Chiral N-alkyl Research Articles

A Nitrogen Out-of-Plane (NOOP) Mechanism for Imine-Based Light-Driven Molecular Motors

Light-driven molecular motors have generated considerable interest due to their potential applications in material and biological systems. Recently, Greb and Lehn reported a new class of molecular motors, chiral N-alkyl imines, which undergo unidirectional rotation induced by light and heat. The mechanism of unidirectional motion in molecular motors containing a C═N group has been assumed to consist of photoinduced torsion about the double bond. In this work, we present a computational study of the photoisomerization dynamics of a chiral N-alkyl imine motor. We find that the location and energetics of minimal energy conical intersections (MECIs) alone are insufficient to understand the mechanism of the motor. Furthermore, a key part of the mechanism consists of out-of-plane distortions of the N atom (followed by isomerization about the double bond). Dynamic effects and out-of-plane distortions are critical to understand the observed (rather low) quantum yield for photoisomerization. Our results provide hints as to how the photoisomerization quantum yield might be increased, improving the efficiency of this class of molecular motors.

Synthesis of New Chiral N-alkyl Betti-base Ligands via One-Pot Three Component Strategy

Background: Betti base derivatives were employed as chiral ligands or as chiral auxiliaries in asymmetric transformations. In particular, N-alkyl Betti base derivatives are gaining importance as excellent ligands in asymmetric reactions. However, no appropriate method exists for regioselective N-alkylation of Betti base. Therefore, there is an urgent need to develop a new protocol for the regioselective N-alkylation of chiral Betti-base. Methods: An efficient method has been developed for one-pot synthesis of chiral N-alkyl Bettibase ligands from (S)-(+)-Betti base, arylglyoxals and cyclic 1,3-C,N-binucleophiles under catalyst- free conditions. Results: The chiral N-alkyl Betti-base ligands were obtained in good yields with excellent enantioselectivities. This method is rapid, clean and does not require a catalyst or chromatographic purification. Conclusion: We have developed an efficient one-pot three component strategy for the synthesis of new chiral N-alkyl Betti-base ligands from (S)-(+)-Betti base, arylglyoxals and cyclic 1,3-C,Nbinucleophiles in the absence of a catalyst.

Exploring the Mechanism of a Chiral N-Alkyl Imine-Based Light-Driven Molecular Rotary Motor at MS-CASPT2//CASSCF and MS-CASPT2//(TD) DFT Levels.

The working mechanism including the photoisomerization and thermal isomerization steps of a chiral N-alkyl imine-based motor synthesized by Lehn et al. are revealed by MS-CASPT2//CASSCF and MS-CASPT2//(TD-)DFT methods. For the photoisomerization process of the imine-based motor, it involves both the bright (π,π*) state and the dark (n,π*) state. In addition, the MECI has similar geometry and energy to the minimum of the S1 state, which shows that the process is barrierless and keeps the unidirectionality of rotation well; the result confirms the imine-based motor is a good candidate for a light-driven molecular rotary motor. For the thermal isomerization process of the imine-based motor, there are two even isomerization paths: one with the mechanism of the in-plane N inversion, the energy barriers of which are 29.6 kcal mol-1 at MS3-CASPT2//CAM-B3LYP level and 29.2 kcal mol-1 at MS3-CASPT2//CASSCF level; the other with the mechanism of ring inversion of the cycloheptatriene moiety, with energy barriers of 28.1 kcal mol-1 at MS3-CASPT2//CAM-B3LYP level and 18.1 kcal mol-1 at MS3-CASPT2//CASSCF level. According to the structural feature of the stator moiety, the imine molecule can be used as a two-step or a four-step light-driven rotary motor.

Light-driven molecular motors: imines as four-step or two-step unidirectional rotors.

Chiral N-alkyl imines undergo unidirectional rotation induced by light and heat, thus providing a new class of molecular motors. Depending on the conformational flexibility of the stator part (the carbonyl residue) and the nitrogen inversion barrier of the rotor part (the amine residue) in the molecule, the operation mode of the motor can be controlled as either a four- or a two-step cycling motion of the rotor part.

Solvent‐ and Temperature‐Dependent Functionalisation of Enantioenriched Aziridines

A highly stereo- and regioselective functionalisation of chiral non-racemic aziridines is reported. By starting from a parent enantioenriched aziridine and finely tuning the reaction conditions, it is possible to address the regio- and stereoselectivity of the lithiation/electrophile trapping sequence, thereby allowing the preparation of highly enantioenriched functionalised aziridines. From chiral N-alkyl trans-2,3-diphenylaziridines (S,S)-1 a,b, two differently configured chiral aziridinyllithiums could be generated (trans-1 a,b-Li in toluene and cis-1 a,b-Li in THF), thus disclosing a solvent-dependent reactivity that is useful for the synthesis of chiral tri-substituted aziridines with different stereochemistry. In contrast, chiral aziridine (S,S)-1 c showed a temperature-dependent reactivity to give chiral ortho-lithiated aziridine 1 c-ortho-Li at -78 °C and α-lithiated aziridine 1 c-α-Li at 0 °C. Both lithiated intermediates react with electrophiles to give enantioenriched ortho- and α-functionalised aziridines. The reaction of all the lithiated aziridines with carbonyl compounds furnished useful chiral hydroxyalkylated derivatives, the stereochemistry of which was ascertained by X-ray and NMR spectroscopic analysis. The usefulness of chiral non-racemic functionalised aziridines has been demonstrated by reductive ring-opening reactions furnishing chiral amines that bear quaternary stereogenic centres and chiral 1,2-, 1,3- and 1,5-aminoalcohols. It is remarkable that the solvent-dependent reactivity observed with (S,S)-1 a,b permits the preparation of both the enantiomers of amines (11 and ent-11) and 1,2-aminoalcohols (13 and ent-13) starting from the same parent aziridine. Interestingly, for the first time, a configurationally stable chiral α-lithiated aziridine (1 c-α-Li) has been generated at 0 °C. In addition, ortho-hydroxyalkylated aziridines have been easily converted into chiral aminoalkyl phthalans, which are useful building blocks in medicinal chemistry.

General Access to Chiral N-Alkyl Terminal Aziridines via Organocatalysis

A three step, one-pot protocol involving enantioselective alpha-chlorination of aldehydes, subsequent reductive amination with a primary amine, and S(N)2 displacement to afford chiral N-alkyl terminal aziridines in 40-65% yield (74-87%/step) and, in most cases, >90% ee is reported.

Novel chiral C1-1′,2′,3′,4′-tetrahydro-1,1′-bisisoquinolines: synthesis, resolution, and applications in catalytic enantioselective reactions

A straightforward synthesis of a structurally constrained C 1-1′,2′,3′,4′-tetrahydro-1,1′-bisisoquinoline 1 is described. Resolution of this compound has been achieved successfully. The preparation of chiral N-alkyl, urea, and thiourea derivatives as potential new chiral ligands, based on the parent compound 1, is reported. Chiral compound 1 induced very good selectivity and yield in the addition of either Et 2Zn (85% ee, 96% yield) or nitromethane (85% ee, 60% yield) to benzaldehyde.

Enantioselective Synthesis of Chiral Tetrahydroisoquinolines by Iridium-Catalyzed Asymmetric Hydrogenation of Enamines

Chiral iridium complexes based on spiro phosphoramidite ligands are demonstrated to be highly efficient catalysts for the asymmetric hydroge- nation of unfunctionalized enamines with an exocy- clic double bond. In combination with excess iodine or potassium iodide and under hydrogen pressure, the complex Ir/ASa,R,R)-3a provides chiral N-alkyl- Anolines in high yields with up to 98% ee. The L/Ir ratio of 2:1 is crucial for obtaining a high reaction rate and enantioselectivity. A deute- rium labeling experiment showed that an inverse iso- tope effect exists in this reaction. A possible catalytic cycle including an iridiumA species bearing two monophosphoramidite ligands is also proposed.

Total synthesis of (+)-batzelladine A and (-)-batzelladine D via [4 + 2]-annulation of vinyl carbodiimides with N-alkyl imines.

A diastereoselective [4 + 2]-annulation of vinyl carbodiimides with chiral N-alkyl imines has been developed to access the stereochemically rich polycyclic guanidine cores of the batzelladine alkaloids. Application of this strategy, together with additional key steps such as long-range directed hydrogenation and diastereoselective intramolecular iodo-amination, led to highly convergent total syntheses of (-)-batzelladine D and (+)-batzelladine A with excellent stereocontrol.

Diastereoselective [4 + 2] Annulation of Vinyl Carbodiimides with N‐Alkyl Imines. Asymmetric Synthetic Access to the Batzelladine Alkaloids.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Role Reversal in a Supramolecular Assembly: a Chiral Cyanine Dye Controls the Helicity of a Peptide−Nucleic Acid Duplex

A new dicarbocyanine dye bearing branched, chiral N-alkyl substituents was synthesized and its ability to form helical aggregates on peptide nucleic acid (PNA) double-helical templates was studied. The dye aggregates less effectively than an analogous dye bearing linear, achiral substituents, presumably due to steric problems with packing the branched substituents compared with the linear substituents. When the PNA duplex has a left-handed helicity, addition of the achiral dye leads to formation of a left-handed dye aggregate. However, when the chiral dye aggregates in the presence of this duplex, a right-handed structure is formed, suggesting that the dye alters the helicity of the underlying template. When a racemic PNA duplex (i.e., equal amounts of right- and left-handed helices) is used, no chirality is observed for the dye aggregate formed by the achiral dye but a right-handed helical aggregate is once again formed by the chiral dye. These results indicate that chirality is transferred from the dye to the PNA, as opposed to other examples of polymer-templated dye aggregation where chirality is transferred from the template to the dye.

Diastereoselective [4+2] Annulation of Vinyl Carbodiimides with N-Alkyl Imines. Asymmetric Synthetic Access to the Batzelladine Alkaloids

A diastereoselective [4+2] annulation of vinyl carbodiimides with chiral N-alkyl imines has been developed to access the stereochemically rich tricyclic core of the batzelladine alkaloids. Its application to the asymmetric synthesis of batzelladine D permitted the use of long-range, directed hydrogenation and stereoselective intramolecular iodoamination as additional key steps to establish the remaining stereocenters within the natural product with excellent stereocontrol.

Oxygen atom transfer from a chiral N-alkyl oxaziridine promoted by acid. The asymmetric oxidation of sulfides to sulfoxides

Chiral N-alkyl oxaziridines may be used as reagents for the asymmetric oxidation of sulfides in an acid-promoted reaction leading exclusively to the corresponding sulfoxides A planar transition state geometry seems consistent with the observed stereochemistry which should result from the steric interactions in the transition state. The influence of the solvent and the acid strength on the oxygen transfer reaction are discussed.

Synthesis of Ferroelectric Liquid Crystalline Polysiloxanes Having a Chiral n-Alkyl Tolansulfinate as the Pendant Group

The synthesis and mesomorphic properties of a series of ferroelectric liquid crystalline polysiloxanes bearing chiral 4-[(4-((n-alkyloxy)sulfinyl)phenyl)ethynyl]phenyl 4-(undecyloxy)benzoate as mesogenic pendant groups are presented. In these polymers, the chirality is introduced via an asymmetric sulfur atom. The synthesis of the materials was possible through three successive polymer-analogous reactions. The last key synthetic step is the polyesterification of the poly((undecyloxy)benzoic acid-methylsiloxane) with the n-alkyl 4-[(4-hydroxyphenyl)ethynyl]benzenesulfinate derivatives, which could be carried out efficiently. Up to 97% overall substitution rates of the siloxane units by the mesogenic moiety could thus be obtained. In this paper are also reported the results of the preliminary investigations of the ferroelectric properties, carried out in the S c * phase of one polymer. Finally, the comparison of the mesomorphic properties of a sulfinate-based polymer and molecule with their carboxylate-based counterparts is briefly reviewed.