Biscarbene-Ruthenium Complexes in Catalysis: Novel Stereoselective Synthesis of (1E,3E)-1,4-Disubstituted-1,3-dienes via Head-to-Head Coupling of Terminal Alkynes and Addition of Carboxylic Acids
Résumé
The reaction of a variety of alkynes RCCH with a variety of carboxylic acids R1CO2H, in the presence of 5% of RuCl(COD)C5Me5, selectively leads to the dienylesters (1E,3E)-RCH1=CH2-CH3=C(R)(O2CR1). The reaction also applies to amino acid and dicarboxylic acid derivatives. It is shown that the first step of the reaction consists of the head-to-head alkyne coupling and of the formation of the metallacyclic biscarbene-ruthenium complex (C5Me5)(Cl)Ru:C(R)-CH=CH-C:(R), isolated for R = Ph and catalyzing the formation of dienylester. D-labeled reactions show that the alkyne protons remain at the alkyne terminal carbon atoms and carboxylic acid protonates the C1 carbon atom. QM/MM (ONIOM) calculations, supporting a mixed Fischer-Schrock-type biscarbene complex, show that protonation occurs preferentially at the carbene carbon C1 adjacent to Ru, in the relative cis position with respect to the Ru-Cl bond, to give a mixed C(1)alkyl-C(4)carbene complex in which the C4 carbene is conjugated with the noncoordinated C2=C3 double bond. This 16-electron intermediate has a weak stabilizing agostic C-H bond. This most stable isomer appears to have a C4 center more accessible to the nucleophilic addition which accounts for the experimentally observed product.