Allylsilanes photochemical

Another interesting example of a photochemi-cally induced domino process is the combination of the photocyclization of aryl vinyl sulfides with an intramolecular addition as described by Dittami et al. [901 as intermediate a thiocarbonyl ylide can be assumed. The domino-Norrish I-Knoevenagel-allyl-silane cyclization developed by us allows the efficient stereoselective formation of 1,2-trans-subsituted five- and six-membered carbocycles.1911 A photochemical cycloaddition of enamino-aldehydes and enamino-ketones with the intermediate formation of an iminium salt followed by addition to allylsilanes gives access to novel bicyclic heterocy-des. New examples of photochemically induced... 

In photochemical reactions, some examples have been known of fragmentation of cation radicals of silyl compounds into carbon radicals. Mizuno and colleagues reported a photochemical addition reaction of allylsilane to dicyanophenylethene. Photochemical electron transfer between excited phenanthrene E50 and dicyano-... 

With the exception of photochemical reactions, the chemistry of allyltin compounds is analogous to that of allylsilanes and translates to the use of acetylenic tin reagents. As a final illustration of the utility of tin reagents to the formation of C-glycosides, the use of acetylenic tin reagents, shown in Scheme 7.20, is addressed. In the cited report, Zhai et al. [98] observed the formation of a-C-acetylenic glycosides in good yields. The reactions were catalyzed by zinc chloride and run in carbon tetrachloride. 

Cr(CO)6 is active in the 1,4-hydrosilylation of 1,3-dienes to yield allylsilanes using a method of synthetic utility [1 ]. Wrighton and co-workers have reported the high catalytic activity of Fe(CO)s upon UV irradiation under mild temperature conditions (0-50 °C) in the hydrosilylation of alkenes [13]. Ru, Os, and Ir are effective photocatalysts of the hydrosilylation of acetone, whereas Fe3(CO)i2, Co2(CO)s, and Co4(CO)i2 are known to be efficient photoactivated catalysts in the hydrosilylation of olefins. It has been also demonstrated that Ru(CO)4PPh3 and Ru(CO)3(PPh3)2 complexes can serve as catalysts for the photochemical hydrosilylation of benzaldehyde [2]. 

Knoevenagel products are highly reactive compounds because of their low energy LUMO. They can act as dienophiles in the normal Diels-Alder reaction, as heterodienes in the hetero Diels-Alder reaction with inverse electron demand,- as dipolarophiles in 1,3-dipolar cycloadditions, as enophiles in the ene reaction and as acceptors for the addition of allylsilanes. Sigmatropic rearrangements and photochemical reactions have been described. 

In studies directed toward the synthesis of members of the Cephalotaxus alkaloid family, irradiation of allylsilane iminium ion (129) provided a 70% yield (at 45% conversion) of spiroazacycle (130). ° It should again be noted that this S-cndo-trig cyclization does not occur without photochemical activation (c/. Scheme 25). 

Chiral oxetanes can be synthesized by the BF3-OEt2 catalyzed [2 + 2] cycloaddition reactions of 2,3-O-isopropylidenealdehyde-D-aldose derivatives with allylsilanes, vinyl ethers, or vinyl sulfides. The regiospecificity and stereoselectivity is greater than in the photochemical reaction fra s-2-alkoxy- and trans-2-phenylthiooxetanes are the resulting products. 

Photochemical oxidations of various substrates such as amines [763], alkenes [769,770,772,775,778-780], fiiran [764,765], sulfide [766,773], phytol [767], bi-adamantylidene [768], thiophenolate [771], catechol [774], allylsilane [776], and imines [777], etc., through reactions with singlet oxygen sensitized by porphyrins have been extensively studied. Photochemical atrop isomerization of picket-fence-type porphyrin should be noted here as another interesting type of photoreaction [793]. 

Photochemical [2 + 2] ring closures have been used to synthesize cyclophanes and related polycyclic ring systems containing silicon in the ring using triplet sensitizers such as benzophenone or dicyanonaphthalene as shown in Scheme 67. The photochemistry of simpler allylsilanes was also investigated, as shown in the scheme. In these reactions the silyl group and its substituents acted as tethers which controlled the orientation of the two carbon-carbon double bonds involved in the photocyclization. 

Selective Electrophilic Addition Reactions. The difference in reactivity between the allylstannane and aUylsilane functionalities incorporated within this bismetaUoid allows for selective reaction with electrophiles (eq 1). The more nucleophilic allylstannane moiety reacts preferentially with electrophiles such as acid chlorides and aldehydes under thermal conditions, thereby providing functionalized allylsilanes (eq 2). Related processes have been induced photochemically. The allylstannane function-... 

Radical Allylation Reactions. Trimethyl 2-[(tributylstannyl) methyl]-2-propenyl silane reacts with organic halides under photochemical conditions (irradiation with a medium pressure mercury lamp through a pyrex filter) at temperatures below 20 °C to give various allylsilanes (eq 7). The ease of the reaction was found to be halide dependent with those affording more electrophilic radicals generally behaving most effectively. 

The role of the light is to generate a thermally active catalyst. This catalytic reaction can also be achieved thermally using benzene Cr(CO)3 at 175°. However, though the allylsilanes obtained are common to the Cr(CO)6 photochemical procedure, the products occur in different ratios and substantial yields of diene dimers are formed (Wrighton and Schroeder, 1974). 

Mella, M., Fasani, E., and Albini, A., The photochemical reaction between aromatic nitriles and allylsilane,/. Org. Chem., 57, 6210,1992. 

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