Directing group-controlled regioselective borylation reactions with HBpin have
been reported by a few groups. Suginome et al. reported an efficient route to
1,2-diborylarenes via boronyl group-directed ortho C–H borylation catalyzed by
[Ir(OMe)(cod)] 2 without an ancillary ligand (Scheme 86a) [214]. The use of
pyrazolylaniline-modified phenylboronic acids (Ph-Bpza) resulted in successful
borylation at ortho positions with both B 2 pin 2 and HBpin as boron sources. Moreover, the Bpza group could be selectively converted to Bpin, OH, and Ar groups
with retention of the newly introduced Bpin group.
Phosphine-directed borylation catalyzed by [Ir(cod) 2 ][BF 4 ] with 3,4,7,8tetramethyl-1,10-phenanthroline (tmphen) ligand was demonstrated by Clark et al.
(Scheme 86b) [211]. The borylated products were isolated as forms of phosphineborane by adding BH 3 ÁTHF after the Ir-catalyzed reaction. Benzylic phosphines,
indole-based phosphines, and a ferrocenyl phosphine were applicable to this
dehydrogenative borylation.
Xu et al. reported pyridine-directed dehydrogenative borylation of benzyl-2pyridines proceeding at room temperature and affording intramolecularly coordinated pyridine-boronate adducts (Scheme 86c) [215]. Borylation at the ortho positions of benzylic moieties was smoothly catalyzed by [Ir(OMe)(cod)] 2 without any
supporting ligands.
Scheme 84 Borylation of indoles at C-7 and C-2 positions
Scheme 85 Borylation of N-Boc-protected heterocycles
Iridium-Catalyzed Dehydrogenative Reactions
53
been reported by a few groups. Suginome et al. reported an efficient route to
1,2-diborylarenes via boronyl group-directed ortho C–H borylation catalyzed by
[Ir(OMe)(cod)] 2 without an ancillary ligand (Scheme 86a) [214]. The use of
pyrazolylaniline-modified phenylboronic acids (Ph-Bpza) resulted in successful
borylation at ortho positions with both B 2 pin 2 and HBpin as boron sources. Moreover, the Bpza group could be selectively converted to Bpin, OH, and Ar groups
with retention of the newly introduced Bpin group.
Phosphine-directed borylation catalyzed by [Ir(cod) 2 ][BF 4 ] with 3,4,7,8tetramethyl-1,10-phenanthroline (tmphen) ligand was demonstrated by Clark et al.
(Scheme 86b) [211]. The borylated products were isolated as forms of phosphineborane by adding BH 3 ÁTHF after the Ir-catalyzed reaction. Benzylic phosphines,
indole-based phosphines, and a ferrocenyl phosphine were applicable to this
dehydrogenative borylation.
Xu et al. reported pyridine-directed dehydrogenative borylation of benzyl-2pyridines proceeding at room temperature and affording intramolecularly coordinated pyridine-boronate adducts (Scheme 86c) [215]. Borylation at the ortho positions of benzylic moieties was smoothly catalyzed by [Ir(OMe)(cod)] 2 without any
supporting ligands.
Scheme 84 Borylation of indoles at C-7 and C-2 positions
Scheme 85 Borylation of N-Boc-protected heterocycles
Iridium-Catalyzed Dehydrogenative Reactions
53