Effects of complexation with 18-crown-6 on the hypernucleofugality of phenyl-λ3-iodanyl groups. Synthesis of vinyl- λ3-iodane·18-crown-6 complex

Article abstract of DOI:10.1021/ol050858z

(Chemical Equation Presented) 4-tert-Butyl-1-cyclohexenyl(phenyl) (tetrafluoroborato)-λ³-iodane forms a discrete supramolecular complex by the reaction with 18-crown-6. Solvolysis of the cyclohexenyl- λ³-iodane in the presence of 18-crown-6 indicates that the complexation with 18-crown-6 tends to decrease the leaving group ability of hypervalent phenyl-λ³-iodanyl groups.

Full text of DOI:10.1021/ol050858z

ORGANIC
LETTERS
2005
Vol. 7, No. 14
2893-2896
Effects of Complexation with
18-Crown-6 on the Hypernucleofugality
of Phenyl-
λ
3-iodanyl Groups. Synthesis
of Vinyl- ‚18-Crown-6 Complex
λ
3-iodane
Masahito Ochiai,*,† Takashi Suefuji,† Kazunori Miyamoto,† and Motoo Shiro‡
Faculty of Pharmaceutical Sciences, UniVersity of Tokushima, 1-78 Shomachi,
Tokushima 770-8505, Japan, and Rigaku Corporation, 3-9-12 Matsubara,
Akishima, Tokyo 196-8666, Japan
mochiai@ph2.tokushima-u.ac.jp
Received April 19, 2005
ABSTRACT
4-tert-Butyl-1-cyclohexenyl(phenyl)(tetrafluoroborato)-
Solvolysis of the cyclohexenyl-
3-iodane in the presence of 18-crown-6 indicates that the complexation with 18-crown-6 tends to decrease the
leaving group ability of hypervalent phenyl-
3-iodanyl groups.
λ
3-iodane forms a discrete supramolecular complex by the reaction with 18-crown-6.
λ
λ
Solvolysis of 4-tert-butyl-1-cyclohexenyl(phenyl)(tetrafluo-
roborato)-λ3-iodane (1a) proceeds at a reasonable rate in
aqueous alcoholic solution even at room temperature and
generates a reactive 1-cyclohexenyl cation 2 with reductive
elimination of iodobenzene (Scheme 1).1 The phenyl-λ3-
a leaving group ability of 106 times greater than the
superleaving group triflate (TfO). The origin of this high
nucleofugality of the phenyl-λ3-iodanyl group is attributed
to the involvement of an energetically favorable reduction
of the hypervalent iodine(III) to the normal valency with octet
structure during the leaving process.2 This process is also
associated with an increase in entropy.1 The high nucleo-
fugality of phenyl-λ3-iodanyl groups makes possible the
unusual vinylic SN2 displacement of â-alkylvinyl(phenyl)-
λ3-iodanes by the reaction with a wide range of nucleophiles.3
Scheme 1
(1) Okuyama, T.; Takino, T.; Sueda, T.; Ochiai, M. J. Am. Chem. Soc.
1995, 117, 3360.
(2) (a) Ochiai, M. In Chemistry of HyperValent Compounds; Akiba, K.,
Ed.; Wiley-VCH: New York, 1999, p 359. (b) Ochiai, M. In Topics in
Current Chemistry; Wirth, T., Ed.; Springer: Berlin, 2003; Vol. 224, p 5.
(3) (a) Ochiai, M.; Oshima, K.; Masaki, Y. J. Am. Chem. Soc. 1991,
113, 7059. (b) Okuyama, T.; Takino, T.; Sato, K.; Ochiai, M. J. Am. Chem.
Soc. 1998, 120, 2275. (c) Okuyama, T. ReV. Heteroat. Chem. 1999, 21,
678. (d) Ochiai, M. J. Organomet. Chem. 2000, 611, 494. (e) Yan, J.; Chen,
Z.-C. Tetrahedron Lett. 1999, 40, 5757. (f) Okuyama, T.; Ochiai, M. J.
Am. Chem. Soc. 1997, 119, 4785. (g) Ochiai, M.; Yamamoto, S.; Sato, K.
Chem. Commun. 1999, 1363. (h) Ochiai, M.; Yamamoto, S.; Suefuji, T.;
Chen, D.-W. Org. Lett. 2001, 3, 2753. (i) Ochiai, M.; Nishi, Y.; Hirobe,
M. Tetrahedron Lett. 2005, 46, 1863.
iodanyl group was found to be an excellent nucleofuge with
† Tokushima University.
‡ Rigaku Corporation.
10.1021/ol050858z CCC: $30.25
© 2005 American Chemical Society
Published on Web 06/08/2005

Recently, we found that 18-crown-6 (18C6) forms discrete
complexes with aryl(tetrafluoroborato)-λ3-iodanes, in which
the iodine(III) contacts with the three adjacent oxygen atoms
of 18C6 through two hypervalent secondary bonding and a
weak interaction.4 The complexation with 18C6 increases
the stability of λ3-iodanes. For instance, the hydroxy(phenyl)-
λ3-iodane PhI(OH)BF4 is thermally labile and decomposes
at room temperature within a few minutes; however, no
decomposition of the crown ether complex PhI(OH)BF4‚
18C6 was detected when it was left standing under ambient
conditions over 10 days.5 We report herein that 1-cyclohex-
enyl(aryl)(tetrafluoroborato)-λ3-iodane 1 forms a supramo-
lecular complex with 18C6. The complexation with 18C6
slows down the rates of solvolysis of the 1-cyclohexenyl-
λ3-iodane 1 in chloroform and methanol and decreases the
leaving group ability of phenyl-λ3-iodanyl groups.
Slow evaporation of a dichloromethane-hexane-ethyl
acetate (1:2:4) solution of a 1:2 mixture of 4-tert-butyl-1-
cyclohexenyl(phenyl)(tetrafluoroborato)-λ3-iodane (1a) and
18C6 in a refrigerator at 4 °C afforded an 82% yield of single
crystals of a 1:1 complex 36 suitable for X-ray analysis.
The solid-state structure of 3 (Figure 1) illustrates that the
cyclohexenyl(phenyl)-λ3-iodanyl group protrudes above one
for I1, C1, C7, O1, O2, and O6) from their least-squares
plane and the sum of the iodine-centered bond angles (Σ°I
) 357.6°). The I1‚‚‚O2 and I1‚‚‚O6 distances (3.023(4) and
3.175(5) Å) are shorter than the van der Waal’s distance (3.50
Å),8 and both C7-I1‚‚‚O2 and C1-I1‚‚‚O6 triads are nearly
linear. Therefore, these close contacts are indicative of
hypervalent secondary interactions, in which each oxygen
atom donates an electron pair into an I-C σ* orbital.4,5
In solution, the complex formation between 1a and 18C6
1
is clearly evident in H NMR experiments: A methylene
singlet of 18C6 in CD2Cl2 (0.01 M) exhibits an upfield shift
of 0.03 ppm by the addition of 1 equiv of 1a. The 13C
resonance of 18C6 at δ 70.7 ppm in CDCl3 is also shifted
to higher field (δ 70.3 ppm) by the addition of 1a. The 1:1
stoichiometry for the complexation in CD2Cl2 solution was
determined from the Job plots by the 1H NMR experiments.9
The complexation-induced shifts of a methylene singlet of
18C6 were small but distinct and reproducible. A plot of
complex concentration versus [1a]/([1a] + [18C6]) shows
a maximum at near 0.5 (Figure 2).
Figure 2. Job plot for complexation between the λ3-iodane 1a
and 18C6 in CD2Cl2 at 23 °C. Concentration: [1a] + [18C6] )
0.01 M.
1
Figure 1. Crystal structure of 1:1 complex 3 (with /2 hexane).
Hexane and BF4 were omitted for clarity. Selected interatomic
distances (Å) and angles (°): I1-C1 2.091(7); I1-C7 2.092(7);
I1‚‚‚O1 3.215(4); I1‚‚‚O2 3.023(4); I1‚‚‚O6 3.175(5); C1-I1-C7
92.5(2); C1-I1‚‚‚O6 167.4(2); C7-I1‚‚‚O2 161.0(2).
The binding constants were measured by 1H NMR
titrations of CD2Cl2 solutions of 18C6 with 1a at 24 °C. The
(6) Selected physical data for complex 3: colorless plates; mp 75-77
°C.; 1H NMR (CDCl3, 400 MHz) δ 7.98 (d, J ) 7.7 Hz, 2H), 7.72 (t, J )
7.3 Hz, 1H), 7.56 (dd, J ) 7.7, 7.3 Hz, 2H), 7.06-6.98 (m, 1H), 3.68 (s,
24H), 2.76-2.59 (m, 2H), 2.57-2.42 (m, 1H), 2.37-2.25 (m, 1H), 1.99-
1.89 (m, 1H), 1.54-1.35 (m, 2H), 0.86 (s, 9H); 13C NMR (CDCl3, 75 MHz)
δ 147.6, 135.9, 132.7, 132.3, 115.8, 112.1, 70.3, 42.0, 35.3, 32.3, 31.8,
27.2, 27.0; IR (KBr) 3560, 2880, 1627, 1475, 1353, 1150-1100 cm-1. Anal.
face of 18C6.7 The iodine(III) contacts with three adjacent
oxygen atoms (O1, O2, and O6) of 18C6, and the complex
adopts a distorted pentagonal planar geometry about the
iodine, with root-mean-square (rms) deviation (0.3560(2) Å
Calcd for C28H46BF4IO6: C, 48.57; H, 6.70. Found: C, 48.49; H, 6.79.
1
(7) Crystallographic data for complex
3
involving
/
hexane:
2
(4) (a) Ochiai, M.; Suefuji, T.; Miyamoto, K.; Tada, N.; Goto, S.; Shiro,
M.; Sakamoto, S.; Yamaguchi, K. J. Am. Chem. Soc. 2003, 125, 769. (b)
Ochiai, M.; Miyamoto, K.; Suefuji, T.; Sakamoto, S.; Yamaguchi, K.; Shiro,
M. Angew. Chem., Int. Ed. 2003, 42, 2191. (c) Ochiai, M.; Miyamoto, K.;
Suefuji, T.; Shiro, M.; Sakamoto, S.; Yamaguchi, K. Tetrahedron 2003,
59, 10153.
(5) (a) Ochiai, M.; Miyamoto, K.; Shiro, M.; Ozawa, T.; Yamaguchi,
K. J. Am. Chem. Soc. 2003, 125, 13006. (b) Ochiai, M.; Miyamoto, K.;
Yokota, Y.; Suefuji, T.; Shiro, M. Angew. Chem., Int. Ed. 2005, 44, 75.
C31H53BF4IO6, M ) 735.46, T ) 93 K, triclinic, space group P-1 (no. 2),
a ) 9.757(9) Å, b ) 10.998(8) Å, c ) 16.41(2) Å, R ) 98.73(8)°, â )
91.47(7)°, γ ) 101.80(7)°, V ) 1700(2) Å3, Z ) 2, Dc ) 1.436 g cm-3, µ
(Mo KR) ) 10.03 cm-1. 19 486 reflections were collected; 9743 were
unique. R1 ) 0.083 [I > 2σ(I)], wR2 ) 0.220 [I > 3σ(I)]. Crystallographic
data have been deposited with the Cambridge Crystallographic Data Centre
as supplementary publication number CCDC-250402. Data can be obtained,
free of charge, on application to CCDC, 12 Union Road, Cambridge CB2
1EZ, U.K. [fax: +44 (0) 1223-336-033 or e-mail: deposit@ccdc.cam.ac.uk]
2894
Org. Lett., Vol. 7, No. 14, 2005

resulting binding curve gave an excellent fit with a 1:1
binding model and was analyzed by the nonlinear least-
squares method to give a binding constant (Ka) value of 2.84
× 102 M-1 in CD2Cl2 (correlation coefficient 0.97).
Recently, Okuyama and co-workers reported that the
thermal decomposition of 1-cyclohexenyl-λ3-iodane 1a in
chloroform generates 1-cyclohexenyl cation 2 in a manner
similar to the solvolysis in an aqueous alcoholic solution.10
Thermolysis of 1a in the presence of a large excess of 18C6
(10 equiv) in chloroform at 45 °C resulted in a comparable
product profile shown in Scheme 2. Nucleophilic attack of
Scheme 2
tetrafluoroborate ion,10 chloroform,11 and iodobenzene to the
initially generated 1-cyclohexenyl cation 2 produces vinyl
fluoride 4a (29%), vinyl chloride 4b (23%), and a mixture
of vinylated iodobenzenes 5 (23%), respectively. Predomi-
nant formation of the ortho isomer of 5 indicates the inter-
vention of an intimate ion-molecule pair of 2 and iodoben-
zene during the thermal decomposition.1
Rates for thermal decomposition of 1-cyclohexenyl-λ3-
iodane 1a (1 × 10-4 M) in the absence and presence of 18C6
at varying concentrations were measured spectrophotometri-
cally in chloroform and methanol. The pseudo-first-order rate
constants kobsd obtained were plotted against the ratios of
[18C6]/[1a] (Figure 3). In methanol at 50 °C (Figure 3b),
an increasing concentration of 18C6 results in a gradual
decrease in the rates of decomposition of 1a, although the
extent of the decrease is very small. Interestingly, however,
both acceleration and retardation were observed in the
decomposition of 1a in chloroform at 45 °C, depending upon
the concentrations of added 18C6. At low concentrations of
18C6, a slight increase in the rates of decomposition of 1a
was detected as shown in the inset of Figure 3a; however,
increased concentrations of 18C6 decreased the rates. A
similar concentration dependence of the rates was also
observed in the thermal decomposition of p-methylphenyl-
(1b) and p-chlorophenyl(4-tert-butyl-1-cyclohexenyl)(tet-
rafluoroborato)-λ3-iodane (1c) (Table 1): slightly increased
rates of decomposition at low concentration (1 × 10-4 M)
Figure 3. Observed rate constants for thermal decomposition of
1a (1 × 10-4 M) in the presence of 18C6. (a) In CHCl3 at 45 °C.
The inset shows details at low concentrations of 18C6. (b) In MeOH
at 50 °C.
of 18C6 and decreased rates at higher concentration (1 ×
10-2 M) were again observed.
From the equation shown in Scheme 3, the association
constant (Ka) and the rate constant (kcomplex) for decomposition
of the 1:1 complex 3 in CHCl3 at 45 °C were estimated by
using the four sets of data in Figure 3a, where [18C6]/[1a]
Table 1. Observed Rate Constants (104 kobsd/s-1) for Thermal
Decomposition of 1 in CHCl3 in the Presence of 18C6 at 45 °Ca
(8) (a) Bondi, A. J. Phys. Chem. 1964, 68, 441. (b) Rowland, R. S.;
Taylor, R. J. Phys. Chem. 1996, 100, 7384.
[18C6]/M
1a
1b
1c
(9) Job, A. Ann. Chim. (Paris) 1928, 9, 113.
1.15
1.63
0.43
0.45
0.50
0.43
4.48
4.77
2.35
(10) (a) Okuyama, T.; Fujita, M.; Gronheid, R.; Lodder, G. Tetrahedron
Lett. 2000, 41, 5125. (b) Fujita, M.; Kim, W. H.; Sakanishi, Y.; Fujiwara,
K.; Hirayama, S.; Okuyama, T.; Ohki, Y.; Tatsumi, K.; Yoshioka, Y. J.
Am. Chem. Soc. 2004, 126, 7548.
(11) White, E. H.; McGirk, R. H.; Aufdermarsh, C. A.; Tiwari, H. P.;
Todd, M. J. J. Am. Chem. Soc. 1973, 95, 8107.
1 × 10-4
1 × 10-2
a The initial concentration of 1 was 1 × 10-4 M.
Org. Lett., Vol. 7, No. 14, 2005
2895

decomposition rates would be negligible at the low concen-
trations of 18C6. It seems reasonable to assume that initial
increase in the decomposition rates in CHCl3 (Figure 3a) is
presumably due to the increased polarity of the medium by
the addition of 18C6. In fact, the rate of thermal decompo-
sition of 1a in alcohol and aqueous solutions is known
Scheme 3
to depend on the solvent polarity and increase with the
1
increasing solvent ionizing power YOTs
.
In conclusion, 1-cyclohexenyl(aryl)(tetrafluoroborato)-λ3-
iodane forms a supramolecular 1:1 complex with 18C6 in
the solid state as well as in solution. The complexation with
18C6 decreases the leaving group ability of phenyl-λ3-iodanyl
groups.
is greater than 10: Ka ) ca. 3.3 × 102 M-1 and kcomplex
)
ca. 3.1 × 10-5 s-1. Comparison of kcomplex with k0 (Table 1)
indicates that the complexation with 18C6 decreases the
decomposition rate constant of 1a in CHCl3 solution to
about 1/4. Thus, the nucleofugality of the phenyl-λ3-iodanyl
group was considerably decreased by the complexation with
18C6.
Acknowledgment. We wish to acknowledge financial
support by a grant from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan.
Supporting Information Available: Typical experimen-
tal procedures and spectral data for 4 and 5; CIF file for
X-ray structure of complex 3. This material is available free
of charge via the Internet at http://pubs.acs.org.
The estimated association constant (Ka) in CHCl3 solution
is rather small, and therefore, at low concentrations ([18C6]/
[1a] < 5) of 18C6, the proportion of the complexed λ3-iodane
3 is quite small and calculated to be less than 15%, which
suggests that the effect of complexation with 18C6 on the
OL050858Z
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Org. Lett., Vol. 7, No. 14, 2005