Reaction of lithium metal with benzil in THF. A kinetic study

Article abstract of DOI:10.1002/poc.649

The kinetics of benzil reduction by lithium metal in THF were investigated under different reaction conditions. The main reaction products were phenylacetophenone and bibenzyl; 1,2-diphenylethanol and diphenylethene were minor products. The first radical anion intermediate formed by electron transfer upon chemisorption of the benzyl on the metal surface was fully characterized and quantitatively determined by ESR spectroscopy. The kinetics of the benzil decay and of the formation of the two main products were followed by GC and ESR. A unique feature of this reaction is the presence of a well-defined and reproducible induction period. A mechanism is proposed which accounts for the main experimental observations. Copyright

Full text of DOI:10.1002/poc.649

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2003; 16: 669–674
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.649
Reaction of lithium metal with benzil in THF.
A kinetic study
1
2
2
Norma Sbarbati Nudelman, * Susana Velurtas and Maria A. Grela
1
Dpto. Qu ´ı mica Org a´ nica, Facultad de Ciencias Exactas, Universidad de Buenos Aires, Ciudad Universitaria, 1428 Buenos Aires, Argentina
Dpto. de Qu ´ı mica, Facultad de Ciencias Exactas, Universidad Nacional de Mar del Plata, B7602AYL Mar del Plata (BA), Argentina
2
Received 20 February 2003; revised 24 March 2003; accepted 26 March 2003
ABSTRACT: The kinetics of benzil reduction by lithium metal in THF were investigated under different reaction
conditions. The main reaction products were phenylacetophenone and bibenzyl; 1,2-diphenylethanol and dipheny-
lethene were minor products. The first radical anion intermediate formed by electron transfer upon chemisorption of
the benzyl on the metal surface was fully characterized and quantitatively determined by ESR spectroscopy. The
kinetics of the benzil decay and of the formation of the two main products were followed by GC and ESR. A unique
feature of this reaction is the presence of a well-defined and reproducible induction period. A mechanism is proposed
which accounts for the main experimental observations. Copyright # 2003 John Wiley & Sons, Ltd.
KEYWORDS: organolithium; benzil; lithium metal; surface reactions; induction period; heterogenous systems
INTRODUCTION
mechanisms of heterogeneous reactions is sparse or al-
most non-existent.
11
Some reactions that occur between organic compounds
and specific metals have long been known, and the
reaction of organic halide derivatives with magnesium,
giving rise to the versatile Grignard chemistry, is one of
the most useful synthetic tools available to organic
chemist to assist in the assembly of complex structures
in the laboratory and also useful in the preparation of
We have recently reported a detailed kinetic study of
the reaction between benzaldehyde and lithium metal in
Tetrahydrofuran (THF): the first radical intermediate was
fully characterized and the reaction rates of all of the
several steps involved in the overall reaction were deter-
12a
mined. Conditions were achieved to make the reaction
useful for the quantitative conversion of benzaldehyde
1
12b
into benzyl benzoate. This paper reports a mechanistic
many industrial chemicals. Because of the continous
search for new synthetic methods, other metal-mediated
study of the reaction of benzil with lithium metal under
well-defined reaction conditions. The reaction progress
was followed both by gas chromatographic (GC) and
electron spin resonance (ESR) methods, and a simplified
reaction scheme is proposed to describe the main experi-
mental observations. A brief preliminary report of this
2
organic transformations are currently being studied. The
reducing ability of indium metal has recently been shown
to have unique features because of its low first ionization
3,4
potential and its mediatory effect in powerful reactions
5
such as the Michael reaction. Reactions with other
6
7
13
reducing metals such as zinc and samarium are also
being developed. Mechanistic studies on these reactions
are still extremely scarce; even the detailed mechanisms
of the reactions of Grignard reagents have been under
scrutiny by three or four generations of researchers and
reaction was recently communicated.
EXPERIMENTAL
8
1
4a
the work is continuing. Very few kinetic studies on the
Materials. THF was purified as described previously
reactions of metals with organic substrates have been
reported. The main problem in studying heterogeneous
reaction mechanisms is that of obtaining reproducible
kinetic information for a clear interpretation. Thus,
although experimental techniques to overcome this pro-
and was distilled from dark blue solutions of sodium
benzophenone ketyl under nitrogen immediately prior
to use. Benzil (Aldrich, 99%) was crystallized from
ethanol, m.p. 94–95 C (lit. ). All glassware, syringes
and needles were dried in a vacuum oven, cooled in a
desiccator and flushed with dry nitrogen immediately
prior to use.
ꢀ
14b
9,10
blem have been designed,
quantitative analysis of the
*
Correspondence to: N. S. Nudelman, Dpto. Qu ´ı mica Org a´ nica,
Facultad de Ciencias Exactas, Universidad de Buenos Aires, Ciudad
Universitaria, 1428 Buenos Aires, Argentina.
E-mail: nudelman@qo.fcen.uba.ar
Contract/grant sponsor: University of Buenos Aires.
Contract/grant sponsor: University of Mar del Plata.
General procedure. Lithium wire (Merck, >99%, 3 mm
diameter) was weighed under ligroin, washed with THF
and cut into pieces 2–3 mm long directly over a reaction
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 669–674

6
70
N. S. NUDELMAN, S. VELURTAS AND M. A. GRELA
flask containing a small portion of boiling THF, to create
a fresh surface in which the reaction could commence.
The reaction flask was capped with an ‘air-tight’ stopper,
and alternately evacuated and flushed with nitrogen
several times, as is usually done for organolithium reac-
15
tions. The flask was equilibrated to the desired tem-
perature for 15 min under vigorous magnetic stirring. A
solution of benzil of the desired concentration in THF
(thermostated at the same temperature) was transferred
by syringe, and a 0.2 ml sample was immediately taken to
verify the initial concentration, [1]0.
Kinetic measurements. Standard techniques for the ma-
nipulation of air- and water-sensitive compounds were
16
strictly followed. A general kinetic procedure, similar
to that developed for the kinetic determinations of the
17
reactions of naphthyllithium, was used. The composi-
tions of aliquots of the reaction mixture withdrawn at
various reaction times were determined by GC and/or
ESR analysis.
Rates of benzil decay and product formation, shown in
Fig. 1, were measured by GC. In this case, the samples
were quenched with a saturated solution of NH Cl and
4
analysed using an OV-101 column (temperature gradient:
ꢀ
5
0–270 C). In separate runs, stable products were iso-
lated and fully characterized by mass spectrometry (using
a Shimadzu QP 5050A GC–MS system), and NMR
spectroscopy (determined on a Brucker 200, 300 or 500
1
spectrometer operating at 200, 300 or 500 MHz for H
13
and 50, 75 or 125 MHz for C).
Figure 1. Reaction of lithium metal with benzil (1) in THF at
2
98 K, [1] ¼ 0.2 M, Li ¼ 428 mg. (A) [1] decay determined by
0
ESR analysis of intermediates. Well-determined volumes
of the reaction mixture were placed in a quartz tube under
argon and their ESR spectra were immediately recorded
at 298 K using a Bruker (Germany) ER 200 X-band
spectrometer. We verified that for the higher concentra-
tions attained in our experiments the spectra of the
aliquots were qualitatively invariant whereas their inten-
sity varied slightly (<10%) over 20 min.
GC analysis of the reaction mixture; experimental points are
shown by (*); error bars are standard deviations of at least
five measurements. (B) Concentration of the main reaction
products, (&) [phenylacetophenone] and (*) [bibenzyl], as a
function of time, determined by GC analysis. In (A) and (B),
the solid lines are the simulated concentration dependence
derived from the numerical integration according to the
mechanism depicted in Scheme 1 and Table 1 (see the text)
Radical concentrations were estimated by comparing
the area under the ESR first-derivative spectrum of the
sample with that of a solution of galvinoxyl, the reference
standard, recorded at the same microwave power, mod-
ulation amplitude and amplification gain.
(
points are experimental determinations and the bars
show the dispersion range observed in at least five
independent runs). As frequently found in heterogeneous
reactions at liquid–solid interfaces, the reaction shows an
induction time after which the reaction seems to proceed
autocatalytically. It is worth remarking that the induction
time was fairly reproducible. The sudden onset of benzil
decay is almost coincident with the abrupt change in the
solution from pale yellow to purple, the characteristic
color of the benzil radical anion. Figure 1(B) shows the
time-dependent appearance of the two main reaction
products determined by GC. The curves also show an
induction period. The lag in time for the formation of
phenylacetophenone is very similar to that shown in
Fig. 1(A) for the decay of 1.
RESULTS AND DISCUSSION
Reaction of benzil (1) with lithium metal in THF under
the reaction conditions described in the Experimental
section affords phenylacetophenone (9) and bibenzyl (12)
as the main products, plus small amounts of 1,2-diphe-
nylethanol (10) and 1,2-diphenylethene (11). The benzil
decay and the rate of appearance of the main products
were followed by GC analysis of the reaction mixture at
different time intervals, as described in the Experimental
section. The representative behaviour of the decay of (1)
for the reaction in THF at 298 K is shown in Fig. 1(A)
The ESR analysis of the reaction mixture at different
time intervals revealed the involvement of the expected
benzil radical anion as the most important radical
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 669–674

REACTION OF LITHIUM WITH BENZIL IN THF
671
Scheme 1
7% of the benzaldehyde remained in solution, the rest
being adsorbed on the metal surface. Adsorption and
desorption are, then, the first phenomena which are
shown in a simplified way in Scheme 1 as the two first
steps. In all the reaction steps shown in Table 1, subscript
S is used to distinguish an adsorbed from a free diffusing
species in solution.
Figure 2. Reaction of lithium metal with benzil (1) in THF at
2
98 K, [1] ¼ 0.2M, Li ¼ 428 mg. (*) Benzil anion radical
0
2
concentration determined by ESR analysis of the reaction
mixture as a function of time. Experimental conditions as in
Fig. 1. The solid line is the simulated concentration depen-
dence derived from the numerical integration according to
the mechanism depicted in Scheme 1 and Table 1 (see the
text)
12
intermediate. The ESR spectrum of the reaction mixture in
THF ([1] ¼ 0.2 M, Li ¼ 428mg) at 298 K after 20min of
It is well known that lithium has a strong tendency to
19
lose ions into a solution. Ion solvation drives this
process, which is relatively easy in THF because of its
known ability for cation solvation. Electron transfer from
the lithium surface to the adsorbed benzil (1S in
1
3
reaction was reported recently. The spectral total width
(0.76mT) and its hyperfine structure are fully consistent
with previous reports on the benzil radical anion ([cou-
pling constants: a ¼ 0.99G (4); 0.36G (4) and 1.12G (2)
Scheme 1; the rate of ET measured by k ) produces the
3
H
18
for the ortho-, meta- and para-H, respectively ]). The
paramagnetic signal appears after an induction time in
close correspondence with that observed for the decay of 1
by GC and it grows steadily. It is worth emphasizing that
the spectral characteristics of the aliquots, taken up to 80%
of benzil disappearance, remain almost the same. This fact
is taken as evidence that, if not the unique, the benzil
radical anion is the overwhelming radical intermediate.
Figure 2 shows the rate of formation of the benzil
radical anion (2); the concentration of 2 was determined
by ESR analysis of the reaction mixture at different time
intervals. An induction period is also observed for the
appearance of the radical anion. The induction time is
apparently shorter than that shown in Fig. 1(A), which is
consistent with the fact that the decay of 1 to give the
radical anion is not detected by GC. It also indicates that
further reaction of 2 to give the precursors to the first
reaction product should be relatively slow.
adsorbed [benzil radical anion–(2)–lithium cation] pair,
symbolized by 2 , that is also strongly solvated by THF.
S
Species 2 partially escapes from the surface (and the
solvent cage), goes into solution and can be detected by
ESR in the reaction solution.
There is still controversy as to whether formation of the
Grignard reagents takes place on the magnesium surface
20
(called the A model) or after the radical has diffused
21
into solution (called the D model). In the present case,
sorption and desorption of the radical anion (processes 5
and 4, respectively, in Table 1) need to be considered to
Table 1. Reaction of benzil with lithium in THF at 298 K:
reaction steps as in Scheme 1
ꢁ1
No.
Reaction step
k (s
)
ꢁ
ꢁ
ꢁ
3
1
4
1
2
3
4
5
6
7
8
9
1 ! 1
8.3 ꢂ 10
1.0 ꢂ 10
2.1 ꢂ 10
1.3 ꢂ 10
1.7 ꢂ 10
6.7 ꢂ 10
1.0 ꢂ 10
1.7 ꢂ 10
2.2 ꢂ 10
5.0 ꢂ 10
3.7 ꢂ 10
3.0 ꢂ 10
5.0 ꢂ 10
2.5 ꢂ 10
5.0 ꢂ 10
s
1 ! 1
s
1 þ S ! 2 þ 2 S
s
s
A complete reaction mechanism able to interpret the
observed results is depicted in Scheme 1. Adsorption of
the reagent on the lithium surface, (symbolized by 1 ; the
ꢁ4
2 ! 2
s
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
2
4
1
2
2
2 ! 2
s
2 ! 3
s
s
s
3 ! 4
rate measured by k ), is postulated to be the initial step in
S
s
s
4 ! 5
s
s
the reaction of 1 with Li. With the current molecular level
understanding of surface reactions, it is known that
5 ! 5 ( ! 9)
s
ꢁ2
1
0
5 ! 6
s
s
10
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
2
2
4
3
6
adsorption plays an essential role. In our previous study
we also observed that adsorption of benzaldehyde on the
lithium surface was very important: determination of the
concentration of benzaldehyde immediately after putting
the THF solution in contact with lithium showed that only
11
12
6 ! 6 ( ! 10)
s
s
6 ! 7
s
s
1
1
1
3
4
5
7 ! 8
s
s
8 ! 8 ( ! 12)
s
7 ! 7 ( ! 11)
s
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 669–674

6
72
N. S. NUDELMAN, S. VELURTAS AND M. A. GRELA
obtain curves that fit the experimental results. As can be
observed, the rate of adsorption of the radical is faster
than its desorption. Most of the rest of the reactions
involve reduction by lithium metal and they are assumed
to occur on the surface metal in the simplified reaction
scheme.
Thus, further successive reductions of 2 by the lithium
metal gives intermediates 3 ! 4, that by loss of lithium
oxide gives intermediate 5, precursor of 9, which is one of
the main reaction products (24% yield in the first 60 min,
final yield 35%). Further reduction of 5, followed by a
Mg, that ‘etched’ it, thus cleaning the surface. Never-
theless, the ‘doctrine of etching’ was recently invalidated
by careful experiments that demonstrate that the presence
of MgBr catalysed the reaction, irrespective of whether
2
24
the Mg had been ‘etched’ or not.
In order to rationalize the experimental results, includ-
ing the autocatalytic behaviour, we set up the mechanism
presented in Scheme 1 and Table 1. The autocatalytic
behaviour is simulated indicating that S, the number of
active sites, is incremented by the reaction. The group of
differential equations for the full reaction mechanism was
solved using a non-commercial numerical integration
program, specially developed for this purpose. The cal-
culated rate constants are listed in Table 1, and they
proved to be adequate to simulate the experimental
concentration profiles of benzil decay, and radical anion
as well as product generation, as indicated by the solid
lines in Figs 1(A) and (B) and 2. As usual, all the kinetic
rates were calculated prior to the quenching of the
reaction mixture, since protonation of the precursors is
very fast. Then, the rate constants k , k , k and k14
second loss of Li O and reduction of the double bond,
2
afford, successively, intermediates 6, 7 and 8, which are
the precursors of the other isolated products. The fact that
only 5% of 12 is produced in the first 60 min of the
reaction, together with the slower formation of bibenzyl
shown in Fig. 1(B), can be considered as additional
evidence that 5 is an intermediate in the formation of
12. The solid lines in Fig. 1 were drawn following the
numerical integrals derived from Scheme 1, that are
gathered in Table 1. The yields of 10, 11 and 12 after
9
11 15
24 h of reaction are 5, 6 and 24%, respectively. The above
results were obtained at 298 K.
geven in Table 1 are the calculated rate constants for the
desorption of the precursors of products 9–12 (intermedi-
ates 5–8), respectively.
Kinetic analysis
Further experiments
The most striking kinetic feature of this reaction is the
presence of a well-defined induction period. The obser-
vation of induction periods is a common situation in
reactions occurring at solid–liquid interfaces, as for
example the synthesis of Grignard and Reformatsky
reagents on magnesium and zinc surfaces, respectively.
The induction period in Grignard reagent formation is
specially annoying since it is erratic, prolonged and, in
In an effort to achieve a better understanding of the nature
of the induction period, a series of experiments were
carried out. The first variable studied was the benzil
concentration. In agreement with the above kinetic treat-
ment of the data, we observed that the induction period
was highly dependent on [1]. As shown in Fig. 3, the
induction period was significantly increased by reducing
21
some cases, apparently infinite. Plausible hypotheses
22
abound, but there is no comprehensive, documented
understanding of the factors that create the induction
21
period or constitute initiation.
A likely explanation is that the initiation consists of the
removal of oxide layers which are generally present on
untreated reactive metals and are passivating. Neverthe-
less, we did not observe any difference in the induction
period when the lithium surface was carefully mechani-
cally cleaned, or when the lithium pellets were very
rapidly treated with dry methanol, immediately prior to
the reaction. Similar results were observed in the
Grignard reagent formation: sometimes reactions with
ordinary magnesium (‘Grignard’ grade) initiated better
than those of purer forms. Traces of transition metal
23
impurities, such as iron, might be responsible.
Whatever the reasons are that determine the presence of
very few active sites where the reaction can initiate, when
the reaction starts it seems that the area of the activated
surface grows exponentially, since an autocatalytic beha-
viour is observed. In the case of magnesium, the observed
Figure 3. Reaction of lithium metal with benzil (1) in THF at
2
98 K, Li ¼ 400 mg, at different benzil concentrations:
[
1] ¼ 88 mM (~) and 40 mM (*); benzil decay as a function
0
of time determined by GC analysis. Solid lines are the
simulated concentration dependences derived from the
numerical integration, according to the mechanism depicted
in Scheme 1 and Table 1 (see the text)
activation by I was proposed to be due to a reaction with
2
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 669–674

REACTION OF LITHIUM WITH BENZIL IN THF
673
Figure 5. Reaction of lithium metal with benzil (1) in THF at
2
98 K; [1] ¼ 200 mM. Circles are experimental data for the
0
Figure 4. Reaction of lithium metal with benzil (1) in THF;
benzil anion radical concentration versus time at 323 K (&)
and 273 K (*), determined by ESR analysis of the reaction
mixture. Other experimental conditions as in Fig. 1
benzil decay under different initial lithium conditions:
Li ¼ 105 mg, 12 pellets (*); Li ¼ 400 mg, 50 pellets (*). The
broken line represents the simplified model prediction
using the data in Table 1, with S proportional to the number
of pellets (S is reduced from 1 to 0.24). The solid line
is a simulation using S ¼ 0.24 and a slightly lower value of
ꢁ4
ꢁ1
s ; the rest of rate constants are as in
the benzil concentration. Note that the simplified
mechanism is able to reproduce the experimental find-
ings, as indicated by the solid lines.
k
3
¼ 1.5 ꢂ 10
Table 1
The temperature effects on the reaction rates (between
73 and 323 K) were then analysed by ESR (see Fig. 4).
2
hypothesis invokes corrosion in Grignard reactions by
20a
Interestingly, we observed that the rate of radical forma-
tion, and also the induction time at 323 K, were nearly
identical with those found at 298 K. However, at 273 K
the onset of radical production was delayed by a few
minutes (see Fig. 4). It is worth noting that this finding
may simply reflect the fact that the radical anion species
spends more time on the surface. In fact, we were able to
detect by GC analysis of the species adsorbed at the metal
surface that the degree of adsorption of benzil is con-
siderably enhanced by reducing the temperature from 298
to 273 K.
direct chemical action. Nevertheless, there seems to be
no evidence that distinguishes local-cell corrosion from
21
direct chemical action.
Several ways to increase the initial rate of a Grignard
reaction have been proposed. A recent report described
the activation of the Mg metal surface that allowed the
initiation of the Grignard reagent formation at or below
ꢀ
20 C, which is very convenient for a safe preparation on
26
a plant scale. Previous studies of the effect of ultra-
sound on the preparation of the magnesium had shown
that presonication (in the absence of the reaction partner)
had no effect on the induction time, but more recent
work indicated that sonication promotes initiation but
does not always influence yields. Scanning electron
microscopy studies confirmed that the number of initia-
tion pits is effectively increased by ultrasound.
27
As a further test, we analysed the influence of the total
mass and the number of lithium pellets on the kinetic
behaviour. If the rate of the reaction were proportional to
the area, a, on lithium, then increasing a would increase
the rate of reduction of 1 and of subsequent reactions and,
eventually, affect the product distribution. However, it
can be seen in Fig. 5 that the main difference observed for
an increase in a is a change in the induction period. This
finding, which may seem apparently contradictory,
clearly suggests that it is the surface modification induced
by the reaction that is rate determining and not the
availability of initiation sites. This surface modification
could be a sort of ‘metallic corrosion;’ if so, then the
28
26
In the present case, activation of the lithium surface by
using a lithium emulsion was attempted but no changes in
rate or in the product distribution were observed, which is
consistent with the conclusion that the only remaining
explanation for the induction period and the autocatalytic
behaviour is surface activation induced by the reaction.
As discussed before, in spite of the heterogenous
system and a complex overall reaction scheme, the
simplified mechanism in Scheme 1 and Table 1 is able
to reproduce qualitatively the experimental results.
Although in complex reactions, especially when dealing
with heterogenous reactions, it is difficult to interpret
every rate constant, some attempts can be made to check
whether the calculated rate constants are in agreement
with the general knowledge about the expected relative
rates of each step. Thus, it is known that adsorption of
þ
local-cell hypotheses could apply: Li ions enter the
solution and electrons leave the metal, being transferred
to the adsorbed reductant. This is consistent with the fact
that the reaction requires a somewhat dipolar solvent to
occur; no reaction is observed in hexane or even in
diethyl ether. Our results are in line with the previous
‘metallic corrosion’-like hypothesis to explain the auto-
catalytic behaviour in Grignard reactions. Another
25
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 669–674

6
74
N. S. NUDELMAN, S. VELURTAS AND M. A. GRELA
neutral species on the surface is more difficult than that of
charged species, whereas the opposite applies with des-
Acknowledgements
10
orption: the values of the pairs of steps 1–2 and 4–5 are
consistent with this. Electron transfer (ET) to carbonyl
compounds has been found to be the determining step in
Financial support from the University of Buenos Aires
and from the University of Mar del Plata is gratefully
acknowledged.
17
several reactions: step 3 implies an ET and is one of the
slowest steps in the scheme. On the other hand, loss of
lithium oxide is relatively easy in these oxyanions and
has similar rates in related species (see steps 8 and 12).
Steps 7 and 10 are relatively fast: this is consistent with
the expectation that unsaturated oxyanions would be
more easily reduced than carbonyl compounds.
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and those of formation of the two main reduction pro-
ducts (phenylacetophenone and bibenzyl) were inter-
preted within a reaction scheme proposed to account
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phenomena on the lithium surface seem to play an
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Formation of the first radical anion intermediate was
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Copyright # 2003 John Wiley & Sons, Ltd.
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