Practical synthesis of chlorosilyl-functionalized triphenylphosphanes

Article abstract of DOI:10.1055/s-2004-816008

A practical synthesis of ortho-dialkylchlorosilyl-functionalized triarylphosphanes has been developed. An X-ray crystal structure analysis of phosphane 3 unambiguously confirmed the constitution of the new phosphane functionalized chlorosilanes. It also showed an interesting P/Si interaction, which may render this compound a model for the early stages of the mechanism of S_N2 nucleophilic displacement at silicon.

Full text of DOI:10.1055/s-2004-816008

PAPER
905
Practical Synthesis of Chlorosilyl-Functionalized Triphenylphosphanes
S
ynthesis of Chlor
e
osilyl-Func
l
tionaliz
p
ed Tripheny
h
lphosphanesine Quintard, Manfred Keller, Bernhard Breit*
Institut für Organische Chemie und Biochemie, Albert-Ludwigs-Universität Freiburg, Albertstr. 21, 79104 Freiburg, Germany
Fax +49(761)2038715; E-mail: bernhard.breit@organik.chemie.uni-freiburg.de
Received 29 January 2004
Synthesis of these compounds starts from ortho-dibro-
mobenzene, which was treated at –120 °C with one equiv-
alent of n-BuLi to undergo smoothly a monohalogen–
metal exchange (Scheme 1).9 The intermediate organo-
Abstract: A practical synthesis of ortho-dialkylchlorosilyl-func-
tionalized triarylphosphanes has been developed. An X-ray crystal
structure analysis of phosphane 3 unambiguously confirmed the
constitution of the new phosphane functionalized chlorosilanes. It
also showed an interesting P/Si interaction, which may render this lithium compound was reacted instantaneously at the
compound a model for the early stages of the mechanism of SN2 nu-
cleophilic displacement at silicon.
same temperature with chlorodiphenylphosphane, which
allowed to suppress elimination to benzyne. After warm-
Key words: phosphorus, silicon, ligands
ing of the reaction mixture to –80 °C during one hour and
aqueous workup, the ortho-bromotriphenylphosphane 1
was obtained in good yield. The reaction has been run on
a 12 g scale.
Phosphanes are an important class of organic compounds
for a variety of applications ranging from their use as
ligands in transition metal catalysis,1 Wittig ylide forma-
tion,2 Mukaiyama redox condensations,3 Mitsunobu reac-
tions,4 and nucleophilic organocatalysis.5 Particularly
useful are triarylphosphanes occupied with a functional
group in ortho-position, which can be attached easily to
organic substrates (Figure 1). Such phosphanes are useful
for the preparation of ligand libraries applicable to homo-
geneous catalysis,6 they may serve as mediators in
Staudinger ligation reactions7 as well as substrate bound
catalyst/reagent-directing groups in organic synthesis and
catalysis.8
PAr2
FG
PAr2
R
Si
Cl
R
Scheme 1 Preparation of new phosphane functionalized chlorosi-
lanes 2 and 3.
known
FG = COOH, CHO, CH2OH, OH, NH2
this work
FG = SiR2Cl
Figure
1
Triarylphosphanes equipped with useful functional
To install a dialkylchlorosilane function, phosphane 1 was
subjected to halogen–metal exchange at room tempera-
ture by reacting with n-butyllithium. The resulting aryl-
lithium intermediate was quenched at the same
temperature with dichlorodimethylsilane to give phosph-
anyl-substituted chlorosilane 2 (Scheme 1).
groups in the ortho-position.
In this respect an interesting, yet unknown, ortho-func-
tionalization of a triarylphosphane would be a silyl chlo-
ride group, since it can be attached easily to protic
functions such as alcohols, phenols, amines, carboxylic
acids etc.
Unfortunately, it was found that the dimethylchlorosilane
2 showed extreme sensitivity towards traces of moisture,
which led to the immediate formation of the siloxane
bridged diphosphane 4 (Scheme 2).
We herein report on a first and practical synthesis of
ortho-dialkylsilyl chloride-functionalized triphenylphos-
phanes and their characterization via X-ray crystal struc-
trure analysis.
PPh2Ph2P
H2O
2
(quant.)
O
Si
Si
Me Me Me Me
SYNTHESIS 2004, No. 6, pp 0905–0908
x
x
.
x
x
.2
0
0
4
4
Advanced online publication: 15.03.2004
DOI: 10.1055/s-2004-816008; Art ID: Z01304SS
© Georg Thieme Verlag Stuttgart · New York
Scheme 2 Hydrolysis of chlorosilane 2.

906
D. Quintard et al.
PAPER
Ph
It was envisioned that incorporation of steric hindrance
into the chlorosilane function could improve stability of
this function towards hydrolysis. Thus, treatment of the
ortho-lithiotriphenylphosphane obtained from compound
1 with diisopropyldichlorosilane at room temperature fur-
nished in quantitative yield the ortho-diphenylphospha-
nyl-functionalized chlorosilane 3 as colorless crystals.
Compound 3 turned out to be rather robust, since it is sta-
ble under air and at room temperature for at least one
month. From a solution of chlorosilane 3 in petroleum
ether suitable single crystals could be obtained which al-
lowed to perform an X-ray-crystal structure analysis
(Figure 2, Table 1,2).
Ph
P
d(PSi) = 3.31 Å
i-Pr
Si i-Pr
Cl
Figure 3 Consideration of chlorosilylphosphane 3 as a model com-
pound for the early stages of the mechanism of SN2 nucleophilic dis-
placement at silicon.
Table 1 Selected Bond Lenghts [Å] of 3 in the Solid State
Bond Lenghts (in Å)
C(41)–Si(1)
C(45)–Si(1)
C(36)–Si(1)
Cl(1)–Si(1)
C(31)–C(36)
C(31)–P(1)
P(1)–Si(1)
1.874(2)
1.882(2)
1.893(2)
2.0999(8)
1.420(3)
1.845(2)
3.3117(8)
Table 2 Selected Bond Angles of 3 in the Solid State
Bond Angles (in degrees)
C(36)–Si(1)–Cl(1)
C(41)–Si(1)–C(45)
C(45)–Si(1)–C(36)
C(41)–Si(1)–C(36)
C(31)–C(36)–Si(1)
C(36)–C(31)–P(1)
P(1)–Si(1)–Cl(1)
104.77(7)
113.98(11)
112.25(10)
113.30(10)
122.99(16)
117.79(16)
162.36(3)
Figure 2 X-ray plot (ORTEP) of the structure of 3 in the solid state.
The X-ray crystal structure of 3 suggests an electronic in-
teraction between the phosphorus and the silicon atom
(see Figure 3). Thus, the orientation of the P-lone pair is
such that an efficient n(P)–s*(SiCl) orbital interaction is
feasible. Consistently, the P--Si–Cl angle is almost linear
with 162.36(0.03)°. As a consequence of this interaction,
the distance between the phosphorus and the silicon atom
is rather small with 3.31 Å, and the silicon atom adopts a
distorted trigonal bipyramidal geometry, which is indicat-
ed by significantly increased C–Si–C-angles (112–114°)
compared to 109.28° for an ideal tetrahedral system. Fi-
nally, the Si–Cl bond distance is elongated with 2.10 Å
compared to the standard Si–Cl bond length expected for
a tetrahedral R3SiCl system of 2.05 Å.10 This interpreta-
tion of the geometrical data of 3 is in agreement with a re-
cent series of X-ray crystal structures of N-
(halogenodimethylsilylmethyl)lactams, which have been
discussed as a model of the SN2 nucleophilic displacement
at the silicon atom.11 Thus, X-ray structure of compound
3 may be regarded as a model for the early stages of the
SN2 nucleophilic displacement reaction at silicon.
Furthermore, NMR studies in solution indicate that the P-
lone-pair orientation of phosphines 2 and 3 is similar to
the one found in the X-ray crystal structure of 3. Thus, the
signal of b-arylcarbon atom (C36, X-ray plot in Figure 2)
of phosphanes 2 and 3 was observed at d = 142–144.
These signals are split to a doublet with a large 2JC,P cou-
pling constant of 47–51 Hz. A correlation between the di-
hedral angle of P-lone-pair P–C–bC and the size of the
2JC,P coupling is well-established and suggests in this case
a dihedral angle of approximately 0° which is in agree-
ment with the geometry found in the crystalline state of
3.12
In summary, we have developed a practical high-yielding
two-step synthesis of two ortho-silyl chloride functional-
ized triarylphosphanes. X-ray crystal structure analysis of
silylphosphane 3 shows an interesting P/Si interaction,
which may allow to regard 3 as a model compound for the
early stages of the mechanism of SN2 nucleophilic dis-
placement at silicon.
Synthesis 2004, No. 6, 905–908 © Thieme Stuttgart · New York

PAPER
Synthesis of Chlorosilyl-Functionalized Triphenylphosphanes
907
Reactions were performed in flame-dried glassware under argon
(purity >99.998%). Petroleum ether used refers to the fraction with
bp 40–60 °C. The solvents were dried by standard procedures, dis-
tilled and stored under argon. All temperatures quoted are not cor-
HRMS: m/z calcd for C20H20ClPSi: 354.07604; found: 354.07601.
Chlorodiisopropyl(2¢-diphenylphosphanyl)phenylsilane (3)
To a solution of bromo-2-diphenylphosphinobenzene (1; 3.41 g, 10
mmol) in a mixture of Et2O–toluene (60 mL, 1:2) was added at r.t.,
a solution of n-BuLi (1.6 M in hexanes, 6.87 mL, 11 mmol). After
15 min, pure dichlorodiisopropylsilane (2.77 g, 15 mmol) was add-
ed. The resulting mixture was stirred for further 2 h, followed by
successive filtration with a filter paper and a syringe filter (0.45mm
porosity; 30 mm diameter). All volatile components were removed
in vacuo to give analytically pure phosphane 3; yield: 4.0 g (97%);
colorless crystals; mp 92 °C.
1
rected. H, 13C NMR spectra: Varian Mercury 300 HFCP, Bruker
AM 400, Bruker DRX 500, with TMS, CHCl3 or C6H6 as internal
standards. 31P NMR spectra: Varian Mercury 300 HFCP with 85%
H3PO4 as external standard. 29Si NMR: Bruker AM 400 with TMS
as internal standard. Melting points: Melting point apparatus by Dr.
Tottoli (Büchi). Elemental analyses: Vario EL (Elementaranalysen
GmbH). Mass spectrometry: Thermo Finnigan MAT 8200 and TSQ
7000. Flash chromatography: silica gel 40–63 mm (230–400 mesh,
Macherey-Nagel).
3
1H NMR (300.00 MHz, C6D6, 25 °C): d = 0.97 [d, 6 H, J1H = 7.3
Hz, CH(CH3)2], 1.28 [d, 6 H, 3J1H = 7.3 Hz, CH(CH3)2], 2.03 [sept,
o-Bromodiphenylphosphinobenzene (1)
3
3
1 H, J6H = 7.3 Hz, CH(CH3)2], 2.04 [sept, 1 H, J6H = 7.3 Hz,
CH(CH3)2], 6.97–7.16 (m, 8 H, ArH), 7.25 (m, 4 H, ArH), 7.36 (m,
1 H, ArH), 8.34 (m, 1 H, ArH).
To a solution of 1,2-dibromobenzene (12.4 g, 52.5 mmol) in Et2O–
THF (100 mL/100 mL) at –120 °C (with a cold bath composed of
ca. 85% Et2O, 10% acetone and 5% pentane and liquid N2) was add-
ed dropwise a solution of n-BuLi (1.47 M in hexanes, 35.7 mL, 49
mmol). The resulting mixture was stirred for further 45 min at –
120 °C followed by the addition of chlorodiphenylphosphine (11.03
g, 50 mmol). The reaction mixture was allowed to warm to –80 °C
during 1 h. At this temperature, a sat. aq solution of NH4Cl (80 mL)
was added. The resulting mixture was then allowed to warm to r.t.
during 1.5 h. After phase separation, the aqueous phase was extract-
ed with Et2O (3 × 75 mL). The combined organic phases were dried
(MgSO4) and all volatile components were removed in vacuo to fur-
nish 18 g of the crude product, which directly crystallized. The
product was dissolved in CH2Cl2 (170 mL) and filtered over a small
plug of silica gel to remove the traces of phosphane oxide. Recrys-
tallization from petroleum ether–Et2O (2:1) gave 12.7 g (76%) of
very fine white crystals of 1; mp 125 °C.
13C NMR (125.7 MHz, C6D6, 25 °C): d = 17.69, 17.80, 18.40,
3
18.42, 18.44, 18.45, 128.75 (2 C), 128.83 (d, JC,P = 6.5 Hz, 4 C),
129.41 (d, 4JC,P = 1.2 Hz), 130.21, 133.48 (d, 2JC,P = 18.1 Hz, 4 C),
4
2
135.90 (d, JC,P = 1 Hz), 137.71 (d, JC,P = 17.6 Hz), 142.08 (d,
1JC,P = 8.8 Hz, 3 C), 142.81 (d, 2JC,P = 51 Hz).
31P NMR (121.5 MHz, C6D6, 25 °C): d = –9.7.
29Si NMR (79.46 MHz, CDCl3, 25 °C): d = 32.6 (d, JSi,P = 7.4 Hz).
MS (EI, 70 eV): m/z (%) = 410 (34), 369 (39), 368 (33), 367 (100),
325 (26), 290 (42), 248 (54), 212 (16), 211 (16), 183 (97), 152 (43),
107 (20).
HRMS: m/z calcd for C24H28ClPSi: 410.1386; found: 410.1387.
Anal. Calcd for C24H28ClPSi (410.97): C, 70.14; H, 6.86. Found: C,
70.05; H, 6.87.
1H NMR (300.00 MHz, CDCl3, 25 °C): d = 6.75–6.81 (m, 1 H,
ArH), 7.16–7.23 (m, 2 H, 2 ArH), 7.30–7.42 (m, 10 H, 2 C6H5),
7.58–7.62 (m, 1 H, ArH).
X-ray Crystal Structure Analysis of Phosphane 313
Colorless crystals of phosphane 3 were grown from a petroleum
ether solution at r.t.: C24 H28 Cl P Si, M = 410.97, a = 10.1647(6),
b = 12.3576(6), c = 17.7690(10) Å, b = 97.319(3)°, V = 2213.8(2)
Å3, Z = 4, dcalcd = 1.233 Mg/m3. Crystal system: monoclinic, space
group P21/c. Data collection and processing: crystal size:
0.3 × 0.1 × 0.08 mm, Enraf-Nonius KappaCCD diffractometer,
l = 0.71073 Å, collected reflections: 8505, independent: 5024
(Rint = 0.048), observed: 3036 [I > 2s(I)], m = 0.306 mm–1, no ab-
sorption correction. Solution by direct phase determination (SIR-
97), full-matrix least-squares refinement on F2 (SHELXL-97), and
hydrogen positions were refined isotropically. Parameters 356, final
indices [I > 2 s(I)]: R = 0.0432, Rw2 = 0.0822, Goodness-of-fit on
F2: 0.915, largest diff. peak: 0.292 eÅ–3.
13C NMR (75.45 MHz, CDCl3, 25 °C): d = 127.6, 128.8 (d,
3JC,P = 7.2 Hz, 4 C), 129.2 (2 C), 130.1 (d, 2JC,P = 30.1 Hz), 130.3,
3
2
133.2 (d, JC,P = 2.3 Hz), 134.2 (d, JC,P = 20.1 Hz, 4 C), 134.6,
136.1 (d, 1JC,P = 11.2 Hz, 2 C), 139.1 (d, 1JC,P = 12.0 Hz).
31P NMR (121.5 MHz, CDCl3, 25 °C): d = –4.5.
Chlorodimethyl(2¢-diphenylphosphanyl)phenylsilane (2)
To a solution of bromo-2-diphenylphosphinobenzene (1; 170.6 mg,
0.5 mmol) in a mixture Et2O–toluene (6 mL, 1:2) was added, a so-
lution of n-BuLi (1.57 M in hexanes, 0.350 mL, 0.55 mmol) at r.t.
After 10 min, pure dichlorodimethylsilane (193 mg, 1.5 mmol) was
added. The resulting reaction mixture was filtered under argon with
a syringe equipped with a syringe filter (0.45 mm porosity, 13 mm
diameter). Evaporation of all volatile components in oil pump vac-
uo furnished 2 quantitatively. Spectroscopical analysis showed the
presence of £10% impurity by siloxane 4 which could not be sepa-
rated.
Degradation Product 4
Exposure of the arylchlorodimethylsilane 2 to air led to the quanti-
tative formation of siloxane 4; colorless crystals; mp 141 °C.
1H NMR (500.00 MHz, CDCl3, 25 °C): d = 0.49 (s, 12 H, 4 CH3),
7.15–7.23 (m, 10 H, ArH), 7.25–7.30 (m, 12 H, ArH), 7.30–7.36
(m, 4 H, ArH), 7.88 [m, 2 H, 2 C(3)-H].
1H NMR (500.00 MHz, CDCl3, 25 °C): d = 0.74 [s, 6 H, Si(CH3)2],
7.13–7.18 (m, 6 H, ArH), 7.21–7.25 (m, 5 H, ArH), 7.29 [app td, 1
H, 3J2H = 7.5 Hz, 4J1H = 1.5 Hz, C(5)-H], 7.34 [app tt, 1 H, 3J2H = 7.5
13C NMR (125.7 MHz, CDCl3, 25 °C): d = 9.6 (2 C), 10.1 (2 C),
128.25 (4 C), 128.41 (d, 3JC,P = 6.4 Hz, 8 C), 128.48 (2 C), 129.49
(2 C), 133.36 (d, 2JC,P = 18.5 Hz, 8 C), 135.06 (d, 2JC,P = 16.4 Hz, 2
4
3
Hz, J2H = 1.5 Hz, C(4)-H], 7.93 [dddd, 1 H, J1H = 7.5 Hz,
3JH,/P = 2.8 Hz, 4J1H = 1.5 Hz, 5J1H = 0.6 Hz, C(3)-H].
1
C), 135.18 (2 C), 138.21 (d, JC,P = 11.5 Hz, 4 C), 141.9 (d,
13C NMR (125,7 MHz, CDCl3, 25 °C): d = 5.78, 5.89, 128.59 (d,
3JC,P = 6.6 Hz, 4 C), 128.62 (2 C), 129.00, 130.54, 133.29 (d,
2JC,P = 18.3 Hz, 4 C), 135.40, 135.53 (d, 2JC,P = 16.4 Hz), 137.22 (d,
1JC,P = 12.0 Hz, 2 C), 148.1 (d, 2JC,P = 47.1 Hz, 2 C).
31P NMR (121.5 MHz, CDCl3, 25 °C): d = –9.94.
1
1JC,P = 9.4 Hz, 2 C), 142.34 (d, JC,P = 10.3 Hz), 144.47 (d,
MS (EI, 70 eV): m/z = 654 (38), 577 (48), 468 (90), 377 (28), 327
(25), 319 (62), 262 (68), 195 (32), 183 (100), 108 (26).
2JC,P = 47.6 Hz).
31P NMR (121.5 MHz, CDCl3, 25 °C): d = –10.4.
HRMS: m/z calcd for C40H40OPSi: 654.2092; found: 654.2090.
MS (EI, 70 eV): m/z (%) = 354 (11), 336 (14), 319 (20), 263 (20),
262 (86), 195 (16), 183 (100), 152 (17), 108 (30), 107 (17), 57 (17).
Synthesis 2004, No. 6, 905–908 © Thieme Stuttgart · New York

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D. Quintard et al.
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(6) (a) Trost, B. M.; Lee, C. Asymmetric Allylic Alkylation
Acknowledgment
Reactions, In Catalytic Asymmetric Synthesis, 2nd ed.;
Wiley-VCH: New York, 2000, Chap. 8E, 593. (b)Degrado,
S. J.; Mizutani, H.; Hoveyda, A. H. J. Am. Chem. Soc. 2001,
123, 755.
We thank the Fonds of the Chemical Industry and the Krupp Foun-
dation (Krupp Award for young university teachers to BB) for fi-
nancial support.
(7) (a) Bertozzi, C. R. Science 2000, 287, 2007. (b) Hang, H.
C.; Yu, C.; Pratt, M. R.; Bertozzi, C. R. J. Am. Chem. Soc.
2004, 126, 6. (c) Merkx, R.; Rijkers, D. T. S.; Kemmink, J.;
Liskamp, R. M. J. Tetrahedron Lett. 2003, 44, 4515.
(8) (a) Breit, B. Acc. Chem. Res. 2003, 36, 264. (b) Breit, B. In
Organic Synthesis Highlights V; Schmalz, H.-G.; Wirth, T.,
Eds.; Wiley-VCH: Weinheim, 2003, 68.
(9) Harder, S.; Brandsma, L.; Kanters, J. A.; Duisenberg, A.;
van Lenthe, J. H. J. Organomet. Chem. 1991, 420, 143.
(10) Kaftory, M.; Kapon, M.; Botoshansky, M. In The Chemistry
of Organic Silicon Compounds, Vol. 2; Rappoport, R.;
Apeloig, Y., Eds.; Wiley: Chichester, 1998, Chap. 5, 181.
(11) (a) Macharashvili, A. A.; Shklover, V. E.; Struchkov, Y. T.;
Oleneva, G. I.; Kramarova, E. P.; Shipov, A. G.; Baukov, Y.
I. J. Chem. Soc., Chem. Commun. 1988, 683. (b) For a
review, see: Bassindale, A. R.; Glynn, S. J.; Taylor, P. G. In
The Chemistry of Organic Silicon Compounds, Vol. 2;
Rappoport, R.; Apeloig, Y., Eds.; Wiley: Chichester, 1998,
Chap. 9, 495.
References
(1) (a) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G.
Principles and Applications of Organotransition Metal
Chemistry; University Science Books: Mill Valley, 1987,
Chap. 3.3, 66. (b) Applied Homogeneous Catalysis with
Organometallic Compounds, Vol. 1; Cornils, B.; Hermann,
W. A., Eds.; VCH: Weinheim, 1996. (c) Comprehensive
Asymmetric Catalysis, Vol. 1; Jacobsen, E. N.; Pfaltz, A.;
Yamamoto, H., Eds.; Springer: Berlin, 1999.
(2) (a) Bestmann, H. J.; Zimmermann, R. In Comprehensive
Organic Synthesis, Vol. 6; Trost, B. M.; Fleming, I.;
Winterfeldt, E., Eds.; Pergamon: Oxford, 1991, 171.
(b) Johnson, A. W. Ylides and Imines of Phosphorus; Wiley:
New York, 1993. (c) Maryanoff, B. E.; Reitz, A. B. Chem.
Rev. 1989, 89, 863.
(3) Mukaiyama, T. Angew. Chem., Int. Ed. Engl. 1975, 14, 801;
Angew. Chem. 1976, 88, 111.
(12) Verkade, J. G.; Quin, L. D. Phosphorus-31 NMR
Spectrospcopy in Stereochemical Analysis; VCH Publishers:
Deerfield Beach, Florida, 1987, 396.
(13) Crystallographic data (excluding structure factors) for the
structure reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary
publication no. CCDC-229247. Copies of the data can be
obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB21EZ, UK [Fax: +44(1223)336033; E-
Mail: deposit@ccdc.cam.ac.uk].
(4) Hughes, D. L. Org. React. 1992, 42, 335.
(5) (a) Dalko, P. I.; Moisan, L. Angew. Chem. Int. Ed. 2001, 40,
3726; Angew. Chem. 2001, 113, 3841. (b) Su, W.; McLeod,
D.; Verkade, J. J. Org. Chem. 2003, 68, 9499. (c) Wang,
L.-C.; Luis, A. L.; Agapiou, K.; Krische, M. J. J. Am. Chem.
Soc. 2002, 124, 2402.
Synthesis 2004, No. 6, 905–908 © Thieme Stuttgart · New York