Carbon-Oxygen Bond-Cleavage Reactions by Electron Transfer. 4. Electrochemical and Alkali-Metal Reductions of Phenoxynaphthalenes

Article abstract of DOI:10.1021/ja00189a010

The reductions of 1- (1) and 2-phenoxynaphthalene (2) to their respective radical anions were observed at -2.42 and -2.48 V vs SCE.Cyclic voltammetric and chronoamperometric studies indicate that both radical anions have half-lives that are greater than 100 s in dry N,N-dimethyloformamide.Controlled-potential elektrolysis of the ethers resulted in relatively slow , regioselective carbon-oxygen bond cleavage to produce phenol and naphthalene .The intermediacy of the naphthyl radical or the naphthyl anion in the reductive pathway was tested by the use of anintramolecular radical trap in the form of a 2-(3'-butenyl) substituent, i.e., 2-(3'butenyl)-1-phenoxynaphthalene (3).Formation of 1-methylbenzindan (4) upon the electrochemical reduction of 3 in the absence and presence of D2O confirmed the presence of the 2-(3'butenyl)naphthyl radical in the radical-anion cleavage mechanism.Reduction of the phenoxynaphthalenes with Na/HMPA in tetrahydrofuran resulted in slow regioselektive cleavage while reduction with Na-K alloy/tetraglyme quantitatively cleaved the ether bond more rapidly.The intermediacy of the naphthyl anion, which is produced via the ether dianion in the Na-K alloy reduction, was demonstrated by the reduction of 3 at -78 and 0 deg C.Reduction of 3 at -78 deg C produced only 2-(3'-butenyl)naphthalene (5) whereas reduction at 0 deg C produced both 5 and 4, the latter by anionic cyclization