Allyl ligand, nucleophilic attack

The reactivity of allyl complexes is included in many of the later chapters of this text. Although examples of reductive eliminations and migratory insertions of allyl complexes are known, the dominant reaction chemistry involves the attack by nucleophiles on the allyl ligand and attack of the allyl ligand on external and coordinated electrophiles. The latter reaction leads to catalytic allylic substitution reactions. The reactions of nucleophiles and electrophiles with allyl complexes are described in Chapters 10 and 11, and the catalytic allylic substitution is described in Chapter 20. 

Depending primarily on the nature of nucleophiles, either attack on the 7r-allyl ligand or attack at Pd has been observed. Thus, for example, the reaction of tt-allylpalladium complexes with soft carbon nucleophiles generally involves attack on the TT-allyl ligands proceeding via inversion at the site of substitution leading to... 

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). 

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. 

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... 

In Lambert s approach, the triarylstannylium ion is generated by the reaction of an electrophile with an allyltri-arylstannane. The bulky aryl groups sterically protect the tin center in the stannylium ion from attack by nucleophiles, yet the allyl ligand permits unhindered conjugate electrophilic displacement of the tin (Equation (42)).145... 

Protonation of the epoxide by AcOH is followed by nucleophilic ring-opening with Pd(0) (SN2-type reaction) to give an allylpalladium(II) complex. The AcO- then attacks the allyl ligand, regenerating Pd(0) and affording the product. 

Fig. 2 Stoichiometric nucleophilic attack on the terminus of an allyl ligand...

After the initial demonstration of stoichiometric nucleophilic attack on 7i-allyl ligands, catalytic allylic substitution reactions were pursued. In 1970, groups from Union Carbide [3, 4], Shell Oil [5], and Toray Industries [6] published or patented examples of catalytic allylic substitution. All three groups reported allylic amination with palladium catalysts. The Toray Industries report also demonstrated the exchange of aryl ether and ester leaving groups, and the patent from Shell Oil includes catalysts based on rhodium and platinum. 

The high concentration of ammonia that is required for selective formation of primary allylic amines suggests that ammonia is much less reactive toward nucleophilic attack on an allyl ligand than the primary allylic amine products. The reactivity of different nucleophiles toward allyl ligands was assessed by treating the allylir-idium complex 6a with ammonia and l-phenylprop-2-en-l-amine. No product from allylation of ammonia was detected from this reaction. In addition, the reaction of an allylic carbonate with the combination of ammonia and l-phenylprop-2-en-l-amine in the presence of lb did not produce detectable amounts of the product from allylation of ammonia. These results confirm that the primary amine is much more... 

A variety of rhodium complexes, including [Rh(CO)2Cl]2 and [Rh(COD)Cl]2 when used in combination with a variety of bisphosphine ligands, will catalyze the ring opening of vinyl epoxides in the presence of aniline nucleophiles [19, 20]. These reactions occur under very mild and neutral conditions (at room temperature or with mild heating) and are highly regio- and stereoselective. In all cases, nucleophilic attack occurs at the allylic epoxide carbon atom and proceeds with inversion of stereochemistry (Scheme 9.11). 

The proposed mechanism for allyhc acetoxylation of cyclohexene is illustrated in Scheme 15. Pd -mediated activation of the allyhc C - H bond generates a Jt-allyl Pd intermediate. Coordination of BQ to the Pd center promotes nucleophilic attack by acetate on the coordinated allyl ligand, which yields cyclohexenyl acetate and a Pd -BQ complex. The latter species reacts with two equivalents of acetic acid to complete the cycle, forming Pd(OAc)2 and hydroquinone. The HQ product can be recycled to BQ if a suitable CO catalyst and/or stoichiometric oxidant are present in the reaction. This mechanism reveals that BQ is more than a reoxidant for the Pd catalyst. Mechanistic studies reveal that BQ is required to promote nucleophilic attack on the Jt-allyl fragment [25,204-206]. 

Transition metal-catalyzed allylic alkylation is generally considered to involve mechanistically four fundamental steps as shown in Scheme 1 coordination, oxidative addition, ligand exchange, and reductive elimination. A key step of the catalytic cycle is an initial formation of a (7r-allyl)metal complex and its reactivity. The soft carbon-centered nucleophiles, defined as those derived from conjugate acids whose pAj, < 25, usually attack the allyl ligand from the opposite side... 

Nucleophilic attack on a rt-allyl ligand of a metal complex occurs in general at one of the terminal carbons to afford allylated products. The attack, however, may be directed to the central carbon atom of the 7i-allyl group to produce cyclopropyl derivatives by appropriate choice of nucleophile, metal ligand and reaction conditions (equation 33). A variety of nucleophiles (pA"a > 20) including ester and ketone enolates and a-sulfonyl carbanions react with... 

A parallel situation appears to obtain for the mixed allyl nitrosyl complex Ru(NO)(C3H5)L2 prepared by Schoonover and Eisenberg (231). This complex which is coordinatively saturated (NO+ and rf -allyl), forms a CO adduct which is assigned a bent nitrosyl structure (231). Further reaction under CO leads to the formation of Ru(CO)3L2 with the possible elimination of acrolein oxime. The coupling of the allyl and nitrosyl ligands can be viewed in this case as nucleophilic attack of NO- on an f/3-allyl species. Unlike in reaction (110), both of the moieties to be coupled lie within the same coordination sphere. The significance of these results is that it lends viability to the notion embodied in (109) in which a migratory insertion of nitrosyl occurs as NO-. 

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