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

The reaction of a variety of alkynes RCdropCH with a variety of carboxylic acids (RCO2H)-C-1, 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 C-1 carbon atom. QM/MM (ONIOM) calculations, supporting a mixed Fischer-Schrock-type biscarbene complex, show that protonation occurs preferentially at the carbene carbon C-1 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 C-4 carbene is conjugated with the noncoordinated C-2=C-3 double bond. This 16-electron intermediate has a weak stabilizing a agostic C-H bond. This most stable isomer appears to have a C-4 center more accessible to the nucleophilic addition which accounts for the experimentally observed product.