Pyrazolo[1,5‐a]pyrimidine‐Dioxaborinine Hybrid Dyes: Synthesis and Substituent Effect in the Photophysical Properties

Abstract

A novel family of pyrazolo [1,5-a]pyrimidine-dioxaborinine (PP-DB) hybrid dyes was synthesized by the direct construction of the dioxaborinine (DB) fragment on the pyrazolo[1,5a]pyrimidine (PP) ring, which implies the formation of four new bonds in a one-pot manner. The dyes' optical properties were investigated and compared with the starting pyrazolo[1,5a]pyrimidines; a study evidencing large fluorescence quantum yields in products (φ f up to 69 %) due to an intramolecular charge transfer (ICT) process from the PP core to a ring of DB (PP!DB) that is absent in precursors (φ f = 0.03-0.30). Timedependent density functional theory (TD-DFT) calculations confirmed the fluorescence process involved in the novel dyes, where their ICT limits the non-radiative process due to the restricted rotation in the DÀ A system. The present work provides insight into how phenyl and DB ring incorporation impact the optical properties of this new group of hybrids dyes based on PP-DB.

Cyano-capped molecules: versatile organic materials

“…Sensing methodologies of cyanide ions based on their nucleophilic addition reactions are signicantly facilitated by the CN-functionalization of the probe structure. [140][141][142] Besides popular DCV-derived sensors, cyanovinyl analogue such as 42 are also feasible (Scheme 5). This push-pull molecule undergoes a nucleophilic addition solely with cyanide ions (LOD = 4.24 × 10 −8 M, Table 10), leading to an interrupted ICT (decolorization) and pronounced emission at 560 nm.…”

Synthesis of structural analogues of Reversan by ester aminolysis: an access to pyrazolo[1,5-a]pyrimidines from chalcones

“…In this line, our group has focused on obtaining diverse functionalized PPs, 7,8 mostly investigating their photophysical properties as a promising approach, 9,10 adding to recurring biological applications of these compounds. [10][11][12][13] PP ring access is usually achieved by constructing the pyrimidine moiety via the cyclocondensation reaction of NH-5-aminopyrazoles with 1,3-bis(electrophiles) such as ynones, b-dicarbonyls, b-enamionones, and b-ketonitriles, which permit the involved unsaturation in products (Fig. 1b).…”

“…1b). [10][11][12][13] However, although chalcones (enones) yield 5,7-diaryl derivatives with greater molecular diversity than similar substrates (e.g., bdiketones such as diaroylmethane), enones are rarely used because mixtures of products with the dihydro derivative (or perhaps some regioisomer) are obtained; more rigorous conditions or further steps are required to obtain the aromatic ring (Scheme 1a). 8,[14][15][16][17][18] The poor aromatic character of the pyrimidine ring inside the PP core possibly is an insignicant driving force that favors p-conjugation in products.…”

“…8,[14][15][16][17][18] The poor aromatic character of the pyrimidine ring inside the PP core possibly is an insignicant driving force that favors p-conjugation in products. [10][11][12][13] As a result, there are even rarer examples for synthesizing PPs from chalcones 2 and NH-5aminopyrazoles bearing an electron-withdrawing group (EWG), such as ethyl-5-amino-1H-pyrazole-4-carboxylate (1), which is a crucial substrate in Reversan synthesis since they are poorly reactive 1,3-bis-(nucleophiles). 19 To date, only four articles where precursor 1 gives 5,7-diarylpyrazolo[1,5-a]pyrimidines 4 have been published; in two of these, an acid-mediated tricomponent reaction with arylaldehydes and arylacetylene was carried out, 20,21 while in the other two, chalcones 2 (ref.…”

“…[24][25][26][27][28][29] Thus, the solventfree direct amidation reaction of carboxylic acids or esters by heterogeneous catalysis using silica or alumina is a suitable method, though little-used, to efficiently access amides due to the value and easy disposal of these solids (Scheme 2a). [29][30][31][32][33] Due to the biological importance of pyrazolo[1,5-a] pyrimidines [10][11][12][13] and the amide functional group, [24][25][26][27][28] there are several examples of synthesis of this N-heterocycle substituted at position 3 with the carboxamide group. However, when the respective ester is used as a starting reagent, the synthesis proceeds by hydrolysis and subsequent amidation under conditions that imply coupling agents, further reaction steps, and poor yields (Scheme 2b).…”