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 RCtbd1;CH with a variety of carboxylic acids R(1)CO(2)H, in the presence of 5% of RuCl(COD)C(5)Me(5), selectively leads to the dienylesters (1E,3E)-RCH(1)=CH(2)-CH(3)=C(R)(O(2)CR(1)). 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 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 alpha 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.