Alkenylation of haloarenes

The application of Heck coupling polycondensation is not limited to the synthesis of poly(arylene vinylene)s via the alkenylation of haloarenes in simple monomer systems but includes a variety of self- and cross-coupling reactions involving reactants with various functionalities. For instance, the polycondensation of diiodoarene with bis(acrylamido)arene by the Pd(OOCCH3)2—P (o-C6H4—CH3)3 catalyst yields respective aromatic polycinnamamide [106] ... 

The cross-coupling between 1-alkenylboronates and haloarenes (Scheme 2-35) is useful for the alkenylation of haloarenes [93, 94]. 

The proposed mechanism for a standard Heck reaction is depicted in Scheme 6.5. Generally, a haloalkene or haloarene undergoes oxidative addition to an in situ generated, coordinatively unsaturated 14-electron palladium(O) complex, but other substrates such as tosylates, triflates or diazonium salts can also be applied. Subsequent, sy -insertion into the C=C double bond of a complexed olefin yields a t7-(j -alkenyl) or (j- aryl)alkylpalladium complex. If no hydrogen atom in a pseudo cis-position relative to the palladium is present, an internal rotation step is required prior to syw-elimination of the olefin to afford the traws-olefin product and a palladium(II) hydride complex. The latter is restored to the initial Pd(0) species by base-induced reductive elimination.137"401... 

In the first step of the sequence (A in Scheme 3-1), a holoalkene or haloarene is commonly assumed to add oxidatively to the coordinatively unsaturated palladium(O) complex, generating ao-alkenyl- or o -arylpalladium(IT) complex [8]. As the electrophilicity of this complex is enhanced, it more readily accepts an alkene molecule in its coordination sphere, probably by exchange for another ligand. 

An important feature of the Suzuki reaction is that it allows cr/Z Z-alkenyl or alkyl-aryl coupling of fi-a//cy/-9-borabicyclo[3.3.3]nonane derivatives with haloalkenes and haloarenes without concomitant p-hydride elimination. The required organoboranes are obtained by hydroboration of the appropriate alkenes or functionally substituted alkenes with 9-BBN. The coupling reaction tolerates the presence of functional groups in both reaction partners, thus circumventing a requirement of their prior pro-tection. ... 

The coupling of arylzinc halides with alkyl (E)-2,3-dibromoalkenoates occurs at C-3. 3-Arylindoles are obtained from 3-indolylzinc halides with haloarenes." A synthesis of 1,4-disubstituted butadienes starts from the hydrozirconation of ethynyl phenyl selenide. After Pd(0)-catalyzed coupling with alkenyl bromides, the seleno group is replaced via Grignard reaction in the presence of (Ph3P)2NiCl2. 

A coordinatively unsaturated 14-electron palladium(0) complex, usually containing two tertiary phosphanes as weakly electron-donating ligands, has been proved to be the catalyUcaUy active species. It is conunonly generated in situ from either from a palla-dium(0) complex or by reduction of relatively inexpensive paUadium(II) acetate or chloride.In the first step of the catalytic cycle ( in Scheme 1), a haloalkene, a haloarene, or a similar substrate is oxidatively added to the coordinatively unsaturated paUadium(O) species to generate a tr-alkenyl- or (r-arylpalladium(II) complex.f t This then coordinates an alkene molecule, and when the latter and the alkenyl (aryl) residue on the palladium are in a cis orientation, the cr-alkenyl- or o--arylpalladium complex can... 

The Heck reaction can be used to couple alkenyl, aryl, allyl (the intramolecular Heck reaction on aUyUc substrates is called a palladaene reaction),f ° t f benzyl, methyl, alkoxycarbonyhnethyl, alkynyl, certain alkyl, and silyl fragments to a variety of alkenes. The nature of the leaving group greatly affects the reaction rate aryl iodides react faster than bromides, and aryl chlorides are notoriously unreactive unless special catalysts or ligands and elevated temperatures are used to enhance the reaction rate. This has been taken to indicate that the oxidative addition of the haloarene (haloalkene) to pal-ladium(O) is the rate-determining step. " It has been shown that the Heck reaction can be performed with aryldiazonium salts, A -nitroso-Af-arylacetamides, and hy-pervalent iodo compounds " at room temperature. 

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