alcohols as well as the α-alkylation of ketones can be achieved via manganesecatalyzed hydrogen autotransfer reactions.
Yu and coworkers used an NN-based tricarbonyl complex containing a
methoxide ligand for β-alkylation of secondary alcohols. A broad variety of aromatic
and aliphatic secondary alcohols could be alkylated using Mn32. The synthetic
importance of the introduced protocol could be demonstrated by selective
β-alkylation of two steroid derivatives [70].
El-Sepelgy, Rueping, and coworkers compared the reactivity of different PNNand PNP-based pincer complexes for the β-alkylation of secondary alcohols using
primary alcohols as alkylation agent, whereas Mn33 showed the highest reactivity of
the given reaction. Apart from that, the coupling of two aliphatic alcohols was
reported [71].
An interesting way of synthesizing cycloalkenes from diols and secondary
alcohols or ketones was reported by the group of Leitner in 2019 employing Mn1.
This procedure allowed the formation of cycloalkanes with a ring size of five to
seven. However, a large excess of diols as alkylation agents had been used in order to
suppress the formation of lactones as side products [72]. Very recently, the same
group reported on the methylation of primary and secondary alcohols, employing
methanol as carbon source. In the case of secondary alcohols as substrate, demethylation could be achieved [73]. An overview of the abovementioned reaction is
given in Scheme 43.
Beller and coworkers employed manganese-catalyzed HAT for the α-alkylation
of ketones with primary alcohols using an aliphatic PNP-supported complex (Mn1)
as it can be seen in Scheme 44. This procedure allowed the use of a broad variety of
different aromatic and aliphatic ketones. In addition to that, benzylic as well as
aliphatic alcohols could serve as carbon source. The potential use of the established
system for fast derivatization of hormones was demonstrated, whereas estrone and
testosterone derivatives could selectively be alkylated in the α-position [74].
The group of Milstein compared the performance of Mn11, Mn23, and Mn26 for
the alkylation of ketones with primary alcohols, whereas Mn26 showed slightly
higher reactivity for the desired reaction. In contradiction to other procedures, the
installed protocol operated with an equimolar amount of ketone substrate and
alcohol which lead to high atom efficiency. The β-alkylation of secondary alcohols
with primary alcohols and consecutive oxidation resulted in the formation of a
ketone group [75].
The investigation of NN and NNN chelating ligands for the phosphine-free
α-alkylation of ketones was reported by Maji and coworkers in 2018. Upon the
investigated ligands, the hydrozone-based NNN ligand revealed the highest reactivity in combination with [Mn(CO) 5 Br] as precursor. Unfortunately, the complexion
of L12 with the precursor had to be done prior to the catalytic reaction resulting a
time-consuming additional step [76].
An ADC of ketones with primary alcohols to yield α,β-unsaturated ketones was
introduced by Guanathan employing Mn15’. Within this context a catalyst loading
as low as 0.3 mol% with the use of Cs 2 CO 3 as weak base was reported [77].
The Role of Metal-Ligand Cooperation in Manganese(I)-Catalyzed. . .
253
Yu and coworkers used an NN-based tricarbonyl complex containing a
methoxide ligand for β-alkylation of secondary alcohols. A broad variety of aromatic
and aliphatic secondary alcohols could be alkylated using Mn32. The synthetic
importance of the introduced protocol could be demonstrated by selective
β-alkylation of two steroid derivatives [70].
El-Sepelgy, Rueping, and coworkers compared the reactivity of different PNNand PNP-based pincer complexes for the β-alkylation of secondary alcohols using
primary alcohols as alkylation agent, whereas Mn33 showed the highest reactivity of
the given reaction. Apart from that, the coupling of two aliphatic alcohols was
reported [71].
An interesting way of synthesizing cycloalkenes from diols and secondary
alcohols or ketones was reported by the group of Leitner in 2019 employing Mn1.
This procedure allowed the formation of cycloalkanes with a ring size of five to
seven. However, a large excess of diols as alkylation agents had been used in order to
suppress the formation of lactones as side products [72]. Very recently, the same
group reported on the methylation of primary and secondary alcohols, employing
methanol as carbon source. In the case of secondary alcohols as substrate, demethylation could be achieved [73]. An overview of the abovementioned reaction is
given in Scheme 43.
Beller and coworkers employed manganese-catalyzed HAT for the α-alkylation
of ketones with primary alcohols using an aliphatic PNP-supported complex (Mn1)
as it can be seen in Scheme 44. This procedure allowed the use of a broad variety of
different aromatic and aliphatic ketones. In addition to that, benzylic as well as
aliphatic alcohols could serve as carbon source. The potential use of the established
system for fast derivatization of hormones was demonstrated, whereas estrone and
testosterone derivatives could selectively be alkylated in the α-position [74].
The group of Milstein compared the performance of Mn11, Mn23, and Mn26 for
the alkylation of ketones with primary alcohols, whereas Mn26 showed slightly
higher reactivity for the desired reaction. In contradiction to other procedures, the
installed protocol operated with an equimolar amount of ketone substrate and
alcohol which lead to high atom efficiency. The β-alkylation of secondary alcohols
with primary alcohols and consecutive oxidation resulted in the formation of a
ketone group [75].
The investigation of NN and NNN chelating ligands for the phosphine-free
α-alkylation of ketones was reported by Maji and coworkers in 2018. Upon the
investigated ligands, the hydrozone-based NNN ligand revealed the highest reactivity in combination with [Mn(CO) 5 Br] as precursor. Unfortunately, the complexion
of L12 with the precursor had to be done prior to the catalytic reaction resulting a
time-consuming additional step [76].
An ADC of ketones with primary alcohols to yield α,β-unsaturated ketones was
introduced by Guanathan employing Mn15’. Within this context a catalyst loading
as low as 0.3 mol% with the use of Cs 2 CO 3 as weak base was reported [77].
The Role of Metal-Ligand Cooperation in Manganese(I)-Catalyzed. . .
253