Symmetric catalysts

Whereas general activities and selectivities for hydrogenations of ketones are similar to those of aldehydes, one big difference exists between the two. The hydrogenation of prochiral ketone carbonyls produces chiral carbons. Over symmetrical catalysts, racemic alcohols are formed however, over unsymmet-rical surfaces, enantioselectivity may occur. Enantioselective hydrogenations of ketones is an increasingly active research held and is covered in Chapter 3. Here we discuss that aspect of stereoselectivity associated with ring systems. 

With MAO activation, Zr- and Hf-FI catalysts 1 and 3 exhibit fairly high reactivity toward propylene and produce propylene oligomers [64, 65], Conversely, the corresponding Ti-FI catalyst/MAO 2 forms semicrystalline PP (1 °C polymerization), which displays a peak melting temperature of 97 °C, indicative of the formation of a stereoregular polymer. To our surprise, microstructural analysis by 13C NMR indicates that the resultant polymer is syndiotactic (rr 19%), and that a chain-end control mechanism is responsible for the observed stereocontrol, regardless of the C2 symmetric catalyst ([28] for the first report on syndiospecific propylene... 

Additionally, research on FI catalysts has provided useful information on high catalytic activity, the formation of highly syndiotactic PPs with C2-symmetric catalysts, the origin of highly controlled living olefin polymerization, and the high incorporation capability for higher a-olefins. 

In the early stages of the metallocene-catalyzed olefin polymerizations the focus of research lay on C2- and CY-symmetric complexes [3, 12, 13], Since the beginning of the 1990s C,-symmetric catalysts have had more and more impact. The reason is... 

Looking at C2-symmetric catalysts, there exist in principle two different metallocene-olefin complexes (Fig. 4). Structure a is energetically favored because the methyl group of the propene is in trans-position relative to the (3-methyl structure of the polymer chain. Both methyl groups in structure b are in cis position to each... 

Fig. 6 Proposed mechanisms for the formation of isotactic polypropylenes I-IV with isolated stereoerrors obtained from -symmetric catalysts [9]...

Inspired by the design of metallocence catalysts, there has been research into the use of C2-symmetric Ni a-diimine catalysts for the preparation of polyolefins with stereoregularity. Such catalysts were shown to afford higher degrees of isotacticity in polypropylenes as compared to the standard C2v-symmetric catalysts, which afford mostly syndiotactic polymer [106], Coates and coworkers have studied... 

The preferred primary and secondary insertions of opposite monomer prochiral faces into isospecific C2-symmetric catalysts, and of a same prochiral face into syndiospecific Cs-symmetric catalysts, have been confirmed by recent characterization studies on propene-ethene-styrene terpolymers.79... 

Chiral, Ci-symmetric (asymmetric) bridged metallocenes, 16 108-109 Chiral, C2-symmetric bridged metallocenes, 16 104-108 Chiral, C2-symmetric catalysts, racemic mixture of, 16 106 Chiral, C2-symmetric unbridged metallocenes, 16 108 Chiral catalysts, 16 395 Chiral centers, in biochemical compounds, 17 402... 

Figure 12. Scheme of stereospecific 1-olefins polymerization with generic C2 and Cs symmetric metallocenes. In the framework of a regular chain migratory mechanism, the C2 and Cs symmetric catalysts lead to iso- and syndiotactic polymers, respectively. In fact, multiple insertions of the same enantioface occur with C2 symmetric metallocenes, while multiple insertions of alternating enantiofaces occur with Cs metallocenes. 

In the framework of the regular chain migratory mechanism of Figure 12, the enantioface selectivities we have calculated for the C2- and Cs-symmetric catalysts of Figures 13 and 14, explain the iso- and syndiospecificity experimentally found for the corresponding real catalysts [89-91]. 

Figure 10.16. Simplified view for syndiotactic polymer formation at a Cs symmetric catalyst...

A new C3-symmetric chiral phase-transfer catalyst that offers multipoint inteaction with a nucleophile has been described (Scheme 7.6) [23]. Thus, various quaternary ammonium salts were prepared through the ring opening of optically active epoxides, followed by quaternization of the resulting amines. Asymmetric benzylation of Schiff s base 20 in the presence of catalyst 24—26 yielded (S)-21 with moderate enantioselectivity. As expected, the C3-symmetric catalyst R,R,R)-26a provided... 

The more recent computational works on the topic of polymerization of substituted olefins have used DFT/MM methods. Since the general picture was defined with the pure MM methods described above, current research has targeted the treatment of specific aspects, and the development of new catalysts. A great deal of this research is related to variations of the catalysts depicted in Fig. 6. A usual QM/MM partition is shown in Fig. 7. It is clear from this that the MM region is crucial, because otherwise the C2- and Cs-symmetric catalysts would be identical. 

Unsymmetrically substituted unsaturated NHC catalysts (silyl ether-mesityl 34 [86,87], perfluoroalkyl-mesityl 35 [86], ester-mesityl 36 [88]) can catalyze the RCM of dienes, however, with lower yields than those obtained with the parent catalyst 2 (Fig. 7). Subsequently, Blechert et al. reported the synthesis of alkyl-mesityl saturated NHC complexes 37 and 38. These catalysts show a generally lower reactivity than the corresponding symmetric catalysts 3 and 15 in RCM and CM [89]. 

The copolymerization parameter rt which indicates how much faster an ethene is incorporated in the growing polymer chain than an a-olefin, when the last inserted monomer was an ethene unit, lies between 1 and 60 depending on the kind of comonomer and catalyst. The copolymerization parameter r2 is the analogous ratio for the a-olefin. The product r r2 is important for the distribution of the comonomer and is close to unity when using C2 symmetric metallocenes, indicating a randomly distributed comonomer. It is less than unity with a more alternating structure for Cs-symmetric catalysts [62-65] (Table 5). 

The microstructures described above correlate to chain migratory insertion. While in the case of a C2 or C2v symmetric metallocene, due to the homotopic nature of the potentially active sites, chain stationary insertion or migratory insertion followed by site isomerization would result in the same microstructure as chain migratory insertion, in the case of Cs symmetric catalysts they result in isotactic blocks. 

Figure 19 Stereoselective insertions of propylene (grey) under catalytic-site control, mediated by the oc,[3 segment of the growing polymer chain (black), for isospecific polymerization by a C2-symmetric catalyst (A, left) and for syndiospecific polymerization by a Cs-symmetric catalyst (B, right).

Bao and Wulff compared catalysts prepared from vaulted biaryls and from bromo-borane dimethylsulfide with those generated from linear biaryls with regard to their capacity to provide enantioselective induction in the Diels-Alder reaction of cyclo-pentadiene and methacrolein (Eqs 6 and 7) [7]. Because the (5) enantiomers of vaulted biaryls result in induction opposite to that resulting from use of the (5) enantiomer of binaphthol, and because effective catalysts cannot be generated from binaphthol and phenylboron dichloride, suggest that the catalysts obtained from vaulted biaryls do not have the same structure as the Cs-symmetrical catalyst produced from binaphthol. 

Figure 7 Schematic representation of the most common symmetries of group 4 olefin polymerization catalysts. Gray rectangles define the space occupied by the organic ligand. Hollow squares represent the coordination positions available to the growing chain and to the monomer. Dashed lines represent the local mirror plane of the two Cs-symmetric catalysts.

The most successful classes of metallocene catalysts studied for low-tacticity iPP are (i) the fluxional bis(2-arylindenyl) metallocenes first conceived and demonstrated by Waymouth and Coates748 and recently reviewed 749,750 (ii) a few examples of C2-symmetric, 3-alkyl-substituted ansa-bis(indenyl) zirconocenes 222,709,751 and (iii) several types of -symmetric catalysts. 

All aspects of syndioselective propagation with ( -symmetric catalysts, the influence of ligand,794,797-807 metal,806,808,809 and counteranion100,104 variations, as well as the influence of the polymerization conditions,805,810 have been studied in detail and reviewed.162,181,209,288,811... 

Figure 31 Melting point of sPPs made with Cs-symmetric catalysts, versus the rr triad content. Note that samples with rr< 75% are amorphous in the second DSC heating scan, and require several days of annealing at room temperature to develop measurable crystallinity.

Cl-symmetric metallocenes present a synthetic advantage over C2-symmetric ones in terms of their application for isospecific polymerization of propylene. As mentioned previously, a problem associated with the synthesis of anra-C2-symmetric metallocenes is that they are almost invariably generated along with their wew-isomers, which are difficult to remove from the catalyst mixture and often produce undesirable low molecular weight atactic PP with certain polymerization activity. The synthesis of pure C2-symmetric catalysts usually requires multiple purification steps with low yields. In contrast, a e o-form does not exist... 

The common feature of Ci-symmetric metallocenes is that their two coordination sites are diastereotopic. On the size basis of the substitution on the cyclopenta-dienyl ligand, Ci-symmetric catalysts can vary in stereoselectivity to form PP that is hemi-isospecific (amorphous hi-PP) to partially isospecific (amorphous or low-crystallinity PP) to isospecific (i-PP with high Tin and high crystallinity). 

Recently, Ewen et al. have designed C2-, Cj-, and Ci-symmetric catalysts bearing heterocycle-condensed Cp ligands, as illustrated in Scheme 9, which add electronic effects to the stereocontrol. ° The complexes, containing isopropylidene-bridged cyclopentadienyl and cyclopentyl thiophene ligands, show activity... 

Figure 6.9. C2-symmetric catalysts for cyclopropanation (a) Pfaltz, 1988 [129] (b) Pfaltz,...

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