UPDATES
Concerning the mechanism of the alkyl radical
formation, the pathway involving the oxidation of the
phosphine seems to be more probable for the following
reasons. Measurement of oxidation potentials sug-
gested that triphenylphosphine (+0.78 V vs. SCE) is
oxidized more readily than the zinc thiolate (cyclo-
hexylSZnCl, +1.48 V vs. SCE). Moreover, ethyl
phosphite P(OEt)3, which is known to react with thiyl
radicals,[8] was ineffective under our conditions (Ta-
ble 1, entry 6). In further agreement, the Stern-Volmer
studies demonstrated fluorescence quenching of the
iridium photocatalyst by triphenylphosphine, but not
by the zinc thiolate.
In summary, a mechanistic scenario for the gener-
ation of alkyl radicals from thiols under the conditions
excluding undesired hydrogen atom transfer is de-
scribed. This is achieved by using an organozinc
reagent to completely remove the thiol hydrogen. The
phosphine serves as a recipient of sulfur, thereby
providing driving force for the cleavage of the carbon-
phosphorus bond.
References
[1] a) M. Yan, J. C. Lo, J. T. Edwards, P. S. Baran, J. Am.
Galliher, D. A. Pratt, C. R. J. Stephenson, Chem. Soc.
[2] a) Radicals in Organic Synthesis (Eds.: P. Renaud, M. P.
Sibi), Wiley-VCH, Weinheim, 2001; b) S. Z. Zard,
Radical Reactions in Organic Synthesis, Oxford Univer-
sity Press, New York, 2004.
[3] a) B. Quiclet-Sire, S. Z. Zard, Pure Appl. Chem. 2011,
83, 519–551; b) B. Quiclet-Sire, S. Z. Zard, Chem. Eur.
[4] a) M. D. Kosobokov, M. O. Zubkov, V. V. Levin, V. A.
9453–9456; b) M. O. Zubkov, M. D. Kosobokov, V. V.
Levin, V. A. Kokorekin, A. A. Korlyukov, J. Hu, A. D.
ova, M. O. Zubkov, V. A. Kokorekin, V. V. Levin, A.
d) B. A. van der Worp, M. D. Kosobokov, V. V. Levin,
[5] Y. Wang, L.-F. Deng, X. Zhang, Z.-D. Mou, D. Niu,
[6] For generation of radicals via substitution at sulfurs, see:
[7] a) F. W. Hoffmann, R. J. Ess, T. C. Simmons, R. S.
b) W. G. Bentrude, in: Reactive Intermediates: Volume 3
(Ed.: R. A. Abramovitch), Springer US, Boston, MA,
1983, pp. 199–298.
[9] For recent reviews, see: a) J. A. Rossi-Ashton, A. K.
Clarke, W. P. Unsworth, R. J. K. Taylor, ACS Catal.
c) J. Maddigan-Wyatt, J. F. Hooper, Adv. Synth. Catal.
2021, 363, 924–936.
Experimental Section
Reaction of Thiols with α-trifluoromethyl Styrenes
(General Procedure)
Photocatalyst (Ir[dF(CF3)ppy]2(dtbpy))PF6 (1.4 mg, 0.25 mol%)
and triphenyl phosphine (262 mg, 1.0 mmol, 2.0 equiv.) were
placed in a tube (Duran cat. no 261351155, Roth cat. no
K248.1, outside diameter=12 mm). The tube was evacuated
and filled with argon. Thiol 1 (0.67 mmol 1.33 equiv.) was
added followed by benzyl zinc chloride (1.9 M in THF, 350 μL,
0.67 mmol, 1.33 equiv.), and the mixture was stirred for 10
minutes at room temperature. Then, Me3SiCl (16 μL,
0.125 mmol, 0.25 equiv.), dichloromethane (1.5 mL) and
styrene 2 (0.50 mmol) were successively added [for 3d, DMF
(200 μL) was added as a co-solvent to mitigate low solubility of
the corresponding zinc thiolate]. The tube was closed with a
screw cap and irradiated for 4 hours by a 450 nm LED chip
(Hontiey royal blue 100 W) operated at 40 Watt for 3a–s, u–x
or for 24 hours operated at 60 Watt for 3t. The distance
between LED chip and the reaction tube was 1 cm. The reaction
[10] Indeed, thiols are frequently used as sources of hydrogen
towards alkyl radicals. For examples, see: a) Y. Wang,
408; b) J. Dong, X. Wang, Z. Wang, H. Song, Y. Liu, Q.
Nagao, A. J. Hoover, D. Hesk, N. R. Rivera, S. L.
Colletti, I. W. Davies, D. W. C. MacMillan, Science
8036; b) L. Zhang, X. Si, Y. Yang, S. Witzel, K. Sekine,
M. Rudolph, F. Rominger, A. S. K. Hashmi, ACS Catal.
[12] a) P. Knochel, N. Millot, A. L. Rodriguez, C. E. Tucker,
in Org. React., John Wiley & Sons, Inc., 2004; b) P.
Knochel, H. Leuser, L. Z. Gong, S. Perrone, F. F.
°
temperature was maintained in a range of 15–20 C by a cooling
bath. For the work-up, the mixture was poured into water
(10 mL) and extracted with hexanes (4×4 mL). The combined
organic phases were dried over Na2SO4, filtered, concentrated,
and the residue was purified by column chromatography on
silica gel.
Acknowledgements
This work was supported by the Russian Science Foundation
(project 20-13-00112).
Adv. Synth. Catal. 2021, 363, 1–6
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