Direct Arylation Mediated by Palladium Complexes with Rigid Phosphine-Functionalized N-Heterocyclic Carbenes

New phosphine-functionalized N-heterocyclic carbene ligand precursors
(L1H·I and L2H·I) were prepared. The
former compound contains a PPh2 moiety, whereas the latter
one has a more electron-donating PCy2 group. A reaction
of L1H···I with PdCl2 in the
presence of base afforded the complex PdL1ClI. An interconversion
process between the two structural isomers of PdL1ClI was
observed by variable-temperature NMR. The coalescence temperature
(Tc) is at 368 K with the free energy
of activation (ΔG⧧) found
to be 68.9 kJ mol−1. A similar reaction of L2H·I with PdCl2 produced a mixture of PdL2ClI, PdL2Cl2, and PdL2I2. Unlike PdL1ClI, PdL2ClI undergoes
a much faster exchange between the two structural isomers. Pure PdLI2 (L = L1 and L2) can be obtained by
adding NaI in the reaction mixture to facilitate ligand exchange.
[PdL(CH3CN)2](BF4)2 were also prepared straightforwardly by salt metathesis reactions.
In total, seven new palladium complexes of L were structurally characterized,
and in each of these structures an anagostic interaction exists between
the palladium center and one of the diastereotopic methylene protons.
The interaction also exists in solution, as evidenced by the downfield
NMR shift of the proton. The new complexes PdLI2 and [PdL(CH3CN)2](BF4)2 show
mediocre activities in benchmark Suzuki coupling reactions under conventional
and microwave heating. They are, however, highly promising in mediating
direct arylation of phenyl halides and diphenylacetylene, affording
9-benzylidene-9H-fluorene in good yields.