10.1002/anie.201711028
Angewandte Chemie International Edition
COMMUNICATION
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Figure 3. Molecular structure of 2j (left) and 2m (right), as obtained by single
crystal X-ray diffraction studies. Selected bond distances [Å]: 2j, 2m C1C8:
1.513(2), 1.518(3); C4C5: 1.509(2), 1.511(3); C5C6: 1.332(2), 1.333(3);
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In conclusion, a range of unique dibenzocyclooctenes has been
synthesized in good to excellent yields via metalloradical
activation of N-tosyl hydrazones 1 using [Co(TPP)] as the
catalyst. The resulting novel base-metal catalysed route to
8-membered rings is very different from known synthetic methods
that typically proceed via closed-shell pathways and rely on the
use of expensive and scarce transition metals. A large variety of
substituents is tolerated, including electron-withdrawing
and -donating groups on both phenyl rings. The reaction
mechanism involves carbene radical formation, which is
supported by PBN radical trapping experiments. 2D-NOESY
NMR and X-ray diffraction studies confirm C=C double bond
migration at the allylic position. This is in agreement with the
proposed mechanism, which also involves 8π-cyclisation of ortho-
quinodimethane intermediates followed by a [1,5]-hydride shift.
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B. de Bruin, Chem. Sci. 2017, DOI: 10.1039/C7SC03909C.
[15] Related ‘open-shell organometallic‘ approaches have been used by the
group of Gansäuer to activate epoxides and aziridines with TiIII
complexes: a) A. Gansäuer, S. Hildebrandt, A. Michelmann, T. Dahmen,
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[16] To obtain additional structural information, compound 1a has also been
hydrogenated to the corresponding dibenzocyclooctane. See ESI.
[17] The observed small variations in product yield upon changing R1 should
probably be ascribed to differences in product loss during the
purification steps.
[18] NMR shows no indications for allylic C=C double bond isomerisation
under the applied reactions conditions.
Acknowledgements
[19] Active participation of a gem-ester moiety in direct carbene insertion
reactions into CaromaticH bonds has been reported, but is not accessible
for carbene radical C and would not explain the double bond migration:
M. R. Fructos, M. Besora, A .A. C. Braga, M. M. Díaz-Requejo, F.
Maseras and P. J. Pérez, Organometallics, 2017, 36, 172-179.
We thank Ed Zuidinga for MS measurements and Jan Meine
Ernsting and Andreas Ehlers for NMR advice. Financial
support from the Netherlands Organization for Scientific
Research (NWO-CW VICI project 016.122.613) and the
University of Amsterdam (Research Priority Area
Sustainable Chemistry) is gratefully acknowledged.
[20] The allyl-phenyl substrate produces an unknown product. Replacing the
ester by a hydrogen atom leads to indene and cyclopropane products.
See ESI.
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Keywords: metalloradical • carbene radical •
dibenzocyclooctenes • ortho-quinodimethanes • 8-membered
ring formation
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