TiO2-photocatalyzed oxidation of toluene (1a), benzyltrimethylsilane (1b), and 4-methoxybenzyltrimethylsilane (1c) has been carried out in acetonitrile under oxygen, under nitrogen, and in the presence of electrophilic alkenes under various conditions (using Ag2SO4 as electron acceptor, adding 2.5% H2O, changing solvent to CH2Cl2). Benzyl radicals, formed via electron transfer and fragmentation, are trapped. A good material balance is often obtained. The overall efficiency of the process depends on the donor Eox, on the rate of fragmentation of the radical cation, and on the acceptor present (Ag+ is an efficient oxidant, an electrophilic alkene a poor one, O2 is intermediate). With ring-unsubstituted benzyl derivatives 1a and 1b, oxidative fragmentation occurs mainly close to the catalyst surface. The benzyl radicals form at a high local concentration and give benzaldehyde under O2, bibenzyl under N2 and dibenzylated derivatives by attack on the alkenes (acrylonitrile, fumaronitrile, maleic acid). In this case, using CH2Cl2O2 enhances the yield of benzaldehyde. With methoxylated 1c, however, the radical cation migrates into the solution before fragmentation and, therefore, the free benzyl radical is formed. This radical in part is oxidized to the cation, giving a considerable amount of benzylacetamide (or of the alcohol with water), and in part is trapped by the alkenes. The last reaction is less efficient in this case and yields monobenzyl derivatives.Key words: photocatalysis, oxidation, alkylation.
TiO2光催化氧化
甲苯(1a)、苄基三甲基
硅烷(1b)和4-甲氧基苄基三甲基
硅烷(1c)已在
乙腈中在
氧气下、在氮气下和在存在亲电烯烃的情况下进行,使用
Ag2SO4作为电子受体,添加2.5%
H2O,将溶剂改变为
CH2Cl2等各种条件。通过电子转移和裂解形成的苄基自由基被捕获。通常可以得到良好的物质平衡。该过程的整体效率取决于给体Eox,自由基阳离子裂解速率以及存在的受体(Ag+是高效氧化剂,亲电烯烃是较差的,O2处于中间位置)。对于环未取代的苄衍
生物1a和1b,氧化裂解主要发生在催化剂表面附近。苄基自由基在高局部浓度下形成,并在
氧气下生成
苯甲醛,在氮气下生成
联苯,在攻击烯烃(
丙烯腈、
己二腈、
马来酸)时生成二苄衍
生物。在这种情况下,使用 -O2可以增加
苯甲醛的产率。然而,对于甲氧基化的1c,自由基阳离子在裂解之前会迁移到溶液中,因此形成自由苄基自由基。这种自由基部分被氧化成阳离子,产生相当数量的苄基乙酰胺(或与
水反应生成醇),部分被烯烃捕获。在这种情况下,最后一种反应效率较低,产生单苄基衍
生物。关键词:光催化,氧化,烷基化。