Phosphine ligands displacement

Phosphine ligands displace one or two carbonyl groups from... 

Attention should be paid to the fact that the ratio of Pd and phosphine ligand in active catalysts is crucial for determining the reaction paths. It is believed that dba is displaced completely with phosphines when Pd2(dba)3 is mixed with phosphines in solution. However the displacement is not eom-plcte[16]. Also, it should be considered that dba itself is a monodentate alkene ligand, and it may inhibit the coordination of a sterically hindered olefinic bond in substrates. In such a case, no reaction takes place, and it is recommended to prepare Pd(0) catalysts by the reaction of Pd(OAc)2 with a definite amount of phosphinesflO]. In this way a coordinatively unsaturated Pd(0) catalyst can be generated. Preparation of Pd3(tbaa)3 tbaa == tribenzylidene-acetylacetone) was reported[17], but the complex actually obtained was Pd(dba)2[l8],... 

Starting from (OC)5MnSiR2H (R = Me, Ph, Cl), the p-silylene complex 70 is accessible via the oxidative addition of the Si —H bond to Pt(C2H4.)(PPh3)2 and Pt(PPh3)4, respectively. Structure 70 can be functionalized by displacement of the phosphine ligands alcoholysis and hydrolysis of the compound 70 leads to silicon-free complexes [175]. 

The vast majority of phosphine/phosphite-substituted products involve F-ligand ligation at late transition metals. In contrast, phosphite ligands displaced rhenium-coordinated CO or acetylene in [RePt3(/i-dppm)3(CO)3(L)l", - which are the... 

If cyanide is present, both the thiolate and the phosphine ligand can be displaced [56]. While the mixed thiol environment displacement is an equilibrium process, the right-hand reaction is much more favored in the reactions of phosphine and cyanide. 

A catalytic cycle proposed for the metal-phosphine complexes involves the oxidative addition of borane to a low-valent metal yielding a boryl complex (35), the coordination of alkene to the vacant orbital of the metal or by displacing a phosphine ligand (35 —> 36) leads to the insertion of the double bond into the M-H bond (36 —> 37) and finally the reductive elimination to afford a hydroboration product (Scheme 1-11) [1]. A variety of transition metal-boryl complexes have been synthesized via oxidative addition of the B-H bond to low-valent metals to investigate their role in cat-... 

Tetraphenylarsacymantrene (74) undergoes several CO ligand displacements without disruption of the arsolyl ligand (Scheme 16).24 It is reported that these displacements take place more readily and in higher yield than those for the corresponding pyrolyl derivatives. However, 108 and 71 are found to be much less reactive than 107 toward nonphotochemical CO displacement by phosphines.37... 

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. 

The problem with carbene formation is that they can displace the phosphine ligands attached to the catalyst and deactivate the catalyst. In general, the active catalyst is a palladium(O) compound and this low oxidation state is best stabilized by very bulky phosphines such as P(lBu)3 mentioned above. 

Green et al. observed that the tantalum complex 101 is converted into the bicyclic complex 109 after prolonged exposure to sodium and trimethyl-phosphine.930 Two phosphine ligands of 101 are easily displaced by butadiene, affording the complex 110, while treatment of 109 with dihydrogen leads to the reduction of the metallic center. In both cases, the metallacycle remains intact.930... 

In many cases, the driving force behind studies of Ru or Os complexes containing phosphine ligands incorporating O-donors is potential application in catalysis. A common feature of many of the P,0-bound ligands described in this section is their partial lability, with the conversion of a P,0- to P-coordinated ligand as another Lewis base such as CO or RNC displaces the O-donor. 

Remarkably, the catalytic cycle is not controlled by the presence of phosphine ligands, but it is controlled by the organo group Y at the cobalt the neutral ligand L is displaced by the substrates in the initial step. Oxidative addition of two acetylenes results in a cobaltacycle that reacts with the nitrile to give the pyridine derivative with regeneration of the active [YCo] species. 

Another simple oligomerization is the dimerization of propylene. Because of the formation of a relatively less stable branched alkylaluminum intermediate, displacement reaction is more efficient than in the case of ethylene, resulting in almost exclusive formation of dimers. All possible C6 alkene isomers are formed with 2-methyl-1-pentene as the main product and only minor amounts of hexenes. Dimerization at lower temperature can be achieved with a number of transition-metal complexes, although selectivity to 2-methyl-1-pentene is lower. Nickel complexes, for example, when applied with aluminum alkyls and a Lewis acid (usually EtAlCl2), form catalysts that are active at slightly above room temperature. Selectivity can be affected by catalyst composition addition of phosphine ligands brings about an increase in the yield of 2,3-dimethylbutenes (mainly 2,3-dimethyl-1-butene). 

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