1,2-disilacyclobut-2-enes: Donor-free four-membered cyclic silenes from reaction of disilenides with vinylbromides

Article abstract of DOI:10.1002/chem.200800919

A study conducted to demonstrate the quantitative conversion of disilenides of vinyl anions into donor-free four-membered cyclic silenes. The donor-free 1,2-disilacyclobut-2-enes were synthesized with planar Si-C double bond from disilenides and vinyl bromides. The study used single crystal X-ray diffraction method to determine the structure of phenyl substituted derivatives in the solid state. The study found that the derivatives show pronounced π conjugation of the Si=C moiety with the phenyl substituents. The study also found that reaction with the vinyl bromide produced a stable compound with a hydrogen atom attached to the carbon atom of the Si=C double bond. The study also observed that these compounds are thermally stable in the absence of oxygen and moisture, and trisilyl-substituted disilacyclobutenes show tri-coordinate Si and C atoms.

Full text of DOI:10.1002/chem.200800919

COMMUNICATION
DOI: 10.1002/chem.200800919
1,2-Disilacyclobut-2-enes: Donor-Free Four-Membered Cyclic Silenes from
Reaction of Disilenides with Vinylbromides
Iulia Bejan,[a] Shigeyoshi Inoue,[b] Masaaki Ichinohe,[b] Akira Sekiguchi,*[b] and
David Scheschkewitz*[a]
In 1981 the report by Brook et al. on the first stable silene
1,[1] that is, a compound with a Si C double bond, contribut-
ꢀ
ed to the change of the paradigm that silicon as a heavier
main group element would not engage in multiple bond-
ing.[2] The unexpectedly long Si=C bond of 1 was attributed
Scheme 1. Ad=1-adamantyl, 3a, 4a: R=Tip =2,4,6-iPrC6H2, R’=Ad;
4b: R=Tip, R’=tBu; 3b, 4c: R=Si(Me)tBu2, R’=Ad.
to the influence of the electron-releasing siloxy group
(Scheme 1). The subsequent synthesis of donor-free 2 with a
considerably shorter Si–C distance[3] further supported a
strong dependency of the electronic and topological charac-
strated that the unprecedented significant pyramidalisation
of the silicon atom of the double bond was not entirely due
to the presence of the electron-releasing oxygen atom next
to the carbon atom of the double bond, but also to the in-
corporation of the Si=C moiety into a four-membered ring.
Early theoretical calculations, however, predicted a planar
topology in case of the related 1,4-dihydro-1,2-disiletes, in
which the endocyclic oxygen atom of the hydrogen substitut-
ed parent of 4 is formally replaced by a CH2 moiety.[10]
These 1,2-disilacyclobutenes are particularly interesting syn-
thetic targets in view of the rich cycloreversion chemistry of
their saturated congeners, that is, 1,2-and 1,3d- isilacyclobu-
tanes, which under photolytic or thermal conditions frag-
ment into transient silenes and were therefore pivotal for
ꢀ
ter of the Si C p bond on the nature of the substituents,
which was later put on a firm theoretical basis.[2c,4] As Ottos-
son and Steel pointed out recently,[2a] the application of the
resulting versatile reactivity of the numerous stable and
transient silenes reported to date is still limited due to the
particular (sometimes forcing) conditions for their genera-
tion: high temperatures or irradiation,[1,5] use of strong
bases,[3,6] or formation of nucleophilic secondary products.[7]
We recently reported the quantitative conversion of disile-
nides 3a,b, disila analogues of vinyl anions,[8] into cyclic si-
lenes 4a–c featuring an endocyclic oxygen atom by reaction
with carboxylic acid chlorides under very mild conditions
(Scheme 1).[9] On grounds of DFT calculations we demon-
ꢀ
the development of Si C double bond chemistry as a
whole.[11]
The recent successful transfer of Si=Si moieties to aromat-
ic substrates by reaction of disilenide 3a with the weakly
electrophilic aryl iodides[12] encouraged us to investigate the
reactivity of 3a,b towards vinyl bromides. We speculated
that intermediately formed vinyl substituted disilenes would
likely not be stable and therefore rearrange to the desired
cyclic derivatives.
Indeed, upon treatment of vinyl bromides H2C=C(R’)Br
with disilenides 3a,b in benzene or toluene at room temper-
ature affords 1,4-dihydro-1,2-disiletes 5a–d in good to excel-
lent yields (Scheme 2). Moreover, reaction of 3b with the
parent vinyl bromide (R’=H) yielded 5e, the first stable
compound with a hydrogen atom directly attached to the
carbon atom of the Si=C double bond.[13,14] All compounds
are thermally stable in the absence of oxygen and moisture
[a] Dipl.-Chem. I. Bejan, Dr. D. Scheschkewitz
Institut für Anorganische Chemie
Julius-Maximilians-Universität Würzburg
Am Hubland, 97074 Würzburg (Germany)
and new address:
Department of Chemistry, Imperial College London
South Kensington Campus, London SW7 2AZ (UK)
E-mail: d.scheschkewitz@imperial.ac.uk
[b] Dr. S. Inoue, Dr. M. Ichinohe, Prof. A. Sekiguchi
Department of Chemistry
Graduate School of Pure and Applied Sciences
University of Tsukuba, Tsukuba, Ibaraki 305-8571 (Japan)
E-mail: sekiguch@chem.tsukuba.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800919.
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7119

Table 1. Comparison of selected NMR and structural data of 1,2-disilacy-
clobut-2-enes 5a–e (exptl) and 6a–f (calcd).
29Si (Si=C)[c]
d
13C (Si=C)[c]
ꢀ
Si=C [Š] Si Si [Š] SSi [8]
d
5a
5b
5c
5d
5e
1.746(2)
2.3154(5) 356.2(1) 75.0
155.2
185.4
159.6
195.6
175.1
171.5
201.9
179.5
224.2
193.6
153.9
–
–
–
–
–
–
98.6
Scheme 2. 5a: R=Tip, R’=Ph; 5b: R=Si(Me)tBu2, R’=Ph; 5c: R=Tip,
R’=SiMe3; 5d: R=SiMetBu2, R’=SiMe3; 5e: R=SiMetBu2, R’=H.
–
–
–
–
–
–
112.5
139.1
112.5
85.5
118.8
124.8
166.0
127.0
99.6
6a[a] 1.762
6b[b] 1.775
6c[a] 1.748
6d[b] 1.760
6e[b] 1.750
6 f[a] 1.738
2.305
2.326
2.314
2.338
2.345
2.318
360.0
360.0
359.9
360.0
360.0
360.0
and were characterized by multinuclear NMR and UV/Vis
spectroscopy. Additionally, the structure of phenyl substitut-
ed derivative 5a in the solid state was confirmed by single
crystal X-ray diffraction (Figure 1).[15]
[a] B3LYP/6-31 g(d,p). [b] B3LYP/6-31 g(d). [c] GIAO/B3LYP/6-311 g-
G
A
scopic features of a hydrogen directly attached to the
carbon atom of the Si=C bond. While a recently reported
1
hydrosilene exhibited a H NMR shift at d 6.06 ppm for the
hydrogen bonded to the silicon atom of the Si=C moiety;[13]
in the case of 5e a signal at much lower field (7.47 ppm) is
observed probably due to the different electronegativities of
silicon and carbon. This resonance can indeed be unambigu-
ously assigned to the hydrogen at the carbon atom of the
ꢀ
Si C double bond on grounds of its triplet splitting due to
coupling with the adjacent protons of the methylene group
(3J=4.8 Hz). This coupling is significantly higher than that
reported for carbon based cyclobutenes (3J=0.9 to
1.4 Hz).[16] Conversely, the 1J coupling constant of the “vinyl-
ic” proton to the Si=C carbon atom is somewhat smaller in
case of 5e (153.4 Hz) than the corresponding value of
parent cyclobutene (168.5 Hz).[16c] This observation can be
rationalized in terms of the larger covalent radii of the sili-
con atoms in the ring as compared to carbon allowing for
larger inner angles at the endocyclic carbon atoms, which by
Figure 1. Structure of 5a in the solid state. All hydrogen atoms and disor-
dered isopropyl groups removed for clarity. Ellipsoids at 30% probabili-
ꢀ
ꢀ
ty. Selected bond lengths [Š] and angles [8]: Si1 C2 1.7459(15), Si1 Si2
ꢀ
ꢀ
ꢀ
ꢀ
2.3154(5), Si2 C1 1.9191(16), C1 C2 1.536(2), C2 C3 1.457(2), Si1 C16
ꢀ
ꢀ
1.8729(15), Si2 C31 1.9111(15), Si2 C46 1.9056(14); C2-Si1-C16
127.43(7), C2-Si1-Si2 80.74(5), C16-Si1-Si2 148.06(5), C1-Si2-Si1 74.45(5),
C3-C2-C1 121.45(13), C3-C2-Si1 134.99(12), C1-C2-Si1 103.53(10), C2-
C1-Si2 100.43(9).
ꢀ
consequence should raise the s character of the C C bond
in the four-membered ring and hence the spin couplings
through that bond.
As expected, the NMR data of 5a–e suggest a significant
influence of the substituents on the electronic structure of
the Si=C bond. Trisilyl-substituted disilacyclobutenes 5b,d
show strongly deshielded tricoordinate Si and C atoms com-
pared to those of 5a,c with triaryl substitution of the Si2
moiety (Table 1). Within these two subsets the most shielded
double bond silicon atoms are those of 5a and 5b, respec-
tively, with a phenyl group attached to the Si=C carbon
atom, which may be explained in terms of a reduced polari-
ty of the double bond due to p conjugation with the sub-
stituent. A comparison of the longest-wavelength absorption
maxima in the UV/Vis spectra of 5a,b shows that the influ-
ence of the substituents at silicon on the HOMO–LUMO
gap is negligible (lmax, 5a: 385, 5b: 389 nm). Conversely, the
effect of the phenyl substituent at the Si=C carbon atom is
much more pronounced and responsible for a significant
red-shift (lmax, 5d: 330, 5e: 322 nm), which is widely appre-
ciated as evidence for conjugation.
Single crystals suitable for an X-ray diffraction study were
obtained in case of phenyl substituted 5a (Figure 1) con-
firming the constitution of a 1,2-disilacyclobut-2-ene. The
Si1–C2 distance of 1.7459(15) Š is within the typical range
of Si=C double bonds[2] Even though the four-membered
ring is slightly twisted (Si2-Si1-C2-C1 7.17(9)8; Si2-C1-C2-
Si1 ꢀ8.69(10)8) the Si=C double bond is essentially planar
(sum of angles Si1 356.2(1), C2 360.0(1)8) in sharp contrast
to the strongly pyramidalized Si=C silicon atom of oxygen
containing 4a–c.[9] The p conjugation of the phenyl group of
5a suggested by the spectroscopic data becomes readily ap-
parent from the co-planar arrangement of the phenyl sub-
stituent with the Si2C2 ring (dihedral angle Si1-C2-C3-C4
5.4(2)8) and the relatively short bond to the ipso-carbon
ꢀ
atom (C2 C3 1.457(2) Š). The larger covalent radius of sili-
con compared with carbon indeed imposes small inner
angles at Si1 and Si2 and larger ones at C1 and C2 (Si2-Si1-
C2 80.74(5), C1-Si2-Si1 74.45(5), Si1-C2-C1 103.53(10), C2-
It is important to note that the half-parent system 5e
offers the unprecedented possibility to study the spectro-
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Chem. Eur. J. 2008, 14, 7119 – 7122

1,2-Disilacyclobut-2-enes
COMMUNICATION
C1-Si2 100.43(9)8) corroborating the above assumption that
was used to explain the observed NMR couplings of 5e.
In order to clarify the electronic nature of 5a–e, we car-
ried out DFT calculations[17] on model compounds 6a–f
(Scheme 3). In previous work on cyclic silenes 4a–c we con-
To conclude, we have synthesized a number of donor-free
ꢀ
1,2-disilacyclobut-2-enes with planar Si C double bonds
from disilenides and vinyl bromides. Derivatives 5a,b show
pronounced p conjugation of the Si=C moiety with the
phenyl substituent. The vast number of readily available
vinyl bromides offers a broad basis for future exploitation of
ꢀ
this new synthetic approach to Si C double bond containing
compounds. The chemistry of the new silenes, particularly
under photochemical conditions, is currently being investi-
gated.
Scheme 3.
Acknowledgements
cluded that B3LYP compared to MP2 overestimates the pyr-
amidalization of the double bond’s silicon atom. Given that
5b exhibited an almost planar coordination environment at
silicon we opted for B3LYP in the present case to avoid un-
necessary simplification of the models, which would have
been mandatory with the more costly MP2 level of theory.
The agreement of calculated and experimental parameters
(Table 1) is indeed good given the still rough approximation
of the experimental substitution patterns. Irrespectively of
the nature of substituents, the Si=C bonds of all calculated
1,2-disilacyclobut-2-enes 6a–f feature a strictly planar geom-
etry. The carbon bonded phenyl groups of 6a,b are almost
co-planar to the double bonds as it had been observed ex-
perimentally in the solid state structure of 5a. The HOMO
We thank the Deutsche Forschungsgemeinschaft (DFG 906/3-2), the
Fonds der Chemischen Industrie (FCI), the Dr. Otto-Rçhm Gedächtnis-
Stiftung and the Ministry of Education, Science, Sports, and Culture of
Japan (Grants-in-Aid for Scientific Research Nos. 18064004, 19105001,
19020012, 19022004, 20038006) for financial support. D.S. is grateful to
Professor H. Braunschweig for his generous support.
Keywords: density functional calculations · multiple bonds ·
silenes · small rings · synthesis
[1] A. G. Brook, F. Abdesaken, B. Gutekunst, G. Gutekunst, R. K. Kal-
lury, J. Chem. Soc. Chem. Commun. 1981, 191.
[2] Silene reviews: a) H. Ottosson, A. M. Eklçf, Coord. Chem. Rev.
2008, 252, 1287; b) H. Ottosson, P. G. Steel, Chem. Eur. J. 2006, 12,
1576; c) T. L. Morkin, W. J. Leigh, Acc. Chem. Res. 2001, 34, 129;
d) T. Müller, W. Ziche, N. Auner in The Chemistry of Organic Sili-
con Compounds, Vol. 2, Part 2 (Eds.: Z. Rappoport, Y. Apeloig),
Wiley, Chichester, 1998, 857; e) J. EscudiØ, C. Couret, H. Ranaivon-
jatovo, Coord. Chem. Rev. 1998, 178–180, 565; f) G. Raabe, J. Michl,
Chem. Rev. 1985, 85, 419; g) H. F. Schaefer III, Acc. Chem. Res.
1982, 15, 283.
ꢀ
of 6a is dominated by the Si C p bond, but nonetheless
highly delocalized across the phenyl substituent with a little
hyperconjugative admixture of s electron density from the
ꢀ
endocyclic methylene group’s C H bonds (Figure 2). The
main contribution to the LUMO is procured from the anti-
bonding Si-C p*-orbital, which participates in bonding inter-
actions to both pending phenyl groups.
[3] N. Wiberg, G. Wagner, G. Müller, Angew. Chem. 1985, 97, 220;
Angew. Chem. Int. Ed. Engl. 1985, 24, 229.
[4] a) Y. Apeloig, M. Karni, J. Am. Chem. Soc. 1984, 106, 6676; b) T.
VeszprØmi, M. Takahashi, J. Ogasawara, K. Sakamoto, M. Kira, J.
Am. Chem. Soc. 1998, 120, 2408; c) H. Ottosson, Chem. Eur. J. 2003,
9, 4144; d) V. G. Avakyan, S. L. Guselnikov, L. E. Gusel’nikov, J. Or-
ganomet. Chem. 2003, 686, 257; e) M.-J. Cheng, S.-Y. Chu, Organo-
metallics 2005, 24, 3746.
[5] a) A. G. Brook, S. C. Nyburg, F. Abdesaken, B. Gutekunst, G. Gute-
kunst, R. Krishna, M. R. Kallury, Y. C. Poon, Y. M. Chang, W. N.
Winnie, J. Am. Chem. Soc. 1979, 101, 6750; b) A. G. Brook, S. C.
Nyburg, F. Abdesaken, B. Gutekunst, G. Gutekunst, R. K. Kallury,
Y. C. Poon, Y.-M. Chang, W. Wong-Ng, J. Am. Chem. Soc. 1982,
104, 5667; c) A. G. Brook, F. Abdesaken, G. Gutekunst, N. Plavac,
Organometallics 1982, 1, 994; d) A. G. Brook, K. D. Safa, P. D. Lick-
iss, K. M. Baines, J. Am. Chem. Soc. 1985, 107, 4338; e) I. El-Sayed,
T. Guliashvili, R. Hazell, A. Gogoll, H. Ottosson, Org. Lett. 2002, 4,
1915.
[6] a) P. R. Jones, T. F. O. Lim, J. Am. Chem. Soc. 1977, 99, 2013; b) N.
Wiberg, G. Wagner, Angew. Chem. 1983, 95, 1027; Angew. Chem.
Int. Ed. Engl. 1983, 22, 1005; c) N. Auner, Z. Anorg. Allg. Chem.
1988, 668, 87; d) G. Delpon-Lacaze, C. Couret, J. Organomet. Chem.
1994, 480, C14; e) M. Mickoleit, R. Kempe, H. Oehme, Chem. Eur.
J. 2001, 7, 987.
Figure 2. Contour plots of the frontier orbitals of 6a at 0.05 cut-off level
(Molekel).[18]
Additionally, an NBO analysis[19] of 6a according to
second-order perturbation theory provides further support
for an effective p conjugation: the Si=C p orbital strongly
donates into antibonding p orbitals of the phenyl group (p
Si1–C2!p* C3–C4: 19.31 kcalmolꢀ1). The reverse donation
is somewhat less pronounced (p C3–C4!p* Si1–C2:
13.53 kcalmolꢀ1) reflecting the electron withdrawing nature
of the phenyl substituent.
[7] a) Y. Apeloig, M. Bendikov, M. Yuzefoviv, M. Nakash, D. Bravo-
Zhivotovskii, D. Bläser, R. Boese, J. Am. Chem. Soc. 1996, 118,
12228; b) K. Sakamoto, J. Ogasawara, Y. Kon, T. Sunagawa, C.
Kabuto, M. Kira, Angew. Chem. 2002, 114, 1460; Angew. Chem. Int.
Chem. Eur. J. 2008, 14, 7119 – 7122
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7121

D. Scheschkewitz, A. Sekiguchi et al.
Ed. 2002, 41, 1402; c) S. Inoue, M. Ichinohe, A. Sekiguchi, Angew.
Chem. 2007, 119, 3410; Angew. Chem. Int. Ed. 2007, 46, 3346; d) M.
Igarashi, M. Ichinohe, A. Sekiguchi, J. Am. Chem. Soc. 2007, 129,
12660.
2003, 686, 84; c) P. E. Hansen, J. J. Led, Org. Magn. Reson. 1981, 15,
288.
[17] Gaussian03 (RevisionC.02), M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery,
Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar,
J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega,
G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R.
Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross,
V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y.
Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg,
V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O.
Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Fores-
man, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski,
B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L.
Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Na-
nayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen,
M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Wallingford
CT, 2004; see Supporting Information.
[8] a) D. Scheschkewitz, Angew. Chem. 2004, 116, 3025; Angew. Chem.
Int. Ed. 2004, 43, 2965; b) M. Ichinohe, K. Sanuki, S. Inoue, A. Seki-
guchi, Organometallics 2004, 23, 3088; c) S. Inoue, M. Ichinohe, A.
Sekiguchi, Chem. Lett. 2005, 34, 1564; d) R. Kinjo, M. Ichinohe, A.
Sekiguchi, J. Am. Chem. Soc. 2007, 129, 26.
[9] I. Bejan, D. Güclü, S. Inoue, M. Ichinohe, A. Sekiguchi, D. Schesch-
kewitz, Angew. Chem. 2007, 119, 3413; Angew. Chem. Int. Ed. 2007,
46, 3349.
[10] a) J. A. Boatz, M. S. Gordon, J. Phys. Chem. 1988, 92, 3037; b) J. A.
Boatz, M. S. Gordon, J. Phys. Chem. 1991, 95, 7244.
[11] Review: L. E. Gusel’nikov, Coord. Chem. Rev. 2003, 244, 149.
[12] I. Bejan, D. Scheschkewitz, Angew. Chem. 2007, 119, 5885; Angew.
Chem. Int. Ed. 2007, 46, 5783.
[13] A recently reported hydrosilene features a hydrogen atom bonded
to silicon: S. Ozaki, T. Sasamori, N. Tokitoh, Organometallics 2008,
27, 2163,.
[14] All attempts to synthesize the corresponding product from parent
vinyl bromide and disilenide 3a were unsuccessful yielding only mix-
tures of products with highfield 29Si NMR signals incompatible with
tricoordinate silicon species.
[15] CCDC 683755 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif
[18] P. Flükiger, H. P. Lüthi, S. Portmann, J. Weber, MOLEKEL 4.0,
2000, Swiss National Supercomputing Centre CSCS, Manno (Swit-
zerland).
[19] E. D. Glendening, J. K. Badenkoop, A. E. Reed, J. E. Carpenter,
J. A. Bohmann, C. M. Morales, F. Weinhold, NBO 5.G, 2001, Theo-
retical Chemistry Institute, University of Wisconsin, Madison WI.
[16] a) A. Fürstner, C. Aissa, J. Am. Chem. Soc. 2006, 128, 6306; b) A.
ChØnedØ, I. Fleming, R. Salmon, M. C. West, J. Organomet. Chem.
Received: May 14, 2008
Published online: July 14, 2008
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