Sandmeyer reactions. Part 6. A mechanistic investigation into the reduction and ligand transfer steps of Sandmeyer cyanation

Article abstract of DOI:10.1039/b200747a

For Sandmeyer cyanation at 298 K in 50% v/v aqueous acetonitrile buffered at pH 8, absolute rate constants have been obtained for the reduction of 4-methoxybenzenediazonium tetrafluoroborate by the cyanocuprate(I) anions Cu¹(CN)₄^3-, Cu¹(NCMe)(CN)₃^2- and Cu¹(NCMe)₂(CN)₂^- as (0.50 ± 0.05), (0.12 ± 0.03) and 0.0 dm³ mol^-1 s^-1, respectively. The relative reactivity of the two reactive cyanocuprates reflects the estimated difference in their standard reduction potentials. Ligand transfer to the aryl radical from the cyanocuprate(II) anions produced in the reaction occurs within the solvent cage. By use of radical clocks, first order rate constants of the order of 1 × 10⁸ s^-1 for ligand transfer between the caged reactants can be evaluated although the transfer rate may vary from one aryl radical to another. No difference was discerned in ligand transferring reactivity between the two cyanocuprate(II) complexes involved.

Full text of DOI:10.1039/b200747a

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Sandmeyer reactions. Part 6.1 A mechanistic investigation into the
reduction and ligand transfer steps of Sandmeyer cyanation
Peter Hanson,*a Simon C. Rowell,a Alec B. Taylor,a Paul H. Waltona and Allan W. Timmsb
2
a Department of Chemistry, University of York, Heslington, UK YO10 5DD
b Great Lakes Fine Chemicals Ltd., Halebank, Widnes, UK WA8 8NS
Received (in Cambridge, UK) 21st January 2002, Accepted 21st March 2002
First published as an Advance Article on the web 19th April 2002
For Sandmeyer cyanation at 298 K in 50% v/v aqueous acetonitrile buffered at pH 8, absolute rate constants have
been obtained for the reduction of 4-methoxybenzenediazonium tetrafluoroborate by the cyanocuprate() anions
Cu(CN)43Ϫ, Cu(NCMe)(CN)32Ϫ and Cu(NCMe)2(CN)2Ϫ as (0.50 0.05), (0.12 0.03) and 0.0 dm3 molϪ1 sϪ1,
respectively. The relative reactivity of the two reactive cyanocuprates reflects the estimated difference in their
standard reduction potentials. Ligand transfer to the aryl radical from the cyanocuprate() anions produced in the
reaction occurs within the solvent cage. By use of radical clocks, first order rate constants of the order of 1 × 108 sϪ1
for ligand transfer between the caged reactants can be evaluated although the transfer rate may vary from one aryl
radical to another. No difference was discerned in ligand transferring reactivity between the two cyanocuprate()
complexes involved.
Although the general mechanism of the Sandmeyer reaction is
with the aryl radical and the efficiency of the ligand transfer
step is increased.
well understood (Scheme 1),2,3 most previous studies in this field
When cyanide is the ligand, only the Sandmeyer product, the
benzonitrile, is obtained in the presence of alkene,20 demon-
strating that the transfer of cyanide from concomitantly formed
Cu is sufficiently fast to prevent competition from other bi-
molecular reactions of the aryl radical. By contrast, for Sand-
meyer hydroxylation, the ligand transfer step is so slow that the
resting state of the copper has to be Cu(OH2)62ϩ and the lower
oxidation state required for reducing the diazonium ion is sup-
plied either from Cu2O in suspension21 or by reduction of a
small part of the Cu by addition of ascorbic acid or other
initiating reductant.6–8
Cowdrey and Davies11 attempted to measure the kinetics of
the chlorination reaction using Cu solutions. They recognised
that in aqueous solutions Cu would form a series of complexes
with chloride ions according to the equilibria shown in Scheme
3 and from their kinetic evidence they deduced CuCl2Ϫ to be the
Scheme 1
have focused on the ligand transfer;3–8 in recent times less work
has been devoted to the initial reduction step.9–12
Customarily, Sandmeyer reactions are carried out in
Cu-containing aqueous solutions in the presence of chloride,
bromide or cyanide ions.13 The Cu needed in the ligand trans-
fer step is produced in situ and the copper cycles between the
two oxidation states.3 The low concentration of Cu under these
conditions constrains the ligand transfer step and it is possible
for the intermediate aryl radical to react alternatively. Indeed, if
reaction is carried out in the presence of an alkene, when
copper() chloride or bromide is used as the reductant, the
intermediate aryl radical can add to the double bond giving a
radical adduct 1 which subsequently reacts with Cu and the
product is the corresponding haloalkylbenzene 2 (Meerwein
reaction, Scheme 2).14–18 As an improvement to synthetic
Scheme 3
most active reducing agent but the mechanism they proposed is
at odds with that now accepted.2,3
Galli12 carried out a study in which he decomposed pairs of
diazonium ions in solutions containing excess of chloride but
less than equivalent amounts of Cu and various reductants
including Cu. He confirmed that diazonium ions bearing elec-
tron withdrawing substituents react the faster, which is consist-
ent with the acceptance of an electron from a reducing agent,
and he suggested the reaction to be an outer sphere reduction
where the rate is dependent on the reduction potential of the
reductant. In non-Sandmeyer (i.e. copper-free) conditions,
Scheme 2
procedure in Sandmeyer halogenations, Galli4,19 has suggested
the use of mixed Cu and Cu salts: the ligand transfer is thereby
no longer dependent on just the Cu formed concomitantly
1126
J. Chem. Soc., Perkin Trans. 2, 2002, 1126–1134
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b200747a

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outer-sphere reductions of diazonium ions by ferrocyanide in
aqueous solution and by decamethylferrocene in MeCN have
been reported by Doyle and co-workers22 and we have previ-
ously inferred comparable behaviour in their reduction by Fe2ϩ
ligated by EDTA in aqueous solution at pH 9.23
No kinetic study of Sandmeyer cyanation has been carried
out, perhaps because of the formation of the intermediate solid
or oily phases which have been reported for the reaction in
aqueous solution.24 Nevertheless, cyanide being a ‘pseudo-
halide’, the reaction is believed to occur in a manner com-
parable to that of the copper() halides and there may well be
more than one active cyanocuprate species. In this paper we
report the evaluation of absolute rate constants for the reduc-
tion of 4-methoxybenzenediazonium ion, 3, by various cyano-
cuprate() complexes in 50% v/v aqueous acetonitrile at pH 8
and 298 K.
(Table 1),29 hence allowing estimation of the rate of reduction
of 3 by each copper complex. (For convenience, only the cyan-
ide ligands on Cu will be indicated whereas, in fact, in aqueous
acetonitrile solutions at low and intermediate pH values,
coordination sites not occupied by CNϪ are occupied by
MeCN.)28,30 Cyanide ions and protons, however, exist in an
equilibrium with hydrogen cyanide (pKa = 9.93 in 50% v/v
aqueous MeCN)28 and so the pH must be known and main-
tained in order to determine accurately the concentration of
cyanide available for co-ordinating Cu. The solution was there-
fore buffered and a pH of 8 was chosen. Practically, a solution
of KH2PO4 (0.1 mol dmϪ3) and K2HPO4 (0.062 mol dmϪ3) was
used to buffer the reaction and, although this mixture gives a
pH of 7 in a purely aqueous medium, it was found that in 50%
v/v aqueous MeCN the co-solvent perturbs the buffering equi-
libria so that a value closer to 8 was measured. This value could
then be adjusted to 8 precisely by the addition of a solution of
H2SO4 or NaOH (in 50% v/v aqueous MeCN) as appropriate.
The pH of the mixture was measured using a pH meter previ-
ously calibrated with aqueous standard solutions and it was
assumed that the measured value obtained for the mixed sol-
vent remained representative of the true oxonium ion concen-
tration. Reaction at higher pH was found not to give good
kinetic results perhaps owing to the intervention of species of
the type Cu(CN)n(OH)m(m ϩ n Ϫ 1)Ϫ which have been suggested to
be present in these conditions.28
Results
(i) Identification of the intermediate phase in Sandmeyer
cyanation
Synthetically, Sandmeyer cyanation is usually carried out in
aqueous solution where the diazonium ion is added to a cyano-
cuprate() solution.13 There have been reports of the formation
of solid or oily intermediate phases, an explanation for which
has been suggested to be the reversible formation of the diazo-
(iii) Measurement of the rates of reduction in the Sandmeyer
cyanation
Experimentally, the rate of reduction of 3 was determined by
measuring the evolution of N2 over times up to ca. 2 h at 298 K,
the amount evolved being quantified continuously by use of an
apparatus adapted from an original design by Crossley and co-
workers.31 The system operates so that the nitrogen produced
displaces from a glass cell an equal volume of water, which is
collected and weighed continuously by an electronic top-pan
balance. The mass of water is equated to the volume of nitrogen
which is evolved and the molar quantity, at 298 K, can hence be
calculated using the ideal gas law. Experiments were restricted
to total concentrations of cyanide between 0.099 and 0.152 mol
dmϪ3 (for [Cuϩ]total = 0.038 mol dmϪ3 and [ArN2ϩ] = 0.0095 mol
dmϪ3) as at lower values a white precipitate of CuCN formed28
and at higher values the solution separated into two phases,
presumably because the salt concentration rises to the point
where MeCN is no longer miscible. It is evident from Fig. 1 and
cyanide (ArN᎐N–CN).24 However, diazonium salts of halo-
᎐
cuprate anions which are relatively stable to oxidation are well
known and the structure of a benzenediazonium bromo-
cuprate() (PhN2ϩ Cu2Br3Ϫ) has been resolved by X-ray crystal-
lography;25 it is hence possible that the intermediate observed in
Sandmeyer cyanation is merely a simple salt of the diazonium
ion and a cyanocuprate anion. Investigations were carried out
to confirm this suggestion by comparing the spectroscopic
properties of the isolated solid with those of appropriate diazo-
cyanide and diazonium and cyanocuprate salts. In order to mini-
mise contamination by decomposition products derived from
the diazonium ion, 3 (as its tetrafluoroborate) was chosen as it
is relatively stable to heterolysis26 and the solid formed in its
Sandmeyer cyanation reaction (using a solution of CuCN in
aqueous KCN with [CNϪ] : [Cuϩ] = 3 : 1) was collected. (E)-4-
Methoxybenzenediazocyanide was synthesised following the
procedure of Ignasiak and coworkers.27 The two solids have
different solubilities in CHCl3 and different IR spectra which
clearly indicate that the isolated solid is not the diazocyanide.
Moreover, the intermediate solid isolated from the Sandmeyer
reaction has IR and solid phase 13C CP-MAS NMR spectra
ϩ
which are consistent with it being the simple salt (ArN2
)
2
2Ϫ
Cu(CN)3 containing some Sandmeyer product, 4-methoxy-
benzonitrile, as contaminant (see Experimental). The suggested
presence of the ion Cu(CN)32Ϫ is based merely on the stoichio-
metric ratio of Cuϩ and CNϪ used and not on rigorous proof
but the observation of a single signal for the CN component in
both the 13C-NMR and IR spectra of the intermediate precipi-
tate and the potassium salt is consistent with this formulation.
(ii) Optimisation of conditions for the measurement of
Sandmeyer cyanation kinetics
Fig.
1
Distribution of Cu species as
a function of cyanide
Ϫ
concentration in aqueous MeCN (1 : 1). 1, Cuϩ; 2, CuCN; 3, Cu(CN)2
;
In order to measure kinetics for the Sandmeyer cyanation reac-
tion, a homogenous solution is required in which the concentra-
tions of the various reducing cyanocuprates are known. A 50%
v/v aqueous acetonitrile solvent provides the requisite condi-
tions: formation constants have been reported28 for the various
cyanocuprate complexes (Scheme 3) and these allow calculation
of the concentration of each species at the start of the reaction
4, Cu(CN)32Ϫ; 5, Cu(CN)4
.
3Ϫ
Table 1 that, in the accessible range of cyanide concentration,
the concentrations of free Cuϩ and CuCN are negligible but
those of the three anionic cyanocuprates are significant and all
should be considered as potential reductants of 3.
J. Chem. Soc., Perkin Trans. 2, 2002, 1126–1134
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Table 1 Initial species distributiona/mol dmϪ3 and observed first order rate constants for Sandmeyer cyanation of 4-methoxybenzenediazonium
tetrafluoroborate in 50% v/v aqueous acetonitrile containing Cuϩ (0.038 mol dmϪ3) and various concentrations of KCN at 298 K and pH 8
[CNϪ]total
106[CNϪ]
104[HCN]
1011[Cuϩ]
107[CuCN]
103[Cu(CN)2
102[Cu(CN)3
]
103[Cu(CN)4
]
103kobs
Ϫ
]
2Ϫ
3Ϫ
b
0.0991
0.107
0.114
0.122
0.130
0.137
0.145
0.152
2.78
6.40
15.4
38.2
72.6
110
2.37
5.45
13.1
32.5
61.8
93.5
136
6240
654
52.5
999
241
15.3
8.48
3.91
1.51
0.705
0.410
0.242
0.161
2.23
2.85
3.15
3.03
2.68
2.36
2.03
1.78
0.333
0.981
2.60
6.22
10.5
13.9
17.5
20.1
2.34 0.11
2.74 0.11
3.80 0.17
6.20 0.32
7.08 0.36
9.12 0.43
9.56 0.62
11.8 0.60
46.3
7.18
1.77
0.679
0.275
0.140
3.26
0.423
0.107
0.0298
0.0115
160
210
179
a Calculated using equilibrium constants for ionic strength 1.0 mol dmϪ3 from ref. 28; the actual ionic strength varied in the range ca. 0.3–0.5 mol
dmϪ3 b The uncertainties are the 95% confidence limits.
.
Table 2 Second order rate constants for various extents of conversion
of 3 into 4-methoxybenzonitrile
If it is assumed that the reduction of 3 by each cyanocuprate
is a second order process overall, the rate of evolution of N2 is
given by eqn. (1):
Conversion of 3 (%)
k3/dm3 molϪ1 sϪ1
k4/dm3 molϪ1 sϪ1
d[N2]/dt = Ϫd[3]/dt = k2[Cu(CN)2Ϫ][3] ϩ
0
25
50
75
100
Mean
0.086 0.044
0.100 0.044
0.116 0.046
0.131 0.047
0.145 0.047
0.116 0.026
0.486 0.054
0.496 0.057
0.502 0.062
0.507 0.068
0.511 0.075
0.500 0.049
k3[Cu(CN)32Ϫ][3] ϩ k4[Cu(CN)43Ϫ][3] = kobs[3]
(1)
i.e.
kobs = k2[Cu(CN)2Ϫ] ϩ k3[Cu(CN)32Ϫ] ϩ k4[Cu(CN)4
3Ϫ
]
(2)
where kobs is the observed combined first order rate constant.
Eqn. (3) follows from the first order condition:
[N2] = [3]0{1 Ϫ exp(Ϫkobst)}
(3)
where [N2] is the amount of N2 (in units of concentration)
produced at time t and [3]0 is the initial concentration of 3. For
decompositions involving 1 mmol of 3, this expression is
adapted to eqn. (4), where νt is the volume of N2, in cm3,
evolved at time t and νm is the millimolar volume at SATP (i.e.
24.790 cm3 mmolϪ1).
100νt/νm = A{1 Ϫ exp(Ϫkobst)} ϩ C
(4)
Eqn. (4) thus expresses the percentage of the theoretical evo-
lution of N2 as a function of time in terms of three constants A,
C and kobs; C represents the amount of nitrogen which does not
displace water immediately the timing commences (t = 0) and A
represents [3]0 adjusted for this inertial factor and for minor
side reactions (ca. 10%) which reduce the theoretical nitrogen
yield.
Fig. 2 Evolution of N2 as a function of time for a run with [Cu] =
0.038 mol dmϪ3, [CNϪ]total = 0.0991 mol dmϪ3 fitted by eqn. (4) with A =
93.75%, C = Ϫ5.25% and kobs = 2.5 × 10Ϫ3 sϪ1
.
and the total Cu concentration remains essentially constant.
However, owing to the limited practical range of cyanide con-
centrations and the need to use a quantity of 3 sufficient to
produce enough N2 for measurement, [CNϪ]total falls signifi-
cantly during the reaction (by ca. 10% in the worst case) as the
ion is diverted to the product, 4-MeOC6H4CN, and such loss
from the inorganic reservoir results in a significant change in
the distribution of Cu species over the course of the reaction.
For each decomposition reaction the amount of diazonium salt
used is known (1 mmol). It is thus possible to calculate the
amounts of cyanide lost from the inorganic reservoir for vari-
ous extents of reaction (assuming all diazonium ion is con-
verted to 4-methoxybenzonitrile, a reasonable assumption since
the yields of the latter are typically ca. 90%). For each chosen
extent of reaction, the distribution of cyanocuprate ions can be
recalculated and the corresponding values of k3 and k4 found
via eqn. (5). The values found for five different percentage
extents of reaction are given in Table 2. Numerically both con-
stants show an apparent upward trend though the value of k4 is
constant within the quoted uncertainty (95% confidence inter-
val); the apparent increase in k3 is somewhat outside this inter-
val. We suggest this variation has its origin in the change in
ionic strength over the course of reaction. However, the mean
values of both constants do not differ significantly from the
Reactions were carried out for solutions of Cu (0.038 mol
dmϪ3) containing various cyanide concentrations within the
homogeneous range and at least two runs were carried out for
each total cyanide concentration. All the kinetic data were fitted
to eqn. (4) using non-linear regression. Excellent fits were
obtained in every case (e.g. Fig. 2) and the average combined
first order rate constants, kobs, found for the various initial total
cyanide concentrations are given in Table 1.
Preliminary estimates of the second order rate constants k2–
k4 were obtained by a multiple linear regression of kobs upon the
initial equilibrium concentrations of cyanocuprate anions
taken from Table 1 [eqn. (2)]. Inclusion of the term for
Cu(CN)2Ϫ led to a value of k2 which is not significantly different
Ϫ
from zero. Hence it is inferred that Cu(CN)2 plays no part in
the reduction process and the dependence of kobs on cyano-
cuprates is given by eqn. (5).
3Ϫ
kobs = k3[Cu(CN)32Ϫ] ϩ k4[Cu(CN)4
]
(5)
In the reaction, Cu is recycled (i.e. Cu formed in the rate-
determining step is rapidly returned to Cu by ligand transfer)
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J. Chem. Soc., Perkin Trans. 2, 2002, 1126–1134

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to equal the ratio of their rate constants. Under the assumption
that the radical clock undergoes cyclisation at the same rate
irrespective of whether or not it is caged with cyanocuprate, a
first order rate constant for the caged ligand transfer may then
be evaluated from the product ratio (Scheme 4). The rate of
fragmentation of the intermediate diazenyl radical is of no con-
sequence as neither of the two competitive reactions can occur
until after it has occurred.
Table 3 gives the molar product ratios obtained using two
radical clocks in Sandmeyer cyanation: the 2-benzoylphenyl
radical, 4, and the stilbene-derived radical, 5, for which cyclis-
ation rate constants, kCyc, have been given previously as (8 0.9)
× 105 and (3 0.5) × 109 sϪ1, respectively.7,34 It is apparent from
the product ratios that neither clock is ideal for obtaining a
ligand transfer rate, 4 cyclising rather slowly relative to ligand
transfer and 5 rather fast with the consequence that both give
product ratios in which the proportion of minor product is
small. Nevertheless, each clock gives a characteristic mean ratio
of aryl nitrile to cyclised product which is independent of the
concentration of the reducing agent and, for 4, of the propor-
tion of copper to ligand. It is easily shown for each clock that
Fig. 3 Plot of kobs versus kcalc given by eqn. (5).
values for 50% conversion and we propose they be taken as the
best estimates of k3 and k4. Fig. 3 shows the plot of kobs versus
kcalc, the latter being the calculated combined first order rate
constant obtained using these values of k3 and k4 and the
cyanocuprate concentrations appropriate at 50% conversion
of 3.
the ligand transfer rate constant, kCN, is given by kCN = kCyc
×
product ratio. Inserting values, 4 gives kCN = (3.5 0.5) × 107 sϪ1
and 5 gives kCN = (1.8 0.3) × 108 sϪ1, values which agree within
a factor of 5 although, given the difference in the substituents
ortho to the radical centre in 4 and 5, the difference is probably
significant; we have shown previously that second order ligand
transfer rate constants of simply substituted aryl radicals are
sensitive to variation in substitution8 and that methylation of
the radical ring of 4 meta to the radical centre doubles the rate
at which it undergoes Sandmeyer hydroxylation.1 The radical 4
reacts in bimolecular chloride ligand transfers with CuCl2 and
CuCl3Ϫ with a statistically corrected second order rate constant
In summary, for the Sandmeyer cyanation of 4-methoxy-
benzenediazonium ion in 50% v/v aqueous acetonitrile at pH 8
and 298 K, of the potential reductants in the system, Cuϩ and
CuCN were not present in significant concentrations in the
Ϫ
conditions employed; Cu(CN)2 was present but unreactive
2Ϫ
3Ϫ
whereas both Cu(CN)3 and Cu(CN)4 reacted with respec-
tive rate constants of (0.12
0.03) and (0.50
0.05) dm3
molϪ1
Cu(CN)3
s
Ϫ1; i.e. Cu(CN)4 reacts some 4.3-fold faster than
3Ϫ
2Ϫ
.
of 1 × 108 dm3 molϪ1 Ϫ1. Since, in buffered conditions, the
s
(iv) Estimation of rates of caged cyanide ligand transfer
product ratio from 4 shows no significant variation as the
[CNϪ]/[Cu] ratio is varied, it appears that the ligand transfer
rate is independent of the particular cyanocuprate with which
the radical is caged. The first order rate constants obtained
are thus global values applicable to caged ligand transfer
from either of the two cyanocuprate() anions which may be
produced together with the particular radical clock.
In previous studies7,8 we have estimated rate constants for
the transfer of ligands from Cu to aryl radicals by use of
the 2-benzoylphenyl radical, 4, as a radical clock. Using a prior
estimate of the rate of cyclisation of the radical, second order
rate constants for ligand transfer were calculated from meas-
urements of the variation in the ratio of the substituted to
cyclised products as a function of the concentration of the Cu
ligand-transfer agent. This procedure cannot be applied to find
second order rate constants for cyanide ligand transfer because
cyanocuprate() complexes are unstable, addition of cyanide to
an aqueous solution of Cu2ϩ resulting in the formation of
cyanogen and cyanocuprate() via transient cyanocuprate() in
a reaction which is of second order in the latter.32,33 It is there-
fore impossible to observe variation in product ratio as a
function of the concentration of added cyanocuprate().
Discussion
(i) Reduction of 4-methoxybenzenediazonium tetrafluoroborate
3؊
by Cu(CN)32؊ and Cu(CN)4
3Ϫ
We have shown that Cu(CN)4 reduces 4-methoxybenzene-
diazonium ion, 3, 4.3-fold faster than Cu(CN)32Ϫ. Galli12 has
previously related the rate of the reduction step for diazonium
ions to the standard reduction potential of the reductant invok-
ing a threshold potential of ca. 1 V above which reduction fails
and suggesting that the lower the reduction potential, the faster
the rate of reduction. In order to ascertain whether this trend is
followed by the cyanocuprates, information on the reduction
2Ϫ
3Ϫ
potentials of Cu(CN)3 and Cu(CN)4 is required as they
have not been measured experimentally in 50% v/v aqueous
MeCN.
From the Nernst equation it follows that the standard reduc-
Under the conditions of Sandmeyer cyanation, cyano-
cuprate() is formed in the first propagation step, i.e. by the
overall rate-determining reduction of diazonium ion by cyano-
cuprate() (Scheme 1) and the fact that the derived aryl radical
is not captured by alkene in a Meerwein reaction20 indicates
that it does not escape from the solvent cage which also con-
tains the concomitantly formed cyanocuprate(). If an aryl rad-
ical clock is produced during Sandmeyer cyanation, the ligand
transfer and the cyclisation of the radical clock can therefore be
regarded as competitive first order reactions of the caged pair
of reactants and, as neither of the competing processes is likely
to be reversible, the molar ratio of their products may be taken
tion potential, En, of a general cyanocuprate redox couple, Cu-
(n
Ϫ
Ϫ 1)Ϫ
(CN)n
eqn. (6):
2)Ϫ/Cu(CN)n(n
in aqueous solution, is given by
En = E{Cu2ϩ/Cuϩ} Ϫ (RT/F) ln (β n/β  )
(6)
n
Where E{Cu2ϩ/Cuϩ} is the standard reduction potential
for the aquated ions, β  and β  are the appropriate overall
n
n
formation constants of the cyanocuprate complexes comprising
the couple, R is the gas constant, T the absolute temperature
and F the Faraday constant.
J. Chem. Soc., Perkin Trans. 2, 2002, 1126–1134
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Table 3 Molar product ratios from competitive caged cyanide ligand transfer and cyclisation of radical clocks 4 and 5
Radical clock 4a
Radical clock 5b
[Cu]/mol dmϪ3
[CNϪ]/mol dmϪ3
[6]/[8]
[Cu]/mol dmϪ3
[CNϪ]/mol dmϪ3
[7]/[9]
0.03
0.04
0.06
0.08
0.042
0.042
0.042
0.042
0.09
0.12
0.18
0.24
0.100
0.117
0.133
0.166
42.9
41.3
42.6
45.5
47.0
42.5
41.4
42.3
0.17
0.25
0.33
0.42
0.50
0.75
1.00
1.25
0.061
0.055
0.061
0.062
Mean 0.060 0.006
Mean 43.2 0.4
a Solvent 50% v/v aqueous MeCN buffered at pH 8. b Solvent 10% v/v aqueous MeCN, unbuffered.
Scheme 4
Hence,
for K4 in 50% aqueous MeCN, then E3 Ϫ E4 = 0.04 V. This
indicates that E4 < E3, and our finding that Cu(CN)43Ϫ reduces 3
more rapidly than Cu(CN)32Ϫ is in agreement with Galli’s sug-
gestion. If the difference in free energies of activation for reduc-
tion of 3 by the two complexes were to equal the difference in
their reduction potentials (i.e. 0.04F = 3.86 kJ molϪ1), all other
things being equal, the relative reduction rate would be 4.75;
our experimental value of 4.3 is therefore of a reasonable
magnitude. Since coordination by MeCN stabilises Cu,30 it is
to be expected that in aqueous solution the aquacyanocuprates,
E3 Ϫ E4 = (RT/F) ln [(β 4/β  )/(β 3/β  )] =
4
3
(RT/F) ln (K 4β  /β  ) (7)
3
4
where K  is the stepwise stability constant for the formation
4
of Cu(CN)4 from Cu(CN)3 and CNϪ. An expression
analogous to eqn. (7) is expected to hold for solutions in 50%
v/v aqueous MeCN. The overall formation constants for
2Ϫ
Ϫ
Cu(CN)3 (β  ) and Cu(CN)4 (β  ) in this solvent are 1022.21
2Ϫ
3Ϫ
3
4
Ϫ
1)Ϫ
Cu(OH2)4 n(CN)n(n
should show somewhat higher reac-
(dm3 molϪ1)3 and 1025.95 (dm3 molϪ1)4, respectively.28 In order to
evaluate K 4 for aqueous solution, Katagiri and co-workers32,33
first used eqn. (6) with an experimental value for E4{Cu-
(CN)42Ϫ/Cu(CN)43Ϫ} of 0.54 V vs. NHE to evaluate β 4 as 1024.0
(dm3 molϪ1)4; however, Beck35 has pointed out that the value
Ϫ
tivity than the corresponding MeCN complexes studied here
if their reactivity is also determined principally by the reduction
potential; on the other hand, Cu(CN)43Ϫ might be expected to
exhibit similar reactivity in both aqueous and mixed solvents
since the inner coordination sphere remains the same in both
media and any difference can stem only from secondary
taken for β  [1030.53 (dm3 molϪ1)4] was too high. Adopting the
4
recommended value35 for β 4 [1027.9 (dm3 molϪ1)4] we re-evaluate
β  as 1021.35 (dm3 molϪ1)4. Then, from this value, after the
solvation effects.
4
2Ϫ
The standard reduction potential for the couple Cu(CN)4
/
manner of Katagiri and co-workers,32,33 by assuming that the
ratios of consecutive stepwise stability constants of cyano-
3Ϫ
Cu(CN)4 (in water) found by Katagiri and co-workers32,33
cuprate() complexes K(n 1)/Kn equal36 the means of those of
was 0.54 V vs. NHE and, on the basis of the argument given
ϩ
2Ϫ
above, that for Cu(CN)3Ϫ/Cu(CN)3 in aqueous MeCN is
chlorocuprate() complexes37 and amminecopper() com-
plexes,38 we estimate the stepwise stability constants of cyano-
cuprate() complexes in water to be: K1 = 106.16 dm3 molϪ1, K2 =
105.54 dm3 molϪ1, K3 = 105.23 dm3 molϪ1 and K4 = 104.43 dm3
molϪ1. If we then further assume that MeCN has negligible
effect in stabilising Cu and adopt the value of 104.43 dm3 molϪ1
Ϫ
0.58 V. These are similar to the values for the couples CuCl3
/
CuCl3 and CuCl2/CuCl2 (0.52 and 0.50 V, respectively, vs.
NHE)8 indicating their reactions with any particular diazonium
ion to be almost isoergonic. We indicated that, other things
being equal, a difference in reduction potential of 0.04 V might,
2Ϫ
Ϫ
1130
J. Chem. Soc., Perkin Trans. 2, 2002, 1126–1134

View Article Online
at most, be manifest as a 4.75-fold difference in reduction rate.
On this basis, these chloro- and cyano-copper() complexes
would be expected to react with a given substrate at rates falling
within the same order of magnitude, yet chlorocuprates reduce
diazonium ions much more rapidly than cyanocuprates. (We
estimate a factor of about 103 for solutions containing compar-
able total concentrations of Cu and chloride or cyanide). Evi-
dently, when comparing the differently ligated cuprates, there is
no correspondence of reactivity with reduction potential
(assuming the various stability constants from which the latter
are derived are soundly based). The question arises as to what is
the mechanism of these reductions. The marked difference in
rates could arise from a variety of circumstances: (i) if the
reduction steps in both Sandmeyer reactions were to occur by
outer-sphere mechanisms but with that in cyanation manifest-
ing a significantly higher intrinsic barrier to electron transfer or,
(ii) if the reduction step of Sandmeyer cyanation were to occur
by an outer-sphere mechanism whereas that of chlorination
were to occur by an inner-sphere mechanism or, (iii) if both
reduction steps were inner-sphere processes but with differing
conductivity of the bridging ligands in electron transfer. In the
ensuing paper, we shall adduce evidence that chlorocuprates
reduce diazonium ions by an inner-sphere mechanism preclud-
ing circumstance (i) but not circumstance (ii). Circumstances
(ii) and (iii) both depend on the relative ability of cyanide and
chloride to act as a bridging ligand, the difference being merely
one of degree. In order for a ligand to act as a bridge, a lone
pair on the ligand is required to participate at a suitable electro-
philic site on the diazonium ion. It is expected that the polaris-
able lone-pair orbitals on chloride should be better able to do
this than the less polarisable lone-pair of sp-character on the
nitrogen atom of cyanide. Furthermore, on account of the con-
tracted, filled d orbitals in d10 Cu complexes, it is likely that in
an inner-sphere process the electron will be conducted via the
σ-framework rather than via π-orbital interactions. Again the
orbitals of chloride are expected to be the better fitted for the
role. Circumstance (iii) could therefore hold. The simplest
interpretation of the available circumstantial evidence (Occam’s
razor) is that it does hold and that both chloro- and cyano-
cuprates reduce diazonium ions by inner-sphere mechanisms in
which the activation barriers differ significantly in favour of the
chlorocuprates on account of their possession of ligands of
higher electron conductivity.† There is a possibility that cyano-
cuprates may react by an outer-sphere mechanism but with only
approximate reduction potentials and no self-exchange rates it
cannot be tested via Marcus theory.
also the ligand which is transferred. The third mechanism is
penetration of the aryl radical into the primary coordination
shell of the metal, formally to give L–Cu–Ar, which then
undergoes reductive elimination to Ar–L and Cu.
The first two mechanisms differ only in the synchrony of
electron and atom transfer. Previously, we have envisaged them
as the end-members of a gradation in the character of ligand
transfer which is dependent on the extent of decoupling of the
transfer of the electron from that of the ligand itself such that
the electron transfer aspect is enhanced by factors such as
nucleophilicity of the aryl radical (influenced by substitution)
or high reduction potential of the Cu complex.8 The third
mechanism differs from the others in the site of attack of the
aryl radical on the Cu complex. Although there is spectro-
scopic evidence of organocopper intermediates formed in the
reaction of alkyl radicals with Cu2ϩaq,41,42 we are aware of no
such evidence regarding aryl radicals. There is evidence that
organocopper intermediates are involved in the production of
by-products associated with Sandmeyer reactions (e.g. azo-
arenes and n,n-biaryls)43,44 but there is no evidence that requires
their involvement in the steps of Sandmeyer reaction proper.
Following on from the above, the transfer of a cyanide ligand
could result from attack by the aryl radical upon a ligand of the
cyanocuprate() which co-occurs within its solvent cage. Since
Sandmeyer cyanation is regioselective yielding only nitrile and
not isonitrile,‡ the attack must be adjacent, i.e. on the carbon
atom which is bonded to the metal. In a wholly inorganic con-
text, adjacent attack on a cyanide ligand does not occur as the
ligating carbon atom possesses no additional lone pair with
which to coordinate a second metal. In the present context,
where the reaction partner is an organic radical, no such restric-
tion need apply, the attacking radical being able to interact at
carbon with both occupied and/or unoccupied bonding orbitals
associated with the ligand, whichever affords the greater stabil-
isation. On the other hand, the regioselectivity of nitrile
formation might be construed as circumstantial evidence of
organometallic involvement. For example, if the aryl radical
were to attack square planar tetracyanocuprate()46 in the axial
direction, reductive elimination from the penta-coordinate
adduct would result in the formation of ArCN. A theoretical
analysis of the relative energetics of different reaction paths
would be informative.
Conclusion
Of the various copper() species present when [Cu(NCMe)4]-
(BF4) and KCN are dissolved in 50% v/v aqueous acetonitrile
at pH 8 only Cu(NCMe)(CN)3 and Cu(CN)4 reduce
4-methoxybenzenediazonium ion and their respective rate
constants are (0.12 0.03) and (0.5 0.05) dm3 molϪ1 sϪ1 at
298 K. The relative reactivity closely reflects the estimated dif-
ference in the reduction potentials of the two ions but the
cyanocuprate ions are significantly less reactive than chloro-
cuprates which have similar reduction potentials. The balance
of circumstantial evidence seems to favour cyanocuprates
reducing diazonium ions by an inner-sphere mechanism but an
outer-sphere mechanism is not ruled out. When a diazonium
ion is reduced by one of the reactive cyanocuprate() complexes,
the derived aryl radical undergoes ligand transfer from the
cyanocuprate() anion with which it shares its solvent cage. The
first order rate constant for ligand transfer within the caged pair
of reactants is ca. 1 × 108 sϪ1 but may vary with substitution
in the aryl radical like bimolecular ligand transfer rates; no
difference in reactivity of the two concerned cyanocuprate()
complexes is detectable.
2Ϫ
3Ϫ
(ii) The mechanism of the ligand transfer step
Although many reagents are capable of reducing diazonium
ions, Cu is virtually unique in its ability to transfer certain
ligands to the derived aryl radicals. Three distinct mechanisms
can be envisaged for the transfer of a ligand L. The first is atom
ؒ
transfer where the aryl radical abstracts L from the coordin-
ation shell of Cu in an SH2 type of process simultaneously
producing a new C–L bond and reducing Cu to Cu. The
second mechanism is an electron transfer from the radical to
the Cu complex which reduces it to Cu and the aryl cation
which results from the electron transfer then captures a ligand
LϪ liberated by the metal centre on its change of oxidation
state; in order to account for the unique transferring ability of
copper, the electron transfer must be an inner-sphere process in
the sense that the ligand which provides the bridge between the
aryl radical and the metal for transmission of the electron is
† An account of the causes of differing conductivities of halide ions as
bridging ligands has been given by Cannon.39 We are not aware of any
comparable analysis of the relative conductivities of chloride and cyan-
ide as bridging ligands but note that in the inner-sphere reduction of
Co(NH3)5X2ϩ by Cuϩaq, the rate for X = ClϪ exceeds that for X = CNϪ
by seven orders of magnitude.40
‡ Synthetically, PhNC has been reported as a contaminant of PhCN45
but this can be ascribed to minor heterolysis of the diazonium pre-
cursor under the high temperature conditions of the procedure. No
isonitrile is detected in the cyanation conditions used here.
J. Chem. Soc., Perkin Trans. 2, 2002, 1126–1134
1131

View Article Online
phosphate buffer [KH2PO4 (0.1 mol dmϪ3), K2HPO4 (0.062 mol
Experimental
dmϪ3), pH 7] to give a solution (225 cm3) of 50% v/v aqueous
MeCN solvent composition. 2-Benzoylbenzenediazonium
tetrafluoroborate (0.74 g) dissolved in solvent of the same com-
position (25 cm3) was added and the mixture was stirred. After
2 h. it was added to a solution of disodium 2-naphthol-3,6-
disulfonate (100 cm3, 0.17 mol dmϪ3) and the organic product
was extracted with ethyl acetate (3 × 25 cm3). Removal of the
solvent and chromatography of the residue on a silica column,
eluting with light petroleum (40–60 ЊC)–ethyl acetate (5 : 1),
gave 2-cyanobenzophenone, mp 78 ЊC, lit.53 84.5–85.5 ЊC
(Found: Mϩ 207.0693. C14H9NO requires 207.0684); m/z 207
(34%, Mϩ), 130 (17), 105 (100), 102 (19), 77 (56), 51 (34);
(i) Materials
Tetrakis(acetonitrile)copper(I) tetrafluoroborate. This was
synthesised from copper() oxide and fluoroboric acid by the
method of Kubas.47
4-Methoxybenzenediazonium tetrafluoroborate, 3. 4-Anis-
idine (12.3 g), dissolved in fluoroboric acid (50%, 34 cm3) and
diluted with water (40 cm3), was diazotised by portion-wise
addition of sodium nitrite (6.9 g) in water (15 cm3) at 0–5 ЊC.
After stirring for 15 min, the thick precipitate was collected and
re-dissolvedinacetone. 4-Methoxybenzenediazoniumtetrafluoro-
borate, 3, (21 g, 95%) was precipitated by addition of diethyl
νmax(Nujol)/cmϪ1 2229 (C᎐N), 1660 (C᎐O); δ (270 MHz,
᎐
᎐
᎐
H
ether, mp 141–142 ЊC, lit.48 142 ЊC; νmax(Nujol)/cmϪ1 2253
CDCl3) 7.50 (m, 2H), 7.68 (m, 4H), and 7.83 (m, 3H); δC(67.9
MHz, CDCl3) 118.8, 117.0, 128.6 (2C), 130.0, 130.2 (2C), 131.3,
132.0, 133.8, 134.2, 135.8, 141.3 and 193.7 in excellent agree-
ment with precedent.54
ϩ
(N᎐N ), 1044br (BF Ϫ); δH[270 MHz, (CD3)2CO] 4.24 (s, 3H),
᎐
᎐
4
7.60 (d, J 9.5, 2H), 8.82 (d, J 9.5, 2H); δC[67.9 MHz, (CD3)2CO]
58.0, 103.6, 118.3 (2C), 137.0 (2C) and 170.6, lit.49,50
(E )-4-Methoxybenzenediazocyanide. This was prepared from
potassium cyanide and 4-methoxybenzenediazonium tetra-
fluoroborate by the method of Ignasiak and co-workers.27 An
aqueous solution of potassium cyanide (1.63 g in 7.5 cm3) was
added slowly to a stirred aqueous solution of 4-methoxy-
benzenediazonium tetrafluoroborate (2.78 g in 10 cm3) at 0 ЊC
until the reaction solution when tested with potassium cyanide
on a piece of filter paper failed to give an orange-coloured
product. The mixture was diluted with ice-water (13 cm3) and
then filtered through a Buchner funnel. The wet, coloured resi-
due was extracted into chloroform and dried over anhydrous
magnesium sulfate. After filtration to remove the desiccant, the
solution was heated on a steam bath [which completes the Z to
E isomerisation of the diazocyanide] and then evaporated. (E)-
4-Methoxybenzenediazocyanide was recrystallised from hexane,
mp 105–108 ЊC, lit.27 124 ЊC [low owing to a remaining trace of
(Z)-form]; νmax(Nujol)/cmϪ1 2172 (CN), 1599, 1258, 1154, 1017,
843, 723; δH[270 MHz, CDCl3] 3.90 (s, 3H), 6.97 (d, 2H), 7.87
(d, 2H); δC[67.9 MHz, CDCl3] 56.6, 115.5, 116.9, 128.2, 148.8
and 167.8, in good agreement with precedent.51
Methyl (E )-3-(2-cyanophenyl)-2-phenylpropenoate, 7. 2-
Cyanobenzaldehyde and phenylacetic acid were reacted by
DeTar’s procedure55 to give (E)-3-(2-cyanophenyl)-2-phenyl-
propenoic acid (47%), mp 184–186 ЊC, lit.56 191.5–192.5 ЊC.
This acid was methylated by refluxing overnight in methanol
acidified with H2SO4. After addition of water, the ester was
extracted into ether, washed with a saturated solution of
Na2CO3, and with water, and the solution dried. On removal of
the solvent methyl (E)-3-(2-cyanophenyl)-2-phenylpropenoate
was obtained as colourless crystals (93%), mp 94.5–95 ЊC;
(Found: Mϩ 263.09450. C17H13NO2 requires 263.09463); m/z
263 (95%, Mϩ), 232 (12), 204 (100), 203 (65), 176 (15), 146 (51)
102 (9), 77 (10), 51 (10); νmax(Nujol)/cmϪ1 2229 (C᎐N), 1703
᎐
᎐
(C᎐O), 1245 (C–O); δ (270 MHz, CDCl ) 3.78 (s, 3H), 6.8 (m,
᎐
H
3
1H), 7.07–7.27 (m, 8H), 7.58 (m, 1H), and 8.00 (s, 1H); δC(67.9
MHz, CDCl3) 52.7, 113.0, 117.0, 127.5, 128.0, 128.4, 128.6,
128.9, 129.3 129.5, 133.4, 135.5, 136.0, 173.2, and 167.0.
(ii) Kinetic measurements
Sandmeyer cyanation. The following procedure is typical of
those carried out to measure rates of the Sandmeyer cyanation
reaction. A master solution of KCN was made (fresh, daily)
by dissolving 5.2 g in water (100 cm3) buffered at pH 7, the
composition of the buffer being KH2PO4 (0.1 mol dmϪ3) and
K2HPO4 (0.062 mol dmϪ3). Tetrakis(acetonitrile)copper()
tetrafluoroborate in an acetonitrile solution (20 cm3, 0.20 mol
dmϪ3) was then added to a chosen aliquot of the cyanide solu-
tion and the volume increased to approximately 90 cm3 by the
addition of more acetonitrile and buffer as necessary to ensure
a final composition of 1 : 1 MeCN–water. The pH was adjusted
to a measured value of 8 by the addition of acid or base (0.01
mol dmϪ3 H2SO4 or NaOH in 1 : 1 MeCN–water). The total
volume was then adjusted to 100 cm3, ensuring that a 1 : 1
MeCN–water ratio was maintained, and the pH re-measured to
ensure it remained 8. The solution was placed in a three necked
flask and stirred from overhead (through a gas tight seal) and
allowed to equilibrate at 298 K. One arm of the flask was con-
nected to the gas displacement cell and the other to a pressure
equalising funnel containing 4-methoxybenzenediazonium
tetrafluoroborate, 3, (0.22 g, 1 mmol) in 1 : 1 MeCN–aqueous
pH 7 buffer (5 cm3).
Once the setup had been made gas-tight (all joints were
sealed with labfilm), the diazonium solution was added to the
cyanocuprate solution giving a total reaction volume of 105
cm3 with [Cu] = 0.038 mol dmϪ3, [CNϪ] according to the aliquot
of master solution taken, [3] = 0.0095 mol dmϪ3 and pH 8. The
volume of evolved nitrogen was measured at suitable intervals
by weighing the water which was displaced from the gas cell
into a beaker placed on a top pan balance. After 2 hours
the reaction mixture was added to a solution of disodium
4-Methoxybenzenediazonium tricyanocuprate, [ArN2؉]2-
I
[Cu (CN)32؊]. A similar procedure was followed to that given for
(E)-4-methoxybenzenediazocyanide in which the potassium
cyanide solution was replaced by an aqueous cyanocuprate
solution (2.24 g CuCN and 3.26 g KCN in 10 cm3). The precipi-
tate was collected by filtration, washed with CHCl3 and dried in
a vacuum desiccator. In Table 4 a comparison of the 13C CP-
MAS NMR and IR spectra of this solid is made with spectra of
3, K2Cu(CN)3 and 3-methoxybenzonitrile.
Dipotassium tricyanocuprate. To an aqueous suspension of
CuCN (1 g, 0.011 mol in 10 cm3) was added KCN (1.45 g, 0.022
mol) with vigorous stirring. The solution was filtered and the
water removed by rotary evaporation. The resultant white
powder was dried at 100 ЊC under vacuum, νmax(Nujol)/cmϪ1
2078 [cf. the CN stretching frequency at 2094 cmϪ1 reported for
disodium tricyanocuprate in aqueous solution];52 δC[67.9 MHz,
D2O] 152.5.
2-Benzoylbenzenediazonium tetrafluoroborate7 and methyl
(E )-3-(2-diazoniophenyl)-2-phenylpropenoate
tetrafluoro-
borate.34 These precursors to the radical clocks 4 and 5 were
prepared as previously described as was methyl phenanthrene-
9-carboxylate,34 9, the cyclisation product of the latter. Fluoren-
9-one, 8, the cyclisation product of 4, was a commercial
material (Lancaster).
2-Cyanobenzophenone, 6. Tetrakis(acetonitrile)copper()
tetrafluoroborate in MeCN solution (48 cm3, 0.2 mol dmϪ3) was
added to KCN (1.95 g) dissolved in mixed MeCN and aqueous
1132
J. Chem. Soc., Perkin Trans. 2, 2002, 1126–1134

View Article Online
Table 4 Spectroscopic comparison of the intermediate precipitate from Sandmeyer cyanation with that of component ions and the Sandmeyer
product
13C Chemical shifts/ppm
IR Frequencies/cmϪ1 a
d
Precipitateb
3c
K2Cu(CN)3
4-MeOC6H4CNe
55.5
Precipitate
3
K2Cu(CN)3
2078
57.0
58.9
99.5
103.1
117.5
2234
2105
1576
1297
1095
2247
58.0
103.6
1583
1293
1098
103.9
114.7
Broad
118.3
137.0
170.6
1009
842
522
999f
840
522
119.2
133.9
134.0
137.3
152.0
162.0
170.0
152.5
162.8
a IR spectra were obtained for Nujol mulls.b CP-MAS spectrum; the absorptions assigned to 4-methoxybenzonitrile appeared during acquisition of
the spectrum. c Spectrum obtained for solution in (CD3)2CO. d Spectrum obtained for solution in D2O. e Spectrum obtained for solution in CDCl3.
Ϫ
f Shoulder on broad absorption due to BF4
.
2-naphthol-3,6-disulfonate (50 cm3, 0.17 mol dmϪ3). The buffer
provided sufficiently basic conditions for azo-coupling to
remove any unreacted 3. The mixture was extracted with diethyl
ether and the extract analysed by GC on an SE-54 capillary
column (temperature program 60 ЊC for 5 min then 16 ЊC minϪ1
until 290 ЊC). The main product (ca. 95%) was 4-methoxy-
benzonitrile (retention time 10.2 min) giving a mass spectrum
agreeing with precedent:57 m/z 133 (100%, Mϩ), 118 (12), 103
(35), 90(62) and 64 (20). Each run was repeated at least once.
5 C. Galli, J. Chem. Soc., Perkin Trans. 2, 1982, 1139.
6 P. Hanson, J. R. Jones, B. C. Gilbert and A. W. Timms, J. Chem.
Soc., Perkin Trans. 2, 1991, 1009.
7 P. Hanson, R. C. Hammond, P. R. Goodacre, J. Purcell and A. W.
Timms, J. Chem. Soc., Perkin Trans. 2, 1994, 691.
8 P. Hanson, R. C. Hammond, B. C. Gilbert and A. W. Timms,
J. Chem. Soc., Perkin Trans. 2, 1995, 2195.
9 E. Pfeil and O. Velten, Justus Liebigs Ann. Chem., 1949, 562, 163.
10 E. Pfeil and O. Velten, Justus Liebigs Ann. Chem., 1949, 565, 183.
11 W. S. Cowdrey and D. S. Davies, J. Chem. Soc., 1949, S48.
12 C. Galli, J. Chem. Soc., Perkin Trans. 2, 1984, 897.
13 See e.g. B. S. Furniss, A. J. Hannaford, V. Rogers, P. W. G. Smith and
A. R. Tatchell, Vogel’s Textbook of Practical Organic Chemistry,
4th Edn., Longman, London, 1979, pp. 689–703.
14 S. C. Dickerman, K. Weiss and A. K. Ingberman, J. Org. Chem.,
1956, 21, 380.
15 S. C. Dickerman, K. Weiss and A. K. Ingberman, J. Am. Chem. Soc.,
1957, 80, 1904.
16 S. C. Dickerman, D. J. DeSouza and N. Jacobson, J. Org. Chem.,
1969, 34, 710.
17 C. J. Rondestvedt, Org. React. (N. Y.), 1960, 11, 189.
18 C. J. Rondestvedt, Org. React. (N. Y.), 1976, 24, 225.
19 C. Galli, Chem. Rev., 1988, 88, 765.
20 J. K. Kochi, J. Am. Chem Soc., 1956, 78, 1228.
21 T. Cohen, A. G. Dietz and J. R. Miser, J. Org. Chem., 1977, 42,
2053.
22 M. P. Doyle, J. K. Guy, K. C. Brown, S. N. Mahapatro, C. M.
VanZyl and J. C. Pladziewicz, J. Am. Chem. Soc., 1987, 109, 1536.
23 B. C. Gilbert, P. Hanson, J. R. Jones, A. C. Whitwood and A. W.
Timms, J. Chem. Soc., Perkin Trans. 2, 1992, 629.
24 D. S. Wulfman, The Chemistry of Diazonium and Diazo groups, Part
1, Ed. S. Patai, Interscience, 1978, p. 293.
25 C. H. R. Rømming and K. Wærstad, J. Chem. Soc., Chem.
Commun., 1965, 299.
26 M. L. Crossley, R. H. Kienle and C. H. Benbrook, J. Am. Chem.
Soc., 1940, 62, 1400.
27 T. Ignasiak, J. Susko and B. Ignasiak, J. Chem. Soc., Perkin Trans. 1,
1975, 2123.
28 K. Kurnia, D. E. Giles, P. M. May, P. Singh and G. T. Hefter,
Talanta, 1996, 43, 2045.
Caged ligand transfer. For the evaluation of the rate con-
stants for caged ligand transfer to 4, buffered cyanocuprate()
solutions in 1 : 1 aqueous MeCN were made up as above but on
a one quarter scale (25 cm3) as measurement of nitrogen evolu-
tion was not required. 2-Benzoylbenzenediazonium tetrafluoro-
borate (0.05 g) in the same solvent was added and the solution
stirred for 2 h after which the solution was added to a solution
of disodium 2-naphthol-3,6-disulfonate to ensure removal of
any traces of unreacted diazonium ion. The products were
extracted into ethyl acetate and analysed by GC as above. The
relative response factor for the products of ligand transfer, 6,
and cyclisation, 8, was determined by use of the authentic
materials.
For evaluation of rates of ligand transfer to 5, a somewhat
different procedure was used: CuCN and KCN were dissolved
in fixed proportion (1 : 2) but different concentrations in 10%
aqueous acetonitrile (25 cm3). As the solution was stirred
vigorously, the diazonium salt (0.025 g) in the same solvent
(5 cm3) was added which resulted in a short-lived orange turbid-
ity. After stirring for 10 minutes, the solution was extracted with
ethyl acetate containing dibenzofuran as internal standard for
GC analysis, which was performed on an SE30 capillary col-
umn at 180 ЊC, the necessary response factors being obtained
on authentic materials.
29 The computer program used, ES4ECI, was kindly provided by
Professor S. Sammartano. C. De Stefano, P. Mineo, C. Rigano and
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Acknowledgements
We thank Great Lakes Fine Chemicals with the EPSRC for a
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for a Research Studentship (A. B. T.).
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