Synthesis Of Dendritic Polyamidoamine Supported 2, 2, 6, 6-tetramethylpiperidine-1-oxyl And Its Catalytic Performance On The Oxidation Of Alcohols With Molecular Oxygen As Oxidant

Selective oxidations of alcohols to the corresponding aldehydes and ketones, respectively, are pivotal reactions in chemical field. The oxidation of alcohols catalyzed by 2, 2, 6, 6-tetramethylpiperidine-1-oxyl（TEMPO）, which is stable in light and to heat, has the advantages of mild reaction condition requirement, good selectivity and high conversion rate. However, TEMPO is expensive, and it has similar boiling point of aldehydes and ketones leading to difficulty of purification of the products. Thus, the immobilization of TEMPO for its recycling in the oxidant of alcohols is of importanance both in theory and in practice.In this paper, a second-generation dendritic polyamidoamine（PAMAM） was prepared via repeated Michael addition and aminolysis of ethylenediamine and methyl acrylate. The obtained PAMAM condensed with 4-oxo-2, 2, 6, 6-tetramethylpiperidinooxy followed by reduction with sodium cyanoborohydride to afford a PAMAM supported TEMPO（PAMAM-TEMPO）. The PAMAM-TEMPO and the intermediates in the preparation of it were characterized by ¹H NMR, ¹³ C NMR and FT-IR. The PAMAM-TEMPO in combination with CuBr₂ and 2, 2’-bipyridine as a catalytic system was applied to the selective oxidation of alcohols with molecular oxygen as an oxidant. The reactions were optimized with benzylic alcohol as substrate. For the catalytic system PAMAM-TEMPO/ CuBr₂/2, 2’-bipyridine, the optimized conditions were: benzylic alcohol 2 mmol, PAMAM-TEMPO 1 mol%,2, 2’-bipyridine 1 mol%,CuBr₂ 3 mol%,K2CO3 2.5 mol%,acetonitrile 1.25 ml, H2 O 1.25 ml, reaction temperature 25 ℃. Under these conditions, the catalytic system showed high activity and selectivity in oxidation of benzyl alcohols and allyl alcohols. The immobilization of TEMPO on PAMAM allowed easy separation of the oxidation products（aldehydes） from the catalytic system, and the recovered catalyst can be recycled for several times directly.