Palladacycles, generation

The acetylene-insertion reaction presumably occurs by the following mechanistic sequence (1) insertion of Pd(0) into the SCB, (2) regioselective yy -silylpalladation of the acetylenic compounds to provide seven-membered l-pallada-4-silacyclic intermediate, and (3) reductive elimination of Pd(0) to afford silacyclohexene. Alternatively, /3-hydride elimination would open the palladacycle, generating a vinylpalladium hydride species that would undergo reductive elimination to yield the ring-opened allylvinylsilane. Isotopic labeling studies provided evidence in support of this mechanistic hypothesis (Scheme 47). 

Generally, monophosphine complexes can be generated by decomposition of suitable precursors, among which the most notable are palladacycles (Section 9.6.3.4.7). A spectacular example makes use of spontaneous disproportionation of a dimeric complex of Pd1 with very bulky ligands to give one of the most reactive catalytic systems known so far, which catalyzes the fast crosscoupling of arylboronic acids with aryl chlorides and hindered aryl bromides at room temperature (Equation (28)) 389... 

The domino carbonylation and Diels-Alder reaction proceed only as an intramolecular version. Attempted carbonylation and intermolecular Diels-Alder reaction of conjugated 2-yne-4-enyl carbonates 101 in the presence of various alkenes as dienophiles give entirely different carbocyclization products without undergoing the intermolecular Diels-Alder reaction. The 5-alkylidene-2-cyclopenten-4-onecarboxy-lates 102 were obtained unexpectedly by the incorporation of two molecules of CO in 82% yield from 101 at 50 °C under 1 atm [25], The use of bidentate ligands such as DPPP or DPPE is important. The following mechanism of the carbocyclization of 103 has been proposed. The formation of palladacyclopentene 105 from 104 (oxidative cyclization) is proposed as an intermediate of 108. Then CO insertion to the palladacycle 105 generates acylpalladium 106. Subsequent reductive elimination affords the cyclopentenone 107, which isomerizes to the cyclopentenone 108 as the final product. 

A plausible mechanism for the one-pot synthesis ofcarbazoles is shown in Scheme 5. It consists of two interlinked catalytic cycles. In the first cycle a classical Buchwald-Hartwig amination reaction occurs to generate an intermediate 5 which then enters the second cycle by oxidative addition to Pd(0). The resulting Pd(II) complex then undergoes intramolecular C-H activation to give a six-membered palladacycle which subsequently yields the carbazole by reductive elimination. 

Treatment of 88 with /-BuOK afforded the corresponding palladacycles 89 in good overall yield, as a mixture of two diastereosiomers (Equation 13). The stereoselective event involved an intramolecular displacement of the iodide by a potassium ester enolate generated in situ <20030M2961>. 

The results imply that the conjugated enallene ester system (l,2,4-alkatriene-3-carboxylate) 127 is required for incoiporation of the second molecule of carbon monoxide, and the following mechanism (Scheme 11-39) has been proposed. The formation of the palladacyclopentene 137 from 136 is suggested as an intermediate of 140. Then carbon monoxide insertion into the palladacycle 137 generates the acylpalladium 138. Subsequent reductive elimination affords the cyclopentenone 139, which isomerizes to give the cyclopentenone 140 as a final product. 

The Pd(0) complexes [Pd(PPh3)4] and [Pd P(OPh)3 4] also react with hexafluoroacetone to give compounds containing a three-membered ring. With [Pd P(OMe)3 4] or [Pd P(OMe)2Ph 4], however, coupling of two ketones occurs to generate a five-membered palladacycle - ... 

These catalysts are more active in some cases than complexes generated from the same ligands and either palladium salts or dba adducts. In general, palladacycles have a higher thermal stability compared to the Pd/phosphine system and precipitation of palladium black is reduced. 

Later in 2010, Wu and co-workers developed a series of carbene adducts of cyclo-palladated ferrocenylimine (Figure 7) and evaluated their activity in the KTC reaction between aryl halides and aryl Grignard reagents. Palladacycle 68 was identified as the most active catalyst, generating a number of di- and tri-orf/io-substituted biaryls in good to excellent yield using 0.5 mol% catalyst loading and 2 equiv. of LiCl (Scheme 6). 

Scheme 6 Reactivity of palladacycle 68 in KTC coupling generating di- and tri-ortho-substituted biaryls...

In the presence of alkyl halides, palladacycle 5 reacts at the C-X bond to generate a proposed palladium(IV) species such as 6. In recent years, palladium(IV) species have become popular catalytic intermediates in the scientific community, and are generally accessed via oxidation of palladium(II) to palladium(IV) with strong oxidizing agents [27, 28], The formal oxidation of 5 to 6 using alkyl halides would be one of the mildest methods to accomplish this feat. 

Catellani found that the interaction of palladacycles such as 5 with aryl halides led to the incorporation of the aryl group in a manner analogous to alkyl halides [39], A similar mechanism could be proposed for the oxidative addition of aryl halides to palladacycles such as 5 to generate a palladium(TV) species, although to date there is no direct evidence for this pathway. An alternative mechanism has been put forward by Echavarren which involves transmetallation between two palladium... 

The first example of an o/t/20-alkylation/Mizoroki-IIcck coupling was reported by Catellani [4] in 1997. Using the PNP dimer as a catalyst in the presence of an aryl halide, norbomene, an alkyl iodide, a terminal olefin and a base at room temperature, 1,2,3-trisubstituted benzenes (Scheme 16), were synthesized through alkylation of a palladacycle of type 35, followed by Mizoroki-Heck coupling with an arylpalladium(II) species of type 36. Although the synthetic scope of the reaction was limited, the importance of the report reveals an unprecedented catalytic transformation where two aryl C-H bonds are converted to sp2-sp3 C-C bonds followed by a standard Mizoroki-Heck coupling. The 1,2,3-trisubstitution pattern generated in the products would be very difficult to obtain via conventional methods. 

In the mechanistic studies of the reactivity of (arylnorbomyl) palladacycles [39], Catellani observed the ort/zo-arylation of the PNP dimer to generate unsymmetrical homobiaryl products. This o/t/io-arylation was combined with the Mizoroki-Heck... 

Also worth mentioning here are studies based around the preparation and use of silica-supported palladacyclic complexes. It was the use of these that gave valuable evidence for the decomposition of half-pincer and SCS pincer " complexes during Heck reactions, generating soluble Pd(0) species that are the true catalysts. 

Interestingly, HR of < -bromobenzaldehyde (93) with acrylate gave the doubly substituted product 94 and the expected product 95 under Jeffery s ligandless conditions [60]. Eormation of 94 is explained by the following mechanism. Insertion of acrylate to 93, followed by oxidative addition of aldehyde generates 96. The palladacycle 97 is formed by decarbonylation, and its reductive elimination gives 98. The final product 94 is obtained by HR of 98 with acrylate. 

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