Vinyl Carbocations
J . Org. Chem., Vol. 66, No. 16, 2001 5565
mmol), in 49% yield: mp 67-69 °C; IR (film, NaCl) 3067 s,
2978 s, 1622 m, 1461 m, 1378 m, 1344 s, 1278 s, 1167 s, 1139
s, 1028 s, 889 s, 694 s, 633 s, cm-1; 1H NMR (CDCl3, 400 MHz)
δ 8.40 (s, 2H), 8.04 (s, H), 6.87 (s, 1H), 2.52 (q, J ) 7.3 Hz,
2H), 2.20 (s, 3H), 1.13 (t, J ) 7.3 Hz, 3H). Anal. Calcd for
stirring, and the pentane layer was examined by gas chroma-
tography and GC-MS and compared to authentic samples.
HCl (2.0 mL, 1.0 M) was then added to the two-layer system
and the system stirred vigorously for 30 min at room temper-
ature. The organic phase was separated and the aqueous phase
extracted with pentane (5 × 3.0 mL). The pentane was dried
(MgSO4) and concentrated under reduced pressure to ca. 3 mL.
The organic layer was again analyzed by GC-MS, concen-
trated further, and chromatographed on silica gel with hexanes
and 98/2 hexanes/EtOAc as eluants; iodobenzene (18, 0.067
g, 53%), 1-phenyl-1,2-butadiene (22, 0.020 g, 25%) and 1-phen-
yl-2-butanone (23, 0.032 g, 35%) were recovered. Neither (E)-
19a nor 1-phenyl-1-butyne (21) was recovered, but their
presence was verified by GC-MS analysis of authentic samples.
1-P h en yl-1,2-bu ta d ien e (22).29 This elimination product
was isolated as described above to provide 0.020 g (25%) of 24
as a light yellow liquid: IR (neat, NaCl) 3044, 3029, 2991,
2921, 1945 (allene), 1595, 1494, 1464, 1370, 1264, 1069, 873;
1H NMR (CDCl3, 300 MHz) δ 7.30-7.20 (m, 5H), 6.12-6.07
(m, 1H), 5.58-5.49 (m, 1H), 1.80-1.77 (dd, J ) 3.0, 7.0 Hz,
3H); 13C{1H} NMR (CDCl3, 125 MHz) δ 206.0 (central carbon),
135.1, 128.5, 128.1, 126.6, 94.0, 89.6, 14.1; GC-MS m/z for
C
14H12F9IO3S: C, 30.12; H, 2.17; S, 5.74. Found: C, 30.39; H,
2.29; S, 5.77.
P r ep a r a tion of (Z)-2-Eth yl-1-h exen yl(4-tr iflu or om eth -
ylp h en yl)iod on iu m Tr ifla te (16e). (Z)-2-Ethyl-1-hexenyl-
(4-trifluoromethylphenyl)iodonium triflate was prepared ac-
cording to the general procedure using cyano(4-trifluoro-
methylphenyl)iodonium triflate, 13 (0.12 g, 0.27 mmol), and
(Z)-2-ethyl-1-hexenyl tri-n-butylstannane, 11d (0.12 g, 0.30
mmol), to afford 16e as a white crystalline solid in 65% yield:
mp 38-40 °C; IR (KBr) 3087, 2963, 2872, 1596, 1466, 1400,
1325, 1277, 1249, 1169, 1128, 1067, 1029, 1000, 823, 771, 680,
1
662, 638 cm -1; H NMR (CDCl3) δ 8.07 (d, J ) 8.4 Hz, 2H),
7.71 (d, J ) 8.4 Hz, 2H), 6.71 (s, 1H), 2.43-2.53 (m, J ) 7.5
Hz, 4H), 1.60-1.67 (m, 2H), 1.26-1.33 (m, 2H), 1.14 (t, J )
7.5 Hz, 3H), 0.90 (t, J ) 7.5 Hz, 3H). Anal. Calcd for C15H19F6-
SO3I: C, 36.10; H, 3.60; S, 6.02. Found: C, 36.39; H, 3.48; S,
5.82.
P r ep a r a tion of (Z)-2-Eth yl-1-h exen yl(3,5-bis(tr iflu o-
r om eth yl)p h en yl)iod on iu m Tr ifla te (17e). (Z)-2-Ethyl-1-
hexenyl(3,5bis(trifluoromethyl)phenyl)iodonium triflate was
prepared according to the general procedure using cyano(3,5-
bis(trifluoromethyl)phenyl)iodonium triflate, 14 (0.120 g, 0.23
mmol), and (Z)-2-ethyl-1-hexenyl tri-n-butylstannane, 11e
(0.11 g, 0.27 mmol), to afford 0.10 g (72%) of 17e as a white
crystalline solid: mp 83-84 °C; IR (KBr) 3015, 2970, 2862,
C
10H10, 130.
Kin et ic P r oced u r es. The alkenyl(aryl)iodonium triflate
(ca. 15 mg) was weighed into an oven-dried NMR tube.
Chloroform-d was filtered through a short plug of basic
aluminum oxide to remove any acidic impurities and then
placed in a separate NMR tube. The variable temperature
probe on the 300 MHz NMR was set to the appropriate
temperature (between 20 and 35 °C) and the chloroform-d was
allowed to equilibrate to the probe temperature. Then the
chloroform-d (0.5 mL) was added to the sample tube via
syringe and the timing begun. Either glyme or 1,2-dibromo-
ethane (2 µL) was then added as an internal standard. The
cap was sealed with Parafilm and the sample was placed in
the NMR spectrometer. Spectra were obtained in timed
intervals through at least one half-life. Integration was used
to determine the concentrations of each species at time t.
The rate constants (k) were determined from the slope of
the first-order rate law plot: ln [A]t/[A]o vs time. The initial
concentration ([A]o) was determined by averaging the total
concentration calculated via integration (i.e., [molstarting material
+ moldecomposed product]/0.5 mL solvent). The concentration at time
t ([A]t) was determined by the relative integration of the
fragemented product to that of the internal standard. The time
was considered to be halfway through the pulse sequence for
each spectrum (e.g., for 256 pulses (13 min 58 s), halfway was
considered to be 128 pulses (6 min 59 s)). Any rate constants
taken from plots with an R2 value exceeding 0.9500 and a rate
constant within 10% of the other values were used in deter-
mining the average rate constant at each temperature.
2369, 1739, 1718, 1436, 1364, 1343, 1277, 1226, 1144, 1082,
-1
1034, 892, 739, 697, 636 cm
;
1H NMR (CDCl3) δ 8.07 (s,
2H), 8.05 (s, 1H), 6.81 (s, 1H), 2.53 (q, J ) 7.3 Hz, 2H), 2.48 (t,
J ) 7.1 Hz, 2H), 1.29-1.38 (m, 4H), 1.14 (t, J ) 7.3 Hz, 3H),
0.90-0.95, (overlapping m, 2H), 0.88 (t, J ) 7.1 Hz, 3H). Anal.
Calcd for C17H18F9SO3I: C, 34.02; H, 3.02; S, 5.32. Found: C,
34.01; H, 3.06; S, 5.52.
P r ep a r a tion of (E)-2-Eth yl-1-h exen yl(4-tr iflu or om eth -
ylp h en yl)iod on iu m Tr ifla te (16f). (E)-2-Ethyl-1-hexenyl(4-
trifluoromethylphenyl)iodonium triflate was prepared via the
general procedure from cyano(4-trifluoromethylphenyl)iodo-
nium triflate, 13 (0.20 g, 0.45 mmol), and (E)-2-ethyl-1-hexenyl
tri-n-butylstannane, 11d (0.35 g, 0.87 mmol), in 63% yield: mp
54-56 °C; IR (KBr) 3076, 2964, 2871, 1594, 1466, 1400, 1323,
1251, 1179, 1128, 1066, 1046, 1025, 1000, 830, 769, 661, 635
cm-1; 1H NMR (CDCl3) δ 8.07 (d, J ) 8.4 Hz, 2H), 7.71 (d, J )
8.4 Hz, 2H), 6.68 (s, 1H), 2.45-2.52 (m, 4H), 1.40-1.55 (m,
2H), 1.25-1.37 (m, 2H), 1.06 (t, J ) 7.5 Hz, 3H), 0.91 (t, J )
7.5 Hz, 3H).
Isola tion of (E)-19a F r a gm en ta tion of (E)-1-P h en yl-1-
bu ten -1-yl Tr iflu or om eth a n esu lfon a te ((E)-15a ) in Ch lo-
r ofor m . Iodonium salt 15a (0.200 g, 0.413 mmol) was weighed
into an oven-dried 25 mL round-bottom flask equipped with a
magnetic stir bar and 2.0 mL of chloroform added. The
homogeneous solution was stirred at room temperature for 12
h, during which time the solution turned dark brown. Silica
gel (ca. 1.5 g) was added and the solvent removed in vacuo on
Ack n ow led gm en t. We are grateful to the Research
Corp. (Grant CC4659), the NSF (CAREER Award:
CHE-9983863), Dow Corp. (scholarship for E.A.R.), and
the Arnold and Mable Beckman Foundation (scholar-
ship for A.J .M.) for support of this work. We also thank
Dr. J . C. Poutsma for G2 calculations of cations 3-10.
a
rotary evaporator. The adsorbed product mixture was
chromatographed on 10 g of SiO2 using hexanes followed by
98/2 hexanes/EtOAc as eluant to provide 0.059 g (51%) of (E)-
19a as a colorless liquid: Rf (98/2 hexanes/EtOAc) 0.29; IR
(neat, NaCl) 3074, 3033, 2981, 2941, 1416, 1242, 1211, 1140,
Su p p or tin g In for m a tion Ava ila ble: Isodesmic reaction
equations, plot of 17e at 20-35 °C; table of G2 data; GC trace
of product mixture from 15a ; 1H and 13C NMR spectra of allene
22 and enol triflate 19a ; a copy of the Hammet plot (15e, 16e,
17e) and representative kinetic plots of disappearance of
iodonium salts 15e, 16e, 17c, and 17e; copies of representative
GC MS traces (15e, 15f, and 16c) of fragmentation. This
material is available free of charge via the Internet at
http://pubs.acs.org.
1
1110, 1074, 1036, 954, 921, 899, 823, 754, 726, 697 cm-1; H
NMR (CDCl3, 300 MHz) δ 7.20-7.50 (m, 5H Ar), 6.60 (s, 1H,
vinyl), 2.61 (q, J ) 7.5 Hz, 2H), 1.21 (t, J ) 7.5 Hz, 3H); 13C-
{1H} (CDCl3, 125 MHz) δ 152.3, 131.6, 127.7, 127.6, 127.1,
120.8, 117.6 (q, J ) 320 Hz, CF3), 23.2, 10.1.
F r a gm en t a t ion of 15a in MeOH. Iodonium salt 15a
(0.300 g, 0.619 mmol) was added to a 25 mL round-bottom
flask equipped with a magnetic stir bar. MeOH (3.0 mL) was
added and the solution stirred for 16 h at room temperature.
Water (2 mL) and pentane (5 mL) were added with rapid
J O015746+