4970 J. Am. Chem. Soc., Vol. 122, No. 20, 2000
Cubbage et al.
into a reactor whose volume controls the residence time, which is a
few seconds. Samples were injected as concentrated solutions in
acetonitrile. After the furnace section, the gases are sent to a GC that
operates at lower temperatures, where starting materials and products
are separated and quantified. Rate constants are extracted from each
run, and multiple injections were made at each temperature. For the
isotope effect measurements, the samples 1 and 2 were measured
alternately at each temperature to ensure accurate measurements of kH/
kD. All sulfones thermalized were greater than 99% purity, as determined
by the observation of a single peak by GC without thermolysis.
Compound Preparation: General. Unless otherwise noted, starting
materials were obtained from Aldrich and used as received. Charac-
terization was carried out on a Bruker Avance DXR NMR operating
at 400 MHz for proton and 100 MHz for carbon. The 13C signals for
CD2 carbons were generally not observed due to the low signal-to-
noise and high multiplicity. MS were obtained on a Finnigan TSQ 700
operating in the EI mode. IR spectra were obtained on a Mattson Galaxy
Series FTIR 3000. Dry THF was freshly distilled from benzophenone
ketyl. Both compounds 1 and 3 are known;27-29 the isotopomer 2 is a
new compound. Modern spectroscopic data for 1 and 2, both prepared
by oxidation sulfoxides already on hand,30 are given in the Supporting
Information.
General Procedure for Preparation of Sulfone from Sulfide (or
Sulfoxide). To an ice-cooled solution of 2-3 mmol of the sulfoxide
(sulfide) in methylene chloride (15 mL) was added 2.2 (1.1) equiv of
m-chloroperbenzoic acid dissolved in 25 mL of methylene chloride
dropwise by means of a dropping funnel. After 2 h, the mixture was
poured into aqueous NaOH (5%, 50 mL) and the layers were separated.
The organic layer was washed with another portion of aqueous NaOH,
then dried with MgSO4 and concentrated in vacuo. Yields were nearly
quantitative and products clean by NMR. Further purification was
carried out as noted.
arbitrarily plotted at the average value, 2.0. Given the limitations
of the experimental data and the necessity to reduce the size of
the molecule for computations, the calculated and experimental
KIEs are taken to be in excellent agreement.
A final experimental consideration is the observed value of
∆Hq, which is inconsistent with the radical mechanism. The
C-S bond dissociation energies (BDEs) of sulfones 1-3 are
expected to be approximately 68 kcal/mol,25,26 significantly
higher than the observed ∆Hq of 53.5 kcal/mol.
The magnitude of ∆Hq for the sulfone Ei reaction is
significantly greater than that for the corresponding sulfoxide
elimination. The activation entropies in Table 1, however, are
in line with literature reports and our observations for the
sulfoxide reaction.18 The calculated ∆H and ∆Hq for sulfoxide
8 are included in Table 2 for comparison. Neither the sulfoxide
nor the sulfone reaction has a transition state that can be
described as particularly early or late. The computed transition
state geometries are substantially similar, save that both the
C-H and H-O distances are 0.02-0.03 Å shorter at the
expense of a 0.05 Å longer C-C distance for 4, compared to
the sulfoxide 8. Both have all 5 key atoms in a nearly coplanar
arrangement.
A large part of the difference in ∆Hq may lie simply in the
fact that the sulfone reaction is substantially more endothermic.
It is also attractive to speculate that the decreased basicity of
the sulfonyl group relative to the sulfinyl group outweighs the
increased nucleofugacity in the transition state. While studies
that compare nucleofugacity are generally system dependent,
we have been unable to find any cases in which sulfones are
any more than modestly better leaving groups than the corre-
sponding sulfoxide.13-15
Finally, given the strong evidence for the Ei reaction of
sulfones, one must ask why this simple thermolytic reaction
has escaped the attention of the chemical community until now.
First, the activation enthalpy is not insubstantial. Many sulfones
that have been pyrolyzed at sufficiently high temperatures for
the Ei reaction to be observed are not physically capable of the
reaction or have substituents that lower a C-S bond dissociation
energy such that it is in the range of the ∆Hq reported here.
Not only are the BDEs for benzyl- and allyl-SO2R bonds low
(55-56 kcal/mol), but the CH3-SO2Ph BDE is reported to be
54-57 kcal/mol.25,26 Such weak bonds would probably make
homolytic reactions very competitive, especially considering the
favorable ∆Sq values for the homolyses. Cyclic compounds may
not have revealed Ei reactivity because the reverse reaction is
likely to be very rapid, with sulfone being overwhelmingly
favored thermodynamically.
Ethyl 2,2,3,3-Tetradeuterio-3-phenylpropionate. In a 250 mL
round-bottom flask, ethyl phenylpropiolate (10.0 g, 57.4 mmol) was
dissolved in diethyl ether (10 mL) and Pd/C (2.0 g) was added. The
mixture was stirred rapidly and D2 was introduced into the chamber as
follows: a three-way valve was attached to the deuterium source, the
reaction flask, and a calibrated U-shaped tube (1.4 L) filled with mineral
oil. The reaction was run until completion as monitored by GC. The
mixture was filtered and concentrated to give the product in 86% yield.
1
The product was clean by NMR and used for subsequent steps. H
NMR (CDCl3) δ 7.3-7.18 (m, 5H), 4.13 (q, J ) 7.2 Hz, 3H), 1.23 (t,
J ) 7.2 Hz, 3H); 13C NMR (CDCl3) δ 173.2, 140.7, 128.7, 126.4, 60.6,
14.4; IR (thin film) 3026, 2981, 2222, 2101, 1732, 1268, 1026, 735,
699 cm-1
.
2,2,3,3-Tetradeuterio-3-phenyl-1-propanol. To a suspension of
lithium aluminum hydride (0.63 g, 16.5 mmol) in dry THF (25 mL)
under Ar at O °C was added ethyl 2,2,3,3-tetradeuterio-3-phenylpro-
pionate (1.0 g, 5.49 mmol). The suspension was allowed to warm to
room temperature. After being stirred 1 h the reaction mixture was
heated to reflux for 5 h. The reaction was quenched by slow, successive
addition of H2O (0.6 mL), aqueous NaOH (0.6 mL), and H2O (1.8
mL). The solution was filtered then poured into ether (30 mL) and
washed with brine (3 × 25 mL). The organic layer was dried (MgSO4)
and concentrated to give 2,2,3,3-tetradeuterio-3-phenyl-1-propanol in
98% yield. The material was clean by NMR and used in the next step
Conclusions
In summary, a new unimolecular reaction of sulfones, Ei
elimination to form alkenes, has been observed. Its activation
enthalpy, though high, is below what is to be expected for C-S
bond rupture. The radical mechanism further is ruled out on
grounds of substituent effects and computations of an Ei
transition state that well reproduces the absolute ∆Hq and KIE.
1
without further purification. H NMR (CDCl3) δ 7.29-7.16 (m, 5H)
3.65 (s, 2H), 1.64 (s, 1H); 13C NMR (CDCl3) δ 141.8, 128.5, 128.5,
125.9, 62.2; IR (thin film) 3346, 3024, 2918, 2876, 2206, 2112, 1604,
1043, 699 cm-1
.
2,2,3,3-Tetradeuterio-3-phenylpropyl p-Toluenesulfonate. In a
(27) Entwilstle, I. K.; Johnstone, R. A. W.; Millard, B. J. J. Chem. Soc.
C 1967, 302-306.
Experimental Section
(28) Oda, R.; Yamamoto, K. J. Org. Chem. 1961, 26, 4679-4681.
(29) Russell, G. A.; Sabourin, E.; Mikol, G. J. J. Org. Chem. 1966, 31,
4-2858.
Instrument. The stirred-flow reactor has a temperature-controlled
furnace and is modeled very closely after the one that has been
previously described.17 It uses He as a carrier gas to bring the sample
(30) Guo, Y. Study on the Photolysis and Thermolysis of Alkyl Aryl
Sulfoxides; Iowa State University: Ames, IA, 1997.
(31) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.;
Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, N.; Su,
S. J.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput. Chem.
1993, 14, 1347-1363.
(25) Benson, S. W. Chem. ReV. 1978, 78, 23-35.
(26) Herron, J. Thermochemistry of Sulfoxides and Sulfones; Patai, S.,
Rappoport, Z., Stirling, C. J. M., Eds.; John Wiley and Sons Ltd.: New
York, 1988; pp 95-105.