Strained alkenes, dimerization

Dimerization is an important mode of reaction of strained alkenes, and two different modes of reaction may be found. With cyclopropene, an ene reaction occurs to give a dimer,and this in turn may further react in the same fashion to give a polymer. 

If alkenes or alkynes are subjected to strain, their jt bonds are weakened, and such compounds often behave chemically as diradicals. Their tendency to dimerize or polymerize will be significantly enhanced, and quick reaction with oxygen will occur in air [18, 19]. Reactions of strained alkenes which lead to a decline of strain, for example Michael additions or cycloadditions, can proceed significantly faster than with related, unstrained alkenes (Scheme 3.6). 

Pauson and Khand discovered the very important class of alkyne/alkene/CO cyclization catalytic reactions catalyzed, once again, by Co2(CO)g see Pauson-Khand Reaction). This reaction produces a, /3-unsaturated cyclopentanones (equation 23). With unstrained alkenes the reaction works best in a stoichiometric setting, but with strained alkenes like norbomadiene the reaction can be made catalytic. These reactions have been fairly extensively studied, and the reaction proceeds through an alkyne-bridged Co2(CO)6 dimer species. Unsymmetrical alkynes lead to mixtures of the various substituted a, /3-unsaturated cyclopentanone products. 

The Meisenheimer rearrangement can compete with the formation of highly strained alkenes. The pyrolysis of the homoadamantylamine oxide (34 equation 15) gave a mixture of products, including hyc carbons believed to have been formed by dimerization of 3-homoadamantene (36), which was trapped by 1,3-diphenylisobenzofuran, and the Meisenheimer product (35). ... 

In the case of Ni, metallacyclic compounds containing the divalent metal may be obtained by alkene dimerization. Not only does this reaction occur with electron-deficient or strained alkenes, but under suitable conditions ethene or 1,7-octadiene reacts to give nickelacyclopentane species ... 

Diels-Alder reaction of the highly strained alkene (48) with benzene is one of the strikingly unusual reactions. The driving force of the unusual cycloaddition would be a favorable electron transfer from benzene to the low-lying LUMO of the highly electron-deficient double bond of 48 [15]. In contrast, the corresponding hydrocarbon (50) dimerizes and polymerizes rapidly at ambient temperature (Scheme 1.43) [16]. 

An isolable CuOTf complex of a highly strained alkene, trans-cycloheptene, is produced by UV irradiation of a hexane solution of cis-cycloheptene in the presence of CuOTf (eq 14).i Photocycloaddition of cycloheptene is also catalyzed by CuOTf. Surprisingly, the major product is not a trans,anti,trans dimer analogous to that formed from cyclohexene (eq 12) but rather a trans,anti,trans,anti,trans trimer (eq 15). ... 

Pentacarbonyliron reacts with alkenes, particularly strained alkenes, to produce carbonyl-inserted dimers (Scheme 27). > When 2-(R5 )-norbomen-5-one is treated... 

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]... 

As in the case of dimerizations, MCP derivatives are known to undergo metal-catalysed [2 + 2] codimerizations with other alkenes in a few cases [2]. The examples are limited to strained olefins, such as norbornadiene (572) (Scheme 79) [152] and cyclobutene (574) (Scheme 80) [153], and to alkyl acrylates (Table 46) [154] and always compete with the alternative [3 + 2] addition of TMM species. 

Several factors and observations support the route proposed in Scheme 10 (1) Due to steric factors, the styrenyl alkene is expected to react preferentially (versus the neighboring disubstituted cyclic olefin see below for further discussion). (2) Involvement of tetracyclic intermediates such as 43 provides a plausible rationale for the reluctance of six-membered ring ethers [46 in Eq. 4] to participate in the catalytic rearrangement and for the lack of reactivity of cyclopen-tenyl substrates [48 in Eq. 5] because of the attendant angle strain, the generation of the tetracyclic intermediate is not favored. (3) Reactions under ethylene atmosphere inhibit dimer formation, since 44 is intercepted with H2CCH2, rather than 41 [19]. 

With ions or dipolar substrates, radical ions undergo nucleophilic or electrophilic capture. Nucleophilic capture is a general reaction for many alkene and strained-ring radical cations and may completely suppress (unimolecular) rearrangements or dimer formation. The regio- and stereochemistry of these additions are of major interest. The experimental evidence supports several guiding principles. 

Bicyclo[2.2.0]hex-l(4)-ene (8) is one of the most highly strained of the alkenes (SE = 87, OS = 35)50 that can be observed at room temperature.51 In dilute solution, it undergoes moderately rapid dimerization at room temperature, presumably giving the pentacyclic propellane 9 as an intermediate in the formation of the diene 10. 

Dipolar cycloaddition of 3//-1.2.4-triazole-3,5(4// )-diones to strained bicyclic alkenes generally leads to the formation of rearranged 1,2,4-triazolidines (Section7.2.10.1.), although bicyclo[2.2.2]-octene does not react and cyclopentadiene dimer gives exclusively the cne reaction product24. 

Although the early examples of the 4ir participation of heterodienes in [4 + 2] cycloaddition reactions describe their reactions widi electron-deficient aJkenes, e.g. the thermal dimerization of a,3 unsaturated carbonyl compounds, the introduction of one or more heteroatoms into the 1,3-butadiene framewoiic does convey electrophilic character to the heterodiene. Consequently, such systems may be expected to participate preferentially in LUMOdiene-controlled Diels-Alder reactions with electron-rich, strained, or simple alkene and alkyne dienophiles. The complementary substitution of the heterodiene with one or more electron-withdrawing substituents further lowers the heterodiene Elumo, accelerates the rate of heterodiene participation in the LUMOdioie-conn-olled Diels-Alder reaction, and enhances the observed regioselectivity of the [4 + 2] cycloaddition reaction. ... 

Simple a,3-unsaturated aldehydes, ketones, and esters (R = C02Me H > alkyl, aryl OR equation l)i, 60 preferentially participate in LUMOdiene-controlled Diels-Alder reactions with electron-rich, strained, and selected simple alkene and alkyne dienophiles, - although the thermal reaction conditions required are relatively harsh (150-250 C) and the reactions are characterized by the competitive dimerization and polymerization of the 1-oxa-1,3-butadiene. Typical dienophiles have included enol ethers, thioenol ethers, alkynyl ethers, ketene acetals, enamines, ynamines, ketene aminals, and selected simple alkenes representative examples are detailed in Table 2. - The most extensively studied reaction in the series is the [4 + 2] cycloaddition reaction of a,3-unsaturated ketones with enol ethers and E)esimoni,... 

In examining the cycloalkenes, one must first recognize that a double bond has considerable inherent strain. For example, the dimerization of ethylene to give cyclobutane is fairly exothermic (—18 kcal mol" ) and if there were a way to readily overcome orbital symmetry restrictions, cyclobutane would be a very common reagent. However, in the following, we will take the conventional view that ethylene is unstrained. Then, in comparing cycloalkanes and cycloalkenes it is helpful to define olefinic strain (OS) as the difference in strain between the alkene and the corresponding alkane ... 

With methyl 2-alkylcycloprop-2-enecarboxylates, on the other hand, only [2-1-1] cycloaddition to the tricyclic dimers of cyclopentadiene (18) occurs. Only the strained double bond in the norbornene substructure is cyclopropanated, and the exo-adducts are formed. The Z/E isomeric ratios (6 1 and 2.5 1) are comparable to the ratios obtained in the reactions with norbornadiene vide supra). A mechanism proposing ring opening of the cyclopropene and attack on the metal-coordinated alkene in a concerted way has been suggested in order to explain the stereoselectivity of the reaction. ... 

Bicyclopropylidene (7), a special MCP derivative with a geminal disubstitution at the alkene, is also capable of forming a [2 + 2] cycloadduct, even with an only moderately strain-activated alkene such as cyclobutene, in a nickel-catalyzed reaction. Cycloocta-1,5-diene is formed as the second major poduct, presumably as a cyclobutene dimerization product. ... 

Dichlorotricyclo[4.2.1.0 ]non-3-ene (20) is even more strained and rearranges on warming to room temperature. The main products were derived from reactions of the unstable bridgehead alkene, 6,9-dichlorobicyclo[4.2.1]nona-l(9),3-diene (21), which was formed as a transient intermediate. This species dimerized to give 22, or could be trapped as a cycloadduct 23 with furan. ... 

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