Palladium-catalysis reaction types

A new type of triaryl phosphine-functionalized imidazolium salt containing cations such as (6) has been prepared. Palladium complexes of (6) generated in situ have been used successfully in Heck-type reactions of aryl halides with acrylates and of 4-bromotoluene with styrene derivatives.34 The first Heck-type reaction of aryl halides with allenes has been reported. 1,3-Double arylations were observed with 3-substituted-l,2-allenyl sulfones, while 1-monoarylation was favoured with 3,3-disubstituted-l,2-allenyl sulfones.35 It has been shown that the a-arylation of methane-sulfonamides (7) may be achieved using palladium catalysis reaction proceeds through the sulfonamide enolates.36 It is also reported that palladium cross-coupling of alkynes with /V - (3 - i odophe n y I an i I i ncs) may lead to the formation of substituted carbazoles.37... 

Oxidative vicinal functionalization of alkenes represents an important reaction among alkene transformations as it allows the introduction of two functional groups within a single transformation. In the area of palladium catalysis, this type of transformation usually consists of a two-step procedure, during which the two heteroatoms are introduced into the carbon framework. Despite this simplicity at first sight, the actual reaction mechanism may be very complex and, in principle, several mechanistic pathways are possible. 

Domino Heck-Diels-Alder type reaction has been demonstrated by the reaction of l,3-dicydopropyl-l,2-propadiene with iodobenzene in the presence of dimethyl maleate under palladium catalysis (Scheme 16.28) [33]. 

The three-component synthesis of benzo and naphthofuran-2(3H)-ones from the corresponding aromatic alcohol (phenols or naphthols) with aldehydes and CO (5 bar) can be performed under palladium catalysis (Scheme 16) [59,60]. The mechanism involves consecutive Friedel-Crafts-type aromatic alkylation and carbonylation of an intermediate benzylpalla-dium species. The presence of acidic cocatalysts such as TFA and electron-donating substituents in ortho-position (no reaction with benzyl alcohol ) proved beneficial for both reaction steps. 

According to a recent Organic Syntheses procedure, fluoromethyl phenyl sul-fone, which can be prepared on a large scale [90] (Eq. 23), undergoes a number of useful reactions. A Wadsworth-Emmons-type procedure affords a-fluoro-vinylsulfones [91], which undergo tin-sulfur exchange under free radical conditions (Eq. 24). The products maybe protodestannylated [92],fluorinated [93],or coupled under palladium catalysis [94]. The difluoromethyl phenyl sulfone also shows some useful chemistry. 

The field of homogeneous palladium catalysis traces its origin to the development of the Wacker process in the late 1950s (Eq. 7) [83]. Since this discovery, palladium-catalyzed reactions have evolved into some of the most versatile reactions for the synthesis of organic molecules [84,85]. Palladium-catalyzed Wacker-type oxidation of alkenes continues to be an active field of research [86-88], and several recent applications of NHC-coordinated Pd catalysts have been reported for such reactions. 

Three research groups discovered almost at the same time that non-C2-symmetrical oxazolines of the type 32 can be even more effective ligands for asymmetric catalysis than type 4 ligands (Fig. 11). For the palladium-catalyzed allylic substitutions on 62, record selectivities could be reached using 32 (X = PPhj) [30]. It seems that not only steric but also electronic factors, which cause different donor/acceptor qualities at the coordination centers of the ligand, seem to play a role here [31]. The reaction products can subsequently be converted to interesting molecules, for example 63 (Nu = N-phthalyl) can be oxidized to the amino acid ester 64 [32]. 

There are a number of commonly used reactions of this type and many are named after the chemists who discovered them. They differ, among other things, in the exact nature of the metallic component. One of the first to be developed extensively was the Stille coupling, which specifically involves the coupling of an arylstannane with an aryl halide or triflate under the action of palladium catalysis (Scheme 10.19). 

The most common means of activating aromatic C-H bonds via palladium catalysis is by electrophilic C-H activation. This proceeds more like a Freidel-Craft type metahation mechanism, followed by rearomatization to form versatile aryl-metal intermediates (Scheme 5) [19]. It can occur with electrophilic palladium(II) catalysts such as Pd(OAc)2, PdCl2, Pd(TFA)2 (Scheme 5a) or on electrophilic aryl-pahadium(II) complexes, that result from oxidative addition of palladium(O) into an aryl halide (Scheme 5b). The resultant aryl-palladium(H) complexes are analogous to those observed in conventional cross-coupling reactions and as such are versatile intermediates in the formation of new C-C bonds. 

The synthesis of heterocycles making use of Heck-type reactions is obviously a growing field of current research, which takes advantage both of the broad applicability of palladium catalysis and of the outstanding importance of heterocycles. Palladium catalysis is even more useful for heterocyclic chemistry when taking into account that the Pd-catalyzed de-rivatization of heterocycles is not included in this overview. 

Another potentially powerfnl sequence arises by combining one or two intramolecular Heck-type couplings with an intra- or intermolecular Diels-Alder addition (for early examples of inter-intermolecular one-pot domino Heck-Diels-Alder reactions see Refs. [49] and [50]). An all-intramolecular version of such a sequence has been shown to proceed reasonably smoothly for terminally alkoxycarbonyl-substituted 2-bromotrideca-l,ll-dien-6-ynes under palladium catalysis at 130 °C. At 80 °C, the sequential reaction stops after the two consecutive Heck-type cyclizations and subsequent /3-hydride elimination to give a 1,3,6-triene apparently only the ( )-isomer undergoes the intramolecular Diels-Alder reaction, as the (Z)-l,3,6-triene is observed accompanying the tetracyclic system obtained at 130 °C (Scheme 36). 

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