Aryl group interchange

Figure 4. Aryl group interchange between molecules of (para-tolyl) diphenylphosphine.(Conditions 7 wt % (paratolyl)diphenylphosphine 1000 ppm Rh in toluene. 120°C 60 psia propylene hydrogen. Solid lines are the computed).

Table 1. Rates of Aryl Group Interchange between triphenylphosphine and tris-para-tolylphosphine catalyzed by different rhodium complexes. Conditions 120 °C iodo ppm of Rh 100 psi of H2 phosphine rhodium mole ratio 20...

The low reactivity of the rhodium clusters is surprising if one assumes that aryl group interchange can occur between vicinal metal centers as is proposed by Kaneda et al. (5). It is possible however that the rhodium clusters used here are rather coordinatively and structurally stable and do not promote aryl interchange at the employed temperatures. At the higher temperatures however, cluster breakdown to reactive mononuclear species and/or coordinatively unsaturated clusters is likely. 

Catalysis of aryl group interchange in triarylphosphines is not unique to rhodium. Other group 8 transition metals show varying degrees of activity (Table 2). 

Table 2 Relative rates of aryl group interchange between triphenylphosphine and tris-para-tolylphosphine cat yzed by group 8 transition metals. Reaction conditions 1000 ppm of metal catalyst phosphine metal mole ratio = 20 100 psi ofH2 C3H6(l l)...

Finally, aryl group interchange between triarylphosphines is mediated by Wilkinson s catalyst at 120 °C, but a near statistical mixture of the exchanged materials is formed along with some byproducts. ... 

During a study of this reaction, we observed a surprisingly facile rhodium-catalyzed interchange of aryl groups of tertiary phosphines (4). Thus we observed that during rhodium-catalyzed propylene... 

Scheme 18 shows the solution of this problem and also that of the stereochemistry of intermediates. We proved that the dilactone had both aryl groups equatorial by interchanging the order of introduction of the piperonyl and veratryl groups. After equilibration with 0.2% HCl/MeOH the two series did not give the same compound, as expected, but removal of the methylthio group in each case, followed by mild acid equilibration gave the same all equatorial dimethoxyacetal, The conditions of equilibration are... 

The C—P bond cleavage processes responsible for the observed interchange has been described in the literature. Novak and coworkers reported on the reversible formation of tetraarylphosphonium ion, via a reductive elimination pathway, as a possible key intermediate in the exchange of aryl groups on the central palladium with aryl groups on the phosphine moiety (Scheme 4.4) [5]. A subsequent oxidative addition of a different C-P bond in the phosphonium ion to the intermediary palladium(O) complex would then create the interchanged arylpaUadi-um(II) complex also observed by Chernard and coworkers [6]. A similar C-P bond... 

Phenolic compounds are weaker nucleophiles and better leaving groups than aliphatic alcohols. They do not yield polyesters when reacted with carboxylic acids or alkyl carboxy lates. The synthesis of polyesters from diphenols is, therefore, generally carried out through the high-temperature carboxylic acid-aryl acetate or phenyl ester-phenol interchange reactions with efficient removal of reaction by-product (Schemes 2.10 and 2.11, respectively). 

We chose the symbol to be explicitly reminiscent of the typographer s symbol for interchanging. By choosing sp3-bonding groups here, we are exempted from current considerations of the energetics of acyl and aryl halides, topics of interest that were discussed in earlier sections. 

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