Metal insertion conjugate addition reactions

Three approaches to zinc enolates are commonly adopted the process associated to the classical Reformatsky reaction is based on the insertion of Zn(0) into the carbon—halogen bond of an a-haloester. Two additional routes involve (i) transmetallation of a lithium enolate with a Zn(II) salt (Section V.A) and (ii) the transition-metal-catalysed conjugate addition of diethylzinc to Michael acceptors (Section V.B). 

Mechanistically, two scenarios have been advanced. The first involves coordination of the nitrile to the Ru center which is followed by nucleophilic attack at the nitrile, while the second entails a sequence of events starting with oxidative addition of water, an alcohol or an amine to the metal center, followed by insertion of the nitrile into the Ru-OH bond. A recent report describes the conjugate addition of alcohols to acrylonitrile compounds catalyzed by a mthenium-acetamido complex [Ru(PCy3)2(CO)(CH3CONH)-(i-PrOH)H] (7) (Scheme 10.13) [17]. The mechanistic investigation of this reaction supports the proposal that the N-coordination of acrylo-... 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

Metal-Halogen Compounds. An unusual example of the addition of a metal halide to a conjugated diene has been reported. The complex formed from palladium chloride and butadiene has been shown to be a dimer of 1-chloromethyl-7r-allylpalladium chloride, (85). Whether this is a true insertion reaction or some type of ionic reaction has not been determined, but its close analogy with the olefin-palladium chloride insertion reaction mentioned above would suggest an insertion mechanism for the diene reaction also. 

To date, several jt-allylmthenium complexes have been prepared and reported. The representative methods for introducing an allyl group to a ruthenium complex are quite similar to those for other transition metals for example, (1) the reaction of ruthenium halides with allyl Grignard reagents (2) the insertion of conjugated dienes into a hydrido-ruthenium bond and (3) the oxidative addition of several allylic compounds to low-valence ruthenium complexes. 

Equations 3.64-3.66 illustrate routes to allyl complexes from dienes, diene complexes, and olefins. Allyl complexes have been prepared by the insertion of a conjugated diene into a metal hydride, alkyl, or acyl linkage, as illustrated for the cobalt complexes in Equation 3.64. ° Alternatively, allyl complexes have been prepared by nucleophilic or electrophilic attack on a coordinated diene. Equation 3.65 shows the formation of allyl complexes by the addition of carbanions to a cationic diene complex, and Equation 3.66 shows the formation of a cationic diene complex by the protonation of a neutral 1,3-diene complex. Allyl complexes have also been formed by the abstraction of an allylic proton from a metal-olefin complex, either by a base or by the metal itself. This reaction has been proposed as a step in the isomerization of olefins (Equation 3.67) and in the allylic oxidation of olefins (Equation 3.68). - ... 

Metal-Catalyzed Reactions The metal-catalyzed bisthiolations are currently limited to Pd, Ni, and Rh complexes, where the stereoselective formation of alkenes 40 with high (Z)-selectivity was observed. For the Ni-catalyzed reaction, it is possible to prepare stereo-chemically defined 1,4-bisthio substituted conjugate dienes 41 through sequential alkyne insertions to Ni—S and Ni—C bonds 42 (Scheme 46.7). The use of silyldisulfides" followed by one-pot desilyation and electrophilic addition... 

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