Abstract
A new and efficient mild Pd/P(t -Bu)3 catalyst for selective reduction of various alkenes under transfer-hydrogenation conditions has been developed leading to the corresponding saturated derivatives in chemical yields varying from 65% to 98%.
Key words
palladium - reduction of alkenes - tri-tert -butyl phosphine - transfer hydrogenation - catalysis
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Selective Reduction of 4-Allylanisole (1 ) under Transfer Hydrogen Conditions; General Procedure (Table 1, Entry 4) : CAUTION: All reagents (solvent, phosphine ligand, formic acid) must be purified before use in order to obtain good conversions and reproducible results. In a two-necked round-bottomed flask placed under argon was added Pd(OAc)2 (5.4 mg, 2.4 × 10-5 mol) at 20 °C in freshly distilled anhyd THF (5 mL). To this solution, P(t -Bu)3 (20 mg, 10-4 mol; purchased from Acros) was successively added and the solution was heated to reflux for 10 min. To this mixture 4-allylanisole (1 ; 148 mg, 10-3 mol) and formic acid (230 mg, 5 × 10-3 mol) were added. The resulting solution was stirred for 15 min to reflux and then cooled to 20 °C for 12 h. After filtration over Celite and evaporation of the solvents, the crude residue was purified by chromatog-raphy on a silica gel column using PE-EtOAc (100:0 → 50:50) as eluent affording the expected product 2 in 95% yield.
10 Kinetic experiments performed on the selective reduction of 4-allylanisole (1 ) under transfer-hydrogenation conditions indicate that such a reaction takes place in less than 4 h. Nevertheless, this reaction time seems to be highly substrate-dependent and we have chosen to perform all experiments for 12 h.