Regioselective dimerization

Dimerization of isoprene, if carried out regioselectively to give the head-to-tail dimer 97, would be useful for terpene synthesis. So far few reports on successful regioselective dimerization of isoprene have been given, and this remains an unsolved problem. 2,6-Dimethyl-1,3,6-octatriene (97), the head-to-tail dimer, can be prepared with high selectivity using Zr [36], Ni [37,38] and Pd [39] catalysts. However, selective functionalization of this dimer to the terpene alcohol 98 is not easy. 

Recently, it was shown that the metathesis catalyst RuCl2(PCy3)2(=CHPh), where Cy is cyclohexyl, reacted in refluxing toluene with phenylacetylene to produce a ruthenium vinylidene species which promoted the regioselective dimerization of phenylacetylene into ( )-1,4-diphenylbutenyne [56]. The addition of 1 Eq acetic acid did not lead to enol esters but to a faster reaction and the stereoselective dimerization of phenylacetylene into the Z dimer. 

The enone (130), the pesticide Coumaphos, undergoes regioselective dimerization on irradiation at wavelengths > 313 nm. The product was identified as the dimer (131). Irradiation of the furocoumarin, Imperatorin (132), in methanol affords a low yield of the (2t2)-dimer (133). This product is accompanied by the dealkylated compound (134) and the rearrangement product (135). This last compound is formed by a photo-Claisen rearrangement. The influence of solvent was studied and the dimer was obtained only in methanol. The other two products were formed in a variety of polar and non polar solvents. In any solvent the dealkylated compound (135. was the main product. ... 

Regioselective dimerization of propene to 2,3-dimethylbutenes (DMBs) is currently operated by Sumitomo and BP Chemicals. Both use P(cyclohexyl)3 as the bulky ligand. In the Sumitomo process [7] very high selectivities in DMBs (up to 85 %) are obtained at 20-50 °C, thanks to a sophisticated, highly efficient, Ziegler-type catalyst system (ten times more efficient than those of conventional catalysts) and by using toluene as a solvent. Isomerization of 2,3-dimethyl-1-butene (DMB-1) into 2,3-dimethyl-2-butene (DMB-2) takes place directly in... 

Regioselective dimerization of propane to 2,3-dimethylbutenes can be performed in a biphasic system using liquid acidic chloroaluminates as the solvent for the nickel-phosphine catalytic system [19]. 

The cationic nickel complex [ /3-allylNi(PR3)]+, already described by Wilke etal. [21], as an efficient catalyst precursor for alkene dimerization when dissolved in chlorinated organic solvents. It proved to be very active in acidic chloroaluminate ionic liquids. In spite of the strong potential Lewis acidity of the medium, a similar phosphine effect is observed. Biphasic regioselective dimerization of propylene into 2,3-dimethylbutenes can then be achieved in chloroaluminates. However, there is a competition for the phosphine between the soft nickel complex and the hard aluminum chloride coming from the dissociation of polynuclear chloroaluminate anions. Aromatic hydrocarbons, when added to the system, can act as competitive bases thus preventing the de-coordination of phosphine ligand from the nickel complex [22 b]. Performed in a continuous way, in IFP pilot plant facilities, dimerization of propene and/or butenes with this biphasic system (Difasol process) compares... 

Many nickel-based systems catalyze the dimerization of olefins. In 1966, Wilke [5] demonstrated that the well-defined cationic ri -allylnickel complex (1) catalyzes the regioselective dimerization of propene in organic chlorinated solvent. The AICI4... 

Metzger and coworkers have studied reactive intermediates of chemical reactions in solution by using a microreactor coupled to an ESI mass spectrometer. The highly stereo- and regioselective dimerization of trows-anethole to give the head-to-head trans, anti, trans-cydobutane initiated by aminium salt proceeds by a radical cation chain mechanism (Scheme 4.4) and this method was further used to study the transient radical cations intermediates in electron transfer-initiated D-A reactions [12-14]. 

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. 

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