Diene dimerization

The major products from the photosensitized dimerization of isoprene predicted. The ratio of diene dimers vary as the sensitizer is varied or not ... 

Photosensitized dienes dimerize to give 2 + 2 and 2 + 4 addition products. For example, triplet butadiene yields the three products in Equation 13.60,... 

The photochemistry of ir-allylpalladium complexes has been studied to a limited extent. Two basic reactions have been observed. Irradiation at 366 nm of ir-allylpalladium complexes produced 1,5-diene dimers, reportedly via a radical coupling mechanism.334 333 Similar irradiations in the presence of species capable of trapping the presumed allyl radical intermediate, such as BrCCb, BrCH2Ph or allyl bromide, now yield alkylated and halogenated allyls, in addition to 1,5-diene dimer. This reaction fails for simple alkyl or aryl halides due to the instability of the associated radical (equations 130 and 131 ).336... 

The hydrocarboxylation of conjugated dienes such as butadiene can yield monocarboxylate, dicarbox-ylate or diene dimerized carboxylated products. The carboxylation reaction is important because it is a potential route to adipic acid. 

Sensitized irradiation of dienes with 1-acetoxy acrylonitrile 29 also gave a sensitizer-dependent product mixture of [4+2] 30 and [2+2] 31 (Sch. 7), with a good correlation with the results of Hammond for 1,3-diene dimerization (Sch. 4), and consistent with triplet sensitization of the diene [33-35]. 

These diene dimerizations might start with the formation of cationic Ru(IV) bis(jt-allyl) complexes rather than ruthenacyclopentanes such as 137 (Scheme 4.49). This was confirmed by the stoichiometric reaction of the 1,3-pentadiene complex... 

Catalyst Adducts Solvent Temp. 1°C] (time [h]) Yield [%] (based on butadiene) Lactones Esters Buta- Ref. diene dimers ... 

Unlike nickel catalysts which form cyclic dimers and trimers (1,5-cyclooctadiene and 1,5,9-cyclododecatriene), palladium compounds catalyze linear dimerization of conjugated dienes. 1,3-Butadiene itself is converted to 1,3,7-octatriene. The reaction most characteristic of palladium is the formation of various telomers. 1,3-Buta-diene dimerizes with incorporation of various nucleophiles to form telomers of the following type ... 

The volatile diene dimerizes readily and is prepared as required by depolymerization of technical dicyclopentadiene (Enjay, high purity, Matheson, purity 85% b.p. 170°, sp. gr. 0.98). The depolymerization is done by heating the dimer carefully under a fractionating column by heating the dimer under a partial condenser containing methylene chloride or by adding the dimer at a suitable rate to mineral oil maintained at 240-270°. ... 

We are going to use a diene dimerization to add more detail to our explanation of the formation of endo products. To make matters even easier we shall look at the dimerization of a cyclic diene—we might almost say the cyclic diene—cyclopentadiene. We introduced the preference for endo products on p. 885 by saying there was a favourable electronic interaction between the conjugating group on the dienophile and the back of the diene. 

Simple 1,3-dienes such as 1,3-butadiene, isoprene, and related compounds undergo efficient metal-catalyzed oligomerization. Under palladium catalysis, diene dimerization is the most common oligomerization reaction observed. Four modes of dimerization have been reported (Scheme 1) (i) [2 + 2] cycloaddition to afford 1,2-divinylcyclobutane (1) (ii) [4 -I- 2] cycloaddition to afford 4-vinylcyclohexene (2) (iii) [4 + 4] cycloaddition to afford 1,4-cyclooctadiene (3) and (iv) linear dimerization to afford 1,3,7-octatriene (4). 

Most of the work on the mechanism of Pd-catalyzed linear diene dimerization has been done on the variant involving dimerization with trapping by a protic trapping reagent H—Y, and a particularly interesting series of papers has been published by Jolly and co-work-ers [io]-[i4] jjj (jjggg gjjj related papers, structural and spectroscopic data are presented that support the catalytic cycle shown in Scheme While the JoUy catalytic cycle is not... 

Recent advances in each of the four Pd-catalyzed linear diene dimerization modes (Scheme 2 dimerization without trapping dimerization with incorporation of protic H—Y trapping reagents dimerization with incorporation of silanes or disilanes and... 

---