Bifunctional activation

Our work on the bifunctional activation of CO insertion was prompted by the thought that strong molecular Lewis acids should be more effective and more general than simple cations. It already had been observed that molecular Lewis acids would promote a molecular Fischer-Tropsch type reaction (5), and that iron diene complexes can be converted to polycyclic ketones by the action of aluminum halides, equation 7,(18), but information on the course of these reactions was sketchy. 

The above observations strongly indicate that O-protonation is an important step in this particular reaction for the reduction of coordinated CO. Recent studies in our laboratory provide other examples of proton induced reduction in metal cluster systems, and an example of proton induced CO reduction has recently been reported by Atwood (44). It thus appears that protons as well as Lewis acids are effective in the bifunctional activation of coordinated CO. 

Scheme 6.117 Proposed bifunctional activation of the reactants through catalyst 117 in the asymmetric Michael-type Friedel-Crafts alkylation of 2-naphthols.

Chen and co-workers presented, in 2007, a Michael-type Friedel-Crafts reaction of 2-naphthols and trans-P-nitroalkenes utilizing the bifunctional activating mode of cinchonine-derived catalyst 117 [277]. The nitroalkene was activated and steri-cally orientated by double hydrogen bonding, while the tertiary amino group interacts with the naphthol hydroxy group to activate the naphthol for the nucleophilic P-attack at the Michael acceptor nitroalkene (Scheme 6.117). 

Scheme 6.130 Proposed mechanism of the 121-catalyzed enantioselective thio-Michael-aldol tandem reaction of 2-mercaptobenzaldehydes with a,P-unsaturated oxazolidinones bifunctional activation through thiourea 121.

Scheme 6.183 Proposed bifunctional activation mode of oxazoline-thiourea catalyst 222.

According to another NMR study, the mechanism of bifunctional activation in the asymmetric aza-Morita-Baylis-Hillman reaction (Scheme 7) involves rate-limiting proton transfer (116) in the absence of added protic species155 (in consonance with the report summarized in Scheme 5144), but exhibits no autocatalysis. Addition of Brpnsted acids led to substantial rate enhancements through acceleration of the elimination step. Furthermore, it was found that phosphine catalysts, either alone or in combination with protic additives, can cause racemization of the reaction product by proton exchange at the stereogenic centre. This behaviour indicates that the spatial arrangement of a bifunctional chiral catalyst for the asymmetric aza-Morita-Baylis-Hillman reaction is crucial not only for the stereodifferentiation within the catalytic cycle but also for the prevention of subsequent racemization.155... 

The absolute configuration of the amine 7 may be explained by a stereochemical model based on the X-ray crystal structure of the chiral BINOL-phosphate (Fig. 4). In the transition state the ketimine is activated by the Brpnsted acid in such a way, that the nucleophile has to approach from the less hindered si face as the re face is effectively shielded by the large aryl substituent of the catalyst (Fig. 4, left). Furthermore, a bifunctional activation seems to be plausible, where next to the ketimine protonation, the dihydropyridine is activated through a hydrogen bond from the Lewis basic oxygen of the phosphoryl group. 

Marks first suggested that the use of bifunctional activators in olefln polymerization might be advantageous. They were... 

Interfacial polycondensations can also be carried out in vapor-liquid systems. Reaction takes place at the interface between an aqueous solution of a bifunctional active hydrogen compound and the vapor of diacid chloride. Interfacial condensation is commercially important in the synthesis of polycarbonates (1-52). Polymerizations based on diacids are always less expensive than those that use diacid chlorides. In the polycarbonate case, however, the parent reactant, carbonic acid, is not suitable and the derived acid chloride, phosgene (COCI2), must be used. 

Methanol conversion to oxygenated compounds has not yet been reported, but has a potential for practical application to produce chemicals from methanol. Further studies are now centered on the characterization of this complicated catalyst to clarify the configuration of bifunctional active sites and to improve selectivity for a desired product. 

Oligomers of this structure were found in polymers prepared at low temperature. [92, 150]. Bifunctional activators, e.g. terephthaloyl-bis-caprolactam, yield molecules with two growing end groups... 

From these results, it is considered that the electrophilic carbon atom of CO is bound to the amino group of PAn and the basic oxygen atom coordinates to the central metal of the complex. Such bifunctional activation of the substrate may lead to the reduction at the electrode potential close to the thermodynamic one. Lactic acid is probably generated through the formation of formaldehyde, the insertion of CO to the C-H bonds to form C-C-C bond and the reduction by... 

This chapter presented the current stage of development in the desymmetrization of mt >o-com pounds and (dynamic) kinetic resolution of racemic compounds in which cinchona alkaloids or their derivatives are used as organocatalysts. As shown in many of the examples discussed above, cinchona alkaloids and their derivatives effectively promote these reactions by either a monofunctional base (or nucleophile) catalysis or a bifunctional activation mechanism. Especially, the cinchona-catalyzed alcoholytic desymmetrization of cyclic anhydrides has already reached the level of large-scale synthetic practicability and, thus, has already been successfully applied to the synthesis of key intermediates for a variety of industrially interesting biologically active compounds. However, for other reactions, there is still room for improvement... 

Explain why bifunctional activation using a basic and a protic center is a viable strategy for catalyst design in the asymmetric aza MBH reaction. 

Since metallic and acidic sites are both created during reduction of NiSMM, it is very difficult to measure exclusively the influence of the metal function on the bifunctional activity of the catalyst. 

The high activity of this catalyst can be ascribed to Br nsted sites of high acidity, vdiich are mainly formed during reduction of lattice nickel. The acidic activity of NiSMM is so strong that, even with 0.7 %w Pd or Pt on the catalyst, effects due to too low a metal activity on the bifunctional activity and selectivity cannot be excluded. 

In summary, the complexity of the HD-1 strain can be stated as follows it contains at least four insecticidal toxin genes, three of the PI type (that specify proteins active on lepidopteran insects and are of high molecular weight) and a P2 gene (whose protein is much smaller and has bifunctional activity on both lepidopteran and dipteran insects). These... 

Organometallic derivatives of polyoxometalates are perfectly characterized at the molecular level and in turn provide models for the grafting of an organometallic catalyst on an oxide surface, especially in the case of the supported derivatives. Beyond, they also display their own reactivity and catalytic activity.Some synergy between the organometallic and oxide parts has been reported in the literature, as well as bifunctional activity. The mobility of organometallic fragments at an oxide surface could also be reproduced on a polyanion. 

Scheme 37 Proposed bifunctional activation in Ti- and Al-catalyzed asymmetric hydrophospho nylation of aldehydes...

Scheme 38 Proposed bifunctional activation in ALB-catalyzed hydrophosphonylation of nitroalkenes...

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