Howard E. Zimmerman

Interpretation of Some Organic Photochemistry

A general introduction to current organic photochemical research is presented. A limited survey of typical organic photochemical reactions, with an emphasis on recent studies, is given. Reactions of the n-π* and π-π* type are included. Some new thoughts are included; among these is the relation of M�bius strip geometry to organic photochemistry.

Orientation in metal ammonia reductions

Abstract The unique orientation effects observed in the alkali metal-ammonia reactions of aromatic compounds are considered from a molecular orbital viewpoint. The Birch rule governing the reduction of substituted aromatics, the peculiar effect of substituents on the ease of hydrogenolysis of benzyl alcohols, the similar effect in the dealkylation and dearylation of alkyl aryl ethers and aryl ethers, the course of dealkoxylation of alkyl aryl ethers, are all found to be reasonably accommodated by molecular orbital theory.

Möbius and hückel systems in the scf and ci approximations

Abstract Over the last 15 years since the introduction of Mobius-Huckel theory, a number of varied questions has accumulated. The most interesting of these deals with the question of whether or not the Mobius-Huckel theory is valid in the SCF and SCF-CI approximation. This paper presents a treatmet which shows that the repulsion and exchange contributions are independent of the Mobius vs Huckel nature of the orbital array. Also it is shown that the one-electron terms are symmetry determined and derive from SCF coefficients. An analytical SCF-CI treatment if given. Several further unanswered questions are also considered.

A Mechanistic Analysis of the Birch Reduction

The Birch Reduction is one of the main reactions of organic chemistry. The reaction involves the reaction of dissolving metals in ammonia with aromatic compounds to produce 1,4-cyclohexadienes. Discovered by Arthur Birch in 1944, the reaction occupies 300 pages in Organic Reactions to describe its synthetic versatility. Thus, it is remarkable that the reaction mechanism has been so very controversial and only relatively recently has been firmly established. Perhaps this is not that surprising, since the reaction also has many unusual and esoteric mechanistic facets. Here, I provide a description of how I have applied ever-evolving levels of quantum mechanics and a novel experimental test to understand details of the mechanism and the origins of the selectivities observed in the Birch reduction. The reaction involves an initial radical anion resulting from introduction of an electron from the blue liquid ammonia solution of free electrons formed by the dissolution of Li or related metals. This radical anion is protonated by an alcohol and then further reduced to a carbanion. Finally, the carbanion is protonated using a second proton to afford a nonconjugated cyclohexadiene. The regiochemistry depends on substituents present. With 18 resonance structures in the case of anisole radical anion, prediction of the initial protonation site would seem difficult. Nevertheless, computational methods from Hückel theory through modern density functional calculations do correctly predict the site of protonation. An esoteric test established this mechanism experimentally. The nature of the carbanion also is of mechanistic interest, and the preponderance of the resonance structure shown was revealed from Hückel calculations involving variable bond orders. For the trianion from benzoic acid, parallel questions about structure are apparent, and have been answered. Some mechanistic questions are answered experimentally and some by modern computations. Recently, our mechanistic understanding has led to a variety of synthetic applications. For example, the preparation of alkyl aromatics from benzoic acids makes use of the intermediates formed in these reactions. This Account provides an overview of both experimental techniques and theoretical methodology used to provide detailed mechanistic understanding of the Birch Reduction.

Stereochemistry as a probe for photochemical reaction mechanisms

Abstract In this publication we have reviewed examples derived from our photochemical investigations where stereochemistry provides information allowing elucidation of the mechanistic details of electronically excited state transformations. The reactions discussed include unimolecular rearrangements of both singlet and triplet excited state species.

Five decades of mechanistic and exploratory organic photochemistry

This exposition summarizes a personal perspective on past and current developments in organic photochemistry. It comprises three portions. The first section describes the beginnings of our photochemical studies and how it was possible to relate photochemical reactivity to excited-state electronic structures. The second selectively relates some of the reactions and concepts developed in the intermediate years. Finally, the third portion describes our recent research.

DI-π-METHANE HYPERSURFACES AND REACTIVITY; MULTIPLICITY AND REGIOSELECTIVITY; RELATIONSHIP BETWEEN THE DI-π-METHANE AND BICYCLE REARRANGEMENTS: MECHANISTIC AND EXPLORATORY ORGANIC PHOTOCHEMISTRY

Abstract The photochemistry of 1,1-dicarbomethoxy-3,3,5,5-tetraphenyl-1,4-pentadiene and 3,3-dicarbomethoxy-1,1,5,5-tetraphenyl-1,4-pentadiene was investigated. Both direct and sensitized photolyses of the 3,3-diester afforded 1,1-dicarbomethoxy-2,2-diphenyl-3-(2′,2′-diphenylvinyl)cyclopropane with quantum yields of 0.42 and 0.32, respectively. Direct irradiation of the 1,1-dicarbomethoxy-pentadiene gave this same diphenylvinyl-cyclopropane product (O = 0.39), while acetophenone sensitization of the 1,1-dicarbomethoxy-pentadiene led to the regioisomeric 1,1,2,2-tetraphenyl-3-{2′,2′-dicarbomethoxyvinyl) cyclopropane (O = 0.92). Thus the unsymmelrically substituted cyclopropyldicarbinyl diradical opened one or the other of two three-ring bonds depending on multiplicity. Direct irradiation of the dicarbomethoxyvinylcyclopropane resulted in a bicycle rearrangement affording the diphenylvinyl-cyclopropane (O = 0.10): additionally, the 1,1-dicarbomethoxy-pentadiene was formed (O = 0.069) in this photolysis. The second product can be viewed as arising from a reverse di-π-methane rearrangement or, alternatively, as deriving from partial bicycling of the diphenylcarbon moiety. To confirm that the bicycling process was intramolecular rather then by disengagement of a diphenylcarbene followed by readdition, a crossover experiment was run using 1,1-dicarbomethoxy-4,4-di-p-tolylbutadiene. However, no diphenylcarbene transfer was seen. Single photon counting was employed to obtain unimolecular rate constants for the S1, excited states undergoing the di-π-methane rearrangements. A plot of the log of the S1 rates for centrally disubstituted 1,1,5,5-tetraphenyl-1,4-pentadienes-having central carbomethoxy, cyano or methyl groups—vs Hammelt ground stale sigma constants proved linear with a rho value of −2.53. The various excited states, intermediate species and the related hypersurfaces were subjected to SCF and SCF-CI calculations. The delta-P treatment showed that excitation is concentrated in the cyclopropyldicarbinyl diradical moiety as the di-π-methane rearrangement proceeds. The calculations also suggest that for a successful bicycle rearrangement excitation energy needs to be distributed into the three-ring bond not conjugated with the vinyl group. Triptych correlation diagrams were constructed using SCF and SCF-CI energies and are used to interpret the photochemistry. The differing singlet versus triplet regiochemistry was found to follow the “small K vs large K” generalization used previously.

Report on recent photochemical investigations

A mechanistic treatment of organic photochemical reactions was proposed by us earlierl wherein the electronic details of molecular transformations occurring ma y be considered in detail. The portion of this hypothesis dealing with unsaturated ketones is the subject of the present talk. In our earlier publications we proposed that mesoionic species of types (I) and (li), and their conjugate acids, are intermediates in many of the photochemical reactions of cross-conjugated cyclohexadienones. This portion of

Heterocyclic Photochemistry in Contrast with Carbon Behavior. Regioselective Photochemical Rearrangement of an Azacyclohexadienone: Mechanistic and Exploratory Organic Photochemistry

The Type-A photochemistry of cyclohexadienones is well-studied and follows a well-established mechanistic pathway. One early example is the rearrangement of santonin to lumisantonin. Another example is the rearrangement of 4,4-diphenylcyclohexa-1,5-dienone. Remarkably, replacement of one carbon by nitrogen alters the reaction course to give a regioselective phenyl migration.

Di-π-methane hypersurfaces and reactivity; multiplicity and regioselectivity; relationship between the di-π-methane and bicycle rearrangements

Abstract The photochemistry of 1,1-dicarbomethoxy-3,3,5,5-tetraphenyl-1,4-pentadiene and 3,3-dicarbomethoxy-1,1,5,5-tetraphenyl-1,4-pentadiene was investigated. Both direct and sensitized photolyses of the 3,3-diester afforded 1,1-dicarbomethoxy-2,2-diphenyl-3-(2′,2′-diphenylvinyl)cyclopropane with quantum yields of 0.42 and 0.32, respectively. Direct irradiation of the 1,1-dicarbomethoxy-pentadiene gave this same diphenylvinyl-cyclopropane product (O = 0.39), while acetophenone sensitization of the 1,1-dicarbomethoxy-pentadiene led to the regioisomeric 1,1,2,2-tetraphenyl-3-{2′,2′-dicarbomethoxyvinyl) cyclopropane (O = 0.92). Thus the unsymmelrically substituted cyclopropyldicarbinyl diradical opened one or the other of two three-ring bonds depending on multiplicity. Direct irradiation of the dicarbomethoxyvinylcyclopropane resulted in a bicycle rearrangement affording the diphenylvinyl-cyclopropane (O = 0.10): additionally, the 1,1-dicarbomethoxy-pentadiene was formed (O = 0.069) in this photolysis. The second product can be viewed as arising from a reverse di-π-methane rearrangement or, alternatively, as deriving from partial bicycling of the diphenylcarbon moiety. To confirm that the bicycling process was intramolecular rather then by disengagement of a diphenylcarbene followed by readdition, a crossover experiment was run using 1,1-dicarbomethoxy-4,4-di-p-tolylbutadiene. However, no diphenylcarbene transfer was seen. Single photon counting was employed to obtain unimolecular rate constants for the S1, excited states undergoing the di-π-methane rearrangements. A plot of the log of the S1 rates for centrally disubstituted 1,1,5,5-tetraphenyl-1,4-pentadienes-having central carbomethoxy, cyano or methyl groups—vs Hammelt ground stale sigma constants proved linear with a rho value of −2.53. The various excited states, intermediate species and the related hypersurfaces were subjected to SCF and SCF-CI calculations. The delta-P treatment showed that excitation is concentrated in the cyclopropyldicarbinyl diradical moiety as the di-π-methane rearrangement proceeds. The calculations also suggest that for a successful bicycle rearrangement excitation energy needs to be distributed into the three-ring bond not conjugated with the vinyl group. Triptych correlation diagrams were constructed using SCF and SCF-CI energies and are used to interpret the photochemistry. The differing singlet versus triplet regiochemistry was found to follow the “small K vs large K” generalization used previously.