Can t-BuOK be a good nucleophile? An ion-pairing answer. Cleavage of aryl ethers in their cationic iron complexes

2006 ARKIVOC  
t-BuOK reacts with cationic 18-electron d 6 Fe II complexes of aryl ethers ArOR as their PF 6 salts to cleave the C-O bond of the aryl ether, yielding Fe II -hexahapto-phenolate complexes and the ethers t-BuOR, only in the presence of a salt, the best one being [n-Bu 4 N] [PF 6 ]. Salt effects in THF result from either enhancement of nucleophilic reactivity of t-BuOwith a large cation or polarization of the aryl ether C-O bond with a THF-insoluble salt such as KBr or NaCl. This nucleophilic
more » ... tion of t-BuOis of special interest for the 8-step synthesis of a triallyl phenol dendron obtained when the cleavage reaction of the complexes of aryl ethers is carried out in the presence of allyl bromide. During this reaction that is carried out without addition of a salt, it is the in situ formation of KBr during the three deprotonation-allylation sequences that further facilitates the crucial aryl ether cleavage at low temperature. ARKAT p.C(CH 2 CHCH 2 ) 3 C 6 H 5 OCH 2 CHCH 2 ][PF 6 ], 6 and [Fe II Cp(η 6 -p.C(CH 2 CHCH 2 ) 3 C 6 H 5 O], dendron A, were synthesized and characterized as described earlier. 10, 12 Reaction of [Fe II Cp(η 6 -PhOMe][PF 6 ], 4a, with t-BuOK. [Fe II Cp(η 6 -PhOMe][PF 6 ], 4a (0.371 g, 1 mmol) and t-BuOK (0.112 g, 1 mmol) were deaerated in a Schlenk tube, then 10 mL distilled THF was introduced. The reaction mixture was stirred for one day at 25°C, the solvent was removed under vacuum, the solid residue was dissolved in 5 mL dichloromethane, and this solution was filtered, then dried over Na 2 SO 4 and the filtered solution was evaporated under vacuum. The residue was chromatographied using an alumina/product mass ratio of 40. No iron complex, but only anisole, resulting from quantitative decomplexation, was obtained and characterized and titrated by 1 H NMR.
doi:10.3998/ark.5550190.0007.412 fatcat:zluybwz4r5fxlcdymnwz5zhpiq