As concluding remarks it has to be said that iridium-catalyzed C–H borylation
keeps being a source of inspiration for chemists around the world that search
efficient and suitable methods to activate C(sp
2 )–H and C(sp
3 )–H bonds and convert
them into versatile C–B bonds. [Ir(μ-OMe)(COD)] 2 can activate B 2 pin 2 as well as
HBpin, in the presence of 4,4
0 -di-tert-butyl bipyridine (dtbpy). But new challenges
have been acquired in recent years by using alternative ligands such as 3,4,7,8tetramethyl-1,10-phenanthroline (Me 4 Phen) and 1,10-phenanthroline (phen), among
others. The selective formation of the new C–B bond can be rationalized by a
conjunction of steric and electronic issues. Essentially, the lack of directing groups
increases the interest of the new catalytic systems able to afford high levels of
chemo-, regio-, and stereoselectivity. The preference of C(sp
3 )–H borylation versus
C(sp
2 )–H borylation opens an unprecedented selective functionalization of C–H
bonds. In mostly of the new approaches, theoretical calculations have deeply studied
all the mechanistic pathways to understand the rate-limiting steps and propose
alternative catalytic cycles.
The future of the field might concern the development of improved catalytic
systems to generalize even more the reaction trends, but the exploration of new
reaction media to improve the sustainability of iridium C–H borylation remains
crucial. Mechanochemistry has recently been applied for the first time to iridium(I)catalyzed C–H borylation by the group of Ito and co-workers [44]. By using either
none or just a catalytic amount of a liquid, the mechanochemical borylation can be
performed in an efficient way. Therefore, the future might be orientated to determine
a careful choice of the reaction milling, jar, ball, and additives that can enable the
development of a solvent-free iridium(I)-catalyzed C–H borylation in air. Alternatively, the immobilization of iridium complexes, functionalized with bpy ligands,
into oligomers has been shown by Madrahimov and co-workers to efficiently
catalyze the C–H borylation of arenes with bis(pinacolato)diboron under mild
conditions to produce a variety of arylboronate compounds [45]. The activity of
this heterogenized catalyst is similar to that of an original non-recyclable catalyst
which allows it to be used under milder conditions than other reported recyclable
catalysts. The challenging oligomer that supports the Ir catalyst can be successfully
recovered through biphasic extraction and reused for several cycles without a loss of
activity.
Globally, the idea to borylate the most abundant yet inert bonds represents an
exciting area of work. But if that strategy allows natural products to be created in a
selective way, all the efforts are justified. Liu and Yuan have collected the most
representative synthetic examples of high degree of molecular complexity from
relatively simple building blocks via C–H borylation with iridium complexes
[46]. The most significant approaches from 2014 are Baran’s total synthesis of
verruculogen and fumitremorgin A [21], Jia’s total synthesis of (À)-goniomitine
[47], Sperry’s total synthesis of scalaridine A [48], or Shibata’s total synthesis of
cis-clavicipitic acid [49].
Iridium-Catalyzed Undirected Homogeneous C–H Borylation Reaction
223
keeps being a source of inspiration for chemists around the world that search
efficient and suitable methods to activate C(sp
2 )–H and C(sp
3 )–H bonds and convert
them into versatile C–B bonds. [Ir(μ-OMe)(COD)] 2 can activate B 2 pin 2 as well as
HBpin, in the presence of 4,4
0 -di-tert-butyl bipyridine (dtbpy). But new challenges
have been acquired in recent years by using alternative ligands such as 3,4,7,8tetramethyl-1,10-phenanthroline (Me 4 Phen) and 1,10-phenanthroline (phen), among
others. The selective formation of the new C–B bond can be rationalized by a
conjunction of steric and electronic issues. Essentially, the lack of directing groups
increases the interest of the new catalytic systems able to afford high levels of
chemo-, regio-, and stereoselectivity. The preference of C(sp
3 )–H borylation versus
C(sp
2 )–H borylation opens an unprecedented selective functionalization of C–H
bonds. In mostly of the new approaches, theoretical calculations have deeply studied
all the mechanistic pathways to understand the rate-limiting steps and propose
alternative catalytic cycles.
The future of the field might concern the development of improved catalytic
systems to generalize even more the reaction trends, but the exploration of new
reaction media to improve the sustainability of iridium C–H borylation remains
crucial. Mechanochemistry has recently been applied for the first time to iridium(I)catalyzed C–H borylation by the group of Ito and co-workers [44]. By using either
none or just a catalytic amount of a liquid, the mechanochemical borylation can be
performed in an efficient way. Therefore, the future might be orientated to determine
a careful choice of the reaction milling, jar, ball, and additives that can enable the
development of a solvent-free iridium(I)-catalyzed C–H borylation in air. Alternatively, the immobilization of iridium complexes, functionalized with bpy ligands,
into oligomers has been shown by Madrahimov and co-workers to efficiently
catalyze the C–H borylation of arenes with bis(pinacolato)diboron under mild
conditions to produce a variety of arylboronate compounds [45]. The activity of
this heterogenized catalyst is similar to that of an original non-recyclable catalyst
which allows it to be used under milder conditions than other reported recyclable
catalysts. The challenging oligomer that supports the Ir catalyst can be successfully
recovered through biphasic extraction and reused for several cycles without a loss of
activity.
Globally, the idea to borylate the most abundant yet inert bonds represents an
exciting area of work. But if that strategy allows natural products to be created in a
selective way, all the efforts are justified. Liu and Yuan have collected the most
representative synthetic examples of high degree of molecular complexity from
relatively simple building blocks via C–H borylation with iridium complexes
[46]. The most significant approaches from 2014 are Baran’s total synthesis of
verruculogen and fumitremorgin A [21], Jia’s total synthesis of (À)-goniomitine
[47], Sperry’s total synthesis of scalaridine A [48], or Shibata’s total synthesis of
cis-clavicipitic acid [49].
Iridium-Catalyzed Undirected Homogeneous C–H Borylation Reaction
223