ELECTRON-TRANSFER-INDUCED PHOTOADDITIONS OF THE SILYL AMINE ET2NCH2TMS TO ALPHA,BETA-UNSATURATED CYCLOHEXENONES - DUAL REACTION PATHWAYS BASED ON ION-PAIR-SELECTIVE CATION-RADICAL CHEMISTRY
ELECTRON-TRANSFER-INDUCED PHOTOADDITIONS OF THE SILYL AMINE ET2NCH2TMS TO ALPHA,BETA-UNSATURATED CYCLOHEXENONES - DUAL REACTION PATHWAYS BASED ON ION-PAIR-SELECTIVE CATION-RADICAL CHEMISTRY
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DOI：
10.1021/ja00232a023
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发表时间：
1988-11-23
影响因子：
15
通讯作者：
KIM, JU
中科院分区：
文献类型：
--
作者：
HASEGAWA, E;WEI, X;KIM, JU
对一系列共轭环己烯酮与叔硅胺Et_2NCH_2TMS的光加成反应进行了探索性和机理研究。通常，在这些体系中发生光加成以适度至良好的产率产生由烯酮-碳和胺硅取代的-碳之间的碳-碳键合产生的加合物。这些过程导致TMS和非TMS加合物的形成，对应于烯酮基团取代胺中的氢或三甲基甲硅烷基基团。用于这些光反应的溶剂的极性和质子性质似乎控制TMS与非TMS加合物的比率，前者在低极性非质子介质（MeCN）中受到青睐，后者在极性质子介质（MeOH）中受到青睐。通过使用4，4-二甲基环己-2-烯-1-酮与Et 2NCH 2 TMS的光加成反应对这种现象进行了更详细的研究，揭示了这种控制的来源。因此，发现该系统中的非TMS与TMS加合物比率直接取决于溶剂极性（ET或AN值）和质子性质。同样，当盐LiCl 〇 4和n-Bu 4 NCl 〇 4存在于光解溶液中时，该比率增加。质子酸也会产生同样的结果。另一方面，当胺浓度增加或加入n-Bu 4 NOH时，产物混合物中TMS加合物的比例增加。这些影响被解释在离子对中间体的选择性形成和化学反应方面。在低极性的非质子溶剂中，由Et_2NCH_2TMS单电子转移到烯酮的三重激发态所产生的溶剂分离离子对（SSIP）迅速坍缩成由烯酮自由基阴离子和胺自由基阳离子组成的接触离子对（CIP）. CIP中从胺阳离子自由基的硅取代的n-碳到烯酮阴离子自由基的氧的快速质子转移产生TMS加合物的自由基对前体。然而，在极性质子溶剂中，当存在质子酸或离子化盐时，SSIP解离产生游离胺阳离子自由基。该中间体经历脱甲硅烷基化或脱质子化，形成相应的非TMS和TMS加合物的自由基前体。虽然脱甲硅烷基化过程是优选的，但当介质中存在高浓度的碱（如胺或n-Bu 4 NOH）时，游离胺阳离子自由基的去质子化变得越来越具有竞争性。加合物的形成时，-氨基自由基以这种方式产生最有可能涉及共轭加成的基态烯酮。支持这一建议的观察，9，10-二氰基蒽作为一个电子转移敏化剂的硅胺加成环己-2-烯-1-酮及其4，4-二甲基类似物在MeCN中发现。电子转移光化学领域已收到在过去的decade 1由于在开发新的激发态有机过程和理解其机理细节的持续兴趣得到了激烈的研究。随着这一领域知识的增长，越来越清楚的是，许多光诱导的单电子转移（SET）过程符合合成实用程序所需的标准。2SET机制激发态转变的一个独特之处是关键的反应中间体是离子和中性自由基
The photoaddition reactions of a series of conjugated cyclohexenones with the tertiary-silyl amine Et2NCH2TMS have been subjected to exploratory and mechanistic investigation. In general, photoadditions in these systems occur to generate in modest to good yields adducts resulting from carbon-carbon bonding between the enone^-carbons and amine silicon-substituted-carbon. These processes lead to formation of TMS and non-TMS adducts corresponding to substitution by the enone grouping for hydrogen or the trimethylsilyl grouping inthe amine. The polarity and protic nature of solvents used for these photoreactions appear to control the TMS to non-TMS adduct ratios, with the former being favored in low-polarity aprotic media (MeCN) and the latter in polar protic media (MeOH). A more detailed investigation of this phenomenon by using the photoadditions of 4, 4-dimethylcyclohex-2-en-1-one with Et2NCH2TMS has uncovered the source of this control. Accordingly the non-TMS to TMS adduct ratio in this system is found to be directly dependent upon the solvent polarity (£ T or AN values) and protic nature. Likewise, thisratio increases when the salts LiC104 and n-Bu4NC104 are present in the photolysis solution. Protic acids bring about the same result. On the other hand, the proportionof TMS adduct in the product mixture is enhanced when amine concentration is increased or by the additionof n-Bu4NOH. These effects are interpreted in terms of selective formation and chemical reaction of ion-pair intermediates. In aprotic solvents of low polarity, the solvent-separated ion pair (SSIP), generated by single electron transfer fromEt2NCH2TMS to the triplet excited state of the enone, rapidly collapses to a contact ion pair (CIP) comprised of the enone radical anionand amine radical cation. Rapid proton transfer in the CIP from the silicon-substituted «-carbon of the amine cation radical to oxygen of the enone anion radical produces the radical-pair precursor of the TMS adduct. However, in polar protic solvent and when protic acids or ionized salts are present, the SSIP dissociates to produce the free amine cation radical. This intermediate undergoes desilylation or deprotonation, forming radical precursors of the respective non-TMS and TMS adducts. While the desilylation processis preferred, deprotonation of the free amine cation radical becomes increasinglycompetitive when high concentrations of base, such as the amine or n-Bu4NOH, are present in the medium. Adduct formation when-amino radicals are produced in this way most probably involves conjugate addition to the ground-state enone. Support for this proposal is found in the observation that 9, 10-dicyanoanthracene serves as an electron-transfer sensitizer for silyl amine additions to cyclohex-2-en-l-one and its 4, 4-dimethyl analogue in MeCN.The area of electron-transfer photochemistry has received in-tense study during the past decade1 owing to a continuing interest in developing new excited-state organic processes and in understanding their mechanistic details. As the body of knowledge in this area has grown, it has become increasingly clear that a number of photoinduced, single-electron-transfer (SET) processes match the criteria required for synthetic utility. 2 A unique feature of excited-state transformations occurring by SET mechanisms is that the key reactive intermediates are ion and neutral radical