Alkoxy-1,3,5-triazapentadien(e/ato) copper(II) complexes: Template formation and applications for the preparation of pyrimidines and as catalysts for oxidation of alcohols to carbonyl products

摘要

Template combination of copper acetate (Cu(AcO) _2·H _2O) with sodium dicyanamide (NaN(C≡N) _2, 2 equiv) or cyanoguanidine (N≡CNHC(=NH)NH _2, 2 equiv) and an alcohol ROH (used also as solvent) leads to the neutral copper(II)-(2,4-alkoxy-1,3,5- triazapentadienato) complexes Cu{NH=C(OR)NC(OR)=NH} _2 (R=Me (1), Et (2), nPr (3), iPr (4), CH _2CH _2OCH _3 (5)) or cationic copper(II)-(2-alkoxy-4-amino-1,3,5-triazapentadiene) complexes Cu{NH=C(OR)NHC(NH _2)=NH} _2(AcO) _2 (R=Me (6), Et (7), nPr (8), nBu (9), CH _2CH _2OCH _3 (10)), respectively. Several intermediates of this reaction were isolated and a pathway was proposed. The deprotonation of 6-10 with NaOH allows their transformation to the corresponding neutral triazapentadienates Cu{NH=C(OR)NC(NH _2)=NH} _2 11-15. Reaction of 11, 12 or 15 with acetyl acetone (MeC(=O)CH _2C(=O)Me) leads to liberation of the corresponding pyrimidines NC(Me)CHC(Me)NCNHC(=NH)OR, whereas the same treatment of the cationic complexes 6, 7 or 10 allows the corresponding metal-free triazapentadiene salts {NH 2C(OR)=NC(NH _2)=NH _2}(OAc) to be isolated. The alkoxy-1,3,5-triazapentadiene/ato copper(II) complexes have been applied as efficient catalysts for the TEMPO radical-mediated mild aerobic oxidation of alcohols to the corresponding aldehydes (molar yields of aldehydes of up to 100 with >99 selectivity) and for the solvent-free microwave-assisted synthesis of ketones from secondary alcohols with tert-butylhydroperoxide as oxidant (yields of up to 97, turnover numbers of up to 485 and turnover frequencies of up to 1170 h ~(-1)). One pot for the lot: Alkoxy-1,3,5-triazapentadien(e/ato)-Cu ~(II) complexes have been synthesised by one-pot template synthesis, used for metal-mediated synthesis of pyrimidines and triazapentadienes, and shown to be effective catalysts in the oxidation of alcohols to carbonyl products (see scheme).