Directed meta-metalation using alkali-metal-mediated zincation

Article abstract of DOI:10.1002/anie.200600720

(Figure Presented) Move over directed ortho-metalation, here comes directed meto-metalation. The unprecedented meto-zincation of two anilides by a synergic mixed sodium/zinc dialkyl-amide base is presented. The orientation of the deprotonation of the anil

Full text of DOI:10.1002/anie.200600720

Angewandte
Chemie
Aromatic Substitution
DOI: 10.1002/anie.200600720
Directed meta-Metalation Using Alkali-Metal-
Mediated Zincation**
David R. Armstrong, William Clegg, Sophie H. Dale,
Eva Hevia, Lorna M. Hogg, Gordon W. Honeyman,
and Robert E. Mulvey*
Transforming a carbon–hydrogen bond of an organic com-
pound into a more useful, more reactive carbon–metal bond
(so-called deprotonative metalation), which, in turn, can be
treated with an electrophile to create a new carbon–carbon or
carbon–heteroatom bond, is one of the most fundamental
synthetic approaches that chemists employ to construct
compounds.[1,2] Many of these reactions involve a special
type of deprotonative metalation, in which an activating
functional group is positioned adjacent to the hydrogen atom
(strictly a proton) that is to be replaced by the metal cation.
[*] Dr. D. R. Armstrong, Dr. E. Hevia, L. M. Hogg, Dr. G. W. Honeyman,
Prof. R. E. Mulvey
WestCHEM
Department of Pure and Applied Chemistry
University of Strathclyde
Glasgow, G11XL (UK)
Fax: (+44)141-552-0876
E-mail: r.e.mulvey@strath.ac.uk
Prof. W. Clegg, Dr. S. H. Dale
School of Natural Sciences (Chemistry)
University of Newcastle
Newcastle upon Tyne, NE17RU (UK)
[**] This work was supported by the EPSRC (UK) through grant award
no. GR/R81183/01 and GR/T27228/01, a Marie Curie Fellowship
(EU) to E.H., and the Royal Society/Leverhulme Trust through a
Fellowship to R.E.M.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Directed ortho-metalation (DOM, in which the metal is
yield.[13–15] Changing the metalation agent from these main-
stream homometallic species to heterometallic [(tmeda)·Na-
(tBu)(tmp)Zn(tBu)]( 2), a sodium TMP/zincate reagent
recently introduced by our group, remarkably switches the
orientation of the deprotonation to the meta site
(Scheme 1).[16,17] The carbanion generated by this regiospe-
usually lithium) represents the best-developed, most heavily
utilized technique of this type.[3,4] Defined as a metalation
(lithiation) of an aromatic ring directed towards a position
adjacent (ortho) to an activating heteroatom-containing
functional group, DOM now probably surpasses classical
electrophilic aromatic substitution as the number
one methodology for constructing regiospecifically
substituted aromatic rings.[5] DOM is therefore
practiced routinely in synthetic laboratories all
across the world, both in academia, for general
preparative purposes, and in industry, for the
manufacture of fine chemicals, pharmaceuticals,
and other biologically important aromatic com-
pounds.
The tools most commonly employed for effect-
ing DOM and the other types of metalation are
single-metal (homometallic) reagents, typically alkyl
lithium or lithium amide reagents. Recently, we and
a few other research groups have begun to explore
Scheme 1. Synergic metalation of N,N-dimethylaniline and its 3-methyl derivative
the behavior of related but mixed-metal (hetero-
bimetallic) reagents with amide-based ligands in this
important area of directed metalation.[6–10] These
initial studies have established that in certain cases
using a sodium TMP/zincate base.
cific meta-proton abstraction is stabilized through coordina-
these mixed-metal alternatives can exhibit a remarkable
chemical synergy that enables them to perform special
metalation reactions that cannot be reproduced by either of
the single-metal components that make up the mixed-metal
reagent.[11] In these synergic metalation reactions, the depart-
ing hydrogen atom is replaced by magnesium or zinc, but the
participation of an alkali metal is mandatory for the reaction
to follow its special course. Metalation reactions of this type
are therefore best regarded as an alkali-metal-mediated
magnesiation (AMMM) or alkali-metal-mediated zincation
(AMMZ).
tion to a bimetallic alkyl amido cation (the residue of the base
following alkyl transfer), which advantageously permits the
metalated product of the reaction [(tmeda)·Na(m-Ar*)(m-
tmp)Zn(tBu)]( 3; Ar* = 3-C6H4NMe2) to be obtained in a
stable crystalline form, which is isolable at room temperature.
Direct zincation of a tertiary anilide is not possible using
mainstream alkyl zinc reagents, so the unprecedented
carbon–hydrogen to carbon–zinc transformation accom-
plished can be attributed to the synergy inherent in AMMZ.
1
Characterized in solution by H and 13C NMR spectroscopic
studies, 3 was also subjected to an X-ray crystallographic
study, which unambiguously established its molecular struc-
ture (Figure 1).[18] A four-element NaNZnC rhomboidal ring
with a mixed TMP Ar* bridging ligand set forms the core,
which is completed by terminal TMEDA (N,N-attached) and
In the context of the new work reported herein, the best
example of synergic metalation to date has been achieved
with the alkyl arene toluene. Conventional metalation
reagents, such as the alkyl lithium BuLi·TMEDA
(TMEDA = N,N,N’,N’-tetramethylethylenediamine),
nor-
mally deprotonate toluene at the methyl (lateral) site to
generate the resonance-stabilized benzyl “carbanion”
ꢀ
(PhCH2 ). In contrast, under AMMM conditions using the
sodium–magnesium
monoalkyl
bisamido
reagent
[(tmeda)·Na(m-Bu)(m-tmp)Mg(tmp)](TMP
= 2,2,6,6-tetra-
methylpiperidide), the site of deprotonation is switched to
the meta site on the ring and the methyl substituent remains
intact.[12] Although the regiospecificity of this deprotonation
is most unusual, it does not infringe any expected DOM
effect, as toluene does not possess a heteroatom-containing
functional group. To overcome any DOM effect and direct
metalation to a normally inaccessible site would be excep-
tional news for chemists that could have major implications
for synthetic chemistry. We report herein two case studies that
prove that such a desirable situation can be engineered using
the special chemistry inherent in AMMZ.
As N,N-dimethylaniline (1) possesses an activating dime-
thylamino substituent, it undergoes directed ortho-metalation
with phenyllithium in poor yield or n-butyllithium in good
Figure 1. Molecular structure of 3 with selective atom labeling. Hydro-
gen atoms, apart from those of the anilide ring, and minor disorder
components are omitted for clarity.
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tBu (C-attached) ligands on the Na and Zn centers, respec-
tively. In general terms, the structure can be categorized as an
ion-contacted heterotrianionic zincate. Its key geometrical
feature concerns how the deprotonated anilide engages
differently with the two metal atoms: Zn forms a short
bond to the meta C(3) atom (bond length: 2.035(4) Š), which
is close to coplanarity with the aryl ring plane (the bond is
inclined at 4.58 to the plane), whereas the Na–C(3) bond is
significantly longer (2.691(4) Š) and inclined at an angle of
76.18 to the same plane. This parallel/perpendicular distinc-
tion to the deprotonated substrate–metal interactions, which
implies more s character for the former and more p character
for the latter, has become a signature feature of these mixed-
metal compounds,[10] which may provide a vital clue to the
origin of the synergy (see below).
The second case study focused on 3-methyl-N,N-dimethyl-
aniline (4), chosen as it has both a methyl (cf toluene) and a
dimethylamino substituent on its aromatic ring. To our
knowledge, 4 has never been previously metalated, though
its 2- and 4-methyl isomers undergo preferential lithiation
ortho to the activating dimethylamino substituent (methyl
deprotonation in the case of the 2-methyl isomer).[19] How-
ever, applying AMMZ to 4 using the same reagent 2, also
achieves a meta-orientated deprotonation (Scheme 1), man-
ifested in the crystalline product [(tmeda)·Na(m-Ar**)(m-
tmp)Zn(tBu)]( 5; Ar** = 5-(3-Me)C6H3NMe2). Character-
ized by NMR spectroscopic and X-ray crystallographic
studies,[16] 5 adopts a molecular structure (Figure 2) remark-
ably similar to that of 3, which includes the parallel (s)/
perpendicular (p) distinction in the aryl–metal bonding
(corresponding angles: 4.5 and 67.88, respectively).
Figure 3. Relative energy sequence of the four theoretical regioisomers
(3A–D) of the experimentally observed product 3.
the energy minimum structure (relative energy: 0.00 kcal
molꢀ1); the ortho isomer is destabilized further (at 4.53 kcal
molꢀ1); and the least stable of all is the methyl isomer (at
8.95 kcalmolꢀ1). Through modeling studies, the overall reac-
tion (2!3B + tBuH) is found to be exothermic by
ꢀ22.21 kcalmolꢀ1. Comparison of the dimensions of 3B and
3C show that it is noticeable that the Na contacts to the aryl C
atoms are invariably shorter in 3B (bond lengths: 2.594, 3.070,
3.212, 3.959, 4.097, 4.403 Š) than in 3C (bond lengths: 2.596,
3.178, 3.239, 4.135, 4.185, 4.596 Š). By contrast the Zn–
C(aryl) bonds are within 0.001 Š of each other (2.084 and
2.083 Š, respectively). In the ortho isomer 3 A, the product
expected from conventional metalation, the Na–C(ortho)
bond is much longer (2.747 Š) and thus weaker than the Na–
C(meta) bond in 3B (2.594 Š) as a result of increased steric
constraints. Therefore, the important distinction lies in max-
imizing the strength of the Na···Cp contacts, and although
they may be weak individually, collectively they must
contribute significantly to the stability of 3B. This favorable
enthalpic effect, coupled with the minimal structural reor-
ganization required by reagent 2 to form product 3 (in effect
there is retention of structure except for the deprotonated
anilide that refills the position vacated by the tBuꢀ base),
appears to be a major factor in the unexpected meta
selectivity observed.
To establish whether these meta-zincated dimethyl aniline
complexes could be intercepted by electrophiles, we have
carried out a preliminary iodolysis reaction of isolated crystals
1
of 3 in a solution of THF with I2. On the basis of H NMR
Figure 2. Molecular structure of 5 with selective atom labeling. Hydro-
gen atoms, apart from those of the anilide ring, and minor disorder
components are omitted for clarity.
experiments with ferrocene as an internal standard, iodina-
tion at the meta position was quantitative within experimental
error (ꢁ 5%), thus producing N,N-dimethyl-3-iodoaniline.[20]
This augurs well for opening up a gateway for meta-
substituted derivatives previously closed to synthetic aro-
matic chemistry.
Theoretical calculations at the DFT level employing the
B3LYP method and the 6-311G** basis set were used to
compute the relative stabilities of the four regioisomers of 3,
in which the anilide ring is deprotonated at the ortho (3A),
meta (3B), para (3C), or methyl (3D) positions (Figure 3).[17]
In support of the experimental findings, the meta isomer 3B is
Experimental Section
Synthesis of 3 and 5: A solution of tBu2Zn (0.358 g, 2 mmol) in hexane
(10 mL) was transferred by cannula to a suspension of NaTMP in
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hexane (prepared in situ by reaction of BuNa (0.16 g, 2 mmol) with
[10]Mixed lithium–Grignard reagents are also making a consider-
able impact in metal–halogen exchange reactions, see: a) A.
Krasovskiy, P. Knochel, Angew. Chem. 2004, 116, 3396; Angew.
Chem. Int. Ed. 2004, 43, 3333; b) S. Kii, A. Akao, T. Iida, T.
Mase, N. Yasuda, Tetrahedron Lett. 2006, 47, 1877.
TMP(H) (0.34 mL, 2 mmol)). This was followed by the addition of a
molar equivalent of TMEDA (0.30 mL, 2 mmol). The resultant
suspension was heated gently to form a yellow solution. At this stage,
one molar equivalent (2 mmol) of the relevant dimethylaniline was
introduced, and the reaction mixture was refluxed at 658C for 2 h.
The resulting yellow solution was transferred to the freezer to aid
crystallization. A crop of transparent crystals formed in solution
which were suitable for X-ray crystallographic analysis in yields of
39% for 3 (0.38 g) and 43% for 5 (0.46 g) for the first batch of isolated
crystals. See Supporting Information for the labeling scheme.
[11]For a perspective article, see: R. E. Mulvey, Organometallics
2006, 25, 1060.
[12]P. C. Andrikopoulos, D. R. Armstrong, D. V. Graham, E. Hevia,
A. R. Kennedy, R. E. Mulvey, C. T. OꢀHara, C. Talmard, Angew.
Chem. 2005, 117, 3525; Angew. Chem. Int. Ed. 2005, 44, 3459.
[13]Activation of N,N-dimethylaniline is primarily through an
inductive, acidifying effect of the N atom; the coordination
effect of the N atom is thought to be less important as a result of
its conjugation with the p system of the ring; see reference [5],
page 45.
3: 1H NMR (400 MHz, 258C, C6D6): d = 7.34 (d, 1H, Hortho
(Ar*)), 7.17 (t, 1H, Hmeta’ (Ar*)), 7.10 (d, 1H, Hortho (Ar*)), 6.54 (m,
1H, Hpara (Ar*)), 2.63 (s, 6H, N(CH3)2 (Ar*)),1.65 (s, 9H, tBu), 1.58
(s, broad, 14H, CH3 (TMEDA) and Hg (TMP)), 1.52 (s, broad, 14H,
CH3 (TMP), Hb (TMP), and CH2 (TMEDA)), 1.16 ppm (s, broad, 6H,
CH3 (TMP)); 13C{1H} NMR (100.63 MHz, 258C, C6D6): d = 170.79
[14]G. Wittig, H. Merkle, Chem. Ber. Dtsch. Chem. Ges. 1942, 75,
1491.
ꢀ
(Zn Cmeta (Ar*)), 150.87 (Cipso (Ar*)), 128.87 (Cmeta’ (Ar*)), 127.18
[15]A. R. Lepley, W. A. Khan, A. B. Giumanini, A. G. Giumanini, J.
Org. Chem. 1966, 31, 2047.
(Cpara (Ar*)), 124.48 (Cortho (Ar*)), 111.18 (Cortho’ (Ar*)), 57.58 (CH2
(TMEDA)), 53.22 (Ca (TMP)), 46.46 (CH3 (TMEDA)), 40.41 (Cb
(TMP)), 41.03 (N(CH3)2 (Ar*)), 36.75 (CH3 (TMP)), 36.01 (CH3
(tBu)), 35.47 (CH3 (TMP)), 21.10 (Cg (TMP)), 21.01 ppm (C (tBu)).
5: 1H NMR (400 MHz, 258C, C6D6): d = 7.08 (d, 1H, Hortho
(Ar**)), 6.96 (s, broad, 1H, Hpara (Ar**)), 6.37 (s, broad, 1H, Hortho’
(Ar**)), 2.68 (s, 6H, N(CH3)2 (Ar**)), 2.25 (s, 3H, CH3 (Ar**)), 1.68
(s, broad, 14H, CH3 (TMEDA) and Hg (TMP)), 1.66 (s, 9H, tBu), 1.58
(s, 4H, CH2 (TMEDA)), 1.53 (s, broad, 6H, CH3 (TMP)), 1.31 (m,
4H, Hb (TMP)), 1.21 ppm (s, broad, 6H, CH3 (TMP)); 13C{1H} NMR
[16]Complexation of N,N-dimethylaniline with [Cr(CO)3]leads to a
mixture of ortho, meta, and para deprotonation upon lithiation;
see: R. J. Card, W. S. Trahanovsky, J. Org. Chem. 1980, 45, 2560.
[17]Special cases of meta deprotonation can exist when a combina-
tion of substituents are present on an aromatic ring. Such cases,
which because of the multiple substitution limit the number of
sites available for deprotonation, are usually forced by steric
constraints and therefore should be clearly distinguished from
examples of monosubstituted aromatic rings in which meta
deprotonation is orders of magnitude more difficult to realize.
For a recent example involving trisubstituted (2,6-dihalophenyl)
silanes, see: a) C. Hess, F. Cottet, M. Schlosser, Eur. J. Org.
Chem. 2005, 5236; for monosubstituted but N,N-crowded aniline
examples, see: E. Baston, R. Maggi, K. Friedrich, M. Schlosser,
Eur. J. Org. Chem. 2001, 3985.
ꢀ
(100.63 MHz, 258C, C6D6): d = 170.01 (Zn Cmeta (Ar**)), 150.77 (Cipso
(Ar**)), 136.54 (Cmeta’ (Ar**)), 128.52 (Cpara (Ar**)), 121.55 (Cortho
(Ar**)), 111.86 (Cortho’ (Ar**)), 57.37 (CH2 (TMEDA)), 52.96 (Ca
(TMP)), 46.23 (CH3 (TMEDA)), 41.22 (N(CH3)2 (Ar**)), 40.76 (Cb
(TMP)), 36.50 (CH3 (TMP)), 35.77 (CH3 (tBu)), 35.32 (CH3 (TMP)),
22.80 (CH3 (Ar**)), 21.05 (C (tBu)), 20.78 ppm (Cg (TMP)).
[18]Crystal data for 3: C27H53N4NaZn, Mr = 522.1, monoclinic, space
group P21/c, a = 16.170(3), b = 11.0651(19), c = 18.683(3) Š, b =
112.614(3)8, V= 3085.8(9) Š3, Z = 4, T= 150 K. 23451 measured
reflections (CCD diffractometer, MoKa radiation, l =
0.71073 Š), 6048 unique (Rint = 0.042), 362 refined parameters
with constrained H atoms and disorder for TMEDA, R = 0.061
for F values of 4622 reflections with F2 > 2s(F2), Rw = 0.152 for
all F2 values, GOF = 1.14, final difference map extremes = + 0.94
and ꢀ0.47 eŠꢀ3. Crystal data for 5: C28H55N4NaZn, Mr = 536.1,
monoclinic, space group P21/c, a = 15.291(7), b = 11.391(5), c =
18.729(8) Š, b = 99.992(7)8, V= 3213(2) Š3, Z = 4, T= 150 K.
24659 measured reflections, 6299 unique (Rint = 0.032), 475
refined parameters with constrained H atoms and disorder for all
ligands except anilide, R = 0.034 for F values of 4948 reflections
with F2 > 2s(F2), Rw = 0.094 for all F2 values, t = 1.08, final
difference map extremes = + 0.43 and ꢀ0.32 eŠꢀ3. CCDC-
297778 and -297779 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. See http://www.ccdc.
cam.ac.uk/products/csd/request/ for details.
Received: February 23, 2006
Published online: May 3, 2006
Keywords: anilides · deprotonation · metalation · sodium ·
.
zincation
[1]M. Schlosser, Organometallics in Synthesis, 2nd ed., Wiley,
Chichester, 2002, chap. 1.
[2]For an authoritative review on hydrogen–metal interconversion
reactions in aromatic systems, see: M. Schlosser, Angew. Chem.
2005, 117, 380; Angew. Chem. Int. Ed. 2005, 44, 376.
[3]P. Beak, V. Snieckus, Acc. Chem. Res. 1982, 15, 306.
[4]“The Directed ortho-Metalation Reaction. A Point of Departure
for New Synthetic Aromatic Chemistry”: C. G. Hartung, V.
Snieckus in Modern Arene Chemistry (Ed.: D. Astruc), Wiley-
VCH, New York, 2002, pp. 330 – 367.
[5]J. Clayden, Organolithiums: Selectivity for Synthesis, Pergamon,
Elsevier Science, Oxford, 2002.
[6]Y. Kondo, M. Shilai, M. Uchiyama, T. Sakamoto, J. Am. Chem.
Soc. 1999, 121, 3539.
[7]P. C. Andrikopoulos, D. R. Armstrong, W. Clegg, C. J. Gilfillan,
E. Hevia, A. R. Kennedy, R. E. Mulvey, C. T. OꢀHara, J.A.
Parkinson, D. M. Tooke, J. Am. Chem. Soc. 2004, 126, 11612.
[8]For the use in synthesis of lithium magnesiate reagents with only
alkyl ligands, see: F. Mongin, A. Bucher, J. P. Bazureau, O. Bayh,
H. Awad, F. TrØcourt, Tetrahedron Lett. 2005, 46, 7989.
[9]The classical “LiCKOR” (Li–C + KOR) superbases are also
heterobimetallic, see: a) M. Schlosser, Mod. Synth. Methods
1992, 6, 227; b) L. Lochmann, Eur. J. Inorg. Chem. 2000, 115;
c) see also reference [1].
[19]R. E. Ludt, G. P. Crowther, C. R. Hauser, J. Org. Chem. 1970, 35,
1288.
[20]The presence of N,N-dimethyl-3-iodoaniline was confirmed by
comparing its 1H NMR spectrum in CDCl3 with that reported by
S. Padmanabhan, N. L. Reddy, G. J. Durant, Synth. Commun.
1997, 27, 691; see the Supporting Information.
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