Dimerization of Functional Alkenes

For example, the tail-to-tail dimerization of methyl acrylate was catalyzed by ruthenium complexes such as RuHCl(C0)(Pz-Pr3)2/CF3S03Ag or even RuC13 and gave dimethyl hexenedioate isomers. Efficient catalytic systems such as Ru(rf-naphthalene)(COD)/CH3CN, where COD is cyclooctadiene, selectively led to the diester 2 in 75% yield [1] (Eq. 1). 

The tail-to-tail dimerization of acrolein [3] and acrylonitrile [4, 5] was also obtained, with a lower reactivity and stereoselectivity. However, the dimerization of acrylonitrile was performed under mild conditions in the presence of molecular hydrogen with Ru(COD)(COT), where COT is cyclooctatetraene, [4] (Eq.2). 

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). 

An initial hydrometallation was also invoked in the dimerization of nor-bornadiene with the catalyst precursor Ru(COD)(COT) to generate penta-cyclotetradeca-4,11-diene 7 in very good yield [7] (Eq. 4). A suggested mechanism for the formation of 7 involves olefin insertion into the preformed Ru-H bond and the cleavage of two C-C bonds. 

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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... 

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). 

The behavior of strained,/Zuorimiret/ methylenecyelopropanes depends upon the position and level of fluorination [34], l-(Difluoromethylene)cyclopropane is much like tetrafluoroethylene in its preference for [2+2] cycloaddition (equation 37), but Its 2,2-difluoro isomer favors [4+2] cycloadditions (equation 38). Perfluoromethylenecyclopropane is an exceptionally reactive dienophile but does not undergo [2+2] cycloadditions, possibly because of stenc reasons [34, 45] Cycloadditions involving most possible combinations of simple fluoroalkenes and alkenes or alkynes have been tried [85], but kinetic activation enthalpies (A/f j for only the dimerizations of tetrafluoroethylene (22 6-23 5 kcal/mol), chlorotri-fluoroethylene (23 6 kcal/mol), and perfluoropropene (31.6 kcal/mol) and the cycloaddition between chlorotnfluoroethylene and perfluoropropene (25.5 kcal/mol) have been determined accurately [97, 98] Some cycloadditions involving more functionalized alkenes are listed in Table 5 [99. 100, 101, 102, 103]... 

The ruthenium carbene catalysts 1 developed by Grubbs are distinguished by an exceptional tolerance towards polar functional groups [3]. Although generalizations are difficult and further experimental data are necessary in order to obtain a fully comprehensive picture, some trends may be deduced from the literature reports. Thus, many examples indicate that ethers, silyl ethers, acetals, esters, amides, carbamates, sulfonamides, silanes and various heterocyclic entities do not disturb. Moreover, ketones and even aldehyde functions are compatible, in contrast to reactions catalyzed by the molybdenum alkylidene complex 24 which is known to react with these groups under certain conditions [26]. Even unprotected alcohols and free carboxylic acids seem to be tolerated by 1. It should also be emphasized that the sensitivity of 1 toward the substitution pattern of alkenes outlined above usually leaves pre-existing di-, tri- and tetrasubstituted double bonds in the substrates unaffected. A nice example that illustrates many of these features is the clean dimerization of FK-506 45 to compound 46 reported by Schreiber et al. (Scheme 12) [27]. 

A very remote secondary H/D isotope effect has been measured for the 2 + 2-cycloaddition of TCNE to 2,7-dimethylocta-2,fran -4,6-triene. The reaction of nitric oxide with iV-benzylidene-4-methoxyaniline to produce 4-methoxybenzenediazonium nitrate and benzaldehyde is thought to proceed via a 2 + 2-cycloaddition between nitric oxide and the imine double bond. A novel mechanism for the stepwise dimerization of the parent silaethylene to 1,3-disilacyclobutane involves a low-barrier [1,2]-sigmatropic shift. Density functional, correlated ab initio calculations, and frontier MO analysis support a concerted 2 + 2-pathway for the addition of SO3 to alkenes. " The enone cycloaddition reactions of dienones and quinones have been reviewed. The 2 + 2-photocycloadditions of homochiral 2(5H)-furanones to vinylene carbonate are highly diastereoisomeric. ... 

Under optimized conditions, cycloisomerizations of a number of functionalized hept-l-en-6-ynes took place in good-to-excellent yields (Table 9.3). Heteroatom substitution was tolerated both within the tether and on its periphery. Alkynyl silanes and selenides underwent rearrangement to provide cyclized products in moderate yield (entries 6 and 7). One example of seven-membered ring formation was reported (entry 5). Surprisingly, though, substitution was not tolerated on the alkene moiety of the reacting enyne. The authors surmize that steric congestion retards the desired [2 + 2]-cycloaddition reaction to the point that side reactions, such as alkyne dimerization, become dominant. 

By far the most widespread type of photocycloaddition in synthetic applications employs the afkene unit of an c. p u nsaturated carbonyl compound as one of the addends. Simpleexamplesof such reactions are the dimerization of cyclopent-2-ennne (2.71), and the photo-addiiion of 2-methylptopene to cyclohex-2-enone (2.72). The function of the carbonyl group is to bring the absorption of the alkene into... 

However, the equilibrium monomer concentrations of disubstituted alkenes is measurable. The equilibrium constants for dimerization, tri-merization, and polymerization of a-methylstyrene have been determined as a function of temperature under anionic conditions [12] similar values should be obtained under cationic conditions. Unfortunately, the equilibrium position can t be determined directly under cationic conditions due to the irreversible side reactions of isomerization and indan and spirobiindan formation (Section II. A). The equilibrium monomer concentrations of isobutene and isopropenyl vinyl ethers should also be relatively high, albeit lower than those of a-methylstyrenes. However, the true equilibrium can t be reached with these monomers due to irreversible side reactions, and reliable data are therefore not available. Nevertheless, the ceiling temperature of isobutene polymerization is apparently between 50 and 150° C. 

Reagents such as magnesium, or low-valency states of titanium dissolving in acid, function by donating an electron to the carbonyl group to form a radical anion. The reductive process may be completed by the dimerization of these radicals to form 1,2-diols (pinacols) in the case of magnesium or alkenes in the case of titanium. 

R. I. Kureshy, S. Singh, N. H. Khan, S. H. R. Abdi, I. Ahmed, A. Bhatt, R. V. Jasra, Environmentally friendly protocol for enantioselective epoxidation of non-functionalized alkenes catalyzed by recyclable homochiral dimeric Mn(III) salen complexes with hydrogen peroxide and UHP adduct as oxidants, Catal. Lett. 107, 127-130 (2006). 

A photophysical study of the interaction of 2-amino-4,6-dimethylpyrimidine in p-CD has been carried out. Calculations have been carried out in an examination of the isomerism of methyl urocanate. Density functional theory techniques have been used to study the dimerization of thymine units in DNA. The effects of both low intensity irradiation (254 nm) or laser photolysis at 266 nm on adenosine have been reported. The photoionization of various purine and pyrimidine derivatives can be brought about by 266 nm nanosecond laser photolysis. Irradiation of alloxan in the presence of alkenes with allylic hydrogens affords adducts of 5-hydroxy-5-alkenyl barbituric acid. When the alkene is... 

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