Chiral selenium-containing compounds have found growing application in asymmetric catalysis over the past few years. The large majority of these ligands are derived from readily available chiral amino alcohols in a few high-yielding synthetic steps. The advantages of using these compounds are described, particularly those regarding ready accessibility, modular nature, formation of strong bonds with soft metals and more rarely with hard ones. By using an appropriate electrophilic or nucleophilic organoselenium fragment, novel chiral organoselenium catalysts or ligands were designed from chiral aziridines, oxazolines, ferrocenes, etc. High regioselectivity and enantioselectivity were achieved through the four following techniques: the enantioselective copper-catalyzed conjugate addition of organometallic reagents to enones, the diorganozinc addition to aldehydes, palladium-catalyzed enantioselective allylic alkylation and asymmetric hydrosilylation or transfer hydrogenations of ketones.
A set of phenethylamines has been successfully prepared via Suzuki-Miyaura cross-coupling of diverse potassium β-aminoethyltrifluoroborates with aryl halides. The potassium β-aminoethyltrifluoroborates were easily prepared via hydroboration of enamine and enamide precursors.Phenethylamines and their structural analogues comprise important substructures of a variety of biologically important compounds including dopamine, tyrosine, amphetamine, and adrenaline. These privileged scaffolds are also widely found as components of alkaloid natural products 1 and often serve as key building blocks in the synthesis of numerous nitrogencontaining complex molecules. Previous methods to introduce an aminoethyl group into an arene have employed the Friedel-Crafts acylation of activated arenes with N-protected amino acid chlorides, 2 Heck arylation of N-vinyloxazolone followed by hydrogenation, 3 and crosscoupling reactions involving β-amino organozinc reagents, which are somewhat unstable to β-elimination. 4 Earlier investigations to access these important units also include the direct coupling of β-aminoethyl organolithiums with aryl-and alkenyl halides. 5 A more broadly applicable Suzuki cross-coupling 6 approach to this interesting class of compounds has been developed by Overman. 7 However, although there are distinct advantages of this one-pot β-aminoethylation procedure in cross-coupling reactions and total synthesis (e.g., the reactions can be performed at room temperature), 8 there are some limitations as well. In particular, the organoborane reagents prepared in situ via hydroboration of benzyl vinyl carbamate cannot be easily isolated and stored, but must be prepared and utilized on a reaction-by-reaction basis.By contrast, potassium organotrifluoroborates have been shown to overcome this particular limitation. These salts are unique organoboron compounds, notable for their stability to moisture and air. 9 They are powders or crystalline solids that are easy to access and handle, and these properties have made them attractive synthetic intermediates. Herein we describe our initial efforts to develop a convenient and practical access to phenethylamines via Suzuki cross-coupling of potassium β-aminoethyltrifluoroborates with aryl electrophiles.To initiate studies on the aminoethylation reactions, a hydroboration protocol was employed that mimicked procedures reported by Overman. 8 Thus, the respective N-vinyl substrates 10 were hydroborated using Snieckus' di(isopropylprenyl)borane (i-PP 2 BH), 11 and the resulting organoborane intermediates were treated with an aqueous solution of KHF 2 to afford the desired β-aminoethyltrifluoroborates 1a-e as depicted in Table 1.
[structure: see text] A set of chiral beta-seleno amides have been efficiently synthesized via the ring-opening reaction of chiral 2-oxazolines by selenium nucleophiles. The present method is applicable to the synthesis of beta-seleno amides containing thioether, alcohol, and ether moieties in good yields. As an application, the synthesis of a selenocysteine derivative has been accomplished. Additionally, these new compounds were evaluated in the palladium-catalyzed asymmetric allylic alkylation, giving the alkylated products in up to 98% ee.
Selenium-based methods have developed rapidly over the past few years and certain features of chiral selenium-containing compounds make these reagents particularly valuable for efficient stereoselective reactions.Recent advances in stereoselective transformations involving one-pot selenenylation-deselenylation sequences, which occur using only catalytic amounts of the optically active diselenides in the synthesis of valuable building blocks will be summarized. Additionally, recent results of catalytic reactions using chiral selenides and diselenides such as the enantioselective copper catalyzed conjugate addition of organometallic reagents to enones, diorganozinc addition to aldehydes, palladium-catalyzed enantioselective allylic alkylation, among other topics will also be addressed.
A set of chiral β-seleno amides were efficiently synthesized by a simple and efficient procedure involving a ring-opening reaction of 2-oxazoline with diorganyl dichalcogenides mediated by indium(I) iodide. As an application, the synthesis of selenocysteine derivatives was accomplished.
The catalytic enantioselective arylation of several aldehydes using boronic acids as the source of transferable aryl groups is described; the reaction is found to proceed in excellent yields and high enantioselectivities (up to 97% ee) in the presence of a chiral amino alcohol.
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