Carbon-Skeletal Anionic and Radical Sigmatropic Rearrangements: Group Migratory Aptitudes as a Probe of Charge Type in the 1,2-Shifts of β-Phenyl-β-(2-pyridyl)- and β-Phenyl-β-(4-pyridyl)ethyl Systems

Article abstract of DOI:10.1021/jo00229a001

In order to probe the occurrence and relative ease of carbon-skeletal <1,2> sigmatropic rearrangements of the free-radical, anionic, or radical-anionic type, derivatives of the β,β-diphenyl-β-(2-pyridyl)- and the β,β-diphenyl-β-(4-pyridyl)ethane systems, PyPh2CCH2E (A), were treated with reagents expected to generate radical or anionic sites.The ensuing, competitive <1,2>-shifts of the phenyl and/or pyridyl groups were then used as a diagnostic sign of the mechanism of rearrangement.Both the treatment of A (E = p-tolyl) with MeLi or KH and the reaction of A (E = Cl) with sodium or lithium in donor solvents caused an exclusive <1,2>-pyridyl shift.Gas chromatographic and mass spectral analyses were able to place the limit of any <1,2>-phenyl shift as under 0.5percent.In such alkali metal reactions, persistent aromatic radical-anions were detected by ESR spectroscopy until the completion of the reaction.Such signals and the significant amounts of carbon-carbon bond cleavage products support the formation of pyridyl radical-anions as precursors for such cleavages and <1,2>-pyridyl rearrangements.That such radical-anions could lead to spiro intermediates that promote the <1,2>-pyridyl migrations wins corroboration from the finding that the methiodide of 1-chloro-2-methyl-2-(4-pyridyl)propane can be reduced with lithium in THF to yield the isolable 1,1,6-trimethyl-6-azaspiro<2.5>octa-4,7-diene.The same two chlorides of A responded differently under other rearrangement conditions: (1) in preparing such chlorides from the corresponding alcohols, PyPh2CCH2OH, with thionyl chloride, the 4-pyridyl isomer underwent a Wagner-Meerwein rearrangement with exclusive <1,2>-phenyl migration; the 2 isomer underwent normal displacement of OH by Cl; (2) toward the free-radical reducing agent, (n-Bu)3SnH, the 2-chloro isomer underwent both <1,2>-phenyl and -pyridyl shifts, while the 4-chloro isomer underwent neither reduction nor rearrangement; it simply induced the formation of hexa-n-butylditin.A similar reducing action was observed with bis(1,5-cyclooctadiene)nickel.These observations are analyzed with the aid of Hueckel molecular orbital theory and the rearrangements observed with reducing agents are assessed in terms of three types of mechanisms: (1) authentic <1,2>-anionic shifts; (2) authentic <1,2>-free-radical shifts; and (3) competing electron transfer from the metal to the chloride center or from the metal to the pyridyl ring, which permits anionic rearrangements to compete with rearrangements mediated by radical-anion or dianions, which latter processes form the crucial spiro intermediate by intramolecular nucleophilic displacement on the CH2Cl group.