Additive dimerization

In addition dimeric species are formed, being in equilibrium with the monomeric RMgX. The Schlenk equilibrium is influenced by substrate structure, the nature of the solvent, concentration and temperature. 

Isobutylene could be dimerized in the presence of an acid catalyst to diisobutylene. The product is a mixture of diisobutylene isomers, which are used as alkylating agents in the plasticizer industry and as a lube oil additive (dimerization of olefins is noted in Chapter 3). 

Kolbe radicals can be added to olefins that are present in the electrolyte. The primary adduct, a new radical, can further react by coupling with the Kolbe radical to an additive monomer I (Eq. 9, path a), it can dimerize to an additive dimer II (path b), it can be further oxidized to a cation, that reacts with a nucleophile to III (path c), or it can disproportionate (path d). 

To some degree the ratio of additive monomer to additive dimer can be infiuenced by the current density. High current densities favor the formation of additive monomers, low ones these of additive dimers (Table 8, Nos. 4, 5). This result can be rationalized according to Eq. 9 At high current densities, which corresponds to a high radical concentration in front of the electrode, the olefin can trap only part of the Kolbe radicals formed. This leads to a preferred coupling to the Kolbe dimer and a combination of the Kolbe radical with the primary adduct to the additive monomer. At low current densities the majority of the Kolbe radicals are scavenged by the olefin, which leads to a preferential formation of the additive dimer. 

No. Carboxylic acid Olefin Additive monomer (%-yield) Additive dimer (%-yield) Ref. 

By the radical pathway l, -diesters, -diketones, -dienes or -dihalides, chiral intermediates for synthesis, pheromones and unusual hydrocarbons or fatty acids are accessible in one to few steps. The addition of the intermediate radicals to double bonds affords additive dimers, whereby four units can be coupled in one step. By way of intramolecular addition unsaturated carboxyhc acids can be converted into five raembered hetero- or carbocyclic compounds. These radical reactions are attractive for synthesis because they can tolerate polar functional groups without protection. 

When the reaction is conducted in the presence of 1,3-dienes, additive dimerization can occur " ... 

In solutions containing more than one compound of the RCOOH type, asymmetric products of the crossed addition of different radicals may form together with the standard additive dimers. 

Oxides of Platinum Metals Anodes of platinum (and more rarely of other platinum metals) are used in the laboratory for studies of oxygen and chlorine evolution and in industry for the synthesis of peroxo compounds (such as persulfuric acid, H2S2O8) and organic additive dimerization products (such as sebacic acid see Section 15.6). The selectivity of the catalyst is important for all these reactions. It governs the fraction of the current consumed for chlorine evolution relative to that consumed in oxygen evolution as a possible parallel reaction it also governs the current yields and chemical yields in synthetic electrochemical reactions. 

In addition, dimer-type black developing fluoran compounds such as 2,2-bis(4-[6 -(/V-cyclohexyl-/V-methylamino)-3 -methylfluoran-2 -yl-amino]phenyl propane (59)60 are also proposed. Fluoran 59 has much lower solubility in organic solvents to improve image stability to plasticizer for use in thermosensitive recording label paper. 

In the latter case an extraneous messenger has to dock at the receptor s extracellular binding site on the cell surface. The information about the occupancy of the corresponding receptor is transmitted through the transmembrane part of the protein into its cytosolic domains by conformational changes. This structural response can be induced by an additional dimerization and results in a covalent modification of intracellular side chains. The new conformation is then recognized by cytosolic partner molecules. In this connection GTP binding pro-... 

Unsymmetrical dimers are accessible through the coelectrolysis of different olefins (Table 7, number 11). Coupling can also be achieved by the addition of anodically generated radicals with subsequent dimerization of the adduct radicals (additive dimerization, see Section 5.3.3). 

If anions R are oxidized in the presence of olefins, additive dimers (24) and substituted monomers (26) are obtained (Scheme 5, Table 8, and Ref. [94]). The products can be rationalized by the following pathway the radical R obtained by a le-oxidation from the anion R adds to the alkene to give the primary adduct (25), which dimerizes to afford the additive dimer (24) with regiospeciflc head-to-head connection of the two olefins, or couples with R to form the additive monomer (26). If the substituent Y in the olefin can stabilize a carbenium ion, (25) is oxidized to the cation (27), which reacts intra- or inter-molecularly with nucleophiles to give (28) or (29). 

In the oxidation of anionized 1,3-dicarbonyl compounds (Table 8, numbers 1-7) at potentials between 0.6 and 1.4 V (see) and in the presence of butadiene, mainly the additive dimer (24) is obtained in the presence of ethyl vinyl ether chiefly the disubstituted monomers (28) or (29) arise. 

Also azide radicals generated by anodic oxidation of sodium azide in the presence of olefins afford in acetic acid additive dimers, products of allylic substitution and... 

If carboxylates are subjected to Kolbe electrolysis in the presence of olefins, the generated Kolbe radicals add to the double bonds to afford mainly additive dimers (Table 8, entries 10-17). 

Upon electrolysis of trifluoroacetate in MeCN-H20-(Pt) in an undivided cell in the presence of an electron, deficient olefins additive dimers and additive monomers are obtained. The selectivity can be controlled by current density, temperature, and the substitution pattern of the olefin [117]. 

Another example of type iii-a is the trifluoromethylation of double bonds by electrolysis of trifluoroacetic acid in the presence of olefins. Methyl vinyl ketone, vinyl acetate, diethyl fumarate, diethyl maleate, Ai-ethylmaleimide, and 2,5-dihydrothiophene-l,1-dioxide were examined as olefins. The products were bis-trifluoromethylated additive dimers (66) (type iii-a) and monomers (67) (type... 

In fact, when a cyclohexene-yne 65a is treated with Ig under an ethylene gas atmosphere, three products are produced. An expected bicyclic compound 66a is obtained in only 14% yield, and a major product (57%) is an unexpected bicyclic compound which turned out to be 66b. In addition, dimeric compound 68 is formed in 26%... 

Electrocyclic ring closure reactions of phenyl ketenimides 331 allow the construction of the quinazolone ring (Scheme 73). For instance, quina-zolones 332 were synthesized some years ago in 4-85% yield by heating 331 (R2 = Ph) in addition, dimerization through a [4 + 2] cycloaddition leading to 333 (30-73% yield) was observed (78S760). Compound 331 (R1 = CF3) has been employed for the preparation of the trifluoromethy-lated quinazolone 334 (71% yield) [90TL(31 )2717]. 

Anodic dehydrodimerization and additive dimerization of electron-rich olefins. 

In the anodic olefin oxidation, the electrode material plays an important role as well. The oxidation of enolethers at platinum anodes mainly leads to the dimethoxylation product [74], while the application of graphite anodes favors the additive dimerization (Eq. 22.33) [39]. 

Photoaddition to heterocyclic systems occurs both as a result of excitation of the heterocycle or of the species undergoing addition. Dimerization will be discussed separately. 

At higher olefin concentrations, additive dimerization of the olefin is also observed 368-... 

The key point of this scenario is to assume a priori that XY promotes the product of opposite configuration to X. In addition, additive dimers and not monomers are considered as catalytically active species, opposing the commonly accepted mechanism for typical dialkylzinc additions to aldehydes catalyzed by Vamino alcohols [ 18]. 

The Kolbe electrolysis is a convenient source for alkyl radicals R (Eq. (94) ). If conducted in the presence of olefins, e.g., butadiene, isoprene, cyclohexadiene, or styrene, the intermediate radicals R add to the double bond to form adducts 49, which dimerize to additive dimers JO or couple with R to disubstituted monomers 51 (Eq. (114) ) 257 26°). 

Additive dimerization is a useful synthetic method since four subunits, two radicals and two olefins, are joined in one step and in a specific way to head-to-head dimers 50a. To obtain usable yields, however, precisely controlled reaction conditions have to be maintained. 

The primary adduct 53 (Eq. (117) ) of the anodically generated radical R undergoes a series of follow-up reactions a) hydrogen abstraction to 54, b) dimerization to additive dimers 55, c) coupling with R to 1,2-disubsti-tuted monomers56, d) le-oxidation to a carbonium ion that either solvolyzes to 57 or, when 1,3-dicarbonyl compounds are added cyclizes intramolecularly to tetra (58) - or dihydrofuran derivatives (59). Product control is possible in some cases by suitable choice of the anode potential. With a high anode potential,... 

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