The mechanism study on Pd(0)-catalyzed cyclization of alkynols

The mechanism study on Pd(0)-catalyzed cyclization of non-terminal alkynols under molecular level shows that the alkynols prefer to form endocyclic ethers in the present of Pd2(dba)3 and Xantphos rather than exocyclic ethers, which is attributed to the steric hindrance of Xantphos. With the help of the large bulk of metal complexe, reaction point can be regiocontrolled so as to synthesize compounds selectively. The vacant d orbital of Pd atom can accommodate electrons, which is the key point for the binding of CO bond. This step has to overcome a free energy barrier of 17.1 kcal/mol to form exocyclic ethers while it only needs 15.0 kcal/mol free energy barrier to to form endocyclic ethers. The analysis of NBO charges and molecular orbital also demonstrates that it is more favorable to form endocyclic ethers with Pd2(dba)3 and Xantphos.