Alkene derivatives ligands

Fig. 5. An alkene-derived ligand acting in a polydentate mode.

The homo- and cross-addition of alkenes catalyzed by a transition-metal provided another economical way of forming C-C bonds.155 These reactions are carried out by using nickel, palladium, or ruthenium phosphine complexes to yield vinylarenes and some can occur in aqueous media. By using carbohydrate-derived ligands, asymmetric hydrovinylations can be carried out in aqueous conditions.156... 

An advantage of using the BPPM-derived ligands was that as a result of the lack of symmetry, Achiwa was able to determine which phosphine group was orientated ds and which one was irons to the -coordinated alkene (cf. Scheme 3). 

Table 6. Asymmetric Catalytic Dihydroxylation of Alkenes in the Presence of Cinchona Derived Ligands"...

In oxidation reactions, however, osmium is significantly more selective than catalysts derived from other transition metals. Osmium-based catalysts for the hydroxylation and amination of alkenes are very widely used in organic synthesis.With alkaloid-derived ligands, the hydroxylation and amination reactions are highly enantioselective see Enantioselectivit. The use of bleach, hydrogen peroxide, ferric cyanide, and oxygen have been reported as secondary oxidants for some of these reactions. 

Bis-oxazolines (Box), azabis-oxazolines (azaBox) and pyridine bis-oxazoline (PyBox) with a C2 symmetry form the great majority of various oxazoline-derived ligands which are used in immobilized catalytic cyclopropanation reactions (Scheme 36). For these reactions, immobilized ruthenium and copper complexes which are able to form carbene intermediates have been tested. Most of the time, the reaction involved styrene and ethyl diazoacetate (EDA). However other alkenes (diphenyl ethylene. ..) and other diazo compounds (i.e. tertiobutyl and ethyl 2-phenyl diazoacetate) have also been investigated (Scheme 36). 

Group 6 metal-carbene complexes are most stable when devoid of (3-hydrogen atoms on the carbene ligand, the latter leading to decomposition by 1,2-H migration and formation of alkene derivatives.The carbene ligand usually bears hydrogen or alkyl substitu-... 

The diazonium salts 145 are another source of arylpalladium com-plexes[114]. They are the most reactive source of arylpalladium species and the reaction can be carried out at room temperature. In addition, they can be used for alkene insertion in the absence of a phosphine ligand using Pd2(dba)3 as a catalyst. This reaction consists of the indirect substitution reaction of an aromatic nitro group with an alkene. The use of diazonium salts is more convenient and synthetically useful than the use of aryl halides, because many aryl halides are prepared from diazonium salts. Diazotization of the aniline derivative 146 in aqueous solution and subsequent insertion of acrylate catalyzed by Pd(OAc)2 by the addition of MeOH are carried out as a one-pot reaction, affording the cinnamate 147 in good yield[115]. The A-nitroso-jV-arylacetamide 148 is prepared from acetanilides and used as another precursor of arylpalladium intermediate. It is more reactive than aryl iodides and bromides and reacts with alkenes at 40 °C without addition of a phosphine ligandfl 16]. 

The bulk of derivatives are obtained by the displacement of CO by other ligands. These include phosphines and other group 15 donors, NO, mer-captans and unsaturated organic molecules such as alkenes, alkynes and cyclopentadienyls. 

Another important reaction associated with the name of Sharpless is the so-called Sharpless dihydroxylation i.e. the asymmetric dihydroxylation of alkenes upon treatment with osmium tetroxide in the presence of a cinchona alkaloid, such as dihydroquinine, dihydroquinidine or derivatives thereof, as the chiral ligand. This reaction is of wide applicability for the enantioselective dihydroxylation of alkenes, since it does not require additional functional groups in the substrate molecule ... 

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