Photochemical Reaction with Silanes

The formation of the adduct 22 is reversible. Although the photochemical preparation of 22 proceeds in yields greater than 90%, solutions containing this complex decompose over a period of one day to regenerate and Re2(CO)jQ [70]. 

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Flash-photolysis studies of the hexacarbonyls Cr(CO)g, Mo(CO)g and W(CO)g are, like previous years, still abundant Nayak and Burkey have found that there are low quantum yields for Cr(CO)g substitution in fluorocarbon solvents - which provides yet more evidence that metal-fluorocarbon interactions are very weak There have been estimates made of solvent-metal bond strengths in (Solvent)M(CO)5 complexes (Solvent = Benzene [M = Mo, W] and Tetrachloromethane [M = Cr]) and the photolysis of W(CO)g in the presence of hex-l-ene has been reported Flash-photolysis studies of the photochemical reactions of silanes with Cr(CO)g have been published Using time-resolved infrared spectroscopy RIR), Turner and co-workers have captured the IR spectrum of the MLCT excited state of W(CO)5(4-cyanopyridine) which rapidly decays to W(CO)5. The excited state of W(CO)5(4-cyanopyridine) is relatively long lived, which makes the experiment possible. This reporter will be interested to see how this technique develops. 

Allylic silanes react with aldehydes, in the presence of Lewis acids, to give an allyl-substituted alcohol. In the case of benzylic silanes, this addition reaction has been induced with Mg(C104)2 under photochemical conditions. The addition of chiral additives leads to the alcohol with good asymmetric induction. In a related reaction, allylic silanes react with acyl halides to produce the corresponding carbonyl derivative. The reaction of phenyl chloroformate, trimethylallylsilane, and AICI3, for example, gave phenyl but-3-enoate. ... 

Lee and coworkers distinguish between sub-critical, critical and supercritical external flash intensities to produce photochemical reaction. For subcritical intensities, free radical production is too low to produce the sequence of events described above. Critical intensities do create these events. At very high, supercritical flash intensities, free radical production thruout the gas volume is so large and subsequent reactions are so rapid that the entire gas volume explodes in what amounts to a constant volume expln Urtiew et al (Ref 7) studied the reaction between silane (SiH4) and tetrafluorohydrazine (N2F4) with cis-2-butane as an inhibitor. The... 

Cycloaddition reactions of acyl silanes appear to be rare, but Brook has shown that a-silyloxy bis(trimethylsilyl)silenes (52), generated photochemically from acyl tris(trimethylsilyl)silanes (vide infra, Section IV.A.4), undergo [2 + 2] and [4 + 2] cycloaddition reactions with ketones, and [4 + 2] cycloaddition reactions with less bulky acyl silanes, as illustrated in Scheme 8717,24 26 72 73,201. They do not, however, react with their parent acyl tris(trimethylsilyl)silanes. 

Takuwa, A., Fujii, N., Tagawa, H., and Iwamoto, H. (1989) Photochemical reaction of benzophenones with allylic silanes. Bulletin of the Chemical Society of Japan, 62, 336-338. 

The photolysis of silacyclopropanes 21 and 22 by irradiation with a high-pressure mercury lamp proceeds simultaneously by two different routes, one leading to the formation of a 1-alkenyl substituted silane via a 1,2-hydrogen shift which has never been observed in the photolysis of the silacyclopropanes produced from methylphenylsilylene with olefins, and the other involving the usual 1,3-hydrogen shift. The photochemical reaction of 21 is shown in Eqs. (27) and (28) as a typical example. 

Table 1 Sellcted Photochemical Reactions of Metal Carbonyls with Silanes...

It is probable that the course of reactions discussed here is photochemical CO ejection followed by oxidative addition of Si—H to the 16-electron metal center this is reflected in the nature of the products (Table 1). Few reactions of mononuclear metal carbonyls with silanes are induced thermally in preference, and these often lead to different products. For example, M(CO)2C5H5-t (M = Co, Rh) and Cl3SiH give M(SiCl3)2(CO)CjH5t at 100"C and RT respectively whereas FefCO), and the same silane provide Fe(SiCl3)2(CO)4 at 140°C and Fe3(CO),2(/<-SiCl2)3 at 160°C . 

Chromium.—Photochemical reactions of, for example, Cr(C He)(CO)3 with silanes in hydrocarbon solvents give the silyl-transition-metal hydrides (18). The mechanism is thought to be dissociative, with loss of one carbon... 

Another methodology is the deoxygenation of nitroxides by (TMSlsSiH, shown in Reaction (22). Indeed, the reaction of this silane with TEMPO, in the presence of thermal or photochemical radical initiators, afforded the corresponding amine in quantitative yield, together with the siloxane (TMS)2Si (H)OTMS. 

The reaction of thermally and photochemically generated tert-butoxyl radicals with trisubstituted silanes [Eqs. (6) and (7)] has been used extensively for the generation of silyl radicals in ESR studies, in time-resolved optical techniques, and in organic synthesis. Absolute rate constants for reaction (7) were measured directly by LFP techniques,56,62,63 whereas the gas phase kinetic values for reactions of Me3SiH were obtained by competition with decomposition of the tert-butoxyl radical.64,65... 

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