Preformed -allyl iron complexes as potent, well-defined catalysts for the allylic substitution

Full text of DOI:10.1002/anie.200901930

Angewandte
Chemie
DOI: 10.1002/anie.200901930
Allylic Substitutions
Preformed p-Allyl Iron Complexes as Potent, Well-Defined Catalysts
for the Allylic Substitution**
Michael Holzwarth, Andrꢀ Dieskau, Misbah Tabassam, and Bernd Plietker*
A key feature for the successful development of
selective catalytic process is the use of defined
catalysts. Knowledge of the electronic and steric
properties at the active metal center offers the
chance to optimize structure–reactivity relation-
ships. Within the past few years, catalysts based on
abundant, inexpensive first-row transition metals,
for example, iron have attracted considerable
interest.[1] For future elaboration of these reactions,
detailed knowledge of the structure of the inter-
mediate catalyst–substrate complexes is important.
However, the high reactivity of the in-situ formed
active iron complex often prohibits a deeper
Scheme 1. Model for the mechanistic dichotomy. MTBE tert-butyl methyl ester,
investigation of the mechanism. The gap between
À
À
Nu H=H CH(CO2iBu)2.
catalysis development[2] and mechanistic know-how
might be reduced by the synthesis and character-
ization of postulated intermediates and comparison
of their catalytic activities[3] against known systems. Herein
we present structurally defined p-allyl iron complexes as
novel, air and moisture stable precatalysts for the allylic
substitution and demonstrate that the reactions in the
presence of aryl-substituted N-heterocyclic carbene ligands
(NHCs) follow a p-allyl mechanism.[4–9]
Based upon the mechanistic hypothesis that p-allyl iron
complexes, such as 3, are intermediates in the catalytic cycle,
we assumed that these compounds, although described to date
as being catalytically inactive, could act as structurally defined
precatalysts.[15] Furthermore we were hoping for a further
proof of our proposed p-allyl mechanism through a direct
comparison of the regioselective courses using preformed p-
allyl iron complexes in either a stoichiometric or a catalytic
allylation.[14]
Our investigations started with an analysis of the regio-
selective course of the allylation of diisobutyl malonate with
stoichiometric amounts of the preformed p-allyl iron com-
plexes 5–7. These complexes are accessible in one step
starting from the corresponding allyl halides (Scheme 2) and
Based upon earlier reports by Roustan et al.[10] and Zhou
et al.[11] we recently developed an efficient method for the
iron-catalyzed allylic substitution using the [Fe(CO)5]-derived
ferrate complex [Bu4N][Fe(CO)3(NO)] (TBAFe). Whereas
early investigations concentrated mainly on the use of PPh3 as
a ligand[12,13] a more efficient method was elaborated recently
by employing NHC ligands instead.[14] Furthermore, depend-
ing on the N-substituent within the ligand core we observed a
significant shift of the regioisomeric ratio for which, based
upon empirical results, we postulated a ligand-dependant
mechanistic dichotomy (Scheme 1).
[*] M. Holzwarth, A. Dieskau, Prof. Dr. B. Plietker
Institut fꢀr Organische Chemie, Universitꢁt Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
Fax: (+49)711-6856-4289
E-mail: bernd.plietker@oc.uni-stuttgart.de
M. Tabassam
Institute of Chemistry, University of the Punjab
Lahore-54590 (Pakistan)
Scheme 2. Preparation of defined p-allyl iron complexes.
[**] We are grateful to the Deutsche Forschungsgemeinschaft, the
Fonds der Chemischen Industrie, the Studienstiftung des Deut-
schen Volkes (PhD grant for A.D.), the Dr.-Otto-Rꢂhm-Gedꢁchtnis-
stiftung, and the Higher Education Commission of Pakistan (grant
for M.T.) for generous financial support.
can be purified by column chromatography.[16] Reaction of the
complexes 5–7 with the malonate anion resulted in the
formation of the corresponding allyl malonates 8, 9, and 4
(Table 1). Owing to the high vapor pressure of 5–7 the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200901930.
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Table 1: Stoichiometric allylation reactions.[a]
The preformed p-allyl iron complexes are transferred into
potent catalysts for the allylic substitution upon addition of
the SIMES ligand. The regioselectivities of the catalytic
reactions are in good agreement with the selectivities
observed in the stoichiometric reactions (compare Tables 1
and 2). Subsequently, a variety of differently substituted p-
allyl iron complexes was prepared and evaluated with respect
to their relative catalytic activities in the allylic substitution of
carbonate 1 (Table 3).
Fe complex
R1
R2
Product
A:B[b]
Yield [%][b]
5
6
7
H
CH3
CH3
H
H
CH3
8
9
4
–
68
59
65
79:21
96:4
Table 3: Relative activity of different p-allyl iron complexes.[a]
[a] All reactions were performed on a 1 mmol scale in MTBE (1 mL)
under a N2 atmosphere. [b] Determined by GC integration.
complexes were used as a stock solution in methyl tert-butyl
ether (MTBE).
Entry
1
Catalyst
4A:4B[b]
Yield [%][b]
84
In analogy to the results reported by Nakanishi et al.[17]
and Mattern and Eberhardt[18], a mixture of two regioisomers
was obtained with the major isomer being formed by attack of
the nucleophile at the sterically less-hindered carbon atom of
the allyl ligand. The regioselectivities are not influenced by
additional ligands. Only the reactivity and stability of the
complexes is significantly altered. Hence, in the presence of
PPh3 the corresponding phosphane complex 5’ (see Scheme 4)
was isolated as an air- and moisture-stable solid. Its corre-
sponding NHC adduct on the other hand is thermally instable
and can not be isolated and does not undergo stoichiometric
reactions.
94:6
2
3
4
93:7
94:6
93:7
94
89
42
Knowing the regioselective course of the stoichiometric
allylation reactions we subsequently wondered whether the
preformed p-allyl iron complexes were catalytically active.
Hence the corresponding allylic carbonates were treated with
the respective allyl complex under our previously established
conditions in the presence or absence of the aryl substituted
NHC ligand SIMES (1,3-dimesitylimidazolin-2-ylidene;
Table 2).
5
94:6
84
6
7
94:6
94:6
69
95
Table 2: p-Allyl iron complexes as catalysts.[a]
[a] All reactions were performed on a 1 mmol scale in MTBE (1 mL)
under a N2 atmosphere. [b] Determined by GC integration using
n-dodecane as internal standard.
Entry
Carbonate
Cat.
5
Ligand
A:B[b]
Yield [%][c]
We were pleased to find that various p-allyl iron
complexes acted as reactive precatalysts. After extensive
optimizations we were finally able to catalyze the allylic
substitution of the model compound 16 with a significantly
reduced catalyst loading of only 1 mol% of complex 7
(Scheme 3).
1
2
3
4
5
6
10
–
–
–
traces
86
<10
78
12
89
(R1=H, R2=H)
SIMES
–
SIMES
–
11
n.d.
78:22
6
(R1=CH3, R2 =H)
1
7
(R1=R2=CH3)
SIMES
94:6
The optimized method is applicable to the alkylation of
various allyl carbonates. Independent on the constitution of
the starting material, identical regioselectivities are observed
with the major isomer being formed by attack of the
[a] All reactions were performed on a 1 mmol scale in MTBE (1 mL)
under a N2 atmosphere. KOtert-Am=KOC(CH3)2CH2CH3, potassium 2-
methyl-2-butoxide. [b] Determined by GC integration; n.d.=not deter-
mined. [c] Yields of isolated product.
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ate, methyl carbonates and tert-butyl carbonates are also
suitable leaving groups (Table 5, entries 8, 9, and 11). In
particular tert-butyl carbonate is important to prevent unde-
sired transesterification processes.[19] However, this suppres-
sion is at the expense of reactivity. Furthermore, the p-allyl
iron complex 7, because of its higher solubility and a smaller
induction period, shows a better performance than with
TBAFe after identical reaction times even at a catalyst
concentration of only 1 mol% (Table 5, entries 1–3, 5, 6, 12–
Scheme 3. The allylic substitution catalyzed by 7 and SIMES.
Table 4: Alkylation of various allyl carbonates.[a]
14). At
a catalyst loading of
2.5 mol%, however both precata-
lysts are of equal activity in the
allylation of problematic substrates
(Table 5, entries 4, 7–9, 11).
Although the monophosphane
adduct 5’ proved to be catalytically
inactive, it is a solid and hence
shows some important advantages
with regard to the overall opera-
tional simplicity of the process. We
tried to reanimate the catalytic
activity of this complex by adding
Entry
Carbonate
R3
Product(A:B)[b]
Yield [%][c]
R1
R2
R4
R5
1[d]
2[e]
3
H
H
CH3
H
H
Ph
CH3
Ph
CH3
H
H
H
H
H
H
H
H
H
H
H
H
CH3
H
H
H
H
H
H
CH3
H
H
Ph
H
CH3
CH3
Ph
CH3
H
H
H
H
H
H
H
H
H
8
81
93
82
94
89
84
96
93
81
95
97
9 (76:24)
9 (26:74)
18
17 (80:20)
17 (19:81)
19
20 (6:94)
20 (95:5)
4 (6:94)
4 (96:4)
SIMES as
a ligand. We were
pleased to find that this approach
proved successful. Although the
turnover numbers are somewhat
lower than for the same reaction in
the presence of complex 7, the
regioselective course of the reaction
remained unchanged (Scheme 4).
Herein we report, for the first
4[e]
5
6
7[d]
8
9
10
11
H
H
CH3
H
CH3
H
H
H
CH3
time, the use of structurally defined
preformed p-allyl iron complexes as
stable precatalysts for allylic substi-
tution reactions. Upon addition of
the N-heterocyclic carbene ligand
SIMES an activation of the inactive
metal complexes occurred leading
to a system that has improved
catalytic activity compared to the
previous TBAFe/SIMES system.
The improvement is a result of the
better solubility of the new system
in organic solvents and the reduced
induction period which lead to an
overall reduction of the catalyst
concentration required down to
12
13
21 (96:4)
21 (96:4)
77
82
14
22 (98:2)
94
15[d]
16[d]
H
H
H
H
H
CH2OBn
H
H
H
23 (94:6)
85
89
BnOCH2
23[f] (4:96)
[a] All reactions were performed on a 1 mmol scale in MTBE (1 mL) under a N2 atmosphere.
[b] Determined by GC integration. [c] Yields of isolated product. [d] 2.5 mol% 7, 5 mol% SIMES·PF6, and
2.5 mol% Bu4NBr. [e] 5 mol% 7, 10 mol% SIMES·PF6, and 5 mol% Bu4NBr. [f] The Z-configured
product was observed as the sole double-bond containing isomer.
nucleophile at the sterically less-hindered terminus of the allyl
ligand (Table 4).
1 mol%. Furthermore by direct comparison of the regiose-
lectivities, both in stoichiometric and catalytic allylic substi-
tutions, direct support for the p-allyl mechanism was
obtained. These results build the base for the future develop-
ment of an allylic substitution according to the principles of a
dynamic-kinetic asymmetric transformation.
Thus for the first time we were able to show that 1) p-allyl
iron complexes are catalytically active intermediates in the
presence of NHC ligands, and that 2) the observed regiose-
lective courses of the catalytic transformations in the presence
of SIMES ligand are the direct consequence of a nucleophilic
attack at a p-allyl iron complex.
Moreover a broad range of pronucleophiles is allylated in
good to excellent yields using the conditions mentioned above
(Table 5). Different functional groups are tolerated. Prelimi-
nary investigations indicated that apart from isobutylcarbon-
Experimental Section
Preparation of (p-allyl)dicarbonylnitrosyl iron complexes (general
procedure): In a 100 mL Schlenk tube [Bu4N][Fe(CO)3(NO)] (1.65 g,
4 mmol) was dissolved in methylene chloride (40 mL) under a
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Communications
Table 5: Allylation of pronucleophiles.[a]
Entry
Nucleophile
EWG1
Product (A:B)[b]
Yield [%][c,d]
EWG2
CO2iBu
COCH3
COPh
SO2Ph
CN
1
2
3
CO2iBu
CO2iBu
CO2iBu
CO2iBu
CO2iBu
SO2Ph
CN
4 (94:6)
95 (81)
78 (63)
82 (74)
58 (53)
79 (74)
85 (83)
96 (92)
67 (65)
91 (90)
24 (93:7)
25 (96:4)
26 (96:4)
27 (64:36)
28 (82:18)
29 (7:93)
30 (58:42)
31 (98:2)
4[e]
5
6
CN
CN
7[e]
8[e,f]
9[e,g]
=
CO2Me
N CPh2
Scheme 4. “Reanimation” of catalytically inactive 5’.
=
(EtO)2P O
CO2Et
10
32 (82:18)
75 (64)
Keywords: allyl ligands · homogeneous catalysis · iron ·
.
nucleophiles · regioselectivity
11[e,f]
33 (97:3)
78 (81)
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14
36 (97:3)
78 (61)
[a] All reactions were performed on a 1 mmol scale in MTBE (1 mL)
under a N2 atmosphere. [b] Determined by GC integration. [c] Yields of
isolated products. [d] Yields of isolated products from the corresponding
reaction using TBAFe as the precatalyst are given in parenthesis.
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=
=
methoxy carbonate CH2 CHC(CH3)2OCO2CH3 was employed. [g] The
tert-butoxy carbonate CH2 CHC(CH3)2OCO2tBu was employed.
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Received: April 9, 2009
Revised: July 3, 2009
Published online: August 22, 2009
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Chemie
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