Phenonium Ions from the Addition of Phenyl Cations to Alkenes. Photochemical Synthesis of (Rearranged) Aminoalkylanilines from Haloanilines in the Presence of Alkenes and Amines

Abstract

beta-Aminoalkylanilines are smoothly obtained by irradiation of 4-chloro- and 4-fluoroanilines (as well as the N,N-dimethyl derivatives) in the presence of alkenes (1-hexene, cyclohexene) and amines (butylamine, piperidine) in polar, protic solvents such as trifluoroethanol (yield 40-75%). The reaction involves photoheterolysis of the haloaniline, addition of the resulting phenyl cation to the alkene and trapping of the phenonium cation by amine. A fraction (up to ca. 20%) of aminoalkylanilines resulting from Wagner-Meerwein rearrangement of the phenonium cation is obtained in some cases. Reduction and direct trapping of the phenyl cation by the amine compete with the above three-component synthesis in a less stabilizing solvent such as acetonitrile, but not in CF(3)CH(2)OH.

Photo‐Cross‐Coupling Reaction of Electron‐Rich Aryl Chlorides and Aryl Esters with Alkynes: A Metal‐Free Alkynylation

“…A radically different approach for the synthesis of aryl acetylenes is presented here, and is based on the use of phenyl cations. In the last few years we have demonstrated that these intermediates (in the triplet state) are obtained smoothly by photolysis of aryl halides substitued with electron‐donating groups (for example, chlorophenol15 or fluoroanilines16), aryl sulfonates, and aryl phosphates (for example, methoxyphenyl mesylate or N , N ‐dimethylaminophenyl phosphate),17 and add to alkenes to form an arylC bond via a phenonium ion 15a. 18 We surmised that phenyl cations would react with triple bonds to yield β‐phenylvinyl cations, or more probably cyclic vinylenebenzenium ions (Scheme ).…”

Looking for a Paradigm for the Reactivity of Phenonium Ions

“…This approach (Scheme , a) is appealing for synthetic applications,12 whereas other methods, such as the ring‐opening of benzocyclobutene in superacids, are of spectroscopic interest 13. Some years ago we demonstrated a novel, mild access to phenonium ions by the addition of triplet phenyl cation IV , photogenerated from phenyl halides or esters,14 to olefins (Scheme , b) 15–17. The first‐formed triplet adduct cation with alkenes has a diradical character ( V ), just like the starting phenyl cation, but then intersystem crossing occurs to the more stable singlet phenethyl cation/phenonium ion system ( I / III ), which, contrary to what happens in the triplet manifold, undergoes addition of nucleophiles.…”

Aromatic Chemistry in the Excited State: Facilitating Metal‐Free Substitutions and Cross‐Couplings

“…Historically, the aryl cation has been subjected to cryogenic spectroscopic characterizations and gas‐phase studies; however, its potential in synthesis was largely overlooked, presumably owing to its inaccessibility from general precursors and indiscriminate electrophilicity towards nucleophiles . Hampered by these ground‐state issues, explorative studies were directed to photochemistry for seeking new reaction patterns of the triplet‐state aryl cation.…”