Allyl cations complexes

A metal exchange was used to prepare a 4-cupro-l,3-dioxane from the corresponding lithium derivative. This copper species reacted with an allyl cation complex to give addition products with excellent stereoselectivity but with poor regioselectivity when an unsymmetrically substituted allyl cation was employed (Equation 39) <2000SL463, 20040BC1719>. 

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. 

Following the usual mechanism (pp. 1330-4), the palladium complexes to the face of the alker.e opposite the bridge and the ester group leaves to give an allyl cation complex. This is attacked by the malonate anion from the opposite face to the palladium. So the overall resrdt is retention, the starting material giving the cis product. 

The racemization comes from the structure of the allyl cation complex. It is, in fact, symmetric.- with a plane of symmetry and attack occurs equally at the two ends of the allyl system giving the nc enantiomers of the product. 

The palladium forms the usual allyl cation complex and the nitrogen nucleophile attacks the ler hindered end also retaining the conjugation. Attack at the triple bond would give an allene. 

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. 

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... 

Among the methods available for these reactions, two have gained prominence the use of Claisen-style [3,3]-sigmatropic rearrangements for the selective allylation of enolates and the more general reactions of palladium allyl cation complexes. 

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. 

In three dimensions the reaction is stereospecific with retention by double inversion. The palladium approaches the alkene from the opposite face to the leaving group and the nucleophile approaches from the opposite face to the palladium. This is clearly shown by reaction of the two diastereoisomers 237 and 240 with malonate. [Note These compounds are of course racemic and attack at either end of the achiral p3 allyl cation complexes 238 or 241 would give racemic material anyway.]... 

The palladium forms a Jt-complex 245 from the top face of the alkene, opposite the carboxylate leaving group. The nucleophile then adds from the opposite face to the palladium. The rf allyl cation complex is unsymmetrical and the nucleophile adds at the less hindered end. The selectivity is very marked in both steps because the starting material is a folded molecule and the palladium much prefers to add to the exo face of the alkene. The nucleophile has to add from the same face as the CH2C02- side chain so it prefers addition to the end of the complex as far from this side chain as possible 246. 

Evidently cis/trans isomerisation of the p3 allyl cation complex is faster than addition. The regioselectivity is remarkable. Either the steric hindrance of MeT OI I next to the alkene is worse than that of the tertiary centre that is actually attacked or the OH group electronically discourages nucleophilic attack as it does in simpler SN2 reactions. We shall see a related regioselectivity in the reactions of diene monoxides in the next section. 

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. 

The carbonate anion loses C02 to give methoxide ion which, like the anion released from a diene monoepoxide, is basic enough to deprotonate the nucleophile, a bis-sulfone 296. The anion from this 295 adds to the less hindered end of the allyl cation complex 294. The product 297 has the skeleton of the prostacyclin analogue and the sulfones simply need to be removed by reduction. 

Addition of alkyl organo-zlnc reagents Palladium Allyl Cation Complexes with Chiral Ligands Summary... 

Palladium Allyl Cation Complexes with Chiral Ligands... 

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. 

Trost established54 that the ligand 236 was the best for many of these reactions. Thus racemic allylic acetate 237 gives a symmetrical allylic cation complex like 233 and the ligand directs the... 

Now the second alkene comes into play as 234 is really an allyl o-complex that prefers to exist as an allyl p3 cation complex 235. If a suitable nucleophile, such as a malonate enolate, is present this will attack the allyl cation complex. It must attack from the opposite face to the palladium and prefers to attack next to the H atom rather than the Me group giving 236. 

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