Helical chirality of pyrrolo[1,2-a][4,5]diazafluoren-5-one derivatives

The treatment of the N -phenacyl-4,5-diazafluorenium-9-one bromides 4a,b and the diesters of acetylenedicarboxylic acid with Et 3 N in methylene chloride at room temperature gave trans - 9,10-dihydro-pyrrolo[1,2-a][4,5]diazafluoren-5-one derivatives 7 along with the corresponding aromatized compounds 8 . The non-planarity of the diethyl ( 8c,e ) and diisopropyl ( 8d,f ) pyrrolodiazafluorenone esters was deduced by H-NMR spectroscopy. The 1,3-dipolar cycloaddition between diazafluorenonium N -ylides, generated from 4 , and an activated olefinic dipolarophile (acrylonitrile) is reported for the first time. The optical properties of pyrrolodiazafluorenone 8 were also investigated.


Introduction
Diazahelicenes are of interest owing to their properties and practical applications in photo-and optoelectronics as well as their ability to act as proton sponges, with the degree of helicity correlated with the base strength. 1,2Since the discovery of "classical" helicenes, the concept of helical chirality or helicity has been extended to non-fully aromatized systems.Therefore, helicity can be induced by intramolecular sterical strain.][17][18][19] The helical chirality of pyrrolophenanthroline derivatives 1 was deduced from NMR experiments performed in solution on ethyl and isopropyl esters containing diastereotopic protons as probe groups.The activation free energy for ring flipping at coalescence temperature The position of the double bond in the pyrroline moiety could be deduced from the chemical shifts of the three carbonyl groups.Thus, the large difference between the two carbonyl ester group shifts (δ = 164.7 ppm and 174.0 ppm) shows that they are attached to a Csp 2 (δ = 164.7 ppm) and a Csp 3 (δ = 174.0ppm), respectively.The deshielding of the carbonyl group of the 4bromobenzoyl moiety (δ = 184.4ppm) by 6 ppm relative to those of the corresponding aromatic compounds 8a is good evidence that the group is attached to a Csp

Scheme 1
The formation of dihydroderivatives 7 may be explained by the regio-and stereoselective prototropic rearrangement of the primary cycloadducts 6 in the presence of an excess of triethylamine.The aromatization of the dihydroderivative to pyrrolodiazafluoren-5-ones 8 was performed by the action of tetrakis-pyridino Co(II) dichromate (TPCD).2][23][24] The pyrrolodiazafluoren-5-one derivatives 8 were also obtained from bromides 4, esters of acetylenedicarboxylic acid, triethylamine and TPCD, in DMF at 90 °C, as well as by the procedure described in literature for compounds 3. 20 As previously reported, 15,16 in the 1 H-NMR spectra of diethyl esters of pyrrolophenanthroline 1 (R=Et), recorded in CDCl 3 at room temperature, the methylenic protons in both ester groups appear as two distinct ABX 3 patterns.Under the same conditions, in the 1 H-NMR spectra of pyrrolodiazafluoren-5-ones 8c,d the methylenic protons of the ester group in the 8 position appear as a quartet, whereas the methylenic protons of the other ester group show as a broad singlet.A similar observation was made for the isopropyl esters.Thus, in the series of isopropyl ARKAT USA, Inc.
esters 1 the methyl groups in each isopropyl group were found to be non-equivalent in 1 H-NMR spectra, each appearing as a doublet with the integral curve corresponding to three protons.In the case of pyrrolodiazafluoren-5-ones 8e,f, the methyl groups from the isopropyl moieties appear as a doublet with the integral curve corresponding to six protons and as a broad doublet, also with an integral curve of six protons.
From these 1 H-NMR data one can conclude that compounds 8c-f possess a certain amount of helical distortion in solution, caused by intramolecular strain.Furthermore, this helical distortion is smaller than that present in pyrrolophenanthrolines 1, as the enantiotopic groups are near coalescence temperature.This assumption is supported by previously reported X-ray data. 18,19,30][27][28][29] In order to study the regioselectivity of the 1,3-dipolar cycloaddition, the corresponding ylides 5a,b were treated with acrylonitrile as a non-symmetrical dipolarophile, affording tetrahydro derivatives 9a,b.However, as these were too unstable to characterize, by the action of TPCD in DMF at 90 ºC, the crude products 9a,b were aromatized to the pyrrolodiazafluorenones 10a,b (Scheme 2).The overall synthesis of compounds 10a,b was therefore performed as a "one pot" reaction between the bromides 4a,b, acrylonitrile and Et 3 N in DMF, followed by heating and the addition of TPCD.The NMR data for compounds 8 and 10 are in good agreement with the assigned structures.In 1 H-NMR spectra of compounds 8b-f, recorded in CDCl 3 , the signal of H-7 (δ~8.50 ppm) is deshielded in comparison with the protons of the diazafluorenone moiety due to the presence of the carboalkoxy group in position 8 of the pyrrole ring.The deshielding effect of the cyano group (compounds 10a,b) on H-7 is smaller by comparison (δ = 7.92 ppm).Also, the proton H-2 is deshielded due to the fact that it is linked to a C=N double bond.
Due to the low solubility of compound 10b, the H-NMR spectrum was recorded in a CDCl 3 +TFA mixture.We found the coupling constant between H-2 and H-3 in CDCl 3 to be 5.2 Hz (compounds 8 and 10a), whereas in the CDCl 3 +TFA mixture the respective constant was found to be 5.8 Hz.The same coupling constant was observed when 1 H-NMR spectra of pyrrolodiazafluorenones were recorded in CDCl 3 +TFA.The difference in the values of the coupling constant between H-2 and H-3 may be explained by the protonation at the nitrogen N1 by TFA.
Representative solvents have been selected for this investigation.A comparison of the effect of substitution on the photochemical properties of compounds 8e and 8f in different solvents is presented.
The UV-VIS absorption spectra of the compounds in the spectral range 240-600 nm exhibit two electronic bands (Figures 1 and 2).Tables 1 and 2 show the absorption band maxima wavelengths in the 240-650 nm range and the corresponding absorbances for 8e in methanol, acetonitrile, dimethylsulfoxide (DMSO), dioxane and dichloromethane (CH 2 Cl 2 ).The positions of the absorption bands and the values of the molar extinction coefficient values evidence the influence of the substituent and the nature of the solvent.By substitution of the 4-bromobenzoyl group (in 8e) with the 4-nitrobenzoyl group (in 8f) a bathochromic shift of the spectra is observed.The position of the substituent is reflected by the shape modification of absorption spectra of the compounds 8e and 8f.
In comparison with other pyrroloazines, the pyrrolodiazafluorenones 8 and 10 did not present fluorescence either in solution or in the solid state.

Conclusions
Six new pyrrolodiazafluorenones were synthesized by the "one pot" reaction between diazafluorenonium salts, dipolarophiles, Et 3 N and TPCD as oxidant in DMF.The regioselectivity of the reaction was evidenced by the novel use of acrylonitrile as dipolarophile.All the compounds were characterized by NMR spectroscopy, complemented by IR spectroscopy in the case of the novel cyano-bearing compounds.
The non-planarity of these diazafluorenones substituted at the 10 position by a phenacyl group was put in evidence by NMR spectroscopy.
The optical properties of some representative compounds were also investigated.