Preparation of Propargyl Hydroperoxides by Regioselective Oxidation of Allenic Zinc Reagents with Molecular Oxygen Toshiro Harada* and Emiko Kutsuwa Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan
[email protected] Received June 1, 2003
Treatment of allenic zinc reagents (R1R2CdCdC(R3)ZnL), generated by the reaction of propargyl derivative (R1R2C(X)CtCH) with triorganozincates (R33ZnLi), under oxygen atmosphere in the presence of ZnCl2 and chlorotrimethylsilane afforded propargyl hydroperoxides (R1R2C(OOH)CtCR3) regioselectively. In this reaction, the use of ZnCl2 and chlorotrimethylsilane as additives is essential for the transformation of the initially generated allenic reagents to more reactive chlorozinc species. Organozinc compounds have been well-known to react with molecular oxygen to provide hydroperoxides and/or alcohols (Scheme 1).1,2 Depending on the reaction conditions, such as solvents, temperatures, and O2 concentration, initially produced zinc peroxides often serve as oxidizing reagent to afford zinc alkoxides.3 The oxidation reaction is of less synthetic utility when the zinc organometallics are prepared from an alky iodide or bromide, derived from the corresponding alcohol. On the other hand, oxidation of organozinc intermediates generated by homologation reaction of organozinc precursors is a synthetically useful transformation, as demonstrated by Knochel et al. in their report of alcohol synthesis from zinc organometallics prepared by hydro- and carbozincation.1c We previously reported the reaction of triorganozincates with propargylic derivatives 1 leading to the efficient generation of homologated allenic zinc reagents 2 (Scheme 2).4 The zinc reagents react with a variety of electrophiles regioselectively at the γ position to give three component coupling products 3. Although organozinc compounds with a C(sp2)-Zn bond, such as vinyl and arylzincs, are reported to show reduced reactivity toward molecular oxygen in comparison with highly reactive alkylzincs,1b oxidation of allenic zinc reagents 1, if it proceeds in an SE2′ manner, would provide us with a method for the preparation of propargyl hydroperoxides5 and/or alcohols. In the present study, we investi(1) (a) Nu¨tzel, K. In Methoden der Organischen Chemie; Thieme: Stuttgart, Germany, 1973; Vol. 13/2a, p 700. (b) Chemla, F.; Normant, J. F. Tetrahedron Lett. 1995, 36, 3157. (c) Klement, I.; Lu¨tjens, H.; Knochel, P. Tetrahedron Lett. 1995, 36, 3161-3164. (d) Klement, I.; Knochel, P. Synlett 1995, 1113. (2) For the reaction of organometallics with molecular oxygen, see: Meher-Detweiler, M. Medhoden der Organischen Chemie; Thieme, Stuttgart, Germany, 1988; Vol. E13, p 175. (3) Boche, G.; Lohrenz, J. C. Chem. Rev. 2001, 101, 697. (4) (a) Harada, T.; Katsuhira, T.; Osada, A.; Iwazaki, K.; Maejima, K. Oku, A. J. Am. Chem. Soc. 1996, 118, 11377. (b) Harada, T.; Oku, A. In Organozinc Reagents. A Practical Approach; Knochel, P., Jones, P., Eds.; Oxford University Press: Oxford, NY, 1999; Chapter 6.
SCHEME 1
SCHEME 2
gated the oxidation of allenic zinc reagents generated by organozincate homologation. Results and Discussion Allenic zinc species 2a was prepared by treatment of propargyl mesylate 1a with 2 equiv of Bu3ZnLi in THF at temperatures from -85 to 0 °C (eq 1). The yield of 2a
is estimated to be 89% by a D2O quench experiment.4a When the resulting mixture was treated under oxygen 10.1021/jo034743p CCC: $25.00 © 2003 American Chemical Society
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J. Org. Chem. 2003, 68, 6716-6719
Published on Web 07/25/2003
Regioselective Oxidation of Allenic Zinc Reagents TABLE 1. O2-Oxidation of Allenic Zinc Reagent
SCHEME 3 a
Generated by the Reaction of Propargyl Mesylate 1a with Bu3ZnLi entry 1 2 3 4 5b 6 7 8 9 a
ZnCl2 (equiv)
TMSCl (equiv)
temp (°C)
yield (%)a 3a 4a 36 6 6 49
13 34 22 5
3 3 3 6 3
-85 -40 -40 -40 -40 -40 -85 -40 -40
54 60 50 21
7
0.5 3 0.5 0.5 0.5 3
