Proline-catalyzed Aldol Reaction Research Articles

Cocatalytic Activity of the Furfuryl and Oxanorbornane-Substituted Guanidines in the Aldol Reaction Catalyzed by (S)-Proline

This work investigated the cocatalytic activity of recently prepared guanidinium salts containing an oxanorbornane subunit in an (S)-proline-catalyzed aldol reaction. The activity was interpreted by the diastereoselectivity of the reaction (anti/syn ratio) and for the most interesting polycyclic guanidinium salt, the enantioselectivity of the reaction was determined. The results indicated a negative impact on the oxanorbornane unit if present as the flexible substituent. For most of the tested aldehydes, the best cocatalysts provided enantioselectivities above 90% and above 95% at room temperature and 0 °C, respectively, culminating in >99.5% for 4–chloro– and 2–nitrobenzaldehyde as the substrate. The barriers for forming four possible enantiomers were calculated and the results for two anti–enantiomers are qualitatively consistent with the experiment. Obtained results suggest that the representatives of furfurylguanidinium and rigid polycyclic oxanorbornane-substituted guanidinium salts are good lead structures for developing new cocatalysts by tuning the chemical space around the guanidine moiety.

Total Synthesis of ( + )-Swainsonine, (–)- Swainsonine, ( + )-8-epi-Swainsonine and ( + )- Dideoxy-Imino-Lyxitol by an Organocatalyzed Aldolization/Reductive Amination Sequence

A tactical combination of either ( S)- or ( R)-proline catalyzed aldol reaction followed by intramolecular reductive amination enabled the synthesis of a chiral pyrrolidine derivative with 3 contiguous stereocenters in only 2 synthetic steps, starting from achiral precursors. This product, obtainable in both enantiomeric forms, was further exploited as a common intermediate in total syntheses of the biologically active iminosugars: ( + )-swainsonine, (–)-swainsonine, ( + )-8- epi-swainsonine, and ( + )-dideoxy-imino-lyxitol.

Effect of Solvents on Proline Modified at the Secondary Sphere: A Multivariate Exploration.

The critical influence of solvent effects on proline-catalyzed aldol reactions has been extensively described. Herein, we apply multivariate regression strategies to probe the influence of different solvents on an aldol reaction catalyzed by proline modified at its secondary sphere with boronic acids. In this system, both in situ binding of the boronic acid to proline and the outcome of the aldol reaction are impacted by the solvent-controlled microenvironment. Thus, with the aim of uncovering mechanistic insight and an ancillary aim of identifying methodological improvements, we designed a set of experiments, spanning 15 boronic acids in five different solvents. Based on hypothesized intermediates or interactions that could be responsible for the selectivity in these reactions, we proposed several structural configurations for the library of boronic acids. Subsequently, we compared the statistical models correlating the outcome of the reaction in different solvents with molecular descriptors produced for each of these proposed configurations. The models allude to the importance of different interactions in controlling selectivity in each of the studied solvents. As a proof-of-concept for the practicality of our approach, the models in chloroform ultimately led to lowering the ketone loading to only two equivalents while retaining excellent yield and enantio- and diastereo-selectivity.

Exploring Free Energy Profiles of Enantioselective Organocatalytic Aldol Reactions under Full Solvent Influence.

We present a computational study on the enantioselectivity of organocatalytic proline-catalyzed aldol reactions between aldehydes in dimethylformamide (DMF). To explore the free energy surface of the reaction, we apply two-dimensional metadynamics on top of ab initio molecular dynamics (AIMD) simulations with explicit solvent description on the DFT level of theory. We avoid unwanted side reactions by utilizing our newly developed hybrid AIMD (HyAIMD) simulation scheme, which adds a simple force field to the AIMD simulation to prevent unwanted bond breaking and formation. Our condensed phase simulation results are able to nicely reproduce the experimental findings, including the main stereoisomer that is formed, and give a correct qualitative prediction of the change in syn:anti product ratio with different substituents. Furthermore, we give a microscopic explanation for the selectivity. We show that both the explicit description of the solvent and the inclusion of entropic effects are vital to a good outcome—metadynamics simulations in vacuum and static nudged elastic band (NEB) calculations yield significantly worse predictions when compared to the experiment. The approach described here can be applied to a plethora of other enantioselective or organocatalytic reactions, enabling us to tune the catalyst or determine the solvent with the highest stereoselectivity.

Probing the gas-phase structure of charge-tagged intermediates of a proline catalyzed aldol reaction - vibrational spectroscopy distinguishes oxazolidinone from enamine species.

An l-proline based catalyst with a charged phenyl-pyridium substituent (1) was used to analyze intermediates of an organocatalyzed aldol reaction by infrared multi-photon dissociation (IRMPD) mass spectrometry after transfer into the gas phase via electrospray ionization (ESI). IRMPD spectra were interpreted with the aid of density functional theory (DFT) computations. A structurally restricted enamine species was used as a reference molecule for the calculated vibrational frequencies. A close correlation between theory and experiment was found for the energetically most favoured oxazolidinone structures.

A reaction energy profile and fragment attributed molecular system energy change (FAMSEC)-based protocol designed to uncover reaction mechanisms: a case study of the proline-catalysed aldol reaction.

A REP-FAMSEC (reaction energy profile-fragment attributed molecular system energy change) protocol designed to explain each consecutive energy change along the reaction pathway is reported. It mainly explores interactions between meaningful polyatomic fragments of a molecular system and, by quantifying energetic contributions, pin-points fragments (atoms) leading to or opposing a chemical change. Its usefulness is tested, as a case study, on the proline-catalysed aldol reaction for which a number of mechanisms have been debated for over four decades. The relative stability of S-proline conformers, their catalytic (in)activity and the superior affinity of the higher energy conformer to acetone is fully explained at atomic and molecular fragment levels, but still appealing to general chemist knowledge. We found that (i) contrary to the generally accepted view, CN-bond formation cannot be explained by the Nδ-, Cδ+ atom pair, but rather by the O-atom of acetone and its strongest inter-molecular attractive interactions with the N-atom as well as the C-atom of the COO group of proline (at this initial stage the lower energy conformer of proline is eliminated) and (ii) the following 'first' H-transfer from N to O atoms of the proline moiety is nearly energy-free even though initially the H-atom interacts three times stronger with the N- than O-atom; a full explanation of this phenomenon is provided.

Liposome Membranes Assist the l-Proline-catalyzed Aldol Reaction of Acetone and p-Nitrobenzaldehyde in Water

An l-Proline (l-Pro)-catalyzed aldol reaction of acetone and p-nitrobenzaldehyde (pNBA) occurs in organic solvents such as DMSO, while the presence of excess water inhibits the reaction. Herein, th...

Enamine/Dienamine and Brønsted Acid Catalysis: Elusive Intermediates, Reaction Mechanisms, and Stereoinduction Modes Based on in Situ NMR Spectroscopy and Computational Studies.

ConspectusOver the years, the field of enantioselective organocatalysis has seen unparalleled growth in the development of novel synthetic applications with respect to mechanistic investigations. Reaction optimization appeared to be rather empirical than rational. This offset between synthetic development and mechanistic understanding was and is generally due to the difficulties in detecting reactive intermediates and the inability to experimentally evaluate transition states. Thus, the first key point for mechanistic studies is detecting elusive intermediates and characterizing them in terms of their structure, stability, formation pathways, and kinetic properties. The second key point is evaluating the importance of these intermediates and their properties in the transition state.In the past 7 years, our group has addressed the problems with detecting elusive intermediates in organocatalysis by means of NMR spectroscopy and eventually theoretical calculations. Two main activation modes were extensively investigated: secondary amine catalysis and, very recently, Brønsted acid catalysis. Using these examples, we discuss potential methods to stabilize intermediates via intermolecular interactions; to elucidate their structures, formation pathways and kinetics; to change the kinetics of the reactions; and to address their relevance in transition states. The elusive enamine in proline-catalyzed aldol reactions is used as an example of the stabilization of intermediates via inter- and intramolecular interactions; the determination of kinetics on its formation pathway is discussed. Classical structural characterization of intermediates is described using prolinol and prolinol ether enamines and dienamines. The Z/E dilemma for the second double bond of the dienamines shows how the kinetics of a reaction can be changed to allow for the detection of reaction intermediates. We recently started to investigate substrate–catalyst complexes in the field of Brønsted acid catalysis. These studies on imine/chiral phosphoric acid complexes show that an appropriate combination of highly developed NMR and theoretical methods can provide detailed insights into the complicated structures, exchange kinetics, and H-bonding properties of chiral ion pairs. Furthermore, the merging of these structural investigations and photoisomerization even allowed the active transition state combinations to be determined for the first time on the basis of experimental data only, which is the gold standard in mechanistic investigations and was previously thought to be exclusively the domain of theoretical calculations.Thus, this Account summarizes our recent mechanistic work in the field of organocatalysis and explains the potential methods for addressing the central questions in mechanistic studies: stabilization of intermediates, elucidation of structures and formation pathways, and addressing transition state combinations experimentally.

Design of thermoresponsive poly(ionic liquid) gels containing proline units to catalyse aldol reaction in water

Ionic liquid monomers and a proline-containing monomer were copolymerised in the presence of cross-linker to form ionic liquid polymer gels that show lower critical solution temperature-type phase change in water. The obtained gels were utilised as catalysts for aldol reaction in water. Since proline-catalysed aldol reaction is composed of three equilibrium steps including dehydration condensation reaction and hydrolysis reaction, yield of product was affected by the water content around the proline unit in the gel. Since thus prepared ionic liquid polymer gels showed the thermo-responsive change in water content, the product yield was found to be affected by the temperature around the phase transition temperature.

Synthesis of Alfaprostol and PGF2α through 1,4-Addition of an Alkyne to an Enal Intermediate as the Key Step.

The veterinary drug Alfaprostol and prostaglandin PGF2α have been synthesized in just nine steps. The strategy involved the conjugate addition of an alkyne to a bicyclic enal, available in three steps by a proline-catalyzed aldol reaction of succinaldehyde. In the case of Alfaprostol, this resulted in the shortest synthesis reported to date. For PGF2α, this approach improved our previous route by making the 1,4-addition and ozonolysis more operationally simple.

Stereoselective de novo synthesis of (5R)-3,4:5,6-di-O-isopropylidene-d-ribo-hexos-5-ulo-5,2-furanose

A concise and stereoselective de novo synthesis of the protected oxidized sugar (5R)-3,4:5,6-di-O-isopropylidene-d-ribo-hexos-5-ulo-5,2-furanose is described. The synthetic sequence involves a stereoselective proline-catalyzed aldol reaction of an orthogonally protected l-glyceraldehyde derivative and 2,2-dimethyl-1,3-dioxan-5-one, to obtain 5-O-acetyl-6-O-benzyl-1,3-isopropylidene-l-psicose as a key intermediate, and the final product in 5 steps and 38% yield.

Efficient Synthesis of Orthogonally Protected Rare l-Hexoses and Derivatives

Stereoselective preparations of protected rare sugars and derivatives, namely <sc>l</sc>-psicose, <sc>l</sc>-altritol, and <sc>l</sc>-talose have been achieved in 70, 44, and 18% yield, respectively, from an orthogonally protected <sc>l</sc>-glyceraldehyde derivative. The key step is the stereoselective proline-catalyzed aldol reaction of this aldehyde with dioxanone.

Parameterization of the ReaxFF reactive force field for a proline-catalyzed aldol reaction.

A parameterization of the ReaxFF reactive FF is performed using a Monte Carlo Simulated Annealing procedure for the modeling of a proline-catalyzed aldol reaction. Emphasis is put on the accurate reproduction of the relative stabilities of several key intermediates of the reaction, as well as, on the description of the reaction path bridging these intermediates based on quantum mechanical calculations. Our training sets include new criteria based on geometry optimizations and short Molecular Dynamics simulations to ensure that the trained ReaxFF potentials adequately predict the structures of all key intermediates. The transferability of the sets of parameters obtained is assessed for various steps of the considered aldol reaction, as well as for different substrates, catalysts, and reagents. This works indeed highlights the challenge of reaching transferable parameters for several reaction steps. © 2016 Wiley Periodicals, Inc.

Kinetics versus thermodynamics in the proline catalyzed aldol reaction.

In this paper the equilibrium properties of the proline catalyzed aldol reaction was studied. The use of well-established methodologies, like reaction progress kinetic analysis and linear free energy relationship analysis, led to the quantification of the reaction reversibility and to its correlation with the substrate electronic activation. Due to these experimental observations, common computational approaches based on a one way transition state analysis become unsuitable. Therefore, a computational model based on the integration of a system of kinetic differential equations associated to the multiple equilibrium reactions was proposed. Such a model was found to successfully rationalize the chemical and stereochemical outcomes of this paradigmatic reaction for the first time.

Accessing the free energy profile of a ring closure in a proline-catalyzed reaction using a reactive force field

The free energy profile of a ring closure involving a carboxylate and an iminium functional group in a proline-derived compound is determined by applying the adaptive biasing force scheme along molecular dynamics simulations. As the reaction considered implies the formation of a covalent bond, the system is modeled with a reactive force field (FF), namely ReaxFF. The impact of the surrounding water molecules on the reaction mechanism is investigated in detail using three FF models for the water molecules. In particular, a hybrid reactive/non-reactive FF is used to assess explicit solvent effects while avoiding solute–solvent reactions. Combined with existing experimental observations, our results provide an explanation for the role of water molecules in the proline-catalyzed aldol reaction, which is one of the hallmark reactions in organocatalysis.

Proline-catalyzed aldol reaction of acetone with (R)- and (S)-2,3-O-alkylideneglyceraldehydes

The results of proline-catalyzed aldol reaction of acetone with O,O′-alkylidene derivatives of glyceraldehyde are determined by the configuration of the chiral center in the aldehyde. The reaction of (S)-2,3-O-alkylideneglyceraldehyde with acetone in the presence of L-proline was characterized by higher diastereoselectivity.

Synthesis of 7-epi-Goniodiol by Proline-Catalyzed Diastereoselective Direct Aldol Reaction

A simple organocatalytic approach was developed for the total synthesis of 7- epi -goniodiol. The strategy involves l -proline-catalyzed diastereoselective direct aldol reaction and Baeyer–Villiger oxidation as key steps for the construction of chiral lactone.

Unexpected Beneficial Effect of ortho-Substituents on the (S)-Proline-Catalyzed Asymmetric Aldol Reaction of Acetone with Aromatic Aldehydes

An intriguing effect of electronegative 2,6-substituents on the stereochemical outcome of (S) -proline-catalyzed aldol reactions between benzaldehyde derivatives and acetone is reported. Remarkably high enantioselectivities can be achieved with such substrates.

Improved Conditions for the Proline-Catalyzed Aldol Reaction of Acetone with Aliphatic Aldehydes

The proline-catalyzed asymmetric aldol reaction between aliphatic aldehydes and acetone has, to date, remained underdeveloped. Challenges in controlling this reaction include avoiding undesired side reactions such as aldol condensation and self-aldolization. In recent years we have developed optimized conditions, which enable high yields and good to excellent enantioselectivities, and which are presented in this communication.

Quantum mechanical investigations on the role of neutral and negatively charged enamine intermediates in organocatalyzed reactions

The proline-catalyzed aldol reaction is the seminal example of asymmetric organocatalysis. Previous theoretical and experimental studies aimed at identifying its mechanism in order to rationalize the outcome of this reaction. Here, we focus on key steps with modern first principle methods, i.e. the M06-2X hybrid exchange–correlation functional combined to the solvation density model to account for environmental effects. In particular, different pathways leading to the formation of neutral and negatively charged enamine intermediates are investigated, and their reactivity towards two electrophiles, i.e. an aldehyde and a benzhydrylium cation, are compared. Regarding the self-aldol reaction, our calculations confirm that the neutral enamine intermediate is more reactive than the negatively charged one. For the reaction with benzhydrylium cations however, the negatively charged enamine intermediate is more reactive.