Alkenes, reductive coupling elimination reactions

Cross-coupling of allylic compounds occurs by transmetallation between 7i-allyl intermediates and organometallic compounds of Mg, Zn, B, Al, Si and Sn, and subsequent reductive elimination. Reaction of the allylic dithioacetal 180 with MeMgBr in the presence of an Ni catalyst affords alkenes 184 bearing a tert-butyl group [90]. In this reaction, generation of the 7i-allylnickel 181 by oxidative addition and subsequent transmetallation with MeMgBr afford 182. Then the methylated product 183 is formed by reductive elimination, and finally the dimethylated product 184 is formed by the sequence of similar reactions. 

Oxidative cross-coupling with alkenes is possible with Pd(OAc)2 [109], The reaction proceeds by the palladation of benzene to form phenylpalladium acetate (164), followed by alkene insertion and elimination of /1-hydrogen. Heteroaromatics such as furan and thiophene react more easily than benzene [109]. Stilbene (177) is formed by the reaction of benzene and styrene. The complex skeleton of paraberquamide 179 was obtained in 80% yield by the Pd(II)-promoted coupling of the indole ring with the double bond in 178, followed by reduction of the intermediate with NaBELt [110]. 

In general, the termination reactions of these polymerizations are not well understood but, depending upon the metal and the monomer, reductive coupling of the metal carbene fragments to give alkene and reduced metal complexes is one possibility. Another termination reaction appears to be initiated by -Hydride Elimination from the carbene complex. These mechanisms have been observed in well-defined catalyst systems, and are possible in the ill-defined systems also. The fact that most catalysts are sensitive to oxygen and moisture (or other proton sources) means that termination of the polymer chain by added or adventitious sources of water is a common problem, especially for the ill-defined catalysts. 

The Pauson-Khand reaction gives the same product as the group 4 metal-mediated reductive coupling and carbonylation, and both reactions proceed by essentially the same mechanism formation of an alkyne-metal tt complex, insertion of an alkene, insertion of CO, and reductive elimination. Some details differ, however. When an alkyne is added to Co2(CO)g, CO evolves, and an isolable, chromatographable alkyne-Co2(CO)6 complex is obtained. This butterfly complex contains four Co(II)-C bonds, and the Co-Co bond is retained. The formation of the alky n e-C o2 (C O) 6 complex involves the formation of an ordinary tt complex of the alkyne with one Co(0) center, with displacement of CO. The tt complex can be written in its Co(II) cobaltacyclopropene resonance structure. The tt bond of the cobaltacyclopropene is then used to form a tt complex to the other Co center with displacement of another equivalent of CO. This second tt complex can also be written in its cobaltacyclopropene resonance structure. The alkyne-Co2(CO)6 complex has two 18-electron Co(II) centers. 

The beneficial role of dba in the formation of the Pd(0) complex is not quite clear, but alkenes are known to favour reductive elimination. See Giovanniiu, R. and Knochel, P. (1998) Ni(II)-catalyzed cross-coupling between polyfunctional arylzinc derivatives and primary alkyl iodides. J. Am. Chem. Soc., 125,11186-7. The alkene in a Heck reaction may play this role and explain why 5 does not react with PhBr until the olefin is added to the mixture , as observed by Herrmann et al. [55a]. 

In the next step, the product-yielding fi-hydride elimination (step D) can occur only after an internal rotation (step C) around the former double bond, as it requires at least one fS-hydrogen to be oriented synperiplanar with respect to the halopalladium residue. In some cases, anti elimination was made possible by suitable substrates or conditions leading to an type mechanism [16, 17]. The subsequent syn elimination yielding an alkene and a hydridopalladium halide is, however, reversible, and therefore, the thermodynamically more stable (E)-alkene is generally obtained when the coupling reaction is performed with a terminal alkene. Reductive elimination of HX from the hydridopalladium halide, aided by the added base, regenerates the active catalyst and thereby (step E) completes the catalytic cycle. 

The same oxidation state ambiguity that we have seen several times before also operates here. Equation 6.54 shows that if the alkenes are considered to be in the metalacyclopropane (X2 or form), the coupling reaction proceeds with formal reduction at the metal and resembles a reductive elimination of two alkyl groups. Parkin has a case of a reductive coupling in Eq. 6.55,... 

The unsaturated c.vo-enol lactone 17 is obtained by the coupling of propargylic acetate with 4-pentynoic acid in the presence of KBr using tri(2-furyl)-phosphine (TFP) as a ligand. The reaction is explained by the oxypalladation of the triple bond of 4-pentynoic acid with the ailenyipailadium and the carbox-ylate as shown by 16, followed by reductive elimination to afford the lactone 17. The ( -alkene bond is formed because the oxypalladation is tnins addition[8]. 

In addition to /3-H elimination, olefin insertion, and protonolysis, the cr-metal intermediate has also proved to be capable of undergoing a reductive elimination to bring about an alkylative alkoxylation. Under Pd catalysis, the reaction of 4-alkenols with aryl halides affords aryl-substituted THF rings instead of the aryl ethers that would be produced by a simple cross-coupling mechanism (Equation (126)).452 It has been suggested that G-O bond formation occurs in this case by yy/z-insertion of a coordinated alcohol rather than anti-attack onto a 7r-alkene complex.453... 

It is anticipated that many of the catalytic Cp2Zr(II) reactions that might have been considered to proceed via oxidative addition and reductive elimination, such as hydrosila-tion [224] and hydrogenation [225], may actually proceed via a couple of o-bond metatheses, i. e. transmetallation and p-H abstraction, as exemplified by the two contrasting mechanisms for the hydrosilation of alkenes (Scheme 1.70). 

Vinylation or arylation of alkenes with the aid of a palladium catalysts is known as the Heck reaction. The reaction is thought to proceed through the oxidative addition of an organic halide, RX onto a zero-valent [PdL2] species followed by coordination of the olefin, migratory insertion of R, reductive elimination of the coupled product and dehydrohalogenation of the intermediate [HPdXL2] (Scheme 6.1). 

Scheme 6.33 illustrates an example of some zinc-induced three-component coupling reactions of alkyl iodides, electron-deficient alkenes, and carbonyl compounds [51]. In this instance, the isopropyl radical is generated by a one-electron reduction of isopropyl iodide followed by elimination of iodide ion. The resulting radical then adds to acrylonitrile to form an a-cyano alkyl radical, which is con-... 

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