Phenylation of benzaldehyde by dizinc organometallics supported by binucleating bis(amidoamine) ligands

Article abstract of DOI:10.1021/om050580e

Several new dizinc organometallics that are supported by dianionic bis(amidoamine) ligands are reported. The reaction of N,N′-bis(2- (diisopropylamino)ethyl)dibenzofuran-4,6-diamine (^iPrLH₂) with excess ZnR₂ (R = Et, Me) i

Full text of DOI:10.1021/om050580e

Organometallics 2005, 24, 5335-5341
5335
Phenylation of Benzaldehyde by Dizinc Organometallics
Supported by Binucleating Bis(amidoamine) Ligands
Mark L. Hlavinka and John R. Hagadorn*
Department of Chemistry and Biochemistry, University of Colorado,
Boulder, Colorado 80309-0215
Received July 11, 2005
Several new dizinc organometallics that are supported by dianionic bis(amidoamine)
ligands are reported. The reaction of N,N′-bis(2-(diisopropylamino)ethyl)dibenzofuran-4,6-
diamine (iPrLH2) with excess ZnR2 (R ) Et, Me) in toluene solution forms the dizinc derivatives
iPrLZn2Et2 (1) and iPrLZn2Me2 (2), which were isolated in 69% and 72% yields, respectively.
Both 1 and 2 feature three-coordinate zinc centers in the solid state. Reaction of 2 equiv of
ZnPh2 with N,N′-bis(2-(dimethylamino)ethyl)dibenzofuran-4,6-diamine (MeLH2) forms MeLZn2-
Ph2 (3), which was isolated in 73% yield. The analogous reaction using iPrLH2 instead of
MeLH2 forms iPrLZn2Ph2 (4), which was isolated in 56% yield. The solid-state structure of 3
(two independent molecules) reveals a parallelogram-shaped [Zn2(µ-Ph)2]2+ core, with the
overall molecule having approximate (noncrystallographic) C2 symmetry and a short
1
intermetal separation (2.7279(6) and 2.7809(6) Å). In contrast, the H NMR spectrum of 3
(CD2Cl2) indicates overall C2v symmetry; this is likely due to cleavage of the phenyl bridges
in solution. The solid-state structure of 4, which features the bulkier iPr-substituted ligand,
reveals the absence of Ph bridges. Instead, each molecule of 4 has a pair of essentially
noninteracting three-coordinate Zn centers. Benzene solutions of 3 react with 1 equiv of
benzaldehyde to form the addition product MeLZn2(Ph)(OCHPh2) (5) in 87% isolated yield.
The solid-state structure of 5 reveals a symmetric and puckered [Zn2(µ-Ph)(µ-OCHPh2)]2+
core with a short Zn-Zn separation of 2.6977(13) Å. In CD2Cl2 solution, the Ph bridge of 5
undergoes reversible cleavage. This process followed by rotation about the Zn-Ph bond leads
to exchange of the inequivalent ortho (and meta) protons of the µ-Ph ligand. Variable-
1
temperature H NMR spectroscopic data indicates that this exchange occurs with ∆Gq )
11.5(1) kcal mol-1 (-50 °C). At 70 °C, benzene solutions of 5 react with 1 equiv of
benzaldehyde to form the double-addition product MeLZn2(OCHPh2)2 (6) in 67% isolated yield.
The related species iPrLZn2(OCHPh2)2 (7) was formed in good yield by the reaction of 2 equiv
of benzaldehyde with 4 at 70 °C for 24 h.
Chart 1
Introduction
The activation of organic carbonyls by Zn-based Lewis
acids is a topic of relevance to several C-C bond-forming
processes, including aldol reactions1 and asymmetric
additions to aldehydes2 and ketones.3 Many of these
reactions are believed to proceed via dizinc reaction
intermediates that use novel ambifunctional mecha-
nisms,4 with one Zn center acting as a Lewis acid and
the other Zn providing the nucleophile. One of the best
defined examples of an ambifunctional mechanism is
Noyori’s (-)-DAIB-catalyzed (DAIB ) 3-exo-(dimethyl-
amino)isoborneol) addition of dialkylzincs to benzalde-
hyde.5 In the first step of this reaction, the dialkylzinc
(R2Zn) reacts with the alcohol group of (-)-DAIB to form
the Zn alkoxide species [(-)-DAIB]ZnR. Interestingly,
this complex does not react with benzaldehyde to give
the addition product unless a second equivalent of R2-
Zn is added. These data and subsequent calculations
and structural studies6 provide strong evidence that the
key intermediate is the dizinc species A (Chart 1), which
efficiently activates the aldehyde toward nucleophilic
attack while increasing the nucleophilicity of the R2Zn
reagent.
(1) (a) Trost, B. M.; Ito, H. J. Am. Chem. Soc. 2000, 122, 12003-
12004. (b) Trost, B. M.; Ito, H.; Silcoff, E. R. J. Am. Chem. Soc. 2001,
123, 3367-3368. (c) Trost, B. M.; Yeh, V. S. C. Angew. Chem., Int. Ed.
2002, 41, 861-863.
(2) (a) Soai, K.; Niwa, S. Chem. Rev. 1992, 92, 833-856. (b) Pu, L.;
Yu, H. Chem. Rev. 2001, 101, 757-824.
(3) (a) Dosa, P.; Fu, G. J. Am. Chem. Soc. 1998, 120, 445-446. (b)
Ramon, D.; Yus, M. Tetrahedron 1998, 54, 5651-5666. (c) Garcia, C.;
LaRochelle, L.; Walsh, P. J. Am. Chem. Soc. 2002, 124, 10970-10971.
(d) Jeon, S.; Walsh, P. J. Am. Chem. Soc. 2003, 125, 9544.
(4) (a) Rowlands, G. Tetrahedron 2001, 57, 1865-1882. (b) Williams,
N.; Takasaki, B.; Wall, M.; Chin, J. Acc. Chem. Res. 1999, 32, 485-
493.
(5) Kitamura, M.; Suga, S.; Kawai, K.; Noyori, R. J. Am. Chem. Soc.
1986, 108, 6071-6072.
(6) (a) Yamakama, M.; Noyori, R. J. Am. Chem. Soc. 1995, 117,
6327-6335. (b) Yamakuma, M.; Noyori, R. Organometallics 1999, 18,
128-133.
10.1021/om050580e CCC: $30.25 © 2005 American Chemical Society
Publication on Web 09/29/2005

5336 Organometallics, Vol. 24, No. 22, 2005
Hlavinka and Hagadorn
through a fritted disk to afford a brown solution. Evaporation
of the solvent left a brown semisolid, which was taken up in
H2O (200 mL) and Et2O (250 mL). The organics were separated
and washed with H2O (3 × 150 mL) and brine (100 mL). The
solution was dried over MgSO4 and filtered through a pad of
Florisil. The yellow solution was concentrated to 75 mL and
cooled to -40 °C to afford the product as pale yellow crystals
(8.25 g, 51.4%). 1H NMR (CDCl3): δ 7.26 (dd, J ) 1.0, 7.6 Hz,
2H), 7.18 (t, J ) 7.6 Hz, 2H), 6.69 (dd, J ) 1.0, 7.6 Hz, 2H),
4.77 (t, J ) 5.0 Hz, 2H, -NH), 3.36 (q, J ) 5.9 Hz, 4H,
-NHCH2), 2.66 (t, J ) 6.2 Hz, 4H, -CH2NMe2), 2.30 (s, 12H,
-NMe2). 13C{1H} NMR (CDCl3): δ 144.5, 134.6, 124.7, 123.5,
109.1, 107.9, 58.2, 45.4 (-NMe2), 41.3. Anal. Calcd (found) for
C20H28N4O: C, 70.56 (70.82); H, 8.29 (8.33); N, 16.46 (16.08).
N,N′-Bis(2-(diisopropylamino)ethyl)dibenzofuran-4,6-
diamine (iPrLH2). This compound was made analogously to
MeLH2, except that N,N-diisopropylethylenediamine was used
instead of N,N-dimethylethylenediamine. The product was
isolated as colorless crystals in 38.0% yield. 1H NMR (CDCl3):
δ 7.27 (dd, J ) 0.8, 7.5 Hz, 2H), 7.19 (t, J ) 7.5 Hz, 2H), 6.71
(dd, J ) 0.8, 7.5 Hz, 2H), 4.97 (t, J ) 5 Hz, 2H, -NH), 3.28 (q,
J ) 6 Hz, 4H, -NHCH2), 3.11 (sept, J ) 6.5 Hz, 4H, -CHMe2,
2.85 (t, J ) 6 Hz, 4H, -CH2NiPr2), 1.08 (d, J ) 6.5 Hz, 24H,
-CHMe2). 13C{1H} NMR (CDCl3): δ 144.8, 135.1, 124.9, 123.6,
108.9, 108.4, 44.7 (-NHCH2), 43.0 (-CHMe2), 42.3 (-CH2-
NiPr2), 22.0 (-CHMe2).
Chart 2
Largely because of the success of Noyori’s system, the
design and development of ambifunctional catalysts
continues to be a topic of great interest. In this context,
we have been developing preorganized binucleating
ligands that are suitable for the preparation of discrete
main-group bimetallic complexes.7 These ligands include
a series of bis(diaminoethanes) that feature a dibenzo-
furan “backbone” (Chart 2). This relatively rigid ligand
system was chosen because it provides some control over
key structural features, such as intermetal separation
and sterics. In this paper we describe the use of these
ligands for the preparation of several new dizinc orga-
nometallics. These discrete, structurally characterized
complexes provide an unusual opportunity to study the
addition of organozincs to benzaldehyde with well-
defined reactants. Solid-state structures are reported for
1, 2, 4, and 5. The structure of 3 was recently com-
municated.8
iPrLZn2Et2 (1). To a toluene (75 mL) solution of iPrLH2 (1.00
g, 2.21 mmol) was added ZnEt2 (0.905 mL, 8.84 mmol), forming
a colorless solution. The reaction mixture was stirred for 24 h
at 60 °C. The solution was cooled to 5 °C to yield the product
as colorless crystals, which were isolated and dried under
Experimental Section
1
General Considerations. Standard Schlenk-line and glove-
reduced pressure (0.978 g, 69.2%). H NMR (CD2Cl2): δ 7.04
box techniques were used unless stated otherwise. 4,6-Di-
(t, J ) 7.6 Hz, 2H), 6.97 (dd, J ) 1.5, 7.6 Hz, 2H), 6.55 (dd, J
) 1.5, 7.6 Hz, 2H), 3.47 (t, J ) 5.6 Hz, 4H, -NCH2), 3.37 (sept,
J ) 6.4 Hz, 4H, -CHMe2), 3.07 (t, J ) 5.6 Hz, 4H, -CH2-
NiPr2), 1.27 (d, J ) 6.4 Hz, 24H, -CHMe2), 1.09 (t, J ) 8 Hz,
6H, -ZnCH2CH3), 0.56 (q, J ) 8 Hz, 4H, -ZnCH2CH3). 13C-
{1H} NMR (CD2Cl2): δ 146.8, 144.8, 125.1, 123.9, 108.1, 104.7,
50.3 (-NCH2), 46.5 (-CHMe2), 46.3, (-CH2NiPr2), 20.7
(-CHMe2), 12.1 (-ZnCH2CH3), 6.23 (-ZnCH2CH3). Anal. Calcd
(found) for 1‚(toluene), C39H60N4OZn2: C, 64.02 (64.47); H, 8.27
(8.42); N, 7.66 (7.65).
iododibenzofuran9 and ZnPh2 were prepared by following
10
literature procedures. Benzaldehyde was distilled from CaH2
under N2 prior to use. N,N-Dimethylethylenediamine, N,N-
diisopropylethylenediamine, ZnEt2, ZnMe2, and OPMe3 were
purchased from commercial sources (Sigma-Aldrich, Alfa,
Lancaster, TCI) and were used as received. Hexanes, Et2O,
toluene, tetrahydrofuran (THF), and CH2Cl2 were passed
through columns of activated alumina and sparged with N2
prior to use. C6D6 and C7D8 were vacuum-transferred from
Na-benzophenone ketyl. CD2Cl2 and CDCl3 were vacuum-
iPrLZn2Me2 (2). The synthesis was carried out analogously
to that of 1, except ZnMe2 was used instead of ZnEt2. The
product crystallized from toluene, yielding colorless crystals
1
transferred from CaH2. Chemical shifts (δ) for H NMR (400
MHz) spectra are given relative to residual protium in the
deuterated solvent at 7.16, 5.32, 7.27, and 2.09 ppm for C6D6,
CD2Cl2, CDCl3, and C7D8, respectively. Elemental analyses
were determined by Desert Analytics and the University of
Michigan elemental analysis laboratory. Analytical data are
provided for at least one representative of each type of
compound reported.
N,N′-Bis(2-(dimethylamino)ethyl)dibenzofuran-4,6-di-
amine (MeLH2). 4,6-Diiododibenzofuran (20.0 g, 47.7 mmol),
K3PO4 (40.5 g, 191 mmol), and CuI (0.908 g, 4.77 mmol) were
combined in a 500 mL round-bottomed flask. 2-Propanol (250
mL) was added to the solids to form a white suspension. Next,
ethylene glycol (10.6 mL, 191 mmol) and N,N-dimethylethyl-
enediamine (10.7 mL, 97.8 mmol) were added by syringe. The
flask was then equipped with a condenser, and the reaction
1
in 72.3% yield. H NMR (CD2Cl2): δ 7.09 (t, J ) 7.6 Hz, 2H),
7.00 (dd, J ) 1.0, 7.6 Hz, 2H), 6.54 (dd, J ) 1.0, 7.6 Hz, 2H),
3.45 (t, J ) 5.6 Hz, 4H, -NCH2), 3.37 (sept, J ) 6.4 Hz, 4H,
-CHMe2), 3.09 (t, J ) 5.6 Hz, 4H, -CH2NiPr2), 1.26 (d, J )
6.4 Hz, 24H, -CHMe2), -0.30 (s, 6H, -ZnCH3). 13C{1H} NMR
(CD2Cl2): δ 146.6, 144.6, 124.9, 124.0, 107.5, 104.7, 49.6
(-NCH2), 46.1 (-CHMe2), 45.9, (-CH2NiPr2), 20.4 (-CHMe2),
-6.23 (-ZnCH3). Anal. Calcd (found) for 2‚1.5(toluene),
C
40.5H60N4OZn2: C, 64.88 (64.79); H, 8.07 (8.08); N, 7.47 (7.58).
MeLZn2Ph2 (3). ZnPh2 (3.24 g, 14.7 mmol) and MeLH2 (2.50
g, 7.35 mmol) were combined in toluene (200 mL) to form a
clear colorless solution. The reaction mixture was heated to
75 °C for 1 h. The hot solution was filtered into a warm 250
mL Schlenk tube. Colorless crystals of the product formed as
the solution was slowly cooled to room temperature. The
solution was cooled to -15 °C overnight. The product was
isolated by filtration and dried under reduced pressure (3.37
g, 73.3%). 1H NMR (CD2Cl2): δ 7.76 (dd, J ) 2.0, 5.6 Hz, 4H),
7.28 (m, 6H), 7.11 (t, J ) 7.6 Hz, 2H), 6.96 (d, J ) 7.6 Hz,
2H), 6.36 (d, J ) 7.6 Hz, 2H), 3.09 (t, J ) 6.0 Hz, 4H, -NCH2),
2.12 (t, J ) 5.6 Hz, 4H, -CH2NMe2), 2.10 (s, 12H, -NMe2).
13C{1H} NMR (CD2Cl2): δ 148.5 (ipso-Zn-Ph), 146.2, 143.9,
142.4, 128.8, 128.5, 127.9, 124.2, 105.9, 103.2, 60.1, 43.2
1
mixture was heated to reflux. After 2 days, H NMR spectro-
scopic analysis of the crude reaction mixture indicated that
the 4,6-diiododibenzofuran was completely consumed. The
reaction mixture was cooled to room temperature and filtered
(7) (a) Hlavinka, M. L.; Hagadorn, J. R. Chem. Commun. 2003,
2686-2687. (b) Clare, B. C.; Sarker, N.; Shoemaker, R.; Hagadorn, J.
R. Inorg. Chem. 2004, 43, 1159-1166.
(8) Hlavinka, M. L.; Hagadorn, J. R. Organometallics 2005, 24,
4116-4118.
(9) Tsang, K.; Diaz, H.; Gracin, N.; Kelly, J. J. Am. Chem. Soc. 1994,
116, 3988-4005.
1
(-NMe2), 43.7. H NMR (CD2Cl2, -90 °C): δ 7.67 (d, J ) 6.4
Hz, 4H), 7.26 (m, 6H), 7.06 (t, J ) 7.6 Hz, 2H), 6.89 (d, J )
7.6 Hz, 2H), 6.29 (d, J ) 7.6 Hz, 2H), 3.0 (br, 4H, -NCH2), 2.0
(10) Ashby, E.; Willard, G.; Goel, A. J. Org. Chem. 1979, 44, 1221-
1232.

Phenylation of Benzaldehyde by Dizinc Organometallics
Organometallics, Vol. 24, No. 22, 2005 5337
Table 1. Crystallographic Data and Collection Parameters
1‚(toluene)
2‚1.5(toluene)
4
5
formula
formula wt
space group
temp (°C)
a (Å)
C
39H60N4OZn2
C
40.5H60N4OZn2
C
40H52N4OZn2
C
39H42N4O2Zn2
731.65
749.66
735.60
729.51
P21/n (No. 14)
-130
10.4858(4)
31.631(1)
11.3696(4)
90
99.613(1)
90
4
3718.1(2)
1.307
1.325
0.45 × 0.2 × 0.2
1.93-27.88
8837/13/426
0.0537
P1h (No. 2)
-129
P21/c (No. 14)
-117
22.577(1)
15.4249(8)
20.916(1)
90
97.819(1)
90
8
7216.1(7)
1.354
1.366
0.4 × 0.31 × 0.11
1.60-27.48
16 553/0/863
0.0776
Cc (No. 9)
-122
19.0274(9)
12.0167(6)
17.0229(8)
90
118.837(1)
90
4
3409.6(3)
1.421
1.447
0.4 × 0.2 × 0.06
2.09-27.88
7141/2/428
0.0777
10.2478(3)
12.9348(4)
15.8925(5)
67.037(1)
81.799(1)
89.405(1)
2
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
Z
V (Å3)
1917.5(1)
1.298
1.287
0.4 × 0.2 × 0.1
1.74-27.88
9060/13/437
0.0579
0.0757, 0.1587
1.026
1.17, -0.79
dcalcd (g/cm3)
µ (mm-1
)
cryst size (mm)
θ range (deg)
no. of data/restraints/params
R1 (for Fo > 4σFo)
R1, wR2 (all data)
GOF
0.0675, 0.1436
1.040
1.09, -0.52
0.1965, 0.2038
0.940
1.29, -0.98
0.1145, 0.1983
0.991
0.87, -0.66
largest peak, hole (e/Å3)
(br, 16H, -CH2NMe2, -NMe2). Anal. Calcd (found) for 3,
) 5.6 Hz, 4H), 0.9 (br, 24H). 13C{1H} NMR (C6D6): δ 147.0,
146.3, 144.5, 129.4, 127.9, 127.3, 126.0, 125.4, 107.2, 104.3,
81.9, 53.5, 52.2, 45.6, 21.8.
X-ray Crystallography. Table 1 lists a summary of crystal
data and collection parameters for all crystallographically
characterized compounds. Table 2 lists selected bond lengths
and angles.
C32H36N4OZn2: C, 61.65 (61.30); H, 5.82 (5.64); N, 8.99 (9.19).
iPrLZn2Ph2 (4). The synthesis was carried out analogously
to that of 3, except that iPrLH2 was used instead of MeLH2 and
the reaction mixture was heated for 12 h. The product was
1
isolated as colorless crystals in 56% yield. H NMR (CD2Cl2):
δ 7.54 (m, J ) 1.6, 6.4 Hz, 4H), 7.1 (m, 10H), 6.67 (dd, J )
1.2, 7.6 Hz, 2H), 3.46 (t, J ) 5.6 Hz, 4H, -NCH2), 3.34 (sept,
J ) 6.8 Hz, 4H, -NCHMe2), 2.71 (t, J ) 5.6 Hz, 4H, -CH2-
NiPr2), 1.06 (d, J ) 6.8 Hz, 24H, -CHMe2). 13C{1H} NMR (CD2-
Cl2): δ 150.4, 146.8, 140.8, 127.4, 127.2, 125.5, 124.1, 109.3,
109.3, 105.0, 50.5, 46.0, 44.8, 20.6 (-CHMe2).
(a) General Procedure. A crystal of appropriate size was
mounted on a glass fiber using Paratone-N oil, transferred to
a Siemens SMART diffractometer/CCD area detector, centered
in the beam (Mo KR; λ ) 0.710 73 Å; graphite monochromator),
and cooled to ca. -123 °C by a nitrogen low-temperature
apparatus. The preliminary orientation matrix and cell con-
stants were determined by collection of 60 10 s frames, followed
by spot integration and least-squares refinement. A minimum
of a hemisphere of data was collected using 0.3° ω scans. The
raw data were integrated and the unit cell parameters refined
using SAINT. Data analysis was performed using XPREP.
Absorption correction was applied using SADABS. The data
were corrected for Lorentz and polarization effects, but no
correction for crystal decay was applied. Structure solutions
and refinements were performed (SHELXTL-Plus V5.0) on
F2.11
(b) Structure of iPrLZn2Et2‚toluene (1‚(toluene)). Needle-
shaped crystals that were suitable for X-ray diffraction studies
were grown from toluene at 20 °C. Preliminary data indicated
a primitive monoclinic cell. Systematic absences indicated
space group P21/n (No. 14), which was confirmed by the
successful solution and refinement of the structure. All non-H
atoms were refined anisotropically. Hydrogens were placed in
idealized positions and were included in structure factor
calculations but were not refined. It was necessary to constrain
the 1,2- and 1,3-bond distances of the cocrystallized toluene
to obtain acceptable metrical parameters.
MeLZn2(Ph)(OCHPh2) (5). Benzaldehyde (19.3 µL, 0.190
mmol) and 3 (0.118 g, 0.190 mmol) were combined in C6H6
(10 mL) to form a pale yellow solution. The mixture was stirred
for 24 h, during which time a white precipitate of the product
formed. This crude product was recrystallized by the vapor
diffusion of hexanes into a CH2Cl2 solution of 5 (0.120 g,
86.7%). 1H NMR (CD2Cl2): δ 7.73 (br, 2H, ortho ZnPh), 7.27-
7.40 (m, 7H, Ar H), 7.08-7.18 (m, 8H, Ar H), 6.92 (dd, J )
0.8, 7.6 Hz, 2H), 6.24 (dd, J ) 0.8, 7.6 Hz, 2H), 6.11 (s, 1H,
-OCHPh2), 2.93 (m, 2H), 2.77 (m, 2H), 2.30 (s, 6H), 1.94 (m,
2H), 1.52 (m, 2H), 1.34 (s, 6H). 13C{1H} NMR (CD2Cl2): δ 147.8,
146.3, 144.5, 136.2 (ipso-Zn-Ph), 133.2, 128.8, 128.7, 128.4,
128.3, 127.4, 124.7, 124.5, 105.3, 102.5, 80.7, 60.2, 47.1, 43.5,
42.1. Anal. Calcd (found) for 5, C39H42N4O2Zn2: C, 64.21
(64.05); H, 5.80 (5.81); N, 7.68 (7.64).
MeLZn2(OCHPh2)2 (6). Benzaldehyde (41.3 µL, 0.406 mmol)
and 3 (0.126 g, 0.203 mmol) were combined in C6H6 (5 mL) in
a 25 mL round-bottomed flask. The solution was heated to 70
°C for 24 h. The yellow solution was cooled to ambient
temperature. The slow addition of hexanes by vapor diffusion
yielded the product as colorless crystals, which were isolated
1
and dried under reduced pressure (0.113 g, 66.6%). H NMR
(c) Structure of iPrLZn2Me2‚1.5(toluene) (2‚1.5(tolu-
ene)). Needle-shaped crystals that were suitable for X-ray
diffraction studies were grown from a toluene at 20 °C.
Preliminary data indicated a triclinic cell. The choice of the
centric space group was confirmed by the successful solution
and refinement of the structure. The structure of 2 contains
1.5 molecules of cocrystallized toluene in the asymmetric unit.
The half-occupancy toluene is disordered and resides on a
crystallographic inversion center. The 1,2- and 1,3-bond
distances were constrained for the disordered molecule, and
it was refined with isotropic thermal parameters. All other
(CD2Cl2): δ 7.20-7.05 (m, 22H, Ar H), 7.01 (dd, J ) 1.2, 7.6
Hz, 2H), 6.30 (d, J ) 7.6 Hz, 2H), 5.99 (s, 2H, -OCHPh2), 2.77
(t, J ) 5.6 Hz, 4H), 1.81 (s, 12H), 1.64 (t, J ) 5.6 Hz, 4H).
13C{1H} NMR (CD2Cl2): δ 147.2, 145.9, 144.5, 128.5, 128.2,
127.5, 125.1, 124.6, 105.8, 102.5, 80.5, 60.8, 45.6, 43.1. Anal.
Calcd (found) for 6, C46H48N4O3Zn2: C, 66.11 (66.40); H, 5.79
(5.88); N, 6.70 (6.66).
iPrLZn2(OCHPh2)2 (7). The synthesis was carried out
analogously to that of 6, except that 4 was used instead of 3.
The product was isolated as colorless crystals in 69.1% yield.
1H NMR (C6D6): δ 7.63 (t, J ) 7.6 Hz, 2H), 7.55 (dd, J ) 0.8
Hz, 7.6 Hz, 2H), 7.11-7.16 (m, 8H, Ar H), 6.88-6.93 (m, 12H,
Ar H), 6.64 (dd, J ) 0.8, 7.6 Hz, 2H), 6.20 (s, 2H, -OCHPh2),
2.79 (t, J ) 5.6 Hz, 4H), 2.69 (sept, J ) 6.8 Hz, 4H), 1.77 (t, J
(11) Sheldrick, G. M. SHELXTL-Plus, a Program for Crystal Struc-
ture Determination, Version 5.1; Bruker AXS, Madison, WI, 1998.

5338 Organometallics, Vol. 24, No. 22, 2005
Table 2. Selected Bond Distances and Angles
Hlavinka and Hagadorn
Compound 1
Zn1-N1
Zn2-N3
2.218(2)
1.897(2)
Zn1-Zn2
Zn1-N2
4.0997(5)
Zn2-N4
2.215(2)
1.955(3)
Zn2-C31
1.952(3)
153.0(1)
1.909(2)
Zn1-C29
N2-Zn1-C29
N4-Zn2-C31
146.9(1)
120.9(1)
N1-Zn1-N2
N3-Zn2-N4
85.74(9)
85.6(1)
N1-Zn1-C29
126.1(1)
N3-Zn2-C31
Compound 2
Zn1-N1
Zn2-N3
2.209(3)
1.903(2)
Zn1-Zn2
Zn1-N2
3.9645(5)
1.905(2)
Zn2-N4
2.218(2)
1.944(3)
Zn2-C30
1.951(3)
151.0(1)
Zn1-C29
N2-Zn1-C29
N4-Zn2-C30
152.4(1)
122.2(1)
N1-Zn1-N2
N3-Zn2-N4
85.8(1)
85.7(1)
N1-Zn1-C29
120.8(1)
N3-Zn2-C30
Compound 3
Zn1-N1
Zn1-C27
Zn2-C27
2.149(3)
2.618(3)
2.008(3)
Zn1-N2
Zn2-N3
1.922(3)
1.940(3)
Zn2-C21
Zn1-C21
2.546(3)
2.013(4)
Zn2-N4
Zn1-Zn2
2.139(3)
2.7279(6)
N1-Zn1-N2
N3-Zn2-N4
85.8(1)
84.8(1)
N1-Zn1-C21
N3-Zn2-C27
115.2(1)
137.4(1)
N2-Zn1-C21
136.9(1)
N4-Zn2-C27
114.9(1)
Compound 4
Zn1-N1
Zn2-N3
1.899(5)
1.901(5)
Zn1-Zn2
Zn1-N2
4.020(1)
2.234(5)
Zn2-N4
2.225(5)
1.954(6)
Zn2-C35
1.945(7)
156.7(2)
Zn1-C29
N2-Zn1-C29
N4-Zn2-C35
121.9(2)
118.3(2)
N1-Zn1-N2
N3-Zn2-N4
85.7(2)
84.2(2)
N1-Zn1-C29
150.9(2)
N3-Zn2-C35
Compound 5
Zn1-O2
Zn1-C21
Zn2-N4
2.029(6)
2.086(8)
2.118(7)
Zn1-N1
Zn2-O2
2.131(7)
1.961(5)
Zn2-C21
Zn1-N2
2.183(9)
1.934(7)
Zn2-N3
Zn1-Zn2
1.948(7)
2.698(1)
O2-Zn1-N1
N1-Zn1-N2
O2-Zn2-N3
115.1(3)
84.5(3)
126.9(3)
N3-Zn2-N4
O2-Zn1-N2
N1-Zn1-C21
85.8(3)
119.5(3)
110.4(3)
O2-Zn2-N4
N3-Zn2-C21
O2-Zn1-C21
117.5(3)
127.8(3)
92.7(3)
N2-Zn1-C21
O2-Zn2-C21
N4-Zn2-C21
135.4(3)
91.7(3)
107.8(3)
Scheme 1
non-H atoms were refined anisotropically. Hydrogens were
placed in idealized positions and were included in structure
factor calculations but were not refined.
(d) Structure of iPrLZn2Ph2 (4). Crystals that were
suitable for X-ray diffraction studies were grown from toluene
at 20 °C. Preliminary data indicated a primitive monoclinic
cell. Systematic absences indicated space group P21/c (No. 14),
which was confirmed by the successful solution and refinement
of the structure. There are two independent molecules in the
asymmetric unit. Analysis using Platon did not reveal any
additional symmetry. All non-H atoms were refined anisotro-
pically. Hydrogens were placed in idealized positions and were
included in structure factor calculations but were not refined.
(e) Structure of MeLZn2(Ph)(OCHPh2) (5). Crystals that
were suitable for X-ray diffraction studies were grown from a
hexanes-CD2Cl2 mixture at ambient temperature. Prelimi-
nary data indicated a C-centered monoclinic cell. Systematic
absences were consistent with space groups Cc and C2/c. The
choice of Cc (No. 9) was confirmed by the successful solution
and refinement of the structure. All non-H atoms were refined
anisotropically. Hydrogens were placed in idealized positions
and were included in structure factor calculations but were
not refined. The Flack parameter was refined to 0.03(2),
indicating that the correct absolute structure was refined.
features an upfield quartet at δ 0.56 ppm for the
R-methylene protons of each Zn-Et group. The reso-
nances assigned to the bis(amidoamine) ligand reveal
that the complex has overall C2v symmetry in solution.
1H NMR spectroscopic data for 2 (CD2Cl2) are similar
and feature a sharp upfield singlet at δ -0.03 ppm for
the two equivalent Zn-Me groups. The solid-state
structures of 1 and 2 (Figure 1) were determined by
single-crystal X-ray diffraction. The overall features of
the two molecules are very similar; therefore, only the
structure of 1 will be described. It features a pair of
essentially independent three-coordinate Zn centers,
each of which is coordinated by an Et anion and a
chelating amidoamine donor. The geometry at Zn is best
described as between trigonal planar and T-shaped. The
sums of the angles for the three ligands bound to Zn1
and Zn2 are 358.7 and 359.5°, respectively. As expected,
the amidoamine ligands are coordinated unsymmetri-
cally, with the Zn-Namido bonds being, on average, 0.31
Å shorter than the dative Zn-Namine bonds. The Zn-Et
bonds of 1.955(3) and 1.952(3) Å are similar to other
reported Zn-C(sp3) bond lengths.12 The Zn-Zn distance
is 4.0997(5) Å. Compounds 1 and 2 are rare examples
of isolable three-coordinate organozincs.12 Closely re-
lated syntheses using simple chelating amidoamine
ligands have yielded only dimeric or oligomeric products
with four-coordinate Zn centers.13 For example, N,N,N′-
trimethylethylenediamine has been reported to undergo
Results and Discussion
The bis(diaminoethane) ligands shown in Chart 2
readily undergo protonolysis with common organozinc
reagents to form isolable bis(amidoamine) derivatives
of the general formula LZn2R2 (R ) alkyl, aryl). For
example, the reaction of iPrLH2 with excess ZnEt2 in
toluene solution at 60 °C formed iPrLZn2Et2 (1), which
was isolated as colorless crystals in 69% yield following
cooling to 5 °C (Scheme 1). The related reaction using
ZnMe2 formed iPrLZn2Me2 (2), which was isolated in 72%
1
yield. The H NMR spectrum of 1 in CD2Cl2 solution

Phenylation of Benzaldehyde by Dizinc Organometallics
Organometallics, Vol. 24, No. 22, 2005 5339
Scheme 2
The composition of 3 is identical with that of 4, except
Figure 1. (A) Structure of 1‚(toluene) drawn with 50%
thermal ellipsoids. Hydrogen atoms and cocrystallized
toluene are omitted. (B) Structure of 2‚1.5(toluene) drawn
with 50% thermal ellipsoids. Hydrogen atoms and cocrys-
tallized toluene are omitted.
that it is supported by the Me-substituted amidoamine
i
ligand (Me
L
2-) instead of the Pr-substituted one. The
use of this less bulky ligand results in a significant
structural change. The solid-state structure of 38 is
shown in Figure 2B. There are two molecules in the
asymmetric unit. Both feature a parallelogram-shaped
[Zn2(µ-Ph)2]2+ core with two strong Zn-C σ bonds
(average 1.99-2.01 Å) and two weak bridging interac-
tions (average 2.55-2.64 Å).14 The Zn-Ph σ bonds of 3
are longer than those of 4 by approximately 0.06 Å.
Thus, it appears that the additional bridging interac-
tions are achieved with some expense to the Zn-Ph σ
bonds. The two independent molecules have similar Zn-
Zn distances of 2.7279(6) and 2.7809(6) Å. In contrast
to the solid-state structure, which has approximate C2
symmetry, the 1H NMR spectrum of 3 (CD2Cl2) indicates
protonolysis with ZnR2 (R ) Me, Et) to form dimeric
[ZnR(µ-NMeCH2CH2NMe2)]2, which feature strong ami-
do bridges between the two Zn centers. Apparently the
steric bulk and rigidity of the iPrL2- ligand prevents
dimerization in 1 and 2 and allows for the isolation of
these relatively Lewis acidic Zn species. Attempts to
prepare analogues of 1 and 2 using the less bulky MeL2-
ligand afforded only insoluble products.
To study the reactivity and structural differences
between alkyl and arylzinc species, we have also pre-
pared a series of bimetallic arylzinc derivatives. The
reaction of iPrLH2 with 2 equiv of ZnPh2 at 75 °C led to
the elimination of benzene and the formation of iPrLZn2-
Ph2 (4) in 56% isolated yield (Scheme 2). MeLZn2Ph2 (3)
was prepared in good yield in a similar fashion. The
solid-state structure of 4 (Figure 2A) is very similar to
those of 1 and 2. Thus, 4 features a pair of essentially
independent three-coordinate Zn centers with geom-
etries that are between trigonal planar and T-shaped.
The T-shaped distortion can be attributed to the small
bite angle of the chelating amidoamine ligands (ca. 85°),
which is incompatible with trigonal-planar geometry.
With the T-shaped distortion, the two stronger donors
(Namido, Ph) occupy positions opposite to each other. This
is seen in the Ph-Zn-Namido angles of 150.9(2) and
156.7(2)° for Zn1 and Zn2, respectively. The Zn-Ph
bond lengths (Zn1-C29, 1.936(5); Zn2-C35, 1.947(6) Å)
are in the range expected for Zn-C(sp2) single bonds.12e
The Zn1-Zn2 distance of 4.022(1) Å is similar to that
of compounds 1 and 2.
(12) (a) Hannat, M.; Schormann, M.; Bochmann, M. Dalton Trans.
2002, 4071-4073. (b) Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J.
Am. Chem. Soc. 2001, 123, 8738-8749. (c) Liang, L.; Lee, W.; Hung,
C. Inorg. Chem. 2003, 42, 5471-5473. (d) Looney, A.; Han, R.; Gorrell,
I.; Cornebise, M.; Yoon, K.; Parkin, G. Organometallics 1995, 14, 274-
288. (e) Prust, J.; Stasch, A.; Zheng, W.; Roesky, H.; Alexopoulos, E.;
Uson, I.; Bohler, D.; Schuchardt, T. Organometallics 2001, 20, 3825-
3828.
Figure 2. (A) Structure of 4 drawn with 50% thermal
ellipsoids. One of two molecules in the asymmetric unit and
hydrogens are omitted. (B) Structure of 3 drawn with 50%
thermal ellipsoids. One of two molecules in the asymmetric
unit and hydrogens are omitted.
(13) Malik, M. A.; O’Brien, P.; Motevalli, M.; Jones, A. C. Inorg.
Chem. 1997, 36, 5076-5081.

5340 Organometallics, Vol. 24, No. 22, 2005
Hlavinka and Hagadorn
Scheme 3
overall C2v symmetry. Accordingly, the four Me groups
of the bis(amidoamine) ligand are equivalent and are
observed as a singlet at δ 2.10 ppm. The higher
symmetry of the solution structure may result from
cleavage of the weak Ph bridges and rotation about the
dibenzofuran-N bonds in solution. Alternatively, rapid
Ph group transfer between the Zn centers may occur to
give time-averaged C2v symmetry. However, at -90 °C
the 1H NMR spectrum did not indicate lower symmetry;
therefore, the simple cleavage of the Ph bridges probably
best explains the symmetric solution structure.
Organozinc reagents are commonly used as nucleo-
philes in addition reactions with unsaturated organics,
such as aldehydes,2 ketones,3 and olefins.15 Consistent
with this, we have found that the arylzinc species 3 and
4 readily react with electrophilic organics. For example,
the addition of 1 equiv of benzaldehyde to a C6H6
solution of 3 at ambient temperature formed the addi-
tion product MeLZn2(Ph)(OCHPh2) (5), which was iso-
lated in 87% yield as colorless crystals from CH2Cl2/
hexanes. The 400 MHz 1H NMR spectrum of 5 dissolved
in CD2Cl2 indicates overall Cs symmetry, which is
consistent with a structure featuring both bridging Ph
and alkoxide ligands. The Me groups of the dimethy-
lamino donors are observed as two sharp singlets at δ
2.30 and 1.34 ppm. There is also a distinct singlet at δ
6.11 ppm, which is assigned to the nonaromatic proton
of the diphenylmethoxide ligand. Resonances assigned
to the µ-Ph ligand are broad at room temperature. At
-70 °C, however, the ortho protons are observed as a
pair of distinct doublets at δ 7.96 and 7.27 ppm with
integrated intensities of one proton each. Thus at low
temperatures rotation about the Zn-Ph bond is slow.
Since this phenomenon was not observed for the related
arylzinc derivatives 3 and 4, this suggests that 5 has a
more robust Ph bridge. At higher temperatures, the
inequivalent ortho (and meta) protons of the Ph group
undergo dynamic exchange, with an observed coales-
cence temperature of -8 °C. At -50 °C, the rate
constant for the exchange was determined to be 25(1)
(2.029(6) Å) is nearly 0.07 Å longer than the Zn2-O2
bond. The µ-Ph group, however, has a bond to Zn1
(2.086(8) Å) that is 0.1 Å shorter than the bond to Zn2.
Thus, the Ph ligand is more strongly bound to Zn1 and
the alkoxide is more strongly bound to Zn2. As expected
for a bimetallic complex with two single-atom bridges,
the Zn-Zn distance is quite short (2.698(1) Å).
The synthesis of 5 involved the room-temperature
addition of one of the two Zn-Ph groups of 3 to
benzaldehyde. The remaining Ph group can also un-
dergo addition to benzaldehyde, but at a much slower
rate. Heating a C6H6 solution of 3 with 2 equiv of
benzaldehyde to 70 °C for 24 h formed the double-
addition product MeLZn2(OCHPh2)2 (6), which was iso-
lated as colorless crystals in 67% yield following the
addition of hexanes. The related iPrLZn2(OCHPh2)2 (7)
was formed in good yield by the reaction of 2 equiv of
benzaldehyde with 4 under similar conditions. The
composition of 6 is supported by NMR spectroscopic data
1
and combustion analysis. The H NMR spectrum of 6
(CD2Cl2) indicates overall C2v symmetry. The nonaro-
matic protons of the diphenylmethoxide ligands are
observed as a sharp singlet at δ 5.99 ppm. For compari-
son, the related proton of 5 was observed at δ 6.11 ppm.
The methyls of the dimethylamino groups are all
equivalent (in 6) and are observed as a singlet at δ 1.81
ppm. These data and intuition suggest that 6 features
two bridging alkoxide ligands in solution. The charac-
terization of 6 by single-crystal X-ray diffraction con-
firmed this structural motif in the solid state, but the
data are not presented here due to difficulties in the
refinement of the data set.
s-1 and ∆Gq was calculated to be 11.5(1) kcal mol-1
.
These values are not affected by the addition of 1 equiv
of benzaldehyde. The H NMR spectroscopic data are
1
consistent with 5 having a permanent µ-diphenyl-
methoxide ligand and a Ph ligand that undergoes
reversible cleavage of its bridging interaction (Scheme
3). This cleavage then allows for free rotation about the
Zn-Ph bond and scrambling of the inequivalent ortho
(and meta) protons. To our knowledge, the low-temper-
ature NMR study of 5 has provided the first observation
of a Ph bridge between Zn centers in the solution state.
The solid-state structure of 5 is shown in Figure 3.
The molecule features a [Zn2(µ-Ph)(µ-OCHPh2)]2+ core
that is significantly puckered. The plane defined by
Zn1-Zn2-C21 intersects that defined by Zn1-Zn2-O2
at an angle of 35.5(2)°. The bond lengths of the core
reveal a significant amount of asymmetry in the bonds
between Zn and the bridging ligands. The Zn1-O2 bond
(14) Other structurally characterized complexes with [Zn2(µ-Ph)2]2+
cores: (a) Markles, P. R.; Schat, G.; Akkerman, O. S.; Bickelhaupt, F.;
Smeets, W. J. J.; Spek, A. L. Organometallics 1990, 9, 2243-2247. (b)
Dickson, R. S.; Fallon, G. D.; Zhang, Q.-Q. Dalton Trans. 2000, 1973-
1974.
(15) (a) Alexakis, A.; Polet, D.; Rosset, S.; March, S. J. Org. Chem.
2004, 69, 5660-5667. (b) Choi, H.; Hua, Z.; Ojima, I. Org. Lett. 2004,
6, 2689-2691.
Figure 3. Structure of 5 drawn with 50% thermal el-
lipsoids. Hydrogens are omitted.

Phenylation of Benzaldehyde by Dizinc Organometallics
Organometallics, Vol. 24, No. 22, 2005 5341
The addition of the Zn-Ph group of 5 to benzaldehyde
may occur by an ambifunctional mechanism similar to
that proposed for DAIB-catalyzed additions (Scheme 1).
Thus, cleavage of the Ph bridge of 5 would be followed
by binding of benzaldehyde to the three-coordinate Zn
center. This forms an intermediate in which the nu-
cleophilic Ph group could attack the electrophilic car-
bonyl with the formation of a cyclic six-membered
transition state.
Acknowledgment. We thank M. J. McNevin for the
structure of 4, Dr. Richard Shoemaker for assistance
acquiring and interpreting NMR spectroscopic data, and
the donors of the Petroleum Research Fund, adminis-
tered by the American Chemical Society, for funding.
NMR instrumentation used in this work was supported
in part by the National Science Foundation CRIF
program, Award No. CHE-0131003.
In conclusion, we have used sterically hindered bis-
(amidoamine) ligands to prepare a new class of coordi-
natively unsaturated dizinc organometallics. Members
of this class of complexes readily undergo addition to
benzaldehyde to form dizinc alkoxide products. These
reactions may be of relevance to asymmetric additions
of dialkylzincs to aldehydes and ketones that are
effectively catalyzed by DAIB and other related ligands.
Supporting Information Available: Crystallographic
information files (CIF) are provided for compounds 1, 2, 4, and
5. This material is available free of charge via the Internet at
http://pubs.acs.org.
OM050580E