Palladium hydrodehalogenation

Palladium NHC systems for the hydrodehalogenation of aryl chlorides and bromides and polyhalogenated aromatic substrates originate from about the same time as the first reports on Ni chemistry, and show many similarities. Initial efforts showed that the combination of PdCdba) (2 mol%), one equivalent of imidazolium chloride and KOMe produced an effective system for the reduction of 4-chlorotolu-ene, especially upon use of SIMes HCl 2 (96% yield of toluene after 1 h at 100°C) [7]. Interestingly, higher ligand to metal ratios severely inhibited the catalysis with only 7% yield of toluene achieved in the same time in the presence of two equivalents of SIMes HCl 2. Variation of the metal source (Pd(OAc)2, Pd(CjHjCN)jClj), alkoxide (NaOMe, KO Bu, NaOH/ ec-BuOH) or imidazolium salt (IMes HCl 1, IPr HCl 3, lAd HCl, ICy HCl) all failed to give a more active catalyst. 

This type of hydrodehalogenation has been performed generally in the presence of organic or inorganic bases to neutralize the hydrogen halides formed. Among published results, the use of rhodium complexes as catalysts dominates, but palladium and ruthenium complexes have also been applied on a frequent basis. 

Rhodium(III) complexes [e.g. (i-Pr,P)2Rh(H)Cl2] in the presence of quaternary ammonium salts are excellent catalysts for the hydrogenolysis of chloroarenes under mild conditions [5] other labile substituents are unaffected. Hydrodehalogenation of haloaryl ketones over a palladium catalyst to give acylbenzenes is also aided by the addition of Aliquat [6]. In the absence of the phase-transfer catalyst, or when the hydrogenation is conducted in ethanol, the major product is the corresponding alkyl-benzene, which is also produced by hydrodehalogenation of the halobenzyl alcohols. 

Schiith, C. and Reinhard, M. Catal34ic hydrodehalogenation of some aromatic compounds using palladium on different support materials, in American Chemical Society - Division ofEnvironmental Chemistry, Preprints of Extended Abstracts, 37 (1) 173-174.1997. 

Hydrodehalogenation of organic halides catalysed by Ru/tppms555 or palladium complexes generated from the ligands 20 (n=l, R=H) and 21 (R=Ph, n=3,4) (depicted in Table 2).124-125... 

The preparation of OPDA from CNA requires two different catalytic steps, i.e. dechlorination and reduction. Both of these reactions requires hydrogen and are catalyzed by Group VIII metals. Supported palladium is considered as one of the most active catalyst both for hydrodehalogenation [2,3] and reduction of the nitro group [2]. 

Surface reactions (1) and (2] have been widely used to prepare silica supported metal complex catalysts [6]. The stabilization of ionic forms of palladium was strurigly increased by the presence of -OLi moiety. Simultaneous presence of -OLi and (-D) Pd surface species resulted in a very active hydrodehalogenation and reduction catalyst. 

Schreier, C. G., and Reinhard, M. (1995). Catalytic Hydrodehalogenation ofChlorinated Ethylenes Using Palladium and Hydrogen forthe Treatment of Contaminated Water. Chemosphere, 31(6), 3475-3487. 

Schiith, C., and Reinhard, M. (1997). Catalytic Hydrodehalogenation of Some Aromatic Compounds Using Palladium on Different Support Materials. Proc., 213th ACS Meeting, San Francisco, CA, 173-175. 

Fig. 25. Hydrodehalogenation progress observed by (+)-MS with intensities over time of all key species bearing the charged tag (Ar=[C6H4CH2PPh3r L=PPh3).The intensities of the palladium-containing intermediates have been multiplied by 100. Inset plot demonstrating the first order kinetics.

Aryl chlorides are widely used for C-C bond creation under Pd(0)-Pd(II) catalytic cycle conditions [62], As a consequence, use of cheaper ArCl in benzoic acid arylation should also be possible if appropriate ligands are used and catalytic cycle for the arylation is changed from Pd(II)-Pd(IV) to a Pd(0)-Pd(II) couple. The method involves use of catalytic palladium acetate in combination with butyl-di-1-adamantylphosphine ligand, cesium carbonate base, and an aryl chloride coupling partner [61]. Benzoic acids of any electronic properties are reactive (Scheme 15). Electron-poor benzoic acids react well, and it is possible to couple them with both electron-poor and electron-rich aryl chlorides. Both electron-poor and electron-rich aryl chlorides can be coupled with electron-rich benzoic acids. However, the combination of an electron-rich aryl chloride with an electron-rich benzoic acid is occasionally problematic due to product decarboxylation or aryl halide hydrodehalogenation. The nitro group is compatible with the reaction conditions, as is the ester group. 

Hydrodehalogenation of aliphatic or benzylic halides were catalyzed by water soluble ruthenium phosphine complexes in the presence of sodium formate as hydrogen donor [194], Hydroxycarbonylations could also be performed with high palladium catalyst activities in biphasic systems [195-197]. 

Poly(V-vinyl-2-pyrrolidone) has been studied as a carrier for palladium in the hydrogenation of oleflns f and nitroaromatics, hydrodehalogenation of organic halides,f t and carbonylation of aUyl halides. - In these cases PVP mostly likely played the role of supporting in situ formed colloidal Pd, rather than that of a well-defined ligand. 

Patzold, E. Ohme, G. (1993) Water-soluble palladium(ll) phosphine complexes as catalysts in the hydrodehalogenation of allyl and benzyl halogenides under biphasic and phase-transfer conditions, J. Prakt. Chem. - Chem. Ztg., 335,181-4. 

Chelucci G, Figus S (2014) NaBH4-TMEDA and a palladium catalyst as efficient legio- and chemoselective system for the hydrodehalogenation of halogenated heterocycles. J Mol Catal A Chem 393 191-209. doi 10.1016 5.molcata.2014.06.012... 

Table 3.3 Hydrodehalogenation of CHCIF2 with hydrogen over various metal fluorides supported palladium catalyst ...

Calo, V., Nacd, A., MonopoU, A., Damascelli, A., leva, E. and QolE, N. (2007) Palladium-nanopartides catalyzed hydrodehalogenation of aryl chlorides in ionic Uc[uids. Journal of Organometallic Chemistry, 692,4397. 

It has also been proposed that reaction CD is the rate limiting step in hydrodehalogenation of aryl halides in the presence of homogeneous and metallocomplex catalysts [7,8]. Based on this knowledge it has been suggested that the introduction of palladium into the support not in metallic but in ionic form should increase the rate of hydrodehalogenation provided the mode of stabilization of the ionic form of palladium can be found. 

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