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

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journal contribution
posted on 12.09.2011, 00:00 by Xiaoyuan Zhou, Richard F. Jordan
The chemistry of palladium alkyl complexes that incorporate the NHC-sulfonate ligand N-(2,6-iPr2-Ph)-N′-2-benzenesulfonate-NHC ([C-O], NHC = cyclo-CNCH2CH2N) is described. The reaction of {C-O}Ag (2) with Pd2(μ-Cl)2Me2(PPh3)2 affords cis-C,C-{C-O}PdMe(PPh3) (3, 79%). The reaction of 2 with Pd2(μ-Cl)2Me2(2,6-lutidine)2 at 25 °C in CH2Cl2 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 κ2-C,O fashion in 3 and 4a,b. 4a isomerizes to 4b by dissociation of the Ar-SO3 unit to form a configurationally labile three-coordinate intermediate. This process is accelerated by hydrogen bond donors (CD3OD, lutidinium) and Lewis acids (B(C6F5)3) that can labilize the sulfonate group. 4a and 4b react with 1 equiv of B(C6F5)3 to yield the O-bound adduct cis-C,C-{C-O-B(C6F5)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(C6F5)3) N-(2,6-iPr2-Ph)-N′-2-benzenesulfonate-imidazolium methyl (6). trans-C,C-4a reacts with CO and tBuNC to yield the insertion products {C-O}Pd{C(O)Me}(2,6-lutidine) (7) and {C-O}Pd{C(NtBu)Me}(tBuNC) (9), in which the acyl and iminoacyl ligands are cis to the NHC ligand, via stereospecific displacement of Ar-SO3 by the substrate followed by migratory insertion. The bis-isocyanide adduct {κ1-C-C-O}PdMe(tBuNC)2, in which the tBuNC ligands are trans, is an intermediate in the formation of 9. tBuNC reversibly displaces the Ar-SO3 ligand of 9 to form {κ1-C-C-O}Pd{C(NtBu)Me}(tBuNC)2 (10). In contract, cis-C,C-4a does not undergo net reaction with CO and reacts with tBuNC via reductive elimination to yield 6. Displacement of the ArSO3 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.