New Strategies for the Development of an Asymmetric Version of the Baylis-Hillman Reaction This work was supported by the Fonds der Chemischen Industrie (Liebig-scholarship and funds for P. L.). P. L. thanks Prof. Dr. A. de Meijere for his support

Unravelling the intramolecular n → σ* interaction in ultra-electron deficient naphthalenediimides and their radical ions

In this work, the role of non-covalent n → σ* intramolecular interactions in bestowing stabilization to exceptionally low-lying LUMO molecules of Naphthalenediimides (NDI) and their radical ions have been investigated utilizing different electronic structure calculation methods at the DFT level of theory and X-ray crystallography. We compared the effect of electron donating groups (EDGs) and electron-withdrawing groups (EWGs), e.g., OMe and F, respectively, at the para-positon of the phenylphosphonium groups integrated at the 2,6-positions of the NDI scaffold on the intramolecular P-O interactions and the evolving electronic effects. The natural bond orbital (NBO) analysis exhibited strong charge transfer from the imide O atoms of the NDI to the phosphorus atom of the phosphonium groups validating a donor-acceptor type of orbital interaction. Atoms in molecule (AIM) analysis also illustrated the strong P-O interaction, as the charge density, ρ(r), and the Laplacian of the density ∇2ρ(rb) at BCP is within the range of well-known non-covalent interactions. These calculations also revealed long-range electronic communication between the EDGs/EWGs at the phenyl groups and the NDI scaffold.

Diastereoselective 1,3-dipolar intramolecular nitrone olefin cycloaddition (INOC) reaction of a sugar-derived allyl alcohol: Synthesis of functionalized aminocyclopentitols

The DIBAL-H reduction of the Baylis-Hillman sugar adduct, obtained from 3-O-benzyl-1,2-isopropylidene-α-D-xylo-pentodialdo-1,4-furanose yielded trisubstituted alkenes by eliminating the β-hydroxyl group. Subsequently, the hydrolysis of the isopropylidene acetal to the corresponding hemiacetal was reacted with N-benzyl hydroxylamine hydrochloride to generate the nitrone, which underwent diastereoselective intramolecular 1,3-dipolar nitrone olefin cycloaddition (INOC) to give an isoxazolidine skeleton. The concomitant reductive cleavage of the N-O bond and benzyl group of the fused isoxazolidines afforded new functionalized aminocyclopentitols in good yields.

Morita–Baylis–Hillman reaction of 3-formyl-9H-pyrido[3,4-b]indoles and fluorescence studies of the products

β-Carboline is a privileged class of the alkaloid family and is associated with a broad spectrum of biological properties. 3-Formyl-9H-pyrido[3,4-b]indole is a such potent precursor belonging to this family which can be tailored for installing diversity at various positions of β-carboline to generate unique molecular hybrids of biological importance. The present work is a step towards this and assimilates the results related to the exploration of 3-formyl-9H-β-carbolines for the synthesis of β-carboline C-3 substituted MBH adducts followed by evaluation of their fluorescent characteristic. The effect of contact time, solvent system, concentration and substituents was also studied during investigation of fluorescence properties of these derivatives.

Brucine Diol-Catalyzed Enantioselective Morita-Baylis-Hillman Reaction in the Presence of Brucine N-Oxide

Brucine diol (BD) catalyzed asymmetric Morita–Baylis–Hillman (MBH) reaction is observed for the first time. Brucine N-oxide (BNO) was found to not have an effective chiral catalyst. Faster reaction rate was obtained using unsaturated ester or aromatic aldehydes in the presence of BNO. 4-Nitrobenzaldehyde and α,β-unsaturated ketone/ester were converted to the MBH adduct in moderate yields (up to 74%) with 70% ee value by this catalytic system. The mechanism of BD catalysis is probably initiated by conjugating the vicinal diol of BD to the carbonyl group of the aromatic aldehyde through hydrogen bonding. The tertiary amine of BD acts as a nucleophile to activate vinyl ketone for coupling with the carbonyl of aldehyde through an intramolecular carbonylated reaction. Finally, the breakdown of the complex caused the formation of the MBH adduct (a benzyl-allyl alcohol). The chirality of the benzyl-allyl alcohol is likely affected by the interaction of the bulky asymmetric plane of BD.

Phosphine-Catalyzed (4+1) Annulation: Rearrangement of Allenylic Carbamates to 3-Pyrrolines through Phosphonium Diene Intermediates

We have developed a phosphine-catalyzed (4+1) annulative rearrangement for the preparation of 3-pyrrolines from allenylic carbamates via phosphonium diene intermediates. We employed this methodology to synthesize an array of 1,3-disubstituted- and 1,2,3-trisubstituted-3-pyrrolines, including the often difficult to prepare 2-alkyl variants. A mechanistic investigation employing allenylic acetates and mononucleophiles unexpectedly unveiled that a phosphine-catalyzed (4+1) reaction for the construction of cyclopentene products, previously reported by Tong, might not occur through a phosphonium diene, as had been proposed, but rather through multiple mechanisms working in concert. Consequently, our phosphine-catalyzed rearrangement is most likely the first transformation to involve the unequivocal formation of a phosphonium diene intermediate along the reaction pathway. To demonstrate the synthetic utility of this newly developed reaction, we have completed concise formal syntheses of the pyrrolizidine alkaloids (±)-trachelanthamidine and (±)-supinidine.

A Novel Two-Dimensional Non-Interpenetrating Coordination Polymer [Ag2.5L(NO3)2.5]∞ with three different coordination modes of AgI (L = diquinoxalino[2,3-a:2′,3′-c]phenazine)

A novel 2-D AgI neutral infinite framework of a large delocalised system diquinoxalino[2,3-a:2′,3′-c]phenazine (L), [Ag2.5L(NO3)2.5]∞ (1), has been synthesised and structurally determined, in which three kinds of different coordination modes are found for the AgI centers.

Studies with Functionally Substituted Enamines: Synthesis of New Aminoazolo-Pyrimidines and -1,2,4-Triazines

3-Aminoacrylonitrile derivatives (1d–g) coupled with aromatic and heteroaromatic diazonium salts yielding arylhydrazonals and pyrazolo[5,1-c]triazines. The enaminonitriles 1d–g condensed to form dienes 6a–c on reflux in acetic acid. The latter underwent Diels-Alder type addition to naphthoquinone. Aminopyrazolopyrimidines were obtained from reaction of 1d–g with heteroaromatic aminoazoles. Enaminonitrile 1d formed pyran 21 with benzylidene malononitrile, and dihydropyrimidine 22 with benzaldehyde and urea.

An Extended Network via Hydrogen Bond Linkage of the Linear Coordination Polymer [Cd(μ-dptz)(NO3)2]∞

The crystal structure of [Cd(μ-dptz)(NO3)2]∞ reveals a neutral one-dimensional coordination polymer exhibiting a two-dimensional architecture through inter-chain hydrogen-bonding.

DABCO- and DBU-promoted one-pot reaction of N-sulfonyl ketimines with Morita-Baylis-Hillman carbonates: a sequential approach to (2-hydroxyaryl)nicotinate derivatives

An intriguing DABCO-catalyzed and DBU-promoted one-pot synthesis of an important class of (2-hydroxyaryl)pyridine derivatives bearing a carboxylate or a nitrile group suitably placed at C3 position of the aza-ring has been achieved in acceptable chemical yields with a broad functional group tolerance. This sequential C-C/C-N bond making process proceeds through a regioselective allylic alkylation/aza-Michael reaction between MBH carbonates derived from an acrylate/acrylonitrile and N-sulfonyl ketimines as C,N-binucleophiles catalyzed by DABCO, followed by elimination of SO2 under the influence of base and subsequent aromatization in an open atmosphere.

Phosphine Organocatalysis

The hallmark of nucleophilic phosphine catalysis is the initial nucleophilic addition of a phosphine to an electrophilic starting material, producing a reactive zwitterionic intermediate, generally under mild conditions. In this Review, we classify nucleophilic phosphine catalysis reactions in terms of their electrophilic components. In the majority of cases, these electrophiles possess carbon-carbon multiple bonds: alkenes (section 2), allenes (section 3), alkynes (section 4), and Morita-Baylis-Hillman (MBH) alcohol derivatives (MBHADs; section 5). Within each of these sections, the reactions are compiled based on the nature of the second starting material-nucleophiles, dinucleophiles, electrophiles, and electrophile-nucleophiles. Nucleophilic phosphine catalysis reactions that occur via the initial addition to starting materials that do not possess carbon-carbon multiple bonds are collated in section 6. Although not catalytic in the phosphine, the formation of ylides through the nucleophilic addition of phosphines to carbon-carbon multiple bond-containing compounds is intimately related to the catalysis and is discussed in section 7. Finally, section 8 compiles miscellaneous topics, including annulations of the Hüisgen zwitterion, phosphine-mediated reductions, iminophosphorane organocatalysis, and catalytic variants of classical phosphine oxide-generating reactions.

Phosphine-Catalyzed Asymmetric Umpolung Addition of Trifluoromethyl Ketimines to Morita-Baylis-Hillman Carbonates

A novel phosphine-catalyzed, highly enantioselective umpolung addition of trifluoromethyl ketimines to Morita-Baylis-Hillman carbonates was developed and it provides facile access to optically active trifluoromethyl amines with a chiral tertiary stereocenter under mild reaction conditions. The salient features of this reaction include general substrate scope, mild reaction conditions, good yields, high enantioselectivity, ease of scale-up to gram scale, and further transformations of the products.

Mechanism and reactivity in the Morita-Baylis-Hillman reaction: the challenge of accurate computations

A systematic density functional theory exploration of various reactive steps together with benchmark coupled cluster results are used to propose an accurate model of the mechanism of the Morita-Baylis-Hillman (MBH) reaction in organic chemistry. This reaction has attracted considerable interest from the synthetic and mechanistic points of view in recent years, with both computational and experimental mechanistic studies. It has recently (R. E. Plata and D. A. Singleton, J. Am. Chem. Soc., 2015, 137, 3811-3826) been correctly pointed out that previous computational studies failed to reproduce known mechanistic features of the reaction. The same study argued that computation is simply not able at the present time to provide accurate models for such reactions. This second claim is shown by our present work to overstate the problem: by using current 'state of the art' methodology, our results are fully consistent with observed behavior within the expected error bars of 1-5 kcal mol^-1, far smaller than the errors reported in Plata and Singleton's study. On the basis of exhaustive calculations reported here, we suggest that our proposed approaches for modeling electronic structure, solvation, and entropy should be able to provide accurate predictions for many more reactions. We also suggest that reactions like the MBH reaction, where solvation and entropy effects are particularly large, are among the hardest for computational mechanistic studies.

Phosphine catalyzed δ-carbon addition and isomerization of alkynones to ketimines: the preparation of 1,3-diene substituted dihydroquinazolinones and 3-aminooxindoles

A novel strategy of the phosphine catalyzed addition of alkynones to ketimines for the synthesis of α-amino diene derivatives has been developed for the first time, thus providing diene incorporating α-amino products in moderate to excellent yields.

Catalytic Asymmetric Synthesis of Butenolides and Butyrolactones

γ-Butenolides, γ-butyrolactones, and derivatives, especially in enantiomerically pure form, constitute the structural core of numerous natural products which display an impressive range of biological activities which are important for the development of novel physiological and therapeutic agents. Furthermore, optically active γ-butenolides and γ-butyrolactones serve also as a prominent class of chiral building blocks for the synthesis of diverse biological active compounds and complex molecules. Taking into account the varying biological activity profiles and wide-ranging structural diversity of the optically active γ-butenolide or γ-butyrolactone structure, the development of asymmetric synthetic strategies for assembling such challenging scaffolds has attracted major attention from synthetic chemists in the past decade. This review offers an overview of the different enantioselective synthesis of γ-butenolides and γ-butyrolactones which employ catalytic amounts of metal complexes or organocatalysts, with emphasis focused on the mechanistic issues that account for the observed stereocontrol of the representative reactions, as well as practical applications and synthetic potentials.

Regioselective addition–elimination of Morita–Baylis–Hillman adducts with 2-naphthol or phenol catalyzed by functionalized ionic liquids: a direct strategy to construct functional alkenes

A highly regioselective addition–elimination of Morita–Baylis–Hillman adducts promoted by base ionic liquids (0.5 mol%) was reported to afford a variety of functional alkenes in excellent yields (up to 99%). This protocol provides a new method to access functional alkenes directly. Recycled base ionic liquids could be reused at least five times and the isolated yield of the product was almost consistent after five runs.

A mechanistic study of carbonyl activation under solvent-free conditions: evidence drawn from the synthesis of imidazoles

An assembly of carbonyls held together by weak-bond interactions (Constitutional Dynamic Chemistry) results in bulk polarisation. Molecules participate in reactions in a cycle rather than as individual moieties leading to higher reaction rates.

Synthesis of furo[3,4-c]quinolin-3(1H)-one derivatives through TMG catalyzed intramolecular aza-MBH reaction based on the furanones

The first TMG catalyzed intramolecular aza-MBH reaction based on furanone derivatives for the construction of furo[3,4-c]quinolin-3(1H)-ones scaffold.

Construction of spirocycles containing highly substituted pyrroli-dine and 1-indanone motifs with spiro quaternary stereogenic centers via 1,3-dipolar cycloaddition of 2-alkylidene-1-indanone and azomethine ylides promoted by simple imidazolium salts

The 1,3-dipolar cycloaddition of 2-alkylidene-1-indanone with azomethine ylides has been successfully developed promoted by simple imidazolium salts to construct a variety of spirocycles, containing highly substituted pyrrolidines, in excellent yields (up to 99%).

Acetaldehyde in asymmetric organocatalytic transformations

This review summarize both the recent developments in the organocatalysed use of acetaldehyde as a substrate in various organic transformations and its application in the synthesis of bioactive molecules.

Advances in nucleophilic phosphine catalysis of alkenes, allenes, alkynes, and MBHADs

In nucleophilic phosphine catalysis, tertiary phosphines undergo conjugate additions to activated carbon-carbon multiple bonds to form β-phosphonium enolates, β-phosphonium dienolates, β-phosphonium enoates, and vinyl phosphonium ylides as intermediates. When these reactive zwitterionic species react with nucleophiles and electrophiles, they may generate carbo- and heterocycles with multifarious molecular architectures. This article describes the reactivities of these phosphonium zwitterions, the applications of phosphine catalysis in the syntheses of biologically active compounds and natural products, and recent developments in the enantioselective phosphine catalysis.