Room-temperature dissociation of 1,2-dibromodisilenes to bromosilylenes

Article abstract of DOI:10.1021/ja209736d

A room-temperature dynamic equilibrium between dibromodisilenes and bromosilylenes has been demonstrated by taking advantage of the steric protection using the fused-ring bulky 1,1,3,3,5,5,7,7-octa-R-s-hydrindacen-4-yl (Rind) groups. Although the bromosilylenes cannot be directly observed by spectroscopic methods, the thermal homolytic cleavage of the Si=Si double bond has been confirmed by a pseudo-first-order kinetics for the trapping with bis(trimethylsilyl)acetylene and a crossover reaction using two kinds of Rind-substituted dibromodisilenes. The addition of 4-pyrrolidinopyridine (PPy) to the dibromodisilene leads to an equilibrium mixture between the dibromodisilene and a PPy adduct of bromosilylene, the latter being isolated and characterized. The substitution of the bromine atom in the dibromodisilene by the Grignard reagent is significantly accelerated by the addition of PPy.

Full text of DOI:10.1021/ja209736d

COMMUNICATION
pubs.acs.org/JACS
Room-Temperature Dissociation of 1,2-Dibromodisilenes to
Bromosilylenes
Katsunori Suzuki,† Tsukasa Matsuo,*,†,§ Daisuke Hashizume,‡ and Kohei Tamao*,†
†Functional Elemento-Organic Chemistry Unit and ‡Advanced Technology Support Division, RIKEN Advanced Science Institute,
2-1 Hirosawa, Wako, Saitama 351-0198, Japan
§PRESTO, JST.
S Supporting Information
b
the corresponding highly reactive bromosilylenes 2, which can be
ABSTRACT: A room-temperature dynamic equilibrium
between dibromodisilenes and bromosilylenes has been
demonstrated by taking advantage of the steric protection
using the fused-ring bulky 1,1,3,3,5,5,7,7-octa-R-s-hydrinda-
cen-4-yl (Rind) groups. Although the bromosilylenes can-
not be directly observed by spectroscopic methods, the
thermal homolytic cleavage of the SidSi double bond has
been confirmed by a pseudo-first-order kinetics for the
trapping with bis(trimethylsilyl)acetylene and a crossover
reaction using two kinds of Rind-substituted dibromodisi-
lenes. The addition of 4-pyrrolidinopyridine (PPy) to the
dibromodisilene leads to an equilibrium mixture between
the dibromodisilene and a PPy adduct of bromosilylene, the
latter being isolated and characterized. The substitution of
the bromine atom in the dibromodisilene by the Grignard
reagent is significantly accelerated by the addition of PPy.
isolatedand characterized asLewis base adducts3 (Scheme 1). We
also report some substitution reactions of the dibromodisilenes by
organolithium and magnesium reagents in the presence or absence
of a Lewis base.
As shown in Scheme 1, the dibromodisilenes 1a and 1b
were readily prepared by the reduction of the tribromosilane
precursors (Rind)SiBr3 with 2 equiv of lithium naphthalenide
(LiNaph)18 and isolated as air- and moisture-sensitive yellow
crystals in moderate yields. The formation of the SidSi bond was
clearly confirmed by the 29Si NMR spectra, in which the
characteristic signal was observed at 74.6 (1a) and 73.2 (1b)
ppm, respectively. Compounds 1a and 1b are stable at room
temperature in the solid state for months under a dry argon
atmosphere with no detectable change as confirmed by the 1HNMR
spectra.
Figure 1A shows the molecular structure of 1b determined by
X-ray crystallography. This molecule has an inversion center at
the midpoint of the SidSi bond. The disilene core adopts a trans-
bent geometry with an E configuration; the trans-bent angle
between the Si1ꢀSi10 vector and the Si1ꢀBr1ꢀC1 plane is
29.0°. The Si atoms are pyramidal with the sum of the surround-
ing angles of 350.8°. These structural features of 1b found
in the crystal are distinct from those of the Eind-substituted
1,2-diphenyldisilene (Eind)PhSidSiPh(Eind), which adopts an
almost planar geometry around the SidSi bond with the trans-
bent angle of 2.7°.19 The SidSi bond length of 1b [2.1795(9) Å]
is comparable to that of the diphenyldisilene [2.1593(16) Å]19
and in the range of those for the typical disilenes.2
ver the past three decades, many unsaturated silicon com-
O
pounds have been extensively developed using bulky substit-
uents based on the concept of kinetic stabilization, which enable us
to study their fundamental chemistry and potential properties as
functional materials.1,2 Theoretical studies suggest that the nature
of the bonding in disilenes (R2SidSiR2) significantly depends on
the singletꢀtriplet energy difference (ΔEST) of the corresponding
silylene fragment (R2Si:).3ꢀ5 Thus, the bond dissociation energy
(BDE) for the SidSi double bond tends to decrease with the
increasing electronegativity of the R-substituents on silicon due to
the greater stabilization of the corresponding singlet silylenes;
disilenes with alkoxy and amino groups or halogen atoms are
predicted to be highly distorted disilenes or to readily dissociate
into the corresponding silylenes, which are stabilized by the
formation of the R-bridged silylene dimer.6,7 Experimentally, a
reversible disilene-silylene equilibrium has been observed for
some sterically overcrowded disilenes8,9 and amino-substituted
disilenes.10ꢀ12
The thermolysis of 1 at 100 °C overnight in bis-
(trimethylsilyl)acetylene (BTMSA) afforded a cycloadduct, i.e.,
the silacyclopropenes 4 as the sole product, isolated in moderate
yields (Scheme 2),20 indicative of the formation of the bromo-
silylenes (Rind)BrSi: (2) as a reactive intermediate through the
homolytic cleavage of the SidSi bond of 1.8,9 Kinetic studies of
the thermal reaction of 1 with BTMSA (ca. 1000 equiv vs 1)
in toluene follow the pseudo-first-order kinetics with ΔHq =
26.64 ( 0.19 kcal molꢀ1 and ΔSq = 9.32 ( 0.59 cal molꢀ1 Kꢀ1 20
,
which are comparable to the previously reported kinetic data for
the thermolysis of the overcrowded disilenes,8b thus suggesting
the facile dissociation of 1 into 2.
Whereas halogen-substituted silylenes and disilenes are regarded as
valuable precursors for the construction of unique unsaturated silicon
frameworks,13ꢀ16 such as disilynes15,16 and substituted disilenes,16 the
dynamic equilibrium between halo-disilenes and halo-silylenes
through cleavage of the SidSi bond has not yet been reported.17
We now present the facile thermal dissociation of 1,2-dibro-
modisilenes 1, stabilized by the fused-ring bulky 1,1,7,7-tetraethyl-
3,3,5,5-tetra-R-substituted s-hydrindacen-4-yl (Rind) groups, into
In fact, equilibrium between 1 and 2 in solution was confirmed
even at room temperature by a crossover reaction between the
Received: October 17, 2011
Published: November 10, 2011
r
2011 American Chemical Society
19710
dx.doi.org/10.1021/ja209736d J. Am. Chem. Soc. 2011, 133, 19710–19713
|

Journal of the American Chemical Society
COMMUNICATION
Scheme 1. Dynamic Equilibrium between Dibromodisilenes and Bromosilylenes
Scheme 2. Thermolysis of 1 in BTMSA
Scheme 3. Crossover Reaction between 1a and 1b
Figure 1. (A) Molecular structure of 1b (50% probability ellipsoids).
Hydrogen atoms and disordered ethyl group are not shown. Selected atomic
distances (Å) and bond angles (deg): Si1ꢀSi10 = 2.1795(9), Si1ꢀC1 =
1.8875(16), Si1ꢀBr1 = 2.2279(6), C1ꢀSi1ꢀSi10 = 126.86(6), C1ꢀSi1ꢀ
Br1 = 115.35(5), Br1ꢀSi1ꢀSi10 = 108.59(3). (B) Molecular structure of 3a
(50% probability ellipsoids). Hydrogen atoms and solvent molecule are not
shown. Selected atomic distances (Å) and bond angles (deg): Si1 N1 =
3 3 3
1.939(2), Si1ꢀBr1 = 2.3922(9), Si1ꢀC1 = 1.960(3), Br1ꢀSi1ꢀN1 =
96.23(7), Br1ꢀSi1ꢀC1 = 109.92(9), N1ꢀSi1ꢀC1 = 97.48(10).
temperature into an equilibrium mixture containing a new disilene
1ab bearing both the EMind and Eind groups, as monitored
by 1H NMR. After 24 h at room temperature, a dynamic equili-
brium was attained in the ratio of 1a:1b:1ab = 1:1:2.9, judging
EMind-substituted 1a and the Eind-substituted 1b (Scheme 3).
Thus, a mixture of 1a and 1b in C6D6 gradually changed at room
19711
dx.doi.org/10.1021/ja209736d |J. Am. Chem. Soc. 2011, 133, 19710–19713

Journal of the American Chemical Society
COMMUNICATION
from integration of the aromatic signals in the 1H NMR
spectrum.20 The 29Si NMR spectrum, reproduced in Scheme 3,
shows a pair of new signals at 71.6 and 74.8 ppm for 1ab, in
addition to the signals due to 1a and 1b. The equilibrium lies
somewhat to the right, probably due to the release of the steric
repulsion between the two bulkier Eind groups in 1b.
Scheme 4. Reactions of 1a with Organometallic Reagents
The facile dissociation of 1 into 2 is supported by the DFT
computations at the B3LYP/6-31G(d,p) level using the Gaussian
03W program package,21 performed for a set of model compounds
having H2Mind (see Scheme 1) groups. The BDE of the SidSi
double bond in the dibromodisilene (H2Mind)BrSidSiBr(H2Mind)
(1c) is estimated to be 19.2 kcal molꢀ1, which is much lower than
that of the corresponding diphenyldisilene (H2Mind)PhSid
SiPh(H2Mind) (34.0 kcal molꢀ1) and between (Me5C5)-
{(Me3Si)2N}SidSi{N(SiMe3)2}(C5Me5) (23.2 kcal molꢀ1 11
)
and (i-Pr2N)2SidSi(i-Pr2N)2 (3ꢀ8 kcal molꢀ1).10c The
ΔEST value for the bromosilylene (H2Mind)BrSi: (2c) (46.4
kcal molꢀ1) is larger than that for the phenylsilylene
(H2Mind)PhSi: (38.2 kcal molꢀ1) and again between that for
The resulting monobromodisilene 5a is thermally stable, in
contrast to the dibromodisilene 1a, both in solution and in the
solid state; the SidSi bond dissociation was not observed in
C6D6 even in the presence of excess PPy, as confirmed by the
NMR monitoring. A possible mechanism for the PPy-promoted
monophenylation of 1a is proposed as follows: The bromosily-
lene-PPy complex 3a, less hindered than 1a, readily reacts
with PhMgBr to give the phenylsilylene (EMind)PhSi: as a
reactive intermediate, which is immediately trapped by 3a to
produce 5a. At the moment, an alternative additionꢀelimination
mechanism16 is not ruled out; thus the PPy coordination to the
Mg center may enhance the reactivity of the PhMgBr that causes
a direct attack on the dibromodisilene 1a to form 5a via an
additionꢀelimination mechanism. While the detailed mechan-
ism remains to be clarified by further studies, the synthetic
potentials of dibromodisilene 1a are noteworthy. Thus, both
1a and 5a readily react with PhLi to form the diphenylated
disilene 6a in high yields, suggesting a useful general route
to a variety of diaryldisilenes, including the unsymmetrical or
heteroaryl-substituted diaryldisilenes, which are hardly accessible
by the existing methods. Further synthetic investigations using
1 for the construction of unique π-conjugated disilene frame-
works are currently in progress and will be reported shortly.
(Me5C5){(Me3Si)2N}Si: (39.6 kcal molꢀ1 11
and (i-Pr2N)2Si:
)
(54.3 kcal molꢀ1).10c
The bromosilylenes 2 could not be directly observed by
spectroscopic methods, but the equilibrium became observable
in the presence of a strong Lewis base (Scheme 1). The addition
of 4-pyrrolidinopyridine (PPy) to a solution of 1 in C6D6
immediately afforded an equilibrium mixture of 1 and the
1
bromosilylene-PPy adducts 3, as monitored by H NMR.20 In
the 29Si NMR spectra, one new resonance appears at 59.3 (3a)
and 63.1 (3b) ppm, respectively. The equilibrium constants
Keq = 350 Mꢀ1 for 3a and 0.64 Mꢀ1 for 3b at 25 °C are quite
different from each other.20 Thus, the steric bulkiness of the Rind
groups significantly affects the coordination of PPy to the Si
center of the bromosilylene 2. The negative enthalpy values
of ΔH = ꢀ15.6 (3a) and ꢀ10.3 (3b) kcal molꢀ1 were derived
from the respective van’t Hoff plots of the equilibrium between
1 and 3,20 which suggest that the PPy adducts 3 are favored at low
temperatures.
Orange crystals of 3a were isolated in 58% yield by crystal-
lization from the equilibrium mixture of 1a and 3a in benzene at
6 °C. The dissolution of the pure crystals of 3a in C6D6 again
afforded the mixture of 1a and 3a. The X-ray structural analysis of
3a demonstrates that the coordination of PPy forms a trigonal
pyramidal geometry around the Si center with the sum of the
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details, crystal-
b
bond angles of 303.6°, as shown in Figure 1B. The Si
N
lographic data for 1b, 3a, and 4a in CIF format, details of the
calculations, and complete ref 21. This material is available free of
charge via the Internet at http://pubs.acs.org.
3 3 3
atomic distance of 3a [1.939(2) Å] is much longer than the
typical SiꢀN single bond length with the tetracoordinate Si(IV)
center (1.74 Å)22 but shorter than that observed in the
SiBr4ꢀpyridine (Py) adduct (SiBr4Py2) [1.981(3) Å].23 The
SiꢀBr and SiꢀC bond lengths in 3a, 2.3922(9) and 1.960(3) Å,
respectively, are elongated relative to those, 2.2279(6) and
1.8875(16) Å, in 1b, because of the increased p character of
the silicon atomic orbitals directed to the bromine atom and the
aryl group.13,24 The solid state 29Si CP-MAS NMR spectrum of
3a shows a strong signal at 62.4 ppm, which is close to that
observed in C6D6 (59.3 ppm), indicative of a solution structure
of 3a similar to the structure found in the crystal.
’ AUTHOR INFORMATION
Corresponding Author
matsuo@riken.jp; tamao@riken.jp
’ ACKNOWLEDGMENT
We thank the Ministry of Education, Culture, Sports, Science,
and Technology of Japan for the Grant-in-Aid for Specially
Promoted Research (No. 19002008). We thank Dr. Y. Hongo
and Mr. T. Nakamura (RIKEN) for their kind help with the mass
spectrometry and solid-state NMR spectroscopy. We are grateful
to the RIKEN chemical analysis team for the elemental analyses
of the samples synthesized in this study. We also thank Professor
N. Tokitoh and Dr. T. Sasamori for their valuable discussions.
The substitution reactions of 1a have also been examined in
the presence or absence of PPy (Scheme 4). Although 1a did not
show any sign of reaction with PhMgBr in THF at room
temperature, the addition of PPy (2 equiv) to the reaction
mixture resulted in the facile, selective formation of a mono-
phenylated product 5a, which was isolated in 61% yield.
19712
dx.doi.org/10.1021/ja209736d |J. Am. Chem. Soc. 2011, 133, 19710–19713

Journal of the American Chemical Society
COMMUNICATION
’ REFERENCES
(21) Frisch, M. J. et al. Gaussian 03, revision E.01; Gaussian, Inc.:
Wallingford, CT, 2004. Complete reference appears as ref S8 in
Supporting Information.
(22) Cambridge Structural Database, version 5.32 (Nov 2010). See:
Allen, F. H. Acta Crystallogr., Sect. B 2002, 58, 380.
(23) Bolte, M.; Hensen, K.; Spangenberg, B. J. Chem. Crystallogr.
2000, 30, 245.
(1) (a) West, R.; Fink, M. J.; Michl, J. Science 1981, 214, 1343.
(b) Brook, A. G.; Abdesaken, F.; Gutekunst, B.; Gutekunst, G.; Kallury,
R. K. J. Chem. Soc., Chem. Commun. 1981, 191.
(2) For recent reviews, see: (a) Kira, M.; Iwamoto, T. Adv. Organomet.
Chem. 2006, 54, 73. (b) Mizuhata, Y.; Sasamori, T.; Tokitoh, N. Chem.
Rev. 2009, 109, 3479. (c) Fischer, R. C.; Power, P. P. Chem. Rev. 2010,
110, 3877. (d) Lee, Y. Va.; Sekiguchi, A. In Organometallic Compounds of
Low-Coordinate Si, Ge, Sn and Pb; John Wiley & Sons, Ltd.: West Sussex,
U.K., 2010. (e) Scheschkewitz, D. Chem. Lett. 2011, 40, 2.
(24) Filippou, A. C.; Chernov, O.; Blom, B.; Stumpf, K. W.;
Schnakenburg, G. Chem.—Eur. J. 2010, 16, 2866.
(3) Krogh-Jespersen, K. J. Am. Chem. Soc. 1985, 107, 537.
(4) Carter, E. A.; Goddard, W. A., III J. Phys. Chem. 1986, 90, 998.
(5) (a) Trinquier, G.; Malrieu, J.-P. J. Am. Chem. Soc. 1987, 109,
5303. (b) Malrieu, J.-P.; Trinquier, G. J. Am. Chem. Soc. 1989, 111, 5916.
(c) Trinquier, G. J. Am. Chem. Soc. 1990, 112, 2130. (d) Trinquier, G.;
Malrieu, J.-P. J. Phys. Chem. 1990, 94, 6184.
(6) Liang, C.; Allen, L. C. J. Am. Chem. Soc. 1990, 112, 1039.
(7) (a) Maxka, J.; Apeloig, Y. J. Chem. Soc., Chem. Commun
1990, 737. (b) Karni, M.; Apeloig, Y. J. Am. Chem. Soc. 1990, 112,
8589. (c) Apeloig, Y.; Muller, T. J. Am. Chem. Soc. 1995, 117, 5363.
(8) (a) Tokitoh, N.; Suzuki, H.; Okazaki, R.; Ogawa, K. J. Am. Chem.
Soc. 1993, 115, 10428. (b) Suzuki, H.; Tokitoh, N.; Okazaki, R. Bull.
Chem. Soc. Jpn. 1995, 68, 2471. (c) Takeda, N.; Suzuki, H.; Tokitoh, N.;
Okazaki, R. J. Am. Chem. Soc. 1997, 119, 1456. (d) Takeda, N.; Tokitoh,
N. Synlett 2007, 2483.
(9) (a) Matsumoto, S.; Tsutsui, S.; Kwon, E.; Sakamoto, K.
Angew. Chem., Int. Ed. 2004, 43, 4610. (b) Tsutsui, S.; Tanaka, H.;
Kwon, E.; Matsumoto, S.; Sakamoto, K. Organometallics 2004, 23, 5659.
(c) Tsutsui, S.; Kwon, E.; Tanaka, H.; Matsumoto, S.; Sakamoto, K.
Organometallics 2005, 24, 4629.
(10) (a) Sakamoto, K.; Tsutsui, S.; Sakurai, H.; Kira, M. Bull. Chem.
Soc. Jpn. 1997, 70, 253. (b) Tsutsui, S.; Sakamoto, K.; Kira, M. J. Am.
Chem. Soc. 1998, 120, 9955. (c) Takahashi, M.; Tsutsui, S.; Sakamoto,
K.; Kira, M.; Muller, T.; Apeloig, Y. J. Am. Chem. Soc. 2001, 123, 347.
(11) Schmedake, T. A.; Haaf, M.; Apeloig, Y.; Muller, T.; Bukalov, S.;
West, R. J. Am. Chem. Soc. 1999, 121, 9479.
(12) Jutzi, P.; Mix, A.; Neumann, B.; Rummel, B.; Schoeller, W. W.;
Stammler, H.-G.; Rozhenko, A. B. J. Am. Chem. Soc. 2009, 131,
12137.
(13) (a) So, C.-W.; Roesky, H. W.; Magull, J.; Oswald, R. B. Angew.
Chem., Int. Ed. 2006, 45, 3948. (b) Ghadwal, R. S.; Roesky, H. W.; Merkel,
S.; Henn, J.; Stalke, D. Angew. Chem., Int. Ed. 2009, 48, 5683. (c) Filippou,
A. C.; Chernov, O.; Schnakenburg, G. Angew. Chem., Int. Ed. 2009, 48, 5687.
(d) Gau, D.; Kato, T.; Saffon-Merceron, N.; Cꢀozar, A. De; Cossío, F. P.;
Baceiredo, A. Angew. Chem., Int. Ed. 2010, 49, 6585.
(14) (a) Wang, Y.; Xie, Y.; Wei, P.; King, R. B.; Schaefer, H. F., III;
Schleyer, P. v. R.; Robinson, G. H. Science 2008, 321, 1069. (b) Sen, S. S.;
Jana, A.; Roesky, H. W.; Schulzke, C. Angew. Chem., Int. Ed. 2009,
48, 8536. (c) Inoue, S.; Epping, J. D.; Irran, E.; Driess, M. J. Am. Chem.
Soc. 2011, 133, 8514. (d) Tanaka, H.; Inoue, S.; Ichinohe, M.; Driess, M.;
Sekiguchi, A. Organometallics 2011, 30, 3475.
(15) Wiberg, N.; Niedermayer, W.; Fischer, G.; Noth, H.; Suter, M.
Eur. J. Inorg. Chem. 2002, 1066.
(16) Sasamori, T.; Hironaka, K.; Sugiyama, Y.; Takagi, N.; Nagase,
S.; Hosoi, Y.; Furukawa, Y.; Tokitoh, N. J. Am. Chem. Soc. 2008,
130, 13856. A thermal 1,2-migration of a bromine atom in 1,2-
dibromodisilene at 100 °C is also described in this paper.
(17) The LiBr-assisted generation of a silylene from 1,2-dibromodi-
silene has recently been reported. Han, J. S.; Sasamori, T.; Mizuhata, Y.;
Tokitoh, N. Chem-Asian. J. In press.
(18) If 3 equiv of LiNaph is used, the tetrasilacyclobutadiene is
obtained: Suzuki, K.; Matsuo, T.; Hashizume, D.; Fueno, H; Tanaka, K.;
Tamao, K. Science 2011, 331, 1306.
(19) (a) Fukazawa, A.; Li, Y.; Yamaguchi, S.; Tsuji, H.; Tamao, K.
J. Am. Chem. Soc. 2007, 129, 14164. (b) Matsuo, T.; Kobayashi, M.;
Tamao, K. Dalton Trans. 2010, 39, 9203.
(20) See Supporting Information for details.
19713
dx.doi.org/10.1021/ja209736d |J. Am. Chem. Soc. 2011, 133, 19710–19713