Oxidation of Heteroleptic Diarylpalladium Compounds withtert-Butyl Hydroperoxide. Substituent Effects in Aromatic Oxidation Reactions

Abstract

A series of heteroleptic diarylpalladium compounds, containing both a naphthyl (1-C10H6CH2NMe2-2 or 1-C10H5CH2NMe2-2-Me-3) and a phenyl (1-C6H4CH2NMe2-2 or 1-C6H3CH2NMe2-2-Me-x, x = 3, 5, 6) monoanionic C,N-bidentate ligand, was reacted with tert-butyl hydroperoxide (TBHP) to give selective oxygen insertion into one of the C−Pd bonds. It was found that the order of reactivity of the palladium−carbon bonds decreases in the order 1-naphthyl−Pd > phenyl−Pd. Introduction of methyl substituents enabled us to fine-tune the regioselectivity of the oxidation reactions (i.e. the ratio of phenyl- versus naphthyl oxygenation was affected) but not to reverse the order. The methyl groups have a deactivating influence on the aromatic substrates that contain them, indicating an oxidatively induced nucleophilic pathway for the actual oxygenation step. The yields of mono-oxygenated product RPdOR‘ varied from 82 to 97% and were independent of the presence of a VO(acac)2 or [RhCl(cod)]2 catalyst, although the reaction rates were strongly enhanced in the presence of these catalysts.

Reactions of m-chloroperoxybenzoic acid with dimeric cyclopalladated complexes derived from 2-phenyl-2-oxazolines

“…Complexes of type 2 were previously described in reactions of dimeric dichloro-bridged CPCs with various oxidants including m-CPBA; they are one of the two types of oxygen-insertion products reported in these studies [38,47,49,50,54]. It appears that type 2 complexes are relatively unstable, especially during chromatographic purification, and gradually produce the corresponding compounds of type 4 as well as, presumably, PdCl 2 .…”

“…Complexes of type 5, in which only one out of two palladacycles in the starting dimer underwent oxygen insertion, were previously reported in oxidation reactions of cis-(j 2 -C,N) 2 Pd complex F with t-BuOOH [47]. In the reaction of CPC 1a with 2.7 equiv.…”

“…1) and from 30 to 60 % for the l-Cl-C,S analogs (type E). Mononuclear C,N, C,N,S and C,N,N azoarene derivatives (complexes of types B-D) provided even higher yields, up to 90 % [44,47]. It appears that all reported reactions of azoarene-derived CPCs, which are likely to have poor solubility in the majority of organic solvents, were performed in MeCN.…”

“…Oxygen insertion into the C-Pd bond of cyclopalladated complexes can be accomplished by any of the following reagents: m-chloroperoxybenzoic acid (m-CPBA) (used alone [34][35][36][37][38][39][40][41][42][43][44][45][46] or with an iron(III) porphyrin catalyst [37]), other peroxy acids [40], t-BuOOH (used alone [47][48][49][50] or with a catalyst [34,35,[47][48][49][50][51][52]), hydrogen peroxide in the presence of an iron(III) porphyrin catalyst [53], pentafluoroiodosylbenzene C 6 F 5 IO (alone [37,52], in a combination with t-BuOOH [37,52] or in the presence of an iron(III) porphyrin catalyst [37,52]), iodosylbenzene C 6 H 5 IO [54], [di(benzoyloxy)iodo]benzene PhI(O 2 CPh) 2 [55] and the molybdenum peroxide MoO(O 2 ) 2 ÁHMPAÁH 2 O (HMPA = hexamethylphosphoric triamide) [14,56]. The mechanisms involved in these transformations appear to be different [5]; however, in all cases, the C-Pd bond is transformed into the C-O-Pd moiety.…”

“…However, in spite of the number of studies focused on reactions of CPCs with m-CPBA, only a few rather similar types of palladacycles have been tested: (1) dinuclear complexes of the CN type derived from azoarenes ( Fig. 1, type A) [36,44,45], (2) mononuclear complexes based on azoarenes and having an additional cyclopentadienyl ligand (B) [20,39], (3) mononuclear complexes based on azoarenes with an SR (C) [40][41][42][43] or other chelating substituent (D) [34,35], (4) dinuclear complexes of the CS type obtained from dibenzyl sulfide, benzyl phenyl sulfide and benzyl phenyl sulfoxide (E) [38] and (5) mononuclear complexes derived from 2-(dimethylamino)methylnaphthalene (F) [47]. In this paper, we report our data for the reactions of m-CPBA with dimeric dichloro-and diacetato-bridged CPCs derived from 2-phenyl-2-oxazolines.…”