Haloarenes carbanion reactions

Reaction of activated haloarenes with cyanide and carbanions... 

Rossi, R.A. and Penenory, A.B. (2003) The photostimulated SRN1 process reaction of haloarenes with carbanions, in CRC... 

Quantum yields of photoinitiated reactions have been used as a qualitative measure of the chain length. The quantum yields for substitution of haloarenes with nitrile-stabilized carbanions range from 7 to 31 in liquid ammonia [24]. Quantum yields from 20 to 50 have been determined for the substitution of iodobenzene by (EtO)2PO ions [25]. 

Haloarenes, such as 2-bromonaphthalene or/ -bromobiphenyl, form in liquid ammonia CTC with acetonitrile or propionitrile carbanions, which are responsible for the photo-stimulated ET of the initiation step. The quantum yields of these reactions (2 > 313 nm) range from 8 to 3132. 

As indicated in Section IV. A. 1, the reactivity of carbanions in photostimulated reactions with haloarenes depends on the nature of the reaction step. In the initiation step the rate of photostimulated ET from the carbanion to the acceptor Phi increases with the pvalue of the corresponding conjugate acids. In the propagation cycle the reactivity of the carbanions would depend on their pKa values as well as on the energy of the SOMO of the radical anion intermediate, that is, on the HOMO-SOMO energy difference (loss in n energy). 

In the presence of an electron rich donor molecule an alternative to direct fission or reaction via an excimer is the formation of an exciplex or radical anion/radical cation ion pair (Eq. 2). The radical anion has been viewed as the key intermediate which undergoes fission to aryl radical and halide ion (Eq. 3). With polyhaloarenes there is an additional option. A polychloroarene radical anion, for example, has two possible modes for bond fission (a) fission to produce aryl radical and chloride ion or (b) fission to form an aryl carbanion and chlorine atom (Scheme 6). The options for fragmentation of a haloarene radical anion... 

Complexes of haloarenes undergo nucleophilic substitution of the halogen by stabilized carbanions to generate the substituted arene complex. - This reaction occurs with nucleophiles that add reversibly to the arene ring because the kinetically preferred site of attack is ortho and meta to the halogen. As noted above, extrusion of the halogen and a productive substitution reaction occurs upon addition to the less-favored carbon located ipso to the halogen (Equation 11.53). 

Photonucleophilic aromatic substitution reactions of phenyl selenide and telluride with haloarenes have also been proven to involve the S jlAr mechanism, with the formation of anion radical intermediates. Another photonucleophihc substitution, cyanomethylation, proves the presence of radical cations in the reaction mechanism. Liu and Weiss have reported that hydroxy and cyano substitution competes with photo substitution of fluorinated anisoles in aqueous solutions, where cation and anion radical intermediates have been shown to be the key factors for the nucleophilic substitution type. Rossi et al. have proposed the S j lAr mechanism for photonucleophihc substitution of carbanions and naphthox-ides to halo anisoles and l-iodonaphthalene. > An anion radical intermediate photonucleophilic substitution mechanism has been shown for the reactions of triphenyl(methyl)stannyl anion with halo arenes in liquid ammonia. Trimethylstannyl anion has been found to be more reactive than triphenylstannyl anion in the photostimulated electron- transfer initiation step. 

The Photostimulated SrnI Process Reaction of Haloarenes with Carbanions... 

Even though photoinduction is the most commonly used initiation method, not many studies on the mechanism involved have been reported. This step can involve ET within an excited charge-transfer complex (CTC) formed between the nucleophile and the substrate. This type of initiation has been proposed for the reaction of CHjCOMe ions with iodobenzene and bromobenzene in DMSO, for nitrile carbanions with haloarenes such as 2-bromonaphthalene or p-bromobiphenyl in liquid ammonia, " and in the reaction of iodobenzene and PhSEt " with (EtO)2PO ions in DMSO and DMF, respectively. Acceleration by KI in the substitution of haloarenes with (EtO)2PO ions or with 2-naph-thoxide ions has been explained on the same basis. Initiation by a CTC has also been suggested for the photoinduced reaction of neophyl iodide with carbanions in DMSO. ... 

For a given aryl moiety, a rough correlation exists between the reduction potential and its reactivity in ET reactions.The order of reduction potential in Hquid ammonia. Phi > PhBr > PhNMe, > PhSPh > PhCl > PhF > PhOPh, coincides with the reactivity order determined under photoinitiation. By using competition experiments of pairs of halobenzenes toward CH2COBu-t ions under photoinitiation, the span in reactivity from fluorobenzene to iodobenzene was found to be about 100,000. Quantum yields of photoinitiated reactions have been used as a quahtative measure of the chain length. The quantum yields for substitution of haloarenes with nitrile-stabilized carbanions range from 7 to 31 in Hquid ammonia. " Quantum yields from 50 to 20 have been determined for the substitution of halonaphthoxides and iodobenzene by (EtOljPO ions. ... 

The synthesis of N,N-disubstituted-a-arylacetamides can be achieved by reaction of haloarenes with carbanions from N,N-disubstituted amides in liquid ammonia under light initiation. The potassium salt of N-acetylpiperidine does not react, probably due to its low solubility in liquid ammonia. The anions derived from acetamide and N-methyl acetamide are also unreactive. ... 

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