Journal of the American Chemical Society
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16a•+, selective C(sp3)–Si coupling may then occur through radi-
(5) Takeda, M.; Shintani, R.; Hayashi, T. J. Org. Chem. 2013, 78, 5007–
5017.
(6) For a nickel/copper catalysis using benzylic pivalates, see: Zarate, C.;
Martin, R. J. Am. Chem. Soc. 2014, 136, 2236–2239.
(7) For a palladium catalysis using benzylic bromides and chlorides, see:
Huang, Z.-D.; Ding, R.; Wang, P.; Xu, Y.-H.; Loh, T.-P. Chem.
Commun. 2016, 52, 5609–5612.
(8) For a review, see: (a) Oestreich, M.; Hartmann, E.; Mewald, M.
Chem. Rev. 2013, 113, 402‒441. For the preparation of the Si–B
reagents 1, see: (b) Suginome, M.; Matsuda, T.; Ito, Y.
Organometallics 2000, 19, 4647‒4649 (1a and 1b). (c) Boebel, T. A.;
Hartwig, J. F. Organometallics 2008, 27, 6013‒6019 (1c).
(9) Scharfbier, J.; Oestreich, M. Synlett 2016, 27, 1274–1276.
(10) Chu, C. K.; Liang, Y.; Fu, G. C. J. Am. Chem. Soc. 2016, 138, 6404–
6407.
(11) For early examples of nickel-catalyzed cross-couplings of benzylic
chlorides and SiCl4, see: Lefort, M.; Simmonet, C.; Birot, M.;
Deleris, G.; Dunogues, J.; Calas, R. Tetrahedron Lett. 1980, 21,
1857–1860.
radical recombination between Cy• and 16a•+, leading to the
desired product 6aa and regenerated copper(I) catalyst 12+ in a
highly exergonic step.23 The rate-limiting step is thus very likely
the C–I bond reduction by complex 14a with a overall free energy
barrier of about 14.0 kcal/mol. When chlorocyclohexane (4a, Cl–
Cy) is used instead, a very similar reaction mechanism is involved
but over a 2.9 kcal/mol higher overall barrier due to relatively
smaller electron affinity of 4a upon dissociative electron
attachment in solution (see Table S6 in the Supporting
Information).
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With this work, we added another example to the still very
short list9,10 of methods for carbon–silicon bond formation by
transition-metal-catalyzed cross-coupling of unactivated alkyl
electrophiles.24 The present copper catalysis proceeds through a
radical mechanism, and the catalytic cycle as well as the release
of the silicon nucleophile from the Si–B pronucleophile were
computed. The quantum-chemical data explain the experimental
observations, including the role of the ligand and the thiocyanate
counteranion in the generation of the catalytically active copper
complex. The radical nature of the coupling allowed for its
combination with Ueno–Stork-type radical cyclizations
terminated by carbon–silicon bond formation, also demonstrating
the high functional-group tolerance of the protocol.
(12) There is also evidence of radical intermediates in the related
copper-catalyzed
carbon–boron
bond
formation
using
bis(pinacolato)diboron: (a) Yang, C.-T.; Zhang, Z.-Q.; Tajuddin, H.;
Wu, C.-C.; Liang, J.; Liu, J.-H.; Fu, Y.; Czyzewska, M.; Steel, P. G.;
Marder, T.; Liu, L. Angew. Chem., Int. Ed. 2012, 51, 528–532. (b)
Ito, H.; Kubota, K. Org. Lett. 2012, 14, 890–893.
(13) We also tested Me2PhSiB(NiPr2)2 but only traces of product
formation were observed for 2a→6aa under the optimized reaction
conditions. Known applications of this Si–B reagent are rare and do
not involve transition-metal catalysts. (a) Suginome, M.; Fukuda, T.;
Nakamura, H.; Ito, Y. Organometallics 2000, 19, 719–721 (isonitrile
insertion into Si–B bond). (b) Matsumoto, A.; Ito, Y. J. Org. Chem.
2000, 65, 5707–5711 (photochemically induced homolytic Si–B
bond cleavage).
ASSOCIATED CONTENT
Supporting Information
Experimental procedures, spectral data for all new compounds as
well as crystallographic and computed data. This material is
(14) The cyclohexyl/TEMPO adduct was verified and quantified by
GC-MS and GLC analysis with an internal standard (see the
Supporting Information for details).
(15) For
a recent review of copper-catalyzed atom-transfer radical
cyclizations, see: Clark, A. J. Eur. J. Org. Chem. 2016, 2231–2243.
(16) The radical cyclization of α-haloacetals derived from allylic alcohols
is often referred to as the Ueno–Stork reaction: (a) Ueno, Y.; Chino,
K.; Watanabe, M.; Moriya, O.; Okawara, M. J. Am. Chem. Soc. 1992,
104, 5564–5566. (b) Stork, G.; Mook Jr., R.; Biller, S. A.;
Rychnovsky, S. D. J. Am. Chem. Soc. 1993, 105, 3741–3742. For a
review, see: (c) Salom-Roig, X. J.; Dénès, F.; Renaud, P. Synthesis
2004, 1903–1928.
AUTHOR INFORMATION
Corresponding Author
Notes
(17) For the use of these cyclization precursors in conventional tin-based
radical cyclizations, see: Pezechk, M.; Brunetiere, A. P.; Lallemand,
J. Y. Tetrahedron Lett. 1986, 27, 3715–3718.
The authors declare no competing financial interest.
ACKNOWLEDGMENT
(18) For nickel-catalyzed radical cyclizations, see: (a) Vaupel, A.;
Knochel, P. J. Org. Chem. 1996, 61, 5743–5753. (b) Phapale, V. B.;
Buñuel, E.; García-Iglesias, M.; Cárdenas, D. J. Angew. Chem., Int.
Ed. 2007, 46, 8790–8795. (c) Guisán-Ceinos, M.; Soler-Yanes, R.;
Collado-Sanz, D.; Phapale, V. B.; Buñuel, E.; Cárdenas, D. J.
Chem.–Eur. J. 2013, 19, 8405–8410. (d) Peng, Y.; Xiao, X. J.; Wang,
Y. Chem. Commun. 2014, 50, 472–474. (e) Huang, C.; Fu, G. C. J.
Am. Chem. Soc. 2014, 136, 3788–3791.
(19) For palladium-catalyzed radical cyclizations, see: (a) Bloome, K. S.;
McMahen, R. L.; Alexanian, E. J. J. Am. Chem. Soc. 2011, 133,
20146–20148. For a general review of radical reactions involving
palladium, see: (b) Liu, Q.; Dong, X.; Li, J.; Xiao, J.; Dong, Y.; Liu,
H. ACS Catal. 2015, 5, 6111–6137.
W.X. thanks the China Scholarship Council (CSC) for a
predoctoral fellowship (2015–2019). Z.-W.Q. and S.G.
acknowledge support by the SFB 813 of the Deutsche
Forschungsgemeinschaft. M.O. is indebted to the Einstein
Foundation (Berlin) for an endowed professorship. We thank Dr.
Elisabeth Irran (TU Berlin) for the X-ray analysis.
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