Photoallylation and Photoreduction of Cyclohexylidenepropanedinitrile by Use of Allyltrimethylsilane via Photoinduced Electron Transfer:  Control of the Product Ratio Depending on pK_a Values of Additives

Abstract

Irradiation of an acetonitrile solution containing cyclohexylidenepropanedinitrile (1) and allyltrimethylsilane (2) in the presence of phenanthrene afforded two kinds of allylated products (3, 4) and a reduction product (5). The product ratio of 3, 4, and 5 dramatically changed depending on the pK(a) values of additives.

Diastereoselective protonation on a radical anion in the photoallylation and photoreduction of 1,1-dicyano-2-methyl-3-phenyl-1-butene by allyltrimethylsilane

“…The product ratio of allylation and reduction (3/4) listed inTable 1are lower than the previous report using cyclohexylidenepropanedinitriles[10,16]. The difference in the product ratio between previous and current studies may be due to the increased steric hindrance to protonation, and also due to the decrease of stoichiometry of carboxylic acids added in order to increase de.…”

“…The formation of 4 was explained by hydrogen abstraction of 5, or an electron transfer from 1 -• to 5 followed by protonation, or disproportionation of 5 to give 1 and 4. We have previously reported pK a -dependency of the photoallylation and photoreduction of cyclohexylidenepropanedinitriles with 2 [10]. In the presence of a relatively strong acid such as carboxylic acids, radical anion is protonated at their b-positions, and the ratios of pathways of hydrogen abstraction from b-position of the radical anion followed by protonation at a-position and hydride transfer from the radical anion increase with decreasing acidity of the additives.…”

“…Photoinduced electron transfer (PET) reaction between organosilicon compounds and electron-deficient molecules is a powerful tool for organic synthesis [1][2][3][4][5][6][7][8][9][10][11]. In these photoreactions, radical anions generated from the electron-deficient molecules and carbon radicals generated from radical cations of the organosilicon compounds are postulated as reactive intermediates.…”

Enantioselective Protonation of Radical Anion Intermediates in Photoallylation and Photoreduction Reactions of 3,3-Diaryl-1,1-dicyano-2-methylprop-1-ene with Allyltrimethylsilane

“…Each photoreaction described above takes place through a process termed photoredox sensitization by phenanthrene (Phen) (Scheme 4) [22,23,24,25,26,27]. In the pathway, the excited singlet state of Phen, generated by light absorption, transfers one electron (SET (single electron transfer)) to the electron-deficient alkene 1 to form the phenanthrene radical cation (Phen •+ ) and the alkene radical anion 1 •− .…”

“…Because radical ions that serve as intermediates in these processes are short-lived and highly reactive, control of the stereochemistry of these reactions is often difficult [15,16,17,18,19,20,21]. We have previously developed photoallylation and photoreduction reactions of electron deficient alkenes with allyltrimethylsilane that occur via PET pathways [22,23,24]. In addition, we also demonstrated that diastereoselectivity of this process can be achieved by steric control of allyl radical or proton addition to radical anions that are generated from electron deficient alkenes (Scheme 1) [25,26,27].…”