Diphosphines reductive elimination

Oxidative addition of a silyl-protected 4-(bromomethyl)phenol precursor to (tme-da)Pd(II)Me2 (tmeda = tetramethylethylenediamine), followed by ethane reductive elimination, resulted in formation of the benzylic complex 16 (Scheme 3.10). Exchange of tmeda for a diphosphine ligand (which is better suited for stabilizing the ultimate Pd(0) QM complex), followed by removal of the protecting silyl group with fluoride anion, resulted in the expected p-QM Pd(0) complex, 17, via intermediacy of the zwitterionic Pd(II) benzyl complex. In this way a stable complex of p-BHT-QM, 17b, the very important metabolite of the widely used food antioxidant BHT20 (BHT = butylated hydroxytoluene) was prepared. Similarly, a Pd(0) complex of the elusive, simplest /)-QM, 17a, was obtained (Scheme 3.10). 

From all the above observations, it was concluded that, for diphosphine chelate complexes, the hydrogenation stage occurs after alkene association thus, the unsaturated pathway depicted in Scheme 1.21 was proposed [31 a, c, 74]. The monohydrido-alkyl complex is formed by addition of dihydrogen to the en-amide complex, followed by transfer of a single hydride. Reductive elimination of the product regenerates the active catalysts and restarts the cycle. The monohydrido-alkyl intermediate was also observed and characterized spectroscopically [31c, 75], but the catalyst-substrate-dihydrido complex was not detected. 

Similarly, a double functionalization can be reached when an activating group is present in close vicinity to the triple bond. Tsuji et al. have discovered that with a diphosphine palladium(O) complex, a carbonate function in the a-position of the alkyne provides by decarboxylation a palladium methoxy species on which the alkyne moiety can be isomerized into an al-lenyl a -bonded group. CO insertion in the Pd - C bond, reductive elimination with the methoxy group and further cyclization with incorporation of a second CO molecule give rise to the corresponding cyclopentenone as shown in Scheme 21 [127]. 

In closely related experiments it was shown that sp C—H activation takes place reversibly within the coordinahon sphere of the electron-rich Ir(I)-diphosphine complex 58 (Scheme 6.9) to form an alkyl-amino-hydrido derivative 57 reminiscent of the CCM intermediate 24 the solid-state structure of 57 is shown in Figure 6.13 [40]. It appears that C—H activation only takes place after coordination of the amine function to the Ir(I) center (complex 58, NMR characterized). Amine coordination allows to break the chloro bridge of 59 and to augment the electron density of the metal center, thus favoring oxidative addihon of the C—H bond. Most importantly, the microscopic reverse of this C—H activation process (i.e. C—H reductive elimination) models the final step of the CCM cycle (see Scheme 6.1) indeed, the reaction of Scheme 6.10 is cleanly reversible at 373 K. 

The reaction of Ni(P—P)2 and PhCN (equation 89) is reversible, the reductive elimination of PhCN being easily induced by refluxing [Ni(CN)(Ph)(P—P)] in the presence of the diphosphine.275... 

The mechanism of the enantioselective 1,4-addition of Grignard reagents to a,j3-unsaturated carbonyl compounds (Scheme 5 R1 = alkyl R2 = alkyl, OR3), promoted by copper complexes of chiral ferrocenyl diphosphines (180), has been explored using kinetic, spectroscopic, and electrochemical analysis. The roles of the solvent, copper halide, and the Grignard reagent have been thoroughly examined. Kinetic studies support a reductive elimination as the rate-limiting step, in which the chiral catalyst,... 

The three-coordinate intermediate in Equation 19 would then allow for greater mobility of the groups from which reductive elimination could proceed.(19) Rearrangement followed by a very rapid reductive elimination the cis isomer is also a possibility. However, our unsuccessful attempts to synthesize cis-aryl-methyl-nickel(ll) complexes still leave open the question of reductive elimination from such stereoisomers. The results we obtained in studies with the bidentate diphosphine ligand (dppe), though qualitatively in this direction, unfortunately lack definitiveness as yet. 

In a related reaction, photolysis of the 18-electron complexes fy -CpMo(dmpe)2H3 and (t -j-PrCp)MoL2H3 (L2 = two P-donor ligands or one chelating diphosphine) induces reductive elimination of H2 to produce the monohydrido complexes . These 16-electron complexes can oxidatively add aryl C-H bonds and catalyze H-D exchange in the aryl ring. 

Our group recently showed that addition of I2 to chelated Pt(II) diaryl complexes resulted in the formation of free iodoarene and Pt(II) aryl iodo complexes, with the exception of the small dmpe ligand system, where stable oxidative addition Pt(IV) complex fra y-(dmpe)Pt(Ar)2l2 was isolated (Scheme 26) [70]. Heating this complex in polar solvents gave the mixture of products the thermodynamically stable cis-isomer and Ar-I reductive elimination products (Scheme 27a) [71]. The isomerization reaction was light-assisted with the light triggering the diphosphine chelate... 

Scheme 27 Competitive aryl-iodide reductive elimination/isomerization reactions involving Pt(IV) diaryl diodo complexes (a) The competition is light-dependent with dmpe as the ligand, (b) No isomerization takes place in the rigid dmphz system suggesting that the diphosphine chelating opening is important for the isomerization reaction...

For instance, a cationic aqua complex/<2c-[PtMe3(OH2)3]" is indefinitely stable in aqueous solutions at elevated temperatures, whereas diphosphine - supported trimethyl platinum(lV) complexes such as/<2c-(dppbz)Pt Me3(OR) and/ac-(dppe) Pt MesCOR) shown in Fig. 5 undergo concurrent C-C and C-O reductive elimination at 120°C [21, 23, 24]. 

The importance of 7t-electrophilic nature of aryl ligands was noted also in C-N, C-0 and C-S reductive ehmination from cw-Pd(C6H4-p-Y)(X)(diphosphine) complexes (X = NR2, OR, SR) [23,24], A common kinetic feature observed for these systems is the enhancement in the reaction rate hy a better jt-accepting Y and a better 0-donor ligand X. A kinetic study reported by Hartwig et al. for C-S reductive elimination is particularly detailed, leading to deep insights into the reaction mechanism (Scheme 9.6) [23a],... 

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