Angewandte
Research Articles
Chemie
[
39]
same counteranion effect was preserved across the initiator
series. For example, we observed lower trans alkene content
when 2e was used with 3c versus 3a (Table 1, entry 14 cf. 12).
Temperature effects were initially less straightforward to
interpret than those of counteranions or solvation. Moreover,
our survey revealed that the influence of reaction temper-
ature was codependent upon other reaction parameters,
which was better understood through DFT calculations (vida
infra). In general, stereoselectivities increased with decreas-
ing temperature, although the magnitudes of the changes
were dependent upon pyrylium counteranion and solvent. For
example, use of 3c in dichloromethane at 22 and À768C gave
nearly identical trans/cis alkene ratios (Table 1, entries 3 and
modynamically controlled reaction. Although, it is possible
that the trans/cis ratio in our system shows better linearity
versus Charton parameter than the plot using log(trans/cis)
because of an imperfect correlation of Charton parameters
with the enol ethersꢀ steric bulk, we speculate that the stereo-
determining step in MF-ROMP is under thermodynamic
control. Notably, the value of y increased with decreasing
reaction temperature, indicating a better selectivity at low
temperature, as expected (Figure 1c and Figure S2). Addi-
tionally, the trans/cis ratio was constant throughout the
polymerization (Figure S3), which is consistent with both
a thermodynamically controlled reaction where the last
alkene can isomerize, as well as, a kinetically controlled
process.
4
), whereas changing to toluene as solvent (entries 5 and 6)
gave trans/cis ratios of 55:45 and 35:65 at 22 and À768C,
respectively (see summary in Figure 1b). When 3a was used
as catalyst in combination with 2e as initiator, the trans/cis
alkene ratio changed from 92:8 to > 98:2 when the temper-
ature was changed from 25 to À768C (entries 12 and 13).
Collectively, we learned that trans selectivity is favored by
using large initiators, small counteranions, dichloromethane
as solvent, and low temperatures, whereas cis selectivity is
aided by small initiators, large counteranions, toluene as
solvent, and low temperatures. Notably, through judicious
choice of simple reaction parameters, we were able to
modulate between 75% cis alkene (Table 1, entry 7) to
The interesting stereoselectivity warranted a deeper un-
derstanding of the mechanism of MF-ROMP. In contrast with
our previously proposed cyclobutane radical cationic inter-
mediate, a revised stepwise mechanism is proposed here
[
26]
(Figure 2).
In the revised mechanistic hypothesis, one
electron oxidation of the enol ether generates the enol ether
radical cation, which subsequently reacts with norbornene to
form intermediate i1. Radical rearrangement in i1 through
a four-coordinate transition state (TS1) produces intermedi-
ate i2. Finally, ring opening of the bicycle via TS2 results in the
formation of i3, setting the stereochemistry of the backbone
olefin. The proposed reaction paths from each diastereomer
of i1 to i3, resulting in either trans or cis products, are shown in
Figure S5.
>
98% trans alkene (Table 1, entry 13) content in the polymer
backbone, with the former showing amorphous structure
while the latter exhibits semi-crystalline behavior as deter-
mined by differential scanning calorimetry (Figure 1d).
Highly trans PNB showing semi-crystalline behavior is rarely
To understand which step determines the stereoselectivity,
quantum chemical calculations were performed at the unre-
[
40]
[41]
stricted B3LYP level of theory and the 6-31G* basis set,
[
9]
[41,42]
reported. A 2007 study indicated that PNB samples with cis
content from 75–95% showed melting transitions, however
Mn and tacticity (i.e., stereochemistry of the cycloalkane
using the Growing String Method (GSM)
to simulate the
reaction pathways for isomerization of i2 and ring opening of
i3. See the Supporting Information for full computational
details. In our calculations, we used an isopropyl group to
represent a simplified model for the polymer. The SMD
solvation model was employed using dichloromethane and
[
10]
units) were not given.
We note that contributions to
physicochemical properties from tacticity were not deter-
mined in this study.
[
44]
We next focused on a deeper understanding of the
mechanistic origins for the observed stereoselectivities. Lin-
ear free energy relationships (LFERs) are a powerful tool to
investigate mechanistic influences of reactant substituents.
With initiators 2a–2e at hand, we investigated LFERs using
Charton steric parameters as an approximation of the steric
bulk of the initiator R group. Similar analyses have been used
to quantify substituentsꢀ steric effects in enantio-
toluene as solvents.
Counteranion binding around the
active chain end was neglected at this stage. The simulated
activation barriers for the forward and reverse reactions from
i2 to i3, as well as the isomerization between all four
diastereomers of i2, were each found to be less than
À1
13.1 kcalmol in dichloromethane (Figure S6) and 14.5 kcal
À1
mol in toluene (Figure S7). These calculations suggest that
selective catalytic reactions as well as in a dy-
[38,39]
namic covalent reaction.
LFERs are gener-
ated by plotting the logarithm of the ratio of
a reaction constant (e.g., rate or equilibrium
constant) versus the parameter value. Using our
data collected at 258C, the plot of log(trans/cis) =
yn, where y is the slope of a linear fit and n is
2
a Charton value, gave an R value of 0.88
(
Figure 1c). In comparison, the plot of trans/cis
2
vs. n gave a linear fit having R = 0.99 (Fig-
ure S2). Anslyn and co-workers reported a sim-
ilar observation, greater linearity from the non-
logarithmic plot, when correlating the diastereo-
meric ratio (dr) to steric parameters for a ther-
Figure 2. Proposed mechanism of MF-ROMP, electron transfer and back electron
transfer with i3 are omitted; (inset) four diastereomers of i2 and their pro-stereo-
isomerism.
Angew. Chem. Int. Ed. 2021, 60, 2 – 9
ꢀ 2021 Wiley-VCH GmbH
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