A study of various nucleophiles in aromatic SRN1 reactions

Various nucleophiles were subjected to SRNl (nucleophilic substitution via a radical chain mechanism) conditions with representative carboaromatic and heteroaromatic substrates. Upon 5-25 min of near-UV irradiation, the carbanions of 2-benzyl-4,4-dimethyl-2-oxazoline, ethyl phenylacetate, t-butyl acetate, N,N-dimethylacetamide and dimethyl methylphosphonate underwent reactions with both 2-bromopyridine and iodobenzene in an S_RN1 manner to give yields of good substitution products. The S_RN1 reaction between N,N-dimethylacetamide enolate ion and 2-chloroquinoline occurred even without photostimulation.

Due to incomplete ionization and the lack of homogeneity of the reaction mixture, potassio-2,4,4-trimethyl-2-oxazoline reacted with 2-bromopyridine to give low yield of the substitution product through a photostimulated S_RN1 pathway. However, the reaction of this carbanion with iodobenzene afforded moderate yield of the substitution product, which was found to be formed via both S_RN1 and AE mechanisms.

The dimethyl phosphite anion reacted slowly with 2-bromopyridine and failed to react with 2-chloroquinoline and chloropyrazine under the influence of near-UV light. This unusual trend of reactivity was attributed to the inability of the product radical anion to restore an electron to the aromatic substrate, a necessary step in the propagation of the S_RN1 cycle.

Treatment of t-butyl α-trimethylsilylacetate with KNH₂ caused cleavage of the C-Si bond. t-butyl acetate and hexamethyldisiloxane were obtained upon quenching. Methylene triphenylphosphorane, sulfur and selenium stabilized carbanions of 1,3-dithiane, 2-phenyl-1,3-dithiane, dimethyl sulfone, t-butyl α-phenylthioacetate, bisphenylthiomethane and t-butyl α-phenylselenoacetate, failed to react with 2-bromopyridine and iodobenzene. The carbanion of dimethyl sulfone remained unreactive towards iodobenzene even upon attempted entrainment by a catalytic amount of acetone enolate.

The dianion of phenylacetic acid reacted with iodobenzene to give biphenylacetic acid and benzene upon irradiation. Similar results were obtained with p-bromotoluene as substrate where substitution occurred only at the para position of the dianion. A non-chain mechanism was proposed to described both of these reactions. The dianion of phenylacetic acid also reacted with 2-bromopyridine to give a complex mixture under photostimulated conditions. 2-Benzylpyridine, bis(2-pyridyl)phenylmethane, p-(2-pyridyl)phenylacetic acid, potassio α-bis(2-pyridyl)phenylacetate, and 2-aminopyridine were found as products. Based on experiments performed with several probable intermediates of this reaction, and S_RN1 mechanism can account for formation of all products.

Preliminary results showed that the reaction of phenylacetamide dianion with iodobenzene also gave rise to the para substituted product. The dianion of 1,3-phenylacetone was found to react with iodobenzene to give 1,1,3-triphenylacetone via a mechanism other than an S_RN1 pathway.