A rigorous test for orbital symmetry control in cation radical/neutral cycloadditions

Abstract

Two potentially distinct mechanistic role senses have been recognized for the cation radical Diels-Alder reaction.1,2 The

The Intramolecular Radical Cation Induced Diels-Alder Reaction in the Diene - Diene Format

“…For the major isomer (10), nuclear magnetic resonance spectra ( 1 H-NMR, 13 C-NMR, COSY, HETCOR, NOESY) and decoupling experiments were consistent with the proposed structure (10) but were unable to verify the trans ring juncture. Conversion of (10) to a bis(dichlorocarbene) adduct using chloroform, KOH and a phase transfer catalyst, followed by slow crystallization from acetonitrile produced crystals of the tetrachloro adduct (11), which by X-ray crystallography confirmed the trans ring juncture. (11) As expected, based on the differences in oxidation potential, the initiator (1) failed to induce Diels-Alder adduct formation with compound (4); no reaction had occurred after stirring for 24 hours at room temperature.…”

“…Conversion of (10) to a bis(dichlorocarbene) adduct using chloroform, KOH and a phase transfer catalyst, followed by slow crystallization from acetonitrile produced crystals of the tetrachloro adduct (11), which by X-ray crystallography confirmed the trans ring juncture. (11) As expected, based on the differences in oxidation potential, the initiator (1) failed to induce Diels-Alder adduct formation with compound (4); no reaction had occurred after stirring for 24 hours at room temperature. In an attempt to expose (4) to more vigorous conditions, this compound was irradiated at 300nm in acetonitrile solution in the presence of the sensitizer 1,4-dicyano-2,3,5,6tetraethylbenzene (E ox =1.96 V) [23].…”

“…Although a [4+1] cycloaddition pathway is symmetry allowed [10], the reaction appears virtually insensitive to symmetry considerations and the products of both [4+1] and [3+2] pathways have been observed [11]. Calculation of a minimum energy reaction pathway for the [3+2] cycloaddition using MINDO/3 revealed the likelihood that the reaction proceeds by a non-synchronous "bis-allylic" pathway [12].…”

Reactive atomistic simulations of Diels-Alder-type reactions: conformational and dynamic effects in the polar cycloaddition of 2,3-dibromobutadiene radical ions with maleic anhydride

“…[17][18][19] A number of studies focused on the question of the conservation of orbital symmetry in these ionic reactions in view of the Woodward-Hoffmann rules which are widely used for rationalizing mechanistic aspects of neutral DA processes. [19][20][21] In Ref., 14 it was discussed that arguments based on orbital symmetry can be misleading for polar cycloadditions. In electronic-symmetry-conserving reactions, orbital concepts should be replaced with an analysis of the symmetries of the electronic states of the various species along the entire reaction path.…”

A computational study of the Diels-Alder reactions between 2,3-dibromo-1,3-butadiene and maleic anhydride

“…Cationic Diels-Alder reactions (polar cycloadditions) are often faster but still show a high degree of stereoselectivity [26,27,28]. There have been some studies on the conservation of orbital symmetry to try to construct rules analogous to the Woodward-Hoffmann rules widely used for the neutral reactions [28,29,30]. Wiest and Donoghe proposed a model in which the electronic state symmetry must be conserved throughout the reaction [14].…”