C O M M U N I C A T I O N S
Table 5. Intramolecular Pd-Catalyzed Oxidative Tetrahydrofuran
and Lactone Ring-Forming Examples
National Science and Engineering Research Council of Canada
(NSERC). V.M.D. is grateful for support from Merck Frosst.
Professor Erik Sorensen (Princeton University) is thanked for
helpful discussion.
Note Added after ASAP Publication. The version of this
paper published February 19, 2008, contained an error in Figure
2. The version published on February 22, 2008 has the correct
information.
Supporting Information Available: Experimental procedures,
spectroscopic data for all new compounds, detailed analysis of the
isotopic labeling experiment, and crystallographic data for 12g in cif
format. This material is available free of charge via the Internet at http://
pubs.acs.org.
References
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Lloyd-Jones, G. C.; Booker-Milburn, K. I. J. Am. Chem. Soc. 2005, 127,
7308. (b) Streuff, J.; Ho¨velmann, C. H.; Nieger, M.; Mun˜iz, K. J. Am.
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Am. Chem. Soc. 2007, 129, 11688. (f) Xu, L.; Du, H.; Shi, Y. J. Org.
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Lee, C.; Sorensen, E. J. J. Am. Chem. Soc. 2005, 127, 7690. (b) Liu, G.;
Stahl, S. S. J. Am. Chem. Soc. 2006, 128, 7179. (c) Desai, L. V.; Sanford,
M. S. Angew. Chem., Int. Ed. 2007, 46, 5737.
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J. Am. Chem. Soc. 2006, 128, 1460. (b) Zhang, Y.; Sigman, M. S. J. Am.
Chem. Soc. 2007, 129, 3076.
(4) For examples of carboetherification and carboamination, see: (a) Wolfe,
J. P.; Rossi, M. A. J. Am. Chem. Soc. 2004, 126, 1620. (b) Lira, R.; Wolfe,
J. P. J. Am. Chem. Soc. 2004, 126, 13906. (c) Nakhla, J. S.; Kampf, J.
W.; Wolfe, J. P. J. Am. Chem. Soc. 2006, 128, 2893. (d) Fritz, J. A.;
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(6) Stang, P. J.; Cao, D. H.; Poulter, G. T.; Arif, A. M. Organometallics 1995,
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a 0.25 mmol scale (0.1 M in wet HOAc), 1.1 equiv of PhI(OAc)2, 2 mol
% of catalyst 4. b Diastereomeric ratio. c Isolated yield. d 1.5 equiv of
PhI(OAc)2 was used. e 5 mol % Pd(TFA)2 and 5.5 mol % dppp were used.
in wet AcOH afforded tetrahydrofuran 13a in good yield as a
mixture of diastereoisomers (78% yield, 1.1:1 dr, Table 4, entry
1). Substrates bearing tertiary alcohol groups also underwent 5-endo
cyclization to generate the corresponding tetrahydrofurans 13b (90%
yield, 1.1:1 dr) and 13c (80% yield) (entries 2 and 3). As shown in
entry 4, 5-phenylpent-4-en-1-ol preferentially undergoes 5-exo
cyclization to form tetrahydrofuran 13d regioselectively (90% yield,
2.3:1 dr). By using Pd(TFA)2/dppp as the catalyst, an oxidative
lactonization occurred to give cyclic ester 13e (85% yield, 1.3:1,
entry 5). We are currently investigating the use of chiral ligands to
improve diastereselectivity and achieve enantiocontrol in these
cyclizations.
In summary, we have developed a novel method to dioxygenate
alkenes using cationic Pd catalysts. In comparison to related vicinal
oxidations, a broad range of olefins can be functionalized in both
inter- and intramolecular processes. The catalyst bears two important
structural features: an electron-rich diphosphine ligand and non-
coordinating counterions. Current studies are underway to elucidate
the effect of catalyst structure on the mechanism of this Pd(II)/
(IV) dioxygenation.
(7) For reviews on cationic Pd(II)-catalysts, see: (a) Mecking, S. Coord.
Chem. ReV. 2000, 203, 325. (b) Strukul, G. Top. Catal. 2002, 19 (1), 33.
(c) Hamashima, Y.; Sodeoka, M. Chem. Rec. 2004, 4, 231. (d) Hamashima,
Y. Chem. Pharm. Bull. 2006, 54 (10), 1351.
(8) Preliminary results from 31P NMR experiments support the existence of
a cationic Pd(II)-aquo compex in acetic acid. The spectrum for catalyst 4
in AcOH-d4 shows a 31P resonance at 16.73 ppm. In contrast, Pd(dppp)-
(OAc)2 resonates at 11.78 ppm. Notably, Pd(dppp)(OAc)2 does not catalyze
the desired transformation under the same reaction conditions. The
independently synthesized dppp dioxide shows a 31P resonance at ∼41
ppm. No significant amount of dppp dioxide was observed after the
complete consumption of 1. However, in the absence or after the
consumption of 1, the oxidation of dppp ligand was observed after heating
the catalyst and PhI(OAc)2 over an extended period of time.
(9) See supporting information for proposed mechanisms for the formation
of unlabeled and doubly labeled hydroxyacetates 3a.
(10) Oikawa, M.; Wada, A.; Okazaki, F.; Kusumoto, S. J. Org. Chem. 1996,
61, 4469.
(11) For reviews of natural products, see: (a) Zeng, L.; Ye, Q.; Oberlies, N.
H.; Shi, G.; Gu, Z. M.; He, K.; McLaughlin, J. L. Nat. Prod. Rep. 1996,
13, 275. (b) Dutton, C. J.; Banks, B. J.; Cooper, C. B. Nat. Prod. Rep.
1995, 12, 165.
(12) For reviews on formation of tetrahydrofurans, see: (a) Boivin, T. L. B.
Tetrahedron 1987, 43, 3309. (b) Wolfe, J. P.; Hay, M. B. Tetrahedron
2007, 63, 261.
Acknowledgment. Funding was provided by the University of
Toronto, the Connaught Foundation, Canadian Foundation for
Innovation, Ontario Ministry of Research and Innovation, and
JA711029U
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2964 J. AM. CHEM. SOC. VOL. 130, NO. 10, 2008