Metal-promoted elimination reaction

Metal-promoted reductive p-elimination reactions in 1,2-difunctionalized substrates have been one of the most important means of generating C=KZ bonds [9]. In the past few years, a number of reductive elimination reactions of 1,2-difunctionalized compounds have been reported under various organometallic reagents, such as Sml2 [10], Zn [11], or In [12], In addition, those low-valent organometallic species exhibit excellent properties for sequential organic reactions and several examples have been described in which a metal-promoted ehmination reaction occurs as a step of a sequential process [13],... 

The formation of vinylboranes and vinylboronate esters during some metal-promoted hydroboration of alkenes has led to the suggestion of an alternative mechanistic pathway. Insertion of the alkene into the metal-boron bond occurs in preference to insertion into the metal-hydride bond.44,51,52 In a competing side-reaction to reductive elimination, f3-H elimination from the resulting borylalkyl intermediate furnishes the vinylborane byproduct.52 There remains however a substantial body of evidence, both experimental53 and theoretical,54 that supports the idea that transfer of hydride to the coordinated alkene precedes transfer of the boryl fragment. 

This preparation illustrates an efficient two-step process for the transformation of a cycloalkenone to the corresponding a-substituted derivative. The first step involves the installation of an a-iodo substituent by a process thought to involve nucleophilic addition of pyridine, iodine capture of the resulting enolate, and pyridine-promoted elimination of pyridine.5 The resulting vinyl iodides are superior to other vinyl halides as participants in a variety of transition-metal catalyzed coupling reactions, illustrated here by the Suzuki coupling with an arylboronic acid. Other coupling partners that... 

These salts can be used for the synthesis of both transition metal and main group element thionyl imides by metathetical reactions, for example, Cp2Ti(NSO)2 and Ph3 As(NSO) (x = 1, 2), respectively. " The M NSO group invariably adopts a cis geometry in these derivatives. A characteristic reaction of thionyl imides is the thermal or base-promoted elimination of SO2 to give the corresponding sulftir dimide. ... 

Carbon-carbon bond formation by reductive elimination from Ni, Pd, or Pt complexes is widespread. In many cases it is presumed to occur as the final step in a catalytic cycle, whereby the organic product is expelled from the metal center, but in others it is a well-defined, mechanistically studied reaction. Elimination takes place from Ni, Pd, and Pt complexes in their - - 2 or + A oxidation states, and it may be promoted by thermolysis, by photolysis, or by nucleophilic attack at the metal center. The reaction may proceed by heterolytic or homolytic metal-carbon bond cleavage, reductive elimination, or dinuclear elimination, and more than one mechanism may operate. 

Almost all of the reactions of metals can be classified into just a few typical reactions, and the reactions that metals promote in organic chemistry are simple combinations of these typical reactions. If you learn these typical reactions, you will have no trouble drawing metal-mediated mechanisms. The typical reactions of metal complexes are ligand addition/ligand dissociation/ligand substitution, oxidative addition/reductive elimination, insertion/j8-elimination, a-insertion/ a-elimination, cr-bond metathesis (including transmetallations and abstraction reactions), [2 + 2] cycloaddition, and electron transfer. 

As judged by organic model reactions, p activation (i.e., coordination of a group on the p carbon atom by a metal) should be an effective means of promoting an enolization or an elimination reaction. 

Mg-Al mixed oxides obtained by thermal decomposition of anionic clays of hydrotalcite structure, present acidic or basic surface properties depending on their chemical composition [1]. These materials contain the metal components in close interaction thereby promoting bifunctional reactions that are catalyzed by Bronsted base-Lewis acid pairs. Among others, hydrotalcite-derived mixed oxides promote aldol condensations [2], alkylations [3] and alcohol eliminations reactions [1]. In particular, we have reported that Mg-Al mixed oxides efficiently catalyze the gas-phase self-condensation of acetone to a,P-unsaturated ketones such as mesityl oxides and isophorone [4]. Unfortunately, in coupling reactions like aldol condensations, basic catalysts are often deactivated either by the presence of byproducts such as water in the gas phase or by coke build up through secondary side reactions. Deactivation has traditionally limited the potential of solid basic catalysts to replace environmentally problematic and corrosive liquid bases. However, few works in the literature deal with the deactivation of solid bases under reaction conditions. Studies relating the concerted and sequential pathways required in the deactivation mechanism with the acid-base properties of the catalyst surface are specially lacking. 

Similar considerations are also applicable for the reductive elimination reactions for the other platinum group metals. Our calculations on reductive eUmination reactions have shown an energetic preference of Rh compounds to Ir , Ru and Os compounds. The fact can be rationaUzed taking into account that the only exothermic promotion energy of -37.6 kcal/moR is expected for s d — s d (Rh —>Rh )- In contrast, s d —>sid (Ir —>Ir ) and s d —>d (Ru — Ru ) are endothermic by 25.1 and 9.2 kcal/mol respectively [13]. 

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