Dimerization activated olefins

The Ni-catalyzed oligomerization of olefins in ionic liquids requires a careful choice of the ionic liquid s acidity. In basic melts (Table 5.2-2, entry (a)), no dimerization activity is observed. FFere, the basic chloride ions prevent the formation of free coordination sites on the nickel catalyst. In acidic chloroaluminate melts, an oligomerization reaction takes place even in the absence of a nickel catalyst (entry (b)). FFowever, no dimers are produced, but a mixture of different oligomers is... 

Many anodic oxidations involve an ECE pathway. For example, the neurotransmitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to leukoadrenochrome. The latter can rapidly undergo electron transfer to form adrenochrome (5). The electrochemical oxidation of aniline is another classical example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a dimerization reaction to yield an easily oxidized p-aminodiphenylamine product. Another example (of industrial relevance) is the reductive coupling of activated olefins to yield a radical anion, which reacts with the parent olefin to give a reducible dimer (7). If the chemical step is very fast (in comparison to the electron-transfer process), the system will behave as an EE mechanism (of two successive charge-transfer steps). Table 2-1 summarizes common electrochemical mechanisms involving coupled chemical reactions. Powerful cyclic voltammetric computational simulators, exploring the behavior of virtually any user-specific mechanism, have... 

A. Nucleophilic Attack on Carbon. —(/) Activated Olefins. A study of triarylphosphine-catalysed dimerization of acrylonitrile to 2-methylene-glutaronitrile (26) and 1,4-dicyano-l-butene (27) has established a balance between phosphine nucleophilicity and protolytic strength of the solvent. The reaction of methyl vinyl ketone with triphenylphosphine in triethyl-silanol gave only 3-methylene-2,6-heptadienone (28). 

A wide variety of activated olefins (126) undergo reductive electrochemical dimerization to compounds of structure 127 (electrolytic hydrodimerization) 129 i. While the product 127 is capable of existing in either dl or meso modifications, relatively little attention has been paid to the stereochemistry of hydrodimers... 

Treatment of 19 with ethy(aluminum sesquichloride or aluminum bromide results in the formation of a new catalyst, which is active for the dimerization of olefins such as ethylene or propene but inactive for the dimerization of cyclooctene. 

Hetero Diels-Alder reactions are very useful for constructing heterocyclic compounds, and many important chiral molecules have thus been synthesized. Although the retro Diels-Alder reaction does not itself involve the asymmetric formation of chiral centers, this reaction can still be used as an important tool in organic synthesis, especially in the synthesis of some thermodynamically less stable compounds. The temporarily formed Diels-Alder adduct can be considered as a protected active olefin moiety. Cyclopentadiene dimer was initially used, but it proved difficult to carry out the pyrrolytic process. Pentamethyl cyclopentadiene was then used, and it was found that a retro Diels-Alder reaction could easily be carried out under mild conditions. 

If no suitable reagent is present, the nitrile oxide immediately dimerizes into furoxane. Such reagents can be activated olefins such as vinyl esters and ethers, acrylonitrile, styrene and cycloolefins. 

With primary halides, dimers (R—R) are formed predominantly, while with tertiary halides, the disproportionation products (RH, R(—H)) prevail. Both alkyl nickel(III) complexes, formed by electrochemical reduction of the nickel(II) complex in presence of alkyl halides, are able to undergo insertion reactions with added activated olefins. Thus, Michael adducts are the final products. The Ni(salen)-complex yields the Michael products via the radical pathway regenerating the original Ni(II)-complex and hence the reaction is catalytic. In contrast to that, the Ni(III)-complex formed after insertion of the activated olefin into the alkyl-nickel bond of the [RNi" X(teta)] -complex is relatively stable. Thus, further reduction leads to the Michael products and an electroinactive Ni"(teta)-species. 

Phenanthroline in the presence of heavy metals acts as an activator of the polymerization of vinyl compounds558,559 and other olefins.560-564 It also assists the dimerization of olefins in the presence of titanium catalysts.565,566 It enhances the metal catalyzed oxidation of ascorbic acid567 and dimethyl sulfoxide.568 On the other hand, on its own it can inhibit several polymerization processes.545,569 It also stabilizes butadiene and isoprene and prevents their dimerization.570 It prevents peroxide formation in ether,571 inhibits the vinylation of alcohol572 and stabilizes cumyl chloride.573 It accelerates the vulcanization of diene rubbers574 and copolymers.575 1,10-Phenanthroline catalyzes the autooxidation of linoleic and ascorbic acids in the absence of metals.567... 

Dimerization of activated olefins or ketones as a means of ring closure was mentioned in Part I reports of more examples have been published.132 A special kind of coupling and ring closure occurs in the reduction of 6-phenyl-2,3-dihydrodiazepinium salt (76) in DMF to the diphenylpyrrolo-diazepine derivative (77). A plausible explanation of the formation of 77 involves dimerization of an initially formed radical, followed by intramolecular displacement of ethylenediamine.133-135... 

The third type of cycloaddition results from the reaction of cyclopropanones with activated olefins. For example, dimethylketene adds to methyl substituted cyclopropanones affording the spiro lactones 153 a—c. 96,n8,i22b) Similarly the ortho ester 154 is formed from 1,1-dimethoxy-ethylene and 2,2-dimethylcyclopropanone 154 dimerizes to 155 upon standing. ng>... 

In addition to the cathodic hydrodimerization of activated olefins and the Kolbe reaction, the anodic dimerization of CH-acidic compounds is another possibility for the electrochemical C—C coupling. Monsanto 281 > has used the anodic dimerization of malonates in a laboratory synthesis of intermediates for useful sequestrants and detergency builders. 

Alternatively, this unwanted side reaction is suppressed when electrolytes with higher positive foot potentials, e.g., acetonitrile 352 DMSO, or DMF 351 ( are used. The dimer yields from nitroaliphatics are often diminished by (5-eliminations of HN02 in the basic electrolyte, which leads to activated olefins 356 (Eq. (159)) that undergo follow-up reactions. The yields are generally better with secondary nitroaliphatics, where such eliminations are suppressed. 

Anodic dimerization of electron-rich olefins, the mirror image process to cathodic hydrodimerization of activated olefins (Sect. 12.2), affords a one-step synthesis for substituted butanes(Eq. (175) ) 268)( dienes (Eq. (176) ) 26S precursors of polyenes (Eq. (177)) 36,385 and 1,4-dicarbonyl compounds (Eq. (178)) 35>36). 

Cathodic coupling proceeds via radicals or radical anions, which are reductively generated from suitable substrates, in general electrophiles, e.g., carbonyl compounds and activated olefins. These intermediates either dimerize (Eq. (182)) or add to activated double bonds to yield 1,4-radical anions, which are subsequently reduced to hydrodimers (Eq. (183) ). 

Other activated olefins that have been satisfactorily dimerized include ... 

Mixed coupling of two dissimilar activated olefins A and B is best rationalized by path 3). To suppress the formation of symmetric dimers AA and BB besides the wanted mixed dimer AB the difference in reduction potential between A and B should be 0,2 to 0,4 V. Cpe at the potential of the more easily reducible olefin A with an excess of B present in the electrolyte yields predominantly AB. With equal amounts of A and B AA and AB are obtained and with small differences in the reduction potentials of A and B all three possible dimers are formed. Thus coreduction of diethyl maleate (Ei/2 = -1,32 V.) and acrylonitrile (E j. 2 = -1,94 V.) by cpe at -1,4 V yielded 15-3 (AA) and 154 (AB). Cpe at -1,7 V of 6 equivalents of AN and one equivalent of cyanobutadiene (Ejy2 =... 

The nucleophile initially adds at the multiple bond, forming carbanion A. Further transformations of A occur in line with electronic and steric effects, depending on the reaction conditions and on the use of nucleophilic catalysis. Several routes are possible, leading to different reaction products. Note that the use of nucleophilic catalysis is a general technique in the chemistry of compounds with electrophilic multiple bonds in particular, it is widely employed for dimerization and trimerization of activated olefins, keteneimines, etc. 

The reaction of an isocyanide containing an acidic hydrogen with copper(I) oxide and an activated olefin or a ketone [Eq. (123)] provides a synthesis of either pyrrolines or oxazolines, respectively (251,252). Addition of allyl bromide gave the coupling product with the allyl carbanion derived from allyl isocyanide. Oxazolines are obtained in yields as high as 957o> not pyrrolines because of competing dimerization... 

A similar treatment of stable keteneiminium triflate 72 with sodium bis(trimethylsilyl)amide generates azomethine ylide 73, together with a small amount of the demethylated product of 72. Ylide 73 undergoes readily dimerizes, leading to l,4-diethyl-2,5-bis[2,2-dimethyl-l-(t-butyl)propyridene]-piperazine when no dipolarophile is present. The trapping of ylide 73 with electron-deficient olefins is completely unsuccessful because these activated olefins cannot withstand such strongly basic conditions. The only dipolar-... 

The molybdenum dimer 547 can be converted, by alkylation, to a cation (548), which can be transformed into an anion (549) by two-electron chemical reduction. Treatment of 547 with excess methyllithium leads, after work-up in air, to the bis (/z-methylthiolate) 550, which can also be prepared by addition of methyllithium to 548. The addition of vinylmag-nesium bromide to 548 yields 551. The anion reacts with alkynes, activated olefines, and an allene to form 552, 553, and 554, respectively (305). 

Signal-time behavior of the ESR response following a current pulse has been calculated by Goldberg and Bard [367] for a number of mechanisms, including first-order decomposition, radical ion dimerization, and radical ion-substrate coupling, and working curves from which rate constants can be calculated were presented. Application of this approach, which is very similar to that taken, for example, in transmission spectroelec-trochemistry, was demonstrated for the reductive dimerization of a series of activated olefins (Fig. 58), a reaction that has been studied by a number of different electrochemical... 

Simonet and coworkers [47-49] also reported in detail the stereochemistry of hydrodimers derived from activated olefins. The d, I dimers were predominant in most cases however, the remarkable substituent effect was not enough explained. 

Lund and co workers [51] reported interesting stereochemical results in the reductive dimerization-cyclization of benzylidenemalononitrile, but the reaction pathway was not stereochemically clarified. Recently, it has been clearly demonstrated by some workers [51-54] that the reductive cyclodimerization of activated olefins in aprotic media affords the 1,2-disubstituted cyclobutanes as shown in Eq. (4). 

Activated olefins were shown to be monoacylated when reduced in the presence of acetic anhydride [237-240] and therefore do not lead to a fast dimerization. 

Dicyclopentadiene is a feedstock for both the fragrance and polymer industries. It forms spontaneously from cyclopentadiene by a Diels-Alder reaction, and a retro-Diels-Alder reaction can be used to regenerate cyclopentadiene from it. A number of minor fragrance ingredients are produced by Diels-Alder reaction of the monomer with a variety of activated olefins in which the activating group X, is usually an aldehyde, ketone, ester or nitrile. However, the main fragrance uses stem from the dimer. 

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