Pd-catalyzed dimerization

The Pd °-catalyzed dimerization of 2 leads to the Diels-Alder adduct 314 as the main product (81%), for whose formation only one allene group has participated in the dimerization process, in addition to the dimer 315 (14%), in which no allene group has survived in addition, several other dimers are produced in trace amounts [131]. 

Dupont and co-workers studied the Pd-catalyzed dimerization [108] and cyclodimerization [109] of butadiene in non-chloroaluminate ionic liquids. The biphasic dimerization of butadiene is an attractive research goal since the products formed, 1,3,5-octatriene and 1,3,6-octatriene, are sensitive towards undesired polymerization, so that separation by distillation is usually not possible. These octa-trienes are of some commercial relevance as intermediates for the synthesis of fragrances, plasticizers, and adhesives. Through the use of PdCl2 with two equivalents of the ligand PPhj dissolved in [BMIM][Pp6], [BMIM][Bp4], or [BMIM][CF3S03], it was possible to obtain the octatrienes with 100 % selectivity (after 13 % conversion) (Scheme 5.2-23) [108]. The turnover frequency (TOP) was in the range of 50 mol butadiene converted per mol catalyst per hour, which represents a substantial increase in catalyst activity in comparison to the same reaction under otherwise identical conditions (70 °C, 3 h, butadiene/Pd = 1250) in THF (TOP = 6 h ). 

Scheme 5.2-23 Biphasic, Pd-catalyzed dimerization of butadiene in [BMiMJfBFJ.

These cyclodimerization and cyclotrimerization reactions are catalyzed by low valent Ziegler-type Ni catalysts (139—144). Large ligands, such as tris-o-biphenylyl phosphite on nickel tend to favor cydooctadiene (COD) formation while smaller ligands favor the linear dimer, 1,3,7-octatriene. The dimer yield at 80°C and 101.3 kPa (1 atm) is 96%. The nickel catalyst can also be placed on a support so that it can be recycled (145). Many other type catalysts have been reported for this reaction (146). The linear 1,3,7-octatriene and its 1,3,6 isomer are also obtained by a Pd catalyzed dimerization (147—151). The kinetics of thermally induced dimerization to COD has also been studied (152). 

Finally, Ling and coworkers [161] have presented an intriguing Pd-catalyzed dimerization-cyclization sequence of alkynones 222 to furnish 3,3 -bifurans 223 with good to high selectivity with respect to the furans 224 in good yields (Scheme 93). 

The Pd-catalyzed dimerization of butadiene in the presence of water yields octa-dienols (cf. Section 2.3.5). This type of reaction is referred to as telomerization and can be performed with high selectivity for 2,6-octadien-l-ol in ionic liquids like 1 leaving the product separated from the reaction medium [35]. 

The oligomerization of olefins is mostly catalyzed by cationic complexes which are very soluble in ionic liquids. The Pd-catalyzed dimerization of butadiene [36] and the Ni-catalyzed oligomerization of short-chain olefins [5, 37], which is also known as the Difasol process [1 d] if chloroaluminate melts are used, can be mn in imidazolium salts 1 [38, 39]. Here, the use of chloroaluminate melts and toluene as the co-solvent is of advantage in terms of catalyst activity, product selectivity, and product separation. Cp2TiCl2 [6] and TiCU [40] in conjunction with alkylaluminum compounds were used as catalyst precursors for the polymerization of ethylene in chloroaluminate melts. Neither Cp2ZrCl2 nor Cp2HfCl2 was catalytically active under these conditions. The reverse conversion of polyethylene into mixtures of alkanes is possible in acidic chloroaluminate melts without an additional catalyst [41]. 

The separation of the products from the IL catalytic mixture can be performed in various cases by simple decanting and phase separation or by product distillation. In this respect, a continuous-flow process using toluene as extractant has been appHed for the selective Pd-catalyzed dimerization of methyl acrylate in ILs [136]. However, in cases where the products are retained in the IL phase, extraction with supercritical carbon dioxide can be used instead of classical liquid-liquid extractions that necessitate the use of organic solvents, which may result in cross-contamination of products. This process was successfully used in catalyst recycling and product separation for the hydroformylation of olefins employing a continuous-flow process in supercritical carbon dioxide-IL mixtures [137]. Similarly, free and immobilized Candida antarctica lipase B dispersed in ILs were used as catalyst for the continuous kinetic resolution of rac-l-phenylethanol in supercritical carbon dioxide at 120°C and 150°C and 10 Mpa with excellent catalytic activity, enzyme stability and enantioselectivity levels (Fig. 3.5-11). 

Scheme 5.3-35 Pd-catalyzed dimerization of methylacrylate in the diphasic system[BMiM][BF4]/toluene.

Water is likely to be present in aU practically relevant catalytic applications unless extreme precautions are taken or the system is self-drying (e.g., due to the fact that strong Lewis-acids or metal alkyls are used as co-catalysts). Water will influence the ionic liquid s thermal stability significantly if any part of the ionic liquid is prone to hydrolysis. Apart from the weU-known hydrolysis lability of tetrafluoroborates and hexafluorophosphates, water will thus also affect the stability of ester functionalities in the ionic liquid, e.g. the stability of alkyl sulfate anions. The presence of Bronsted acidity in the reaction system will further promote this kind of thermally induced hydrolysis reaction. Additionally, in strong Lewis-acidic ionic liquids care has to be taken to avoid incompatibilities between oxygen and nitrogen functionalities in the reactants or impurities and the ionic liquid s Lewis acidic group (usually a complex anion). It is for example obvious that the Pd-catalyzed dimerization of methylacrylate caimot be carried out in acidic chloroaluminate ionic hquids since the ionic liquid s anion would decompose in an irreversible reaction with the substrate methylacrylate. 

Finally, any cross-coupling procedures should, in principle, be applicable to the synthesis of homodimers. In addition, other Pd-catalyzed protocols specifically aimed at the synthesis of homodimers have also been developed. Most of them involve Pd-catalyzed dimerization of organometals or organic electrophiles, rather than the reaction of an organometal with an organic electrophile. Some of these homocoupling procedures can be applied to the synthesis of cycUc CToss-dimers. These reactions are discussed in Sect. 

One of the first papers dealing with homocoupling of sp -C halides was a mechanistic study of Pd-catalyzed dimerization of aryl iodides in the presence of tiialkylamine as the... 

The Pd-catalyzed dimerization of butadiene " or isoprene " with trapping by water is of exceptional interest. This is especially trae in the patent literature. 

The mechanism of this process is unclear. Particularly, the source of the PdH species remains obscure (cf. Sect. B.iv), the addition of which to the triple bond must have triggered the whole process. This process may have a relation to industrially important Pd-catalyzed dimerization of butadiene in the presence of water giving octadienols. ... 

Bergamini et al. reported an unusual variation on the linear dimerization, wherein Pd-catalyzed dimerization in acetonitrile/water and in the presence of CO2 affords 37 selectively (52% yield, turnover number (TON) = 420). Under these conditions, only small amounts of the water-trapped product and 1,3,7-octatriene are observed as side products (Scheme 11). The formal intramolecular [3 + 2] cycloaddition product 37, particularly the 1,4-diene subunit imbedded in it, is reminiscent of the products obtained from cyclization of Tp-allylpalla-dium intermediates onto pendant alkenes, a process termed a palladium-ene reaction by Oppolzer and Gavdin, which has also been investigated by Negishi et al. and Trost and Luengo. One can account for the formation of 37 from the chelated Tr-allylpalladium intermediate 39 by ligand insertion to 40 followed by /3-hydride elimination. 

A highly atom-economical cascade transformation employs Trost s Pd-catalyzed dimerization of alkynes [15] and consequent Pd-catalyzed benzannulation reaction [16] (Scheme 14.20). A formal [2 - - 2 - - 2] cycloaddition reaction proceeded through the formation of 2,4-enyne followed by a formal [4 - - 2] benzannulation reaction with diyne, representing a highly chemo- and regioselective trimerization reaction of alkynes. Homo-dimerization of terminal acetylenes followed by a benzannulation... 

SCHEME 14,20 Pd-catalyzed dimerization of acethylenes followed by an intramolecnlar cross-benzannulation reaction. 

Because Pd complexes would catalyze dimerization of EDA to give diethyl fu-marate and maleate, it might be possible that the polymers were obtained by radical polymerization of the diesters generated by the Pd-catalyzed dimerization. However, whereas the polymer obtained by AIBN-initiated radical polymerization of diethyl fumarate exhibited signals in MALDI-TOF-MS spectra with peak intervals of m/z = 172 (CHC02Et x 2), the intervals of the spectra of polymers from EDA were m/z = 86, suggesting that the polymers were not obtained by the radical polymerization of the diesters [38] (Scheme 15). 

Shirakawa et al. reported that addition of catalytic In(OTf)3 is required for the Pd-catalyzed dimerization of vinylarenes [280] (Figure 8.126). The In (111) was proposed to activate the vinylarenes for the nucleophilic attack of the Pd(0) complexes, likely through the formation of benzylic carbocation. 

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