Tmp4Zr: An atom-economical base for the metalation of functionalized arenes and heteroarenes

Article abstract of DOI:10.1002/anie.201003558

Zirconium bases reach lots of places: Only 25 % of zirconium metal is required for the metalation of various functionalized aromatic compounds and heterocycles. All four 2,2,6,6-tetramethylpiperidyl (tmp) groups of the new base tmp₄Zr.4 MgCl

Full text of DOI:10.1002/anie.201003558

Communications
DOI: 10.1002/anie.201003558
Directed Zirconation
tmp4Zr: An Atom-Economical Base for the Metalation of
Functionalized Arenes and Heteroarenes**
Masilamani Jeganmohan and Paul Knochel*
The functionalization of aromatic and heterocyclic ring
systems is of central importance in synthetic organic chemis-
try, beacuse polyfunctional unsaturated units are present in
numerous pharmaceuticals and advanced materials.[1]
Directed metalation is a convenient method for performing
such synthetic tasks. Whereas directed lithiations have been
very popular,[2] the recent development of kinetically highly
active 2,2,6,6-tetramethylpiperidyl (tmp) bases[3,4] of various
transition metals (Mn,[5] Fe,[6] and Ln[7]) that can be solubi-
lized by complexation with lithium chloride allows a unique
and simple preparation of polyfunctional Mn, Fe, or La
unsaturated organometallic complexes. Early-transition-
metal organometallic complexes have a particular reactivity
pattern.[8,9] Especially, complexes derived from Ti and Zr are
attractive due to their unusual reaction chemistry, the low cost
of these metals, and their low toxicity.[9] Herein, we report a
new THF-soluble kinetically highly active zirconium base:[10]
tmp4Zr·4MgCl2·6LiCl (abbreviated as tmp4Zr (1)) which for
the first time allows a direct zirconation of various function-
alized aromatics and heterocycles of type 2 (scheme 1).
Remarkably, this base is especially atom economical regard-
ing the amount of transition-metal (0.25 equiv) and the use of
tmp groups.[11] All four tmp groups are used effectively in the
metalation process allowing the preparation of a range of new,
highly functionalized tetraorganozirconium derivatives of
type 3 under mild reaction conditions (Scheme 1). Thus, the
reaction of ethyl 3-fluorobenzoate (2a, 1 equiv) with 1
(0.25 equiv) at 258C for 45 min provides the tetraarylzirco-
nium reagent 3a in over 95% yield (as indicated by iodolysis
or allylation of reaction aliquots). The zirconated reagent 3a
displays a remarkable reactivity. Its allylation with ethyl (2-
bromomethyl)acrylate (1.05 equiv, CuCN·2LiCl (5 mol%),
08C, 1 h) furnished the diester 5a, isolated in 88% yield
(Scheme 2). The addition to benzaldehyde (1.05 equiv, 08C,
2 h) led to the lactone 5b in 86% yield. A direct palladium-
catalyzed cross-coupling without further transmetalation[12a]
using [Pd(PPh3)4] (3 mol%, 258C, 8 h) afforded the biphenyl
derivative 5c in 77% yield. Styrene oxide was opened by the
zirconium reagent 3a at 508C within 1 h providing after
workup the annelated lactone 5d in 65% yield. Interestingly,
the new tetraarylzirconium species (3a) reacts smoothly with
CO2 (1 atm, 508C, 8 h) furnishing the corresponding acid 5e,
isolated in 84% yield.[12b,c] Similarly, a range of zirconated
aromatic substrates undergo this carboxylation reaction as
well leading to the polyfunctional benzoic acids (5 f–j) in 79–
89% yields (Scheme 2).
A range of substituted benzoates were zirconated with 1
(0.25 equiv) at temperatures between À158C and 258C (see
entries 1–10 of Table 1). The resulting zirconated aromatics
2b–j react with various electrophiles. Thus, a copper(I)-
catalyzed acylation[12d] with 2-chlorobenzoyl chloride
(1.0 equiv, CuCN·2LiCl (25 mol%), À208C, 1 h) gave the
benzophenone 5k in 82% yield (entry 1 of Table 1). The
addition of 2b to cyclohexane carbaldehyde (1.0 equiv, 08C,
2 h) led to the lactone 5l in 76% yield (Table 1, entry 2). A
palladium-catalyzed cross-coupling of the zirconated aromat-
ics 2c–e with various aryl iodides proceeded in each case
directly without the need of an additional transmetalation,
furnishing the functionalized biphenyls 5m–o in 78–81%
yields (Table 1, entries 3–5). The stannylation of the tetraar-
ylzirconium reagent derived from 2 f using Me3SnCl
(1.0 equiv, 08C, 2 h) gave the arylstannane 5p in 75% yield
(Table 1, entry 6). Whereas, the zirconation of 3-fluoroanisole
(2g) with 1 at 258C led to a substantial formation of
benzyne,[9d] this decomposition pathway can be avoided by
performing the zirconation of 2g at À208C (0.25 equiv of 1,
3 h). Quenching of the resulting tetraarylzirconium com-
pound with thiophene-2-carbonyl chloride (1.0 equiv, CuCN·
2LiCl (25 mol%), À208C, 1 h) led to the ketone 5q in 83%
yield (Table 1, entry 7). The benzonitrile 2h was metalated by
1 (0.25 equiv, 08C, 45 min) in position 2. A cross-coupling
Scheme 1. Preparation of 1 and its reaction with aromatics or hetero-
cycles of type 2.
[*] Dr. M. Jeganmohan, Prof. Dr. P. Knochel
Department Chemie, Ludwig-Maximilians-Universitꢀt Mꢁnchen
Butenandtstrasse 5–13, Haus F, 81377 Mꢁnchen (Germany)
Fax: (+49)89-2180-77680
E-mail: Paul.Knochel@cup.uni-muenchen.de
[**] We thank the European Research Council (ERC) for financial
support. M.J. thanks the Humboldt Foundation for a fellowship. We
also thank BASF AG (Ludwigshafen), W. C. Heraeus GmbH
(Hanau), and Chemetall GmbH (Frankfurt) for the generous gift of
chemicals. tmp4Zr=tmp4Zr·4MgCl2·6LiCl; tmp=2,2,6,6-tetrame-
thylpiperidyl.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003558.
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the bromoester 2j leading to the biphenyl 5t
in 83% yield after palladium-catalyzed cross-
coupling with 4-trifluoromethyl-1-iodoben-
zene (0.9 equiv, 258C, 8 h; Table 1, entry 10).
Sulfamates such as 2k,l are also readily
metalated in the position ortho to the sulfa-
mate with 1. After copper-catalyzed acylation
with various acid chlorides, the polyfunctional
ketones 5u,v were obtained in 79–80% yields
(Table 1, entries 11 and 12).
The functionalization of heterocycles is of
key importance for the preparation of novel
pharmaceuticals and agrochemicals.[13a]
A
range of heterocyclic structures could be
zirconated using 1. Thus, chromone (2m)
reacted smoothly with 1 (0.25 equiv, À358C,
30 min) affording the expected zirconium
derivative 3m in approximately 90% yield.
Copper-catalyzed allylation[12d] with cinnamyl
chloride (À408C, 1 h) provided the SN2-sub-
stitution product 6a as a single regioisomer in
76% yield (Scheme 3).
The functionalization of electron-poor
pyridines is especially challenging, because
of the high tendency of such metalated
pyridines to polymerize.[13b,c] Also, the regio-
selective metalation of substituted pyridines
can also be complicated.[13d,e] Treatment of
3-bromopyridine (2n) with 1 at À108C for
Scheme 2. Preparation of tetraarylzirconium reagent 3a using 1 and its reactivity with
various electrophiles.
with 4-chloro-1-iodobenzene (0.8 equiv, 258C, 8 h) using
[Pd(PPh3)4] (3 mol%) gave the biphenyl 5r in 73% yield
(Table 1, entry 8). By treating the ester-substituted aromatic
nitrile 2i with 1 (0.25 equiv) at À158C for 1.5 h, a regiose-
lective zirconation at the position ortho to the ester function
led, after addition of 4-methoxybenzaldehyde, to the lactone
5s in 79% yield (Table 1, entry 9). A regioselective metal-
ation with 1 (0.25 equiv, 08C, 45 min) was also observed for
45 min provided a regioselective zirconation in position 2
leading to the tetrapyridylzirconium species 3n. After a
palladium-catalyzed cross-coupling with 4-trifluoromethyl-1-
iodobenzene (508C, 8 h), the expected pyridine 6b was
obtained in 79% yield. The sensitive 4-cyanopyridine (2o)
also reacted smoothly with 1 within 15 min at À408C furnish-
Scheme 4. BF3-mediated metelation of N-heterocycles using
Scheme 3. Direct zirconation of various heterocycles with 1.
tmp4Zr·4BF3.
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Table 1: Regioselective zirconation of arenes with 1 and reactions with electrophiles.
Entry
Substrate[a]
Electrophile
Product[b]
Entry
Substrate[a]
Electrophile
Product[b]
1
2
2b (25, 0.45)
2b (25, 0.45)
5k: 82%[c]
7
8
2g (À20, 3)
2h (0, 0.75)
5q: 83%[c]
5l: 76%
5r: 73%[d]
3
4
2c (25, 0.30)
2d (25, 2.5)
5m: 79%[d]
9
2i (À15, 1.5)
2j (0, 0.75)
5s: 79%
5n: 78%[d]
10
5t: 83%[d]
5
6
2e (0, 2)
5o: 81%[d]
11
12
2k (25, 2)
5u: 79%[c]
2 f (0, 2)
5p: 75%
2l (25, 4)
5v: 80%[c]
[a] The reaction conditions for the metalation with 1 are given in parentheses (T [8C], t [h]). [b] Yield of isolated analytically pure product. [c]
CuCN·2LiCl (1.0m in THF, 0.25 mL, 0.25 mmol) was added. [d] [Pd(PPh3)4] (3 mol%) was used as catalyst.
ing regioselectivly the tetra(2-pyridyl)zirconium derivative
3o which, after copper-catalyzed allylation with
3-bromocyclohexene, gave the 2,4-disubstituted pyridine 6c
in 80% yield (Scheme 3).
Recently, we have shown that BF3·OEt2 is compatible with
the presence of strong Lewis bases, such as tmpMgCl·LiCl or
tmp2Zn·2MgCl2·2LiCl.[14] The Lewis pairs formed[15] are
reversibly cleaved in the presence of a substrate, such as a
pyridine or a quinoline. The new tmp4Zr base 1 displays a
similar behavior. Thus, the reaction of 6-methoxyquinoline
the 3,4-disubstituted pyridine 6e is obtained in 87% yield
(Scheme 4). This direct formation of N-heteroaromatic tri-
fluoroborate complements the well-established preparation
of these boron intermediates as described by Molander and
Canturk.[16] A number of heterocycles, such as coumarin (2r,
Table 2, entry 1), 2-phenyl-1,3,4-oxadiazole (2s, Table 2,
entry 2), electron-deficient 3-substituted pyridines (2n, 2t,
2u; Table 2, entries 3–6), quinoline (2v, Table 2, entry 7),
3-bromoquinoline (2w, Table 2, entry 8), and 2-methylthio-
pyrazine (2x, Table 2, entry 9) could be zirconated either
directly or in the presence of BF3·OEt2 (Table 2). The
corresponding zirconated species efficiently reacted with
various electrophiles, such as allylic halides, aryl iodides, or
iodine, to give polyfunctional heterocycles in high yields
(Table 2, entries 1–9).
(2p) with
a mixture of 1 (0.25 equiv) and BF3·OEt2
(1.05 equiv, 08C, 45 min) provides the mixed Zr–B species
3p (Scheme 4). The formation of a 2-quinolyltrifluoroborate
3p was confirmed by 13C-, 19F-, and 11B NMR spectroscopy
(see the Supporting Information). Palladium(0)-catalyzed
cross-coupling with ethyl 4-iodobenzoate furnished the
2-functionalized quinoline 6d in 79% yield. Also, the
sensitive 3-carbethoxypyridine 2q was converted at À788C
(15 min) by the reaction with 1 (0.25 equiv) into the
pyridyltrifluoroborate 3q. After copper-catalyzed allylation,
In summary, we have developed a directed zirconation of
functionalized aromatic and heterocyclic compounds using
the new base tmp4Zr·4MgCl2·6LiCl (1). In contrast to most
other aryl organometallic complexes, the resulting aryl
zirconated species display an excellent reactivity toward
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2.05 mmol based on tmp) was added dropwise and the mixture was
Table 2: Zirconation of functionalized heterocycles with the base 1 and
subsequent reactions with electrophiles.
stirred at 258C for 45 min. The reaction mixture was cooled to 08C,
then CuCN·2LiCl (1m in THF, 0.1 mL, 0.1 mmol) and ethyl
2-(bromomethyl)acrylate (400 mg, 2.1 mmol) were added and stirred
for 1 h at 08C. The reaction mixture was quenched with a saturated
aqueous NH4Cl solution (25 mL), extracted with diethyl ether (3 ꢀ
50 mL), and dried over anhydrous MgSO4. After filtration, the
solvents were evaporated in vacuo. The crude product was purified by
flash chromatography (silica gel; pentane/diethyl ether, 10:1) to give
5a (449 mg, 88%) as a colorless oil.
Entry
Substrate[a]
Electrophile
Product[b]
1
2
2r (À40, 30)
2s (0, 20)
6 f: 78%[c]
6g: 79%[c]
Received: June 11, 2010
Published online: September 13, 2010
Keywords: copper catalysis · cross-coupling · frustrated bases ·
palladium · zirconium amides
.
3
4
2t (0, 30)
6h: 82%[d]
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6
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7
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9
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smooth palladium-catalyzed cross-couplings (without further
transmetalation) leading to a range of new polyfunctional
unsaturated compounds. Further extension of the metalation
capabilities of 1 are currently under investigation.
Experimental Section
Typical Procedure: Synthesis of 5a: A dry argon-flushed 25 mL
Schlenk-flask, equipped with a magnetic stirring bar and a septum,
was charged with ethyl 3-fluorobenzoate (2a; 336 mg, 2 mmol) in dry
THF (2 mL). The tmp4Zr·4MgCl2·6LiCl (1, 0.5m in THF, 4.2 mL,
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