Phosphonium sila-ylide: Reaction pathway different from ammonium sila-ylide but similar to phosphonium ylide [3]

Full text of DOI:10.1021/ja0160602

9210
J. Am. Chem. Soc. 2001, 123, 9210-9211
Phosphonium Sila-ylide: Reaction Pathway Different
from Ammonium Sila-ylide but Similar to
Phosphonium Ylide
Akio Toshimitsu,* Tomoyuki Saeki, and Kohei Tamao*
Institute for Chemical Research
Kyoto UniVersity, Uji, Kyoto 611-0011, Japan
ReceiVed April 21, 2001
Figure 1. X-ray structure of 1b in stereoview at 50% probability level.
All hydrogen atoms are omitted for clarity. Selected distances [Å] and
angles [deg]: Si1-F1 1.651(2), Si1-Si2 2.348(1), Si1‚‚‚P1 2.9866(9),
Si1-C1 1.901(3), F1-Si1‚‚‚P1 175.89(8), F1-Si-Si2 98.09(8),
Si2-Si1-C1 122.43(9), Si2-Si1-C11 113.9(1), C1-Si1-C11
116.1(1), Si1-C1-C2 113.5(2).
ReVised Manuscript ReceiVed July 11, 2001
Recently, we reported the novel reaction of an amine-coor-
dinated silylene or an ammonium sila-ylide1 bearing an 8-(di-
methylamino)-1-naphthyl group with diphenylacetylene to afford
the cyclized product containing the 1-silaphenalene skeleton.2
During this cyclization reaction, the acetylene carbons were in-
corporated between the silicon atom and the naphthyl carbon atom
bearing the amino group accompanied by the migration of the
amino group to the silicon center. Reported herein is a completely
different reaction pathway of a phosphine-coordinated silylene3
or a phosphonium sila-ylide1 bearing an 8-phosphino-1-naphthyl
group during the reaction with diphenylacetylene. The observed
behavior reflects the propensity of phosphorus, the heavier con-
gener of nitrogen, to form a pentavalent phosphonium ylide or
phosphorane.
Scheme 1
The pentacoordinate fluorodisilanes 1a and 1b bearing an
8-diethylphosphino- and 8-diphenylphosphino-1-naphthyl group,
respectively, the precursors of the phosphonium sila-ylide 2, were
prepared by the reaction of 8-phosphino-1-naphthyllithium with
1,1-difluoro-1,2-dimethyl-2,2-diphenyldisilane in a way similar
to that for the amine-coordinated counterpart 1c.2 The coordi-
nation of the phosphorus atom to the silicon atom has been
shown in the solid state and in solution by the X-ray crystal-
lographic analysis of 1b (Figure 1)4-7 and by NMR spectroscopy,8
respectively.
All of the reaction courses observed during the thermal
degradation of 1a and 1b are summarized in Scheme 1. When a
solution of pentacoordinate fluorodisilane 1a bearing the dieth-
ylphosphino group in toluene was heated at 110 °C for 12 h under
(1) In this paper, we call this species the sila-ylide for simplicity.
(2) (a) Tamao, K.; Asahara, M.; Saeki, T.; Feng, S.-G.; Kawachi, A.;
Toshimitsu, A. Chem. Lett. 2000, 660-661. (b) Review: Tamao, K.; Asahara,
M.; Saeki, T.; Toshimitsu, A. J. Organomet. Chem. 2000, 600, 118-123.
(3) Phosphine-coordinated silylenes: (a) Mu¨ller, G.; Waldkircher, M.; Pape,
A. In Organosilicon Chemistry III From Molecules to Materials; Auner, N.,
Weis, J., Eds.; Wiley-VCH: Weinheim, Germany, 1998; pp 452-459 and
references therein. (b) Karsch, H. H.; Witt, E. In Organosilicon Chemistry III
From Molecules to Materials; Auner, N., Weis, J., Eds.; Wiley-VCH:
Weinheim, Germany, 1998; pp 65-69.
(4) Crystal data for 1b: orthorhombic, P212121, colorless, a ) 17.2502(8)
Å, b ) 17.8934(6) Å, c ) 9.8489(2) Å, T ) 173 K, Z ) 4, R ) 0.036, Rw
0.049, GOF ) 0.94.
)
(5) The phosphine coordination in the fluorodisilane 1b has been found to
be similar to the amine coordination in analogous 1c2 as follows. The geometry
of Si1 adopts a pseudotrigonal bipyramid as shown by the %TBPe6 of 76%
with fluorine atom and phosphino group at the two pseudoapical posi-
tions, having the P1‚‚‚Si1-F1 angle of 175.89(8)° and P1‚‚‚Si1 distance of
2.9866(9) Å which is ca. 0.9 Å shorter than the sum of the van der Waals
radii.7
nitrogen, the R-elimination product, fluoromethyldiphenylsilane,
was obtained in almost quantitative yield.9 The thus produced
phosphonium sila-ylide 2a was trapped by diphenylacetylene to
afford a six-membered cyclic product 4 as a mixture of stereo-
isomers (ca. 2:1) in the total yield of 78%, which contains an
ethylydene bridge between the silicon atom and the phosphorus
atom and a (Z)-diphenylethenyl group on the silicon atom,
indicative of the hydrogen migration from the methylene in the
diethylphosphino group to the alkenyl carbon atom.
(6) Tamao, K.; Hayashi, T.; Ito, Y.; Shiro, M. Organometallics 1992, 11,
2099-2114.
(7) Bondi, A. J. Phys. Chem. 1964, 68, 441-451.
(8) In 13C NMR of 1a at room temperature, two kinds of signals are
observed for each of the methylene and methyl carbons of the diethylphosphino
group. This result indicates that the coordination of phosphorus to silicon is
strong enough to prevent the equilibration of the two ethyl groups via the
rotation about the phosphino-naphthyl bond. Moreover, spin-spin couplings
1
through the silicon-phosphorus bond are observed in the H, 13C, 19F, 29Si,
and 31P NMR spectra of 1a and 1b as follows. 1a: 1J(P,Si) ) 15 Hz, 2J(P,Si)
2
2
2
) 51 or 38 Hz [one of which is J(F,Si)], J(P,F) ) 108 Hz, J(P,C) ) 16
Hz, J(P,C) ) 6 Hz, J(P,H) ) 12 Hz. 1b: 2J(P,Si) ) 48 or 32 Hz [one of
which is 2J(F,Si)], 2J(P,F) ) 89 Hz, 2J(P,C) ) 15 Hz, 3J(P,C) ) 7 Hz, 3J(P,H)
) 10 Hz.
3
3
(9) It should be noted that this temperature is 30 °C lower than that required
for the thermal degradation of the amine-coordinated 1c.2
10.1021/ja0160602 CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/24/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 37, 2001 9211
ylide 7. The formed ylide carbanion nucleophilically attacks the
silicon atom causing the silicon-phosphorus bond fission, resulting
in the formation of the six-membered cyclic product 4. The
reaction course of 3a is different from that of the nitrogen
analogue 3c, presumably because the proton abstraction from the
dimethylamino group in 3c is not possible due to the lower ability
of nitrogen to stabilize the ylide.
For the diarylphosphino derivative 3b (XR2 ) PAr2) where
the proton abstraction is not possible from the diarylphosphino
group, the reaction course is still different from that of 3a and
3c. Thus, as shown in Scheme 1, the alkenyl anion 3 nucleo-
philically attacks the phosphorus atom to form a pentavalent
phosphorane intermediate 8 (or a transition structure), which
undergoes ligand coupling of the aryl group and the silicon atom
on the phosphorus center,14-16 resulting in the formation of the
seven-membered cyclic product 5 and 5′. This route seems to
have emerged due to the intrinsic ability of phosphorus to form
pentavalent species.
Figure 2. X-ray structure of 5′ in stereoview at 40% probability level.
All hydrogen atoms are omitted for clarity. Selected distances [Å] and
angles [deg]: Si1-C1 1.888(2), Si1-C11 1.868(2), Si1-C12 1.880(2),
Si1-C24 1.888(2), Si1‚‚‚P1 3.0593(7), C11-Si1‚‚‚P1 167.87(8),
C1-Si1-C12 112.10(8), C1-Si1-C24 107.63(8), C11-Si1-C12
106.43(9), C12-Si1-C24 112.48(8).
It is significant to compare the present sila-ylide chemistry with
the carbon protocol ylide chemistry. In the carbon chemistry, a
similar aryl group migration from phosphorus to carbon has been
observed during the reaction of a triarylphosphonium ylide with
benzyne.17 This reaction may also be explained by the ligand
coupling in the phosphorane species.
It may be concluded that the reaction pathway of the phos-
phonium sila-ylide with acetylene is similar to that of the carbon
protocol phosphonium ylide and different from that of the am-
monium sila-ylide.18 Thus, the reaction pathways of the sila-ylides
with acetylenes clearly reflect the character of the cationic center.
Further studies on the reaction pathways of sila-ylides bearing
other cationic moieties should contribute to clarifying the character
of the element.
Thermal degradation of pentacoordinate fluorodisilane 1b
bearing the diphenylphosphino group was carried out at 140 °C,
and the generated phosphonium sila-ylide 2b was trapped by
diphenylacetylene to afford a seven-membered cyclic product 5
in the yield of 67%, in which the diphenylacetylene has been
incorporated between the silicon atom and the phosphorus atom,
and one phenyl group has migrated from the phosphorus to silicon.
The P-to-Si phenyl migration has been confirmed by a similar
experiment using the di(4-fluorophenyl)phosphino analogue, in
which the 4-fluorophenyl group has migrated to afford 5′. The
structures of these products were confirmed by X-ray crystal-
lographic analyses, the structure of 5′ being shown in Figure 2.10,11
The cyclization reactions described above are best understood
by the nucleophilic attack of the phosphonium sila-ylide 2 on
the diphenylacetylene to form the zwitterionic intermediate 3
bearing alkenyl anion and phosphonium ion moieties as also
shown in Scheme 1.12,13 For the diethylphosphino derivative 3a
[XR2 ) P(C2H5)2], the alkenyl anion abstracts a proton from the
methylene of the diethylphosphino group to form the phosphonium
Acknowledgment. We thank the Ministry of Education, Culture,
Sports, Science and Technology, Japan, for the Grant-in-Aids for COE
Research on Elements Science, No. 12CE2005.
Supporting Information Available: Experimental details and spec-
troscopic data (PDF) and X-ray crystallographic files (CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
(10) Crystal data for 5: monoclinic, P21/c, colorless, a ) 16.667(1) Å, b
) 9.2935(6) Å, c ) 17.903(1) Å, â ) 94.538(3)°, T ) 173 K, Z ) 4, R )
0.065, Rw ) 0.085, GOF ) 1.21. Crystal data for 5′: triclinic, P-1, colorless,
a ) 12.4740(7) Å, b ) 13.1328(6) Å, c ) 11.3443(5) Å, R ) 107.217(3)°,
â ) 115.824(3)°, γ ) 63.616(2)°, T ) 173 K, Z ) 2, R ) 0.043, Rw ) 0.065,
GOF ) 1.07.
JA0160602
(14) (a) Wittig, G.; Maercker, A. Chem. Ber. 1964, 97, 747-767. (b)
Seyferth, D.; Fogel, J.; Heeren, J. K. J. Am. Chem. Soc. 1966, 88,
2207-2212. (c) Newkome, G. R.; Hager, D. C. J. Am. Chem. Soc. 1978,
100, 5567-5568. (d) Oae, S.; Uchida, Y. Acc. Chem. Res. 1991, 24, 202-
208.
(11) The seven-membered ring in 5 (and 5′) adopts a puckered structure
with the aryl group on the silicon atom and the unshared electron pair of the
phosphorus located at the “axial” positions. Judging from this direction, the
unshared electron pair of the phosphorus does not seem to coordinate to the
silicon center, despite the short P1‚‚‚Si1 distance [2.9416(9) and 3.0593(7)
Å] and the linear P1‚‚‚Si1-F1 arrangement [168.1(1)° and 167.87(8)°].
(12) Pioneering work of Seyferth et al. has demonstrated the nucleophilic
character of the phosphine-coordinated silylene in the reaction with carbonyl
compounds; Seyferth, D.; Lim, T. F. O. J. Am. Chem. Soc. 1978, 100, 7074-
7075.
(15) The ligand coupling reaction has been clarified to occur between two
apical groups by careful analyses of the thermolysis of pentaarylantimony
compounds.16 For the phosphorane 8, if it is involved as an intermediate, the
reaction pathway is expected to be the apical-equatorial coupling because three
ligands are incorporated in the rigid bicyclic structure.
(16) Akiba, K. Pure Appl. Chem. 1996, 68, 837-842.
(17) Zbiral, E. Tetrahedron Lett. 1964, 3963-3967; Monatsh. Chem. 1964,
95, 1759-1780.
(13) Judging from this reactivity, we feel it better to describe this species
(18) To the best of our knowledge, in carbon chemistry, the reaction of an
ammonium ylide with acetylenes has not been reported.
as a phosphonium sila-ylide.1