Efficient access to substituted silafluorenes by nickel-catalyzed reactions of biphenylenes with et2SiH2

Article abstract of DOI:10.1002/asia.201402599

The reaction of biphenylene (1) with Et₂SiH₂ in the presence of [Ni-(PPhMe₂)₄] results in the formation of a mixture of 2-diethylhydrosilylbiphenyl [2(Et₂HSi)] and 9,9,-diethyl-9-silafluorene (3). Sil

Full text of DOI:10.1002/asia.201402599

FULL PAPER
DOI: 10.1002/asia.201402599
Efficient Access to Substituted Silafluorenes by Nickel-Catalyzed Reactions
of Biphenylenes with Et2SiH2
Jens Michael Breunig, Puneet Gupta, Animesh Das, Samat Tussupbayev,
Martin Diefenbach, Michael Bolte, Matthias Wagner, Max C. Holthausen,* and
Hans-Wolfram Lerner*[a]
Abstract: The reaction of biphenylene
(1) with Et2SiH2 in the presence of [Ni-
ACHTUNGTRENNUNG
of Et2SiH2 and 1-methylbiphenylene.
By contrast, no selectivity could be
found in the Ni-catalyzed reaction be-
tween Et2SiH2 and the biphenylene de-
rivative that bears tBu substituents in
the 2- and 7-positions. Therefore, two
pairs of isomers of tBu-substituted sila-
fluorenes and of the related diethylhy-
drosilylbiphenyls were formed in this
reaction. However, a subsequent dehy-
drogenation of the diethylhydrosilylbi-
phenyls with Wilkinsonꢀs catalyst yield-
ed a mixture of 2,7-di-tert-butyl-9,9-di-
ethyl-9-silafluorene (8) and 3,6-di-tert-
butyl-9,9-diethyl-9-silafluorene (9). Si-
lafluorenes 8 and 9 were separated by
column chromatography.
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
yield. The underlying reaction mecha-
nism was elucidated by DFT calcula-
tions. 4-Methyl-9,9-diethyl-9-silafluor-
ene (7) was obtained selectively from
ꢀ
Keywords: C C activation · density
ꢀ
functional calculations · nickel · Si
H activation · silanes
the [NiACHTUNGTRENNUNG(PPhMe2)4]-catalyzed reaction
Introduction
ient access to silafluorenes with an asymmetric substitution
pattern in the 4-positions is lacking as of yet.
Silafluorenes are being increasingly recognized as p-conju-
gated materials with promising potential for device applica-
tions, and important contributions have been made in the
areas of electro- and photoluminescence.[1–5] With their typi-
cally low lying LUMO energy level originating from inter-
A number of synthetic routes towards 9-silafluorenes
have been reported earlier (cf. Scheme 1): Silafluorenes
ꢀ
were prepared i) by Pd-catalyzed C C coupling reactions of
diaryl silanes ArAr’SiR2 carrying a triflate group in the 2-
position of the aryl substituent Ar, whereby bond formation
occurs with the corresponding unsubstituted 2-position of
the aryl substituent Ar’;[16,17] ii) by ring closure via a silylium
ꢀ
ference of the exocyclic Si C s* orbital and the p* orbital
of the biphenyl fragment,[6–9] this class of compounds can be
used as electron-transporting material for emitters in organ-
ic light-emitting diodes.[10] In this context, the ability to con-
trol the electronic nature of these chromophores by varia-
tion of the functional groups is critical to optimize and fine-
tune their properties.
We have a long-standing interest in boron-doped aromat-
ics[11–15] and have developed inter alia a protocol for the syn-
thesis of 9-borafluorenes by treatment of the corresponding
9-silafluorene derivatives with BBr3.[12,13] However, conven-
[a] J. M. Breunig, P. Gupta, Dr. A. Das, Dr. S. Tussupbayev,
Dr. M. Diefenbach, Dr. M. Bolte, Prof. Dr. M. Wagner,
Prof. Dr. M. C. Holthausen, Dr. H.-W. Lerner
Institut fꢁr Anorganische Chemie
Goethe-Universitꢂt Frankfurt
Max-von-Laue-Strasse 7, 60438 Frankfurt (Germany)
Fax : (+49)69-79829260
E-mail: lerner@chemie.uni-frankfurt.de
max.holthausen@chemie.uni-frankfurt.de
Scheme 1. Known preparation routes of 9-silafluorenes. i) [Pd], Et2NH,
dimethylacetamide, 1008C, 24 h; ii) Ph3CBACHTUNGTRNEGN(U C6F5)4, 1,6-lutidine, CH2Cl2,
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201402599.
RT, 1 h or [Rh], 1,4-dioxane, 1358C, 15 min; iii) [Ir], +R3CCR3, Bu2O,
1108C, 24 h; iv) E=Li, Mg; +R2SiCl2.
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Max C. Holthausen, Hans-Wolfram Lerner et al.
ion formed in the reaction of HSi-functionalized 2-silylbi-
phenyl derivatives 2
(R2HSi) and the trityl cation[18] or by
In our search for alternative, more efficient catalysts, we
identified nickel complexes that were reported previously to
AHCTUNGTRENNUNG
[35–42]
[43–45]
ꢀ
ꢀ
Rh-catalyzed H2 elimination from 2
(R2HSi) and subsequent
activate both C C
to continue our efforts by employing the well-defined nickel
catalyst [Ni
(PPhMe2)4].[46] Treatment of 1 with Et2SiH2 and
5 mol% [Ni(PPhMe2)4] in toluene at 1108C resulted in
and Si H bonds,
and we chose
[19–21]
ꢀ
C C coupling;
iii) by Ir-catalyzed [2+2+2] cycloaddi-
tion of silicon-bridged 1,6-diynes with alkynes;[22] and iv) by
ACHTUNGTRENNUNG
metathesis reactions of 2,2’-dimetalated biphenyls and
ACHTUNGTRENNUNG
[8,23–26]
1
R2SiX2
or triorganyl silanes R3SiX.[27–29] Furthermore,
a fast reaction with concomitant release of H2. The H NMR
spectrum of the reaction solution revealed complete con-
sumption of biphenylene after 4 h at 1108C and formation
a Pd-catalyzed reaction starting from 2,2-dihalogenated bi-
phenyls and Et2SiH2 has also been reported.[30] However,
each of these protocols has specific disadvantages.[31]
In search of alternative synthetic concepts we asked our-
selves whether it is possible to gain access to 9-silafluorenes
via transition metal catalyzed routes from biphenylenes and
of silylated biphenyl 2
of 40:60 (Scheme 2). We were able to separate the two prod-
ucts by column chromatography and isolated 2(Et2HSi) and
ACHTUNGTERN(NUNG Et2HSi) and silafluorene 3 in a ratio
AHCTUNGTRENNUNG
3 in 36.9 and 37.5% yield, respectively. The outcome of this
reaction is independent of the solvent (e.g., THF, toluene,
benzene, or 1,4-dioxane).
To assess mechanistic aspects of this reaction, we conduct-
ed a series of further experiments. Firstly, the [NiACHTNUTRGNEUNG(PPhMe2)4]
ꢀ
ꢀ
diethylsilane through C C and Si H bond-activation reac-
tions with concomitant dehydrogenation. For biphenylenes
bearing bulky substituents in the 1,8- or 2,7-positions, oxida-
tive addition of the metal should occur selectively into the
ꢀ
sterically less hindered C C bond of the four-membered
catalyst was mixed with Et2SiH2 in the absence of bipheny-
lene, but no reaction was observed up to 1108C. Secondly,
the [NiACHTNUTGRENUNG(PPhMe2)4] catalyst was mixed with biphenylene in
the absence of Et2SiH2, and biphenylene dimerization to tet-
raphenylene was observed at 1108C with no reaction at
lower temperatures. In this context we note that Johnson
and Beck[35] obtained tetraphenylene in a similar reaction at
ring of biphenylenes.[32] In this way, the selective preparation
of 9-silafluorene derivatives with sophisticated substitution
patterns should be possible.
As relevant precedent, we noticed a recent report of Mat-
suda et al. on the Pd-catalyzed hydrosilylation of bipheny-
lenes with triorganyl silanes to give 2-silylbiphenyls.[33]
ꢀ
Hence, without doubt, such products were formed in C C
908C by employing the nickel catalyst [NiACTHNGUTERNNU(G cod)2]/PiPr3
ꢀ
and Si H activation reactions, but unfortunately the use of
(cod=1,5-cyclooctadiene). In contrast, when the reaction
was conducted at room temperature, the dinuclear species B
and C (Scheme 3) were isolated, but no tetraphenylene was
dialkyl silanes was not considered by these authors. Here we
report an efficient approach for the synthesis of substituted
9-silafluorenes based on the nickel-catalyzed reaction of bi-
phenylenes with Et2SiH2.
Results and Discussion
Following the approach of Matsuda et al., we first investi-
gated the Pd-catalyzed hydrosilylation of biphenylene with
Et2SiH2, but after heating the reaction mixture to 1108C for
24 h we found only 16% conversion of biphenylene (20%
after 6 d at 1108C), and the 2-silylbiphenyl 2ACTHUNTRGNE(UNG Et2HSi) was
formed in a mixture with the desired 9-silafluorene 3
(Scheme 2).[34] Isolation of 3 from this reaction mixture
would further lower the yield drastically.
Scheme 3. Ni-catalyzed synthesis of tetraphenylene. i) R=iPr, +bipheny-
lene, toluene, 30 min, 258C; ii) R=iPr, 6 h, 258C, toluene; iii) R=iPr,
908C, toluene.[35]
formed under these conditions. On the basis of these find-
ings, it was concluded that tetraphenylene formation at
908C occurs via dinuclear intermediates B and C, while the
mononuclear pathway was proposed to be less favorable.[35]
We further investigated the possibility of interconversion
between the products 2
ACHTUNGTRENNUNG
Scheme 2. Pd- and Ni-catalyzed reaction of biphenylene with Et2SiH2.
nation/addition in the presence of the [NiACTHUNGTRENNUNG
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Max C. Holthausen, Hans-Wolfram Lerner et al.
lyst. In
A
a
reaction of [Ni-
(Et2HSi) car-
ried out at 1108C, no 3 was
formed by dehydrogenation
even in the presence of a hydro-
gen scavenger (3,3-dimethyl-1-
butene). We then examined the
reactivity of 3 towards dihydro-
gen (1 or 130 bar) at 1108C in
the presence of [Ni
but no 2(Et2HSi) was formed.
Thus, we conclude that 2-
(Et2HSi) and 3 are not intercon-
ACHTUNGTRENNUNG(PPhMe2)4],
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
vertible under any reaction con-
ditions relevant to their forma-
tion.
To elucidate the mechanistic
details underlying the experi-
mental observations, we carried
out a computational study on
[NiACHTUNGTRENNUNG(PMe3)4] and Me2SiH2 as
smaller molecular models. For
selected pathways we per-
formed additional calculations
using the realistic catalyst [Ni-
ACHTUNGTRENNUNG(PPhMe2)4]. The dispersion-cor-
rected BP86-D2 functional was
chosen in combination with the
def2-TZVP basis set; the per-
formance of this level of density
functional theory (DFT) was fa-
vorably tested in
a careful
benchmark study against high-
level coupled-cluster results for
a set of model reactions rele-
vant to the present study (see
the Supporting Information for
details). All relative energies
reported in this work are based
on Gibbs free energies in kcal
molꢀ1 at 298.15 K. In the fol-
lowing, we report our results
ꢀ
Scheme 4. Activation of Ni catalyst A0 and its insertion into the C C bond of biphenylene. Gibbs free ener-
gies DGꢁ298 [kcalmolꢀ1] are relative to A0.
for the NiACHTUNGTRENNUNG(PMe3)4-catalyzed di-
merization of biphenylene and for the reaction between bi-
phenylene and Me2SiH2.
The computed reaction paths commence with activation
of the coordinatively saturated complex [NiACTHUNTRGNE(UNG PMe3)4] (A0)
by liberation of PMe3 to create a vacant coordination site
(Scheme 4). Dissociation of the first PMe3 ligand to give
(referenced to A0). Subsequent liberation of a second PMe3
ligand from B1 leads to the 16-electron complex B2
(DrGꢁ298 =7 kcalmolꢀ1). We investigated an obvious mononu-
clear pathway, that is, oxidative addition of another bipheny-
lene molecule to B2 followed by reductive elimination of
tetraphenylene, but we regarded such a process as irrelevant
because of its unreasonably high activation barrier (see Sup-
porting Information, Section 5 and Scheme 2S). Instead, we
identified a dinuclear route that nicely explains tetrapheny-
lene formation, including all pertinent experimental findings
reported by Johnson and Beck[35] (Scheme 3).
[NiACHTUNGTRENNUNG(PMe3)3] (A1) can readily occur under the reaction con-
ditions (DrGꢁ298 =15 kcalmolꢀ1), whereas detachment of
a second phosphine ligand would be a substantially more en-
dergonic step (DrGꢁ298 =36 kcalmolꢀ1 relative to A0). Ener-
getically favored, in fact, is the activation of one of the
ꢀ
strained central C C bonds in biphenylene by oxidative ad-
dition to A1, which leads to exergonic formation of metalla-
In our computational evaluation of the dinuclear route we
adopted E1, the structural analogue of the experimentally
characterized species B (Scheme 3), as starting point. We
cycle B1 with an overall activation barrier of 22 kcalmolꢀ1
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Max C. Holthausen, Hans-Wolfram Lerner et al.
relate its formation to dimerization of two molecules of B2
on release of another phosphine ligand per unit to give E1
(DrGꢁ298 =ꢀ9 kcalmolꢀ1 for this step). This dimerization
We hence investigated the influence that our choice of
PMe3 ligands as molecular model has on the computed reac-
tion paths compared to the PPhMe2 ligands used in our ex-
periments. In the realistic catalyst model, liberation of one
ꢀ
occurs asymmetrically, and E1 features, apart from an Ni
Ni s bond, one formal NiIII ion engaged in three Ni C
of the phosphine ligands from the [NiACHTNGUTRENNU(G PPhMe2)4] complex is
ꢀ
I
s bonds and a formal Ni ion involved in one Ni C s bond
significantly endergonic by 23 kcalmolꢀ1, and the activation
ꢀ
and h2 coordination with one of the phenyl rings. This
formal mixed-valence description is moderately pronounced,
as indicated by CM5 charges[47] of +0.393 and +0.288 calcu-
lated for the NiIII and NiI ions, respectively. The two nickel
barrier for subsequent insertion into the C C bond of bi-
ꢀ
phenylene is 9 kcalmolꢀ1 (see Supporting Information, Sec-
tion 5 and Scheme 3S). The overall activation barrier of this
insertion step is 32 kcalmolꢀ1, which is clearly efficiently sur-
mountable only at elevated temperatures. The main contri-
bution to the increase in the total barrier to insertion stems
from the energy needed for the detachment of PPhMe2, and
we attribute the stronger binding of PPhMe2 to additional
stabilizing p-stacking interactions among the phenyl rings in
ꢀ
centers in E1 cooperatively promote the first C C coupling
step through transition state TSE1-E2 to yield metallacycle
E2, which formally contains two NiI ions, in an exergonic
step (DrGꢁ298 =ꢀ12 kcalmolꢀ1). This reductive elimination
proceeds with a moderate energy barrier of 16 kcalmolꢀ1,
which indicates that it is kinetically feasible and irreversible
at room temperature (reverse barrier: 27 kcalmolꢀ1).
the reactant complex [NiACTHNUGTRENNUG(PPhMe2)4].
We studied two pathways for the initial phase of the reac-
ꢀ
The second C C coupling step resulting in E3, in which
tion, which commence with oxidative addition of the silane
tetraphenylene is coordinatively bound to two Ni0 centers, is
also exergonic (DrG2ꢁ 98 =ꢀ12 kcalmolꢀ1). However, it has
a significantly higher energy barrier of 26 kcalmolꢀ1, which
or biphenylene to the nickel catalyst. Initial Si H bond acti-
ꢀ
vation by A1 leads to silyl hydride complex H1 and, after
decoordination of another phosphine ligand, to H2 in a sub-
stantially endergonic sequence (Scheme 5). Subsequent oxi-
dative addition of biphenylene to H2 via TSH2-B3 yields B3
with an overall reaction barrier of 36 kcalmolꢀ1 relative to
A0.[48] With its high barrier, this process is kinetically not
competitive with the alternative route commencing with the
ꢀ
is most probably related to the loss of Ni Ni interaction via
TSE2-E3. The Ni atoms readily coordinate free phosphine li-
gands in the next steps to liberate tetraphenylene (E6) with
reformation of the initial catalyst A0 in a highly exergonic
reaction sequence (DrGꢁ298 =ꢀ83 kcalmolꢀ1). These results
are nicely in line with the findings of Johnson and Beck,
ꢀ
oxidative addition of the C C bond in biphenylene to form
ꢀ
who reported the first C C coupling to proceed at room
complex B1 in an exergonic step. In view of the expected
substantial entropic penalty, we did not investigate pathways
involving dinuclear species.[49] After loss of PMe3 from B1,
B3 is formed by oxidative addition of the silane with only
temperature and the second, yielding tetraphenylene, to
occur only on heating. With an overall activation barrier of
26 kcalmolꢀ1, the dinuclear pathway is clearly more favora-
ble than the mononuclear route (31 kcalmolꢀ1, see the Sup-
porting Information). Nevertheless, DFT predicts a rather
ꢀ
a small activation barrier, so that the initial C C bond acti-
vation barrier of 22 kcalmolꢀ1 (with respect to A0) repre-
sents the overall kinetic bottleneck for this route. Hence,
fully in line with our experimental observations detailed
moderate barrier of 22 kcalmolꢀ1 for the initial C C bond
activation of biphenylene, in contrast to our experimental
results, in which no reaction was observed at room tempera-
ture.
ꢀ
ꢀ
above, initial Si H bond activation is clearly disfavored and
ꢀ
biphenylene C C bond activation is the initial step in the
Scheme 5. Two alternative pathways for the formation of B3. Gibbs free energies DGꢁ298 [kcalmolꢀ1] are relative to A0.
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Max C. Holthausen, Hans-Wolfram Lerner et al.
mined by the relative barrier
heights associated with the two
routes leading to P1 and P2
(i.e., TSB4-B5 and TSB4-B6, respec-
tively). With nearly barrierless
formation of P1 and a barrier
of 2 kcalmolꢀ1 associated with
the formation of P2, the com-
putational results do not quan-
titatively account for the exper-
imental observation that the
two products are formed in ap-
proximately equal yield, but
this small difference in barrier
heights falls within the accuracy
limitations of the computational
method chosen (2 kcalmolꢀ1, cf.
the benchmark section provided
in the Supporting Information).
However, only minute barriers
occur along reaction paths (a)
and (b), and we find that the in-
ꢀ
itial C C bond activation step
via TSA1-B1 is rate-limiting. Sub-
sequent to formation of B1, oxi-
dative addition of the silane
occurs with an effective barrier
of only 12 kcalmolꢀ1 (B1!
TSB2-B3, Scheme 5). We find this
route to be favored over the
first kinetically relevant steps of
Scheme 6. Pathways leading to the products 2-dimethylhydrosilylbiphenyl (P1) and 9-silafluorene (P2). Gibbs
free energies DG2ꢁ 98 [kcalmolꢀ1] are relative to A0.
the oxidative addition of
second biphenylene unit,
which occurs with an effective
a
overall course of the reaction, also in the presence of dime-
thylsilane.
barrier of 16 kcalmolꢀ1 (E1!TSE1-E2, Scheme 4). Hence,
nicely in line with the experimental observations, dimeriza-
tion of biphenylene to form tetraphenylene does not occur
in the presence of dimethylsilane.
From B3, a silyl shift from the Ni ion onto the carbon
framework leads to the formation of B4, in which one of the
phenyl rings has been silylated in ortho position (Scheme 6).
At this point two pathways branch off: a) a low-barrier hy-
drogen atom transfer via TSB4-B5 leads to the p-coordinated
complex B5 and coordination of PMe3 ligands finalizes the
The computed reaction mechanism indicates no obvious
way to improve silafluorene yields in the product distribu-
tion. This can be accomplished, however, by adopting the
known dehydrogenation of 2-hydrosilylbiphenyls with Wil-
ꢀ
reaction sequence leading to product P1; b) Si H bond acti-
kinsonꢀs catalyst.[21] However, [Rh
ACTHUNGETRN(NUG PPh3)3Cl] cannot just be
ꢀ
vation with concomitant H H bond formation through mul-
added to the reaction mixture, because this catalyst is pois-
oned by PPhMe2 and/or Et2SiH2. Consequently, we conduct-
ed a brief aqueous workup and flash column chromatogra-
ticenter transition state[51] TSB4-B6 leads to the formation of
dihydrogen complex B6, which readily eliminates H2 to give
ꢀ
B7. The second Si C bond is formed on reductive elimina-
tion of silafluorene P2, which involves intermediate p-
bonded complex B8 and ligand substitution by PMe3.
phy prior to the conversion of silylbiphenyl 2
silafluorene 3 (yield of 3: 49%).
To gain access to silafluorene derivatives, our approach
ACHTUNGTRENUN(GN Et2HSi) to the
ꢀ
As illustrated in Scheme 6, both pathways starting from
B4 have moderately low energy barriers and most of the
steps are exergonic. With prohibitively high reverse activa-
tion barriers of 34 and 39 kcalmolꢀ1, respectively, products
P1 and P2 are not readily interconvertible under the reac-
tion conditions. Hence, the resulting product mixture is not
thermodynamically controlled and, independent of the rela-
tive stability of P1 and P2, the product distribution is deter-
relies on the steric hindrance of one of the two C C single
bonds in the four-membered rings of biphenylenes. Substitu-
ents in the 1- and 8-positions of biphenylenes should prevent
oxidative addition of the catalyst. Accordingly, we examined
the [NiACHTNUTGRENUNG(PPhMe2)4]-catalyzed reaction of 1-methylbipheny-
lene (4) and 1,8-dimethylbiphenylene (5) with Et2SiH2 with
regard to their reactivity and selectivity (Scheme 7). Where-
as 4 reacted nicely with Et2SiH2 in the presence of [Ni-
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Max C. Holthausen, Hans-Wolfram Lerner et al.
Scheme 7. Influence of the substitution patterns on the selectivity of sila-
fluorene formation. i) [Ni(PPhMe2)4], C6D6, 1208C, 2 h; ii) [Ni-
(PPhMe2)4], C6D6, 1208C, 2 h; iii) 1) [Ni(PPhMe2)4], toluene, 1108C,
5.5 h ; 2) H2O ; 3) [Rh(PPh3)3Cl], toluene, 1358C, 3 d.
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG(PPhMe2)4], biphenylene 5 showed no reactivity at all.
Clearly, steric repulsion of the two methyl groups prevents
ꢀ
C C bond activation during fluorene formation. Surprising-
ly, the steric hindrance introduced by a single methyl group
suffices to obtain selectivity in the [NiACTHNURGTNEUNG(PPhMe2)4]-catalyzed
reaction of 4 with Et2SiH2 (Scheme 7). 4-Methyl-9,9-diethyl-
9-silafluorene (7) is a colorless liquid, the constitution of
which was determined by 2D NMR experiments. Concomi-
tant with the formation of 7 also the two isomeric biphenyl
species 2-diethylsilyl-6-methylbiphenyl and 2-diethylsilyl-2’-
methylbiphenyl were produced in this reaction. Takai et al.
showed that treatment of 2-dimethylsilyl-2’-methylbiphenyl
with Wilkinsonꢀs catalyst gave a silaphenanthrene derivative
ꢀ
by C H activation of the arylmethyl group of the silylated
methylbiphenyl.[21]
For the nickel-mediated reaction of biphenylene carrying
bulky tert-butyl groups in the 2- and 7-positions, no selectivi-
ty was observed. However, by means of our protocol estab-
lished before to improve the yield of the parent silafluorene,
we converted the product mixture to an isomeric mixture of
2,7-di-tert-butyl-9,9-diethyl-9-silafluorene (8) and 3,6-di-tert-
butyl-9,9-diethyl-9-silafluorene (9; Scheme 7).
The isomeric silafluorenes 8 and 9 were separated by
column chromatography and crystallized by slow evapora-
tion of CHCl3 solutions (see Figure 1 and the Supporting In-
formation for more details). Silafluorene 8 crystallizes in the
orthorhombic space group Pbca with one molecule in the
asymmetric unit. Compound 9 crystallizes in the monoclinic
space group P21/n with one molecule in the asymmetric
unit. X-ray quality crystals of 3 (monoclinic, Pc, two mole-
Figure 1. Molecular structures of 3, 8, and 9.
cules in the asymmetric unit) were obtained by storing a con-
centrated hexane solution at ꢀ408C (see Figure 1 and the
Supporting Information for more details).
By cooling a concentrated diethyl ether solution of [Ni-
A
tained (see Supporting Information). [Ni
G
ꢀ
lized in the triclinic space group P1 with one molecule in
the asymmetric unit. The structure of [NiACTHNURGTENUNG(PPhMe2)4] exhib-
its perpendicular edge-to-face (T-shaped) configurations of
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Max C. Holthausen, Hans-Wolfram Lerner et al.
two phenyl rings of the phosphine ligands.[52,53] One interac-
tion of the phenyl rings of PPhMe2 ligands is intramolecular
and is located between H12 and the center of gravity of
phenyl ring C41–C46 with a distance of 2.582 ꢄ. The second
interaction of the phenyl rings of the PPhMe2 ligands is in-
termolecular (Figure 2) with a distance of 2.857 ꢄ.
Scheme 8. Reaction of 4 with Et3SiH in the presence of [NiACHTUNGTRENNUNG
yielding an isomeric mixture of 10 and 11. i) [NiCAHTUNGTRENNNUG
2 h.
In addition we also investigated the [NiACTHNURGTENUNG(PPhMe2)4]-cata-
lyzed reaction of biphenylene with R3SiH (Table 1; R3Si=
Summary and Conclusions
Convenient access to silafluorenes with sophisticated substi-
tution patterns was established. Reaction of biphenylene
with Et2SiH2 in the presence of [Ni
ACHTUNGRTEN(NUNG PPhMe2)4] resulted in
the formation of a mixture of 2(Et2HSi) and 3, which were
AHCTUNGTRENNUNG
separable by column chromatography in decent yields.
Under the same conditions, 7 was obtained selectively by
employing 1-methylbiphenylene as reactant. In contrast, no
selectivity was found in the Ni-catalyzed reaction between
Et2SiH2 and the 2,7-tBu2-substituted biphenylene derivative,
and two tBu-substituted silafluorene isomers 8 and 9 were
obtained in a mixture with the related diethylhydrosilylbi-
phenyls. However, by subsequent dehydrogenation of the di-
ethylhydrosilylbiphenyls with Wilkinsonꢀs catalyst, a mixture
of isomeric silafluorenes 8 and 9 was obtained and separated
by column chromatography. Further, we investigated the
ꢀ
Figure 2. Packing diagram of [Ni
(PPhMe2)4] (triclinic, P1). H atoms on
the methyl groups are omitted for clarity. Dashed lines indicate intra-
and intermolecular T-shaped CH–p interactions.
Table 1. Yields of the reactions of biphenylene with triorganyl silanes in
the presence of [NiACHTUNGTRENNUNG(PPhMe2)4].
[Ni
R3SiH (R3Si=Et3Si, nPr3Si, iPr3Si, tBu3Si, Me2PhSi, Ph3Si),
and the corresponding 2-silylbiphenyls 2(R3Si) (R3Si=Et3Si,
ACHTUNGRTEN(UNNG PPhMe2)4]-catalyzed reaction of biphenylene with
AHCTUNGTRENNUNG
nPr3Si, Me2PhSi, Ph3Si) were obtained in good yields when
the mixtures were heated to 1108C for 4 h. The constitutions
of these 2-silylbiphenyls were confirmed by NMR spectros-
copy. Surprisingly, no hydrosilylation of biphenylene was ob-
served with the silanes iPr3SiH and tBu3SiH, and only tetra-
phenylene was obtained.
R3SiH
2
Yield [%]
Et3SiH
2
2
2
2
G
93
90
85
72
nPr3SiH
Me2PhSiH
Ph3SiH
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
The underlying reaction mechanism was elucidated by
means of DFT calculations. In keeping with earlier experi-
mental findings, we identified a dinuclear pathway for tetra-
phenylene formation, which is observed for the reaction of
the nickel catalyst with biphenylene in the absence of si-
lanes. This route starts with the activation of the nickel cata-
lyst by detachment of a phosphine ligand and subsequent in-
Et3Si, nPr3Si, iPr3Si, tBu3Si, Me2PhSi, Ph3Si). In these reac-
tions the corresponding 2-silylbiphenyls (R3Si) were
formed in good yields when the mixtures were heated to
1108C for 4 h (cf. Table 1).
Surprisingly, the Ni-catalyzed reaction with triisopropylsi-
lane and tri-tert-butylsilane resulted only in conversion of bi-
phenylene to tetraphenylene, just as found in the reaction of
2ACHTUNGTRENNUNG
ꢀ
sertion into the strained C C bond of biphenylene to give
nickel biphenylene intermediate B2. This intermediate di-
1 with [Ni
G
merizes with concomitant detachment of two more phos-
ꢀ
the steric bulk introduced by bulky alkyl groups in these
two silanes makes their Si H bonds inaccessible for oxida-
tive addition to the nickel biphenylene metallacycle B2,
such that its dimerization and thus tetraphenylene formation
becomes favored.
phine ligands and two C C coupling steps, cooperatively
mediated by the two nickel ions, leading to tetraphenylene
formation.
ꢀ
In the presence of silane, oxidative addition of the Si H
bond to the nickel ion in B2 is kinetically favored over tetra-
phenylene formation. The resulting hexacoordinate nickel
species readily undergoes reductive elimination to form a si-
lylated phenyl ring in intermediate B4. Two competing path-
ways branch off at this point and lead to the formation of
the two products observed experimentally with low activa-
tion barriers: 2-dimethylhydrosilylbiphenyl (P1) is formed
Finally we found that the [NiACHTNUGRTNEUNG(PPhMe2)4]-catalyzed reac-
tion of 4 with Et3SiH (Scheme 8) yielded an inseparable
mixture of the isomeric 2-triethylsilyl-6-methylbiphenyl (10)
and 2-triethylsilyl-2’-methylbiphenyl (11).
ꢀ
by C H bond formation through reductive elimination,
Chem. Asian J. 2014, 9, 3163 – 3173
3169
ꢃ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Max C. Holthausen, Hans-Wolfram Lerner et al.
ꢀ
C6D6 (0.5 mL) was added and the tube was sealed. After heating to
1108C for 2 h the reaction mixture was investigated by NMR spectrosco-
whereas Si H bond activation and subsequent elimination
of dihydrogen leads to the 9,9-dimethyl-9-silafluorene (P2).
The two products are formed with high exergonicity and are
not interconvertible under the reaction conditions.
py, and only the starting materials Et2SiH2 and [Ni
servable.
ACHTUNGRTENN(NUG PPhMe2)4] were ob-
Reaction of biphenylene (1) with [Ni
ACHTUNGTRENNUNG
charged with (20 mg, 0.13 mmol) and [NiACTHUNGTRENNNUG
1
0.13 mmol). C6D6 (0.5 mL) was added and the tube was sealed. After
heating to 1108C for 2 h the reaction mixture was investigated by NMR
spectroscopy.
Experimental Section
Reaction of 2
An NMR tube was charged with 2
(PPhMe2)4] (6 mg, 0.01 mmol), and 3,3-dimethyl-1-butene (0.14 mL).
C6D6 (0.5 mL) was added and the tube was sealed. After heating to
1108C for 13 h the reaction mixture was investigated by NMR spectros-
copy.
ACHUTGTNERNNG(U Et2HSi) with [NiACTHUNGTRENNNUG
Computational Details
AHCTUNGTRENNUNG
Geometry optimizations and Hessian calculations were carried out with
Gaussian 09 (Revision D.01)[54] by employing the BP86[55, 56] density func-
tional including Grimmeꢀs D2 empirical dispersion correction[57] in com-
bination with the def2-TZVP[58] basis set obtained from the EMSL basis
set exchange library.[59–61] To accelerate calculations we made use of the
resolution-of-identity[62–64] approximation, and automatically generated
auxiliary basis sets were employed as provided by the auto[65,66] keyword
implemented in Gaussian09. This functional was selected after a careful
benchmark study (see Supporting Information). All stationary points
were characterized as minima or first-order saddle points by eigenvalue
analysis of the computed Hessians. The connectivities of transition states
and the respective minima implied in the presentation were validated by
means of energy minimizations following small geometry displacements
along the reaction coordinate obtained from the vibrational frequency
analyses of transition-state structures. XYZ coordinates of optimized
minima and transition states are provided in the Supporting Information.
Unscaled zero-point vibrational energies as well as thermal and entropic
correction terms were added to total energies to obtain Gibbs free ener-
gies (298 K) by using the standard procedures implemented in the Gaus-
sian program.
AHCTUNGTRENNUNG
Reaction of 3 with [Ni
ACHTUNGTRENNUNG
(120 mg, 0.503 mmol) and [NiAHCTNUGTRENNNUG
solved in toluene (20 mL). This mixture was heated in an autoclave
under an H2 atmosphere (130 bar) to 1108C for 16 h. An aliquot of this
mixture was investigated by NMR spectroscopy.
Reaction of biphenylene (1) with Et2SiH2 and [Ni
mosphere: Compound (100 mg, 0.657 mmol), Et2SiH2 (87 mg,
0.99 mmol), and [Ni(PPhMe2)4] (20 mg, 0.033 mmol) were dissolved in
ACHTUNGTRENN(NUG PPhMe2)4] in an H2 at-
1
AHCTUNGTRENNUNG
toluene (20 mL). This mixture was heated to 1108C for 3 h in an auto-
clave under an H2 atmosphere (130 bar). An aliquot of this mixture was
investigated by NMR spectroscopy. The product ratio of 2ACTHNUTRGNE(UNG Et2HSi):3 was
still 4:6, but the major product of the reaction was biphenyl.
Reaction of 1 with iPr3SiH: An NMR tube was charged with 1 (50 mg,
0.33 mmol), [NiACTHUNTRGNE(UNG PPhMe2)4] (10 mg, 0.016 mmol), and iPr3SiH (62 mg,
0.39 mmol). C6D6 (0.5 mL) was added and the NMR tube was sealed.
After heating to 1108C for 4 h the reaction mixture was investigated by
NMR spectroscopy.
General Remarks
Reaction of 1 with tBu3SiH: An NMR tube was charged with 1 (40 mg,
0.26 mmol), [NiACTHUNTRGNE(UNG PPhMe2)4] (8 mg, 0.01 mmol), and tBu3SiH (80 mg,
0.40 mmol). C6D6 (0.5 mL) was added and the NMR tube was sealed.
After heating to 1108C for 4 h the reaction mixture was investigated by
NMR spectroscopy.
All manipulations were carried out under a nitrogen atmosphere by
using standard Schlenk techniques or in an argon-filled glovebox. NMR
spectra were recorded on Bruker DPX-250, Avance 300, Avance 400, or
Avance 500 spectrometers. Chemical shifts are referenced to (residual)
solvent signals (1H, 13C) or to external 85% H3PO4 (31P) and tetramethyl-
silane (29Si). [Ni
ACHTUNGTRENNUNG(cod)2], PPhMe2, Et3SiH, nPr3SiH, iPr3SiH, Ph3SiH,
Synthesis of 2-triethylsilylbiphenyl [2CAHTUNGTERN(NUGN Et3Si)]: Compound 1 (50 mg,
Me2PhSiH, and Et2SiH2 are commercially available and were used as re-
ceived. tBu3SiH was prepared according to the published procedure.[67–69]
For syntheses of biphenylene and its derivatives, see the Supporting In-
formation. HPLC was either performed with reversed phase (Reprosil-
Pur C18-AQ, 10 mm, 250ꢅ20 mm) or normal phase (Nucleosil 100 Si,
10 mm, 250ꢅ20 mm).
0.33 mmol) and [Ni(PPhMe2)4] (10 mg, 0.016 mmol) were charged in an
AHCTUNGTRENNUNG
NMR tube. Et3SiH (115 mg, 0.989 mmol) and C6D6 (1.5 mL) were added
and the tube was sealed. After 2 h at 1108C the 1H NMR spectrum
showed quantitative formation of 2ACHTNUGRTENUNG(Et3Si). The reaction mixture was
quenched with water and extracted into ethyl acetate (twice). The com-
bined organic layers were dried (Na2SO4) and filtered. All volatile sub-
stances were removed in vacuo. The crude product was purified by flash
column chromatography (silica gel/hexane). Yield: 93%. The NMR data
were in accordance with those found in the literature.[33] 29Si INEPT
NMR (59.6 MHz, C6D6): d=3.5 ppm.
Synthesis
Synthesis of [Ni
G
ACHTUGNRTEN(NUGN PPhMe2)4] was prepared by a modified
literature procedure.[46] [Ni
AHCTUNGTRENNUNG
(2.19 g, 15.9 mmol) were mixed in a Schlenk tube, whereupon the mix-
ture turned intensely red and warmed up. Et2O (15 mL) was added and
the resulting orange solution was stirred for 2 d at room temperature.
The solution was concentrated to about 2 mL and cooled to ꢀ788C.
Orange X-ray-quality crystals formed. The supernatant was removed by
cannula and the orange solid was dried to constant mass in vacuo. Yield:
Synthesis of 2-tripropylsilylbiphenyl [2
ACHTUNGRTEN(NUNG nPr3Si)]: An NMR tube was
charged with 1 (50 mg, 0.33 mmol), [Ni(PPhMe2)4] (12 mg, 0.020 mmol),
AHCTUNGTRENNUNG
nPr3SiH (55 mg, 0.35 mmol), and C6D6 (1.5 mL). The tube was sealed and
kept at 1108C. After 4 h the reaction mixture was quenched with water
and the organic compounds in the aqueous layer were extracted into
ethyl acetate. The combined organic layers were dried over Na2SO4 and
filtered. The solvent was removed in vacuo. Purification by flash column
1
1.964 g (84%). H NMR (300.0 MHz, C6D6): d=7.41–7.35 (m, 8H, ArH),
7.15–7.04 (m, 12H, ArH), 1.30 ppm (s, 24H, Me); 31P{1H} NMR
(121.5 MHz, C6D6): d=ꢀ9.7 ppm (cf. ref. [70]); 13C{1H} NMR (75.4 MHz,
C6D6): d=148.0 (m, ArC), 129.9 (m, ArC), 128.0 (m, ArC), 126.8 (s,
ArC), 22.2 ppm (m, Me).
(silica gel/hexane) yielded 2
1H NMR (400.1 MHz, CDCl3): d=7.56 (dd, 3J
1.6 Hz, 1H, ArH), 7.40–7.32 (m, 5H, ArH), 7.31–7.27 (m, 2H, ArH), 7.21
(dd, 3J(H,H)=7.2 Hz, 4J
(H,H) 1.6 Hz, 1H, ArH), 1.24–1.14 (m, 6H,
CH2), 0.85 (t, 3J
(H,H)=7.2 Hz, 9H, CH3), 0.47–0.43 ppm (m, 6H, CH2);
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
Reaction of biphenylene (1) with Et2SiH2 and [Pd
NMR tube was charged with 1 (40 mg, 0.26 mmol), Et2SiH2 (35 mg,
0.40 mmol), [Pd(dba)2] (dba=trans,trans-dibenzylideneacetone, 11 mg,
ACHTUNGRTEN(NNUG dba)2]/DavePhos: An
13C{1H} NMR (100.6 MHz, CDCl3): d=149.7, 144.7, 136.0, 135.8, 129.8,
129.4, 128.4, 127.7, 127.2, 126.2 (ArC), 18.6, 17.6, 16.3 ppm
(CH2CH2CH3); 29Si INEPT NMR (59.6 MHz, C6D6): d=ꢀ1.6 ppm; ele-
mental analysis calcd (%) for C21H30Si: C 81.22, H 9.74; found: C 81.01,
H 9.53.
ACHTUNGTRENNUNG
0.019 mmol), and DavePhos (10 mg, 0.025 mmol). C6D6 (0.5 mL) was
added and the tube was sealed. After heating to 1108C for 24 h the reac-
tion mixture was investigated by NMR spectroscopy.
Reaction of Et2SiH2 with [Ni
H
Synthesis of 2-dimethylphenylsilylbiphenyl [2
ACHTUNGTRENNUNG
with Et2SiH2 (20 mg, 0.23 mmol) and [Ni
G
1 (50 mg, 0.33 mmol), Me2PhSiH (45 mg, 0.33 mmol), and [NiACHTUNGTRENNUNG
Chem. Asian J. 2014, 9, 3163 – 3173
3170
ꢃ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Max C. Holthausen, Hans-Wolfram Lerner et al.
(12 mg, 0.020 mmol) were dissolved in C6D6 (1.5 mL). The NMR tube
was sealed and the mixture was heated to 1108C for 4 h. After quenching
with water and extraction of the product into ethyl acetate, the crude
H1), 7.45 (dvt, 3J
(H,H)=7 Hz, 4J(H,H)=1 Hz, 1H, H7), 7.24 (dm, 3J
(H,H)=7 Hz, 1H, H2), 2.76 (s, 3H, ArMe), 1.01–0.83 ppm
(H,H)=7.5 Hz, 4J
ACHUTGTNRENNUG CAHTUNGTRENNUNG
N
G
ACHTUNGTRENNUNG
3
H3), 7.18 (vt, J
ACHTUNGTRENNUNG
product was purified by column chromatography (silica gel/hexane) to
(m, 10H, CH2CH3); 13C{1H] NMR (100.6 MHz, CDCl3): d=150.6 (C5A),
147.0 (C4A), 139.0, 138.9 (C1A,8A), 134.4 (C3), 134.3 (C4), 133.3 (C8), 131.0
(C1), 130.0 (C6), 126.8 (C2), 126.4 (C7), 125.8 (C5), 24.5 (ArMe), 7.7,
4.0 ppm (CH2CH3); 29Si HMBC (400.1 MHz, 79.5 MHz, CDCl3): d=
4.2 ppm; elemental analysis calcd (%) for C17H20Si: C 80.89, H 7.99;
found: C 80.59, H 8.09.
1
afford 2
A
accordance those reported by Matsuda et al.[33] 29Si HMBC (250.1 MHz,
49.7 MHz, CDCl3): d=ꢀ8.0 ppm.
Synthesis of 2-triphenylsilylbiphenyl [2ACTHNUTRGEN(UNG Ph3Si)]: A sealable NMR tube
was charged with 1 (50 mg, 0.33 mmol), Ph3SiH (86 mg, 0.33 mmol), and
[Ni(PPhMe2)4] (12 mg, 0.020 mmol). After adding C6D6 (1.5 mL), the
ACHTUNGTRENNUNG
Synthesis of 2,7-di-tert-butyl-9,9-diethyl-9-silafluorene (8) and 3,6-di-tert-
butyl-9,9-diethyl-9-silafluorene (9): 2,7-Di-tert-butylbiphenylene (6;
350 mg, 1.32 mmol), Et2SiH2 (263 mg, 2.98 mmol), and [NiACHTUNTRGNEUNG(PPhMe2)4]
(53 mg, 0.087 mmol) in toluene (20 mL) were heated to 1108C for 5.5 h,
whereupon the color of the mixture changed from yellow to dark red
with a yellow hue. H2O (20 mL) was added, the two phases were separat-
ed, and the product in the aqueous layer was extracted into ethyl acetate
(3ꢅ30 mL). The combined organic layers were dried over MgSO4 and fil-
tered. All volatile substances were removed in vacuo. Flash column chro-
matography yielded a mixture of 8 and 9 (442 mg). A vial was charged
tube was sealed and kept at 1108C for 4 h. The reaction mixture was
quenched with water and the product in the aqueous layer was extracted
into ethyl acetate. The combined organic layers were dried over Na2SO4
and filtered. All volatile substances were removed in vacuo. Flash
column chromatography (silica gel/hexane) yielded
72%) as a colorless solid. 1H NMR (400.1 MHz, CD2Cl2): d=7.58 (ddd,
3J(H,H)=7.5 Hz, 4J(H,H)=1.5 Hz, 5J
(H,H)=0.6 Hz, 1H, ArH), 7.50
(dvt, 3J(H,H)=7.5 Hz, 4J
(H,H)=1.4 Hz, 1H, ArH), 7.44–7.41 (m, 6H,
2ACTHNGUETRN(NUG Ph3Si) (98 mg,
A
R
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ArH), 7.35–7.23 (m, 11H, ArH), 6.95–6.88 (m, 3H, ArH), 6.83–6.79 ppm
(m, 2H, ArH); 13C{1H} NMR (100.6 MHz, CD2Cl2): d=151.2, 143.7,
138.9, 136.5, 135.8, 132.8, 131.1, 130.0, 129.8, 129.4, 128.0, 127.6, 126.9,
126.5 ppm (ArC); 29Si HMBC (400.1 MHz, 79.5 MHz, CD2Cl2): d=
ꢀ15.2 ppm; elemental analysis calcd (%) for C30H24Si: C 87.33, H 5.86;
found: C 85.25, H 5.93.
with 310 mg of this product mixture, [RhACTHNUTRGNEUNG(PPh3)3Cl] (65 mg, 0.070 mmol),
3,3-dimethyl-1-butene (0.60 mL), and toluene (22 mL). The vial was
sealed and the mixture was heated to 1358C for 3 d, whereupon the color
of the mixture changed to black. After aqueous workup (20 mL) and ex-
traction of the H2O phase with toluene (3ꢅ20 mL), all volatile substances
were removed in vacuo to yield the products as a dark red oil. Column
chromatography (silica gel/hexane) yielded analytically pure 8 (Rf =0.36,
98 mg, 30%) and crude 9 (Rf =0.44, 82 mg, 25%). Analytically pure 9
Synthesis of 2-diethylsilylbiphenyl [2
(3): A Schlenk flask was charged with 1 (1.087 g, 7.142 mmol), Et2SiH2
(1.00 g, 11.3 mmol), and [Ni(PPhMe2)4] (0.22 g, 0.36 mmol). Toluene
ACHTUNGTERN(NUNG Et2HSi)] and 9,9-diethylsilafluorene
AHCTUNGTRENNUNG
was obtained by normal-phase HPLC (n-heptane, flow rate: 3 mLminꢀ1
,
(60 mL) was added and the resulting yellow solution was heated to reflux
for 4 h. Within 10 min after starting heating, the solution turned red.
After an additional hour the color of the mixture turned to black with
a yellow hue. After quenching of the mixture with water (40 mL), the
product in the aqueous layer was extracted into ethyl acetate (3ꢅ40 mL).
The combined organic phases were dried over MgSO4 and filtered. All
volatile substances were removed in vacuo to give the crude product as
retention time: 20.5 min). Analytical data of 8: 1H NMR (300.0 MHz,
3
4
CDCl3): d=7.70 (d, J
(H,H)=8.1 Hz, 2H, ArH), 7.60 (d, J
G
3
4
2H, ArH), 7.43 (dd, J
(H,H)=8.1 Hz, J
U
18H, tBu), 1.06–1.01 (m, 6H, CH3), 0.97–0.89 ppm (m, 4H, CH2); 13C{1H}
NMR (75.4 MHz, CDCl3): d=149.5, 146.0, 137.2, 130.0, 127.2, 120.2
(ArC), 34.8, 31.6 (ArtBu), 7.9, 4.1 ppm (CH2CH3); 29Si INEPT
(59.6 MHz, CDCl3): d=4.8 ppm; elemental analysis calcd (%) for
C24H34Si: C 82.22, H 9.77; found: C 82.46, H 9.89. Analytical data of 9:
a
yellow oil. Column chromatography (silica gel/hexane) yielded 2-
ACHTUNGTRENNUNG(Et2HSi) as a colorless oil (Rf =0.66, 634 mg, 36.9%) and 3 as a colorless
1H NMR (500.2 MHz, CDCl3): d=7.87 (d, 4J
7.54 (d, 3J(H,H)=7.5 Hz, 2H, ArH), 7.29 (dd, 3J
(H,H)=1.6 Hz, 2H, ArH), 1.40 (s, 18H, tBu), 1.03–1.00 (m, 6H, CH3),
ACHTUNGTRENNUNG
solid (Rf =0.57, 639 mg, 37.5%). X-ray-quality crystals of 3 were grown
from a concentrated hexane solution at ꢀ408C. The 1H and 13C{1H}
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
NMR of 2ACHTUNGTRENNUNG(Et2HSi) and 3 are in accord with the data reported by Takai
0.92–0.87 ppm (m, 4H, CH2); 13C{1H} NMR (125.8 MHz, CDCl3): d=
153.1, 148.8, 134.6, 133.1, 124.4, 117.5 (ArC), 35.1, 31.5 (ArtBu), 7.8,
4.0 ppm (CH2CH3); 29Si{1H} NMR (99.4 MHz, CDCl3): d=3.9 ppm;
HRMS (MALDI): m/z calcd for C24H34Si+: 350.2430 [M+]; found:
350.2422.
et al.[21] 29Si INEPT (59.6 MHz, C6D6): d=ꢀ4.8 [2
ACHTUNGTREN(NUGN Et2HSi)], 5.5 ppm (3).
Optimized synthesis of 9,9-diethylsilafluorene (3): Compound 1 (393 mg,
2.58 mmol), [NiACHTUNGTRENNUNG(PPhMe2)4] (80 mg, 0.13 mmol), and Et2SiH2 (0.38 g,
4.3 mmol) in toluene (25 mL) were heated to reflux for 4 h. The color of
the solution turned from yellow to black with a yellow hue. H2O (20 mL)
was added and the two layers were separated. The product in the aque-
ous layer was extracted into ethyl acetate (3ꢅ20 mL). The combined or-
ganic layers were dried over MgSO4 and filtered. All volatile substances
were removed in vacuo to give crude 3 as a yellow oily residue (681 mg).
Flash column chromatography (silica gel/hexane) yielded a colorless oil
Reaction of 1-methylbiphenylene (4) with triethylsilane:
NMR tube was charged with 4 (30 mg, 0.18 mmol), Et3SiH (32 mg,
0.28 mmol), and [Ni(PPhMe2)4] (5 mg, 0.008 mmol). C6D6 (0.5 mL) was
A sealable
AHCTUNGTRENNUNG
added and the NMR tube was sealed. After heating to 1208C for 2 h, the
orange-yellow solution was treated with water. The product in the aque-
ous layer was extracted into ethyl acetate (twice). All volatile substances
were removed in vacuo to yield 58 mg of a yellow oil. Reversed-phase
HPLC (MeOH, flow rate: 3 mLminꢀ1, retention time: 38 min) yielded an
mixture of isomeric products 2-triethylsilyl-6-methylbiphenyl (10) and 2-
triethylsilyl-2’-methylbiphenyl (11) (40 mg, 78%). 1H NMR (300.0 MHz,
CDCl3): d=7.61–7.58 (m, 1H, ArH, 10 or 11), 7.43–7.28 (m, 7H, ArH, 10
and 11), 7.24–7.09 (m, 8H, ArH, 10 and 11), 2.07 (s, 3H, ArMe, 10 or
11), 1.99 (s, 3H, ArMe 10 or 11), 0.83–0.77 (m, 18H, CH2CH3, 10 and
11), 0.47–0.34 ppm (m, 12H, CH2CH3 10 and 11); 13C{1H} NMR
(75.4 MHz, CDCl3): d=148.8, 148.7, 143.8, 143.0, 136.2, 135.9, 135.8,
135.7, 135.6, 133.3, 130.6, 130.1, 129.9, 129.8, 129.7, 128.3, 127.9, 127.5,
127.1, 126.6, 126.0, 124.9 (ArC, 10 and 11), 21.2, 20.6 (ArMe, 10 and 11),
7.7, 7.6, 4.1, 3.8 ppm (CH2CH3, 10 and 11); 29Si INEPT (59.6 MHz,
CDCl3): d=3.6, 3.4 ppm (10 and 11); elemental analysis calcd (%) for
C19H26Si: C 80.78, H 9.28; found: C 80.39, H 9.11. The 1H and 13C NMR
spectra of 10 and 11 are similar to those reported for the trimethylsilyl
analogues.[71, 72]
(458 mg). This oil was dissolved in toluene (30 mL), and [RhACTHUNTRGNE(UNG PPh3)3Cl]
(83 mg, 0.09 mmol) and 3,3-dimethyl-1-butene (1.3 mL, 10 mmol) were
added. After heating this mixture to reflux for 1 d, H2O (20 mL) was
added and the two layers were separated. The organic compounds in the
aqueous layer were extracted into toluene (2ꢅ30 mL). Drying the com-
bined organic layers over MgSO4, removal of all volatile substances in
vacuo, and flash column chromatography (silica gel/hexane) yielded 3
(300 mg, 49%).
Synthesis of 4-methyl-9,9-diethyl-9-silafluorene (7): 1-Methylbiphenylene
(4; 30 mg, 0.18 mmol), Et2SiH2 (24 mg, 0.27 mmol), and [NiACHTUNTRGNEUNG(PPhMe2)4]
(5 mg, 0.008 mmol) in C6D6 (0.5 mL) were heated to 1208C for 2 h in
a sealed vial. The reaction mixture was quenched with water and the
product was extracted into ethyl acetate (three times). Drying over
MgSO4, filtering, and removing all volatile substances in vacuo gave
crude 7 as a yellow oil. Reversed-phase HPLC (MeOH, flow rate:
3 mLminꢀ1, retention time: 33.3 min) yielded 7 as a colorless oil (22 mg,
48%). 1H NMR (400.1 MHz, CDCl3): d=8.07 (d, 3J
ACHTUNGTRENNUNG
H5), 7.66 (ddd, 3J
H8), 7.49 (ddd, 3J
N
G
N
Single-crystal X-ray diffraction: The data for 3 were measured on
a STOE IPDS-II diffractometer equipped with a sealed tube with MoKa
Chem. Asian J. 2014, 9, 3163 – 3173
3171 ꢃ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Max C. Holthausen, Hans-Wolfram Lerner et al.
radiation. An empirical absorption correction with the program
PLATON[73] was performed. The data for remaining structures ([Ni-
ACHTUNGTRENNUNG(PPhMe2)4], 8, and 9) were measured on a STOE IPDS-II diffractometer
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using a Genix Microfocus X-ray source with mirror optics and MoKa radi-
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by direct methods by using the program SHELXS and refined against F2
with full-matrix least-squares techniques by using the program
SHELXL.[75] The crystal of 3 was a nonmerohedral twin with a fractional
contribution of 0.228(5) for the minor domain. The absolute structure
could not be determined reliably, Flack x parameter: ꢀ0.3(2). In 8, both
tert-butyl groups are disordered over two sites with a site occupation
factor of 0.582(16) and 0.642(7) for the major occupied site, respectively.
The disordered atoms were refined isotropically. CCDC 1006499 (3),
1006496 (5), 1006495 (6), 1006503 (8), 1006502 (9), 1006501(12), 1006497
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Acknowledgements
bulky groups such as isopropyl. Furthermore, for the synthesis of un-
ꢀ
symmetrically substituted silafluorenes the Pd-catalyzed C C cou-
This work was supported by the Beilstein-Institut, Frankfurt/Main (Ger-
many), within the research collaboration NanoBiC (projects eNet and
pOligo). Quantum-chemical calculations were performed at the Center
for Scientific Computing (CSC) Frankfurt on the Fuchs and LOEWE-
CSC high-performance computer clusters.
pling reactions between C(Tf) and C(H) of diaryl silanes ArAr’SiR2
have poor selectivity when Ar’ bears an additional substituent in the
3-position (e.g., OMe or F). The trityl-cation-induced silafluorene
synthesis yielding an unsymmetrically 2,7-substituted silafluorene
(reaction (ii) in Scheme 1) has so far only been described for one ex-
ample in the literature. An Rh-catalyzed coupling of the 2’-methyl-
2-silylbiphenyl derivative leads to a product mixture. The major
product of this reaction is a silaphenanthrene derivative, which is
formed by dehydrogenative coupling of the hydrosilyl moiety with
the methyl group. Route (iii) is limited to substitution patterns in
the 2,3-positions. In the end, the reaction of type (iv) (Scheme 1)
runs into selectivity problems if substituents are introduced in inap-
propriate positions of biphenyls.
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