Tetrathia[7]helicene-based phosphanes thiaheliphos (2a), nPr-thiaheliphos (2b) and di-nPr-thiaheliphos (2c) have been prepared from the 2,13-dilithio derivatives of thiahelicenes 1a–c by reaction with an excess of Ph2PCl. Protection of the air-sensitive products as BH3 adducts, from which the phosphanes 2a–c are easily regenerated on heating with ethanol, is described. New rhodium(I) complexes 8 and 9 were obtained by reaction of 2c with [Rh(COD)2]+[BF4]– and [BARF]–, respectively, which were converted by oxidation into the chelating phosphane–phosphane oxide RhI complexes 10 and 11. The monophosphane 5 was similarly prepared, andreaction of the dilithio species with ClP(O)(OEt)2 gave the diphosphonate 17, which was characterized by X-ray crystallography. Comparison of data from DFT calculations (B3LYP/SVP) on 2c, 8 and 10 and X-ray data for 17 reveals a strong tension within the thiaheliphos ligands that is partially relaxed by increasing the Rh–P distances and the bite angle of the chelating ligand. Compound (P)-(+)-1c, obtained by chromatographic resolution, was transformed into optically pure (P)-(+)-2c, which was used in the asymmetric hydrogenation of itaconic acid ester and methyl 2-acetamidoacrylate (product ees up to 40%)
The synthesis of new functionalized 1,2-bis(benzodithienyl)ethenes as well as the preparation of the new tetrathia[7]helicenes are described. The helicenes reported are new chiral pushpull molecules, with potential application in optoelectronics.[n]Heterohelicenes are very interesting all conjugated aromatic systems in which heteroaromatic units alternates with arene rings in an extended p-system. 1 When the number of ortho condensed rings [n] exceeds three, these systems are no more planar and they become chiral. The interconversion barrier between the two enantiomers depends on the number of aromatic rings and, for [n] greater than four, they are stereochemically stable compounds which means that their enantiomers can be separated and stored. 2The presence of heterocyclic rings can give some advantages with respect to carbohelicenes. In principle, it is possible to functionalize the heteroaromatic system regioselectively, thus opening the way for obtaining modified helicenes targeted for specific uses, and the tuning and monitoring of their properties. In particular, quite recently, tetrathia[7]helicenes ([7]THs) have attracted a great attention as potential new molecules with interesting optoelectronic properties. 3 In fact, both theoretical calculations 3b,c and experimental work 3a indicate them as new promising candidates for applications in non-linear optics. In particular, the computational study suggests that the presence of some appropriate electron-withdrawing and electron-donating substituents in the positions 2, 7, 8 and 13 of the helical system should improve their optoelectronic responses. 3c In addition to the above-mentioned potentialities, suitable functionalized [7]THs could be very interesting also as new chiral ligands to be use in catalysis. 4 For all these reasons, the search of new methods for the preparation of [7]THs and their functionalization is a highly desirable goal. Recently, 5 we have set up a new efficient synthesis of substituted [7]THs based on a Suzuki coupling of diboronic esters derivatives. More recently 6 we have studied new silyl substituted [7]THs, useful for the access to variously functionalized derivatives.In this paper we describe the preparation of [7]THs decorated with both electron-donating and electron-withdrawing substituents, in view of studying their non-linear optical properties. In particular, we focused our attention on the introduction of electron-donating and electronScheme 1 Strategies for the synthesis of substituted tetrathia[7]helicenes.
LICANDRO, E.; RIGAMONTI, C.; TICOZZELLI, M. T.; MONTEFORTE, M.; BALDOLI, C.; GIANNINI, C.; MAIORANA*, S.; Synthesis 2006, 21, 3670-3678; Dip. Chim. Org. Ind., Univ. Stud. Milano, CNR, I-20133 Milano, Italy; Eng.) -H. Toeppel
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