Catalyst precursors, allylic alkylations

Fig. 2.21 Chiral binap-based catalysts or catalyst precursors for the enantioselective allylic alkylation of aUylphosphates...

Fig. 2.22 Combination of chiral imidazolidin-2-ylidenes and biphenyl linkers in the chiral catalysts or catalyst precursors for the asymmetric allylic alkylations...

In the present chapter, we focus on the catalyst nature in solution using well-defined metal NPs as catal 4 ic precursors it means, soluble (or dispersible) heterogeneous pre-catalysts, as stated by Finke [6]. Some experiments described in the literature concerning the distinction between homogeneous and heterogeneous catalysts are discussed (see Section 3), followed by a particular case studied by us with regard to the catalyst nature in the allylic alkylation reaction, using preformed palladium NPs as catalytic precursors (see Section 4). 

Our study on the synthesis, structure and catalytic properties of rhodium and iridium dimeric and monomeric siloxide complexes has indicated that these complexes can be very useful as catalysts and precursors of catalysts of various reactions involving olefins, in particular hydrosilylation [9], silylative couphng [10], silyl carbonylation [11] and hydroformylation [12]. Especially, rhodium siloxide complexes appeared to be much more effective than the respective chloro complexes in the hydrosilylation of various olefins such as 1-hexene [9a], (poly)vinylsiloxanes [9b] and allyl alkyl ethers [9c]. 

Similarly tetravalent Ti and Zr dihydride catalysts are formed from alkyl or aryl precursors. A wide range of Group 8-10 metal hydride catalysts has been isolated or formed in situ from precursor allyl complexes. The systems are generally quite active because the catalysts are necessarily ligand deficient with sites available for substrate coordination. For an 17 -allyl precursor, equation (s) initial dihydride addition to an M(r) -allyl) intermediate appears very plausible ", cf. equations (i)-(l). 

Palladium Catalysts Palladium catalysts are effective and powerful for C—H bond functionalization. Carbene precursors and directing groups are commonly used strategies. Generally, sp3 C—H bond activation is more difficult than sp2 C—H bond activation due to instability of potential alkylpalladium intermediates. By choosing specific substrates, such as these with allylic C—H bonds, palladium catalytic systems have been successful. Both intramolecular and intermolecular allylic alkylation have been developed (Scheme 11.3) [18]. This methodology has presented another alternative way to achieve the traditional Tsuji-Trost reactions. 

The allylpalladium chloride dimer (1) has been widely used as catalyst precursor for various Pd-catalyzed reactions such as allylic alkylations and cross-coupling reactions. It has also found applications in hydrovinylation, hydrosilylation, hyroamination, and reduction reactions. 

The allylation of cyclic acetates such as cyclopentenyl or cyclo-hexenyl and cycloheptenyl derivatives also has been widely studied. For exart ile, the allyl palladium chloride dimer (1) is a useful catalyst precursor in the kinetic resolution of 2-cyclohexenyl acetate using Trost s chiral ligands pockets in conjunction to the planar chirality of ferrocene (eq 46). 1 The conversion was stopped at 54%, which allowed high enantiomeric excess for both the alkylated derivative and the chiral allylic acetate. 

A significant contribution to the use of iridium precursors for allylic alkylations has been provided by Takeuchi and co-workers, who demonstrated how the selectivity achieved by using iridium catalysts complements that obtained with palladium complexes. Fast combinatorial colorimetric screening has been used to individuate Ir(l) catalysts active for the allylic substitution reaction. Fundamental advancements in this field were achieved by Helmchen and co-workers who obtained high regio- and enantioselectivity in asymmetric allylic alkylations of achiral or racemic substrates with chiral phosphinooxazolines and phosphoramidites as... 

Scheme 28 Allylic alkylation of allylic phosphonates with AlMc3 using Zn-L5 catalyst precursor...

Addition to linear 1,1-disubstituted allylic acetates is slower than addition to monosubstituted allylic esters. Additions to allylic trifluoroacetates or phosphates are faster than additions to allylic carbonates or acetates, and reactions of branched allylic esters are faster than additions to linear allylic esters. Aryl-, vinyl, alkynyl, and alkyl-substituted allylic esters readily undergo allylic substitution. Amines and stabilized enolates both react with these electrophiles in the presence of the catalyst generated from an iridium precursor and triphenylphosphite. 

S-Allyl dithiocarbonates on treatment with Pd° catalyst expel COS and serve as precursors to RS-nucleophiles. Alkyl-substituted precursors show preferential attack at the more substituted allyl terminus, while aryl-substituted precursors give exclusive attack at the less substituted allyl terminus (equation 286).222... 

When the alkylation was performed with ethyl allyl carbonate as the precursor of the it-allyl intermediate, only 32% ee was obtained, indicative of a subtle proton-transfer process involved in the catalytic process such as in Scheme 8E.39. The chiral rhodium catalyst was shown to be the primary source of the asymmetric induction because the same reaction in the absence of the rhodium catalyst generated a racemic product in 91% yield. It is interesting that the use of only half an equivalent of the chiral ligand together with half an equivalent of achiral ligand (dppb) with respect to [Pd + Rh] was sufficient to give a high enantioselectivity (93% ee). 

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