Allenes Heck reaction

Several routes to the pyrrolo[l,2-f][l,2,3]triazole skeleton have been described. Intramolecular dipolar cycloaddition of azido-alkenes or alkynes seems to be the most convenient process, although the cyclization efficiency seems to be highly substrate dependent (Scheme 16) <2002JA2134, 2003T1477, 2005SL2187, 2005TL8639>. The formation of this bicyclic system by an intramolecular Heck reaction is an attractive alternative. The recent syntheses of sulfamides by intramolecular cyclization of alkenes or allenes offer a complementary route to the classical... 

As shown in the previous sections, a (cr-allenyl)palladium species, which is formed from a propargyl electrophile and a Pd(0) catalyst, reacts with a hard carbon nucleophile in a manner analogous to the Pd-catalyzed cross-coupling reaction to give a substituted allene. The results indicate that the reactivity of the (cj-allenyl)palladium species is similar to that of an alkenylpalladium intermediate. Indeed, it was found that the (cr-allenyl)palladium species reacted with olefins to give vinylallenes, a reaction process that is similar to that of the Heck reaction of alkenyl halides [54]. 

Palladium/silver-catalyzed Heck reactions have usually involved vinyl or aryl halides and alkenes, but these reaction conditions were also extended to allenes. Indeed, Zenner and Larock65 showed that simple alkyl allenes readily reacted with aryl and vinyl iodide derivatives in the presence of palladium acetate or chloride and silver phosphate. Moreover, the reaction could be rendered asymmetric using chiral ligands the best one was a bisoxazolidine derivative (Scheme 10.37). 

A recent paper explored the 1,2-functionalization of the allene moiety in monocyclic P-lactam allenynol derivatives under palladium-catalyzed reaction conditions <02CEJ1719>. Remarkably, a novel domino process, namely allene cyclization/intramolecular Heck reaction... 

The Pd(0)-catalyzed reactions of propargylic compounds so far discovered can be classified into four types, I, II, III, and TV, from a mechanistic viewpoint. The allenyl intermediate complex 8 undergoes three types of transformation, depending on reactants. The reactions of Type I proceed by insertion of unsaturated bonds into the a-bond between palladium and sp carbon in 8. This a-bond has a reactivity similar to the a-bond formed by the oxidative addition of alkenyl halides to Pd(0) in the Heck reaction [3]. Therefore, reactions similar to those observed in the Heck reaction are expected to occur witli the intermediate 8. Alkenes and carbon monoxide are known to insert into the palladium-carbon a-bond. The allene derivatives 9 are formed by these reactions (Scheme 11.3). 

Heterocycle formation, for example, dihydropyrans from allenes and (Z)-3-iodo-2-propenols, and isoquinolines from alkynes and A-f-butyKo-iodobenzylidenelimines," is an extension of the versatile Heck reaction. 

Muller and coworkers have recently developed a coupling-isomerization reaction, initially identified as a side reaction which occurred under standard Sonogashira conditions [79]. As demonstrated below, the coupling reaction is followed by a shuffling of oxidation states via an alkyne-allene isomerization [80]. The product, a,P-unsaturated ketone 146, is reminiscent of a product which would be obtained from a Heck reaction. The utility of this reaction was further demonstrated when diamine 147 was added to the reaction pot. Following a conjugate addition reaction and imine formation, compound 148 resulted from the three-component, one-pot reaction sequence enabled by the coupling-isomerization reaction. 

A special type of Heck coupling was demonstrated vstith 1,2-allenyl carboxylic acids, which led to butenolides. Ma et al. [146] demonstrated a dramatic dependence of the reaction on the base and solvent used. In acetonitrile, intramolecular addition of the carboxylic acid to the adjacent allenic group proceeded without a subsequent Heck reaction. In toluene as solvent, especially in the presence of Bu4NBr, the desired butenohde was obtained. 

A facile synthesis of 2-aryl-1,3-dienes has been realized from aryl halides and allenes. Ionic liquids prove to be a superior media for the Heck reaction with aryl... 

Heck reactions. The o-oxazolinylphenyl(diphenyl)phosphine ligand 11 is an xcellent ligand to direct enantioselective Heck reactions. On the other hand, the iioxazolinylmethane derivative 12 has been used for the annulation of allenes by means of jnctionalized aryl iodides. ... 

Carbopalladation is the reaction of a cr-bonded organopalladium complex I with an unsaturated molecule (such as an alkene 2) to yield the migratory insertion product 3 (Scheme 1). The reaction is tremendously flexible, allowing for a wide variety of structural types for both reactants 1 and 2. The precursors of palladium complexes 1 are commonly alkenyl or aryl halides or triflates (8 and 9, respectively), the reaction of which is more commonly termed the Heck reaction. Allylic systems 10, which react to provide -Tr-allylpalladium complexes, can participate in the reaction as can benzylic precursors 11. Acylpalladium complexes 12 also react and are commonly generated in the same reaction vessel by Pd-catalyzed carbonylation. Their unsaturated reaction partners include alkenes 2, alkynes 4, dienes 6, allenes, and arenes, all of which can be electron rich or poor. Carbopalladation occurs in a syn fashion allowing the installation of stereocenters (2- 3) or control of alkene geometry (4- 5). 

Scheme 5.16 presents two conceptually interesting examples of Mizoroki-Heck reactions. In the first one, Ma and Negishi [41] showed that allenes are also suitable for intramolecular Mizoroki-Heck reactions 75a was cyclized in a 5-endo- g mode, providing benzofulvene (76a) in 69% yield (75a 76a, Scheme 5.16). Its cognate, 75b,...

Ma and Negishi [41] also extended the scope of c do-selective Mizoroki-Heck reactions of allenes to the formation of medium-sized rings and macrocycles. A selection of representative reactions is depicted in Scheme 5.39. Cyclizations of precursors 228a and 228b provided the corresponding eight- and nine-membered rings in 56% and 62% yields... 

In most of the palladium-catalysed domino processes known so far, the Mizoroki-Heck reaction - the palladium(0)-catalysed reaction of aryl halides or triflates as well as of alkenyl halides or triflates with alkenes or alkynes - has been apphed as the starting transformation accordingly to our classification (Table 8.1). It has been combined with another Mizoroki-Heck reaction [6] or a cross-coupling reaction [7], such as Suzuki, Stille or Sonogashira reactions. In other examples, a Tsuji-Trost reaction [8], a carbonylation, a pericyclic or an aldol reaction has been employed as the second step. On the other hand, cross-couphng reactions have also been used as the first step followed by, for example, a Mizoroki-Heck reaction or Tsuji-Trost reactions, palladation of alkynes or allenes [9], carbonylations [10], aminations [11] or palladium(II)-catalysedWacker-type reactions [12] were employed as the first step. A novel illustrative example of the latter procedure is the efficient enantioselective synthesis of vitamin E [13]. 

Similarly, in the reaction of 6, a four- or a six-membered ring could be obtained in the first step with clear preference towards the formation of the six-membered ring. Thus, one of the early examples of two successive Mizoroki-Heck reactions is the synthesis of the condensed bicyclic compound 7 from the acyclic precursor 6 by Overman and coworkers [15] (Scheme 8.2). Later, it has been shown by Negishi that a multitude of rings as in 9 can be formed according to this scheme, starting from 8 [16] (Scheme 8.2). The cycloisomerization of enynes and allenes catalysed by palladium has also proven to be an interesting process for the synthesis of tricycle 13, as shown by Trost and coworkers [17]. 

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