Beta-lactam-forming photochemical reactions of N-trimethylsilylmethyl- and N-tributylstannylmethyl-substituted alpha-ketoamides

Two mechanisms have been proposed for the beta-lactam-forming photochemical reactions of alpha-ketoamides. One, suggested by Aoyama, involves excited-state H-atom abstraction while the other, put forth by Whitten, follows a sequential SET-proton-transfer route. The photochemical properties of N-trimethylsilylmethyl- and N-tributylstannylmethyl-substituted alpha-ketoamides were explored in order to gain information about the mechanism of this process and to develop a regioselective method for beta-lactam formation. The results of this effort show that (1) photoreactions of N-trimethylsilylmethyl-substituted alpha-ketoamides proceed by competitive H-atom abstraction and sequential SET-desilylation pathways and (2) a sequential SET-destannylation pathway is preferentially followed in photochemical reactions of the tributylstannylmethyl-substituted alpha-ketoamides.     

Superacid-promoted reactions of pyrazolecarboxaldehydes and the role of dicationic electrophiles

Pyrazolecarboxaldehydes react in the Brønsted superacid triflic acid (CF₃SO₃H, TfOH) to generate electrophilic intermediates capable of reacting with benzene in condensation reactions. Appropriately substituted pyrazoles may also undergo intramolecular Friedel-Crafts-type reactions. It is proposed that the pyrazolecarboxaldehydes and related systems form diprotonated intermediates in these reactions.     

Isocyanide based multicomponent reactions of oxazolidines and related systems

N-Alkyloxazolidines react in a multicomponent reaction with carboxylic acids and isocyanides to give N-acyloxyethylamino acid amides. The previously reported reaction conditions were improved using a design of experiments approach (DoE). Under the optimised conditions, good yields of the N-acyloxyethylamino acid amide products are obtained both via a three- or four-component approach from N-alkylethanolamines, aldehydes/ketones, isocyanides and carboxylic acids. The reaction of oxazolidines without a nitrogen substituent was found to give either the expected Ugi products or the N-acyloxyethylamino acid amides depending on the choice of reaction conditions. Optimised reaction conditions were also developed for the ring-expansion of oxazolidines to morpholin-2-ones via reaction with an isocyanide followed by hydrolysis. The mechanistic pathway of the multicomponent reaction was briefly investigated using an ¹⁸O labelling experiment. The carboxylic acid component can be replaced by a range of other acidic nucleophiles including thiobenzoic acid, thiophenol or 5-phenyltetrazole, which are incorporated via an alternative pathway. These latter reactions can also be applied to 2-aminotetrahydrofurans, 2-aminotetrahydropyrans or 4-hydroxybut-2-one, further extending the structural diversity of the multicomponent reaction products.     

Superacid-promoted synthesis of indolizidine derivatives

A series of amido-acetals were reacted with the Brønsted superacid, CF₃SO₃H, to provides indolizidine derives by a cyclization cascade. A mechanism is proposed involving formation of a vinylogous enol which undergoes a 6π-electrocyclization reaction with an adjacent N-acyl iminium ion group. With aryl substituents, there is a strong tendency for the N-acyl iminium ion group to undergo Friedel-Crafts type cyclizations with the aryl group. The synthetic methodology was used to prepare the alkaloid natural product, ipalbidine.     

Intramolecular de Mayo reactions leading to zizaane and related terpencid ring systems

Further aspects of the regioselectivity of intramolecular photocycloadditions amongst enolised cyclopentane-1,3-diones are defined; elaboration of the cycloadduct ($\text{15}$) derived from ($\text{14}$) provides an expeditious route to the zizaane ring system ($\text{17}$).     

Synthesis of alpha-ketoamides from a carbamoylsilane and acid chlorides

Treatment of acid chlorides with a carbamoylsilane affords alpha-ketoamides. In some instances, in situ reaction of additional carbamoylsilane with these products yielded alpha-organyl-alpha-siloxymalonamides.     

Palladium-based catalytic systems for the synthesis of conjugated enynes by sonogashira reactions and related alkynylations

Conjugated alkynes are recurring building blocks in natural products, a wide range of industrial intermediates, pharmaceuticals and agrochemicals, and molecular materials for optics and electronics. The palladium-catalyzed cross-coupling between sp(2)-hybridized carbon atoms of aryl, heteroaryl, and vinyl halides with sp-hybridized carbon atoms of terminal acetylenes is one of the most important developments in the field of alkyne chemistry over the past 50 years. The seminal work of the 1970s has initiated an intense search for more general and reliable reaction conditions. The interest in the catalytic activation of demanding substrates, the need to minimize the consumption of depletive resources, and the search for easy access to an increased variety of functionalized enynes has led to the current generations of high-turnover catalysts. This Review gives an overview of the highly efficient palladium catalyst systems for the direct alkynylation of C(sp(2)) halides with terminal alkynes, both in homogeneous and heterogeneous phases.     

One-pot route to new alpha,alpha-difluoroamides and alpha-ketoamides

New alpha,alpha-difluoroamides (4a-d, 6a-d) and alpha-ketoamides (5a-d, 7a-d) result from one-pot reactions of alpha-ketoacids, RCOCO(2)H (R = C(6)H(5), CH(3), CH(3)CH(2), thienyl) (1a-d) with bis(2-methoxyethyl)aminosulfur trifluoride [(CH(3)OCH(2)CH(2))(2)NSF(3)] (2) (Deoxofluor) or diethylaminosulfur trifluoride [(CH(3)CH(2))(2)NSF(3))] (3) (DAST). Product yields depend on reaction times and the ratio of reagents used. Longer reaction times ( approximately 36 h) with a 1:2 ratio of alpha-ketoacids and 2 or 3 gave major yields of the alpha,alpha-difluoroamides, and shorter reaction times ( approximately l h) produced alpha-ketoamides as the major products. Reactants in a 1:1 ratio resulted in alpha-ketoamides only.     

The asymmetric Heck and related reactions

The asymmetric inter- and intramolecular Heck and related reactions are comprehensively reviewed, from their original development to recent advances in terms of substrate scope, reactivity, regio- and enantioselectivity. Their reaction mechanisms/catalytic cycles are discussed in order to understand the concepts underpinning the significant recent developments of these processes. The design and application of new chiral ligands has been pivotal to reaction enhancements and, for each Heck and related process, we classify ligands by the nature of the denticity, chirality and donor atoms involved. In this manner, the continued development of ligand architectural design and application can be more easily monitored for each process. Significant improvements in reaction times, a disadvantage in many Heck reactions, have been addressed through a combination the use of microwave-assisted protocols and ligand design. The asymmetric Fujiwara-Moritani and oxidative boron Heck-type reactions, recent additions to Heck type processes, will also be discussed in this critical review (149 references).     

Some reactions of diphosphenes and related species

Abstract

Some reactions of diphosphenes and the related species involving oxidation, reduction, alkylation, sulfurization, and ⁶η-coordination are described.     

Gas-phase reactions of lithium dimethylaminoborohydride and related species

[reaction: see text] Ab initio and density functional theory (DFT) calculations were used to examine the mechanisms of reduction and amination of chloromethane by lithium dimethylaminoborohydride (LAB) in the gas phase. For comparison, the amination of chloromethane by lithium dimethylamide and the reduction by borane, diborane, and borohydride ions were also examined. The reduction of chloromethane by LAB occurred most readily from a conformation that allowed coordination of the lithium atom to the chloride leaving group, and the most favorable amination pathway occurred by a backside attack of the nitrogen nucleophile on chloromethane.     

Oscillatory reactions in polymer systems

Modified Belousov–Zhabotinskii (BZ) oscillation reactions were studied in the systems of ferroin or tris(2,2′-bipyridyl)ruthenium(II) dichloride or the polymerized ruthenium complex as electron transfer agents and sulfuric acid or polyethylenesulfonic acid (HPES) as acidic media. HPES was a good acidic medium for the chemical oscillation in spite of a long induction period. Moreover, chemical oscillation was observed at low concentration of HPES, where no oscillation was observed in the solution of sulfuric acid. On the other hand, no oscillation was observed in the solution of polystyrenesulfonic acid (HPSS). These results were discussed from the standpoint of the nature of polyelectrolytes.     

Alkaloid-fullerene systems through photocycloaddition reactions

The photocycloaddition of tertiary amines to ?60fullerene (C(60)) is an interesting and useful reaction. We wished to extend the applications of this type of reaction through an investigation of the photoaddition of alkaloids to C(60) for the purpose of synthesizing novel and complex photoadducts that are difficult to obtain by usual methods. Irradiation of tazettine (2) or gramine (3) with C(60) in toluene leads to formation of one monoadduct (6 or 7), whereas scandine (1a) or 10-hydroxyscandine (1b) reacts with C(60) photochemically to give two products, the expected ?6,6 monoadduct (5a, 5b) and a new type of monoadduct with a bis-?6, 6 closed structure (4a, 4b). These new structures were characterized by UV-vis, FT-IR, (1)H NMR, (13)C NMR, (1)H-(1)H COSY, ROESY, HMQC (heteronuclear multiple-quantum coherence), and HMBC (heteronuclear multiple-bond connectivity) spectroscopy. The techniques of time-of-flight secondary ion MS (TOF-SIMS) and field desorption MS (FD-MS) were used for the mass determination. (3)He NMR analysis of the product mixture from photoaddition of 1a to C(60) containing a (3)He atom ((3)He@C(60)) led to two peaks at -9.091 and -11.090 ppm relative to gaseous (3)He, consistent with formation of a ?6, 6-closed monoadduct and a bis-?6,6 closed adduct. Presumably, the bis-?6, 6 closed adducts are formed by an intramolecular ?2 + 2 cycloaddition of the vinyl group to the adjacent 6,6-ring junction of C(60) after the initial photocycloaddition.     

Acylation and related reactions under microwaves. 4. Sulfonylation reactions of aromatics.

Solvent-free sulfonylation of benzene and its activated or deactivated derivatives were carried out under microwave (MW) irradiation and a catalytic amount of iron(III) chloride, which, under these conditions, is more active than other metallic salts. With more reactive and/or nonvolatile reagents (anisole, xylenes, mesitylene) expeditious conditions (short reaction time at constant MW power without control of the temperature) were used. With less reactive and/or low-boiling reagents (benzene, toluene, halobenzenes), the rise in temperature and the increase of reaction time were controlled either by sequential MW irradiation or by a temperature order. It was shown that MWs cause preferential interactions with polar species present in the reaction, especially the aryl sulfone and its FeCl3-complexed form. A MW nonthermal effect was not observed when identical temperature gradients were produced by classical heating and MW irradiation, and if reaction temperature was strictly controlled.     

Synthesis of fluorinated monomers through sulfinatodehalogenation and related reactions

Using sulfinatodehalogenation and related reagent systems several groups of perfluoroalkylated compounds including olefins, dienes, phenolic compounds and aromatic dicarboxylic acids were synthesized which may be useful as potential monomers.     

Photochemical reactions of aqueous plutonium systems

Photochemical effects on four aqueous plutonium perchlorate solutions have been observed and reported. The photochemical reduction of PuO₂²⁺ and of Pu⁴⁺ to Pu³⁺ have been observed, and the quantum yields were determined to be 0·02 and 0·03, respectively. A reversible photochemical shift in the equilibrium of the Pu⁴⁺ disproportionation was observed, and the equilibrium coefficient increased by a factor of three when the sample was irradiated. Light has been observed to increase the rate and extent of depolymerization of Pu(IV) polymer.