Phosphines, reduction reaction

Several combinations of promoters showed particular advantageous results. For example, addition of small amounts of group VI-B elements to tin, lithium or phosphine promoted reactions sharply increase the catalyst activity (Table III). Only the metals or the carbonyls are active for this function. It is presumed that part of the function of these elements is to assist in the reduction part of the catalyst cycle especially in the initial phase of the reaction. 

Several synthetic procedures have been developed for the preparation of complexes with phosphines, arsines and stibines, and the most convenient or historically important ones are summarized in Table 3. They can be grouped in three main types of reaction the direct reactions of nickel with ligands, ligand replacement reactions and reduction reactions of nickel(H) complexes. 

Reduction reactions of nickel(If) compounds. The reduction of nickel(II) compounds to yield nickel(0) phosphine complexes has been carried out using a variety of reducing agents such as sodium amalgam, sodium sand, sodium borohydride, sodium naphthalenide and aluminum trialkyls. In some cases the phosphine ligand itself was found to act as the reducing agent. 

The findings of Sen and Halpern (36, see Reaction 7) suggest alternative pathways for the Rh-catalyzed co-oxidation, in which the phosphine is oxidized by liberated peroxide and the ketone is formed during the reduction of Rh(III) to Rh(I) (a Wacker cycle). Read and Walker (87) ruled out Wacker chemistry, since addition of water in their system (OH is needed for the reduction, Reactions 7 and 8) decreased ketone yield somewhat. More convincing evidence for the absence of Wacker type chemistry comes from isotope studies using H2018, work just published by Tang et al. (89). 

In later work, Green et al. (230) used the reducing ability of NiH(BH4)-fPCy3)2 to prepare PdHCl(PR3)2 and PdH(BHi)(PR3)2 (R - Cy or Pr ). This method has been used to prepare a wide range of hydro and borohydro complexes of nickel and palladium, many of which have been solely characterized by their NMR spectra. It was considered (766) that in all cases the first step in the reaction was reduction, Eq. (3), which was followed by phosphine exchange reactions to form a mixture of six hydro complexes (M = Ni, Pd R = Et, Pr , Bu" all complexes have tram stereochemistry) ... 

It follows that reduction reactions of the Pd and Pt phosphine complexes are often complicated by destructive splitting of the phosphine ligands, resulting in non-... 

Synthesis of the rhodium(I) complexes of TTP and TTX is not as straightforward as, for example, that of the Rh(I) complexes of phosphines. Substitution reactions, such as the refluxing of [Rh(l,5-cyclo-octadiene)2Cl]2 with TTP, generally failed. Also the Rh(III) in RhCls 3H2O is not simultaneously reduced and chelated by TTP (JO). The only successful routes that we foimd involve the reduction of the previously characterized Rh(III) complexes. Voltammetry in acetonitrile solution on [Rh(TTP)Cl2]Cl showed a single cathodic process at —0.72 V (vs. AgVAgCl) which represents a two-electron reduction. Small samples of the Rh(I) salt were prepared by electrolysis, but reduction with sodium borohydride in methanol was used to prepare most of the salts reported here. 

Reduction Reaction. DIBAL has been shown to be a practical reagent to reduce secondary phosphine oxides (eq 36). In most cases, this reaction is carried out in hydrocarbon solvent at room temperature and was demonstrated to be a convenient large scale synthesis. 

Bao X, von Deak D, Biddinger EJ, Ozkan US, Haddad CM (2010) A computational exploration of the oxygen reduction reaction over a carbon catalyst containing a phosphinate fimctiraial group. Chem Commun 46 8621-8623... 

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