Formation of acylsilenolates from Bis(acyl)trisilanes as the silicon analogues of acylenolates

Article abstract of DOI:10.1021/om100643d

Reactions of bis(acyl)trisilanes with tris(trimethylsilyl)silyllithium in THF, followed by treatment of the resulting dark red solutions with alkyl halides, gave the Si-alkylated products and tetrakis(trimethylsilyl)silane, indicating the formation of lithium acylsilenolates via Si-Li exchange. Exothermic formation of lithium acylsilenolate was demonstrated by DFT calculations on a model reaction.

Full text of DOI:10.1021/om100643d

Organometallics 2010, 29, 4199–4202 4199
DOI: 10.1021/om100643d
Formation of Acylsilenolates from Bis(acyl)trisilanes as the Silicon
Analogues of Acylenolates
Joji Ohshita,*,† Hiroyuki Kawamoto,† Atsutaka Kunai,† and Henrik Ottosson‡
†Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University,
Higashi-Hiroshima 739-8527, Japan, and ‡Department of Biochemistry and Organic Chemistry,
Box 576, Uppsala University, 751 23 Uppsala, Sweden
Received July 3, 2010
Summary: Reactions of bis(acyl)trisilanes with tris(trimethyl-
(Scheme 1). Later on, the crystal structure of a crown ether
complex of potassium tert-butylbis(trimethylsilyl)silenolate
was determined by an X-ray diffraction study, which re-
vealed that the potassium atom was located closer to the
silyl)silyllithium in THF, followed by treatment of the resulting
dark red solutions with alkyl halides, gave the Si-alkylated
products and tetrakis(trimethylsilyl)silane, indicating the for-
mation of lithium acylsilenolates via Si-Li exchange. Exo-
thermic formation of lithium acylsilenolate was demonstrated
by DFT calculations on a model reaction.
˚
carbonyl oxygen (K-O = 2.846 A) rather than to the center
˚
silicon atom (K-Si = 3.714 A) and that the central silicon
atom of the potassium silenolate was markedly pyramidal.7
We have also revealed through density functional theory
(DFT) computations that an uncomplexed silenolate is best
described by a resonance structure corresponding to an acyl-
substituted silyl anion rather than by a resonance structure
corresponding to an idealized silenolate.8 According to these
calculations, a potassium ion solvated by THF molecules
leaves the silenolate structure rather unaffected. Recently,
Apeloig and co-workers reported the crystal structures of
donor-free lithium silenolates with completely planar Si
centers.9 Lithium silenolates react readily with several kinds
of electrophiles to form substitution products,10 including bis-
(acyl)trisilanes from the reactions with acyl chlorides, as the
first example of silicon analogues of 1,3-diones (Scheme 2).11
In this paper, we report the formation of lithium acylsileno-
lates from bis(acyl)trisilanes, as the first example of silicon
analogues of acylenolates.12
Introduction
Functionalized silyl anions are of importance because of
their usefulness as synthetic tools for variously substituted
organosilicon compounds. To date, alkoxy-,1 amino-,2 fluoro-,3
arylthio-,4 and boryl-substituted5 silyl anions have been pre-
pared, and their chemical behaviors were explored. Previou-
sly, we found that the interaction of acyltris(trimethylsilyl)-
silanes with tris(trimethylsilyl)silyllithium ((Me3Si)3SiLi) or
methyldiphenylsilyllithium (MePh2SiLi) in THF afforded
lithium silenolates via Li-SiMe3 exchange, as the silicon
analogues of lithium enolates.6 The structures of the lithium
silenolates were first characterized by NMR spectroscopic
analysis of the reaction mixtures, which indicated that the
center Si-C(O) bonds possess double-bond character, de-
pending on the substituent at the carbonyl carbon. Thus, the
NMR spectra of aryl-substituted silenolates were understood
in terms of lithium silenolates rather than acylsilyllithiums,
while alkyl substitution increased the sp3 character of the
center silicon, increasing the contribution of acylsilyllithiums
Results and Discussion
Formation of Lithium Acylsilenolates. When 2,2-dimesi-
toylhexamethyltrisilane (1a) was treated with 1.2 equiv of
(Me3Si)3SiLi in THF at -80 °C, the pale yellow solution
immediately became dark red and tetrakis(trimethylsilyl)-
silane ((Me3Si)4Si) was formed quantitatively. After the
mixture was stirred for 1 h at this temperature, alkyl halides
were added to the dark red solution to produce Si-substituted
products, dimesitoylalkyldisilanes 2aa-2ad, together with
small amounts of 1,2-dione 3a, as summarized in Table 1
(Scheme 2). The reaction with methyl iodide proceeded
*To whom correspondence should be addressed. Tel: þ81-82-424-
7743. Fax: þ81-82-424-5494. E-mail: jo@hiroshima-u.ac.jp.
(1) (a) Tamao, K.; Kawachi, A. Angew. Chem., Int. Ed. Engl. 1995,
34, 818. (b) Tamao, K.; Kawachi, A. Organometallics 1995, 14, 3108.
(c) Kawachi, A.; Tamao, K. Organometallics 1996, 15, 4653. (d) Kawachi,
K.; Maeda, H.; Nakamura, H.; Doi, N.; Tamao, K. J. Am. Chem. Soc. 2001,
123, 3143.
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(3) Molev, G.; Bravo-Zhivotovskii, D.; Karni, M.; Tumanskii, B.;
Botoshansky, M.; Apeloig, Y. J. Am. Chem. Soc. 2006, 128, 2784.
(4) Kawachi, A.; Oishi, Y.; Kataoka, T.; Tamao, K. Organometallics
2004, 23, 2949.
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Chem., Int. Ed. 2003, 42, 1640.
(8) (a) Ottosson, H. Chem.;Eur. J. 2003, 9, 4144. (b) Eklof, A. M.;
€
Ottosson, H. Tetrahedron 2009, 65, 5521.
(9) Dobrovetsky, R.; Zborovsky, L.; Sheberla, D.; Botoshansky, M.;
Bravo-Zhivotovsky, D.; Apeloig, Y. Angew. Chem., Int. Ed. 2010, 49,
4084.
(5) Kajiwara, T.; Takeda, N.; Sasamori, T.; Tokitoh, N. Organome-
tallics 2008, 27, 880.
(10) (a) Ohshita, J.; Masaoka, S.; Morimoto, Y.; Sano, M.; Ishikawa,
M. Organometallics 1997, 16, 1123. (b) Ohshita, J.; Masaoka, S.; Morimoto,
Y.; Ishikawa, M. Organometallics 1997, 16, 910. (c) Ohshita, J.; Tamai, M.;
Sakurai, H.; Kunai, A. Organometallics 2001, 20, 1065.
(11) Ohshita, J.; Tokunaga, Y.; Sakurai, H.; Kunai, A. J. Am. Chem.
Soc. 1999, 121, 6080.
(6) (a) Ohshita, J.; Masaoka, Y.; Masaoka, S.; Ishikawa, M.; Tachi-
bana, A.; Yano, T.; Yamabe, T. J. Organomet. Chem. 1994, 473, 15.
(b) Ohshita, J.; Masaoka, S.; Masaoka, Y.; Hasebe, H.; Ishikawa, M.;
Tachibana, A.; Yano, T.; Yamabe, T. Organometallics 1996, 15, 3136.
(c) Ohshita, J.; Sakurai, H.; Tokunaga, Y.; Kunai, A. Organometallics
1999, 18, 4545. (d) Bravo-Zhivotovskii, D.; Apeloig, Y.; Ovchinnikov, Y.;
Igonin, V.; Struchkov, Y. T. J. Organomet. Chem. 1993, 446, 123.
(12) Preliminary results for the formation of lithium acylsilenolates
have been reported. See ref 11.
r
2010 American Chemical Society
Published on Web 08/19/2010
pubs.acs.org/Organometallics

4200 Organometallics, Vol. 29, No. 18, 2010
Ohshita et al.
cleanly, and the formation of methylated product 2aa in high
yield was confirmed by the mass and NMR spectra of the
reaction mixture. However, 2aa readily decomposed during
the workup process and hence could not be separated from
the mixture. With bulkier alkyl halides, many unidentified
byproducts were detected in low yields in the reaction
mixtures, lowering the yields of the alkylated products, in
contrast to the reaction with methyl iodide. Products
2ab-2ad were more stable than 2aa and could be isolated,
but still underwent decomposition to an extent by subjecting
them to column chromatography. Similar reactions of 1b
also proceeded to give the corresponding alkylated products
2ba-2bd, together with 3b as a minor product. Methylated
product 2ba could not be isolated, again.
Li-SiMe3 exchange (Scheme 2). However, lithium acylsile-
nolates were thermally rather unstable, and standing the
solutions of 4a and 4b overnight at -80 °C followed by
quenching with methyl iodide led to no formation of 2aa or
2ba, but produced complex mixtures containing 3a and 3b in
38% and 31% yield, respectively. This is in marked contrast
to lithium silenolate 5 reported previously (Scheme 2), which
was stable even at room temperature for a few days. The
route leading to 3a and 3b is still unclear. However, the fact
that compound 3a was not detected at all in the reaction
mixtures of 1b indicated that 3b arose from intramolecular
thermal decomposition of 4b. We examined also the reac-
tions of 1a and 1b with (Me3Si)3SiLi, followed by quenching
of the resulting acylsilenolates 5 with methanol. However,
the reactions gave complex mixtures, from which no major
products could be separated.
The formation of (Me3Si)4Si and alkylated products 2
clearly indicates the formation of lithium acylsilenolates by
Computational Studies. To understand the electronic
structure of lithium acylsilenolates, we carried out DFT
calculations at the B3LYP/6-31þG(d)//B3LYP/6-31G(d)
level on model reactions shown in Scheme 3. For all the
lithium compounds examined in the computational studies,
three THF molecules are coordinated to the lithium atom.
Addition of one more THF molecule in the input geometries
always resulted in the displacement of one of the THF
molecules during the optimization. In these reactions, two
kinds of stable isomers are obtained as the products, and
oxylithiums 4c-1 and 5c-1 are more stable than silyllithiums
4c-2 and 5c-2, respectively. It is also noted that the formation
of 4c-1 and 4c-2 from 1c is more exothermic than that of 5c-1
and 5c-2 from 6c, respectively. Table 2 lists the structural
parameters of 4c and 5c derived from the calculations, and
Figure 1 represents the optimized geometries of 4c-1 and 4c-
2. Compound 4c-1 possesses a bidentate structure with
slightly unsymmetrical Li-O bonding of 1.937 and 1.999
Scheme 1. Lithium Silenolate (left) and Acylsilyllithium (right)
Scheme 2. Formation and Reactions of Lithium Acylsilenolates
˚
A. Such chelation seems to be responsible for the more
exothermic formation of 4c-1 than that of 5c-1. The central
˚
Si-C(O) bonds of 4c-1 are 1.904 and 1.900 A, in between
˚
˚
those of 5c-1 (1.864 A) and 4c-2 (1.947, 1.943 A), indicating
that the double-bond character of the center Si-C bonding
increases in the order 4c-2 < 4c-1 < 5c-1. The sum of the
Si-C and Si-Si bond angles around the center silicon atom
of 4c-1 is 324.7°, larger than those of 4c-2 (317.7°) and
(Me3Si)3SiLi(THF)3 (309.3°) obtained by the calculations,
being also indicative of the sp2 character of 4c-2 to an extent,
although this is still in the range of sp3 bonding and smaller
than that of 5c-1 (330.7°). Molecule 4c-2 has a monodentate
structure without evident Li-O interactions, similarly to 5c-
Table 1. Formation and Reactions of Lithium Acylsilenolates
R
R0X
product yield/%a
Mes (1a)
MeI
iPrI
allylBr
PhCH2Br
MeI
nBuBr
allylBr
PhCH2Br
2aa
2ab
2ac
2ad
2ba
2bb
2bc
2bd
93
3a
5
5
6
5
3
2
2
3
42 (35)
51 (40)
46 (38)
91
46 (38)
48 (40)
41 (33)
Ad (1b)
3b
˚
2. In fact, the CdO (1.239, 1.238 A) and Si-C(O) (1.947,
˚
1.943 A) bond lengths are
a Yields were determined by 1H NMR spectra of the reaction mix-
tures. Isolated yields are given in parentheses.
a little different from,
˚
but close to, those of 1c (both 1.225 A for CdO and 1.961,
Scheme 3. Reaction Heats of Formation of Lithium Silenolates (4c and 5c), Derived from DFT Calculations at the B3LYP/6-31þG(d)//
B3LYP/6-31G(d) Level

Note
Organometallics, Vol. 29, No. 18, 2010 4201
Table 2. Structural Parameters of Lithium Acylsilenolate 4c and
Silenolate 5c, Derived from DFT Calculationsa
temperature for 1 h, 2.0 mmol of isopropyl iodide was added and
the mixture was allowed to warm to room temperature. The
mixture was hydrolyzed with water and the organic layer was
separated. The organic layer was dried over anhydrous magne-
sium sulfate. After evaporation of the solvent, the residue was
analyzed by 1H NMR spectroscopy as being 2ab (42% yield), 3a
(5% yield), and (Me3Si)4Si (quant). Compounds 2ab and 3a
were isolated by preparative GPC, eluting with toluene. Data for
2ab: MS m/z 438 (Mþ); 1H NMR (δ in C6D6) 0.27 (s, 9H, Me3Si),
1.14 (d, 6H, J = 7.22, Me2CH-), 1.59 (sept, 1H, J = 6.99 Hz,
Me2CH-), 2.02 (s, 6H, p-Me), 2.15 (s, 12H, o-Me), 6.52 (s, 4H,
ring H); 13C NMR (δ in C6D6) 0.3, 15.2, 19.5, 19.7, 21.0, 129.1,
132.7, 138.3, 144.8, 245.2; exact MS calcd for C26H38O2Si2 (Mþ)
P
compd
Si-C(dO)/A
(Si bond angles)/degb
˚
˚
CdO/A
4c-1
4c-2
5c-1
5c-2
1.904, 1.900
1.947, 1.943
1.864
1.256, 1.254
1.239, 1.238
1.281
324.7
317.7
330.7
315.7
1.952
1.241
a At the B3LYP/6-31þG(d)//B3LYP/6-31G(d) level. b Sum of Si-C
and Si-Si bond angles around the center silicon atom.
438.2410, found 438.2398. Data for 3a: MS m/z 147 (Mþ
-
COMes); 1H NMR (δ in C6D6) 2.04 (s, 12H, o-Me), 2.15 (s, 6H,
p-Me), 6.61 (s, 4H, ring H); 13C NMR (δ in C6D6) 20.2, 21.0,
129.0, 134.8, 135.7, 139.7, 198.3. Anal. Calcd for C20H22O2: C,
81.60; H, 7.53. Found: C, 81.30; H, 7.23.
Other substitution reactions of 1a and 1b were carried out in a
fashion similar to that above. Data for 2aa: MS m/z 410 (Mþ);
1H NMR (δ in C6D6) 0.18 (s, 9H, Me3Si), 0.48 (s, 3H, MeSi),
2.03 (s, 6H, p-CH3), 2.11 (s, 12H, o-CH3), 6.55 (s, 4H, ring H).
Data for 2ac: MS m/z 436 (Mþ); 1H NMR (δ in C6D6) 0.21 (s,
9H, Me3Si), 2.03 (s, 6H, p-Me), 2.15 (s, 12H, o-Me), 2.17 (d, 2H,
J = 7.98, H2CdCHCH2-), 4.81 (dd, 2H, J = 13.75, 1.44 Hz,
H2CdCHCH2-), 5.73-5.84 (m, 1H, H2CdCHCH2-), 6.54 (s,
4H, ring H), 13C NMR (δ in C6D6) -0.6, 19.6, 21.0, 21.2, 115.4,
129.1, 132.7, 133.5, 138.5, 144.4, 243.8; exact MS calcd for
C26H36O2Si2 (Mþ) 436.2254, found 436.2260. Data for 2ad:
Figure 1. Optimized geometries of lithium acylsilenolates 4c-1
(left) and 4c-2 (right) derived from DFT calculations at the
B3LYP/6-31G(d) level. Hydrogen atoms are omitted for clarity.
1
MS m/z 486 (Mþ); H NMR (δ in C6D6) 0.08 (s, 9H, Me3Si),
2.03 (s, 6H, p-CH3), 2.09 (s 12H, o-CH3), 2.77 (s, 3H, PhCH2-),
6.55 (s, 4H, Mes ring H), 6.89-7.01 (m, 5H, Ph); 13C NMR (δ in
C6D6) -0.7, 19.6, 21.0, 22.9, 125.3, 128.6, 129.1, 129.4, 132.8,
138.4, 138.6, 144.4, 244.0; exact MS calcd for C30H38O2Si2 (Mþ)
486.2410, found 486.2409. Data for 2ba: MS m/z 426 (Mþ); 1H
NMR (δ in C6D6) 0.30 (s, 9H, Me3Si), 0.83 (s, 3H, MeSi), 1.57
(br s, 6H, Ad), 1.87 (br s, 9H, Ad), 2.03 (s, 3H, p-Me), 2.18 (s, 6H,
o-Me), 6.58 (s, 2H, ring H). Data for 3b: MS m/z 468 (Mþ); 1H
NMR (δ in C6D6) 1.76 (br s, 6H, Ad), 2.08 (br s, 6H, Ad), 2.18 (s,
6H, o-Me), 2.27 (s, 3H, p-Me), 6.83 (s, 2H, ring H). Data for 2bb:
MS m/z 468 (Mþ); 1H NMR (δ in C6D6) 0.30 (s, 9H, Me3Si), 0.83
(t, 3H, J = 7.24, CH3(CH2)3-), 1.16-1.53 (m, 6H, CH3-
(CH2)3-), 1.57 (br s, 6H, Ad), 1.87 (br s, 9H, Ad), 2.03 (s, 3H,
p-Me), 2.18 (s, 6H, o-Me), 6.58 (s, 2H, ring H); 13C NMR (δ in
C6D6), -0.2, 13.7, 14.1, 19.7, 20.9, 27.0, 27.5, 28.2, 36.7, 37.4,
52.1, 129.1, 132.4, 138.3, 145.0, 243.9, 246.1; exact MS calcd for
C28H44O2Si2 (Mþ) 468.2880, found 438.2897. Data for 2bc: MS
m/z 452 (Mþ); 1H NMR (δ in C6D6) 0.28 (s, 9H, Me3Si), 1.57 (s,
6H, Ad), 1.82-1.89 (m, 9H, Ad), 2.03 (s, 3H, p-Me), 2.09 (m,
1H, H2CdCHCH2-), 2.13 (s, 6H, o-Me), 2.09 (ddt, 1H, J =
13.5, 8.1 1.0 Hz, H2CdCHCH2-), 4.86 (dd, 1H, J = 10.1, 1.0
Hz, H2Cd), 4.92 (dd, 1H, J = 17.1, 1.7 Hz, CH2d), 5.86-5.97
(m, 1H, H2CdCHCH2-), 6.60 (s, 2H, ring H); 13C NMR (δ in
C6D6) -0.2, 19.8, 21.0, 21.4, 28.3, 36.8, 37.3, 52.1, 115.3, 129.2,
132.4, 134.2, 138.4, 144.6, 242.9, 245.1; exact MS calcd for
˚
1.959 A for Si-C(O)). It is likely, therefore, that inductive
electron-withdrawing effects of the acyl groups are primarily
responsible for the more exothermic formation of 4c-2 than
that of 5c-2, and no significant stabilization by extra coordina-
tion through acyl-Li interaction is involved in this compound.
In conclusion, on the basis of the results described above,
we demonstrate the smooth formation of lithium acylsileno-
lates by the reactions of bis(acyl)trisilanes with (Me3Si)3SiLi
via Li-SiMe3 exchange. Treatment of the resulting lithium
acylsilenolates with electrophiles provides a route to var-
iously substituted bisacyldisilanes. DFT calculations on
model reactions were performed, which suggested that the
formation of lithium acyl silenolate is more exothermic than
that of lithium silenolate, due to the extra coordination of the
acyl oxygen to the lithium atom and inductive electron-
withdrawing effects of the acyl groups. However, experi-
mental observation clearly indicated that lithium acylsileno-
late is thermally less stable than lithium silenolate, probably
due to the intramolecular reactions leading to many decom-
position products including 1,2-diones 3a and 3b. Studies on
the reactivity of lithium acylsilenolates toward other electro-
philes are under way.
Experimental Section
(13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.;
Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.;
Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.;
Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo,
C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin,
A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma,
K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.;
Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A.02; Gaussian, Inc.:
Wallingford, CT, 2009.
General Procedures. All reactions were carried out in dry
nitrogen. THF and ether were distilled from sodium-benzophe-
none ketyl immediately before use. NMR spectra were recorded
on a JEOL model LA-400 spectrometer. Low-resolution mass
spectra were measured on a Shimadzu model QP-5050A spec-
trometer, while high-resolution mass spectra were obtained on a
JEOL model SX-102A spectrometer. Compounds 1a and 1b
were prepared as reported in the literature.11
Reactions of 1a and 1b with (Me3Si)3SiLi, Followed by Alkyl
Halides. To a solution of 94 mg (0.20 mmol) of 1a in 1.0 mL of
THF was added a solution of 0.24 mmol of (Me3Si)3SiLi in 2 mL
of THF/ether at -80 °C. After the mixture was stirred at this

4202 Organometallics, Vol. 29, No. 18, 2010
Ohshita et al.
C27H40O2Si2 (Mþ) 452.2567, found 452.2556. Data for 2bd: MS
m/z 502 (Mþ); 1H NMR (δ in C6D6) 0.14 (s, 9H, Me3Si), 1.56 (s,
6H, Ad), 1.83-1.89 (m, 9H, Ad), 2.05 (s, 3H, p-Me), 2.09 (s, 6H,
o-Me), 2.64 (d, 1H, J = 13.75 Hz, PhCH2-), 3.06 (d, 1H, J =
13.75 Hz, PhCH2-), 6.58 (s, 2H, Mes ring Hs), 6.90-7.07 (m,
5H, Ph); 13C NMR(δ in C6D6) -0.1, 19.8, 21.0, 22.2, 28.3, 36.8,
37.4, 52.3, 125.2, 128.6, 129.3, 129.5, 132.7, 138.6, 139.2, 144.5,
243.1, 245.2; exact MS calcd for C31H42O2Si2 (Mþ) 502.2723,
found 502.2739.
DFT Calculations. All calculations were performed with
DFT, using the Gaussian09 program package.13 The B3LYP
hybrid density functional theory method and the 6-31G(d) basis
set were employed for the geometry optimization, and the
energies were obtained at the B3LYP/6-31þG(d) level.14
Acknowledgment. Financial support from Shorai
Foundation for Science and Technology and the Swedish
Research Council is gratefully acknowledged.
Supporting Information Available: Cartesian coordinates for
optimized geometries of all the calculated model compounds.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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