Catalysts ligand

A second question involves the influence of ligands on the rate and selectivity of the Lewis-acid catalysed Diels-Alder reaction in water. In Chapter 3 we have demonstrated that nearly all the ligands studied induce a significant decrease in the affinity of the catalyst for the dienophile. This effect is accompanied by a modest reduction of the rate of the Diels-Alder reaction of the ternary dienophile -catalyst - ligand complex. 

Inhibitors are often iacluded ia formulations to iacrease the pot life and cute temperature so that coatings or mol dings can be convenientiy prepared. An ideal sUicone addition cure may combine iastant cure at elevated temperature with infinite pot life at ambient conditions. Unfortunately, real systems always deviate from this ideal situation. A proposed mechanism for inhibitor (I) function is an equUibtium involving the inhibitor, catalyst ligands (L), the sUicone—hydride groups, and the sUicone vinyl groups (177). 

Moreover, 99 has been used as a catalyst ligand within the dihydroxylation of styrene [99JA1317]. Combined semiempirical and Hartree-Fock studies are presented for the formation of substituted pyridazines and some heterobetaines [99JOC9001,00H1065]. 

Entry R Catalyst ligand Yield of cis-aziridines (%) ee of cis-aziridine (%) cis trans aziridine... 

Monomers 1 Catalyst/Ligand Monomers 1 Catalyst/Ligand... 

Macroinitiator1 Monomer 2a Catalyst /Ligand Solvent Temp. °C... 

In the hydroformylation of lower alkenes using a modified cobalt catalyst complex separation is achieved by distillation. The ligands are high-boiling so that they remain with the heavy ends when these are removed from the alcohol product. Distillation is not possible when higher alcohols or aldehydes are produced, because of decomposition of the catalyst ligands at the higher temperatures required. Rhodium complexes can usually also be removed by distillation, since these complexes are relatively stable. 

It was described that the use of the combination of Pd(OAc)2/dppp /THE resulted in the highest yield. It is not clear whether such a combination of catalyst, ligand and solvent is restricted to the hydrothiocarboxylation of conjugated enynes or also may be applied to other alkynes. Furthermore, it remains to be explored whether, in the Pt-catalyzed reaction performed in CHjCN [see Eq. (7.18)], conjugated enynes can be used. 

A comparison of several of the biphenylphosphine ligands has provided some insight into the mechanism of catalyst activation.168 The results of this study suggest that dissociation of the diphosphino to a monophosphino complex is an essential step in catalyst activation, which would explain why some of the most hindered phosphines are among the best catalyst ligands. This study also indicated that deprotonation of the amine ligand is an essential step. Finally, in catalyst systems that are based on Pd(II) salts, there must be a mechanism for reduction to the active Pd(0) species. In the case of amines, this may occur by reduction by the amine ligand. 

Catalyst Ligand Additive (mol%) Aldol (symanti) 1,4%-Reduction... 

Entry Catalyst Ligand added Reaction time (k) Solvent Temperature TO 333 (%) 23b (%) Catalyst concentration (mol%)... 

The polymerization of 2-(diethylamino)ethyl methacrylate, DEAEMA, was studied under different conditions. It was shown that the best system providing narrow molecular weight distribution polymers involved the use of p-toluenesulfonyl chloride/CuCl/HMTETA as the initiator/catalyst/ligand at 60 °C in methanol [72]. Taking advantage of these results, well-defined PDEAEMA-fr-PfBuMA block copolymers were obtained. The synthesis was successful when either fBuMA or DEAEMA was polymerized first. Poor results with bimodal distributions were obtained when CuBr was used as the catalyst. This behavior was attributed to the poor blocking efficiency of PDEAEMA-Br and the incomplete functionalization of the macroinitiator. 

Systematic variation in chirality at both the chelate backbone and the terminal groups revealed a remarkable effect on the enantioselectivity of the catalysts. Ligand (109) generates chiral cooperativity between the backbone and the terminal moieties in Pt-catalyzed hydroformylation. The highest ee (65%) for 2-phenylpropanal was found for the ligand R-bis(S)-(110) in combination with Pt. The chemoselectivities with all ligands described in association with Pt were rather low. The comparative... 

The main conclusions to be drawn from this study are that the reactor design works well, and that steady state continuous flow operation requires excellent mixing of the gases and two liquid phases and high conversions. Improvements in the catalyst (ligand) are required to reduce leaching still further, but commercialisation will also require a different reactor design or more than one CSTR in series. 

It would be tempting to apply the same rationale to the metathesis of 2-pentene isomers, but clearly, the steric requirements of methyl and ethyl are much less than that of isopropyl, and trade-offs involving cis-1,2-disubstitution vs. axial orientation are not clear neither is the important role of catalyst ligand influence. 

The results of the kinetic analysis for the investigated systems are summarized in Table 10.2, the substrate concentration used being the same for all trials. In the case of methyl- and cyclohexyl-substituted ligands the Michaelis constant is smaller than the initial substrate concentration of [S]o=0.06666 mol L-1 (Table 10.2). However, a description of the hydrogenations with other catalyst ligands as first-order reactions shows that in each of these cases the Michaelis constant must be much greater than the experimentally chosen substrate concentration. 

Catalyst/ligand SCRb Reaction conditions [Solvent temp., H2 pressure, time] ee [%] (config.) Refer- ence... 

Cyclic alkynyl carbonates undergo carbonylation in the presence of a palladium catalyst and carbon monoxide (5 MPa) in MeOH to give allenic carboxylates (Eq. 9.118) [92], Bu3P proved superior to Ph3P as the catalyst ligand. An enynyl cyclic carbonate underwent double vicinal carbonylation at 80 °C to produce a five-membered lactone product in 52% yield (Eq. 9.119). When the reaction was performed at 50 °C, the bicyclic enone lactone was produced in 75% yield along with 10% of the y-lactone. 

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