Evidence for Single-Electron Transfer in the Reactions of Unhindered Saturated Primary Alkyl Iodides with Lithium Aluminum Hydride. Reactions of 1-Halooctanes with Lithium Aluminum Deuteride

Article abstract of DOI:10.1021/jo961871n

Evidence for single-electron transfer (SET) in the reactions of lithium aluminum hydride (LAH) with unhindered, saturated primary alkyl halides has not been available. In this study, experiments have been carried out that provide convincing evidence that the unhindered, saturated primary alkyl iodide 1-iodooctane reacts with lithium aluminum deuteride (LAD) in a 1:0.2 ratio to form 1-deuteriooctane with up to 38% incorporation of protium. Since the protium incorporation increased as the reaction proceeded, studies involving the reaction of 1-iodooctane with AlD₃ (one of the byproducts in the reaction of LAD with alkyl halides) were carried out in order to determine the effect of AlD₃. With AlD₃, protium incorporations as high as 84% were observed, indicating that the reaction of AlD₃ with 1-iodooctane proceeds via a pathway that involves at least 84% radical formation. The study also included the effect of reaction vessel surface, concentration, and leaving group on the course of the reaction. Factors previously suggested by Newcomb to explain the results of the reactions of LiAlH₄ with cyclizable radical probes, e.g., a halogen atom radical-chain process, can be excluded as a valid argument in the present study.

Full text of DOI:10.1021/jo961871n

J . Org. Chem. 1997, 62, 4829-4833
4829
Evid en ce for Sin gle-Electr on Tr a n sfer in th e Rea ction s of
Un h in d er ed Sa tu r a ted P r im a r y Alk yl Iod id es w ith Lith iu m
Alu m in u m Hyd r id e. Rea ction s of 1-Ha loocta n es w ith Lith iu m
Alu m in u m Deu ter id e
Catherine O. Welder and E. C. Ashby*
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332
Received October 3, 1996X
Evidence for single-electron transfer (SET) in the reactions of lithium aluminum hydride (LAH)
with unhindered, saturated primary alkyl halides has not been available. In this study, experiments
have been carried out that provide convincing evidence that the unhindered, saturated primary
alkyl iodide 1-iodooctane reacts with lithium aluminum deuteride (LAD) in a 1:0.2 ratio to form
1-deuteriooctane with up to 38% incorporation of protium. Since the protium incorporation increased
as the reaction proceeded, studies involving the reaction of 1-iodooctane with AlD3 (one of the
byproducts in the reaction of LAD with alkyl halides) were carried out in order to determine the
effect of AlD3. With AlD3, protium incorporations as high as 84% were observed, indicating that
the reaction of AlD3 with 1-iodooctane proceeds via a pathway that involves at least 84% radical
formation. The study also included the effect of reaction vessel surface, concentration, and leaving
group on the course of the reaction. Factors previously suggested by Newcomb to explain the results
of the reactions of LiAlH4 with cyclizable radical probes, e.g., a halogen atom radical-chain process,
can be excluded as a valid argument in the present study.
Sch em e 1
In tr od u ction
In the past, we have found evidence for extensive
cyclization in the reaction of the hindered alkyl iodide
radical probe, 6-iodo-5,5-dimethyl-1-hexene (1), with Li-
AlH4 (eq 1) and have proposed that this reaction proceeds
by a single-electron transfer (SET) process involving
radical intermediates (Scheme 1).1 Newcomb has re-
ported in several papers his objections to this proposed
mechanism and instead has contended that the reactions
we have reported as SET are actually SN2 in nature.2
We have also previously reported the results of experi-
ments involving the unhindered, unsubstituted cyclizable
radical probe, 6-iodo-1-hexene (8), with LAH (eq 2).1c In
that study, no evidence of a radical intermediate was
found. Therefore, in 1987 when Newcomb reported that
he also had found no evidence of a radical intermediate
in the reaction of an unhindered primary alkyl halide
with LAH,2d we were not surprised (eq 3).
We modified Newcomb’s probe 9 to probe 10 in order
to remove the undesirable R,â-unsaturated nitrile moiety,
which is reactive toward LAH, but incorporated New-
comb’s idea3 that a substituted radical probe should
(1) (a) Ashby, E. C.; Pham, T. N.; Amrollah-Madjdabadi, A. J . Org.
Chem. 1991, 56, 1596. (b) Ashby, E. C. Acc. Chem. Res. 1988, 21, 414.
(c) Ashby, E. C.; DePriest, R. N.; Goel, A. B.; Wenderoth, B.; Pham, T.
N. J . Org. Chem. 1984, 49, 3545. (d) Tolbert, L. M.; Sun, X. J .; Ashby,
E. C. J . Am. Chem. Soc. 1995, 117, 2681.
(2) (a) Newcomb, M.; Curran, D. P. Acc. Chem. Res. 1988, 21, 206.
(b) Newcomb, M.; Kaplan, J .; Curran, D. P. Tetrahedron Lett. 1988,
29, 3451. (c) Newcomb, M.; Sanchez, R. M.; Kaplan, J . J . Am. Chem.
Soc. 1987, 109, 1195. (d) Park, S. U.; Chung, S. K.; Newcomb, M. J .
Org. Chem. 1987, 52, 3275.
X Abstract published in Advance ACS Abstracts, J une 15, 1997.
S0022-3263(96)01871-3 CCC: $14.00 © 1997 American Chemical Society

4830 J . Org. Chem., Vol. 62, No. 14, 1997
Welder and Ashby
Ta ble 1. Effect of Stoich iom etr y in th e Rea ction of
1-Iod oocta n e (14) w ith LAD in THF a
cyclize faster than the corresponding unsubstituted
compound. We also modified the halide from bromide
to iodide because we have always observed more evidence
of radical intermediates using alkyl iodide probes.1 Upon
reaction with LAH, alkyl iodide 10 was found to yield
cyclized product 11 in up to 18% yield (eq 4),4 but
products
expt
1
RI:LAD
1:5
time, h
0.5
% 14
% 15 (% D)
MB
100
0
100 (99)
2
3
1:1
0.25
0
100 (99)
100
1:0.2
0.25
1
2.5
8
77
72
66
64
63
20 (96)
21 (93)
25 (87)
27 (86)
29 (85)
97
93
91
91
92
24
a
All reactions were carried out at a concentration of 0.070 M
with respect to 14 in an Ar atmosphere glovebox in the absence of
light at room temperature in used Pyrex flasks. Teflon-coated stir
bars were employed.
Newcomb argues5 that the cyclized product may arise
from an equilibrium of tertiary radical 13 with primary
radical 12, which he suggested was initially formed from
an unknown impurity, through an iodine atom exchange
(Scheme 2).
was observed in the reaction of 1-iodooctane (14) with
LAD in THF was very surprising to us.
Th e Effect of Stoich iom etr y on th e Rea ction of
1-Iod oocta n e (14) w ith LAD. The data in Table 1 show
the effect of variation of stoichiometry on the reaction of
1-iodooctane (14), an unhindered primary alkyl iodide,
with LAD. As in the case of the hindered primary probes
discussed previously,6 lower stoichiometries of LAD
should lead to lower deuterium incorporation, if the
resulting hydrocarbon were formed via a radical inter-
mediate.
Sch em e 2
In both the 1:5 RI:LAD reaction (experiment 1) and
the 1:1 reaction (experiment 2), complete deuterium
incorporation was observed in the product n-octane (99%
D), which was formed in 100% yield, in both experiments.
When the stoichiometry was further reduced to 1:0.2, so
that less than 1 equiv of LAD was employed in the
reaction, siginificant hydrogen atom abstraction was
observed only after 20% reaction (i.e., after reaction of
one deuteride from the 0.2 equiv of LiAlD4).
In experiment 3, after 0.25 h, 20% n-octane (15)
containing 96% D was formed. After 24 h, 29% n-octane
was formed containing 85% D. Using eq 6, the percent
deuterium in the 9% octane formed from the 0.25 h
aliquot to the 24 h aliquot can be calculated. As shown,
In the present study, we have eliminated the pathway
involving radical formation via a halogen atom radical-
chain process (Scheme 2) by removing the double bond
in the radical probe and carrying out studies with the
unhindered, saturated, unsubstituted primary alkyl io-
dide, 1-iodooctane (14). We have dealt in detail with
Newcomb’s arguments in our previous paper6 in regard
to hindered primary alkyl halides and herein extend the
scope of the SET interpretation from hindered to unhin-
dered primary alkyl iodides. We have studied the fol-
lowing effects in the reaction of 1-halooctanes with
LAD: (1) stoichiometry, (2) involvement of byproduct
AlD3, (3) influence of the reaction vessel, (4) concentra-
tion, and (5) leaving group.
Resu lts a n d Discu ssion
the average deuterium content in the 9% octane formed
after 0.25 h was 61% D. It seems reasonable to conclude
that no evidence of SET was observed with LAD; how-
ever, the byproduct (AlD3) reacted with 1-iodooctane (14)
in a fashion to produce a radical intermediate.
Reduction of 1-iodooctane (14) with LAD to form
substantial amounts of protio product 15 cannot be
explained by a SN2 process or the presence of impurities
followed by a halogen atom radical-chain process (eq 5).
Removal of the hindrance of the dimethyl groups
present in the 1-halo-2,2-dimethylhexanes studied ear-
lier6 resulted in an unhindered system (1-halooctanes)
that was no longer SN2 impeded. Thus, the balance
between SET and SN2 that was present in the neopentyl
type probes should be shifted to favor SN2 in the
unhindered probes. When 0.2 equiv of LAD was em-
ployed, after 20% reaction, AlD3 is the major hydride
species in solution. It is apparently the AlD3 that is
responsible for the decrease in the deuterium content of
the product as the reaction proceeds beyond 20% reaction.
Effect of Alu m in u m Deu ter id e on th e Rea ction
of 1-Iod oocta n e (14) w ith LAD. Since evidence for a
radical intermediate in the reaction of 14 with LAD was
observed only when less than 1 equiv of LAD was
present, a study of the reaction of AlD3 (the expected
In the present study, we do indeed find the formation of
protio n-octane in up to 38% yield in the reaction of 14
with LiAlD4. The extent to which protium incorporation
(3) Park, S. U.; Chung, S. K.; Newcomb, M. J . Am. Chem. Soc. 1986,
108, 240.
(4) Ashby, E. C.; Pham, T.; Madjdabadi, A. A. J . Org. Chem. 1988,
53, 6156.
(5) Newcomb, M.; Varick, T. R.; Choi, S. Y. J . Org. Chem. 1992, 57,
373 and references cited therein.
(6) Ashby, E. C.; Welder, C. O. J . Org. Chem. 1997, 62, 3542.

Reactions of 1-Halooctanes with LAD
J . Org. Chem., Vol. 62, No. 14, 1997 4831
Ta ble 2. Effect of Alu m in u m Deu ter id e in th e Rea ction
of 1-Iod oocta n e (14) w ith LAD in THF a
Ta ble 3. Effect of Rea ction Vessel Su r fa ce in th e
Rea ction of 1-Iod oocta n e (14) w ith LAD in THF a
products
products
expt
1
nuc
time, h
% 14
% 15 (% D)
MB
expt
1
reaction vessel
used Pyrex
time, h
% 14
% 15 (% D)
MB
LAD
0.25
1
2.5
8
77
72
66
64
63
20 (96)
21 (93)
25 (87)
27 (86)
29 (85)
97
93
91
91
92
0.25
1
2.5
8
77
72
66
64
63
20 (96)
21 (93)
25 (87)
27 (86)
29 (85)
97
93
91
91
92
24
24
2
AlD3
AlD3
0.08
0.17
0.25
0.5
1
20
93
90
93
89
94
82
1.6 (93)
2.1 (83)
2.5 (72)
3.1 (56)
4.9 (37)
18 (16)
95
92
96
92
99
2
Teflon
quartz
0.25
1
5
8
24
70
74
67
60
64
21 (94)
24 (92)
28 (80)
31 (74)
30 (75)
91
98
95
91
94
100
3
0.25
1
5
8
24
78
75
68
65
63
18 (93)
20 (87)
27 (67)
29 (63)
31 (62)
96
95
95
94
94
3b
0.25
1
2.5
5
95
94
96
83
81
1.3 (83)
2.2 (76)
2.8 (51)
8 (35)
96
96
99
91
93
24
12 (27)
a
All reactions were carried out at a concentration of 0.070 M
with respect to 14 with a RI:LAD ratio of 1:0.2. Also, all reactions
were carried out in an Ar atmosphere glovebox in the absence of
light at room temperature in the absence of Teflon-coated stir bars.
a
All reactions were carried out at a concentration of 0.070 M
with respect to 14 with a RI:Nuc ratio of 1:0.2. Also, all reactions
were carried out in an Ar atmosphere glovebox in the absence of
light at room temperature in used Pyrex flasks, and Teflon-coated
b
stir bars were employed. The AlD3 used in experiment 3 was
made with excess sulfuric acid, ensuring that no LAD was present
in the reaction of 14 with AlD3.
The deuterium incorporation in the n-octane was
similar for experiments 2 and 3 (Table 2). When an
excess of H2SO4 was used to prepare the AlD3 solution
(experiment 3, Table 2), the resulting octane still had
high deuterium incorporation in the first aliquot (experi-
ment 3, Table 2, 0.25 h), although the deuterium content
(83%) was more than in experiment 2 (Table 2, 0.25 h)
(72%). However, the trend of decreasing deuterium
incorporation over the course of the reaction resembled
that of experiment 2 (Table 2). Therefore, the initially
high deuterium incorporation in the octane observed in
the early aliquots of the AlD3 experiment was also
observed when LAD was rigorously excluded from the
reaction. The rapid decrease in deuterium incorporation
as the reaction proceeds (experiments 2 and 3, Table 2)
is due to the fact that the concentration of AlD3 decreases
as the reaction proceeds, and therefore, the generated
radicals have a better chance of abstracting a hydrogen
atom from THF than abstracting a deuterium atom from
AlD3.
byproduct of the reaction of LAD with 14) with 14 at a
1:0.2 14:AlD3 ratio was carried out. As shown in experi-
ment 2, Table 2, even the earliest aliquot, in which only
1.6% octane (containing 93% D) was formed, showed
evidence of protium incorporation and hence a radical
intermediate. The amount of hydrogen atom abstraction
increased rapidly as the reaction proceeded. After 20 h,
18% n-octane was formed that contained only 16% D.
Therefore, the average deuterium content in the octane
formed between 0.08 and 20 h was 8.5% D as calculated
from eq 7. It appears that the reaction is proceeding
exclusively by a radical intermediate that can only be
explained by a SET process. The initiation of the reaction
by impurities followed by a halogen atom radical-chain
process cannot explain these results.
Th e Effect of Rea ctor Su r fa ces on th e Rea ction
of 1-Iod oocta n e (14) w ith LAD. Once an appropriate
stoichiometry for observing SET in the unhindered
primary alkyl iodide system had been determined (RI:
LAD ) 1:0.2), reactions were carried out in used Pyrex,
Teflon, and quartz, in order to study the effect of the
AlD3 solutions were prepared by the addition of 100%
H2SO4 to a homogeneous solution of LAD.7 If any LAD
remained in the AlD3 solution, it would be expected to
react quickly to yield n-octane (15) with a very high
deuterium content on the basis of earlier results (Table
1). For this reason, a reaction was carried out to ensure
that the initially high deuterium incorporation noted in
the AlD3 experiment (Table 2, experiment 2) was not due
to a small amount of LAD that remained in the AlD3
solution. Any remaining LAD could have reacted with
1-iodooctane (14) to give completely deuterated product
followed by SET reduction of 14 by AlD3 with significant
hydrogen atom abstraction. Therefore, an AlD3 solution
was prepared by adding a 10% excess of 100% sulfuric
acid to a LAD solution, thus ensuring that no LAD was
present in the resulting AlD3 solution. It was this
solution of AlD3 that was used in experiment 3 (Table
2).
surface of the vessel on the reaction (Table 3).
A
comparison of the reactions carried out in used Pyrex
(experiment 1, Table 3), Teflon8 (experiment 2, Table 3),
and quartz (experiment 3, Table 3) reveals that the rates
of the reaction in these three vessels were similar,
although more hydrogen atom abstraction occurred in the
reaction carried out in quartz.
The deuterium incorporation in the second aliquot of
the reaction carried out in quartz (experiment 3, Table
3, 1 h) contained 87% D in the 20% n-octane formed. This
deuterium incorporation (87%) is too low for a direct
displacement of iodide by deuteride, as in a SN2 reaction.
Therefore, it appears as if some radicals were formed in
solution during the first 20% of product formation.
(8) We have previously shown that Teflon is not a suitable reaction
vessel for reactions involving LiAlH4. Ashby, E. C.; Welder, C. O.
Tetrahedron Lett. 1995, 36, 7171.
(7) (a) Brown, H. C.; Yoon, N. M. J . Am. Chem. Soc. 1966, 88, 1464.
(b) Brown, H. C.; Krishnamurthy, S. J . Org. Chem. 1980, 45, 849.

4832 J . Org. Chem., Vol. 62, No. 14, 1997
Welder and Ashby
Ta ble 4. Effect of Con cen tr a tion in th e Rea ction of
1-Iod oocta n e (14) w ith LAD in THF a
Ta ble 5. Rea ction s of 1-Ha loocta n es w ith LAD in THF a
products
products
expt
1
X
time, h
% RX
% 15 (% D)
MB
expt [14], M 14:LAD time, h % 14 % 15 (% D) MB
I
0.25
1
2.5
8
77
72
66
64
63
20 (96)
21 (93)
25 (87)
27 (86)
29 (85)
97
93
91
91
92
1
2
3
0.14
1:0.2
1:0.2
1:0.2
1:0.1
0.08
0.25
1
75
77
76
67
21 (98)
22 (96)
21 (94)
33 (81)
96
99
97
20
100
24
0.070
0.035
0.035
0.08
0.25
1
84
82
82
72
13 (98)
16 (96)
18 (90)
28 (68)
97
98
100
100
2
Br
0.25
1
20
93
83
77
7 (98)
17 (98)
21 (97)
100
100
98
20
a
All reactions were carried out at a concentration of 0.070 M
with respect to RX at a RX:Nuc ratio of 1:0.2. Also, all reactions
were carried out in an Ar atmosphere glovebox in the absence of
light at room temperature in used Pyrex flasks, and Teflon-coated
stir bars were employed.
0.08
0.25
1
84
79
74
71
15 (97)
20 (96)
22 (90)
28 (74)
99
99
96
99
20
4
1
20
41
80
76
74
11 (81)
14 (66)
15 (66)
91
90
89
allowed to react with LAD (Table 5). The data show that
only 21% octane was formed after 20 h when the bromide
was allowed to react with 0.2 equiv of LAD, indicating
that the byproducts of LAD (e.g., AlD3) do not easily
reduce the alkyl bromide 16, unlike the alkyl iodide 14
that was further reduced by the byproducts of LAD.
The reaction of bromide 16 with LAD (Table 5, experi-
ment 2) can be compared to the reaction of 1-bromo-2,2-
dimethylhexane (17) with LAD.6 Even though in the
unhindered primary alkyl bromide reaction no evidence
for SET was observed, evidence for SET was significant
in the case of the hindered primary bromide 17 yet still
much less than in the case of the corresponding iodide
1.
The results with 1-bromooctane are very significant in
that, if the results of the experiment with 1-iodooctane
and LiAlD4 (experiment 1, Table 5) can be described by
an ionic process (eqs 8-10), then more protium incopo-
ration should have been observed in experiment 2 than
in experiment 1 (Table 5) since AlD2Br is a stronger
Lewis acid than AlD2I and, therefore, should have
produced more carbocation 18.
a
All reactions were carried out in an Ar atmosphere glovebox
in the absence of light at room temperature in used Pyrex flasks,
and Teflon-coated stir bars were employed.
Quartz gave somewhat stronger evidence supporting SET
during the first 20% reaction when 0.2 equiv of LAD was
allowed to react with 1-iodooctane.
Importantly, lower deuterium incorporation (62% after
24 h) in the product from the quartz experiment indicates
that more radicals were trapped by a hydrogen atom
donor in quartz than in any other vessel studied. Since
more hydrogen atom abstraction is observed in quartz,
reactions in used Pyrex vessels should represent a
minimum amount of protium incorporation products,
which are indicative of a radical intermediate. More
importantly, the results in quartz, in particular, show
clearly that the reaction of LAD with a simple unhin-
dered primary alkyl iodide proceeds, to some extent, via
a radical intermediate, and therefore, SET is involved
in the reduction of unhindered primary alkyl iodides with
LAD.
Th e Effect of Con cen tr a tion on th e Rea ction of
1-Iod oocta n e (14) w ith LAD. As explained previously
for the hindered primary alkyl iodide system,6 a decrease
in the concentration of the reactants should lead to more
hydrogen atom abstraction product for a radical reaction
as compared to more concentrated reactants under
otherwise identical conditions. As reported for the reac-
tion of LAD with iodide 1, more hydrogen atom abstrac-
tion was also observed at lower reactant concentrations
in the reaction of iodide 14 with LAD (Table 4).
In the most concentrated reaction studied (experiment
1, Table 4, 0.14 M), the deuterium content of the product
ranged from 98% to 81% over a 20 h period. When the
concentration was decreased from 0.14 to 0.070 M
(experiment 2, Table 4), the deuterium content of the
resulting n-octane ranged from 98% to 68%. A further
decrease in the concentration to 0.035 M (experiment 3,
Table 4) had very little effect on the reaction. In a further
attempt to minimize the deuterium content of the n-
octane, the stoichiometry of 14 to LAD was reduced from
1:0.2 to 1:0.1 (experiment 4, Table 4). The deuterium
content of the product at the lower stoichiometry signifi-
cantly decreased, in the 11-15% product range, more
than the previous experiments, as expected.
Effect of Tr a n sition Meta l Im p u r ities in In itia t-
in g th e Rea ction s of LiAlH4 w ith Alk yl Ha lid es. In
1978, we reported a study entitled, “Transition Metal
Catalyzed Reactions of Lithium Aluminum Hydride with
Alkyl and Aryl Halides.”9 The data show that in the
reaction of LiAlH4 with 1-iododecane, the reaction pro-
ceeds at approximately the same rate and yield when 10
mol % FeCl2, CoCl2, NiCl2, and TiCl3 are added as when
no additives are present. In the reaction of 1-bromode-
cane, the yield is 92% with pure LiAlH4, considerably
lower in the presence of 10 mol % VCl3, CrCl3, and MnCl2,
and only slightly higher in the presence of FeCl2, CoCl2,
NiCl2, and TiCl3. The data show that as much as 10 mol
% of several transition metal impurities have little effect
(except detrimental) in the reactions of LiAlH4 with
1-iodo- and 1-bromoalkanes. Any transition-metal im-
Th e Effect of th e Lea vin g Gr ou p on th e Rea ction
of 1-Ha loocta n es w ith LAD. In order to observe the
effect of the leaving group, 1-bromooctane (16) was

Reactions of 1-Halooctanes with LAD
J . Org. Chem., Vol. 62, No. 14, 1997 4833
purities in LiAlH4 would have to be in the small ppm or
ppb range, especially since these halides in most cases
would be reduced to the metal (e.g., TiCl3 is reduced to
Ti0 in the presence of LiAlH4), which is insoluble in THF
and would be removed in the filtration process. (Clear,
colorless solutions of LiAlH4 in THF were used in all of
these studies.)
In all of the studies,9 FeCl2 showed similar activity to
CoCl2, NiCl2, and TiCl3. We reported in an earlier paper1c
that recrystallized LiAlH4, redissolved in THF, showed
no difference in reactivity toward the alkyl iodide, 6-iodo-
5,5-dimethyl-1-hexene, in the presence or absence of 10
mol % FeCl3, indicating once again that transition metal
impurities in the ppm and ppb range are not affecting
the reaction.
Su m m a r y a n d Con clu sion s
Evidence for the SET nature of the reaction of LAH
with hindered primary alkyl iodides is overwhelming.
The present report shows for the first time that SET is
also involved in the reaction of LAH with unhindered and
unsubstituted primary alkyl iodides. In the case of the
latter reaction with unhindered and unsubstituted pri-
mary alkyl iodides, it appears that a true competition
between the SET and SN2 pathways is possible, although
we have no evidence that the SN2 pathway is involved
at all.1d In the past, high deuterium incorporation (99%)
in the product from the reaction of 1-iodooctane with LAD
would have been interpreted as evidence for a SN2
pathway; however, we know from the present work that
when the ratio of alkyl halides:LAD is 1:0.2 and the
reaction is carried out in a quartz vessel, that deuterium
incorporation in the product is as low as 62%, which
indicates that the reaction proceeds to a significant extent
via a radical intermediate.
P r op osed Mech a n ism of Red u ction of 1-Iod ooc-
ta n e (14) by LAD a n d Its Byp r od u ct, AlD3. The
above data are consistent with the reaction mechanism
shown in Scheme 3. It is proposed that LAD can react
Sch em e 3
Exp er im en ta l Section
Ma t er ia ls. Tetrahydrofuran (THF) was purchased from
Fisher Scientific and distilled from NaAlH4 prior to use.
LiAlD4 was purchased from Aldrich, allowed to stir in THF
overnight, and then filtered in an argon atmosphere glovebox
to obtain a homogeneous solution that was titrated before use.
1-Iodooctane (Aldrich) and 1-bromooctane (Fisher) were dis-
tilled prior to use. An authentic sample of n-octane (Eastman)
was obtained in order to verify its formation in the mechanistic
studies. Solutions of aluminum deuteride were prepared
according to literature procedures.7 Techniques for titration
of LAD solutions and calculation of deuterium content were
reported previously.6
In str u m en ta tion . GLC analyses were performed on a
Varian 3700 gas chromatograph with a J & W DB-5 fused
silica column (30 m, 0.25 mm i.d., 25 µm film thickness)
equipped with a flame ionization detector. Mass spectra were
obtained from a VG 70-SE mass spectrometer equipped with
a double sector magnetic analyzer.
Typ ica l Red u ction of 1-Ha loocta n es. To a flask in an
Argon atmosphere glovebox at room temperature was added
1-iodooctane (0.034 g, 0.14 mmol), decane (15 µL, an internal
standard for GC), and THF (1.93 mL). To this solution was
added a homogeneous solution of LiAlD4 in THF (91 µL, 0.31
M, 0.028 mmole). The reaction vessel was then covered in
aluminum foil to shield the reaction from light. At the desired
time, an aliquot (0.1-0.2 mL) was removed and quenched in
an approximately equal volume of water. The organic com-
pounds were extracted with diethyl ether (0.2 mL), and the
organic layer was analyzed by GC and/or GC/MS. Unless
otherwise noted in the tables, experiments were carried out
in used Pyrex flasks employing a Teflon-coated stir bar.8
with alkyl iodide 14 by either an SN2 pathway (step a)
to give 1-deuteriooctane (15d 1) or a SET pathway (step
b) to give the radical anion 19. Loss of iodide from 19
yields radical 20 (step c). Radical 20 either abstracts a
hydrogen atom from the solvent (THF) to form the
nondeuterated product 15 (step d) or it abstracts a
-
deuterium atom from AlD4 to form deuterated octane
15d 1 (step e) and the AlD3 radical anion. As shown in
step f, the AlD3 radical anion can act as an electron donor
to alkyl iodide 14 in a radical-chain process to yield 19
and AlD3. As shown in step g, AlD3 can react with 14 to
yield 19 and the AlD3 radical cation. Radical anion 19
can lose iodide (step h) to form 20, which then abstracts
a deuterium atom from AlD3 to yield 15d 1 and the AlD2I
radical anion. The AlD2I radical anion can then react
with 14 (step j) to form 19 and AlD2I in yet another
radical-chain process. It is clear from the results of these
studies that the reaction of 1-iodooctane with LiAlD4, and
by comparison, LiAlH4, proceeds to a significant extent
by a SET pathway involving AlH3.
Ack n ow led gm en t. We are indebted to Sarah J .
Shealy and David E. Bostwick for their mass spectrom-
etry assistance. This work was supported by the
National Science Foundation (Grant No. CHE-8914309)
and the Georgia Institute of Technology, School of
Chemistry and Biochemistry.
J O961871N
(9) Ashby, E. C.; Lin, J . J . J . Org. Chem. 1978, 43, 1263.