Dimerization of acrylates

A variety of substrates have been catalytically hydrogenated at room temperature and 1 -atm. hydrogen pressure by pentacyanocobaltate(ll) anion. Conjugation is required for the reduction of C=C bonds The effects of detailed molecular structure on reducibility and of cyanide-cobalt ratio on mode of reduction have been noted Poisoning and reactivation of the catalyst as well as the effect of alkali are described, and mechanisms are tentatively proposed for these phenomena It is concluded that the aging reaction of pentacyanocobaltate(ll) is reversible A dimerization of acrylic acids at elevated temperatures was found ... 

One of the oldest ruthenium-catalyzed C=C bond coupling reactions deals with the selective dimerization of functionalized alkenes, especially the dimerization of acrylates [ 1,2]. It usually involves either an initial hydrometallation process, oxidative coupling, or vinyl C-H bond activation (Scheme 1). 

Recently, a selective head-to-tail dimerization of acrylic or a,/J-unsaturated carbonyl compounds was performed with Cp RuH3(PCy3) catalyst, where Cp is pentamethylcyclopentadienyl and Cy is cyclohexyl, and was expected to occur via hydrometallation [6] (Eq. 3). 

The dimerization of acrylonitrile is a cheaper route to the synthesis of highly valuable hexamethylenediamine, which is one component of the starting materials for nylon-6,6 [ 16,35] In some cases of the dimerizations of acrylic acid... 

Dimerization of acrylates.2 Acrylates and methacrylates undergo tail-to-tail dimerization at moderate temperatures in the presence of this catalyst. Added lithium tet-rafluoroborate increases the rate and prolongs catalyst life. This additive can be omitted in dimerizations conducted in nitromethane. 

The bottoms from the foreruns column are fed to the product column where the glacial acrylic acid of commerce is taken overhead Bottoms from the product column are stripped to recover acrylic acid values and the high boilers are burned The principal losses of acrylic acid in this process are to the aqueous raf finate and to the aqueous layer from the dehydration column and to dimerization of acrylic acid to 3-acryloxypropionic acid If necessary, the product column bottoms stripper may include provision for a short-contact-time cracker to crack tliis dimer back to acrylic acid (60). 

Dimerization of acrylonitrile is a cheaper way to synthesize highly valuable hexamethylenediamine [30]. In some cases of dimerization of acrylic acid esters, acrylonitriles, and acroleins, the direct C-H bond cleavage step seems to be involved in the catalytic reaction. At an early stage of catalytic dimerization of acrylonitrile, czs-l,4-dicyanobut-l-ene is formed as the major product, not trans-isomer [31]. This high czs-selectivity is suggested to indicate selective cleavage of C-H cis to CN by the metal coordinated to the nitrile group in side-on fashion. 

Catalysts of this type have been studied extensively for the polymerization of butadiene [32], the oligomerization of styrene [33], and the dimerization of acrylates [34]. Indeed it was my becoming aware of this work that led to all of our subsequent discoveries. 

Scheme 10.17. Baylis-Hillman dimerization of acrylic nitriles.

At approximately 20°C, the rate of formation of this acyloxypropionic acid is said to be negligible. The problem appears to be acute primarily during out-of-doors storage of acrylic acid in the summer months. Acryloxy-propionic acid does not seem to inhibit or interfere otherwise with the polymerization of acrylic acid, but, of course, its presence represents a variable impurity in the monomer. There appears to be no method of inhibiting the dimerization of acrylic acid. 

The dimerization of functionalized olefins by transition metal catalysts is a well studied reaction. A first review was given by Lefebvre and Chauvin in 1970 [40]. In 1974 Hidai and Misono described in detail the dimerization of acrylic compounds [41]. These starting olefins have found considerable attention from the industrial point of view, because they can be produced at high quantities and at low costs by the petrochemical industry. For instance, acrylonitrile, acrylates and allyl esters are favorable starting molecules. 

Also the dimerization of acrylates has found great interest in munerous academic and industrial research groups. Alderson and co-workers of Du Pont [48] investigated the dimerization of methyl acrylate in the presence of rhodium chloride. By heating a solution of methyl acrylate in methanol to 140 °C some dimethyl 2-hexenedioate (= dimethyl-a-dihydromuconate) was formed (Equation 42). 

Catalytic C-C bond-forming reactions are often catalyzed by iridium hydrides. As an example, the head-to-tail dimerization of acrylates and vinyl ketones is promoted by [IrCl(cod)]2 in Bu"OH in the presence of small amounts of NaaCOs and P(OMe)3. The alcohol and the carbonate serve to generate in situ an iridium hydride species which is thought to be the catalyst capable of dimerizing acrylates. ... 

Scheme 18.30 Dimerization of acrylates, according to Matsuoka and coworkers and Glorius...

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