Towards a correlation of absolute configuration and chiroptical properties of alkyl aryl sulfoxides: A coupled-oscillator foundation of the empirical Mislow rule?

Article abstract of DOI:10.1002/1521-3765(20010105)7:1<72::AID-CHEM72>3.0.CO;2-1

The absorption and circular dichroism (CD) data for a series of alkyl aryl sulfoxides 1-16 of known S configuration have been analyzed. The strong bathochromic effect exerted by the nitro group in the para position of the phenyl sulfoxides indicates that the sulfur atom acts as an electron donor moiety towards the phenyl ring. Such behavior requires a significant 2p(C)-3sp³(S) overlap, and therefore the phenyl (and p-substituted phenyl) sulfoxides 1-12, as well as the 2-naphthyl sulfoxides 15 and 16, must assume a conformation which permits such orbital overlap. The steric effect of the peri hydrogen in 1-naphthyl-substituted compounds 13 and 14 does not allow a conformation of this type, and in these compounds the above-mentioned 2p(C) and 3sp³(S) orbitals are positioned in almost orthogonal planes. This conformational difference is clearly shown by the absorption spectra: compounds 1-12, 15, and 16 show the lowest energy σ → σ* transition of the sulfoxide chromophore at approximately 250 nm, indicating the existence of a conjugated S=O chromophore. In contrast, the corresponding absorption in 13 and 14 occurs at about 200 nm, indicating the presence of an isolated S=O chromophore. The CD spectra of 13 and 14 show a negative, couplet-like feature between 250 and 200 nm. This spectral feature can be interpreted in terms of exciton coupling between the allowed σ → σ* transition of the isolated S=O chromophore at 200 nm and the ¹B transition of the naphthalene chromophore. In fact, the Harada-Nakanishi rule predicts a negative CD couplet for an S-configured sulfoxide in the conformation found by UV analysis, as found experimentally. The CD spectrum of 13 is quantitatively reproduced by DeVoe coupled-oscillator calculations, strongly implying that a coupled-oscillator mechanism is operative in determining the optical activity of 13 and 14. This approach has also tentatively been extended to the conjugated sulfoxides 1-12, taking into account the coupling of the benzene chromophore ¹L_a transition with the σ → σ* transition of the S=O chromophore. In this case the Harada-Nakanishi rule also predicts a negative CD couplet for the S-configured sulf-oxides, as found experimentally.

Full text of DOI:10.1002/1521-3765(20010105)7:1<72::AID-CHEM72>3.0.CO;2-1

FULL PAPER
Towards a Correlation of Absolute Configuration and Chiroptical Properties
of Alkyl Aryl Sulfoxides: A Coupled-Oscillator Foundation of the Empirical
Mislow Rule?
Carlo Rosini,* Maria Irene Donnoli, and Stefano Superchi[a]
This paper is dedicated to Professor Kurt Mislow
Abstract: The absorption and circular
dichroism (CD) data for a series of alkyl
aryl sulfoxides 1 ± 16 of known S config-
uration have been analyzed. The strong
bathochromic effect exerted by the nitro
group in the para position of the phenyl
sulfoxides indicates that the sulfur atom
acts as an electron donor moiety towards
the phenyl ring. Such behavior requires
shown by the absorption spectra: com-
pounds 1 ± 12, 15, and 16 show the lowest
energy s ! s* transition of the sulfoxide
chromophore at approximately 250 nm,
indicating the existence of a conjugated
SˆO chromophore. In contrast, the cor-
transition of the naphthalene chromo-
phore. In fact, the Harada ± Nakanishi
rule predicts a negative CD couplet for
an S-configured sulfoxide in the confor-
mation found by UV analysis, as found
experimentally. The CD spectrum of 13
is quantitatively reproduced by DeVoe
coupled-oscillator calculations, strongly
implying that a coupled-oscillator mech-
anism is operative in determining the
optical activity of 13 and 14. This
approach has also tentatively been ex-
tended to the conjugated sulfoxides 1 ±
responding absorption in 13 and 14
occurs at about 200 nm, indicating the
presence of an isolated SˆO chromo-
3
a significant 2p(C) ± 3sp (S) overlap, and
therefore the phenyl (and p-substituted
phenyl) sulfoxides 1±12, as well as the
phore. The CD spectra of 13 and 14
show a negative, couplet-like feature
between 250 and 200 nm. This spectral
feature can be interpreted in terms of
2
-naphthyl sulfoxides 15 and 16, must
assume a conformation which permits
such orbital overlap. The steric effect of
the peri hydrogen in 1-naphthyl-substi-
tuted compounds 13 and 14 does not
allow a conformation of this type, and in
exciton coupling between the allowed
12, taking into account the coupling of
1
s ! s* transition of the isolated SˆO
the benzene chromophore L transition
a
1
chromophore at 200 nm and the
B
with the s ! s* transition of the SˆO
chromophore. In this case the Harada ±
Nakanishi rule also predicts a negative
CD couplet for the S-configured sulf-
oxides, as found experimentally.
Keywords: asymmetric synthesis ´
circular dichroism ´ configuration
determination ´ conformation anal-
ysis ´ sulfoxides
these compounds the above-mentioned
3
2
p(C) and 3sp (S) orbitals are posi-
tioned in almost orthogonal planes. This
conformational difference is clearly
understood. In a series of seminal papers, Mislow et al.[4]
examined the chiroptical properties of several alkyl phenyl
sulfoxides of known configuration and formulated an empiri-
cal rule to correlate CD data and absolute configuration, but
only Gottarelli et al. addressed the problem of a nonempir-
ical understanding of the optical activity of the aliphatic
Introduction
Despite the recognized importance of enantiopure sulfoxides
[
1]
[2]
as bioactive compounds, valuable synthetic intermediates,
[
3]
[5]
and ligands for asymmetric synthesis, reliable and general
methods for configurational assignment of these compounds
are still lacking. In particular, no safe correlations between
circular dichroism (CD) data and structure have been
described, presumably because the origin of the chiroptical
properties of the sulfoxide chromophore is not yet well
episulfoxide chromophore. After these papers, a few re-
ports[
6, 7]
have appeared in which the absolute configurations
at the sulfur stereogenic centers of some alkyl aryl sulfoxides
have been assigned simply by comparing CD spectra of
structurally related compounds, without addressing the issue
of the understanding of the mechanism that induces optical
activity in these compounds. This deficiency has recently been
pointed out by Gawronski, who noted[ that until now the
absolute configurations of aryl sulfoxides have not been
derived from CD spectra. The intention of this paper is to
[
a] Prof. C. Rosini, Dr. M. I. Donnoli, Dr. S. Superchi
Dipartimento di Chimica, Universit aÁ della Basilicata
Via Nazario Sauro 85, 85100 Potenza (Italy)
Fax : (‡ 39)0971-202223
8]
E-mail: rosini@unibas.it
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Chem. Eur. J. 2001, 7, No. 1

72±79
analyze the absorption and CD spectra of several alkyl aryl
sulfoxides of different structures and known absolute config-
urations, with the aim of identifying the main mechanism
responsible for the induction of optical activity in this
chromophore and to formulate a simple and reliable, non-
empirical relationship between spectrum and structure.
chemical yields and ees up to 80% for alkyl aryl sulfoxides
and up to 99% for aryl benzyl sulfoxides. The absolute
configuration was assigned by comparison of [a] in the case
D
[10]
of the known compounds 1 ± 10, 13, and 15, while for the
remaining derivatives 11, 12, 14, and 16 it was assigned[ by
comparison of their CD curves with those of similar com-
pounds of known configuration. All the compounds 1 ± 16 are
S-configured.
7]
Results and Discussion
Absorption and CD spectraÐthe empirical Mislow rule: So
that the absorption and CD data of alkyl aryl sulfoxides might
be discussed in a concise and clear manner, compounds 1 ± 16
were divided into four classes, according to the nature of their
aromatic and alkyl moieties (Scheme 1). Class A comprises
alkyl phenyl sulfoxides; the aromatic moiety is kept constant
Synthesis: The subjects of this investigation are compounds
± 16 (Scheme 1). They were prepared in optically active form
by following a previously described procedure:
1
[
7, 9]
prochiral
alkyl aryl sulfides were stereoselectively oxidized, by using
tBuOOH in the presence of a chiral complex formed in situ
from [Ti(iPrO) ], H O, and (R,R)-1,2-diphenylethane-1,2-
4
2
diol, to give the corresponding sulfoxides in 60 ± 80%
Abstract in Italian: Sono stati analizzati gli spettri di
assorbimento e di dicroismo circolare (DC) degli alchil aril
solfossidi 1 ± 16, di configurazione assoluta S. Un gruppo nitro
in posizione para del fenil solfossido esercita un forte effetto
batocromo, indicando che lꢀatomo di zolfo agisce come
elettron donatore nei confronti dellꢀanello benzenico. Tale
fenomeno richiede una significativa sovrapposizione degli
3
orbitali 2p(C) ± 3sp (S), per cui sia i solfossidi fenilici p-
sostituiti 1 ± 12 che quelli 2-naftilici 15 e 16 devono avere una
conformazione tale da permettere la sovrapposizione di questi
orbitali. Lꢀeffetto sterico dellꢀidrogeno peri nei composti
1
-naftil sostituiti 13 e 14 non permette una conformazione di
questo tipo e quindi in questi derivati gli orbitali 2p(C) e
3
3
sp (S) sono su piani ortogonali. Questa differenza conforma-
zionale › chiaramente mostrata dagli spettri di assorbimento: i
composti 1 ± 12,15 e16 presentano la transizione s ! s* di piœ
bassa energia del cromoforo solfossidico a circa 250 nm,
suggerendo la presenza di un cromoforo SˆO coniugato. Al
Scheme 1. Alkyl aryl sulfoxides 1 ± 16, divided into four classes.
contrario, in 13 e 14 lo stesso assorbimento si trova a circa
and the alkyl residue is varied. Class B encompasses
sulfoxides with a p-substituted phenyl moiety, together with
either a methyl or a benzyl group; the para substitution on the
aromatic ring is varied, while the alkyl residues remain the
same. 1-Naphthyl alkyl and 2-naphthyl alkyl sulfoxides were
separated into classes C and D, respectively. The main
features of the absorption and CD spectra of 1 ± 16 are
collected in Table 1, while the absorption and CD spectra of
compounds 2, 8, 9, 13, and 15 (representative examples of
each one of the classes above) are given in Figure 1, Figure 2,
Figure 3, Figure 4, and Figure 5 (see below), respectively. We
first analyzed the CD and UV spectra of phenyl and p-
substituted phenyl alkyl sulfoxides belonging to classes A and
B. The spectra of all these compounds presented similar
features; in the range of 195 ± 300 nm they showed two
absorption maxima withÐin the CD spectraÐtwo corre-
sponding Cotton effects of opposite sign. For example, the
spectra of 2 (Figure 1) and 8 (Figure 2) featured absorption
bands at about 200 ± 220 nm (e ꢀ 12000) and about 250 nm
2
00 nm, indicando la presenza di un cromoforo SˆO isolato.
Gli spettri di DC di 13 e 14 presentano una specie di couplet
eccitonico negativo tra 250 e 200 nm. Tale andamento spettrale
pu oÁ essere attribuito allꢀaccoppiamento eccitonico tra la
transizione permessa s ! s* a 200 nm del cromoforo SˆO
1
isolato e la transizione B del cromoforo naftalenico. Infatti,
per un solfossido di configurazione assoluta S ed avente la
conformazione che abbiamo ricavato dallꢀanalisi UV, la regola
di Harada ± Nakanishi prevede un couplet negativo nello
spettro di DC, come osservato sperimentalmente. Lo spettro
DC di 13 › stato riprodotto quantitativamente adoperando il
modello ad oscillatori accoppiati dovuto a DeVoe, conferman-
do che, effettivamente, lꢀattività ottica di 13 e 14 › determinata
da un fenomeno di accoppiamento eccitonico. Questo tratta-
mento › stato tentativamente esteso anche ai solfossidi coniu-
gati 1 ± 12, prendendo in considerazione lꢀaccoppiamento della
1
transizione L del cromoforo benzenico con la transizione
a
s ! s* del cromoforo SˆO. Anche in questo caso la regola di
Harada e Nakanishi prevede, per solfossidi di configurazione
S, un couplet negativo, come trovato sperimentalmente.
(
e ꢀ 8000), while Cotton effects were observable in their CD
spectra at 210 ± 220 nm (De  25 ± 30) and 245 nm (De � 15).
Chem. Eur. J. 2001, 7, No. 1
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73

C. Rosini et al.
FULL PAPER
Table 1. Main CD data[a] (in acetonitrile).
Ar
R
l [nm] (De)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
Ph
Ph
Ph
Ph
Ph
p-MeC
p-MeOC
p-BrC
p-NO
p-MeC
p-MeOC
p-NO
1-naphthyl
1-naphthyl
2-naphthyl
2-naphthyl
Me
Et
nBu
iPr
PhCH
Me
Me
Me
Me
PhCH
PhCH
PhCH
Me
PhCH
Me
244 (� 13.9); 216 (‡28.5)
247 (� 14.2); 217 (‡25.8)
247 (� 15.7); 218 (‡27.8)
250 (� 12.0); 216 (‡25.2)
257 (� 30.4); 226 (‡48.1)
245 (� 15.4); 220 (‡23.1)
246(� 23.6); 225 (‡26.2)
249 (� 14.8); 222 (‡27.9); 196 (� 20)
2
6
H
4
6
H
4
4
6
H
4
2
C
6
H
4
293 (� 2.9); 234 (‡5.1); 217 (� 3.6); 208 (‡1.1); 197 (� 9.2)
254 (� 20.8); 222 (‡29.1)
1
1
1
1
1
1
1
6
H
4
2
2
2
6
H
255 (� 34.0); 230 (‡36.6)
2
C
6
H
4
300 (� 8.2); 247 (‡8.3); 230 (� 7.0); 216 (‡7.3); 197 (� 15.1)
288 (� 8.9); 225 (� 44.5); 199 (‡34.0)
292 (� 14.4); 237 (� 39.6); 210 (‡28.9)
247 (� 11.2); 223 (‡34.0)
2
2
PhCH
256 (� 23.5); 242 (� 17.9); 225 (‡51.3); 198 (‡14.9)
[
a] Data corrected to 100% ee.
bands, both in absorption and CD spectra, with the exception
of the NO group, which gives rise to a strong bathochromic
2
shift of both bands (compare the data for 1, 6, 7, and 8 with
those of 9 in Table 1 and Figure 2 with Figure 3). It is
important to note that all the compounds studied, possessing
Figure 1. The electronic absorption (UV) and circular dichroism (CD)
spectra of 2 in acetonitrile.
Figure 3. The electronic absorption (UV) and circular dichroism (CD)
spectra of 9 in acetonitrile.
the same S absolute configuration, show the same negative/
positive sequence in the sign of their Cotton effects, on
moving from longer to shorter wavelengths. This correlation
was first formulated[ in 1965 by Mislow et al., who examined
the CD spectra of some alkyl phenyl sulfoxides, differing only
in the size of the R group (Me, Et, iPr, etc.). The data
presented here also extend the validity of this rule to p-
phenyl-substituted compounds. In order to understand the
origin of such spectral features, a more detailed analysis of the
transitions involved is needed. The absorption and CD spectra
of alkyl phenyl sulfoxides have been studied in the past by
Mislow et al.,[ who assigned the 255 nm transition to an
excitation of an electron of the SˆO lone pair into a higher
4]
Figure 2. The electronic absorption (UV) and circular dichroism (CD)
spectra of 8 in acetonitrile.
4]
Regarding compounds of class A, we may observe that the
nature of the alkyl R group does not affect the shape either of
the absorption or of the CD spectra, at least in the wavelength
range studied (see Table 1 and Figures 1 and 2). Also, p-
substitution, which varies in compounds of class B, has only
negligible effects on the position and intensity of the above
energy s* orbital. This absorption band is in fact shifted
towards the blue on changing from a nonprotic (e.g.,
acetonitrile) to a protic (e.g., methanol) solvent, the absorp-
tion bands exhibiting behavior typical of lone-pair excitations.
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Chem. Eur. J. 2001, 7, No. 1

Alkyl Aryl Sulfoxides
72±79
The experimental and theoretical work of Gottarelli et al.,[5]
has also shown that, in dialkyl sulfoxides, the lowest energy
s ! s* transition, situated at approximately 210 nm, is
allowed both magnetically and electrically, clearly pointing
out that this transition is very similar to the n ! p* one in
ketones. For alkyl aryl sulfoxides, we can therefore relate the
transition. The absorption and CD data for the 2-naphthyl
sulfoxide 15 (Figure 5) look very different from those of 13. In
fact, we are now dealing with a b-substituted naphthalene
chromophore, and the absorption spectrum shows all the
[12]
1
features of such. The b-substitution mainly affects the L
b
1
1
and B transitions (long-axis polarization) rather than the L
b
a
transition at 245 nm to a conjugated sulfoxide chromophore.
transition (short-axis polarization), so very weak absorptions
1
The 220 nm band may instead be assigned to the L transition
a
of the benzene chromophore; in a strongly perturbed benzene
chromophore (benzaldehyde, methyl benzoate, thioanisole,
[
11]
etc. ) this is allowed, with polarization along the CAr-
perturber axis. The absorption spectrum of the a-naphthyl
substituted compound 13 reveals features typical of the a-
[
12]
substituted naphthalene chromophore (Figure 4),
with a
band at 290 nm and a second absorption at 225 nm. The first
Figure 5. The electronic absorption (UV) and circular dichroism (CD)
spectra of 15 in acetonitrile.
1
are visible between 300 and 330 nm ( L ), while the allowed
b
1
1
L and B transitions can be observed at about 280 nm and
a
225 nm, respectively. The most striking difference between
the absorption spectra of 13 and 15 is that the latter displays
an allowed absorption in the 230 ± 270 nm range, whereas no
such bands are present in the spectrum of 13. The 250 nm
band in the absorption spectrum of 15 is of the same nature
Figure 4. The electronic absorption (UV) and circular dichroism (CD)
spectra of 13 in acetonitrile.
(
s ! s* of the conjugated sulfoxide chromophore) as the
1
band corresponds to the allowed L transition, which, being
analogous band in compounds 1 ± 12; indeed, on changing
a
polarized along the short axis, is strongly affected by a
from CH CN to methanol as solvent, this absorption is
3
substituent in the 1-position and red-shifted with respect to
significantly blue-shifted. Clearly, the intense band at 223 nm
is associated with the naphthalene B transition. In the CD
1
1
the analogous absorption in the parent naphthalene. The L
b
transition, which occurs at 310 nm in naphthalene, is not
spectrum of 15 (Figure 5), Cotton effects associated with both
1
1
observable because, as generally happens in the a-substituted
the L and the L transition are observable between 320 and
b a
1
compounds, it is covered by the more intense L band. The
270 nm, while two bands of opposite sign (negative/positive
sequence for S absolute configuration at the sulfur atom) are
present between 270 and 200 nm, and are associated with the
a
intense absorption at about 220 nm is associated with the
1
allowed B transition (polarized along the long axis of the
b
1
naphthalene nucleus). As far as the CD spectrum of 13 is
sulfoxide s ! s* transition and the naphthalene B transition,
concerned, a negative Cotton effect (De ꢀ� 9) is found,
respectively. It is interesting to note that in 15 the sulfoxide
1
1
corresponding to the L transition (288 nm), followed by an
s ! s* transition occurs at longer wavelengths than B, while
a
intense negative Cotton effect that corresponds to the
the opposite sequence is found in the spectrum of 13. In this
respect, the CD behavior of 15 and 16 strictly resembles that
of compounds 1 ± 12. This simple phenomenological analysis
of the absorption and CD spectra of the sulfoxides 1 ± 16
strongly endorses the validity of the Mislow rule, and
increases its scope considerably.
1
naphthalene B transition, and by a positive band at 199 nm.
[
6b]
The latter absorption was assigned by Baker et al. to the
s ! s* transition of the SˆO chromophore, although no
experimental proof for this assignment was provided. How-
ever, we observed that the positive Cotton effect, at 199 nm in
CH CN, is clearly blue-shifted to 191 nm in methanol, thus
3
confirming the s ! s* character of the absorption in this
Conformational considerations: Since the aim of this inves-
tigation is to formulate a nonempirical correlation between
CD spectrum and absolute configuration, knowledge of the
most populated conformations of compounds 1 ± 16 is re-
quired. From this point of view, the examination of the UV
spectra carried out in the previous paragraph provides very
spectral region. To summarize, in both the similar S sulfoxides
1
3 and 14 we have, in the range 250 ± 200 nm, a sequence of
negative/positive CD bands, with the negative one associated
1
with the naphthalene chromophore B transition and the
positive one (200 nm) with the sulfoxide chromophore s ! s*
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C. Rosini et al.
FULL PAPER
useful results. As a matter of fact, within the series 1±12, only
similar in number, sign, position, and intensity of their Cotton
effects, clearly establishing the similar conformational situa-
tion of these two compounds. It is also well known that, in
aromatic amides derived from 1-(1-naphthyl)ethylamine, the
hydrogen linked to the chiral center is directed[ towards the
peri hydrogen. Furthermore, the solid state structure of 1-(2-
bromonaphthyl)tert-butyl sulfoxide shows that the lone pair,
p-NO substitution dictates a strong bathochromic shift of the
2
absorption bands, in comparison with the unsubstituted
compound (see Table 1 and Figure 1 vs Figure 3). In contrast,
p-Me, p-Br, and p-OMe substituents have negligible effect
17]
(see Table 1 and Figure 1 vs Figure 2). Behavior of this kind
has previously been observed in the case of alkyl phenyl
[
13]
sulfides and was interpreted as a strong indication that the
sulfur atom acts as an electron donor, by using its lone pair
electrons, but not as an electron acceptor, which would
involve the vacant d orbitals. In the case at hand, the
significant red shift, caused solely by substitution with a
strongly electron-withdrawing group, must mean that only
these groups can extend the conjugation, that is, a negative
which is the smallest substituent, points towards the bromine,
which is the largest atom.[
18, 19]
The conformational analysis
developed so far deserves some additional comment. The
chosen conformations of compounds 1 ± 16 are determined by
a compromise between electronic (orbital overlap and, there-
fore, stabilization by resonance) and repulsive steric factors.
The above analysis, although completely qualitative, is
supported by clear experimental evidence. It is interesting
to point out that, in the case of 13, such conformational
findings agree with the result of ab initio calculations reported
by Benassi et al.,[ while in the case of alkyl phenyl sulfoxides
there are some slight differences. In fact, in the ab initio
[
14]
charge is placed on the para C atom. Clearly, this effect will
3
operate only if there is an efficient 2p(C) ± 3sp (S) overlap. In
[
15]
order to have such an overlap, the structure of the alkyl
15]
phenyl sulfoxides 1 ± 12 must be that described in Scheme 2
(left) for a sulfoxide of S absolute configuration. The second
3
structures, the 2p(C) ± 3sp (S) overlap seems not to play a
large role. However, it has to be noticed that very recent
density functional theory (DFT) calculations on vinyl sulf-
oxides have clearly indicated that some interactions between
the sulfur lone pair and the double bond contribute signifi-
cantly to stabilization of a specific conformation, overwhelm-
3
[20]
Scheme 2. The 2p(C) ± 3sp (S) overlap in alkyl phenyl sulfoxides 1 ± 12
ing even repulsive steric interactions. This particular aspect
deserves further theoretical investigation.
(
(
left), and the absence of overlap in 1-naphthyl compounds 13 and 14
right).
CD analysis and formulation of a nonempirical configuration
correlation: Once the nature of the electronic transitions
present in the absorption spectra of compounds 1 ± 16 has
been understood, and a reasonable conformational assign-
ment made for compounds 1 ± 12, 15, and 16 on the one hand
and compounds 13 ± 14 on the other, a nonempirical analysis
of the CD spectra can be attempted. Let us start our analysis
by considering first the case of compounds 13 ± 14 in the 200 ±
250 nm range. In their UV spectra (between 200 and 250 nm),
these compounds have a couple of electronically allowed
transitions to which, in the CD spectra, correspond to two
Cotton effects of almost equal intensity and opposite sign. As
we have previously shown, the SˆO and the naphthyl
observation concerns compounds 13 ± 16. It has been ob-
served that the absorption spectra of a-naphthyl derivatives
1
3 and 14 are significantly different from those of their b-
naphthyl counterparts 15 and 16, which show an allowed
1
transition in between the naphthalene L (l ˆ 290 nm) and
a
1
B (l ˆ 220 nm) bands at about 255 nm. Clearly, according to
b
its frequency position, intensity, and solvent effects, this band
is associated with the s ! s* transition of a conjugated SˆO
chromophore, as discussed above. In contrast, this band is
absent between 230 and 280 nm in the absorption spectrum of
1
3 and 14, while an allowed transition is present on the high-
1
energy side of B . This last band, taking into account its
b
energy position, intensity, and behavior on changing the
solvent, has been assigned to the s ! s* transition of an
isolated SˆO chromophore (vide supra). Therefore, while we
chromophores do not overlap, and so we are entitled to treat
this system using the coupled-oscillator model for nondegen-
[
21]
erate transitions.
We shall employ such a model at a
22]
have a conjugated sulfoxide chromophore for compounds 1 ±
qualitative level, using the aromatic chirality rule[ of Harada
and Nakanishi. The conformational situation is that reported
in Scheme 2 (right) and two chromophores are present: the
sulfoxide and the a-naphthalene. The isolated sulfoxide
chromophore, following the analysis of Gottarelli et al.,[
has a s ! s* transition located at about 210 nm and polarized
along the C ± C direction of the C �(SˆO)�C moiety. In this
1
2, 15, and 16, in the case of 13 ± 14 we have an isolated SˆO
chromophore. As far as compounds 13 and 14 are concerned,
owing to the steric hindrance exerted by the peri hydrogen,
the sulfoxide is forced into a conformation in which the
smallest group on sulfur points towards the hydrogen in the
5]
8
-position, and the most stable conformation for an S
1
2
1
2
1
sulfoxide is that represented in Scheme 2 (right). In this
situation, the lone pair cannot give rise to overlap with the
range, the a-naphthalene chromophore shows the B tran-
b
sition, polarized along the long axis. As show in Figure 6, in
which an S-configured compound is depicted, these dipole
moments define a negative chirality, and so a negative couplet
is to be expected in the corresponding spectral range, as
observed experimentally. The qualitative analysis discussed
above can be put on a quantitative basis by using coupled-
oscillator calculations of optical activity, by means of DeVoeꢁs
2
p orbitals of the aromatic nucleus, which lie in an orthogonal
plane. This conformational choice can be supported as
follows: first of all, it has been shown by NMR spectroscopy
that the above-described conformer is the most stable in
[
16]
[6b]
1
-naphthyl-tert-butyl sulfoxide. This compound and 13,
which have the same configuration, have spectra completely
76
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Alkyl Aryl Sulfoxides
72±79
1
structure, the L transition dipole and the sulfoxide transition
a
moment define a negative chirality (Figure 7) and, thus, a
negative CD couplet should be observed, as found exper-
imentally. Compounds 15 and 16 have two possible extremes
Figure 6. In 1-naphthyl methyl sulfoxide of S absolute configuration, the
transition dipole moment of the 210 nm s ! s* excitation of the SˆO
1
chromophore, polarized along the CAr ± C
R
direction, and the B
b
transition
Figure 7. In an alkyl p-substituted phenyl sulfoxide of S absolute config-
dipole moment of the naphthalene chromophore, polarized along the long
axis, define a negative chirality. A negative couplet is therefore to be
expected between 200 and 250 nm.
uration, the transition dipole moment of the s ! s* excitation of the SˆO
1
chromophore, polarized along the CAr ± C
R
direction, and the L
a
transition
dipole moment of the substituted benzene chromophore, define a negative
chirality. A negative couplet is therefore to be expected between 200 and
2
80 nm.
all-order polarizabity model.[23, 24] This model has been
successful in formulating spectra/structure relationships for
for ªconjugatedº conformations, as represented below in
Figure 8, differing in the relative orientation of the oxygen
and the H atom on C : transoid (top) and cisoid (bottom). In
[
25]
[26]
organic molecules,
high-molecular-weight compounds,
and coordination compounds.[27] DeVoe calculations were
carried out on the 1-naphthyl substituted sulfoxides, choosing
1
1
these structural situations, the electric dipole of the
B
1
-naphthyl methyl sulfoxide as a model compound and
transition of the naphthalene chromophore and that of the
allowed transition of the sulfoxide moiety define a positive
and negative chirality, respectively. As a negative couplet is
present in the CD spectrum, our coupled-oscillator analysis
indicates that the transoid conformation is the prevailing one,
even if we cannot at present provide any explanation of this
fact.
arbitrarily assuming S configuration at the stereogenic sulfur
center. The molecular geometry represented in Scheme 2
(right) and Figure 6 was employed as input geometry. A single
oscillator, polarized along the long axis of the naphthalene
2
ring and with a polarizability of 40 D centered at 220 nm was
1
employed to describe the naphthalene B transition. At the
b
same time, a single oscillator polarized along the C ± CAr
Me
2
direction, with a polarizability of 10 D , was employed to
describe the 210 nm transition of the sulfoxide chromophore.
The relative orientation of the two dipoles in the sulfoxide
molecule is reported in Figure 6. In this manner, the
absorption spectrum is satisfactorily reproduced, l ˆ 225
(
e
ꢀ 41000), 210 nm (e ꢀ 20000), while the experimental
max
max
values are l ˆ 225 (e ꢀ 50000), 210 nm (e ꢀ 30000). The
max
max
calculated CD spectrum shows extremes at 225 nm (De � 88)
and 206 nm (De ‡95), which compare satisfactorily with the
experimental ones (l 225 nm (De � 45), l 200 nm (De ‡35)).
These results clearly indicate that the coupled-oscillator
mechanism is the main source of optical activity in this range,
and that other mechanisms such as electric magnetic cou-
[
28]
pling
can be disregarded. Considering that the UV/CD
Figure 8. The two conjugated conformations allowed for 15. In the
1
features of 1 ± 12, 15, and 16 are very similar to those of 13 ± 14
analyzed above (i.e., two electronically allowed transitions
are present in the electronic spectrum, with two correspond-
ing Cotton effects of opposing sign and similar intensity in the
CD spectrum), it is tempting to use the coupled-oscillator
model for the former compounds as well. Even if this model
can correctly be applied only to systems with negligible orbital
overlap and electron exchange between the interacting
transoid conformation (top), the B transition dipole of the allowed s !
s* transition of the sulfoxide chromophore defines a negative chirality,
while in cisoid conformation (bottom) the same dipoles define the opposite
chirality.
Conclusion
[
21]
chromophores, it has also been used in cases in which such
orbital overlap cannot be disregarded, such as distorted
Even if this analysis does not constitute an exhaustive
interpretation of the chiroptical properties of alkyl aryl
sulfoxides, a few important conclusions can be drawn. First
of all, a number of different alkyl aryl sulfoxides have become
available in optically active form by means of our efficient and
versatile enantioselective oxidation methodology, allowing
comparative study of the UV/CD spectra of differently
substituted compounds. Secondly, such comparative and
[
29]
[30]
dienes, in 1,1'-binaphthyls with small dihedral angles, and
[
25p]
in twisted 2,2'-bipyridines,
always providing qualitatively
correct answers. We feel justified then in tentatively subjecting
the conjugated sulfoxide chromophores 1 ± 12, 15, and 16 to
the same treatment, by means of the exciton model, at least at
[
31]
a qualitative level. For S-configured sulfoxides 1±12, the
conformation in Scheme 2 (left) is allowed. In such a
Chem. Eur. J. 2001, 7, No. 1
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C. Rosini et al.
FULL PAPER
systematic analysis has enabled us to define the conformations
of these species, distinguishing between conjugated sulfoxides,
which possess a typical allowed transition at 250 nm, and
nonconjugated compounds in which the above band is
used without further purification. Analytical and preparative TLC were
performed on Merck 60 F-254 0.2 mm and 2.0 mm silica gel plates,
respectively, and column chromatography was carried out with Merck 60
silica gel (80 ± 230 mesh). Enantiomerically pure (R,R)-1,2-diphenylethane-
[32]
1,2-diol was prepared by asymmetric dihydroxylation of (E)-stilbene.
missing. In the former case, there is a significant 2p(C) ±
Racemic sulfoxides were prepared by oxidation of the corresponding
sulfides with 30% hydrogen peroxide, according to literature procedures.[
33]
3
3
sp (S) overlap that is completely absent in the latter. In
General procedure for catalytic oxidation of sulfides. [Ti(iPrO)
.08 mmol) and H O (28.8 mL, 1.6 mmol) were added dropwise and
sequentially to a suspension of (R,R)-diphenylethane-1,2-diol (34.3 mg,
.16 mmol) in CCl (5 mL). The sulfide (1.61 mmol) was added to the
4
] (23.6 mL,
other words, we have pointed out the possible importance of
electronic effects in defining the conformational behavior of
these compounds, and then the dilemma, raised by Mislow in
0
2
0
4
[
4b]
his seminal 1965 paper, about the role played by a 2p(C) ±
resulting homogeneous solution and was with stirred at room temperature
for 15 min. The solution was then cooled to 08C, and TBHP (70% in water,
3
3
sp (S) versus 2p(C) ± 3d(S) overlap has been resolved in
4
40 mL, 3.22 mmol) was added. The mixture was left stirring at 08C for 2 h,
then diluted with CH Cl , and dried over Na SO for a few minutes. After
filtration and evaporation of solvent, the residue was immediately purified
by preparative TLC (Et O as eluent) or column chromatography (EtOAc),
isolating the pure sulfoxide.
favor of the former. It is interesting to note that this significant
result has been achieved mainly by the use of ultraviolet
absorption spectroscopy. This technique is the method of
choice here, where we have conjugated and nonconjugated
sulfoxide chromophores, depending on steric effects, that is, a
typical case of steric hindrance to resonance. This shows how
UV spectroscopy, a method underused in structural analysis,
gives, at least on some occasions, a quick and sure answer.
From a stereochemical point of view, we have demonstrated
that, at least in the case of the nonconjugated sulfoxides, the
coupled-oscillator mechanism is the source of optical activity
2
2
2
4
2
Acknowledgement
This paper is dedicated to Professor Kurt Mislow, in recognition of his
fundamental contribution to the development of modern organic stereo-
chemistry. Financial support from MURST (Roma) and the UniversitaÁ
della Basilicata is gratefully acknowledged. The authors wish to thank
Professor Philip J. Stephens, Chemistry Department, University of South-
ern California (USA), for ongoing and stimulating exchange of information
on this topic.
(at least in the 250 ± 200 nm range); this allows safe and
reliable correlation between CD spectrum and absolute
configuration at the sulfur stereogenic center. We have also
proposed the extension of the same treatment to the
conjugated sulfoxidesÐat least at a qualitative levelÐtaking
into account the fact that some conjugated molecules have
[
25p, 29, 30]
[1] a) P. Pitchen, Chem. Ind. 1994, 636; b) M. Tanaka, H. Yamazaki, H.
Hakasui, N. Nakamichi, H. Sekino, Chirality 1997, 9, 17; c) S.
von Unge, V. Langer, L. Sjölin, Tetrahedron: Asymmetry 1997, 8,
been treated previously
in terms of such a model and
the correct answer obtained (vide supra). As we stated at the
beginning of this paragraph, this paper does not pretend to be
a conclusive treatment of the relationships between chirop-
tical properties and stereochemistry in the alkyl aryl sulf-
oxides, because some questions are still open: i) the role
played by electronic effects in determining the structure
would merit subjection to theoretical studies by means of
quantum mechanical techniques, in order to understand the
electronic nature of the C�S bond in depth; ii) the question of
1
967; d) H. L. Holland, F. M. Brown, B. G. Larsen, Tetrahedron:
Asymmetry 1994, 5, 1129; e) C. A. Hutton, J. M. White, Tetrahedron
Lett. 1997, 38, 1643; f) S. Morita, J. Matsubara, K. Otsubo, K. Kitano,
T. Ohtani, Y. Kawano, M. Uchida, Tetrahedron: Asymmetry 1997, 8,
3
5
707; g) H. L. Holland, F. M. Brown, Tetrahedron: Asymmetry 1998, 9,
35; h) T. Nishi, K. Nakajima, Y. Iio, K. Ishibashi, T. Fukurawa,
Tetrahedron: Asymmetry 1998, 9, 2567.
[2] a) M. C. Carre nÄ o, Chem. Rev. 1995, 95, 1717; b) G. Solladi e , Synthesis
1
981, 185; c) K. K. Andersen in The Chemistry of Sulfones and
Sulfoxides (Eds.: S. Patai, Z. Rappoport, C. J. M. Stirling), Wiley,
Chichester, 1988, Chapter 3, pp. 53 ± 94; d) G. H. Posner in The
Chemistry of Sulfones and Sulfoxides (Eds.: S. Patai, Z. Rappoport,
C. J. M. Stirling), Wiley, Chichester, 1988, Chapter 16, pp. 823 ± 849;
e) F. Colobert, A. Tito, N. Khiar, D. Denni, M. A. Medina, M. Martin-
Lomas, J. L. G. Ruano, G. Solladi eÁ , J. Org. Chem. 1998, 63, 8918; f) P.
Bravo, M. Crucianelli, A. Farina, S. V. Meille, A. Volonterio, M.
Zanda, Eur. J. Org. Chem. 1998, 435.
the extension of the exciton model to the conjugated systems
represents another important challenge. Even here, the
assistance of more sophisticated calculations will certainly
be important in defining the mechanisms responsible for the
optical activity. This would certainly solve the problem of the
optical activity of the conformationally nonhomogeneous
sulfoxides 15 and 16. Work is in progress along these lines.
[3] a) K. Hiroi, Y. Suzuki, Tetrahedron Lett., 1998, 39, 6499; b) R. Tokuyo,
M. Sodeska, L. Aol, Tetrahedron Lett. 1995, 36, 8035; c) N. Khior, I.
Fernandez, F. Alcudia, Tetrahedron Lett. 1993, 34, 123.
[
4] a) K. Mislow, A. L. Ternay, Jr., J. T. Melillo, J. Am. Chem. Soc. 1963,
Experimental Section
8
5, 2329; b) K. Mislow, M. M. Green, P. Laur, J. T. Melillo, T.
Simmons, A. L. Ternay, Jr., J. Am. Chem. Soc. 1965, 87, 1958; c) J.
Jacobus, K. Mislow, J. Am. Chem. Soc. 1967, 89, 5228; d) F. D. Saeva,
D. R. Rayner, K. Mislow, J. Am. Chem. Soc. 1968, 90, 4176.
General: HPLC analyses were performed at room temperature on Daicel
Chiralcel OB (cellulose tribenzoate) or Chiralcel OJ (cellulose tris(p-
methylbenzoate)) columns.[ Melting points were determined with a Kofler
7]
[5] a) G. L. Bendazzoli, P. Palmieri, G. Gottarelli, I. Moretti, G. Torre, J.
Am. Chem. Soc. 1976, 98, 2659; b) I. Moretti, G. Torre, G. Gottarelli,
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Gottarelli, I. Moretti, G. Torre, G. Gazz. Chim. Ital. 1979, 109, 19.
[6] a) T. Sagae, S. Ogawa, N. Furukawa, Bull. Chem. Soc. Jpn. 1991, 64,
3179; b) R. W. Baker, G. K. Thomas, D. S. Ren, M. V. Sargent, Aust. J.
Chem. 1997, 50, 1151.
[7] M. I. Donnoli, S. Superchi, C. Rosini, Enantiomer, 2000, 5, 181.
[8] J. Gawronski in Methoden Org. Chem. (Houben ± Weyl) 4th ed., 1995,
Vol. 1, Chapter 4.4, p. 499.
1
hot-stage apparatus and are uncorrected.
H
NMR (300 MHz) and
1
3
C NMR spectra were recorded in CDCl
3
. Optical rotations were
measured with a JASCO DIP-370 digital polarimeter. Absorption and
CD spectra were recorded on a JASCO J600 spectropolarimeter at RT in
�
3
acetonitrile (c ꢀ 6 Â 10 m) in 0.1 and 1.0 mm cells. During measurement,
the instrument was thoroughly purged with N . CCl was distilled from
CaH and stored over activated 4 Š molecular sieves. [Ti(iPrO) ] was
distilled prior to use under N atmosphere. Commercially available tert-
butyl hydroperoxide (TBHP, 70% in water) was purchased (Aldrich) and
2
4
2
4
2
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Chem. Eur. J. 2001, 7, No. 1

Alkyl Aryl Sulfoxides
72±79
[
9] a) S. Superchi, C. Rosini, Tetrahedron: Asymmetry 1997, 8, 349; b) S.
Superchi, M. I. Donnoli, C. Rosini, Tetrahedron Lett. 1998, 39, 8541;
c) M. I. Donnoli, S. Superchi, C. Rosini, J. Org. Chem. 1998, 63, 9392.
dipole-allowed transition, defined by the polarization direction e
the complex polarizability a (n)‡I (n). I (n) is obtainable from
(n) ˆ R
the experiment, that is, from the absorption spectra of the compounds
that can be considered good models of the subsystems, and R (n) can
be calculated from I (n) by means of a Kronig ± Kramers transform.
i
and
i
i
i
i
[
10] a) J.-M. Brunel, P. Diter, M. Duetsch, H. B. Kagan, J. Org. Chem. 1995,
0, 8086; b) P. Pitchen, E. Du nÄ ach, M. N. Deshmukh, H. B. Kagan, J.
i
6
i
Am. Chem. Soc. 1984, 106, 8188; c) H. Sakuraba, K. Natori, Y. Tanaka,
J. Org. Chem. 1991, 56, 4124; d) T. Sugimoto, T. Kokubo, J. Miyazaki,
S. Tanimoto, M. Okano, J. Chem. Soc. Chem. Commun. 1979, 402.
11] J. Murrel in The Theory of Electronic Spectra of Organic Molecules,
Methuen, London, 1963, Chapter 10.
12] a) H. H. Jaffe, M. Orchin in The Theory and Application of UV
Spectroscopy, Wiley, New York (USA), 1962; b) S. Suzuki, T. Fujii, H.
Baba, J. Mol. Spectrosc. 1979, 47, 243.
From the general formulation of the DeVoe model, retaining only the
terms to first order in G12 (physically, this means considering the
electric dipole on the i chromophore to be caused only by the external
electro magnetic field plus the dipolar fields of the other dipole
polarized by the external field), the following expression can be
deduced in the case of two different chromophores, each possessing
only one electrically allowed transition which provides CD as a
[
[
2
2
frequency function: De(n) ˆ 0.014p Ne
1
Xe
2
R
12
G
12n [I
1
(n)R
and e
2
(n) ‡
3
[
[
13] See ref. [12a] pp. 480 ± 481.
I
2
(n)R
1
(n)]; G12 ˆ (1/r12) [e
1
.e
2
� 3(e
1
e
12)(e
2
e
12)]. Here, e
1
2
are
14] It is interesting that, on the basis of IR studies, it was shown that the
MeSO group acts as a net resonance donor (N. C. Coutress, T. B.
Grindley, A. R. Katritsky, R. D. Topsom, J. Chem. Soc. Perkin II 1974,
the unit direction vectors of the transition dipole moments of the first
and second chromophore, respectively, R12 is the distance between
them, G12 is the point-dipole ± point-dipole interaction term, and n is
�
1
263); however, the above interpretation was found inconsistent with
the frequency expressed in cm . This expression gives rise to a
couplet-like feature if the absorption maxima of chromophores 1 and
the NMR findings of Buchanan et al. (G. W. Buchanan, C. Rayes-
Zamora, D. E. Clarke, Can. J. Chem. 1974, 52, 3895).
2
are close in frequency (ªquasi-degenerateº coupled-oscillator
[
15] It should be noted that the structure of Scheme 2 (left) is slightly
different from the most stable conformation of phenyl methyl
sulfoxide found by Benassi (R. Benassi, U. Folli, D. Iarossi, A. Mucci,
L. Schenetti, F. Taddei, J. Chem. Soc. Perkin Trans 2 1989, 517; R.
Benassi, A. Mucci, L. Schenetti, F. Taddei, J. Mol. Struct. 1989, 184,
system).
[25] a) M. Zandomeneghi, J. Chem. Phys. 1979, 83, 2926; b) M. Zando-
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493; d) P. Salvadori, C. Bertucci, C. Rosini, M. Zandomeneghi, G.
Gallo, E. Martinelli, P. Ferrari, J. Am. Chem Soc. 1981, 103, 5553; e) C.
Rosini, G. Giacomelli, P. Salvadori, J. Org. Chem. 1984, 49, 3394; f) C.
Rosini, C. Bertucci, P. Salvadori, M. Zandomeneghi, J. Am. Chem.
Soc. 1985, 107, 17; g) A. M. Caporusso, C. Rosini, L. Lardicci, C.
Polizzi, P. Salvadori, Gazz. Chim. Ital. 1986, 116, 467; h) W. H. Pirkle,
T. J. Sowin, P. Salvadori, C. Rosini, J. Org. Chem. 1988, 53, 826; i) M.
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Asymmetry 1998, 9, 55; p) C. Rosini, C. Bertucci, C. Botteghi, R.
Magarotto, Enantiomer, 1998, 3, 365.; q) C. Rosini, S. Superchi,
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2
81), in which the SˆO bond is almost coplanar with the phenyl ring.
[
16] D. Casarini, E. Foresti, F. Gasparrini, L. Lunazzi, D. Macciantelli, D.
Misiti, C. Villani, J. Org. Chem. 1993, 58, 5674.
[
[
17] W. H. Pirkle, C. J. Welch, M. M. Hyun, J. Org. Chem. 1983, 48, 5022.
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19] It is noteworthy that the conformation of Sceme 2 (right) is very
similar to the most stable structure of o-substituted phenyl methyl
sulfoxides (see ref. [15]).
[
[
20] L. F. Tietze, A. Schuffenhauer, P. R. Schreiner, J. Am. Chem. Soc.
1
998, 120, 7952.
[
21] Treatments of the coupled-oscillator model and its application to
organic stereochemistry: a) S. F. Mason, Quart. Rev. 1962, 17, 20;
b) ªTheory IIº, S. F. Mason, in Optical Rotatory Dispersion and
Circular Dichroism in Organic Chemistry (Ed.: G. Snatzke), Wiley,
London, 1967, Chapter 4, p. 71; c) G. Gottarelli, S. F. Mason, G. Torre,
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and Chiral Discrimination, Cambridge University Press, Cambridge,
1
982.
[
22] a) N. Harada, K. Nakanishi, Acc. Chem. Res. 1972, 5, 257; b) N.
Harada, K. Nakanishi in Circular Dichroic Spectroscopy: Exciton
Coupling in Organic Stereochemistry, University Science Books, Mill
Valley, CA, 1983; c) K. Nakanishi, N. Berova, in Circular Dichroism:
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[
[
23] a) H. DeVoe, J. Chem. Phys. 1964, 41, 393; b) H. DeVoe, J. Chem.
Phys. 1965, 43, 3199.
[28] J. A. Schellman, Acc. Chem. Res. 1968, 1, 194.
[29] A. Rauk, H. A. Peoples, J. Comput. Chem. 1980, 1, 240.
[30] See ref. [22b] pp. 194 ± 195.
[31] One could argue that 1 ± 12, 15, and 16 should be considered as
intrinsically dissymmetric chromophores, requiring an MO treatment.
We intend to begin such a type of analysis.
[32] H. C. Kolb, M. S. Van Nieuwenhze, K. B. Sharpless, Chem. Rev. 1994,
94, 2483.
24] In the DeVoe model, a molecule is considered to be composed of a set
of subsystems: the chromophores. These are polarized by the external
electromagnetic radiation and are coupled with each other by their
own dipolar oscillating fields. The optical properties (absorption,
refraction, optical rotatory dispersion, and circular dichroism) of the
molecule under study can be calculated by taking into account the
interaction of the subsystems. Therefore, this treatment requires a
division of the molecule into a set of subsystems that have to be
suitably characterized. Each group is represented in terms of one (or
more) classical oscillator(s); each oscillator represents an electric-
[33] P. Lupattelli, R. Ruzziconi, P. Scafato, A. DeglꢁInnocenti, A.
Belli Paolobelli, Synth. Commun. 1997, 27, 441.
Received: April 26, 2000 [F2445]
Chem. Eur. J. 2001, 7, No. 1
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79