Synthesis of 3,8‐Dichloro‐6‐ethyl‐1,2,5,7‐tetramethyl–BODIPY from an Asymmetric Dipyrroketone and Reactivity Studies at the 3,5,8‐Positions

Abstract

The asymmetric BODIPY 1a (BODIPY=4,4-difluoro-4-bora-3a,4a-diaza-s-indacene), containing two chloro substituents at the 3,8-positions and a reactive 5-methyl group, was synthesized from the asymmetric dipyrroketone 3, which was readily obtained from available pyrrole 2a. The reactivity of 3,8-dichloro-6-ethyl-1,2,5,7-tetramethyl-BODIPY 1a was investigated by using four types of reactions. This versatile BODIPY undergoes regioselective Pd0-catalyzed Stille coupling reactions and/or regioselective nucleophilic addition/elimination reactions, first at the 8-chloro and then at the 3-chloro group, using a variety of organostannanes and N-, O-, and S-centered nucleophiles. On the other hand, the more reactive 5-methyl group undergoes regioselective Knoevenagel condensation with an aryl aldehyde to produce a monostyryl-BODIPY, and oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) gives the corresponding 5-formyl-BODIPY. Investigation of the reactivity of asymmetric BODIPY 1a led to the preparation of a variety of functionalized BODIPYs with λmax of absorption and emission in the ranges 487–587 and 521–617 nm, respectively. The longest absorbing/emitting compound was the monostyryl-BODIPY 16, and the largest Stokes shift (49 nm) and fluorescence quantum yield (0.94) were measured for 5-thienyl-8-phenoxy-BODIPY 15. The structural properties (including 16 X-ray structures) of the new series of BODIPYs were investigated.

Synthesis and Spectroscopic Investigation of a Series of Push–Pull Boron Dipyrromethenes (BODIPYs)

“…The starting 2,3,5,6,8-pentachloro-BODIPY 1 was prepared from 8-chloro-BODIPY, as previously reported. 13a Based on the known regioselectivity of the pentachloro-BODIPY (reactivity order: 8-Cl > 3,5-Cl > 2,6-Cl), 13a different functional groups can be introduced at the BODIPY periphery allowing for the design of various push-pull BODIPYs through versatile Pd(0)-catalyzed cross-coupling reactions. Furthermore, the unsubstituted 1,7-positions of BODIPY 1 are expected to favor relatively small dihedral angles between the substituents and the BODIPY core, potentially leading to increased electronic interactions.…”

“…Compared with the Pd(PPh 3 ) 4 catalyst, Pd(PCy 3 )G2 greatly increased the yield of the cross-coupling reactions at the 2,6-positions, due to the more reactive complex formed by the bulkier ligands of the Pd(PCy 3 )G2 catalyst. 13a, 15 For comparison purposes, BODIPY 3 was also prepared from 2 using 2-(tributylstannyl)thiophene and Pd(PCy 3 )G2 in refluxing toluene , in 62% yield. The structures of the new BODIPYs were confirmed by 1 H, 13 C, and 11 B NMR spectroscopy, and by HRMS (see Supporting Information, Figures S1–27).…”

“…In the absence of 1,7-substituents, the dihedral angles are significantly smaller (by about 30°) compared with 1,7-substituted BODIPYs (for example in 1,7-dimethyl-BODIPYs) in which the 8-substituent is almost perpendicular with the BODIPY core. 13c, 16 This decrease in dihedral angle could lead to better conjugation and electronic communication between the 8-aryl substituent and the BODIPY core, therefore enhancing the push-pull effect. The dihedral angles between the 3,5-aryl substituents and the BODIPY core are also in the range of 45–65° for all the compounds, except for BODIPY 7 , which shows dihedral angles of 70.3 and 86.1° (Figure S32), due to the presence of the 2,6-thienyl groups.…”

“…All platforms feature an electron-withdrawing group at the 8-position and electron-donating groups at the 3 and 5 positions. In addition, since the introduction of electron-rich thienyl groups, particularly at the 2 and 6 positions, induces large bathochromic and Stokes’ shifts on BODIPYs, 13 two thienyl donors were also installed at the 2 and 6 positions, to give platform V (Figure 1). In order to further investigate the effects of push-pull groups at the 2,6-positions on the spectroscopic and electrochemical properties of BODIPYs, an additional set of push-pull moieties was regioselectively introduced at the 2 and 6 positions to give platform VI (Figure 1).…”

Synthesis and spectroscopic properties of β,β′-dibenzo-3,5,8-triaryl-BODIPYs

“…Various methods have been investigated for the functionalization of the 3,5-positions of BODIPYs, 1,7 including metal mediated cross-coupling reactions, 7,16 aromatic substitutions, 7,16,17 Knoevenagel condensations, 8,18 C–H arylations 19 and radical arylations. 20 Among these, the Pd(0)-catalyzed Suzuki coupling reactions are particularly convenient 32 due to the variety of commercially available boronic acids and the ready availability of 3,5-dichloro- and 3,5-dibromo-BODIPYs.…”