Hemilabile systems

There has been an increased interest in mixed-donor and hemilabile ligands owing to their successful use in catalysis owing to their electronic asymmetry or ability to provide access to an unsaturated site via dissociation of one of the termini. Typical ligands used for catalysis that make use of their hemilability involve a phosphine and an oxygen donor, such as an ether. A weak donor, such as an ether (if it binds at all), is relatively easily displaced, and rapture of the M-0 bond and retention of the M-P bond is expected to provide an ideal hemilabile system. With P,N- ligands, either... 

In this chapter, we focus on P,N-chelating ligands and some of their transition metal complexes because their intrinsic properties, immense structural diversity, and broad range of applications attract increasing attention [3]. Such heterofunctional ligands associate hard nitrogen donor(s) with soft phosphoms donor(s) and offer considerable chemical and structural diversity. They may also generate hemilabile systems endowed with enhanced reactivity. 

Figures 12.2 and 12.4 illustrate the activation steps for the Cp mode of III and IV, whereas Figures 12.3 and 12.5 illustrate the steps for the C mode according to Scheme 12.1. The relative energies for the optimized structures are summarized in Table 12.2. Activation step 3 is not shown for the hemilabile systems because of its...

Although the hemilabile behaviour of the imino functional group together with an apparent isomerisation process on palladium may make it difficult to achieve high chiral resolution (ee) in asymmetric catalysis. Bonnet and Douthwaite developed an imino functionalised carbene ligand system based on the diamino cyclohexane framework that has achieved ees of up to 92% in palladium catalysed allylic alkylatiou reactions using ( )-l,3-diphenylprop-... 

Mixed hemilabile Ugands have been su ested as effective for increasing the activity of a catalytic system. In theory, the substrate molecule can easily displace the weakly bound donor ligand moiety. Furthermore, the transition state can be stabilized with a net energetic gain for the overall reaction. In reality, however, it is much more complex to predict the activity of such ligands as they can stabUize both the ground state and the transition state. 

Dimerization of methyl acrylate may open up a new way to production of adipic acid. The reaction was carried out in a continuous flow biphasic system where the catalyst (obtained from [Pd(acac)2] and the hemilabile (C4H9)2P-CH2-CH2-N(CHs)2 ligand) was dissolved in [bmim][BF4] together with some HBF4, and the substrate methyl acrylate was applied in toluene solution. At 80°C, no sign of catalyst deactivation was seen for 10 h, and the overall TON after 50 h was more than 4000, with a selectivity to A -dihydromuconate more than 90%... 

An important advantage of the Cp tethered to NHC ligands compared to the nonlinked systems is that chelation does not consume an extra coordination site, leaving the metal complexes with an additional site for catalysis (Fig. 10.1). As we have already pointed out, the tethered NHC unit could not only increase the stability of the half-sandwich metal-NHC complexes by chelation, but also could enhance the catalytic activity of their metal complexes by the stabilization of intermediate active species. In addition, the hemilabile dynamic behavior of the Cp-NHC ligand may allow to efficiently control the reactivity and stability of the catalytically active center. 

Using the hemilabile noninnocent ligand 4,6-di-ferf-butyl-(2-methylthiophenylimino)-o-benzosemiquinone Qy (which was developed during efforts to modify a radical/metal/radical three-spin interaction) [18], we could obtain and investigate a system (Eq. 49.4) to prove a predominantly ligand-based electron transfer during the reversible coordination of a thioether sulfur donor [19]. 

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