Rh(I) Complexes with Hemilabile Thioether-Functionalized NHC Ligands as Catalysts for [2 + 2 + 2] Cycloaddition of 1,5-Bisallenes and Alkynes

The [2 + 2 + 2] cycloaddition
of 1,5-bisallenes and alkynes
under
the catalysis of Rh(I) with hemilabile thioether-functionalized N-heterocyclic
carbene ligands is described. This protocol effectively provides an
entry to different trans-5,6-fused bicyclic systems
with two exocyclic double bonds in the cyclohexene ring. The process
is totally chemoselective with the two internal double bonds of the
1,5-bisallenes being involved in the cycloaddition. The complete mechanism
of this transformation as well as the preference for the trans-fusion over the cis-fusion has been rationalized
by density functional theory calculations. The reaction follows a
typical [2 + 2 + 2] cycloaddition mechanism. The oxidative addition
takes place between the alkyne and one of the allenes and it is when
the second allene is inserted into the rhodacyclopentene that the trans-fusion is generated. Remarkably, the hemilabile character
of the sulfur atom in the N-heterocyclic carbene ligand modulates
the electron density in key intermediates, facilitating the overall
transformation.