By combining an organocatalyzed Michael addition of 1,3‐diketones to hydroxy‐nitroolefins with a subsequent, innovative iron(0)‐catalyzed fragmentation, key enantioenriched ketones possessing additional nitro and protected alcohol functions could efficiently be prepared under excellent enantiocontrol (typically 95% ee). The reaction was made possible by the discovery that photochemical activation of iron pentacarbonyl [Fe(CO)5], a cheap widely available complex, efficiently catalyzed a Claisen‐type fragmentation under mild conditions, avoiding decomposition of the intermediates and products.
2,2‐Difluoro‐1,3‐diketones are introduced as gem‐difluoroenolate precursors for the first example of an organocatalytic asymmetric aldol addition with N‐benzylisatins to form 3‐difluoroalkyl‐3‐hydroxyoxindoles.
We have achieved a rare‐metal‐free photo‐aerobic intramolecular dehydrogenative coupling reaction from two C—H bonds of indole with malonate. This catalytic system proceeded at room temperature under visible light irradiation with oxygen in the air as a terminal oxidant, and the cyclization products were obtained in good to excellent yields.
2,3‐Dihydroindenes and isoindolines are important skeletons present in medicinal and synthetic chemistry. In this paper, an acetyl bromide promoted metal‐free construction of 2,3‐dihydroindenes and isoindolines with high synthetic efficiency is developed.
We report here that a cobalt catalyst generated from a cobalt(II) salt and a ligand using magnesium metal is capable of promoting hydroarylation reactions through chelation‐assisted C−H activation. With the appropriate choice of the ligand and other reaction conditions, ketimine‐ or aldimine‐directed branched selective hydroarylations of styrenes and ketimine‐directed hydroarylations of vinylsilane have been achieved. The reported catalytic systems may serve as more convenient alternatives to previously developed catalytic systems employing Grignard reagents as reducing agents.
An iron(III)‐catalyzed C‐3 functionalization of flavones has been achieved using tert‐butyl peroxybenzoate (TBPB)/potassium persulphate (K2S2O8) oxidant combinations with a suitable solvent. In the presence of iron(III)/tert‐butyl peroxybenzoate/potassium persulphate, the reaction of flavones in cycloalkanes afforded exclusive C‐3 cycloalkylation via C –C coupling, whereas the solvent N,N‐dialkylformamide provided C‐3 amidation via C –C coupling. Under identical reaction conditions just by switching the solvent to chlorobenzene, C‐3 methylated flavones were obtained where tert‐butyl peroxybenzoate (TBPB) served as the source of the methyl group.
N‐Propargyl‐ and N‐homoallenyl‐2‐bromo‐β‐tryptamines undergo gold(I)‐catalyzed dearomatizing cyclizations to afford 2‐bromospiroindolenines that are in situ hydrolyzed to furnish spirooxindoles in a one‐pot process. Tryptophane derivatives (R2=CO2Et) led upon cyclization to chiral spirooxindoles in excellent diastereoselectivities.
The sodium iodide/hydrogen peroxide‐mediated oxidation/lactonization of indolepropionic acids was achieved, affording the corresponding spirocyclic oxindole‐lactones in moderate to high yields. This metal‐free procedure features mild reaction conditions, non‐toxicity and easy handling, with hydrogen peroxide (H2O2) as a clean oxidant.
The copper‐catalyzed reaction of 5‐substituted penta‐1,4‐diyn‐3‐yl acetates with P(O)H compounds to efficiently give a new class of phosphonyl diynes is reported. The reaction may take place through a regioselective nucleophilic attack of phosphorus nucleophiles on Cu‐allenylidene intermediates to form allenyl intermediates followed by a rapid allene‐alkyne isomerization process. The synthetic utility of the obtained products is demonstrated.
1‐Alkynylcyclopropyl tosylates react with alkylmagnesium halides in the presence of catalytic ferric acetylacetonate [Fe(acac)3] under net propargylic substitution; allene formation, which is the prevalent reactivity mode of propargylic substrates otherwise, was noticed as a side reaction only when branched alkyl‐ or aryl‐Grignard reagents were used. These transformations represent the first successful iron‐catalyzed cross‐coupling reactions of tert‐alkyl electrophiles.
An efficient, metal‐free, silicon–hydrogen bond functionalization based on the microwave‐assisted reaction of readily available enynones and silanes is reported. This process seemingly proceeds through a 2‐furyl carbene species, a particularly elusive intermediate. Preliminary studies on the metal‐free oxygen–hydrogen and nitrogen–hydrogen bond functionalization of representative alcohols, azoles and sulfonamides are also provided.
A regio‐ and enantioselective copper‐catalyzed 1,4‐conjugate addition of trimethylaluminium to linear δ‐aryl‐substituted α,β,γ,δ‐unsaturated alkyl ketones was developed. A series of γ,δ‐unsaturated alkyl ketones were obtained in good yields with high regio‐ and enantioselectivity (up to 88% ee and 96:4 dr). Expansion of the reaction scope to substrates containing aromatic heterocycles also afforded good yields and enantioselectivities (up to 91% ee) with very high regioselectivities, exclusively providing the single 1,4‐products.
A highly efficient chiral N,N′‐dioxide‐zinc(II) complex catalytic system has been developed for the enantioselective aza‐Friedel–Crafts reaction of isatin‐derived ketimines with indoles. A series of enantiomerically enriched 3‐indolyl‐3‐aminooxindoles containing a tetrasubstituted stereocenter were obtained in up to 99% yield with up to 96% ee. Furthermore, control experiments provide a fundamental sight into the mechanism of the reaction.
A natural quinine‐catalyzed, efficient and practical asymmetric α‐hydroxylation of 4‐substituted pyrazolones has been developed, delivering a broad spectrum of pyrazolones bearing an oxygen‐attached carbon stereocenter at C‐4 in high yields and excellent enantioselectivities. The broad substrate scope, ready availability of the catalyst, ease of operation, and valuable transformation of the product highlight the practical utility of this process.
A facile method to synthesize 3‐halobenzo‐heterocyclic‐2‐carbonyl compounds is described. Mechanistic studies suggested that a halo‐cyclization process, which generated the unstable spiro‐acetal transition state and readily convertible to the corresponding carbonyl compound might be involved. Diverse 3‐halobenzo‐heterocyclic‐2‐carbonyl compounds could be synthesized with up to 95 % yield in mild conditions with inexpensive starting materials.
The bis(N‐heterocyclic carbene)(diphenylacetylene)palladium complex [Pd(ITMe)2(PhC≡CPh)] (ITMe=1,3,4,5‐tetramethylimidazol‐2‐ylidene) acts as a highly active pre‐catalyst in the diboration and silaboration of azobenzenes to synthesize a series of novel functionalized hydrazines. The reactions proceed using commercially available diboranes and silaboranes under mild reaction conditions.
We report a radical cation [4+2] annulation of arylalkenes to afford naphthalene derivatives using an organic photosensitizer (9‐mesityl‐10‐methylacridinium perchlorate) under visible light photocatalysis. In the presence of oxygen (in the air), the oxidative dimerization/intramolecular [4+2] cycloaddition of two alkene molecules provides 3,4‐dihydronaphthalen‐1(2H)‐ones in good to high yields. Under a nitrogen atmosphere, (dihydro)naphthalenes were attained in moderate to excellent yields by using Selectfluor as the oxidant. The transformation proceeds via a tandem dimeric electrophilic addition/Friedel–Crafts cyclization/radical coupling/elimination sequence. This approach represents a mild and straightforward assembly of the naphthalene skeleton using a visible light photocatalytic cascade strategy.
A simple, efficient and practical iron‐catalyzed azidoarylthiation of alkenes has been developed at room temperature, and the corresponding products containing ortho‐sited sulfide and azide units were obtained in moderate to good yields with good tolerance of functional groups. The protocol uses readily available 1‐(alkylthio)pyrrolidine‐2,5‐diones and trimethylsilyl azide as the alkylthiation and azidation reagents, respectively, inexpensive and environmentally friendly iron chloride as the catalyst without addition of any ligand and additive.
