Chiral ions in the gas phase. 1. Intramolecular racemization and isomerization of O-protonated (S)-trans-4-hexen-3-ol

Article abstract of DOI:10.1021/ja960212p

The acid-catalyzed racemization and regioisomerization of (S)-trans-4- hexen-3-ol (1S) has been investigated in gaseous CH₄ and C₃H₈ at 720 Torr and in the 40-120 °C temperature range. The contribution to the racemization and isomerization products by free 1-methyl-3-ethylallyl cations, arising from unimolecular fragmentation of excited O-protonated (S)-trans-4-hexen-3- ol (IS), was evaluated by generating them from protonation of isomeric 2,4- hexadienes and by investigating their behavior toward H2¹⁸O under the same experimental conditions. The rate constant of the gas phase racemization of IS (1.4-21.3 x 10⁶ s^-1) was found to exceed that of its isomerization (1.0-9.9 x 10⁶ s^-1) over the entire temperature range. The experimental results, combined with ab initio theoretical calculations on the model [C₃H₅+/H₂O] system, are consistent with a gas phase intramolecular 1S racemization and isomerization involving the intermediacy of structured ion- molecule complexes, wherein the H₂0 molecule is coplanarly coordinated to the hydrogen atoms of the 1-methyl-3-ethylallyl moiety. The rate of formation of these structured complexes, their relative stability, and the dynamics of their evolution to the racemized and isomerized products depend, in the gas phase, on the specific conformation of IS. The relevant activation parameters point to transition structures wherein a substantial fraction of the positive charge is located on the allyl moiety. The results obtained in the present gas phase investigation confirm previous indications about the occurrence and the role of intimate ion-molecule pairs in acid-catalyzed racemization and isomerization of optically active alcohols, including allylic alcohols, in solution.

Full text of DOI:10.1021/ja960212p

J. Am. Chem. Soc. 1997, 119, 4525-4534
4525
Chiral Ions in the Gas Phase. 1. Intramolecular Racemization
and Isomerization of O-Protonated (S)-trans-4-Hexen-3-ol
Anna Troiani,† Francesco Gasparrini,† Felice Grandinetti,‡ and
Maurizio Speranza*,†
Contribution from the Facolta` di Farmacia, Dipartimento di Studi di Chimica e Tecnologia delle
Sostanze Biologicamente AttiVe, UniVersita` degli Studi di Roma “La Sapienza”,
P.le A. Moro 5, 00185 Rome, Italy, and the Dipartimento di Scienze Ambientali,
UniVersita` della Tuscia, Viterbo, Italy
ReceiVed January 22, 1996X
Abstract: The acid-catalyzed racemization and regioisomerization of (S)-trans-4-hexen-3-ol (1S) has been investigated
in gaseous CH4 and C3H8 at 720 Torr and in the 40-120 °C temperature range. The contribution to the racemization
and isomerization products by free 1-methyl-3-ethylallyl cations, arising from unimolecular fragmentation of excited
O-protonated (S)-trans-4-hexen-3-ol (IS), was evaluated by generating them from protonation of isomeric
2,4-hexadienes and by investigating their behavior toward H218O under the same experimental conditions. The rate
constant of the gas phase racemization of IS (1.4-21.3 × 106 s-1) was found to exceed that of its isomerization
(1.0-9.9 × 106 s-1) over the entire temperature range. The experimental results, combined with ab initio theoretical
calculations on the model [C3H5+/H2O] system, are consistent with a gas phase intramolecular IS racemization and
isomerization involving the intermediacy of structured ion-molecule complexes, wherein the H2O molecule is
coplanarly coordinated to the hydrogen atoms of the 1-methyl-3-ethylallyl moiety. The rate of formation of these
structured complexes, their relative stability, and the dynamics of their evolution to the racemized and isomerized
products depend, in the gas phase, on the specific conformation of IS. The relevant activation parameters point to
transition structures wherein a substantial fraction of the positive charge is located on the allyl moiety. The results
obtained in the present gas phase investigation confirm previous indications about the occurrence and the role of
intimate ion-molecule pairs in acid-catalyzed racemization and isomerization of optically active alcohols, including
allylic alcohols, in solution.
Introduction
cular dissociation (SN1, eq iii of Scheme 1) and intramolecular
isomerization of the involved allylic oxonium intermediates
(SNi′, eq iv of Scheme 1). For instance, in 760 Torr of CH4 at
37.5 °C, up to 12% of intramolecular interconversion (eq iv
of Scheme 1) was observed in the oxonium intermediates
with R/R′ ) H/Me, trans-Me/H, and cis-Me/H prior to substitu-
tion by methanol.3a,b Under the same conditions, 11% is
the maximum extent of interconversion observed prior to
nucleophilic attack in the fraction (g81%) of oxonium inter-
mediates from protonation of 1-methyl-2-cyclohexen-1-ol
and 3-methyl-2-cyclohexen-1-ol surviving unimolecular frag-
mentation.3c
The natural continuation of this study is the assessment of
the intrinsic stereochemistry of the bimolecular reactions i and
ii of Scheme 1 in the gas phase (i.e., without interference from
solvation and ion-pairing effects) and its comparison with
theoretical predictions which normally refer to isolated species.
The use of chiral oxonium ions and the unequivocal definition
of their structure immediately before bimolecular substitution
are required to achieve this goal. The present study addresses
this latter point by analyzing the kinetics and the mechanisms
of unimolecular isomerization and racemization of a model
chiral oxonium ion under typical gas phase nucleophilic
substitution conditions.
Despite the recent development of novel approaches for chiral
recognition by mass spectrometric methods,1 the study of gas
phase reactions involving optically active ions with a single
chiral center still remains a virtually unexplored facet of the
gas phase ion chemistry field.2 The present investigation is a
first attempt to fill this gap, the emphasis being on the kinetics
and the dynamics of intramolecular rearrangements in chiral
oxonium ions.
The study was stimulated from a recent gas phase investiga-
tion of the mechanism of acid-induced nucleophilic substitution
on some allylic alcohols using radiolytic methods.3 It was
demonstrated that the reaction proceeds through the concerted
SN2′ pathway (eq ii of Scheme 1), in competition with the
classical SN2 one (eq i of Scheme 1). Unequivocal evidence
for these mechanisms was achieved only after careful weighing
of the unimolecular processes which normally accompany
bimolecular allylic substitution in solution,4 namely unimole-
† Universita` degli Studi di Roma “La Sapienza”.
‡ Universita` della Tuscia.
X Abstract published in AdVance ACS Abstracts, April 15, 1997.
(1) Sawada, M.; Takai, Y.; Yamada, H.; Hirayama, S.; Kaneda, T.;
Tanaka, T.; Kamada, K.; Mizooku, T.; Takeuchi, S.; Ueno, K.; Hirose, K.;
Tobe, Y.; Naemura, K. J. Am. Chem. Soc. 1995, 117, 7726 and references
therein.
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T. H. J. Am. Chem. Soc. 1981, 103, 2416.
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M. J. Chem. Soc., Chem. Commun. 1995, 547. (b) Renzi, G.; Lombardozzi,
A.; Dezi, E.; Pizzabiocca, A.; Speranza, M. Chem. Eur. J. 1996, 2, 316. (c)
Dezi, E.; Lombardozzi, A.; Renzi, G.; Pizzabiocca, A.; Speranza, M. Chem.
Eur. J. 1996, 2, 323.
(4) (a) DeWolfe, R. H.; Young, W. G. Chem. ReV. 1956, 56, 753. (b)
Bordwell, F. G. Acc. Chem. Res. 1970, 3, 281. (c) McLennan, D. J. Acc.
Chem. Res. 1976, 9, 281. (d) Magid, R. M. Tetrahedron 1980, 36, 1901.
(e) Young, W. G.; Winstein, S.; Goering, H. L. J. Am. Chem. Soc. 1951,
73, 1958. (f) D’Incan, E.; Viout, P. Bull. Soc. Chim. Fr. 1971, 9, 3312. (g)
Meislich, H.; Jasne, S. J. J. Org. Chem. 1982, 47, 2517. (h) Kepner, R. D.;
Winstein, S.; Young, W. G. J. Am. Chem. Soc. 1949, 71, 115.
S0002-7863(96)00212-0 CCC: $14.00 © 1997 American Chemical Society

4526 J. Am. Chem. Soc., Vol. 119, No. 19, 1997
Troiani et al.
Scheme 1
Scheme 2
The selected chiral oxonium ion was IS (Scheme 2), obtained
in the gas phase from O-protonation of the S-enantiomer
of trans-4-hexen-3-ol (1S). To this purpose, stationary con-
centrations of the protonating agents have been generated
from γ-radiolysis of gaseous mixtures, containing either CH4
or C3H8 as the bulk component (720 Torr), together with
traces of the chiral alcohol 1S (0.7-1.0 Torr), of O2 as a
radical scavenger (10 Torr), and of (MeO)3PO as a base
(0.5-0.9 Torr) with a proton affinity (PA) of 212.0 kcal
It is hoped that this study, which represents the first kinetic
and mechanistic investigation on optically active cations in the
gas phase, may provide kinetic information upon the unimo-
lecular rearrangement of the free, unsolvated oxonium inter-
mediate IS, as well as on the dynamics of the H2O molecule
moving around the allyl ion backbone as a function of the
conformation of the rearranging species. Moreover, a com-
parison of the gas phase results with those from analogous
solvolytic studies may contribute to unravelling the role of
intimate ion-molecule pairs in the racemization and isomer-
ization of optically active alcohols and ethers in acidic
solutions.6-15
mol-1 5
. Under such conditions, the high collisional frequency
(ca. 1010 s-1) with the bulk gas molecules favors stabili-
zation and thermal equilibration of IS prior to its rearrange-
ment, whose course is inferred from the yield and composition
of the neutral rearranged products (i.e., 1R, 2R, 2S, 3R, and
3S (Scheme 2)), formed by neutralization of their ionic
precursors (i.e., IR, IIR, IIS, IIIR, and IIIS) with the base
(MeO)3PO.
(6) For reviews, see: (a) Samuel, D.; Silver, B. AdV. Phys. Org. Chem.
1965, 3, 128. (b) Raber, D. J.; Harris, J. M.; Schleyer, P. v. R. Ions and
Ion Pairs in Organic Reactions Szwarc, M., Ed.; Wiley: New York, 1974;
Vol. 2.
(7) Herlihy, K. P. Aust. J. Chem. 1982, 35, 2221.
(8) Allen, A. D.; Kanagasabapathy, V. D.; Tidwell, T. T. J. Am. Chem.
Soc. 1984, 106, 1361.
(9) Dietze, P. E.; Jencks, W. P. J. Am. Chem. Soc. 1987, 109, 2057.
(5) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R.
D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17, Suppl. 1.

Chiral Ions in the Gas Phase. 1
J. Am. Chem. Soc., Vol. 119, No. 19, 1997 4527
MEGADEX 5 fused silica column, operated at the same conditions as
reported above. The products were identified by comparison of their
retention volumes with those of authentic standard compounds, and
their identity confirmed by GLC-MS, using a Hewlett-Packard 5890
A gas chomatograph in line with a HP 5970 B mass selective detector.
Their yields were determined from the areas of the corresponding eluted
peaks, using the internal standard (i.e., 3-methylpentan-3-ol) method
and individual calibration factors to correct for the detector response.
Blank experiments were carried out to ascertain the occurrence and
the extent of thermal isomerization and racemization of 1S at any given
reaction temperature. The yields of the radiolytic products from gas
phase protonation of 1S were corrected accordingly.
Computational Details. The ab initio calculations were performed
using a IBM RISC/6000 version of the Gaussian 9218a and Gaussian
9418b sets of programs. The geometries of the investigated species were
optimized at the MP2(full)/6-31G* level of theory, and their real
character on the surface (minimum, transition structure, higher-order
saddle point) ascertained by computing the corresponding analytical
vibrational frequencies. In particular, the transition structures con-
necting selected energy minima along the appropriate reaction paths
were located using the synchronous transit-guided quasi-Newton method
implemented by Schlegel and co-workers,18c available as a standard
routine (QST3) in Gaussian 94. In addition, the zero-point energies
(ZPEs) of all of the investigated species were calculated using the MP2-
(full)/6-31G* vibrational frequencies. Finally, single-point calculations
at the MP4/6-311G** and QCISD(T)/6-311G** computational levels
were performed in correspondence of the MP2 geometries to better
estimate the influence of the correlation energy on the relative stability
of the various [C3H5+/H2O] structures.
Experimental Section
Materials. Methane, propane, and oxygen were high-purity gases
from Matheson Co. and used without further purification. H218O (18O-
content > 97%), (MeO)3PO, triethylamine, trans,trans-2,4-, cis,trans-
2,4-, and trans-1,3-hexadiene were research grade chemicals from
Aldrich Chemical Co. The racemate of (S,R)-trans-4-hexen-3-ol
(henceforth denoted by 1S-1R) and (S,R)-trans-3-hexen-2-ol (henceforth
denoted by 2S-2R) were prepared according to well-established
procedures16 and purified by semipreparative HPLC on a LiChrosorb
Si-60 column, 5 µm, 250 × 10 mm i.d., eluent 85/15 (v/v) ) hexane/
ethyl acetate, flow rate 5.0 mL min-1, detection by refractive index,
retention factors k′(1-ester) ≈ k′(2-ester) ) 0.33 and k′(1-alcohol) ≈ k′(2-alcohol)
) 2.46.
Kinetic resolution of the 1S-1R and 2S-2R racemates was carried
out by enantioselective biotransformations according to the following
procedure. Lipase (54 mg) from Pseudomonas species type XIII (Sigma
Chemical Co.), supported on a modified silica matrix,17 was added to
a magnetically stirred solution of 1S-1R (or 2S-2R) (300 mg, 3 mmol)
and acetic anhydride (0.423 mL, 4.5 mmol) in benzene (7.5 mL). The
reaction mixture was stirred at room temperature. Periodically, 0.5
µL aliquots of the liquid phase were withdrawn and analyzed by GLC
to follow the progress of the reaction. After 24 h, approximately 50%
conversion of the alcohol in acetic ester was reached, and the reaction
was stopped. The enzymic suspension was filtered, and the filtrate
was evaporated to dryness. The residue, consisting of the S-enantiomer
of the starting alcohol 1S (or 2S) and the acetic ester of the R-enantiomer
of the starting alcohol 1R (or 2R), was recovered. The R-ester was
then separated from the S-alcohol by semipreparative HPLC on the
LiChrosorb Si-60 column, under the conditions described above. The
enantiomeric excess (ee) of the purified (S)-trans-4-hexen-3-ol (1S)
(or (S)-trans-3-hexen-2-ol (2S)) was 98.5%, as determined by chiral
capillary GLC (25 m long, 0.25 mm i.d. MEGADEX 5 (30%
dimethylpentyl-â-cyclodextrin in OV 1701) fused silica column,
operated at temperatures ranging from 50 to 80 °C, 3 °C min-1).
The residue, containing the acetic ester of R-enantiomer of trans-
4-hexen-3-ol (1R) (or trans-3-hexen-2-ol (2R)), was dissolved in 95/5
(v/v) ) water/acetone solution (5 mL) containing 0.18 mL of 0.3 N
NaOH. The solution was stirred at room temperature for 20 h and
periodically analyzed by GLC. After the hydrolysis was completed,
the solvent was evaporated and the residue was purified by semi-
preparative HPLC on the LiChrosorb Si-60 column, under the conditions
described above. The ee of the purified 1R (or 2R) was 99.0%, as
determined by chiral capillary GLC.
Results
Radiolytic Experiments. Table 1 reports the composition
of the irradiated mixtures and the absolute yields of the products
formed from the gas phase protonation of 1S, in the presence
of the base (B ) (MeO)3PO). The absolute yields of the
products are given as G(M) values, defined as the number of
molecules of product M formed per 100 eV of absorbed energy.
The product pattern is characterized by the formation of the
enantiomer 1R of the starting substrate, accompanied by the
racemate of their trans-isomer 2R-2S, in proportions depending
upon the experimental conditions. No formation of the cis-
enantiomers 3R and 3S is observed under the same conditions
(G(M) < 0.001). The cis,trans-2,4-, trans,trans-2,4-, and trans-
1,3-hexadiene (“dienes” in Table 1) are also major products with
an overall yield increasing with the temperature. In the systems
with C3H8 as the bulk gas, these products are accompanied by
the formation of trans-3-isopropoxy-4-hexenes and trans-2-
isopropoxy-3-hexenes, in yields independent of temperature
(Table 1).
Ancillary experiments have been carried out under the same
conditions of Table 1 to verify the tendency of free 1-methyl-
3-ethylallyl cations to add water, yielding 1R, 1S, 2R, 2S, 3R,
and 3S. Free exo-1-methyl-exo-3-ethylallyl cations have been
generated in 720 Torr of bulk gas at 40-120 °C by radiolytic
protonation of trans,trans- or cis,trans-2,4-hexadiene (0.5-1.3
Torr) in the presence of ca. 2 Torr of H218O (18O-content >
97%) (Table 2). These free allyl cations were found to add to
Procedure. The gaseous mixtures were prepared by introducing
fragile ampules, containing weighed amounts of the chiral alcohol 1S,
into carefully outgassed 135 mL Pyrex bulbs, equipped with a break-
seal tip and connected to a greaseless vacuum line. Following the
introduction of the other gaseous components at the desired partial
pressures, the bulbs were cooled to the liquid nitrogen temperature and
sealed off. The fragile ampules were then broken, and the gaseous
components were allowed to mix before being irradiated. The
irradiations were carried out at a temperature ranging from 40 to 120
°C in a 220 Gammacell from Nuclear Canada Ltd. to a dose of 2 ×
104 Gy at a rate of 104 Gy h-1, as determined by a neopentane dosimeter.
Control experiments, carried out at doses ranging from 1 × 104 to 1 ×
105 Gy, showed that the relative yields of products are largely
independent of the dose. The radiolytic products were analyzed by
GLC, using a Perkin-Elmer 8700 gas chromatograph equipped with a
flame ionization detector (FID), on the 25 m long, 0.25 mm i.d.
(10) Merritt, M. V.; Bronson, G. E.; Baczynskyj, L.; Boal, J. R. J. Am.
Chem. Soc. 1980, 102, 346.
(18) (a) GAUSSIAN 92, Revision A; Frish, M. J.; Trucks, G. W.; Head-
Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B.
G.; Schlegel, H. B.; Robb, M. A.; Repogle, E. S.; Gomperts, R.; Andres, J.
L.; Ragavachari, K.; Binkley, J. S.; Gonzales, C.; Martin, R. L.; Fox, D. J.;
Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A.; Gaussian, Inc.:
Pittsburgh, PA, 1992. (b) GAUSSIAN 94, Revision C.2; Frish, M. J.; Trucks,
G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.;
Cheeseman, J. R.; Keith, T. A.; Petersson, G. A.; Montgomery, J. A.;
Ragavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.;
Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.;
Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.;
Andres, J. L.; Repogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.;
Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Head-Gordon,
M.; Gonzales, C.; Pople, J. A.; Gaussian, Inc.: Pittsburgh, PA, 1995. (c)
Peng, C.; Schlegel, H. B. Isr. J. Chem. 1993, 33, 449.
(11) Merritt, M. V.; Bell, S. J.; Cheon, H. J.; Darlington, J. A.; Dugger,
T. L.; Elliott, N. B.; Fairbrother, G. L.; Melendez, C. S.; Smith, E. V.;
Schwartz, P. L. J. Am. Chem. Soc. 1990, 112, 3560.
(12) Merritt, M. V.; Anderson, D. B.; Basu, K. A.; Chang, I. W.; Cheon,
H. J.; Mukundan, N. E.; Flannery, C. A.; Kim, A. Y.; Vaishampayan, A.;
Yens, D. A. J. Am. Chem. Soc. 1994, 116, 5551.
(13) Thibblin, A. J. Chem. Soc., Perkin Trans. 2 1987, 1987.
(14) Thibblin, A. J. Chem. Soc., Chem. Commun. 1990, 697.
(15) Thibblin, A. J. Chem. Soc., Perkin Trans. 2 1992, 1195.
(16) Coburn, E. R. Organic Syntheses; Wiley: New York, 1995; Collect.
Vol. 3, p 696.
(17) Cernia, E.; Palocci, C.; Gasparrini, F.; Misiti, D.; Fagnano, N. J.
Mol. Catal. 1994, 89, L11.

4528 J. Am. Chem. Soc., Vol. 119, No. 19, 1997
Troiani et al.
Table 1. Absolute Yields of the Products from the Gas Phase Protonation of 1S in the presence of (MeO)3PO
absolute product yields, G(M)b
system compositiona
G(2R-2S)
reactn temp
bulk gas
1S (Torr)
(MeO)3PO (Torr)
(°C)
G(1R)
G(2R)
G(2S)
G(dienes)c
G(ethers)d
CH4
CH4
CH4
CH4
C3H8
C3H8
0.69
0.84
1.05
0.84
0.70
0.76
0.53
0.88
0.84
0.88
0.62
0.64
40
70
100
120
40
0.060
0.153
0.175
0.065
0.085
0.110
0.025
0.053
0.067
0.016
0.037
0.037
0.027
0.053
0.068
0.018
0.036
0.037
0.302
0.520
0.809
0.985
0.090
0.890
0.215
0.215
100
a Bulk gas: 720 Torr. O2 : 4 Torr. Radiation dose 2 × 104 Gy (dose rate: 1 × 104 Gy h-1). b G(M) as the number of molecules M produced
per 100 eV of absorbed energy. Each value is the average of several determinations, with an uncertainty level of ca. 5%. c Combined yields of
trans,trans-2,4-, cis,trans-2,4-, and trans-1,3-hexadiene. d Combined yields of trans-3-isopropoxy-4-hexene and trans-2-isopropoxy-3-hexene.
Table 2. Absolute Yields and 18O-Content of Allylic Alcohols from Protonation of Isomeric 2,4-Hexadienes in the Presence of H218O
system compositiona
2,4-hexadiene (Torr)b
18O-content, (%)d
reactn temp
overall absolute yield,
bulk gas
H218O (Torr)
(°C)
G(M)c
1R-1S
2R-2S
CH4
CH4
CH4
CH4
C3H8
C3H8
0.63 (0.65)
0.60 (0.59)
0.53 (0.58)
0.68 (0.73)
1.26 (1.05)
1.10 (0.96)
2.0 (1.9)
2.0 (1.8)
1.9 (2.1)
2.1 (2.0)
2.7 (2.5)
2.4 (2.2)
40
70
100
120
40
0.30 (0.32)
0.12 (0.14)
0.05 (0.04)
<0.001 (<0.001)
0.21 (0.24)
53 (54)
54 (54)
54 (55)
n.d. (n.d.)e
35 (36)
38 (37)
52 (55)
54 (52)
55 (53)
n.d. (n.d.)e
40 (40)
40 (41)
100
0.04 (0.04)
a Bulk gas: 720 Torr. O2: 4 Torr. Radiation dose 2 × 104 Gy (dose rate: 1 × 104 Gy h-1). b The trans,trans-isomer. The results concerning
experiments with cis,trans-isomer are given in parentheses. c See footnote b in Table 1. d Uncertainty level, ca. 5%. e n.d. ) below detection limit
(ca. 2%).
Table 3. Absolute Yields and 18O-Content of Allylic Alcohols from Protonation of 1S in the Presence of H218O
system compositiona
absolute yieldsb
G(1R) G(2R-2S)
18O-content (%)c
reactn temp
overall product yield,
bulk gas
1S (Torr)
H218O (Torr)
(°C)
G(M)b
1R
2R-2S
CH4
CH4
CH4
CH4
C3H8
C3H8
0.63
0.67
0.53
0.58
0.62
0.63
2.0
2.1
2.0
2.0
2.0
1.8
40
70
100
120
40
0.04
0.04
0.11
0.06
0.05
0.07
0.05
0.06
0.09
0.04
0.06
0.07
0.09
0.10
0.20
0.10
0.11
0.14
18
25
24
23
n.d.d
23
9
17
13
n.d.d
16
4
100
a Bulk gas: 720 Torr. O2: 4 Torr. Radiation dose 2 × 104 Gy (dose rate: 1 × 104 Gy h-1). b See footnote b in Table 1. c Uncertainty leve, ca.
5%. d n.d. ) below detection limit (ca. 2%).
H218O yielding equal amounts of 1S-1R and 2S-2R, in absolute
yields decreasing by increasing the temperature, and again, no
formation of the cis-enantiomers 3R and 3S was observed. The
overall G(M) values of 1S-1R and 2S-2R, measured in these
systems, range from 0.3 at 40 °C to <0.001 at 120 °C. The
18O-content in these products, as determined by mass spec-
trometry from the relative abundances of their [M - 29]+ (m/z
) 71(16O); 73(18O)) and [M - 15]+ (m/z ) 85(16O); 87(18O))
fragments, amounts to 54 ( 2% in CH4 and 38 ( 3% in C3H8
and appears essentially independent of temperature (last two
columns of Table 2). In a similar set of experiments with H218O
(18O-content > 97%) carried out with 1S instead of a 2,4-
hexadiene as the starting substrate, the 1R and 2S-2R alcohols
were obtained as well, in yields displaying no monotonic
dependence with temperature (Table 3). Their 18O-content is
appreciably lower than that of the same products from the
corresponding experiments with exo-1-methyl-exo-3-ethylallyl
cations and is found to decrease monotonically by increasing
the reaction temperature (Table 3).
Theoretical Calculations. The structure, the stability, and
the interconversion processes of several [C3H5+/H2O] adducts
have been investigated by ab initio theoretical calculations at
the post-SCF level of theory. Exploration of the MP2(full)/6-
31/G* potential energy surface of [C3H5+/H2O] around different
conceivable regions led to the location of seven different critical
points, unambiguously characterized as true minima or transition
structures by analytical computation of the corresponding
vibrational frequencies. The connectivity and the main geo-
metrical parameters of these ions are shown in Figure 1 and
their absolute and relative energies, at the various computational
levels, are collected in Table 4.
As a general observation, the MP2(full)/6-31G* relative
stability of the investigated [C3H5+/H2O] structures is only
modestly affected at the MP4/6-311G**//MP2/6-31G* compu-
tational level and does not change appreciably by upgrading
the calculation at the QCISD(T)/6-311G**//MP2/6-31G* level.
Thus, only the values obtained at this latter computational level
will be discussed.
Ion A, formally obtained by interacting a water molecule with
the carbon atom of one of the CH2 moieties of a free allyl cation,
is the most stable among the investigated [C3H5+/H2O] struc-
tures. Its optimized geometrical parameters (Figure 1a) suggest
+
a significant interaction between C3H5 and H2O. The C-O
bond distance (1.575 Å) is only slightly longer than the carbon-
oxygen bond in CH3OH2+ (i.e., 1.516 Å). The C1 possesses a
significant degree of pyramidalization, as suggested by the
O-C1-C2 (105.9°) and H1-C1-H2 (112.4°) bond angles (cf.,
this latter value with the H1-C1-H2 angle of 117.2° in the
free allyl cation D (Figure 1d)). In addition, the interaction
between C3H5+ and H2O significantly perturbs the π-system of
the free C3H5+ ion D, whose C1-C2 and C2-C3 bond distances
(1.382 Å) become 1.470 and 1.343 Å, respectively, in ion A.
These variations are consistent with a high degree of localization
of the π-bond between C2 and C3. Adding the thermal
contribution (300 K) to the total energies reported in Table 4,
+
the enthalpy change for the dissociation of ion A into C3H5
and H2O is evaluated to be as large as 26 kcal mol-1, in excellent
agreement with previous ab initio estimates.19 Combining this

Chiral Ions in the Gas Phase. 1
J. Am. Chem. Soc., Vol. 119, No. 19, 1997 4529
Figure 1. MP2(full)/6-31G*-optimized geometries of the investigated [C3H5+/H2O] system: (a) global minimum A; (b) local minimum B; (c)
local minimum C; (d) allyl cation D; (e) first-order saddle point TSAC; (f) first-order saddle point TSAB; (g) first-order saddle point TSS; (h)
first-order saddle point TSC. Bond lengths are in angstrom, and bond angles are in degrees.
+
computed binding energy with the experimental heats of
formation of the allyl cation (226.0 kcal mol-1)5 and of H2O
(-57.8 kcal mol-1),5 a theoretical heat of formation of 142 kcal
mol-1 is estimated for ion A.
Two additional [C3H5+/H2O] energy minima, henceforth
denoted as B and C, have been located on the MP2/6-31G*
potential energy surface (Figure 1b,c). From Table 4, they are
less stable than A by 8.3 and 10.5 kcal mol-1, respectively,
and their dissociation enthalpies into C3H5+ and H2O (i.e., 15.3
and 13.1 kcal mol-1, respectively) decrease accordingly. The
optimized geometrical parameters of B and C are consistent
with those of structured ion-dipole complexes between C3H5
and H2O, held together by hydrogen-bond-like electrostatic
interactions, with no appreciable covalent bonding between the
two interacting moieties. In the C1-symmetric structure B
(Figure 1b), this is demonstrated by the rather long O-H1
(2.191 Å) and O-H5 (2.124 Å) bond distances and by the
observation that the geometrical parameters of the C3H5 moiety
+
do not change appreciably relative to those of free C3H5 ion
D (Figure 1d). We note here that the conceivable existence of
C2V-symmetric structures close to B was checked by imposing
the proper symmetry constraints in the geometry optimization.
A novel fully planar critical point was actually located, but
characterized as a first-order saddle point, unstable with respect
(19) Nobes, R. H.; Radom, L. Org. Mass Spectrom. 1984, 19, 385.

4530 J. Am. Chem. Soc., Vol. 119, No. 19, 1997
Troiani et al.
Table 4. Computed MP2, MP4, and QCI Absolute (au) and Relative Energies (kcal mol-1, including ZPEs) and Zero-Point Energies (ZPE,
kcal mol-1) of [C3H5+/H2O] Structures
structure
MP2
ZPE
MP4
QCI
∆E(MP2)
∆E(MP4)
∆E(QCI)
S(eu)
A
B
C
TSAC
TSAB
TSS
TSC
D
-192.80441
-192.78504
-192.78064
-192.77788
-192.77988
-192.76217
-192.77821
-116.55762
-76.19924
62.2
59.6
59.1
58.7
59.0
59.2
58.9
43.9
13.5
-192.98609
-192.96860
-192.96433
-192.96155
-192.96341
-192.94351
-192.96162
-116.66487
-76.27607
-192.98731
-192.96986
-192.96557
-192.96286
-192.96479
-192.94393
-192.96283
-116.66602
-76.27607
0.0
+10.6
+11.7
+13.1
+12.2
+23.5
+13.1
0.0
+8.4
+10.6
+11.9
+11.0
+23.7
+12.0
0.0
+8.3
+10.5
+11.8
+10.9
+24.2
+12.1
70.8
78.4
80.7
80.5
77.2
74.0
76.8
+25.0
+23.5
+23.6
}
H2O
to the out-of-plane rotation of the H atoms of water. Ion C
(Figure 1c) formally arises by interacting a water molecule with
the H2 and H3 atoms of the free allyl cation. The O-H2 (2.243
Å) and O-H3 (2.433 Å) bond distances are comparatively
greater than those of structure B (2.124-2.192 Å), thus
suggesting relatively weaker interactions, that with H2 being
more intense than that with H3. Consistently, the variation of
the H2-C1-C2 bond angle (119.5°), relative to that of the free
C3H5+ ion D (121.8°; Figure 1d), is more pronounced than that
of the H3-C2-C1 one (119.3° vs 121.2°). Finally, we note
here that, despite careful searching around this region of the
MP2/6-31G* potential energy surface, no energy minima were
located with the water molecule coordinated between the H1
and H2 centers of the allylic moiety, thus pointing to unfavorable
energy requirements for four-center arrangements.
Discussion
The Ionic Reagents. γ-Radiolysis of gaseous CH4 and C3H8
produces known yields of CnH5 (n ) 1, 2) (G(CnH5+) ) 1.9
+
(n ) 1); 0.9 (n ) 2))20a and sC3H7 (G(sC3H7+) ) 3),20b-e
+
respectively. The large excess (over ca. 100:1) of the bulk gas
over the n-type nucleophiles present in the mixtures favors
collisional thermalization of the radiolytic ions before their attack
+
on the substrates. Thermal CH5 (Hf° ) 216 kcal mol-1),5
C2H5+ (Hf° ) 216 kcal mol-1),5 and sC3H7+ (Hf° ) 190.9 kcal
mol-1)5 are powerful Brønsted acids (PA(CH4) ) 131.6 kcal
mol-1; PA(C2H4) ) 162.6 kcal mol-1; PA(C3H6) ) 179.5 kcal
+
mol-1).5 In 720 Torr of pure CH4 and at T e 120 °C, the CH5
ions are present predominantly (>80%) in the monosolvated
[CH5+•CH4] form (Hf° ) ca. 191 kcal mol-1).21 Under the same
conditions, the C2H5+ ions are instead mostly naked (>90%).22a
The search of transition structures connecting the above
energy minima led to four distinct first-order saddle points (i.e.,
TSAC, TSAB, TSS, and TSC (Figure 1e-h)). The transition
structure TSAC (imaginary frequency 97.1i cm-1) connects ion
A with intermediate C and lies 11.8 kcal mol-1 above A. The
structure of TSAC indicates that the A f C rearrangement is
allowed only after a substantial elongation of the C1-O bond
from 1.575 Å (A, Figure 1a) to 2.877 Å (TSAC, Figure 1e).
Consistently, in TSAC, the C-C bond distances of the allylic
moiety (1.386 (C1-C2) and 1.378 Å (C2-C3)) are only slightly
different from those of free C3H5+ ion D (1.382 Å, Figure 1d),
while diverging substantially from the corresponding ones in
structure A (by -0.084 and +0.035 Å, respectively). The
+
In 720 Torr of pure C3H8 and at T e 120 °C, the sC3H7 ions
are present in the monosolvated [sC3H7+•C3H8] form (Hf° )
ca. 152 kcal mol-1).22b The [CH5+•CH4] and C2H5+ ions may
readily protonate all n-type nucleophiles present in the mixtures,
including H2O (PA ) 166.5 kcal mol-1),5 which is a major
ubiquitous impurity of the reaction mixture, either initially
introduced in the system together with its bulk components or
formed from its radiolysis. On the contrary, the [sC3H7+•C3H8]
ions are unable to transfer a proton to H2O, since the process is
ca. 27 kcal mol-1 endothermic. The average stationary con-
centration of H2O in the radiolytic systems is estimated of the
order of ca. 0.3-0.6 mol % (Vide infra). When the relatively
minor concentrations of 1S and (MeO)3PO (e0.14 mol %) and
the substantial diffusion rate of H2O in the gaseous medium
are taken into account, most of the CnH5+(n ) 1, 2) ions
transition structute TSAB (imaginary frequency 119.0i cm-1
)
connects ion A with B and lies 10.9 kcal mol-1 above A. Also,
the A f B rearrangement requires a substantial elongation of
generated in the CH4 samples are able to attack H2O producing
the C1-O bond distance, which increases up to 2.799 Å (TSAB
,
+
predominantly H3O+ in yields approaching those of its CnH5
-
Figure 1f). The transition structure TSS (Figure 1g) refers to
the resonant suprafacial H2O shift from C1 to C3 of ion A.
However, Table 4 indicates that this rearrangement is highly
energy demanding, with a computed energy barrier of 24.2 kcal
mol-1, which approaches closely the dissociation enthalpy of
(n ) 1, 2) precursors (G(CnH5+(n ) 1, 2)) ) 2.8).23 As
mentioned before, the [sC3H7+•C3H8] reactant, generated in the
C3H8 systems, cannot protonate H2O,23 but it is able to alkylate
(20) (a) Ausloos, P.; Lias, S. G.; Gorden, R., Jr. J. Chem. Phys. 1963,
39, 3341. (b) Ausloos, P.; Lias, S. G. J. Chem. Phys. 1962, 36, 3163. (c)
Lias, S. G.; Ausloos, P. J. Chem. Phys. 1962, 37, 877. (d) Sandoval, I. B.;
Ausloos, P. J. Chem. Phys. 1963, 38, 2452. (e) Freeman, G. R. Radiat.
Res. ReV. 1968, 1, 1.
+
ion A into C3H5 and H2O (26 kcal mol-1). This arises from
the pronounced structural distortion in TSS, relative to A,
wherein the contribution of allylic resonance to the stabilization
energy is strongly reduced by the unfavorable 180° f 169.6°
variation of the H1-C1-C2-H3 dihedral angle. Angle distor-
tion is accompanied by a pronounced variation of the C1-O
bond distance, which changes from 1.575 to 2.968 Å. The
observation that, in TSS, the C1-O bond distance is much
longer than the C2-O one (2.117 Å) suggests that water
interacts mostly with the central C2 carbon of the allylic moiety.
Finally, the transition structure for the resonant H2O shift from
C1 to C3 of ion C was located. This process occurs through
the C2V-symmetric structure TSC (Figure 1h). The activation
energy required for this degenerate rearrangement is as low as
1.6 kcal mol-1, as reported in Table 4. This low energy barrier
reflects the limited structural reorganization accompanying the
C f TSC transition, consisting essentially in the change of the
H3-O bond distance from 2.433 to 1.994 Å.
(21) Hiraoka, K.; Kebarle, P. J. Am. Chem. Soc. 1975, 97, 4179.
(22) (a) Hiraoka, K.; Mori, T.; Yamabe, S. Chem. Phys. Lett. 1993, 207,
178. (b) Sunner, J. A.; Hirao, K.; Kebarle, P. J. Phys. Chem. 1989, 93,
4010.
(23) In methane, the [CH5+•CH4] and C2H5 and, in propane,
+
[sC3H7+•C3H8] ions may attack the H2O impurity yielding H3O+, C2H5-
OH2+, and sC3H7OH2+. In the 40-120 °C temperature range and in the
presence of several Torrs of H2O, these ion may be present in multisolvated
forms, e.g. [H3O+•(H2O)3] (cf.: Lau, Y. K.; Ikuta, S.; Kebarle, P. J. Am.
Chem. Soc. 1982, 104, 1462). All of these ions are able to protonate 1S
(ref 24). Extensive ion clustering does not appreciably affect the estimate
of the kinetic results of Table 5 and of the corresponding activation
parameters. For instance, with [H3O+•(H2O)3] as the protonating agent,
treatment of the experimental results of Tables 1-3, as described in the
text, leads to log kB ) (11.4 ( 0.4) - [(7.3 ( 0.5) × 103]/2.303RT
(correlation coeff ) 0.965) and log kC ) (12.4 ( 0.5) - [(8.7 ( 0.6) ×
103]/2.303RT (correlation coeff ) 0.964). Essentially the same activation
parameters are obtained by considering bare H3O+ or [sC3H7+•C3H8] as
the Brønsted acid catalyst (see text).

Chiral Ions in the Gas Phase. 1
J. Am. Chem. Soc., Vol. 119, No. 19, 1997 4531
it and to protonate and alkylate all of the other stronger nucleo-
philes present.24 Exothermic protonation may take place on
either the oxygen atom of 1S, yielding IS, or on its π-bond
(with CnH5+(n ) 1, 2)), producing a 3-hydroxyhexan-4-yl
cation.24 The first process by far kinetically predominates, as
testified by the absence of isomeric hexanones among the
radiolytic products of Table 1. Their O-protonated forms are,
in fact, the most stable structures which the hypothetical
3-hydroxyhexan-4-yl cation, if formed, would isomerize to via
a very fast 1,2-hydrogen shift.25 While undergoing quenching
by multiple collisions with the bulk gas, oxonium intermediate
IS may rearrange to its isomeric structures IR and IIS-IIR (but
apparently not to the IIIS-IIIR ones) or lose the H2O moiety
yielding the corresponding 1-methyl-3-ethylallyl cations.
Since we are interested in investigating the gas phase
intramolecular racemization and isomerization reactions in the
oxonium intermediate IS (path iv of Scheme 1), it is crucial to
recognize unequivocally the origin of radiolytic products,
whether formed from IS (path iv of Scheme 1; R ) Me, R′ )
Et) or from 1-methyl-3-ethylallyl cations (path iii of Scheme
1), and to determine the relative contribution of these two
processes to their yield. Besides, any conceivable role of the
H2O moiety, released by H3O+-protonation of 1S in the CH4
systems, in the conversion of IS to the final products must be
assessed (paths i and ii of Scheme 1; NuH ) H2O).
Nature of the Radiolytic Products. The conditions used
in the radiolytic experiments, in particular the very limited
concentration of 1S (e0.14 mol %) diluted in a large excess of
the bulk gas, rule out direct radiolysis of the starting allylic
alcohol as a significant route to the products of Table 1. Their
exclusive ionic origin is ensured by the presence in the mixtures
of an efficient radical scavenger (i.e., O2 (ca. 0.5 mol %) and
testified by the disappearance of the alcoholic 1R and 2S-2R
products and by the exclusive formation of isomeric hexadienes,
by adding 0.2 mol % of a powerful base, such as NEt3 (PA )
232.3 kcal mol-1)5 to the gaseous system.
acid catalyst (Table 1), provides strong evidence against any
conceivable role of the H2O moiety generated in methane from
protonation of 1S by H3O+ in the IS f IR, IIS, and IIR
conversion (paths i and ii in Scheme 1, NuH ) H2O).26 In
addition, the same observation indicates that the extent of
racemization and isomerization of IS depend neither on the
exothermicity of O-protonation of 1S nor on the cooling
efficiency of the bulk gas but only on the temperature of the
gaseous systems. This corroborates the hypothesis of rapid
quenching of the IS intermediates, excited by the exothermicity
of their formation processes, by multiple unreactive collisions
with the bulk gas well before their rearrangement.
In the intramolecular rearrangement of IS (path iv of Scheme
1), no interchange between the moving H2O and the external
H2O is expected. Therefore, having ruled out paths i and ii of
Scheme 1 (NuH ) H2O) as being involved in the IS racem-
ization and regioisomerization, recovery of the labeled 1S-1R
and 2S-2R products from the 1S/H218O systems (Table 3) can
be only accounted for by partial dissociation of excited IS to
give the free 1-methyl-3-ethylallyl cation and a H2O molecule
(path iii of Scheme 1), which will mix with H218O present in
the bulk gas. Free 1-methyl-3-ethylallyl cations eventually react
with all of the nucleophiles present in the mixture, including
the H2
18O itself, producing a [1S-1R]/[2S-2R] ) 1 mixture in
decreasing yield by increasing the reaction temperature from
40 to 100 °C (Table 2). The 18O/16O distribution in the allylic
products ()1.2 ( 0.1 (CH4), 0.6 ( 0.1 (C3H8); Table 2)) reflects
approximately the [H218O]/[H216O] ratio in the corresponding
irradiated mixtures. This means that the average content of
water impurity in the irradiated mixtures ranges around 2-4
Torr (i.e., 0.3-0.6 mol %).
The low degree of 18O-incorporation in the 1S-1R and 2S-
2R products recovered in the 1S/H218O systems (Table 3), as
compared to that of the corresponding products from attack of
free 1-methyl-3-ethylallyl cations on H218O (Table 2), indicates
that both paths iii and iv of Scheme 1 concur to their formation.
Path iii is expected to lead to 1S-1R and 2S-2R with significant
18O-content (i.e., 54 ( 2% (CH4) and 38 ( 3% (C3H8) (Table
2)). Path iv is expected to yield the same products but with no
incorporation of the 18O-label. Accordingly, the fraction φ(M)
factor of the 1R and 2S-2R products of Table 1 arising from
addition of free exo-1-methyl-exo-3-ethylallyl cations to H2O
(path iii in Scheme 1, NuH ) H2O)27 present as an ubiquitous
impurity in the radiolytic mixtures can be calculated from the
ratio between their 18O-content measured independently from
protonation of 1S in the presence of H218O (Table 3)27 and that
measured from protonation of 2,4-hexadienes under the same
conditions (Table 2). The remaining fraction 1 - φ(M) comes
As already pointed out, kinetic investigation of the gas phase
intramolecular racemization and isomerization reactions in the
oxonium intermediate IS (path iv of Scheme 1) requires
unequivocal determination of (a) the attainment of thermal
equilibration between the gaseous medium and the oxonium
intermediate IS before its rearrangement, (b) the role of the H2O
moiety, arising from O-protonation of 1S by H3O+ in the CH4
systems, in the IS racemization and isomerization (paths i and
ii of Scheme 1; NuH ) H2O), and (c) the origin of radiolytic
products, whether formed from IS (path iv of Scheme 1; R )
Me, R′ ) Et) or from 1-methyl-3-ethylallyl cations (path iii of
Scheme 1), and the evaluation of the relative contribution of
these two processes to their yield.
(26) Further pieces of evidence against occurence of the bimolecular paths
i and ii of Scheme 1 (NuH ) H218O, Table 3), arise from (a) the [18O-
1R]/[18O-2S-2R] yield ratios, calculated from Table 3 ()0.58 ( 0.11 (CH4),
0.51 ( 0.07 (C3H8)), which coincide within the uncertainty range with those
from Table 2 ()0.50 ( 0.02 (CH4), 0.45 ( 0.02 (C3H8)) (ref 27); (b) the
absolute yields of products of Table 3 and their 18O-content which do not
exhibit any monotonic increase with temperature, as would be expected if
bimolecular nucleophilic displacements i and ii were responsible for their
formation; and (c) the absence of any appreciable occurrence of the quasi-
resonant bimolecular H218O-to-H216O displacement in closely related allylic
alcohols (i.e., O-protonated 1-buten-3-ol and isomeric 2-buten-1-ols (ref
2).
(27) This is true as far as the hypothetical bimolecular pathways generate,
in the 1S/H218O systems, unbalanced yields of 18O-labeled 1S-1R and 2S-
2R products. Otherwise, the occurrence and the extent of conceivable
bimolecular pathways cannot be discriminated from the unimolecular
pathway iii on the simple basis of the 18O-content in the radiolytic products.
Yet, in this latter special case, the fraction φ(M) of 1R and 2S-2R products
of Table 1, calculated from the ratio between their 18O-content from Tables
3 and 2 (see further in the text) does still express the relative efficiency of
processes other than the intramolecular rearrangement iv of IS (Scheme
1), irrespective of whether they are unimolecular (path iii) or bimolecular
(paths i and ii) in character.
Concerning the first two points, the observation that the yield
and distribution of the 1R, 2S, and 2R products obtained from
protonation of 1S in C3H8, where formation of H3O+ is
thermochemically prevented, are fully comparable to those
formed from the same process in CH4, where H3O+ is the major
(24) Exact determination of the thermochemistry of these processes is
presently inaccessible, owing to the lack of experimental thermochemical
data for the ionic species involved. An approximate estimate of the PA of
1S (ca. 204 kcal mol-1) can be obtained from a correlation between the
ionization energies and the proton affinities of alcohols (ref 5). The heat of
formation of the 3-hydroxyhexan-4-yl cation (ca. 140 kcal mol-1) is
estimated by means of a modified isodesmic substitution (ref 25). This leads
to a site PA value for the π-bond of 1S of ca. 176 kcal mol-1. Accordingly,
+
the exothermicity of protonation at the oxygen atom of 1S by ROH2 (R
) H, C2H5, and sC3H7) would be around 39, 16, and 13 kcal mol-1
,
respectively. The same process from [sC3H7+•C3H8] is exothermic by 10
kcal mol-1. Protonation of the π-bond of 1S by H3O+ is 11 kcal mol-1
exothermic. The same process by [sC3H7+•C3H8], C2H5OH2+, or sC3H7-
OH2 is endothermic by 18, 12, and 15 kcal mol-1, respectively.
+
(25) Bowen, R. D.; Williams, D. H. J. Am. Chem. Soc. 1978, 100, 7454.

4532 J. Am. Chem. Soc., Vol. 119, No. 19, 1997
Troiani et al.
Table 5. Rate Constants of Intramolecular Racemization and Isomerization of IS
reactn extenta
rate constants (10-6 s-1
)
reactn timeb treactn
c
c
d
e
bulk gas
reactn temp (°C)
Y1R
Y2R-2S
(×108 s)
k1R
k2R-2S
kC
kB
CH4
CH4
CH4
CH4
C3H8
C3H8
40
70
100
120
40
0.040
0.173
0.271
0.394
0.043
0.346
0.028
0.099
0.158
0.207
0.029
0.203
2.9
2.0
2.3
2.3
2.5
3.0
1.4
9.5
13.8
21.3
1.8
1.0
5.2
7.5
9.9
1.2
7.5
1.9 (6.27)
14.2 (7.15)
21.1 (7.32)
37.0 (7.57)
2.4 (6.38)
2.0 (6.29)
11.0 (7.04)
16.5 (7.25)
22.7 (7.36)
2.4 (6.38)
100
14.0
22.2 (7.34)
17.2 (7.23)
a Reaction extent, YM, expressed by G(M)(1 - φ)/G° (see text). b Reaction time, treactn, calculated from the reciprocal of the first-order collision
-1
-1
constant between IS and (MeO)3PO (see text). c k ) treactn ln(1 - YM)-1 (see text). d kC ) treactn ln[1 - 2(Y1R - 0.5Y2R-2S)]-1 (see text), log kC
-1
-1
in parentheses. e kB ) treactn ln(1 - 2Y2R-2S
)
(see text), log kB in parentheses.
from intramolecular rearrangement of IS (path iv of Scheme 1;
R ) Me, R′ ) Et). On the basis of this information, the yield
of the 1R and 2S-2R alcohols of Table 1 actually arising from
intramolecular H2O shifts in the collisionally stabilized IS
intermediates (path iv in Scheme 1) can be estimated from the
corresponding G(M)(1 - φ(M)) products.
Under the experimental conditions of Table 1, the extent of
the acid-catalyzed IS f IR racemization and IS f IIS-IIR
regioisomerization can be expressed as the ratio YM between
the corresponding G(M)(1 - φ(M)) factor and the G° terms,
defined as number of protonating species formed by absorption
of 100 eV energy, which is able to generate stable IS
intermediates by O-protonation of 1S. Within the plausible
hypothesis that the exothermic attack on 1S and B()(MeO)3PO)
by the protonating species is very efficient and totally unselec-
tive, G° can be adequately expressed by the following equation
Derivation of the activation parameters for intramolecular
racemization and isomerization of IS from the data of Table 5
requires adequate modeling of the individual rearrangement
processes.
The Regioisomerization and Racemization Model. Com-
parison between the [1S-1R]/[2S-2R] ≈ 1 distribution from
addition of free exo-1-methyl-exo-3-ethylallyl cations on external
H2O molecules and that (i.e., [1S-1R]/[2S-2R] ) 2.3-3.8) from
protonation of 1S (Table 1) excludes that, at any temperature,
the two pathways involve the same intermediate, namely a
loosely bound complex between 1-methyl-3-ethylallyl cations
and H2O. Modeling of the gas phase intramolecular IS f IR
racemization and IS f IIR-IIS isomerization necessarily
implies the intervention of tightly bound reaction complexes,
wherein the migrating H2O and the delocalized π-system of the
allylic residue become orthogonal. In the tightly bound sym-
metric intermediate involved in the IS f IR racemization, equal
is the probability that the H2O moiety rebonds to the original
side of the orbital (internal return to IS) or moves to the other
side, yielding the enantiomer IR. In the tightly bound sym-
metric intermediate involved in the IS f IIR-IIS regioisomer-
ization, rebonding of H2O to the allylic carbons would yield
both the IS-IR and IIS-IIR racemates in proportions depending
on the corresponding activation free energies. Alternative
hypothesis for intramolecular isomerization, such as that based
on the complete racemization of the starting IS to IS-IR
followed by the slow IS-IR f IIS-IIR regioisomerization,
contrasts with the limited racemization (Y1R; Table 5) and
isomerization yield factors (Y2R-2S; Table 5) measured under
all experimental conditions and with the observed increase of
their ratio with temperature.31
G° ) {[G(protonating species)k1[1S]]/
[(k1[1S] + k2[B]]} - G(dienes) - G(ethers) (1)
where G(protonating species) is taken as equal to either G(H3O+)
= G(CnH5+(n ) 1, 2)) ) 2.823 for the methane systems or
G(sC3H7+) ) 320b-e for the propane systems, G(dienes)28
represents the overall absolute yield of the isomeric hexadienes
formed in the irradiated mixtures, G(ethers) is the combined
absolute yield of trans-3-isopropoxy-4-hexenes and trans-2-
isopropoxy-3-hexenes produced in the propane samples (Table
1), and k1 and k2 are the collision rate constants of the
protonating catalyst toward 1S and B()(MeO)3PO), respec-
tively, estimated at any given temperature by using the trajectory
calculation method.29
The intermediacy of tightly bound structured complexes in
intramolecular IS racemization and isomerization finds further
support in the ab initio theoretical analysis of the critical
structures of the model [C3H5+/H2O] system (Figure 1). Here,
the transition structure TSAB separates the starting oxonium ion
A from ion B, wherein the H2O moiety is located in the plane
of the allyl cation between H1 and H5. The transition structure
TSAC connects instead isomer A with ion C, wherein the H2O
moiety lies again in the same plane but between H2 and H3 of
the allylic moiety. The latter structure is separated from the
degenerate one, wherein H2O is placed between H3 and H4
(denoted as C′), by the symmetric transition structure TSC lying
slightly above TSAC. Therefore, in the unimolecular rearrange-
ment of O-protonated allyl alcohol depicted in Scheme 3,
structure B governs both its resonant isomerization, when the
When it is taken into account that exothermic proton transfer
from gaseous Brønsted acids, including the oxonium ions of
Scheme 2, to powerful bases, such as (MeO)3PO, is generally
fast, the first-order rate constants for the formation of 1R (k1R
)
and 2S-2R (k2S-2R) at any given temperature can be expressed
-1
-1
-1
by k1R ) treactn ln(1 - Y1R
)
and k2S-2R ) treactn ln(1 -
Y2S-2R
)
-1, with treactn corresponding to the collision interval of
IS with the base B30 under the assumption that the trapping
reaction occurs with unit efficiency. The relevant results are
listed in Table 5.
(28) The isomeric hexadienes of Table 1 may as well arise in part from
(MeO)3PO-induced bimolecular E2 and E2′ elimination in IS or in its ionic
isomers. However, the conceivable interference from these bimolecular
elimination processes does not affect the estimate of the rate constants of
Table 5, provided that they occur to the same extent irrespective of the
specific isomer (cf., ref 3c). In this case, in fact, occurrence of bimolecular
E2 and E2′ eliminations may affect the absolute yields of the racemization
and isomerization products, but not their relative distribution.
(30) The reaction time, treactn, is taken as the inverse of the first-order
collision rate constant k ) k3[(MeO)3PO] between IS and the (MeO)3PO
base, with the collision rate constant k3 calculated according to ref 29 (109k3
cm3 molecule-1 s-1 (temperature in °C) ) 2.11 (40); 2.05 (70); 1.99 (100);
1.96 (120)).
(31) The same trend, coupled with the substantial activation barrier
expected for the suprafacial 1,3-shift of H2O in IS (24.2 kcal mol-1 in the
model [C3H5+,H2O] system), excludes the possibility of slow isomerization
of the starting IS to IIR, followed by fast IIR f IIS racemization.
(29) Valves for 109k1 in cm3 molecule-1 s-1 (acid catalyst; temperature
in °C): 2.84 (H3O+; 40); 2.70 (H3O+; 70); 2.58 (H3O+; 100); 2.51 (H3O+;
120); 1.66 ([sC3H7+•C3H8]; 40); 1.51 ([sC3H7+•C3H8]; 100). Valves for 109k2
in cm3 molecule-1 s-1 (acid catalyst; temperature in °C) 3.97 (H3O+; 40);
3.84 (H3O+; 70); 3.72 (H3O+; 100); 3.67 (H3O+; 120); 2.21 ([sC3H7+•C3H8];
40); 2.08 ([sC3H7+•C3H8]; 100); Su, T.; Chesnavitch, W. J. J. Chem. Phys.
1982, 76, 5183.

Chiral Ions in the Gas Phase. 1
J. Am. Chem. Soc., Vol. 119, No. 19, 1997 4533
Scheme 3
Figure 2. Arrhenius plots for the IS f B-like (a) and IS f C-like (b)
intramolecular rearrangements in 720 Torr of methane (open circles)
or propane (solid circles).
the only accessible rearrangement path open to C. Thus, the
racemization route cr of Scheme 3 is the preferred pathway open
to structure C.
Therefore, the structured complexes governing rearrangement
in IS are those depicted in Scheme 4, namely B-like and C-like.
The IS f IIR-IIS isomerization is controlled only by the B-like
structure (route bi), whereas the IS f IR-IS racemization is
governed by both the B-like (route br) and the C-like structures
(route cr).
Treatment of the YM factors of Table 5 to derive the activation
parameters governing IS unimolecular rearrangements requires
an estimate of the relative contribution of pathways br and cr of
Scheme 4 to the yield of racemization product 1R (Y1R in Table
5). A way is to consider the [1S-1R]/[2S-2R] ≈ 1 distribution
from addition of free exo-1-methyl-exo-3-ethylallyl cations to
H2O as evidence of essentially equal stability for their oxonium
precursors IS-IR and IIR-IIS. Thereby, following the Ham-
mond postulate, approximately equal free energy activation
barriers can be expected for the conversion of B-like (Scheme
4) into the more stable IR-IS and IIR-IIS intermediates. Along
this line, the contribution of route br of Scheme 4 to the IS f
IR process is taken equal to one-half of the combined 2S-2R
isomerization yield (i.e., 0.5Y2R-2S). Obviously, the contribution
of route cr of Scheme 4 to the IS f IR process is calculated
from Y1R - 0.5Y2R-2S. As a consequence, the formation rate
Scheme 4
of the B-like (Scheme 4) structure can be expressed by kB )
-1
treactn ln(1 - 2Y2R-2S)
-1. Similarly, considering equal the
probability of conversion of the C-like structure (Scheme 4) to
the IR and IS (internal return) enantiomers, the formation rate
of the C-like structure is expressed by kC ) treactn-1 ln[1 - 2(Y1R
- 0.5Y2R-2S)]-1. The values of the kB and kC rate constants,
obtained in the 40-120 °C range, are listed in Table 5. The
corresponding Arrhenius plots are shown in Figure 2. The
observation that the kB and kC rate constants from the propane
systems at 40 and 100 °C fit the Arrhenius correlation obtained
from the methane samples demonstrates that the kinetics of the
gas phase IS racemization and regioisomerization are indepen-
dent of the nature and the strength of the Brønsted acid catalyst,
whether H3O+ or [sC3H7+•C3H8], and of the heat capacity of
H2O moiety moves to both sides of the C3 one (route bi in
Scheme 3), and racemization, when the H2O moiety internally
returns to both sides of the C1 carbon (route br in Scheme 3).
Concerning structure C, it may evolve either to C′ via TSC or
to A via TSAC (route cr in Scheme 3). The uncertainty (g2%)
in the theoretical approaches used to describe TSC and TSAC
does not allow to predict the most favored C isomerization
pathway whether C f TSC f C′ or C f TSAC f A. However,
in the C-like structure of IS (Scheme 4), the presence of a
methyl group on the cis-C3 position adjacent to the C2-H bond
inhibits the hypothetical formation of a hydrogen-bonded C′-
like structure.32 As a consequence, the conceivable C T TSC
T C′-like equilibrium in IS would be strongly shifted on the
left side, thus making the C f TSAC f A racemization pathway
(32) Kebarle and co-workers (Hiraoka, K.; Grimsrud, E. P.; Kebarle, P.
J. Am. Chem. Soc. 1974, 96, 3359) demonstrated that replacement of a H
atom by an alkyl group in an oxonium ion prevents hydrogen bonding on
that position by n-type molecules and, thus, the buildup of the corresponding
cluster.

4534 J. Am. Chem. Soc., Vol. 119, No. 19, 1997
Troiani et al.
the bulk gas. This implies that IS racemization and regioi-
somerization kinetics are only temperature-dependent and that
the oxonium intermediate IS is in thermal equilibrium with the
bath gas well before its racemization and regioisomerization.
In addition, the same observation confirms the previous indica-
tions that the H2O moiety arising from proton transfer from
H3O+ to 1S does not play any appreciable kinetic and
mechanistic role in the IS rearrangement in the methane systems.
Regression analysis of all the data of Figure 2a leads to the
equation log kB ) (11.6 ( 0.4) - [(7.4 ( 0.5) × 103]/2.303RT,
with a correlation coefficient of 0.975. The same procedure
applied to the data of Figure 2b leads to the equation log kC )
(12.4 ( 0.5) - [(8.6 ( 0.6) × 103]/2.303RT, with a correlation
coefficient of 0.976.23
for the ISsyn f C-like and ISanti f B-like processes wherein a
substantial fraction of the positive charge is delocalized over
the allyl moiety.
Comparison with Solution Chemistry Studies. Demonstra-
tion of the occurrence of the intimate B-like and C-like ion-
dipole structures in the gas phase provides further insight into
the occurrence and the role of ion-molecule pairs in acid-
catalyzed racemization and isomerization of optically active
alcohols, including allylic alcohols,34 in solution.6-12 In the
elucidation of this mechanism, the rates of 18O exchange in water
of labeled alcohols (or in H218O of unlabeled alcohols) of
suitable structure were compared with the rates of loss of optical
activity and of various competing rearrangements.6-9 Within
the hypothesis of a pure unimolecular mechanism for racem-
ization and isomerization, the kinetic data can be accommodated
by a reaction pattern involving the intermediacy of intimate ion-
molecule pairs where the departing water does not equilibrate
immediately with the solvent but remains closely associated with
the carbocation.10-12 In this way, the leaving water has a greater
chance of being recaptured by the carbocation than the water
molecules from the bulk of the solution. However, the premises
on which this model is based are weakened by the observation
that the rates at which the leaving moiety moves within the
solvation sphere around the carbocation skeleton or equilibrates
with the bulk solvent depend on the relative lifetimes of the
carbocation and of its solvent shell which, in turn, depend on
the structure of the carbon skeleton6-12 and the nature of the
solvent and of the leaving moiety.13-15 Unfortunately, these
same factors may influence the reaction mechanism itself, so
that the incursion of a bimolecular SN2 pathway and of
dehydration-rehydration processes to the formation of the
racemization and exchange products cannot be completely
excluded.11-12,34
The important role of ion-neutral complex in gas phase ion
chemistry and the strict analogies to ion-molecule pairs in
solution is now well recognized.35 After careful evaluation of
the contribution of free allyl ions to the formation of the
racemization and isomerization products, the present gas phase
study demonstrates that, in a model O-protonated chiral allylic
alcohol, the leaving H2O moiety does not readily interchange
with external H2O molecules (including the conjugate base of
the H3O+ acid catalyst), but rather moves intramolecularly
around the allyl cation backbone producing eventually the
racemization and regioisomerization products. Unlike solution
studies, this intramolecular pattern is discernible in the gas phase,
owing to the absence of bulk H2O solvent around the oxonium
ion and, therefore, of a rapid interchange of the moving moiety
with the reaction medium. In agreement with previous circum-
stantial evidence obtained in solution,6-12 the dynamics of the
gas phase racemization and isomerization reactions is governed
by the occurrence and the relative stability of structured ion-
neutral complexes (i.e., B-like and C-like), with the moving
H2O electrostatically coordinated to the in-plane hydrogens of
the allylic moiety. A significant role in determining the
formation of these structured intermediates is played by con-
formational factors in their oxonium precursors (ISsyn vs ISanti).
A similar role, which is expected to be operative in any media,
has not been detected in solution yet.
The difference between the experimental activation energy
barriers for the IS f C-like and the IS f B-like rearrangements
‡
‡
(EC - EB ) (8.6 ( 0.6) - (7.4 ( 0.5) ) 1.2 ( 1.1 kcal
mol-1) compares well with the energy gap between the transition
structures TSAC and TSAB (11.8 - 10.9 ) 0.9 kcal mol-1
)
computed for the [C3H5+/H2O] system (Table 4). The lower
absolute values of the energy barriers for the IS intramolecular
rearrangements parallel the expectedly lower C-O bond energy
in IS, relative to O-protonated allyl alcohol.
Conformational Effects. On the basis of the above models,
the dynamics of the gas phase rearrangement of IS depends
upon the specific structure (B-like vs C-like in Scheme 4)
involved in the reaction coordinate, which in turn is determined
by the particular conformation of the starting oxonium inter-
mediate. Ab initio conformational analysis of O-protonated allyl
alcohol indicates that the most stable of its rotamers are those
with the C-O bond coplanar with the π-bond of the allyl
moiety, where p-π conjugative stabilization and charge delo-
calization between the H2O moiety and the allyl cation skeleton
are maximized. In fact, the transition structure with the C-O
bond orthogonal to the π-bond of the allyl moiety is found to
lie ca. 30 kcal mol-1 above the most stable coplanar conforma-
tion. It follows that radiolytic protonation of 1S leads essentially
to two non-interconverting conformers of IS, namely ISsyn
where the double bond is syn to the ethyl group, and ISanti
,
,
where the double bond is anti to the ethyl group (Scheme 4),
in proportions reflecting the relative population of the corre-
sponding 1S rotamers in their encounter complexes with the
protonating agent. In ISsyn, the quasi-coplanar hydrogens are
H2 and H3, and thus, the C-like structure governs the intramo-
lecular H2O shift. In ISanti, instead, the quasi-coplanar hydro-
gens are H1 and H5, and thus, the B-like structure governs the
intramolecular H2O motion. In other words, ISsyn follows
exclusively the racemization path cr of Scheme 4, whereas two
rearrangement channels are open to ISanti, namely the isomer-
ization path bi and racemization path br of Scheme 4. According
to the B-like structure, the isomerization path bi would generate
exclusively the trans-isomers IIR-IIS and not the cis ones IIIR-
IIIS, as actually confirmed by the experimental results (Table
1). This coincidence is somewhat reassuring on the soundness
of the rearrangement models adopted.
Further support is obtained from inspection of the activation
parameters inferred from the Arrhenius plots of Figure 2.
Indeed, the measured 1.2 ( 1.1 kcal mol-1 difference between
the activation energy of the ISsyn f C-like and the ISanti
f
Acknowledgment. This research was supported by the
Italian Ministero dell’Universita` e della Ricerca Scientifica e
Tecnologica (MURST). We thank F. Cacace for his continuous
encouragement and pertinent suggestions.
B-like rearrangements reflects the difference between the energy
gap of the relevant transition structures and that of the ISsyn
and ISanti pair. In view of the expected stability trend ISsyn
<
ISanti, the 1.2 ( 1.1 kcal mol-1 activation energy difference
conforms to the ca. 3 kcal mol-1 stability gap between exo-1-
methyl-exo-3-ethylallyl cation and exo-1-methyl-endo-3-ethy-
lallyl cation, estimated on the basis of ab initio calculations on
strictly related systems.33 This points to transition structures
JA960212P
(33) Mayr, H.; Forner, W.; Schleyer, P. v. R. J. Am. Chem. Soc. 1979,
101, 6031.
(34) Young, W. G.; Franklin, J. S. J. Am. Chem. Soc. 1966, 88, 785.
(35) McAdoo, D. J.; Morton, T. H. Acc. Chem. Res. 1993, 26, 295.