Reaction of dilithiosilane R2SiLi2 and dilithiogermane R2GeLi2 (R = SiMetBu 2) with MesBCl2 (Mes = 2,4,6-trimethylphenyl): Evidence for the formation of silaborene R2S

Article abstract of DOI:10.1246/cl.2005.582

1,1-Dilithiosilane and -germane, R₂SiLi₂ and R ₂GeLi₂ (R = SiMe^tBu₂) reacted with MesBCl₂ to give the unexpected seven-membered ring products, [1,3,2|oxasila- and -germaborepanes

Full text of DOI:10.1246/cl.2005.582

582
Chemistry Letters Vol.34, No.4 (2005)
Reaction of Dilithiosilane R2SiLi2 and Dilithiogermane R2GeLi2 (R = SiMetBu2)
with MesBCl2 (Mes = 2,4,6-trimethylphenyl): Evidence for the Formation
of Silaborene R2Si=BMes and Germaborene R2Ge=BMes
Norio Nakata, Rika Izumi, Vladimir Ya. Lee, Masaaki Ichinohe, and Akira Sekiguchiꢀ
Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571
(Received January 5, 2005; CL-050014)
1,1-Dilithiosilane and -germane, R2SiLi2 and R2GeLi2
tBu2MeSi
Li
SiMetBu2
Li
(R = SiMetBu2) reacted with MesBCl2 to give the unexpected
seven-membered ring products, [1,3,2]oxasila- and -germabore-
panes, through the intermediate formation of >Si=B– and
>Ge=B– doubly bonded species.
THF
E
MesBCl2
tBu2MeSi
tBu2MeSi
1: (E = Si)
2: (E = Ge)
Mes
E
B
O
Until very recently, the chemistry of compounds having a
double bond between elements of Groups 13 and 14 was limited
to the examples of boraalkenes R2C=BR0; that is, compounds
featuring a >C=B– bond, which have been intensively studied
over the past two decades.1 In contrast, the heavy analogues of
boraalkenes of the type R2E=BR (E = Si, Ge) were unknown,
partially because of the relative weakness of the >Si=B– bond,
which was calculated to be only half as strong (26.0 kcal/mol) as
the corresponding >C=B– ꢀ-bond (51.8 kcal/mol).2 Mean-
while, we have recently synthesized very effective coupling re-
agents, 1,1-dilithiosilane 13 and 1,1-dilithiogermane 2,4 interac-
tion of which with 1,1-bifunctional electrophiles resulted in the
very fast and clean formation of a variety of doubly bonded de-
rivatives containing heavier Group 14 elements.5 Using this syn-
thetic approach, quite recently we succeeded in the synthesis and
characterization of 1,3-disila-2-gallata- and -indataallenic
anions, representing the first examples of double bonds between
the heavier Group 13 and 14 elements.6 Here we report the rather
unusual reaction of 1,1-dilithiosilane and -germane 1 and 2 with
MesBCl2 in THF, leading to the formation of novel seven-mem-
bered ring compounds through the intermediate formation of the
silaborene >Si=B– and germaborene >Ge=B– species.
3: (E = Si, 58%)
4: (E = Ge, 50%)
Scheme 1.
ring. The X-ray crystallographic analysis revealed that 3 and 4
are isomorphous, with very similar molecular geometries
(Figure 1).10 The seven-membered rings of both 3 and 4 adopt
a twist boat-like conformation, in contrast to oxacycloheptane,11
which has a twist chair-like conformation. The Si1–B1 bond
ꢀ
length (2.1249(18) A) of 3 is significantly longer than that of
t
lithium silylborates [Li(Ph2RSi–BH3); R = Ph, Bu] (1.984–
12
ꢀ
1.993 A), while the Ge1–B1 bond length (2.1647(17) A) of 4
ꢀ
is close to those of lithium germylborates [Li(Et3Ge–BPh3)]
13
ꢀ
(2.145–2.152 A), which can be explained by the different
degree of steric crowding around the E–B bond.
The formation of 3 and 4 can be reasonably explained by the
7
The reaction of 1 with an equivalent amount of MesBCl2
(Mes = 2,4,6-trimethylphenyl) in dry THF immediately afford-
ed a pale yellow reaction mixture, the 1H NMR spectrum of
which showed four distinct resonances for methylene protons
at 1.19, 1.61, 1.83, and 4.51 ppm, as well as signals due to the
tBu2MeSi- and mesityl groups. After evaporation of solvent,
the reaction mixture was separated by HPLC equipped with a re-
cycling reverse phase ODS column to give the seven-membered
ring compound 3, 2-(2,4,6-trimethylphenyl)-3,3-bis[di-tert-
butyl(methyl)silyl][1,3,2]oxasilaborepane, as colorless crystals
in 58% yield (Scheme 1).8 The cyclic compound 4, 2-(2,4,6-tri-
methylphenyl)-3,3-bis[di-tert-butyl(methyl)silyl][1,3,2]oxager-
maborepane, was also obtained in 50% yield by reaction of
dilithiogermane 2 with MesBCl2 under similar conditions
(Scheme 1).9
Si2
C23
Si3
B1
O1
Si1
C4
Figure 1. ORTEP drawing of 3 (30% thermal ellipsoids). Hy-
drogen atoms are omitted for clarity. Selected bond lengths
ꢀ
(A) and angles (deg): Si1–B1 = 2.1249(18), Si1–Si2 =
The structures of both 3 and 4 were unambiguously deter-
mined by NMR spectral data and X-ray crystallography. Thus,
the 29Si NMR spectrum of 3 displayed two signals at ꢁ63:0
and 12.3 ppm, of which the high field signal (ꢁ63:0 ppm) was
assigned to the endocyclic silicon atom in the seven-membered
2.4726(6), Si1–Si3 = 2.4611(6), Si1–C4 = 1.9409(16), B1–
O1 = 1.362(2), Si2–Si1–Si3 = 121.01(2), Si2–Si1–C4 =
106.49(5), Si3–Si1–B1 = 104.63(5), C4–Si1–B1 = 102.55(7),
Si1–B1–O1 = 120.08(12), Si1–B1–C23 = 128.16(12), O1–
B1–C23 = 111.23(14).
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.4 (2005)
583
6
7
8
N. Nakata, R. Izumi, V. Ya. Lee, M. Ichinohe, and A.
Sekiguchi, J. Am. Chem. Soc., 126, 5058 (2004).
W. Schacht and D. Kaufmann, Chem. Ber., 120, 1331
(1987).
tBu2MeSi
tBu2MeSi
δ− δ+
E
B
Mes
1 or 2
MesBCl2
5: (E = Si), 6: (E = Ge)
Dilithiosilane 1 (0.10 mmol) was reacted with dichloro-
(2,4,6-trimethylphenyl)borane (15 mg, 0.072 mmol) in dry
THF (1.0 mL) for 15 min at room temperature to form a pale
yellow solution. After removal of the solvent in vacuo, the
reaction mixture was separated by HPLC equipped with a re-
verse phase ODS column (eluent: MeOH:tBuOMe = 1:1) to
afford colorless crystals of 3 (23 mg, 58%); mp 199–202 ꢂC
(dec.), 1H NMR (C6D6) ꢁ 0.31 (s, 6 H), 1.11 (s, 18 H), 1.18
(s, 18H), 1.19 (m, 2H), 1.61 (m, 2H), 1.83 (m, 2H), 2.13 (s,
3H), 2.44 (s, 6H), 4.51 (m, 2H), 6.74 (s, 2H); 13C NMR
(C6D6) ꢁ ꢁ2:0, 11.1, 21.2, 22.8, 23.7, 24.8, 29.8, 31.0,
31.2, 67.3, 128.3, 137.7, 140.1; 29Si NMR (C6D6) ꢁ ꢁ63:0,
12.3. The ipso carbon atom in the 13C NMR spectrum was
not observed, probably due to coupling with the 11B
nucleus, leadingꢂto broadening of the peak.
THF
tBu2MeSi
tBu2MeSi
tBu2MeSi
Mes
E
B
Mes
B
3 or 4
E
tBu2MeSi
O
O
Scheme 2.
initial formation of silaborene 5 or germaborene 6 as the key re-
active intermediates, which are stabilized by the coordination of
a THF molecule to the vacant 2pz-orbital on the B atom
(Scheme 2). Because of the different substitution pattern around
the doubly bonded atoms—the electrondonating tBu2MeSi
group on Si (or Ge) atoms and the electronwithdrawing Mes
group on B atoms of intermediates 5 and 6—both >Si=B–
1
9
4: mp 214–215 C (dec.), H NMR (C6D6) ꢁ 0.31 (s, 6 H),
1.10 (s, 18 H), 1.17 (s, 18H), 1.40 (m, 2H), 1.65 (m, 2H),
1.99 (m, 2H), 2.13 (s, 3H), 2.44 (s, 6H), 4.52 (m, 2H), 6.74
(s, 2H); 13C NMR (C6D6) ꢁ ꢁ1:9, 13.2, 21.2, 23.0, 23.7,
25.6, 30.0, 30.8, 30.9, 67.6, 128.3, 137.8, 139.9; 29Si NMR
(C6D6) ꢁ 19.7. The ipso carbon atom in the 13C NMR spec-
trum was not observed.
and >Ge=B– double bonds are greatly polarized: >Eꢁꢁ=B
–
ꢁþ
10 Crystal data for 3: C31H61BOSi3, MW = 544.88, monoclin-
ic, space group P21=c (No. 14), a ¼ 11:3740ð3Þ, b ¼
(E = Si, Ge). Consequently, the nucleophilic Si and Ge atoms
attack the ꢂ-carbons of a coordinated THF molecule accompa-
nied with the cyclic C–O bond cleavage to form finally the sev-
en-membered ring products 3 and 4, respectively (Scheme 2).14
Indeed, the appreciable polarization of the >E=B– bond was
clearly demonstrated by NPA (natural population analysis) cal-
culations on the model compounds, (Me3Si)2E=BPh [E = Si
(7), Ge (8)], which revealed the accommodation of a large part
of the electron density on the Si (ꢁ0:246) and Ge (ꢁ0:335)
atoms, whereas the B atom has a positive charge (+0.286 for
7, +0.295 for 8, respectively).15
18:0810ð9Þ, c ¼ 16:8290ð9Þ A, ꢃ ¼ 103:517ð3Þꢂ, V ¼
ꢀ
ꢀ 3
3365:1ð3Þ A , Z ¼ 4, Dcalc ¼ 1:076 g cmꢁ3
,
R1 ðI >
2ꢄðIÞÞ ¼ 0:0409, wR2 (all data) = 0.1113 for 7535 reflec-
tions and 342 parameters, GOF = 0.968. 4: C31H61-
BGeOSi2, MW = 589.38, monoclinic, space group P21=c
(No. 14), a ¼ 11:4140ð2Þ, b ¼ 18:1010ð6Þ, c ¼ 16:8330ð5Þ
ꢂ
ꢀ
ꢀ 3
A, ꢃ ¼ 103:588ð2Þ , V ¼ 3380:44ð16Þ A , Z ¼ 4, Dcalc
¼
1:158 g cmꢁ3, R1 ðI > 2ꢄðIÞÞ ¼ 0:0318, wR2 (all data) =
0.0851 for 7977 reflections and 342 parameters, GOF =
ꢀ
1.045. Selected bond lengths (A) and angles (deg) for 4:
Ge1–B1 = 2.1647(17), Ge1–Si1 = 2.5001(4), Ge1–Si2 =
2.4914(4), Ge1–C4 = 2.0279(15), B1–O1 = 1.359(2),
This work was supported by a Grant-in-Aid for Scientific
Research (Nos. 147078204, 16205008, 16550028) from the
Ministry of Education, Culture, Sports, Science and Technology
and COE (Center of Excellence) program.
Si1–Ge1–Si2 = 121.375(15),
Si1–Ge1–C4 = 106.57(4),
Si2–Ge1–B1 = 104.56(5), C4–Ge1–B1 = 101.51(6), Ge1–
B1–O1 = 120.06(11), Ge1–B1–C23 = 127.34(11), O1–
B1–C23 = 112.13(13).
References and Notes
1
a) A. Berndt, Angew. Chem., Int. Ed. Engl., 32, 985 (1993).
b) J. J. Eisch, Adv. Organomet. Chem., 39, 355 (1996). c)
P. P. Power, Chem. Rev., 99, 3463 (1999), and references
cited therein.
11 P. Luger, J. Buschmann, and C. Altenhein, Acta Crystallogr.,
C47, 102 (1991).
12 W. Lippert, H. Noth, W. Ponikwar, and T. Seifert, Eur. J.
¨
Inorg. Chem., 1999, 817.
2
3
P. v. R. Schleyer and D. Kost, J. Am. Chem. Soc., 110, 2105
(1988).
a) M. Ichinohe, Y. Arai, A. Sekiguchi, N. Takagi, and S.
Nagase, Organometallics, 20, 4141 (2001). b) A. Sekiguchi,
R. Izumi, V. Ya. Lee, and M. Ichinohe, J. Am. Chem. Soc.,
124, 14822 (2002).
a) A. Sekiguchi, R. Izumi, S. Ihara, M. Ichinohe, and
V. Ya. Lee, Angew. Chem., Int. Ed., 41, 1598 (2002). b) A.
Sekiguchi, R. Izumi, V. Ya. Lee, and M. Ichinohe, Organo-
metallics, 22, 1483 (2003).
V. Ya. Lee and A. Sekiguchi, Organometallics, 23, 2822
(2004).
13 M. Nanjo, K. Matsudo, and K. Mochida, Inorg. Chem. Com-
mun., 6, 1065 (2003).
14 The possibility of the initial formation of R2ELi–BClMes
(THF) (E = Si, Ge) followed by the intramolecular THF
ring-opening to form finally 3 (or 4) seems unfavorable,
since coordination of THF to B atom attached to an anionic
E-center, is unlikely due to the preferential interaction of
anionic center to B empty p-orbital. Instead, ꢃ-elimination
of LiCl would rather take place resulting in the formation
of >E=B– bond.
4
5
15 The calculations were carried out using the Gaussian 98
program at the B3LYP/6-31G(d) level.
Published on the web (Advance View) March 19, 2005; DOI 10.1246/cl.2005.582