Hydride abstraction directing effects

If the nucleophilic site (HOMO) involves a nonbonded pair of electrons (path a), a stable covalently bonded complex will form. If the HOMO is a a bond, direct reaction is unlikely unless the bond is high in energy and sterically exposed, as in a three-membered ring, but if the bond is to H, hydride abstraction may occur (path b, steps 1 and 2) or a hydride bridge may form (path 6, step 1). The last two possibilities are discussed further in Chapter 10. If the HOMO is a n bond, a n complex may result (path c, step 1), or, more commonly, donation of the n electrons results in the formation of a a bond at the end where the n electron density was higher, the other end becoming Lewis acidic in the process (path c, steps 1 and 2). The effects of substituents on olefin reactivity were discussed in Chapter 6. 

In a related study,the effect of attaching para-substituted phenyl groups directly to the 2-position of the 4-phenyl-l,3-dithiolium cation has been examined. As expected, electron-rich aryl groups tend to stabilize the cations in water. The methods used for the preparation of the 2-aryl-l,3-dithiolium salts (66) are of interest. These include the action of Grignard reagents on 2-methoxy-4-phenyl-l,3-dithiole (65) followed by hydride abstraction with triphenylmethyl perchlorate, and the reaction of 4-phenyl-1,3-dithiolium perchlorate (67) with appropriately activated benzene derivatives (e.g. phenol, anisole, and aniline). Treatment of 2-p-hydroxy-phenyl-4-phenyl-l,3-dithiolium perchlorate with triethylamine yields the red quinonoid compound (68). 

Water participates in carbocation formation59,60 by generating the strong acid H[Al(OH)Hlg3],6,59 which forms a carbocation through an alkyl halide intermediate60 or via direct hydride ion abstraction.6 Experiments with heavy water by Pines further proved its cocatalytic effect indicating that HC1 (HBr) formed is not the real activator.60,61... 

Abstract. In article the analysis of an overall HHP performance from the thermodynamic and thermalphysic point of view is carried out. For perfection of HHP operation is necessary to improve hydrides, processes in hydride beds, processes heat and mass transfer in the sorbers working in consisting of HHP. The various constructive measures directed on increase of HHP efficiency are considered. It is shown that mathematical modelling can become one of fast and effective ways of HHP designing for various technical applications. 

The completion of the substitution process to produce a substituted arene free of the metal has not been worked out in general. Direct abstraction of the endo hydride of the r] -cyclohexadienyl intermediate with trityl cation is not effective, but an interesting thermal rearrangement has been observed at 130-150 °C to create an isomeric cyclohexadienyl ligand that bears an exo (easily abstracted) hydrogen substituent (equation... 

The direct dimerization of a 17-electron hydride has rarely been documented. One example is provided by the high yield synthesis of [OsH(CO)4]2 Os-Os), upon hydrogen atom abstraction from OsH2(CO)4 by PhjC . Although the 17-electron OsH(CO)4 intermediate was not directly observed, stopped-flow kinetic and isotope effect evidence is consistent with the slow step being a H atom abstraction [33-35]. 

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