A new pathway in the reaction of disilene with carbonyl compounds: An 'ene' reaction instead of cycloaddition

Article abstract of DOI:10.1039/b106801f

The 3H-disilagermirene, (^tBu₂MeSi)₄GeSi₂, reacts with α-hydrogen containing carbonyl compounds (acetophenone, butane-2,3-dione) by an 'ene'-reaction pathway followed by isomerization or insertion reactions, representing a new mode in the reaction of disilenes with carbonyl compounds.

Full text of DOI:10.1039/b106801f

View Article Online / Journal Homepage / Table of Contents for this issue
A new pathway in the reaction of disilene with carbonyl compounds:
an ‘ene’ reaction instead of cycloaddition
Vladimir Ya. Lee, Masaaki Ichinohe and Akira Sekiguchi*
Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki, 308-8571, Japan.
E-mail: sekiguch@staff.chem.tsukuba.ac.jp
Received (in Cambridge, UK) 26th July 2001, Accepted 7th September 2001
First published as an Advance Article on the web 2nd October 2001
The 3H-disilagermirene, (tBu2MeSi)4GeSi2, reacts with a-
hydrogen containing carbonyl compounds (acetophenone,
butane-2,3-dione) by an ‘ene’-reaction pathway followed by
isomerization or insertion reactions, representing a new
mode in the reaction of disilenes with carbonyl com-
pounds.
determined by all spectral data† and finally confirmed by X-ray
analysis.‡ From the crystal structure, the three-membered ring
skeleton with a trans-arrangement of H- and H2CNC(Ph)O-
substituents can be clearly seen. The bond lengths lie in the
normal range and the Si–Si bond length of the three-membered
skeleton is 2.3383(6) Å, which is very similar to those of
disilagermirane derivatives that we have previously reported
(2.342(2)5 and 2.3269(6) Å6). The formation in the first step of
the cis-isomer 2 agrees with the previously established
mechanism for the reaction of disilenes with alcohols, which
proceeds as syn-addition.8
The stable dimetallenes containing MNM double bonds (M =
heavier Group 14 elements) have now become common, easily
accessible and a rather important class of organometallic
compounds of Group 14 elements.1 The reactivity of such
dimetallenes has been widely studied, including a variety of
addition and cycloaddition reactions. Thus, it is well established
that disilenes with a SiNSi double bond react with carbonyl
compounds, such as benzaldehyde, acetone, acetophenone and
dibenzyl, by [2 + 2] or [4 + 2] cycloaddition reaction modes.2 In
contrast, the reaction of carbonyl compounds having a-
hydrogen atoms with compounds containing polar bonds, such
as SiNC or GeNP, proceeded as an ‘ene’ reaction resulting in the
formation of enol ether products.1,2 Quite recently, we prepared
3H- and 1H-disilagermirenes; that is, three-membered ring
compounds with endocyclic metal–metal double bonds.3 We
have found that such compounds exhibit a high reactivity of
both the double bond and the strained three-membered ring
skeleton.4–6 In this paper, we report the reactions of a 3H-
disilagermirene with carbonyl compounds containing a-hydro-
gen atoms. They unexpectedly produce enol ether type
products, which subsequently undergo further interesting
isomerization and insertion reactions.
The reaction of 3H-disilagermirene 1 with an excess of the
dicarbonyl compound butane-2,3-dione also occurred smoothly
and cleanly in C6D6 at room temperature to produce quantita-
tively a single product in 30 minutes. Its structure was
established by spectral data§ and X-ray crystallography¶ to be
1,2,3,3-tetrakis(di-tert-butyl(methyl)silyl)-4-methyl-5-methyl-
ene-6,7-dioxa-1,2-disila-3-germabicyclo[2.2.1]heptane
(Scheme 2).
4
X-Ray analysis of 4 showed a bicyclic structure with the
norbornane-type skeleton (Fig. 1). Compound 4 has two oxygen
atoms incorporated in the rings, one of which occupies the
bridging 7-position. Formation of the norbornane 4 can be
rationalized as the following reaction sequence. In the first step,
nucleophilic attack of the carbonyl oxygen lone pair on the
doubly-bonded Si atom results in the formation of the cis-
adduct–enol ether 5. The primarily formed 5 may isomerize to
a thermodynamically more favorable 6, due to the lower steric
repulsions of the bulky silyl substituents in the trans-isomer. In
the last step, the second carbonyl group in 6 undergoes an
intramolecular insertion into the endocyclic Si–Ge bond of the
three-membered ring to form the final norbornane 4. The
proposed mechanism satisfactorily explains the structure of
compound 4, in which the H atom on the Si atom is directed
towards the bridge; that is, it occupies the exo-position. Such an
arrangement of the H atom in 4 can be obtained from the trans-
isomer 6 only by geometrical reasons, and the last compound
can be imagined as the result of isomerization of the initially
formed cis-isomer 5.
The reaction of 3H-disilagermirene 1 with a slight excess of
acetophenone in C6D6 at room temperature was complete in two
hours to form quantitatively a single product. It was identified
by NMR spectroscopy data as cis-1,2,3,3-tetrakis(di-tert-butyl-
(methyl)silyl)-1-(1-phenylvinyloxy)disilagermirane 2 (Scheme
1). The formation of this compound can be considered the result
of nucleophilic attack of the carbonyl oxygen atom on the sp2-Si
atom through either a zwitterionic intermediate or six-centered
transition state. This resulted in the formation of the product of
a formal 1,2-addition of 1-phenylvinyl alcohol to a SiNSi double
bond of 1. The enol ether 2 was not stable at room temperature
and spontaneously isomerized to the thermodynamically more
stable trans-isomer 3. After one day the ratio of cis- and trans-
isomers of 2 and 3 was 1+1, whereas after two days 2 was
completely isomerized to 3. The driving force for such
isomerization is most likely to be the decrease of the steric
hindrance on going from the cis- to the trans-isomer.7
The trans-isomer 3 was isolated quantitatively as pale-yellow
crystals by recrystallization from hexane. Its structure was
Scheme 1
Scheme 2
2146
Chem. Commun., 2001, 2146–2147
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b106801f

View Article Online
128.6, 138.5, 159.3 (CNCH2); dSi (59.6 MHz, C6D6) 2142.2, 217.6, 17.2,
23.0, 28.8, 32.4; Anal. Calcd for C44H92OSi6: C, 60.17; H, 10.56. Found: C,
60.50; H, 10.45%.
‡ Crystal data for 3: C44H92GeOSi6, M
= 878.31, monoclinic, a =
11.0980(2), b = 20.9340(5), c = 22.8640(6) Å, b = 96.8960(10) °, V =
5273.5(2) Å3, T = 120 K, space group = P21/c, Z = 4, rcalcd = 1.106 g
cm23. The final R factor was 0.0348 (Rw = 0.0953 for all data) for 10698
reflections with I > 2s(I). GOF = 1.028. CCDC 170581.
§ Spectral data for 4: mp 132–134 °C; dH (300 MHz, C6D6) 0.40 (s, 3H,
Me), 0.45 (s, 3H, Me), 0.53 (s, 3H, Me), 0.55 (s, 3H, Me), 1.19 (s, 9H, tBu),
1.20 (s, 9H, tBu), 1.218 (s, 18H, 2tBu), 1.222 (s, 9H, tBu), 1.24 (s, 9H, tBu),
1.28 (s, 18H, 2tBu), 2.20 (s, 3H, C-Me), 3.46 (s, 1H, Si-H), 4.13 (d, J = 1.1
Hz, 1H, CNCH2), 4.39 (d, J = 1.1 Hz, 1H, CNCH2); dc (75.5 MHz, C6D6)
26.2, 22.8, 21.2, 20.3, 21.6, 21.9, 22.2, 22.4, 22.6, 24.1, 24.4, 24.5, 28.9,
29.4, 29.7, 30.0, 30.8, 31.5, 32.1, 32.16, 32.21, 83.7 (C-O), 85.7 (CNCH2),
168.1 (CNCH2); dSi (59.6 MHz), C6D6) 290.2, 5.4, 19.9, 26.7, 27.3,
41.8.
¶ Crystal data for 4: C40H90GeO2Si6, M = 844.25, triclinic, a = 12.431(1),
b = 12.879(1), c = 16.286(1) Å, a = 81.534(6), b = 84.167(5), g =
¯
86.007 (6)°, V = 2561.6(4) Å3, T = 120 K, space group = P1, Z = 2, rcalcd
= 1.095 g cm23. The final R factor was 0.0784 (Rw = 0.2356 for all data)
for 7486 reflections with I > 2s(I). GOF = 1.038. Diffraction data were
collected on a Mac Science DIP2030K Image Plate Diffractometer
employing graphite-monochromatized Mo-Ka radiation (l = 0.71070 Å).
The structure was solved by the direct method and refined by the full-matrix
least-squares method using SHELXL-97 program. CCDC 160777.
Fig. 1 Structure of 4 (hydrogen atoms are omitted for clarity). Selected bond
distances [Å] are: Ge(1)–Si(2) 2.460(1), Si(1)–Si(2) 2.369(1), Ge(1)–C(3)
2.086(5), Si(1)–O(1) 1.701(4), O(1)–C(2) 1.394(6), C(2)–C(3) 1.503(7),
Si(1)–O(2) 1.685(3), C(3)–O(2) 1.459(6). Selected bond angles [°] are:
C(3)–Ge(1)–Si(2) 95.0(1), Ge(1)–Si(2)–Si(1) 86.1(0), C(3)–C(2)–O(1)
110.6(4), Si(1)–O(1)–C(2) 108.1(3), Si(1)–O(2)–C(3) 104.9(3).
1 Recent reviews on metallenes and dimetallenes of Group 14 elements: M.
Weidenbruch, Eur. J. Inorg. Chem., 1999, 373; P. P. Power, Chem. Rev.,
1999, 99, 3463; J. Escudie´ and H. Ranaivonjatovo, Adv. Organomet.
Chem., 1999, 44, 113.
2 M. J. Fink, D. J. DeYoung, R. West and J. Michl, J. Am. Chem. Soc.,
1983, 105, 1070; R. West, Angew. Chem., Int. Ed. Engl., 1987, 26, 1201;
G. Raabe and J. Michl, The Chemistry of Organic Silicon Compounds, S.
Patai and Z. Rappoport, Eds., Wiley, Chichester, 1989, Part 2, ch. 17.
3 V. Ya. Lee, M. Ichinohe, A. Sekiguchi, N. Takagi and S. Nagase, J. Am.
Chem. Soc., 2000, 122, 9034.
4 V. Ya. Lee, M. Ichinohe and A. Sekiguchi, J. Am. Chem. Soc., 2000, 122,
12604.
5 V. Ya. Lee, T. Matsuno, M. Ichinohe and A. Sekiguchi, Heteroatom.
Chem., 2001, 12, 223.
6 V. Ya. Lee, M. Ichinohe and A. Sekiguchi, Chem. Lett., 2001, 728.
7 The mechanism of such isomerization may involve the nucleophilic
assistance of an excess of carbonyl compound, due to the attack of
carbonyl oxygen atom on the back side of the silicon atom bearing
oxygen substituent, which results in the formation of a trans-isomer.
8 A. Sekiguchi, I. Maruki and H. Sakurai, J. Am. Chem. Soc., 1993, 115,
11460.
9 An example of [2 + 2] cycloaddition reaction of SiNSi and CNO double
bonds to form the corresponding disilaoxetane followed by its isomeriza-
tion to the corresponding disilyl enol ether under photochemical
conditions has previously been reported: A. Scha¨fer and M. Weiden-
bruch, J. Organomet. Chem., 1985, 282, 305.
Both reactions of 3H-disilagermirene 1 with acetophenone
and butane-2,3-dione represent the first examples of an ‘ene’-
type reaction of disilenes with carbonyl compounds, which
were previously commonly recognized to react only by
cycloaddition pathways to form cyclic compounds.2,9 The cis–
trans isomerization of the enol ether was also previously
unknown, as well as the combination of 1,2-addition and
insertion pathways in one reaction.
This work was supported by a Grant-in-Aid for Scientific
Research (Nos. 12042213, 13440185, 13029015) from the
Ministry of Education, Science and Culture of Japan, and
TARA (Tsukuba Advanced Research Alliance) fund.
Notes and references
† Spectral data for 3: mp 179–181 °C; dH (300 MHz, C6D6) 0.23 (s, 3H,
Me), 0.39 (s, 3H, Me), 0.41 (s, 3H, Me), 0.47 (s, 3H, Me), 1.22 (s, 9H, tBu),
1.23 (s, 9H, tBu), 1.26 (s, 9H, tBu), 1.28 (s, 9H, tBu), 1.29 (s, 9H, tBu), 1.30
(s, 9H, tBu), 1.31 (s, 18H, 2tBu), 3.01 (s, 1H, Si-H), 5.07 (d, J = 1.6 Hz, 1H,
CNCH2), 5.14 (d, J = 1.6 Hz, 1H, CNCH2), 7.07 (d, J = 7.2 Hz, 1H, ArH),
7.19 (t, J = 7.2 Hz, 2H, ArH), 7.69 (d, J = 7.2 Hz, 2H, ArH); dC (75.5 MHz,
C6D6) 24.24, 24.21, 20.2, 0.1, 22.3 (2C), 22.5 (2C), 22.7, 23.1, 23.5, 25.0,
30.4, 30.5, 30.6, 30.7, 31.1, 31.3, 31.5, 32.0, 94.1 (CNCH2), 126.8, 128.3,
Chem. Commun., 2001, 2146–2147
2147