Edge Article
Chemical Science
to the pendant alkyne to construct the spirocyclic structure. The
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newly formed vinyl radical 9 can abstract a C4–H hydrogen atom
+
+
of the HE radical cation (HE c). This is because HE c has a low
homolytic bond-dissociation free energy for the C4–H bond
(
ꢀ1
44
this energy requirement is 31.4 kcal mol for acetonitrile).
Deuterium labeling experiments also shows that THF may offer
a H source for hydrogen atom transfer (Fig. 4). The in situ- 11 J. M. Birmingham, A. K. Fischer and G. Wilkinson,
produced Ti–oxygen bond is protonated by the HP cation and
Naturwissenschaen, 1955, 42, 96.
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IV
III
Ti is released as well as the cyclization product. Finally, Ir
III
can regenerate Cp Ti Cl for the next catalytic cycle.
2
1
3 J. Justicia, J. L. Oller-L ´o pez, A. G. Campa n˜ a, J. E. Oltra,
J. M. Cuerva, E. Bu n˜ uel and D. J. C ´a rdenas, J. Am. Chem.
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Conclusions
In summary, we report a novel and greener dual catalyst system 14 J. Friedrich, M. Dolg, A. Gans ¨a uer, D. Geich-Gimbel and
consisting of titanocene and the organic dye 4CzIPN for the T. Lauterbach, J. Am. Chem. Soc., 2005, 127(19), 7071.
radical spirocyclization of epoxides under photocatalytic 15 J. Friedrich, K. Walczak, M. Dolg, F. Piestert, T. Lauterbach,
conditions. This operationally simple, scalable, and efficient
method provides a strategically distinct manner for the
D. Worgull and A. Gans ¨a uer, J. Am. Chem. Soc., 2008, 130(5),
1788–1796.
assembly of heterospirocycles featuring a spiro all-carbon 16 A. Gans ¨a uer, D. Worgull, K. Knebel, I. Huth and
quaternary stereocenter. Unlike the metal reduction catalytic
G. Schnakenburg, Angew. Chem., Int. Ed., 2009, 48(47),
8882–8885.
conditions, mechanistic studies suggest that under photo-
III
catalytic conditions, Ti is present as Cp
2
TiCl which is readily 17 A. Gans ¨a uer, C. Kube, K. Daasbjerg, R. Sure, S. Grimme,
accessible to epoxides. As expected, the photocatalytic condi-
tions showed exceptional high reactivity for the assembly of
G. D. Fianu, D. V. Sadasivam and R. A. Flowers, J. Am.
Chem. Soc., 2014, 136(4), 1663–1671.
heterospirocycles. In this respect, the photocatalytic approach 18 J. Gordon, S. Hildebrandt, K. R. Dewese, S. Klare,
complements the metal reduction approach. Development of
new transformations based on photocatalytic reactions is
currently under way in our laboratory.
A. Gans ¨a uer, T. V. RajanBabu and W. A. Nugent,
Organometallics, 2018, 37(24), 4801–4809.
´
19 N. M. Padial, E. Roldan-Molina, A. Rosales, M. Alvarez-
Corral, I. Rodr ´ı guez-Garc ´ı a, M. Mu n˜ oz-Dorado and
J. E. Oltra, Studies in Natural Products Chemistry, 2018, vol.
Conflicts of interest
55, pp. 31–71.
There are no conicts to declare.
20 S. P. Morcillo, D. Miguel, A. G. Campana, L. Alvarez De
Cienfuegos, J. Justicia and J. M. Cuerva, Org. Chem. Front.,
2
014, 45(25), 15–33.
Acknowledgements
2
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We are grateful to the Fundamental Research Funds for the
2010, 49(48), 9250–9253.
Central Universities (No. DUT19LK31). We thank WATTCAS 22 J. Twilton, C. C. Le, P. Zhang, M. H. Shaw, R. W. Evans and
CHEM-TECH Co., Ltd. for kindly providing the photoreactor.
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