Synthesis of chiral diaryliodonium salts, 1,1'-binaphthyl-2- yl(phenyl)iodonium tetrafluoroborates: Asymmetric α-phenylation of β-keto ester enolates [8]

Full text of DOI:10.1021/ja992236c

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:
mochiai@ph2.tokushima-u.ac.jp.
† 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,
D. H. R.; Finet, J.-P. Tetrahedron 1988, 44, 3039. (d) Transition-metal-
catalyzed reactions: Palucki, M.; Buchwald, S. L. J. Am. Chem. Soc. 1997,
119, 11108. Hamann, B. C.; Hartwig, J. F. J. Am. Chem. Soc. 1997, 119,
12382 and references therein.
(8) For organo-λ3-iodanes bearing chiral carbon ligands, see: (a) Ochiai,
M.; Takaoka, Y.; Masaki, Y.; Nagao, Y.; Shiro, M. J. Am. Chem. Soc. 1990,
112, 5677. (b) Rabah, G. A.; Koser, G. F. Tetrahedron Lett. 1996, 37, 6453.
(c) Kitamura, T.; Lee, C. H.; Taniguchi, Y.; Fujiwara, Y. J. Am. Chem. Soc.
1997, 119, 619.
(9) For Pd-catalyzed asymmetric R-arylation of enolates (up to 98% ee),
see: Ahman, J.; Wolfe, J. P.; Troutman, M. V.; Palucki, M.; Buchwald, S. L.
J. Am. Chem. Soc. 1998, 120, 1918.
(10) (a) Koser, G. F.; Wettach, R. H.; Smith, C. S. J. Org. Chem. 1980,
45, 1543. (b) Ochiai, M.; Sumi, K.; Takaoka, Y.; Kunishima, M.; Nagao, Y.;
Shiro, M.; Fujita, E. Tetrahedron 1988, 44, 4095. (c) Stang, P. J.; Zhdankin,
V. V. J. Am. Chem. Soc. 1990, 112, 6437.
(11) (a) Collette, J.; McGreer, D.; Crawford, R.; Chubb, F.; Sandin, R. B.
J. Am. Chem. Soc. 1956, 78, 3819. (b) Beringer, F. M.; Chang, L. L.; Fenster,
A. N.; Rossi, R. R. Tetrahedron 1969, 25, 4339.
(5) (a) Beringer, F. M.; Galton, S. A.; Huang, S. J. J. Am. Chem. Soc.
1962, 84, 2819. (b) Chen, K.; Koser, G. F. J. Org. Chem. 1991, 56, 5764. (c)
Kita, Y.; Okunaka, R.; Kondo, M.; Tohma, H.; Inagaki, M.; Hatanaka, K. J.
Chem. Soc., Chem. Commun. 1992, 429.
(6) (a) Umemura, K.; Matsuyama, H.; Watanabe, N.; Kobayashi, M.;
Kamigata, N. J. Org. Chem. 1989, 54, 2374. (b) Dolling, U.-H.; Davis, P.;
Grabowski, E. J. J. J. Am. Chem. Soc. 1984, 106, 446. (c) Rein, T.; Reiser,
O. Acta Chem. Scand. 1996, 50, 369. (d) Dai, W.-M.; Wu, J.; Huang, X.
Tetrahedron: Asymmetry 1997, 8, 1979.
10.1021/ja992236c CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/18/1999

9234 J. Am. Chem. Soc., Vol. 121, No. 39, 1999
Communications to the Editor
Table 1. Asymmetric Phenylation of the â-Keto Ester 7a with
λ3-Iodanes
entry
iodane
solvent
T/°C
8a, % yielda
% ee
1
2
3
4
5
6
7
8
9
2
2
2
2
2
4
5
5
6
t-BuOH
25
70
-78
25b
25c
25
65
69
15
71
63
68
30
63
51
37
38
40
37
40
34
53
44
37
t-BuOH
t-BuOH-THF
t-BuOH
t-BuOH
t-BuOH
t-BuOH
THF
25
25b
30
t-BuOH
a Isolated yields. b 2 equiv of Ph2CdCH2 was used. c 1.1 equiv of
18-crown-6 was used.
Figure 1. PLUTO representation of the iodonium salts (S)-(-)-2 with
-
an extra BF4 anion from the second molecule.
angle of 16.0°, similar to the shifted, stacked structure of a
benzene dimer.17
Exposure of the potassium enolate of 2-(methoxycarbonyl)-1-
indanone (7a) (generated by the reaction with t-BuOK) to the
chiral diaryliodonium salt (S)-2 (t-BuOH at room temperature,
20 h) gave selectively the R-phenylated indanone 8a in 65% yield.
No formation of O-phenyl derivative was observed. This reaction
is highly regioselective in the sense that there is no evidence for
transfer of the binaphthylyl group of 2 to the cyclic â-keto ester
7a. This selectivity is in marked contrast to the reported ortho
steric effects that the unsymmetrical diaryliodonium salts experi-
ence: nucleophilic ipso substitutions occur preferentially at the
aryl groups with sterically demanding ortho substituents, so as
to provide maximum relief of the steric strain.18 The chiral (S)-
2-iodo-1,1′-binaphthyl was easily recovered in higher than 80%
yields, without loss of optical purity, and reused.
demonstration of asymmetric arylation of metal enolates that does
not rely on a transition metal catalyst. The results are summarized
in Table 1. Changing the reaction temperature from -78 to 70
°C and the solvents from t-BuOH to THF, DMF, dichloromethane,
or methanol showed negligible effects on the degree of asym-
metric induction (33-40% ee). A similar level of ee was obtained
in the reaction with 2′-methyl-λ3-iodane (S)-4; however, the
sterically more demanding 2′-benzyl-λ3-iodane (S)-5 resulted in
a higher asymmetric induction up to 53% ee (entry 7). Bisiodo-
nium salt (R)-6 gave the R-phenylated indanone 8a in 37% ee
(entry 9). The absolute configuration of the major enantiomer in
these reactions was determined to be R by single-crystal X-ray
analysis of the dithioacetal derivative 9a, obtained by repeated
fractional recrystallization. The degree of chiral induction achieved
by using several chiral â-keto esters 7b-f and (S)-2 is comparable
to that of methyl ester 7a: 11-47% de.
Thus, in the present study, not only do we demonstratesthe
first synthesis of chiral diaryliodonium salts, via BF3-catalyzed
tin-λ3-iodane exchange, but also, even though asymmetric syn-
theses using chiral λ3-organoiodanes were very limited, we were
able to achieve direct asymmetric R-phenylation of enolate anions
derived from cyclic â-keto esters by the reaction with binaph-
thylyl(phenyl)iodonium salts.
The degree of asymmetric induction of 8a (37% ee), determined
1
by H NMR (400 MHz) analysis in the presence of the chiral
shift reagent Eu(hfc)3, is only moderate, but it is the first
(12) The optical purity of (S)-(-)-2 was determined from the 1H NMR
spectra of the corresponding R-methoxy-R-(trifluoromethyl)phenylacetic acid
(MTPA) derivative, prepared by the following ligand exchange sequence: (i)
ligand exchange with KI in MeOH-H2O yielding the diaryliodonium iodide;
(ii) formation of the acetoxyiodane by reaction with silver acetate; and (iii)
ligand exchange with (R)-MTPA in chlorobenzene under reduced pressure,
leading to the formation of 2-((2-methoxy-(2-trifluoromethyl)phenylacetoxy)-
phenyliodo)-1,1′-binaphthyl.13
(13) Stang, P. J.; Boehshar, M.; Wingert, H.; Kitamura, T. J. Am. Chem.
Soc. 1988, 110, 3272.
(14) Irie, M.; Doi, Y.; Ohsuka, M.; Aso, Y.; Otsubo, T.; Ogura, F.
Tetrahedron: Asymmetry 1993, 4, 2127.
(15) Brown, K. J.; Berry, M. S.; Waterman, K. C.; Lingenfelter, D.;
Murdoch, J. R. J. Am. Chem. Soc. 1984, 106, 4717.
(16) The X-ray structure of 2 revealed a fluorine-bridged polymeric structure
of (F-B-F-I-)n. The bond angles C(1)-I-C(21), C(1)-I-F(1), C(21)-
I-F(2′), and F(1)-I-F(2′) are 93.8(2), 76.4(2), 100.0(2), and 85.7(2)°,
respectively.
Supporting Information Available: Experimental procedures for the
synthesis, characterization, and reaction of diaryliodonium salts; X-ray
structural information on (S)-2 and (S)-9a (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
(17) Jorgensen, W. L.; Severance, D. L. J. Am. Chem. Soc. 1990, 112,
4768.
(18) (a) Yamada, Y.; Okawara, M. Bull. Chem. Soc. Jpn. 1972, 45, 1860.
(b) Lancer, K. M.; Wiegand, G. H. J. Org. Chem. 1976, 41, 3360.
JA992236C