Synergistic Cu/Pd-Catalyzed Asymmetric Allenylic Alkylation of Azomethine Ylides for the Construction of α-Allene-Substituted Nonproteinogenic α-Amino Acids

An unprecedented asymmetric allenylic alkylation of readily available imine esters, which was enabled by a synergistic Cu/Pd catalysis, has been developed. This dual catalytic system possesses good substrate compatibility, delivering a diverse array of nonproteinogenic α-allenylic α-mono- or α,α-disubstituted α-amino acids (α-AAs) with high yields and generally excellent enantioselectivities. Furthermore, the scalability and practicability of the current synthetic protocol were proven by performing gram-scale reactions and by the first catalytic asymmetric synthesis of naturally occurring (S)-γ-allenic α-amino acid, respectively.     

Zinc metal-promoted nucleophilic addition of nonactivated alkyl iodides to aromatic aldimines in the presence of chlorotrimethylsilane in ethyl acetate-DMI

A variety of aromatic aldimines react with nonactivated alkyl iodides in the presence of zinc powder and a catalytic amount of chlorotrimethylsilane (TMSCl) in ethyl acetate-DMI to afford the corresponding α-alkylbenzylamine derivatives in good to excellent yields.     

Rhodium-catalyzed addition of arylstannanes to carbon-heteroatom double bond

The addition of arylstannanes to the carbon-heteroatom double bond in the presence of a catalytic amount of a cationic rhodium complex ([Rh(cod)(MeCN)₂]BF₄) was examined. The reactions of aldehydes, α-dicarbonyl compounds, and N-substituted aldimines with the arylstannanes gave corresponding alcohols, α-hydroxy carbonyl compounds, and amines, respectively. An arylrhodium complex generated by the transmetalation with the arylstannane was probably the active catalytic species.     

N‐Methylacridinium Salts: Carbon Lewis Acids in Frustrated Lewis Pairs for σ‐Bond Activation and Catalytic Reductions

N‐methylacridinium salts are Lewis acids with high hydride ion affinity but low oxophilicity. The cation forms a Lewis adduct with 4‐(N,N‐dimethylamino)pyridine but a frustrated Lewis pair (FLP) with the weaker base 2,6‐lutidine which activates H₂, even in the presence of H₂O. Anion effects dominate reactivity, with both solubility and rate of H₂ cleavage showing marked anion dependency. With the optimal anion, a N‐methylacridinium salt catalyzes the reductive transfer hydrogenation and hydrosilylation of aldimines through amine–boranes and silanes, respectively. Furthermore, the same salt is active for the catalytic dehydrosilylation of alcohols (primary, secondary, tertiary, and ArOH) by silanes with no observable over‐reduction to the alkanes.     

Synthesis of β-lactams and β-aminoesters via high intensity ultrasound-promoted Reformatsky reactions

Reformatsky reactions of an imine, an α-bromoester, zinc dust and a catalytic amount of iodine in dioxane under high intensity ultrasound (HIU) irradiation from an ultrasonic probe are explored. A series of 16 aldimines with varying electronic demands is evaluated as potential electrophiles for reactions with three α-bromoesters of differing steric demands. This HIU method is successful for both enolizable and non-enolizable imines affording in short reaction times high yields of a β-lactam, the corresponding β-aminoester or a mixture of the two products depending on the identity of the imine and α-bromoester.     

Phosphinite- and phosphite-based Type I palladacycles as highly active catalysts for addition reactions of arylboronic acids with aldehydes, α,β-unsaturated ketones, α-ketoesters, and aldimines

Phosphinite- and phosphite-based type I palladacycle-catalyzed additions of arylboronic acids with aldehydes, α,β-unsaturated ketones, α-ketoesters, and aldimines are described. Our study showed that readily available phosphinite- and phosphite-based type I palladacycles could possess higher catalytic activity than phosphine-based palladacycles and were highly active, practical catalysts. Our study may pave the road for development of optically active phosphinite- and phosphite-based palladacycles for asymmetric catalysis.     

Etude de l'hydrogenation de l'α-terpinene et du dihyro-2,3 anisole catalysee par le phenanthrene chrome tricarbonyle

The hydrogenation of α-terpinene and 2,3-dihydroanisole catalysed by phenanthrenechromium tricarbonyl in cyclohexane is described. The catalytic reaction is accelerated when small amounts of ketone are added to the solution. Attempts to induce asymmetric synthesis by adding (+)-3-menthylcyclohexanone or l-carvone to the reaction mixture are discussed. Isomerization of cis-Δ²-methene and α-phellandrene is also presented.     

醛亚胺的不对称Strecker反应研究进展

α-氨基腈不仅可以很容易地转化为α-氨基酸,而且是合成许多具有生物活性的天然产物和药物的重要中间体.醛亚胺的不对称Strecker反应作为制备光学活性α-氨基腈的直接而有效的方法之一,已被广泛接受.作者介绍了醛亚胺的不对称Strecker反应研究进展.     

Reactivity of α-chloro-aldimines

A series of secondary N-1-(2-chloroalkylidene)amines has been prepared by condensation of disubstituted acetaldehydes with primary amines followed by chlorination with N-chlorosuccinimide in carbontetrachloride. A study of the reactivity of these N-homologues of α-chloroaldehydes is described. Treatment of the title compounds with sodium methoxide in methanol gave high yields of α,β-unsaturated aldimines. However, N-1-(2-chloro-2-methylpropylidene)amines afforded a mixture of elimination and rearrangement products, which proceeded via an aziridine intermediate. On the other hand, α-phenyl-substituted α-chloro aldimines on treatment with methoxide in methanol underwent α-substitution, consistent with an S_N1 mechanism. Powerful nucleophiles such as sodium thiophenolate in methanol and sodium azide in acetone caused α-substitution. Reaction of α-chloro aldimines with Grignard reagents produced coupling of two aldimine units or α-alkylation. Finally the reactivity of α-chloro aldimines was compared with the reactivity of the corresponding oxygen-analogues, i.e. α-chloro aldehydes.     

Synthesis and properties of 3-chloro- and 3,7-dichloro-3,4-dihydro-1-hydroxycarbostyrils and related heterocyclic compounds

3-Chloro- and 3,7-dichloro-3,4-dihydro-1-hydroxycarbostyrils were synthesized by the catalytic hydrogenation of the α-chloro- and α,4-dichloro-β-(o-nitrophenyl)propionic acids in strong acidic solution over platinum-on-carbon sulfided catalyst. However, the catalytic hydrogenation of α-bromo-β-(o-nitrophenyl)propionic acid yielded 3,4-dihydro-1-hydroxycarbostyril under the same experimental conditions. The 3-chloro-3,4-dihydro-1-hydroxycarbostyril and the α-chloro-β-(o-nitrophenyl)propionic acid underwent facile dehydrochlorination in mild alkaline solution to give 1-hydroxycarbostyril and o-nitrocinnamic acid, respectively. Selective reduction of 3-chloro-3,4-dihydro-1-hydroxycarbostyril and 1-hydroxycarbostyril to the corresponding lactams, 3-chloro-3,4-dihydrocarbostyril and carbostyril, was effected by catalytic hydrogenation in hydrochloric acid over platinum black catalyst. The structures of the substituted carbostyril derivatives were correlated with their proton nmr spectra.     

Heterocyclen aus α-nitroolefinen. XII . 3-Acetyl-4,5-dihydro-5-(methylenamino)furane und 3-acylpyrrole aus α-nitroolefinen und β-diketonen

From α-nitroolefines 1 and β-diketones 2 3-acetyl4,5-dihydro-5-(methy1eneamino)furans 5 are prepared. Furans 5 react in acid medium to yield 1-unsubstituted 3-acylpyrroles 6 , catalytic hydrogenation yields 1-substituted 3-acylpyrroles 9 . The ¹H-nmr and ¹³C-nmr investigations prove the constitutions.     

Reaction du gem-chloro(methyl)allyllithium sur les aldimines et les cetimines: nouvelle methode de synthese d'aziridines α-ethyleniques N-substituees

N-substituted α-ethylenic aziridines are easily prepared by treating gem-chloro-(methyl)allyllithium with aldimines and ketimines.     

Reductive C−C Coupling from α,β‐Unsaturated Nitriles by Intercepting Keteniminates

We present an atom‐economic strategy to catalytically generate and intercept nitrile anion equivalents using hydrogen transfer catalysis. Addition of α,β‐unsaturated nitriles to a pincer‐based Ru?H complex affords structurally characterized κ‐N‐coordinated keteniminates by selective 1,4‐hydride transfer. When generated in situ under catalytic hydrogenation conditions, electrophilic addition to the keteniminate was achieved using anhydrides to provide α‐cyanoacetates in high yields. This work represents a new application of hydrogen transfer catalysis using α,β‐unsaturated nitriles for reductive C?C coupling reactions.     

Promoting Frustrated Lewis Pairs for Heterogeneous Chemoselective Hydrogenation via the Tailored Pore Environment within Metal–Organic Frameworks

Frustrated Lewis pairs (FLPs) have recently been advanced as efficient metal‐free catalysts for catalytic hydrogenation, but their performance in chemoselective hydrogenation, particularly in heterogeneous systems, has not yet been achieved. Herein, we demonstrate that, via tailoring the pore environment within metal–organic frameworks (MOFs), FLPs not only can be stabilized but also can develop interesting performance in the chemoselective hydrogenation of α,β‐unsaturated organic compounds, which cannot be achieved with FLPs in a homogeneous system. Using hydrogen gas under moderate pressure, the FLP anchored within a MOF that features open metal sites and hydroxy groups on the pore walls can serve as a highly efficient heterogeneous catalyst to selectively reduce the imine bond in α,β‐unsaturated imine substrates to afford unsaturated amine compounds.     

Knoevenagel-kondensationen mit TiCl4/base—III : Umsetzungen von ketonen und α-halogenketonen mit malonester

In the presence of titanium tetrachloride and pyridine in THF, diethyl malonate undergoes condensation with aliphatic and aromatic ketones. The yields of the resulting α,β-unsaturated compounds are good even in cases, where other condensation methods are unsuccessful. Products, resulting from α-mono-, α,α-, α,α′-di- and α,α,α-trihalogeno ketones are probably of special interest as useful intermediates for the preparation of heterocycles.     

Synthetic studies on (−)-scabronine A

This Letter describes synthetic studies on (−)-scabronine A utilizing a new chiral building block successfully prepared via the catalytic asymmetric intramolecular cyclopropanation (IMCP) of an α-diazo-β-keto sulfone. The crucial transformations in this study are the coupling reaction of two fragments between the positions adjacent to a quaternary carbon center, the intramolecular aldol reaction, the C14 hydroxyl-directed hydrogenation, and the ring-expansion reaction to furnish the 5-6-7 tricyclic cyathane skeleton.     

19F and 1H NMR of aldimines of fluoroamino acids with pyridoxal-5′-phosphate

¹H and ¹⁹F NMR spectra were obtained for six Schiff bases (aldimines) formed by pyridoxal-5′-phosphate (PLP) with four fluorinated or their two parent non-fluorinated α-amino acids (phenylalanine and α-aminobutyric acid). pK_A Values were derived from ¹H and ¹⁹F titration curves. The imine nitrogen of the aldimines is very basic (～13) and sensitive to the electron withdrawing effect of fluorine. The pyridine nitrogen is less basic in the aldimines (～7) than in PLP (8.12) and is less sensitive to the electron withdrawing effect of the fluorine than is the imine nitrogen. The phosphate group has a pK in the same range (～6) and the chemical shifts of some nuclei are sensitive to both pK values. Protonation of the aldimine causes the ¹H signal to shift downfield at the methyl protons of the pyridine ring and at the aldehydic proton of the aldimine for the high pK value (except for the aldimines prepared from the β-fluorophenylalanine), but upfield at the aromatic proton and at the aldehydic proton of the aldimine for the low pK. Protonation of the aldimine causes the ¹⁹F signal of an aryl fluorine to shift downfield but gives an upfield shift at a β-fluorine. These data are related to the highly conjugated electronic structure of the Schiff bases.     

Zinc-mediated highly α-regioselective prenylation of imines with prenyl bromide

A highly α-regioselective prenylation of imines has been successfully developed. The efficiency of this approach is confirmed by a wide range of imines including N- and C-aryl aldimines, N-alkyl aldimines, C-alkyl aldimines, and N- and C-aryl ketimines. The approach uses prenyl bromide as the prenyl source and inexpensive and convenient zinc as the mediator as well as environmentally benign 1,3-dimethyl-2-imidazolidinone (DMI) as the solvent.     

Efficient catalytic asymmetric synthesis of α-substituted phenyloxyacetyloxy and aroyloxy phosphonates

Optically active α-substituted phenyloxyacetyloxy and aroyloxy phosphonates have been synthesized via catalytic asymmetric hydrogenation of the corresponding prochiral α,β-unsaturated phosphonates using Rh(I)/(R,R)-Me-DuPhos as the catalyst in methanol at 18 °C. The asymmetric hydrogenation reaction exhibits excellent enantioselectivity with enantiomeric excesses from 91 to 96%.     

A Palladium‐Nanoparticle and Silicon‐Nanowire‐Array Hybrid: A Platform for Catalytic Heterogeneous Reactions

We report the development of a silicon nanowire array‐stabilized palladium nanoparticle catalyst, SiNA‐Pd. Its use in the palladium‐catalyzed Mizoroki‐Heck reaction, the hydrogenation of an alkene, the hydrogenolysis of nitrobenzene, the hydrosilylation of an α,β‐unsaturated ketone, and the C‐H bond functionalization reactions of thiophenes and indoles achieved a quantitative production with high reusability. The catalytic activity reached several hundred‐mol ppb of palladium, reaching a TON of 2 000 000.