Palladium chemistry phosphines

There is certain similarity in the order of reactivities between SnAt displacement reactions and oxidative additions in palladium chemistry. Therefore, the ease with which the oxidative addition occurs for these heteroaryl chlorides has a comparable trend. Even a- and y-chloroheterocycles are sufficiently activated for Pd-catalyzed reactions, whereas chlorobenzene requires sterically hindered, electron-rich phosphine ligands. 

In reactions requiring palladium 0), formation of the active complex may be achieved more conveniently by reduction of a palladium(ll) complex, for example, Pd 0Ac)2- Any phdsphine may then be used in the reaction, without the need to synthesize and isolate the corresponding palladium 0)-phosphine complex. Only 2-3 equivalents of phosphine may be needed, making the palladium(O) complex coordinatively unsaturated and therefore very reactive. The reduction of palladium li)to palladium(o) can be achieved with amines, phosphines, alkenes, and organometailics such as DIBAL-H, butyl lithium, or trialkyl aluminium. The mechanisms are worth giving as they illustrate the basic steps of organometallic chemistry. 

A very recent example of carbene/palladium chemistry is reported by Stahl in which (NHC)2Pd add molecular oxygen to form a peroxo complex. Oxidative chemistry involving this type of complex as catalyst is expected to follow. This represents a fundamental departure of NHC ligands from phosphines due to their stability to oxidation Konnick MM, Guzei lA, Stahl SS (2004) J Am Chem Soc 126 10212... 

Nickel and palladium complexes also catalyze the formation of the carbon-phosphorus bonds in phosphorus(V) and phosphorus(III) compounds. Indeed, this chemistry has become a common way to prepare phosphine ligands by the catalytic formation of phosphine oxides and subsequent reduction, by the formation of phosphine boranes and subsequent decomplexation, or by the formation of phosphines directly. The catalytic formation of both aryl and vinyl carbon phosphorus bonds has been accomplished. 

Catalytic asymmetric hydrosilylation of prochiral olefins has become an interesting area in synthetic organic chemistry since the first successful conversion of alkyl-substituted terminal olefins to optically active secondary alcohols (>94% ee) by palladium-catalyzed asymmetric hydrosilylation in the presence of chiral monodentate phosphine ligand (MOP, 20). The introduced silyl group can be converted to alcohol via oxidative cleavage of the carbon-silicon bond (Scheme 8-8).27... 

The phosphine sulfide CYANEX 471X has been proposed as an alternative to the foregoing reagents for palladium platinum separation. Here, the faster extraction of palladium affords separation factors of 10 1, with a 10 min contact time. The chemistry of the process is similar to that of the alkyl sulfides. Various other separations have been published for example, TBP will extract platinum(IV) chloroanions preferentially to pal-ladium(II). 

Studies regarding the nature of the catalytically active species for NHC complexes in Heck-type reactions have focused on the Mizorvki—Heck reaction and have consistently revealed a palladium(O) species as the active catalyst. The induction period is shortened upon addition of a reducing agent,and postulated intermediates of the reaction were isolated and characterized as well as employed in stoichiometric and catalytic reactions. Theoretical studies using DPT calculations showed the mechanism for NHC complexes to most likely he in agreement with phosphine chemistry. ... 

More recent review Hayashi, T. Asymmetric Grignard Cross-Coupling Catalyzed by Chiral Phosphine-Nickel and Phosphine-Palladium Complexes, in Asymmetric Reactions and Processes in Chemistry (Eliel, E. L., Otsuka, S. ed.), American Chemical Soc. Symp. Series Vol. 185, p. 177 (1982)... 

Palladium(I) complexes are in general dimeric or oligomeric and consequently, although they have a d9 configuration, they are usually diamagnetic. The chemistry of this oxidation state is discussed in Section 51.3. Unlike most transition metals, the chemistry of low valent palladium is not dominated by carbonyls [Pd(CO)4] is only stable at 80 K in a matrix. As with platinum, the most common complexes are those containing phosphines, where complexes of the type [PdL ] (n = 2, 4) have been isolated. The chemistry of palladium(O) is dealt with in Section 51.2 and elsewhere.2... 

The chemistry of phosphite, phosphonite and phosphinite complexes of palladium(II) generally resembles that of their phosphine analogues. Its description here will therefore be brief, attention being paid to those areas where differences in behaviour are apparent. 

Significant progress has been made in enantioselective C—C bond formation in ir-allylpalladium chemistry, where the source of the chirality is the phosphine ligand on the palladium. A number of potential difficulties must be addressed in order for this approach to result in synthetically useful optical yields. Racemization can occur by a variety of mechanisms. Intermediates not bearing substituents at both allyl termini can undergo syn-anti isomerization, which accomplishes racemization (equation 347). 

Pd(0)/phosphine complexes, or their precursors, in the presence of a suitable co-base, have also been shown to promote, in good yields (66-100%), the formation of allylic carbamates from various primary and secondary aliphatic amines, pressurized C02 and allylic chlorides, in THF, at ambient temperature [87a]. The choice of the added co-base (Base), used for generating the carbamate salt RR NC02 (BaseH)+, was found to be critical for high yields of O-allylic urethanes. The use of a guanidine (CyTMG) or amidine (DBU) base was optimal for this system (see also Section 6.3.1). ft is assumed that this chemistry passes catalytically through a mechanism similar to that illustrated in Scheme 6.19. This involves nucleophilic attack by carbamate anion on a (tt-allyl) palladium species, formed by the oxidative addition of the allylic chloride to a palladium(O) intermediate. 

The coordination chemistry of optically pure, chiral phosphetanes has been studied with special attention to the preparation and characterization of complexes since they are suitable for asymmetric catalytic reactions. The optically active P-menthylphosphetanes showed similar reactivities with usual trivalent phosphines to afford stable palladium(n) and ruthenium complexes, under usual reaction conditions. Similarly, the Pd-allyl complex 28 <1997JOM(529)465> has been prepared from [(allyl)PdCl]2 and was characterized by X-ray crystallography. Reaction of the P(R),C(3 )-2-benzyl-3,3,4,4-tetramethyl-l-menthylphosphetane 64 with Ru3(C0)12/HC02H proceeds normally to give the formato bridged dimer 65 (Figure 11) <1998S1539>. 

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