Effect of Lewis acids and low temperature initiators on the allyl transfer reaction involving phthalimido-N-oxyl radical

Article abstract of DOI:10.1016/j.tetlet.2014.10.129

Previously, we reported allyl transfer reactions of allyl bromide and allyl phthalimido-N-oxyl substrates with hydrocarbons that result in CC bond formation. In both cases, efficient chain transfer processes along with high reaction yields were observed. Since PINO chemistry leads to an environmentally friendly method of hydrocarbon functionalization, additional studies were performed in order to improve the process. To expand the utility of this reaction, we carried out experiments to optimize reaction conditions and tested the effect of Lewis acids and low temperature initiators. Although allyl-PINO substrates reacted slightly slower than the bromides, the reactions were cleaner with little or no side products. The chain lengths for these reactions were compromised at lower temperatures, attributable to the high activation energy required for the hydrogen atom abstraction by PINO. The addition of a Lewis acid catalyst (AlCl₃) improves the product yield and reaction rate, possibly due to the formation of a PINO/AlCl₃ complex which lowers the activation energy for hydrogen abstraction step.

Full text of DOI:10.1016/j.tetlet.2014.10.129

Tetrahedron Letters 55 (2014) 7029–7033
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Effect of Lewis acids and low temperature initiators on the allyl
transfer reaction involving phthalimido-N-oxyl radical
⇑
Shradha Patil, Liang Chen, James M. Tanko
Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Previously, we reported allyl transfer reactions of allyl bromide and allyl phthalimido-N-oxyl substrates
with hydrocarbons that result in CAC bond formation. In both cases, efficient chain transfer processes
along with high reaction yields were observed. Since PINOÅ chemistry leads to an environmentally
friendly method of hydrocarbon functionalization, additional studies were performed in order to improve
the process. To expand the utility of this reaction, we carried out experiments to optimize reaction con-
ditions and tested the effect of Lewis acids and low temperature initiators. Although allyl-PINO substrates
reacted slightly slower than the bromides, the reactions were cleaner with little or no side products. The
chain lengths for these reactions were compromised at lower temperatures, attributable to the high
activation energy required for the hydrogen atom abstraction by PINOÅ. The addition of a Lewis acid cat-
alyst (AlCl3) improves the product yield and reaction rate, possibly due to the formation of a PINOÅ/AlCl3
complex which lowers the activation energy for hydrogen abstraction step.
Received 19 September 2014
Revised 20 October 2014
Accepted 24 October 2014
Available online 30 October 2014
Keywords:
H atom abstraction
Lewis acid
Allyl transfer
Phthalimido-N-oxyl radical
Chain reaction
Published by Elsevier Ltd.
Introduction
undergoes subsequent b-cleavage to form the final product 3 and
regenerates XÅ. Both BrÅ and PINOÅ proved particularly effective as
A new free radical-based condensation reaction that achieves
hydrocarbon functionalization and CAC bond formation in a single
step was previously reported.1–3 Utilizing an appropriately substi-
tuted allyl bromide or allyl phthalimido-N-oxyl (allyl-PINO)
substrate as the key reactant, the net result of this transformation
is the transfer of an allyl group to the hydrocarbon: RH + CH2@
C(Z)CH2X ? RCH2C(Z)@CH2 + HX (X = Br, PINO). This allyl transfer
reaction was especially fascinating because unlike other methods
that effect this transformation, this functionalization was accom-
plished without the use of heavy metals such as tri-n-butyl tin
hydride, distannanes, or alkyl mercuric halides,4 which are often
needed in radical-based synthetic methods. Similarly, strong acidic
or basic reaction conditions which are needed in electrophilic or
nucleophilic methods are not required. This method demonstrates
that high chemical yields do not necessarily have to be compro-
mised with the adoption of green chemical technologies.5–7
The mechanism of this reaction is depicted in Scheme 1. The
chain carrier (XÅ) abstracts a hydrogen atom from hydrocarbon
R–H such as toluene (which has a relatively weak, benzylic, CAH
bond) generating an intermediate benzyl radical, RÅ. Addition of
RÅ to the alkene of allyl substrate 1 generates a radical 2, which
XÅ in this reaction because they abstract a hydrogen atom from
the benzylic position with high selectivity,8,9 and the intermediate
b-radicals readily undergo b-cleavage. Reaction yields and kinetic
chain lengths (i.e., the rate of product formation relative to the rate
of initiator disappearance) are both improved when the substitu-
ent Z was electron withdrawing, and the relative rates for radical
addition were found to be 180 (Z = CN) > 110 (Z = CO2R) > 65
(Z = Ph) >> 1 (Z = H) when X = Br.2 This reactivity order parallels
the relative rates of addition of PhCHÅ2 to substituted alkenes.10
More importantly, as a hydrogen atom abstractor, PINOÅ has
been shown to be even less reactive and more selective than the
bromine atom.11 Allyl-PINO compounds 8 and 9 (Fig. 1) are excel-
lent substrates for this chemistry (Scheme 2), offering several
+
X
R-H
H-X + R
R-CH2C(Z)-CH2X
R
+ CH2=C(Z)-CH2-X
Propagation
1
2
R-CH2C(Z)=CH2
+
2
X
3
R-CH2C(Z)=CH2 H-X
+
Overall Reaction
CH2=C(Z)-CH2-X
R-H
+
4
5
⇑
Corresponding author. Tel.: +1 540 231 6687; fax: +1 540 231 3255.
E-mail address: jtanko@vt.edu (J.M. Tanko).
Scheme 1. Propagation steps and overall allyl transfer reaction.
http://dx.doi.org/10.1016/j.tetlet.2014.10.129
0040-4039/Published by Elsevier Ltd.

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S. Patil et al. / Tetrahedron Letters 55 (2014) 7029–7033
CO2Et
Ph
lower than for the allyl bromides, the reduced chain lengths for the
allyl-PINO substrates do not have a deleterious effect on the prod-
uct yields or mass balance of the reaction. These results and others
(vide infra) lead to the suspicion that for X = PINO, the hydrogen
abstraction or b-fragmentation step may not be as efficient as is
the case for X = Br.
O
O
Ph
CO2Et
Br
O
O
Br
N
N
O
O
6
7
8
9
Figure 1. Substrates used for allyl transfer reaction.
As noted, one of advantages of using di-tert-butyl peroxide
(DTBPO) as an initiator was that tert-butoxyl radical does not read-
ily add to double bonds and is an excellent hydrogen atom abstrac-
tor. On the other hand, the drawback is the high temperature
(120 °C) required for initiation which does not allow exploring
regio- and stereoselectivity. To address this issue, initiators which
work effectively at low temperatures such as triethylborane and
di-tert-butylhyponitrite (DTBHN) were considered for allyl transfer
reactions at low temperatures.
R1
R2
O
DTBPO
R2
Z
Z
20 mol%
R1
Ph
+
PINOH (↓)
O
N
+
Sealed tube
120 oC
10a-10f
Up to 90% yield
O
8 Z=Ph
9
(neat, 40 eq.
based on
8 or 9
)
Z=CO2Et
Scheme 2. DTBPO (Di-tert-butyl peroxide) initiated reactions of allyl-PINO sub-
strates (8,9) with benzylic hydrocarbons (For Z = Ph, 10a R1 = R2 = H; 10b- R1 = H,
Low temperature reactions using triethylborane/O2 (TEB)
R
2 = CH3, 10c R1 = R2 = CH3, For Z = CO2Et, 10d- R1 = R2 = H; 10e- R1 = H, R2 = CH3,
10f R1 = R2 = CH3).
The primary goal behind performing the allyl transfer reaction
at low temperature was to investigate the regio- and stereoselec-
tivity of the radical addition step. As observed in various cases, ste-
reoselectivity depends directly on the temperature of the reaction
and is generally found to be enhanced at lower temperatures.13–16
In case of allyl transfer processes, low reaction temperatures might
provide an opportunity to achieve an enantioselective radical addi-
tion if a prochiral radical or chiral auxiliary on the electrophile is
used,17,18 but the biggest challenge for the radical processes at
low temperatures is finding an initiator which works efficiently
at those temperatures. There are a few reported initiators like tri-
ethylborane/O219 and dimethylzinc/O220 which have been success-
fully used at very low temperatures. Triethylborane (a precursor of
EtÅ and/or EtOOÅ) was selected for allyl transfer reactions at low
temperature owing to its established use in free radical reactions
at low temperatures.19,21–25
advantages over the corresponding allyl bromides.3 This paper dis-
cusses our efforts to lower the temperature of this reaction and the
results provide insight into the issues related to the propagation
step/s (e.g., a hydrogen abstraction by PINOÅ etc.) in the allyl trans-
fer process.
Comparison of allyl bromides and allyl-PINO substrates
To confirm whether the replacement of PINOÅ with BrÅ leads to
cleaner and more efficient reactions, reactions of allyl bromide
and allyl-PINO substrate with hydrocarbons were studied. Results
are summarized in Table 1. For each of these experiments, the reac-
tion conditions were identical with regard to time, temperature,
etc. High mass balances were observed for the reactions of 7 and
9 with hydrocarbons. Overall, the reactions utilizing the allyl-PINO
substrates were considerably cleaner than the analogous reactions
with allyl bromides. Although allyl bromides tended to react faster
under comparable conditions, the mass balances were lower and
undesirable side-products were formed.
In this section, we discuss the reactions of allyl-PINO substrates
with hydrocarbons using triethylborane/O2 as an initiator. Allyl-
PINO substrates 8 and 9 and allyl-bromide substrate 6 were
allowed to react with neat toluene, ethyl benzene, and cumene
in the presence of triethylborane.
To probe this further, kinetic chain lengths (i.e., the rate of prod-
uct formation relative to the rate of initiator disappearance,
ꢀ(o[product]/ot)/(2o[DTBPO]/ot))12 were determined by following
product yields as a function of time for Z = CO2Et. Although the ini-
tial chain lengths for the allyl-PINO compounds were consistently
The low temperature reactions were first performed with allyl-
bromide substrate 6 and hydrocarbons at varying concentrations of
initiator and reaction temperature (Table 2). The reaction of tolu-
ene with 6 (entry 1) using 20 mol % of initiator at 0 °C led to no for-
mation of product 10b. Increasing the temperature of this reaction
to room temperature led to a negligible 1% product formation
(entry 2). Similarly, gradually increasing the concentration of initi-
ator and reaction temperature (entries 2–4) led to
a slight
Table 1
Comparison: Reactions of allyl-PINO and allyl bromide substrates with hydrocarbons
improvement in product yield (up to 10%). Finally, increasing the
reaction temperature to 80 °C led to a respectable yield of product
(entry 5). Use of ethyl benzene and cumene (entries 6 and 7)
showed similar results, which led to the conclusion that a respect-
able product yield could only occur at high temperatures (80 °C).
The data in Table 3 show results of reactions of the allyl-PINO
substrates 8 and 9 with hydrocarbons. However, even in this case,
no product formation was observed at ꢀ78 °C, 0 °C or room tem-
perature. Similar to the results in Table 2, a significant increase
in product yields was observed only at high temperatures.
R1
R2
R2 CO2Et
CO2Et
DTBPO
120 oC Ph
R1
+
+
H-X
X
X= Br 7
X=PINO
10d-10f
9
Entry R1
R2
X = Br
X = PINO
%
%
Mass
%
%
Mass
7
10d–
f
balance (%)
9
10d–
f
balance (%)
Since Et3B/O2 generates an EtÅ and EtOOÅ,25,26 it was conceivable
that the mediocre results obtained with this initiator were because
of the possibilities; (1) either EtÅ and/or EtOOÅ does not abstract a
hydrogen (or add to the double bond) at the rate sufficient enough
to efficiently initiate the reaction, (2) hydrogen atom abstraction
by PINOÅ is very slow at low temperature, or (3) the radical addition
step is slow at low temperature. Based on the hypothesis that a
Lewis acid could be used to activate the substrate (especially 9)
1a
2b
3b
H
H
H
CH3
0
0
35
70
35
70
77
21 48
44 56
75 20
69
100
95
CH3 CH3 41 36
Reactions performed at 120 °C using 20 mol % DTBPO (di-tert-butyl peroxide) in
neat hydrocarbons. %Mass balance = %9 or 7 + %11.
a
Reaction time 42 h.
Reaction time 3 h (Hydrocarbon: 6.0 M,
b
7 or 9: 0.15 M, DTBPO 0.03 M),
(Z = CO2Et, 10d R1 = R2 = H, 10e R1 = H, R2 = CH3, 10f R1 = R2 = CH3).

S. Patil et al. / Tetrahedron Letters 55 (2014) 7029–7033
7031
Table 2
Reactions of 6 with hydrocarbons at low temperature using Et3B/O2
TEB/O2 (20 mol%)
AlCl3 (5 mol %)
Toluene (40 eq.)
CO2Et
OPIN
Ph
Ph
TEB 20-50 mol%
0- 80 oC
No reaction
R-H
+
-78 oC or rt
12-48 h
+
HBr
R
Br
9
6
10b
Scheme 3. Lewis acid catalyzed hydrocarbon functionalization using TEB/O2.
Entry R–H
Initiator (TEB/O2)
mol %
Temp
(°C)
Time
(h)
GC yield
(%)
1
2
3
4
5
6
7
PhCH3
20–50
20
50
50
50
0
24
24
24
12
12
12
12
N/o
PhCH3
PhCH3
PhCH3
PhCH3
PhCH2CH3
PhCH(CH3)2 50
Rt
Rt
50
80
80
80
1 ( 0)
2 ( 0)
10 ( 1)
30 ( 1)
40 ( 1)
35 ( 3)
50
Hydrocarbon 8.0 M, 6 0.2 M, Et3B(TEB) (0.5 equiv or 50 mol %) 0.07 M.
and accelerate the radical addition step, we decided to explore the
possible Lewis acid catalyzed allyl transfer reaction and address
the issue related to possibility (3) mentioned above.
Low temperature reactions using triethylborane and a Lewis
acid
Figure 2. Triethylborane/O2 initiated reaction of cumene and 9 with Lewis acid
(AlCl3) compared to the reaction without Lewis acid (9 0.15 M, TEB 0.07 M, cumene
6.0 M, AlCl3 0.007 M).
Lewis acids have been reported to activate various functional-
ities and accelerate the rate of the reaction. In 1999, Sibi et al.
developed a highly stereoselective method for the conjugate addi-
balances and high yields were observed in all reactions at 120 °C,
but it was noteworthy that reactions in the presence of a Lewis
acid at high temperature were much cleaner than reactions with-
out the Lewis acid under similar conditions.
tion of carbon radicals to chiral a,b-unsaturated N-enoyloxazolidi-
nones using tributyltin hydride (Bu3SnH) as a chain carrier and
triethylborane/O2 (Et3B/O2) as radical initiator.27 In the presence
of a Lewis acid (Yb(OTf)3) optimized results for both chemical yield
and diastereoselectivity were observed. It was confirmed that
Lewis acid (Yb(OTf)3) plays an important role in controlling the
conformation by chelation to both carbonyl groups and greatly
activate the substrate reactivity at low temperature (ꢀ78 °C).27 In
this case, the Lewis acid also makes the alkene more electrophilic
and accelerates the addition of the nucleophilic alkyl radical to
an alkene.
Based on the literature precedence, commercially available
Lewis acid aluminum chloride (AlCl3) was selected for this acceler-
ation of the radical addition step. A series of experiments were per-
formed using AlCl3 at low temperatures (ꢀ78 °C, 0 °C, and room
temperature) but, no improvement in yields was observed
(Scheme 3). Once again the allyl transfer reaction was observed
to work effectively only at high temperatures (120 °C). High mass
To investigate the exact role of the Lewis acid in the allyl trans-
fer process at high temperature, reactions were performed with
and without AlCl3 at 120 °C. If the Lewis acid is participating in
the process and facilitating the radical addition by activating 9,
then a dramatic difference may be observed in product yields.
Finally, significant improvement in product yields was observed
for reactions with Lewis acid (Figs. 2 and 3). This data suggested
that the Lewis acid accelerates the rate of the reaction by activating
9 at high temperature only. The reactions were cleaner, with neg-
ligible side products, and higher chain lengths when Lewis acid
was used. The product yield was calculated using GC and plot of
% product yield as a function of time for the reaction of 9 with
cumene (Fig. 2) and 9 with toluene (Fig. 3) was used to observe
the role of AlCl3.
Although these reactions showed rapid product formation in
the presence of a Lewis acid, they still might be good chain reac-
tions even in the absence of a Lewis acid. Especially from Figure 3,
Table 3
Reactions of 6 and 8 with hydrocarbons at low temperature using Et3B/O2
Ph
Z
TEB 50 mol%
R-H
+
+
PINOH
X
R
0- 80 o
C
10a-10f
Time (h)
6
8
X=Br, X=PINO
Entry
Substrate
R–H
Temp °C
GC yield (%)
1
2
3
4
5
6
7
8
9
8
8
8
8
8
8
6
6
6
PhCH3
PhCH3
PhCH3
PhCH3
PhCH(CH3)2
PhCH2CH3
PhCH3
0
12
24
12/24
12
12
12
12
12
12
N/o
5 ( 1)
Rt
50
80
80
80
80
80
80
35 ( 1)
40 ( 2)
45 ( 1)
42 ( 1)
36 ( 2)
40 ( 1)
36 ( 1)
PhCH(CH3)2
PhCH2CH3
Figure 3. Triethylborane/O2 initiated reaction of toluene and 9 with Lewis acid
(AlCl3) compared to the reaction without Lewis acid. (9 0.15 M, TEB 0.07 M, toluene
6.0 M, AlCl3 0.007 M).
(0.5 equiv or 50 mol % of initiator TEB was used for all the reactions, hydrocarbon
8.0 M, 6 or 8 0.2 M, TEB 0.07 M.)

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S. Patil et al. / Tetrahedron Letters 55 (2014) 7029–7033
LA
O
O
O
N
N
O
Vs.
N
O
N
O
14
O
O
Figure 4. Structure of di-tert-butylhyponitrite (DTBHN).
15
16
Figure 5. Proposed complexation of a Lewis acid with PINOÅ.
Table 4
Reactions of 7 and 9 (CH2@C(CO2Et)CH2Br) with hydrocarbons (neat) initiated by
DTBHN
CO2Et
CO2Et
DTBHN 20 mol%
which is known to abstract hydrogen readily at room temperature
(k = 1.9 ꢁ 105 Mꢀ1 sꢀ1) for hydrogen abstraction from toluene at
25 °C; Ea = 3.5 kcal/mol),31 so there is little doubt that hydrogen
abstraction by the initiating radical is efficient. Consequently, at
low temperatures, there must be a problem with one of the prop-
agation steps, presumably hydrogen abstraction by PINOÅ or b-
elimination. To investigate this, reported rate constants for a
hydrogen atom abstraction by BrÅ and PINOÅ were compared.32
+
X-H
X
R
+
R-H
25 oC-65 oC
Sealed tube
(neat)
9 Z=PINO
Z=Br
10d-10f
7
Entry
R–H
PhCH3
Substrate
Temp (°C)
Time (h)
% Product yield
1
2
3
4
5
6
9
9
9
9
7
7
25
25
25
65
25
65
96
96
168
12
96
12
31 ( 1)
36 ( 2)
23 ( 3)
65 ( 1)
48 ( 3)
69 ( 3)
PhCH2CH3
PhCH(CH3)2
PhCH2CH3
PhCH3
kbr= 8 X 105 M-1s-1
PhCH
PhCH
2+ HBr
Br
+
3
ð1Þ
PhCH3
kpn=0.620 M-1s-1
PhCH
2 + NHPI
PINO + PhCH3
(7, 9 0.15 M, DTBHN 0.03 M, R–H 6.0 M.)
Reported rate constants for hydrogen abstraction by PINOÅ and BrÅ
from toluene at room temperature is given in Eq. (1).
From Eq. (1), it is clear that hydrogen atom abstraction by BrÅ is
faster than PINOÅ which presumably explains the lower chain
lengths with allyl-PINO substrates. From the results shown in
Table 4 and the respectable product formation at low temperature,
it is clear that radical addition and b-fragmentation steps work effi-
ciently at low temperature. Whereas the hydrogen abstraction
step, which is critical in order to achieve good chain lengths, may
be slow at low temperature. Finally, using these results, it was con-
cluded that the hydrogen abstraction is the rate limiting step. This
data and results from Figures 2 and 3 also led to a speculation that
the Lewis acid probably forms a complex 16 with PINOÅ which
forces the lone pair on nitrogen to tie up even better in the reso-
nance. This effect stabilizes the spin density on oxygen making
PINOÅ more reactive toward hydrogen atom abstraction (Fig. 5).
Table 5
Kinetic chain length data with 20 mol % of 14 at 45 °C
R2
R1
O
14
9
+
+ HO N
45 oC
R1 R2
COOEt
O
Entry
R–H
Kinetic chain length
1a
2a
R1 = R2 = H
R1 = H, R2 = CH3
1.4 ( 1)
1.9 ( 1)
a
Kinetic chain length measurement at 45 °C, (9 0.3 M, 14 0.06 M, hydrocarbon
9.0 M).
the effect of Lewis acid can be quantitated approximately. From
slopes, it can be postulated that the formation of the product as
a function of time is accelerated twice as much with the Lewis acid
than the product formation without the Lewis acid (Fig. 4).
As mentioned earlier, one of the reasons for poor yields at low
temperature may be inefficient initiation by TEB/O2 initiator (EtOOÅ
or EtÅ). To investigate this further, we employed a new initiator di-
tert-butylhyponitrite28,29 (14), which was synthesized using a liter-
ature procedure. Fortunately, through the use of di-tert-butylhyp-
onitrite (14), it was possible to generate tert-butoxyl radical at
room temperature30 and allyl transfer reactions were performed
using 14 at low temperature (Table 4).
The product yields for the reaction of 9 with the alkyl aromatic
hydrocarbons at room temperature were poor. At longer reaction
time (168 h), further drop in the product yields was observed
(entry 3). A respectable yield was only obtained when the reaction
temperature was elevated to 65 °C (entries 4 and 6). To address
this further, the kinetic chain lengths for the reaction of 9 with tol-
uene and ethyl benzene, initiated by 14 at 45 °C, were measured.
This temperature was chosen for the chain length calculation
because of the known decomposition rate constant for DTBHN
(14) at that temperature.
Conclusions
In summary, attempts to reduce the temperature of the reac-
tion, while unsuccessful, provided valuable insight into this chain
reaction. Use of BEt3/O2 as initiator only gave acceptable results
at elevated temperatures, and it is unlikely that it is the nature
of the initiating radical that is causing the problem. This was con-
firmed with the use of BuON@NOtBu (14) as initiator, which just
t
like di-tert-butyl peroxide (DTBPO), generates tert-butoxyl but at
much lower temperatures. Because tert-butoxyl radical readily
abstracts hydrogen from alkylaromatics at lower temperature,
benzyl radicals are being produced that can enter the propagation
steps of this reaction. Lewis acid data confirmed that allyl transfer
process is chain reaction with or without Lewis acid as long as
reactions are performed at high temperature (120 °C). Although
at low temperatures, the hydrogen abstraction step is likely to con-
trol the rate of the reaction, the triethylborane/O2 data suggests
that the kinetics of this reaction could be improved using Lewis
acid to activate the hydrogen abstraction process.
Acknowledgment
In both cases, the chain lengths were slightly greater than unity:
(Table 5). For all practical purposes, these results show that at
lower temperatures, this is not an efficient free radical chain pro-
cess. Di-tert-butylhyponitrite (14) generates tert-butoxyl radical
Acknowledgement is made to the donors of the American
Chemical Society Petroleum Research Fund – United States for sup-
port of this research.

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