Ligands telomerizations

Riou JF et al. (2002) Cell senescence and telomere shortening induced by a new series of speciflc G-quadruplex DNA ligands. Proc Natl Acad Sci USA 99(5) 2672-2677... 

In order to explain the competitive formation of the 1 1 and 1 2 adducts and the formation of the 2,6-octadienyl rather than the 1,6-oc-tadienyl chain, a mechanism was proposed (62, 69) in which the insertion of one mole of butadiene to the Pd—H bond gives the 77-methallyl complex (68) at first, from which 1-silylated 2-butene is formed. At moderate temperature and in the presence of a stabilizing ligand, further insertion of another molecule of butadiene takes place to give C5-substituted n-allyl complex 69. The reductive elimination of this complex gives the 1 2 adduct having 2,6-octadienyl chain. In the usual telomerization of the nucleophiles, the reaction of butadiene is not stepwise and the bis-n--allylic complex 20 is formed, from which the l, 6-octadienyl chain is liberated. 

The results in Table VI were obtained by the reactions at 80°C with Pd(acac)2 using different ligands in a mixture of methanol and isopropyl alcohol. From these results, it seems likely that reaction temperature has large influence on the regiospecificity of the telomerization. As another example, a mixture of isomeric telomers was obtained by the reaction of methanol and isoprene at 100° (95). 

Multiphase homogeneous catalysis (continued) hydroformylation, 42 483-487, 498 hydrogenations, 42 488-491 metal salts as catalysis, 42 482-487 neutral ligands, 42 481 82 organic reactions, 42 495 0X0 synthesis, 42 483-487 ring-opening metathesis polymerization and isomerization, 42 492-494 telomerizations, 42 491-492 diols as catalyst phase, 42 496 fluorinated compounds as catalyst phase, 42 497... 

The results of this analysis of the product and catalyst distribution show that only a limited range of systems may be apphcable for the telomeriza-tion of butadiene and carbon dioxide. The reaction was performed in the biphasic systems EC/2-octanol, EC/cyclohexane and EC/p-xylene. The yield of 5-lactone reached only 3% after a reaction time of 4 hours at 80 °C. hi the solvent system EC/2-octanol triphenylphosphine was used as the hgand. With the ligand bisadamantyl-n-butyl-phosphine even lower yields were achieved in a single-phase reaction in EC or in the biphasic system EC/cyclohexane. The use of tricyclohexylphosphine led to a similar result, only 1% of the desired product was obtained in the solvent system EC/p-xylene, which forms one homogeneous phase at the reaction temperature of 80 °C. Even at a higher temperature of 100 °C and a longer reaction time of 20 hours no improvement could be observed. Therefore, we turned our interest to another telomerization-type process. 

Suitable thermomorphic solvent systems were appUed to the telomeriza-tion of butadiene with ethylene glycol or with carbon dioxide, to the isomer-izing hydroformylation of trans-4-octene and to the hydroaminomethylation of 1-octene with morpholine. Further investigations for the carboxytelomer-ization and for the synthesis of 4-nitrodiphenylamine were also carried out. In addition to common Ugands, PEG-modified ligands and fiuorous Ugands were used for the telomerization and the carboxytelomerization. 

Table 2 Influence of the nature of the ligand on the selectivity of the butadiene/glycerol telomerization...

To circumvent the formation of ditelomers and to attempt recycling of the catalysts, the telomerization of polyols was studied in the presence of water using water soluble catalysts such as Pd/TPPTS (TPPTS = tris(m-sulfonato-phenyl) phosphine trisodium salt) [9, 12, 16, 17]. Behr et al. studied the telomerization of ethylene glycol under biphasic conditions. Under such reaction conditions, 80% of mono-telomer are formed and only traces of ditelomer and butadiene dimers are detected (Fig. 4). This is attributed to the solubility of the monomer in the catalyst phase. However, the catalyst is unstable and decomposes rapidly, leading to almost inactive catalyst after three runs. This is due to TPPTS oxidation during the work-up of the reaction and can be avoided by addition of 2.5 equiv. ligand in the solution prior to each run. 

Finally, a third means of ligand formation from an imidazolium cation, described by Dupont and co-workers, should be mentioned here [34]. They investigated the hydrodimerization/telomerization of 1,3-butadiene with palladium(II) compounds in [BMIM][BF4] and described the activation of the catalyst precursor complex [BMIM]2[PdCl4] by a palladium(lV) compound formed by oxidative addition of the imidazolium nitrogen atom and the alkyl group with cleavage of the C-N bond of the [BMIM] ion, resulting in bis(methyHmidazole) dichloropalladate (Scheme 5.2-5). However, this reaction was only observed in the presence of water. 

Palladium complexes generated from the sulfonated ligand 5 (Table 2 R=Me, n=0,l,2) were also used to catalyse the telomerization of 1,3-butadiene with water108 and from the aminophosphines 83 (Table 5 R=Ph R =H,Me R"=Me,iPr n=2,3) with MeOH to give l-methoxy-2,7-octadiene in selectivities up to 78% at conversions ranging from 52 to 80%.232 Pd/83 (R=Ph R =H,Me R"=Me,iPr n=2,3) catalysts are more active but less selective in the latter reaction compared to their tppts counterparts.232 9... 

The telomerization of dienes in a two-phase system was first described in a patent (100). Water was used as the solvent for the catalyst, with sulfonated phosphane ligands providing the water solubility. Water, alcohols, phenols, acids, amines, and acetylacetic add were used as nucleophiles. 

Water-soluble quaternary ammonium phosphanes have been used as ligands for palladium in the telomerization with methanol under two-phase conditions (101). 

Scheme 5.23 Proposed mechanism for the Pd(0)-catalyzed telomerization of 1,3-butadiene with C02 (example with hemi-labile phosphino-nitrile ligands) [71].

Alkyl ethers of sucrose have been prepared by reaction with long-chain alkyl halides to provide mixtures of regioisomers and products of different degree of substitution.82,83 A similar reaction with chloromethyl ethers of fatty alcohols provides formaldehyde acetals.84,85 Alkenyl ethers of various carbohydrates, and notably of sucrose, can also be obtained by palladium-catalyzed telomerization of butadiene (Scheme 6).86 88 Despite a low-selectivity control, this simple and clean alternative to other reactions can be carried out in aqueous medium when sulfonated phosphines are used as water-soluble ligands. 

Typical Catalysts and Ligands Used in Telomerization Reactions. 52... 

The focus of this review will be on those recent and older contributions to the telomerization field that primarily deal with the palladium-catalyzed telomerization with multifunctional oxygenates. First, a short glossary of commonly used ligands and catalysts is given, followed by a description of the mechanistic intricacies of the process and, finally, the different classes of multifunctional, renewable telogens that are treated in detail. This review complements two other excellent overviews of the telomerization reaction, each with its own primary focus. Behr and co-workers published an extensive review article summarizing the research on telomerization done in the period 1984—2008 with a particular focus on process developments [25]. 

Fig. 2 Some examples of ligands used in the telomerization reaction...

The novel catalyst system based on palladium(O) /V-heterocyclic carbene complexes was developed by the group of Beller, in part prompted by the strong patent position of Dow on phosphine-based palladium catalysts [8]. The catalyst [37], either generated in situ from the corresponding imidazolium salt or the molecularly defined divinyldisiloxane complex [Pd(Imes)(dvds)] (Fig. 3), was used in the telomerization of 1,3-butadiene with methanol [38—40]. The /V-heterocyclic carbenes are in general better a-donor ligands and come with considerably different steric requirements than the phosphines. The [Pd(Imes)(dvds)] complex resembles the final telomer-palladium product complex and thus offers a facile and clean entry into the catalytic cycle. The metal carbene complex was shown to be... 

Generally, octatriene formation is favored by higher temperatures, higher phosphine and/or butadiene concentrations and, importantly, by an increase in steric bulk of either the ligand or the nucleophile. Indeed, Harkal et al. showed a selectivity switch from telomerization products to 1,3,7-octatriene formation by altering the steric demand of the /V-heterocyclic carbene ligand in the reaction of butadiene with isopropanol under further identical reaction conditions [48]. For the more basic nucleophiles, such as the alcohols, the telomer products are stable under experimental conditions, i.e. product formation is irreversible, but for more acidic substrates such as phenol, product formation is reversible and more 1,3,7-octatriene will be formed after the substrate has been depleted. 

Some authors have proposed mechanisms based on bimetallic palladium species. Keim and colleagues, for instance, proposed a reaction mechanism for the telomerization of acetic acid with butadiene (Scheme 10), where the key intermediate is a bispalladium compound such as Pd2([i-1,2,3,6,7,8-r 6-octa-2,7-dien-l, 8-diyl)((j,-OAc)2] [65]. It was shown that these bimetallic compounds are able to catalyse telomerization in the presence of 1,3-butadiene, acetic acid and a phosphine ligand. 

Parvulescu et al. noted an interesting change in EG telomer product selectivity upon immobilization of an Pd/TPPTS catalyst on a basic support [58]. In an attempt to address the issues associated with recovery and reuse of the telomerization catalyst, the anionic TPPTS ligand was immobilized on various layered double hydroxides by ion exchange methods (Scheme 11). The use of these catalysts in the telomerization of methanol and ethylene glycol resulted in a remarkable shift in... 

Table 2 Selected results of the ligand screening for the Pd-catalyzed telomerization of 1,3-butadiene with ethylene glycol by Grotevendt et al. [78]...

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