Thermolyses of tert-butyl .DELTA.1,1'-dicyclohexenylperoxyacetate. Electrocyclic and other transformations of a pentadienyl radical

Ab Initio Study of 1,4-Pentadienyl Electrocyclic Reactions

“…The experimental study by Lehr et al on substituted pentadienyls found that fewer than 13% of the products had the stereochemistry of the disrotatory mechanism …”

“…They concluded that steric factors are likely to play a dominant role in favoring the conrotatory path in the pentadienyl ring-closing. 6 Fox et al reported an AM1 activation barrier some 45 kJ/mol higher for the disrotatory mechanism than for the conrotatory. 10 We carried out a cursory three-dimensional search of the AM1 potential energy surface in the area of the transition state, fixing the parameters φ eab1 , φ aed6 , and r ae and optimizing all other parameters.…”

“…The experimental study by Lehr et al on substituted pentadienyls found that fewer than 13% of the products had the stereochemistry of the disrotatory mechanism. 6 Lehr et al noted that a disrotatory pathway is predicted using Woodward and Hoffman rules guided by the HOMO molecular orbital symmetry, but that this reasoning conflicts with other predictive methods in the case of the analogous cyclization of allyl radical to cyclopropyl. They concluded that steric factors are likely to play a dominant role in favoring the conrotatory path in the pentadienyl ring-closing.…”

“…The 1,4-pentadienyl radical ( 1 ) is an archetypal delocalized system, combining the resonance stabilization of allyl with the conformational degrees of freedom of the polyenes. Numerous theoretical and experimental studies have been directed at the structures, formation enthalpies, and conformational dynamics of pentadienyl and its derivatives, − including several studies focusing on their activity as intermediates in the action of lipoxygenases. ,, Despite a resonance stabilization energy for pentadienyl of 60−80 kJ/mol, , electrocyclic formation of the less-studied cyclopenten-3-yl radical ( 2 ) from the ( E , E )-pentadienyl chain 1a is exothermic by about 48 kJ/mol. , The resulting radical 2 is similarly stabilized by a three-electron, three-carbon π system; Benson and co-workers calculated the resonance stabilization of cyclopenten-3-yl to be 53 kJ/mol . The mechanism for this cyclization reaction has consequently been a candidate for computational and experimental investigation for over a decade.…”

“…The 1,4-pentadienyl radical ( 1) is an archetypal delocalized system, combining the resonance stabilization of allyl with the conformational degrees of freedom of the polyenes. Numerous theoretical and experimental studies have been directed at the structures, formation enthalpies, and conformational dynamics of pentadienyl and its derivatives, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] including several studies focusing on their activity as intermediates in the action of lipoxygenases. 9,17,18 Despite a resonance stabilization energy for pentadienyl of 60-80 kJ/mol, 2,8 electrocyclic formation of the less-studied cyclopenten-3-yl radical (2) from the (E,E)-pentadienyl chain 1a is exothermic by about 48 kJ/ mol.…”