Feringa studies

The enantioselective conjugate addition of dialkylzinc to nitroalkenes using other phosphoramidite,79,79a 83a sulfonamide,84 and binaphthol-based thioether ligands65 has also been studied in the past few years. Particularly noteworthy are the efficient chiral monodentate phosphoramidite ligands (S,R,R)-29 and (A,A)-55 developed by Feringa et al. and Alexakis et al., respectively, for this reaction. (S,R,R)-29 provided excellent enantioselectivities (up to 98% ee) for acyclic nitroalkenes (Scheme 25).80 It also worked well for other nitroolefin substrates such as 3-nitrocoumarin 7068 and methyl 3-nitropropenoate 7185. 

For an initial study using the phosphora-midite MonoPhos, see M. van den Berg, A.J. Minnaard, R.M. Haak, M. Leeman, E.P. Schudde, A. Meetsma, B.L. Feringa,... 

A subsequent study by Feringa and co-worker (105) revealed an interesting effect when the reaction was conducted in the presence of anthraquinone, Eq. 86. 

In another study Feringa et al. [20] reported a catalytic enantioselective three-component tandem conjugate addition-aldol reaction of dialkyl zincs. Here, zinc enolates were generated in situ via catalytic enantioselective Michael addition of dialkylzinc compounds to cydohexenone in the presence of a chiral Cu catalyst. Their diastereoselective reaction with an aldehyde then gave trans-2,3-disubstituted cyclohexanones in up to 92% yields and up to >99% ees (Scheme 9.11). 

Zijlstra, R. W.J., Grozema F. C., Swart M., Feringa B. L. and Duijnen R Th. van, Solvent Induced Charge Separation in the Excited States of Symmetrical Ethylene A Direct Reaction Field Study. J.Phys.Chem.A (2001) 105 3583-3590. 

We have briefly surveyed the chirality in photochromism. As there are a variei of strategies to incorporate chirality in photochromic systems, it is difficult describe them in a systematic manner. In this chapter, we categorized the chi photochromic systems in terms of the reaction modes and compounds in ord< to show the latest trends of the relevant photochromic compounds. Those wh wish to study this field should refer to general reviews and books on photochri mism [1,93,94] and become familiar with the compounds first. The book b] Feringa [95] should be consulted also. 

Kellogg and Feringa have conducted extensive work on cyclic bis-urea molecules derived from fran5-l,2-diaminocyclohexane and 1,2-diaminoben-zene. In particular they studied aliphatic, aromatic and ester derivatives and demonstrated that some of these molecules gel organic solvents below 1 w/v%. 

The model reaction to be mediated by hybrid catalysts comprised of 32/serum albumins, was the Diels-Alder reaction of the H O-soluble aza-chalcone 33 with cyclopentadiene 34 with formation of chiral adducts 35. This reaction had been originally devised by Engberts [123, 124], who used Cu -complexes of amino acids in aqueous medium ee up to 74%). Later it was employed by Feringa in the study of Cu -conjugates of DNA as catalysts [125]. 

The achiral bisurea derivatives 152-154 possessing terminal benzyl groups and 155-157 with terminal alkyl chains of variable lengths were studied by van Esch and Feringa (Scheme 22) [106,107]. The compounds exhibited efficient gelation of 1- and 2-octanol, toluene, tetralin, -butyl acetate, cyclohexanone, and hexadecane. Gelators were prepared in high yields by reaction of alkyl or benzyl isocyanates with the respective amines. 

However, a dominant disadvantage of the above first generation of unidirectional, light-driven molecular rotors is that the upper part of the molecules is too close to the lower part, that is, the gap of the Qord region is too small. Thermal isomerization of the molecule will meet a considerable steric hindrance, making it take several hours to complete the rotation process and limit the speed of the rotation. In the following studies, Feringa et al. proposed two solutions ... 

A light-activated unidirectional molecular motor was synthesized this contained an oligo(phenylene ethynylene) chassis and an axle system with four carboranes incorporated with Feringa s molecular motor as the chassis. This kind of molecular motor indeed rotates upon irradiation with 365 nm light as evidenced by kinetics studies in solution, providing us with a motorized nanocar (Figure 97). 

In 2014, Feringa, Barta and co-workers reported an IV-alkylation reaction of amines with aliphatic alcohols catalyzed by an iron cyclopentadienone complex 16 [130]. A catalytic cycle was proposed based on in situ NMR studies (Scheme 25). Complex 16 was first transformed to the active complex 17 by addition of oxidant MejNO. Complex 17 was then reduced to 18 by the alcohol. 16 and 18 acted as the catalysts to dehydrogenate the alcohol and hydrogenate the imine intermediates. In 2015, Zhao and co-workers realized an AgF-assisted amination of secondary alcohols using complex 16 as the catalyst [131]. In the same year. Wills and coworkers demonstrated a similar iron complex for A-alkylation of amines with alcohols [132]. 

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