- Reversed-polarity synthesis of diaryl ketones via palladium-catalyzed cross-coupling of acylsilanes
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Acylsilanes serve as acyl anion equivalents in a palladium-catalyzed cross-coupling reaction with aryl bromides to give unsymmetrical diaryl ketones. Water plays a unique and crucial activating role in these reactions. High-throughput experimentation techniques provided successful reaction conditions initially involving phosphites as ligands. Ultimately, 1,3,5,7-tetramethyl-6-phenyl-2,4,8-trioxa-6-phosphaadamantane was identified as giving a longer-lived catalyst with higher turnover numbers. Its use, in conjunction with a palladacycle precatalyst, led to optimal reaction rates and yields. Scope and limitations of this novel method are presented along with initial mechanistic insight.
- Schmink, Jason R.,Krska, Shane W.
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p. 19574 - 19577
(2012/01/13)
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- Photochemical behavior of cyclopropyl-substituted benzophenones and valerophenones
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p-Cyclopropylbenzophenone, 20, gives no photoreduction when irradiated in i-PrOH solvent. This is a general phenomenon and a number of cyclopropyl-substituted benzophenones, including 4-(endo-6-bicyclo[3.1.0]hexyl) benzophenone, 19, 4-(cis-2,3-dimethylcyclopropyl)benzophenone, 21, 4-(cis-2-vinylcyclopropyl)benzophenone, 22, and 4-(endo-7-bicyclo[4.1.0]hept-2- enyl)benzophenone, 23, also fail to undergo photoreduction. Instead these latter compounds undergo cis-trans isomerization when irradiated. A mechanism involving formation of an (n, π*) triplet, which subsequently fragments the strained cyclopropane bond to give a lower energy and unreactive open triplet, has been suggested. p-Cyclopropylvalerophenone, 25, and p-(endo-6-bicyclo[3.1.0]hexyl)valerophenone, 24, also undergo photoisomerization and fail to undergo the Norrish Type II photoreactions. Triplet energy dissipation by fragmentation of the cyclopropane bond is also proposed. In addition to the Norrish Type II reaction, p-cyclobutylvalerophenone, 27, undergoes a photofragmentation to give ethylene and p-vinylvalerophenone, 60, by an energy dissipation mechanism involving a 1,4-biradical derived from cyclobutane bond fragmentation.
- Creary, Xavier,Hinckley, Jenifer,Kraft, Casey,Genereux, Madeleine
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experimental part
p. 2062 - 2071
(2011/05/28)
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