Organothorium-catalyzed hydroalkoxylation/cyclization of alkynyl alcohols. Scope, mechanism, and ancillary ligand effects

Article abstract of DOI:10.1021/om300881b

Organothorium complexes bearing amide or alkyl proligands are active toward the highly selective hydroalkoxylation/cyclization of alkynyl alcohols. Substrates include primary and secondary alcohols, as well as terminal and internal alkynes. Catalysts with strongly binding ligation such as pentamethylcyclopentadienyl (Cp* = C₅Me₅) or constrained geometry catalysts (CGC = Me₂Si(η ⁵-Me₄C₅)(^tBuN)) remain soluble throughout the reaction, with the more sterically open (CGC)Th(NMe ₂)₂ (1) exhibiting higher activity than Cp*₂Th(CH₂TMS)₂ (2). The use of precatalyst [(Me₃Si)₂N]₂Th[κ²- (N,C)-CH₂Si(CH₃)₂N(SiMe₃)] (3) leads to precipitation upon the addition of alcohol substrates, although catalytic activity is retained. The substrate scope for 1 includes primary and secondary alcohols as well as terminal and internal alkynes. In situ¹H NMR spectroscopic monitoring indicates that the rate law is zero-order in [substrate] and first-order in [catalyst]. The rates of primary alcohols and terminal alkynes are significantly more rapid than their more sterically hindered counterparts, suggesting that steric demands dominate the hydroalkoxylation/cyclization transition state. Turnover frequencies as high as 49 h^-1 at 60 C are observed, producing exclusively the exo-methylene products. For internal alkyne substrates, alkenes with E-orientation are formed with complete selectivity. Activation parameters ΔH^? = 27.9(0.4) kcal/mol, ΔS^? = -3.0(1.1) eu, and E _a = 28.6(0.4) kcal/mol are largely in accord with observations for other f-element-mediated insertive hydroelementation processes, and an ROH/ROD kinetic isotope effect of 0.97(0.02) is observed. The reactivity patterns, kinetics, and activation parameters are consistent with a pathway proceeding via turnover-limiting alkyne insertion into the Th-O bond, with subsequent, rapid Th-C protonolysis, regenerating the initial Th-OR species.

Full text of DOI:10.1021/om300881b

Article
pubs.acs.org/Organometallics
Organothorium-Catalyzed Hydroalkoxylation/Cyclization of Alkynyl
Alcohols. Scope, Mechanism, and Ancillary Ligand Effects†
Stephen D. Wobser and Tobin J. Marks*
Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, United States
S
* Supporting Information
ABSTRACT: Organothorium complexes bearing amide or alkyl
proligands are active toward the highly selective hydroalkoxylation/
cyclization of alkynyl alcohols. Substrates include primary and
secondary alcohols, as well as terminal and internal alkynes.
Catalysts with strongly binding ligation such as pentamethylcyclo-
pentadienyl (Cp* = C5Me5) or “constrained geometry catalysts”
(CGC = Me2Si(η5-Me4C5)(tBuN)) remain soluble throughout the
reaction, with the more sterically open (CGC)Th(NMe2)2 (1)
exhibiting higher activity than Cp*2Th(CH2TMS)2 (2). The use of
precatalyst [(Me3Si)2N]2Th[κ2-(N,C)-CH2Si(CH3)2N(SiMe3)]
(3) leads to precipitation upon the addition of alcohol substrates, although catalytic activity is retained. The substrate scope
for 1 includes primary and secondary alcohols as well as terminal and internal alkynes. In situ 1H NMR spectroscopic monitoring
indicates that the rate law is zero-order in [substrate] and first-order in [catalyst]. The rates of primary alcohols and terminal
alkynes are significantly more rapid than their more sterically hindered counterparts, suggesting that steric demands dominate the
hydroalkoxylation/cyclization transition state. Turnover frequencies as high as 49 h−1 at 60 °C are observed, producing
exclusively the exo-methylene products. For internal alkyne substrates, alkenes with E-orientation are formed with complete
selectivity. Activation parameters ΔH‡ = 27.9(0.4) kcal/mol, ΔS‡ = −3.0(1.1) eu, and Ea = 28.6(0.4) kcal/mol are largely in
accord with observations for other f-element-mediated insertive hydroelementation processes, and an ROH/ROD kinetic isotope
effect of 0.97(0.02) is observed. The reactivity patterns, kinetics, and activation parameters are consistent with a pathway
proceeding via turnover-limiting alkyne insertion into the Th−O bond, with subsequent, rapid Th−C protonolysis, regenerating
the initial Th−OR species.
relatively mild conditions and is highly atom-efficient, in
principle producing no side products.1e,3 Several methodologies
for the catalytic alkyne hydroalkoxylation with alcohols have
been reported in the literature, employing iridium,4 rhodium,5
gold,6 mercury,7 palladium,8 molybdenum,9 ruthenium,10
tungsten,11 silver,12 platinum,13 and other metal catalysts.1d,14
However, these transformations are often necessarily coupled
to other processes, including oxidation or isomerization.
Furthermore, the rate and selectivity of these processes are
often highly sensitive to the structural and electronic nature of
the substrate. Therefore, pursuing alternative strategies to
produce valuable oxygen-containing heterocycles in an efficient,
atom-economical catalytic process is highly desirable.
INTRODUCTION
■
Carbon oxygen linkages, particularly in oxygen-containing
heterocycles, are ubiquitous in natural products and biologically
active molecules and are important in fine chemicals.1 For this
reason, recent research efforts have been directed toward
developing new methods of heterocycle syntheses and
developing a better understanding of these synthetic processes.2
One method with promising applicability is catalytic hydro-
alkoxylation. Hydroalkoxylation (eq 1), formally defined as the
Organolanthanide catalysts are well known for their activity
and selectivity in hydrofunctionalization processes,15 and this
laboratory recently reported the use of Ln[N(TMS)2]3
complexes (Ln = La, Sm, Y, Lu) as efficient precatalysts for
intramolecular hydroalkoxylation/cyclization (Scheme 1).
These Ln catalysts are active for a wide variety of mono- and
disubstuted alkynes and allenes.16 Moreover, this process was
addition of a hydrogen−oxygen bond across a carbon−carbon
unsaturation, offers several attractions compared to traditional
carbon−oxygen bond-forming reactions. In contrast to
alternative approaches, catalytic hydroalkoxylation requires
Special Issue: Recent Advances in Organo-f-Element Chemistry
†This paper was scheduled to appear in the Recent Advances in
Organo-f-Element Chemistry special issue but was inadver-
tently omitted.
Received: September 13, 2012
Published: November 14, 2012
© 2012 American Chemical Society
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ligand23 (CGC) in many catalytic hydroelementation reactions
is now demonstrated to be applicable to organothorium-
catalyzed hydroalkoxylation. The empirical rate law, activation
energetic parameters, and operative kinetic isotope effects are
determined, and a mechanism is proposed. These results are
then compared and contrasted with related organolanthanide
systems, providing insight into the differences between the
closely related 4f and 5f series. We also identify the substrate
features affecting rate and selectivity and discuss them in
relation to the proposed catalytic cycle.
Scheme 1. General Lanthanide-Mediated
Hydroalkoxylation/Cyclization Process
The scope and activity of various substrates are examined, as
is the effect of temperature on reaction rate. Activation
parameters are determined empirically and discussed in relation
to other hydroelementation processes and metal centers. This
contribution, overall, broadens the scope of hydroelementation
reactions mediated by f-elements and enhances the under-
standing of the unique reactivity of these metal ions in the
context of a catalytic process. This insight should contribute to
the understanding of the behavior of f-elements with polar
substrates and assist in the rational design of new catalysts and
catalytic processes.
argued to proceed through a two-step mechanism involving
alkyne insertion into a Ln−OR bond, followed by rapid
protonolysis of the newly formed Ln−C bond by another
molecule of the alcoholic substrate, simultaneously freeing the
desired cyclic ether and regenerating the Ln−OR species.17
While lanthanide catalysts have long been known to undergo a
variety of rapid insertion processes into Ln−X bonds,15e,r,18 this
marked the first ever report of carbon−carbon bond insertion
into a Ln−OR linkage. Inspired by the discovery of this new
reactivity, we have examined several related metal centers
differing substantially in charge and ionic radius (Th4+, Sc3+,
Zr4+, and Al3+) and report the results of this study here.
Organoactinides have been employed to catalytically effect
many demanding organic transformations.19 The distinctive
reactivity of the 4f lanthanide and 5f actinide series20 is a result
of several factors, including the high electrophilicity and kinetic
lability of the metal centers, the general resistance toward
oxidative-addition/reductive-elimination processes, and the
large ionic radii with high coordination numbers. Actinides
exhibit high reactivity for a variety of σ-bond metathesis and
insertion processes and have demonstrated tolerance for an
increasing variety of polar functionalities.18a,19a,21 Indeed, the
actinide literature over the last several years is rich in studies of
the structure and reactivity of actinide-oxygen complexes.22
This work contributes to a growing toolbox from which
fundamental steps can be assembled into useful catalytic
processes. The present report constitutes the first implementa-
tion of organoactinides in the catalytic hydroalkoxylation of
alkynes by alcohols in a highly efficient and selective manner.
A variety of ligand architectures is examined in this study
(Chart 1 below), and cyclopentadienyl-based (Cp) ligation is
determined to be essential for catalyst solubility in the reaction
mixture. The established efficacy of the “constrained geometry”
EXPERIMENTAL SECTION
■
General Considerations. All manipulations of air-sensitive
materials were carried out with rigorous exclusion of O2 and moisture
in flame- or oven-dried Schlenk-type glassware either on a dual-
manifold Schlenk line, interfaced to a high-vacuum manifold (10−6
Torr), or in an N2-filled MBraun glovebox with a high-capacity
recirculator (<1 ppm O2). Argon (Airgas) was purified by passage
through a MnO column to remove O2 and a column of Davison 4A
molecular sieves to remove water immediately before use. Solvents
used for catalytic reactions were stored over Na/K alloy in resealable
containers and vacuum-transferred immediately prior to use. All liquid
or volatile solid substrates for catalytic experiments were dried over a
series of three or more beds of freshly activated Davison 4A molecular
sieves as solutions in benzene-d6 or toluene-d8 (Cambridge Isotope
Laboratories, 99+ atom % D). Substrate solutions were degassed by
freeze−pump−thaw methods. Solid substrates were purified by
sublimation under high vacuum and were stored at −35 °C in a
glovebox. The NMR internal integration standard, methyltriphenylsi-
lane, was sublimed and exposed to high vacuum overnight before
storage in the glovebox. The precatalysts Sc[N(TMS)2]3,24 Cp*2Th-
25
(CH2TMS)2,19t and (CGC)Th(NMe2)2 were prepared as reported
in the literature. Precatalysts [Al(NMe2)3]2 and Zr(NMe2)4 were
purchased from Strem, purified by sublimation, and stored in a
glovebox. Substrates 4, 6, 8, 18, and 20 were purchased from Sigma-
Aldrich. Substrate 16 was purchased from TCI. Substrates 10,26 12,27
14,28 20,29 and 2230 were prepared as reported in the literature.
Product NMR spectroscopic data agree well with the published
literature spectra.16b,c
Physical and Analytical Measurements. NMR spectra were
1
recorded on a Mercury 400 (400 MHz, H; 100 MHz, 13C; 61 MHz,
2H), an Inova 500 (500 MHz, 1H; 125 MHz, 13C), or a Bruker Avance
1
III 500 (500 MHz, H; 125, 13C) instrument. Chemical shifts (δ) for
2
1H, H, and 13C are referenced to internal solvent resonances and
Chart 1. Organoactinide Complexes Used As Precatalysts in
the Present Work for Alkyne Hydroalkoxylation with
Alcohols
reported relative to SiMe4. NMR scale experiments on air-sensitive
samples were conducted in Teflon valve-sealed J. Young tubes.
[(Me3Si)2N]2Th[κ2-(N,C)-CH2Si(CH3)2N(SiMe3)]. The following
modification of the procedure of Dormond et al.31 was employed. A
flip-frit apparatus with a magnetic stir bar was charged in the glovebox
with ThCl4 (3.0 g, 8.0 mmol) and Na[N(SiMe3)2] (5.9 g, 32 mmol)
and then removed from the glovebox. Approximately 100 mL of dry,
deoxygenated toluene was added. The reaction mixture was next
stirred for 1 h at 90 °C under Ar and then for 90 min at reflux, then
allowed to cool to room temperature before flip filtration. The solid
that collected was washed with toluene. All volatiles were removed by
vacuum, and pentane was condensed onto the resulting residue. The
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mixture was stirred for several minutes, and then the volatiles were
removed by vacuum. This process was repeated a second time, and the
resulting off-white powder was exposed to high vacuum overnight
before being taken into the glovebox. The powder was taken up in a
minimal amount of pentane and filtered again. The resulting off-white
powder, [(Me3Si)2N]2Th[κ2-(N,C)-CH2Si(CH3)2N(SiMe3)], was ob-
tained in 72% yield. The 1H NMR spectral features correspond to the
literature values.31
over the course of 1 week at 120 °C. Thus, these complexes
adhere to the general trend observed for lanthanide-mediated
hydroalkoxylation and hydroamination, where smaller ionic
radii correlate with lower turnover frequencies. While not
lanthanide complexes, the extension of these observations to
groups 3 and 4 d0 complexes provides an informative
comparison. The precatalyst (CGC)Th(NMe2)2 is also found
to cleanly produce the desired cyclization product with Nt =
0.32 h−1, prompting further investigation. In addition to metal
ionic radii, formal oxidation states appear to have a significant
influence on the hydroalkoxylation rate. While Zr(IV) is slightly
smaller than Sc(III), under the same reaction conditions it
exhibits a higher Nt for this reaction. Similarly, while Th(IV)
has a slightly smaller ionic radius than La(III), it effects this
reaction at the highest rate of any metal ion examined in the
present study.
In typical lanthanide-catalyzed hydroalkoxylation/cycliza-
tions, the active species was determined to be a homoleptic
metal alkoxide, generated by the rapid cleavage of protonolytic-
ally labile amido proligands (Scheme 1).16c For the purpose of
comparison to published f-element data, amido ligands were
also used in the present study, with the exception of the Th
precatalyst. Thorium is known to form oligomeric alkoxides,
frequently insoluble in hydrocarbon solvents (vide infra).33
Comparative hydroalkoxylation/cyclization rate data are
compiled in Table 1.
Kinetic Studies of Hydroalkoxylation Reactions. In a typical
experiment, in the glovebox, a weighed amount of the precatalyst (ca.
10 μmol, 5.7 mg) and the internal standard Ph3SiMe (ca. 50 μmol,
13.5 mg) were transferred to a glass vial and dissolved in a measured
quantity of toluene-d8. The resulting solution was then pipetted into a
Teflon-valved J. Young NMR tube. In the case of solid substrates, the
substrate (200 μmol, 20 equiv) was also dissolved in toluene-d8 and
added to the NMR tube in the glovebox. The NMR tube was then
removed from the glovebox and interfaced to a high-vacuum manifold
via a glass adapter with a secondary Teflon valve. The NMR tube was
then cooled to −78 °C and exposed to high vacuum. In the case of
liquid substrates, the upper portion of the valve was then backfilled
with Ar, and a 1.0 M solution of the substrate in C6D6 or C7D8 was
added by syringe. In cases where the substrate reacted within 24 h, the
sample was immediately placed in an NMR spectrometer pre-
equilibrated to the appropriate temperature ( 0.1 °C, calibrated
with an ethylene glycol standard). Data were obtained with a single
transient to avoid signal saturation. In cases where substrate
conversion required greater than 24 h, the reactions were heated in
temperature-controlled oil baths and removed periodically for NMR
spectroscopic analysis. Substrate and/or product concentrations were
determined relative to the intensity of the internal standard resonance
over three or more half-lives.
Thorium-Mediated Hydroalkoxylation/Cyclization Ex-
periments. The results of the catalytic hydroalkoxylation/
cyclization of alkynyl alcohol substrates mediated by organo-
thorium precatalysts bearing cyclopentadienyl and non-cyclo-
pentadienyl ancillary ligation were next investigated. The
transformation 6 → 7 (Table 2) was studied for precatalysts
(CGC)Th(NMe2)2, 1; Cp*2Th(CH2TMS)2 (Cp* = C5Me5),
2; and [(Me3Si)2N]2Th[κ2-(N,C)-CH2Si(CH3)2N(SiMe3)], 3.
Data were fit by least-squares analysis according to eq 2, where t is
time and [product] is the concentration of product at time t. The
turnover frequency (Nt) was calculated according to eq 3, where
[catalyst]0 is the initial catalyst concentration.
[product] = mt
(2)
m
Nt(h−1) =
[catalyst]0
(3)
Table 2. Comparison of Th-Mediated Hydroalkoxylation/
a
Cyclization Rates as a Function of Th Ligation
RESULTS
■
Initial screening of the various metal complexes assayed
catalytic hydroalkoxylation activity using 5-hexyn-1-ol as a
test substrate (Table 1). We find that Zr(NMe2)4 and
Sc[N(TMS)2]3 exhibit moderate catalytic activity for the
formation of 2-methylenetetrahydropyran (Nt = 0.03 and
0.017 h−1, respectively), while [Al(NMe2)3]2 shows no reaction
Table 1. Catalyst Screening for the Hydroalkoxylation/
a
Cyclization of 5-Hexyn-1-ol
b
entry
precatalyst
ionic radius
Nt, (h−1, 120 °C)
1
2
3
4
5
La[N(TMS)2]3
(CGC)Th(NMe2)2
Sc[N(TMS)2]3
Zr(NMe2)4
1.160
1.05
0.1116c
0.32
0.870
0.84
0.017
0.030
N.R.
[Al(NMe2)3]2
0.535
a
Reactions carried out in Teflon-valved J. Young NMR tubes with
b
[metal] = 14.3 mM; [substrate]0 = 286 mM. Data are presented to
indicate the relative metal ionic radii and were tabulated by Shannon.32
Data are for eight-coordinate species, except for Al, which is reported
for six-coordinate species.
a
Reactions carried out in Teflon-valved J. Young NMR tubes with
b
[Th] = 14.3 mM; [substrate]0 = 286 mM. Precipitate observed
during reaction.
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In each case the reaction was complete by NMR spectroscopy
in less than 1 h at 60 °C. None of the catalysts display any
evidence of an induction period, indicating essentially
instantaneous protonolysis of the monodentate amide ligands
by the alcohol substrates. This is consistent with the known
oxophilicity and kinetic lability of Th(IV) and the large
difference in Th−O bond enthalpy versus the precatalyst Th−
N bonds.33a,34
Where cyclopentadienyl-based ancillary ligands are employed
(Table 2, entries 1 and 2), the reaction proceeds to completion
as a homogeneous solution. However, when the more labile
proligands34 are used (Table 2, entry 3), small amounts of
suspended solid are observed immediately upon mixing the
substrate and precatalyst, with free HN(TMS)2 observed by 1H
NMR spectroscopy. This suggests a classic σ-bond protonolysis
reaction pattern for metal amides with alcohols.16,34,35 By the
end of the reaction using precatalyst 3, copious pale yellow
precipitate is observed. Despite the precipitate, NMR
spectroscopic monitoring of the reaction was continued in
this case, and a turnover frequency of 37 h−1 is obtained. In this
case, Nt was derived using data only up to 50% theoretical
conversion due to deviation from linearity toward the end of
the reaction (Figure 1). Notably, in this case, the reaction
rate on catalyst concentration. Thus, a study of the hydro-
alkoxylation/cyclization of 4-pentyn-1-ol at various catalyst
concentrations reveals first-order rate dependence on the
concentration of catalyst 1 (Figure 2). In all cases, the reaction
maintains a zero-order dependence on substrate and the
reaction proceeds cleanly to completion. The overall empirical
rate law is therefore expressed in eq 4.
rate = k[substrate]0[catalyst]1
(4)
Figure 2. Reaction rates for the hydroalkoxylation/cyclization of 4-
pentyn-1-ol by precatalyst 1 as a function of catalyst concentration.
Activation Parameters. Kinetic studies were conducted
over a 60−100 °C temperature range. Reactions were
1
monitored by in situ H NMR spectroscopy for the hydro-
alkoxylation/cyclization of 4-pentyn-1-ol by 1, and the results
are displayed in Figure 3. The rate of product formation in each
Figure 1. Conversion versus time plots for the hydroalkoxylation/
cyclization of 2-ethynylbenzyl alcohol mediated by the indicated 5 mol
% Th catalysts in toluene at 60 °C.
progress plateaus at ∼72% of the theoretical yield. Complex 1
was found to have the most active ancillary ligand set
investigated and achieved a turnover frequency of Nt = 49
h−1, exceeding the rate of the La[N(TMS)2]3-catalyzed process
by ∼5×. The CGC ligand has two binding sites, one through an
amide and the other through a heavily alkylated cyclo-
pentadienyl ring. Note that, unlike CGC complex 1, when
either 2 or 3 is exposed to alcoholic substrates, ancillary ligand
Figure 3. Time dependence of product formation for the catalytic
hydroalkoxylation/cyclization of 4-pentyn-1-ol mediated by 1 at the
indicated temperatures.
case is constant and indicates a zero-order rate dependence on
substrate concentration over the given temperature range.
Standard Eyring (Figure 4) and Arrhenius (Supporting
Information) plots are then used to derive the activation
parameters ΔH‡ = 27.9(0.4) kcal/mol, ΔS‡ = −3.0(1.1) eu, and
Ea = 28.6(0.4) kcal/mol.36 Furthermore, an −OH versus −OD
isotopic labeling study assayed by 1H and 2H NMR
spectroscopy indicates a kinetic isotope effect (KIE) of kH/kD
= 0.97 (0.02) (Table 3, entries 9 and 10). Due to the known
tendency of f-element catalysts to activate terminal alkynyl
RCC−H bonds,16b,21f,g,37 the kinetics of 2-(2-
phenylethynyl)benzyl alcohol, an internal alkyne, were studied
to avoid the complication of H/D scrambling between the
alkynyl and alcoholic hydrogen atoms.
1
protonolysis is observable by H NMR spectroscopy, arguing
that the bidentate CGC ligand is essential for persistent
ancillary ligation. In the case of each precatalyst, the
monodentate amide and alkyl ligands are rapidly protonolyzed.
For the organothorium-catalyzed alkyne hydroalkoxylation/
cyclization reactions, excepting those involving precatalyst 3,
where extensive precipitation is observed, a constant turnover
rate is observed throughout the course of the reaction,
demonstrating zero-order rate dependence on substrate
concentration. This observation is consistent with results for
related lanthanide-mediated hydroalkoxylation/cyclization and
f-element-mediated hydroamination/cyclization processes, both
of which are proposed to proceed via a rate-limiting
intramolecular insertion step. Further support for this turn-
over-limiting step comes from the first-order dependence of the
Proposed Catalytic Cycle. Scheme 2 illustrates the
proposed catalytic cycle for organothorium-mediated alkynyl
alcohol hydroalkoxylation/cyclization. The cycle is based on the
rate law dependence on substrate and catalyst concentrations,
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to generate the product and simultaneously regenerate the Th−
O active species (step iii). This two-step insertion/protonolysis
cycle is consistent with the empirical reaction rate shown in eq
4, where intramolecular step ii is turnover-limiting.
Hydroalkoxylation/Cyclization Substrate Scope. The
scope of alkynyl alcohol substrates examined in the present
study is summarized in Table 3. The intramolecular hydro-
alkoxylation/cyclization of alkynyl alcohols could, in principle,
produce either of two distinct regioisomers (Chart 2). In one
case the oxygen is fused to the proximate alkynyl carbon atom,
generating a cyclic exo-methylene product. In the case of
terminal alkynes, this is the Markovnikov product. Alternatively,
the newly formed bond may be between the oxygen and the
distal alkynyl carbon atom, generating a cyclic product with an
endocyclic double bond. For terminal alkynes this is the anti-
Markovnikov product. With all substrates in Table 3, the exo
product is observed exclusively. The reactions are found to
proceed to >95% completion in all cases and to have constant
rates of product formation over three half-lives. Several
interesting observations can be made from the data in Table
3. It can be seen that five-membered rings close more quickly
than six-membered rings (Table 3, 8 → 9 vs 4 → 5 and 6 → 7
vs 10 → 11). The data in Table 3 also demonstrate that either
an aliphatic or aromatic linker may be incorporated between
the alkyne and alcohol functionalities. Introducing an ortho-
disubstituted aromatic linker enhances the rate up to 300-fold
(Table 3, 8 → 9 vs 6 → 7 and 4 → 5 vs 10 → 11). The
substrate scope includes both primary and secondary alcoholic
substrates, with primary alcohols cyclizing more rapidly than
secondary alcohols (Table 3, 4 → 5 vs 12 → 13).
Figure 4. Eyring plot for the catalytic hydroalkoxylation/cyclization of
4-pentyn-1-ol mediated by 1 over a temperature range 60−100 °C.
Table 3. Scope of Hydroalkoxylation/Cyclization Reactions
a
Catalyzed by Precatalyst 1
Precatalyst 1 catalyzes both terminal alkynyl and internal
alkynyl substrate conversions. However, the internal alkynyl
substrates require elevated reaction temperatures of >120 °C to
achieve catalytic rates comparable to those observed for
terminal alkynyl substrates at 60−70 °C (8 → 9 vs 16 →
17). In the case of internal alkynyl substrates, the substituent
can potentially be located in two different orientations (E or Z)
at the newly formed CC bond. For all internal alkyne
substrates, the E isomer is generated exclusively, with no
1
quantities of Z-oriented substitution detectable by H NMR
spectroscopy.
The TMS-substituted alkyne 18 is a unique case in which the
reaction does not proceed cleanly to completion, and only trace
product is detectable, even after 1 week at 120 °C. Several
unidentified side products are detectable by 1H NMR
spectroscopy, in addition to trimethyl(5-(trimethylsilyl)pent-
4-ynyloxy)silane. This observation is not unique to actinide
catalysts, but is consistent with processes identified for
lanthanide catalysts as well, doubtless facilitated by the
formation of a reactive metal-alkynyl species (Scheme 3).16b
Diene Substrates. Allene hydoalkoxylation/cyclization has
the potential to be a valuable tool for accessing various
heterocyclic products.1a,b,2c,38 Allene double bonds are known
to lie 10 kcal/mol higher in energy than isolated double
bonds,39 and for this and possibly steric reasons, the
hydroalkoxylation/cyclization of allenic double bonds is
significantly more facile than for isolated alkenes.15i,j The
reaction of 4,5-hexadiene-1-ol mediated with 5 mol % of
(CGC)Th(NMe2)2 (1) in toluene-d8 at 120 °C is found to
produce 6-methyl-3,4-dihydro-2H-pyran and 2-methylenetetra-
hydropyran, the latter of which undergoes quantitative
conversion to 6-methyl-3,4-dihydro-2H-pyran under the stand-
ard reaction conditions (Scheme 4). Although a colorless
a
Reactions carried out in J. Young NMR tubes catalyzed by precatalyst
b
1 (5 mol %), using C6D6/C7D8 solvent. o-Xylene-d10 solvent.
as well as the observed regiochemistry, KIE, and activation
parameters. Upon activation of the precatalyst by protonolysis
of the amide/alkyl bonds, the active Th−OR catalytic species is
generated (Scheme 2, step i). Next, the alkyne approaches and
undergoes insertion into the Th−O bond, via a four-center
transition state (step ii).17 This is expected to be the most
thermodynamically demanding and the turnover-limiting step
along the reaction coordinate. The newly generated Th−C
bond is highly susceptible to protonolysis and is rapidly cleaved
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Scheme 2. Proposed Catalytic Cycle for Organothorium-Catalyzed Hydroalkoxylation/Cyclization
Chart 2. Possible exo and endo Products of
Hydroalkoxylation/Cyclization
Scheme 4. Course of 4,5-Hexadien-1-ol Hydroalkoxylation/
Cyclization Mediated by Precatalyst 1
1
precipitate forms upon heating the reaction mixture, in situ H
NMR spectroscopy indicates that the hydroalkoxylation
reaction continues, but that substrate consumption is nonlinear
with time. After two days, the reaction reaches ∼40%
completion. In contrast to these results, a solution of 4,6-
heptadiene-1-ol with 5 mol % of (CGC)Th(NMe2)2 (1) in o-
xylene-d10 is found to be unreactive with respect to hydro-
alkoxylation, even at 145−155 °C over the course of 1 week.
Again, a colorless precipitate is observed upon heating 4,6-
heptadiene-1-ol with the catalyst. Precipitation has also been
observed under similar reaction conditions in the hydro-
amination of both allenes and conjugated alkenes using catalyst
1. This is likely an effect of the strong interaction of actinides
with π-systems. For example, the electron-rich diene terminus
of one ligand could conceivably coordinate to a second metal
center, thus facilitating oligomerizaion and precipitation of
intermediate species. No CGC ligand cleavage is observed by
NMR spectroscopy.
species (M = La, Sc, or Al; R = Me or TMS) follow the trend
La > Sc > Al. This trend approximately parallels the ordering in
ionic radii of the metal ions (Table 1), indicating that larger,
more sterically open metal sites are more reactive in
hydroalkoxylation/cyclization. This trend is also observed
within the lanthanide series Ln[N(TMS)2]3, where the effect
is proposed to reflect steric demands in the four-center insertive
transition state (Scheme 2, step ii).17 The larger, more open
metal centers more easily facilitate the approach and insertion
of the unsaturation into the M−O bond. The insertion step is
rate-limiting, so the relative steric openness of the metal centers
dominates the overall reaction rate. Similarly, it is observed for
the metals in oxidation state +4 that (CGC)Th(NMe2)2
catalyzes the reaction 4 → 5 far more rapidly than does
Zr(NMe2)4 (Nt = 0.32 vs 0.030 h−1, respectively). This
difference is also observed in hydroamination/cyclizations
mediated by group 4 and actinide ions with identical CGC,
amide, and/or chloride ligands.35b
DISCUSSION
■
Metal Identity Effects on Turnover Frequency. The
relative rates of hydroalkoxylation/cyclization depend heavily
on the identity of the catalyst metal. It is found that in the
conversion 4 → 5 the reaction rates mediated by M(NR2)3
When comparing metals of similar size, but different formal
oxidation states, for example Zr(IV) (IR = 0.84 Å) versus
Scheme 3. Catalytic Cycle for Organolanthanide-Mediated Reaction of TMS-Alkynyl Alkynols
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Sc(III) (IR = 0.87 Å), note that Zr(IV) is a slightly more active
hydroalkoxylation/cyclization catalyst than Sc(III). Among
other factors,35b a plausible explanation is that alkoxide and
alcohol ligands have roughly similar steric demands and that
hard, oxophilic ions of similar size will likely have approximately
the same total number of bound alkoxide/alcohol ligands. In
this situation, it is reasonable that CC insertion processes
would be favored at metal centers accommodating greater
numbers of metal−alkoxide σ-bonds (four vs three here).
Overall, the trends among catalyst metal ions in this study are
consistent with related hydroamination results in which the
general activity of tetravalent actinide catalysts is comparable to
that of trivalent lanthanide catalysts and substantially exceeds
that of tetravalent group 4 catalysts, regardless of the ancillary
ligation.35 These qualitative comparisons between groups 3 and
4, lanthanide, and actinide complexes should provide useful
guidance to future efforts to design new hydroalkoxylation
catalysts.
Diene Substrate Scope. Both 1,2- and 1,3-dienes were
investigated as potential hydroalkoxylation substrates. These
polyunsaturated substrates offer access to new product
structures suitable for subsequent functionalization and, in
some cases, higher turnover frequencies than olefins in f-
element-catalyzed hydroamination.35a,40 This is due to the
greater thermodynamic driving force for additions to 1,2-dienes
and the potential coordination of both CC unsaturations in
the case of 1,3-dienes.41 Nevertheless, they are found to be
poor substrates for the present organothorium-catalyzed
hydroalkoxylation process. In both dieneol cases, a precipitate
forms immediately upon heating the substrates with (CGC)-
Th(NMe2)2. This may reflect stronger M−(CC) π-
interactions for actinides versus lanthanides,42 leading to
stronger intermolecular interactions (e.g., Figure 5), aggrega-
precipitous fall in turnover frequency associated with
proceeding from a primary to a secondary alcohol suggests
that far higher reaction temperatures would be required to
effect tertiary alcohol hydroalkoxylation on a reasonable time
scale. Note also that literature reports of organolanthanide-
mediated hydroalkoxylation16,43 and group 4 metal and organo-
f-element-catalyzed hydrothiolation15c,18a,21g do not include
examples of tertiary alcohols or thiols. With few exceptions,
secondary functionalities proceed at a slower rate than primary
functionalities in each of these catalytic systems.
The Baldwin ring-closure rules state that five-membered
rings close more rapidly than six-membered rings,44 and this
trend is ubiquitous in f-element-catalyzed intramolecular
hydroelementations as in the present 8 → 9 vs 4 → 5 and 6
→ 7 vs 10 → 11 transformations (Table 3).15e,16b,18b,35b
Greatly enhanced reactivity is observed in the present study for
substrates in which the alcohol and alkyne functionalities are
linked by a relatively rigid ortho-disubstitued arene versus those
with a flexible aliphatic linker as in conversions 8→ 9 vs 6 → 7
and 4 → 5 vs 10 → 11 (Table 3). It is reasonable that the
reduced degrees of freedom with the more rigid linkers poise
the functionalities in a preorganized structure favorable to the
cyclization transition state. In the case of substrate 6, there is
also likely transition state stabilization by the alkyne−arene
conjugation. Evidence for such an electronic contribution is
more explicitly isolated in the case of organothorium-catalyzed
intermolecular hydrothiolation reactions, in which 1-ethynylcy-
clohexene is more reactive than cyclohexylacetylene toward
insertion into LnTh−SR bonds (eq 5).21g
Alkyne Substituent Effects. Substitution at the CC
linkage increases the steric demands of the hydroalkoxylation
transition state, strongly depressing the turnover frequency (16
→ 17 vs 8 → 9 in Table 3). Achieving the requisite four-center
transition state for the insertion process (Scheme 2, step ii) is
more energetically demanding when substituents are directly
joined to the alkyne or the alcohol carbon atom. A result is that
the products formed from the terminal alkyne substrates show
exclusively E-selectivity at the CC bond (Table 3, entries 7−
10). As shown in Figure 6, the sterically congested coordination
sphere likely repels the alkyne substituent and directs addition
toward E-selectivity for double-bond formation.
Figure 5. Possible mode of intermolecular coordination of dienyl
alcohols to organothorium complexes.
tion, and eventually precipitation. Free H2CGC ligand is not
1
observed in the H NMR spectrum of the reaction mixture.
Interestingly, similar precipitation is also noted in some cases
for organothorium-catalyzed diene hydroamination.35a The
difference in the present case versus similar lanthanide-
catalyzed hydroamination and hydroalkoxylation processes
provides an interesting distinction in Ln/An reactivity.
Reaction Mechanism. Kinetic studies of the present
hydroalkoxylation processes indicate that the rate law has
zero-order dependence on substrate concentration and first-
order dependence on catalyst concentration. This observation
supports the catalytic cycle depicted in Scheme 2, in which
turnover-limiting CC insertion into the Th−O bond is
followed by rapid protonolysis by free or bound substrate. The
observation that reaction rate is heavily influenced by substrate
CC substituents and product ring size argues against rate-
limiting protonolysis. The assertion is further supported by data
indicating a negligible kinetic isotope effect (0.97). Addition-
ally, note that steric demands tend to dominate the transition
states of f-element insertive processes,15c,e,17 so it is not
unexpected that secondary alcohols turn over more slowly than
primary alcohols, as in 4 → 5 vs 12 → 13 (Table 3). The
While steric demands strongly influence the present
cyclization rates, electronic contributions are in some cases
important as well. The only alkyne substrate that fails to react
cleanly to completion is 5-trimethylsilylpent-4-yn-1-ol, 18. This
Figure 6. Two possible insertive transition states leading to double
bonds with either E-orientation (left) or Z-orientation (right).
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substrate is proposed to undergo a process involving TMS
group transfer to the alcohol oxygen, driven by the large Si−O
bond enthalpy, generating a silyl ether. An analogous process
was previously described in lanthanide-mediated silylalkyne
chemistry16b (Scheme 3), and the key step, σ-bond metathesis
involving an M−X bond and a TMS−acetylenyl bond, has been
observed previously for organoactinides.21f,37 While the alkynyl
TMS substituent inhibits the present hydroalkoxylation/
cyclization process, the electronics of the phenyl substituent
(16 → 17) likely accelerate it. Figure 7 depicts a likely arene−
significant energetic challenge in translating known LnM−N,
LnM−P, or LnM−C bond insertive processes to those involving
LnM−O bonds. Traditionally, alkoxide and thiolate function-
alities were sparsely investigated in organo-f-element catalytic
processes such as hydroelementation because of the expected
thermodynamic unfavorability.52 Alcohols and thiols were
assumed to form unreactive species due to the established
stabilities of the LnM−OR and LnM−SR linkages.19r,34,53
Recently it was observed that [η5-1,2,4-(Me3C)3C5H2]2ThE
(E = S or O) complexes undergo [2 + 2] cycloaddition
reactions with Ph2CO and Ph2CS, but are unreactive
toward alkynes.22a Similarly, disruption of An−O σ-bonds is
typically observed only with the concurrent formation of new
An−O bonds, doubtless reflecting the large thermodynamic
barrier.21h,54 The present study represents the first report of
insertion into an An−OR σ-bond by carbon−carbon
unsaturation, driven by the exothermicity of additions to
CC bonds, as illustrated in Scheme 2.19d
Figure 7. Interaction of an organothorium center with aryl
substituents on the alkyne substrates.
The representative transformation 8 → 9 was examined over
a broad temperature range using catalyst 1. From these data,
activation parameters were derived by Eyring (Figure 4) and
Arrhenius (Supporting Information) analysis.36 The values
ΔH‡ = 27.9(0.4) kcal/mol, ΔS‡ = −3.0(1.1) eu, and Ea =
28.6(0.4) kcal/mol are largely in accord with expectations
based on values and trends observed in other f-element-
catalyzed hydroelementation/cyclization processes (Table 4).
The values of ΔH‡ and Ea in alkyne hydroamination processes
are known to be larger for An versus Ln centers in
hydroamination (Table 4), and this trend holds for the present
hydroalkoxylation/cyclization results as well. Additionally, ΔH‡
and Ea values are greater for lanthanide-mediated hydro-
alkoxylation/cyclization versus hydroamination/cyclization pro-
cesses. Again, the relatively large ΔH‡ and Ea values for the
present study follow a similar trend in being somewhat larger
than the corresponding values for organoactinide-catalyzed
hydroamination. Comparison of ΔS‡ values indicates a less
negative value for hydroalkoxylation versus hydroamination
using the same metal center, following literature trends and
likely indicating increased degrees of freedom on proceeding to
the four-center transition state.15m,16b However, the ΔS‡ for the
present study is significantly less negative than for the
analogous organolanthanide-catalyzed transformation. This
trend parallels aminodiene cyclization, where An catalysts
proceed with a ΔS‡ value 17 eu less negative than analogous Ln
catalysts.35a,40a A broader examination of the literature reveals a
lack of consistent trends in ΔS‡ values for An versus Ln
catalysts employed for similar catalytic processes.18c,40b,55 In
terms of mechanism, this may reflect less change in the degrees
of freedom as the insertive transition state is approached (or
the transition state lies on a different place on the reaction
coordinate) in the present hydroalkoxylation/cyclizations
relative to hydroamination/cyclization processes.
electrophilic 5f center π-interaction, which serves to prearrange
the substrate in an orientation favorable for cyclization. Note
that a rate enhancement is also observed when aryl-substituted
alkynes are compared to methyl-substituted alkynes in
organolanthanide-catalyzed hydroalkoxylation.16b,45 Studies of
organolanthanide-catalyzed intermolecular hydroamination in-
voke a similar explanation for the inverted regioselectivity
observed for styrenes versus aliphatic olefins (Scheme 5).46
Scheme 5. Proposed Origin of Inverted Regioselectivity in
Organolanthanide-Catalyzed Styrene Hydroamination
This is attributed to the strong structural influence of Ln−arene
interactions.47 Additional stabilization can plausibly be
attributed to stabilization of the δ− charge by the electron-
withdrawing phenyl group.46a
Comparison to Other f-Element-Catalyzed Hydro-
elementation Processes. While organo-f-element catalysts
are known to be highly active for hydroelementation processes
such as hydrosilylation,20a,48 hydrophosphination,49 and hydro-
amination,15e,k,m,q,25,35 only recently has the utility of oxy-
gen16a,30,43c,50 and sulfur15c,18a,21g,51 substrate functionalities
been investigated. The bond enthalpies for a series of f-element
M−X bonds are summarized in Figure 8 and illustrate the
Ligation and Structure. The first report of organo-
lanthanide-catalyzed hydroalkoxylation/cyclization used Ln[N-
(TMS)2]3 complexes as precatalysts.16c The labile proligands
were intended to be rapidly protonolyzed by alcoholic
substrates to afford catalytically active homoleptic metal−
alkoxide species. To study organothorium-catalyzed hydro-
alkoxylation/cyclizations, a different ligand set is necessary due
to the low solubility of homoleptic thorium alkoxides.33,56 The
literature reflects an infamous dearth of structural certainty,
with the first crystallographic characterization of a homoleptic
aliphatic thorium alkoxide remaining elusive until nearly four
Figure 8. Selected organo-f-element metal−ligand bond disruption
enthalpies.33
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Table 4. Activation Parameters for Selected f-Element-Catalyzed Hydroelementation/Cyclization Processes Involving CC
Bonds15m,16b,35a
Scheme 6. Moderation/Suppression of Thorium Alkoxide Oligomerization by Pyridine59
decades after Bradley and co-workers’ initial reports of their
syntheses.57 Thorium alkoxides consist of varying degrees of
alkoxide-bridged oligomeric species, with the degree of
oligomerization heavily dependent on the nature of the alkyl
group.33,56,58 Therefore, it is plausible that the active catalytic
species of the present study have oligomeric structures highly
dependent on the specific substrate used. Nevertheless, the
kinetic rate law is not consistent with rapid, reversible
dissociation of oligomeric species prior to the turnover-limiting
step.
significant precipitation during the course of the catalytic
reaction. Nevertheless, alkynol hydroalkoxylation/cyclization
catalyzed by 3 proceeds rapidly until no starting material is
1
observable by H NMR spectroscopy, arguing that while there
may be varying degrees of oligomerization, some catalytically
active species are present in the solutions. Using an internal 1H
NMR standard, it is determined that the reaction proceeds only
to ∼72% of the theoretical yield, at which point substrate is no
longer detectible in the NMR spectrum, indicating that a
significant portion of the substrate has been removed from the
solution in the precipitation.
The solubility of the active alkoxide species in the present
work is maintained by using strongly binding/sterically
demanding Cp* and CGC ligands.18a,42c,d While amide and
alkyl proligands are instantly protonolyzed by alcohols, 5f Cp*
derivatives are significantly more resistant to protonolysis. Each
CONCLUSIONS
■
Several organothorium complexes are found to display high
catalytic activity with respect to alkynyl alcohol hydro-
alkoxylation/cyclization. Of these, it is shown that the lack of
strongly binding/sterically demanding (methylCp-based) liga-
tion produces insoluble and marginally active catalysts. The
scope of substrates includes primary and secondary alcohols, as
well as terminal and internal alkynes. However, allenes and 1,3-
dienes are found to be poor substrates that lead to precipitation
upon heating with (CGC)Th(NMe2)2. For example, the rate of
(CGC)Th-mediated allenyl alcohol hydroalkoxylation/cycliza-
tion slows with increasing catalyst precipitation. For alkynyl
alcohols the catalytic cycle is proposed to involve turnover-
limiting alkyne insertion into the Th−O bond, generating a
transitory Th−C species. The ensuing and rapid protonolysis
by alcohol substrate generates the cyclized product and
regenerates the Th−O-bonded catalyst. The activation
parameters ΔH‡ = 27.9(0.4) kcal/mol, ΔS‡ = −3.0(1.1) eu,
1
of the characteristic H NMR spectroscopic resonances of the
free H2CGC ligand are absent in spectra during the course of
these catalytic reactions. Rapid, reversible dissociation is
unlikely, given the sluggish reaction rates for H2CGC
protonolytic reactivity in the synthesis of (CGC)Th(NMe2)2.25
These factors argue for active catalytic species in which the
CGC ligand remains bound in a normal η1,η5-Th fashion.
It has been reported that the formation of oligomeric
thorium alkoxides can be moderated or significantly suppressed
by the addition of strongly σ-binding ligands.59 In these reports,
pyridine serves to moderate oligomer size by cleaving and
“capping” the thorium tetraalkoxides (Scheme 6).59,60 The
propensity of these complexes to undergo cleavage by a Lewis
base, as well as the degree of oligomerization achievable, is
strongly influenced by the nature of the alkoxide ligand. Thus,
precatalyst 3, which has no Cp*-type binding sites, undergoes
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and Ea = 28.6(0.4) kcal/mol largely follow the trends observed
with related f-element insertive hydroelementation processes.
This work provides an enhanced understanding of the
activity and selectivity of organoactinide catalysts and expands
the frontier of known organoactinide reactivity. Although
actinide−oxygen bonds were traditionally believed to be
unreactive, this work and recent literature demonstrate the
potential utility of such species. The rate-limiting step of this
reaction (Scheme 2, step ii) is the first example of CC bond
insertion into an actinide−oxygen bond. The demonstrated
necessity of strongly binding/sterically encumbered ancillary
ligands will likely provide groundwork for developing new
organo-f-element-mediated reactions involving polar substrate
functionalities.
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ASSOCIATED CONTENT
* Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
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AUTHOR INFORMATION
Corresponding Author
*E-mail: t-marks@northwestern.edu.
■
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We thank NSF (Grants CHE-0809589 and CHE-1213235) for
financial support. We also thank Dr. M. Weberski and Dr. C.
Weiss for helpful discussions, and Dr. B. Stubbert for samples
of several catalysts.
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