Metal hydrides pyridines

Metal-free initiators, 14 258-259 Metal fullerenes, 2 718-719 Metal-halogen exchange, in pyridine chemistry, 22 107-108 Metal hydrazides, 23 567 Metal hydrides, 23 611-613 amines by reduction, 2 493 hydrogen storage and, 23 851 nitriding, 27 206-207 storage of, 23 786... 

Pyridine-functionalized N-heterocyclic carbene Rh and Ir complexes have also been described as active precatalysts for C=0 bond TH. For example, Peris and coworkers observed the formation of metal hydrides by C—H oxidative addition of a pyridine-N-substituted imidazolium salt such as N-"Bu-N -(2-pyridylmethyl-imidazolium) hexafluorophosphate in the reaction leading to M-pyNHC complexes, that is [lr(cod)H(pyNHC)Cl] (58) [54]. Transmetallation from silver carbene... 

It is quite difficult to reduce benzene or pyridine, because these are aromatic stmctures. However, partial reduction of the pyridine ring is possible by using complex metal hydrides on pyridinium salts. Hydride transfer from lithium aluminium hydride gives the 1,2-dihydro derivative, as predictable from the above comments. Sodium borohydride under aqueous conditions achieves a double reduction, giving the 1,2,5,6-tetrahydro derivative, because protonation through the unsaturated system is possible. The final reduction step requires catalytic hydrogenation (see Section 9.4.3). The reduction of pyridinium salts is of considerable biological importance (see Box 11.2). 

LiAIH(OEt)3,345 DIBALH,346 and NaAIH4.347 The metal hydride method is useful for aliphatic and aromatic nitriles. Reduction to the aldehyde has also been accomplished by treatment of the nitrile with sodium hypophosphate and Raney nickel in aqueous acetic acid-pyridine or formic acid,348 and with zinc and a Cob(I)alamin catalyst in aqueous acetic... 

The reduction of quinolines, unlike that of isoquinolines, with complex metal hydrides can occur with attack of the hydride at the 2- or 4-position. As with the pyridines, the predominant attack appears to be at the 2-position to form 1,2-dihydroquinolines the... 

All reactions and sample preparations are carried out in an inert-atmosphere enclosure under dry nitrogen. Solvents and reagents are dried in the following manner. Benzene, tetrahydrofuran, and n-pentane are freshly distilled from lithium aluminum hydride pyridine is distilled over barium oxide and tetramethylethylenediamine is distilled over calcium hydride. Solvents used in preparing nmr and infrared samples are degassed by a freeze-thaw technique. Nmr spectra are obtained with torch-sealed nmr tubes. The commercial transition metal carbonyl complexes are recrystallized and vacuum-dried before use. Glassware is routinely flame-dried. 

Those heterocyclic systems attacked by complex metal hydrides are required to be relatively electron deficient. Nitrogen heterocycles in which the heteroatom contributes a single electron to the n system are considered electron deficient (e.g., 1). However, systems where the nitrogen atom contributes two electrons are considered electron rich (e.g., 2) and are not normally attacked by metal hydrides. Aromatic species that contain both a pyridine-like (1) and a pyrrole-like (2) heteroatom (e.g., 3) exhibit be-... 

The base-catalyzed rearrangement of aromatic ort/70-acyloxyketones to the corresponding aromatic (3-diketones is known as the Baker-Venkataraman rearrangement. 3-Diketones are important synthetic intermediates, and they are widely used for the synthesis of chromones, flavones, isoflavones, and coumarins. The most commonly used bases are the following KOH, potassium fert-butoxide in DMSO, Na metal in toluene, sodium or potassium hydride, pyridine, and triphenylmethylsodium. 

In an effort to understand the high reactivity of electron rich transition metal hydrides, Edelbach and Jones [59] have studied the mechanism of the reaction of (C5Me5)Rh(PMe3)H2 with perfluoroaromatics in pyridine/benzene solution. 

In the case of HDN, an additional interesting possibility also consistent with the heterolytic mechanism arises, since substrates like the pyridines -or intermediate alkyl or aryl amines- are sufficiently basic to promote the activation of hydrogen so as to form a metal hydride plus a protonated base (e.g. a pyridinium or an alkylammonium cation). Furthermore, some of the most widely accepted amine HDN mechanisms include the initial protonation of the amine nitrogen, followed by elimination of ammonia from the ammonium cation. Therefore, it is very easy to combine the idea of a heterolytic hydrogen activation promoted by, say n-pentylamine, with a subsequent degradation by a Hoffmann mechanism, to conform a reasonable HDN catalytic cycle. A simplified representation of this idea is given in Fig. E4. 

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