Oxidative addition monodentate ligands

The P-H oxidative addition, acrylonitrile insertion, and C-H reductive elimination steps were observed directly with the dcpe catalyst, and the potential intermediates Pt(diphos)(PHMes )(CH2CH2CN) (7, diphos = dppe, dcpe) were shown not to undergo P-C reductive elimination. The generality of this proposed mechanism for less bulky phosphine substrates, or for Pt catalysts supported by monodentate ligands, remains to be investigated [9]. 

The Co complexes of the o-phenylenediamine-linked dioxime Hdmg2Ph bearing a variety of monodentate ligands (halides and pseudo halides) in the axial coordination sites have been synthesized.1176 B12 model complexes [RCo(dmg2Ph)(L)]+ (R = Me, Et, Me2CH, Bz, ch L = py, H-im, or PPh3) were prepared by the oxidative addition reactions of the alkyl halide to the in situ-generated monovalent dibromo complex. 

The MOP series of ligands59 (see Section 9.5.4.2) in conjunction with standard palladium precursors has been reported to catalyze the addition of HBcat to 1,3-enynes. With 1 mol.% catalyst produced by combination of Pd2(dba)3 and the monodentate ligand (Y)-MeO-MOP (22), axially chiral allenyl-boranes are formed (Equation (3)). Subsequent oxidation affords the corresponding alcohols with moderate ee values.60... 

Mechanistically, the latter sequence rationalizes as follows (Scheme 47). The Pd-catalyzed arylamination with the bulky monodentate phosphane stops at the stage of the N-aryl allyl benzene 128. Upon addition of the chelating ligand dpephos, a subsequent cycle is opened that commences with the oxidative addition of the aryl bromide to the Pd-dpephos chelate. After the complexation of 128 to the Pd complex, the insertion of the alkene takes place to furnish an alkyl aryl Pd species that, due to the chelating dpephos, undergoes reductive elimination rather than /1-hydride elimination to furnish the product 127. 

There is an extensive chemistry of tertiary phosphine rhodium(III) complexes. However, there are comparatively few complexes of monodentate tertiary arsines, although the complexes of ditertiary arsines are more numerous. There are virtually no tertiary stibine complexes. The two main preparative routes to the complexes described in this section are (i) direct reaction pf the ligands with rhodium trichloride, which usually yields trichloro complexes and (ii) oxidative addition to rhodium(I) tertiary phosphine complexes, which gives rise to more diverse products of the type [RhXYZ(PRj) ], Metathetical reactions on the complexes prepared by either method (i) or (ii) have been used to prepare most of the remaining compounds. 

The most numerous complexes of this general type are those containing two bidentate neutral ligands, which are discussed in Section 48.6.3.7 below. These can easily be obtained by the oxidative addition of two anionic ligands to the numerous [Rh(LL)2]X complexes. There are, however, a considerable number of complexes which contain monodentate neutral ligands. These include the catalytically important [RhH2(PR3)2(solv)2]X complexes,1017-1019 and a number of dioxygen complexes.1030... 

There are many complexes containing one bidentate and two monodentate neutral ligands. A preliminary crystal study has been made on one of them.1034 It may finally be noted that the oxidative addition of sundry reagents to rhodium(I) complexes is not an infallible preparative method for complexes of this type. Nitronium tetrafluoroborate, for example, gives only a five-coordinate cation (equation 230).1035... 

Others have investigated the kinetics of amination reactions mediated by catalyst systems employing the new electron-rich monodentate ligands. In particular, Hartwig has shown that for catalysis by a 1 1 palladium to Xn tert-butyl)phosphine system, a mechanism in which oxidative addition of aryl chlorides follows coordination of base to the palladium competes with the standard nonanionic pathway. Finally, Caddick, Cloke, and coworkers have studied amination reactions of aryl chlorides performed by palladium complexes of N-heterocyclic carbene ligands. They found the rate to be limited by the oxidative addition step, which occurs first through the dissociation of an NHC ligand. 

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