Dimerization of 1,3-dienes

Chloroaluminate-free ionic liquids have been used for dimerization of dienes, as these ionic liquids are more stable and easier to handle than the moisture-sensitive ionic liquid chloroaluminate(III). In one example, a mixture of [BMIM]BF4/water (1 1 v/v) was used as the medium in the hydrodimerization of 1,3-butadiene catalyzed by [BMIM]2 PdCl4] (237). In addition to the dimer, 1,3,6-octatriene and 2,7-octadienol were produced. However, by using PdCl2/PPh3 as a catalyst in [BMIMJPC] (X = BF4, PFe, CF3SO3), the dimer, 1,3,6-octatriene, was obtained exclusively (238). 

Thus, the observations that (a) dienes quench the photoreduction reaction (b) the isomerization and dimerization of dienes is sensitized by the it,n carbonyl triplet and (c) there is a lack of photocycloaddition products with dienes, taken in conjunction with the relative energy levels of carbonyl compounds and dienes, form a consistent picture. 

Tetramerization of alkynes 5-52 Dimerization of dienes 5-53 Addition of two alkyl groups to an alkyne... 

Some homogeneously catalyzed telomerizations, i.e., dimerizations of dienes coupled with the addition of a nucleophile [Eq. (11)], have been carried out in two-phase systems. One example has found industrial application, the synthesis of 1,7-octadienol from butadiene and water (Section VI). 

In this context two additional aspects should be noted. First, our high-pressure experiments [64] provided detailed information about the PET dimerization of 17a in nonpolar solvents which confirm the triplex mechanism (cf. Sect. 4 and Ref. [65]). Second, analogeous experiments with triplet sensitizers indicate the existence of a Marcus Inverted Region for triplet energy transfer. Third, the mechanism described here for the PET dimerization of dienes resembles that by Mattes and Farid [66] for the dimerization of 1,1-diphenylethene. This latter example will be discussed in more detail in Sect. 3.2 of this article. 

Polymerization and dimerization of dienes are competing reactions that occur simultaneously during the course of the polymerization. The dimerization of BD occurs by a Diels-Alder reaction which results in the formation of vinylcyclohexene (VCH), a non-conjugated diene (Scheme 14). 

SCHEME 4.3 Observed products associated with the dimerization of dienes 8a-d. 

The use of dienes such as piperylene in the quenching of triplet states of carbonyl compounds is well known. However, in many photochemical studies, attempts to quench the carbonyl Ti state are often complicated by the formation of oxetane photoproducts.Since the photosensitized dimerization of diene triplets is well known, synthetic applications utilizing dienes in the Patemo-BUchi photocycload dition are rather limited. However, the diene addend not only allows for rapid assemblage of functionality but remains a continuing challenge in terms of experimental efficiency and generality. 

Synthetic application of Paterno-Bilchi reaction of simple dienes with carbonyl compounds is rare. While seemingly an extension of the photocycloaddition of olefins and carbonyl compounds, the reaction between dienes and carbonyls is often complicated by the fact that triplet excited states of carbonyl compounds are quenched by dienes, although the formation of oxetanes can be observed during these reactions. Recall also that the photosensitized dimerization of diene triplet excited states is also a well known reaction vide infra), these two observations would seem to naturally limit the synthetic potential of this process. Kubota and coworkers found that irradiation of propanal in the presence of 1,3-cyclohexadiene produced oxetanes 164a and 164b in a 4 1 ratio (Scheme 37). ... 

Dimerization of dienes. The reagent converts 1,3-cyclooctadiene into the dimer (3).2 It dimerizes isoprene to a mixture of 2,6-dimethyl-2,6-octadiene (4) and 2,7-dimethyl-2,6-octadiene (5).3... 

The ability of the nitrosyl ligand to behave as an electron pair reservoir has also been considered to play an important part in certain catalytically active systems. The vacant site provided by isomerization of the ligand could enable an unsaturated organic molecule to enter the transition metal s coordination sphere, thus forming an active intermediate. Examples of catalysis by nitrosyl complexes include the hydrogenation of alkenes by Rh(NO)L3 species and the dimerization of dienes in the presence of Fe(CO)2(NO)2 or Fe(n-C3Hs)(CO)2NO. Certain molybdenum dinitrosyl complexes, such as MoCljfNOljfPPhjlj, have also been found to provide very efficient alkene dismutation catalysts. ... 

As an example, this apply to enols or tautomeric enols such as maleic acid derivatives. While with a chemical reagent (cerium ammonium nitrate) the only process occurring is oxidative dimerization, when aromatic nitriles are used as the photochemical oxidant, selective trapping of the radicals by an electrophilic alkenes or by the nitrile itself occurs. Under these conditions, both the alkylation of alkenes and the oxidative alkylation/dimerization of dienes have been smoothly obtained (see Scheme 8) and side processes such as double alkylation or polymerization often occurring with other methods have been avoided. A three-component (Nucleophile-Olefin Combination, Aromatic Substitution) process is also possible. ... 

Bis(1,5-CyclOOCtadiene)nickel(0). This complex can be used for the oligomerization of acetylene and the dimerization of dienes such as butadiene. It is prepared by the reaction of nickel(II) (2,4-pentanedionate) and triethylalu-minum with cyclooctadiene (7). 

The concerted mechanism, in which the two new bonds form synchronously (Fig. 7), is probably less common than generally assumed. A concerted non-synchronous mechanism can involve diradicals or zwitterions, which means more or less dissymmetry, geometrical and/or electronic, in the bond formation, which can be increased by the presence of catalysts, such as Lewis acids, especially lithium salts,26 or solvent effects.27 Ionization of one of the reactants (Fig. 8), frequently the dienophile, is efficient in promoting cycloadditions with unreactive reagents, e.g., the [4+2] dimerization of dienes, by a selective transformation to the reactive radical cations ("hole" catalysis). ... 

The unsensitized dimerization of dienes has not been investigated extensively. It is significant, however, that preliminary results indicate that among the dimers formed upon irradiation of butadiene is a compound which is not obtained by the sensitized (triplet state) route, Eq. (81), (Turro et al., rmpublished results). 

An alternative procedure for the cyclization is oxidative/reductive dimerization of dienes on low-valent metal centers. It is followed by the cleavage of the formed metal-carbon bonds to release the product and the catalytically active metal species that again joins the catalytic cycle. There are several possible reaction pathways to achieve this process and they depend on the nature of the transition metal. 

Independently, from product analyses of triplet-sensitized dimerization of dienes, Hammond and Liu recognized the absence of interconversion of conformers in the triplet state. Thereby, the term isomeric triplets was coined. ... 

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