Theoretical studies on the reaction mechanism of palladium(0)-catalyzed addition of thiocyanates to alkynes

Abstract

The reaction mechanism of the Pd(0)-catalyzed alkyne cyanothiolation reaction is investigated by MP2, CCSD(T) and the density functional method B3LYP. The overall reaction mechanism is examined. The B3LYP results are consistent with the results of CCSD(T) and MP2 methods for the isomerization, acetylene insertion and reductive elimination steps, but not for the oxidative addition step. For the oxidative addition, the bisphosphine and monophosphine pathways are competitive in B3LYP, while the bisphosphine one is preferred for CCSD(T) and MP2 methods. The electronic mechanisms for the oxidative addition of thiocyanate HS–CN to Pd(PH₃)₂ and Pd(PH₃) and for the acetylene insertion into Pd–S and Pd–CN are discussed in terms of the electron-donation and back-donation. The chemo-selectivity that acetylene inserts into the Pd–S bond rather than into the Pd–CN bond is due to the involvement of the S p orbital. It is the doubly occupied S p unhybridized orbital that donates an electron to the alkylene π* anti-bonding orbital, which makes insertion into Pd–S bond more favorable than into the Pd–CN bond. During the insertion into the Pd–S bond, the S sp² hybrid orbital and unhybridized p orbital transform into each other, while the C sp hybrid orbital shifts its direction for insertion into Pd–CN bond. By using the monosubstituted acetylenes (CN, Me and NH₂), the influence of substituents at acetylene on the chemo- and regio-selectivities is analyzed.