Advanced Synthesis & Catalysis ( IF 4.4 ) Pub Date : 2024-07-19 , DOI: 10.1002/adsc.202400584 Adrian Luguera Ruiz 1 , Valentina Benazzi 1 , Federico Tucci 1 , Francesca Rizzo 1 , Daniele Merli 1 , Stefano Protti 1 , Maurizio Fagnoni 1
|
Introduction
Photochemical generation of carbon-centered radicals is currently a hot topic as it enables the forging of C−X bonds in a sustainable manner compared to traditional methodologies, typically employing stoichiometric amounts of reductants or oxidants, toxic species, and harsh conditions.1 Organosilicon compounds have found application in the (photo)generation of reactive intermediates2 also thanks to their lower toxicity compared to organostannanes.1g, 3 In this context, photogenerated silyl radicals are important in XAT reactions,4 benzoyldiisopropylchlorosilanes were purposely designed as photocleavable protecting groups for alcohols5 or silanols were used to generate alkyl radicals via β-scission of a LMCT complex by using CeIII salts.6 However, one of the main advantages of having a silicon atom in organic derivatives is the profound effect it exerts on their electrochemical behaviour when they contain π-systems and/or heteroatoms.7 Thus, in compounds having heteroatoms such as oxygen, nitrogen, and sulfur, the presence of a silyl group makes them significantly easier to be oxidized. This is apparent from the EOX values of ethers (e.g. I) and (protected) amines (e.g. II) in comparison with the corresponding α-silyl ethers (Ia) or (protected) α-silylamines (IIa, see Figure 1a).7 This has important implications in photoredox catalysis since easily oxidizable silanes were used for the release of carbon radicals. The silyl moiety functions as a redox auxiliary group and is able to promote the formation of the corresponding radical cation which subsequently fragments, releasing the radical of interest.8 Typical cases are the release of α-oxy radicals (from α-silyl ethers Ia9), α-amino radicals (from α-silylamines III10), allyl radicals (from allyl silanes IV11), benzyl radicals (from benzyl silanes V12) and acyl radicals (from acyl silanes VI,13 Figure 1b). On the contrary, the reactivity of unfunctionalized tetraalkylsilanes VII towards monoelectronic oxidation is very low, since the oxidation potential of such compounds is >2.5 V7c, 14 thus forcing harsh conditions.
a) Comparison of the oxidation potential of organic compounds with those incorporating a trialkylsilyl groups. b) Silicon based derivatives used in photoredox catalysis and in this work.
Nevertheless, it was described in the early 90s detailing the photochemical alkylation of pyrylium salts15 and of aromatic nitriles (e. g. 1,2,4,5 tetracyanobenzene, TCB)16 via photoinduced electron transfer with tetralkylsilanes.
In this case, the high reduction potential in the excited state of these aromatics allowed for the oxidation of R4Si and the radical released from the resulting radical cation is then able to couple with the aromatic radical anion. Despite this was an interesting case of aromatic carbon-carbon ipso-substitution reaction,17 the only fate of the radical is the functionalization of the absorbing species. A recent and elegant strategy for the generation of alkyl radicals makes use of hypervalent bis-catecholato silicates VIII (EOX <1 V vs SCE1c, 18 Figure 1b) including Martin silicates IX (EOX ca. 1.5 V vs SCE19). However, the preparation of these charged silicon derivatives is not so trivial.
On the other hand, despite alcohols are very difficult to oxidize (e. g. EOX EtOH >3.5 V vs SCE20) their conversion into silyl ethers was found to be beneficial (the EOX EtOSiMe3 was reported to be lower with respect to the corresponding alcohol; >2.5 V vs SCE21). The oxidative capability of the silyl ether depends on the alcohol and not on the silyl groups (as an example the EOX Et3Si−H and Bu3Si−H are 2.15 V vs SCE22 and ca. 2.5 V vs SCE,23 respectively).
In the frame of finding new neutral radical precursors,24 we deemed then worthwhile to investigate a more accessible class of silyl derivatives namely silyl ethers as alkyl radical precursors.
Whereas the (direct) photochemistry of such compounds has received only few attentions,25-27 sparse examples were reported on the photocatalyzed generation of alkyl radicals from silyl ethers by using cyanoarenes as photooxidants. Thus, irradiation of TCB (320 nm), in the presence of tert-butyldimethyl(octyloxy)silane led to the release of a tert-butyl radical that coupled with the generated radical anion of the cyanoarene to afford tert-butylated tricyano benzene as the exclusive product.28 However, when an electron-poor olefin was present in the mixture, Giese-type reaction competes with the functionalization of the cyanoarene, resulting in a hydroalkylation reaction.29
In view of these premises, we thus reconsidered the potential of silyl ethers as precursors of C and Si radicals under photoredox catalyzed conditions, by taking advantage on the oxidizability of the Si−O bond in trialkylalkoxysilanes.
中文翻译：
使用甲硅烷基醚作为自由基前体的有机光催化 C−Si 键断裂
介绍
以碳为中心的自由基的光化学生成目前是一个热门话题，因为与传统方法相比，它能够以可持续的方式形成CX键，通常采用化学计量的还原剂或氧化剂、有毒物质和恶劣的条件。 1有机硅化合物已在（光）生成反应性中间体2中得到应用，这也归功于其与有机锡烷相比毒性较低。 1g, 3在这种情况下，光生甲硅烷基自由基在 XAT 反应中很重要， 4苯甲酰基二异丙基氯硅烷被特意设计为醇的光可裂解保护基团5或硅烷醇用于通过使用 Ce III盐对 LMCT 络合物进行 β 断裂来生成烷基自由基。 6然而，有机衍生物中含有硅原子的主要优点之一是，当它们含有 π 系统和/或杂原子时，硅原子会对它们的电化学行为产生深远的影响。 7因此，在含有氧、氮和硫等杂原子的化合物中，甲硅烷基的存在使它们更容易被氧化。与相应的 α-甲硅烷基醚 ( Ia ) 或（保护的）α-甲硅烷基胺（ IIa ，参见图 1a）相比，从醚（例如I ）和（受保护的）胺（例如II ）的 E OX值可以明显看出这一点。7这对光氧化还原催化具有重要意义，因为容易氧化的硅烷用于释放碳自由基。甲硅烷基部分起到氧化还原辅助基团的作用，能够促进相应自由基阳离子的形成，该自由基阳离子随后断裂，释放出感兴趣的自由基。 8典型情况是释放 α-氧基（从 α-甲硅烷基醚Ia 9 ）、α-氨基（从 α-甲硅烷基胺III 10 ）、烯丙基（从烯丙基硅烷IV 11 ）、苄基（从苄基硅烷） V 12 ) 和酰基自由基（来自酰基硅烷VI , 13图 1b）。相反，未官能化的四烷基硅烷VII对单电子氧化的反应性非常低，因为此类化合物的氧化电位为>2.5 V 7c, 14 ，从而迫使条件苛刻。
在图查看器PowerPoint中打开
a) 有机化合物与含有三烷基甲硅烷基的有机化合物的氧化电位的比较。 b) 用于光氧化还原催化和本工作的硅基衍生物。
然而，在20世纪90年代初，详细描述了吡喃鎓盐15和芳族腈（例如1,2,4,5四氰基苯，TCB） 16通过光诱导电子转移与四烷基硅烷的光化学烷基化。
在这种情况下，这些芳族化合物的激发态下的高还原电势允许R 4 Si的氧化，并且从所得自由基阳离子释放的自由基然后能够与芳族自由基阴离子偶联。尽管这是芳香族碳-碳本身取代反应的一个有趣的例子， 17自由基的唯一命运是吸收物质的功能化。最近一种优雅的烷基自由基生成策略利用高价双儿茶酚硅酸盐VIII （E OX <1 V 与 SCE 1c，18图 1b），包括马丁硅酸盐IX （E OX约 1.5 V 与 SCE 19 ）。然而，这些带电硅衍生物的制备并不是那么简单。
另一方面，尽管醇非常难以氧化（例如 E OX EtOH >3.5 V 与 SCE 20 ），但它们转化为硅醚被发现是有益的（据报道，E OX EtOSiMe 3相对于 SCE 20 而言较低）相应的酒精；>2.5 V vs SCE 21 ）。甲硅烷基醚的氧化能力取决于醇而不是甲硅烷基（例如，E OX Et 3 Si−H 和 Bu 3 Si−H 分别为 2.15 V vs SCE 22和 ca. 2.5 V vs SCE, 23分别）。
在寻找新的中性自由基前体的框架中， 24我们认为值得研究一类更容易获得的甲硅烷基衍生物，即作为烷基自由基前体的甲硅烷基醚。
虽然此类化合物的（直接）光化学只受到很少的关注，但关于使用氰基芳烃作为光氧化剂从甲硅烷基醚光催化生成烷基自由基的例子报道了25-27 个。因此，在叔丁基二甲基（辛氧基）硅烷存在下，TCB（320 nm）的照射导致叔丁基自由基的释放，该自由基与产生的氰基芳烃自由基阴离子偶联，得到叔丁基三氰基苯作为独家产品。 28然而，当混合物中存在缺电子烯烃时，吉斯型反应会与氰基芳烃的官能化竞争，导致加氢烷基化反应。 29
鉴于这些前提，我们因此重新考虑了硅醚在光氧化还原催化条件下作为 C 和 Si 自由基前体的潜力，利用三烷基烷氧基硅烷中 Si−O 键的氧化性。
京公网安备 11010802027423号