Pd-allyl cations

We shall consider reactions catalysed by two different types of pro-catalyst the first (type A) employs Pd-allyl cations ([Pd(a]lyl)(PCy3)]+/Et3SiH or [Pd(allyl)(MeCN)2] + ), and the second (type B) employs Pd-alkyl or chloro complexes ([(phen)Pd(Me)(MeCN)]+, where phen = phenanthroline, and [(RCN)2PdCl2]). These two types of catalysts give very different products in the cyclo-isomerisation of typical 1,6-dienes such as the diallyl-malonates (10), Scheme 12.6. Since there is known to be a clear order of thermodynamic stability 11 < 12 <13, with a difference of ca. 3-4 kcal mol 1 between successive pairs, any isomerisation of products under the reaction conditions will tend towards production of 12 and 13 from 11 and 13 from 12. Clearly, when 11 is the major product (as with pro-catalysts of type A), it must be the kinetic product (see Chapter 2 for a discussion of kinetic and thermodynamic control of product distributions). However, when 12 is generated selectively, as it is with pro-catalysts of type B, there is the possibility that this is either generated by rapid (and selective) isomerisation of 11 or generated directly from 10. 

The Simmons-Smith cyclopropanation reaction Stereochemically controlled epoxidations Regio- and Stereocontrolled Reactions with Nucleophiles Claisen-Cope rearrangements Stereochemistry in the Claisen-Cope rearrangement The Claisen-Ireland rearrangement Pd-catalysed reactions of allylic alcohols Pd-allyl acetate complexes Stereochemistry of Pd-allyl cation complexes Pd and monoepoxides of dienes The control of remote chirality Recent developments Summary... 

Lipases have also been combined with palladium catalysts to provide the dynamic aspect of the kinetic resolution.30 In the example below the unreacted allyl acetate (5)-49 is racemised. The formation and reactions of Pd-allyl cations are discussed in chapter 18. 

Tandem Asymmetric Heck and Pd-Allyl Cation Reactions... 

Other examples that involve intermediate allyl cations are illustrated in Scheme 1.4. The cationic palladium(II) complex [Pd(dppp)(PhCN)2](BF4)2 coordinates the carbonyl oxygen of benzaldehyde and the activated carbonyl carbon attacks the isoprene, forming the allyl cation 10 which then cyclizes to give the 4-methyl-6-phenyl-5,6-dihydro-2H-pyran [22]. 2-Oxopropyl acrylate 11, in the presence of trimethylsilyltrifluoromethane sulfonate (TMSOTf) and methoxytrimethylsilane (MeOSMT), generates the cation 11a which is an efficient dienophile that reacts easily with the cyclohexadiene to give the Diels-Alder adduct in good yield [23]. 

In 2007, Fernandez et al. demonstrated that transition-metal complexes with heterobidentate S/C ligands based on imidazopyridin-3-ylidene and thioether functionalities could be readily prepared from the corresponding azolium salts by reaction with Ag20 and transmetalation of the resulting silver carbenes with appropriate metal sources. The cationic Pd(allyl)(carbene-S) complexes have proven to be active catalysts in the test reaction, reaching enantioselectivities of... 

The Pd(0)-catalyzed allylic alkylation developed by Tsuji and Trost is useful for creating organic frameworks that have a variety of polar functional groups (197). The reaction is formally viewed as a combination of an allylic cation and a carbanion. A number of allylic compounds that have an electronegative leaving group can be coupled with stabilized cafbanions of pKa less than 16 under mild reaction conditions (Scheme 84). Nucleophilic attack of Pd(0) species on an allylic substrate... 

You can represent the palladium it-allyl cation complex in two ways. Either you draw a neutral ally) group complexed to Pd+ or you draw an aliyl cation complexed to neutral Pd. Though the counting is different (Pd+ has only 9 electrons the neutral aliyl has 3 but the aliyl cation only 2), both come out as it316-electron species, which isjustasweil as they are different ways of drawing the same thing. 

Tertiary phosphine and benzonitrile adducts of PdCl afford allyl complexes when reacted with allylmagnesium halide. The complex (PhCN)Pd( j -C3H5)Cl formed in Eq. (c) is mentioned above. Cationic tertiary phosphine complexes containing a Pd-allyl group are isolated from ... 

In chapter 19 you will meet palladium allyl cations as useful reagents and Evans and Robinson38 have combined the Pauson-Khand reaction with allyl cation complexes using rhodium as a compromise between Pd and Co. The enolate 132 combines with the Rh(I) cation complex from the allylic carbonate 133 to give the enyne 134 that gives the Pauson-Khand product 135 in 87% yield with the same catalyst but at higher temperatures. 

Allylic alcohols 218 can be converted into their acetates 227 by standard methods, e.g. AczO/pyridine or DMAP, without any allylic rearrangement. Reaction with Pd(0) gives initially a jr-complex 228 but this loses acetate as the Pd atom donates a pair of electrons to form an rf allyl cation complex 229 of Pd(II). 

These allyl cation complexes 229 are electrophilic and react with a variety of nucleophiles, most notably with the stabilised enolates of P-dicarbonyl compounds such as malonates. The immediate product is again a Jt-complex of Pd(0) 230 but there is now no leaving group so the Pd(0) drops off and is available for a second cycle of reactions. Though the reaction strictly requires Pd(0), the more convenient Pd(II) compounds are often used with phosphine ligands. Reduction to Pd(0) occurs either because the phosphine is a reducing agent or by oxypalladation and p-elimination. 

The reaction starts with an allylic carbonate 291 that gives the usual T 3 allyl cation complex 294 with Pd(0) though the ligand is the less usual chelating diphos 292 (Ph2PCH2CH2PPh2). The leaving group is a carbonate anion 293. 

In chapter 19 we discussed the uses of palladium allyl cation complexes as electrophiles. We established that Pd(0) adds to the opposite face of the allylic system to the leaving group232 to form an t 3 cation complex 233 and that the nucleophile attacks from the opposite face to the Pd so that the two inversions lead to retention. We established that regioselectivity and diastereoselectivity can be well controlled. If this seems unfamiliar we suggest you read the relevant section of chapter 19 before proceeding. 

Radicals and carbenes ch37 ch38 metal to carbon are key steps Carbon monoxide inserts into metal-carbon bonds Palladium is the most important metal C-C, C-0, and C—N bonds can be made with Pd catalysis Cross-coupling of two ligands is common Allyl cation complexes are useful electrophiles ... 

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