Angewandte
Research Articles
Chemie
in polymers because the acceptor orbital at the boron would
To synthesize the new targeted bora[1]ferrocenophanes
2a–c (Figure 2), the improved conditions of the recent study
almost exclusively be involved in p-bonding with nitrogen.
To put these speculations to the test, we targeted bora-
[1]ferrocenophanes with aryl or alkyl substituents at the
boron. It was expected that these targeted FCPs, compared to
the known compounds B and C, will exhibit an increased
electrophilicity at the boron, which should increase their
reactivity, in particular with respect to ROP. In this work, we
report the first examples of boron-bridged [1]FCPs with aryl
and alkyl substituted three-coordinated boron centers. It will
be shown that these new monomers can lead to helical
metallopolymers.
1
2
of 2R R were first applied (Scheme 1).[21,22] Accordingly, the
planar-chiral dibromoferrocene derivative 1MeMe was lithiated
(following Scheme 1), after 30 min, the ice bath was replaced
by a preheated oil bath of 508C, and after 10 min, the
respective RBCl2 solution (0.1m in hexanes) was added
dropwise. However, the obtained results were different for
1
each of the three boron dichlorides RBCl2. H NMR spec-
troscopy from aliquots of the reaction mixtures revealed that
2a (R = Mes)[23] was not formed, but the targeted [1]FCP 2b
(R = Tip)[23] was produced under the same conditions. On the
contrary, the salt-metathesis reaction with TsiBCl2[23] resulted
in the formation of 2c along with its 1,1’-bis(boryl)ferrocene
(3c) counterpart. When the same reaction was carried out at
a lower temperature (08C), the unwanted 3c was obtained
almost exclusively. The latter result was expected since it
matched with our previously investigated salt-metathesis
reactions mentioned above (Scheme 1). In contrast, when
TipBCl2 was applied, lowering the temperature from 50 to
08C increased the conversion towards the targeted [1]FCP
2b; surprisingly, no significant change was observed using the
same temperature change in the attempted synthesis of the
Mes-substituted species 2a.
Results and Discussion
In our recent report on the new family of bora-
[1]ferrocenophanes, we thoroughly investigated the outcome
and mechanism of the salt-metathesis reaction that is
illustarted in Scheme 1.[21,22] The starting dibromoferrocene
The latter results clearly indicate that the formation of the
Tip-substituted [1]FCP 2b is more favored under slower
kinetics. Moreover, to our surprise, over the course of 1–
2 days the amount of 2b in an NMR sample that was taken
from an aliquot of a reaction mixture increased. This must
have resulted from a slow reaction between leftover starting
materials in the NMR solvent C6D6. This triggered our
motivation to change the reaction conditions in the prepara-
tion of 2b. After the lithiation of 1MeMe in a solvent mixture of
Et2O/hexanes (1:9) was completed, the solvent was replaced
by benzene, resulting in an orange suspension (method A,
Scheme 2). After the addition of a benzene solution of
TipBCl2 and stirring over a period of 24 h, the color of the
reaction mixture changed from orange to red-brown to dark
Scheme 1. Recently investigated preparation of chiral boron-bridged
[1]FCPs.
1
2
derivatives 1R R were applied as enantiopure species with
(Sp,Sp) planar chirality so that the resulting [1]FCPs were
obtained as chiral compounds.[21,22] We discovered that the
1
2
1,1’-bis(boryl)ferrocene species 3R R (Scheme 1) is the main
1
2
byproduct in the synthesis of the strained [1]FCPs 2R R . Due
to an understanding of the reaction mechanism, reaction
conditions could be improved in order to increase the
conversion towards the targeted strained compounds
1
purple. As shown in Figure 3, the H NMR spectrum of the
reaction mixture revealed an almost quantitative conversion
towards the targeted bora[1]ferrocenophane 2b.
1
2
2R R
.
Interestingly, the same method, when applied to MesBCl2,
gave a similarly clean reaction mixture with 2a as the main
product. In this case, the dark purple color developed much
faster than in the case of the Tip-substituted [1]FCP 2b, and
the reaction was already completed after 1 h. Applying the
same conditions (method A) to TsiBCl2 did not result in a high
conversion to the strained [1]FCP 2c. Instead, even after 48 h,
significant amounts of unreacted TsiBCl2 were present. Given
the fact that 2c could be synthesized more successfully using
the reaction conditions developed for amino-substituted
[21,22]
Figure 2 shows the targeted new bora[1]ferrocenophanes
2a–c. We selected bulky substituents at the bridging boron
atom because such groups often facilitate the formation of the
strained compounds in the commonly applied salt-metathesis
reaction. Additionally, the application of ortho-substituted
benzene derivatives, in particular in the form of mesityl
groups,[23] is widespread in conjugated boron-containing
materials.[24]
1
2
bora[1]ferrocenophanes 2R R (Scheme 1), we further
tweaked the conditions by increasing the reaction temper-
ature (50 to 608C) and reducing the speed of addition of
1
TsiBCl2 (10 to 20 min; method B, in Scheme 2). A H NMR
spectrum of the reaction mixture showed a significant im-
provement in the conversion towards the targeted strained
compound 2c.
Figure 2. Sterically stabilized bora[1]ferrocenophanes (this work).
Angew. Chem. Int. Ed. 2019, 58, 2 – 10
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