Allyl complexes nucleophilic attacks

The loss of stereospecificity in the addition of bis(sulfone) anions to cyclohexenyl allylic acetates was attributed to a scrambling of the stereochemistry of the starting acetate. The ability of Pd° catalyst to effect this epimerization was confirmed in the absence of added nucleophile. This epimerization was attributed to the ability of the acetate to return to add to the ir-allyl complex via attack at the metal center (equation 177).167 This suggestion was confirmed by treatment of a preformed allylpalladium acetate dimer with CO, which resulted in cis migration of the acetate from Pd to the allyl ligand (equation 178).164... 

A wide range of transition metal-allyl complexes are known to react with many types of nucleophiles. In most cases, these reactions occur between cationic allyl complexes and amines or stabilized, anionic carbon nucleophiles. The reaction typically occurs between the nucleophile and the form of the allyl complex, and attack usually occurs at the face of the allyl ligand opposite the metal. However, there are exceptions to these trends. For example, several experiments suggest that unstabilized carbon nucleophiles react first at the metal center, and C-C bond formation occurs between the alkyl and the allyl group by reductive elimination. In addition, a recent study has shown through deuterium labeling that attack of malonate anion on a molybdenum-allyl complex occurs with retention of configuration. ... 

Diene complexes give allyls on nucleophilic attack. Note how the cisoid conformation of the butadiene in Eq. 8.18 gives rise to an anti methylallyl (in the nomenclature of allyl complexes, a substituent is considered as syn or anti with respect to the central CH proton). Equation 8.19 is interesting in that the amine acts in this case as a carbon, not as a nitrogen nucleophile. ... 

Optically active barbiturates important in pharmacy, are obtained by classical synthesis and resolution procedures [14]. A new approach to the synthesis of optically active barbiturates is enantioselective catalysis with Pd complexes. An example is shown in Scheme 10 on the right side. The synthesis of the enantiomers of a N-methyl barbiturate containing a quaternary asymmetric carbon atom is achieved by allylation of the corresponding precursor with allyl acetate. Pd/triphenylphosphine complexes are efficient catalysts for this reaction which give a racemic mixture of the product. The reaction proceeds via a jt-allyl complex of Pd as an intermediate (Scheme 10, left side). This 7t-allyl complex is attacked by the nucleophile, the anion of N-... 

EMene complexes give allyls on nucleophilic attack. Note how the cisoid conformation of the butadiene in Eq. 8.18 gives rise to an anti methylallyl (in the... 

Typical nucleophiles known to react with coordinated alkenes are water, alcohols, carboxylic acids, ammonia, amines, enamines, and active methylene compounds 11.12]. The intramolecular version is particularly useful for syntheses of various heterocyclic compounds[l 3,14]. CO and aromatics also react with alkenes. The oxidation reactions of alkenes can be classified further based on these attacking species. Under certain conditions, especially in the presence of bases, the rr-alkene complex 4 is converted into the 7r-allylic complex 5. Various stoichiometric reactions of alkenes via 7r-allylic complex 5 are treated in Section 4. 

Two monomeric and dimeric 2-substituied 7r-allylic complexes (548 and 549) are obtained by treatment of allene with PdCl2(PhCN)2. They are formed by the nucleophilic attack at the central carbon of allene[493, 494],... 

The stereochemistry of the Pd-catalyzed allylation of nucleophiles has been studied extensively[5,l8-20]. In the first step, 7r-allylpalladium complex formation by the attack of Pd(0) on an allylic part proceeds by inversion (anti attack). Then subsequent reaction of soft carbon nucleophiles, N- and 0-nucleophiles proceeds by inversion to give 1. Thus overall retention is observed. On the other hand, the reaction of hard carbon nucleophiles of organometallic compounds proceeds via transmetallation, which affords 2 by retention, and reductive elimination affords the final product 3. Thus the overall inversion is observed in this case[21,22]. 

Based on the above-mentioned stereochemistry of the allylation reactions, nucleophiles have been classified into Nu (overall retention group) and Nu (overall inversion group) by the following experiments with the cyclic exo- and ent/n-acetales 12 and 13[25], No Pd-catalyzed reaction takes place with the exo-allylic acetate 12, because attack of Pd(0) from the rear side to form Tr-allyl-palladium is sterically difficult. On the other hand, smooth 7r-allylpalladium complex formation should take place with the endo-sWyWc acetate 13. The Nu -type nucleophiles must attack the 7r-allylic ligand from the endo side 14, namely tram to the exo-oriented Pd, but this is difficult. On the other hand, the attack of the Nu -type nucleophiles is directed to the Pd. and subsequent reductive elimination affords the exo products 15. Thus the allylation reaction of 13 takes place with the Nu nucleophiles (PhZnCl, formate, indenide anion) and no reaction with Nu nucleophiles (malonate. secondary amines, LiP(S)Ph2, cyclopentadienide anion). 

Lewis acids, particularly the boron trifluroride diethyl ether complex, are used to promote the reaction between allyl(trialkyl)- and allyl(triaryl)stannanes and aldehydes and ketones52-54. The mechanism of these Lewis acid promoted reactions may involve coordination of the Lewis acid to the carbonyl compound so increasing its reactivity towards nucleophilic attack, or in situ transmetalation of the allyl(trialkyl)stannane by the Lewis acid to generate a more reactive allylmetal reagent. Which pathway operates in any particular case depends on the order of mixing of the reagents, the Lewis acid, temperature, solvent etc.55- 58. 

Togni s [38] approach was therefore to test the ability of sparteine to act as an ancillary ligand in Pd(II)-allyl complexes—susceptible to nucleophilic attack by stabihzed anions such as Na[CH(COOMe)2]—which could be employed as catalyst precursors. In addition he speculated that the rather rigid and bulky sparteine would be able to induce significant differentiation between the two diastereotopic sites of 1,3-disubstituted allyl hgand, thus leading to enantioselection upon nucleophilic attack. 

Intermolecular hydroalkoxylation of 1,1- and 1,3-di-substituted, tri-substituted and tetra-substituted allenes with a range of primary and secondary alcohols, methanol, phenol and propionic acid was catalysed by the system [AuCl(IPr)]/ AgOTf (1 1, 5 mol% each component) at room temperature in toluene, giving excellent conversions to the allylic ethers. Hydroalkoxylation of monosubstituted or trisubstituted allenes led to the selective addition of the alcohol to the less hindered allene terminus and the formation of allylic ethers. A plausible mechanism involves the reaction of the in situ formed cationic (IPr)Au" with the substituted allene to form the tt-allenyl complex 105, which after nucleophilic attack of the alcohol gives the o-alkenyl complex 106, which, in turn, is converted to the product by protonolysis and concomitant regeneration of the cationic active species (IPr)-Au" (Scheme 2.18) [86]. 

Simpler chiral pyrrolidine thioethers, reported in 2004 by Skarzewski et al., proved to be effective ligands in the test reaction. The sense of the stereoinduction was in agreement with the nucleophilic attack directed at the allylic carbon located trans to the sulfur atom in the intermediate complex (Scheme 1.40). 

Stoichiometric reaction with matched S-carbamate having the D atom in the Z-position 733) in the presence of S,S-ligand 64 without a nucleophile solely formed (no other isomer was observed by NMR) the Mo-complex 74 without transposition of the label. The structure of 74 was probed based on NMR studies by comparison with NMR studies and the X-ray structure of the protio complex 71. Nucleophilic attack of sodium malonate on the Mo complex 74 provided the S-product 75, where the D atom remained at the Z-position. On the other hand, stoichiometric reaction with mismatched R-carbamate having the D atom in the Z-position 76 without a nucleophile generated the Mo complex 80 as sole product, based on NMR studies. The structure of the complex 80 was elucidated by NMR. In 80, Mo is located on the same face as in 74 but the D atom is transposed from the Z to the E position. The transposition could be explained as follows. Initially the n-allyl Mo-complex 77 (unobserved) must form with retention. Mo complex 77 is equilibrated into the more stable Mo complex 80, where the D atom is moved... 

Ethenylcyclopropyl tosylates 131 and 2-cyclopropylideneethyl acetates 133, readily available from the cyclopropanone hemiacetals 130, undergo the re-gioselective Pd(0)-catalyzed nucleophilic substitution via the unsymmetrical 1,1-dimethylene-jr-allyl complexes. For example, reduction with sodium formate affords a useful route from 131 to the strained methylenecyclopropane derivatives 132. The regioselective attack of the hydride is caused by the sterically... 

Green demonstrated (48) that a 7r-allyl complexed to W or Mo can undergo a nucleophilic attack by H- on the central carbon of the tj3 allylic group, forming a stable metallocyclobutane. [See Eq. (20).] Cyclopropane and propylene were evolved (49) on heating the metallocyclic... 

Osborn and Green s elegant results are instructive, but their relevance to metathesis must be qualified. Until actual catalytic activity with the respective complexes is demonstrated, it remains uncertain whether this chemistry indeed relates to olefin metathesis. With this qualification in mind, their work in concert is pioneering as it provides the initial experimental backing for a basic reaction wherein an olefin and a metal exclusively may produce the initiating carbene-metal complex by a simple sequence of 7r-complexation followed by a hydride shift, thus forming a 77-allyl-metal hydride entity which then rearranges into a metallocyclobutane via a nucleophilic attack of the hydride on the central atom of the 7r-allyl species ... 

Palladium-catalyzed oxidation of 1,4-dienes has also been reported. Thus, Brown and Davidson28 obtained the 1,3-diacetate 25 from oxidation of 1,4-cyclohexadiene by ben-zoquinone in acetic acid with palladium acetate as the catalyst (Scheme 3). Presumably the reaction proceeds via acetoxypalladation-isomerization to give a rr-allyl intermediate, which subsequently undergoes nucleophilic attack by acetate. This principle, i.e. rearrangement of a (allyl)palladium complex, has been applied in nonoxidative palladium-catalyzed reactions of 1,4-dienes by Larock and coworkers29. Akermark and coworkers have demonstrated the stereochemistry of this process by the transformation of 1,4-cyclohexadiene to the ( r-allyl)palladium complex 26 by treatment... 

The Tsuji-Trost reaction is the palladium-catalyzed allylation of nucleophiles [110-113]. In an application to the formation of an A-glycosidic bond, the reaction of 2,3-unsaturated hexopyranoside 97 and imidazole afforded A-glycopyranoside 99 regiospecifically at the anomeric center with retention of configuration [114], Therefore, the oxidative addition of allylic substrate 97 to Pd(0) forms the rc-allyl complex 98 with inversion of configuration, then nucleophilic attack by imidazole proceeds with a second inversion of configuration to give 99. 

A one-pot synthesis of 3,3-disubstituted indolines was achieved by taking advantage of a sequential carbopalladation of allene, nucleophile attack, intramolecular insertion of an olefm and termination with NaBPh4 (Scheme 16.6) [10]. First, a Pd(0) species reacts with iodothiophene selectively to afford ArPdl, probably because the oxidative addition step is facilitated by coordination with the adjacent sulfur atom. Second, the ArPdl adds to allene, giving a Jt-allylpalladium complex, which is captured by a 2-iodoaniline derivative to afford an isolable allylic compound. Under more severe conditions, the oxidative addition of iodide to Pd(0) followed by the insertion of an internal olefm takes place to give an alkylpalladium complex, which is transmetallated with NaBPh4 to release the product. 

A bromoallene was demonstrated to act as an allyl dication equivalent. When treated with Pd(0) in an alcoholic solvent, an ei-hydroxybromoallene provides a mediumsized heterocycle (Scheme 16.101) [106]. The oxidative addition of a bromoallene to Pd(0) generates an allenylpalladium species, which is successively transformed into a Jt-allylpalladium complex through the attack of the hydroxyl group on the sp carbon followed by the protonation of the resulting Pd-carbene complex. Finally, the products are provided as a mixture of regioisomers by the nucleophilic attack of the external methanol. 

Palladium(II) is one of the most important transition metals in catalytic oxidations of allenes [1], Scheme 17.1 shows the most common reactions. Transformations involving oxidative addition of palladium(O) to aryl and vinyl halides do not afford an oxidized product and are discussed in previous chapters. The mechanistically very similar reactions, initiated by nucleophilic attack by bromide ion on a (jt-allene)pal-ladium(II) complex, do afford products with higher oxidation state and are discussed below. These reactions proceed via a fairly stable (jt-allyl)palladium intermediate. Mechanistically, the reaction involves three discrete steps (1) generation of the jt-allyl complex from allene, halide ion and palladium(II) [2] (2) occasional isomeriza-... 

Good diastereoselectivity was obtained with BQ as the oxidant in acidic media but the reaction times were relatively long (1-2 days at 40 °C). Using the copper(II)-oxy-gen system in slightly basic media permits a much faster reaction (0.5-1 h at 20 °C) with better isolated yields but with poor or even reversed diastereoselectivity. The slower reaction with BQ as oxidant is due to the fact that this oxidant requires an acidic medium, which lowers the nucleophilicity of the acid moiety. It is also likely that BQ or copper(II) has to coordinate to palladium(II) before the second nucleophile can attack to make the Jt-allyl complex more electrophilic. Coordination of cop-per(II) would make a more electrophilic intermediate than coordination of BQ. The relation between reaction time and diastereoselectivity supports a mechanism analogous to that in Scheme 17.7. 

The reaction of an allene with an aryl- or vinylpalladium(II) species is a widely used way of forming a Jt-allyl complex. Subsequent nucleophilic attack on this intermediate gives the product and palladium(O) (Scheme 17.1). Oxidative addition of palladium ) to an aryl or vinyl halide closes the catalytic cycle that does not involve an overall oxidation. a-Allenyl acids 27, however, react with palladium(II) instead of with palladium(O) to afford cr-vinylpalladium(II) intermediates 28 (Scheme 17.12). These cr-complexes than react with either an allenyl ketone [11] or with another alle-nyl acid [12] to form 4-(3 -furanyl)butenolides 30 or -dibutenolides 32, respectively. 

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