Subject olefin dimerization

Electron-rich olefins are nucleophilic and therefore subject to thermal cleavage by various electrophilic transition metal complexes. As the formation of tetraaminoethylenes, i.e., enetetramines, is possible by different methods, various precursors to imidazolidin-2-ylidene complexes are readily available. " Dimerization of nonstable NHCs such as imidazolidin-2-ylidenes is one of the routes used to obtain these electron-rich olefins [Eq. (29)]. The existence of an equilibrium between free NHC monomers and the olefinic dimer was proven only recently for benzimidazolin-2-ylidenes. In addition to the previously mentioned methods it is possible to deprotonate imidazolidinium salts with Grignard reagents in order to prepare tetraaminoethylenes. " The isolation of stable imidazolidin-2-ylidenes was achieved by deprotonation of the imidazolidinium salt with potassium hydride in THF. ... 

It is well known that olefins dimerize on contact with a nickel oxide catalyst at elevated temperature. Since the discovery of their specific activity in catalysing dimerization reactions, nickel-based catalysts have been the subject of intensive mechanistic and kinetic studies directed toward identifying and understanding the active sites responsible for this property. Earlier nickel catalysts were homogeneous however, nickel deposited on different supports has shown high selectivity towards alkene dimerization (see Table 5, page 276). 

The rate law does, of course, not decide on the exact pathway by which the olefin dimer is formed from the excited 1 2 complex. One possibility is a radical-radical-dimerization with intermediate formation of a five membered metallacycle that could form the product by reductive elimination. Such a sequence is not subject to the restrictions of a concerted electrocyclic mechanism and the final stereochemistry of the cycloaddition product would be largely determined by the favored stereochemical arrangement in the 1 2 complex. An isolated and structurally characterized intermediate which is cited for support is the Ir complex 5, shown in Scheme 4, formed from [(COD)IrCl]2 and NBD followed by metathesis with 2,5-pentane-dionate [17]. 

To gain insight into this remarkably efficient cyclization reaction (77—>76), and to determine the extent to which existing stereogenic centers preorganize diene precursor 77, we examined the catalytic RCM of 79 and 82. When 79 was subjected to 25 mol% 2 (50°C, 18 h), <2% 80 was formed (Scheme 20). Instead, dimer 81 was obtained in 52% yield (3 1 mixture of olefin isomers identity of major product not determined). When diene 82 was treated with identical conditions, macrolactam 83 was obtained in 41% isolated yield [33] along with 20% of the... 

If carboxylates are subjected to Kolbe electrolysis in the presence of olefins, the generated Kolbe radicals add to the double bonds to afford mainly additive dimers (Table 8, entries 10-17). 

Miller[72] has described a process for the production of hydrocarbons in the lube base stock range. The process involves the use of a Ni-ZSM5 zeolite as catalyst for dimerization of C5-C11 olefins into a first product containing C10-C22 olefins. This first product is subjected to an additional dimerization step, using the same or similar catalyst giving a second product that includes hydrocarbons in the lube base stock range. 

According to the reaction mechanism in Fig. 6, olefins coordinate axially to the dinuclear Pt111 complexes. Whether olefins actually coordinate to Pt111 is the subject of further research. Pt11 is known to coordinate various olefins, whereas PtIV does not coordinate any of them. Therefore, an attempt was made to isolate the olefin -complex of the Ptm dimer in order to prove the proposed mechanisms in Fig. 6. While no olefin -complex was obtained despite our intensive efforts, pent-4-en-l-ol and ethylene glycol vinyl ether... 

The bridging chloride ligands in these [Ir(olefin)2Cl]2 compounds are susceptible to metathesis reactions, yielding new dimeric compounds of the form [Ir(olefin)2B]2 where B represents a new bridging ligand. AUcoxides, thiolates, and carboxylates have all been employed successfully in the replacement of chloride. The complexes with B = Br, I have also been prepared, both by metathesis reactions and by direct reaction of cyclooctene or cyclooctadiene with IrBrs or Iris The olefin complexes also provide excellent starting materials for the syntheses of arene and cyclopentadienyl iridium complexes, a subject that will be discussed in the next section. 

Although a variety of platinum(II)-ethylene complexes have been described (268), those most frequently encountered are Zeise s salt, K[(C2H4)PtCls] H20, and Zeise s dimer, [(C2H4)PtCl2]2- These have been employed for preparation of many other platinum(II)-olefin complexes and have been the subject of much research. 

A more detailed consideration of the Woodward-Hofimann postu-ulates for olefinic systems in the presence of a transition metal indicates that the thermally forbidden dimerization of two ethylene molecules to cyclobutane becomes allowed if the orbitals of the olefins can interact symmetrically with the dxt and dyz orbitals of the transition metal catalyst (53). One would consequently also expect transition metal complexes to catalyze the conversion of quadricyclene (IV) back to norbornadiene. This has been reported to be the case (54). The reactions leading to the formation of VI, XXX, and XXXI are examples of processes in which thermally allowed sigmatropic reactions become subject to catalysis by transition metal complexes. The catalysts thus display the dual role of removing symmetry restrictions and of generally lowering activation energies. 

If carboxylates are subjected to Kolbe electrolysis in the presence of olefins, the generated radicals add to the double bonds to afford mainly additive dimers (Table 8, entries 12-20). In vicinal disubstituted styrenes, upon addition of the Kolbe radical Me02CCH2, the yields of adducts decrease with increasing size of the /f-substituent H = 42%, Me = 27%, Et = 11%, /Pr = 5%, tBu = 2% [125]. The ratio of additive dimer 87 (Eq. 11) to monomer 89 can be changed to some extent by the current density i. Upon electrolysis of trifluoroacetate in MeCN-H20-(Pt) in an undivided cell in the presence of electron-deficient olefins, additive dimers and additive monomers are obtained. The selectivity can be controlled by current density, temperature and the substitution pattern of the olefin [126]. Trifluoromethylation of various aromatic compounds with -M substituents has been achieved in satisfactory yield via electrolysis of pyridinium trifluoroacetate in acetonitrile [127]. 

Interestingly, in the absence of external olefins as cycloaddition partners, indoles appear to undergo photodimerization upon being subjected to ultraviolet light irradiation. Both head-to-head (33, major) and head-to-tail (34, minor) dimerization products have been observed [24] (Scheme 7). 

Cyclopentadiene [542-92-7] (CPD), CsHe, (1), and its more stable dimer, dicy-clopentadiene [77-73-6] (DCPD), C10H12, (2), are the major constituents of hydrocarbon resins, cyclic olefin polymers, and a host of specialty chemicals. They can be transformed into many chemical intermediates used in the production of pharmaceuticals, pesticides, perfumes, flame retardants, and antioxidants. Because of their wide industrial uses, their chemistry has been extensively investigated and documented. Numerous reviews (1-12) have been published on the subject. The production processes and industrial uses of CPD and DCPD are summarized in Reference 13. In addition to the classical organic reactions, CPD forms organic metallic complexes, ferrocene, with transition metals (14). Some of these complexes have been established as excellent olefin polymerization catalysts. Several reviews have been published on this rapid growing field (15-19) (see Single-Site Catalysts). 

Deactivation. One of the factors that complicates the quantification of active-site concentration (135) is the fact that metallocene cations are subject to equilibria between catalytically active and inactive forms. In situations in which intramolecular coordination of an arene group can occur, this process competes with monomer coordination in styrene (136) and possibly olefin polymerization. Another dormant state invoked to explain catalyst decay is the dimeric structure [Cp2Zr(CH3)(/u.-CH3)Zr(CH3)Cp2]+ in which a methyl group bridges two metallocene fragments. This has been characterized by NMR for the reaction of Cp2Zr( CH3)2 with MAO and other cocatalysts (136). 

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