Through Phosphine Elimination

The elimination of phosphine (arsine) E15Me3 occurs through the transition states TS-1. The activation barriers of this process for betaines of the phosphorus series are higher than those for arsenic betaines. They increase and the thermal effect of the reaction decreases in the series... 

Finally, Cristau and coworkers have reported on a quite efficient preparation of triphenylphosphine oxide (Figure 2.13) by a similar addition-elimination reaction of red phosphorus with iodobenzene in the presence of a Lewis acid catalyst followed by oxidation of an intermediate tetraarylphosphonium salt.42 This approach holds the potential for the preparation of a variety of triarylphosphine oxides without proceeding through the normally used Grignard reagent. Of course, a variety of approaches is available for the efficient reduction of phosphine oxides and quaternary phosphonium salts to the parent phosphine, including the use of lithium aluminum hydride,43 meth-ylpolysiloxane,44 trichlorosilane,45 and hexachlorodisilane.46... 

Some experimental evidences are in agreement with this proposed mechanism. For example, coordinating solvents like diethyl ether show a deactivating effect certainly due to competition with a Lewis base (149). For the same reason, poor reactivity has been observed for the substrates carrying heteroatoms when an aluminum-based Lewis acid is used. Less efficient hydrovinylation of electron-deficient vinylarenes can be explained by their weaker coordination to the nickel hydride 144, hence metal hydride addition to form key intermediate 146. Isomerization of the final product can be catalyzed by metal hydride through sequential addition/elimination, affording the more stable compound. Finally, chelating phosphines inhibit the hydrovinylation reaction. 

Similarly, decreasing the electron density on the metal will enhance its willingness to undergo reductive elimination and become more electron rich. A typical demonstration of this tendency is the nickel catalyzed coupling of sp3 hybridized carbon atoms. Here, if a nickel-phosphine catalyst is used, the catalytic cycle is very slow, due to disfavoured reductive elimination. The use of additives that deplete the electron density of the nickel atom through coordination (e.g. fluorostyrenes) leads to a marked increase of the turnover frequency.18... 

Studies on catalytic asymmetric aza-Baylis-Hillman reaction has shown that the reaction involves rate-limiting proton transfer in the absence of added protic species, but exhibits no autocatalysis.41 Brpnsted acidic additives lead to substantial rate enhancements through acceleration of the elimination step. Furthermore, it has been found that phosphine catalysts, either alone or in combination with protic additives, can cause racemization of the aza-Baylis-Hillman product by proton exchange at the stereogenic centre. 

The data presented in Tables I-III demonstrate that through proper utilization of substitutents it is possible to direct carbonyl olefination through elimination of a phosphinate phosphorus atom (PO) or a phosphonate phosphorus atom (P02). Condensation of... 

Complexes of the [Tc(N)(/c2-PS)2] type were also synthesized at noncarrier added level and their biological behavior was studied [46]. These compounds exhibited high initial heart uptake, and elimination through liver and kidneys. The washout kinetics from heart was dependent on the nature of the lateral R groups on the phosphine thiol ligands. Extraction of the activity from myo-... 

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