Carbon-oxygen bond-cleavage reactions by electron transfer. 4. Electrochemical and alkali-metal reductions of phenoxynaphthalenes

Abstract

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 iV,7V-dimethylformamide. Controlled-potential electrolysis 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 an intramolecular radical trap in the form of a 2-(3'-butenyl) substituent, i.e., 2-(3'-butenyl)-l-phenoxynaphthalene (3). Formation of 1 -methylbenz[e]indan (4) upon the electrochemical reduction of 3 in the absence and presence of D20 confirmed the presence of the 2-(3,-butenyl)naphthyl radical in the radical-anion cleavage mechanism. Reduction of the phenoxynaphthalenes with Na/F1MPA in tetrahydrofuran resulted in slow regioselective 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 °C. Reduction of 3 at -78 °C produced only 2-(3'-butenyl)naphthalene ( 5) whereas reduction at 0 °C produced both 5 and 4, the latter by anionic cyclization.

Synthesis and Regiochemistry of [60]Fullerenyl 2-Methylmalonate Bisadducts and their Facile Electron-Accepting Properties

“…From the reported value for the reductive potential E red p of naphthalene (Np → Np ·− ) as −2.52 V vs SCE,18 C 60 4− (or partially C 60 5− as well) might be formed in the presence of an excess amount (>6.0 equiv) of sodium naphthalenide 19,20. Thus, under the condition to give bisadducts using 2.2 equiv of sodium naphthalenide relative to C 60 , it is obvious that C 60 2− is the main anion species formed in situ .…”

“…As far as the reaction mechanism of C 60 2− is concerned, C 60 was known to undergo consecutive electron reductions up to C 60 6− with the reduction potentials of E 1red 1/2 = −0.36 V vs SCE, E 2red 1/2 = −0.83, E 3red 1/2 = −1.42, E 4red 1/2 = −2.01, and E 5red 1/2 = −2.60 . From the reported value for the reductive potential E red p of naphthalene (Np → Np •− ) as −2.52 V vs SCE, C 60 4− (or partially C 60 5− as well) might be formed in the presence of an excess amount (>6.0 equiv) of sodium naphthalenide. , Thus, under the condition to give bisadducts using 2.2 equiv of sodium naphthalenide relative to C 60 , it is obvious that C 60 2− is the main anion species formed in situ . The reaction mechanism of C 60 2− with alkyl halide, such as benzyl bromide, have been proposed by Fukuzumi and Kadish et al to proceed by an electron transfer from C 60 2− to alkyl halide in the first step followed by the radical coupling between C 60 •− and R • forming RC 60 − .…”

Synthetic Organic Electrochemical Methods Since 2000: On the Verge of a Renaissance

“…Huang and co-workers developed a method to cleave C–O bonds in aryl and benzyl ethers electrochemically (Figure F) . In the 1980s, Kariv-Miller has used a mercury cathode to effect the reductive cleavage of aryl ether linkages through a Birch-type process; together with several other subsequent studies, − these early examples required oxygen-free conditions . To this end, Huang et al discovered that NaBH 4 could promote the C–O cleavage in an undivided cell under galvanostatic electrolysis where minimal precautions are necessary.…”

Electrocatalytic Hydrogenation of Biomass-Derived Organics: A Review

“…It is worth noting that most of the studies discussed in this section explored the reductive conversion of phenolic compounds and ethers at potentials more positive than those needed for direct electron transfer and do not include radical mechanisms, which enable C–O bond cleavage and C–C bond coupling. ,− …”