Elimination reactions photoextrusion

Very recently, a promising new reaction has been discovered, alkane borylation, illustrated by Eq. 2.41 [123]. This can be driven by light, or, being exothermic, it can be carried out as a thermal process. Photoextrusion of CO from the tungsten carbonyl precursor is believed to be followed by oxidative addition of the alkane CH bond, followed by reductive elimination of the R-BR2product (Eq. 2.42) [123a]. 

Photochemical alkane carbonylation with RhCl(CO)(PMe3)2 is also possible. This seems to operate by initial photoextrusion of CO from the catalyst, oxidative addition of the alkane C—H bond, addition of CO to the metal, followed by insertion, and then reductive elimination as shown in Figure 3. Preferential reaction at the 1 ° or 2° C—H bond is found. Here the initial product does not seem to isomerize, but Norrish type II photoreactions tend to degrade the aldehyde product. Moving to longer wavelengths minimizes the Norrish degradation problem, but the selectivity of the catalytic system then falls off No more than 30 turnovers have been observed to date (e.g. equation 24)... 

Silylene complexes have been prepared by coordination of free silylene, extrusion of dihydrogen from a dihydrosilane, a-hydrogen elimination from a hydrosilyl complex, extraction of hydride, halide, or pseudo-haUde from a metal-silyl complex, and photoextrusion of silylene from a disilanyl complex. Examples of these reactions are shown in Equations 13.39-13.45. 

In spite of its great importance, reductive elimination has received less detailed study than oxidative addition. The reaction is most often seen in higher oxidation states, because the formal oxidation state of the metal is reduced by two units in the reaction. The reaction is especially efficient for intermediate oxidation states, such as the d metals, Ni(II), Pd(II), and Au(III), and the d metals, Pt(IV), Pd(IV), Ir(III), and Rh(III). Reductive elimination can also be stimulated by photolysis the case of photoextrusion of H2 from dihydrides is the best known (Section 12.3). 

The general synthetic methods for [2.2] CPs have been already estabhshed. Most of them employ the ring contraction reactions of the 2,11-diheteroatom-substituted [3.3]CPs. Photochemical ehmination of sulfur or selenium atoms from [3.3] CP-2,11-disulfide or diselenide and flash vacuum pyrolysis of sulfur dioxide obtained by the oxidation of the corresponding sulfides are the most general methods. 2,11-Diaza[3.3]CPs are converted to the corresponding [2.2]CPs via their nitroso derivatives by reductive elimination of nitrogen. " Photoextrusion reactions of COj from cychc diesters can also be appHed to the synthesis of [2.2]paracyclophanes (PCPs)"- and [2.2]heterophanes, as described below. "... 

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