Expedient Iron-Catalyzed C-H Allylation/Alkylation by Triazole Assistance with Ample Scope

Article abstract of DOI:10.1002/anie.201509603

Triazole assistance set the stage for a unified strategy for the iron-catalyzed C-H allylation of arenes, heteroarenes, and alkenes with ample scope. The versatile catalyst also proved competent for site-selective methylation, benzylation, and alkylation with challenging primary and secondary halides. Triazole-assisted C-H activation proceeded chemo-, site-, and diastereo-selectively, and the modular TAM directing group was readily removed in a traceless fashion under exceedingly mild reaction conditions. One for all: A unified strategy for iron-catalyzed C-H allylation and alkylation was developed by the use of a triazole directing group that could be cleaved under exceedingly mild conditions.

Full text of DOI:10.1002/anie.201509603

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International Edition: DOI: 10.1002/anie.201509603
German Edition: DOI: 10.1002/ange.201509603
Iron Catalysis
À
Expedient Iron-Catalyzed C H Allylation/Alkylation by Triazole
Assistance with Ample Scope
Gianpiero Cera, Tobias Haven, and Lutz Ackermann*
À
Abstract: Triazole assistance set the stage for a unified strategy
reactions, 2) a versatile catalyst that enabled a plethora of C
À
for the iron-catalyzed C H allylation of arenes, heteroarenes,
H allylation and alkylation reactions of arenes and alkenes,
and 3) a novel protocol for the removal of the TAM auxiliary
under exceedingly mild reaction conditions.
and alkenes with ample scope. The versatile catalyst also
proved competent for site-selective methylation, benzylation,
and alkylation with challenging primary and secondary
At the outset of our studies, we probed the unprecedented
À
halides. Triazole-assisted C H activation proceeded chemo-,
use of inexpensive and readily available allyl chlorides 2 in the
À
site-, and diastereo-selectively, and the modular TAM directing
group was readily removed in a traceless fashion under
exceedingly mild reaction conditions.
iron-catalyzed C H functionalization of triazolyldimethyl
(TAM) amide 1a (Table 1, see also Tables S1–S4 in the
[a]
À
Table 1: Optimization of the iron-catalyzed C H allylation.
T
he selective ortho-functionalization of carboxylic acid
derivatives is a major challenge in organic synthesis. In
contrast to established strategies,[1] transition-metal-catalyzed
À
C H alkylation has emerged as a particularly versatile
[2]
À
À
approach for step-economical C C bond formation. C H
alkylation and allylation reactions have mostly involved
catalysts based on precious 4d transition metals; however,
these protocols often suffer from long reaction times, high
reaction temperatures, and/or moderate selectivities.[3,4] Very
recently, considerable progress has been made with inex-
Entry
[Fe]
Ligand
3aa [%][b]
1
2
3
4
5
6
7
[Fe(acac)3]
[Fe(acac)3]
FeCl2
FeCl3
[Fe(acac)3]
–
dppen
dppbz
dppe
dppe
dppe
–
17
14
75
85
84
–
À
pensive iron complexes to establish novel C C forming
processes.[5] A particularly powerful approach involves the
use of a bidentate directing group developed by Daugulis and
co-workers.[6] This directing group derived from 8-amino-
[Fe(acac)3]
dppe
12[c]
[a] Reaction conditions: 1a (0.20 mmol), [Fe] (10 mol%), ligand
(15 mol%), PhMgBr (0.75 mmol), 2a (0.75 mmol), THF (0.1m), 658C,
30 min. [b] Yield of the isolated product. [c] The AQ-derived auxiliary was
used instead of the TAM group. acac=acetylacetonate, Bn=benzyl,
dppen=cis-1,2-bis(diphenylphosphino)ethylene, dppe=1,2-bis(diphe-
nylphosphino)ethane, dppbz=1,2-bisdiphenylphosphinobenzene.
À
quinoline (8-AQ) enables ortho-selective C H allylation and
alkylation reactions, as were devised by Nakamura and co-
workers, with important contributions by Cook and co-
workers.[7] These transformations were restricted to the AQ
motif, the structural modification of which is challenging.
More importantly, the removal of the bidentate directing
group generally called for harsh reaction conditions, namely,
concentrated HCl at 1308C.[6,7] To address these limitations,
our research group has recently established a novel family of
highly modular bidentate directing groups,[8] which are
Supporting Information).[9] The iron-catalyzed C H allyla-
À
tion with substrate 2a proceeded most efficiently with
catalysts derived from FeCl3 or Fe(acac)3 and the ligand
dppe (Table 1; entries 1–5). A test reaction verified that the
iron catalyst was essential. Notably, the AQ-derived auxiliary
was found to be ineffective under otherwise identical reaction
conditions (Table 1; entry 7), thus illustrating the unique
features of the TAM directing group. Besides allyl chlorides 2,
À
particularly effective in promoting iron-catalyzed C H trans-
formations.[8a,c]
À
Within our program on sustainable C H functionaliza-
tion, we have now devised a unified strategy for iron-
catalyzed C H allylation/alkylation through triazole assis-
tance. Notable features of our approach include 1) the use of
À
allylic phosphonates, sulfonates, carbonates, and acetates
proved to be viable electrophiles for the C H allylation.
[9]
À
À
inexpensive iron compounds for challenging C H activation
The optimized catalyst proved amenable to the trans-
formation of various triazole-functionalized benzamides
1 (Table 2). The spiro substitution pattern in the amide
backbone of substrate 1b did not diminish the yield (Table 2,
entry 1), and also N-alkylated or N-arylated triazoles pro-
vided the desired products 3ca and 3da efficiently (entries 2
and 3). Notably, our triazolylmethylamide (TAH)[8b,d] direct-
[*] Dr. G. Cera, T. Haven, Prof. Dr. L. Ackermann
Institut für Organische und Biomolekulare Chemie
Georg-August-Universität
Tammannstrasse 2, 37077 Gçttingen (Germany)
E-mail: Lutz.Ackermann@chemie.uni-goettingen.de
Homepage: http://www.ackermann.chemie.uni-goettingen.de
À
ing group (in 1e) was found to be unsuitable for the C H
transformation (Table 2, entry 4).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201509603.
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Table 2: Influence of the substitution pattern.[a]
products 3sa and 3ta occurred site-selectively, and the
unprecedented synthesis of ferrocenyl derivative 3ra was
also possible. Notably, the optimized catalyst was also
À
applicable to the first iron-catalyzed C H allylation of
alkenes. Thus, products 3ua–za were accessible in good
yields with excellent diastereoselectivity.[9]
Subsequently, we explored the versatility of the optimized
catalytic system with differently substituted allyl chlorides 2
(Scheme 2). Thus, with (E)-crotyl chloride (2b), the branched
Entry
R1
R2
R3
3
1
2
3
4
-(CH2)5-
Bn
nPr
PMP
nHex
3ba: 64%
3ca: 77%
3da: 78%
3ea: –
Me
Me
H
Me
Me
H
[a] Reaction conditions: 1 (0.20 mmol), [Fe(acac)3] (10 mol%), dppe
(15 mol%), PhMgBr (0.75 mmol), 2a (0.75 mmol), THF (0.2m), 658C,
30 min. Yields are for the isolated products. PMP=p-methoxyphenyl.
In terms of the substrate scope, benzamides 1 with ortho-,
meta-, or para-substituents were smoothly converted into the
desired products 3 fa–qa (Scheme 1). The allylation of
heteroaromatic thiophene and pyrrole derivatives to give
À
Scheme 2. Iron-catalyzed C H allylation with substituted allyl chlorides
2.
allylated benzamide 3ab was obtained in good yield with
moderate regioselectivity.[10] To gain insight into the reaction
mechanism, we next used the secondary allyl chloride 2c,
which furnished the product 3ac with a comparable regiose-
lectivity. These findings provided strong support for the
formation of an h3-allyl intermediate, whereby the regiose-
lectivity is dominated by the nature of the phosphine ligand,
in resemblance of Plietkerꢀs observations on iron-catalyzed
coupling reactions.[11] Thereafter, several E-configured allyl
chlorides were tested. For instance, substrates 2d–f furnished
the corresponding branched allylated benzamides 3ad–af by
chelation assistance. Furthermore, the use of a Z-allyl
chloride, 2g, resulted in the formation of product 3ag with
good selectivity for the branched product. The b-substituted
allyl chloride 2h also proved to be a viable substrate for the
À
Scheme 1. Iron-catalyzed C H allylation with allyl chloride (2a).
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À
À
iron-catalyzed C H activation. Finally, we explored the
influence of the triazole auxiliary in the synthesis of arenes
3bb and 3db, which were obtained in high yields and with
good selectivity for the branched isomers.
were alkylated at the less congested C H bond, and
thiophenes 5sk and 5ssk were also accessible.
Intriguingly, more challenging secondary alkyl bromides
6a–e also proved to be competent substrates under the
optimized reaction conditions (Scheme 4a). A variety of sec-
alkylated benzamides 7aa–ae were thus synthesized in a site-
selective fashion. The exo-norbornyl bromide 6c gave selec-
tively the exo product 7ac. Also, acyclic 2-bromopropane (6e)
was identified as a suitable substrate. The protocol proved to
We investigated the reaction mechanism by carrying out
competition experiments with isotopically labeled substrates
and found considerable intra- and intermolecular kinetic
isotopic effects (KIEs) of kH/kD ꢀ 1.8 and 2.1, respectively,[9]
À
thus suggesting that C H cleavage occurs as or prior to the
À
turnover-limiting step. Moreover, the use of the radical
scavenger 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) in
stoichiometric amounts led only to a slight decrease in the
yield to 65%, which indicates that a radical-based reaction
mechanism is less likely to be operativehere.
be widely applicable, and hence allowed for the C H
activation of meta-substituted benzamides to furnish products
In consideration of the unique catalytic activity of our
À
TAM-assisted iron-catalyzed C H allylation, we tested the
À
feasibility of challenging C H alkylation reactions
(Scheme 3). To our delight, the direct alkylation of benzamide
À
Scheme 3. TAM-assisted iron-catalyzed C H alkylation with primary
alkyl halides 4.
1 proceeded well with various alkyl bromides 4a–f to deliver
compounds 5aa–af under our standard reaction conditions.
Furthermore, methyl iodide (4g) was identified as a suitable
organic electrophile. With this reagent, benzamide 5ag was
formed in good yield with excellent monoalkylation selectiv-
ity.[12] Furthermore, C H benzylation was observed with
À
benzyl chloride (4h). The robustness of the iron catalyst was
reflected by its tolerance of ether and ester functionalities,
among others. Benzamides 1 f,o,p bearing meta-substituents
À
Scheme 4. Iron-catalyzed C H alkylation with secondary alkyl halides 6
and mechanistic findings.
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6-bromohex-1-ene (4l) to the optimized reaction conditions
and observed the formation of the cyclized product 5al.
Second, the use of cyclopropylmethyl bromide (4m) afforded
the homoallylated product 5am. Next, the presence of radical
inhibitors, such as TEMPO, led in this case to a significantly
decreased conversion of 20%.[9] Furthermore, the reaction of
diastereomerically pure cis-1-bromo-4-methylcyclohexane
(6 f) with 1a delivered the alkylated benzamide 7af with
almost complete erosion of the stereochemical information.
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[15]
(Scheme 5).[14] Thus, NOBF4 promoted TAM cleavage at
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Scheme 5. Mild removal of the TAM directing group.
À
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À
iron-catalyzed C H allylation, benzylation, methylation, and
alkylation with primary as well as secondary alkyl halides.
À
Thus, triazole assistance enables the site-selective C H
functionalization of arenes, heteroarenes, and alkenes. Our
strategy features the use of a removable directing group,
which was cleaved in a traceless fashion under exceedingly
mild reaction conditions.
Acknowledgements
Generous support by the Alexander von Humboldt Founda-
tion (fellowship to G.C.) and the European Research Council
under the Seventh Framework Program of the European
Community (FP720072013; ERC Grant Agreement No.
307535) is gratefully acknowledged.
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À
Keywords: alkylation · allylation · C H activation · iron ·
triazoles
How to cite: Angew. Chem. Int. Ed. 2016, 55, 1484–1488
Angew. Chem. 2016, 128, 1506–1510
À
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[9] For details, see the Supporting Information.
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Received: October 13, 2015
Published online: December 11, 2015
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