Allyl complexes with electrophiles

Both stoichiometric and catalytic reactions of allylic compounds via 7r-allyl complexes are known. Reactions of nucleophilic 71-allyl complexes with electrophiles involve oxidation of metals and hence constitutes stoichiometric reactions. 7i-Allyl complexes of Ni, Fe, Mo, Co and others are nucleophilic and undergo the stoichiometric reaction with electrophiles. However, electrophilic 71-allyl complexes react with nucleophiles, accompanying reduction of metals. For example, 71-allylnickel chloride (2) reacts with electrophiles such as aldehydes, generating Ni(II), and hence the reaction is stoichiometric. In contrast, electrophilic 7i-allylpalladium chloride (3) reacts with nucleophiles such as malonate and Pd(0) is generated. Thus repeated oxidative addition of allylic compounds to Pd(0) constitutes a catalytic reaction. 

The reactions of T) -allyl complexes with electrophiles are less common, but examples of the reactions of isolated Ti -allyl complexes with electrophiles, such as aldehydes, are known (Equations 12.59 and 12.60). Although these reactions are initiated with Tj -allyl... 

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. 

These rc-allyl complexes are electrophilic in character and undergo reaction with a variety of nucleophiles. After nucleophilic addition occurs, the resulting organopalladium intermediate usually breaks down by elimination of Pd(0) and H+. 

Other reports of (jj -allyl)Fe(CO)3R complexes are much more scattered and much less systematic. At least three general types of reactions have been observed in more than one case. First, the reaction of (diene)Fe(CO)3 complexes with electrophilic alkenes gives aUyhron complexes in two different ways. If the diene in question is acylic, electrophilic attack at C-1 of the diene gives compounds of type (158) (equation 33). In the case of substituted rj -cycloheptatriene-or azepine-Fe(CO)3 complexes, reaction... 

Cycloaddition reactions of 18-electron transition metal ti -allyl complexes with unsaturated electrophiles to form five-membered rings have been extensively investigated. These transformations constituted a family of metal-assisted cycloaddition reactions in which the metal functions as an electron-donor center. These are typically two-step processes that involve the initial formation of a dipolar metal r) -alkene intermediate (2) and subsequent internal cyclization (equation 2). The most extensively investigated application of this methodology has been with dicarbonyl-ii -cyclopentadienyliron (Fp) complexes from the laboratory of Rosenblum. These (ri -allyl)Fp complexes are available either by metallation of allyl halides or tosylates with a Fp anion, or by deprotonation of (alkene)Fp cations. ... 

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. 

Coupling of Allyl Halide Derived Nickel-Allyl Complexes with Alkyl Halides and Other Electrophiles... 

Addition of a 1,3-diene to a solution of the acyltetracarbonylcobalt complex 4.213 results in insertion, with formation of an ti -allyl complex 4.214 (Scheme 4.77) acylation occurring at the less-hindered terminus of the diene. ° In the presence of a base, the allyl complex undergoes elimination to give an acyl diene 4.215. As the allyl complex is electrophilic, it can also be attacked by some nucleophiles, such as malonate anions and nitronates (see Chapter 9, Section 9.1). Allenes can be used in place of the 1,3-diene (Scheme 4.78). The reaction is especially useful in an intramolecular sense. The products are useful Michael acceptors. ... 

Two resonance forms can be depicted for this type of complex as exemplified in Equation (50), and the contribution of the allylic form is higher than the contribution of the palladacyclobutanone. Interestingly, the reverse situation is found for the analogous platinum complexes. " The contribution of the Ty -allyl form can be enhanced in polar media, and its importance is manifest in the facile reactivity of these palladium complexes with electrophiles. " The reaction of a bis-imidazolium salt with Pd(OAc)2 affords another example where a zwitterionic neutral imidazolium fragment is bound to the metal in an j/ -oxatrimethylenemethane form 102. " ... 

These 7r-allyl complexes are electrophilic in character and undergo reaction with a variety of nucleophiles. 

In this reaction, the tethered alkene functionality of the aniline derivatives form a Tr-allyl complex with the Pd(II) catalyst, and then the cyclization takes place via intramolecular nucleophilic attack by the N to form another Pd-complex, which reacts with CO as the electrophilic carbon source in the presence of MeOH to afford substituted bicy-clic pyrrolidine derivatives. However, these products were not stereoselective in nature. A similar type of Pd-catalyzed nonstereoselective cyclizations were also reported by Weinreb et al. and by Larock and Yang for the construction of bicyclic and bridged aza-cycles." ... 

Allylic metal compounds useful for further transformations can be prepared by Pd-catalyzed reactions of allylic compounds with bimetallic reagents. By this transformation, umpolung of nucleophilic 7r-allylpalladium complexes to electrophilic allylmetal species can be accomplished. Transfer of an allyl moiety from Pd to Sn is a typical umpolung. 

The zirconium TMM complexes also react with organic electrophiles, including unactivated ones, according to Scheme 99 under formation of the corresponding substituted allyl complexes. ... 

This regio- and stereochemistry in these reactions can be accounted for as shown in Scheme 17.26 When coordinating electrophiles like ketones and aldehydes are used, the equilibrium between ij1- and 3-allyl complexes shifts to rj1, resulting in the formation of the least substituted -complex 52 preferentially. Carbon-carbon bond formation takes place via a six-membered ring transition state 53, leading to the formation of the branched homoallylic alcohols 54 with //-diastereoselectivity. 

Feringa and coworkers [258] and O Doherty et al. [259] independently reported palladium-catalyzed glycosylations of 2-substituted 6-acyl-2H-pyran-3(6H)-one derivatives and alcohols (Scheme 5.98). This reaction presumably involves electrophilic Pd 7t-allyl complex intermediate, which was generated by the reaction of 2-substituted 6-acyl-2Ff-pyran-3(6H)-one and Pd(0)/PPh3. It is noteworthy that 2-substituted 6-acyl-2H-pyran-3(6H)-one derivatives were stereoselectively converted into 2-substituted 6 - a I k o x y - 2 H - p y r an - 3 (6 H) -o n e derivatives with complete retention of configuration by this reaction. A two-step reduction/oxidation manipulation after the glycosylation can install new stereocenters in the obtained glycosides. 

Electrophilic carbene complexes can also react with organic halides to yield halonium ylides. Reaction of acceptor-substituted carbene complexes with allyl... 

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. 

A second type of organopalladium intermediates are 7r-allyl complexes. These complexes can be obtained from Pd(II) salts and allylic acetates and other compounds with potential leaving groups in an allylic position.79 The same type of 7i-allyl complexes can be prepared from alkenes by reaction with PdCl2 or Pd(02CCF3)2.80 The reaction occurs by electrophilic attack on the n electrons followed by loss of a proton. The proton loss probably proceeds via an unstable species in which the hydrogen is bound to... 

AUyRc aluminum ate complexes. Carbanions of allylic sulfides and ethers generally react with electrophiles at both the a- and the y-position. If the carbanion is converted into an ate complex -with triethylaluminum, electrophiles react predominately at the a-position. After the reaction is completed the trialkylaluminum is destroyed with aqueous methanol. Boron ate complexes are not so generally useful for control of regioselectivity.1 Example ... 

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