J. Am. Chem. Soc. 1999, 121, 9233-9234
9233
their derivatives. These chiral diaryliodonium salts allow asym-
metric R-phenylation of cyclic â-keto esters.9
Synthesis of Chiral Diaryliodonium Salts,
1,1′-Binaphthyl-2-yl(phenyl)iodonium
Tetrafluoroborates: Asymmetric r-Phenylation of
â-Keto Ester Enolates
Among the most general methods for the regioselective
synthesis of organoiodonium salts is Lewis acid-catalyzed group
14 metal-iodine(III) exchange.10 Attempts at Si-I(III) exchange
of racemic 2-(diacetoxyiodo)-1,1′-binaphthyl (1)8a with phenyl-
trimethylsilane in the presence of BF3-Et2O, however, did not
result in formation of the desired 1,1′-binaphthyl-2-yl(phenyl)-
iodonium tetrafluoroborate (2) but, instead, gave a cyclic five-
membered iodolium salt 3, presumably produced via the more
facile intramolecular cyclization at the C-2′ position.11 Tetraphen-
ylgermane also afforded 3; however, use of the more reactive
organostannane dramatically changed the reaction course and
resulted in Sn-I(III) exchange under mild conditions. Treatment
of (S)-(+)-1 with tetraphenylstannane (1 equiv) in the presence
of BF3-Et2O (2 equiv) in dichloromethane at room temperature
for 18 h in nitrogen afforded the chiral diaryliodonium salt (S)-
Masahito Ochiai,*,† Yutaka Kitagawa,† Naoko Takayama,†
Yoshikazu Takaoka,‡ and Motoo Shiro§
Faculty of Pharmaceutical Sciences, UniVersity of
Tokushima, 1-78 Shomachi, Tokushima 770-8505, Japan
Gifu Pharmaceutical UniVersity
5-6-1 Mitahora Higashi, Gifu 502-0003, Japan
Rigaku Corporation, 3-9-12 Matsubara
Akishima, Tokyo 196-8666, Japan
ReceiVed June 29, 1999
Diaryliodonium salts are versatile reagents in organic synthesis
and serve as highly activated species of aryl halides in nucleophilic
aromatic substitutions at the ipso positions.1 Their high reactivity
is probably due to the excellent nucleofugality of the aryliodonio
group, which shows a leaving group ability about 106 times greater
than that of triflate.2 Simple unactivated aromatic halides tend to
be nonreactive in bimolecular displacement reactions and, con-
sequently, fail to react with metal enolates under standard
experimental conditions;3,4 however, it has been well established
that diaryliodonium salts undergo transfer of one of the aryl groups
to enolate anions under mild conditions to give the R-arylated
carbonyl compounds.5
(-)-2 as colorless prisms (mp 236 °C; [R]24 -47.3° (c 0.91,
D
acetone); g98% ee)12 in 76% yield. Tin-λ3-iodane exchange of
(S)-2-(diacetoxyiodo)-2′-methyl-1,1′-binaphthyl, prepared from
(S)-2-bromo-2′-methyl-1,1′-binaphthyl14 via bromine-iodine ex-
change, followed by sodium perborate oxidation in acetic acid,
afforded the (S)-2′-methylbinaphthylyliodonium salt 4 (82%).
Similarly, (S)-2′-benzyl-λ3-iodane 5 was prepared from the known
(S)-2′-benzyl-2-bromo-1,1′-binaphthyl.15 Reaction of C2 chiral (R)-
2,2′-bis(diacetoxyiodo)-1,1′-binaphthyl8a with tetraphenylstannane
(2 equiv) gave the bisiodonium salt (R)-(-)-6 (hygroscopic
colorless powder; mp 222 °C; [R]25D -234.6° (c 0.55, acetone))
as a 1:1 inclusion complex with diethyl ether in 90% yield.
Although some chiral onium reagents such as sulfonium,
ammonium, phosphonium, and arsonium salts are employed in
asymmetric synthesis,6 to date there have been no reports of the
synthesis of optically active diaryliodonium salts or of their use
in asymmetric synthesis.7,8 We report herein, for the first time,
the synthesis and characterization of the chiral diaryliodonium
salts, 1,1′-binaphthyl-2-yl(phenyl)iodonium tetrafluoroborates and
* Corresponding author. Tel.: 88-633-7281. Fax: 88-633-9504. E-mail:
† University of Tokushima.
‡ Gifu Pharmaceutical University.
§ Rigaku Corporation.
(1) For reviews, see: (a) Koser, G. F. The Chemistry of Functional Groups,
Supplement D; Wiley: New York, 1983; Chapter 25. (b) Moriarty, R. M.;
Vaid, R. K. Synthesis 1990, 431. (c) Stang, P. J. Angew. Chem., Int. Ed. Engl.
1992, 31, 274. (d) Varvoglis, A. The Organic Chemistry of Polycoordinated
Iodine; VCH Publishers: New York, 1992. (e) Koser, G. F. The Chemistry
of Functional Groups, Supplement D2; Wiley: New York, 1995; Chapter 21.
(f) Stang, P. J.; Zhdankin, V. V. Chem. ReV. 1996, 96, 1123. (g) Kitamura,
T.; Fujiwara, Y. Org. Prep. Proc. Int. 1997, 29, 409. (h) Zhdankin, V. V.;
Stang, P. J. In Chemistry of HyperValent Compounds; Akiba, K., Ed.; Wiley-
VCH: New York, 1999; Chapter 11. (i) Ochiai, M. In Chemistry of
HyperValent Compounds; Akiba, K., Ed.; Wiley-VCH: New York, 1999;
Chapter 12.
Both the structure and the absolute configuration of (S)-(-)-2
were unambiguously established by single-crystal X-ray analysis.
The PLUTO representation of Figure 1, which includes the
counteranion (BF4-) of another molecule, exhibits an essentially
square-planar arrangement with four bonds to iodine [I-C(1),
I-C(21), I-F(1), and I-F(2′)].16 Notably, the phenyl ring and
one of the naphthylyl rings are almost parallel, with a dihedral
(2) Okuyama, T.; Takino, T.; Sueda, T.; Ochiai, M. J. Am. Chem. Soc.
1995, 117, 3360.
(3) Caine, D. Carbon-Carbon Bond Formation; Marcel Dekker: New
York, 1979; Chapter 2.
(7) For organo-λ3-iodanes bearing chiral oxygen ligands, see: (a) Imamoto,
T.; Koto, H. Chem. Lett. 1986, 967. (b) Hatzigrigoriou, E.; Varvoglis, A.;
Bakola-Christianopoulou, M. J. Org. Chem. 1990, 55, 315. (c) Ray, D. G.;
Koser, G. F. J. Am. Chem. Soc. 1990, 112, 5672.
(4) For R-arylation of enolates, see: (a) Photostimulated SRN1 reactions
with aryl halides: Rossi, R. A.; Rossi, R. H. Aromatic Substitution by the
SRN1 mechanism; ACS Monograph 178; American Chemical Society: Wash-
ington, DC, 1983. (b) Aryllead(IV) triacetates: Pinhey, J. T. Aust. J. Chem.
1991, 44, 1353. (c) Arylbismuth(V) reagents: Abramovitch, R. A.; Barton,
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10.1021/ja992236c CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/18/1999