chelotropic reactions

CHELETROPIC REACTIONS
 Group members
 Wajid khan
 Junaid Mashwani
 Arshad Ali
 Nasir Mehmood

Background
"We define as cheletropic reactions those processes
in which two σ bonds which terminate at a single
atom are made, or broken, in concert."
Woodward, R.B.; Hoffman, R. Angew. Chem. Int. Ed.
Engl. 1969, 8, 781–853.

Cheletropic reactions are a separate class of
pericyclic reactions that are subject to orbital
symmetry analysis. They must obey the Woodward-
Hoffman rules the same way that cycloadditions
and sigmatropic rearrangements do.

THEORITICAL ANALYSIS.
In the pericyclic transition state, a small molecule donates two electrons to the ring.
The reaction process can be shown using two different geometries. The small
molecule can approach in a linear or non-linear fashion. In the linear approach,
the electrons in the orbital of the small molecule are pointed directly at the π-
system. In the non-linear approach, the orbital approaches at a skew angle. The π-
system's ability to rotate as the small molecule approaches is crucial in forming new
bonds. The direction of rotation will be different depending on how many π-
electrons are in the system. Shown below is a diagram of a two-electron fragment
approaching a four-electron π-system using frontier molecular orbitals.

The rotation will be disrotatory if the small molecule approaches linearly and
conrotatory if the molecule approaches non-linearly. Disrotatory and
conrotatory are sophisticated terms expressing how the bonds in the π-system
are rotating. Disrotatory means opposite directions while conrotatory means the
same direction. This is also depicted in the diagram below.
Using Hückel's Rule, one can tell if the π-system is aromatic or anti-aromatic. If
aromatic, linear approaches use disrotatory motion while non-linear approaches
use conrotatory motion. The opposite goes with an anti-aromatic system. Linear
approaches will have conrotatory motion while non-linear approaches will have
disrotatory motion

CHELETROPIC REACTIONS INVOLVING SO2
THERMODYNAMICS.
In 1995, Suarez and Sordo showed that sulfur dioxide
when reacted with butadiene and isoprene gives
two different products depending on the
mechanism. A kinetic and thermodynamic product
are both possible, but the thermodynamic product is
more favorable.

The kinetic product arises from a Diels-Alder reaction,
while a cheletropic reaction gives rise to a more
thermodynamically stable product.
.
The cheletropic pathway is favored because it gives rise
to a more stable five-membered ring adduct. The
scheme below shows the difference between the two
products
 CONTINUED

The path to the right shows the more stable
thermodynamic product, while the path to the left
shows the kinetic product.
 continued

Kinetics.
The cheletropic reactions of 1,3-dienes with
sulfur dioxide have been extensively investigated
in terms of kinetics.In the first quantitative
measurement of kinetic parameters for this
reaction, a 1976 study by Isaacs and Laila
measured the rates of addition of sulfur dioxide to
butadiene derivatives. Rates of addition were
monitored in benzene at 30 °C with an initial
twentyfold excess of sulfur dioxide, allowing for a
pseudo first-order approximation

CONTINUED.
The disappearance of SO2 was followed spectrophotometrically at
320 nm The reaction showed pseudo first-order kinetics. Some
interesting results were that electron-withdrawing groups on the
diene decreased the rate of reaction. Also, the reaction rate was
affected considerably by steric effects of 2-substituents, with more
bulky groups increasing the rate of reaction. The authors attribute
this to the tendency of bulky groups to favor the cisoid
conformation of the diene which is essential to the reaction.

(SEE TABLE BELOW). IN ADDITION, THE RATES AT FOUR
TEMPERATURES WERE MEASURED FOR SEVEN OF THE
DIENES PERMITTING CALCULATIONS OF THE ENTHALPY
OF ACTIVATION (ΔH‡
) AND ENTROPY OF ACTIVATION
(ΔS‡
) FOR THESE REACTIONS THROUGH THE ARRHENIUS
EQUATION.
 continued

-Butadiene
104
k /min−1
(30 °C) (±
1-2%) absolute
104
k /min−1
(30 °C) (± 1-2%)
relative
ΔH‡
/kcal mol−1
ΔS‡
/cal mol−1
K−1
2-methyl 1.83 1.00 14.9 -15
2-ethyl 4.76 2.60 10.6 -20
2-isopropyl 13.0 7.38 12.5 -17
2-tert-butyl 38.2 20.8 10.0 -19
2-neopentyl 17.2 9.4 11.6 -18
2-cloro 0.24 0.13 N/A N/A
2-bromoethyl 0.72 0.39 N/A N/A
2-p-tolyl 24.7 13.5 10.4 -19
2-phenyl 17.3 9.45 N/A N/A
2-(p-bromophenyl) 9.07 4.96 N/A N/A
2,3-dimethyl 3.54 1.93 12.3 -18
cis-1-methyl 0.18 0.10 N/A N/A
trans-1-methyl 0.69 0.38 N/A N/A
1,2-dimethylene-cyclohexane 24.7 13.5 11.4 -16
2-methyl-1,1,4,4-d4 1.96 N/A N/A N/A

More recently, a 2002 study by Monnat, Vogel, and Sordo
measured the kinetics of addition of sulfur dioxide to 1,2-
dimethylidenecycloalkanes. An interesting point presented
in this paper is that the reaction of
1,2dimethylidenecyclohexane with sulfur dioxide can give
two different products depending on reaction conditions.
The reaction produces the corresponding sultine through a
hetero-Diels-Alder reaction under kinetic control (≤ -60 °C),
but, under thermodynamic control (≥ -40 °C), the reaction
produces the corresponding sulfolene through a
cheletropic reaction.

The activation enthalpy for the hetero-Diels-Alder reaction is about
2 kcal/mol smaller than that for the corresponding cheletropic
reaction. The sulfolene is about 10 kcal/mol more stable than the
isometric sultine in CH2Cl2/SO2 solution

Solvent effect.
The effect of the solvent of the cheletropic reaction of 3,4-dimethyl-
2,5-dihydrothiophen-1,1-dioxide (shown at right) was kinetically
investigated in 14 solvents. The reaction rate constants of the
forward and reverse reaction in addition to the equilibrium
constants were found to be linearly correlated with the ET(30)
solvent polarity scale.
Reactions were done at 120 °C and were studied by 1H-NMR
spectroscopy of the reaction mixture. The forward rate k1 was found
to decrease by a factor of 4.5 going from cyclohexane to
methanol:

. THE REVERSE RATE K 1− WAS FOUND TO INCREASE BY A FACTOR OF
53 GOING FROM CYCLOHEXANE TO METHANOL, WHILE THE
EQUILIBRIUM CONSTANT KEQ DECREASED BY A FACTOR OF 140. IT IS
SUGGESTED THAT THERE IS A CHANGE OF THE POLARITY DURING
THE ACTIVATION PROCESS AS EVIDENCED BY CORRELATIONS
BETWEEN THE EQUILIBRIUM AND KINETIC DATA. THE AUTHORS
REMARK THAT THE REACTION APPEARS TO BE INFLUENCED BY
THE POLARITY OF THE SOLVENT, AND THIS CAN BE EXPLAINED BY
THE CHANGE IN THE DIPOLE MOMENTS WHEN GOING FROM
REACTANT TO TRANSITION STATE TO PRODUCT. THE AUTHORS
ALSO STATE THAT THE CHELETROPIC REACTION DOESN’T SEEM
TO BE INFLUENCED BY EITHER SOLVENT ACIDITY OR BASICITY.

The results of this study lead the authors to expect the following
behaviors.
1. The change in the solvent polarity will influence the rate less than the equilibrium.
2. The rate constants will be characterized by opposite effect on the polarity: k1 will
slightly decrease with the increase of ET(30), and k−1 will increase under the same
conditions.
3. The effect on k−1 will be larger than on k1

Carbene Additions to Alkenes.
A) The Orbitals for Singlet
Carbenes.
B) Non-linear Approach of
a) Carbene sp2 Orbital and
b) Carbene p Orbital.

CARBENE ADDITIONS TO ALKENES.
One of the most synthetically important cheletropic reactions is the
addition of a singlet carbene to an alkene to make a cyclopropane
(see figure at left). A carbene is a neutral molecule containing a
divalent carbon with six electrons in its valence shell. Due to this,
carbenes are highly reactive electrophiles and generated as
reaction intermediates. A singlet carbene contains an empty p
orbital and a roughly sp2
hybrid orbital that has two electrons.

SINGLET CARBENES ADD STEREOSPECIFICALLY TO ALKENES, AND
ALKENE STEREOCHEMISTRY IS RETAINED IN THE CYCLOPROPANE
PRODUCT. THE MECHANISM FOR ADDITION OF A CARBENE TO AN
ALKENE IS A CONCERTED [2+1] CYCLOADDITION (SEE FIGURE).
CARBENES DERIVED FROM CHLOROFORM OR BROMOFORM CAN BE
USED TO ADD CX2 TO AN ALKENE TO GIVE A
DIHALOCYCLOPROPANE, WHILE THE SIMMONS-SMITH REAGENT ADDS
CH2 INTERACTION OF THE FILLED CARBENE ORBITAL WITH THE ALKENE
Π SYSTEM CREATES A FOUR-ELECTRON SYSTEM AND FAVORS A NON-
LINEAR APPROACH.

IT IS ALSO FAVORABLE TO MIX THE CARBENE EMPTY P
ORBITAL WITH THE FILLED ALKENE Π ORBITAL. FAVORABLE
MIXING OCCURS THROUGH A NON-LINEAR APPROACH (SEE
FIGURE AT RIGHT). HOWEVER, WHILE THEORY CLEARLY
FAVORS A NON-LINEAR APPROACH, THERE ARE NO
OBVIOUS EXPERIMENTAL IMPLICATIONS FOR A LINEAR VS.
NON-LINEAR APPROACH.

A) The Orbitals for Singlet Carbenes B) Non-linear Approach of a) Carbene sp2
Orbital and b) Carbene p Orbital

References
^ Chelotropic reaction IUPAC GoldBook
^ a b c d e f
Eric V. Anslyn and Dennis A. Dougherty Modern Physical Organic Chemistry University Science
Books, 2006.
^ Ian Fleming. Frontier Orbitals and Organic Chemistry Reactions. Wiley, 1976.
^ a b
Suarez, D.; Sordo, T. L.; Sordo, J. A. (1995). "A Comparative Analysis of the Mechanisms of Cheletropic
and Diels-Alder Reactions of 1,3-Dienes with Sulfur Dioxide: Kinetic and Thermodynamic Controls". J. Org.
Chem. 60 (9): 2848–2852. doi:10.1021/jo00114a039.
^ Isaacs, N. S.; Laila, A. A. R. (1976). "Rates of addition of sulphur dioxide to some 1,3-dienes". Tetrahedron
Lett. 17 (9): 715–716. doi:10.1016/S0040-4039(00)74605-3.
^ a b
Monnat, F.; Vogel, P.; Sordo, J. A. (2002). "Hetero-Diels-Alder and Cheletropic Additions of Sulfur Dioxide
to 1,2-Dimethylidenecycloalkanes. Determination of Thermochemical and Kinetics Parameters for Reactions
in Solution and Comparison with Estimates From Quantum Calculations". Helv. Chim. Acta. 85 (3): 712–732.
doi:10.1002/1522-2675(200203)85:3<712::AID-HLCA712>3.0.CO;2-5.
^ Fernandez, T.; Sordo, J. A.; Monnat, F.; Deguin, B.; Vogel, P. (1998). "Sulfur Dioxide Promotes Its Hetero-
Diels−Alder and Cheletropic Additions to 1,2-Dimethylidenecyclohexane". J. Am. Chem. Soc. 120 (50):
13276–13277. doi:10.1021/ja982565p.
^ Desimoni, G.; Faita, G.; Garau, S.; Righetti, P. (1996). "Solvent effect in pericyclic reactions. X. The
cheletropic reaction". Tetrahedron. 52 (17): 6241–6248. doi:10.1016/0040-4020(96)00279-7.
^ John McMurry Organic Chemistry, 6th Ed. Thomson, 2004.
^ Robert B. Grossman The Art of Writing Reasonable Organic Reaction Mechanisms Springer, 2003.

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