Induction precatalyst

There was an induction period whose length depended on reaction temperature. Further evaluation of the PdCy precatalysts showed that the reaction temperatures required were significantly lower for processes involving reactions of ArPdX with allenes... 

The hydrogen consumption and enantioselectivities for the asymmetric hydrogenation of dimethyl itaconate with various substituted catalysts of the basic type [Rh(PROPRAPHOS)COD]BF4 are illustrated in Figure 10.13 [61]. The systems are especially suitable for kinetic measurements because of the rapid hydrogenation of COD in the precatalyst. There are, in practice, no disturbances due to the occurrence of induction periods. 

Such induction periods can, for example, result from transferring a precatalyst into the active species. For the asymmetric hydrogenation this is described in detail in (a) W. Braun, A. Salzer, H.-J. Drexler, A. Spannenberg, D. Heller, Dalton Trans. 2003, 1606 (b) H.-J. Drexler, W. Baumann, A. Spannenberg, C. Fischer, D. Heller,/. Organomet. Chem. 2001, 623, 89 ... 

Enantioselective hydrogenation of 1,6-enynes using chirally modified cationic rhodium precatalysts enables enantioselective reductive cyclization to afford alky-lidene-substituted carbocycles and heterocycles [27 b, 41, 42]. Good to excellent yields and exceptional levels of asymmetric induction are observed across a structurally diverse set of substrates. For systems that embody 1,2-disubstituted alkenes, competitive /9-hydride elimination en route to products of cycloisomerization is observed. However, related enone-containing substrates cannot engage in /9-hydride elimination, and undergo reductive cyclization in good yield (Table 22.12). 

Dauben et al. (15) applied the Aratani catalyst to intramolecular cyclopropanation reactions. Diazoketoesters were poor substrates for this catalyst, conferring little asymmetric induction to the product, Eq. 10. Better results were found using diazo ketones (34). The product cyclopropane was formed in selectivities as high as 77% ee (35a, n = 1). A reversal in the absolute sense of induction was noted upon cyclopropanation of the homologous substrate 34b (n = 2) using this catalyst, Eq. 11. Dauben notes that the reaction does not proceed at low temperature, as expected for a Cu(II) precatalyst, but that thermal activation of the catalyst results in lower selectivities (44% ee, 80°C, PhH, 35a, n = 1). Complex ent-11 may be activated at ambient temperature by reduction with 0.25 equiv (to catalyst) DIBAL-H, affording the optimized selectivities in this reaction. The active species in these reactions is presumably the aluminum alkoxide (33). Dauben cautions that this catalyst slowly decomposes under these conditions. 

The popularity of Cu(acac)2, where acac = acetylacetonato, as a precatalyst in alkene cyclopropanation using diazoesters has led to the investigation of chiral 1,3-dicarbonyls as a source of asymmetric induction in this process. Mathn et al. (26) report a selective cyclopropanation of styrene with a dimedone-derived diazocarbonyl in the presence of a camphor-derived diketone, Eq. 12. The reaction is con-... 

Although the Chalk-Harrod mechanism has been widely accepted,69 some phenomena (include an induction period for many precatalysts and the formation of vinylsilanes) cannot be explained well by the Chalk-Harrod mechanism. An alternative mechanism to the Chalk-Harrod mechanism involves insertion of the alkene into the M-Si bond instead of insertion of the alkene into the M-H bond (Fig. 5).70... 

The precatalyst 17 produced (S)-benzoin (6, Ar = Ph) in very good yield (83%) and enantioselectivity (90% ee). The condensation of numerous other aromatic aldehydes 4 yielded the corresponding a-hydroxy ketones 6 with excellent ee-values of up to 95%. (For experimental details see Chapter 14.20.2). Electron-rich aromatic aldehydes gave consistently higher asymmetric inductions than electron-deficient (i.e., activated) aromatic aldehydes, with lower reaction temperatures or lower amounts of catalyst leading to slightly higher enantioselectivities coupled with lower yields. 

Bulky ligands accomplish the desired, well-defined monolanthanide precatalyst species and their fine tuning directly affects the reactivity, as impressively demonstrated by tied-back cyclopentadienyl complexes and even water-stable BINOL systems. Simultaneously, strongly chelating ligands provide a sterically rigid ligand frame ( spectator area ) which is a prerequisite for induction of asymmetry at the lanthanide center. 

Drawing from their success with catalytic [4 + 2] cycloaddition, Lectka group developed another highly enantioselective cycloaddition of o-quinone methide (o-QM) with silyl ketene acetals, using a chiral cinchona alkaloid derived ammonium, N-(3-nitrobenzyl)quinidinium fluoride Is, as a precatalyst. The free hydroxyl group of the cinchona alkaloid moiety was crucial to high optical induction. A variety of silyl ketene acetals had been screened to afford the cycloadducts 22 with good ee (72-90%) and excellent yield (84—91%) (Scheme 10.26) [35]. 

This new generation of iron precatalysts was tested for the reduction of ketones and imines, in both ATH and, most recently, ADH. In both cases, no induction period... 

Ferrocenyl analogues 188, and a precatalyst version combined with a silver salt, have been tapped forenantioselective aza-Claisen rearrangement. Excellent asymmetric induction... 

Recently, the asymmetric hydrogenation of two related substrates with precatalysts of the type [Rh(DuPHOS)(diolefin)]BF4 has been investigated (4). It was demonstrated that the COD-containing complex required an induction time for the COD ligand to be removed from the precatalyst. This manifested itself as very low initial rate of hydrogen uptake, which increased as more catalyst became available. Conversely, for the NBD precatalyst there was no observable induction time and the hydrogenation reactions were complete in a fraction of the time required for the COD containing systems. Furthermore, at the end of the reactions they were able to show that in the case of the COD systems, approximately half the precatalyst introduced to the reaction system had not been converted to active catalyst. 

Much effort has been spent to ascertain whether these soluble precatalysts generate active heterogeneous catalysts. It is currently thought that most or all of these complexes actually do form heterogeneous arene hydrogenation catalysts. Finke has shown that the catalysts react with induction periods that are characteristic of the association of small particles to generate the true catalyst. ... 

The treatment of the dienediol bisnonaflate 309 containing an ( >— 1)-alkenyl substituent with the typical Heck precatalyst cocktail in the presence of an external alkene, such as an acrylate, gives rise to the formation of the bicyclic tetraene 310 by an intramolecular Heck coupling followed by an intermolecular Heck coupHng (Scheme 8.66) [239]. This reaction can be performed using chiral catalysts to achieve asymmetric induction with up to 30% ee (cf. Scheme 8.77). 

Phillips Chromox Catalyst. Impregnation of chromium oxide into porous, amorphous silica-alumina followed by calcination in dry air at 400-800°C produces a precatalyst that presumably is reduced by ethylene during an induction period to form an active polymerization catalyst (47). Other supports such as silica, alumina, and titanium-modified silicas can be used and together with physical factors such as calcination temperature will control polymer properties such as molecular weight. The precatalyst can be reduced by CO to an active state. The percent of metal sites active for polymerization, their oxidation state, and their structure are the subject of debate. These so-called chromox catalysts are highly active and have been licensed extensively by Phillips for use in a slurry loop process (Fig. 14). While most commonly used to make HDPE, they can incorporate a-olefins to make LLDPE. The molecular weight distributions of the polymers are very broad with PDI > 10. The catalysts are very sensitive to air, moisture, and polar impurities. 

In order to collect more information about the mechanism of the reaction, we devised three independent experiments in which the three reactants (alkyne, silane, and catalyst) were incubated two by two at 60 °C for 3h, before addition of the third component at 20 °C [35]. The results of these experiments are presented in Figure 5.20. As can be seen, the addition of complex 45 to a mixture of alkyne and silane displays a kinetic profile identical to what was observed previously (curve A). However, when the silane was added to the alkyne, previously incubated with the precatalyst, a considerable reduction of the catalytic activity occurred (curve B). It thus transpires that the alkyne triggers somehow the deactivation of the catalyst. In stark contrast, a dramatic acceleration of the reaction rate, concomitant with the disappearance of the induction period, was observed when the catalyst was heated with the silane prior to addition of the alkyne (curve C). This last effect is reminiscent of what was observed upon repeated addition of fresh reactants during the hydrosilylation of alkenes (see Figure 5.7). Therefore, treating the precatalyst with a silane before adding the alkyne leads to a particularly active and selective catalyst. 

No full mechanistic description of the whole hydroformylation cycle with NHC-Rh catalysts has been available to date. Only Trzeciak and Ziotkowski provided evidence that a Rh(NHC) - hydride complex is the active catalyst, which is formed from the corresponding halide precursor [71]. The formation of such a hydride complex is consistent with the induction period usuaUy observed when such precatalysts are used. In contrast, induction periods have not been observed when recycled catalysts have been reused. 

Indeed, organometallic precatalysts can be transformed during an induction period into catalyticaUy active species that do not contain metal-carbon bonds. For example, molybdenum [7a] and tungsten [7b] carbonyls catalyze aerobic photooxygenation of cyclohexane to cyclohexyl hydroperoxide (primary product) and cyclohexanol and cyclohexanone (Fig. 1.4). The proposed mechanism is shown in Fig. 1.5. It includes the formation during the induction period of an oxo derivative. Complexes CpFe( r-PhH)BF4 and ( 7r-durene)2Fe(BF4)2 also catalyzed the aerobic alkane photooxygenation [7c]. The mechanism has not been studied. 

---