Organometallics
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stereospecific addition is in contrast to the reaction between
cyclopropanes and diborane19 but resembles the well-studied
hydroboration of alkenes and alkynes.24
and the element at the β position.27 It is likely that the release
of ring strain in TS (the C1−C3−C2 angle increases from 60°
in cyclopropane to 86° in TS) substantially stabilizes the
transition state and does not require an additional interaction
between B1 and C2, thus avoiding a six-coordinated carbon
center in the transition state. Natural bond orbital (NBO)
analysis of TS shows that 0.67 e− charge transfers from the σ-
bonding orbital of C1−C2 to the empty p orbital of B1 and
0.29 e− charge transfers from the occupied σ-bonding orbital of
B1−H1 to the σ*-antibonding orbital of C1−C2 (Figure S39).
This result indicates that the electrophilicity of the borenium
center plays a paramount role in this hydroboration reaction,
consistent with our observation that the weaker Lewis acid
HB(C6F5)2 does not react with cyclopropane. It is noteworthy
that similar unequal electron charge transfers were also
observed in the transition state of the reaction between
alkylborane and H2 via σ-bond metathesis.13b Further NBO
analysis revealed that in TS the (C1)H2 and (C3)H2 fragments
are almost neutral with charges of −0.08 and +0.05 e,
respectively. On the other hand, the (C2)H2 fragment bears a
considerable positive charge of +0.51 e. This is likely the cause
of the observed regioselectivity of this hydroboration, as
substituents on the C2 atom can help to delocalize the positive
charge, thus lowering the energy of the transition state and
rendering the transfer of hydrogen to the most substituted
carbon favorable. Analogous arguments were also applied in
the explanation of anti-Markovnikov hydroboration of
alkenes.28 Indeed, the formation of [IMe4BCH(BrC6H4)-
(Me)]+, the isomer of 2b, from 1 and 1-bromo-4-cyclo-
propylbenzene needs to overcome a free energy barrier of 37.0
kcal mol−1. On the other hand, the free energy barrier for the
formation of 2b is found to be only 18.7 kcal mol−1 (Figure
S40). Interestingly, after this transition state, we identified an
intermediate, which is 15.7 kcal mol−1 above free 1 and 1-
bromo-4-cyclopropylbenzene. Given that the free energy
barrier from this intermediate to 2b is only 2.1 kcal mol−1,
the existence of this intermediate is likely experimentally
irrelevant.
Intrigued by the concerted activation of C−C bonds with
the hydroborenium complex 1, we investigated the mechanism
of hydroboration of cyclopropane with 1 by density functional
theory (DFT (M06-2X)) calculations.25,26 Our studies suggest
that the thermodynamic driving force for this reaction is
substantial with ΔG° = −27.7 kcal mol−1 at 298 K. The
hydroboration takes place via a four-centered transition state
without prior coordination of cyclopropane with 1. The
calculated free energy barrier is 27.4 kcal mol−1, consistent
with our observation that this reaction takes place under
ambient conditions. In the transition state (TS; Figure 2),
After the establishment of direct hydroboration of cyclo-
propanes with 1, we set out to examine the reactivity of the
resulting alkylboreniums against H2. 2b was chosen as the
substrate. While 2b shows no reactivity at room temperature
against H2 (4 bar) in C6D5Br, under harsher conditions (80
°C, 80 bar of H2) 2b can be quantitatively converted to 1-
bromo-4-n-propylbenzene after 12 h. However, the formed 1
appeared to be unstable under such conditions and
decomposed to several unidentified species. A controlled
experiment carried out with 1 afforded the same decom-
position products after heating its C6D5Br solution to 80 °C
for 12 h. Although the decomposition of 1 at elevated
temperature seems to suggest the unsuitability of 1 as a
hydrogenolysis catalyst, we found that alkylborenium 2b is very
stable in C6D5Br and shows no decomposition after 12 h at 80
°C. Therefore, the 1 formed upon hydrogenolysis of
alkylboreniums can immediately react with cyclopropanes to
regenerate the more stable alkylboreniums, which will be the
resting state of the catalyst. Indeed, with 15 mol % of 1 as the
catalyst, 1-bromo-4-cyclopropylbenzene can be converted to 1-
bromo-4-n-propylbenzene in 82% NMR yield with C6D5Br as
solvent at 80 °C under 80 bar of H2. Since the rather high
boiling point of bromobenzene rendered the isolation of pure
1-bromo-4-n-propylbenzene difficult, we switched the solvent
to the more volatile benzene for preparative-scale reactions and
Figure 2. Geometry of TS with WBI values. Hydrogen atoms except
H1 and those in cyclopropane are omitted for clarity.
while the B1 and H1 atoms remain closely bonded with a
distance of 1.21 Å, the C1−C2 bond is substantially elongated
from 1.51 Å in cyclopropane to 2.06 Å. Meanwhile, the C1
atom is bonded to the B1 atom with a distance of 1.67 Å, and a
weak bonding interaction between C2 and H1 (1.828 Å) is
also observed. Accordingly, the Wiberg bond indices (WBI)
revealed fractional C1−C2 (0.25), B1−H1 (0.79), B1−C1
(0.81), and C2−H1 (0.13) interactions. Although the
geometry of TS resembles a typical four-membered σ-bond
metathesis transition state, the lack of interaction between B1
and C2 atoms (2.46 Å, WBI = 0.01) is in sharp contrast to σ-
bond metathesis transition states of H−E bond activation,
which have a strong interaction between the central element
C
Organometallics XXXX, XXX, XXX−XXX