Phosphorus ligands, alkyl substituted

However, considerable amounts of 2,3-dihydrofuran 50 and tetrahydro-furan-2-carbaldehyde 53 were present because of an isomerization process. The isomerization takes place simultaneously with the hydroformylation reaction. When the 2,5-dihydrofuran 46 reacts with the rhodium hydride complex, the 3-alkyl intermediate 48 is formed. This can evolve to the 2,3-dihydrofuran 50 via /3-hydride elimination reaction. This new substrate can also give both 2- and 3-alkyl intermediates 52 and 48, respectively. Although the formation of the 3-alkyl intermediate 48 is thermodynamically favored, the acylation occurs faster in the 2-alkyl intermediates 52. Regio-selectivity is therefore dominated by the rate of formation of the acyl complexes. The modification of the phosphorus ligand and the conditions of the reaction make it possible to control the regioselectivity and prepare the 2- or 3-substituted aldehyde as the major product [78]. As far as we know, only two... 

Catalysts lacking phosphorus ligands have also been used as catalysts for allylic substitutions. [lr(COD)Cl]2 itself, which contains a 7i-accepting diolefin ligand, catalyzes the alkylation of allylic acetates, but the formation of branched products was only favored when the substitution reaction was performed with branched allylic esters. Takemoto and coworkers later reported the etherification of branched allylic acetates and carbonates with oximes catalyzed by [lr(COD)Cl]2 without added ligand [47]. Finally, as discussed in Sect. 6, Carreira reported kinetic resolutions of branched allylic carbonates from reactions of phenol catalyzed by the combination of [lr(COE)2Cl]2 and a chiral diene ligand [48]. 

Iron complexes containing bidentate alkyl and aryl phosphorus ligands cleave a variety of C-H bonds under mild conditions, Hydrido acetylide complexes were prepared by oxidative addition of primary acetylenes in the Fe(DPPE)2 and the Fe(DMPE)2 systems [DPPE = bis(diphenylphos-phino)ethane, DMPE = bis(dimethylphosphino)ethane]. The Fe(DMPE)2 system also cleaves C-H bonds of activated methyl groups, aromatic compounds, and certain other sp hybridized molecules. The C-H cleavage reactions are reversible, resulting in equilibrium mixtures of isomeric products in many cases. Studies of substituted benzenes show that while product stability is favored by electron withdrawing substituents, steric effects play a predominant role in the determination of product distribution. 

Nemoto and Hamada [50] has described the development of a new class of chiral phosphorus ligand - aspartic acid-derived P-chirogenic diaminophosphine oxides, DIAPHOXs - and their application to several Pd-catalyzed asymmetric allylic substitution reactions. Pd-catalyzed asymmetric allylic alkylation was initially examined in detail using diaminophosphine oxides 77, resulting in the highly enantioselective construction of quaternary stereocenters. With the use of the Pd-DIAPHOX catalyst system, asymmetric allylic alkylation, asymmetric allylic amination, and enantioselective construction of quaternary carbons were achieved with high ee (up to 97-99% in many cases) (Scheme 24). 

In the framework of this concept a series of new mixed P/N ligands was prepared, employing the phosphorus-analogous Mannich reaction (Scheme 9). This transformation permits the substitution of primary or secondary amines by methylenephos-phine residues -CH2PRR (R, R = alkyl, aryl) through reaction with a secondary phosphine and formaldehyde (46). Based on the... 

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