Transition metals catalytic allylic alkylations

Transition metal-catalyzed allylic alkylation is generally considered to involve mechanistically four fundamental steps as shown in Scheme 1 coordination, oxidative addition, ligand exchange, and reductive elimination. A key step of the catalytic cycle is an initial formation of a (7r-allyl)metal complex and its reactivity. The soft carbon-centered nucleophiles, defined as those derived from conjugate acids whose pAj, < 25, usually attack the allyl ligand from the opposite side... 

Scheme 8E.1. Catalytic cycle of transition metal catalyzed allylic alkylation.

A unique feature of the transition-metal-catalyzed allylic alkylation is its ability to convert starting materials of various symmetry types, such as racemic, meso- and achiral compounds, into optically pure material, Strategies to effect such transformations derive from recognition of the stereochemical courses in each step of the catalytic cycle and analysis of symmetry elements in the substrate or intermediate. Figure 8E.1 summarizes potential sources of enantio-discrimination in transition-metal-catalyzed allylic alkylation. 

In general, the catalytic cycle for the transition-metal catalyzed allylic substitution reactions involves initial attack of the metal at the double bond followed by oxidative insertion into the antiperiplanar C-0 bond to afford the Ti-allyl system. At this point, depending on whether soft or hard nucleophiles are used, however, the alkylation reaction proceeds through distinctly different pathways (Scheme 10). With soft nucleophiles, where Pd is often the metal center of choice. 

Direct catalytic intermolecular a-allylic alkylation of aldehydes and cyclic ketones has been achieved using a one-pot combination of a transition metal catalyst, Pd(PPh3)4, and an organocatalyst a secondary amine which facilitates enamine catalysis.300... 

Whilst electrophilic attack is commonly observed for tj -allyl ligands, the reactivity of rj3-allyl ligands towards nucleophiles has been intensely studied because the enormous synthetic utility this offers for allylic functionalization (allylic alkylation). The metal of choice for such applications is generally palladium (PE = 2.2), which may be employed in numerous catalytic processes. The versatility of palladium arises from the general tolerance of late transition metal centres to polar functional... 

These hydroxy-1,1 -binaphthyl functionalised NHC ligands can be used in asynunetric catalysis. Catalytic reagents performed with transition metal catalysts carrying these ligands include olefin metathesis [19,80,86], allylic alkylation [17,18,88] and hydrosilylation of ketones [85]. 

Transition-metal-mediated C-O bond cleavage reactions are interesting in view of environmentally benign halogen-free chemical processes [59]. Zerovalent ruthenium complexes are also active toward C-O bond-deavage reactions, and a number of catalytic processes have been developed in this respect. For example, Ru(l,5-COD)(l,3,5-COT) catalyzes allylic alkylation of carbon nucleophiles with allylic carbonates in basic solvent (Scheme 14.24) [60]. 

Oxidative addition of the carbon-halogen bond is a well-documented reaction for Group 10 transition metal complexes, but it is relatively limited for ruthenium. The example given here involves the reversible oxidative addition of allyl halide to RuCp(CO)2X to produce RuCp(p -allyl)X2 [78]. Oxidative addition of allyl halide to a Ru(0) complex Ru(l,5-COD)(l,3,5-COT) is also reported, but the product yield was poor [79]. Nevertheless, a catalytic Heck-type alkenylation of bromostyrene with methyl acrylate by Ru(l,5-COD)(l,3,5-COT) proceeded smoothly [80]. A cross-coupling reaction of alkenyl halide with Grignard reagents or alkyl lithium also pro-... 

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