Basis routes, complex reactions

For linear mechanisms we have obtained structurized forms of steady-state kinetic equations (Chap. 4). These forms make possible a rapid derivation of steady-state kinetic equations on the basis of a reaction scheme without laborious intermediate calculations. The advantage of these forms is, however, not so much in the simplicity of derivation as in the fact that, on their basis, various physico-chemical conclusions can be drawn, in particular those concerning the relation between the characteristics of detailed mechanisms and the observable kinetic parameters. An interesting and important property of the structurized forms is that they vividly show in what way a complex chemical reaction is assembled from simple ones. Thus, for a single-route linear mechanism, the numerator of a steady-state kinetic equation always corresponds to the kinetic law of the overall reaction as if it were simple and obeyed the law of mass action. This type of numerator is absolutely independent of the number of steps (a thousand, a million) involved in a single-route mechanism. The denominator, however, characterizes the "non-elementary character accounting for the retardation of the complex catalytic reaction by the initial substances and products. 

The photoaquations of X in fra .v-[Ir(X)2(en)2]+ provide a useful route for the rapid and clean preparation of various fraM.T-[Tr(X)(Y)(en)2] + and trans-[lr(X )2 (en)2]+ complexes (reactions 67-70).229,237 The products obtained were those expected on the basis of the frans-effect behaviour in such systems. 

On the basis of all the experiments with vinyl-substituted silanes and siloxanes with heteroatom-functionalized alkenes, catalyzed by ruthenium complexes, we were able to propose general synthetic routes. The reaction of vinyl-substituted silanes with vinyl-substituted heteroorganic (N,0,S) compounds proceeds effectively and yields, under optimum conditions (usually a five-fold excess of alkene 80-110 °C) and in the presence of ruthenium complexes containing or generating Ru-H and/or Ru-Si bonds, l-silyl-2-N(0 or S)-substituted ethenes with a high preference for the -isomer, according to Eq. 4, where R3 = Mea, Me2Ph, or (OEt)3. 

UV irradiation. Indeed, thermal reaction of 1-phenyl-3,4-dimethylphosphole with (C5HloNH)Mo(CO)4 leads to 155 (M = Mo) and not to 154 (M = Mo, R = Ph). Complex 155 (M = Mo) converts into 154 (M = Mo, R = Ph) under UV irradiation. This route was confirmed by a photochemical reaction between 3,4-dimethyl-l-phenylphosphole and Mo(CO)6 when both 146 (M = Mo, R = Ph, R = R = H, R = R" = Me) and 155 (M = Mo) resulted (89IC4536). In excess phosphole, the product was 156. A similar chromium complex is known [82JCS(CC)667]. Complex 146 (M = Mo, R = Ph, r2 = R = H, R = R = Me) enters [4 -H 2] Diels-Alder cycloaddition with diphenylvinylphosphine to give 157. However, from the viewpoint of Woodward-Hoffmann rules and on the basis of the study of UV irradiation of 1,2,5-trimethylphosphole, it is highly probable that [2 - - 2] dimers are the initial products of dimerization, and [4 - - 2] dimers are the final results of thermally allowed intramolecular rearrangement of [2 - - 2] dimers. This hypothesis was confirmed by the data obtained from the reaction of 1-phenylphosphole with molybdenum hexacarbonyl under UV irradiation the head-to-tail structure of the complex 158. 

For further contributions on the dia-stereoselectivity in electropinacolizations, see Ref. [286-295]. Reduction in DMF at a Fig cathode can lead to improved yield and selectivity upon addition of catalytic amounts of tetraalkylammonium salts to the electrolyte. On the basis of preparative scale electrolyses and cyclic voltammetry for that behavior, a mechanism is proposed that involves an initial reduction of the tetraalkylammonium cation with the participation of the electrode material to form a catalyst that favors le reduction routes [296, 297]. Stoichiometric amounts of ytterbium(II), generated by reduction of Yb(III), support the stereospecific coupling of 1,3-dibenzoylpropane to cis-cyclopentane-l,2-diol. However, Yb(III) remains bounded to the pinacol and cannot be released to act as a catalyst. This leads to a loss of stereoselectivity in the course of the reaction [298]. Also, with the addition of a Ce( IV)-complex the stereochemical course of the reduction can be altered [299]. In a weakly acidic solution, the meso/rac ratio in the EHD (electrohy-drodimerization) of acetophenone could be influenced by ultrasonication [300]. Besides phenyl ketone compounds, examples with other aromatic groups have also been published [294, 295, 301, 302]. 

Facile routes for the convenient syntheses of similar complexes of the aluminum(II) subhalides do not exist at all. These compounds [X2(L)A1-A1(L)X2 (X = Cl, Br, I L = NMe2SiMe3, OEt2, PEt3, MeOPh)] were obtained in low yields by the treatment of metastable solutions of aluminum monohalides A1X with the corresponding donor substances similar to the reactions described in Eq. (3).33 35 A12C14 was supposed to be formed from elemental A1 and A1C13 at 120 °C in a solvent, however, it was not isolated in a pure form and was characterized on the basis of some not really specific reactions.36 In view of the results obtained recently with respect to the synthesis and stability of aluminum subhalides,33 35 it seems to be rather implausible that indeed considerable concentrations of a subhalide had formed. Quantum-chemical calculations... 

Either fusion with alkali metals or reaction with aUcali-metal complexes with aromatic hydrocarbons will break down most fluorocarbon systems, due to the high electron affinities of these systems. Such reactions form the basis of some methods of elemental analysis [13], the fluorine being estimated as hydrogen fluoride after ion exchange. Surface defluorination of PTFE occurs with alkali metals and using other techniques [14]. Per-fluorocycloalkanes give aromatic compounds by passage over hot iron and this provides a potential route to a variety of perfluoroaromatic systems (Chapter 9, Section IB). 

---