Palladium leaving groups

Allylic acetoxy groups can be substituted by amines in the presence of Pd(0) catalysts. At substituted cyclohexene derivatives the diastereoselectivity depends largely on the structure of the palladium catalyst. Polymer-bound palladium often leads to amination at the same face as the aoetoxy leaving group with regioselective attack at the sterically less hindered site of the intermediate ri -allyl complex (B.M. Trost, 1978). 

The transfer of simple alkyl groups (R in the table—mostly -Bu or Me), from tin to palladium complex 6 is a very slow process, and the substituent R (see table) is transferred selectively. The leaving group X on the coupling component... 

An approach to isobacteriochlorins1 ln-e makes use of Pd(II) or metal-free bilatrienes 1 as starting materials. Cyclization of the corresponding bilatriene derivatives is induced by base in the presence of palladium(II) or zinc(II) which exercise a template effect. Zinc can be readily removed from the cyclized macrotetracycles by acid whereas palladium forms very stable complexes which cannot be demetalated. Prior to the cyclization reaction, an enamine is formed by elimination of hydrogen cyanide from the 1-position. The nucleophilic enamine then attacks the electrophilic 19-position with loss of the leaving group present at the terminal pyrrole ring. 

For unsymmetrical allylic systems both the regiochemistry and stereochemistry of the substitution are critical issues. The palladium normally bonds anti to the acetate leaving group. The same products are obtained from 2-acetoxy-4-phenyl-3-butene and 1 -acetoxy-l-phenyl-2-butene, indicating a common intermediate. The same product mixture is also obtained from the Z-reactants, indicating rapid ,Z-equilibration in the allylpalladium intermediate.118... 

Allyl sulphones can be converted to dienes by alkylation and elimination of sulphinic acid under basic conditions (equation 64)105. Several vitamin A related polyenes have been synthesized following this two-step protocol (Table 10)106. The poor leaving-group ability of the arylsulphonyl group requires treatment with strong base for elimination. However, elimination of the allylsulphonyl group takes place readily under palladium catalysis (equation 65)107. Vinyl sulphones can be converted to dienes via Michael addition, alkylation with allyl halides and elimination of sulphinic acid sequence (equation 66)108. 

Enantio- and diastereoselective syntheses of a variety of heterocycles were accomplished by combining the ruthenium-catalyzed Alder-ene reaction with a palladium-catalyzed asymmetric allylic alkylation (AAA) (Scheme 7). For the AAA, y>-nitrophenol was found to function as a suitable leaving group and yet was stable to the Alder-ene conditions. Extensive solvent studies were performed to determine the best conditions for the one-pot procedure. 

The invention of the triflate (trifluoromethylsulfonyl) group — one of the world s best leaving groups — has led to its use in palladium chemistry [42]. Conway and Gribble described the synthesis of 3-indolyl triflate 34 [12] and 2-indolyl triflate 35 from oxindole [43]. Mdrour synthesized the N-phenylsulfonyl derivative 36 by employing a Baeyer-Villiger oxidation of the appropriate indolecarboxaldehyde [44],... 

If allenes bear a potential leaving group in the a-position to the cumulene system, they are very attractive substrates for palladium-catalyzed substitutions. Examples are a-allenic acetates and particularly a-allenic phosphates, which react under palladium(O) catalysis with carbanions derived from /3-diesters, /i-keto esters, a-phenylsulfonyl esters and glycine ester derivatives. They lead to /3-functionalized allenes such as 86, 89 and 93 (Eqs. 14.9-14.11) [45 18]. 

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. 

Although several Lewis Acids were evaluated, including titanium(lV) chloride, aluminum(lll) chloride and tin(lV) chloride, ferric(lll) chloride proved to be the most effective co-catalyst. We believe that in the presence of a Lewis Acid, the rate of j3-palladium hydride elimination (H-Pd-X) from the -allyl carbomethoxy palladium complex 4 can be enhanced. A good leaving group such as iodide attached to -allyl carbomethoxy palladium complex 4 would facilitate iodopalladium hydride elimination to selectively form methyl, -pentadienoate (Equation 11.). 

In 1992, Legros and Fiaud found palladium-catalyzed benzyiic alkylation of naphthylmethyl and 1-naphthylethyl esters 103 with sodium dimethyl malonate 104 in dimethylformamide (DMF) to give the corresponding benzyiic alkylated products 105 in high yields (Equation (41)). " When trifluoroacetyl group is used as a leaving group of the ester partner, catalytic alkylation proceeds quite smoothly even at room temperature. In this reaction system, no reaction occurs with benzyiic acetates. 

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