No abstract

Synthetic polymers are widely used materials, as attested by a production of more than 200 millions of tons per year, and are typically composed of linear repeat units. They may also be branched or irregularly crosslinked. Here, we introduce a two-dimensional polymer with internal periodicity composed of areal repeat units. This is an extension of Staudinger's polymerization concept (to form macromolecules by covalently linking repeat units together), but in two dimensions. A well-known example of such a two-dimensional polymer is graphene, but its thermolytic synthesis precludes molecular design on demand. Here, we have rationally synthesized an ordered, non-equilibrium two-dimensional polymer far beyond molecular dimensions. The procedure includes the crystallization of a specifically designed photoreactive monomer into a layered structure, a photo-polymerization step within the crystal and a solvent-induced delamination step that isolates individual two-dimensional polymers as free-standing, monolayered molecular sheets.

We report the synthesis and physical study of a series of 1,1‐dicyano‐4‐[4‐(diethylamino)phenyl]buta‐1,3‐dienes in which the number and position of additional CN substituents along the 1,1‐dicyanobuta‐1,3‐dienyl fragment is systematically varied. While X‐ray analysis provided unambiguous information about molecular geometries in the crystal, UV/Vis and electrochemical measurements, by cyclic voltammetry (CV) and rotating disk voltammetry (RDV), revealed that introduction of additional cyano groups in the C2‐ and C4‐positions most affected the optical properties of these molecules in solution, in terms of intramolecular charge‐transfer absorption energy and intensity. A comparison with structurally related chromophores indicates that the shift of the anilino donor from position 2/3 to 4 along the butadiene scaffold results in a remarkable bathochromic shift of the ICT absorption maxima, mainly due to the higher planarity in the present series. These findings are further corroborated by density functional theory calculations. Preliminary nonlinear optical (NLO) measurements confirm the promise of the new push‐pull chromophores as third‐order nonlinear‐optical molecular materials.

In memoriam A. I. Meyers 3 ), who paved the way for the use of heterocycles in stereoselective organic synthesis 4 )Reaction of 2-phenylacetaldehyde with the Me 3 Si ether of diphenyl-prolinol, with removal of H 2 O, gives a crystalline enamine (1). The HBF 4 salts of the MePh 2 Si ether of diphenyl-prolinol and of 2-(tertbutyl)-3-methyl-and 5-benzyl-2,2,3-trimethyl-1,3-imidazolidin-4-one react with cinnamaldehyde to give crystalline iminium salts 2, 3, and 4. Single crystals of the enamine and of two iminium salts, 2 and 3, were subjected to X-ray structure analysis (Figs. 1, 2, and 6), and a 2D-NMR spectrum of the third iminium salt was recorded (Fig. 7). The crystal and NMR structures confirm the commonly accepted, general structures of the two types of reactive intermediates in organocatalysis with the five-membered heterocycles, i.e., D, E (Scheme 2). Fine details of the crystal structures are discussed in view of the observed stereoselectivities of the corresponding reactions with electrophiles and nucleophiles. The structures 1 and 2 are compared with those of other diphenyl-prolinol derivatives (from the Cambridge File CSD; Table 1) and discussed in connection with other reagents and ligands, containing geminal diaryl groups and being used in enantioselective synthesis (Fig. 4). The iminium ions 3 and 4 are compared with N-acylated imidazolidinones F and G (Figs. 9, 12, and 13, and Table 3), and common structural aspects such as minimalization of 1,5-repulsion (the A 1,3 -effect), are discussed. The crystal structures of the simple diphenyl-prolinol · HBF 4 salt ( Fig. 3) and of Boc-and benzoyl-(tert-butyl)methyl-imidazolidinone (Boc-BMI and Bz-BMI, resp.; Figs. 10 and 11) are also reported. Finally, the crystal structures are compared with previously published theoretical structures, which were obtained from high-level-oftheory DFT calculations (Figs. 5 and 8, and Table 2). Delicate details including pyramidalization of trigonal N-atoms, distortions around iminium C¼N bonds, shielding of diastereotopic faces, and the pinteraction between a benzene ring and a Me group match so well with, and were actually predicting the experimental results that the question may seem appropriate, whether one will soon start considering to carry out such calculations before going to the laboratory for experimental optimizations. 2000 5 ) In this expanding field of organic chemistry, we have to refer to selected reviews and a few seminal original articles [1], and the enumeration of reactions given here cannot be complete. 6 ) Besides proline itself, derivatives such as a-methyl-proline, proline amides, a tetrazole [1r], sulfonylimides, and (aminomethyl)-pyrrolidines [1m] have been used as catalysts. 7 ) See also the use of chiral H-bond donors [1j], of Brønsted acids [1t], and of heterocyclic carbenes [1p] as catalysts. 8 ) See summary of a 1990 review article entitled Organic Synthesis -Where now? [2]. 9 ) Cf. the title In the Golden Age of Organocatalysis [1c]; Asymmetric Organocatalysis: From Infancy to Adolescence [1u]...