Metal-based nucleophile

Metal-based nucleophiles may react with activated haloarenechromium complexes leading to substitution of the halides. The reaction of Collmarfs reagent Na2[Fe(CO)4] with chloro-arenetricarbonylchromium complexes in THF/N-methylpyrrolidinone produces the yellow anionic dinuclear complexes 91a in 45 % yield (Scheme 40) as a result of an ipso SNAr [64]. Spectroscopic data suggest that complex 91a adopts the rj6 structure as opposed to the alternative //5 -cyclohexadieny 1 carbene structure 92. Similarly, it has been reported by the same group that the potassium salt of [CpFe(CO)2] participates in ipso nucleophilic attack on chloroarenechromium substrates, producing dinuclear complexes 91b in 92 % yield (Scheme 40) [65]. 

The scope of the review 2 Introduction to metal complexes as nucleophiles 2 Types of metal-based nucleophiles 5 The scale of nucleophilicity 6 The influence of ligands on the nucleophilicity 7 The nucleophilicity of metal complexes 12 The influence of the solvent on the nucleophilicity 14 Stereochemical changes at a saturated carbon centre 18 Reaction of coordinatively-saturated complexes 18 Reactions of coordinatively-unsaturated complexes 22 Reactions of MCjSn 27... 

The aza-Henry reaction is the nucleophilic addition of nitroalkanes to imines to give nitroamine derivatives. This reaction was also studied with metal-based catalysts [164]. 

Transition metal oxides represent a prominent class of partial oxidation catalysts [1-3]. Nevertheless, materials belonging to this class are also active in catalytic combustion. Total oxidation processes for environmental protection are mostly carried out industriaUy on the much more expensive noble metal-based catalysts [4]. Total oxidation is directly related to partial oxidation, athough opposes to it. Thus, investigations on the mechanism of catalytic combustion by transition metal oxides can be useful both to avoid it in partial oxidation and to develop new cheaper materials for catalytic combustion processes. However, although some aspects of the selective oxidation mechanisms appear to be rather established, like the involvement of lattice catalyst oxygen (nucleophilic oxygen) in Mars-van Krevelen type redox cycles [5], others are still uncompletely clarified. Even less is known on the mechanism of total oxidation over transition metal oxides [1-4,6]. 

The first class of amine-based nucleophilic catalysts to give acceptable levels of selectivity in the KR of aryl alkyl. yec-alcohols was a series of planar chiral pyrrole derivatives 13 and 14, initially disclosed by Fu in 1996 [25, 26]. Fu and co-workers had set out to develop a class of robust and tuneable catalysts that could be used for the acylative KR of various classes of. yec-alcohols. Planar-chiral azaferrocenes 13 and 14 seemed to meet their criteria. These catalysts feature of a reasonably nucleophilic nitrogen and constitute 18-electron metal complexes which are highly stable [54-58]. Moreover, by modifying the substitution pattern on the heteroaromatic ring, the steric demand and hence potentially the selectivity of these catalysts could be modulated. 

Condensation reactions exhibit several characteristics such as (typically) expulsion of a smaller molecule on reaction leading to a repeat unit containing fewer atoms than the sum of the two reactants, and most reactions can be considered in terms of polar (Lewis acid-base nucleophilic-electrophilic) mechanisms. The reaction site can be at the metal atom (that is adjacent to the atom)... 

Hydrogen bonding to substrates such as carbonyl compounds, imines, etc., results in electrophilic activation toward nucleophilic attack (Scheme 3.1). Thus, hydrogen bonding represents a third mode of electrophihc activation, besides substrate coordination to, for example, a metal-based Lewis acid or iminium ion formation (Scheme 3.1). Typical hydrogen bond donors such as (thio)ureas are therefore often referred to as pseudo-Lewis-acids. ... 

The formation of imine or Schiff base metal complexes can be effected very readily by arranging for reaction of the amine with the carbonyl compound to take place in the presence of the metal ion. Nucleophilic attack of the amine on the carbonyl group is enhanced by polarization of the carbonyl group by coordination of the oxygen atom to the metal ion. Furthermore, the reaction is reversible unless steps are taken to effect dehydration of the intermediate hydroxy amine, and coordination of the product imine is an important factor in the promotion of the reaction by suitable metal ions. 

To start with the addition of y-dienolates to aldehydes, the so-called vinylogous Mukaiyama aldol reaction, Campagne et al. studied the applicability of different types of catalyst when using the silyldienolate 115 as nucleophile [121]. In general, many products obtained by means of this type of reaction are of interest in the total synthesis of natural products. It should be added that use of CuF-(S)-TolBinap (10 mol%) as metal-based catalyst led to 68% yield and enantioselectivity up to... 

As discussed in Section 10.1, asymmetric epoxidation of C=C double bonds usually requires electrophilic oxygen donors such as dioxiranes or oxaziridinium ions. The oxidants typically used for enone epoxidation are, on the other hand, nucleophilic in nature. A prominent example is the well-known Weitz-Scheffer epoxidation using alkaline hydrogen peroxide or hydroperoxides in the presence of base. Asymmetric epoxidation of enones and enoates has been achieved both with metal-containing catalysts and with metal-free systems [52-55]. In the (metal-based) approaches of Enders [56, 57], Jackson [58, 59], and Shibasaki [60, 61] enantiomeric excesses > 90% have been achieved for a variety of substrate classes. In this field, however, the same is also true for metal-free catalysts. Chiral dioxiranes will be discussed in Section 10.2.1, peptide catalysts in Section 10.2.2, and phase-transfer catalysts in Section 10.2.3. 

Another type of metal-carbon bond, the metal carbene bond (with carbene of an electrophilic or nucleophilic character), appears to be the active bond in transition metal-based catalysts for the ring-opening metathesis polymerisation of cycloolefins. Such a bond, which is co-originated with metal by the sp2-hybridized carbon atom, possesses a a, n double bond character (Mt = C) [34,35], The enchainment of the coordinating cycloolefin at the active site... 

As discussed in this chapter, there are three direct modes of activation that metal ions can provide for hydrolysis reactions, i.e., by Lewis acid, nucleophile and leaving group. In addition, metal coordinated water molecules can act as general acid catalysts and metal coordinated hydroxides can act as general base catalysts. We are mainly concerned with the three direct modes of activation since there is no particular advantage to using metal-based general acids and bases. 

A complete survey of polar coupling reactions might include all types of organic ligands, n- and generally useful synthesis methodology based on polar activation by transition metals is nucleophile addition to... 

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