Formation of organozincate anions in LiCl-mediated zinc insertion reactions

Article abstract of DOI:10.1021/om800947t

Tetrahydrofuran solutions of the products formed in LiCl-mediated zinc insertion reactions into various organic halides RHal were analyzed by anion-mode electrospray ionization (ESI) mass spectrometry. In all cases, organozincate anions were observed. The

Full text of DOI:10.1021/om800947t

Organometallics 2009, 28, 771–779
771
Formation of Organozincate Anions in LiCl-Mediated Zinc Insertion
Reactions
Konrad Koszinowski* and Petra Bo¨hrer
Department Chemie und Biochemie, Ludwig-Maximilians-UniVersita¨t Mu¨nchen, Butenandtstr. 5-13,
81377 Mu¨nchen, Germany
ReceiVed October 1, 2008
Tetrahydrofuran solutions of the products formed in LiCl-mediated zinc insertion reactions into various
organic halides RHal were analyzed by anion-mode electrospray ionization (ESI) mass spectrometry. In
all cases, organozincate anions were observed. The reactions with RHal, Hal ) Br and I, yielded
-
predominantly mononuclear complexes, such as ZnRHal2 and ZnRHalCl-, whereas for the reaction
-
-
with benzylchloride abundant polynuclear organozincates, such as Zn2Bn2Cl3 and Zn3Bn3Cl4 , were
detected. The equilibria governing the stoichiometry and aggregation state of these complexes appear to
be mainly controlled by the nature of the halide ions present in solution. It seems likely that the formation
of organozincate complexes also changes the reactivity of the organozinc species, thus offering a rationale
for the recently found pronounced effect of LiCl in organozinc chemistry. Additional preliminary studies
suggest that organozincate anions as well as organozinc cations may moreover form in the absence of
LiCl.
toward transmetalation.10 Furthermore, LiCl promotes important
reactions in organomagnesium11-14 and organoindium chem-
istry.15
1. Introduction
Because of their excellent chemoselectivity and good compat-
ibility with functional groups, organozinc compounds are
valuable reagents in organic synthesis.1,2 Functionalized orga-
nozinc halides R-Zn-Hal can be accessed via zinc insertion
into the respective alkyl and aryl halides RHal (Hal ) Br and
I). However, this method suffers from some limitations in that
not all alkyl and aryl halides readily undergo zinc insertion. In
general, alkyl bromides only react with Rieke-zinc but not with
commercially available zinc powder.3,4 Rieke-zinc is also
required for the conversion of aryl iodides3 unless elevated
temperatures are applied.5
Although the beneficial effects of LiCl in zinc insertion and
subsequent transmetalation reactions are thus well documented,
their molecular mode of action is unknown. Knochel and co-
workers surmised that LiCl leads to the formation of R-Zn-
Hal · LiCl complexes that are well-soluble in tetrahydrofuran
(THF), the solvent of choice for zinc insertion reactions.6 The
formation of these dissolved complexes supposedly regenerates
the free zinc surface, which can then react with further RHal
molecules.6 Similarly, Oshima and co-workers conjectured that
LiCl enhances the reactivity of organozinc halides by coordina-
tion to the Zn center and formation of monomeric R-Zn-Hal ·
LiCl species.10 These monomeric zinc species are expected to
have a higher tendency toward transmetalation than larger
aggregates. While the formulation as R-Zn-Hal · LiCl delib-
erately avoids a specific structural assignment of this putative
intermediate, the well-known properties of zinc offer some clues
to its likely structure. Zn(II) complexes with two ligands have
an unsaturated coordination sphere and behave as strong Lewis
acids that readily add a Lewis base. For instance, diorganylzinc
species ZnR2 react with organolithium compounds RLi to yield
Recently, Knochel and co-workers have found that the
addition of stoichiometric amounts of lithium chloride greatly
facilitates zinc insertion and significantly broadens the scope
of this reaction.6-9 In the presence of LiCl, alkyl bromides react
smoothly with zinc powder,6 and even allyl7 and benzyl
chlorides9 give the respective organozinc chlorides. Recent
findings of Oshima and co-workers moreover suggest that the
addition of LiCl not only facilitates zinc insertion but also
enhances the reactivity of the resulting organozinc halides
* To whom correspondence should be addressed. E-mail:
Konrad.Koszinowski@cup.uni-muenchen.de.
(1) Knochel, P.; Singer, R. D. Chem. ReV. 1993, 93, 2117.
(2) Knochel, P.; Perea, J. J. A.; Jones, P. Tetrahedron 1998, 54, 8275.
(3) Zhu, L.; Wehmeyer, R. M.; Rieke, R. D. J. Org. Chem. 1991, 56,
1445.
(4) Rieke, R. D.; Hanson, M. V.; Brown, J. D.; Niu, Q. J. J. Org. Chem.
1996, 61, 2726.
(5) Ikegami, R.; Koresawa, A.; Shibata, T.; Takagi, K. J. Org. Chem.
2003, 68, 2195.
(6) Krasovskiy, A.; Malakhov, V.; Gavryushin, A.; Knochel, P. Angew.
Chem. 2006, 118, 6186. Krasovskiy, A.; Malakhov, V.; Gavryushin, A.;
Knochel, P. Angew. Chem., Int. Ed 2006, 45, 6040.
(7) Ren, H.; Dunet, G.; Mayer, P.; Knochel, P. J. Am. Chem. Soc. 2007,
129, 5376.
(8) Boudet, N.; Sase, S.; Sinha, P.; Liu, C.-Y.; Krasovskiy, A.; Knochel,
P. J. Am. Chem. Soc. 2007, 129, 12358.
(9) Metzger, A.; Schade, M.; Knochel, P. Org. Lett. 2008, 10, 1107.
(10) Ochiai, H.; Jang, M.; Hirano, K.; Yorimitsu, H.; Oshima, K. Org.
Lett. 2008, 10, 2681.
(11) Krasovskiy, A.; Knochel, P. Angew. Chem. 2004, 116, 3396.
Krasovskiy, A.; Knochel, P. Angew. Chem., Int. Ed. 2004, 43, 3333.
(12) Krasovskiy, A.; Straub, B. F.; Knochel, P. Angew. Chem. 2006,
118, 165. Krasovskiy, A.; Straub, B. F.; Knochel, P. Angew. Chem., Int.
Ed 2006, 45, 159.
(13) Piller, F. M.; Appukkuttan, P.; Gavryushin, A.; Helm, M.; Knochel,
P. Angew. Chem. 2008, 120, 6907. Piller, F. M.; Appukkuttan, P.;
Gavryushin, A.; Helm, M.; Knochel, P. Angew. Chem., Int. Ed. 2008, 47,
6802.
´
(14) Garc´ıa-Alvarez, P.; Graham, D. V.; Hevia, E.; Kennedy, A. R.;
Klett, J.; Mulvey, R. E.; O’Hara, C. T.; Weatherstone, S. Angew. Chem.
´
2008, 120, 8199. Garc´ıa-Alvarez, P.; Graham, D. V.; Hevia, E.; Kennedy,
A. R.; Klett, J.; Mulvey, R. E.; O’Hara, C. T.; Weatherstone, S. Angew.
Chem., Int. Ed. 2008, 47, 8079.
(15) Chen, Y.-H.; Knochel, P. Angew. Chem. 2008, 120, 7760. Chen,
Y.-H.; Knochel, P. Angew. Chem., Int. Ed. 2008, 47, 7648.
10.1021/om800947t CCC: $40.75
2009 American Chemical Society
Publication on Web 01/06/2009

772 Organometallics, Vol. 28, No. 3, 2009
Koszinowski and Bo¨hrer
Typical procedure:6 Zinc powder (7.0 mmol, Sigma-Aldrich) was
placed in a flask and flame-dried under high vacuum. The flask
was filled with Ar and allowed to cool down before the procedure
was repeated twice. A total of 10 mL of a 0.5 M solution of
anhydrous LiCl (5.0 mmol) in THF was added, and the resulting
suspension was concentrated to a volume of 5 mL. The zinc was
activated by the addition of 1,2-dibromoethane (0.020 mL), followed
by a short boiling-up of the suspension, and of chlorotrimethylsilane
(0.020 mL), again followed by a short boiling-up. The alkyl or
aryl halide (5.0 mmol) was added dropwise and the suspension was
stirred overnight at room temperature. The remaining zinc powder
was allowed to precipitate, and an aliquot of the overlaying solution
was diluted with THF before subjecting it to ESI mass spectrometric
analysis. A solution of benzylzinc bromide in THF was prepared
according to the method of Berk et al.28
16
-
lithium triorganozincates Li+ZnR3
;
these triorganozincates
are more reactive than the corresponding diorganyl zinc com-
pounds,17 and they have therefore found applications in various
important synthetic transformations, including Michael-type
addition18 and halogen-zinc exchange reactions.19 One may
expect organozinc halides R-Zn-Hal to react with LiCl in a
similar way to produce Li+ZnRClHal- complexes, in which the
zinc center adopts a trigonal coordination geometry. In a strongly
coordinating solvent such as THF, these complexes may at least
partly dissociate into free Li+ and ZnRClHal- ions (eq 1).
R-Zn-Hal + Li+Cl- f Li+ZnRClHal- h
Li++ZnRClHal- (1)
-
Complexes of the type ZnRHal2 have been observed in
electrochemical studies,20 but otherwise only very little was
known about such species. This lack of information reflects
experimental difficulties in the selective detection of charged
organometallics. The advent of electrospray ionization (ESI)
mass spectrometry has changed this situation, however, and
made possible the identification of a manifold of organometallic
ions.21-23 Using this method, we could show that the reactions
ESI Mass Spectrometry. The ESI mass spectrometric experi-
ments were analogous to those described previously in more
detail.24 In brief, sample solutions were transferred into a gastight
syringe and administered into the ESI source of a TSQ 7000
multistage mass spectrometer (Thermo-Finnigan) at flow rates of
approximately 10 µL min-1. Nitrogen was used as sheath gas, and
ESI voltages ranging from 3.0 to 4.3 kV were applied. In order to
avoid unwanted fragmentation of possibly labile complexes, gentle
ESI conditions were chosen (heated capillary at 60 °C, low potential
difference between the heated capillary and the following electro-
optical lens). The m/z ratios of the ions were then established by
scanning the first quadrupole mass filter.
For the gas-phase reactivity studies, the first quadrupole mass
filter was used to mass-select the organozincate ions of interest,
which then passed an 18 cm long octopole ion guide. For studying
the unimolecular reactivity of a mass-selected ion in a collision-
induced dissociation (CID), argon (Linde, 99.998% purity) was
added as collision gas into the octopole (p(Ar) ≈ 0.6 mtorr as
measured with a Convectron). The collision energy ELABORATORY
was controlled by adjusting the voltage offset of the octopole. For
the bimolecular reactions of the organozincate ions with methyl
iodide and formic acid, these substrates were purified by repeated
freeze-pump-thaw cycles before adding them into the octopole
(p ≈ 0.4 mtorr, uncorrected reading of the Convectron). The m/z
ratios of the product ions formed by CID or ion-molecule reactions
were then established by scanning the second quadrupole mass filter
before the ions reached the detector.
n
s
of ZnCl2 with 1 equiv of RLi (RLi ) CH3Li, BuLi, BuLi,
tBuLi, and 2-lithiothiophene) in THF produced free ZnRCl2
-
ions (eq 2), along with related polynuclear complexes.24 We
also investigated the gas-phase reactivity of the ZnBuCl2- anions
found and compared it with that of their ZnBu2Cl- and ZnBu3
-
congeners24 as well as with the behavior of previously studied
organomagnesate25 and organocuprate ions.26,27
-
ZnHal2 + RLi f Li+ + ZnRHal2
(2)
In our present contribution, we apply ESI mass spectrometry
to demonstrate that similar organozincate anions are formed in
LiCl-mediated zinc insertion reactions. For comparison, we also
report preliminary results on organozinc ions produced by zinc
insertion into benzylbromide in the absence of LiCl and analyze
solutions of LiCl in THF. In addition, we probe the gas-phase
reactivity of selected organozincate anions in order to achieve
a more complete characterization.
2. Experimental Section
3. Results
Synthetic Methods. The synthetic procedures were similar to
those described by Knochel and co-workers6,28 with the exception
that we found it more convenient to use a solution of anhydrous
LiCl in THF (distilled from potassium/benzophenone) rather than
neat LiCl.
3.1. Stoichiometry of Organozincate Anions. The LiCl-
mediated zinc insertion reaction was studied for a variety of
i
i
n
alkyl and aryl halides RHal (RHal ) CH3I, PrBr, PrI, BuI,
BnCl, BnBr, and 4-EtOOC-C6H4I). In order to ensure full
comparability of results, in all cases, 1 mM THF solutions (with
respect to reactant RHal as well as to LiCl) were analyzed under
very similar ESI conditions. In addition, we probed solutions
of Bn-Zn-Br in THF and of LiCl in THF.
(16) Tochtermann, W. Angew. Chem. 1966, 78, 355. Tochtermann, W.
Angew. Chem., Int. Ed. Engl. 1966, 5, 351.
(17) Maclin, K. M.; Richey, H. G., Jr J. Org. Chem. 2002, 67, 4602.
(18) Isobe, M.; Kondo, S.; Nagasawa, N.; Goto, T. Chem. Lett. 1977,
679.
(19) Kondo, Y.; Fujinami, M.; Uchiyama, M.; Sakamoto, M.; Sakamoto,
T. J. Chem. Soc., Perkin Trans. 1 1997, 799.
(20) Habeeb, J. J.; Osman, A.; Tuck, D. G. J. Organomet. Chem. 1980,
185, 117.
(21) Lipshutz, B. H.; Keith, J.; Buzard, D. J. Organometallics 1999,
18, 1571.
(22) Plattner, D. A. Int. J. Mass Spectrom. 2001, 207, 125.
(23) Chen, P. Angew. Chem. 2003, 115, 2938. Chen, P. Angew. Chem.
Int. Ed. 2003, 42, 2832.
(24) Koszinowski, K.; Bo¨hrer, P. Organometallics, in press.
(25) O’Hair, R. A. J.; Vrkic, A. K.; James, P. F. J. Am. Chem. Soc.
2004, 126, 12173.
(26) James, P. F.; O’Hair, R. A. J. Org. Lett. 2004, 6, 2761.
(27) Rijs, N.; Khairallah, G. N.; Waters, T.; O’Hair, R. A. J. J. Am.
Chem. Soc. 2008, 130, 1069.
(28) Berk, S. C.; Yeh, M. C. P.; Jeong, M.; Knochel, P. Organometallics
1990, 9, 3053.
LiCl-Mediated Reaction of CH3I with Zn. Anion-mode ESI
of a solution of the products formed in the LiCl-mediated zinc
insertion into CH3I yielded, along with smaller amounts of I-,
-
several different zincate ions, such as Zn(CH3)ICl-, Zn(CH3)I2 ,
ZnI2Cl-, and ZnI3- (Figure 1a). These zincate complexes could
be easily identified thanks to their characteristic isotopic
patterns,29 as illustrated for Zn(CH3)I2 and ZnI2Cl- (Figure
-
-
1b). The identity of the organometallic Zn(CH3)I2 ion was
further corroborated by a CID experiment, which produced I-
as ionic fragment together with neutral Zn(CH3)I (Supporting
Information Table S1, entry 1).
(29) Isotope patterns can be conveniently calculated with the help of
web-based resources, such as Yan, J. Isotope Pattern Calculator, v4.0; http://
www.geocities.com/junhuayan/pattern.htm.

Organozincate Anions
Organometallics, Vol. 28, No. 3, 2009 773
Figure 3. Anion-mode ESI mass spectrum of a 1 mM solution (with
i
respect to reactant PrI and to LiCl) of the products formed in the
LiCl-mediated reaction of isopropyliodide with zinc powder in THF.
All of the mononuclear zincates listed above yielded pre-
dominantly Br- upon fragmentation (Supporting Information
Table S1, entries 2-6). For the heavier ions, however, additional
fragment ions were found that pointed to the presence of
-
LiZnBrCl3-, LiZniPrBrCl2-, and LiZnBr2Cl2 as parent ions
(Supporting Information Table S1, entries 7-9). For instance,
CID of the ion at m/z ) 259 produced small amounts of ionic
-
fragments at m/z ) 171 and 173, which we assign to ZnCl3
.
Obviously, ZnCl3- cannot originate from ZnBrCl2- but instead
-
is formed by fragmentation of isobaric LiZnBrCl3
(M(Li35Cl37Cl-) ) M(79Br-)). The parent ions at m/z ) 259
Figure 1. Anion-mode ESI mass spectrum of a 1 mM solution (with
respect to reactant CH3I and to LiCl) of the products formed in the
LiCl-mediated reaction of methyliodide with zinc powder in THF:
(a) overview and (b) comparison of observed (black) and expected
isotopic patterns (red) for the low mass range.
thus correspond to a mixture of ZnBrCl2 and LiZnBrCl3-. A
-
careful analysis of the measured isotopic pattern suggests that
-
-
ZnBrCl2 strongly prevailed over LiZnBrCl3 (I(ZnBrCl2-):
-
-
I(LiZnBrCl3-) g 9:1). Similarly, ZniPrBr2 and ZnBr3 were
-
only slightly contaminated by LiZniPrBrCl2- and LiZnBr2Cl2
species, respectively. The fact that the latter nevertheless left
their signature in the CID experiments can be ascribed to their
higher tendency toward dissociation. Our previous studies on
organozincate ions showed that polynuclear zincate complexes
fragment much more readily than their mononuclear conge-
ners.24
LiCl-Mediated Reaction of iPrI with Zn. Anion-mode ESI
mass spectra obtained for this reaction closely resemble those
i
for the analogous reaction of PrBr. The most abundant ions
-
detected, ZnI3-nCln- (n ) 0, 1, and 2), ZniPrICl-, and ZniPrI2
(Figure 3), are all direct counterparts of the corresponding
bromozincate complexes. In contrast to the latter, however, the
iodozincate complexes apparently do not form higher aggregates,
as no ions were observed at m/z > 460.
-
Figure 2. Anion-mode ESI mass spectrum of a 1 mM solution (with
respect to reactant iPrBr and to LiCl) of the products formed in the
LiCl-mediated reaction of isopropylbromide with zinc powder in
THF.
The organozincates ZniPrICl- and ZniPrI2 were also sub-
jected to CID. Both complexes produced I- as ionic fragment
(Supporting Information Table S1, entries 10 and 11) and thus
again behaved in complete analogy to the corresponding
bromozincates.
LiCl-Mediated Reaction of iPrBr with Zn. Analysis of the
reaction solution by anion-mode ESI mass spectrometry resulted
in the detection of various zincate anions (Figure 2). Besides
the purely inorganic complexes ZnBr3-nCln- (n ) 0, 1, and 2),
n
LiCl-Mediated Reaction of BuI with Zn. Analysis of the
reaction solution by anion-mode ESI mass spectrometry again
permitted the detection of the inorganic zincate complexes
ZnI3-nCln- (n ) 0 and 1) as well as of the organometallic species
-
the organometallic species ZniPrBrCl- and ZniPrBr2 were
ZnnBuICl- and ZnnBuI2 (Figure 4). The latter two ions both
-
gave I- as ionic fragment upon CID (Supporting Information
Table S1, entries 12 and 13).
observed in significant abundance. Additional ions were detected
at higher m/z ratios. Although their low signal intensities
prevented their unambiguous identification, it seems very likely
that these ions correspond to zincate complexes in higher
aggregation states.
LiCl-Mediated Reaction of BnCl with Zn. Anion-mode ESI
of a solution of the products formed in the LiCl-mediated
reaction between benzylchloride and zinc gave the expected

774 Organometallics, Vol. 28, No. 3, 2009
Koszinowski and Bo¨hrer
Figure 4. Anion-mode ESI mass spectrum of a 1 mM solution (with
respect to reactant nBuI and to LiCl) of the products formed in the
LiCl-mediated reaction of n-butyliodide with zinc powder in THF.
Figure 6. Anion-mode ESI mass spectrum of a 1 mM solution (with
respect to reactant BnBr and to LiCl) of the products formed in
the LiCl-mediated reaction of benzylbromide with zinc powder in
THF.
Figure 5. Anion-mode ESI mass spectrum of a 1 mM solution (with
respect to reactant BnCl and to LiCl) of the products formed in the
LiCl-mediated reaction of benzylchloride with zinc powder in THF.
Figure 7. Anion-mode ESI mass spectrum of a 1 mM solution (with
respect to reactant EtOOC-C6H4I and to LiCl) of the products
formed in the LiCl-mediated reaction of 4-iodo-ethylbenzoate (RI)
with zinc powder in THF.
-
-
mononuclear zincate anions ZnCl3 and ZnBnCl2 (Figure 5).
In addition to these, abundant species occurred at higher m/z
ratios.
LiCl-Mediated Reaction of BnBr with Zn. Anion-mode ESI
mass spectra obtained for this reaction show the presence of
-
-
ZnBr3-nCln (n ) 0, 1, and 2), ZnBnBrCl-, and ZnBnBr2
The most prominent of these ions could be identified as
-
(Figure 6) in the sampled solution. As expected, CID of these
ions gave Br- as ionic fragment (Supporting Information Table
S1, entries 19-23). The detection of traces of additional
fragment ions that cannot arise from these parent ions, however,
again indicated the presence of isobaric contaminants, such as
Zn2Bn2Cl3-, LiZn2Bn2Cl4-, Zn3Bn3Cl4-, and LiZn3Bn3Cl5
.
These assignments are based on a comparison between the
observed and calculated isotopic patterns29 and are also fully
consistent with CID experiments performed for all of these ions
(Supporting Information Table S1, entries 14-18). In compari-
son to their mononuclear counterparts, the polynuclear zincate
complexes showed a much more diverse fragmentation behavior,
which allows their unequivocal identification. For example,
Zn3Bn3Cl4- as the most abundant zincate anion observed yielded
three different fragment ions upon CID ( Supporting Information
Figure S1) (eq 3a-3c).
-
-
-
LiZnCl4 , LiZnBnCl3 , LiZnBr2Cl2 , and LiZnBnBrCl2- (Sup-
porting Information Table S1, entries 24-27). A careful analysis
of the measured isotopic patterns suggests that these isobaric
contaminants account for only negligibly small fractions of the
ion populations in question, though.
Additional ions observed at higher m/z ratios did not exhibit
sufficient signal intensities for CID experiments and unambigu-
ous identification. However, the measured isotopic patterns are
consistent with those calculated for polynuclear zincate com-
Zn3Bn3Cl4- f ZnCl3- + Zn2Bn3Cl
fZn2BnCl4- + ZnBn2
(3a)
(3b)
(3c)
-
plexes, such as LiZnBr3Cl-, LiZnBnBr2Cl-, Zn2BnBr4-nCln
fZn2Bn2Cl3- + ZnBnCl
-
(n ) 1, 2, and 3), and Zn2Bn2BrCl2
.
It is interesting to note that the dissociation reactions tend to
produce ionic fragments enriched in chloride ligands whereas
the concomitantly formed neutrals contain a relatively higher
number of benzyl groups. Similar fragmentation patterns were
also observed for the other polynuclear zincates.
LiCl-Mediated Reaction of 4-EtOOC-C6H4I with Zn.
Anion-mode ESI mass spectrometry revealed that the LiCl-
mediated zinc insertion into 4-iodo-ethylbenzoate (RI) afforded
-
-
ZnI3-nCln (n ) 0, 1, and 2), ZnRICl-, and ZnRI2 as major
ionic products (Figure 7). As expected, the latter produced I-

Organozincate Anions
Organometallics, Vol. 28, No. 3, 2009 775
In general, an increase of the analyte concentration translates
into higher absolute ESI signal intensities, although at the
relatively high concentrations sampled in the present work
saturation effects may come into play.30 We indeed observed
an increase in the overall absolute ESI signal intensities as a
function of analyte concentration, but here we are interested in
the relatiVe ESI signal intensities of the different zincate
complexes. For calculating these relative signal intensities, we
considered the major isotopologues of the most abundant zincate
ions detected (zincates of lower abundance were neglected). It
is important to note that the ratio between the (relative) ESI
signal intensities of different zincate complexes does not
necessarily reflect their relative concentrations in solution
because the ESI signal intensities also depend on the relative
ESI response factors of the zincates. It is unknown how these
ESI response factors differ for different zincate ions. Hence,
we do not compare the relative ESI signal intensities of different
zincates but instead analyze the concentration dependence for
each zincate species separately.
LiCl-Mediated Reaction of nBuI with Zn. Even with
extremely gentle ESI conditions, hardly any polynuclear zincates
were detected for product solutions of c ) 1 mmol L-1 (with
respect to reactant nBuI and to LiCl). At higher concentrations,
however, new species occurred at high m/z ratios (Supporting
Information Figure S3a). Their pronounced isotopic patterns
(Supporting Information Figure S3b) point to the presence of
multiple zinc and chlorine atoms as ingredients of polynuclear
zincate complexes.
Figure 8. Anion-mode ESI mass spectrum of a 1 mM solution of
Zn-Bn-Br in THF (without addition of LiCl).
as ionic fragment upon CID (Supporting Information Table S1,
entry 28). No ions of significant intensity were detected for m/z
> 480.
Reaction of BnBr with Zn in the Absence of LiCl. Analysis
of the reaction solution by anion-mode ESI mass spectrometry
-
resulted in the detection of ZnBr3 and abundant polynuclear
zincates (Figure 8). On the basis of their isotopic patterns and
CID experiments (Supporting Information Table S1, entries 29
and 30), the most prominent of these zincates could be identified
-
as Zn2Br5-, Zn2BnBr4-, Zn3Bn2Br5-, and Zn3Bn3Br4
.
LiCl in THF. Anion-mode ESI mass spectra of solutions of
LiCl in THF gave only very weak genuine analyte signals,
whereas, even after extensive cleaning and purging of the inlet
The observed isotopic patterns of the two most prominent
polynuclear zincates are best compatible with assignments as
n
n
Zn2 BuI2Cl2- and Zn2 BuI3Cl- (Supporting Information Figure
-
system and the ESI source, contaminants such as ZnCl3 and
S3b). These assignments also appear plausible in that the
I- remained visible. Only with optimized ESI conditions,
n
n
proposed polynuclear complexes Zn2 BuI2Cl2- and Zn2 BuI3Cl-
are closely related to the abundant mononuclear zincates
ZnI2Cl-, ZnnBuI2-, and ZnI3-. Unfortunately, the rather low
signal intensities of the polynuclear ions did not permit
confirmation of the given assignments by CID experiments. The
poor signal-to-noise ratios also caused statistic fluctuations,
which help to explain the deviations between the measured and
the expected isotopic patterns. A particularly conspicuous
deviation is seen for m/z ) 601. Presumably, the higher than
expected experimental signal intensity resulted from the presence
acceptable signal intensities could be obtained for Cl-, LiCl2
and Li2Cl3 (Supporting Information Figure S2).
,
-
-
3.2. Concentration Dependence. All of the ESI mass
spectrometric experiments presented so far sampled solutions
with concentrations of c ) 1 mmol L-1, both in reactant RHal
and in LiCl (except for the additional experiments on Bn-Zn-Br
and LiCl solutions). The concentration of these species sup-
posedly has a decisive influence on the equilibria between the
different aggregation states, which the zincate ions adopt in
solution. If ESI mass spectrometry really is capable of faithfully
probing the aggregation state of the zincate complexes in
solution, variation of the concentration should produce discern-
ible effects in the ESI mass spectra. We thus investigated the
concentration dependence of the relative ESI signal intensities
of the different zincate ions to test our methodology and to learn
more about the equilibria operative in solution. In doing so, we
focused on the LiCl-mediated zinc insertion reactions into
n-butyliodide, benzylchloride, and 4-iodo-ethylbenzoate.
The experiments presented above already applied gentle ESI
conditions in order to avoid unwanted fragmentation of weakly
bound complexes, particularly of polynuclear zincates, which
are quite susceptible to dissociation.24 Despite the gentle ESI
conditions applied, we cannot rigorously rule out that fragmen-
tations during the ESI process led to the relative scarcity of
polynuclear zincates observed in all cases except for the insertion
reactions into BnCl and BnBr. For the studies of the concentra-
tion dependence, we therefore chose even milder ESI conditions
(lower potential difference between the heated capillary and the
following electro-optical lens). While these extremely gentle
ESI conditions reduced the overall signal intensity, they should
permit the detection of intact labile polynuclear zincates
complexes.
of a second species besides 64Zn2 BuI335Cl-. The fact that
n
considerable signal intensities were observed at the lower
neighboring m/z ratios of m/z ) 597 and 599 supports this
assessment. We refrain from proposing assignments for this
second species and for other additional ions at high m/z ratios.
The relative ESI signal intensities of the polynuclear zincates
increased as a function of concentration (Figure 9). For the
mononuclear zincates, the trend was less clear: the signal
intensities of the purely inorganic complexes ZnI2Cl- and
ZnI2Cl- first declined and then leveled off with increasing
concentration, whereas that of ZnnBuI2- exhibited a pronounced
maximum at an intermediate concentration of c ) 10 mmol
L-1
.
LiCl-Mediated Reaction of BnCl with Zn. For this reaction,
abundant polynuclear zincates were already observed at a
concentration of c ) 0.1 mmol L-1 (with respect to reactant
BnCl and to LiCl). The relative ESI signal intensity of
Zn3Bn3Cl4- considerably increased as a function of concentra-
tion, while those of Zn2Bn2Cl3- and LiZn2Bn2Cl4- showed only
slight overall increases (Figure 10). In marked contrast, the
(30) Cech, N. B.; Enke, C. G. Mass Spectrom. ReV. 2001, 20, 362.

776 Organometallics, Vol. 28, No. 3, 2009
Koszinowski and Bo¨hrer
Figure 11. Cation-mode ESI mass spectrum of a 1 mM solution
of Zn-Bn-Br in THF (without addition of LiCl). The solvated
Figure 9. Relative signal intensities of zincate complexes produced
by anion-mode ESI of 1, 10, and 50 mM solutions (with respect to
Li+ and Na+ ions result from a contamination. Inset: Enlarged part
n
reactant BuI and to LiCl) of the products formed in the LiCl-
+
of the mass spectrum showing the isotopic pattern of ZnBn(THF)3
.
mediated reaction of n-butyliodide with zinc powder in THF. Open
symbols represent mononuclear complexes, and filled symbols
represent polynuclear complexes.
mass spectra measured for THF solutions of Bn-Zn-Br.
Although for this case the zinc insertion occurred in the absence
of LiCl and no sodium-containing compound was added either,
+
Li(THF)3 and Na(THF)n+, n ) 2 and 3, yielded the highest
signal intensities (Figure 11). We assume that these species
stemmed from residual traces of Li+ and Na+ in the inlet system
and/or ESI source.
In addition, small amounts of cations were observed at higher
m/z ratios. The ions with m/z ) 371-375 displayed an isotopic
pattern indicative of a mononuclear zinc complex (Figure 11,
inset). CID resulted in the subsequent loss of up to three THF
molecules and thus established the identity of this species as
the organometallic cation ZnBn(THF)3+. In contrast, the other
ions at high m/z ratios, such as NaZnBr2(THF)3+, did not bear
organyl groups.
3.4. Bimolecular Gas-Phase Reactivity. For the gas-phase
Figure 10. Relative signal intensities of zincate complexes produced
by anion-mode ESI of 0.1, 1, and 10 mM solutions (with respect
to reactant BnCl and to LiCl) of the products formed in the LiCl-
mediated reaction of benzylchloride with zinc powder in THF. Open
symbols represent mononuclear complexes, and filled symbols
represent polynuclear complexes.
reactivity studies, we focused on the mononuclear organozin-
-
cates ZnnBuI2-, ZnBnCl2-, and ZnRI2 (derived from RI )
4-EtOOC-C6H4I) as selected examples. None of these species
reacted with methyliodide (a prototypical carbon electrophile)
or formic acid (a prototypical proton donor) at a measurable
rate. From this finding, it can be estimated that the ion-molecule
reactions of the organozincates both with CH3I and HCOOH
have efficiencies of less than 1% (less than 1 out of 100
collisions is productive; see ref 24 for details).
relative signal intensities of the mononuclear zincate complexes
-
-
ZnCl3 and ZnBnCl2 decreased as function of concentration.
LiCl-Mediated Reaction of 4-EtOOC-C6H4I with Zn. No
polynuclear zincate ions were detected even at the extremely
gentle ESI conditions applied at a concentration of c ) 50 mmol
L-1 (with respect to reactant 4-EtOOC-C6H4I and to LiCl). The
relative ESI signal intensities of the mononuclear zincates
remained almost constant over the concentration range inves-
tigated (1 e c e 50 mmol L-1, data not shown).
4. Discussion
General. For all LiCl-mediated zinc insertion reactions
investigated, we observed the formation of organozincate anions.
-
Together with purely inorganic ZnHal3 complexes, these
organozincates dominate the ESI mass spectra recorded. We
therefore conclude that organozincate complexes represent major
anionic constituents of THF solutions of the products from LiCl-
mediated zinc insertion reactions. Preliminary results suggest
that organozincate ions also form in zinc insertion reactions in
the absence of LiCl. However, our experiments do not probe
the absolute concentration of organozincate ions in solution nor
the ratio between anionic and neutral organozinc species.
Mononuclear Organozincate Complexes. Mononuclear or-
ganozincate anions of the type ZnRHal2- were produced in all
reactions studied. These species correspond to those formed in
the transmetalation reactions between ZnHal2 and RLi (eq 2).24
Both LiCl-mediated zinc insertion and transmetalation reactions
3.3. Observed Cations. The cation-mode ESI mass-spectra
of the products from the LiCl-mediated zinc insertion reactions
were completely dominated by Li(THF)3+ and Li(THF)4+ ions
(compare Supporting Information Figure S4 for the case of
Bn-Zn-Br · LiCl). Very similar results had previously been
obtained for the reaction of ZnCl2 with RLi (eq 2).24 ESI of
+
solutions of LiCl in THF also yielded Li(THF)3 as the main
+
peak but in addition gave the dinuclear species Li2Cl(THF)n
,
n ) 3 and 4, and Li2Br(THF)4+ (Supporting Information Figure
S5). The latter apparently originated from a contamination of
the inlet system and/or ESI source.
The high propensity of alkaline metal cations to form
complexes with THF molecules is also evident from the ESI

Organozincate Anions
Organometallics, Vol. 28, No. 3, 2009 777
thus apparently afford completely analogous organozinc species.
Moreover, the ZnRHal2- or, more specifically, the ZnRHalCl-
anions observed can be linked to the R-Zn-Hal · LiCl com-
plexes proposed by Knochel and co-workers as intermediates
in LiCl-mediated zinc insertion reactions.6 Our present findings
provide support for the hypothesis of these authors and further
suggest that the R-Zn-Hal · LiCl intermediates should be
viewed as lithium zincates, which at least partly dissociate in
THF (eq 4).
all cases. However, no ZnR2Hal- ions were found, which should
be formed concomitantly with ZnHal3 complexes according
-
to eq 5. It seems rather unlikely that the absence of ZnR2Hal-
in the mass spectra could be due to exceptionally low ESI
response factors or to the complete formation of contact ion
pairs between ZnR2Hal- and Li+. More probably, the lack of
ZnR2Hal- ions observed reflects the nonoccurrence of the
-
disproportionation according to eq 5. For ZnRHal2 obtained
from transmetalation reactions, we did not find evidence for
disproportionation reactions either and only for the dithiophe-
nylzincate complex Zn(C4H3S)2Cl- such a reaction appeared
to take place;24 for neutral organozinc species, similar dispro-
portionation reactions are reported to be negligible in most,32
but not in all, cases studied.33 The ZnHal3- complexes observed
in the present experiments instead are ascribed to the partial
decomposition of organozinc species by residual traces of
moisture and/or oxygen (see below).
Polynuclear Organozincate Complexes. For the LiCl-
mediated zinc insertion into n-butyliodide, significant amounts
of polynuclear organozincates were observed only at higher
analyte concentrations. This concentration dependence is con-
sistent with the expected equilibria for the corresponding
association reactions, such as eq 6a and 6b.
Li+ZnRHalCl- h Li+ + ZnRHalCl-
(4)
A more detailed analysis takes into account the nature of the
halogen. In the transmetalation reactions according to eq 2, only
a single type of halogen is involved. The same holds true for
the LiCl-mediated zinc insertion into benzylchloride. However,
the situation is more complicated for the LiCl-mediated reactions
of zinc with organic bromides and iodides, where two different
halogen atoms are present. The original hypothesis of Knochel
and co-workers assumed that R-Zn-Hal · LiCl intermediates
contain the halogen from the reactant RHal (Hal ) Br and I)
as well as one chlorine atom stemming from LiCl. We indeed
observed the corresponding ZnRHalCl- anions but, particularly
for Hal ) I, in considerably lower signal intensities than their
-
ZnRHal2 counterparts. Similarly, the signal intensities of the
purely inorganic ZnHal3-nCln- complexes are the higher the less
chloride ligands they contain, with this trend being more
n
-
ZnI2Cl- + ZnnBuCl h Zn2 BuI2Cl2
(6a)
(6b)
-
pronounced for the ZnI3-nCln- series than for the ZnBr3-nCln
n
ZnI3- + ZnnBuCl h Zn2 BuI3Cl-
series. The lower signal intensities of the chlorine-rich zincates
may reflect lower ESI response factors and/or lower concentra-
tions of these species in solution. Note that a putative lower
concentration of chlorine-rich zincates could not simply result
from a lower overall concentration of ionogenic chlorine
compared to bromine or iodine, respectively, because in all
experiments equimolar amounts of reactant RHal and LiCl and
thus of Hal and Cl were employed (for incomplete conversion
and remaining RHal, the concentration of ionogenic chlorine
would be even higher than that of ionogenic bromine or iodine,
respectively). Instead, we surmise that the concentration of free
chlorine-rich zincate anions that are detectable by ESI mass
spectrometry is significantly reduced by ion pairing with lithium
cations (eq 4 for the case of Li+ZnRHalCl-). The lithium cation
is well-known to be a hard Lewis acid, and so supposedly it
preferentially interacts with hard Lewis bases. Because chloride
is a harder Lewis base than bromide and iodide, the lithium
cation should bind more strongly to chlorine-rich than to
chlorine-free zincate anions. This stronger binding directly
translates into a higher equilibrium concentration of contact ion
pairs and thus a smaller fraction of free ions in the case of the
chlorine-rich zincates. The strong tendency of lithium cations
to form higher aggregates with chloride is also reflected in the
low equivalent conductivity of LiCl in THF, which amounts to
only 2% of the corresponding equivalent conductivity of LiBr
(for c(LiHal) ≈ 10 mmol L-1 at 298 K).31
The parallel decline of the relative ESI signal intensities of the
mononuclear ZnI2Cl- and ZnI3- complexes at higher concentra-
tions is also in line with this interpretation, whereas the
-
intermediate increase observed for ZnnBuI2 clearly is not.
-
Interestingly, the signal intensity of the analogous ZnnBuCl2
zincate generated from the transmetalation reaction of ZnCl2
with nBuLi displayed very similar nonmonotonic concentration
dependence.24 We rationalized this behavior by the occurrence
of hydrolysis and/or oxidation processes due to residual
contamination by traces of moisture and/or oxygen. The
depletion of the organozincate species due to these reactions
should be most significant for low absolute concentrations. At
higher concentrations, these decomposition processes affect only
a small fraction of the organozincate ions whereas now depletion
because of association reactions sets in.
The successive formation of higher aggregation states for
increasing analyte concentrations can be seen even more clearly
in the LiCl-mediated reaction between zinc and benzylchloride.
Here, the increase of the relative signal intensity for the
-
trinuclear organozincate Zn3Bn3Cl4 is mirrored by decreases
-
observed for mononuclear ZnBnCl2 and ZnCl3-. The signal
intensity for the intermediate dinuclear zincate Zn2Bn2Cl3- (as
-
well as the related LiZn2Bn2Cl4 ) exhibits only weak concentra-
tion dependence because formation and depletion of this
complex according to eq 7 apparently balance each other.
The simultaneous presence of different zincate complexes,
-
-
such as ZnRHalCl-, ZnRHal2 , ZnHal2Cl-, and ZnHal3 , points
to an extensive equilibration via exchange of halide ligands.
Similar equilibration processes might conceivably also lead to
the exchange of the organyl R (eq 5).
-
ZnBnCl2- + 2 ZnBnCl h Zn2Bn2Cl3
+
-
ZnBnCl h Zn3Bn3Cl4 (7)
Similar to the case of the transmetalation reaction between ZnCl2
and nBuLi,24 the observed concentration dependences thus well
correlate with the expected shifts of the equilibria between
-
2 ZnRHal2- h ZnR2Hal- + ZnHal3
(5)
At first sight, such Schlenk-type equilibria seem to rationalize
the presence of ZnHal3 complexes, which were observed in
-
(32) Evans, D. F.; Fazakerley, G. V. J. Chem. Soc. A 1971, 182.
(33) Blake, A. J.; Shannon, J.; Stephens, J. C.; Woodward, S.
Chem.sEur. J. 2007, 13, 2462.
(31) Das, D. J. Solution Chem. 2008, 37, 947.

778 Organometallics, Vol. 28, No. 3, 2009
Koszinowski and Bo¨hrer
different aggregation states in solution. This finding gives us
further confidence that ESI mass spectrometry can be used to
probe the different zincate complexes actually present in
solution.
a dependence of the exact stoichiometry of the polynuclear
organozincates produced on the nature of the organyl R.24 These
findings indicate that already subtle effects may influence the
formation of polynuclear zincate complexes in a not easily
predictable manner. In line with this assessment, a considerable
variety of structural motifs is known for organozinc compounds
in the literature.35 However, no structural information is
available that is directly relevant to the complexes found in the
present work.
Although the reactions both with n-butyliodide and benzyl-
chloride lead to the formation of polynuclear zincate anions,
important differences exist. Whereas for the former reaction
significant amounts of polynuclear zincates were only detected
for concentrations of c g 10 mmol L-1 (with respect to reactant
RHal and to LiCl), the latter gave abundant ESI signal intensities
already for c ) 0.1 mmol L-1. Moreover, the reaction with
n-butyliodide did not produce zincate complexes containing
more than two metal centers, whereas for the reaction with
Cationic Species. In contrast to the anionic complexes
observed, all of the detected cations contained at least two
molecules of THF. Essentially the same behavior could be seen
for transmetalation reactions between zinc halides and organyl
lithium compounds in THF.24 This difference clearly shows that
the Lewis base THF has a high affinity toward cations whereas
its interaction with anions apparently is not sufficiently strong
to survive the ESI process, despite the mild conditions applied.24
The high affinity of THF toward cations also explains the
formationoftheorganozinccationZnBn(THF)3+ fromBn-Zn-Br.
This species closely resembles complexes of alkylzinc cations
coordinated by dimethylformamide (DMF) molecules, which
Caggiano et al. recently observed by ESI mass spectrometry of
solutions of the corresponding alkylzinc iodides in DMF.36
Theoretical calculations by these authors suggest that three
molecules of DMF are necessary to achieve ionization of
-
benzylchloride the trinuclear complex Zn3Bn3Cl4 gave the
highest signal intensity. A comparison of these two reactions
alone cannot differentiate whether these different tendencies
toward formation of aggregates are governed by the organyl or
the halogen of the RHal reactant. The fact that even for c ) 50
mmol L-1 no polynuclear zincates were observed for the reaction
of 4-iodo-ethylbenzoate can be viewed as a first indication that
it is the exclusive presence of chlorine that facilitates the
detection of higher aggregates.
This assessment is fully borne out by a comparison with the
results from our previous study on organozincates generated
by transmetalation according to eq 2. For the reaction of ZnCl2
CH3-Zn-I.36 In the resultant ZnCH3(DMF)3 complex, the
+
n
with BuLi, we observed much more abundant polynuclear
ZnCH3+ cation is stabilized by O-coordination.36 It is remarkable
that the system Bn-Zn-Br/THF apparently behaves very
similarly although the two solvents DMF and THF have rather
different properties (compare, e.g., ε(DMF) ) 36.7 and ε(THF)
) 7.4). Presumably, the ionization of Bn-Zn-Br is facilitated
by the ability of the benzyl group to stabilize the cationic zinc
center by its positive mesomeric effect.
zincate complexes24 than for the LiCl-mediated zinc insertion
into nBuI, and this difference obviously results from the different
properties of the two halogens. Reactions of ZnCl2 with other
alkyl lithium compounds RLi also led to polynuclear zincates.24
All of these complexes displayed R/Zn ratios of e1, suggesting
that a minimum number of chloride ligands is necessary for
the formation of polynuclear zincates.24 No polynuclear zincates
were observed for the reactions of ZnBr2 and ZnI2 with nBuLi,
although these reactions did yield lithium halide clusters of the
We believe that the generation of the solvated ZnBn+ cation
is intimately linked to the formation of organozincate anions
and can be understood as a disproportionation of neutral
Bn-Zn-Br (eq 8).
24
-
type LinHaln+1
.
Apparently, chloride is more effective in
forming bridging bonds between two zinc atoms than the heavier
halides. This interpretation is also in line with findings on other
halide-containing zinc complexes.34 Note, however, that the zinc
insertion into benzylbromide in the absence of LiCl led to
significant amounts of polynuclear organozincates as well. This
observation demonstrates that bromide is also capable of forming
bridging bonds between zinc atoms if competition by chloride
is excluded.
The fact that no abundant polynuclear zincates were detected
when both chloride and bromide or iodide, respectively, were
present in solution is probably related to the low relative ESI
signal intensities also observed for mononuclear chlorine-rich
zincates in these cases (see above). Presumably, the proposed
higher tendency toward the formation of contact ion pairs is
not limited to mononuclear chlorine-rich zincates but also applies
to their polynuclear congeners and thus suppresses their ESI
signal intensities in comparison to those of the (mononuclear)
bromo- and iodozincates, respectively.
-
n Bn-Zn-Br h ZnBn+(solv) + Znn-1Bnn-1Brn+1 (8)
In line with this interpretation, the organozincates observed in
general contained a higher number of bromine atoms than benzyl
groups. For the LiCl-mediated zinc insertion reactions, the
situation is different because here the addition of extra LiCl
permits the formation of an organozincate complex without
abstraction of a halide ion from the organylzinc compound.
Indeed, no organozinc cations were detected for the LiCl-
mediated zinc insertion reactions.
Gas-Phase Reactivity. In their unimolecular fragmentation
reactions, the mononuclear organozincate complexes of the type
ZnRHal2 almost exclusively lost ZnRHal to produce Hal- as
-
ionic fragment (eq 9).
ZnRHal2- f Hal- + ZnRHal
(9)
Similarly, the polynuclear organozincates from the LiCl-
mediated reaction of zinc with benzylchloride fragmented in
such a way that the chloride ligands preferentially remained in
the ionic fragment whereas the benzyl groups accumulated in
the neutral fragment. Analogous fragmentation patterns were
observed for polynuclear organozincates produced in transmeta-
Besides the halide ligands, the organyl groups also have some
influence on the tendency toward aggregation. While for the
LiCl-mediated zinc insertion into n-butyliodide polynuclear
zincates were detected at higher analyte concentrations, analo-
gous complexes were not observed for the reaction with 4-iodo-
ethylbenzoate. Similarly, the transmetalation reactions between
ZnCl2 and different organyl lithium compounds RLi also showed
(35) Melnik, M.; Skorsˇepa, J.; Gvo¨ryova´, K.; Holloway, C. E. J.
Organomet. Chem. 1995, 503, 1.
(36) Caggiano, L.; Jackson, R. F. W.; Meijer, A. J. H. M.; Pickup, B. T.;
Wilkinson, K. A. Chem.sEur. J. 2008, 14, 8798.
(34) Castin˜eiras, A.; Arquero, A.; Masaguer, J. R.; Martinez-Carrera,
S.; Garcia-Blanco, S. Z. Anorg. Allg. Chem. 1986, 539, 219.

Organozincate Anions
Organometallics, Vol. 28, No. 3, 2009 779
lation reactions.24 This asymmetric fragmentation maximizes
the stabilization of the anionic fragment by virtue of the electron-
withdrawing effect of the halogen atoms. Interestingly, these
heterolytic dissociations differ from the behavior of the recently
studied MgBnCl2- anion. As O’Hair and co-workers reported,37
this ion undergoes a homolytic bond cleavage upon CID (eq
10).
chlorides to the formation of polynuclear zincates points to the
ability of chlorine to bridge different zinc centers.
The observed coordination of halides ions to organozinc
species is likely to influence the reactivity of the latter. Indeed,
Oshima and co-workers found the addition of LiCl to accelerate
the transmetalation of hexylzinc iodide and rationalized this
effect by a putative deaggregation of the organozinc species.10
In contrast, our present results indicate that chloride ions
facilitate the formation of polynuclear organozincate complexes
instead of causing deaggregation. Further work is needed to
resolve this apparent contradiction.
MgBnCl2- f MgCl2·-+Bn·
(10)
The gas-phase experiments furthermore showed a rather low
bimolecular reactivity of ZnRHal2- complexes. This finding is
fully consistent with the results of our previous gas-phase studies
Preliminary studies of the zinc insertion into benzylbromide
show that organozincates also form in the absence of LiCl. The
formation of organozincate anions most likely involves a
disproportionation of neutral Bn-Zn-Br, thus generating
organozinc cations as coproducts. This hypothesis is directly
24
-
on analogous ZnRCl2 species generated by transmetalation
and is also in line with the well-known moderate reactivity of
organozinc compounds in solution.1,2
+
supported by the observation of the ZnBn(thf)3 ion. The
5. Conclusions
ionization of Bn-Zn-Br in THF mimics the recently reported
heterolytic dissocation of alkylzinc iodides in DMF.36 We are
currently investigating zinc insertion reactions in the absence
of LiCl in more detail.
Anion-mode ESI mass spectrometry of THF solutions of the
products from the LiCl-mediated zinc insertion reactions into
organic halides RHal resulted in the detection of abundant
-
organozincate anions, such as ZnRHalCl- and ZnRHal2 (for
Finally, the gas-phase reactivity of the organozincate com-
plexes probed in the present work closely resembles that of
analogous species investigated previously.24 Mono- and poly-
nuclear organozincates fragment by elimination of neutral zinc
species. The partitioning of the organyl groups and halogen
atoms in the ionic and neutral fragment is largely controlled by
thermochemistry. The bimolecular gas-phase reactivity of the
organozincate anions sampled is rather low, and this finding is
in line with the moderate reactivity characteristic of organozinc
compounds in solution.1,2
Hal ) Br and I). We identify the former with the dissociated
form of the R-Zn-Hal · LiCl intermediate proposed by Knochel
and co-workers.6 The present results hence support the hypoth-
esis put forward by these authors. The fact that not only
-
ZnRHalCl- but also ZnRHal2 ions were observed indicates
extensive equilibration of the organozincate complexes by
exchange of the halide ligands. The thus formed ZnRHal2- ions
yielded even higher ESI signal intensities than their ZnRHalCl-
counterparts. This difference is ascribed to a smaller concentra-
tion of free ZnRHalCl- anions, of which presumably a larger
fraction is tied up in contact ion pairs with Li+. The nature of
the halogen present in the zincate complexes also has a strong
influence on their aggregation state. Whereas the reactions with
iodides predominantly or exclusively afforded mononuclear
zincates, abundant polynuclear zincates were detected for the
LiCl-mediated zinc insertion into benzylchloride. The relative
signal intensities of these polynuclear zincates increased as a
function of concentration (relative to reactant BnCl and to LiCl),
thus apparently reflecting the corresponding association equi-
libria operative in solution. The propensity of the organozinc
Acknowledgment. We thank Prof. Ulrich Koszinowski
for generously making his TSQ 7000 mass spectrometer
available to us and are grateful to Prof. Paul Knochel and
Matthias Schade for stimulating discussions. We gratefully
acknowledge support by Prof. Herbert Mayr, Prof. Thomas
Carell, the Munich Center for Integrated Protein Science
CIPSM, the Fonds der Chemischen Industrie, and the Deut-
sche Forschungsgemeinschaft (SFB 749).
Supporting Information Available: Table of CID experiments
performed and figures showing additional mass spectra. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
(37) Thum, C. C. L.; Khairallah, G. N.; O’Hair, A. J. O. Angew. Chem.
2008, 120, 9258. Thum, C. C. L.; Khairallah, G. N.; O’Hair, A. J. O. Angew.
Chem., Int. Ed. 2008, 47, 9118.
OM800947T