Nonvolatile Me3P-like P-donor ligand: Synthesis and properties of 4-phenyl-1-phospha-4-silabicyclo[2.2.2]octane

Article abstract of DOI:10.1021/ol0349099

(Matrix presented) A new trialkylphosphine ligand with Me ₃P-like steric and electronic properties, 4-phenyl-1-phospha-4-silabicyclo[2.2.2]octane (Ph-SMAP), was synthesized. Given a phenyl group at the silicon atom, the Ph-SMAP ligand displayed nonvolatility with retention of Me₃P-like properties. The new ligand was air-stable, crystalline, and easy to handle.

Full text of DOI:10.1021/ol0349099

ORGANIC
LETTERS
2003
Vol. 5, No. 15
2671-2674
Nonvolatile Me3P-like P-Donor Ligand:
Synthesis and Properties of
4-Phenyl-1-phospha-4-silabicyclo[2.2.2]octane
Atsuko Ochida, Kenji Hara, Hajime Ito, and Masaya Sawamura*
DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity,
Sapporo 060-0810, Japan
sawamura@sci.hokudai.ac.jp
Received May 23, 2003
ABSTRACT
A new trialkylphosphine ligand with Me3P-like steric and electronic properties, 4-phenyl-1-phospha-4-silabicyclo[2.2.2]octane (Ph-SMAP), was
synthesized. Given a phenyl group at the silicon atom, the Ph-SMAP ligand displayed nonvolatility with retention of Me3P-like properties. The
new ligand was air-stable, crystalline, and easy to handle.
Trialkylphosphines have found wide application in coordina-
tion chemistry and organometallic chemistry as metal-
coordinating ligands with strong σ-donating ability. One
ligand with an extremely low steric demand is trimeth-
ylphosphine (Me3P), which is especially important in the
metal-mediated C-H bond activation of hydrocarbons.1 We
have designed a new Me3P-like trialkylphosphine ligand 1
(SMAP, named after silicon-constrained monodentate alkyl-
phosphine) (Figure 1). A new feature of this ligand is the
presence of a site for functionalization at the backside of
the P-lone pair, which is not the case for Me3P. The SMAP
ligand 1 contains phosphorus and silicon atoms at each
bridgehead of the bicyclo[2.2.2]octane framework. The
molecular constraint of the bicyclic framework makes the
steric demand around the phosphorus center as small as that
of Me3P and projects the P-lone pair and the Si-substituent
(R) in diametrically opposite directions on the straight line
defined by the two bridgehead atoms (see Figure 1). P-Donor
ligands that can be functionalized with such a directional
constraint are rarely found and are limited to phosphaalkynes
(2),2 phosphabenzenes (3),3 bicyclic phosphites (4),4 and
phosphatriptycenes (5).5 To the best of our knowledge, no
analogous trialkylphosphine ligand exists.6 We expected that
the rigidity of this framework would promote molecular
(1) (a) Jones, W. D. In Topics in Organometallic Chemistry 3: ActiVation
of UnreactiVe Bonds and Organic Synthesis; Murai, S., Ed.; Springer:
Berlin, 1999; pp 9-46. (b) Kakiuchi, F.; Murai, S. In Topics in Organo-
metallic Chemistry 3: ActiVation of UnreactiVe Bonds and Organic
Synthesis; Murai, S., Ed.; Springer: Berlin, 1999; pp 47-79.
Figure 1. Structures of SMAP and related P-donor ligands.
10.1021/ol0349099 CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/20/2003

assembly into a crystal phase, yielding an odorless, crystalline
SMAP stable against air oxidation. We report the synthesis
and characterization of the first example of SMAP (1a, Ph-
SMAP), which contains a phenyl group on the Si atom that
provides a property of nonvolatility.
contained both trans- and cis-isomers (major:minor ) ca.
60:40). Upon heating with excess 1-octene in refluxing DME,
the isomeric mixture of 9 was transformed into phosphonium
salt 10 in 88% yield.9,10 Subsequent reductive cleavage of
the P-Ph bond of 10 with lithium naphthalenide followed
by reaction with sulfur afforded phosphine sulfide 11 in 40%
yield. Alternatively, addition of BH3‚SMe2 instead of S8 gave
borane complex 12 in 46% yield. Finally, desulfuration of
11 through S-methylation followed by treatment with
HMPT11 afforded white crystalline solid Ph-SMAP (1a) in
68% yield: mp 90.5-90.7 °C (in a sealed tube); sublimes
at 40 °C/0.04 mmHg; 31P NMR (C6D6, 85% H3PO4) δ -59.2
(+3.0 relative to Me3P/C6D6).12
The synthesis of Ph-SMAP (1a) is illustrated in Scheme
1. Phenyltrivinylsilane (6) was converted into triol 7 in 68%
Scheme 1. Synthesis of Ph-SMAPa
Solid Ph-SMAP is highly air-stable, with no detectable
oxidation observed after exposure to air for several days.13
Moreover, being almost odorless, Ph-SMAP does not
produce the noxious phosphine odor characteristic of volatile
phosphines.
Single-crystal X-ray diffraction analysis revealed a rod-
like shape of Ph-SMAP (1a) and Ph-SMAP-BH3 (12)
(Figure 2).14,15 Analyses also showed that the bicyclic cage
a Reagents and conditions: (a) (i) (Ipc)2BH, THF, -25 °C f
rt; (ii) H2O2, NaOH. (b) (i) MsCl, Et3N, THF, 0 °C; (ii) NaI, acetone,
0 °C f reflux. (c) BH3‚SMe2/PhPH2/BuLi (1:1:2), THF, -78 °C
f rt. (d) 1-Octene, DME, reflux. (e) (i) Lithium naphthalenide,
toluene, 40 °C; (ii) S8, toluene, 80 °C. (f) (i) MeOTf, CH2Cl2, rt;
(ii) P(NMe2)3, CH2Cl2, rt. (g) (i) Lithium naphthalenide, toluene,
40 °C; (ii) BH3‚SMe2, 0 °C.
yield through 3-fold hydroboration with diisopinocampheyl-
borane followed by H2O2/NaOH oxidation.7 The hydro-
boration proceeded with complete regioselectivity to give
the primary alcohol against the electronic requirement of the
silicon atom to induce the opposite selectivity. Mesylation
of 7 and subsequent substitution with iodide anion afforded
tris(2-iodoethyl)phenylsilane (8) in 82% yield. Then, [5 +
1] annulation between the triiodide (8) and dilithium salt8
of PhPH2-BH3 produced the borane complex of monocyclic
tertiary phosphine (9) carrying a 2-iodoethyl substituent at
the Si atom (47%). NMR analysis indicated that the product
Figure 2. ORTEP drawings of the molecular structures of Ph-
SMAP (1a, left) and Ph-SMAP-BH3 (12, right).
possesses some flexibility and twists toward chiral C3-
symmetric conformations. In free phosphine 1a, the values
for the average C-P-C and P-C-C-Si dihedral angles
and the P-Si distance are 100.9°, 15.5°, and 3.105 Å,
(2) (a) Markovskii, L. N.; Romanenko, V. D. Tetrahedron 1989, 45,
6019-6090. (b) Nixon, J. F. Chem. Soc. ReV. 1995, 24, 319-328.
(3) Le Floch, P.; Mathey, F. Coord. Chem. ReV. 1998, 178-180, 771-
791.
(4) Wadsworth, W. S., Jr.; Emmons, W. D. J. Am. Chem. Soc. 1962, 84,
610-617.
(5) (a) Jongsma, C.; De Kleijn, J. P.; Bickelhaupt, F. Tetrahedron 1974,
30, 3465-3469. (b) Rot, N.; De Wijs, W.-J. A.; De Kanter, F. J. J.; Dam,
M. A.; Bickelhaupt, F.; Lutz, M.; Spek, A. L. Main Group Met. Chem.
1999, 22, 519-526.
(6) For a diphosphine with a related structure, 1,4-diphosphabicyclo-
[2.2.2]octane, see: Hinton, R. C.; Mann, F. G. J. Chem. Soc. 1959, 2835-
2843.
(7) Brown, H. C.; Desai, M. C.; Jadhav, P. K. J. Org. Chem. 1987, 47,
5065-5069.
(8) Bourumeau, K.; Gaumont, A.-C.; Denis, J.-M. J. Organomet. Chem.
1997, 529, 205-213.
(9) Uziel, J.; Riege, N.; Aka, B.; Figuie`re, P.; Juge´, S. Tetrahedron Lett.
1997, 38, 3405-3408.
(10) High yield indicates that both isomers were converted into the
phosphonium salt 10.
(11) Omelanczuk, J.; Mikolajczyk, M. Tetrahedron Lett. 1984, 25, 2493-
2496.
(12) In the 13C{1H} NMR spectra of cage compounds 10-12 and 1a,
the signals for the ipso-carbons of the Si-phenyl groups were observed as
4
doublets with JC-P coupling constants of 3.5, 3.5, 3.4, and 4.5 Hz,
4
respectively. In contrast, no JC-P coupling was observed for monocyclic
compound 9. The long-range electronic interaction through the cage may
suggest that the electron-donating power of a SMAP ligand can be controlled
by a Si-substituent.
2672
Org. Lett., Vol. 5, No. 15, 2003

respectively. In BH3 complex 12, the P atom bonds to the B
atom with a distance of 1.922(2) Å.16 The average C-P-C
angle is enlarged to 104.2°. Such a slight enlargement of
the angles around the P atom is typical for the metal
coordination of a P-donor ligand. The BH3 coordination also
causes shrinkage of the cage as indicated by enlargement of
the P-C-C-Si dihedral angles (22.3°, averaged) and
shortening of the P-Si distance (3.031 Å). Although the cage
possesses some flexibility for twisting and stretching, almost
no bending of the longest molecular axes was observed for
both 1a and 12.
the sheets are then stacked along the b-axis so that the cage
and the aromatic ring are alternatively arranged to allow van
der Waals contacts. In contrast, the sheet of 12 is stacked
through C-H‚‚‚π interactions between neighboring aromatic
rings.
DFT calculations [B3LYP/6-31G(d,p)] indicated that Ph-
SMAP possesses an electron-donating ability as strong as
that of Me3P, and replacement of the Si atom of Ph-SMAP
with a carbon atom drastically decreases the donor power.
We optimized the geometry of Ph-SMAP and evaluated
donor ability by the value of the molecular electrostatic
potential minimum Vmin (kcal/mol) according to Koga’s
method.18 A larger negative Vmin value corresponds to a
stronger electron-donating ability of a phosphine. For
comparison, we also performed calculations for 4-phenyl-
1-phosphabicyclo[2.2.2]octane (13), an analogue of Ph-
SMAP that has a bridgehead carbon atom instead of the Si
atom.19 As shown in Table 1, the Vmin (-43.14 kcal/mol) of
Figure 3 represents densely packed crystal structures of
1a and 12. The former consists of the two enantiomeric
Table 1. Results of DFT Calculations for Various Tertiary
Phosphines
Vmin
average C-P-C angle
entry
phosphine
(t-Bu)3P
(i-Pr)3P
Et3P
Ph-SMAP (1a )
Me3P
Me2PhP
13
(kcal/mol)
(deg)a
1b
2b
3b
4
-45.48
-44.47
-43.51
-43.14
-43.02
-40.41
-39.06
-36.76
107.5
101.6
99.5
99.7
99.4
5b
6b
7
Figure 3. Crystal packing of Ph-SMAP (1a, left) and Ph-SMAP-
BH3 (12, right).
96.2
8b
MePh2P
a Values of optimized structures. b Data were taken from ref 18.
molecules (P21/n), while only a single enantiomer is involved
in the latter (chiral space group: P21).17 The feature common
to the crystal packing of 1a and 12 is the columnar stacking
along the a-axis through van der Waals contacts between
neighboring 1-phospha-4-silabicyclo[2.2.2]octane cages. In
both cases, the one-dimensional columns are further stacked
along the longest molecular axis in a head-to-tail manner to
form a sheet structure on the a,c plane. In the case of 1a,
Ph-SMAP is much more negative than the value of monoaryl-
dialkylphosphine PhMe2P and is in the range for trialkyl-
phosphines, being between the values of Me3P and Et3P.
However, the Vmin of 13 is less negative than that of PhMe2P.
The drastic decrease in donor ability upon placement of a
carbon atom at the bridgehead may be due to the increase
in s-character of the P-lone pair caused by the strain in the
1-phosphabicyclo[2.2.2]octane cage. The strain is evident
from the comparison of the C-P-C angles of the optimized
structures; the avarage angle of 13 (96.2°) is much smaller
than that of Ph-SMAP (99.7°), and the latter is almost the
same as the values of Me3P (99.4°) and Et3P (99.5°).
(13) To our surprise, Ph-SMAP showed considerable air-stability even
in solutions. A solution of Ph-SMAP in C6D6 prepared without exclusion
1
of air underwent no oxidation detectable by H NMR after standing for 3
days in the air. Similar experiments with CD2Cl2 and acetone-d6 produced
only a trace amount (<3%) of the corresponding phosphine oxide after 1
h.
(14) Crystal data for 1a: monoclinic, P21/n (#14), a ) 6.3438(3) Å, b
) 18.0866(5) Å, c ) 10.4720(6) Å, â ) 100.732(1)°, V ) 1180.52(9) Å3,
Z ) 4. Data collection: Rigaku RAXIS-RAPID Imaging Plate diffracto-
meter, T ) -153 °C. 2θmax ) 54.9°, R ) 0.041, Rw ) 0.076, I > 1.5σ(I),
GOF ) 1.96.
We presented a new trialkylphosphine Ph-SMAP with
steric and electronic features that guarantee its wide applica-
tion as a robust ligand for transition metal coordination.
SMAP derivatives with various Si-substituents can be easily
synthesized either by starting with the corresponding orga-
nosilicon compounds or by transforming Ph-SMAP through
(15) Crystal data for 12: monoclinic, P21 (#4), a ) 6.3632(3) Å, b )
7.6482(3) Å, c ) 13.6844(7) Å, â ) 660.93 Å3, Z ) 2. Data collection:
Rigaku RAXIS-RAPID Imaging Plate diffractometer, T ) -153 °C, 2θmax
) 54.9°, R ) 0.028, Rw ) 0.037, I > 3σ(I), GOF ) 1.26.
(16) P-B bond length of 12 (1.922(2) Å) is apparently longer than that
of Me3P-BH3 (1.901 Å) as determined by microwave spectroscopy. See:
Bryan, P. S.; Kuczkowski, R. L. Inorg. Chem. 1972, 11, 553-559. The
reason of the elongation is not clear at present. Correlation between steric/
electronic properties of phosphines and P-B bond lengths is not clear in
general. For a review, see: Brunel, J. M.; Faure, B.: Maffei, M. Coord.
Chem. ReV. 1998, 178-180, 665-698.
(18) Suresh, C. H.; Koga, N. Inorg. Chem. 2002, 41, 1473-1578.
(19) Phosphine oxide form of 1-phosphabicyclo[2.2.2]octane was syn-
thesized, and its strain around the P atom was discussed. However, it has
not been converted to the free phosphine. See: Wetzel, R. B.; Kenyon, G.
L. J. Am. Chem. Soc. 1974, 96, 5189-5198.
(17) This is a case of chiral crystallization of an achiral molecule with
chiral conformations.
Org. Lett., Vol. 5, No. 15, 2003
2673

Si-Ph bond cleavage. The molecular rigidity and flexibility
of functionalization allows the preparation of a sophisticated
series of SMAP ligands providing useful components for
supramolecular architectures based on coordination chem-
istry.
program of JST. We thank D. Sato and T. Kato for their
help in the early stages of this work and N. Kobayashi for
assistance in the X-ray structure analysis.
Supporting Information Available: Experimental pro-
cedures and spectroscopic data for all products. This material
is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. The present research was supported
by a Grant-in-Aid for Scientific Research (No. 13874095),
a grant from Sumitomo Foundation, and the PRESTO
OL0349099
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Org. Lett., Vol. 5, No. 15, 2003