Organic Letters
Letter
Single crystals of 4d were fortuitously obtained after one of
the column chromatography fractions (eluted with EtOAc/
CH Cl solvent mixture) was left to stand at room temperature
overnight. Compound 4d crystallizes in I4 space group with
̅
two molecules per unit cell. Its molecular structure is shown in
Our observation of the solvent-filled channels in the crystal
structure of 5b and 4d prompted us to examine the porosity of
isolated cyclotetrabenzoin esters through gas sorption experi-
ments. Disappointingly, miniscule nitrogen sorption suggested
2
2
11
that the samples activated at 60 °C for 14 h are not porous.
The presence of alkyne moieties in the cyclotetrabenzoins 4c
12
and 5c opens many opportunities for further modifications.
In this work, we attempted one of them: hexacarbonyl dicobalt
complexation of triple bonds in 4c/5c. The reaction of the
mixture of 4c and 5c with Co (CO) in CH Cl smoothly
2
8
2
2
proceeded to give complex 6, which was isolated as a red solid
in 54% yield (Scheme 2, only 4c is shown for simplicity). In
4
c by Complexation with Co (CO) Groups
2 6
comparison with 4c/5c, compound 6 shows significant visible
light absorption between 300 and 400 nm with the maximum
at 310 nm and an additional band at 260 nm. IR spectra of 6
−
1
shows the appearance bands at 2092, 2053, and 2003 cm
13
related to the cobalt carbonyls, and the disappearance of the
−
1
low-intensity 2220 cm band, associated with the CC
vibration in the starting materials. High resolution electrospray
ionization mass spectrometry (HR-ESI MS) provided strong
evidence in determining the composition of 6 as
Figure 2. X-ray crystal structure of 4d (A) and its packing diagram
(
B), viewed along the crystallographic c axis. Element colors: C, gray;
C H Co O . HR-ESI MS spectra in negative mode showed
a peak at m/z = 2375.565, which was assigned to the [M + I]
H, white; O, red. Solvent molecules were removed for clarity.
96 48
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36
−
adduct, with iodine stemming from the added CsI. Even more
−
stereogenic centers. The overall shape of 4d, defined by the
four corners represented by benzoin CHOAc carbon atoms, is
that of a puckered square (more symmetric than that observed
for 5b) with angles of 86.7° and sides of 8.63 Å. At the same
time, naphthalene “walls” are very much distorted from a
parallel arrangement: those on the opposite sides of the
molecule form an angle of 52.2° with each other, while those
on the neighboring sides stand at an angle of 78.9°. The crystal
packing diagram of 4d is shown in Figure 2B and reveals two
kinds of one-dimensional channels running in parallel when
viewed along the crystallographic c axis: a larger one with
approximate dimensions of 8.4 × 7.3 Å and a smaller one,
measuring ∼6.5 × 5.4 Å. Close contacts established between
molecules of 4d include [C−H···O] hydrogen bonds between
the ester carbonyl oxygen and the benzoin hydrogen (2.69 Å)
as well as the α-hydrogen on the naphthalene nucleus (2.61 Å).
Despite extensive experimentation, we were unable to
produce crystals of 4c, 5c, or 5d suitable for X-ray diffraction.
diagnostic was a series of fragment peaks [M + I − 28n] ,
where n indicates the number of lost CO molecules. We have
observed the sequential loss of all CO molecules, i.e. up to n =
1
24. In the H NMR spectra, strong downfield shift of aromatic
1
3c
signals is observed, together with expected peak broadening
due to the presence of the metal. Signals at ∼199 ppm in the
1
3
C NMR spectra additionally confirm the presence of CO
groups. Unfortunately, our attempts to obtain single crystals of
6 were unsuccessful.
In conclusion, the work presented in this contribution
advances the chemistry of cyclobenzoins in three significant
ways. We have (a) shown that these macrocycles can be
prepared using environmentally friendly NHC catalysts, (b)
expanded the family of cyclotetrabenzoins with larger
members, and (c) postsynthetically modified functional groups
within the cyclobenzoin skeletons. The roughly square-shaped
cavities of 4d and 5b are about 60 and 125% greater in volume,
respectively, that those of 2a/4a. We presume that they will be
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Org. Lett. 2021, 23, 781−785