Catalysts control

Reaction conditions must be controlled since HF is also an excellent polymerization catalyst. Controlled reaction conditions can alternatively lead to vinyl fluoride or to HFC-152a (CH2CHF2). The latter can be thermally cracked to form vinyl fluoride. 

Baum, R. Elastomeric Polypropylene Oscillating Catalyst Controls Microstructure, Chemical and Engineering News, Jan., 16, 1995, pp. 6-7. 

A new iterative strategy for enantio- and diastereoselective syntheses of all possible stereoisomers of 1,3-polyol arrays has been described by Shibasaki. This strategy relies on a highly catalyst-controlled epoxidation of a, 3-unsaturated morpholi-nyl amides promoted by the Sm-BIN0L-Ph3As=0 complex, followed by the con-... 

Fig. 4.15), are active for ATRP of both styrene and methylmethacrylate (MMA) [46]. Polymerisation was well controlled with polydispersities ranging from 1.05 to 1.47. The rates of polymerisation 1 x 10 s ) showed the complexes to be more active than phosphine and amine ligated Fe complexes, and were said to rival Cu-based ATRP systems. It was quite recent that Cu(I) complexes of NHCs were tested as ATRP catalysts [47]. In this work, tetrahydropyrimidine-based carbenes were employed to yield mono-carbene and di-carbene complexes 42 and 43 (Fig. 4.15), which were tested for MMA polymerisation. The mono-carbene complex 42 gave relatively high polydispersities (1.4-1.8) and a low initiation efficiency (0.5), both indicative of poor catalyst control. The di-carbene complex 43 led to nncontrolled radical polymerisation, which was ascribed to the insolubility of the complex. 

Examples of catalyst control of stereoselectivity have been encountered in the course of the use of the ene reaction to elaborate a side chain on the steroid nucleus. The steroid 4 gave stereoisomeric products, depending on the catalysts and specific aldehyde that were used.33 This is attributed to the presence of a chelated structure in the case of the SnCl4 catalyst. 

The synthesis in Scheme 13.40 features a catalytic asymmetric epoxidation (see Section 12.2.1.2). By use of me30-2,4-dimethylglutaric anhydride as the starting material, the proper relative configuration at C(4) and C(6) is ensured. The epoxidation directed by the (+)-tartrate catalyst controls the configuration established at C(2) and C(3) by the epoxidation. Although the epoxidation is highly selective in... 

Scheme 10.34. Catalyst controlled synthesis of 12 stereoisomers of emetine.

Porco s synthesis of ( )-kinamycin C (3) constituted the first reported route to any of the diazofluorene antitumor antibiotics. This synthesis invokes several powerful transformations, including a modified Baylis-Hillman reaction, a catalyst-controlled asymmetric nucleophilic epoxidation, and a regioselective epoxide opening to establish the D-ring of the kinamycins. The tetracyclic skeleton was constructed by an... 

The choice of catalyst controls the intramolecular cyclisation of 2-(l-alkynyl)benzoic acids, with AgNCh effecting efficient formation of 3-substituted isocoumarins, but Ag powder favouring a 5-exo-dig-cyclisation to the phthalide (Scheme 45) <00T2533>. 

As described hitherto, diastereoselectivity is controlled by the stereogenic center present in the starting material (intramolecular chiral induction). If these chiral substrates are hydrogenated with a chiral catalyst, which exerts chiral induction intermolecularly, then the hydrogenation stereoselectivity will be controlled both by the substrate (substrate-controlled) and by the chiral catalyst (catalyst-controlled). On occasion, this will amplify the stereoselectivity, or suppress the selectivity, and is termed double stereo-differentiation or double asymmetric induction [68]. If the directions of substrate-control and catalyst-control are the same this is a matched pair, but if the directions of the two types of control are opposite then it is a mismatched pair. 

Striking examples of this phenomenon are presented for allyl and homoallyl alcohols in Eqs. (5) to (7). The stereodirection in Eq. (5) is improved by a chiral (+)-binap catalyst and decreased by using the antipodal catalyst [60]. In contrast, in Eq. (6) both antipode catalysts induced almost the same stereodirection, indicating that the effect of catalyst-control is negligible when compared with the directivity exerted by the substrate [59]. In Eq. (7), the sense of asymmetric induction was in-versed by using the antipode catalysts, where the directivity by chiral catalyst overrides the directivity of substrate [52]. In the case of chiral dehydroamino acids, where both double bond and amide coordinate to the metal, the effect of the stereogenic center of the substrate is negligibly small and diastereoface discrimination is unsuccessful with an achiral rhodium catalyst (see Table 21.1, entries 9 and 10) [9]. 

In the hydrogenation of diketones by Ru-binap-type catalysts, the degree of anti-selectivity is different between a-diketones and / -diketones [Eqs (13) and (14)]. A variety of /1-diketones are reduced by Ru-atropisomeric diphosphine catalysts to indicate admirable anti-selectivity, and the enantiopurity of the obtained anti-diol is almost 100% (Table 21.17) [105, 106, 110-112]. In this two-step consecutive hydrogenation of diketones, the overall stereochemical outcome is determined by both the efficiency of the chirality transfer by the catalyst (catalyst-control) and the structure of the initially formed hydroxyketones having a stereogenic center (substrate-control). The hydrogenation of monohydrogenated product ((R)-hydroxy ketone) with the antipode catalyst ((S)-binap catalyst) (mis-... 

In principle, asymmetric synthesis involves the formation of a new stereogenic unit in the substrate under the influence of a chiral group ultimately derived from a naturally occurring chiral compound. These methods can be divided into four major classes, depending on how this influence is exerted (1) substrate-controlled methods (2) auxiliary-controlled methods (3) reagent-controlled methods, and (4) catalyst-controlled methods. 

CHIRAL CATALYST-CONTROLLED ASYMMETRIC ALDOL REACTION... 

Chiral Catalyst-Controlled Asymmetric Aldol Reaction 155... 

Hydrogenation of diacetyl (5) catalyzed by (S)-l-Ru gives a 74 26 mixture of meso-6 and S,S-6. Evidently in this reduction catalyst control favoring formation of meso-diols dominates over substrate control favoring formation of / or d-diols. 

Five-coordinate aluminum alkyls are useful as oxirane-polymerization catalysts. Controlled polymerization of lactones102 and lactides103 has been achieved with Schiff base aluminum alkyl complexes. Ketiminate-based five-coordinate aluminum alkyl (OCMeCHCMeNAr)AlEt2 were found to be active catalyst for the ring-opening polymerization of -caprolactone.88 Salen aluminum alkyls have also been found to be active catalysts for the preparation of ethylene carbonate from sc C02 and ethylene oxide.1 4 Their catalytic activity is markedly enhanced in the presence of a Lewis base or a quaternary salt. 

In this section, you learned that chemical reactions usually proceed as a series of steps called elementary reactions. You related the equations for elementary reactions to rate laws. You learned how the relative speed of the steps in a reaction mechanism help to predict the rate law of an overall reaction. Finally, you learned how a catalyst controls the rate of a chemical reaction hy providing a lower-energy reaction mechanism. In this chapter, you compared activation energies of forward and reverse reactions. In the next unit, you will study, in detail, reactions that proceed in both directions. 

Diastereoselective hydrogenations of this type have been reported by Burgess and coworkers [54—59] using chiral-protected and -unprotected allylic and homo allylic alcohols as substrates with their carbene catalyst lr(9). Catalyst control was found to be dominant, but depending on the position and nature of the oxygen substituents, moderate to strong match/ mismatch effects were observed. 

A reasonably large difference in diastereomeric excess was observed between product 47b with an adjacent methyl ester and 48b with a primary alcohol in the equivalent position [57]. It was noted by the authors that in cases involving a 1,3 system, changing the pendant group from a primary allylic alcohol to a methyl ester caused a reversal of facial selectivity [54, 58]. The same effect was absent in the 1,2 systems 51b and 52b studied. The diastereomeric ratio in the latter case was attributed mainly to catalyst control [58]. 

Although all of the reported Rh-catalyzed reactions of allenynes were of type A, an Ir catalyst resulted in a different regioselectivity. That is, when allenynes with two substituents on the allene terminus were used under a low partial pressure of CO, the type B reaction proceeded exclusively such that bicyclic cyclopentenones with an aLkylidene substituent were obtained (Scheme 11.22) [34]. However, when [RhCl(CO)(PPh3)2] was used as a catalyst under the same reaction conditions in place of [IrCl(CO)(PPh3)2], the type A reaction was predominant These results imply that the metal centers of the catalysts control the regioselectivity of two ole-finic moieties of allene to some extent... 

Scheme 12.7 Catalyst control of product distributions/mechanistic implications.

The use of chiral Br0nsted acids is illustrated in Eq. 93 as a method for catalyst-controlled double diastereoselective additions of pinacol allylic boronates. Aside from circumventing the need for a chiral boronate, these additions can lead to very good amplification of facial stereoselectivity. For example, compared to both non-catalyzed (room temperature, Eq. 90) and SnCU-catalyzed variants, the use of the matched diol-SnCU enantiomer at a low temperature leads to a significant improvement in the proportion of the desired anti-syn diastereomer in the crotylation of aldehyde 117 with pinacolate reagent (Z)-7 (Eq. 93). Moreover, unlike reagent (Z)-ll (Eq. 91) none of the other diastereomers arising from Z- to E-isomerization is observed. 

On the other hand, it is generally accepted that the redox properties of the selective oxidation catalysts control the oxygen activation as well as the surface stabilization of the oxygen activated species and their reactivity (19), In particular, the stabilization of active oxygen forms requires the presence of reduced sites on the surface. In fact, the peculiar behaviour of Mo, V and Fe oxides in selective oxidation reactions is strictly linked with the stabilization of reduced states (19), This point has stimulated a growing interest in providing correlation between the degree of reduction (32) or the extent of reduced sites (20) and the reactivity in... 

This chapter surveys the polymerization of substituted acetylenes focusing on the research during this decade. Monomers and polymers, polymerization catalysts, controlled polymerizations, and functional polyacetylenes are discussed. Readers are encouraged to access other reviews and monographs on the polymerization of substituted acetylenes, and a,cj-diynes. ... 

With meso-conflgured dialdehyde precursors, the enantiotopic nature of the termini must give rise to a conflgurational terminus differentiation upon twofold chain extension because the catalyst-controlled diastereoselective aldol additions will break the inherent o symmetry. While the two enantiotopic termini cannot... 

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