Reactions of an isolable dialkylsilylene with ketones

Article abstract of DOI:10.1021/om1004644

The reactions of isolable dialkylsilylene 2,2,5,5-tetrakis(trimethylsilyl)- 1-silacyclopentane-1,1-diyl with different types of ketones such as adamantanone, acetone, benzophenone, and cyclopropenones proceed smoothly at room temperature to afford the cor

Full text of DOI:10.1021/om1004644

5526 Organometallics 2010, 29, 5526–5534
DOI: 10.1021/om1004644
Reactions of an Isolable Dialkylsilylene with Ketones†
Shintaro Ishida, Takeaki Iwamoto,* and Mitsuo Kira*
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku,
Sendai, 980-8578, Japan
Received May 14, 2010
The reactions of isolable dialkylsilylene 2,2,5,5-tetrakis(trimethylsilyl)-1-silacyclopentane-1,1-diyl
with different types of ketones such as adamantanone, acetone, benzophenone, and cyclopropenones
proceed smoothly at room temperature to afford the corresponding siloxirane, silyl enol ether, 2-oxa-
silacyclopentene, and unusual cyclopropenylsilanes, respectively, without formation of any second-
ary or side-reaction products. The electronic structure of various types of model carbonyl silaylides
was investigated using DFT calculations. The diversity of the reaction modes is explained by invoking
the substituent effects on the electronic nature of the initially formed carbonyl silaylides.
Introduction
number of substituted silylenes such as di-tert-butylsilylene,6
phenylmethylsilylene,7 diphenylsilylene,8 and other diaryl-
silylenes9 are also available as transient silylenes using rela-
ted methods as above. It is therefore natural that the reac-
tions of the transient silylenes with aldehydes and ketones
have been investigated extensively by many authors10 since
the late 1970s as one of the fundamental reactions of sily-
lenes. The reactions were found however not to be straight-
forward, giving diverse types of final products depending on
the structures of silylenes and carbonyl compounds. Typi-
cally, Ando et al. have found that the reactions of thermally
or photochemically generated dimethylsilylene with benzo-
phenone10a and adamantanone10b afford mainly 1-sila-2-
oxa-4,5-benzocyclopentene 1 and a mixture of 1:2- and 2:2-
cycloadducts (2 and 3), respectively. Siloxirane 4 was isolated
through the reaction of photochemically generated dimesi-
tylsilylene with 1,1,3,3-tetramethyl-2-indanone, and its struc-
ture was determined by X-ray crystallography.10d,11 Belzner
A number of organosilylenes (divalent organosilicon
compounds) have been generated as transient species since
the discovery of dimethylsilylene extrusion via high-tem-
perature thermolysis of dibenzo-7-silanorbornadienes by
Gilman et al. more than 40 years ago.1,2 Among useful
methods for the generation of dimethylsilylene are the
thermolysis of 1,2-dimethoxytetramethyldisilane at 225 °C3
and hexamethylsilirane at >60 °C4 and photolysis of linear
and cyclic permethylpolysilanes at lower temperatures.5 A
† Part of the Dietmar Seyferth Festschrift. Dedicated to Professor Dietmer
Seyferth in honor of his outstanding contributions as Editor-in-Chief of
Organometallics.
*To whom correspondence should be addressed. E-mail:
iwamoto@m.tains.tohoku.ac.jp; mkira@mail.tains.tohoku.ac.jp.
€
(1) For recent reviews on silylenes, see: (a) Driess, M.; Grutzmacher,
H. Angew. Chem., Int. Ed. Engl. 1996, 35, 828. (b) Jutzi, P.; Burford, N.
Chem. Rev. 1999, 99, 969. (c) Tokitoh, N.; Okazaki, R. Coord. Chem. Rev.
2000, 210, 251. (d) Gaspar, P. P.; West, R. In The Chemistry of Organic
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Y.; Sasamori, T.; Tokitoh, N. Chem. Rev. 2009, 109, 3479.
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pubs.acs.org/Organometallics
Published on Web 07/22/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 21, 2010 5527
Scheme 1
Chart 1
et al. have reported that the reaction of a diarylsilylene,
bis[2-{(dimethylamino)methyl}phenyl]silylene (Ar2Si:), with
adamantanone gives the corresponding siloxirane 5 as an iso-
lable compound.10j They found that the reactions of Ar2Si:
with tetraphenylcyclopentadienone and fluorenone gave 6
and 7, respectively, as major products. Enolizable ketones
have been reported to react with various silylenes, giving
the corresponding silyl enol ethers such as 8.10a,h,12a Jutzi
et al. have found that decamethylsilicocene 9, the first stable
divalent silicon(II), gives 1,3-dioxasilolane 10 during the
reaction with benzaldehyde.10k Although 2, 7, and 10 are
all 1:2 cycloadducts, the ring structure of 7 and 10 is isomeric
to that of 2. Recently, a number of interesting and useful
synthetic reactions via the addition of dimesitylsilylene12 and
di-tert-butylsilylene13 to various carbonyl compounds have
been explored.
Among reaction mechanisms proposed, those involving
intermediary formation of the corresponding carbonyl silay-
lides 11 are dominant, and actually, the existence of a car-
bonyl silaylide has been evidenced spectroscopically during
the reaction of dimesitylsilylene with 1,1,3,3-tetramethyl-2-
indanone in low-temperature matrixes.10g However, the ori-
gin of the diversity of the reactions has not yet been well dis-
cussed until now. To draw an entire picture of the reactions
of silylenes with carbonyl compounds, the electronic struc-
ture of carbonyl silaylides, especially its dependence on the
substituents on silicon and carbon atoms in the skeleton,
should be elucidated. As shown in Scheme 1, we can expect
that five resonance forms contribute to the electronic struc-
ture of a carbonyl silaylide as a silicon congener of carbonyl
ylide,15,16 a well-known 1,3-dipole. The five forms are classi-
fied into three types: type I, with nucleophilic silicon; type II,
with electrophilic silicon; and type III with a 1,3-biradical
nature. The relative importance among types I-III may de-
pendontheelectronicnatureofsilylenesandketones, although
only type I is usually used for descriptive purposes. The
carbonyl silaylide may be equilibrated with the correspond-
ing siloxirane. These factors should influence the diversity of
the reaction modes.
Because isolable dialkylsilylene 12 (Chart 1), which is
sterically well protected but electronically similar to di-
methylsilylene, is now available,17 we have had an oppor-
tunity to investigate the reactions of 12 with different
types of ketones such as adamantanone, acetone, benzo-
phenone, and two cyclopropenones. All the reactions pro-
ceed smoothly at room temperature to afford single pro-
ducts in high yields without formation of any secondary or
(12) (a) Sakai, N.; Fukushima, T.; Minakata, S.; Ryu, I.; Komatsu,
M. Chem. Commun. 1999, 1857. (b) Sakai, N.; Fukushima, T.; Okada, A.;
Ohashi, S.; Minakata, S.; Komatsu, M. J. Organomet. Chem. 2003, 686,
368. (c) Minakata, S.; Ohashi, S.; Amano, Y; Oderaotoshi, Y.; Komatsu, M.
Synthesis 2007, 16, 2481.
(13) (a) Franz, A. K.; Woerpel, K. A. J. Am. Chem. Soc. 1999, 121,
949. (b) Franz, A. K.; Woerpel, K. A. Acc. Chem. Res. 2000, 33, 813.
ꢀ
ꢀ
(c) Cirakovic, J.; Driver, T. G.; Woerpel, K. A. J. Org. Chem. 2004, 69, 4007.
(d) Clark, T. B.; Woerpel, K. A. J. Am. Chem. Soc. 2004, 126, 9522.
(e) Calad, S. A.; Woerpel, K. A. J. Am. Chem. Soc. 2005, 127, 2046. (f)
Calad, S. A.; Woerpel, K. A. Org. Lett. 2007, 9, 1037. (g) Howard, B. E.;
Woerpel, K. A. . Org. Lett. 2007, 9, 4651. (h) Bourque, L. E.; Woerpel, K. A.
Org. Lett. 2008, 10, 5257. Di-tert-butylsilylene has been generated by
treating the corresponding siliranes with carbonyl compounds in the presence
of catalytic amounts of silver or copper reagents, and hence, the silylene may
not be free. Dimethyl- and di-tert-butylsiliranes are known to react directly
with aldehydes and ketones to give the corresponding insertion products.14
(14) (a) Seyferth, D.; Duncan, D. P.; Shannon, M. L. Organometallics
1984, 3, 579. (b) Bodnar, P. M.; Palmer, W. S.; Ridgway, B. H.; Shaw, J. T.;
Smitrovich, H.; Woerpel, K. A. J. Org. Chem. 1997, 62, 4737. (c) Franz,
A. K.; Woerpel, K. A. Acc. Chem. Res. 2000, 33, 813.
(16) (a) Houk, K. N.; Rondan, N. G.; Santiago, C.; Gallo, C. J.;
Gandour, R. W.; Griffin, G. W. J. Am. Chem. Soc. 1980, 102, 1504.
(b) Yamaguchi, Y.; Schaefer, H. F., III; Alberts, L. J. Am. Chem. Soc. 1993,
115, 5790. (c) Benchouk, W.; Mekelleche, S. M.; Aurell, M. J.; Domingo,
L. R. Tetrahedron 2009, 65, 4644.
(17) (a) Kira, M.; Ishida, S.; Iwamoto, T.; Kabuto, C. J. Am. Chem.
Soc. 1999, 121, 9722. (b) Kira, M. J. Organomet. Chem. 2004, 689, 4475.
(c) Kira, M.; Ishida, S.; Iwamoto, T. Chem. Rec. 2004, 4, 243. (d) Kira, M.;
Iwamoto, T.; Ishida, S. Bull. Chem. Soc. Jpn. 2007, 80, 258. (e) Kira, M.
Chem. Commun. 2010, 46, 2893.
(15) (a) Huisgen, R. Angew. Chem., Int. Ed. Engl. 1977, 16, 572.
(b) Padwa, A.; Hornbuckle, S. F. Chem. Rev. 1991, 91, 263. (c) Padwa, A.
Helv. Chim. Acta 2005, 88, 1357.

5528 Organometallics, Vol. 29, No. 21, 2010
Ishida et al.
Figure 1. ORTEP drawing of siloxirane 13. Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms are omitted for
˚
clarity. Selected bond lengths (A) and angles (deg): Si1-C1 = 1.849(2), Si1-O1 = 1.666(1), C1-O1 = 1.523(2), Si1-C11 = 1.881(2),
Si1-C14 = 1.894(2), C1-C2 = 1.528(2), C1-C3 = 1.529(2); Si1-O1-C1 = 70.71(8), C1-Si1-O1 = 51.03(6), O1-C1-Si1 = 58.26(7).
Figure 2. ORTEP drawing of 15. Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms are omitted for clarity.
˚
Selected bond lengths (A) and angles (deg): Si1-C3 = 1.908(3), Si1-O1 = 1.701(2), C1-O1 = 1.380(3), Si1-C14 = 1.891(3),
Si1-C17 = 1.892(3), C1-C2 = 1.352(4), C1-C8 = 1.469(4), C2-C3 = 1.527(4), C2-C7 = 1.451(4), C3-C4 = 1.500(4), C4-C5 =
1.335(4), C5-C6 = 1.451(5), C6-C7 = 1.337(4); O1-Si1-C3 = 92.4(1), Si1-O1-C1 = 113.4(2), O1-C1-C2 = 116.1(2), C1-
C2-C3 = 115.0(3), C2-C1-C8 = 130.6(3), Si1-C3-C2 = 102.0(2), C2-C3-C4 = 110.5(2), Si1-C3-C4 = 125.5(2).
side-reaction products. With the hope of understanding the
origin of the diversity, the electronic structure of various
model carbonyl silaylides has been investigated using DFT
calculations.
the air probably due to effective steric protection around the
siloxirane ring.
The preparation of carbonyl silaylides through the addi-
tion of silylenes to carbonyl compounds would be less limited
than that of carbonyl ylides, which are usually generated by
the ring-opening of the corresponding oxiranes with elec-
tron-withdrawing substituents15a and by the transition-me-
tal-catalyzed reactions of diazomethane derivatives with
carbonyl compounds.15b,c
1
The structure of 13 was determined by H, 13C, and 29Si
NMR spectroscopies, elemental analysis, and X-ray crystal-
lography. The molecular structure of 13 determined by X-ray
analysis is shown in Figure 1. The structural parameters for
the central siloxirane ring are almost the same as those found
for 4;10d the Si-C, Si-O, and C-O bond lengths in the
Results
Reaction of 12 with Adamantanone. Dialkylsilylene 12
reacted with an equimolar amount of adamantanone at
room temperature in benzene to give siloxirane 13 as the
sole product (eq 1). Compound 13 was very stable toward
water and oxygen and remained intact more than 3 years in
˚
siloxirane ring are 1.849(2), 1.666(1), and 1.523(2) A, respec-
tively, and the C-Si-O, Si-C-O, and C-O-Si bond
angles are 51.03(6)°, 58.26(7)°, and 70.71(8)°. Interestingly,

Article
Organometallics, Vol. 29, No. 21, 2010 5529
Figure 3. ORTEP drawing of 16. Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms are omitted for clarity.
˚
Selected bond lengths (A) and angles (deg): Si1-C3 = 1.893(2), Si1-O1 = 1.670(1), C1-O1 = 1.432(2), Si1-C14 = 1.903(2),
Si1-C17 = 1.892(2), C1-C2 = 1.514(3), C1-C8 = 1.515(3), C2-C3 = 1.400(3), C2-C7 = 1.394(3), C3-C4 = 1.398(3), C4-C5 =
1.387(3), C5-C6 = 1.392(3), C6-C7 = 1.376(3); O1-Si1-C3 = 91.90(7), Si1-O1-C1 = 117.0(1), O1-C1-C2 = 107.9(2),
C1-C2-C3 = 115.8(2), C2-C1-C8 = 112.1(2), Si1-C3-C2 = 106.7(1), C2-C3-C4 = 117.4(2), Si1-C3-C4 = 135.6(2).
not only are the C11-Si1-C14 plane and the C2-C1-C3
plane in the trispiro compound perpendicular to the central
siloxirane ring, but these two planes are almost coplanar
together with the Si1-C1 bond in the central ring; the sum of
bond angles except for oxygen bonds around Si1 and C1 is
358.71(6)° and 359.64(12)°, respectively. The angle between
the planes C11-Si1-C14 and C2-C1-C3 is 15.80°. In
addition to the severe steric repulsion between bulky sub-
stituents on Si1 and C1, an increased π-complex character
between the SidC double bond and negative O may con-
tribute to its origin, although the ring Si-C and Si-O bond
lengths are normal.18
Reaction of 12 with Acetone. The reaction of silylene 12
with a large excess acetone at room temperature in hexane
gave the corresponding silyl enol ether 14 in a high yield
(eq 2), similarly to the reactions of dimethylsilylene and di-
mesitylsilylene with acetone and other enolizable ketones.10a,12a
The structure was determined using 1H, 13C, and 29Si
NMR, MS, and elemental analysis.
around C1 = 359.9(7)°]. Obviously, an aromatic ring in ben-
zophenone is destroyed in the product 15.
The reaction mode is similar to that for the addition of
transient silylenes to benzophenone,10a,j while usually the
corresponding aromatized products are obtained in the
latter. Compound 15 was able to be stored in a refrigerator
in a glovebox but was not very stable toward oxygen and
moisture, isomerizing gradually to aromatized product 16 at
room temperature (eq 4). The molecular structure of com-
pound 16 was determined by X-ray crystallography as shown
in Figure 3. A similar type of cyclic adduct to 15 was obtained
by Jutzi et al.10k during the reactions of decamethylsilicocene
with benzophenone and benzaldehyde, but the adducts are
reported not to aromatize through 1,3-hydrogen migration
even if they were heated at 100 °C for 6 days.
Reaction of 12 with Benzophenone. The reaction of silylene
12 with an equimolar amount of benzophenone at room
temperature in benzene gave 2-oxasilacyclopentene 15 as
yellow crystals (eq 3). The structure of 15 was unequivocally
determined by X-ray structural analysis. As shown in Figure
2, the molecular structure of 15 was characterized especially
by the shorter C1-C2, C4-C5, and C6-C7 bond lengths
Reaction 12 with Di(tert-butyl)- and Diphenylcycloprope-
nones. The reaction of silylene 12 with an equimolar amount
of di(tert-butyl)cyclopropenone at room temperature in
benzene gave the quite unusual product 17 as a single
product. Although there are two chiral centers in 17, only
the R*,R*-isomer was obtained (eq 5). Similarly, the reac-
tion of 12 with diphenylcyclopropenone produced adduct
18 with the same stereochemistry as that of 17 (eq 6). The
structure of 17 and 18 is rather unexpected but was con-
firmed by 1H, 13C, and 29Si NMR spectroscopies, MS, and
˚
[1.352(4), 1.335(4), and 1.337(4) A], longer C2-C3, C2-C7,
and C3-C4 bond lengths [1.527(4), 1.451(4), and 1.500(4)
A], and planarity around the C1 atom [sum of bond angles
˚
(18) (a) Grev, R. S.; Schaefer, H. F. J. Am. Chem. Soc. 1987, 109,
6577. (b) Cremer, D.; Kraka, E. J. Am. Chem. Soc. 1985, 107, 3800.

5530 Organometallics, Vol. 29, No. 21, 2010
Ishida et al.
Figure 5. ORTEP drawing of 18. Thermal ellipsoids are shown
at the 30% probability level. Hydrogen atoms are omitted for
clarity. Selected bond lengths (A) and angles (deg): Si1-O1 =
1.620(1), Si1-C17 = 1.850(2), Si1-C1 = 1.856(2), Si1-C4 =
1.887(2), C1-C2 = 1.336(2), C2-C3 = 1.507(3), C3-C4 =
1.582(3), Si5-O1 = 1.639(2), C17-C18 = 1.303(3), C18-
C19 = 1.491(3), C17-C19 = 1.552(3); C1-Si1-C4 = 96.49(9),
Si1-O1-Si5 = 159.1(1).
Figure 4. ORTEP drawing of 17. One of the three crystal-
lographically independent molecules (17A) is shown. The other
molecular structures of 17 are described in the Supporting
Information. Thermal ellipsoids are shown at the 30% prob-
ability level. Hydrogen atoms are omitted for clarity. Selected
˚
˚
bond lengths (A) and angles (deg): Si1-O1 = 1.618(2), Si1-
C17 = 1.856(3), Si1-C1 = 1.861(3), Si1-C4 = 1.889(3), C1-
C2 = 1.337(5), C2-C3 = 1.521(5), C3-C4 = 1.578(4), Si5-
O1 = 1.640(2), C17-C18 = 1.309(4), C18-C19 = 1.480(4),
C17-C19 = 1.550(4); C1-Si1-C4 = 114.0(1), Si1-O1-Si5 =
172.0(2).
Discussion
Several interesting issues emerge about the structural
discussion of the carbonyl silaylides that will play key roles
in the reactions of silylenes with ketones: (1) Is there a facile
equilibrium between a carbonyl silaylide, the corresponding
siloxirane, and a set of its fragments, silylene þ ketone? (2) Is
there any significant substituent dependence of the geometric
and electronic structure of the carbonyl silaylide? (3) Do the
substituents on the carbonyl silaylide affect the reaction
modes? To obtain a basic idea about the electronic structures
of carbonyl silaylides, we have performed DFT calculations
for model carbonyl silaylides, dimethylcyclopropenone (19a),
acetone (19b), acetophenone (19c), benzophenone (19d), and
cyclopentadienone silaylide (19e), and the related singlet
carbonyl ylide 20 at the theoretical level of B3LYP/6-31þG-
(d) (Chart 2),19 where the electron-withdrawing ability of the
substituents on the skeletal carbon atom increases in the
order 19a<19b<19c<19d<19e; a phenyl group is more
electron-accepting than a methyl group, and the important
contribution of the following resonance forms exists for
cyclopropenone and cyclopentadienone (Chart 3). The struc-
tural parameters of 19a-19e and 20 and their energies
relative to the corresponding siloxiranes 19a0-19e0 and 200
are compared in Table 1.
finally X-ray crystallography. The reaction mechanism for
formation of 17 and 18 is not straightforward and should
involve the migration of a trimethylsilyl group on dialkyl-
silylene 12. The mechanism is discussed in detail in the
following section.
Our theoretical calculations indicate that the ring closure
of carbonyl silaylides 19b-19e to the corresponding siloxir-
anes 19b0-19e0 is more than 20 kcal/mol exothermic. Siloxi-
rane 19a0 expected for the cyclization of cyclopropenyl-
substituted carbonyl silaylide 19a was not located as a local
minimum in our calculations. Instead, the corresponding
ring-opened silyloxide 19a00 was found as a minimum
(eq 7). Silyloxide 19a00 would be more stable than siloxirane
Single crystals of 17 and 18 suitable for X-ray crystallogra-
phy were obtained by the recrystallization from THF. There
were three crystallographically independent molecules (17A,
17B, and 17C) in an asymmetric unit of a single crystal of 17,
and disorder was observed in a molecule of 17C (17C:17C0 =
81:19). The molecular structures of 17A and 18 are shown in
Figures 4 and 5. The detailed discussion of the X-ray structures
of 17 is given in the Supporting Information.
(19) All calculations were carried out with Gaussian 03 package
programs; full reference of Gaussian 03 is shown in the Supporting
Information.

Article
Organometallics, Vol. 29, No. 21, 2010 5531
Chart 2
Chart 3
Scheme 1 would be largest in 19a among these carbonyl
silaylides. On the other hand, in 19e, which has a cyclopen-
tadienylidene substituent, the Si-O bond length is the short-
est and close to the standard Si-O single bond length, while
P
19a0 due to the aromatic stability of cyclopropenium ion in
the former; 19a0 would be a short-lived intermediate or a
transition state between 19a and 19a00. The reaction of 19a to
19a00 is exothermic with a similar reaction energy to those for
the ring-closure of carbonyl silaylides 19b-19e. Although
the exothermicity (ΔE in Table 1) for the ring closure of
carbonyl silaylides is about 60% of that of a reference car-
bonyl ylide 20 to 200 (44 kcal/mol), the ring-closing energies
of 19b-19e would be large enough to hamper the inverse
reactions from siloxirane to carbonyl silaylide at room
temperature.
the C-O bond length and @Si for 19e are the largest. The
charge on silicon is even positive in 19e. The contribution of
type II in Scheme 1 is expected to be largest in 19e. Although
silylene adducts of carbonyl compounds are often drawn by
type I structures, the actual electronic structure should be
taken to be remarkably different depending on the substit-
uents. Thus, the contribution of type I decreases in the order
19a > 19b > 19c > 19d > 19e, while that of type II in-
creases, in the order 19a<19b<19c<19d<19e. Related
substituent effects on carbonyl ylides have been investigated
using theoretical calculations.16a
The diverse reaction modes of silylenes with carbonyl
compounds may be explained by taking into account the
initial formation of the corresponding carbonyl silaylide as a
reactive intermediate and remarkable substituent effects on
the carbonyl silaylides revealed by the DFT calculations.
A typical reaction of a silylene with a nonenolizable
dialkylketone would be the formation of the corresponding
carbonyl silaylide and its intramolecular cyclization to the
siloxirane. Actually, the corresponding siloxiranes have been
obtained by the reactions of 12, Belzner’s Ar2Si:,10j and
dimesitylsilylene10d with adamantanone and tetramethyl-2-
indanone. Because the intramolecular cyclization reactions
of carbonyl silaylides are largely exothermic, no inverse
reaction would occur at room temperature. However, during
the reaction of dimethylsilylene with adamantanone giving 2
and 3 at high temperatures,10b the equilibrium between the
carbonyl silaylide and the siloxirane may be feasible.
The reaction of 12 with benzophenone afforded a five-
membered silacycle 15 as an isolable product. The formation
of 15 would be explained by the intramolecular [2þ3] cyc-
loaddition between an aromatic CdC double bond and car-
bonyl silaylide as a 1,3-dipole, similarly to the reactions of
benzophenone with other silylenes such as dimethylsilylene,
Ar2Si:,10j MePhSi:,10i and (Me5C5)2Si:.10k
During the reaction of silylene 12 with di(tert-butyl)- and
diphenylcyclopropenones, adducts 17 and 18, having unu-
sual structure, were obtained in high yields. A plausible re-
action pathway is shown in Scheme 2. The reactivity is ex-
plained by invoking a rather extreme electronic structure of
the corresponding carbonyl silaylides 21, which has a nu-
cleophilic silicon character due to strong contribution of type
I in Scheme 1. As found theoretically, the corresponding silo-
xiranes 22 would be less stable than the ring-opened cyclo-
propenium-substituted silyloxides 23, which leads to the
final products via 1,3-trimethylsilyl migration.20 The stereo-
chemical outcome giving only the R*,R*-isomer of 17 and 18
is compatible with the mechanism.
It is noted that the structural parameters of tetramethyl-
carbonyl silaylide 19b is significantly different from those of
its carbon congener 20, as shown in Table 1. The C-O bond
˚
length for 19b (1.259 A) is much shorter than that of 20 (1.328
˚
A). The Mulliken charges are small negative and large
positive on skeletal Si and C atoms in 19b, respectively, while
the charges are both positive on the two terminal carbon
atoms in 20. These characteristics suggest that in carbonyl
silaylide 19b the contribution of type I in Scheme 1 is more
important than that of type II.
Geometry and Mulliken charges of carbonyl silaylides are
however strongly dependent on the substituents. Evidently,
with increasing electron-accepting ability of substituents on
the carbonyl carbon of carbonyl silaylide, i.e., going from
19a to 19e, the Si-O bond length decreases, the C-O bond
length increases, and the sum of bond angles around silicon
P
of the skeleton ( @Si) increases. In the same order, the
charge on the carbonyl carbon decreases from þ0.57 to
-0.11 and that on the silicon increases from -0.14 to
þ0.35. Remarkable dependence of the geometrical para-
meters and charges on the substituents indicates that the
electronic structure of carbonyl silaylides is modified
strongly by the substituents. In 19a, with a cyclopropenyli-
dene substituent, the C-O bond length is very close to that of
the normal CdO double bond, while the Si-O bond is very
long compared to the standard Si-O single bond. The
oxygen approaches from the vertical direction of the silylene
C-Si-C plane, and the C-Si-O angles are close to 90°;
hence, sumΣ@Si of 283° of 19a is the smallest among the
carbonyl silaylides in Table 1. The negative charge on the
silicon of 19a is the largest. The contribution of type I in

5532 Organometallics, Vol. 29, No. 21, 2010
Ishida et al.
Table 1. Selected Structural Parameters of 19a-19e and 20 and Their Relative Energies Calculated at the B3LYP/6-31Gþ(d) Level
Mulliken group charge
P
˚
˚
ylide
d(Si-O) /A
d(C-O) /A
@Sia/deg
Sib
Oc
Cd
ΔE/(kcal/mol)e
19a
19b
19c
19d
19e
20
2.049
1.897
1.810
1.767
1.691
1.328g
1.245
1.259
1.283
1.285
1.312
1.328
283.4
293.5
302.7
306.7
325.4
352.0h
-0.14
-0.05
þ0.05
þ0.07
þ0.35
þ0.11i
-0.69
-0.36
-0.23
-0.19
-0.24
-0.22
þ0.57
þ0.39
þ0.19
þ0.12
-0.11
þ0.11
-25.58f
-25.67
-22.40
-23.62
-22.12
-44.18j
a Sum of bond angles around the silicon atom unless otherwise noted. b Group charge of the Me2Si moiety. c Atomic charge on oxygen. d Group charge
of the R2C moiety. Etotal(19) - Etotal (190) in kcal/mol unless otherwise noted. f The siloxirane was not found as a local minimum. Instead, an
e
intramolecular ion pair 19a00 formed via the C-O bond cleavage of the siloxirane was located as an energy minimum. The ΔE is given as that based on the
energy of 19a00. d(C-O)/A. h Sum of bond angles around the carbonyl ylide carbon. i Group charge of the Me2C moiety. j Energy (kcal/mol) of 20 based
g
˚
on the corresponding oxiranes.
Scheme 2
Scheme 3
the resonance forms 24 similar to type II in Scheme 1 may
be predominant due to the cation-stabilizing effects of the
Cp rings, although steric effects also favor the observed
regioselectivity.21
Formation of six-membered silacycle 6 (Scheme 3) by the
reaction of Ar2Si: with tetraphenylcyclopentadienone10j is
not unexpected if the corresponding carbonyl silaylide has a
strong contribution of type II resonance forms in Scheme 1,
i.e., if it is represented by the structure 25. Intramolecular
electrophilic aromatic addition of silyl cations in 25 to a
neighboring phenyl substituent will lead to 6 via an inter-
mediate 26.22
The reaction of silylene 12 with acetone gave the corre-
sponding silyl enol ether 14 as the sole product, suggesting
that the reaction is more rapid than the competitive cycliza-
tion into the corresponding siloxirane. Although the pre-
ferred formation of the silyl enol ethers is common for the
reaction of enolizable ketones with dimethylsilylene, dime-
sitylsilylene, etc., no such reaction has been found for (Me5-
C5)2Si:,10k which is reported to give typically a cyclic com-
pound with a similar ring structure to 7 by the reaction with
acetone. The results may suggest that a silyl enol ether is
formed via the [2þ3] reaction between the corresponding
carbonyl silaylide and an intramolecular C-H bond, and the
reaction requires the strong contribution of type I resonance
forms (Scheme 1) in the carbonyl silaylide because of the
C(-)-H(þ) polarity (Scheme 4).
Notably, the regioselectivity of the 1:2 adducts 7 and 10
(Scheme 1) has been reported to be different from that of 2.
These three adducts are regarded to form via the [2þ3]
cycloaddition between carbonyl and carbonyl silaylide.
The reversed regioselectivity of 7 and 10 compared with that
of 2 is compatible with the inversed polarity of the cyclopen-
tadienylidene-substituted carbonyl silaylide 19e. In a carbo-
nyl silaylide formed from the reaction of (Me5C5)2Si:,10k
(20) (a) Eaborn, C.; Safa, K. D. J. Organomet. Chem. 1982, 234, 7.
(b) Damrauer, R.; Eaborn, C.; Happer, D. A. R.; Mansour, A. I. J. Chem.
Soc., Chem. Commun. 1983, 348. (c) Al-Mansour, A. I.; Al-Gurashi, M. A.
M. R.; Eaborn, C.; Fattah, F. A.; Lickiss, P. D. J. Organomet. Chem. 1990,
393, 27. A review: Kira, M.; Iwamoto, T. In The Chemistry of Organic
Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; John-Wiley: Chiche-
ster, 2001; Vol. 3, p 853.
Conclusion
(21) The origin of the regioselectivity and stereoselectivity for the 1:2
adducts may be more complicated than that proposed here. Woerpel
et al. have reported that the regioselectivity for the 1:2 adducts obtained
by the reactions of metal-coordinated di-tert-butylsilylene with benzal-
dehyde and n-butanal is different from each other.13h In addition, the
stereochemistry between two phenyl groups in the former adduct is syn,
while it is anti in 1010k (Chart 1).
The reactions of isolable dialkylsilylene 12 with adaman-
tanone, acetone, and benzophenone proceed smoothly at
(22) Aromatic electrophilic substitution of silyl cations has been
reported recently by Kawashima et al.: Furukawa, S.; Kobayashi, J.;
Kawashima, T. J. Am. Chem. Soc. 2009, 131, 14192.

Article
Organometallics, Vol. 29, No. 21, 2010 5533
Scheme 4
vacuo. Silyl-enol ether 14 was isolated in 70% yield (66 mg,
0.15 mmol). An attempted isolation of 14 by HPLC using
ethanol as eluent failed, because during the HPLC procedure
compound 14 was converted to 27 quantitatively probably due
to the addition of ethanol to 14 catalyzed by a small amount of
acid in the silica gel column (eq 8). Analytically pure 12 was
obtained by GPC (toluene). 14: colorless crystals; mp 76 °C; 1H
NMR (C6D6, δ) 0.21 (s, 18H), 0.30 (s, 18H), 1.75 (s, 3H),
1.70-2.05 (m, 4H), 4.12 (brs, 1H), 4.36 (brs, 1H), 5.60 (brs,
1H); 13C NMR (C6D6, δ) 2.6 (SiMe3), 3.3 (SiMe3), 9.5 (C),
32.2 (CH2), 43.6 (CH3), 90.4 (CH2), 156.7 (C); 29Si NMR (C6D6,
δ) 2.9 (SiMe3), 3.5 (SiMe3), 18.5 (Si); MS (70 eV, EI) m/z 430
(Mþ, 11), 415 (Mþ - 15, 18), 387 (40), 373 (90), 73 (100). Anal.
Calcd for C19H46OSi5: C, 52.95; H, 10.76. Found: C, 53.02; H,
10.70.
room temperatures to afford the corresponding siloxirane,
silyl enol ether, and 2-oxa-silacyclopentene without forma-
tion of any secondary or side-reaction products. The diverse
reaction modes are similar to those found previously in the
reactions of dimethylsilylene and related organosilylenes.
Rather unexpectedly, the reactions of 12 with di-tert-butyl-
and diphenylcyclopropenones in similar reaction conditions
to those above afford cyclopropenylsilanes 17 and 18 invol-
ving the migration of a trimethylsilyl group in 12 to the
cyclopropenone oxygen. DFT calculations for various types
of carbonyl silaylides, possible intermediates of the reac-
tions, revealed the remarkable substituent effects on the
electronic structure; the silicon atom in the carbonyl silay-
lides is either nucleophilic or electrophilic depending on the
substituents on the skeletal carbon atom. The diversity of the
reactions of organosilylenes including 12 with different car-
bonyl compounds is in accord with the diverse nature of the
carbonyl silaylides. Further works including the search for
the transition states will be required for elucidating the
detailed reaction mechanisms of these reactions.
27: colorless crystals; mp 85 °C; 1H NMR (C6D6, δ) 0.26 (s,
18H), 0.33 (s, 18H), 1.13 (t, J = 7.1 Hz, 3H), 1.38 (s, 6H),
1.90-2.00 (m, 4H), 3.30 (q, J = 7.1 Hz, 2H), 5.51 (s, 1H); 13
C
NMR (C6D6, δ) 2.6 (SiMe3), 3.6 (SiMe3), 8.5 (C), 15.9
(CH2CH3), 27.5 (CH3), 32.1 (CH2), 58.5 (OCH2CH3), 101.3
(C); 29Si NMR (C6D6, δ) 2.5 (SiMe3), 3.7 (SiMe3), 7.4 (Si); MS
(70 eV, EI) m/z (%) 476 (Mþ, 1), 461 (2), 403 (5), 373 (9), 232
(13), 73 (100). Anal. Calcd for C21H52O2Si5: C, 52.87; H, 10.99.
Found: C, 52.58; H, 11.23.
Experimental Section
All synthetic experiments were performed under argon or
nitrogen in a standard vacuum system unless otherwise noted.
1H (300 MHz),13 C (75 MHz), and 29Si (59 MHz) NMR spectra
were recorded on a Bruker AC-300P spectrometer. Mass spectra
were obtained on a JEOL JMS-600W mass spectrometer.
Sampling of silylene 12 and other air-sensitive materials was
carried out in a VAC MO-40-M glovebox. Di(tert-butyl)-
cyclopropenone23 was prepared according to a literature proce-
dure. Adamantanone and benzophenone were sublimed under
reduced pressure prior to use.
Reaction of Silylene 12 with Adamantanone. In an NMR tube,
silylene 1 (59 mg, 0.16 mmol) and adamantanone (27 mg, 0.16
mmol) were placed and then deoxygenated. Dry benzene-d6 (0.5
mL) was transferred to the tube by using a vacuum line. After
the color of the solution disappeared, NMR spectra were
recorded. Quantitative formation of siloxirane 13 was evidenced
by NMR spectroscopies. Analytically pure 13 was obtained in
87% yield (73 mg, 0.14 mmol) by HPLC (ethanol). Single
crystals suitable for X-ray crystallography were obtained by
recrystallization from ethanol. 13: colorless crystals; mp 200 °C;
1H NMR (C6D6, δ) 0.23 (s, 18H), 0.30 (s, 18H), 1.51-1.62 (brs,
2H), 1.74 (brs, 2H), 1.80-1.95 (m, 8H), 2.05-2.10 (m, 4H),
2.70-2.75 (brs, 2H); 13C NMR (C6D6, δ) 2.5 (SiMe3), 3.2
(SiMe3), 5.9 (C), 27.4 (CH), 28.4 (CH), 32.1 (CH2), 33.8
(CH2), 36.5 (CH2), 37.2 (CH), 38.1 (CH2), 74.2 (C); 29Si NMR
(C6D6, δ) 2.4 (SiMe3), 4.4 (SiMe3), 4.7 (Si); MS (70 eV, EI) m/z
(%) 507 (Mþ - 15, 2), 373 (46), 299 (35), 225 (21), 135 (100), 73
(69). Anal. Calcd for C26H54OSi5: C, 59.69; H, 10.40. Found: C,
59.43; H, 10.28.
Reaction of Silylene 12 with Benzophenone. The reaction of 12
(166 mg, 0.45 mmol) with benzophenone (79 mg, 0.45 mmol)
was performed in a similar manner to the reaction of 12 with
adamantanone. Compound 15 was obtained as yellow crystals
in 80% yield (199 mg, 0.36 mmol). 15: pale yellow crystals; 1H
NMR (C6D6, δ) 0.170 (s, 9H), 0.171 (s, 9H), 0.27 (s, 9H), 0.29 (s,
9H), 1.72-2.15 (m, 4H), 3.77 (brs, 1H), 5.75 (m, 1H), 6.02 (m,
1H), 6.24 (m, 1H), 6.76 (m, 1H), 7.04-7.25 (m, 5H); 13C NMR
(C6D6, δ) 3.6 (SiMe3), 3.7 (SiMe3), 4.0 (SiMe3), 4.8 (SiMe3), 10.0
(C), 16.6 (C), 32.4 (CH2), 33.6 (CH2), 34.0 (CH), 115.2 (C), 122.6
(CH), 124.8 (CH), 125.7 (CH), 125.8 (CH), 128.6 (CH), 128.5
(CH), 128.3 (CH), 135.3 (C), 150.2 (C); 29Si NMR (C6D6, δ) 1.0
(SiMe3), 2.0 (SiMe3), 4.17 (SiMe3), 4.21 (SiMe3), 57.2 (Si).
Compound 15 was stored intact for a long time in a refrigerator
in a glovebox but reacted with oxygen and moisture to gradually
isomerize into 16 at room temperature. 16: colorless crystals; mp
174-176 °C; 1H NMR (C6D6, δ) 0.10 (s, 9H), 0.11 (s, 9H), 0.18
(s, 9H), 0.31 (s, 9H), 2.02-2.13 (m, 4H), 6.17 (s, 1H), 6.92 (d, J =
7.2 Hz, 1H), 7.00-7.15 (m, 5H), 7.25 (m, 2H), 7.99 (d, J = 7.2
Hz, 1H); 13C NMR (C6D6, δ) 4.1 (SiMe3), 4.2 (SiMe3), 4.23
(SiMe3), 4.24 (SiMe3), 13.1 (C), 14.1 (C), 32.6 (CH2), 33.2 (CH2),
83.0 (CH), 124.9 (CH), 126.9 (CH), 127.7 (CH), 128.5 (C), 129.3
(CH), 129.8 (CH), 134.2 (CH), 136.9 (CH), 142.8 (C), 152.4 (C);
29Si NMR (C6D6, δ) 1.9 (SiMe3), 2.6 (SiMe3), 3.5 (SiMe3), 3.9
(SiMe3), 41.1 (Si); MS (70 eV, EI) m/z (%) 554 (Mþ, 18), 539
(17), 341 (42), 105 (42), 73 (100). Anal. Calcd for C29H50OSi5: C,
62.74; H, 9.08. Found: C, 62.97; H, 9.15.
Reaction of Silylene 12 with Di(tert-butyl)cyclopropenone.
Reaction of silylene 12 (85 mg, 0.23 mmol) with di(tert-bu-
tyl)cyclopropenone (41 mg, 0.23 mmol) was performed in a
similar manner to the reaction of 12 with adamantanone.
Compound 17 was obtained in 77% yield (99 mg, 0.18 mmol).
When monitored by NMR, the reaction was very clean and no
other products than 17 appeared during the reaction. 17: color-
less crystals; mp 65-68 °C; 1H NMR (C6D6, δ) 0.14 (s, 9H), 0.17
(s, 9H), 0.28 (s, 9H), 0.35 (s, 9H), 1.06 (s, 9H), 1.12 (s, 9H), 1.53
(s, 1H), 2.51 (m, 2H), 6.96 (m, 1H); 13C NMR (C6D6, δ) 0.4
(SiMe3), 2.0 (SiMe3), 2.8 (SiMe3), 3.8 (SiMe3), 12.4 (C), 29.1
Reaction of Silylene 12 with Acetone. To silylene 12 (83 mg,
0.22 mmol) in a Schlenk tube (10 mL) was transferred dried and
deoxygenated acetone (10 mL). After the yellow color of the
silylene in the solution disappeared, volatiles were removed in
(23) Ciabattoni, J.; Nathan, E. C.; Feiring, A. E.; Kocienski, P. J.
Organic Synthesis; Wiley: New York, 1988; Collect. Vol. VI, p 991. We
thank Prof. Kenkichi Sakamoto and Dr. Yoshihiro Kon, Tohoku
University, for the preparation of these compounds.

5534 Organometallics, Vol. 29, No. 21, 2010
Ishida et al.
(CH3), 30.1 (CH3), 32.9 (C), 33.8 (C), 37.2 (CH2), 37.7 (CH),
112.3 (C), 144.6 (C), 151.5 (CH), 159.2 (C); 29Si NMR (C6D6, δ)
-7.4 (SiMe3), 1.6 (SiMe3), 2.9 (Si), 3.1 (SiMe3), 7.0 (SiMe3); MS
(70 eV, EI) m/z (%) 523 (Mþ - 15, 9), 481 (100), 387 (100), 73
(79). Anal. Calcd for C27H58OSi5: C, 60.15; H, 10.84. Found: C,
60.14; H, 10.62. Single crystals suitable for X-ray crystallogra-
phy were obtained by the recrystallization from THF.
(SiMe3), 11.6 (C), 27.2 (CH), 38.5 (CH2), 117.4 (C), 125.8 (CH),
126.9 (CH), 128.5 (CH), 128.8 (CH), 129.5 (CH), 129.7 (CH),
130.7 (C), 131.2 (C), 144.8 (C), 146.5 (CH), 161.2 (C); 29Si NMR
(C6D6, δ) -6.5 (SiMe3), -1.2 (SiMe3), 2.7 (Si), 3.6 (SiMe3), 9.5
(SiMe3); MS (70 eV, EI) m/z (%) 578 (Mþ, 0.7), 563 (2), 387
(100), 73 (33). Anal. Calcd for C31H50OSi5: C, 64.29; H, 8.70.
Found: C, 64.53; H, 8.50.
Reaction of Silylene 12 with Diphenylcyclopropenone. Reac-
tion of silylene 12 (97 mg, 0.26 mmol) with diphenylcyclopro-
penone (48 mg, 0.23 mmol) was performed in a similar manner
to the reaction of 12 with adamantanone. Compound 18 was
obtained in 81% yield (108 mg, 0.19 mmol). When monitored by
NMR, the reaction was very clean, and no other products than
18 appeared during the reaction. 18: colorless crystals; mp
Acknowledgment. This work was supported by the
Ministry of Education, Culture, Sports, Science, and Tec-
hnology of Japan (Grants-in-Aid for Specially Promoted
Research No. 17002005 (M.K. and T.I.)).
1
Supporting Information Available: X-ray analysis of 17, de-
tails for the calculations of 19a-19e, 19b0-19e0, 19a00, 20, and
200, and X-ray crystallographic data of 13, 15, 16, 17, and 18 in
CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
113-115 °C; H NMR (C6D6, δ) -0.02 (s, 9H), 0.16 (s, 9H),
0.20 (s, 9H), 0.29 (s, 9H), 2.67 (m, 2H), 3.12 (s, 1H, cyclopropene
ring), 6.98 (m, 1H), 7.08 (t, J = 7.2 Hz, 3H), 7.18 (t, J = 6.8 Hz,
3H), 7.35 (d, J = 7.2 Hz, 2H), 7.56 (d, J = 6.8 Hz, 2H); 13C
NMR (C6D6, δ) -0.2 (SiMe3), 1.7 (SiMe3), 2.0 (SiMe3), 2.2