Cleavage and reorganization of Zr-C/Si-C bonds leading to Zr/Si-N organometallic and heterocyclic compounds

Article abstract of DOI:10.1021/ja1074932

The t-BuCN-stabilized zirconacyclopropene-azasilacyclopentadiene complexes 2 are generated in situ in high yields from the Si-tethered diynes, Cp ₂Zr(II) species, and 2 equiv of t-BuCN via a coordination-induced Zr-C/Si-C bond cleavage and reorganization. Complexes 2 have been demonstrated to be synthetically very useful. A variety of novel Zr/Si organo-bimetallic compounds and Si/N heterocyclic compounds, such as azasilacyclopentadienes, azasilacyclohexadienes, and allenylazazirconacycles, are obtained in high yields when complexes 2 are treated with ketones, carbodiimides, alkynes, elemental sulfur (S₈), acid chlorides, or nitriles. In this chemistry, t-BuCN behaves as both an initiator and a brake/release handle to initiate and control the reaction process.

Full text of DOI:10.1021/ja1074932

Published on Web 09/22/2010
Cleavage and Reorganization of Zr-C/Si-C Bonds Leading to Zr/Si-N
Organometallic and Heterocyclic Compounds
Shaoguang Zhang,† Wen-Xiong Zhang,† Jing Zhao,†,‡ and Zhenfeng Xi*,†,§
Beijing National Laboratory of Molecular Sciences (BNLMS) and Key Laboratory of Bioorganic Chemistry and
Molecular Engineering of Ministry of Education, College of Chemistry, Peking UniVersity, Beijing 100871, China,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China, and State Key Laboratory of
Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, China
Received August 19, 2010; E-mail: zfxi@pku.edu.cn
in high yields from the zirconocene-mediated reaction of its
Abstract: The t-BuCN-stabilized zirconacyclopropene-aza-
corresponding Si-tethered diyne 3.8 Because of the concomitance
silacyclopentadiene complexes 2 are generated in situ in high
yields from the Si-tethered diynes, Cp2Zr(II) species, and 2 equiv
presence of two Zr-C bonds and two Si-C bonds in the skeleton,
this compound 1 is structurally unique and should display novel
of t-BuCN via a coordination-induced Zr-C/Si-C bond cleavage
reaction chemistry and synthetic applications.6 When we treated 1
and reorganization. Complexes 2 have been demonstrated to be
with 2 equiv of t-BuCN, an unprecedented skeletal rearrangement
synthetically very useful. A variety of novel Zr/Si organo-bimetallic
took place to afford the compounds 2 in 55-75% isolated yields
compounds and Si/N heterocyclic compounds, such as azasila-
cyclopentadienes, azasilacyclohexadienes, and allenylazazir-
(Scheme 2). An X-ray analysis of 2b unambiguously revealed
2
2
the structure of the Csp -Csp -linked zirconacyclopropene-aza-
silacyclopentadiene (Figure 1). The dihedral angle of 88.10°
between the two cyclic planes demonstrates a nearly perpendicular
conformation. The azasilacyclopentadiene species,9 though structur-
ally and chemically interesting, are very rare in terms of synthetic
methods and reaction chemistry study. This transformation of
silacyclobutenes to azasilacyclopentadienes represents an unprec-
edented and useful reaction pattern of silacycles.10
conacycles, are obtained in high yields when complexes 2 are
treated with ketones, carbodiimides, alkynes, elemental sulfur
(S8), acid chlorides, or nitriles. In this chemistry, t-BuCN behaves
as both an initiator and a brake/release handle to initiate and
control the reaction process.
Among the most fundamental reactions of organometallic
compounds, the reaction initiated by the incoming coordinating
ligand L is central to virtually all organometallic reactions of great
significance for organic synthesis.1 In particular, besides commonly
observed ligand substitution, the coordination of ligand L may
greatly alter the steric and electronic environment around the metal
center,2,3 thus activating the whole compound, resulting in novel
skeletal rearrangement or cleavage of chemical bonds (Scheme 1).
Depending on its steric or electronic property, L may behave as a
brake handle to stabilize the L-coordinated complexes. When the
coordinating L is substituted by a different substrate S, the whole
complex will become reactive again (Scheme 1). Consequently, in
this way, reactivity control, synthetic applications, and otherwise
unavailable reaction patterns can be expected. The steric and
electronic match (or co-operation) between the metal M and L is
essential to realize such a process.4-6 Herein we report an exciting
example following the process model shown in Scheme 1.
A proposed mechanism involving coordination-induced formation
of silacyclopropene-zirconacyclopropene species 5 is given in
Scheme 2. Zirconacyclopropene species are known to be very
reactive.11 However, in this case, one t-BuCN is coordinated to
the zirconium center and thus deactivates the zirconacyclopropene
moiety.11 A second t-BuCN is inserted into the C-Si bond of the
reactive silacyclopropene moiety in 5 to form the azasilacyclopen-
tadiene moiety in 2.
Scheme 2. t-BuCN-Induced Formation of
Zirconacyclopropene-Azasilacyclopentadienes 2
Scheme 1. Initiating-Braking-Releasing Process Modela
a An incoming ligand L behaves as both an initiator and a brake/release
handle. The different shapes around the metal center indicate different
structure and bonding.
The zirconacyclobutene-silacyclobutene fused compound 1, first
reported by Takahashi and co-workers,7 could be readily generated
† Peking University.
This successful transformation could be attributed to the strong
coordinating ability and the steric effect of t-BuCN (as the L in
‡ Institute of Chemistry, Chinese Academy of Sciences.
§ Shanghai Institute of Organic Chemistry.
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10.1021/ja1074932  2010 American Chemical Society

C O M M U N I C A T I O N S
zirconacyclopropene ring afforded the corresponding oxazircona-
cyclopentene derivative 6.11d,12 The compound 6a was isolated in
72% yield, and its structure was determined by single-crystal X-ray
structural analysis (see Supporting Information). Although the
azasilacyclopentadiene moiety did not participate in this transforma-
tion, the cooperation between the azasilacycle and the zirconacycle
resulted in unprecedented cyclization chemistry upon hydrolysis.
Hydrolysis of 6 afforded the butadiene-fused aminotetrahydrofuran
derivatives 7, which are useful but not accessible by other means.13
The single-crystal structure of 7b and a proposed mechanism for
the hydrolysis process of 6 are given in the Supporting Information.
Similarly, selective insertion of a CdN double bond of N,N′-
diisopropylcarbodiimide was also observed to afford the complex
8 in 86% isolated yield.14
In addition to the above CdO and CdN double bond insertion
reactions, the CtC triple bond of alkynes was also found to react
smoothly and selectively with the zirconacyclopropene moiety to
afford its corresponding zirconacyclopentadiene derivative 9.15
When 2 was treated with elemental sulfur (S8), demetalation of
2 took place to afford the alkynylazasilacyclopentadienes 10 in
excellent yields (Scheme 3).9 The structure of 10a, as the first case
of azasilacyclopentadiene derivatives,9 was determined by single-
crystal X-ray structural analysis. As far as we know, no other
method could efficiently afford alkynylazasilacyclopentadienes.
Compound 10 was also obtained when 2 was treated with CO or
I2.16
Figure 1. ORTEP drawing of 2b with 30% thermal ellipsoids. Hydrogen
atoms are omitted for clarity. Selected bond lengths (Å): Zr1-C11 2.197(6),
Zr1-C12 2.289(7), C11-C12 1.322(8), C12-C13 1.480(9), C13-C15
1.358(9), C13-C14 1.547(8), C14-N2 1.281(8), Si1-C15 1.875(7),
Si1-N2 1.760(6), C36-N1 1.150(9).
Scheme 1). t-BuCN behaved as both an initiator and a brake handle
(Scheme 2). The reactivity of the resulting zirconacyclopropene
moiety in 2 is controlled (or shut down) by the coordinating t-BuCN.
The compound 2 has been demonstrated to be indeed very
reactive and synthetically useful. Whenever the coordinating
t-BuCN, functioning as the brake handle in 2, is substituted by a
different substrate (as the S in Scheme 1), the stabilized zircona-
cyclopropene moiety will become reactive, thus generating diversi-
fied zirconacycles (Scheme 3) or even initiating further reactions
of the whole molecule, including the azasilacyclopentadiene moiety,
generating heterocycles of novel structures (Scheme 4).
Scheme 4. Reactions Initiated by the Release of the
Zirconacyclopropene Moiety of 2
Scheme 3. Reactions of the Zirconacyclopropene Moiety of 2
Summarized in Scheme 4 are examples showing the reactions
of the whole molecule 2, including the azasilacyclopentadiene
moiety, initiated by the substitution of the coordinating t-BuCN
with other substrates. The reaction of 2 with acid chloride gave
the formally silacycle ring expansion products azasilacyclohexa-
dienes 11 in good to excellent isolated yields. The single-crystal
structure of 11a confirmed its six-membered silacycle bonding with
the oxychlorozirconocene moiety (Figure 2). Both aromatic and
aliphatic acid chlorides showed high efficiency. To the best of our
knowledge, this is the first example of such azasilacyclic skeletons.
The isolated azasilacyclopentadienes 10 did not show any reaction
with RCOCl. Thus, we assume the RCOCl replaces the coordinating
t-BuCN and reacts with the zirconacyclopropene moiety as the first
step. The azasilacyclopentadiene moiety then takes part (or cooper-
ates) in a further skeletal rearrangement to generate 11 (see
Supporting Information for detailed mechanistic discussion).
The cooperative effect between the azasilacyclopentadiene moiety
and the zirconacycle showed an unexpected and very interesting
impact on the reaction of 2 with nitriles. When 2 was further treated
with other nitriles, the incoming nitrile substituted the t-BuCN and
initiated an unprecedented reaction process. As shown in Scheme
4, the reaction of PhCN with 2b resulted in the formation of 12a
in 86% isolated yield. The structure of 12a, unambiguously
Summarized in Scheme 3 are examples showing independent
reactions of the zirconacyclopropene moiety with different sub-
strates. The azasilacyclopentadiene moiety in 2 does not participate
in these cases. First, ketones were used to substitute the coordinating
t-BuCN. Selective insertion of the CdO double bond into the
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C O M M U N I C A T I O N S
spectra for all isolated compounds; crystallographic data, in CIF format,
for 2b, 6a, 7b, 8, 9, 10a, 11a, and 12a. This material is available free
of charge via the Internet at http://pubs.acs.org.
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In summary, we have demonstrated a synthetically useful
protocol in which t-BuCN behaves as both an initiator and a brake/
release handle. The t-BuCN-stabilized zirconacyclopropene-aza-
silacyclopentadiene complexes 2, generated in situ in high yields
from their corresponding Si-tethered diynes 3, a low-valent zir-
conocene species, and 2 equiv of t-BuCN, have been demonstrated
to be synthetically very useful and can be readily transformed into
a wide variety of novel organometallic compounds and heterocyclic
compounds. Among them, the generation and characterization of
the azasilacyclopentadienes, azasilacyclohexadienes, and allenyla-
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intermediates reported in this Communication are in progress.
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Acknowledgment. This work was partially supported by the
National Natural Science Foundation of China and National High-
Tech Research and the Development Program of China (863
Program, 2007AA06A407). Eli Lilly China and BASF are gratefully
acknowledged.
Supporting Information Available: Details for proposed mecha-
nisms, experimental details, characterization data, and 1H and 13C NMR
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