 1 H), 7.42–7.77
(m, 5 H) ppm; 13C NMR (CDCl3) d = 25.61, 26.24, 26.29,
26.48, 26.52, 26.69, 28.18, 28.21, 32.99, 33.67, 124.11,
125.17, 128.60, 128.79, 131.37, 131.42, 133.20, 133.36
ppm; 31P NMR (CDCl3) d = 13.12 ppm (m); Anal for
C12H20BP Calcd: C 69.94, H 9.78. Found: C 69.72, H 9.82.
i-Propylphenylphosphine-borane (2g). Yield 64%; 1H NMR
(CDCl3) d = –0.05–1.40 (br m, 3 H), 1.09–1.36 (m, 6 H),
2.16–2.39 (m, 1 H), 5.30 (dq, JP-H = 364 Hz, 1 H), 7.42–7.60
(m, 3 H), 7.63–7.78 (m, 2 H) ppm; 13C NMR (CDCl3) d =
17.45, 17.99, 23.31, 24.01, 124.13, 125.19, 128.63, 128.82,
131.48, 133.19, 133.35 ppm; 31P NMR (CDCl3) d = 17.03
ppm (m); Anal for C9H16BP Calcd: C 65.12, H 9.71. Found:
C 64.91, H 9.57.
(1) Brunel, J. M.; Faure, B.; Maffei, M. Coord. Chem. Rev.
1998, 178-180, 665.
(2) Ohff, H.; Holz, J.; Quirmbach, M.; Börner, A. Synthesis
1998, 1391.
(3) Carboni, B.; Monnier, L. Tetrahedron 1999, 55, 1197.
(4) Engel, R. In Synthesis of Carbon-Phosphorus Bonds; CRC
Press, Inc.: Boca Raton FL, 1987.
(5) (a) Haynes, R. K.; Freeman, R. N.; Mitchell, C. R.;
Vonwiller, S. C. J. Org. Chem. 1994, 59, 2919. (b) Au-
Yeung, T. L.; Chan, K. Y.; Chan, W. K.; Haynes, R. K.;
Williams, I. D.; Yeung, L. L. Tetrahedron Lett. 2001, 42,
453.
(6) Drabowicz, J.; Łyżwa, P.; Omelańczuk, J.; Pietrusiewicz, K.
M.; Mikołajczyk, M. Tetrahedron: Asymmetry 1999, 10,
2757.
(7) Wolfe, B.; Livinghouse, T. J. Am. Chem. Soc. 1998, 120,
5116.
(8) (a) Imamoto, T.; Osehiki, T.; Onozawa, T.; Kusumoto, T.;
Sato, K. J. Am. Chem. Soc. 1990, 112, 5244. (b) Imamoto,
T.; Tsuruta, H.; Wada, Y.; Masuda, H.; Yamaguchi, K.
Tetrahedron Lett. 1995, 36, 8271. (c) Miura, T.; Yamada,
H.; Kikuchi, S.; Imamoto, T. J. Org. Chem. 2000, 65, 1877.
(9) Beres, J.; Dodds, A.; Morabito, A. J.; Adams, R. M. Inorg.
Chem. 1971, 10, 2072.
Synlett 2003, No. 7, 1012–1016 ISSN 1234-567-89 © Thieme Stuttgart · New York

1016
M. Stankevič, M. Pietrusiewicz
LETTER
Di-p-anisylphosphine-borane (2o). Yield 73%. 1H NMR
(CDCl3) d = 0.15–1.87 (br m, 3 H), 3.87 (s, 6 H), 6.30 (dq,
JP-H = 380 Hz, 1 H), 6.93–7.05 (m, 4 H), 7.53–7.68 (m, 4 H)
ppm; 13C NMR (CDCl3) d = 55.37, 114.55, 114.78, 134.38,
134.59 ppm; 31P NMR (CDCl3) d =–1.18 ppm (m); Anal for
C14H18BO2P Calcd: C 64.65, H 6.98. Found: C 64.54, H
6.89.
Phenyl-p-tolylphosphineborane (2h). Yield 76%. 1H NMR
(CDCl3) d = 0.21–2.02 (br m, 3 H), 2.44 (s, 3 H), 6.34 (dq,
JP-H = 380 Hz, 1 H), 7.39–7.48 (m, 2 H), 7.43–7.79 (m, 7 H)
ppm; 13C NMR (CDCl3) d = 21.50, 121.80, 122.93, 125.98,
127.12, 128.60, 128.80, 128.99, 129.39, 129.62, 129.83,
130.49, 130.54, 130.69, 130.72, 130.77, 131.34, 131.39,
131.51, 132.58, 132.76, 132.94, 142.06, 142.11 ppm; 31
P
NMR (CDCl3) d = 1.90 ppm (m); Anal for C13H16BP Calcd:
Di-(3,5-dimethyl)-phenylphosphine-borane (2p). Yield
70%. 1H NMR (CDCl3) d = 0.15–1.88 (br m, 3 H), 2.37 (s,
12 H), 6.21 (dq, JP-H = 380 Hz, 1 H), 7.16 (br s, 2 H), 7.27
(br s, 2 H), 7.33 (br s, 2 H) ppm; 13C NMR (CDCl3) d =
21.26, 130.30, 130.50, 133.27, 133.32, 138.60, 138.82 ppm;
31P NMR (CDCl3) d = 3.23 ppm (m); Anal for C16H21BP
Calcd: C 75.03, H 8.66. Found: C 74.84, H 8.72.
1-Naphthylphenylphosphine-borane (2q). Yield 68%; 1H
C 72.95, H 7.53. Found: C 72.89, H 7.61.
Phenyl-o-tolylphosphine-borane (2i). Yield 18%; 1H NMR
(CDCl3) d = 0.23–2,00 (br m, 3 H), 2,42 (s, 3 H), 6,46 (dq,
JP-H = 380 Hz, 1 H), 7.25–7.80 (m, 9 H) ppm; 13C NMR
(CDCl3) d = 20.97, 21.08, 125.31, 126.33, 126.56, 128.86,
128.91, 129.06, 130.93, 131.08, 131.32, 131.37, 131.91,
131.96, 132.61, 132.79, 134.13, 134.40, 141.62 ppm; 31
P
NMR (CDCl3) d = –3.83 ppm (m); Anal for C13H16BP Calcd:
NMR (CDCl3) d = 0.28–2.13 (br m, 3 H), 6.84 (dq, JP-H
=
C 72.95, H 7.53. Found: C 72.88, H 7.61.
380 Hz, 1 H), 7.37–7.74 (m, 8 H), 7.90–8.00 (m, 1 H), 8.00–
8.19 (m, 3 H) ppm; 13C NMR (CDCl3) d = 121.79, 122.86,
125.05, 125.10, 125.18, 125.37, 125.55, 126.56, 126.67,
127.50, 128.87, 129.08, 129.22, 131.33, 131.37, 132.47,
132.67, 133.07, 133.12, 133.19, 133.47, 133.60, 134.81,
135.08 ppm; 31P NMR (CDCl3) d = –5,73 ppm (m); Anal for
C16H16BP Calcd: C 76.84, H 6.45. Found: C 76.64, H 6.31.
[(2-Methyl)-1-naphthyl]phenylphosphine-borane (2r).
Yield 20%; 1H NMR (CDCl3) d = 0.20–1.95 (br m, 3 H),
2.71 (s, 3 H), 7.09 (dq, JP-H = 386 Hz, 1 H), 7.32–7.56 (m, 5
H), 7.61–7.75 (m, 2 H), 7.80–7.98 (m, 3 H), 8.09–8.19 (m, 1
H) ppm; 13C NMR (CDCl3) d = 23.31, 23.47, 125.48, 125.68,
127.14, 127.56, 128.86, 128.91, 129.00, 129.06, 129.26,
129.44, 131.32, 131.37, 132.07, 132.19, 132.24, 132.83,
133.02 ppm; 31P NMR (CDCl3) d = –22,11 ppm (m).
Di-n-hexylphosphine-borane (2s). Yield 26%; 1H NMR
(CDCl3) d = –0.30–1.45 (br m, 3 H), 0.84–1.03 (m, 6 H),
Di-c-Hexylphosphine-borane (2k). Yield 41%; 1H NMR
(CDCl3) d = –0.45–1.50 (br m, 3 H), 1.20–1.59 (m, 10 H),
1.64–2.02 (m, 12 H), 4.15 (dq, JP-H = 348 Hz, 1 H) ppm; 13
C
NMR (CDCl3) d = 25.65, 26.21, 26.43, 26.63, 27.60, 28.48,
29.15, 29.21, 29.27 ppm; 31P NMR (CDCl3) d = 18.94 ppm
(m); Anal for C12H26BP Calcd: C 67.95, H 12.35. Found: C
68.11, H 12.35.
Phenyl-i-propoxyphosphine-borane (2l). Yield 36%; 1H
NMR (CDCl3) d = 0.05–1.73 (br m, 3 H), 1,31 (t, 6 H), 4.45
(m, 1 H), 7.18 (dq, JP-H = 400 Hz, 1 H), 7.47–7.68 (m, 3 H),
7.76–7.90 (m, 2 H) ppm; 13C NMR (CDCl3) d = 23.50,
73.38, 73.50, 128.75, 128.95, 131.79, 132.01, 132.54,
132.59 ppm; 31P NMR (CDCl3) d = 93.30 ppm (m); Anal for
C12H26BOP Calcd: C 59.39, H 8.86. Found C 59.31, H 8.78.
(15) Typical procedure for the reduction of secondary phosphine
oxides with BH3·THF complex:
To a solution of secondary phosphine oxide (0.5 mmol) in 10
mL of dry THF was added water (about 5 mmol) and after
homogenization of the reaction mixture 10 mmol of
BH3·SMe2 complex was added dropwise via syringe. The
mixture was stirred at room temperature for 8 min–1.5 h (see
Table 2). Then reaction mixture was evaporated to dryness
and the crude product was purified by column
chromatography (hexane:ethyl acetate 6:1).
1.22–1.51 (m, 13 H), 1.51–1.90 (m, 7 H), 4.57 (dq, JP-H
=
352 Hz, 1 H) ppm; 13C NMR (CDCl3) d = 14.02, 20.13,
20.84, 22.43, 24.38, 24.45, 30.27, 30.50, 30.93, 31.25 ppm;
31P NMR (CDCl3) d = –7.04 ppm (m); Anal for C12H30BP
Calcd: C 66.68, H 13.99. Found: C 66.91, H 13.87.
(16) Imamoto, T.; Matsuo, M.; Nonomura, T.; Kishikawa, K.;
Yanagawa, M. Heteroatom Chem. 1993, 4, 475.
(17) Oae, S.; Uchida, Y. Acc. Chem. Res. 1991, 24, 202.
Compounds data:
Synlett 2003, No. 7, 1012–1016 ISSN 1234-567-89 © Thieme Stuttgart · New York