One-pot transformation of cyclic phosphine oxides to phosphine-boranes by dimethyl sulfide-borane

Article abstract of DOI:10.1039/b005380p

Different types of cyclic phosphine oxides, such as tetrahydrophosphole oxide 1, phosphabicyclo[3.1.0]hexane 3-oxide 8 and phosphabicyclo[2.2.1]heptene 7-oxides 10 and 12 were efficiently converted to phosphine-boranes 2,9,11 and 13, respectively, under relatively mild conditions by reaction with 4.4 equivalents of dimethyl sulfide-borane. The more strained hetero-ring the starting phosphine oxide (in general 16) has, the easier to accomplish the change in the P-function, that takes place through the corresponding phosphine intermediate (20). It is noteworthy that the imide carbonyl groups in starting materials 10 and 12 were fully reduced by the borane to give 11 and 13 respectively. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b005380p

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One-pot transformation of cyclic phosphine oxides to phosphine–
boranes by dimethyl sulfide–borane
György Keglevich,*a Melinda Fekete,a Tungalag Chuluunbaatar,a András Dobó,b
Veronika Harmatc and László To´´ k e d
1
a Department of Organic Chemical Technology, Budapest University of Technology and
Economics, 1521 Budapest, Hungary. E-mail: keglevich@oct.bme.hu
b Hungarian Academy of Sciences, Chemical Research Center, 1525 Budapest, Hungary
c Department of Theoretical Chemistry, Eötvös University, 1518 Budapest, Hungary
d Research Group of the Hungarian Academy of Sciences at the Department of Organic
Chemical Technology, Budapest University of Technology and Economics, 1521 Budapest,
Hungary
Received (in Cambridge, UK) 5th July 2000, Accepted 25th September 2000
First published as an Advance Article on the web 10th November 2000
Different types of cyclic phosphine oxides, such as tetrahydrophosphole oxide 1, phosphabicyclo[3.1.0]hexane 3-
oxide 8 and phosphabicyclo[2.2.1]heptene 7-oxides 10 and 12 were efficiently converted to phosphine–boranes 2, 9, 11
and 13, respectively, under relatively mild conditions by reaction with 4.4 equivalents of dimethyl sulfide–borane. The
more strained hetero-ring the starting phosphine oxide (in general 16) has, the easier to accomplish the change in the
P-function, that takes place through the corresponding phosphine intermediate (20). It is noteworthy that the imide
carbonyl groups in starting materials 10 and 12 were fully reduced by the borane to give 11 and 13 respectively.
Introduction
A survey of the recent literature confirms that the P-hetero-
cyclic derivatives of phosphine–boranes are of interest.1–7
Phosphine–boranes can be regarded as protected phosphines
and hence they are precursors of phosphines.8,9 On the other
hand, the complexation at the phosphorus atom may affect
the reactivity of the other functions in the molecule.10 The
most general way to synthesize phosphine–boranes involves the
reaction of phosphines, obtained from the phosphine oxides by
deoxygenation, with dimethyl sulfide–borane (BMS) or with
tetrahydrofuran–borane.8 The possibilities for the application
of ring enlargements in the preparation of cyclic phosphine–
boranes have also been explored.4 It was most interesting to find
Scheme 1
that the bridging P᎐O moiety of the phosphole oxide dimers
᎐
could easily be converted to a phosphine–borane unit by reac-
tion with three equivalents of the BMS reagent.11 Noteworthy is
that the transformation was selective; the electron-poor double
bond of the starting material remained intact. In other
instances, the BMS reagent could, however, be well utilised in
the selective reduction of the double bond of cyclic and acyclic
vinylphosphine oxides.12,13
The purpose of this paper is to discuss the scope and limit-
ations of the application of dimethyl sulfide–borane in the
synthesis of cyclic phosphine–boranes from the P-oxides.
Fig. 1 Perspective view of 2; hydrogen atoms are shown, but not
labeled.
Results and discussion
2. The relative P-configuration in 2 was assigned on the basis of
single crystal X-ray analysis (Fig. 1). The same stereochemistry
was assumed to exist in starting dihydrophosphole oxide 1. The
selected bond parameters for phosphine–borane 2 are listed
in Table 1. The steric crowding due to the two methyl groups on
the same side of the five membered ring distorts the ring to a
twisted conformation, hence the C1–C2–C3–C4 torsion becomes
gauche with C5-Me in a pseudo-equatorial position and with
C6–Me occupying a pseudo-axial position. The C1–P1–C4–C3
and C4–P1–C1–C2 torsion angles are of 17.8 and 8.8Њ, respec-
tively (Table 1). The phosphorus atom lies at a distance of
Testing the simplest model, we found that the reaction of
dimethyl(phenyl)tetrahydrophosphole oxide 1 with a consider-
able excess (4.4 equivalents) of the BMS reagent afforded
phosphine–borane 2 after three days’ heating at the boiling
point of chloroform (Scheme 1). The prolonged reaction time
was crucial from the point of view of the quantitative con-
version. Column chromatography furnished product 2 (in
86% yield) which was characterised by 31P, 11B, 13C and 1H
NMR, as well as mass spectroscopic methods. The 13C and 1H
NMR spectra were consistent with the symmetry of compound
DOI: 10.1039/b005380p
J. Chem. Soc., Perkin Trans. 1, 2000, 4451–4455
This journal is © The Royal Society of Chemistry 2000
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Table 1 Selected bond lengths (Å) angles (Њ) and torsion angles (Њ)
for 2
Table 2 Selected bond lengths (Å) angles (Њ) and torsion angles (Њ) for
9
P(1)–C(7)
P(1)–C(1)
P(1)–C(4)
P(1)–B(1)
1.796(4)
1.810(4)
1.838(4)
1.886(5)
P(1)–C(7)
P(1)–C(2)
P(1)–C(3)
P(1)–B(1)
1.792(3)
1.806(4)
1.830(3)
1.897(5)
C(7)–P(1)–C(1)
C(7)–P(1)–C(4)
C(1)–P(1)–C(4)
C(7)–P(1)–B(1)
C(1)–P(1)–B(1)
C(4)–P(1)–B(1)
B(1)–P(1)–C(7)–C(8)
C(1)–P(1)–C(4)–C(3)
C(4)–P(1)–C(1)–C(2)
C(1)–C(2)–C(3)–C(4)
108.3(2)
107.2(2)
95.5(2)
113.7(2)
115.6(2)
114.9(3)
Ϫ13.9(4)
17.8(3)
C(7)–P(1)–C(2)
C(7)–P(1)–C(3)
C(2)–P(1)–C(3)
C(7)–P(1)–B(1)
C(2)–P(1)–B(1)
C(3)–P(1)–B(1)
B(1)–P(1)–C(7)–C(12)
C(2)–P(1)–C(3)–C(4)
C(3)–P(1)–C(2)–C(1)
109.6(2)
111.2(2)
96.0(2)
116.5(2)
108.9(2)
112.9(2)
26.9(4)
23.6(3)
Ϫ26.6(2)
8.8(3)
47.5(4)
0.030 Å from the plane defined by the C1, C2, C3 and C4 atoms
(rms deviation of 0.191 Å). Despite their different size, the two
substituents of the phosphorus atom are of similar distance
from the ring plane (1.524 and 1.533 Å for P–C7 and P–B,
respectively).
A similar reaction of 2,5-dihydrophosphole oxide 3 with
dimethyl sulfide–borane led to a mixture of the borane deriva-
tive of the dihydrophosphole (4) and that of the tetrahydro-
phosphole consisting of isomers 2 and 2Ј as suggested by the 31
P
NMR spectrum and the FAB-MS measurement (Scheme 2).
Scheme 2
Fig. 2 Perspective view of 9; hydrogen atoms are shown, but not
labeled.
Traces of tetrahydrophosphole oxide 1 could also be detected.
Compounds 1 and 4 are possible intermediates for the isomers
(2 and 2Ј) of the tetrahydrophosphole–borane. The 4 →
2 ϩ 2Ј transformation may take place through hydroboration.
No efforts were made to separate the components of the crude
reaction mixture.
oxide of a five- or a six-membered ring in reaction with borane.
We learnt that phosphine oxide 8 was efficiently (in 81% yield)
transformed to borane 9 under the conditions applied above
(Scheme 4). The spectral parameters of product 9 were identical
Applying similar reaction conditions, we were not able
to convert phosphinane oxide 5 to borane 6; even after three
days’ reflux in chloroform, oxide 5 was recovered unchanged
(Scheme 3). Phosphine–borane 6 could, however, be prepared
through phosphine 7. The phosphine (7) was obtained by a
conventional deoxygenation of the oxide (5) using trichloro-
silane,14 to give after reaction with borane the desired product
(6) characterised by 31P, 11B and 13C NMR, as well as FAB-MS
(Scheme 3).
Scheme 4
with those of an authentic sample.4 The relative configuration
in 9 was confirmed by single crystal X-ray analysis (Fig. 2)
suggesting that the sterochemistry of the phosphorus atom in
starting material 8 was preserved.15 Selected bond parameters
for product 9 can be found in Table 2. Similarly to phosphine–
borane 2, compound 9 also exhibits a significantly shorter
exocyclic P–C bond as compared to the endocyclic ones. The
five-membered hetero-ring of 9 has an envelope conformation
with the phosphorus atom on the flap; its distance from the
C2–C1–C4–C3 plane is 0.541 Å (Table 2). The phenyl group
is placed in a pseudo-equatorial position, while the borane
unit occupies the pseudo-axial position. The angle between the
planes of the phenyl ring and the five-membered ring (110.4Њ) is
different from that found in 2 (101.1Њ), bringing about the steric
proximity of C3 and the edge of the phenyl ring in 9.
From the above observations it is clear that the ring strain
Scheme 3
of the starting cyclic phosphine oxide is critical; the P᎐O group
᎐
It seemed to be interesting to examine whether phospha-
of strained tetrahydro- and dihydrophospholes (1 and 3) can
bicyclo[3.1.0]hexane 3-oxide 8 behaves as a cyclic phosphine
be converted to a P→BH3 function, at the same time, the
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phosphinane oxides (e.g. 5), obviously lacking any considerable
ring strain, resist the refunctionalisation effect of borane. It
is, however, worth mentioning that while the conversion of
the phosphole oxide derivatives (e.g. 1 or 3) required forcing
reaction conditions (a considerable excess of the borane and
prolonged reaction times), the transformation of the more
strained phosphole oxide dimers proceeded with a relative
ease.11 We wished to evaluate if phosphabicyclo[2.2.1]heptene
derivatives, such as 10 and 12, could also be converted readily
to the corresponding boranes. We found that the use of 4.4
equivalents of the borane under mild conditions (1., 22 h at
25 ЊC, 2., 5 h at 63 ЊC) led to boranes 11 and 13 (Scheme 5).
Only a single example, the deoxygenation of triphenylphos-
phine oxide, is known from the literature for the reduction of
phosphine oxides to phosphines by borane.17 The deoxygen-
ation of phosphine oxides by different silanes,14 and quite
recently by alanes,18,19 is, however, a well-known method. This is
the first time that phosphine oxides have been converted under
relatively mild conditions (63 ЊC) to the boranes by borane,
moreover in a one-pot manner. The synthesis described earlier
uses trialkylamine–boranes and a reaction temperature higher
than 120 ЊC.17 Unfortunately, no details of the experimental
procedure and the yield were provided. In our case, the only
requirement is that the starting cyclic phosphine oxide should
have a notable ring strain. For the phosphabicyclo[2.2.1]-
heptenes, a C–P–C angle of ca. 83.0Њ was reported.20 The
driving force for the deoxygenation is the decrease of the
ring strain in five-coordinate intermediate 18. The example of
the conversion of triphenylphosphine oxide to triphenyl-
phosphine–borane by borane (T ≥ 120 ЊC)17 shows clearly the
effect of the lack of ring strain in the starting P-oxide.
The complete reduction of an imide (or amide) function
under the conditions of the reaction with borane is unusual and
hence is of novelty.
It can be concluded that cyclic phosphine oxides, such as
dihydro- and tetrahydrophosphole 1-oxides, as well as phos-
phabicyclo[3.1.0]hexane 3-oxides and phosphabicyclo[2.2.1]-
heptene 7-oxides can be efficiently transformed to the corre-
sponding phosphine–boranes by reaction with 4.4 equivalents
of borane in a one-pot synthesis. The only criterion of the novel
refunctionalisation taking place through a pentacoordinate
P-intermediate is that the starting heterocycle should be of
considerable ring strain. The P᎐O → P→BH transformation
can probably be extended to non-cyclic compounds provided
that an elevated reaction temperature is applied. This is the
first case that simple cyclic phosphine–boranes have been
characterised by single crystal X-ray analysis.
Scheme 5
It is interesting that both imide carbonyl groups were reduced
to methylene moieties. The boranes (11 and 13) were charac-
terised by 31P, 13C and 11B NMR, as well as FAB-MS. The
molecular ions for compounds 11 and 13 appeared in un-
protonated forms.
The use of less than 4.4 equivalents of the borane (3 equiva-
lents) led to mixtures containing predominantly 11 (or 13)
and only a little amount of the expected product 14 (or 15)
was formed, as shown by 31P NMR and FAB-MS.16
᎐
3
Experimental
The 31P, 13C, 1H and 11B NMR spectra were taken on a Bruker
DRX-500 spectrometer operating at 202.4, 125.7, 500 and 160.4
MHz, respectively. Chemical shifts are downfield relative to
85% H3PO4, TMS or F3BؒOEt2. The couplings are given in Hz.
EI-mass spectra were obtained on a MS-902 spectrometer
at 70 eV. FAB measurements were conducted on a reverse
geometry VG ZAB-2SEQ instrument using a 30 kV Csϩ ion
gun and 8 kV accelerating voltage.
The first step of the above transformations is probably the
Preparation of the starting materials
nucleophilic attack of the oxygen atom of the P᎐O group in
᎐
16 on the boron atom of the borane to afford species 17 which
is stabilised by a hydride anion shift. The key intermediate
(18) so formed with a pentacoordinate phosphorus atom
and with a trigonal bipyramidal geometry loses the elements
of BH2OH resulting in the formation of phosphine 20 and
then final product 21 by reaction with the excess of borane
(Scheme 6).
Tetrahydrophosphole oxide 1 was obtained by the catalytic
hydrogenation of the dihydro derivative 3 (1.8 g, 8.74 mmol) at
50 ЊC and 11 bar in methanol (50 mL) using 10% Pd/C (0.9 g)
and by subsequent purification by column chromatography
(silica gel, 2% methanol in chloroform). Yield: 1.2 g (66%) of 1;
31P NMR (CDCl3) δ 59.4; MS, m/z (rel. int.) 208 (Mϩ, 100),
193 (M Ϫ Me, 36), 125 (PhPO ϩ H, 63); Mϩfound = 208.0995,
C12H17OP requires 208.1017.
Phosphinane oxide 5 and phosphabicyclo[3.1.0]hexane 8
were prepared as described earlier.21,22
Phosphabicyclo[2.2.1]heptene 12 was synthesized according
to an earlier procedure.23 Compound 10 was prepared in a
similar way via the trapping of 3-methyl-1-phenylphosphole 1-
oxide (0.013 mmol) by N-phenylmaleimide (5.0 g, 0.029 mmol)
at 60 ЊC. The phosphole oxide was generated in situ from 3,4-
dibromo-3-methyl-1-phenyltetrahydrophosphole oxide (4.57 g,
0.013 mmol) by triethylamine (4.4 mL, 0.032 mmol) in benzene
(100 mL) solution. Flash column chromatography (silica gel,
3% methanol in chloroform) of the crude product obtained
after the evaporation of the filtrate afforded 1.14 g (25%)
of 10. 31P NMR CDCl3) δ 83.9; FAB, 364 (M ϩ H); (M ϩ
H)ϩfound = 364.1072, C21H19NO3P requires 364.1103.
Scheme 6
J. Chem. Soc., Perkin Trans. 1, 2000, 4451–4455
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General procedure for the synthesis of phosphine–boranes from
the oxides
with 1.4 mL of 2 M dimethyl sulfide–borane in THF (2.80
mmol) at room temperature. After a 2 h reaction time, 1.0 mL
of water was added and the mixture stirred for 5 min. The
precipitated material was removed by filtration and the organic
phase dried (Na2SO4). The crude product obtained after
evaporating the volatile components of the filtrate was purified
by column chromatography (2% methanol in chloroform, silica
gel) to give 0.20 g (41%) of borane 6 as a 55:45 mixture of two
isomers; FAB, 207 (M ϩ H); (M ϩ H)ϩfound = 207.1454,
C12H21BP requires 207.1474 for the 11B isotope.
To 1.12 mmol of the phosphine oxide (1, 8, 10 and 12) in 20 mL
of absolute chloroform was added 4.4 equivalents (2.5 mL)
of 2 M dimethyl sulfide–borane in THF, and the solution was
stirred at 25–63 ЊC for 27–72 h as shown in Schemes 1, 4
and 5, respectively. After the addition of 2.0 mL of water,
the mixture was stirred for 10 min and then filtered. The
organic phase of the filtrate was separated and dried (Na2SO4).
The crude product obtained after evaporating the volatile
components was purified by column chromatography (silica
gel, 2% methanol in chloroform) to give the products (2, 9,
11 and 13) as crystalline or semicrystalline solids. The purity
of the phosphine–boranes (2, 9, 11 and 13) was indicated by
TLC.
61. 31P NMR (CDCl3) δ 9.1; 11B NMR (CDCl3) δ Ϫ33.2; 13
C
NMR (CDCl3) δ 21.3 (J = 35.1, C6), 21.6 (J = 7.5, C5), 24.8
(J = 10.7, Me), 28.5 (J = 7.1, C3), 30.0 (J = 33.8, C2), 34.9
(J = 3.6, C4), 129.1 (J = 8.9, C3Ј*), 129.2 (J = 50.5, C1Ј), 130.5
(J = 2.0, C4Ј), 130.8 (J = 8.3, C2Ј*), *may be reversed.
3,4-Dimethyl-1-phenyl-2,3,4,5-tetrahydrophosphole–borane 2.
Yield: 92%, mp 54–55 ЊC; 31P NMR (CDCl3) δ 28.3; 11B NMR
(CDCl3) δ Ϫ33.8; 13C NMR (CDCl3) δ 15.9 (J = 5.6, Me),
33.0 (J = 35.6, C2), 39.6 (C3), 128.8 (J = 9.6, C3Ј*), 131.0
(J = 2.0, C4Ј), 131.2 (J = 8.7, C2Ј*), 132.0 (J = 47.1, C1Ј), *may be
62. 31P NMR (CDCl3) δ 4.8; 11B NMR (CDCl3) δ Ϫ37.9; 13
C
NMR (CDCl3) δ 22.0 (J = 2.1, C5), 23.0 (J = 34.7, C6), 24.9
(J = 13.5, Me), 29.2 (C3), 31.8 (J = 33.5, C2), 35.6 (J = 4.7, C4),
128.9 (J = 9.7, C3Ј*), 131.3 (J = 2.4, C4Ј), 131.3 (J = 8.4, C2Ј*),
*may be reversed.
1
reversed; H NMR (CDCl3) δ 1.06 (d, J = 6.7, 6H, Me), 1.88–
1.97 (m, 2H, CH), 2.14–2.23 (m, 2H, CH), 2.40–2.52 (m, 2H,
CH), 7.40–7.77 (m, 5H, Ar); MS, m/z (rel. int.) 192 (M Ϫ BH3,
100), 177 (192 Ϫ Me, 12).
Crystal data for 2 and 9†
X-Ray diffraction data of single crystals of 2 and 9 were
collected at 293 K.
6,6-Dichloro-1-methyl-3-phenyl-3-phosphabicyclo[3.1.0]-
hexane(P–B)borane 9. Yield: 65%, mp 98–100 ЊC; 31P NMR
(CDCl3) δ 59.1 (JPB = 60.5), lit.4 δ 59.0 (JPB = 64.7); MS, m/z 271
(Mϩ Ϫ H).
Crystal data for 2: C12H20BP, M = 206.06, triclinic, space
¯
group P1, a = 9.361(6) Å, b = 11.236(6) Å, c = 6.912(2) Å, α =
104.73(3)Њ, β = 107.85(2)Њ, γ = 65.89(2)Њ, V = 624.6(5) Å3, Z = 2,
Dc = 1.096 g cmϪ1, µ(Mo-Kα) = 0.182 mmϪ1
.
Crystal data for 9: C12H16BCl2P, M = 272.93, monoclinic,
space group P21/n, a = 6.851(2) Å, b = 17.570(4) Å, c =
11.462(3) Å, β = 103.59(4)Њ, V = 1341.0(5) Å3, Z = 4, Dc = 1.352 g
8-Methyl-4,10-diphenyl-4-aza-10-phosphabicyclo[5.2.1.02,6]-
dec-8-ene(P–B)borane 11. Yield: 45%; 31P NMR (CDCl3)
δ 130.6; 11B NMR (CDCl3) δ Ϫ35.2; 13C NMR (CDCl3) δ 20.6
cmϪ1, µ(Mo–Kα) = 0.573 mmϪ1
.
a
a
(Me), 42.6 (J = 18.0, C2 ), 42.9 (J = 17.2, C6 ), 44.1 (J = 33.4,
Structure solutions with direct methods were carried out with
the teXsan package.24 Refinements were carried out using the
SHELXL-93 program.25 Final R indices for 2 are R = 0.1213,
Rw = 0.2613 (for 1394 unique reflections) R = 0.0757, Rw =
0.1990 (I > 2σ(I)); those for 9 are R = 0.0700, Rw = 0.1494 (for
1518 unique reflections) R = 0.0465, Rw = 0.1238 (I > 2σ(I)).
Final difference maps: 0.365 and Ϫ0.398 e ÅϪ3 for 2; 0.389 and
Ϫ0.340 e ÅϪ3 for 9.
b
b
C1), 48.9 (J = 33.2, C7), 51.0 (J = 11.2, C5 ), 51.4 (J = 10.8, C3 ),
112.8 (C3Љ), 116.7 (C4Љ), 125.4 (C9), 128.5 (J = 9.2, C3Јc), 129.2
(C2Љ), 130.7 (C4Ј), 132.0 (J = 8.0, C2Јc), 143.9 (C8), a–cmay be
reversed; 1H NMR (CDCl3) δ 1.72 (s, 3H, Me), 2.96–3.41
(m, 4H, e), 3.48–3.59 (m, 2H, e), 3.79–3.93 (m, 2H, e), 6.71
e
(dd, J1 = J2 = 7.2, 1H, C9-H), 7.17–7.52 (m, 10H, Ar), skeletal
hydrogen atom(s); FAB, 333 (M); Mϩfound = 333.1783,
C21H25BNP requires 333.1818 for the 11B isotope.
Acknowledgements
The authors are grateful for the OTKA support of the work
(Grant No. T 029039).
8,9-Dimethyl-4,10-diphenyl-4-aza-10-phosphabicyclo-
[5.2.1.02,6]dec-8-ene(P–B)borane 13. Yield: 39%; 31P NMR
(CDCl3) δ 124.4; 11B NMR (CDCl3) δ Ϫ34.9; 13C NMR
(CDCl3) δ 17.1 (Me), 42.5 (J = 18.1, C2), 49.2 (J = 33.8, C1), 50.8
(J = 11.3, C5), 112.8 (C3Љ), 116.6 (C4Љ), 128.3 (J = 10.1, C3Ј*),
129.1 (C2Љ), 130.5 (C4Ј), 131.3 (J = 7.5, C2Ј*), 133.8 (C4), *may
be reversed; FAB, 347 (M); Mϩfound = 347.1947, C22H27BNP
requires 347.1974 for the 11B isotope.
† CCDC reference number 207/488. See http://www.rsc.org/suppdata/
p1/b0/b005380p/ for crystallographic files in .cif format.
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of compound 2 (δP = 28.9, M ϩ H = 207) and 33% of isomer 2Ј
(δP = 29.5, M ϩ H = 207).
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14 R. Engel, in Handbook of Organophosphorus Chemistry,
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17 R. Köstner and Y. Morita, Angew. Chem., Int. Ed. Engl., 1965, 4, 593.
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