Group 4 Metal Based Catalysts

Group 4 metal based catalysts have been studied intensively in hydroamination reactions involving alkynes and allenes [77 81], but (achiral) hydroamination reac tions involving aminoalkenes were only recently reported [82 84]. The reactivity of these catalysts is significantly lower than that of rare earth, alkali, and alkaline earth metal based catalysts. In most instances, gem dialkyl activation [37] of the aminoalk ene substrate is required for catalytic turnover. 

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Surprisingly, the polymerization rate has practically a zeroth-order dependence on the concentration of the monomer, which is a rare example for a group 4 metal-based catalyst. Although the reason for the zeroth-order dependence is unclear at the current time, one possible explanation is that, under the conditions examined, the cationic complex virtually exists as a (higher a-olefm)-coordinated form, presumably due to the highly electrophilic and sterically open nature of the cationic active species. 

We and others have revealed that syndiospecific propylene polymerization is exclusively initiated by 1,2-insertion followed by 2,1-insertion as the principal mode of polymerization [64]. This is the first example of a predominant 2,1-insertion mechanism for chain propagation exhibited by a group 4 metal-based catalyst. The unusual preference for 2,1-regiochemistry displayed by the Ti-FI catalysts compared with the Zr- and Hf-FI catalysts is apparently inconsistent with the crys-tallographically characterized structures, which indicate that the Ti is shielded more by the phenoxy-imine ligands and thus possesses higher steric compression. The reason for the unusual preference in the regiochemistry of Ti-FI catalysts is unclear at the present time. 

Fig. 8 Selected group-4-metal-based catalysts for hydroamination/cyclization of aminoalkenes [52, 55,57,61, 115-118]...

The development of group-4-metal-based catalysts for intramolecular hydro-amination of aUcenes has also led to several advanced systems for asymmetric hydroamination (Fig. 19). Most group 4 metal catalyst systems exhibit inferior reactivity and substrate scope (Table 19) in comparison to most rare earth metaland alkaline earth metal-based catalyst systems. They typically require high catalyst loadings and elevated reaction temperatures. However, the recent development of zwitterionic zirconium catalysts with significantly improved reactivities and selectivities [60, 118] promises to close this gap. 

A dimeric proline-derived diamidobinaphthyl dilithium salt has been introduced as the first example of a chiral main group metal-based catalyst for asymmetric hydroami-nation-cyclization reactions of aminoalkenes.256... 

Addendum Recent Achievements in Polymerisation with Main Group Metal-based Catalysts... 

The proline derived diamidobinaphthyl dilithium salt (S,S,S) 41, which is dimeric in the solid state and can be prepared via deprotonation of the corresponding tetraamine with nBuLi, represents the first example of a chiral main group metal based catalyst for asymmetric intramolecular hydroamination reactions of aminoalk enes [72]. The unique reactivity of (S,S,S) 41, which allowed reactions at or below ambient temperatures with product enantioselectivities of up to 85% ee (Scheme 11.12) [76], is believed to derive from the proximity of the two lithium... 

These examples illustrate that main group metal-based catalysts have the potential to be viable alternatives to rare earth metal catalysts. However, significant research efforts are necessary to improve these systems further. 

It is noteworthy that all group -metal-based catalysts described above can only be used for primary amines, as opposite to most late transition metal-based systems. While the stoichiometric reaction of Ti(NMe2)4 with phenylacetylene was shown to produce some enamine hydroanunation product [196], a catalytic process was only facilitated using the tethered zirconium bis(ureate) complex 35 (29) [57]. 

On these transition metal-based catalysts, the selechve hydrogenahon of the C=0 group is very difficult because C=C double bond hydrogenahon is both thermodynamically and kinehcally favored, especially in the case of small molecules (e.g., acrolein, crotonaldehyde) where addihonal steric effects are not important [62, 71, 72]. 

The same group of coordination polymerisations in which alkene undergoes re complex formation with the metal atom includes the copolymerisation of ethylene, a-olefins, cycloolefins and styrene with carbon monoxide in the presence of transition metal-based catalysts [54-58], In this case, however, the carbon monoxide comonomer is complexed with the transition metal via the carbon atom. Coordination bond formation involves the overlapping of the carbon monoxide weakly antibonding and localised mostly at the carbon atom a orbital (electron pair at the carbon atom) with the unoccupied hybridised metal orbitals and the overlapping of the filled metal dz orbitals with the carbon monoxide re -antibonding orbital (re-donor re bond) [59], The carbon monoxide coordination with the transition metal is shown in Figure 2.2. 

As stated above, the carbonylation oxidative polycondensation of bisphenol in the presence of transition metal-based catalysts leads to aromatic polycarbonate [scheme (18)] [6]. The reaction of bisphenol (HOArOH, e.g. Ar = p-C6H4 CMe2—C6H4—), carried out under CO and O2 pressure in a chlorohydrocarbon solvent under anhydrous conditions, using a group 8 metal-based catalyst (e.g. a PdBr2 complex) and a redox catalyst (e.g. Mn(II) (benzoinoxime)2, L vMn) in the presence of a base (e.g. 2,2,6,6,N-pentamethylpiperidine, R3N), involves most probably the pathway shown schematically below ... 

On the other hand, late transition metal-based catalyst systems that had been identified by the early 1990s were characterized by low activity but high functional group tolerance, especially toward water and other protic solvents. These features led to reinvestigations of ruthenium systems and, ultimately, to the preparation of the first well-defined, ruthenium-carbene olefin metathesis catalyst (PPh3)2(Cl)2Ru=CHCH=Ph2 (Ru-1) in 1992 [5]. 

As already mentioned, there has been significant progress in the development of chiral catalysts for asymmetric hydroamination reactions over the last decade. However, significant challenges remain, such as asymmetric intermolecular hydro aminations of simple nonactivated alkenes and the development of a chiral catalyst, which is applicable to a wide variety of substrates with consistent high stereochemical induction and tolerance of a multitude of functional groups as well as air and moisture. Certainly, late transition metal based catalysts show promising leads that could fill this void, but to date, early transition metal based catalysts (in particular, rare earth metals) remain the most active and most versatile catalyst systems. 

Candidate CO Control Technologies Catalytic Oxidation. The CO abatement unit uses a catalyst with a platinum group metal base to react the CO at high temperature with the oxygen present in the flue gas to produce carbon dioxide. The flue gas temperature range required for satisfactory operation is not as critical as with the SCR process. Temperatures in the range of 260—... 

Other active single-site early transition-metal-based catalysts for nonpolar monomers correspond to different bis-cyclopentadienyl group 3 and lanthanide complexes, such as L2MR (M = Sc, Y, La, Nd, Sm, Lu R = alkyl or H). Usually, these systems do not require the activator component to generate high activity catalysts for olefin polymerizations [3],... 

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