Coupling homocoupling product

A general problem of alkene cross-metathesis is the formation of self-condensation products from the starting alkenes. On the solid phase, dimerization of polymer-bound alkene should be miniinized by the use of excess alkene in solution combined with the effective dilution of the resin-bound alkene by site isolation effects (see Suzuki coupling). Homocoupled products of the solution phase alkene are simply washed way during the resin washes. 

The alkynyl iodide 359 undergoes cross-coupling with a terminal alkyne to give the 1,3-diyne 360[264]. No homocoupling product is formed. This reaction offers a good synthetic method for unsymmetrical 1,3-diynes. 

The reaction is considered to proceed via a silyl anion mechanism, although the possibility of a radical-based mechanism has also been discussed.115,125 In order to clarify the mechanism, coupling experiments on a 1 1 mixture of chlorotrimethylsilane, 27 (reduction potential <—3.0 V),126 and chlorotriphenylsilane, 28 (reduction potential vs. standard calomel electrode (SCE) < —3.0 V),120 were performed, in which the mixed coupling product 1,1,1-trimethyl-2,2,2-triphenyldisilane, 29, and the homocoupling product hexaphenyldisilane, 30, only, were found,125 as indicated in Scheme 15. 

Reactions of aryllead triacetates with olefins (Heck-type reactions) proceed similarly but do not require Cul as co-catalyst. From the numerous reported reactions, that of phenyllead triacetate with 2,3-dihydrofuran is mentioned as a typical example. This affords the C-C coupling product 27 in 68% yield, together with 10% of the homocoupling product (Equation (ll)).47... 

Oxidation of arylolefins, enolethers, or dienes yields intermolecular homocoupling products in moderate to good yield (see Sect. 13.2.1.4) however, no pronounced diastereoselectivity was observed. This is also due to the fact that the coupling sites do not tolerate substituents that would make up a prostereogenic center. Furthermore, the fairly stable cations of the dimerized radical cation solvolyze stereounselectively. The same holds for the intermolecular coupling of aromatic compounds, in... 

Intermolecular coupling of a vinyl ether with styrene at a carbon anode in methanol is successful, giving a mixture of the cross coupled product and the two homocoupled products [49], Intramolecular coupling between an enol ether and an alkene centre, as in 24 and 25, proceeds to give the cyclized product in good yield [50], Five and six membered rings can be constructed in this way. An easily oxidised vinyl ether group is necessary to initiate the reaction and the second alkene... 

The formation of the undesired homocoupling product could be decreased using a larger excess of silane and reducing the reaction temperature to 60 °C. This catalytic system could also mediate the coupling of aryl halides with vinyltrimethoxysilane to form substituted styrenes in good yields. 

J.W. Morzycki and co-workers described the synthesis of dimeric steroids to be used as components of artificial lipid bilayer membranes. The key coupling of two steroid derivatives was achieved by the Wurtz reaction. The steroid primary alkyl iodide was dissolved in anhydrous toluene and treated with an excess of sodium metal. After 20h of reflux, the desired homocoupled product was obtained in moderate yield along with a considerable amount (36%) of the reduced compound. 

Additionally, Suzuki-Miyaura cross coupling reactions can be performed on similar supported iodides. Aryl bromides could be employed in this reaction as well. After optimization of conditions, it turned out that a hydroxyl-derived IL was the best solvent for this reaction. Namely, reaction completion was obtained after 12 h at room temperature in [N11130H][NTf2] in the presence of 1% Pd(OAc)2 and using potassium carbonate as abase. Under these conditions, less than 1% of homocoupling product is observed and easily eliminated by washing with diethyl ether prior to transesterification with methanol. Overall, biphenyls were isolated under analytically pure form in 90-95% yields [126],... 

Halichomycin is a structurally unprecedented tricyclic hemimacrolactam with strong antitumor effects. In the course of a synthetic study on a putative intermediate of halichomycin, tandem hydrostannation/coupling was exploited. On cross-coupling, however, a significant amount of homocoupling product of the stannane was produced, and the yield of the desired adduct was not satisfactory (Scheme 12.109) [211]. 

In principle, linker 96 can be regarded as a safety-catch linker. Prior to activation by alkylation it is completely stable towards the coupling conditions, while after alkylation 97 undergoes efficient cleavage. The principle has been used to synthesize a small library of biarylmethanes 98 [115]. A disadvantage is the observed formation of homocoupling products. Pure products were only obtained after chromatography on silica gel. 

In spite of the considerable difference in the rates of the reactions of organometallic reagents with 2-pyridyl and 2-quinolinylsulfoxides, a cross-coupling reaction was successfully performed with different combination of mixtures of sulfoxides [(155) + (156)]. This reaction is not selective and affords mixtures of cross-coupling products (157) and homocoupling products (158) and (159). 

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