Modular cyclodiphosph(V)azanes are synthesised and their affinity for chloride and actetate anions were compared to those of a bisaryl urea derivative (1). The diamidocyclodiphosph(V)azanes cis-[{ArNHP(O)(μ-tBu)}2 ] [Ar=Ph (2) and Ar=m-(CF3 )2 Ph (3)] were synthesised by reaction of [{ClP(μ-NtBu)}2 ] (4) with the respective anilines and subsequent oxidation with H2 O2 . Phosphazanes 2 and 3 were obtained as the cis isomers and were characterised by multinuclear NMR spectroscopy, FTIR spectroscopy, HRMS and single-crystal X-ray diffraction. The cyclodiphosphazanes 2 and 3 readily co-crystallise with donor solvents such as MeOH, EtOH and DMSO through bidentate hydrogen bonding, as shown in the X-ray analyses. Cyclodiphosphazane 3 showed a remarkably high affinity (log[K]=5.42) for chloride compared with the bisaryl urea derivative 1 (log[K]=4.25). The affinities for acetate (AcO(-) ) are in the same range (3: log[K]=6.72, 1: log[K]=6.91). Cyclodiphosphazane 2, which does not contain CF3 groups, exhibits weaker binding to chloride (log[K]=3.95) and acetate (log[K]=4.49). DFT computations and X-ray analyses indicate that a squaramide-like hydrogen-bond directionality and Cα H interactions account for the efficiency of 3 as an anion receptor. The Cα H groups stabilise the Z,Z-3 conformation, which is necessary for bidentate hydrogen bonding, as well as coordinating with the anion.
SummaryTen novel hydrogen-bonding catalysts based on open-chain PV-amides of BINOL and chinchona alkaloids as well as three catalysts based on rigid cis-PV-cyclodiphosphazane amides of N
1,N
1-dimethylcyclohexane-1,2-diamine have been developed. Employed in the asymmetric Michael addition of 2-hydroxynaphthoquinone to β-nitrostyrene, the open-chain 9-epi-aminochinchona-based phosphorus amides show a high catalytic activity with almost quantitative yields of up to 98% and enantiomeric excesses of up to 51%. The cyclodiphosphazane catalysts show the same high activity and give improved enantiomeric excesses of up to 75%, thus representing the first successful application of a cyclodiphosphazane in enantioselective organocatalysis. DFT computations reveal high hydrogen-bonding strengths of cyclodiphosphazane PV-amides compared to urea-based catalysts. Experimental results and computations on the enantiodetermining step with cis-cyclodiphosphazane 14a suggest a strong bidentate H-bond activation of the nitrostyrene substrate by the catalyst.
Six collected phenidates, i.e. 4-methylmethylphenidate, 3,4-dichloromethylphenidate, ethylphenidate, 3,4-dichloroethylphenidate, ethylnaphthidate and N-benzyl-ethylphenidate were fully characterized by means of X-ray, NMR, GC-MS, ESI-MS Running title: Analytical characterization of six phenidate analogs
Keywords: Umpolung / Asymmetric catalysis / Cross benzoin coupling / Lithium phosphonatesTwo new classes of terpenol-based lithium phosphonates, i.e. phenylfenchyl phosphonates and 2,2Ј-biphenyldiylbis(terpenyl) phosphonates, are employed as umpolung catalysts for the enantioselective cross benzoin coupling. The structural characteristics of the phosphonates were investigated by means of X-ray, 31 P-NMR analyses as well as DFT computations. The chiral lithium phosphonates were found to catalyze the cross benzoin coupling with enantioselectivities up
Enantioselective Michael additions of 4-hydroxycoumarin to β-nitrostyrenes are catalyzed by different chiral, bifunctional hydrogen-bonding catalysts, based on thiourea- and squaramide motifs. The scope of the catalysis is tested by employing a series of substituted β-nitrostyrenes as well as different solvents. The 3,5-bis(trifluoromethyl)phenyl- and quinine-substituted squaramide catalyst is shown to be the most selective catalyst, resulting in 78% yield and 81% ee. Computational analyses of transition structures with different binding modes show that the most favored transition structure exhibits squaramide (NH)2 binding to an oxygen atom of the enolate nucleophile, while the nitroalkene coordinates via hydrogen bonding to the ammonium function of the quinuclidine unit of the catalyst. Hence, the canted directionality of the squaramide (NH)2 motif, favoring one-atom binding, might be decisive for the selectivity of the reaction. The absolute configuration of the major (-)-(R) enantiomer of the product is assigned computationally according to its optical rotation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.