differences in static quenching constants K S could be observed, likely due to the
formation of a diastereotopic tight ion pair between deprotonated 5 and the chiral
base (i.e. RO
À
ÁÁÁHNR 3
+
), the ratio of K S (R–S)/K S (R–R) ¼ 1.40 indicating that bis
(BODIPY) 5 may have potential for use as an enantioselective molecular sensor
(Fig. 6.4).
A final example of a BODIPY covalently attached to a chiral moiety was
disclosed by Huszthy et al. in which two mono(BODIPY)-linked azacrown ethers
6 and 7 were prepared through the reaction of 3-chloro-5-methoxyBODIPY with the
corresponding enantiopure 1-aza-18-crown-6 ethers [35]. Although 6 and
7 performed admirably as fluorescent sensors of divalent metal cations, attempts to
employ either as enantioselective molecular sensors of chiral ammonium cations
(e.g. (R)- or (S)-α-phenylethylammonium perchlorate) proved unsuccessful
(Fig. 6.5).
6.2.2 BODIPYs Containing a Chiral Boron Centre
In 2010 the group of Ulrich and Ziessel published one of the first examples of a
BODIPY based around a chiral boron atom, termed a B
à -BODIPY [36]. In order to
obtain a stable chiral B
à -BODIPY, Ziessel et al. proposed the following design
requirements: (1) lateral differentiation of the dipyrromethene core; (2) a polar
group attached via the 3/5-position capable of intramolecular association with the
central boron atom; and (3) a polyaromatic residue causing moderate steric congestion around the boron atom (also aiding in chromatographic resolution on a chiral
solid phase). Therefore racemic B
à -BODIPY 8 (Fig. 6.6) was prepared starting from
the parent BODIPY through displacement of single fluoride with
N
B N
N B
N
F
F
F
F
Et
Et
Et
Et
OH HO
5
Fig. 6.4 BINOL-based bis
(BODIPY) 5
N B
N
Ph
F F
OMe
N
O
O
O
O
O
R
R
R = Me; 6
R = i-Bu; 7
Fig. 6.5 Aza-crown-etherbased mono(BODIPY)s
6 and 7
122
M. J. Hall and S. de la Moya