Di-pi-methane rearrangement
Encyclopedia
The di-pi-methane rearrangement is a photochemical reaction
of a molecular entity
comprising two π-systems
, separated by a saturated carbo
n atom (a 1,4-diene or an allyl-substituted aromatic analog), to form an ene- (or aryl-) substituted cyclopropane
. The rearrangement reaction
formally amounts to a 1,2 shift of one ene group (in the diene
) or the aryl group (in the allyl-aromatic analog) and bond formation between the lateral carbons of the non-migrating moiety.
to give semibullvalene. Once the mechanism was recognized as general by Zimmerman
in 1967, it was clear that the structural requirement was having two pi groups attached to an sp3-hybridized carbon, and then a variety of further examples was obtained. One was the photolysis of the Mariano Compound, 3,3-methyl-1,1,5,5-tetraphenyl-1,4-pentadiene. Another was the reaction of the Pratt diene
Equation 1. The mechanism of the Mariano diene rearranging
In contrast, in the case of the Pratt diene rearranging, there are two possible regiochemistries - a and b. Process a is preferred since it leaves benzhydryl odd-electron stabilization.
Equation 2. The mechanism of the Pratt diene rearranging; note the regioselectivity
The barrelene rearrangement is now presented. It is a bit more complex than the Mariano and Pratt examples since there are two sp3-hybridized (i.e. methane) carbons. Each such bridgehead carbon has three (ethylenic) pi bonds while two are needed for the di-pi-methane rearrangement. Another difference is that the barrelene reaction requires the triplet excited state
while the Mariano and Pratt acyclic dienes used the excited singlet. Thus acetone
is used in the barrelene reaction; acetone captures the light and then delivers triplet excitation to the barrelene reactant. In the final step of the rearrangement there is a spin-flip, termed intersystem-crossing
(ISC) to provide paired electrons and a new sigma bond
.
Equation 3. The mechanism of the Barrelene to Semibullvalene transformation
The dependence of the success of the Di-pi-Methane rearrangement on singlet versus triplet multiplicity arises primarily from the Free-Rotor Effect The triplet acyclic 1,4-dienes are free to undergo cis-trans interconversion of the diene double bonds (i.e. free-rotation) thus inhibiting the Di-pi-Methane process. The cis-trans isomerization proceeds by weakening of a pi-bond and then twisting. The singlet excited states don't rotate and then are free to undergo the Di-pi-Methane mechanism.
For cyclic dienes, as in the barrelene example, the ring structure prevents cis-trans isomerization and the Di-pi-Methane can then occur.
Mechanistic organic photochemistry
Mechanistic organic photochemistry is that aspect of organic photochemistry which seeks to explain the mechanisms of organic photochemical reactions. The absorption of ultraviolet light by organic molecules very often leads to reactions. In the earliest days sunlight was employed while in more...
of a molecular entity
Molecular entity
According to the IUPAC Gold Book a molecular entity is "any constitutionally or isotopically distinct atom, molecule, ion, ion pair, radical, radical ion, complex, conformer, etc., identifiable as a separately distinguishable entity"....
comprising two π-systems
Pi bond
In chemistry, pi bonds are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital...
, separated by a saturated carbo
Carbo
Carbo was a plebeian family within the gens Papiria of ancient Rome. Several members were notable:* Gaius Papirius Carbo, praetor 168 BC* Gaius Papirius Carbo, consul 120 BC* Gnaeus Papirius Carbo, consul 113 BC...
n atom (a 1,4-diene or an allyl-substituted aromatic analog), to form an ene- (or aryl-) substituted cyclopropane
Cyclopropane
Cyclopropane is a cycloalkane molecule with the molecular formula C3H6, consisting of three carbon atoms linked to each other to form a ring, with each carbon atom bearing two hydrogen atoms...
. The rearrangement reaction
Rearrangement reaction
A rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. Often a substituent moves from one atom to another atom in the same molecule...
formally amounts to a 1,2 shift of one ene group (in the diene
Diene
In organic chemistry a diene or diolefin is a hydrocarbon that contains two carbon double bonds.Conjugated dienes are functional groups, with a general formula of CnH2n-2. Dienes and alkynes are functional isomers...
) or the aryl group (in the allyl-aromatic analog) and bond formation between the lateral carbons of the non-migrating moiety.
Discovery & mechanism
This rearrangement was originally encountered in the photolysis of barreleneBarrelene
Barrelene is a bicyclic organic compound with chemical formula C8H8 and systematic name bicyclo[2.2.2]octa-2,5,7-triene. First synthesized and described by H. E. Zimmerman in 1960 the name derives from the obvious resemblance with a barrel, with the staves being three ethylene units attached to two...
to give semibullvalene. Once the mechanism was recognized as general by Zimmerman
Howard Zimmerman
Howard E. Zimmerman is a professor of chemistry at the University of Wisconsin–Madison. He was elected to the National Academy of Sciences in 1980 and the recipient of the 1985 American Institute of Chemists Pioneering Award....
in 1967, it was clear that the structural requirement was having two pi groups attached to an sp3-hybridized carbon, and then a variety of further examples was obtained. One was the photolysis of the Mariano Compound, 3,3-methyl-1,1,5,5-tetraphenyl-1,4-pentadiene. Another was the reaction of the Pratt diene
Equation 1. The mechanism of the Mariano diene rearranging
In contrast, in the case of the Pratt diene rearranging, there are two possible regiochemistries - a and b. Process a is preferred since it leaves benzhydryl odd-electron stabilization.
Equation 2. The mechanism of the Pratt diene rearranging; note the regioselectivity
The barrelene rearrangement is now presented. It is a bit more complex than the Mariano and Pratt examples since there are two sp3-hybridized (i.e. methane) carbons. Each such bridgehead carbon has three (ethylenic) pi bonds while two are needed for the di-pi-methane rearrangement. Another difference is that the barrelene reaction requires the triplet excited state
Triplet state
A spin triplet is a set of three quantum states of a system, each with total spin S = 1 . The system could consist of a single elementary massive spin 1 particle such as a W or Z boson, or be some multiparticle state with total spin angular momentum of one.In physics, spin is the angular momentum...
while the Mariano and Pratt acyclic dienes used the excited singlet. Thus acetone
Acetone
Acetone is the organic compound with the formula 2CO, a colorless, mobile, flammable liquid, the simplest example of the ketones.Acetone is miscible with water and serves as an important solvent in its own right, typically as the solvent of choice for cleaning purposes in the laboratory...
is used in the barrelene reaction; acetone captures the light and then delivers triplet excitation to the barrelene reactant. In the final step of the rearrangement there is a spin-flip, termed intersystem-crossing
Intersystem crossing
Intersystem crossing is a radiationless process involving a transition between two electronic states with different spin multiplicity.-Singlet and triplet states:...
(ISC) to provide paired electrons and a new sigma bond
Sigma bond
In chemistry, sigma bonds are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most clearly defined for diatomic molecules using the language and tools of symmetry groups. In this formal approach, a σ-bond is...
.
Equation 3. The mechanism of the Barrelene to Semibullvalene transformation
The dependence of the success of the Di-pi-Methane rearrangement on singlet versus triplet multiplicity arises primarily from the Free-Rotor Effect The triplet acyclic 1,4-dienes are free to undergo cis-trans interconversion of the diene double bonds (i.e. free-rotation) thus inhibiting the Di-pi-Methane process. The cis-trans isomerization proceeds by weakening of a pi-bond and then twisting. The singlet excited states don't rotate and then are free to undergo the Di-pi-Methane mechanism.
For cyclic dienes, as in the barrelene example, the ring structure prevents cis-trans isomerization and the Di-pi-Methane can then occur.