Heterobimetallic Rebound: A Mechanism for Diene-to-Alkyne Isomerization with M - -Zr Hydride Complexes (M = Al, Zn, and Mg)

Article abstract of DOI:10.1021/acs.organomet.7b00908

The reaction of a series of M·Zr heterobimetallic hydride complexes with dienes and alkynes has been investigated (M = Al, Zn, and Mg). Reaction of M·Zr with 1,5-cyclooctadiene led to diene isomerization to 1,3-cyclooctadiene, but for M = Zn also result in an on-metal diene-to-alkyne isomerization. The resulting cyclooctyne fragment is trapped between Zr and Zn metals in a heterobimetallic species that does not form for M = Mg or Al. The scope of diene isomerization and alkyne trapping has been explored leading to the isolation of three new heterobimetallic slipped metallocyclopropene complexes. The mechanism of diene-to-alkyne isomerization was investigated through kinetics. While the reaction is first-order in Zn·Zr at high diene concentration and proceeds with ΔH^? = +33.6 ± 0.7 kcal mol^-1, ΔS^? = +23.2 ± 1.7 cal mol^-1 K^-1, and ΔG^?_{298 K} = +26.7 ± 1.2 kcal mol^-1, the rate is dependent on the nature of the diene. The positive activation entropy is suggestive of involvement of a dissociative step. On the basis of DFT calculations, a heterobimetallic rebound mechanism for diene-to-alkyne isomerization has been proposed. This mechanism explains the origin of heterobimetallic control over selectivity: Mg - -Zr complexes are too strongly bound to generate reactive fragments, while Al - -Zr complexes are too weakly bound to compensate for the contrathermodynamic isomerization process. Zn - -Zr complexes have favorable energetics for both dissociation and trapping steps.

Full text of DOI:10.1021/acs.organomet.7b00908

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Cite This: Organometallics XXXX, XXX, XXX−XXX
Heterobimetallic Rebound: A Mechanism for Diene-to-Alkyne
Isomerization with M‑--Zr Hydride Complexes (M = Al, Zn, and Mg)
M. J. Butler, A. J. P. White, and M. R. Crimmin*
Department of Chemistry, Imperial College London, South Kensington, London SW7 2AZ, United Kingdom
*
S Supporting Information
ABSTRACT: The reaction of a series of M·Zr heterobimetallic hydride complexes with
dienes and alkynes has been investigated (M = Al, Zn, and Mg). Reaction of M·Zr with
1
,5-cyclooctadiene led to diene isomerization to 1,3-cyclooctadiene, but for M = Zn also
result in an on-metal diene-to-alkyne isomerization. The resulting cyclooctyne fragment is
trapped between Zr and Zn metals in a heterobimetallic species that does not form for M
=
Mg or Al. The scope of diene isomerization and alkyne trapping has been explored
leading to the isolation of three new heterobimetallic slipped metallocyclopropene
complexes. The mechanism of diene-to-alkyne isomerization was investigated through
kinetics. While the reaction is first-order in Zn·Zr at high diene concentration and
‡
−1
‡
−1 −1
⧧
−1
proceeds with ΔH = +33.6 ± 0.7 kcal mol , ΔS = +23.2 ± 1.7 cal mol K , and ΔG
= +26.7 ± 1.2 kcal mol , the rate
98 K
2
is dependent on the nature of the diene. The positive activation entropy is suggestive of involvement of a dissociative step. On
the basis of DFT calculations, a heterobimetallic rebound mechanism for diene-to-alkyne isomerization has been proposed. This
mechanism explains the origin of heterobimetallic control over selectivity: Mg---Zr complexes are too strongly bound to generate
reactive fragments, while Al---Zr complexes are too weakly bound to compensate for the contrathermodynamic isomerization
process. Zn---Zr complexes have favorable energetics for both dissociation and trapping steps.
6
,7
INTRODUCTION
[Cp* TaH ]. The reaction involves 2 equiv of 1,3-butadiene,
2 3
■
1
equiv of which is hydrogenated to give 1-butene and the
Diene-to-alkyne isomerization is a contrathermodynamic
2
1
other forms [Cp* TaH(η -MeCCMe)]. The observation of
2
reaction that has limited experimental precedent. A related
1
2
a series of hydride intermediates by H NMR spectroscopy
reaction, alkene-to-alkyne dehydrogenation is equally as rare.
allowed the authors to conclude that a metal hydride pathway
for isomerization was likely to be operating. Brinkmann et al.
have documented a similar reaction. The isomerization of an
η -allyl to a σ-vinyl ligand occurs on a titanocene fragment and
The dearth of examples can be explained by considering the
thermodynamics of isomerization. Calculations of the relative
stabilities for C H rings are shown in Figure 1.
6
8
8
12
3
2
proceeds through an η -propyne intermediate. In both these
cases, the alkyne adduct is only a reaction intermediate, and the
diene-to-alkyne isomerization event is merely part of a more
complex network of reactions.
In 2016, we reported an example of diene-to-alkyne that
involves the reaction of either 1,3- or 1,5-cyclooctadiene with a
heterobimetallic hydride complex and results in the formation
9
of a trapped cyclooctyne isomer. We showed that the unusual
on-metal isomerization only occurs for zinc---zirconium hydride
complexes and is not observed for analogous magnesium---
zirconium or aluminum---zirconium hydride complexes. Here
we provide a detailed analysis of the mechanism. We conclude
that diene-to-alkyne isomerization occurs by a heterobimetallic
rebound mechanism. This new mechanism is supported by
kinetic analysis, computational studies and a thorough
investigation of the solution dynamics of the complexes
involved. We explain the origin of the heterobimetallic effect
Figure 1. Thermodynamics of cyclooctadiene isomerization calculated
by DFT.
The calculated values are consistent with experimental
3
thermochemical data. Due to the incorporation of increased
2
angle strain as carbon centers are converted from sp - to sp-
hybridized form, diene-to-alkyne isomerization is unfavorable
4
,5
within small or medium rings.
9
reported in our preliminary communication.
As part of an extensive study modeling potential
intermediates in the Fischer−Tropsch hydrocarbon chain
lengthening process, Bercaw and co-workers have shown that
diene-to-alkyne isomerization can be effected by
Received: December 26, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00908
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RESULTS AND DISCUSSION
formation of the trapped cyclooctyne complex Zn·1 (Scheme
■
1
b). This reactivity is unique for the Zn-analogue of the series,
Reactions of M·Zr heterobimetallics with dienes and
alkynes. Reaction of 1,5-COD with the bimetallic complexes
Mg·Zr, Al·Zr, and Zr·Zr (Scheme 1a) consistently give
and no evidence for trapped alkyne complexes were observed
during reactions of Mg·Zr or Al·Zr.
Isomerization is barely affected by reducing the ring size from
8 to 7 carbons, and Zn·2 is formed in 85% yield from reaction
of 1,3-cycloheptadiene (1,3-CHD) with Zn·Zr (Scheme 1b).
Addition of of Zn·Zr to either 1,3- or 1,4-cyclohexadiene did
not lead to an alkyne adduct formation but a catalytic
a
Scheme 1. Structures and Reactions
1
0
redistribution reaction to form cyclohexene and benzene.
Reactions of freshly prepared 1,2-cyclononadiene
1
1
or
commercial samples of 1,7-octadiene with Zn·Zr did not lead
to tractable products but instead unidentified mixtures.
A series of reactions between Zn·Zr and alkynes were also
12
investigated. Addition of 4,4-dimethylpent-2-yne to Zn·Zr,
led directly to the alkyne adduct Zn·3. A similar reaction is
observed with 1-trimethylsilylpropyne to form Zn·4. Colorless
crystals of Zn·3 were obtained from the reaction mixture at 298
K. The structure is presented in Figure 2, the short C−C and
Zn−C bond length and the slipped metallocyclopropane
binding mode is consistent with that previously reported for
9
Zn·1 and that reported herein for Zn·2.
13
The binding mode has precedent for both Mg and B/Al/
14,15
Ga analogues,
bimetallics is the first structurally characterized of its kind.
Despite the possibility of forming different regioisomers of Zn·
but this series of zinc---zirconium hetero-
1
6
1
2
3
or Zn·4, with the alkyne orientated such that either C or C
is in a bridging role, exclusive formation of a single isomer is
observed. The reaction of oct-4-yne with Zn·Zr did not yield a
heterobimetallic product but gave the zirconacyclopentadiene,
1
7
5
, from the oxidative coupling of two alkynes. Monomeric Zn
18
is formed as a byproduct in this reaction (Scheme 1c). In all
reactions reported for Zn·Zr transfer hydrogenation of the
substrate accompanies product formation, generating cyclo-
octene, cycloheptene, 4,4-dimethylpent-2-ene, trimethyl(prop-
1
-en-1-yl)silane, or octene alongside Zn·1−4 or 5.
Thermodynamics of Diene-to-Alkyne Isomerization. To
a
(
a) Structures of hydride reagents used in this study. (b) Reaction of
investigate if transfer hydrogenation is a requirement for the
observed reactivity, the thermodynamics of diene and alkyne
binding to the M---Zr heterobimetallic fragments were
calculated by DFT (Scheme 2, eq 1−4). While the on-metal
isomerization of 1,5-COD is close to thermoneutral (eq 2), that
of 1,3-COD is slightly uphill (eq 3). Transfer hydrogenation of
a further equiv of substrate results in a more thermodynamically
favorable process (eq 4), and although not a strict requirement
for diene-to-alkyne isomerization, provides an additional
thermodynamic driving force for this reaction to occur.
Zn·Zr with dienes and alkynes to form Zn·1-4. (c) Reaction of Zn·Zr
with oct-4-yne to form 5.
mixtures of 1,3-COD, cyclooctene, and trace cyclooctane.
The details of these catalytic experiments were included in a
preliminary communication of this work and are not repeated
here. A number of zirconium hydride complexes are known to
catalyze diene isomerization. In contrast, reaction of Zn·Zr
with 1,5-COD leads to transfer hydrogenation of the diene and
9
10
Figure 2. Crystal structure of (a) Zn·2 and (b) Zn·3. Hydrogen atoms are omitted save the hydride ligand. Selected bond lengths [Å] are listed in
c).
(
B
DOI: 10.1021/acs.organomet.7b00908
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Scheme 2. Calculated Gibbs Free Energies for Formation of
M·1 from COC, 1,5-COD, and 1,3-COD (COE =
cyclooctene)
fragments into the geometries observed in M·1 were compared
against the interaction energy of these fragments (E ). We
int
have previously shown that for Zn·1 the distortion energies are
9
aptly compensated for by the large interaction energy. Metal
coordination of the cycloalkyne therefore appears to be the key
thermodynamic driving force in diene-to-alkyne isomerization.
While a near identical value of distortion of the bimetallic
fragment was calculated for Mg·1 the interaction energy is
more favorable than that calculated for Zn·1. In contrast, for Al·
1
the distortion of both the cyclooctyne and the organometallic
fragment is more unfavorable than for Zn·1 (and Mg·1). These
data begin to explain the experimental observations; the
analysis would seem to suggest that the formation of Al·1 is
disfavored based on the contorted geometry of both the alkyne
21
and heterobimetallic fragment required for alkyne binding.
Kinetics of Diene-to-Alkyne Isomerization. Two simple
experiments provide evidence for a mechanism that involves
chain-walking and not reversible hydrogenation/dehydrogen-
ation of the diene. Hence, Zn·Zr reacts with both 1,5-COD and
The thermodynamics of the corresponding processes for the
two other main group metals (M·Zr, M = Mg, Al) were also
calculated. While ligand exchange reactions to form analogues
of Zn·1 are favorable in all instances (eq 1), the isomerization
reactions are significantly endergonic for Al·Zr (eqs 2 and 3) in
the absence of transfer hydrogenation of the substrate (eq 4).
The factors that affect the stability of the theoretical M·1 (M
1
,3-COD to form Zn·1 in similar yields with similar byproducts
but does not form the same product upon reaction with
cyclooctene. The latter alkene is the product of transfer
hydrogenation and its inability to re-enter the synthetic
pathway suggests that it is not an intermediate in the formation
of Zn·1.
=
Mg, Zn, and Al) complexes were quantified by breaking the
1
The reaction of Zn·Zr with 1,3-COD was monitored by H
molecule into organometallic and cyclooctyne fragments
NMR spectroscopy. Under pseudo-first-order conditions
(Scheme 3). This treatment is reminiscent of the activation−
(
excess diene) at 353 K, the reaction follows the empirical
0
1
rate law: Rate = k [1,3-COD] [Zn·Zr] . Varying the excess of
a
obs
Scheme 3. Activation Strain Analysis of M·1
diene (1.5−64 equiv) had no effect on the kinetics with k ≈ 1
obs
−
4 −1
×
10
s
in all cases. An Eyring analysis between 343 and 358
⧧
K gave the activation parameters for the isomerization as ΔH
−
1
⧧
−1 −1
=
+33.6 ± 0.7 kcal mol , ΔS = +23.2 ± 1.7 cal mol K ,
−1
⧧
and ΔG
= +26.7 ± 1.2 kcal mol (Figure 3a,b). Rates of
98 K
2
reaction for addition of 1,5-COD to Zn·Zr have not been
measured. Chain-walking mechanisms for alkene isomerization
10
with zirconium hydride reagents are well-proven, and the
choice to focus on 1,3-COD during the mechanistic analysis is
based on the assumption that 1,5-COD readily converts to 1,3-
COD by a well established mechanism under the reaction
conditions.
a
Single point SCF energies in kcal mol−1.
The activation parameters allow immediate elimination of a
series of plausible rate-limiting steps for diene-to-alkyne
isomerization. Rate-limiting insertion of alkenes into a Zr−H
bond in 16-electron transition metal complexes [Cp* ZrH ]
1
9,20
strain or distortion−interaction model.
The distortion
energies that are required to contort the ground state structures
2
2
1
) and cycloalkyne (E2strain)
of the organometallic (E
has been found to be extremely facile and has negative entropy
strain
Figure 3. Kinetic analysis of the reactions of Zn·Zr with alkynes and dienes. (a) Kinetics for the reaction of Zn·Zr + 1,3-COD under pseudo-first-
order conditions. (b) Eyring analysis and (c) first-order rate constants for the reaction of Zn·Zr with dienes and an alkyne at 343 K.
C
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−1
Figure 4. Calculated pathway for the reaction of Zr with 2 equiv of 1,3-COD. Gibbs energies in kcal mol .
1
2
of activation. The implication is the formation of an ordered
four-membered transition state, and similar measurements have
been recorded for the reaction of lanthanide and group 5
pentadiene 5 (Scheme 1c) and the positive activation entropy
from the Eyring analysis all suggest that dissociation of Zn·Zr is
likely under the reaction conditions. Furthermore, a detailed
analysis of the solution dynamics of M·Zr (see the Supporting
Information) allows the characterization of both intramolecular
and intermolecular fluxional processes that occur due to
reversible formation of 3-center, 2-electron M−H−Zr bonds.
The strength of the M−H−Zr bonds increase across the series
22−24
metallocene hydrides with alkenes.
elimination, the microscopic reverse of alkene insertion, is also
expected to proceed with a similar increase in order of the
Rate-limiting β-hydride
25−29
transition state relative to the reagents.
For example, the
⧧
−1 −1
values of ΔS range from −11 ± 2 to −10 ± 1 cal mol
K
27
32
for β-hydride reactions of [Cp* ScR] complexes.
Mg > Zn > Al.
2
Due to the requirement for transfer hydrogenation of diene
en route to the formation of Zn·1, the reaction cannot be run
with an excess of Zn·Zr. As a result, the rate-dependence on
diene concentration cannot be measured using pseudo-first-
order conditions (excess Zn·Zr). An alternative approach must
be used. With diene or alkyne in excess, kobs was measured for
the reaction of a series substrates with Zn·Zr (Figure 3c). If the
diene or alkyne is involved in the rate-limiting step, then kobs
should be dependent on the nature of the substrate. Different
rate constants were recorded for the reaction of 1,3-COD, 1,3-
CHD and 4,4-dimethylpent-2-yne with Zn·Zr (Figure S10−
S12).
DFT studies have been published on the hydrozirconation of
33−36
alkenes and fluoroalkenes by [Cp ZrH ] and [Cp* ZrH ].
2
2
2
2
Similarly, mechanisms of [Cp ZrH ] catalyzed hydroalumina-
2
2
3
7
38
tion and hydrosilylation have been investigated by
computational methods. These studies have clearly concluded
that the lowest energy pathway for hydrozirconation of the
alkene occurs via coordination of the alkene in between the
wedge created by the H−Zr−H bonds and subsequent
migratory insertion in to one of the Zr−H bonds. The result
is readily understandable from the frontier MO picture of
[Cp ZrH ] as the LUMO of this molecule exists between the
2
2
39
two hydride ligands.
The reaction of the 16-electron complex Zr
40−44
In combination, the data suggest that the rate-limiting step
for the reaction of 1,3-COD with Zn·Zr involves a significant
amount of bond breaking, an increase in disorder of the
transition state compared with the ground state, and is
dependent on the nature (if not the concentration) of the
diene substrate. It is worth noting that Sita and co-workers
reported an unusual set of activation parameters for the
isomerization of a cationic group 4 alkyl complex, for which a
stepwise pathway involving β-hydride elimination and hydro-
with 2 equiv
of 1,3-COD to form a COC adduct was investigated by DFT.
Transfer hydrogenation of 1,3-COD to form cyclooctene by
[Cp ZrH ] is calculated to occur by the established pathway,
2
2
with coordination forming Int-1 which undergoes sequential
hydrozirconation, σ−π isomeriation, and hydrozirconation
steps ultimately producing Int-5. Int-5 may undertake a
conformational change of the cyclooctene ring to form the
lower energy intermediate Int-5′ (Figure 4). Int-5 and Int-5′
differ in the conformation of the 8-membered hydrocarbon ring
(Figure S18). The formally 16-electron intermediates (Int-2
30
⧧
zirconation steps was proposed. The modest ΔH = 21.8 ±
−1
⧧
0
.5 kcal mol was accompanied by a positive ΔS = +8.1 ± 0.5
cal mol K−1 and explained by invoking a difference in the
degree of ion-pairing between the ground state and transition
state. In the current case, a transition state that is formed by a
−1
and Int-4) are stabilized by β-agostic interactions. Ligand
45
exchange of Int-5 with a further equivalent of 1,3-COD is close
to thermoneutral and forms Int-6. Diene-to-alkyne isomer-
ization proceeds from the metallocyclopropane adduct Int-6 via
a relatively flat potential energy surface eventually generating
the metallocyclopropene adduct Int-12 as the thermodynamic
product. The hydrocarbon substrate remains bound to a series
31
reaction sequence that involves initial dissociation of Zn·Zr,
followed by a key bond breaking or making event would
rationalize both the activation parameters and the rate
dependence on the nature of the substrate.
2
DFT Studies of Diene-to-Alkyne Isomerization. The
mechanism of diene-to-alkyne isomerization of 1,3-COD with
Zn·Zr was investigated by DFT. The proclivity of Zn·Zr to
form monomeric zirconium complexes, such as zirconacyclo-
of Zr-intermediates through either η or σ-coordination modes
throughout the mechanism. Diene-to-alkyne isomerization
proceeds by a series of β-hydride elimination, rotation and
hydrozirconation steps (Figure 4).
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2
Figure 5. Geometries of key transition states in the on-metal diene-to-alkyne isomerization: sp C−H β-hydride elimination (TS-6 and TS-9) and
intramolecular hydrozirconation (TS-11). Values in blue are the difference in the NPA charges between the TS and its preceding ground state.
It is important to note that Int-6 exists as two enantiomers,
while Int-12 possesses pro-chiral protons in the β-position of
the bound cyclooctene ring. The hydrozirconation and β-
hydride elimination steps are all stereospecific, and as such
stereochemistry becomes an important consideration in
defining the diene-to-alkyne reaction pathway. An alternative
pathway originating from the enantiomer of Int-6, was also
calculated. The pathway is broadly similar to that presented
above with the caveat that the important bond breaking and
making steps are marginally higher in energy (Figure S19).
While the fundamental steps for the diene-to-alkyne
isomerization all have extensive precedent, their combination
positive element. It is noteworthy that as the diene-to-alkyne
isomerization progresses the key transition states show
2
increasingly short Zr---C distances to the η -bound C unit
2
and increasingly obtuse C−C−C angles; both are consistent
with increasing metallocyclopropene character of the hydro-
carbon ligand.
Heterobimetallic Rebound Mechanism. The transfer
hydrogenation and diene-to-alkyne isomerization on mono-
metallic zirconium complexes are, however, only part of the
mechanistic picture. Low-energy zirconocene intermediates on
the potential energy surface can potentially reversibly bind to
the molecular zinc hydride Zn. This coordination can either
occur as a weakly exergonic (or indeed endergonic) binding
through 3-center, 2-electron Zr−H−Zn interactions or the
formation of more strongly bound heterobimetallic metal-
locyclo-propane or -propene adducts as observed in the solid
state structures of Zn·1−3 (Scheme 4).
30,44
as applied to diene-to-alkyne isomerization is unknown.
The highest energy transitions states on the potential energy
surface TS-6, TS-9 and TS-11 all occur within a similar energy
⧧
−1
span ΔG
= 26−29 kcal mol and involve either β-hydride
298K
elimination or the microscopic reverse hydrozirconation
(
Figure 5). Rather unusually, however, the β-hydride
The reversible trapping and stabilization of reaction
intermediates by a heterobimetallic rebound mechanism is
represented in the potential energy surface in Figure 6 and is
reminiscent of the mechanism of atom-transfer radical
elimination steps occur from metallocyclopropane intermedi-
2
ates and involve the breaking of an sp C−H bond. Similarly,
hydrozirconation occurs across the remote double bond of a
metal allenyl fragment.
2
There is limited experimental precedent for sp C−H β-
a
Scheme 4. Heterobimetallic Rebound Events
hydride elimination. For example, Bercaw and co-workers have
reported the rearrangement of group 4 alkenyl derivatives by a
pathway involving β-H elimination from an sp -hybridized
2
4
6
carbon.
While both TS-6 and TS-9 could also be
conceptualized in terms of a migration of a zirconium(II)
fragment {Cp Zr} from the alkene moiety to the adjacent C−H
2
bond with bond breaking occurring by oxidative addition, β-
hydride elimination from a zirconium(IV) metallocyclopropane
intermediate is also a fair description. Both TS-6 and TS-9
contain long C---H distances, short Zr---H distances, and
charge-accumulation on the hydrogen atom, all suggestive of a
large degree of C−H bond breaking and Zr−H bond making.
Intramolecular hydrozirconation of an allenyl intermediate
occurs through similar bond making and breaking events in TS-
1
1. Here hydride transfer occurs from zirconium to carbon and
a
is accompanied by charge-depletion of the H atom as it
migrates from the more electropositive to the less electro-
The binding of M to {Zr} intermediates. Values are Gibbs free
−1
energies in kcal mol .
E
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Figure 6. Potential energy surface for the reaction of Zr with 2 equiv of 1,3-COD. Solid line = PES from {Zr} intermediates. Dotted line = PES with
heterobimetallic rebound of {Zr} intermediates only exergonic binding events considered.
4
7
polymerization. The trapping of Int-12 as its zinc hydride
adduct forms the experimentally isolated Zn·1 (= Zn-Int-12).
This binding event is of crucial importance in isolating this
metallocyclopropene complex.
atom-transfer radical polymerization mechanisms and more
broadly could be considered a highly specialized ligand
dissociation/association step. Mankad and co-workers have
concluded that similar mechanisms may be in operation in
catalytic applications of heterobimetallic Cu−Fe and Ag−Ru
This mechanism explains the kinetic data observed under
saturation of diene. Dissociation of a heterobimetallic Zn---Zr
intermediate is on the way to the highest energy transition
states (TS-6, TS-9, or TS-11) and is consistent with the
reaction being first-order in [Zn·Zr], having a large and positive
activation entropy (ΔS ), and a large activation enthalpy
ΔH ). The proposed mechanism also explains the observed
rate dependence on the nature of the diene, as hydrocarbon
derived fragments are involved in the highest energy transition
states. The calculated Gibbs activation energies for the highest
transition states on the heterobimetallic rebound mechanism
= 35−36 kcal mol . Because of the small
calculated energy difference between TS-6, TS-9, and TS-11
50−53
complexes.
The thermodynamics of the binding events
explains the unusual heterobimetallic effect, only the zinc---
zirconium hydride complexes possess binding energies that are
suitable for reversible capture and release of the isomerization
products under the reaction conditions.
⧧
⧧
(
We believe that these results have two important
implications. First, the reversible generation of low concen-
trations of monometallic reactive intermediates through
trapping as a heterobimetallic complexes should be considered
in future mechanistic analysis of heterobimetallic complexes in
catalysis. Second, diene-to-alkyne isomerization may well be a
hidden pathway in the reaction of dienes with transition metal
hydrides. This reaction is contrathermodynamic with respect to
the hydrocarbon fragment but can be rendered exergonic by a
suitable exothermic event (such as the trapping as a
heterobimetallic as reported herein). If the new alkyne moiety
could be exploited in further reactions, then perhaps tandem
isomerization−functionalization processes could be developed.
⧧
−1
are ΔG
DFT
−1
(
ΔG ∼ 2.5 kcal mol ), it cannot be determined which of these
steps, if any, is rate-limiting.
Origin of Heterobimetallic Selectivity. The observation that
the on-metal diene-to-alkyne isomerization only occurs for Zn·
Zr and not Al·Zr or Mg·Zr can be explained by this new
mechanism. Comparison of the binding energies of the
heterobimetallics (Scheme 4) reveals that Mg·Zr is too strongly
bound to dissociate and promote the isomerization reaction of
the diene under mild reactions conditions <353 K, while Al·1 is
too weakly bound to allow effective trapping of the cyclooctyne
complex. The thermodynamics of formation of both Zn·Zr and
Zn·1 are just right for both events to be favorable. Hence, the
observed heterobimetallic effect is both thermodynamic and
kinetic in origin, with Mg·Zr being too kinetically stable to
access significant quantities of the requisite Zr and Mg
ASSOCIATED CONTENT
■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00908.
Experimental procedures, details of the DFT studies,
single crystal X-ray data and multinuclear NMR spectra
4
8
intermediates and Al·1 not being thermodynamically stable
enough to compensate for the contrathermodynamic diene-to-
alkyne isomerization and prevent the expected catalytic
(PDF)
Full coordinates for calculated structures (MOL)
49
isomerization of 1,5-COD to 1,3-COD.
Accession Codes
CCDC 1587265−1587266 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
CONCLUSIONS
■
In summary, we report the reaction of metal---zirconium
hydride heterobimetallics with a series of dienes and alkynes. In
the case of zinc---zirconium hydride complexes, a highly
unusual on-metal diene-to-alkyne isomerization is observed.
Through a combined experimental and computational
approach, we propose a new heterobimetallic rebound
mechanism for diene-to-alkyne isomerization. The proposed
heterobimetallic rebound mechanism has direct parallels with
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: m.crimmin@imperial.ac.uk.
F
DOI: 10.1021/acs.organomet.7b00908
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
ORCID
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by Ligand-Induced Reductive Elimination.Bis(η -Cyclopentadienyl)-
5
M. R. Crimmin: 0000-0002-9339-9182
Notes
The authors declare no competing financial interest.
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clohexane also led to transfer hydrogenation of the alkene, the Zn/Zr
containing products of these reactions are yet to be fully characterized.
ACKNOWLEDGMENTS
■
(19) Bickelhaupt, F. M.; Houk, K. N. Analyzing Reaction Rates with
We are grateful to the Royal Society for generous support in the
form of a University Research Fellowship and the EPSRC for
project support (EP/L011514/1).
the Distortion/Interaction-Activation Strain Model. Angew. Chem., Int.
Ed. 2017, 56, 10070−10086.
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H
DOI: 10.1021/acs.organomet.7b00908
Organometallics XXXX, XXX, XXX−XXX