Kinetic parameters for radical reductions involving chiral nonracemic (1R,2S,5R)-menthyl substituted stannanes

Article abstract of DOI:10.1016/j.tetasy.2004.01.016

Absolute rate constants and Arrhenius parameters for hydrogen atom abstraction by primary alkyl radicals from chiral nonracemic, (1R,2S,5R) -menthyl (Men) substituted stannanes, 1-3, as well as stannanes containing both the menthyl as well as the 8-dimethylaminonaphthyl (L) or 2-[(1S)-1- dimethylaminoethyl]phenyl (L*) substituents, namely MenPhLSnH 4 and MenPhL*SnH 5 have been determined in tert-butylbenzene through utilization of the '5-hexenyl radical clock' reaction. At 80°C, MenPh ₂SnH, Men₂PhSnH and Men₃SnH react with primary alkyl radicals with rate constants that fall in the range, 0.61-1.36 × 10⁷ M^-1 s^-1, while similar values for MenPhLSnH and MenPhL*SnH are 1.30 and 2.74 × 10⁷ M ^-1 s^-1 (80°C), respectively. Arrhenius expressions have been determined for all reactions studied; values for log (A/M ^-1 s^-1) are found to lie in the range: 9.54-9.95, with activation energies determined to be: 15.8-19.7 kJ mol^-1. Menthyl substitution appears to affect both the energy and entropy of activation. The effect of intramolecular coordination on the rate constants for the reactions involving MenPhLSnH and MenPhL*SnH is discussed and computational data presented.

Full text of DOI:10.1016/j.tetasy.2004.01.016

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 15 (2004) 995–999
Kinetic parameters for radical reductions involving chiral
nonracemic (1R,2S,5R)-menthyl substituted stannanes
Le Zeng, V. Tamara Perchyonok and Carl H. Schiesser*
School of Chemistry, Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Victoria 3010, Australia
Received 16 December 2003; accepted 13 January 2004
Abstract—Absolute rate constants and Arrhenius parameters for hydrogen atom abstraction by primary alkyl radicals from chiral
nonracemic, (1R,2S,5R)-menthyl (Men) substituted stannanes, 1–3, as well as stannanes containing both the menthyl as well as the
8-dimethylaminonaphthyl (L) or 2-[(1S)-1-dimethylaminoethyl]phenyl (L*) substituents, namely MenPhLSnH 4 and MenPhL*SnH
5 have been determined in tert-butylbenzene through utilization of the Ô5-hexenyl radical clockÕ reaction. At 80 ꢀC, MenPh2SnH,
Men2PhSnH and Men3SnH react with primary alkyl radicals with rate constants that fall in the range, 0.61–1.36 · 107 Mꢀ1 sꢀ1, while
similar values for MenPhLSnH and MenPhL*SnH are 1.30 and 2.74 · 107 Mꢀ1 sꢀ1 (80 ꢀC), respectively. Arrhenius expressions have
been determined for all reactions studied; values for log (A/Mꢀ1 sꢀ1) are found to lie in the range: 9.54–9.95, with activation energies
determined to be: 15.8–19.7 kJ molꢀ1. Menthyl substitution appears to affect both the energy and entropy of activation. The effect of
intramolecular coordination on the rate constants for the reactions involving MenPhLSnH and MenPhL*SnH is discussed and
computational data presented.
ꢁ 2004 Elsevier Ltd. All rights reserved.
1. Introduction
of providing single enantiomer outcomes for a variety of
transformations of synthetic and commercial signifi-
cance.34;35
In free-radical chemistry, few reagents have had the
same impact as tributyltin hydride.1–3 As a chain-
carrying reagent, tributyltin hydride is cheap, readily
available, has favourable rate constants for delivery of
hydrogen atom to carbon-centred and other radicals4–10
while the corresponding stannyl radical reacts readily
with a variety of free-radical precursors.10–23 Formation
of carbon–carbon bonds through the use of inter- and
intramolecular homolytic addition chemistry,1–3;24–29 and
carbon–heteroatom bonds through the use of homolytic
substitution chemistry30;31 are key chemical reactions of
synthetic significance. The impact of this chemistry has
been made possible through significant advances in our
understanding of the factors that govern radical reac-
tivity together with a knowledge of the important rate
constants involved in the overall chain process.1–10
MenPh SnH
2
1
Me2N
Sn(H)PhMen
NMe2
Men PhSnH
2
= Men
2
Sn(H)PhMen
4
5
Men SnH
3
3
An example of this chemistry is shown in Scheme 1, in
which (R)-naproxen ethyl ester 6 is prepared in 99%
enantioselectivity (ee) from racemic bromide 7 by
reduction with bis[(1R,2S,5R)-menthyl]phenyltin hy-
dride (Men2PhSnH) 2 at )78 ꢀC in the presence of
magnesium bromide.34
Recently, we reported the preparation of chiral, non-
racemic, (1R,2S,5R)-menthyl (Men) containing stan-
nanes 1–5 for use in enantioselective free-radical
reduction chemistry32;33 and demonstrated that in con-
junction with Lewis acids, these stannanes are capable
CH3
Br
Men PhSnH
2
OEt
OEt
O
O
MgBr / –78°
2
MeO
MeO
99% ee (R)
7
6
* Corresponding author. Tel.: +61-3-9344-4180; fax: +61-3-9347-5180;
e-mail: carlhs@unimelb.edu.au
Scheme 1.
0957-4166/$ - see front matter ꢁ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2004.01.016

996
L. Zeng et al. / Tetrahedron: Asymmetry 15 (2004) 995–999
Table 1. Selected rate constant data (0.10 M) for the reactions of
primary alkyl radicals with (1R,2S,5R)-menthyl substituted stannanes
1–5 in tert-butylbenzene at 80 ꢀC and comparative data for tributyl-
and triphenyltin hydride
Given that a knowledge of rate constants is crucial to
the design of free-radical reactions of synthetic signifi-
cance, we have also been determining these important
parameters for our recently developed chiral reagents.
Herein we report that stannanes 1–5 react with primary
alkyl radicals with rate constants that fall in the range,
Stannane
%8
%9
kc=kH
(M)a
kH
(·107 Mꢀ1 sꢀ1
)
0.61–2.74 · 107 Mꢀ1 sꢀ1, with associated log (A/Mꢀ1 sꢀ1
)
Ph3SnH
––
––
––
>1.50b
1.36
and activation energy (kJ molꢀ1) that lie in the ranges:
9.54–9.95 and 15.8–19.7, respectively. Menthyl substi-
tution appears to affect both energy and entropy terms,
while intramolecular coordination in stannanes 4 and 5
activate these stannanes towards homolytic hydrogen
transfer.
MenPh2SnH 1
Men2PhSnH 2
Men3SnH 3
Bu3SnH
55.2
42.7
35.6
––
44.8
57.3
64.4
––
12.3
7.9
0.82
0.61
0.60c
5.5
––
MenPhLSnH 4
MenPhL*SnH 5
47.4
70.0
52.6
30.0
11.1
22.7
1.30
2.50
a Average of three experiments.
b Lit.45
c Lit.9
2. Results and discussion
1-bromo-5-hexene were initially performed at 80 ꢀC
under Ôpseudo-first-orderÕ conditions as described in our
previous work.37 Selected results of this study are sum-
marized in Table 1 together with available data for
tributyl- and triphenyltin hydride.45;46
The menthyl-substituted stannanes used herein were
prepared as described previously by us.32;33 Absolute
rate constants for the delivery of a hydrogen atom to
primary alkyl radicals in tert-butylbenzene were deter-
mined through application of the well-established Ô5-
hexenyl radical clockÕ36 reaction as described by us.37 An
example of the use of the ÔclockÕ as applied to the use of
(1R,2S,5R)-menthyldiphenyltin hydride (MenPh2SnH) 1
is provided in Scheme 2. Provided that the ÔclockÕ rate
constant (kc) is well established for any given tempera-
ture, then the rate equation (Scheme 2) will provide a
value for the hydrogen transfer rate constant (kH). It
should be noted that several published Arrhenius
parameters exist for the ring closure of the 5-hexenyl
radical.38–44 Kinetic EPR spectroscopy and competitive
experiments provide Arrhenius expressions, with values
of 9.5–10.7 for log (A/Mꢀ1 sꢀ1), and activation energies
The remaining kinetic data for reactions at temperatures
other than 80 ꢀC were also obtained under Ôpseudo-first-
orderÕ conditions. Systematic variations in temperature
(60–120 ꢀC) reveal a pleasing linear correlation between
log kH and reciprocal temperature. All kinetic data are
averages of three experiments and errors in log A and
activation energy (Ea) are expressed to 95% confidence
and account for random, but not systematic errors. The
Arrhenius data obtained in this manner are summarized
in Table 2 together with the available data for tributyltin
hydride.46
of 25.5–32.6 kJ molꢀ1 43;44
with the ÔbestÕ values being
,
Table 2. Kinetic parameters for the reactions of the 5-hexenyl radical
with (1R,2S,5R)-menthyl substituted stannanes 1–5 in tert-butylbenz-
ene (60–120 ꢀC) and comparative data for tributyltin hydride
a
10.4 0.3 and 28.7 1.8.9 In our previous work,37 we
calibrated the Ôhexenyl radical clockÕ in tert-butylbenz-
ene and determined the Arrhenius expression in that
solvent to be similar to the expressions determined in
other solvents, namely:
Stannane
log A/Mꢀ1 sꢀ1a Ea (kJ molꢀ1
)
kH (80 ꢀC)b
(·107 Mꢀ1 sꢀ1
)
MenPh2SnH 1
Men2PhSnH 2
Men3SnH 3
9.95 0.22
9.79 0.13
9.70 0.12
9.07 0.24
9.54 0.33
9.77 0.31
19.08 1.52
19.52 0.90
19.70 0.84
15.45 1.34
16.44 2.26
15.80 2.17
1.36
0.81
0.61
0.60
1.30
2.74
log kc=sꢀ1 ¼ ð10:13 ꢁ 0:42Þ ꢀ ð27:6 ꢁ 2:6Þ=h;
ð1Þ
Bu3SnHc
where h is 2:3RT kJ molꢀ1. We therefore have chosen to
use this expression for the Ôclock reactionÕ throughout
this work. Reactions of the various stannanes 1–5 with
MenPhLSnH 4
MenPhL*SnH 5
a Error limits are expressed to 95% confidence but include random and
not systematic variations.
b Calculated from the Arrhenius parameters.
c Ref. 9.
MenPh2Sn
k
c
Br
What is immediately clear from the kinetic data pre-
sented in Tables 1 and 2 is that the reaction rate con-
stant is largely affected by the number of phenyl
substituents on tin, and to a lesser extent by the steric
bulk associated with the number of menthyl substituents
present. Where intramolecular coordination is possible,
stannanes 4 and 5, this phenomenon serves to activate
the stannane towards hydrogen transfer; this effect
manifests itself in an observed increase in the rate con-
stant. The order of reactivity at 80 ꢀC is clearly
5 > Ph3SnH > 4 > 1 > 2 > 3 ꢂ Bu3SnH. However, the data
k
1
1
H
8
9
d[9]
d[8]
k
c
rate equation:
=
k
H[1]
Scheme 2.

L. Zeng et al. / Tetrahedron: Asymmetry 15 (2004) 995–999
997
in Table 2 suggests that these observations are not solely
the result in differences in activation energy (Ea), but
rather the subtle interplay between entropy (log A) and
energy terms. It is clear that the menthyl-substituted
stannanes react with log A values that lie in a narrow
range (9.54–9.95) and are very similar within experi-
mental error, but are significantly higher than the value
for tributyltin hydride. Despite the similarity in log A
values, the data does indicate a trend: the more menthyl
substituents present on tin, the lower (more negative)
the entropy of activation becomes. This suggests that
the steric influence of the menthyl group is such that it
increases Ea, while at the same time providing increasing
order to the transition state in the reaction (decreasing
log A).
to that of tributyltin hydride at the same temperature.
Our Arrhenius data suggests that this outcome may well
be entropy driven. As might be expected on steric
grounds, 3 has an activation energy some 4.3 kJ higher
than Bu3SnH. However, 3 also has a higher entropy of
activation (DSy), which at )60.6 J Kꢀ1 molꢀ1 is about
12 J Kꢀ1 molꢀ1 higher than that determined for Bu3SnH.
These two kinetic parameters operate in a manner that
leads to the same rate constant as that of Bu3SnH at
80 ꢀC. We speculate that a similar phenomenon involv-
ing transition state disorder is partly responsible for the
reported data involving trimesityltin hydride.
In order to provide further insight into the effect of
intramolecular coordination on the stannane reductions
in this study, the transition state 11 involved in the
transfer of the hydrogen atom from stannane 10 to
methyl radical was modeled using the AM1 molecular
modeling technique (Scheme 3). This method has been
used previously to provide good qualitative transition
state data for stannane reductions.35;47 The AM1 opti-
mized transition state 11 is displayed in Figure 1 along
with that of 12, calculated for the analogous reaction
involving phenylstannane (PhSnH3) using the same
computational technique.
Stannanes 4 and 5, containing potentially intramolecu-
lar coordinating ligands on tin, would appear to react
significantly more rapidly with primary alkyl radicals
than the other stannanes in this study. Indeed, Men-
PhL*SnH 5, with a rate constant of 2.74 · 107 Mꢀ1 sꢀ1
,
may even react more rapidly than triphenyltin hydride
(>1.5 · 107 Mꢀ1 sꢀ1) at 80 ꢀC. The data presented in
Table 2 suggest that intramolecular coordination affects
most significantly the activation energies of these two
reactions, with log A values for 4 and 5, at 9.54 and 9.77,
that differ little from those determined for the other
menthyl-substituted systems.
It is worthwhile comparing the AM1 data herein to
those from previous work. The calculated transition
ꢀ
ꢀ
state distance of about 1.79 A (C–H) and 1.68 A (Sn–H)
compare favourably with those calculated for other
unsubstituted stannanes,48 reinforcing our previous
observation that substitution at the reacting tin or car-
bon centres has little influence on the transition state
geometry; overall CTS–SnTS separations are invariably
It is interesting to compare these data to those reported
for reactions of other stannanes with the 5-hexenyl
radical. Values of log A generally range between 8.8 and
10.0, with activation energies in the range 13–
18 kJ molꢀ1 46
Interestingly, Hershberger and co-work-
.
ꢀ
ers reported that sterically crowded trimesityltin hydride
reacts with the 5-hexenyl radical with a rate constant of
1.1 · 107 Mꢀ1 sꢀ1 at 50 ꢀC, and concluded that there is no
appreciable steric hindrance restricting the approach of
the hexenyl radical to the (supposedly) crowded stann-
ane.46 Unfortunately, no Arrhenius parameters are
available for this reaction involving the mesityl-substi-
tuted stannane.
close to 3.5 A with a preferred co-linear arrangement of
attacking and leaving groups.35;47
Interestingly, AM1 would appear to be capable of
reproducing the observed activating affect of intra-
molecular nitrogen coordination. The data presented in
Figure 1 clearly indicates that, in good agreement with
the experimental data (vide supra), AM1 predicts that
nitrogen coordination serves to lower the energy barrier
in these hydrogen transfer reactions. Stannane 10 is
calculated to react with a methyl radical with a barrier
some 3.7 kJ molꢀ1 lower than the analogous process
involving PhSnH3. In comparison, stannane 5 has been
determined to react with the 5-hexenyl radical with an
Apart from being intriguing, this result is in good
agreement with those in this current study, namely that
the sterically crowded tris[(1R,2S,5R)-menthyl]tin
hydride 3 reacts with the 5-hexenyl radical with an
almost identical rate constant (6 · 106 Mꢀ1 sꢀ1 at 80 ꢀC)
H
H
H2N
Sn
H CH3
NH2
SnH2
NH2
∆E
CH4
+
+
CH3
SnH3
10
11
Scheme 3.
The log A values presented in Table 2 translated into entropies of activation (DSy) of )55.8, )58.9, )60.6, )72.7, )63.7, )59.3 J Kꢀ1 molꢀ1 in moving
down the table from 1 to 5.

998
L. Zeng et al. / Tetrahedron: Asymmetry 15 (2004) 995–999
1.677Å
179.2°
2.659Å
1.793Å
Sn
N
Sn
174.3°
1.774Å 1.674Å
∆E‡ = 11.8 kJ.mol-1
∆E‡ = 8.1 kJ.mol-1
-1
-1
ν
TS = 530i cm
νTS = 539i cm
Figure 1. AM1 calculated transition states 11 (left) and 12 (right).
energy barrier about 3.3 kJ molꢀ1 lower than the unco-
ordinated analogue 1.
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butylbenzene was placed in a small pyrex tube, 1-bro-
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Acknowledgements
We thank the Australian Research Council and Chiro-
gen Pty. Ltd. for financial support.
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