7908 J. Am. Chem. Soc., Vol. 121, No. 34, 1999
Shaler et al.
â-fluoroisopropyl cation), corresponds to a classical secondary
cation, but the â-fluoro substituent destabilizes its positive
charge. The ion to the right, 1-fluoro-1-propyl cation (or
R-fluoro-n-propyl cation), is isoelectronic with propionaldehyde
by virtue of the resonance structure CH3CH2CHdF+. Here the
R-fluoro substituent contributes a net stabilization of the positive
charge. Consequently, the R-fluoro-n-propyl cation should be
somewhat more stable than the â-fluoroisopropyl cation.11 The
R-fluoro-n-propyl cation can form from the 3-fluoro-1-propyl
cation either by a 1,3-hydrogen shift, as depicted, or via
successive 1,2-shifts. We describe experiments that reveal the
relative rates of these rearrangements.
Stereochemical studies of diazonium ions in solution confirm6
that the kinetics of 1,2- and 1,3-shifts within cyclic systems do
not represent an unbiased measure of the competition between
these pathways. By contrast, flexible acyclic precursors rapidly
equilibrate among a range of conformations and provide a fairer
test. In the gas phase fluoropropyl cations exhibit the following
characteristics: (1) as mentioned above, the 1,3-shift product
(R-fluoro-n-propyl cation) is calculated to be more stable than
the 1,2-shift product (â-fluoroisopropyl cation),10,11 (2) the two
isomeric 1-fluoropropyl cations interconvert within ion-neutral
complexes, transposing (but not randomizing) hydrogens be-
tween the central carbon and the one attached to fluorine,12 and
(3) free 1-fluoropropyl cations rearrange on the millisecond time
scale to 2-fluoropropyl cations, (CH3)2CF+, the global minimum
on the C3H6F+ surface. This last result has been ascertained by
preparing 1-fluoropropyl ions from propionaldehyde via fluorine-
oxygen metathesis12 (the ion-molecule reaction depicted as the
first step of eq 3) followed by deprotonation in a subsequent
Over the past 30 years a number of research groups have
devoted considerable effort to ascertaining which C6H6O•+
tautomer forms when an ionized phenyl ether expels alkene-
phenol radical cation versus an ionized cyclohexadienone.14
While some conflicting data remain to be resolved, the
preponderance of evidence supports the former alternative. In
other words, the proton transfers to the oxygen of the phenoxy
radical, as depicted in the second step of eq 1.
The identity of the base affects the distribution of alkenes
from gas-phase Brønsted acid-base reactions. In bimolecular
proton transfers, for instance, the 2-methylbutene isomer ratio
from deprotonation of CH3CH2C(CH3)2+ depends on the proton
affinity of the base.15 That ratio turns out to be the same (within
experimental uncertainty) regardless of whether the deprotona-
+
tion takes place in an ion-neutral complex [CH3CH2C(CH3)2
PhO•] or in a bimolecular reaction with a neutral ether that has
roughly the same basicity as phenoxy radical.16 The experi-
mental proton affinity of the phenoxy radical lies in the range
845-860 kJ mol-1 (owing to the uncertainty in the heat of
formation of the neutral radical-∆fH°298 ) 38-55 kJ mol-1, a
range of values that ab initio calculations have not yet served
to narrow),17 comparable to the proton affinity of di-n-propyl
ether, 846 kJ mol-1 18
The ion-neutral complex gives a
.
2-methyl-1-butene:2-methyl-2-butene isomer ratio of 1.3 (
0.15,16 while the bimolecular reaction with di-n-propyl ether
gives a ratio of 1.2.15 These ratios contrast to the nearly statistical
ratio of 2.5 ( 0.5 that results from a bimolecular reaction in
which a tertiary amine acts as base.15
Experimental Section
3-Fluoro-1-phenoxypropane (1) was prepared by reduction of
3-phenoxypropionic acid (Aldrich) with lithium aluminum hydride,
followed by conversion of the resulting alcohol to the fluoride by means
of diethylaminosulfur trifluoride (DAST): 19F NMR (280 MHz, CDCl3)
-222.3 ppm (tt, J ) 47.0, 25.2 Hz). The 3,3-dideuterated analogue
(1-3,3-d2) was synthesized in a similar fashion using LiAlD4 to prepare
the dideuterated alcohol, C6H6OCH2CH2CD2OH: 1H NMR (300 MHz,
CDCl3) δ 1.86 (br s, 1H), δ 2.04 (t, 5.9 Hz, 2H), δ 4.12 (t, 5.9 Hz,
2H), δ 6.85-7.0 (m, 3H), δ 7.2-7.4 (2H); GC/MS (70 eV) m/z (rel
intensity, not corrected for 13C natural abundance): 154 (M•+, 14), 95
(15), 94 (100), 77 (10), 66 (10), 65 (10), 51 (10), 43 (6), 40 (7). The
dideuterated alcohol was converted to 1-3,3-d2 by dropwise addition
of 0.81 g (5 mmol) of DAST to a -78 °C solution of 0.73 g (4.7 mmol)
C6H6OCH2CH2CD2OH in CH2Cl2 under nitrogen, slow warming to
room temperature, recooling to -78 °C, quenching by slow addition
of saturated aqueous NaHCO3 at that temperature, and separation and
distillation of the organic layer to afford 0.71 g of 1-3,3-d2 (bp 45-50
°C, 0.2 Torr; 96% yield): 19F NMR (280 MHz, CDCl3) -222.6 ppm
(t of I ) 1 quintets, J ) 25.2, 7.2 Hz); 1H NMR (300 MHz, CDCl3) δ
2.15 (dt, J ) 25.2, 5.9 Hz, 2H); δ 4.10 (t, 5.9 Hz, 2H), δ6.8-7.0 (m,
3H), δ 7.2-7.3 (2H); 2H NMR (48 MHz, CDCl3) δ 4.60 (d, JDF ) 7.2
Hz); GC/MS (70 eV) m/z (rel intensity, not corrected for 13C natural
abundance) 156 (M•+, 24), 96 (4), 95 (54), 94 (100), 77 (21), 66 (18),
65 (21), 63 (10), 51 (21), 43 (12), 41 (14), 40 (10). Within our limits
of detection the labeled compounds were completely deuterated at the
3-position, i.e., >99 atom %D at that carbon.
(3)
collision with propionaldehyde.13 Since a free C3H6F+ ion
formed as a fragment of 70 eV electron ionization of PhOCH2-
CH2CH2F (1) would be expected to have an internal energy at
least as great as that of CH3CH2CHF+ from the first step of eq
3, our previously published experimental data provide an
unambiguous way to differentiate neutral fluoropropenes pro-
duced by free fluoropropyl ions from those formed via the
intermediacy of ion-neutral complexes. Free fluoropropyl ions
yield 2-fluoropropene as the predominant neutral product, while
(as will be presented below) 1-fluoropropyl ions in ion-neutral
complexes do not undergo skeletal rearrangement.
The second step of eq 1 affords the products observed from
ion-neutral complexes: C6H6O•+ (seen in the mass spectrom-
eter) and neutral alkenes, which are recovered in a specially
constructed Electron Bombardment Flow (EBFlow) reactor.
EBFlow experiments19 and NMR analyses were performed as
previously described, with 19F NMR integrations corrected for pulse
(9) (a) McAdoo, D. J.; Morton, T. H. Acc. Chem. Res. 1993, 26, 295-
302 and references therein. (b) Shaler, T. A.; Morton, T. H. J. Am. Chem.
Soc. 1994, 116, 9222-9226.
(10) Stams, D. A.; Thomas, T. D.; MacLaren, D. C.; Ji, D.; Morton, T.
H. J. Am. Chem. Soc. 1990, 112, 1427-1434.
(11) Our most recent DFT calculations (B3LYP/6-311G**, including
zero-point correction) place the 0 K heat of formation of 1-fluoro-1-propyl
cation 15 kJ mol-1 below that of 1-fluoro-2-propyl cation.
(12) Shaler, T. A.; Morton, T. H. J. Am. Chem. Soc. 1989, 111, 6868-
6870; 1990, 112, 4090.
(13) Eyler, J. R.; Ausloos, P.; Lias, S. G. J. Am. Chem. Soc. 1974, 96,
3673-3675.
(14) (a) Morton, T. H. Tetrahedron 1982, 38, 3195-3243 and references
therein. (b) Weddle, G. H.; Dunbar, R. C.; Song, K.; Morton, T. H. J. Am.
Chem. Soc. 1995, 117, 2573-2580. (c) Taphanel, M. H.; Morizur, J. P.;
Leblanc, D.; Borchardt, D.; Morton, T. H. Anal. Chem. 1997, 69, 4191-
4196.
(15) Marinelli, W. J.; Morton, T. H. J. Am. Chem. Soc. 1978, 100, 3536-
3539; 1979, 101, 1908.
(16) Morton, T. H. J. Am. Chem. Soc. 1980, 102, 1596-1602.
(17) Wu, Y.-D.; Lai, D. K. W. J. Org. Chem. 1996, 61, 7904-7910.
(18) Lias, S. G.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard,
W. G. J. Phys. Chem. Ref. Data 1988, 17, suppl. 1.