Alkenyl halides alkenes

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. 

Alkynes react with many of the same electrophilic reagents that add to the carbon-carbon double bond of alkenes Hydrogen halides for example add to alkynes to form alkenyl halides... 

Alkenyl alcohols, (E) disubstituted, 55, 66 ALKYL HALIDES, alkenes from, 55, 103 C-ALKYLATION, phase transfer catalysis... 

The coupling reaction of aryl-alkenyl halides with alkenes in the presence of a palladium catalyst and a base is known as the Heck coupling (Scheme 9.4).6 Since the early 1980s, this type of coupling reaction has been used for die syndiesis of poly(arylenevinylene) and related polymers by polymerization of AB- or AA/BB-type of monomers (Scheme 9.5).7... 

The Pd-catalyzed coupling reaction of aryl-alkenyl halides widi alkenes in the... 

The general catalytic cycle for the coupling of aryl-alkenyl halides with alkenes is shown in Fig. 9.6. The first step in this catalytic cycle is the oxidative addition of aryl-alkenyl halides to Pd(0). The activity of the aryl-alkenyl halides still follows the order RI > ROTf > RBr > RC1. The olefin coordinates to the Pd(II) species. The coordinated olefin inserts into Pd—R bond in a syn fashion, p-Hydrogen elimination can occur only after an internal rotation around the former double bond, as it requires at least one /I-hydrogen to be oriented syn perpendicular with respect to the halopalladium residue. The subsequent syn elimination yields an alkene and a hydridopalladium halide. This process is, however, reversible, and therefore, the thermodynamically more stable (E)-alkene is generally obtained. Reductive elimination of HX from the hydridopalladium halide in the presence of a base regenerates the catalytically active Pd(0), which can reenter the catalytic cycle. The oxidative addition has frequently assumed to be the rate-determining step. 

The Heck reaction consists in the Pd(0)-catalysed coupling of alkenes with an aryl or alkenyl halide or triflate in the presence of a base to form a substituted alkene (Scheme 7.1). The reaction is performed in the presence of an organopalladium catalyst. The halide or triflate is an aryl or a vinyl compound and the alkene contains at least one proton. 

Another general process involves the reaction of Pd(0) species with halides or sulfonates by oxidative addition, generating reactive intermediates having the organic group attached to Pd(II) by a ct bond. The oxidative addition reaction is very useful for aryl and alkenyl halides, but the products from saturated alkyl halides often decompose by (3-elimination. The a-bonded species formed by oxidative addition can react with alkenes and other unsaturated compounds to form new carbon-carbon bonds. The... 

Transition metal-catalyzed transformations are of major importance in synthetic organic chemistry [1], This reflects also the increasing number of domino processes starting with such a reaction. In particular, Pd-catalyzed domino transformations have seen an astounding development over the past years with the Heck reaction [2] - the Pd-catalyzed transformation of aryl halides or triflates as well as of alkenyl halides or triflates with alkenes or alkynes - being used most often. This has been combined with another Heck reaction or a cross-coupling reaction [3] such as Suzuki, Stille, and Sonogashira reactions. Moreover, several examples have been published with a Tsuji-Trost reaction [lb, 4], a carbonylation, a pericyclic or an aldol reaction as the second step. 

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

C-C bond formation using the Heck reaction allows the introduction of functional groups to obtain new organic structures on solid supports. This reaction between an alkene with an alkenyl or an aryl halide has been widely employed in various in-tra- and inter-molecular versions on solid-phase because of the readily accessibility of starting materials. The Heck reaction was performed on immobilized aryl or alkenyl halides with soluble alkenes and vice versa (Scheme 3.11). 

Heck in one of his first papers already demonstrated the feasibility of applying the palladium-catalyzed crosscoupling of aryl and alkenyl halides with alkenes repetitively on appropriate oligofunctional substrates. For example, twofold coupling of 1,4-diiodobenzene with styrene furnished 1,4-distyrylbenzene in 67% yield (Scheme 1). Since then, a large number of ortho-, meta-, and / r< -dihaloarenes and -heteroarenes have been subjected to twofold Heck reactions with various alkenes (Schemes 2-4). 

The Pd-catalyzed reaction of alkynylzincs with alkenyl electrophiles is generally dependable and widely applicable. However, recent studies with 1,1-dihalo-l-alkenes, 1,2-dihaloalkenes, and other alkenyl halides have indicated that the presence of two proximal... 

The Heck reaction involves the coupling of an organopalladium species formed by oxidative addition to an alkene followed by /S-hydride elimination. The product is an alkene in which a vinyl hydrogen on the original alkene is replaced by die organic group on palladium. Thus aryl and alkenyl halides can be coupled to alkenes. 

Terminal alkynes 170 undergo the substitution with aryl and alkenyl halides to form arylalkynes and enynes in the presence of Cul, as described in Section 3.3.1. However, the insertion of terminal alkynes 170 occurs in the absence of Cul, and the alkenylpalladium complex 171 is formed as an intermediate, which cannot be terminated by itself and must undergo further reactions, such as alkene insertion or anion capture. These reactions of terminal and internal alkynes with intermediates 172 and 173 are summarized in Schemes 3.7 and 3.8. 

The stereodefined alkenyl halides are of prime importance due to the recent developments of di- or trisubstituted alkene synthesis by cross-coupling reactions between organometallics and alkenyl halides catalyzed by transition metal compounds 171). These alkenyl halides can be conveniently obtained from alkenylboranes or alkeneboronic acids. B-Alk enylcatecholboranes undergo rapid hydrolysis when stirred with excess water at 25 °C (Eq. 109)102). The alkeneboronic acids are usually crystalline solids of low solubility in water and can be easily isolated and handled in air without significant deterioration. 

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