Phenylpalladium halides

Among many substrates used for Pd(0)-catalyzed reactions, organic halides are most widely used. In Grignard reactions, Mg(0) metal reacts with organic halides of sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand, Pd(0) complexes react more easily with halides attached to sp carbons, namely aryl and alkenyl halides. In addition, several pseudohalides are used as well. They undergo facile oxidative addition to Pd(0) to form Pd complexes which have cr-Pd-carbon bonds. Scheme 3.1 summarizes the oxidative addition of phenyl halides and pseudohalides to form phenylpalladium halides. Aryl iodides and bromides have been used widely. 

The phenylpalladium halides 1 are living species and undergo several further transformations before termination. Insertion of unsaturated bonds is one of them, as summarized in Scheme 3.2. Alkene insertion is followed by /i-H elimination to yield arylalkenes 7. Insertion of 1,2- and 1,3-dienes generates the jr-allylpalladium... 

Another method for the hydrogenoiysis of aryl bromides and iodides is to use MeONa[696], The removal of chlorine and bromine from benzene rings is possible with MeOH under basic conditions by use of dippp as a ligand[697]. The reduction is explained by the formation of the phenylpalladium methoxide 812, which undergoes elimination of /i-hydrogen to form benzene, and MeOH is oxidized to formaldehyde. Based on this mechanistic consideration, reaction of alcohols with aryl halides has another application. For example, cyclohex-anol (813) is oxidized smoothly to cyclohexanone with bromobenzene under basic conditions[698]. 

Innovations in the chemistry of aromatic compounds have occurred by recent development of many novel reactions of aryl halides or pseudohalides catalysed or promoted by transition metal complexes. Pd-catalysed reactions are the most important [2,29], The first reaction step is generation of the arylpalladium halide by oxidative addition of halide to Pd(0). Formation of phenylpalladium complex 1 as an intermediate from various benzene derivatives is summarized in Scheme 3.1. 

Cupric chloride, if present in concentrations above ca. 0.5 M, may cause side reactions to occur in the olefin aiylation reaction similar to those that occur with cupric chloride in the vinyl acetate synthesis mentioned above. The side reaction produces 2-arylethyl chlorides and these products may be made the major ones if cupric chloride is present to the extent of about 2M in 10% aqueous acetic acid solution 29>. The mixed solvent is required to obtain the necessary solubility of the cupric chloride. This is a general reaction useful for producing a variety of 2-arylalkyl halides. For example, 3-phenyl-2-chloropropionaldehyde is obtained in 63% yield by the reaction of phenylpalladium chloride , cupric chloride and acrolein. 

Scheme 3.1 Formation of phenylpalladium intermediates from organic halides and pseudohalides by oxidative addition.

Catellani and Lautens have independently reported unique palladium/ norbornene-catalyzed reactions of aryl halides, which mechanistically involve a reversible alkene insertion/p-carbon elimination process [11]. For example, iodobenzene reacted with 1-iodobutane and methyl acrylate to form the multiply-alkylated benzene 29 (Scheme 7.9) [12]. The following mechanism is proposed oxidative addition of phenyl iodide onto palladium generates phenylpalladium(ll) iodide. A double bond of norbornene inserts into the C-Pd bond to form an alkylpalladium species, which cleaves a C-H bond nearby to form the palladacycle 25. -Butyl iodide then reacts with 25 to form the Pd(IV) intermediate 26, which undergoes reductive elimination. Repetition of the cyclometalation/alkylation process leads to the formation of 27. Then, P-carbon elimination affords the arylpalladium species 28 together with norbornene. Subsequently, a Heck-type reaction takes place with methyl acrylate, giving rise to 29. 

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