146
Y. Kubota
Under method (3), the Paal-Knorr reaction of 2-acylacetophenones with an ammonium salt affords diisoindolomethenes (Fig. 5.29c) (Ulrich et al. 2011). In this reaction, isoindole intermediates react with isoindolenine intermediates, followed by
the elimination of formaldehyde via a retro-Aldol reaction. Subsequent air oxidation gives diisoindolomethenes (Speck and Magauer 2013). Boron complexation
of diisoindolomethenes gives the corresponding benzo-[a]-fused BODIPY dyes. In
2016, Kubo and co-workers reported the first synthesis of naptho-[a]-fused BODIPY
dyes by using this method (Yamazawa et al. 2016).
In method (4), a Vilsmeier–Haack reaction of phthalimidine gives isoindolenine
derivatives and subsequent basic hydrolysis forms 3-halogeno-1-formylisoindoles
(Fig. 5.29d) (Diana et al. 2011). Jiao and co-workers reported asymmetric halogenated benzo-[a]-fused BODIPY dyes via condensation reactions of 3-halogeno1-formylisoindoles with α-unsubstituted pyrroles under POCl 3 catalyzed condition
with subsequent boron complexation (Jiao et al. 2010). Pd-catalyzed cross-couplings
or S N Ar reactions of the halogenated benzo-[a]-fused BODIPY dyes give various
asymmetric benzo-[a]-fused BODIPY dyes (Jiao et al. 2010). The one-pot synthesis
of an isoindole-BODIPY dye having a 2-pyrrolyl group is also reported; the reaction
of 3-chloro-1-formylisoindole with excessive α-unsubstituted pyrrole in the presence
of POCl 3 and subsequent boron complexation yields the isoindole-BODIPY dyes
(Yu et al. 2012). In the reaction, unexpected S N Ar of the chlorinated dipyrromethene
intermediate with the α-unsubstituted pyrrole occurs with the assistance of POCl 3 ,
which acts as a Lewis acid.
As a synthetic method, except for the method (1) to (4), phenanthrene-fused
BODIPY dyes are synthesized by the introduction of two biphenyl groups at C2
and C6 positions of 2,6-dichloro-BODIPY dyes via Suzuki coupling reaction,
followed by bromination at the 1,7-positions and then Pd(0)-catalyzed intramolecular
cyclization (Zhao et al. 2017a).
Synthesis of [b]-fused BODIPY
Synthetic methods to produce [b]-fused BODIPY dyes can be roughly classified into two types: (1) application of classical synthetic methods to ring-fused
pyrroles (Figs. 5.30 and 5.31) and (2) intramolecular annulation of BODIPY dyes.
Under method (1), Burgess and co-workers reported the synthesis of benzofuran-,
benzothiophene- and dialin-fused BODIPY dyes by the reaction of benzofuro[3,2b]pyrrole, thianaphtheno[3,2-b]pyrrole and 4,5-dihydro-1H-benz[g]indole with acyl
chloride, followed by boron complexation, respectively (Fig. 5.30) (Chen et al. 2000).
Ohe and co-workers described the synthesis of spirofluorene-indene-fused BODIPY
dyes by the reaction of spiro[fluorene-9,4
(1
H)-indeno[1,2-b]pyrrole] and aromatic
aldehydes, followed by oxidation and boron complexation (Kowada et al. 2010).
A benzo[e]indole-fused BODIPY dye was synthesized from the reaction of naphthobipyrrole and triethylorthoformate in the presence of POCl 3 , followed by boron
complexation (Sarma et al. 2013). TFA-mediated condensation between thieno[2,3b]pyrrole and TFA anhydride, followed by boron complexation, gives a meso-CF 3
substituted thieno[2,3-b]pyrrole type BODIPY dye (Wang et al. 2016a). Similarly,
furan (Matsui et al. 2011)-, thiophene (Kubota et al. 2019)-, acenaphthylene (Jiang
Y. Kubota
Under method (3), the Paal-Knorr reaction of 2-acylacetophenones with an ammonium salt affords diisoindolomethenes (Fig. 5.29c) (Ulrich et al. 2011). In this reaction, isoindole intermediates react with isoindolenine intermediates, followed by
the elimination of formaldehyde via a retro-Aldol reaction. Subsequent air oxidation gives diisoindolomethenes (Speck and Magauer 2013). Boron complexation
of diisoindolomethenes gives the corresponding benzo-[a]-fused BODIPY dyes. In
2016, Kubo and co-workers reported the first synthesis of naptho-[a]-fused BODIPY
dyes by using this method (Yamazawa et al. 2016).
In method (4), a Vilsmeier–Haack reaction of phthalimidine gives isoindolenine
derivatives and subsequent basic hydrolysis forms 3-halogeno-1-formylisoindoles
(Fig. 5.29d) (Diana et al. 2011). Jiao and co-workers reported asymmetric halogenated benzo-[a]-fused BODIPY dyes via condensation reactions of 3-halogeno1-formylisoindoles with α-unsubstituted pyrroles under POCl 3 catalyzed condition
with subsequent boron complexation (Jiao et al. 2010). Pd-catalyzed cross-couplings
or S N Ar reactions of the halogenated benzo-[a]-fused BODIPY dyes give various
asymmetric benzo-[a]-fused BODIPY dyes (Jiao et al. 2010). The one-pot synthesis
of an isoindole-BODIPY dye having a 2-pyrrolyl group is also reported; the reaction
of 3-chloro-1-formylisoindole with excessive α-unsubstituted pyrrole in the presence
of POCl 3 and subsequent boron complexation yields the isoindole-BODIPY dyes
(Yu et al. 2012). In the reaction, unexpected S N Ar of the chlorinated dipyrromethene
intermediate with the α-unsubstituted pyrrole occurs with the assistance of POCl 3 ,
which acts as a Lewis acid.
As a synthetic method, except for the method (1) to (4), phenanthrene-fused
BODIPY dyes are synthesized by the introduction of two biphenyl groups at C2
and C6 positions of 2,6-dichloro-BODIPY dyes via Suzuki coupling reaction,
followed by bromination at the 1,7-positions and then Pd(0)-catalyzed intramolecular
cyclization (Zhao et al. 2017a).
Synthesis of [b]-fused BODIPY
Synthetic methods to produce [b]-fused BODIPY dyes can be roughly classified into two types: (1) application of classical synthetic methods to ring-fused
pyrroles (Figs. 5.30 and 5.31) and (2) intramolecular annulation of BODIPY dyes.
Under method (1), Burgess and co-workers reported the synthesis of benzofuran-,
benzothiophene- and dialin-fused BODIPY dyes by the reaction of benzofuro[3,2b]pyrrole, thianaphtheno[3,2-b]pyrrole and 4,5-dihydro-1H-benz[g]indole with acyl
chloride, followed by boron complexation, respectively (Fig. 5.30) (Chen et al. 2000).
Ohe and co-workers described the synthesis of spirofluorene-indene-fused BODIPY
dyes by the reaction of spiro[fluorene-9,4
(1
H)-indeno[1,2-b]pyrrole] and aromatic
aldehydes, followed by oxidation and boron complexation (Kowada et al. 2010).
A benzo[e]indole-fused BODIPY dye was synthesized from the reaction of naphthobipyrrole and triethylorthoformate in the presence of POCl 3 , followed by boron
complexation (Sarma et al. 2013). TFA-mediated condensation between thieno[2,3b]pyrrole and TFA anhydride, followed by boron complexation, gives a meso-CF 3
substituted thieno[2,3-b]pyrrole type BODIPY dye (Wang et al. 2016a). Similarly,
furan (Matsui et al. 2011)-, thiophene (Kubota et al. 2019)-, acenaphthylene (Jiang