Organonickel(II) complexes containing aryl ligands with chiral pyrrolidinyl ring systems; syntheses and use as homogeneous catalysts for the Kharasch addition reaction

Abstract

Abstract.New chiral organonickel(I1) complexes 3a-d have been synthesized containing monoanionic terdentate ligands of the type 2,6-bis[(l-pyrrolidinyl)methyl]phenyl. Chirality is intro- The catalytic ability of the nickel complexes in the addition reaction of polyhalogenated compounds with alkenes (Kharasch addition) is markedly influenced by the geometry around the nickel atom and only 3a and 3b prove to be effective catalysts. The highest enantiomeric excesses of 16-17% obtained with 3b and L-menthyl methacrylate in this catalytic reaction indicate that the C , chiral fence produced by the Me-substituted pyrrolidinyl rings is not very effective for the transfer of chiral information.

A new platform for NCN dimethylamino pincer complexes: Synthesis and structural studies

“…For example, modification of the methylene groups of the pincer ligand A can yield complexes of type C (Chart 2) [20]. Alternatively, one can build asymmetry into the N-donor site as examples D and E demonstrate (Chart 2) [21,22].…”

Novel enantiopure non-C2-symmetric NCN-pincer palladium complexes with l-proline chiral auxiliaries: mer η3-N,C,N versus square planar η4-N,C,N,O coordination

“…19 In all three molecules of 2a and 3a, the coordinated nitrogen atoms have an (R N )-configuration, while the C a -atoms have the expected (S C )-configuration. The palladium centres have slightly distorted square-planar coordination environments with angles at palladium amounting to C(1)-Pd-N(1,2) 81.09(13)-84.43 (9) (Table 3). Solution IR spectra of 2b and 3a in dichloromethane also showed the presence of the two distinct carbonyl vibrations at similar wavenumbers (see Table 3).…”

“…To assign the different resonances for each remaining diastereoisomer, the chemical shifts of the stereogenic hydrogen were used and compared to a structurally related NCN-pincer nickel(II) system. 9 Based on the specific integrals of identical groups of the different diastereoisomers the relative ratio of these isomers could be determined. The resonances of the methyl groups are well resolved and give a clear indication of this molar ratio, which amounts to 1(S N S N ):1(R N S N ):0.6(R N R N ).…”

“…14 Depending on the metal of choice, these complexes have been applied in different catalytic reactions, such as aldol condensations, 10b,g,h,11,13a,c Michael additions, 10f,14 Diels-Alder reactions, 10b cyclopropanation, 10a allylation of aldehydes, 12d allylic alkylation, 10b alkylation of aldimines, 10e hydrogen transfer, 13f and the Kharasch addition. 9 The enantioselectivities obtained vary from moderate (ee's below 35%) 9,10a,b,f,11,13a,f to high (ee's up to 80%). 10d,e,g,h,13b,c, 14 We have now incorporated ester derivatives of the natural amino acid L L-proline (Chart 2) as N-donor groups into the NCN-pincer backbone.…”

“…8 There are, however, relatively few asymmetric pincer complexes known. They contain either a chiral auxiliary, such as pyrrolidines (structure B), 9 oxazolines (structure C), 10 phosphines or sulfoxides (structure D), 11,12 or a stereogenic centre at the benzylic groups of the pincer backbone (structure E). 13 Takenaka et al recently published pincer palladium complexes bearing pyrroloimidazolone auxiliaries (structure F) with stereogenic centres present on the proline rings as well as at the benzylic groups of the pincer backbone.…”