Palladium-catalyzed transformation

Another possibility of allowing differentiation is preference in the formation of different ring sizes in the different steps, as in the reaction of 6/1-6, where a four- or a six-membered ring could be formed in the first step. 

an early example of two successive Heck reactions is the formation of the condensed bicyclic compound 6/1-7 from the acyclic precursor 6/1-6 by Overman 

The variety of products can be increased by an anion-capturing process of tbe intermediate Pd-compounds, and tbis has been intensively explored by Grigg and coworkers [18]. 

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The basic method of Heravi et al. <1996IJB1260, 2002PS2491> is the palladium-catalyzed transformation of the Tpropargyl compound 314 its treatment with a Pd(ll) catalyst resulted in the ring closure to the thiazole part containing an -methylene moiety 315. The related selenazole 316 was also synthesized <1998IJB587, 2001PS87>. 

Because of the usually high temperatures required to rearrange propargyl sulfi-nates to allenyl sulfones, a report about such palladium-catalyzed transformations seems to be promising [106], Even at low temperatures the synthesis and isomerization of trifluoromethanesulfinate 51 are possible if, during the reaction of the ter-... 

Novel palladium-catalyzed transformations of allylic alcohol and its derivatives are developed by Yamamoto and his co-workers. Bis(7r-allyl)palladium complexes are considered to be the key intermediates for the allylation of benzyl-idenemalonitrile and benzyl chloride (Scheme 29) (for examples see Refs 176,176a-176d). Asymmetric version of these reactions is being awaited. 

Furane derivatives, like pyrroles, couple effectively with acetylenes. In case both the a-, and / -position are available for the reaction, like in the case of 4,5-dibromo-furfural in 6.45., the cross-coupling takes place preferentially in the former position65 The observed selectivity is in line with other palladium catalyzed transformations of dihalofuranes, such as their Stifle coupling.66... 

Palladium-catalyzed transformations greatly enhance the scope of solid-phase synthetic chemistry. A number of fundamental pharmacophores are accessible through a variety of reliable manipulations that may be performed in high yield under mild conditions. This area continues to grow, as solution-phase chemistry is adapted to provide better methods for carbon-carbon bond formation in combinatorial chemistry. We view these advances as central to the field and look forward to future developments. 

The Ullmann coupling, being a copper-catalyzed N-arylation, is known to be more sluggish than the corresponding palladium-catalyzed transformation. Even so, Wu and coworkers succeeded to accelerate reaction times down to only one hour with preserved chemoselectivity (Scheme 26) [93]. A set of aromatic azaheterocycles produced yields of 49-91% after 1 -22 hours of microwave heating. 

The copper-mediated C(aryl)-0, C(aryl)-N, and C(aryl)-S bond formation can be used as an alternative to many palladium-catalyzed transformations <2003AGE5400, 2006T4435>. 

Bideau and coworkers found that vinylepoxysilanes 169 underwent a palladium-catalyzed transformation to give siloxybutadienes 170 and y8,y-unsaturated-o -silyl ketones... 

Scheme 12 Various palladium-catalyzed transformations of open-chain precursors leading to 1,2-dialkylidenecycloalkanes 83 [55,57-60,62-75]...

Yet another palladium-catalyzed transformation leading to 1,2-dialkyl-idenecycloalkanes was established by Trost et al. when investigating a catalytic Alder-ene reaction (path D in Scheme 12). They showed that two different catalyst systems are capable of cycloisomerizing enynes 92 to either cyclic 1,4-dienes 96—the products of regular Alder-ene reactions— or the 1,3-dienes 95 (Scheme 15) [66-68]. Starting from palladium acetate, the reaction presumably occurs by coordination of both unsaturated moieties (intermediate 93) and subsequent cycloisomerization to the ring-... 

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