Evidence for reversible ylide formation: Equilibrium between free alkylidenecarbenes and ethereal solvent-alkylidenecarbene complexes (oxonium ylides)

Article abstract of DOI:10.1021/ja961797q

The free alkylidenecarbene, 2-butyl-1-hexenylidene 1 (R = Bu), generated by triethylamine-induced α-elimination of 2-butyl-1-hexenyliodonium tetrafluoroborate 7 in tetrahydrofuran undergoes regioselective 1,5-C-H insertions, 1,2-shifts of the butyl group, and electrophilic attack on the tertiary amine followed by protonation to give the cyclopentene 8, the alkyne 9, and the vinylammonium salt 10, respectively. In addition to these free carbene-derived products, the reaction affords the three-component coupling product, the vinyl ether 11, produced through nucleophilic attack of tetrahydrofuran on 1 (R = Bu) generating the oxonium ylide 2 (R = Bu), followed by protonation with subsequent ring-opening of the resulting cyclic oxonium salt 15 (R = Bu) by nucleophilic attack of triethylamine. Reactions were observed to be temperature dependent, as reflected in variations in the product profiles: decreasing the reaction temperature tended to decrease the yields of free alkylidenecarbene-derived products, i.e., cyclopentenes alkynes, and vinylammonium salts, and to increase those of the vinyloxonium ylide-derived products, vinyl ethers. This temperature dependence is explained in terms of reversible oxonium ylide formation. No evidence was observed to suggest the existence of an equilibrium between the free alkylidenecarbene and the sulfonium ylide in the reaction in tetrahydrothiophene. These results are consistent with quantum calculations using the MOPAC 93 program.

Full text of DOI:10.1021/ja961797q

J. Am. Chem. Soc. 1996, 118, 10141-10149
10141
Evidence for Reversible Ylide Formation: Equilibrium between
Free Alkylidenecarbenes and Ethereal
Solvent-Alkylidenecarbene Complexes (Oxonium Ylides)
Takuya Sueda, Takema Nagaoka, Satoru Goto, and Masahito Ochiai*
Contribution from the Faculty of Pharmaceutical Sciences, UniVersity of Tokushima,
1-78 Shomachi, Tokushima 770, Japan
ReceiVed May 28, 1996X
Abstract: The free alkylidenecarbene, 2-butyl-1-hexenylidene 1 (R ) Bu), generated by triethylamine-induced
R-elimination of 2-butyl-1-hexenyliodonium tetrafluoroborate 7 in tetrahydrofuran undergoes regioselective 1,5-
C-H insertions, 1,2-shifts of the butyl group, and electrophilic attack on the tertiary amine followed by protonation
to give the cyclopentene 8, the alkyne 9, and the vinylammonium salt 10, respectively. In addition to these free
carbene-derived products, the reaction affords the three-component coupling product, the vinyl ether 11, produced
through nucleophilic attack of tetrahydrofuran on 1 (R ) Bu) generating the oxonium ylide 2 (R ) Bu), followed
by protonation with subsequent ring-opening of the resulting cyclic oxonium salt 15 (R ) Bu) by nucleophilic attack
of triethylamine. Reactions were observed to be temperature dependent, as reflected in variations in the product
profiles: decreasing the reaction temperature tended to decrease the yields of free alkylidenecarbene-derived products,
i.e., cyclopentenes, alkynes, and vinylammonium salts, and to increase those of the vinyloxonium ylide-derived
products, vinyl ethers. This temperature dependence is explained in terms of reversible oxonium ylide formation.
No evidence was observed to suggest the existence of an equilibrium between the free alkylidenecarbene and the
sulfonium ylide in the reaction in tetrahydrothiophene. These results are consistent with quantum calculations using
the MOPAC 93 program.
Although no stable and isolable oxonium ylide has been
reported in the literature1,2 and no spectroscopic data are
available to establish the intermediacy of oxonium ylides in the
reaction of carbenes or carbenoids with ethers, they have been
postulated as intermediates to explain the products observed on
either transition metal-catalyzed, thermal, or photochemical
reaction of diazocompounds.3
the ring-opening with t-BuOH.5 Base-induced R-elimination
of 2-methylpropenyl triflate in THF similarly produced the
oxonium ylide-derived enol ethers, along with the formation of
the alkylidenecarbene-derived enol ether.6,7
Alkylidenecarbenes possess a singlet ground state and are
electrophilic in nature.4 When they are generated in ethereal
solvents, the transient formation of the solvent-alkylidenecar-
bene complex, i.e., oxonium ylide, has also been proposed; in
1980, Gilbert and Weerasooriya reported that the generation of
1-diazoethene by t-BuOK-promoted reaction of dimethyl dia-
zomethylphosphonate with acetone in tetrahydrofuran (THF)
gave the enol ether 3 (R ) Me) in 15% yield, via the formation
of THF-alkylidenecarbene complex 2 (R ) Me) followed by
X Abstract published in AdVance ACS Abstracts, October 1, 1996.
(1) For reviews of carbene-derived oxonium ylides, see: (a) Padwa, A.;
Hornbuckle, S. F. Chem. ReV. 1991, 91, 263. (b) Padwa, A.; Krumpe, K.
E. Tetrahedron 1992, 48, 5385. (c) Jones, M.; Moss, R. A. Carbenes;
Wiley-Interscience: New York, 1973.
On the other hand, detailed measurements by Gilbert and
Giamalva of the F value for the cycloaddition of 2-methyl-1-
propenylidene 1 (R ) Me) derived from 1-diazoethene to
substituted styrenes as a function of solvent molecules such as
THF and Et2O showed evidence against the intervention of an
ethereal solvent-alkylidenecarbene complex. Their results
indicated that the carbene is free of encumbrance with the
solvent molecules, at least in the sense that the latter are
functioning as Lewis bases and that bulk properties of the
solvent, not complexation, are responsible for the observed
variation in F values as a function of solvent.8
(2) For stable carbene-derived sulfonium ylides, see: Ando, W. Acc.
Chem. Res. 1977, 10, 179.
(3) (a) Nozaki, H.; Takaya, H.; Noyori, R. Tetrahedron Lett. 1965, 2563.
(b) Nozaki, H.; Takaya, H.; Moriuti, S.; Noyori, R. Tetrahedron 1968, 24,
3655. (c) Ando, W.; Kondo, S.; Nakayama, K.; Ichibori, K.; Kohoda, H.;
Yamato, H.; Imai, I.; Nakaido, S.; Migita, T. J. Am. Chem. Soc. 1972, 94,
3870. (d) Iwamura, H.; Imahashi, Y.; Kushida, K. Tetrahedron Lett. 1975,
1401. (e) Kirmse, W.; Chiem, P. V. Tetrahedron Lett. 1985, 26, 197. (f)
Pirrung, M. C.; Werner, J. A. J. Am. Chem. Soc. 1986, 108, 6060. (g)
Roskamp, E. J.; Johnson, C. R. J. Am. Chem. Soc. 1986, 108, 6062. (h)
Shields, C. J.; Schuster, G. B. Tetrahedron Lett. 1987, 28, 853. (i) Doyle,
M. P.; Bagheri, V.; Harn, N. K. Tetrahedron Lett. 1988, 29, 5119. (j)
Kirmse, W.; Kund, K. J. Am. Chem. Soc. 1989, 111, 1465. (k) Kido, F.;
Sinha, S. C.; Abiko, T.; Yoshikoshi, A. Tetrahedron Lett. 1989, 30, 1575.
(l) Eberlein, T. H.; West, F. G.; Tester, R. W. J. Org. Chem. 1992, 57,
3479. (m) West, F. G.; Eberlein, T. H.; Tester, R. W. J. Chem. Soc., Perkin
Trans. 1 1993, 2857.
(5) (a) Gilbert, J. C.; Weerasooriya, U. Tetrahedron Lett. 1980, 21, 2041.
(b) Gilbert, J. C.; Weerasooriya, U. J. Org. Chem. 1983, 48, 448. (c)
Newman, M. S.; Beard, C. D. J. Am. Chem. Soc. 1970, 92, 7564.
(6) (a) Oku, A.; Harada, T.; Hattori, K.; Nozaki, Y.; Yamaura, Y. J.
Org. Chem. 1988, 53, 3089. (b) Oku, A.; Harada, T.; Nozaki, Y.; Yamaura,
Y. J. Am. Chem. Soc. 1985, 107, 2189. (c) Ohira, S.; Noda, I.; Mizobata,
T.; Yamato, M. Tetrahedron Lett. 1995, 36, 3375.
(7) Formation of three-component coupling products consisting of a
carbene [phenylcarbene and ethoxycarbonylcarbene], THF, and an alcohol
has been reported. See: Oku, A.; Kimura, K.; Ohwaki, S. Acta Chem.
Scand. 1993, 47, 391.
(4) (a) Stang, P. J. Chem. ReV. 1978, 78, 383. (b) Stang, P. J. Acc. Chem.
Res. 1982, 15, 348.
(8) Gilbert, J. C.; Giamalva, D. H. J. Org. Chem. 1992, 57, 4185.
S0002-7863(96)01797-0 CCC: $12.00 © 1996 American Chemical Society

10142 J. Am. Chem. Soc., Vol. 118, No. 42, 1996
Sueda et al.
Table 1. Effects of Reaction Temperature on Reaction of Vinyliodonium Salt 7 with Triethylamine in THF
product, % yielda
ratio
(8 + 9 + 10):11c
Et3N
(equiv)
entry
conditions
60 °C (10 h)
40 °C (10 h)
20 °C (10 h)
0 °C (10 h)
-20 °C (2 days), 0 °C (10 h)
-40 °C (2 days), 0 °C (10 h)
-60 °C (2 days), 0 °C (10 h)
40 °C (10 h)
20 °C (10 h)
0 °C (10 h)
8
9
10
11
totalb
8:9d
1
2
3
4
5
6
7
8
9
10
11
12
13
1.2
1.2
1.2
1.2
1.2
1.2
1.2
2.0
2.0
2.0
2.0
2.0
2.0
74
75
75
71
48
44
44
75
71
71
54
34
41
14
11
8
3
2
2
3
11
7
2
<1
<1
1
1
1
1
3
1
2
2
2
2
4
2
9
90
95
99
98
78
87
92
98
95
97
84
90
92
98:2
84:16
87:13
90:10
96:4
97:3
95:5
94:6
87:13
91:9
97:3
97:3
97:3
95:5
90:10
85:15
77:23
65:35
54:46
54:46
89:11
84:16
78:22
68:32
42:58
51:49
15
23
27
40
42
11
15
22
27
53
45
-20 °C (2 days), 0 °C (10 h)
-40 °C (2 days), 0 °C (10 h)
-60 °C (2 days), 0 °C (10 h)
1
1
2
1
a Yields of 8 and 9 were determined by GC. Yields of 10 and 11 were determined by H NMR. b Total yields of 8, 9, 10, and 11. c Ratios of
the free alkylidenecarbene-derived products (8 + 9 + 10) versus the vinyloxonium ylide-derived product 11. d Ratios of 1,5-C-H insertions versus
1,2-shifts.
These results on the reactions of alkylidenecarbenes can be
explained, we believe, in terms of a rapid equilibrium between
free alkylidenecarbenes 1 and vinyloxonium ylides 2 in which
free alkylidenecarbenes 1 are responsible for cycloaddition to
styrenes. In the present paper we show that the ratio of the
free alkylidenecarbene-derived and the oxonium ylide-derived
products is temperature dependent, which strongly suggests
reversible oxonium ylide formation.
bombardment (FAB) mass spectrometry. The vinyl ether 6 that
is a 7:3 mixture of E- and Z-isomers was found to be a coupling
product consisting of three components: alkenyliodonium salt
4, THF, and triethylamine.
Results and Discussion
Recently we reported that base treatment of alkenyl(phenyl)-
iodonium tetrafluoroborates leads to reductive R-elimination of
iodobenzene under mild conditions to generate alkylidenecar-
benes, because of the very high leaving group ability of the
phenyliodonio groups.9,10 The findings of complete stereocon-
vergence for intramolecular 1,5-carbon-hydrogen insertions of
the resulting alkylidenecarbenes9a and a small F value of -0.56
obtained by a Hammett study for the cycloaddition of 2-methyl-
1-propenylidene 1 (R ) Me) derived from an alkenyliodonium
salt to ring-substituted styrenes11,12 suggest that the reactive
intermediate involved is a free carbene and not a carbenoid.
To uncover the effects of reaction temperature on the
formation of the three-component coupling products as well as
to gain some insight into the nature of alkylidenecarbenes in
THF, we synthesized simple â,â-dibutylvinyliodonium tet-
rafluoroborate 7, bearing the same alkyl substituent at the
â-carbon atom, from 1-hexyne by the reaction with lithium bis-
(trimethylsilyl)cuprate and butyl iodide,15 and subsequently
allowed the resulting vinylsilane to react with iodosylbenzene
in the presence of BF3-Et2O.16 The results on the temperature
dependence of the reaction of 7 with triethylamine in THF are
summarized in Table 1. When the reaction with 1.2 equiv of
triethylamine was carried out at 60 °C for 10 h in nitrogen,
analysis by gas chromatography (GC) showed the formation of
two major products, the cyclopentene 8 (74%) and the rear-
ranged alkyne 9 (14%). 1H NMR also detected the presence
of the vinylammonium salt 10 (<1%) and the triethylammonium
vinyl ether 11 (2%) (Table 1, entry 1). Table 1 clearly shows
that the yields of both the 1,5-C-H insertion product 8 and of
the rearranged product 9 decrease with a decrease in the reaction
temperature while that of the three-component coupling product
11 increases. For instance, by starting at -60 °C for 2 days
and then gradually raising the temperature to 0 °C, 44% of 8,
3% of 9, 3% of 10, and 42% of 11 were produced.17 A similar
temperature dependence was observed in the reaction using 2
equiv of triethylamine (Table 1, entries 8-13).
Reaction of the alkenyliodonium salt 4 with triethylamine or
t-BuOK in THF at room temperature has been shown to result
in the selective formation of bicyclo[3.3.0]octene 5 in 80-84%
yields, through regioselective 1,5-carbon-hydrogen insertions
of the alkylidenecarbene formed by R-elimination.9a Surpris-
ingly, however, when the reaction using triethylamine as a base
was carried out under the same conditions, except for the
reaction temperature of -78 °C, the triethylammonium vinyl
ether 6 was obtained as the major product (43% yield) in
addition to the formation of 5 (39%). The structure of 6 was
determined by spectroscopy using two-dimensional (2D) NMR
techniques, i.e., 1H,1H-COSY, 13C,1H-COSY, and nuclear Over-
hauser enhancement spectroscopy (NOESY), and fast atom
Because the yields of the vinylammonium salt 10 were very
low, it was difficult to measure the effects of temperature on
(9) (a) Ochiai, M.; Takaoka, Y.; Nagao, Y. J. Am. Chem. Soc. 1988,
110, 6565. (b) Ochiai, M.; Kunishima, M.; Tani, S.; Nagao, Y. J. Am.
Chem. Soc. 1991, 113, 3135.
(10) For a very high leaving group ability of phenyliodonio groups, see:
Okuyama, T.; Takino, T.; Sueda, T.; Ochiai, M. J. Am. Chem. Soc. 1995,
117, 3360.
(11) Ochiai, M.; Sueda, T.; Uemura, K.; Masaki, Y. J. Org. Chem. 1995,
60, 2624.
(12) Stang and Mangum reported that the free alkylidenecarbene is mildly
electrophilic in nature and shows a relatively small negative F value,13
whereas the carbenoid shows a very large degree of electrophilicity and a
large negative F value.14
(13) Stang, P. J.; Mangum, M. G. J. Am. Chem. Soc. 1975, 97, 6478.
(14) (a) Newman, M. S.; Patrick, T. B. J. Am. Chem. Soc. 1969, 91,
6461. (b) Patrick, T. B.; Haynie, E. C.; Probst, W. J. J. Org. Chem. 1972,
37, 1553.
(15) (a) Fleming, I.; Newton, T. W. J. Chem. Soc., Perkin Trans. 1 1984,
1805. (b) Fleming, I.; Newton, T. W.; Roessler, F. J. Chem. Soc., Perkin
Trans. 1 1981, 2527.
(16) (a) Ochiai, M.; Sumi, K.; Takaoka, Y.; Kunishima, M.; Nagao, Y.;
Shiro, M.; Fujita, E. Tetrahedron 1988, 44, 4095. (b) Hinkle, R. J.; Poulter,
G. T.; Stang, P. J. J. Am. Chem. Soc. 1993, 115, 11626.
(17) The reaction at -60 °C was found to be very slow, since, even
after 2 days at the temperature, 70% of 7 was recovered unchanged.

EVidence for ReVersible Ylide Formation
J. Am. Chem. Soc., Vol. 118, No. 42, 1996 10143
Table 2. Effects of Temperature on Reaction of Vinyliodonium
Salt 12 with Amines in THF
ecarbene 1 (R ) Bu), which undergoes regioselective 1,5-C-H
insertions yielding the cyclopentene 819 as well as 1,2-shift of
the Bu group to the carbenic center to give the alkyne 9.20 The
electrophilic nature of alkylidenecarbenes4 makes the nucleo-
philic attack of the tertiary amine possible, which produces the
triethylammonium ylide 16 (R ) Bu).21 The follow-up proto-
nation with ammonium tetrafluoroborate produced in situ affords
the vinylammonium salt 10.22 On the other hand, nucleophilic
attack of nonbonding electrons of THF oxygen on 1 produces
the vinyloxonium ylide 2 (R ) Bu). After protonation with
the ammonium tetrafluoroborate, ring-opening of the resulting
cyclic oxonium salt 15 (R ) Bu) by the nucleophilic attack of
triethylamine gives the three-component coupling product, the
vinyl ether 11.23
conditions T/°C
(t/h)
%
ratioa
product yielda,b 13:14
entry base (equiv)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Et3N (1.2)
Et3N (1.2)
Et3N (1.2)
Et3N (2)
Et3N (2)
Et3N (2)
25 (17)
13a, 14a
62
55
63
60
62
63
76
68
73
87
58
89
85
76
52
87
35:65
22:78
14:86
45:55
29:71
19:81
55:45
12:88
10:90
55:45
43:57
39:61
47:53
5:95
-20 (5), 25 (12) 13a, 14a
-78 (5), 25 (10) 13a, 14a
25 (17)
13a, 14a
-20 (5), 25 (12) 13a, 14a
-78 (5), 25 (10) 13a, 14a
DMAP (1.2) 25 (24)
DMAP (1.2) -20 (5), 25 (19) 13b, 14b
DMAP (1.2) -78 (5), 5 (19)
DMAP (2)
DMAP (2)
DMAP (2)
DBN (1.2)
DBN (1.2)
DBU (1.2)
DBU (1.2)
13b, 14b
13b, 14b
13b, 14b
25 (24)
-20 (5), 25 (19) 13b, 14b
-78 (5), 25 (19) 13b, 14b
25 (24)
13c, 14c
As an alternative process leading to the formation of the
vinylammonium salts, we may consider nucleophilic substitu-
tions of the alkenyliodonium salts 7 and 12 and the cyclic
alkenyloxonium salt 15 with amines at the vinylic carbon atoms.
This process, however, does not seem to occur perhaps because
(1) SN1 type reactions of these onium salts involving the
intermediacy of primary vinyl cations will not be important
(there are no known primary vinyl cations probably because of
their low thermodynamic stability)24 and (2) there has been no
well established direct SN2 displacement at a vinylic carbon
atom, which has been calculated to be more difficult energeti-
cally than displacement at an sp3 carbon atom.25,26
-78 (5), 25 (48) 13c, 14c
25 (24) 13d, 14d
-78 (5), 25 (48) 13d, 14d
31:69
3:97
1
a Yields and ratios were determined by H NMR. b Total yields of
13 and 14.
the formation of the vinylammonium salt. 2-Methyl-1-prope-
nyliodonium tetrafluoroborate 1211 was the substrate of choice
to investigate this temperature effect because 2-methyl-1-
propenylidene 1 (R ) Me) generated from 12 does not undergo
the intramolecular 1,5-C-H insertions yielding cyclopentenes,
one of the most facile reactions of alkylidenecarbenes, and thus
the reaction may lead to increased yields both of the vinylam-
monium salts and of the vinyl ethers. In fact, exposure of the
vinyliodonium salt 12 to 1.2 equiv of triethylamine in THF at
25 °C resulted in the formation of a large amount of the
vinylammonium salt 13a (22%) and, in addition, the vinyl ether
14a (40%) was obtained as a major product. In this reaction,
yields of the rearranged alkyne, 2-butyne, were not determined.
As a base, 4-(dimethylamino)pyridine (DMAP), 1,5-diazabicyclo-
[4.3.0]non-5-ene (DBN), and 1,8-diazabicyclo[5.4.0]undec-
7-ene (DBU) were also used; the results are summarized in
Table 2.
(18) (a) Hermecz, I. AdV. Heterocycl. Chem. 1987, 42, 83. (b) Cowley,
A. H. Acc. Chem. Res. 1984, 17, 386. (c) Oedidiger, H.; Moller, F.; Eiter,
K. Synthesis 1972, 591. (d) Reed, R.; Reau, R.; Dahan, F.; Bertrand, G.
Angew. Chem., Int. Ed. Engl. 1993, 32, 399. (e) McCoy, L. L.; Mal, D. J.
Org. Chem. 1981, 46, 1016. (f) Alder, R. W.; Sessions, R. B. Tetrahedron
Lett. 1982, 23, 1121. (g) Schwesinger, R.; Willaredt, J.; Schlemper, H.;
Keller, M.; Schmitt, D.; Fritz, H. Chem. Ber. 1994, 127, 2435.
(19) For intramolecular 1,5-C-H insertion of alkylidenecarbenes, see:
(a) Walsh, R. A.; Bottini, A. T. J. Org. Chem. 1970, 35, 1086. (b) Baumann,
M.; Kobrich, G. Tetrahedron Lett. 1974, 1207. (c) Karpf, M.; Dreiding,
A. S. HelV. Chim. Acta 1979, 62, 852. (d) Gilbert, J. C.; Giamalva, D. H.;
Weerasooriya, U. J. Org. Chem. 1983, 48, 5251. (e) Gilbert, J. C.;
Blackburn, B. K. Tetrahedron Lett. 1984, 25, 4067. (f) Gilbert, J. C.;
Giamalva, D. H.; Baze, M. E. J. Org. Chem. 1985, 50, 2557. (g) Gilbert,
J. C.; Blackburn, B. K. J. Org. Chem. 1986, 51, 3656. (h) Gilbert, J. C.;
Blackburn, B. K. J. Org. Chem. 1986, 51, 4087. (i) Baird, M. S.; Baxter,
A. G. W.; Hoorfar, A.; Jefferies, I. J. Chem. Soc., Perkin Trans. 1 1991,
2575. (j) Ohira, S.; Okai, K.; Moritani, T. J. Chem. Soc., Chem. Commun.
1992, 721. (k) Kunishima, M.; Hioki, K.; Tani, S.; Kato, A. Tetrahedron
Lett. 1994, 35, 7253. (l) Kim, S.; Cho, C. M. Tetrahedron Lett. 1994, 35,
8405. (m) Williamson, B. L.; Tykwinski, R. R.; Stang, P. J. J. Am. Chem.
Soc. 1994, 116, 93. (n) Tykwinski, R. R.; Stang, P. J.; Persky, N. E.
Tetrahedron Lett. 1994, 35, 23. (o) Schildknegt, K.; Bohnstedt, A. C.;
Feldman, K. S.; Sambandam, A. J. Am. Chem. Soc. 1995, 117, 7544.
(20) For 1,2-shift of alkylidenecarbenes, see: (a) Bothner-By, A. A. J.
Am. Chem. Soc. 1955, 77, 3293. (b) Wolinsky, J. J. Org. Chem. 1961, 26,
704. (c) Erickson, K. L.; Wolinsky, J. J. Am. Chem. Soc. 1965, 87, 1142.
(d) Newman, M. S.; Gromelski, S. J. J. Org. Chem. 1972, 37, 3220. (e)
Kobrich, G.; Merkel, D.; Thiem, K.-W. Chem. Ber. 1972, 105, 1683. (f)
Wolinsky, J.; Clark, G. W.; Thorstenson, P. C. J. Org. Chem. 1976, 41,
745. (g) Kowalski, C. J.; Fields, K. W. J. Am. Chem. Soc. 1982, 104, 321.
(h) Gilbert, J. C.; Weerasooriya, U. J. Org. Chem. 1982, 47, 1837. (i)
McDougall, J. J. W.; Schlegel, H. B.; Francisco, J. S. J. Am. Chem. Soc.
1989, 111, 4622. (j) Walsh, R; Untiedt, S.; Stohlmeier, M.; Meijere, A.
Chem. Ber. 1989, 122, 637. (k) Ochiai, M.; Ito, T.; Takaoka, Y.; Masaki,
Y.; Kunishima, M.; Tani, S.; Nagao, Y. J. Chem. Soc., Chem. Commun.
1990, 118. (l) Kunishima, M.; Hioki, K.; Ohara, T.; Tani, S. J. Chem.
Soc., Chem. Commun. 1992, 219. (m) Fischer, D. R.; Williamson, B. L.;
Stang, P. J. Synlett 1992, 535.
As shown in Table 2, formation of the vinylammonium salt
13a exhibited a temperature dependence similar to that of the
1,5-C-H insertion product 8 and the rearranged product 9;
decreasing temperature trends to decrease the yield of 13a while
increasing that of the three-component coupling product 14a,
as observed in the reaction of 7. This tendency was further
confirmed in the reaction with DMAP, which yielded a mixture
of 13b and 14b. Concentration of a base also affects the ratios
of the vinylammonium salts 13 to the vinyl ethers 14; thus,
increasing the concentration of a base by using 2 equiv of
triethylamine of DMAP increased the relative amounts of 13a,b.
(Compare entries 1-3 with 4-6, and entries 7-9 with 10-12
of Table 2.) Note that DBN and DBU, known as nonnucleo-
philic strong bases,18 can act both as nucleophiles and as bases
in this reaction. Here, they afforded a mixture of 13c,d and
14c,dsnotably, the reaction at -78 °C gave the vinyl ethers
14c,d with more than 95% selectivity. At 25 °C, again, these
bicyclic imidines afforded a large amount of the vinylammonium
salts 13c,d. The structure of 13 and 14 was determined by
2D NMR and NOESY.
(21) For generation of carbene-derived ammonium ylides, see ref 1a.
(22) Generation of alkylidenecarbenes in the presence of secondary
amines and alcohols has been reported to produce enamines and enols,
respectively.5
See, also: (a) Gilbert, J. C.; Weerasooriya, U.; Wiechman,
B.; Ho, L. Tetrahedron Lett. 1980, 21, 5003. (b) Ohira, S. Synth. Commun.
1989, 19, 561. (c) Miwa, K.; Aoyama, T.; Shioiri, T. Synlett 1994, 109.
(23) Generation of 2-methyl-1-propenylidene 1 (R ) Me) in THF in the
presence of alcohols has been reported to result in formation of the similar
three-component coupling products albeit in low yield (11-15%), via the
intermediacy of the oxonium ylide 2 (R ) Me).5,6
Triethylamine-induced reductive R-elimination of iodoben-
zene from the alkenyliodonium salt 7 generates the alkyliden-
(24) Stang, P. J.; Rappoport, Z.; Hanack, M.; Subramanian, L. R. Vinyl
Cations; Academic Press, New York, 1979.

10144 J. Am. Chem. Soc., Vol. 118, No. 42, 1996
Sueda et al.
Scheme 1
Table 3. Reaction of Vinyliodonium Salt 12 with Amines in THTa
because no reaction of 14a with DBN and triethylamine in THF
conditions productb % yieldc 18:19
was observed.
entry
base (equiv)
1
2
3
Et3N (1.2)
DMAP (1.2)
(i-Pr)2NEt (1.2) 25 °C (3 h)
25 °C (24 h) 18, 19a
25 °C (24 h) 18, 19b
100
94
100
75:25
60:40
100:0
18, 19c
1
a Yields and ratios were determined by H NMR. b R of 18 and 19
) Me. c Total yields of 18 and 19.
The temperature dependence of the product profiles described
above (that decreasing temperature has a tendency to decrease
the yield of the free alkylidenecarbene-derived products, i.e.,
cyclopentenes, alkynes, and vinylammonium salts and increase
that of the vinyloxonium ylide-derived products, vinyl ethers)
might be attributed to a rapid equilibrium between the free
alkylidenecarbene 1 and the oxonium ylide 2.27,28 Based on
the assumption that the process leading to the formation of the
oxonium ylide 2 from the alkylidenecarbene 1 and THF is
associated with a decrease in entropy,29 decreasing the reaction
temperature can reasonably be expected to yield a higher
concentration of the oxonium ylide 2, which, in turn, increases
the relative amount of the three-component coupling products,
as was indeed observed. The reaction pathways of Scheme 1
suggest that the concentration of amines has some influence on
the yield of vinylammonium salts and vinyl ethers; the yield of
vinylammonium salts will depend upon the concentration of
amines used and will increase with the higher concentrations
of amines, as shown in Table 2.
The fact that neither 13c nor 14c was detected by exposure
of the vinylammonium salt 13a to DBN at room temperature
in THF, and that 13a was recovered unchanged, clearly implies
that regeneration of the alkylidenecarbene 1 (R ) Me) from
13a does not proceed to any appreciable extent under these
reaction conditions. Similarly, 13a was recovered in the reaction
with triethylamine in THF. No deuterium incorporation of
R-vinylic proton of 13a was observed in the reaction with
butyllithium, phenyllithium, or lithium diisopropylamide, fol-
lowed by quenching with D2O. Furthermore, the possibility
for regeneration of the oxonium ylide 2 (R ) Me) from the
vinyl ether 14a under these conditions was also ruled out
In marked contrast to the cyclic oxonium ylide 2, it seems
reasonable to assume that the corresponding cyclic sulfonium
ylide 1730,31 would not establish an equilibrium with the free
alkylidenecarbene 1, given that no evidence for formation of
the alkylidenecarbene-derived products was obtained when the
reaction was carried out in tetrahydrothiophene (THT) instead
of THF as a solvent. Thus, reaction of 7 with triethylamine in
THT at 25 °C for 10 h afforded only sulfonium ylide-derived
products, the sulfonium salt 18 (R ) Bu; 87%) and the three-
component coupling product 19a (R ) Bu; 7%). Careful GC
1
and H NMR analyses of the crude reaction mixture showed
no evidence of formation of the alkylidenecarbene-derived
products 8, 9, and 10. These results indicate that relatively rapid
intramolecular 1,5-C-H insertions of alkylidenecarbenes yield-
ing cyclopentenes32 cannot compete with nucleophilic attack
of the sulfur atom of THT on the electron-deficient carbenic
center. Similar results were obtained in the reaction of 12 with
triethylamine in THT at 25 °C, which produced, quantitatively,
a 75:25 mixture of 18 (R ) Me) and 19a (R ) Me). Use of
the more sterically demanding diisopropylethylamine resulted
in selective formation of the sulfonium salt 18 (R ) Me)
(Table 3).
The mechanism involving the intermediacy of the alkyliden-
ecarbene 1 and the sulfonium ylide 17 will predict deuterium
incorporation at the R-vinylic position of 18, if the reaction in
THT is carried out in the presence of relatively acidic deuterium
atoms. In the presence of methanol-d4, the reaction of the
vinyliodonium salt 7 with diisopropylethylamine in THT
resulted in 61% deuterium incorporation at the R-vinylic position
of 18 (R ) Bu) in an 88% yield, as expected. Since no evidence
for deuterium exchange of the R-vinylic hydrogen of 18 (R )
Bu) was observed under our reaction conditions, we interpret
these results to mean that neither an SN1 and SN2 mechanism
nor an addition-elimination mechanism for this onium transfer
reaction of 7sboth of which predict retention of the R-vinylic
hydrogen atom of 7sis involved.
(25) (a) Kelsey, D. R.; Bergman, R. G. J. Am. Chem. Soc. 1971, 93,
1953. (b) Bunnett, J. F.; Zahler, R. E. Chem. ReV. 1951, 49, 273. (c)
Bunnett, J. F. Quart ReV. 1958, 12, 1. (d) Glukhovtsev, M. N.; Pross, A.;
Radom, L. J. Am. Chem. Soc. 1994, 116, 5961. (e) Lucchini, V.; Modena,
G.; Pasquato, L. J. Am. Chem. Soc. 1995, 117, 2297.
(26) Reaction of 1-decenyl(phenyl)iodonium salt with triethylamine
affords 1-decyne exclusively and no formation of vinylammonium salts is
detected.9a
Furthermore, the involvement of free alkylidenecarbenes was
firmly established by the stereochemical outcome of this onium
(27) Equilibrium between Rh-carbenoids and oxonium ylides has been
proposed for intramolecular C-H insertion of carbenoids.3m
(28) The equilibrium constant of the reaction of (biphenyl-4-yl)-
chlorocarbene with THF yielding oxonium ylide has been determined by
Naito and Oku to be 0.5 M-1 at 293 K in 2,2,4-trimethylpentane. See:
Naito, I.; Oku, A.; Otani, N.; Fujiwara, Y.; Tanimoto, Y. J. Chem. Soc.,
Perkin Trans. 2 1996, 725. Kinetic studies for oxonium ylide formation
from carboalkoxycarbenes by the reaction with ethers have been reported.
See: (a) Toscano, J. P.; Platz, M. S.; Nikolaev, V.; Popic, V. J. Am. Chem.
Soc. 1994, 116, 8146. (b) Wang, J.-L.; Toscano, J. P.; Platz, M. S.;
Nikolaev, V.; Popik, V. J. Am. Chem. Soc. 1995, 117, 5477.
(30) 1,2-Shifts of R-phenylsulfenyl groups of alkylidenecarbenes yielding
alkynyl sulfides were proposed to involve generation of sulfonium ylides
produced by nucleophilic attack of lone pair electrons on the sulfur atoms
to the electron-deficient carbenic center.9b See, also: (a) Delavarenne, S.
Y.; Viehe, H. G. Chem. Ber. 1970, 103, 1209. (b) Viehe, H. G.;
Delavarenne, S. Y. Chem. Ber. 1970, 103, 1216.
(31) For sulfonium ylides, see: Trost, B. M.; Melvin, L. S. Sulfur Ylides;
Academic Press: New York, 1975.
(32) Ochiai, M.; Uemura, K.; Masaki, Y. J. Am. Chem. Soc. 1993, 115,
2528.
(29) (a) Lucht, B. L.; Collum, D. B. J. Am. Chem. Soc. 1995, 117, 9863.
(b) Strong, J.; Tuttle, T. R., Jr. J. Phys. Chem. 1973, 77, 533.

EVidence for ReVersible Ylide Formation
J. Am. Chem. Soc., Vol. 118, No. 42, 1996 10145
Figure 1. Reaction coordinates for onium ylide formation with selected geometrical parameters: (a) reaction of 1 (R ) Me) with THF; (b)
reaction of 1 (R ) Me) with trimethylamine: and (c) reaction of 1 (R ) Me) with THT. Bond distances in Å; angles in deg.
transfer reaction; i.e., treatment of the E-vinyliodonium salt 20
with diisopropylethylamine in THT afforded a mixture of
stereoisomers of the vinylsulfonium salt 21 with E-isomer as a
major product, and the ratio of E- and Z-isomers was determined
by 1H NMR analysis to be 62:38. The E-sulfonium salt 21 was
1 (R ) Me) and the corresponding onium ylides were carried
out with the quantum calculations using the MOPAC 93
program.34 THF, trimethylamine, and THT were used as
model nucleophiles for our calculations. Energy calculations,
performed with the PM3 Hamiltonian, are summarized in
Figure 1.
For the oxonium ylide 2 (R ) Me: the reaction coordinate,
1.36 Å), the amount of polarization, i.e., the partial charges of
the alkylidenecarbene and THF portions, were estimated as the
au values of -0.359 and +0.359. These polarization values
were found to be less than half of those of the trimethylam-
monium and the sulfonium ylides ((0.774 and (0.755,
respectively), implying that the trimethylammonium and the
sulfonium ylides 17 (R ) Me) were more efficiently stabilized
than the oxonium ylide 2 (R ) Me). The relatively small
intrinsic barrier of formation (6.63 kcal/mol) and dissociation
(10.91 kcal/mol) of the oxonium ylide 2 (R ) Me) probably
suggests that the free alkylidenecarbene 1 (R ) Me) and the
cyclic oxonium ylide 2 (R ) Me) is in equilibrium in THF.
This does not seem to hold for the ammonium ylides, given
that 2-methylpropenyl(trimethyl)ammonium ylide shows a large
dissociation barrier of 34.42 kcal/mol. Similarly, a large energy
barrier of 49.57 kcal/mol is obtained for dissociation of the
sulfonium ylide 17 (R ) Me). These values are in good
agreement with the experimental results and are consistent with
the reaction pathway discussed above.
As shown in Table 1, the ratios of intramolecular 1,5-C-H
insertions versus 1,2-shifts of the alkyl group in the reaction of
7 in THF vary with reaction temperature to a small but
discernible extent, and the relative amount of the insertion
product 8 increases with a decrease in reaction temperature. We
attribute these findings to the difference in entropy of their
transition states; that is, 1,5-C-H insertions of 1 (R ) Bu) may
require a more conformationally limited transition state than
that for 1,2-shifts of the alkyl group.
also obtained as a major product (E:Z ) 64:36) in the reaction
of the Z-vinyliodonium salt 20. Similar stereoconvergence with
E-isomer 22 as a major product was observed in the reaction
with diphenyl sulfide. Since reactions of alkylidenecarbenoids
with nucleophiles have been shown to proceed in favor of
inversion of configuration,33 the high degree of stereoconver-
gence of the olefin geometry in this onium transfer reaction
that we observed strongly indicates intermediacy of the free
alkylidenecarbene 23. This stereoselectivity is most likely a
consequence of the steric differences between the two R-sub-
stituents of the alkylidenecarbene 23.
The variation in behavior of THF and THT toward the
alkylidenecarbenes is probably attributable to the difference in
nucleophilicities of the oxygen and sulfur atoms and in the
inherent stability of their onium ylides.3c,i Computational sim-
ulations of the reaction path between the free alkylidenecarbene
Conclusion
Reactions of alkylidenecarbenes generated by amine-induced
R-elimination of vinyliodonium salts in tetrahydrofuran were
observed to be temperature dependent: decreasing the reaction
temperature tended to decrease the yields of free alkylidenecar-
bene-derived products and to increase those of the vinyloxonium
(33) (a) Duraisamy, M.; Walborsky, H. M. J. Am. Chem. Soc. 1984, 106,
5035. (b) Rachon, J.; Goedken, V.; Walborsky, H. M. J. Am. Chem. Soc.
1986, 108, 7435. (c) Oku, A.; Yamaura, Y.; Harada, T. J. Org. Chem.
1986, 51, 3730. (d) Topolski, M.; Duraisamy, M.; Rachon, J.; Gawronski,
J.; Gawronska, K.; Goedken, V.; Walborsky, H. M. J. Org. Chem. 1993,
58, 546. (e) Oku, A. J. Synth. Org. Chem., Jpn. 1995, 53, 2.
(34) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209.
(35) Tanaka, K.; Shiraishi, S.; Nakai, T.; Ishikawa, N. Tetrahedron Lett.
1978, 34, 3103.

10146 J. Am. Chem. Soc., Vol. 118, No. 42, 1996
Sueda et al.
HRFAB MS Calcd for C16H24I[(M - BF4)+] 343.0923. Found
343.0915. Anal. Calcd for C16H24BF4I: C, 44.68; H, 5.62. Found:
C, 44.28, H, 5.50.
ylide-derived products. Although all attempts to detect directly
the intermediacy of the vinyloxonium ylide were fruitless, this
temperature dependence strongly suggests reversible oxonium
ylide formation. In marked contrast, no evidence was observed
to suggest the existence of an equilibrium between the free
alkylidenecarbene and the vinylsulfonium ylide.
Synthesis of (E)-Phenyl(2-methyl-3-phenyl-1-propenyl)iodonium
Tetrafluoroborate (E)-(20). (E)-2-Methyl-3-phenyl-1-(trimethylsilyl)-
prop-1-ene was prepared stereoselectively from 3-phenyl-1-propyne by
the reaction with lithium bis(trimethylsilyl)cuprate and methyl iodide
in 72% yield according to the procedure developed by Fleming.15 To
a stirred suspension of iodosylbenzene (1.58 g, 7.20 mmol) and the
vinylsilane (917 mg, 4.50 mmol) in dichloromethane (50 mL) was added
dropwise BF3-Et2O (1.02 g, 7.20 mmol) at 0 °C in nitrogen and the
mixture was stirred for 0.25 h at 0 °C. After the addition of a saturated
aqueous solution of sodium tetrafluoroborate (9.9 g, 90 mmol), the
mixture was stirred for 15 min. Extraction with dichloromethane,
filtration, and then concentration under aspirator vacuum gave an oil,
which was washed several times with hexane by decantation at -78
°C. Recrystallization from dichloromethane-diethyl ether gave the
iodonium salt (E)-20 (774 mg, 48%) as colorless prisms: mp 104-
105 °C; IR (KBr) 3073, 1598, 1495, 1472, 1453, 1443, 1260, 1058
Experimental Section
General Information. IR spectra were recorded on JASCO IRA-1
and Perkin Elmer 1720 FT-IR spectrometers. 1H and 13C NMR were
recorded in CDCl3 on a JEOL JNM-FX 200 or JNM-GX 400
spectrometer. Chemical shifts were reported in parts per million (ppm)
downfield from internal Me4Si. Mass spectra (MS) were obtained on
a JEOL JMS-DX300 spectrometer. Analytical gas chromatography
(GC) was conducted on a Shimadzu GC-14A gas chromatograph with
10% or 20% Silicone GE SF-96 on a Chromosorb W-AWDMCS
column (3 m) and on a Shimadzu GC-15A gas chromatograph with an
0.25 mm × 50 m FFS ULBON HR-20M capillary column. Preparative
GC was performed on a Shimadzu GC-14A gas chromatograph with
10% or 20% Silicone GE SF-96 on a Chromosorb W-AWDMCS
column. Preparative thin-layer chromatography (TLC) was carried out
on precoated plates of silica gel (Merck, silica gel F-254). Kieselgel
60 (Merck, 230-400 mesh) was used for flash chromatography.
Melting points were determined with a Yanaco micro melting points
apparatus and are uncorrected.
1
(br), 745, 704, 682 cm-1; H NMR δ 7.89 (d, J ) 8.1 Hz, 2 H), 7.59
(t, J ) 7.4 Hz, 1 H), 7.45 (dd, J ) 8.1, 7.4 Hz, 2 H), 7.35-7.05 (m,
5 H), 6.80 (s, 1 H), 3.75 (s, 2 H), 2.11 (s, 3 H); FAB MS m/z 335 [(M
- BF4)+]; HRFAB MS Calcd for C16H16I [(M - BF4)+] 335.0297.
Found 335.0319. Anal. Calcd for C16H16BF4I: C, 45.54; H, 3.82.
Found: C, 45.24; H, 3.83.
Synthesis of (Z)-Phenyl(2-methyl-3-phenyl-1-propenyl)iodonium
Tetrafluoroborate (Z)-(20). (Z)-2-Methyl-3-phenyl-1-(trimethylsilyl)-
prop-1-ene was prepared stereoselectively from propyne by the reaction
with lithium bis(trimethylsilyl)cuprate and benzyl bromide in 68% yield
according to the procedure developed by Fleming.15 To a stirred
suspension of iodosylbenzene (3.57 g, 16.2 mmol) and the vinylsilane
(2.07 g, 10.1 mmol) in dichloromethane (30 mL) was added dropwise
BF3-Et2O (2.23 g, 16.2 mmol) at 0 °C in nitrogen, and the mixture
was stirred for 0.5 h at 0 °C. After the addition of a saturated aqueous
solution of sodium tetrafluoroborate (22.2 g, 203 mmol), the mixture
was stirred for 15 min. Extraction with dichloromethane, filtration,
and then concentration under aspirator vacuum gave an oil, which was
washed several times with hexane by decantation at -78 °C. Recrys-
tallization from dichloromethane-diethyl ether gave the iodonium salt
(Z)-20 (3.0 g, 70%) as colorless prisms: mp 91-92 °C; IR (KBr)
3092, 1596, 1495, 1471, 1445, 1261, 1050 (br), 752, 705, 651, 521
cm-1; 1H NMR δ 7.89 (d, J ) 8.0 Hz, 2 H), 7.57 (t, J ) 7.4 Hz, 1 H),
7.41 (dd, J ) 8.0, 7.4 Hz, 2 H), 7.30-7.15 (m, 3 H), 7.07-6.93 (m,
2 H), 6.86 (s, 1 H), 3.80 (s, 2 H), 2.06 (s, 3 H); FAB MS m/z 335 [(M
- BF4)+]; HRFAB MS Calcd for C16H16I [(M - BF4)+] 335.0297.
Found 335.0320. Anal. Calcd for C16H16BF4I: C, 45.54; H, 3.82.
Found: C, 45.32; H, 3.89.
Dichloromethane was dried over CaH2 and distilled. BF3-Et2O was
distilled from CaH2 under nitrogen. THF was distilled from sodium
benzophenone ketyl under nitrogen.
Substrates. Phenyl(2-methyl-1-propenyl)iodonium tetrafluoroborate
(12) was prepared by silicon-iodonium exchange reaction of 2-methyl-
1-(trimethylsilyl)prop-1-ene with iodosylbenzene.11
Synthesis of (E)-Phenyl(3-cyclopentyl-2-methyl-1-propenyl)iodo-
nium Tetrafluoroborate (4). (E)-3-Cyclopentyl-2-methyl-1-(trimeth-
ylsilyl)prop-1-ene was prepared stereoselectively from 3-cyclopentyl-
1-propyne by the reaction with lithium bis(trimethylsilyl)cuprate and
methyl iodide in 54% yield according to the procedure developed by
Fleming.15 To a stirred suspension of iodosylbenzene (3.84 g, 17.5
mmol) and the vinylsilane (2.14 g, 10.9 mmol) in dichloromethane (50
mL) was added dropwise BF3-Et2O (2.48 g, 17.5 mmol) at 0 °C in
nitrogen, and the mixture was stirred for 0.3 h at 0 °C. After the
addition of a saturated aqueous solution of sodium tetrafluoroborate
(23 g, 0.21 mol), the mixture was stirred for 15 min. Extraction with
dichloromethane, filtration, and then concentration under aspirator
vacuum gave an oil, which was washed several times with hexane by
decantation at -78 °C. Further purification by decantation using diethyl
ether at -78 °C gave the iodonium salt 4 (3.7 g, 82%) as colorless
needles: mp 69-71 °C (recrystallized from dichloromethane-diethyl
ether-hexane); IR (KBr) 3080, 2950, 2860, 1608, 1568, 1473, 1443,
1380, 1260, 1200-1000, 995, 745, 683 cm-1; 1H NMR δ 7.91 (d, J )
8.1 Hz, 2 H), 7.64 (t, J ) 7.5 Hz, 1 H), 7.50 (dd, J ) 8.1, 7.5 Hz, 2
H), 6.67 (s, 1 H), 2.49 (d, J ) 7.4 Hz, 2 H), 2.20 (s, 3 H), 2.06 (septet,
J ) 7.4 Hz, 1 H), 1.78-1.50 (m, 6 H), 1.13-1.04 (m, 2 H); FAB MS
m/z 327 [(M - BF4)+]. Anal. Calcd for C14H20BF4I: C, 43.51; H,
4.87. Found: C, 43.15; H, 4.82.
Reaction of (E)-Phenyl(3-cyclopentyl-2-methyl-1-propenyl)iodo-
nium Tetrafluoroborate (4) with Triethylamine. To a solution of
the iodonium salt 4 (41 mg, 0.10 mmol) in 1 mL of THF was added
triethylamine (12 mg, 0.12 mmol) at -78 °C under nitrogen, and the
mixture was stirred for 5 h. The reaction mixture was gradually warmed
to 0 °C over 4 h and stirred for another 10 h at the temperature. After
addition of 1 mL of water and 2 mL of dichloromethane to the mixture,
analytical GC using a column of 20% Silicone GE SF-96 (3 m, 100
°C) with nonane as an internal standard showed the formation of the
bicyclo[3.3.0]octene 5 in 39% yield. Extraction with dichloromethane,
filtration, and then concentration under aspirator vacuum gave an oil,
which was washed several times with pentane by decantation to give
the triethylammonium vinyl ether 6, contaminated with a small amount
of impurity (ca. 4%). 1H NMR spectra with nitrobenzene as an internal
standard showed formation of 6 in 43% yield. A pure sample of 6
(E:Z ) 7:3) was obtained by preparative TLC (dichloromethane-
methanol 9:1). (E)- and (Z)-6: pale yellow oil; IR (film) 2920, 2840,
1705, 1670, 1460, 1390, 1160, 1130-930, 790 cm-1; 1H NMR δ 5.80
(br s, CdCH, Z-isomer), 5.78 (br s, CdCH, E-isomer), 3.74 (t, J )
5.6 Hz, OCH2, E-isomer), 3.70 (t, J ) 5.7 Hz, OCH2, Z-isomer), 3.31
(q, J ) 7.1 Hz, N(CH2CH3)3, E-isomer), 3.31 (q, J ) 7.1 Hz, N(CH2-
CH3)3, Z-isomer), 3.26-3.18 (m, 2 H), 2.02 (d, J ) 7.7 Hz, CCH2,
Z-isomer), 1.94 (septet, J ) 7.7 Hz, 1 H, CHCH2), 1.85-1.45 (m,
NCH2CH2, 8 H), 1.82 (d, J ) 7.7 Hz, CCH2, E-isomer), 1.56 (br s,
CCH3, E-isomer), 1.52 (br s, CCH3, Z-isomer), 1.33 (t, J ) 7.1 Hz, 9
H, (CH2CH3)3), 1.2-1.0 (m, 2 H); FAB MS m/z 296 [(M - BF4)+].
Synthesis of Phenyl(2-butyl-1-hexenyl)iodonium Tetrafluorobo-
rate (7). 2-Butyl-1-(trimethylsilyl)hex-1-ene was prepared from 1-hex-
yne by the reaction with lithium bis(trimethylsilyl)cuprate and 1-io-
dobutane in 64% yield.15 To a stirred suspension of iodosylbenzene
(3.65 g, 16.6 mmol) and the vinylsilane (2.2 g, 10 mmol) in
dichloromethane (50 mL) was added dropwise BF3-Et2O (2.35 g, 16.6
mmol) at 0 °C in nitrogen, and the mixture was stirred for 0.25 h at 0
°C. After the addition of a saturated aqueous solution of sodium
tetrafluoroborate (23 g, 0.21 mol), the mixture was stirred for 15 min.
Extraction with dichloromethane, filtration, and then concentration under
aspirator vacuum gave an oil, which was washed several times with
hexane by decantation at -78 °C. Recrystallization from dichlo-
romethane-diethyl ether gave the iodonium salt 7 (3.8 g, 86%) as
colorless plates: mp 77-78 °C; IR (KBr) 3032, 2959, 2871, 1605,
1
1565, 1472, 1441, 1267, 1060 (br), 991, 739, 680, 649, 534 cm-1; H
NMR δ 7.95 (d, J ) 8.0 Hz, 2 H), 7.61 (t, J ) 7.4 Hz, 1 H), 7.47 (dd,
J ) 8.0, 7.4 Hz, 2 H), 6.71 (s, 1 H), 2.54-2.33 (m, 4 H), 1.59-1.15
(m, 8 H), 0.99-0.76 (m, 6 H); FAB MS m/z 343 [(M - BF4)+];

EVidence for ReVersible Ylide Formation
J. Am. Chem. Soc., Vol. 118, No. 42, 1996 10147
HRFAB MS Calcd for C19H38ON [(M - BF4)+] 296.2953. Found
296.2959. 1H NMR signals of (E)- and (Z)-6 were assigned by 1H,1H-
and 13C,1H-COSY spectra. The stereochemistry of (E)-6 was estab-
lished by the observation of a nuclear Overhauser effect (NOE)
enhancement between the vinylic proton and the allylic methylene
protons. The stereochemistry of (Z)-6 was established by the observa-
tion of an NOE enhancement between the vinylic proton and the allylic
methyl group. A pure sample of 5 was obtained by preparative GC
using a column of 20% Silicone GE SF-96 (3 m, 70 °C) in a separate
experiment. 5: colorless oil; IR (film) 2930, 2855, 1598, 1443, 1373
6 H), 1.99 (s, 3 H), 1.92 (s, 3 H), 1.34 (t, J ) 7.6 Hz, 9 H); MS m/z
(relative intensity) 156 [7, (M - BF4)+], 127 (41), 112 (100), 98 (21),
56(81); FAB MS m/z 156 [(M - BF4)+]; HRMS Calcd for C10H22N
[(M - BF4)+] 156.1753. Found 156.1762. Anal. Calcd for C10H22-
BF4N: C, 49.41; H, 9.12; N, 5.76. Found: C, 49.26; H, 9.11; N, 5.59.
14a: oil; IR (film) 2905, 1685, 1480, 1400, 1090 (br), 795 cm-1; 1H
NMR δ 5.78 (br s, 1 H), 3.79-3.61 (m, 2 H), 3.38-3.10 (8 H), 1.88-
1.47 (10 H), 1.33 (t, J ) 6.6 Hz, 9 H); 13C NMR δ 139.72, 110.84,
70.09, 56.67, 52.95, 26.12, 19.45, 18.51, 15.07, 7.37; MS m/z (relative
intensity) 228 [5, (M - BF4)+], 128 (9), 101 (9), 86 (100), 72 (20), 55
(19); FAB MS m/z 228 [(M - BF4)+]; HRMS Calcd for C14H30NO
[(M - BF4)+] 228.2326. Found 228.2333.
1
cm-1; H NMR δ 5.06 (m, 1 H), 3.09 (m, 1 H), 2.90-2.30 (m, 2 H),
2.10-1.20 (7 H), 1.67 (m, 3 H); MS m/z (relative intensity) 122 (59,
M+), 107 (22), 94 (78), 93 (100), 91 (25), 80 (33), 79 (47), 77 (23), 67
(11); HRMS Calcd for C9H14(M+) 122.1096. Found 122.1096.
13b: colorless crystals; mp 97-98 °C (recrystallized from dichlo-
romethane-diethyl ether-hexane); IR (Nujol) 2900, 1635, 1560, 1435,
1
1390, 1180, 1030 (br), 820 cm-1; H NMR δ 7.84 (d, J ) 7.8 Hz, 2
General Procedure for the Reaction of Phenyl(2-butyl-1-hexenyl)-
iodonium Tetrafluoroborate (7) with Triethylamine in THF. To a
solution of the iodonium salt 7 (43 mg, 0.10 mmol) in 1 mL of THF
was added triethylamine (12 mg, 0.12 mmol) under nitrogen. The
mixture was stirred under the conditions described in Table 1. After
addition of 1 mL of water and 2 mL of dichloromethane, the reaction
mixture was analyzed by GC using a column of 15% FFAP (3 m, 70
°C) with undecane as an internal standard, which showed the formation
of 1-butyl-3-methylcyclopentene (8) and 5-decyne (9). The yields of
8 and 9 are reported in Table 1. Extraction with dichloromethane,
filtration, and then concentration under aspirator vacuum gave an oil,
which was washed several times with pentane by decantation to give
a mixture of the vinylammonium salt 10 and the triethylammonium
vinyl ether 11. The yields of 10 and 11 were determined by 1H NMR
analyses of the mixture with nitrobenzene as an internal standard and
are reported in Table 1. Pure samples of the cyclopentene 8 and the
alkyne 9 were obtained by preparative GC using a column of 15%
FFAP (3 m, 70 °C) in a separate experiment. 8:19d colorless oil; IR
H), 6.98 (d, J ) 7.8 Hz, 2 H), 6.64 (br s, 1 H), 3.29 (s, 6 H), 1.91 (d,
J ) 1.5 Hz, 3 H), 1.70 (d, J ) 1.2 Hz, 3 H); FAB MS m/z 177 [(M -
BF4)+]; HRMS Calcd for C11H17N2 [(M - BF4)+] 177.1391. Found
177.1379.
14b: yellow oil; IR (film) 3080, 2920, 2870, 1690, 1655, 1575, 1440,
1
1405, 1170, 1060 (br), 830 cm-1; H NMR δ 8.07 (d, J ) 7.8 Hz, 2
H), 6.90 (d, J ) 7.8 Hz, 2 H), 5.76 (br s, 1 H), 4.20 (t, J ) 7.8 Hz, 2
H), 3.68 (t, J ) 6.0 Hz, 2 H), 3.24 (s, 6 H), 2.11-1.83 (m, 2 H),
1.72-1.37 (m, 2 H), 1.57 (d, J ) 0.7 Hz, 3 H), 1.52 (d, J ) 1.0 Hz,
3 H); 13C NMR δ 156.37, 141.89, 139.84, 110.76, 108.17, 70.68, 57.96,
40.17, 27.92, 26.04, 19.46, 15.10; FAB MS m/z 249 [(M - BF4)+];
HRMS Calcd for C15H25N2O [(M - BF4)+] 249.1967. Found 249.1970.
13c: orange oil; IR (film) 2950, 1660, 1525, 1440, 1380, 1310, 1060
(br), 810 cm-1; 1H NMR δ 6.03 (br s, 1 H), 3.85 (t, J ) 7.6 Hz, 2 H),
3.57-3.39 (m, 4 H), 2.86 (t, J ) 7.9 Hz, 2 H), 2.30-2.05 (m, 4 H),
1.76 (d, J ) 1.2 Hz, 3 H), 1.72 (d, J ) 1.2 Hz, 3 H); 13C NMR δ
165.71, 139.02, 123.13, 54.71, 46.89, 42.28, 31.11, 21.64, 19.04, 18.02,
17.57; MS m/z (relative intensity) 179 [100, (M - BF4)+], 177 (90),
163 (62), 149 (61), 135 (55), 109 (65), 55 (83); HRMS Calcd for
C11H19N2 [(M - BF4)+] 179.1549. Found 179.1550.
14c: orange oil; IR (film) 2915, 1665, 1440, 1310, 1060 (br), 1055
cm-1; 1H NMR δ 5.77 (sept, J ) 1.5 Hz, 1 H), 3.78 (t, J ) 7.3 Hz, 2
H), 3.69 (t, J ) 6.1 Hz, 2 H), 3.54-3.35 (m, 6 H), 3.02 (t, J ) 7.9 Hz,
2 H), 2.32-2.07 (m, 4 H), 1.88-1.48 (m, 4 H), 1.59 (br s, 3 H), 1.54
(br s, 3 H); 13C NMR δ 164.69, 139.84, 110.84, 70.73, 54.31, 53.04,
44.26, 42.25, 30.37, 26.51, 24.20, 19.47, 18.91, 18.12, 15.05; FAB MS
m/z 251 [(M - BF4)+]; HRFAB MS Calcd for C15H27N2O [(M - BF4)+]
251.2123. Found 251.2092.
1
(CHCl3) 2950, 2920, 2850, 1600, 1455, 910 cm-1; H NMR δ 5.25-
5.16 (m, 1 H), 2.80-2.59 (m, 1 H), 2.30-1.90 (m, 5 H), 1.50-1.16
(m, 5 H), 0.98 (d, J ) 6.8 Hz, 3 H), 0.90 (t, J ) 6.8 Hz, 3 H); MS m/z
(relative intensity) 138 (33, M+), 123 (30), 81 (100), 67 (76), 55 (34).
9:35 colorless oil; IR (CHCl3) 2950, 2915, 2850, 1660, 1600, 1457,
1370, 910 cm-1; 1H NMR δ 2.20-2.07 (m, 4 H), 1.59-1.23 (m, 8 H),
0.90 (t, J ) 6.9 Hz, 6 H); 13C NMR δ 140.16, 119.34, 70.22, 56.91,
53.13, 31.10, 30.45, 29.98, 26.67, 26.23, 22.75, 22.44, 18.78, 13.98,
7.53; MS m/z (relative intensity) 138 (17, M+), 123 (14), 81 (100), 67
(73), 54 (13). A pure sample of the triethylammonium vinyl ether 11
was obtained by preparative TLC (dichloromethane-methanol 9:1).
11: pale yellow powder; IR (KBr) 2957, 2946, 2889, 2863, 1674, 1467,
1
1100 (br), 798 cm-1; H NMR δ 5.78 (s, 1 H), 3.72 (t, J ) 5.3 Hz, 2
13d: orange oil; IR (film) 2920, 2850, 1620, 1510, 1445, 1320, 1200,
1060 (br), 820 cm-1; 1H NMR δ 6.05 (br s, 1 H), 3.74-3.60 (m, 4 H),
3.51 (t, J ) 5.9 Hz, 2 H), 2.90-2.77 (m, 2 H), 2.26-2.11 (m, 2 H),
1.94-1.54 (m, 6 H), 1.79 (d, J ) 1.2 Hz, 3 H), 1.69 (d, J ) 1.2 Hz,
3 H); 13C NMR δ 167.28, 138.99, 124.10, 55.61, 49.15, 47.69, 29.85,
28.83, 25.97, 22.83, 21.59, 19.78, 17.57; FAB MS m/z 207 [(M -
BF4)+]; HRFAB MS Calcd for C13H23N2 [(M - BF4)+] 207.1861. Found
207.1871.
H), 3.39-3.11 (m, 8 H), 2.01 (t, J ) 7.1 Hz, 2 H), 1.85 (t, J ) 7.4 Hz,
2 H), 1.79-1.48 (m, 4 H), 1.43-1.15 (m, 17 H), 0.89 (t, J ) 6.6 Hz,
6 H); FAB MS m/z 312 [(M - BF4)+]; HRFAB MS Calcd for C20H42-
NO [(M - BF4)+] 312.3266. Found 312.3289.
To obtain a pure sample of the vinylammonium salt 10, reaction of
the iodonium salt 7 (215 mg, 0.50 mmol) with triethylamine (1.0 g, 10
mmol) was carried out in dichloromethane (5 mL) at 25 °C for 9 h
under nitrogen. Preparative TLC (dichloromethane-methanol 9:1) gave
10 (3 mg, 2%) as a pale yellow oil: IR (film) 2925, 1450, 1380, 1060
14d: orange oil; IR (film) 2970, 2900, 1700, 1623, 1540, 1450, 1340,
1
1170, 1060 (br) cm-1; H NMR δ 5.78 (m, 1 H), 3.80-3.46 (m, 10
1
(br) cm-1; H NMR δ 5.39 (br s, 1 H), 3.59 (q, J ) 7.2 Hz, 6 H),
H), 2.91-2.75 (m, 2 H), 2.22-2.04 (m, 2 H), 1.90-1.40 (m, 10 H),
1.58 (br s, 3 H), 1.53 (br s, 3 H); 13C NMR δ 166.62, 139.87, 110.72,
70.71, 55.26, 53.78, 49.12, 47.02, 28.65, 28.30, 26.44, 25.99, 25.41,
22.99, 20.03, 19.47, 15.05; FAB MS m/z 279 [(M - BF4)+]; HRMS
Calcd for C17H31N2O [(M - BF4)+] 279.2436. Found 279.2407.
2.39-2.11 (m, 4 H), 1.80-1.18 (m, 17 H), 1.06-0.79 (m, 6 H); FAB
MS m/z 240 [(M - BF4)+]; HRFAB MS Calcd for C16H34N [(M -
BF4)+] 240.2691. Found 240.2706.
General Procedure for the Reaction of Phenyl(2-methyl-1-
propenyl)iodonium Tetrafluoroborate (12) with Base in THF. To
a solution of the iodonium salt 12 (35 mg, 0.10 mmol) in 1 mL of
THF was added a base (0.12 or 2.0 mmol) under nitrogen. The mixture
was stirred under the conditions described in Table 2. The organic
solvent was evaporated off in vacuo to give an oil, which was washed
several times with hexane by decantation to give a mixture of the
vinylammonium salt 13 and the vinyl ether 14. The yields of 13 and
14 were determined by 1H NMR analyses of the mixture with
dichloromethane as an internal standard and are reported in Table 2.
Pure samples of the vinylammonium salt 13 and the vinyl ether 14
were obtained by preparative TLC (dichloromethane-methanol 8:2).
Reaction of (2-Methyl-1-propenyl)triethylammonium Tetrafluo-
roborate (13a) with DBN in THF. Vinylammonium salt 13a (19 mg,
0.08 mmol) was treated with DBN (10 mg, 0.08 mmol) and iodobenzene
(16 mg, 0.08 mmol) in 1.5 mL of THF at room temperature for 2 days
under nitrogen. Extraction with dichloromethane and decantation with
hexane recovered the vinylammonium salt 13a (16 mg, 87%). Forma-
tion of the vinylammonium salt 13c and the vinyl ether 14c was not
1
detected by H NMR analyses of the crude reaction mixture.
Reaction of (2-Methyl-1-propenyl)triethylammonium Tetrafluo-
roborate (13a) with Triethylamine in THF. Vinylammonium salt
13a (38 mg, 0.16 mmol) was treated with triethylamine (16 mg, 0.16
mmol) and iodobenzene (33 mg, 0.16 mmol) in 3 mL of THF at room
temperature for 2 days under nitrogen. Extraction with dichloromethane
and decantation with hexane recovered the vinylammonium salt 13a
13a: colorless plates: mp 250-254 °C (recrystallized from dichlo-
romethane-diethyl ether-hexane); IR (KBr) 2990, 1461, 1403, 1070
(br), 792, 522 cm-1; 1H NMR δ 5.54 (br s, 1 H), 3.56 (q, J ) 7.6 Hz,

10148 J. Am. Chem. Soc., Vol. 118, No. 42, 1996
Sueda et al.
(35 mg, 93%). Formation of the vinyl ether 14a was not detected by
1H NMR analyses of the crude reaction mixture.
H), 3.44-3.23 (m, 2 H), 2.61-2.23 (m, 4 H), 2.14 (s, 3 H), 2.05 (s, 3
H); 13C NMR δ 160.83, 109.53, 47.38, 28.72, 25.95, 21.21; FAB MS
m/z 143 [(M - BF4)+]; HRFAB MS Calcd for C8H15S [(M - BF4)+]
143.0894. Found 143.0896. Anal. Calcd for C8H15BF4S: C, 41.76;
H, 6.57. Found: C, 41.93; H, 6.45.
19a (R ) Me): pale yellow oil; IR: (film) 2900, 1625, 1480, 1450,
1395, 1370, 1280, 1170, 1060 (br), 855, 800 cm-1; 1H NMR δ 5.57 (s,
1 H), 3.31 (q, J ) 7.3 Hz, 6 H), 3.24-3.11 (m, 2 H), 2.69 (t, J ) 6.4
Hz, 2 H), 1.90-1.56 (m, 4 H), 1.78 (s, 3 H), 1.73 (s, 3 H), 1.34 (t, J
) 7.0 Hz, 9 H); FAB MS m/z 244 [(M - BF4)+]; HRFAB MS Calcd
for C14H30NS [(M - BF4)+] 244.2099. Found 244.2101.
Reaction of 2-Methyl-1-propenyl Ether 14a with DBN in THF.
Vinyl ether 14a (8.1 mg, 0.026 mmol) was treated with DBN (3.7 mg,
0.03 mmol) in 0.3 mL of THF at room temperature for 2 days under
nitrogen. Extraction with dichloromethane and decantation with hexane
recovered the vinyl ether 14a (6.5 mg, 80%). Formation of the
vinylammonium salt 13c and the vinyl ether 14c was not detected by
1H NMR analyses of the crude reaction mixture.
Reaction of 2-Methyl-1-propenyl Ether 14a with Triethylamine
in THF. Vinyl ether 14a (10 mg, 0.03 mmol) was treated with
triethylamine (3 mg, 0.03 mmol) and iodobenzene (6 mg, 0.03 mmol)
in 0.3 mL of THF at room temperature for 2 days under nitrogen.
Extraction with dichloromethane and decantation with hexane recovered
the vinyl ether 14a (9.7 mg, 95%). Formation of the vinylammonium
19b (R ) Me): pale yellow oil; IR (film) 2910, 1715, 1650, 1570,
1
1440, 1405, 1280, 1235, 1180, 1060 (br), 830 cm-1; H NMR δ 8.06
(d, J ) 7.7 Hz, 2 H), 6.89 (d, J ) 7.7 Hz, 2 H), 5.56 (s, 1 H), 4.18 (t,
J ) 7.3 Hz, 2 H), 3.24 (s, 6 H), 2.63 (t, J ) 6.8 Hz, 2 H), 2.11-1.49
(m, 4 H), 1.76 (s, 3 H), 1.71 (s, 3 H); 13C NMR δ 156.38, 141.89,
135.11, 117.33, 108.20, 57.86, 40.22, 33.10, 29.67, 26.37, 25.22, 19.66;
FAB MS m/z 265 [(M - BF4)+]; HRFAB MS Calcd for C15H25N2S
[(M - BF4)+] 265.1738. Found 265.1767.
1
salt 13a was not detected by H NMR analyses of the crude reaction
mixture.
Reaction of Phenyl(2-butyl-1-hexenyl)iodonium Tetrafluorobo-
rate (7) with Triethylamine in THT. To a solution of the iodonium
salt 7 (43 mg, 0.10 mmol) in 1 mL of THT was added triethylamine
(12 mg, 0.12 mmol) at room temperature under nitrogen and the mixture
was stirred for 10 h. After addition of 1 mL of water and 2 mL of
dichloromethane, the reaction mixture was analyzed by GC using a
column of 15% FFAP (3 m, 70 °C). Formation of 1-butyl-3-
methylcyclopentene (8) and 5-decyne (9) was not detected. Extraction
with dichloromethane, filtration, and then concentration under aspirator
vacuum gave an oil, which was washed several times with hexane by
decantation to give a mixture of the vinylsulfonium salt 18 (R ) Bu;
87%) and the triethylammonium vinyl sulfide 19a (R ) Bu; 7%). The
Reaction of (E)-Phenyl(2-methyl-3-phenyl-1-propenyl)iodonium
Tetrafluoroborate (E)-(20) with Diisopropylethylamine in THT. To
a solution of the iodonium salt (E)-20 (42 mg, 0.10 mmol) in 3 mL of
THT was added diisopropylethylamine (16 mg, 0.12 mmol) at room
temperature under nitrogen, and the mixture was stirred for 3 h. After
addition of 1 mL of water, extraction with dichloromethane, filtration,
and then concentration under aspirator vacuum gave an oil, which was
washed several times with hexane by decantation to give a mixture of
stereoisomers of the vinylsulfonium salt 21 (19.3 mg, 63%). The E:Z
1
ratio of 21 was determined to be 62:38 by H NMR analyses. (E)-
1
yields of 18 and 19a were determined by H NMR analyses. A pure
and (Z)-21: IR (film) 3043, 3013, 2943, 2913, 1623, 1594, 1493, 1448,
1
1428, 1378, 1288, 1060 (br), 743, 703 cm-1; H NMR δ 7.43-7.05
sample of 18 was obtained by preparative TLC (dichloromethane-
methanol 9:1). 18 (R ) Bu): pale yellow oil; IR (film) 2950, 2860,
(m, 5 H), 6.18 (s, CdCH, Z-isomer), 6.09 (s, CdCH, E-isomer), 3.95-
3.68 (m, 2 H), 3.79 (s, PhCH2, Z-isomer), 3.57 (s, PhCH2, E-isomer),
3.45-3.22 (m, 2 H), 2.63-2.21 (m, 4 H), 2.04 (s, Me, E-isomer), 1.99
(s, Me, Z-isomer); FAB MS m/z 219 [(M - BF4)+]; HRFAB MS Calcd
for C14H19S [(M - BF4)+] 219.1207. Found 219.1214. The stereo-
chemistry of (E)-21 was established by the observation of an NOE
enhancement between the vinylic and benzylic protons. The stereo-
chemistry of (Z)-21 was established by the observation of an NOE
enhancement between the vinylic proton and the methyl group.
Reaction of (Z)-Phenyl(2-methyl-3-phenyl-1-propenyl)iodonium
Tetrafluoroborate (Z)-(20) with Diisopropylethylamine in THT.
Iodonium salt (Z)-20 (42 mg, 0.10 mmol) was treated with diisopro-
pylethylamine (16 mg, 0.12 mmol) in 3 mL of THT at room temperature
for 3 h under nitrogen. Extraction with dichloromethane and decanta-
tion with hexane gave a mixture of (E)- and (Z)-21 (17.3 mg, 56%) in
a ratio 64:36.
1
1610, 1460, 1380, 1280, 1060 (br), 880 cm-1; H NMR δ 6.00 (s, 1
H), 3.93-3.70 (m, 2 H), 3.41-3.20 (m, 2 H), 2.66-2.20 (m, 8 H),
1.57-1.20 (m, 8 H), 0.95 (t, J ) 6.7 Hz, 3 H), 0.92 (t, J ) 6.7 Hz, 3
H); FAB MS m/z 227 [(M - BF4)+]; HRFAB MS Calcd for C14H27S
[(M - BF4)+] 227.1833. Found 227.1844. A pure sample of 19a (R
) Bu) was obtained by the reaction of 18 (R ) Bu) with triethylamine
in dichloromethane at 50 °C for 3 days. 19a (R ) Bu): colorless
crystals; mp 99-101 °C (recrystallized from dichloromethane-
hexane); IR (KBr) 2965, 2928, 2872, 2851, 1621, 1461, 1397, 1301,
1060 (br), 803, 535, 523 cm-1; 1H NMR δ 5.55 (s, 1 H), 3.30 (q, J )
7.2 Hz, 6 H), 3.23-3.10 (m, 2 H), 2.70 (t, J ) 6.1 Hz, 2 H), 2.18-
1.96 (m, 4 H), 1.85-1.63 (m, 4 H), 1.45-1.20 (m, 17 H), 0.97-0.84
(m, 6 H); FAB MS m/z 328 [(M - BF4)+]; HRFAB MS Calcd for
C20H42NS [(M - BF4)+] 328.3038. Found 328.3054. Anal. Calcd
for C20H42BF4NS‚1/2 H2O: C, 56.59; H, 10.21; N, 3.30. Found: C,
56.96; H, 10.15; N, 3.20.
Reaction of (E)-Phenyl(2-methyl-3-phenyl-1-propenyl)iodonium
Tetrafluoroborate (E)-(20) with Diisopropylethylamine and Diphe-
nyl Sulfide. To a solution of the iodonium salt (E)-20 (42 mg, 0.10
mmol) and diphenyl sulfide (22 mg, 0.12 mmol) in 1 mL of
dichloromethane was added diisopropylethylamine (16 mg, 0.12 mmol)
at room temperature under nitrogen, and the mixture was stirred for
0.5 h. After addition of 1 mL of water, extraction with dichlo-
romethane, filtration, and then concentration under aspirator vacuum
gave an oil, which was washed several times with hexane by decantation
to give a mixture of stereoisomers of the vinylsulfonium salt 22 (20.3
Reaction of Phenyl(2-butyl-1-hexenyl)iodonium Tetrafluorobo-
rate (7) with Diisopropylethylamine in THT and Methanol-d4.
Iodonium salt 7 (43 mg, 0.10 mmol) was treated with diisopropylethy-
lamine (16 mg, 0.12 mmol) in 0.5 mL of THT and 1.5 mL of methanol-
d4 at room temperature for 0.5 h under nitrogen. The reaction gave
the vinylsulfonium salt 18 (R ) Bu; 88%), in which 61% deuterium
1
incorporation at the R-vinylic position was observed by H NMR.
No deuterium incorporation at the R-vinylic position of 18 (R )
Bu) was observed in the reaction with diisopropylethylamine in THT
and methanol-d4 (1:3) at room temperature for 0.5 h under nitrogen.
General Procedure for the Reaction of Phenyl(2-methyl-1-
propenyl)iodonium Tetrafluoroborate (12) with Base in THT. To
a solution of the iodonium salt 12 (35 mg, 0.10 mmol) in 1 mL of
THT was added a base (0.12 mmol) under nitrogen. The mixture was
stirred under the conditions described in Table 3. The organic solvent
was evaporated off in vacuo to give an oil, which was washed several
times with hexane by decantation to give the vinylsulfonium salt 18
(R ) Me) and the vinyl sulfide 19 (R ) Me). The yields of 18 and 19
1
mg, 50%). The E:Z ratio of 22 was determined to be 69:31 by H
NMR analyses. A pure sample of (E)- and (Z)-22 was obtained by
fractional recrystallization from dichloromethane-diethyl ether-hex-
ane. (E)-22: colorless plates; mp 195-197 °C; IR (KBr) 3060, 1620,
1479, 1445, 1387, 1055 (br), 757, 742, 710, 681, 469 cm-1; 1H NMR
(CD3CN) δ 7.85-7.63 (m, 10 H), 7.45-7.20 (m, 5 H), 6.62 (s, 1 H),
3.76 (s, 2 H), 2.16 (s, 3 H); FAB MS m/z 317 [(M - BF4)+]; HRFAB
MS Calcd for C22H21S [(M - BF4)+] 317.1364. Found 317.1357. Anal.
Calcd for C22H21BF4S: C, 65.36; H, 5.24. Found: C, 64.94; H, 5.22.
The stereochemistry of (E)-22 was established by the observation of
an NOE enhancement between the vinylic and benzylic protons. (Z)-
22: colorless solid; IR (KBr) 3060, 1620, 1600, 1580, 1479, 1445,
1
(R ) Me) were determined by H NMR analyses of the mixture with
dichloromethane as an internal standard and are reported in Table 3.
Pure samples of 18 and 19 (R ) Me) were obtained by preparative
TLC (dichloromethane-methanol 85:15).
1
1387, 1289, 1073, 757, 742, 710, 681 cm-1; H NMR δ 7.94-7.78
18 (R ) Me): colorless crystal; mp 59-62 °C (recrystallized from
dichloromethane-ethyl acetate); IR (KBr) 2944, 1630, 1441, 1306,
(m, 4 H), 7.77-7.58 (m, 6 H), 7.36-7.18 (m, 3 H), 7.13-7.00 (m, 3
H), 3.95 (s, 2 H), 2.25 (d, J ) 0.9 Hz, 3 H); FAB MS m/z 317 [(M -
BF4)+]; HRFAB MS Calcd for C22H21S [(M - BF4)+] 317.1364. Found
1
1060 (br), 540, 523 cm-1; H NMR δ 6.03 (s, 1 H), 3.88-3.67 (m, 2

EVidence for ReVersible Ylide Formation
J. Am. Chem. Soc., Vol. 118, No. 42, 1996 10149
317.1350. The stereochemistry of (Z)-22 was established by the
observation of an NOE enhancement between the vinylic proton and
the methyl group.
MNDO Calculations. MNDO calculations were carried out using
the MOPAC 93 program.34 THF, trimethylamine, and THT were used
as model nucleophiles for the calculations. Energy calculations were
performed with the PM3 Hamiltonian, and the stable and transient
structures were initially built with the general parameters of bond
lengths, bond angles, and dihedral angles, and refined with the
eigenvector following (EF) optimization method. Their reaction
coordinates were defined by the distance (Å) of the atomic centers
between carbene and the heteroatoms of THF, trimethylamine, and
THT.
Reaction of (Z)-Phenyl(2-methyl-3-phenyl-1-propenyl)iodonium
Tetrafluoroborate (Z)-(20) with Diisopropylethylamine and Diphe-
nyl Sulfide. Iodonium salt (Z)-20 (42 mg, 0.10 mmol) was treated
with diisopropylethylamine (16 mg, 0.12 mmol) and diphenyl sulfide
(22 mg, 0.12 mmol) in 1 mL of dichloromethane at room temperature
for 0.5 h under nitrogen. Extraction with dichloromethane and
decantation with hexane gave a mixture of (E)- and (Z)-22 (22.1 mg,
55%) in a ratio 71:29.
JA961797Q