Dienes trimerization

Conjugated dienes can be dimerized or trimerized at their 1,4 positions (formally, [4 4- 4] and [4 4-4 4-4] cycloadditions) by treatment with certain complexes or other transition metal compounds. " Thus butadiene gives 1,5-cyclooctadiene and 1,5,9-cyclododecatriene. " The relative amount of each product can be controlled by use of the proper catalyst. For example, Ni P(OC6H4—o-Ph)3 gives predominant dimerization, while Ni(cyclooctadiene)2 gives mostly trimerization. The products arise, not by direct 1,4 to 1,4 attack, but by stepwise mechanisms involving metal-alkene complexes. " ... 

In the presence of catalytic amounts of Bp3.0Et2 aromatic aldehydes such as benzaldehyde are converted by bis(trimethylsilyl)selenide 604 into hexamethyldi-siloxane 7 and the corresponding trimers, for example 611 in up to 90% yield. On heating with 1,3-dienes such as 2,3-dimethylbutadiene trimers such as 611 react to give the Diels-Alder product 612 [155] (Scheme 5.49). 

A further hetero-Diels-Alder reaction with inverse electron demand between o-QM 3 as the dienophile and either of the two diastereomers of spiro dimer 9 as the diene provided the spiro trimers 31 and 32 (Fig. 6.25). The absolute configuration was derived from NMR experiments. It was moreover shown that only two of the four possible stereoisomeric trimers were formed in the hetero-Diels-Alder reaction the attack of the o-QM 3 occurred only from the side syn to the spiro ring oxygen.28... 

Recently, catalytic asymmetric Diels-Alder reactions have been investigated. Yamamoto reported a Bronsted-acid-assistcd chiral (BLA) Lewis acid, prepared from (R)-3-(2-hydroxy-3-phcnylphenyl)-2,2 -dihydroxy-1,1 -binaphthyl and 3,5A(trifluoromethy I) - be nzeneboronic acid, that is effective in catalyzing the enantioselective Diels-Alder reaction between a,(3-enals and various dienes.62 The interesting aspect is the role of water, THF, and MS 4A in the preparation of the catalyst (Eq. 12.19). To prevent the trimerization of the boronic acid during the preparation of the catalyst, the chiral triol and the boronic acid were mixed under aqueous conditions and then dried. Using the catalyst prepared in this manner, a 99% ee was obtained in the Diels-Alder reaction... 

Tetrazine (171) and its derivatives are electron-deficient cycloaddends, which undergo [4 + 2] cycloadditions with inverse electron demand. When bicyclopropylidene (3) was added to a dichloromethane solution of 171, its red color disappeared within 1.5 h at room temperature. The white crystalline product isolated in 86% yield turned out to be a mixture of at least two stereoisomeric compounds 174, trimers of the 8,9-diazadispiro[2.0.2.4]deca-7,9-diene (173) evidently formed via the normal [4 + 2]-cycloadduct 172 after nitrogen extrusion (Scheme 25) [13b]. 

The products of electrochemical oxidation of conjugated dienes are considerably affected by the reaction conditions such as the material of the electrode, the supporting electrolyte and the solvent. The oxidation of butadiene with a graphite or carbon-cloth anode in 0.5 M methanolic solution of NaClCU mainly yields dimerized products along with small amounts of monomeric and trimeric compounds (equation 5)1. The use of platinum or glassy carbon mainly gives monomeric products. Other dienes such as isoprene, 1,3-cyclohexadiene, 2,4-hexadiene, 1,3-pentadiene and 2,3-dimethyl-l,3-butadiene yield complex mixtures of isomers of monomeric, dimeric and trimeric compounds, in which the dimeric products are the main products. 

The nickel-catalyzed [4 + 4]-cycloaddition of butadiene to form cyclooctadiene was first reported by Reed in 1954.90 Pioneering mechanistic and synthetic studies largely derived from the Wilke group advanced this process to an industrially important route to cyclodimers, trimers, and other molecules of interest.91-94,943 95,96 While successful with simple dienes, this process is not useful thus far with substitutionally complex dienes as needed in complex molecule synthesis. In 1986, Wender and Ihle reported the first intramolecular nickel-catalyzed [4 + 4]-reaction of... 

Another potential use of homogeneous hydrogenation catalysts is the hydrogenation of dienes and trienes to monoolefins, where they display high specificity. Such an example is the conversion of the easily available butadiene dimers and trimers to polymer intermediates6. 

But we succeeded in co-trimerizing butadiene and 2-mono- or 2,3-diaza-dienes to twelve-membered heteroring systems (Fig. 2 in Scheme 2,3-4). Alkylated C=N double bonds react like ethylene with butadiene to 1 2 adducts (Fig. 3, Scheme 2.3-4). Hydrogen-transfer reactions, as mentioned before, are favored in this ca over N-C-bond-forming reactions, so that open-chain instead of heterocyclic products are formed. 

Homogeneous nickel complexes proved to be versatile catalysts in dimerization and trimerization of dienes to yield different oligomeric products.46-55 Depending on the actual catalyst structure, nickel catalyzes the dimerization of 1,3-butadiene to yield isomeric octatrienes, and the cyclodimerization and cyclotrimerization to give 1,5-cyclooctadiene and all-trans-l,5,9-cyclododecatriene, respectively46 56 [Eq. (13.13)]. Ziegler-type complexes may be used to form cis,trans,trans-1,5,9-cyclododecatriene37,57 58 [Eq. (13.14)], which is an industrial intermediate ... 

Conversion of the trimer (80) to the seven-membered ring system (81) occurs readily with cesium fluoride [3,75], whereas in the presence of TAS fluoride, the di-anion (82) is trapped. These observations lead to the most likely mechanism for rearrangement as that in Scheme 37 [3]. The cyclisation step shown in Scheme 37 is made more easily accepted by the fact that the diene (83) (Scheme 38), undergoes rapid rearrangement in the presence of fluoride ion, giving the cyclic system (86) [77]. The ready cyclisation of a crowded anion (84) to give what appears to be a sterically unfavourable intermediate (85) would not be easily predictable ... 

Besides other homo and hetero dimers and trimers of 6, the trapping products 11 and 12 were obtained, with the conjugated diene 12 presumed to be a secondary product formed by thermal equilibration of the [2+2] cydoadduct 11 of 7 and 10. 

The products are normally the cyclodimers, 4-vinylcyclohexene and cycloocta-1,5-diene, together with some trimer, cyclododeca-l,5,9-triene and possibly a small amount of divinylcyclobutane depending on the conditions and the particular catalyst, i.e.,... 

Butadiene could also be trimerized to give cyclododecatriene. The trimer is again used by Hulls to manufacture nylon 12 and Vestamid . The codimerization of butadiene and ethylene is used by DuPont to manufacture 1,4-hexadiene, one of the monomers of EPDM (ethylene, propylene, diene, monomers) rubber. The role of the diene monomer in EPDM rubber is to provide with two double bonds of different reactivities. The more reactive, terminal double bond takes part in the polymerization with ethylene and propylene. The less reactive internal one is used later on for cross-linking. These important catalytic reactions are shown in Fig. 7.6. 

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