Core-modified hexaphyrin: Synthesis and characterization of 31,34-disilahexaphyrinoid

Article abstract of DOI:10.1039/c2ob07134g

Condensation of 16-silatripyrrane with pentafluorobenzaldehyde under catalytic conditions followed by DDQ oxidation leads to 31,34- disilahexaphyrinoid - a four times reduced derivative of 31,34-disilahexaphyrin which contains two built-in silole units fl

Full text of DOI:10.1039/c2ob07134g

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PAPER
Core-modified hexaphyrin: synthesis and characterization of
31,34-disilahexaphyrinoid†
Janusz Skonieczny, Lechosław Latos-Grażyński* and Ludmiła Szterenberg
Received 19th December 2011, Accepted 29th February 2012
DOI: 10.1039/c2ob07134g
Condensation of 16-silatripyrrane with pentafluorobenzaldehyde under catalytic conditions followed by
DDQ oxidation leads to 31,34-disilahexaphyrinoid – a four times reduced derivative of 31,34-
disilahexaphyrin which contains two built-in silole units flanked by four tetrahedrally hybridized meso
carbons. In the preferred folded macrocyclic conformation the silole rings remain perpendicular to each
other. The steric hindrance of bulky substituents at silicon atoms and β-positions of siloles prevented
aromatization. Only one meso diastereomer (5S, 15S, 20R, 30R) has been isolated and subsequently
identified by 1D and 2D NMR techniques. The density functional theory (DFT) has been applied to
model the molecular structure of 31,34-disilahexaphyrinoid consistent with constraints imposed by NOE
experiments. The total energies calculated at the B3LYP/6-31G**//B3LYP/6-31G** level for four feasible
meso diastereomers clearly demonstrated the energetic preference for the meso diastereomer
(5S, 15S, 20R, 30R).
Ni(II) or Cu(II) in a hexagonal M3O3 manner.27 Coordination of
Introduction
phosphorus(V) favours a formation of [4n]π Möbius aromatic
and [4n + 2]π Möbius antiaromatic systems.28 On the other hand
insertion of boron(III) induced a skeletal rearrangement of hexa-
phyrin(1.1.1.1.1.1) to bis-boron hexaphyrin(2.1.1.0.1.1).29
Rearrangements of porphyrinoids, especially contractions, trig-
gered by boron(III), phosphorus(V) and silicon(IV) ions were pre-
viously examined by our group.30–32
In construction of a nontrivial macrocyclic environment for
organometallic chemistry, the heteroatom confusion (X-con-
fusion) concept, originally exemplified by a porphyrin – 2-aza-
21-carbaporphyrin couple,33,34 has been applied.2,6 The analo-
gous approach namely interchanging of nitrogen atoms in hexa-
phyrin frame with a β-methine group transforms the regular
hexaphyrin into the singly or doubly N-confused isomers.35,36
Porphyrinoids can be derived from the regular porphyrin using
contraction, isomerisation, expansion or/and core modification
concepts.1–10 In the group of expanded porphyrins, hexaphyrins
are macrocycles containing six heterocyclic units connected by
meso-carbon bridges. Apart from regular hexaphyrins
(1.1.1.1.1.1)11,12 some non-regular systems have been also syn-
thesized bearing their own common names: amethyrin
(1.0.0.1.0.0),13 rosarin(1.0.1.0.1.0),14 rubyrin(1.1.0.1.1.0)15 or
cyclo[6]pyrrole.16
Geometry of the hexaphyrin(1.1.1.1.1.1) 1-NH (Chart 1) skel-
eton highly depends on meso-aryl substituents, both in the sol-
ution and in the solid state.17–19 Organometallic complexes of
1-NH (Hg(II), Ni(II), Pd(II), Pt(II), Ag(III), Au(III), Cu(III) and
Rh(III)) reveal mono and bimetallic, homo and heteronuclear
modes of coordination.20–24 Owing to the conformational flexi-
bility hexaphyrins are able to form two types of binuclear com-
plexes of substantially different geometry: almost flat (e.g. bis-
gold(III) complex with inverted pyrroles)20 and gable structured
(e.g. complex where bis-copper(II) ions are coordinated by six
nitrogens without inversion of pyrroles).25 Calix[3]dipyrrin,26
which is the three times meso reduced form of regular hexa-
phyrin, simultaneously accommodates three metal ions such as
Department of Chemistry, University of Wrocław, 14 F. Joliot-Curie St.,
Wrocław 50 383, Poland. E-mail: lechoslaw.latos-grazynski@
chem.uni.wroc.pl
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2ob07134g
Chart 1 Hexaphyrin and 31,34-diheterohexaphyrin.
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Interestingly, confusion also occurs during the metalation reac-
tion of regular hexaphyrin with copper(II) chloride in pyridine.37
Diheterohexaphyrins 1-X (Chart 1) are formally related to
regular hexaphyrin by a replacement of NH fragments by het-
eroatoms (X), typically from 16 group of the periodic table. A
common synthetic route leading to modified expanded porphyrin
involves an acid catalysed condensation of 16-heterotripyr-
rane.38,39 The MacDonald type synthesis of converted heterotri-
pyrranes (furan-, thiophene-, or selenophene-analogues) with
TSA and arylaldehyde in dichloromethane using chloranil as an
oxidant leads to appropriate diheterohexaphyrins.40,41 The con-
formations of these diheterohexaphyrins are critically dependent
on the nature of meso substituents and the state of protonation.
For instance di-p-benzihexaphyrin undergoes three-level topolo-
gical interconversions (planar Hückel, Möbius and figure-eight
Hückel) triggered by solvent, temperature and acid–base
control.42,43 Finally calix[2]metallocenyl[4]phyrin, where metal-
locene is (ferrocene or ruthenocene) replaces the pyrrole ring,
belongs to carbahexaphyrin44 demonstrating the structural
analogy to both calix[6]phyrin45 and the expanded
heterohexaphyrins.46
a mixture of diastereomers (3-RR, 3-SS and meso form 3-SR),
which could not be separated by chromatography. The diastereo-
mers have been readily identified by NMR spectroscopy because
of the apparent difference in the molecular symmetry (3-RR, C2;
3-SR, Cs).
Accordingly
a single silicon bound methyl resonance
(−0.40 ppm) has been detected for 3-RR (SS) but two (0.00 and
−0.85 ppm) were detected for 3-SR. The 1H–29Si scalar coupling
detected by HMBC experiment confirmed the assignment
(see ESI, Fig. S1†). The impressive chemical shift difference
reflects the mutual orientation of the MeSi, p-tolyl and 2-pyrrolyl
groups as shown in Fig. 1. In the extreme case of 3-SR the upfield
shifted resonance corresponds to the methyl located in the shield-
ing zone of two adjacent aryls where its counterpart positioned
on the other side of the silole ring stays away from this influence.
Consistently, the 3-RR (SS) spectrum reveals the shielding
due to a single meso-aryl yielding the intermediary chemical
shift. An analogous situation was observed in the case of 1,1-
dimethyl-3,4-diphenyl-2,5-bis(p-tolylhydroxymethyl)silole 2.53
Diheterosapphyrins were one of the first synthesized meso-
aryl expanded heteroporphyrins, where two pyrrole rings are
converted to furan, thiophene or selenophene respectively.47,48
The recent report by Matano et al. on the synthesis of phospha-
dithiasapphyrin presented the replacement of nitrogen by phos-
phorous for the very first time in an expanded porphyrin
environment.49 This remarkable step in the field of core
modified porphyrin provides a stimulus to search for expanded
porphyrinoids which contain a built-in non-trivial heteroatom.
Herein, we report the synthesis and characterization of 31,34-
disilahexaphyrinoid which contains two built-in silole units.
Synthesis and characterization of 31,34-disilahexaphyrinoid
The synthetic route leading to 31,34-disilahexaphyrinoid is sum-
marized in Scheme 2.
The synthesis involves the one-pot reaction of 16-silatripyr-
rane 3 with pentafluorobenzaldehyde (1 : 1 molar ratio), carried
out in dichloromethane in low temperature, catalysed by metha-
nesulfonic acid, which is followed by oxidation with DDQ. The
procedure follows the slightly modified methodology previously
utilized for the synthesis of meso-aryl expanded porphyrins54
and the intended product was aromatic 31,34-disilahexaphyrin
(1-SiMe2). Instead of the expected 31,34-disilahexaphyrin, a
non-aromatic macrocyclic product was isolated with 1% yield
after chromatographic workup: 31,34-disilahexaphyrinoid 4a.
The preference for reduced 31,34-disilahexaphyrin formation
is consistent with the reported chemistry of hexaphyrinogens.55
Hexaphyrinogens are reduced hexaphyrin derivatives which
Results and discussion
Synthesis of 7,8-diphenyl-16,16-dimethyl-5,10-bis(p-tolyl)-16-
silatripyrrane
A key step in the synthesis of 31,34-disilahexaphyrinoid is the
construction of the 16-silatripyrrane which is a suitable synthon
for introducing the silole ring into a hexaphyrin-like skeleton.
The one-pot silole cyclization method has been applied to obtain
2,5-dilithiosilole,50–52 which can be easily modified at the 2,5
positions leading to 1,1-dimethyl-3,4-diphenyl-2,5-bis(p-tolyl-
hydroxymethyl)silole 2. We have previously described this syn-
thetic procedure.53 Compound 2 reacts with an excess of pyrrole
in the presence of trifluoroacetic acid as shown in Scheme 1.
Acid neutralisation and removing of pyrrole afforded 7,8-
diphenyl-16,16-dimethyl-5,10-bis(p-tolyl)-16-silatripyrrane 3 as
Fig. 1 Drawing of the 3-SR and 3-RR structures (top: front projections,
bottom: side projections) as obtained from the molecular mechanics cal-
culations (MM+).
Scheme 1 Synthesis of 16-silatripyrrane.
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Scheme 2 Synthesis of disilahexaphyrinoid 4a.
NMR investigations
The identities of 4a have been confirmed by high-resolution
mass spectrometry and NMR spectroscopy. Thus the assign-
ments of the 31,34-disilahexaphyrinoid resonances, which are
given above selected groups of peaks (Fig. 3) have been made
on the basis of relative intensities and detailed two-dimensional
1
NMR studies (1H,1H (COSY, NOESY), 19F,19F (COSY), H,13C
1
(HMQC, HMBC) and H,29Si (HMBC)) carried out at 240 K in
dichloromethane-d2.
1
The H NMR spectrum of 4a (240 K) exhibits two AB pat-
terns and four methyl signals of two non-identical siloles. Two
different sets of meso-p-tolyl ring resonances have been detected.
This multiplets have been readily correlated via COSY and
NOESY with particular methyl resonances. The fast rotation has
been detected for 5,30- and 15,20-p-tolyls which are adjacent to
the silole moieties as documented by a single doublet for the
ortho and meta positions in each case. Actually the phenyl rings
attached at β-silole positions revealed the different dynamic be-
havior. Namely the fast rotation has been detected for all
β-phenyl resonances in the whole temperature range. Such sets
of resonances reveals the specific symmetry of disilahexaphyri-
noid molecule. The structural formula of 4a shown in Scheme 2
demonstrated merely the projection facilitating a presentation. To
Fig. 2 The electronic spectrum of 4a in dichloromethane.
contain six pyrroles bridged by six sp3 meso-carbon atoms. In
most cases they are treated just as unstable intermediates and
readily oxidized to the targeted porphyrinoids without isolation.
However, if an appropriate pyrrole with sufficiently bulky β-sub-
stituents e.g. mesityl, 2,6-Cl2C6H3 or C6F5 groups is use for syn-
thesis, hexaphyrinogens can be easily isolated. They are stable
and resistant to oxidation with DDQ either under reflux in
toluene or in the presence of an acid such as TSA and
BF3·OEt2.56
The UV-vis specturm of 4a is shown in Fig. 2. 31,34-Disila-
hexaphyrinoid shows broad absorption peaks below 500 nm and
less intensive above 300 nm. There are no bands in the regions
typically assigned to the Soret-like and Q bands of aromatic
hexaphyrin57 or diheterohexaphyrin.40 In fact the electronic
spectrum is very similar to non-aromatic calixphyrin58 which has
a smaller chromophore but contains the same sp3 meso-bridges
and conjugated system of two pyrrole subunits.
The steric barrier which occurs in 31,34-disilahexaphyrinoid
is primarily imposed by the substitution of 31,34-silicon atoms
with methyls (vide infra). Two SiMe2 groups cannot be simul-
taneously placed towards the hexaphyrin centre because the
packing is too tight. Both of the silole units must remain perpen-
dicular to each other. Furthermore, the steric hindrance also
created by phenyl rings located at the β-silole positions is the
reason for the parallel position of the conjugated dipyrrolic parts
with the plane consisting of two silicon atoms and four MeSi
carbon atoms getting in the way of planarization and conse-
quently aromatization.
1
account for the H NMR characteristic of 4a a non-planar struc-
tural scaffolds have been considered (Fig. 4 and 5).
In principle the different mutual orientations of dipyrro-
methene units of 4a could generate three major macrocyclic con-
formers: two boats and one chair (Fig. 4). Dihedral angles
between dipyrromethene planes are markedly different as deter-
mined from MM+ models (Fig. 4): 25–33° (31–35° for DFT,
boat), 157–162° (chair) and 288–303° (boat-I conformers).
Similar conformations have been adopted by calix[6]phyrin,27
calix[2]metallocenyl[4]phyrin,44
tetraazuliporphyrin
tetra-
cation,59 tetraphenylmetacyclophanes60,61 or squaraine rotax-
anes.62,63 In fact we have readily demonstrated that the NOE
determined fundamental connectivity pattern excludes chair and
boat-I geometries.
Consequently, DFT studies have been used to visualize the
suggested structures of 31,34-disilahexaphyrinoid and to assess
the degree of the macrocycle distortion (Fig. 5). The geometry of
4a was optimized at the B3LYP/6-31G** level of theory. The
optimized structure of 4a highlights the fact that the silole rings
are almost mutually perpendicular. Consequently the internal
methyl group (31-i-MeSi) is wedged between methyl substitu-
ents of the opposite silole.
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Fig. 3 1H NMR spectrum of 4a (dichloromethane-d2, 240 K). Inset: 1H–29Si NMR correlations.
Fig. 4 The conformers of 31,34-disilahexaphyrinoid (p-tolyl, β-phenyl and hydrogen substituents omitted for clarity; geometries obtained from the
molecular mechanics calculations (MM+)).
Fig. 5 The DFT optimized structure of 4a (left: side views, middle: front views, right: perspective views with aromatic and p-Me hydrogens
removed for clarity).
Additionally, one might consider a variety of different mutual
spatial orientations of methyl siloles, but, consistent with the
considered geometry, only one methyl group 31-i-MeSi is close
enough to produce the detectable dipolar 31-i-MeSi ↔ 34-o-
MeSi (2.64 Å) and 31-i-MeSi ↔ 34-i-MeSi (2.66 Å) couplings
(the shortest H–H distances from DFT optimized geometry are
given in parentheses). The distances to hydrogen atoms of 31-o-
MeSi (>5.89 Å) are too large to allow any efficient dipolar coup-
ling to 34-MeSi groups. These structural features have been
reflected in the NOESY map by two well defined cross peaks
(see ESI, Fig. S2†). The straightforward assignment of 31,34-
MeSi and H(5,30 and 15,20) resonances via 1H–29Si scalar
Subsequently the HMBC studies revealed the cross-correlation
which involved o-H(5-Tol) ↔ C(5,30 and 15,20) and H(5,30
and 15,20) ↔ o-C(5,30-Tol and 15,20-Tol) couples. Once
assigned this particular set of resonances has been used as a
starting point for NOE studies.
The fundamental relays of NOE connectivities are shown in
Fig. 6. The structural constraints determined by NOE have been
built in the DFT optimized structure.
31,34-Disilahexaphyrinoid contains four sp3 meso-carbon
stereocentres which could potentially generate 16 stereoisomers.
A molecule with an even number of chiral carbon atoms and a
plane of symmetry can yield only 4 pairs of enantiomers and two
meso forms. Eventually we have realized that the 31,34-disila-
hexaphyrinoid macrocycle is folded, acquiring the boat-like con-
formation (Fig. 4). The imposed lowering of symmetry increases
the number of hypothetical stereoisomers twofold. Notably, the
reduced number of macrocyclic frame signals and the
1
coupling (Fig. 3, inset) provided the initial step for the H NMR
analysis. Significantly the complementary HMQC experiment
allowed the unambiguous identification of the unique C(5,30
and 15,20) resonances revealing their tetrahedral geometry con-
sistent with the 46.7 and 47.9 ppm chemical shifts respectively.
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Fig. 6 The established NOE connectivities for 4a. Only the analytically useful hydrogens are shown. For clarity, half of molecule has been plotted
translucently. The spatial proximities (in Å) are in parentheses.
1
identification of four MeSi signals in H NMR spectrum of 4a
have readily indicated the plane symmetry passing through two
silicon atoms and four MeSi carbon atoms, preserving, however,
the concurrent differentiation of two silole units. Consequently
only four meso diastereomers of 4 have been further considered
(Fig. 8). These meso forms share common structural motif of
boat-like macrocyclic conformation where two conjugated dipyr-
rolic moieties take shape of a stem and a stern, and the C4 plane
(defined by the four meso carbon atoms) constructs a bottom.
Considering in detail the spatial proximity of the H(15,20) ↔
H(13,22), HN ↔ H(5,30) and H(31-i-MeSi) ↔ H(5,30) as found
at the DFT optimized structure of four boat-like diastereomers,
only the geometry of 4a satisfactory reproduces quantitatively
the constrains defined by NOE experiments. The relative intensi-
ties of NOE cross peaks follow the expected r−6 relation in the
series H(15,20) ↔ H(13,22) (2.48 Å) > HN ↔ H(5,30) (2.63 Å)
> H(34-i-MeSi) ↔ H(5,30) (3.44 Å) > H(31-i-MeSi) ↔ H(5,30)
(4.48 Å) firmly confirming the structural analysis. The diagnostic
spatial proximities determined for DFT optimized structures 4b,
4c and 4d of 31,34-disilahexaphyrinoid meso diastereomers dis-
close the significant mismatch with the corresponding NOE data.
The five 19F resonances: −140 (10,25-o-C6F5); −141 (10,25-
o′-C6F5); −154 (10,25-p-C6F5); −162 (10,25-m′-C6F5)
and −163 ppm (10,25-m-C6F5) serve as a fingerprint of the
pentafluorophenyl moiety (see ESI, Fig. S3†). The detected
differentiation of ortho and meta resonances seen for 10,25-pen-
tafluorophenyl suggests that their rotation with respect to the
Fig. 7 Linear correlation between calculated and experimental values
of chemical shifts for 4a.
C
meso–Cipso bond is slow below 240 K and obviously the macro-
cycle is not planar. Remarkably, the 19F chemical shifts of these
fluorine atoms match those of typical meso-pentafluorophenyl
porphodimethene (−138, −153 and −162 ppm).64 Indirectly, the
presence of fluorine atoms is suggested by a significant broaden-
ing of corresponding carbon signals (see ESI, Fig. S4†).
1H NMR chemical shifts calculated for 4a using the GIAO
B3LYP/6-31G** method are given in ESI, Table S1.† There is a
satisfactory qualitative agreement for the available set of theor-
etical and experimental data readily demonstrated by linear cor-
relations between the calculated and experimental chemical
shifts as shown for 4a (Fig. 7).
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Fig. 8 The DFT optimized structures of 4a, 4b, 4c, 4d (left: side views, middle: front views, right: perspective views with aromatic and p-Me hydro-
gens removed for clarity). Right views preserve the orientation of structures and numbering of sp3 meso-bridges in Chart 2.
DFT calculations
The optimized structures of 4a, 4b, 4c and 4d are shown in
Fig. 8. The calculated total energies, using the B3LYP/6-31G**//
B3LYP/6-31G** approach (Table 1), readily demonstrated that
4a isomer of 31,34-disilahexaphyrinoid is the most stable. The
relative energies of diastereomers increase in a series: 4a < 4b <
4c < 4d.
The condensation reaction leads to only one of four possible dia-
stereomers of 31,34-disilahexaphyrinoid.
1A reasonable hypothesis is that going to the smallest steric
hindrance is the driving force leading to this preference. It seems
that the weakest interaction between meso and β-silole aryls is
for the diastereomer 4a. In order to address this problem we have
carried out DFT calculations. Density functional theory (DFT)
has been successfully used to describe the properties of porphyr-
ins and related systems, providing information on their ener-
getics, conformational behavior, tautomerism, and aromaticity.65
In particular, we were able to reproduce the experimental values
of proton chemical shifts, conformations of the macrocycle ring
and also the extent of π-conjugation for di-p-benzihexaphyrin.43
Conclusion
The acid-catalyzed condensation of 16-silatripyrrane with
pentafluorobenzaldehyde yields the first 31,34-disilahexaphyri-
noid. The macrocycle is related to reduced form derived from the
[26]hexaphyrin(1.1.1.1.1.1) frame and contains two silole rings
replacing the pyrroles. The DFT optimised geometry reflected all
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Chart 2 Four diastereomers of non-planar 31,34-disilahexaphyrinoid (see Fig. 8).
Table 1 The calculated total energies (B3LYP/6-31G**//B3LYP/
6-31G**
Materials
)
Dichloromethane and pyrrole were purified by standard pro-
cedures. Pentafluorobenzaldehyde, MSA and BF3·OEt2 (Aldrich)
were used as received.
Diastereomers of 31,34-
disilahexaphyrinoid
Energies (kcal mol−1) vs.
4a
4a
4b
4c
4d
0.00
9.41
11.85
17.59
1,1-Dimethyl-3,4-diphenyl-2,5-bis(p-tolylhydroxymethyl)-silole
(2)
The synthetic procedure was previously described.53
structural constrains derived from 1D and 2D NMR experiments.
The condensation reaction affords only one isolable product of
four possible meso diastereomers of 31,34-disilahexaphyrinoid
point for the smallest steric hindrance is the driving force of this
preference.
7,8-Diphenyl-16,16-dimethyl-5,10-bis(p-tolyl)-16-silatripyrrane
(3)
3,4-Diphenyl-1,1-dimethyl-2,5-bis(p-tolylhydroxymethyl)-silole
2 (0.5 g, 1.0 mmol) was dissolved in pyrrole (2.8 mL,
40.0 mmol) and the solution was purged with nitrogen for
20 min. Subsequently, BF3·OEt2 (0.01 mL, 0.13 mmol) was
added. Reaction was protected from light and stirred for 30 min
in room temperature. Than dichloromethane (50 mL) was added
and mixture was neutralized with 40% water solution of NaOH.
Organic layer was separated and washed with water (2 × 25 mL)
before dried with Na2SO4. Excess of pyrrole was removed under
reduced pressure. 16-Silatripyrrane was obtained as a yellow
Experimental section
Instrumentation
NMR spectra were recorded on a Bruker Avance 500 MHz spec-
trometer. The absorption spectrum was recorded on a Varian
Cary 50 Bio spectrometer. The mass spectrum was recorded on a
Bruker micrOTOF-Q spectrometer using the electrospray
technique.
1
dark oil. Yield (3) (mixture of diastereomers): 0.54 g (90%). H
NMR (500 MHz, CD2Cl2, 298 K) δ 7.76 (s, 4H, 15,17-NH);
7.14–6.99 (36H, 7,8-o,m,p-Ph, 5,10-o,m-Tol); 6.65, 6.62 (2m,
4H, 1,14); 6.15, 6.10 (2m, 4H, 2,13); 6.00, 5.92 (2m, 4H, 3,12);
4.96 (2s, 4H, 5,10); 2.35, 2.32 (2s, 12H, 5,10-p-Me); 0.00,
−0.40, −0.85 (3s, 12H, MeSi). 29Si NMR (99 MHz, CD2Cl2,
298 K) δ 6.45 (SR); 6.38 (RR(SS)).
Computational chemistry
Density functional theory calculations were performed using
Gaussian 03.66 Geometry optimizations were carried out within
unconstrained C1 symmetry, with starting coordinates preopti-
mized using semiempirical methods. Becke’s three-parameter
exchange functional67 with the gradient-corrected correlation
formula of Lee, Yang, and Parr (B3LYP)68 was used with the 6-
31G** basis set. The chemical shift values were subsequently
calculated relative to tetramethylsilane (TMS, absolute shielding:
31.75 ppm).
31,34-Disilahexaphyrinoid (4a)
A 500 mL round-bottomed flask equipped with a stirring bar,
topped with a tube filled with CaCl2 and a nitrogen inlet, was
charged with freshly distilled dichloromethane (400 mL).
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16-Silatripyrrane 3 (0.5 g, 0.83 mmol), and pentafluorobenzalde-
hyde (0.103 mL, 0.83 mmol) were added and the solution was
purged with nitrogen for 20 min. The flask was placed in ice-
water bath and protected from light. Subsequently, MSA
(0.011 mL, 0.2 mmol) was added. After stirring at 0 °C for 2 h,
a portion of DDQ (0.681 g, 3 mmol) was added and stirring was
continued for a further 5 min. The solvent was removed by
means of a vacuum rotary evaporator. The dark residue was dis-
solved in dichloromethane and subjected to preliminary chroma-
tographic separation on a short basic alumina (grade II) column.
The fast-moving fraction was collected and separated again by
chromatography on a basic alumina (grade II) column using a
mixture of dichloromethane–hexane (ratio 5 : 5 v/v) as the
eluent. Compound 4a eluted first as a brown-yellow. The crude
product was further purified by column chromatography on a
basic alumina (grade II, dichloromethane–hexane 4 : 6 v/v).
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1
Yield of 4a: 6.6 mg (1%). H MR (500 MHz, CD2Cl2, 240 K) δ
11.87 (s, 2H, NH); 7.18 (d, 4H, 5,30-o-Tol); 7.16 (d, 2H, 17,18-
p-Ph); 7.08 (m, 2H, 5,3-m′-Ph); 7.05 (m, 4H, 17,18-o′-Ph,
17,18-m-Ph); 7.03 (2d, 8H, 15,20-o-Tol, 15,20-m-Tol); 6.99 (t,
2H, 2,3-p-Ph); 6.94 (m, 2H, 17,18-m′-Ph); 6.92 (d, 4H, 5,30-m-
Tol); 6.81 (d, 2H, 17,18-o-Ph); 6.76 (m, 2H, 2,3-m-Ph); 6.73 (d,
2H, 2,3-o′-Ph); 6.37 (d, 2H, 12, 23); 6.34 (d, 2H, 2,3-o-Ph); 6.26
(d, 2H, 13, 22); 6.10 (d, 2H, 8, 27); 5.67 (d, 2H, 7, 28); 5.01 (s,
2H, 15, 20); 4.84 (s, 2H, 5, 30); 2.26 (s, 6H, 15,20-p-Me); 2.23
(s, 6H, 5,30-p-Me); 0.79 (s, 3H, 34-i-MeSi); 0.33 (s, 3H, 31-o-
MeSi); −0.88 (s, 3H, 34-o-MeSi); −1.05 (s, 3H, 31-i-MeSi). 13
NMR (126 MHz, CD2Cl2, 240 K): δ 166.6 (14, 21); 159.5 (6,
C
1
29); 156.4 (17, 18); 155.4 (2, 3); 145.0 (10,25-C6F5, J(F,C)
≈
2
1
250 Hz, J(F,C) ≈ 20 Hz); 144.5 (10,25-C6F5, J(F,C) ≈ 250 Hz,
2J(F,C) ≈ 20 Hz); 142.3 (11, 24); 140.5 (1, 4); 139.8 (16, 19);
1
139.6 (C1-15,20-Tol); 139.4 (10,25-C6F5, J(F,C) ≈ 250 Hz);
138.7 (C1-17,18-Ph); 138.4 (C1-5,30-Tol); 138.3 (C1-2,3-Ph);
137.4(2 × 10,25-C6F5, J(F,C) ≈ 250 Hz); 136.5 (9, 26); 136.1
1
(C4-15,20-Tol); 136.0 (C4-5,30-Tol); 129.6 (12, 23); 129.2 (2,3-
o-Ph); 129.1 (5,30-m-Tol); 129.0 (17,18-o-Ph); 128.9 (15,20-o-
Tol, 15,20-m-Tol); 128.7 (2,3-o′-Ph); 128.6 (17,18-m′-Ph); 128.3
(5,30-o-Tol); 127.8 (17,18-p-Ph); 127.8 (2,3-m-Ph); 127.6
(17,18-o′-Ph); 127.0 (2,3-m-Ph); 126.6 (17,18-m-Ph); 126.1
(2,3-p-Ph); 125.3 (8, 27); 123.1 (13, 22); 121.5 (10, 25); 116.5
(7, 28); 111.3 (C1-10,25-C6F5); 47.9 (15, 20); 46.7 (5, 30); 21.2
(5,30-p-Me); 21.0 (15,20-p-Me); −0.3 (34-i-MeSi); −2.4 (31-o-
MeSi); −7.6 (34-o-MeSi); −8.1 (31-i-MeSi). 19F NMR
(470 MHz, CD2Cl2, 240 K) δ −140 (10,25-o-C6F5); −141
(10,25-o′-C6F5); −154 (10,25-p-C6F5); −162 (10,25-m′-C6F5);
−163 (10,25-m-C6F5). 29Si NMR (99 MHz, CD2Cl2, 298 K): δ
6.35 (34-Si); 5.39 (31-Si). UV-vis (CH2Cl2, 298 K): λmax = 318,
450 nm. HRMS (ESI): m/z calcd for C98H75N4F10Si+:
1553.5365, found: 1553.5275 [M + H]+.
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Acknowledgements
Financial support from the Ministry of Science and Higher Edu-
cation (Grant N N204 021939) is kindly acknowledged.
Quantum chemical calculations have been carried out at the
Poznań Supercomputer Center (Poznań) and Wrocław Super-
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