Hydrogenolysis hydride sources

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 Pd-catalyzed hydrogenolysis of allylic compounds by various hydrides gives alkenes. From terminal allylic compounds, either 1-alkenes or 2-alkenes are formed depending on the hydride sources [360a]. 

Hydrogenolysis over Palladium with Other Hydride Sources... 

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. 

In 1973, Hey and Arpe first reported Pd-catalyzed hydrogenolysis of aUyUc esters, phenyl ether, and amine using formic acid as a hydride source. In their attempt to prepare allylic formates from various allylic acetates by Pd-catalyzed transesterification, they obtained a mixture of alkenes instead of the expected allylic formatesf (Scheme 2). 

Pd-catalyzed hydrogenolysis. An additional example to show the difference of the regio-selectivity depending on the hydride sources is depicted for allylic nitro compound (Scheme 5). 

C. PALLADIUM-CATALYZED HYDROGENOLYSIS WITH OTHER HYDRIDE SOURCES... 

AUylic heteroatom functional groups are cleaved by Pd-catalyzed hydrogenolysis using other hydride sources such as LLAlH4,f f " borohydride, " hydrosilanes, BusSnH, -butylzinc chloride/ ° A -propyl-l,4-dihydronicotinamide, Smlj/i-propanol, and electrolysisJ Some examples are shown in Scheme... 

Pd-catalyzed hydrogenolysis of allylic compounds with formates is an efficient and mild method. The hydride generated from the palladium formate attacks the more substimted side of the allylic system to give less substituted olefins in contrast to the case with other hydride sources. Pd-catalyzed hydrogenolysis of propargylic compounds affords either aUenes or alkynes depending on the structure of the propargylic compounds. 

The potential participation of an alternative route, involving a binuclear elimination reaction between a metal-acyl and a metal-hydride has also been probed [73]. In Rh-catalysed cydohexene hydroformylation, both [Rh4(CO)i2] and [Rh(C(0)R)(C0)4] are observed by HP IR at steady state, the duster species being a potential source of [HRh(CO)4] by reaction with syn-gas. The kinetic data for aldehyde formation indicated no statistically significant contribution from binudear elimination, with hydrogenolysis of the acyl complex dominant. For a mixed Rh-Mn system. 

Hydrogenolysis of the toluene-4-sulfonate of an alcohol may be carried out with a nucleophilic hydride such as lithium aluminium hydride. There are also a series of radical methods based on the reduction of alkyl halides with tri- -butyltin hydride (BUjSnH). Finally, the source of the hydrogen may be the electrophilic proton, exemplified by the decomposition of organometallic reagents such as the Grignard reagent with water. 

---