Dimerization functionalized olefin

Of the cyclic olefins, norbornadiene replaces two CO groups from one Co to yield a labile complex 159, 160, 235), cyclooctatetraene replaces the axial CO ligands from all three cobalt atoms 53) and is itself replaced by other Lewis bases 330), and cyclopentadiene forms the unusual complex [95] with Co3(CO)gCMe 159,160). A few catalytic reactions were observed with methinyltricobalt enneacarbonyls including the dimerization of norbornadiene 160, 235) and the polymerization of functional olefins 312) with different Co3(CO)9CX. 

Perhaps the most basic form of the olefin metathesis reaction is the cross metathesis (CM) of acyclic olefins to yield new acyclic olefins (Fig. 4.11). The ratio of CM products may be controlled by steric and electronic factors to provide one product preferentially, rather than a statistical mixture, which is key to the synthetic utility of this reaction. For example, various functionalized olefins, dimers with bioactive substituents, and trisubstituted olefins have all been made by CM [33], and one of the industrial applications is the synthesis of insect pheromones [34]. 

The dimerization of functionalized olefins is of general technical importance. For example, the dimerization of methylacrylate (MA) to A -dihydrodimethylmuconate (DHM) leads to a highly interesting intermediate which can be transformed to both fine chemicals (such as cyclopentenones) and adipic acid. A continuous, liquid-liquid biphasic version of this Pd catalyzed reaction was realized by Tkatchenko et al. for the first time using [BMIM][BF4]/toluene as reaction system (Scheme 5.3-35) [252]. 

Olefin Dimerization and Polymerization The homogeneous catalytic dimerization of olefins, e.g., ethylene to butene, is another important reaction that can proceed via the addition, insertion, cleavage sequence. Among the catalysts that effect ethylene dimerization are RhCl3, PdCl2, and combinations of alkylaluminum halides with divalent iron, cobalt, and nickel compounds. The rhodium system has been thoroughly studied (177) and almost certainly proceeds by the sequence shown below (S = solvent). The nickel system probably functions similarly, but other mechanisms have been considered for the iron-, cobalt-, and palladium-catalyzed reactions. 

The dimerization of functionalized olefins by transition metal catalysts is a well studied reaction. A first review was given by Lefebvre and Chauvin in 1970 [40]. In 1974 Hidai and Misono described in detail the dimerization of acrylic compounds [41]. These starting olefins have found considerable attention from the industrial point of view, because they can be produced at high quantities and at low costs by the petrochemical industry. For instance, acrylonitrile, acrylates and allyl esters are favorable starting molecules. 

Besides acrylonitrile and acrylates other functional olefins such as vinyl or allyl compounds can also be dimerized. An important example for vinyl compounds is styrene, which can be dimerized to 1,3-diphenyH-butene. Owing to its high tendency to poljmerize spontaneously, the reaction conditions must be chosen carefuUy. Thus the dimerization catalyzed by PdCl2 at 100 °C yielded only 33% of a dimer fraction with more than 60% of a dark polymeric residue [49]. Using Ni( 1/ -03115)2 as the catalyst, styrene is converted to l,3-diphenyl-/mn5-l-butene [58, 59]. With [PdCl r] -C,U,)] dimers and trimers are obtained (Equation 43) [60]. 

A great number of Kolbe dimerizations have been tabulated in refs. [9, 17-19]. Here no comprehensive coverage is intended, but to demonstrate with selected examples the range and limitations of Kolbe dimerization. In the following discussion and in Table 2 the carboxylates are arranged according to their functional groups in the order alkyl-, ester-, keto-, halo- and olefinic substituents. 

The behavior of 3 toward ether or amines on the one hand and toward phosphines, carbon monoxide, and COD on the other (Scheme 2), can be qualitatively explained on the basis of the HSAB concept4 (58). The decomposition of 3 by ethers or amines is then seen as the displacement of the halide anion as a weak hard base from its acid-base complex (3). On the other hand, CO, PR3, and olefins are soft bases and do not decompose (3) instead, complexation to the nickel atom occurs. The behavior of complexes 3 and 4 toward different kinds of electron donors explains in part why they are highly active as catalysts for the oligomerization of olefins in contrast to the dimeric ir-allylnickel halides (1) which show low catalytic activity. One of the functions of the Lewis acid is to remove charge from the nickel, thereby increasing the affinity of the nickel atom for soft donors such as CO, PR3, etc., and for substrate olefin molecules. A second possibility, an increase in reactivity of the nickel-carbon and nickel-hydrogen bonds toward complexed olefins, has as yet found no direct experimental support. 

Sternberg et al. (28a) postulated that [HPe(CO)4] can exist as a dimer because (among other reasons) of its ability to function as a donor of molecular hydrogen. The ion was shown to be a catalyst for olefin isomerization shaking 1-hexene at room temperature with an ether solution of [HFe(CO)4] for 24 hours isomerized all the 1-hexene to 2-and 3-hexenes. Similar isomerizations also have been reported to be catalyzed by H2Fe(CO)4 (28b) and by Fe(CO)j (29). 

Wittig-type olefinations can also be performed using selenoaldehydes. Phosphorus ylides initially attack the carbon atom of the selenocarbonyl functionality.405 Aromatic selenoketones undergo reductive dimerization with organo-lithium reagents probably via an electron transfer mechanism.406 Also the addition of organolithium reagents takes... 

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