Chirality transfer through sulfur or selenium to chiral propellers

Article abstract of DOI:10.1039/c5ra14072b

The mechanism of chirality transfer from a chiral alkyl substituent to a trityl moiety through sulfur or selenium atoms is analysed and discussed on the basis of ECD measurements, DFT structure and ECD spectra calculations. It is shown that the presence o

Full text of DOI:10.1039/c5ra14072b

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Chirality transfer through sulfur or selenium to
chiral propellers†
Cite this: RSC Adv., 2015, 5, 69441
a
b
b
a
Paweł Skowronek,* Jacek S´ cianowski, Agata J. Pacuła and Jacek Gawronski
´
Received 16th July 2015
Accepted 10th August 2015
DOI: 10.1039/c5ra14072b
www.rsc.org/advances
The mechanism of chirality transfer from a chiral alkyl substituent to a similar to C symmetry structure of the transition state between
2
trityl moiety through sulfur or selenium atoms is analysed and dis- oppositely twisted propeller-like structures. Such a pattern was
7
cussed on the basis of ECD measurements, DFT structure and ECD observed also for chiral N-tritylamines.
spectra calculations. It is shown that the presence of a chalcogen
3
We reasoned that the length of the Ph C–X bond in trityl
atom is manifested by elongation of the distance between the chiral chalcogenides signicantly increases from oxygen to selenium
and the trityl moieties as well as by the change of electronic properties (Table 1). In the case of chiral trityl substituted thiols and
of the trityl chromophore while maintaining its chiroptical response to selenols, this elongation of the distance between chiral
the chirality of the molecule.
substituent and trityl group should lower steric hindrance and
lead to less congested structures. Indeed, available data from X-
ray diffraction determined crystal structures show a 27% longer
The trityl (triphenylmethyl) group is known to exist in two
enantiomeric M, P conformations possessing C
8
9
C–X bond in dimethyl sulde compared to dimethyl ether and
3
symmetry
a further 7% bond length increase on going from dimethyl
(Fig. 1). Due to steric repulsion the three phenyl rings cannot be
10
disulde to dimethyl diselenide. In ditrityl disulde C–X bond
simultaneously perpendicular to the plane dened by the three
ipso carbon atoms nor can be coplanar. On the other hand, in
trityl cation coplanarity of the phenyl rings is crucial for efficient
length increase is even larger (31%), compared to ditrityl
peroxide.
In order to observe the effect of chirality transmission to
trityl groups we have chosen the prepare of three chiral suldes
11
conjugation and charge distribution. When the trityl group is
+
connected to a single atom (i.e. H, Cl or Br) or to CH
groups of C or C
3
, NH
3
(1a, 2a, 3a) and selenides (1b, 2b, 3b) with chiral substituents
N
3
symmetry, respectively, phenyl rings adopt a
derived from p-menthane, carane and pinane terpenes.
propeller shape and both enantiomeric M and P forms are
equally abundant.
Optically active terpenyl trityl suldes (1a, 2a, 3a) were
obtained by the reaction of the corresponding thiols with trityl
chloride in the presence of pyridine. Using this method we
could not obtain the corresponding terpenyl trityl selenides.
Selenides 1b, 2b, 3b were prepared by the reaction of terpene
selenolates, generated in situ from the terpene selenols and
NaH, with trityl chloride. Starting terpene thiols and selenols
were prepared according to the known procedures from ter-
Investigations of dynamic stereochemistry of trityl group was
1–4
started by Mislow and followed by other groups. It was shown
by dynamic NMR studies that both enantiomeric forms readily
interchange through a low energy barrier what makes the sepa-
5
ration of enantiomeric propeller-shaped conformers impossible.
Introduction of substituent having symmetry different than
C
N
or C
3
leads to signicant conformational changes in the
12–15
penyl tosylates (Scheme 1).
trityl group. For trityl ethers we observe breakdown of C
3
symmetry, i.e. the two dihedral angles Cortho–Cipso–C–X are of
6
opposite sign to the sign of the third. Trityl group adopts a
conformation which can be described as bevel gear” and is
a
Department of Chemistry, A. Mickiewicz University, Pozna n´ , Poland. E-mail: Pawel.
Skowronek@amu.edu.pl
b
Department of Organic Chemistry, Nicolaus Copernicus University, 7 Gagarin Street,
8
†
1
7-100 Torun, Poland. E-mail: jsch@chem.umk.pl
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra14072b
Fig. 1 Two enantiomeric conformations of triphenylmethyl group.
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ꢀ
Table 1 Lengths of the C–X bonds (in A) in dimethyl and ditrityl mono- Table 2 Experimental ECD data in D3 (nm) for trityl derivatives 1a–3a
8
–11
and dichalcogenides based on X-ray diffraction data
and 1b–3b
sh
Me–X–Me
Me–X–X–Me
Ph
3
C–X–X–CPh
3
1a
ꢀ43.5 (187)
+43.6 (202)
ꢀ13.8 (203)
ꢀ11.1 (202)
+58.6 (208)
ꢀ2.3 (200)
ꢀ4.6 (202)
+20.7 (216)
ꢀ2.8 (236)
ꢀ2.0 (231)
+0.5 (234)
—
—
—
2a
+11.7 (184)
+11.1 (186)
ꢀ75.2 (189)
—
ꢀ5.5 (215)
sh
X ¼ O
X ¼ S
X ¼ Se
1.41
1.79
n.a.
n.a.
1.806
1.948, 1.938
1.45
1.90
n.a.
3a
1b
2b
ꢀ5.6 (216)
+24.5 (225)
ꢀ1.9 (223)
ꢀ4.1 (224)
3b
+7.1 (188)
Measured ECD spectra of trityl suldes and selenides show
characteristic pattern of Cotton effects in the absorption range
below 230 nm, corresponding to the trityl chromophore absorp-
ꢀ
1
Table 3 Calculated conformers relative energies (kcal mol ) and
twist of the phenyl rings, defined as Cortho–Cipso–C–X angle
6
tion, i.e. +/+/ꢀ for 1a, 1b and ꢀ/ꢀ/+ for 2a, 3a, 3b (Table 2). Very
DG
[kcal mol
Abundance
[%]
Cortho–Cipso–C–X
ꢀ1
large Cotton effects were recorded for neomenthyl derivatives 1a
]
[deg]
3
and 1b in which the X–CPh substituent occupies an axial posi-
1
a
a
Conf 1
Conf 2
Conf 1
Conf 2
0.00
0.16
0.00
0.14
57
42
56
44
ꢀ80, ꢀ36, ꢀ38
ꢀ10, 42, 70
tion (see below). Smaller Cotton effects were seen for the less
sterically congested trityl derivatives 2a, 2b, 3a, 3b. For caranyl
trityl selenide 2b measured ECD spectrum is weak (Table 2). This
may be due to increased distance between the stereogenic center
and the trityl group in the selenide 2b, compared to sulde 2a.
The C–S and C–Se bond lengths in 2a and 2b are not available,
2
ꢀ80, ꢀ36, ꢀ45
ꢀ5, 51, 67
however reference bond lengths are available for published X-ray may suggest a strong preference toward one type of conforma-
diffraction measurements for dimethyl and ditrityl chalcogens tion. Molecular modelling using Monte Carlo search followed by
3
(Table 1). Surprisingly, even if the X–CPh group is separated DFT (B3LYP/6-311g(d,p)) method structure optimizations were
from the chiral substituent by a methylene group, as in pinane performed for molecules 1a and 1b. Conformational rigidity of
derivatives 3a and 3b, ECD spectra are still signicant. However, the neomenthyl moiety and axial position of the chalcogen atom
no obvious correlation between the ECD spectra and the struc- strongly restrict the number of available low energy conformers.
ꢀ1
tures of trityl derivatives could be proposed, although they were For 1a only two conformers within 2 kcal mol energy window
6
7
ꢀ1
previously established for trityl ethers and trityl amines.
were found, with almost the same energy (0.16 kcal mol energy
For neomenthyl derivatives 1a and 1b strong couplets (A ¼ 87 difference) and abundance. For higher energy conformer 2 of 1a
1
and 134) at the wavelength of phenyl group B band absorption the trityl group has approximately C symmetry, similar to that
s
observed earlier for chiral trityl ethers. The lowest energy
conformer 1 has a propeller-like conformation of the trityl group.
Planes of all three phenyl rings are twisted in the same direction.
The phenyl ring sterically closest to the neomenthyl moiety is the
ꢁ
most twisted (up to ꢀ80 ) whereas next two have almost the
same twist. Conformation of the selenide derivative 1b follows a
similar pattern (Table 3 and Fig. 2).
Reliability of the conformers structures was conrmed by the
calculation of their CD spectra. For the conformers of 1a and 1b
Boltzmann averaged calculated CD spectra satisfactorily repro-
duce the experimental ones (Fig. 3).
The observed Cotton effects for the trityl suldes and sele-
nides are signicantly larger when compared to the trityl ethers.
Scheme 1 Synthesis of chiral tritylated sulfides and selenides.
Fig. 2 Lowest energy (left) and next in energy (right) conformers of 1a.
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entire molecule 1a. As expected, neomenthyl part of molecule 1a
displays a negligible effect on the overall CD spectrum (dashed
line in Fig. 4). The pattern and intensity of the spectrum of
conformer 1 of 1a can be in satisfactory way reproduced when
entire aliphatic moiety of the neomenthyl group is replaced by
just a methyl group. This behaviour can be explained by
conjugation of the lone pairs of the sulfur or selenium atom
with triphenylmethyl chromophore. The lone pairs located on
sulfur atom are very different in character. One, lower in energy,
have dominant contribution of s orbital whereas second one,
being the highest occupied NBO, constitutes of almost pure p
orbital (see ESI, Table X2 and Fig. X2†). Higher energy lone pair
orbital on sulfur atom interacts with antibonding orbitals
3
located between sp carbon atom of the trityl group and ipso
carbon atom of adjacent phenyl rings. Weaker interaction
ꢀ
1
(
lower than 1 kcal mol ) are estimated between higher energy
lone pair orbital and NBOs located on closest Cortho–Hortho and
Cipso–Cortho bonds on phenyl rings. Many transitions contrib-
uting to CD spectrum have signicant participation of excita-
tion from HOMO to orbitals placed on trityl phenyl rings. Thus
CD spectrum of 1a depends on relative position between chal-
cogen lone pairs and phenyl rings of the trityl group. Similar
effect is observed for methyl trityl ether of the conformation
resembling that of conf 1 of 1a.
Fig. 3 Experimental (solid line) and calculated (dashed line) CD
spectra of 1a (top) and 1b (bottom).
For comparison, there is seen only a weak effect of the
heteroatom on the UV spectra of compounds of the structure
Ph CXH, where X ¼ O, S, NH, compared to UV spectrum of
3
triphenylethane (Ph CCH ), see ESI.† Increased absorption at
3
3
Such behaviour can be explained by the electronic effect of
sulfur and selenium atom. Calculated CD spectrum of triphe-
nylmethane having the same conformation as in conformer 1 of
around 260 nm was observed for Ph
electronic absorption involving n orbitals of the sulfur.
In chiral trityl derivatives, Ph CXR*, chirality of the stereo-
3
CSH and ascribed to
3
1a showed different pattern and lower intensity of the Cotton
genic carbon atom in R* is efficiently transferred to the trityl
group via the chalcogen atom X ¼ S, Se. This is demonstrated by
the CD spectra as well as by calculations of conformer structures
and rotational strengths of their electronic transitions at the
DFT level. CD active electronic transitions involve molecular
orbitals located both on the phenyl rings and on the chalcogen
atom. Less intense CD spectra are observed with trityl selenides,
compared to trityl suldes, on account of longer carbon–chal-
cogen bond in the former. Weaker steric demands result
in preferred propeller type conformation of the trityl
chromophore.
effects (dotted line in Fig. 4) compared to that calculated for the
Acknowledgements
This work was supported by Grant no. 2011/03/B/ST5/01011
from National Center for Science (NCN), Poland. All calcula-
tions were performed at Poznan Supercomputing and
Networking Center.
Notes and references
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sulfide (dotted-dashed line), methyl trityl ether (long-dashed line). The
conformation of triphenylmethyl moiety in all cases 2–5 was the same
as in conformer 1 of 1a.
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