Ligands allyl, with electrophiles

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. 

Electrophilic attack has also been shown to occur at the 7-position of several t) es of organometallic ligands. Electrophilic attack at the 7-position of T -allyls may be the most common. These reactions form substituted olefin complexes, as shown generally in Equations 12.53 and 12.54. Although fewer examples of the reactions of electrophiles with allyl complexes have been reported than the reactions of nucleophiles with allyl complexes, a number of catalytic processes have been developed that are likely to occur by electrophilic attack at the 7-position of allyl complexes, and a few examples of the reactions of electrophiles with isolated T -allyl complexes have been reported. " ... 

Cu-catalysed additions of ZnEt2 to Baylis-Hillman-derived allylic electrophiles with BINOL-based thioether ligand. 

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... 

Reactions of allylic electrophiles with stabilized carbon nucleophiles were shown by Helmchen and coworkers to occur in the presence of iridium-phosphoramidite catalysts containing LI (Scheme 10) [66,69], but alkylations of linear allylic acetates with salts of dimethylmalonate occurred with variable yield, branched-to-linear selectivity, and enantioselectivity. Although selectivities were improved by the addition of lithium chloride, enantioselectivities still ranged from 82-94%, and branched selectivities from 55-91%. Reactions catalyzed by complexes of phosphoramidite ligands derived from primary amines resulted in the formation of alkylation products with higher branched-to-linear ratios but lower enantioselectivities. These selectivities were improved by the development of metalacyclic iridium catalysts discussed in the next section and salt-free reaction conditions described later in this chapter. 

Propene is also proposed as a product of the reaction of the same Rh-siloxy organometaUic fragment with CO, as a consequence of an electrophilic attack of a proton in or near the coordination sphere of rhodium on one of the allyl ligands (Scheme 7.3). 

The reactivity of the organosilanes65 and organostannanes66 towards electrophiles is dependent on the characteristics of the organic ligands. Typically, the alkylsilanes and alkylstannanes are unreactive, which is a consequence of the weakly polarized carbon-silicon and carbon-tin cr-bonds (C8-—Ms+). However, allylsilanes67 and allylstannanes are highly reactive to electrophiles because of extensive ct-tt (C—Si or C—Sn) conjugation in the ally metals and the 0-carbonium ion stabilization effect of the metal center. Consequently, electrophiles add exclusively with allylic transposition. 

As noted in the introduction, in contrast to attack by nucleophiles, attack of electrophiles on saturated alkene-, polyene- or polyenyl-metal complexes creates special problems in that normally unstable 16-electron, unsaturated species are formed. To be isolated, these species must be stabilized by intramolecular coordination or via intermolecular addition of a ligand. Nevertheless, as illustrated in this chapter, reactions of significant synthetic utility can be developed with attention to these points. It is likely that this area will see considerable development in the future. In addition to refinement of electrophilic reactions of metal-diene complexes, synthetic applications may evolve from the coupling of carbon electrophiles with electron-rich transition metal complexes of alkenes, alkynes and polyenes, as well as allyl- and dienyl-metal complexes. Sequential addition of electrophiles followed by nucleophiles is also viable to rapidly assemble complex structures. 

Complementary to the conjugate substitution reaction in which the nucleophile is transferred directly from the tetraalkyl ferrate to the allylic ligand, preformed low-valent Fe complexes can form reactive allyl-iron complexes via an SN2 -type mechanism (path C, Equations (7.8) and (7.9), Scheme 7.16], These complexes react with incoming nucleophiles and electrophiles in a substitution reaction. Depending on the nature ofthe iron complex employed in the reaction, either o- or Jt-allyl complexes are generated. 

A palladium-based method has been developed for the alkylation of the phenolic oxygen of tyrosine residues. Fig. 5f (61). In this reaction, allylic carbonates, esters, and carbamates are activated by palladium(O) complexes in aqueous solution to form electrophilic pi-allyl complexes. These species react at pH 8-10 with the phenolate anions of tyrosine residues, which results in the formation of an aryl ether and the regeneration of the Pd(0) catalyst. The reaction requires P(m-C6H4S03 )3 as a water-soluble phosphine ligand. Activated pi-allyl complexes that do not react with tyrosine residues undergo P-hydride elimination under the basic conditions to yield diene by-products. A particularly attractive feature of this method is its ability to use substrates with charged groups in the allylic positions. This ability allows hydrophobic substrates, such as lipids, to be solubilized to facilitate protein modification. 

Allyl complexes are susceptible to nucleophilic and electrophilic attack. A typical reaction of an allyl complex with a nucleophile is illustrated in Eq. (5) [17]. Nucleophilic attack at one of the terminal carbons of an allyl ligand most often occurs on the face of the allyl group opposite the metal, as shown in Eq. (5) however, the nucleophile may bind to the metal center initially if there is a vacant site available and, then, transfer to the endo face of the allyl group. 

Compared with well-established electrophilic it-allylpalladium chemisty, the catalytic asymmetric reaction via umpolung of jt-allylpalladium has received very limited exploration [93]. Zhou and co-workers investigated the Pd-catalyzed asymmetric umpolung allylation reactions of aldehydes [22a, 94], activated ketones [95], and imines [96] by using chiral spiro ligands (5)-18e, (S)-17c, and (5)-17a, respectively. One representative example is that of the Pd/(5)-18e-catalyzed umpolung allylation of aldehydes with allylic alcohols and their derivatives, which provided synthetically useful homoallylic alcohols from readily available allylic alcohols, with high yields and excellent enantioselectivities (Scheme 33). 

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. 

Metal-carbon (M—C) bonds are thermodynamically unstable with regard to their hydrolysis products. Water can attack M—C bonds either by proton transfer (H+, electrophilic reaction) or via the oxygen (OH2 or OH, nucleophilic reaction). Examples are shown in Scheme 1. Ligands such as carbon monoxide and ethylene are activated toward nucleophilic attack upon coordination to (low-valent) metals, e.g., Pd2+. A number of C—C-bond forming reactions derive from this activation. Allyl ligands are generated by proton attack to the terminal 1,3-diene carbon... 

Early investigations on the asymmetric palladium(0)-catalyzed allylation of prostereogenic nucleophiles with allyl phenolates and acetates as electrophiles achieved up to 10% ee using Diop [4,5-bis(diphenylphosphinomethyl)-2,2-dimethyl-l,3-dioxolane] as the chiral ligand6. Other ligands like Prophos [1,3-bis(diphenylphosphino)propane]7 and sparteine8 also provided only poor stereoselectivities. 

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