[reaction: see text] A simple and efficient method for the synthesis of unsymmetrical diaryl sulfones using the palladium-catalyzed coupling of aryl boronic acids and arylsulfonyl chlorides has been developed. High product yields, a short reaction time, and mild reaction conditions are important features of this method.
An unprecedented vanadyl(V) methoxide complex 4 derived from 3,5-dibromo-N-salicylidene-l-tert-leucinate enables highly enantioselective aerobic oxidations of alpha-hydroxyphosphonates at ambient temperature with selectivity factors ranging from 3 to >99.
A series of chiral vanadyl(V) methoxides bearing 3-t-butyl-5-substituted N-salicylene-L-valinate and L-t-leucinate as chiral auxiliaries has been prepared. In all cases except the 3,5-di-t-butyl analogue, they exist as monomers both in solution and in the single crystal state. In the case of the 3,5-di-t-butyl analogue, the architectural nature of the vanadyl(V) complex highly depends on the base used during the complex formation event. A pentanuclear C4-symmetric complex was formed when potassium salts were employed instead of the corresponding sodium salts. A central vanadate(V) unit serves to grip four identical chiral monomeric vanadyl(V) units together, by which a potassium ion sits on top of the four flanking units through carbonyl coordinations and serves to hold the whole cluster by cooperation with the central vanadate(V) unit. In comparison with the corresponding monomeric vanadyl(V) methoxide complex, the cluster complex was utilized to facilitate the asymmetric aerobic oxidations of various racemic alpha-hydroxyesters, -amides, and -thioesters with excellent selectivity factors (krel 40 to >500).
Reaction of sulfur ylide with chiral non-racemic imine afforded the desired aziridine in excellent yield. Diastereomeric ratios of >95:5 were obtained. Both enantiomeric lines of the butanediacetal-protected chiral non-racemic sulfinyl imines were examined. The sulfur ylides were generated in situ upon thermal decarboxylation of carboxylmethyl betaine functionality. The enantiomeric pairs of D-mannitol with (S)-(-)-2-methyl-2-propane sulfinamide and ascorbic acid with (R)-(-)-2-methyl-2-propane sulfinamide resulted in the highest levels of diastereocontrol when performing aziridination reactions.Previously communicated from our laboratory was a novel protocol for the preparation of methylphenylsulfonium methylide ii (Scheme 1). 1 When prepared in situ, this sulfur ylide has been shown to effectively trap aldehydes and ketones to form terminal epoxides and imines to form terminal aziridines. 2 The ease by which sulfur ylide ii is generated is one key advantage of our method over other existing technologies. 3 Generation of the sulfur ylide occurs upon decarboxylation of carboxylmethyl betaine functionality. The process does not require the use of strong and often pyrophoric bases such as butyl lithium or sodium hydride, does not require expensive metal catalysts, and is compatible with ecologically benign solvent systems. However, the 'ease by which the sulfur ylide is generated' as stated above does come at a price. That is, while proficient when considering both scope and levels of conversion, the shortcomings of a protocol which relies on refluxing THF for complete conversion of π-acceptor to heterocycle are notable when considering asymmetry.Our entry into stereoselective methylene transfers began with the use of chiral non-racemic sulfinyl imines (Scheme 2). 2bReaction of thioacetate derivative 1 with a series of aryl substituted chiral non-racemic sulfinyl imines afforded the corresponding aziridines in high yield and good stereoselection. A total of seven systems were examined which consisted of both electron deficient and electron releasing aryl substituted chiral non-racemic imines. Upon in situ generation of the sulfur ylide via the thermally induced decarboxylation of carboxymethylsulfonium betaine functionality, a drop in the diastereomeric ratio was observed when going from electron deficient to electron releasing aryl substituted imines.Correspondence to: David C. Forbes, dforbes@jaguar1.usouthal.edu.
NIH Public Access
Author ManuscriptChem Commun (Camb) When working at reaction temperatures below refluxing THF, a moderate increase in stereoselection was observed. This was, however, at the expense of percent conversion and still below the level of diastereocontrol reported when working with the less stabilized dimethylsulfonium methylide under identical reaction conditions (dr = 93:7 with dimethylsulfonium methylide (84% isolated yield at room temperature) and dr = 90:10 with methylphenylsulfonium methylide ii (50% conversion at room temperature)).In our search for alternative protocols f...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.