Formation of organozincate species from diorganozinc compounds and salts

Article abstract of DOI:10.1039/b009126j

Stirring a benzene solution of a diorganozinc compound with an insoluble alkali metal alkoxide (MeOK or MeONa) or potassium amide (PhN(Me)K) formed a solution containing a species having a salt: R₂Zn composition of 1:2. A salt (t-BuOK) that is soluble in benzene also forms a 1:2 species and, when the salt to R₂Zn ratio is sufficiently high, forms a 1:1 species. Controlled heating at reduced pressure of solutions prepared from an alkali metal alkoxide and an excess of a diorganozinc compound leaves materials with alkoxide:R₂Zn ratios of 1:2 or 1:1. Reactions of benzene solutions of diorganozinc compounds with (p-MeC₆H₄)SK or Ph ₂PK form solutions of composition R₃ZnK. The Royal Society of Chemistry 2001.

Full text of DOI:10.1039/b009126j

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Formation of organozincate species from diorganozinc compounds
and salts†
Ronaldo M. Fabicon and Herman G. Richey, Jr.*
Department of Chemistry, The Pennsylvania State University, University Park,
Pennsylvania, 16802, USA. E-mail: hgr@chem.psu.edu
Received 14th November 2000, Accepted 17th January 2001
First published as an Advance Article on the web 16th February 2001
Stirring a benzene solution of a diorganozinc compound with an insoluble alkali metal alkoxide (MeOK or MeONa)
or potassium amide (PhN(Me)K) formed a solution containing a species having a salt : R2Zn composition of 1 : 2.
A salt (t-BuOK) that is soluble in benzene also forms a 1 : 2 species and, when the salt to R2Zn ratio is sufficiently
high, forms a 1 : 1 species. Controlled heating at reduced pressure of solutions prepared from an alkali metal alkoxide
and an excess of a diorganozinc compound leaves materials with alkoxide : R2Zn ratios of 1 : 2 or 1 : 1. Reactions of
benzene solutions of diorganozinc compounds with (p-MeC6H4)SK or Ph2PK form solutions of composition R3ZnK.
Solutions with organozincate compositions such as R3ZnK and
R3ZnLi undergo reactions not significant with R2Zn or RZnX.1
Addition of appropriate salts to diorganozinc compounds
also might form organozincates (e.g., eqn. (1)), and indeed a few
moved downfield (≈0.4 ppm). When >0.5 equivalent of MeOK
was used undissolved solid remained. 1H NMR absorption
areas indicated that the MeOK : Et2Zn ratio in solution never
exceeded 0.5 : 1, even when a large excess of MeOK was used.
MeOK also dissolved in solutions of Bu2Zn or s-Bu2Zn to form
solutions in which the MeOK : R2Zn ratio reached but never
exceeded 0.5 : 1.5 The ratio of 0.5 : 1 persisted when mixtures
prepared with an excess of MeOK were heated at 70 ЊC or
subjected to ultrasonic radiation. MeONa, also insoluble
in benzene, formed solutions having MeONa : Et2Zn ratios
≤0.5 : 1 and NMR spectra almost identical to those of solutions
prepared with MeOK.
Solutions prepared in benzene or diethyl ether from MeOK
and an excess of Et2Zn were heated at reduced pressure
to remove volatile materials.6 With temperatures in the range
60–90 ЊC and pressures in the range 1–5 Torr viscous liquids
remained. These liquids dissolved in benzene-d6, and 1H NMR
spectra of the solutions showed a MeOK : Et2Zn ratio of
0.5 : 1. Similar experiments with Et2Zn and MeONa (benzene),
Bu2Zn and MeOK (benzene), and Bu2Zn and MeONa (diethyl
ether) also resulted in oils that dissolved in benzene-d6;
1H NMR spectra of these solutions indicated MeOM : R2Zn
ratios of 0.5 : 1.7
R2Zn ϩ MϩAϪ → R2ZnAϪMϩ
(1)
reports have indicated that some salts enhance reactivity.1,2
When we began this work the principal studies of compositions
and structures that resulted from combining R2Zn and a
salt involved hydride as the anion.3 Solids that had
salt : R2Zn compositions of 1 : 1 and 1 : 2 were obtained with
LiH or NaH. In an earlier report we described preliminary
results of a study of R2Zn compounds and potassium
alkoxides.4 A solid was obtained that X-ray diffraction
analysis showed to contain the organozincate dianion 1 (R = Et,
RЈ = t-Bu). Less direct evidence suggested that solutions of
R2Zn compounds and potassium alkoxides contain species
with alkoxide : R2Zn ratios of 1 : 1 and 1 : 2. This paper presents
additional information and adds studies of R2Zn with other
classes of salts.
Formation of species having the stoichiometry of one MeOK
or MeONa to two R2Zn is suggested by repeated observations
of this composition in both solutions formed using an excess
of an alkoxide salt and oils formed by removing excess of
R2Zn.
Results and discussion
R2Zn with MeOK and MeONa
R2Zn with t-BuOK
t-BuOK is significantly soluble8 in benzene and forms solutions
having higher RЈOK : R2Zn ratios than does MeOK. In 1H
NMR studies with Et2Zn, Bu2Zn, s-Bu2Zn, or (Me3SiCH2)2Zn
and varying amounts of t-BuOK only one t-BuO absorption
and one set of R absorptions were seen. Some broadening but
no additional absorption was evident in spectra (toluene-d8
solutions) at Ϫ80 ЊC. Therefore R and t-BuO of all species
present must equilibrate rapidly relative to the NMR timescale.
The 1H NMR absorption of CH2Zn moved upfield with
increasing t-BuOK : Et2Zn ratio but, particularly as the reactant
ratio approached 1 : 1, formation within a few minutes of a
precipitate (the solid shown4 by X-ray diffraction to contain
Although MeOK is virtually insoluble in benzene, it dissolved
completely when stirred with a benzene solution containing ≥2
equivalents of Et2Zn. Except where noted, benzene (or
benzene-d6) was the solvent in this work; an amount of R2Zn
was used that would form a 0.2–0.6 M solution. The 1H NMR
spectra of all solutions in which MeOK had dissolved showed
only one MeO absorption and one set of Et absorptions. As the
MeOK : Et2Zn ratio in solution increased the CH2Zn absorp-
tion moved upfield (≈0.1 ppm) and the CH3CH2Zn absorption
† This paper is dedicated to the memory of Ron Snaith.
DOI: 10.1039/b009126j
J. Chem. Soc., Dalton Trans., 2001, 783–788
783
This journal is © The Royal Society of Chemistry 2001

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Fig. 1 Plots of the upfield shifts of the 1H NMR absorption of CH2Zn
versus the t-BuOK : R2Zn ratio for R = butyl or Me3SiCH2. The R2Zn
concentration in all solutions is ≈0.4 M.
Fig. 2 13C NMR spectrum (75 MHz) of a benzene-d6 solution pre-
pared from t-BuOK, 18C6, and Bu2Zn (0.75 : 0.75 : 1). Absorptions of
butyl are labeled A, of t-butoxy B, and of 18C6 C.
organozincate dianion 1 (R = Et, RЈ = t-Bu)) limited study.
When R was Bu, s-Bu, or Me3SiCH2, solutions prepared
using various t-BuOK : R2Zn ratios did not form precipitates.
Fig. 1 shows typical plots of upfield shifts (∆δ) of the 1H
NMR absorption of CH2Zn versus the t-BuOK : R2Zn ratio.
As t-BuOK : R2Zn increases from 0 to 0.5 : 1 (A–B) the absorp-
tion position changes linearly. From 0.5 to 1 : 1 (B–C) the
change continues but to a lesser degree. Above 1 : 1 (C–D) the
absorption remains constant (its position unchanged at a t-
BuOK : R2Zn ratio of 5 : 1). A species having a t-BuOK : R2Zn
ratio of 1 : 2 must form in region A–B and incorporate most of
the t-BuOK in the solution. At C most solute must be in a
species having a t-BuOK : R2Zn ratio of 1 : 1. The constant
absorption position in region C–D is most readily interpreted
by assuming that essentially all R2Zn is incorporated into the
1 : 1 species which, however, exchanges relatively rapidly with
additional t-BuOK.9 If that is the case, then in region C–D the
Adding a coordinating agent for Kϩ, such as 18-crown-6
(18C6), is another possible way of slowing equilibration.
The 1H NMR spectrum of
a solution in which t-
BuOK : 18C6 : Bu2Zn was 0.75 : 0.75 : 1 had two t-BuO and
two CH2CH2Zn absorptions, and the position of the 18C6
absorption was typical of 18C6 coordinated to Kϩ; the
13C spectrum (Fig. 2) had two absorptions for all C of butyl
except CH3.11 These results are consistent with the presence
of two species having t-BuOK : Bu2Zn ratios of 1 : 2 and
1 : 1.Whent-BuOK : 18C6 : Bu2Znwas0.4 : 0.4 : 1or1.5 : 1.5 : 1,
1
only single sets of H and 13C NMR absorptions were noted
1
for Bu and t-BuO (although in the latter case the H NMR
absorption of t-BuO and the 13C NMR absorption of CO of
t-BuO were broad). At ratios <0.5 : 1 the 1 : 2 species must
equilibrate rapidly with excess of Bu2Zn and at ratios >1 : 1 the
1 : 1 species must equilibrate relatively rapidly with excess of
t-BuOK.
1
position of the H NMR absorption of t-BuO should move
from that of the 1 : 1 species toward that of free t-BuOK.
Unfortunately, the t-BuO absorption is virtually the same at all
reactant ratios. If only species having t-BuOK : R2Zn ratios of
1 : 2 and 1 : 1 are present, then the B–C region should be linear.
Owing to the limited range of chemical shifts, the presence
in that region of another component (t-BuOK : R2Zn between
0.5 and 1 : 1) cannot be excluded.
The small range of CH2Zn NMR absorptions (0.5 ppm for
Me3SiCH2, 0.3 ppm for Bu) is an obvious limitation. 13C NMR
spectra of the Bu2Zn system were investigated, but the CH2Zn
absorptions spanned a range of only 0.3 ppm. Nevertheless,
several considerations suggest the validity of the indications
that species form with t-BuOK : R2Zn ratios of 1 : 1 and 1 : 2. (1)
The observations are reproducible; various experiments gave
data having the features seen in Fig. 1. (2) Plots4 by the Job
method10 of data from the Me3SiCH2 system indicate one
species having a t-BuOK : (Me3SiCH2)2Zn ratio of 1 : 1 and
a second having a ratio of approximately 1 : 2. (3) Solutions
having compositions in the B–C region show NMR absorptions
for two species when treated in ways described below that might
slow exchange.
A second liquid phase formed when 18C6 was added to
solutions prepared from t-BuOK and (Me3SiCH2)2Zn, and
essentially all solute was in the lower phase when equimolar
amounts of the three components were used. This likely indi-
cates that the solute is ionic, since organometallic ions in
aromatic solvents often reside in such lower phases.13 In fact,
1H NMR spectra of the lower phases show an absorption
characteristic for 18C6 coordinated with Kϩ and, when the
ratio of t-BuOK and 18C6 to (Me3SiCH2)2Zn is less than 1 : 1,
generally show two t-BuO absorptions and two sets of Me3-
SiCH2 absorptions.11
A viscous liquid indicated by its 1H NMR spectrum
(benzene-d6) to have a t-BuOK : Et2Zn ratio of 1 : 2 remained
when volatile material was removed at 60–65 ЊC and 5 Torr
from a solution prepared from t-BuOK and excess of Et2Zn
in diethyl ether. Subjecting the viscous liquid to 120–130 ЊC at
0.1 Torr resulted in a white crystalline solid identical to the
1 : 1 precipitate from Et2Zn and t-BuOK in benzene. A similar
viscous oil and solid were obtained by subjecting a solution
prepared in benzene first to 40 ЊC and 2 Torr and then to 90 ЊC
and 0.1 Torr. Yet higher temperatures and lower pressures
resulted in no observable changes until gray solid (Zn metal)
formed when the temperature reached 250 ЊC. A viscous liquid
(solidified at 0 ЊC) indicated by its 1H NMR spectrum (benzene-
d6) to have a t-BuOK : Bu2Zn ratio of 1 : 2 also remained when
volatile material was removed at 30–40 ЊC and 0.5 Torr from a
Lowering the temperatures of solutions prepared in toluene-
1
d8 led to more absorptions. The H NMR spectrum at Ϫ80 ЊC
of a solution prepared with a t-BuOK : (Me3SiCH2)2Zn ratio
of 0.74 : 1 (to fall in the B–C region) showed two t-BuO absorp-
tions, ca. 1 : 2 in intensity, and two sets of Me3SiCH2
absorptions, ca. 1 : 1 in intensity (intensities are approximate
because absorptions were broad and the t-BuO absorptions
overlapped). The absorptions could be due to species having
t-BuOK : (Me3SiCH2)2Znratiosof1 : 2and1 : 1.Absorptionsof
a similar solution prepared from Bu2Zn broadened at lower
temperatures and at Ϫ80 ЊC two absorptions (approximately
1 : 1) were resolved for CH3CH2.
1
solution of t-BuOK and excess of Bu2Zn in benzene. The H
NMR spectrum (benzene-d6) of the solid (mp 84–89 ЊC) that
remained after subjecting this oil to 75 ЊC and 0.01 Torr showed
a t-BuOK : Bu2Zn ratio of 1 : 1. A similar viscous liquid and
solid were obtained by similar treatment of a solution prepared
in diethyl ether.
784
J. Chem. Soc., Dalton Trans., 2001, 783–788

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Conclusions about R2Zn–alkoxide preparations
Although all R2Zn–alkoxide solutions exhibit only single sets
of R and alkoxide NMR absorptions at ambient temperature,
several pieces of evidence indicate that species form that have
alkoxide : R2Zn ratios of 1 : 1 and 1 : 2. (1) Plots of absorption
position versus alkoxide : R2Zn ratio indicate the presence of
1 : 1 and 1 : 2 species. (2) Solutions prepared using alkox-
ide : R2Zn ratios between 0.5 and 1 : 1 have multiple NMR
absorptions at low temperatures. (3) Solutions prepared using
alkoxide–18C6 : R2Zn ratios between 0.5 and 1 : 1 have multiple
NMR absorptions. (4) MeOK and MeONa dissolve in R2Zn
solutions to form solutions having precisely 1 : 2 compositions.
(5) Removing solvent and R2Zn from solutions prepared with
excess of R2Zn tends to produce materials with alkoxide : R2Zn
ratios of 1 : 2 or 1 : 1.14 (6) Greater stability of R2Zn solutions
when alkoxide salts are dissolved in them suggests the for-
mation of new species. Pure s-Bu2Zn or its solutions in hydro-
carbon solvents, for example, decompose slowly at ambient
temperature. When two solutions of s-Bu2Zn, one also con-
taining 0.5 equivalent of t-BuOK, were heated at 70 ЊC for
30 min decomposition in the solution without alkoxide was
evident by deposition of gray solid (Zn) and appearance
Scheme 1
1
solid that did not dissolve on heating in benzene. H NMR
spectra of the materials remaining in the upper phases when
varying amounts of reactants were used indicated that the
lower phases had a Bu4NCl : Et2Zn ratio of 1 : 1. Experiments
using chloroform-d as the solvent also produced solids that did
not dissolve significantly when heated with the solvent. Bu2Zn
and Bu4NCl in benzene formed a dense phase which remained
liquid. 1H NMR absorptions of this phase were very broad, but
the areas of the CH2N and CH2Zn absorptions indicated a
Bu4NCl : Bu2Zn ratio of 1 : 1.
Bu4NBr and Bu2Zn in benzene produced homogeneous solu-
tions. As the Bu4NBr : Bu2Zn ratio increased from 0 to 0.5 : 1 the
1H NMR absorption of CH2Zn moved (0.25 ppm) linearly
downfield (rather than upfield as observed with alkoxides).
As the ratio increased further the absorption moved further
downfield (0.04 ppm between ratios of 0.5 and 1 : 1, and 0.03
ppm more between 1 and 4 : 1). A 1 : 2 species must form, but
the subsequent changes in absorption position are too small
to interpret. Bu4NCl and (Me3SiCH2)2Zn resulted in homo-
geneous solutions and similar NMR observations: the CH2Zn
absorption moved ≈0.15 ppm downfield as the Bu4NCl : (Me3-
SiCH2)2Zn ratio went from 0 to 0.5 : 1, and ≈0.05 ppm as it went
from 0.5 to 1.0 : 1.
1
of significant new H NMR absorptions (e.g. for ᎐CH ); the
᎐
2
solution containing alkoxide did not deposit a solid and had
an unchanged NMR spectrum.
The 1 : 1 species in solution presumably have structures like
that (1) of the solid obtained from Et2Zn and t-BuOK. The
1 : 2 species probably have structures such as 2. In solutions
in which alkoxide : R2Zn <0.5 : 1 only 2 and free R2Zn might
be present. Conceivably, species that contain more R2Zn
per alkoxide also could be present. The observation that the
1H NMR absorption of MeO and the 13C NMR absorption of
CO of t-BuO drift somewhat downfield as the RЈO : R2Zn ratio
increases from 0 to 0.5 : 1 could be an indication that not
all RЈO is incorporated into 2 until the ratio reaches 0.5 : 1.
The ranges of RЈO absorption positions are small, however,
and could be due to effects of the changing medium on the
absorptions of 2 rather than to the presence of an additional
species.
PhN(Me)K. This salt is quite insoluble in benzene, but some
dissolved when stirred with benzene solutions of Et2Zn,
i-Bu2Zn, (CH3CH2CHEtCH2)2Zn, or (Me3SiCH2)2Zn. The H
1
NMR absorption of CH2Zn moved upfield (maximum of
0.47 ppm) with increasing intensity of the PhNMe absorptions.
The spectra of solutions obtained after stirring for 24 h indi-
cated a PhN(Me)K : R2Zn ratio of 1 : 2. Using more PhN(Me)K
did not increase the ratio and heating mixtures in an effort to
introduce more of the salt resulted in decomposition, evidenced
by formation of gray solid and appearance of new absorptions
1
in H NMR spectra. Neither sonication nor addition of 18C6
significantly increased the PhN(Me)K : Et2Zn ratio.
The multiple absorptions in the presence of 18C6 probably
are due to decreased rates of exchange between species similar
to those present in its absence. With t-BuOK : R2Zn ratios <0.5
or >1 : 1, however, only single t-BuO and R absorptions are seen
in the presence of 18C6. Averaged absorptions are expected for
ratios <0.5 : 1 since exchange of a species such as 2 with free
R2Zn should be rapid. For ratios >1 : 1 in the presence of 18C6
the 1 : 1 species (presumably 1) must exchange rapidly with
excess of t-BuO groups. Yet the possibility that exchange occurs
by initial cleavage of 1 to form R2Zn or R2ZnO-t-BuϪ probably
is ruled out by the evidence that at lower t-BuOK : R2Zn ratios
the 1 : 1 and 1 : 2 species do not exchange rapidly. Perhaps
exchange occurs (Scheme 1) by cleavage of only one Zn–O
bridge bond followed by addition of t-BuO to the less sub-
stituted Zn, loss of one of the two equivalent t-BuO, and
regeneration of the Zn–O bridge bond, a sequence in which the
two R2Zn of 1 never separate.
TolSK and Ph2PK. White solid remained when a benzene
solution of Et2Zn was stirred at ambient temperature with
TolSK (Tol = p-methylphenyl) or Ph2PK. The Et absorptions
1
in H NMR spectra of the resulting solutions, however, were
shifted in position from those of Et2Zn. When, after continued
stirring, the NMR spectra showed no further changes (for
experiments with TolSK this also required heating at 70 ЊC
for 20–30 min), no Tol or Ph absorptions were present. The
1H NMR absorptions of all such solutions were essentially
identical, δ Ϫ0.09 (CH2) and 1.60 (CH3) (absorptions of Et2Zn
are δ 0.12 and 1.11).
Several pieces of evidence indicate that solutions of Et3ZnK
(eqns. 2 and 3) are formed. (1) Analysis15 shows that a basic
3 Et2Zn ϩ 2 TolSK → 2 Et3ZnK ϩ (TolS)2Zn↓ (2)
3 Et2Zn ϩ 2 Ph2PK → 2 Et3ZnK ϩ (Ph2P)2Zn↓ (3)
R2Zn with other salts
Bu4N؉X؊. Bu4NCl is relatively soluble in benzene. Com-
bining Et2Zn and Bu4NCl in that solvent resulted in formation
of a second, dense, liquid phase which turned into a gum-like
group (“EtϪ”), Zn, and K are in 3 : 1 : 1 ratio. (2) The
NMR spectra of the solutions are identical to those observed
J. Chem. Soc., Dalton Trans., 2001, 783–788
785

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for an Et3ZnK solution prepared1 in an established way.
(3) A dense phase formed when 18C6 was added to a solu-
tion prepared from TolSK or Ph2PK or to a solution of
under a nitrogen atmosphere. Diethyl ether was distilled from
sodium–benzophenone immediately prior to use. Diorganozinc
compounds were synthesized from organic halides using
1
1
authentic Et3ZnK. The 18C6 absorption (δ 3.00) in H NMR
standard procedures. Their H NMR absorptions (needed for
spectra of these phases is characteristic of 18C6 coordinated
comparison with spectra obtained in this work) are available12
except for those of (CH3CH2CHEtCH2)2Zn: (benzene-d6)
δ 0.37 (d, CH2Zn), 0.92 (t, CH3), 1.10 (m, CH2CH) and 1.48
(m, CH). N-Methylaniline was distilled from NaOH. t-BuOK
was sublimed (100 ЊC, <0.1 Torr). The other reactants (Aldrich)
were used as received. To prepare potassium N-methylanilide,
the mineral oil was removed from a mineral oil suspension of
KH (35%, 5.89 g corresponding to 2.0 g of KH, 0.050 mol) by
repeatedly washing the mixture with benzene and decanting
the supernatant liquid. Benzene (45 mL) was added followed
by dropwise addition to the stirred suspension of a solution
of N-methylaniline (10.7 g, 0.10 mol) in benzene (45 mL). Gas
(H2) evolution was immediately evident. The suspension was
stirred for 24 h and then transferred to centrifuge tubes and
centrifuged. The supernatant liquid was withdrawn using a
syringe and replaced with an equal volume of benzene. The
mixture was agitated by sonication until the suspension was
well mixed; the suspension was centrifuged and the benzene
removed. Washing the solid with additional benzene was
repeated. The remaining solid was heated (80 ЊC) at reduced
pressure for 24 h to remove residual solvent (5.1 g, 0.035 mol,
70%). Potassium p-methylbenzenethiolate (96%) and potassium
diphenylphosphide (84%) were prepared in the same manner
except that only 0.06 mol of p-methylbenzenethiol or diphenyl-
phosphine was used.
to Kϩ.
1
R3ZnK solutions exhibiting only H NMR absorptions of
R of R2Zn also were obtained using TolSK with i-Bu2Zn, (Me3-
SiCH2)2Zn, or (Tol)2Zn and Ph2PK with i-Bu2Zn, (CH3CH2-
CHEtCH2)2Zn, (Me3SiCH2)2Zn, or (Tol)2Zn. A dense phase
formed when 18C6 was added to the solution of (Me3SiCH2)3-
1
ZnK or i-Bu3ZnK. In H NMR spectra of the dense phases
the 18C6 absorption is characteristic of K(18C6)ϩ and the
absorption areas indicate that R and 18C6 are in 3 : 1 ratio.
Conclusion
An R2Zn compound in benzene and an alkali metal alkoxide,
tetraalkylammonium halide, or potassium amide form species
having a 2 : 1 ratio of the reactants. A 1 : 1 species also forms
when the salt to R2Zn ratio is sufficiently high, but only the
2 : 1 species is observed when the salt itself is insoluble.
Zinc lacks a partially filled d shell and in many respects
resembles magnesium, so comparison with results from reac-
tions of R2Mg and salts is pertinent. NMR studies with sodium
or potassium alkoxides demonstrated that when RЈOM : R2Mg
is 1 : 1, the highest ratio found in mixed species, the dominant
species in solution is 3.16 A second set of absorptions is seen
when RЈOM : R2Mg is less than 1 : 1,17 but sharp absorptions of
3 continue to be evident until the reactant ratio drops below
0.5 : 1. (s-Bu)2Mg–alkoxide solutions even have separate absorp-
tions for stereoisomers of structure 3 (containing different
arrays of R and S s-Bu groups). The discrete absorptions
seen for 3 indicate that, unlike its zinc counterpart 1, it
does not exchange rapidly relative to the NMR timescale with
other solution components. The more rapid exchanges in R2Zn
solutions may reflect the lesser tendency18 of R2Zn than of
R2Mg to form bonds to donor oxygen atoms. Equilibrium
constants for combining with an alkoxide are smaller for R2Zn
than for R2Mg and exchange between species in solution is
more rapid. Formation of a second phase with a tetraalkyl-
ammonium halide, found for R2Zn in this work, also has been
noted for R2Mg.19
NMR studies
In the glove box the salt was weighed into a vial equipped with a
magnetic stirring bar. Benzene-d6 (ordinarily 0.5 mL) was
added and the mixture stirred for a few minutes. The dialkylzinc
compound then was weighed into the vial and the contents
were stirred for 5–10 min. If solid was present then stirring was
continued for 12–24 h. For the preparations with potassium
p-methylbenzenethiolate a 20–30 min period of heating at
60–70 ЊC was added. The preparation then ordinarily was
transferred with a pipette to an NMR tube which had been
modified by adding an extension of routine glass tubing to
facilitate sealing with a flame. The tube was closed with a
septum, removed from the glove box, frozen in liquid nitrogen,
and sealed at the extension. When solid remained, either the
preparation was centrifuged before addition to the NMR tube
or, after addition, the NMR tube was centrifuged to collect
the solid at the unobserved (by the spectrometer) end. When a
preparation resulted in two liquid phases, each was transferred
to an NMR tube; those preparations had to be done on a
somewhat larger scale.
Listed below for representative preparations are (1) any
notable features, (2) (in parentheses) the salt : R2Zn ratio
(18C6 : salt : R2Zn when 18C6 also is present), and (3) the NMR
spectra. The listed ratio ordinarily is based on the amounts
of reagents used. For preparations having undissolved solid,
however, it is based on the relative intensities of 1H NMR
absorptions. For the solutions used to obtain plots (e.g. Fig. 1)
of ∆δ versus salt : R2Zn ratio the listed ratios also are based on
the relative intensities of 1H NMR absorptions, but closely
match the ratios expected from the amounts of compounds
used to prepare the solutions. Except where noted, spectra were
taken at ambient temperature, the solvent is benzene-d6, and
the amount of R2Zn used in the preparation would provide a
0.2–0.6 M concentration of this compound. The field strength
of the spectrometer is given only when the description is field
dependent.
R2Zn compounds and TolSK or Ph2PK form solutions
of composition R3ZnK. Low solubility20 of (TolS)2Zn and
(Ph2P)2Zn must be crucial in leading to formation of R3ZnK
rather than of species incorporating TolSK or Ph2PK.
Experimental
Procedures involving organometallic compounds were per-
formed under a nitrogen atmosphere using Schlenk techniques,
a glove box, and a vacuum line. 1H NMR absorption positions
are relative to internal C6D5H (δ 7.15) for benzene-d6 solutions
and internal CD5CD2H (δ 2.09) for toluene-d8 solutions.
Notations used are: s, singlet; d, doublet; t, triplet; q, quartet;
m, a more complex multiplet; c, complex overlapping absorp-
tions; br, broad. 13C NMR absorption positions are relative to
internal C6D6 (δ 128.0). Samples to be treated ultrasonically
were sealed in glass tubes and placed in a 60 kHz E/MC Corp.
Ultrasonic Cleaner, model 450.
Hydrocarbon solvents, including deuteriated solvents, were
distilled from CaH2 and stored over molecular sieves (4 Å)
MeOK–Et2Zn. (0.5) 1H NMR: δ Ϫ0.02 (q, CH2), 1.51
(t, CH3CH2) and 3.57 (s, CH3O).
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MeONa–Et2Zn. (0.5) 1H NMR: δ Ϫ0.05 (q, CH2), 1.53
(t, CH3CH2) and 3.51 (s, CH3O).
(s and s, CH3Si), 1.58 and 1.78 (s and s, CH3CO) and 3.01
(s, 18C6). (1.5 : 1.5 : 1) δ Ϫ0.49 (s, CH2), 0.48 (s, CH3Si), 1.55
(s, CH3CO) and 2.98 (s, 18C6).
MeOK–Bu2Zn. (0.5) 1H NMR: δ 0.02 (t, CH2Zn), 1.13
(t, CH3CH2), 1.25 (m, CH2CH3), 1.87 (m, CH2CH2Zn) and 3.49
(s, CH3O).
Bu4NCl–Bu2Zn. (1) 1H NMR (lower phase): δ 0.6 (br,
CH2Zn), 3.2 (br, CH2N) and 1.0–2.2 (c, all other H).
Bu4NCl–(Me3SiCH2)2Zn. (0.5) 1H NMR (300 MHz): δ Ϫ0.50
(s, CH2Zn), 0.47 (s, CH3Si), 0.95 (t, CH3CH2), 1.38–1.42
(c, CH2CH2CH2N) and 2.88 (br m, CH2N). (1) δ Ϫ0.44
(s, CH2Zn), 0.48 (s, CH3Si), 0.93 (t, CH3CH2), 1.37–1.41
(c, CH2CH2CH2N) and 2.85 (br m, CH2N).
1
MeOK–s-Bu2Zn. (0.5) H NMR: δ 0.12 (m, CHZn), 1.17 (t,
CH3CH2), 1.36 (d, CH3CH), 1.79 (m, CH2) and 3.52 (s, CH3O).
t-BuOK–Et2Zn. A solution resulted when t-BuOK (1.80 g,
16 mmol) and Et2Zn (1.98 g, 16 mmol) were stirred in toluene
(8.0 mL), but white crystals soon formed. The sample was
heated to 80 ЊC to dissolve the solid. Crystals formed again
on slow cooling to ambient temperature. The sample was then
cooled at Ϫ5 ЊC for 24 h. The liquid was decanted and the
crystals were washed twice with benzene and allowed to dry
(mp 145–148 ЊC). A 6 mg sample of the crystals dissolved
Bu4NBr–Bu2Zn. (0.5) 1H NMR (300 MHz): δ 0.51 (t,
CH2Zn), 0.8–1.9 (c, overlapping absorptions of CH3CH2CH2-
CH2Zn and CH3CH2CH2CH2N) and 3.02 (br m, CH2N).
(1) δ 0.56 (t, CH2Zn), 0.9–1.9 (c, overlapping absorptions of
CH3CH2CH2CH2Zn and CH3CH2CH2CH2N) and 2.99 (br m,
CH2N).
1
completely in 0.5 mL of benzene-d6 (intensities of H NMR
absorptions indicated a 1 : 2 ratio of t-BuO and Et groups).
1H NMR: δ Ϫ0.16 (q, CH2), 1.18 (s, CH3CO) and 1.62 (t,
CH3CH2). 13C NMR: δ 7.0 (CZn), 14.2 (CH3), 33.8 (CCO) and
69.1 (CO).
PhN(Me)K–Et2Zn. (0.5) 1H NMR (300 MHz): δ Ϫ0.02
(q, CH2), 1.51 (t, CH3CH2), 2.66 (s, CH3N), 6.04–6.20 (c, o-H
and p-H) and 6.70 (t, m-H).
1
t-BuOK–Bu2Zn. (0.5) H NMR: δ Ϫ0.04 (t, CH2Zn), 1.09
PhN(Me)K–i-Bu2Zn. (0.5) 1H NMR (300 MHz): δ 0.12
(d, CH2Zn), 1.16 (d, CH3CH), 2.15 (m, CH), 2.63 (s, CH3N),
6.16–6.21 (c, o-H and p-H) and 6.95 (t, m-H).
(s, CH3CO), 1.25 (t, CH3CH2), 1.29 (m, CH2CH3) and 1.87
(m, CH2CH2Zn). 13C NMR: δ 14.4 (CH2Zn), 17.3 (CH3CH2),
30.5 (CH2CH3), 32.6 (CH2CH2Zn), 35.3 (CH3CO) and 68.4
(CO). (1) 1H NMR: δ Ϫ0.09 (t, CH2Zn), 1.12 (s, CH3CO), 1.20
(t, CH3CH2), 1.27 (m, CH2CH3) and 1.95 (m, CH2CH2Zn). 13
C
PhN(Me)K–(CH3CH2CHEtCH2)2Zn. (0.5) 1H NMR (300
MHz): δ Ϫ0.10 (d, CH2Zn), 1.15 (t, CH3CH2), 1.20–1.85
(c, CH(CH2CH3)2), 2.60 (s, CH3N), 6.15–6.23 (c, o-H and p-H)
and 6.94 (t, m-H).
NMR: δ 14.6 (CH2Zn), 17.4 (CH3CH2), 30.7 (CH2CH3), 32.8
(CH2CH2Zn), 35.5 (CH3CO) and 67.9 (CO). (0.75) (toluene-d8)
1H NMR: δ 0.00 (t, CH2Zn), 1.10 (s, CH3CO), 1.17 (t,
CH3CH2), 1.25 (m, CH2CH3) and 1.87 (m, CH2CH2Zn).
1H NMR (Ϫ80 ЊC): δ Ϫ0.27 (t, CH2Zn), 1.09 (s, CH3CO), 1.60
(t, CH3CH2), 1.40 and 1.52 (m and m, CH2CH3) and 2.24
(m, CH2CH2Zn).
PhN(Me)K–(Me3SiCH2)2Zn. (0.5) 1H NMR (300 MHz):
δ Ϫ1.01 (s, CH2), 0.32 (s, CH3Si), 2.68 (s, CH3N), 6.06–6.17
(c, o-H and p-H) and 6.89 (t, m-H).
t-BuOK–18C6–Bu2Zn. (0.4 : 0.4 : 1) 1H NMR: δ 0.55 (t, CH2-
Zn), 1.20 (t, CH3CH2), 1.60 (s, CH3CO), 1.75 (m, CH2CH3),
2.15 (m, CH2CH2Zn) and 3.04 (s, 18C6). 13C NMR: δ 17.1
(CH2Zn), 21.5 (CH3CH2), 27.4 (CH2CH3), 32.4 (CH2CH2Zn),
35.4 (CH3CO), 68.1 (CO) and 70.0 (18C6). (0.75 : 0.75 : 1)
1H NMR: δ 0.42 (t, CH2Zn), 1.19 (t, CH3CH2), 1.55 and 1.59
(s and s, CH3CO), 1.73 (m, CH2CH3), 2.09 and 2.20 (m and
m, CH2CH2Zn) and 3.06 (s, 18C6). 13C NMR: δ 17.2 and 17.3
(CH2Zn), 21.6 (CH3CH2), 27.7 and 28.2 (CH2CH3), 32.4 and
32.6 (CH2CH2Zn), 35.5 (CH3CO), 68.3 (CO) and 69.9 (18C6).
TolSK–Et2Zn. 1H NMR: δ Ϫ0.08 (q, CH2) and 1.62 (t, CH3).
TolSK–18C6–Et2Zn. 1H NMR (lower phase): δ 0.28 (q, CH2),
1.49 (t, CH3) and 3.00 (s, 18C6).
1
TolSK–i-Bu2Zn. H NMR: δ Ϫ0.06 (d, CH2), 1.18 (d, CH3)
and 2.20 (m, CH).
1
(1.5 : 1.5 : 1) H NMR: δ 0.40 (t, CH2Zn), 1.17 (t, CH3CH2),
TolSK–18C6–i-Bu2Zn. 1H NMR (lower phase): δ 0.46 (d,
CH2), 1.34 (d, CH3), 2.22 (m, CH) and 2.99 (s, 18C6).
1.49 (br, CH3CO), 1.71 (m, CH2CH3), 2.09 (m, CH2CH2Zn)
and 3.03 (s, 18C6). 13C NMR: δ 17.3 (CH2Zn), 21.6 (CH3CH2),
28.2 (CH2CH3), 32.7 (CH2CH2Zn), 35.1 (CH3CO), 68.1 (CO)
and 70.0 (18C6).
TolSK–(Me3SiCH2)2Zn. 1H NMR: δ Ϫ1.13 (s, CH2) and 0.24
(s, CH3).
1
t-BuOK–s-Bu2Zn. (0.5) H NMR: δ 0.18 (m, CHZn), 1.02
(t, CH3CH2), 1.08 (s, CH3CO), 1.32 (d, CH3CH) and 1.68 (m,
CH2). (1) H NMR: δ 0.03 (m, CHZn), 1.07 (s, CH3CO), 1.18
(t, CH3CH2), 1.40 (d, CH3CH) and 1.86 (m, CH2).
TolSK–18C6–(Me3SiCH2)2Zn. 1H NMR (lower phase):
δ Ϫ0.50 (s, CH2), 0.43 (s, CH3) and 2.98 (s, 18C6).
1
TolSK–(Tol)2Zn. 1H NMR: δ 2.15 (s, CH3), 7.05 (d, m-H) and
7.99 (d, o-H).
1
t-BuOK–(Me3SiCH2)2Zn. (1) H NMR: δ Ϫ1.13 (s, CH2),
0.32 (s, CH3Si) and 1.09 (s, CH3CO). (0.5) 1H NMR: δ Ϫ1.01 (s,
CH2), 0.28 (s, CH3Si) and 1.08 (s, CH3CO). (0.74) (toluene-d8)
1H NMR: δ Ϫ1.01 (s, CH2), 0.35 (s, CH3Si) and 1.09 (s,
CH3CO). (Ϫ80 ЊC) δ Ϫ1.12 and Ϫ1.30 (s and s, CH2), 0.38 and
0.42 (s and s, CH3Si), 1.18 and 1.20 (s and s, CH3CO).
Ph2PK–Et2Zn. 1H NMR: δ Ϫ0.09 (q, CH2) and 1.59 (t, CH3).
Ph2PK–18C6–Et2Zn. 1H NMR (lower phase): δ 0.30 (d,
CH2), 1.51 (t, CH3) and 2.99 (s, 18C6).
t-BuOK–18C6–(Me3SiCH2)2Zn.(0.4 : 0.4 : 1)1HNMR(lower
phase): δ Ϫ0.50 and Ϫ0.58 (s and s, CH2), 0.44 and 0.47 (s and
s, CH3Si), 1.58 and 1.76 (s and s, CH3CO) and 2.98 (s, 18C6).
(0.8 : 0.8 : 1) δ Ϫ0.41 and Ϫ0.46 (s and s, CH2), 0.46 and 0.49
1
Ph2PK–i-Bu2Zn. H NMR: δ Ϫ0.04 (d, CH2), 1.18 (d, CH3)
and 2.20 (m, CH).
J. Chem. Soc., Dalton Trans., 2001, 783–788
787

View Article Online
Ph2PK–(Me3SiCH2)2Zn. 1H NMR: δ Ϫ1.12 (s, CH2) and
(<0.5 Torr) to remove more R2Zn from materials having
MeOM : R2Zn ratios of 0.5 : 1 led to insoluble, white solids. The
Et2Zn–MeOK oil, for example, gave such a solid when heated at
100–110 ЊC at 0. 1 Torr for 24 h. Even higher temperatures (≥200 ЊC)
led to obvious decomposition and gray solid (Zn).
8 Its solubility, for example, is 2.27 g/100 g at 25–26 ЊC in toluene:
L. F. Fieser and M. Fieser, Reagents for Organic Synthesis, Wiley,
New York, 1967, p. 911.
9 NMR absorptions of R in the C–D region were not much broader
than in the other regions. Particularly when t-BuOK : R2Zn >1.2 : 1,
however, t-BuO absorptions were broader, suggesting that not
all t-BuO groups exchanged very rapidly relative to the NMR
timescale.
0.24 (CH3).
Ph2PK–(CH3CH2CHEtCH2)2Zn. 1H NMR: δ Ϫ0.16 (d,
CH2Zn), 1.18 (t, CH3), 1.26 (m, CH2CH) and 1.55 (m, CH).
Ph2PK–(Tol)2Zn. 1H NMR: δ 2.17 (s, CH3), 7.03 (d, m-H) and
7.96 (d, o-H).
Acknowledgements
10 R. Sahai, G. L. Loper, S. H. Lin and H. Eyring, Proc. Natl. Acad.
Sci. USA., 1974, 71, 1499; V. M. S. Gil and N. C. Oliveira, J. Chem.
Educ., 1990, 67, 473 and references therein.
We thank the National Science Foundation for supporting this
work.
11 None of the absorptions is due to an R2Zn(18C6) rotaxane since
18C6 does not form significant amounts of a rotaxane with Bu2Zn
or (Me3SiCH2)2Zn.12
12 R. M. Fabicon, M. Parvez and H. G. Richey, Jr., Organometallics,
1999, 18, 5163.
Notes and references
1 C. A. Musser and H. G. Richey, Jr., J. Org. Chem., 2000, 65, 7750
and references therein.
2 For example: M. Chastrette and R. Amouroux, Tetrahedron Lett.,
1970, 5165; M. W. Rathke and H. Yu, J. Org. Chem., 1972, 37,
1732. Also see T. Shiota and T. Yamamori, J. Org. Chem., 1999,
64, 452.
3 G. J. Kubas and D. F. Shriver, J. Am. Chem. Soc., 1970, 92, 1949;
G. J. Kubas and D. F. Shriver, Inorg. Chem., 1970, 9, 1951;
E. C. Ashby and R. G. Beach, Inorg. Chem., 1971, 10, 2486;
E. C. Ashby and J. J. Watkins, Inorg. Chem., 1973, 12, 2493. Also
see D. F. Shriver, G. J. Kubas and J. A. Marshall, J. Am. Chem. Soc.,
1971, 93, 5076.
13 J. L. Atwood, in Inclusion Compounds, eds. J. L. Atwood,
J. E. Davies and J. E. D. MacNicol, Academic Press, London, 1984,
vol. 1, ch. 9.
14 For the 1 : 2 materials that are liquids this composition cannot be
due to chance interactions within a crystalline solid.
15 An aliquot of a reaction (carried out on a larger scale than the
NMR preparations) was hydrolysed and titrated for base. Another
aliquot was hydrolysed and then heated to remove most of the
benzene. Following appropriate dilutions of this solution and
of commercial standard solutions, K and Zn were determined by
AA analysis.
4 R. M. Fabicon, M. Parvez and H. G. Richey, Jr., J. Am. Chem. Soc.,
1991, 113, 1412.
16 E. M. Hanawalt and H. G. Richey, Jr., J. Am. Chem. Soc., 1990, 112,
5 Separate 1H NMR absorptions are seen for the diastereotopic
methylene hydrogens of s-Bu2Zn. As the ROK : s-Bu2Zn ratio
increases the absorptions move closer together, although without
obvious broadening. At ratios of ≈0.4 : 1 and higher only one
absorption is seen.
4983.
17 As the ratio decreases below 1 : 1 species such as (R2Mg)2(ORЈ)Ϫ
probably are increasingly significant and these exchange rapidly with
excess of R2Mg.
18 H. Tang, M. Parvez and H. G. Richey, Jr., Organometallics, 2000,
19, 4810 and references therein.
19 E. M. Hanawalt, Ph.D. Thesis, The Pennsylvania State University,
1989; J. Farkas, Ph.D. Thesis, The Pennsylvania State University,
1985.
20 (PhS)2Zn, for example, is insoluble in a wide range of solvents and
is thought to have a polymeric structure: F. F. Said and D. G. Tuck,
Inorg. Chim. Acta, 1982, 59, 1.
6 An excess of a solution (0.8–1.2 M) of R2Zn was added to the
alkoxide. The resulting suspension was stirred and then centri-
fuged. A flask containing the supernatant liquid was immersed
in a bath; the temperature and pressure were gradually adjusted to
the desired values and the flask was left under those conditions for
12–24 h.
7 Use of higher temperatures (100–170 ЊC) and lower pressures
788
J. Chem. Soc., Dalton Trans., 2001, 783–788