Pyridines reductive elimination

The intermolecular Heck reaction of halopyridines provides an alternative route to functionalized pyridines, circumventing the functional group compatibility problems encountered in other methods. 3-Bromopyridine has often been used as a substrate for the Heck reaction [124-126]. For example, ketone 155 was obtained from the Heck reaction of 3-bromo-2-methoxy-5-chloropyridine (153) with allylic alcohol 154 [125]. The mechanism for such a synthetically useful coupling warrants additional comments oxidative addition of 3-bromopyridine 153 to Pd(0) proceeds as usual to give the palladium intermediate 156. Subsequent insertion of allylic alcohol 154 to 156 gives intermediate 157. Reductive elimination of 157 gives enol 158, which then isomerizes to afford ketone 155 as the ultimate product This tactic is frequently used in the synthesis of ketones from allylic alcohols. 

A formally -elimination reaction occurs when [Pt2Et3(jw-dppm)2]+ is thermolyzed to give [Pt2Et2( i-H)( i-dppm)2]+. This reaction across a biplatinum complex follows first-order kinetics, and is not retarded by added dppm. The -elimination step is not rate determining.56 Photolysis of [Pt2Me3(/r-dppm)2]+ in pyridine gives PtMe dppm) and [PtMe(py)dppm]+, but in MeCN, acetone or CH2C12 solvent, reductive elimination of ethane occurs.5 ... 

Complex XV undergoes an interesting binuclear reductive elimination reaction losing H2 upon reacting with ligands such as tertiary phosphines see Equation 6) (15), but pyridine and simple alkenes such as ethylene will not induce this reaction. 

An electrophilic palladation by a phenyl palladium intermediate at C(3) and a C(3) to C(2) migration of a palladium species, followed by reductive elimination, is indicated. 2-Phenylpyridine has been formed by the reaction of pyridine and iodobenzene at 150 °C in the presence of phosphido-bridged ruthenium dimer complexes.49 A catalytic cycle involving one of the complexes in the system was proposed. Optimum conditions for the efficient and regioselective palladium-catalysed C(2) arylation of ethyl 4-oxazolecarboxylate (47) with iodobenzene have been presented.50... 

Analogous experimental conditions (i.e. Cl, 0.1-1 Torr) allowed for the detection by tandem mass spectrometry of the collision complexes formed in the ion-molecule reactions of several aromatic radical cations M+ (M = C6H5X, X= Me, N02, Cl pyridine, furan, thiophene) and neutral iodoalkanes RI (R= n-Pr, 2-Pr, n-Bu, 2-Bu, etc.) en route to areni-um ions34,35. The collision complexes are covalently bonded species, namely nonisomeriz-ing iodonium radical positive ions 4 which dissociate to arenium ions 5 via reductive elimination of I (Scheme 7)34. 

Oxidative addition of C2 - H bonds of imidazolium salts to low valent metals was first observed by Nolan and coworkers in 2001, who proposed a NHC - Pd - H intermediate in the catalytic cycle of the dehalogenation of aryl halides with Pd(dba)2 in the presence of imidazolium salts [154]. More direct evidence of this process was described by Crabtree and coworkers two years later [155]. The reaction between a pyridine-imidazolium salt and Pd2(dba)3 afforded the preparation of bis-NHC - Pd(II) complexes by C2 - H oxidative addition (Scheme 40). The presumed Pd - H intermediates were not detected. The authors proposed a mechanism via two successive C - H oxidative additions followed by reductive elimination of H2 [ 155]. 

The cyclo addition of the alkene to the ruthenium vinylidene species leads to a ruthenacyclobutane which rearranges into an allylic ruthenium species resulting from / -elimination or deprotonation assisted by pyridine and produces the diene after reductive elimination (Scheme 16). This mechanism is supported by the stoichiometric C-C bond formation between a terminal alkyne and an olefin, leading to rf-butatrienyl and q2-butadienyl complexes via a ruthenacyclobutane resulting from [2+2] cycloaddition [62]. 

Photochromic anthrapyridones (IIB1, IIB2) were produced by cyclization of 1-chloracetylamino-4-(or -5)-phenoxyanthraquinones with the subsequent reductive elimination of the pyridine group from the products obtained in pyridine (Scheme 2).38 Photochromic chloro derivatives of pyridines (IIC) were synthesized by the interaction of the resultant photochromic anthrapyridines with phosphorous chlor-oxide.38... 

Phenoxy-substituted 2H, 8 -naphtha [l,2,3-de]-benz[/j]-2,8-quinolinedione (IIIC, R2 = hydrogen, R3 = CH3) was synthesized by cyclization of TV-chloro-acetyl-6-aminonaphthacenequinone in pyridine to the corresponding naphthacenepyrido-nyl-3-pyridinium chloride, with the subsequent reductive elimination of the pyridi-nium residue.39 The 3-acetyl derivative was obtained by the interaction of 6-amino-ll-chloro-5,12,naphthacenequinone with acetic anhydride in phenol in the presence of anhydrous sodium carbonate.39... 

Such a reactivity has been observed previously during the pyridine-promoted solvolysis of acetyltetracarbonyl cobalt(I) (56). Both effects in the case of ruthenium suggest that the carbonylated product does not leave the metal center by reductive elimination of acyl iodide, but rather by nucleophilic displacement. 

A radical species may also be generated by reduction of an electron-deficient compound and a classical entry to 4,4 -bipyridines is the reduction of a pyridine by sodium and subsequent rearomatization. Figure 8a illustrates the use of such a reduction in order to prepare the precursor of a sodium-ion molecular switch [30], A more general route, derived from the ancient copper-catalyzed Ullmann coupling, is the metal-induced dimerization of an aryl halide. The key step is a reductive elimination within the coordination sphere of the metal. A nickel(O) complex, in stoichiometric quantities, is usually selected for this purpose. Constable and Ward have used such a reaction to prepare a bis-terpyridine from an interesting synthon, which would have otherwise required a more specialized strategy with dedicated intermediates (Figure 8b) [31]. 

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