Competing 1,3- and 1,2-hydrogen shifts in gaseous fluoropropyl cations

Article abstract of DOI:10.1021/ja990567j

An initial 1,2-hydrogen shift versus a 1,3-deuterium shift in emerging FCD₂CH₂CH₂⁺ ions lead to differing patterns of deuteration in the neutral products that are recovered following reaction with a Bronsted base. Transient dideuterated fluoropropyl ions have been produced in [C₃H₄D₂F⁺ PhO·] ion-neutral complexes by 70 eV electron impact on FCD₂CH₂CH₂OPh and the resulting fluoropropenes collected in a specially designed electron bombardment flow (EBFlow) reactor. Under these conditions ion-neutral complexes greatly predominate over free fluoropropyl cations. Free ions afford products that are easily distinguished from those that result from ion-neutral complexes, in which the phenoxy radical serves as the Bronsted base. 1,2-Hydrogen shift yields ions containing CH₃-groups, while the ions from 1,3-shift have CDH₂-groups. Allyl fluoride and 1-fluoropropene arise from ion-neutral complexes, and the extent and positions of deuteration have been determined by ¹⁹F NMR. Six deuterated variants of trans-1-fluoropropene can be resolved in the NMR spectrum of the neutral products collected from the EBFlow. The proportions of CH₃ and CDH₂ measured by integration imply a branching ratio of 1,2-hydrogen versus 1,3-deuterium shift of approximately 94:6.

Full text of DOI:10.1021/ja990567j

J. Am. Chem. Soc. 1999, 121, 7907-7913
7907
Competing 1,3- and 1,2-Hydrogen Shifts in Gaseous Fluoropropyl
Cations
Thomas A. Shaler, Dan Borchardt, and Thomas Hellman Morton*
Contribution from the Department of Chemistry, UniVersity of California,
RiVerside, California 92521-0403
ReceiVed February 23, 1999
Abstract: An initial 1,2-hydrogen shift versus a 1,3-deuterium shift in emerging FCD2CH2CH2+ ions lead to
differing patterns of deuteration in the neutral products that are recovered following reaction with a Brønsted
base. Transient dideuterated fluoropropyl ions have been produced in [C3H4D2F+ PhO•] ion-neutral complexes
by 70 eV electron impact on FCD2CH2CH2OPh and the resulting fluoropropenes collected in a specially designed
electron bombardment flow (EBFlow) reactor. Under these conditions ion-neutral complexes greatly
predominate over free fluoropropyl cations. Free ions afford products that are easily distinguished from those
that result from ion-neutral complexes, in which the phenoxy radical serves as the Brønsted base. 1,2-Hydrogen
shift yields ions containing CH3-groups, while the ions from 1,3-shift have CDH2-groups. Allyl fluoride and
1-fluoropropene arise from ion-neutral complexes, and the extent and positions of deuteration have been
determined by 19F NMR. Six deuterated variants of trans-1-fluoropropene can be resolved in the NMR spectrum
of the neutral products collected from the EBFlow. The proportions of CH3 and CDH2 measured by integration
imply a branching ratio of 1,2-hydrogen versus 1,3-deuterium shift of approximately 94:6.
Unimolecular hydrogen migration from an sp3 center to a
nearby carbon takes place frequently in carbocations1 but only
rarely in free radicals2 or carbanions.3 In positive ions 1,2-shifts
correspond to an orbital symmetry-allowed thermal sigmatropic
rearrangement. 1,3-Hydrogen shifts to sp2 cationic centers occur
in terpene biosyntheses4 as well as in other reactions of natural
products5 where 1,2-shifts cannot happen. They also compete
with 1,2-shifts in sterically constrained cyclohexyl6 cations and
operate to the exclusion of sequential 1,2-shifts in appropriately
substituted cycloheptyl systems.7 A recent upsurge of interest
in 1,3-shifts within reactive intermediates2,3,5,7,8 prompts the
question as to how well they compete with 1,2-shifts in acyclic
cations. This paper reports a study of neutral products from such
transpositions in the gas phase, an unbiased comparison between
a 1,3-deuterium shift and a 1,2-hydrogen shift.
noxyfluoroalkanes (R′-OPh•+).9 This bond cleavage forms ion-
neutral complexes containing a fluoroalkyl cation (R+) plus a
phenoxy radical, as schematically depicted in eq 1, in preference
(1)
to free R+. The complex lives on the order of a nanosecond
before undergoing proton transfer from the charged to the
uncharged partner, yielding neutral alkene and ionized phenol
as the observed products. The C-O bond heterolyzes with
concomitant rearrangement of the alkyl group (R′ f R+). The
effects of fluorine (e.g., its capacity to stabilize positive charge
when directly attached to a primary, positively charged carbon)
have thus been revealed to an extent not hitherto observed in
condensed phases. In the present case the starting structure, R′
) 3-fluoro-1-propyl, would form a primary cation if it did not
rearrange.
Over the past decade we have been examining monofluori-
nated cations formed by heterolysis of ionized, gaseous 1-phe-
(1) Shubin, V. G. Top. Curr. Chem.1984, 116/117, 267-341.
(2) (a) For an upper bound to 1,2- and 1,3-hydrogen shifts in simple
alkyl radicals, see: Yamauchi, N.; Miyoshi, A.; Kosaka, K.; Koshi, M.;
Matsui, H. J. Phys. Chem. A 1999, 103, 2723-2733. (b) Plausible 1,2- and
1,3-hydrogen shifts in photoexcited, matrix-isolated hexyl radicals have
recently been reported by Takada, T.; Koizumi, H.; Ichikawa, T. Chem.
Phys. Lett. 1999, 300, 233-256.
(3) For one of the rare examples of a unimolecular 1,3-hydrogen shift
in gas-phase carbanion chemistry, see: Brauman, J. I.; Sannes, K. A. J.
Am. Chem. Soc. 1995, 117, 10088-10092.
Equation 2 illustrates the cations (R+) produced by competing
(2)
(4) Croteau, R. in Biosynthesis of Isoprenoid Compounds; Porter, J. W.,
Spurgeon, S. L., Eds.; Wiley: New York, 1981; Vol. 1 pp 225-282.
(5) For recent examples, see: (a) Alchanati, I.; Patel, J. A.; Liu, J.;
Benedict, C. R.; Stipanovic, R. D.; Bell, A. A.; Yunxing, C.; Magill, C. W.
Phytochemistry 1998, 47, 961-967. (b) Berrier, C.; Jacquesy, J.-C.;
Jourannetaud, M.-P.; Lafitte, C.; Vidal, Y.; Zunino, F.; Duflos, A.
Tetrahedron 1998, 54, 13761-13770.
hydrogen migrations starting from a nominal 3-fluoro-1-propyl
cation, the ion at the center of the equation. That structure
corresponds to an unstabilized primary alkyl cation and does
not represent a stable geometry. Hydrogen transpositions via
1,2-shifts or 1,3-shifts convert it to species that constitute local
minima on the C3H6F+ potential energy surface. Ab initio
calculations10 predict that the resulting cations have comparable
stabilities. The ion to the left, 1-fluoro-2-propyl cation (or
(6) Maskill, H.; Whiting, M. C. J. Chem. Soc., Perkin Trans. 2 1976,
1462-1470.
(7) (a) Angle, S. R.; Mattson-Arnaiz, H. L. J. Am. Chem. Soc. 1992,
114, 9782-9786. (b) Angle, S. R.; Hossain, M. A. Tetrahedron Lett. 1994,
26, 4519-4522.
(8) (a) Coates, R. M.; Ho, J. Z.; Klobus, M.; Zhu, L. J. Org. Chem.
1998, 63, 9166-9176. (b) Hudson, C. E.; McAdoo, D. J. J. Am. Soc. Mass
Spectrom. 1998, 9, 130-137.
10.1021/ja990567j CCC: $18.00 © 1999 American Chemical Society
Published on Web 08/12/1999

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.

Hydrogen Shifts in Fluoropropyl Cations
J. Am. Chem. Soc., Vol. 121, No. 34, 1999 7909
Table 1. Proportions of Rearranged Monofluoropropenes from 70
EV EBFlow Radiolysis of PhOCH2CH2CD2F, with Upfield
Chemical Shifts (∆δ>0) Relative to Their Undeuterated
Isotopomersa
and CH3CFdCH2 in proportions of 0.16, 0.13, and 0.009,
respectively. No fluorocyclopropane was detected. The yield
of CH3CFdCH2 is so low that the 1-fluoropropene yields do
not need to be corrected for the contribution from free
fluoropropyl cations.
In the mass spectrometer ionized phenol (as produced by eq
1) accounts for more than half of the total ion current from 70
eV electron ionization of 3-fluoropropyl phenyl ether; about
20% of the total ion current comes from further decomposi-
tions of ionized phenol; and the molecular ion 1•+ represents
7% of the total ion current. Free C3H6F+ constitutes only about
3% of the total ion current, a level consistent with the low
recovery of CH3CFdCH2 in the EBFlow experiment. The
intermediacy of [C3H6F+ PhO•] ion-neutral complexes accounts
for virtually all of the 1-fluoropropenes. While these experiments
were performed at just one ionizing voltage, EBFlow studies
of n-butyl phenyl ether have shown that isomer ratios do not
vary as electron energy is decreased to 20 eV (although this
does diminish the net yield).19 Since CH3CFdCH2 is re-
covered only to the extent that one would expect on the basis
of the proportion of free fluoropropyl fragment ions, it is clear
that the ion-neutral complexes do not live long enough for
skeletal rearrangement to take place, a result consistent with
the absence of linear f branched rearrangement that we have
found in EBFlow studies of n-butyl phenyl ether at all ionizing
energies.
Assessing the relative contributions of 1,2- and 1,3-shifts
requires the deuterium labeling experiment summarized in eq
4. Isotopic label that ends up on the methyl undergoes no further
(4)
a Calculated values reflect relative fractions 0.06 1,3-shift and 0.94
1,2-shift using Scheme 1 with γ ) 0.25 and η ) 0.5.
transposition. Hydrons on the other carbons can move back and
forth. The neutral product distribution was assayed using 19F
NMR. We find that EBFlow radiolysis of 1-3,3-d2 gave a
mixture of C3H4DF and C3H3D2F, 49( 2% of which was CH2d
CHCD2F (whose 19F chemical shift is 1.24 ppm upfield from
that of CH2dCHCH2F). That particular labeled allyl fluoride
can come from vicinal elimination, some of which might have
occurred from the neutral precursor. Our discussion focuses on
the fluoropropenes that must have arisen via eq 4. Table 1
summarizes their relative proportions (and chemical shifts).
Figure 1 reproduces the 467 MHz spectra of the cis- and trans-
1-fluoropropenes. Isotopically induced chemical shift differences
permit the resolution and quantitation of six different trans
products. The position of the deuterium label was determined
on the basis of observed coupling constants compared with those
reported for the undeuterated analogue,20 as well as by the
splitting patterns observed under decoupling conditions. Some
of the deuterated cis products and allyl fluorides (3-fluoropro-
penes) could not be resolved from one another, and they are
grouped together in Table 1. The 1-fluoropropenes from the
droop and relaxation time effects.12 Total ionizing electron currents
were on the order of 0.1-0.25 mA for run durations of 45-90 min.
The total C3H5F recovery from EBFlow radiolysis of 1 was gauged by
adding a known amount of hexafluorobenzene as an NMR integration
standard to the reaction mixture following one EBFlow radiolysis, from
which the fluoroalkene yield was measured as 4 µmol/A-sec. Reported
relative yields are averages of duplicate runs.
Density functional (DFT) calculations were performed using GAUSS-
IAN94 on the Cray C90 mainframe at the San Diego Supercomputing
center. Normal modes calculations were performed on optimized
geometries, and zero-point energy differences are based on unscaled
harmonic vibrational frequencies.
Results
Bombardment of 3-fluoro-1-phenoxypropane (FCH2CH2CH2-
OPh, 1) with 70 eV electrons in a specially constructed electron
bombardment flow (EBFlow) reactor12,19 affords allyl fluoride
as the major neutral product (0.7 of recovered C3H5F). Allyl
fluoride could arise either from ionization or from thermal
decomposition of the neutral starting material. The remaining
isomers, however, result from rearrangments that only cations
might have undergone: cis-CH3CHdCHF, trans-CH3CHdCHF,
(19) Morton, T. H. In Techniques for the Study of Ion-Molecule
Reactions, Techniques of Chemistry XX; Farrar, J. M., Saunders, W. H.,
Jr., Eds.; Wiley-Interscience: New York, 1988; pp 119-164.
(20) Beaudet, R. A.; Baldeschwieler, J. D. J. Chem. Phys. 1962, 9, 30-
41.

7910 J. Am. Chem. Soc., Vol. 121, No. 34, 1999
Shaler et al.
Figure 1. 467 MHz 19F NMR spectrum of an acetone-d6 solution of the neutral products recovered from 70 eV EBFlow radiolysis of 10-4 Torr
1-3,3-d2 (0.1 mA for 90 min) showing the deuterated trans- (spectrum A) and cis- (spectrum B) 1-fluoropropenes.
1,3-shift contain deuterium in their methyl groups. The peaks
assigned to CDH2CHdCHF might overlap with those from
small amounts of isotopomers with CD2H-groups, but 1H-
decoupling of the cis isomer shows a 1:1:1 triplet, indicating
that the methyl is predominantly monodeuterated.
The distribution of deuterium in the recovered neutral
products agrees with the 2.1:1 PhOH•+:PhOD•+ ion abundance
ratio observed in the 70 eV mass spectrum of 1-3,3-d2, provided
we suppose that nearly all of the recovered CH2dCHCD2F
(5)
comes from molecular ion dissociation. The ratio of CDH2:CH3
in trans-1-fluoropropenes, 0.07, represents the contribution of
monodeuterated and undeuterated fluoropropenes. A double
1,3-shift to the formation of those products. The proportion of
exchange would form undeuterated fluoropropenes. While we
CDH2-containing products appears to be slightly higher in the
do not detect undeuterated trans-1-fluoropropene, the chemical
cis-1-fluoropropenes (judging from the one isotopomer that can
shift of undeuterated cis-1-fluoropropene is the same as that of
be well resolved), while 1,3-shift products constitute ap-
its 3-d1-analogue. The splitting pattern that we observe for the
proximately 0.01 of the recovered allyl fluorides.
four peaks in this region of the NMR (132.3-132.6 ppm) is
A substantial body of evidence suggests that 1,3-shifts in
more complicated than would be expected for CH2DCHdCHF
simple carbocations take place via the intermediacy of corner-
and may result from the superposition of that signal with the
protonated cyclopropanes, which randomize isotopic label via
signal from a small amount of the undeuterated analogue.
corner-to-corner proton shifts.21 The distribution of neutral
Portraying the isomerization of the fluoropropyl group as
products in the present case argues against complete isotopic
cationic rearrangements of free C3H6F+ conveys a partial
scrambling. Suppose, for example, randomization had taken
theoretical picture of 1,2- and 1,3-shifts. DFT calculations show
place in the small percentage of ions that undergo a 1,3-shift.
that the primary 3-fluoro-n-propyl cation is unstable and enjoys
Subsequent ring opening to R-fluoro-n-propyl cation, 2 (the most
two barrier-free pathways to stable isomers: a 1,2-hydride shift
stable linear cation) would have yielded a deuterium distribution
(to â-fluoroisopropyl cation) or closure to a corner-protonated
corresponding to a statistical ratio CD2HCHdCHF:CDH2CHd
fluorocyclopropane (which has a mirror plane of symmetry, with
CHF:CH2DCD)CHF:CH2DCHdCDF equal to 2:2:2:1, which
the C-F bond out of the plane of the ring). Geometry
differs substantially from the distribution of those products
optimizations and normal modes calculations were performed
among the trans-1-fluoropropenes.
at B3LYP/6-311G**, since this level has been reported to
One additional isotopic variant might arise via chain-ring
reproduce geometries of simple halogenated cations optimized
exchange, as represented in eq 5. This is a minor process that
at much higher computational levels.23 The calculated 0 K heat
has been observed to take place through distonic ions formed
of formation of corner-protonated fluorocyclopropane is 44 kJ
via six-member cyclic transition states.22 In the present case
chain-ring exchange would have to occur via seven-member
cyclic transition states. A single exchange, followed by forma-
tion of ion-neutral complexes, would lead ultimately to
mol-1 higher than that of the â-fluoroisopropyl cation. Subse-
quent isomerization of the corner-protonated fluorocyclopropane
to R-fluoro-n-propyl ion is equivalent to a net 1,3-shift (except
for the position of the isotopic label), for which the calculated
barrier is ∆H‡ ) 20 kJ mol-1 (∆H‡ ) 22 kJ mol-1 for the
(21) Corma, A.; Boronat. M.; Virula, P. Appl. Catal. A 1996, 207-223
and references therein.
(22) Nguyen, V.; Cheng, X.; Morton, T. H. J. Am. Chem. Soc. 1992,
114, 7127-7132.
(23) van Alem, K.; Sudho¨lter, E. J. R.; Zuilhof, H. J. Phys. Chem. A
1998, 102, 10860-10868.

Hydrogen Shifts in Fluoropropyl Cations
J. Am. Chem. Soc., Vol. 121, No. 34, 1999 7911
deuteration pattern corresponding to 1-3,3-d2). The observed
kinetic preference for the 1,2-shift agrees qualitatively with our
calculations, but it would be surprising if the 1,3-deuterium shift
could operate to any detectable extent in competition with a
barrier-free 1,2-hydride shift. A more accurate calculation would
have to take into account the C-O bond heterolysis concurrent
with the R′ f R+ isomerization, which should introduce a
barrier to 1,2-shift.
initially. However, in both cases the isomeric cations intercon-
vert as portrayed by eq 6 (with k symbolizing the first-order
rate constant for shifting one hydrogen from position 1 of the
â-fluoroisopropyl cation and k′ the rate constant for shifting
one hydrogen from position 2 of R-fluoro-n-propyl cation).
(6)
Discussion
EBFlow radiolysis of PhOCH2CH2CH2F (1) yields a product
distribution that reflects kinetic control. Of the three principal
isomers that are recoveredsallyl fluoride, cis-1-fluoropropene,
and trans-1-fluoropropenesallyl fluoride is the least stable
thermodynamically,24 yet it is the most abundant. cis-1-
Fluoropropene is more stable than the trans, but the ratio
recovered from 70 eV electron ionization of 1 (1.3) is less than
the equilibrium ratio at temperatures e1000 K.
Our kinetic analysis supposes that the conformers of the
intermediate cations equilibrate rapidly but that the Brønsted
acid-base reactions with the phenoxy radical inside the complex
have rate constants with magnitudes comparable to k and k′.
Equation 6 summarizes the conformational equilibrium constants
previously extracted from the experimental EBFlow product
distribution from CH3CDFCH2OPh. Ab initio calculations
predict that the â-fluoroisopropyl cation is planar, with stable
s-cis and s-trans conformers, while the R-fluoro-n-propyl cation
has only one stable planar (s-cis) and one nonplanar conformer
(skew, which is chiral). Deprotonation of the skew can yield
either cis- or trans-1-fluoropropene, while each planar conformer
can form only one geometric isomer.
The results of EBFlow radiolysis of 1 show that eq 1 (where
R′ ) FCH2CH2CH2) operates in the gas-phase decomposition
of 1•+. A vicinal alkene elimination via a four-member cyclic
transition state (such as that which dominates the decomposition
of ionized sec-alkyl phenyl ethers25) would have produced allyl
fluoride as the only neutral product. While vicinal elimination
does take place to some extent, production of 1-fluoropropene
implies the intervention of [C3H6F+ PhO•] ion-neutral com-
plexes. Since the cis:trans ratio is the same as has been reported
Our previously reported steady-state analysis of 13C-labeled
CH3CDFCH2OPh measured not only the two conformational
equilibrium constants shown in eq 6, but also six branching
ratios and three kinetic isotope effects. These give the relative
rate constants corresponding to the numerical coefficients for
the sequence of 1,2-shifts and deprotonation steps summarized
in Scheme 1. We find that those relative rate constants cannot
explain the product distribution recovered from EBFlow radi-
olysis of PhOCH2CH2CD2F. This is not surprising, since the
internal energy of the ion-neutral complex ought to depend
on the structure of the precursor. However, if we assume that
the Brønsted acid-base reaction rates are the only ones affected
by changing the precursor, the relative rates from CH3CDFCH2-
OPh give a reasonable quantitative fit to the distribution of
rearranged products recovered from PhOCH2CH2CD2F. If we
guess that deprotonations to form 1-fluoropropene are slowed
from gas-phase deprotonation of free CH3CH2CHF+ ions,12
a
pathway involving such ions is called for. As discussed in the
Introduction, free ions cannot be contributing substantially to
the EBFlow product yield, since the majority of them undergo
the skeletal rearrangement depicted in eq 3. Nor could free
fluoropropyl ions (which constitute only a small fraction of the
total ionization from 1) account for the comparatively high yield
of neutral fluoropropenes. Ion-neutral complexes represent
transient intermediates that reconcile the formation of ion-
molecule reaction products with unimolecular kinetics of their
production.
Isotopic substitution of the starting material at position 3
provides much more information. On one hand, intervention of
a distonic intermediate (with subsequent expulsion of a neutral
diradical) might conceivably account for the isomer distribution
from unlabeled 1. On the other hand, the deuterium distribution
in the neutral products from EBFlow radiolysis of PhOCH2-
CH2CD2F (1-3,3-d2) cannot be rationalized by any distonic
pathway but is efficiently explained in terms of unimolecular
cation rearrangements that find precedent in the previously
published EBFlow radiolysis of CH3CDFCH2OPh.12
Within ion-neutral complexes intramolecular 1,2-hydrogen
shifts operate reversibly between carbons that can bear a positive
charge with comparable stability. In the case of ionized PhOCH2-
CH2CD2F this leads to deuteration of either or both vinylic
carbons in the recovered 1-fluoropropene. We have observed
this reversibility in ionized CH3CDFCH2OPh, as well. That prior
study entered the 1-fluoropropyl cation potential surface via
competing irreversible 1,2-methyl and 1,2-fluoride shifts, which
form R-fluoro-n-propyl and â-fluoroisopropyl cations, respec-
tively, with an initial ratio of approximately 2:1. Ionization of
1 enters the same manifold predominantly via an irreversible
1,2-hydride shift, which forms only the â-fluoroisopropyl cation
1
by a factor of γ ) /4 and that deprotonations to form allyl
1
fluoride are slowed by a factor of η ) /2, we calculate the
product distribution summarized in the right-hand column of
Table 1. According to this calculation, the proportion of CH2d
CHCD2F from interconverting ions should amount to 0.12 of
the total fluoropropene yield and that a mixture of CH2d
CDCH2F and CHDdCDCH2F should represent a fraction of
0.0006. The former is recovered in much higher yield than
predicted, while the latter isotopomers lie below our detection
limit.
The relative proportions of H- versus D-transfer measured
by EBFlow agree with the mass spectrometric intensity ratio
(m/z 95 + m/z 67)/(m/z 94 + m/z 66) only if we suppose that
all of the recovered CH2dCHCD2F comes from decomposition
of ionized PhOCH2CH2CD2F. As a consequence, the mole
fraction of ionized PhOCH2CH2CD2F decomposing via vicinal
elimination to form CH2dCHCD2F must be 0.42 ( 0.02 (as
compared to a mole fraction of 0.1 for vicinal elimination from
ionized CH3CDFCH2OPh12).
Neither Scheme 1 nor vicinal elimination (nor interconver-
sions of distonic intermediates) can explain the formation of
neutral products containing CH2D-groups. We therefore infer
that initial 1,3-deuterium shift competes with 1,2-hydride shift,
as represented by eq 4. The 1,3-shift initially forms an R-fluoro-
n-propyl cation, which is assumed to undergo the same sort of
(24) Dolbier, W. R., Jr; Medinger, K. S.; Greenberg, A.; Liebman, J. F.
Tetrahedron 1982, 38, 2415-2420.
(25) Traeger, J. C.; Luna, A.; Tortajada, J. C.; Morton, T. H. J. Phys.
Chem. A 1999, 103, 2348-2358.

7912 J. Am. Chem. Soc., Vol. 121, No. 34, 1999
Shaler et al.
Scheme 1
interconversions as drawn in Scheme 1. Relative proportions
of 0.94:0.06 1,2-hydride shift:1,3-deuterium shift were used to
derive the calculated product abundances in the right-hand
column of Table 1, along with the relative rate constants for
interconversion and Brønsted acid-base reactions summarized
in Scheme 1. The normal isotope effect for the 1,2-hydride shift
is k′H/k′D )1.8, while the proton-transfer isotope effect is 1.3.
If we guess that the isotope effect on 1,3-shift is the mean of
those two values, then the ratio of 1,2-hydride shift to 1,3-
hydrogen shift in ionized 1 should be 10:1.
We conclude that three decomposition pathways compete in
fluoroalkene expulsion from ionized PhOCH2CH2CD2F (mole
fractions in parentheses): formation of ion-neutral complexes
via a 1,2-hydride shift (0.54), formation of ion-neutral com-
plexes via a 1,3-deuterium shift (0.035), and vicinal elimina-
tion (0.42). Since 1 is a mixture of conformational isomers,
one may inquire whether these mole fractions somehow re-
flect a conformational equilibrium in the neutral precursor.
While there are some data showing that this might be the case
in phenyl n-propyl ether,26 there is no evidence for such an ef-
fect in PhOCH2CD2F, a shorter homologue of 1-3,3-d2.27 With
regard to compound 1 the answer to this question remains to
be sought.
A qualitative picture of the transition state for the 1,2-hydride
shift is easily imagined. The C-O bond of ionized 1 stretches
until enough positive charge builds up on carbon to permit a
sigmatropic rearrangement. The reaction coordinate has the
character of a C-O bond heterolysis, in which the electron pairs
that end up on the phenoxy oxygen lie in the same plane as the
itinerant hydrogen, while the unpaired electron remains in an
orbital parallel to the p-orbitals of the benzene ring. Maximum
orbital overlap constrains both migration termini and the itinerant
hydrogen to lie in the same plane as that of the benzene ring,
as depicted by the transition state represented by structure 2.
The transition state for 1,3-shift is less evident. One alternative
is that the shift occurs concurrently with fission of the C-O
bond of 1.+, as suggested by eq 4. The other alternative supposes
that the fluoromethyl bridges, forming an ion-neutral complex
containing a corner-protonated cyclopropane. This transition
state is represented by structure 3. Here all eight carbon atoms
should be nearly coplanar with the oxygen. After this barrier is
passed, rapid transfer of a proton from the fluorinated corner
to one of the unfluorinated corners forms the much more stable
R-fluoro-n-propyl ion, an exothermic isomerization (∆H ) -59
kJ mol-1 by DFT) for which DFT calculations give a 20 kJ
mol-1 barrier. If competition between the transition states
represented by 2 and 3 determines the product ratio, then
deuteration does not affect the branching ratio by virtue of the
2 kJ mol-1 difference that we compute for the barrier height
difference between H- and D-transfer. Deuterium substitution
would instead exert a secondary isotope effect on fluoromethyl
bridging versus a 1,2-hydride shift. Methyl bridging without
isotopic scrambling has been invoked as a minor pathway in
expulsion of propene from ionized phenyl n-propyl ether.28
Given the symmetry that we have calculated for corner-
protonated fluorocyclopropane, this mechanism stipulates that
the two undeuterated carbons of ionized PhOCH2CH2CD2F
should become equivalent in the course of forming 1,3-shift
products, an experimentally testable hypothesis. These reac-
tion coordinates are the subject of ongoing computational
exploration.
Conclusions
The competition between 1,2- and 1,3-shifts of hydrogen in
an acyclic gaseous cation is reported here for the first time.
While the 1,3-shift initially forms a cation (R-fluoro-n-propyl)
that is more stable thermodynamically, it is kinetically less
favorable than the 1,2-shift to form â-fluoroisopropyl cation
(26) Song, K.; van Eijk, A.; Shaler, T. A.; Morton, T. H. J. Am. Chem.
Soc. 1994, 116, 4455-4460.
(27) Kohler, B. E.; Morton, T. H.; Nguyen, V.; Shaler, T. A. J. Phys.
Chem. A 1999, 103, 2302-2309.
(28) Traeger, J. C.; Morton, T. H. J. Am. Chem. Soc. 1996, 118, 9661-
9668.

Hydrogen Shifts in Fluoropropyl Cations
(which is less stable by 15 kJ mol-1 11
by approximately a factor
J. Am. Chem. Soc., Vol. 121, No. 34, 1999 7913
)
ments described here, gas-phase cationic rearrangements within
ion-neutral complexes provide a picture of fast isomerization
processes that become obscured on longer observational time
scales.
of 0.1. The cations produced by either shift interconvert with
one another during the lifetime of their ion-neutral complexes
with phenoxy radical. However, the complex formed from
ionized 1 does not live long enough for skeletal rearrangement
to 2-fluoroisopropyl cation (which occurs extensively in free
fluoropropyl cations). The recovered neutral products from
proton transfer within the complex exhibit the same cis:trans
ratio of 1-fluoropropenes as do the neutral products from depro-
tonation of free CH3CH2CHF+. As exemplified by the experi-
Acknowledgment. This work was supported by NSF grant
CHE 9522604. NMR instrumentation was acquired with funding
from NSF-MRI grant CHE 9724392.
JA990567J