Synergistic coordination reaction

As shown in Table 4.38, three major reaction pathways are available to hypova-lent metals in the presence of an alkene (A) and (C) dative and synergistic coordination (B) carbocation formation and (D) and (E) metallacyclic and migratory insertions. The latter types are of particular importance in metal-catalyzed alkene polymerizations and will be given primary attention in the discussion that follows. 

In order to enhance the activity of coordination catalysts we typically add a cocatalyst. The cocatalyst works synergistically with the catalyst to allow us to tailor the tacticity and molecular weight of the product while also enhancing the rate of the reaction. An example of a commercially used cocatalyst is methylaluminoxane used in conjunction with metallocene catalysts. 

D) Oxidative addition (insertion). In the limit of strong synergism, the H—H bond can be broken and two new M—H bonds formed. Although termed oxidative addition, this reaction does not necessarily deplete the metal of electron density (i.e., oxidize the metal). Rather, this reaction is fundamentally an insertion of the metal into the H—H bond. In the process, a lone pair of electrons at the metal and the electron pair of the H—H bond are uncoupled and then recoupled to form two M—H bonds. Whether synergistic H2 coordination or insertion prevails depends largely on the relative... 

This excellent regiocontrol was exploited by subjecting terminal alkenes and hydroxyalkynoates to ruthenium catalysis conditions to afford butenolides and pentenolides (Equation (23)).36 The Alder-ene reaction occurs preferentially to form the G-G bond at the alpha-carbon of the alkynoate. The unusually high regioselectivity is attributed to a synergistic effect derived from an enhanced coordination of the hydroxyl group to the ruthenium. 

Both THC and ethanol increase reaction time, and produce decrements in standing steadiness and psychomotor coordination (Belgrave et al. 1979). Whereas the peak effects of ethanol appeared quickly and wore off quickly (after 280 minutes), THC s effects were slower in onset and longer in duration. The effects of combined THC and ethanol are additive and not synergistic, and THC did not alter blood-ethanol levels. Other studies have shown an interaction between ethanol and THC on psychomotor skills necessaiy for driving, although there were no interactions on the subjective "high," heart rate acceleration, or THC plasma concentration (Perez-Reyes et al. 1988). 

Figure 7. Synergistic effect of Mo-W mixed-coordinated heteropolyacids for the oxidation of cyclopentene with hydrogen peroxide [26] 1 h, 2 h, and 3 h are the reaction times. GA = glutaraldehyde...

Entelis assumes the formation of an activated alcohol-isocyanate binary complex during the catalysis of the methanol-phenyl isocyanate reaction by dibutyltin dilaurate (DBTDL) (3, 5) Activated alcohol-isocyanate-catalyst ternary complexes have also been proposed by others. However, significant differences can be noted in the structures of either the postulated one (2, 4, 6, 7) or two (8) coordination positions of the isocyanate to the metal. To explain the synergistic effects observed when tertiary amine and organometallic compounds are combined, several authors suggest the formation of an activated quaternary complex I, II or III (2, 6, 9, 10, 11, 27). 

There is some evidence of the involvement of the coordinatively unsaturated bimetallic compound [CoRh(CO)v] as an active species in hydroformylation reactions.l A synergistic effect was observed in the hydroformylation of pentafluorostyrene. i Excellent regioselectivities were reported for the hydroformylation step of the hydroformylation-amidocarbonylation of trifluoropropene using a [Co2(CO)8]/[Rh6(CO)i6] system in which [CoRh(CO)7] could have been generated. A mixed-metal [Co.vRh (CO).] species has previously been postulated as an active catalyst in the hydrocarbonylation of diketene. " Hydroformylation of 1-hexene and 3,3-dimethylbut-l-ene catalyzed by [Co2Rh2(CO)i2] and of... 

Reactions between R02 and Go(II) or Mn(II) are discussed in terms of the methods of rate constant determination, the values of the rate constants and the effect of media on rate constants. A synergistic effect of Mn(II) addition is also discussed. The effect of the media is discussed in terms of the coordination ability of the solvent with Go(II) ion as well as the proton donor property of the solvent. It is concluded that the presence of a proton donor in the coordination sphere of Co(n) is required for hydroperoxide formation but at the same time this prevents the peroxy radical from reacting with the cobalt(II) ion. As a result, a bell-shaped dependence of the rate constant on the acetic acid and water concentration is observed. Our data explain some of the controversial literature data on solvent effects for the kinetics of metal bromide catalysis. 

As mentioned in Sect. 1, the field of cooperative or synergistic behaviour is very broad and this chapter is certainly not the appropriate place to survey the area. Having said that, mention was made of Jacobsen s quadratic systems in Sect. 2.2.1 although there are some dissimilarities with monometallic CBER. Other groups of reactions which have attracted this author s attention are the Pt-Sn hydroformylation systems and the Ir-Ru Cativa process for acetic acid [87]. A common theme in the Pt-Sn and Ir-Ru systems appears to be the need of the second metal (Sn or Ru) in order to abstract a halogen from the first metal, thereby freeing a coordination site. Catalytic bimetallic systems where a second metal is needed to abstract a halogen should, at some level, exhibit a bilinear term to reflect the abstraction. 

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