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Copper(II) acetylacetonate immobilized in ionic liquids efficiently catalyzes the aza-Michael reaction of amines with a,b-unsaturated carbonyl compounds to produce the corresponding b-amino carbonyl compounds with great alacrity in excellent yields. The reactions are far more facile than those reported earlier. The recovered ionic liquid phase containing the copper catalyst can be reused for several cycles with consistent activity.Keywords: amides; amines; b-amino ketones; aza-Michael reaction; copper(II) acetylacetonate; esters; ionic liquids; a,b-unsaturated carbonyl compoundsThe aza-Michael reaction is one of the important reactions in organic chemistry especially for the synthesis of C À N heterocycles [1,2] containing the b-amino carbonyl functionality. Such a functionality [3] not only constitutes a component of biologically active natural products but also serves as an essential intermediate in the synthesis of b-amino ketones, b-amino acids and b-lactam antibiotics, in addition to its use in the fine chemicals and pharmaceutical sectors.[4] Because of the intrinsic importance of b-amino carbonyl compounds, they have attracted sustained attention in organic synthesis, and the methods of construction of the functionality have undergone a metamorphosis from the classic Mannich-type reactions to the more widely used conjugated addition of nitrogen nucleophiles to a,b-unsaturated carbonyl compounds [5] which is commonly known as the aza-Michael reaction. The classical Mannich-type reactions are certainly very powerful but need quite severe reaction conditions and are rather sluggish thereby limiting their use in practice.[6] The conjugated addition reactions are, in the contrast, atom economic and quite easy to operate. However, these reactions require either basic conditions [7] or acidic catalysts [8] which seem to be detrimental to the desired synthesis. In order to overcome some of the disadvantages, a good number of alternative procedures have been reported over the past few years using Yb(OTf) 3 , [9] and heterogeneous solid acids, [16] but the need for an environmentally benign and facile protocol still exists. Room temperature ionic liquids are acknowledged as eco-benevolent alternatives to volatile organic solvents and have also other useful properties like very low vapor pressure, wide liquid range, high thermal stability and possess highly conductive solvation ability for a variety of organic substrates and catalysts including Lewis acids and enzymes. [17,18] Recently, ionic liquids [19] and quaternary ammonium salts in water [19b] have been reported to efficiently catalyze the aza-Michael reaction. Considering all these, it was thought that a combination of a suitable catalyst immobilized in an ionic liquid would enable facile conjugate additions, as desired. Incidentally, our attention was drawn to this problem at a juncture when we were engaged [20] in the aziridination of olefins catalyzed by Cu(acac) 2 immobilized in ionic liquids. To the best of our knowledge, aza-Michael reaction...

“Equatorial” energy transfer: Orientation control of intramolecular energy transfer (EnT) is found in the donor–acceptor system shown, light absorbed by the “peripheral” anthracene is efficiently transferred to the porphyrin, but that by the “axial” anthracene subunits is not.

The rates of electronic energy transfer (EET) reactions mediated by both dipole ± dipole and electron-exchange mechanisms have long been postulated to be critically dependent, amongst other things, on the distance and mutual orientation between the donor and acceptor moieties. [1, 2] Recent experimental validation of these theoretical predictions comes from energy-transfer studies carried out with rationally designed, photochemically active donor ± acceptor (D ± A) assemblies. [3] Amongst such D ± A systems, those based on the porphyrinoid class of chromophores are of relevance to the present work. A number of studies carried out with the covalently/noncovalently linked porphyrin ± porphyrin [3b,c, 4] and porphyrin ± nonporphyrinic chromophore [5] assemblies have dealt with the effect of D ± A distance on the rates of EET reactions but, relatively less attention has been paid to the corresponding orientation effects. In addition, to our knowledge, orientation dependence of EET has not been unequivocally demonstrated in a porphyrinbased system where the donor and the acceptor subunits are disposed at two distinctly different orientations in a given D ± A ensemble. Here, we demonstrate the orientation dependence of energy transfer in a simple, porphyrin-based, D ± A system 3 where the donor anthracene subunits are linked both at the axial and peripheral sites of a tin(iv) porphyrin scaffold, Scheme 1. Fluorescence-emission and excitation spectra reveal that light absorbed by the ªperipheralº anthracene unit of 3 is efficiently transferred to the porphyrin but, that absorbed by the ªaxialº anthracene subunits is not.The syntheses of 3 and the corresponding ªreferenceº compounds 1 (where the anthracene subunit occupies only a peripheral position) and 2 (where the anthracene subunits occupy only the axial sites) are illustrated in Scheme 1. These new porphyrins have been characterized by elemental analysis, UV/Vis, 1 H and 13 C NMR spectroscopy, and electrochemical methods. In the 1 H NMR spectra, the spacer methylene protons connecting the porphyrin and anthracene chromophores of 1 resonate at d 6.33, whereas the corresponding axial methylene protons of 2 resonate at d 5.65 a result of the ring-current effect exerted by the basal porphyrin macrocycle. In the spectrum of 3, resonances arising from the two peripheral and the four axial methylene protons are at d 6.32 and 5.62, respectively. The UV/Vis spectra of 1, 2 and 3 (in CH 2 Cl 2 ) are nearly equivalent to the summation of the with amide carbonyl oxygen atoms is likely to lead to strong mixing of the np* and pp* transitions in those amide groups.In conclusion, some helical structures formed by simple polypeptides in solution may have unusually short main-chain hydrogen bonds. Our calculations indicate that the helicity of such peptides will be over-estimated by CD analysis based on literature calibration of helical content. Whilst much is still to be established, if short hydrogen bonds can form under certain conditions, the use of CD to estimate helicity wil...

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