Efficient iron-catalyzed direct β-alkylation of secondary alcohols with primary alcohols

Article abstract of DOI:10.1002/adsc.201000907

The efficient iron-catalyzed direct β-alkylation of secondary alcohols with primary alcohols is described. In the presence of the commercially available iron catalyst (ferrocenecarboxaldehyde, 1b) and a catalytic amount of base, the reactions give β-alkylated higher alcohols in high yields in the absence of any sacrificial agents (hydrogen acceptors or hydrogen donors) and nitrogen or phosphorus ligands. For the first time, iron is employed as an inexpensive and environmentally benign alternative with high atom efficiency to noble metal-based catalysts in this type of reaction. Copyright

Full text of DOI:10.1002/adsc.201000907

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DOI: 10.1002/adsc.201000907
Efficient Iron-Catalyzed Direct b-Alkylation of Secondary
Alcohols with Primary Alcohols
Jian Yang,a,b Xin Liu,a Da-Li Meng,b Hong-Yan Chen,a Zhi-Hui Zong,a
Ting-Ting Feng,a and Kai Suna,*
a
School of Pharmaceutical Sciences, Jilin University, Changchun 130021, Peopleꢀs Republic of China
Fax : (+86)-431-8561-9803; phone: (+86)-431-8561-9152; e-mail: thomassk@jlu.edu.cn
School of Traditional Medicine, Shenyang Pharmaceutical University, Shenyang 110016, Peopleꢀs Republic of China
b
Received: December 2, 2010; Revised: September 20, 2011; Published online: February 9, 2012
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000907.
Abstract: The efficient iron-catalyzed direct b-alky-
lation of secondary alcohols with primary alcohols
is described. In the presence of the commercially
available iron catalyst (ferrocenecarboxaldehyde,
Scheme 1. The classic b-alkylation of a secondary alcohol.
1b) and a catalytic amount of base, the reactions
give b-alkylated higher alcohols in high yields in the
absence of any sacrificial agents (hydrogen accept-
ors or hydrogen donors) and nitrogen or phospho-
rus ligands. For the first time, iron is employed as
an inexpensive and environmentally benign alterna-
tive with high atom efficiency to noble metal-based
catalysts in this type of reaction.
an environmentally benign route with high atom effi-
ciency.
On the other hand, the realm of transition metal or-
ganic chemistry has provided quite a few excellent re-
agents to realize the alkylation of a series of nucleo-
philic agents with alcohols, which has received in-
creasing attention.[5,6] Several groups have reported
some examples[7–9] of direct b-alkylation of secondary
alcohols with primary alcohols as the environmentally
benign electrophilic alkylating agents, which give
higher alcohols along with water as a sole by-product
(Scheme 2). Among them, Ru complexes are one of
À
Keywords: alkylation; C C coupling; iron; primary
alcohols; secondary alcohols
À
C C bond formation is a pivotal method to construct important classes of catalysts widely used (method
complex molecules from some simple substrates, with A). The first successful example of such a direct b-al-
the electrophilic alkylation of a given starting material kylation was reported with the RuCl2ACTHNUTRGEN(UGN PPh3)3/KOH
being one of the most useful procedures.[1] Alcohols system, requiring excess 3 (200.0 mol% referred to 2),
are one of the most basic and important organic ma- stoichiometric KOH (300.0 mol% referred to 2), a
terials owing to their wide variety of uses in industrial large amount of sacrificial hydrogen acceptor
and laboratory chemistry. Although a plethora of syn- (500.0 mol% of 1-dodecene referred to 2) and hydro-
thetic methods of alcohols has been developed,[2] the gen donor (dioxane solvent).[7a] Then complex [(p-
straightforward routes to the creation of a variety of cymene)RuCl2]2 and Grubbsꢀ catalyst RuCl2=CHPh-
alcohols having intricate structures through alkylation
ACHTUNGTREN(UNNG PCy3)2 were mentioned to be able to realize the b-al-
of simple alcohols remain very unusual. For example, kylation of 1-phenylethanol with benzyl alcohol, how-
the classic b-alkylation of a secondary alcohol leading ever, without any other substrate pair being stud-
to a higher alcohol usually requires tedious processes ied.[7b] Although RuCl2 (DMSO)4 was described as an
producing a lot of waste: oxidation, alkylation and re- active catalyst exhibiting high yields for this b-alkyla-
duction (Scheme 1), which cause some disadvantages tion, the reaction was completed in 7 days with
in the atom economic[3] and environmental[4] points of 200.0 mol% of KOH.[7c] Other Ru catalytic sys-
view. Thus any new strategy for the catalytic direct b- tems[7d–h] containing complex nitrogen or phosphor li-
alkylation of secondary alcohols to create higher alco- gands have also been developed, most of which
hols would be highly desirable, all the more so if it is worked in the presence of a stoichiometric base with
328
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Adv. Synth. Catal. 2012, 354, 328 – 334

Efficient Iron-Catalyzed Direct b-Alkylation of Secondary Alcohols with Primary Alcohols
Scheme 2. Various methods of catalytic direct b-alkylation of secondary alcohols with primary alcohols to give higher alco-
hols.
only one exception[7g] utilizing a slight excess of 3 and a catalytic amount of base without any sacrificial
(120.0 mol% referred to 2). Another class of impor- agents (hydrogen acceptors or hydrogen donors) and
tant catalysts for this reaction is Ir complexes nitrogen or phosphorus ligands (Scheme 2).
(method B).[8] However, they are very expensive, and
As a starting point, we initiated our research with
in most of cases the reactions proceeded with stoi- equimolecular amounts of 1-phenylethanol (2a) and
chiometric base[7h,8a–c] and complex nitrogen li- benzyl alcohol (3a) to give the corresponding higher
gands.[7f,h,8b,c]. Very recently, the first direct b-alkyla- alcohol (4a) under a catalytic amount of NaOH
tion of a secondary alcohol with a primary alcohol (20.0 mol%) as the reaction model from the atom
catalyzed by a Pd complex with pincer-type N-hetero- economical and environmentally benign viewpoints.
cyclic carbene ligands to give a higher alcohol was And the reaction was performed in p-xylene at
published (method C).[9] Although the reaction re- 1258C, according to the previous reports,[7–9] to screen
quired excess 2 (200.0 mol% referred to 3), it could various commercially available ferric salts (5.0 mol%,
be carried out in the presence of only a catalytic
amount of base. Nevertheless, so far all of the reports
Table 1. Selected optimization of iron catalysts for b-alkyla-
tion of 1-phenylethanol with benzyl alcohol.[a]
focus on group 8 noble metals, which are considerably
expensive and precious. Therefore, it becomes a great
challenge to search for more readily available, more
environmentally benign and less expensive catalytic
systems with higher atom efficiency for this type of b-
alkylation.
As one of the most abundant metals on earth, the
numerous merits of iron have made it very attractive
for organic synthesis.[10] Considerable studies have
been directed to iron catalysts and a variety of iron-
À
catalyzed organic transformations, such as C H bond
Entry
Catalyst
Yield of 4a [%][b]
oxidation,[11] Friedel–Crafts reactions,[12] cross-cou-
pling reactions,[13] cycloaddition reactions,[14] and so
on. Recently, there have also been some efforts[15] to
1
2
3
4
5
6
7
8
9
none[c]
FeCl3
FeCl3·6H2O
FeCl2
3
36
10
16
12
65
45
78
13
À
À
develop C C bond-forming reactions based on C H
functionalization, in which transition metals have tra-
ditionally played important roles.[16] To the best of our
knowledge, however, the catalytic direct b-alkylation
of secondary alcohols with primary alcohols based on
iron to give higher alcohols has not been reported as
yet. Herein, we present our preliminary experimental
results – the first examples of an efficient, iron-cata-
lyzed, direct b-alkylation of secondary alcohols with
primary alcohols producing higher alcohols, which is
catalyzed only by the inexpensive and commercially
available iron catalyst (ferrocenecarboxaldehyde, 1b)
FeCl2·4H2O
FeSO4·7H2O
ferrocene (1a)
ferrocenecarboxaldehyde (1b)
ferrocenemethanol (1c)
[a]
Reaction conditions: 2a (2.0 mmol), 3a (2.0 mmol), iron
catalyst (5.0 mol%) and NaOH powder (20.0 mol%) in
p-xylene (2.0 mL) were refluxed at 1258C under an
argon atmosphere for 12 h.
[b]
[c]
GC yields.
The control experiment without any catalysts.
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329

Jian Yang et al.
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Table 2. Optimization of the reaction conditions of the reac-
for the later research. In the respect of base, only
strong alkaline metal hydroxides could produce the
desired product in our case, and NaOH exhibited the
best result (entries 1 and 5–9). It was also found that
20.0 mol% of NaOH was the preferred amount (en-
tries 1 and 10–12). Furthermore, we recognized that
the temperature had an important effect on the yield
and the reaction rate, which was consistent with the
results of other groups.[6d,7c,8a] The reaction at 1208C
resulted in a considerably low yield (entry 13). How-
ever, when the temperature was raised up to 1308C,
we were very glad to observe the most satisfying
result with 99% yield (entry 14).
To investigate the substrate scope, we started to ex-
amine the b-alkylation of a variety of secondary alco-
hols with primary alcohols (Table 3). This catalytic
system could be tolerant of various functional groups.
Based on the standard conditions, the most of corre-
sponding b-alkylated higher secondary alcohols
(except for entry 24) were able to be obtained in
good to excellent yields with subtle tuning of the reac-
tion conditions. The minimum loadings of catalyst and
base were, respectively, 2.5 mol% (entry 16) and
15.0 mol% (entries 6–9, 22). All reactions were com-
pleted in 12–24 h. Alcohol 2a was b-alkylated with
benzyl alcohols bearing an electron-donating or elec-
tron-withdrawing substituent to produce the corre-
sponding higher alcohols in high yields (entries 2–6).
Even with the heterocyclic alcohol (entry 7), the alky-
lation of 2a still proceeded efficiently. And the reac-
tions of 3a and 1-arylethanols could also be per-
formed smoothly (entries 8, 12, 17 and 18). Together
tion model.[a]
Entry Temp. [8C] Solvent
Base
(x mol%)
Yield of
4a [%][b]
1
2
3
4
5
6
7
8
125
125
125
125
125
125
125
125
125
125
125
125
120
130
p-xylene
toluene
1,4-dioxane NaOH (20.0)
solvent free NaOH (20.0)
NaOH (20.0)
NaOH (20.0)
78
75
31
65
0
72
0
0
0
75
70
71
p-xylene
p-xylene
p-xylene
p-xylene
p-xylene
p-xylene
p-xylene
p-xylene
p-xylene
p-xylene
LiOH (20.0)
KOH (20.0)
Li2CO3 (20.0)
Na2CO3 (20.0)
K2CO3 (20.0)
NaOH (10.0)
NaOH (15.0)
NaOH (30.0)
NaOH (20.0)
NaOH (20.0)
9
10
11
12
13
14
52
99 (97)[c]
[a]
Reaction conditions: 2a (2.0 mmol), 3a (2.0 mmol), 1b
(5.0 mol%) and base were refluxed in solvent (2.0 mL)
under an argon atmosphere for 12 h.
[b]
[c]
GC yields.
The value in parenthesis indicated the isolated yield.
Table 1). The control experiment (entry 1), in which with the reactions of 1-arylethanols and substituted
no catalyst was present, gave the desired product 4a benzyl alcohols (entries 9–11 and 13–16), it was appar-
in a very low yield. We first focused on the simple in- ent that the position and the electronic nature of the
organic ferric salts. The preliminary results showed substituent on the aromatic rings of 2 and 3 did not
that most of them produced 4a in considerably low affect product yields. For the reactions of the aliphatic
yields (entries 2–5), however, FeSO4·7H2O gave a secondary alcohol, a higher temperature seemed to be
moderate yield (entry 6), which indicated that some necessary in our case. With the benzyl alcohol
ferric salts did have considerable potential for this al- (entry 19), the desired product was obtained at 1808C
kylation. This positive result pushed us to extend the in excellent yield. Unfortunately, with the aliphatic
catalysts to some inexpensive and commercially avail- primary alcohol (entry 24), this catalytic system could
able ferrocene salts (1a–1c, entries 7–9). To our great only offer moderate yields, although the temperature
delight, finally we found that 1b, usually acting as a was raised up to 1908C. It should be pointed out that
reagent or starting material for the synthesis of other the b-alkylation with aliphatic primary alcohols (en-
ferrocene derivatives,[17] was the most active for this tries 20–24) could be run in the absence of solvents,
alkylation with 78% yield for the first try (entry 8).
among which the reactions of 1-arylethanols (en-
This interesting finding promised a bright prospect tries 20–23) gave the desired higher alcohols with sat-
and made us start to optimize other different parame- isfying yields, though slightly lower than the cases
ters with 1b as the catalyst (Table 2). As far as sol- with benzyl alcohols.
vents were concerned, in the cases of 1, 4-dioxane
During the parameter optimization of our reaction
and toluene, the reactions could also proceed with 1b model, trace or small amounts of 5–8 (Scheme 3)
to give 4a (entries 2 and 3), although the yields were were often determined in the reaction mixtures,
lower than in the case of p-xylene. And it should be which showed us some clues on the reaction inter-
noted that even without any solvent, 4a could still be mediates. To obtain more information concerning the
formed in a slightly lower yield under the catalysis of reaction mechanism, we studied the a-alkylation of
1b and NaOH (entry 4), which showed us some hints ketone 5 with 3a [Eq. (1)], the b-alkylation of 2a with
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Efficient Iron-Catalyzed Direct b-Alkylation of Secondary Alcohols with Primary Alcohols
Table 3. b-alkylation of secondary alcohols with primary alcohols catalyzed by 1b.[a]
Entry
R1
R2
1b loading
[mol%]
Temp.
[8C]
NaOH
[mol%]
Time
[h]
Product
Yield
[%][b]
1
2
3
4
2a: R1 =Ph
2a
2a
2a
3a: R2 =Ph
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
7.5
5.0
5.0
5.0
2.5
7.5
7.5
7.5
7.5
5.0
5.0
7.5
7.5
130
130
135
130
130
130
130
130
135
130
130
130
130
130
130
140
130
130
180
135
140
140
150
190
20.0
20.0
20.0
20.0
30.0
15.0
15.0
15.0
15.0
20.0
20.0
30.0
20.0
20.0
20.0
20.0
20.0
37.5
30.0
30.0
40.0
15.0
30.0
30.0
12
24
12
24
12
12
12
12
24
24
12
12
24
12
12
12
12
12
12
12
12
12
12
12
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
4q
4r
97
82
77
81
81
87
81
80
83
87
82
81
86
80
72
92
95
72
94
72
75
70
78
54
3b: R2 =4-MeC6H4
3c: R2 =4-ClC6H4
3d: R2 =4-MeOC6H4
5
2a
2a
2a
3e: R2 =3-ClC6H4
6
3f: R2 =3,4-(MeO)2C6H3
7[c]
8
3g: R2 =2-furyl
2b: R1 =4-MeC6H4
3a
3b
3c
3d
3a
3b
3c
3d
3e
3a
3a
3a
9
2b
2b
2b
10
11
12
13
14
15
16
17[d]
18[d]
19[e]
20[f]
21[g]
22[h]
23[i]
24[i]
2c: R1 =4-ClC6H4
2c
2c
2c
2c
2d: R1 =2-MeC6H4
2e: R1 =4-FC6H4
2f: R1 =phenethyl
4s
4t
4u
4v
4w
4x
2a
2b
2c
2a
2f
3h: R2 =n-Pr
3h
3h
3i: R2 =t-Bu
3i
[a]
Reaction conditions: 2 (2.0 mmol), 3 (2.0 mmol), NaOH powder (referred to 3) and 1b (referred to 3) were refluxed in p-
xylene (2.0 mL) under an argon atmosphere, unless other stated.
The isolated yields.
2 (1.0 mmol) and 3 (1.0 mmol) were used with p-xylene (1.0 mL).
With p-xylene (0.5 mL).
2 (1.0 mmol) and 3 (1.0 mmol) were used with p-xylene (0.25 mL).
2 (10.0 mmol) were used without any solvent.
2 (3.5 mmol) were used without any solvent.
[b]
[c]
[d]
[e]
[f]
[g]
[h]
[i]
2 (4.0 mmol) were used without any solvent.
2 (1.0 mmol), 3 (2.0 mmol), NaOH powder (referred to 2) and 1b (referred to 2) were used without any solvent.
aldehyde 6 [Eq. (2)], and the transfer hydrogenation aldehyde group of 1b under the basic conditions,
of ketone 7 [Eqs. (3) and (4)] and a,b-unsaturated which results in the hydride abstraction by iron cation
ketone 8 [Eqs. (5) and (6)], respectively, with alcohols of 1b to generate an iron hydride species with the oxi-
2a and 3a under the standard conditions. These ex- dation of primary and secondary alcohols to the cor-
periments exhibited that it was possible for 5–8 to responding aldehyde and ketone. And then base-
produce the desired higher alcohol 4a with our cata- mediated cross-aldol condensation occurs to produce
lytic system, which indicated the possibility for 5–8 to an a,b-unsaturated ketone, which undergoes transfer
be the intermediates in this catalytic reaction, al- hydrogenation of C=O and C=C double bonds with
though other pathways could not be fully ruled out at that iron hydride species to give the desired product
this stage.
and regenerate the catalyst. The hydride species gen-
Based on these results, a simplified plausible mech- erated at the stage of the oxidation of alcohols are
anism for the 1b-catalyzed, direct b-alkylation of sec- consumed at the stage of the final reduction, which
ondary alcohols with primary alcohols was formulated follows the reported “hydrogen autotransfer process”
as described in Scheme 4. At first, the reaction in- or “borrowing hydrogen” process.[5] So the sacrificial
volves the nucleophilic attack of alkoxy anions to the agents are not required for our catalytic system.
Adv. Synth. Catal. 2012, 354, 328 – 334
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Jian Yang et al.
COMMUNICATIONS
Scheme 3. Results obtained in the study on the intermediates of 1b-catalyzed b-alkylation of secondary alcohols with pri-
mary alcohols.
In summary, we have developed a novel iron-cata- Experimental Section
lyzed, direct, b-alkylation of secondary alcohols with
primary alcohols to produce higher alcohols with high General Procedure A for the b-Alkylation of Various
atom efficiency. It should be noted that this reaction Secondary Alcohols with Benzyl Alcohols and
system only required inexpensive and commercially Heterocyclic Alcohols
available 1b and a catalytic amount of NaOH without
any sacrificial agents (hydrogen acceptors or hydro-
b-Alkylation of 2a with 3a as an example: Under an atmos-
phere of argon in
a heavy-walled glass reactor, 2a
gen donors) and complex nitrogen or phosphorus li-
gands. It is the first time for iron to be employed as
an inexpensive and environmentally benign alterna-
tive to noble metal-based catalysts in this b-alkylation
of secondary alcohols with primary alcohols giving
higher alcohols. Based on the theoretical calculations,
the exploration of the details on the mechanistic as-
pects for this reaction is underway, and other reac-
tions catalyzed by inexpensive metals, especially iron
catalysts, to activate alcohols as electrophilic alkylat-
ing agents are also currently being pursued in our lab-
oratory. When we were preparing this report, an very
interesting alkaline metal base-promoted direct b-al-
kylation of secondary alcohols with primary alcohols
in air, based on Oppenauer oxidation and Meerwein–
Ponndorf–Verley reduction, was published.[18] Un-
fortunately, It was not a catalytic reaction, and stoi-
chiometric base (100.0 mol% of KOH or NaOH) was
required.
(2.0 mmol), 3a (2.0 mmol), 1b (5.0 mol%), NaOH powder
(20.0 mol%), p-xylene (2.0 mL) and a Teflon-coated stirring
bar were placed. The reactor was sealed, and the mixture
was magnetically stirred at 1308C for 12 h in a silicon oil
bath. It was cooled down to room temperature and directly
purified by column chromatography on silica gel (n-hexane/
ethyl acetate=from 100/1 to 30/1) to give 4a; yield:
411.6 mg (1.94 mmol, 97%).
General Procedure B for the b-Alkylation of Various
Secondary Alcohols with Aliphatic Primary Alcohols
b-Alkylation of 2a with 3h as an example: Under an atmos-
phere of argon in
a heavy-walled glass reactor, 2a
(10.0 mmol), 3h (2.0 mmol), 1b (7.5 mol% referred to 3h),
NaOH powder (30.0 mol%, referred to 3h) and a Teflon-
coated stirring bar were placed. The reactor was sealed, and
the mixture was magnetically stirred at 1358C for 12 h in a
silicon oil bath. It was cooled down to room temperature
and directly purified by column chromatography on silica
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Adv. Synth. Catal. 2012, 354, 328 – 334

Efficient Iron-Catalyzed Direct b-Alkylation of Secondary Alcohols with Primary Alcohols
Scheme 4. The simplified plausible mechanism for b-alkylation of secondary alcohols with primary alcohols catalyzed by 1b.
gel (n-hexane/ethyl acetate=from 100/1 to 40/1) to give 4t;
yield: 256.8 mg (1.44 mmol, 72% referred to 3h).
Compounds 4b–4s were synthesized in a similar way to
general procedure A, and compounds 4u–4x were obtained
in a similar way to general procedure B. All the compounds
were characterized by MS and NMR (1H and 13C). Data are
given in the Supporting Information.
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Acknowledgements
This work was financially supported by Foundation of Sci-
tech Development Project of Jilin Province (Scientific Re-
search Foundation for Youth Scholars, No. 20100135) and
Basic Professional Foundation for Scientific Research of Jilin
University (No. 200903351). K. Sun greatly thanks Prof. C.-J.
Li (Tulane University & McGill University) for his valuable
suggestions on the plausible mechanism of this reaction.
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