Reactions of ZnR2 compounds with dibenzoyl: Characterisation of the alkyl-transfer products and a striking product-inhibition effect

Article abstract of DOI:10.1002/chem.201101997

The first systematic theoretical and experimental studies of reaction systems involving ZnR₂ (R=Me, Et or tBu) with dibenzoyl (dbz) as a non-innocent ligand revealed that the character of the metal-bonded R group as well as the ratio of the reagents and the reaction temperature significantly modulate the reaction outcome. DFT calculations showed four stable minima for initial complexes formed between ZnR₂ and dbz and the most stable structure proved to be the 2:1 adduct; among the 1:1 adducts three structural isomers were found of which the most stable complex had the monodentate coordination mode and the chelate complex with the s-cis conformation of the dbz unit appeared to be the least stable form. Interestingly, the reaction involving ZnMe₂ did not lead to any alkylation product, whereas the employment of ZntBu₂ resulted in full conversion of dbz to the O-alkylated product [tBuZn{PhC(O)C(OtBu)Ph}] already at -20-°C. A more complicated system was revealed for the reaction of dbz with ZnEt₂. Treatment of a solution of dbz in toluene with one equivalent of ZnEt ₂ at room temperature afforded a mixture of the O- and C-alkylated products [EtZn{PhC(O)C(OEt)Ph}] and [EtZn{OC(Ph)C(O)(Et)Ph}], respectively. The formation of the C-alkylated product was suppressed by decreasing the initial reaction temperature to -20-°C. Moreover, in the case of the dbz/ZnEt ₂ system monitoring of the dbz conversion over the entire reaction course revealed a product inhibition effect, which highlights possible participation of multiple equilibria of different zinc alkoxide/ZnEt₂ aggregates. Diffusion NMR studies indicated that dbz forms an adduct with the O-alkylated product, which is a competent species for executing the inhibition of the alkylation event. It all depends on R: The first systematic theoretical and experimental study of ZnR₂ (R=Me, Et or tBu) with dibenzoyl revealed that the character of the metal-bonded alkyl group as well as the reaction conditions (e.g., ratio of the reagents, temperature) modulate the reaction outcome (see scheme). Monitoring the dibenzoyl conversion over the reaction course revealed a product-inhibition effect, which highlights possible participation of multiple equilibria of different zinc alkoxide/ZnR₂ aggregates.

Full text of DOI:10.1002/chem.201101997

FULL PAPER
DOI: 10.1002/chem.201101997
Reactions of ZnR2 Compounds with Dibenzoyl: Characterisation of the
Alkyl-Transfer Products and a Striking Product-Inhibition Effect
Izabela Dranka,[a] Marcin Kubisiak,[a] Iwona Justyniak,[b] Michał Lesiuk,[a]
Dominik Kubicki,[a] and Janusz Lewin´ski*[a, b]
Dedicated to Professor Janusz Jurczak on the occasion of his 70th birthday
Abstract: The first systematic theoreti-
cal and experimental studies of reac-
tion systems involving ZnR2 (R=Me,
Et or tBu) with dibenzoyl (dbz) as a
non-innocent ligand revealed that the
character of the metal-bonded R group
as well as the ratio of the reagents and
the reaction temperature significantly
modulate the reaction outcome. DFT
calculations showed four stable minima
for initial complexes formed between
ZnR2 and dbz and the most stable
structure proved to be the 2:1 adduct;
among the 1:1 adducts three structural
isomers were found of which the most
stable complex had the monodentate
coordination mode and the chelate
complex with the s-cis conformation of
the dbz unit appeared to be the least
stable form. Interestingly, the reaction
involving ZnMe2 did not lead to any al-
kylation product, whereas the employ-
ment of ZntBu2 resulted in full conver-
sion of dbz to the O-alkylated product
products
[EtZnACTHNGUTERNNU{G PhC(O)CACHTUNGTRENN(UNG OEt)Ph}]
and [EtZn{OC(Ph)C(O)(Et)Ph}], re-
spectively. The formation of the C-al-
kylated product was suppressed by de-
creasing the initial reaction tempera-
ture to ꢀ208C. Moreover, in the case
of the dbz/ZnEt2 system monitoring of
the dbz conversion over the entire re-
action course revealed a product inhib-
ition effect, which highlights possible
participation of multiple equilibria of
different zinc alkoxide/ZnEt2 aggre-
gates. Diffusion NMR studies indicated
that dbz forms an adduct with the O-al-
kylated product, which is a competent
species for executing the inhibition of
the alkylation event.
[tBuZnACTHNUTRGENNU{G PhC(O)CHCATUNGTREN(NUGN OtBu)Ph}] already
at ꢀ208C. A more complicated system
was revealed for the reaction of dbz
with ZnEt2. Treatment of a solution of
dbz in toluene with one equivalent of
ZnEt2 at room temperature afforded a
mixture of the O- and C-alkylated
Keywords: alkoxides · alkylation ·
ketones · structure–activity relation-
ships · zinc
Introduction
structures essential in the autocatalytic cycle and the am-
plification process. It is also peculiar that in another funda-
mental reaction, the asymmetric alkylation of benzaldehyde
by ZnEt2,[3] the noticed influence of the conversion on the
enantioselectivity of the alkoxide product was attributed to
product inhibition, but without a clear structural model.[1c]
In this regard, our group implied the possible participation
of multiple equilibria, including a dimeric, tetranuclear ag-
gregate formed between the alkoxide product and ZnR2
moieties in the donor-functionalised, alcohol-promoted addi-
tion of dialkylzinc to aldehydes, which was supported by
structurally well-characterised model complexes.[4] Undoubt-
edly, further systematic elucidation of the mechanisms by
which carbonyls react with organozinc species is a challeng-
ing task and may help to broaden our understanding of
these complex reaction systems.
The influence of a zinc alkoxide product on the rate and/or
course of various reactions involving dialkylzinc reagents
and carbonyl compounds is an important issue of synthetic
chemistry.[1] The most spectacular example concerns the Soi
autocatalytic asymmetric addition of diisopropylzinc to pyri-
midinyl aldehydes,[1b,d] where both the role of the zinc alkox-
ide product and the structural model of the catalytic centre
have been the subject of intense investigations in the last
decade.[2] In the latter system, the presence of dimers and
higher aggregates of the alkoxides and their adducts with
diisopropylzinc have been assumed as the key molecular
[a] Dr. I. Dranka, M. Kubisiak, M. Lesiuk, D. Kubicki,
´
Prof. Dr. J. Lewinski
Surprisingly, a-diketones have not yet been explored in
reactions with ZnR2 or main group-metal alkyls in contrast
to the related rich chemistry of their non-innocent N,N-type
analogues, that is, a-diimines.[5] Formally, a-diketones can
act as bidentate ligands or undergo oxidative addition reac-
tions accepting one or two electrons. Nevertheless, only two
metal complexes with a neutral a-diketone ligand were
structurally characterised hitherto[6] and the first complex
Faculty of Chemistry Warsaw University of Technology
Noakowskiego 3, 00-664 Warsaw (Poland)
Fax : (+48)22-234-7279
E-mail: lewin@ch.pw.edu.pl
´
[b] Dr. I. Justyniak, Prof. Dr. J. Lewinski
Institute of Physical Chemistry Polish Academy of Sciences
Kasprzaka 44/52, 01-224 Warsaw (Poland)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101997.
Chem. Eur. J. 2011, 17, 12713 – 12721
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with a singly reduced form of an a-diketone was isolated
only recently.[7] In the course of our systematic investigation
on the structure and reactivity of organozinc complexes, we
report herein on the first systematic experimental and theo-
retical studies of reaction systems involving ZnR2 (R=Me,
Et or tBu) with dibenzoyl (dbz). We provide computational
evidences for intriguing thermodynamic preference for ini-
tial adducts formed between ZnR2 and dibenzoyl, as well as
the first structurally characterised examples of products of
the alkyl transfer reaction from zinc to an a-diketone. More-
over, we demonstrate a striking zinc alkoxide product inhib-
ition effect with its mechanistic picture. The study also pro-
vides an efficient access to alkylzinc enolates,[8] which poten-
tially may be further exploited as valuable reagents in or-
ganic synthesis[9] or initiators in polymerisation processes.[10]
the O- and C-alkylated products, [EtZnACTHNGURTENN{UG PhC(O)CACTHNUGTREN(NGNU OEt)Ph}]
(2) and [EtZn{OC(Ph)C(O)(Et)Ph}] (2’), in a 1.5:1 ratio
1
(Scheme 1); as the H NMR spectrum of the reaction mix-
ture after the reaction is complicated and not informative,
the alkylation products were identified spectroscopically
after hydrolysis of the crude product (see the Supporting In-
formation for more details). However, the formation of the
C-alkylated product was suppressed by decreasing the initial
reaction temperature to ꢀ208C followed by slow warming
up to ambient temperature, which resulted in a highly selec-
tive formation of the O-alkylated product 2 (2/2’ ratio
>19:1). The observed divergent pathways of the alkylation
reaction affected by the character of the R group are similar
to those found in a-diimine/R2Zn systems, where the tert-
butyl reagent tends to alkylate the nitrogen, whereas the
ethyl reagent leads to a mixture of N- and C-alkylated pro-
ducts.[5a]
Results and Discussion
Strikingly, the corresponding experiment in an NMR tube
showed that only approximately half of the amount of
ZnEt2 is consumed initially in the first few minutes (Fig-
ure 1a) and in the next 2–3 days the signals assigned to
Treatment of a solution of dibenzoyl in toluene with one
equivalent of ZnMe2 resulted in a colour change from light
yellow to orange; this observation was the only indication of
a complex formation between those reagents and IR spec-
troscopy appeared to be not a sensitive probe for detecting
an equilibrium between the chelated and free dibenzoyl (see
the Supporting Information). The reaction did not lead to
any methylation products, even after extended periods of
time at room temperature. In contrast, employment of
ZntBu2 resulted in a full conversion of dbz upon mixing of
the substrates in toluene already at ꢀ208C (Scheme 1). This
Scheme 1. Schematic representation of the reaction products formed in
the dbz/ZnR2 system at 208C.
fast reaction led to the O-alkylated product [tBuZn-
ACHTUNGTRENNUNG{PhC(O)CACHTUNGTRENNUNG(OtBu)Ph}] (1), which was isolated with high
yield after a standard work up. The NMR data are fully con-
sistent with the anticipated formula of 1 (see the Supporting
Information), which was confirmed by X-ray crystallography
(see below).
Unexpectedly, a more complicated system was revealed
when we set to investigate the reaction of dbz with ZnEt2.
Treatment of a solution of dbz in toluene with one equiva-
lent of ZnEt2 at room temperature afforded a mixture of
Figure 1. 1H NMR spectra for the alkylation of dbz with ZnEt2: a) after
30 min from mixing of the substrates at ꢀ208C and warming up to 208C;
b) the crude product 2 (* indicates the residual signal of [D8]toluene
residual signal).
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Reactions of ZnR2 Compounds with Dibenzoyl
FULL PAPER
ZnEt2 gradually decrease with concomitant increase of
those associated with 2. The 1H NMR spectrum of 1 in
[D8]toluene showed broad signals for the ethoxide group at
d=1.15 and 3.78 ppm, associated with the ethoxy-enolate
ligand, and the corresponding broad signals of the ZnEt
moiety at d=0.46 and 1.51 ppm (Figure 1b). The observed
broadness of all the ethyl group signals apparently reflects
fluxional behaviour of the resulting alkylzinc–alkoxide ag-
gregates in solution. Reaction progress for alkylation of di-
benzoyl with ZnEt2 is best expressed by in-situ-monitored
conversion of dibenzoyl as a function of time (Figure 2).
Figure 2. Conversion versus time diagram for the alkylation of dbz with
an equimolar amount of ZnR2 in [D8]toluene.
The observed complicated character of the conversion plot
of dibenzoyl with an initial steep slope followed by a sharp
change to a slower rate could be reasonably explained by
the assumption that product 2 forms an adduct with ZnEt2,
which effectively leads to the inhibition of the alkylation re-
action. Previously we have implied the possible participation
of multiple equilibria of different Zn aggregates in RZn-
Figure 3. a) Optimised structures of the ZnEt2 complexes with dbz
(ZPE=zero-point energy). b) DFT-calculated relationships between the
complexes of ZnEt2 with dbz.
ACHTUNGTRENNUNG(O,N)/ZnR2 systems and suggested the involvement of ad-
ducts between alkylzinc alkoxides and ZnR2 moieties, which
was supported by structurally well-characterised model com-
plexes.[4] In light of the assumed ZnEt2 deficiency in the re-
action mixture, we subsequently treated dibenzoyl with an
excess of ZnEt2. Surprisingly, when two or more equivalents
of ZnEt2 were used, the alkylation reaction was further in-
hibited and repeatedly only a very low conversion (<5%
for the ZnMe2/dbz system (for the corresponding data see
Figures S3 and S4 in the Supporting Information), therefore
below we discuss in detail the diethylzinc adducts. The cal-
culations show four stable minima for adducts formed be-
tween ZnEt2 and dbz, and the most stable structure proves
to be the 2:1 adduct I (Figure 3a). Remarkably, among the
1:1 adducts three structural isomers were found, of which
the most stable complex II has a monodentate coordination
mode and is 28.7 kJmolꢀ1 higher in energy than the 2:1
adduct. Another stable minimum for the 1:1 system ap-
peared to be the semi-chelated complex III stabilised by a
number of cooperative weak interactions, which was found
to be only 1.6 kJmolꢀ1 higher than the monodentate adduct
II. Surprisingly, the chelate complex IV with the s-cis confor-
mation of the dbz unit appeared to be the least stable and
was calculated as 28.6 kJmolꢀ1 higher in energy than the
monodentate adduct II. Puzzling feature of this calculation
is the observed anticlinal conformation of the dbz unit in ad-
ducts I–III, which is essentially the same as that found for
the free dbz in its ground state[12] (Figure 3b). This intrigu-
ing conformation preference was inspected by computing
found by NMR spectroscopy) of dibenzoyl to
2 was
found.[11] This observation clearly indicated that it is not the
ZnEt2 deficiency that blocks the alkylation of dbz but rather
its excess. The results forced us to embark on the examina-
tion of less obvious parameters.
In order to explore the nature of the species formed in so-
lution between ZnR2 and dibenzoyl in more detail, we have
used density functional theory. Calculations of the structures
and their relative energies for the 2:1 and 1:1 adducts were
carried out by using the Gaussian 09 program package with
the M06-2X functional and the aug-cc-pVTZ basis set level
of theory.[23] The energy profile for the ZnEt2/dbz system is
shown in Figure 3a and the optimised geometries of the key
species are given in Figure 3b. Similar data were obtained
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´
J. Lewinski et al.
ꢀ
ð2Þ
the barriers for rotation about the central C C bond, which
M ¼ ð4prM31NAÞ=3
showed that the anticlinal conformation is more stable (by
39 kJmolꢀ1) than the planar s-cis conformation. Thus, this
thermodynamic preference of the anticlinal conformation is
a key for understanding the energetic relationships among
the ZnEt2 and dibenzoyl adducts and causes the planar s-cis
chelate adduct IV to be the least stable structure.
where rH and rM are the hydrodynamic and the molar radius,
respectively, h is the viscosity and 1 is the density of the
liquid.[13] 1H DOSY spectra (measured at 293 K) of the solu-
tion exhibiting approximately 50% conversion of dbz to 2,
show that all distinct signals assigned to the alkoxide com-
plex have comparable diffusion coefficients D=5.42ꢂ
10ꢀ10 m2 sꢀ1, which gave an estimate of the hydrodynamic
radius of 4 ꢁ and further the calculated molecular weight of
an aggregate likely formed by 2 of 866 gmolꢀ1. The latter
value is significantly higher than the formula weight calcu-
lated for a dimeric structure of 2 (667.46 gmolꢀ1). However,
with the formula weights of dbz and ZnEt2 being 210.23 and
123.50 gmolꢀ1, respectively, one can clearly see that the pre-
sumptive dimeric structure 22 (2ꢂ(210.23+123.50)=
667.46 gmolꢀ1) with one dbz
ꢀ
Furthermore, all complexes feature relatively long Zn O
distances with the lower limit of 2.50 ꢁ and an essentially
linear structure of the ZnEt2 molecules (with the C-Zn-C
angle ranging from 168.18 in the chelate complex to 171.88
in the 2:1 adduct). Some adducts are additionally stabilised
by hydrogen-bonding interactions between the p-electron
density of the phenyl residues and the hydrogen atoms of
the ZnEt moieties (see adduct I and II, Figure 3b). In our
opinion, taking all these features into account, it is justified
to call adducts formed between ZnR2 and dbz noncovalent
complexes. Finally, it should be stressed that thermodynami-
cally the process of their formation is exothermic by
35.4 kJmolꢀ1 for the monodentate complex I, or by
5.3 kJmolꢀ1 for the chelate complex IV in the gas phase,
and that it is not significantly affected by the nature of R
(the corresponding value for analogous adducts with ZnMe2
are 24.2 and 5.6 kJmolꢀ1, respectively). The energy differ-
ence among the structural isomers is below 60 kJmolꢀ1,
which indicates that a multitude of species could be simulta-
neously present in equilibrium. The results demonstrate that
the formation of the 2:1 adduct is favoured, which corrobo-
rates with the experimental details, that is, an excess of
ZnEt2 should shift the equilibrium towards this dinuclear,
monodentate complex, which may nicely explain the ob-
served inhibition of the alkylation reaction with increasing
amount of ZnEt2. Thus, in these circumstances our prelimi-
nary assumption that the formation of an adduct involving
product 2 and ZnEt2 leads to inhibition of the alkylation re-
action rather does not match. It is likely that in the mono-
molecule involved, gives the
total
of
667.46+210.23=
2A(dbz)
877.69 gmolꢀ1 for the 2
CTHUNGTRENNUNG
adduct, which fairly resembles
the experimental data; the pu-
tative structure of this adduct is
shown on Scheme 2. Thus, the
results indicate that dibenzoyl
forms an adduct with the alkox-
ide product 2, thereby strongly
Scheme 2. Proposed structure
for the 22A(dbz) adduct as struc-
tural model for the dormant
species present in the dbz/
ZnEt2 system.
CTHUNGTRENNUNG
suggesting that the
22ACTHNGUTERNN(UG dbz)
adduct is a competent species
for executing the inhibition of
the alkylation event; the re-
vealed nature of the product inhibition effect for the addi-
tion of ZnEt2 to dbz is opposite in its character to the ob-
served effect for the addition of phenylacetylene to carbon-
yls promoted by Me2Zn where the carbonyl substrate itself
was implicated as a ligand facilitating the zinc-mediated ad-
dition of alkynes to the substrate.[14]
dentate, three-coordinate adduct {PhC(O)C(O)Ph}
the a-diketone unit acts as an innocent ligand and the for-
mation of the chelate complex {k2
(O,O)-PhC(O)C(O)Ph}-
ACHTUNGERTN(NUNG ZnEt2)
The presumed structure of the 22ACTHUNTGRNEUNG(dbz) adduct resembles
to some degree the ones observed for 22 and 1,2-bis(4-pyri-
AHCTUNGTRENNUNG
ZnEt2 is crucial for further transformations to proceed (see
below).
dyl)ethane (see below). This adduct features a non-centro-
symmetric dimer with the Zn2ACHTNUGRTENUNG(m-O)2 bridges formed by the
To shed more light on the putative role of the zinc alkox-
ide product 2 in the observed course of the equimolar reac-
tion, diffusion-ordered NMR spectroscopy (DOSY) investi-
gations on the dibenzoyl/ZnEt2 system were performed.
1H DOSY measurements provide a reliable evidence of the
approximate size of the species existing in solution. Very re-
cently, it was demonstrated that the molecular weight of the
species in question can be straightforwardly obtained by
combining the Stokes–Einstein equation [Eq. (1)]:
enolate oxygen atoms, one chelating and one dangling eno-
late ligand, and the coordination environment of one zinc
centre is completed by a chelating dibenzoyl ligand. It is
likely that the adduct formed between the alkoxide product
2 and dbz is additionally stabilised by intramolecular nonco-
valent interactions involving the phenyl rings and various
oxygen centres (see below). It is also worthy to note that
the formation of this type of adduct simultaneously supports
the formation of the 2:1 adduct between ZnEt2 and dbz in
the parent reaction mixture, according to the Equation (3),
which cooperatively should suppress the alkylation reaction.
D ¼ ðkTÞ=ð6phrHÞÞ
ð1Þ
4 dbzþ4 Et2Zn ! 22ðdbzÞþðdbzÞðEt2ZnÞ2
ð3Þ
and the relationship between the molecular weight M and
the molar radius [Eq, (2)]:
The observed number of salient features of the dbz/ZnR2
system allows for mechanistic rationalisation of this intrigu-
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Reactions of ZnR2 Compounds with Dibenzoyl
FULL PAPER
ing reaction system. As it was mentioned above, the forma-
tion of the chelate complex {k2
ACTHUNTRGENNG(U O,O)dbz}ZnR2 is the first
crucial step for the alkylation process. Only for this type of
highly organised, four-coordinate chelate complexes the
non-innocent character of the dibenzoyl ligand is preserved
as a result of the accessibility and symmetry properties of
the p* molecular orbitals, which allows single-electron trans-
ꢀ
fer (SET) from the Zn C bond (Scheme 3). The subsequent
alkyl radical transfer to carbonyl oxygen atom (or carbon
atom) of the ligand leads to the alkylated product.
Scheme 3. Alkyl radical transfer following the SET in the ZnR2/a-di-
ketone chelate complex, which leads to the formation of the alkylation
products.
All processes in these first stages are fast for ZnEt2 and
ZntBu2, and resemble those proposed for the a-diimine/
ZnR2 system.[5] The observed lack of dbz alkylation by
ꢀ
ZnMe2 is likely due to the character of the Zn Me bond,
which is less prone to the single-electron transfer (e.g., a
similar effect was observed for the reactions with a-di-
ACHTUNGTRENNUNG
imines[15]) as the formation of the initial chelate complex is
not significantly affected by the nature of R (see above).
Furthermore, in the second slow distinct stage, the alkylated
product traps dbz to form an aggregate, which leads to the
observed inhibition of the alkylation reaction. In our opin-
ion, the observed differences in the dbz conversion mediat-
ed by ZnEt2 and ZntBu2 are an additional instance that
highlights the role of an intermediate adduct between the
resulting O-alkylated species and the carbonyl substrate.
The putative intermediate aggregate between 12 and dbz is
likely to be unstable due to steric encumbrances and does
not affect the alkylation reaction. These observations are in
line with another type of product inhibition effect disclosed
recently by our group in the course of the oxygenation of al-
kylzinc chelate complexes, where the resulting alkylperoxide
species and the parent alkylzinc complex formed an
adduct.[16] Only destabilisation of this type of adducts, for
Figure 4. a) ORTEP diagram of the molecular structure of 12 with ther-
mal ellipsoids set at 35% probability; hydrogen atoms have been omitted
for clarity. b) 1D channels of 12 along the c axis with entrapped toluene
molecules (shown in space-filling mode). Selected bond lengths [ꢁ] for
ꢀ
ꢀ
ꢀ
ꢀ
12: Zn1 C1 1.991(2), Zn1 O1 1.999(1), Zn1 O1’ 2.032 (2), Zn1 O2’
ꢀ
2.257(1), Zn1 O2 2.257(1).
idyl)ethane at ambient temperature afforded the crystalline
Lewis acid–base adduct {[22]
ACHTUNGTRENNUNG
Compound 12 is a centrosymmetric dimer with the Zn CHTUNGTRENNUNG
O)2 bridges formed by the enolate oxygen atoms and with
each zinc having a distorted tetrahedral geometry (Fig-
ꢀ
ure 4a). The Zn O bridging bonds fall into the narrow
ꢀ
example, by introduction of steric encumbrances led to the
range of 1.999–2.032 ꢁ and the Zn O distance (2.257(1) ꢁ)
[17]
ꢀ
complete oxygenation of the Zn C bonds.
involving the chelating OtBu group is significantly longer.
The C=C bond with 1.347(3) ꢁ is characteristic for a normal
double bond observed for zinc enolate complexes.[8]Analysis
of the crystal structure of 3 revealed that coordination-
driven self-assembly of the dimeric alkoxide [EtZn-
The identity of the alkylated products has been confirmed
by X-ray crystallography (Figure 4). Although the tert-buty-
lated product 1 easily crystallises from a solution of THF/
hexane, attempts to grow crystals of the O-ethylated product
2 were unsuccessful due to its excellent solubility in non-co-
ordinating solvents. Nevertheless, our group has recently
demonstrated that a combination of N,N-linkers with well-
ACHUTNRGEN{NUG PhC(O)CCAHTUNGTREN(NUNG OEt)Ph}]2 units with the bipyridine spacer of an
anti conformation leads to a novel zigzag-like coordination
polymer (Figure 5b).
soluble organometallic entities provides
a
convenient
In the 22 unit the O-alkylated dibenzoyl acts as a mono-
method for entrapment of elusive species in form of various
extended assemblies.[18] Given this experience, treatment of
a solution of 2 with 0.5 equivalent (per zinc) of 1,2-bis(4-pyr-
ACHTUNGTRENaNGNU nionic ligand with a bridging enolate oxygen atom and a
ꢀ
ꢀ
chelating ethoxide group; the average C C and C O distan-
ces within this ligand are very close to those found for 1.
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J. Lewinski et al.
teractions between aromatic rings lead to 3D networks with
slightly gated voids or open channels, respectively, filled by
solvent molecules. The corresponding supramolecular ar-
rangements are emphasised in the skeletal space filling dia-
grams presented in Figures 4b and 5b. Thus, the results ad-
ditionally demonstrate that alkylzinc enolates derived from
dibenzoyl can potentially act efficiently as predesigned sec-
ondary building blocks for the construction of coordination
polymers and novel microporous architectures by coordina-
tion-driven and non-covalent interactions-driven self-assem-
bly.
Conclusion
In conclusion, we have performed for the first time system-
atic experimental and theoretical investigations of ZnR2
(R=Me, Et and tBu) with dibenzoyl as a non-innocent a-di-
ketone ligand and revealed that the character of the metal-
bonded R substituent as well as the reaction conditions, like
temperature or the ratio of the reagents, significantly modu-
late the outcome of the reaction. The reaction involving
ZnMe2 did not lead to any alkylation products at ambient
temperature, whereas the employment of ZntBu2 resulted in
full conversion of dbz to the O-alkylated product already at
ꢀ208C. The addition of ZnEt2 to dibenzoyl appeared partic-
ularly complex and monitoring of the dibenzoyl conversion
over the entire course of the alkylation reaction revealed
the product inhibition effect, which highlights possible par-
ticipation of multiple equilibria of different zinc alkoxide ag-
gregates. The reported results should be helpful for a ration-
al organozinc reagent design and shall stimulate further ex-
perimental and theoretical investigations to clarify the cor-
relation between structure and activity of organozinc spe-
cies. In addition, the alkylzinc alkoxides derived from
dibenzoyl appear intriguing metallotectons for the construc-
tion of coordination polymers and novel microporous archi-
tectures by coordination-driven and non-covalent interac-
tions-driven self-assembly. Further studies on the reactivity
of a-diketone/ZnR2 systems and factors controlling the
degree of aggregation of alkylzinc alkoxides are in progress.
Figure 5. a) ORTEP diagram of the molecular structure of 3 with thermal
ellipsoids set at 35% probability; hydrogen atoms have been omitted for
clarity. b) 1D channels of 3 along the c axis with entrapped toluene mole-
cules (shown in space-filling mode). Selected bonds lengths [ꢁ] for 3:
ꢀ
ꢀ
ꢀ
ꢀ
Zn1 C1 1.976(8), Zn1 O1 2.025(5), Zn1 O2 2.669(6), Zn1 O3 2.125(5),
ꢀ
ꢀ
ꢀ
ꢀ
Zn2 O1 2.120(6), Zn2 O3 2.020(6), Zn2 O4 2.674(6), Zn1 N1 2.101(7),
ꢀ
Zn2 N2 2.065(7).
The pyridine ligands complete the coordination sphere of
ꢀ
the EtZn centres with an average Zn N bond length of
ꢀ
2.083 ꢁ. The Zn O distances within the central Zn
2A
ring in 3 differ slightly and range from 2.020–2.125 ꢁ. The
distances between the zinc centres and the ethoxide oxygen
atoms are rather large with an average of 2.669 ꢁ (drawn by
dashed lines on Figure 5a) and they should be rather de-
scribed as incipient bonding interactions.[19] Not surprisingly,
this interaction essentially does not affect the coordination
geometry of the zinc centres, which can be best described as
severely distorted tetrahedrons. It should also be emphas-
ised that compound 3 represents the first example of a coor-
dination polymer with an alkylzinc–alkoxides aggregate as a
node. Furthermore, analysis of the crystal structure of 12 and
3 revealed that in both cases non-covalent interactions-
Experimental Section
General methods: All reactions were conducted under a nitrogen atmos-
phere by using standard Schlenk techniques. Diethylzinc (pure), dime-
thylzinc (2m solution in toluene), 1,2-bis(4-pyridyl)ethane and dibenzoyl
of commercial grade (Sigma–Aldrich). were used. Di-tert-butylzinc (1.4m
solution in toluene) was prepared according to the literature proce-
dure.[20] Solvents were carefully dried and distilled prior to use. NMR
spectra were acquired on a Varian Mercury 400 MHz spectrometer. Low
temperature and diffusivity measurements were performed on a Varian
Inova 500 MHz instrument.
Caution: Zinc reagents (Me2Zn, Et2Zn, tBu2Zn) used in the reactions de-
scribed below are pyrophoric compounds reacting violently with oxygen
and water. Special care must be taken when operating with these com-
pounds.
ꢀ
driven self-assembly processes involving C H–p and p–p in-
12718
www.chemeurj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 12713 – 12721

Reactions of ZnR2 Compounds with Dibenzoyl
FULL PAPER
Diffusivity measurement: Application of diffusion-ordered spectroscopy
(DOSY), which has happened to become a very routine NMR technique
lately, provided us with a reliable evidence of approximate size of the
species existing in solution. As already discussed in antecedent works[13]
the molecular weight can be straightforwardly obtained by combining
Stokes–Einstein equation [Eq. (1)] and the relationship between molecu-
lar weight M and molar radius [Eq. (2)]. It has also been shown that
these quantities are much alike for small molecules, which is the case in
our system. DOSY measurements were performed in dry and degassed
[D8]toluene as we exploited the fact that the physical properties of the
diluted solution, namely density and viscosity, deviate only slightly from
the properties of the pure solvent.[13] Solutions were prepared so that the
concentration of the resulting species emerging in the course of the reac-
tion was approximately 0.01m. A pulsed field gradient double-stimulated
echo convection-compensated sequence with total of 15 diffusion encod-
ing gradients (ranging from 3–45 Gcmꢀ1, sine shaped, equal steps in gra-
dient squared) was used and the total width of the gradient pulse was op-
timised to achieve attenuation of about 90% of the initial intensity of the
signals. Overall, the key acquisition parameters were as follows: total
length of gradient encoding pulses gradient: 2 ms, diffusion delay: 50 ms,
gradient recovery delay: 1 ms, relaxation delay: 2 s. Steady-state scans in
number of 16 were performed prior to data acquisition. The sample was
allowed to equilibrate in the set temperature (298 K), controlled by a VT
module. Spinner was turned off. Raw data were processed with a power-
ful DOSY toolbox, which is extensively described in the literature.[21]
(m, 18H; CACHTUNGTREU(NNG CH3)3), 7.51 (m, 4H; CHAr), 7.69 (m, 2H; CHAr), 7.97 ppm
(m, 4H; CHAr).
Low temperature reaction of dibenzoyl with Et2Zn (2): Et2Zn (0.09 mL,
0.111 g, 0.90 mmol) was added to a yellowish solution of dibenzoyl
(0.189 g, 0.90 mmol) in diethyl ether (6 mL) at ꢀ208C. The reaction mix-
ture was slowly warmed up to room temperature over 1 h and the reac-
tion mixture was stirred for further 36 h at room temperature. The sol-
vent was removed from the resulting intensively yellow reaction mixture
under reduced pressure to give the crude product 2 as a white solid.
1H NMR (500 MHz, [D8]toluene, ꢀ308C): d=0.40 (2H; Zn(1)CH2), 0.60
(brs, 2H; Zn(2)CH2), 0.92 (brs, 3H; Zn(2)CH2CH3), 1.10 (3H;
Zn(1)CH2CH3), 1.64 (3H; O(1)CH2CH3), 1.70 (brs, 3H; O(2)CH2CH3), 3.54
(brs, 2H; O(2)CH2CH3), 3.67 (2H; O(1)CH2CH3), 6.70–8.20 ppm (m, 10H;
CHAr); 13C NMR (500 MHz, [D8]toluene, 208C): d=0.01 (br), 6.22 (br),
6.90, 10.49 (br), 13.37 (br), 15.16, 65.84, 80.69, 133.6, 133.8, 134.3 (d),
137.4, 139.4, 139.7, 142.0, 146.6, 208.8, 209.8 ppm. Then NaHCO3 was
added to the crude product and the resulting mixture was stirred for
30 min. Toluene (5 mL) was added and after evaporation of the solvents
under reduced pressure the organic residue was analysed by 1H NMR
spectroscopy (for the data description see the text below and Figure S1a
in the Supporting Information).
Room-temperature reaction of dibenzoyl with Et2Zn (2 and 2’): Et2Zn
(0.09 mL, 0.111 g, 0.90 mmol) was added to a yellowish solution of diben-
zoyl (0.189 g, 0.90 mmol) in diethyl ether (6 mL) at room temperature
and the reaction mixture was stirred for 36 h. The solvent was removed
from the resulting intensively yellow reaction mixture under reduced
pressure to give a mixture of crude product 2 and 2’ as a white solid. The
1H NMR spectrum of the reaction mixture was complicated and not in-
formative, the alkylation products were identified spectroscopically after
hydrolysis: NaHCO3 was added to the crude product and the resulting
mixture was stirred for 30 min. Toluene (5 mL) was added and after
evaporation of solvents under reduced pressure the organic residue was
analysed by 1H NMR spectroscopy (for the data description see the text
below and Figure S1b in the Supporting Information). The resulting
1H NMR spectra of the organic residue obtained from hydrolysis of reac-
tion mixture confirm the anticipated O-alkylated and C-alkylated prod-
ucts: 2-hydroxy-1,2-diphenylbutan-1-one (2) and 2-ethoxy-1,2-diphenyl-
Computational details: The geometry of all molecules was optimised by
density functional theory[22] by using the Gaussian 09[23] package. Geome-
try optimisations were carried out by using standard gradient methods
(Berny algorithm) with the M06-2X functional[24] and the Dunning-like
aug-cc-pVDZ[25] double-zeta basis set. Optimisations were performed
without constrains apart from the s-cis conformation of dibenzoyl, where
the O-C-C-O dihedral angle was frozen (0.08) (as a consequence, this ge-
ometry does not correspond to a minimum at the potential energy sur-
face). To validate locations of stationary points, all freely optimised geo-
metries were subjected to frequency calculations (by using analytical
methods) at the same level of theory and in the same basis set as optimi-
sations were performed. As a result, zero-point energies for all species
apart from the s-cis conformation of dibenzoyl, were obtained. For all
freely optimised geometries nuclear hessian had only positive eigenvalues
(corresponding to energy minima). To improve the accuracy of the ener-
getic results, single point DFT energy recalculations were performed on
previously optimised structures by using the same exchange–correlation
functional but with extension of the basis set to triple zeta quality—aug-
cc-pVTZ.[25] These re-calculations were performed by using automatic
correction for BSSE (basis set superposition error) with a counterpoise
(Boys–Bernardi) method.[26, 27] During all optimisations the aug-cc-pVDZ
basis set was used. Spherical harmonics basis set functions were used
throughout and the linear dependence of the basis set functions was not
observed. To provide reliable results of both geometry optimisations and
energy calculations grid used for numerical integration of the DFT equa-
tions (both self-consistent field and gradient/hessian) was extended to
126 radial points and 770 angular points per atom (which roughly corre-
sponds to extension by a factor of two compared to the default Gaussi-
an 09 options). At the stage of geometry optimisations, no correction for
BSSE was introduced. This approximation is fully justified by further
single-point energy re-calculations corrected for BSSE—in all cases
BSSE was small (usually less than 4 kJmolꢀ1), so its influence on the re-
sulting geometries is clearly insignificant. Final energetic relations were
obtained by using results from re-calculations at the aug-cc-pVTZ level
(corrected for BSSE) and ZPE computed at aug-cc-pVDZ level. Solvent
effects were not included at any stage of the calculations (for the results
of the DFT calculations see the Supporting Information).
AHCTUNGERTGeNNUN thanone (2’). The latter compound exists in a keto–enol equilibrium in a
ratio of 1:1.5. 2-hydroxy-1,2-diphenylbutan-1-one (2): 1H NMR
(400 MHz, [D6]benzene, 208C): d=0.80 (t, J=7.3 Hz, 3H; CH3), 2.24 (m,
2H; CH2), 4.34 (brs, 1H; OH), 6.50–7.30 ppm (m, 10H; CHAr); 2-
ethoxy-1,2-diphenylethanone (2’), keto form: 1H NMR ([400 MHz,
3
[D6]benzene, 208C): d=1.05 (t, J
N
CH2), 5.41 (s, 1H; CH) 6.50–7.30 ppm (m, 10H; CHAr); enol form:
1H NMR (400 MHz, [D6]benzene, 208C): d=0.99 (t, 3J
(H,H)=7.3 Hz,
3H; CH3), 4.10 (q, 3J
(H,H)=7.3 Hz, 2H; CH2), 5.77 (brs, 1H; OH),
6.50–7.30 ppm (m, 10H; CHAr).
{[12][1,2-bis(4-pyridyl)ethane]}n (3): Et2Zn (0.45 mL, 0.90 mmol, 2m solu-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
tion in THF) was added to a solution of dibenzoyl (0.189 g, 0.90 mmol)
and 1,2-bis(4-pyridyl)ethane (0.083 g, 0.45 mmol) in THF (5 mL) at
ꢀ788C . The reaction mixture was warmed up to room temperature.
During this time precipitation of light yellow crystals was observed. The
reaction mixture was stirred at room temperature for 2 h. The solvents
were removed under reduced pressure. The Remaining solid was dis-
solved in THF (1.5 mL) and hexane (1 mL) was added. The solution was
left for crystallisation at room temperature. Colourless crystals suitable
for X-ray analysis were obtained after 24 h in 75% yield. 1H NMR
(400 MHz, CDCl3, 208C): d=0.85 (4H; ZnCH2CH3), 1.26 (6H;
ZnCH2CH3), 1.30 (6H; OCH2CH3), 2.97 (m, 4H; CH2CH2), 3.88 (4H;
OCH2CH3), 7.10 (m, 8H; CHAr), 7.11 (m, 4H; CHpyridyl), 7.13 (m, 4H;
CHAr), 7.45 (m, 8H; CHAr), 8.46 ppm (m, 4H; CHpyridyl).
X-ray structure determination: The data were collected at 100(2) K on a
Nonius Kappa CCD diffractometer[28] by using graphite monochromated
MoKa radiation (l=0.71073 ꢁ). The crystals were mounted in a nylon
loop in a drop of silicon oil to prevent the possibility of decay of the crys-
tals during data collection. The unit cell parameters were determined
from ten frames, then refined on all data. The data were processed with
DENZO and SCALEPACK (HKL2000 package).[29] The structure was
ACHTUNGTRENNUNG[tBuZnACHTUNGTRENNUNG{PhC(O)CACHTUNGTRENNUNG(OtBu)Ph}] (1): tBu2Zn (87.7 mg, 0.49 mmol) in THF
(0.35 mL) was added to a solution of dibenzoyl (0.104 g, 0.49 mmol) in
THF (5 mL) at ꢀ788C. The reaction mixture was left to warm up to
room temperature and stirred for 2 h. After 24 h of crystallisation from
THF/hexane at ꢀ158C colourless crystals suitable for X-ray analysis
were obtained in 70% yield. 1H NMR (400 MHz, CDCl3, 208C): d=1.28
Chem. Eur. J. 2011, 17, 12713 – 12721
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12719

´
J. Lewinski et al.
solved by direct methods by using the SHELXS97[30] program and was re-
fined by full matrix least-squares on F2 by using the program
SHELXL97.[31] All non-hydrogen atoms were refined with anisotropic
displacement parameters. The hydrogen atoms were introduced at geo-
metrically idealised coordinates with a fixed isotropic displacement pa-
rameter equal to 1.5 (methyl groups) times the value of the equivalent
isotropic displacement parameter of the parent carbon. CCDC-789748
(12) and CCDC-787086 (3) 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. For selected bond lengths see Tables S1 and S2 in the Sup-
porting Information.
[5] See for example: a) G. van Koten, K. Vrieze, Adv. Organomet.
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12: C44H59O4Zn2·C6H5CH3; M=871.80; crystal dimensions 0.52ꢂ0.38ꢂ
0.28 mm3, monoclinic; space group C2/c (no. 15); a=11.8700(4), b=
17.6520(10), c=22.2180(11) ꢁ; b=94.885(3)8; V=4638.4(4) ꢁ3; Z=4; F-
ACHTUNGTRENNUNG ACHTUNGTRENNUGN
(000)=1848; 1calcd =1.248 gm3; m(MoKa)=1.08 mmꢀ1; qmax =27.488; 5102
unique reflections; refinement converged at R1=0.0734; wR2=0.0928
for all data and 263 parameters (R1=0.0395, wR2=0.0846 for 3502 re-
flections with I>2s(I)); goodness-of-fit on F2 was equal 0.998; a weight-
2
2
ing scheme w=[s ACHTUNGTRENNUNG ACHUTNGTRENNNUG
2(Fo +(0.0418P)2+3.1964P]ꢀ1 where P=(Fo +2Fc2)/3
was used in the final stage of refinement; residual electron density= +
0.60/ꢀ0.50 eꢁꢀ3
.
3: C48H52N2O4Zn2·3C4H8O; M=1068.01; crystal dimensions 0.42ꢂ0.34ꢂ
0.24 mm3, monoclinic; space group P21/c (no. 14); a=10.5689(4), b=
19.6178(9), c=26.3051(9) ꢁ; b=97.050(2)8; V=5412.8(4) ꢁ3; Z=4; F-
ACHTUNGTRENNUNG ACHTUNGTRENNUGN
(000)=2264; 1calcd =1.311 gm3; m(MoKa)=0.94 mmꢀ1; qmax =23.828; 8287
unique reflections; refinement converged at R1=0.1488; wR2=0.1822
for all data and 654 parameters (R1=0.0689, wR2=0.1526 for 4370 re-
flections with I>2s(I)); goodness-of-fit on F2 was equal 1.058; a weight-
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This work was supported by the Ministry of Science and Higher Educa-
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 12713 – 12721

Reactions of ZnR2 Compounds with Dibenzoyl
FULL PAPER
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Received: June 28, 2011
Published online: September 28, 2011
Chem. Eur. J. 2011, 17, 12713 – 12721
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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