Pd II Catalysis

TTomogeneous catalysis has made considerable progress in the past decade. Even so there is still uncertainty as to the detailed mechanism of many catalytic reactions. This is particularly true of Pd (II) catalysis. The advances in organic synthesis by Pd (II) catalytic chemistry have been impressive a recent review (I) of two volumes had few references before 1963. However the advances have mainly been in synthetic chemistry, and relatively little effort has been expended on studies of mechanism in this chemistry. To fill this gap we have undertaken a systematic study of the basic mechanisms of Pd (II) catalysis. Some choice of the many catalytic reactions available for study had to be made. For several reasons the reactions chosen for initial study were olefin oxidation and exchange reactions of vinylic and allylic esters and chlorides. Work has concentrated on the PdCl2-NaCl or LiCl systems in... 

A review of the use of Pd(II) catalysis for the acceleration of the Cope, Claisen, and aza-Claisen rearrangements has focused on the mechanism of catalysis, stereochemical (g) outcome and synthetic applications of these reactions.4... 

Jung, K. W. Mild and efficient formation of symmetric biaryls via Pd(II) catalysis and Cu(II) oxidants. Tetrahedron Lett. 2001, 42, 7729-7731. 

The first step of this reaction is the known ring cleavage of the cyclopropanol 106 under Pd(II) catalysis [94]. Next, the authors suggest either a Wacker-type process for the Pd(II)-catalyzed reaction leading to 2,3-dihydro-lff-pyridin-4-ones 107 after a /J-hydride elimination step, or... 

From cyclopropenylketones, either 2,3,4- or 2,3,5-trisubstituted furans are available with high regioselectivities, depending on the choice of the catalyst <2003JA12386>. Pd(ii)-catalysis leads to the formation of the 2,3,5-isomers, whereas with Cul as a catalyst 2,3,4-isomers are obtained (Scheme 8). 

Often a base such as NaOAc is necessary to make the reaction go smoothly (281), Basic solvents such as A, A -dimethylformamide (DMF) also aid in the formation of 77-allyl compounds from 1-olefins (200) in this case [(DMF)2H][Pd2Cle] is also formed. In a recent study of the formation of 77-allylic complexes from 1-olefins in DMF under mild conditions, it was suggested that bases may not only shift the equilibrium to the right by neutralizing the acid but may also aid in removal of the proton from the allylic position (40). One possible mode of promotion might be the stabilization of intermediate Pd(IV) hydrido species. However, more work is required before the role of basic agents in Pd(II) catalysis is finally understood. 

Since Pd(II)-hydride is generally not stable to decomposition to Pd(0) and the intermediate hydride has not been isolated. Stable Pd(II) hydrides have been reported, but this is not direct evidence they are intermediates in the oxidation. However, Maitlis, in his model system for Pd(II) catalysis, has been able to demonstrate Pd(II)-hydride elimination to give a stable hydride (133, 182) (see Section V, A). 

One potentially very useful synthetic reaction in Pd(II) catalysis is the vinyl and allylic exchange reactions,... 

Pyridyl triflates in the benzenoid 3-position readily couple with terminal acetylenes (88JOC386). When the phenylacetylene is substituted in the phenyl ring by an o-amino group, the alkynylated product (56) can be cyclized by Pd(II)-catalysis to an indole, in this case to 2-(3-pyridyl)indole (89TL2581). 

Scheme 24). Reductive deoxygenation of the derived tetrazole provided 177 which was N-benzylated and demethylated with hot aqueous HBr to give tetrahydroisoquinoline 178. Esterification with acid chloride 60 gave ester 179 which smoothly underwent intramolecular coupling under Pd(II) catalysis to give the axially prostereogenic lactone 180. Reduction of 180... 

In 1987, Oppolzer and Gaudin reported an intramolecular version of the reaction of a tt-allylpalladium complex with an alkene. The reactions of various 2,6-octadienyl acetate derivatives were achieved in the presence of Pd(dba)2-PPhj at 70-80 C leading to 2-vinyl-1-alkenyUdenecyclopentane derivatives in yields ranging from 20% to 98% (Schemes 4 and 5). With one or two methyl groups attached at the alkenyl terminus, the acyclic alkeny-lallyl acetates under Pd(II) catalysis cycUzed to yield l-alkenyl-2-vinylcyclopentane derivatives (Scheme A 7r-allylpalladium intermediate generated from an allyl ester can... 

An alternative sequence was the addition of allyl bromides to the thioamides in the presence of potassium carbonate, and heating [52, 80]. It involved isomerization of the S-allyl thioimidates to the S-allyl keteneaminothioacetal, prior to the rearrangement, which explains the drastic conditions required (160-200 °C). Similarly, allylation on nitrogen could be achieved under Pd(II) catalysis [52, 80] (without initial isomerization). 

Fig. 3 Generic Pd(0)/Pd(II) catalysis cycle for oxidative C-H cross-coupling...

Many questions remain to be addressed regarding the mechanisms of these amidation reactions. Primarily, are high-valent Pd intermediates involved or are classical Pd(0)/Pd(II) catalysis cycles operative If single-electron oxidation affords Pd(III) intermediates, does C-N bond formation proceed directly from... 

V Pd-CATALYZED REACTIONS INVOLVING NUCLEOPHILIC ATTACK ON LIGANDS C. REACTIONS VIA Pd(II) CATALYSIS... 

Reactions via Pd(II) catalysis involve a nucleophilic addition to the tenninal position of the 1,3-diene to give a 7r-allylpalladium intennediate (Scheme 6). Subsequent nucleophihc attack on the TT-allyl complex gives a 1,4-addition product The reaction is carried out in the presence of an oxidant and in this way Pd(ll) is regenerated in the second reaction step. 

The utility of the Heck reaction in its many forms has been extensively studied for the construction of useful synthons and complex molecules. Advances in C—H functionalization have enabled the synthesis of Heck products directly from unactivated arenes via Pd(II) catalysis. Olefination of electron-rich and -deficient arenes has been reported. Approaches to olefination of arenes have included directed olefination with haloolefins. ... 

When the use of nucleophiles other than water in the presence of terminal alkenes under Pd(II) catalysis Wacker-type products frequently predominate. Use of White s electrophilic ftts-sulfinyl Pd(ll) acetate catalyst with terminal alkenes in the presence of acetic acid enabled the preparation of terminal acetoxylated olefins via allylic C—H oxidation and subsequent regioselective nucleophilic trapping of a Pd 7i-allyl complex (Table 3.3). The substrates screened afforded the desired end products with high levels of fi-selectivity and in moderate yields with excellent linear branched ratios. Recent updates to this method include the use of A-tosylcarbamates as nucleophiles. ... 

Oxidative Heck reactions via Pd(II) C—H functionalization of terminal alkenes with pinacol boranes have been described for the preparation of styrenes and derivatives through electrophilic Pd(II) catalysis (Scheme 3.20). ° Treatment of a functionalized allylic precursor with the Pd(II) catalysts listed facilitated an allylic C—H activation. Subsequent transmetallation of the aryl boronic acid and reductive elimination afforded the desired olefin with excellent stereoselectivity. The scope of the transformation allows for a variety of activating and deactivating substituents on the aryl boronic acid as well as numerous functional groups on the starting alkene. A tandem allylic C—H oxidation/vinylic arylation protocol has also been reported. " ... 

Approaches to the preparation of functionalized sucdnimides and their ring-opened analogs via intra-moiecular carbonylation of fluoroamide-directed sp -hybridized C—C bonds represent an important step forward for Pd(II) catalysis and can provide direct access toward the preparation of differentiated 1,4-dicarbonyl compounds a common structural motif in complex molecule synthesis (Schane 3.27). ... 

A novel type of coupling was introduced in 2006 when Pd(II) was directly used tmder an atmosphere of dioxygen to perform oxidative Pd(II) catalysis [63]. This methodology, which connects alkenylboron derivatives 71 to olefins 72 (even highly substituted or cyclic ones) in the absence of base, works at moderate temperatures and in short times, minimizing undesired side-reactions. The dienic esters are recovered stereoselectively in good to high yields (Scheme 38). 

Later, a similar cascade reaction combining amino and Pd(II) catalysis was established by Yu et al. [50], whereby PdCl was utilized as the metal salt catalyst instead of PdCPPhj) (Scheme 9.56). This metal-organic binary catalytic system gave access to synthetically useful cyclopentenes, with comparable results with respect to yields and selectivities. 

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