Photoallylation

Scheme 47 Reductive photoallylation of mono-cyanonaphthalines and dicyanonaph-th alines.

Mizuno et al. successfully synthesized benzotricyclo[4.2.1.0 ]nonenes by reductive photoallylation of monocyanonaphthalines and dicyanonaphthalines with allyltrimethylsilane, followed by an intramolecular [2 + 2] photocycloaddition (Scheme 47). 

For example, allylic silanes react with photoexcited iminium salts by ET resulting in photoallylation. ° Key steps in such reactions are the cleavage of the C—Si... 

In addition to regiospecificity, appropriate substrates also show stereoselectivity. For example, the photoallylation of the rigid styrene (139) with phenanthrene as cosensitizer in acetonitrile, produced a 3 1 ratio of trans- to cis-allylated products. ... 

More recently Mizuno and coworkers studied the photoallylation of various cyanon-aphthalenes with allyltrimethylsilane, using added phenanthrene which, in some cases, facilitated the formation of the benzotricyclic compound 338, as shown in equation 34174. 

Protti, S., Fagnoni, M. and Albini, A. (2005) Expeditious synthesis of bioactive allylphenol constituents of the genus Piper through a metal-free photoallylation... 

Similar to ketones, quinones have also been known for a long time to undergo PET processes with several donors such as aromatic hydrocarbons, olefins and amines etc. [10b, 11]. It is pertinent here to illustrate one of the synthetically important quinone-olefin [186] reactions which has been utilized for the preparation of Benz (a) anthracene 7,12-dione derivatives (237). In the present example, the excited state of naphthaquinone (234) reacts with ethene (235) to give 236, provided the electron transfer is thermodynamically allowed (ca. AGet < 0). In the follow up processes, (236) is transformed to (237) in 30-50% yield. Another report from the same group has described [187] a novel method of photoallylation of naphthaquinone by allyl stannane. 

Table 1. Effect of Substituents Bonded to Silyl Groups on the Photoallylation of p-DCB with CH2=CHCH2SiR3...

The product distribution in flie photoreactions of organosilicon compounds with electron-deficient compounds often depends on the reaction media. " In nonpolar solvents, the photocycloaddition of allylsilanes to electron-deficient aromatic compounds occurs to give cyclobutanes via exciplex intermediates. In contrast, photoallylation of the aromatic compounds or the carbonyl group takes place via free radical ions in polar solvents. Some examples are shown in Schemes 5 and 6. 

Table 3. Effect of Sensitizers on Photoallylation of p-DCB with Allyltrimethylsilane...

The photoreactions of electron-deficient polyaromatic compounds with allylic silanes afford reductive photoallylation products efficiently. Mono- and di-cyanonaphthalenes regioselectively reacted with allyltrimethylsilane to give reductive allylation products. Similar photoreactions of naphthalene dicarboxylates with allyl-... 

In 1982, Ohga and Mariano first reported the photoallylation of iminium salts. The photoreaction of iminium salts with allyl-trimethylsilane in acetonitrile affords allylated compounds via the addition of an allyl radical to an a-aminomethyl radical. Allyl radicals are produced via the cleavage of C-Si bonds of allylsilane radical cations and the a-aminomethyl radicals are generated by one-electron reduction of iminium salts as shown in Scheme 4. 

The photoreaction of l,l-dicyano-2-phenylefliene with allylsilane in the presence of phenanthrene in acetonitrile affords 5,5-dicyano-4-phenyl-l-pentene in good yield. Phenanthrene acts as a redox sensitizer in this process and allylation occurs at the position P to the cyano group. On the other hand, a similar photoallylation reaction of cyclo-... 

Arakawa, R., Lu, J., Mizuno, K., Inoue, H., Doe, H., Matsuo, T. (1997) On-line Electrospray Mass Analysis of Photoallylation Reactions of Dicyanobenzenes by Allylic Silanes via Photoinduced Electron Transfer. Int. J. Mass Spectrom. Ion Proc. 160 371-376. 

Maeda et al. reported the photoinduced electron-transfer photoallylation and photoreduction of an electrodeficient alkene (141) and allyl-trimethylsilane (142) in the presence of nucleophiles. The diastereoselec-tivity of the products (143) and (144) was dependent on the structures and... 

The same on-line photochemical ESI-MS methodology was employed in the analysis of photoallylation reactions of dicyanobenzene (DCB) by aUyhc silanes via photoinduced electron transfer [48]. Brum and Dell Orco [32] reported on investigations of the photolysis of a substituted triphenylethylene derivative using the reactor depicted in Figure 5.4. The approach allowed the monitoring of the parent species and five relevant reaction products simultaneously. The rate constants for photoconversion could be derived from the time-ion current data. Herein, Schuster, and coworkers used ESI-MS to study offline some photoadditions to Cgo [49]. 

---