Dual role of allylsamarium bromide as a Grignard reagent and a single electron transfer reagent in the one-pot synthesis of terminal olefins

Article abstract of DOI:10.1039/c3cc45611k

The utility of allylsamarium bromide, both as a nucleophilic reagent and a single-electron transfer reagent, in the reaction of carbonyl compounds with allylsamarium bromide in the presence of diethyl phosphate is reported in this communication. From a synthetic point of view, a simple one-pot method for the preparation of terminal olefins is developed.

Full text of DOI:10.1039/c3cc45611k

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Dual role of allylsamarium bromide as a Grignard
reagent and a single electron transfer reagent in the
one-pot synthesis of terminal olefins†
Cite this: Chem. Commun., 2013,
49, 10635
Received 24th July 2013,
Accepted 25th September 2013
Ying Li, Yuan-Yuan Hu and Song-Lin Zhang*
DOI: 10.1039/c3cc45611k
www.rsc.org/chemcomm
The utility of allylsamarium bromide, both as a nucleophilic reagent synthesis of terminal olefins using aldehyde as a substrate
and a single-electron transfer reagent, in the reaction of carbonyl has not been reported. So, developing a new method using an
compounds with allylsamarium bromide in the presence of diethyl aldehyde and an allyl metal reagent as substrates to obtain
phosphate is reported in this communication. From a synthetic point terminal olefins is desirable.
of view, a simple one-pot method for the preparation of terminal
olefins is developed.
In this continuing work using allylsamarium bromide in
organic synthesis by our group, we think that realization of two
roles (Grignard reagent and single electron transfer reagent) for
Since 1977, when Kagan1 introduced samarium diiodide into allylsamarium bromide in one reaction was definitely meaningful
organic synthesis, a lot of important individual transformations and challenging. Herein, we reported the efficient synthesis of
have been carried out using it as a mild, neutral, selective and terminal olefins by the reaction of allylsamarium bromide with
versatile single-electron transfer (SET) reducing2 and coupling carbonyl compounds in the presence of diethyl phosphate.
reagent.3 For example, samarium diiodide-mediated Barbier,
Initially, the reaction between benzaldehyde 1a and allyls-
radical-alkene/alkyne, Reformatsky, carbonyl-alkene/alkyne, amarium bromide was selected as a model reaction to check
Pinacol-type and other reactions have been successfully realized. this possibility (Table 1, entries 1–13). Firstly, a series of
Besides, samarium diiodide has been efficiently applied in the organophosphorus additives were examined at room temperature
total synthesis of many natural compounds.4 The active study of and the results were summarized in Table 1. From the results,
SmI2 further promoted research of the other samarium reagents
such as metal samarium, SmI3 and organosamarium reagents.5
In 1996, Zhang first reported the convenient preparation of Table 1 Optimization of reaction conditions
allylsamarium bromide and used it successfully in Grignard-
type reactions.6 The subsequent research of allylsamarium
bromide as a Grignard reagent has grown rapidly and received
much attention from the synthetic community.7 However, the
2
Time Temp. Yieldb
possibility that allylsamarium bromide (like SmI2) can play the
role of a single electron transfer reagent has been ignored for
over one decade until the first report from our group appeared.8
The terminal double bond is a very important group in
organic compounds and can be functionalized in many ways.9
Because of the importance of terminal olefins, a lot of methods
have been developed to synthesize these kinds of compounds.10
This kind of transformation could be realized by addition of
allylsilanes or allylstannanes to organic electrophiles with
Lewis acids as catalysts.11 To the best of our knowledge,
Entrya Additive (equiv.)
(equiv.) (h)
(1C)
(%)
1
2
Ph3P (1.0)
(EtO)3P (1.0)
3
3
3
3
3
3
3
3
3
3
3
2
4
24
24
24
24
24
24
6
12
12
12
18
12
12
r.t
r.t
r.t
r.t
r.t
r.t
65
65
65
65
65
65
65
None
None
None
None
12
23
48
51
55
64
56
3
4
5
6
7
8
9
10
11
12
13
(EtO)3PO (1.0)
(EtO)2P(O)CH2COOEt (1.0)
(PhO)2P(O)H (1.0)
(EtO)2P(O)H (1.0)
(EtO)2P(O)H (1.0)
(EtO)2P(O)H (1.0)
(EtO)2P(O)H (1.5)
(EtO)2P(O)H (2.0)
(EtO)2P(O)H (2.0)
(EtO)2P(O)H (2.0)
(EtO)2P(O)H (2.0)
23
64
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry,
Chemical Engineering and Materials Science, Soochow University, Suzhou 215123,
P. R. China. E-mail: zhangsl@suda.edu.cn
a
Unless noted, to a solution of 1a (0.5 mmol) in THF (3 mL) was added
2 (X mmol) in THF under a nitrogen atmosphere at room temperature,
and the mixture was stirred for 1 h, then the additive (Y mmol) was
b
† Electronic supplementary information (ESI) available: Experimental procedures,
characterization and spectral data of the products. See DOI: 10.1039/c3cc45611k
added to this mixture and stirred at 65 1C. Isolated yield based on 1a
after silica gel chromatography.
c
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Table 2 Reaction of aldehyde compounds with allylsamarium bromide
we could see that the reaction was dramatically influenced by the
additives. With Ph3P, (EtO)3P, (EtO)3PO and (EtO)2P(O)CH2COOEt
as additives, no product was obtained (Table 1, entries 1–4). With
(EtO)2P(O)H as an additive, product 3a could be obtained in low
yield (Table 1, entry 6). To our delight, the yield could be further
improved to 48% through increasing the temperature from room
temperature to 65 1C (Table 1, entry 7). When we prolonged the
reaction time to 12 h, the yield was not obviously increased
(Table 1, entry 8). Then, the molar ratio of 1a/additive/2 was also
investigated. To our delight, upon increasing the amount of
diethyl phosphate, a yield of 64% was achieved (Table 1, entries
10 vs. 8 and 9). Thus, (EtO)2P(O)H as an additive and 65 1C as the
reaction temperature with a 1/2/3 molar ratio of 1a/additive/2
proved to be the optimal reaction conditions for the reaction.
With the optimized reaction conditions in hand, the substrate
scope of the reaction of various aldehydes with allylsamarium
bromide was subsequently explored and the results were sum-
marized in Table 2. As shown in Table 2, good yields were
obtained with aromatic aldehydes bearing electron-donating groups
on the aryl ring, such as 3-methyl-, 4-methoxy-, 4-methylthio-,
4-isopropyl-, 4-tertiarybutyl-, 4-methyl-, and 3,5-dimethoxy-
benzaldehyde (Table 2, entries 2–8). Piperonyl aldehyde also
gave the product 3i in good yield (Table 2, entry 9). With
aromatic aldehydes bearing electron-withdrawing groups
on the aryl ring, such as 4-chloro-, 4-bromo-, 3-chloro-, and
3,4-dichlorobenzaldehyde, the products were obtained in
moderate yields (Table 2, entries 10–13). The above results
showed that electron-donating groups on the aryl ring were
more favorable for the reaction than electron-withdrawing
groups. The reactions of 1-naphthaldehyde 1n, 2-naphthaldehyde
1o and pyrene-1-carboxaldehyde 1p with allylsamarium bromide
showed quite good performance giving the desired products
3n–3p in moderate to good yields (Table 2, entries 14–16).
Interestingly, terephthalaldehyde 1q could also be well tolerated
and the product 3q was obtained in good yield (Table 2, entry 17).
To our delight, aliphatic aldehydes 1r–1t could also react with
allylsamarium bromide providing the products 3r–3t in moderate
yields (Table 2, entries 18–20).
Entrya Substract
Product
Yieldb (%)
1
2
64
75
3
4
79
83
5
70
6
7
66
63
8
69
9
76
59
10
11
12
55
60
13
68
Then, we turned our attention to the reaction of allylsamarium
bromide with ketones 4a and 4b. Unexpectedly, the conjugated
olefin products 5a and 5b were obtained (Table 2, entries 21 and 22).
To investigate the role of diethyl phosphate and allylsamarium
bromide in the reaction, studies similar to our previous work12
were carried out under the optimum reaction conditions.
The reaction of diethyl phosphate with allylsamarium bromide
indicated that diethyl phosphate, allylsamarium bromide
diethoxy(oxo)phosphate 7 or dialkylphosphinylallylsamarium
bromide 8 might be the accelerant for the reaction (Scheme 1).
To further identify which may be the real accelerant in the
reaction, 1,1-diphenylbut-3-en-1-ol 6 was synthesized and tested
in control experiments. We found that the reaction of 6 with 7
gave the desired product in moderate yield, but the reaction of 6
with (EtO)2P(O)H or 8 failed to give the product (Scheme 2).
Based on the above results, a possible mechanism is
proposed. Firstly, aldehyde undergoes allylation leading to
the formation of intermediate 9, then, the oxygen atom of
intermediate 9 coordinates with 7 to afford transition state 10,
14
15
88
85
16
17
62
71
18
19
42
51
20
42
c
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Table 2 (continued )
of allylsamarium bromide with ketones and its mechanism are
underway in our laboratory and will be reported in due course.
We gratefully acknowledge the project funded by the Priority
Academic Program Development of Jiangsu Higher Education
Institutions and the National Natural Science Foundation of
China (No. 21072143) for financial support.
Entrya Substract
Product
Yieldb (%)
Notes and references
21
91
89
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a
Unless noted, to a solution of carbonyl compounds (0.5 mmol) in THF
(3 mL) was added the allylsamarium reagent (1.5 mmol) in THF under a
nitrogen atmosphere at room temperature, and the mixture was stirred
for 1 h, then diethyl phosphate (1.0 mmol) was added to this mixture
b
and stirred at 65 1C for 12 h. Isolated yield based on carbonyl
compounds after silica gel chromatography.
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Scheme 2 Control experiments.
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which is followed by a sequential single-electron transfer process
to give the intermediate 11. At last, proton capture by inter-
mediate 11 from (EtO)2P(O)H or (EtO)2P(O)D affords the target
product 3 or [D]-3 (Scheme 3).
In summary, a series of terminal alkenes were synthesized
through the reaction of allylsamarium bromide with carbonyl
compounds. In these reactions, single-electron transfer and
nucleophilic properties of allylsamarium bromide were fully
investigated. It is an efficient and experimentally simple one-
pot method to obtain terminal alkenes. Studies on the reaction
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 10635--10637 10637