Palladium catalysts hydrovinylation

It was shown earlier that palladium-catalyzed hydrovinylation of styrene using phosphino ester-type ligands leads to isomerization of the external al-kene (kinetic product) to the internal alkene (thermodynamic product) at higher substrate conversion. In this regard, the idea was to suppress this isomerization by running the reaction at lower conversion in a CFMR system in order to minimize the catalyst-substrate contact time. 

For some of them, the use of membrane reactors for their recovery or application in continuously operated reactors has been demonstrated. Examples include the use of dendrimer-bound nickel catalysts for the Kharasch addition [54, 59] and dendritic palladium catalysts for an allylic substitution [73, 60]. The membrane reactor concept has also been transferred to reactions at higher pressure, as shown for the hydrovinylation of styrene (cf. Section 3.3.3) [75]. Modem ultra-and nanofiltration membranes allow an effective recovery of the homogeneously soluble catalyst. However, in some cases the long-term stability of the catalyst under operating conditions has to be improved. 

Finally, the preparation and subsequent use of a-phosphino substituted ACTCs has been reported by Salzer and co-workers [41]. The three complexes 56-58 were investigated as ligands for a palladium-catalysed hydrovinylation employing styrene and ethylene as substrates. The active catalyst was formed in situ from an achiral palladium source and the respective ligand. Scheme 11. 

The retention of the free [Go]-ligand system (without palladium) was 85%, the molecular weight of the actual catalyst is much larger (Mw = 2868 Da versus 1314 Da for the Go-system), which should be sufficient for initial experiments. The result of this continuous hydrovinylation is shown in Figure 4.21. 

It is frequently assumed that the mechanism of the hydrovinylation reaction is identical for catalysts containing the same metal, irrespective of the nature of the metal precursor. However, it is questionable whether this assumption can be extended to different metals and it should not, for example, be assumed that the nickel-catalyzed reactions have mechanisms identical to those of the palladium-catalyzed reactions. 

Palladium(II) catalysts can also be applied to stereo- and regioselective syn hydroarylation and hydrovinylation of symmetrically and unsymmetrically substituted alkynes8 9. Thus, alkyl 4-hydroxy-2-alkynoates in the presence of Pd(OAc)2(PPh3)2, formic acid, and tributylamine in dimethyl formamide or acetonitrile are converted with aryl iodides in a one-pot syn hydroary-lation/cyclization reaction to give functionalized substituted butenolides. The reactions occur with high syn stereoselectivities and with good regioselectivities8. 

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