Remarkably robust monomeric alkylperoxyzinc compounds from tris(oxazolinyl)boratozinc alkyls and O2

Article abstract of DOI:10.1021/ja303440n

Metal alkylperoxides are remarkable, highly effective, yet often thermally unstable, oxidants that may react through a number of possible pathways including O-O homolytic cleavage, M-O homolytic cleavage, nucleophilic O-atom transfer, and electrophilic O-atom transfer. Here we describe a series of zinc alkyl compounds of the type To^MZnR (To^M = tris(4,4-dimethyl-2-oxazolinyl)phenylborate; R = Et, n-C₃H ₇, i-C₃H₇, t-Bu) that react with O₂ at 25 °C to form isolable monomeric alkylperoxides To^MZnOOR in quantitative yield. The series of zinc alkylperoxides is crystallographically characterized, and the structures show systematic variations in the Zn-O-O angle and O-O distances. The observed rate law for the reaction of To^MZnEt (2) and O₂ is consistent with a radical chain mechanism, where the rate-limiting S_H2 step involves the interaction of ^?OOR and To^MZnR. In contrast, To^MZnH and To^MZnMe are unchanged even to 120 °C under 100 psi of O ₂ and in the presence of active radical chains (e.g., ^?OOEt). This class of zinc alkylperoxides is unusually thermally robust, in that the compounds are unchanged after heating at 120 °C in solution for several days. Yet, these compounds are reactive as oxidants with phosphines. Additionally, an unusual alkylperoxy group transfer to organosilanes affords To^MZnH and ROOSiR₃'.

Full text of DOI:10.1021/ja303440n

Article
pubs.acs.org/JACS
Remarkably Robust Monomeric Alkylperoxyzinc Compounds from
Tris(oxazolinyl)boratozinc Alkyls and O2
Debabrata Mukherjee, Arkady Ellern, and Aaron D. Sadow*
Department of Chemistry and United States Department of Energy Ames Laboratory, Iowa State University, Ames, Iowa 50011,
United States
S
* Supporting Information
ABSTRACT: Metal alkylperoxides are remarkable, highly
effective, yet often thermally unstable, oxidants that may
react through a number of possible pathways including O−O
homolytic cleavage, M−O homolytic cleavage, nucleophilic O-
atom transfer, and electrophilic O-atom transfer. Here we
describe a series of zinc alkyl compounds of the type ToMZnR
(ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate; R = Et, n-
C3H7, i-C3H7, t-Bu) that react with O2 at 25 °C to form
isolable monomeric alkylperoxides ToMZnOOR in quantitative yield. The series of zinc alkylperoxides is crystallographically
characterized, and the structures show systematic variations in the Zn−O−O angle and O−O distances. The observed rate law
for the reaction of ToMZnEt (2) and O2 is consistent with a radical chain mechanism, where the rate-limiting SH2 step involves
the interaction of •OOR and ToMZnR. In contrast, ToMZnH and ToMZnMe are unchanged even to 120 °C under 100 psi of O2
and in the presence of active radical chains (e.g., •OOEt). This class of zinc alkylperoxides is unusually thermally robust, in that
the compounds are unchanged after heating at 120 °C in solution for several days. Yet, these compounds are reactive as oxidants
with phosphines. Additionally, an unusual alkylperoxy group transfer to organosilanes affords ToMZnH and ROOSiR3′.
an organic substrate. A radical chain could also give discrete
[Zn]OOR intermediates, or alternatively such alkylperoxyzinc
species could form via electron-transfer steps that exclude the
chain reaction. Low-temperature reactions of L2ZnR2 (L = 4-
methylpyridine, 1,4-diazabutadiene, 2,2′-(1′-pyrrolinyl)pyrrole)
and O2 that yield isolable and crystallographically characterized
alkylperoxyzinc species provide support for the latter
mechanism.9c,10,11 However, the products of these reactions
invariably contain multimetallic bridging alkoxides and/or
alkylperoxides that can obscure the reaction mechanism.
Highly reactive, alkylperoxy main-group compounds
[M]OOR (M = Mg,12 Zn,9c,10,13 Ga,14 In)15 have been
prepared from organometallics and O2, although these systems
are not amenable to quantitative kinetic investigations. For
1. INTRODUCTION
Reactions of alkylzinc reagents and O2 provide environmentally
and economically appealing approaches for useful oxidations,
including epoxidations,1 hydroxylations,2 and peroxidations.3 In
addition, alkylzinc compounds and their reactions with oxygen
are fundamentally interesting because zinc occupies a unique
position as a nontransition-element and nonredox active
divalent metal center whose chemistry nonetheless bears
resemblance to the 3d metals. Reactions of (nonmetallic)
alkylboranes and O2 provide a close well-studied main-group
system and are proposed to follow a radical chain based on
compelling evidence from kinetic studies, inhibition by radical
traps such as galvinoxyl, stereochemical studies, and spectro-
scopic radical trapping experiments. Galvinoxyl similarly
inhibits the reactions of ZnR2 and O2,4 and additional evidence
for open-shell alkyl, alkoxy, and alkylperoxy intermediates is
provided by EPR spectroscopy of trapped species.5 Oxygen
initiates carbozincations, which is taken as evidence for a radical
chain pathway.6,7 However, direct kinetic support for a radical
chain reaction is limited,4 and a rate law for the reaction of
alkylzinc compounds and O2 has not even been reported.
Furthermore, detailed mechanistic investigations of reactions
between organotransition-metal compounds and O2 have
provided a number of mechanisms, including the radical
chain, O2 coordination/protonation, and direct insertion.8
Likewise, several pathways have been proposed for oxidations
and metal alkylperoxide formations involving main-group
example, Parkin’s seminal Tpt‑BuMgOOR compounds (Tpt‑Bu
=
HB(N2C3H2t-Bu)3; R = Me, Et, i-C3H7, t-Bu) are proposed to
form through a radical chain pathway based on galvinoxyl
inhibition, and the relative rates follow the expected stability of
•R.12 Interestingly, the analogous Tpt‑BuZnR compounds and
O2 do not provide detectable quantities of Tpt‑BuZnOOR.16
The diketinimate ligand supports a monomeric aluminum tert-
butylperoxide formed from alkane elimination with HOOt-
Bu,17 but bridging alkylperoxy-magnesium and zinc species are
formed from (diketiminate)MR and O2 [M = Mg, R =
CH2Ph;12c M = Zn, R = Et].13 Monomeric structures may have
enhanced kinetic stability because [M]R and [M]OOR can
organometallic systems including zinc.4,5,9 At one extreme, R
•
Received: April 10, 2012
or •OOR, formed through a radical chain, could directly oxidize
© XXXX American Chemical Society
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comproportionate through multimetallic structures to metal
alkoxides.2 Still, monomeric, divalent, tetrahedral 3d transition-
metal alkylperoxides, stabilized by bulky tris(pyrazolyl)borate
ligands, decompose rapidly by homolysis at room temper-
ature.18
Kinetic investigations of reactions of [Zn]R and O2, then,
may be simplified by monomeric alkylperoxyzinc products. The
tridentate monoanionic tris(4,4-dimethyl-2-oxazolinyl)-
phenylborate ligand [ToM] supports monomeric zinc hydride,
alkyls, amides, and alkoxides,19 and therefore this ancillary
ligand might also stabilize zinc alkylperoxides. Here, we report
the synthesis and characterization of a series of remarkably
robust, monomeric, terminal ToMZnOOR compounds and
kinetic studies of their formation from reactions of O2 and
ToMZnR as well as their slow decomposition and mild atom-
and group-transfer reactivity.
2. RESULTS AND DISCUSSION
2.1. Synthesis and Characterization of ToMZnOOR (R =
Et, n-C3H7, i-C3H7, t-Bu, CMe2Ph). A series of zinc alkyl
compounds ToMZnR [R = Me (1),19b Et (2), n-C3H7 (3), i-
C3H7 (4), t-Bu (5), Ph (6), CH2Ph (Bn; 7)] is synthesized by
metathesis from Tl[ToM] and ZnR2, protonolysis of ZnR2 with
HToM, or salt elimination of ToMZnCl and RLi.20 Some of
these compounds are precursors to the targeted ToMZnOOR
species. Compounds 1−7 are characterized by spectroscopic
and analytical methods as well as single crystal diffraction for all
compounds except 4. Compounds 1−7 are all pseudo-C3v
Figure 1. ORTEP diagram of ToMZnEt (2). Significant interatomic
distances (Å): Zn1−C22, 1.994(2); Zn1−N1, 2.058(1); Zn1−N2,
2.084(1); Zn1−N3, 2.075(1). Significant interatomic angles (°): Zn1−
C22−C23, 118.7(1); N1−Zn1−C22, 128.83(6); N2−Zn1−C22,
120.53(7); N3−Zn1−C22, 126.35(7).
room temperature 1H NMR spectra of 8−11 contain
resonances assigned to equivalent oxazoline groups that suggest
pseudo-C3v symmetry for the ToM ligand. Downfield [Zn]-
OOCH resonances of 8−10 replace the upfield [Zn]CH signals
of 2−4. However, tert-butyl 5 and tert-butylperoxy 11 are
distinguished only by the 13C{1H} NMR resonance for the
CMe3 that shifts from 35.82 to 77.60 ppm.
1
symmetric, as indicated by equivalent oxazoline groups in H
and 13C NMR spectra. This spectroscopy is consistent with
tridentate coordination of the tris(oxazolinyl)borate ligand to a
single zinc center. Additionally, the 1H NMR spectra contained
diagnostic upfield resonances assigned to the α-CH of the Zn−
R moiety. These spectral data are consistent with the
monomeric structures and four-coordinate zinc centers
confirmed by single crystal X-ray diffraction studies (see Figure
1 for 2 and Figures S-13−16 in the Supporting Information
(SI) for 3 and 5−7).
Compounds 1−7 are resistant to thermal decomposition; for
example, compound 2 was recovered quantitatively after
thermolysis at 170 °C for 24 h in a sealed NMR tube. Thus,
initiation of the radical chain mechanism, proposed for
reactions with O2 (vide infra), is unlikely to involve
spontaneous Zn−C bond homolysis in this system.10a
17O NMR spectra are obtained from samples of 17O-enriched
8-11 that are synthesized by treatment of compounds 2−5 with
17O2 in benzene-d6. The data are listed in Table 1. Interestingly,
Table 1. 17O NMR Chemical Shifts of ToMZnOOR and
Tpt‑BuMgOOR (vs H2O)
δ([M]OOR)
δ([M]OOR)
ToMZnOOEt (8)
319
407
322
304
373
284
323
169
130
167
193
159
204
183
a
Tpt‑BuMgOO(Et)
ToMZnOO(n-Pr) (9)
ToMZnOO(i-Pr) (10)
Compounds 2−5 react with O2 (1 atm) in benzene-d6 over 3
days at room temperature, 6−8 h at 60 °C, or 30 min at 120 °C
to form ToMZnOOR [R = Et (8), n-C3H7 (9), i-C3H7 (10), t-
Bu (11)] as the only species detected (eq 1). These synthetic
a
Tpt‑BuMgOO(i-Pr)
ToMZnOO(t-Bu) (11)
a
Tpt‑BuMgOO(t-Bu)
a
Data from ref 12b.
the chemical shifts for the two resonances in 8-17O2 are similar
to the shifts for 9-17O2. The chemical shift differences for the
two resonances from 10 (Δ(δO) = δOα − δOβ = 111 ppm) are
smaller than the differences in 8 and 9 (Δ(δO) = 150 and 155
ppm, respectively). The tert-butyl compound 11 gives even
smaller differences in 17O NMR chemical shifts (Δ(δO) = 80
ppm). A similar observation was reported for Tpt‑BuMgOOR
compounds, and by comparison we assign the downfield signal
as [Zn]OOR, whereas the upfield resonance is attributed to
[Zn]OOR. Interestingly, the ZnOOR signals are upfield relative
to the chemical shifts of Tpt‑BuMgOOR, and the [Zn]OOR is
conditions highlight the remarkable thermal stability of 8−11
and strongly contrast the low-temperature preparation and
kinetic lability typically associated with 3d transition-metal and
many main-group alkylperoxides.
Compounds 8−11 form quantitatively, and evaporation of
the reaction mixtures provides analytically pure products. The
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downfield with respect to the corresponding magnesium
alkylperoxide.
Additionally, compounds 11 and ToMZnOOCMe2Ph (12)
are readily prepared by reaction of ToMZnH (13) with HOOt-
Bu and HOOCMe2Ph, respectively (eq 2).
As noted in the Introduction, the related Tpt‑BuZnEt is not
oxidized by O2 even at 100 °C.21 Surprisingly, only starting
materials are evident after treatment of ToMZnMe (1) and
ToMZnH (13) with O2 from room temperature to 120 °C and
from 1 atm to 100 psi for 12 h. Addition of O2 to mixtures of 2
and 1 or 13 converts 2 to 8, while 1 or 13 remains unreacted.
Alternatively, ToMZnPh (6) and ToMZnBn (7) react under O2
at 120 °C to form (κ2-ToM)2Zn (14) in benzene over 48 h (see
Figure S-22, SI). Compound 14 is prepared independently
from Tl[ToM] and 13. Neither light nor the radical initiator
AIBN facilitates reactions of 1, 6, 7, and 13 with O2 to provide
isolable organoperoxyzinc species.
Figure 2. ORTEP diagram of ToMZnOOEt (8); ellipsoids are plotted
at 35% probability, and hydrogen atoms and a toluene solvent
molecule are not plotted. Significant interatomic distances (Å): Zn1−
O4, 1.877(1); Zn1−O5, 2.630(2); Zn1−N1, 2.027(1); Zn1−N2,
2.045(1); Zn1−N3, 2.033(1); O4−O5, 1.466(2). Significant intera-
tomic angles (°): Zn1−O4−O5, 103.1(1); N1−Zn1−O4, 121.34(6);
N2−Zn1−O4, 119.87(6); N3−Zn1−O4, 128.53(6).
Table 2. Comparison of Interatomic O−O and Zn−O (Å)
Distances and Zn−O−O Angles (°) for Compounds 8 and
10−12
Reactions of oxygen and alkylzinc compounds are known to
often yield alkoxides rather than peroxides, and the synthesis of
alkylperoxyzinc compounds typically requires carefully con-
trolled conditions to avoid formation of alkoxides.3,9,10
ToMZnOR is not formed in these reactions based on the
combustion analyses of 8−12, the X-ray structures (see below),
the poorer benzene solubility of ToMZnOR (R = Et, n-C3H7, i-
C3H7) in comparison to 8−10, and the nonequivalence of the
1H NMR spectra of 8−11 to spectra obtained from treatment
of ToMZnH with ROH (R = Et, n-C3H7, i-C3H7, t-Bu).
compound
O−O
∠Zn−O−O
Zn−Oβ
ToMZnOOEt (8)
1.466(2)
1.441(3)
1.490(2)
1.477(3)
103.1(1)
105.1(1)
97.11(9)
103.4(2)
2.630(2)
2.644(2)
2.534(2)
2.637(2)
ToMZnOO(i-C3H7) (10)
ToMZnOOt-Bu (11)
ToMZnOOCMe2Ph (12)
10 < 8 < 12 < 11 that inversely tracks the Zn−Oα−Oβ angles
and the Zn−Oβ distances 11 < 8 ∼ 12 < 10. All of the Zn−Oβ
distances in 8 and 10−12 are within the sum of Zn and O van
der Waals radii (2.91 Å) but well outside the sum of covalent
radii for Zn and oxygen (1.88 Å).22 The nature of that
interaction may be considered in the context of the
systematically monomeric structures for 8−12 that contrast
dimeric or oligomeric [ToMZnOR]n (R = Et, n-C3H7, i-C3H7)
compounds. As noted above, the spectroscopic and physical
properties of ToMZnOR are more consistent with oligomeric
structures. Thus, the Zn−Oβ interaction likely perturbs the
atomic distances to a small but important degree to stabilize
compounds 8−12 as the first structurally characterized
monomeric zinc alkylperoxides.
2.2. Mechanistic Investigations and Kinetics of
ToMZnR and O2. General observations for the reactions of
ToMZnR and O2 allow comparison to alkylzinc/O2 systems that
are less amenable to rate law determinations. Reactions of
ToMZnEt (2) and O2 (50 psi) performed under ambient
lighting and in the dark give equivalent conversion of 2 and
yield (93%) of 8 after 32 h. Galvinoxyl inhibits the conversion.
The ToMZnOOR compounds are the only detectable products
from reactions of 2, 3, 4, or 5 with O2 in the presence or
absence of azobis(isobutyronitrile) (AIBN). Species, such as
C4H10, C2H6, ToMZnCMe2CN, ToMZnOOCMe2CN (or
related products from 3−5), that might be expected from
radical initiation or termination processes are not observed for
reactions of oxygen and ToMZnEt.
1
Interestingly, these reactions of alcohols give broad H NMR
resonances in benzene; upon evaporation of volatile materials,
benzene-insoluble white solids are obtained that suggest
oligomeric structures. (The bulkier ToMZnOt-Bu is monomeric
and soluble in benzene, and it is also spectroscopically distinct
from 11.)19a The monomeric structures of 8−12 are verified by
X-ray crystallography (see Figure 2 for 8 and Figures S-18−21,
SI, for 9-12). The alkylperoxy group of 9 is disordered over two
positions and will not be discussed.
The Zn−Oα distances in 8 and 10−12 are similar (from
1.873(2) Å in 10 to 1.877(1) Å in 8). Bridging
[diketiminatoZn(μ2-OOEt)]2 contains longer Zn−O distances
[1.971(1) and 2.044(1) Å],13 and triply bridging Zn3(μ3-
OOMe) Zn−O distances are much longer (2.132 Å average) in
the tetrameric cube [(MeZn)4(μ3-OOMe)2(μ3-OZnMeL)2] (L
= diazabutadiene).10 However, the related alkoxide ToMZnOt-
Bu contains a shorter Zn−O distance of 1.835(1) Å.19a
Additionally, the general structural features of 8−12 are roughly
similar to those of crystallographically characterized monomeric
tris(pyrazolyl)borato transition-metal alkylperoxides.18 These
3d transition-metal alkylperoxides are typically synthesized
from hydroperoxides which limits the diversity of readily
accessible alkyl groups. The preparation of 8−11 from O2, as
well as the availability of cumylperoxy 12, provides a range of
alkylperoxy zinc compounds for structural comparison.
Interestingly, the compound with the longest O−O distance
contains the smallest Zn−O−O angle and shortest Zn−Oβ
distance (Table 2). Thus, the O−O distances follow the trend
Rate law measurements for the reaction of 2 and O2 from
45−96 °C and 30−100 psi O2 were thwarted by variable
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induction periods and inconsistent concentration dependencies
for both O2 and ToMZnEt. However, in the presence of AIBN
(0.2−1.3 equiv), plots of [2] vs time follow an exponential
species [(κ2-ToM)Zn(μ-OH)]3 (15) that deposits as white
crystals over 24 h (eq 3). The organic carbonyl products are
decay providing the pseudofirst-order rate constant kobs
.
Notably, kobs values are equivalent within error at O2 pressures
of 30, 50, 70, 80, and 100 psi, showing zero-order oxygen
dependence (Figure S-3, SI). A linear correlation of [AIBN]1/2
vs kobs passes through the origin ([2]ini = 25 mM, [AIBN]ini
5.4−33.9 mM, 54 °C), giving the rate law −d[2]/dt =
k′[2]1[O2]0[AIBN]1/2 (k′ = 3.0 0.1 × 10−3 M−1/2 s−1) which
appears valid over at least three half-lives of the reaction time
course. This rate law is consistent with a radical chain
mechanism (Scheme 1) and is similar to empirical rate laws
MeCHO, Me2CO, and EtCHO from 8, 9, and 10, respectively.
Compound 11 forms Me2CO, while the cumylperoxyzinc 12
forms acetophenone, methane, and the epoxide 2-methyl-2-
phenyloxirane.
X-ray crystallography of disordered 15 establishes its
connectivity as a trimeric [Zn(μ-OH)]3 structure. Compound
15 is soluble in methylene chloride but insoluble in benzene,
Scheme 1. Proposed Radical Chain Mechanism for
ToMZnOOEt Formation from ToMZnEt and O2
1
toluene, and tetrahydrofuran. The room temperature H NMR
spectrum of 15 in methylene chloride-d2 was broad. The
spectrum resolved at −53 °C indicated local Cs symmetry with
equivalent zinc centers that are related by a rotation axis that is
assumed to be C3 based on the solid-state structure. Bidentate
coordination of ToM to Zn in 15 is further supported by two
oxazoline νCN bands in the IR spectrum at 1577 (coordinated)
and 1623 cm−1 (noncoordinated) in an approximately 2:1 ratio.
The formation of zinc hydroxide and an oxidized organic
species is consistent with O−O homolysis; for example, thermal
decomposition of Cp*2Hf(Ph)OOR gives Cp*2Hf(Ph)OH,25
and photolysis of dialkylperoxides is well-known to provide two
•OR that decompose to similar products.26 Compound 15 is a
likely intermediate in the thermolysis of alkylperoxides 8−12.
That process also likely involves homolytic O−O bond cleavage
because thermal treatment of 15 (in benzene-d6) gives 14 (8 h,
120 °C). Conversion of 15 to 14 is faster than the overall rate
of alkylperoxide decomposition, making that step kinetically
competent for the overall conversion. Thus, the rate-
determining step of conversion of 8−12 to 14 is O−O bond
homolysis.
for autoxidation of organic compounds, organoboranes,2 and a
few O2 insertions into transition-metal alkyls. Thus, our studies
provide the first rate law-based support for a radical chain
process for the interaction of alkylzinc species and O2.
The postulated radical chain mechanism includes a
bimolecular homolytic substitution process (SH2) at zinc that
likely involves an electron transfer from the HOMO (the Zn−
•
•
C bond) to OOEt to form a Zn−O bond and Et. The inert
nature of ToMZnMe (1) and ToMZnH (13) with O2 under
these conditions suggests that OOR (R = H, Me) is not able
•
to oxidize the Zn−H and Zn−Me bonds even in the presence
•
of AIBN and initiated radical chains (i.e., OOEt).
Previously, the weaker EtZn−Et bond vs MeZn−Me
provided a rationalization for the slower reaction of oxygen
and ZnMe2 in comparison to ZnEt2.5 Although the bond
dissociation energies (BDE’s) for ToMZn−R have not yet been
determined, the values are expected to follow the trends of
RZn−R, which have been determined from statistical
unimolecular reaction calculations (RRKM theory): EtZn−Et
< MeZn−Me (EtZn−CH2CH3 = 219 8 kJ/mol (52.4 2.0
kcal/mol); MeZn−CH3 = 266.5 6.3 kJ/mol (63.7 1.5 kcal/
mol).23 However, the experimental intrinsic BDE (determined
from the ion beam method) for [Zn−H]+ is much smaller than
[Zn−CH3]+ BDE: [Zn−H]+ = 231 13 kJ/mol (55.2 3.1
kcal/mol); [Zn−CH3]+ = 295 13 kJ/mol (70.6 3.2 kcal/
mol).24 The intrinsic BDE’s also provide the BDE for MeZn−
CH3 as 290 13 kJ/mol (69.3 3.2 kcal/mol) for comparison
between the two sets of values. Regardless, the inert nature of
ToMZnH toward O2 is not readily rationalized by known
BDE’s.
The thermal stability of the [Zn]OOR moieties is remarkable
in comparison to related main-group and transition-metal
species, and this may be at least partly related to the
monomeric nature of compounds 8−12. We therefore further
investigated their reactivity for comparison with zinc alkylper-
oxides known to have multimetallic or unknown structures. For
example, rapid comproportionation of alkylzinc and alkylper-
oxyzinc compounds to alkoxyzinc species starkly contrasts the
1
present system. H NMR spectra of mixtures of alkylperox-
yzincs 8−11 with the corresponding tris(oxazolinyl)boratozinc
alkyl (2, 3, 4, or 5) or hydride 13 only contained signals
assigned to starting materials, even after the mixtures were
heated at 80 °C for 12 h.
Despite their thermal stability, 8−12 are reactive in oxidation
processes and group transfer chemistry. Compounds 8−12
readily react with phosphines, such as PH2Ph, PPh3, P(p-
C6H4Me)3, and PMe3, to form the corresponding phosphine
oxides via stoichiometric O-atom transfer (eq 4). Kinetic
2.3. Oxidation Reactivity of ToMZnOOR. The concen-
tration of ToMZnOOEt in benzene-d6 is unchanged after 24 h
at 120 °C in a sealed NMR tube. After 3 days at 140 °C, 63% of
ToMZnOOEt is converted to (κ2-ToM)2Zn (14). Tert-
butylperoxide 11 is even slower to form 14 (30% after 3 d at
135 °C), and cumylperoxy 12 is unchanged after 1 d at 165 °C.
Photolysis of the alkylperoxy zinc compounds 8−12 (350 nm,
benzene, room temperature) provides a trimeric zinc hydroxide
D
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studies of stoichiometric phosphine oxidations, particularly on a
series of isolable monomeric alkylperoxymetal compounds, are
surprisingly rare even though a range of mechanisms are
conceivable (and proposed in related metal−alkylperoxide
systems), including radical chains via O−O bond homolysis to
give alkoxyradical intermediates,27 nucleophilic attack of the
phosphine on an η2-peroxyzinc, and coordination of phosphine
to zinc followed by nucleophilic attack by the alkylperoxide.28,29
Reaction rates for ToMZnOOt-Bu and PR3 follow the trend
PPh3 < P(p-C6H4Me)3 < PMe3 < PH2Ph; PPh3 reacts in less
than 2 h at 60 °C, whereas PMe3 reacts in less than 5 min at
ambient temperature. The second-order rate law −d-
[ToMZnOOR]/dt = k[ToMZnOOR][P(p-C6H4Me)3] empha-
sizes the mononuclear nature of alkylperoxyzinc species and the
bimolecular nature of the reaction. The rate constants follow
parameters for the reaction of 8 and BnMe2SiH, ΔH‡ = 12.6
0.7 kcal·mol−1 and ΔS‡ = −26 2 cal·mol−1K−1, are consistent
with an ordered transition state associated with a second-order
process.30 Primary isotope effects for the reactions of
ToMZnOOR and BnMe2SiH or BnMe2SiD are unity [kH/
(8)
(10)
kD = 1.10(7), kH/kD
= 1.11(9)].
For comparison, the second-order Si−O bond forming
reaction of the aryloxyzinc ToMZnOC6H3Me2 and PhMeSiH2
occurs with similar activation parameter values (ΔH‡ = 13
kcal·mol−1; ΔS‡ = −27 cal·mol−1K−1) and an isotope effect of
1.3(1).19b The isotope effect for the σ-bond metathesis reaction
of CpCp*ClHf-SiH2Ph and PhSiH3 or PhSiD3 is 2.7(2), with
ΔH‡ = 19 kcal·mol−1 and ΔS‡ = −33 cal·mol−1K−1.35 Rare
earth-mediated Si−C bond formations provide isotope effects
closer to unity.36 For example, Cp*2ScMe and Ph2SiH2 or
Ph2SiD2 react with an isotope effect of 1.15(5) and activation
parameters of ΔH‡ = 6.6 kcal·mol−1 and ΔS‡ = −43
cal·mol−1K−1. Additionally, the reaction of ToMMgNHt-Bu
and PhMeSiH2 involving Si−N bond formation occurs with an
isotope effect of 1.0(2), ΔH‡ = 5.7(2) kcal·mol−1, and ΔS‡ =
−46.1(8) cal·mol−1K−1.37 Although a number of variables are
not constant between these experiments (including variations
in organosilane and metal center as well as various Si-E bond
formations including Si−O, Si−N, and Si−C bonds), a rough
empirical trend of isotope effect and activation parameters may
be noted from the kinetic parameters associated with these
reactions. In particular, metathesis reactions involving isovalent
metal-mediated group transfer to silicon in which Si−H or Si−
D bonds are broken have isotope effects closer to unity in
highly ordered transition states (ΔS‡ ≪ 0) and small enthalpic
activation barriers. While these parameters are not correlated,38
a negative ΔS‡ and a small ΔH‡ for a second-order elementary
step together with a primary kinetic isotope effect ∼1 may
indicate an early transition state in which little bond cleavage
has occurred. More experiments, as well as theoretical
treatments, that probe isotope effects and activation parameters
of concerted Si−H bond cleavages/Si−E bond formations are
still needed before significant conclusions may be drawn from
these observations.
the trend: 8 > 9 > 10 ≫ 11 (k259K(8) = 8.8 0.3 × 10−2, k259K
(9)
= 7.5 0.2 × 10−2, k259K(10) = 6.3 0.2 × 10−3, k294K(11) = 1.22
0.04 × 10−3 M−1s−1; [P(p-C6H4Me)3] = 30 mM) showing
steric effects in the reactivity of the alkylperoxyzinc reagents.
Eyring analysis of the reaction between 8 and P(p-C6H4Me)3
provides ΔH‡ = 9.5
0.3 kcal·mol−1 and ΔS‡ = −27
1
cal·mol−1·K−1.30 The second-order rate law, activation param-
eters, and reaction conditions rule out a unimolecular
mechanism for phosphine oxidation by ToMZnOOR involving
oxygen−oxygen bond homolysis.27 The rate of P(p-C6H4Me)3
oxidation by 8−11 follows the size of the peroxyalkyl group (t-
Bu < i-C3H7 < n-C3H7 < Et), but no relationship with Zn−O−
O angles or O−O distances (obtained from the X-ray data)
could be identified. Faster reaction rates with smaller alkyl
groups suggest the mechanism involving nucleophilic attack of
phosphine on an electrophilic alkylperoxide.
2.4. Alkylperoxy Group Transfer Reactivity with
Hydrosilanes. In addition to the phosphine oxidation via O-
atom transfer, metal peroxides are also known to oxidize C−H
and Si−H bonds to give alcohol and silanol (SiOH) groups,
respectively.19b,29,31 In contrast, the ToMZnOOR compounds
8−12 react with organosilanes (HSiR′3) by peroxy-group
transfer to silicon. For example, 8 and HSiMe2Bn react
according to eq 5. This alkylperoxy-group transfer reactivity
This metathical pathway is apparently accessible because
Zn−O and O−O bond homolysis pathways, which are
common for transition-metal alkylperoxides, are not fast in
this monomeric zinc system at moderate temperatures.
Comproportionation reactions of peroxyzinc and alkylzinc
compounds also are inhibited by the tris(oxazolinyl)borate
ligand. Once these decomposition pathways are blocked, the
reaction chemistry of peroxyzinc compounds provides several
intriguing reactions and observations including a nonoxidative
peroxy group transfer.
resembles previously observed alkoxy-group transfer to silicon
from zinc alkoxides,19b and that reaction is likely important in
zinc-catalyzed dehydrocoupling of alcohols and silanes as well
as hydrosilylation of carbonyls.32 Related Si−O bond
formations may be involved in transition-metal, rare earth,
and main-group metal catalyzed hydrosilylations.33 While zinc
alkylperoxides may be hydrolyzed to alkyl hydroperoxides,3,34
the hydrolysis product is [Zn]OH. In the reactions here with
organosilanes, ToMZnH is the product, and that gives a
possibility for (future) catalysis (e.g., zinc-catalyzed hydro-
silylation of O2).
3. CONCLUSION
Aspects of the chemistry of ToMZnOOR 8−12 are distinct
from transition-metal alkyl peroxides. A particularly interesting
comparison is with monomeric d0 Cp2ClTiOOt-Bu, in which
Cp2ClTiO• is easily formed.27 Neither Ti(IV) nor Zn(II) can
be further oxidized, yet the apparent homolysis of the O−O
bond in the two compounds occurs at very different rates and
provides different products. Thus, Cp2ClTiOOt-Bu produces
tert-butanol, whereas homolysis reactions of 11 generate
Me2CO. The putative intermediates ToMZnO• and Cp2ClTiO•
seem to have divergent chemistry that may be associated with
empty orbitals on Ti(IV) versus stabilized occupied orbitals on
Zn(II). With 3dn transition elements, metal-centered oxidation
The empirical rate law for the reaction of 8 and BnMe2SiH of
−d[ToMZnOOEt]/dt = k[ToMZnOOEt][BnMe2SiH] (k309K
=1.64 0.09 × 10−2 M−1s−1) was measured from 288 to 320
K. The steric effects in this reactivity of the zinc alkyl peroxides
also follow a similar trend as observed in phosphine oxidation:
8 > 9 > 10 (k309K(8) = 1.64 0.09 × 10−2, k309K(9) = 1.17 0.02
(10)
× 10−2, k309K
= 1.9 0.02 × 10−3 M−1s−1). The activation
E
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Article
4.2. Synthetic Methods. 4.2.1. ToMZnEt (2). Diethylzinc (73.2
μL, 0.0882 g, 0.714 mmol) was added to a solution of HToM (0.274 g,
0.714 mmol) in 10 mL of benzene in a dropwise fashion. The solution
was stirred for 12 h at room temperature, and then the volatiles were
evaporated under reduced pressure. The resulting white solid was
washed with pentane to provide analytically pure ToMZnEt (2) as a
crystalline white solid (0.304 g, 0.638 mmol, 89.3%). X-ray quality
crystals were obtained from a concentrated toluene solution that was
allowed to stand at −30 °C. 1H NMR (400 MHz, benzene-d6): δ 0.68
(q, 2 H, ZnCH2CH3), 1.02 (s, 18 H, CNCMe2CH2O), 1.81 (t, 3 H,
ZnCH2CH3), 3.48 (s, 6 H, CNCMe2CH2O), 7.35 (t, 3JHH = 4.0 Hz, 1
H, para-C6H5), 7.53 (t, 3JHH = 4.0 Hz, 2 H, meta-C6H5), 8.34 (d, 3JHH
= 4.0 Hz, 2 H, ortho-C6H5). 13C{1H} NMR (175 HMz, benzene-d6): δ
−2.08 (ZnCH2CH3), 15.36 (ZnCH2CH3), 28.33 (CNCMe2CH2O),
65.61 (CNCMe2CH2O), 80.78 (CNCMe2CH2O), 126.19 (para-
C6H5), 127.21 (meta-C6H5), 136.48 (ortho-C6H5), 142.92 (br, ipso-
C6H5), 190.02 (br, CNCMe2CH2O). 11B NMR (128 MHz, benzene-
d6): δ −17.3. 15N{1H} NMR: δ −155.5. IR (KBr, cm−1): 2969 (s),
2929 (m), 2896 (m), 1603 (s, νCN), 1461 (m), 1385 (m), 1366 (m),
1350 (m), 1268 (s), 1193 (s), 1159 (s), 953 (s), 892 (w), 741 (m),
706 (s). Anal. calcd for C23H34BN3O3Zn: C, 57.95; H, 7.19; N, 8.81.
Found: C, 57.77; H, 7.10; N, 8.82. Mp 167−170 °C (dec).
may further contribute to the destabilization of alkylperoxides.
The tris(pyrazolyl)borato transition-metal alkylperoxides
clearly show these effects, and the resulting compounds can
oxidize CH bonds.
A second interesting comparison is with the autocatalytic
chain reactions of Tp*PtHMe2 and O2, where Tp*Me2PtOO•
reacts with the thermodynamically stronger Pt−H bond rather
than the Pt−Me moiety.39 H-atom abstraction, however, is
•
more plausible than CH3 abstraction, and in this sense the
Tp*PtHMe2 oxidation is similar to autocatalytic oxidation of
organic compounds (where weaker C−C bonds are less
reactive than C−H bonds). While the zinc−carbon BDE’s in
1−5 are expected to follow the trend 1 > 2 > 3 > 4 > 5 and the
reaction rates with O2 follow 5 > 4 ∼ 3 ∼ 2 (with 1 not
reacting), the unreactive Zn−H bond in 13 is expected to be
weaker than the Zn−C bonds. The reaction rate for Zn-Me,
Zn-Et, and Zn-CMe3 follows the expected trend from the
BDE’s, while Zn−H does not. Thus, unlike the selectivity (i.e.,
relative rates) of radical chains in autoxidation of hydrocarbyl
species that are thought to be governed by C−H BDE’s
(tertiary > secondary > primary), the radical chains of these
reactions are not entirely dominated by cleavage of the weakest
Zn−E bond.
Synthesis of 3 is given here as a representative example.
Compounds 3−5 and 7 are synthesized following identical procedures.
Full experimental details for 4, 5, and 7 are provided in the SI.
4.2.2. ToMZn(n-C3H7) (3). A suspension of Zn(OMe)2 (0.174 g, 1.36
mmol) in Et2O (20 mL) was cooled to 0 °C. n-C3H7MgBr (1.4 mL, 2
M in Et2O) was added, and the resulting mixture was allowed to warm
to room temperature and stir for 1 h. The mixture was filtered to
separate the insoluble magnesium salts from (n-C3H7)2Zn, and the
solution was added to Tl[ToM] (0.200 g, 0.341 mmol). Immediately, a
black mixture formed. This mixture was stirred for 20 min, the volatile
materials were evaporated under reduced pressure, and the residue was
extracted with benzene (10 mL). Evaporation of the colorless extracts
provided a white solid that was washed with pentane (3 × 5 mL) and
dried under vacuum. Analytically pure ToMZn(n-C3H7) (0.143 g,
0.291 mmol, 85.3%) was obtained as a white crystalline solid. X-ray
quality crystals were grown by slow diffusion of pentane into a
4. EXPERIMENTAL SECTION
4.1. General Procedures. All reactions were performed under a
dry argon atmosphere using standard Schlenk techniques or under a
nitrogen atmosphere in a glovebox, unless otherwise indicated.
Benzene, toluene, pentane, diethyl ether, and tetrahydrofuran were
dried and deoxygenated using an IT PureSolv system. Benzene-d6 and
toluene-d8 were heated to reflux over Na/K alloy and vacuum
transferred. Dichloromethane-d2 was vacuum transferred from CaH2.
ToMZnMe (1),19b ToMZnH (13),19a H[ToM],20b Tl[ToM],20a and
dialkylzincs were synthesized according to literature procedures.40
Grignard reagents were purchased from Sigma-Aldrich and transferred
to flasks equipped with resealable Teflon valves for storage. t-BuOOH
(5.5 M in decane), PhMe2COOH (80% technical grade), and AIBN
were purchased from Sigma-Aldrich and stored inside a glovebox
freezer at −30 °C. Benzyldimethylsilane was purchased from Gelest.
Benzyldimethylsilane-d1 was synthesized by reduction of benzyldime-
thylchlorosilane with LiAlD4.
1
concentrated toluene solution standing at −30 °C. H NMR (400
MHz, benzene-d6): δ 0.70 (m, 2 H, ZnCH2CH2CH3), 1.02 (s, 18 H,
3
CNCMe2CH2O), 1.46 (t, JHH = 7.2 Hz, 3 H, ZnCH2CH2CH3), 2.08
(m, 2 H, ZnCH2CH2CH3), 3.47 (s, 6 H, CNCMe2CH2O), 7.37 (t,
3
3JHH = 6.8 Hz, 1 H, para-C6H5), 7.56 (t, JHH = 7.6 Hz, 2 H, meta-
C6H5), 8.36 (d, 3JHH = 7.2 Hz, 2 H, ortho-C6H5). 13C{1H} NMR (175
HMz, benzene-d6 ):
δ 10.92 (ZnCH2 CH2CH3 ), 23.29
1H, 13C{1H}, 11B, and 17O NMR spectra were collected on a Bruker
DRX400 or an Avance II 600 spectrometer. 15N chemical shifts were
(ZnCH2CH2CH3), 24.93 (ZnCH2CH2CH3), 28.33 (CNCMe2CH2O),
65.67 (CNCMe2CH2O), 80.78 (CNCMe2CH2O), 126.22 (para-
C6H5), 127.24 (meta-C6H5), 136.49 (ortho-C6H5), 146.10 (br, ipso-
C6H5), 190.32 (br, CNCMe2CH2O). 11B NMR (128 MHz, benzene-
d6): δ −17.3. 15N{1H} NMR (71 MHz, benzene-d6): δ −155.8. IR
(KBr, cm−1): 3074 (w), 3045 (w), 2966 (s), 2935 (s), 2825 (m), 1601
(s, νCN), 1495 (w), 1462 (s), 1432 (w), 1384 (m), 1365 (m), 1354
(m), 1272 (s), 1251 (w), 1195 (s), 1161 (s), 957 (s), 896 (w), 867
(w), 841 (w), 816 (m), 746 (s), 703 (s), 668 (s). Anal. calcd for
C24H36BN3O3Zn: C, 58.74; H, 7.39; N, 8.56. Found: C, 58.98; H,
7.59; N, 8.43. Mp 180−184 °C (dec).
1
determined by H−15N HMBC experiments on a Bruker Avance II
700 spectrometer with a Bruker Z-gradient inverse TXI 1H/13C/15N 5
mm cryoprobe; 15N chemical shifts were originally referenced to an
external liquid NH3 standard and recalculated to the CH3NO2
chemical shift scale by adding −381.9 ppm. Elemental analyses were
performed using a Perkin-Elmer 2400 Series II CHN/S in the Iowa
State Chemical Instrumentation Facility.
Caution! High-pressure glass apparatuses, reactions of oxygen and
reduced compounds, and peroxide-containing materials must be
handled with care. Isolated alkylperoxide compounds 8−12 were
tested for possible explosive properties, and they did not ignite with
attempted initiation under thermal, physical, or electrical stress.
Regardless, only small quantities of alkylperoxides were prepared at a
single instance. Thick-walled NMR tubes equipped with J. Young-style
resealable Teflon valves (pressured to 100 psi with O2) were obtained
from Wilmad-Labglass and attached to a high-pressure steel manifold
through commercial Swagelock fittings.8c The tubes were handled in
protective jackets for safety concerns. Due to the potentially
pyrophoric and explosive nature of silylalkylperoxides,26 the
BnMe2SiOOR (R = Et, n-C3H7, i-C3H7 and t-Bu) species were not
isolated; these compounds were characterized in solution and
compared with the corresponding BnMe2SiOR using 1H, 13C{1H},
and 29Si NMR spectroscopy.
4.2.3. ToMZnPh (6). ToMZnCl (0.206 g, 0.426 mmol) and PhLi
(0.036 g, 0.430 mmol) were dissolved in THF (12 mL) and stirred for
8 h at ambient temperature. A white solid was obtained by evaporation
of the volatile materials, and the residue was extracted with 12 mL of
benzene. Evaporation of the benzene, followed by pentane washes (3
× 5 mL) and drying under vacuum provided crystalline, analytically
pure ToMZnPh (0.207 g, 0.394 mmol, 94.5%). X-ray quality single
crystals were grown from a concentrated toluene solution of ToMZnPh
at −30 °C. 1H NMR (400 MHz, benzene-d6): δ 1.03 (s, 18 H,
CNCMe2CH2O), 3.46 (s, 6 H, CNCMe2CH2O), 7.38 (m, 1 H, para-
C6H5), 7.38 (m, 1 H, para-ZnC6H5), 7.51 (vt, J = 7.6 Hz, 2 H, meta-
3
ZnC6H5), 7.58 (vt, J = 7.2 Hz, 2 H, meta-C6H5), 8.09 (d, JHH = 7.6
3
Hz, 2 H, ortho-ZnC6H5), 8.38 (d, JHH = 7.2 Hz, 2 H, ortho-C6H5).
13C{1H} NMR (175 HMz, benzene-d6): δ 28.40 (CNCMe2CH2O),
F
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65.86 (CNCMe2CH2O), 81.06 (CNCMe2CH2O), 126.32 (para-
C6H5), 126.66 (para-C6H5) 127.28 (meta-C6H5), 127.95 (meta-
ZnC6H5), 136.49 (ortho-C6H5), 140.41 (ortho-ZnC6H5), 142.52 (br,
ipso-C6H5), 154.40 (ipso-ZnC6H5), 190.52 (br, CNCMe2CH2O). 11B
NMR (128 MHz, benzene-d6): δ −18.1. 15N{1H} NMR: δ −157.4. IR
(KBr, cm−1): 3048 (w), 2966 (m), 2928 (w), 2891 (w), 1594 (s, νCN),
1460 (m), 1389 (m), 1369 (m), 1352 (m), 1275 (s), 1196(s), 1158
(m), 1071 (w), 948 (m), 895 (w), 842 (w), 817 (m), 758 (s), 746
(m), 727 (s), 704 (s). Anal. calcd for C28H36BN3O3Zn: C, 61.80; H,
6.53; N, 8.01. Found: C, 61.25; H, 6.30; N, 8.01. Mp: 197−200 °C
(dec).
and heated to 85 °C for 48 h. A shiny metallic black precipitate
appeared during the course of the reaction. A colorless solution was
obtained upon filtration. Removal of the volatiles materials from the
filtrate gave a white solid that was washed with pentane (3 × 5 mL)
and dried under vacuum, affording analytically pure bis(κ2-ToM)Zn
(0.417 g, 0.502 mmol, 90.1%) as a white powdery solid. X-ray quality
single crystals were grown from a slow pentane diffusion into a
concentrated toluene solution of 14 at −30 °C. 1H NMR (benzene-d6,
400 MHz): δ 0.94 (s, 6 H, CNCMe2CH2O), 1.12 (s, 6 H,
CNCMe2CH2O), 1.23 (s, 6 H, CNCMe2CH2O), 1.31 (s, 18 H,
CNCMe2CH2O), 3.29−3.38 (m, 6 H, ZnNCMe2CH2O), 3.51 (d, 3JHH
= 8.4 Hz, 2 H, ZnNCMe2CH2O), 3.69 (s, 4 H, CNCMe2CH2O), 7.25
(t, 3JHH = 7.2 Hz, 2 H, para-C6H5), 7.44 (t, 3JHH = 7.2 Hz, 4 H, meta-
Synthesis of 8 is given here as a representative example.
Compounds 8−10 are prepared following related procedures. Full
experimental details for 9 and 10 are provided in the SI.
3
C6H5), 8.05 (d, JHH = 7.2 Hz, 4 H, ortho-C6H5). 13C{1H} NMR
4.2.4. ToMZnOOEt (8). A 100 mL resealable Teflon-valved flask was
charged with a benzene solution (20 mL) of ToMZnEt (0.450 g, 0.944
mmol). The solution was degassed with freeze−pump−thaw cycles,
placed under an atmosphere of O2, and then sealed. The solution was
heated at 60 °C for 12 h and then allowed to cool to room
temperature. The volatile materials were removed under reduced
pressure. The resulting white residue was washed with pentane (3 × 5
mL) and dried under vacuum yielding analytically pure ToMZnOOEt
(8; 0.436 g, 0.857 mmol, 90.8%). X-ray quality crystals were grown by
allowing pentane to slowly diffuse into a saturated toluene solution of
(benzene-d6, 100 MHz): δ 25.80 (ZnNCMe2CH2O), 27.84
(ZnNCMe2 CH2 O), 28. 64 (ZnNCMe2 CH2 O), 28. 66
( Z n N C M e 2 C H 2 O ) , 2 9 . 5 7 ( C N C M e 2 C H 2 O ) , 6 6 . 4 4
(ZnNCMe2 CH2 O), 67. 30 (ZnNCMe2 CH2 O), 68. 32
(CNCMe2CH2O), 77.35 (CNCMe2CH2O), 78.67 (ZnNCMe2CH2O),
79.18 (ZnNCMe2CH2O), 126.15 (para-C6H5), 127.74 (meta-C6H5),
134.56 (ortho-C6H5), 147.40 (ipso-C6H5), 194 (br, CNCMe2CH2O).
11B NMR (benzene-d6, 128 MHz): δ −17.0. 15N{1H} NMR (benzene-
d6, 71 MHz): δ −120.6 (CNCMe2CH2O), −178.9 (CN(Zn)-
CMe2CH2O). IR (KBr, cm−1): 2966 (s), 2928 (w), 2878 (w), 1604
(m, νCN), 1560 (s, νCN), 1490 (w), 1464 (m), 1432 (w), 1369 (m),
1359 (m), 1278 (m), 1250 (m), 1199 (m), 1152 (m), 1002 (s), 969
(s), 892 (w), 848 (w). Anal. calcd for C42H58B2N6O6Zn: C, 60.78; H,
7.04; N, 10.13. Found: C, 60.91; H, 7.38; N, 9.95. Mp 190−195 °C
(dec).
1
8 cooled to −30 °C. H NMR (400 MHz, benzene-d6): δ 1.14 (s, 18
3
H, CNCMe2CH2O), 1.40 (t, JHH = 6.8 Hz, 3 H, ZnOOCH2CH3),
3
3.48 (s, 6 H, CNCMe2CH2O), 4.34 (q, JHH = 6.8 Hz, 2 H,
ZnOOCH2CH3), 7.37 (t, 3JHH = 7.2 Hz, 1 H, para-C6H5), 7.55 (vt, J =
3
7.6 Hz, 2 H, meta-C6H5), 8.32 (d, JHH = 7.2 Hz, 2 H, ortho-C6H5).
13C{1H} NMR (175 HMz, benzene-d6): δ 14.85 (ZnOOCH2CH3),
28.15 (CNCMe2CH2O), 65.67 (CNCMe2CH2O), 71.94
(ZnOOCH2CH3), 81.19 (CNCMe2CH2O), 126.47 (para-C6H5),
127.35 (meta-C6H5), 136.36 (ortho-C6H5), 142.03 (br, ipso-C6H5),
190.66 (br, CNCMe2CH2O). 11B NMR (128 MHz, benzene-d6): δ
−17.1. 15N{1H} NMR (71 MHz, benzene-d6): δ −159.2. 17O NMR
4.2.7. (ToMZnOH)3 (15). A benzene solution of ToMZnOOEt (0.210
g, 0.413 mmol) was photolyzed in a Rayonet chamber at 350 nm for
24 h at ambient temperature. Colorless, X-ray quality crystals were
formed during the photolysis. The crystals were isolated by decanting
of the supernatant solution. Further grinding and washing with
pentane (3 × 5 mL) followed by drying under reduced pressure
provided analytically pure ToMZnOH (0.123 g, 0.265 mmol, 64.1%) as
a trimeric species. 1H NMR (400 MHz, methylene chloride-d2, 25 °C):
δ 1.26 (s, br, 18 H, CNCMe2CH2O), 3.88 (s, br, 6 H,
(81 MHz, benzene-d6):
δ 319 (ZnOO(CH2CH3)], 169
(ZnOO(CH2CH3)]. IR (KBr, cm−1): 3083 (w), 3042 (w), 2969
(m), 2929 (w), 2885 (w), 1592 (s, νCN), 1495 (w), 1462 (s), 1435
(w), 1387 (m), 1368 (m), 1351 (m), 127 (s), 1198 (s), 1164 (s), 963
(s), 898 (w), 819 (w), 744(w). Anal. calcd for C23H34BN3O5Zn: C,
54.30; H, 6.73; N, 8.26. Found: C, 54.52; H, 6.74; N, 8.12. Mp: 208−
211 °C.
3
CNCMe2CH2O), 7.13 (m, 3 H, para- and meta-C6H5), 7.24 (d, JHH
= 6.4 Hz, 2 H, ortho-C6H5). 13C{1H} NMR (175 MHz, methylene
chloride-d2, 25 °C):
δ 29.59 (CNCMe2CH2O), 66.78
(CNCMe2CH2O), 79.19 (CNCMe2CH2O), 125.92 (para-C6H5),
127.88 (meta-C6H5), 133.57 (ortho-C6H5), 149.42 (br, ipso-C6H5),
191.09 (br, CNCMe2CH2O). 11B NMR (128 MHz, methylene
chloride-d2): δ −18.2. 15N{1H} NMR (71 MHz, methylene chloride-
d2, 25 °C): δ −160.7. IR (KBr, cm−1): 3069 (w), 2969 (m), 2930 (w),
1623 (m, νCN), 1577 (s, νCN), 1492 (w), 1462 (m), 1433 (w), 1383
(w), 1367 (m), 1280 (m), 1197 (m), 1160 (m), 1121 (w), 999 (s),
970 (s), 944 (m) 879 (w), 846 (w), 732 (m), 706 (m), 651 (w). Anal.
calcd for C63H90B3N9O9Zn3: C, 54.28; H, 6.51; N, 9.04. Found: C,
54.22; H, 6.30; N, 8.69. Mp 260−263 °C.
Synthesis of 11 is representative of the procedure for preparation of
12, which is provided in the SI.
4.2.5. ToMZnOO(t-Bu) (11). A solution of ToMZnH (0.550 g, 1.23
mmol) in 12 mL of benzene was treated with t-BuOOH (0.23 mL, 5.5
M in decane). The mixture was allowed to stir at room temperature for
1 h. The solvent was then evaporated to obtain a white residue that
was washed with pentane (3 × 5 mL) and dried under vacuum
affording crystalline, analytically pure 11 (0.549 g, 1.02 mmol, 83.4%
yield) as a white solid. X-ray quality single crystals were grown from a
concentrated toluene solution of ToMZnOO(t-Bu) at −30 °C. 1H
4.2.8. General Synthesis of BnMe2SiOOR (R = Et, i-C3H7, t-Bu).
BnMe2SiH (0.012 g, 0.08 mmol) and ToMZnOOR (0.02 mmol) were
allowed to react in benzene-d6 (0.6 mL). Upon completion of the
reaction (R = Et, 2 h; R = i-C3H7, 6 h; R = t-Bu, 12 h at 80 °C), the
NMR (400 MHz, benzene-d6): δ 1.15 (s, 18 H, CNCMe2CH2O), 1.59
3
(s, 9 H, ZnOOCMe3), 3.48 (s, 6 H, CNCMe2CH2O), 7.37 (t, JHH
=
7.4 Hz, 1 H, para-C6H5), 7.55 (vt, J = 7.6 Hz, 2 H, meta-C6H5), 8.32
3
1
(d, JHH = 7.6 Hz, 2 H, ortho-C6H5). 13C{1H} NMR (175 MHz,
products were identified and characterized by H, 13C{1H}, and 29Si
benzene-d6): δ 27.57 (ZnOOCMe3), 28.16 (CNCMe2CH2O), 65.69
(CNCMe2CH2O), 77.60 (ZnOOCMe3), 81.20 (CNCMe2CH2O),
126.44 (para-C6H5), 127.34 (meta-C6H5), 136.37 (ortho-C6H5),
142.12 (br, ipso-C6H5), 190.49 (br, CNCMe2CH2O). 11B NMR (128
MHz, benzene-d6): δ −17.1. 15N{1H} NMR (71 MHz, benzene-d6): δ
−158.8. 17O NMR (81 MHz, benzene-d6, obtained from ToMZnCMe3
and 17O2): δ 284 (ZnOOCMe3), 204 (ZnOOCMe3). IR (KBr, cm−1):
3078 (w), 3048 (w), 2967 (m), 2927 (w), 2897 (w), 2870 (w), 1598
(s, νCN), 1496 (w), 1464 (s), 1433 (w), 1367 (m), 1353 (s), 1278 (s),
1198 (s), 1164 (s), 963 (s), 898 (w), 819 (w), 744 (w). Anal. calcd for
C25H38BN3O5Zn: C, 55.94; H, 7.14; N, 7.83. Found: C, 55.85; H,
7.38; N, 7.83. Mp: 193−195 °C.
NMR spectroscopy. Equimolar ToMZnH and BnMe2SiOOR were
formed in each reaction. Additionally, the products’ spectra were not
equivalent with spectra of BnMe2SiH starting material and
BnMe2SiOR (synthesized below).
4.2.9. General Synthesis of BnMe2SiOR (R = Et, i-C3H7, t-Bu).
BnMe2SiH (0.014 g, 0.09 mmol) and ROH (equimolar with respect to
BnMe2SiH) were added to ToMZnH (5 mg, 0.01 mmol) dissolved in
benzene-d6 (0.6 mL). The resulting solution was heated in a Teflon-
valved NMR tube (R = Et, 60 °C, 4 h; R = i-C3H7, 80 °C, 10 h; R = t-
Bu, 135 °C, 65 h). Upon completion of the reaction, the products
were identified and characterized based on their 1H, 13C{1H}, and 29Si
NMR spectra.
4.2.6. (κ2-ToM)2Zn (14). ToMZnH (0.250 g, 0.557 mmol) and
Tl[ToM] (0.327 g, 0.557 mmol) were dissolved in benzene (15 mL)
4.2.10. BnMe2SiOOEt. 1H NMR (600 MHz, C6D6): δ 0.14 (s, 6 H,
3
SiMe2), 1.04 (t, 3 H, JHH = 7.2 Hz, OOCH2CH3), 2.26 (s, 2 H,
G
dx.doi.org/10.1021/ja303440n | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
3
PhCH2), 3.91 (q, 2 H, JHH = 7.2 Hz, OOCH2CH3), 7.06 (m, 3 H,
4.5. Determination of Rate Dependence on PO . A benzene-d6
2
para- and meta-C6H5), 7.15 (m, 2 H, ortho-C6H5). 13C{1H} NMR
(150 MHz, C6D6): δ −2.93 (SiMe2), 13.70 (SiOCH2CH3), 25.70
(PhCH2), 72.76 (SiOCH2CH3), 125.14 (para-C6H5), 129.01 (meta-
C6H5), 129.22 (ortho-C6H5), 139.12 (ipso-C6H5). 29Si NMR (120
MHz, C6D6): δ 21.63.
stock solution containing cyclooctane (9.6 mM), ToMZnEt (33 mM),
and AIBN (14.1 mM) was prepared. A known volume of this solution
(0.6 mL) was added to a thick-walled J. Young NMR tube. The tube
was charged with a measured pressure of O2 (30−100 psi) using a
high-pressure manifold. The tube was shaken vigorously and then
placed in a preheated NMR spectrometer probe (54 °C). The
integrated intensities of ToMZnEt were measured over the reaction
1
4.2.11. BnMe2SiOEt. H NMR (600 MHz, C6D6): δ 0.01 (s, 6 H,
3
SiMe2), 1.05 (t, 3 H, JHH = 7.2 Hz, OCH2CH3), 2.06 (s, 2 H,
PhCH2), 3.44 (q, 2 H, 3JHH = 7.2 Hz, OCH2CH3), 7.0 (m, 3 H, para-
and meta-C6H5), 7.13 (m, 2 H, ortho-C6H5). 13C{1H} NMR (150
MHz, C6D6): δ −1.96 (SiMe2), 19.09 (SiOCH2CH3), 27.34 (PhCH2),
58.85 (SiOCH2CH3), 124.92 (para-C6H5), 128.94 (meta-C6H5),
129.08 (ortho-C6H5), 139.91 (ipso-C6H5). 29Si NMR (120 MHz,
C6D6): δ 12.50.
time course. The observed pseudofirst-order rate constants kobs
,
determined from nonlinear least-squares analysis, were identical within
error for each O2 pressure (Figure S-3, SI).
4.6. General Description of 1H NMR Kinetic Experiments for
the Reactions Between ToMZnOOR and P(p-C6H4Me)3. A
toluene-d8 stock solution containing known concentrations of
cyclooctane (9.9 mM) and ToMZnOOR (20 mM) was prepared. A
measured quantity of this stock solution (0.6 mL) was placed in a
septa-capped NMR tube, and the tube was cooled to −78 °C.
Tris(para-tolyl)phosphine was dissolved in a minimal amount (50 μL)
of toluene-d8, and this solution was added through the septa using a
microliter syringe, and the hole was sealed with silicone grease. The
sample was placed in a precooled NMR spectrometer probe. Single
scan spectra were acquired automatically at preset time intervals. The
concentrations of ToMZnOOR, P(p-C6H4Me)3, and OP(p-C6H4Me)3
were determined by comparison of corresponding integrated
resonances to the known concentration of the internal standard.
The second-order rate constants (kobs) were obtained by a
nonweighted linear least-squares fit of the data to the second-order
rate law:
4.2.12. BnMe2SiOO(i-C3H7). 1H NMR (600 MHz, C6D6): δ 0.15 (s,
3
6 H, SiMe2), 1.11 (d, 6 H, JHH = 6.0 Hz, OOCHMe2), 2.27 (s, 2 H,
3
PhCH2), 4.13 (sept, 1 H, JHH = 6.0 Hz, OOCHMe2), 7.05 (m, 3 H,
para- and meta-C6H5), 7.14 (m, 2 H, ortho-C6H5). 13C{1H} NMR
(150 MHz, C6D6): δ −2.79 (SiMe2), 20.70 (OCHMe2), 25.80
(PhCH2), 78.19 (SiOCHMe2), 125.11 (para-C6H5), 128.99 (meta-
C6H5), 129.24 (ortho-C6H5), 139.23 (ipso-C6H5). 29Si NMR (120
MHz, C6D6): δ 21.26.
4.2.13. BnMe2SiO(i-C3H7). 1H NMR (600 MHz, C6D6): δ 0.05 (s, 6
3
H, SiMe2), 1.07 (d, 6 H, JHH = 6.0 Hz, OCHMe2), 2.11 (s, 2 H,
3
PhCH2), 3.80 (sept, 1 H, JHH = 6.0 Hz, OCHMe2), 7.03 (m, 3 H,
para- and meta-C6H5), 7.16 (m, 2 H, ortho-C6H5). 13C{1H} NMR
(150 MHz, C6D6): δ −1.32 (SiMe2), 26.37 (OCHMe2), 27.84
(PhCH2), 65.56 (SiOCHMe2), 124.90 (para-C6H5), 128.89 (meta-
C6H5), 129.12 (ortho-C6H5), 140.01 (ipso-C6H5). 29Si NMR (120
MHz, C6D6): δ 10.38.
[P(p‐C6H4Me)3]
[P(p‐C6H4Me)3]o
ln
= ln
+ kobsΔot
[ToMZnOOR]
[ToMZnOOR]o
(6)
4.2.14. BnMe2SiOO(t-Bu). 1H NMR (600 MHz, C6D6): δ 0.16 (s, 6
H, SiMe2), 1.21 (s, 9 H, OCMe3), 2.28 (s, 2 H, PhCH2), 7.07 (m, 3 H,
para- and meta-C6H5), 7.14 (m, 2 H, ortho-C6H5). 13C{1H} NMR
(150 MHz, C6D6): δ −2.55 (SiMe2), 25.80 (PhCH2), 26.55
(SiOCMe3), 81.41 (SiOCMe3), 125.08 (para-C6H5), 128.80 (meta-
C6H5), 129.19 (ortho-C6H5), 139.48 (ipso-C6H5). 29Si NMR (120
MHz, C6D6): δ 20.45.
(Figures S-4−8, SI).
4.7. General Description of 1H NMR Kinetic Experiments for
the Reactions Between ToMZnOOR and BnMe2SiH. A toluene-d8
stock solution containing known concentrations of cyclooctane (15.7
mM) and ToMZnOOEt (20.7 mM) was prepared. A measured
quantity of this stock solution (0.6 mL) was placed in a septa-capped
NMR tube, and the tube was cooled to −78 °C. BnMe2SiH (100 mM)
was added through the septa using a microliter syringe, and the hole
was sealed with silicone grease. The sample was placed in a precooled
NMR spectrometer probe. Single scan spectra were acquired
automatically at preset time intervals. The concentrations of
ToMZnOOEt, BnMe2SiH, ToMZnH, and BnMe2SiOOEt were
determined by comparison of the corresponding integrated resonances
to the known concentration of the internal standard. The second-order
rate constants (kobs) were obtained by a nonweighted linear least-
squares fit of the data to the second-order rate law:
1
4.2.15. BnMe2SiO(t-Bu). H NMR (600 MHz, C6D6): δ 0.10 (s, 6
H, SiMe2), 1.16 (s, 9 H, OCMe3), 2.14 (s, 2 H, PhCH2), 7.04 (m, 3 H,
para- and meta-C6H5), 7.17 (m, 2 H, ortho-C6H5). 13C{1H} NMR
(150 MHz, C6D6): δ 1.09 (SiMe2), 29.49 (PhCH2), 32.47 (SiOCMe3),
65.56 (SiOCMe3), 125.02 (para-C6H5), 128.80 (meta-C6H5), 129.19
(ortho-C6H5), 140.42 (ipso-C6H5). 29Si NMR (120 MHz, C6D6): δ
4.35.
4.3. Kinetic Experiments of ToMZnEt + O2. Reactions were
monitored with 1H NMR spectroscopy using a Bruker DRX-400
spectrometer. The concentrations of NMR-active reactants, initiators,
and products were determined by comparison of corresponding
integrated resonances to the known concentration of the internal
standards. The experiments were performed under pseudofirst-order
conditions with excess O2, and the [ToMZnEt] was monitored for at
least three half-lives.
[BnMe2SiH]
[ToMZnOOEt]
[BnMe2SiH]o
[ToMZnOOEt]o
ln
= ln
+ kobsΔot
(7)
ASSOCIATED CONTENT
* Supporting Information
■
4.4. Determination of Rate Dependence on [AIBN]. A
benzene-d6 stock solution was prepared to contain known
concentrations of cyclooctane (5.2 mM) as an internal standard and
ToMZnEt (25 mM). An experiment was initiated by adding a known
quantity of AIBN to a measured volume (0.6 mL) of the stock
solution; the [AIBN] was then verified by comparison with the
S
Preparations of compounds 4, 5, 7, 9, 10, and 12, plots of
kinetic data, and crystallographic data files. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
sadow@iastate.edu
1
internal standard in the integrated H NMR spectrum. The resulting
■
solution was pressurized with O2 (50 psi) in a high-pressure NMR
tube using a high-pressure manifold. The mixture was shaken
vigorously and then inserted into a NMR probe that was preheated
to 54 °C. The [AIBN] was varied from 5.4 to 31.5 mM. The integrated
intensities of ToMZnEt were measured over the reaction time course.
For each experiment, the −d[ToMZnEt]/dt followed an exponential
decay (Figure S-1, SI) to give kobs. The half-order dependence on
[AIBN] was obtained by a nonweighted linear least-squares fit of the
kobs values against [AIBN]1/2 (Figure S-2, SI).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support for this work was provided by the U.S.
Department of Energy, Office of Basic Energy Sciences,
■
H
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Journal of the American Chemical Society
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