Mechanism of the Swern oxidation: Significant deviations from transition state theory

Article abstract of DOI:10.1021/jo101636w

Deprotonation of the alkoxysulfonium intermediate has been shown to be rate-determining in the Swern oxidation of benzyl alcohol. Directly following this rate-determining step is the intramolecular syn-β-elimination of the ylide. In the present study, int

Full text of DOI:10.1021/jo101636w

pubs.acs.org/joc
Mechanism of the Swern Oxidation: Significant Deviations from
Transition State Theory
Thomas Giagou and Matthew P. Meyer*
School of Natural Sciences, University of California, Merced, P.O. Box 5200 North Lake Road, Merced,
California 95343, United States
mmeyer@ucmerced.edu
Received September 3, 2010
Deprotonation of the alkoxysulfonium intermediate has been shown to be rate-determining in the Swern
oxidation of benzyl alcohol. Directly following this rate-determining step is the intramolecular syn-
β-elimination of the ylide. In the present study, intramolecular 2H kinetic isotope effects (KIEs) are used
to gain insight into this syn-β-elimination step. As a result of the stereogenic sulfur center in the ylide
intermediate, two diastereomeric transition states (endo-TS1 and exo-TS1) must be assumed to contribute
to the intramolecular KIE. The intramolecular 2H KIE determined at -78 °C is 2.82 ( 0.06. Attempts to
reproduce this measurement computationally using transition state theory and a Bell tunneling correction
yielded a value (1.58) far below that determined experimentally. Computational analysis is complicated by
the existence of two distinct transition structures owing to the stereogenic center. Two extremes of
Curtin-Hammett kinetics are explored using energies, vibrational frequencies, and moments of inertia
from computed transition structures. Neither Curtin-Hammett scenario can reproduce the observed KIE
to any acceptable degree of fidelity. Evidence based upon previous kinetics measurements and calculations
upon a model system suggests that the stereogenic sulfur center is not likely to undergo inversion to a
significant degree at the temperatures at which the Swern oxidation is performed here. Proceeding under the
assumption of no stereoinversion at the sulfur center, calculations predict a nearly linear Arrhenius plot for
the KIE;even with the inclusion of a one-dimensional tunneling correction. By contrast, the experimen-
tally determined temperature dependence shows a significant concave upward curvature indicative of the
influence of tunneling. Notably, KIEs measured in CCl4, CHCl3, CH2Cl2, dichloroethane, and
chlorobenzene at -23 °C showed little variance. This finding discounted the possible influence from
dynamical effects due to incomplete vibrational relaxation. An ad hoc amplification of the imaginary
frequencies corresponding to the first-order saddle points corresponding to endo-TS1 and exo-TS1
allowed us to reproduce the experimental temperature dependence of the KIE using only two
adjustable parameters applied to a kinetic scenario that involves four isotopomeric transition states.
The cumulative data and computational modeling strongly suggest that, even though the intramo-
2
lecular H KIE observed in these experiments is small, this reaction requires a multidimensional
description of the tunneling phenomenon to accurately reproduce experimental trends.
8088 J. Org. Chem. 2010, 75, 8088–8099
Published on Web 11/02/2010
DOI: 10.1021/jo101636w
2010 American Chemical Society
r

Giagou and Meyer
JOCArticle
Introduction
SCHEME 1. Consensus Mechansism for the Swern and Related
Oxidations
The Swern oxidation is one of the most useful methods for
the conversion of primary and secondary alcohols to alde-
hydes and ketones, respectively.1-3 Excellent mechanistic
work by Swern and others has yielded a cogent mechanistic
model (Scheme 1) for this oxidation.4-10 The rate-determin-
ing step, under standard conditions, is the deprotonation of
1 to yield the ylide. The following step, an intramolecular
syn-β-elimination, is perhaps the most interesting elementary
reaction step in the mechanism for the Swern oxidation.
The β-elimination step, which liberates dimethyl sulfide
and the product aldehyde (in the oxidation of 1° alcohols),
involves the participation of six electrons in a cyclic array.
This aspect raises a number of important questions: (1) Why
is the syn-β-elimination in the Swern oxidation significantly
more facile than other syn-β-eliminations that proceed by
five-membered transition states (Scheme 2)? (2) How impor-
tant is tunneling to this reaction step? (3) Is the five-mem-
bered transition state aromatic? The challenge of answering
these questions is exacerbated by the fact that the rate-
determining deprotonation precedes the step of interest.9
This precludes measurements of substituent and solvent
effects upon rate as a means to probing charge distribution
in the transition state, which could otherwise lend insight into
the question of an aromatic transition state. Bach, Kwart, and
others have performed kinetic isotope effect (KIE) measure-
ments upon other syn-β-eliminations.11-23 These studies
raise questions of how transition state geometry affects the
facility of tunneling and raise the possibility that traditional
hydrogen tunneling models are inappropriate for some of
these systems. In a related vein, pericyclic reactions that in-
volve hydrogen transfer have also required the invocation of
tunneling models that explicitly treat more than one dimen-
sion quantum mechanically. Exploration of the temperature
SCHEME 2. Intramolecular syn-β-Eliminations with Five-
Membered Transition States
SCHEME 3. Competing Reactions That Yield the Intramole-
cular 2H KIEs Measured Here
2
dependence of the intermolecular H KIE can lend insight
intotheimportanceandnatureofhydrogentunneling.However,
(1) Mancuso, A. J.; Swern, D. Synthesis 1981, 165–185.
(2) Tidwell, T. T. Org. React. 1990, 39, 297–572.
(3) Tidwell, T. T. Synthesis 1990, 857–870.
(4) Albright, J. D.; Goldman, L. J. Am. Chem. Soc. 1965, 87, 4214–4216.
(5) Omura, K.; Swern, D. Tetrahedron 1978, 34, 1651–1660.
(6) Pfitzner, K. E.; Moffat, J. G. J. Am. Chem. Soc. 1965, 87, 5661–5670.
(7) Fenselau, A. H.; Moffat, J. G. J. Am. Chem. Soc. 1966, 88, 1762–1765.
(8) Mancuso, A. J.; Huang, S.-L.; Swern, D. J. Org. Chem. 1978, 43,
2480–2482.
(9) Marx, M.; Tidwell, T. T. J. Org. Chem. 1984, 49, 788–793.
(10) Isaacs, N. S.; Laila, A. H. J. Phys. Org. Chem. 1991, 4, 639–642.
(11) Bach, R. D.; Andrzejewski, D. J. Am. Chem. Soc. 1971, 93, 7118–
7120.
measurements of intermolecular KIEs as a function of
temperature do not report upon the syn-β-elimination. To
circumvent these limitations, we have utilized intramolecular
2H KIEs to serve as a point of contact with values computed
from transition structure models.
Intramolecular KIEs have traditionally been employed in
systems where the competing positions are equivalent by sym-
metry.24-26 The presence of a chiral sulfur center in the
ylide makes the benzylic positions diastereotopic and therefore
inequivalent. This situation does not preclude the use of intra-
molecular KIEs, but computational treatments must take the
four energetically distinct transition states and their enantio-
mers into account. As a result, the experimentally determined
KIE is a composite of rate constants k1-k4 (Scheme 3).
(12) Bach, R. D.; Bair, K. W.; Andrzejewski, D. J. Am. Chem. Soc. 1972,
94, 8608–8610.
(13) Janssen, J. W. A. M.; Kwart, H. J. Org. Chem. 1977, 42, 1530–1533.
(14) Kwart, H.; George, T. J.; Louw, R.; Ultee, W. J. Am. Chem. Soc.
1978, 100, 3927–3928.
(15) Kwart, H.; Brechbiel, M. J. Am. Chem. Soc. 1981, 103, 4650–4652.
(16) Kwart, H.; George, T. J.; Horgan, A. G.; Lin, Y. T. J. Org. Chem.
1981, 46, 5143–5147.
(17) Kwart, H.; Brechbiel, M. J. Am. Chem. Soc. 1981, 103, 4650–4652.
(18) Yoshimura, T.; Tsukurimichi, E.; Iizuka, Y.; Mizuno, H.; Isaji, H.;
Shimasaki, C. Bull. Chem. Soc. Jpn. 1989, 62, 1891–1899.
(19) Bach, R. D.; Braden, M. L. J. Org. Chem. 1991, 56, 7194–7195.
(20) Bach, R. D.; Knight, J. W.; Braden, M. L. J. Am. Chem. Soc. 1991,
113, 4712–4714.
(24) Grdina, M. B.; Orfanopoulos, M.; Stephenson, L. M. J. Am. Chem.
Soc. 1979, 101, 3111–3112.
(25) Singleton, D. A.; Hang, C.; Szymanski, M. J.; Meyer, M. P.; Leach,
A. G.; Kuwata, K. T.; Chen, J. S.; Greer, A.; Foote, C. S.; Houk, K. N. J. Am.
Chem. Soc. 2003, 125, 1319–1328.
(21) Bach, R. D.; Gonzalez, C.; Andres, J. L.; Schlegal, H. B. J. Org.
Chem. 1995, 60, 4653–4656.
(22) Komaromi, I.; Tronchet, J. M. J. J. Phys. Chem. A 1997, 101, 3554–
3560.
(23) Cubbage, J. W.; Guo, Y.; McCulla, R. D.; Jenks, W. S. J. Org. Chem.
2001, 66, 8722–8736.
(26) Singleton, D. A.; Szymanski, M. J. J. Am. Chem. Soc. 1999, 121,
9455–9456.
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Questions of stereochemical inversion at the sulfur center
further complicate the expression of the observed KIE in
terms of rate constants k1-k4. Fortunately, these difficulties
can be overcome using both theory and experiment in con-
cert. In this paper, we present measurements of the intramo-
lecular 2H KIE for the Swern oxidation of benzyl-d1 alcohol
as a function of temperature and as a function of solvent.
From these measurements and complementary computa-
tional efforts, we attempt to elucidate intriguing aspects of
the detailed mechanism of the syn-β-elimination step of the
Swern reaction.
FIGURE 1. Use of intramolecular KIEs in the (A) Baeyer-Villiger
reaction (ref 26) and the (B) singlet oxygen ene reaction (2H KIEs,
see ref 24; 13C KIEs, see ref 25).
Results and Discussion
2
Experimental Intramolecular H KIE. The Swern oxida-
tion is typically performed at -60 °C. We found that the
reaction proceeded to completion at -78 °C in under 1 h
after addition of triethylamine (Figure 1). Measurement of
the ratios of benzaldehyde-d 4 to benzaldehyde 5 resulting
from oxidation at -78 °C yielded an intramolecular 2H KIE
of 2.82 ( 0.06. In considering what this measurement means
in terms of the factors that contribute to its magnitude, it is
useful to consider the intrinsic meaning of traditional in-
tramolecular KIEs. Two excellent examples of the applica-
tion of intramolecular KIEs are represented in mechanistic
studies of the singlet oxygen ene reaction and the Baeyer-
Villiger oxidation. In an early example of the power of intra-
molecular KIEs, Grdina et al. reported a very elegant study
of the singlet oxygen ene reaction. Using isotopomers of
tetramethylethylene-d6 as mechanistic probes, they provided
evidence that suggested the formation of a symmetric perep-
oxide intermediate during the rate-determining step.24 This
result was at variance with early computational work by
Goddard et al., who found that the perepoxide possessed too
much potential energy to be a viable intermediate within the
reaction pathway for the reaction.27 Recently, Singleton et al.
resolved this discrepancy by measuring intramolecular 13C
KIEs and computing the potential energy surface using high-
level ab initio methods in conjunction with density func-
tional theory.25 Their findings interpret the intramolecular
KIEs as arising from the presence of a valley-ridge inflection
instead of a stable perepoxide intermediate. Structurally, the
valley-ridge inflection found in this study does resemble the
putative perepoxide. In an application of intermolecular and
intramolecular 2H and 13C KIEs, Singleton and Szymanski
provided evidence for rate-limiting addition of peroxyacid
to the cyclohexanone followed by product-determining mi-
FIGURE 2. Two distinct first-order saddle points for the syn-β-
elimination step investigated here computed with B3LYP/6-31þG-
(d,p): (A) endo-TS1 and (B) exo-TS1.
TABLE 1. Structural and Energetic Metrics of the endo-TS1 and exo-
TS1 Transition Structures
r1 (A) r2 (A) r3 (A) r4 (A) r5 (A) iν‡(cm-1
)
Erel
a
˚
˚
˚
˚
˚
endo-TS1 2.241 1.657 1.816 1.189 1.337
exo-TS1 2.225 1.656 1.802 1.189 1.335
-390
-367
0.5
0.0
aRelative computed gas phase [B3LYP/6-31þG(d,p)] energies in
kcal/mol.
between the exo-TS1 and endo-TS1 was substantially larger
than the energy differences that give rise to the primary
2H KIE, then no intramolecular KIE would be observed.
Abstraction of the pro-S or pro-R positions would be man-
dated by the stereochemistry of the sulfur ylide. As can be seen
in Table 1, the two competing transition structures do indeed
have similar structural and energetic properties. As a conse-
quence, the effect of isotopic substitution upon the relative free
energies of reaction manifest themselves in the observed KIE.
As a first approximation of the magnitude of isotope
effects that one might expect, it is instructive to compute
the ratio of the primary and secondary 2H KIEs for both the
endo-TS1 and exo-TS1 structures. These ratios are 2.19 and
2.26, respectively. These values contain a tunnel correction
computed using the first term of the Bell28 expression and a
frequency scaling factor of 0.9806.29 The computed ratios
2
gration.26 Intramolecular H KIEs in the Swern oxidation
would be simple to interpret if the sulfur center were planar in
2 and 3 (making them equivalent). In such a situation, the
observed intramolecular 2H KIE would simply be the ratio of
the 1°-KIE to the 2°-KIE at the benzylic position.
The Swern oxidation poses additional challenges regard-
ing the interpretation of the observed experimental intramo-
lecular KIEs. Because of the stereogenic sulfur center present
in the ylide, abstraction of the pro-S and pro-R benzylic
hydrogen atoms represents distinct first-order saddle points
on the potential energy surface for this reaction. These two
transition structures can be classified as endo-TS1 and exo-
TS1 (Figure 2A,B, respectively). If the energy difference
(27) Harding, L. B.; Goddard, W. A., III. J. Am. Chem. Soc. 1977, 99,
4520–4523.
(28) Bell, R. P. The Tunnel Effect in Chemistry; Chapman and Hall:
New York, 1980.
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SCHEME 4. Kinetic Scheme Employed for Testing the Two
Limiting Curtin-Hammett Scenarios
deviate quite strongly from the measured intramolecular
KIE. In considering the effects that both transition states
have upon the observed KIE, it is instructive to consider two
limiting Curtin-Hammett kinetic regimes (Scheme 4).30 The
first kinetic scheme (Figure 3A) we employ assumes forbid-
den stereoinversion for the sulfur ylides (2 and 3). The other
limiting case is facile stereoinversion at the sulfur center for 2
and 3 (Figure 3B).
FIGURE 3. Limiting Curtin-Hammett scenarios: (A) forbidden
stereoinversion and (B) facile stereoinversion of ylides 2 and 3.
for the unstabilized ylide derived from Eastman’s sulfonium
salt yields an enthalpic barrier of 28.0 kcal/mol.32 Other
measurements of sulfur ylide inversion barriers have yielded
similar values.33 Calculations of the potential energy differ-
ences between the ground state of 7 and the transition structure
for stereoinversion yielded a similar value of 36.3 kcal/mol.
Estimates of the inversion barriers for these two ylides
suggest that the barrier for inversion between 2 and 3 is high
enough to prohibit any substantial stereoinversion at any of
the temperatures explored in the present study.
½4ꢀ k2 þ k4
KIE ¼
¼
ð1Þ
½5ꢀ k1 þ k3
½4ꢀ k1k2 þ 2k2k4 þ k3k4
¼
KIE ¼
ð2Þ
½5ꢀ k1k2 þ 2k1k3 þ k3k4
Under the assumption of facile stereoinversion of the
sulfur center in 2 and 3 (kinv . k1 - k4), the ratio of
benzaldehyde-d to benzaldehyde is given by eq 1. The
2
computed intramolecular H KIE under this assumption is
2.21, which is substantially smaller than the experimen-
tally determined value. Under the assumption of forbidden
stereoinversion, the intramolecular KIE is given by eq 2. The
2
computed intramolecular H KIE under this assumption is
Experimental data from other researchers and computa-
tonal data collected in the present study suggest that inter-
conversion between 2 and 3 is likely to be attenuated to such
a degree that would allow it to be ignored. It is surprising,
then, that the forbidden stereoinversion yields a computed
KIE that deviates so strongly from the experimentally deter-
mined value. Several possible reasons for this disagreement
are plausible. First, it is conceivable that the energy differ-
ence between endo-TS1 and exo-TS1 is not estimated accu-
rately by the B3LYP34 functional when paired with the
6-31þG(d,p)35 basis set. Second, though the syn-β-elimina-
tion step studied here appears to be pericyclic (vide infra), it
may be that a proper solvent model would change the fre-
quencies corresponding to the transition structures in such a
1.58, which is egregiously lower than the experimentally
determined value. It would be a reasonable hypothesis to
assume that these results imply that stereoinversion of the
sulfur ylide is, in fact, quite facile in the Swern oxidation and
that tunneling beyond what might be computed using a
simple one-dimensional correction might be the cause of
disagreements between the experimental and calculated
KIEs. Such a supposition, if it were true, would seem contra-
dictory to the expectation that chiral sulfur ylides are con-
formationally stable. An early measurement of the rate
constant for stereoinversion of stabilized sulfur ylides found
that the rate constant for the stereoinversion of 6 is 3.52 ꢁ
10-5 s-1 at 25 °C in dichloromethane.31 Temperature de-
pendence of the rate constant for inversion yielded an en-
thalpy of activation of 23.3 kcal/mol. A similar measurement
(32) Roush, D. M.; Heathcock, C. H. J. Am. Chem. Soc. 1977, 99, 2337–
2338.
(33) Scartazzini, R.; Mislow, K. Tetrahedron Lett. 1967, 2719–2722.
(34) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (b) Stephens,
P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98,
11623–11627.
(35) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; DeFrees,
D. J.; Pople, J. A.; Gordon, M. S. J. Chem. Phys. 1982, 77, 3654–3665.
(29) Irikura, K. K.; Johnson, R. D., III; Kacker, R. N. J. Phys. Chem. A
2005, 109, 8430–8437.
(30) Seeman, J. L. Chem. Rev. 1983, 83, 83–134.
(31) Darwish, D.; Tomlinson, R. L. J. Am. Chem. Soc. 1968, 90, 5938–
5939.
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FIGURE 4. Dependence of the computed intramolecular KIE
upon the difference in electronic energies (kcal/mol) of endo-TS1
and exo-TS1.
FIGURE 5. Overlays of endo-TS1 and exo-TS1 structures com-
puted in the gas phase and optimized using a polarizable continuum
with a dielectric constant corresponding to dichloromethane.
way that the intramolecular KIE is more exalted. A third
possibility is that dynamical effects that are not included in
our semiclassical transition state theory treatment are op-
erative. A fourth possibility is that the effects of hydrogen
tunneling upon the KIE in this reaction step are not amen-
able to a simple Bell correction. Below, we will systematically
explore these possibilities.
Thus, it appears that changing the relative electronic energies
of the two distinct transition structures cannot produce a com-
puted KIE as large as the experimentally determined value.
Another consideration in our discussion of disparities
between computed versus measured intramolecular 2H KIEs
is the validity of utilizing density functional theory to treat
the present system. The endo-TS1 structure places the S-Me
group somewhat near the face of the phenyl ring in the sub-
strate. Dispersion interactions, which are mediated by elec-
tron correlation, are not reproduced by the hybrid B3LYP
functional. Inclusion of dispersion interactions may be ex-
pected to alter the differences between Eendo-TS1 and Eexo-TS1
relative to what is found using B3LYP. This consideration is
raised in light of Figure 4, which illustrates that the difference
between Eendo-TS1 and Eexo-TS1 can have profound effects
upon the computed KIE. In fact, using Grimme’s B97D38
functional with a dispersion correction alters the energy
difference between endo-TS1 and exo-TS1 such that the
endo-TS1 becomes slightly lower in potential energy than
exo-TS1. However, the change is not substantial, and the
overall effect is a lowering of the expected intramolecular
KIE under the assumption of no stereoinversion at the sulfur
center to 1.47 (versus 1.52 for B3LYP) in the gas phase.
Calculations involving a polarizable continuum model for
CH2Cl2 yield 1.42 using the B97D functional and 1.63 using
the B3LYP functional. In summary, it appears that disper-
sion forces do affect the relative energies corresponding to
the two energetically distinct transition structures but do not
appreciably affect the computed intramolecular 2H KIE.
Solvent Effects upon Intramolecular KIEs. Another possi-
ble cause of disagreement between experiment and theory is
the use of gas phase calculations to model reactions occur-
ring in solution. If there is considerable charge buildup or
attenuation at the transition states, it might be that optimi-
zation of the transition structures in the presence of a
continuum model would yield transition structures that
differ substantially from those optimized in the gas phase.
Figure 5 shows overlays of the endo-TS1 and exo-TS1
structures optimized in the presence of a dielectric con-
tinuum model of dichloromethane and in the gas phase. For
these calculations, the IEFPCM model was employed.39
A discrete sphere was employed for the transferred hydrogen
The B3LYP functional has proven to be very good at
reproducing frequencies.36 In fact, the vast majority of KIE
calculations, which rely heavily upon accurate force constant
estimates, utilize the B3LYP functional.37 Computing intra-
molecular KIEs here requires that accurate estimates of the
relative energies of the saddle points are obtained. The
B3LYP functional has proven only moderately useful in
the prediction of barrier heights in pericyclic reactions. How-
ever, the problem treated here is less exacting than the cal-
culation of barrier heights. Instead, we are computing the
relative electronic energies of the first-order saddle points
represented by endo-TS1 and exo-TS1. Because these two
transition structures are the same in kind and differ only
conformationally, it is likely that relative energy calculations
are reasonably accurate. In other words, it is likely that the
errors inherent in estimating the electronic energies of the two
transition states are of similar origin and magnitude. As a con-
sequence, calculations of the differences in energy between
these two structures are likely to benefit from canellation of
error. It is worthwhile, though, to explore the degree to which
the computed KIE under the assumption of forbidden stereo-
inversion varies as a function of the difference between Eendo-
TS1 and Eexo-TS1 (Figure 4). The maximum intramolecular KIE
under the assumption of forbidden inversion is 2.20, and this
value is obtained when Eendo-TS1 and Eexo-TS1 are equivalent.
The intramolecular KIE under the assumption of facile inver-
sion is nearly invariant to the energy difference between the
two competing transition structures. This situation is obtained
because the ratios of primary to secondary 2H KIEs for both
endo-TS1 and exo-TS1 are nearly equivalent (k2/k1 ≈ k4/k3).
(36) Salomon, O.; Reiher, M.; Hess, B. A. J. Chem. Phys. 2002, 117, 4729–
4737.
(37) A few examples: (a) Beno, B. R.; Houk, K. N.; Singleton, D. A.
J. Am. Chem. Soc. 1996, 118, 9984–9985. (b) Meyer, M. P.; DelMonte, A. J.;
Singleton, D. A. J. Am. Chem. Soc. 1999, 121, 10865–10874. (c) Christian,
C. F.; Takeya, T.; Szymanski, M. J.; Singleton, D. A. J. Org. Chem. 2007, 72,
6183-6189. (d) Saettel, N. J.; Wiest, O.; Singleton, D. A.; Meyer, M. P.
J. Am. Chem. Soc. 2002, 124, 11552–11559. (e) Stafford, S. E.; Meyer, M. P.
Tetrahedron Lett. 2009, 50, 3027–3030. (f) Saavedra, J.; Stafford, S. E.;
Meyer, M. P. Tetrahedron Lett. 2009, 50, 1324–1327. (g) Zhu, H.; Clemente,
F. R.; Houk, K. N.; Meyer, M. P. J. Am. Chem. Soc. 2009, 131, 1632–1633.
(38) Grimme, S. J. Comput. Chem. 2006, 27, 1787–99.
(39) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999–
3094.
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TABLE 2. Structural and Energetic Metrics of the endo-TS1 and
exo-TS1 Transition Structures Optimized within a Polarizable Continuum
Model of CH2Cl2 (Structural Parameters Correspond to Those Shown in
Table 1)
TABLE 4. Intramolecular 2H KIEs Measured at -23 °C as a Function
of Solvent, Computed KIEs under Assumptions of No Stereoinversion and
Facile Stereoinversion, and Ratios of Primary to Secondary 2H KIEs for
the Two Distinct Transition Structures
a
r1 (A) r2 (A) r3 (A) r4 (A) r5 (A) iν‡ (cm-1
)
Erel
CH2Cl2
8.9
1.14
CCl4
PhCl C2H4Cl4 CHCl3
˚
˚
˚
˚
˚
ε
μ
2.2
0.00
5.6
1.54
10.1
1.80
4.7
endo-TS1 2.296 1.649 1.864 1.174 1.343
exo-TS1 2.273 1.649 1.866 1.175 1.345
-286
-272
0.4
0.0
1.15
1.98
1.52
1.70
1.67
1.76
2H KIE (expt.)
2.08 ( 0.07 2.17 ( 0.04 2.14
2.08
1.52
1.64
1.61
1.71
aRelative computed gas phase [B3LYP/6-31þG(d,p)] energies in
2H KIE (no inversion) 1.53
1.51
1.81
1.75
1.86
1.41
1.68
1.64
1.74
kcal/mol.
2H KIE (inversion)
1.65
(1°/2°) KIE (endo-TS1) 1.62
(1°/2°) KIE (exo-TS1) 1.72
TABLE 3. Comparison of Charges Obtained from Natural Population
Analysis on the Component Atoms of the Five-Membered endo-TS1 and
exo-TS1 Transition Structures Optimized in the Gas Phase and Using a
Polarizable Continuum Model of CH2Cl2
polarizable continuum in both transition structures are the
oxygen atom and the hydrogen-abstracting carbon atom. The
screening effect of the dielectric allows for the development of
greater negative charge with concomitant lengthening of the
S-O bond. Effectively, the transition state is later due to this
change, but this effect is not propagated into the nascent and
breaking bonds to the transferred hydrogen atom. In sum-
mary, the data provided by the PCM calculations suggest that
disparities between calculation and experiment cannot be
rectified by including electrostatic solvent effects in the calcu-
lations.
gas phase structures
CBenzyl
H
CDMS
S
O
endo-TS1
exo-TS1
-0.090
-0.090
0.204
0.206
-0.803
-0.807
0.719
0.724
-0.685
-0.689
polarizable continuum (CH2Cl2) structures
O
CBenzyl
H
CDMS
S
endo-TS1
exo-TS1
-0.097
-0.100
0.203
0.206
-0.766
-0.774
0.720
0.726
-0.737
-0.737
A third possible hypothesis for the observed deviations of
experimental numbers from transition state theory calcula-
tions is dynamical corner-cutting.41,42 Conceptually, this
could arise in the following way: The rate-limiting step pre-
cedes the syn-β-elimination step being studied here. The ylide
produced from the rate-limiting step could retain vibrational
energy in crossing the dividing surface between the ylide and
the products. The resulting alteration of the geometry at which
most reaction flux occurs will change the manner in which
isotopic labeling affects the rate. Some recent studies on
small molecule gas phase hydrogen abstractions have shown
that both primary and secondary kinetic isotope effects can
deviate strongly from transition state theory predictions;
largely as a result of dynamical avoidance of the first-order
saddle point.43,44 Perhaps the best way to test this hypothesis
is to perform intramolecular KIE measurements in solvents
that have different effects upon vibrational relaxation life-
times. The physical properties of a solvent such as the
dielectric constant, dipole moment, and vibrational density
of states can drastically affect the rate of vibrational relaxa-
tion in solutions. Solvents with the smallest degree of cou-
pling to the reacting species should yield results that deviate
most strongly from transition state theory. Solvents with a
high density of states should facilitate vibrational relaxation
in going from the transition state for deprotonation to the
ylide intermediate.
atom. Table 2 shows how structural and energetic elements
within the two transition structures are altered.
There are three main alterations that are induced by
optimization using a polarizable continuum model. Structur-
ally, it appears that the only bond that changes appreciably is
the S-O bond, which becomes longer due to dielectric
screening of the positive charge upon sulfur and the negative
charge upon oxygen. Perhaps the most substantial change
can be seen in the imaginary frequencies. The presence of a
polarizable continuum reduces the imaginary frequencies of
both endo-TS1 and exo-TS1 transition structures by approx-
imately 100 cm-1. Finally, the energies of the two transition
structures become closer by a small amount. Given the sub-
stantially smaller magnitude of the imaginary frequencies for
transition structures optimized within a polarizable con-
tinuum as compared to gas phase frequencies, it seems reason-
able to expect the absolute magnitude of the computed KIE
to decrease. In fact, the intramolecular 2H KIE decreases to
1.89 under the facile stereoinversion scenario. This dimin-
ished value results from the attenuation of the ratio of the
primary and secondary 2H KIEs in both transition structures
to 1.72 and 1.85 for exo-TS1 and endo-TS1, respectively.
Due to a smaller difference in electronic energy between both
endo-TS1 and exo-TS1 transition structures, the intramolec-
2
ular H KIE increases to 1.63 under the forbidden stereo-
inversion scenario. This value, while higher than the value of
1.58 computed using gas phase calculations, is still quite
distant from the experimental value of 2.82. Analysis of
the charge distribution using natural population analysis40
offers another means of understanding the effects of the
polarizable continuum upon endo-TS1 and exo-TS1 transi-
tion structures. Table 3 shows the partial charges upon the
five atoms undergoing substantial bonding changes in gas
phase and polarizable continuum structures. The only atoms
whose charges are altered substantially by the presence of a
Table 4 shows the intramolecular KIEs measured in
different chlorinated solvents. We performed five replicates
of measurements in both CH2Cl2 and CCl4 based on the
knowledge that the dipole moments, dielectric constants,
and vibrational densities of states of these two solvents are
(41) Parr, C. A.; Polanyi, J. C.; Wong, W. H. J. Chem. Phys. 1973,
58, 5–20.
(42) Hartke, B.; Manz, J. J. Am. Chem. Soc. 1988, 110, 3063–3068.
(43) Marinkovic, M.; Gruber-Stadtler, M.; Nicovich, J. M.; Soller, R.;
€
€
Mulhauser, M.; Wine, P. H.; Bache-Andreassen, L.; Nielsen, C. J. J. Phys.
Chem. A 2008, 112, 12416–12429.
€
€
(44) Gruber-Stadtler, M.; Mulhauser, M.; Sellevag, S. R.; Nielsen, C. J.
(40) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83,
735–746.
J. Phys. Chem. A 2008, 112, 9–22.
J. Org. Chem. Vol. 75, No. 23, 2010 8093

Giagou and Meyer
JOCArticle
!
!
H,H
k
k
1
1
endo
H,H
exo
þ
2° - KIEexo 2° - KIEendo
½4ꢀ
½5ꢀ
KIE ¼
¼
ð3Þ
H,H
endo
H,H
exo
k
k
1
1
þ
1° - KIEexo 1° - KIEendo
½4ꢀ
KIE ¼
½5ꢀ
!
!
H,H
H,H
exo
H,H
endo
1° - KIEexo
2° - KIEendo
1° - KIEendo 1° - KIEexo 1° - KIEendo
k
k
k
k
endo
H,H
exo
þ 2
þ
2o - KIEexo
2° - KIEendo 2° - KIEexo
!
!
¼
ð4Þ
H,H
endo
H,H
exo
1° - KIEexo
2° - KIEendo
1° - KIEendo
2° - KIEexo
k
k
H,H
k
k
exo
H,H
endo
þ2 þ
very different. We performed these experiments at -23 °C,
which is the freezing point of carbon tetrachloride, the solvent
with the highest freezing point of the solvents tested. We found
that, while there was some difference in intramolecular KIEs
measured in dichloromethane and carbon tetrachloride, the
differences were not substantial. To explore solvent effects
in ethyl trichloroacetate were shown to differ by a factor of 2
between the two solvents.46 Given the fundamentally differ-
ent way in which these solvents facilitate vibrational relaxa-
tion, it seems reasonable to have expected a much larger
solvent effect upon the intramolecular 2H KIE if inefficient
vibrational relaxation were responsible for the substantial
differences between measured and computed KIEs.
2
further, we then measured intramolecular H KIEs once in
dichloroethane, chlorobenzene, and chloroform. The intramo-
Temperature Dependence of Intramolecular KIEs. KIEs are
among the most successful experimental diagnostics for hydro-
gen tunneling phenomena.47,48 Perhaps one of the most often
employed metrics for the importance of tunneling to hydrogen
transfer is the magnitude of the primary 2H KIE. The maximum
2
lecular H KIE measured in chlorobenzene was close to the
average value determined in carbon tetrachloride. This is
surprising given the marked difference between the two solvents
in terms of dipole moment and vibrational density of states. By
contrast, chloroform and dichloromethane yielded different
2
primary H KIE to be expected from semiclassical transition
2
intramolecular H KIEs but have similar dielectric constants
state theory is approximately kH/kD = 7 at 25 °C. When this
value is extrapolated to -78 °C under the assumption that the
isotope effect largely results from isotopic perturbation of ΔHq, a
value of 19 is obtained. While the intramolecular KIEs reported
here are not fully expressed primary 2H KIEs, the kinetic
expressions for the observed intramolecular 2H KIE measured
here can be expanded into expressions that are dominated by
primary 2H KIE factors. Equation 3 is the expression for the
intramolecular KIE under the assumption of facile inver-
sion, and eq 4 is the same expression under the assumption of
no stereoinversion. The kH,H factors are rate constants
corresponding to the perprotiated transition structures.
The measured intramolecular KIE reported here (2.82 (
0.07) is an order of magnitude smaller than the semiclassical
maximum extrapolated to -78 °C. On the basis of the magni-
tude of the intramolecular KIE alone, there is no reason to
and dipole moments. In total, it became apparent that solvent
effects upon the experimental intramolecular 2H KIE appear to
be small and yield no observable trend.
It is useful to question whether one should expect a solvent
effect within the array of solvents employed in the measure-
ments reported in Table 4. To answer this question, one should
consider the ways in which solvent differences might impact the
observed KIEs. Solvent dipole moment or dielectric constant
could certainly be expected to perturb transition structure;
however, this perturbation does not appear to be substantial,
given that polarizable continuum models do not predict sub-
stantial differences among the solvents in ratios of primary to
secondary 2H KIEs for the endo-TS1 and exo-TS1 transition
structures. As these values ultimately largely determine the
2
observed intramolecular H KIE, it could be expected that
dipole moment and dielectric constant should not yield distinct
differences among KIEs measured in these solvents.
(46) Navarro, R.; Hernanz, A.; Bratu, I. J. Chem. Soc., Faraday Trans.
1994, 90, 2325–2330.
The question inherent to understanding potential dy-
namical differences among the solvents can be related to other
researchers’ measurements of vibrational relaxation in chlo-
rinated solvents. Measurements of vibrational relaxation of
the carbonyl chromaphore in tungsten hexacarbonyl in CCl4
and CHCl3 show very different behaviors between the two
solvents.45 In fact, the temperature dependence of the vibra-
tional lifetimes of the carbonyl chromaphore is opposite for
CCl4, where the relaxation time decreases with increasing
temperature, compared to CHCl3, where the relaxation time
increases with increasing temperature. Similarly, in another
study, the vibrational lifetimes of the carbonyl chromaphore
(47) Reviews: (a) Kohen, A.; Klinman, J. P. Chem. Biol. 1999, 6, R191–
R198. (b) Kohen, A. Prog. React. Kinetics Mech. 2003, 28, 119–156. (c) Pu, J.;
Gao, J.; Truhlar, D. G. Chem. Rev. 2006, 106, 3140–3169. (d) Caldin, E. F.
Chem. Rev. 1969, 69, 135–156. (e) Nagel, Z. D.; Klinman, J. P. Chem. Rev.
2006, 106, 3095–3118. (f) Gunay, A.; Theopold, K. H. Chem. Rev. 2010, 110,
1060–1081. (g) German, E. D.; Sheintuch, M. J. Phys. Chem. 2010, 114,
3089–3097. (h) Hammes-Schiffer, S. Acc. Chem. Res. 2006, 39, 93–100.
(i) Kohen, A.; Klinman, J. P. Acc. Chem. Res. 1998, 31, 397–404.
(48) Recent examples: (a) Wang, Z.; Kohen, A. J. Am. Chem. Soc. 2010,
132, 9820–9825. (b) Edwards, S. J.; Soudackov, A. V.; Hammes-Schiffer, S.
J. Phys. Chem. B 2010, 114, 6653–6660. (c) Cho, J.; Woo, J.; Nam, W. J. Am.
Chem. Soc. 2010, 132, 5958–5959. (d) Panay, A. J.; Fitzpatrick, P. F. J. Am.
Chem. Soc. 2010, 132, 5584–5585. (e) McCusker, K. P.; Klinman, J. P. J. Am.
Chem. Soc. 2010, 132, 5114–5120. (f) Yoon, M.; Song, H.; Hakansson, K.;
Marsh, E. N. G. Biochemistry 2010, 49, 3168–3173. (g) Finnegan, S.;
Agniswamy, J.; Weber, I. T.; Gadda, G. Biochemistry 2010, 49, 2952–2961.
(h) Fukuzumi, S.; Kobayashi, T.; Suenobu, T. J. Am. Chem. Soc. 2010, 132,
1496–1497.
(45) Moore, P.; Tokmakoff, A.; Keyes, T.; Fayer, M. D. J. Chem. Phys.
1995, 103, 3325–3334.
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Giagou and Meyer
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TABLE 5. Intramolecular 2H KIEs Measured in Dichloromethane As a
Function of Temperature, Computed KIEs under Assumptions of No
Stereoinversion and Facile Stereoinversion, And Ratios of Primary to
Secondary 2H KIEs for the Two Distinct Transition Structures (All
Structures Used To Compute KIEs Were Optimized in the Presence of a
Polarizable Continuum Model for Dichloromethane)
-78 °C
-63 °C
-47 °C
-23 °C
2H KIE (expt.)
2H KIE (sim.)a
2H KIE
2.82 ( 0.06 2.37 ( 0.05 2.22 ( 0.02 2.08 ( 0.07
2.76
1.63
2.44
1.60
2.23
1.57
2.04
1.53
(no inversion)
2H KIE (inversion) 1.89
1.81
1.77
1.74
1.70
1.65
1.62
(1°/2°) KIE
(endo-TS1)
(1°/2°) KIE
(exo-TS1)
1.85
2.00
1.90
1.82
1.72
aComputed intramolecular KIE under the assumption of no stereo-
inversion at the stereogenicsulfur center with amplification factors of 2.3
and 2.6 for the imaginary frequencies corresponding to deuterium
abstraction (k1 and k3) and protium abstraction (k2 and k4), respectively.
FIGURE 6. Arrhenius plots of the computed expressions for the
intramolecular 2H KIE as given by eqs 1 (open squares) and 2
(closed squares) and the ratios of primary to secondary 2H KIEs for
both endo-TS1 (closed circles) and exo-TS1 (open circles). Black
lines are linear fits to each set of data.
believe that tunneling is contributing substantially to the
observed KIE. Furthermore, the reduced masses for the
imaginary modes in the transition structures for the syn-
β-elimination involved in the oxidation of benzyl alcohol are
4.39 for exo-TS1 and 3.62 for endo-TS1. By contrast, most
hydrogen transfer reactions have reduced masses near unity.
Neither the magnitude of the observed intramolecular KIE nor
the computed reduced mass corresponding to the imaginary
mode in the computed transition structures suggest that tunnel-
ing should be responsible for the disagreement between the
computed and measured intramolecular KIEs reported here.
2
The temperature dependence of primary H KIEs is also
frequently usedasadiagnosticforthe importance oftunneling
to the magnitude of the observed KIE.28,47,49,50 Curvature in
plots of ln(1°-KIE) versus 1/T is indicative of tunneling
having a significant effect upon the observed KIE. Upon the
basis of previous work, it is quite reasonable to expect tunnel-
FIGURE 7. Arrhenius plots of the experimentally determined in-
tramolecular 2H KIE (closed circles), computed KIEs given by eqs 1
(open squares) and 2 (closed squares) and KIEs computed from eq 2
employing static multipliers of 2.3 and 2.6 for the imaginary
frequencies corresponding to deuterium and protium transfer,
respectively (open circles). The black line is a linear fit to the
experimental KIE measurements.
2
ing to affect the primary H KIEs to a greater degree than
secondary 2H KIEs.51,52 Given that eqs 3 and 4 are dominated
by factors that effectively resemble the ratio of a primary to a
2
secondary H KIE, one would expect a plot of ln(KIEIntra
)
transition structures has a very small upward curvature. This
is not surprising, given that there is a tunneling correction
applied to these KIE calculations. What is surprising is that the
Bell tunneling correction is nearly the same for transfer of H
and D in the endo-TS1 (Qt,H =1.171; Qt,D =1.170) and exo-
TS1 (Qt,H = 1.188; Qt,D = 1.187) transition structures opti-
mized in a polarizable continuum model for dichloromethane.
The same trend is present in gas phase calculations and in other
polarizable continuum model solvents and appears to be a
general feature of transition structures for the Swern oxida-
tion. Presumably because of the composite nature of eqs 3 and
4 upon individual KIE ratios, computed expectations of the
intramolecular KIE under assumptions of facile inversion and
forbidden inversion have a slight downward curvature.
versus 1/T to be significantly curved if tunneling factored into
the observed intramolecular 2H KIE significantly. Of course,
one possible origin of the minute curvature observed in the
Arrhenius plot could be the composite nature of the expres-
sions for the intramolecular KIE (eqs 1 and 2). To gain insight
into how one might expect the expressions in eqs 1 and 2 to
depend upon temperature, we constructed Arrhenius plots of
the computed composite expressions for the natural logarithm
of the intramolecular 2H KIE versus inverse temperature. As a
point of comparison, we also plotted the natural logarithms of
the ratios of the primary to secondary 2H KIEs for both endo-
TS1 and exo-TS1 versus inverse temperature, as these ratios
are component parts of the expressions shown in eqs 3 and 4
(Figure 6). The ratio of primary to secondary KIEs for the two
Having exhausted all viable hypotheses, except that the
primary KIE is exalted beyond what is computed using
semiclassical transition state theory augmented with a one-
dimensional tunnel correction, we explored the temperature
(49) Limbach, H.-H.; Lopez, J. M.; Kohen, A. Philos. Trans. R. Soc.
London, Ser. B 2006, 361, 1399–1415.
(50) Kwart, H. Acc. Chem. Res. 1982, 15, 401–408.
(51) Rickert, K.; Klinman, J. P. Biochemistry 1999, 38, 12218–12228.
ꢀ
2
dependence of the intramolecular H KIE (Table 5 and
Figure 7). We also attempted to compute the KIE under the
(52) Ostovic, D.; Roberts, R. M. G.; Kreevoy, M. M. J. Am. Chem. Soc.
1983, 105, 7629–7631.
J. Org. Chem. Vol. 75, No. 23, 2010 8095

Giagou and Meyer
JOCArticle
occurs at greater donor/acceptor distances than deuterium
tunneling. Likewise, the tunneling pathway for protium pro-
ceeds under regions of greater potential energy than those
corresponding to deuterium transfer. These two dominant
characteristics of corner-cutting tunneling directly affect
what might be thought of as an effective imaginary frequency
corresponding to the point on the potential energy surface at
which the tunneling pathways cross from the reactant por-
tion of the surface to the products region.
Comparison with Other Reactions. In addition to the Cope
elimination13,15,18,21,22 and sulfoxide elimination,13,23 the
Wittig modification of the Hofmann elimination11,12,17 has
been shown to proceed via five-membered syn-β-elimation in
many cases. Kwart has argued for a similar transition state in
the base-promoted thermolyses of nitrates; however, this
argument was based entirely upon the magnitude of the pri-
mary 2H KIE exhibited.56 How do intramolecular 2H KIEs
in these systems compare with the syn-β-elimation step in the
Swern oxidation?
FIGURE 8. Pictorial representation of corner-cutting tunneling.
The blue path represents the minimum energy path, while the red
path denotes a representative corner-cutting tunneling pathway.
assumption of no inversion by amplifying the magnitudes of νq
corresponding to deuterium transfer (k1 and k3) by a factor of
2.3 and νq corresponding to protium transfer (k2 and k4) by a
factor of 2.6. While this is an ad hoc correction, it is satisfying
that, by using only two adjustable parameters, we can repro-
duce the temperature dependence of a complex kinetic expres-
sion that involves four transition states.
The Cope elimination has been studied using intramolec-
2
ular H KIEs and computational methods. Bach et al. re-
Fitting the experimental data shown in Figure 7 to the
Arrhenius equation yields AH/AD =0.71 and DEa - HEa =
0.52 kcal/mol. If we consider these parameters to largely
port transition structures for the Cope elimination of butyl-
amine oxide that are similar in kind to the endo-TS1 and exo-
TS2 structures reported here. These small model systems
were optimized using MP2 theory with the 6-31G* basis
2
reflect the primary H KIE, then one might say that the
isotope effect upon the Arrhenius prefactor, AH/AD, is close
to what is normally accepted as a lower limit for evidence of
tunneling.53 The difference in Ea for the two isotopes is well
within what might be expected owing only to differences in
zero-point energy. As Kreevoy has pointed out, this is an
overly simplistic view. Tunneling is no more exotic than zero-
point energy, as both of these effects are merely results of
wave/particle duality.54 More to the point of mechanistic
inquiry is the question of whether semiclassical transition
state theory is a suitable model. In other words, is it sufficient
to include quantum effects arising from zero-point energy
only and to assume that tunneling has only a negligible effect
upon observables such as rates or KIEs? Assuming that the
transition structures presented here are ample representa-
tions of the average geometry of the transition state ensem-
ble, it seems evident that semiclassical transition state theory
fails to reproduce the temperature dependence of the intra-
molecular 2H KIE measured here. The question then becomes
one of whether tunneling is best represented as occurring along
the minimum energy path or whether the average tunneling
path might be expected to deviate from this path.
How can the ad hoc imaginary frequency amplification
discussed above be understood conceptually? Frequency
amplification can be thought of as an effective reduction in
the reduced mass, an increase in the force constant magni-
tude, or some combination thereof.55 As such, this simple
correction reproduces in a curve-fitting fashion what might
be expected from tunneling corrections that take into ac-
count deviations from the minimum energy path. In what has
often been called corner-cutting tunneling (Figure. 8), the
pathway that describes the average trajectory for protium
transfer deviates from that which describes deuterium trans-
fer. In particular, the critical configuration for transfer of the
two isotopes differs in such a way that protium tunneling
2
set.21 These transition structures yield primary H KIEs of
3.37 at 120 °C including a one-dimensional Wigner tunneling
correction.57 Komaromi and Tronchet computed primary
2H KIEs for the Cope elimination of ethylamine oxide also
using MP2 theory but with a slightly larger basis set, 6-31G**.22
They report a computed primary 2H KIE of 2.56 at 125 °C not
including a tunnel correction. Unfortunately, they do not
report imaginary frequencies for the reaction coordinate at
the transition state as a function of isotope, so estimation of
the Wigner or Bell tunneling correction is precluded. How-
ever, applying the same tunnel correction used in the study
by Bach et al., the primary 2H value is elevated to 2.97. Bach
and Braden performed a very elegant measurement of both
the primary and secondary intramolecular 2H KIEs using of
intramolecular 2H KIEs in the Cope elimination of 8.18 They
2
report a primary and secondary H KIE of 2.23 and 1.22,
respectively, at 120 °C. Extrapolated to -78 °C, these values
are 5.03 and 1.49, respectively. Kwart and Brechbiel report a
measurement for the intramolecular KIE as kH/kD =2.778
for the Cope elimination of 9 at 120 °C.15 Extrapolated to
-78 °C, this value becomes 7.833. The value measured by
Kwart and Brechbiel is actually the ratio of primary to
2
2
secondary H KIEs. In a system where the secondary H
KIE might be expected to be normal and substantial, this
implies a sizable primary KIE. In any course, it can be seen
that the extrapolated KIEs are well in excess of those
measured here.
(53) Schneider, M. E.; Stern, M. J. J. Am. Chem. Soc. 1972, 94, 1517–
1522.
(54) Kim, Y.; Kreevoy, M. M. J. Am. Chem. Soc. 1992, 114, 7116–7123.
(55) Truhlar, D. G. J. Phys. Chem. A 2003, 107, 4006–4007.
Thermolyses of sulfoxides to yield an alkene and sulfenic
acid are quite analogous to the syn-β-elimination step in the
8096 J. Org. Chem. Vol. 75, No. 23, 2010

Giagou and Meyer
JOCArticle
Swern oxidation. Kwart and co-workers have investigated
this reaction by performing intramolecular 2H KIE measure-
elimination. Bach and co-workers have examined this reac-
tion in cyclooctylammonium salts using a number of strong
2
2
ments14 on 10 and intermolecular H KIE measurements13
bases to create the ylide.11,12,20 Primary H KIEs typically
using 11 and 11-d9 as competing reactants. Unfortunately,
no mention is made of how the measured KIE is corrected for
elimination from the ethyl substituent in the competition
between 11 and 11-d9. If reported intermolecular KIEs do
not have this reaction pathway factored into the result, the
net effect would be that the actual KIE would be larger than
the one measured by monitoring the disappearance of 11 and
11-d9 versus an internal standard. In perhaps a more appro-
priate system for the exploration of intermolecular isotope
effects, Yoshimura et al. measured KIEs using 12 and 12-d3
as competing reactants.18 Intermolecular KIEs measure-
ments upon 11 and 12 yielded values of 5.4 at 112 °C and
5.15 at 80 °C, respectively. KIEs estimated from computed
transition structures produced values that were approxi-
mately 20% too small in each case; however, the computed
KIEs did not include any correction for tunneling. An intra-
molecular KIE of 3.17 at 130 °C was reported for the ther-
molysis of 10.14 The intramolecular KIE measured via the
conversion of 10, however, is more complex than the com-
petition between two pathways that differ only in the posi-
tion of the isotopic label. This system shares the complexity
inherent to the syn-β-elimination step in the Swern oxida-
tion. Four rate constants are likely to play a role in the ther-
molysis of 10, as well. It is worth noting that the only system
for which the computed 2H KIEs of Cubbage et al.23 agree
with experiment are those for which ethyl phenyl sulfoxide is
used as a model for 2-heptyl-d1 phenyl sulfoxide. It is not our
intention to question the choice of ethyl as a substitute for
heptyl. Instead, it is important to point out that agreement is
likely spurious for the following reasons. The calculations do
not include tunneling or relevant diastereomeric transition
states. This is, to some degree, reflected in the fact that the
computed KIE at higher temperatures is larger than the
measured value and smaller than the measured value at lower
temperatures. If tunneling were responsible for differences in
experiment and theory, one might reasonably expect com-
puted and measured values to converge asymptotically at
higher temperatures barring substantial contributions from
variational effects. In spite of the difficulties associated with
obtaining reliable interpretations of the physical processes
giving rise to KIEs measured in sulfoxide eliminations, it can
at least be surmised that the primary KIEs in sulfoxide
eliminations are of far greater magnitude at high tempera-
tures (ca. 100 °C) than those contributing to the intramolec-
ular KIEs in the Swern oxidation at low temperatures.
vary between 1.47 and 2.07 for eliminations leading to (Z)-
cyclooctene using KNH2 in liquid ammonia (-33 °C). Ex-
trapolated to -78 °C, these limits become 1.61 to 2.45. The
corresponding limits for eliminations leading to (E)-cyclooc-
tene are 2.53 and 5.89. Reactions that utilize alkyllithiums as
the base (at 25 °C) yield primary 2H KIEs between 1.26 and
1.52. Extrapolated to -78 °C, these values become 1.42 to
1.90. Gas phase calculations of the primary KIE upon a simple
model system yield estimates of kH/kD =1.69 at -33 °C and
1.53 at 25 °C.21 These estimates include a Wigner tunneling
correction but are still on the low end of the distribution of
measured values. Unfortunately, the temperature dependence
of this isotope effect was not investigated. It is worth noting that
this system does exhibit vanishingly small primary 2H KIEs. It
is likely that these values are even smaller than the primary
2H KIEs that are inherent in the intramolecular KIE mea-
surements reported here. The principle difference between
the transition structure models computed for the Wittig
modification of the Hofmann elimination and the Cope
elimination is the degree of hydrogenic motion involved in
the normal mode corresponding to the imaginary frequency.
In this regard, the former reaction exhibits a noteworthy
similarity to the syn-β-elimination step in the Swern oxida-
tion. This seems to point to a scenario in the former reaction
whereby the donor and acceptor atoms are moving together
in concert with hydrogen transfer.
Results for the Wittig modification of the Hofmann elimi-
nation serve as a bridge to a pericyclic reaction in which
multidimensional tunneling has been implicated. More than
€
four decades ago, Roth and Konig measured the primary
deuterium KIE for the [1,5] sigmatropic rearrangement of
1,3-pentadiene (5.1 at 200 °C).58 The surprisingly large value
of this KIE at an elevated temperature led theorists to
develop numerous models capable of reproducing this mea-
surement. Some 40 years later, Doering and Zhao explored
the temperature dependence of the [1,5] sigmatropic rear-
rangement in a compound that possesses an conformation-
ally rigid s-cis conformation of the double bonds involved in
the rearrangement.59 Surprisingly, Doering and Zhao found
2
that the temperature dependence of the primary H KIE
produced a linear Arrhenius plot. In early theoretical work,
Liu et al. found that small curvature tunneling corrections
were essential to reproducing the KIE measured by Roth and
Konig.60 In recent work, Shelton et al. have reproduced the
€
temperature dependence observed by Doering and Zhao and
have found that multidimensional tunneling is, in fact,
important to reproducing the temperature dependence of
the KIE.61 Curvature in the Arrhenius plot of the intramo-
lecular 2H KIE measured in the Swern oxidation (Figure 7) is
obvious and substantial. There are points at which the
physical behavior of the [1,5] sigmatropic rearrangement
and the syn-β-elimination in the Swern oxidation diverge.
Phenomenologically, the [1,5] sigmatropic rearrangement
The Wittig modification on the Hofmann elimination has
been found to proceed by at least two mechanisms. One is the
E2 pathway which directly involves the base in the rate-
limiting step. The other mechanism involves deprotonation
to form a nitrogen ylide followed by rate-limiting syn-β-
€
(58) Roth, W. R.; Konig, J. Liebigs Ann. Chem. 1966, 699, 24–32.
(59) Doering, W. v. E.; Zhao, X. J. Am. Chem. Soc. 2006, 128, 9080–9085.
(60) Liu, Y.-P.; Lynch, G. C.; Truong, T. N.; Lu, D.-H.; Truhlar, D. G.;
Garrett, B. C. J. Am. Chem. Soc. 1993, 115, 2408–2415.
(61) Shelton, G. R.; Hrovat, D. A.; Borden, W. T. J. Am. Chem. Soc.
2007, 129, 164–168.
(56) Kwart, H.; George, T. J.; Horgan, A. G.; Lin, Y. T. J. Org. Chem.
1981, 46, 5143–5147.
(57) Wigner, E. Trans. Faraday Soc. 1938, 34, 29–41.
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Giagou and Meyer
JOCArticle
value for sulfoxide thermolysis is of substantially smaller
magnitude than those for the syn-β-elimination step in
the Swern oxidation but still might be classified as having
substantial aromatic character. Cubbage et al. attribute the
behavior observed in sulfoxide eliminations to the fact that
the sulfur-oxygen bond in sulfoxides is best represented as a
single bond that is shortened due to Coulombic attraction
between the positively charged sulfur atom and the nega-
tively charged oxygen atom.23 By contrast, the ylide S-C
˚
double bond in both 2 and 3 changes by less than 0.001 A
upon changing from optimizations in the gas phase to those
including a polarizable continuum. This leads to the conclu-
sion that the ylides 2 and 3 are best represented as possessing
a S-C double bond versus the alternative single-bonded
resonance structure. Natural bond orbital66 (NBO) calcula-
tions support our assertion that ylides 2 and 3 are best re-
presented by their double-bonded resonance structure. In
contrast, NBO calculations of the model sulfoxide shown
above yield only one occupied orbital connecting the sulfur
and oxygen atoms. In conjunction with NICS values shown
in Figure 9, these observations suggest that the syn-β-elim-
ination step in the Swern oxidation is pericyclic.
Other well-studied pericyclic reactions involving hydrogen
transfer offer a point of comparison with the NICS values
reported in Figure 9. The [1,5] sigmatropic hydrogen shift
that occurs upon heating (Z)-1,3-pentadiene has served as a
previous comparison to the syn-β-elimination step in the
Swern oxidation. This reaction has been well-studied com-
putationally and has served as an exemplum in early discus-
sions of aromatic transition states.67 The NICS value com-
puted at the geometric center of the six-membered transition
structure for this reaction optimized at B3LYP/6-311þG
(d,p) is -16.6.68 This NICS value is smaller in magnitude
than that found for the pericyclic hydrogen transfer explored
here but is well in excess of that computed for the sulfoxide
elimination shown in Figure 9C. The transition structure for
the prototypical ene reaction between propene and ethylene
yields a NICS value of -24.4 in a structure optimized using
the B3LYP functional in conjunction with the 6-311þG(d,p)
basis set.69 The transition structures for these two inconically
pericyclic reactions yield NICS values that bracket those
computed for both endo-TS1 and exo-TS1 structures, while
they well exceed the NICS value computed for the transition
structure for sulfoxide elimination. These comparisons further
suggest that the syn-β-elimination step in the Swern oxidation is
pericyclic, yielding some insight into why this fundamental
reaction step is so facile.
FIGURE 9. NICS values for the (A) endo-TS1, (B) exo-TS1, and
(C) sulfoxide elimination (TS2) transition structures computed
using B3LYP/6-31þG(d,p).
2
has a substantially larger primary H KIE. In terms of the
physical origins that give rise to these differences, it seems
likely that the reduced mass for the [1,5] sigmatropic is much
smaller than that for the syn-β-elimination. Perhaps the best
correspondence in the literature at this time is the Wittig
modification to the Hofmann elimination. Unfortunately,
the temperature dependence of the primary KIE for this
reaction has not yet been determined.
Transition State Aromaticity and the β-Elimination. Given
the cyclic six-electron transition states inherent to five-
membered syn-β-elimination transition states, it is worth-
while to consider which aspects lead to aromaticity in the
transition state. We have employed the nuclear-independent
chemical shift (NICS) method developed by Schleyer and co-
workers to analyze the transition states for the syn-β-elim-
ination step in the Swern oxidation and the intramolecular
elimination of sulfoxides (see Scheme 2).62,63 Stable aromatic
rings are present in each structure to provide a NICS value
against which the NICS value at the center of the five-
membered transition state may be compared (Figure 9). Here,
we have utilized the GIAO formalism for computing NICS
values for the bystander aromatic ring and the geometric
center of the active atoms in the five-membered transition
structures.64 In order to compare the transition structures for
the Swern oxidation to other reactions, we attempted to
control as many factors as possible. The sulfoxide thermolysis
shown below allows direct computational comparison with a
reaction that may plausibly described as pseudopericyclic.23,65
Concluding Comments
In the study presented here, we have attempted to convey
several notions. First, the use of intramolecular KIEs as a
mechanistic probe is not limited to systems that exhibit sym-
metry. Computational analysis of the intramolecular KIE
Geometrically, the transition structure for the thermolysis
(TS2) shown in Figure 9 is similar to exo-TS1. The NICS
(65) Ross, J. A.; Seiders, R. P.; Lemal, D. M. J. Am. Chem. Soc. 1976, 98,
4325–4327.
(66) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88,
(62) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; van Eikema
Hommes, N. J. R. J. Am. Chem. Soc. 1996, 118, 6317–6318.
(63) Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer,
P. v. R. Chem. Rev. 2005, 105, 3842–3888.
(64) Wolinski, K.; Hilton, J. F.; Pulay, P. J. Am. Chem. Soc. 1990, 112,
8251–8260.
899–926.
(67) Woodward, R. B.; Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1969,
8, 781–932.
(68) Jiao, H.; Schleyer, P. v. R. J. Phys. Org. Chem. 1998, 11, 655–662.
(69) Manojkumar, T. K. J. Mol. Struct. (THEOCHEM) 2009, 909,
96–101.
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Giagou and Meyer
JOCArticle
becomes more complicated in asymmetric systems because of
the influence that relative barrier heights can have upon the
observed value. However, if the transition structures that
give rise to the observed KIE are similar in nature, then
computed relative barrier heights should be more reliable
than the estimation of an isolated barrier height or the com-
parison of barrier heights for two disparate reactions. Sec-
ond, we have provided compelling evidence for multidimen-
sional tunneling in a reaction that does not exhibit a 2H KIE
of large magnitude. Finally, we have attempted to under-
stand the facility with which the syn-β-elimination in the
Swern oxidation occurs. It appears that this fundamental
reaction step is likely to proceed via an aromatic transition
state. Furthermore, in comparison to sulfoxide elimination,
it appears that the nonzwitterionic resonance forms for the
ylides 2 and 3 best represent the reactivity of these species.
Here, we provide reliable experimental measurements of the
intramolecular 2H KIE for the Swern oxidation of benzyl
alcohol. We have also attempted to understand the fundamental
reactivity of ylides 2 and 3 as a function of temperature and
solvent. Our measurements provide an entry into further under-
standing of hydrogen transfer phenomena, multidimensional
tunneling, and aromatic transition states involving hydrogen
transfer. Study of the syn-β-elimination in the Swern oxidation
provides a point of comparison with the few existing mechanistic
studies of other syn-β-eliminations and other reactions with
aromatic transition states that involve hydrogen transfer. We
hope that these measurements will stimulate theoreticians to
undertake more detailed studies of the phenomena reported here.
four enantiomeric pairs. The intramolecular 2H KIE measured
in the current study is simply the ratio of 4 to 5 in the product.
2
Here, we compute the anticipated H KIE under two limiting
assumptions. The first assumption is rapid inversion of the
stereogenic sulfur center in 1. Equation 1 shows how the ratio of
4 to 5 is determined in terms of operative rate constants under this
assumption. Equation 5 illustrates how this expression can be
decomposed into an expression in terms of single rate constant
ratios. The second limiting regime is that under which the barrier
for inversion at the stereogenicsulfur center prohibitsinversion on
the time scale of the syn-β-elimination step. Under this assump-
tion, the ratio of 4 to 5 is given by eq 2. Equation 6 is an expression
of the KIE in terms of rate constant ratios which allows for the
calculation of the KIE from computed transition structures.
Derivations of eqs 1-6 are provided in the Supporting Information.
k2 þ k4
1
1
¼
þ
k2 k2 k4 k4
ð5Þ
k1 k3 k1 k3
þ
k1 þ k3
þ
k4
k1
k2 k4 k2
þ 2
2 þ
þ
k2 k4
k1k2 þ 2k2k4 þ k3k4
k1k2 þ 2k1k3 þ k3k4
k3 k1 k3
¼
ð6Þ
þ
k3 k1
Because the rate constant ratios, k1/k2 and k3/k4, reflect
isotopomeric ratios, they can be computed by employing the
Redlich-Teller product rule (eq 7).70 Because k2 and k3 (and k1
and k4) correspond to structurally and energetically distinct
transition states, the ratios k2/k3 and k4/k1 must be computed
using the ratios of the full expression for the molecular partition
function pertaining to the relevant transition states (eq 8). Be-
cause the reactants involved in the rate constant ratio expres-
sions are the same or diastereomeric by virtue of isotopic sub-
stitution in all cases, only partition functions for the transition
states need to be considered. Vibrational partition function
ratios of reactants are found to be very near unity for isotopic
diastereomers of the ylides. All structures used to compute rate
constant ratios were optimized using the B3LYP34 functional in
conjunction with the 6-31þG(d,p)35 basis set. Frequency calcula-
tions wereperformed at the same level of theory and used the same
basis set. For all rate constant ratio calculations, a one-dimen-
sional Bell tunneling correction was applied (eq 9).
Experimental Section
Intramolecular 2H KIEs. The substrate for this study, benzyl-
d1 alcohol was synthesized by reducing benzaldehyde with
NaBD4 in ethanol (see Supporting Information). Two series of
intramolecular 2H KIE measurements were performed. One
series explored the temperature dependence of the intramolec-
ular 2H KIE. The other explored the effect of solvent upon the
2
intramolecular H KIE at -23 °C. The choice of temperature
was necessitated by the melting temperature of CCl4. Intramo-
lecular 2H KIEs are reported as the ratio of benzaldehyde-d to
benzaldehyde as determined by quantitative 1H NMR using
calibrated 90° pulse widths and delays of 5ꢁT1. Longitudinal
relaxation times were determined using an inversion recovery
experiment. The intramolecular 2H KIE is computed from the
integrations of the aromatic peaks and the aldehyde peaks. The
ratio of benzaldehyde-d to benzaldehyde was corrected for con-
tamination of the benzyl-d1 alcohol substrates with perprotiated
3Nq
q
q
- 7 ui,2expðuiq,1=2Þ 1 - expðui,2
Þ
Þ
k2
k1
υ2q
υ1q
Y
Qt,2
Qt,1
¼
ð7Þ
uqi,1expðuqi,2=2Þ 1 - expðuiq,1
i ¼ 1
qqrot,2qq
υ2q
υq3 qqrot,3qvib,3
Qt,2
Qt,3
k2
k3
vib,2
q
¼ exp½ðEqElec,3 - EEqlec,2Þ=RTꢀ
ð8Þ
ð9Þ
hυq=2kBT
1
substrate using quantitative H NMR spectra of the benzyl-d1
Qt ¼
sinðhυq=2kBTÞ
alcohol substrates. Swern oxidations of benzyl-d1 alcohol at
-78, -63, -47, and -23 °C were performed using a dry ice/
acetone bath and the melting transitions of CHCl3, m-xylene,
and CCl4 as thermostatting mechanisms, respectively. It should
be noted here that no evidence of the Pummerer rearrangement
product was observed at any of the temperatures explored. Five
replicates of each experiment were performed. The effect of
Acknowledgment. M.P.M. thanks the NIGMS (GM87706-
01) for financial support. This research was also supported in
part by the National Science Foundation through TeraGrid
resources provided by PSC. M.P.M. thanks Charles Perrin
(UCSD) and David F. Kelley (UCM) for useful discussions
and Michael Colvin (UCM) for the use of his Linux cluster.
2
solvent upon the intramolecular H KIE was explored by per-
forming the Swern oxidation of benzyl-d1 alcohol in chloroben-
zene, dichloroethane, CHCl3, CH2Cl2, and CCl4. Five replicates
were performed for CH2Cl2 and CCl4, and one measurement
was taken for the other three solvents. Detailed experimental
procedures are provided in the Supporting Information.
Calculation of KIEs. The syn-β-elimination step that converts
the diasteromeric ylides (2 and 3) to a mixture of benzaldehyde-
d (4) and benzaldehyde (5) has eight transition states that exist as
Supporting Information Available: Details of the experi-
mental and computational methods, derivations of eqs 1-6, and
Cartesian coordinates for all computed structures. This material
is available free of charge via the Internet at http://pubs.acs.org.
(70) Wilson, E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrations;
Dover: New York, 1980.
J. Org. Chem. Vol. 75, No. 23, 2010 8099