Aryl bromides hydrogenolysis

Another method for the hydrogenolysis 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 /3-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]. 

Hydrogenolysis Aryf Chlorides 4.5.2.4 Hydrogenolysis Aryl Bromides... 

Bar,R., Y. Sasson, and Y. Blum, Hydrogenolysis of Aryl Bromides by Sodium Formate and Palladium-Phosphone Catalyst Under Phase Transfer Conditions, J.Mol.Catal.,l6, 175 (1982). 

D. G. Putnam described di- or mono-allylamine as a masked ammonia equivalent.92 The amination proceeds well with aryl bromides or halopyridines using either (DPPF)PdCl2 or [(o-tol)3P]2PdCl2. Deprotection was accomplished using standard Pd/C hydrogenolysis conditions. 

Reductions-Electrolysis has been reported to effect the selective hydrogenolysis of aryl bromide moieties in the presence of other reducible groups. The yield was 94%, compared to 13% with EtsSnH. Amides may also be reduced electro-chemically on a 0.05 mole scale to the corresponding aldehydes and alcohols, depending on conditions, in 50-97% yield. 

Examples of the formylation of aryl halides with synthesis gas catalyzed by palladium complexes are summarized in Equation 19.90. These reactions relied upon the development of ligands with particular steric and electronic properties. The dia-damantyl-n-butyl phosphine shown in the equation, in combination with palladium acetate, leads to the formation of aromatic aldehydes in high yields from electron-rich and electron-poor aryl bromides. Reactions of nitroarenes and 2-bromopyridine provided the aldehydes in low yield, but other examples occurred in satisfactor) yield with only 0.1-0.75 mol % catalyst. The identity of the base is important in this process, and TMEDA was the most effective base. The mechanism of this process was not proposed in the initial work, but is likely to occur by oxidative addition of the aryl halide, insertion of the carbon monoxide into the palladium-aryl bond, and a combination of hydrogenolysis of the acyl intermediate and elimination of hydrogen halide to regenerate palladium(O). The base would then be involved in the hydrogenol5 sis and consumption of hydrogen halide. 

P (p-anisyl)3 >> PMePh2, dppe) whose activities decrease with increasing ligand basicity as shown . The hydrogenolysis of aryl bromides by benzyl alcohol is catalysed by a Pd(II) complex under phase transfer conditions (reaction 2) . 

Hydrolysis of the hydrazone followed by catalytic hydrogenation of the olefin and hydrogenolysis of the aryl bromide gave keto-phenol 25. Treatment of 25 with bromine in acetic acid, followed by reaction of the intermediate tribromide with 2,4-dinitrophenylhydrazine, gave 26. Hydrolysis of the hydrazone was followed by reduction of the enone to provide codeine (3). Demethylation of codeine provided morphine (1). This is a remarkable synthesis given tools that were available at the time. 

In the reaction of Q,/3-unsaturated ketones and esters, sometimes simple Michael-type addition (insertion and hydrogenolysis, or hydroarylation, and hydroalkenylation) of alkenes is observed[53,54]. For example, a simple addition product 56 to methyl vinyl ketone was obtained by the reaction of the heteroaromatic iodide 55[S5]. The corresponding bromide affords the usual insertion-elimination product. Saturated ketones are obtained cleanly by hydroarylation of o,/3l-unsaturated ketones with aryl halides in the presence of sodium formate, which hydrogenolyses the R—Pd—I intermediate to R— Pd—H[56]. Intramolecular hydroarylation is a useful reaction. The diiodide 57 reacts smoothly with sodium formate to give a model compound for the afla-toxin 58. (see Section 1.1.6)[57]. Use of triethylammonium formate and BU4NCI gives better results. 

An interesting case is the hydrogenolysis of aryl chlorides, bromides and iodides in respective yields of 72%, 72% and 82% on irradiation in isopropyl alcohol [310]. 

The hydrogenolysis of aryl halides proceeds more readily than the cleavage of alkyl-halogen bonds. Aryl chlorides are rather stable to hydrogenolysis in neutral medium and bromides are dehalogenated only to a moderate extent 24,132 but iodides are readily lost. Aryl fluorides, however, are hydrogenolyzed only under... 

When sodium formate is used as a terminating agent, hydrogenolysis of aryl-palladium occurs [10]. Reaction of iodobenzene, n-propyl bromide, and sodium formate afforded 1,3-di-n-propylbenzene (35) in 78% yield. In this case, the 2,6-di-n-propylphenylpalladium 30 was hydrogenolyzed with sodium formate, and 35 was obtained via Pd formate 33 and hydridopalladium 34. This is not only very interesting, but also useful, because meta dialkylation of benzene can be achieved in this way, which is difficult to carry out by conventional methods. 

This method offers high functional group tolerance as electron-withdrawing groups and free hydro)g ls are tolerated. In addition to aryl and vinyl iodides, aiyl bromides may also be used but typically provide lower yields. A variant employing aryl boronic acids in place of aryl halides was also reported. Additionally, primary sulfonamides are accessible by a hydrogenolysis/Zn reduction sequence (Scheme 13.18). ... 

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