Homolytic reactivity of ligated boranes toward alkyl, alkoxyl, and peroxyl radicals

Full text of DOI:10.1021/jo951707k

J . Org. Chem. 1996, 61, 1161-1164
1161
reactions is important, for both synthetic applications and
mechanistic studies, because radical-based synthetic
procedures usually involve chain reactions wherein a
series of propagation steps must be appropriately timed.6
In this paper we wish to report the results of a kinetic
investigation of the generation of ligated boryl radicals
by reactions with a primary alkyl radical, with t-BuO‚,
and with peroxy radicals.
Hom olytic Rea ctivity of Liga ted Bor a n es
tow a r d Alk yl, Alk oxyl, a n d P er oxyl
Ra d ica ls
Marco Lucarini,* Gian Franco Pedulli, and
Luca Valgimigli
Dipartimento di Chimica Organica “A. Mangini”, Universita`
di Bologna, Via S. Donato 15, I-40127 Bologna, Italy
Resu lts a n d Discu ssion
Received September 18, 1995
Kin etic Stu d y of th e Rea ction of LBH3 w ith a
P r im a r y Alk yl Ra d ica l. The determination of the
Arrhenius parameters for the reaction of LBH3 with a
primary alkyl radical was done by choosing as free-
radical clock the neophyl rearrangement (eq 2), which
has been studied in detail by Franz et al.7 and has been
already employed with success for similar purposes.9
In tr od u ction
Interest in organic radical reactions has increased over
the last years as radical-based methods for organic
synthesis have evolved.1 In a series of papers,2 Roberts
described the investigation of the structures and elemen-
tary reactions of a variety of ligated boryl radicals L f
B‚H2 (L ) R3N, R3P, R2S) in fluid solution using EPR
spectroscopy. Although ligated boryl radicals are reactive
species toward various organic functional groups, the
corresponding boranes are rather poor hydrogen atom
donors toward alkyl radicals due to the strength of the
B-H bond.3 However, electrophilic carbon radicals (such
as those formed by addition of an alkyl radical to an
electron-deficient alkene) were found by Roberts4 to react
sufficiently rapidly with Bu3PBH2Ph to support chain
reactions. For instance, this propagation step, together
with halogen abstraction from RI and addition of the
resulting alkyl radical to the CdC bond, is involved in
the chain reaction between Bu3PBH2Ph, an alkyl iodide,
and ethyl acrylate according to eq 1.
The neophyl radical was formed from the correspond-
ing bromide and the borane in benzene by a thermally
initiated radical chain reaction. Relationship 3, where
kH is the rate constant for the hydrogen abstraction from
the primary alkyl radical, is valid provided that the
borane is the only source of hydrogen and that its
concentration does not change significantly during an
experiment.
[PhCMe3]
k
H[LBH3]
kr
(3)
Bu3PBH2Ph + RI + CH2dCHCOOEt f
RCH2CH2COOEt + Bu3PBHIPh (1)
)
[PhCH2CHMe2]
It also been shown that reactions of t-BuO‚ with ligated
boranes to generate the corresponding ligated boryl
radicals is very fast,2 although no quantitative data were
given. This favorable feature permits the use of ligated
boranes as donor polarity reversal catalyst for reactions
in which tert-butoxyl radicals are not able to abstract
electron-deficient hydrogen from carbon.5
Despite the interest in the properties of ligated boryl
radicals, only a limited number of rate constants for the
reactions used for their generation are reported in the
literature.2,5 Knowledge of the rate constants of radical
To check the validity of eq 3, a number of experiments
were carried out in which di-tert-butyl peroxide (0.03 M),
1-bromoadamantane (0.1 M), and the ligated borane (L
) Me3N, Et3N, Bu3P) at concentrations of 0.5-1.0 M were
photolyzed at 120 °C for 3 h in a sealed glass tube using
tert-butylbenzene free of oxygen as solvent. In the case
of Bu3PBH3 a chain reaction ensued,10 affording adaman-
tane in a yield of 91% based on bromoadamantane. With
Et3NBH3 and Me3NBH3 no significant amounts of reduc-
tion products were instead observed, indicating that these
reagents do not support chain reactions under these
conditions. Since the halogen abstraction is a fast
process11 it can be deduced that amine-boranes are very
poor hydrogen atom donors toward alkyl radicals.
On this basis, we could perform the radical-clock
experiments only for Bu3PBH3. By measuring the rela-
tive yields of tert-butylbenzene and isobutylbenzene at
(1) (a) Giese, B. Radicals in Organic Synthesis: Formation of
Carbon-Carbon Bonds; Pergamon Press: Oxford, 1986. (b) Curran,
D. P. Synthesis 1988, 417, 489.
(2) (a) Dang, H. S.; Diart, V.; Roberts, B. P.; Tocher, D. A. J . Chem.
Soc., Perkin Trans. 2 1994, 1039. (b) J ohnson, K. M.; Kirwan, J . N.;
Roberts, B. P. J . Chem. Soc., Perkin Trans. 2 1990, 1125. (c) Kaushal,
P.; Mok, P. L. H.; Roberts, B. P. J . Chem. Soc., Perkin Trans. 2 1990,
1663. (d) Kirwan, J . N.; Roberts, B. P. J . Chem. Soc., Chem. Comm.
1988, 480. (e) Baban, J . A.; Roberts, B. P. J . Chem. Soc., Perkin Trans.
2 1987, 497. (f) Marti, V. P. J .; Roberts, B. P. J . Chem. Soc., Perkin
Trans. 2 1986, 1613. (g) Baban, J . A.; Roberts, B. P. J . Chem. Soc.,
Perkin Trans. 2 1986, 1607. (h) Green, I. G.; Roberts, B. P. J . Chem.
Soc., Perkin Trans. 2 1986, 1597. (i) Baban, J . A.; Goddard, J . P.;
Roberts, B. P. J . Chem. Research (S), 1986, 30. (j) Baban, J . A.; Marti,
V. P. J .; Roberts, B. P. J . Chem. Soc., Perkin Trans. 2 1985, 1723. (k)
Baban, J . A.; Roberts, B. P. J . Chem. Soc., Perkin Trans. 2 1984, 1717.
(l) Giles J . R.; Roberts, B. P. J . Chem. Soc., Perkin Trans. 2 1983, 743.
(3) Russell has demonstrated that NaBH4 does not react with alkyl
radicals with a measurable rate constant. Russell, G. A.; Guo, D.
Tetrahedron Lett. 1984, 25, 5239.
(6) Newcomb, M. Tetrahedron 1993, 49, 1151 and references therein.
(7) The temperature-dependent function for neophyl rearrangment
is:8 log kr (s-1) ) (11.55 ( 0.32) - (11.82 ( 0.48)/θ, where θ ) 2.3RT
kcal mol-1
.
(8) Franz, J . A.; Barrows, R. D.; Camaioni, M. J . Am. Chem. Soc.
1984, 106, 3964.
(9) For an example see: (a) Lustzyk, J .; Maillard, B.; Ingold, K. U.
J . Org. Chem. 1986, 51, 2457. (b) Chatgilialoglu, C.; Ferreri, C.;
Lucarini, M. J . Org. Chem. 1993, 58, 249.
(10) Evidence for a free-radical chain mechanism was provided by
the observation that in the absence of initiator in the dark no reactions
was detectable under the same conditions.
(4) Baban, J . A.; Roberts, B. P. J . Chem. Soc., Perkin Trans. 2 1988,
1195.
(5) Paul, V. P; Roberts, B. P.; Willis, C. R. J . Chem. Soc., Perkin
Trans. 2 1989, 1953.
(11) Boryl radicals abstract the halogen atom rapidly from alkyl
bromides. By combining data reported by Roberts,2i the rate constant
for the bromine abstraction from ethylbromide by (i-Prop)2EtNB‚H2
at 255 K is calculated as 2 × 107 M-1 s-1
.
0022-3263/96/1961-1161$12.00/0 © 1996 American Chemical Society

1162 J . Org. Chem., Vol. 61, No. 3, 1996
Notes
Ta ble 2. Kin etic P a r a m eter s for th e Rea ction of
P r im a r y Alk yl Ra d ica ls w ith Bor on , Silicon , a n d Tin
Hyd r id es
Ta ble 1. Kin etic Da ta for th e Rea ction of Neop h yl
Ra d ica ls w ith Bu 3P BH3 a t Va r iou s Tem p er a tu r es
T/K
[Bu3PBH3]a/M
(kH/kr)/M-1
log A
Ea
kH (353 K)
313
333
353
373
0.61-1.35
0.41-1.05
0.41-1.07
0.61-1.35
0.336 ( 0.070
0.284 ( 0.058
0.177 ( 0.044
0.131 ( 0.022
hydride
(M-1 s-1
)
(kcal mol-1
)
(M-1 s-1
)
ref
Me3NBH3
Et3NBH3
Bu3PBH3
Et3SiH
(Me3Si)3SiH
Bu3SnH
<1 × 103
this work
this work
<1 × 103
8.46
8.66
8.86
9.07
8.03
7.98
4.47
3.69
3.0 × 103 this work
a
Range of concentration employed.
5.2 × 103 9b
1.2 × 106 16
6.1 × 106 17
various borane concentrations, under experimental con-
ditions in which borane is the only source of hydrogen12
and its concentration does not change significantly during
an experiment, we obtained the following Arrhenius
expression in the temperature range 313-373 K:
Ta ble 3. Absolu te Ra te Con sta n ts for th e Rea ction of
ter t-Bu toxyl Ra d ica ls w ith Sever a l Su bstr a tes a t 298 K
kH (298 K)
hydride
solvent
(M-1 s-1
)
ref
4.1 × 107
2.1 × 108
2.6 × 107
1.3 × 108
2.1 × 107
6.1 × 107
1.2 × 107
6.3 × 107
2.3 × 105
1.0 × 107
1.0 × 108
2.1 × 108
this work
this work
this work
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19
log(kH/kr) (M) ) (-3.09 ( 0.70) + (3.79 ( 1.09)/θ (4)
Me3NBH3
t-butylbenzene
pyridine
t-butylbenzene
pyridine
benzene
pyridine
benzene
pyridine
benzene
benzene
Et3NBH3
Bu3PBH3
Ph3PBH3
where θ ) 2.3RT kcal mol-1 and the errors correspond
to 95% confidence limit. The detailed results of the
individual experiments are reported in Table 1.
The absolute values of the rate constant for hydrogen
abstraction, kH, from phosphine-borane by the primary
alkyl radical can be obtained by combining eq 4 with the
Arrhenius equation for the neophyl rearrangement.7
These data are shown in Table 2 together with the
PhCH3
Ph3SiH
(Me3Si)3SiH
Bu3SnH
19
20
19
benzene
benzene
‚
analogous values for the reaction of PhC(Me2)CH2 with
ligated boranes (see Table 3).21 The reported data
confirm the high reactivity of ligated boranes toward tert-
butoxy radicals predicted by Roberts2 for the hydrogen
abstraction.
This high reactivity was attributed by Roberts and co-
workers4 to polar effects causing lower than expected
activation energies; actually, the alkoxy radical being
highly electrophilic and the hydrogens attached to the
boron atom relatively electron rich, polar structures like
1, may contribute significantly to the transition state.
some organometallic hydrides. The Arrhenius preexpo-
nential factors are all the same and lie in the expected
range.13 The order of reactivity is R3NBH3 < R3PBH3 =
Et3SiH < (Me3Si)3SiH < Bu3SnH, and this order follows
the differences in bond strength14 which are manifest in
the enthalpies of activation.
Rea ction of LBH3 w ith ter t-Bu toxyl Ra d ica ls. The
reactivities of ligated boranes toward t-BuO‚ radicals
have been obtained by GC analysis by measuring the
concentrations of the reagents at different times during
the photochemically initiated reaction between di-tert-
butyl peroxide and either the borane and Ph3SiH or the
borane and (Me3Si)3SiH (eqs 5 and 6).
[t-ButO-H•H2B-L+]
1
Me3COOCMe3
9
hν8 2Me3CO•
This hypothesis is strongly supported by the remark-
able increase of the rate constant for hydrogen abstrac-
tion observed by carrying out the reaction in a solvent
like pyridine,22 characterized by a larger dielectric con-
stant than benzene (see Table 3).
Au toxid a tion of Liga ted Bor a n es. With the aim of
estimating the rates of reaction of boranes with peroxyl
radicals, we have also studied the autoxidation of Me3-
NBH3, Et3NBH3, and Bu3PBH3 in the presence of a
radical initiator in air under controlled conditions.23 At
variance with trialkylboranes which undergo autoxida-
tion so easily that they inflame in air,24 ligated boranes
are much more stable. In fact, an attempted autoxidation
of Me3NBH3, Bu3PBH3, and Bu3PBH2Ph was tried previ-
ously,4 but no oxygen consumption was noted in the
conditions employed.
k5
9
Me3CO• + LBH3
Me3CO• + R3SiH
8 Me3COH + LB˙ H2
(5)
(6)
k6
9
8 Me3COH + R3Si•
Values of k5/k6 were calculated from the loss of starting
material by using the method of Ingold and Shaw.18 By
using k6 ) 1.1 × 107 for Ph3SiH19 and k6 ) 1.0 × 108 for
(Me3Si)3SiH20 we obtained average values of k5 at 298 K
for the reaction of tert-butoxyl radicals with the studied
(12) The value obtained in this way does not distinguish between
sites of interaction of borane with primary alkyl radicals.
(13) Benson, S. W. Thermochemical Kinetics, 2nd ed.; Wiley-Inter-
science: New York, 1976.
(14) Bond dissociation energies for the metal-hydrogen bond15 of
We have carried out the autoxidation by following the
oxygen uptake with a method based on the variation of
Bu3SnH, (Me3Si)3SiH, and Et3SiH are 308, 331, and 376 kJ mol-1
,
respectively. The approximate XH2B-H bond calculated dissociation
energies4 of H3NBH3 and H3PBH3 are 431 and 389 kJ mol-1
.
(15) Kanabus-Kaminska, J . M.; Hawari, J . A.; Griller, D.; Chatgil-
ialoglu, C. J . Am. Chem. Soc. 1987, 109, 5267.
(16) Chatgilialoglu, C.; Dickhaut, J .; Giese, B. J . Org. Chem. 1991,
56, 6399.
(21) The rate constant for hydrogen abstraction from Me3NBH2Thx5
(Thx ) Me2CHCMe2-) and Et2SBH32b have been previously estimated
to be 4.7 × 107 at 189 K and 1 × 106 M-1 s-1 at 165 K, respectively.
(22) Although, in principle, exchange with pyridine to give
a
(17) Chatgilialoglu, C.; Ingold, K. U.; Scaiano, J . C. J . Am. Chem.
Soc. 1981, 103, 7739.
(18) Ingold, C. K.; Shaw, F. R. J . Chem. Soc. 1927, 2918.
(19) Chatgilialoglu, C.; Ingold, K. U.; Lustzyk, J .; Nazran, A. S.;
Scaiano, J . C. Organometallics 1983, 2, 1332.
pyridine-borane could take place, the gaschromatographic analysis
of the reaction mixture did not show any signal with the same retention
time of an authentic sample of PyrBH3.
(23) Howard, J . A. In Free Radicals; Kochi, J . K., Ed.; Wiley
Interscience: New York, 1973; Vol. II, Chapter 12, pp 3-62.
(24) Davies, A. G. J . Organomet. Chem. 1980, 200, 87.
(20) Chatgilialoglu, C.; Rossini, S. Bull. Soc. Chim. Fr. 1988, 298.

Notes
J . Org. Chem., Vol. 61, No. 3, 1996 1163
an EPR spectral line of a nitroxide spin probe dissolved
in solution.25 For a given borane a likely mode of reaction
is that one represented by the following steps (eqs
7-10):
initiation
Ri
9
8 LB˙ H2
(7)
(8)
propagation
LB˙ H2 + O2 f LBH2OO•
kp
LBH2OO• + LBH3
termination
98 LBH2OOH + LB˙ H2 (9)
kt
2LBH2OO• 98 products
(10)
Here, Ri is the rate at which the initiating radicals are
produced and kp and kt represent the propagation and
termination rate constants, respectively. Quantitative
autoxidation studies have been performed by keeping Ri
constant by thermally decomposing the azo initiator
azoisobutyronitrile (AIBN).
Figure 1 reports the decrease with time of the oxygen
concentration observed during the autoxidation at 50 °C
of the three ligated boranes in benzene containing AIBN
and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as
spin probe.25
The ease of oxidation of the boranes has been estimated
using a parameter independent on the concentration of
the oxidizable substrate LBH3 and of the azo initi-
ator, that is the oxidizability, kp/(2kt)1/2 (eq 11),23 whose
values for the various ligated boranes are reported in
Table 4.26
F igu r e 1. Time dependence of [O2] during the AIBN (27.5
mM)-initiated autoxidation of (b) Bu3PBH3 (1 M), (1) Et3NBH3
(1 M), and (9) Me3NBH3 (1 M) at 50 °C in benzene containing
TEMPO (5 × 10-5 M). (O) Dependence of oxygen concentration
on time measured during the decomposition of AIBN (27.5
mM) in air-saturated benzene at 50 °C in the presence of
R-tocopherol (5 × 10-3 M) and TEMPO (5 × 10-5 M). Ri ) 1.13
× 10-7 M/s.
Ta ble 4. Oxid iza bility [kp /(2kt)1/2] Va lu es for Liga ted
Bor a n es Mea su r ed a t 50 °C in Ben zen e
substrate
oxidizability/M-1/2 s-1/2
Me3NBH3
Et3NBH3
Bu3PBH3
8.10 × 10-4
7.01 × 10-4
2.56 × 10-3
alkyl substituents a different behavior would have been
expected for the two amine-boranes.
kp
-d[O2]/dt - Ri
)
(11)
Con clu sion s
2k
x
[LBH ] R
x
t
3
i
Ligated boranes containing amine or phosphine ligands
give rise to a hydrogen transfer reaction slow with alkyl
radicals but very fast with alkoxy radicals. The rate of
reaction with alkyl radicals is larger for the phosphine-
borane than for the amine-boranes and, thus, is es-
sentially determined by the strength of the B-H bond.
Accordingly, Bu3PBH3 is able to reduce bromoalkanes in
a thermally or photochemically initiated radical chain
reaction, while under similar conditions, amine-boranes
do not afford reduction products.
On the other hand, hydrogen transfer from the ligated
boranes to tert-butoxyl radicals, in addition to being very
fast, is scarcely affected by the nature of the ligand. An
explanation of this behavior previously proposed in terms
of polar effects14 is strongly supported by the kinetic data
obtained in benzene and pyridine.
In principle, oxidizability values can not be related to
changes in the nature of the substrate, since the ratio
kp/(2kt)1/2 can be influenced by the reactivity of the
substrate, the reactivity of the peroxy radical, and the
absolute magnitude of the termination rate constant.
Nevertheless, in the present case it seems reasonable to
assume that the different boron peroxy radicals exhibit
similar reactivity in the propagation and termination
reaction so that differences in the overall rate of oxidation
are mainly due to differences in the reactivity of the
substrate, i.e., in the B-H bond strength. Indeed, the
greater reactivity is found experimentally with Bu3PBH3,
that is with the borane characterized by the lower bond
dissociation energy.14
It is also worth pointing out that the oxidizability is
almost the same in the two amine-boranes examined,
confirming that the site of attack of the peroxy radical
to the ligated borane is the B-H bond. If the hydrogen
abstraction would have occurred at a C-H bond of the
In the reaction of the boranes with peroxyl radicals the
actual rate constants could not be determined; however,
the oxidizability values indicate a behavior intermediate
between those for the reactions with alkyl and with
alkoxy radicals. This suggests that polar effects are less
important than in the reaction with alkoxyl radicals.
(25) (a) Cipollone, M.; Di Palma, C.; Pedulli, G. F. Appl. Magn.
Reson. 1992, 3, 99. (b) Pedulli G. F. In Free Radicals and Antioxidants
in Nutrition; Corongiu, F., Banni, S., Dessi, M. A., Rice-Evans, C., Eds.;
Richelieu Press: London, 1993; pp 211. (c) Pedulli, G. F.; Lucarini,
M.; Pedrielli, P.; Sagrini, M.; Cipollone, M. Res. Chem. Intermed. 1996,
22, 1-14.
(26) The rates of initiation, Ri, were measured by following the
oxygen consumption during the thermal decomposition of AIBN, in
the presence of R-tocopherol. Under these conditions oxygen is removed
from the solution at the constant rate Ri by the alkyl radicals generated
from the azo initiator, and the resulting peroxyl radicals, in turn, are
trapped by R-tocopherol.
Exp er im en ta l Section
Ma ter ia ls. Neophyl bromide27 and di-tert-butyl hyponitrite28
were prepared following the literature procedures. All other
materials were commercially available.
(27) Fainberg, A. H; Winstein, J . J . Am. Chem. Soc. 1956, 78, 2763.
(28) Mendenhall, G. D. Tetrahedron Lett. 1983, 24, 451.

1164 J . Org. Chem., Vol. 61, No. 3, 1996
Notes
Gen er a l P r oced u r e for Kin etic Mea su r em en ts of th e
Rea ction of P r im a r y Alk yl Ra d ica ls w ith Liga ted Bor a n es.
A solution of neophyl bromide, Bu3PBH3 (in a ratio of ca. 1:20),
a radical initiator, and an internal GC standard, in benzene,
was degassed and sealed under argon in Pyrex ampules. The
reaction mixture was thermolyzed in the temperature range
313-373 K. tert-Butyl hyponitrite or dibenzoyl peroxide was
used as the radical initiator depending on the reaction temper-
ature. The products of the reactions were analyzed by GC using
a 30 m × 0.53 mm HP-5 column with temperature programming
from 50 to 250 °C using a HP Series II chromatograph. The
products of interest were identified by comparison of their
retention times with authentic materials.
quartz ampoules. The reaction mixture was photolyzed at room
temperature for 15-30 min, and the disappearance of the
products was analyzed by GC.
Au toxid a tion of Liga ted Bor a n es. A solution of the borane
(0.5-1.0 M), 2,2′-azobis(2-methylpropionitrile) (0.0275 M), and
2,2,6,6-tetramethylpiperidine-1-oxyl (5 × 10-5 M) in benzene was
air saturated at room temperature and introduced (ca. 200 µL)
into a capillary tube with the internal diameter of ca. 1.85 mm.
A second capillary tube (external diameter of 1.60 mm) sealed
at one end was introduced into the sample tube so to leave very
little dead volume space. The oxygen uptake was followed by
an EPR method previously described.25
Gen er a l P r oced u r e for Kin etic Mea su r em en ts of th e
Ack n ow led gm en t. Financial support from MURST
and CNR is gratefully acknowledged. We also thank
Mrs. C. Vignoli for assistance in preparing some samples.
Rea ction of Bu toxyl Ra d ica ls w ith Liga ted Bor a n es.
A
solution of di-tert-butyl peroxide (0.2-0.4 M), the ligated borane
(ca. 1 × 10-2 M), the silane, and an internal GC standard, in
benzene or pyridine, was degassed and sealed under argon in
J O951707K