Head-to-Tail Dimerization of Acrylates

The dimerization of functional alkenes such as acrylates and acrylonitrile represents an attractive route to obtain bifunctional compounds such as dicarboxylates and diamine, respectively. The head-to-tail dimerizahon of acrylates and vinyl ketones was catalyzed by an iridium hydride complex generated in situ from [IrCl(cod)]2 and alcohols in the presence of P(OMe)3 and Na2C03 [26]. The reaction of butyl acrylate 51 in the presence of [IrCl(cod)]2 in 1-butanol led to a head-to-tail dimer, 2-methyl-2-pentenedioic acid dibutyl ester (53%), along with butyl propionate (35%) which is formed by hydrogen transfer from 1-butanol. In order to avoid 

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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). 

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. ... 

The reaction may proceed as follows (see Scheme 10.4). An acrylate 51 coordinates to the iridium-hydride complex generated in situ, and then inserts into the Ir-H bond to form a o-lr complex 55 the coordination and insertion of another acrylate to 55 leads to an iridium complex, 56. A (5-hydride elimina-hon of the iridium-hydride from the intermediate 56, followed by isomerization of the double bond to a more stable internal aUcene, results in a head-to-tail dimer. 

The hydrodimerization of acrylic acid to adipic acid is a tail-to-tail addition hydrodimerization of methacrylic acid and crotonic acid produces mainly the head-to-head and head-to-tail dimers [48]. 

Tandem GM-RGM (see Scheme 20) can be employed for the synthesis of complex ring systems from simple starting materials. A very noteworthy example of this reaction serves as the cornerstone for the total synthethis of the natural product cyclindrocyclophane. The GM-RGM reaction of diene 182 was completely selective for formation of the head-to-tail isomer 183. The selectivity was attributed to thermodynamic control the head-to-head dimer 184 is considerably less stable. In an effort to synthesize pyrenophorin derivatives, the formation of cyclic dimers (e.g., 186) and trimers (e.g., 187) from treatment of acrylate ester-alkene derivatives (e.g., 185) with various ruthenium metathesis catalysts was examined.The product distribution was very time and concentration dependent higher concentrations favor the trimer over the dimer. 

In a related work, Vittal and coworkers reported on salts with cations, as opposed to anions, that undergo photodimerizations wherein the stereochemistry was anion controlled. Specifically, formation of the trifluoroacetate and sulfate salts of ( )-3-(4-pyridyl)acrylic acid (4-pa) and subseqnent photodimerization generated head-to-tail and head-to-head dimers, respectively (Figure 20a and b). The C=C bonds were separated on the order of 3.7 A. Head-to-tail and head-to-head geometries were dictated by N-H- 0, O-H- O, and C-H- -O hydrogen forces. 

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