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An improved synthetic procedure for the intramolecular ketene [2+2] cycloaddition was developed for the preparation of 1‐heteroatom‐substituted bicyclo[2.1.1]hexan‐5‐ones. It was found that the use of the Mukaiyama reagent (2‐chloro‐N‐methyl‐pyridinium iodide) was key to efficiently generate the α‐heteroatom substituted homoallyl ketene intermediate for the cycloaddition reaction. The synthetic utility of the resulting bicyclic ketone was demonstrated through the preparation of a saturated variant of vortioxetine. [ABSTRACT FROM AUTHOR]

Herein, a visible-light-induced chiral N-heterocyclic carbene (NHC)-catalyzed asymmetric amination of ketenes has been developed. This strategy provides a convenient and facile synthetic protocol for the efficient construction of α-chiral amides under mild reaction conditions with moderate to excellent yields and stereoselectivities (up to 95% yields and 97 : 3 e.r.). Furthermore, a chiral 2-phenyl-1-propylamine derivative and (R)-ibuprofen could be obtained after simple transformations of the chiral products. [ABSTRACT FROM AUTHOR]

Thermolysis of 5-aryl-4-trichloroacetylfuran-2,3-diones in the presence of cyclic ketones yielded 6-aryl-2-spiro-substituted 5-trichloroacetyl-4H-1,3-dioxin-4-ones. The obtained compounds were screened for antimicrobial activity. [ABSTRACT FROM AUTHOR]

The ring‐expansion of arylthiiranes and phenoxyacetyl chloride was performed in the presence of triethylamine and provided a novel strategy for the synthesis of 4,5‐dihydrobenzo[d]thiepin‐2(1H)‐ones, benzo[d]‐ϵ‐thiolactones, directly without catalysts or additives. Its linear free energy analysis was conducted on the basis of competitive experiments. The results indicate that both reaction constants are close to zero (0.14 for σm and 0.20 for σp), supporting the proposed sulfur‐shifted ene mechanism. The current reaction features atom and step‐economic one‐pot characteristic via a tandem sequence of in situ ketene generation, sulfur‐shifted ene reaction, and aromatization. [ABSTRACT FROM AUTHOR]

A novel Au-allenylidene promoted decarboxylative annulation by intramolecular α-nucleophilic addition has been disclosed. The unsaturated cyclic ethynylethylene carbamates/carbonates can be converted to unique nucleophiles attached with alkylidene ketenes by sequential decarboxylation and oxidation processes. Such alkylidene ketenes can be rapidly trapped by intramolecular α-attacking annulation to generate potential biological active unsaturated γ-lactams/lactones with broad scope, facile post-modification, high regioselectivity and efficiency. [ABSTRACT FROM AUTHOR]

Diverse chiral trans and cis-3-chloro/oxo/thio-4-styryl-β-lactams were efficiently synthesized using chiral 3′-phenylallylidene-[(R)-1′-phenylethyl] amine and substituted ethanoic acids. Moore ketenes derived from chloro/phenythio/benzylthio ethanoic acids (Cl/PhS/BzS) affords both chiral trans- and cis β-lactams, with a preference for trans-stereochemistry. However, Bose-Evans ketenes obtained from 2-methoxy/phenoxyethanoic acids (MeO/PhO), led to the exclusive formation of cis-diastereomers only. Further, individual diastereomers of trans- and cis-β-lactams were separated via efficient column chromatography. The relative cis and trans configuration of the C-3 and C-4 protons in chiral β-lactams was established using coupling constant value ranging from J = 1.4 to 2.1 Hz (in case of trans) and J = 4.3 to 5.2 Hz (in case of cis) respectively. Structural confirmation of β-lactams was done using FT-IR, NMR (1H,13C,1H-1H COSY and 1H-13C HSQC), elemental analysis, ESI-MS and X-ray crystallography. [ABSTRACT FROM AUTHOR]

Pd2dba3·CHCl3 (2.5 mol%)-BINAPHANE (5 mol%) was used to promote the first catalytic enantioselective allylation of disubstituted ketenes to give α-allyl esters. The ester products were formed in good to excellent yields (61–93% yield for 13 examples, 16 examples in all), with moderate to good enantioselectivity (68–80% ee for 7 examples). [ABSTRACT FROM AUTHOR]

In recent years, significant progress has been made in the synthesis of cyclobutenones. The unique structural features and reactivity of cyclobutenones, making them of great values in organic synthesis. Therefore, the development of new strategies for synthesizing the cyclobutenones has been a major focus of organic chemists. This review summarizes the synthetic methods for the cyclobutenones in the past several decades and provides an overview of the characteristics and advantages of these strategies. The aim of this review is to provide valuable insights for the synthesis of cyclobutenones. [ABSTRACT FROM AUTHOR]

In this paper we report a [2 + 2] cycloaddition reaction between ketenes and benzils, characterized by an unusual double photochemical activation triggered by visible light. Employment of a flow system and optimization of reaction conditions through Design of Experiments resulted in moderate to good yields of the corresponding β-lactones. A thorough computational analysis allowed to elucidate the mechanism of the reaction and justify the observed diastereoselectivity. The reaction was also successfully tested with mixed benzils, showing complete regioselectivity. [ABSTRACT FROM AUTHOR]

The complex [(dpp-bian)2−Mg2+(THF)3] (1) (dpp-bian is 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) reacts with Ph2C=C=O to form the product of insertion into the Mg—N bond, [(dpp-bian)(Ph2C=C—O)Mg(THF)2] (2), whereas the reaction of 1 with another heteroallene, Ph3SnN=C=O, gives the reduction product [(dpp-bian) Mg(DME)(NCO)]2 (4) and hexaphenyldistannane. Complex 1 reduces Ph2N—N=O, giving [(dpp-bian)MgNPh2(THF)] (3). Complex 2 is the first structurally characterized nucleophilic addition product of ketenes at the Mg—N bond. The new compounds were characterized by IR, EPR, and NMR spectroscopy and elemental analysis. The molecular structures of complexes 2–4 were established by X-ray diffraction. [ABSTRACT FROM AUTHOR]

Multistage mass spectrometry (MSn) experiments were used to explore extrusion-insertion (ExIn) reactions of the palladium complex [(phen)Pd(O2CPh)]+ (phen, 1,10-phenanthroline). Under collision-induced dissociation (CID) conditions, the organopalladium cation [(phen)Pd (Ph)]+ was formed via decarboxylation and was found to react with phenylmethylketene to yield the enolate [(phen)Pd(CPhMeC(O)Ph)]+ via an insertion reaction. A further stage of CID revealed that the enolate fragments via loss of styrene to form the acyl complex [(phen)Pd (C(O)Ph)]+. Formation of both the coordinated enolate and acyl anions is supported by density functional theory (DFT) calculations. Attempts to develop a palladium-mediated one-pot synthesis of ketones from 2,6-dimethoxybenzoic acid as the key substrate and the ketene substrates R¹R²"C"O (R¹ = Ph, R² = Me; R¹ = R² = Ph) proved challenging owing to low yields and side product formation. [ABSTRACT FROM AUTHOR]

4-Hydroxy-2-pyrones are of interest as potential biorenewable molecules for a sustainable transition from biomass feedstock to valuable chemical products. This review focuses on the methodologies for the synthesis of 4-hydroxy-2-pyrones published over the last 20 years. These pyrones as polyketides are widespread in Nature and possess versatile bioactivity that makes them an attractive target for synthesis and modification. Biosynthetic paths of the pyrones are actively developed and used as biotechnological approaches for the construction of natural and unnatural polysubstituted 4-hydroxy-2-pyrones. The major synthetical methods are biomimetic and are based on the cyclization of tricarbonyl compounds. Novel chemical methods of de novo synthesis based on alkyne cyclizations using transition metal complexes and ketene transformations allow for straightforward access to 4-hydroxy-2-pyrones and have been applied for the construction of natural products. Possible directions for further pyrone ring modification are discussed. [ABSTRACT FROM AUTHOR]

The development of intermolecular [2+2] cycloadditions between allenamides and diarylketenes is described. α-Aryldiazo arylketones are employed as ketene precursors that react smoothly with allenamides in the presence of a P(C6F5)3 promoter. High diastereoselectivity (dr > 20 : 1) with endo-regioselectivity can be achieved when two aryl groups of ketenes have opposite electronic properties. The role of P(C6F5)3 is to increase the reaction yields, but also enhancing stereoselectivity. [ABSTRACT FROM AUTHOR]

Meldrum's acid mediated ketene chemistry (MAMKC) is applied to synthesis of poly(ester amide)s in this work. An MA-functionalized phenolic amide compound (MA_PhOH-Am) is synthesized from the reaction between a MA-functionalized amine and 3,4-dihydrocoumarine (DHC, a biomass-based chemical) through amine-lactone addition reaction. Polymerization of MA_PhOH-Am results in the corresponding poly(ester amide) (P(Es-Am)), through MA thermolysis reaction (generating the corresponding ketene groups) and ketene-phenol addition reaction. Both MA_PhOH-Am and P(Es-Am) have been characterized with spectral methods and thermal analysis. P(Es-Am) has an inherent viscosity of 0.49 dL g-1, a molecular weight of 76,500 Da, a glass transition temperature of 145°C (tand peak in dynamic mechanical analysis), a thermal stability of about 320°C, and a storage modulus at 50°C of 1.0 GPa. In the stress-strain measurement, P(Es-Am) exhibits a Young's modulus, a tensile strength, and an elongation at break of 10.6 ± 0.7 GPa, 165 ± 40 MPa, and 1.83 ± 0.41%, respectively. An effective approach for the synthesis of high-performance poly(ester amide)s has been demonstrated. [ABSTRACT FROM AUTHOR]

The enantioselective addition of potent nucleophiles to ketenes poses challenges due to competing background reactions and poor stereocontrol. Herein, we present a method for enantioselective phosphoric acid catalyzed amination of ketenes generated from α‐aryl‐α‐diazoketones. Upon exposure to visible light, the diazoketones undergo Wolff rearrangement to generate ketenes. The phosphoric acid not only accelerates ketene capture by amines to form a single configuration of aminoenol intermediates but also promotes an enantioselective proton‐transfer reaction of the intermediates to yield the products. Mechanistic studies elucidated the reaction pathway and explained how the catalyst expedited the transformation and controlled the enantioselectivity. [ABSTRACT FROM AUTHOR]

Design, synthesis and application of benzene bioisosteres have attracted a lot of attention in the past 20 years. Recently, bicyclo[2.1.1]hexanes have emerged as highly attractive bioisosteres for ortho‐ and meta‐substituted benzenes. Herein we report a mild, scalable and transition‐metal‐free protocol for the construction of highly substituted bicyclo[2.1.1]hexan‐2‐ones through Lewis acid catalyzed (3+2)‐cycloaddition of bicyclo[1.1.0]‐butane ketones with disubstituted ketenes. The reaction shows high functional group tolerance as documented by the successful preparation of various 3‐alkyl‐3‐aryl as well as 3,3‐bisalkyl bicyclo[2.1.1]hexan‐2‐ones (26 examples, up to 89 % yield). Postfunctionalization of the exocyclic ketone moiety is also demonstrated. [ABSTRACT FROM AUTHOR]

Acyl(imidoyl)ketenes are highly reactive heterocumulenes that enable diversity-oriented synthesis of various drug-like heterocycles. Such ketenes, bearing heterocyclic substituents, afford angularly fused pyridin-2(1H)-ones in their [4+2]-cyclodimerization reactions. We have utilized this property for the development of a new synthetic approach to pharmaceutically interesting pyrido[2,1-b][1,3]benzothiazol-1-ones via the [4+2]-cyclodimerization of acyl(1,3-benzothiazol-2-yl)ketenes generated in situ. The thermal behaviors of 3-aroylpyrrolo[2,1-c][1,4]benzothiazine-1,2,4-triones and 3-benzoylpyrrolo[2,1-b][1,3]benzothiazole-1,2-dione (two new types of [e]-fused 1H-pyrrole-2,3-diones reported by us recently) have been studied by thermal analysis and HPLC to elucidate their capability to be a source of acyl(1,3-benzothiazol-2-yl)ketenes. As a result, we have found that only 3-aroylpyrrolo[2,1-c][1,4]benzothiazine-1,2,4-triones are suitable for this. The experimental results are supplemented with computational studies that demonstrate that thermolysis of 3-aroylpyrrolo[2,1-c][1,4]benzothiazine-1,2,4-triones proceeds through an unprecedented cascade of two thermal decarbonylations. Based on these studies, we discovered a novel mode of thermal transformation of [e]-fused 1H-pyrrole-2,3-diones and developed a new pot, atom, and step economic synthetic approach to pyrido[2,1-b][1,3]benzothiazol-1-ones. The synthesized drug-like pyrido[2,1-b][1,3]benzothiazol-1-ones are of interest to pharmaceutics, since their close analogs show significant antiviral activity. [ABSTRACT FROM AUTHOR]

The reactivity of a literature‐known, ring‐strained bismuth amide cation towards a range of unsymmetric heterocumulene substrates has been investigated. Reactions with ketenes R2C=C=O (R=Me, Ph), isocyanates R'N=C=O, and isothiocyanates R'N=C=S (R'=Ph, 4‐CF3‐C6H4) proceed via facile insertion of the heterocumulene in the Bi−N bond of the cationic bismuth amide. Unexpectedly pronounced differences in the regioselectivity of these insertion reactions have been observed, yielding a rich variety of heterocycle motifs (BiC2NC2, BiC2NCO, BiC2NCS, BiC2NCN), some of which are unprecedented. Parameters that control the regioselectivity of the insertion reactions have been identified and are discussed based on experimental and theoretical investigations. Analytical techniques applied in this work include heteronuclear and two‐dimensional NMR spectroscopy, IR spectroscopy, elemental analysis, single‐crystal X‐ray diffraction analyses, and DFT calculations. [ABSTRACT FROM AUTHOR]

We present a theoretical analysis of the reaction mechanisms on Lithium Carbenoids LiCH2X (X = F, Cl y Br) and ketenes at B3LYP/6-311++G(d,p) level at the IRC. We have considered the implicit solvation effects of dichloromethane using the PCM methodology. For the reaction coordinate analysis, we have used the reaction force analysis and the theoretical tools coming from conceptual DFT, NBO populations and NCI. The reaction mechanism has a barrierless step corresponding to the interaction of the carbenoid and the hydrogen of the ketene α carbon and a single kinetic step representing the bonding of the carbon carbenoid to the ketene α carbon, the activation of the reaction proceeds mostly by electrostatic interactions perpendicular to the molecular plane followed by a rotation of the ketene showing the influence of the ionic nature of the carbenoid, and the following formation of the bond with the ketene β carbon atom. All three reactions are highly exothermic leading to a decomposition product that is far lower in energy than the cyclopropanone. The tendency of the reaction barriers is LiCH2Br < LiCH2Cl < LiCH2F, indicating that the bromine carbenoid is the most reactive molecule of this group. [ABSTRACT FROM AUTHOR]

Gold-catalyzed oxidations of alkynes by N-oxides offer direct access to reactive α-oxo gold carbene intermediates from benign and readily available alkynes instead of hazardous diazo carbonyl compounds. Despite various versatile synthetic methods developed based on this strategy, one of the hallmarks of α-oxo carbene/carbenoid chemistry, that is, the Wolff rearrangement, has not been realized in this context. This study discloses the first examples that show the Wolff rearrangement can be readily realized by α-oxo gold carbenes oxidatively generated from TBS-terminated alkynes (TBS=tert-butyldimethylsilyl). The thus-generated silylketenes can be either isolated pure or subsequently trapped by various internal or external nucleophiles in one pot to afford α-silylated carboxylic acids, their derivatives, or TBS-substituted allenes.

Although NH‐sulfoximines are known to react with carbenes by deamidation pathways, they can be combined if the latter are generated as transients from diazo compounds under blue light irradiation to undergo rapid Wolff rearrangement to generate reactive ketenes. Subsequent addition of sulfoximines or sulfonimidamides lead to N‐acylated products in good to excellent yields. Beneficial are the mild reaction conditions, which neither require the addition of a base nor an oxidant or a metal catalyst. [ABSTRACT FROM AUTHOR]

Searching for photoconvertible or photoactivatable dyes, we considered trimethine (Cy3) and pentamethine (Cy5) fluorophores with a 4‐amino‐1‐diazo‐2‐butanone fragment (H2N(CH2)2COCH=N2) attached to the meso (γ) position of the polymethine chain via the amino group. The Cy3 derivative was prepared. All Cy5 derivatives with the basic amino group at this position decomposed with breaking the polymethine chain. However, amino‐squaraine based Cy5 dyes were found to be stable and accessible. Photolysis of Cy3 and squaraine‐based Cy5 dyes with a 4‐amino‐1‐diazo‐2‐butanone fragment was accompanied with Wolff rearrangement and led to pyrrolidones (in methanol and aprotic solvents) formed via intramolecular cyclization of the intermediate ketenes. In aqueous acetonitrile, ketenes reacted with water and gave carboxylic acids. Both products were non‐fluorescent, though the model Cy5 dyes (without the squaraine fragment) with acylated amino and N‐methylamino groups were found to be fluorescent. [ABSTRACT FROM AUTHOR]

Various 9,10-cyclopropanated phenanthrenes have been shown previously to be viable photochemical precursors to many different types of carbenes. There is also one known example in the literature of a related approach used to generate diphenylketene from a phenanthrene-derived cyclobutanone in a laser flash photolysis study. In an effort to broaden the scope of this promising yet under-utilized method of ketene generation, the synthesis and characterization of several phenanthrene-based cyclobutanones, and their subsequent photolysis to produce a variety of ketenes, are presented in this report. The ketenes are produced, along with phenanthrene, by a formal 2 + 2 cyclo-reversion of the cyclobutanone moiety and have been intercepted by benzyl alcohol and benzyl amine to form the corresponding benzyl esters and amides respectively. [Display omitted] • Straightforward approaches to phenanthrene-based cyclobutanones are described. • Photolysis of these phenanthrene-based cyclobutanones in solution gives ketenes. • Adamantylketene persists in solution at ambient temperature for several hours. [ABSTRACT FROM AUTHOR]

2-cyclohexen-1-ol and 2-cyclohexen-1-one are very important chemical raw materials, and it is a challenge to develop an environmental and efficient synthesis route with titanium-containing catalyst to obtain high yield of allylic oxidation product. Herein, we present an intensive study on the allylic oxidation of cyclohexene over titanosilicates/H 2 O 2 system. Ti-MCM-41 is found as an efficient catalyst. The reaction parameters, including cyclohexene/H 2 O 2 molar ratios, solvent, catalyst amount, titanium content, reaction temperature and reaction time, were studied in detail. And a high cyclohexene conversion (∼20.2 %) and total selectivity of 2-cyclohexen-1-one and 2-cyclohexen-1-ol (∼90.0 %) was achieved under optimized reaction conditions. Further the allylic oxidation of cyclohexene proceeds a radical mechanism as demonstrated by the addition of radical scavenger BHT. This will make it possible for the highly selective green generation of enols and ketenes. Ti-MCM-41 can efficiently catalyze cyclohexene to yield allyl products using H 2 O 2 as oxidant, and the selectivity reach 90 % under optimized conditions. [Display omitted] • Ti-MCM-41/H 2 O 2 system exhibits efficient catalysis for cyclohexene allylic oxidation. • The optimized conditions of cyclohexene allylic oxidation were determined. • The radical mechanism of allyl pathway was confirmed. • A reaction network of cyclohexene oxidation was proposed. [ABSTRACT FROM AUTHOR]

The capacity of transition metals to bind and transform carbon monoxide (CO) is critical to its use in many chemical processes as a sustainable, inexpensive C1 building block. By contrast, only few s- and p-block element compounds bind and activate CO, and conversion of CO into useful carbonyl-containing organic compounds in such cases remains elusive. We report that metalated phosphorus ylides provide facile access to ketenyl anions ([RC=C=O]–) by phosphine displacement with CO. These anions are very stable and storable reagents with a distinctive electronic structure between that of the prototypical ketene (H2C=C=O) and that of ethynol (HC≡C–OH). Nonetheless, the ketenyl anions selectively react with a range of electrophiles at the carbon atom, thus offering high-yielding and versatile access to ketenes and related compounds. [ABSTRACT FROM AUTHOR]

Ketene (CH2=C=O) has been postulated as a key intermediate for the first olefin production in the zeolite‐catalyzed chemistry of methanol‐to‐olefins (MTO) and syngas‐to‐olefins (STO) processes. The reaction mechanism remains elusive, because the short‐lived ethenone ketene and its derivatives are difficult to detect, which is further complicated by the low expected ketene concentration. We report on the experimental detection of methylketene (CH3−CH=C=O) formed by the methylation of ketene on HZSM‐5 via operando synchrotron photoelectron photoion coincidence (PEPICO) spectroscopy. Ketene is produced in situ from methyl acetate. The observation of methylketene as the ethylene precursor evidences a computationally predicted ketene‐to‐ethylene route proceeding via a methylketene intermediate followed by decarbonylation. [ABSTRACT FROM AUTHOR]

4-Acyl-1H-pyrrole-2,3-diones fused at [e]-side with a heterocyclic moiety are suitable platforms for the development of a hetero-Diels–Alder-reaction-based, diversity-oriented approaches to series of skeletally diverse heterocycles. These platforms are known to react as oxa-dienes with dienophiles to form angular 6/6/5/6-tetracyclic alkaloid-like heterocycles and are also prone to decarbonylation at high temperatures resulting in generation of acyl(imidoyl)ketenes, bidentate aza- and oxa-dienes, which can react with dienophiles to form skeletally diverse products (angular tricyclic products or heterocyclic ensembles). Based on these features, we have developed an approach to two series of skeletally diverse 4H-1,3-oxazines (tetracyclic alkaloid-like 4H-1,3-oxazines and 5-heteryl-4H-1,3-oxazines) via a hetero-Diels–Alder reaction of 4-acyl-1H-pyrrole-2,3-diones fused at [e]-side with cyanamides. The products of these transformations are of interest for drug discovery, since compounds bearing 4H-1,3-oxazine moiety are extensively studied for inhibitory activities against anticancer targets. [ABSTRACT FROM AUTHOR]

The recent advances in the N‐heterocyclic carbene (NHC)‐organocatalyzed generation of azolium enolate intermediates and their subsequent interception with electrophiles are highlighted. The NHC‐bound azolium intermediates are generated by the addition of NHCs to suitably substituted aldehydes, acid derivatives or ketenes. A broad range of coupling partners can intercept the azolium enolates to form [2+n] cycloadducts (n=2,3,4) and various α‐functionalized compounds. The enantioselective synthesis of the target compounds are achieved with the use of chiral NHCs. Herein, we summarized the development that occurred in this subclass of NHC catalysis. [ABSTRACT FROM AUTHOR]

Transition metal-catalysed carbene- and nitrene transfer to the C1-building blocks carbon monoxide and isocyanides provides heteroallenes (i.e. ketenes, isocyanates, ketenimines and carbodiimides). These are versatile and reactive compounds allowing in situ transformation towards numerous functional groups and organic compounds, including heterocycles. Both one-pot and tandem processes have been developed providing valuable synthetic methods for the organic chemistry toolbox. This review discusses all known transition metal-catalysed carbene- and nitrene transfer reactions towards carbon monoxide and isocyanides and in situ transformation of the heteroallenes hereby obtained, with a special focus on the general mechanistic considerations. [ABSTRACT FROM AUTHOR]

Ketenes (R2C=C=O) are important reactive intermediates in organic chemistry which are often challenging to access in stable form due to their polarized cumulenic structure and inherent high reactivity. Ketenyl anions (R(M)C=C=O) or metal ynolates (R−C≡C−OM) represent alternative, highly attractive synthetic targets, but are typically prepared in situ and their structure and synthetic access are little investigated. Now, a fundamentally new strategy utilizing an exchange reaction of a phosphine with carbon monoxide at a metalated ylide gives rise to a series of novel ketenyl anions. These compounds are flexible reagents and can easily be transferred with electrophiles into more complex building blocks for synthetic applications. Since carbon monoxide is an inexpensive C1 building block, the new exchange strategy is an elegant concept and a valuable new entry in the synthetic toolbox of organic synthesis. [ABSTRACT FROM AUTHOR]

We report herein the synthesis of β-lactams via a three-component reaction of N-hydroxyanilines, diazo compounds and cyclobutenones, which proceeds through a sequential rhodium-catalyzed imine formation and a [2+2] cycloaddition with the in situ generated ketenes from the ring-opening of cyclobutenones. The reaction features broad substrate scope and simplicity of operation, affording densely functionalized β-lactams in moderate to good yields with excellent diastereoselectivity. [ABSTRACT FROM AUTHOR]

The arylation of carboxylic acid derivatives via Smiles rearrangement has gained great interest in recent years. Both radical and ionic approaches, as well as radical-polar crossover concepts, have been developed. In contrast, a reversed polar-radical crossover approach remains underexplored. Here we report a simple, efficient and scalable method for the preparation of sterically hindered and valuable α-quaternary amides via a polar-radical crossover-enolate oxidation-aryl migration pathway. A variety of easily accessible N-alkyl and N-arylsulfonamides are reacted with disubstituted ketenes to give the corresponding amide enolates, which undergo upon single electron transfer oxidation, a 1,4-aryl migration, desulfonylation, hydrogen atom transfer cascade to provide α-quaternary amides in good to excellent yields. Various mono- and di-substituted heteroatom-containing and polycyclic arenes engage in the aryl migration reaction. Functional group tolerance is excellent and substrates as well as reagents are readily available rendering the method broadly applicable. The α-arylation of amides via aryl migration has attracted considerable interest in recent years. Here, the authors report a method for the preparation of bulky α-quaternary amides via a polar-radical crossover enolate oxidation-aryl migration cascade. [ABSTRACT FROM AUTHOR]

A versatile asymmetric synthesis of bicyclic pyrazolidinones through alkaloid‐catalyzed formal [3+2]‐ and [3+2+2]‐cycloadditions of ketenes with azomethine imines is described. The methodology was found to be tolerant of ketene and a variety of monosubstituted ketenes (R=alkyl, OAc). The products were formed in good to excellent yields (71–99 % for 24 examples, 39 examples in all), with good to excellent diastereoselectivity in many cases (dr 3 : 1 to 27 : 1 for 22 examples), and with excellent enantioselectivity for most examples (≥93 % ee for 34 products). In the case of most disubstituted ketenes, the reaction proceeded through a [3+2+2]‐cycloaddition to form structurally interesting bicyclic pyrazolo‐oxadiazepinediones with moderate diastereoselectivity (dr up to 3.7 : 1) and as racemic mixtures (3 examples). The method represents the first unambiguous example of an enantioselective reaction between ketenes and a 1,3‐dipole. [ABSTRACT FROM AUTHOR]

Diazo compounds such as α‐diazocarbonyls, diazoalkanes, and diazoarenes are well known compounds in organic chemistry. Their synthetic utility is of paramount importance. They are commonly used as precursors of reactive intermediates like ketenes and carbenoids. Their reactions have been traditionally carried out under thermal or photochemical conditions in inert atmosphere. The synthesis and chemistry of such compounds under microwave irradiation have seen steady growth. The present article critically reviews the application of microwave irradiation in synthesis and chemistry of diazoalkanes and α‐diazocarbonyls. The microwave‐assisted reactions via ketenes and carbenes with alcohols, thiols, phenols, amines, imines, alkenes, alkynes, and three‐membered heterocyclic compounds, etc. are described. These reactions furnish synthetically useful building blocks and also biologically relevant heterocyclic compounds. Most reactions are accelerated by the use of microwave irradiation. There are also some reports revealing that the reactions did not occur by conventional heating but yielded products on irradiation with microwaves. The mechanisms of the reactions where necessary and synthetic and biological relevance of the products are highlighted. [ABSTRACT FROM AUTHOR]

α‐Oxoketenes and cyclohexadiene α‐oxoketenes are reactive intermediates, particularly the latter due to their high re‐aromatization potential. In this communication, we report photolytic studies on the generation of such species from 4‐OMe and 4‐OMOM protected resorcylate dioxinones and their trapping to give resorcylate esters and amides as well as the formation of adducts with enol‐ethers as trapping reagents. [ABSTRACT FROM AUTHOR]

In the presence of a chiral spiro phosphoric acid catalyst, the asymmetric reaction of disubstituted ketenes with N–H pyrroles occurred to afford enantioenriched C-acylated pyrroles bearing α-stereogenic carbon centres. The described reaction constitutes a rare example of a catalytic asymmetric reaction of ketenes with carbon-based nucleophiles. [ABSTRACT FROM AUTHOR]

Alkylidene ketenes typically display high intrinsic reactivity, impeding isolation on a preparative scale. Herein, we report the synthesis of alkylidene ketenes by reaction of imidazole-based diazoolefins with carbon monoxide. The good thermal stability of these heterocumulenes allows for characterization by single crystal X-ray diffraction. N-Heterocyclic alkylidene ketenes can be used as C-donor ligands for transition and main group metals, as evidenced by the isolation of complexes with AuCl, RhCl(CO)2, PdCl(C3H5) and GaCl3. [ABSTRACT FROM AUTHOR]

To commemorate the centenary of the birth of modern polymer science, a review of the life and accomplishments of Hermann Staudinger is given within the framework of the first half of the twentieth century. Staudinger is remembered for his discovery of ketenes and the Staudinger reaction, but his greatest contribution to chemistry was in developing the concept of macromolecules, for which he received the Nobel Prize in 1953. [ABSTRACT FROM AUTHOR]

An enantioselective [4+2] cycloadditions of 2-alkylidenetrimethylene carbonates with photochemically generated ketenes was first accomplished by utilizing readily available chiral palladium catalysts. This unique protocol provided a variety of chiral lactones bearing a chiral all-carbon quaternary stereocenter in generally high reaction efficiency and selectivity under mild conditions. [Display omitted] [ABSTRACT FROM AUTHOR]

A DBU-catalyzed desymmetric [3+2] cycloaddition between paraquinamines and photogenerated ketenes was developed for the first time. Under the irradiation of low-energy blue LEDs, a variety of hydroindoles bearing all-carbon quaternary centers were produced with good reaction efficiency and complete diastereoselectivity (34 examples, 45-99% yields and 495 : 5 dr). This protocol represents a new approach to synthetically significant hydroindoles, and features broad substrate scope, high functional group compatibility and mild reaction conditions. [ABSTRACT FROM AUTHOR]

A novel visible‐light‐driven catalytic activation of C=O bonds by exploiting the photoredox chemistry of 1,3,2‐dioxaphospholes, readily accessible from α‐diketones and trialkyl phosphites, is reported. This mild and environmentally friendly strategy provides an unprecedented and efficient access to the Wolff rearrangement reaction which traditionally entails α‐diazoketones as precursors. The resulting ketenes could be precisely trapped by alcohols/thiols to give α‐aryl (thio)acetates and by imines to afford the valuable β‐lactams in up to 99 % yields. [ABSTRACT FROM AUTHOR]

The review summarizes the data on the synthesis of heterocyclic compounds by means of [3,3]-sigmatropic transformations, which include the Fischer, Claisen, and Cope rearrangements, as well as their numerous variations. These reactions have proved to be a convenient and efficient method for the synthesis of complex and poorly available heterocyclic systems with several chiral centers due to the highly ordered transition state of the process. Methods for the construction of five- and six-membered and medium size oxygen-, nitrogen-, and sulfur- containing heterocycles are described, as well as their use in the synthesis of condensed systems and natural compounds. [ABSTRACT FROM AUTHOR]

[Display omitted] • Dual reactivity of indoles-Schiff bases towards S/Se/Cl substituted ketenes. • Novel hybrids of S/Se/Cl-indole-β-lactams. • trans -Stereochemistry was established based on SC-XRD analyses. • 5a and 5b with Cl substitutions suppress Gram +ve and -ve bacterial strain addressing "magic chloro effect". • Molecular docking predicted hydrogen and hydrophobic interactions in ligand-receptor complex. A convenient methodology for C -4 indole-β-lactam hybrids with chloro, sulphur and seleno substitutions through dual site reactivity of indole-3-Schiff bases towards ketenes has been developed. The reaction proceeded in a stereospecific manner with the exclusive formation of trans -β-lactams assigned with respect to C 3-H and C 4-H. The synthesized novel β-lactams have been characterized with the help of elemental analysis (CHNS) and spectroscopic techniques viz. 1H NMR, 13C NMR, DEPT 135, HSQC and IR. The trans configuration was further estabilished based on X-ray crystallographic data. Examination of antibacterial properties unveiled that only derivatives 5a and 5b , featuring chloro substitution, exhibited potent activities, underscoring the emergence of the recently coined term "magic chloro effect". Molecular docking analysis provided additional support for the observed in vitro antibacterial activities of compounds 5a-b. [ABSTRACT FROM AUTHOR]

• The competing mechanisms of chiral phosphoric acid (CPA)-catalyzed Friedel-Crafts-like reaction of ketene and pyrrole has been systematically studied. • FMO analysis was performed to reveal the CPA can promote the reaction by lowering the energy gap of FMOs. • The origin of stereoselectivity has been discovered by using NCI and AIM analyses. DFT calculations have been carried out to explore the competing mechanisms and origin of selectivities of chiral phosphoric acid (CPA)-catalyzed Friedel-Crafts-like reaction of ketene and pyrrole. Two kinds of pathways including O-protonation pathway and C-protonation pathway have been investigated and compared in theory for the first time. The O-protonation pathway is confirmed as the most favorable pathway, and contains several stages: (1) combination of CPA and ketene, (2) complexation with pyrrole, (3) O-protonation, (4) stepwise CPA-assisted [1,3]-proton transfer, and (5) dissociation of CPA. Frontier molecular orbital (FMO) analysis indicates the CPA can promote the reaction by lowering the energy gap of FMOs, which is remarkably different from the role of Lewis acid/Lewis base catalysts by changing the FMO overlap modes. Local nucleophilic activity indexes have been computed to explore the origin of regioselectivity. Furthermore, atoms-in-molecules (AIM) analysis on the stereoselectivity-determining CPA-assisted [1,3]-proton transfer process demonstrates that the stronger C–H⋯O interaction and more C–H⋯π interactions are key for controlling the stereoselectivity. The obtained insights should be helpful for recognizing the general nature and regio-/stereo-selectivities of the CPA-catalyzed activation and transformation reactions of ketenes. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] Reactivity of ytterbium and magnesium complexes of gallylenes against carbon dioxide and diphenylketene has been assessed. Ytterbium complex of redox-active gallylene [{(dpp-bian)Ga} 2 Yb(DME) 2 ] (dpp-bian = 1,2-bis[(2,6-diiso-propylphenyl)imino]acenaphthene) on treatment with CO 2 gives complex [(dpp-bian)Ga(DME)Me], while the treatment of magnesium-stabilized gallylene[{(dpp-bian)Ga} 2 Mg(DME) 2 ] with Ph 2 CCO affords cycloadduct [(dpp-bian)(Ph 2 CCO)GaMe]. [ABSTRACT FROM AUTHOR]

The Staudinger synthesis of β‐lactams from imines and ketenes has been conducted photochemically only in very limited cases. Recent advances in the visible light mediated Wolff rearrangement of diazoketones allowed us to perform, for the first time, a 3‐component Staudinger reaction, by mixing aldehydes, amines and diazoketones in the dark and switching the light on after imine formation. The new photoinduced reaction parallels the classic methods in terms of efficiency/selectivity and adds to the existing multicomponent approaches for the preparation of combinatorial libraries of compounds. A thorough optimization of the reaction conditions was performed both for the classic two component reaction and for the novel three component version of the photoinduced Staudinger reaction. [ABSTRACT FROM AUTHOR]

The first examples of Lewis base adducts of the parent boraphosphaketene (H2B‐PCO) and their cyclodimers are prepared. One of these adducts is shown to undergo mild decarbonylation and phosphinidene insertion into a B−C bond of a borole, forming very rare examples of 1,2‐phosphaborinines, B/P isosteres of benzene. The strong donor properties of these 1,2‐phosphaborinines are confirmed by the synthesis of their π complexes with the Group 6 metals. [ABSTRACT FROM AUTHOR]

Little‐explored hydrosilylation of ketenes promoted by main‐group catalysts is reported. The boron Lewis acid tris(pentafluorophenyl)borane accelerates the slow uncatalyzed reaction of ketenes and hydrosilanes, thereby providing a convenient access to the new class of β,β‐di‐ and β‐monoaryl‐substituted aldehyde‐derived silyl enol ethers. Yields are moderate to high, and Z configuration is preferred. The corresponding silyl bis‐enol ethers are also available when using dihydrosilanes. The related trityl‐cation‐initiated hydrosilylation involving self‐regeneration of silylium ions is far less effective. [ABSTRACT FROM AUTHOR]