Palladate complexes

An unactivated methyl group can be functionalized by the cyclopalladation of oximes. The equatorial methyl of geminal methyls in steroids or hexapyr-anosides is selectively aceto.xylated by the reaction of the palladation complex 523 of the 3-oxime with lead tetraacetate[467,468]. 

As mentioned in the introduction, the idea is to first synthesize a hydrosol (PdO, Sn02), and then contacting it with a stannate (or palladate) complex aqueous solution. The initial oxide suspension acts as a nano carrier for the second element. 

Similar orf/to-palladated complexes 137, 138 (Scheme 75) were presented by Espinet and Coco [141]. 

Scheme 75 Palladated complex 137, 138 containing two dibenzo[18]crown-6 units...

The n-arylpalladium bonds in these complexes are reactive and undergo insertion and substitution reactions. The reactions offer useful methods for regiospecific functionalization of aromatic rings, although the reactions are difficult to make catalytic in most cases. Insertion of styrene to AA-dimcthylbcnzylaminc complex (180) to form the stilbene derivative 185 occurs smoothly at room temperature in AcOH [5]. The reaction has been extended to the functionalization of the dopamine analogue (A,A-dimethyl-2-arylethylamine) 187 via the six-membered o/Y/zo-palladated complex 186 [118]. 

The flexibility of such approach can be further exploited for the design of mesogens with innovative shapes. Pucci and coworkers prepared ortho-palladated complexes with a molecular shape which can be considered at the crossover point between calamities and discotics [70],... 

Cyclopalladation of 4 -(functionalized)-6 -phenyl-2)2,-bipyridine ligands (HL) afforded several new [PdLCl] complexes liquid crystalline complex 50 exhibits thermotropic nematic mesomorphism, which represents the first example of an organometallic mesogen with a terdentate o-C deprotonated ligand. Complex 50 is photoluminescent at 77 K [48]. Other researchers have reported the applications of orf/m-platinated or -palladated complexes bearing azobenzene or imine ligands as liquid crystalline materials [49]. 

Other structurally related complexes have also been prepared by cleavage of dinuclear ort/zo-palladated complexes with acac (19 [28], 20 [29], 21 [30]), with various chelating 0,0 monoanionic ligands [31], or with cyclopentadienyl groups (Cp) [30, 32]. It appears that in most cases, and for the appropriate chain length, the mononuclear complexes usually give rise to a nematic phase, and/or a Sa phase, and at lower temperatures than the dinuclear precursors. 

Fig. 1. Schematic diagram of the syn and anti isomers possible for di-/i-halo-bridged ortho-palladated complexes...

Dimethylpalladium(ll) complex with PEt2Ph ligand undergoes alkylation by MeLi or PhLi to afford palladate complexes which are characterized by NMR spectra of the solution. Detailed NMR studies of mixtures of the complex and MeLi or PhLi in various ratios revealed stepwise conversion of the neutral dimethyl complex into monoanionic palladate and then into homoleptic dianionic tetraorganopalladate complexes [38] (Scheme 5.6). 

Several series of dinuclear or /zo-palladated complexes of symmetric azines were prepared with various bridges, and the mesomorphism analyzed accordingly ((270) X = OAc, Cl, Br, SCN, n = 10). In the complexes where X = C1 and Br, only the trans isomer was observed (IH NMR spectroscopy), while for X = SCN, two isomers were observed in a 60 40 ratio and it was assumed that these were cis and trans In each of these examples the complex was assumed to be planar by comparison with related structures in the literature. However, where X = OAc, the situation was more complex and tram and cis isomers in the ratio 3 1 were produced consistently. IH NMR studies went on to show that the tram isomer was optically active and hence the structure had to be that of an open book (Figure 105, R = Me), although in the synthesis a racemic mixture was produced. 

As above (242), the dinuclear ort/ o-palladated complexes derived from (S)-2-(2-hydroxyaryl) oxazoline Schiflf-base ligands ((277) X = OAc, Cl x = 1 R = CHMez x = 1,2 R = C HCH3CH2CH3) showed a broad SmA phase. In the planar, chloro-bridged complexes (cis. trans mixture 1 2), the phase existed from 50 °C to 170 °C, and up to 250 °C for the biphenyl derivative as far as the non-planar acetato-bridged complexes (only tram isomer) were concerned, only the biphenyl derivative was mesomorphic (g 71 SmA 121 Cr 179 SmA 232 I). Used as chiral dopants (ca. lOmol.%), a chiral nematic phase was induced systematically in both the chloro- and acetato-ljridged series, with the suppression of the SmA phase in the chloro-bridged systems (x = 1). 

The effects of the nature of the nitrogen donor atom vs. sp ) upon the properties and chemistry of palladated complexes with cr(Pd-G(i/) )), ferrocene) bonds were studied by Lopez et Mossbauer spectra show that... 

NMR spectroscopy was applied in the enantiomeric excess determination of (iV,iV-dimethyl-(2,2,2-trifluoro-l-phenylethyl)amine-C,A)palladium complexes of a-amino acids.Dunina et al. used NMR spectroscopy and a P -chiral phosphapalladacycle for the enantiomeric purity determination of a-amino acids. P NMR spectroscopy was also used in the enantiomeric excess determination of N,C o///5< -palladated complexes of P-chiral phosphines. A ferrocene-based reagent (i p,successfully designed for the chiral recognition of /3-hydroxyphosphines and the discrimination of ( )-bis(diphenylphosphino)-1,1 -binaphthyl. 

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