Linear-branched selectivity

Of particular importance for industry is the hydroformylation of alk-l-enes and one of the best processes developed so far for C2-C5 olefins employs a biphasic system in combination with a hydrosoluble rhodium complex bearing TPPTS as ligand [69-71], From this statement, Wasserscheid et al. investigated in the early 2000s the possibility of using ILs supported catalysts for this reaction [71-75], owing to the discovery by Chauvin that good linear/branched selectivities could be achieved... 

When using 5.0 run Rh(0) nanoparticles, the olefins are converted into aldehydes (Table 11.13, entries 1 and 4) and the addition ofXantphos leads to linear/branched selectivities up to 25 (Table 11.13, entry 2). With smaller colloids (2.7nm), the... 

The hydroformylation of the water-soluble substrates, 4-penten-l-ol and 3-buten-l-ol in aqueous solution using HRh(CO)(TPPTS)3 as the catalyst was investigated. Activation parameters and reaction selectivity for the hydroformylation of 4-penten-l-ol were found to depend on the ionic strength of the solution. As sodium sulfate was added, the activation energy increased. The linear-branched selectivity was strongly influenced by the ionic strength and temperature. The reaction could be directed to yield a product distribution of modest linearity (75%) or an exceptionally high ratio of the branched product (98%) as a cyclic 2-hydroxy-3-methyltetrahydropyran [33]. 

Rhodium nanoparticles were used as catalyst precursors for the solventless hydroformylation of 1-alkenes. Linear/branched selectivities up to 25 were achieved by adding xantphos to the catalyst system [46]. 

The basic flow sheet for the flotation-concentration of nonsulfide minerals is essentially the same as that for treating sulfides but the family of reagents used is different. The reagents utilized for nonsulfide mineral concentrations by flotation are usually fatty acids or their salts (RCOOH, RCOOM), sulfonates (RSO M), sulfates (RSO M), where M is usually Na or K, and R represents a linear, branched, or cycHc hydrocarbon chain and amines [R2N(R)3]A where R and R are hydrocarbon chains and A is an anion such as Cl or Br . Collectors for most nonsulfides can be selected on the basis of their isoelectric points. Thus at pH > pH p cationic surfactants are suitable collectors whereas at lower pH values anion-type collectors are selected as illustrated in Figure 10 (28). Figure 13 shows an iron ore flotation flow sheet as a representative of high volume oxide flotation practice. 

Recently, a new bidentate hemispherical chelating bisphosphite ligand based on a calixarene backbone has been designed for linear selective hydroformylation of alkenes (Scheme 9) [54], Excellent levels of regioselectivity have been observed, and even the intrinsic branched-selective hydroformylation of styrene could be overruled by this system. However, the system suffers from low catalytic activity. 

Selectivity refers to the fraction of raw material alkene that is converted to product aldehyde, but since hydroformylation typically gives both a linear and branched isomer, selectivity also refers to the relative amounts of each. The linear branched (l b) ratio is highly catalyst dependant. One must simultaneously consider whether the proposed catalyst will give the desired l b selectivity and also whether the proposed catalyst is feasible for use with the catalyst/product separation technologies. For example, water extraction of a polar product, such as in the hydroformylation of allyl alcohol to give 4-hydroxybutanal, would not work well with a sodium salt of a sulfonated phosphine since both are water soluble. 

The improvements made in hydroaminomethylation technology suggest that certain variants of this reaction are sufficiently developed for the potential production of amines. The synthesis of linear tertiary and secondary amines from terminal alkenes shows promise in this regard. Belief s recent contributions towards hydroaminomethylation using ammonia to produce linear primary amines, which are of industrial significance due to their abundance, suggest a bright future for this reaction. Branched selective hydroaminomethylation remains relatively underdeveloped and needs further study. 

Although early catalysts were based on cobalt, nowadays, rhodium catalysts are preferred because they require lower pressure and afford higher chemo- and regioselectivity [1,2]. In recent years, extensive research into the production of only linear aldehydes has provided impressive results. The application of phosphines with a wide bite angle in the rhodium catalyzed hydroformylation of terminal alkenes enable the regioselectivity to be almost totally controlled [3]. Branched selective hydroformylation, al-... 

Reactions of allylic electrophiles with stabilized carbon nucleophiles were shown by Helmchen and coworkers to occur in the presence of iridium-phosphoramidite catalysts containing LI (Scheme 10) [66,69], but alkylations of linear allylic acetates with salts of dimethylmalonate occurred with variable yield, branched-to-linear selectivity, and enantioselectivity. Although selectivities were improved by the addition of lithium chloride, enantioselectivities still ranged from 82-94%, and branched selectivities from 55-91%. Reactions catalyzed by complexes of phosphoramidite ligands derived from primary amines resulted in the formation of alkylation products with higher branched-to-linear ratios but lower enantioselectivities. These selectivities were improved by the development of metalacyclic iridium catalysts discussed in the next section and salt-free reaction conditions described later in this chapter. 

Unstabilized enolates react with allylic carbonates in the presence of metalacyclic iridium-phosphoramidite catalysts. Although ketones and aldehydes have not yet been used directly as pronucleophiles with this catalyst system, silyl enol ethers [80] and enamines [81] react with linear allylic carbonates to form, after workup, p-branched, y-8 unsaturated ketones (Scheme 13). Both methods form products in high yield, branched selectivity, and enantioselectivity for a range of cinnamyl and alkyl-substituted allylic carbonates. However, the silyl enol ethers derived from aliphatic ketones reacted in lower yields than enamines derived from the same ketones. 

Under optimum reaction conditions (See Table IV.), selectivity to linear dimer is controlled by the choice of temperature, solvent and tertiary phosphine. Toluene and tetrahydrofuran are the best solvents. Temperatures between 25 to 60 C with a triphenyl or tributylphosphine/palladium acetate catalyst give linear dimer selectivities in the 80 s. At 25 C in toluene, a palladium acetate/tributylphosphine catalyst gave 98.7% conversion and 89.6% linear, 4.7% branched, 1.9% cyclic, and 3.8% heavies selectivity. The linear dimerization reaction was second order in diene with a 3.6 Kcal/mole activation energy. 

Catalytic systems to afford linear esters selectively are scant.306,309 A report in 1995 disclosed that palladium complexes based on l,l -bis(diphenylphosphine)fer-rocene showed excellent regioselectivity for the formation of linear a,p-unsaturated esters.309 The results with phenylacetylene are remarkable because this compound is known to exhibit a distinct preference for the formation of the branched products on palladium-catalyzed carboxylations. Mechanistic studies indicate that the alkoxycarbonylation of alkynes proceeds via the protonation of a Pd(0)-alkyne species to give a Pd-vinyl complex, followed by CO insertion and alcoholysis.310... 

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