Jacobsen-type catalysts

Fueled by the success of the Mn (salen) catalysts, new forays have been launched into the realm of hybrid catalyst systems. For example, the Mn-picolinamide-salicylidene complexes (i.e., 13) represent novel oxidation-resistant catalysts which exhibit higher turnover rates than the corresponding Jacobsen-type catalysts. These hybrids are particularly well-suited to the low-cost-but relatively aggressive-oxidant systems, such as bleach. In fact, the epoxidation of trans-P-methylstyrene (14) in the presence of 5 mol% of catalyst 13 and an excess of sodium hypochlorite proceeds with an ee of 53%. Understanding of the mechanistic aspects of these catalysts is complicated by their lack of C2 symmetry. For example, it is not yet clear whether the 5-membered or 6-membered metallocycle plays the decisive role in enantioselectivity however, in any event, the active form is believed to be a manganese 0x0 complex <96TL2725>. 

In conclusion, mention should be made of dendronised chiral salen ligands and their Co2+ and Ni2+ complexes, which were prepared for use as Jacobsen-type catalysts by diaza-Cope rearrangement [24]. However, instead of the meso compounds produced, the enantiomers will have to be synthesised directly in enantioselective manner (Fig. 4.69). 

Regardless of the mechanism, the chiral (salen)Mn-mediated epoxidation of unfunctionalized alkenes represents a methodology with constantly expanding generality. Very mild and neutral conditions can be achieved, as illustrated by Adam s epoxidation of chromene derivatives 12 using Jacobsen-type catalysts and dimethyldioxirane as a terminal oxidant [95TL3669]. Similarly, periodates can be employed as the stoichiometric oxidant in the epoxidation of cis- and tram-olefins [95TL319],... 

An alternative approach for the epoxidation of simple olefins has been developed by Pietikainen and co-workers and Katsuki and co-workers by using the Jacobsen-type catalysts (Fig. 23) and in situ addition of imidazole or N-methylimidazole (122,129). The highest cc-values obtained were 60 % for... 

Berkessel and co-workers synthesized a library of structurally diverse tertiary amine-functionalized catalyst candidates incorporating a chiral 1,2- or 1,4-diamine chiral backbone [231, 232, 246]. Structure-efficiency studies through sequential modification of the diamine backbone, the tertiary amine functionality, the (thio) urea N-substituents as well as of the amide substituent pattern, exemplarily illustrated a Jacobsen-type 1,2-diamine-based structure (figure 6.24), identified... 

The early salen-type catalysts 143 of Aratani gave moderate results when menthyl diazoacetate was used,32 but more modern versions, such as the salen-Co catalyst 145 of Katsuki33 are much more impressive. You saw such catalysts in the last chapter under Jacobsen epoxidation. 

Scheme 8.28. Formally group-selection insertion of oxygen into enantiotopic C-H bonds, (a) An asymmetric Kharasch reaction [124], The catalyst is similar to that shown in Scheme 8.12, except that each oxazoline bears two methyl substituents at C-5. (b) Kinetic resolution of dihydronaphthalenes [125]. The reaction uses a Jacobsen epoxidation catalyst (Scheme 8.6, type A).

On the other hand, chiral primary amine-thiourea catalysts 85 and 90 developed by Tsogoeva [125] and Jacobsen [130], respectively, show an opposite sense of relative stereoinduction in the conjugate addition of acyclic ketones to nitroolefins (see Scheme 2.41 for 90). These anti selective catalysts stand in contrast to the usually obtained results which lead to selective formation of the i yn-conflgured diastereoiso-mers. The unexpected situation suggests participation of a Z-enamine intermediate. Moreover, with respect to the electrophile activation by the urea-type catalysts, it is also demonstrated that only one oxygen of the nitro group is bound to the thiourea moiety in an out-of-plane arrangement [125,130]. 

Another important type of transformation, which is has inspired the above developments, is the nitro-Mannich (or aza-Henry) reaction catalyzed by Jacobsen thiourea catalysts (Scheme 4.16) [77]. The reaction is highly 5yn-selective, and the corresponding products were isolated in high yields and ees. 

Chiral Catalysts Containing Group 7 Metals (Mn, Tc, and Re). Most of the chiral manganese complexes belong to the Mn(III)-salen-type complexes (Fig. 17), which are effective catalysts in asymmetric epoxidation (147). (The most widely used one is the Jacobsen s catalyst, iV,Ar -bis(3,5-di-terf-butylsalicylidene)-l,2-cyclohexanediamino-manganese(III) chloride.) These types of catalysts are also efficient for enantioselective aziridination (148), kinetic resolution of racemic allene via enantiomer differentiating oxidation (149), and enantiotopic selective... 

In the presence of methyl, tert-butyl, or (-)-menthyl esters of 2-oxocyclopentanecarboxylic acids, the cobalt(II)-salen type complexes and Jacobsen-type manganese(ni) complexes shown in Figure 17 are active catalysts for alkene epoxidation with dioxygen . 

The asymmetric Strecker-type reaction developed by the Jacobsen group is suitable for both aUphatic and aromatic imines, giving high enantiomeric excesses for a wide range of substrates. In this reaction the urea derivative also acts as the catalyst (Scheme 36). 

Jacobsen et al. reported that a different type of dintrogen ligand (48), fe[(2,6-dichlorophenyl)-methylideneaminojcyclohexane, was an efficient chiral ligand for copper-mediated asymmetric aziridination (Scheme 35).154 The reactions of conjugated c/.v-olefins show high enantioselectivity with this catalyst, but enantioselectivity of the reactions of simple olefins such as styrene and indene is moderate. 

The computations suggested that the enantioselectivity of the cyanosilylation arose from direct interactions between the ketone substrate and the amino-acid derived unit of the catalyst type represented by thiourea 72. On the basis of this insight, the Jacobsen group designed thiourea catalysts 73 and dipepetide thiourea catalyst 74 [67]. These optimized catalysts gave access to a broader spectrum of silylated cyanohydrins (e.g., 1-6) and proved to be more active (88-97% yield) and more enantioselective (98-98% ee) than 72 (Scheme 6.85) [242]. 

Several different immobilization methods are currently under investigation in order to immobilize various types of Jacobsen complexes in the mesopores of Al-MCM-48, Al-MCM-41 and Al-SBA-15 types of support materials. The novel chiral heterogeneous catalysts obtained will be characterized and their activity in different test reactions will be investigated. 

Jacobsen and Finney are to be credited with a major contribution to the comparison of different catalytic technologies by explicitly listing their evaluation criteria and by comparing quantitative data on a common basis for catalysts of different types and sources. Good criteria for catalyst evaluation should allow a clear and rapid decision in cases where all the data are available. Excellent performance with... 

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