Deprotonation pathway in the reaction of Me6Si2 with MeLi

Article abstract of DOI:10.1021/om9908175

A mixture of 3 mol of Me₆Si₂ and 1 mol of MeLi in the presence of P(O)(NMe₂)₃ produced Me₃SiLi as the initial product. This then deprotonated excess Me₆Si₂ and started an unprecedented transformation leading to (Me₃Si)₂(SiMe₂H)CLi (3), whose quench by [(η⁵-C₅H₅)Fe(CO)₂PPh ₃⁺] produced {η⁴-exo-[(Me₃Si)₂C-(SiMe ₂H)]C₅H₅}Fe(CO)₂(PPh₃) (2e). In the literature the reaction of Me₆Si₂ and MeLi gives Me₃SiLi and/or Me₃SiSiMe₂Li (4). The lithium compound 3 is the major product when a previously unnoticed deprotonation pathway is enhanced by use of an excess of Me₆Si₂ and a longer reaction time.

Full text of DOI:10.1021/om9908175

374
Organometallics 2000, 19, 374-376
Dep r oton a tion P a th w a y in th e Rea ction of Me6Si2 w ith
MeLi
Ling-Kang Liu*,†,‡ and Lung-Shiang Luh†
Institute of Chemistry, Academia Sinica, Nankang, Taipei, Taiwan 11529, ROC,
and Department of Chemistry, National Taiwan University, Taipei, Taiwan 10767, ROC
Received October 13, 1999
Summary: A mixture of 3 mol of Me6Si2 and 1 mol of
MeLi in the presence of P(O)(NMe2)3 produced Me3SiLi
as the initial product. This then deprotonated excess Me6-
Si2 and started an unprecedented transformation lead-
ing to (Me3Si)2(SiMe2H)CLi (3), whose quench by
[(η5-C5H5)Fe(CO)2PPh3+] produced {η4-exo-[(Me3Si)2C-
(SiMe2H)]C5H5}Fe(CO)2(PPh3) (2e). In the literature the
reaction of Me6Si2 and MeLi gives Me3SiLi and/ or Me3-
SiSiMe2Li (4). The lithium compound 3 is the major
product when a previously unnoticed deprotonation
pathway is enhanced by use of an excess of Me6Si2 and
a longer reaction time.
SiLi. The color of the solution changed gradually from
black to orange-red during the addition of R3SiLi,
sometimes with formation of a yellow precipitate that
redissolved as the reaction proceeded. For the silyl
anions with at least one aryl group, the Cp ring
silylation products (η4-exo-R3SiC5H5)Fe(CO)2(PPh3) (R3
) Ph3 (2a , 51%), MePh2 (2b, 36%), Me2Ph (2c, 45%))
Organosilanes have been used to enhance reactivity
and selectivity in chemical transformations.1 In one
synthetic approach, the addition of silyl anions to a
variety of organic electrophiles results in formation of
the needed Si-C bond.2 Among such silyl anions, R3-
SiLi can be generated in situ by reaction of R3SiCl with
Li, of (R3Si)2Hg with Li, or of R3SiSiR3 with R′Li.3 We
have used the electrophile [(η5-C5H5)Fe(CO)2PPh3+] (1)
to quench the generated silyl anions. The results
revealed an unprecedented conversion of Me3SiLi to
(Me3Si)2(SiMe2H)CLi and gave retro-chemical evidence
for a deprotonation pathway in the Me6Si2 reaction with
MeLi.
The R3SiLi anions were generated by reaction of the
respective disilane with MeLi. The resulting solutions
were cooled to -78 °C and transferred dropwise by
cannula to a solution of 1:1 (η5-C5H5)Fe(CO)2I/PPh3 in
THF at -78 °C, the practical equivalent to [1][I], after
chemical initiation with a trace of lithiated reagent,4
which in the present cases is the first few drops of R3-
were isolated as major products after column chroma-
tography.5 Thus, the Si-based nucleophiles add at the
Cp ring of 1, similar to what occurred with C-based
nucleophiles6 and different from O-based nucleophiles
(5) Manipulations were carried out under N2 with dry degassed
reagents. Preparation of 2a (typical): a 100 mL two-necked flask was
charged with Ph3SiCl (5 mmol), and fine-cut Li wire (20 mmol) and
then THF (30 mL) was added. The solution became turbid after stirring
for several minutes and the color changed gradually from yellow to
brown to black. After it was stirred for 6 h, the resulting solution was
cooled to -78 °C and filtered through a pad of Celite. The filtrate was
added dropwise to a mixture of (η5-C5H5)Fe(CO)2I (3 mmol) and PPh3
(3 mmol) in THF (100 mL), also at -78 °C. The color of solution
changed from black to orange-red during the addition, accompanied
by the formation of a yellow precipitate, which redissolved when the
addition was completed. The reaction mixture was quenched with H2O
(200 mL) and extracted with Et2O (100 mL × 2) after it was stirred
overnight. The organic layers were combined, dried over MgSO4, and
then evaporated to dryness under vacuum. The oily residue was
purified by SiO2 column chromatography with 1/15-20 EtOAc/hexane
as eluent to give yellow-orange 2a (51%). IR (CH2Cl2): νCO 1962 (s),
* To whom correspondence should be addressed. E-mail: liuu@
chem.sinica.edu.tw.
† Academia Sinica.
1903 (s) cm-1
2H), 6.96-7.53 (m, 30H). 31P NMR (C6D6): δ 72.1 (s). 29Si NMR
(C6D6): δ -22.6 (s). FAB MS: m/z 698 (M+). Anal. Calcd for C43H35
FeO2PSi: C, 73.92; H, 5.05. Found: C, 73.60; H, 4.99. 2b: yield 36%.
IR (CH2Cl2): νCO 1963 (s), 1902 (s) cm-1 1H NMR (C6D6): δ 0.32 (s,
.
1H NMR (C6D6): δ 2.71 (b, 2H), 3.90 (b, 1H), 5.10 (b,
‡ National Taiwan University.
(1) (a) Fleming, I. In Comprehensive Organic Chemistry; Barton, E.,
Ollis, W. E., Eds.; Pergamon Press: Oxford, U.K., 1979. (b) Colvin, E.
Silicon in Organic Synthesis; Butterworth: Boston, 1981. (c) Weber,
W. P. Silicon Reagents in Organic Synthesis; Springer-Verlag: New
York, 1983. (d) Patai, S., Rappoport, Z., Eds. The Chemistry of Organic
Silicon Compounds; Wiley: New York, 1989. (e) Hwu, J . R.; Wang, N.
Chem. Rev. 1989, 89, 1599. (f) Rappoport, Z.; Apeloig, Y., Eds. The
Chemistry of Organic Silicon Compounds; Wiley: New York, 1998; Vol.
2.
(2) Oshima, K. In Advances in Metal-Organic Chemistry; Liebeskind,
L. S., Ed.; J AI Press: London, 1991; Vol. 2, pp 101-141.
(3) (a) Tamao, K.; Kawachi, A. Adv. Organomet. Chem. 1998, 38, 1
and references cited herein. (b) Wiberg, E.; Stecher, O.; Andrascheck,
H. J .; Kreubichler, L.; Staude, E. Angew. Chem., Int. Ed. Engl. 1963,
2, 507. (c) Fujita, M.; Hiyama, T. J . Synth. Org. Chem. J pn. 1984, 42,
293. (d) Vyazankin, N. S.; Razuvaev, G. A.; Gladyshev, E. A.; Korneva,
S. P. J . Organomet. Chem. 1967, 7, 353. (e) Gladyshev, E. N.; Fedorova,
E. A.; Yuntila, L. A.; Razuvaev, G. A.; Vyazankin, N. S. J . Organomet.
Chem. 1975, 96, 169.
-
.
3H), 2.57 (b, 2H), 3.51 (b, 1H), 5.18 (b, 2H), 6.97-7.52 (m, 25H). 31P
NMR (C6D6): δ 71.6 (s). 29Si NMR (C6D6): δ -17.3 (d, J PSi ) 10.0 Hz).
FAB MS: m/z 636 (M+). Anal. Calcd for C38H33FeO2PSi: C, 71.70; H,
5.23. Found: C, 71.58; H, 5.22. Orange side product (η5-C5H5)Fe(CO)C-
(O)SiMePh2(PPh3): yield 12%. IR (CH2Cl2): νCO 1906 (s), 1574 (m)
cm-1. 1H NMR (C6D6): δ 1.22 (s, 3H), 4.07 (s, 5H), 6.96-7.77 (m, 25H).
31P NMR (C6D6): δ 75.7 (s). 29Si NMR (C6D6): δ -36.5(s). Anal. Calcd
for C38H33FeO2PSi: C, 71.70; H, 5.23. Found: C, 71.87; H, 5.15. 2c:
yield 45%. IR (CH2Cl2): νCO 1960 (s), 1901 (s) cm-1
.
1H NMR (C6D6):
δ -0.04 (s, 6H), 2.46 (b, 2H), 3.02 (b, 1H), 5.18 (b, 2H), 6.95-7.48 (m,
20H). 31P NMR (C6D6): δ 71.8 (s). 29Si NMR (C6D6): δ -11.5 (d, J PSi
)
8.0 Hz). FAB MS: m/z 574 (M+). Anal. Calcd for C33H31FeO2PSi: C,
69.00; H, 5.44. Found: C, 69.16; H, 5.33.
(6) (a) Liu, L.-K.; Luh, L.-S. Organometallics 1994, 13, 2816. (b) Luh,
L.-S.; Liu, L.-K. Bull. Inst. Chem., Acad. Sin. 1994, 41, 39. (c) Liu,
L.-K.; Luh, L.-S.; Chao, P.-C.; Fu, Y.-T. Bull. Inst. Chem., Acad. Sin.
1995, 42, 1. (d) Luh, L.-S.; Eke, U. B.; Liu, L.-K. Organometallics 1995,
14, 440.
(4) Gipson, S. L.; Liu, L.-K.; Soliz, R. U. J . Organomet. Chem. 1996,
526, 393.
10.1021/om9908175 CCC: $19.00 © 2000 American Chemical Society
Publication on Web 01/21/2000

Communications
Organometallics, Vol. 19, No. 4, 2000 375
F igu r e 1. Molecular plot of 2b.
F igu r e 2. Molecular plot of 2e.
which react at the CO ligand.7 The single-crystal X-ray
structure of 2b confirmed that the MePh2Si group is exo
to Fe (Figure 1),8 indicative of a direct silyl attack.
Compounds 2a -c were desilylated upon acidification
with HBF4(aq) or HCl(aq) to re-form the cationic com-
nucleophilic base easily breaks the Si-Si bond.11 When
the reaction is carried out on a larger scale, the C-Si
bond in Me6Si2 is often observed to be cleaved in a side
reaction, which leads to the formation of Me3SiSiMe2Li
(4)12 (Scheme 1).13 Obviously, the negative charge on
the silicon atom is stabilized by the second silicon atom
in the R-position.14 Still another pathway that has not
1
pound [1][X] (X ) BF4, Cl), as shown by IR and H and
31P NMR spectroscopic studies.
Me3SiLi was prepared by treating Me6Si2 with MeLi
in the presence of P(O)(NMe2)3 for 15 min at 0 °C.9 The
reagent solution was then quenched with 1:1 (η5-C5H5)-
Fe(CO)2I/PPh3 at -78 °C in THF, with the expectation
that {η4-exo-(Me3Si)C5H5}Fe(CO)2(PPh3) (2d ) would be
formed. IR monitoring of the reaction solution indicated
a quantitative formation of an η4 product. However,
after column chromatography, only a low-yield product
(<10%) was obtained with spectroscopic data as follows
(cf. 2a -c): IR νCO stretching bands at 1967 (s) and 1908
(s) cm-1, a 31P NMR resonance (C6D6) at δ 76.1 (s), and
1H NMR resonances (C6D6) at δ 2.58 (b, 2H), 3.65 (b,
1H), and 5.03 (b, 2H). Nevertheless, the proton integra-
tion for silyl-Me was not correct, nor was the elemental
analysis. A single-crystal X-ray analysis confirmed the
structure as being {η4-exo-[(Me3Si)2C(SiMe2H)]C5H5}-
Fe(CO)2(PPh3) (2e), which is actually a Cp ring alkyla-
tion product (Figure 2).10
It seemed possible that 2d might have been produced
as the initial product but was desilylated or decomposed,
since 2a -c are acid-sensitive. However, the generation
of Me3SiLi by the literature procedure9 must have also
produced, at the same time, a small amount of (Me3-
Si)2(SiMe2H)CLi (3) before the quench (vide infra). We
suggest that the in situ generation of deep red Me3SiLi,
from MeLi and Me6Si2, is facile because a strong
(10) Preparation of 2e. A solution of Me3SiSiMe3 (5 mmol) in
anhydrous P(O)(NMe2)3 (4 mL) was cooled to 0 °C. MeLi (4 mmol) was
added via syringe, and the resulting deep-red solution was stirred for
15 min. THF (30 mL) was added, and the solution was cooled to -78
°C. The solution was transferred dropwise via cannula into the mixture
of (η5-C5H5)Fe(CO)2I (3 mmol) and PPh3 (3 mmol) in THF (100 mL) at
-78 °C. The color of solution gradually changed from black to orange,
accompanied by the formation of a yellow precipitate that redissolved
when the addition was completed. The solution was warmed to room
temperature and stirred overnight before the mixture was quenched
with H2O (200 mL) and extracted with Et2O (100 mL × 2). The
combined organic layers were dried over MgSO4 and then evaporated
to dryness under vacuum. The oil-like residue was purified by SiO2
column chromatography with 1:12 (v/v) EtOAc/hexane as eluent to give
yellow-orange 2e (6%). Improved procedure: The treatment of
Me3SiSiMe3 (15 mmol) in anhydrous P(O)(NMe2)3 (12 mL) with MeLi
(5 mmol) and stirring for 2 h, (otherwise the same procedure as above)
resulted in 2e (60%). IR (CH2Cl2): νCO 1967(s), 1908(s) cm-1 1H NMR
.
(C6D6): δ 0.14 (s, 18H), 0.21 (d, 3J HH = 4 Hz, 6H), 2.58 (b, 2H), 3.65 (b,
3
1H), 4.30 (hept, J HH = 4 Hz, 1H), 5.03 (b, 2H), 6.98-7.53 (m, 15H).
31P NMR (C6D6): δ 76.1(s). 29SiNMR (C6D6): δ -15.9 (s), -16.5 (s).
FAB MS m/z: 656 (M+). Anal. Calcd for C34H45FeO2PSi3: C, 62.18; H,
6.90. Found: C, 61.99; H, 6.81. Crystal data of 2e: C34H45FeO2PSi3,
triclinic P1h, a ) 11.283(2) Å, b ) 12.591(2) Å, c ) 14.276(3) Å, a )
66.63(1)°, â ) 72.17(2)°, γ ) 76.70(1)°, V ) 1758.5(5) Å3, Z ) 2, Dcalcd
) 1.240 g/cm3, 5238 reflections (I > 2.5σ(I)), 374 parameters, R ) 0.033,
Rw ) 0.045, GOF ) 2.31.
(11) Marschner, C. Eur. J . Inorg. Chem. 1998, 221.
(12) (a) Hudrlik, P. F.; Waugh, M. A.; Hudrlik, A. M. J . Organomet.
Chem. 1984, 271, 69. (b) Nadler, E. B.; Rappoport, Z. Tetrahedron Lett.
1990, 31, 555. (c) Allred, A. L.; Smart, R. T.; Van Beek, D. A., J r.
Organometallics 1992, 11, 4225. (d) Hwu, J . R.; Wetzel, J . M.; Lee, J .
S.; Butcher, R. J . J . Organomet. Chem. 1993, 453, 21. (e) Krohn, K.;
Khanbabaee, K. Angew. Chem., Int. Ed. Engl. 1994, 33, 99.
(13) It is believed that the C-based nucleophile MeLi is transformed
to the Si-based nucleophile Me3SiLi before other second-stage reactions.
As an indirect clue, the formation of 4 was only observed in large-
quantity preparations.
(14) Armitage, D. A. In Comprehensive Organometallic Chemistry:
The Synthesis, Reactions and Structures of Organometallic Compounds;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press:
Oxford, U.K., 1982; Vol. 2, pp 1-203.
(7) Liu, L.-K.; Eke, U. B.; Mesubi, M. A. Organometallics 1995, 14,
3958.
(8) Crystal data for 2b: C38H33FeO2PSi, triclinic, P1h, a ) 9.619(1)
Å, b ) 10.424(1) Å, c ) 16.566(3) Å, R ) 87.22(1)°, â ) 77.06(1)°, γ )
82.19(1)°, V ) 1603.5(3) Å3, Z ) 2, Dcalcd ) 1.318 g/cm3, 4739 reflections
(I > 2.0 σ(I)), 389 parameters, R ) 0.031, Rw ) 0.039, GOF ) 1.92.
(9) Still, W. C. J . Org. Chem. 1976, 41, 3063.

376 Organometallics, Vol. 19, No. 4, 2000
Communications
Sch em e 1
nation in Scheme 2 must be Me3SiLi. Thus, it takes
overall 3 mol of Me6Si2 in order for 1 mol of MeLi to
produce 1 mol of 3; therefore, an increase in stoichio-
metric ratio between Me6Si2 and MeLi should favor the
deprotonation pathway. The isolated yield of 2e was
improved to 60% when a 3:1 mixture of Me6Si2/MeLi
was allowed to react for a longer time (2 h), resulting
in a color change from deep red to orange before the
quench. To our knowledge, this is the first example of
the transformation of a Me anion to a silyl anion and
then back to a carbanion, starting with a simple
disilane. The present deprotonation pathway in the
reaction of Me6Si2 with MeLi is intermolecular. The
known intramolecular transfer of the organolithium
function in 1-Me3Si-8-Li-C10H6 to form 1-Me2SiCH2Li-
Sch em e 2
17
C10H7 is a similar process (C10H6 ) 1,8-disubstituted
naphthalene skeleton; C10H7 ) 1-substituted naphtha-
lene skeleton).
The speculative mechanism shown in Scheme 2 was
tested with different organic electrophiles in order to
provide evidence that 3 actually is formed under the
reaction conditions. When a 3:1 mixture of Me6Si2/MeLi
was quenched with Me3SiCl, for instance, the expected
(Me3Si)3CSiMe2H18 could be isolated (ca. 30%, not
optimized) (29Si NMR (C6D6) δ -16.4 (SiMe3) and -16.1
1
(SiMe2H); H NMR (C6D6) δ 0.24 (s, SiMe3, 27H), 0.29
2
2
(d, J HH ) 4.0 Hz, SiMe2H, 6H), 4.31 (hept, J HH ) 4.0
Hz, SiMe2H, 1H)). Spectroscopic evidence for the forma-
tion of Me3SiH also was obtained. In a sealed NMR tube
experiment, a 3:1 mixture of Me6Si2/MeLi with P(O)-
1
(NMe2)3 in d8-THF gave H NMR peaks at δ 4.61 (hept,
yet been reported for Me6Si2 is deprotonation of a
methyl substituent, which results in a lithiated carban-
ion Me3SiSiMe2CH2Li (5) (Scheme 2). The R-SiMe2
group stabilizes the polar C-Li bond. The â-SiMe3
group, however, destabilizes the C-Li bond (a â-Si atom
normally stabilizes a carbonium ion).14 Thus, intramo-
lecular Me3Si group migration15 from the silicon atom
to the carbon atom results, followed by a 1,2-proton
shift. This gives the lithiated carbanion Me3Si(SiMe2H)-
CHLi (6), which is isomeric with 5 and is stabilized by
two R-silyl groups. An extra 1 equiv of Me6Si2 is
attacked by 6 to cleave the Si-Si bond and regenerate
Me3SiLi, which deprotonates the (Me3Si)2(SiMe2H)CH
thus formed to give the final lithiated species, 3, which
is stabilized by three R-silicon atoms. The nucleophilic
alkylation of the Cp ring of 1 by 3 affords 2e.16
2J HH ) 4.0 Hz), assigned to the unique SiH of 3, and at
2
δ 4.00 (decatet, J HH ) 4.0 Hz), assigned to the unique
SiH of HSiMe3, in the correct molar ratios. The corre-
sponding 29Si NMR data were δ -28.5 (SiMe3) and
-27.9 (SiMe2H) for 3 and δ -16.1 for Me3SiH.
In conclusion, the reaction of Me6Si2 and MeLi results
in Me3SiLi (and 4), plus the previously unnoticed 3. The
latter is the more important product when excess Me6-
Si2 is used.
Ack n ow led gm en t. Thanks are due to the National
Science Council of the ROC for financial support, to Mr.
Yuh-Sheng Wen for X-ray data collection, and to Prof.
J ih-Ru R. Hwu and Prof. Hiromi Tobita for fruitful
discussions.
As the characteristic deep red color of Me3SiLi in
solution is clearly observed, the base effecting deproto-
Su p p or tin g In for m a tion Ava ila ble: Details of the single-
crystal structure analyses for 2b,e including tables of posi-
tional parameters and bond lengths and angles. This material
is available free of charge via the Internet at http://pubs.acs.org.
(15) (a) Brook, A. G.; Bassindale, A. R. In Rearrangements in Ground
and Excited States; De Mayo, P., Ed.; Academic Press: New York, 1980;
Vol. 2, pp 149-227. (b) Eisch, J . J .; Tsai, M.-R. J . Organomet. Chem.
1982, 225, 5.
OM9908175
(16) Occasionally {η4-exo-[(Me3Si)CH(SiMe2H)]C5H5}Fe(CO)2(PPh3),
the speculative Cp-ring alkylation product of 6 and 1, could be detected
in trace amount in the 1H NMR spectrum of 2e. The pure complex
has not been isolated for complete characterization.
(17) Wroczynski, R. J .; Baum, M. W.; Kost, D.; Mislow, K.; Vick, S.
C.; Seyferth, D. J . Organomet. Chem. 1979, 170, C29.
(18) Dua, S. S.; Eaborn, C.; Happer, D. A. R.; Hopper, S. P.; Safa,
K. D.; Walton, D. R. M. J . Organomet. Chem. 1979, 178, 75.