Photochemical reduction alkanes

TABLE 6. Alkanes through photochemical reductive deacetoxylation... 

Most reductive elimination reactions are induced thermally, but some important reductive elimination reactions are induced photochemically. Most examples of such reductive eliminations form dihydrogen from metal-dihydride complexes. Two examples of the photochemical reductive elimination of dihydrogen to generate intermediates that add alkanes are shown in Equations 8.6 and 8.7. In these examples, the photochemical irradiation provides the energy to induce the reductive elimination process. 

Alkanes are formed when the radical intermediate abstracts hydrogen from solvent faster than it is oxidized to the carbocation. This reductive step is promoted by good hydrogen donor solvents. It is also more prevalent for primary alkyl radicals because of the higher activation energy associated with formation of primary carbocations. The most favorable conditions for alkane formation involve photochemical decomposition of the carboxylic acid in chloroform, which is a relatively good hydrogen donor. 

The origin of the persistent radicals which are produced in low yields is of some interest. Simple photochemical silicon-carbon bond cleavage lacks precedent and is not consistent with the failure to observe any alkane or 1-alkene in the photolysis mixture. While the latter could not be expected to survive in the irradiated solution, the former would. A possible route involving photochemical 1,1-reductive elimination is described below. 

DUiydride photochemistry is not limited to reductive elimination. CpRe(PPh3)2H2 is known to photochemically catalyze H/D exchange between CeDe and other arenes or alkanes. Photochemical studies of the mechanism of this process have ruled out loss of phosphines and instead postulate photochemical transfer of one or both hydrides to the cyclopentadienyl ligand yielding a 14-e intermediate, ( -C5H7)Re(PPh3)2, as a hkely intermediate. ... 

Reductions of 2-halobicyclo-[n. 1.0]alkanes and 2-halobenzobicyclo[n. 1. OJalkenes by tributyltin hydride, with photochemical initiation, led to the formation of 3-methylcycloalkenes. The substrates 15129,18129 2129.74... 

Haloxenated Alkanes.- A previous study of the photochemical behaviour of the Insecticide Bromodmn (39) showed that there was a solvent dependency on the outcome of the reaction. Thus irradiation in hexane usinx a quartz filter xave products (40) and (41) resultinx from the reductive dehaloxenation of the double bond. In methanol dehaloxenation of the double bond also took place affordinx products (41) and (42). In a mixture of acetone and hexane as solvent a more complex mixture of products... 

The photochemical aspects of carbonyl photochemistry remain important subjects of research. Wagner and Thomas have used CIDNP to elucidate radical formation from a,a,a-trifluoroacetophenone. Irradiation of benzophenone and its derivatives in the presence of molecules with abstractable hydrogen atoms can give rise to intensely fluorescent compounds. This effect may interfere with the observation of nanosecond-domain kinetics.Quantum yields and kinetic isotope effects in nanosecond flash studies of the reduction of benzophenone by aliphatic amines have been measured by Inbar et Rate constant data are given in Tables 13 and 14. Winnik and Maharaj have studied the reaction of benzophenone with n-alkanes through hexane to hexatriacontane is 3.9 0.2kcal for all chain lengths.The effects of substituents on the benzophenone on these reactions have also been examined. The reactions of phenylacetophenone when used as polymerization initiator have been reviewed by Merlin and Fouassier. ... 

Important work by Chen et al. [123b] has shown how borylation of alkanes can be achieved both photochemically and thermally from diboron reagents to give alkylboranes (Eq. 2.43). The best catalysts, [CpRh(ethylene)2] and [Cp Rh(r/ -CeMce)], are active at 150°C. The B-B bond oxidatively adds to the metal probably followed by CH oxidative addition. Reductive eMmination gives rise to a new B-C bond being formed. Functionalization occurs at the terminal position of a linear alkane as in the alkane chemistry described above. Since C-B bonds are in principle precursors to a wide variety of functional groups, this reaction has great promise for future development. 

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)... 

Reaction of Cp Re(CO)2(Bpin)2 (16), prepared from Cp Re(CO)j (15) and puiaBa, led to the regiospecific formation of 1-borylpentane in quantitative yield under irradiation of light in pentane. Thus, the catalytic cycle involves oxidative addition of pin2B2 to Cp Re(CO)j with photochemical dissociation of CO, oxidative addition of C-H bond to Cp Re(CO)2(Bpin)2 (16) giving a rhenium(V) intermediate (17), and finally reductive elimination of an alkylboronate with association of CO (Scheme 2.4) [51]. The interaction required for C-H activation of alkane with 16 is not known but higher reactivity of primary over secondary C-H bonds has been reported in both oxidative addition (17) and bond metathesis (18) processes [52]. Isomerization of a sec-alkyl group in Cp Re(H)(R)(CO)(Bpin)2 (17) to an n-alkyl isomer before reductive elimination of pinB-R is another probable process that has been reported in metal-catalyzed hydroboration of internal alkenes [15c]. 

Reactions.— Nitriles can be reduced to aldehydes in aqueous media by photochemically generated hydrated electrons. The reduction of nitriles to amines by sodium borohydride is catalysed by Raney nickel. The transition metal promoted reductive decyanation of alkyl nitriles to homologous hydrocarbons with one less carbon atom is reported, and has been shown by van Tamelen to proceed by a different mechanism from that of a similar reduction by alkali metals in ammonia, where a process of stepwise two-electron transfer has been proposed. In the former reaction, where use is made of ferric acetylacetonate and sodium, the proton required for alkane production is derived exclusively from the acetylacetonate ligand, alkane being formed before a proton source is added this suggests a mechanism which involves initial co-ordination of the nitrile to iron, followed by reductive cleavage of the nitrile 1,2-carbon bond and proton transfer to this area, as pictured in Scheme 42. 

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