Pd/P(t-Bu)₃: A Mild Catalyst for Selective Reduction of Alkenes under ­Transfer-Hydrogenation Conditions

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

References and Notes

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  Nevertheless, in the present study even if no evidence has been obtained for the exclusion, or no formation, of such Pd colloids, it seems that our catalytic system was totally homogeneous. Studies are now underway to determine the exact nature of the catalyst and will be reported in due course.

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9

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.