Photoinversion of sulfoxides as a source of diversity in dynamic combinatorial chemistry

Article abstract of DOI:10.1021/ol102715p

Photochemical interconversion of the two diastereoisomers cis- and trans-thianthrene dioxide (1) can be considered an example of photodynamic combinatorial chemistry (PDCC) in which the interconversion among diastereomeric equilibrating species is brought

Full text of DOI:10.1021/ol102715p

ORGANIC
LETTERS
2011
Vol. 13, No. 1
142-145
Photoinversion of Sulfoxides as a
Source of Diversity in Dynamic
Combinatorial Chemistry
Stefano Di Stefano,*,†,‡ Marco Mazzonna,†,‡ Enrico Bodo,† Luigi Mandolini,†,‡ and
Osvaldo Lanzalunga*,†,‡
Dipartimento di Chimica and Istituto CNR di Metodologie Chimiche-IMC, Sezione
Meccanismi di Reazione c/o Dipartimento di Chimica, UniVersita` di Roma La
Sapienza, P.le A. Moro 5, 00185 Rome, Italy
stefano.distefano@uniroma1.it; osValdo.lanzalunga@uniroma1.it
Received November 9, 2010
ABSTRACT
Photochemical interconversion of the two diastereoisomers cis- and trans-thianthrene dioxide (1) can be considered an example of photodynamic
combinatorial chemistry (PDCC) in which the interconversion among diastereomeric equilibrating species is brought about by electromagnetic
irradiation. Photoequilibrium can be shifted by irradiation at different wavelengths or by addition of SnCl2 that binds cis-1 more efficiently than
trans-1.
The investigation of reactions carried out under thermody-
namic control has received considerable attention in recent
years, mainly because of the increasing interest in dynamic
combinatorial chemistry (DCC).1 The related prospects of
“proof reading and editing” via repeated bond dissociation-
recombination processes and of amplification of a desired
component of the equilibrated mixture via specific interaction
with a complexing entity (template) make DCC particularly
appealing. Several reversible reactions leading to covalent
bonds have been employed in DCC such as olefin metath-
esis,2 imine3 and hydrazone4 formation, transesterification,5
thiol-disulfide interchange,6 transacetalation,7 and so on. All
of the above reactions are carried out in the absence of any
interaction between electromagnetic field and matter. In this
paper, we report an example of photodynamic combinatorial
(3) (a) Leung, K. C.-F.; Arico´, F.; Cantrill, S. J.; Stoddart, J. F. J. Am.
Chem. Soc. 2005, 127, 5808–5810. (b) Storm, O.; Lu¨ning, L. Chem.sEur.
J. 2002, 8, 793–798. (c) Ziach, K.; Jurczak, J. Org. Lett. 2008, 10, 5158–
5162. (d) Giuseppone, N.; Lehn, J.-M. Chem.sEur. J. 2006, 12, 1715–
1722.
(4) (a) Lam, R. T. S.; Belenguer, A.; Roberts, S. L.; Naumann, C.;
Jarrosson, T.; Otto, S.; Sanders, J. K. M. Science 2005, 308, 667–669. (b)
Bulos, F.; Roberts, S. L.; Furla´n, R. L. E.; Sanders, J. K. M. Org. Biomol.
Chem. 2007, 5, 3092–3093. (c) Voshell, S. M.; Lee, S. J.; Gagne´, M. R.
J. Am. Chem. Soc. 2006, 128, 12422–12423.
† Dipartimento di Chimica, Universita` “La Sapienza”.
‡ Istituto CNR di Metodologie Chimiche, IMC.
(5) (a) Rowan, S. J.; Hamilton, D. G.; Brady, P. A.; Sanders, J. K. M.
J. Am. Chem. Soc. 1997, 119, 2578–2579. (b) Rowan, S. J.; Brady, P. A.;
Sanders, J. K. M. Angew. Chem., Int. Ed. Engl. 1996, 35, 2143–2145.
(6) (a) Otto, S.; Furlan, R. L. E.; Sanders, J. K. M. Science 2002, 297,
590–593. (b) Otto, S.; Kubik, S. J. Am. Chem. Soc. 2003, 125, 7804–7805.
(c) Corbett, P. T.; Tong, L. H.; Sanders, J. K. M.; Otto, S. J. Am. Chem.
Soc. 2005, 127, 8902–8903. (d) Leclaire, J.; Vial, L.; Otto, S.; Sanders,
J. K. M. Chem. Commun. 2005, 1959–1961. (e) Sarma, R. J.; Otto, S.;
Nitschke, J. R. Chem.sEur. J. 2007, 13, 9542–9546. (f) Rodriguez-
Docampo, Z.; Otto, S. Chem. Commun. 2008, 5301–5303.
(1) (a) Corbett, P. T.; Leclaire, J.; Vial, L.; West, K. R.; Wietor, J.-L.;
Sanders, J. K. M.; Otto, S. Chem. ReV. 2006, 106, 3652–3711. (b) Dynamic
Combinatorial Chemistry; Reek, J. N. H., Otto, S., Eds.; Wiley-VCH Verlag
GmbH & Co. KGaA: Weinheim, 2010.
(2) (a) Brandli, C.; Ward, T. R. HelV. Chim. Acta 1998, 81, 1616–1621.
(b) Marsella, M. J.; Maynard, H. D.; Grubbs, R. H. Angew. Chem., Int. Ed.
Engl. 1997, 36, 1101–1103. (c) Speller, S.; Nolte, R. J. M.; Rowan, A. E.
Chem. Commun. 2005, 3535–3537.
10.1021/ol102715p  2011 American Chemical Society
Published on Web 12/01/2010

chemistry (PDCC), herein defined as the DCC, in which the
interconversion among equilibrating species is brought about
by electromagnetic irradiation instead of thermal bond-
breaking and bond-forming processes.8
Scheme 1. Photochemical Interconversion of the Two
Diastereoisomers cis- and trans-Thianthrene Dioxide (1)
Sulfoxides are attracting continuous attention for their
importance as bioactive compounds and drugs, as valuable
intermediates in organic synthesis, and as ligands for
asymmetric transformations.9 An attractive characteristic of
sulfoxides is their well-known stereochemical stability.
Consequently, experimental conditions leading to stereomu-
tation of sulfoxides and in particular photochemically induced
(direct or sensitized) racemization of enantiopure sulfoxides
are the object of intensive investigation.10,11 Stereomutation
of the sulfur center causes racemization in the absence of
other stereogenic centers, but diastereomeric interconversions
can occur in the presence of other chiral centers as found in
the photochemical studies of (S,R) sulfoxides of penicillin
derivatives.12 Very recently, axial to equatorial change in
1,9-dithiaalkane-bridged thianthrene 10-oxides was accom-
plished by photoinduced S-O configuration change.13
When two or more sulfinyl groups are present in the same
molecule, inversion of one of them leads to interconversion
among diastereomers. Accordingly, we felt it worthwhile to
analyze the photochemical interconversion of the two dias-
tereoisomeric cis- and trans-thianthrene dioxides (1) (Scheme
1). While the trans-1 isomer exists in two identical endo-
exo conformers, the cis-1 isomer exists in two conformers
of different stability, namely endo-endo and exo-exo. The
latter is less stable than the former by 11 kcal/mol and,
therefore, not appreciably populated at room temperature.14
To the best of our knowledge, inversion of configuration
of a tetrahedral stereogenic center, although potentially useful
to interconvert molecules with different properties, was never
used as a diversity source in DCC.
Upon UV irradiation (λ ) 280 nm) of a 3.0 mM solution
of trans-1 in C6D6, a partial isomerization to cis-1 was
observed by H NMR analysis (Figure 1a and Figure S1 in
1
Figure 1. Photoisomerization of thianthrene dioxide 1 (irradiation
at λ ) 280 nm) in C6D6 at 25 °C (a) starting from trans-1 and (b)
starting from cis-1.
the Supporting Information). The process occurs with the
absence of side products at short times, but after prolonged
irradiation (>120 min), partial deoxygenation15 of 1 to give
thianthrene oxide was observed (see the experimental details
in the Supporting Information). The photoisomerization
follows first-order kinetics with a rate constant (ktrans + kcis)
of 5.4 × 10-4 s-1. The reverse process (cis-1 f trans-1) is
also observed upon irradiation at 280 nm in C6D6 and shows
a very similar first-order rate constant (Figure 1b). The same
final photostationary ratio of diastereoisomers ([cis-1]/[trans-
1] ) 2.4) is obtained in both experiments.
Photoisomerization of trans-1 to cis-1 was also carried
out in different solvents (CD2Cl2, CD3CN, and CD3OD)
under continuous irradiation at 280 nm. In all cases,
isomerization follows a first-order process leading to a
solvent-dependent [cis-1]/[trans-1] ratio. Rate constants ktrans
and kcis and the pertinent [cis-1]/[trans-1] photoequilibrium
ratios are listed in Table 1.
(7) (a) Fuchs, B.; Nelson, A.; Star, A.; Stoddart, J. F.; Vidal, S. Angew.
Chem. 2003, 115, 4352–4356. (b) Cacciapaglia, R.; Di Stefano, S.;
Mandolini, L. J. Am. Chem. Soc. 2005, 127, 13666–13671. (c) Di Stefano,
S. J. Phys. Org. Chem. 2010, 23, 797–805. (d) Berkovich-Berger, D.;
Lemcoff, N. G. Chem. Commun. 2008, 1686–1688.
(8) For a previous example of PDCC, see: Ingerman, A. L.; Waters,
M. L. J. Org. Chem. 2009, 74, 111–117. Here the azobenzene light-induced
trans-cis inversion is used to interconvert the members of a dynamic li-
brary.
(9) (a) Pellissier, H. Tetrahedron 2006, 62, 5559–5601. (b) Legros, J.;
Dehli, J. R.; Bolm, C. AdV. Synth. Catal. 2005, 347, 19–31. (c) Bentley, R.
Chem. Soc. ReV. 2005, 34, 609–624. (d) Fernandez, I.; Khiar, N. Chem.
ReV. 2003, 103, 3651–3705. (e) Carreno, M. C. Chem. ReV. 1995, 95, 1717–
1760.
(10) (a) Vos, B. W.; Jenks, W. S. J. Am. Chem. Soc. 2002, 124, 2544–
2547. (b) Lee, W.; Jenks, W. S. J. Org. Chem. 2001, 66, 474–480. (c)
Tsurutani, Y.; Machida, S.; Horie, K.; Kawashima, Y.; Nakano, H.; Hirao,
K. J. Photochem. Photobiol. A:Chem. 1999, 122, 161–168. (d) Guo, Y.;
Jenks, W. S. J. Org. Chem. 1995, 60, 5480–5486. (e) Mislow, K.; Axelrod,
M.; Rayner, D. R.; Gotthardt, H.; Coyne, L. M.; Hammond, G. S. J. Am.
Chem. Soc. 1965, 87, 4958–4960
.
Since the UV spectra of cis-1 and trans-1 differ markedly
in the position and shape of the absorption bands (Figure
S2, Supporting Information),16 we analyzed the effect of
(11) (a) Aurisicchio, C.; Baciocchi, E.; Gerini, M. F.; Lanzalunga, O.
Org. Lett. 2007, 9, 1939–1942. (b) Cooke, R. S.; Hammond, G. S. J. Am.
Chem. Soc. 1970, 92, 2739–2745. (c) Mislow, K.; Axelrod, M.; Rayner,
D. R.; Gotthardt, H.; Coyne, L. M.; Cooke, R. S.; Hammond, G. S. J. Am.
Chem. Soc. 1969, 92, 2739–2745. (d) Cooke, R. S.; Hammond, G. S. J. Am.
Chem. Soc. 1968, 90, 2958–2959
.
(15) (a) Korang, J.; Grither, W. R.; McCulla, R. D. J. Am. Chem. Soc.
2010, 132, 4466–4476. (b) McCulla, R. D.; Jenks, W. S. J. Am. Chem.
Soc. 2004, 126, 16058–16065. (c) Gregory, D. D.; Wan, Z. H.; Jenks, W. S.
J. Am. Chem. Soc. 1997, 119, 94–102. (d) Thomas, K. B.; Greer, A. J.
Org. Chem. 2003, 68, 1886–1891.
(12) (a) Spry, D. O. J. Am. Chem. Soc. 1970, 92, 5006–5008. (b) Archer,
R. A.; Kitchell, B. S. J. Am. Chem. Soc. 1966, 88, 3462–3463.
(13) Suwabe, S.; Okuhara, A.; Sugahara, T.; Suzuki, K.; Kunimasa, K.;
Nakajima, T.; Kumafuji, Y.; Osawa, Y.; Yoshimura, T.; Morita, H.
Tetrahedron Lett. 2009, 50, 1381–1384.
(16) Mislow, K.; Schneider, P.; Ternay, A. L. J. Am. Chem. Soc. 1964,
86, 2957–2958.
(14) Deubel, D. J. Org. Chem. 2001, 66, 2686–2691.
Org. Lett., Vol. 13, No. 1, 2011
143

high photoisomerization quantum yield clearly indicates that
a significant nonradiative decay of the excited state occurs
by stereomutation at the sulfinyl group. It is interesting to
note that the photoisomerization process is not influenced
by the presence of oxygen; photoisomerization of trans-1 to
cis-1 occurred with the same rate (ktrans ) 3.6 × 10-4 s-1)
in N2- and O2-saturated CD3CN solutions. Moreover, we
found that photoisomerization of trans-1 to cis-1 occurred
to a very small extent by irradiating at 280 nm a 3.0 mM
solution of trans-1 in acetone.21 The lack of photo-stereo-
mutation with triplet sensitization suggests that a nonradiative
decay of a singlet excited state of 1 occurs as found for the
photoinduced racemization of aryl methyl sulfoxides.22
The photoequilibration properties related to trans-1 and
cis-1 can be exploited to generate a minimal dynamic library
consisting of two interconverting compounds. This is a small
dynamic system in which the interconversion among species
is guaranteed by irradiation. An important characteristic of
a dynamic system is its ability to modify the product
distribution by changing the factors that rule the equilibrium.
Actually, changes in the cis-trans distribution of 1 at
photoequilibrium were observed at different irradiation
wavelengths. Moreover the addition of a complexing species
able to tightly and selectively bind one of the photoequili-
brating diastereomers should shift the photoequilibrium
toward the complexed species.
Table 1. Rate Constants for the Photoisomerization of trans-1 to
cis-1 (ktrans)a and of cis-1 to trans-1 (kcis)a and [cis-1]/[trans-1]
Photoequilibrium Ratios Determined under Irradiation of 3.0
mM Solutions of Thianthrene Dioxide at 25 °C
ktrans
kcis
(10-4 s-1
b
b
solvent λ (nm) (10-4 s-1
)
)
[cis - 1]/(trans - 1]
C6D6
280
280
280
280
250
259
315
3.83
3.33
2.50
3.61
0.83
1.25
1.83
1.61
0.53
0.67
0.69
1.33
1.36
n.d.
2.4
6.3
3.8
5.2
0.63
0.92
>100c
CD2Cl2
CD3OD
CD3CN
a Calculated from kobs (ktrans+kcis) and final photostationary [cis-1]/[trans-
1] ratios (ktrans/kcis). b Error (10%. c No traces of trans-1 have been detected.
wavelength variations on the photoisomerization process.
Thus, kinetic measurements were carried out by irradiating
3.0 mM solutions of trans-1 in CD3CN17 at 280, 259, and
250 nm where the molar absorptivity of trans-1 is higher,
equal, and lower, respectively, than that of cis-1. Photoir-
radiation was also carried out at 315 nm where only trans-1
has a significant absorption. In Table 1 are reported the
photoisomerization rate constants ktrans and kcis determined
at the four wavelengths and the corresponding [cis-1]/[trans-
1] photoequilibrium ratios.
The [cis-1]/[trans-1] ratio varies significantly by changing
the irradiation wavelength. As expected, irradiation of 1 at 280
and 250 nm shifted the equilibrium (1) toward the cis-1 and
trans-1 isomers, respectively. Irradiation at 259 nm, where the
two isomers have the same molar absorptivity, led to compa-
rable amounts of cis-1 and trans-1. Finally, irradiation at 315
nm, where cis-1 has no significant absorption, leads to an almost
complete isomerization of trans-1 to cis-1.18 On the basis of
these results it is possible to rationalize the effect of changing
the nature of the solvent on the photostationary equilibrium
observed by irradiation at 280 nm (Table 1). The different [cis-
1]/[trans-1] ratios observed depend on the differences of the
molar absorptivity of the two isomers in the four solvents
investigated, in that the [cis-1]/[trans-1] ratio increases on
increasing the εtrans-1/εcis-1 at 280 nm in the four solvents (Figure
S3, Supporting Information).
With the aim of finding a suitable complexing species,
we studied the effect of the presence of several metal cations
in acetonitrile solution of trans-1 and cis-1. We discovered
that SnCl2 binds to both trans-1 and cis-1. When a 10-fold
excess of SnCl2 is added to an equimolar solution (1.5 mM)
1
of trans-1 and cis-1 in CD3CN, H NMR aromatic signals
of both trans-1 and cis-1 show a significant downfield shift
which is higher for trans-1 (0.15 ppm) than for cis-1 (0.05
ppm) (see Figure S4, Supporting Information). When pho-
toirradiation (λ ) 280 nm) of a 3.0 mM solution of trans-1
in CD3CN is carried out in the presence of SnCl2 (1.5 mM,
0.5 equiv), 1H NMR analysis of the reaction mixture shows
that the photostationary equilibrium is significantly shifted
toward the cis isomer.23 This effect is stronger at increasing
SnCl2 concentration (Table 2).
On addition of 1 or 2 equiv of SnCl2, no traces of trans-1
were detected in the 1H NMR spectra of the photoequilibrated
(21) Acetone triplet energy (ET ) 82 kcal/mol) should be higher than
that of 1 since for a large number of aryl and diaryl sulfoxides ET < 81
kcal/mol. Jenks, W. S.; Lee, W.; Shutters, D. J. Phys. Chem. 1994, 98,
2282–2289.
We have also determined the quantum yield (Φiso) for the
photoisomerization of trans-1 to cis-1 in acetonitrile at 280
nm using the photolysis of azoxybenzene to form 2-hy-
droxyazobenzene as actinometer.19 A high value of Φiso
(0.61) was obtained which is not surprising since values even
higher than 0.8 were found for the quantum yields of the
loss of optical rotation in enantiopure aryl sulfoxides.20 The
(22) The evidence in favor of the existence of such mechanisms for
photochemical isomerization that does not produce homolytic R-cleavage
but involves a nonradiative decay from an exited state was discussed in
detail by Jenks and co-workers (ref 10b). Although experimental evidence
is accumulating, the precise details of the inversion mechanisms are still
unknown. A pathway which is consistent with all of the experimental
findings involves the following: (i) the initial formation of an electronically
excited state in the pyramidal geometry, (ii) its geometric relaxation to a
configuration which is approximately trigonal on sulfur, and (iii) a
subsequent nonradiative decay to the ground state in a planar geometry
(structurally similar to the thermal transition state) which evolves into one
of the two possible configurations. The multiplicity of the excited state is
still debated, although there is some evidence in favor of a singlet state as
found in the present case.
(17) Acetonitrile was chosen because of its transparency in the UV
spectral region investigated.
(18) The photoinduced axial to equatorial S-O configuration change
in 1,9-dithiaalkane-bridged thianthrene 10-oxides was accounted for in terms
of the weak UV absorption of the axial isomer and the negligible absorption
of the equatorial isomer (ref 13).
(19) Bunce, N. J.; LaMarre, J.; Vaish, S. P. Photochem. Photobiol. 1984,
39, 531–533.
(23) It was verified that the presence of SnCl2 (1.5 mM) has a negligible
influence on the relative absorption of 3.0 mM solutions of cis-1 and trans-1
in CD3CN at 280 nm.
(20) Guo, Y.; Jenks, W. S. J. Org. Chem. 1997, 62, 857–864.
144
Org. Lett., Vol. 13, No. 1, 2011

is transformed into the SnCl2 complex of cis-1 upon
irradiation, the latter cannot revert to the former.26
Table 2. Photoisomerization (λ ) 280 nm) of trans-1 Solutions
(3.0 mM) in CD3CN at 25 °C in the Presence of SnCl2
Interestingly, the trans-1 f cis-1 photoinversion process
is also more rapid in the presence of SnCl2 and becomes
increasingly faster on increasing the SnCl2 concentration (see
Table 2).27 This effect might be due to a photosensitizing
effect of the Sn(II) salt for the sulfoxide photoinversion. In
order to check this hypothesis, we have investigated the effect
of SnCl2 on the photostereomutation process of a system
containing only one sulfinyl group, namely, (S)-methyl
p-tolyl sulfoxide, (S)-2. When a 3.5 mM solution of (S)-2 in
CH3CN is irradiated at the absorbance maximum (243 nm)
in the presence of an equimolar amount of SnCl2 a significant
decrease of the photoracemization rate is observed (see
details and Figure S8, Supporting Information).
[SnCl2] (mM)
t1/2 (s)
[cis-1]/[trans-1]a
1.8 × 103
1.8 × 103
1.2 × 103
7.5 × 102
5.2b
1.50
3.00
6.00
16b
>100c
>100c
a Measured at photoequilibrium. b Error (10%. c No traces of trans-1
1
were detected by H NMR analysis.
solutions, thus indicating that complexation of SnCl2 by cis-1
is far more effective than complexation by trans-1 in the
reaction conditions.24 This result can be explained by the
formation of a chelated adduct between cis-1 and SnCl2 in
which the tin ion is complexed by the two sulfinyl oxygen
atoms of cis-1 in its exo-exo conformation (Figure 2a).25 In
This finding suggests that SnCl2 is not a photosensitizer for
the trans-1 f cis-1 photoisomerization but its effect on the
photoinversion rate is probably due to a stabilizing chelate binding
of the TS, the structure of which is similar to that of the thermal
TS22 and most likely resembles that of the cis isomer.
In the present paper, we have demonstrated that photo-
equilibration of trans-1 and cis-1 can be used as source of
diversity in the generation of a minimal dynamic library.
Photoequilibrium in Scheme 1 can be shifted by irradiation
of the solution at different wavelengths or by the presence
of SnCl2 that binds cis-1 more efficiently than trans-1. These
results suggest that the exploitation of photodynamic pro-
cesses as a further degree of freedom in the constitution of
dynamic libraries can offer an additional route to the
development of DCC. Furthermore, photoreversible processes
appear to be highly suitable for a “multiple reactions
approach” to DCC.6e,f,8,28 Systems endowed with a higher
number of stereocenters, giving rise to more complex
photodynamic libraries, are currently under investigation. The
results of these studies will be reported in due time.
Figure 2. Chelated complex between SnCl2 and cis-1 in its exo-
exo conformation.
Figure 2b is reported the structure of the tin(II) complex of
cis-1 calculated at the B3LYP/6-31+G(d)/sddall level of
theory (see Figure S7, Supporting Information).
Even though cis-1 is much more stable in its endo-endo
conformation,13 theoretical calculations indicate that the exo-exo
tin complex is ca. 83 kcal/mol more stable than the endo-endo tin
complex in a vacuum (see Figure S6, Supporting Information).
It seems likely that complexation of SnCl2 acts as a kinetic
trap for cis-1. It hinders the pyramidal inversion at the sulfur
stereogenic centers so that once the SnCl2 complex of trans-1
Acknowledgment. “La Sapienza University” of Rome is
thanked for financial support (project AST 2008). E. B.
acknowledges computational support of the CASPUR
Supercomputing Center.
Supporting Information Available: Instrumentation,
materials and methods, photoisomerization of trans-1 and
cis-1 in the absence and in the presence of SnCl2, photora-
cemization of (S)-methyl p-tolyl sulfoxide in the absence and
in the presence of SnCl2, and theoretical calculations. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(24) All of the attempts to directly measure the association constants of
cis-1 and trans-1 with SnCl2 failed because of the interference of tiny but
unavoidable amounts of water in the acetonitrile solution. Even a sample
of freshly distilled CD3CN contains a water concentration >2 mM. The
water 1H NMR signal changes in shape (broader) and in chemical shift
(downfield shifted) when SnCl2 is added to the solution during the titration,
thus demonstrating interactions between water and the tin salt. As a
consequence, any quantitative binding measurement concerning the as-
sociation between SnCl2 and both the thianthrene dioxide isomers does not
seem allowed. Obviously, this interference increases when the concentration
of the species to be titrated decreases.
(25) Formation of complexes of tin ions with sulfoxides by interaction
with the sulfinyl oxygen is well documented: (a) Calligaris, M. Coord. Chem.
ReV. 2004, 24, 8–351. (b) Hsu, C. C.; Geanangel, R. A. Inorg. Chem. 1980,
19, 110–119. The structures of chelated complexes of disulfoxides with
triphenyltin chloride with bonding occurring through the sulfinyl oxygen
atoms have been also reported: Filgueiras, C. A. L.; Celso, C.; Marques,
E. V.; Johnson, B. F. G. Inorg. Chim. Acta 1982, 59, 71–74.
OL102715P
(26) Theoretical calculations at the B3LYP/6-31+G(d)/sddall level have
been carried out for the structure of Sn2+ complexes of cis-1, trans-1, and
thermal transition state (see Figure S7 in the Supporting Information). The
transition state has now a geometry much more similar to the exo-exo
conformer of cis-1 than to the endo-exo of trans-1. This result, together
with the assumption that the photochemical mechanism described in ref 22
does not change in the presence of Sn2+ should justify qualitatively the
preferential formation of cis-1.
(27) Since no clear first-order kinetic profiles were observed for the
photoisomerization in the presence of SnCl2, t1/2 values have been reported.
(28) Christinat, N.; Scopelliti, R.; Severin, K. Angew. Chem., Int Ed.
2008, 47, 1848–1852
.
Org. Lett., Vol. 13, No. 1, 2011
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