Hydrogenolysis aryl halides

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. 

Many examples of insertions of internal alkynes are known. Internal alkynes react with aryl halides in the presence of formate to afford the trisubstituted alkenes[271,272]. In the reaction of the terminal alkyne 388 with two molecules of iodobenzene. the first step is the formation of the phenylacetylene 389. Then the internal alkyne bond, thus produced, inserts into the phenyl-Pd bond to give 390. Finally, hydrogenolysis with formic acid yields the trisubstituted alkene 391(273,274], This sequence of reactions is a good preparative method for trisubstituted alkenes from terminal alkynes. 

Formate is an excellent hydride source for the hydrogenolysis of aryl halides[682]. Ammonium or triethylammonium formate[683] and sodium formate are mostly used[684,685]. Dechlorination of the chloroarene 806 is carried out with ammonium formate using Pd charcoal as a catalyst[686]. By the treatment of 2,4,6-trichloroamline with formate, the chlorine atom at the /iiara-position is preferentially removed[687]. The dehalogenation of 2,4-diha-loestrogene is achieved with formic acid, KI, and ascorbic acid[688]. 

Palladium-catalyzed aminations of aryl halides is now a well-documented process [86-88], Heo et al. showed that amino-substituted 2-pyridones 54 and 55 can be prepared in a two-step procedure via a microwave-assisted Buchwald-Hartwig amination reaction of 5- or 6-bromo-2-benzyloxypyri-dines 50 and 51 followed by a hydrogenolysis of the benzyl ether 52 and 53, as outlined in Fig. 9 [89]. The actual microwave-assisted Buchwald-Hartwig coupling was not performed directly at the 2-pyridone scaffold, but instead at the intermediate pyridine. Initially, the reaction was performed at 150 °C for 10 min with Pd2(dba)3 as the palladium source, which provided both the desired amino-pyridines (65% yield) as well as the debrominated pyridine. After improving the conditions, the best temperature and time to use proved... 

The complexes [PdCl2(133)2] catalyze the hydrogenolysis of 1-chloromethylnaphthalene with K[HCOO] or Na[HCOO]. Both the solid methanoates and their aqueous solutions could be used. Addition of [R4N]+X phase transfer agents significantly accelerated the hydrogenolysis of aryl halides with methanoates.347... 

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]. 

Unexpectedly, the arylsilane 383 can be prepared by the Pd-catalyzed reaction of aryl halides with the hydrosilanes 382, without giving the expected hydrogenolysis product 384 [186],... 

This chapter surveys the reduction of saturated alkyl halides to alkanes. Reductive -eliminations of vicinal dihalides to alkenes are also described briefly. Reduction of vinyl and aryl halides is covered in this volume. Chapter 4.5 hydrogenolysis of allyl and benzyl halides is covered in this volume. Chapter 4.7, and reduction of a-halo-substituted carbonyl compounds CX—CO to carbonyl compounds CH—CO is covered in this volume. Chapter 4.8. 

HYDROGENOLYSIS OF THE TRIGONAL CARBON-HALOGEN BOND IN ARYL HALIDES... 

Because of the different rates of hydrogenolysis, aryl halides containing different halogens can be reduced selectively. 1 -Bromo-2-chlorobenzene is reduced to chlorobenzene in 96% yield on treatment for 2 h at room temperature with a complex reducing agent prepared from nickel(II) chloride, sodium hydride and t-pentyl alcohol. l-Bromo-4-chlorobenzene is converted to chlorobenzene on treatment with... 

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... 

Reduction of aryl halides. Aryl halides have generally been considered to be resistant to hydrogenolysis by lithium aluminum hydride. However, Karabatsos and Shone2 found that certain aromatic halides are reduced by the reagent in diglyme at 100°. The order of reactivity is 1 > Br > Cl > F. These chemists concluded, however, that triphenyltin hydride is superior for this purpose. 

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