Carbon-Skeletal Anionic Rearrangements and the ?-Orbital Overlap Constraint: The Question of Nucleophilic Attack versus Electron Transfer

Article abstract of DOI:10.1021/jo00267a011

In order to evaluate the geometrical requirements and the actual electronic nature of apparent <1,2> anionic rearrangements of metalated aromatic hydrocarbons, amines, and ethers, cyclic structural types of such anions were generated as lithium salts by proton abstraction from C-H bonds by RLi or by C-Cl bond cleavage by Li.The cyclic systems examined were anions of 9,9-dimethyl-, 9-methyl-9-benzyl-, and 9-benzyl-9-phenylfluorenes; 9-methyl-9-phenyl-, 9,9-diphenyl-, and 9,9-(2,2'-biphenylene)-9,10-dihydrophenanthrenes; 5-methyl- and 5-phenyl-5,6-dihydrophenathridines;and 9H-dibenzopyran.The anions generated from 9-methyl-9-benzylfluorene, 9-benzyl-9-phenylfluorene, 5-methyl-5,6-dihydrophenanthridine, and 9H-dibenzopyran, as well as 9-methyl-9-(lithiomethyl)fluorene, did not undergo skeletal rearrangement when heated between 40 and 120 deg C for protracted periods.However, the anions derived from the 9-methyl-9-phenyl-, 9,9-diphenyl-, and 9,9-(2,2'-biphenylene)-9,10-dihydrophenanthrenes did undergo rearrangement with a <1,2> shift of the 9-aryl group.With the anion of 5-phenyl-5,6-dihydrophenanthridine, some <1,2> shift of the 5-phenyl was observed, but the principal rearrangement was ring contraction with the formation of N-phenyl-9-fluorenylamine.By noting which anions underwent skeletal rearrangement and which competing migrating groups in a given anion underwent a <1,2> shift preferentially, we have formulated an appropriate geometrical view of the transition states involved.Furthermore, by generating the 2,2,2-triphenylethyl anion (as its lithium salt) from (a) 2-chloro-1,1,1-triphenylethane and Li, (b) 2-bromo-1,1,1-triphenylethane and n-BuLi, and (c) bis(2,2,2-triphenylethyl)mercury and n-BuLi, we attempted to learn whether such <1,2> shifts were truly nucleophilic or whether SET processes were involved.Evidence for SET processes was obtained for the generation of (2,2,2-triphenylethyl)lithium by method a, but no ESR or CIDNP evidence for radical intermediates was observable when (2,2,2-triphenylethyl)lithium was produced by method c.