Palladium complexes phosphane

Terminal palladium-complexed, phosphane-functionalised carbosilane dendrimers have been used as potential catalysts in membrane reactors [87]. 

Catalytic enantioselective synthesis of 4,4-dimethyl-l-phenyl-l,2-pentadiene from 4,4-dimethyl-1,2-pentadiene and iodobenzene using 0.4 to 1 mol % of palladium complexes containing chiral phosphane ligands as the catalyst for the enantioselective cross coupling134 is the only example of substoichiometric transition metal catalyzed enantioselective allene synthesis. 

Yam, V.W.-W. and Lu, X.-X.C.-C. (2003) First observation of alkali metal ion induced trans-cis isomerization of palladium(ll) phosphane complexes containing crown ether moieties. Angeiv. Chem., 115, 3507-3510. 

Biich F1M, Binger P, Benn R, Kruger C, Rufinska A (1983) Phosphane-induced, stepwise rearrangement of an octadienediyl palladium complex to seven- and nine-membered metallacycles. Angew Chem Int Ed 22 774-775... 

The arenetellurolate-phosphane-palladium complexes are orange or brown solids that were obtained in yields of approximately 50%. ... 

The PflN ligands 10a,b as well as the corresponding palladium complexes were synthesized as depicted in Fig. 2.14. Treatment of 2-(chloromethyl)pyridine with one equivalent of bis(2-methylphenyl)phosphane in the presence of potassium tert-butylate yielded ligand 10 a which was directly converted into the palladium diiodide complex 11b. Purification of 11b was achieved by column chromatography on sihca. 

A third means of isomerization can be the anti attack of a free palladium(O) phosphane species onto the Ti-allyl-metal complex. Additionally, the 7r-allylpalladium complex can suffer epimer-ization through n-a-n rearrangement, especially, if one allyl terminus is unsubstituted. The degree of stereoselectivity is also a function of the reactivity of the nucleophile as well as of the substrate, the medium, the ligands employed and the catalyst used. Furthermore, the stereointegrity in the alkylation of some acylic substrates depends on the ionization from a single conformation. 

More recently, Bouwman carried out a detailed study on the carbonylation of nitrobenzene in methanol with palladium bidentate phosphane complexes as catalysts [29-31]. After a careful analysis of the reaction, mixtures revealed that besides the frequently reported reduction products of nitrobenzene [methyl phenyl carbamate (MFC), A, 7/-diphenylurea (DPU), aniline, azobenzene (Azo) and azoxyben-zene (Azoxy)], large quantities of oxidation products of methanol were co-produced (dimethyl carbonate (DMC), dimethyl oxalate (DMO), methyl formate (MF), H2O, and CO). They proposed the Pd-imido species P2Pd = NPh, which is the central key intermediate that can link together all the reduction products of nitrobenzene and all the oxidation products of methanol into one unified mechanistic scheme. 

Nishikata, T. Yamamoto, Y. Miyama, N. Conjugate Addition of Aryl Boronic Acids to Enones Catalyzed by Cationic Palladium(ll)-Phosphane Complexes. Angew. Chem., hit. Ed. 2003,42,2768-2770. 

An interesting regiochemistry has been observed in the palladium-catalyzed allylic alkylation of ( )-3-substituted-2-propene acetate and 1-substituted 2-propenyl acetate. In this allylic substitution catalyzed by a palladium complex prepared from 1 in the presence of (/ )-MeO-MOP catalyst (P/Pd = 2/1), a selective substitution at the position originally substituted with acetate was observed (eq 62). Studies with 3-deuterio-2-cyclohexenyl acetate revealed that neutral phosphane Pd-complex is formed during the process. 

With 1,1-difluorocyclopropabenzene and a range of nickel(O) complexes, nickelabicy-clobutanes 118 (84—93%) are formed by loss of olefin or phosphane ligands and addition of the nickel atom across the bridge bond (Scheme 20)256-266. The products appear to be stable at ambient temperatures but are oxygen sensitive the majority revert to cycloproparene in solution even below -20 °C. With (j -allyl)( s-cyclopentadienyl(palladium in the... 

The preparation of a mixed NHC/phosphane palladium(II) complex can be achieved by simply reacting [PdCPPhj) ] with a 2-chloro-imidazohum salt [259]. Oxidative addition of the C-Cl bond yields the palladium(II) coordinated carbene and eliminates 2 mol of phosphane (see Figure 3.83). The cationic and neutral paUadium(n) carbene complexes are in equilibrium with each other documenting the comparative labUity of the Pd-PPhj bond. 

Figure 3.84 A conventional route to mixed NHC/phosphane palladium(ll) complexes.

Note Herrmann et al. observed an equilibrium between the mixed NHC/phosphane palladiumill) complex and the two homoleptic complexes [Pd(PR )JJ and [Pd(NHC)JJ. The ratio is given as 7 3. This would mean that three different palladium(II) complexes are potentially present in a catalytic reaction. 

We conclude that the addition of phosphanes to palladium(II)-NHC complexes is advantageous with respect to reaction times, yield and completion ratio for a wide range of catalytic coupling reactions including Tsuji-Trost, allylic amination, Suzuki-Miyaura, Heck and Stille. 

Note Reaction of the dimethyl palladium(U) carbene complex with excess methyl iodide leads to decomposition of the compound by reductive elimination of an imidazolium salt that remains pendant on the phosphane anchor [283]. 

Naturally, it is possible to synthesise a similar ligand system without central chirality and in fact without the unnecessary methylene linker unit. A suitable synthesis starts with planar chiral ferrocenyl aldehyde acetal (see Figure 5.30). Hydrolysis and oxidation of the acetal yields the corresponding carboxylic acid that is transformed into the azide and subsequently turned into the respective primary amine functionalised planar chiral ferrocene. A rather complex reaction sequence involving 5-triazine, bromoacetal-dehyde diethylacetal and boron trifluoride etherate eventually yields the desired doubly ferrocenyl substituted imidazolium salt that can be deprotonated with the usual potassium tert-butylate to the free carbene. The ligand was used to form a variety of palladium(II) carbene complexes with pyridine or a phosphane as coligand. 

Metal complex chemistry, homogeneous catalysis and phosphane chemistry have always been strongly connected, since phosphanes constitute one of the most important families of ligands. The catalytic addition of P(III)-H or P(IV)-H to unsaturated compounds (alkene, alkyne) offers an access to new phosphines with a good control of the regio- and stereoselectivity [98]. Hydrophosphination of terminal nonfunctional alkynes has already been reported with lanthanides [99, 100], or palladium and nickel catalysts [101]. Ruthenium catalysts have made possible the hydrophosphination of functional alkynes, thereby opening the way to the direct synthesis of bidentate ligands (Scheme 8.35) [102]. 

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