Phosphole-helicene

Fig. 38 Top Structure of the Pd-bis(phosphole-helicene) complex and a comparison of the BHLYP/SV(P) computed and experimental CD spectra. Bottom Two-dimensional structure of the phosphole-helicene ligand as well as the experimental UV-Vis vs the Boltzmann averaged spectrum computed at the BHLYP/SV(P) level of theory, red-shifted 0.25 eV. Data to prepare the plots were taken from [260]...

Fig. 38. The agreement between simulated and experimental CD spectra for the PD complex was very good after a red-shift of 0.25 eV was applied to the excitation energies. Analysis of the computed spectrum showed the intense bands to originate predominately from n-to-n transitions within an extended n framework of the phosp-hole-helicene ligands. Unlike initially expected, the various bands in the CD spectrum cannot be assigned to transitions centered separately on the helicene and phosphole moieties, respectively. The experimentally measured molar rotation of the Pd complex was 23.1 103 deg cm2 dmol 1 2% in dichloromethane. For an analogous Cu complex it was 13.1 103 2%, a staggering 104 deg cm2 dmol 1 lower.

At this stage, only 151c bears a helicene moiety that is configurationally stable. The last step is the synthesis of the phosphole derivatives 152a—e in 27—73% yields (Scheme 40). In solution, a rapid inversion of conf yiration is observed for phospholes 152a,b,d, while for 152e, two diastereomers are observed in the P-NMR spectrum. 

The method consists of the one-step synthesis of (Z)-l,2-(bis-benzo-dithienyl)ethenes 10 using double Suzuki coupling between stereochemi-caUy defined diboronic acid esters 11 and 2-iodo-benzodithiophene 12. The (Z)-alkenes thus obtained can be easily and efficiently photochemicaUy cyclized to the corresponding substituted tetrathia[7]helicenes. This methodology can also be applied to the synthesis of stilbenoid derivatives with two different heterocyclic systems such as the phosphole 13 (Figure 3) (2014CEJ12373). 

Nozaki and co-workers reported -phospha[7]helicene 99 via palladium-catalyzed C—P bond formation (Scheme 23.39) [47], The cross-coupling of 3,3 -t>iphenanthlyl-2,2 -ylene disulfonate 97 with ethyl phenylphosphinate gave monophosphorus compound 98 as a diastereomer mixture. Reduction of 98, the subsequent palladium-catalyzed intramolecular P-arylation, and oxidation provided X -phospha[7]helicene 99 with a phosphole-oxide moiety. 

Interestingly, the X-ray diffraction study of complex 7h (Scheme 11.6) and 8h,i (Scheme 11.7) revealed that the twist angle between the phosphole ring and the azahelicene (<50°) allows an electronic interaction between these two 7r-units to take place. [46, 45] The helical shape of the aza[4]helicene part is not perturbed upon complexation since the twist angle between the two terminal aromatic rings of the aza[4]helicene (33.1°) is comparable to that of the free ligand (30.1°) [45,46]. This result shows that mixed phosphole-azahelicene derivatives can act as... 

Graule S, Rudolph M, Vanthuyne N, Autschbach J, Roussel C, Crassous J, R6au R (2009) Metal-Bis(helicene) assemblies incorporating pi-conjugated phosphole-azahelicene ligands impacting chiroptical properties by metal variation. J Am Chem Soc 131 3183... 

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