Constantin-Gabriel Daniliuc

Functional‐Group Tolerance in Frustrated Lewis Pairs: Hydrogenation of Nitroolefins and Acrylates

Apart from molecular hydrogen (H2), [1] other small molecules, such as CO2, [2] N2O, [3] NO, and SO2, [5] were targeted by frustrated Lewis pairs (FLPs) and resulted in the fixation or activation of these small molecules by Lewis acid interaction. Consequently, functionalized molecules, such as a,b-unsaturated esters, sulfoxides, or nitro compounds, are challenging substrates for FLP-catalyzed hydrogenations, and strategies toward higher functional-group tolerance are topic of current research. So s et al. demonstrated that bulky mesityl-substituted boranes could attain a level of functional-group tolerance according to the size exclusion principle. Other modifications of boranes to achieve stronger and weaker Lewis acidity in conjunction with FLP chemistry were reported. In particular, weaker Lewis acids appear to be favorable for the reduction of electron-deficient double bonds, such as a,b-unsaturated ketones, owing to the pronounced nucleophilicity of the corresponding hydridoborate anion. However, reduced Lewis acidity might also suppress the H2 activation, and a careful balance must be met. To date, there has been no report on the FLP-catalyzed hydrogenation of nitroolefins and acrylates. Only recently, B(C6F5)3 (1) was applied for the reduction of strongly electrophilic malonates at elevated temperatures and pressure (80 8C, 60 bar H2). [10a] Herein we report the reactivity of B(2,6-F2-C6F3)3 (2) as Lewis acid in FLP-catalyzed hydrogenations and its unique structural features in solid state and solution. The unprecedented FLP-mediated hydrogenation of nitroolefins and a,b-unsaturated esters under mild conditions (RT or 40 8C; 4 bar H2) was established. In contrast to the size-exclusion concept, in this case functional-group tolerance was solely achieved by modification of the electronic nature of the Lewis acid and the Lewis base. We initiated our studies with the synthesis of the borane B(2,6-F2-C6H3)3–THF adduct (2·THF) according to Naumann. Surprisingly, 2 has not been studied in FLP chemistry to date, even though it resembles an electronically modified B(C6F5)3 with identical steric shielding. The solidstate structure of 2 was established and displays similar structural features to BPh3 (Figure 1a). [14]

Mild metal-free tandem α-alkylation/cyclization of N-benzyl carbamates with simple olefins.

Easy does it! The chemoselective oxidative α-C(sp(3))-H alkylation/cyclization reaction of N-benzyl carbamates using simple mono-, di-, and trisubstituted olefins provides functionalized N-heterocycles such as oxazinones. A TEMPO oxoammonium salt serves as the oxidant, making it possible to carry out the reaction at low temperatures. Neither a metal catalyst nor preactivation in the α-position to the nitrogen group are needed.

Hydroalumination of Ketenimines and Subsequent Reactions with Heterocumulenes: Synthesis of Unsaturated Amide Derivatives and 1,3-Diimines.

The series of differently substituted ketenimines 1 was hydroluminated using di-iso-butyl aluminum hydride. For the sterically congested ketenimine 1a, preferred hydroalumination of the C═N-bond was proven by X-ray crystallography (compound 5a). In situ treatment of the hydroaluminated ketenimines 5 with various heterocumulenes like carbodiimides, isocycanates, isothiocyanates and ketenimines as electrophiles and subsequent hydrolytic workup resulted in novel enamine derived amide species in case of N-attack (sterically less hindered ketenimines) under formation of a new C-N-bond or in 1,3-diimines by C-C-bond-formation in case of bulky substituents at the ketenimine-nitrogen atom. Furthermore, domino reactions with more than 1 equiv of the electrophile or by subsequent addition of two different electrophiles are possible and lead to polyfunctional amide derivatives of the biuret type which are otherwise not easily accessible.

C3-Symmetric Tricyclo[2.2.1.02,6]heptane-3,5,7-triol

A straightforward access to a hitherto unknown C3 -symmetric tricyclic triol both in racemic and enantiopure forms has been developed. Treatment of 7-tert-butoxynorbornadiene with peroxycarboxylic acids provided mixtures of C1 - and C3 -symmetric 3,5,7-triacyloxynortricyclenes via transannular π-cyclization and replacement of the tert-butoxy group. By refluxing in formic acid, the C1 -symmetric esters were converted to the C3 -symmetric formate. Hydrolysis gave diastereoisomeric triols, which were separated by recrystallization. Enantiomer resolution via diastereoisomeric tri(O-methylmandelates) delivered the target triols on a gram scale. The pure enantiomers are useful as core units of dopants for liquid crystals.

1,3,5-Triazapentadienes by Nucleophilic Addition to 1,3- and 1,4-Dinitriles—Sterically Constrained Examples by Incorporation into Cyclic Peripheries: Synthesis, Aggregation, and Photophysical Properties

1,3,5-Triazapentadienes usually show U- or twisted S-shaped conformations along the N-C-N-C-N skeleton due to dominating n/π* interactions. If, however, the 1,3,5-triazapentadiene unit is part of a ring, its W conformation might be restricted to the plane. Here, we describe the synthesis of 13 new 1,3,5-triazapentadienes 10-12, which are sterically restrained by incorporation into six- or seven-membered ring systems, by addition of a lithiated primary amine or hydrazine 5 to a dinitrile 7, 8, or 9 with the two cyano groups in 1,3 or 1,4 distance. These novel compounds show very strong tendency for aggregation due to hydrogen bonding, especially to form homodimers as seen from X-ray data in the solid state. Additional hydrogen bonding generates also linear chains in the crystal. Several of the new compounds show fluorescence in solution. Quantum chemical DFT calculations were used for evaluation of the dimerization energies and for interpretation of the photophysical properties.