Dimerizations regioselective

The dimerization of isoprene is possible, but the reaction of isoprene is slower than that of butadiene. Dimerization or telomerization of isoprene, if carried out regioselectively to give a tail-to-liead dimer 18 or a head-to-tail... 

The reaction of isoprene with MeOH catalyzed by Pd(acac)2 and Ph3P is not regioselective, giving a mixture of isomers[37]. However, l-methoxy-2,6-dimethyl-2,7-octadiene (35), the head-to-tail dimer, was obtained in 80% yield, accompanied by the tail-to-tail dimer (15%) using 7r-allylpalladium chloride and BU3P. On heating, 35 was converted into 2.6-dimethyl-1,3,7-octatriene (36) by an elimination reaction[38]. 

To be applied industrially, performances must be superior to those of existing catalytic systems (activity, regioselectivity, and recyclability). The use of ionic liquid biphasic technology for nickel-catalyzed olefin dimerization proved to be successful. 

Monosubstituted ketenes dimerize into 1,3-cyclobutandiones. The regioselectivity is believed to be determeined by the steric repulsions of the substituents. Catalysts change the regioselectivity. 

However, with 1-hexyne or phenylacetylene, the thorium catalyst induces a dramatic inversion in regioselectivity giving imines with various amounts of dimerized alkyne (e.g., Eq. 4.84) [301],... 

FIGURE 6.27 Regioselectivity of spiro-pyrano link formation upon dimerization of the o-QMs derived from twin-tocopherol 33. 

Encapsulated Cu—chlorophthalocyanines oxidize hexane at C-l using 02 and at C-2 using H202 as oxidants. The dimeric structure of copper acetate is intact when it is incorporated into the zeolite. This is a regioselective aromatic hydroxylation catalyst, which mimics the specificity of the monooxygenase enzyme tyrosinase.82,89 Zeolite NaY catalysts made with a tetranuclear Cu(II) complex were synthesized and characterized.90... 

Grignon-Dubois and coworkers have shown that reduction of a quinoline using zinc and acetic acid in THF gives the dimeric compound 3-177 via intermediate 3-176 (Scheme 3.47) [70J. Usually, a mixture of the syn- and anti-products is formed the substituent has some influence on the regioselectivity of the dimerization and cydization step. With R = H and R = 6-Me, only the benzazepine 3-177 were produced, by a head-to-head dimerization. 

Such reactions are initiated by a single electron transfer from the dialkylzinc moiety to the diazabutadiene ligand to afford radical pairs 109, which collapse by the regioselective transfer of the organic radical to the nitrogen 110 or carbon 111 atoms of the diazabutadiene moiety. If the alkyl radical escapes from the organozinc radical, the latter dimerizes reversibly to 112 via the formation of a very long yet credible carbon-carbon bond. 

Sensitized cross-dimerizations form a second group with a few examples shown in Eqs. 24—27. Most examples involve electron-deficient olefins as one addend. The reactions are again highly regioselective... 

Triplet photoaddition of simple non-cyclic monoolefins is unknown. The sensitized dimerization of ethyl vinyl ether gives exclusively head-to-head adducts, Eq. 21, and probably should not be classed as an example of simple acyclic olefin. Usually the triplets have high energies and are severly twisted. 55> Some cyclic rigid molecules, Eq. 20, that do dimerize 63> do not incorporate substituents that allow regioselectivity to be determined. Butadiene gives principally head-to-head dimerization, Eq. 19, concordant with the PMO prediction, and so does indene, Eq. 22. The anti dimer that is formed would not be expected from a singlet excimer reaction. 

The product of the dimerization of an unsymmetrical substituted olefin, such as propene, is dependent on the direction of coupling of the two olefinic units in general, a mixture of isomers is obtained. In order to obtain a high yield of a particular dimer, the control of the regioselectivity of the individual addition steps in the catalytic cycle or even the reversal of the direction of addition in consecutive steps is required (Section IV,E). 

The nature and distribution of the products of the dimerization or oligomerization of unsymmetrical olefins, such as propene, will depend, among others, on the direction of addition of the hydrido- and alkylnickel species to the olefin, i.e., on the regioselectivity of the catalyst (see Section 1 V,E). In order to define the direction of addition of hydrido- or alkylnickel species to terminal olefins, we shall adopt the convention nickel-to-C, addition [Eq. (12)] (Ni —> Cj) ... 

The regioselectivity in the dimerization of allyl radicals has been studied by a variety of methods. One of the earliest investigations into this field employed the Kolbe electrolysis... 

Notes ak (dimer)/fcobs (monomer) where "dimer" is the dinuclear complex and monomer is a related monomer complex. bSeveral substrates were measured. cThe effect of monomer complex is too small to be detected. dlmportant regioselectivity was detected. eSelf-hydrolysis. 

Formation of a titanacyclopentane via cyclic dimerization of ethylene was reported as early as 197644 (Scheme 17). In marked contrast with the corresponding Zr-promoted reactions discussed later in this chapter, the intramolecular version of the Ti-promoted alkene-alkene coupling reactions does not appear to have been well developed. Consequently, detailed aspects of pair -selectivity and regioselectivity still remain largely unknown. 

If the cycloaddition and cycloreversion steps occurred under the same conditions, an equilibrium would establish and a mixture of reactant and product olefins be obtained, which is a severe limitation to its synthetic use. In many cases, however, the two steps can very well be separated, with the cycloreversion under totally different conditions often showing pronounced regioselectivity, e.g. for thermodynamic reasons (product vs. reactant stability), and this type of olefin metathesis has been successfully applied to organic synthesis. In fact, this aspect of the synthetic application of four-membered ring compounds has recently aroused considerable attention, as it leads the way to their transformation into other useful intermediates. For example aza[18]annulene (371) could be synthesized utilizing a sequence of [2 + 2] cycloaddition and cycloreversion. (369), one of the dimers obtained from cyclooctatetraene upon heating to 100 °C, was transformed by carbethoxycarbene addition to two tetracyclic carboxylates, which subsequently lead to the isomeric azides (368) and (370). Upon direct photolysis of these, (371) was obtained in 25 and 28% yield, respectively 127). Aza[14]annulene could be synthesized in a similar fashion I28). 

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