Communications
DOI: 10.1002/anie.200704939
N-Heterocyclic Silylenes
Dichotomic Reactivity of a Stable Silylene toward Terminal Alkynes:
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Facile C H Bond Insertion versus Autocatalytic Formation of
Silacycloprop-3-ene**
Shenglai Yao, Christoph van Wüllen, Xiao-Ying Sun, and Matthias Driess*
Dedicated to Professor Philip P. Power
bulky organic groups (A, B). Recently we described the
Silylenes, the silicon analogues of carbenes, are key inter-
mediates in numerous thermal and photochemical reactions.[1]
They are indispensable building blocks for the synthesis of
stable ylide-like silylene 1, in which significant participation
of the 6p-aromatic resonance form 1’ is found in the electronic
ground state (Scheme 1).[3]
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organosilanes through X Y insertion reactions (X Y= C H,
Owing to its unusual ylide-like character, silylene 1 shows
a remarkably distinct reactivity toward both electrophiles and
nucleophiles, in comparison with A and B, respectively.
Striking results include the facile formation of silyliumylidene
[1b–h]
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C C , C O, C Cl, Si H, O H, for example).
Until two
decades ago, silylenes were generally considered to be
extremely unstable species, decomposing readily at low
temperature (> ꢀ1968C). The situation changed profoundly
in 1994, when West et al. described the synthesis of the first
stable N-heterocyclic silylenes A with two-coordinate silicon
which were persistent at room temperature (Scheme 1).[2a,b]
Following the successful isolation of stable silylenes A, only a
few other stable silylenes have been reported as yet,[2c–e]
including the remarkable nonsupported dialkylsilylene B
(Scheme 1).[2f] The latter systems benefit from p-donor
stabilization of the low-valent silicon by the nitrogen atoms,
pseudoaromaticity (A), and/or steric protection through
cations by addition of H+ or other Lewis acid centers at the
[4a]
=
nucleophilic carbon atom of the terminal C Cmoiety in 1,
synthesis of a stable siloxysilylene through heterolytic addi-
tion of water to 1,[4b] and insertion of the divalent silicon atom
[4c]
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in 1 into P P bonds of P4.
Another specific feature
represents the addition of simple electrophiles RX (R = H,
Me3Si; X = halogen, OSO2CF3) to 1, which resulted in the
isolation of the corresponding 1,1-adducts (insertion prod-
ucts).[3] Similarly, the 1,4-dipolar nature of 1 could for the first
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time facilitate a marked preference for C H insertion of
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divalent silicon versus C C-p bond addition with terminal
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alkynes. To our knowledge, neither facile silylene C H bond
insertion for terminal alkynes[5] nor formation of isolable
silacycloprop-3-enes, starting from isolable silylenes A have
been reported.[2f] Herein we report the remarkably different
reactivity of 1 toward terminal alkynes. although conversion
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at room temperature results solely in the formation of the C
H insertion adducts 2a, 2b, and 3 as thermodynamic products,
the same starting materials surprisingly undergo [2+1] cyclo-
addition after a induction period of a few hours at low
temperature (ꢀ788C) to furnish the corresponding silacyclo-
prop-3-enes 4a and 4b in high yield (Scheme 2).
Scheme 1. Stable silylenes A and B, and mesomeric forms of 1.
R=2,6-iPr2C6H3.
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Exposure of yellow solutions of 1 in hexane to HC C H at
room temperature leads to colorless solutions within a few
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minutes from which the C H insertion product 2a can be
[*] Dr. S. Yao, Prof. Dr. M. Driess
isolated in the form of colorless crystals in 82% yield.[6]
Additionally, prolonged reaction time with molar excess of
1 or conversion of 2a with 1 in molar ratio of 1:1 affords the
double insertion product 3 which was isolated in 85% yield.
Technische Universität Berlin
Institute of Chemistry: Metalorganic and Inorganic Materials
Sekr. C2, Strasse des 17. Juni 135, 10623 Berlin (Germany)
Fax: (+49)30-314-29732
E-mail: matthias.driess@tu-berlin.de
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Furthermore, addition of PhC C H to1 leads exclusively to
the corresponding alkynylsilane 2b, which was isolated in
93% yield. The composition and constitution of the products
was confirmed by NMR spectroscopy (1H, 13C, 29Si), mass
spectrometry, and elemental analysis (C, H, N). The six-
membered C3N2Si rings in 2a and 3 are only slightly puckered,
and the Si Cand C Cbond lengths (Figures 1 and 2) are
similar to those reported for other alkynylsilanes.[7] As
expected, the most important geometrical features of 2b are
practically identical to those of 2a.[6]
Prof. Dr. C. van Wüllen, Dr. X.-Y. Sun
Technische Universität Kaiserslautern
Fachbereich Chemie
Erwin-Schrödinger-Strasse, 67663 Kaiserslautern (Germany)
[**] Financial support from the Deutsche Forschungsgemeinschaft and
the Cluster of Excellence “Unifying Concepts in Catalysis” (EXC 314/
1) are gratefully acknowledged.
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Supporting information for this article is available on the WWW
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 3250 –3253