Modifying alkylzinc reactivity with 2,2′-dipyridylamide: Activation of tBu-Zn bonds for para-alkylation of benzophenone

Article abstract of DOI:10.1002/anie.201302426

Undercover agent: Masquerading as a simple donor-acceptor complex (1), sodium amide substoichiometrically activates tBu₂Zn for the challenging 1,6-addition of a tert-butyl group to benzophenone. In contrast, the nonactivated tBu₂Zn is ineffectual. Copyright

Full text of DOI:10.1002/anie.201302426

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Angewandte
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DOI: 10.1002/anie.201302426
Homogeneous Catalysis
Modifying Alkylzinc Reactivity with 2,2’-Dipyridylamide: Activation of
À
tBu Zn Bonds for para-Alkylation of Benzophenone**
David R. Armstrong, Jennifer A. Garden, Alan R. Kennedy, Robert E. Mulvey,* and
Stuart D. Robertson
Many of the recent eye-catching advances in zincate executed
metallation chemistry have centered on the secondary
monoamide 2,2,6,6-tetramethylpiperidide (TMP). Developed
by Kondo, Uchiyama, and co-workers, the original TMP
have investigated replacing monofunctional TMP with multi-
functional 2,2’-dipyridylamide [dpa, (2-NC H ) N]. Offering
5
4 2
three potential ligating N sites, this N-bridged bis(N hetero-
cycle) finds utility in a diverse range of applications including
[
3]
[4]
[5]
zincate reagent [LiZn(TMP)tBu ] is an effective base for both
medicine, catalysis, photoluminescence, and supramolec-
2
[
1]
[6]
aromatic and heteroaromatic substrates. Knochel and co-
workers have since compiled a library of new TMP-based
metalating agents, including the zinc pivalate [(TMP)Zn-
ular chemistry. Herein we report a remarkable set of novel
structures incorporating dpa into neutral zinc, sodium zincate,
and potassium zincate modifications containing tert-butyl
ligands. Preliminary reactivity studies reveal that the mixed
sodium-zinc dpa complexes can tert-butylate the ketone
benzophenone at the para (1,6-addition) position, which is
challenging, and hint that the reaction can be made sub-
stoichiometric in the sodium component.
(
OPiv)·LiCl] (OPiv = pivalate), which like [LiZn(TMP)tBu ],
2
displays strong deprotonating power and exceptional func-
tional-group tolerance but has the added advantage that its
[
2]
arylzincated derivatives boast prolonged air stability. The
area of zincate metalation chemistry that is best structurally
defined is that involving the sodium TMP reagent
The neutral, heteroleptic zinc complex [{(dpa)Zn(tBu)}2]
(2) was synthesized straightforwardly through the 1:1 stoi-
[
(TMEDA)Na(TMP)(tBu)Zn(tBu)]
(1;
TMEDA =
N,N,N’,N’-tetramethylethylenediamine). This reagent, which
exhibits enhanced reactivity over the aforementioned lithium
zincates, and many of its arylzincated derivatives adopt
similar structures, which are designed on an architectural
template of a Na–anion–Zn–anion ring carrying terminal
ligands, namely a neutral donor and an anion on Na and Zn,
respectively (Figure 1). To break this template and develop
new structural motifs that could stimulate new reactivity we
chiometric combination of tBu Zn and the amine dpa(H) in
2
a hexane solution. NMR studies of the filtrate revealed the
absolute yield was higher than the 70% yield of the isolated
crystals. Possessing an attractive hour-glass-shaped core, the
centrosymmetric molecular structure of 2 (Scheme 1) is
[
7]
dimeric with the distorted tetrahedral zinc atom bonded to
the deprotonated N of one amido unit, which occupies the di-
pyridyl pocket of the other and completes its coordination
with a terminal tBu ligand. In the hour-glass description (see
the Supporting Information), each bulb comprises a puckered,
six-atom (NCNCNZn) ring with Zn situated 1.0991(3) ꢀ from
the nearest NCNCN plane and 1.7143(3) ꢀ from the second
NCNCN plane. This motif bears a strong resemblance to that
of the isoelectronic neutral zinc dimer [{MeZnC(H)-
[8]
Py } ](Py=2-pyridyl). To our knowledge, 2 represents the
2
2
first crystallographically characterized Zn/dpa complex show-
ing a ZnÀ(bridgehead)N bond, although it has been noted in
alkylated dimeric derivatives wherein the mode of dimeriza-
[
9]
[10]
tion is distinct from that in 2. The anti/anti conformation
i.e., both pyridyl N atoms directed away from the bridgehead
N) within 2 also contrasts with the syn/anti conformations
Figure 1. General structural motif observed for heteroleptic ate bases.
Here the subordinate metal is zinc.
(
[
11]
found in polymorphs of dpa(H) and the aforementioned
alkylated dpa derivatives.
[12]
[*] Dr. D. R. Armstrong, J. A. Garden, Dr. A. R. Kennedy,
A simple transamination reaction between TMP/zin-
cate 1 and dpa(H) in a hexane solution was anticipated, but
instead the isolated crystalline product was the disodium
monozinc species [(TMEDA) Na (m-dpa) Zn(tBu) ] [3; 70%
Prof. R. E. Mulvey, Dr. S. D. Robertson
WestCHEM, Department of Pure and Applied Chemistry
University of Strathclyde, Glasgow (UK)
2
2
2
2
E-mail: r.e.mulvey@strath.ac.uk
yield based on the dpa(H) stoichiometry]. Formally trans-
amination has occurred but the 2:1 Na/Zn stoichiometry of 3
is inconsistent with the 1:1 stoichiometry of the reaction. Its
molecular structure (Figure 2) can be viewed as a cocomplex
between a TMEDA-complexed sodium amide dimer and
a bis(alkyl) zinc monomer with the connection being two
dative ZnÀN(pyr) bonds completing a distorted tetrahedral
[
**] This work was supported by the EPSRC (grant award EP/K001183/
), the Royal Society (Wolfson Merit Award to R.E.M.), the Royal
Society of Edinburgh (BP Fellowship to S.D.R.), and George Fraser
studentship to J.A.G.). We also thank Dr. Eva Hevia and Dr. Tobias
1
(
Blꢀmke (University of Strathclyde) for stimulating discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201302426.
7190
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chemie
a higher-order zincate (conforming to
the empirical formula Na ZnR R’ ), 3 is
2
2
2
best interpreted as a neutral donor-
acceptor bis(alkyl)zinc complex with
a metalloligand as a donor. Supporting
this view, the structure of 3 is novel as it
bears little resemblance to the common
Weiss motif of dialkali-metal zincate
[
14]
and related ate structures (Figure 3)
but shows more in common with homo-
nuclear zinc complexes such as
[15]
[
tBu Zn{(iPrN(H)CH CH N(H)iPr}]
2
2
2
16]
[
or [(tBu Zn) (C H N ) ].
2
3
4
4
2 4
Surprisingly when the same metal
components in 3 were mixed together in
the presence of THF and in the absence
of TMEDA, the structural outcome was
remarkably different (Scheme 1). Sub-
stituting TMEDA by THF can be trivial
Scheme 1. Synthesis of the neutral zinc dimer 2, disodium zincate 3, sodium zinczincate 4, and
potassium zinczincate 5.
Figure 3. Graphical representation of a Weiss motif structure.
in organoalkalimetal chemistry with one bidentate TMEDA
being replaced by two monodentate THF molecules without
altering the basic structure of the complex, but the reactivity
[
17]
may be changed.
However, here the structural effect is
more profound. Unlike the contact ion-pair 3 with its 2:1 Na/
Zn stoichiometry, the isolated product is the solvent-sepa-
+
rated monosodium dizinc complex [{Na(THF) } {Zn(tBu) -
6
2
[
26]
À
Figure 2. Molecular structure of 3 with thermal ellipsoids at 50%
probability level. Hydrogen atoms omitted for clarity. Selected bond
lengths (ꢁ) and angles (8): Zn1ÀC1 2.048(2), Zn1ÀC5 2.044(2),
Zn1ÀN2 2.185(2), Zn1ÀN3 2.157(2), C10ÀN1 1.371(2), C10ÀN2
(
dpa)Zn(tBu) } ] (4). The octahedrally coordinated cation of
is known but its anionic moiety (Figure 4) is unprece-
2
[18]
4
1
.347(2), C15ÀN1 1.373(2), C15ÀN3 1.346(2); C1-Zn1-C5 130.31(7),
C1-Zn1-N2 111.19(6), C1-Zn1-N3 110.41(6), N2-Zn1-N3 82.42(6),
N2-Zn1-C5 108.19(6), N3-Zn1-C5 103.65(7), C10-N1-C15 125.1(2).
(
C N ) zinc environment. To bind to Zn, one dipyridylamide
2 2
must adopt an anti/anti disposition with its pyridine N atoms
oriented away from the [{Na(amido)N} ] ring, while the
2
second has a syn/syn conformation with each Na bonded to
the amido N and one or other of the pyridine N atoms.
Resonance delocalization within the dipyridylamide scaffold
is usually associated with shorter C-N(amido)-C bridges (e.g.,
about 1.34 ꢀ each), longer C-N(pyr) bonds (e.g., about 1.38 ꢀ),
and a dihedral angle between the two pyridine ring planes
approaching 08 as ascertained by data from several neutral
[
26]
Figure 4. Anion of the solvent-separated ion pair structure of 4 with
thermal ellipsoids at 50% probability level. Hydrogen atoms and
disordered components omitted for clarity. The anionic moiety of 5 is
effectively identical.
dented. Two tBu Zn monomers connect through one dpa
2
[
11,13]
dpa(H) and anionic dpa complexes
(see the Supporting
anion which, lying flat in an anti/anti conformation, presents
a chelating pyridyl pocket to one Zn atom, thus making it four
coordinate, and the bridgehead amido N to the other Zn, thus
making it three coordinate. Unfortunately, the anion lies on
a crystallographic center of symmetry, and as this symmetry
can only be approximately satisfied by the dpa ligand, the
Information). The corresponding dimensions within 3 [e.g.,
C10ÀN1, 1.371(2) ꢀ; C15ÀN1, 1.373(2) ꢀ; C10ÀN2,
1
.347(2) ꢀ; C15ÀN3, 1.346(2) ꢀ; dihedral angle of unit
bonded to Zn, 22.98] suggest the degree of delocalization is
small. This prompts the thought that though formally
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Table 1: Reaction of zinc reagents with benzophenone in a hexane solution for 18 h.
[
a]
Entry Reagent
T
Yield [%]
[8C]
para addi- Carbonyl
tion(1,6) addition(1,2) tion(1,4)
ortho addi- benzhydrol
total
À
(H addition)
Stoichiometric
1
2
3
4
5
6
tBu Zn (1 equiv)
RT
RT
RT
RT
RT
RT
1
1
1
58
40
42
0
0
1
14
6
13
0
0
0
3
0
0
0
0
0
0
11
3
1
1
2
75
57
58
2
tBu Zn·2pyridine (1 equiv)
2
tBu Zn·TMEDA (1 equiv)
2
[(TMEDA)Na(TMP)(tBu)Zn(tBu)] (1; 1 equiv)
[(TMEDA) Na (m-dpa) Zn(tBu) ] (3; 1 equiv)
2
2
2
2
+
[{Na(THF)6}
À
{
Zn(tBu) (dpa)Zn(tBu) } ] (4; 1 equiv)
2 2
Substoichiometric
7
8
9
0
tBu Zn (1 equiv)
75
75
75
11
33
52
33
1
8
12
7
0
0
0
0
8
6
7
9
20
47
71
49
2
tBu Zn (1 equiv)+[(TMEDA)Na(TMP)] (0.1 equiv)
2
tBu Zn (1 equiv)+[(TMEDA)Na(dpa)] (0.1 equiv)
2
2
+
À
1
tBu Zn (1 equiv)+[{Na(THF) } {Zn(tBu) (dpa)Zn(tBu) } ] (0.1 equiv) 75
2 6 2 2
1
[
a] Yields were determined by H NMR spectroscopy using hexamethylbenzene (10 mol%) as an internal standard.
[
24]
structure is disordered over two sites. This prevents any
discussion of dimensions, though the atomic connectivity is
definite. We therefore turned to a DFT study using the
B3LYP method and the 6-311G (d,p) basis set in which
the anionic moiety of 4 was modeled. Its dimensions (see the
Supporting Information for full details) revealed slightly
more delocalization than that implied in 3. Key indicators are
isolated crystalline enolato intermediate.
Table 1 reveals
that the three sodium zincates 3, 4, and 1 give competitive
yields of the challenging para-addition product 4-tbutylben-
zophenone in contrast to the near 0% yields witnessed for the
homonuclear zinc species. A striking comparison is that the
metalloligand solvated tBu Zn produces 40 times as much
para product as unsolvated tBu Zn or its pyridine and
[19]
[20]
2
2
the CÀN(
bond lengths [1.363 ꢀ; 1.372 ꢀ (mean) in 3] and
TMEDA solvates. Olah and co-workers reported that react-
ing tBuLi with benzophenone in THF solvent gave a 52%
yield of ring alkylation products (1:9, ortho/para ratio), but
only at À1008C using toxic thionyl chloride as oxidant as an
aqueous work-up dropped the combined yield to 288C and
running the reaction in hexane at ambient temperature
switched the major product to that of 1,2 addition. Clearly
the zincate systems are advantageous since they give com-
parable yields at ambient temperature using a mild aqueous
work-up. The most intriguing result came when a 1:1 mixture
amido)
the dihedral angle between the N ÀC(H) bonds [C-N···N-C
(
pyr)
is 3.98 in 4; 26.08 in 3]. Also within the pyridyl rings there are
long CÀN bonds and two relatively short CÀC bonds akin to
a N-C=C-C=C-C-N pattern (bond lengths 1.345/1.345, 1.380/
1
.380, 1.401/1.401, 1.374/1.374, 1.429/1.428, and 1.360/1.359 ꢀ)
[
23]
similar to that found in a series of [Me MPy N] complexes,
where M is Al, Ga, In, or Tl.
as a sodium zinczincate by combining a neutral tBu Zn unit
with a [Zn(dpa)tBu2] ate, though the true electronic
2
2
n
[13b]
Formally 4 could be regarded
[
21]
2
À
distribution, as reflected by the theoretical calculations, lies
between these two extremes. We also successfully prepared
of tBu Zn and benzophenone was treated with only 10 mol%
2
of the sodium amide [{(TMEDA)Na(dpa)} ], presumably
2
+
the potassium congener [{K(THF) } {Zn(tBu) (dpa)Zn-
generating 10 mol% of 3 in situ in a hexane solution. At
6
2
À
(
tBu) } ] (5; see the Supporting Information for details).
ambient
temperature,
4-tert-butylbenzophenone
was
2
Alkylselective addition to ketones by Grignard reagents
obtained in a low yield of 11% but this remarkably rose to
52% under reflux conditions, thus implying that the sub-
stoichiometric sodium amide is recycling in some way.
To conclude, this study has uncovered a set of alkylzinc-
based compounds with unprecedented structures by utilizing
a secondary amide equipped with two pyridyl appendages as
a structural template-changing co-ligand. Unlike conven-
tional alkylzinc reagents these heteroleptic compounds can
directly alkylate (tert-butylate) benzophenone in the chal-
lenging para position. Most intriguingly, substoichiometric
has been much studied with benzophenone, which is often
used as a benchmark as its reactions can have several
outcomes. Excellent progress has been made in carbonyl
1,2) additions most recently by Ishihara and co-workers who
showed that catalytic amounts of ZnCl2 (thus generating
(
R ZnMgCl intermediates) can greatly enhance such reac-
3
[
22]
tions. Hevia and co-workers also reported that adding just
+
1
[
0 mol% of the magnesium-zinc hybrid [(THF) Mg Cl ] -
6 2 3
À
Zn Et ] to the reaction of EtMgCl and benzophenone gave
2 5
[
23]
the 1, 2-addition product in 90% yield. These bimetallic
zinc-mediated successes inspired us to test the tert-butylation
performance of 3 and 4 in a hexane solution, noting that 1 had
previously delivered a tBu nucleophile to the para (1,6)
position of benzophenone albeit on the sole evidence of an
amounts of sodium amide [{(TMEDA)Na(dpa)} ] can acti-
2
vate tBu Zn towards this tert-butylation through the inter-
2
mediacy of the zincate 3, the structure of which masquerades
as a neutral donor-acceptor complex.
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Received: March 22, 2013
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Helm, J. Organomet. Chem. 1986, 299, 67 – 84.
Keywords: alkali metal · alkylation · amides ·
.
homogeneous catalysis · zinc
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group P2 /c, a = 9.4833(3), b = 10.4377(4), c = 14.0888(5) ꢀ, b =
1
3
À3
105.844(4)8, V= 1341.58(8) ꢀ , Z = 2, Rcalcd = 1.449 Mgm ,
[
[
R1 = 0.0413 [for 2840 reflections with I >2s(I)], wR2 = 0.1039
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66 10
2
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1
3
2
1.6403(4), c = 18.3837(3) ꢀ; b = 93.636(2)8, V= 4571.26(16) ꢀ ,
À3
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5
0
92
3
6
2
PÀ1, a = 10.0075(11), b = 11.1283(9), c = 13.3084(15) ꢀ; a =
3
6
1
9.353(9), b = 85.491(9), g = 84.723(8)8; V= 1379.3(2) ꢀ , Z =
À3
, Rcalcd = 1.186 Mgm , R1 = 0.0606 [for 5614 reflections with
[
[
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1985 – 1987.
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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