Transformation promoters

The use of chiral diaminocarbenes as transition metal hgands for catalyzed asymmetric synthesis is certainly an emerging field of research. They are relatively easy to prepare and they allow munerous structural modifications. Their transition metal complexes shows very usefull properties such as the thermal and air stability. Even if there is only a few reports of effective asymmetric transformations promoted by these class of catalyst, all these pioneering works open the route to the discovery of efficient new catalysts. 

Foreign protein Culture type Plant species Transformation Promoter Leader sequence Production level Reference... 

In connection with reactions where a solvent is required, it must be noted that all transformations promoted by chiral Mo catalysts may be carried out in toluene (in addition to benzene) or various alkanes (e.g., n-pentane) with equal efficiency (see below for specific examples). Moreover, although 5 mol % catalyst is typically used in our studies, 1-2 mol % loading often delivers equally efficient and selective transformations. 

The first report of direct carbanion formation by acyclic C—N bond cleavage has exploited the benzotriazolyl leaving group in a novel transformation promoted by lithium in THF (Scheme ll).176... 

Abstract A description of selected types of reactions catalyzed by heme peroxidases is given. In particular, the discussion is focused mainly on those of potential interest for fine chemical synthesis. The division into subsections has been done from the point of view of the enzyme action, i.e., giving emphasis to the mechanism of the enzymatic reaction, and from that of the substrate, i.e., analyzing the type of transformation promoted by the enzyme. These two approaches have several points in common. 

Abstract Ruthenium holds a prominent position among the efficient transition metals involved in catalytic processes. Molecular ruthenium catalysts are able to perform unique transformations based on a variety of reaction mechanisms. They arise from easy to make complexes with versatile catalytic properties, and are ideal precursors for the performance of successive chemical transformations and catalytic reactions. This review provides examples of catalytic cascade reactions and sequential transformations initiated by ruthenium precursors present from the outset of the reaction and involving a common mechanism, such as in alkene metathesis, or in which the compound formed during the first step is used as a substrate for the second ruthenium-catalyzed reaction. Multimetallic sequential catalytic transformations promoted by ruthenium complexes first, and then by another metal precursor will also be illustrated. 

Table 1.1 Selected biochemical key transformations promoted (catalyzed) by metal ions comparison... 

A range of organic transformations promoted by lithium bromide and triethy-lamine under neat reaction conditions have been reported. As the reagents ben-zaldehyde and triethylamine are liquids, these reactions may not be entirely solvent free, just without an added solvent. The product distribution (or class of reaction) was affected by the solvent used in the reaction work up (Figure 2.14), and therefore a wide range of products can be obtained using a very simple approach. 

Sigmatr( c Feairangements (e.g. ene and Qaisen reactions) are beyond the scope of this review however, there are specific examples of this transformation, promoted by organoaluminum reagents. 

Olefin metathesis (OM) has proven to be one of the most important advances in catalysis in recent years based on the application of this chemistry to the synthesis of polymers and biologically relevant molecules [1-10]. This unique transformation promotes chain and condensation polymerizations, namely ring opening metathesis polymerization and acyclic diene metathesis polymerization (ADMET). Applications of metathesis polymerization span many aspects of materials synthesis from cell-adhesion materials [11] to the synthesis of linear polyethylene with precisely spaced branches [12]. 

The literature is filled with various processes and catalyst compositions and systems for these transformations. Promoted platinum and sulfided platinum are the most selective group VIII metal catalysts but depending on reaction conditions and the nature of the halogenonitrobenzene, some undesirable halo-azo and azoxy compounds are left in the product (refs. 3, 11). 

The type of the electrochemical cell (divided or undivided) can influence the EOI values especially for the treatment of benzene derivatives containing a -NO2 substituent. A typical example is the electrochemical treatment of p-Nitro Toluene Sulfonic acid (p-NTS) low EOI values (- 0,1) were obtained in the divided cell contrary to the undivided cell where high EOI values (0,5) were obtained. The increase of EOI values in the undivided cell is due to the cathodic reduction of -NOg group to -NH2 group, this transformation promotes the electrochemical oxidation as the substituent constant (a) for -NH2 has negative value (favouring the electrophilic attack on the benzene ring) contrary to the -NO2 substituent which has positive value (see 4 i). 

In this section, various transformations promoted by the use of stoichiometric phosphazenes are discussed, classified by the types of the phosphazenes P1-P4. 

This chapter will initially cover several aspects of dihapto-coordination of aromatic molecules, including the scope of the dearomatization agent and the aromatic substrate. The primary focus of this work, however, will be the fundamental organic reactions of these complexes with electrophiles and the subsequent reactions of those products. Several applications of this methodology will also be illustrated. Owing largely to its earlier discovery, the majority of the organic transformations reviewed will be with osmium(II), however, recent arene transformations promoted with rhenium(I) and molybdenum(O) will also be discussed, with an emphasis on differences in reactivity compared to those of osmium. 

Scheme 19.5 Various transformations promoted by Jacobsen s thiourea catalysts.

We will focus particularly on such transformations induced by ferric salts. This review is thus not intended to present exhaustive work done in carbohydrate chemistry with these salts. It will rather focus on a few transformations promoted by ferric salts used as a Lewis acid at our laboratories at Orsay and Gif sur Yvette. Dedicating this very specific topic to Andre Lubineau seems most appropriate. 

Iron salts are easily accessible, inexpensive and abundant and the metal itself is non-toxic. Their use should therefore become attractive from an economic and environmental point of view in a wide variety of carbohydrate transformations, in either stoichiometric applications or as a catalyst. As stated in the introduction, this review concentrates on a few transformations promoted by ferric salts used as Lewis acids in our laboratories and does not present exhaustive work done in carbohydrate chemistry with these salts. Many more other applications have been reported. However, their uses could be far more developed for fast and selective transformations of carbohydrates to useful new molecular constructs. Besides the acidic properties of iron(iii) presented here, iron chemistry is rich and could be particularly fruitful with carbohydrates in generating new types of complexes for regioselective transformations or in carbon-carbon forming reactions based on iron-catalyzed cross-coupling reactions. The glycochemistry community should certainly expect many more useful accomplishments in the near future. 

It is the aim of this chapter to present in detail a few selected examples of useful organic transformations promoted by Group 4-11 (Ti-Cu) metals rather than to give a comprehensive listing of all possible transformations, as this information is available in several other excellent books. - The protocols are selected to demonstrate the most common oxygenation (addition of O atoms) or oxidation (removal of H atoms) pathways encountered in transition metal-promoted reactions of organic substrates. 

Gold- and rhodium-catalyzed acylations of ammonia have been reported [112, 132], but similar transformations promoted by homogeneous mthenium catalysts still remain unknown. Such a reaction could only be performed employing a heterogeneous ruthenium-based system under aerobic conditions. However, a completely different mechanism operates in this case (see Sect. 6) [133]. 

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