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Notes and references
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4 For selected examples of aza Diels–Alder reactions of sulfinimines, see:
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(b) S. Kobayashi, T. Furuya, T. Otani and T. Saito, Tetrahedron, 2008,
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5 See ESI for further details on the solvents screening.
6 F. A. Davis, A. J. Friedman and E. W. Kluger, J. Am. Chem. Soc., 1974,
96, 5000.
Fig. 2 X-Ray structures of disulfoxide 4a and 4b.
spectroscopic data, 1,2-disulfoxide is isolated from our reaction
conditions as a stochastic mixture of diastereomers. Racemic
C2 symmetric disulfoxide, (R,R)-4a (and its antipode), and the meso
compound were identified by 1H-NMR and 13C-NMR spectroscopy.
Isolation of suitable crystals allowed us to perform X-ray diffraction
analysis although just the meso compound afforded crystals
suitable for the above mentioned analysis (Fig. 2). For comparative
purposes, already known disulfide 3a was crystallised and analysed
under the same conditions.14
The compound meso-4a has a bond length between the
sulfur atoms of 2.110 Å, the sulfur oxygen bond is 1.261 Å with
a O–S–S–O dihedral angle of 1801 to minimise dipole repulsion.
Comparing to related disulfoxides, disulfoxide 4a presents a
shorter bond distance between sulfur atoms that could explain
the unusual stability observed for our compound. Calculations
previously reported on similar substrates predicted the same spatial
disposition in order to minimise the dipole–dipole interaction
together with a bond length for the S–S bond of 2.30 Å.
As previously observed for disulfoxide 4a, although 4b was
isolated as a mixture of rac-(R,R)-4b and meso 4b, only the
non chiral molecule afford crystals suitable for X-ray analysis.
Compared to the analogous 4a, the more electron rich disulfoxide
4b bears a longer sulfur–sulfur bond length of 2.128 Å and a
sulfur–oxygen atom distance of 1.392 Å. The longer sulfur–sulfur
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8 Geminate recombination of radicals is well know and has been
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9 A similar suggestion, involving the intervention of solvent, has been
made in the computational study of the isomerisation of sulfenyl-
sulfinates to thiosulfonate carried out by Gregory and Jenks. D. D.
Gregory and W. S. Jenks, J. Phys. Chem. A, 2003, 107, 3414.
10 (a) G. Medes, Biochem. J., 1937, 31, 1330; (b) G. Medes and N. Floyd,
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distance could be explained by the lower electron withdrawing 11 (a) M. S. Chen, N. Prabagaran, N. A. Labenz and M. C. White, J. Am.
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effect of mesityl group, compared to tolyl, on the sulfur atom and
could also be an explanation for the lower stability in solution
M. C. White, J. Am. Chem. Soc., 2007, 129, 7274; (e) S. A. Reed and M. C.
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noticed for this compound. Furthermore, the two oxygen atoms in
the molecule display an antiplanar disposition in order to minimise
dipole–dipole repulsion (O–S–S–O dihedral angle of 1801). Our
results show again that such instability was overestimated and 12 For selected studies on the reactivity of 1,2-disulfoxides, see: (a) F. Freeman,
Chem. Rev., 1984, 84, 117; (b) M. M. Chau and J. L. Kice, J. Am. Chem. Soc.,
1976, 98, 7711; (c) F. Freeman and C. N. Angeletakis, J. Am. Chem. Soc.,
1982, 104, 5766; (d) F. Freeman and C. N. Angeletakis, J. Am. Chem.
vicinal disulfoxides are the actual compound obtained, together
with disulfide and the corresponding nitrile, on the thermal
decomposition of sulfinimines.
Soc., 1983, 105, 4039; (e) F. Freeman, C. N. Angeletakis and T. Maricich,
J. Am. Chem. Soc., 1981, 103, 6232; ( f) F. Freeman, C. N. Angeletakis, W. J.
Pietro and W. J. Hehre, J. Am. Chem. Soc., 1982, 104, 1161.
We have unambiguously confirmed the structure of the
products obtained from the thermolysis of sulfinimines as
vicinal sulfoxides. Furthermore the isolation and characterisation
of these novel species opens the door to studies of the reactivity of
these structures and the design and synthesis of new ligands for
transition metals. Studies along these lines are ongoing in our
laboratories.
13 (a) A. Ishii, M. Nakabayashi and J. Nakayama, J. Am. Chem. Soc., 1999, 121,
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14 See ESI for further details.
Xunta de Galicia (JAS), University of Nottingham and EPSRC
(EP/E055346) are acknowledged for funding.
12632 | Chem. Commun., 2014, 50, 12630--12632
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