Rare-Earth Catalyzed C?H Bond Alumination of Terminal Alkynes

Article abstract of DOI:10.1002/chem.202000325

Organoaluminum reagents’ application in catalytic C?H bond functionalization is limited by competitive side reactions, such as carboalumination and hydroalumination. Herein, rare-earth tetramethylaluminate complexes are shown to catalyze the exclusive C?H bond metalation of terminal alkynes with the commodity reagents trimethyl-, triethyl-, and triisobutylaluminum. Kinetic experiments probing alkyl-group exchange between rare-earth aluminates and trialkylaluminum, C?H bond metalation of alkynes, and catalytic conversions reveal distinct pathways of catalytic aluminations with triethylaluminum versus trimethylaluminum. Most significantly, kinetic data point to reversible formation of a unique [Ln](AlR₄)₂?AlR₃ adduct, followed by turnover-limiting alkyne metalation. That is, C?H bond activation occurs from a more associated organometallic species, rather than the expected coordinatively unsaturated species. These mechanistic conclusions allude to a new general strategy for catalytic C?H bond alumination that make use of highly electrophilic metal catalysts.

Full text of DOI:10.1002/chem.202000325

A Journal of
Accepted Article
Title: Rare Earth Catalyzed C–H Bond Alumination of Terminal Alkynes
Authors: Uddhav Kanbur and Aaron David Sadow
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Rare Earth Catalyzed C–H Bond Alumination of Terminal Alkynes
Uddhav Kanbur[b] and Aaron D. Sadow*[a]
[a]
U. Kanbur, Prof. A. D. Sadow
Department of Chemistry
Iowa State University
1605 Gilman Hall, 2415 Osborn Dr., Ames, IA, 50011, United States
E-mail: sadow@iastate.edu
Supporting information for this article is given via a link at the end of the document.
Abstract: Organoaluminum reagents’ application in catalytic C–H
bond functionalization is limited by competitive side reactions, such
as carboalumination and hydroalumination. Here, we demonstrate
that rare-earth tetramethylaluminate complexes catalyze the
exclusive C–H bond metalation of terminal alkynes with the
commodity reagents trimethyl-, triethyl-, and triisobutylaluminum.
Kinetic experiments probing alkyl group exchange between rare
earth aluminates and trialkylaluminum, C–H bond metalation of
alkynes, and catalytic conversions reveal distinct pathways of
catalytic aluminations with triethylaluminum vs. trimethylaluminum.
Most significantly, kinetic data point to reversible formation of a
unique [Ln](AlR4)2·AlR3 adduct, followed by turnover-limiting alkyne
metalation. That is, C–H bond activation occurs from a more
associated organometallic species, rather than the expected
coordinatively unsaturated species. These mechanistic conclusions
allude to a new general strategy for catalytic C–H bond alumination
that make use of highly electrophilic metal catalysts.
triethylaluminum
to
generate
diethylphenylaluminum.[13]
Interestingly, alumination of unactivated aryl C–H bonds, making
use of diketiminate aluminum hydride, is catalyzed by palladium
at room temperature.[14, 15]
The more common transition-metal-catalyzed reaction
pathways of alkylaluminums and unsaturated organic
compounds, namely hydroaluminations and carboaluminations
involving the addition of organoaluminum compounds to alkenes
and alkynes,[16] limit the direct metalation chemistry. Instead,
these reactions provide valuable organic nucleophiles for
synthesis and are industrially important in Ziegler-type
polymerizations.[17-20] In reactions between terminal alkynes and
alkylaluminum compounds, organometal-catalyzed routes to
alkynylaluminum species were unknown, prior to the present
work. The latter species, which are useful alkynyl transfer
agents,[21] complement the reactivity of lithium and copper
acetylides in carbon-alkyne bond forming reactions and have
even been utilized in the synthesis of valuable biomolecules,
such as prostaglandins.[22] Alkynyldialkylaluminums are
conventionally generated in situ, via salt elimination reactions of
lithium acetylides with dialkylaluminum halides, along with solid
waste byproducts that may further influence reactivity.[23, 24] Few
aliphatic alkynylaluminum species have been isolated and
characterized by modern spectroscopic techniques. Catalytic
routes to alkynylaluminums could facilitate their isolation and
characterization, as well as provide easier access to these
useful nucleophiles.
Metalation of arylalkynes by triphenylaluminum gives
alkynyldiphenylaluminum, but this approach is limited by the
availability of AlPh3 and the fact that aluminum phenyl and
alkynyl groups in alkynyldiphenylaluminum are both highly
reactive in subsequent group transfer reactions.[25] On the other
hand, uncatalyzed reactions of trialkylaluminums and terminal
aliphatic alkynes typically generate mixtures of carboalumination,
hydroalumination, and metalation products.[26] This poor intrinsic
selectivity improves in the presence of amines, either in
stoichiometric or catalytic quantities. Binger originally showed
that tertiary amine-coordinated adducts R2AlH⋅NR3 selectively
metalate 1-alkynes, giving alkynylaluminum compounds and
dihydrogen as a byproduct.[27] More recently, Micouin and
Introduction
Organoaluminum compounds are versatile and sustainable
reagents finding utility in synthetic organic, organometallic, and
polymer chemistry for more than fifty years. The high elemental
abundance of aluminum in the earth’s crust (8%) and relatively
low toxicity combined with straightforward, atom-economical and
scaleable preparation from aluminum, hydrogen, and alkenes
have resulted in commercial applications of trialkylaluminum
reagents.[1] Moreover, organoaluminums combine nucleophilicity
and Lewis acidity to offer reactivity, such as conjugate addition
and facile transmetalation, that is complementary to other useful
reagents, including organo-magnesium, -zinc, -copper and -
lithium compounds. Unfortunately, syntheses of complex
organoaluminum compounds instead typically involve salt
metathesis reactions that rely upon other alkylating agents and
corrosive aluminum halides. Ideally, direct metalation of C–H
bonds by inexpensive trialkylaluminums could greatly increase
the availability of functional-group containing organoaluminum
compounds, but C–H bond alumination is underdeveloped
compared to other catalytic C–H bond functionalization methods
such as borylation[2-7] or silylation.[8-10]
coworkers
eliminations from the combination of trimethylaluminum and
terminal alkynes.[28] Although competing
atom transfer
developed
Lewis
base-catalyzed
methane
Instead, most C–H bond alumination reactions rely on the
inherent reactivity of organoaluminum species. Intramolecular
activations, pioneered by Eisch and coworkers, convert mixed
arylaluminum compounds into benzaluminoles.[11] Stoichiometric
aluminations of aryl and heteroaryl compounds are
accomplished with heterobimetallic aluminate complexes.[12]
Another heterobimetallic, the niobium aluminate Cp2Nb(µ-
H)2AlEt2, mediates metalation of one equiv. of benzene by
H
reactions of higher alkylaluminums containing β-hydrogen limit
this approach to trimethylaluminum, these studies suggest that
four-coordinate aluminum species favor metalation over addition
or insertion to unsaturated organics. For example, highly
electrophilic metal centers such as those of rare earth elements
1
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containing more tightly associated aluminate centers, that
{R'2Al(OxMe2)2PhBCp}Ln(AlMe4)2 (2Y, La, Nd, R' = Et; 3Y, La, Nd, R' =
disfavor insertion, could catalyze alumination of terminal alkynes. iBu) are synthesized from the in-situ generated κ2-
Lanthanide aluminates, upon activation, are typically
catalyst precursors for alkene and diene polymerization and are
also used to generate highly reactive species such as masked
alkylidenes and cationic alkyls.[29-40] In the few reports of
aluminum addition chemistry, piano-stool scandium compounds
catalyze the carboalumination of ether-containing alkynes,[41]
Ph(C5H5)B(OxMe2)2AlR'2 (R' = Et, iBu).
O
Ph
N
benzene
Ph B
O
OxMe2
N
B
R'2Al
AlR'3
+
H
N
25 °C, 15 min
– HR'
and
bent-sandwich
yttrium
compounds
catalyze
O
H
hydroalumination,[42] together suggesting that the appropriate
ancillary ligands also are required to promote alumination
pathways. In the initial phase of this work, we communicated
H
R' = Me, Et, iBu
Ln(AlMe4)3
toluene
25 °C, 6 h
– CH4, – AlMe3
that
a
neodymium
aluminate
ligand,
supported
by
a
cyclopentadienylborate
{Me2Al(OxMe2)2BPh(C5H4)}-
Nd(AlMe4)2 (1Nd; OxMe2 = 4,4-dimethyl-2-oxazoline), is a catalyst
for alumination of terminal alkynes.[43] The paramagnetic
neodymium center, however, limited NMR spectroscopic studies
and kinetic measurements that are needed to establish solution-
phase structural dynamics and mechanistic features leading to
selective C–H bond activation. Here we report a series of
diamagnetic and paramagnetic zwitterionic rare earth complexes
that provide trends in activity, access to mechanistic data that
implicate pathways that vary with the trialkylaluminum reactant,
and lead to superior catalysts that provide access to isolable
alkynylaluminum compounds.
O
Ph
B
2Y: R' = Et, Ln = Y, 76%
2La: R' = Et, Ln = La, 74%
2Nd: R' = Et, Ln = Nd, 86%
N
Me
Me
R'2Al
AlMe2
N
Ln
O
Me
3Y: R' = iBu, Ln = Y, 52%
3La: R' = iBu, Ln = La, 73%
3Nd: R' = iBu, Ln = Nd, 70%
Me
Al
Me2
Scheme 2. Synthesis of heteroleptic heterobimetallic cyclopentadienylborato
rare earth aluminates with mixed alkylaluminum species.
The reaction of Ln(AlMe4)3 and H{PhB(OxMe2)2C5H5} of
Scheme 1 is significantly faster than the corresponding reaction
with dialkylaluminum-bonded κ2-Ph(C5H5)B(OxMe2)2AlR'2 in
Scheme 2. Based upon this observation, a likely pathway to 1,
shown in Scheme 3, involves oxazoline donor-induced cleavage
of the lanthanide aluminate to form [Ln]–Me and oxazoline-
coordinated trimethylaluminum.[44] The latter species eliminates
methane to give the bis(4,4-dimethyl-2-oxazolinyl)borate
aluminumdimethyl moiety. Then, C5H5 metalation by the
Results and Discussion
Zwitterionic Piano-stool Rare Earth Aluminate Catalysts.
The complexes {Me2Al(OxMe2)2PhBCp}Ln(AlMe4)2 (Ln = Y, 1Y; Ln
=
La, 1La) are synthesized from H{PhB(OxMe2)2C5H5} and
Ln(AlMe4)3 in toluene at room temperature in respectable yields
(Scheme 1), similar to our previously reported preparation of
{Me2Al(OxMe2)2PhBCp}Nd(AlMe4)2 (1Nd).[43] All three aluminum
atoms from the tris(tetramethylaluminate) precursors are
incorporated into the heteroleptic products, which contains a
bis(oxazoline)-coordinated dimethylaluminum and two AlMe4
ligands. Methane, formed in the protonolysis steps, is the only
byproduct detected in 1H NMR spectra of micromole-scale
reactions performed in benzene-d6.
(AlMe4)2LnMe gives 1Ln
.
donor induced cleavage
and AlMe3 protonolysis
AlMe4
AlMe4
Ln(AlMe4)3
– CH4
Ph
B
OxMe2
N
Ph
B
OxMe2
N
H
Ln Me
+
AlMe2
O
O
O
Ph
B
O
N
– CH4
Ph
Me
Me
Me2Al
N
AlMe2
B
N
Ln
O
Me
Me
Me
toluene
Me2Al
AlMe2
Ph
B
OxMe2
N
N
Me
+ Ln(AlMe4)3
Ln
O
Me
H
Ln-Me protonolysis
25 oC, 45 min
– 2 CH4
Al
Me2
Me
O
1Y, 62%
1La, 75%
1Nd, 91%
Al
Me2
Scheme 3. Proposed pathway to 1Ln
.
Scheme 1. Synthesis of heteroleptic heterobimetallic cyclopentadienylborato
rare earth aluminates.
NMR spectra of the half-sandwich compounds 1Ln, 2Ln, and
1
3Ln reveal a few common characteristics. First, similar H NMR
signals corresponding to the diastereotopic AlR'2 were observed
in related diamagnetic yttrium and lanthanum congeners. In
general, peaks from methylaluminum or methylenealuminum in
B(OxMe2)2AlR'2 appeared at lower frequency than those from
AlR3 or AlMe4 species. Second, NMR spectra were consistent
with Cs-symmetric complexes, on the basis of a single set of
diastereotopic methyl and methylene signals from the two
Compounds 1Ln can also be accessed via the reaction of
Ln(AlMe4)3
and
dimethylaluminum-coordinated
bis(oxazolinyl)borate. κ2-Ph(C5H5)B(OxMe2)2AlMe2 itself is formed
by treatment of H{PhB(OxMe2)2C5H5} with 0.5 eq. of (AlMe3)2.[43]
This preparation may be extended to triethyl- and
triisobutylaluminum (Scheme 2), and the complexes
2
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oxazolines, two signals for diastereotopic dialkylalumino groups,
and two signals assigned to C5H4. That is, the two oxazoline
groups appear to be equivalent. The gas phase DFT-calculated
Yttrium chemical shifts for compounds 1Y, 2Y, and 3Y
(Figures 2A, S16, and S33), determined through 1H-89Y{1H}
HSQC experiments that revealed cross-peaks with signals from
AlMe4, were almost identical to Y(AlMe4)3 (394 ppm).[33] The IR
spectra of the heteroleptic tetramethylaluminate complexes in
the solid state revealed two distinct bands between 1530-1600
cm–1 that were assigned to νC=N of two inequivalent oxazolines,
owing to the likely coordination of only one to the lanthanide
center via oxygen.
model of 1Y (see below) and crystal structure of 1Nd [43]
in
,
contrast, reveals an oxygen from an oxazoline coordinating to
the neodymium center, giving a static C1 symmetric structure
with inequivalent oxazolines. Likely, the Ln–Ooxazoline bond is
highly labile, enabling exchange of oxazolines via rotation about
the B–Cpipso bond.
The exchange of free and associated alkylaluminum
species may be important for accessing catalytically active
species, either coordinatively-unsaturated or highly associated
intermediates. A single, broad resonance (24 H), assigned to
AlMe4 groups, was observed in the spectra for all the
compounds (Y: –0.24 ppm; La: –0.17 to –0.19 ppm). The AlMe4
signals of the paramagnetic neodymium species 2Nd and 3Nd
manifested as broad peaks centered around ~6.5 ppm, similar to
the spectrum of 1Nd. The AlMe4 groups are highly fluxional, and
bridging Ln-Me-Al and terminal Al-Me undergo rapidly exchange
even at 193 K. In contrast, these groups are resolved in
homoleptic analogues in spectra acquired at low temperature.[33]
Although bridging and terminal groups do not resolve at low
temperature, the broad AlMe4 resonance sharpens in a variable
A
O
Ph
N
B
Me
Me2Al
AlMe2
N
O
Me
Y
Me
Me
Al
Me2
1
temperature H NMR study of 1Y, and a doublet (2JYH = 1.9 Hz)
corresponding to averaged coupling of bridging and terminal
methyls can be observed at 253 K (Figure 1). The dynamic
behavior of aluminates 1-3Ln, which is apparent in 1H NMR
spectra at a distinctly lower temperature regime than those of
89
Y : 394 ppm
homoleptic Ln(AlMe4)3 (Ln
=
Y, La), may also affect
intermolecular exchange rates with AlR3 which are key to the
proposed catalytic mechanisms (see below).
0.5
0.0
- 0.5
B
89
Y : 367 ppm
O
Ph
B
N
Me
Me2Al
AlR2
N
O
Y
Et
Et
Et
Al
R2
O
Ph
B
N
Et
Me2Al
AlR2
N
O
Y
Et
Et
Et
Al
R2
Figure 1. 1H NMR spectra of {Me2Al(OxMe2)2PhBC5H4}Ln(AlMe4)2 (1Y),
acquired in toluene-d8 from 223-283 K.
89
Y : 390 ppm
0.5
0.0
- 0.5
1
H NMR (ppm)
1
In a 2D H NOESY experiment on 2La, the signal assigned
to AlMe4 correlated with both signals of the C5H4 group and
methylene and methyl groups of the oxazoline (Figures S22-23).
Through-space interactions were also detected between AlMe4
and the B(OxMe2)2AlEt2. Similarly, a NOESY experiment on 3Y
revealed close contacts between AliBu2 groups and AlMe4. In
contrast, correlations between AlMe2 and AlMe4 groups in 1Y
were not detected. The spatial proximity of the AlMe4 and AlEt2
or AliBu2 groups is likely responsible for the slower catalytic
performances of 2Ln and 3Ln compared to 1Ln (see below).
Figure 2. 1H-89Y{1H} NMR spectra of (A) 1Y and (B) 1Y + 10 equiv. of
triethylaluminum.
A gas-phase model 1Y-CALC was optimized using DFT
methods (LANL2DZ + ECP(Al, Y)/M06-2X-D3),[45-47] starting with
an initial geometry based on the atomic coordinates of 1Nd
composed of O-coordinated oxazoline and two bidentate (µ-
Me)2AlMe2 units. Interestingly, the DFT-optimized geometry of
3
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1Y-CALC reveals inequivalent coordination modes of the AlMe4
ligands, with one forming a bidentate (µ-Me)2AlMe2 structure and
the second giving monodentate coordination (µ-Me)AlMe3
(Figure 3). The former Y–C distances are calculated to be 2.723
and 2.626 Å, with a longer third distance (dashed bond in Figure
3) of 3.01 Å. The calculated Y–C distance in the monodentate
tetramethylaluminate is 2.464 Å. The structure also features an
oxazoline coordinating to yttrium through its oxygen. The
calculated νCN of this O-coordinated oxazoline, which is part of
for alkynylalumination, it is even slower than 1Nd [43]
. Together, it
seems unlikely that the trend in catalytic activity results from
chemical reactions directly involving the dialkylaluminum-
coordinated bis(oxazolinyl)borate. Instead, the steric effects from
the bulkier dialkylaluminum, as suggested by nOe studies above,
are most likely responsible for the change in catalytic
performance.
Reactions catalyzed by 1Y and 1La also exhibit improved
selectivity, with respect to side reactions involving higher
alkylaluminum reactants, compared to 1Nd
. For example,
the
bis(oxazolinyl)borate
moiety
coordinated
to
dimethylaluminum, is 1598 cm–1, whereas the calculated νCN of
the other oxazoline, coordinated only to aluminum, is 1532 cm–1.
These calculated values are in reasonable agreement with
experimental νCN of 1594 and 1560 cm–1.
products of side carboalumination processes are not detected in
1Y-catalyzed reactions with triethylaluminum, as is observed with
1Nd or Ln(AlMe4)3 (Ln = Y, La) as catalysts. In addition, products
of hydroalumination with triisobutylaluminum, which occurs via
β-hydride transfer processes in the absence of a catalyst, are
also not detected.
Trimethylaluminum reacts more sluggishly compared to the
higher alkylaluminum analogues, as expected on the basis of
previously observed trends for reactions of AlR3 and
phenylacetylene or benzophenone[25] and for 1Nd-catalyzed
reactions.[43] The products nBuC≡C–AlEt2 and nBuC≡C–AliBu2
are formed within two hours in reactions of triethyl- or
triisobutylaluminum with 1-hexyne catalyzed by 1Y, whereas
reactions of trimethylaluminum under similar conditions are more
than 3× longer.
Figure 3. Optimized gas-phase geometry for 1Y-CALC
.
Table 1. Comparisons of catalysts and reagents in the alumination of terminal
alkynes
H
AlR2
3 mol % cat.
Catalytic Alumination of Terminal Alkynes. The compounds
1Ln are effective precatalysts for alumination reactions of
terminal alkynes (R1C≡CH) with AlR3 (R = Me, Et, iBu; R1 = alkyl
or aryl), giving dimeric alkynylaluminums (R1C≡C–AlR2)2 and the
alkane byproduct with high selectivity at 60 ° C (Table 1). A few
important points are revealed by the present studies. First, 1Y
and 1La afford significantly faster catalysts than our previously
reported 1Nd in comparable alkynylalumination reactions.[43]
Second, neither Y(AlMe4)3 nor La(AlMe4)3 provide active
catalysts for the metalation of 1-hexyne with trimethylaluminum
at 60 °C after 24 h (only starting materials are observed in 1H
NMR spectra), in contrast to catalytically active Nd(AlMe4)3.
Ln(AlMe4)3 is unselective and affords both ethylalumination and
alkyne C–H alumination in reactions of triethylaluminum and 1-
hexyne. That is, the cyclopentadienyl ligand enhances the
catalytic properties of yttrium and lanthanum to a greater extent
than those of neodymium, and catalytic rates for reaction of
AlMe3 increase following the trend: Ln(AlMe4)3 (Ln = Y, La) <<
Nd(AlMe4)3 < 1Nd < 1Ln (Ln = Y, La).
Interestingly, the peripheral dialkylaluminum groups
coordinated to the bis(oxazolinyl)borate also have a significant
effect on reaction rates, following the trend (1Ln > 2Ln ~ 3Ln). For
example, the rate of alumination of 3,3-dimethyl-1-butyne by
triethylaluminum is much faster with the dimethylalumino
bis(oxazolinyl)borate ligand in 1Y than with diethylaluminum 2Y
and diisobutylaluminum 3Y catalysts, based on time needed to
achieve high conversion under equivalent conditions. The AlR'2
groups in 2Ln or 3Ln do not undergo detectable chemical
exchange with free trialkylaluminums or with lanthanide
aluminates, as shown by an 1H-1H EXSY experiment, or upon
treatment of 1-3Ln with a different trialkylauminum. Although the
aluminum-only ligand κ2-Ph(C5H5)B(OxMe2)2AlMe2 is a catalyst
+
AlR3
benzene-d6
R1
R1
60 °C, – R-H
R1
R
cat.
Time (h)
yield (%)
nBu
nBu
nBu
nBu
nBu
nBu
nBu
nBu
nBu
tBu
Me
Me
Me
Me
Me
Me
Et
Y(AlMe4)3
La(AlMe4)3
Nd(AlMe4)3
1Y
24
24
24
6
< 2
< 2
< 2
> 99
> 99
> 99
> 99
> 99
73 [a]
> 99
1La
6
1Nd
18
2
1Y
iBu
Et
1Y
1.5
8
Y(AlMe4)3
Et
Ln(AlMe4)3
(Ln = Y, La)
< 1
tBu
tBu
tBu
Et
Et
Et
1Y
2Y
3Y
< 1
5
> 99
> 99
> 99
6
[a] The remaining products (27%) are vinylaluminum species produced from
ethylalumination.
A series of alkynylaluminum products R1C≡C–AlR2 were
isolated from these reactions in good yield (Table 2). Terminal
4
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10.1002/chem.202000325
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alkynes with either alkyl or aryl substituents react readily. As an
example, bulky tert-butyl acetylene affords the alkynylaluminum
product after short reaction times. An internal, conjugated alkene
in cyclohexenyl acetylene is also tolerated in the reaction without
addition to the double or triple bond. p-Tolylacetylene requires
slightly longer reaction time than phenylacetylene under similar
conditions; however, reaction rates are highly sensitive to
trialkylaluminum and alkyne concentration (see below). These
reactions have been performed on moderate scales, giving 0.25
g of hexynylaluminum and 0.3 g of phenylethynylaluminum, for
example.
characterization was typically limited to titration of the alkynyl
moiety present. In other cases, alkynylaluminums were
quenched by reactions with carbonyl compounds in lieu of
isolation. Here, we provide previously lacking spectroscopic data
for this class of organoaluminums. The 1H and 13C{1H} NMR
chemical shifts of the AlMe2 groups of the alkynylaluminum are
higher frequency with respect to free trimethylaluminum (–0.39
and –7.1 ppm for 1H and 13C NMR, respectively). The 13C{1H}
NMR chemical shifts of sp hybridized alkynyl carbons are
significantly higher in frequency than the alkyne starting
materials, and the high frequency shift for the signal associated
with the β-carbon is much larger than that of the α-carbon (Table
3). For example, the terminal and internal alkynyl carbons of 1-
hexyne appeared at 68 and 85 ppm in the 13C{1H} NMR
spectrum, respectively. These values were 91 (and broadened
due to 27Al) and 140 ppm for nBuC≡C–AlMe2. In addition,
intense bands in the IR spectra from 2050 to 2100 cm–1 were
assigned to νC≡C, in contrast to the less intense and higher
energy νC≡C of terminal alkynes.
Table 2. Catalytic synthesis of alkynylaluminum compounds.
Product[a]
Time (h)
4
Isolated yield (%)
87
nBu
AlMe2
AlMe2
4
6
4
97
91
95
Table 3. 13C NMR spectroscopic data for isolated alkynylaluminum species
H3C
AlMe2
R1CCAlR2
R1CCAlR2
Species
Δ(R1CCAlR2 – R1CCH)
13C (ppm)
13C (ppm)
13C (ppm)
AlMe2
nBu
AlMe2
90.09
97.11
139.97
120.25
53.97
36.25
AlMe2
4
2
2
80
94
95
AlMe2
nBu
AlEt2
96.76
90.61
117.33
139.79
33.33
56.19
H3C
AlMe2
AlEt2
AlMe2
tBu
AlEt2
1
1
88
91
AlMe2
94.03
85.86
93.33
138.12
140.91
120.39
55.12
54.91
36.39
nBu
AliBu2
nBu
AlEt2
Me2Al
Me2Al
(CH2)2
6
91
AlEt2
[a] Reaction conditions: 3 mol % 1Y, 60 °C, benzene.
tBu
AlEt2
81.92
87.22
149.88
142.25
55.88
56.25
Reaction of 1,7-octadiyne with an equivalent of
trimethylaluminum in the presence of 1Y yields a mixture of
Me2AlC≡C(CH2)4C≡CAlMe2, Me2AlC≡C(CH2)4C≡CH and starting
materials. The monoaluminated alkyne was not isolated in this
nBu
AliBu2
Me2Al
Me2Al
(CH2)2
90.64
138.83
54.83
reaction; however, addition of
a
second equivalent of
trimethylaluminum results in full conversion to new
bis(dimethylaluminum)diyne Me2AlC≡C(CH2)4C≡CAlMe2, which
is isolated as a brown-red solid (91%). In contrasting and
Ground state geometries and IR frequencies of dimeric
alkynylaluminum species (R1C≡CAlR2)2-calc (R1 = CyCH2, tBu,
nBu) were computed at the 6-31G**/M06-2X-D3 level of theory.[46,
complementary
chemistry,
polystyrene-supported
methylpiperidine-catalyst mediates monoalumination of 1,7-
octadiyne exclusively to Me2AlC≡C(CH2)4C≡CH, and the
bis(dimethylaluminum) product is not observed.[48]
The catalytic products, alkynyldialkylaluminum dimers, are
isolable as analytically pure species and are fully characterized
by NMR and IR spectroscopy. Many of these compounds were
only previously generated and used in-situ, and their
47, 49]
The C2-symmetric, acetylide-bridged dimeric starting
geometries optimize to C1 species that contained asymmetrically
bridging alkynyl units, as expected on the basis of crystal
structures of (R1C≡CAlR2)2.[24, 50] As a result, two 13C NMR shifts
for the alkynyl carbon and two νC≡C are calculated. Trends in the
calculated 13C NMR chemical shifts (DFT-GIAO; 6-31G**/B3LYP-
5
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10.1002/chem.202000325
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49, 51, 52]
D3)[47,
were in good agreement with experimental
proportional to monomer concentration [AlR3] will be also directly
proportional to the square root of dimer concentration, scaled by
observations, particularly the deshielded R1C≡CAlR2.The νC≡C of
(R1C≡CAlMe2)2-calc are ca. 50 cm–1 lower than calculated
values for R1C≡CH-calc, accurately matching the experimental
trend (Table 4).
the equilibrium constant: rate ~ [Al2R6]1/2 = ([AlR3)]2/Keq)1/2
.
Because these experiments have mixtures of methyl and ethyl
aluminum groups with unknown equilibrium constants, and
because the rate will be directly proportional to [AlR3]total and
only scaled by unknown (Keq)1/2, the rate constants are given
with respect to total trialkylaluminum concentration.
Table 4. Calculated NMR and IR spectroscopic properties of gas-phase
dimeric alkynylaluminum species.
R1
C
O
R
R
R
R
Al
Al
Ph
B
O
Ph
B
N
C
N
Me
Me
AlEt3
Me2Al
AlMe2
N
Me2Al
Y
O
Me
N
Y
O
AlMemEt4-m
R1
AlMe3
Me
AlMenEt4-n
1Y
Al
R1
Calculated
R1CCAlR2
13C (ppm)
Calculated
R1CCAlR2
13C (ppm)
Calculated
R1CCAlR2
Calculated
R1CCH
νCC (cm–1
)
Me2
n and m range from 0 - 4;
multiple aluminate
compositions are present
and undergo exchange
νCC (cm–1
)
AlEt3
CH2Cy
tBu
81.95, 95.19
76.87, 92.16
84.96, 93.63
122.07, 139.60
132.32, 147.40
128.31, 137.39
2140, 2148
2130, 2138
2141, 2144
2188
2181
2189
O
N
AlMe3 + AlEt3
R'2Al
B
nBu
N
Et
Et
O
Me
Ln
Al
Et
Me
Me Al
AlMe2Et + AlEt2Me
Al
Mechanistic Studies. Possible intermediates in rare-earth-
catalyzed alumination reactions of terminal alkynes R1C≡CH and
AlR3 to give R1C≡C–AlR2 include the mixed alkylaluminate
Me
Me
Me
Me
Me
A
{R'2Al(OxMe2)2PhBCp}Ln(AlRnMe4-n)2,
lanthanide
acetylide
AlMe3
species [Ln]–C≡CR1, lanthanide alkyls derived from AlR3, and
R1C≡C–AlMe2. The viability of such species, in terms of their
formation and reactivity, are discussed here.
O
O
Ph
Ph
B
N
B
N
AlMe2Et
The reaction of 1Y and triethylaluminum at room
temperature leads to the highly fluxional mixed-alkyl aluminate
species collectively assigned as {Me2Al(OxMe2)2PhBCp}-
Y(AlMenEt4–n)2. These species and free alkylaluminum
compounds (AlMe3–nEtn)2 exchange rapidly (Scheme 4), with
species populations affected by the relative initial concentrations
of 1Y and triethylaluminum. For example, a 1H-89Y{1H} HSQC
experiment on mixtures of 1Y and 10 equiv. of triethylaluminum
revealed two distinct yttrium species. One compound contains
only bridging Y-Et-Al, while the other has both Y-Me-Al and Y-
Et-Al moieties (Figure 2B above). The alkylaluminum and
lanthanide alkylaluminate species undergo chemical exchange,
Et
Et
Me2Al
AlMe2
N
Me
Et
Me2Al
AlEt2
N
Y
O
Me
Y
O
Me
AlMeEt2
Me
Me
Al
Me2
Al
Me2
Scheme 4. Plausible steps in aluminate/alkylaluminum exchange. Variable
temperature EXSY experiments implicate an associative exchange of
trialkylaluminum and lanthanide bis(aluminate), via proposed intermediate A.
Repeating similar EXSY experiments at several
temperatures (298-333 K) provides activation parameters ΔS‡ (–
43±1 cal·mol–1·K–1) and ΔH‡ (4.1±0.4 kcal·mol–1; Figure S75) for
ethyl group exchange between [Ln](AlR3Et) and R2AlEt. The
relatively large and negative activation entropy for alkyl
1
confirmed by crosspeaks in a 2D H EXSY experiment between
ethyl groups (methylene and methyl) in the two moieties.
The rate of chemical exchange occurring in the mixture
obtained from reaction of 1Y and one equiv. of triethylaluminum
in benzene-d6 (kon+koff; kon = 2.08 M–1s–1, corresponding to
exchange at the lanthanide center suggests
associated transition state while ruling out
a
a
strongly
terminal
alkyllanthanide intermediate such as [Ln]–Et. On the basis of
this associative exchange and the readily accessible
monodentate aluminate from DFT calculations, we propose the
mechanism for alkyl group exchange depicted in Scheme 4, with
exchange of methyl and ethyl groups occurring via intermediate
A. For comparison, terminal and bridging methyls in cationic
[Cp2Zr(AlMe4)]+ exchange by dissociation of AlMe3.[57] In a
remarkable contrast with zirconium, [Cp2Hf(AlMe4)]+ and AlMe3
exchange by an associative process.[58]
Addition of 1 equiv. of tBuC≡CH to this mixture at room
temperature does not generate detectable quantities of the
aluminated product tBuC≡CAlEt2 or the alkynylaluminate.[59]
Instead, heating the reaction mixture to 60 °C affords tBuC≡C–
AlEt2 and ethane and reforms 1Y (Scheme 5).
transfer of ethyl from AlEtnMe3–n to [Y](AlMenEt4–n 2
)
and koff
=
3.60 M–1s–1, the reverse process) was extracted from H EXSY
NMR experiments performed with varying mixing times (see
Figure S74).[53] Note that trialkylaluminum species are rapidly
exchanging mixtures of monomers and dimers, with the
1
dimer/monomer equilibrium constants Keq
varying with temperature and alkyl group. For example at 60 °C,
=
[AlR3]2/[Al2R6]
Me
Et
iBu
Keq = 4.75 × 10–7 M, Keq = 8.47 × 10–5 M, and Keq = 20.47
M for AlMe3, AlEt3 and AliBu3, respectively.[54-56] The lowest
energy configuration of trimethyl- and triethylaluminum is the
dimer, but the exchange involves monomers because the [AlR4]
moiety contains only one aluminum, which correspond to half-
order rate dependence on [Al2R6] or first-order dependence on
[AlR3]. The rate of an exchange process that is directly
6
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O
k1k2 [1Y]1[tBuC≡CH]1[AlEt3]1
k1[AlEt3] + k–1 + k2[tBuC≡CH]
Ph
B
– d[tBuC≡CH]
N
=
(4)
Me
Me
dt
Me2Al
AlMe2
N
O
Y
O
Me
Ph
N
B
Me
H
C
C
1Y
The rate constant k2 may be determined by fitting saturation
curves obtained from initial rate measurements. The initial rates
Me2Al
N
Al
Me2
Y
O
AlMemEt4-m
60 °C
AlMenEt4-n
+
C2H6
+
Et2Al
C
C
of product formation were measured over
a series of
experiments in which triethylaluminum concentration was varied.
The initial rate increases as [AlEt3] increases in a nonlinear
manner toward saturation (Figure 4). This data is fit to the
Scheme 5. Selective extrusion of ethane and formation of tBuC≡CAlEt2.
equation d[tBuC≡CAlEt2]/dt
= A[AlEt3]/{[AlEt3] + B} using
nonlinear least-squares regression analysis, where
A
=
It was not possible to determine the rate law for the reaction
k2[1Y][tBuC≡CH] and B = {k–1 + k2[tBuC≡CH]}/k1. From this fit, k2
is calculated to be 0.34 ± 0.03 M–1s–1 at 60 °C. A rough estimate
of k2 can also be calculated by substituting experimental values
for initial rate at a particular concentration into eq 6, which
affords k2 = 0.45 M–1s–1 at 60 °C in good agreement with the
result from the [AlEt3] saturation experiments.
of
Scheme
5
because
the
)
mixed
alkylaluminum
{Me2Al(OxMe2)2PhBCp}Y(AlMenEt4-n 2
is generated in the
presence of excess AlRnMe3–n and the exchange processes
described above complicate NMR integration. The reaction of 1Y
and tBuC≡CH at 60 °C, however, follows a second-order rate
law of equation (1), k = 0.041 M–1s–1:
– d[1Y]
= k[1Y]1[tBuC≡CH]1 (1)
dt
In addition, the initial rates of reaction of tBuC≡CH with 1Y
(alone) or with 1Y in the presence of 10.4 equiv of AlMe3 are
equivalent within error. The rate of metalation is ca. two orders
of
magnitude
slower
than
the
trialkylaluminum/tetraalkylaluminate exchange. Together, the
rapid alkylaluminum/aluminate exchange at room temperature
and the requirement of elevated temperature for alumination and
alkane formation provide compelling evidence that alkyne
metalation is the turnover-limiting step during catalytic
alumination.
The time-dependent concentrations of tBuC≡CH and
triethylaluminum starting materials and the tBuC≡C–AlEt2
1
product were monitored in situ by H NMR spectroscopy for the
alumination reaction catalyzed by 1Y in benzene-d6 heated at
60 °C. Non-linear least squares regression analysis of
[tBuC≡CH] versus time to the equation for a second-order rate
law, namely equation (2), reveals first-order dependence on both
[AlEt3] and [tBuC≡CH], where Δo = [AlEt3]o – [tBuC≡CH]o,
determined prior to heating the reaction mixture.
Figure 4. Plots of initial rate of product formation vs. [AlEt3] concentration for
1Y-catalyzed reactions of tBuC≡CH or tBuC≡CD at 60 °C, showing rate
saturation in [AlEt3]. The curve represents a nonlinear least-squares fit to the
equation d[tBuC≡CAlEt2]/dt = A[AlEt3]/{[AlEt3] + B}, where A = k2[1Y][tBuC≡CH]
and B = (k–1 + k2[tBuC≡CH])/k1. [AlEt3] = 0.194-1.419 M; [tBuC≡CH] = 0.096
M; [1Y] = 0.012 M.
Δo[tBuC≡CH]o
[tBuC≡CH] =
(2)
[AlEt3]oeΔ k t – [tBuC≡CH]o
o
obs
A plot of kobs vs [1Y] is linear (Figure S77), indicating first-
order dependence on catalyst concentration and providing the
third-order experimental rate law of equation (3). Because a
termolecular elementary step is highly unlikely, this rate law is
better interpreted following the Michaelis-Menten-type model,
modified for a ternary (two-substrate) reaction shown in the rate
expression of equation (4), in which the catalyst and first
substrate reversible form an adduct that subsequently reacts
with the second substrate.[60] Thus, the experimental ternary-
order rate constant (k') corresponds to a composite of the
product of forward rate constants divided by the sum of the rates
of individual steps, which are dependent on [AlEt3] and
[tBuC≡CH].
The rates for 1Y-catalyzed alkynylalumination, determined
for the isotopically labelled tBuC≡CD, show the expected
saturation at higher [AlEt3]. Nonlinear least-squares regression
analysis provides k2 (Figure 4), and k2(H)/k2 = 4.1 ± 0.4. This
large primary isotope effect further indicates that C–H bond is
broken during the turnover-limiting step.
(D)
(D)
A catalytic cycle that is consistent with these data involves
a reversible addition of rare earth tetraalkylaluminate catalyst
and
trialkylaluminum
to
form
transient
mixed-alkyl
heterobimetallic A containing four aluminums. Three aluminums
come from Et/Me-exchanged 1Y and one comes from AlR3. This
adduct reacts by an irreversible and turnover-limiting alkyne
metalation at an aluminum ethyl to yield a monoalkynyl
– d[tBuC≡CH]
= k' [1Y]1[tBuC≡CH]1[AlEt3]1 (3)
dt
7
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FULL PAPER
O
B
N
A.
B.
R'2Al
N
R1
R2Al
O
O
B
Ln
k1
R
R
N
R'2Al
R Al
R
R
R
Al R
R
AlR3
N
k–1
O
Ln
Me
R = Et,iBu
Me
Me Al
Me
R1
Me
Me2Al
Al Me
Me
R1
H
O
Me
N
R'2Al
B
N
O
N
R
O
R
R'2Al
R
B
O
B
Ln
Al
N
R
R
N
R
MeH
Al
O
R
R
AlMe3
R'2Al
R
R
R Al
R
R
N
Ln
Al
R
R
R
Al
A
O
CR1
R
C
R Al
R
R
CR1
Ln
Me
R
Me
Me Al
C
Me
Al Me
Me
k2
Me
R1
RH
H
Scheme 6. A. Proposed catalytic cycle for alumination of tBuC≡CH with AlEt3, by ethyl-exchanged 1Y. R may be a methyl group from 1Y, but ethyl groups
preferentially react with alkyne. B. Proposed catalytic cycle for the alumination of R1C≡CH with trimethylaluminum by 1Y.
aluminate species and ethane, which subsequently extrudes
the product (Scheme 6A).
context, we note that the rate law for this C–H bond
alumination contrasts that of carboalumination and
polymerization (in the presence of alkylaluminums). The rates
of the carboalumination processes have inverse dependence
on [AlR3], implying that trialkylaluminum must dissociate prior
to insertion.[62] In addition, as noted above, exchange of
The evidence in support of this mechanism includes (i) a
ternary rate law showing first order-concentration
dependences on 1Y, tBuC≡CH, and AlEt3, (ii) saturation upon
addition of excess AlEt3, and (iii) a large, primary kinetic
isotope effect on k2 in the catalytic C–H (or C–D) bond
cleavage step. In this mechanism, the proposed resting state
is the mixed rare earth bis(tetraalkylaluminate), which
undergoes fast exchange with the trialkylaluminum species
present in the reaction mixture. This exchange process is
associative (Scheme 4), and the intermediate species in this
exchange is also postulated to be the intermediate A that
metalates alkyne during catalytic alkynylalumination. Note
that k1 and kon are identically associated with the formation of
transient adduct A (the precise values of k1 and kon may vary
depending on R); in contrast, koff and k–1 are each associated
with unique processes. koff corresponds to the exchange of
ethyl from [Ln](AlMenEt4-n) and R2AlEt while k–1 involves
dissociation of AlR3 from [Ln](AlMenEt4-n)·AlR3 (A). Because
A is transient, k–1 is much larger than than koff and k1.
Interestingly, the base-catalyzed alumination of alkynes is
fastest at ca. 50 mol % NEt3 and slower at 100 mol %,
implying that the central Me3Al·NEt3 adduct is further
promoted by the presence of AlMe3,[61] and there may be
some mechanistic analogy between the two systems.
bridging
and
terminal
methyls
in
zirconium
tetramethylaluminate is dissociative and suggests a zirconium
methyl intermediate.[57]
A
third mechanism, also ruled out by these data
specifically for reactions of AlEt3, and likely eliminated for
AliBu3, but plausible for AlMe3, involves reaction of terminal
alkyne and bis(tetraalkylaluminate) rather than the adduct A
(ethyl-exchange 1Y and AlR3). Such a mechanism, shown in
Scheme 6B, appears plausible for AlMe3 on the basis of the
observed stoichiometric reaction of 1Y and tBuC≡CH. In fact,
reactions of 1Y and tBuC≡CH produce methane and new
yttrium alkynylaluminate species. The catalytic product
tBuC≡C–AlMe2 is formed only upon addition of AlMe3 to this
yttrium alkynylaluminate; however, this displacement of
tBuC≡C–AlMe2 by AlMe3 is rapid at room temperature.
In the case of the pathway of Scheme 6B when R = Et,
iBu, the resting state would be an alkynylated rare earth
aluminate, and exchange between this species and
{Me2Al(OxMe2)2PhBCp}Y(AlMenEt4-n 2
responsible for catalytic rate law showing
)
would have to be
first-order
a
A few other mechanisms are ruled out by these data.
dependence on [AlEt3]. This idea is somewhat ruled out by
the immediate and irreversible extrusion of tBuC≡CAlEt2 upon
reaction of 1Y, AlEt3, and tBuC≡CH, which indicates that the
pair of species [Y](AlR4) + R2AlC≡CR is not in equilibrium with
[Y](R3AlC≡CR) + AlR3.
Most significantly, the reaction of 1Y and tBuC≡CH is not
kinetically competent for the catalytic reactions involving AlEt3.
The rate constant for metalation of tBuC≡CH by 1Y is 8-11×
smaller than the rate constant (k2) for metalation during 1Y-
First,
comparisons
of
rate
constants
for
alkylaluminum/aluminate exchange and alkyne metalation
establish the sequence as reversible trialkylaluminum
association followed by alkyne metalation rather than
reversible alkyne coordination prior to metalation by
trialkylaluminum.
Second,
mechanisms
involving
alkylaluminum dissociation prior to alkyne metalation are
eliminated by the first-order dependence on [AlR3]. In this
8
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catalyzed reaction of tBuC≡CH and AlEt3 (at 60 °C, saturated
in AlEt3). This difference matches the approximate 10-fold
difference in catalytic rates of alkynyl alumination (–
d[tBuC≡CH]/dt) for AlMe3 and AlEt3, normalized for catalyst,
alkyne, and trialkylaluminum concentrations. One possibility
is that these differences are simply a function of inequivalent
rates of alkyne metalation by aluminum-methyl compared to
aluminum-ethyl species. Unfortunately, we are not able to
measure the experimental rate law or determine k2 for
catalytic metalations using AlMe3 due to overlapping signals
in the broad 1H NMR spectra of reaction mixtures. Instead,
we measured initial rates for the reaction of 1Y and tBuC≡CH,
which are equivalent in the presence or absence of additional
equivalents of AlMe3, indicating zero-order dependence on
[AlMe3]. That is, there is no rate enhancement of metalation
by formation of 1Y·AlMe3 suggesting that this metalation does
not require the excess trimethylaluminum under catalytic
conditions. Thus, the catalyst for metalation is 1Y rather than
1Y·AlMe3, and the rate laws and mechanisms for catalytic
alumination with AlMe3 (Scheme 6B) and AlEt3 (Scheme 6A)
are distinct. Likely, the faster catalytic chemistry of AlEt3 and
AliBu3 is related to the much higher reactivity of
compared to R1C≡CH. We are currently working to apply this
insight and the versatile catalytic preparation of R1C≡CAlR2 to
develop their reactions with electrophiles, in carboalumination,
and in polymerizations. In this context, comparisons with
carboalumination catalysts and mechanism become even
more important to enable the design of new transformations
involving aluminum-carbon bond formations.
Experimental Section
General. All manipulations were carried out under inert conditions,
either using Schlenk techniques or in gloveboxes under a nitrogen
atmosphere, unless stated otherwise. Dry and degassed solvents
were used throughout. Benzene and toluene were sparged with
nitrogen, passed through activated alumina columns, and stored
under nitrogen. Deuterated benzene and toluene were degassed via
three consecutive freeze-pump-thaw cycles, dried over Na/K alloy,
vacuum transferred, and stored over molecular sieves under nitrogen.
[43]
H[PhB(OxMe2)2C5H5],[66] Ln(AlMe4)3 (Ln = Y, La, Nd),[33] and 1Nd
were synthesized according to corresponding literature procedures.
Trimethylaluminum, triethylaluminum, and triisobutylaluminum were
purchased from Aldrich and were used as received. Terminal alkynes
were obtained from Aldrich, dried over anhydrous MgSO4, distilled,
and stored over molecular sieves in the glovebox.
trialkylaluminum adduct
A
in comparison to the
tetraalkylaluminate resting state. Although the proposed
structure of transient A is currently based only on its
kinetically-determined composition, we speculate that its
enhanced reactivity could results from a unique bridging Al–
R–Al moiety not present in 1 or the catalytic resting state.
1H, 13C{1H} and 11B NMR spectra were obtained on a Brucker
Avance III 600 MHz spectrometer. 15N chemical shifts obtained via
15N-1H HMBC experiments, originally referenced to liquid NH3, were
re-referenced to CH3NO2 by subtracting 381.9 ppm. 89Y chemical
shifts were obtained via 89Y{1H}-1H HSQC experiments and were
referenced internally to tetramethylsilane (TMS) to give the 1H NMR
frequency, and the yttrium chemical shift is calibrated on the basis of
the known relationship of 1H to 89Y frequencies.
Conclusion
{Me2Al(OxMe2)2PhBCp}Y(AlMe4)2 (1Y). Toluene was added to
a
mixture of H[PhB(OxMe2)2(C5H5)] (0.150 g, 0.428 mmol) and Y(AlMe4)3
(0.150 g, 0.428 mol), leading to immediate evolution of methane and
a yellow solution. The solution was stirred vigorously for 45 min. All
volatiles were removed in vacuo, and the solid residue was washed
with pentane and dried to afford {Me2Al(OxMe2)2PhBCp}Y(AlMe4)2 as a
highly air- and moisture-sensitive pale yellow powder (0.177 g, 0.266
mmol, 62%). The room temperature spectra of isolated materials are
broad. Therefore, NMR data is given for in-situ generated 1Y. 1H NMR
(benzene-d6): δ 7.40 (vq, J = 8.5 Hz, 2 H, m-C6H5), 7.32 (vt, J = 7.3
The catalytic properties of compounds 1-3Ln contrast the
typical behavior of transition metals in reactions of
alkylaluminums and unsaturated organic compounds, which
generally result in formal insertion into aluminum-carbon
bonds. Carboalumination, the Aufbaureaktion,[63] and olefin
polymerization (especially coordinative chain transfer
polymerization)[64] all involve generation of transition-metal
alkyls from interaction with alkylaluminum species, a series of
one or many alkene insertions, followed by exchange with a
new alkyl aluminum. The highly electrophilic lanthanide
catalysts here do not access coordinatively unsaturated
species, and metalation is observed rather than insertion.
Because aluminum alkyls are known to react via three
pathways with alkynes (carboalumination, hydroalumination,
Hz, 2 H, o-C6H5), 7.18 (vt, J = 6.6 Hz, 1 H, p-C6H5,), 6.33 (br s, 2 H
2
C5H4), 6.21 (br s,
2
H, C5H4), 3.75 (d, JHH
=
8.6 Hz,
2
H,
2
CNCMe2CH2O), 3.30 (d, JHH = 8.6 Hz, 2 H, CNCMe2CH2O), 0.92 (s,
6 H, CNCMe2CH2O), 0.82 (s, 6 H, CNCMe2CH2O), –0.24 (br s, 24 H,
AlMe4), –0.45 (s, 3 H, AlMe2), –0.69 (s, 3 H, AlMe2). 13C{1H} NMR
(benzene-d6, 150 MHz): δ 133.51 (m-C6H5), 133.19 (ipso-C5H4),
128.92 (o-C6H5), 127.48 (p-C6H5), 121.76 (C5H4), 117.11 (C5H4),
81.41 (CNCMe2CH2O), 66.64 (CNCMe2CH2O), 27.92 (CNCMe2CH2O),
25.99 (CNCMe2CH2O), 1.53 (br, AlMe4), –6.01 (AlMe2), –6.80 (AlMe2).
11B NMR (benzene-d6, 192 MHz): δ –16.7. 15N{1H} NMR (benzene-d6,
60 MHz): δ –177.7 (CNCMe2CH2O). 89Y{1H } NMR (benzene-d6, 29
MHz): δ 394. IR (KBr, cm–1): 3069 w, 3047 w, 2926 s, 2827 m, 1594
m (C=N), 1560 m (C=N),1465 m, 1432 w, 1394 w, 1373 m, 1291 w,
1262 m, 1192 s, 1052 s, 974 m, 804 m, 697 s, 518 w, 419 w, 397 w..
Anal. Calcd for C31H55BN2O2Al3Y: C, 55.71; H, 8.23; N, 4.19. Found:
C, 55.93; H, 7.94; N, 4.01. Mp: 93-96 °C, dec.
and
C–H
metalation),
the
selective
C–H
bond
functionalization here represents an important breakthrough.
Moreover, given the propensity of multimetallic species to
mediate metalations of nonpolar C–H bonds,[65] the present
chemistry with rare earth aluminates suggests strategies for
catalytic alumination of less polar C–H bonds.
The products of this catalysis, monoalkynylaluminums,
are now readily accessible for higher alkylaluminums. The
alkylaluminum starting materials are directly synthesized from
aluminum, hydrogen, and alkenes. Thus, this catalytic
development may facilitate the applications of R1C≡C–AlR2 in
new chemical transformations. Already, spectroscopic
characterization of isolated R1C≡CAlR2 reveal an increase in
the allowedness of νC≡C and a generally smaller HOMO-
LUMO gap of R1C≡C–AlR2 for alkynylated C≡C bonds
{Me2Al(OxMe2)2PhBCp}La(AlMe4)2 (1La). Toluene was added to a
mixture of H[PhB(OxMe2)2C5H5] (0.150 g, 0.429 mmol) and La(AlMe4)3
(0.172 g, 0.429 mmol), leading to immediate evolution of methane
and a yellow solution. The solution was stirred vigorously for 45 min.
All volatiles were removed in vacuo, and the solid residue was
washed
{Me2Al(OxMe2)2PhBCp}La(AlMe4)2 as
sensitive yellow powder (0.228 g, 0.317 mmol, 74%). NMR data for
with
pentane
and
dried
to
afford
a
highly air and moisture
9
This article is protected by copyright. All rights reserved.

10.1002/chem.202000325
Chemistry - A European Journal
FULL PAPER
the in-situ generated compound is presented; isolated material gives
spectra with identical chemical shifts but broad signals. 1H NMR
(benzene-d6): δ 7.39 (m, 2 H, m-C6H5), 7.36 (vt, J = 7.0 Hz, 2 H, o-
C6H5), 7.20 (vt, J = 6.3 Hz, 1 H, p-C6H5), 6.38 (br s, 2 H C5H4), 6.25
(br s, 2 H, C5H4), 3.56 (d, 2JHH = 8.4 Hz, 2 H, CNCMe2CH2O), 3.31 (d,
2JHH = 8.4 Hz, 2 H, CNCMe2CH2O), 0.89 (s, 6 H, CNCMe2CH2O), 0.82
(s, 6 H, CNCMe2CH2O), –0.19 (br s, 24 H, AlMe4), –0.43 (s, 3 H,
AlMe2), –0.77 (s, 3 H, AlMe2). 13C{1H} NMR (benzene-d6, 150 MHz): δ
133.30 (m-C6H5), 128.92 (o-C6H5), 127.44 (p-C6H5), 124.41 (C5H4),
119.13 (C5H4), 80.96 (CNCMe2CH2O), 70.48 (CNCMe2CH2O), 66.75
(CNCMe2CH2O), 27.96 (CNCMe2CH2O), 26.04 (CNCMe2CH2O), 2.55
(AlMe4), –6.09 (AlMe2), –6.99 (AlMe2). 11B NMR (benzene-d6, 192
MHz): δ –16.57. 15N{1H} NMR (benzene-d6, 60 MHz): δ –177.9
(CNCMe2CH2O). IR (KBr, cm–1): 3070 w, 3049 w, 2969 m, 2929 s,
2827 w, 1594 s (C=N), 1557 m (C=N), 1489 w, 1464 m, 1433 w, 1394
w, 1373 m, 1291 m, 1260 m, 1197 s, 1052 m, 1037 m, 970 m, 939 w,
889 w, 845 w, 775 w, 698 s, 626 w, 579 m, 520 w, 419 w. Anal. Calcd
for C31H55BN2O2Al3La: C, 51.83; H, 7.66; N, 3.90. Found: C, 49.67; H,
7.87; N, 3.78. Mp: 95-100 °C, dec.
{Et2Al(OxMe2)2PhBCp}Y(AlMe4)2 (2Y). In the glovebox, (AlEt3)2 (39.2
µL, 0.143 mmol) was added via syringe to a toluene solution (5 mL) of
H[PhB(OxMe2)C5H5] (0.100 g, 0.286 mmol). This solution was stirred
for 15 min. A toluene solution (5 mL) of Y(AlMe4)3 (0.100 g, 0.286
mmol) was slowly added to the first solution. The resulting solution
was stirred vigorously for 6 h, and then all volatiles were removed in
vacuo. The residue was washed with pentane and dried to obtain
{Et2Al(OxMe2)2PhBCp}Y(AlMe4)2 as a pale yellow powder (0.151 g,
0.217 mmol, 76%). 1H NMR (benzene-d6): δ 7.41 (vd, J = 7.3 Hz, 2 H,
δ –179.4 (CNCMe2CH2O). IR (KBr, cm–1): 3072 w, 3047 w, 2929 s,
2729 w, 1594 s (C=N), 1542 m (C=N), 1464 m, 1433 w, 1412 w, 1373
m, 1293 m, 1261 m, 1194 s, 1134 w, 1050 m, 1037 m, 966 s, 933 m,
888 w, 845 m, 794 m, 700 s, 644 m, 591 m, 538 w, 499 w, 419 w.
Anal. Calcd for C33H59BN2O2Al3La: C, 53.10; H, 7.91; N, 3.75. Found:
C, 53.04; H, 8.46; N, 3.56. Mp: 97-100 °C, dec.
{Et2Al(OxMe2)2PhBCp}Nd(AlMe4)2 (2Nd). (AlEt3)2 (39.2 µL, 0.143
mmol) and H[PhB(OxMe2)C5H5] (0.100 g, 0.286 mmol) were mixed in
toluene (5 mL) and allowed to stir for 15 min. A toluene solution (5
mL) of Nd(AlMe4)3 (0.116 g, 0.286 mmol) was slowly added to the
reaction mixture, and the resulting solution was stirred vigorously for 6
h. All volatiles were removed in vacuo, and the residue was washed
with pentane and dried to obtain {Et2Al(OxMe2)2PhBCp}La(AlMe4)2 as
a pale green powder (0.185 g, 0.246 mmol, 86%). 1H NMR (benzene-
d6): δ 13.15 (br s, 2 H, C5H4), 6.46 (br s, 24 H, AlMe4), 4.48 (s, 1 H, p-
C6H5), 3.68 (br s, 2 H, C5H4), 1.34 (br s, 2 H, CNCMe2CH2O), 0.05 (br
s, 3 H, AlCH2CH3), –0.08 (s, 2 H, C6H5), –0.26 (s, 2 H, C6H5), –1.67
(br s, 2 H, AlCH2CH3), –1.75 (s, 6 H, CNCMe2CH2O), –1.88 (s, 6 H,
CNCMe2CH2O), –1.94 (br s, 3 H, AlCH2CH3), –2.84 (br s, 2 H,
CNCMe2CH2O), –3.09 (s, 2 H, AlCH2CH3). 13C{1H} NMR (benzene-d6,
150 MHz): δ 125.43 (p-C6H5), 124.18 (C6H5), 123.29 (C6H5), 67.92
(CNCMe2CH2O), 63.71 (CNCMe2CH2O), 23.96 (CNCMe2CH2O),
22.95 (CNCMe2CH2O), 8.51 (AlCH2CH3), 6.76 (AlCH2CH3), 0.71 (br,
AlCH2CH3), –1.33 (AlCH2CH3). 11B NMR (benzene-d6, 192 MHz): δ –
40.0. IR (KBr, cm–1): 3071 w, 3050 w, 2929 s, 2868 s, 1595 s (C=N),
1539 m (C=N), 1492 w, 1463 m, 1434 w, 1411 w, 1373 m, 1291 m,
1261 m, 1194 s, 1132 w, 1053 m, 1038 m, 966 s, 936 w, 888 w, 869
w, 844 w, 802 m, 775 w, 703 s, 644 m, 592 m, 572 m, 499 w, 419 w,
398 w, 384 w. Anal. Calcd for C33H59BN2O2Al3Nd: C, 52.73; H, 7.85;
N, 3.72. Found: C, 53.49; H, 8.35; N, 3.26. Mp: 110-115 °C, dec.
{iBu2Al(OxMe2)2PhBCp}Y(AlMe4)2 (3Y). AliBu3 (72.2 µL, 0.286 mmol)
and H[PhB(OxMe2)C5H5] (0.100 g, 0.286 mmol) were mixed in toluene
(5 mL) and stirred for 15 min. A toluene solution (5 mL) of Y(AlMe4)3
(0.100 g, 0.286 mmol) was slowly added to this solution, and the
resulting mixture was stirred vigorously for 6 h. All volatiles were
removed in vacuo to obtain {iBu2Al(OxMe2)2PhBCp}Y(AlMe4)2 as a
pale yellow solid (0.112 g, 0.149 mmol, 52%). 1H NMR (benzene-d6):
δ 7.39 (vt, J = 7.4 Hz, 2 H, o-C6H5), 7.32 (vt, J = 7.4 Hz, 2 H, m-C6H5),
7.19 (vt, J = 7.3 Hz, 1 H, p-C6H5), 6.31 (br s, 2 H, C5H4), 6.11 (br s, 2
m-C6H5), 7.32 (vt, J = 7.5 Hz, 2 H, o-C6H5), 7.18 (vt, J = 7.5 Hz, 1 H,
2
p-C6H5), 6.32 (br s, 2 H, C5H), 6.19 (br s, 2 H, C5H4), 3.75 (d, JHH
=
2
8.6 Hz,
2
H, CNCMe2CH2O), 3.31 (d, JHH
=
8.6 Hz,
2
H,
3
CNCMe2CH2O), 1.24 (t, JHH = 8.0 Hz, 3 H, AlCH2CH3), 0.97 (s, 6 H,
CNCMe2CH2O), 0.95 (t, JHH = 8.0 Hz, 3 H, AlCH2CH3), 0.88 (s, 6 H,
CNCMe2CH2O), 0.12 (q, JHH = 8.0 Hz, 2 H, AlCH2CH3), –0.10 (q,
3
3
3JHH = 8.0 Hz, 2 H, AlCH2CH3), –0.24 (br s, 24 H, AlMe4). 13C{1H}
NMR (benzene-d6, 150 MHz): δ 133.55 (m-C6H5), 133.15 (ipso-C5H4),
127.46 (o-C6H5), 127.21 (p-C6H5), 121.63 (C5H4), 117.11 (C5H4),
81.26 (CNCMe2CH2O), 66.84 (CNCMe2CH2O), 27.56 (CNCMe2CH2O),
26.02 (CNCMe2CH2O), 9.90 (AlCH2CH3), 9.75 (AlCH2CH3), 2.61 (br,
AlCH2CH3), 2.14 (br, AlCH2CH3), 1.57 (br, AlMe4). 11B NMR
(benzene-d6, 192 MHz): δ –16.71. 15N{1H} NMR (benzene-d6, 60
MHz): δ –179.1 (CNCMe2CH2O). 89Y{1H} NMR (benzene-d6, 29 MHz):
δ 395. IR (KBr, cm–1): 3071 w, 3049 w, 2932 s, 2927 s, 2867 s, 1591
s (C=N), 1552 m (C=N), 1464 m, 1433 w, 1411 w, 1374 m, 1292 m,
1260 m, 1195 s, 1052 m, 1038 s, 987 m, 970 s, 891 w, 845 w, 794 m,
702 s, 643 m, 593 w, 466 w. Anal. Calcd for C33H59BN2O2Al3Y: C,
56.92; H, 8.48; N, 4.02. Found: C, 57.32; H, 8.64; N, 3.99. Mp: 102-
105 °C, dec.
2
2
H, C5H4), 3.80 (d, JHH = 8.6 Hz, 2 H, CNCMe2CH2O), 3.26 (d, JHH
=
3
8.6 Hz, 2 H, CNCMe2CH2O), 1.95 (sept, JHH = 6.5 Hz, 1 H,
AlCH2CHMe2), 1.58 (sept, 3JHH = 6.5 Hz, 1 H, AlCH2CHMe2), 1.17 (d,
3JHH = 6.5 Hz, 6 H, AlCH2CHMe2), 0.99 (s, 6 H, CNCMe2CH2O), 0.97
3
(s, 6 H, CNCMe2CH2O), 0.77 (d, JHH = 6.4 Hz, 6 H, AlCH2CHMe2),
3
2
0.18 (d, JHH = 7.0 Hz, 2 H, AlCH2CHMe2), –0.04 (d, JHH = 6.7 Hz, 2
H, AlCH2CHMe2), –0.24 (br s, 24 H, AlMe4). 13C{1H} NMR (benzene-
d6, 150 MHz): 133.72 (m-C6H5), 133.27 (ipso-C5H4), 127.38 (o-C6H5),
127.16
(p-C6H5),
121.50
(C5H4),
117.07
(C5H4),
81.24
{Et2Al(OxMe2)2PhBCp}La(AlMe4)2 (2La). (AlEt3)2 (39.2 µL, 0.143
(CNCMe2CH2O), 66.91 (CNCMe2CH2O), 28.94 (AlCH2CHMe2), 28.71
(AlCH2CHMe2), 27.99 (AlCH2CHMe2), 27.05 (AlCH2CHMe2), 27.00
(CNCMe2CH2O), 26.29 (AlCH2CHMe2), 26.20 (CNCMe2CH2O), 25.42
(AlCH2CHMe2), 24.89 (AlCH2CHMe2), 24.70 (AlCH2CHMe2), 1.75
(AlMe4). 11B NMR (benzene-d6, 192 MHz): δ –16.74. 15N{1H} NMR
(benzene-d6, 60 MHz): δ –177.0 (CNCMe2CH2O). 89Y{1H } NMR
(benzene-d6, 29 MHz): δ 393. IR (KBr, cm–1): 3071 w, 3048 w, 2948 s,
2886 s, 1588 m (C=N), 1551 m (C=N), 1534 m, 1464 m, 1433 w,
1397 w, 1374 m, 1362 m, 1317 w, 1293 m, 1258 w, 1194 s, 1137 w,
1052 s, 1038 s, 970 m, 889 w, 844 w, 797 m, 698 s, 593 m, 523 w,
439 w. Anal. Calcd for C37H67BN2O2Al3Y: C, 59.06; H, 8.91; N, 3.72.
Found: C, 58.26; H, 8.82; N, 3.59. Mp: 93-95 °C, dec.
mmol) was added via syringe to
a toluene solution (5 mL) of
H[PhB(OxMe2)C5H5] (0.100 g, 0.286 mmol), and the solution was
stirred for 15 min. A toluene solution (5 mL) of La(AlMe4)3 (0.115 g,
0.286 mmol) was slowly added to the first solution. The resulting
solution was stirred vigorously for 6 h, and then all volatiles were
removed in vacuo. The solid residue was washed with pentane and
dried to obtain {Et2Al(OxMe2)2PhBCp}La(AlMe4)2 as a yellow powder
(0.158 g, 0.212 mmol, 74%). 1H NMR (benzene-d6): δ 7.41 (vd, J =
7.1 Hz, 2 H, o-C6H5), 7.37 (t, J = 7.4 Hz, 2 H, m-C6H5), 7.20 (t, J = 7.4
Hz, 1 H, p-C6H5), 6.37 (br s, 2 H, C5H4), 6.24 (br s, 2 H, C5H4), 3.56 (d,
2
2JHH = 8.4 Hz, 2 H, CNCMe2CH2O), 3.30 (d, JHH = 8.4 Hz, 2 H,
3
CNCMe2CH2O), 1.23 (t, JHH = 8.1 Hz, 3 H, AlCH2CH3), 0.93 (s, 6 H,
{iBu2Al(OxMe2)2PhBCp}La(AlMe4)2 (3La). AliBu3 (72.2 µL, 0.286
CNCMe2CH2O), 0.87 (m, 9 H, CNCMe2CH2O, AlCH2CH3), 0.12 (q,
3JHH = 8.1 Hz, 2 H, AlCH2CH3), –0.19 (br m, 26 H, AlCH2CH3, AlMe4).
13C{1H} NMR (benzene-d6, 150 MHz): δ 133.21 (o-C6H5), 128.68 (m-
C6H5), 127.47 (p-C6H5), 124.26 (C5H4), 119.16 (C5H4), 80.79
(CNCMe2CH2O), 66.94 (CNCMe2CH2O), 27.53 (CNCMe2CH2O),
26.01 (CNCMe2CH2O), 9.87 (AlCH2CH3), 9.60 (AlCH2CH3), 2.83 (br,
AlMe4), 2.55 (br, AlCH2CH3), 2.05 (br, AlCH2CH3). 11B NMR
(benzene-d6, 192 MHz): δ –16.7. 15N{1H} NMR (benzene-d6, 60 MHz):
mmol) was added via syringe to a toluene solution (5 mL) of
H[PhB(OxMe2)C5H5] (0.100 g, 0.286 mmol), and the reaction mixture
was stirred for 15 min before a toluene solution (5 mL) of La(AlMe4)3
(0.115 g, 0.286 mmol) was slowly added. The resulting solution was
stirred vigorously for 6 h, and all volatiles were removed in vacuo, and
the residue was washed with pentane and dried to obtain to obtain
{iBu2Al(OxMe2)2PhBCp}La(AlMe4)2 as a pale yellow solid (0.167 g,
0.209 mmol, 73%). 1H NMR (benzene-d6): δ 7.40-7.35 (m, 4 H, C6H5),
10
This article is protected by copyright. All rights reserved.

10.1002/chem.202000325
Chemistry - A European Journal
FULL PAPER
7.20 (vt, J = 6.9 Hz, 1 H, p-C6H5), 6.36 (br s, 2 H, C5H4), 6.18 (br s, 2
63 μmol) were allowed to react for 6 h at 60 °C. The product was
2
2
H, C5H4), 3.59 (d, JHH = 7.9 Hz, 2 H, CNCMe2CH2O), 3.26 (d, JHH
=
isolated per the aforementioned procedure (0.254 g, 1.83 mmol, 87%).
3
7.9 Hz, 2 H, CNCMe2CH2O), 1.94 (m, 1 H, AlCH2CHMe2), 1.49 (m, 1
1H NMR (benzene-d6):
δ
1.79 (t, JHH
=
6.8 Hz,
2
H,
3
H, AlCH2CHMe2), 1.16 (d, JHH = 6.3 Hz, 6 H, AlCH2CHMe2), 0.97 (s,
AlC≡CCH2CH2CH2CH3), 1.12 (m, 4 H, AlC≡CCH2CH2CH2CH3), 0.66 (t,
3JHH = 7.2 Hz, 3 H, AlC≡CCH2CH2CH2CH3), –0.11 (br s, 6 H, AlMe2).
6 H, CNCMe2CH2O), 0.94 (s, 6 H, CNCMe2CH2O), 0.72 (d, 3JHH = 6.3
3
Hz, 6 H, AlCH2CHMe2), 0.17 (d, JHH = 6.8 Hz, 2 H, AlCH2CHMe2), –
13C{1H}
NMR
(benzene-d6,
150
MHz):
δ
139.97
3
0.05 (d, JHH = 6.8 Hz, 2 H, AlCH2CHMe2), –0.17 (br s, 24 H, AlMe4).
(AlC≡CCH2CH2CH2CH3), 90.09 (AlC≡CCH2CH2CH2CH3), 29.77
(AlC≡CCH2CH2CH2CH3), 23.36 (AlC≡CCH2CH2CH2CH3), 21.06
(AlC≡CCH2CH2CH2CH3), AlC≡CCH2CH2CH2CH3), –6.40 (br, AlMe2).
IR (KBr, cm–1): 3335 w, 3327 w, 3016 w, 2961 s, 2937 s, 2893 m,
2875 m, 2825 w, 2141 w, 2094 s (C≡C), 1593 w, 1465 m, 1427 w,
1381 w, 1322 w, 1303 w, 1250 w, 1190 s, 1106 w, 1057 w, 1011 w,
986 w, 939 w, 845 w, 796 w, 695 s, 634 m, 569 m, 462 w, 438 w. Anal.
Calcd for C8H15Al: Al, 19.53. Found: Al, 19.66.
13C{1H} NMR (benzene-d6, 150 MHz): δ 133.21 (p-C6H5), 129.67
(ipso-C6H5), 128.89 (C6H5), 127.42 (C6H5), 126.03 (C5H4), 124.17
(C5H4), 119.10 (C5H4), 80.76 (CNCMe2CH2O), 70.48 (CNCMe2CH2O),
66.95 (CNCMe2CH2O), 29.08 (AlCH2CHMe2), 28.94 (CNCMe2CH2O),
28.70 (CNCMe2CH2O), 28.10 (AlCH2CHMe2), 27.90 (AlCH2CHMe2),
27.03 (CNCMe2CH2O), 26.14 (CNCMe2CH2O), 25.42 (AlCH2CHMe2),
3.13 (AlMe4) 11B NMR (benzene-d6, 192 MHz): δ –16.64. 15N{1H}
NMR (benzene-d6, 60 MHz): δ –175.1 (CNCMe2CH2O). IR (KBr, cm–
1): 3071 w, 3049 w, 2948 s, 2922 s, 2866 s, 2864 s, 1591 s (C=N),
1535 m, 1463 m, 1374 m, 1362 w, 1317 w, 1292 w, 1258 w, 1185 s,
1132 w, 1052 m, 1037 w, 967 s, 938 w, 887 w, 842 w, 816 w, 797 w,
776 w, 702 s, 592 m, 441 w. Anal. Calcd for C37H67BN2O2Al3La: C,
55.38; H, 8.35; N, 3.49. Found: C, 55.01; H, 7.95; N, 3.52. Mp: 105-
110 °C, dec.
Dimethyl(2-phenylethynyl)aluminum. This compound was previously
characterized
by
analysis
of
hydrolysis
products.[25]
Trimethylaluminum (200 μL, 2.10 mmol), phenylacetylene (230 µL,
2.10 mmol), and 1Y (0.0420 g, 63 µmol) were allowed to react for 6 h
at 60 °C. The product was isolated per the aforementioned procedure
3
(0.321 g, 2.03 mmol, 97%). 1H NMR (benzene-d6): δ 7.32 (d, JHH
=
3
7.5 Hz, 2 H, o-C6H5), 6.90 (vt, JHH = 7.5 Hz, 1 H, p-C6H5), 6.79 (vt,
3JHH = 7.7 Hz, 2 H, m-C6H5), 0.03 (br s, 6 H, AlMe2). 13C{1H} NMR
(benzene-d6, 150 MHz): δ 134.15 (o-C6H5), 131.92 (p-C6H5), 129.17
(m-C6H5), 120.26 (ipso-C6H5), 116.56 (PhC≡CAlMe2), 97.12
(PhC≡CAlMe2), –6.45 (br, AlMe2). IR (KBr, cm–1): 3078 w, 3058 w,
3031 w, 3018 w, 2933 m, 2891 w, 2851 w, 2822 w, 2128 m, 2073 s
(C≡C), 1595 w, 1572 w, 1486 m, 1443 m, 1204 m, 1069 w, 1026 m,
976 m, 917 w, 812 m, 800 m, 783 m, 758 s, 689 s, 577 w, 550 m, 536
m, 420 m, 397 w, 387 w. Anal. Calcd for C10H11Al: Al, 17.06. Found:
Al, 17.27.
{iBu2Al(OxMe2)2PhBCp}Nd(AlMe4)2 (3Nd). AliBu3 (72.2 µL, 0.286
mmol) was added via syringe to
a toluene solution (5 mL) of
H[PhB(OxMe2)C5H5] (0.100 g, 0.286 mmol), and the reaction mixture
was stirred for 15 min before a toluene solution (5 mL) of Nd(AlMe4)3
(0.116 g, 0.286 mmol) was slowly added. The resulting solution was
stirred vigorously for 6 h. All volatiles were removed in vacuo, and the
residue was washed with pentane and dried to obtain to obtain
{iBu2Al(OxMe2)2PhBCp}Nd(AlMe4)2 as a light green solid (0.162 g,
0.200 mmol, 70%). 1H NMR (benzene-d6): δ 13.49 (br s, 2 H, C5H4),
10.39 (br s, 2 H, C5H4), 6.52 (br s, 24 H, AlMe4), 4.28 (s, 1 H, p-C6H5),
3.71 (s, 2 H, C6H5), 2.26 (s, 2 H, AlCH2CHMe2), 1.34 (br s, 2 H,
CNCMe2CH2O), 0.44 (br s, 1 H, AlCH2CHMe2), 0.17-0.16 (m, 6 H,
AlCH2CHMe2), –0.48 (br s, 2 H, CNCMe2CH2O), -1.48 (s, 2 H, C6H5),
–1.56 (br m, 18 H, CNCMe2CH2O, AlCH2CHMe2), –2.54 (br s, 1 H,
AlCH2CHMe2), –3.05 (s, 2 H, AlCH2CHMe2). 13C{1H} NMR (benzene-
d6, 150 MHz): δ 124.62 (p-C6H5), 123.78 (C6H5), 122.88 (C6H5), 68.02
(CNCMe2CH2O), 63.91 (CNCMe2CH2O), 31.08 (AlCH2CHMe2), 27.91
(AlCH2CHMe2), 26.27 (CNCMe2CH2O), 25.65 (CNCMe2CH2O), 25.24
(AlCH2CHMe2), 24.58 (AlCH2CHMe2), 23.05 (AlCH2CHMe2), 22.98
(AlCH2CHMe2), 22.52 (AlCH2CHMe2), 21.50 (AlCH2CHMe2). 11B NMR
(benzene-d6, 192 MHz): δ –40.12. IR (KBr, cm–1): 3072 w, 3049 w,
2948 s, 2887 m, 2863 m, 1593 m (C=N), 1555 m, 1463 m, 1432 w,
1374 m, 1361 m, 1317 w, 1292 m, 1257 w, 1196 s, 1161 m, 1070 m,
1053 m, 1037 m, 968 m, 888 w, 842 w, 801 w, 773 w, 702 s, 668 s.
Anal. Calcd for C37H67BN2O2Al3Nd: C, 55.01; H, 8.30; N, 3.46. Found:
C, 55.02; H, 8.83; N, 3.46. Mp: 110-115 °C, dec.
Dimethyl(p-tolylethynyl)aluminum. The compound was previously
69]
generated and used in situ.[68,
Trimethylaluminum (100 μL, 1.05
mmol), 4-ethynyltoluene (132.5 µL, 1.05 mmol), and 1Y (0.0210 g,
31.5 µmol) were allowed to react for 6 h at 60 °C. The product was
isolated per the aforementioned procedure (0.165 g, 0.95 mmol, 91%).
3
1H NMR (benzene-d6): δ 7.30 (d, JHH = 8.0 Hz, 2 H, C6H4), 6.64 (d,
3JHH = 8.0 Hz, 2 H, C6H4), 1.87 (s, 3 H, 4-MeC6H4), 0.05 (br s, 6 H,
AlMe2). 13C{1H} NMR (benzene-d6, 150 MHz): δ 142.84 (MeCPhipso),
136.11 (C≡CCPhipso), 134.36 (3’-C6H4), 130.04 (2’-C6H4), 117.33
(C≡CAlMe2), 96.76 (C≡CAlMe2), 21.82 (Me-C6H4), –6.43 (AlMe2). IR
(KBr, cm–1): 3031 w, 2935 m, 2891 w, 2853 w, 2821 w, 2127 w, 2079
s (C≡C), 1913 w, 1651 w, 1603 m, 1505 m, 1459 w, 1179 s, 1105 w,
1039 w, 1019 w, 980 m, 951 w, 816 s, 732 s, 694 s, 569 m, 548 m,
535 w, 494 w. Anal. Calcd for C11H13Al: Al, 15.67. Found: Al, 15.86.
Dimethyl(2-cyclohexylmethylethynyl)aluminum.
Trimethylaluminum
(20 μL, 0.21 mmol), 3-cyclohexyl-1-propyne (30.2 µL, 0.21 mmol),
and 1Y (0.0042 g, 6.3 µmol) were allowed to react for 6 h at 60 °C.
The product was isolated per the aforementioned procedure (0.0360 g,
General Procedure for Catalytic Aluminations. Caution:
Trialkylaluminum and alkynyldialkylaluminum compounds are
pyrophoric liquids and must be handled under inert atmospheres. The
terminal alkyne and trialkylaluminum reactants were added via
syringe to a solution of the catalyst (3 mol %) in benzene-d6 at room
temperature, and the reaction mixture was placed in a Teflon-sealed J
Young-style NMR tube. The NMR tube was heated at 60 °C.
Conversion of the starting materials was monitored by 1H NMR
spectroscopy. Following full conversion, the volatile materials were
removed in vacuo. Anhydrous n-pentane (2 mL) was added to the
crude mixture of product and catalyst, and the resulting suspension
was stirred for 10 min. The suspension was filtered, pentane was
removed at reduced pressure, and the product dried under vacuum
for 12 h. The isolated alkynylaluminum products were characterized
by 1H, 13C{1H} and FT-IR spectroscopy. Elemental analysis was
performed using inductively coupled plasma-optical electronic
spectroscopy (ICP-OES) to determine Al weight %. CHNS analysis
did not provide consistent % C and % H values, likely as a result of
the air sensitivity of organoaluminum compounds.
3
0.20 mmol, 95%). 1H NMR (benzene-d6): δ 1.79 (d, JHH = 6.70 Hz, 2
H, CyCH2C≡CAlMe2), 1.54 (m,
4
H, (-CH2CH2CH2CH2CH2CH-
H, (-CH2CH2CH2CH2CH2CH-
H, (-CH2CH2CH2CH2CH2CH-
)CH2C≡CAlMe2), 1.22 (m,
)CH2C≡CAlMe2), 1.01 (m,
1
4
)CH2C≡CAlMe2),
0.76
(m,
2
H,
(CH2CH2CH2CH2CH2CH-
)CH2C≡CAlMe2), –0.03 (br s, 6 H, AlMe2). 13C{1H} NMR (benzene-d6,
150 MHz): δ 139.79 (CyCH2C≡CAlMe2), 90.61 (CyCH2C≡CAlMe2),
30.88 (-CH2CH2CH2CH2CH2CH-), 36.87 (-CH2CH2CH2CH2CH2CH-),
32.97 (-CH2CH2CH2CH2CH2CH-), 29.13 (CyCH2C≡CAlMe2), 26.47 (-
CH2CH2CH2CH2CH2CH-), –6.25 (AlMe2). IR (KBr, cm–1): 2926 s, 2853
s, 2141 w, 2093 m (C≡C), 1590 w, 1554 w, 1450 m, 1426 w, 1372 w,
1322 w, 1263 w, 1199 m, 1067 w, 1053 w, 1010 w, 986 m, 930 w, 891
w, 843 w, 777 w, 696 s, 576 w, 519 w, 457 w, 444 w, 419 w. Anal.
Calcd for C11H19Al: Al, 15.14. Found: Al, 15.24.
2-(1-Cyclohexen-1-yl)ethynyl)dimethylaluminum. The compound was
previously prepared and used in situ.[70] Trimethylaluminum (20 μL,
0.21 mmol), 1-ethylnyl-1-cyclohexane (24.5 μL, 0.21 mmol), and 1Y
(0.0042 g, 6.3 µmol) were allowed to react for 6 h at 60 °C. The
product was isolated per the aforementioned procedure (0.0272 g,
1-Hexyn-1-yl-dimethylaluminum. This compound was previously
synthesized by salt metathesis and used in situ.[67] Trimethylaluminum
(200 μL, 2.10 mmol), 1-hexyne (240 μL, 2.10 mmol), and 1Y (0.0420 g,
0.17 mmol, 80%). 1H NMR (benzene-d6):
CH2CH2CH2CH2CH=C-)CH2C≡CAlMe2), 1.92
δ
6.24 (m,
(m,
1
H, (-
2
H, (-
11
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FULL PAPER
CH2CH2CH2CH2CH=C-)CH2C≡CAlMe2),
CH2CH2CH2CH2CH=C-)CH2C≡CAlMe2), 1.18-1.07 (m,
1.58
(m,
2
H,
H, (-
CH2CH2CH2CH2CH=C-)CH2C≡CAlMe2), –0.01 (br s, 6 H, AlMe2).
(-
12 H, AlCH2CHMe2), 1.23-1.13 (m, 4 H, CH3CH2CH2CH2C≡C), 0.71 (t,
3JHH = 7.3 Hz, 3 H, CH3CH2CH2CH2C≡C), 0.60 (d, 3JHH = 6.9 Hz, 4 H,
4
AlCH2CHMe2). 13C{1H} NMR (benzene-d6, 150 MHz):
δ
142.59
29.96
13C{1H}
NMR
(benzene-d6, 150
MHz):
(C≡CAlMe2),
(C≡CAlMe2),
δ
145.21
119.92
28.42
(-
(-
(-
(C≡CAlCH2CHMe2), 87.22 (C≡CAlCH2CHMe2),
CH2CH2CH2CH2CH=C-),
CH2CH2CH2CH2CH=C-),
138.12
94.03
(CH3CH2CH2CH2C≡C), 28.69 (AlCH2CHMe2), 27.30 (AlCH2CHMe2),
25.68 (AlCH2CHMe2), 22.38 (CH3CH2CH2CH2C≡C), 21.33
CH2CH2CH2CH2CH=C-), 26.38 (-CH2CH2CH2CH2CH=C-), 22.16 (-
CH2CH2CH2CH2CH=C-), 21.27 (-CH2CH2CH2CH2CH=C-), –6.10
(AlMe2). IR (KBr, cm–1): 3748 w, 3675 w, 3027 w, 2934 s, 2889 m,
2860 m, 2059 s (C≡C), 1618 m (C=C), 1588 w, 1449 m, 1435 m,
1350 w, 1268 w, 1186 m, 1153 w, 1137 w, 1078 w, 1042 w, 967 w,
920 m, 859 m, 847 m, 798 w, 765 w, 693 s, 566 m, 530 m, 480 m,
451 m, 418 m. Anal. Calcd for C10H15Al: Al, 16.63. Found: Al, 16.76.
1-Hexyn-1-yl-diethylaluminum. The compound was previously
prepared and used in situ.[71] Triethylaluminum (30 μL,0.21 mmol), 1-
hexyne (24 μL, 0.21 mmol), and 1Y (0.0042 g, 6.3 μmol) were allowed
(CH3CH2CH2CH2C≡C), 13.76 (CH3CH2CH2CH2C≡C). IR (KBr, cm–1):
2956 s, 2872 m, 2143 w, 2091 w (C≡C), 1466 m, 1397 w, 1365 w,
1321 w, 1182 w, 1064 m, 1023 m, 946 w, 909 w, 851 w, 823 w, 668 m,
457 w, 419 w. Anal. Calcd for C14H27Al: Al, 12.13. Found: Al, 12.29.
1,8-bis(dimethylalumino)-1,7-octadiyne. Trimethylaluminum (40.0 μL,
0.42 mmol), 1,7-octadiyne (27.7 μL, 0.21 mmol), and 2 (0.0042 g, 6.3
μmol) were reacted for 6 h at 60 °C. The product was isolated per the
aforementioned procedure (0.0417 g, 0.19 mmol, 91%). 1H NMR
(benzene-d6): δ 1.64 (m, 4 H, C≡CCH2CH2CH2CH2C≡C), 1.09 (m, 4 H,
C≡CCH2CH2CH2CH2C≡C), –0.11 (AlMe2). 13C{1H} NMR (benzene-d6,
to react for
2
h
at 60 °C. The product was isolated per the
150
MHz):
δ
138.83
(C≡CCH2CH2CH2CH2C≡C),
90.64
aforementioned procedure (0.0328 g, 0.20 mmol, 94%). 1H NMR
(C≡CCH2CH2CH2CH2C≡C), 26.60 (C≡CCH2CH2CH2CH2C≡C), 20.65
(C≡CCH2CH2CH2CH2C≡C), –6.28 (AlMe2). IR (KBr, cm–1): 3331 w,
3016 w, 2937 s, 2891 m, 2867 m, 2822 w, 2095 s (C≡C), 1595 w,
1553 w, 1459 m, 1415 m, 1325 m, 1188 s, 1019 w, 978 w, 922 w, 693
s, 567 m, 458 w, 419 w. Anal. Calcd for C12H20Al2: Al, 24.72. Found:
Al, 24.77.
3
(benzene-d6): δ 1.85 (vt, JH,H = 7.0 Hz, 2 H, C≡CCH2CH2CH2CH3),
3
1.46 (t, JH,H
=
8.1 Hz,
6
H, AlCH2CH3), 1.15 (m,
4
3
H,
H,
C≡CCH2CH2CH2CH3), 0.68 (vt, 3JH,H
=
7.1 Hz,
3
C≡CCH2CH2CH2CH3), 0.51 (q, JH,H = 8.1 Hz, 4 H, AlCH2CH3).
13C{1H} NMR (benzene-d6, 150 MHz): δ 140.91 (C≡CCH2CH2CH2CH3),
85.86 (C≡CCH2CH2CH2CH3), 30.16 (C≡CCH2CH2CH2CH3), 22.35
Description of kinetics experiments
(C≡CCH2CH2CH2CH3),
21.02
(C≡CCH2CH2CH2CH3),
13.74
Stoichiometric reaction of 1Y and tBuC≡CH. A solution (0.5 mL
total volume) of 1Y (0.157 M), tBuC≡CH (0.155 M), and
hexamethylbenzene (0.068 M) standard in C6D6 was prepared. The
NMR probe was pre-heated to 60 °C, and the reaction mixture was
placed in the probe. Single-pulse scans were acquired every 30
seconds. Consumption of starting materials was monitored by
integration of corresponding signals in the 1H NMR spectra, and
concentrations were determined by the proportionality to the standard.
A linear plot of ln{[1Y]/[tBuC≡CH]} vs time provides the second-order
rate constant.
Catalytic rate law determination. A solution (0.5 mL) containing
tBuC≡CH (0.487 M), triethylaluminum (0.731 M), hexamethylbenzene
standard (0.048 M), and 3 mol % 1Y (0.014 M) in C6D6 was prepared
and placed in a J-Young-style NMR tube. The NMR probe was
preheated to 60 °C, and the NMR tube was placed in the probe.
Single-repetition scans were acquired every 30 seconds. The signals
of triethylaluminum and alkyne reagents were monitored over the
reaction, and concentrations were determined by integration relative
to the internal standard. Plots of [tBuC≡CH] vs time fit by nonlinear
least squares regression analysis to obtain second-order rate
constants kobs. Alternatively, plots of ln{[AlEt3]/[tBuC≡CH]} vs time are
(C≡CCH2CH2CH2CH3), 10.08 (AlCH2CH3), 3.42 (AlCH2CH3). IR (KBr,
cm–1): 3326 w, 2959 s, 2935 s, 2900 s, 2862 s, 2790 w, 2722 w, 2142
w, 2090 s (C≡C), 1591 w, 1553 w, 1463 m, 1408 w, 1375 w, 1321 w,
1294 w, 1228 w, 1197 m, 1104 w, 1054 m, 988 w, 949 m, 919 m, 898
w, 845 w, 785 w, 728 w, 654 s, 634 s, 540 w, 456 w, 447 w, 419 w.
Anal. Calcd for C10H19Al: Al, 16.23. Found: Al, 15.99.
Diethyl(2-phenylethynyl)aluminum. The compounds was previously
generated and used in situ.[72] Triethylaluminum (30 μL,0.21 mmol),
phenylacetylene (23 μL, 0.21 mmol), and 1Y (0.0042 g, 6.3 μmol)
were allowed to react for 2 h at 60 °C. The product was isolated per
the aforementioned procedure (0.0372 g, 0.20 mmol, 95%). 1H NMR
3
(benzene-d6): δ 7.40 (m, 2 H, m-C6H5), 6.91 (vt, JHH = 7.5, 1 H, p-
3
3
C6H5), 6.81 (vt, JHH = 7.5, 2 H, o- C6H5), 1.50 (t, JHH = 8.1 Hz, 6 H,
3
AlCH2CH3), 0.64 (q, JHH = 8.1 Hz, 4 H, AlCH2CH3). 13C{1H} NMR
(benzene-d6, 150 MHz): δ 136.43 (PhC≡CAlEt2), 134.34 (m-C6H5),
131.90 (p-C6H5), 129.21 (o-C6H5), 120.39 (ipso-C6H5), 93.33
(PhC≡CAlEt2), 10.17 (AlCH2CH3), 3.63 (AlCH2CH3). IR (KBr, cm–1):
3080 w, 3061 w, 2936 s, 2899 s, 2861 s, 2789 w, 2724 w, 2130 m,
2073 s (C≡C), 1596 w, 1487 m, 1446 m, 1406 w, 1393 w, 1373 w,
1290 w, 1208 m, 1166 w, 1100 m, 1053 m, 1027 m, 988 m, 951 w,
918 m, 898 m, 868 w, 811 m, 798 m, 781 m, 758 s, 689 s, 654 s, 564
m, 537 m, 442 w, 420 w. Anal. Calcd for C12H15Al: Al, 14.49. Found:
Al, 14.63.
(3,3-dimethylbut-1-yn-1-yl)diethylaluminum. The compound was
previously synthesized by salt metathesis.[73] Triethylaluminum (300
μL, 2.10 mmol), 3,3-dimethyl-1-butyne (260 μL, 2.10 mmol), and 1Y
(0.0420 g, 63 μmol) were reacted for 2 h at 60 °C. The product was
isolated per the aforementioned procedure (0.3072 g, 1.85 mmol,
88%). 1H NMR (benzene-d6): δ 1.43 (t, 3JHH = 8.0 Hz, 6 H, AlCH2CH3),
linear and provide equivalent values for kobs
.
Experiments to determine the order in lanthanide pre-catalyst were
conducted using 0.5 mL solutions of tBuC≡CH (0.487 M),
triethylaluminum (0.731 M), hexamethylbenzene (0.048 M) as above,
with a series of concentrations of 1Y (4-13 mM) in C6D6 at 60 °C.
Second-order plots were used to obtain kobs values at the different [1Y]
concentrations.
A
linear correlation confirms the first-order
dependence on the pre-catalyst.
Saturation and isotope effects. tBuC≡CH and nBuLi (2.5 M in
hexanes) were allowed to react at –30 °C in pentane for 1 h, followed
by solvent removal in vacuo to generate tBuC≡CLi. tBuC≡CLi (0.360
g, 4.09 mmol) and D2O (70 µL, 3.88 mmol) were allowed to react in
C6D6 (4 mL). The reaction mixture was filtered and dried over P2O5 to
yield a 0.430 M solution of tBuC≡CD in C6D6, which was then used for
isotope effect studies.
Saturation experiments were performed with solutions containing
tBuC≡CH (0.096 M), 1Y (0.012 M), and hexamethylbenzene (0.127
M), while concentration of triethylaluminum (0.194-1.419 M) was
varied. These mixtures were prepared with equivalent total volumes
(0.5 mL) in C6D6, placed in J-Young type NMR tubes, and inserted
into the preheated NMR probe and allowed to react at 60 °C.
Concentration of starting materials were followed by integration of 1H
NMR signals, and these changes were used to calculate initial
reaction rates (10-15% of conversion). A similar procedure was
1.01 (s, 9 H, CMe3), 0.46 (q, JHH = 8.0 Hz, 4 H, AlCH2CH3). 13C{1H}
NMR (benzene-d6, 150 MHz):
3
δ 149.88 (Me3CC≡CAl), 81.92
(Me3CC≡CAl), 30.13 (Me3CC≡CAl), 30.02 (Me3CC≡CAl), 10.04
(AlCH2CH3), 3.89 (AlCH2CH3). IR (KBr, cm–1): 3327 w, 3317 w, 2974
s, 2936 s, 2900 s, 2863 s, 2790 w, 2723 w, 2128 w, 2077 s (C≡C),
1594 w, 1553 w, 1459 m, 1407 w, 1366 m, 1247 m, 1201 m, 1103 w,
1054 w, 990 m, 951 w, 920 w, 898 w, 789 w, 722 m, 654 s, 633 w,
539 m, 458 m, 436 m. Anal. Calcd for C10H19Al: Al, 16.23. Found: Al,
16.48.
1-Hexyn-1-yl-diisobutylaluminum.[74] Triisobutylaluminum (53.4 μL,
0.21 mmol), 1-hexyne (24 μL, 0.21 mmol), and 2 (0.0042 g, 6.3 μmol)
were reacted for 1 h at 60 °C. The product was isolated per the
aforementioned procedure (0.0425 g, 0.19 mmol, 91%). 1H NMR
3
(benzene-d6): δ 2.25 (v sept, JHH = 6.6 Hz, 2 H, AlCH2CHMe2), 1.90
3
3
(t, JHH = 7.0 Hz, 2 H, CH3CH2CH2CH2C≡C), 1.27 (d, JHH = 6.6 Hz,
12
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10.1002/chem.202000325
Chemistry - A European Journal
FULL PAPER
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K. T. Smith, S. Berritt, M. González-Moreiras, S. Ahn,
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employed for saturation and kinetic isotope effect studies with the
deuterated alkyne tBuC≡CD (0.079 M), 1Y (0.010 M),
hexamethylbenzene (0.084 M) and 0.206-1.208 M triethylaluminum in
0.5 mL C6D6). Saturation curves for protio and deuterated systems
were obtained by plotting normalized initial rates (initial rates divided
by {[1Y][alkyne]}) against triethylaluminum concentrations. The true
[8]
[9]
(H)
(D)
second-order rate constants k2
and k2
were obtained from
[10]
[11]
[12]
nonlinear least-squares fits to the equation d[Et2Al≡CtBu]/dt
A[AlEt3]/{[AlEt3] + B}, where A = k2[1Y][HC≡CtBu] and B = (k–1
k2[HC≡CtBu])/k1.
=
+
EXSY. 1H EXSY experiments were conducted using a solution of 1Y
(0.114 M) and triethylaluminum (0.120 M) in C6D6 (0.5 mL). The total
rate of exchange k' = kon+koff was obtained by conducting experiments
at different mixing times (tm = 200-400 ms) as described by Perrin.[53]
Repeating this procedure at different temperatures (25-60 °C)
provided a series of k' values which enabled the calculation of
activation parameters ΔS‡ and ΔH‡ from an Eyring plot.
[13]
[14]
[15]
[16]
[17]
Computational Methods
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A. C. M. Baroni, C. C. P. Arruda, D. B. Carvalho, in
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Chemistry of Functional Groups (Ed.: Z. Rappoport),
2016.
The ground state geometry optimization and IR frequency calculations
on 1Y-CALC were performed using Truhlar’s Minnesota 06-2X[46] meta-
GGA functional, as implemented in NWChem.[75] The Los Alamos
National Laboratory double-ζ valence basis set (LANL2DZ)[45] was
used along with effective core potentials (ECPs) for Al and Y.
Grimme’s dispersion corrections were empirically added through a
long-range contribution (DFT-D3) for all calculations.[47] Ground state
geometry optimizations and IR frequency calculations on the
[19]
[20]
[21]
alkynylaluminum
compounds
(CyCH2C≡CAlMe2)2-calc,
[22]
[23]
[24]
[25]
[26]
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3316.
frequencies were corrected by the scaling factors of 0.960 (1Y-CALC
)
and 0.961 ((CyCH2C≡CAlMe2)2-calc, (tBuC≡CAlEt2)2-calc) and
(nBuC≡CAlMe2)2-calc. The NMR chemical shift calculations of
alkynylaluminums were performed using the GIAO method,[51] using
the 6-31G** basis set for all atoms and dispersion corrected Becke’s
three-parameter hybrid density functional (B3LYP-D3).[52] The 1H and
13C chemical shifts were calculated by taking the difference of the
isotropic shielding values of the compounds of interest with
tetramethylsilane (TMS), also calculated at the same level of theory.
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2006.
Acknowledgements
The authors gratefully thank the National Science Foundation
(CHE-1900393) for financial support, and K. Basemann and T.
Windus for assistance with DFT calculations, and A.
Venkatesh and A. Rossini for help implementing
heteronuclear 2D NMR.
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Keywords: rare earth • catalysis • CH bond activation •
saturation kinetics • aluminate
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Entry for the Table of Contents
Alkynylaluminum compounds are synthesized directly from terminal alkynes and abundant, inexpensive trialkylaluminum reagents via
catalytic C-H bond functionalization using rare earth aluminate compounds, in contrast to transition-metal complexes which catalyze
carboaluminations. A saturated lanthanide aluminate adduct, implicated by kinetic studies, provides a rationale for C-H bond
alumination occurring rather than addition.
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