Photoinduced, Ionic Meerwein Arylation of Olefins

Abstract

Irradiation of 4-chloroaniline or of its N,N-dimethyl derivative in polar solvents generates the corresponding triplet phenyl cations. These are trapped by alkenes yielding arylated products in medium to good yields. B3LYP calculations show that the triplet cation slides with negligible activation energy to a bonded adduct with ethylene, whereas it forms only a marginally stabilized CT complex with water (chosen as a representative sigma nucleophile). The structure of the final products depends on the preferred path from the adduct cation with the alkene. In the case of aryl olefins, this deprotonates to stilbene derivatives, while, from 2,3-dimethyl-2-butene and allytrimethylsilane, allylanilines are obtained by elimination of an electrofugal group in gamma. In the case of mono- and disubstituted alkenes the cation adds chloride rather than eliminating and beta-chloroalkylanilines are obtained. The regio- and sterochemistry of the addition across the alkene are best understood with a phenonium ion structure for the adduct. The nucleophile entering in beta can be varied under conditions in which the adduct cation is trapped more efficiently than the starting phenyl cation. Thus, beta-methoxyalkylanilines are formed when the irradiation is carried out in methanol. beta-Iodoalkylanilines are obtained in acetonitrile containing iodide and unsubstituted alkylanilines in the presence of sodium borohydride. A case of intramolecular nucleophilic trapping is found with 4-pentenoic acid. The reaction is a wide-scope ionic analogue of the radicalic Meerwin arylation of olefins.

Revealing Phenylium, Phenonium, Vinylenephenonium, and Benzenium Ions in Solution

“…Formation of a second C À C bond and intersystem crossing led to the strongly stabilized (singlet) phenonium ion. [10] This mechanism fitted nicely with the sequence of events observed in the flash photolysis experiments performed in the presence of DMB (0.2 m). Thus, complete quenching of 3 …”

“…The development of a mild method for their generation greatly enlarges the choice of conditions available for their exploitation, and thus, the control over the final outcome, which hopefully will allow the use of these versatile intermediates for preparative purposes. [10] Experimental Section…”

“…EPR spectra of the phenyl cation have been obtained by photolysis of some phenyl diazonium ions in EtOH at 77 K, [8] and only recently, the IR spectrum of the phenyl cation was obtained by photolysis of phenyl iodide in an argon matrix at 4 K in which the phenyl radical is formed and gives the cation by ensuing electron transfer. [9] Recently, a class of reactions has emerged in which C À C bond formation has been achieved by the photolysis of electron-donating substituted aryl halides [10] or esters [11] in the presence of carbon nucleophiles, for example, alkenes, alkynes, or (hetero)aromatics, with little competing solvolysis. These reactions have been rationalized in terms of trapping of the triplet phenylium ion (I, Scheme 1), which generates further cationic intermediates, for example, the phenonium ion (the cyclopropanespirocyclohexadienyl cation, II) in the case of alkenes.…”

“…These reactions have been rationalized in terms of trapping of the triplet phenylium ion (I, Scheme 1), which generates further cationic intermediates, for example, the phenonium ion (the cyclopropanespirocyclohexadienyl cation, II) in the case of alkenes. [10] This latter species is an intermediate in the solvolysis of phenethyl derivatives, [12] but has mostly been directly detected under superacid conditions. [13] As mentioned above, with benzene, the benzenium ion III is expected to be formed by trapping the phenylium ion with benzene.…”

“…[10] Thus, in polar solvents, heterolysis of the triplet state of 4-chloroaniline led to the triplet cation 4-NH 2 -C 6 H 4 + , which is the lowest-energy spin state of this derivative. Contrary to the singlet cation localized at C1 (p 6 s 0 structure), which reacted unselectively with both n (e.g., water) and p nucleophiles (e.g., alkenes), the triplet cation had a p 5 s 1 structure; this gave C1 a triplet carbene character (one of the unpaired electrons in the molecular plane, the other one delocalized in the p system) with the charge distributed over the ring.…”