Alkenes ligand displacement

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],... 

Of the many different preparative methods known, the standard preparation of [RhCl(PPh3)3] is not of wide applicability to other phosphines (equation 16). The best general method for RhX(QR3) (Q = P, As, Sb) is by displacement of alkene ligands from dimeric rhodium(I) complexes (equation 17). This method also avoids using rare and expensive tertiary phosphines as mere reductants. 

The first examples of a thermal [2jt + 6n] cycloaddition, made possible by coordination to iron, have appeared. When [(cycloheptatriene)Fe(CO)3] is irradiated in THF at — 78°C, the light removed, and diphenylacetylene added, the hydrocarbon (387) can be isolated, after warming to room temperature, in 25% yield. There is strong evidence that the light allows displacement by solvent of one of the alkene ligands and perhaps also one of the COs. The acetylene then replaces one THF molecule and an intramolecular [2 + 6] cycloaddition gives an endo-complex which loses the iron. ... 

The bulk of derivatives are obtained by the displacement of CO by other ligands. These include phosphines and other group 15 donors, NO, mer-captans and unsaturated organic molecules such as alkenes, alkynes and cyclopentadienyls. 

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-... 

As before, K20s02(0H)4 is in equilibrium with OSO3. TsNCl adds to Os, which uses a lone pair to displace Cl from N and give the key Os(VHE) intermediate. Coordination of the Sharpless ligand creates a complex that adds rapidly to the alkene. Hydrolysis of the Os(VI) product regenerates 0s02(0H)2 and provides the product. 

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