Synthesis, cis/trans Isomerization, and Reactivity of Palladium Alkyl Complexes That Contain a Chelating N-Heterocyclic-Carbene Sulfonate Ligand

The chemistry of palladium alkyl complexes that incorporate the NHC-sulfonate ligand N -(2,6- i Pr 2 -Ph)- N ′-2-benzenesulfonate-NHC ([C-O] −, NHC = cyclo -CNCH 2 CH 2 N) is described. The reaction of {C-O}Ag ( 2 ) with Pd 2 (μ-Cl) 2 Me 2 (PPh 3 ) 2 affords cis-C,C- {C-O}PdMe(PPh 3 ) ( 3, 79%). The reaction of 2 with Pd 2 (μ-Cl) 2 Me 2 (2,6-lutidine) 2 at 25 °C in CH 2 Cl 2 gives a 4/1 mixture of trans-C,C -{C-O}PdMe(2,6-lutidine) ( 4a ) and cis-C,C -{C-O}PdMe(2,6-lutidine) ( 4b ), which were isolated in 62% and 18% yield, respectively, by recrystallization. The NHC-sulfonate ligands bind in a κ 2 - C,O fashion in 3 and 4a, b . 4a isomerizes to 4b by dissociation of the Ar-SO 3 – unit to form a configurationally labile three-coordinate intermediate. This process is accelerated by hydrogen bond donors (CD 3 OD, lutidinium) and Lewis acids (B(C 6 F 5 ) 3 ) that can labilize the sulfonate group. 4a and 4b react with 1 equiv of B(C 6 F 5 ) 3 to yield the O-bound adduct cis-C,C -{C-O-B(C 6 F 5 ) 3 }PdMe(2,6-lutidine) ( 5 ). 5 decomposes at 40 °C by C–C reductive elimination to afford (in the presence of pyridine to trap the B(C 6 F 5 ) 3 ) N -(2,6- i Pr 2 -Ph)- N ′-2-benzenesulfonate-imidazolium methyl ( 6 ). trans-C,C - 4a reacts with CO and t BuNC to yield the insertion products {C-O}Pd{C(═O)Me}(2,6-lutidine) ( 7 ) and {C-O}Pd{C(═N t Bu)Me}( t BuNC) ( 9 ), in which the acyl and iminoacyl ligands are cis to the NHC ligand, via stereospecific displacement of Ar-SO 3 – by the substrate followed by migratory insertion. The bis-isocyanide adduct {κ 1 - C -C-O}PdMe( t BuNC) 2, in which the t BuNC ligands are trans, is an intermediate in the formation of 9 . t BuNC reversibly displaces the Ar-SO 3 – ligand of 9 to form {κ 1 - C -C-O}Pd{C(═N t Bu)Me}( t BuNC) 2 ( 10 ). In contract, cis-C,C - 4a does not undergo net reaction with CO and reacts with t BuNC via reductive elimination to yield 6 . Displacement of the ArSO 3 – ligand of cis-C,C - 4b by potential substrates yields adducts in which the substrate and Me group are trans and insertion is not possible.