Resendiz Marino J E, Family Farnosh, Fuller Kerrian, Campos Luis M, Khan Saeed I, Lebedeva Natalia V, Forbes Malcolm D E, Garcia-Garibay Miguel A
Department of Chemistry and Biochemistry, University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, California 90095-1569, USA.
J Am Chem Soc. 2009 Jun 24;131(24):8425-33. doi: 10.1021/ja900781n.
The solution photochemistry of bis(phenylpyrrolidinonyl)ketones (R,R)-1b and (S,S)-1b exhibited a remarkably high memory of chirality. Stereospecific decarbonylation to products (R,R)-3b and (S,S)-3b, respectively, occurred with an ee of ca. 80%. The reaction is thought to occur along the single state manifold by sequential Norrish type-I alpha-cleavage, decarbonylation, and radical-radical combination in a time scale that is comparable to that required for the radical intermediate to expose its other enantiotopic face by rotation about an axis perpendicular to that of the p orbital (ca. 3-7 ps). The absolute configuration of a key intermediate and that of ketone (R,R)-1b were determined by single-crystal X-ray diffraction and the ee values of the photochemical products with the help of chiral shift reagent (+)-Eu(tfc)(3) and chiral LC-MS/MS. On the basis of the ee and de values at 25 degrees C, it could be determined that ca. 70% of the bond forming events occur with double memory of chirality, ca. 21% occur after rotation of one radical to form the meso product (R,S)-3b, and only 9% occur after double rotation to form the opposite enantiomer. This report represents the first example of a doubly enantiospecific Norrish type-I and decarbonylation reaction in solution and illustrates potentially efficient ways to obtain compounds with adjacent stereogenic quaternary centers.
双(苯基吡咯烷酮基)酮(R,R)-1b和(S,S)-1b的溶液光化学表现出极高的手性记忆。分别立体定向脱羰生成产物(R,R)-3b和(S,S)-3b,对映体过量(ee)约为80%。该反应被认为是通过单重态反应历程,依次进行Norrish I型α-裂解、脱羰以及自由基-自由基结合,其时间尺度与自由基中间体通过绕垂直于p轨道轴的旋转来暴露其另一个对映异位面所需的时间尺度相当(约3 - 7皮秒)。借助手性位移试剂(+)-Eu(tfc)₃和手性液相色谱-串联质谱,通过单晶X射线衍射确定了关键中间体和酮(R,R)-1b的绝对构型以及光化学产物的ee值。根据25℃下的ee和非对映体过量(de)值,可以确定约70%的成键事件发生时具有双重手性记忆,约21%的事件发生在一个自由基旋转形成内消旋产物(R,S)-3b之后,只有9%的事件发生在双重旋转形成相反对映体之后。本报告代表了溶液中双重对映体特异性Norrish I型脱羰反应的首个实例,并展示了获得具有相邻手性季碳中心化合物的潜在有效方法。