Carbon–Carbon Bond Forming by Ball Milling - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 41

The syntheses of 2,2 0 -dihydroxy-1,1 0 -binaphthyl via oxidative coupling of
2-naphthol was realized in a planetary ball mill (Pulverisette 7, Fritsch
GmbH), giving 87% yield (Scheme 2.11, method A). 26 The substrate was ball
milled with FeCl 3 6H 2 O and NaCl for 60 min at ambient temperature.
This reaction was improved by Shayesteh and co-workers. 25 2-Naphthol
and FeCl 3 6H 2 O were placed in stainless steel capsules and shaken with a
vibration mill at 50 s 1 (Scheme 2.11, method B). After 8 min, 95% yield was
achieved. The reaction was controlled with the help of TLC and UV spec-
troscopy. Additionally, the reaction method was compared to classic syn-
theses in solvents to emphasize the good reaction conditions (short time,
room temperature).

2.3.2 Glaser Coupling
Investigations of Sonogashira reactions have noted that there is a small
amount of the homo-coupling side product. Replacing the Pd-catalyst with a
copper species, the product (1,4-diphenylbuta-1,3-diyne) was achieved after
10 min in 93% isolated yield (Scheme 2.12). 10 A mixer mill from Retsch was
used (MM301) in this reaction. Extensive investigations of the reagent sys-
tem showed that KF/Al 2 O 3 is most suitable for the homo-coupling. 27 Thereby
different modifications and prior treatment of the additives were examined.
Furthermore the influence of KF amount and other bases were tested.
A catalyst screening with various metal salts revealed that, beside Cu-
catalysts, Ni- and Co-salts can promote the reaction while with Fe-salts no
conversion could be observed.
The Glaser coupling also took place using a Pd-catalyst. 28 Application of
[Pd(PPh 3 ) 4 ] gave 74% yield and with [PdCl 2 (PPh 3 ) 2 ], and an excess of PPh 3 ,
90% homo-coupling product was generated. Reactions were performed in
stainless steel or Teflon s milling vials, but the material showed no signifi-
cant effect on the yield. Different phenylacetylene derivatives could be con-
verted using this method in 62-82% yield (Scheme 2.13).

2 equiv. FeCl 3 . 6H 2 O
NaCl
method A: NaCl, PBM, 60 min
method B: MBM, 50 s -1 , 8 min

OH

2

HO

OH

Scheme 2.11 Iron( III )-catalyzed homo-coupling of 2-naphthol.

5 mol% CuI
1.25 equiv. DABCO
KF/Al 2 O 3
MBM
30 s -1 , 10 min

2

Scheme 2.12 Glaser-coupling with KF/Al 2 O 3 as milling auxiliary.