Aryl/vinyl halides

The first step in the cycle, analogous to the cross-coupling reactions, is the oxidative addition of an aryl (vinyl) halide or sulfonate onto the low oxidation state metal, usually palladium(O). The second step is the coordination of the olefin followed by its insertion into the palladium-carbon bond (carbopalladation). In most cases palladium is preferentially attached to the sterically less hindered end of the carbon-carbon double bond. The product is released from the palladium in a / -hydrogen elimination and the active form of the catalyst is regenerated by the loss of HX in a reductive elimination step. To facilitate the process an equivalent amount of base is usually added to the reaction mixture. 

The palladium catalyzed iminoannulation and carboxyannulation of alkynes and an appropriate aryl/vinyl halide is an efficient tool to construct six membered nitrogen and oxygen heterocycles. The process encompasses the concomitant formation of a carbon-carbon and a carbon-heteroatom bond. 

The transition metal catalyzed carbon-carbon bond formation between organomagnesium reagents and aryl (vinyl) halides has been one of the pioneering entries into cross-coupling chemistry. The reaction has been widely utilized since than in azine chemistry,22 with the limitation that the functional group tolerance of Grignard reagents is only moderate. Here only some of the more recent developments will be mentioned. 

Photochemical reactions of aryl-vinyl halides also proceed via a vinylic cation [62-64], The amounts of radical versus ionic products are affected by... 

Vinyl cations [1], the dicoordinated unsaturated analogs of divalent carbenium ions, were first detected by Grob and coworkers in the early 1960s in solvolysis reactions of a-aryl vinyl halides [2]. The direct NMR detection of vinyl cations in superacidic media was achieved in 1992 at temperatures below -100 °C [3]. We recently reported a convenient synthesis of unusually stable vinyl cations at room temperature [4, 5]. One reason for the unusual high thermodynamic stabilization of these vinyl cations is the presence of two 3-silyl substituents. [4]. We report here details of the X-ray structure of the vinyl cation 1 and discuss the structural and spectroscopic consequences of 3-SiC hyperconjugation [6]. 

Besides the traditional coupling between alkenes and aryl (vinyl) halides, other functionalized aryl derivatives can also couple with alkenes in the Heck reaction, including aryl silanes,stannanes, bismuth, antimony,triflates, boric acid, phosphonic acid, carboxylic acid, and diazonium salt. ... 

The palladium-catalysed Mizoroki-Heck reaction is the most efficient route for the vinyla-tion of aryl/vinyl halides or triflates. This reaction, in which a C—C bond is formed, proceeds in the presence of a base (Scheme 1.1) [1, 2], Nonconjugated alkenes are formed in reactions involving cyclic alkenes (Scheme 1.2) [le, 2a,c,e,g] or in intramolecular reactions (Scheme 1.3) [2b,d-g] with creation of stereogenic centres. Asymmetric Mizoroki-Heck reactions may be performed in the presence of a chiral ligand [2], The Mizoroki-Heck reaction has been intensively developed from a synthetic and mechanistic point of view, as expressed by the impressive number of reviews and book chapters [1,2]. 

In addition to intermolecular carbonylations, there are intramolecular reactions that allow for the synthesis of various heterocycles. As a prime example, the intramolecular alkoxy- or aminocarbonylation (cyclocarbonylation) of hydroxyl-or amino-substituted aryl/vinyl halides enables the synthesis of lactones, lactams, oxazoles, thiazoles, imidazoles, etc. [229]. 

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