Hydrogenolysis with Various Hydrides

Hydrogenolysis of aryl and alkenyl halides and triflates proceeds by the treatment with various hydride sources. The reaction can be explained by the transmetallation with hydride to form palladium hydride, which undergoes reductive elimination. Several boro hydrides are used for this purpose[680], Deuteration of aromatic rings is possible by the reaction of aryl chlorides with NaBD4681]. 

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]. 

The enone 807 is converted into the dienol triflatc 808 and then the conjugated diene 809 by the hydrogenolysis with tributylammonium for-mate[689,690]. Naphthol can be converted into naphthalene by the hydrogenolysis of its triflate 810[691-693] or sulfonates using dppp or dppf as a ligand[694]. Aryl tetrazoyl ether 811 is cleaved with formic acid using Pd on carbon as a catalyst[695]. 

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]. 

Aryl and alkenyl triflates 8 and 9 were hydrogenolyzed with EtsSiH in DMF [4]. 

As a convenient process, Rawal used Pd(OAc)2 combined with As(o-Tol)3 or P(o-To1)3 as a catalyst, and hydroquinone as a reductant in DMA in the presence of CS2CO3 for the coupling of mainly aryl iodides. The coupling product 10 was obtained in 95 % yield. Intramolecular coupling of the diiodide 11 afforded 12 in 70% yield [6]. 

The coupling catalyzed by Pd/C in the presence of Zn powder proceeds smoothly at room temperature in a mixed solvent of acetone and water as the best solvent. Venkatraman and Li carried out the reductive reaction even under an air atmosphere, and claimed that the Pd catalyst is not sensitive to oxygen [7]. 

Pd(0)-Catalyzed Reactions of Ally lie Compounds via n-Allylpalladium Complexes 

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Palladium-Catalyzed Hydrogenolysis of Allylic and Propargylic Compounds with Various Hydrides. Tsuji, J. Mandai, T. Synthesis 1996,1. 

Pd-catalyzed hydrogenolysis of allylic compounds with various hydride sources is important not only for the preparation of alkenes, but also for a deprotection of aUyl-derived protecting group. The latter will be discussed in Sect. V.2.3.2. 

Hydrogenolysis of Allylic and Propargylic Compounds with Various Hydrides. 

Boron substituents in the [l,3,2]diazaborolo[l,5- ]pyridine derivative 109 were studied. This compound was obtained via reduction of its precursor 108 with sodium amalgam (Scheme 27). The bromide attached to the boron atom was further displaced with various halide, hydride, sulfur, and carbon nucleophiles <2001JCD378>. [l,2,5]thiadiazolo[2,3- ]pyridine derivative 110 was deprotected (R = Cbz to R=H) by classical hydrogenolysis <2002AGE3866>. 

Another limitation is seen when extra strain is included in the compound to be reduced. Dehalogenation of 3,3-dichlorobicyclo[2.2.0]hexan-2-one with zinc/ammonium chloride in methanol gave, at best, a 25% yield of 3-chlorobicyclo[2.2.0]hexan-2-one (14) together with cyclohexenone and 6-chlorohex-5-enoic acid.128 The best results were achieved with the zinc/ acetic acid system, while addition of water, silver-promoted zinc reduction in methanol, tri-butyltin hydride reduction or hydrogenolysis with palladium in methanol did not result in formation of 14, but various other ring-opened products. 

Oxidative addition of aryl and alkenyl halides, and pseudohalides, followed by transmetallation with various metal hydrides generates Ar—M—FI species, reductive elimination of which results in hydrogenolysis of halides. In the main, Pd is used as an efficient catalyst for the hydrogenolysis. 

Reaction 1 appears to result solely in termination. In hydrogenolysis experiments with various chelates we have observed precipitation of lithium hydride in all cases at room temperature. Attempts to generate chelated LiH in situ by adding hydrogen during ethylene polymerization also caused a rapid, irreversible loss of activity. Since there is no evidence that lithium hydride can add to ethylene under moderate polymerization conditions, it is unlikely that any significant chain transfer occurs via this mechanism. Potassium alkyls readily eliminate olefin with the formation of metal hydride, and sodium alkyls do so at elevated temperatures (56). It was noted earlier that chelation of lithium alkyls makes them more like sodium or potassium compounds, so it is quite probable that some termination occurs by eliminating LiH. It is conceivable that this could be a chain transfer mechanism with more reactive monomers than ethylene because addition to lithium hydride would be more favorable. 

Chrysene. Orlow and Lichatschew (8) found that with 70 atm H2 the hydrogenolysis reaction products (by weight) were 25% methane, 35% coke, with the remaining 40% containing phenanthrene, naphthalene, benzene, and various hydrides of each. The pyrolysis products were hydrogen and methane with traces of acetylene in the gas and solid carbon. 

From these data, some key information can be drawn in both cases, the couple methane/pentane as well as the couple ethane/butane have similar selectivities. This implies that each couple of products (ethane/butane and methane/pentane) is probably formed via a common intermediate, which is probably related to the hexyl surface intermediate D, which is formed as follows cyclohexane reacts first with the surface via C - H activation to produce a cyclohexyl intermediate A, which then undergoes a second C - H bond activation at the /-position to give the key 1,3-dimetallacyclopentane intermediate B. Concerted electron transfer (a 2+2 retrocychzation) leads to a non-cychc -alkenylidene metal surface complex, C, which under H2 can evolve towards a surface hexyl intermediate D. Then, the surface hexyl species D can lead to all the observed products via the following elementary steps (1) hydrogenolysis into hexane (2) /1-hydride elimination to form 1-hexene, followed by re-insertion to form various hexyl complexes (E and F) or (3) a second carbon-carbon bond cleavage, through a y-C - H bond activation to the metallacyclic intermediate G or H (Scheme 40). Under H2, intermediate G can lead either to pentane/methane or ethane/butane mixtures, while intermediate H would form ethane/butane or propane. 

Pd-catalyzed hydrogenolysis of allylic compounds with formates is an efficient and mild method. The most important feature of the hydrogenolysis is that the hydride generated from the palladium formate attacks the more substituted side of the allylic system to give less substituted olefins. Various terminal allylic compounds are converted to 1-alkenes. - ... 

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