Malonate compounds allylic electrophiles

The molybdenum and tungsten complexes catalyze reactions of soft nucleophiles, such as malonates, related 1,3-dicarbonyl compoimds, and nitroalkanes. Azlactones are also soft carbanions, and Trost has shown that complexes formed from molybdenum and the bis(pyridine) ligands catalyze enantioselective and diastereoselective allylation of azlactones with allylic phosphates to form quaternary amino acids (Equation 20.40). In these reactions, the nucleophile adds to the more substituted position of the allylic electrophile, and a stereocenter is formed at both the allyl carbon and the azlactone carbon. One route to the protease inhibitor tipranavir by the molybdenum-catalyzed allylation with 1,3-dicarbonyl compounds was demonstrated by Trost (Equation 20.41), and the Merck process group used related allylation chemistry with Trost s bis(pyridine) ligand to prepare the cyclopentanone precursor to various analogs of tipranavir (Equation 20.42). 

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. 

Nucleophilic attack on ( -alkene)Fp+ cations may be effected by heteroatom nucleophiles including amines, azide ion, cyanate ion (through N), alcohols, and thiols (Scheme 39). Carbon-based nucleophiles, such as the anions of active methylene compounds (malonic esters, /3-keto esters, cyanoac-etate), enamines, cyanide, cuprates, Grignard reagents, and ( l -allyl)Fe(Cp)(CO)2 complexes react similarly. In addition, several hydride sources, most notably NaBHsCN, deliver hydride ion to Fp(jj -alkene)+ complexes. Subjecting complexes of type (79) to Nal or NaBr in acetone, however, does not give nncleophilic attack, but instead results rehably in the displacement of the alkene from the iron residue. Cyclohexanone enolates or silyl enol ethers also may be added, and the iron alkyl complexes thus produced can give Robinson annulation-type products (Scheme 40). Vinyl ether-cationic Fp complexes as the electrophiles are nseful as vinyl cation equivalents. ... 

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 addition to allylic organolithiums reacting with conventional electrophiles such as akyl halides and carbonyl compounds, activated olefins have also been employed to provide Michael addition products [86,87,105]. Both mono- and diactivated olefins including a,p-unsaturated ketones, esters, malonates, malo-nitriles and nitro compoimds are compatible (Scheme 49). Addition of the orga-nolithium to cyclic a,p-unsaturated carbonyl compoimds occurred with retention of configuration, whereas addition to other activated olefins proceeds with inversion. 

As you can see from the carbonyl groups in these compounds, it is pretty obvious which is the new bond to be made. In both cases, the electrophile will need to be an allylic hahde. These are good electrophiles for Sn2 reactions so they will work well here. We need to use the electrophile twice in the first case and the enolate is that of diethyl malonate. The second case will require an enol or enolate equivalent to prevent self-condensation a silyl enol ether (p. 595 in the textbook) or an enamine (p. 591 in the textbook) is ideal If you use a silyl enol ether, don t forget the Lewis acid ... 

Starting from oxygen-linked 1,3-dicarbonyl compounds (malonates or acetoacetates), Tietze and co-workers have demonstrated an allylic substitution at the a-position of varions snbstrates (aUyl acetates, carbonates, and chlorides). Under the conditions employed, bisalkylation was observed in all cases. Since the acetoacetates could be alkylated by hard electrophiles at the y-position, a broad spectrum of compounds might be obtained. The cleavage from the resin was performed using DIBAL-H to obtain the corresponding diols (Scheme 24). 

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