Oxidation and Reduction by Solid Oxidants and Reducing Agents using Ball-Milling - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 74

OH

O HO OH

Zn/ZnCl 2
m.m. (30-58.3 Hz)
or grinding (1.7 Hz)
r.t. 0.5-1 h

R 1 R 1 R 1

H

R 1 R 2

R 2 R 2

R 2

(20-90%) (0-28%)

Scheme 3.17 Mechanochemical reaction of aromatic ketones and aldehydes in the
presence of Zn/ZnCl 2 .

R

O

N

(a)

R

N

Bi metal
m.m. (30 Hz)
r.t.; 1.5 h

R

R

N

NO 2

(b)

N

R

Scheme 3.18 Solvent free reduction of nitroarenes to azoxy and azoarenes using

bismuth shots; milling apparatus used: Retsch mixer mill, MM200
(Retsch GmbH, Haan, Germany).

temperatures. Milled samples possess much higher surface areas than both
unmilled mixtures and mixtures of separately milled components. 21
Wang and Liu have reported that the mechanochemical reaction of aro-
matic ketones and aldehydes in the presence of Zn-ZnCl 2 in a high-speed
vibration mill (Retsch MM200 mixer mill and Retsch RM100 mortar grinder)
gave the pinacol coupling product and reduction product in varying yields
(Scheme 3.17). Of the three possible activation methods, the gentler Retsch
RM100 mortar grinder technique afforded a much higher yield and im-
proved pinacol coupling product selectivity. However, notably, the pinacol
coupling products were formed predominantly or exclusively in all cases. 22
A solvent-free, one-step reduction of nitroarenes to azoxy and azoarenes
using bismuth shots or lead shots under milling conditions was first de-
scribed by Suzuki and co-workers. 23 The reaction was highly chemoselective,
and gave good yields in short reaction times (Scheme 3.18). Although the
reaction with Bi shot gave azoxyarenes as major products in aerobic con-
ditions, azoarenes were predominantly formed under the protection of a
nitrogen atmosphere (Table 3.11).
Interestingly, the addition of a drop of an inert organic solvent such as
hexane or benzene to the nitroarene bismuth mixture prior to milling led to
the complete formation of azoxyarenes. Replacement of Bi with Pb selectively
provided azoarenes instead. Scheme 3.19 shows the proposed pathway for the
deoxygenative dimerization of nitroarenes on the activated bismuth surface.
Nitroarenes were adsorbed and deoxygenated on the newborn bismuth
surface to form nitrosoarenes as the initial products, which underwent di-
merization to afford azoxyarenes. In the presence of oxygen or an additive,
part of the active metal surface is destroyed or occupied by these molecular