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amount of pure 1a and 2a (R = C16H33) were isolated, the
majority of the product was obtained as a mixture (90%
combined yield). An accurate assessment of the relative
product ratios was difficult as a result of their very similar
polarities.
We found that an electron-withdrawing substituent (such
as a tert-butyl ester group) on the monomer significantly
slowed down the metathesis reaction. Although monomer 7a
substituted with alkyl group successfully formed 1a and 2a
under catalysis by 8, tBoc-protected monomer 7b only yielded
oligomers with a broad molecular weight distribution under
the same conditions. The predominant formation of products
1b and 2b was achieved using the triphenolsilane-based
complex 9 (Scheme 1) which had a slightly higher catalytic
activity than 8.[26] These results suggest that the success of
dynamic covalent assembly is predicated on the ability of
catalysts to rapidly reach the point of equilibrium.
The assignment of complexes 1a/1b consisting of two
monomer units was straightforward. Based on their simple
1H NMR spectra (see the Supporting Information, Figures S4
and S5), which indicate high structural symmetry, the com-
pounds were assigned as the C3-symmetric dimers 1a/1b. In
contrast to the sharp resonance signals of protons on dimers
1
1a/1b, the H NMR spectra of the species containing four
monomer units are quite broad and complicated (Figure S8).
Similar line broadening and signal complication were
observed previously by Cooper et al. when a monomeric
imine-linked cage was converted into triply interlocked
complexes.[14] Generally, the interlocked complexes lose the
original symmetry of the monomeric cage upon catenation,
leading to the complication of their proton resonance signals.
Based on the literature precedent and careful inspection, we
assigned 2a/2b as the interlocked dimer complexes. Con-
formational heterogeneity was then expected for such
mechanically connected complexes. Indeed, at low temper-
ature, we detected much sharper and well resolved signals in
À ꢀ À
Scheme 1. Synthesis of interlocked complexes: a) PPT C C H, [Pd-
(PPh3)2Cl2], CuI, THF, piperidine, RT; b) Pinacolborane, [Pd(PPh3)2Cl2],
triethylamine (TEA), toluene, reflux; c) 5a, [Pd(PPh3)4], Na2CO3, tolu-
ene–EtOH–H2O (v/v/v, 25:3:3), 908C; d) 4b, [Pd(PPh3)4], Na2CO3,
toluene–EtOH–H2O (v/v/v, 25:3:3), 908C.
Methyl-substituted alkynes are generally preferred substrates
because they are synthetically facile to use and the 2-butyne
byproduct can be easily removed under vacuum or using
molecular sieves as scavengers.[25–28] However, in our study, we
used benzoylbiphenyl-substituted alkynes 7a/7b as the sub-
strates to drive the equilibrium by precipitation of bis-
1
the H NMR spectra of 2a/2b, whereas the coalescence of
multiple peaks was detected at higher temperature (Fig-
ure S9). This indicates that the conformational exchange
occurs within the NMR timescale and causes the broadening
and complexity of NMR resonance signals. The hydrody-
namic radii of 2b and 1b (r1b/r2b = 0.72), estimated through
a diffusion-ordered NMR spectroscopic experiment (DOSY),
also agree well with the expected size difference between
a dimer and an interlocked complex. Gel permeation
chromatography (GPC) traces of 1b and 2b show a sharp
single peak with retention volumes of 16.25 mL and 16.55 mL,
respectively, indicating a larger size of 2b compared to the
cage 1b (Figure 2a). Additionally, the possible presence of
higher-order interlocked complexes containing three or four
molecules of the dimer 1b in the MALDI-TOF mass spectra
of the crude product mixture (Figure 2b) also supports the
formation of interlocked dimers.
[29,30]
À ꢀ À
(benzoylbiphenyl)acetylene (PPT C C PPT).
We had
difficulty in separating tris-propyne-substituted monomers
from di-, or mono-substituted products. The installation of
benzoylbiphenyl substituents facilitates the purification of the
monomers by increasing their polarity.[31]
Next, we explored the covalent assembly of the monomer
7a/7b through reversible alkyne metathesis. The metathesis
of 7a/7b was performed in CCl4/CHCl3 at slightly elevated
temperature (40–558C). A high-activity multidentate molyb-
denum carbyne complex (8 or 9, Scheme 1) was used as the
catalyst.[26,32] Upon the addition of the catalysts, precipitation
of the metathesis byproduct bis(benzoylbiphenyl)acetylene
occurs. After stirring overnight, the precipitates were filtered
off and the crude product left in solution was analyzed by
MALDI-TOF mass spectrometry. We were intrigued to
detect mass signals corresponding to two, four, six, and
eight monomer units, with the species containing four
monomer units (2a/2b) being predominant. We were able
to isolate the relatively major products 1b (6%) and 2b
(59%), where R = tert-butoxycarbonyl (tBoc), in pure form
using flash column chromatography. Although a small
Our structure assignment was further substantiated by the
subsequent decatenation experiments in which the inter-
locked complexes are broken down to release independent
dimer cages. Upon cleavage of alkyne bonds in 2b through
ruthenium- or iron-catalyzed oxidation (RuO2/oxone or
FeCl3/H2O2), we detected the appearance of signals corre-
2
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Angew. Chem. Int. Ed. 2015, 54, 1 – 6
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