Copper catalysed Gomberg-Bachmann-Hey reactions of arenediazonium tetrafluoroborates and heteroarenediazonium o-benzenedisulfonimides. Synthetic and mechanistic aspects

Article abstract of DOI:10.1016/j.tet.2020.131632

Gomberg-Bachmann-Hey reactions were carried out in the presence of copper as a catalyst and gave rise to biaryls or heterobiaryls in good yields and in mild reaction conditions. A computational study of some key points of the reaction was performed. The results are coherent with the experimental data and confirm some aspects of the mechanism. The reaction free energies for the reduction in benzene by CuI of a set of 40 (hetero)arenediazonium tetrafluoroborates were calculated. Both the experiments and the calculations showed that in the coupling with substituted solvents (toluene, bromobenzene, nitrobenzene and anisole) the binding to the ortho position was always favoured.

Full text of DOI:10.1016/j.tet.2020.131632

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Copper catalysed Gomberg-Bachmann-Hey reactions of arenediazonium
tetrafluoroborates and heteroarenediazonium o-benzenedisulfonimides. Synthetic and
mechanistic aspects
Achille Antenucci, Margherita Barbero, Stefano Dughera, Giovanni Ghigo
PII:
S0040-4020(20)30839-5
https://doi.org/10.1016/j.tet.2020.131632
TET 131632
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 23 July 2020
Revised Date: 19 August 2020
Accepted Date: 4 September 2020
Please cite this article as: Antenucci A, Barbero M, Dughera S, Ghigo G, Copper catalysed Gomberg-
Bachmann-Hey reactions of arenediazonium tetrafluoroborates and heteroarenediazonium o-
benzenedisulfonimides. Synthetic and mechanistic aspects, Tetrahedron, https://doi.org/10.1016/
j.tet.2020.131632.
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1
Copper catalysed Gomberg-Bachmann-Hey reactions of arenediazonium tetrafluoroborates
and heteroarenediazonium o-benzenedisulfonimides. Synthetic and mechanistic aspects.
Achille Antenucci, Margherita Barbero, Stefano Dughera*, Giovanni Ghigo*
Dipartimento di Chimica, Università di Torino, Via P. Giuria 7, 10125 Turin, Italy.
* Corresponding authors
Tel: +39(011)6707823; e-mail: stefano.dughera@unito.it
Tel: +39(011)6707872; e-mail: giovanni.ghigo@unito.it
Keywords: Diazonium salts. Gomberg-Bachmann-Hey reaction. Copper catalysis. Mechanistic
studies. C-H bond activation.
Abstract: Gomberg-Bachmann-Hey reactions were carried out in the presence of copper as a
catalyst and gave rise to biaryls or heterobiaryls in good yields and in mild reaction conditions. A
computational study of some key points of the reaction was performed. The results are coherent
with the experimental data and confirm some aspects of the mechanism. The reaction free energies
for the reduction in benzene by CuI of a set of 40 (hetero)arenediazonium tetrafluoroborates were
calculated. Both the experiments and the calculations showed that in the coupling with substituted
solvents (toluene, bromobenzene, nitrobenzene and anisole) the binding to the ortho position was
always favoured.
1. Introduction
The preparation of biaryls by reactions of arenediazonium salts with aromatic or heteroaromatic
compounds in an alkaline environment is referred to as the well-known Gomberg–Bachmann-Hey
(GBH) reaction,1 which was first published in 1924.1a This reaction is quick and easy to carry out
with good functional group tolerance; with regard to the mechanism its homolytic nature has been
widely demonstrated (Scheme 1).2

2
Scheme 1. GBH reaction.
However, the yields, due to a number of side-reactions, are not so high and the regioselectivity is
very scarce. Mainly for these reasons, over the years, metal catalysed cross-coupling reactions have
supplanted GBH reaction.3 However, as direct activation of C-H bonds are always interesting for
present-day organic synthesis,4 as the biaryl scaffold is a significant substructure in several
bioactive molecules5 and as very stable diazonium salts, including tetrafluoroborates6 and o-
benzenedisulfonimides7 are easily available nowadays, GBH reaction has recently been
rediscovered1d with the development of new improved protocols, mainly involving the use of a wide
variety of inducers or catalysts, including phase-transfer catalysts,8 photoredox catalysts,9
electrolytic systems10 and other organic catalysts, such as ascorbic acid11a and phenalenyl
derivatives,11b-g able to inducing the formation of aryl radicals from the diazonium salts.
Moreover, several methods, involving the use of some complexes of transition metals including
Ti,12 Au (under photoredox conditions),13 Fe14 and Co15 as catalysts have been reported; however,
only one study has been carried out on the Cu catalysed GBH reactions. Interestingly, copper
enjoyed a resurgence at the beginning of the 21st century, due to its lower cost and toxicity and to
its high abundance.
On this basis, in 2011 Akamanchi16 presented a Cu catalysed synthetic protocol in order to
synthesize biaryls via diazotization of anilines with p-toluenesulfonic acid and NaNO2, in aromatic
solvents at 50°C. It must be stressed that there are no mechanistic insights in the only Cu-catalyzed
GBH reaction known in the literature though it shows two examples of experimental and theoretical
studies on copper-catalysed arylation of heterocycles (e.g. pyrroles17 and benzoxazoles18) through
arenediazonium salts.
In the light of these, in this paper we wish to report a comprehensive synthetic and theoretical study
on the reactions between various arenediazonium tetrafluoroborates or heteroarenediazonium o-
benzenedisulfonimides and several arenes in the presence of cesium carbonate and a catalytic
amount of CuI in order to obtain biaryls or heterobiaryls, under the GBH protocol (Scheme 2).

3
I. Previous work
Ar-NH2
4-CH3C6H4SO3H/NaNO2
TEOF/CuCl; 50°C
Ar'-H
Ar-N2+ 4-CH3C6H4SO3
Ar-Ar'
16 examples; average yield: 58%16
II. This work
Ar-N2+ BF4
Ar-Ar'
HetAr-Ar'
CuI/Cs2CO3; rt
Ar'-H
-
O2
S
HetAr N2
N
S
O2
33 examples; average yield: 72%
Scheme 2. Cu-catalysed GBH reactions.
2. Results and discussion
In
order
to
optimize
the
reaction
conditions, a model reaction between 4-
methoxybenzenediazonium tetrafluoroborate (1a) and benzene (2a) was studied under various
conditions (Table 1).
Differing amounts of benzene 2a and various bases (NaOH, Na2CO3, K3PO4, pyridine, Cs2CO3)
were tested (Table 1). The optimal combination was 20 equiv of 2a, 2 mol% of CuI as a a catalyst
and Cs2CO3 as a base (1 equiv; Table 1; entry 19). Cesium carbonate led to far better results than
the other tested bases due to its better solubility in organic solvents. 4-Methoxybiphenyl (3a) was
obtained in good yield (72%) at room temperature and was easily purified from the only by-
product (anisole) using a short chromatographic column. Crucially, control experiments in the
absence of Cu catalyst (Table 1; entries 25, 27, 28, 29) and base (Table 1; entry 26) resulted in no or
low conversion. Moreover, optimisation studies showed that some excess of benzene 2a is
advantageous (20 equiv) but greater excess (30 equiv; Table 1; entry 24) does not lead to substantial
yield improvements. Even heating to 50 °C (Table 1; entry 21), a higher amount of CuI (Table 1;
entry 22) or Cs2CO3 (Table 1; entry 23) did not give better results. It must be stressed that, under
optimal condition (Table 1, entry 19), the reactions were carried out in benzene which, being in
large excess, also acts as a solvent. In fact, in some collateral tests, the reaction was carried out,
with unsatisfactory results, using a stoichiometric amount of benzene in both polar aprotic (MeCN
and DMSO. Table 1; entries 14‒15) and protic solvents (EtOH. Table 1; entry 16) Moreover, in the
reactions carried out with an excess of benzene, the presence of EtOH (Table 1; entries 18, 20) as a
solvent did not allow to increase the yield of 3a.

4
The formation of small amounts of 3a without the presence of copper (entries 27‒29) indicates the
occurrence of a reaction mechanism similar to classical GBH. However, yields are low and the
results with copper are more promising, suggesting a more competitive reaction mechanism (see
below).
Table 1 Trial reactions: optimization of reaction conditions.
Entry Ratio 1a: 2a (equiv)
Base
NaOH
NaOH
NaOH
NaOH
NaOH
Na2CO3
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
Pyridine
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Time (h)
Yield of 3a (%)a,b
1
2
1:1
1:5
8
8
8
8
8
8
8
8
4
4
1
8
8
8
8
8
3
traces
traces
traces
traces
traces
-
3
1:10
1:15
1:50
1:1
4
5
6
7
1:1
traces
25
8
1:5
9
1:20
1:30
1:30
1:20
1:1
55
10
11
12
13
14
15
16
17
52
45c
25
23
1:1
20c
1:1
22d
1:1
33e
1:10
65

5
18
19
20
21
22
23
24
25
26
27
28
29
1.10
1:20
1:20
1:20
1:20
1:20
1:30
1:20
1:20
1:20
1:20
1:20
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
-
3
3
3
3
3
3
3
3
8
8
3
4
67e
72
71f
54g
72h
70i
72
Tracesj
Tracesk
15l
Cs2CO3
Cs2CO3
NaOH
25m
27m
a
All the reactions were performed at rt, unless explicitly reported, with 2 mol% of CuI catalyst, 1
equiv. of chosen base and without any solvent unless explicitly reported. b Yields refer to pure and
isolated 3a, purified from anisole, MS (EI) m/z = 108 [M]+ (only by-product) on a silica gel
chromatography column. The eluent was petroleum ether.c The reaction was carried in MeCN (15
mL) as a solvent. d The reaction was carried out in DMSO (15 mL) as a solvent. e The reaction was
carried out in EtOH as a solvent (15 mL).f The reaction was carried out with 20 equiv of 2a in
h
EtOH as a solvent (15 mL).g The reaction temperature was 50 °C. The reaction was carried out
with 5 mol% of CuI catalyst.i The reaction was carried out with 1.5 equiv. of Cs2CO3. j The reaction
was carried out without CuI catalyst. k The reaction was carried out without base. l The reaction was
carried out without CuI catalyst and with 1 equiv. of Cs2CO3. mThe reaction was carried out without
CuI catalyst and with 1 equiv. of bases at 50°C.
Encouraged by these results and with optimised conditions in hand, a number of other diazonium
salts 1, bearing either electron withdrawing or electron donating groups, were reacted.
Good yields of target biaryls 3 were obtained (Table 2); however, the electron-rich 1a, 1d reacted
more slowly than 1b, 1c, 1e, 1f with decreasing yields observed in the target biaryls 3. Moreover,
the influence of steric hindrance was evident: lower yields of 3i were obtained when two ortho
substituents to the diazonium group were present (salt 1i).

6
In order to expand the scope of this protocol, we also decided to test heteroaromatic diazonium
salts. However, since tetrafluoroborates are rather unstable and difficult to isolate, we synthesized
four more stable and easy to handle heteroarenediazonium o-benzenedisulfonimides 4a-d19 and, as
reported in Table 3, the reactions carried out in the presence of 4a-d and 2 provided heterobiaryls
5a-d in good yields.
Table 2. Arenediazonium tetrafluoroborate 1 scope.
a
Yields refer to pure and isolated 3, purified from corresponding Ar-H (only by-products) on a
silica gel chromatography column. The eluent was always petroleum ether. b Reaction time was 3h.c
Reaction time was 1 h. d Reaction time was 1.5 h

7
Table 3. Heteroarendiazonium o-benzenedisulfonimides 4 scope.
a Yields refer to pure and isolated 5, purified from corresponding HetAr-H (only by-products) on a
silica gel chromatography column. The eluent was always petroleum ether. b Reaction time was 1h.c
Reaction time was 1.5 h.
As noted above, the electron poor (hetero)arenediazonium react faster. This aspect seems to be
coherent with the mechanism proposed in literature for this,1d,20,21 and similar copper-catalysed
reactions, such as Sandmeyer protocol, where the first step22,23,24 is the rate-determining
arenediazonium reduction by copper (step 1, ET - Electron Transfer in Scheme 3).
ET
+
-
I
BF4CuIII
+
Ar N2 BF +
Ar N2
Cu I
(1)
4
1
+
Ar
N2
Ar N2
(2)
(3)
Ar
+
Ar
H
2
Ar
Ar
H
ET
-
BF4CuIII
CuII
(4')
+
+
BF4
H
and/or
Ar
ET
Ar
-
+
-
(4")
+
+
Ar N2
BF4
Ar N2 BF4
H
H
1
Ar
- + CO3
--
-
-
(5)
BF4
+ HCO3 + BF4
Ar
H
3
Scheme 3. Proposed mechanism.
The mechanism of this step is far from being fully understood and appears to be very complex,
considering similar cases.24,25 Our preliminary studies applying the Marcus’outer-sphere model
p-NO2
p-CH3O
gave unrealistic results (k1
/ k1
> 107, see Table S0 of the Supplementary data).
Therefore, for what concerns step 1, the present study is limited to thermochemical calculations,

8
and their qualitative comparison with experimental data is discussed herein; further details can be
found in Appendix 1.
Table S1 of the Supplementary data compares the calculated free energy for step 1 in benzene
(ΔGET) with the yields and reaction times for the reactions reported in Tables 2 and 3. The
qualitative correlation between the data is evident: the larger negative ΔGET correspond to the
higher yields and/or shorter times. However, the correlation is far from being quantitative as
Figures S2 clearly shows: the R2 is only 0.29, testifying that the picture is quite complex. We must
keep in mind that small amounts of product are obtained even without copper iodide (see Table 1,
entries 25,27−29).
The products of the reduction of the arenediazoniums are the arenediazenyl radicals which give rise
to an unimolecular decomposition generating N2 and the aryl radicals (step 2, Scheme 3). Initially,
an upper limit of 108 s-1 was proposed26 for the rate constants of this process. Later, the activation
parameters and the rate constants were experimentally estimated in cyclopropane at low
temperatures ( -117 to -48°C) for some arenediazenyl radicals.27 The agreement between our
calculated activation energies and the experimentals is not exceptional (see Table S3 and Figure S4
of the Supplementary data), however, we must remember that the calculations are performed for
different reaction conditions (solvent and temperature). In any case, the estimated rate constants are
very high (104 ‒ 107 s-1 ) confirming that, with all due caution on the interpretation of calculations
and considering the long reaction times (hours), step 2 is probably not rate-determining. Although
not fully comparable, we can also report that the bimolecular rate constants measured for the
reduction of aryldiazonium
by FeII complexes,
range
from
10-4
to 101
M-1 s-1, depending also on the ligand (cyanide or cyclopentadienyl anion).25b
Step 3 is the addition of the aryl radicals to benzene. This is the key step in the coupling reaction.20
The experimental bimolecular rate constant in Freon 113 for this reaction has been estimated to be
4.5 • 105 M-1 s-1.28 Later, the reaction and activation energies for the bimolecular addition of the 4-
chlorophenyl radical to benzene (and other aromatics) were calculated by Hoffmann et al.1c. Here,
we calculate the energies and rate constants for the addition of the aryl radicals using the π complex
between them and benzene because, as the latter are in a such concentration as to be considered as a
solvent and thus already in contact with the radicals, the reaction can be assumed to be mono-
molecular. This approach allowed us to estimate the reaction rate constants for step 3 which range
from 104 to almost 109 sec-1 (see Table S5 of the Supplementary Data). The formation of the radical
adducts is irreversible, being, in all cases, exoergonic by 15 ‒ 22 kcal mol-1 in term of energies
(similar to that found in literature1c) and 13 ‒ 20 kcal mol-1 in terms of free energies.

9
The oxidation of the radical adducts formed in step 3 could take place either by reduction of CuII
(step 4’) or by a second arenediazonium (step 4”). Literature1c,29 suggests the latter as the oxidation
properties of the arenediazoniums have been known since 1991.30 However, without step 4’, the
regeneration of CuII that would close the catalytic circle, would remain unexplained. In any case,
this process consists of an ET which, for the moment, we do not computationally explore.
Step 6 yields the final products. This plausible role for base is coherent with the observation that
without it reaction yields are very poor (entry 26, Table 1).
With these optimised conditions in hand, an arene (or heteroarene) scope was also carried out
(Table 4). Suitable arenes were evaluated; p-dichlorobenzene (2b), mesitylene (2c) and p-xylene
(2e) coupled with good yields, but the steric hindrance did not facilitate the reaction between 2d and
2,6-dimethylbenzenediazonium tetrafluoroborate (1i). Interestingly, the reactions carried out with
biphenyl (2d), were regioselective; only 3n and 3o were obtained.
Table 4. Arene (heteroarene) scope.

10
a Yields refer to pure and isolated 3, purified from corresponding Ar-H (only by-products) and from
b
the excess of 2 on a silica gel chromatography column. The eluent was always petroleum ether.
c
d
Reaction time was 1.5h. Reaction time was 3h. Reaction time was 1h.e Reactions were carried
out in MeCN as a solvent.
On the border of GBH and Meerwein reactions,31 we also used N-methylpyrrole (2f) and N-
methylindole (2g) as heteroarenes. The yields of target 3r-u were good, with excellent
regioselctivity. On the other hand, predictably, biaryls are formed as a mixture of o-/m-/p-isomers
3v-ae, with the clear prevalence of the ortho regioisomers in the coupling carried out with
monosubstitued arenes 2h-k (Table 5).
The mild regioselectivity observed in the formation of 3v-ae is reproduced quite well by the
computations (see Table S6 in the Supplementary Data). A good reproduction was also achieved in
other works.1c,17 The activated toluene is only slightly more reactive than benzene towards the
radical addition. The ratio between the experimental rate constants28 for the addition of the phenyl
radical to toluene (1.7 • 106 M-1 s-1 ) and benzene (4.5 • 105 M-1 s-1) in Freon 113 is 3.8 while that
between the calculated rate constants in the respective solvents (toluene, 5.4 • 107 s-1, entry 13 in
Table S5 and benzene, 2.9 • 107 s-1, entry 4 in Table S6) is 1.9.
Table 5. Arene scope.

11
a Yields refer to pure and isolated 3, purified from corresponding Ar-H (only by-products) and from
b
the excess of 2 on a silica gel chromatography column. The eluent was always petroleum ether.
c
d
Reaction time was 1.5h. Reaction time was 3h. Reaction time was 1h.e Reactions were carried
out in MeCN as a solvent.f The o,m,p ratio was determined by GC analysis by comparison of
retention times with standard o,m,p regioisomers and area percent. Standard regioisomers were
synthesized with a Pd(OAc)2 catalysed Suzuki reaction.
From inspecting Table S6 we can also observe that the slowest radical addition takes place between
the two nitrosubstituted aryls (3ab, entry 7) while the fastest ones are the additions of the p-bromo
and p-nitrophenyl radicals to toluene (3w and 3x, entries 2 and 3). Other cases for the coupling
with toluene and anisole have been calculated and reported in the Table S7 of the Supplementary
data. Tables S6 and S7 show that the ortho position is always favoured.
Even in this set of reactions, electronic effects confirmed the same general trend as above: the
diazonium salts bearing electron withdrawing groups react faster than the diazonium salts bearing
electron donating groups and generally lead to higher biaryl yields.

12
3. Conclusions
We have proposed a mild, easy and efficient Cu catalysed GBH coupling of arenediazonium
tetrafluoroborates 1 and heteroarenediazonium o-benzenedisulfonimides 4. The target products,
biaryls 3 or heterobiaryls 5 were generally obtained in satisfactory yields (33 positive examples,
80% average yield). The results of the computational study are coherent with the experimental data
and confirm that: a) the higher yields (and the shorter reaction times) are shown by the
arenediazonium salts to be easier to reduce; b) after the reduction of the arenediazonium salts, the
aryl radicals are rapidly generated and quickly react with the solvent generating the precursor of the
final product; c) in the case of a substituted benzene as the solvent (toluene, bromobenzene,
nitrobenzene and anisole), the binding to the ortho position is always preferred. The study of the
other steps of the reaction mechanism and, in particular, the electron transfer steps (1 and 4 in
Scheme 3) is currently in progress.
In the light of point a, the reaction free energies for the reduction in benzene by CuI of a set of 40
(hetero)arenediazonium tetrafluoroborates have been calculated in view of further application of the
copper catalysed Gomberg-Bachmann-Hey reactions.
4. Experimental
4.1.
General Information
All the reactions were carried out in open air glassware. Analytical grade reagents and solvents
(purchased from Sigma Aldrich or Alfa-Aesar) were used and reactions were monitored by GC,
GC-MS and TLC. Column chromatography and TLC were performed on Merck silica gel 60 (70-
230 mesh ASTM) and GF 254, respectively. Petroleum ether refers to the fraction boiling in the
range 40–70 °C. Room temperature (rt) is 22 °C. Mass spectra were recorded on an HP 5989B mass
selective detector connected to an HP 5890 GC with a methyl silicone capillary column. GC
analyses were performed on a Perkin Elmer AutoSystem XL GC with a methyl silicone capillary
1
13
column. H NMR and C NMR spectra were recorded on a Jeol ECZR spectrometer at 600 and
150 MHz respectively. Arenediazonium tetrafluoroborates were prepared as reported in the
literature.32 Dry heteroarenediazonium o-benzenedisulfonimides 4 were prepared as described
previously by us.17 Structures and purity of biaryls 3 and heterobiaryls 5 were confirmed by their
spectral (NMR, MS) and physical data, substantially identical to those reported in the literature.
Yields of pure (GC, GC-MS, TLC and NMR) isolated biaryls 3 are collected in Tables 2 and 4;
yields of pure (GC, GC-MS, TLC and NMR) isolated heterobiaryls 5 are collected in Table 3; their
NMR spectra are reported in Supplementary Data.

13
4.2.
Mixture of 4-Methoxy-2’-methylbiphenyl; 4-Methoxy-3’-methylbiphenyl; 4-Methoxy-4’-
methylbiphenyl (3v); typical procedure.
4-Methoxybenzenediazonium tetrafluoroborate (1a, 1.10 g, 5 mmol) was added at rt to a stirred
suspension of cesium carbonate (1.63 g, 5 mmol) and copper iodide (19 mg, 0.1 mmol) in toluene
(2h, 9.2 g, 100 mmol, 8 mL). Immediately, the colour of the solution turned black and slight
evolution of nitrogen was observed. The mixture was stirred at room temperature for 3.5 h; the
completion of the reaction was confirmed by the absence of azo coupling with 2-naphthol. Then,
the reaction mixture was poured into diethyl ether/water (100 mL, 1:1). The aqueous layer was
separated and extracted with diethyl ether (50 mL). The combined organic extracts were washed
with water (50 mL), dried with Na2SO4 and evaporated under reduced pressure. GC-MS analyses of
the crude residue showed a mixture of 4- methoxy2’-/3’/4’-methylbiphenyl (3a, MS (EI): m/z = 198
[M]+ each) as the major products, besides traces of anisole, MS (EI) m/z = 108 [M]+. The crude
residue (tarry solid) was purified on a short column, eluting with petroleum ether. The only isolated
product was title compounds (3v, 0.71 g, 72% yield). The ratio of 4-methoxy-2’-methylbiphenyl/4-
methoxy-3’-methylbiphenyl/4-methoxy-4’-methylbiphenyl was respectively 81% (retention time
11.08)/13% (retention time 11.83)/6% (retention time 11.97) and was determined by GC analysis by
comparison of retention times (initial temperature: 70°C; final temperature: 250°C; rate 20°C/min)
with standard 2’/3’/4’ regioisomers and area percent. Standard 4-methoxy-2’-methylbiphenyl, 4-
methoxy-3’-methylbiphenyl, 4-methoxy-4’-methylbiphenyl were synthesized with a Pd(OAc)2
catalysed Suzuki reaction by reacting 1a respectively with 2-tolyl, 3-tolyl, 4-tolylboronic acid and
they had the following retenction times (70-250°C; rate 20°C min): 4-methoxy-2’-methylbiphenyl
11.09 min; 4-methoxy-3’-methylbiphenyl 11.84 min; 4-methoxy-4’-methylbiphenyl 11.97 min. All
other biaryls 3 and heterobiaryls 5 were prepared according to this procedure, with the exception of
the reactions carried out with the solid biphenyl 2d. In this case, they were performed with 20 eq of
biphenyl (15.4 g) using 10 ml of MeCN as a solvent.
4.3.
Characterization
4.3.1. 4-Methoxybiphenyl (3a)
3a was obtained as a grey solid (0.66 g, 72% yield); mp 89–90 °C (MeOH; lit.33 mp 91°C); Rf
1
(petroleum ether ) 0.21; H NMR (600 MHz, CDCl3): δ 7.64−7.62 (m, 4H ), 7.44 (t, J = 7.2 Hz,
13
2H), 7.36 (t, J = 7.2 Hz, 1H), 7.06 (d, J = 9.0 Hz, 2H), 3.82 ppm (s, 3H); C NMR (150 MHz,
CDCl3): δ 159.4, 140.4, 131.2, 128.9, 128.7, 128.3, 127.3, 114.6, 56.0 ppm; MS (EI): m/z 184 (100,
M+)
4.3.2. 4-Bromobiphenyl (3b)

14
3b was obtained as a grey solid (0.99 g, 85% yield); mp 90–91 °C (MeOH; lit.34 mp 91°C); Rf
1
(petroleum ether ) 0.35; H NMR (600 MHz, CDCl3): δ 7.64−7.62 (m, 4H ), 7.52 (d, J = 7.2 Hz,
13
2H), 7.44 (t, J = 7.2 Hz, 2H), 7.36−7.35 ppm (m, 1H); C NMR (150 MHz, CDCl3): δ 140.5,
140.4, 131.6, 128.9, 128.6, 128.3, 127.3, 122 ppm; MS (EI): m/z 232 (100, M+), 234 (100, M+ +2).
4.3.3. 4-Nitrobiphenyl (3c)
3c was obtained as a pale yellow solid (0.81 g, 81% yield); mp 114–115 °C (MeOH; lit.35 mp 112–
1
113 °C); Rf (petroleum ether) 0.15; H NMR (600 MHz, CDCl3): δ 8.32 (d, J = 9.0 Hz, 2H), 7.90
(d, J = 9.0 Hz, 2H), 7.62−7.61 (m, 2H), 7.47−7.45 (m, 2H), 7.37−7.36 ppm (m, 1H); 13C NMR (150
MHz, CDCl3): δ 148.1, 144.9, 140.4, 129.1, 128.9, 128.3, 127.3, 124.5 ppm; MS (EI): m/z 199
(100, M+).
4.3.4. 4-Methylbiphenyl (3d)
3d was obtained as a pale yellow solid (0.55 g, 65% yield); mp 44–45 °C (MeOH; lit.36 mp 45–47
°C); Rf (petroleum ether) 0.31; 1H NMR (600 MHz, CDCl3): δ 7.63 (d, J = 6.6. Hz, 2H), 7.57 (d, J
= 7.2 Hz, 2H), 7.46−7.43 (m, 2H), 7.37−7.35 (m, 1H), 7.31 (d, J = 7.2. Hz, 2H), 2.36 ppm (s, 3H);
13C NMR (150 MHz, CDCl3): δ 148.4, 138.2, 136.9, 129.6, 128.9, 128.3, 127.3, 127.2, 21.1 ppm;
MS (EI): m/z 168 (100, M+).
4.3.5. 4-Acetylbiphenyl (3e)
3e was obtained as a yellow solid; (0.79 g, 81% yield); mp 118–120 °C (MeOH; lit.37 mp 120°C);
1
Rf (petroleum ether) 0.18; H NMR (600 MHz, CDCl3): δ 7.99 (d, J = 7.8. Hz, 2H), 7.77 (d, J =
7.8 Hz, 2H), 7.66-7.65 (m, 2H), 7.47−7.45 (m, 2H), 7.39-7.37 (m, 1H), 2.54 (s, 3H) ppm; 13C NMR
(150 MHz, CDCl3): δ 197.3, 140.4, 139.3, 137.8, 128.9, 128.6, 128.3, 127.3, 27.8 ppm; MS (EI):
m/z 196 (85, M+).
4.3.6. 4-Phenylbenzonitrile (3f)
3f was obtained as a grey solid (0.75 g, 84% yield); mp 87–88 °C (MeOH; lit.37 mp 86−89°C); Rf
1
(petroleum ether) 0.21; H NMR (600 MHz, CDCl3): δ 7.80 (d, J = 7.8. Hz, 2H), 7.75 (d, J = 7.8
13
Hz, 2H), 7.64-7.62 (m, 2H), 7.48−7.46 (m, 2H), 7.40−7.38 ppm (m, 1H); C NMR (150 MHz,
CDCl3): δ 144.8, 140.4, 133.7, 131.2, 128.9, 128.3, 127.3, 119.1, 115.7 ppm; MS (EI): m/z 179
(100, M+).
4.3.7. 2-Nitrobiphenyl (3g)
1
3g was obtained as a waxy solid (0.69 g, 69% yield); Rf (petroleum ether) 0.15; H NMR (600
MHz, CDCl3): δ 8.31 (d, J = 7.2 Hz, 1H), 7.87−7.79 (m, 2H), 7.63−7.58 (m, 3H), 7.46−7.44 (m,
13
2H), 7.38−7.36 ppm (m, 1H); C NMR (150 MHz, CDCl3): δ 148.5, 136.2, 133.2, 131.7, 130.3,
129.0, 127.9, 127.8, 127.6, 126.7 ppm; MS (EI): m/z 199 (100, M+).
4.3.8. 3-Nitrobiphenyl (3h)

15
3h was obtained as a grey solid (0.87 g, 87% yield); mp 57–58 °C (MeOH; lit.38 mp 56−58°C); Rf
1
(petroleum ether) 0.15; H NMR (600 MHz, CDCl3): δ 8.59 (s, 1H), 8.24 (d, J = 7.8 Hz, 1H), 8.00
13
(m, 1H), 7.72−7.69 (m, 1H), 7.65−7.64 (m, 2H), 7.46−7.44 (m, 2H), 7.38−7.36 ppm (m, 1H); C
NMR (150 MHz, CDCl3): δ 149.7, 145,1, 140.2, 136.5, 128.9, 128.5, 128.3, 127.1, 125.1, 124.7
ppm; MS (EI): m/z 199 (100, M+).
4.3.9. 2,6-Dimethylbiphenyl (3i)
3i was obtained as a pale yellow solid (0.49 g, 54% yield); mp 64–65 °C (MeOH; lit.39 mp 64–65
1
°C); Rf (petroleum ether) 0.34; H NMR (600 MHz, CDCl3): δ 7.59−7.57 (m, 2H), 7.46−7.43 (m,
2H), 7.38−7.36 (m, 1H), 7.27−7.26 (m, 1H), 7.20−7.19 (m, 2H), 2.33 ppm (s, 6H); 13C NMR (150
MHz, CDCl3): δ 140.9, 140.4, 134.3, 128.8, 128.5, 127.9, 127.1, 126.7, 20.6 ppm; MS (EI): m/z 182
(100, M+).
4.3.10. 2,5-Dichlorobiphenyl (3j)
1
3j was obtained as a viscous oil (0.72 g, 65% yield); Rf (petroleum ether) 0.38; H NMR (600
MHz, CDCl3): δ 7.58−7.57 (m, 1H), 7.52−7.51 (m, 2H), 7.46−7.44 (m, 3H), 7.41−7.40 (m, 1H),
7.34−7.33 ppm (m, 1H); 13C NMR (150 MHz, CDCl3): δ 140.0, 136.7, 134.5, 131.2, 130.7, 129.3,
128.8, 128.2, 127.8, 126.8 ppm; MS (EI): m/z 222 (100, M+).
4.3.11. 2,5-Dichloro-4’-methoxy-1,1’-biphenil (3k)
3k was obtained as grey solid (0.91 g, 72% yield); mp 53–54 °C MeOH; lit.40 mp 51–52 °C); Rf
1
(petroleum ether) 0.25; H NMR (600 MHz, CDCl3): δ 7.64 (s, 1H), 7.60 (d, J = 7.8 Hz, 2H)
13
7.41−7.40 (m, 2H), 7.33−7.31 (m, 2H), 7.05 (d, J = 7.8 Hz, 2H), 3.82 ppm ; C NMR (150 MHz,
CDCl3): δ 159.4, 140.0, 134.5, 131.2, 130.7, 129.5, 129.3, 129.2, 128.2, 114.7, 56.0 ppm; MS (EI):
m/z 252 (100, M+).
4.3.12. 2,4,6-trimethyl-4’-nitro-1,1‘-biphenyl (3l)
3l was obtained as a white solid (0.52 g, 43% yield); mp 93–94 °C (MeOH; lit.41 mp 90–93 °C); Rf
(petroleum ether) 0.33; 1H NMR (600 MHz, CDCl3): δ 8.28 (d, J = 9.0 Hz, 2H), 7.32 (d, J = 9.0 Hz,
13
2H), 6.94 (s, 2H), 2.34 (s, 3H), 1.98 ppm (s, 6H); C NMR (150 MHz, CDCl3): δ 148.6, 146.7,
137.8, 136.9, 135.3, 130.5, 128.5, 123.9, 21.1, 20.7 ppm; MS (EI): m/z 241 (100, M+).
4.3.13. [1,1';4',1'']Terphenyl (3n)
3n was obtained as a white solid (0.82 g, 71% yield); mp 209–210 °C (MeOH; lit.42 mp 210–212
1
°C); Rf (petroleum ether) 0.41; H NMR (600 MHz, CDCl3): δ 7.82−7.81 (m, 4H), 7.65−7.64 (m,
13
4H), 7.46−7.44 (m, 4H), 7.37−7.36 ppm (m, 2H); C NMR (150 MHz, CDCl3): δ 140.4, 138.8,
129.0, 128.9, 128.3, 127.3 ppm; MS (EI): m/z 230 (100, M+).
4.3.14. 2-Nitro-[1,1';4',1'']terphenyl (3o)

16
3o was obtained as a white solid (0.90 g, 65% yield); mp 130–131 °C (MeOH; lit.43 mp 129–130
°C); Rf (petroleum ether) 0.24; 1H NMR (600 MHz, CDCl3): δ 8.35 (d, J = 7.2 Hz, 1H), 7.88−7.87
13
(m, 1H), 7.83−7.77 (m, 5H), 7.65−7.61 (m, 3H), 7.46−7.44 (m, 2H), 7.38−7.36 ppm (m, 1H); C
NMR (150 MHz, CDCl3): δ 148.5, 140.4, 139.4, 133.2, 133.0, 131.9, 131.7, 130.3, 128.9, 128.3,
127.9, 127.3, 127.2, 126.7 ppm; MS (EI): m/z 275 (100, M+).
4.3.15. 2,5-Dimethyl-4’-nitro-1,1‘-biphenyl (3p)
3p was obtained as a white solid (0.92 g, 81% yield); mp 88–89 °C (MeOH; lit.44 mp 87–88 °C) Rf
1
(petroleum ether) 0.28; H NMR (600 MHz, CDCl3): δ 8.28 (d, J = 9.0 Hz, 2H), 7.47 (d, J = 9.0
13
Hz, 2H), 7.19-7.17 (m, 1H), 7.13−7.12 (m, 1H), 7.03 (s, 1H), 2.35 (s, 3H), 2.19 ppm (s, 3H); C
NMR (150 MHz, CDCl3): δ 149.1, 139.5, 135.7, 131.9, 130.8, 130.1, 129.3, 125.7, 123.6, 119.5,
20.9, 20.0 ppm ; MS (EI): m/z 227 (100, M+).
4.3.16. 2,5-Dimethyl-2’-nitro-1,1‘-biphenyl (3q)
1
3q was obtained as a viscous oil (0.82 g, 72% yield); Rf (petroleum ether) 0.30; H NMR (600
MHz, CDCl3): δ 8.21 (d, J = 6.6 Hz, 1H), 7.97−7.96 (m, 1H), 7.62-7.51 (m, 1H), 7.49−7.47 (m,
1H), 7.33-7.32 (m, 1H), 7.10−7.09 (m, 1H), 6.91 (s, 1H), 2.43 (s, 3H), 2.31 ppm (s, 3H); 13C NMR
(150 MHz, CDCl3): δ 149.2, 137.4, 136.7, 132.5, 132.4, 129.9, 129.8, 129.4, 129.0, 128.3, 124.1,
123.6, 21.0, 19.5 ppm ; MS (EI): m/z 227 (100, M+).
4.3.17. N-Methyl-2-(4-methoxyphenyl)pyrrole (3r)
3r was obtained as a brown solid (0.51 g, 55% yield); mp 58–59 °C (MeOH; lit.45 mp 59–60 °C) Rf
(petroleum ether) 0.32; 1H NMR (600 MHz, CDCl3): 7.58 (d, J = 7.2 Hz, 2H), 7.04 (d, J = 7.2 Hz,
2H), 6.73-6.71 (m, 1H), 6.48−6.47 (m, 1H), 6.25−6.23 (m, 1H), 3.82 (s, 3H), 3.45 ppm (s, 3H); 13C
NMR (150 MHz, CDCl3): δ 158.6, 132.6, 127.7, 125.9, 125.2, 114.7, 110.1, 109.0, 56.0, 36.9 ppm;
MS (EI): m/z 187 (35, M+).
4.3.18. 3-(N-Methyl-pyrrol-2-yl)pyridine (3s)
1
3s was obtained as a viscous oil (0.53 g, 67% yield); Rf (petroleum ether) 0.26; H NMR (600
MHz, CDCl3): δ 8.59 (s, 1H), 8.61 (d, J = 7.8 Hz, 1H), 8.02−8.00 (m, 1H), 7.46−7.44 (m, 1H),
13
6.76−6.75 (m, 2H), 6.26−6.23 (m, 1H), 3.61 ppm (s, 3H); C NMR (150 MHz, CDCl3): δ 148.9,
148.7, 137.9, 132.9, 129.8, 125.9, 124.9, 112.7, 110.1, 37.0 ppm; MS (EI): m/z 158 (65, M+).
4.3.19. N-Methyl-3-phenylindole (3t)
3t was obtained as a brown solid (0.77 g, 74% yield); mp 66–67 °C (MeOH; lit.46 mp 64–65 °C); Rf
1
(petroleum ether) 0.26; H NMR (600 MHz, CDCl3): δ 7.71 (s, 1H), 7.66−7.65 (m, 2H), 7.44−7.41
13
(m, 2H), 7.34−7.29 (m, 2H), 7.27−7.25 (m, 2H), 7.16−7.12 ppm (m, 1H); C NMR (150 MHz,
CDCl3): δ 137.0, 136.9, 128.7, 127.3, 127.2, 126.9, 123.8, 122.9, 122.3, 121.3, 109.6, 35.9 ppm;
MS (EI): m/z 207 (65, M+).

17
4.3.20. 3-(4-Methoxyphenyl)-N-methylindole (3u)
3u was obtained as a pale brown solid (0.78 g; 66% yield); mp 96–97 °C (MeOH; lit.46 mp 95–96
1
°C); Rf (petroleum ether) 0.18; H NMR (600 MHz, CDCl3): δ 7.64−7.61 (m, 3H), 7.32−7.31 (m,
1H), 7.29−7.25 (m, 1H), 7.16−7.15 (m, 1H), 7.13−7.12 (m, 1H), 7.04 (d, J = 7.8 Hz, 2H), 3.79 (s,
13
3H), 3.75 ppm (s, 3H); C NMR (150 MHz, CDCl3): δ 157.1, 136.9, 130.8, 128.9, 126.7, 123.7,
123.0, 122.9, 121.3, 114.01, 109.5, 56.0, 35.8 ppm ; MS (EI): m/z 237 (80, M+).
4.3.21. Mixture of 4-Nitro-2’methylbiphenyl; 4-Nitro-3’methylbiphenyl; 4-Nitro-4’methylbiphenyl
(3w)
3w was obtained as a pale brown solid (0.86 g; 81%); MS (EI): m/z 213 (100, M+). The ratio (%) of
4-nitro-2’-methylbiphenyl/4-nitro-3’-methylbiphenyl/4-nitro-4’-methylbiphenyl was respectively
82:13:5.
4.3.22. Mixture of 4-Bromo-2’-methylbiphenyl; 4-Bromo-3’-methylbiphenyl; 4-Bromo-4’-
methylbiphenyl (3x)
3x was obtained as a grey solid (0.89 g; 72%); MS (EI): m/z 246 (100, M+), 248 (100, M+ +2). The
ratio (%) of 4-bromo-2’-methylbiphenyl/4-bromo-3’-methylbiphenyl/4-bromo-4’-methylbiphenyl
was respectively 65:20:15.
4.3.23. Mixture of 2-methylbiphenyl; 3-methylbiphenyl; 4-methylbiphenyl (3y)
3y was obtained as a waxy solid (0.68 g; 81%); MS (EI): m/z 168 (100, M+). The ratio (%) of 2-
methylbiphenyl/3-methylbiphenyl/4-methylbiphenyl was respectively 69:18:13.
4.3.24. Mixture of 2-bromo-4'-methoxylbiphenyl; 3-bromo-4'-methoxylbiphenyl; 4-bromo-4’-
methoxybiphenyl (3z)
3z was obtained as a pale yellow solid (0.98 g; 74%); MS (EI): m/z 262 (100, M+), 264 (100, M+
+2). The ratio (%) of 2-bromo-4'-methoxylbiphenyl/3-bromo-4'-methoxylbiphenyl/4-bromo-2-
methoxybiphenyl was respectively 75:18:7.
4.3.25. Mixture of 2-bromo-4'-nitrobiphenyl; 3-bromo-4'-nitrobiphenyl; 4-bromo-4’-nitrobiphenyl
(3aa)
3aa was obtained as a grey solid (1.09 g; 78%); MS (EI): m/z 279 (100, M+), 281 (100, M+ +2).
The ratio (%) of 2-bromo-4'-nitrobiphenyl; 3-bromo-4'-nitrobiphenyl; 4-bromo-4’-nitrobiphenyl
was respectively 80:12:8.
4.3.26. Mixture of 2,4'-dinitrobiphenyl; 3,4'-dinitrobiphenyl; 4,4'-dinitrobiphenyl (3ab)
3ab was obtained as a pale red solid (0.95 g; 78%); MS (EI): m/z 244 (100, M+). The ratio (%) of
2,4'-dinitrobiphenyl; 3,4'-dinitrobiphenyl; 4,4'-dinitrobiphenyl was respectively 85:8:7.
4.3.27. Mixture of 4-bromo-2’nitrobiphenyl; 4-bromo-3’nitrobiphenyl; 4-bromo-4'-nitrobiphenyl
(3ac)

18
3ac was obtained as a grey solid (1.27 g; 91%); MS (EI): m/z 279 (100, M+), 281 (100, M+ +2).
The ratio (%) of 2-bromo-4'-nitrobiphenyl; 3-bromo-4'-nitrobiphenyl; 4-bromo-4’-nitrobiphenyl
was respectively 87:7:6.
4.3.28. Mixture of 2'-methoxy-2-nitrobiphenyl; 3’-methoxy-2-nitrobiphenyl; 4'-methoxy-2-
nitrobiphenyl (3ad)
3ad was obtained as a pale orange solid (0.63 g; 55%); MS (EI): m/z 229 (100, M+). The ratio (%)
of 2'-methoxy-2-nitrobiphenyl; 3’-methoxy-2-nitrobiphenyl; 4'-methoxy-2-nitrobiphenyl
respectively 82:14:4.
was
4.3.29. Mixture of 2-4’dimethoxybiphenyl; 3,4’-dimethoxybiphenyl; 4,4'-dimethoxybiphenyl (3ae)
3ae was obtained as a grey solid (0.76 g; 71%); MS (EI): m/z 214 (100, M+). The ratio (%) of 2-
4’dimethoxybiphenyl; 3,4’-dimethoxybiphenyl; 4,4'-dimethoxybiphenyl was respectively 74:15:11.
4.3.30. 3-Phenylpirydine (5a)
1
5a was obtained as a viscous oil (0.56 g, 72% yield); Rf (petroleum ether) 0.25; H NMR (600
MHz, CDCl3): δ 8.79 (s, 1H), 8.57 (d, J = 6.6 Hz, 1H), 7.97 (d, J = 7.2 Hz, 1H), 7.62 (d, J = 6.6 Hz,
13
1H), 7.46-7.35 (m, 4H); C NMR (150 MHz, CDCl3): δ 148.9, 147.9, 138.0, 136.3, 131.3, 128.9,
127.8, 126.9, 125.4 ; MS (EI): m/z 155 (60, M+).
4.3.31. 3-Phenylquinoline (5b)
5b was obtained as a grey solid (0.71 g, 69% yield); mp 50–52 °C (MeOH; lit.47 mp 49–51 °C); Rf
1
(petroleum ether) 0.21; H NMR (600 MHz, CDCl3): δ 8.97 (s, 1H), 8.42 (s, 1H), 8.09 (d, J = 7.2
13
Hz, 1H), 7.73 (d, J = 7.2 Hz, 1H), 7.64–7.58 (m, 3H), 7.47–7.42 (m, 3H), 7.36–7.34 (m, 1H); C
NMR (150 MHz, CDCl3): δ 148.7, 148.4, 138.1, 135.2, 135.1, 132.1, 129.9, 129.2, 129.0, 128.7,
128.4, 126.8 126.4; MS (EI): m/z 205 (70, M+).
4.3.32. 2-Phenylbenzo[d]thiazole (5c)
5c was obtained as a pale yellow solid (0.78 g, 74% yield); mp 112–113 °C (MeOH; lit.48 mp 114–
1
115 °C); Rf (petroleum ether) 0.18; H NMR (600 MHz, CDCl3): δ 8.05−8.03 (m, 1H), 8.02−7.99
13
(m, 1H), 7.69−7.68 (m, 2H), 7.49−7.47 (m, 2H), 7.44−7.42 (m, 1H), 7.36−7.34 (m, 2H); C NMR
(150 MHz, CDCl3): δ 162.4, 152.5, 136.3, 134.4, 133.0, 129.4, 127.2, 126.9, 125.5, 123.7, 122.7;
MS (EI): m/z 211 (65, M+).
4.3.33. Methyl 3-phenylthiophene-2-carboxylate (5d)
5d was obtained as a pale yellow solid (0.94 g, 81% yield); mp 60–61 °C (MeOH; lit.49 mp 60–61
°C); Rf (petroleum ether) 0.18; δH (600 MHz, CDCl3) 7.50-7.49 (m, 1H), 7.46-7.45 (m, 1H),
7.39−7.36 (m, 1H), 7.27−7.25 (m, 2H), 7.16−7.15 (m, 1H), 7.07 (m, 1H), 3.74 (s, 3H) ; δC (150
MHz, CDCl3) 162.5, 148.5, 135.4, 131.7, 129.3, 128.0, 127.9, 126.9, 52,1; MS (EI): m/z 232 (25,
M+).

19
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
Acknowledgments
This work has been supported by the University of Torino and by Ministero dell'Università e della
Ricerca Scientifica
Appendix A: Calculated reaction free energies for the reduction of arenediazonium
tetrafluoroborates by CuI in benzene.
The results here collected and the literature suggest that a primary role in this copper catalysed
Gomberg-Bachmann-Hey reactions is the reduction by CuII of the arenediazonium, i.e. the first step
in the proposed reaction mechanism as reported in Scheme 2 and repeated here as reaction 6:
+
-
.
(6)
Ar-N2 BF4 + CuII → Ar-N2 + BF4CuIII
As noted above, the more the reaction 6 is exoergic the larger are the final yields (or the shorter are
the times). Therefore, the estimation of the thermochemistry of this reaction can be an useful
indication before to undertake the synthesis with other arenediazonium.
The reaction free energy (ΔGET) of this process can be estimated by the experimental reduction
potential of the copper ions and of the arenediazonium tetrafluoroborates:
ΔGET = E1/2(CuII) - E1/2(Ar-N2 BF4 )
+
-
(7)
The value for copper (0.16 eV) is taken from ref. 20 and that for the aryldiazonium tetrafluoroborate
in the non-polar sulfolane (the solvent more similar to the benzene used in this work) from Elofson
and Gadallah.50 Reduction potential measured in polar solvents are also variable (nitromethane,51
acetone,52 and dimethylformamide,53 and acetonitrile54). The paper by Elofson and Gadallah
reports the data for a small number of systems but the reaction energies can now be calculated for
virtually any system by computational DFT method. The computational method was chosen on the
basis of the good result obtained in a previous work on copper catalyzed reaction55 and tested for
the present case. We calculated the ΔGET in Benzene for the aryldiazonium whose reduction
potential (vs SCE) are available from Elofson and Gadallah and obtained the values reported in
Table 5 along the experimental values.
Table 5. Experimental and calculated reaction free energies for the reduction of the arenediazonium
tetrafluoroborates by copper iodide.
a
ΔGET exp.b,c
ΔGET calc.b
errord
+
-
E1/2
Ar-N2 BF4

20
p-NO2
p-CN
+0.450
+0.433
+0.410
+0.350
+0.410
+0.495
+0.429
+0.383
+0.383
+0.295
+0.285
+0.250
+0.228
+0.140
+0.153
-0.095
-0.290
-0.273
-0.213
-0.190
-0.250
-0.335
-0.269
-0.223
-0.223
-0.135
-0.125
-0.090
-0.068
+0.020
+0.007
+0.255
-0.384
-0.370
-0.296
-0.222
-0.295
-0.336
-0.319
-0.199
-0.175
-0.131
-0.132
-0.046
-0.060
+0.074
+0.048
+0.270
-0.094
-0.097
-0.083
-0.032
-0.045
-0.001
-0.050
+0.024
+0.048
+0.004
-0.007
+0.044
+0.008
+0.054
+0.041
+0.015
m-Cl
p-Cl
o-Cl
2,6-DiCl
m-Br
p-Br
p-I
none
m-CH3
p-CH3
o-CH3
p-OCH3
o-OCH3
p-N(CH3)2
a in V, from ref. 24; b in eV; c from eq. 7; d ΔGET calc ‒ ΔGET exp.
Although the non-polar solvents used in the experiments (sulfolane) and in the calculations
(benzene) are not the same, the calculation are in good agreement with the experimental data. The
correlation between the experimental and calculated data is quite good ( R2 = 0.952 ) as also shown
by Figure S8 of the Supplementary data.
Encouraged by these results we extended the calculation of the ΔGET for a large set of
arenrdiazonium tetrafluoroborates in view of further application of the copper catalysed Gomberg-
Bachmann-Hey reactions. The results are collected in Table 6.
Table 6. Calculated reaction free energies (ΔGET, eV) for the reduction of the arenediazonium
tetrafluoroborates by copper iodide in benzene.
+
-
+
-
ΔGET
ΔGET
Ar-N2 BF4
Ar-N2 BF4
4-NO2-C6H4-
3-NO2-C6H4-
2-NO2-C6H4-
(1b) -0.384
(1h) -0.458
(1g) -0.356
4-(CH3)2N-C6H4-
3-(CH3)2N-C6H4-
4-C6H5-C6H4-
0.270
-0.013
-0.048

21
4-CN-C6H4-
(1f) -0.370
-0.296
4-CH3CO-C6H4-
4-CH2=CH-C6H4-
4-CH3OCO-C6H4-
2-Naphthyl
(1e) -0.273
-0.080
3-Cl-C6H4-
4-Cl-C6H4-
-0.222
-0.283
2-Cl-C6H4-
-0.295
-0.123
2,6-Cl2-C6H3-
2,5-Cl2-C6H3-
3-Br-C6H4-
-0.336
1-Naphthyl
-0.069
-0.439
4-Pyridyl
-0.432
-0.319
3-Pyridyl
(4a) -0.293
-0.350
4-Br-C6H4-
(1c) -0.199
-0.175
2-Piridyl
4-I-C6H4-
5-Quinolinyl
-0.130
C6H5-
-0.131
4-Quinolinyl
-0.344
4-CH3-C6H4-
3-CH3-C6H4-
2-CH3-C6H4-
2,6-(CH3)2-C6H3-
4-CH3O-C6H4-
3-CH3O-C6H4-
2-CH3O-C6H4-
(1d) -0.046
-0.132
3-Quinolinyl
(4b) -0.172
-0.370
2-Quinolinyl
-0.060
4-isoQuinolinyl
3-isoQuinolinyl
1-isoQuinolinyl
Benzothiazol-2-yl
2-MeOOC-thien-2-yl
-0.166
(1i)
0.088
0.074
-0.183
0.048
-0.160
(1a)
-0.297
(4c) -0.714
(4d) -0.246
Appendix B. Supplementary data
Supplementary data to this article can be found online at https://doi.
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Copper catalysed Gomberg-Bachmann-Hey reactions of arenediazonium tetrafluoroborates
and heteroarenediazonium o-benzenedisulfonimides. Synthetic and mechanistic aspects.
Achille Antenucci, Margherita Barbero, Stefano Dughera*, Giovanni Ghigo*
Highlights
•
•
•
•
Gomberg-Bachmann-Hey reactions were carried out in the presence of copper as a catalyst
Good yields of biaryls or heterobiaryls in mild reaction conditions
Computational study of some key points of the reaction
Aryl radicals are key intermediates to generate the target products

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: