Tertiary phosphines, ancillary ligand

Scheme 1. Chemistry typical of d8, d20 Pt metal systems M = Rh(I), Jr(/), Ni(0), Pd<0), Pt<0)9 with ancillary ligands L, typically a tertiary phosphine type ligand, CO, etc. (4, 33, 34)...

The oligomerization product thus formed is a mixture of olefins with a Schulz-Flory distribution of molecular weights (see also Section 4.7.2), whose composition can be modified, for example by adding an excess of tertiary phosphine or by changing the ancillary ligand. 

To this end, monodentate phosphine or bidentate PX (X=P, N, O) ligands have usually been employed as ancillary ligands for transition-metal-catalyzed reactions, with bulky tertiary alkyl phosphines proving particularly effective. Significant advances have been achieved in the use of less active aryl chlorides (bond strength C-Cl>C-Br>C-I) as chemical feedstock [5], with a number of processes mediated by palladium-bulky phosphine systems. This success is often explained by the effect of bulk and electron richness at the metal center along the catalytic cycle depicted in Fig. 1 [6]. 

In 2007, Glueck s group reported a catalytic DKR process in which secondary phosphines were converted into the corresponding enantio-enriched tertiary phosphines through palladium-catalysed asymmetric hydrophosphination of aryliodides using secondary phosphines. The key intermediates were diaster-eomeric phosphide complexes with chiral ancillary ligands (L Pd PRR ). Their relative rates of P-inversion and phosphorus-carbon bond formation controlled the enantioselectivity of the prodnct formation. As shown in Scheme 2.63, the reaction allowed moderate enantioselectivities of up to 70% ee to be achieved. 

At the onset of our investigations, tertiary phosphines were selected as the ancillary ligand because of their long and successful history in this field. Therefore, addition of different phosphines to the Karstedt complex (1) afforded the corresponding monophosphine platinum(O) complexes 13 bearing one dvtms ligand (Table 5.1) [14]. 

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