A selective convenient ruthenium-mediated synthesis of mixed acetals

Abstract

Addition of alcohols and phenols to allyl ethers catalyzed mainly by ruthenium complexes was studied. Complexes of ruthenium generated in situ from precursors such as {[RuCl 2(1,5-COD)] x } or [Ru 3(CO) 12] and from external ligands such as phosphines (e.g. PPh 3, PBu 3, BINAP) or phosphites (e.g. P(OPh) 3, P(OMe) 3) were found to be particularly efficient catalysts of the studied reactions. Transacetalization reaction could be practically completely eliminated by the addition of a base (particularly Na 2CO 3) to the catalytic systems. It was observed that the selectivity of mixed acetals formation increases with increasing value of Θ parameter of phosphines. Especially interesting results (0–5% of transacetalization) have been obtained for catalytic systems generated from {[RuCl 2(1,5-COD)] x } or [Ru 3(CO) 12], phosphines (PPh 3, BINAP, dppe, tris(2,4,6-tri-metylphenyl)phosphine, or dppf) and Na 2CO 3. The mechanism of mixed acetals formation has been investigated using deuterated reagents. It is postulated that the examined reaction is a nucleophilic addition of ROH to a hydrido-π-allyl complex formed during oxidative addition of allyl substrate to metal complex. As a result, a new, selective, and convenient method of the synthesis of symmetrical and, in particular, unsymmetrical (mixed) acetals has been developed. Mixed acetals CH 3CH 2CH(OR 1)(OR 2) may be obtained in the reaction of R 1- O-allyl with R 2OH or R 1OH with R 2- O-allyl, depending on the structure of R 1 and R 2.