Jerome A. Berson

Discoveries missed, discoveries made: creativity, influence, and fame in chemistry

Abstract The paper describes some of the historical background of the discoveries of the diene synthesis and the orbital symmetry conservations rules. It examines the circumstances that prevented these discoveries from being made by earlier investigators and speculates about the mental qualities conductive to creativity in science.

Non-kekulé molecules derived conceptually by heteroatom-for-carbon substitution in alternant hydrocarbons: M-Quinomethane and M-Naphthoquinomethane

Abstract Qualitative theoretical consideration suggest that substitution of a heteroatom for a C atom in an alternant hydrocarbon might results in a non-Kekule with a triplet ground state, despite the nondegeneracy of the Huckel MBMOs of the heteroatom derivatives. The syntheses of 3-methylene-1-phenoxyl and 3-methylene-1-naphthoxyl are described. These compounds. biradical valence tautomers of m-quinomethanes, are characterized by EPR spectroscopy and are assigned triplet ground states. Optical resolution of 6-methylenebicyclo[3.1.0]hex-3-ene-one, the precursor of 3-methylene-1-phenoxyl, is described. Repeated irradiation of the active ketone a 20K with intermittent warming to room temperature followed by re-isolation gives ketone of undimished activity, which suggests that re-cyclization of presumably achiral) 3-methylene-1-phenoxyl does not occur under the conditions.

The stereochemistry and mechanism of the [σ2 + σ2 +σ2] cycloreversion☆

Abstract The search for concert in the cycloreversion of a 6-membered ring to three pi-bonded fragments uses three mechanistic criteria: an enhanced rate as compared to model compounds, the absence of ring-closure products, and stereospecificity. By these criteria, the decompositions of 1,2- diazacyclohexa-1,4-dienes (57) and 2,3-diazabicyclo [4.1.0] hept-2-enes (58) are clearly concerted. The decomposition of 2,3-diazabicyclo [4.2.0] oct-2-enes (59), although it shows only a small rate enhancement, nevertheless meets the product and stereospecificity criteria and probably is concerted. The data suggest that the degree of concert gradually declines as the pi-character of the participating ring bond decreases. The cycloreversions of both sets of bicyclic compounds 58 and 59 follow an “inward-disrotatory” path undoubtedly because their transition states have the most efficient orbital overlap. An analysis of the [2 + 2 + 2] cycloreversion reveals an ambiguity in nomenclature that is a general feature of the Woodward-Hoffmann Rules in their 2-electron-component form. For a reaction involving any given number of such components, all allowed stereochemical pathways have the same set of descriptions.

Synthesis of the covalent benzene-carbon monoxide cycloadduct, norborna-2,5-dien-7-one: Correlation of kinetic and thermodynamic stabilities in cycloreversion reactions

endo-Tricyclo[4.2.1.02,5]non-7-ene-3,4,9-trione (14) is prepared from the 5,5-diethoxy-cyclopcntadiene-diethyl maleate Diels-Alder adduct. Photolysis of the trione in CD2Cl2 solution at 192 K forms norborna-2,5-dien-7-one (2). The latter substance is thermally unstable and decomposes to benzene and CO with ΔG‡ =15 kcal mol-1. This appears to be the lowest activation energy for a cycloreversion yet measured. A correlation is found for the kinetic and thermodynamic stabilities in a series of 18 orbital symmetry allowed cycloreversions by plotting ΔG‡ vs ΔHr.

Π-Conjugated Non-Kekule Molecules and the Limits of Hund's Rule

Abstract Whether certain theoretical restrictions on the applicability of Hunds rule must be kept in mind for the proper design of high-spin organic molecules as potential building blocks for ferromagnetic materials is a subject of intense current concern. It has been almost fifty years since E. Huckel pointed out that a biradical constituted by union of inactive sites of two monoradicals might have a singlet ground state. Experimental tests of this conjecture (in its recent more sophisticated formulations by Borden and Davidson and by Ovchinnikov) now have led to the synthesis and characterization of several new ground state singlet biradicals.

Identity of 3,4-dimethylenethiophene reactive intermediates from diazene and bis-allene precursors. Correlation of singlet biradical reactivity in the tetramethyleneethane and trimethylenemethane series.

Abstract The relative reactivities of the 3,4-dimethylenethiophene intermediates 1 from the deazetation of the diazene 5 and from the intramolecular cyclization of bisallenyl sulfide 3 , determined by competition experiments, serve as a “fingerprint” to show that the same reactive transient is generated from both precursors.

Quantitative measurement of endo preference in homodienyl hydrogen shifts

Abstract From the kinetics of the gas-phase pyrolysis of a series of cis -1,1-dimethyl-2-alkenylcyclopropanes, the preference for the endo transition state in the homodienyl hydrogen shift (retro-ene reaction) is estimated to be at least 12 kcal/mol.

Generation and reactivity of sterically hindered bicyclo[3.1.0]hex-1-enes

Abstract 2-t-butylbicyclo[3.1.0]hex-1-ene dimerizes primarily by a [σ+ π] way, whereas 2,5-di-t-butylbicyclo[3.1.0]hex-1-ene undergoes an unusual hydrogen shift reaction to 1,3-di-t-butyl-2-methylcyclopentadiene.

1-bar|t-butyl-5-isopropylidenebicyclo[2.1.0]pentane

Abstract The title compound, synthesized by a carbenoid cyclization-rearrangement route, is the most stable member of the bicyclo[2.1.0]pentane series yet prepared.

Experimental test of an alternative hypothesis for isotope-position scrambling in the photochemical wolff rearrangement of zibenzil

Abstract Photolysis of azibenzil1 13 CO in methanol gives partially scrambled methyl diphenulacetate and unscrambled recovered azibenzil.