ABSTRACT
Herein we describe concise enantioselective chemical syntheses of (-)-viridin and (-)-viridiol. Our convergent approach couples two achiral fragments of similar complexity and employs an enantioselective intramolecular Heck reaction to set the absolute stereochemical configuration of an all-carbon quaternary stereocenter. To complete the syntheses of these base- and nucleophile-sensitive natural products, we conduct carefully orchestrated site- and diastereoselective oxidations and other transformations. Our work is the first to generate these targets as single enantiomers.
Subject(s)
Androstenediols/chemical synthesis , Androstenes/chemical synthesis , Bacteriocins/chemical synthesis , Androstenediols/chemistry , Androstenes/chemistry , Bacteriocins/chemistry , Molecular Structure , StereoisomerismABSTRACT
Cationic palladium(II) complexes have been found to be highly reactive towards aromatic C-H activation of arylureas at room temperature. A commercially available catalyst [Pd(MeCN)4](BF4)2 or a nitrile-free cationic palladium(II) complex generated in situ from the reaction of Pd(OAc)2 and HBF4, effectively catalyzes C-H activation/cross-coupling reactions between aryl iodides, arylboronic acids and acrylates under milder conditions than those previously reported. The nature of the directing group was found to be critical for achieving room temperature conditions, with the urea moiety the most effective in promoting facile coupling reactions at an ortho C-H position. This methodology has been utilized in a streamlined and efficient synthesis of boscalid, an agent produced on the kiloton scale annually and used to control a range of plant pathogens in broadacre and horticultural crops. Mechanistic investigations led to a proposed catalytic cycle involving three steps: (1) C-H activation to generate a cationic palladacycle; (2) reaction of the cationic palladacycle with an aryl iodide, arylboronic acid or acrylate, and (3) regeneration of the active cationic palladium catalyst. The reaction between a cationic palladium(II) complex and arylurea allowed the formation and isolation of the corresponding palladacycle intermediate, characterized by X-ray analysis. Roles of various additives in the stepwise process have also been studied.
ABSTRACT
Palladium-catalysed cross-couplings, in particular Heck, Suzuki-Miyaura and Negishi reactions developed over three decades ago, are routinely carried out in organic solvents. However, alternative media are currently of considerable interest given an increasing emphasis on making organic processes 'greener'; for example, by minimising organic waste in the form of organic solvents. Water is the obvious leading candidate in this regard. Hence, this review focuses on the application of micellar catalysis, in which a 'designer' surfactant enables these award-winning coupling reactions to be run in water at room temperature.
ABSTRACT
An environmentally benign surfactant (TPGS-750-M), a diester composed of racemic α-tocopherol, MPEG-750, and succinic acid, has been designed and readily prepared as an effective nanomicelle-forming species for general use in metal-catalyzed cross-coupling reactions in water. Several "name" reactions, including Heck, Suzuki-Miyaura, Sonogashira, and Negishi-like couplings, have been studied using this technology, as have aminations, C-H activations, and olefin metathesis reactions. Physical data in the form of DLS and cryo-TEM measurements suggest that particle size and shape are key elements in achieving high levels of conversion and, hence, good isolated yields of products. This new amphiphile will soon be commercially available.
Subject(s)
Metals/chemistry , Succinates/chemistry , Temperature , Vitamin E/analogs & derivatives , Water/chemistry , Catalysis , Hydrophobic and Hydrophilic Interactions , Micelles , Polyethylene Glycols , Sulfonamides/chemistry , Surface-Active Agents/chemistry , Thiadiazoles/chemistry , Vitamin E/chemistryABSTRACT
Cationic palladium(II) catalyst realized facile C-H activation of aryl urea with arylboronic acids at room temperature. This reaction is extremely mild to carry out aromatic C-H activations through electrophilic substitution.
Subject(s)
Hydrocarbons, Aromatic/chemical synthesis , Organometallic Compounds/chemistry , Palladium/chemistry , Temperature , Catalysis , Cations/chemistry , Hydrocarbons, Aromatic/chemistry , Molecular StructureSubject(s)
Urea/chemistry , Water/chemistry , Aniline Compounds/chemistry , Carbon/chemistry , Catalysis , Palladium/chemistry , TemperatureABSTRACT
Pd-catalyzed couplings involving several heteroaromatic halides (bromides and chlorides) as well as boronic acids can be done under exceedingly mild conditions (between rt and 40 degrees C) in pure water using commercially available Pd catalysts and PTS, a nanomicelle-forming amphiphile.
Subject(s)
Heterocyclic Compounds/chemistry , Micelles , Water/chemistry , Boronic Acids/chemistry , CatalysisABSTRACT
Use of a dilute aqueous solution containing a nonionic amphiphile allows efficient Suzuki-Miyaura cross-couplings of arylboronic acids with a wide array of aryl halides and pseudohalides, including sterically hindered and lipophilic substrates, in most cases at room temperature.
