Mechanistic organic photochemistry
Encyclopedia
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 modern times ultraviolet lamps are employed. Organic photochemistry has proven to be a very useful synthetic tool. Complex organic products can be obtained simply. Over the last century and earlier an immense number of photochemical reactions have been uncovered. In modern times the field is quite well understood and is used in organic synthesis and industrially. The utility of organic photochemistry has arisen only by virtue of the available mechanistic treatment; reactions which appear unlikely in ground-state understanding become understandable and accessible in terms of electronic excited-state consideration.
. Using steroid numbering, we note that the C-3 carbonyl group has moved to C-2, the C-4 methyl has moved to C-1, and the C-10 carbon has been inverted.
An comparably bizarre example was uncovered by Egbert Havinga in 1956. The curious result was activation on photolysis by a meta nitro group in contrast to the usual activation by ortho and para groups.
Over the decades, many interesting but puzzling organic photochemical reactions were discovered that did not proceed by ordinary organic ground state
processes. Rather, they arose from the electronic excited state
s. The real problem was that, at the time, organic chemists were not versed in quantum mechanics and physical chemists were not versed in organic chemistry. Real mechanistic treatments were not possible.
Starting in 1961 it was found that one could understand organic photochemical reactions in the context of the relevant excited states."A Mechanistic Approach to Organic Photochemistry," Zimmerman, H. E., Seventeenth National Organic Symposium of the Am. Chem. Soc., Abstracts, Bloomington, Indiana, 1961, pgs. 31-41. DOI: 10.1021/cen-v039n016.p084"Mechanistic Organic Photochemistry. IV. Photochemical Rearrangements of 4,4-Diphenylcyclohexadienone," Zimmerman, H. E.; Schuster, D. I. J. Am. Chem. Soc., 1962, 84, 4527-4540
One example is the n-pi* excitation of mono-carbonyl compounds, the simplest being that of formaldehyde. The structure was first described by Mulliken. The three-dimensional representation (top drawing) is simplified in the second line using a two-dimensional representation, which facilitates arrow pushing
.
In this early research simple Hückel
computations were used to get excited state electron densities and bond-orders. The stereochemistry
in Scheme 1 is shown three-dimensionally. The Hückel computations revealed that the beta-carbons (i.e. C2 and C5) of the cyclohexadienone ring had a large bond-order. As seen in the scheme a beta-beta bond is formed. Subsequent to this, radiationless decay leads to a zwitterion
ground state. The final rearrangement leads to lumisantonin as can be discerned by comparing the three-dimensional drawing with the earlier two-dimensional representation.
Scheme 1. The santonin to lumisantonin rearrangement in three-dimensions
excited state undergoes the same beta-beta bonding. This is followed by intersystem crossing
(i.e. ISC) to form the singlet ground state which is seen to be a zwitterion
. The final step is the rearrangement to the bicyclic photoproduct. The reaction is termed the type A cyclohexadienone rearrangement.
the case of 4,4-diphenylcyclohexenone is presented here. It is seen that the rearrangement is quite different; thus two double bonds are required for a type A rearrangement. With one double bond one of the phenyl groups, originally at C-4, has migrated to C-3 (i.e. the beta carbon)."Mechanistic and Exploratory Organic Photochemistry, IX. Phenyl Migration in the Irradiation of 4.4-Diphenylcyclohexenone," Zimmerman, H. E.; Wilson, J. W. J. Am. Chem. Soc., 1964, 86, 4036-4042.
It is of considerable interest that when one of the aryl groups has a para-cyano or para-methoxy group, that substituted aryl group migrates in preference. Inspection of the alternative phenonium-type species, in which an aryl group has begun to migrate to the beta-carbon, reveals the greater electron delocalization with a substituent para on the migrating aryl group and thus a more stabilized pathway.
."Unsymmetrical Substitution and the Direction of the Di-pi-Methane Rearrangement; Mechanistic and Exploratory Organic Photochemistry. LVI," Zimmerman, H. E.; Pratt, A. C. J. Am. Chem. Soc., 1970, 92, 6259-6267 Two further early examples were the rearrangement of 1,1,5,5-tetraphenyl-3,3-dimethyl-1,4-pentadiene (the "Mariano" molecule) and the rearrangement of barrelene
to semibullvalene. We note that, in contrast to the cyclohexadienone reactions which used n-pi* excited states, the di-pi-methane rearrangements utilize pi-pi* excited states.
Hammond pursued the role of multiplicity on reactivity. The importace of triplet excited species was emphasized. The triplets tend to be longer lived than singlets and of lower energy than the singlet of the same configuration.
Hammond had summarized the energies of triplets of the more common molecules. He noted that triplets may arise from (A) conversion of the initially formed singlets or by (B) interaction with a higher energy triplet (sensitization).
Another contribution of Hammond was the determination of triplet reaction rates. Finally, with low energy triplets present it was shown possible to quench a triplet reaction.
The ground work had been laid by very earlier studies by Terenin and Ermolaev in Russia who demonstrated intermolecular triplet transfer at low temperatures and described the kinetics.
from the Cal. Tech. group of George Hammond, a group of remarkable researchers. Nicholas (Nick) Turro was a graduate student in the Hammond group along with Angelo Lamola,Peter Leermakers, Jack Saltiel, Robert Liu, Angelo Lamola and a number of others. However, the emphasis of the Cal. Tech. group remained on observation of kinetic and multiplicity dependence rather than the effect of the electronic structure of the excited species in controlling charges, bond-orders and thus reactivity.
, the Norrish Type II
, the racemization
of optically active biphenyls, the type A cyclohexadienone rearrangement, the type B cyclohexenone rearrangement, the di-pi-methane rearrangement
, the type B bicyclo[3.1.0]hexanone rearrangement to phenols, photochemical electrocyclic
processes, the rearrangement of epoxyketones to beta-diketones, ring opening of cyclopropyl ketones, heterolysis of 3,5-dimethoxylbenzylic derivatives, and photochemical cyclizations of dienes. Some of these have been described above and with this being an encyclopedic survey only a selected few are considered here.
History
One of the earliest photochemical studies dealt with the natural product santonin. In the 19th century it had been observed by Ciamician that in Italian sunlight santonin gave several photoproducts. The structure of santonin was first correctly described by Clemo and Hayworth in 1929. The initial photoproduct obtained from santonin is lumisantonin. As depicted in Eqn. 1, the photoreaction involves a rearrangementRearrangement 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...
. Using steroid numbering, we note that the C-3 carbonyl group has moved to C-2, the C-4 methyl has moved to C-1, and the C-10 carbon has been inverted.
An comparably bizarre example was uncovered by Egbert Havinga in 1956. The curious result was activation on photolysis by a meta nitro group in contrast to the usual activation by ortho and para groups.
Over the decades, many interesting but puzzling organic photochemical reactions were discovered that did not proceed by ordinary organic ground state
Ground state
The ground state of a quantum mechanical system is its lowest-energy state; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state...
processes. Rather, they arose from the electronic excited state
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
s. The real problem was that, at the time, organic chemists were not versed in quantum mechanics and physical chemists were not versed in organic chemistry. Real mechanistic treatments were not possible.
Starting in 1961 it was found that one could understand organic photochemical reactions in the context of the relevant excited states."A Mechanistic Approach to Organic Photochemistry," Zimmerman, H. E., Seventeenth National Organic Symposium of the Am. Chem. Soc., Abstracts, Bloomington, Indiana, 1961, pgs. 31-41. DOI: 10.1021/cen-v039n016.p084"Mechanistic Organic Photochemistry. IV. Photochemical Rearrangements of 4,4-Diphenylcyclohexadienone," Zimmerman, H. E.; Schuster, D. I. J. Am. Chem. Soc., 1962, 84, 4527-4540
One example is the n-pi* excitation of mono-carbonyl compounds, the simplest being that of formaldehyde. The structure was first described by Mulliken. The three-dimensional representation (top drawing) is simplified in the second line using a two-dimensional representation, which facilitates arrow pushing
Arrow pushing
Arrow pushing or electron pushing is a technique used to describe the progression of organic chemistry reaction mechanisms. In using arrow pushing, "curved arrows" or "curly arrows" are superimposed over the structural formulae of reactants in a chemical equation to show the reaction mechanism...
.
In this early research simple Hückel
Hückel method
The Hückel method or Hückel molecular orbital method proposed by Erich Hückel in 1930, is a very simple linear combination of atomic orbitals molecular orbitals method for the determination of energies of molecular orbitals of pi electrons in conjugated hydrocarbon systems, such as ethene,...
computations were used to get excited state electron densities and bond-orders. The stereochemistry
Stereochemistry
Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms within molecules. An important branch of stereochemistry is the study of chiral molecules....
in Scheme 1 is shown three-dimensionally. The Hückel computations revealed that the beta-carbons (i.e. C2 and C5) of the cyclohexadienone ring had a large bond-order. As seen in the scheme a beta-beta bond is formed. Subsequent to this, radiationless decay leads to a zwitterion
Zwitterion
In chemistry, a zwitterion is a neutral molecule with a positive and a negative electrical charge at different locations within that molecule. Zwitterions are sometimes also called inner salts.-Examples:...
ground state. The final rearrangement leads to lumisantonin as can be discerned by comparing the three-dimensional drawing with the earlier two-dimensional representation.
Scheme 1. The santonin to lumisantonin rearrangement in three-dimensions
4,4-Diphenylcyclohexadienone rearrangement
Quite parallel to the santonin to lumisantonin example is the rearrangement of 4,4-diphenylcyclohexadienone Here the n-pi* tripletMultiplicity (chemistry)
Multiplicity in quantum chemistry is used to distinguish between several degenerate wavefunctions that differ only in the orientation of their angular spin momenta. It is defined as 2S+1, where S is the angular spin momentum....
excited state undergoes the same beta-beta bonding. This is followed by 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:...
(i.e. ISC) to form the singlet ground state which is seen to be a zwitterion
Zwitterion
In chemistry, a zwitterion is a neutral molecule with a positive and a negative electrical charge at different locations within that molecule. Zwitterions are sometimes also called inner salts.-Examples:...
. The final step is the rearrangement to the bicyclic photoproduct. The reaction is termed the type A cyclohexadienone rearrangement.
The contrasting case of 4,4-diphenylcyclohexenone, one double bond missing
To provide further evidence on the mechanism of the dienone in which there is bonding between the two double bonds,the case of 4,4-diphenylcyclohexenone is presented here. It is seen that the rearrangement is quite different; thus two double bonds are required for a type A rearrangement. With one double bond one of the phenyl groups, originally at C-4, has migrated to C-3 (i.e. the beta carbon)."Mechanistic and Exploratory Organic Photochemistry, IX. Phenyl Migration in the Irradiation of 4.4-Diphenylcyclohexenone," Zimmerman, H. E.; Wilson, J. W. J. Am. Chem. Soc., 1964, 86, 4036-4042.
It is of considerable interest that when one of the aryl groups has a para-cyano or para-methoxy group, that substituted aryl group migrates in preference. Inspection of the alternative phenonium-type species, in which an aryl group has begun to migrate to the beta-carbon, reveals the greater electron delocalization with a substituent para on the migrating aryl group and thus a more stabilized pathway.
Pi-pi* reactivity
Still another type of photochemical reaction is the di-pi-methane rearrangementDi-pi-methane rearrangement
The di-pi-methane rearrangement is a photochemical reaction of a molecular entity comprising two π-systems, separated by a saturated carbon atom , to form an ene- substituted cyclopropane...
."Unsymmetrical Substitution and the Direction of the Di-pi-Methane Rearrangement; Mechanistic and Exploratory Organic Photochemistry. LVI," Zimmerman, H. E.; Pratt, A. C. J. Am. Chem. Soc., 1970, 92, 6259-6267 Two further early examples were the rearrangement of 1,1,5,5-tetraphenyl-3,3-dimethyl-1,4-pentadiene (the "Mariano" molecule) and the rearrangement of barrelene
Barrelene
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 semibullvalene. We note that, in contrast to the cyclohexadienone reactions which used n-pi* excited states, the di-pi-methane rearrangements utilize pi-pi* excited states.
Parallel studies on multiplicity; the role of triplets
Parallel to the structural studies of the Zimmerman group as described above, the Cal. Tech. group with GeorgeHammond pursued the role of multiplicity on reactivity. The importace of triplet excited species was emphasized. The triplets tend to be longer lived than singlets and of lower energy than the singlet of the same configuration.
Hammond had summarized the energies of triplets of the more common molecules. He noted that triplets may arise from (A) conversion of the initially formed singlets or by (B) interaction with a higher energy triplet (sensitization).
Another contribution of Hammond was the determination of triplet reaction rates. Finally, with low energy triplets present it was shown possible to quench a triplet reaction.
The ground work had been laid by very earlier studies by Terenin and Ermolaev in Russia who demonstrated intermolecular triplet transfer at low temperatures and described the kinetics.
Further on the subsequent years
After the basics had been established organic photochemistry seemed to accelerate exponentially. One facet derivedfrom the Cal. Tech. group of George Hammond, a group of remarkable researchers. Nicholas (Nick) Turro was a graduate student in the Hammond group along with Angelo Lamola,Peter Leermakers, Jack Saltiel, Robert Liu, Angelo Lamola and a number of others. However, the emphasis of the Cal. Tech. group remained on observation of kinetic and multiplicity dependence rather than the effect of the electronic structure of the excited species in controlling charges, bond-orders and thus reactivity.
Common organic photochemical reactions
Among the more common organic photochemical reactions there are the Norrish Type INorrish reaction
The Norrish reaction in organic chemistry describes the photochemical reactions taking place with ketones and aldehydes. This type of reaction is subdivided in Norrish type I reactions and Norrish type II reactions...
, the Norrish Type II
Norrish reaction
The Norrish reaction in organic chemistry describes the photochemical reactions taking place with ketones and aldehydes. This type of reaction is subdivided in Norrish type I reactions and Norrish type II reactions...
, the racemization
Racemization
In chemistry, racemization refers to the converting of an enantiomerically pure mixture into a mixture where more than one of the enantiomers are present...
of optically active biphenyls, the type A cyclohexadienone rearrangement, the type B cyclohexenone rearrangement, the di-pi-methane rearrangement
Di-pi-methane rearrangement
The di-pi-methane rearrangement is a photochemical reaction of a molecular entity comprising two π-systems, separated by a saturated carbon atom , to form an ene- substituted cyclopropane...
, the type B bicyclo[3.1.0]hexanone rearrangement to phenols, photochemical electrocyclic
Electrocyclic reaction
In organic chemistry, an electrocyclic reaction is a type of pericyclic rearrangement reaction where the net result is one pi bond being converted into one sigma bond or vice-versa...
processes, the rearrangement of epoxyketones to beta-diketones, ring opening of cyclopropyl ketones, heterolysis of 3,5-dimethoxylbenzylic derivatives, and photochemical cyclizations of dienes. Some of these have been described above and with this being an encyclopedic survey only a selected few are considered here.