P,O ligands

Only recently (2002) have the very first examples of the transition metal-catalyzed incorporation of acrylate monomers into linear polyethylene been demonstrated. In our opinion the most notable report is that of Drent and coworkers [48] who describe the use of a neutral palladium catalyst with a chelating P-O ligand to generate linear copolymers that included the incorporation of acrylate monomers (Drent s catalyst and proposed catalytic cycle are shown in Scheme 2). In these early results, there was only minor acrylate incorporation (limited to some 3-17 mol%) and the resulting polymers were of very low molecular weight (Mn 4000-15,000). 

Alkenes and carbon monoxide are currently copolymerised in the presence of homogeneous Pd catalysts to give thermoplastic materials vith a perfectly alternating structure (Scheme 7.1a) [1, 2]. The non-perfect alternation of monomers (Scheme 7.1b) has been uniquely observed for ethene/CO copolymerisation reactions catalysed by Pd precursors vith anionic P-O ligands [3]. H NMR... 

The early stages in the chain growth process have been mimicked by Braunstein with the use of a Pd-Me complex supported by an acetamido-derived P-O ligand. Four catalytic intermediates were intercepted by the sequential addition of CO-ethene-CO-ethene, and the occurrence of reversible and irreversible steps was established (Scheme 7.9). Unlike diphosphine ligands [10], the insertion of ethene into the y-chelate acyl complex was found to be a facile process occurring without the intervention of CO [25]. 

Triazasilatrane 195 behaves as a bidentate (P,P ) or a tridentate (P.P. O) ligand to form chelate complexes with transition metal compounds411. On reaction with sulfur or methyl iodide, compound 195 is converted into the corresponding derivatives 196 and 197 (equations 183 and 184)411. 

The insertion of ethylene into Pdn allyl complexes with hemilabile P—O ligands gives (l -T -pentenyl complexes.187 The insertion of alkenes into M—C bonds of T -iminoacyl and rf-pyridyl,188 into Rh—B bonds of boryl complexes,189 into M—Si,190 M—P,191 and M—O bonds is also known, an example for the latter being... 

Iridium complexes such as IrCl(COD)(L-L), [Ir(COD)(L-L)]+ and [Ir(COD)-(py)(PR3)2]+ are active catalysts for the hydrogenation of a variety of substrates, particularly tetrasubstituted alkenes, e.g. [Ir(COD)(py)(PCy3)] known as Crabtree s catalyst, hydrogenates tetramethylethylene. On the other hand, [Ir(COD)(py)2]+ is inactive, apparently since it does not add H2. The hydrido cation [Ir(H)2(COD)(i72-Pr,2PCH2CH2OMe)]+, with a chelating P—O ligand, selectively hydrogenates phenylacetylene to styrene, via Ir—CH=CHPh intermediates.2... 

Figure 14 A proposed mechanism for the Ni-catalyzed trimerization of ethylene to 1-hexene using a hemilahile P O ligand.

The Ni complex incorporating mixed-donor P/O-ligands find industrial application in the Shell Higher Olefin Process (SHOP) to yield a-alkenes, while the complex (71) has been shown to be active in the polymerization of olefins and will also tolerate functionalized monomers such as methyl methacrylate. Grubbs and coworkers have adapted the standard SHOP catalyst to yield a highly active family of catalysts (eg. 72) for the polymerization of low-branched polyethylene. This is in contrast to the diimine catalysts (Section 5.2) which lead to a more highly branched polyethylene. 

The bidentate P, O ligand complexes ZrX2[OC(But)2CH2PPh2]2 (X = C1, Me, CH2Bu ) and ZrX[OC(But)2CH2PPh2]3 (X = C1, Me) have been prepared and characterized by Baxter and Wolczanski. NMR spectroscopy revealed fluxional behavior in solution.832... 

Ligand mode assignments have been proposed from the IR spectra of Pt(dach)(L), where dach = irans-( ) 1,2-diaminocyclohexane, H2L = -substituted L-glutaric or L-aspartic acids.517 Similar data were given for [Pt4 (p2-isonic)4(dppf)4]4+, where isonic = isonicotinate, dppf = l,l -bis (diphenylphosphine)ferrocene.518 The IR spectrum of (83) includes vCOO of the P.O-ligand at 1638 cm-1.519... 

These nickel-hased compounds with P-O ligands also proved to he capable of ter-polymerizing norbornenes and ethylene with 1-alkenes. As in copolymerizations with ethylene, the level of incorporation of 5- -butylnorbornene in the terpoly-mers was lower than that of norbornene. Additionally, the molecular weights obtained were lower, suggesting that the incorporation of the 1-alkene facilitates chain termination. 

In view of many potential applications, the synthesis of a largely linear polyethylene with some degree of crystallinity in aqueous emulsion would be of interest Recently, Mecking et al. and Spitz et al. independently reported nickel(II)-cata-lyzed polymerization of ethylene to linear material in aqueous emulsion [65, 66]. Neutral nickel(II) complexes 6 and 7 (Scheme 7.7) based on known bidentate P-O-ligands [67-70] were found to be suitable catalyst precursors. 

The P.O-ligand developed by A. S. Guram (L9) is a versatile ligand for the amination of aryl chlorides as most classes of amines work well.43 Primary amines have a more limited scope and tend to couple best with electron neutral to rich aryl chlorides, especially those with ortho substitution. 

Phosphinated polystyrene Phosphinated polystyrene Phosphinated polystyrene Gyanomethylated polybenzimidazole P/O ligand on polystyrene Nitroxide-functionalized polystyrene... 

Guram P-O Ligands Guram s P-O Ugands (Figure 3.9) were used for example for the coupling of bromoacetophenones with piperidine [106]. 

Rageot, D. Woodmansee, D. H. Pugin, B. Pfaltz, A. Proline-based P, O ligand/ iridium complexes as highly selective catalysts Asymmetric hydrogenation of trisubstituted aUtenes. Angew. Chem. Int. Ed. 2011,50,9598-9601. 

The reactivity of the Mn -oxyl moiety was explored by considering the nucleophilic attack of one water molecule in the AB mechanism (Fig. 4). The reaction takes place in a single step, in which water transfers a proton to the closest p-O ligand and the incipient hydroxide forms the 0—0 bond with the oxyl. The transition state involves a five-membered MnOsH ring (Fig. 6). On the reactants side, water is FI-bound to the p-O ligand, whereas, on the products side, water appears spHt across the new p-OH and k-OOH... 

Note that the arrow pushing mechanism takes into account that, while formally the Ni-Ph and Ni-0 bonds are covalent bonds, the Ni-P bond is a dative bond. Chelating ligands having the backbone of the type present in 6.46 are often referred to as (P, O) ligands. 

The steric and electronic characteristics of the chelating (P, O) ligand exercise considerable influence on the stabilities of all the intermediates and transition states. Thus modifications of the chelating (P, O) ligand, as in 6.54 and 6.55, have notable effects on the product profile and the activity. 

Explain why (a) Cossee mechanism is not the only possible mechanism for chain propagation in ethylene oligomerization reaction (b) selectivity of the oligomerization of ethylene is expected to be a function of the electronic and steric properties of the P, O ligands (c) conversion of 6.46 to 6.47 is accompanied by the formation of styrene (d) the reaction of Ni(COD)j with appropriate phosphonium ylide and PPhj gives 6.46 (e) in ethylene oligomerization, product formation may take place by chain transfer (f) on replacement of PPhj in 6.46 by PMOj, there is an almost complete inhibition of the catalytic reaction. 

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