Rhodium(II)-catalyzed transylidation of aryliodonium ylides: Electronic effects of aryl groups determine their thermodynamic stabilities

Article abstract of DOI:10.1021/ol800211x

(Chemical Equation Presented) Intermolecular transylidations between aryliodonium ylides under catalytic conditions were developed. Heating a solution of phenyliodonium bis(trifluoromethylsulfonyl)methylide in a large amount (80 equiv) of a substituted iodobenzene in the presence of 5 mol % of rhodium(II) acetate as a catalyst transfers the bis(trifluoromethylsulfonyl)- methylidene group to the iodine(I) atom and affords a substituted aryliodonium ylide in a good yield. Reversible nature of the catalytic intermolecular transylidation makes it possible to evaluate the thermodynamic stability of aryliodonium ylides.

Full text of DOI:10.1021/ol800211x

ORGANIC
LETTERS
2008
Vol. 10, No. 7
1425-1428
Rhodium(II)-Catalyzed Transylidation of
Aryliodonium Ylides: Electronic Effects
of Aryl Groups Determine Their
Thermodynamic Stabilities
Masahito Ochiai,* Takuya Okada, Norihiro Tada, and Akira Yoshimura
Graduate School of Pharmaceutical Sciences, UniVersity of Tokushima,
1-78 Shomachi, Tokushima 770-8505, Japan
mochiai@ph.tokushima-u.ac.jp
Received January 31, 2008
ABSTRACT
Intermolecular transylidations between aryliodonium ylides under catalytic conditions were developed. Heating a solution of phenyliodonium
bis(trifluoromethylsulfonyl)methylide in a large amount (80 equiv) of a substituted iodobenzene in the presence of 5 mol % of rhodium(II)
acetate as a catalyst transfers the bis(trifluoromethylsulfonyl)-methylidene group to the iodine(I) atom and affords a substituted aryliodonium
ylide in a good yield. Reversible nature of the catalytic intermolecular transylidation makes it possible to evaluate the thermodynamic stability
of aryliodonium ylides.
Because of the hyper-leaving group ability of aryl-λ3-iodanyl
groups,1 aryliodonium ylides serve as excellent progenitors
for generation of singlet carbenes (or carbenoids) under
thermal conditions2 and transfer the alkylidene groups to
heteroatom nucleophiles such as nitrogen heterocycles and
sulfides.3,4 Use of copper(I) and (II) catalysts or photochemi-
cal irradiation accelerates the transylidation with nucleo-
philes, providing a useful preparative method for pyridinium,
phosphonium, sulfonium, sulfoxonium, and arsonium ylides,
etc.5,6
In 1992, Dai and co-workers reported the first example
of transylidations between aryliodonium ylides under copper-
catalyzed conditions; the alkylidene group of aryliodonium
ylide 1 was transferred in an intramolecular fashion to the
close-neighboring iodine(I) atom attached to the aromatic
moiety via the formation of active Cu-carbenoid species,
yielding the cyclic iodonium ylide 2 (Scheme 1).7 In spite
Scheme 1
(1) (a) Ochiai, M. In Chemistry of HyperValent Compounds; Akiba, K.-
y., Ed.; Wiley-VCH: New York, 1999; Chapter 12. (b) Okuyama, T.;
Takino, T.; Sueda, T.; Ochiai, M. J. Am. Chem. Soc. 1995, 117, 3360.
(2) Camacho, M. B.; Clark, A. E.; Liebrecht, T. A.; DeLuca, J. P. J.
Am. Chem. Soc. 2000, 122, 5210.
(3) (a) Neiland, O.; Karele, B. Zh. Org. Khim. 1965, 1, 1854. (b)
Hatzigrigoriou, E.; Bakola-Christianopoulou, M.; Varvoglis, A. J. Chem.
Res. S 1987, 374.
(4) Reviews: (a) Muller, P. Acc. Chem. Res. 2004, 37, 243. (b) Zhdankin,
V. V.; Stang, P. J. Chem. ReV. 2002, 102, 2523. (c) Varvoglis, A. The
Organic Chemistry of Polycoordinated Iodine; VCH: New York, 1992.
(d) Koser, G. F. In The Chemistry of Functional Groups, Supplement D.;
Patai S., Rappoport. Z., Eds.; Wiley: New York, 1983; Chapter 18.
of the surging interest and activity in the ylide chemistry,
the intermolecular version of transylidation between aryl-
iodonium ylides remains unknown.8 In the reaction of
phenyliodonium bis(phenylsulfonyl)methylide with methyl
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© 2008 American Chemical Society
Published on Web 02/28/2008

and primary alkyl iodides (EtI and n-BuI), which affords
C-I insertion products, the intermediacy of labile alkyl-
iodonium ylides produced via transylidation has been
proposed but the alkyliodonium ylides were neither detected
nor isolated.9,10
We report herein the first example of well-characterized
intermolecular transylidations between aryliodonium ylides
3 under rhodium(II)-catalyzed conditions. The transylidation
reaction makes it possible to evaluate the thermodynamic
stability of aryliodonium ylides 3 with a wide range of
substituents at the para position.
Attempted thermal decomposition of phenyliodonium bis-
(trifluoromethylsulfonyl)methylide (3a)6 in a large excess (80
equiv) of p-iodotoluene at 90 °C for 5 h showed no evidence
for thermal transylidation reactions and the ylide 3a was
recovered unchanged. Heating at 170 °C for 2 h resulted in
the partial transfer of bis(trifluoromethylsulfonyl)methylidene
group to the iodine(I) atom, yielding p-tolyliodonium ylide
3b, albeit in a very low yield. Prolonged heating at that
temperature resulted in the extensive decomposition of 3a
(Table 1, entries 1-3).
idation temperature and afforded modest yields (33-38%)
of 3b at 90 °C for 5 h. CuI and CuCN were less effective.
Rhodium(II) acetate was found to be the catalyst of choice
for carbenoid capture by p-iodotoluene, yielding the iodo-
nium ylide 3b in 97% yield after purification by preparative
TLC using 1:1 hexane-ethyl acetate (Table 1, entry 8). To
the best of our knowledge, this is the first successful example
of the intermolecular transylidation between aryliodonium
ylides.
Rhodium(II) acetate-catalyzed transfer of the bistriflyl-
methylidene group to substituted iodobenzenes provides a
direct route for the synthesis of a variety of aryliodonium
ylides 3 (Table 2). Thus, p-methoxy, o-methyl, m-methyl,
Table 2. Rh(II)-Catalyzed Transylidation of Iodonium Ylide
3aa
entry
iodoarene
t (h)
3 yield (%)b
Table 1. Transylidation of Iodonium Ylide 3a to
p-Iodotoluenea
1
2
3
4
5
6
7
8
p-MeOC6H4I
o-MeC6H4I
m-MeC6H4I
3,5-Me2C6H3I
p-FC6H4I
5
5
5
5
5
3c 84
3d 92
3e 92
3f 82
3g 89
3h 78
3i 77
3j 77
3k 38d
3l -d
3m -
p-ClC6H4I
7
p-BrC6H4I
p-CF3C6H4I
p-NO2C6H4Ic
C6F5I
10
24
24
24
24
9
10
11
entry
additiveb
temp (°C)
t (h)
yield (%)c
CF3 (CF2)2CH2I
1
2
3
4
5
6
7
8
-
-
-
CuCl
CuI
CuCN
Cu(acac)2
Rh2(OAc)4
90
170
170
90
90
90
5
2
24
5
5
5
- (94)
7 (80)
- (4)
38 (38)
14 (50)
11 (85)
33 (38)
97 (3)
a Reaction conditions: an aryl iodide (80 equiv)/Rh2(OAc)4 (5 mol %)/
90 °C/Ar. b Isolated yields. c In 1,2-dichloroethane. d Recovered ylide 3a:
49% (entry 9) and 81% (entry 10).
3,5-dimethyl, p-fluoro, p-chloro, p-bromo, and p-trifluoro-
methyl substituted aryliodonium ylides 3c-j were prepared
from the ylide 3a in the presence of 5 mol % of rhodium(II)
acetate at 90 °C in high yields. Use of iodobenzenes with
electron-withdrawing substituents (Cl, Br, and CF3) slows
down the rate of transylidation and requires longer reaction
times (7-24 h, Table 2, entries 6-8). Carbenoid capture by
electron deficient p-nitroiodobenzene was carried out in 1,2-
dichloroethane solution at 90 °C because of its higher melting
point (175-177 °C), and this afforded the ylide 3k in a
modest yield (38%). Pentafluoroiodobenzene and hepta-
fluoroiodobutane showed no evidence for formation of the
corresponding iodonium ylides 3l and 3m, respectively.
A reaction mechanism involving the formation of a metal-
carbenoid complex as an active species is depicted in Scheme
90
90
5
5
a Reaction conditions: p-iodotoluene (80 equiv)/Ar. b 5 mol %. c Isolated
yields. Parentheses are yields of recovered ylide 3a.
Use of a small amount (5 mol %) of traditional copper
catalysts such as CuCl and Cu(acac)2 lowered the transyl-
(5) (a) Karele, B.; Neiland, O. Zh. Org. Khim. 1966, 2, 1680. (b)
Hadjiarapoglou, L.; Spyroudis, S.; Varvoglis, A. J. Am. Chem. Soc. 1985,
107, 7178. (c) Hadjiarapoglou, L.; Varvoglis, A. Synthesis 1988, 913. (d)
Hood, J. N. C.; Lloyd, D.; MacDonald, W. A.; Shepherd, T. M. Tetrahedron
1982, 38, 3355.
(6) (a) Zhu, S.-Z. Heteroatom Chem. 1994, 5, 9. (b) Zhu, S.-Z.; Chen,
Q.-Y. J. Chem. Soc., Chem. Commun. 1990, 1459. (c) Hackenberg, J.;
Hanack, M. J. Chem. Soc., Chem. Commun. 1991, 470.
(7) Yang, R.-Y.; Dai, L.-X.; Chen, C.-C. J. Chem. Soc., Chem. Commun.
1992, 1487.
(8) Recently, we reported the intermolecular transfer of the alkylidene
groups of arylbromonium ylides to aryl halides (iodides, bromides, and
chlorides) yielding a variety of aryliodonium, bromonium, and chloronium
ylides under thermal or metal-catalyzed conditions. See: Ochiai, M.; Tada,
N.; Okada, T.; Sota, A.; Miyamoto, K. J. Am. Chem. Soc. 2008, 130, 2118.
(9) Gogonas, E. P.; Nyxas, I.; Hadjiarapoglou, L. P. Synlett 2004, 2563.
(10) For transylidations between a diazo compound and a halonium ylide,
see: (a) Sheppard, W. A.; Webster, O. W. J. Am. Chem. Soc. 1973, 95,
2695. (b) Janulis, E. P.; Arduengo, A. J. J. Am. Chem. Soc. 1983, 105,
3563. (c) Moriarty, R. M.; Bailey, B. R.; Prakash, O.; Prakash, I. J. Am.
Chem. Soc. 1985, 107, 1375.
1426
Org. Lett., Vol. 10, No. 7, 2008

Scheme 2
Table 3. Competitive Transylidations to Iodobenzene and a
Substituted Iodobenzenea
entry substrate p-RC6H4I (R) product (yield %)b ratioc
1
2
3
4
5
6
7
8
3a
3c
3a
3b
3a
3h
3a
3j
MeO
MeO
Me
Me
Cl
Cl
CF3
CF3
3a (14) 3c (67)
3a (13) 3c (75)
3a (27) 3b (59)
3a (23) 3b (58)
3a (68) 3h (18)
3a (66) 3h (19)
3a (77) 3j (4)
17:83
14:86
31:69
28:72
79:21
78:22
95:5
2. Nucleophilic attack of an iodine atom of aryl iodide to
rhodium-carbenoid 4, produced from 3a via reductive
elimination of a phenyl-λ3-iodanyl hyper-leaving group,1 will
afford the complex 5. Regeneration of rhodium(II) catalyst
from 5 produces the substituted phenyliodonium ylides 3b-
k.
3a (86) 3j (4)
96:4
a Reaction conditions: each iodobenzene (50 equiv)/CF3Ph/Rh2(OAc)4
(5 mol %)/125 °C/10 h/Ar. b 1H NMR yields. c Ratios of unsubstituted 3a
to a substituted iodonium ylide.
It has been shown to be difficult to generalize the effects
of substituents of the aromatic ring on thermal stability of
aryliodonium ylides, since the same substituent variation may
operate in opposite directions in a different class of iodonium
ylides.4d,11 Therefore, it seems to be desirable to evaluate
the substituent effects on the thermodynamic stability of
aryliodonium ylides 3 to gain some insight into the intrinsic
nature of the ylides. The mechanism of our transylidations,
shown in Scheme 2, would expect the reversible nature of
the reactions, which makes it possible to assess the thermo-
dynamic stability of iodonium ylides 3.
Product ratios of the catalyzed transylidations to a mixture
of an unsubstituted and a substituted iodobenzene with
p-MeO, p-Me, p-Cl, or p-CF3 group were measured in
refluxing (trifluoromethyl)benzene as a solvent for 10 h by
competitive reactions using 1H NMR, in which a mixture of
a 50-fold excess of iodobenzene and a substituted iodo-
benzene was used (Table 3). The competitive transylidation
of unsubstituted ylide 3a to iodobenzene and p-methoxy-
iodobenzene afforded a 17:83 mixture of ylides 3a and 3c
in a high yield; in addition, a comparable ratio (14:86) of
these ylide mixtures was also produced in the rhodium(II)-
catalyzed transfer of the bistriflylmethylidene group using
substituted ylide 3c (Table 3, entries 1 and 2). Thus, both
transylidation reactions resulted in the formation of p-
methoxyphenyliodonium ylide 3c as a major product with
greater than 80% selectivity. Decreased selectivity (69-72%)
for p-methyl-substituted ylide 3b in the competitive trans-
ylidation to iodobenzene and p-methyliodobenzene was
evaluated (Table 3, entries 3 and 4). In marked contrast,
competition with electron deficient p-chloro- and p-(tri-
fluoromethyl)iodobenzene afforded the unsubstituted phenyl-
iodonium ylide 3a as a major product with 78-79% and
95-96% selectivity, respectively. These results clearly
demonstrate the reversible nature of our rhodium(II)-
catalyzed transylidations between aryliodonium ylides 3
under the conditions.
Furthermore, the results of Table 3, showing similar ratios
of the ylide products 3 in each set of the competitive
rhodium(II)-catalyzed transylidations to two competing
substrates, most likely indicate that equilibrium is established
under the conditions, and the ratios of the ylides 3 reflect
the differences in their thermodynamic stabilities. Thus, the
equilibrium constants K between unsubstituted 3a and
substituted aryliodonium ylides 3 were estimated from their
average ratios (Table 3): 3c (p-MeO), 5.5; 3b (p-Me), 2.4;
3a (H), 1.0; 3h (p-Cl), 0.27; 3j (p-CF3), 0.047. A Hammett
plot (Figure 1) showed an excellent correlation of log K with
substituent constants σp and gave a F value of -2.5 (r )
1.0).12 These results clearly demonstrate that the thermo-
(11) For instance, p-nitrophenyliodonium dinitromethylide is more stable
to thermal decomposition than phenyliodonium dinitromethylide, but the
reverse is true for the iodonium ylides derived from dimedone. See Ref 4d.
Figure 1. Hammett plot of log K vs σp constants.
Org. Lett., Vol. 10, No. 7, 2008
1427

dynamic stability of ylides 3 is mostly determined by the
electronic nature of para substituents of the aromatic ring:
the stability of aryliodonium ylides 3 increases with the
increasing electron-donating nature of the para substituents,
which probably decreases the positive charge localized on
the iodine(III) atom to some extent.
substituents on the aromatic ring would contribute to
determine the thermodynamic stability of all of the aryl-λ3-
iodanes to some extent.
Interesting examples of a reverse type of transylidation
reaction, where a phenyliodonium ylide transfers its phenyl-
λ3-iodanyl group (PhI+) to an active methylene compound,
have been reported (Scheme 4).5b,14 In other words, this is a
The large negative F value (-2.5) evaluated here can be
compared with that (F ) -0.91) reported for the competitive
rhodium(II)-catalyzed carbenoid transfer from arylbromo-
nium ylide 6 to aryl iodides at 40 °C (Scheme 3),8 in which
Scheme 4
Scheme 3
ligand exchange reaction on the iodine(III) atom.13,15 Here,
the carbon acid (PhCOCH2COPh) liberated from the initial
iodonium ylide must be less acidic than the active methylene
compound (in this case, dimedone).4c Such transylidation
reactions also allow at least a qualitative assessment of
relative iodonium ylide stabilities.4d
In conclusion, we have shown unambiguous experimental
evidence for the intermolecular transylidation between aryl-
iodonium ylides under rhodium(II)-catalyzed conditions. The
reaction provides a new tool for the synthesis of iodonium
ylides with a variety of aryl-λ3-iodanyl groups that show
various degrees of leaving group ability.1 The relative
thermodynamic stabilities of aryliodonium ylides were evalu-
ated.
a relatively small positive charge is assumed to be developed
on the iodine atom in the product-determining transition state
7. At 40 °C, no catalyzed transylidation between aryl-
iodonium ylides 3 takes place.
Acknowledgment. We gratefully acknowledge the Min-
istry of Education, Culture, Sports, Science, and Technology
of Japan for financial support in the form of a grant.
Intrinsically, a partial positive charge develops on the
central iodine atom in hypervalent three-center four-electron
bonding of aryl-λ3-iodanes (ArIXY).13 Therefore, it seems
reasonable to assume that this type of electronic effect of
Supporting Information Available: Typical experimen-
tal procedures and spectral data. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL800211X
(12) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165.
(13) Ochiai, M. In Topics in Current Chemistry; Wirth, T., Ed.;
Springer: Berlin, 2003; Vol. 224, p 5.
(14) Prikule, D. E.; Neiland, O. Zh. Org. Khim. 1971, 7, 2441.
(15) Finet, J.-P. Ligand Coupling Reactions with Heteroatomic Com-
pounds; Pergamon: Oxford, 1998.
1428
Org. Lett., Vol. 10, No. 7, 2008