Organic halides, homocoupling

The Organohalogen Component. Just as for Mg and the solvent, the organic halide must be dry (less than 0.02 wt % of H Ol and free of CK The relative reactivity of the halogens is reflected in the rate of disappearance of Mg. which follows ihc general ordei I > Br > Cl > F. Unfortunately, the rate of disappearance Mg of does not always correlate with the formation of active Grignard. Typically, the more reactive the RX is. ihe higher ihc probability of forming a homocoupled product. Therefore. 

Scheme 8.10. Possible mechanisms of the Pd-catalyzed reduction and homocoupling of organic halides. X= halide, triflate Y = R2(T, NHR, O. ...

The homocoupling of aryl Grignard reagents triggered by organic halides in the presence of catalytic quantities of cobalt chloride or other transition metal halogenides was studied initially by Kharasch and Fields in 1941 [36]. They... 

Homocoupling reactions of organic halides such as bromobenzene and benzyl bromide were successfully carried out in the presence of an Ni(II) complex catalyst in [bmim][NTf2] (Equation 12.4) [11]. 

Before the discovery of the nickel-phosphine catalyst, the cross-coupUng of Grignard reagents with organic halides was seldom used in synthetic practice. This fact was mainly due to the formation of homocoupling products along with significant elimination side reactions. With the use of this new catalyst, the reaction now has wide application in the synthesis of unsymmetrical allca-nes and alkenes. 

Nickel-catalyzed electroreductive homocoupling reaction of organic hahdes and coupling reaction between organic halide and activated olefins could be carried out with iron anode in ionic liquid. With addition of small amount of DMF, coupling compounds were obtained at room temperature in good to high... 

We reported that smooth oxidative addition of organic halides such as aryl, benzyl, and allyl halides to metallic nickel proceeded to afford organonickel halides under mild conditions, which yielded homocoupled products [11, 41] or ketones by the reaction with acid chlorides [42] or alkyl oxalyl chlorides [43]. We describe here a new method for the preparation of 3-aryl-2-hydroxy-l-propanones (4) in good yield by the Grignard-type addition of benzyl halides to 1,2-diketones mediated by metallic nickel under neutral conditions [44]. 

Homocoupling can involve either two organic halides (or their equivalent) or two organometallic compounds (Scheme 2.131). Palladium is often employed for this transformation. Neither reaction is of itself catalytic, but each can be rendered catalytic by the inclusion of either a reducing agent or an oxidizing agent. 

Figure 2. Mechanism of Homocoupling of Organic Halides in Nonpolar Solvents in the Presence of Large Amounts of Ni(0) Reagents.

The homocoupling of aryl halide to diaryl compounds, known as Ull-mann coupling, is a synthetically useful reaction and has wide applications in material research. Such couplings have been studied in aqueous conditions. In 1970, arylsulfinic acids were coupled with Pd(II) in aqueous solvents to biaryls (Eq. 6.25).53 However, the reaction required the use of a stoichiometric amount of palladium. In the presence of hydrogen gas, aryl halides homocoupled to give biaryl compounds in moderate yields (30-50%) in an aqueous/organic microemulsion (Eq. 6.26).54... 

Application in organic synthesis of pentacoordinated triorganodifluorosilicate anions, such as [Bu4N][Ph3SiF2] 825, have been extended to palladium(0)-catalyzed cross coupling reactions (solvents DMF, TFIF, dioxane) with arene halides (Scheme 111).825 This method is tolerant to various palladium(O) catalysts and provides excellent yields of mainly heterocoupled products and only small amounts of homocoupled byproducts. 

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