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.