Dearomatization and Functionalization of Terpyridine Ligands Leading to Unprecedented Zwitterionic Meisenheimer Aluminum Complexes and Their Use in Catalytic Hydroboration

Article abstract of DOI:10.1021/acscatal.8b04096

This paper reports the first example of dearomatization of ubiquitous terpyridine (tpy) ligands via 2′/6′-, 3′/5′-, or 4′-selective alkylation of the central pyridine ring. The reaction is mediated by the most abundant metal in the Earth's crust, aluminum (Al), and depending on the conditions employed, exhibits ionic or radical character as suggested by experimental and computational analysis. In the latter case, intermediate formation of an Al^III complex supported by π-radical monoanionic ligand (tpy^?)^1- is apparent. The 3′/5′-alkylation leads to unprecedented zwitterionic Meisenheimer Al^III complexes, which were identified as efficient precatalysts for the selective hydroboration of C=O and C-C functionalities. Turnover numbers (TONs) up to ～1000 place the corresponding complexes in the category of the most efficient Al catalysts reported to date for the title reaction. The acquired data suggest that aluminum monohydrides, or more likely dihydrides, could be relevant catalytic species. Alternatively, one can also imagine a mechanistic scenario in which the dearomatized "chemically noninnocent" ligand acts as hydride donor, and a detailed investigation of this is warranted in the future.

Full text of DOI:10.1021/acscatal.8b04096

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Article
Dearomatization and Functionalization of Terpyridine Ligands
Leading to Unprecedented Zwitterionic Meisenheimer
Aluminum Complexes and Their Use in Catalytic Hydroboration
Guoqi Zhang, Jing Wu, Haisu Zeng, Michelle C Neary,
Matthew Devany, Shengping Zheng, and Pavel A. Dub
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04096 • Publication Date (Web): 14 Dec 2018
Downloaded from http://pubs.acs.org on December 17, 2018
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Dearomatization and Functionalization of Terpyridine Ligands
Leading to Unprecedented Zwitterionic Meisenheimer Aluminum
Complexes and Their Use in Catalytic Hydroboration
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Guoqi Zhang,1,* Jing Wu,1,2 Haisu Zeng,1,2 Michelle C. Neary,2 Matthew Devany,2 Shengping
Zheng,2,* and Pavel A. Dub3,*
1Department of Sciences, John Jay College and PhD in Chemistry Program, The Graduate
Center of City University of New York, New York, 10019, New York, United States.
2Department of Chemistry, Hunter College, City University of New York, New York, 10065,
New York, United States.
3Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States.
*Correspondence to: guzhang@jjay.cuny.edu; szh0007@hunter.cuny.edu; pdub@lanl.gov.
Abstract: This paper reports the first example of dearomatization of ubiquitous terpyridine
(tpy) ligands via 2′/6′-, 3′/5′- or 4′-selective alkylation of the central pyridine ring. The reaction
is mediated by the most abundant metal in the Earth’s crust, aluminum (Al), and depending on
the conditions employed, exhibits ionic or radical character as suggested by experimental and
computational analysis. In the latter case, intermediate formation of an AlIII complex supported
by π-radical monoanionic ligand (tpy·)1– is apparent. The 3′/5′-alkylation leads to
unprecedented zwitterionic Meisenheimer AlIII complexes in which the ligands are both redox-
and chemically non-innocent. These isolable compounds were identified as efficient
precatalysts for the selective hydroboration of C=O and C≡C functionalities. Turnover numbers
(TONs) up to ~1000 place the corresponding complexes in the category of the most efficient
Al catalysts reported to date for the title reaction. The acquired data suggests that aluminum
monohydrides, or more-likely dihydrides, are relevant catalytic species.
Keywords: terpyridine • aluminum • dearomatization • hydroboration • alkynes • redox non-
innocent • ligand radical
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Introduction.
Homogeneous catalysis with transition metal complexes is an integral part of the scientific-
technological revolution in the 20th century.1 The properties of these complexes are the result
of the interactions between the metal center and its surrounding ligands. Not only do these
ligands contribute to the electronic and steric properties of the metal center, but they can even
be directly involved in the catalytic reaction. Examples include cleaving or forming bonds,
gaining or losing electrons and stabilizing transition states via non-covalent interactions.2 The
last decade of homogeneous catalysis was marked by an extensive development and effective
use of transition metal complexes bearing so-called “redox non-innocent” ligands.3 Catalysis
with complexes containing these ligands relies on the ligand accepting/releasing electrons or
forming/breaking chemical bonds of a substrate with the help of the ligand radical.2c, 3c The
tridentate ligand 2,2′:6′,2"-terpyridine (tpy) along with its 4′-substituted (Xtpy) analogues4 are
some of the most widely used “redox-active” ligands, as shown in Fig. 1.5
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N
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
Ni
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Co
2 N

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N
N

eff = 1.64 B in THF
(Evans method)
NiII (S = 1)
Vicic, 2004[5a,5b]
Si
Xtpy
CoII (S = 0)
Chirik, 2014[5c]
Figure 1. Structure and nomenclature for the Xtpy ligand and examples of isolable complexes
based on the π-radical monoanionic ligand (tpy·)1–. Red arrows: α-spin electron; blue arrow:
β-spin electron.5
While many homogeneous catalysts have been developed using precious metals,6 a
rational approach is needed to design metal complexes that make use of Earth-abundant
elements.7 Over the last ten years, various structurally well-defined catalysts based on first-row
transition metal elements have been developed and discussed in some reviews7d, 8 and a Special
Issue in Acc. Chem. Res. 2015.9 Recent demands in molecular catalysis science provoked a
renaissance of studies with a non-transition metal, Al.10 In fact, Al was among the first
homogeneous catalysts to be discovered (Friedel-Crafts, 1877).11 Until recently,12 most
applications of this metal have relied almost exclusively on the use of its halide/triflate salts
(Lewis acid catalysis), rather than on catalysis with well-defined complexes. Not only is Al the
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most abundant metal in the Earth′s crust (8%), it is also inexpensive and much less toxic than
heavy metals, which could make it ideal for applications in catalysis.10a The results presented
in this work were obtained serendipitously as a result of our initial goal to investigate possible
catalytic activity of putative AlIII complexes based on redox non-innocent Xtpy ligands.13
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Results and Discussion.
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In an attempt to prepare complex 1, which would contain a one-electron reduced ligand
(Phtpy·)1–, the popular precursor L114 and commercial AliBu3 were reacted under conditions
(a) as shown in Scheme 1A. However, the reaction instead resulted in a formal 1,3-migration
i
of one of the three Bu groups from Al to the 2′ or 6′-position on the central pyridine ring of
L1. The corresponding AlIII complex 2 was isolated in 56% yield and characterized via X-ray
diffraction, Elem. Analysis, FTIR and solution NMR.15 The proposed nature of the bonding of
the anionic amido moiety in the “1,3-product” 2 is supported by the relatively short Al(1)–
N(12) bond distance of 1.884(8) Å [1.94 Å by density functional theory (DFT) computations].
This can be compared to the dative interactions Al(1)–N(11) (2.134(8) Å) [DFT: 2.11 Å] and
Al(1)–N(13) (2.132(8) Å) [DFT: 2.13 Å]. Formation of 2, however, is not entirely unexpected.
A similar dearomatization in a lutetium(III) complex has one precedent in the literature,16 and
several examples were reported for related conjugated C–N ligands such as 2,6-
diiminepyridine (dimpy).17 Inspired by these findings, the reaction conditions were further
changed to (b) in Scheme 1 in order to prepare a putative complex similar to 2 with a –
CH2Si(CH3)3 substituent. To our further surprise, the reaction instead led to the isolation and
characterization (X-ray diffraction, Elem. Analysis, FTIR and liquid NMR) of an
unprecedented zwitterionic AlIII complex, 3, in 55% yield, as shown in Scheme 1A.15 By
analogy with 2, the latter can be formally viewed as a “1,4-product” of dearomatized ligand L1
functionalized at the 3′ or 5′-position. However, in contrast to the neutral complex 2, 3 features
a zwitterionic “Meisenheimer-type” nature.18 Meisenheimer complexes in classical organic
chemistry are well-known intermediates in nucleophilic aromatic substitution, and some stable
isolable examples have been reported.18
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A
1
<1%
2
3
4
5
20%
Ph
17%
<1%
Ph
Si
H
2%
26%
9%
21%
N
1%
24%
N
6
22%
3%
7
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9
"1,4-product"
"1,3-product"
Ph
Ph
Ph
Ph
Si
H
N
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iBu·
(a)
(b)
N
N
N
N
iBu·
N
N
N
N
N
N
N
Al
Al
Al
L1
Si
Si
a
1
3
: targeted
, y. 55%
2
, y. 56%
(green crystals)
(deep-green crystals)
3
1
2
B
"1,3-product"
X
R
N
N
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5
N
"1,4-product"
Al
R
R
Cl
X
N
Si
H
N
4: X = H, R = iBu, y. 65%
5: X = Cl, R = iBu, y. 60%
6: X = Cl, R = Me, y. 46%
7: X = Ph, R = Me, ~y. 30%
(a)
(b)
N
N
N
N
Al
"1,5-product"
L1
L2
L3
L4
: X = Ph
: X = H
: X = Cl
: X = 2-pyridyl
Si
Si
N
8: y. 68%
6
7
N
N
N
Al
2%
9[26]
8
N
24%
2%
24%
4%
N
26%
9
Scheme 1. Reactions of ligand L1 with Al precursors under different conditions: (a) AliBu3
(1.2 eq.), Et2O, rt, 2 h.; (b) (i) AlCl3, THF, 24 h, rt;19 (ii) LiCH2SiMe3, Et2O, rt, 2 h; B. Scope
and limitations of the reaction between various Xtpy ligands L1–4 and commercial AlR3
reagents under conditions (a) and (b) in A. X-ray structures are shown at 30% thermal ellipsoid
probability level. HOMO Plots and Corresponding Atomic Contributions are Shown for the
Optimized Structures of 2, 3 and 9 Computed at the ωB97X-D/6-311++G**/SMD(diethyl
ether) Level of Theory.15 H-atoms are omitted for clarity. aIsolable complex 1 has not been
unequivocally identified as an intermediate to formation of 2, but the identity of the former
complex is suggested by experimental and computational analyses.
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Complexes of type 3 were actually predicted for the first time by Budzelaar for similar
conjugated C–N imine dimpy ligands in 2006,17a, 20 and very recently observed for the first time
by Cámpora et al. for Mg/dimpy as a mixture of two isomers,21 but this is the first example for
non-imine tpy ligand. While the dative Al(1)–N(1) (2.0752(17) Å) [DFT: 2.10 Å] and Al(1)–
N(3) (2.0866(17) Å) [DFT: 2.12 Å] bond lengths are similar to the analogous distances in
compound 2, the Al(1)–N(2) (1.9774(17) Å) [DFT: 2.00 Å] bond length is ~0.1 Å longer than
the central Al–N bond distance for 2. This supports the zwitterionic characterization of 3 in
which the negative charge remains delocalized on the ligand. The identity of 2 and 3 as neutral
and zwitterionic species, respectively, is also supported by the shapes of the computed Highest
Occupied Molecular Orbitals (HOMOs) of these complexes, as shown in Scheme 1A. The
liquid NMR spectra for both complexes 2 and 3 are in full agreement with the solid-state
structures.15
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Stoichiometric experiments and DFT computations have been further performed in order
to explore the origin of the selective formation of “1,3-” versus “1,4-products” under
conditions (a) and (b), respectively. During the work-up process for 2, we noticed that several
pieces of well-shaped green crystals spontaneously crystallized on the vial wall prior to the
isolation of 2.15 X-ray crystallography identified these crystals to be the targeted complex 1,
which is thus either a reaction by-product or an intermediate, as shown in Scheme 1A. The
i
ligand within complex 1 represents an anion-radical,22 whose combination with Bu· radical
generated in the reaction mixture under conditions (a) may explain the formation of complex
2. Attempts to prepare 2 in the presence of a radical trap were unsuccessful,15 and some free
ligand and complex 1 were recovered; this is consistent with the radical nature of the reaction
leading to 2. Computational analysis has been further performed to explore the reaction
between 1 and iBu· as shown in Fig. 2A.
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9.7
9.5
Ph
0.0
N
Ph
N
N
Ph
Al
0.32
H
– 0.14
0.22
– 0.10
0.17
– 20.7
– 29.0
N
N
N
N
2(1,3)
N
N
0.10
Al
Al
– 27.6
Ph
1
0.07
2(1,4)
N
0.27
N
N
Al
2(1,5)
B
Si
9.1
Ph
3.7
Si
N
N
15%
11%
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1%
Ph
Al
Ph
Si
Si
H
N
Si
3.4
N
3(1,3)
N
N
Al
1.1 Si
Ph
N
LUMO
N
Si
Si
Al
N
1+
20%
5%
N
N
Si
Si
Al
3(1,4)
Si
Si
3(1,5)
LUMO+1
║ ΔG°298K, 1 M, kcal·mol– 1
non-covalent interactions
vibrational vector
Figure 2. A. Energy Profile for the Radical Reaction Between 1/iBu·and Mulliken Spin
Density Plot for 1 (yellow: α-spin; red: β-spin); B. Energy Profile for the Ionic Reaction
Between 1+/(CH3)3SiCH2 and LUMO/LUMO+1 Plots for 1+. Computed at the ωB97X-D/6-
–
311++G**/SMD(diethyl ether) Level of Theory (for the Optimized Structures, Non-critical H
atoms are Omitted for Clarity).23 Experimentally isolated complexes are highlighted in green.
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1
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9
The C–C bond formation leading to the experimentally obtained “1,3-product” 2(1,3) takes
place with an activation barrier of 9.7 kcal·mol–1 (ΔG°298K, 1 M). The activation barrier
becomes higher (12.8 kcal·mol–1) for the C–C bond formation leading to an isomeric
Meisenheimer-type “1,4-product” 2(1,4). This is consistent with the Mulliken spin density
analysis of 1, which has limited electron density localized on the meta-C atom of the central
pyridine ring. Interestingly, 2(1,4) is 6.9 kcal·mol–1 more stable than 2(1,3) (or 5.8 kcal·mol–1 on
an E scale). Therefore, 2(1,3) is formed under kinetic control, and the reaction is irreversible due
to an activation barrier of 30.4 kcal·mol–1 for the reverse reaction. It is also interesting to note
that according to the same Mulliken spin density analysis, the unpaired electron of the redox
non-innocent ligand within 1 is partially localized on the para-C atom of the central pyridine
ring. Corresponding C–C bond formation may potentially lead to a “1,5-product” 2(1,5). The
latter is 8.3 kcal·mol–1 more stable than 2(1,3) (6.1 kcal·mol–1 on E scale), and slightly more
stable or even isoenergetic with 2(1,4) (1.4 kcal·mol–1 on G°298K scale or 0.3 kcal·mol–1 on E
scale). The activation barrier leading to 2(1,5) is computed as isoenergetic with the one leading
to 2(1,3) (ΔΔG°298K = –0.2 kcal·mol–1,ΔΔE = –0.1 kcal·mol–1). Taking into account the accuracy
of DFT (at least ~3 kcal·mol–1)24 and the nature of calculations themselves (conformational
effects, explicit solvation, uncertainty in free energy calculations2b), these calculations suggest:
1) the “1,4-product” 2(1,4) is likely not formed or formed in very minor quantities under
conditions (a), and 2) along with 2(1,3), one may also expect formation of the neutral “1,5-
product” 2(1,5). Though we did not observe 2(1,5) experimentally, this might be possible by
varying the nature of the tpy ligand (e.g. changing the substituents), Al reagent, solvent, and
other conditions used for the reaction. In fact, with respect to the current reaction conditions,
the relatively low isolated yield for 2(1,3) (56%) does not preclude accumulation of 2(1,5) in the
original mixture prior to crystallization as 2(1,3)/2(1,5) could have very different solubility due to
different molecular symmetry.
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In contrast to the radical reaction we project to form 2, we propose that 3 is formed from
–
the reaction between 1+ generated in the reaction mixture15 and the anion of (CH3)3SiCH2 .
Similar to the 1/iBu· radical reaction described above, one may anticipate three isomeric
products 3(1,3)–3(1,5) as shown in Fig. 2B. The relative computed energy of these isomeric
complexes is as follows: “1,4-product” ≤ “1,5-product” < “1,3-product”,25 similar to the trend
for products 2(1,3)–2(1,5), where the “1,3-product” is clearly the least-stable one for both cases.
Additionally, formation of the “1,3-product” 3(1,3) needs to overcome the highest activation
barrier, 5.4 kcal·mol–1 higher than that required for the formation of experimentally observed
3(1,4). According to the analysis of Lowest Unoccupied Molecular Orbitals (LUMOs), it is
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LUMO+1 that interacts with the anion of (CH3)3SiCH2 leading to 3(1,4). Interestingly,
formation of a hypothetical “1,5-product” 3(1,5) takes place with a slightly lower computed
activation barrier (3.7 kcal·mol–1, or 3.2 kcal·mol–1 on E scale) than the one leading to 3(1,4) and
–
1
2
3
4
5
6
–
this happens as a result of the interaction between LUMO and the anion of (CH3)3SiCH2 . The
energy difference between transition states (TSs) leading to 3(1,4) and 3(1,5) is, however, within
DFT noise. In addition, as discussed above, conformational effects, explicit solvation and other
factors could have dramatic effects in tuning the energy difference between the corresponding
TSs in favor of the formation of 3(1,4). Similar to the radical reaction described above, relatively
low isolated yield for 3(1,4) (55%) does not preclude accumulation of 3(1,5) in the original
mixture prior to crystallization as 3(1,4)/3(1,5) could also have very different solubility due to
different molecular symmetry. To conclude, computations about the ionic reaction between
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–
1+/(CH3)3SiCH2 suggest that the “1,3-product” is likely not formed, whereas one may
anticipate the formation of a “1,5-type” product in addition to the experimentally observed
“1,4-type” zwitterionic product by changing the reaction conditions discussed above.
To explore the scope and limitations of the reactions shown in Scheme 1, we further tested
various commercially available Xtpy ligands L1–4 and AlR3 reagents under conditions (a) or
(b), respectively, as shown in Scheme 1B. Various “1,3-type” 4–7 or “1,4-type” products 8
were isolated in appreciable yields between ~30–68% and fully characterized by IR, solution
NMR spectroscopies, elemental analysis and X-ray crystallography.15 Interestingly, in the
reaction of AliBu3 with L4, one of the crystals was identified as the “1,5-product” 9 based on
X-ray crystallography, whereas the overall mixture is composed of two components.26 The
anionic amido moiety of 9 is suggested by the relatively short Al(1)–N(2) bond distance of
1.8967(15) Å [DFT: 1.92 Å], which is comparable to the Al–Namido interaction in the “1,3-
product” 2 (vide supra). In contrast, the dative Al–N bond lengths in 9 are 2.1012(15) [DFT:
2.14 Å] and 2.1026(15) Å [DFT: 2.15 Å], i.e. nicely comparable to those found in 2 and 3,
respectively. The identity of 9 as a neutral species is also supported by the shape of the
computed HOMO as shown in Scheme 1B. Experimental isolation of 9 further supports our
conclusions gleaned from the computational analysis that “1,5-type” products can be
potentially obtained and isolated in these reactions.
The five-coordinate zwitterionic Meisenheimer AlIII complex 8 was further tested in the
catalytic hydroboration of acetophenone and phenylacetylene with freshly distilled
pinacolborane (HBpin), as shown in Scheme 2A and Scheme 3A, respectively.
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A
O
1
2
3
4
O
B
[cat]
O
O
O
O
H
+
H B
conditions
5
6
7
8
9
H Mes
H
Mes
N
B
Al
N
Cl
Si
H
N
N
N
iPr
iPr
Al
N
N
iPr
Mes
Mes H H
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37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
N
H
N
N
N
H
Inoue, 2016[12f]
Al
Al
N
OTf
iPr
N
N
Al
S/C = 25
C6D6, 60 C, 6 h
y. 53% (NMR)
Si
Si
Roesky, 2015[12g]
8
this work
S/C = 1000
neat, rt, 3 h
y. 98% (GC)
Nembenna, 2016[28]
S/C = 50
neat, rt, 3 h
S/C = 50
N
Al
Li
C6D6 (2 M), rt, 6 h
y. 51% (NMR)
H
2
y. 93% (isol.)
Mulvey, 2018[12b,12c]
S/C = 20
N
(97% y. NMR)
toluene, 110 C, 2 h
Me2N
NMe2
Ni(bpy)
Li
iPr
H
B
Ph
N
iPr
Sn
iPr
King, 2016[35b]
S/C = 3333
Me2
Si CHPh2
iPr
iPr
Ph
Ph
N
H
Okuda, 2016[31]
S/C = 100000
THF, rt, 1.5 h
conv. 100%
N
N
C6D6, rt, 1 h
Ph2HC
iPr
y. 93% (NMR)
iPr
iPr
OtBu
Cu
Jones, 2014[33]
S/C = 200
La[N(SiMe3)2]3
Marks, 2017[34]
S/C = 1000
C6D6, rt, 15 min
Mankad, 2016[35a]
S/C = 1000
Et2O, rt, 1 h
y. >99% (NMR) for
[Mg(THF)6][HBPh3]2
Okuda, 2016[32]
S/C = 10000
C6D6, rt, 2.5 h
y. 95% (NMR) for
benzophenone
THF-d8, rt, 1 h
conv. >99%
y. 92% (NMR)
4'-Iodoacetophenone
C
O
OH
8
1) [ ] neat, rt, 3 h
O
+
H B
R2
2) silicagel, rt
S/C = 1000
R1
R2
R1
O
11
HBpin
10 mmol
OH
15 mmol
OH
OH
OH
OH
O2N
11c
(88%)[a]
: 1.30 g
11b
: 1.54 g
(92%)
11d
11e
11a
: 1.36 g
: 1.82 g
: 1.15 g
(94%)
(86%)[a]
(92%)[a]
OH
OH
OH
O
OH
N
HO
MeO2C
11f:
S
N
11j
: 1.23 g
11g
11i
: 1.26 g
(94%)
: 1.18 g
(95%)[a]
1.49 g
(90%)
11h:
1.09 g
(96%)
(82%)[a]
Scheme 2. A. Hydroboration of acetophenone with HBpin; B. Comparison of catalytic
efficiencies of precatalyst 8 with selected, most efficient examples for reaction in A catalyzed
by alkali, alkaline–earth, main-group, lanthanide and transition metals; C. Functionalized
ketone hydroboration with 8. [a] Reaction run for 16 h. S/C = substrate-to-catalyst ratio.
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A
B
1
2
3
4
5
6
7
8
O
B
[cat]
O
O
+
H B
O
conditions
Cl
Si
H
Et
H Mes
H
Mes
N
Et
H
N
H
N
N
Al
N
Al
H
Al
Li
Al
N
N
N
H
Et
9
N
N
Al
2
N
N
2
Mes
Mes H H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Al
Cowley, 2016[12e]
S/C = 10
Et
Inoue, 2016[12f]
Mulvey, 2018[12b],[12c]
Si
Si
S/C = 25
C6D6, 80
C, 10 h
Roesky, 2016[12d]
S/C = 10
toluene,
110 C, 2 h
y. 71% (NMR)
toluene,
110 C, 2 h
y. 73% (isol.)
8
this work
S/C 33

S/C = 1000
CDCl3, 30 C,
y. 80% (NMR)
NaOtBu (1 mol%)
12 h
toluene, 80 oC, 16 h
y. 91% (GC)
y. 73% (NMR)
iPr
Et
Et
Bertrand, 2017[39b]
S/C = 40
CH3CN, rt, 2 h
y. 84% (isol.)
Trovitch, 2017[39c]
S/C = 100
C6D6, rt, 2 h
N
PPh2
H
Co
N
N
N
tBu2P
PtBu2
Fe
i
Cu
Pr
PPh2
H
y. 96% (isol.)
OPh
Nishibayashi, 2017[39a]
S/C = 100
hexane, rt, 16 h
y. 88% (NMR)
C
R2
O
H
8
[ ]
H B
Bpin
R1
+
R1
R1
5 mmol
C [alkyne] = 2 M 6 mmol
O
NaOtBu (1 mol %)
toluene, 80 oC, 16 h
S/C = 1000
HBpin
12
Bpin
Bpin
Bpin
O
Bpin
O
Bpin
F
12b
: 1.09 g
(88%)
12a
: 0.98 g
(85%)
12c
: 1.22 g
(84%)
12d
: 0.91 g
12e
: 1.17 g
O
(94%)
(90%)
Bpin
Bpin
O
Bpin
12f
: 1.10 g
(92%)
12h
: 1.07 g
(96%)
12g
: 0.99 g
(95%)
Scheme 3. A. cis-Selective anti-Markovnikov hydroboration of phenylacetylene with HBpin;
B. All reported examples of aluminum-catalyzed and select recent examples for transition
metal-catalyzed transformation in A; C. Functionalized alkyne hydroboration with 8. S/C =
substrate-to-catalyst ratio.
Ketones and alkynes are important raw materials, and their hydroboration (or
dehydrogenative borylation of terminal alkynes) represents a powerful method to generate
boric and boronic esters (vinylboronates or alkynylboronates), respectively. These
hydroboration products are versatile building blocks in the synthesis of fine chemicals in
pharmaceutical, material, and agrochemical industries.27 Over the last five decades, various
uncatalyzed, base- and acid-catalyzed, and metal-catalyzed (alkali, alkaline–earth, main-group,
lanthanide and transition-metals) reactions have been reported using Brown’s catecholborane
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ACS Catalysis
and Knochel’s more robust HBpin reagents.27 The need to access boric and boronic esters of
many differing substitution patterns, stereochemistries, and functional groups has triggered the
development of well-defined molecular catalysts, typically based on transition metal
complexes.
1
2
3
4
5
6
Pioneering studies by Roesky12g and others12b, 12c, 12f, 28 represent the only few examples,
to the best of our knowledge, of aluminum-catalyzed ketone/aldehydes hydroboration. These
catalysts are shown in Scheme 2B and feature turnover numbers (TONs)29 of up to ~50 for
ketones as substrates. Zwitterionic Meisenheimer complex 8 was found to catalyze the
hydroboration of acetophenone producing the final product with >98% yield and a TON of 980
under neat conditions after 3 h at rt (Scheme 2B).30 While the catalytic efficiency is lower than
the most efficient examples of alkali,31 alkaline–earth,32 main-group,33 lanthanide34 and
transition-metal35 catalysts, it is comparable to others and outperforms all reported Al catalysts
(Scheme 2B).
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Further evaluation of the catalytic activity of 8 has been conducted on the catalytic
hydroboration of phenylacetylene with HBpin.36,37 A diversity of products in catalytic
hydroboration of phenylacetylene is expected,38 and the cis-selective anti-Markovnikov
hydroboration represents one possibility as shown in Scheme 3A. We found that 8 catalyzes
cis-selective anti-Markovnikov hydroboration of phenylacetylene with appreciable TON of
910 as shown in Scheme 3B. This can be compared to TONs of ~10–33 reported for all other
Al catalysts,12b, 12d-f as well as TONs of ~40–100 for some recently reported first row transition
metals (Scheme 3B).39 In order to evaluate the scope and selectivity, various functionalized
ketones/aldehydes and alkynes were hydroborated with HBpin in the presence of zwitterionic
Meisenheimer Al complex 8, as shown in Schemes 2C and 3C. All the reactions were
performed on a 5–10 mmol (gram) scale to demonstrate the potential practical utility of
zwitterionic Meisenheimer Al complex 8. For both reaction types, the catalyst features
excellent selectivities, leaving C=C of alkenes and arenes, CO2Me, NO2 and C=N functional
groups unaffected. All the products, secondary alcohols and vinylboronates as a result of anti-
Markovnikov cis-addition, were isolated in 84–96 % yield after column chromatography.15
Intrigued by the impressive catalytic activity of precatalyst 8 in the hydroboration of
ketones and internal/terminal alkynes,40 preliminary stoichiometric NMR experiments and
parallel computational analyses have been performed in order to elucidate the catalytically
relevant species. No visible reaction occurs when 8 and 2 equiv. of PhC(O)CH3 or PhCCH
alone are treated in THF-d8 according to the 1H and 13C{1H} NMR within 1 hour. In contrast,
the same reaction with 2 equiv. of HBpin leads to ~30% conversion of 8 into new complex 10
and pinBCH2Si(CH3)3 in a 1:1 ratio as shown in Scheme 4.41
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H
O
B
O
1
2
3
4
5
6
Cl
Si
H
N
Cl
H
H
2 equiv
N
N
N
N
N
THF-d8, rt, 1h
Al
Al
7
8
9
Si
O
Si
Si
Si
Si
B

10
8
O
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
30%
30%
Scheme 4. Outcome of the reaction between 8 and 2 equiv. of HBpin in THF-d8 according to
the 1H and 13C{1H} NMR spectra recorded ~1 hour after mixing the reagents (C8 ~ 0.02 M).
Several characteristic features exist in the 1H NMR spectrum of 8 including the signal of
the dearomatized CH proton at δ/ppm 4.50 (dd, 3JHH = 10 Hz, 3JHH = 2 Hz), and signals for two
diastereotopic CH2 protons at δ 1.03 (1H, dd, 3JHH = 15 Hz, 3JHH = 10 Hz) and 0.65 (1H, dd,
3JHH = 15 Hz, 3JHH = 2 Hz) for the CH2Si(CH3)3 group attached to the stereogenic carbon atom
of the central pyridine ring of the ligand. Additionally, four diastereotopic CH2 protons for two
CH2Si(CH3)3 groups attached to the metal center appear as a set of second-order signals at –
0.80 to –1.00 (4H).15 For complex 10, two dearomatized CH2 protons are chemically equivalent
and resonate at δ 2.91 (2H, s), whereas the CH2 protons of the CH2Si(CH3)3 groups overlap
with the CH3 protons at δ/ppm 0.00 (22H, s). The dearomatized carbon atom resonates at δ
37.7 and presumably at δ ~24 ppm (overlapped with THF-d8) for 8 and 10, respectively in the
13C{1H} NMR spectrum. The methylene carbon of the CH2Si(CH3)3 group attached to the
stereogenic carbon atom of the central pyridine ring of the ligand in 8 resonates at δ 14.3. All
other C-atoms of the CH2Si(CH3)3 groups attached to the metal center resonate in the region
from δ 1 to –3 for both complexes respectively in the 13C{1H} NMR spectra.
Figure 3 shows the computed mechanism for the formation of 10 from 8, as well as the
free energy profile for the further putative σ-bond metathesis reaction42 of 10 with HBpin to
afford monohydride 11 and dihydride 12. Complexes 11 and 12 can be assumed to be the likely
catalytic species based on stoichiometric studies of related complexes,43 as well as reports of
Roesky,12d, 12g Thomas & Cowley12a, 12e and others.12b, 12c, 28
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ACS Catalysis
1
2
3
4
5
6
7
8
9
TS2
29.2
TS1
24.2
HBpin
TS3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
22.1
Si
O
O
TS4
18.4
B
Si
O
O
17.8
B
H
B
Si
O
O
Cl
Si
O
O
HBpin
HBpin
B
N
0.0
N
N
Al
Si
Si
Cl
– 16.8
– 17.5
8
H
H
int1
– 20.0
Cl
H
H
N
N
N
N
Al
N
N
H
H
H
Al
12
Si
ΔG°298K, kcal·mol– 1 (1 M)
11
10
Figure 3. Computed at the ωB97X-D/6-311++G**/SMD(THF) level of theory mechanism for
the formation of 10 from 8, as well as the free energy profile for the further σ-bond metathesis
reaction of 10 with HBpin to afford monohydride 11 and dihydride 12, respectively. For the
Optimized Structures, Non-critical H atoms are Omitted for Clarity. Solid line = intrinsic
reaction coordinate (IRC)44 path; dotted line = non-IRC path.
The formation of 10 from 8 takes place via a classical SN1 mechanism.45 Complex 10
–
undergoes “slow” dissociation via transition state TS1 to produce the (CH3)3SiCH2 anion and
an analog of 1+ cation described above in Figure 2. This step is characterized by an activation
≠
–
barrier ∆G°298K of 24.2 kcal·mol–1. (CH3)3SiCH2 further reacts with HBpin to afford the
contact ion-pair complex int1 consisting of the tetrahedral anion of pinB(H)(CH2Si(CH3)3)–
and the aforementioned cation, (∆G°298K = 17.8 kcal·mol–1). The anion of
pinB(H)(CH2Si(CH3)3)– further rapidly transfers the hydride atom to the cation via low-lying
TS2 to afford 10 and pinBCH2Si(CH3)3 (∆∆G°298K≠ = 11.4 kcal·mol–1 relative int1). The overall
transformation 8 → 10 is very exergonic, and characterized by the ∆G°298K = –16.8 kcal·mol–
1. Favorable formation of 10 from 8 results from a destabilization initially present in 8 due to
steric constraint imposed by the bulky (CH3)3SiCH2 group attached to the central pyridine ring.
Indeed, three carbon atoms of the central pyridine ring, including the dearomatized-one,
significantly deviate from the plane of the 3′/5′-functionalized tpy ligand in 8 as observed in
the X-ray structure. This geometry is fully reproduced in calculations as shown in Figure 3. In
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contrast, all carbon atoms of the 3′/5′-functionalized tpy ligand are planar in the optimized
geometry of 10. Finally, the step-wise reaction of 10 with two molecules of HBpin affords
monohydride 11, then dihydride 12, along with two pinBCH2Si(CH3)3. These steps are
characterized by favorable thermodynamics of –0.7 kcal·mol–1 and –3.5 kcal·mol–1 (∆∆G°298K
relative to 10), respectively. The computed activation barriers of 38.9 kcal·mol–1 and 35.9
1
2
3
4
5
6
7
8
≠
9
kcal·mol–1 (∆∆G°298K relative 10 and 11), respectively, however, the values are somewhat
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
unsettling due to their considerable magnitudes. Qualitatively, however, this is in agreement
with the NMR experiment in THF-d8 as only the partial reaction leading to 10 was observed
(computed ∆∆G°298K≠ = 24.2 kcal·mol–1 for the “slowest step”), at least within ~1h after mixing
the reagents. In the catalytic reaction pool,46 these barriers could be lowered by means of
involvement of other molecules in the process (e.g. chemically or via specific solvation),
including impurities present in substrates. Alternatively, hydrido species 11 and/or 12 could be
generated via different σ-bond metathesis mechanisms, for example, via heterolytic Al–
CH2Si(CH3)3 bond dissociation following hydride transfer from the pinB(H)(CH2Si(CH3)3)–,
–
obtained from association of (CH3)3SiCH2 and HBpin. One can also imagine Al–CH2Si(CH3)3
to Al–X bond substitution, where X is a better leaving group and an anionic ligand, generated
from the reagents and/or impurities. This fragment can further be replaced by H leading to the
hydrido species.
Based on the presently accumulated data in this work as well as works by others,12a-e, 12g,
28, 43 and the experimental fact that both terminal and internal alkynes can be hydroborated, it
is reasonable to propose the plausible mechanisms of ketones and alkynes anti-Markovnikov
cis-selective hydroboration catalyzed by 8 as shown in Scheme 5.
O
B
H
R1
R2
O
O
H
O
H
H
O
O
B
O
B
B
R1
R2
O
O
Cl
Cl
Cl
H
H
H
H
H
H
I
II
N
N
N
N
N
N
N
N
N
Al
Al
Al
R2
X
O
H
X
H
X
A
R1
R2
H
B
R1
O
C
R1
R2
R1
R2
n pinBH (n = 2, 3)
 pinBCH2Si(CH3)3 (n = 3, 3)
X = H, CH2Si(CH3)3 etc.
8
Scheme 5. Plausible mechanisms of ketones (I) and alkynes anti-Markovnikov cis-selective
(II) hydroboration catalyzed by 8. The active species are monohydride and/or more-likely
dihydride complex A, obtained from the reaction of precatalyst 8 with HBpin.
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ACS Catalysis
1
2
3
4
5
6
7
8
9
For both reactions, the key intermediate is the mono-hydrido, or most-likely dihydrido
complex A, obtained from 8 and HBpin as discussed above. For ketone hydroboration (catalytic
cycle I), inner- or outer-sphere hydride transfer47 from A to a ketone would afford alkoxo
species C as an active intermediate or an off-loop species. Further reaction with HBpin via σ-
bond metathesis or other mechanism would regenerate A and produce the reaction product. For
alkyne hydroboration (catalytic cycle II), the first step is hydroalumination27e yielding alkenyl
species B. B then reacts with HBpin via σ-bond metathesis or an alternative mechanism to
produce the anti-Markovnikov cis-addition product and regenerate A. In summary,
unprecedented catalytic activity of aluminum precatalyst 8 seems to be attributed to its unusual
electronic and structural properties. The Meisenheimer-type anionic ligand is simultaneously
both redox and chemically non-innocent. Overall the ligand seems to increase the positive
charge at the aluminum center, which implies a strong Lewis acidic character. A detailed
investigation of the nature of bona fide catalytic species, effects of the ligand, and catalytic
reaction mechanisms for these two reactions is warranted in the future.48
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Conclusions.
We report the unprecedented selective dearomatization of popular terpyridine (tpy) ligands
via alkylation at various (2′/6′ vs 3′/5′ and 4′) positions of the central pyridine ring mediated by
AlIII. Although dearomatization and 2′-functionalization of Xtpy ligands has one precedent in
the literature,16 and similar dearomatizations are reported for dimpy or related imine ligands by
Budzelaar, Cámpora, van Koten and others,17, 21, 49 this work demonstrates the first example of
dearomatization and functionalization of Xtpy at 3′/5′ or 4′-positions, respectively.
Experimental and computational analysis suggest that the nature of these reactions depends on
the conditions used, and ionic or radical mechanisms are possible. In the latter case the
reactions seems to take place via intermediate formation of complexes supported by π-radical
monoanionic ligands (Xtpy·)1–. The 3′/5′-functionalized/dearomatized Xtpy ligands leading to
a zwitterionic Meisenheimer complexes are of particular interest due to their unusual structures.
Indeed, structure is often a key component of a molecular catalyst’s design, responsible for
achieving high selectivities and turnover efficiencies. Studies in this work indicate that the
zwitterionic Meisenheimer complex 8 in particular is an efficient precatalyst in the selective
hydroboration of ketones and alkynes with pinacolborane with a turnover number of ~1000,
placing the title complexes in the category of the most efficient Al catalysts developed to date
for catalytic hydroboration. Preliminary investigations suggest that precatalyst 8 undergoes
reaction with HBpin to afford thermodynamically more stable Meisenheimer complex 10,
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which ultimately yields possible relevant catalytic species, the aluminum(III) monohydride or
1
2
3
more likely dihydride complexes supported by both chemically and redox non-innocent ligands.
4
5
6
ASSOCIATED CONTENT
7
8
9
Supporting Information. Syntheses, characterizations, NMR spectra/charts, X-ray data for all
complexes reported in this work, catalytic experiments, stoichiometric NMR experiments,
computational details, three orthogonal views of HOMO’s for complexes 2, 3 and 9 as well as
other details see SI. Metrical parameters for the solid-state structures are available free of
charge from the Cambridge Crystallographic Data Centre under the reference numbers CCDC
1833905 (1), 1833906 (2), 1585885 (3), 1833907 (4), 1833909 (5), 1833908 (6), 1833910 (7),
1585884 (8), 1833911 (9) and 1834247 ([(L3)Al(H2O)3]Cl3(H2O)4).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
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35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
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60
ACKNOWLEDGMENT
We are grateful to donors of the American Chemical Society Petroleum Research Fund for
partial support of this work (54247-UNI3). We also acknowledge the support from the CUNY
Collaborative Research Incentive Program, the PSC-CUNY awards (60328-0048) and the Seed
grant from the Office for Advancement of Research at John Jay College. Partial support from
the National Science Foundation (CHE-1464543) is also gratefully acknowledged.
Computations were performed by using Darwin Computational Cluster (LANL).
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and other details see SI.
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(19) The room temperature reaction between terpy ligands L1–4 and AlCl3 in dry THF
afforded white solids after 24 h. These solids are extremely air-sensitive and insoluble in
most organic solvents such as acetone, acetonitrile, chloroform and toluene, but are soluble in
methanol, leading to rapid decomposition. No satisfactory elemental analysis results could be
obtained. For ligand L3 in particular, elemental analysis C 45.13%, H 3.81%, N 8.66%
indicates the complicated nature of the product with a possible stoichiometry of
(L3)AlCl3(THF)n. In a separate experiment, the white solid (20 mg) from the reaction of L3
and anhydrous AlCl3 (131.9 mg, 1.00 mmol) in dry THF in a glovebox was brought out and
redissolved in MeOH (3 mL) and acetone (3 mL). The colorless solution was slowly
evaporated in the air for 3 days to give colorless crystals. The crystals were then
characterized by X-ray diffraction. The complex was identified as [(L3)Al(H2O)3]Cl3•(H2O)4.
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The white solids obtained in step (i) are directly used in the following step (ii) after a small
work-up.15
(20) Budzelaar, P. H. M. Radical Chemistry of Iminepyridine Ligands. Eur. J. Inorg. Chem.
2012, 2012, 530-534.
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(21) Sandoval, J. J.; Palma, P.; Álvarez, E.; Cámpora, J.; Rodríguez-Delgado, A.
Mechanism of Alkyl Migration in Diorganomagnesium 2,6-Bis(imino)pyridine Complexes:
Formation of Grignard-Type Complexes with Square-Planar Mg(II) Centers.
Organometallics 2016, 35, 3197-3204.
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(22) The N(1)−C(35), C(35)−C(41) and C(41)−N(2) distances of 1.355(3), 1.456(3), and
1.377(3) Å, respectively, are consistent with one electron reduction of the terpyridine
chelate.5c
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(23) On a computed G° profile, association of two ions 1+/(CH3)3SiCH2 is exergonic. For
example, the transition state leading to 3(1,2) and 3(1,2) itself are computed to be –21.1
kcal·mol–1 and –47.5 kcal·mol–1 relative to "free" ions, respectively. In a solution, "free" ions
do not exist (each cation and anion are strongly solvated by a solvent), and the association of
two ions is expected to have some activation barrier. In order to avoid possible confusion,
only relative energies are reported for the ionic reactions in Fig. 2B.
(24) Laury, M. L.; Wilson, A. K. Performance of Density Functional Theory for Second
Row (4d) Transition Metal Thermochemistry. J. Chem. Theory Comput. 2013, 9, 3939-3946.
(25) Complex 3(1,5) is 1.1 kcal·mol–1 less stable than 3(1,4) on G°298K scale but 0.4 kcal·mol–1
more stable on E scale, indicating that both have similar energy.
(26) The identity of the second component in the mixture is now under investigation.
Tentatively, it can be identified as a 9(1,3) product by comparing the NMR data with those of
the structurally related complex 2. This assignment is also consistent with the Energy Profile
in Fig. 2A.
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(30) Hydroboration of acetophenone with neutral precatalyst 5 under conditions of Scheme
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8.
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background reaction, but only affording the product with ~10% yield under identical
conditions, in agreement with studies of Wu, Liu and Zhao.37
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(40) Complex 8 outperforms all reported Al catalysts by at least one order of magnitude in
terms of TON, see Schemes 3–4.
(41) see SI for the full description of the experiment, NMR spectra/charts and peak
assignments in THF-d8. 1H NMR for 10: δ 0.0 (s, 22H), 2.91 (s, 2H), 7.27 (m, 2H), 7.60 (m,
1H), 7.79 (dt, J = 8 Hz, J = 2 Hz, 1H), 7.85 (dt, J = 8 Hz, J = 2 Hz, 1H), 8.22 (d, J = 8 Hz,
1H), 8.37 (d, J = 8 Hz, 1H), 8.52 (d, J = 8 Hz, 1H), 8.61 (vt, 1H); 1H NMR for
pinBCH2Si(CH3)3: δ 1.17 (s, 12H), –0.10 (s, 11H); 13C{1H} NMR for pinBCH2Si(CH3)3: δ –
1.4 (s, 3C), 23.0 (s, 1C), ~24.0 (s, 4C), 80.9 (s, 2C). Further progress of the reaction has not
been studied for the present contribution.
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(48) One can also imagine a mechanism in which the hydride is actually transferred to/from
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organic syntheses. J. Organom. Chem. 1983, 250, 49-61.
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