Bisphosphine palladium

The asymmetric hydrosilylation that has been most extensively studied so far is the palladium-catalyzed hydrosilylation of styrene derivatives with trichlorosilane. This is mainly due to the easy manipulation of this reaction, which usually proceeds with perfect regioselectivity in giving benzylic silanes, 1-aryl-1-silylethanes. This regioselectivity is ascribed to the formation of stable 7t-benzylpalladium intermediates (Scheme 3).1,S Sa It is known that bisphosphine-palladium complexes are catalytically much less active than monophosphine-palladium complexes, and, hence, asymmetric synthesis has been attempted by use of chiral monodentate phosphine ligands. In the first report published in 1972, menthyldiphenylphosphine 4a and neomenthyldiphenylphosphine 4b have been used for the palladium-catalyzed reaction of styrene 1 with trichlorosilane. The reactions gave l-(trichlorosilyl)-l-phenylethane 2 with 34% and 22% ee, respectively (entries 1 and 2 in Table l).22 23... 

It is reasonable to assume that the identical complex will be generated whether it be done stoichiometrically from an alkene, to give a chloride or carboxylate dimer followed by the addition of 2 equiv. of a phosphine per Pd, or by the addition of an allyl-X compound to give a phosphine-Pd0 complex. This assumption is supported by the fact that complexes generated in either manner have been found to exhibit identical reaction profiles.380 Furthermore, for the vast majority of allylpalladium reactions studied, it is most likely that the reactive species is a cationic bisphosphine-palladium complex (13).13 Calculations... 

The neutral pathway differs from the cationic pathway in the absence of a vacant coordination site in the square-planar four-coordinate palladium(II) intermediate prior to alkene coordination. The key question is then how does alkene coordination take place. Early studies pointed out that Heck reactions of aryl or vinyl halides promoted by (bisphosphine)palladium complexes could be sluggish, and this sluggishness was attributed to a reluctance of one of the phosphines of the... 

Phosphines that are tightly chelated to the metal center often achieve high selectivity of secondary over tertiary amine products in reactions of primary alkylamines with unhindered aryl halides. The chelation helps prevent competing /3-hydrogen elimination of the aryl palladium amido intermediate (vide infra). Additionally, the greater steric hindrance of bisphosphine palladium complexes, when compared to mono phosphine palladium complexes, prevents diarylation. Some ligands originally introduced by... 

A dendritic spacer of poly(aryl benzyl ether) was used for incorporation of phosphine ligands onto a PS resin support [116,117]. Thus, the condensation of 4-(diphenylphosphino)benzoic acid with Wang resin bearing poly(aryl benzyl ether) (generation 1-3) gave the PS-supported dendritic phosphine 86. Treatment of the dendritic phosphine 86 with Pd(dba)2 in THF afforded the bisphosphine-palladium complex 87 (Scheme 29) [ 118]. The PS-supported palladium complex exhibited a positive dendritic influence on the Heck reaction of bromobenzene with methyl acrylate. 

Scheme 5 Fluorination of 3-keto esters with chiral bisphosphine palladium catalysts, (a) After aminolysis with benzylamine. (b) With 5 mol% catalyst and 50 mol% lutidine...

Our dispersion-corrected computations at ZORA-BLYP-D3/TZ2P (the computational details are described in [62], except that we have now also included dispersion corrections using Grimme s third-generation DFT-D3 method, as described in [86]) revealed that all halogen-substituted bisphosphine palladium complexes... 

The addition of anions to the reaction mixture may affect the rate of oxidative addition, creating a more polar medium or bind directly to the palladium and thereby changing the mechanism [38, 39]. The neutral bisphosphine palladium(O) complex is intrinsically more reactive than the anionic analogous due to the more naked central metal, but the concentration of the anionic species is significantly higher in polar media [2, 37]. Hence, the reaction will predominantly proceed through the tricoordinated anionic species, when polar solvents are used generating an intermediary pentacoordinated anionic palladium(ll) complex. 

The identity of the electrophile also affects the nature of the active catalyst when operating with extremely hindered monodentate phosphines. Accordingly, activation of aryl iodides, do not require an initial ligand dissociation, as they undergo oxidative addition to bisphosphine palladium(0) complexes, while the more reluctant aryl chlorides are only accessed through the more reactive monophosphine compounds [36]. Nevertheless, in both cases the tricoordinated T-shaped paUadium(II) intermediate is formed. 

The hydroboration of enynes yields either of 1,4-addition and 1,2-addition products, the ratio of which dramatically changes with the phosphine ligand as well as the molar ratio of the ligand to the palladium (Scheme 1-8) [46-51]. ( )-l,3-Dienyl-boronate (24) is selectively obtained in the presence of a chelating bisphosphine such as dppf and dppe. On the other hand, a combination of Pdjldba), with Ph2PC6p5 (1-2 equiv. per palladium) yields allenylboronate (23) as the major product. Thus, a double coordination of two C-C unsaturated bonds of enyne to a coordinate unsaturated catalyst affords 1,4-addition product On the other hand, a monocoordination of an acetylenic triple bond to a rhodium(I)/bisphosphine complex leads to 24. Thus, asymmetric hydroboration of l-buten-3-yne giving (R)-allenyl-boronate with 61% ee is carried out by using a chiral monophosphine (S)-(-)-MeO-MOP (MeO-MOP=2-diphenylphosphino-2 -methoxy-l,l -binaphthyl) [52]. 

The first palladium-catalyzed formation of aryl alkyl ethers in an intermolecular fashion occurred between activated aryl halides and alkoxides (Equation (28)), and the first formation of vinyl ethers occurred between activated vinyl halides and tin alkoxides (Equation (29)). Reactions of activated chloro- and bromoarenes with NaO-Z-Bu to form /-butyl aryl ethers occurred in the presence of palladium and DPPF as catalyst,107 while reactions of activated aryl halides with alcohols that could undergo /3-hydrogen elimination occurred in the presence of palladium and BINAP as catalyst.110 Reactions of NaO-/-Bu with unactivated aryl halides gave only modest yields of ether when catalyzed by aromatic bisphosphines.110 Similar chemistry occurred in the presence of nickel catalysts. In fact, nickel catalysts produced higher yields of silyl aryl ethers than palladium catalysts.108 The formation of diaryl ethers from activated aryl halides in the presence of palladium catalysts bearing DPPF or a CF3-subsituted DPPF was also reported 109... 

Palladium catalyzed P-C ctoss coupling [58] between primary or secondary phosphines and appropriate aryl iodides made possible the preparation of several aminophenyl-phosphines with the general formula 70 and also the bisphosphine 71. 

By using three equivalents of diphenylphosphine oxide (relative to al-kyne), the palladium-catalyzed reaction affords bisphosphine oxides in high... 

The generality of the carbon monoxide insertion reaction is clear from reports that methylcyclopentadienyliron dicarbonyl (16), ethylcyclopentadienylmolylbde-num tricarbonyl (66), alkylrhenium pentacarbonyls (50), alkylrhodium dihalo carbonyl bisphosphines (34), allylnickel dicarbonyl halides (35), and mono-and di-alkyl derivatives of the nickel, palladium, and platinum bisphosphine halides (P), also undergo the reaction. The reaction of Grignard reagents (24), and of boron alkyls (51) with carbon monoxide probably takes place by the same mechanism. 

A key feature of the cationic mechanism is that removal (or dissociation) of an anionic ligand from the palladium coordination sphere allows alkene complexation to occur while maintaining coordination of both phosphines of a bisphosphine ligand. That both phosphines can be accommodated in a square-planar four-coordinate intermediate during the insertion step has provided a simple rationalization for the higher enantioselectivities often observed for the cationic pathway. Concrete information on the enantioselective step of asymmetric Heck reactions proceeding by the cationic pathway has not been reported to date. It is likely to be either coordination of the alkene to generate 20.S or the insertion step (20.5 —> 20.6, Scheme 8G.20). 

Palladium complexes with monocoordinated phosphine ligands were synthesized by mixing the corresponding phosphine dendrimer with [Pd(cod)MeCl], which afforded exclusively trans dendrimer(MePdCl) complexes (P/Pd = 2). Complexes in which the ligand is coordinated in a bidentate fashion were synthesized by reaction of the respective bisphosphine den-... 

Scheme 10. Overall mechanism for aryl halide amination catalyzed by bisphosphine-ligated palladium complexes.

The ligands to be eliminated must be cis to one another for reductive elimination to occur. This is because the process is concerted. Two examples from palladium chemistry make this point clear. Warming in DMSO causes ethane production from the first palladium complex because the two methyl groups are cis in the square planar complex. The more elaborate second bisphosphine forces the two methyl groups to be trans and reductive elimination does not occur under the same conditions. Reductive elimination is one of the most important methods for the removal of a transition... 

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