Reactions of Trimethylaluminium: Modelling the Chemical Degradation of Synthetic Lubricants

Article abstract of DOI:10.1002/chem.201604553

In investigating and seeking to mimic the reactivity of trimethylaluminium (TMA) with synthetic, ester-based lubricating oils, the reaction of methyl propionate 1 was explored with 1, 2 and 3 equivalents of the organoaluminium reagent. Spectroscopic analysis points to the formation of the adduct 1(TMA) accompanied only by the low level 1:1 production of Me₂AlOCEtMe₂2 and Me₂AlOMe 3 when an equimolar amount of TMA is applied. The deployment of excess TMA favours reaction to give 2 and 3 over 1(TMA) adduct formation and spectroscopy reveals that in hydrocarbon solution substitution product 2 traps unreacted TMA to yield 2(TMA). The¹H NMR spectroscopic observation of two Al?Me signals not attributable to free TMA and in the ratio 1:4 suggests the formation of a previously only postulated, symmetrical metallacycle in Me₄Al₂(μ²-Me)(μ²-OCEtMe₂). In the presence of 3, 2(TMA) undergoes thermally induced exchange to yield Me₄Al₂(μ²-OMe)(μ²-OCEtMe₂) 4 and TMA. The reaction of methyl phenylacetate 5 with TMA allows isolation of the crystalline product Me₂AlOCBnMe₂(TMA) 6(TMA), which allows the first observation of the Me₄Al₂(μ²-Me)(μ²-OR) motif in the solid state. Distances of 2.133(3) ? (Al?Me_bridging) and 1.951 ? (mean Al?Me_terminal) are recorded. The abstraction of TMA from 6(TMA) by the introduction of Et₂O has yielded 6, which exists as a dimer.

Full text of DOI:10.1002/chem.201604553

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Title: Reactions of trimethylaluminium: modelling the chemical
degradation of synthetic lubricants
Authors: Andrew Wheatley, Jonathan Slaughter, and Andrew Peel
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10.1002/chem.201604553
Chemistry - A European Journal
Reactions of trimethylaluminium: modelling the chemical
degradation of synthetic lubricants
Jonathan Slaughter,[a] Andrew J. Peel,[a] and Andrew E. H. Wheatley*[a]
[a]
J. Slaughter, A. J. Peel, Dr. A. E. H. Wheatley
Department of Chemistry, University of Cambridge
Lensfield Road, Cambridge, CB2 1EW (UK)
Fax: (+) 44 1223 336362
E-mail: aehw2@cam.ac.uk
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201604553.
Abstract
In investigating and seeking to mimic the reactivity of trimethylaluminium (TMA) with
synthetic, ester-based luricating oils the reaction of methyl propionate 1 was explored with 1,
2 and 3 eq. the organoaluminium reagent. Spectroscopic analysis points to the formation of
the adduct 1(TMA) accompanied only by the low level 1:1 production of Me2AlOCEtMe2 2
and Me2AlOMe 3 when an equimolar amount of TMA is applied. The deployment of excess
TMA favours reaction to give 2 and 3 over 1(TMA) adduct formation and spectroscopy
reveals that in hydrocarbon solution substitution product 2 traps unreacted TMA to yield
2(TMA). The 1H NMR spectroscopic observation of two AlMe signals not attributable to free
TMA and in the ratio 1:4 suggests the formation of a previously only postulated, symmetrical
metallacycle in Me4Al2(2-Me)(2-OCEtMe2). In the presence of 3, 2(TMA) undergoes
thermally induced exchange to yield Me4Al2(2-OMe)(2-OCEtMe2) 4 and TMA. Reaction of
methyl phenylacetate 5 with TMA allows isolation of the crystalline product
Me2AlOCBnMe2(TMA) 6(TMA), which allows the first observation of the Me4Al2(2-
Me)(2-OR) motif in the solid state. Distances of 2.133(3) Å (Al–Mebridging) and 1.951 Å
(mean Al–Meterminal) are recorded. The abstraction of TMA from 6(TMA) by the introduction
of Et2O has yielded 6, which exists as a dimer.
Introduction
Ideas on the atmospheric reaction of chlorofluorocarbons (CFCs) have existed for more than
40 years[1] and are well documented.[2] While legislation has been implemented aimed at
eliminating their use,[3] the effects of substitute refrigerants such as perfluorocarbons (PFCs)
and hydrofluorocarbons (HFCs) have been the subject of subsequent scrutiny[4] and
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regulation.[5] More specifically, with emissions from (automobile) air conditioning units
representing a growing climate control concern[6] action has been initiated[7] to avoid the use
of refrigerants with a global warming potential (GWP = 100 year warming potential of one kg
of a gas relative to one kg CO2)[8] >150. This has had the effect of phasing out greenhouse
gases such as R-134a (1,1,1,2-tetrafluoroethane, GWP = >1000).[8] However, illicit HFC use
remains a problem, with R-40 (chloromethane; GWP = 13) having been used as a counterfeit
refrigerant.[9] This raises issues of reactivity with aluminium components in refrigeration
units. Whilst the reaction of alkyl chlorides with aluminium under the influence of an
aluminium halide catalyst is well established,[10] it is known that reaction also proceeds in the
absence of catalyst.[10b,11] In this vein, in our hands the autocatalytic formation of
trialkylaluminium and (catalyst) AlCl3 from an alkyl chloride-aluminium mixture has been
initiated by heat only.[12] The products of reaction between R-40 itself and aluminium include
trimethylaluminium (TMA), which is potentially reactive with respect to other chemicals
present. These include proprietary compound oils (e.g. RL 32H)[13] formulated for use in
conjunction with HFC refrigerants. They comprise synthetic polyolesters (POEs) which, in RL
32H itself, have a pentaerythritol core.[14]
Although the interaction of organoaluminium compounds with esters has been studied[15] the
specifics of the mechanism remain surprisingly obscure and, in particular, reaction
intermediates are incompletely understood. AlEt3 has been reacted with esters in an equimolar
ratio to give ’ate complexes that rearrange to give ketones and aldehydes.[16] Studies using
TMA have explored the formation of donor-acceptor complexes and their derivatization with
excess aluminium reagent.[17] Moreover, the deployment of excess TMA at high temperature
has incurred double methylation and tertiary alcohol formation. However, mechanistic
insights were limited to the alternative use of Me2AlCl.[18] While the formation of
hemialkoxides has been postulated based on the derivatization of ketones and aldehydes using
alkylaluminiums,[19] these species have not hitherto been recorded in ester-based systems.
Meanwhile, the use of excess TMA has been reported in the alkylation of acetates,[20] and the
ketonization of heteroaromatc esters using 1 eq. TMA has been reported.[21,22] Reaction
selectivity has been investigated, with TMA used at low temperature[23] in the stereoselective
reduction of cyclic ketones[24] to give neoliacinic acid.[25] Reaction was done in the presence
of ancillary ester groups, with competing transesterification proving controllable.[26]
The expected by-product of ester reaction with TMA, Me2AlOR, has been the subject of
extensive study.[27] However, this has tended to focus not upon its synthesis as a by-product of
ketonization reactions but rather on the oxophilic derivatization of Al–C bonds[28] by
moisture[29] or oxygen.[30] From a structural point of view, aluminium organooxide
formation[31] and dimer-/trimerization is well established,[32] e.g. the simple aluminium
alkoxide Me2AlOMe has been shown to be trimeric.[33,34]
In this work we model the reaction of TMA with synthetic POEs and elucidate intermediates
along the reaction pathway between TMA and esters in general for the first time. Structure
and stability are monitored for intermediate complexes and solution data clarifies the reaction
stoichiometry.
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Results and discussion
The ability of alkylaluminiums to be autocatalytically generated through the action of alkyl
chlorides on aluminium metal has led us to seek to model the potential reactivity of lubricant
oils used in industrial refrigeration units with respect to TMA. Reactions involving a simple
aliphatic ester were undertaken whereby TMA in toluene was initially added dropwise to
methyl propionate 1 (1:1) under a N2 atmosphere at –78 °C. Though this system failed to
readily produce isolable products, the observation of a pale green colour upon heating, which
disappeared when left to cool to room temperature, suggested the interaction of ester and
TMA and led to further investigations. Accordingly, an excess of TMA (see ESI Figure S1)
was added to 1 (3:1 TMA:1) under N2 at –78 °C. After reaching room temperature the
solution was stirred for 2 hours, whereupon the NMR spectra of an aliquot were collected. 1H
NMR spectroscopy and COSY suggested the formation of two species (Figure 1, top), with
13C NMR spectroscopy confirming the complete absence not only of ester but of C=O groups
from each species (see Figure S3). These data suggest that 2:1 reaction of TMA with ester has
occurred, one equivalent of TMA expelling methoxide to induce the formation of a reactive
EtMeC=O intermediate alongside Me2AlOMe 3 (1H  3.06, –0.59 ppm)[32] before a second
equivalent of TMA has reacted with the ketone to give the dimethylaluminium alkoxide
Me2AlOCEtMe2 2. Integrals of peaks at  0.61 ppm (2) and 3.06 (3) suggest the two products
to be present in a 1:1 ratio. Lastly, the observation that signals at  –0.47 and –0.59 ppm
reveal relative integrals of 2:1 leads us to speculate that 2 traps the final (unreacted)
equivalent of TMA present to give Me2AlOCEtMe2(TMA) 2(TMA) (Scheme 1). Based on
these spectroscopic data we attribute what would be an unusual 4-membered Al2OC
metallacycle to this adduct, in which four Me groups are equivalent, with the terminal groups
(Met) resonating at  –0.47 ppm while the unique bridging group (Meb) resonates at  0.09
ppm. This view is reinforced by 13C NMR spectroscopy (Figure S3), which reveals a sharp
signal at  –4.1 ppm due to the bridging methyl in 2(TMA) and broad signals for terminal
AlMe groups at  –6.7 and –10.7 ppm in 2(TMA) and 3, respectively. Lastly, it is consistent
with 27Al NMR spectroscopic evidence, which reveals a broad signal at  156.2 ppm (see
Figure S3) attributable to 4-coordinate aluminium.[35]
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Figure 1. The 1H spectra of aliquots from the reaction between TMA and methyl propionate in
toluene employing 3:1 (top), 2:1 (middle) and 1:1 (bottom) stoichiometries. The solvent is
[D6]benzene. *Free TMA.
Scheme 1.
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To further clarify the co-formation of putative 2(TMA) and 3, the same synthetic process was
repeated using 2:1 and 1:1 TMA:1 ratios (Figure 1, middle and bottom). The 1H NMR
spectrum of the last of these systems is dominated by the formation of a complex between 1
and TMA (Figure 1, bottom), with signals from unreacted 1 at  3.32 (s), 1.99 (q) and 0.93 (t)
ppm (cf. Figure S2) moved to  2.98 (m), 1.97 (m) and 0.65 (m) ppm, whilst coordinated
TMA is revealed downfield ( –0.33 ppm) of free TMA ( –0.35 ppm, Figure S1). 13C NMR
spectroscopy reveals retention of a modified ester in 1(TMA) ( 181.4 ppm, Figure S4; cf. 
173.9 ppm in 1, Figure S2) and the presence of coordinated TMA ( –7.7 ppm). However,
consistent with previous reports,[20,26] negligible conversion of the complex 1(TMA) into
addition product is observed in this system. In contrast, the 2:1 TMA:1 system reveals not
only the interaction of 1 with TMA to yield adduct 1(TMA) but also the development of two
further species (Figure 1, middle). Hence, signals attributable to 1(TMA) are noted in the 1H
NMR spectrum at  2.94, 1.95, 0.63 and –0.32 ppm and 13C NMR spectroscopy reveals
retention of the ester function at  181.8 ppm (Figure S5). However, this adduct is now less
populous in solution than two other distinct species. One of these presents signals at  3.08
and –0.60 ppm and is attributable to evolving Me2AlOMe 3, while the other is consistent with
the alkoxide 2. The high field region of the spectrum demonstrates 1H NMR resonances at 
0.09 and –0.49 ppm (3H and 12H, respectively), neither of which correspond to unreacted
TMA. This reinforced the view already expressed (Figure 1, top) that an adduct, 2(TMA),
exists in solution, that high field signals are attributable to one bridging (AlMeb) and four
terminal (AlMet) AlMe groups, respectively, in Me2Al(2-Me)AlMe2 and that 2(TMA) is
symmetrical, metallacyclic Me4Al2(2-Me)(2-OCEtMe2) (Scheme 1). In a similar vein, 13C
NMR spectroscopy showed resonances at  –4.6 and –7.7 ppm due to AlMeb and AlMet,
respectively, the latter representing the superposition of signals attributable to both 1(TMA)
and 2(TMA). Meanwhile, 3 was now clearly shown by the presence of a signal at  –11.1
ppm. Both 2(TMA) and 3 were retained in the 3:1 system, with 1(TMA) now completely
absent and a small amount of unreacted TMA identified at  –0.36 ppm (Figure 1, top and
Figure S3). 27Al NMR spectroscopy evidenced the trend from 1(TMA) towards the formation
of 2(TMA) and 3 by the gradual replacement of a dominant signal at  185.0 ppm in the 1:1
system (carbonyl-bonded 4-coordinate Al) with a signal at  156.2 ppm in the 3:1 system
(alkoxide-bonded 4-coordinate Al; Figures S3-S5).
Spectroscopy points to an Me4Al2(2-Me)(2-OCEtMe2) formulation based on a symmetrical
OAl2C metallacycle for 2(TMA). However, while this is similar to motifs previously
proposed,[19] the thermal stability of such a motif has not hitherto been reported. With this in
mind, the reaction mixture resulting from the introduction of TMA in toluene to 1 in a 3:1
ratio (spectroscopically characterized as ostensibly a 1:1 mixture of 2(TMA) and 3, Figure 1,
top) was heated to reflux for 4 hours. NMR spectroscopic analysis of aliquots obtained after t
= 0, 1, 2, 3 and 4 hours revealed a gradual thermal rearrangement (Figures 2 and S6), with the
spectra demonstrating the in situ reformation of free TMA ( –0.36 ppm (1H after 4 hours))
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alongside that of the new complex 2(3) = Me4Al2(2-OMe)(2-OCEtMe2) 4 (Scheme 2).
Evidence for the symmetry of an O2Al2 ring in 4 comes from the development of a single Met
signal at  –0.48 ppm. Meanwhile, residual 2(TMA) ( –0.47 ppm) and 3 ( –0.59 ppm)
remain clearly identifiable. Integrals suggest that thermal rearrangement of 2(TMA) + 3 to
give 4 + TMA proceeds to ca. 54% completion after 4 hours under the conditions used.
Scheme 2.
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Figure 2. The 1H spectra (locked using [D6]benzene) of aliquots from the thermally induced
reaction between 2(TMA) and 1 eq. 3 in toluene after time (t) = 0-4 hours.
Further investigation of the thermal rearrangement of 2(TMA) focused on the reaction
mixtures resulting from the 1:1 and 2:1 reaction of TMA with 1 (spectroscopically
characterized as ostensibly 1(TMA) (Figure 1, bottom) and a mixture of 1(TMA), 2(TMA)
and 3 (Figure 1, middle), respectively) being heated to reflux for 2 hours. Data elucidate the
processes in Eqns. 1 and 2;
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The use of 1:1 1:TMA without heating results in very limited reaction, with only traces of
2(TMA) and 3 existing alongside (dominant) 1(TMA) ( 2.98 ppm, Figure 1, bottom). Even
heating fails to completely consume 1 and instead ca. 50% unreacted 1 can clearly be seen
after 2 hours ( 3.32 ppm, Figure 3, bottom). This is explained by viewing 1 as reacting with 3
eq. TMA to yield 2(TMA) and 3, which then undergoes thermal exchange to give 4 + TMA.
This latterly generated TMA can then react with remaining 1, eventually converting half the
available 1 into 4. In the 1:2 1:TMA system, the greater amount of TMA present aids the
formation of 2(TMA) + 3 (Eqn. 1 and Figure 3, top). These then act as a source of further 4 +
TMA (Eqn. 2). The eventual consequence of this cycle is the complete removal of both
1(TMA) and TMA from the system. This explains the formation of 4 only in this system.
However, remnant 3 is also observed. This can be understood by the appearance of a further
species ( 1.50, 1.15, 0.68, –0.40 ppm), which we attribute to 2. (We speculate that this
product forms a dimer or trimer in solution based on the observation of a 27Al NMR signal
that, at  155.1 ppm, is consistent with tetracoordinate metal.) It appears then that 4 is in
equilibrium with it constituents (Eqn. 3); a fact most clearly suggested by noting the 1:1 ratio
of the signals at  –0.40 and –0.59 ppm (2 and 3, respectively). Based on 1H NMR
spectroscopy, this equilibrium lies heavily on the side of 4 (ca. 88:12 4:(2 + 3)).
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Figure 3. The 1H spectra (locked using [D6]benzene) of aliquots from the thermally induced
reaction of mixtures resulting from the 2:1 (top) and 1:1 (bottom) TMA:1 systems obtained
after 2 hours.
Repeated attempts to isolate crystalline products of reaction between methyl propionate 1 and
TMA proved fruitless on account of a low melting point and led to the replacement of 1 with
methyl phenylacetate 5 (Figure S7) in an attempt to crystallographically verify the
identities/structures of ester decomposition products. Hence, TMA in toluene was added
dropwise to 5 (1:1, 2:1 or 3:1 TMA:5). NMR spectroscopic analysis of the resulting mixture
revealed similar behaviour to that noted for the methyl propionate system, with the formation
of initial adduct 5(TMA) in the presence of 1 eq. TMA followed by reaction to give 6(TMA)
and 3 in the presence of more than 1 eq. TMA (Scheme 1 and Figures S8-S10). As with
2(TMA), the capture of excess TMA by 6 could be inferred from the 1H NMR spectroscopic
observation of Al-bonded Me groups at highfield ( –0.42 ppm (Met) and  0.13 ppm (Meb))
in a 4:1 ratio alongside retention of the singlet at  –0.59 ppm due to 3 (see above). 13C NMR
spectroscopy reinforced the co-presence of 3 alongside 6(TMA) through the observation of a
broad high field resonance at  –11.1 ppm (3) alongside signals at  –4.5 (6(TMA), Meb) and
–7.0 ppm (6(TMA), Met). For the 3:1 TMA:5 combination, the liquid remaining after reaction
was reduced in volume and stored at 4 °C for 1 day to produce colourless crystals that
analyzed as a mixture of Me2AlOCBnMe2(TMA) (6(TMA); Bn = CH2Ph) and 3. It was now
possible to confirm the identity of 6(TMA) as Me4Al2(2-Me)(2-OCBnMe2), with X-ray
diffraction establishing the symmetry of the Al2OC ring formed by the capture of TMA and
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the presence of the expected terminal (Met) and 2-bridging (Meb) methyl groups (Figure 5).
The result is the observation of two distinct classes of Al–Me interaction; Al–Meb 2.133(3) Å,
Al–Met 1.951 Å (mean). The only previous report of which we are aware of diffraction data
for the symmetrical metallacyclic motif reported here lies with the electron diffraction analysis
of the hemialkoxide Me2AlOBut(TMA) in the gas phase (Al–Meb 2.103(10) Å, Al–Met
1.948(7) Å (mean)).[36,37] In the solid state the Al2OC motif has only very rarely been
recorded, with a search of the Cambridge Crystallographic Database returning just 7 results.
Of these, only 5 show trapped TMA, demonstrating the highly unusual nature of this
phenomenon. The nearest analogues of 6(TMA) are based on asymmetric bis(oxyphenyl)
structures of type I (Figure 4) demonstrated by tetraaluminium
bis(bis(oxyphenyl)methyl)anthracene and -dibenzofuran complexes[38] and the bisaluminium
derivative of a 1,1´-bis-2,2´-oxynaphthyl ligand.[39] A type I motif has been recorded once also
in heterobimetallic Al-Ti chemistry.[40]
Figure 4.
Figure 5. Molecular structure of 6(TMA), H-atoms omitted. Selected bond lengths (Å) and
angles (°): O1–Al1 1.8240(15), C1–Al1 1.952(3), C2–Al1 1.949(3), C3–Al1 2.133(3), C4–O1
1.468(4), Al1–C3–Al1A 79.50(12), Al1–O1–Al1A 96.79(10), C3–Al1–O1 90.97(8), C1–Al1–
C2 120.17(12).
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The repeated observation that both 2(TMA) and 6(TMA) form alongside Me2AlOMe 3 and
that crystalline 6(TMA) is isolated contaminated by 3 led to attempts to separate the
components. Efforts here took two forms. In one set of experiments, the solvent mixture was
modified post-synthetically. Hence, the recrystallization of 6(TMA) (leading to the crystal
structure shown in Figure 5) gave a crystalline material that analyzed by 1H NMR
spectroscopy as an approximately 1:1 mixture of 6(TMA) and 3 (see Experimental Section,
Co-synthesis of 6(TMA) and 3, Method 1). This ratio accurately reflected that of the two
products generated in the reaction, which point was simply evidenced by analysing an aliquot
of the reaction mixture (see Experimental Section, Spectroscopic Characterization of 5 + 3
TMA). In contrast, the introduction of hexane prior to recrystallization vastly improved the
purity with which crystalline 6(TMA) could be isolated (10:1 6(TMA):3 by 1H NMR
spectroscopy; see Experimental Section, Co-synthesis of 6(TMA) and 3, Method 2). An
alternative approach involved attempting to solvate one component of the 3/6(TMA) mixture
using a Lewis base. With this in mind, TMA in toluene was added to methyl phenylacetate in
a 3:1 ratio under N2 at –78 °C. Removal of toluene was followed by the addition of excess
Et2O. This resulted in the precipitation of a white solid, which was recrystallised by heating to
give a solution and then storing at room temperature to produce colourless blocks. 1H NMR
spectroscopic analysis suggested the presence of Ph but not of Et2O and high field signals
previously attributed to TMA were absent. These data suggest the abstraction of TMA as an
ether solvate,[41] leading to the crystallization of 6. This was confirmed crystallographically by
the observation of a simple dimer based on an (AlO)2 core of a type common in aluminium
organooxide chemistry (Figure 6).[27b,28a]
Figure 6. Molecular structure of 62, H-atoms and minor disorder omitted. Selected bond
lengths (Å) and angles (°): O1–Al1 1.8457(10), O1–Al2 1.8493(10), O2–Al1 1.8526(10), O2–
Al2 1.8447(10), C1–Al1 1.9522(16), C2–Al1 1.9559(16), C3–Al2 1.9565(15), C4–Al2
1.9492(15), C5–O1 1.434(10), C15–O2 1.4615(16), Al1–O1–Al2 98.76(5), Al1–O2–Al2
98.68(5), O1–Al1–O2 81.20(4), O1–Al2–O2 81.32(4).
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Conclusions
In summary, the autocatalytic nature of the reaction between alkylchlorides and aluminium to
give AlCl3 and TMA has led us to study the reactions of TMA with model esters that mimic
synthetic lubricants of the type used in industrial refrigeration units. Reaction has been found
to be heavily dependent on stoichiometry. Hence, the treatment of either methyl proionate or
methyl phenylacetate with 1 eq. TMA gave predominantly the corresponding ester-TMA
adducts 1(TMA) or 5(TMA) with only nominal reaction occurring to give a 1:1 mixture of
Me4Al2(2-Me)(2-OCRMe2) (R = Et 2(TMA) or Bn 6(TMA)) alongside Me2AlOMe 3. In
either case reaction was encouraged by adding more TMA, with full conversion occurring for
a 3:1 TMA:ester ratio. Spectroscopy clarified the trapping of 2 and 6 by TMA, suggesting the
structures of the resulting adducts to be based on symmetrical OAl2C metallacycles. For
2(TMA), the presence of concurrently formed 3 induced thermal exchange to yield a more
stable metallacycle in Me4Al2(2-OMe)(2-OCEtMe2) 4. The ability to isolate 6(TMA) from a
mixture of reaction products proved highly solvent dependent, with by-product Me2AlOMe 3
largely retained in hexane solution whilst 6(TMA) crystallized, allowing confirmation the rare
OAl2C heterocycle. Efforts are now underway to extend this study to the use of more complex
diesters and pentaerythritol-based esters, the latter representing a close analogue of bona fide
POEs.
Experimental section
General synthetic and analytical details.
Reactions and manipulations were carried out under dry nitrogen, using double manifold and
glove-box methods. Solvents were distilled off sodium-potassium amalgam (Et2O, hexane)
immediately before use. Methyl propionate (99%) and methyl phenylacetate (>99%) were
purchased from Sigma-Aldrich and stored over molecular sieve (4 Å). TMA (2.0 M in
toluene) was purchased from Sigma-Aldrich and used as received. Elemental analysis was
carried out on a Perkin Elmer 240 Elemental Analyser. NMR data were collected on a Bruker
Avance III HD 400 MHz Smart Probe FT NMR spectrometer (400.130 MHz for 1H, 100.613
MHz for 13C, 104.261 for 27Al). Spectra were obtained at 25°C. For 1H and 13C, chemical
shifts are internally referenced to deuterated solvent and calculated relative to TMS. For 27Al,
an external reference was used (1 M AlCl3(H2O)6 in D2O). Chemical shifts are expressed in 
ppm. The following abbreviations are used: br = broad, m = multiplet, q = quartet, s = singlet,
sh = shoulder, t = triplet.
Crystallographic details.
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Crystals were transferred from the mother liquor to a drop of perfluoropolyether oil mounted
upon a microscope slide under cold nitrogen gas.[42] Suitable crystals were attached to the
goniometer head via a MicroLoopTM, which was then centred on the diffractometer. Data were
collected on a Bruker D8 Quest (Cu-K,  = 1.54184 Å), equipped with an Oxford
Cryosystems low-temperature device. Structures were solved using SHELXT,[43] with
refinement, based on F2, by full-matrix least squares.[44] Non-hydrogen atoms were refined
anisotropically and a riding model with idealized geometry was employed for the refinement
of H-atoms. For 62 one BnMe2C group was modelled as disordered, though separate positions
for the phenyl group could not be refined satisfactorily. The occupancy was refined, with
restraints placed upon both the 1,2- and 1,3-distances and upon the anisotropic atomic
displacement parameters. Crystallographic data have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publications CCDC 1504652 and 1504653.
Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: +44 1223 336033; e-mail: deposit@ccdc.cam.ac.uk).
Spectroscopic characterization of EtC(O)OMe 1 + TMA reaction mixtures
TMA (1.5, 3.0 or 4.5 ml, 3, 6 or 9 mmol, 2.0 M in toluene) was added dropwise to methyl
propionate 1 (0.29 ml, 3 mmol) under a N2 atmosphere at –78 °C before being allowed to
reach room temperature. The resulting solution was stirred for 2 hours at this temperature. An
aliquot (0.1 ml) was mixed with [D6]benzene (0.7 ml) and analyzed by NMR spectroscopy,
revealing 1(TMA), 2(TMA) and 3.
1:1 1:TMA. 1H NMR spectroscopy (400 MHz, [D6]benzene):  3.08 (s, 0.2 H; 3 OMe), 2.98
(m, 3 H; 1(TMA), OMe), 1.97 (m, 2 H; 1(TMA), CH2), 1.42 (q, 0.1 H; 2(TMA) CH2), 1.07 (s,
0.3 H; 2(TMA) Me), 0.65 (m, 3.2 H; 1(TMA) + 2(TMA) CH2Me), 0.16 (s, 0.1 H; 2(TMA)
Meb), –0.33 (m, 9 H; 1(TMA), AlMe), –0.49 (s, 0.3 H; 2(TMA) Met), –0.60 ppm (s, 0.4 H; 3
Me); 13C NMR (100 MHz, [D6]benzene):  181.4 (1(TMA) CO), 79.5 (2(TMA) CO), 53.8
(1(TMA) OMe), 50.4 (3 OMe), 36.6 (2(TMA) CH2), 27.7 (1(TMA) CH2Me + 2(TMA) Me),
8.8 (2(TMA) CH2Me), 8.2 (1(TMA) CH2Me), –7.7 ppm (1(TMA) AlMe); 27Al NMR (104
MHz, [D6]benzene):  185.0 (1(TMA)), 157.7 ppm (2(TMA) + 3).
1:2 1:TMA. 1H NMR spectroscopy (400 MHz, [D6]benzene):  3.08 (s, 3 H; 3 OMe), 2.94 (s,
2.4 H; 1(TMA), OMe), 1.95 (q, 1.6 H; 1(TMA), CH2), 1.42 (q, 2 H; 2(TMA) CH2), 1.07 (s, 6
H; 2(TMA) Me), 0.63 (t, 5.4 H; 1(TMA) + 2(TMA) CH2Me), 0.09 (br, s, 3 H; 2(TMA) Meb),
–0.32 (s, 7.1 H; 1(TMA), AlMe), –0.49 (s, br, 12 H; 2(TMA) Met), –0.60 ppm (s, 6H, 3 Me);
13C NMR (100 MHz, [D6]benzene):  181.8 (1(TMA) CO), 79.5 (2(TMA) CO), 54.0
(1(TMA) OMe), 50.4 (3 OMe), 36.6 (2(TMA) CH2), 27.7 (1(TMA) CH2Me + 2(TMA) Me),
8.8 (2(TMA) CH2Me), 8.2 (1(TMA) CH2Me), –4.6 (2(TMA) Meb), –7.7 (br, 1(TMA) +
2(TMA) Met), –11.1 ppm (3 Me); 27Al NMR (104 MHz, [D6]benzene):  179.7 (sh.,
1(TMA)),153.9 ppm (2(TMA) + 3).
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1:3 1:TMA. 1H NMR spectroscopy (400 MHz, [D6]benzene):  3.06 (s, 3 H; 3 OMe), 1.41 (q,
2 H; 2(TMA) CH2), 1.05 (s, 6 H; 2(TMA) Me), 0.61 (t, 3 H; 2(TMA) CH2Me), 0.09 (s, 3 H;
2(TMA) Meb), −0.36 (s, 3.5 H; TMA), –0.47 (s, 12 H; 2(TMA) Met), –0.59 ppm (s, 6 H; 3
Me); 13C NMR (100 MHz, [D6]benzene):  79.9 (2(TMA) CO), 50.8 (3 OMe), 37.0 (2(TMA)
CH2), 28.1 (2(TMA) Me), 9.2 (2(TMA) CH2Me), –4.1 (2(TMA) Meb), –6.7 (br, 2(TMA)
Met), –10.7 ppm (br, 3 Me); 27Al NMR (104 MHz, [D6]benzene):  156.2 ppm (2(TMA) + 3).
Thermal stability of 1:3 EtC(O)OMe 1:TMA reaction mixture
The reaction mixture at the end of the 1:3 reaction of 1 with TMA in toluene (to give 2(TMA)
and 3; see above) was heated to reflux for 4 hours using an oil bath to form EtMe2CO(2-
AlMe2)2OMe 4 and TMA. Aliquots (0.1 ml) were diluted with [D6]benzene (0.7 ml) and
analyzed by NMR spectroscopy after time, t, = 0 (before heating), 1, 2, 3 and 4 hours.
t = 0 hr. 1H NMR spectroscopy (400 MHz, [D6]benzene):  3.07 (s, 3 H; 3 OMe), 1.41 (q, 2
H; 2(TMA) CH2), 1.06 (s, 6 H; 2(TMA) Me), 0.62 (t, 3 H; 2(TMA) CH2Me), 0.10 (s, 3 H;
2(TMA) Meb), –0.36 (s, br, 1.5 H; TMA), –0.48 (s, 12 H; 2(TMA) Met), –0.60 ppm (s, 6 H; 3
Me); 13C NMR (100 MHz, [D6]benzene):  79.5 (2(TMA) CO), 50.4 (3 OMe), 36.6 (2(TMA)
CH2), 27.7 (2(TMA) Me), 8.8 (2(TMA) CH2Me), –4.5 (2(TMA) Meb), –7.4 (br, 2(TMA)
Met), –10.9 ppm (br, 3 Me).
t = 1 hr. 1H NMR spectroscopy (400 MHz, [D6]benzene):  3.06 (s, 2 H; 3 OMe), 3.00 (s, 1 H;
4 OMe), 1.44 (q, 0.7 H; 4 CH2), 1.41 (q, 1.3 H; 2(TMA) CH2), 1.09 (s, 2 H; 4 Me), 1.05 (s, 4
H; 2(TMA) Me), 0.65 (t, 1 H; 4 CH2Me), 0.61 (t, 2 H; 2(TMA) CH2Me), 0.09 (s, 2 H;
2(TMA) Meb), –0.36 (s, br, 3 H; TMA), –0.47 (s, 8 H; 2(TMA) Met), –0.48 (s, 4 H; 4 Met), –
0.59 ppm (s, 4 H; 3 Me); 13C NMR (100 MHz, [D6]benzene):  79.5 (2(TMA) CO), 77.0 (4
CO), 50.4 (3 OMe), 48.4 (4 OMe), 37.1 (4 CH2), 36.6 (2(TMA) CH2), 28.1 (4 Me), 27.7
(2(TMA) Me), 8.9 (4 CH2Me), 8.8 (2(TMA) CH2Me), –4.5 (2(TMA) Meb), –7.3 (4 Met), –7.5
(2(TMA) Met), –9.5 ppm (3 Me).
t = 2 hr. 1H NMR spectroscopy (400 MHz, [D6]benzene):  3.07 (s, 1.7 H; 3 OMe), 3.01 (s,
1.3 H; 4 OMe), 1.43 (q, 0.8 H; 4 CH2), 1.41 (q, 1.2 H; 2(TMA) CH2), 1.09 (s, 2.5 H; 4 Me),
1.06 (s, 3.5 H; 2(TMA) Me), 0.66 (t, 1.3 H; 4 CH2Me), 0.62 (t, 1.7 H; 2(TMA) CH2Me), 0.10
(s, 1.5 H; 2(TMA) Meb), –0.36 (s, br, 4.5 H; TMA), –0.48 (s, 7 H; 2(TMA) Met), –0.49 (s, 5
H; 4 Met), –0.60 ppm (s, 3.5 H; 3 Me); 13C NMR (100 MHz, [D6]benzene):  79.5 (2(TMA)
CO), 77.0 (4 CO), 50.4 (3 OMe), 48.3 (4 OMe), 37.1 (4 CH2), 36.6 (2(TMA) CH2), 28.1 (4
Me), 27.7 (2(TMA) Me), 8.9 (4 CH2Me), 8.8 (2(TMA) CH2Me), –4.5 (2(TMA) Meb), –7.4
(br, 4 Met + 2(TMA) Met), –9.5 ppm (3 Me).
t = 3 hr. 1H NMR spectroscopy (400 MHz, [D6]benzene):  3.07 (s, 1.5 H; 3 OMe), 3.01 (s,
1.5 H; 4 OMe), 1.44 (q, 1 H; 4 CH2), 1.41 (q, 1 H; 2(TMA) CH2), 1.10 (s, 3 H; 4 Me), 1.06 (s,
3 H; 2(TMA) Me), 0.66 (t, 1.5 H; 4 CH2Me), 0.62 (t, 1.5 H; 2(TMA) CH2Me), 0.10 (s, 1.5 H;
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2(TMA) Meb), –0.35 (s, br, 4.5 H; TMA), –0.47 (s, 6 H; 2(TMA) Met), –0.48 (s, 6 H; 4 Met),
–0.59 ppm (s, 3 H; 3 Me); 13C NMR (100 MHz, [D6]benzene):  79.5 (2(TMA) CO), 77.0 (4
CO), 50.4 (3 OMe), 48.3 (4 OMe), 37.1 (4 CH2), 36.6 (2(TMA) CH2), 28.1 (4 Me), 27.7
(2(TMA) Me), 8.9 (4 CH2Me), 8.8 (2(TMA) CH2Me), –4.5 (2(TMA) Meb), –7.5 (br, 4 Met +
2(TMA) Met), –9.5 ppm (3 Me).
t = 4 hr. 1H NMR spectroscopy (400 MHz, [D6]benzene):  3.07 (s, 1.4 H; 3 OMe), 3.02 (s,
1.6 H; 4 OMe), 1.43 (q, 1.1 H; 4 CH2), 1.41 (q, 0.9 H; 2(TMA) CH2), 1.10 (s, 3.3 H; 4 Me),
1.06 (s, 2.7 H; 2(TMA) Me), 0.66 (t, 1.6 H; 4 CH2Me), 0.62 (t, 1.4 H; 2(TMA) CH2Me), 0.10
(s, 1.4 H; 2(TMA) Meb), –0.36 (s, br, 5 H; TMA), –0.47 (s, 5.5 H; 2(TMA) Met), –0.48 (s, 6.5
H; 4 Met), –0.59 ppm (s, 2.8 H; 3 Me); 13C NMR (100 MHz, [D6]benzene):  79.5 (2(TMA)
CO), 77.0 (4 CO), 50.4 (3 OMe), 48.3 (4 OMe), 37.1 (4 CH2), 36.6 (2(TMA) CH2), 28.1 (4
Me), 27.7 (2(TMA) Me), 8.9 (4 CH2Me), 8.8 (2(TMA) CH2Me), –4.5 (2(TMA) Meb), –7.5 (4
Met + 2(TMA) Met), –9.5 ppm (3 Me).
Thermal stability of 1:1 and 1:2 EtC(O)OMe 1:TMA reaction mixtures
The reaction mixture at the end of the 1:1and 1:2 reactions of 1 with TMA in toluene (to give
1(TMA), 2(TMA) and 3; see above) was heated to reflux for 2 hours using an oil bath.
Aliquots (0.1 ml) were diluted with [D6]benzene (0.7 ml) and analyzed by NMR spectroscopy.
1:1 EtC(O)OMe 1:TMA, t = 2 hr. 1H NMR spectroscopy (400 MHz, [D6]benzene):  3.32 (s,
2.4 H; 1 OMe), 3.08 (s, 0.8 H; 3 OMe), 3.02 (s, 3 H; 4 OMe), 1.99 (q, 1.6 H; 1 CH2), 1.50 (q,
0.4 H; 2 CH2), 1.44 (q, 2 H; 4 CH2), 1.15 (s, 1 H; 2 Me), 1.10 (s, 6 H; 4 Me), 0.93 (t, 2.4 H; 1
Me), 0.69 (t, 0.5 H; 2 CH2Me), 0.66 (t, 3 H; 4 CH2Me), –0.42 (s, 1 H; 2 AlMe), –0.50 (s, 12
H; 4 Met), –0.61 ppm (s, 1.6 H; 3 Me); 13C NMR (100 MHz, [D6]benzene):  173.4 (1 CO),
77.3 (2 CO), 77.0 (4 CO), 50.5 (1 OMe), 50.3 (3 OMe), 48.3 (4 OMe), 37.1 (4 CH2), 37.0 (2
CH2), 28.1 (4 Me + 2 Me), 26.9 (1 CH2), 9.0 (2 CH2Me), 8.8 (4 CH2Me + 1 CH2Me), –9.5
ppm (br, 4 Me + 3 Me); 27Al NMR (104 MHz, [D6]benzene):  156.7 (2 + 3 + 4), 8.1 ppm
(trace, unidentified).
1:2 EtC(O)OMe 1:TMA, t = 2 hr. 1H NMR spectroscopy (400 MHz, [D6]benzene):  3.06 (s,
0.5 H; 3 OMe), 3.01 (s, 3 H; 4 OMe), 1.50 (q, 0.3 H; 2 CH2), 1.43 (q, 2 H; 4 CH2), 1.15 (s, 0.9
H; 2 Me), 1.09 (s, 6 H; 4 Me), 0.68 (t, 0.4 H; 2 CH2Me), 0.66 (t, 3 H; 4 CH2Me), –0.40 (s, 0.9
H; 2 AlMe), –0.48 (s, 12 H; 4 Met), –0.59 ppm (s, 0.9 H; 3 Me); 13C NMR (100 MHz,
[D6]benzene):  77.3 (2 CO), 77.0 (4 CO), 50.4 (3 OMe), 48.3 (4 OMe), 37.1 (4 CH2), 37.1 (2
CH2), 28.1 (4 Me + 2 Me), 9.0 (2 CH2Me), 8.9 (4 CH2Me), –6.1 (w, br, 2 Me), –9.5 ppm (br, 3
Me + 4 Me); 27Al NMR (104 MHz, [D6]benzene):  155.1 ppm (2 + 3 + 4).
Spectroscopic characterization of BnC(O)OMe 5 + TMA reaction mixtures
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As for 1 + TMA but using methyl phenylacetate 5 (0.42 ml, 3 mmol) to give 5(TMA),
6(TMA) and 3. An aliquot (0.1 ml) was mixed with [D6]benzene (0.7 ml) and analyzed by
NMR spectroscopy.
1:1 5:TMA. 1H NMR spectroscopy (400 MHz, [D6]benzene):  7.13-6.86 (m, 5.5 H; 5(TMA)
+ 6(TMA) Ph), 3.34 (s, 2 H; 5(TMA) CH2), 3.07 (s, 0.4 H; 3 OMe), 2.89 (s, 3 H; 5(TMA)
OMe), 2.86 (s, 0.2 H; 6(TMA) CH2), 1.12 (s, 0.6 H; 6(TMA) Me), –0.24 (m, 9 H; 1(TMA),
AlMe), –0.43 (s, 1.2 H; 6(TMA) Met), –0.59 ppm (s, 0.8 H; 3 Me); 13C NMR (100 MHz,
[D6]benzene):  178.1 (5(TMA) CO), 136.6, 131.7, 130.3, 128.1, 127.6 (5(TMA + 6(TMA)
Ph), 79.5 (6(TMA) CO), 54.0 (5(TMA) OMe), 50.7 (6(TMA) CH2), 50.4 (3 OMe), 40.7
(5(TMA) CH2), 28.1 (6(TMA) Me), –7.5 (6(TMA) AlMe), –9.3 (5(TMA) Me), –11.0 ppm (3
Me); 27Al NMR (104 MHz, [D6]benzene):  186.3 (5(TMA)), 157.8 ppm (3 + 6(TMA)).
1:2 5:TMA. 1H NMR spectroscopy (400 MHz, [D6]benzene):  7.13-6.86 (m, 12.5 H;
5(TMA) + 6(TMA) Ph), 3.36 (s, 2 H; 5(TMA) CH2), 3.09 (s, 4.5 H; 3 OMe), 2.89 (s, 3 H;
5(TMA) OMe), 2.88 (s, 3 H; 6(TMA) CH2), 1.13 (s, 9 H; 6(TMA) Me), 0.14 (s, br, 4.5 H;
6(TMA) Meb), –0.30 (m, 9 H; 5(TMA), AlMe), –0.44 (s, br, 18 H; 6(TMA) Met), –0.61 ppm
(s, 9 H; 3 Me); 13C NMR (100 MHz, [D6]benzene):  178.8 (5(TMA) CO), 136.6, 131.4,
130.3, 129.2, 128.6, 128.0, 126.7 (5(TMA + 6(TMA) Ph), 79.5 (6(TMA) CO), 54.3 (5(TMA)
OMe), 50.7 (6(TMA) CH2), 50.4 (3 OMe), 40.7 (5(TMA) CH2), 28.0 (6(TMA) Me), –4.5
(6(TMA) Meb), –7.0 (6(TMA) Met), –7.5 (5(TMA) Me), –11.2 ppm (3 Me); 27Al NMR (104
MHz, [D6]benzene):  180.5 (sh, 5(TMA)),153.6 ppm (3 + 6(TMA)).
1:3 5:TMA. 1H NMR spectroscopy (400 MHz, [D6]benzene):  7.15-6.84 (m, 5 H; 6(TMA)
Ph), 3.06 (s, 3 H; 3 OMe), 2.86 (s, 2 H; 6(TMA) CH2), 1.11 (s, 6 H; 6(TMA) Me), 0.13 (s, 3
H; 6(TMA) Meb), –0.42 (s, 12 H; 6(TMA) Met), –0.59 ppm (s, 6 H; 3 Me); 13C NMR (100
MHz, [D6]benzene):  136.7, 130.3, 128.0, 126.7 (6(TMA) Ph), 79.5 (6(TMA) CO), 50.7
(6(TMA) CH2), 50.4 (3 OMe), 28.0 (6(TMA) Me), –4.5 (6(TMA) Meb), –7.0 (6(TMA) Met),
–11.1 ppm (3 Me); 27Al NMR (104 MHz, [D6]benzene):  154.0 ppm (3 + 6(TMA)).
Co-synthesis and characterization of BnMe2COAlMe2(TMA) 6(TMA) and 3
Method 1) TMA (4.5 ml, 9 mmol, 2.0 M in toluene) was added dropwise to methyl
phenylacetate (0.42 ml, 3 mmol) under a N2 atmosphere at –78°C and allowed to reach room
temperature. The resulting solution was stirred and generated heat. After 2 hours the solution
was placed under vacuum to remove the toluene. The remaining liquid was stored at 4°C for 1
day, producing colourless crystals of 6(TMA) and 3. Combined yield of 6(TMA) and 3: 910
mg (83% of the total mass expected); m.p. <30°C; 1H NMR spectroscopy (400 MHz,
[D6]benzene):  7.05-6.84 (m, 5 H; 6(TMA) Ph), 3.06 (s, 3 H; 3 OMe), 2.86 (s, 2 H; 6(TMA)
CH2), 1.11 (s, 6 H; 6(TMA) Me), 0.13 (s, 3 H; 6(TMA) Meb), –0.43 (s, 12 H; 6(TMA) Met), –
0.60 ppm (s, 6 H; 3 Me); 13C NMR (100 MHz, [D6]benzene):  136.7, 130.3, 128.0, 126.7
(6(TMA) Ph), 79.5 (6(TMA) CO), 50.7 (6(TMA) CH2), 50.4 (3 OMe), 28.0 (6(TMA) Me), –
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4.5 (6(TMA) Meb), –7.0 (6(TMA) Met), –11.1 ppm (3 Me); 27Al NMR (104 MHz,
[D6]benzene):  153.8 ppm (3 + 6(TMA)); satisfactory elemental analysis could not be
achieved; X-ray crystal data: C15H28Al2O, M = 278.33, orthorhombic, space group Pbcm, a =
7.3370(4), b = 17.3975(8) c = 13.8976(6) Å, V = 1773.97(15) Å3, Z = 4, calcd = 1.042 g cm–3,
 = 1.378 mm–1, T = 180(2) K. 6976 data (1327 unique, Rint = 0.0377,  < 59.03 ) were
2
2 2
collected. wR2 = {[w(Fo Fc2)2]/[w(Fo ) ]}1/2 = 0.1024, conventional R = 0.0412 on F
values of 1071 reflections with F2 > 2(F2), GoF = 1.072, 94 parameters. Max. peak/hole
0.282 eÅ–3.
Method 2) As for Method 1 but after stirring the reaction mixture for 2 hours the solution was
placed under vacuum to remove the toluene. The remaining liquid was treated with hexane (1
ml) and the resulting solution stored at –20°C for 1 day, producing a small quantity of
colourless crystals. Combined yield of 6(TMA) and 3: 240 mg (22% of the total mass
expected); m.p. 68-70°C; 1H NMR spectroscopy (400 MHz, [D6]benzene):  7.05-6.84 (m, 5
H; 6(TMA) Ph), 3.05 (s, 0.3 H; 3 OMe), 2.86 (s, 2 H; 6(TMA) CH2), 1.11 (s, 6 H; 6(TMA)
Me), 0.12 (s, 3 H; 6(TMA) Meb), –0.42 (s, 12 H; 6(TMA) Met), –0.58 ppm (s, 0.6 H; 3 Me);
13C NMR (100 MHz, [D6]benzene):  136.7, 130.3, 128.1, 126.7 (6(TMA) Ph), 79.5 (6(TMA)
CO), 50.7 (6(TMA) CH2), 28.0 (6(TMA) Me), –4.5 (6(TMA) Meb), –7.0 ppm (6(TMA) Met);
27Al NMR (104 MHz, [D6]benzene):  155.4 ppm (3 + 6(TMA)); satisfactory elemental
analysis could not be achieved.
Synthesis and characterization of BnMe2COAlMe2 6
TMA (4.5 ml, 9 mmol, 2.0 M in toluene) was added dropwise to methyl phenylacetate (0.42
ml, 3 mmol) under a N2 atmosphere at –78°C before being allowed to attain room
temperature. The resulting solution was stirred and generated heat. After 2 hours the solution
was placed under vacuum to remove the toluene. The remaining liquid was treated with Et2O
(3 ml) to give a white precipitate that dissolved upon gentle heating. Colourless prismatic
crystals formed as the mixture cooled to room temperature and over a period of 1 day
produced a large crop of 6. Yield 417 mg (67% wrt TMA); m.p. 124-126°C; 1H NMR
spectroscopy (400 MHz, [D6]benzene):  7.07-6.92 (m, 5 H; Ph), 3.00 (s, 2 H; CH2), 1.22 (s, 6
H; Me), –0.28 ppm (s, 6 H; AlMe); 13C NMR (100 MHz, [D6]benzene):  137.1 (i-Ph), 130.4
(o-Ph), 128.3 (m-Ph), 126.6 (p-Ph), 77.6 (CO), 51.2 (CH2), 28.5 (Me), –5.7 ppm (AlMe); 27Al
NMR (194 MHz, [D6]benzene):  179.8 ppm; elemental analysis calcd (%) for C24H38Al2O2:
C 69.88, H 9.29; found: C 68.73, H 9.70; X-ray crystal data: C24H38Al2O2, M = 412.50,
monoclinic, space group P21/n, a = 11.9568(5), b = 8.7121(4), c = 23.7485(10) Å,  =
98.577(2) °, V = 2446.19(18) Å3, Z = 4, calcd = 1.120 g cm–3,  = 1.182 mm–1, T = 180(2)K.
34306 data (4334 unique, Rint = 0.0301,  < 66.73 ) were collected. wR2 =
2
2 2
{[w(Fo Fc2)2]/[w(Fo ) ]}1/2 = 0.0944, conventional R = 0.0340 on F values of 4013
reflections with F2 > 2(F2), GoF = 1.063, 285 parameters. Max. peak/hole 0.281 eÅ–3.
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Acknowledgements
This work was supported by Cambridge Refrigeration Technology (J. S.) and thanks go to Dr.
Richard Lawton for discussions and insights. The U.K. EPSRC are acknowledged for grant
EP/J500380/1 (A. J. P.). Detailed supporting data for this paper are available at the University
of Cambridge data repository (see http://dx.doi.org/10.17863/CAM.6118).
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10.1002/chem.201604553
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19
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Figure Legends
Figure 1. The 1H spectra of aliquots from the reaction between TMA and methyl propionate in
toluene employing 3:1 (top), 2:1 (middle) and 1:1 (bottom) stoichiometries. The solvent is
[D6]benzene. *Free TMA.
Figure 2. The 1H spectra (locked using [D6]benzene) of aliquots from the thermally induced
reaction between 2(TMA) and 1 eq. 3 in toluene after time (t) = 0-4 hours.
Figure 3. The 1H spectra (locked using [D6]benzene) of aliquots from the thermally induced
reaction of mixtures resulting from the 2:1 (top) and 1:1 (bottom) TMA:1 systems obtained
after 2 hours.
Figure 4.
Figure 5. Molecular structure of 6(TMA), H-atoms omitted. Selected bond lengths (Å) and
angles (°): O1–Al1 1.8240(15), C1–Al1 1.952(3), C2–Al1 1.949(3), C3–Al1 2.133(3), C4–O1
1.468(4), Al1–C3–Al1A 79.50(12), Al1–O1–Al1A 96.79(10), C3–Al1–O1 90.97(8), C1–Al1–
C2 120.17(12).
Figure 6. Molecular structure of 62, H-atoms and minor disorder omitted. Selected bond
lengths (Å) and angles (°): O1–Al1 1.8457(10), O1–Al2 1.8493(10), O2–Al1 1.8526(10), O2–
Al2 1.8447(10), C1–Al1 1.9522(16), C2–Al1 1.9559(16), C3–Al2 1.9565(15), C4–Al2
1.9492(15), C5–O1 1.434(10), C15–O2 1.4615(16), Al1–O1–Al2 98.76(5), Al1–O2–Al2
98.68(5), O1–Al1–O2 81.20(4), O1–Al2–O2 81.32(4).
Scheme Legends
Scheme 1.
Scheme 2.
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Table of Contents Headline and Text
Attack of the alkylaluminiums
The reaction of trimethylaluminium (TMA) with ester mimics of synthetic lubricants has been
investigated with results pointing to adduct formation followed by reaction with 2 further
equivalents of TMA and the trapping of intermediate Me2AlOCRMe2(TMA) en route to
Me2AlOCRMe2(Me2AlOMe), each of which demonstrate a symmetrical OAl2X (X = C, O)
metallacycle.
Keywords
Ester
NMR spectroscopy
Nucleophilic addition
Trimethylaluminium
Solid-state structure
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