Reaction of titanacyclobutene-silacyclobutene fused-ring complexes with nitriles via formal insertion of the C-N triple bond of nitrile into the silacyclobutene ring

Article abstract of DOI:10.1021/om401065c

The reaction of titanacyclobutene-silacyclobutene fused-ring complexes with nitriles was investigated. Formal insertion of the C-N triple bond of nitriles into the silacyclobutene ring was observed, affording their corresponding titanacyclobutene and six-

Full text of DOI:10.1021/om401065c

Communication
pubs.acs.org/Organometallics
Reaction of Titanacyclobutene−Silacyclobutene Fused-Ring
Complexes with Nitriles via Formal Insertion of the C−N Triple Bond
of Nitrile into the Silacyclobutene Ring
Jing Zhao,† Shaoguang Zhang,‡ Wen-Xiong Zhang,*,‡ and Zhenfeng Xi*,‡,§
†Beijing National Laboratory of Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing
100190, People’s Republic of China
‡Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry, Peking
University, Beijing 100871, People’s Republic of China
§State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry (SIOC), Chinese Academy of Sciences,
Shanghai 200032, People’s Republic of China
S
* Supporting Information
ABSTRACT: The reaction of titanacyclobutene−silacyclobutene fused-ring
complexes with nitriles was investigated. Formal insertion of the C−N triple
bond of nitriles into the silacyclobutene ring was observed, affording their
corresponding titanacyclobutene and six-membered azasilacycle fused-ring
complexes. This reaction pattern with nitriles is totally different from that of
their analogous zirconacyclobutene−silacyclobutene fused-ring complexes.
The size effect of the central metals (Ti vs Zr) is considered as the major
reason for the different reaction patterns.
products 6 were obtained in high yields. This reaction pattern
of the Cp2Ti complexes 5 with nitriles is totally different from
that of their analogous Cp2Zr complexes 1. The size effect of
the central metals (Ti vs Zr) is considered to be the major
reason for the different reaction patterns.
itanacycles and zirconacycles are readily available metal-
1
Tlacycles.
Reaction chemistry studies of these metalla-
cycles with various substrates have proved that they are
synthetically very useful.1−3 Among them, the zirconacyclobu-
tene−silacyclobutene fused-ring complexes 1 and their
analogous titanacyclobutene−silacyclobutene fused-ring com-
plexes 5 (Scheme 1) are structurally unique and can be easily
generated from the reaction between bis(alkynyl)silanes and
low-valent Cp2ZrII and Cp2TiII species, respectively.4,5 We have
systematically investigated the reaction chemistry of the
zirconocene complexes 1.6,7 The reaction of 1 with nitriles is
particularly interesting and useful.7 As shown in Scheme 1,
depending on the nitrile, different reaction patterns of 1 were
observed, affording different types of zirconacycles 2−4. In all
these reactions, the original zirconacyclobutene ring in 1 is not
maintained and is skeletally reorganized. The expected ring-
expansion products such as I−III and their regioisomers were
never observed.
The complex 5a could be readily prepared in high yield by
the reaction of the Rosenthal complex [Cp2Ti(η2-
Me3SiC2SiMe3)] with bis(alkynylphenyl)dimethylsilane, as
reported by Mach and co-workers.5 When this isolated complex
5a was treated with 1.5 equiv of PhCN, the titanacyclobutene
and six-membered azasilacycle fused-ring product 6a was
obtained as the sole isomer in 67% isolated yield (Scheme
2). Similarly, the formal ring-expansion complexes 6b,c could
be also isolated in high yields. In contrast, when 5a was treated
i
t
with PrCN or BuCN, no reaction was observed under the
present reaction conditions. None of the types of products 2−4
(Scheme 1) were obtained. All of these results are in sharp
contrast to the results obtained for the reaction of the
zirconocene complexes 1 with nitriles.7
Previous reports on group 4 metallocenes in the literature
have demonstrated that titanocene complexes, in many cases,
behave differently from zirconocene complexes.1−3 The
titanacyclobutene−silacyclobutene fused-ring complexes 5
(Scheme 1) can be readily synthesized by the reaction of
bis(alkynyl)silanes with a titanocene−bis(trimethylsilyl)ethyne
complex (Rosenthal complex, Cp2Ti(η2-Me3SiC2SiMe3)).5 We
envisioned that a different reaction pattern would result when
nitriles were treated with the Cp2Ti complexes 5. Herein we
discuss a formal insertion of the CN triple bond of nitriles
into the silacyclobutene ring of 5. The unprecedented
titanacyclobutene and six-membered azasilacycle fused-ring
Single crystals of complex 6a suitable for X-ray structural
analysis were grown in THF at room temperature, and the
ORTEP drawing is presented in Figure 1. X-ray structural
analysis of 6a reveals that it consists of two fused rings: one
four-membered titanacyclobutene and one six-membered
azasilacycle. The Si1−N1 distance (1.7476(16) Å) is
comparable with that found in 5-tert-butyl-2,2-dimethyl-3-p-
tolyl-4-(p-tolylethynyl)-2H-1,2-azasilole (1.760(3) Å).7c The
Received: November 1, 2013
Published: December 17, 2013
© 2013 American Chemical Society
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Organometallics
Communication
Scheme 1. Totally Different Reactions of
Zirconacyclobutene−Silacyclobutene Fused-Ring Complexes
1 and Titanacyclobutene−Silacyclobutene Fused-Ring
Complexes 5 with Nitriles
Figure 1. ORTEP drawing of 6a with 30% probability thermal
ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond
lengths (Å) and angles (deg): Ti1−C11 2.1256(19), Ti1−C13
2.1421(19), C12−C13 1.484(2), C11−C12 1.364(3), C12−C15
1.505(2), C13−C14 1.361(3), Si1−C14 1.8705(19), Si1−N1
1.7476(16), N1−C15 1.283(2); N1−Si1−C14 104.47(8).
distances in [Ti(η5-C5Me5)2{Ph2C4(SiMe2)}] (2.190(4) and
1.967(8) Å).6
Our previous study on the reactivity of the Cp2Zr complexes
has demonstrated that these complexes 1 are very sensitive to
the bulkiness of substituents on these complexes and of
approaching substrates.7 For example, the reaction of 1b (with
a SiEt2 moiety; Scheme 3) with PhCN required a much longer
Scheme 3. Formation of Products 6d,e from Complexes 5b,c
and PhCN
Scheme 2. Formation of Titanacyclobutene and Six-
Membered Azasilacycle Fused-Ring Compounds 6a−c from
Complex 5a and Nitriles
reaction time and higher temperature. Notably, when 1c (with
a SiPh2 moiety) was treated with nitriles, only a trace amount of
the product was obtained, with most of the complex 1c being
unreacted. This finding indicated that the steric environment
around the silicon center has a strong impact on the reactivity
of the Cp2Zr complexes 1. However, in contrast, their
titanocene analogues 5b,c did react with PhCN smoothly and
cleanly, affording their corresponding silacyclobutene ring-
expansion products 6d,e in 66% and 79% isolated yields,
respectively. The structures of products 6d,e were both
determined by single-crystal X-ray structural analyses (Figure
2). These results again demonstrate that the reactivity of nitriles
bond lengths of Ti1−C11 (2.1256(19) Å) and Ti1−C13
(2.1421(19) Å) are comparable with the related Ti−C
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Organometallics
Communication
with the Cp2Ti complexes 5 is very much different from that of
the Cp2Zr complexes 1.
nitriles with zirconacyclobutene−silacyclobutene fused-ring
complexes 1. The strained fused-ring skeletons, the sterically
demanding environments around the metal centers and the silyl
groups, and the different sizes of the metals are all considered
to contribute to the different reactivities. Further investigation
into the reaction mechanism and synthetic applications for the
synthesis of silacycles are ongoing.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and CIF files giving crystallographic data,
full experimental procedures, NMR spectra, and details for the
preparation of complexes 6. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: wx_zhang@pku.edu.cn (W.-X.Z.); zfxi@pku.edu.cn
(Z.X.).
■
Figure 2. ORTEP drawings of 6d (left) and 6e (right) with 30%
probability thermal ellipsoids. Hydrogen atoms and THF in 6e are
omitted for clarity. Selected bond lengths (Å) and angles (deg) are as
follows. For 6d: Ti1−C17 2.131(2), Ti1−C3 2.1724(19), C2−C3
1.481(3), C2−C17 1.363(3), C1−C2 1.500(3), C3−C4 1.364(3),
Si1−C4 1.873(2), Si1−N1 1.7484(17), N1−C1 1.281(3); N1−Si1−
C4 105.72(9). For 6e: Ti1−C3 2.133(2), Ti1−C4 2.154(2), C2−C4
1.481(3), C2−C3 1.366(3), C1−C2 1.491(3), C4−C5 1.353(3), Si1−
C5 1.859(2), Si1−N1 1.7446(19), N1−C1 1.285(3); N1−Si1−C5
106.80(9).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Major State Basic Research
Development Program (2012CB821600) and the Natural
Science Foundation of China.
Although a direct insertion of the C−N triple bond into the
silacyclobutene ring of complex 5 cannot be totally excluded,
the proposed pathway shown in Scheme 4 for the formation of
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