Reactivities of chlorotrisylsilylenoid with ketones

Article abstract of DOI:10.1039/c0dt00147c

Chlorotrisylsilylenoid 1 reacted with both acetophenone and benzil to give the corresponding 2,5-dioxasilolanes and silylethers, respectively. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0dt00147c

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The pinnacle of the ancient sundial is pointing
a dioxasilolane, a functional silaheterocycle,
which is produced from the reaction of a stable
chlorotrisylsilylenoid with acetophenone. The sundial
metaphorically represents that silylenoid (R2SiMX,
M = metal, X = halogen) chemistry is in progress.
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Title: Reactivities of chlorotrisylsilylenoid with ketones
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www.rsc.org/dalton | Dalton Transactions
Reactivities of chlorotrisylsilylenoid with ketones†
Hyeon Mo Cho, Young Mook Lim and Myong Euy Lee*
Received 16th March 2010, Accepted 4th June 2010
DOI: 10.1039/c0dt00147c
Chlorotrisylsilylenoid 1 reacted with both acetophenone
and benzil to give the corresponding 2,5-dioxasilolanes and
silylethers, respectively.
Silylenoids (R2SiMX) are species in which an electropositive
metal (M, usually alkali metal) and a leaving group (X, usually
halogen) are bound to the same silicon atom, and have amphiphilic
properties. The reactivities of silylenoids are similar to those
of silylenes (R2Si:).1–6 Only a few stable silylenoids including
(tBuO)Ph2SiLi,2 (Mes)2Si(SMes)Li,3 (Me3Si)2Si(OMe)K·crown-
ether4 and (R3Si)2SiFLi5 (R3Si = t-Bu2MeSi) have been reported
and there are no reports on the reaction of a silylenoid with
ketones. Recently we reported the synthesis of halosilylenoids
TsiSiX2Li (Tsi (trisyl) = C(SiMe3)3, X = Br, Cl) and their
reactivities with various alcohols, dienes, aldehydes, alkyl halides,
silyl halides, aryllithiums, arylmagnesium bromides and lithium
naphthalenide.6 Since stable halosilylenoids have two halogens
and one lithium, reactions such as substitution, addition, trans-
metalation and reduction are all possible. In this paper we report
unprecedented examples of the reaction of a chlorotrisylsilylenoid
with a monoketone and a diketone.
Fig. 1 ORTEP drawing of 2 shown with 30% thermal ellipsoid. Hydrog◦en
˚
atoms are omitted for clarity. Selected bond lengths (A) and angles ( ):
Si1–Cl1 = 2.091(1), Si1–C1(Tsi) = 1.847(3), Si1–O1 = 1.640(2), Si1–O2 =
1.646(2); O1–Si1–O2 = 96.65(9).
To the stable chlorosilylenoid 1 having a bulky Tsi group, which
was formed by the reduction of trichlorotrisylsilane with two
equivalents of lithium naphthalenide, was added an excess of
acetophenone at -78 ◦C. Following 1 h stirring at that temperature,
treatment of the reaction mixture with excess MeOH gave the
products 2,5-dioxasilolane (2) and silylalkylether (3) in 43% and
17% yields (Scheme 1).‡
having a trans–trans conformation of Tsi-Pha–Phb was observed.
This stereo- and regio-selectivity is evidently due to the bulky
substituents.7
A possible mechanism for the formation of 3 can be explained
by the Brook rearrangement8 (Scheme 2). Nucleophilic attack of
silicon atom of 1 to acetophenone generated a silylalkoxide, which
underwent further reaction via a [1,2]-silyl migration from carbon
to oxygen to give carboanion 4. Species 4 did not undergo an
intramolecular cyclization, possibly due to the large Si–O–C angle.
Treatment of 4 with excess MeOH and MeOD gave products 3 and
deuterated 3.
Scheme 1 Reaction of chlorosilylenoid with acetophenone.
Compound 2, which was stable to air and moisture, was
obtained as a colorless single crystal from recrystallization in
n-hexane at room temperature.§ The molecular structure of 2
was characterized by X-ray crystallography (Fig. 1). All atoms
except a quaternary carbon in the Tsi group appeared to be
disordered with two possible orientations in a ratio of 0.77 to
0.23. The sum of the interior angles of 2 was 533.0◦, indicating
a nearly planar structure of the five-membered 2,5-dioxasilolane
ring. Interestingly, only one of the possible isomeric structures,
Department of Chemistry & Medical Chemistry, College of Science and
Technology, Yonsei University, Wonju, Gangwondo, 220-710, Korea. E-mail:
melgg@yonsei.ac.kr; Fax: +82-33-760-2182; Tel: +82-33-760-2237
† CCDC reference numbers 770202 and 770203. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/c0dt00147c
Scheme 2 Possible mechanisms for the formation of 2 and 3.
9232 | Dalton Trans., 2010, 39, 9232–9234
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The Royal Society of Chemistry 2010
©

The formation of 2 can be explained by two plausible mecha-
nisms. Firstly, a highly reactive intermediate oxasilacyclopropane9
was generated by intramolecular cyclization of the silylalkoxide
(pathway i), followed by a direct insertion of an acetophenone
into a Si–C bond of the oxasilacyclopropane to give compound 2.
A silacarbonyl ylide9c–f,10 (TsiClSi-–O+ CMePh), formed through
=
the heterolytic cleavage of oxasilacyclopropane, might be involved
in producing the 2,5-dioxasilolane. However, the previous DFT
calculation revealed that the silacarbonyl ylide ((H3Si)3CClSi-–
O+ CMeH) was less stable than the corresponding oxasila-
=
cyclopropane formed by the reaction of the silylenoid with
acetaldehyde.6d Jutzi and co-workers also suggested that oxasila-
cyclopropanes formed from the reaction of decamethylsilicocene
with aldehydes, would be key intermediates in silylene chemistry,
rather than silacarbonyl ylides.9b An alternative pathway to 2
is the formation of silylether alkoxide by nucleophilic attack of
carboanion of 4 on an acetophenone, followed by intramolecular
cyclization (pathway ii).
In the reaction of chlorosilylenoid 1 with the a-diketone benzil,
the reaction mixtures were quenched by H2O and MeOH to give
compounds 5 and 6 in 56% and 62% yields (Scheme 3). Compound
6 was isolated through flash column chromatography and then
obtained as colorless crystals by recrystallization from n-hexane.
The structure of 6 was characterized by X-ray crystallography
(Fig. 2). The sum of the interior angles around the ring of 6
was 523.3◦, indicating a slightly distorted structure of the five
membered heterocycle.
Fig. 2 ORTEP drawing of 6 shown with 30% thermal ellipsoid. Hydrog◦en
˚
atoms are omitted for clarity. Selected bond lengths (A) and angles ( ):
Si1–Cl1 = 2.080(2), Si1–C16(Tsi) = 1.834(4), Si1–O1 = 1.650(3), Si1–O2 =
1.668(3); Cl1–Si1–C16(Tsi) = 111.6(2).
different result from that with H2O. Addition of MeOH to the
reaction mixtures containing intermediate 7 afforded 1,3-dioxa-
2-silacyclopentane, 6, which was formed through the nucleophilic
addition of LiOMe to the unreacted carbonyl carbon followed by
cyclization. To prove this proposed mechanism, we carried out the
reaction of the isolated compound 5 with LiOMe. As anticipated,
compound 6 was formed.
It is worth mentioning that these reactions of a silylenoid with
diketone are different from those of silylenes. In the reactions
of silylenes with a-diketones, 1,3-dioxa-2-silacyclopent-4-enes
were formed by a concerted [2 + 1] cycloaddition process.11 By
comparison, the silylenoid reaction took place stepwise to produce
a silylether type compound which underwent further reactions
to give 2,5-dioxasilolane. Our ongoing work will address the
reactivities of silylenoid with various ketones, such as acetone,
b-diketones, naphthoquinone and a,b-unsaturated ketones. We
wish to establish new synthetic routes for functional silahetero-
cylcles.
Scheme 3 Reaction of chlorosilylenoid with benzil.
For the reaction of 1 with benzil, we propose the following
mechanism (Scheme 4). Organolithium intermediate 7 was initially
generated through a similar mechanism to the formation of
4. From the trapping experiment of species 7 with H2O and
D2O, the hydrolyzed products 5 and deuterated 5 were obtained,
respectively. Interestingly the trapping of 7 by MeOH showed a
Acknowledgements
This work was supported by WCU (World Class University)
Program through the National Research Foundation of Korea
funded by the Ministry of Education, Science and Technology
(R33-10082). We thank Professor Robert West, distinguished
professor in the WCU Program for his valuable discussions. We
thank Professor Moon-Gun Choi (Yonsei University), Professor
Ki-Min Park (Gyeongsang National University), Mr Kang Mun
Lee (KAIST) and Professor Youngkyu Do (KAIST) for help
solving the crystal structures.
Scheme 4 Possible mechanisms for the formation of 5 and 6.
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 9232–9234 | 9233
©

§ Crystal and structure refinement data for 2: C26H43ClO2Si4, M = 535.41,
Notes and references
˚
monoclinic, P21/n, a = 11.513◦2(4), b = 9.1353(3), c = 28.9123(10) A,
◦
3
˚
‡ Synthesis of 2 and 3: naphthalene (1.28 g, 10 mmol) dissolved in THF
(40 mL) was added to Li (0.060 g, 8.7 mmol) at room temperature. After
stirring for 3 h at room temperature, LiNp (lithium naphthalenide) was
obtained as a dark green solution. LiNp in THF was added slowly to
TsiSiCl3 (1.46 g, 4.0 mmol) in THF (40 mL) at -78 ◦C using cannula
technique within 10 min. The solution was stirred for 12 h at the
same temperature; all the starting reactants were consumed to give
chlorosilylenoid 1. To the dark brown reaction mixture was added an
excess of acetophenone (4.7 mL, 40 mmol) at -78 ◦C. After 1 h, to
the reaction mixture was added an excess of MeOH. The final reaction
mixture was slowly warmed to room temperature, and then the solvent was
evaporated under reduced pressure. After the sublimation of naphthalene,
to the resulting viscous light yellow oil was added n-hexane (100 mL) and
then precipitated species were removed by filtration. The crude material
was purified by silica gel chromatography (n-hexane) to afford 2 (0.92 g,
1.7 mmol) as a white powder and 3 (0.31 g, 0.68 mmol) as a colorless
oil. Spectral data for 2: colorless crystal (mp = 194–196 ◦C). 1H NMR
(400 MHz, chloroform-d): d 0.47 (s, 27H), 1.36 (s, 3H), 1.39 (s, 3H),
7.27–7.66 (m, 10H). 13C NMR (100 MHz, chloroform-d): d 5.3 (SiCH3),
6.5 (C(Si(CH3)3)3), 29.8 (CH3), 31.3 (CH3), 88.0 (OC), 88.8 (OC), 125.5,
126.7, 126.9, 127.0, 127.7, 127.8 (Ph-C), 145.5, 145.8 (Ph-ipso-C). GC-
MS: m/z (%) 534 (M+, 3.0), 519 (14.7), 295 (100), 275 (34.2), 187 (46.4),
105 (50.4), 73 (23.2). HRMS: C26H43ClO2Si4 534.2029 (calcd), 534.2028
(found). Anal. Calcd for C26H43ClO2Si4: C, 58.32; H, 8.09; O, 5.98. Found:
C, 58.29; H, 8.11; O, 5.89. Spectral data for 3: Colorless oil. 1H NMR
(400 MHz, chloroform-d): d 0.30 (s, 27H), 1.60 (d, J = 6.44, 3H), 5.29 (q,
J = 6.44, 1H), 7.28–7.39 (m, 5H). 13C NMR (100 MHz, chloroform-d): d
4.3 (SiCH3), 7.5 (C(Si(CH3)3)3), 24.8 (CH3), 74.1 (OC), 128.2, 127.8, 128.2
(Ph-C), 143.0 (Ph-ipso-C). GC-MS: m/z (%) 449 (M+-1, 1.3), 331 (3.4),
295 (3.9), 207 (6.0), 105(100), 73 (12.3). HRMS: C18H35Cl2OSi4 449.1142
(M+-1, calcd), 449.1143 (found). Anal. Calcd for C18H36Cl2OSi4: C, 47.86;
H, 8.03; O, 3.54. Found: C, 47.90; H, 8.09; O, 3.57.
a = g = 90.00 , b = 90.532(2) , V = 3040.76(18) A , T = 296 K, Z =
46 436 independent reflections (R(int) = 0.0346), R1 = 0.0588 (observed
data), wR2 = 0.1806 (all data). Crystal and structure refinement data
for 6: C25H41O3Si4Cl, M = 536.38, monoclinic, P21/c, a = 17.846(2), b =
◦
◦
˚
9.1122(12), c = 18.625(2) A, a = g = 90.00 , b = 91.569(2) , V = 3027.6(7)
3
˚
A , T = 273 K, Z = 44 583 independent reflections (R(int) = 0.0400), R1 =
0.0669 (observed data), wR2 = 0.2182 (all data). Several atoms (Si1, Si3,
Si4, C22, C25, and Cl1) appeared to be disordered (a ratio of each of
disorder components = 0.81 : 0.19).
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Synthesis of 5 and 6: to the product mixture of chlorosilylenoid 1 was added
an excess of benzil (4.2 g, 20 mmol) in a manner similar to that described
above. After 6 h, to the reaction mixture was added an excess of H2O
and MeOH. The crude material was purified by silica gel chromatography
(n-hexane) to afford 5 and 6 in 56% (1.2 g, 2.2 mmol), and 62% (1.3 g,
2.5 mmol), respectively. White powder 6 was recrystallized from n-hexane
to give colorless crystals. Spectral data for 5: colorless oil. 1H NMR
(400 MHz, chloroform-d): d 0.32 (s, 27H), 6.55 (s, 1H), 7.29–7.93 (m, 10H).
13C NMR (100 MHz, chloroform-d): d 4.2 (SiCH3), 7.9 (C(Si(CH3)3)3),
128.6, 128.7, 128.8, 128.9 (Ph-C), 133.2 (CH), 135.0, 136.0 (Ph-ipso-C),
+
=
194.3 (C O). GC-MS: m/z (%) 525 (M -15, 3.5), 504 (3.4), 435 (28.4),
195 (56.5), 167 (54.9), 105 (100), 73 (43.2). HRMS: C24H39Cl2O2Si4 (M++1)
541.1404 (calcd), 541.1409. Anal. Calcd for C24H38Cl2O2Si4: C, 53.20; H,
7.07; O, 5.91. Found: C, 53.08; H, 7.11; O, 5.85. Spectral data for 6:
colorless crystal. 1H NMR (400 MHz, chloroform-d): d 0.49 (s, 27H), 3.03
(s, 3H), 5.04 (s, 1H), 7.31–7.54 (m, 10H). 13C NMR (100 MHz, chloroform-
d): d 4.6 (SiCH3), 4.7 (C(Si(CH3)3)3), 50.4 (OCH3), 84.4 (OCH), 106.1
(OCO), 127.4, 127.5, 127.8, 128.0, 128.1, 128.3, 128.9, 129.3 (Ph-C), 135.9,
139.8 (Ph-ipso-C). GC-MS: m/z (%) 536 (M+, 13.5), 521 (5.5), 400 (10.8),
295 (47.5), 167 (88.9), 73 (100). HRMS: C25H41O3Si4Cl 536.1821 (calcd),
536.1823 (found). Anal. Calcd for C25H41O3Si4Cl: C, 55.88; H, 7.69; O,
8.93. Found: C, 55.77; H, 7.72; O, 8.85.
9234 | Dalton Trans., 2010, 39, 9232–9234
This journal is
The Royal Society of Chemistry 2010
©