BCl3-mediated ene reaction of sulfur dioxide and-unfunctionalized alkenes

Article abstract of DOI:10.1002/chem.201000164

The first ene reactions of SO₂ and unfunctionalized alkenes are reported. Calculations suggest that the endergonic ene reactions of SO ₂ with alkenes can be used to generate β,γ-unsaturated sulfinyl and sulfonyl compounds. Indeed, in the presence of one equivalent of BCl₃, the unstable sulfinic acid form stable sulfinic acid-BCl ₃ complexes that can be reacted in situ with NCS to generate corresponding sulfonyl chlorides, or with a base to generate corresponding sulfinates. The latter can be reacted with electrophiles to generate sulfones, or with silyl chloride to form β,γ-unsaturated silyl sulfinates. The sulfinic acid-BCl₃ complexes can be reacted with ethers that act as oxygen nucleophiles to produce corresponding sulfinic esters. Thus one-pot, three-component synthesis of β,γ-unsaturated sulfonamides, sulfinyl esters and sulfones have been developed starting from alkenes and sulfur dioxide (reagent and solvent).

Full text of DOI:10.1002/chem.201000164

FULL PAPER
DOI: 10.1002/chem.201000164
BCl3-Mediated Ene Reaction of Sulfur Dioxide and
Unfunctionalized Alkenes
Dean Markovic´,*[a] Chandra M. R. Volla,[a] Pierre Vogel,*[a]
Adriꢀn Varela-ꢁlvarez,*[b] and Josꢂ A. Sordo[b]
Abstract: The first ene reactions of
NCS to generate corresponding sulfo-
nyl chlorides, or with a base to gener-
ate corresponding sulfinates. The latter
can be reacted with electrophiles to
generate sulfones, or with silyl chloride
to form b,g-unsaturated silyl sulfinates.
SO2 and unfunctionalized alkenes are
reported. Calculations suggest that the
endergonic ene reactions of SO2 with
alkenes can be used to generate b,g-un-
saturated sulfinyl and sulfonyl com-
pounds. Indeed, in the presence of one
equivalent of BCl3, the unstable sulfinic
acid form stable sulfinic acid·BCl3 com-
plexes that can be reacted in situ with
The sulfinic acid·BCl3 complexes can
be reacted with ethers that act as
oxygen nucleophiles to produce corre-
sponding sulfinic esters. Thus one-pot,
three-component synthesis of b,g-unsa-
turated sulfonamides, sulfinyl esters
and sulfones have been developed
starting from alkenes and sulfur diox-
ide (reagent and solvent).
ꢀ
Keywords: C H activation
reaction · sulfonamides · sulfones ·
· ene
sulfur dioxide
Introduction
with Na2SO3 to give the corresponding sodium sulfonates,
which then must be converted into corresponding sulfonyl
chlorides with PCl5 or POCl3.[4] This approach presents sev-
eral problems including harsh conditions and incomplete se-
lectivity in the allylic monochlorination,[5] allylic rearrange-
Sulfonamides and sulfones exhibit a broad range of biologi-
cal activities and are important pharmacophores.[1] The
direct ene reaction of sulfur dioxide[2] with alkenes to pro-
duce b,g-unsaturated sulfinic acids and their conversion into
sulfinates or sulfone intermediates would be short and eco-
nomical entry to this type of compounds.[3]
The classical approach to b,g-unsaturated sulfonamides is
the reaction of corresponding sulfonyl chlorides with pri-
mary and secondary amines. The classical method for the
synthesis of b,g-unsaturated sulfonyl chlorides imply chlori-
nation or bromination of allylic C–H, followed by reaction
[6]
ment for the reaction with Na2SO3 and problematic con-
version of sulfonates into sulfonyl chlorides.[7]
The intermolecular Alder-ene reaction is a useful and
completely atom-efficient carbon–heteroatom and carbon–
carbon bond-forming reaction. The ene reactions of eno-
philes such as: 1O2 (1Dg),[8] HN=NH,[9] H–N=O,[10] and
[11]
SeO2 were extensively studied. For example, Houk, Sin-
gleton, and co-workers studied the ene reaction of singlet
oxygen and have shown theoretical evidence which supports
the possibility of a two-step no-intermediate mechanism.[12]
Sharpless and co-workers reported evidence in favor of the
initial ene reaction in the selenium dioxide oxidations of al-
kenes.[13] Several reports on the use of sulfur diimines and
N-sulfinyl arenesulfonamides as enophiles were reported by
the Kresze,[14] Sharpless[15] and Deleris[16] groups, respective-
ly, but the corresponding products are rather unstable and
´
[a] Dr. D. Markovic, Dr. C. M. R. Volla, Prof. Dr. P. Vogel
Contribution from the Laboratoire de glycochimie
et de synthꢀse asymꢁtrique (LGSA)
Swiss Federal Institute of Technology of Lausanne (EPFL)
1015 Lausanne (Switzerland)
E-mail: dean.markovic@epfl.ch
pierre.vogel@epfl.ch
readily undergo subsequent [2,3]-sigmatropic shift to form
[17]
[b] A. Varela-ꢂlvarez, Prof. Dr. J. A. Sordo
Laboratorio de Quꢃmica Computacional
Departamento de Quꢃmica Fꢃsica y Analꢃtica
Universidad de Oviedo, Principado de Asturias (Spain)
E-mail: adrian@klingon.uab.es
ꢀ
C N bonds. Interestingly, SO3 does not undergo the ene
reaction, but adds to alkenes at ꢀ608C to give the corre-
sponding b-sultones.[18]
Despite its elemental character, ene reactions of sulfur di-
oxide with alkenes have not been developed because it is
endergonic above ꢀ1008C. b,g-Unsaturated sulfinic acids
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000164.
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5969

are generally unstable[19] and undergo quick desulfinylation
at room temperature. Nevertheless, the formation of the
latter as intermediates has been suggested in the hydrolysis
of sulfinamides,[20] the reduction of b-ketosulfones,[21] and
the oxidation of allylic thiols.[22]
Table 1. Effect of various Lewis acids on the reaction of SO2 with 1-
octene (1).[a,b]
Contrarily to previous reports,[23] we have shown that
SO2-mediated isomerization of alkenes do not always imply
an ene reaction to form the b,g-unsaturated sulfinic acid in-
termediates followed by [1,3]-sigmatropic shift and retro-
ene reaction, but imply a radical process.[24] Firstly, SO2 and
the alkene generate a polysulfone polymer that is responsi-
ble for allylic hydrogen atom abstraction from the alkenes.
Then the allyl radical intermediate abstracts hydrogen atom
from sulfinic acid to form the most stable alkene
isomer.[25,26]
Entry
LA
(1 equiv)
t
T
[8C]
Yield ene
[%][b]
Yield poly-
mers [%]
G
A
1
2
3
4
5
6
BF3
NbCl5
BCl3
AlCl3
PhBCl2
<5
30
<5
20
120
760
ꢀ78
67
72
95
63
14
n.r.
33
28
n.o.
37
n.o.
ꢀ50
ꢀ78
ꢀ78
ꢀ20
B
N
ꢀ78!20
[a] For a complete list of tested Lewis acids see Supporting Information.
[b] Because of signals overlaps the E/Z ratio could not be evaluated;
n.r.=no reaction; n.o.=not observed.
In 1995 Young measured kinetic and thermodynamic pa-
rameters for the retro-ene reaction of prop-2-enesulfinic
acid.[27] Recently, we have performed detailed kinetic and
computational studies of the same reaction in the presence
of various additives.[28] To our surprise, when an excess of
BF3·Me2O was added to the prop-2-enesulfinic acid the de-
sulfinylation was completely inhibited. Calculations showed
that such a behavior should be attributed to the formation
entry 3). The ene reaction did not occur if solvent (CH2Cl2)
of the reaction was changed to THF, ether or hexane.
We then explored the ene reaction of SO2 + BCl3 with
various alkenes. The results of these experiments are sum-
marized in Table 2. High yields of the desired alk-2-enesul-
finic acid·BCl3 complexes were obtained. Evaporation of
solvent and excess of SO2 gave crude products of type
of
a stable tetramolecular complex of type (CH2=
CHCH2SO2H·BF3)2, that is calculated to be 15.5 kcalmolꢀ1
more stable than the prop-2-enesulfinic acid dimer+BF3
and 9.9 kcalmolꢀ1 more stable than propene+SO2 +BF3.
Herein we describe the first BCl3-mediated ene reactions
of SO2 with alkenes. The reaction can be applied to a wide
range of unfunctionalized alkenes to obtain products in ex-
cellent yields. Ab initio calculations predicted the formation
of stable sulfinic acid·BCl3 tetramolecular complex. Indeed,
use of BCl3 in the reaction gave complexes of type (CH2 =
CHCH2SO2H·BCl3)n and prevented the copolymerization of
alkenes with SO2. This newly observed stabilization of sul-
finic acid by boron Lewis acids allowed a generally ender-
gonic ene reaction of SO2 to occur. In addition we have
been able to transform these complexes to a range of sulfi-
nyl and sulfonyl derivatives. In a one-pot three-component
process, sulfonyl chlorides, sulfonamides, sulfones, sulfonic
esters, sulfinic esters and silyl sulfinates can be prepared in
high yields.
(R(R’)C=CHCH2ACHTUNGTRNEG(UN R’’)SO2H·BCl3)n in excellent yields.
Table 2. Yields in alk-2-enesulfinic acid·BCl3 complexes obtained by ene
reaction of SO2 and monoalkenes 3–10.
Entry
Alkene
Yield [%]
Entry
5
Alkene
Yield [%]
95
1
97
2
3
4
93
6
7
8
92
91
quant.
95
quant.
Results and Discussion
The reaction of cyclohexene (9) or (Z)-but-2-ene (10),
(entries 7 and 8) led to the exclusive formation of secondary
sulfinic acids·BCl3 complexes. These complexes can be used
for further transformations without purification. Interesting-
ly, SO2 alone can be used as a solvent for ene reactions at
ꢀ208C. When 1-octene was added to pure SO2 correspond-
ing sulfinic acid·BCl3 complexes precipitated from solution.
SO2 recovery by simple evaporation provided crude product
in quantitative yield. The recovered SO2 is pure enough to
be employed in a second ene reaction. The sulfinic acid·BCl3
complexes can be stored at ꢀ208C without degradation
after three months.
The reaction of 1-octene (1) with SO2 was selected as a pro-
totype reaction (Table 1). Interestingly, BACTHNURGTNEUNG(OMe)3 or Lewis
acids typically used in the Friedel–Crafts acylation such as
FeCl3, TiCl4, SbCl3, or SbF3 did not provide any sulfinic·
Lewis acid complexes 2. We observed the formation of com-
plexes of type 2 in the presence of a several other Lewis
acids such as BF3, AlCl3, NbCl5. However, the reactions
were accompanied by polymerization. To our delight, the
use of BCl3 in CH2Cl2 afforded the expected sulfinic·Lewis
acid complexes at low temperature (ꢀ78 to ꢀ208C) and in
excellent yield without any polymer formation (Table 1,
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Ene Reaction
FULL PAPER
The 1H, 13C and 11B NMR spectra of prop-2-enesulfinic
acid·BCl3 complex 11 at 258C in [D2]tetrachloroethane are
given in Supporting Information. The 1H NMR spectra
(258C, C2D2Cl4) of 11 showed (CH2=CH-) protons resonat-
ing between dH = 6.57–6.34 ppm whereas methylene pro-
tons (CH2) were seen at dH = 4.42–3.80 ppm. The 13C NMR
gave sets of peaks dC = 130.4–129.1, 120.9–119.5 and 59.0–
moiety demonstrating that the ethereal moiety is the nucleo-
philic partner that displaces the OH group of the sulfinic
acid under activation with BCl3. It is not the sulfinic acid
moiety or a SO2/BCl3 moiety that act as nucleophile onto a
carbon center of the ether activated through coordination
with BCl3. These findings explain why tBuOMe, that gives
exclusively the methyl sulfinate 21, reacts as a methoxy nu-
cleophile with tBu+ as electrofugal group rather than as a
tBuOꢀ nucleophile and methyl cation electrofugal group.
Silyl sulfinates are newly developed reagents for the non-
basic, selective silylation of alcohols, phenols and acids.[31]
Bubbling NH3 into a solution of crude 11 followed by addi-
tion of Me3SiCl at ꢀ308C gave a trimethylsilyl prop-2-ene-
sulfinate 23 in 55% yield.
Our one-pot conversion of alkenes into b,g-unsaturated
sulfonyl chlorides (e.g., 24, 27, 29, 31), sulfonamides (e.g.,
25, 28, 30, 32) and sulfonic esters (e.g., 25) is illustrated in
Scheme 3 for isobutylene (4), 1-octene (1), 1-hexene (5),
and allylbenzene (6). Isobutylene (4) provided 1-methyl-
prop-2-enesulfonyl chloride (24) in 95% yield under ene re-
action with SO2 and subsequent treatment with NCS. Con-
version of sulfonyl chloride 24 into sulfonamide 25 and sul-
fonic acid ester 26 were realized in 92 and 91% yield, re-
spectively. Similarly, crude complexes of type 2 derived from
1-octene (1) + SO2 + BCl3 were oxidized by NCS (ꢀ20 to
208C) to provide a 81:19 mixture of (E)- and (Z)-oct-2-ene-
sulfonyl chloride (27, 92%) and 1-hexene (5) was converted
into a 87:13 mixture of (E)- and (Z)-hex-2-enesulfonyl chlo-
ride (29, 95%). The reaction of sulfonyl chlorides 27, 29 or
31 with secondary amines provided sulfonamides 28 (E/Z
83:17), 30 (E/Z 90:10), 32 (E/Z 97:3) in 81, 82, 85 and 62%
yield, respectively.
ꢀ
58.5 ppm for CH2=CH-, CH2=CH- and =CH CH2. The C(1)
signal is broad with linewidth 35 Hz. Additionally, the base-
line around C(2) and C(3) carbons showed significant
broadening. The 11B NMR spectrum shows several signals in
the region from 8.1–5.5 and 5.0–3.76 ppm. These data sug-
gest that in addition to the threo-11 and other tetramolecu-
lar complexes, oligomeric structures of sulfinic acid·BCl3
complexes 11’ are formed (Scheme 1). The possible forma-
tion of protic acids that can eliminate HCl cannot be exclud-
ed at this stage. When these complexes were heated to 608C
polymerization occurred.
Scheme 1. Preparation of prop-2-enesulfinic acid·BCl3 complexes (11).
We then explored the reactivity of these complexes
(Scheme 2). One-pot chlorination of complex 11 with NCS
afforded sulfonyl chloride 12 in 89% yield after filtration
through a silica gel plug. One-pot conversion of 11 into 12
and subsequent reaction with Et2NH under basic conditions
(pyridine) gave sulfonamide 13 (83%). Similarly, use of
We have developed also an one-pot conversion of alkenes
into b,g-unsaturated sulfones as illustrated in Scheme 4, for
3-methylbut-1-ene (7), but-1-ene (8), cyclohexene (9) and
(Z)-but-1-ene (10). For instance, 7 reacts with SO2/BCl3.
Slow addition of formed complexes to prepared solution of
10% aqueous NaOH, nBu4NCl (10%) and 2-bromoaceto-
phenone afforded sulfone 33 (87%) (Scheme 4). Similarly,
crude sulfinic acids·BCl3 complexes obtained from ene reac-
tion of but-1-ene (8), cyclohexene (9) and (Z)-but-1-ene
allylACHTUNGTRENNUNGamine, n-dodecylamine or ammonia gave the corre-
sponding sulfonamides 14 (84%), 15 (87%)[29] and 16
(81%), and with isopropanol, sulfonic ester 17 (85%) yield.
Neutralization of 11 with NaOH/H2O/CH2Cl2 gave sodium
prop-2-enesulfinate which was reacted with BnBr under
phase-transfer conditions to form sulfone 18 (83%). Inter-
estingly, the reaction of 11 with
n-octyl ether and (iPr)2O led to
ꢀ
etheral C O bond cleavage and
provided sulfinic acid esters 19
and 20 in 79 and 67% yield, re-
spectively. With tBuOMe, 11
gave exclusively the methyl
ester 21 (57%) and with THF,
22 was obtained in 71% yield.
We obtained 17O-enriched
THF by reaction of H217O with
1,4-diiodobutane[30] and use it
to prepare 17O-labeled 22.
17O NMR spectroscopy showed
that all labels of THF are found
ꢀ
in the sulfinate S O alkyl Scheme 2. Reactions of crude prop-2-enesulfinic acid·BCl3 complexes.
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´
D. Markovic, P. Vogel, A. Varela-ꢂlvarez et al.
the self-consistent reaction field (SCRF) method augmented by atomic
surface tensions.[36] The so-called solvation model 5.43 (SM5.43R)[37,38]
was employed. This model is specially suitable in the present case as it
was parametrized to predict the free energy of solvation of solutions with
species containing H, C, O, and S atoms in organic solvents (including
CH2Cl2) using B3LYP/6-31G(d) method to describe the electronic struc-
ture of the solute. Calculations were carried out for CH2Cl2 as a solvent,
thus simulating experimental conditions. The ion convention standard
state (1m)[35] was used to compute the changes in Gibbs free energies.
The thermodynamic functions (DH, DS, and DG) were estimated within
the ideal gas, rigid rotor, and harmonic oscillator approximations.[39]
A
temperature of 258.15 K was assumed. The Gaussian03 package of pro-
grams[40] was used to carry out the electronic structure calculations.
Scheme 3. One-pot conversion of alkenes into b,g-unsaturated sulfonyl
chlorides, sulfonamides and sulfonic esters.
Mechanistic Considerations
Ene reactions of alkenes with SO2 were theoretically pre-
dicted to occur in the presence of boron trihalides by using
the ab initio methodology.[28] To elucidate the mechanism
for the ene reaction of SO2/BCl3 with alkenes, we computed
possible reaction pathways, intermediates and evaluated
free energy profile for the ene reaction of SO2/BCl3 and pro-
pene at the CCSD/6-311+GACTHNUTRGENNG(U d,p)//B3LYP/6-31+GACHTUNGTRENNUNG(d,p) +
SM5.43R level. The reaction proceeds by a concerted mech-
anism with free energy barrier of 16.8 kcalmolꢀ1 (Figure 1).
The complex 38 is 8.7 kcalmolꢀ1 more stable than reactants
(propene, SO2 + BCl3) and 14.3 kcalmolꢀ1 more stable than
sulfinic acid dimer 40. The boron center of BCl3 interacts
with the non-bonding electrons of the sulfinic acid oxygen
and the chloride of BCl3 interacts by H-bonding to another
sulfinic acid.
Scheme 4. One-pot conversion of alkenes into b,g-unsaturated sulfones.
(10) were reacted with NaOH/H2O/Bu4NCl and
PhCOCH2Br or BnBr to provide sulfones 34–36 in excellent
yields. The E/Z ratio of sulfone 34 was established as 78:22.
The transition structure for the ene reaction of SO2 and
propene (3) in the presence of BCl3 was located on potential
Theoretical Methods
Geometry optimizations were carried
out using density functional theory
(DFT)[32] calculations with the Becke
three-parameter
(B3LYP) hybrid functional and
Popleꢅs 6-31+G
(d,p) basis set.[34] All
Lee–Yang–Parr[33]
ACHTUNGTRENNUNG
geometrical parameters were fully op-
timized and all structures located on
the potential energy surfaces were
characterized as minima or transition
structures by computing the corre-
sponding Hessian matrices and exam-
ining the number of imaginary fre-
quencies from them. Graphical analy-
sis of the imaginary frequencies of the
transition structures as well as intrinsic
reaction coordinate (IRC) calculations
allowed us to interconnect the differ-
ent structures located on the potential
energy surfaces and then construct the
corresponding energy profiles.
The energy predictions were improved
by performing single-point CCSD(T)/
6-311+GACHTUNGTRENNUNG(d,p)//B3LYP/6-31+GACHUTNGTREN(NUGN d,p)
Figure 1. CCSD(T)/6-311+GACTHNUTRGNENGU(d,p)//B3LYP/6-31+GACHTUGNTREN(NUGN d,p) Gibbs free energies (DG) profile including solvent ef-
calculations.[34,35] Solvent effects were
estimated through quantum mechani-
cal free energy calculations based on
fects (DGsolv), for the ene reaction of propene+SO2 +BCl3 in the presence of BCl3. DU, DH and DS are calcu-
lated at 258.15 K and 1atm. DG and DGsolv are calculated using standard state 1m. All energy values are given
in kcalmolꢀ1
.
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Ene Reaction
FULL PAPER
free energy surface as shown in Figure 2a. This transition
ꢀ
structure realizes a six-membered ring with H C(1) bonds
ꢀ
both staggered with S O, and S=O bonds. Furthermore, if
one considers the plane realized by S, C(1) and C(3), BCl3
coordinates to the O(2) of sulfur dioxide on the same side
ꢀ
as carbon atom C(2) (Figure 2b). The oxygen center of S
O(1) is above this plane similarly to C(2), so that it resem-
bles a distorted chair-type of structure with an axial S=O
group, in a similar way to calculated structures for propene
(3)+ SO2 + BF3.[28]
Formation of the tetramolecular complex 38 is accom-
plished by assembly of two molecules of prop-2-ene sulfinic
acids and BCl3 (Figure 2c). The complex 38 closes into a
twelve-membered ring in a preferred conformation adopting
a center of inversion as symmetry element.
Conclusion
Selective functionalization of alkenes represents one of the
fundamental aspects in organic chemistry. First BCl3-mediat-
ed ene reactions of SO2 with unfunctionalized alkenes were
realized. Terminal and internal alkenes smoothly underwent
ene reaction to give b,g-unsaturated sulfinic acids·BCl3 com-
plexes at low temperatures and high yields.
As presented above, transformation of these complexes
into sulfinates or sulfone intermediates is a new, short and
economical entry to this type of compounds. The use of
BCl3 makes those endergonic reactions possible. Pure sulfur
dioxide can be used as a solvent for the reaction. The ene
reaction of SO2 and unfunctionalized alkenes performed at
0.25 mol scale provided the corresponding sulfinic acid·BCl3
complexes in quantitative yield. The excess of sulfur dioxide
can be recovered and reused for the same reaction. Thus
one-pot three-component procedures for the synthesis of
b,g-unsaturated sulfonyl chlorides, sulfonamides, sulfonic
acid esters and sulfones have been developed.
ꢀ
After initial C H activation, sulfinic acid·BCl3 complexes
ꢀ
react selectively with ethers to give sulfinic esters by C O
bond activation. These complexes can also be reacted with
THF to provide corresponding d-chlorobutyl sulfinate
esters. Interestingly, the reaction with 17O-enriched THF
Figure 2. B3LYP/6-31+GACTHNUTRGNE(NUG d,p) transition structure TS (a and b) and
structure of prop-2-enesulfinic acid·BCl3 tetramolecular complex 38 (c)
located on the potential energy surface of the ene reaction of SO2 and
propene (3) in the presence of BCl3. Some selected bond lengths are
17
ꢀ
showed complete incorporation of O into the esteric S O
bond. This is one of the rare examples where sulfinic acid
does not act as a nucleophile, but exhibits electrophilic char-
acter. In the case of long chain terminal alkenes correspond-
ing sulfonyl derivatives were obtained in excellent yields
with good stereoselectivities. The reaction has been further
extended to internal alkenes to prepare corresponding sul-
fones.
given in ꢆngstrom. In parentheses, CCSD(T)/6-311+G
31+G(d,p) Gibbs free energy differences, including solvent effects, with
respect to propene (3), SO2 and BCl3.
ACHTUNGTREN(NUGN d,p)//B3LYP//6-
AHCTUNGTRENNUNG
The realization of endergonic ene reaction was achieved by
the stabilization of sulfinic acids with Lewis acids which was
an entirely new concept.
Sulfones, sulfonamides, sulfinic and sulfonic acid esters
are used as pharmaceuticals and bulk chemicals. In that
light, we expect that b,g-unsaturated sulfonyl derivatives ob-
tained by this method will find useful applications.
High level ab initio calculations predicted the viability of
direct ene reaction of sulfur dioxide and unfunctionalized al-
kenes to produce b,g-unsaturated sulfinic acids. The reaction
proceeds by concerted mechanism with the low free energy
of activation. Most importantly, the ene reaction is feasible
due to the formation of tetramolecular complexes of sulfinic
acid and BCl3 that was predicted by ab initio calculations.
Chem. Eur. J. 2010, 16, 5969 – 5975
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5973

´
D. Markovic, P. Vogel, A. Varela-ꢂlvarez et al.
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Experimental Section
For full details, see the Supporting Information.
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Acknowledgements
This project was supported by the Swiss National Science Foundation,
MEC (Madrid, Spain, Project No. CTQ 2007-67234-C02-01/BQU), and
FICYT (Gobierno del Principado de Asturias, Project No. IB08-023). We
thank Dr. R. Gomperts (SGI) and Dr. V. M. Rayꢇn (Universidad de Val-
ladolid) for carrying out some calculations that exceeded Universidad de
Oviedoꢅs computer capabilities. We also thank Prof. Lothar Helm for the
donation of H217O and Mr. Martial Rey and Franscisco Sepulveda for
technical help. J.A.S. wishes to dedicate this contribution to his former
student and then colleague, as well as co-worker (project no. IB08-023),
Prof. M. A. Blanco, who passed away recently.
[13] D. Arigoni, A. Vasella, K. B. Sharpless, H. P. Jensen, J. Am. Chem.
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5974
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Ene Reaction
FULL PAPER
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Received: January 21, 2010
Published online: April 15, 2010
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5975